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Applied Catalysis A: General 246 (2003) 4968

Oxidative dehydrogenation of isobutane over V2O5-based catalystsprepared by grafting vanadyl alkoxides on TiO2SiO2 supports

V. Iannazzo a, G. Neri a, S. Galvagno a, M. Di Serio b, R. Tesser b, E. Santacesaria b,

a Dipartimento di Chimica Industriale e Ingegneria dei Materiali dellUniversit di Messina, Salita Sperone 31, Messina 98166, Italyb Dipartimento di Chimica dellUniversit di Napoli Federico II, Complesso di M. te S. Angelo, Via Cinthia, Napoli 80126, Italy

Received 31 July 2002; received in revised form 9 December 2002; accepted 10 December 2002

Abstract

Following four different procedures many vanadium-based catalysts have been prepared by using the grafting technique and

have been tested on the oxidative dehydrogenation of isobutane. Thebest results of selectivity have been obtained withcatalysts

prepared by grafting bimetallic vanadiumtitanium alkoxides directly on silica. The alkoxide precursors have been obtained

by partially hydrolysing titanium alkoxide, dissolved in isopropanol, with a stoichiometric amount of water and reacting

then with vanadyl tri-isopropoxide, or alternatively by mixing the two mentioned alkoxides in isopropanol and submitting

both to controlled partial hydrolysis. The bimetallic alkoxide grafted on silica show a prevalence of isolated VOTi bonds

with respect to polyvanadylic VOV bonds that are prevalent, on the contrary, when vanadyl tri-isopropoxide dissolved in

n-hexane is grafted on a TiO2SiO2 support. Catalysts characterised by the prevalence of VOTi bonds are slightly less

active but two times more selective than catalysts in which VOV bonds prevail. The preparation of vanadium-based catalystswith a favourable TiO2 environment has been largely simplified by avoiding the use of a TiO2SiO2 support obtaining, in the

meantime, a remarkable improvement in the selectivity.

2003 Elsevier Science B.V. All rights reserved.

Keywords: Isobutane; Oxidative dehydrogenation; Grafting alkoxides; V2O5; Hydrolysis

1. Introduction

Isobutene is an important feedstock for petrochem-

ical, polymer and chemical industries [1,2]. Light

olefins can be produced by dehydrogenation, at hightemperature, of the corresponding alkanes. However,

the catalytic dehydrogenation still suffers from a

number of limitations including high energy input and

catalyst deactivation. The light alkane oxidative dehy-

drogenation (ODH) represents an alternative for the

production of these chemicals, provided that highly

selective catalysts are developed. A number of studies

on the ODH of isobutane to isobutene are reported

Corresponding author.

in the literature [39]. It can be pointed out that sup-

ported V2O5, a well-established catalyst for the ODH

of propane [1016], was less investigated in the ODH

of isobutane. With conventional supported V2O5 cat-

alysts, low selectivity to the desired olefines werereported. Hoang et al. [17] obtained over V2O5/Al2O3catalysts a selectivity of less than 15%, at 7% of

isobutane conversion. However, a very recent paper

of Zhang et al. [18] reports that vanadium-containing

MCM-41 catalysts prepared by a direct hydrothermal

(DHT) method show selectivities to isobutene higher

than 40%, at a conversion of about 10%. On the other

hand, it has been clearly shown that activities and

selectivities of vanadia-based catalysts in the ODH

of alkane strongly depend on the VOx environment,

0926-860X/03/$ see front matter 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0926-860X(02)00668-3

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50 V. Iannazzo et al. / Applied Catalysis A: General 246 (2003) 4968

on the VOx dispersion, on the acidbase properties

of the support, and therefore, on the method of cata-

lyst preparation [19]. Activities and selectivities also

depend on the size of the alkane molecule, being dif-ferent for the same catalyst for respectively ethane,

propane and butane [20].

Vanadia catalysts prepared by grafting vanadyl

tri-isopropoxide on a titaniasilica support were in-

vestigated with the aim of developing catalysts for

the selective ODH of isobutane to isobutene. It is

well known that titania allows to obtain a very good

dispersion of V2O5 on its surface [21,22], but the

resulting samples show low surface area and low re-

sistance to sintering. On the contrary, SiO2 weakly

interacting with V2O5 favours the thermally induced

agglomeration of VOx species on the surface [22].

Hence, the preparation of mixed TiO2/SiO2 supports

by co-precipitation, by impregnation of titanium salts

on silica or by grafting titanium alkoxides on sil-

ica surface, is a common practice. In particular, the

grafting technique is an interesting route to obtain

TiO2/SiO2 supports [2327] with:

(i) a surface area higher than that usually obtained

with titania (typically 50100 m2/g);

(ii) a high resistance to sintering and good mechani-

cal properties;(iii) a higher dispersion of the surface active species.

The grafting method, can also be used then to an-

chor vanadyl chloride or vanadyl tri-isopropoxide, in

a water-free solvent, on the hydroxyl groups of a sup-

port. This technique has been used to obtain the VOxphase on respectively: SiO2 [28,29], Al2O3, [28,29],

TiO2 [2832], TiO2/SiO2 [3335].

In the present work, vanadium-based catalysts have

been prepared by using four different grafting proce-

dures:

(a) grafting vanadyl tri-isopropoxide, dissolved in

n-hexane, on TiO2/SiO2;

(b) grafting vanadyl tri-isopropoxide, dissolved inn-hexane and partially hydrolysed by a controlled

procedure before grafting on TiO2/SiO2;

(c) grafting vanadyl tri-isopropoxide, dissolved in

isopropanol and partially hydrolysed by a con-

trolled procedure before grafting on TiO2/SiO2;

(d) grafting directly on SiO2 mixtures of titanium and

vanadylic alkoxides, dissolved in isopropanol and

submitted to partial hydrolysis, performed in dif-

ferent ways, before grafting.

