ARTICLE pubs.acs.org/IECR
Synthesis of Stainless-Steel-Net Supported TS-1 Catalyst and Its Catalytic Performance in Liquid-Phase Epoxidation Reactions Yuting Zheng, Yingtian Zhang, Zhendong Wang, Yueming Liu,* Mingyuan He, and Peng Wu* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, North Zhongshan Road 3663, Shanghai 200062, China
bS Supporting Information ABSTRACT: TS-1 particles were loaded on stainless-steel-net (SSN) to form an integrated material (TS-1/SSN) by secondary hydrothermal synthesis. This avoided completely the filtration separation in both catalyst preparation and catalytic applications in liquid-phase reactions. Seeding-assisted secondary crystallization prevented zeolite from detaching from the support during the synthesis process. The TS-1 particles were coated on the SSN support uniformly, and their loading amount was adjustable in the range of 7 25 wt % by repeating the seeding and crystallization processes. UV visible and IR spectra verified that the titanium species in TS-1/SSN mostly occupied the tetrahedral coordination sites in the zeolite framework. TS-1/SSN was comparably active to conventional TS-1 particles in batchwise epoxidation of 1-hexene with H2O2. The zeolite particles maintained the catalytic activity and did not detach from the SSN after five reuses. Moreover, TS-1/SSN proved to be efficient and reusable in continuous epoxidation of allyl chloride where the separation of products and catalyst occurred easily.
1. INTRODUCTION The first titanosilicate with the MFI topology (TS-1) was discovered by EniChem in 1983. As a novel selective oxidation catalytic material with H2O2 as green oxidant, TS-1 has received in the past decades increasing research interests ranging from developing new synthesis methods to finding possible catalytic applications in selective oxidations. TS-1 exhibits a high performance in a variety of industrially important oxidation reactions among which phenol hydroxylation, cyclohexanone ammoximation, and propylene oxidation have already come into commercial use.1 It is well-known that the crystal size of TS-1 plays an important role in its catalytic activities. Small crystals would shorten the diffusion path for substrate molecules and enhance the accessibility of Ti sites located within pores. Many researchers reported that nanosized TS-1 particles up to 300 nm were highly efficient, while micrometer scaled TS-1, e.g., with a particle size larger than 1 μm, is less efficient because it proposed significant diffusion limitations to molecules.2,3 However, the nanoparticles encounter serious separation difficulties in both synthesis procedures and reaction processes using either fixed bed or slurry reactors. Recently researchers are trying to find out ways useful for ameliorating these problems. Additives have been used to make the nanoparticles formed in synthetic gels assemble and aggregate into integrated zeolite particles of micrometer size. H2O2 and surfactants were shown to prompt the assembly of nanocrystals and meanwhile introduce mesoporosity into the bulk zeolites.4,5 On the other hand, preparing extruded catalysts could be an efficient way to obtain integrated catalyst. Different binders and extrusion methods have been attempted to prepare extruded TS-1 catalysts which are applicable to the epoxidation of propylene in a fixed bed reactor.6 Similarly, silica sol was used as binder to prepare stick-shaped extruded TS-1 catalyst for propylene epoxidation.7 TS-1 powder was also mixed with diatomite and then crushed into supported r 2011 American Chemical Society
catalyst of 0.30 0.45 mm grains.8 This material was used to investigate the intrinsic kinetics and reaction mechanism in the hydroxylation of phenol operated in a fixed-bed reactor. Different from aluminosilicates, the catalytic performance of which is seldom affected by binders, the active components of titanosilicates are delicate in physicochemical properties and receive easily negative influence from binders. Another approach to solve the separation problem is to load TS-1 nanoparticles on proper supports. Typically, two techniques have been developed to prepare supported zeolite membranes. The first one is in situ crystallization where the bare support is soaked directly in synthetic gel, and then the nucleation and growth of the zeolite phase