Ind. Eng. Chem. Res. 2009, 48, 4293–4299
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Silicalite-1 Zeolite Membrane Reactor Packed with HZSM-5 Catalyst for meta-Xylene Isomerization Chun Zhang, Zhou Hong, Xuehong Gu,* Zhaoxiang Zhong, Wanqin Jin, and Nanping Xu* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, China
A silicalite-1/R-Al2O3 zeolite membrane with high para-xylene (PX) selectivity was tested for isomerization of meta-xylene (MX) to PX. Two HZSM-5 catalyst packing methods were compared in the membrane reactor: (1) catalyst in contact with the Al2O3 substrate and (2) catalyst in contact with the membrane layer. The second packing method exhibited higher efficiency for MX isomerization. Compared with a conventional reactor, the membrane reactor with the second catalyst packing method had increased rates of 26.0% and 31.2% for the PX yield and selectivity, respectively, at a temperature of 270 °C. PX selectivity and yield increased as the flow rate of the sweep gas increased because of the enhanced PX flux through the zeolite membrane. Increasing the gas hourly space velocity (GHSV) could reduce the contribution of membrane permeation to PX productivity. It was also revealed that MFI zeolite membranes exhibit a high stability for xylene isomerization. 1. Introduction Para-xylene (PX) is a main chemical stock for the synthesis of pure terephthalic acid (PTA), which is used in the production of polyester resins and fibers. Industrially, PX is mainly produced from the catalytic reforming of naphthas and the disproportionation of toluene. Extensive side products such as ortho-xylene (OX), meta-xylene (MX), and ethyl benzene (EB) are formed in the process. The isomerization of the less-used MX and OX is generally employed to recover more PX in the subsequent industrial stage. During the past decade, continuous efforts have been directed toward improving the PX yield and selectivity in xylene isomerization1-5 and reducing the costs of PX separation from xylene isomers. Because of their similar boiling points, xylene isomers are extremely difficult to separate by distillation. Industrially, PX separation is carried out by cryogenic crystallization or selective adsorption from xylene isomers, which are highly energyintensive because they are batch operations. Recently, MFItype zeolite membranes have been studied for PX separation based on shape selectivity. MFI zeolite has a well-defined structure with an average pore size of 5.5 Å, which is similar to the kinetic diameter of PX (∼5.8 Å), but smaller than those of OX and MX (∼6.8 Å). Thus, PX exhibits as higher diffusivity through MFI zeolite channels than MX and OX as a result of configurational effects. In principle, defect-free MFI zeolite membranes are capable of separating PX from the other two isomers with high permselectivity. Over the past two decades, many studies have been conducted on the preparation of MFI zeolite membranes and their uses.6-16 The earlier reports10-12 showed considerable difficulty in the use of MFI zeolite membranes for xylene separation. It is commonly accepted that the microstructure of MFI zeolite membranes (e.g., defects, intercrystalline pores, thickness, and orientation) is vital to the membrane performance for xylene separation. In addition, xylene separation by MFI zeolite membranes is also strongly related to the operating conditions such as temperature and feed pressure because of sorbate-sorbate and sorbate-framework interactions. With the improvement of * To whom correspondence should be addressed. Tel.: (86)2583172268. Fax: (86)25-83172268. E-mail:
[email protected].
