Nitrogen-incorporated TS-1 zeolite: Synthesis, characterization and application in the epoxidation of propylene Hao Li a,b , Qian Lei a,c , Xiaoming Zhang a,⇑ , Jishuan Suo a a Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR China b Graduate School of Chinese Academy of Sciences, Beijing, PR China c Jiaxing Center of Green Chemistry and Engineering, Chinese Academy of Sciences, Jiaxing, PR China article info Article history: Received 23 March 2011 Received in revised form 14 May 2011 Accepted 30 May 2011 Available online 12 June 2011 Keywords: Nitrogen-incorporated TS-1 Nitridation Epoxidation of propylene abstract A novel nitrogen-incorporated TS-1 zeolite (N-TS-1) was successfully synthesized by direct calcinating the as-synthesized TS-1 powder in NH 3 flow at high temperature. The samples were characterized by XRD, FTIR, UV–Vis, NH 3 -TPD, ICP-AES, CHN, XPS and 29 Si MAS NMR techniques. The results showed that nitrogen was incorporated into the framework of TS-1, and N-TS-1 preserved the MFI structure well. The fresh N-TS-1 samples showed low H 2 O 2 conversion in the epoxidation of propylene with dilute H 2 O 2 , which was due to the coverage of the active titanium sites by unstable nitrogen species. By refluxing the fresh N-TS-1 samples with methanol, the unstable nitrogen species were washed out and the conver- sion of H 2 O 2 increased markedly. The stable nitrogen species incorporated into the zeolite framework could effectively decrease the acidity of TS-1 zeolite and inhibit the side reactions, thereby improving the propylene oxide (PO) selectivity. After the 20th run, the N-TS-1-850-5 catalyst gave a H 2 O 2 conver- sion of 91.5%, a H 2 O 2 selectivity of 92.0%, and a PO selectivity of 90.9%. Finally, a model for nitridation of the as-synthesized TS-1 powder at high temperature was proposed. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Titanium silicalite-1 (TS-1) is an active and highly selective cat- alyst in the epoxidation of olefins with dilute H 2 O 2 [1]. The classi- cal TS-1 is synthesized using tetrapropylammonium hydroxide (TPAOH) as the template, but the synthesis requires stringent con- ditions (alkali-free TPAOH solutions) and high cost, which restrains its industrial application [2]. Instead, using tetrapropylammonium bromide (TPABr) to replace TPAOH, the synthesis of TS-1 becomes facile and reproducible [3,4]. However, the weak acidity of TS-1 zeolite, especially those prepared with TPABr as template and silica sol as silica source [5–7], usually caused side reactions during het- erogeneous catalytic oxidation [5–19]. To overcome this drawback, considerable efforts have been made to improve the catalytic selectivity of TS-1 in the epoxidation of olefins with dilute H 2 O 2 in the last decade. It has been reported that the selectivity to epoxides can be improved by treating TS-1, before or during the epoxidation reaction, with a neutralizing agent selected from organic derivatives of silicon of the type X- Si(R) 3 [8] or hydrosoluble substances deriving from cations of groups I and II with a different base strength [7–10]. Thiele [11] has found that a neutral salt or acid, which was selected from Na 2 SO 4 , (NH 4 ) 2 SO 4 , NH 4 NO 3 or NaH 2 PO 4 , can also be used as neu- tralizers to treat TS-1 before or during the epoxidation reaction in order to improve the epoxides selectivity. Crocco and Zajacek [12] have adopted other neutralizing agents to treat TS-1 during the epoxidation reaction, i.e., a non-base salt selected from lithium chloride, sodium nitrate, potassium sulfate or ammonium phos- phate. Moreover, Arca et al. [13] have systematically studied the ion exchange treatment of TS-1 with metal cations (M n+ ), and en- larged the scale of the neutralizers. When the metal salts or organic derivatives of silicon mentioned above were added before or dur- ing the epoxidation reaction, TS-1 afforded good H 2 O 2 conversion and epoxides selectivity. However, these processes have disadvan- tages deriving from the fact that these catalytic systems have a short duration of the catalytic cycle and consequently require fre- quent regeneration. Instead, non-ionic compounds which have been used as neutralizing agents, such as a non-ionic tertiary