Preparation of a Catalyst Composite Silica Membrane Reactor for

Apr 21, 2006 - A silica membrane prepared by a counterdiffusion CVD method using tetramethyl orthosilicate and O2 was applied to a steam reforming ...
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Preparation of a Catalyst Composite Silica Membrane Reactor for Steam Reforming Reaction by Using a Counterdiffusion CVD Method Mikihiro Nomura,*,† Masahiro Seshimo,‡ Hitoshi Aida,† Katsuya Nakatani,† Suraj Gopalakrishnan,† Takashi Sugawara,† Toru Ishikawa,‡ Mitsutaka Kawamura,‡ and Shin-ichi Nakao† Department of Chemical System Engineering, Faculty of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Department of EnVironmental Chemical Engineering, Kogakuin UniVersity, 1-24-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo 163-8677, Japan

A silica membrane prepared by a counterdiffusion CVD method using tetramethyl orthosilicate and O2 was applied to a steam reforming reaction of methane. This silica membrane showed hydrothermal stability for more than 80 h at 773 K under H2O/N2 ) 3. The H2/H2O permeance ratio was about 290 after the hydrothermal stability test. Rh or Ni catalyst was dipped on a porous alumina substrate before chemical vapor deposition (CVD). As a result, a composite catalytic membrane of a hydrogen permselective silica layer and a catalyst layer was obtained. This catalyst composite membrane reactor was applied to steam reforming reaction to extract hydrogen. Rh catalyst showed better stability than that for Ni catalyst. Methane conversion was increased to 64.5% from the equilibrium value (31.4%) at 773 K under S/C ) 2 by the Rh-dipped membrane reactor. High conversion of methane was due to high selectivity of H2/H2O that was confirmed by the simulation evaluation. Introduction It is important to develop an efficient hydrogen production method for a future energy system. Application of a hydrogen permselective membrane reactor to hydrogen production reaction (e.g. steam reforming of methane) is one of the solutions for efficient operations. There are many reports concerning a hydrogen permselective membrane reactor using palladium or palladium alloy membranes.1 Palladium or palladium alloy membranes are superior to ceramic membranes for their high hydrogen flux and hydrogen selectivity. However, there should be the cost limitation for palladium membranes due to the total amounts of palladium on the earth. A membrane module cost is a major factor in a membrane reactor system. In this report, we had paid attention to ceramic membranes such as silica membranes for hydrogen separation. Silica membranes have been prepared by sol-gel or chemical vapor deposition (CVD) methods. There are several reports that silica membranes prepared by the sol-gel method were used as a hydrogen permselective membrane reactor,2-5 and these silica membranes were composite membrane reactors that catalysis was deposited on porous alumina substrates of the silica membranes. Tsuru et al.5 reported that methane conversion was over 80% at 773 K under S/C (steam/carbon) ) 3 by using a composite membrane reactor prepared by the sol-gel method. However, important properties for application to a steam reforming reaction are hydrothermal stability at high temperature and steam selectivity over hydrogen. High steam selectivity ()H2 permeance/H2O permeance) is required for a silica membrane to obtain high conversion for steam reforming reaction. Silica membranes prepared by sol-gel method5 showed H2 selectivity over H2O of ca. 10. Hydrocarbon conversion can be improved by using a membrane having high H2/H2O permeance ratio, because H2O is one of the reactants of the reaction. CVD methods are classified into one side geometry6-10 and counterdiffusion * To whom correspondence should be addressed. Tel. and Fax: +813-5859-8160. E-mail: [email protected]. † The University of Tokyo. ‡ Kogakuin University.

