Mesoporous catalysts for ammoxidation of acrolein to acrylonitrile · 2018-05-24 · mixture state...

Mesoporous catalysts for ammoxidation of acrolein toacrylonitrile

Thèse

Thanh-Binh Nguyen

Doctorat en génie chimiquePhilosophiæ doctor (Ph.D.)

Québec, Canada

© Thanh-Binh Nguyen, 2018

Résumé

L’acrylonitrile est une matière première importaite de l’industrie des polymères, produite àgrande échelle à partir de matériaux d’origine fossile. Les tendances de recherche actuellespour une industrie chimique plus écologique favorisent l’utilisation de molécules plate-formes d’origine biologique telles que le glycérol. De plus, la conception de catalyseursest un élément essentiel pour développer ces produits. Les catalyseurs hétérogènes, enparticulier les catalyseurs à base d’oxydes métalliques mésoporeux jouent un rôle majeurdans l’industrie pétrochimique. Par conséquent, l’objectif de cette thèse est de dévelop-per des catalyseurs nouveaux, efficaces et utiles, à base d’oxydes métalliques mixtes pourl’ammoxydation de l’acroléine ex-glycérol en acrylonitrile.

Sur la base des catalyseurs traditionnels pour l’ammoxydation du propène/propane en acry-lonitrile, une série de catalyseurs à base de molybdates et d’antimonates supportés sur unesilice mésoporeuse a été développée. Tout d’abord, les molybdates de bismuth ont été sup-portés sur la silice mésoporeuse KIT-6 en utilisant une méthode de gabarit solide (hard tem-plate). Différentes phases de molybdates de bismuth ont été synthétisées, caractérisées ettestées pour l’ammoxydation de l’acroléine en acrylonitrile. Les conditions réactionnellesont été soigneusement optimisées à différentes températures, débits et rapports molaires deréactifs. Les catalyseurs obtenus ont montré une bonne activité catalytique, une sélectivité etune stabilité, en particulier les échantillons contenant des phases mixtes de molybdates debismuth.

Deuxièmement, une série de mélanges de molybdates et d’antimonates supportés sur unesilice mésoporeuse à l’aide d’une méthode de gabarit flexible (soft-template) a égalementété étudiée. Cette nouvelle méthode de soft template a été développée en utilisant la tech-nique d’auto-assemblage induite par évaporation (EISA) et de tensioactifs comme agentsstructurants. Les catalyseurs obtenus présentaient une surface spécifique élevée et un grandvolume de pores. De plus, les résultats catalytiques indiquent que les molybdates mixtesjouent un rôle majeur dans l’ammoxydation de l’acroléine.

Certains des catalyseurs ont été choisis pour étudier le mécanisme de réaction de l’ammoxydationde l’acroléine en acrylonitrile. Parce que l’oxygène (l’air) et l’ammoniac sont des réactifs dansce procédé, les effets des lacunes d’oxygène et de la réduction par l’ammoniac sur l’activité

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catalytique ont ensuite été étudiés. Les résultats obtenus ont démontré que les catalyseursayant plus de lacunes d’oxygène et qui étaient facilement réduits par l’ammoniac présen-taient une activité catalytique plus élevée. Tous les catalyseurs contenant des molybdatesont montré une bonne activité catalytique et une bonne sélectivité pour l’ammoxydation del’acroléine. Ainsi, un nouveau mécanisme de réaction a été proposé pour l’ammoxydationde l’acroléine sur les catalyseurs à base de molybdates.

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Abstract

Acrylonitrile is a raw material in polymer industry with a large scale demand and it has beenproduced from fossil origin. Current research trends for a greener chemical industry are pro-moted by using platform molecules of biological origin such as glycerol. Designing catalystsbecomes an essential part to develop these products. Heterogeneous catalysts, especiallymesoporous metal oxide catalysts, play a major role in petrochemical industry. Therefore,the scope of this thesis is to develop new, effective and useful mesoporous catalysts for am-moxidation of ex-glycerol acrolein to acrylonitrile.

Based on the traditional catalysts for propene/propane ammoxidation to acrylonitrile, a se-ries of molybdates and antimonates based catalysts supported in mesoporous silica was de-veloped. First, bismuth molybdate oxides were supported in mesoporous silica KIT-6 usingthe hard-templating method. Different phases of bismuth molybdates were synthesized,characterized and tested for ammoxidation of acrolein to acrylonitrile. The reaction con-ditions were carefully optimized at different temperatures, flow rates and reactant ratios.The obtained catalysts showed good catalytic activity, selectivity and stability, especially, thesamples containing mixed phases of bismuth molybdates.

Second, a series of molybdate and antimonate mixtures supported on mesoporous silicausing a soft-templating method was also studied. This new soft-templating method wasdeveloped based on the evaporation induced self-assembly (EISA) technique and dual sur-factants as structure directing agents. The obtained catalysts exhibited high specific surfacearea and large pore volume. In addition, the catalytic results indicated that molybdates inmixture state play a major role in acrolein ammoxidation.

Some of the above catalysts were chosen to study the reaction mechanism of acrolein am-moxidation to acrylonitrile. Because oxygen (air) and ammonia are reactants in this process,the effects of oxygen vacancies and ammonia reduction on catalytic activity were then in-vestigated. The obtained results demonstrated that the catalysts having more oxygen vacan-cies and being readily reduced by ammonia showed higher catalytic activity. All catalystscontaining molybdates showed good catalytic activity and selectivity for acrolein ammoxi-dation. Thus, a new reaction mechanism was proposed over molybdates oxides as catalysts.

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Contents

Résumé iii

Abstract v

Contents vii

List of Tables xi

List of Figures xiii

List of Abbreviations xvii

Acknowledgements xix

Chapter 1: Introduction 11.1 Background and problem identification . . . . . . . . . . . . . . . . . . . . 11.2 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3 Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Chapter 2: Ammoxidation Catalysts 72.1 Heterogeneous catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2 Ammoxidation of propene/propane to acrylonitrile . . . . . . . . . . . . . 9

2.2.1 Ammoxidation of propene to acrylonitrile . . . . . . . . . . . . . . 92.2.2 Ammoxidation of propane to acrylonitrile . . . . . . . . . . . . . . 11

2.3 Acrylonitrile from glycerol . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3.1 One step-ACN from glycerol . . . . . . . . . . . . . . . . . . . . . . 132.3.2 Two steps-ACN from glycerol via acrolein . . . . . . . . . . . . . . 152.3.3 Three steps-ACN from glycerol via acrolein . . . . . . . . . . . . . . 15

2.4 Acrolein ammoxidation to acrylonitrile . . . . . . . . . . . . . . . . . . . . 162.5 Methods for synthesis of mesoporous metal oxide catalysts . . . . . . . . . 21

2.5.1 Soft-templating method . . . . . . . . . . . . . . . . . . . . . . . . . 212.5.2 Hard-templating method . . . . . . . . . . . . . . . . . . . . . . . . 252.5.3 Supporting metal oxides on mesoporous materials . . . . . . . . . 28

Chapter 3: Experimental methods 333.1 Catalytic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333.2 Ammonia temperature programmed desorption and pulse chemisorption 36

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Chapter 4: Ammoxidation of acrolein to acrylonitrile over bismuth molybdatecatalysts 39

Résumé . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.2.1 Synthesis of bismuth molybdate catalysts . . . . . . . . . . . . . . . 424.2.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424.2.3 Catalysts test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434.3.1 N2 physisorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444.3.2 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474.3.3 Catalytic tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 534.5 Supporting information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Chapter 5: Molybdate/antimonate as key metal oxide catalysts for acrolein am-moxidation to acrylonitrile 61

Résumé . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62


5.2.1 Synthesis of antimonate molybdate catalysts . . . . . . . . . . . . . 645.2.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655.2.3 Catalytic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

5.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665.3.1 Characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665.3.2 Catalytic tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 725.5 Supporting information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Chapter 6: NH3 adsorption as related to mechanism of acrolein ammoxidationover molybdate catalysts 85

Résumé . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85


6.2.1 Synthesis of catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . 876.2.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 876.2.3 Temperature programmed desorption of ammonia (TPD) and am-

monia chemisorption . . . . . . . . . . . . . . . . . . . . . . . . . . . 886.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 886.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956.5 Supporting information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

Chapter 7: Conclusions and Future work 1057.1 Overview of contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1057.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

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Appendix 107.1 Nitrogen physisorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107.2 X-ray diffraction (XRD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108.3 Transmission electron microscopy (TEM) . . . . . . . . . . . . . . . . . . . 109.4 Inductively coupled plasma (ICP) . . . . . . . . . . . . . . . . . . . . . . . . 111.5 X-ray photoelectron spectroscopy (XPS) . . . . . . . . . . . . . . . . . . . . 112.6 Thermal analysis (TGA/DTA) . . . . . . . . . . . . . . . . . . . . . . . . . . 112.7 Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.8 Gas chromatography (GC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

Bibliography 115

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List of Tables

2.1 Commercial propene ammoxidation catalysts . . . . . . . . . . . . . . . . . . . 102.2 Propane ammoxidation catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . 122.3 Conversion of glycerol over different catalysts . . . . . . . . . . . . . . . . . . 142.4 First order rate constants and selectivity of bismuth molybdate catalysts . . . 172.5 Catalyst performance observed for ammoxidation of acrolein . . . . . . . . . 202.6 Summary reaction parameters of acrolein ammoxidation reaction . . . . . . . 212.7 Some surfactants for synthesis of metal oxides . . . . . . . . . . . . . . . . . . 232.8 Non-silica metal oxides prepared by the soft-templating method . . . . . . . . 242.9 Several nanocasting materials with different porous hard templates . . . . . . 26

4.1 Texture properties and structure of the catalysts . . . . . . . . . . . . . . . . . 454.2 Reactants feed flow rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544.3 Effect of temperature on catalysts activity and selectivity . . . . . . . . . . . . 554.4 Effect of contact time on catalytic conversion . . . . . . . . . . . . . . . . . . . 564.5 Effect of reactant molar ratio on catalytic activity and selectivity . . . . . . . . 56

5.1 Textural properties and structure of the SbnMo10-nOx/SiO2 catalysts . . . . . . 66

6.1 Summary of catalytic and textural properties of the samples . . . . . . . . . . 886.2 Volume of adsorbed/reacted ammonia on catalysts . . . . . . . . . . . . . . . 93

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List of Figures

1.1 Applications of acrylonitrile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Global demand of acrylonitrile . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1 Course of a heterogeneously catalyzed gas-phase reaction . . . . . . . . . . . 72.2 Proposed mechanism of propene and propane ammoxidation . . . . . . . . . 112.3 Reaction pathway from glycerol to acrolein . . . . . . . . . . . . . . . . . . . . 162.4 (a) Initial reaction rates vs PO2

. Conditions: temperature 340 ◦C, catalyst 1(g),PAC 0.014-0.015 (atm), PNH3

0.107-0.115 (atm), PH2O 0.205-0.220 (atm) ; (b) Ini-tial reaction rates vs PNH3

. Conditions: temperature 340 ◦C, catalyst 1(g), PAC0.013-0.016 (atm), PO2

0.093-0.119 (atm), PH2O 0.189-0.240 (atm) . . . . . . . . . 182.5 Scheme reaction pathway of AC to ACN . . . . . . . . . . . . . . . . . . . . . . 192.6 Soft-templated process for synthesizing ordered mesoporous materials . . . . 222.7 Mesostructured pore models with different symmetries: (A) p6mm, (B) Ia3d,

(C) Pm3n, (D) Im3m, (E) Fd3m, and (F) Fm3m . . . . . . . . . . . . . . . . . . 222.8 Schematic representation for the hard templated process . . . . . . . . . . . . 252.9 Distribution of the precursors into the pores . . . . . . . . . . . . . . . . . . . . 262.10 Schematic diagram of contact angle . . . . . . . . . . . . . . . . . . . . . . . . . 272.11 Effect of support on sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.12 Representation of self-assembly of surfactants-salt-silica species (liquid crys-

talline and soft matter; red domains are silica; stars are salts pieces, left) andmeso SiO2-metal oxides- thin film (after calcination, right) . . . . . . . . . . . 30

3.1 Schematic diagram of the catalytic ammoxidation reactor system . . . . . . . 333.2 Position of columns in GC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353.3 Scheme of temperature programmed chemisorption apparatus, TCD-thermal

conducting detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.4 Pulse ammonia chemisorption using loop of 250 µl . . . . . . . . . . . . . . . . 36

4.1 N2 physisorption isotherms of the catalysts and KIT-6 support . . . . . . . . . 444.2 NLDFT pore size distributions (adsorption branch) of the catalysts and KIT-6

support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464.3 X-ray patterns of the catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474.4 AC non-converted and ACN selectivity in blank tests run absence of catalyst,

TOS=1.5 h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484.5 Influence of temperature on catalytic activity . . . . . . . . . . . . . . . . . . . 494.6 AC conversion as a function of flow rate at 400 ◦C and molar reactant ratio

AC/ NH3/ O2/ N2 = 1.0/ 1.25/ 16.5/ y (y values as reported in Table 4.2),TOS=3.5 h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

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4.7 Effect of oxygen on AC conversion at 450 ◦C and F =66 cc/min over catalyst n3 514.8 Effect of oxygen in gas feed on catalyst structure. The numbers below patterns

indicate molar ratio of the O2/AC, TOS=3.5 h . . . . . . . . . . . . . . . . . . . 524.9 Catalytic activity stability test . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53S4.1 Flammability limits for the AC/ O2/ N2 and NH3/ O2/ N2 mixtures. . . . . . 57S4.2 Flammability limits; ? Our data points for (AC+NH3)/ O2/ N2, • Our data

points for AC/O2/ N2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58S4.3 Raman spectra of catalysts n1, n2 and n3 . . . . . . . . . . . . . . . . . . . . . . 59S4.4 N2 physisorption isotherm and NLDFT pore size distribution of SiO2 and n3-

SiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

5.1 TEM images of the SbnMo10-nOx/SiO2 (18 wt%) catalysts . . . . . . . . . . . . 675.2 X-ray diffraction pattern of the SbnMo10-nOx/SiO2 (18 wt%) catalysts . . . . . 685.3 Activity and selectivity of the SbnMo10-nOx/SiO2, 18 wt% (solid symbol) and

n0-KIT-6 (up-half symbol) catalysts; at 450 ◦C and GHSV=15700 h−1, TOS=3.0 h 695.4 Activity and selectivity of the MoO3/SiO2 catalysts at different loadings; at

450 ◦C and GHSV=15700 h−1, TOS=3.0 h . . . . . . . . . . . . . . . . . . . . . 705.5 n0-18 (MoO3/SiO2, 18 wt%) catalyst test at different temperatures, GHSV=15700

h−1, TOS=3.0 h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715.6 Stability of n0-18 catalyst (MoO3/SiO2, 18 wt%) at 450 ◦C and GHSV=15700 h−1 725.7 Stability of n13.4-18 catalyst (Sb1.34Mo8.66Ox/SiO2, 18 wt%); at 450 ◦C and

GHSV=15700 h−1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73S5.1 X-ray diffraction patterns of n0 (MoO3/SiO2) at different loadings . . . . . . . 74S5.2 Hysteresis loops of the SbnMo10-nOx/SiO2catalysts SiO2, 18 wt% . . . . . . . . 75S5.3 Pore size distribution of the SbnMo10-nOx/SiO2catalysts SiO2, 18 wt% . . . . . 76S5.4 TGA curve of the dried and non-calcined Mox/SiO2 under air atmosphere . . 77S5.5 TGA curve of the dried and non-calcined SbOx/SiO2 under air atmosphere . 77S5.6 Raman spectra of n0, n13.4 and n100 catalysts . . . . . . . . . . . . . . . . . . . 78S5.7 Mo3d XPS spectra of (fresh and used) n13.4 and n90 catalysts . . . . . . . . . 79S5.8 Sb3d XPS spectra of fresh (left) and used (right) Sb1.34Mo8.66Ox/SiO2 catalyst

(left to right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80S5.9 Sb3d XPS spectra of fresh n90 (Sb9MoOx/SiO2) catalyst . . . . . . . . . . . . . 81S5.10Hysteresis loops of n0 (MoO3/SiO2) at different loadings . . . . . . . . . . . . 82S5.11Pore size distribution of n0 (MoO3/SiO2) at different loadings . . . . . . . . . 83S5.12Fresh and used catalyst MoO3/SiO2 (18 wt%) . . . . . . . . . . . . . . . . . . . 83

6.1 Color of the samples before (b) and after (a) reduction by ammonia at 450 ◦C 896.2 Mo3d spectra of samples (1-4) before (b) and after (a) reduction by ammonia

at 450 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 916.3 Pulse reduction of sample (2) (Bi2O3. 3 MoO3-KIT6) by ammonia at different

temperatures (a) loop 100 µl; (b) loop 250 µl . . . . . . . . . . . . . . . . . . . . 946.4 Proposed reaction scheme for ammoxidation of acrolein to acrylonitrile over

MoO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956.5 Reoxidization and NH3 adsorption on MoO3 oxide . . . . . . . . . . . . . . . 96S6.1 N2 physisorption of all samples (a) isothermal hysteresis loop; (b) pore size

distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96S6.2 Effect of degassing on the color of the samples; a-after; b-before degassing in

vacuum at 150 ◦C for 6 h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

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S6.3 XRD patterns of sample (1) (MoO3/KIT-6) before (b) and after (a) reductionby ammonia at 450 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

S6.4 XRD patterns of sample (2) (Bi2O3. 3 MoO3-KIT6) before (b) and after (a) re-duction by ammonia at 450 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

S6.5 XRD patterns of sample (3) (SbMo9Ox/SiO2) before (b) and after (a) reductionby ammonia at 450 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

S6.6 XRD patterns of sample (4) (Sb9MoOx/SiO2) before (b) and after (a) reductionby ammonia at 450 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

S6.7 XRD patterns of sample (5) (Bi2O3/KIT-6) before (b) and after (a) reduction byammonia at 450 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

S6.8 XRD patterns of sample (6) (Sb2O3/SiO2) before (b) and after (a) reduction byammonia at 450 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

S6.9 Mo3p3/2 (N1s) XPS of sample (2) (Bi2O3. 3 MoO3-KIT6) before (b) and after(a) reduction by ammonia at 450 ◦C . . . . . . . . . . . . . . . . . . . . . . . . 103

A1 Types of physisorption isotherm (left) and hysteresis loop (right) . . . . . . . 107A2 X-rays exhibit constructive interference in Bragg’s law reflection . . . . . . . . 109A3 Main interaction of incident electron beam with specimen in transmission

electron microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110A4 Diagram of inductively coupled plasma . . . . . . . . . . . . . . . . . . . . . . 110A5 Schematic energy level diagram for photoemission . . . . . . . . . . . . . . . . 111A6 Propene oxidation and ammoxidation mechanism over bismuth molybdate

catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

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List of Abbreviations

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ABS Acrylonitrile-butadiene-styreneAC AcroleinACE AcetonitrileACN AcrylonitrileAKC Asahi Kasei Chemicals CorporationBET Brunauer-Emmett-TellerBrij56 Poly(ethyleneoxide)-based nonionic diblock CnH2n-1(OCH2CH2)yOH, n/y = 16/10Brij58 Poly(ethyleneoxide)-based nonionic diblock CnH2n-1(OCH2CH2)yOH, n/y = 16/20CTAB Cetyltrimethylammonium bromideDTA Differential thermal analysisEISA Evaporation induced self-assemblyGHSV Gas hourly space velocityICPMS Inductively coupled plasma mass spectroscopyIUPAC International Union of Pure and Applied ChemistryKIT-6 Mesoporous silica Korean Institute of Technology number 6KLE Poly(hydrogenatedpoly-b-butadiene-coethylene)-block-poly(ethyleneoxide)MCM-41 Mobil Corporation Material number 41MCM-48 Mobil Corporation Material number 48MMs Mesostructured materialsMT Metric tonsNLDFT Non-local density functional theoryLLC Lyotropic liquid crystalOMMs Ordered mesoporous materialsPI-b-PEO Poly(isoprene-block-ethylene oxide)PEO Poly(ethylene oxide), -(CH2CH2O)-Pluronic F127 Triblock copolymer, PEO106PPO70PEO106Pluronic P123 Triblock copolymer, PEO20PPO70PEO20PPO Poly(propylene oxide), -[CH2CH(CH3)O]-QSDFT Quenched solid density functional theorySAN Styrene-acrylonitrileSAXS Small angle X-ray scatteringSBA-15 Mesoporous silica Santa Barbara number 15SBA-16 Mesoporous silica Santa Barbara number 16SEM Scanning electron microscopySSA Specific surface areaTCD Thermal conductivity detectorTEM Transmission electron microscopyTG TriglyceridesTGA Thermogravimetric analysisTOS Time on streamTPD Temperature-programmed desorptionWHSV Weight hourly space velocityXRD X-ray diffractionXPS X-ray photoelectron spectroscopy

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Acknowledgements

First, I would like to thank Prof. Serge Kaliaguine for giving me a chance to work in catalysisfor this project, which combines academic and industrial interests. I have been very luckyto have Serge Kaliaguine as my supervisor who always provided me with constant supportand help during my study period. For his excellent advice in science, encouragement andpatience, I am truly and greatly indebted.

I would like to thank Dr. Jean-Luc Dubois for his advices and many discussions for ourpublications. He gave some new ideas to improve the catalytic setup. In addition, he alsoprovided several catalysts for initial test of the reaction.

I would like to thank all technicians and assistants for their help during my study. I wouldlike to thank Dr. Bendaoud Nohair, Jean-Nicolas Ouellet, Jérôme Noël and Marc Lavoie forrepairing and setting up the catalyst testing unit. I would like to thank Mr. Gilles Lemayfor his guidance of my first steps in the lab and even after his retirement he still came torepair and to assist in some process. I would like to thank Yann Giroux for his help in TGAmeasurement. I thank Dr. Alain Adnot for his help and discussion in XPS analysis, Mr. JeanFrenette for XRD measurement, and Mr. Richard Janvier for TEM. I also thank ProfessorDongyuan Zhao and Mr. Wang Shuai of Fudan University for providing TEM mapping.

I also thank all members in our lab for their help and numerous discussions on the projectand exciting life too. In particular, I would like to thank Dr. Hoang Vinh-Thanh and Dr.Hoang Yen for their advices and discussions about the catalysts. I would also like to thankTien-Binh Nguyen for his help and several ideas to improve my setup for the catalytic test.I thank all lab members and all Vietnamese students in Laval University for their friendshipand encouragement during my study in Québec city.

I thank The Consortium de Recherche et d’Innovation en Biotechnologie Industrielle duQuébec and MITACS for financial support.

Last but not least, I would like to express my sincere gratitude to members of my family fortheir support and understanding, especially my partner, best friend, and husband, Trung-Thien, during this project. He always gave me the inspiration and the courage that I neededto pursue my goals. With my deepest sense of gratitude and love, this thesis is dedicated to

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him and to my family.

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To my familyTo my husband Trung-Thien

Chapter 1

Introduction

1.1 Background and problem identification

Acrylonitrile (ACN) is one of the most important worldwide chemical compounds (withinthe top 20), which has been applied in many fields as shown in Fig. 1.1 [1–5]. ABS (Acry-lonitrile Butadiene Styrene) with 15-35 wt% of ACN has been used to manufacture manyproducts that work in a wide range of environmental temperature (-20 to 80 ◦C) such aspipes, bumpers, auto parts, and etc. Transparent SAN (Styrene Acrylo Nitrile resin) hassimilar applications as ABS. Moreover, 90 % of carbon fiber production is produced frompolyacrylonitrile. As well known, the carbon fibers have been widely applied in space shut-tle and auto parts because of their light mass and high stability. In addition, polyacrylonitrile(more than 85 wt% of ACN; Mw ∼ 100000) has been used to produce acrylic fibers, that canbe used to make clothes and dye receptor. Furthermore, ACN is used to produce adiponitrilein order to manufacture hexamethylenediamine, which is used in Nylon-6,6 [6]. Therefore,ACN touches us everyday in several ways. That is the reason for the high demand of ACNnowadays as shown in Fig. 1.2, it is produced at about seven million tons per year andexpected to reach eight million tons per year by 2020.

There were several ways to produce commercial acrylonitrile before the discovery of So-hio process in 1960 [7, 8]. The first method based on acetylene/HCN route was practicedby BASF, Du Pont, American Cyanamid, and Monsanto. Another one based on ethyleneoxide/HCN was studied by BASF, Union Carbide, and American Cyanamid. Other routeswere based on acetaldehyde/HCN dehydration, propionitrile dehydrogenation, and propeneoxidation with NO. These routes, however, could not be applied in large scale because oftheir high raw material cost, environmentally unfriendly properties, and complex proce-dure. In 1960, Sohio (now BP-America) was successful in commercializing ACN producedby ammoxidation of propene. This process then became the most important and useful ap-plication to revamp the above routes. Their reactions were carried out in a fluid-bed reactorwith temperatures ranging from 400 ◦C to 450 ◦C and pressures ranging between 1.5 and

1

Figure 1.1. Applications of acrylonitrile [2]

Figure 1.2. Global demand of acrylonitrile

2

3.15 x 105 Pa over multi-component metal oxide catalysts. The ACN product prices werehighly dependent on that of propene. The prices of propene were predicted to be more andmore increasing in the next decade, so that the ACN world demand was affected [9]. For thisreason, the ammoxidation of propane to ACN became a cheaper alternative, which was dis-covered and developed by Asahi Kasei Chemicals Corporation (AKC). This process requiredtemperatures ranging from 450 ◦C to 500 ◦C. Moreover, the catalysts contained vanadiumto activate propane oxidative dehydrogenation to propene. As the results of this reaction,the ACN yields ranged from 36 % up to 62 % [7, 8, 10]. However, both processes were highlydepending on the fossil feedstock that could be exhausted one day and become more ex-pensive in future. Therefore, finding a renewable and greener source to produce ACN is acritical task.