amine or tertiary amine oxide [14], organic molecules comprising an amide group substituted on the nitrogen atom [15] and nitro- genated organic bases [16], are emerged recently. By adding the non-ionic compounds during the epoxidation reaction, TS-1 showed high catalytic selectivity and stability. For example, when n-propylamine was used as the neutralizer for a continuous reac- tion of 40 h, TS-1 gave a H 2 O 2 conversion of 91.7% and a PO selec- tivity of 97% at 333 K [16]. Interestingly, Le-Khac [17] has reported that the PO selectivity can be improved by using the vinylpyridine polymer-encapsulated TS-1 catalysts in the epoxidation of propyl- 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.05.035 ⇑ Corresponding author. Tel.: +86 28 85226215; fax: +86 28 85223978. E-mail address: [email protected](X. Zhang). Microporous and Mesoporous Materials 147 (2012) 110–116 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Transcript
Microporous and Mesoporous Materials 147 (2012) 110–116
Nitrogen-incorporated TS-1 zeolite: Synthesis, characterization and applicationin the epoxidation of propylene
Hao Li a,b, Qian Lei a,c, Xiaoming Zhang a,⇑, Jishuan Suo a
a Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR Chinab Graduate School of Chinese Academy of Sciences, Beijing, PR Chinac Jiaxing Center of Green Chemistry and Engineering, Chinese Academy of Sciences, Jiaxing, PR China
a r t i c l e i n f o
Article history:Received 23 March 2011Received in revised form 14 May 2011Accepted 30 May 2011Available online 12 June 2011
Keywords:Nitrogen-incorporated TS-1NitridationEpoxidation of propylene
1387-1811/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.micromeso.2011.05.035
A novel nitrogen-incorporated TS-1 zeolite (N-TS-1) was successfully synthesized by direct calcinatingthe as-synthesized TS-1 powder in NH3 flow at high temperature. The samples were characterized byXRD, FTIR, UV–Vis, NH3-TPD, ICP-AES, CHN, XPS and 29Si MAS NMR techniques. The results showed thatnitrogen was incorporated into the framework of TS-1, and N-TS-1 preserved the MFI structure well. Thefresh N-TS-1 samples showed low H2O2 conversion in the epoxidation of propylene with dilute H2O2,which was due to the coverage of the active titanium sites by unstable nitrogen species. By refluxingthe fresh N-TS-1 samples with methanol, the unstable nitrogen species were washed out and the conver-sion of H2O2 increased markedly. The stable nitrogen species incorporated into the zeolite frameworkcould effectively decrease the acidity of TS-1 zeolite and inhibit the side reactions, thereby improvingthe propylene oxide (PO) selectivity. After the 20th run, the N-TS-1-850-5 catalyst gave a H2O2 conver-sion of 91.5%, a H2O2 selectivity of 92.0%, and a PO selectivity of 90.9%. Finally, a model for nitridationof the as-synthesized TS-1 powder at high temperature was proposed.
� 2011 Elsevier Inc. All rights reserved.
1. Introduction
Titanium silicalite-1 (TS-1) is an active and highly selective cat-alyst in the epoxidation of olefins with dilute H2O2 [1]. The classi-cal TS-1 is synthesized using tetrapropylammonium hydroxide(TPAOH) as the template, but the synthesis requires stringent con-ditions (alkali-free TPAOH solutions) and high cost, which restrainsits industrial application [2]. Instead, using tetrapropylammoniumbromide (TPABr) to replace TPAOH, the synthesis of TS-1 becomesfacile and reproducible [3,4]. However, the weak acidity of TS-1zeolite, especially those prepared with TPABr as template and silicasol as silica source [5–7], usually caused side reactions during het-erogeneous catalytic oxidation [5–19].
To overcome this drawback, considerable efforts have beenmade to improve the catalytic selectivity of TS-1 in the epoxidationof olefins with dilute H2O2 in the last decade. It has been reportedthat the selectivity to epoxides can be improved by treating TS-1,before or during the epoxidation reaction, with a neutralizingagent selected from organic derivatives of silicon of the type X-Si(R)3 [8] or hydrosoluble substances deriving from cations ofgroups I and II with a different base strength [7–10]. Thiele [11]has found that a neutral salt or acid, which was selected from
ll rights reserved.
: +86 28 85223978.