membrane synthesis and operation optimizations, the MFI zeolite membranes with high PX separation performance have recently been achieved by several groups.13-16 Keizer et al.13 obtained a PX/OX separation selectivity of about 200 at 127 °C on a disk-shaped MFI zeolite membrane. Lai et al.14 reported vapor permeation separation of xylene isomers with a b-oriented MFI zeolite membrane. The membrane showed a high PX permeance of 2 × 10-7 mol/m2 · s · Pa with a PX/OX separation factor of up to 500. Yuan et al.15 synthesized silicalite membrane by a secondary growth method without a template and achieved high PX selectivity using the pervaporation method. Gu et al.16 used a tubular MFI zeolite membrane having an ideal PX/OX selectivity of 72.9 to separate simulated industrial mixtures. A PX/(MX + OX) selectivity of 7.71 was achieved at 250 °C and atmospheric pressure for the separation of an eightcomponent mixture. Meanwhile, several groups have shown a strong research interest in a new MFI membrane reactor that integrates xylene isomerization and PX separation.17-21 Immediate and selective removal of PX from the reaction system could significantly enhance the reaction toward high selectivity and yield.17 van Dyk et al.18 demonstrated xylene isomerization in an MFI zeolite membrane reactor packed with commercial catalyst. Improvement in both PX productivity and PX selectivity was achieved in the membrane reactor compared with a conventional fixedbed reactor. Haag et al.19 also investigated a catalytic HZSM-5 membrane for xylene isomerization. Although the membrane exhibited a modest separation performance, increases of ∼15% in MX conversion and ∼10% in PX selectivity were found for the membrane reactor compared to a conventional reactor. By simulation of a membrane reactor for xylene isomerization, Deshayes et al.20 predicted an increase of PX production and suggested that membranes with high PX selectivities were necessary for this process. In the same group,21 Ba2+ ionexchanged ZSM-5 zeolite membranes were prepared recently, and an increase of 28% in PX production was achieved in a membrane reactor for xylene isomerization. These results provide strong evidence for the potential utilization of MFI zeolite membrane reactors in xylene isomerization; however, understanding of the new process is still limited based on the current studies. Better knowledge of the
10.1021/ie801606s CCC: $40.75 2009 American Chemical Society Published on Web 03/27/2009
4294 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009
Figure 1. Schematic diagram of the experimental apparatus used for membrane separation and reaction. (Disk: membranes or R-Al2O3 substrates with a full covering of dense glass enamel.)
new process is important to advance our understanding of the optimization of reactor configurations and operating conditions. In this work, we synthesized high-quality silicalite-1 membranes based on our previous work.16 An extensive study on the effects of operating conditions and membrane stability was performed on the zeolite membranes for xylene isomerization using catalyst packing methods. Commercial HZSM-5 catalyst was used in this study because it has a phase structure similar to that of the membrane layer. A catalyst containing Al atoms in the zeolite framework provided catalytically active acid sites for the isomerization reaction, and the membrane layer was all-silica MFI zeolite, which can be used only for PX separation. 2. Experimental Section 2.1. Membrane Preparation. Silicalite-1 zeolite membranes were synthesized on disk-shaped R-Al2O3 substrates by in situ hydrothermal crystallization method reported in our previous work.16,22 The substrates had a diameter of 27 mm, a thickness of 2 mm, and an average pore size of ∼100 nm with a porosity of ∼30%. Before hydrothermal synthesis, the substrate was polished by 600-mesh SiC, and the substrate edges were covered with glass enamel for sealing purposes. The active membrane area was about 2.54 cm2. The synthesis solution was prepared by dissolving 5 g of fumed SiO2 (99.98%, Aldrich) in a solution containing 25 mL of 1 M tetrapropylammonium hydroxide (TPAOH, Aldrich) and 0.35 g of NaOH pellets (99.998%, Aldrich) at 80 °C. Hydrothermal crystallization was carried out at 180 °C for 5 h. The as-made membranes were then washed with deionized water and dried in an oven overnight. To remove the occluded template in the zeolite framework, the membranes were further fired at 450 °C for 8 h. 2.2. Membrane Separation and Reaction. The PX (99%) and MX (99%) used in this study were purchased from Alfa Aesar. High-purity helium (99.999%) was used as a carry gas or sweep gas. Commercial HZSM-5 catalyst (20-40 mesh, Nankai University Catalyst Co., Ltd.) with Si/Al ) 50 was used forxyleneisomerization.ThecatalysthadaBrunauer-Emmett-Teller (BET) surface area of ∼345.2 m2/g. Before the isomerization reaction, the catalyst was activated in air at 450 °C for 5 h. Figure 1 shows a schematic diagram of the experimental apparatus used for membrane separation and reaction. A zeolite membrane was sealed by graphite rings and mounted in a stainless steel cell. A helium stream was saturated with xylene vapor in a saturator and diluted by another helium stream. The resulting stream containing xylene vapor was introduced onto one side of the zeolite membrane (feed side). The other side of the zeolite membrane (permeate side) was swept with a helium stream. The products obtained from separation or reaction tests were analyzed with an online gas chromatograph (GC, GC2014, Shimadzu) equipped with a flame ionization detector (FID) and a packed column of 5% Rt-1200/5% Bentone 34 on 100/120 Silcopt W (Restek). The pipe connected to the GC was wrapped
Figure 2. Different operating modes for xylene isomerization. (MR1 and MR2, packed-bed membrane reactors; CR, conventional reactor.)