Biodiesel is a renewable and alternative fuel, which has drastically increased in quantityduring the last decade. As reported in OECD FAO Agricultural Outlook [11], the productionof biodiesel rose from 5 million metric tons in 2005 to 25 million metric tons in 2014 and isexpected to double by 2020. Biodiesel is produced via transesterification of fatty acid (fromplants oil or animal fat) with methanol using base catalysts. Consequently, the availabilityof glycerol, a 10 wt% co-product of biodiesel process, has increased as well. In particular,the pure glycerol price was about 900-1000 USD/ton in 2013, whereas that of the crude onewas just about 150 USD/ton [12]. Therefore, developments in the conversion of glycerolinto valuable products or chemicals have been focused. Glycerol is traditionally used inmany applications such as food 11 wt%, personal care 16 wt%, drugs 18 wt%, alkyd resins 8wt%, polyols 14 wt%, and others [13, 14]. Recently, a number of publications is dealing withthe dehydration of glycerol to acrolein and the conversion of glycerol to acrylonitrile viaacrolein [15–22], but these processes have not been well investigated yet to be commercial.These topics were also registered in patents [21, 22]. Glycerol may be converted to ACNthrough two major ways: (i) an indirect route (dehydration of glycerol to acrolein, thenammoxidation of acrolein to ACN); (ii) a direct way-one step, in which, glycerol is convertedto ACN via acrolein. In the first way, the dehydration of glycerol to acrolein is using someacid catalysts at temperatures ranging from 200 to 300 ◦C [21, 22]. In the second step of thefirst way, the ammoxidation of acrolein to ACN over metal oxides as acid-base catalysts wasused at temperatures ranging from 350 to 450 ◦C. Unfortunately, the second way (directlyconverting glycerol to ACN) was found to be difficult to choose a suitable catalyst and tocontrol the reaction conditions. Hence, the indirect method seems to be easier in application.However, its second step of ammoxidation of acrolein (AC) to ACN was not well studiedto obtain the high ACN yield. Thus, this process will be further studied for developingcatalysts and catalytic reaction mechanism.

Multi-component catalysts based on molybdates or antimonates are viewed as the most ef-fective candidates for the ammoxidation of acrolein to acrylonitrile. However, these catalysts

3

are not active enough because of their poor network structure, low specific surface area, andlow dispersion of active components [15, 16, 18–27]. As well-known, mesostructured materi-als have been widely applied as catalyst supports because of their high specific surface area,large pore volume, and high chemical and thermal stabilities. In addition, these materialshelp to tune mass transfer of the reactants and products. Therefore, these catalysts are ap-propriate for application in the ammoxidation of AC [28–30]. Based on these great potentialproperties, this study focuses on developing mesostructured molybdate/antimonate oxidebased catalysts for ammoxidation of AC to ACN.

1.2 Objectives

The scope of this work is to develop mesoporous molybdate/ antimonate oxide based cat-alysts, then to study their catalytic properties in AC ammoxidation to ACN and reactionmechanism. These catalysts will be synthesized based on simple soft- and hard-templatingmethods for building their mesostructures with high specific surface area, large pore size,large pore volume and proper catalytic properties. Therefore, this study can be divided intothree major parts: (1) synthesize mesoporous molybdate/antimonate based catalysts withthe high texture properties of mesoporous materials; (2) characterize the physicochemicalproperties and catalytic properties of the synthesized catalysts; (3) study reaction mecha-nism in order to further understand this reaction and design good catalysts.

1.3 Thesis outline

The thesis is organized as follows.

— Chapter 1 shows the motivation, scope and structure of the thesis.— Chapter 2 is divided into three parts. First, fundamentals of heterogeneous catalysts

are introduced briefly. Second, state of the art about ammoxidation catalysts, espe-cially propene/ propane/ acrolein ammoxidation, is summarized. Finally, two mainmethods for synthesizing metal oxide catalysts including hard- and soft-templatingmethods are described. Furthermore, several methods for preparation of supportedcatalysts are also introduced.

— Chapter 3 introduces two main experimental methods that will be applied in thisthesis. First for the catalytic test, the ammoxidation reactor system and treatment ofcatalytic data are described in detail. In second method, ammonia temperature pro-grammed desorption and pulse chemisorption are used to study for ammonia reduc-tion effects. The other experimental techniques including N2 physisorption, surfaceanalysis (TEM, XPS), thermoanalysis (TGA/DTA and TPC), phase analysis (XRD andRaman) and catalyst compositions (ICP) are described in the appendix section.

4

— Chapter 4 is a scientific paper that was published in Apply Catalysis A: General [31].The different bismuth molybdate phases supported in mesoporous silica KIT-6 weresynthesized based on the dual-solvent and solid-liquid impregnation methods. Theobtained catalysts exhibited high specific surface area, large pore size, and large porevolume that are key factors to be effective in AC ammoxidation. The blank test andoptimization reaction conditions were carefully investigated in a quartz fixed-bed re-actor. The results indicated that the specific surface area and synergistic phases havestrong effects on catalytic activity and selectivity.

— Chapter 5: Two groups of metal oxides have been used as main catalysts for ammox-idation reaction including molybdate and antimonate based oxides. Their powderswere synthesized by a simple and new route via soft-templating method. The ef-fects of metal oxides (molybdenum oxide and antimony oxide) to their mixtures werestudied. This chapter is a paper that was published in Catalysis Letters [32].

— Chapter 6: Several synthesized catalysts that were tested for the AC ammoxidationin Chapters 4 and 5, were selected for studying the reaction mechanism. The effectsof two reactants (oxygen and ammonia) were investigated by thermal desorption.The obtained results indicated that the catalysts having more oxygen vacancies andwhich are readily reduced by ammonia showed good catalytic activities. Based on theinteraction between adsorbed ammonia on molybdenum oxide (=N-H) and aldehydegroup of AC (-CHO), a reaction mechanism for AC ammoxidation over MoO3 wasproposed. This chapter will be submitted soon to a scientific journal [33].

— Chapter 7 concludes the thesis with a brief discussion and future direction.

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Chapter 2

Ammoxidation Catalysts

2.1 Heterogeneous catalysts

Catalysts can accelerate chemical reactions without being used up in the process. Catalystsalso help to speed-up the rate of a thermodynamically feasible reaction without affecting theposition of thermodynamic equilibrium. As shown in Fig. 2.1, a catalyst provides a tunnelinstead of climbing a mountain with sufficient energy to overcome the reaction activationbarrier. This way significantly enhances the reaction rate. Due to this vitally important thing,catalysts have been used in many chemical processes with numerous industrial applicationsin food, chemical, pharmaceutical, and petrochemical industries [34–36].

Figure 2.1. Course of a heterogeneously catalyzed gas-phase reaction [37]

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Catalysts can be classified into two groups based on the physical phases of reactants andcatalysts: (i) heterogeneous catalysts and (ii) homogeneous catalysts. Heterogeneous cata-lysts are different physical phases than reactants, so that they are easily separated from thereaction products either in liquid solution or in gas mixture. Generally, inorganic solids suchas metals, metal oxides, sulfides, acids, carbides and salts have been used as heterogeneouscatalysts in chemical processes [35, 38]. During a catalytic reaction, reactants and productsare passing through several steps over a catalyst, including [34, 39]:

1. Diffusion of reactants to boundary layer around the catalyst

2. Diffusion of reactants through the catalyst pores to active sites

3. Adsorption of reactants onto active sites

4. Surface reactions

5. Desorption of products

6. Diffusion of products through the catalyst pores

7. Diffusion of products through the boundary layer around the catalyst particle

As mentioned before, catalysts affect the reaction rate by providing a way to change theactivation energy. In fundamental principle, the activation energy Ea can be derived from anArrhenius plot, which expresses rate constants as a function of temperature as shown in Eq.2.1:

lnk(T) = lnν− Ea

RT(2.1)

where ν is the pre-exponential factor, k is the rate constant, R is the gas constant, and T is theabsolute temperature (K).

At low temperature, the diffusion rates (steps 1, 2, 6, and 7) are faster than the surface reac-tion rate (step 4) and the overall reaction rate is controlled by the intrinsic kinetic reactionrates (steps 3, 4, and 5). When the temperature is increased, the surface reaction rate in-creases more rapidly than the diffusion ones. In addition, the temperature also has a signifi-cant effect on the mass and heat transfer. Therefore, there are various factors controlling theintrinsic reaction kinetic rates. In general, a good catalyst [34, 36, 40–45] should have threemain features:

— Good selectivity: enhancing desirable products and minimizing undesirable by-products.Many aspects will affect on the catalytic selectivity such as catalyst composition, struc-ture and phase, supports and reaction conditions.

— High catalytic activity: this point is defined as amount of reactants transformed toproducts per unit of time or unit of volume reactor. In addition, the catalytic activity

8

relates to interactions between catalyst surface and adsorbed species. If these interac-tions are too weak, the catalysts require a high activation energy and may have slighteffect on the reaction conditions. In contrary, if they are too strong, the adsorbates willblock active sites. Therefore, these interactions should be neither too strong nor tooweak in order to obtain high catalytic activity. Moreover, these catalysts react withabsorbed species on their surfaces, thus their surface areas will play a key role on thecatalytic activity.

— Stability or long life reaction: in order to increase catalyst stability, not only the cata-lysts composition must be considered but also the support materials.

2.2 Ammoxidation of propene/propane to acrylonitrile

Ammoxidation is a reaction, in which ammonia interacts with reducible organic materials inpresence of oxygen (air) and a suitable catalyst. Traditionally, the organic materials can bean alkene, alkane, or aromatic. Mixed oxides are typically used as catalysts.

2.2.1 Ammoxidation of propene to acrylonitrile

CH2−−CH−CH3 + NH3 +32

O2 → CH2−−CH−CN + 3H2O (2.2)

Ammoxidation of propene to ACN (Eq. 2.2) is the most important process in industry forsynthesizing ACN [7–9, 24, 25, 46–53]. The catalysts developed [7] for this commercial pro-cess are summarized in Table 2.1. Initially, the catalysts based on bismuth-molybdate-iron,iron-antimonate, or uranium-antimonate oxides supported on silica (50 wt%) achieved 55to 70 % ACN yields. Then, several additives such as Cr, Mg, Rb, K, Cs, P, B, Ce, etc. wereapplied in order to increase the ACN yields up to 80 % or more [54]. In the first mechanisticcycle (left, Fig. 2.2), ammonia is interacting with the bifunctional active sites located on thecatalyst (oxo sites) to form isoelectronic NH2− surface moieties and water. Then, propene iscoordinated to create an adsorbed surface complex. After multiple rearrangement and oxi-dation steps, the surface complex is transformed to a nitrile molecule. Then, this molecule isdesorbed and released into the gas phase. Finally, the catalyst active sites are reoxidized bylattice oxygen.

There are some key elements to improve the catalytic properties. For example, Bi3+, Sb3+, orTe4+ are responsible for α-H abstraction; Mo6+ or Sb5+ plays a role for propene chemisorp-tion and oxygen or nitrogen insertion component; Fe2+/Fe3+ or Ce3+/Ce4+ enhances thetransfer of lattice oxygen between surface and bulk of the catalyst. In addition, the catalystscan achieve high catalytic selectivity based on the seven principles [7–9, 46, 47, 54–59] shownbelow:

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Table 2.1. Commercial propene ammoxidation catalysts

Early catalysts ACN in tank-yields Company Ref.Bi9PMo12O55 –SiO2 ∼55 % Sohio [60]Fe4.5Bi4.5PMo12O55 –SiO2 ∼65 % Sohio [61]FeSb8.6Oz –SiO2 ∼65 % Nitto Corporation [62]USb4.6Oz –SiO2 ∼70 % Sohio [63]Advanced multicomponentcatalystsKa (NiCo)9Fe3BiPMo12Oz –SiO2 ∼75 % Sohio [64, 65](KCs)a (NiCoMn)9.5 (FeCr)2.5

BiPMo12Oz –SiO2 78-80 % Sohio [66](KCs)a(NiMgMn)7.5(FeCr)2.5Bi0.5

Mo12Oz –SiO2 >80 % Sohio [67]Na0.3(NiCuMnZn)0-4(VW)0.05-1Mo0.1-2.5

Te0.2-5Fe10Sb13-20Oz –SiO2 ∼75 % Nitto Corporation [62]

1. Type of surface lattice oxygen: this factor affects the conversion of a hydrocarbon toa corresponding nitrile and the reoxidizing by dioxygen from the gas phase. Certainlattice oxygens from multivalent metal oxides are better for catalytic selectivity thanoxygen from gas phase.

2. Metal-oxygen bond strength: this strength relates to the chemisorption process, beingreadily reducible and reoxidizable. As discussed above, if the M-O bond is too strong,it could be inactive, whereas it may lead to complete combustion if it is too weak.

3. Host structure: this point has several tasks in order to not only reduce the cost ofcatalysts but also increase the activity, selectivity, mechanical property, and life time.Metals or metal oxides (guest) are supported on the host materials to obtain high dis-persion. In addition, the host and guest materials also have physical and chemicaleffects such as electronic effects, adhesive forces, formation of new phases, and forma-tion of reduced support species [68].

4. Redox activity can enhance the transfer of lattice oxygen from the bulk to the catalystsurface.

5. Multi-functionality of active sites: each active site plays a role in the reaction, so that themulti-functionality of active sites is important for the catalytic activity and selectivity.

CH2−−CH−CH3 + O2 → CH2−−CH−CHO + H2O (2.3)

CH2−−CH−CH3 + 3O2 → 3CO + 3H2O (2.4)

CH2−−CH−CH3 +92

O2 → 3CO2 + 3H2O (2.5)

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Figure 2.2. Proposed mechanism of propene and propane ammoxidation [54]

6. Site isolation is regarding to the spatial isolation of active sites in order to obtain highreaction selectivity. For example, stoichiometry in the ammoxidation of propene (Eq.2.2) indicates that three oxygens are required to form one acrylonitrile molecule andthree water molecules. As shown in Eq. 2.3, one acrolein and one water molecule areformed from two oxygen atoms and one propene molecule (the mechanism of propeneoxidation to AC is shown in Fig. A6). On the other hand, six or nine oxygen atoms cre-ated three CO or three CO2 via the propene oxidation, respectively (Eqs. 2.4 and 2.5).However, a single oxygen atom can only abstract an α-H to form an allyl and a hy-droxide group on the surface. Different site isolations in bismuth molybdate phases,corresponding to different Mo coordinations such as tetrahedral and octahedral coor-dination geometry, have high effect on the catalytic activity.

7. Phase co-operation: two or more phases are in an intimate contact with each otherin order to facilitate their co-operation on an atomic scale. The cooperations betweendifferent bismuth molybdate phases affect the reaction yield. α-Bi2Mo3O12 phase hasScheelite structure, while γ-Bi2MoO6 phase has Koechlinite layer structure. β-Bi2Mo2O9

phase is a combination between α and γ phases [8]. The highest ACN yield in the am-moxidation reaction was obtained with the β phase. Based on this concept, usefulcatalysts may be designed.

2.2.2 Ammoxidation of propane to acrylonitrile

CH3−CH2−CH3 + NH3 + 2O2 → CH2−−CH−CN + 4H2O (2.6)

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Table 2.2. Propane ammoxidation catalysts

Early catalysts ACN in tank-yields Company Ref.VMoxMyOz M=Bi, Te, Ga, Nb, Ta, Ce, Ag Mitsubishi [69, 70]Mo0.6V0.16-0.21Nb0.06-0.10Te0.10-0.18Oz 55.1 % ACN yield (86.7 % conv., 63.5% sel.) Misubishi [69]Mo0.6V0.187Nb0.085Te0.14Oz 61.8 % ACN yield (86 % conv., 72 % sel.) Sohio [71]MoV0.3Nb0.06Sb0.20G0.30Oz 44.7 % ACN yield (86 % conv., 52 % sel.) BP [72]Advanced multicomponentcatalystsVSbxMyOz M=W, Te, Nb, Sn, B, Bi, Al, Ti Sohio [73]VSb5W0.5TeSn0.5Oz 39.1% ACN yield (68.8 % conv., 56.7 % sel.) Sohio [73–75]Oxy-nitride VAlON 36% ACN yield (55%conv., 66%sel.) - [76]

Propene feedstock in the manufacture of acrylonitrile costs about 67 % of the full cost of pro-duction [77]. On the other hand, the difference between the prices of propene and propaneare estimated to be around 360 USD per ton in 2007 [6]. Thus, the ammoxidation of propanehas been found attractive to revamp that of propene. For example, Asahi Kasei Corpora-tion produced about 70,000 tons of ACN per year from propane. Large companies such asBP, Mitsubishi, Rhodia, BASF, Nitto and Monsanto already started plants using this process[6, 7].

Two main catalytic systems based on vanadium-antimonate and multi-component molyb-date (Mo/V/Nb/Te/O) have been proposed in the literature (see Table 2.2) [69–76, 78–85].Many factors such as composition, procedure of synthesis, activation process and modalityof doping affect directly the catalyst properties. Based on these factors, Mitsubishi and AsahiKasei produced their catalysts with high ACN yield up to 62 % [86, 87].

The propane ammoxidation (Eq. 2.6) mechanism over molybdate catalysts is shown on theright part of Fig. 2.2. This reaction occurs mainly via an intermediate formation of propene.That is the reason for the presence of vanadium in the catalysts in order to activate propaneto propene by oxidative dehydrogenation. Therefore, there are two different active sites: (i)to activate propane then to oxi-dehydrogenate to propene, and (ii) to ammoxidate propene.The detailed mechanism over different catalysts was described in reference [8].

In general, the ammoxidation of propane is carried out at higher temperature than that ofpropene by about 50 ◦C, at atmospheric pressure or more, and using a series of reactors dueto a large amount of un-reacted propane. This process needs to be improved in order toobtain higher ACN yield.

ACN has been obtained from the ammoxidations of propene or propane, thus these pro-cesses are highly dependent on fossil sources. The reaction byproducts are hydrocyanic acid(HCN), acetonitrile (CH3CN, ACE), and oxides of carbon (CO, CO2). HCN has been usedin the synthesis of methyl methacrylate. ACE has limited application as a solvent. The am-

12

moxidations of propane and propene are, however, highly exothermic (-632 kJ/mol and -516kJ/mol, respectively) [7, 8, 10], so that they are difficult to control and to reactor set-up [22].

2.3 Acrylonitrile from glycerol

Sustainable and greener sources and easy setting-up of the reaction systems will be attrac-tive. Glycerol can dehydrate to AC [12, 88, 89]. Furthermore, literature results [85, 90, 91]demonstrated that acrolein is an intermediate product in propene ammoxidation. Nilssonand Andersson studied the transient response to identify possible reaction mechanism overSb-V-oxide catalyst [85]. The reaction was carried out in a quartz reactor, 200 mg of catalyst,at 480 ◦C and 460 ◦C for propane and propene ammoxidations, respectively. These authorsused a model to analyze the transient response upon a step change from an inert gas to re-actant feed gases. The obtained results indicated that the ammoxidation of propane to ACNoccurred via propene as an intermediate product of oxidative dehydrogenation. Propene isthen transformed into adsorbed AC and then reacts with NHx on the catalyst surface to formACN.

Acrylonitrile from glycerol processes are classified into three groups:

1. One step-ACN from glycerol (Eq. 2.7)

2. Two steps-ACN from glycerol via acrolein: dehydration of glycerol to AC and ammox-idation of AC to ACN (Eqs. 2.8 and 2.9)

3. Three steps-ACN from glycerol via acrolein: (i) dehydration of glycerol to AC; (ii)purification; (iii) ammoxidation of AC to ACN

CH2OH−CHOH−CH2OH + NH3 +12

O2 → CH2−−CH−CN + 4H2O (2.7)

CH2OH−CHOH−CH2OH→ CH2−−CH−CHO + 2H2O (2.8)

CH2−−CH−CHO + NH3 +12

O2 → CH2−−CH−CN + 2H2O (2.9)

As mentioned before, AC is an intermediate product. We suggest that the catalysts of thepropane/propene ammoxidations could be used for the AC ammoxidation with simplercompositions and better ACN yields.

2.3.1 One step-ACN from glycerol

One step-ACN from glycerol was first studied by Guerrero-Perez and Bañares [18] using V,Nb, Sb, VSb and VSbNb supported on Al2O3. The reaction was carried out at 400 ◦C in

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Table 2.3. Conversion of glycerol over different catalysts ∗ [18]

Catalyst Sb/Al Nb/Al V/Al VSb/Al VSbNb/AlConversion (%) 10.4 16.2 87.2 71.6 82.6

Selectivity (%)

CO 0.5 0.2 0.6 0.7 1.1CO2 7.7 6.8 7.7 2.9 4.4CH4 0 0 0.1 0.5 0.2C2H6 - - 1.5 - -C2H4 - - 1.2 - -Propane - - 0.1 - -Propylene - - 0.1 - -1,2-propanediol 28.1 28.8 27.1 5.7 6.9ACE 19.2 1.5 1.1 1 0.8Propanal 14.4 34.7 53.5 4.3 2.1AC 28.9 26.7 6.9 28.9 26.2ACN 1.2 1.3 - 56 58.3

∗: measured at 400 ◦C; Reactants include 30 ml/h of glycerol; 25 % O2, 8.6 % NH3diluted in He (volumetric percentages)

a fixed-bed quartz reactor with 50 mg catalyst diluted in 250 mg CSi. The reactants wereglycerol (no water), 25 % O2, 8.6 % NH3 diluted in He. 87.2 % of glycerol conversion and53.5 % of propanal selectivity were obtained over the V/Al2O3 catalyst. Thus, the authorsbelieved that vanadium provided active oxidation sites, antimony increased the selectivitiesto AC and ACN, and niobium enhanced the catalyst acidity and facilitated the dehydrationof glycerol. The effects of these elements are summarized in Table 2.3. The best catalyst wasVSbNb/Al2O3 with 82.6 % of glycerol conversion and 58.3 % of ACN selectivity, however, itdeactivated after 2h time on stream (TOS). The authors explained that forming coke (poly-acrylonitrile or polyacrolein) on the catalyst surface could be the reason for fast deactivation.The optimized reaction conditions to improve catalytic properties were not reported. Liebiget al. [15] reproduced the same catalysts for this process with 100 % of glycerol conversionand only 2 % of ACN selectivity. The Guerrero-Perez’s group [92] criticized the method forsynthesizing catalysts, and selectivity of the reaction. First, they indicated that the Sb-V-Ocatalysts were produced by a different method compared to their method. Thus, the cata-lysts had a dramatic difference in specific surface area (SSA) (2 vs. 130 m 2/g), that was a keyfor the reaction. In addition, the Holderich’s group did not mention a mistake they madeabout the selectivity. Glycerol was 100 % converted to 2 % ACN, 50 % CO2, 9 % acetic acid,6 % ACE, and 3 % propionitrile with 96 % carbon balance, therefore, leaving 31 % selectivityof unknown product. Then, the Holderich’s group replied to the above comments [93]. Thisgroup one more time concluded that the catalysts were produced according to the methodreported by Guerrero-Perez et al. [18], however, the catalysts were different in SSA due tounknown type of γ-alumina support. Furthermore, this group agreed about the sum of se-lectivity and they believed that the chromatographic methods had limitation in identifyingthe unknown products. Finally, the dispute could not be solved.

14

In conclusion, producing ACN from glycerol in one step is interesting since it requires onlyone reactor. This process still remains a challenge in order to obtain high glycerol conversion,high ACN selectivity and long life of catalysts.

2.3.2 Two steps-ACN from glycerol via acrolein

Liebig et al. [15] also investigated the conversion of glycerol to acrylonitrile via acrolein intwo steps. Two reactors in tandem were run in plug-flow and loaded with granule catalysts.The first reactor was used to dehydrate glycerol to AC at 280 ◦C and contact time of 0.35 s.5 g of WO3/TiO2 was loaded and fed with 4.54 % glycerol, 92.74 % water, and 2.72 % O2.The results showed high glycerol conversion and AC selectivity (95 and 81 %, respectively).However, the glycerol conversion decreased exponentially from 100 to 50 % over 96 h ofTOS. The ammoxidation of acrolein to ACN over FeSbOx catalysts was carried out in thesecond reactor at 400 ◦C and contact time of 0.12 s. The feed was 86.6 % water, 7.7 % O2, 3.3% NH3 and 2.2 % acrolein. Reasonable 84 % AC conversion and 44 % ACN selectivity wereobtained, however, only 36 % ACN yield was reported.