Na2SO4, (NH4)2SO4, NH4NO3 or NaH2PO4, can also be used as neu-tralizers to treat TS-1 before or during the epoxidation reaction inorder to improve the epoxides selectivity. Crocco and Zajacek [12]have adopted other neutralizing agents to treat TS-1 during theepoxidation reaction, i.e., a non-base salt selected from lithiumchloride, sodium nitrate, potassium sulfate or ammonium phos-phate. Moreover, Arca et al. [13] have systematically studied theion exchange treatment of TS-1 with metal cations (Mn+), and en-larged the scale of the neutralizers. When the metal salts or organicderivatives of silicon mentioned above were added before or dur-ing the epoxidation reaction, TS-1 afforded good H2O2 conversionand epoxides selectivity. However, these processes have disadvan-tages deriving from the fact that these catalytic systems have ashort duration of the catalytic cycle and consequently require fre-quent regeneration. Instead, non-ionic compounds which havebeen used as neutralizing agents, such as a non-ionic tertiaryamine or tertiary amine oxide [14], organic molecules comprisingan amide group substituted on the nitrogen atom [15] and nitro-genated organic bases [16], are emerged recently. By adding thenon-ionic compounds during the epoxidation reaction, TS-1showed high catalytic selectivity and stability. For example, whenn-propylamine was used as the neutralizer for a continuous reac-tion of 40 h, TS-1 gave a H2O2 conversion of 91.7% and a PO selec-tivity of 97% at 333 K [16]. Interestingly, Le-Khac [17] has reportedthat the PO selectivity can be improved by using the vinylpyridinepolymer-encapsulated TS-1 catalysts in the epoxidation of propyl-
H. Li et al. / Microporous and Mesoporous Materials 147 (2012) 110–116 111
ene. One of his catalysts, poly(2-vinylpyridine-co-styrene)-encap-sulated TS-1 (TS-1: 50 wt.%), gave a H2O2 conversion of 94.0%and a PO selectivity of 96.1% after a reaction of 0.5 h at 343 K[17]. Dakka et al. [18] have proposed that the catalytic selectivityof TS-1 can be improved by performing the epoxidation in the pres-ence of carbon dioxide under supercritical conditions. Unfortu-nately, the conversion of H2O2 decreased during aging tests. Liuet al. [19] have studied the heat treatment of TS-1 in the vaporat high temperature and found that the surface acidity of TS-1 pro-duced by aluminum impurity decreased and PO selectivity in-creased, but the improvement was poor.
Recently, we have successfully prepared a novel nitrogen-incor-porated TS-1 zeolite (designated as N-TS-1) by calcinating the as-synthesized TS-1 powder in NH3 flow at high temperature [20],nitrogen atoms were incorporated into the framework of TS-1 bysubstituting oxygen atoms in the lattice, and the acidity of TS-1zeolite was effectively decreased and the selectivity of the epoxidewas improved greatly. In this work, we present detailed studies onthe preparation, characterization and catalytic performance of thisnovel N-TS-1 zeolite.
Fig. 1. XRD spectra of TS-1 and N-TS-1: (A) TS-1; (B) N-TS-1-800-5; (C) N-TS-1-850-5; (D) N-TS-1-900-5; (E) N-TS-1-850-10; (F) N-TS-1-850-20.
2. Experimental
2.1. Synthesis
The as-synthesized TS-1 precursor was prepared in the samemanner as in previous reports [20,21]. In a typical synthesis,12.8 g of TPABr and 92.7 g of silica sol (30 wt.%) were dissolvedin 192 g of deionized water, and 3.15 g of tetrabutylorthotitanate(TBOT) dissolved in 6.4 g of aqueous H2O2 solution (30 wt.%) wasadded dropwise. Then 20.7 g of n-butylamine (NBA) was addedto adjust the alkalinity of the solution (pH = 11.5). The molar com-position was SiO2–0.02TiO2–0.6NBA–0.1TPABr–30H2O. Finally,1.2 g of TS-1 powder (commercially purchased) was added as theseeds. After stirring at room temperature for 2 h, the gel was trans-ferred into a Teflon-lined autoclave and heated at 448 K under aut-ogeneous pressure for 72 h. Finally, the solid product wasrecovered by centrifugation, washed with deionized water, driedovernight at 373 K. For nitridation, 8.0 g of the as-synthesized TS-1 powder was placed in a silica boat and inserted into a quartz tubefurnace. Prior to thermal treatment, the quartz tube was purgedwith N2 (80 ml/min) for 30 min, then the temperature of the fur-nace was increased at a ramp rate of 3 K/min to the expected tem-perature and maintained for given hours under the NH3
atmosphere (60 ml/min). Finally, the furnace was cooled to 353 Kand purged again with N2 (80 ml/min) for 30 min in order to re-move the physically adsorbed NH3. The nitridized samples weredenoted as N-TS-1-T-t, where T and t represent nitridation temper-ature (�C) and nitridation time (h), respectively.
2.2. Pretreatment of the fresh N-TS-1 with methanol
The methanol reflux was achieved by refluxing a suspension of1 g fresh N-TS-1 sample in 15 ml methanol at 353 K for 3 h. Thesample was then filtered, washed with deionized water and driedin air at 373 K for 3 h before the activity test.