takes place on the support under hydrothermal conditions.9 The interaction and combination between two phases are critical to inducing an oriented crystal growth on chosen supports. The second method used more frequently is to carry out a secondary growth. This technique includes two steps, that is, first depositing premade zeolite seeds on the support and then hydrothermally synthesizing the zeolite particles covering the seeds and support.10,11 The choice of support is an important factor for constructing mechanically stable and catalytically active integrated catalysts regardless of zeolite loading processes. The porous R-Al2O3 tube once was adopted as the support to prepare the TS-1 membrane reactor for the hydroxylation of phenol.12 However, the Al species in the alumina support may strongly impact the TS-1 membrane composition and properties.13,14 The Al ions isomorphously substituted in titanosilicate would generate strong acid sites and increase the electron density of framework, which lowers the Received: April 9, 2011 Accepted: July 7, 2011 Revised: July 5, 2011 Published: July 07, 2011 9587
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Scheme 1. Preparation Procedures of Methods A and B
catalytic activity of Ti active sites and the product selectivity as well by causing acid-catalyzed side reactions.15 Thus, inert supports such as carbon nanofiber are preferred to load TS-1, but the catalytic performance of resultant materials are not mentioned.14 Considering the fact that carbon materials are unable to endure the calcination in air at high temperature, that is, the fundamental conditions for regenerating used catalysts, thermally stable supports are required from a practical point of view. Thus, silicon-based supports are widely used to load TS-1. The supports include porous mullite tube,9 silica pellet,16 silica nanofibers,17 and Si wafer.18 Partial dissolving of supports is unavoidable as the preparation is conducted in the zeolite synthetic gel with a strong basity. In addition to these materials, stainless steel is also taken as a possible support.10,11,19 In this study, we have carried out loading of TS-1 zeolite on the stainless-steel-net (SSN) by direct hydrothermal synthesis. SSN was used to support ZSM-5 aluminosilicate successfully for adsorbing and separating heavy metal salts from aqueous solutions.20 The use of SSN allows one to omit the filtration procedure in zeolite synthesis. More importantly, possessing a high flexibility, the integrated materials of zeolites and SSN can be bent into the shapes suitable for packing in different reactors. We have optimized the preparation procedures for loading TS-1 on SSN. The H2O/SiO2 molar ratio in gel was decisive based on the crystal size and catalytic performance of TS-1, while the secondary growth technique was effective to increase its loading amount. TS-1/SSN was highly active and regenerable in the liquid-phase epoxidation of alkenes and showed potential applications in continuous reactions.
2. EXPERIMENTAL SECTION 2.1. Source of Chemicals and Materials. Tetraethyl orthosilicate (TEOS) was from Sinopharm Chemical Reagent Co., Ltd.; tetrabutyl orthotitanate (TBOT) was from Chinasun Specialty Products Co., Ltd.; tetrapropylammonium hydroxide (TPAOH) was from Hunan Jianchang Sinopec Co., Ltd. All chemicals were used as received without further purification. Stainless-steel-net was from Shanghai Hai Tang Strainer and Filters Co., Ltd. (500 mesh; 316 L stainless steel; stainless steel diameter, 25 μm; pore size, 26 μm 26 μm). 2.2. Preparation of TS-1 Seeds and Conventional TS-1. In a typical synthesis, a mixture of TEOS and TBOT was dropped into TPAOH solution under stirring. The alcohols formed were removed by evaporation at 353 K to form a synthetic gel having the following molar composition: 1.0SiO2/0.025TiO2/ 0.18TPAOH/18H2O. The gel was transferred into a 120 mL