As discussed above, because of the rapid catalyst deactivation in the first step, a large amountof un-reacted glycerol will go through the second reactor, which may create coke on the cat-alyst surface. Consequently, less acrolein will be fed to the second reactor and the reactantratios will be changed. Hence, the optimized reaction conditions and catalytic properties canbe affected. Moreover, the ACN yield was still low.

2.3.3 Three steps-ACN from glycerol via acrolein

Three-step process from glycerol to acrylonitrile, in which one-step purification was sand-wiched between two-step reactions, was studied by Dubois [22]. In the first step, the de-hydration of glycerol to AC was carried out in a fixed-bed reactor loaded with WO3/ZrO2

(9.3/90.7 %) at 300 ◦C. This catalyst was used in milled or/ and pellet form with particle sizeabout 0.5 to 1 mm. The feed gases, including aqueous solution of glycerol and oxygen, wereintroduced. The results showed that glycerol was completely converted with 54 % AC selec-tivity. The byproducts were acetaldehyde (∼9 %), propanaldehyde (∼2 %) and acetone (0.1%). As shown in Eq. 2.8, one glycerol molecule produces two water molecules. This meansthat a large amount of water from the feed gases and products was present in the out-let ofthe first reactor. Therefore, this out-let needs to be separated into two parts: (i) heavy prod-ucts including polymers and water and (ii) light products containing acetone, acetaldehyde,propanaldehyde, gases (inert, CO and CO2), and AC. Consequently, a purification step wasapplied.

In the third step, the ammoxidation of AC to ACN was performed at 420 ◦C and HSV (hourlyspace velocity) 1200 h−1. A pyrex reactor was charged with 6.578 g of Fe-Sb-O catalystsdiluted in 7 ml of SiC with particle size about 0.125 mm. The feed gases, containing 4.5 %

15

-H2O-H2O

H2O

Glycerol 3-hydroxypropanal Acrolein

Figure 2.3. Reaction pathway from glycerol to acrolein

AC, 8.7 % oxygen, 5.4 % ammonia, 15 % water and 66.4 % nitrogen, were introduced. Finally,the highest ACN yield of 60 % was reported. This process can solve the problem of catalystlife (deactivation after the short time on stream).

In the three-step process, the first step of dehydration of glycerol to AC has been well studied(Fig. 2.3) [22, 88, 89, 94]. In this reaction pathway, one glycerol eliminates one water toform 3-hydroxypropanal and then, dehydration of another water to form AC. This processwas performed at temperatures ranging from 250 to 340 ◦C using several catalysts such aszeolites, zirconia, heteropolyacids, and mixed oxides. The third step of ammoxidation ofAC to ACN was, however, still yielding low ACN yield. Unfortunately, up to now only afew publications dealing with this subject were reported. Herein, we will focus on studyingthoroughly the ammoxidation of AC to ACN.

2.4 Acrolein ammoxidation to acrylonitrile

Literature on AC ammoxidation to ACN can be divided into two eras: (i) before 1975 withsome examples of successful catalysts and (ii) recent reports showing quite low ACN yield.

In 1951, Bellringer et al. (The Distillers Company Limited) [95] claimed that several unsatu-rated nitriles were created from α, β -unsaturated aliphatic aldehydes or aldehydes in pres-ence of oxidative catalysts with ammonia and oxygen. The catalysts might be selected fromthe following metals, their oxides, or mixture: copper, chromium, vanadium, manganese,iron, cobalt, nickel, molybdenum, silver, zinc, cadmium, tin, tungsten, lead, platinum, gold,aluminum, palladium, rhodium, bismuth, and uranium oxides. These elements were sup-ported on silica gel, zeolites, or alumina. The feed gases, including acrolein, ammonia, oxy-gen, with nitrogen as a diluent, were investigated at different molar ratios. The reaction wasoperated at temperatures ranging between 350 and 500 ◦C and contact times about 0.1 to 20seconds, preferably less than 10 seconds. The results indicated that the catalysts containingmolybdenum or its compounds were suitable for producing unsaturated nitriles. In addi-tion, the authors claimed that ACN was the main product when AC was used as the startingorganic material. Using molybdenum oxides on Kieselguhr pellets, the AC ammoxidationto ACN showed the highest ACN yield of 80 %.

In 1963, The Distillers Company Limited reported a production of ACN in vapor phase [96].

16

Table 2.4. First order rate constant and selectivity of bismuth molybdate catalysts [98]

Catalyst Reaction mixture Feed ratio k1 at 400 ◦C% selectivity forC3H3N(ACN)

C3H4O(AC)

UBM4

C3H6 - 6.4±0.6×10-5 (C3H6) - 85-90C3H6/O2 1:1 2.9±0.3×10-4 (C3H6) - 95C3H6/NH3/O2 1:1:1 3.1±0.3×10-4 (C3H6) 50 1-2C3H4O/NH3/O2 1:1:1 9±1×10-3 (C3H4O) 70 -C3H6/NH3 1:1 5.8×10-5 (C3H6) 35-40 -

GS1

C3H6 - 2.0±0.2×10-4 (C3H6) - 95C3H6/O2 1:1 9±1×10-4 (C3H6) - 95C3H6/NH3/O2 1:1:1 1.7±0.1×10-3 (C3H6) 95 -C3H4O/NH3/O2 1:1:1 4.5±0.4×10-2 (C3H4O) 95 -C3H6/NH3 1:1 7±0.7×10-4 (C3H6) 75-80 -

k1 is the specific first order rate constant in the equation -dn/dt = k1P in mole/(min x g xatm)

Mixed oxides Sb-Zn-O were used as catalysts in form of granules or pellets. This processwas carried out in a fixed-bed reactor at temperatures ranging from 300 ◦C to 550 ◦C andcontact times ranging from 1 to 30 seconds. The feed gases, including 2 % of acrolein, 87 %of air, 8 % of nitrogen, and 3 % of ammonia in volume, were introduced. The best 74.3 %ACN yield was obtained.

In 1971, Cathala and Germain studied this reaction using bismuth molybdate oxide (Bi/Mo=1/1molar ratio) in a fixed bed reactor at 460 ◦C [97]. The gas mixture ratio between acrolein, am-monia, and air was 1/ 2.4/ 46 with the total flow rates between 7 and 55 l/h. 150-500 mg ofthe catalyst was diluted in 25 g of quartz powder. The results showed that the AC conver-sions were varied from 60 to 100 % with the highest 87 % of ACN selectivity. The byproductswere acetonitrile (0.85 %), ethylene (0.80 %), hydrogen cyanide (>0.5 %), and carbon oxides(CO, CO2; 7.5 %). In the blank tests (without catalysts), 50 % AC was converted to car-bon oxides (91 %), ethylene (7 %), and ACN (2 %). This revealed that carbon oxides andethylene were created by thermal degradation. Furthermore, the authors claimed that theAC ammoxidation to ACN was faster and obtained higher ACN selectivity than that of thepropene ammoxidation.

Wragg et al. [98] investigated two catalysts including bismuth molybdate (UBM4) with Bi:Mo= 0.73: 1 and Koechlinite phase of Bi2MoO6 (GS1) for the ammoxidations of propeneand acrolein. The specific surface areas of UBM4 and GS1 were 1 and 4 m 2/g, respectively.As shown in Table 2.4, both UBM4 and GS1 catalysts exhibited good ACN selectivity. 95% of ACN selectivity was obtained with GS1, while this value was only 70 % lower forUBM4. The authors claimed that the AC ammoxidation exhibited higher ACN selectivityand reaction rate than propene ammoxidation. The reaction rate of AC ammoxidation was

17

Lo

g R

i, lo

g (

g.m

ol/

min

)

, log (atm) , log (atm)

Figure 2.4. (a) Initial reaction rates vs PO2. Conditions: temperature 340 ◦C, catalyst 1(g), PAC 0.014-

0.015 (atm), PNH30.107-0.115 (atm), PH2O 0.205-0.220 (atm) ; (b) Initial reaction rates vs PNH3

. Condi-tions: temperature 340 ◦C, catalyst 1(g), PAC 0.013-0.016 (atm), PO2

0.093-0.119 (atm), PH2O 0.189-0.240(atm) [99]

30 folds higher compared to that of propene ammoxidation. Bismuth molybdates could bepotential candidates for these ammoxidations. The first order reaction rate (k1) and (ACNand AC) selectivities are shown in Table 2.4.

Oka et al. [99] also studied the ammoxidations of AC and propene in a fixed bed reactorover Fe2O3-Bi2O3-P2O5 (44.5: 44.5: 11.0 mol%) catalysts. The reactions were carried out attemperatures ranging from 310 ◦C to 380 ◦C and contact times ranging from 0.3 to 1.8 s.Partial pressures of reactants were as follows: acrolein 0.003-0.16 atm, oxygen 0.039-0.358atm, ammonia 0.029-0.300 atm, and water 0.18-0.24 atm (total pressure of 1 atm, using nitro-gen as diluent). The mass of catalyst was varied from 1 to 3 g, preferably 1 g with particlesizes of 0.15-0.42 mm. This is the first publication considering the effect of water as reactanton reaction rate. This effect can be significant because of the presence of water in glycerol,which appears as a product in the glycerol conversion to ACN via AC, as mentioned above.

Effects of reactants (oxygen and ammonia) on initial reaction rates were shown in Fig. 2.4.The reaction rates were increased by increasing the partial pressure of oxygen, and then keptconstant. In the acrolein consumption and acrylonitrile formation, these parameters wereonly dependent on the oxygen partial pressure at low value. For the CO2 formation, this

18

ACACN

Figure 2.5. Scheme reaction pathway of AC to ACN

rate was independent of the partial pressure of ammonia because CO2 was created by burn-ing out acrolein and hydrocarbons. In the case of acetonitrile, this ACE rate was increasedlinearly with increasing the partial pressure of ammonia. In addition, the authors reportedthat the AC conversion was 1000 times faster than that of propene ammoxidation. The high-est ACN selectivity and AC conversion were 70 % and 30 %, respectively, at PO2

/PAC = 15 orPNH3

/PAC = 2-3.5. This reaction could not be observed at temperatures higher than 380 ◦Cbecause its reaction rate was too fast to measure. In addition, the authors also demonstratedthat AC was an intermediate product during the conversion of propene to ACN as shown inFig. 2.5.

In Fig. 2.5, one acrolein is converted to one imine, then reacts with one oxygen to create oneACN. The authors suggested that the first step was very fast and the second one determinedthe overall reaction rate. Therefore, the ACN selectivity will be increased with increasingdensity of active oxygen on the surface of the catalyst.

Recently, Hoeldrich’s group studied the AC ammoxidation over a series of catalysts includ-ing mixtures of SbFeO, SbVO, pure MoO3, MoO3/TiO2, and MoVSbO/SiO2 [15, 100]. Allreaction tests were carried out in a fixed bed reactor using 2-8 g of catalysts at temperaturesranging from 300 ◦C to 500 ◦C and 1 atm. The out-let and in-let lines were carefully heatedto 200 ◦C in order to avoid acrolein polymerization. In the blank tests, 3 % of AC was con-sumed, but no ACN was formed. This could be related to the accuracy limitation of thesystem, and the thermal activation had no effect. The authors investigated several factors(such as temperature, molar ratio, specific surface area, particle size and presence of waterin feed) that could have affected the catalytic properties. The reactant ratios of NH3/AC andO2/AC were adjusted from 0.5 to 1.5 and 0.5 to 6.5, respectively. The authors demonstratedthat the particle sizes ranging 0.25-0.5 and 0.5-1 mm had no influence on the catalytic prop-erties and there was no limitation on the diffusion of reactants and products. Moreover, thespecific surface areas of catalysts were also changed to adjust the catalytic activity. Further-more, the presence of water in the feed was studied on SbFeO and MoO3/TiO2 catalysts. Theobtained results were totally different on both catalysts. Over SbFeO, the presence of watercan increase the AC conversion from 25 % to 50 %. By contrast, the ACN selectivity wasreduced from 30 % to 15 % over MoO3/TiO2, whereas, both their ACN selectivity and ACconversion were slightly increased with time on stream. The authors did not, however, giveany explanation for this case. In conclusion, all catalysts worked for this AC ammoxidation

19

Table 2.5. Catalyst performance observed for ammoxidation of acrolein

Catalyst H2O/ACTemp.◦C

YACNS1

ACNRef.

Prio

rto

1975

MoO3 - 365-405 81.5 [101]MoO3/SiO2 - 334-359 77.2 [101]SbSnO - 421 74.3 [96]BiMoO - 400 701 [98]α-BiMoO - 400 951 [98]BiMoO - 460 75 [97]FeBiPO 14.5 380 44.0 [99]

Rec

entr

epor

ts

Co/Ni/Mg/Fe/K/P (BiMoO) - 390 ≤59 [16]

SbFeO- 400 25.0

[15, 100]

14.0 wt% 400 ∼40.0SbVO - 400 ∼13.0MoO3 - 400 18.0

MoO3 /TiO2- 400 30.0

14.0 wt% 400 20.0MoVSbO/SiO2 - 400 27.0

and MoO3/TiO2 was reported as the best catalysts.

Ghalwadkarl et al. [16] studied the AC ammoxidation over multi-element based Bi-Mo-O catalysts. These solids were synthesized by using a co-precipitation method in order toobtain mixtures of bismuth molybdate with different valent metals: (i) bivalent metals suchas Co, Ni, Mg; (ii) trivalent metals such as Fe, Cr and Al; (iii) alkaline metal (K); (iv) non-metal: P. The specific surface areas of these catalysts were approximately 20 m 2/g. The ACreaction was carried out in a fixed-bed reactor at 390 ◦C and 1 atm. The reactant ratios wereselected as NH3/AC = 1.75 and O2/AC = 2.7. The highest ACN yield in this study was59 % and similar to the result reported by Dubois [22]. The authors demonstrated that thebivalent/trivalent metals were promoting the reaction yield due to the efficient oxygen oncatalyst surface and the redox process.

The summarized results in Table 2.5 indicate that several single and mixed metal oxides canbe good candidates for the AC ammoxidation. In the recent reports, two main groups, in-cluding molybdate and antimonate based catalysts, were used for this reaction, however, theACN yields were still low. Indeed, the reported catalysts exhibited low specific surface areas,low pore volumes, and low dispersion of active components [15, 16, 98–100]. As a rule forheterogeneous catalysts, the active surface area plays an essential role in the catalytic perfor-mance [34]. Therefore, developing a catalyst with high texture properties could enhance thecatalyst yield [102]. We have thus introduced some methods for synthesizing metal oxidesand their supports with high surface area, high pore volume, narrow pore size distribution,and large pore size.

20

Table 2.6. Summary reaction parameters of acrolein ammoxidation reaction

Parameter ValueTemperature ( ◦C) 350-500Pressure 1 atmMolar ratio NH3/AC 1.0-1.5Molar ratio O2/AC 1.5-15

Based on the above reports, the AC ammoxidation conditions can be controlled by adjustingseveral parameters as shown in Table 2.6.

2.5 Methods for synthesis of mesoporous metal oxide catalysts

Mesostructured materials (MMs) with pore sizes ranging from 2 to 50 nm have great poten-tial applications in sorption, catalysis, and separation. These materials show narrow poresize distributions, high specific surface area, large pore volume, nano-scale pores, high ther-mal and mechanical stabilities [28–30, 103–108]. Mesostructured materials can be synthe-sized by two main methods: hard-templating and soft-templating methods.

2.5.1 Soft-templating method

Soft-templating is a process in which organic molecules serve as a ‘mold’ and a frameworkof target materials is built-up around. Removing these organic molecules can result in acavity, which retains the same morphology and structure of the organic molecules. The softtemplate is usually in molten or liquid (solution) state. During soft-templating processes,the sol-gel or evaporation induced self-assembly (EISA) techniques are typically involvedfor synthesizing ordered mesoporous materials (OMMs) [103, 103, 109–117].

A typical synthetic scheme of OMMs is described in Fig. 2.6. In the first step, the mesostruc-ture is produced by a sol-gel/self-assembly process or a liquid crystal replication if the con-centration of non-ionic surfactant is high enough. In the second step, mesostructured hy-brid is consolidated under aging process or hydrothermal treatment. During this period,the polycondensation is continued along with localized solution and precipitation of the gelnetwork, then the thickness of interparticle necks is increased, whereas the porosity is de-creased. Solvent is then removed from interconnected pore network by drying. In the finalstep, mesoporous framework is formed during high thermal treatment and after removingthe organic template.

Evaporation-induced self-assembly (EISA) [114, 119] via soft-templating method has beenwidely used for synthesizing mesomaterials. This method is related to a liquid crystal tem-plate. The solution must contain a volatile solvent (such as ethanol) in order to promotethe evaporation. During this process, the solution will become more concentrated and a

21

critical micelle concentration is reached. After further loss of solvent, a liquid crystallinephase is formed. Different pore structures, including two-dimensional hexagonal (p6mm)and three-dimensional cubic (Im3m, Pm3n, Ia3d, Fm3m, etc.), were synthesized as shown inFig. 2.7. The textural properties and morphologies can be controlled by various synthesisconditions (pH, time, and temperature), surfactants, gel compositions, and organic additives[103, 111, 114, 119].

Surfactants

Surfactants consist of two parts: (i) a hydrophobic region designated as the tail and (ii) ahydrophilic region called the head. Due to hydrophobic effects, the different organizationsof micelles are formed such as spherical, cylindrical, bilayer, reverse, bicontinuous cubic,and vesicular-liposomes micelles.

Surfactant micelles are used as a ’mold’ of the mesostructures and have a significant effecton the pore size and pore structure. Based on the nature of the surfactants (anionic, cationic,or non-ionic), the surfactants and inorganic metal oxide precursors interact following S+I-,S+X-I+, S0I0 hybrid interfaces (where I is an organic precursor; S is a surfactant head group;

36

Figure 2.6. Soft-templated process for synthesizing ordered mesoporous materials (modified from[118])

Figure 2.7. Mesostructured pore models with different symmetries: (A) p6mm, (B) Ia3d, (C) Pm3n,(D) Im3m, (E) Fd3m, and (F) Fm3m

22

Table 2.7. Some surfactants for synthesis of metal oxides

Name Surfactants Pore size (nm) Pore structure Ref.KIT-6 P123 (EO20PO70EO20) 4-12 3D cubic Ia3d [120]SBA-15 P123 4-15 2D hexagonal p6mm [109]MCM-41 Cn(CH3)3N+ Br– or Cl– ; n=8-20 1.5-10 2D hexagonal p6mm [121, 122]MCM-48 Cn(CH3)3N+ Br– or Cl– ; n=12-20 1.5-4.5 3D cubic Ia3d [122, 123]

TiO2

F127, Brij581-8

Cubic[124]P123, Brij56 Hexagonal

P123, Brij56 LamellarZrO2 F127, P123 7-15 Cubic [125]Fe2O3 F127, P123 11 Lamellar [125]MoO3 F127, P123 10 Lamellar [125]WO3 F127, P123 12 Lamellar [125]

• SBA-15 mesoporous silica Santa Barbara number 15• KIT-6 mesoporous silica Korean Institute of Technology number 6• MCM-41 Mobil Corporation Material number 41• MCM-48 Mobil Corporation Material number 48• P123 triblock copolymer, PEO20PPO70PEO20• F127 triblock copolymer, PEO106PPO70PEO106• Brij56 poly(ethyleneoxide)-based nonionic diblock CnH2n-1(OCH2CH2)yOH, n/y =

16/10• Brij58 poly(ethyleneoxide)-based nonionic diblock CnH2n-1(OCH2CH2)yOH, n/y =

16/20.

and X– is halide anion such as Cl– or Br– ). The material pores are created from the hy-drophobic groups of surfactant, so that their lengths affect directly the pore diameter andpore structure. For instance, the pore size of MCM-41 is increased with increasing the car-bon chain of structure directing agent (CnTAB, n increases from 8 to 18) [108]. In addition,the pore size and pore structure are also dependent on the concentration of surfactants. Nor-mally, the ionic surfactants can create a small pore. Table 2.7 described the effects of varioussurfactants on the synthesis of mesostructured materials.

Temperature and time

Temperature and time of the synthesis and/or aging processes are parameters for adjustingthe hydrophobic cores and hydrophilic regions of micelles. Higher temperature and longertime can enhance the polymerization degree of inorganic species, so that the pore size isincreased, whereas the wall thickness is slightly reduced [126, 127]. Mesoporous silica suchas SBA-15, SBA-16 and KIT-6 are often synthesized at temperatures ranging from 35 to 45◦C, while the aging temperatures are ranging from 50 to 150 ◦C. Both synthesis and agingprocesses can affect directly and significantly the pore diameter and wall thickness [109, 128–130].

23

Table 2.8. Non-silica metal oxides prepared by the soft-templating method

Sample Organic templateSpecific surface area

m 2/gPore volume

cm 3/gRef.

Al2O3P123 209-410 0.4-0.8 [137]P123 152 0.42 [138]

TiO2 F127 191 0.48 [138]Bi2O3 KLE 140 - [139]Fe2O3 CTAB 94-274 - [140]MoO2 Bis(trimethylsilyl)dodecylamine - - [141]MoOx PEO 212 - [142]

• KLE poly(hydrogenatedpoly-b-butadiene-coethylene)-block-poly(ethyleneoxide)• CTAB cetyltrimethylammonium bromide

pH and electrolytes

Mesostructured silica or metal oxides are often synthesized under acid or base conditions.pH affects the sol-gel process and polymerization of the structure directing agents. More-over, adding salts (KCl, NaCl, and K2SO4) also has electrolyte effects on the interaction be-tween surfactants and inorganic metal oxides [131–133].

Organic additives

Organic additives can be soluble in hydrophobic region of micelles and will then expandthe pore size of mesostructured silica. Docedane, tri-methyl-benzene, tri-isopropyl-benzene,and tertiary amines are often used as organic additives. Moreover, solvent or cosolute polar-ity (for example, butanol is used for synthesis of KIT-6) are also applied for the same purpose[130, 134–136].

Non-siliceous mesostructured materials have been attractive for catalysis applications. Theirprocess syntheses are susceptible to hydrolysis (using precursors such as chloride or alkox-ide), redox reaction or phase transition by the thermal breakdown of structure [106, 125].Therefore, these processes are more difficult to control using several synthetic parameterssuch as pH, temperature, time, surfactants, and organic additives as compared to those ofthe synthesis of silicous materials. Recently, many mesostructured metal oxides were syn-thesized such as titania, alumina, zicornia, and tin [125]. Other oxides of transition metalsuch as Fe, Co, Mo, Ni, and W still need improvement of the synthesis process to obtainmesoporous structures [103, 106, 125, 143, 144]. Table 2.8 summarizes several mesostruc-tured non-silica metal oxides that were prepared by the soft-templating method.

In conclusion, the syntheses of mesostructured transition metal oxides are still complex andunpredictable because there are many effective synthesis factors including temperature, re-action time, structure directing agents, pH, and additives. In this work, the soft-templating

24

37

Figure 2.8. Schematic representation for the hard templated process (modified from [108])

method will be applied for synthesizing mesoporous silica KIT-6 as a hard-template andmetal oxides SbMoSiOx as catalysts for AC ammoxidation.

2.5.2 Hard-templating method

Nanocasting or hard-templating has been considered as a promising synthetic pathway tocreate mesoporous materials with various chemical compositions and predicted pore topolo-gies as shown in Fig. 2.8 and Table 2.9. In this method, the pores of hard templates can bemore or less filled by the metal oxide precursors [30, 103, 112, 145–148]. This process involvesthree steps as shown in Fig. 2.8. In the first step, the precursors are filled inside the poresof hard templates (generally, the templates are either mesoporous silicas or mesoporous car-bons). Because the infiltration step is the most important one in the nanocasting method,the precursors need to satisfy several requirements. The precursors must easily diffuse intothe template pores. Thus, the precursors should be gaseous, highly soluble, or liquid state.In addition, the precursors should undergo no chemical reaction with the hard templatesand easily convert into the designed composition. Metal nitrate, metal chloride, and metalammonium are often used as metal oxide precursors for synthesizing mesosmaterials. Theprecursors interact with the hard templates via hydrogen bonding, Coulombic interaction,van der Waals force, and/or coordination of metal ions. Therefore, functionalizing the hardtemplates with certain groups such as -OH, -NH2, -CH=CH2 may improve the interactionbetween them [149–151]. In the second step, a thermal treatment is introduced in order toconvert the precursors into the designed materials. Finally, the desired mesoporous mate-rials are created after removing the hard templates [30]. For example, silica templates aredigested either by hot NaOH solution (at least 2M) or by diluted HF solution. Carbon tem-plates are removed at high thermal treatment in air. Therefore, the hard templates must bestable under thermal treatment and easily removed without disrupting the replica structures.

During impregnation and drying processes, there are many factors including interaction be-tween precursor and hard template, viscosity of solution, and drying temperature that can

25

Table 2.9. Several nanocasting materials with different porous hard templates

Template ReplicaSpecific surface area

m 2/gPore volume

cm 3/gRef.