2.3. Characterization
The samples were characterized by X-ray diffraction (PhilipsX’PERT, nickel filtered Cu-Ka radiation), FTIR (Nicolet MX-1E 560,KBr pellet technique) and UV–Vis spectroscopy (Hitachi Model U-3010 Spectrophotometer). The chemical composition of the sam-ples was determined by ICP-AES analysis (Perkin–Elmer OptimaDV 2000) and CHN analysis (Carlo Erba 1106 CHN Elemental Ana-
lyzer). The X-ray photoelectron spectra measurement was per-formed on a Thermo VG MultiLab 2000 X-ray photoelectronspectrometer using Mg-Ka radiation as the excitation source. Theshift of binding energy due to relative surface charging was cali-brated using C 1s level at 284.8 eV as an internal standard. TheNMR analysis was performed on a Bruker Avance II 400WB spec-trometer equipped with a 4.0 mm MAS probe. NH3-TPD spectrawas measured in a fixed-bed reactor connected to a thermal con-ductivity detector (GC 2000II). The sample (�100 mg) was initiallyactivated at 673 K for 1 h in ultra high purity argon flow (25 ml/min). It was then cooled to 373 K and pure NH3 (20 ml/min) wasadsorbed for 1.5 h. Then the sample was flushed with pure argon(25 ml/min) for 1 h at 373 K. The NH3-TPD curves of the sampleswere achieved by increasing the temperature from 373 to 873 Kat a ramp rate of 10 K/min under a pure argon flow of 25 ml/min.
2.4. Catalytic activity
The epoxidation of propylene with dilute H2O2 was carried outat 318 K in a stainless-steel autoclave reactor. For a typical run,2.0 g of N-TS-1 catalyst, 5.0 ml of �30 wt.% H2O2 and 158 ml meth-anol were fed into the reactor. Then propylene was charged at con-stant pressure (0.4 MPa). After heating the mixture at 318 K underagitation for 1.5 h, the residual H2O2 was checked by iodometrictitration. The product of the reaction was analyzed on a GC7890F chromatography. PO was the main product, and propyleneglycol mono-methyl ethers (MME) were the by-products. Theselectivity of H2O2 was defined as moles of PO formed with regardto the oxidant consumed. The selectivity of PO and MME weredetermined by GC analysis.
3. Results and discussion
3.1. Physicochemical properties of nitrogen-incorporated TS-1
Fig. 1 shows the XRD patterns of TS-1 and N-TS-1 zeolites. Thesamples show the characteristic of MFI topology without impurephase. With increasing the nitridation temperature or prolongingthe nitridation time, the peak intensity of samples changes little,indicating that nitridation at high temperature does not result inthe structural destruction. Comparing with TS-1, the BET surfaceareas and the micropore volumes of nitridized samples changedlittle upon nitridation, indicating that the pore structure is wellpreserved [20]. The nitridized samples contained only isolatedTi(IV) species in the framework as evidenced by the predominantband at �220 nm in the UV–Vis spectrum (Fig. 2), but TS-1 shows
Fig. 2. UV–Vis spectra of TS-1 and N-TS-1: (A) TS-1; (B) N-TS-1-800-5; (C) N-TS-1-850-5; (D) N-TS-1-900-5; (E) N-TS-1-850-10; (F) N-TS-1-850-20.
Fig. 3. IR spectra of TS-1 and N-TS-1: (A) TS-1; (B) N-TS-1-800-5; (C) N-TS-1-850-5;(D) N-TS-1-900-5; (E) N-TS-1-850-10; (F) N-TS-1-850-20.
112 H. Li et al. / Microporous and Mesoporous Materials 147 (2012) 110–116
another absorption peak at 270–280 nm (Fig. 2), which was attrib-uted to Ti4+ ions in an octahedral coordination with two watermolecules in the coordination sphere or small hydrated oligomericTiOx species [7]. Comparing with TS-1, N-TS-1 show a small bandat �1400 cm�1 attributed to the vibration of NH from adsorbedNHþ4 [22] (Fig. 3), and the bands at �1630 cm�1 and �3440 cm�1
assigned to Si–OH groups (mOH vibrations) [22] drop shortly withincreasing the nitridation temperature or prolonging the nitrida-tion time (Fig. 3). Moreover, the intensity of the band at�960 cm�1 for N-TS-1 decreases slightly with prolonging thenitridation time, while it changes little upon increasing the nitrid-ation temperature (Fig. 3). It was suggested that the 960 cm�1 bandof TS-1 is the sum of the Si–OH stretching mode and the TiO4 unitasymmetric stretching mode, while the latter contribution is pre-dominant [23]. These results indicate that the Si–OH groups areconsumed gradually during nitridation.
