Ind. Eng. Chem. Res. 2010, 49, 12423–12428
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Synthesis of Ni-SiO2/Silicalite-1 Core-Shell Micromembrane Reactors and Their Reaction/Diffusion Performance Easir A. Khan,† Arvind Rajendran,*,† and Zhiping Lai*,‡ School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, N1.2, 62 Nanyang DriVe, Singapore 637459, and DiVision of Chemical and Life Sciences and Engineering, Center of AdVanced Membranes and Porous Materials, King Abdullah UniVersity of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia
Core-shell micromembrane reactors are a novel class of materials where a catalyst and a shape-selective membrane are synergistically housed in a single particle. In this work, we report the synthesis of micrometer -sized core-shell particles containing a catalyst core and a thin permselective zeolite shell and their application as a micromembrane reactor for the selective hydrogenation of the 1-hexene and 3,3-dimethyl-1-butene isomers. The bare catalyst, which is made from porous silica loaded with catalytically active nickel, showed no reactant selectivity between hexene isomers, but the core-shell particles showed high selectivities up to 300 for a 1-hexene conversion of 90%. 1. Introduction A membrane reactor combines reaction with separation, leading to a more compact process with enhanced reaction performances in terms of conversion or yield.1-4 Implementation of this concept often requires a highly efficient separation layer. Further, the harsh reaction conditions often impose stringent requirements on membrane materials in terms of thermal and chemical stabilities, which often exclude the application of conventional materials such as polymers. Zeolite membranes, a novel inorganic membrane technology developed in the last 20 years, exploit the regular molecular-sized pores as well as the inorganic nature of zeolites. These membranes have shown not only high thermal, hydrothermal, and chemical stabilities but also excellent separation performances for a wide range of systems including mixtures of simple gases, linear/branched hydrocarbons, hydrocarbon isomers, etc.5,6 Not surprisingly, application of these materials in membrane reactors has attracted a significant amount of interest in recent years.7,8 A typical design for zeolite membrane reactors is a packed bed based on the configuration where catalytic particles are filled inside tubular membrane units whose dimension is of the order of millimeters or larger.9-13 However, such a design suffers from several drawbacks. First, synthesis of defect-free zeolite membranes for large-scale units is still a technical challenge. Very often, it results in poor reproducibility, low production rate, and high cost.14,15 Second, the diffusion rate through the membrane layer is often not sufficiently high because of low packing densities of the membrane units that are often in the range of ≈100 m2/m3. Miniaturizing the membrane structure may provide a solution to increase the packing density as well as the membrane reproducibility. One approach to the fabrication of zeolite micromembrane reactors is the use of techniques that have been widely applied in “lab-on-a-chip” devices.16-19 However, this route is complex and difficult to scale up. The more attractive alternative is the so-called “bottom-up” approach based on chemical synthesis to develop nanostructures. One idea that has * Authors to whom correspondence should be addressed. E-mail:
[email protected] or
[email protected]. † Nanyang Technological University. ‡ King Abdullah University of Science and Technology.