geometry.11-14 For a counterdiffusion CVD method, two kinds of reactants are supplied at both sides of the substrate. These reactants diffuse into a porous substrate, and deposition occurs in the pores of the substrate. The deposition finishes by the limitation of supply of the reactants by the deposition. Xomeritakis et al.15 reported the distribution of pore size by the counterdiffusion CVD method. This method is also useful for the posttreatment of the other type of ceramic membranes.16 Recently, stable silica membrane was prepared by the counterdiffusion CVD method using tetramethyl orthosilicate and O2.17 This silica membrane kept its permeation properties under 75% of steam at 773 K for 21 h. This is a typical steam condition of steam reforming reaction for hydrogen permselective membrane reactor. In this report, a steam stable silica membrane prepared by the counterdiffusion CVD method was applied to steam reforming reaction of methane. Catalysts were dipped on alumina substrates for the composite membrane reactor. The experimental results were also discussed with the simulation results. Experimental Section 1. Catalysts Dipping on Porous Alumina Substrates. Ni or Rh catalysts were dipped onto porous alumina substrates before a CVD treatment for a composite membrane reactor. Porous R-alumina capillary substrates (effective membrane area, φ )2.7 mm; L ) 50 mm; pore size, 0.1 µm) were provided by NOK Co. (Japan). The γ-alumina layer having 4 nm pores was dipped on the R-alumina substrates by a sol-gel method and calcined at 873 K for 3 h in air. The thickness of the γ-alumina layer was ca. 5 µm on a porous R-alumina substrate. Nickel nitrate or rhodium chloride was employed as a metal precursor. A 2 wt % amount of the metal precursor was prepared for catalyst dipping. The catalyst dipping procedure was carried out for 10 s, and the dipped substrates were dried at room temperature overnight. The dried substrates were calcined at 873 K for 3 h in the air. The metal-dipped alumina substrates were reduced at 773 K under hydrogen atmosphere for 2 h before the steam reforming reaction. 2. Preparation of Silica Membranes by the Counterdiffusion CVD Method. Silica membranes were prepared by a

10.1021/ie051345z CCC: $33.50 © 2006 American Chemical Society Published on Web 04/21/2006

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Figure 1. Schematic diagram of a membrane reactor for steam reforming.

Figure 3. Permeances through a silica membrane prepared by the counterdiffusion CVD method.

Figure 2. Schematic diagram of a composite catalytic membrane reactor.

counterdiffusion chemical vapor deposition method using tetramethyl orthosilicate and oxygen as reactants. CVD was carried out on porous γ-alumina substrates at 873 K for 2 h. The detailed preparation procedures were described elsewhere.17,18 A steam stability test was employed for a silica membrane without catalyst dipping at 773 K under a H2O/N2 ratio of 3. Steam was supplied from the outer side of the silica membrane, and the other side of the membrane was vacuumed. The steam permeance during the stability test was also measured by a pressure change method. 3. Hydrogen Permselective Membrane Reactor for Steam Re-forming of Methane. Figure 1 shows a schematic diagram for a hydrogen permselective membrane reactor. A silica membrane prepared by the counterdiffusion CVD method was set in the stainless membrane module. Both sides of the membrane were fixed by Viton O-rings. The temperature of the membrane module was controlled between 723 and 823 K by the thermocouple placed around the membrane. The flow rate of the methane was controlled by a mass flow meter between 1.5 and 15 mL min-1 through a water bubbler. Two kinds of configurations were employed for the membrane reactor test. One is a traditional packed bed reactor test. In this case, steam and methane were supplied into the outer side of the membrane. Commercial Ni catalyst (3.0 g) was packed around the membrane. Inside of the membrane was swept by Ar gas at a rate of 200 mL min-1. The other is a catalytic membrane reactor in which metal catalyst was dipped on the porous alumina substrate. Figure 2 shows the schematic diagram of a catalytic membrane reactor. The top layer is the silica layer for H2 selective separation deposited by the counterdiffusion CVD method in a porous γ-alumina layer. The porous γ-alumina layer is 4 nm in pore diameter with ca. 5 µm thickness on the porous R-alumina layer. The base porous R-alumina layer is ca. 100 nm in pore diameter for the mechanical support. The thickness of the R-alumina layer is about 500 µm, where the metal catalyst was dipped. Steam was supplied through a water bubbler into the inside of the membrane for the catalytic membrane reactor. The outer side of the membrane was swept by Ar gas at a rate of 200 mL min-1. For both cases, the temperature of the water bubbler was controlled at 362 K to keep the S/C ratio at 2. Concentrations of methane, CO, and CO2 after a cold trap were measured by the gas chromatography (Shimadzu: GC-14B). The total flow rates of the feed side and permeation side were