Partial hydrolysis of vanadyl tri-isopropoxide has the

objective of inducing a moderate molecular aggrega-

tion, before grafting, with the aim of modifying vana-

dium dispersion on the surface and verify, therefore,

the effect of vanadia dispersion on the catalytic ac-

tivity and selectivity in the ODH of isobutane. Partial

hydrolysis of mixtures of titanium and vanadyl alkox-

ides brings, on the contrary, to bimetallic alkoxides in

isopropanol solution that have been anchored directly

on SiO2 support. In this way, the catalyst preparation

of a vanadium-based catalyst having an intimate TiO2environment is largely simplified without loosing, as

it will be seen, activity but increasing selectivity. Anincreased selectivity has been observed also for cata-

lysts in which vanadyl tri-isopropoxide is grafted on

silica coated with a sub-monolayer of TiO2. As it will

be seen, the conclusion is that for obtaining good se-

lectivities we need a good dispersion of vanadia and a

prevalence of VOTi bonds with respect to VOV

ones.

A simplified kinetic approach has shown that a com-

plex reaction scheme is operative in which CO and

CO2 are produced by both isobutane and isobutene.

The influence of the reaction parameters such as, tem-perature, isobutane and oxygen partial pressure and

contact time on the reaction products distribution, over

all the mentioned catalysts, is reported.

The structure of both the titaniasilica supports,

prepared by a multi-step grafting procedure, and

the vanadia catalysts were characterised by several

techniques including scanning electron microscopy

(SEM) with EDX, X-ray diffraction (XRD), BET

surface area measurements, temperature programmed

reduction and oxidation (TPR, TPO), and FTIR- and

UV-diffuse reflectance. In the present work, an effort

has been made to correlate the observed properties

with the obtained performances.

2. Experimental

2.1. Catalysts and supports preparation methods

The TiO2SiO2 support was prepared in three

steps by grafting, first of all, titanium isopropoxide

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V. Iannazzo et al. / Applied Catalysis A: General 246 (2003) 4968 51

dissolved in toluene on commercial SiO2 (furnished

by Grace, type S 432). The amount of titanium iso-

propoxide, dissolved in toluene and used for the first

step of grafting, roughly corresponds to a monolayeror to a moderate excess with respect to a monolayer,

by assuming a conventional stoichiometry of one

hydroxyl for one alkoxide molecule. Silica, after cal-

cination at 500 C for 28 h, was contacted with the

mentioned solution by refluxing, at the boiling point

of the solvent for 6 h. The density of the OH groups

on the silica surface was determined by means of

thermogravimetric analysis (TGA), as previously re-

ported [27]. The solid obtained after the first grafting

step was recovered by filtration, washed with toluene,

dried at about 105 C, steamed at 190 C for 2 h, for

eliminating residual alkoxide groups from the surface

by hydrolysis and finally calcined at 500 C for 2h.

The same procedure was repeated other two times to

obtain a support of silica coated with a multi-layer of

TiO2. The properties of the silica coated supports in

correspondence of each of the three different graft-

ing steps are reported in Table 1, together with some

properties of the solids obtained after each step. The

differences observed in the two series of TiO2SiO2supports prepared and reported in Table 1 correspond

to the differences in the adopted calcination times (2 h

at 500 C for the TS11 to TSM1 series and 8 h forTS1 to TSM series) and in the amounts of titanium

alkoxide used for each step. A support TsmS contain-

ing a sub-monolayer of TiO2 has also been prepared.

The supports of silica coated with multi-layers of ti-

tania were then contacted with different solutions of

vanadyl tri-isopropoxide in n-hexane or isopropanol

at room temperature for 24 h under He atmosphere.

After reaction the samples were filtered, washed in

Table 1

Properties of the prepared supports TSM1, TSM and TsmS and of the solids obtained during multi-step titanium alkoxide grafting on silica

Sample Acronyms Grafting

step

TiO2(wt.%)

Metal initial

amount (mmol/g)

SSA (m2 /g) Pore volume

(cm3/g)

OH density

(mmol/g)

SiO2 S 0 282 1.02 0.92

TiO2SiO2 TS11 1 7.0 1.37

TiO2SiO2 TS21 2 13.7 1.37

TiO2SiO2 TSM1 3 17.8 1.37 289

TiO2SiO2 TS1 1 5.9 0.95 237 0.23 0.71

TiO2SiO2 TS2 2 9.7 0.95 267 0.26 0.79

TiO2SiO2 TSM 3 11.3 0.95 299 0.27 0.82

TiO2SiO2 TsmS 1 2.3 0.30 245 0.81

n-hexane, dried at 105 C, steamed at 190 C for 2h

and calcined at 500 C for 2 h. In Table 2, a list of all

the prepared catalysts is reported together with the

conditions adopted for the preparation, the loading ofTiO2 and V2O5 and other properties. The acronyms

of the catalysts are easily interpretable and summarise

both the preparation methods and the compositions.

We have named for example: S = SiO2, T = TiO2,

TSM and TSM1silica coated with a multi-layer

of TiO2, TsmSsilica coated with a sub-monolayer

of TiO2, Hn-hexane solvent of the precursor, I

isopropanol solvent. Suffix h corresponds to a hydrol-

ysed alkoxide. Catalysts of VH/TSM and VH/TSM1

series, for example, were prepared by grafting vanadyl

tri-isopropoxide dissolved in n-hexane on TiO2SiO2supports. Grafting was made under an inert and dry

atmosphere, at room temperature, by contacting the

support with the stirred solution. The grafting yields

in this case is almost quantitative.