with heating tapes and kept at 125 °C to avoid condensation of the products. Three reactor configurations (MR1, MR2, and CR) were investigated for xylene isomerization, as shown in Figure 2. The as-synthesized zeolite membranes, which exhibit strong chemical bonds between the zeolite layer and the substrate, were used for the construction of membrane reactors. MR1 and MR2 are typical packed-bed membrane reactors, in which MFI zeolite membranes were packed with HZSM-5 catalyst particles for xylene isomerization. The surfaces of the substrate and zeolite film were in contact with the catalyst beds of MR1 and MR2, respectively. CR is a conventional reactor packed with HZSM-5 catalyst. Dense glass enamel was fused to one surface of an R-Al2O3 substrate and used for catalyst loading. To avoid shortcut of feed flow, a stainless steel tube with 2-mm o.d. was used to introduce the feed stream to the center of the catalyst bed in the chamber of the reactors. The reaction products of MX isomerization over HZSM-5 catalyst mainly included PX, OX, toluene (TL), and trimethyl benzene (TB). For membrane reactions, the product compositions from the retentate and permeate were both analyzed by GC. It was found that TB in the permeate product was under the detection level of the GC because its molecular size (kinetic diameters: 1,3,5-TB, ∼7.5 Å; 1,2,4-TB, ∼6.8 Å) is much larger than the pore size of MFI zeolite membranes. Figure 3 shows typical GC analysis patterns for the retentate and permeate in the membrane reactor. The conversion, selectivity, and yield for membrane reactions were calculated by combination of the analysis results from the retentate and permeate. The membrane separation factor for component i over component j is defined as
Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4295
Ri/j )
yi /yj xi /xj
where xi and xj are the mole compositions of components i and j, respectively, in the feed stream and yi and yj are the corresponding mole compositions in the permeate stream. The membrane ideal selectivity for component i over component j is defined as Si/j )
Pi Pj
where Pi and Pj are the single-component permeances (mol/ m2 · s · Pa) for components i and j, respectively, through the zeolite membranes. 2.3. Characterization. The structures of prepared samples were determined by X-ray diffraction (XRD, D8-Advance, Bruker). The textures of the membranes were observed by scanning electron microscopy (SEM, Quanta200, LEO). The carbon deposition on HZSM-5 catalyst was characterized by thermogravimetric analysis (TGA, STA409, Netzsch). The acidic centers of the catalyst were determined by NH3 temperature-programmed desorption (TPD, Chembet-3000, Quantachrome).
Figure 3. GC patterns of the retentate and permeate components from the membrane reactor.
3. Results and Discussion 3.1. Xylene Separation. Figure 4 shows the separation results of PX/OX and PX/MX binary mixtures as a function of temperature (250-400 °C). The flow rates of the feed gas and sweep gas were kept at 20 mL/min. The separation layer of zeolite membrane was facing toward the sweep side. The feed partial pressures (kPa) for the PX/MX and PX/OX mixtures were 0.70/0.67 and 0.70/0.54, respectively. The permeance and ideal selectivity obtained from the single-component permeations of PX and MX are also included in Figure 4. In the tested temperature range, bell-shape profiles for PX permeance vs temperature were observed for binary-component and singlecomponent permeations, whereas the MX and OX permeances decreased monotonically with temperature. Thus, the separation factors or ideal selectivity for PX over other isomers exhibited an increasing trend at lower temperatures and then a declining trend when the temperature was increased further, as can be seen in Figure 4a. For the separation of the PX/MX mixture, a maximum separation factor of 16.2 was achieved at 375 °C with a corresponding PX permeance of 1.9 × 10-8 mol/m2 · s · Pa. The single-component permeations of PX and MX exhibited tendencies similar to those of binary permeation. However, because of the interaction between PX and MX in the zeolite channels, the PX permeance for binary-component permeation was lower than that for single-component permeation, whereas the MX permeance in binary-component permeation was higher. A similar phenomenon was observed in our previous study on tubular MFI zeolite membranes for xylene separation.16 The maximum ideal selectivity for PX over MX occurred at 350 °C and was 66.8. The PX permeance at that temperature was about 2.3 × 10-8 mol/m2 · s · Pa. The separation results suggested that a reasonably good-quality membrane had been achieved. For the separation of the PX/OX mixture, the separation factor was slightly higher than that for the separation of the PX/MX mixture, but it was much lower than ideal PX/MX selectivity. The separation factor for PX over MX had a maximum value of 16.2 achieved at 375 °C, which exhibited a PX permeance of 1.9 × 10-8 mol/m2 · s · Pa. It should be pointed out that the zeolite membrane underwent several thermal cycles to perform
Figure 4. Binary permeations for PX/MX and PX/OX mixtures and singlecomponent permeations for PX/MX as a function of temperature.
