Applied Catalysis A: General 236 (2002) 17

Epoxidation of propylene using supportedtitanium silicalite catalysts

Gang Li, Xiangsheng Wang, Haisheng Yan, Yihui Liu, Xuewu LiuState Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, ChinaReceived 19 October 2001; received in revised form 2 February 2002; accepted 2 April 2002

Abstract

The propylene epoxidation using supported titanium silicalite TS-1 catalyst was systematically investigated in a fixed bedreactor. Different supports such as SiO2 and Al2O3 were used and two molding methods were adopted to prepare the catalyst.Strip TS-1 catalyst of 1 mm 2 mm dimensions is obtained by the extruding method. Use of SiO2 as the support rather thanAl2O3 is better because the latter has strong acidity that promotes the side reaction of propylene oxide (PO) with the solvent.With the decrease of the amount of TS-1 in TS-1/SiO2, the PO selectivity increases. The lamina TS-1 catalyst, the core of whichis an inert ball used as the support and the shell of which is a TS-1 lamina of 0.10.2 mm, exhibits better performance than thestrip TS-1 catalyst owing to its shorter diffusion path for reaction heat, raw material and product. The lamina TS-1 catalystwas used in the fixed bed reactor for about 200 h. The results indicate that it is a promising catalyst in propylene epoxidation.The effect of aluminum impurity was also investigated. It has been shown acid sites originating from framework aluminumcan catalyze the side reaction and a small amount of base can inhibit it. In conclusion, diffusion and the concentration of acidsites are the two main factors influencing the performance of titanium silicalite catalyst in propylene epoxidation. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Propylene; Epoxidation; Titanium silicalite; TS-1; Supported catalyst

1. Introduction

Propylene oxide (PO) is an important chemicalproduct. Nowadays PO is produced in industry usingthe old method involved chlorine and lime or usingthe process developed by Halcon. But the formerproduces much pollution and the latter produces alarge amount of side products such as t-butyl alco-hol or styrene. The epoxidation of propylene usinghydrogen peroxide as the oxidant offers a clean andeconomically viable alternative to existing processes.

Corresponding author. Tel.: +86-411-368-9065;fax: +86-411-368-9065.E-mail address: guoxw@dlut.edu.cn (G. Li).

The synthesis of titanium silicalite TS-1 was firstreported by Taramasso et al. [1] in 1983. TS-1 hasreceived considerable interest during the last decadebecause of its unique catalytic properties in oxidationreactions involving hydrogen peroxide as the oxidant.TS-1 is an active and selective catalyst in the epoxida-tion of olefins [2,3]. Almost quantitative yields of POare produced in propylene epoxidation at near roomtemperature, by dilute methanol solutions of hydrogenperoxide [4]. But there are still at least two obstaclesthat impede attempts to industrialize the propyleneepoxidation process catalyzed by titanium silicaliteTS-1. The first is the cost of the catalyst and thesecond is the difficulty in molding the powdery TS-1into particles that can be used in industry reactors.

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2 G. Li et al. / Applied Catalysis A: General 236 (2002) 17

In the classical synthesis of TS-1 [1], alkali-freetetrapropylammonium hydroxide (TPAOH) is used asthe template and TS-1 is produced under severe con-ditions and at high cost, which makes the synthesisdifficult to be industrialized. In order to reduce thecost of TS-1, tetrapropylammonium bromide (TPABr)can be used as the template to synthesize TS-1 [58],instead of using the expensive template TPAOH.N-Butylamine, ammonia, diethylamine etc. can beused as the base to adjust the alkalinity of the gel inthe synthesis using TPABr as the template [8]. Thus,obtained powdery TS-1 exhibits good performance inpropylene epoxidation in an intermittent reactor. Butit is difficult to separate the powder of TS-1 from theproducts and the process is not continuous.