Teflon-lined stainless-steel autoclave that was heated statically at 443 K for 1 day. The suspension containing about 10 wt % TS-1 particles of approximately 300 nm in size was used as the seeding solution for synthesizing stainless-steel-net supported TS-1. A part of the product was collected by centrifugation, washed with deionized water, dried at 353 K overnight, and finally calcined in air at 823 K for 10 h, leading to conventional TS-1. 2.3. Preparation of TS-1/SSN Catalysts. In a typical preparation, stainless-steel-net was dipped in the TS-1 seeding suspension for 1 min. Seeded SSN (seeds/SSN) was calcined in air at 823 K for 10 h. It was then immersed in the synthetic solution with a chemical composition of 1.0SiO2/0.025TiO2/0.18TPAOH/xH2O (x = 18, 60, 100, 140, 180, 220) for secondary growth at 443 K for 1 day. The material loaded with TS-1 was taken out directly and subjected to ultrasonic treatment (500 W, 1 h) in water, washed with deionzied water, dried at 353 K, and calcined at 823 K for 10 h. The samples were denoted as TS-1/SSN-x, where x represents the H2O/Si ratio used in the synthesis. Two methods were used to increase the loading amount of TS-1 (Scheme 1). In method A, the seeded SNN after calcination was dipped again in seeding suspension. After the seeding and calcination procedures were repeated several times, the secondary growth of TS-1 in synthetic gel was carried out as above. The samples were denoted as TS-1/SSN-A-y, where y represents the times of seeding and calcination. In method B, the whole preparation procedures, i.e., seeding, calcination, and hydrothermal recrystallization were repeated. The samples were denoted as TS-1/SSN-B-y, where y represents repeated times. The weight of loaded TS-1 was quantified by deducting the SSN weight from that of TS-1/SSN. 2.4. Characterization Methods. The X-ray diffraction (XRD) patterns were collected on a Rigaku Ultima IV diffractometer using CuKR radiation and a nickel filter in the 2θ angle range of 5 35° at 35 kV and 25 mA. The scanning electron microscopy (SEM) images were taken on a Hitachi S-4800 microscope. FTIR spectra were recorded on a Nicolet infrared spectrometer (NEXUS 670). The spectrum of conventional TS-1 powder was measured using the KBr technique, whereas that of TS-1/SSN was measured directly by penetrating the net sheet with a laser beam because the TS-1 powder was hardy scraped off. UV visible measurements were performed on a Shimadzu UV-2400PC using the diffuse reflectance mode with BaSO4 as a reference. 2.5. Catalytic Reactions. The epoxidation of 1-hexene with H2O2 was adopted as a probe reaction to measure the catalytic activity and reusability of TS-1/SSN. The reaction was carried out under vigorous stirring in a 50 mL glass flask connected to a cooling condenser. In a typical run, 10 mL of methanol solvent, 9588
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Table 1. Effect of Preparation Procedure on TS-1 Loading on SSN seeding
hydrothermala
TS-1 loading amountb
no.
of SSN
calcination
regrowth
1 2
no no
no yes
yes yes
3
yes
no
yes
0
4
yes
yes
yes
∼7 wt %
0 0
a
Crystallization conditions: gel molar composition, 1.0SiO2/0.025TiO2/ 0.18TPAOH/18H2O; T = 443 K; t = 1 day. b After ultrasonic treatment.
Figure 1. Flowchart of epoxidation of allyl chloride in a continuous reactor.
10 mmol of 1-hexene, 10 mmol of H2O2 (30 wt % aqueous solution), and a desirable amount of catalyst were mixed in the flask and heated under stirring at 333 K for 2 h. After removal of the catalyst powder, the products were analyzed and quantified with gas chromatography (GC) (Shimadzu, flame ionization detector (FID) and 30 m OV-1 column) with cyclohexanone as an internal standard. The amount of unconverted H2O2 was quantified by titration with Ce(SO4)2 solution. The catalytic performance of TS-1/SSN was further investigated in the epoxidation of allyl chloride (ACL). The reaction was carried out continuously using a homemade fixed-bed reactor equipped with feeding pump, condenser, and overflow outlet (Figure 1). The reaction temperature was controlled at 333 K using a water bath. TS-1/SSN sheets (containing 0.6 g of net active component of TS-1 particles) were curled into rolls and jammed into the reactor (30 mL). The mixture of methanol, ACL, and H2O2 (30 wt %) were fed into the reactor at a molar ratio of 30:3:1 using a peristaltic pump at a rate of 10 g h 1. The weightfeed rate (W/F) relative to the TS-1 catalyst was 0.34 and 2.37 h for ACL and H2O2, respectively. The reaction mixture overflow from the outlet of the reactor was subjected to GC analysis and titration with Ce(SO4)2 solution at different time intervals. After regeneration by calcination in air at 823 K for 10 h, the spent catalyst was reused in continuous epoxidation of ACL.