been explored in several reports20-24 is to synthesize zeolite core-shell membrane reactors where a catalytic core is encapsulated in a zeolite shell. For example, Nishiyama et al. synthesized Pt-TiO2/silicalite-1 core-shell catalysts that showed good reactant selectivity (∼30 with a 70% of 1-hexene conversion) for the hydrogenation of 1-hexene and 3,3-dimethyl1-butene (3,3-DMB).22 Zhong et al. reported the synthesis of zeolite 4A/Pt-γAl2O3 core-shell structures and studied the selective oxidation of an equimolar mixture of CO and n-butane. In the presence of the micromembrane reactor, CO reacted completely, whereas n-butane showed no conversion because of size exclusion.24 However, in most of these reports the core-shell particles are usually in millimeter or sub-millimeter scale, and the shell thickness is larger than 20 µm.20-24 In this report, we present a synthesis method that allowed us to prepare much smaller core-shell particles (∼50 µm) with a much thinner zeolite shell (∼1.5 µm). When applied as micromembrane reactor for the hydrogenation of hexene isomers, the reaction performance was significantly improved in terms of both selectivity and conversion compared to the previous reports. The zeolite shell synthesized in this work is silicallite-1, which has MFI-type zeolite framework containing two types of interconnected channels: straight channels along the b-axis with pore size of 0.55 nm and sinusoidal channels along the a-axis with pore size of 0.54 nm.25 These pore sizes can be used to separate many industrially important mixtures. For example, the above-mentioned pore size should allow transport of 1-hexene (kinetic diameter 0.43 nm), but prevent the transport of its isomer, 3,3-dimethyl-1-butene (3,3-DMB, kinetic diameter 0.63 nm). In this report we consider this system as a prototype to demonstrate an important application of micromembrane reactors; i.e, when the core-shell particles are exposed to a mixture of reactants, the shell allows only the reactant of interest to pass through and react on the core. Hence, the core-shell particles will have additional reactant selectivity that is not possessed by the bare core particles. 2. Experimental Methods 2.1. Catalyst Synthesis. The catalyst used in this study is nickel-loaded silica gel. The silica gel (SiO2, grade 633, 60 Å) was purchased from Sigma-Aldrich, Singapore. It was loaded
10.1021/ie101850j 2010 American Chemical Society Published on Web 11/19/2010
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Figure 1. Microflow reactor for continuous-flow catalytic reaction systems (Newton & Stocks, Singapore).
with catalytically active nickel by dissolving 2.34 g of NiNO3 · 6H2O (∼98%, Alfa-Aesar) into 40 mL of deionized water and then mixed with 4.48 g of silica gel. Subsequently, 10 mL of NH4OH (25% in H2O, Sigma-Aldrich) was added slowly into the mixture while stirring. The mixture was stirred for 2 h followed by filtration and washing. The solid filtrate was dried overnight at 333 K, after which they were calcined at 573 K for 5 h under air and activated at 573 K for another 5 h under H2 flow. 2.2. Synthesis of Silicalite-1 Seeds. The synthesis solution was prepared by mixing tetraethyl orthosilicate (TEOS, ∼98%, Sigma-Aldrich), tetrapropyl ammonium hydroxide (TPAOH, ∼40% in H2O, Merck Chem. Ltd.) and deionized water with a final molar composition of 5:1:16:250 SiO2:TPAOH:EtOH:H2O. The mixture was stirred at room temperature for about 5 h until a clear solution was formed. The resulting clear mixture was transferred to a Teflon-lined stainless steel autoclave. The autoclave was loaded into a heating oven equipped with a rotary frame with a speed of 30 rpm and the temperature was maintained at 423 K for duration of 5 h. After being cooled, the suspension was washed with deionized water and separated by centrifugation (5000 rpm, 5 min). The seed particles were then dried overnight at 333 K and calcined at 813 K for 6 h with a temperature ramp of 1 K/min under air flow. The calcined seeds were dispersed into deionized water to make a coating suspension with a zeolite content of about 1 wt %. The pH of the suspension was adjusted to ∼9.5 by adding a few drops of 0.1 M NH4OH to stabilize the suspension. 2.3. Synthesis of Ni-SiO2/Silicallite-1 Core-Shell Particles. A seeded growth method developed in our previous report26 was used to synthesize Ni-SiO2/silicallite-1 core-shell particles. The procedure mainly includes two steps: The first step is to coat a layer of silicallite-1 seeds on the surface of core particles, and the second step is to grow the seed layer into a compact zeolite shell. Two types of polyelectrolyte coating solutions were prepared: one made from positively charged polydiallyldimethylammonium chloride (PDDA; Mw ∼ 100000-200000, 20% in water, Aldrich); the other from negatively charged sodium polystyrenesulfonate (PSS; Mw ∼ 70000, Aldrich). A certain amount of