the separation tests. It was found that the zeolite membrane kept a repeatable separation performance, indicating a high thermal stability. During separation tests over a wide range of operating temperature, no isomerization products were detected, which suggests that the silicalite-1 zeolite membrane was inert to xylene isomerization. 3.2. Xylene Isomerization. The as-synthesized zeolite membrane was used as a membrane reactor for MX isomerization. In this case, 1.2 g of HZSM-5 catalyst was packed on the substrate surface or the membrane surface. The packing modes
4296 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 Table 1. Productivities for the Products in CR, MR1, and MR2a product
CR
PX OX TL TM
6.78 6.61 3.50 2.87
productivity (×10-9 mol/s · gcat) MR1 MR2 7.86 7.06 1.67 1.80
8.15 6.97 1.34 1.22
a Operating temperature, 300 °C; feed flow rate, 20 mL/min; sweep flow rate in MR1 and MR2, 20 mL/min.
Figure 5. Reactive results of MR1, MR2, and CR as a function of temperature. (Feed flow rate, 20 mL/min; sweep flow rate, 20 mL/min.)
for the membrane reactor are denoted as MR1 and MR2, respectively. The membrane was generated by firing it in air at 450 °C for 8 h each time operation modes (MR1 and MR2) were changed. A conventional packed-bed reactor (CR) using the same amount of catalyst was investigated for comparison. MX vapor with a partial pressure of 0.32 kPa in 20 mL/min helium was fed to the reactors. For MR1 and MR2, the permeate sides of the zeolite membranes were swept by 20 mL/min helium. The results of MX isomerization in MR1, MR2, and CR at various temperatures are shown in Figure 5. It was observed that higher PX yield and selectivity were achieved in the membrane reactors, indicting an enhancement of xylene isomerization toward PX by the immediate removal of PX through the zeolite membrane. The MX conversions for MR1, MR2, and CR increased with increasing temperature in the temperature range of 270-390 °C, as shown in Figure 5.
The PX selectivity in the both membrane reactors showed a decreasing trend with temperature after a slight increase below 300 °C. However, an increase in PX selectivity was found in the CR when the temperature was increased from 300 to 360 °C. The results imply that the membrane could play an important role in xylene isomerization. Because the PX permeance through zeolite membrane changed with temperature, the degree of enhancement in xylene isomerization by membrane separation could also be related to temperature. It is shown in Figure 4b that the PX permeance decreased with increasing temperature at higher temperature, which would reduce the contribution of the membrane to PX generation. As a result, the PX yield for the membrane reactors varied in a smaller range at the tested temperatures of 270-390 °C compared with that of the CR. It is interesting to note that, although the PX yields achieved from the three reactors followed the sequence MR2 > MR1 > CR, the MX conversions showed the reverse result (CR > MR1 > MR2). The phenomenon of reduced MX conversion in the membrane reactors was different from the previous report by Haag et al.19 In their work, an improved xylene conversion was observed in a membrane reactor compared to a conventional reactor. They ascribed the results to higher catalytic activity of the HZSM-5 membrane material. Table 1 summarizes the productivities for all products in the three reactors at 300 °C. It was found that the productivities of TL and TM produced from MX disproportionation in the membrane reactors were obviously lower than those obtained in the CR. In fact, the PX and OX productivities in our experiments were both enhanced significantly for the membrane reactions. Therefore, the reduction in MX conversion was attributed to the suppressed disproportionation reaction. The reason for the decrease in disproportionation products could be the permeation of