SBA-15, KIT-6, MCM-48NiFe2O4 187-296 0.32-0.76CuFe2O4 [155]

SBA-15, KIT-6LaFeO3

110-155 0.1-0.2LaCoO3 [156]LaMnO3

SBA-15, KIT-6 SiC 430-720 0.42-0.79 [157]

affect precursor distribution in the pores (Fig. 2.9) [152]. For example, an egg-shell shapecan be created under high adsorption of precursor on the surface of hard templates, highviscosity of solution, and low drying temperature. A uniform distribution of the precursorsis formed either with equal interaction between precursor and hard template or weak inter-action and low drying temperature. On the other hand, an egg-yolk shape can be formedunder the strong interaction and low drying temperature. This method is powerful for syn-thesizing mesostructured transition metal oxides because it could be easy to control andpredict the pore size and shape. However, the particle sizes of the final materials are oftensmaller than those of the mother hard templates [153, 154] (for instance, the particle sizes arereduced from few hundred micrometer to 50-300 nanometer).

Non-siliceous ordered mesoporous materials are often synthesized by using four main im-pregnation methods: wet impregnation, incipient wetness, dual-solvent, and solid-liquid[145, 158–160].

1. Wet impregnation: hard template (powder) is dispersed in a diluted solution. Theprecursors are dissolved in the same solution, then diffuse into the template pores,where they are adsorbed on the pore walls. This method is limited by the loading ofprecursors inside the pores, thus the impregnation step needs to be repeated several

Egg-shell Egg-yolkUniform

Figure 2.9. Distribution of the precursors into the pores [152]

26

SolidɣL-S

ɣG-L

ɣG-SϴLiquid

Gas

Figure 2.10. Schematic diagram of contact angle, where θ: contact angle; γG−S, γG−L, and γL−S:interfacial tensions

times. As reported in [161], only 6-11 vol% of the template pores are replaced by thetarget metal oxides, when the precursors are completely filling these pores (100 vol%).

2. Incipient wetness: a precursor solution, the volume of which is equal to the pore vol-ume of the template, is impregnated into the pores. Therefore, in this method if there isonly weak interaction of the precursor in solution with the support internal surface it isexpected that the precursors are only deposited into the pores of the template (not onthe outer template surface). This method can yield higher loading of target materialsthan the wet impregnation [162, 163].

3. Dual-solvent: two different types of solvent are used to separate the dispersion of tem-plate and precursor. First, the support is dispersed in a pure less wetting solvent (thesolvent has low polarity index such as heptane, hexane, pentane, etc.). The precursorsolution is then impregnated to the above pre-wetted support. Indeed, the impreg-nation process is considered as replacement of a solid-gas interface by a solid-liquidinterface [152]. As shown in Fig. 2.10 and Eq. 2.10, in case of low wettability of solvent,cos(θ) ∈ {0; 1} the surface tension of liquid-solid (γL−S) is smaller than that of gas-solid (γG−S). Hence, this method helps to improve the filling efficiency of the poroustemplates and homogeneity of the products [164, 165].

γG−S = γL−S + γG−L ∗ cos(θ) (2.10)

4. Solid-liquid: the solid precursors are mixed with the hard templates without usingany solvent. At high thermal treatment, these precursors are melted and filled into thepores of template.

In this report, Bi-Mo-O supported in mesoporous silica KIT-6 as catalysts were synthesizedby combining the dual-solvent and solid-liquid methods [31]. The support KIT-6 was first

27

Figure 2.11. Effect of support on sintering [167]

dispersed in a hydrophobic solvent, then the metal oxide precursors were filled into thetemplate pores as molten salts at a certain temperature.

2.5.3 Supporting metal oxides on mesoporous materials

Many catalysts are supported ones. They have high stability, desired shape, porous struc-ture, and low cost. Non-supported catalysts have high cost for the same performance, theysinter under harsh reaction conditions (as shown in Fig. 2.11), and are difficult to recover[28, 68, 166, 167]. Therefore, the supported catalysts are widely used in catalytic applica-tions.

The interaction between a metal oxide catalyst and its support affects the catalytic activityin different ways such as molecular structure, electronic structure and formation of mixedoxide phases [168, 169]. This interaction depends not only on the nature of the support butalso on the synthesis process of the supported catalyst. Therefore, a suitable support andsynthesis method will play important roles to achieve high catalytic properties. In general,alumina, silica, and activated carbon have been most frequently used as supports. Alu-mina is favorable because of its low price, high thermal and mechanical stabilities, and richchemistry. In addition, alumina has several structures such as nonporous, crystallographicordered of α-Al2O3, and the porous γ-Al2O3 and η-Al2O3, so that alumina is used for differ-ent applications. Moreover, transition aluminas contain a large number of surface hydroxylgroups, which can increase the interaction with the oxide precursors. For example, γ-Al2O3

28

has about 10-15 hydroxyl groups per nm 2. On the other hand, silica has been well-knownas a low bulk density, high thermal stability, and inert surface. Silica also contains numer-ous silanol groups (Si-OH) on its surface (about 4-5.5 hydroxyl groups per nm 2). Activatedcarbon has very high specific surface area and extremely high thermal stability in absence ofoxidizing agents. This support is highly hydrophobic, hence, it is quite difficult to incorpo-rate with oxide precursors [107].

Nowadays, porous supports are widely used in order to improve mass and heat transfer. Theporous materials can be divided into three classes such as microporous (pore sizes < 2 nm),mesoporous (pore sizes range from 2 to 50 nm), and macroporous (pore sizes > 50 nm). Thepore sizes have significant effects in catalytic applications. For example, the microporousmaterials such as zeolites can be used for molecular selectivity in the petrochemistry andorganic synthesis fields based on their pore sizes [35]. The mesoporous and macroporousmaterials are widely applied for improving mass and heat transfer [40, 68, 170, 171]. Inaddition, the pore structures also have influence on the catalytic properties. In general, twoor three dimensional pore structures (2D or 3D) can avoid the limitation of mass transport.

There are two main ways for producing supported catalysts [36, 40, 152, 172]:

1. Co-precipitating active components (e.g. metal oxide precursors) with the supportsto obtain their mixture. After drying and calcination, the designed porous materials,which contain both the metal oxides and supports, are formed. This method is knownas co-precipitation and often applied for producing low price metal oxide precursors.

2. Loading metal oxide precursors into pre-existing mesoporous support materials. Thereare two main methods including impregnation and deposition precipitation. Thismethod is preferred for synthesizing noble metal oxide catalysts.

Co-precipitation

Co-precipitation (also called as one-pot process) is a simple technique to introduce metaloxides into the porous framework of OMMs. In this method, metal oxide precursors andsupports are dissolved in a homogeneous solution with a precipitating agent. These sup-ported metal oxides are formed in a single step mixing with high metal oxide loadings up to70 wt/wt%. However, the supported metal oxide materials often have pore size and shapewith undesirable textures. Furthermore, with this process it is difficult to obtain homoge-neous dispersion as the two components are simultaneously precipitated. Thus, variousfactors, including temperature gradients, insufficient mixing, concentration gradients, andpH, need to be adjusted in this process [36, 40].

Karakaya et al. [166] introduced a combined approach of lyotropic liquid crystal (LLC)mesophase concept and evaporation induce self-assembly (EISA) to generate a mesostruc-tured metal oxide-silica film. In this method, the authors used two different types of surfac-

29

tants including CTAB as ionic surfactant and F127 or P123 as non-ionic surfactant. The ionicsurfactant was used to incorporate metal oxide precursors, while the non-ionic surfactantserved as a structure directing agent for silica. A mixture of metal oxide precursors, silica,and surfactants was adjusted to achieve LLC mesophases. Then, the final mesoporous mate-rials are formed during EISA of the above mixture. In addition, metal salts with low meltingtemperature were used as metal oxide precursors in order to obtain metal oxide nano-islandson the mesopore walls of the mesoporous silica as shown in Fig. 2.12 (right).

Impregnation

Impregnation is related to the nanocasting method. Instead of removing mesoporous sup-ports after thermal treatment, the obtained metal oxides are kept inside the mesopores. Thisprocess is quite simple, but it is difficult to get a uniform structure. The target metal oxidesare densely packed in these pores. The distribution of metal oxides is depending not onlyon the nature of the precursors and supports, but also on the synthesis conditions. For ex-ample, in order to achieve a high loading and uniform structure (e.g. egg-shell distributionas shown in Fig. 2.9), the interaction between precursors and supports should be enhanced.In addition, the drying process should be carried out at low temperature and long time. Insome cases, vacuum is applied for impregnation and drying [163].

Figure 2.12. Representation of self-assembly of surfactants-salt-silica species (liquid crystalline andsoft matter; red domains are silica; stars are salts pieces, left) and meso SiO2-metal oxides- thin film(after calcination, right) [166]

30

Deposition-precipitation

In this method, metal precursors are dissolved in an appropriate solvent with pH adjust-ment in order to achieve a complete precipitation of metal hydroxides. Thus, these hydrox-ides are subsequently deposited on the surface of supports. The target materials are formedafter calcination. This method has been applied for several supported metal oxide materialssuch as molybdenum, iron, nickel and copper oxides with high loading and uniformity [40].However, the final materials are often containing nanoparticles that are intimately connectedtogether. Hence, the supported metal oxides could sinter and rapidly deactivate under reac-tion conditions [152, 173, 174].

As mentioned above, the catalysts supported in ordered mesoporous materials have severaladvantages for the AC ammoxidation to ACN. In order to obtain the supported catalysts,two main methods can be used and each method has different benefits. A first series of cat-alyst, bismuth-molybdate supported in mesoporous silica KIT-6 was prepared by impreg-nation method. The popular precursors for preparing bismuth molybdate include bismuthnitrate (Bi(NO3)3 · 5 H2O) and ammonium molybdate tetrahydrate ((NH4)6Mo7O24 · 4 H2O).For the impregnation methods (wet-impregnation, wetness impregnation, and dual-solvent),they require a solvent that can dissolve the above two precursors. However, these two pre-cursors are only possible in different solvents. Acidified water (pH < 1) is the only mediumwhich can dissolve both precursors. This could be too acidic for the support. Therefore,the solid-liquid method seems to be the only technique for synthesizing bismuth-molybdatecatalysts. There is another problem due to the high melting point of (NH4)6Mo7O24 · 4 H2O.Therefore, none of the four major impregnation techniques mentioned above could be ap-plied for preparing the bismuth molybdate catalysts.

Much efforts currently devoted to synthesize bismuth-molybdate catalysts based on the ini-tial method [155] that was successful with low melting point precursors. Our oxide pre-cursors were first mixed well by mechanical technique (in mortar), so that the high meltingpoint precursor ((NH4)6Mo7O24 · 4 H2O) were brought into the template pores together withthe low melting point precursor (Bi(NO3)3 · 5 H2O). In addition, this method can create thebismuth molybdate mixed phases instead of single ones.

The other series of catalyst is molybdate-antimonate supported in mesoporous silica, thatwere prepared in one-step, co-precipitation. Our method was developed from the Karakya’smethod [166] and this could be applied for both low and high melting point of the precur-sors. In addition, the dispersion of the oxides on the support was related to electrostaticinteraction instead of the precursor melting point.

31

Chapter 3

Experimental methods

3.1 Catalytic test

In this research, the catalysts for AC ammoxidation are loaded into a fixed-bed quartz reactor(inner diameter and length are 8 mm and 420 mm, respectively) that is connected on-linewith a gas chromatograph (GC, HP 5890). The reactor system is shown in Fig. 3.1.


O2 → CH2−−CH−CN + 2H2O (3.1)

1

GC, TCD

detector

T

Quartz reactor

Furnace

MFCN2

Air

NH3

P

MFC

Vent-out

N2

SolventMFC

MFC

Sample loop

Vent-out HeV5

V3

V1

V4

V2AC

MFC

Figure 3.1. Schematic diagram of the catalytic ammoxidation reactor system. MFC: mass flow con-troller, P: pressure gauge, T: Thermocouple, V2: two way ball valve, the other valves: three way ballvalves

33

AC ammoxidation reaction is described by Eq. 3.1. However, AC (in liquid state) reacts im-mediately with NH3 to create polyacrolein (yellow color and high viscosity), which becomessolid after contacting with air. Furthermore, in presence of ammonia, ACN is easily trans-formed to polyacrylonitrile. These two polymers are difficult to be digested or removed byusual solvents such as water, alcohol, or acetone, causing blocking of the system. Therefore,avoiding the formation of these polymers is the first requirement to stabilize the system. Sev-eral techniques are applied to solve the above problem. In the first technique, acrolein anda mixture of air, ammonia, and nitrogen, are fed to the reactor through two separated inlets.In the first inlet, vaporized AC (95 % aqueous solution) at 0 ◦C is brought to the reactor by25 cc/min of nitrogen. The second inlet contains other reactants including air, ammonia andnitrogen diluent. In the second technique, the system lines (red-lines) are heated at 180 ◦Cin order to prevent forming of the polymers. On the other hand, three way valves , namely(V3), (V4), and (V5), are used to reduce risk for the sample-injection six port valve becauseof its very small inner diameter. Thus, the out-let of the reactor can be passed through twodirections: (i) (V3), then vent-out at (V4). At the same time, N2 goes to (V5), then vent-outat the six port valve; (ii) (V3) to (V4), then to six port valve, and inject to GC. During thecatalytic test, steps (i) and (ii) are repeated for 20 and 10 min, respectively. Therefore, thesystem is always cleaned by N2 flow for 20 min after each injection of the products in theGC.

After the catalytic test, the system is cleaned completely in three steps. During 1 h for coolingdown the reactor from reaction temperature (350-500 ◦C) to room temperature, N2 gas (70cc/min) goes to (V2), then passes to (V3), vent-out at (V4). By the way, N2 gas with (V5)direction is still flown over the six port valve. After removing the catalyst, 10 cc/min of N2

brings water, then methanol (each solvent flows in the above direction for 30 min). This stepaims at removing the remaining AC and to clean the out-let line between reactor-out and(V3), that could not be done during the catalytic test. In the final step, the solvent (water,then methanol) fed with the from syringe pump is passed to (V3), (V4), and vented-out atthe six port valve for 3 h.

The reactant and product compositions are analyzed by a GC equipment with a HayesepP (C1) and molecular sieve 13X (C2) columns and a TCD detector. The Hayesep P columnoperates at 115 ◦C and 30 psi of He with the different retention times of air, COx, NH3, H2O,MeOH, ACE, AC, and ACN being 0.25, 0.34, 0.79, 0.79, 4.34, 5.15, 6.85 (min), respectively.NH3 and H2O under the above conditions have however same retention time. Therefore,they should be separated at 50 ◦C and 30 psi of He to have retention time of air, NH3, andwater as 0.65, 2.58, 5.05 (min), respectively. In molecular sieve 13 X column at 50 ◦C and30 psi of He, the retention time of O2, N2, and CO are 0.73, 1.12, 2.4 (min), respectively. Inorder to use both columns with only one detector, we set the columns as shown in Fig. 3.2.A metering valve is set in another position of the six port valve and is run in parallel with

34

1

GC, TCD

detector

C1TCD

Metering valve

Sample

C1: Haysep P column (it can separate air, CO2, CH3OH, NH3, H2O, C3, ACE,

AC, ACN)

C1: molecular sieve 13X (it can separate N2 and O2)

TCD: thermal conducting detector

Figure 3.2. Position of columns in GC. C1: hayesep P column , C2: molecular sieve 13X column, TCD:thermal conductivity detector; position 1: C1-C2-TCD; position 2: C1-metering valve-TCD

molecular sieve 13X in order to maintain a constant pressure on the TCD. The temperatureof two columns is controlled by a program (1 min at 50 ◦C, then increasing to 115 ◦C at ramprate 10 ◦C/min). After 1 min, the six port valve is switched from position (1) to position (2)to make sure that only air is entered to C2. When the C1 separation process is finished, thesix port valve is turned back to position (1) for separation of O2 and N2. During catalytictest, only the Hayesep P is used at 115 ◦C and 30 psi of He.

The effects of contact time, temperature, and molar ratio on the catalytic properties werecarefully tested outside the flammability regions (see detail in supporting information ofChapter 4). A carbon balance is calculated based on all detected products including car-bon dioxide, acrolein, acetonitrile (ACE), and acrylonitrile. The other possible byproducts,including HCN, carbon monoxide and acrylic acid, could not be detected. This is advan-tageous because the purification process will be simple and economic. The reported con-version and selectivity values (calculated as Eq. 3.2 and Eq. 3.3, respectively) are obtainedby averaging from three to six measurements made at carbon balance (Eq. 3.4) higher than94-95 %.

Conversion(%) =nAC,in − nAC,out

nAC,in∗ 100 (3.2)

Selectivity(%) =nACN

nAC,in − nAC,out∗ 100 (3.3)

Carbon balance(%) =3nACN + 3nAC,out + 2nACE + nCO2

3nAC,in∗ 100 (3.4)

35

NH3/He or Ar (TPD, ammonia chemisorption)

Cold trap

Figure 3.3. Scheme of temperature programmed chemisorption apparatus, TCD-thermal conductingdetector

3.2 Ammonia temperature programmed desorption and pulsechemisorption

Ammonia temperature programmed desorption (TPD-NH3) is a popular method for charac-terizing acid sites based on the amount of desorbed ammonia. The TPD-NH3 system (XRM-100) is simplified as shown in Fig. 3.3. Metal oxide catalysts are pretreated in air (25 cc/min)at 300 ◦C for 1 h in order to remove humidity and coke on the surface. Then, 25 cc/min of

Figure 3.4. Pulse ammonia chemisorption using loop of 250 µl

36

argon is passed to the powders for 30 min at 300 ◦C before being cooled down to 100 ◦C.The catalysts are then contacted with flowing ammonia (20 vol% in argon) at 100 ◦C. Ar-gon is then flowed to the samples for several hours to remove physisorbed ammonia. Theamount of desorbed ammonia on stream from 150 to 850 ◦C at a heating rate of 5 ◦C/min ismeasured by a TCD. During this test, the temperature of a cold trap is maintained at about-50 ◦C to avoid water to reach the TCD. This cold trap contains a mixture of dry-ice andethanol.

In ammonia chemisorption tests, the above pretreatment process is performed on the cat-alyst. An Ar flow (10 cc/min) brings a certain amount of ammonia from a sampling loop(loop volumes ranging from 100 to 250 µl) to the catalytic reactor at different constant tem-peratures up to 450 ◦C. This process could directly allow measuring the amount of ammoniathat is adsorbed on or reacted with the catalysts.

For example, five injections of a given amount of ammonia are shown in Fig. 3.4. By com-paring the TCD detector signal area after the passage of a pulse (black-line) with the signalobtained in absence of catalyst (red-line), the amount of chemisorbed ammonia on the cata-lyst could be measured.

37

Chapter 4

Ammoxidation of acrolein toacrylonitrile over bismuth molybdatecatalysts

Nguyen Thanh-Binh1, Jean-Luc Dubois2, Serge Kaliaguine∗1

1Department of Chemical Engineering, Laval University, 1065, avenue de la Médecine, G1V0A6, Québec, Canada

2ARKEMA, 420 Rue d’Estienne d’Orves, F-92705 Colombes, France

Applied Catalysis A: General 520 (2016) 7-12

Résumé

Le présent travail porte sur un processus potentiellement significatif convertissant l’acro-léine (AC) d’origine verte en l’acrylonitrile (ACN) en utilisant des catalyseurs mésoporeuxà base de molybdate de bismuth. Les catalyseurs d’ammoxydation ont été caractérisés parla physisorption de l’azote, la diffraction des rayons X, et des essais catalytiques dans diffé-rentes conditions de température de réaction, de temps de contact et de rapports molairesdes réactifs. Les résultats obtenus ont indiqué que l’activité catalytique était proportionnelleà la surface spécifique, et dépendait des phases de molybdate de bismuth et de la concentra-tion d’oxygène dans l’alimentation de gaz. La sélectivité en ACN des catalyseurs dépendaituniquement de la température de réaction. La sélectivité d’ACN obtenue à 350-400 ◦C étaitde 100 % et se réduisait à 97 % à 450 ◦C.

39

Abstract

The present work deals with a potentially significant process converting acrolein (AC) ofgreen origin to acrylonitrile (ACN) using mesoporous bismuth molybdate catalysts. Theammoxidation catalysts were characterized by N2 physisorption, X-ray diffraction, and cat-alytic tests under various conditions at different temperatures, contact times, and reactantmolar ratios. The results indicated that the catalytic activity was proportional to the spe-cific surface area, and depended on the bismuth molybdate phases, and the concentration ofoxygen in the gas feed. The ACN selectivity of the catalysts only depended on the reactiontemperature. The ACN selectivity obtained at 350-400 ◦C was 100 % and reduced to 97 % at450 ◦C.

4.1 Introduction

Acrylonitrile (ACN) is a key monomer of the polymer industry. The global market of ACNis about 6 Mt in 2015 and expected to reach 8 Mt in 2020 [1, 2, 175]. However, the majorsource of ACN is propylene or propane from fossil origin [8]. Current research trends fora greener chemical industry is based on using platform molecules of biological origin suchas glycerol. This tricarbon trialcohol is a 10 wt% by-product of biodiesel production. Asreported in [12] the annual production of biodiesel increases steadily and reached 27 Mtin 2015. A process was developed by Dubois [22, 176] and Takaaki and Minoru [177] toconvert glycerol to ACN using direct or indirect processes. For the indirect process, the firststep is dehydration of glycerol to acrolein (AC) over acidic catalysts such as WO3/TiO2 orFePO4 [22, 100]. The second one is the ammoxidation of acrolein to acrylonitrile using mixedmetal oxides such as bismuth molybdate or antimonate catalysts. For the direct process, bothabove steps were combined by using catalysts comprising Sb, Nb, and V [100]. This directprocess was also previously examined by Guerrero-Pérez et al. both in gas [17–19] and liquidphases [20]. Most importantly the indirect process allows controlling the water content inthe acrolein ammoxidation reactor. This is done by using a condenser between the tworeactors. A strict control of high water to ammonia ratio suppresses the risk of acrylonitrilehydrolysis to acrylic acid. The indirect method yields an easier control of temperature andhigher ACN yields. Therefore, the indirect process seems preferable for the synthesis ofACN from renewable glycerol.

Only a few references discussed ammoxidation of acrolein to acrylonitrile [15, 16, 97–99,101, 178, 179]. Mixed metal oxides based on molybdates, antimonates, tin oxide were usedas catalysts for ammoxidation of acrolein. The results showed that the ammoxidation ofacrolein is faster than ammoxidation of propylene over the same catalysts. The reaction wascarried out at temperatures around 300 to 500 ◦C under gas feeds including AC, ammonia,oxygen, and nitrogen. The results showed that molybdate catalysts are better catalysts than

40

antimonate ones. The catalysts have however really low specific surface area of about 1-4m2/g. Recently, Hoelderich’s group researched on this reaction [15, 100]. SbVO, SbFeO,and MoO3 were investigated for ammoxidation of AC to ACN. The highest ACN yield washowever only 40 %. This study was conducted in the presence of water vapor and at low ACpartial pressure. They demonstrated that higher specific surface area catalysts yield higheractivity. Hence, in the present work simple catalysts with high specific surface area werestudied.

The bismuth molybdate catalysts are the simplest and most promising candidates for am-moxidation of acrolein. In general, the two major factors to achieve a good heterogeneouscatalyst for ammoxidation of acrolein are a large active surface area and the right composi-tion yielding not only high activity but also high selectivity [8, 180, 181]. High surface areaoxidation catalysts are prone to overheating and require special reactor design to enhancelocal heat transfer. In 1972, Batist et al. reported several ways of synthesizing of Bi-Mo-Obased on Bi(NO3)3 · 5 H2O, (NH4)6Mo7O24 · 4 H2O, and H2MoO4 and the method to avoidprecipitation of the two precursors by using strong acid HNO3 [182]. Recently, the bismuthmolybdates were obtained by several methods such as combining complexation and spraydrying [183, 184], surfactant assisted hydrothermal treatment [185], hydrothermal treatment[186], reflux and solid state [187], and hard-templating [188]. However, the specific surfacearea of the bismuth molybdate catalysts are still low (most specific surface areas are around2-4 m2/g). Therefore, preparing bismuth molybdate with high surface area requires atten-tion. On the other hand, the different phases of bismuth molybdate also show differenteffects on catalytic performance [8, 55]. Bismuth molybdates have three phases including α

(Bi2Mo3O12), β (Bi2Mo2O9), and γ (Bi2MoO6). For ammoxidation of propylene, the α phaseshows good NH3 activation and α-H abstraction. The γ phase is good for the reoxidationprocess. Finally, the β phase combines α and γ structures, so this phase is the most effectivefor ammoxidation of propylene. No report has however described the effect of the phase onthe ammoxidation of acrolein to acrylonitrile yet.