The acidities of the samples were studied by NH3-TPD and theamounts of desorbed ammonia are summarized in Table 1. Theacid amount of TS-1 (sample A) is 0.15 mmol/g. When the nitrida-tion temperature increased from 600 to 900 �C, the acid amounts ofthe nitridized samples decrease from 0.11 to 0.09 mmol/g. Whenthe nitridation time increased from 2 to 20 h, the acid amountsof the nitridized samples decrease from 0.13 to 0.07 mmol/g. Theseresults indicate that nitridation of the as-synthesized TS-1 powdercan effectively decrease the acidity of the TS-1.
Table 1Characterizations and catalytic properties of TS-1 and N-TS-1.
a The samples A, B, C, D, E, F, G, H and I were TS-1, N-TS-1-800-5, N-TS-1-850-5, N-TS850-2, respectively.
b The total acidity was determined by quantifying the desorbed NH3 with NH3-TPD.c The nitrogen content was determined by CNH elemental analysis (given in weight pe
XPS analysis are shown in brackets.d The N/Ti molar ratio of the bulk (determined by ICP-AES and CHN analyses) and the s
in brackets.e The catalytic results for propylene epoxidation over fresh samples and samples after
The elemental compositions of the samples were studied byCHN, ICP-AES and XPS analyses, and the results are shown in Table1. The titanium content of the TS-1 and the nitridized samplesdetermined by ICP-AES analysis kept constant (1.44 wt.%), andthe surface titanium content of TS-1 determined by XPS analysiswas 2.99 at.%. With increasing the nitridation temperature or pro-longing the nitridation time, the nitrogen content of the nitridizedsamples increases, and the surface nitrogen content and N/Ti molarratio are higher than that of the bulky, which indicated that nitrid-ation mainly occurs at the surface of the as-synthesized TS-1 pow-der. Interestingly, the surface titanium content of nitridizedsamples diminished with increasing the nitridation temperature(ranging from 2.66 to 1.33 at.%) or prolonging the nitridation time(ranging from 1.50 to 1.20 at.%), which might result from the cov-erage of the titanium sites by nitrogen species.
The effect of nitridation temperature on the nitrogen and car-bon contents is shown in Fig. 4. Obviously, the precursor exhibitshigh nitrogen and carbon contents, which was due to the presenceof template (TPABr). During nitridation, there were two processesoccurred simultaneously, one was thermal decomposition of thetemplate and the other was nitridation of the precursor. Whenthe nitridation temperature increased from 300 to 600 �C, the car-bon and nitrogen contents decrease markedly, indicating that thethermal decomposition of template is the major reaction. Whenthe nitridation temperature increased further, the carbon contentis near zero, while the nitrogen content increases, suggesting that
-1-900-5, N-TS-1-850-10, N-TS-1-850-20, N-TS-1-600-5, N-TS-1-700-5 and N-TS-1-
rcent) and XPS analysis (given in atomic percent), respectively. The data obtained by
urface (determined by XPS analysis), respectively. The data of the surface are shown
methanol reflux, respectively. The data shown in brackets are the results of samples0 wt.%), 5 ml; propylene pressure, 0.4 MPa; temp., 318 K; time, 1.5 h.
Fig. 4. The nitrogen and carbon contents of N-TS-1 (fresh and after methanol reflux)as a function of the nitridation temperature.
Fig. 5. The nitrogen contents of N-TS-1 (fresh and after methanol reflux) as afunction of the nitridation time.
Fig. 6. XPS spectra (Ti 2p XPS spectra (a) and N 1s XPS spectra (b)) of samples: (A)TS-1; (B) N-TS-1-800-5; (C) N-TS-1-850-5; (D) N-TS-1-900-5; (E) N-TS-1-850-10;(F) N-TS-1-850-20; (G) N-TS-1-600-5.
H. Li et al. / Microporous and Mesoporous Materials 147 (2012) 110–116 113
the nitridation is the major reaction at higher temperatures. Theeffect of the nitridation time on the nitrogen content is shown inFig. 5. With prolonging the nitridation time, the nitrogen contentincreases at an exponential rate, and the increasing trend was sim-ilar to that of mesoporous molecular sieves [24,25]. These resultsare in consistent with the results of NH3-TPD analysis.