Catalysts of the VhH/TSM1 series were pre-

pared by flowing moistured nitrogen in a vanadyl

tri-isopropoxide solution in n-hexane, for different

times, inferior to the time necessary for obtain-

ing a precipitate of vanadium oxide hydrate (about

1617 min). Partially hydrolysed vanadium alkox-

ides, so obtained, are then grafted on the TiO2SiO2

support by contacting solution and solid at roomtemperature, for 24 h under a dry helium atmosphere.

After grafting reaction, the catalyst was recovered

by filtration, washed with the used solvent, dried at

105 C, steamed for 2 h at 150 C and finally calcined

at 500 C for 2 h.

Catalysts of the VhI/TsmS and VhI/TSM type were

prepared by grafting partially hydrolysed vanadyl

tri-isopropoxide, dissolved in 2-propanol, on the

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Table 2

List of the prepared catalysts, their properties and preparation modalities

Catalyst Acronyms TiO2(mmol/g)

V2O5(wt.%)

Preparation modalities BET surface

area (m2/g)

V2O5/TiO2SiO2 VH/TSM 1.41 0.80 Grafting in n-hexane 249

V2O5/TiO2SiO2 VH/TSM1(1) 2.20 0.65 Grafting in n-hexane 243

V2O5/TiO2SiO2 VH/TSM1(2) 2.20 2.00 Grafting in n-hexane 250 V2O5/TiO2SiO2 VH/TSM1(4) 2.20 3.60 Grafting in n-hexane 258

V2O5/TiO2SiO2 VhH/TSM1 (5)a 2.20 1.03 Grafting of partially

hydrolysed vanadyl

tri-isopropoxide in n-hexane

280


hydrolysed vanadyl


269


hydrolysed vanadyl


239

V2O5/TiO2SiO2 (H2O/Vh = 1) VhI/TSM 1.41 0.90 Grafting of partially

hydrolysed vanadyl

tri-isopropoxide in

isopropanol

314

V2O5/TiO2SiO2sub-monolayer of TiO2(H2O/Vh = 1)

VhI/TsmS 0.30 0.56 Grafting of partially

hydrolysed vanadyl

tri-isopropoxide in

isopropanol

V2O5TiO2/SiO2 (H2O/Vh = 1) (Vh-T)I/S (Ti/V = 4) 0.22 0.80 Grafting of partially

hydrolysed vanadyl

tri-isopropoxide reacted with

titanium tetra-isopropoxide

in isopropanol

335

V2O5TiO2/SiO2 (H2O/(V-T)h = 1) (V-T)h I/S (Ti/V = 12) 0.91 0.80 Grafting of partially

hydrolysed vanadyl

tri-isopropoxide in mixture

with titaniumtetra-isopropoxide in

isopropanol

303

V2O5TiO2/SiO2 (H2O/Th = 1) (Th-V)I/S (Ti/V = 12) 0.35 0.90 Grafting of partially

hydrolysed titanium

tetra-isopropoxide reacted

with vanadyl tri-isopropoxide

in isopropanol

348

a Hydrolysis time in minutes.

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corresponding supports, again by taking the solution

and the solid in contact at room temperature for 24 h

under a dry helium atmosphere. Hydrolysis was car-

ried out by dissolving 1 ml of vanadyl tri-isopropoxidein 4 ml of 2-propanol and by adding a stoichiomet-

ric amount of water (1 mol of water/1 mol of vanadyl

tri-isopropoxide) containing traces of HCl acting as

catalyst. The operation has been made at room temper-

ature for 5 h, under stirring, always by keeping the so-

lution under a dry helium atmosphere. After the graft-

ing reaction, the catalyst was recovered by filtration,

washed with the used solvent, dried at 105 C, steamed

for 2 h at 150C and finally calcined at 500 C for 2 h.

The grafting yields in these cases are not quantita-

tive for the influence of the solvent (the parent alco-

hol) on the following equilibrium:

surface OH + Me(OR)n

surface O Me(OR)n1 +ROH (1)

As a consequence, small amounts of vanadium oxide

(

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Carboxen 1000) using a TCD detector and another

one suitable for the hydrocarbons analysis (a col-

umn of fused silica Plot with Al2O3 /KCl of 50m

length, 0.53 mm of diameter, using a stationary phaseChrompack) using a FID detector. Blank runs, per-

formed in the empty reactor at 500 C, allowed to rule

out the occurrence of homogeneous reactions to a

significant extent. Conversions and selectivities were

defined as it follows:

Y=

molisobutane reacted

molisobutane in the feed

100

Si =

molisobutane converted to i

molisobutane reacted

100

where i = i-C4H8, CO, CO2 and others (CH4, C2H6,C3H8, C3H6, n-C4H10, etc.).

Catalytic runs have been performed by changing for

each catalyst the temperature and the residence time.

In some runs the partial pressure of the reagents have

been changed too.