Metal oxides supported on mesostructured silica are widely applied in the catalysis fieldbecause of large specific surface area, high pore volume, large pore size, and high stability[102, 189]. The bismuth and molybdate precursors precipitate from their solutions uponmixing, so that a suitable method should be considered. Several reports indicated that in astrong acid, base, or in glycerol, the mixture remains homogeneous [182, 190, 191]. However,strong acids or bases will change silica surface properties. In addition, glycerol has highviscosity, making it difficult for the precursors to diffuse into the pores of the mesoporoussilica. Yen et al. synthesized mesostructured metal oxides using a hard-templating techniquebased on a dual-solvent and solid-liquid impregnation method [155]. A non-polar solvent(heptane in our work) was used for pre-wetting, so as to reduce surface tension (surfacetensions of solid-gas are larger than surface tensions of solid-liquid) [155, 192]. Therefore,

41

metal salts can be easily transported inside the pores of the mesoporous silica. In addition,using this method in absence of water will reduce precipitation of precursors.

Hence, the effect of surface area and the phases of the bismuth molybdate catalysts could bestudied for ammoxidation of acrolein to acrylonitrile by using Yen et al. one-step impreg-nation catalyst preparation method. The catalysts including supported on mesoporous 3DKIT-6 and non-supported bismuth molybdate will be investigated in this work.

4.2 Experimental

4.2.1 Synthesis of bismuth molybdate catalysts

Ordered mesoporous silica KIT-6 with large specific surface area, large pore size (8.1 nm),and 3D mesopore connectivity structure, synthesized, at 100 ◦C aging temperature, accord-ing to a previous report [120] were used as support. The catalysts designed as differentmixtures between Bi2O3 and MoO3 were synthesized based on the method developed byYen et al. [155]. Specifically, the mixtures Bi2O3.nMoO3 with n= 1, 2, and 3 supported onKIT-6 were designated as n1, n2, and n3, respectively. First, 1.25 g evacuated KIT-6 waspre-wetted by n-heptane, and then pre-mixed with 0.94 g of the precursors including calcu-lated weights of bismuth nitrate Bi(NO3)3 · 5 H2O and ammonium molybdate tetrahydrate(NH4)6Mo7O24 · 4 H2O. The mixture was transferred to a round bottom flask and heated at85 ◦C overnight. The solids were filtered and dried at 50 ◦C, and then calcined at 550 ◦C for3 h in air. In order to compare supports, catalyst C3 was synthesized using commercial po-rasil silica (Milipore Corporation, 34 Maple street, Milford, MA 01757) as support and sameelemental composition as catalyst n3 active phase.

In preparing the non-supported catalyst a similar impregnated silica was prepared and theKIT-6 silica support was then removed by digestion in 2M NaOH solution at room tem-perature (three times over one day). A total amount of 4.5 g of the mixed precursors wasimpregnated on 1.25 g of KIT-6. The resulting solids were filtered and calcined at 550 ◦C for3 h in air before silica removal. The final bismuth molybdate mixed oxide was then washedfor several times in water and aqueous ethanol and dried at 100 ◦C.

4.2.2 Characterization

N2 physisorption isotherms at 77 K were measured using a Quantachrome Nova 2000 seriesinstrument. The samples were preliminarily degassed in vacuumn at 150 ◦C for 6h. Specificsurface area of the catalysts was calculated using the linear part of the BET plot (0.05-0.2 inrelative pressure). The pore size distribution is calculated from the adsorption branch follow-ing the NLDFT (non-local density functional theory) method for cylinder shape. The porevolume is taken as the adsorbed nitrogen volume at 0.99 relative pressure. Wide-angle X-ray

42

diffraction (XRD) analysis was performed with a Siemens 80 Model D5000diffractometer us-ing CuKα radiation (λ=0.15496 nm). In addition, the catalysts phases were monitored by Ra-man spectroscopy LABRAM HR800 (Horiba Jobin Yvon, Villeneuve d’Ascq, France) coupledwith an Olympus BX30 fixed stage microscope using Ar+ laser (514.5 nm) as an excitationlight source (Coherent, INNOCA 70C Series Ion Laser, Santa Clara, CA). The chemical com-position of the samples was established by Inductively Coupled Plasma-Optical EmissionSpectroscopy (ICP-OES) using a Perkin Elmer Optima 4300DV spectrometer.

4.2.3 Catalysts test

50 mg of the catalyst was loaded in a fixed-bed quartz reactor (inner diameter and length 8mm and 420 mm, respectively) at atmospheric pressure, on-line connected with a gas chro-matograph (GC, HP 5890). The low catalyst mass allows avoiding hot spots. The reactantsand products were analyzed using a TCD detector and Hayesep P and molecular sieve 13Xcolumns. The reactor system is shown in Fig. 3.1. Gas feeds including acrolein, air, am-monia, and nitrogen enter the quartz reactor through two separate inlets in order to avoidpolymerization at the contact between ammonia and acrolein. One feed comprises air, am-monia (0.47 cc/min), and nitrogen diluent. Acrolein is vaporized from a 95 % aqueous so-lution at 0 ◦C. The acrolein vapor diluted in 25 cc/min nitrogen is fed separately. In orderto investigate the effect of total flow rate (contact time) on catalytic reaction, reactant molarratio (AC/NH3/O2) was fixed whereas the diluent nitrogen flow rate was varied. All inletgas compositions were selected outside of the flammability region (see Figs. S4.1 and S4.2in supporting information). On the other hand, studying the effect of reactant molar ratioon catalytic performance, the flow rates of air and diluent nitrogen were changed to keeptotal flow rate constant (detail information in Table 4.2). In order to avoid condensation ofpolyacrolein and polyacrylonitrile, the system lines are heated (red-lines) to 180 ◦C and sev-eral three way valves (V1, V3, V4, V5) were used. After each catalytic test, methanol is flownthrough part of the system for cleaning. In order to protect the GC sampling loop, the reactorexhaust is send to the vent using valves V3 and V4 between sampling.

A carbon balance was calculated based on all detected products including carbon dioxide,acrolein, acetonitrile (ACE), and acrylonitrile (HCN, acrylic acid, and carbon monoxide wasnever detected). The reported conversion and selectivity values were obtained by averaging3-6 measurements made at carbon balance better than 94-95%. Their catalytic propertieswere calculated using the Eqs. (3.2, 3.3 and 3.4) that were described in Chapter 3.

4.3 Results and discussion

The Mo/Bi atomic ratio of the catalysts was established by ICP. The results are displayed inTable 5.1. They show Mo/2Bi ratios rather close to the target values.

43

4.3.1 N2 physisorption

0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 00

1 0 02 0 03 0 04 0 05 0 06 0 07 0 0

+ 5 0

Adso

rptio

n volu

me (c

c/g)

R e l a t i v e p r e s s u r e ( P / P 0 )

K I T - 6 n o n - s u p p o r t e d n 3 n 2 n 1

+ 1 0 0

Figure 4.1. N2 physisorption isotherms of the catalysts and KIT-6 support

44

Table 4.1. Texture properties and structure of the catalysts

CatalystsCatalyst

componentsSBET

(m2/g)Pore size

(nm)Pore volume

(cc/g) XRD Mo/2Bi∗

SiO2 269 13.9 1.04KIT-6 823 8.1 1.09

non-supported Bi2O3 · 2 MoO3 17 7.0 0.13 α + γ 1.57n1 Bi2O3 ·MoO3/KIT−6 389 7.6 0.61 γ 0.82n2 Bi2O3 · 2 MoO3/KIT−6 360 7.6 0.62 γ 1.64n3 Bi2O3 · 3 MoO3/KIT−6 378 7.6 0.63 α + γ 2.48C3 Bi2O3 · 3 MoO3/SiO2 183 13.9 0.73 α + γ 2.42

n3(used)O2/AC=16.5O2/AC=0.5O2/AC=0

Bi2O3 · 3 MoO3/KIT−6330280270

7.37.07.0

0.530.500.50

α + γγ+MoO2+Bi

MoO2+Bi2O3+Bi

∗ determined by ICP

45

0 5 1 0 1 5 2 0

0 . 0 0

0 . 0 1

0 . 0 2

0 . 0 3

0 . 0 4

0 . 0 5

- 0 . 0 0 1

+ 0 . 0 0 1 5+ 0 . 0 0 1 2

dV/dD

(g/cm

3 .nm)

P o r e w i d t h ( n m )

K I T - 6 n o n - s u p p o r t e d n 3 n 2 n 1

+ 0 . 0 0 9

Figure 4.2. NLDFT pore size distributions (adsorption branch) of the catalysts and KIT-6 support

As shown in Table 5.1, the specific surface area of the supported catalysts are the highestreported so far [183–188]. The hysteresis loops of the supported catalysts (n1-n3) are similarto the hysteresis loop of KIT-6 as presented in Fig. 4.1. This indicates that the pore structureof the supported catalysts are similar to the bi-continuous pore structure of KIT-6. The sup-ported catalysts have however no micropore and the mesopore size was shifted to slightlysmaller sizes compared to the KIT-6 template as shown in Fig. 4.2. This data indicates thatmetal oxides fill all micropores and cover the mesopore walls KIT-6. As discussed above,specific surface area (SSA) plays a key role for catalysts performance. Hence, the n1-n3 andC3 (Fig. S4.4) catalysts are suitable candidates for ammoxidation of acrolein to acrylonitrile.There are not much SSA difference between the supported catalysts. On the other hand, thenon-supported catalyst has low SSA and pore volume. After the silica template removal,the non-supported catalyst has crystal domain size of about 15 nm (based on X-ray diffrac-tion data). The pores of the non-supported catalyst are likely to be external pores betweennanoparticles.

46

4.3.2 X-ray diffraction

X-ray diffraction patterns of the catalysts are shown in Fig. 4.3. The non-supported, n3, andC3 catalysts have α (JCPSD 21-0103) and γ (JCPSD 72-1524) phases. The n2 and n1 catalystshave only γ phase. The phase composition of the catalysts was confirmed by Raman spec-troscopy (Fig. S4.3) showing same result of X-ray diffraction. Le at al. showed that obtainingthe three phases of bismuth molybdate depends not only on the weight ratio of the precur-sors but also on calcination temperature [193]. For example, α phase is created at 500-650◦C, whereas 600-660 ◦C is required for β phase, and 500-600 ◦C for γ phase. As mentionedabove, the β phase would be the best performing catalyst, but it is also known to be toofragile under ammoxidation conditions [8, 55]. It may thus be expected that the n3, C3, andnon-supported catalysts would perform better in the ammoxidation of acrolein.

1 0 2 0 3 0 4 0 5 0 6 0

∗ α p h a s e♦ K o e c h l i n i t e ( γ)

n o n - s u p p o r t e dC 3n 3n 2

♦

♦

♦

♦♦

♦♦

♦♦♦♦♦

∗∗∗∗∗∗Co

unts

∗♦

n 1

2 - T h e t a

Figure 4.3. X-ray patterns of the catalysts

47

3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 00

1 02 03 04 05 06 07 08 09 0

1 0 0

ACN selectivity (%)

T e m p e r a t u r e ( o C )

A C N w i t h o u t N H 3 A C N w i t h N H 3

0

2

4

6

8

1 08 6 8 3 8 1

4 65 5

6 8

Non-c

onve

rted A

C (%

)

A C w i t h o u t N H 3 A C w i t h N H 3

8 0

0

2

4

6

8

1 0

Figure 4.4. AC non-converted and ACN selectivity in blank tests run absence of catalyst, TOS=1.5 h

4.3.3 Catalytic tests

Prior to catalytic tests, the blank experiments were conducted to assess thermal stability ofAC in absence of ammonia or in its presence. Both kinds of tests were performed from300-550 ◦C. They were run in the reactor setup shown in Fig. 3.1 in absence of catalyst(substituted with quartz wool) at the same flow rate as the catalytic tests. In absence ofammonia, the feed gas composition was AC/ NH3/ O2/ N2 = 1/ 0/ 12/ 160 and in presenceof ammonia 1/ 1.25/ 12/ 158. In each case the blank test results shown in Fig. 4.4 wereobtained at steady state after about 1.5 h on line.

In the absence of ammonia, AC underwent some combustion (mostly above 500 ◦C, about5-8 %) and no ACN was formed. This first test confirmed the thermal stability of AC evenin the presence of oxygen. In the presence of NH3, below 450 ◦C, some AC was reacted (<20 %) which gradually increased to 54 % at 550 ◦C. AC was mostly converted above 450 ◦Cyielding large amount of water vapor likely associated with AC oligomerization under thecatalytic activity of gaseous NH3 or polymerization involving AC and NH3. The possibleoligomerization/ polymerization products were undetected in the GC analysis [194, 195].

The effect of temperature on catalyst activity was tested from 300 to 450 ◦C (Fig. 4.5). At

48

300 ◦C, however the carbon balance was low (about 60-80 %). There are two reasons for thisphenomenon: (i) coke condenses on catalyst surface, (ii) un-known products that cannot beseparated by Hayesep P column or yield very small peaks, are obtained under these condi-tions. Conversion of AC continuously increases with temperature. As shown in supportinginformation, ACN selectivity decreases from 100 % to 96-98 % when the temperature risesup to 450 ◦C and acetonitrile (ACE) and CO2 are the only by-products (Table S2). The non-supported catalyst yields only 5 % AC conversion at 450 ◦C and no conversion of AC attemperature 350 ◦C. The n1 and n2 catalysts show similar trends for conversion and selec-tivity variation with temperature. AC conversion varies from 13 to 39 % and 18 to 33 % withn1 and n2, respectively. C3 catalytic activity is better than those of n1,n2, and non-supportedcatalysts. However, its activity is significantly lower than that of n3 catalyst, which has highAC conversion increasing from 30% to 84 % as temperature increases from 350 to 450 ◦C.This catalyst n3 yields the highest conversion and selectivity compared to the other catalysts

3 5 0 4 0 0 4 5 00

1 02 03 04 05 06 07 08 09 0

Conv

ersion

(%)


n o n - s u p p o r t e d n 3 n 2 n 1 C 3

Figure 4.5. Influence of temperature on catalytic activity at reactant molar ratio AC/ NH3/ O2/ N2= 1.0/ 1.25/ 16.5/ 158.3, F = 66 cc/min, TOS = 3.5 h

49

reported here or in literature [15]. For example, the best SbFeO catalyst reported in [15, 100]shows an ammoxidation conversion of 80 % at 400 ◦C and 0.11 s contact time, but a selec-tivity of 30-40 % to ACN. Using catalyst n3 we observed a 84 % conversion at 0.16 s contacttime (calculated as 1/GHSV) but the selectivity was 97-98 %. The results shown in Fig. 4.5indicate that both surface area and bismuth molybdate phase play key roles for catalyticactivity.

The flow rate or space velocity affects directly catalytic conversion and selectivity as well.As expected, increasing contact time will decrease selectivity and increase conversion. Theresults shown in Fig. 4.6 were obtained at 400 ◦C and reactant molar ratio AC/ NH3/ O2 =1.0/ 1.25/ 16.5 (oxygen being fed as air). The AC conversion reduces gradually with increas-ing flow rate from 56 cc/min to 76 cc/min but the ACN selectivity remains 100 % (Table 4.4).This result indicates that the reaction is limited to the primary reaction with no secondaryACN conversion (Eq. 5.1). At 400 ◦C, the selectivity is not dependent on contact time. As

5 6 6 0 6 4 6 8 7 2 7 60

1 02 03 04 05 06 07 08 09 0

Conv

ersion

(%)

F l o w r a t e ( c m 3 / m i n )

n o n - s u p p o r t e d n 3 n 2 n 1

Figure 4.6. AC conversion as a function of flow rate at 400 ◦C and molar reactant ratio AC/ NH3/O2/ N2 = 1.0/ 1.25/ 16.5/ y (y values as reported in Table 4.2), TOS=3.5 h

50

0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 2 22 03 04 05 06 07 08 09 0

Conv

ersion

(%)

M o l a r r a t i o O 2 / A C

Figure 4.7. Effect of oxygen on AC conversion at 450 ◦C and F =66 cc/min over catalyst n3

discussed above only increasing temperature to 450 ◦C affected selectivity.


O2 −−→ CH2−−CH−C−−−N + 2 H2O (4.1)

The influence of the oxygen concentration on AC ammoxidation was investigated over cat-alyst n3 at 450 ◦C and 66 cc/min total flow rate. The molar ratio of the gas feed is AC/NH3/ O2/ N2 = 1.0/ 1.25/ x/ y (see Table 4.2 for y values). Nitrogen flow rate was adjustedto keep the volume flow rate constant. Then the GHSV remains constant but the WHSVchanged slightly from 304 to 309 h−1 due to slight differences in gas density with composi-tion as molar ratio of O2/AC was varied from 0 to 21.5 (Tables 4.2 and 4.5). As shown in Fig.4.7 , AC conversion showed a sharp increase with x ranging from 0 to 2.5. It then increasedslowly when x got higher than 2.5 and it was almost unchanged with x above 9.5. The resultsindicate that oxygen concentration has a strong effect on catalytic reaction rate. Catalyst n3is still active in absence of oxygen in the gas feed and the observed conversion was constantover 3.5 h. This activity was found equal to the one obtained at stoichiometric ratio x=0.5. It

51

is therefore likely that the reaction makes use of the catalyst oxygen atoms (Mars-Van Krev-elen mechanism) and that the role of gaseous oxygen is to reoxidize the catalyst. A quickcalculation of the number of atoms of oxygen consumed over 3.5 h yields are estimate of1.4 ×10−3O atoms. The total number of oxygen atoms in the 50 mg of catalyst n3 (15 mg ofBi2Mo3O12) is estimated to 0.2 ×10−3 O atoms. It is therefore likely that the traces of waterpresent in the feed contributed to catalyst reoxidation.

It was also observed that at low oxygen content (x≤ 0.5) the color of the catalyst turnedto black over 30 min. This change may be due either to catalyst reduction or coking. Thepresence of coke is also accompanied by a decrease in SSA and pore volume of the catalystas observed experimentally (see Table 5.1). X-ray diffraction patterns of the fresh catalyst andthe ones used at different O2/AC ratios are shown in Fig. 4.8. The pattern of the fresh (n3)catalyst is same as the one obtained after a test at high amount of oxygen in feed (x=16.5).However, reducing oxygen in feed to the stoichiometric value of 0.5, bismuth molybdateswere partly reduced to MoO2 (JCPDS 76-1807) and Bi (JCPDS 44-1246). In the absence of

1 0 2 0 3 0 4 0 5 0 6 00

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

♥ ♥0 . 0

0 . 5

♥

( α)

#

♥

♥ ♥ �

�

�

∗

∗∗

∗

∗∗

♦

♦♦♦

♦♦♦

♦

♦

Coun

ts

2 T h e t a

∗♦

�♥

#

K o e c h l i n i t e ( γ )B i 2 M o 3 O 1 2 B iM o O 2B i 2 O 3

f r e s h n 31 6 . 5

Figure 4.8. Effect of oxygen in gas feed on catalyst structure. The numbers below patterns indicatemolar ratio of the O2/AC, TOS=3.5 h

52

0 2 4 6 8 1 0 1 20

2 0

4 0

6 0

8 0

1 0 0Co

nvers

ion (%

)

T i m e o n s t r e a m ( h )

Figure 4.9. Catalytic activity stability test; reactant molar ratio AC/ NH3/O2/N2=1.0/1.25/16.5/158.3, 450 ◦C, F=66 cc/min

oxygen, the solid contains only Bi, Bi2O3 (JCPDS 27-0050), and MoO2, and no mixed bismuthmolybdate phases. These results demonstrated that the mixed α and γ phases are necessaryfor catalytic activity.

The stability of the n3 catalyst was tested at 450 ◦C and 66 cc/min with molar ratio AC/NH3/ O2/ N2=1.0/ 1.25/ 16.5/ 158.3 (GHSV=22500 h−1) (Fig. 4.9). Catalyst n3 reachessteady state after about 1 h on stream. AC conversion and ACN selectivity keep constant toabout 84 % and 97 %, respectively. After 12h, n3 catalyst showed no sign of deactivation.

4.4 Conclusions

In summary, simple bismuth-molybdate supported on KIT-6 catalysts were synthesized show-ing high activity, selectivity, and stability compared to previously reported catalysts such asSbVO4, AsFeO, and SbMoO. The n3-supported catalyst with large specific surface area andmixed phases, showed the highest AC conversion (84 %) and ACN selectivity (97 %). This

53

demonstrated that the surface active and synergistic phases play a vital role for catalytic am-moxidation of AC to ACN. In addition, the reaction temperature also has some influenceon catalytic activity and selectivity. Other factors such as contact time and molar ratio ofreactants affect only reaction rate as expected.

Acknowledgments

The authors gratefully acknowledge financial support of The Consortium de Recherche enBiotechnologie Industrielle du Québec and MITACS. We also thank Dr. Hoang Vinh-Thangand Mr. Gilles Lemay for help in setting up the system and for interesting discussions.

4.5 Supporting information

Table 4.2. Reactants feed flow rates

DesignationMolar ratioAC/ NH3/ O2/ N2

Flow rate (cc/min)AC NH3 O2 N2 Total

1 1.0/1.25/16.5/131.5 0.37 0.47 6.15 49.01 56.02 1.0/1.25/16.5/158.3 0.37 0.47 6.15 59.01 66.03 1.0/1.25/16.5/185.2 0.37 0.47 6.15 69.01 76.04 1.0/1.25/0.0/174.8 0.37 0.47 0.00 65.16 66.05 1.0/1.25/0.5/174.3 0.37 0.47 0.19 64.98 66.06 1.0/1.25/2.5/172.3 0.37 0.47 0.93 64.23 66.07 1.0/1.25/5.5/169.3 0.37 0.47 2.05 63.11 66.08 1.0/1.25/9.2/165.8 0.37 0.47 3.35 61.81 66.09 1.0/1.25/16.5/158.3 0.37 0.47 6.15 59.01 66.0

10 1.0/1.25/21.5/153.3 0.37 0.47 8.01 57.15 66.0

#2 (Figs. 4.5 and 4.9) ; #1 and 3 (Fig. 4.6); #4-10 (Fig. 4.7)

54

Table 4.3. Effect of temperature on catalysts activity and selectivity

Catalyst300 ◦C 350 ◦C 400 ◦C 450 ◦C

Conv.(%)

Sel.(%)

Carbonbalance

Conv.(%)

Sel.(%)

Carbonbalance

Conv.(%)

Sel.(%)

Carbonbalance

Conv.(%)

Sel.(%)

Carbonbalance

non-supported - - - 0 - - 3 100 94 5 100 94n1 - - - 13 100 97 33 100 100 39 97* 100n2 25 12 64 18 100 99 28 100 100 33 98* 100n3 20 15 80 29 100 99 64 100 95 84 98* 102C3 - - - 30 100 97 36 100 95 46 97* 96

mcatalyst = 50 mg; F=66 cc/min; AC/ NH3/ O2/ N2 = 1.0/ 1.25/ 16.5/ 158.3; TOS = 3.5 h; *By-products include acetonitrile and CO2

55

Table 4.4. Effect of contact time on catalytic conversion

SamplesConversion (%) atflow rate (cc/min)56 66 76

non-supported 7 5 1n3 70 64 52n2 37 28 23n1 35 33 29

mcatalyst = 50 mg, temperature: 400 ◦C, AC/ NH3/ O2/ N2 = 1.0/ 1.25/ 16.5/ 158.3, TOS =3.5 h. Selectivity: 100 %; carbon balance: 95±4

Table 4.5. Effect of reactant molar ratio on catalytic activity and selectivity

Molar ratioAC/NH3/O2/N2

Conversion(%)

ACN Selectivity(%)

ACN Yield(%)

Carbon balance(%)

1.0/1.25/ 0.0/174.8 20-34 100 20-34 80-921.0/1.25/ 0.5/174.3 24-32 100 24-32 92-991.0/1.25/ 2.5/172.3 74-75 96-97 71-73 93-1021.0/1.25/ 5.5/169.3 77-79 96-98 74-76 94-991.0/1.25/ 9.2/165.8 81-83 96-98 78-80 94-1001.0/1.25/ 16.5/158.3 82-83 96-98 79-80 95-1001.0/1.25/ 21.5/153.3 83-86 96-98 80-83 95-101

Catalyst n3, mcatalyst = 50 mg, temperature: 450 ◦C, F=66 cc/min (GHSV=22500 h−1), TOS =3.5 h

The flammability limits for the ternary gas blends AC/ O2/ N2 and NH3/ O2/ N2 are re-ported in Fig. S4.1. These data are estimates for AC/O2/N2 at 250 ◦C and for NH3/ O2/ N2

at 25 ◦C, found in references [196–198]. Nothing replaces actual measurement of flamma-bility limits of acrolein-ammonia mixture. However, the Le Chatelier law could be used todetermine a good approximation of the flammability limits of the mixture. Such a diagramwould be very close to the acrolein limits. Acrylonitrile flammability limits would also fitwithin acrolein flammability limits.