As shown in Fig. 4, the nitrogen content of the nitridized sam-ples after methanol reflux decrease. The decreasing extents of thenitrogen content for N-TS-1-500-5, N-TS-1-600-5 and N-TS-1-700-5 were 64.0%, 60.4% and 62.3%, respectively, while their finalnitrogen contents are nearly the same, indicating that an amountof N species of the samples nitridized at low temperatures areunstable and can be washed out by methanol reflux. When thenitridation temperature increases further, the decreasing extentof nitrogen content was varied from 51.7% to 31.7%. These resultssuggest that, the higher nitridation temperature, the more stablenitrogen species of the samples are kept after methanol reflux.As can be seen in Fig. 5, upon methanol reflux, the nitrogen contentof the samples decrease and the curve is nearly in parallel with thatof the fresh one. With prolonging the nitridation time from 2 to20 h, the decreasing extent of nitrogen content was varied from50.0% to 29.3%. This indicates that, the longer nitridation time,the more stable nitrogen species of the samples are kept aftermethanol reflux. Moreover, it should be noted that the acid amountof sample C after methanol reflux (0.10 mmol/g) increased slightlycomparing with that of the fresh one (Table 1).
Fig. 6a shows the XPS spectra in the Ti 2p region for the sam-ples. Sample A shows a sharp peak at �458.1 eV in the Ti 2p3/2 re-gion, which could be fitted with three doublets, including Ti(IV) inoctahedral coordination, Ti(IV) in tetrahedral coordination and aTi(III) chemical state [26]. Comparing with sample A, the spectrain Ti 2p3/2 region of sample G changes little. When the nitridationtemperature was above 800 �C, the peak in the Ti 2p3/2 region isbroadened and shifts towards lower binding energy (ranged from457.1 to 456.6 eV), and the trends are reinforced with increasingthe nitridation temperature or prolonging the nitridation time.The shift toward lower binding energy upon nitridation indicatedthat the successful incorporation of nitrogen into the zeolite frame-work [27,28], while the broadening of the Ti 2p3/2 peaks for the N-TS-1 zeolites was due to the generation of the titanium oxynitridespecies [29].
The N 1s XPS spectra (Fig. 6b) further confirms the incorpora-tion of nitrogen into the framework of zeolite following nitridation.As expected, no peaks were observed for the TS-1 [20]. Sample Gshows a broad peak at �399.3 eV. However, when the nitridationtemperature was above 800 �C, the Peak 1 shifts slightly to�398.5 eV, and a new peak at �395.4 eV appears. Peak 1 was as-signed to NHx (x = 1 or 2) species produced by the dissociativeadsorption of ammonia [22,30], while Peak 2 was attributed tothe oxynitride species [22,29]. With increasing the nitridation tem-perature or prolonging the nitridation time, the area of N 1s peaksincreases drastically, indicating that the degree of nitridation is en-hanced and the nitrogen content increases.
Fig. 7. 29Si NMR spectra of TS-1 and N-TS-1: (A) TS-1; (C) N-TS-1-850-5; (F) N-TS-1-850-20.
114 H. Li et al. / Microporous and Mesoporous Materials 147 (2012) 110–116
The results of 29Si MAS NMR analysis are shown in Fig. 7. Twomain peaks at ��112.7 and ��116.0 ppm, together with a weakpeak at ��103.3 ppm are well resolved for sample A. The peakat ��112.7 and ��116.0 ppm were characteristics of Q4 [(SiO)4Si]species in MFI-type zeolites having an orthorhombic symmetry,and the peak at ��103.3 ppm was attributed to Q3 [(SiO)3SiOH]species [1]. After nitridation, a new peak at ��92.1 ppm attributedto Si–NH–Si species [31] is detected for samples C and F, and thepeak intensity increases slowly with prolonging the nitridationtime, while the peak at ��103.3 ppm assigned to Q3 species de-creases markedly. These results suggest that silanols react withammonia to form –NH2 and –NH– species during nitridation,which is in good agreement with the results of IR and XPS analyses.
As reported in a previous contribution [20], the contents of so-dium and aluminum did not change upon nitridation of the as-syn-thesized TS-1 powder, indicating that the dealumination reactionhas not occurred during nitridation of the zeolite. This might bedue to the presence of NH3 which could heal defects and preventthe dealumination of zeolites [32]. During the nitridation of Y zeo-lite in NH3 flow at high temperature, the substitution reaction oc-curred preferentially at the sites next to aluminum sites [33]. Inother words, the nitridation might target the Brönsted acid sites(if any) and neutralize these sites first.