3. Results

3.1. Catalytic activities and selectivities

Results obtained in the ODH of isobutane over

the investigated catalysts have shown that, under the

experimental conditions adopted, isobutene, carbon

monoxide and carbon dioxide were the main reaction

products. Only small amounts (

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Table 3

Catalytic runs performed at different temperatures

Catalyst code V2O5 (wt.%) T (C) Isobutane conversion (%) Selectivities (%)

i-C4H8 CO CO2

VH/TSM 0.8 325 3.3 25.2 30.5 41.2

0.8 340 5 17.5 36.8 44.8

0.8 380 12.3 10 48.2 40.3

0.8 430 16.9 8.3 53.2 37.4

VH/TSM1(1) 0.65 380 3.7 13.3 43 41.2

0.65 420 7.9 9 46.7 41.6

0.65 450 11.9 8.7 52.5 36

0.65 520 22.6 8.2 60.6 27.4

VH/TSM1(2) 2 335 2.6 9.3 36.1 52.1

2 380 5.2 11.9 36.3 46.6

2 390 9.1 11.6 42.4 41.6

2 450 21 9 57 31.1

VH/TSM1(4) 3.6 310 3.4 20.2 37.6 39.4

3.6 330 5.8 14.6 41.9 42.3

3.6 350 6.9 13.6 42 42.7

3.6 500 34.9 3.7 64.4 31.3

VhI/TSM 0.9 340 3.6 18.1 35.6 44.3

0.9 350 4.5 14.3 39.7 44.1

0.9 370 7.9 11.2 44.6 42.1

0.9 380 16.6 8.3 54.6 35

0.9 470 36.1 10.1 61.1 26.5

VhI/TsmS 0.56 405 2.4 24 32.5 35.1

0.56 420 3.1 26.4 37.6 28.2

0.56 455 4.9 24.5 40.4 27.90.56 480 5.9 23.6 41.3 28.8

(Th-V)I/S 0.9 380 3.8 29 41.6 27

0.9 420 6.2 23.4 43.6 30.6

0.9 470 9.4 20.9 47.4 30

0.9 490 11.2 19.7 48.3 30.2

(Vh-T)I/S 0.8 380 3.1 15.3 39.6 37.3

0.8 420 6.5 12.6 47.8 35.6

0.8 465 11.2 12.4 53.4 30.3

0.8 500 14.2 13.9 59.5 22.7

(V-T)hI/S 0.8 380 4.4 29.5 35.5 30.9

0.8 410 6.1 26.9 40.3 29.4

0.8 450 9.4 23 45.3 300.8 480 9.8 25.2 46.6 27.3

0.8 515 16.4 19.4 50.1 28.8

Other reaction conditions have been kept constant: catalyst amount = 0.4 g; Ftot = 300 ml/min.

the examined catalysts, two different ranges of selec-

tivity and this is probably related to the prevalence

on the surface of two different catalytic sites. The

most selective catalysts have in the temperature range

350500 C selectivities changing from 30 to 20% by

increasing the conversion, that is about the double of

the less selective ones. From the runs of Table 4 other

interesting information can be obtained. In Fig. 4, for

example, it is possible to appreciate the evolution of

the conversions and selectivities with the residence

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Table 4

Catalytic runs made at 450C by changing residence time and reagents partial pressures

Catalyst code Catalyst

weight (g)

V2O5

(wt.%)

Tot (ml/min) Pi-C4H10 (kPa) PO2 (kPa) Isobutane

conversion (%)

Selectivities (%)

i-C4H8 CO CO2

VH/TSM1(1) 0.3 0.65 200 7.2 13.17 9.5 8.7 54.4 35.4

0.3 0.65 300 7.69 13.04 7.6 10.6 54.8 32.7

0.3 0.65 500 7.09 13.17 9.4 10.2 54.3 33

0.1 0.65 200 7.2 13.17 6.1 10.2 48.7 36.5

0.1 0.65 300 7.77 13.17 4.9 19.1 43.7 30.2

0.1 0.65 490 6.2 12.41 4.3 20.5 37.4 28.7

VH/TSM1(2) 0.3 2 200 7.6 13.17 28.8 7 51.7 38.8

0.3 2 500 7.09 13.17 14.6 12.3 50.4 33.5

VH/TSM1(4) 0.1 3.6 200 7.6 13.17 24.3 8.4 58.5 31.4

0.1 3.6 300 7.62 12.91 18.2 10 59.6 28.8

(Th-V)I/S 0.4 0.9 200 9.63 13.07 10.3 23.5 47.7 27.80.4 0.9 300 6.01 13.07 6.4 29.5 42.5 26.8

0.4 0.9 380 7.95 13.07 4.1 43.9 33.8 21.3

0.4 0.9 540 8.26 13.07 4.6 42 35.3 20.8

(Th-V)I/S 0.4 0.9 300 13.3 3.1 5.1 40.3 33.9 25.2

0.4 0.9 300 13.3 7.6 7 29.4 41.1 28.7

0.4 0.9 300 13.3 9.9 7.5 26.6 44.8 27.7

0.4 0.9 300 13.3 12.9 7.4 26.1 41.8 31

(Th-V)I/S 0.4 0.9 300 5.9 7.85 8.5 29 43.3 27.6

0.4 0.9 300 7.6 7.85 9.4 21.5 25.9 52.1

0.4 0.9 300 10.2 7.85 7.4 31.9 27.6 20.7

0.4 0.9 300 14.5 7.85 5.2 41.6 33.6 23.8

(V-T)hI/S 0.4 0.8 200 7.2 13.17 9.1 29.2 39.4 29.10.3 0.8 200 7.2 13.17 7 29 40.1 27.2