3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of TS-1/StainlessSteel-Net. Table 1 shows the effects of pretreatment and
hydrothermal crystallization on loading TS-1 on SSN. When as-received SSN was immersed in synthetic gel and then hydrothermally heated at 443 K for 1 day, the crystallization of TS-1 was readily realized in mother gel solution as evidenced by the XRD pattern. However, no weight gain was observed with the treated SSN (Table 1, no. 1). The SEM image also showed that the SNN surface was still bare without any zeolite phase. This implies that it is necessary to increase the interaction between SSN and zeolite in order to realize in situ growth of TS-1 on SSN. SSN was thus calcined at 823 K for 10 h to make its surface partially oxidized. It was then subjected to hydrothermal synthesis. However, its weight still did not increase (Table 1, no. 2).
Figure 2. XRD patterns of (a) SSN support, (b) seeds/SSN after calcination, (c) TS-1/SSN, and (d) conventional TS-1. The inset shows magnified patterns of SSN support and seeds/SSN.
Even when SSN was seeded with premade suspension containing TS-1 nanocrystals but without calcination, the regrowth of zeolite did not take place on SSN but only in the liquid phase (Table 1, no. 3). The coated seeds dropped off easily from SSN when immersed in water if without calcination. When the seeded SSN was first calcined and then hydrothermally treated in synthetic gel, a weight gain of ∼7 wt % was observed even after ultrasonic treatment (Table 1, no. 4). The XRD patterns of SSN, seeds/SSN, TS-1/SSN, and conventional TS-1 are shown in Figure 2. SSN lacked any diffraction in the 2θ range of 5 35° (Figure 2a). After SSN was seeded with TS-1 suspension and calcination, the XRD pattern was almost the same (Figure 2b). However, it showed around 2θ of 23° a very weak diffraction which was characteristic of the MFI phase (see magnified pattern), indicating that SSN was coated by zeolite seeds but with a very low loading. The XRD pattern taken directly by setting the sheet material of TS-1/SSN on the sample holder of the diffractometer was identical to the powder pattern of conventional TS-1 (Figure 2c,d), indicating successful in situ growth of TS-1 crystals on SSN. Compared with conventional TS-1, the diffractions of TS-1/SSN were comparably intensive but did not show orientation to specific diffraction planes. Unlike crystal growth on disk or plate supports, TS-1 crystals seemed to grow randomly on present net materials. The SEM images show that as-received SSN were woven from the stainless steel columns (25 μm diameter) with a relatively smooth surface (Figure 3a,b). The grid meshes were of about 26 μm 26 μm square-shaped holes. Surface roughness was observed after calcination in air (Figure 3c), which can be taken as a proof for the formation of metal oxides. After seeding and calcination, one layer of TS-1 seeds with a secondary particle size of ∼300 nm covered the steel columns (Figure 3d). These seeds existed almost independently with extremely weak interconnection, 9589
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Figure 5. FT-IR spectra of (a) TS-1 loaded on SSN and (b) conventional TS-1 both after calcination.
Figure 3. SEM images of (a) SSN column surface, (b) SSN overview, (c) SSN column surface after calcination, (d) seeds/SSN surface, (e) TS-1/ SSN overview, and (f) TS-1/SSN surface after ultrasonic treatment.
Figure 4. UV vis spectra of (a) TS-1 loaded on SSN and (b) conventional TS-1 both after calcination.