The molding of the powdery TS-1 is one of themost important steps in the industry use of propyleneepoxidation. Bellussi et al. [9] reported the prepa-ration of titanium silicalite catalyst microspheresconstituted by oligomeric silica and titanium silicalitecrystal based on the use of an aqueous solution ofsilica and tetraalkylammonium hydroxide obtainedby hydrolyzing tetraethyl orthosilicate (TEOS) inan aqueous solution of TPAOH. The method seemscomplicated and expensive. Two molding methodswere adopted in this paper to prepare the titaniumsilicalite catalyst used in the fixed bed reactor. One isthe simpler and cheaper extruding method in whichstrip TS-1 catalyst is obtained. The other is a newmolding method in which lamina TS-1 catalyst ismade by spraying the powdery TS-1 on a small inertball. Both the strip TS-1 and the lamina TS-1 wereused to catalyze propylene into propylene oxide inthe fixed bed reactor. The effects of the supports andthe aluminum impurity in TS-1 on the properties oftitanium silicalite catalyst were also investigated.

2. Experimental

Titanium silicalite TS-1 was prepared according to[8]. Colloidal silica (30%) and tetra-butyl-orthotitanate(TBOT) were used as silicon and titanium source, re-spectively. Tetrapropylammonium bromide (TPABr)was used as the template. TS-1 samples crystallizedfrom gels with the following molar composition:

SiO2 a TiO2 b n-butylaminec TPABr d H2O

where 0 < a < 0.03, 0 < b < 1.0, 0 < c < 0.1 and20 < d < 100. The gel was transferred into theautoclave and heated for 210 days at 423463 K.The solid obtained was filtered, washed with distilledwater, dried at 373 K in static air and finally calcinedat 813 K. Thus, the powdery TS-1 was obtained. Thechemical composition of the samples was obtained ona Bruker SRS 3400x spectrometer. The crystal sizeof TS-1 was determined on a Japan JEM-1200EXscanning electron microscope. The surface area andpore size of changes were obtained on an USAQuantachrome AUTOSORB-1 autosorber using BETmethod.

The strip TS-1 catalyst was prepared by mixing thepowdery TS-1 with a given amount of support such ascolloidal silica or Al2O3, and extruding the mixture.The resulting product was dried at room temperatureand calcined at 823 K for 5 h. The strip TS-1 was cutinto the cylindrical pieces of 1 mm 2 mm to be usedin the fixed bed reactor.

The lamina TS-1 catalyst was prepared accordingto [10]. Thus, obtained lamina TS-1 catalyst was driedat 393 K for 3 h and calcined at 823 K for 5 h. Thethickness of the TS-1 lamina in the catalyst was de-termined on a Japan JEM-1200EX scanning electronmicroscope after some catalyst particles were smashedinto pieces.

The epoxidation of propylene was carried out ina continuous-flow fixed bed micro-reactor [11]. Theamount of catalyst used in the reaction was 410 g.The typical conditions were: solvent methanol;temperature 323 K; the initial H2O2 concentration0.85 mol/l; the ratio of H2O2 to C3H6 was 1:4.17;pressure 3.0 MPa; the WHSV of propylene is 0.1 h1.

The epoxidation of propylene was also carried outin a stainless-steel reactor, which was immersed intoa bath temperature-controlled at the required temper-ature. In a typical run, 0.4 g of powdery TS-1 cata-lyst, 2.0 ml 26 wt.% H2O2 and 31.6 ml of methanolwere fed into the reactor. Then propylene was chargedat constant pressure (0.4 MPa) and the mixture washeated at 333 K under magnetic agitation for 60 min.

The residual H2O2 was measured by iodometrictitration. The product of each reaction was analyzedon a SHIMADZU GC-8A gas chromatography us-ing a flame ionization detector and a column (40 m0.25 mm) containing polyethylene glycol 20 M as thestationary phase. Propylene oxide (PO) was the main

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G. Li et al. / Applied Catalysis A: General 236 (2002) 17 3

product. Propylene glycol (PG) and its mono-methylethers (MMEs) were the by-products.

The result of the reaction was given using thesecriteria:

XH2O2 =n0H2O2 nH2O2

n0H2O2 100

UH2O2 =nPO + nMME+nPGn0H2O2 XH2O2

100

SPO = nPOnPO + nMME + nPG 100

SMME = nMMEnPO + nMME + nPG 100

XH2O2 , UH2O2 , SPO and SMME stand for the con-version of H2O2, the utilization of H2O2 (H2O2efficiency), the selectivity of PO and the selectivityof MME, respectively. The nPO, nMME and nPG standfor the number of moles of PO, MME and PG, re-spectively. The n0H2O2 and nH2O2 stand for the initialmole content and the final mole content of H2O2,respectively.