measured by a soap flow meter. Samplings of the reactor were conducted after steady state (more than 2 h), and the carbon balance of the reactor was kept within 5% during the experiment. 4. Simulation of the Membrane Reactor. The experimental results were discussed with the simulation results. The simulation procedure was followed as discussed by the former article.5 One-dimensional simulation was conducted using 100 cells. The rates of steam reforming reactions of methane on Ni catalysts were employed from previous literature results.19 The reactions for steam reforming of methane are described below.

CH4 + H2O ) CO + 3H2

-206.1 kJ mol-1

CO + H2O ) CO2 + H2

+41.15 kJ mol-1

CH4 + 2H2O ) CO2 + 4H2

-165.0 kJ mol-1

The concentrations of each molecule in the one cell were assumed to be uniform. Plug flow in the feed and the permeation side was assumed under isothermal conditions in the cells. The pressure drops in the cells were not considered for the calculation. Feed side compositions were calculated on the basis of the rates of reaction. The permeation amounts through the membrane were calculated by the partial pressure differences (after the composition calculation in the cell) between the silica membrane and permeance through the silica membrane for each molecule. H2 permeances were employed from the experimental values just before the membrane reactor tests. H2/H2O permeance ratios were parameters for the calculation, while H2/ CO, H2/CO2, and H2/CH4 were kept constant at 100. Figure 3 shows the experimental permeation results for a silica membrane prepared by the counterdiffusion CVD method. H2 permeance at 873 K was 4.38 × 10-8 mol m-2 s-1 Pa-1. The H2/N2 permeance ratio at 873 K was ca. 3000. These results are similar to those shown in the previous report.17 H2 permeation ratios over CH4 and CO2 at 873 K were 2500 and 6700, respectively. The molecular weights of N2 and CO are the same, and the same permeation ratio for H2/N2 and H2/CO was employed for the simulation in the former article.5 Thus, it is reasonable to use the permeation ratio of 100 for H2/CH4, H2/CO, and H2/ CH4. The effective catalyst coefficient of Ni was decided by the traditional packed bed reaction experiment. There were no reports found for the rates of reactions on Rh catalysts, so the same rates were used for the Ni catalysts in this article. The detailed calculation conditions were summarized in Table 1. There were no other fitting parameters.

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Table 1. Parameters Used for the Simulation parameters H2 permeance H2/CH4 permeance ratio H2/CO permeance ratio H2/CO2 permeance ratio H2/H2O permeance ratio feed pressure steam/Carbon ratio methane flow rate permeation pressure temperature effective catalysis coefficient