the above two polyelectrolytes were dissolved into 0.1 M NaCl solution to form clear coating solutions with polyelectrolyte content around 0.1 wt %. The surface charge of the core Ni-SiO2 particles was first rendered positive using a layer-bylayer self-assembly method. The basic procedure of this method is to coat the above two types of polyelectrolytes alternatively on the surface of the core particles. In this study, we used the following sequence: Ni-SiO2/PDDA/PSS/PDDA. In a typical coating process, 0.5 g of Ni-SiO2 particles were immersed into 40 mL of a polyelectrolyte solution and allowed to rest for 20 min. Particles were collected by centrifuging at 6000 rpm and washed with deionized water at least three times, and then they were dipped into another solution and the same procedure was repeated. Next, the core particles were immersed into a mixture of 10 mL of seed suspension that was prepared in the previous step and 1 mL of 0.5 M NaCl; the resulting solution was allowed to rest for 20 min to complete the deposition of seed particles on the silica surface. Afterward, the seed-coated Ni-SiO2 particles were filtered, washed with deionized water, and dried at 363 K overnight. The synthesis solution intended for secondary growth was prepared by adding 4 g of TPAOH aqueous solution (∼40% in H2O) and 8 g of TEOS (∼98%) in 144 g of deionized water with the final molar composition of 5:1:1000:20 SiO2:TPAOH: H2O:EtOH.27 The mixture was stirred at room temperature for about 4 h until a clear solution was obtained. Approximately 40 mL of synthesis solution was mixed with 0.2 g of seedcoated Ni-SiO2 particles and the resulting mixture was then transferred to a 45 mL Teflon-lined autoclave. The autoclave was loaded into a heating oven under rotation at 30 rpm at a preheated temperature of 448 K for a duration of 24 h. After being cooled, the resulting particles were separated by filtration and washed with deionized water. The core-shell particles were finally calcined at 753 K for 10 h with a ramping rate of 0.5 K under air flow. 2.4. Catalytic Test. The gas-phase hydrogenation of 1-hexene (∼97%, Sigma-Aldrich) and 3,3-dimethyl-1-butene (3,3DMB, ∼95%, Sigma-Aldrich) was performed in a continuousflow fixed-bed reactor operating at atmospheric pressure. The setup for the reaction system is schematically shown in Figure
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q*i ) HiCi
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(3)
Integrating eq 1, after combining with eqs 2 and 3 and inserting the definition for conversion at the reactor outlet, Xi as Xi ) 1 - (Ci(z))/(Ci0) yields ln(1 - Xi) ) Figure 2. Scheme of a packed-bed column loaded with core-shell micromembrane reactors.
1. A tubular stainless steel reactor (42 cm length and 10 mm i.d.) was used to conduct the experiment. Liquid 1-hexene and 3,3-DMB were pumped through a HPLC micropump (PerkinElmer S200) into a mixer where the liquid 1-hexene and 3,3DMB were gasified and mixed with hydrogen. The reaction was carried at 383 K. All the other parts including the mixer and connecting tubing are also maintained at 383 K using a temperature controller with accuracy of (0.1 K. Prior to the measurements, the catalyst sample was activated at 573 K with a temperature ramp of 1 K/min under hydrogen flow of 30 mL/min for 2 h. The reactants and products were analyzed by means of online gas chromatography (Agilent 7890 GC) equipped with a flame ionization detector (FID) and an Agilent HP-PLOT-Al2O3 column. 3. Mathematical Modeling of a Packed-Bed Reactor Loaded with Core-Shell Micromembrane Reactors The reaction process can be considered as a fixed-bed column loaded with core-shell particles as shown in Figure 2. The average radius of the core-shell particles is rp with a shell thickness of δ. The bed has cross-sectional area A, length L, and void fraction ε. The feed stream enters the bed with a superficial velocity u and with the concentration of component i being Ci. To derive the mass balance, the following assumptions are made. 1 The system is at steady state and at a constant pressure and temperature. 2 The core-shell particles are spherical and monodispersed with a uniform shell thickness. 3 Axial dispersion is negligible. 4 The reaction on the core is instantaneous; i.e., the concentration of the 1-hexene and 3,3-DMB on the core equals zero and process is mainly controlled by the diffusion through the shell. Under these assumptions the mass balance at steady state is given by -u
dCi 1-ε ) aJi dz ε
(1)
where Ji is the flux of the component i through the shell and a ) 3/rp is the surface area per unit volume of the core-shell particles. At steady state, the flux Ji can be written as Ji ) DCi
q*i - 0 δ
(2)