0.4 0.8 265 7.26 13 5.1 38.3 32.4 27.2

0.3 0.8 300 7.77 17.17 4.9 35.9 38.8 26.4

0.4 0.8 480 6.33 17.72 5.4 36.4 33.2 26.8

0.3 0.8 490 6.2 12.41 4.6 31.8 40.5 32.5

(Vh-T)I/S 0.3 0.8 200 7.2 13.17 23.9 11.5 54.8 28.6

0.3 0.8 300 7.77 13.17 5.1 25.6 44.2 24.7

0.3 0.8 500 7.09 13.17 8.1 22.6 46.1 22.7

VhH/TSM1 (15) 0.25 0.98 200 7.2 13.17 13.8 12 46.8 33.4

0.25 0.98 490 6.2 12.41 7.1 14.6 43.4 28.7

VhH/TSM1 (10) 0.31 1.05 200 7.2 13.17 14.6 10.1 50.7 35.2

0.31 1.05 300 7.77 13.17 10.7 15.5 46.8 33.7VhH/TSM1 (5) 0.3 1.03 200 7.2 13.17 14.9 12.2 53 31.7

0.3 1.03 300 7.77 13.17 4.9 19.1 43.7 30.2

VhI/TsmS 0.4 0.56 200 7.2 13.17 8.2 22.2 38.1 27.6

0.4 0.56 300 7.77 13.17 6.7 23.5 38.2 33.6

0.4 0.56 500 7.09 13.17 3.9 33.1 38.7 34.1

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Fig. 3. Evolution of the selectivity with the temperature for dif-

ferent catalysts.

time for a given catalyst (VH/TSM1(1)), at a fixed

temperature of 450 C. Similar trends are observable

also for other catalysts, that is, conversion obviously

increases with the residence time, while selectivity to

isobutene slightly decreases favouring the formation

of CO and CO2. In Fig. 5, it is possible to appreciate

the effect of vanadium loading on the activity of the

catalysts of the VH/TSM1 type. The activity seems

to increase about linearly with the vanadium load in

agreement with the very high dispersion of these cat-

alysts. Fig. 6, reporting selectivity to isobutene as afunction of the conversion for different catalysts, con-

firms the existence of two ranges of selectivity for the

examined catalysts and the most selective catalysts

resulted again (Th-V)I/S and (V-T)hI/S.

Fig. 4. An example of the evolution of conversion with residence

time for the VH/TSM1(1) catalyst.

Fig. 5. Influence of the vanadium load on the activity for VH/TSM1

type catalysts.

A kinetic approach applied to the overall isobutene

conversion has then been made, derived from the Mars

and Van Krevelen model [38], simplified by adopt-

ing a pseudo-first-order kinetic law in agreement with

the suggestion of Boisdron et al. [39]. We have in-

terpreted with this model the kinetic runs of some

of the catalysts reported in Tables 3 and 4. The ob-

tained results are reported in Table 5. As it can be

seen, the apparent activation energy for the overall re-action resulted about 914 Kcal/mol with significant

changes from one to another catalyst. It is also inter-

esting to observe that the activation energy is much

smaller than the one obtained for propane on similar

Fig. 6. Selectivity as a function of conversion for different catalysts.

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Table 5

Kinetic parameters and fitting errors on conversion

Catalyst Activation

energy(Kcal/mol)

Pre-exponential

factor(mol/gcat./h/atm)

Mean absolute

percent erroron isobutane

conversion

VH/TSM1(1) 14.3 5229 24.6

VH/TSM1(4) 11.8 1881 12.7

(Th-V)I/S 8.7 53.6 15.8

(V-T)hI/S 8.8 44.7 21.6

VhI/TsmS 13.8 1236 14.9

catalysts [40] (about 20 Kcal/mol) in agreement with a

more easy hydrogen abstraction from the hydrocarbon

molecule. Moreover, the high average errors given by

the model on the conversions calculations introducesome doubts on the correctness of the kinetic analysis.

As a matter of fact, by extrapolating to zero contact

time the selectivities of the main reaction products

(see, for example, Fig. 4), it is easy to observe that

high values are obtained not only for isobutene but

also for CO and CO2. This means that a complex ki-

netic scheme is operative in which CO and CO2 are

obtained not only from isobutene but also from isobu-

tane. The kinetic approach could be deepened, there-

fore, only by performing much more kinetic runs in

order to determine the kinetic parameters of the sev-eral parallel-consecutive occurring reactions.

3.2. Supports and catalysts characterisation

3.2.1. Specific surface area and pores distributions

The support TiO2SiO2 has been obtained, as pre-

viously described, by repeating three times the graft-

ing procedure. The amount of grafted TiO2 for each

grafting step is reported in Table 1 together with the

specific surface area, the pore volume and the surface

hydroxyls density (obtained by TGA as described else-where [27]). It is interesting to observe, first of all, that

the specific surface area of TiO2SiO2 remain very

high after titania grafting, comparable with the spe-

cific surface area of the original silica support. Pore

volume is, on the contrary, reduced for the appearance

of micropores (2030% of the pore volume) that was

not present in the original silica support. However, a

TiO2SiO2 support of high surface area and stable to

thermal treatment is obtained. As mentioned before,

the difference in TSM and TSM1 supports consists

only in the duration time of SiO2 calcination and in the

different concentration of the titanium alkoxide solu-

tion put in contact with the solid during the successive

grafting steps. OH density does not change too muchin the first two steps of grafting. A 2/2 stoichiom-

etry can, therefore, be suggested as prevalent in the

grafting reaction, i.e. two hydroxyls reacting with two

alkoxide groups of a molecule of titanium alkoxide.

Vanadyl alkoxide grafting in some cases reduces

further the specific surface area, but this remains al-

ways high and comparable with that of the original

support, in other cases an increase of the surface area

is observed, as it can be seen in Table 2. In particular,

catalysts prepared by grafting bimetallic alkoxide di-

rectly on silica do not show the strong decrease of pore

volume observed in the case of TiO2SiO2 supports.