3. Results and discussion

3.1. Effect of the support type

Different supports such as SiO2, Al2O3, TiO2 andMgO were used to prepare the supported titanium sil-icalite catalyst using the extruding method. But onlyTS-1/SiO2 and TS-1/ Al2O3 exhibited high mechani-cal strength. The strength of other supported titaniumsilicalite catalysts is very weak and they cannot beused in the fixed bed reactor.

The comparison of TS-1/SiO2 catalyst andTS-1/Al2O3 catalyst in propylene epoxidation isshown in Table 1. The two catalysts exhibited nearlythe same activity in propylene epoxidation, which ischaracterized by H2O2 conversion rate XH2O2 , butthe H2O2 efficiency of the former is higher than thatof the latter. The TS-1/SiO2 catalyst exhibited thehigher PO selectivity, while the TS-1/Al2O3 catalystexhibited the higher selectivity in mono-methyl etherof propylene glycol (MME).

The following reactions may occur in the system ofpropylene, hydrogen peroxide and methanol.

Table 1The comparison of TS-1/SiO2 catalyst and TS-1/Al2O3 catalystin propylene epoxidation

Catalyst XH2O2 (%) UH2O2 SPO(%) SMME (%)TS-1/SiO2 94.27 87.05 73.90 25.12TS-1/Al2O3 96.00 71.93 32.39 64.31

Reaction condition: temperature 333 K, pressure 3.0 MPa,C3H6/H2O2 (molar ratio) 1.9, H2O2 concentration 0.85 mol/l;methanol is solvent; total WHSV is 14.0 h1; reaction time is 8 h.

(1)

(2)

(3)

H2O2OH H2O+ 0.5O2 (4)

More acid sites are introduced when Al2O3 is usedas the support, and such acid sites can catalyze theside reaction of PO with the solvent. So the PO selec-tivity in propylene epoxidation is much higher usingTS-1/SiO2 than using TS-1/Al2O3.

Thus, SiO2 is the preferred support when one pre-pares the supported titanium silicalite catalyst.

3.2. Effect of the amount of TS-1 in supportedtitanium silicalite catalyst

The TS-1/SiO2 catalysts with different TS-1 con-tents were used in propylene epoxidation (Table 2).The fumed silica is added to reduce the ratio of TS-1and SiO2 in the catalyst. There is not obvious differ-ence in H2O2 conversion rate and H2O2 efficiency. Butwith the decrease of the amount of TS-1 in TS-1/SiO2,the PO selectivity increases. The TS-1 synthesized us-ing colloidal silica as silicon source contains somealuminum impurity (Table 3). So the more TS-1 inthe TS-1/SiO2 catalyst, the more chances there are forPO to react further, catalyzed by acid sites, before itdiffuses out the catalyst particle.

The TS-1/SiO2 catalyst C with the least TS-1 con-tent exhibited the highest PO selectivity in propylene

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4 G. Li et al. / Applied Catalysis A: General 236 (2002) 17

Table 2Performance in propylene epoxidation of TS-1/SiO2 catalyst withdifferent TS-1 content

TS-1 content inTS-1/SiO2 catalyst(wt. %)

XH2O2(%)

UH2O2(%)

SPO(%)

SMME(%)

10 95.89 78.63 76.40 22.1550 96.89 78.95 60.32 38.1290 95.42 79.86 45.83 53.62

Reaction condition: temperature 333 K, pressure 3.0 MPa,C3H6/H2O2 (molar ratio) 1.9, H2O2 concentration 0.85 mol/l;methanol is solvent; WHSV is 5.6 h1; reaction time is 24 h.

Fig. 1. SEM pictures of powdery TS-1.

epoxidation. But the strength of the catalyst is weakand it cannot be used in an industry reactor. Soother means have to be taken to increase the POselectivity.