where DCi is the effective diffusion coefficient of component i, through the shell of the core-shell particles, and qi* is the solidphase concentration of component i, which will be at equilibrium with the fluid-phase concentration Ci. The relationship between Ci and qi* is given by the adsorption isotherm, which in the present case is assumed to be linear, i.e,
3(1 - ε)HiDCi L rpδ εu
(4)
Finally, by assuming the gas phase behaves ideally, an expression for the mass of catalyst per unit flow rate, W/F, can be written as W L FRT ) F εu P
( )( )
(5)
where F is the packing density of the core-shell particles. Finally, eqs 4 and 5 can be combined to obtain an expression that relates the W/F to the conversion at the reactor outlet. ln(1 - Xi) ) -
3(1 - ε)HiDCi P W rp δ FRT F
(6)
In the packed-bed micromembrane reactor, the concentration of the reactants varies along the length of the column, thereby resulting in a variation of the permeance. Hence, an average permeance can be defined as
ji ) P
∫
Ji dz pi
L
0
∫
Z
0
(7)
dz
where pi represents the partial pressure of the reactant. By combining the above equation with eqs 2 and 3, we obtain ji ) P
DCiHi δRT
(8)
4. Results and Discussion The core particle used in this study is porous silica powder, which has a specific surface area of 456 m2/g. The particles have irregular geometries with an average particle size of 50 µm. The porous silica is immersed into a solution of NiNO3 to incorporate catalytically active nickel within the pores of silica particles. The Ni content obtained from EDX elemental analysis is about (5.5 ( 0.5)% (w/w). Figure 3a shows the core particles coated with silicalite-1 seed layers and the inset of Figure 3a shows a zoomed-in image of the surface of one particle where the round disk shaped particles are the silicallite-1 seeds with a planar dimension of about 500 nm and with thickness of 200 nm. From Figure 3a, one can see that although the shape of the porous silica is very irregular, a uniform seed layer with good coverage can be obtained. This indicates that the layer-by-layer self-assembly technique used in our synthesis is very flexible and facile. Figure 3b shows the particles after secondary growth where a dense zeolite shell is formed, fully covering the entire surface of the silica core. Figure 3c shows the cross section of the core-shell particle. The shell thickness is measured from the cross section, which is about 1.5 µm. The roughness of the top layer is about 0.5 µm, whereas the bottom compact layer has a thickness of 1 µm.
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Figure 4. Hydrogenation activity of 1-hexene and 3,3-DMB on core Ni-SiO2 catalyst as a function of space time, W/F (g-cat · h/mol), 100 mg of catalyst, feed molar ratio, 1-hexene:3,3-DMB:hydrogen:helium ) 1:1: 10:2.
Figure 3. Scanning electron microscopy images of (a) seed-coated Ni-SiO2 (inset shows the silicalite-1seed monolayer), (b) Ni-SiO2/silicalite-1 (inset shows the surface of the shell), and (c) cross-sectional view of a broken core-shell particle.
4.1. Hydrogenation Activities of Bare Ni-SiO2 Catalyst. Three types of control experiments were first conducted on the tubular reactor. The first control experiment used silicalite-1 particles that were synthesized from the same precursor solution used for preparing the zeolite shell; the second one used SiO2/ silicalite-1 core-shell particles without loading nickel catalyst; and the third one used bare nickel-loaded SiO2 particles (Ni-SiO2), as catalyst, that is, without the silicalite-1 shell. One hundred milligrams of these particles was packed into the reactor. A gas mixture with molar ratio 1-hexene:3,3-DMB:H2: Ar ) 1:1:10:2 was fed to the reactor with a flow rate from 0.06 to 0.5 mol/h. The corresponding catalyst-to-feed ratio (W/F) is from 1.66 to 0.2 g-cat · h/mol. The reaction temperature was maintained at 383 K. In the first two types of control experiments, no reaction was observed over the entire studied range of W/F ratio, indicating that both the silicalite-1 shell and the SiO2 particles are inert to the reaction system. In the third one, both 1-hexene and 3,3-DMB were reacted completely in the entire investigated W/F range as shown in Figure 4. However, if the amount of the bare catalyst reduced to 50 mg, then at W/F ) 0.25 g-cat · h/mol, the conversions of 1-hexene and 3,3DMB both reduced to 91%. No reactant selectivity was observed on the Ni-SiO2 bare catalyst. 4.2. Hydrogenation Activities of Core-Shell Micromembrane Reactors. For reactions with the core-shell particles, typical reaction conditions were the following: 200 mg of
Figure 5. Effect of hydrogenation activity of 1-hexene and 3,3-DMB on Ni-SiO2/silicalite-1 core-shell catalyst as a function of space time, W/F (g-cat · h/mol). Symbols represent experimental data. Lines represent the calculated values obtained by fitting eq 6 to the experimental data.