On the basis of the above results and discussion, a model fornitridation of the as-synthesized TS-1 powder at high temperaturewas proposed (Scheme 1). At low temperatures (below 800 �C),the formation of T–NH2 (T = Si or Ti) species starting from terminalhydroxyl groups (Scheme 1a) were dominant [33–35]. At highertemperatures (above 800 �C), the ammonia could react not only with
Scheme 1. Proposed model for the nitridation of the as-synthesized TS-1 powder atlow temperature (a) and higher temperature (b).
terminal hydroxyl groups, but also with bridging hydroxyl groups(Si–OH–Al groups) and siloxane bonds (Scheme 1b), while the bridg-ing hydroxyl groups would be substituted preferentially [22,33–35].At the same time, the concomitant formation of Si–NH–Si or Si–NH–Ti species through the condensation between the Si–NH2 and T–OH(T = Si or Ti) species could also occur. The incorporation of nitrogeninto the framework of TS-1 can effectively suppress the acidity of theTS-1 zeolite, and this novel nitrogen-incorporated TS-1 zeolite willfind potential application as selective oxidation catalyst, basic cata-lyst as well as catalyst support.
3.2. Catalytic tests
In this paper, TS-1 was successfully synthesized by using TPABr,silica sol and NBA as template, silica source and base, respectively.However, impurities (such as Al3+ or Na+) which stem from the sil-ica sol were unavoidably introduced into the TS-1 zeolite [20]. Theincorporation of the trace Al3+ into the framework of TS-1 can formBrönsted acid sites, leading to the ring-opening reaction of PO toform MME through acid catalysis during propylene epoxidation[5]. Therefore, TS-1 shows low H2O2 selectivity and low PO selec-tivity in the epoxidation of propylene (Table 1). As shown in Table1, after nitridation, the fresh nitridized samples show low H2O2
conversion, and the H2O2 conversion decreases with increasingthe nitridation temperature or prolonging the nitridation time.However, the H2O2 selectivity of the fresh nitridized samples reachup to 97.5%, and the PO selectivity of the fresh nitridized samplesreach 100.0%. During the study of catalytic recycles over the freshnitridized samples, we have found that the conversion of H2O2 in-creased markedly in the 2nd run for propylene epoxidation withdilute H2O2 (data not shown). As discussed above, the nitrogencontent of the nitridized samples treated by methanol reflux de-creased comparing with that of the fresh ones. These results indi-cated that the unstable N species can be washed out by the solvent(methanol) during the reaction conditions (the 1st run). Therefore,we treated the fresh nitridized samples by methanol reflux.
From the above results, a possible speculation was proposed toexplain why the samples nitridized at different temperatures andtimes show the low H2O2 conversion: the active sites of the nitri-dized samples for selective oxidation (framework Ti) were coveredby the unstable N species in the surface, which led to the low con-version of H2O2, while the stable N species with weak basicity sup-pressed the acidities of samples and inhibited the side reactions,resulting in the high H2O2 and PO selectivity.
The results of activity tests using the nitridized samples treatedby methanol reflux are shown in Table 1. Comparing with the freshnitridized samples, the samples treated by methanol reflux exhibithigh H2O2 conversion, high H2O2 selectivity and high PO selectiv-ity, although the H2O2 and PO selectivity decrease slightly compar-ing with that of the fresh ones. With increasing the nitridationtemperature from 600 to 700 �C, the H2O2 conversion changes lit-tle, while the H2O2 and PO selectivity increase slightly. When thenitridation temperature increased from 700 to 850 �C, the H2O2
conversion changes little, while the H2O2 and PO selectivity first in-crease markedly and then increase slightly. Further increasing thenitridation temperature to 900 �C, the activity and selectivity of thesample begin to decrease, indicating that higher nitrogen contentof the sample is not appropriate. Interestingly, the catalytic selec-tivity of sample B is comparable to that of sample C, this might beexplained by that the samples have the similar nitrogen contents,N species and acid amounts. Thus, the proper nitridation tempera-ture should be 800–850 �C. With prolonging the nitridation timefrom 2 to 5 h, the H2O2 conversion changes little, while the H2O2
and PO selectivity increase markedly (Table 1). Further prolongingthe nitridation time, the H2O2 conversion, H2O2 selectivity and POselectivity both decrease. Therefore, the proper nitridation time
Fig. 8. Catalyst recycles in the epoxidation of propylene with dilute H2O2 oversample C. Reaction conditions were the same as in Table 1. In every run, the catalystwas centrifuged from the reaction mixture and used as the catalyst of next rundirectly.