The ternary diagram also shows the composition line corresponding to air as the only sourceof nitrogen and the composition line corresponding to the stoichiometric ratio AC/O2 = 0.5or NH3/ O2 = 0.5. On the left side of the latter line O2 would not be in sufficient content forcomplete AC conversion. Fig. S4.1 shows that acrolein is much more flammable than am-monia so that to be perfectly safe, the inlet gas blend should have composition lying abovethe AC flammability limit zone. Working in this composition range also allows avoidinggas phase oxidation of reactants and products. Actually, it is possible to operate within theflammable composition range, especially when using a fluidized bed reactor, since in thiscase oxygen/air and reactant are fed at different locations, thus reducing the risk of explo-sion. In fixed bed reactor, operation within flammability limits is possible provided safety

56

Figure S4.1. Flammability limits for the AC/ O2/ N2 and NH3/ O2/ N2 mixtures.

devices such as rupture disks are installed. In Fig. S4.1 graph the composition domain fornon-flammable mixtures is therefore shown in green color. Fig. S4.2 shows the upper part ofFig. S4.1 diagram with the location of the experimental inlet gas compositions used in thisstudy. All our data points lie in the non-flammable region. An even more severe test wouldbe obtained by plotting the molar fraction of (AC+NH3) on the AC scale which would re-sult in considering NH3 flammability to be as high as that of AC. Even under this drastichypothesis, our data points are still all in the safe region.

Only n3 catalyst shows some band of Raman shift at 121.5, 906.0, 929.4 cm−1 correspondingto the α phase of bismuth molybdate [199]. These Raman spectra confirm the results of X-raydiffraction indicating that n3 has mixed (α+γ) phases and n1, n2 only have the γ phase (Fig.S4.3).

57

Figure S4.2. Flammability limits; ? Our data points for (AC+NH3)/ O2/ N2, • Our data points forAC/O2/ N2

58

2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0

n 3

n 2Inten

sity (

a.u.)

R a m a n S h i f t ( c m - 1 )

n 1

Figure S4.3. Raman spectra of catalysts n1, n2 and n3

59

0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 00

1 0 02 0 03 0 04 0 05 0 06 0 07 0 0

0 1 0 2 0 3 0 4 0 5 0 6 0


Adso

rptio

n volu

me (c

c/g)

R e l a t i v e p r e s s u r e ( P / P 0 )

S i O 2 C 3

Figure S4.4. N2 physisorption isotherm and NLDFT pore size distribution of SiO2 and n3-SiO2

60

Chapter 5

Molybdate/antimonate as key metaloxide catalysts for acroleinammoxidation to acrylonitrile


1Department of Chemical Engineering, Laval University, 1065, avenue de la Médecine, G1V0A6, Québec, Canada


Catalysis Letters 147 (2017) 2826-2834

Résumé

L’acrylonitrile, un important produit chimique utilisé dans l’industrie des polymères peutêtre produit par une ammoxidation de l’acroléine, ce dernier pouvant être obtenu par unedéshydratation du glycérol. Cela constituerait une synthèse verte de l’acrylonitrile par rap-port aux pratiques industrielles actuelles, qui impliquent une ammoxydation du propylène(ou du propane) d’origine fossile. Traditionnellement, les catalyseurs à base d’antimonate etde molybdate sont utilisés dans l’ammoxydation du propylène à l’acrylonitrile, et devraientégalement être actifs dans la conversion de l’acroléine. Dans ce travail, nous rapportons uneméthode simple pour synthétiser une série de catalyseurs mixtes antimonate/molybdate àdifférents rapports molaires supportés sur les silices mésostructurées avec des porosités etdes surfaces spécifiques élevées. Les résultats obtenus indiquent que les oxydes de molyb-dène ont joué un rôle majeur pour l’ammoxydation de l’acroléine par rapport à celui desoxydes d’antimoine. Les conversions d’acroléine et les sélectivités d’acrylonitrile ont été ré-duites avec une augmentation de la fraction d’oxyde d’antimoine. Les catalyseurs obtenus

61

ont été caractérisés par la physisorption d’azote, la diffraction des rayons X, l’analyse thermogravimétrique, l’absorption atomique, la spectroscopie de photoélectrons aux rayons X, lamicroscopie électronique à transmission et les essais catalytiques.

Abstract

Acrylonitrile, a large tonnage chemical used in the polymer industry may be produced by anammoxidation of acrolein, in which the latter being possibly obtained by a glycerol dehydra-tion. This would provide a green synthesis of acrylonitrile as compared to the present indus-trial practices, which involve an ammoxidation of propylene (or propane) with fossil origin.Traditionally, the antimonite and molybdate based catalysts are used in the ammoxidationof propylene to acrylonitrile, so that should be also active in the acrolein conversion. Inthis work, we report a simple method for synthesizing a series of the mixed antimonite andmolybdate catalysts at different molar ratios supported on the mesostructured silicas withhigh porosities and specific surface areas. The obtained results indicated that the molyb-denum oxides played a major role for the ammoxidation of acrolein as compared to that ofthe antimonyoxides. The acrolein conversions and acrylonitrile selectivities were reduced atincreasing the fraction of antimony oxide. The obtained catalysts were characterized by N2

physisorption, X-ray diffraction, thermal gravimetric analysis, inductively coupled plasma,X-ray photoelectron spectroscopy, transmission electron microscopy, and catalytic tests.

5.1 Introduction

Acrylonitrile (ACN) is a key monomer used in the polymer industry, with large application.Nowadays, ACN global production is about 6 million tons per year and is predicted to in-crease in near future [1, 2]. The traditional fossil carbon source of ACN [8] is propylene andfinding a greener production pathway would be a significant step toward a greener polymerindustry.

Some recent works have dealt with ammoxidation of acrolein (AC) based on glycerol toACN [12]. The conversion of glycerol to ACN may be effected by two different pathways: (i)direct and (ii) indirect ammoxidation. The direct ammoxidation uses a two-in-one catalystfor dehydration of glycerol to AC and ammoxidation of AC to ACN [17–19, 22, 176]. Thisprocess still has the remaining drawbacks of difficulty to control reaction temperature andlow ACN yield. In the indirect path way, a first step of glycerol dehydration to AC is carriedout at between 250-300 ◦C. The second step of ammoxidation of AC to ACN is, however,run at 350-450 ◦C. Controlling temperature for the two-in-one catalyst process is difficult.In addition, catalysts for these two steps exhibit different properties. Hence, the indirectprocess is considered more interesting in terms of yield and technical problems [15, 16, 31,97–99, 101, 178, 179].

62

Some research has demonstrated that AC was an intermediate product in ammoxidation ofpropane or propylene to ACN [85, 90, 91, 200]. This explains why traditional catalysts basedon molybdate or antimonate have been used for AC ammoxidation to ACN. The literature onAC ammoxidation to ACN may be divided in two different eras. The work reported priorto 1975 show some examples of successful catalysts. British Distillers company patentedthe use of MoO3 and silica-supported MoO3 reaching up to 80 % ACN yield [101]. BritishDistillers moreover proposed the use of tin antimonate (SnSbO) catalysts, also showing upto 75 % ACN yield [96]. The early work of the Germain [97] and Oka [99] groups dealt withBiMoO reaching up to 70 % ACN yield whereas the group of Wragg et al. using FeBiPOmaterials [98] only obtained a maximum yield of 44 %.

More recent work from the Bañares group dealt with antimonates such as VSbO/Al2O3 andVSbNbO/Al2O3 with a maximum ACN yield of 37-52 % [18, 19]. Recently Dubois studiedthis reaction using SbFeO and SbVO and obtained up to 60 % ACN yield [21, 22]. Hoelderichet al. also investigated several catalysts including SbVO, SbFeO, MoO3, MoO3/TiO2 andMoVSbO/SiO2 with the highest ACN yield of 40 % [15, 100]. The recent work from thegroup of Dumeignil examined the possible modification of BiMoO doped with Co, Ni, Mg,Fe, K and P. The highest ACN yield was of about 65 % [16]. Thanh-Binh et al. used differentphases of BiMoO supported on mesostructured silica KIT-6 and achieved up to 82 % ACNyield [31].

Thus from prior literature it seems that antimonates and molybdates are both appropriateactive catalysts for this reaction. The comparison in terms of activity, selectivity to ACN andstability under reaction conditions are, however, not easy to make on the basis of existingliterature reports. Moreover, it seems that silica and mesostructured silica supports havebeneficial effects on the catalytic properties of these materials. Therefore, studying the roleof Sb and Mo oxides in mesoporous materials for this reaction is needed.

Mesoporous materials have been applied widely in heterogeneous catalysis because of theirhigh SSA and regular and high porosity [29]. Generally, metal oxides supported on meso-porous materials may be synthesized either by hard or soft-template. Using a hard template,it is easy to control the catalysts pore structure and to prepare highly crystalline metal/metaloxides. The hard templating method involves, however, multiple steps, while being timeconsuming and expensive. In addition, metal oxides could collapse or sinter at high reactiontemperature [30, 201]. On the other hand, the soft-templating method alleviates some draw-backs of the hard templating one. The soft-templating method is carried out in one-step andis applicable to a wide range of materials. This method however is not straightforward be-cause many factors (temperature, surfactant, molar ratio of reactants, etc.) affect the synthe-sis so that controlling pore size, shape, and morphology may be cumbersome [202, 203]. Thesoft-templating approach may also be used in one-pot condensation of a metal oxide and sil-ica yielding a simple method to produce supported oxides. This method of Stucky’s group

63

[125] allowed synthesizing various mesostructured metal oxides (supported or not) usingblock copolymers as structure directing agents. However, this was only successful with cer-tain hydrolysable metal oxide precursors and could not be applied to synthesize mesostruc-tured antimony oxides or molybdenum oxides. Grosso et al. [119] gave an overview ofevaporation-induced self-assembly to prepare silica and non-silica oxides. This method alsohas a limitation on the type of metal precursors, which should be hydrolysable. In 2011,Karakaya et al. [166] combined both above methods for synthesis of metal oxides supportedon silica. Low melting point metal nitrates were used as precursors of metal oxides. Dualsurfactants including non-ionic and ionic ones were used as structure directing agents forsilica and the metal oxide, respectively. The latter method was found appropriate for prepa-ration of mesostructured silica supported zinc and cadmium oxides, the usual precursors ofwhich are not hydrolysable.

Based on the Karakya et al. [166] synthesis using dual surfactants evaporation inducedself-assembly, a new method was developed in the present paper using both high and lowmelting point chlorides of molybdenum and antimony, respectively. In this approach, thedispersion of the final supported oxides is not related to melting point but is due to electro-static interaction of the Mo5+ and Sb3+ with the ionic hexadecyltrimethylammonium bro-mide (CTAB) surfactant. This new method is applicable to a wide range of non hydrolysablemetal oxide precursors.

5.2 Experimental

5.2.1 Synthesis of antimonate molybdate catalysts

All reagents were purchased from Sigma Aldrich and Alfa Aesar and used without purifica-tion. Two surfactants including CTAB (hexadecyltrimethylammonium bromide, 97 %) andF127 (Pluronic F-127, EO106PO70EO106) were used as structure directing agents for antimonyor molybdenum oxide and silica, respectively. Molybdenum (V) chloride (MoCl5), antimony(III) chloride (SbCl3), and TMOS (tetramethyl orthosilicate, 98 %) were used as precursorsfor metal oxides. Different molar ratios SbnMo10-nOx/SiO2 materials were prepared withthe value of n being determined by ICP (Inductively Coupled Plasma). The catalysts weredesignated as na-b (a and b refering to % atomic ratio Sb/(Sb+Mo) and wt% loading, re-spectively). Initial solutions were prepared at molar ratio CTAB/ F127/ TMOS/ HCl (37%)/ EtOH/ H2O/ MCly = 1/ 0.05/ 1.0/ 0.469/ 100.5/ 514.5/ ∼ 1.15. In a typical synthesis,0.7871 g of CTAB was mixed with the above molar ratio of F127, HCl, EtOH, and distilledwater. After obtaining a homogeneous mixture, TMOS was added to the mixture. After 3 h,the chloride salts were slowly poured in and stirred for 24 h. Subsequently, the mixture wastransferred to a Petri dish and dried in an oven at 35 ◦C. The catalysts were calcined at 550◦C in air for 3h (ramp 1 ◦C/min) . In addition, pure MoO3 supported on silica (n0-b) was

64

synthesized at different loadings using the same approach.

n0-KIT-6, pure MoO3 supported on mesoporous silica KIT-6 was prepared by impregnationusing the same method as in our previous study [31]. Typically, 0.4599 g Mo7O24(NH4)6 · 4H2Owas mixed in a mortar with 1.25 g KIT-6 that was pre-wetted in n-heptane. The mixture wasthen transferred to a round bottom flask and heated at 85 ◦C overnight under reflux. Thesolid was filtered, dried at 50 ◦C, and then calcined at 550 ◦C for 3 h in air.


N2 physisorption isotherms at -196 ◦C were measured using a Quantachrome Nova 2000series instrument. The samples were preliminarily degassed in vacuum at 150 ◦C for 6h. Thespecific surface area of the catalysts was calculated using the linear part of the BET plot (0.05-0.2 in relative pressure). The pore size distribution is calculated from the adsorption branchfollowing the NLDFT (non-local density functional theory) method using the SiO2 kernel forcylindrical pores. The pore volume is taken as the adsorbed nitrogen volume at 0.99 relativepressure. Wide-angle X-ray diffraction (XRD) analysis was performed using a Siemens 80Model D5000 diffractometer with CuKα radiation (λ=0.15496 nm). The chemical composi-tion of the samples was established by Inductively Coupled Plasma-Optical Emission Spec-troscopy (ICP-OES) using a Perkin Elmer Optima 4300DV spectrometer. Catalysts structureand morphology were established by transmission electron microscopy (TEM, JEOL JEM1230 operated at 120 kV). Furthermore, valence states of Mo and Sb were obtained by X-rayPhotoelectron Spectroscopy (XPS) using a photoelectron spectrometer (Kratos Axis-Ultra,evacuated to 10−9 Torr) equipped with a focused X-ray source (Al Kα, hν = 1486.6 eV). In ad-dition, the catalyst template degradation was tested by thermal gravimetric analysis (TGA)on a model Q5000IR (New Castle DE 19720, USA).

5.2.3 Catalytic test

Catalytic tests were performed in the system shown in Fig. 3.1. More detail on the perfor-mance of this system, is described in the previous contribution [31]. Gas feeds includingacrolein, air, ammonia, and nitrogen enter the quartz reactor through two separate inlets inorder to avoid polymerization at the contact between ammonia and acrolein. The first linecontains compressed air, ammonia, and nitrogen diluent. Acrolein vapor (95 % aqueous so-lution) at 0 ◦C and another nitrogen flow were fed to the reactor in the second line. Thesystem lines were heated to about 180 ◦C (red-line). Moreover, a set of three-way valveswas used in order to avoid condensation or polymerization in the gas sampling loop. Thissystem also allowed cleaning the lines by flowing methanol between tests.

About 55 mg catalyst was loaded in a fixed-bed reactor (8 mm in inner diameter and 420mm in length) at atmospheric pressure. According to SEM and TEM images, the particlesize is about 20-25 µm. The samples were used without any dilutent. Based on conditions

65

optimized in a previous study [31], the catalysts were tested at GHSV=15700 h−1 (calculatedbased on volume of the catalyst in the bed, 4.5 cm high) under various temperatures rangingfrom 350 to 550 ◦C. The molar ratio of the reactants in feed gases was fixed at AC/ NH3/O2/ N2 = 1/ 1.25/ 12.0/ 158 a composition which is out of the flammability zone limits (seesupporting information in [31]). All reactants and products were analyzed online using athermal conductivity detector (TCD) and Hayesep P and molecular sieve 13X columns.

A carbon balance was calculated based on all detected products including carbon dioxide,acrolein, acetonitrile (ACE), and acrylonitrile (carbon monoxide, HCN and acrylic acid werenever detected). The conversion, selectivity, and carbon balance were calculated using Eqs.(3.2, 3.3 and 3.4) as described in the previous part in Chapter 3. Error bar was calculated asthe standard deviation.

Table 5.1. Textural properties and structure of the SbnMo10-nOx/SiO2 catalysts

Catalyst designation Catalyst components Loading(wt%)

SBET(m2/g)

Poresize(nm)

Porevol-ume(cc/g)

KIT-6 823 8.1 1.09n0-KIT-6 MoO3/KIT−6 23 344 8.1 0.729

n0-1 MoO3/SiO2 1 689 5.9 0.613n0-5 MoO3/SiO2 5 574 4.9 0.538n0-10 MoO3/SiO2 10 260 4.9 0.407

n0-18 (fresh) MoO3/SiO2 18 197 7.0 0.445n0-18 (used 400 ◦C) MoO3/SiO2 18 159 6.8 0.347n0-18 (used 350 ◦C) MoO3/SiO2 18 76 6.8 0.217

n0-26 MoO3/SiO2 26 167 6.8 0.398n0-40 MoO3/SiO2 40 160 5.9 0.312

n13.4-18 Sb1.34Mo8.66Ox/SiO2∗ 18 258 1.2/4.9 0.346

n33.6-18 Sb3.36Mo6.64Ox/SiO2 18 310 1.2/4.2 0.411n58.5-18 Sb5.85Mo4.15Ox/SiO2 18 504 1.2/4.2 0.398n90-18 Sb9MoOx/SiO2 18 529 1.2/4.0 0.461n100-18 Sb2O5/SiO2 18 533 1.2/3.5 0.414

∗ The Sb/(Sb+Mo) % atomic ratio (for example 1.34/(1.34+8.66)) given for samples n13.4-18 to n100-18

were established by ICP and correspond to target values 1/9, 3/7, 5/5, 9/1, 10/0.


5.3.1 Characterizations

MoO3 supported on silica at different loadings (n0-a) exhibit high specific surface area (SSA)and pore volume as shown in Table 5.1 and high crystalline (Fig. S5.1) . The SSA and porevolume are however lower than those of MoO3 supported on KIT-6 at similar loading. TheSSA and pore volume of the catalysts SbnMo10-nOx/SiO2 increased with increasing Sb con-

66

(a) n0 (b) n13.4 (c) n33.6

(d) n58.5 (e) n90 (f) n100

Figure 5.1. TEM images of the SbnMo10-nOx/SiO2 (18 wt%) catalysts

tent in the mixture. The SSA raised from 197 to 533 m2/g when the catalyst content waschanged from MoO3/SiO2 (n0) to Sb2O5/SiO2 (n100) at 18 wt% loading. The detailed N2

isotherms and pore size distributions of the these samples are reported in supplementaryinformation as Figs. S5.2 and S5.3 respectively. The micropore volume increases as the Sbcontent is raised, whereas the mesopore average diameter is significantly reduced. In gen-eral these micro- and small mesopores are main contributors to specific surface area and porevolume. They are however likely to become plugged by organic residues during reaction. Inaddition, molybdenum and antimony oxides thermal stability was measured by TGA (Figs.S5.4 and S5.5) and found stable up to 800 ◦C in air.

TEM images of the SbnMo10-nOx/SiO2 samples show that the synthesis procedure used pro-duced mesostructured catalysts (Fig. 5.1). At increasing Sb content, the structure progres-sively changes from wormhole like to better ordered. The XRD patterns shown in Fig. 5.2indicate that MoO3 appears progressively, first as an amorphous phase at n ranging from3.36 (n33.6) to 9 (n90) and as crystal phase (MoO3, JCPSD 05-0508) at higher Mo content(0 < n < 3.36). The large signal at 22-30 ◦ is related to amorphous silica (22.5 ◦) possiblyenlarged by contribution of an amorphous Sb2O5 phase (22 plus 29 ◦, JCPDS 33-0111). In ad-dition, Raman spectroscopy spectra (Fig. S5.6) confirmed that amorphous Sb2O5 was highly

67

1 0 2 0 3 0 4 0 5 0 6 0

♦

♦♦♦♦♦♦♦♦♦♦

♦

♦

♦♦♦

♦

♦

♦

n 1 0 0n 9 0

n 5 8 . 5n 3 3 . 6n 1 3 . 4

Coun

ts

2 - T h e t a

n 0

♦

M o O 3

Figure 5.2. X-ray diffraction pattern of the SbnMo10-nOx/SiO2 (18 wt%) catalysts

dispersed on silica [204] and mixed oxides (MoO3 and Sb2O5) were deposited as separateoxidic phases. The spectrum of the Sb1.34Mo8.66Ox/SiO2 is identical to that of MoO3. XPSdata allowed confirming the presence of Mo (VI) and Sb (V) in these samples [205] (see Figs.S5.7,S5.8, and S5.9 in supporting information). In addition, XPS spectra indicated that theoxidation state of Mo and Sb was not affected after reaction.

5.3.2 Catalytic tests

Based on the blank test in the previous chapter, the ammoxidation reaction tests of reaction(5.1) were therefore mostly conducted at the optimal temperature of 450 ◦C with carefulmonitoring of the carbon balance.


O2 −−→ CH2−−CH−C−−−N + 2 H2O (5.1)

68

Figure 5.3. Activity and selectivity of the SbnMo10-nOx/SiO2, 18 wt% (solid symbol) and n0-KIT-6(up-half symbol) catalysts; at 450 ◦C and GHSV=15700 h−1, TOS=3.0 h

The AC conversion vs Sb content curve is shown in Fig. 5.3. A 5 % increase is first observed atlimited Sb content in the mixed oxides (Sb/(Sb+Mo)=13.4), and then AC conversion reducessharply from 90 to 25 % with increasing Sb content. A limited content of Sb could accelerateMo reoxidation step thus accelerating the ammoxidation process. MoO3 supported on silica(n0-18) exhibits a slightly higher catalytic activity compared to n0-KIT-6 even though its SSAis lower. MoO3 supported on the silica prepared using the soft-templating method showedmore efficient. As shown in our previous study [31], the best BiMoO supported on KIT-6catalyst yielded an AC conversion of about 84 % under the same conditions. The three cat-alysts n0-18, n0-KIT-6 and the best performing BiMo/KIT-6 having similar supported oxidecontent only showed 2 % difference in catalytic conversion. These results indicated that thesupported MoO3 phase bears the significantly active sites for the AC ammoxidation to ACN.Fig. 5.3 also shows that the ACN selectivity was essentially stable to about 80 % from n0 ton58.5, and then decreased to 25 and 10 % at n90 and n100, respectively. Increasing Sb molarratio above 58.5 %, AC conversion and ACN selectivity are both reduced. Sb2O5/SiO2 (n100)showed low conversion and selectivity. As shown in Fig. 5.3, carbon balance for this catalystgets lower (75 %) likely associated with condensation of organic residues in the micropores.This possible effect was already discussed above.

69

0 1 0 2 0 3 0 4 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

A C c o n v e r s i o n A C N s e l e c t i v eAC

conv

ersion

(%)

2 03 04 05 06 07 08 09 01 0 0

Selectivity & Carbon balance (%)

L o a d i n g ( w t % )

C a r b o n b a l a n c e

Figure 5.4. Activity and selectivity of the MoO3/SiO2 catalysts at different loadings; at 450 ◦C andGHSV=15700 h−1, TOS=3.0 h

The effects of MoO3 content in silica on catalyst activity and selectivity is shown in Fig.5.4. AC conversion increases from 35 % to 85 % when MoO3 loading rises to 10 wt%. Athigher loading, catalytic activity remains stable. ACN selectivity stays between 75 and 87 %until the MoO3 loading reaches 40 % at which value it decreases to 68 %. At this value theSSA, average pore size and pore volume are considerably reduced whereas the microporeshave completely disappeared (see Figs. S5.10 and S5.11). The low observed carbon balancesuggests that at this loading heavier products are formed either by secondary reaction ofACN or by competitive conversion of AC. These results call for studies of activity stabilityof supported MoO3 described below (see Figs. 5.6 and 5.7).

n0-18 catalyst was tested at different temperatures ranging from 350 to 540 ◦C. After eachtest, the catalyst was activated in air at 550 ◦C for 1.5 h. As shown in Fig. 5.5, AC conversionexhibits a volcano shape. At 450 ◦C, AC conversion is maximum at about 85 %. At temper-ature lower than 450 ◦C, selectivity and carbon balance are both very low because of cokedeposition on catalyst. N2 physisorption of the used catalysts at 350 and 400 ◦C are shownin Table 5.1. As expected, AC conversion increased when temperature was raised from 350to 450 ◦C. The specific surface area, pore size, and pore volume of the used catalysts were

70

3 5 0 4 0 0 4 5 0 5 0 0 5 5 03 0

4 0

5 0

6 0

7 0

8 0

9 0

A C c o n v e r s i o n A C N s e l e c t i v i t y


AC co

nvers

ion (%

)

1 02 03 04 05 06 07 08 09 01 0 0

Selectivity & Carbon balance (%) C a r b o n b a l a n c e

Figure 5.5. n0-18 (MoO3/SiO2, 18 wt%) catalyst test at different temperatures, GHSV=15700 h−1,TOS=3.0 h

reduced compared with the fresh sample. Above 500 ◦C, catalytic activity and ACN selec-tivity are both reduced. Only acetonitrile and CO2 were detected as byproducts at about2 % each. These various results show that the organic material left in the pores at lowertemperature (350 ◦C) are consumed by reaction with oxygen and/ or NH3 when tempera-ture is raised. This clearly explains why the carbon balance is much better in our optimizedconditions. In addition, this effect can be visualized by the color of the catalyst (Fig. S5.12).It is noteworthy that as temperature gets over 450 ◦C the conversion shown in Fig. 5.5 areonly slightly higher than the ones observed in the blank test in the presence of ammonia. Asdiscussed in dealing with blank tests, the side reaction accounted for about 32 and 54 % ACconversion at 500 and 550 ◦C, respectively. This suggests that in these conditions most ofthe AC conversion is associated with thermal reaction thus decreasing selectivity.