core-shell particles (equivalent to 118 mg of core catalyst because the catalyst contributed to 59% of the total weight of the core-shell particles) were packed into the reactor; the packing density was 0.44 g/cm3; the void fraction of catalyst bed was 0.48; the molar composition of feed gas mixture was again 1-hexene: 3,3-DMB:hydrogen:helium ) 1:1:10:2; the total feed pressure was fixed at 1 atm and the reaction temperature was set to 383 K. The selectivity S is defined as S)
CP1 /CP2 CF1 /CF2
(9)
where CiF is the concentration of component i at the feed gas and CiP is the concentration after reaction. The catalytic activity and selectivity are shown in Figure 5. It is worth noting that the selectivity calculated from eq 9 is an apparent selectivity, not the intrinsic selectivity of the zeolite shell as the composition of the gas phase changes along the length of the catalyst bed. For each data point in Figure 5, the catalyst was activated at 573 K for 2 h under H2 flow before the reaction. As the results will show later (c.f. Figure 6), this activation temperature is high enough to regenerate the catalyst to its fresh state and eliminate effects from the previous experiments. Data was taken 1 h after the reaction started to ensure a pseudo steady state diffusion is established within the zeolite shell. As shown in
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Figure 6. Effect of on-stream time on hydrogenation activity of 1-hexene and 3,3-DMB on Ni-SiO2/silicalite-1 core-shell catalyst as a function of time at W/F ) 0.71 g-cat · h/mol.
Figure 5, a reactant selectivity up to 70 with a 1-hexene conversion rate of 88% was achieved at W/F ≈ 0.36 (g-cat · h/ mol). The conversion of 1-hexene increased with the increase of W/F and reached almost 100% at W/F ≈ 0.7 g-cat · h/mol and consequently the apparent reaction selectivity decreased. This is because when conversion of 1-hexene increases, the partial pressure of 1-hexene in the gas phase gradually decreases along the catalyst bed, and hence decreases the diffusion driving force for 1-hexene, whereas the concentration of 3,3-DMB in the gas phase will not change much as its conversion is very low. Hence, the apparent reaction selectivity decreases when the conversion of 1-hexene increases. Compared with the results obtained in the literature,22 where a maximum reactant selectivity of 30 with 70% conversion of 1-hexene was reported, the reaction selectivity is doubled in the current work with conversion of 1-hexene improved to 90%. A simple packed-bed reactor model developed in eq 6 can be used to describe the current reaction system. The model assumes that the diffusion through the zeolite shell is the ratelimiting step of the entire process. This assumption can be justified based on the fact that when the same amount of bare catalyst was used in the control experiments, both 1-hexene and 3,3-DMB were fully converted. Hence, the reaction rate is faster than the diffusion rate. Because of the lack of kinetic data for 1-hexene and 3,3DMB, we assume all the transport properties of 1-hexene are the same as n-hexane, whereas those for 3,3-DMB are identical to those of 2,2-dimenthylbutane (2,2-DMB). The adsorption isotherm data was obtained from the literature.28 By fitting eq 6 to the experimental data in Figure 5, which is shown as solid lines in Figure 5, we can obtain the diffusivity of 1-hexene and 3,3-DMB through the zeolite shell to be 3.4 × 10-14 and 5 × 10-16 m2/s, respectively. On the basis of these diffusivities, the permeance of 1-hexene through the silicalite-1 shell is 1.3 × 10-7 mol/(m2 · s · Pa) and the permselectivity of 1-hexene to 3,3DMB is 142 at a reaction temperature of 383 K. These values are comparable to the experimental permeance data (permeance of n-hexane ≈ 1 × 10-7 mol/(m2 · s · Pa) and selectivity ≈ 350 at 398 K) that were reported on compact silicalite-1 tubular membranes for separation of n-hexane and 2,2-DMB mixtures.29,30 4.3. Effect of On-Stream Time on the Performance of Core-Shell Micromembrane Reactors. The lowest selectivity in Figure 5 is about 28, obtained at the highest studied W/F ratio, i.e., 0.71 g-cat · h/mol. It was found that the selectivity could significantly increase with process time. Figure 6 shows the effect of the on-stream time on the performance of the micromembrane reactors. In this experiment the W/F ratio was maintained at 0.71 g-cat · h/mol and the reaction was started from