3.2.2. XRD analyses

XRD analyses have been performed on different

supports and catalysts such as: TSM, TSM1, VH/

TSM, VhI/TSM, (Vh-T)I/S, (V-T)hI/S and (Th-V)I/S.

For the support TSM and the catalysts of the type

VH/TSM and VhI/TSM, XRD analyses show a signal

corresponding to the presence of small crystallites

of TiO2 in the form of anatase. The size of these

crystallites is somewhat higher in the presence of

vanadium oxide. However, the amorphous part ofTiO2 is largely predominant. The catalysts of the type

(Vh-T)I/S, (V-T)hI/S and (Th-V)I/S resulted com-

pletely amorphous. It is important then to point out

that we have never observed crystalline V2O5 in all

the examined catalysts. The conclusion is that TiO2coating is mainly composed of amorphous TiO2 with

some small crystallites of anatase, while V2O5 is

always highly dispersed and completely amorphous.

More details about the properties of the TiO2SiO2support are reported elsewhere [27].

3.2.3. Morphological analyses at SEM and EDX

SEM analyses performed on the TSM and TSM1

supports have shown the presence of particles of irreg-

ular shape having a wide size distribution (in the range

160m). No large cluster of TiO2 were observed on

the surface. This, coupled with EDX elemental map-

ping analysis, indicates an homogeneous distribution

of grafted titanium species on the surface of silica.

Catalysts prepared by grafting vanadium on sup-

ports TSM and TSM1 through procedures ac show

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Fig. 7. SEM micrograph showing the morphology of the sample VhI/Tsm S.

the same morphological features observed on the par-

ent supports (see micrograph in Fig. 7 taken on thesample VhI/TsmS). Grafted vanadium species are then

supposed to be highly dispersed on the carriers.

On the contrary, on catalysts of (V-T)I/S series, pre-

pared by procedure d, has been also noticed the pres-

ence of separate aggregates composed, according to

EDX analysis mainly of titanium and vanadium (Fig. 8

referred to catalyst (Th-V)I/S). These aggregates show

no crystalline shape, suggesting they are likely amor-

phous.

3.2.4. Spectroscopic analysesFTIR and DRIFT spectra have been collected for

many supports and catalysts reported in Tables 1 and 2.

It is known from the literature [41] that pure V2O5shows FTIR absorption bands at respectively 515,

603, 827, 950, 1019, 1600, 3666 cm1. The absorp-

tion bands between 550 and 670 cm1 are attributed

to the rocking modes of the VOV bonds, while the

bands between 670 and 770 cm1 correspond to the

stretching of the same bonds. The absorption band

at 980 cm1 is normally attributed to the symmetric

stretching of V=O bond in amorphous V2O5, while

the band at 1020 cm

1 corresponds to the vibration ofthe same bond and is characteristic of the crystalline

V2O5. At last, the band at 3670 cm1 is associated to

the vibration of the bond VOH. For monolayers of

V2O5, the band at 1020 cm1 disappears and that at

980 is enhanced [41]. So it is possible to recognise

the presence of crystalline V2O5 in a catalyst. For

very low charge of V2O5, as in our case, crystalline

V2O5 is completely absent and the band at 1020 cm1

is never observed, while absorption at 960990 cm1

confirms the formation of polyvanadylic species of

low nuclearity [42]. The FTIR spectra of the sup-ports TSM and TSM1 show four absorption bands.

The most intense, at 1104 cm1, corresponds to the

asymmetric stretching vibration of the bonds SiOSi

for the tetrahedric SiO4 units [4345]. Other bands

are observable at 1200 cm1 (asymmetric stretching

vibration of the SiO bonds) [46], 800cm1 (sym-

metric stretching vibration of the same bond) and

at about 950 cm1 corresponding to the asymmetric

stretching vibration of the SiOTi bond [45,47,48].

SiO2, the original support shows an intense band of

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Fig. 8. SEM micrographs and EDX elemental analysis of the sample (Th-V)I/S. Numbers refer to regions where corresponding EDX

patterns have been collected.

absorption at 3747 cm1 corresponding to isolated

silanol groups SiOH [49,50]. This peak disappears

as a consequence of the titanium alkoxide grafting,

as it can be appreciated in Fig. 9 where the spectra

for SiO2 and TiO2SiO2 (TSM) are compared. In the

same figure are also reported, for comparison, the

spectra of (V-T)hI/S and of VH/TSM. As it can be

seen, the spectrum for VH/TSM is quite similar to the

one for TSM, while the spectrum for (V-T)hI/S show

many silanol groups in agreement with the fact that

in this case a bimetallic alkoxide of vanadium and ti-tanium is directly grafted on silica without forming a

monolayer. The presence of intense bands character-

istic of the support and the presence of small amounts

of loaded V2O5 in the prepared catalysts, give place

to DRIFT spectra of low intensity in the wavenumber

ranges that are peculiar of vanadium bonds. How-

ever, some useful information can equally be derived

from the observation of these spectra made in the

range 4002000 cm1. In Fig. 10, the DRIFT spectra

for the catalysts of the VH/TSM1 type, containing

different amounts of V2O5, from 0.65 to 3.6 wt.%,

are reported. In this figure, it is possible to observe

a slight increase of the VOV absorption bands in

the range 550800 cm1 suggesting an increase of

the aggregation degree and a consequent decrease

of the V2O5 dispersion. The effect of hydrolysis of

vanadyl alkoxide in n-hexane solution performed

for different times (catalysts VhH/TSM1 (5,10,15))

has the effect of forming vanadium aggregates and

we also observe in this case a slight increase of the

VOV bands in the range 550800 cm

1

. Hydroly-sis in a polar solvent such as isopropanol, has a more

relevant effect that can be appreciated in Fig. 11,

where spectra of catalysts obtained from precursors

either subjected to hydrolysis or not are compared. It

is interesting to observe, for catalysts obtained from

bimetallic precursors subjected to hydrolysis (cata-

lysts (V-T)h/I/S, (Vh-T)/I/S), the disappearance of the

polyvanadylic VOV absorption bands. This means

that the adopted procedure of preparation prevents

the V2O5 aggregation and gives place to isolated

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Fig. 9. Disappearance of silanol SiOH groups as a consequence of titanium alkoxide grafting as it can be seen by DRIFT analysis.