3.3. Effect of the molding method

The powdery TS-1, whose crystal size is about6m 2m 1m (Fig. 1) and whose specific sur-face area is 361.8 m2/g, was molded into the catalystused in the fixed bed reactor using two methods. Oneis the traditional extruding method; here strip TS-1

Table 3The chemical composition of titanium silicalite TS-1 containing different amounts of aluminum impurity

Sample SiO2 (wt.%) TiO2 (wt.%) Al2O3 (wt.%) SiO2/TiO2 (mol/mol) SiO2 /Al2O3 (mol/mol)TS-1-a 94.8 3.75 0.240 33.6 671.5TS-1-b 95.4 4.21 0.034 30.2 4690

Table 4The comparison of strip TS-1 and lamina TS-1 catalyst in propy-lene epoxidation

Catalyst XH2O2 (%) UH2O2 (%) SPO (%) SMME (%)Strip TS-1 95.89 78.63 76.40 22.15Lamina TS-1 97.70 95.63 84.37 15.63

Reaction conditions: temperature 333 K, pressure 3.0 MPa,C3H6/H2O2, (molar ratio) 4.17; solvent is methanol; H2O2 con-centration is 0.85 mol/l; WHSV is 5.6 h1; reaction time is 24 h.

catalyst of 1 mm 2 mm is obtained. The other is tospray the powdery TS-1 on a small inert ball [10]; thusthe lamina TS-1 catalyst, the core of which is an inertball used as the support and the shell of which is aTS-1 lamina of 0.10.2 mm (Fig. 2), can be prepared.Although the specific surface area of the lamina TS-1catalyst is only 54.76 m2/g, the pores of 56 exist inthe zeolite channel and the pores with different sizesexist between the zeolite crystals (Fig. 3). Unlikethe catalyst prepared using the traditional moldingmethod, here all the pores exist in the outer laminaof the catalyst and the core is a solid ball withoutpores.

The strip TS-1 catalyst and the lamina TS-1 cata-lyst, containing the same amount of titanium silicalite,were used to catalyze propylene into PO under thesame reaction conditions. The comparison of their cat-alytic performance in propylene epoxidation is shownin Table 4.

The two catalysts exhibited nearly the same ac-tivity in propylene epoxidation, but the lamina TS-1catalyst had the higher H2O2 efficiency and PO selec-tivity. Two reasons may explain this difference. Thepropylene epoxidation is a fast and highly exothermicreaction. If the heat released by the reaction cannotbe transmitted in time from the reaction site to theenvironment, the temperature of the reaction site mayrise. It has been shown in a previous study that for thehigher reaction temperatures, the lower PO selectivity

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G. Li et al. / Applied Catalysis A: General 236 (2002) 17 5

Fig. 2. SEM pictures of lamina TS-1 catalyst.

results [12]. The support in lamina TS-1 catalyst isa good heat radiator and it will help to transmit thereaction heat. Thus, the temperature of the reaction siteis stable, which may result in the higher PO selectivityusing lamina TS-1 catalyst.

On the other hand, PO easily reacts with methanolor water to form monomethyl ethers or glycol, fur-ther catalyzed by acidic or basic impurity in thesystem. So the longer the diffusion path, the morechances there are for PO to contact with acid sitesin the catalyst and form the further by-products. Inorder for one to obtain high PO selectivity, PO has toquickly diffuse from catalyst into solution. Due to the

Fig. 3. Pore size of lamina TS-1 catalyst and its support (curve a) lamina TS-1 catalyst; (curve b) support.

better diffusion for PO using lamina TS-1 catalyst,the process of catalytic solvolysis of PO is not easy toobtain.

Hydrogen peroxide is a reagent that easily decom-poses. The longer the path for it to diffuse into thepore of catalyst to react with propylene, the larger theamount of it which may decompose and the lowerthe H2O2 efficiency which may result. Owing to thebetter diffusion for H2O2, the H2O2 efficiency is alsohigher using the lamina TS-1 catalyst than that usingthe strip TS-1.