xylene isomers through the zeolite membrane, which resulted in reduced reactant concentrations and insufficient contact with the catalyst for the permeated xylene isomers. It should be noted that the catalytic activity of the catalyst used by Haag et al. was slightly different from that in our study, as they did not observe disproportionation products in their study. Figure 6 presents a comparison between MR2 and CR in terms of the PX/OX ratio (P/O) in the products and the disproportionation/isomerization (D/I) ratio. The D/I value was obtained by calculating the ratio of disproportionation products (TB + TL) to isomerization products (PX + OX) in the products. It is clearly observed that the P/O ratio obtained in MR2 was higher than it in CR, but the D/I ratio in MR2 was lower. These results suggest that the membrane reaction configuration was favorable to the isomerization of MX to PX. Figure 7 shows the increasing rates of yield and selectivity for MR2 versus CR at different temperatures. The rate for PX selectivity increased as the temperature increased from 270 to 300 °C and then decreased gradually as the temperature further increased from 300 to 390 °C. The rate for PX yield was found to decrease monotonically with temperature in the tested temperature range. The variation was mainly related to the PX permeability of the zeolite membrane and the catalytic activity of the HZSM-5 catalyst. At 270 °C, the rates for PX yield and
Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4297 Table 2. Reactive Results of MR2 at Various Sweep Gas Flow Ratesa flow rate MX PX PX PX flux membrane of sweep conversion selectivity yield (×10-6 mol/ contributionb 2 gas (mL/min) (%) (%) (%) m · s) (%) 20 40 60
47.8 47.8 47.6
44.6 45.1 45.4
21.3 21.5 21.6
2.02 2.37 2.40
7.09 8.54 8.76
a Operating temperature, 330 °C; feed flow rate, 20 mL/min. of PX permeated to PX productivity.
Figure 6. P/O and D/I ratios for MR2 and CR as a function of temperature. (P/O, PX/OX ratio in the products; D/I, disproportionation/isomerization ratio.)
Figure 7. Increasing rate of PX yield and selectivity for MR2 vs CR as a function of temperature.
selectivity were 26.0% and 31.2%, respectively, which are comparable to the results reported by Tarditi et al.21 A high PX yield of 21.3% was achieved at 300 °C in MR2, which was 19.7% higher than the yield obtained in CR. Comparing the configurations of MR1 and MR2, we found from Figure 5 that MR2 had a higher PX selectivity and yield. In fact, the difference between MR1 and MR2 was dependent on the PX permeate direction through the membrane, i.e., PX permeation from the substrate to the membrane layer for MR1 and from the membrane layer to the substrate for MR2. Liang et al.23 compared palladium composite membranes with different hydrogen permeation directions. They found that, in the separation of mixed gas (H2/N2), the hydrogen permeation rate for the permeation direction from the membrane layer to the substrate was higher than that for the direction from the substrate to the membrane layer. This was because of the diffusion resistance caused by N2 exerted on hydrogen transport through the porous substrate when permeating from the substrate to the membrane layer. Similarly, in our case, components such as MX, OX, TB, and TL had higher concentrations in the porous substrate of MR1, which resulted in a stronger effect on the diffusion of PX through the substrate pores. Therefore, the higher PX productivity of MR2 is probably attributable to more effective removal of PX using the configuration of MR2. To further reveal the effects of the membrane flux on the membrane reaction, xylene isomerization in MR2 was investi-
b
Ratio