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a fresh batch of core-shell particles. As shown in Figure 6, initially nearly complete conversion of 1-hexene was obtained with a selectivity of 28. With longer on-stream time, the conversion of both isomers decreased gradually while the selectivity increased dramatically. After 24 h of reaction, the conversion of 1-hexene dropped by 10%, whereas the selectivity increased almost eightfold. The reaction was stopped at this point and the catalyst was regenerated at 473 K for 2 h with a steady flow of H2 by simply increasing the reactor temperature and switching the feed gas. After regeneration, the conversion of 1-hexene was recovered to about 97% while the initial selectivity stayed above 100; during the second cycle the conversion still gradually dropped with time with a corresponding increase in selectivity. However, as can be seen in Figure 6, successive regenerations at 473 K resulted in fairly reproducible behaviors to the second cycle in terms of conversion and selectivity. A maximum selectivity of 300 can be observed in Figure 6 with a 1-hexene conversion above 90%. If the regeneration temperature increases to 573 K, again under the flow of hydrogen, then the catalyst performance will be restored back to its fresh state. As mentioned above, this was utilized in Figure 5 to ensure that each data point was obtained from the fresh state of the catalyst. The behavior observed in Figure 6 might be explained by the special interactions of linear hydrocarbons with the silicalite-1 zeolite framework that could cause expansion of the silicalite-1 framework upon adsorption. This effect has been observed on silicalite-1 zeolite membranes,30,31 and further elaborated by Yu et al. using the concept of “molecular valve”.32 Basically, the zeolite shell contains not only zeolitic pores but also defects such as grain boundaries.33 1-Hexene or n-hexane can pass through both zeolitic pores and defects, whereas 3,3DMB can pass only through the defects.34 Both n-hexane and 1-hexene adsorb strongly on the silicalite-1 shell, leading to two consequences. First, it can lead to pore blockage owing to capillary condensation, thereby reducing the flux of other molecules.35 Second, it leads to the expansion of the zeolitic frameworks, thereby shrinking the nonzeolitic pathways and effectively blocking the transport of 3,3-DMB through the defect. Both effects will contribute to the increase in selectivity with increasing on-stream time. At the beginning of the experiments where there were no feed molecules present, both zeolitic and nonzeolitic pathways were available for transport and hence the flux is high enough for both 1-hexene and 3,3DMB to convert almost completely. As the experiment proceeds, defects are closed due to adsorption of 1-hexene (or n-hexane as well), which reduces the permeability of both 1-hexene and 3,3-DMB, with 3,3-DMB dropping much more. Regeneration at 473 K can remove only part of the adsorbed 1-hexane or 3,3-DMB that might be trapped inside the defects, and therefore, high selectivity can be maintained. When the regeneration temperature is higher than 573 K, then all the adsorbed molecules can be removed and the catalyst can be brought back to its fresh state. 5. Conclusions We have presented a synthesis method that allowed the successful synthesis of ultrathin well intergrown zeolite shell on commercially available core particles with size in the tens of micrometers. The core-shell micromembrane reactors showed excellent shape selectivity for the hydrogenation of C6 isomers while maintaining high conversion of 1-hexene. The conversion decreases to some extent with on-stream time, but the selectivity significantly improved. On the basis of this behavior, a proper