and they were detached easily from the support by ultrasonic treatment. However, the grid meshes were occupied entirely by zeolite particles after secondary growth in synthetic gel, and the zeolite phase on the columns also increased in thickness (Figure 3e). A close view of surface showed that the zeolite phase was composed of strawberry-like nanocrystals of 100 500 nm (Figure 3f). The crystal surface remained rugged as TS-1 seeds, and its primary particle size became slightly smaller. These supported crystals were closely connected with each other to construct a hierarchical structure mechanically stable against ultrasonic treatment. The above results implied that the loading of TS-1 on SSN in the present system required simultaneously the procedures of seeding, calcination, and hydrothermal regrowth. Especially, the calcination of seeded SSN was indispensable. This was different from previous reports where the secondary growth was carried out with the seeded support without calcination. In the case of porous stainless steel plate, the uneven surface provides anchorage for the formation of zeolite membrane.10,11 However, the surface of the stainless steel column was too smooth to anchor the TS-1 particles with a much smaller crystal size. The calcination would make the SSN surface partially oxidized to form a metal oxide phase as evidenced by the SEM image (Figure 3c), which is presumed to enhance the interaction between the SSN and the external silanol groups of the TS-1 seeds. This then made an orientated crystal regrowth take place possibly on SSN without exfoliation.
UV visible and FT-IR spectra were measured to investigate the state of incorporated Ti species. Both TS-1/SSN and conventional TS-1 showed the main band at 220 nm in UV visible spectra (Figure 4). It is attributed to the isolated Ti species tetrahedrally coordinated in the zeolite framework.21 The absence of the 330 nm band implied that they were almost free of the anatase phase. There was no obvious band at 260 nm, which demonstrated that both samples were almost free of octahedral Ti species.22 In the region of framework vibration, the FT-IR spectra exhibited two characteristic bands at 960 and 550 cm 1 for both TS-1/SSN and conventional TS-1 (Figure 5). Although there are still some arguments about the assignment of the 960 cm 1 band, it is probably assignable to the stretching of Si O or Si O Ti in the (Si O)3SiOTi configuration in the framework, while the 550 cm 1 band is considered as the fingerprint of the pentasil structure with the MFI topology.23 As the spectra of two samples were measured in different ways, that is, with the KBr technique for the conventional TS-1 powder and directly for the TS-1/SSN sheets, they were used only for qualitative analysis but not for quantitative comparison. In agreement with the above UV visible spectra, the IR spectra also confirmed that the Ti ions were successfully incorporated in the MFI framework, which may contribute to a good catalytic performance in liquid-phase oxidation with H2O2. 3.2. Increasing TS-1 Loading of TS-1/SSN. For the purpose of practical application, it is preferable to load the active component of TS-1 crystals on SSN as much as possible. The H2O/Si ratio is reported to play an important role in zeolite synthesis, especially in the preparation of zeolite membranes.24 We have investigated the influence of water on TS-1 loading in the secondary growth process. The H2O/Si ratio of parent gels was changed in the range of 18 220. The percentage of loaded TS-1 increased gradually with an increasing H2O/Si ratio and reached 24 wt % at a H2O/Si of 220 (Table 2). Dilute gels were thus in favor of regrowing TS-1 crystals on SSN support. The crystallization of TS-1 crystals occurred simultaneously in the liquid-phase and on the support in concentrated gels, for example, at a lower H2O/Si ratio of 18. The concentration of structure-directing agent and Si species turned to be much lower in dilute gels. This retarded the crystallization rate of TS-1 in the liquid phase. In contrast, the crystallization on the SSN surface turned to be preferable with the assistance of seeds existing therein. Moreover, a lower OH concentration in dilute gels may reduce the possibility of dissolving the seeds from the support, leading to a higher loading amount of zeolite. With respect to the catalytic performance of TS-1/SSN, the epoxidation of 1-hexene was used as a probe reaction to evaluate its catalytic activity. The reaction was carried out in methanol which was shown to be the favorable solvent for TS-1.25 The 9590
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Table 2. Catalytic Activity of Supported TS-1 Synthesized at Different H2O/Si Ratios in the Epoxidation of 1-Hexenea 1-hexene epoxidation (%) H2O/Si ratio
TS-1 loading (wt %)
crystal size (μm)
1-hexene conversion
epoxide selectivityb
H2O2 conversion
H2O2 selectivity
18
7.0
0.4
22.0
>95%
30.8
71.4
60
8.9
0.6
18.7
>95%
29.7
63.0
100
14.5
0.8
7.3
>95%
10.2
71.5
140
15.2
1.0
7.4
>95%
11.4
64.3
220
24
1.5
4.5
>95%
6.6
70.2
a
Reaction conditions: TS-1 active component, 0.05 g; 1-hexene, 10 mmol; H2O2 (30 wt %), 10 mmol; methanol, 10 mL; T = 333 K; t = 2 h. b Main product, 1,2-epoxyhexane; byproducts, hexane-1,2-diol, 1-methoxyhexan-2-ol, and 2-methoxyhexan-1-ol (see Scheme S1 in the Supporting Information).