The stability of MoO3/SiO2 catalyst was confirmed by long reaction tests as shown in Fig.5.6. The catalyst is quite stable over 11 h on stream with conversion , selectivity, and carbonbalance of about 85, 80, and 85 %, respectively. In order to check the effect of Sb, catalystn13.4-18 was also tested in long time on stream as shown in Fig. 5.7. After 1h on streamAC conversion became essentially stable for 10h whereas both ACN selectivity and carbon

71

0 1 2 3 4 5 6 7 8 9 1 0 1 10

2 0

4 0

6 0

8 0

1 0 0

A C c o n v e r s i o n A C N s e l e c t i v i t y C a r b o n b a l a n c e


AC co

nvers

ion (%

)

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0 ACN selectivity & Carbon balance (%)

Figure 5.6. Stability of n0-18 catalyst (MoO3/SiO2, 18 wt%) at 450 ◦C and GHSV=15700 h−1

balance decreased for about 4h and then remained almost stable. This behaviour is puzzlingand likely related to some natural rearrangement of both Sb and Mo during the first 4h. Thiskind of rearrangement is suggested by XPS analysis (Figs. S5.7, S5.8, and S5.9). Actually theXPS spectrum of fresh n13.4-18 sample yielded a Mo/Sb molar ratio of 12 whereas the samematerial after 10h on stream had a Mo/Sb ratio of 2.59. A work in progress in our laboratoryis aiming at elucidating these structural changes and their effects on catalytic behaviour.

5.4 Conclusions

Antimonate-molybdate supported on silica using a new preparation method exhibited highSSA and high pore volume with the metal oxides highly dispersed on silica. Pure molyb-date and mixed Sb-Mo with minor antimonate content supported on silica both displayedhigh AC conversion up to 87 % and high ACN selectivity of about 80 %. The catalysts withdifferent molar ratios of antimonate and molybdate demonstrated that molybdenum plays amajor role in controlling both activity and selectivity for acrolein ammoxidation to acryloni-trile.

72

0 1 2 3 4 5 6 7 8 9 1 00

1 02 03 04 05 06 07 08 09 0

1 0 0

Conv

. & Se

l. & C

. bala

nce (

%)


A C c o n v e r s i o n A C N s e l e c t i v i t y C a r b o n b a l a n c e

Figure 5.7. Stability of n13.4-18 catalyst (Sb1.34Mo8.66Ox/SiO2, 18 wt%); at 450 ◦C and GHSV=15700h−1

Acknowledgments

The authors gratefully acknowledge the financial support of NSERC and CRIBIQ. We alsothank Mr. Yann Giroux of Laval University for help in TGA measurements and ProfessorDongyuan Zhao and Mr.Wang Shuai of Fudan University for providing TEM.


73

1 0 2 0 3 0 4 0 5 0 6 0

♦

♦ ♦♦♦♦♦

♦♦♦

♦♦

♦

♦

♦

♦

♦

Coun

ts

2 - T h e t a

n 0 - K I T 6 n 0 - 4 0 n 0 - 1 0 n 0 - 2 6 n 0 - 5 n 0 - 1 8 n 0 - 1

♦

M o O 3

Figure S5.1. X-ray diffraction patterns of n0 (MoO3/SiO2) at different loadings

74

0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0

n 5 8 . 5

n 1 0 0n 9 0

n 3 3 . 6n 1 3 . 4

Volum

e (cc

/g)

P / P 0

n 0

Figure S5.2. Hysteresis loops of the SbnMo10-nOx/SiO2catalysts SiO2, 18 wt%

75

0 5 1 0 1 5 2 0

n 1 0 0n 9 0n 5 8 . 5n 3 3 . 6n 1 3 . 4Vo

lume a

dsor

bed (

cc/g.

nm)


S b n M o 1 0 - n O x / S i O 2

n 0 - 1 8

Figure S5.3. Pore size distribution of the SbnMo10-nOx/SiO2catalysts SiO2, 18 wt%

76

1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 02 0

4 0

6 0

8 0

1 0 0


Weigh

t (%)

M o O x / S i O 2

- 0 . 50 . 00 . 51 . 01 . 52 . 02 . 53 . 03 . 54 . 0

Deriv. weight (%/ oC)

Figure S5.4. TGA curve of the dried and non-calcined Mox/SiO2 under air atmosphere

1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 02 0

4 0

6 0

8 0

1 0 0


Weigh

t (%)

- 0 . 50 . 00 . 51 . 01 . 52 . 02 . 53 . 03 . 54 . 0

Deriv. weight (%/ oC)

Figure S5.5. TGA curve of the dried and non-calcined SbOx/SiO2 under air atmosphere

77

1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 00

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0

6 0 0 0

7 0 0 0

Inten

sity (

a.u.)

R a m a n s h i f t ( c m - 1 )

S b 1 . 3 4 M o 8 . 6 6 O x / S i O 2 M o O 3 / S i O 2 S b 2 O 5 / S i O 2

Figure S5.6. Raman spectra of n0, n13.4 and n100 catalysts

78

2 4 2 2 4 0 2 3 8 2 3 6 2 3 4 2 3 2 2 3 0 2 2 8

n 1 3 . 4 u s e d

2 3 5 . 8 4 2 3 2 . 7 1

2 3 6 . 7 32 3 3 . 5 9

2 3 5 . 1 2

3 d 5 / 2

CPS

B i n d i n g e n e r g y , e V

3 d 3 / 2

2 3 1 . 9 7

M on 9 0

n 1 3 . 4 f r e s h

Figure S5.7. Mo3d XPS spectra of (fresh and used) n13.4 and n90 catalysts

79

Laboratoire d'analyse de surface-CERPIC-Université Laval

Sb 3d3/2 Sb 3d5/2

SiO2

oxide

Residual STD = 2.63966

Name

Sb 3d3/2

Sb 3d5/2

SiO2

oxide

Pos.

541.28

531.94

533.35

530.70

FWHM

1.50

1.50

1.80

1.80

L.Sh.

GL(30)

GL(30)

GL(58)

GL(59)

%Area

0.06

0.09

99.62

0.23

x 103

10

20

30

40

50

CP

S

544 540 536 532 528Binding Energy (eV)


Sb 3d3/2 Sb 3d5/2

SiO2

oxide


Name

Sb 3d3/2

Sb 3d5/2

SiO2

oxide

Pos.

541.04

531.70

532.92

530.58

FWHM

1.71

1.71

1.89

1.89

L.Sh.

GL(30)

GL(30)

GL(50)

GL(50)

%Area

1.58

2.39

92.68

3.34

x 103

5

10

15

20

25

30

35

40

45

CP

S


Figure S5.8. Sb3d XPS spectra of fresh (left) and used (right) Sb1.34Mo8.66Ox/SiO2 catalyst (left toright)

80


Sb3d_87_F

Sb 3d3/2Sb 3d5/2

O1s

Oxide


Name

Sb 3d3/2

Sb 3d5/2

O1s

Oxide

Pos.

541.32

531.98

533.19

530.14

FWHM

2.15

2.15

2.02

2.02

L.Sh.

GL(30)

GL(30)

GL(50)

GL(50)

%Area

9.47

14.30

74.41

1.82

x 103

5

10

15

20

25

30

35

40

CP

S


Figure S5.9. Sb3d XPS spectra of fresh n90 (Sb9MoOx/SiO2) catalyst

81

0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 00

5 01 0 01 5 02 0 02 5 03 0 03 5 04 0 0

+ 4 0Volum

e (cc

/g)

P / P 0

n 0 - 1 n 0 - 5 n 0 - 1 0 n 0 - 1 8 n 0 - 2 6 n 0 - 4 0

+ 5 0

Figure S5.10. Hysteresis loops of n0 (MoO3/SiO2) at different loadings

82

0 5 1 0 1 5 2 0 2 5

0 . 0 0

0 . 0 5

0 . 1 0

0 . 1 5

0 . 2 0

0 . 2 5Vo

lume a

dsor

bed (

cc/g.

nm)

P o r e s i z e ( n m )

n 0 - 1 n 0 - 5 n 0 - 1 0 n 0 - 1 8 n 0 - 2 6 n 0 - 4 0

Figure S5.11. Pore size distribution of n0 (MoO3/SiO2) at different loadings

Used at 350 oC Used at 400 o Fresh

Figure S5.12. Fresh and used catalyst MoO3/SiO2 (18 wt%)

83

84

Chapter 6

NH3 adsorption as related tomechanism of acrolein ammoxidationover molybdate catalysts


1Department of Chemical Engineering, Laval University, 1065, avenue de la Médecine, Québec,G1V 0A6, Canada


Résumé

Ce travail a porté sur le mécanisme de l’ammoxydation de l’acroléine en acrylonitrile mettanten oeuvre les lacunes d’oxygène et l’ammoniac adsorbé sur les catalyseurs, à base d’oxydesmésostructurés de bismuth, d’antimoine, de molybdène, et leurs mélanges. Les résultats ob-tenus ont révélé que les catalyseurs qui présentent le plus de lacunes d’oxygène et qui sontfacilement réduits par l’ammoniac montrent de meilleures activités catalytiques. Un schémaréactionnel a été proposé basé sur une interaction simple entre l’ammoniac adsorbé (=N-H)et le groupement aldéhyde (-CHO) de l’acroléine sur le catalyseur MoO3.

Abstract

This work studies the reaction mechanism of acrolein ammoxidation to acrylonitrile takinginto account the oxygen vacancies and adsorbed ammonia on mesostructured oxides of bis-muth, antimony, molybdenum, and their mixtures as catalysts. The obtained results indicatethat the catalysts having more oxygen vacancies and which are readily reduced by ammoniashowed good catalytic activities. A new reaction scheme was proposed based on a sim-

85

ple interaction between adsorbed ammonia (=N-H) and aldehyde group (-CHO) over MoO3

catalyst.

6.1 Introduction

Ammoxidation of ex-glycerol acrolein (AC) to acrylonitrile (ACN) may have a major im-pact in green chemistry because of the wide application of acrylonitrile and the rapid de-velopment of glycerol production either as a co-product of biodiesel synthesis or as productof C5, C6 sugars hydrogenolysis. In recent research, the AC ammoxidation process overmolybdate- and antimonate-based catalysts yielded significant progress in increasing theACN yield [15–22, 31, 32]. However, the reaction mechanism has not yet been elucidated.

In 1971, Cathala and Germain studied the AC ammoxidation over Bi-Mo-O catalysts [97].The authors only proposed a simple reaction scheme, in which AC is converted to ACNwithout any intermediate. In 1975, Oka et al. also studied the chemical reaction kinetics ofAC ammoxidation over Fe-Bi-P oxide catalysts [99]. The authors confirmed that the reactionscheme was following Hadley’s process (AC contacted with NH3 to create allylic interme-diate, then converted to ACN). In addition, the reaction rate was independent on the con-centration of oxygen in the feed gas. The authors assumed that oxygen was taken from thesurface of catalyst. Moreover, they demonstrated that the reaction rate of the AC ammoxi-dation was 1000 times faster than that of propene ammoxidation. These results were relatedto the effects of reactant concentrations on the reaction rate, but there was no explanationabout the role of catalysts in the reaction scheme.

Recent work [15, 16, 31, 32] investigated the AC ammoxidation over molybdate-based orantimonate-based catalysts. The molybdate-based catalysts exhibited better catalytic prop-erties than antimonate-based ones. The obtained results indicated that the oxygen/ammoniamolar ratio in feed gases has significant effects on the catalytic reaction rate. Thanh-Binh etal. [31] established that Bi-Mo-O powders were still active in the absence of oxygen in thefeed gases. The surface oxygen of the catalyst was participating in the reaction followinga Mars-van Krevelen mechanism. These results were in agreement with previous reports[99, 206], in which, the reaction mechanism was however not mentioned. Several reportshad shown that AC was an intermediate product in the propene/propane ammoxidationto ACN [85, 90, 91, 200, 207]. It is also noteworthy that the complex bismuth molybdatebased catalyst used in industry of ACN production from propene is also active and usedindustrially for propene oxidation to acrolein [8]. Licht et al. [207] investigated the reactionmechanism and kinetics of the propene ammoxidation over α bismuth molybdate. The au-thors concluded that AC was a by-product in the conversion of propene to ACN, however,its production was limited. Based on these discussions, there was still not a clear reactionscheme for the AC ammoxidation to ACN.

86

Multicomponent metal oxides were often used as catalysts in ammoxidation reactions. Theactive sites contained at least two vicinal metal oxide moieties, which had optimal metal-oxygen bond strengths and could be readily reducible by ammonia and reoxidizable byoxygen (air) [7–9, 46, 54, 55, 57–59]. One other suggestion was that the catalysts shouldhave anion vacancies that can increase the mobility of cations between anions. Therefore,this work will study the reaction mechanism of the AC ammoxidation in terms of reductionand reoxidation over several catalysts that were investigated in our previous contributions[31, 32].

6.2 Experimental

6.2.1 Synthesis of catalysts

Metal oxides MoO3, Bi2O3 and Bi2O3. 3MoO3 supported on mesoporous silica KIT-6 (18 wt%of loading) were synthesized as in the previous study [31]. Bi(NO3)3 · 5 H2O, (NH4)6Mo7O24.H2O were used as metal oxide precursors. First, the precursors were pre-mixed with pre-wetted KIT-6 in n-heptane. Then, the mixture was transferred to a round bottom flask andheated overnight with reflux at 90 ◦C. Finally, the powders were dried at 35 ◦C after fil-tration and then calcined at 550 ◦C for 3 h. The other catalysts, including Sb2O3, SbMo9Ox

and Sb9MoOx supported on silica, were prepared by a soft-templating method as describedin the previous report [32]. Dual-surfactants, including cetyltrimethylammonium bromide(CTAB) and triblock copolymer-PEO106PPO70PEO106 (F127), were used as structure direct-ing agents. The metal oxide precursors (MoCl5 and SbCl3), silica precursors (tetramethylorthosilicate, TMOS), and surfactants were homogeneously mixed together, transferred to apetri dish, then evaporated in air at 35 ◦C for 24 h. Subsequently, the powders were obtainedafter calcination at 550 ◦C for 3h. All catalysts were designated with a and b correspondingto after and before conducting degassing or ammonia reduction tests.


N2 physisorption isotherms at 77 K were determined using a Quantachrome Nova 2000 se-ries instrument. The samples were preliminarily degassed in vacuum at 150 ◦C for 6h. Thespecific surface areas were calculated from the linear part of the BET plot (P/P0 = 0.05 - 0.2).The pore size distributions were determined from the adsorption branch using the NLDFT(non-local density functional theory) method for silica with cylinder pore shape. The porevolumes were determined from the volume of physisorbed nitrogen at the relative pressureof 0.99. X-ray diffraction (XRD) analysis was performed with a Siemens 80 Model D5000diffractometer using CuKα radiation (λ=0.15496 nm) in order to identify the structure ofcatalysts before and after ammonia adsorption. Furthermore, the effects of the adsorbedammonia on the valence states of Mo and Sb were studied by X-ray photoelectron spec-

87

Table 6.1. Summary of catalytic and textural properties of the samples [31, 32]

No. Composition∗AC conv.∗∗

(%)ACN sel. ∗∗

(%)SBET

(m2/g)Pore size

(nm)Pore volume

(cc/g)1 MoO3/KIT-6 82 90 340 8.1 0.7292 Bi2O3. 3MoO3/KIT-6 84 97 380 7.6 0.6303 SbMo9Ox/SiO2 90 76 240 1.2/4.2/6.6 0.3594 Sb9MoOx/SiO2 42 25 530 1.3/4.0 0.4615 Bi2O3/KIT-6 3 - 510 7.6 0.7396 Sb2O5/SiO2 26 9 530 1.3/2.5-3.5 0.4147 KIT-6 0 - 810 8.1 1.040

∗: 18 wt% of metal oxide loading on the support∗∗: measured at 450 ◦C; reactant molar ratio AC/ NH3/ O2/ N2 = 1.0/ 1.25/ 16.5/158, F = 66 cc/min, TOS = 3.5 h

troscopy (XPS) using a photoelectron spectrometer (Kratos Axis-Ultra, evacuated to 10−9

Torr) equipped with a monochromatized X-ray source (Al Kα, hν = 1486.6 eV).

6.2.3 Temperature programmed desorption of ammonia (TPD) and ammoniachemisorption

In a typical NH3-TPD experiment, 50 mg powder of catalysts was loaded in a U-shapedquartz reactor. In the first step, the powders were pretreated under an oxygen/argon mix-ture flow (20 vol% in argon, 25 cc/min) at 300 ◦C for 1h in order to remove humidity andcoke. Then, only argon flow was passed through the reactor at the same temperature for 30min. After this pretreatment, the catalysts were saturated with flowing ammonia (20 vol%in argon) at 100 ◦C for 30 min. Subsequently, argon was again flown over the catalysts at100 ◦C for 6 h in order to remove physisorbed ammonia. Ammonia TPD analyses were thencarried out by increasing temperature from 150 to 850 ◦C at a heating rate of 5 ◦C/min.The amount of desorbed ammonia was monitored on stream, using a thermal conductivitydetector (TCD).

In ammonia chemisorption tests, the same NH3-TPD reactor system was used to measurethe amount of adsorbed ammonia on stream. A certain loop of ammonia (loop volumesranging from 100 to 250 µl) was pulsed to the reactor at different temperatures up to 450 ◦C.This process could directly allow measuring the amount of ammonia that was adsorbed onor reacted with the catalysts by comparing the TCD detector signal area after the passage ofa pulse with the signal obtained in absence of catalyst.


The catalyst morphological properties including specific surface area, average pore sizeand pore volume are reported in Table 6.1. These were derived from the nitrogen adsorp-

88

tion/desorption isotherms shown in supporting information Fig. S6.1 along with the poresize distributions established from these curves using DFT calculations. In this table, cat-alytic results on the AC ammoxidation obtained in our previously reported work [31, 32] arealso reported. Six catalysts, including MoO3/KIT-6 (1), Bi2O3. 3MoO3/KIT-6 (2), SbMo9Ox/SiO2

(3), Sb9MoOx/SiO2 (4), Bi2O3/KIT-6 (5), and Sb2O5/SiO2 (6), were selected from the previ-ous studies [31, 32]. As shown in Table 6.1, the catalytic activities and ACN selectivities maybe divided into two groups: (A) including samples (1-3) that exhibited good catalytic prop-erties on both AC conversion and ACN selectivity; (B) including samples (4-6) with low oreven no-activity.

The catalyst behaviors in the AC ammoxidation will obviously relate to their interaction withthe reactants including ammonia and oxygen (air). Some observations were made during theN2 physisorption tests. As the first step of the N2 physisorption, the sample powders weredegassed in vacuum at 150 ◦C for 6 h. After degassing, the colors of the powders of all sam-ples in group (A) had changed, whereas in group (B) no differences in color were observed(Fig. S6.2). It seems thus that there is a correlation between this change and what makes agood or poor catalyst activity for the AC ammoxidation. As shown in Fig. S6.2, the color ofthe powder of sample (1) changed from white to light blue. The light yellow color of sample(2) changed to light gray after degassing. In the case of sample (3), its color was shifted fromnavy blue to light blue. These changes in color could be related to the presence of oxygenvacancies on the catalyst surface which would change color upon degassing. Dasgupta etal. [208] showed that MoO3 oxide had many oxygen vacancy defects. Grasselli et al. [46]demonstrated that the oxygen vacancies were primarily located on the catalyst’s surface, onwhich the reoxidation process could be rapid. In general, having numerous oxygen vacan-cies will increase catalytic activity of an oxide in any reaction involving an oxidation process[206, 209–211]. Therefore, the oxygen vacancy surface concentration should be a key factoraffecting the activity of an AC ammoxidation catalyst. This actually is in line with the pro-posed Mars-Van Krevelen mechanism for AC ammoxidation on bismuth molybdate basedcatalysts, in which the kinetics is of zero order for oxygen [31].

Ammonia interaction with active sites located at the surface of a catalyst can be measured

1b 1a 6b5b4b

1b

3b2b 2a 3a 4a 5a 6a

Figure 6.1. Color of the samples before (b) and after (a) reduction by ammonia at 450 ◦C

89

by using the NH3-TPD method. In this study, all samples were submitted to the NH3-TPDtest and there were no peaks corresponding to desorbed ammonia in the NH3-TPD curvesrecorded over the temperature range from 150 to 850 ◦C. There may be a priori two reasonsfor this behavior: (i) no ammonia was adsorbed or (ii) ammonia interacted so strongly withthe substrate that it either cannot desorb over that temperature range or react with the sur-face forming products not dectected in the experiment. Usually the adsorption of NH3 on anoxide is associated with the presence of surface acid groups which interact with it, formingquaternary ammonium ions. Previous reports [212, 213] have mentioned that, as expected,the silica KIT-6 support (7) did not contain acid sites on its surface. Bi2O3 oxide in sample(5) has a low surface concentration of weak acid sites [214, 215], while Sb2O3 in sample (6)shows very minor density of acid sites [216, 217]. These acid properties correspond with ourNH3-TPD observations. Similar to the N2 physisorption results for the samples in group (a)with good catalytic activity, the color of the powders of samples (1-3) changed from whiteand light yellow to black, respectively, after the NH3-TPD test. As mentioned above thesecolor changes are associated with the formation of surface oxygen vacancies and reductionof some surface molybdenum atoms. Moreover, after the NH3-TPD test a densification ofthe sample particles was observed for group (A) samples. On the other hand, there was nochange in the powder’s color and state for the three samples (4-7) from group (B).

Thus it seems that even though no desorbed ammonia was observed during NH3-TPD test,some surface reaction has happened with ammonia at a temperature comprised between 150to 850 ◦C. In order to clarify this point, some ammonia reduction tests of samples (1-6) wereperformed at 450 ◦C which was found to be the optimum temperature for AC ammoxidationin our previous works [31, 32]. These tests were performed under the same flowing gasmixture (20 % NH3 in Ar) as in the NH3-TPD tests, for 15 min. Fig. 6.1 illustrates the changein powder color of samples (1-6). Samples (1-3) which originally had bright colors (white,light yellow, and navy blue, respectively) became black. In sample (4), the color changedfrom light blue to light violet, while the white color of sample (5) was converted to lightgray. The color of the powder of sample (6) remained unchanged. As shown in Table 6.1,our samples (1-4) contained molybdenum in a variety of mixed oxides. As reported in ourprevious study [31], the changing color may be related to a reduction of metal oxides byammonia. Therefore, Mo6+ in MoO3 oxide could be reduced to lower valence states such asMo5+, Mo4+, or Mo3+, while Bi3+ in sample (5) might also be converted to lower valences.On the other hand in sample (6), Sb2O5 oxide may not have been reduced since no change incolor was observed upon contact with ammonia. Interestingly, these changing color resultsseem to be related to the observed catalytic activities in AC ammoxidation reaction (Table6.1).

In order to confirm the above hypotheses, the phase structures and valence states of samples(1-6) before and after the ammonia reduction test were further analyzed by XRD and XPS

90

Inten

sity (

cps)

B i n d i n g e n e r g y ( e V )

( )

Figure 6.2. Mo3d spectra of samples (1-4) before (b) and after (a) reduction by ammonia at 450 ◦C

91

methods. As shown in the XRD results of samples (1-3) in group (A) (Figs. S6.3-S6.5), MoOx

oxides were changed to MoO2. In the XRD patterns of sample (1), MoO3 (JCPDS 05-0508)was converted to MoO2 (JCPDS 76-1807 and 73-1807) after reaction with ammonia at 450 ◦Cfor 15 min (Fig. S6.3). For sample (2), two new phases, including Bi (JCPDS 44-1246) andMoO2 (JCPDS 76-1807), were created from the parent bismuth molybdate phases (Bi2MoO6

with JCPDS 21-0102 and Bi2Mo3O12 with JCPDS 21-0103), as shown in Fig. S6.4. In the caseof sample (3) (Fig. S6.5), MoO3 (JCPDS 05-0508) was partly converted to MoO2 (JCPDS 76-1807) and no signals of SbOx and SbMo9Ox were detected. On the other hand, there wasno difference in the XRD patterns of samples (4-6) before and after coming into contact withammonia as shown in Figs. S6.6-S6.8. The broad feature ranging from 17 to 30 ◦ of 2θ inthe patterns of samples 4 and 6 reflects the presence of an amorphous phase comprisingSiO2 and Sb2O5 materials. For sample (5), two phases, including Bi2O3 (JCPDS 78-1793) andBi2SiO5 (JCPDS 36-0287), are present before and after interaction with ammonia. The Bi2SiO5

pattern increase in intensity indicates a solid-solid reaction of bismuth oxide with silica. Bi isnot reduced under these conditions. This result indicated that Bi2O3 in the KIT-6 support (5)was not active in contrast to its behavior in the mixed phase with MoO3 (sample (2)). Thismay explain the result obtained in reference [32] showing no catalytic activity of sample (5)in AC ammoxidation.