H. Li et al. / Microporous and Mesoporous Materials 147 (2012) 110–116 115
was 5 h. The above results demonstrate that the nitrogen contentand N species of N-TS-1 after methanol reflux have a great influ-ence on the catalytic activity and selectivity of the catalyst.
Through the above discussion, the speculation was confirmed. Itindicates that, chemisorbed NH3 (or NHþ4 ) and partial unstable NH2
species of the fresh N-TS-1, which are unstable, can be washed outby methanol reflux [36]. This was further proved by the slightly in-crease of the acid amount of sample C after methanol reflux.
The lifetime of a catalyst is critical to decide whether it can beused in the industry. In this work, aging tests were conducted bysimple phase separation and reused directly in the next run.Fig. 8 shows the catalytic results for the epoxidation of propyleneover sample C. The H2O2 conversion decreases slightly throughthe run 1 to run 11 and keeps stable along with further increasingthe recycles, while the H2O2 and PO selectivity decrease shortlyduring the run 1 to run 9 and keep stable in the latter 11 recycles.After the 20th run, sample C still kept high H2O2 conversion(91.5%), high H2O2 selectivity (92.0%) and high PO selectivity(90.9%). The catalytic results are comparable to that of obtainedby adding neutralizer [7,16] during the reaction. In addition, itshould be noted that the sample used in the latter 10 runs wasstored under ambient conditions for 30 days. However, the cata-lytic activity changes little, suggesting the sample has a satisfac-tory storage.
3.3. Catalytic mechanism
In the epoxidation of lower olefins with dilute H2O2 and TS-1,Clerici and Ingallina [37] proposed that the selective oxidation iscarried out through the five-member-ring intermediate. Maximumand minimum concentrations of the five-member-ring intermedi-ate are achieved in acid and basic solutions, respectively. However,the acidity of TS-1, even if modest, is sufficient to catalyze consec-utive solvolytic reactions on the epoxide, leading to the low selec-tivity of epoxide. Because the nitrogen species in the nitrogen-incorporated zeolites possess basicity, they can decrease the acidsites as evidenced by the NH3-TPD analysis and suppress thering-opening reaction of PO to form MME through acid catalysis[5,7,16]. A small amount of nitrogen will not influence the activityof N-TS-1, but excessive amount of nitrogen can lead to the lowcatalytic activity and PO selectivity (Table 1). Thus, the nitrogencontent of the sample after methanol reflux should be appropriatein the epoxidation of propylene, because it has a great effect on thecatalytic activity and selectivity of the sample.
The nitrogen and carbon contents of sample C treated by meth-anol reflux were 0.33 and 0.14 wt.%, respectively (Fig. 4). After the20th reaction, the nitrogen and carbon contents of sample C deter-mined by CHN analysis were 0.32 and 2.50 wt.%, respectively.Comparing with the sample C after methanol reflux, the nitrogencontent of sample C after the 20th reaction changed slightly, whichis responsible for the little change of the acid amount betweenthem (both were 0.10 mmol/g). This demonstrates that the nitro-gen species incorporated in the TS-1 zeolite are not leaching underoperative conditions. Although small concentration of carbonattributed to the bulky organic by-products was detected for thespent sample, sample C still kept high catalytic activity, indicatingthat sample C is not deactivated [38]. Therefore, the remaining partof the nitrogen species incorporated into the zeolite frameworkafter methanol reflux (Si–NH–Si, Si–NH–Ti, Si–NH–Al and partialstable –NH2 species), which are more stable and can adjust the sur-face acidity of TS-1, are responsible for the high catalytic selectivityand stability of sample C.
4. Conclusions
N-TS-1 zeolite was successfully prepared by nitridation of theas-synthesized TS-1 powder in NH3 flow at high temperature.The results of various kinds of characterization techniques showedthat nitrogen was incorporated into the framework of TS-1, and N-TS-1 preserved the MFI structure well. By introducing nitrogen intothe framework of TS-1 zeolite, the acid sites of TS-1 zeolite werereduced effectively. The amount and chemical nature of the incor-porated N species were controlled by the nitridation temperatureand nitridation time. At higher temperatures, the ammonia couldreact not only with terminal hydroxyl groups, but also with bridg-ing hydroxyl groups and siloxane bonds, while the bridging hydro-xyl groups would be substituted preferentially. By refluxing thefresh N-TS-1 samples with methanol, the unstable N species werewashed out, leaving the stable N species which could adjust theacidity of TS-1 zeolite. The N-TS-1 showed high catalytic selectivityand stability for propylene epoxidation. This strategy can be ap-plied to prepare a series of nitrogen-incorporated Ti-containingmolecular sieves with special properties.
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