Figure 7. Percentage of loaded TS-1 after repeated syntheses.
Figure 6. SEM images of TS-1/SSN hydrothermally synthesized at a H2O/Si ratio of (a) 18, (b) 60, (c) 100, (d) 140, (e) 180, and (f) 220.
reaction gave 1,2-epoxyhexane as the main product together with byproducts of hexane-1,2-diol, 1-methoxyhexan-2-ol, and 2-methoxyhexan-1-ol as a result of solvolysis of epoxide with water and methanol. The conversion of 1-hexene decreased with an increasing H2O/Si ratio when the reaction was carried out using the same amount of TS-1 active component (Table 2). The SEM images showed that both the morphology and the crystal size of loaded TS-1 changed significantly with the H2O/Si ratio (Figure 6). Nanosized crystals were formed at a H2O/Si of 18, while the crystals of micrometer size were obtained at higher H2O/Si ratios. Moreover, the materials turned to be blockshaped crystals with a smooth surface with an increasing H2O/ Si ratio. It is generally accepted that TS-1 with a high catalytic activity has a crystal size distribution around 300 nm.2 The activity drops down when the particle size is larger than 1 μm because of a longer diffusion path for the substrates inside the pores.26 The TS-1 crystals of TS-1/SSN-100, TS-1/SSN-140, TS-1/SSN-180, and TS-1/SSN-220 were about 1 μm and had a cubic morphology. TS-1 with this kind of crystal size and morphology is considered to be less efficient in oxidation. As a result, the H2O/Si ratio of 18 was used as the optimized condition for synthesizing highly active TS-1/SSN catalysts although with a low loading of active component.
In order to increase the loading amount of TS-1 at a H2O/Si of 18, two repeated preparation methods have been attempted as shown in Scheme 1. In the method A, the preparation was just repeated in seeding and calcination procedures. In the method B, the procedures of seeding, calcination, and secondary hydrothermal synthesis were repeated. The loading amount of TS-1 increased after repeated syntheses (Figure 7). Both method A and method B were effective to increase the loading amount of the zeolite phase. At first synthesis, there was no obvious difference in the loading amount of TS-1 between TS-1/SSNA-1 and TS-1/SSN-B-1. However, after the second synthesis, the increment of TS-1/SSN-B was larger than that of TS-1/SSN-A owing to extra regrowth. After the third synthesis, the loading amount of TS-1 reached 24 wt % in TS-1/SSN-B-3, which was about three times that of TS-1/SSN-B-1. Meanwhile, the loading amount of TS-1 in TS-1/SSN-A-3 was 17 wt %. The SEM images showed that the steel columns of SSN were still observed clearly for TS-1/SSN-A-3 (Figure 8a). On the other hand, the columns were difficult to identify for TS-1/SSN-B-3 with a higher coverage of the zeolite phase (Figure 8c). The thickness of the TS-1 crystal layers in TS-1/SSN-B-3 was about 6 μm. These two samples exhibited slightly different morphology for the crystals loaded. TS-1/SSN-A-3 had the same morphology and crystal size as conventional TS-1. The secondary particles of TS-1/SSN-B-3 were larger and were composed of aggregated crystals with a cubic shape (Figure 8d). Nevertheless, their primary crystals were still nanosized. When they were tested for the epoxidation of 1-hexene with H2O2 by fixing the amount of TS-1 active component at 50 mg, the conversion was in the range of 22.4 23.6%. Thus, they were comparably active to conventional TS-1 powder in the epoxidation of 1-hexene (23.7%). 3.3. Catalytic Properties of TS-1/SSN in Epoxidation of Alkenes with H2O2. The epoxidation of 1-hexene with H2O2 was carried out to check the stability and reusability of TS-1/SSN which was prepared at a H2O/Si ratio of 18 (TS-1/SSN-18 with 9591
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Figure 8. SEM images of (a, b) TS-1/SSN-A-3 and (c, d) TS-1/SSN-B-3.