gated at three sweep gas flow rates (20, 40, and 60 mL/min). The membrane reactor was operated at 330 °C, and the feed stream was controlled at 20 mL/min with an MX partial pressure of 0.32 kPa. The reactive results are presented in Table 2. An increase in the PX flux from 2.02 × 10-6 to 2.40 × 10-6 mol/ m2 · s was observed when the flow rate of sweep gas was increased from 20 to 60 mL/min, which was attributed to the increase of the transmembrane pressure resulting from the reduction of the PX partial pressure on the permeate side. The increasing PX flux caused an enhanced PX removal from the isomerization products. As seen in Table 2, the mole fraction of PX removed over the PX productivity increased with increasing sweep gas flow rate. Thus, an increase in both the PX yield and the PX selectivity was observed when the sweep gas flow rate was increased. Similarly, a slight drop in MX conversion was also obtained. However, the rate of PX removal was essentially low (less than 10%) for the membrane used in this study. High-flux zeolite membranes are necessary to improve the efficiency of MFI zeolite membrane reactors for xylene isomerization. Table 3 summarizes the reactive results of CR and MR2 at different values of the gas hourly space velocity (GHSV, h-1). The reactors were all operated at a temperature of 330 °C. The GHSV were adjusted by changing the flow rate of the feed stream at a constant catalyst loading of 1.2 g. It was found that, with increasing GHSV, the MX conversion decreased but the PX selectivity increased in CR and MR2. As a result, a higher PX yield was achieved at lower GHSV. Moreover, the increased PX yield for MR2 compared to CR was found to decrease with GHSV. This was due to the fact that the ratio of permeated PX to produced PX was lower at higher GHSV, which essentially resulted in a reduced contribution to the xylene isomerization for the membrane permeation. 3.3. Stability. Figure 8 shows XRD patterns of the fresh membrane and the used membrane after xylene isomerization. The XRD pattern of the particles collected from the bottom of the autoclave for membrane synthesis is also included. These results indicate that the used membrane kept the pure MFI phase of the fresh membrane. To further investigate the membrane stability, an MFI zeolite membrane was tested in the atmosphere of MX isomerization at 300 °C for 120 h using the MR2 operation mode. The helium streams for carrying MX vapor and sweeping were both kept at 20 mL/min. SEM images of a fresh membrane and the tested membrane are shown in Figure 9. A well-intergrown zeolite film with a thickness of 2-3 µm can be observed in the SEM images shown in Figure 9a,b. After the 120-h isomerization reaction, the tested membrane showed a clear and integral crystal morphology that was similar to that of the fresh membrane. These characterization results suggest that the MFI zeolite membranes have good structural stability for use in xylene isomerization. Similarly, SEM and XRD analyses were conducted on the substrate side for the used membrane operated with MR1. No obvious chemical interaction or chemical growth was observed between the catalyst and the R-alumina substrate.
4298 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 Table 3. Reactive Results for CR and MR2 at Various GHSVsa CR
MR2
GHSV (h-1)
conversion (%)
selectivity (%)
yield (%)
conversion (%)
selectivity (%)
yield (%)
PX increment
permeate PX/product PX
1574 3148 4722
51.9 43.6 36.5
35.7 43.0 47.3
18.5 18.8 17.3
47.8 42.1 36.1
44.6 47.3 49.6
21.3 19.9 17.9
15.1 5.85 3.47
7.09 7.78 8.71
a
Operating temperature, 330 °C; catalyst amount, 1.2 g.
Figure 8. XRD patterns of the MFI zeolite powders and the MFI disk membrane.
Figure 9. SEM images of MFI disk membranes: (a) surface of a fresh membrane, (b) cross section of a fresh membrane, (c) surface of a used membrane.
NH3 TPD characterization was carried out on the HZSM-5 catalyst before and after reaction. The catalysts used for MR2 and CR had similar catalytic conditions. The samples were first
Figure 10. TGA profiles of fresh and used catalysts.