unit 10-8

mol

m-2 s-1

comment Pa-

Pa mL mim-1 Pa K

Results and Discussions 1. Steam Permeation Test of a Silica Membrane at 773 K. Figure 4 shows the steam stability test at 773 K for a silica membrane without catalyst dipping. Usually, steam reforming is carried out around 1000-1100 K. The target reaction temperature of the membrane reactor is at 773 K by removing H2 from the reactor. Steam was provided to the outer side of the membrane at 75 kPa partial pressure. H2 and N2 permeances were almost constant for 82 h, keeping H2/N2 permeance ratios. This is the same trend in the former report in which the silica membrane was stable for 21 h.17 Both H2 and N2 permeances slightly decreased at the first few hours of steam treatment. This must be the densification of the silica by the steam treatment. However, the H2/N2 permeance ratio was almost the same throughout the treatments. The silica membrane was stable for more than 80 h under 75 kPa of steam at 773 K. Steam permeances were also plotted in the figure. The steam permeances were almost the same values as the N2 permeances. The H2/H2O permeance ratio after 82 h of the stability test was ca. 290, which is much larger than those (ca. 10) through the silica membranes prepared by the sol-gel method.5 Further stability tests (the longer time tests, the higher pressure tests, etc) are required to realize the membrane reactor system using the silica membrane. However, the silica membranes prepared by the counterdiffusion CVD method have a potential for the application to the steam reforming reaction by steam stability and by high H2/H2O permeance ratio. The activation energy of H2 through the silica membranes prepared by the sol-gel method and by the counterdiffusion CVD method was similar at around 20 kJ mol-1, and both membranes showed a H2/N2 permeance ratio over 100. However, only H2O permeances were different from each other. Further structural analysis is required for the detailed discussions for the difference of the silica membrane prepared by the sol-gel method and by the counterdiffusion CVD method.

Figure 4. Hydrothermal stability tests through the silica membrane. Steam conditions: 773 K, 75 kPa.

2.54-6.82 100 100 100 1-100 1.013 × 105 2 1.5-15 1.013 × 105 773 0.006

pure H2 permeance fixed parameter fixed parameter fixed parameter

Ar sweep: 200 mL min-1 isothermal fixed parameter

2. Traditional Packed Bed Reactor. First, the traditional packed bed test was conducted using the silica membrane and commercial Ni catalyst. Figure 5 shows the methane conversion and operating temperature with or without H2 extraction by the silica membrane. Methane and steam were supplied to the outer side of the membrane. The methane flow rate was kept at 15 mL min-1. The solid line in the figure shows the equilibrium value of the methane conversion, and the dotted line shows the calculated line by the 100 cells simulation at the H2/H2O permeance ratio of 100. The experimental results without H2 extraction were almost the same as the equilibrium values, showing that the catalyst activity was enough. The methane conversions for the membrane reactor were slightly higher than those without H2 extraction. These values were similar to the simulation results, while the difference of the conversions was not significant due to the high methane flow rate compared with the H2 permeance throughout the membrane. However, the simulation results agreed with the experimental values, indicating that the simulation assumptions are reasonable for this experiment. 3. Ni Catalyst and Rh Catalyst for the Catalytic Membrane Reactor. Catalysts comparison for Ni and Rh was carried out using catalytic membrane reactors at 773 K under S/C ) 2 without H2 extraction. Amounts of Ni or Rh dipped on the membrane were calculated by the void of the R-alumina layer. All the void of the R-alumina layer is assumed to be filled by the dipping solution. The deposited amounts were calculated by the concentration of the metal solution. The amounts were 0.0023 and 0.0016 g for Ni and Rh, respectively. Figure 6 shows that steady-state methane conversions plotted as a function of W/F (catalyst weight/methane flow rate) [g-cat. h mol-1] through the metal (Ni or Rh) doped membrane reactors. The amounts of metal catalysts on the substrates were kept constant, and the methane flow rate was changed from 5.0 to 15.0 mL min-1.

Figure 5. Traditional packed bed membrane reactor using Ni catalyst: commercial Ni catalyst, 3.0 g; CH4 flow rate, 15 mL min-1.

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Figure 6. Catalysts activity test on R-alumina substrates without H2 extraction: Ni catalyst, 0.0023 g; Rh catalyst, 0.0016 g; CH4 flow rate, 5-15 mL min-1; temperature, 773 K.

Figure 7. Catalysts stability test on R-alumina substrates without H2 extraction: Ni catalyst, 0.0023 g; Rh catalyst, 0.0016 g; CH4 flow rate, 5-15 mL min-1; temperature, 773 K.