Fig. 10. DRIFT spectra for three VH/TSM1 catalysts characterised by a different load of vanadium.

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Fig. 11. A comparison of DRIFT spectra obtained for different catalysts.

monovanadylic species or conversely to a prevalence

of VOTi bonds with respect to VOV ones.

Some supports and catalysts have also been sub-

mitted to DR-UV analysis. In Fig. 12 examples of thespectra obtained are reported. The effect of V2O5 in

altering the band of reflectance shown by the support

TSM1 is clear. The band between 250 and 350 nm

corresponds to titanium ion in octahedral coordination

[51,52]. The broadening of the band is due to V2O5 ab-

sorbing at wavelengths higher than TiO2 and broader.

Crystalline V2O5 absorbs at 500550 nm [41], and

as this band is never present in the examined cata-

lysts we can exclude the presence crystals of V2O5in agreement with the XRD analyses. V5+ in octahe-

dral coordination exhibits a charge transfer transitionat 400480 nm, while tetrahedral coordination shows

absorption bands at 300350 nm, absorption bands at

270300 nm are characteristic of isolate V5+ in tetra-

hedral form [5357]. It is clear from the observation

of Fig. 10 that in our dispersed catalysts vanadium

is mainly present in a tetrahedral form. This is more

evident in Fig. 13 where it is possible to appreci-

ate the effect of vanadium loading on the reflectance.

In this spectrum the reflectance of the support has

been subtracted. As it can be seen, the vanadium load

changes both the intensity of the band and the po-

sition of the maximum falling in the range typical

of tetrahedral coordination. In Fig. 14, a compari-

son between VH/TSM1(1) and VhH/TSM1 (for threedifferent times of hydrolysis) is reported. It is inter-

esting to observe the strong difference in the maxi-

mum position between VH/TSM1(1) and VhH/TSM1

catalysts.

3.2.5. TPR and oxygen uptake by pulse

technique

Different catalysts of the ones reported in Table 2

have been submitted to thermal programmed reduc-

tion (TPR) with hydrogen, showing in some cases

very different behaviours. Samples reduced at about550 C with hydrogen and then frozen in Helium at

370 C have been re-oxidised with oxygen pulses at

370 C so determining the oxygen uptake for each

vanadium atom. By assuming a stoichiometry O/V =

1, chemisorbed oxygen becomes a roughly evaluation

of V2O5 dispersion [58], even if some authors do not

consider this measure reliable because it depends on

many parameters [59].

Examples of TPR plots obtained are reported

in Figs. 15 and 16. In Fig. 15, TPR obtained

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Fig. 12. A comparison of DR-UV spectra obtained for respectively the TSM1 support and different vanadia catalysts.

Fig. 13. DR-UV spectra obtained for VH/TSM1 catalysts containing different amount of vanadia. Spectrum of the support has been

subtracted.

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Fig. 14. A comparison of DR-UV spectra respectively obtained for a VH/TSM type catalyst and three V hH/TSM catalysts.

for respectively VH/TSM, VhI/TSM, (Vh-T)I/S,

(Th-V)I/S and (V-T)hI/S catalysts are compared,

while in Fig. 16 a comparison between the TPR of two

VH/TSM1 catalysts with different vanadium load is re-

Fig. 15. A comparison of TPR plots obtained for different catalysts.

ported. Reduction initiates at about 250280 C in all

cases. In Fig. 15, it is possible to observe that the ob-

tained curves correspond to the addition of two or three

different peaks, with a maximum falling at different

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Fig. 16. Effect of the vanadium load on the TPR plots.

temperatures, that are reasonably related to more

or less reducible catalytic sites. Curve related to

(V-T)hI/S, for example, can be dissociated in twopeaks, one greater having a maximum at about

445 C and another one smaller at 565 C. Catalyst

(Th-V)I/S shows three different peaks at respectively

320, 410 and 465 C, that is, the catalyst surface

is more heterogeneous and the different vanadium

species anchored on the surface are more reducible.

Two peaks are also present at respectively 400 and

540 C also for VH/TSM catalyst but with a large

predominance of the less reducible sites. A dispersion

index for the different catalysts expressed as oxygen

uptake for vanadium atom is reported in Table 2 forthe catalysts submitted to TPR analysis. As it can be

seen, all the catalysts are largely dispersed, in par-

ticular, (V-T)hI/S and (Th-V)I/S showing a complete

accessibility of the vanadium atoms. The large dis-

persion of catalyst VH/TSM1(1) is probably due to

the very low vanadium charge. In Fig. 16 the effect

of vanadium loading on TPR behaviour can be appre-

ciated. As it can be seen, by increasing the amount of

vanadium on the surface the catalyst becomes more

reducible.

4. Discussion and conclusions

Many vanadium-based catalysts have been pre-pared following different preparation procedures. All

the prepared catalysts have been tested in the ODH

of isobutane giving place to great differences of both

activities and selectivities.