In conclusion, the lamina TS-1 catalyst affords theshorter diffusion path for reaction heat, raw material

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6 G. Li et al. / Applied Catalysis A: General 236 (2002) 17

Table 5Performance of TS-1 containing different amount of aluminumimpurity in propylene epoxidation in autoclave

Sample XH2O2 (%) SPO (%) SMME (%) UH2O2 (%)TS-1-a 98.5 5.1 91.9 96.4TS-1-b 97.6 89.6 10.4 92.4

Reaction conditions: temperature, 333 K; H2O2, 0.50 mol/l; propy-lene pressure, 0.4 MPa; catalyst, 12 g/l; time 1.5 h; solvent,methanol.

and product and exhibits the better performance inpropylene epoxidation than the strip TS-1 catalystdoes.

3.4. Effect of the amount of aluminum impurity

TS-1 samples containing different amounts ofaluminum impurity have been synthesized in thisresearch (Table 3). The effect of the amount of alu-minum impurity was studied both in autoclave andin fixed bed reactor. The two TS-1 samples exhibitednearly equal activity in propylene epoxidation in auto-clave (Table 5), but there was a large difference in POselectivity. A similar phenomenon can be observed if

Fig. 4. Performance in propylene epoxidation of TS-1/SiO2 catalyst prepared using TS-1 containing different amounts of aluminum impurity.Reaction condition: temperature 333 K, pressure 3.0 MPa, C3H6/H2O2, (molar ratio) 4.17; H2O2 concentration 0.85 mol/l; methanol issolvent; total WHSV 3.36 h1; reaction time 54 h.

the two TS-1 samples were molded into TS-1/SiO2catalyst and used in a fixed bed reactor (Fig. 4). Thesephenomena can be attributed to the acid sites thatoriginate from aluminum impurity, because the tracealuminum may be incorporated into the frameworkof titanium silicalite and form Brnsted acid sites[13]. So the amount of the aluminum impurity hasan obvious influence on the product distribution ofpropylene epoxidation catalyzed by titanium silicalitecatalyst. Some weak base can be added in the mediumto neutralize the acidity originating from frameworkaluminum, but a slightly excessive amount of basecan deactivate the titanium silicalite catalyst [14].

The lamina TS-1 catalyst prepared using the twoTS-1 samples was used in a fixed bed reactor to cat-alyze the propylene epoxidation for about 200 h. Thecatalyst containing more aluminum impurity exhib-ited low PO selectivity, even if more base was added(Fig. 5). But both catalysts have good performance inthe propylene epoxidation. When the time on streamexceeds 200 h, the H2O2 conversion rate XH2O2 , thePO selectivity SPO and the H2O2 efficiency UH2O2all remain at more than 90%. Such results show thatthe lamina TS-1 catalyst is a promising catalyst inpropylene epoxidation.

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G. Li et al. / Applied Catalysis A: General 236 (2002) 17 7

Fig. 5. Performance in propylene epoxidation of lamina TS-1catalyst prepared using TS-1 containing different amounts of alu-minum impurity. (a) TS-1-a; (b) TS-1-b; reaction conditions: sol-vent methanol, temperature 323 K, H2O2 concentration 0.85 mol/l,pressure 3.0 MPa, total WHSV 0.609 h1, propylene/H2O2 (mo-lar ratio) 4.17. The concentration of basic additive: (a)09 h, 0.2 mM; 980 h, 0.4 mM; 80200 h, 0.5 mM; (b) 0200 h,0.1 mM.

4. Conclusion

Diffusion and the concentration of acid sites in thecatalyst are the two main factors influencing the prop-erty of supported titanium silicalite catalyst. The cata-lyst having the shorter diffuse path and fewer acid sitesexhibits better performance in propylene epoxidation.

Acknowledgements

The financial support of the National Natural Sci-ence Foundation of China (No. 29792071) and KeyLaboratory of Catalysis, China National PetroleumCo. is gratefully acknowledged.

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Epoxidation of propylene using supported titanium silicalite catalystsIntroductionExperimentalResults and discussionEffect of the support typeEffect of the amount of TS-1 in supported titanium silicalite catalystEffect of the molding methodEffect of the amount of aluminum impurity

ConclusionAcknowledgementsReferences