Figure 10. Catalytic performance of TS-1/SSN-A (A) and TS-1/SSN-B (B) in continuous epoxidation of allyl chloride. Reaction conditions: catalyst, 0.6 g; methanol/ACL/H2O2 molar ratio = 30:3:1; feeding rate = 10 g h 1; T = 333 K.
Figure 9. The conversion of 1-hexene and H2O2 conversion of reused TS-1/SSN catalyst. Reaction conditions: TS-1/SSN, 0.7 g (containing TS-1 active component, 0.05 g); 1-hexene, 10 mmol; H2O2 (30 wt %), 10 mmol; methanol, 10 mL; T = 333 K; t = 2 h.
7 wt % TS-1 loading as shown in Table 2). After regeneration by calcination at 823 K for 10 h in air, the spent TS-1/SSN catalyst was reused under same reaction conditions. During five recycles, the conversion of 1-hexene and that of H2O2 was on average 21% and 27%, respectively (Figure 9), implying a very constant catalytic performance. Meanwhile, The TS-1/SSN catalyst (initial weight of 0.7 g with ∼7 wt % TS-1 loading) did not show weight loss after repeated reactions. The mass remained in the range of 0.703 ( 0.002 g, without a noticeable trend (Figure S1 in the Supporting Information). Thus, the exfoliation of loaded TS-1 particles and the leaching of Ti species both hardly occurred in catalytic reuse. Possessing a high mechanical strength, TS-1/SSN can serve as a robust and regenerable catalyst for liquid-phase oxidation reactions. Under the same conditions, the conventional TS-1 powder (Si/Ti = 40, 0.05 g) showed 23.7% of 1-hexene conversion and 27.3% of H2O2 conversion. The activity of conventional TS-1 powder and TS-1/SSN was at the same level with respect to the active component. The selectivity of H2O2 of TS-1/ SSN was a little lower. This is probably due to nonproductive decomposition of H2O2 catalyzed by the metal oxides formed on SSN during calcination. As a result, TS-1/SSN with an integrated structure and a good catalytic activity is potentially applicable to fixed bed reactions. 3.4. Application of TS-1/SSN to Continuous Epoxidation of Allyl Chloride in A Fixed Bed Reactor. TS-1/SSN-A-3 and TS-1/SSN-B-3 with high TS-1 loadings were applied to continuous epoxidation of allyl chloride (ALC) in a designed fixed bed reactor (Figure 1). The reaction mixture was pumped continuously into the reactor inside which the TS-1/SSN catalyst was
packed, whereas the reaction mixture separated automatically with the catalyst and overflowed from the outlet. This reaction is industrially important for producing raw material for epoxy resins. In the case of TS-1/SSN-A-3, the conversion of ALC was about 25% at the beginning and decreased gradually with time on stream (Figure 10A). The conversion decreased to 15% at a time on stream (TOS) of 20 h. The selectivity of epichlorohydrin (ECH) was over 80%. Also the H2O2 conversion was always kept at about 90%. In comparison to TS-1/SSN-A-3, TS-1/ SSN-B-3 showed a better catalytic performance in terms of stability and epoxide selectivity. The conversion of ALC was maintained at ∼25% during 30 h of TOS (Figure 10B). The ECH selectivity and H2O2 conversion were around 85% and 90%, respectively. As shown in Scheme 1, TS-1/SSN-B-3 and TS-1/SSN-A-3 were prepared following different procedures of seeding and crystallization. The post-treatment with TPAOH was demonstrated to be extremely effective to enhance the catalytic activity of TS-1 as a result of a so-called framework rearrangement.27 Two extra hydrothermal syntheses in the presence of TPAOH adopted for TS-1/SSN-B-3 are presumed to induce that kind of structural modification to the TS-1 crystals gained in the previous synthesis and then contribute positively to oxidation reactions. Catalytic deactivation was observed for TS-1/SSN-A-3 in the first run. After