A first group of catalysts has been prepared, for

example, by grafting different amounts of vanadyl

tri-isopropoxide, dissolved in n-hexane, on a support

of silica coated with a multi-layer of TiO2. The graft-

ing reaction, in this case, is almost quantitative and

allows to obtain catalysts containing an increasing

amount of vanadium on the surface (see catalysts ofVH/TSM1 and VH/TSM type reported in Table 2).

The activities of these catalysts resulted roughly pro-

portional to the vanadium content, as it can be seen in

Fig. 5. This behaviour suggests a good dispersion for

all the considered catalysts. This is not contradictory

with the DRIFT analyses for these catalysts showing

the presence of polyvanadylic groups because the ten-

dency of V2O5 to give monolayered structures on TiO2surfaces [22] until it reaches a complete coverage is

well known. The apolar solvent n-hexane, favours the

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molecular aggregation giving place in solution at least

to dimeric structures of the type:

as it has been shown for vanadyl tri-ethoxide [60].

Selectivities to isobutene of these catalysts remain

relatively low (from 10 to 30%). Moreover, by extrap-

olating selectivity data to zero contact time (see Fig. 4)

it is possible to observe high values for all the main

reaction products, that is, isobutene, CO and CO2 Thismeans that CO and CO2 are produced not only by

isobutene but also by isobutane, even if in a less ex-

tent. A similar behaviour has been observed also for

the other catalysts. It is possible to suggest, therefore,

a reaction scheme of the type:

This complex scheme of reaction well explains the

poor adherence of the pseudo-first-order kinetic ap-

proach to the experimental behaviour.

Another group of catalysts has been prepared start-

ing from a defined amount of vanadyl tri-isopropoxide,

always dissolved in n-hexane and submitted to an

appropriate flow of moistured nitrogen for different

times. The times of exposure to moisture has beenchosen 5, 10 and 15 min, considering that 1617 min

is the time occurring for observing the formation of a

precipitate. In this way, hydrolysis and condensation

of vanadyl tri-isopropoxide is induced and catalysts

obtained have the same vanadium load (about 1 wt.%,

see catalysts of type VhH/TSM1 in Table 2) but would

have different dispersions. Activities and selectivities

are not particularly affected by the described treat-

ment. Probably, condensation occurs but giving place

to bidimensional structures that leave all the vanadium

atoms accessible without changing, therefore, neither

the activity nor the selectivity. Other two catalysts have

been prepared by grafting partially hydrolysed vanadyl

tri-isopropoxide, dissolved in isopropanol and treatedwith a stoichiometric amount of water (1:1), on re-

spectively silica coated with a multi-layer of TiO2 and

silica coated with a sub-monolayer of TiO2. It is in-

teresting to observe that in the case of using the sup-

port of silica coated with a sub-monolayer of TiO2,

partially hydrolysed vanadium alkoxide is grafted ex-

clusively on the islands of TiO2 and not on silica as

shown by SEM with EDX analysis. Catalyst VhI/TSM

is one of the most active catalysts but it is not much

selective. On the contrary, VhI/TsmS is less active but

much more selective.

The last group of catalysts have been prepared

by grafting directly on silica, a vanadiumtitanium

bimetallic alkoxide, dissolved in isopropanol, pre-

pared by following three different alternative routes.

In the first case, vanadium alkoxide has been partially

hydrolysed and then reacted with titanium alkoxide

(Vh-T)I/S. In the second case, titanium alkoxide has

been partially hydrolysed and reacted with vanadium

alkoxide (Th-V)I/S. In the last case, both the alkox-

ides of vanadium and titanium in mixture have been

treated with a stoichiometric amount of water before

grafting on silica (V-T)hI/S. With the exception of(Vh-T)I/S, the catalysts of this group resulted as the

most selective ones and activities are inferior but

comparable with those of the most active catalysts.

It is interesting to observe that the best catalysts

(Th-V)I/S and (V-T)hI/S do not show at the DRIFT

analysis the presence of polyvanadylic groups in the

wavenumber range 550800 cm1, while these groups

are present for both VH/TSM and VhI/TSM catalysts.

This can be interpreted with the formation of isolated

vanadium oxide groups directly bounded to titanium

oxide grafted on the silica support. Therefore, theprevalence of VOTi bonds in these catalysts with

respect to VOV bonds in the others seems to be

the reason of the observed high selectivity. Similar

results are obtained for both (Th-V)I/S and (V-T)hI/S

catalysts probably because the same active sites are

formed. This occurs because in the polar solvent,

isopropanol hydrolysis is strongly contrasted by the

solvent, acting negatively on the equilibrium (1). Ti-

tanium alkoxide reacts more fastly than vanadium

alkoxide giving place to mononuclear species of

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the type:

Ti(OR)4 +H2O Ti(OR)3OH +ROH

Then, Ti(OR)3OH can react with vanadyl tri-isoprop-oxide to give:

1, 2 o r 3 T i(OR)3OH + OV(OR)3

OV[OTi(OR)3]1,2 o r 3

Being all the catalysts examined well dispersed,

the differences in both activity and selectivity are too

many, probably due to differences in the active sites.

This is confirmed by both the TPR plots reported in

Fig. 15, where the most selective catalysts (Th-V)I/S

and (V-T)hI/S show a reduction peak at lower temper-

ature and also by the apparent activation energies re-ported in Table 5 that are lower for the same catalysts.

It is worth pointing out in conclusion that the

preparation method of vanadium-based catalysts with

a TiO2 favourable environment has been realised in

a simpler and cheaper way by using a silica support

and by obtaining in the meantime a remarkable im-

provement of the selectivity in the ODH of isobutane.

Acknowledgements

Thanks are due to MIUR for the financial support.

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