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regeneration protocol can be found to reproducibly restore the high conversion while maintaining high selectivity. The separation performance through the zeolite shell is comparable to that of tubular zeolite membranes, but since the dimension of the micromembrane reactor is an order of magnitude smaller than conventional membrane reactors, the specific surface area for diffusion also improved to the same extent. Acknowledgment Financial supports provided by the NTU start-up funding M58120001 and AcRF tier-1 funding RG26/07, and KAUST distribution fund allocated to Z.P. Lai are gratefully acknowledged. Literature Cited (1) McLeary, E. E.; Jansen, J. C.; Kapteijn, F. Zeolite based films, membranes and membrane reactors: Progress and prospects. Microporous Mesoporous Mater. 2006, 90, 198. (2) Westermann, T.; Melin, T. Flow-through catalytic membrane reactors--Principles and applications. Chem. Eng. Process 2009, 48, 17. (3) Tarditi, A. M.; Horowitz, G. I.; Lombardo, E. A. Xylene isomerization in a ZSM-5/SS membrane reactor. Catal. Lett. 2008, 123, 7. (4) Sirkar, K. K.; Shanbhag, P. V.; Kovvali, A. S. Membrane in a reactor: A functional perspective. Ind. Eng. Chem. Res. 1999, 38, 3715. (5) Tsapatsis, M.; Xomeritakis, G.; Hillhouse, H. W.; Nair, S.; Nikolakis, V.; Bonilla, G.; Lai, Z. P. Zeolite Membranes. CATTECH 2000, 3, 148. (6) Tavolaro, A.; Drioli, E. Zeolite Membranes. AdV. Mater. 1999, 11, 975. (7) Caro, J.; Noack, M. Zeolite membranes - Recent developments and progress. Microporous Mesoporous Mater. 2008, 115, 215. (8) Coronas, J.; Santamaria, J. State-of-the-art in zeolite membrane reactors. Top. Catal. 2004, 29, 29. (9) Zhang, C.; Hong, Z.; Gu, X. H.; Zhong, Z. X.; Jin, W. Q.; Xu, N. P. Silicalite-1 Zeolite Membrane Reactor Packed with HZSM-5 Catalyst for meta-Xylene Isomerization. Ind. Eng. Chem. Res. 2009, 48, 4293. (10) Kong, C. L.; Lu, J. M.; Yang, H. H.; Wang, J. Q. Catalytic dehydrogenation of ethylbenzene to styrene in a zeolite silicalite-1 membrane reactor. J. Membr. Sci. 2007, 306, 29. (11) Aboudheir, A.; Akande, A.; Idem, R.; Dalai, A. Experimental studies and comprehensive reactor modeling of hydrogen production by the catalytic reforming of crude ethanol in a packed bed tubular reactor over a Ni/Al2O3 catalyst. Int. J. Hydrogen Energy 2006, 31, 752. (12) Dyk, L. V.; Lorenzen, L.; Miachon, S.; Dalmon, J.-A. Xylene isomerization in an extractor type catalytic membrane reactor. Catal. Today 2005, 104, 274. (13) Maloncy, M. L.; Gora, L.; McLeary, E. E.; Jansen, J. C.; Maschmeyer, T. Hydroisomerization of hexane within a reactor composed of a tubular silicalite-1 membrane packed with Pt-loaded chlorided alumina catalyst. Catal. Commun. 2004, 5, 297. (14) Snyder, M. A.; Tsapatsis, M. Hierarchical Nanomanufacturing: From Shaped Zeolite Nanoparticles to High-Performance Separation Membranes. Angew. Chem., Int. Ed. 2007, 46, 7560. (15) Saracco, G.; Vesteeg, G. F.; van Swaaij, W. P. M. Current hurdles to the success of high-temperature membrane reactors. J. Membr. Sci. 1994, 95, 105. (16) Yeung, K. L.; Kwan, S. M.; Lau, W. N. Zeolites in Microsystems for Chemical Synthesis and Energy Generation. Top. Catal. 2009, 52, 101. (17) Mateo, E.; Lahoz, R.; de la Fuente, G. F.; Paniagua, A.; Coronas, J.; Santamarı´a, J. Preparation and application of silicalite-1 micromembranes on laser-perforated stainless steel sheets. J. Membr. Sci. 2008, 316, 28.
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ReceiVed for reView September 3, 2010 ReVised manuscript receiVed October 22, 2010 Accepted October 25, 2010 IE101850J