treated in a helium stream at 500 °C for 1 h before TPD analysis. NH3 TPD of the samples was carried out by heating the samples at a heating rate of 10 °C/min from 100 to 700 °C. The stream was monitored continuously with a thermal conductivity detector (TCD) to determine the rate of ammonia desorption. There were two desorption peaks in the tested temperature range. The lowtemperature peak between 190 and 270 °C was attributed to the weak acid sites, and the high-temperature peak between 410 and 500 °C was attributed to the strong acid sites. It was found that the TPD curve of the used MR2 catalyst was similar to that of the fresh catalyst, indicating that the acid sites of the MR2 catalyst well maintained after the reaction. However, an obvious loss of acid sites was found for the catalyst used for CR. This loss of acid sites could be due to coke deposition during xylene isomerization. To further clarify this phenomenon, TGA analysis was carried out on the catalyst samples. The samples were heated from 40 to 1000 °C at a heating rate of 10 °C/min in an O2 stream. The flow rate of O2 was 25 mL/min. As shown in Figure 10, the TGA curve of MR2 catalyst had a similar trend as the fresh catalyst. The weight loss before 400 °C was attributed to desorption of water in the sample. A significant weight loss was found between 450 and 600 °C for the CR catalyst, which was attributed to the burning of deposited coke. The TGA results are consistent with the NH3 TPD results. It is speculated that the membrane reaction mode could be effective to reduce coke deposition in the loaded catalyst. The reaction mechanism, however, is not clear. One possible explanation is that the immediate removal of PX for the membrane reactor could be beneficial in suppressing side reactions such as disproportionation, which were observed in our experiments. Further studies are necessary to reveal the reaction mechanism in the membrane reactor. 4. Conclusion MFI zeolite membrane reactors with packed HZSM-5 catalyst have been demonstrated for xylene isomerization. PX selectivity and yield can be significantly enhanced in the membrane reactors compared to a conventional reactor. The maximum increase in PX yield was achieved at 270 °C, which was about 26.0%.
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Quick removal of PX could be beneficial to MX isomerization toward high PX yield. The enhancement for the membrane reactor was found to be dependent on the membrane flux. Highflux zeolite membranes are required to improve the efficiency of membrane reactors for xylene isomerization. It is also suggested that a bilayered membrane with high-performance catalytic and separating layers could be more suitable for xylene isomerization because of the reduced diffusion resistance of PX through the catalyst bed. Acknowledgment This work was sponsored by the National Basic Research Program of China (No. 2009CB623403), Joint Founds of NSFCGuangdong (No. U0834004), National Natural Science Foundation of China (No. 20706030), and the Science & Technology Support Program (Industry) of Jiangsu Province of China (No. BE2008141). Literature Cited (1) Guisnet, M.; Gnep, N. S.; Morin, S. Mechanisms of Xylene Isomerization over Acidic Solid Catalysts. Microporous Mesoporous Mater. 2000, 35-36, 47–59. (2) Zheng, S.; Jentys, A.; Lercher, J. A. Xylene Isomerization with Surface-modified HZSM-5 Zeolite Catalysts: An in Situ IR Study. J. Catal. 2006, 241, 304–311. (3) Iliyas, A.; Al-Khattaf, S. Gas-phase Isomerization of meta-Xylene over USY Zeolite in a Riser Simulator: A Simplified Kinetic Model. Chem. Eng. J. 2005, 107, 127–132. (4) Araujo, A. S.; Domingos, T. B.; Souza, M. J. B.; Silva, A. O. S. m-Xylene Isomerization in SAPO-11/HZSM-5 Mixed Catalyst. React. Kinet.Catal. Lett. 2001, 73, 283–290. (5) Laforge, S.; Martin, D.; Guisnet, M. m-Xylene Transformation over H-MCM-22 Zeolite. 2. Method for Determining the Catalytic Role of the Three Different Pore Systems. Microporous Mesoporous Mater. 2004, 67, 235–244. (6) Hedlund, J.; Sterte, J.; Anthonis, M.; Bons, A.; Carstensen, B.; et al. High-flux MFI Membranes. Microporous Mesoporous Mater. 2002, 52, 179–189. (7) Tarditi, A. M.; Irusta, S.; Lombardo, E. A. Xylene Isomerization in a Membrane Reactor Part I: The Synthesis of MFI Membranes for the p-Xylene Separation. Chem. Eng. J. 2006, 122, 167–174. (8) Sakai, H.; Tomita, T.; Takahashi, T. p-Xylene Separation with MFItype Zeolite Membrane. Sep. Purif. Technol. 2001, 25, 297–306. (9) Xomeritakis, G.; Lai, Z.; Tsapatsis, M. Separation of Xylene Isomer Vapors with Oriented MFI Membranes Made by Seeded Growth. Ind. Eng. Chem. Res. 2001, 40, 544–552.
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ReceiVed for reView October 23, 2008 ReVised manuscript receiVed February 18, 2009 Accepted February 25, 2009 IE801606S