the spent catalyst was regenerated by calcination at 823 K in air and reused under the same reaction conditions, the catalyst almost recovered the activity by showing an initial ALC conversion of 22% (Figure S2 in the Supporting Information). In the third reuse, the conversion of ALC was slightly lowered to 20%. The selectivity of epichlorohydrin and the H2O2 conversion were kept at the same level as the first run. The weight of TS-1/SSN-A-3 did not decrease after the long run and regeneration by calcination. The deactivation is thus considered to be mainly due to the deposition of heavy byproducts inside the pores. The results suggest that by combining secondary growth technique and regeneration, TS-1/SSN is potentially applicable to continuous 9592
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4. CONCLUSIONS We loaded TS-1 zeolite on stainless-steel-net successfully via a secondary growth technique. The TS-1 crystals completely covered the stainless-steel-net after seeding, calcination, and recrystallization processes. The morphology, crystal size, and amount of loaded TS-1 were adjustable by controlling the H2O/SiO2 ratio in synthetic gels and repeating the crystal regrowth. The secondary synthesis incorporated the Ti species mainly in the framework of TS-1. The TS-1 crystals loaded at an optimum H2O/SiO2 ratio of 18 showed catalytic properties similar to conventional TS-1 particles in the liquid-phase epoxidation of alkene. The TS-1 crystals loaded on SSN were mechanically stable against ultrasonic treatment and were hardy detached in catalytic reuse. the integrated catalysts of TS-1/SSN showed a potential to oxidation in fixed bed reactors when maintaining a stable activity for a long time in the continuous epoxidation of allyl chloride. ’ ASSOCIATED CONTENT
bS
Supporting Information. Product distribution and catalyst reuse in alkene epoxidation. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (P.W.);
[email protected] (Y.L.). Fax: +86-21 62232292.
’ ACKNOWLEDGMENT We gratefully acknowledge the NSFC of China (Grants 20890124, 20925310, 20873043), the Science and Technology Commission of Shanghai Municipality (Grant 09XD1401500), 863 Program (Grant 2008AA030801), and the Shanghai Leading Academic Discipline Project (Grant B409). ’ REFERENCES (1) Perego, C.; Carati, A.; Ingallina, P.; Mantegazza, M. A.; Bellussi, G. Production of titanium containing molecular sieves and their application in catalysis. Appl. Catal., A 2001, 221, 63–72. (2) Tuel, A. Crystallization of titanium silicalite-1 (TS-1) from gels containing hexanediamine and tetrapropylammonium bromide. Zeolites 1996, 16, 108–117. (3) van der Pol, A. J. H. P.; Verduyn, A. J.; Van Hooff, J. H. C. Why are some titanium silicalite-1 samples active and others not. Appl. Catal., A 1992, 92, 113–130. (4) Shan, Z.; Lu, Z.; Wang, L.; Zhou, C.; Ren, L.; Zhang, L.; Meng, X.; Ma, S.; Xiao, F.-S. Stable bulky particles formed by TS-1 zeolite nanocrystals in the presence of H2O2. ChemCatChem 2010, 2, 407–412. (5) Shan, Z.; Wang, H.; Meng, X.; Liu, S.; Wang, L.; Wang, C.; Li, F.; Lewisc, J. P.; Xiao, F.-S. Designed synthesis of TS-1 crystals with controllable b-oriented length. Chem. Commun. 2011, 47, 1048–1050. (6) Serrano, D. P.; Sanz, R.; Pizarro, P.; Moreno, I.; de Frutos, P.; Blazquez, S. Preparation of extruded catalysts based on TS-1 zeolite for their application in propylene epoxidation. Catal. Today 2009, 143, 151–157. (7) Liu, H.; Lu, G.; Guo, Y.; Guo, Y.; Wang, J. Chemical kinetics of hydroxylation of phenol catalyzed by TS-1/diatomite in fixed-bed reactor. Chem. Eng. J. 2006, 116, 179–186.
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