The above XRD results demonstrated that ammonia was involved in a reduction reaction ofboth Mo and Bi at 450 ◦C, which is the temperature where samples (1-3) were found activein the ammoxidation reaction. Mo6+ and Bi3+ were reduced to Mo4+ and Bi0, respectively.These samples, after reaction with ammonia, were treated with oxygen in an air flow at 450◦C to examine their re-oxidizability. The obtained XRD patterns (not shown here) provedthat these catalysts returned to their original active state. Interestingly, samples (1-3) werereadily reducible and re-oxidizable, which should be related to the fact that they are activeand selective catalysts in the AC ammoxidation.

Samples (1-6) were further investigated by the XPS spectroscopy to establish the effect ofammonia on the valence states of Mo, Bi and Sb at the surface of catalysts. The XPS spectraof sample (5) and (6) are not reported here because the ammonia reduced materials showedno difference compared to the initial samples. It was also observed that the Mo3d lines ofsamples (1-4) (Fig. 6.2) already reflected the presence of some (minor) reduced surface Moatoms even before reduction. These were mostly in the Mo5+ valence state likely correspond-ing to initial oxygen vacancies on the surface. The rather intense Mo3d lines of Mo6+ in thereduced samples suggest that this reduction was only partial and located at the surface ofthe supported active phase. The Mo6+ and Mo5+ in samples (1-4) were partly reduced toMo5+/Mo4+/Mo3+ [218–221]. After reduction with ammonia, the Mo6+ and Mo4+ stateslines were present in the XPS spectra of samples (1, 3, and 4), while sample (2) contained allfour Mo valence states including Mo6+, Mo5+, Mo4+, and Mo3+. Thus, sample (2) which is

92

Table 6.2. Volume of adsorbed/reacted ammonia on catalysts

Temp. (oC)Volume of adsorbed/reacted ammonia (µl)/ 0.05 (g) catalyst(5) Bi2O3/KIT-6 (6) Sb2O3/SiO2 (2) Bi2O3. 3 MoO3/KIT-6

25 - - 303 (S)200 - - 347 (S)300 - - 377 (S)325 - - 488 (S)350 - - 2750 (N-S)400 - - 3265 (N-S)450 80 (S) 70 (S) 3845 (N-S)

S: saturated; N-S: non-saturated

the most active ammoxidation catalyst, is also the most easily reduced by ammonia. Theseobservations are also in line with the XRD patterns of samples (1-3) which show that thereduction of these phases was not complete after the reaction.

Nagai et al. [219] and Leung et al. [220] reported that unsupported MoO3 (Mo6+) wasreduced to MoO2 (Mo4+), Mo2N (Mo1.5+), Mo2N0.78 (Mo1.17+) or Mo0 in the presence ofammonia above 350 ◦C as established by XRD and XPS analysis. The authors concludedthat nitrogen (N1s) was present on the reduced catalysts and combined with Mo. In our XPSspectra of samples (1-4) no N1s lines in the survey and no N1s Auger peaks (≈ 1113 eV)could be detected. This may be related to a low surface concentration of nitrogen bearingspecies in the ammonia reduction conditions. Therefore, high resolution N1s lines wereacquired, an example of which is reported in Fig. S6.9 for sample (2). Unfortunately, the N1sline which according to [220] should appear at 397-399, is strongly interfering with the Mo3p3/2 line (400.0 eV). As shown in Fig. S6.9, the Mo3p line of sample (2) is enlarged afterammonia reduction and a shoulder (S) appears on the low binding energy side of the peak.This may be an indication of the presence of some nitrogen bearing surface species. The XPSspectra of bismuth (Bi4f, samples (2 and 5)) and antimony (Sb3d, samples (3,4 and 6)) (notshown here) indicated no change after ammonia reduction. This result also indicates thatbismuth and antimony were not bearing chemisorbed ammonia contrarily to molybdenum.

Fig. 6.3 reports results of pulse reduction experiments in which sample (2) being underflowing argon (10 cc/min) at constant temperature was submitted to successive pulses ofammonia. The integrated area of the TCD signal is compared with the area of a signal ob-tained with the same loop volume but in absence of adsorbent. When after a few pulses theNH3 signal is back to the full loop volume, the integration of numbers of moles of ammoniaabsorbed under the previously passed injections corresponds to saturation. These valuesare reported in Table 6.2. The minor amounts of adsorbed NH3 measured in these condi-tions fill a small volume reasonably commensurate with the volume of residual micropores.

93

These correspond to delayed physisorption. At temperatures exceeding 325 ◦C, saturationcould not be reached. The values reported in Table 6.2 at temperatures 350-450 ◦C are thoserecorded using the 250 µl loop after 14 injections. They do not correspond to a saturatedadsorbent indicating that a gas-solid reduction reaction started occurring at these tempera-tures. In reference [31], it was observed that the activity of this catalyst in AC ammoxidationappeared over the same range of temperature which again suggests that the catalyst surfacereduction is involved in the catalytic process.

The above results are consistent with an ammoxidation mechanism we suggest, as depictedin Fig. 6.4. Some oxygen vacancy initially present even before interaction with ammonia issuggested as the site for initial ammonia adsorption and oxidation using oxygen ions whichmigrate from the bulk (Fig. 6.5). In Fig. 6.4, the oxygen vacancy is pictured as associatedwith a Mo4+ species but it could also be shared between two Mo5+ ions. Prolonged reactionwith ammonia would result in deeper Mo reduction. Assuming the formation of a dissocia-tively chemisorbed ammonia intermediate, the amine like surface species formed could reactwith the aldehyde group of AC. The surface complex undergoes electronic rearrangementand the ACN formation results in a reduction step similar to the one observed in ammoniareduction experiments. Of course, the lattice oxygen ion must be regenerated by reaction ofgaseous oxygen with some other surface site of the oxidic Mo phase to complete the Mars-van Krevelen process.

The proposed mechanism has similarities with the accepted mechanism for propene ammox-idation of bismuth molybdate catalysts [8]. They both consider a Mo6+NH intermediate cre-

0 6 0 0 1 2 0 0 1 8 0 0 2 4 0 0 3 0 0 0T i m e ( s )

4 5 0 o C 3 0 0 o C 4 0 0 o C 2 0 0 o C 3 5 0 o C 2 5 o C 3 2 5 o C

(a)

0 6 0 0 1 2 0 0 1 8 0 0 2 4 0 0 3 0 0 0 3 6 0 0

3 5 0 o C

4 0 0 o C

T i m e ( s )

4 5 0 o C

(b)

Figure 6.3. Pulse reduction of sample (2) (Bi2O3. 3 MoO3-KIT6) by ammonia at different temperatures(a) loop 100 µl; (b) loop 250 µl

94

ated by simultaneous dissociative chemisorption of NH3 and generation of H2O. The Mo4+

is however considered to be generated by reduction of a Mo6+ surface species with propeneforming AC. Therefore, in this process AC formation is considered a parallel reaction to ACNdirect formation from propene. It is, however, established that the ammoxidation of AC isroughly three orders of magnitude faster than that of propene. We therefore suggest thatthe two reactions yielding ACN from propene may actually be successive instead of parallelprocesses. In this case the mechanism shown in Fig. 6.4 would describe the second step inthe commercially significant propene ammoxidation process.

6.4 Conclusions

In this work straightforward experiments involving observations of color changes upon de-gassing and ammonia reduction of selected AC ammoxidation catalysts, as well as a system-atic XRD and XPS study associated with these changes were performed. The results werefound coherent with a rather simple AC ammoxidation reaction mechanism. The latter in-volves some surface oxygen vacancy interacting with both oxygen ions from the bulk andgaseous ammonia in a Mars-Van Krevelen process. The surface NH formed can directly reactwith the aldehyde group of AC yielding ACN and regenerating the oxygen vacancy site.


Figure 6.4. Proposed reaction scheme for ammoxidation of acrolein to acrylonitrile over MoO3

95

Figure 6.5. Reoxidization and NH3 adsorption on MoO3 oxide

0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 00

1 0 02 0 03 0 04 0 05 0 06 0 07 0 08 0 0

Volum

e (cc

/g)

P / P 0

( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 )

(a)

0 6 1 2 1 80 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

dV/dD

(cc/g

.nm)

P o r e s i z e ( n m )

( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 )

(b)

Figure S6.1. N2 physisorption of all samples (a) isothermal hysteresis loop; (b) pore size distribution

96

1a 1b

6a 6b5a 5b4a 4b

3a 3b2a 2b

Figure S6.2. Effect of degassing on the color of the samples; a-after; b-before degassing in vacuum at150 ◦C for 6 h

1 0 2 0 3 0 4 0 5 0 6 00

1 0 0

2 0 0

3 0 0

4 0 0

1 b

M o O 3♦

♦♦♦♦

∗

∗∗

♦♦♦♦

♦

♦

♦

∗

Coun

ts

2 - T h e t a

♦

M o O 2

1 a

Figure S6.3. XRD patterns of sample (1) (MoO3/KIT-6) before (b) and after (a) reduction by ammoniaat 450 ◦C

97

1 0 2 0 3 0 4 0 5 0 6 0

2 b

♥

♥

♥

⊕⊕

⊕

⊕ ⊕⊕ ⊕ ⊕ ⊕⊕⊕

⊕

♥

♥♥♥♥♥

♥

♥

⊕⊕⊕

⊕⊕

⊕∗

∗∗

♦

♦♦♦

♦♦♦

♦♦

Coun

ts

2 - T h e t a

♦

∗

⊕

B iM o O 2B i 2 M o O 6

♥B i 2 M o 3 O 1 2

2 a

Figure S6.4. XRD patterns of sample (2) (Bi2O3. 3 MoO3-KIT6) before (b) and after (a) reduction byammonia at 450 ◦C

98

1 0 2 0 3 0 4 0 5 0 6 00

1 0 0

2 0 0

3 0 0M o O 3

∗

∗

♦

♦♦♦♦

♦♦♦

♦

♦

♦

♦ ∗

∗

Coun

ts

2 - T h e t a

M o O 2

3 b

3 a

Figure S6.5. XRD patterns of sample (3) (SbMo9Ox/SiO2) before (b) and after (a) reduction by am-monia at 450 ◦C

99

4a

4b

Figure S6.6. XRD patterns of sample (4) (Sb9MoOx/SiO2) before (b) and after (a) reduction by am-monia at 450 ◦C

100

5a

5b

Figure S6.7. XRD patterns of sample (5) (Bi2O3/KIT-6) before (b) and after (a) reduction by ammoniaat 450 ◦C

101

1 0 2 0 3 0 4 0 5 0 6 00

5 0

1 0 0

1 5 0

2 0 0

2 5 0

6 b

Coun

ts

2 - T h e t a

6 a

Figure S6.8. XRD patterns of sample (6) (Sb2O3/SiO2) before (b) and after (a) reduction by ammoniaat 450 ◦C

102

2a

2b

Figure S6.9. Mo3p3/2 (N1s) XPS of sample (2) (Bi2O3. 3 MoO3-KIT6) before (b) and after (a) reductionby ammonia at 450 ◦C

103

Chapter 7

Conclusions and Future work

7.1 Overview of contributions

In this work, we designed a series of mixed metal oxide catalysts with high efficiency foracrolein ammoxidation to acrylonitrile. These catalysts were prepared by two simple meth-ods with hard- and soft-templating techniques. Our samples from both methods showedhigh specific surface, large pore size, narrow pore size distribution and large pore volumehigher than those of recently reported ones. In addition, this report was the first one consid-ering the AC ammoxidation reaction conditions caring for flammability region.

In Chapter 4, a series of bismuth molybdates supported in mesoporous silica KIT-6 wassynthesized by the hard-templating method. These metal oxide catalysts containing threedifferent phases of bismuth molybdates (α, β and γ) were easily prepared and then theircatalytic properties investigated in the acrolein ammoxidation to acrylonitrile under condi-tions outside of the flammability region. Based on the blank and several catalytic tests, thereaction temperatures were chosen in the range from 350 to 450 ◦C. At temperatures above500 ◦C, 50 % of acrolein was converted to non-desired by-products. Our catalysts exhib-ited high catalytic activity, ACN selectivity, and stability as compared to previously reportedones such as SbVO4, AsFeO, and SbMoO. The specific surface areas and synergistic phaseswere found to have major effects on the catalytic process. Other parameters such as reactantratios and contact time also affected the ACN selectivity. Bi2O3. 3 MoO3/KIT-6 was the bestcatalyst with the highest AC conversion of 84 % and ACN selectivity of 97 %.

In Chapter 5, other series of molybdate antimonates supported on mesoporous silica wasprepared by a new simple soft-templating method. Beside high specific surface areas andpore volumes, the metal oxides of our catalysts were highly dispersed in mesoporous silicasupports over only one step. This method can be applied to synthesize mesostructured metaloxides. The results showed that single molybdenum oxide (MoO3) and mixed Sb-Mo withminor Sb content yielded high AC conversion up to 87 % and high ACN selectivity of 80 %.

105

Molybdate content played a major role in both AC conversion and ACN selectivity for ACammoxidation to ACN.

Chapter 6 is the first report studying the reaction mechanism of AC ammoxidation. Severalcatalysts with high and low catalytic activities in AC ammoxidation reaction, prepared inthe works reported in Chapters 4-5, were selected for further investigation. Based on theobservations of color changes upon degassing or reaction with ammonia, as well as somechanges in XRD and XPS spectra of these catalysts, it was shown that high concentrationof oxygen vacancies, easy reduction by ammonia, and reoxidation by oxygen are associatedwith the catalytic activity in AC ammoxidation. The catalysts containing molybdenum ox-ides show high catalytic activity in the AC ammoxidation to ACN. A new AC ammoxidationreaction mechanism was proposed based on a simple interaction between -NH and -CHO(in acrolein) groups over MoO3 catalysts.

7.2 Future work

There are some works that could be done in the future to continue this investigation

1. Reaction mechanism of the AC ammoxidation to ACN based on bismuth molybdateand antimonate molybdate oxides could be further studied by the in-situ FT-IR. Thecatalysts will firstly be contacted with an ammonia flow at 450 ◦C and then reactedwith AC at different temperatures ranging from 300 to 450 ◦C. All changes in thesurface functional groups may be recorded on-line by FT-IR.

2. As shown in our results, bismuth molybdate oxides are promising candidates for theAC ammoxidation. However, the AC conversion over these catalysts is still limited to82 %. Some additional elements such as P, Co, Ni, and Fe could enhance their catalyticactivities. Therefore, combining these elements with the bismuth molybdate oxidesmay be a promising challenge for the AC ammoxidation to ACN.

3. Support materials also play a significant role for catalytic properties. Mesostructuredalumina shows high specific surface area, large pore volume and large pore size. Inaddition, alumina contains a large number of hydroxyl groups on its surface that canenhance interaction between catalysts and supports. Hence, mesostructured aluminamay have interesting properties as a catalyst support for AC ammoxidation to ACN.

4. Catalysts containing macropores and mesopores exhibit easy diffusion of reactantsto active sites. Thus, combining these different kinds of pores in the preparation ofmacro/meso pore bearing materials will be a promising synthesis strategy to enhanceactivity of mixed metal oxides.

106

Appendix

.1 Nitrogen physisorption

Nitrogen physisorption has been widely used for the characterization of pore size, poresize distribution, specific surface area, and pore volume of mesomaterials. The adsorptionisotherm is corresponding to the number of adsorbed molecules as a function of pressure ata given temperature [222–224]. Nitrogen and argon have been often used as adsorbates atcryogenic temperature (77 and 87 K, respectively).

The International Union of Pure and Applied Chemistry (IUPAC) classified the adsorptionisotherms into six types, corresponding to their internal pore sizes as shown in Fig. A1.Micropore size is less than 2 nm. Mesopores range between 2 and 50 nm. The pore size ofmacropores is larger than 50 nm. Type I isotherm is considered as respresenting purely mi-

Figure A1. Types of physisorption isotherm (left) and hysteresis loop (right) [223, 224]

croporous materials. Type II and type III isotherms are designating as nonporous or macrop-orous materials. Type IV isotherm is occurring on porous adsorbents possessing pore radiusranging approximately from 15 to 1000 Å. This slope is increased at higher elevated pres-sures corresponding to a large amount of adsorbate uptake. Type V isotherm is regarded ascharacterizing small adsorbate-adsorbent interaction potentials that are similar to the typeIII isotherm. Type V isotherm is, however, also associating with pores in the same rangeas those of the type IV isotherms. Type VI isotherm is a new rare type of isotherm, whichexhibits a series of steps [224].

At relative pressure (P/P0) less than 0.3, the pore radii would be smaller than 15 Å. At P/P0

above 0.3, the IUPAC classification scheme showed four types of hysteresis loops as shownin Fig. A1. H1 hysteresis loop indicated a narrow pore size distribution such as cylindricalpores. H2 hysteresis loop is associated with pore blocking or percolation. H3 hysteresis loopdescribes no limitation adsorption at high P/P0 because the pores are formed by a non-rigidaggregation of plate-like particles or assemblages of slit-shaped pores. H4 hysteresis loopexhibits micropores and mesopores.

Normally, specific surface area is determined by the Brunauer-Emmett-Teller (BET) methodfollowing Eq. 1:

SBET = nmLσ (1)

where L is the Avogadro constant, σ is the effective molecular cross-sectional area in a com-plete monolayer, and nm (the capacity of the monolayer) is calculated from the linear BETplot within relative pressures ranging from 0.05 to 0.3. The pore size distribution is com-monly determined using either BJH model (named after its discoverers Barrett, Joyner, andHalenda) or non-local density functional theory (NLDFT) [222]. The NLDFT principle is of-ten applied for complete nanopores ranging from 0.5 to 100 nm, so this method has beenwidely used more than the BJH method.

In this work, the samples are measured the texture propeties by using a Quantachrome Nova2000 series instrument. They are first degassed in in vacuumn at 150 ◦C for 6h, then run N2

physisorption isotherms at 77 K (maintain that temperature in liquid nitrogen). Specificsurface area of the catalysts is calculated using the linear part of the BET plot (from 0.05 to0.2 of relative pressure). The pore size distribution is calculated from the adsorption branchfollowing the NLDFT (non-local density functional theory) method for cylinder shape ofsilica. The pore volume is taken as the adsorbed nitrogen volume at 0.99 relative pressure.

.2 X-ray diffraction (XRD)

X-ray diffraction technique could provide some important information about geometric struc-ture of crystalline phases, crystalline size, bulk defects, shapes, and absence of doping ele-

108

Figure A2. X-rays exhibit constructive interference in Bragg’s law reflection

ments in the lattice [225, 226]. Inter-atomic distances in a crystal are on the same scale as thewavelength of X-rays and bound electrons are acted as a diffraction grating. XRD providesthe region distance (distance from one crystal plane to the next one) with the highest electrondensity. Scattering occurs to each atom layer of the crystal resulting in characteristic sets ofsharp lines as described in Fig. A2.

The Bragg Law Basic equation is showed in Eq. 2:

nλ = 2dsinθ (2)

where λ is the wavelength of the X-ray, n is the integer number, d is the distance between theadjusted crystal planes, and θ is the Bragg angle. The unit cell parameter can be determinedbased on θ. However, XRD cannot detect the particles that are either too small or amorphous.In addition, small-angle X-ray scattering (SAXS) (angle ranging from 0 to 5) technique is apopular analytic method to investigate mesomaterials because it can enhance the resolutionand exactly identify the mesostructures.

.3 Transmission electron microscopy (TEM)

Transmission electrons microscopy (TEM) is a popular and useful technique to obtain accu-rate data of morphology, size, structure, and defects [227]. In principle, the highly energeticelectrons (100-300 kV) are transmitted through an ultra-thin specimen. During this pro-cess, the electrons are reacted with the specimen as shown in Fig. A3. Most electrons arepassed through to the specimen that required thin enough about 5-75 nm. Some electronsare absorbed on the specimen as a function of the specimen thickness and composition. Inaddition, the other electrons are scattered over small angles that caused phase contrast in theimage. In crystalline specimen, some electrons are scattered in distinct directions (diffracted

109

e- (absorbed)

Electron beam

Specimen

Reflected electrons

Transmitted beam(without interaction)

Scattered beam

5-75 nm

Diffracted beam

Figure A3. Main interaction of incident electron beam with specimen in transmission electron mi-croscopy

Plasma Monochromator Detector

Figure A4. Diagram of inductively coupled plasma

contrast in the image). Moreover, some electrons are also reflected. Transmitted electronsbeam can be counted by an energy loss during this process. The energy loss could carryinformation about the elemental, chemical, and electronic states of the sample atoms.

110

.4 Inductively coupled plasma (ICP)

Inductively coupled plasma (ICP) is an analytical method to determine trace elements inenvironment. In principle, a plasma source (Ar gas at high temperature about 6000-10000K) is dissociated the sample of constituent atoms or ions at higher energy level. Then theyare returned to the ground state or to the lower energy state and then emitted light energy[228]. A wavelength for each element is unique, so that the element is detected based on thelight energy. In addition, the intensity of emitted light is proportional to the concentration ofelement. Therefore, this method can identify about 80 elements in both quality and quantity.Diagram of ICP is simplified as shown in Fig. A4. In this work, 15 mg of our metal oxidesis digested in 100 ml of solvent including 25 ml of HCl 3M, 1 ml of concentrated HF andwater. The chemical composition of the metal oxide catalysts is established by InductivelyCoupled Plasma-Optical Emission Spectroscopy (ICP-OES) using a Perkin Elmer Optima4300DV spectrometer.

X-ray

hν

Fermi level

Vacuum level

Photoelectron

Photoemission

Ek

Eb

ᵩhν

Figure A5. Schematic energy level diagram for photoemission

111

.5 X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) uses a soft X-ray source (AlKα-1486.6 eV or MgKα-1253.6 eV) to ionize electrons on the surface solid. First, an atom is absorbed a photon energy(hν) causing emitted electrons with kinetic energy Ek (Fig. A5). According to the conserva-tion of energy law:

Eb = hν− Ek − ϕ (3)

where h is the Planck’s constant; ν is the frequency of the exciting radiation; Ek is the kineticenergy of ejected electrons on the surface of sample, which is measured by a detector; Eb

is the binding energy of photoelectron with respect to the Fermi level of the sample; ϕ isthe work function of the spectrometer. Based on Eq. 3, the binding energy of electrons,Eb, is measured, then elemental composition and chemical/ electronic state are identified[229]. This method is applied to our research for studying the chemical state and surfacecomposition of the metal oxide catalysts by using a photoelectron spectrometer (Kratos Axis-Ultra, evacuated to 10−9 Torr) equipped with a focused X-ray source (Al Kα, hν = 1486.6 eV).

.6 Thermal analysis (TGA/DTA)

Thermogravimetric analysis (TGA) is a technique for measurement weight loss of substancewith a function of temperature [230, 231]. The effects of temperature on the raw metal oxides(before calcination) are investigated. These tests are carried out at the conditions that aresimilar to those of the sample calcining (ramp rate 5 ◦C/min in air). Two major weightlosses are observed: (i) from 100-150 ◦C physisorbed water and solvent ; (ii) from 200-400 ◦Cdecomposition of salts or organic groups. In parallel with TGA, differential thermal analysis(DTA) is characterized by any energy taking in place during thermal treatment. Based onthese data, the metal oxides phases are determined [230, 231]. All tests in this report areconducted in TGA Q5000 V3.17 Build 265 in Department of Chemical Engineering, LavalUniversity.

.7 Raman spectroscopy

Raman spectroscopy is a spectroscopic technique that provides molecular vibration spectra.Based on the molecule fingerprints, the sample can be identified and quantified. A laser inthe visible, near infrared, or near ultraviolet range is used to interact with molecular vibra-tions or phonons. The major scattered light is occurred as Rayleigh (elastic scattering) withthe same frequency of excitation sources. The other scattered lights including stocks andantistocks (inelastic scattering) are shifted in energy. The stock lines are recorded as Ramanspectrum that corresponds to vibration of functional groups [199, 232, 233].

112

In this study, Raman spectroscopy is used to identify the metal oxide phases. This techniquewill be useful for small and well dispersed particles that can not be determined by XRD.

.8 Gas chromatography (GC)

Gas chromatography (GC) is a popular method for separation of volatile products basedon the difference of partitioning behaviors between a mobile phase and a stationary phase[234]. The mobile phase is comprised of volatile gases and an inert gas (carrier gas) suchas helium, argon, or nitrogen. The stationary phase as solid with high specific surface areais packed into a column. In fact, vapor or gas mixture are separated by physisorption instationary phase. Therefore, the separation process is depended on the polarity of stationaryphase, temperature, carrier gas flow, column length, and concentration of volatile gases.In this report, two chromatography columns including molecular sieve 13X and HayesepP are used to separate O2, N2, air, carbon mono-oxide, carbon dioxide, water, ammonia,methanol, acetonitrile, acrolein and acrylonitrile that are the reactants and products of ACammoxidation. The columns are set up in an oven of the GC (HP 5890).

Figure A6. Propene oxidation and ammoxidation mechanism over bismuth molybdate catalysts [8]

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