S/C was kept at 2 during the experiment. The solid line in the figure shows the equilibrium conversion. Methane conversions for Rh catalyst were almost the same as the equilibrium conversion, indicating that catalytic activity for Rh catalyst was enough. Methane conversions for Ni catalyst were slightly lower than those for Rh catalyst. However, the difference of the initial conversions for Ni and Rh was not significant for this experimental condition. Figure 7 shows catalyst stability tests under S/C ) 2 at 773 K for Ni and Rh catalysts. Conversions for Rh catalyst were lower than the equilibrium conversion due to higher methane flow rates than those shown in Figure 6. The Rh catalyst showed catalytic activity after introduction of 0.80 mol of methane. On the other hand, methane conversion on Ni catalyst decreased with increasing amount of methane supply, and there was no catalytic activity found after introduction of 0.80 mol of methane. It is obvious that Rh catalyst shows better stability than Ni catalyst under S/C ) 2 at 773 K. Rh catalyst was dipped on the membrane for the following discussions. 4. Membrane Reactor Test Using a Catalyst Composite Silica Membrane. Figure 8 shows methane conversions by extracting H2 through the silica membrane. The solid line shows the equilibrium conversion without H2 extraction. The dotted line shows the simulation results assuming a H2/H2O permeance ratio of 100. Pure H2 permeances just before the membrane reactor tests were measured at 773 K. These H2 permeances were employed for each simulation. The x-axis shows W/F [g-cat. h mol-1] calculated by assuming a Rh deposition amount

Figure 8. Composite catalytic membrane reactor tests with Rh catalyst: Rh catalyst, 0.0016 g; CH4 flow rate, 5-15 mL min-1; temperature, 773 K.

(0.0016 g). The methane flow rate was changed from 1.5 to 5.0 mL min-1. As indicated in the former section, methane conversion was the same as the equilibrium conversion at the lower W/F without H2 extraction. The methane conversions increased with increasing W/F for the membrane reactor tests. The relative H2 extraction ratio should be increased at the higher W/F, because the flow rate of methane was lower at the higher W/F. All the methane conversions for the membrane reactor were in agreement with the simulation results, showing that the simulation assumptions were reasonable. The methane conversion was increased to 64.5% at W/F ) 0.41 g-cat. h mol-1 from the equilibrium conversion (31.4%). Pure H2 permeance before the membrane reactor test through the catalytic membrane was 2.54 × 10-8 mol m-2 s-1 Pa-1. This is about half of the initial H2 permeance (6.78 × 10-8 mol m-2 s-1 Pa-1), and much smaller than that shown in Figure 4. The experimental period for this membrane reactor test was within 16 h. The difference between the hydrothermal stability test shown in Figure 4 and the membrane reactor tests in Figure 8 was the geometry of the supply side of the steam vapor. Steam was supplied from the outer side (the silica layer side) of the membrane for Figure 4, and steam was supplied from the inner side of the membrane for Figure 8 (cf. Figure 2). The porous γ-alumina layer was damaged under a steam condition at 773 K.20 Thus, the silica layer might cover the γ-alumina layer for the case of Figure 4. Further investigation is required for the difference of Figure 4 and Figure 8. The high methane conversion was successfully obtained using the composite catalytic membrane reactor. The H2/H2O permeance ratios during the membrane reactor tests were discussed. Figure 9 shows sensitivity analyses by changing H2/H2O permeance ratios by simulation calculation. The solid line shows the experimental conversion at W/F ) 0.12 g-cat. h mol-1 as indicated in Figure 8. The dotted line shows the simulation results. The methane conversion by the simulation increased with increasing H2/H2O permeance ratios below H2/H2O ratios of 30. The methane conversion was 45.8% for H2/H2O ) 30, while only 0.2% of methane conversion was increased until H2/ H2O ) 100. It is important to obtain a H2/H2O ratio over 30 for the silica membrane. The experimental conversion was the same as the simulation results around H2/H2O ) 35, showing that the silica membrane employed for the experiment was over 35 for the H2/H2O permeance ratio. This high H2/H2O permeance ratio during membrane reactor tests agrees with the H2O permeance test shown in Figure 4.

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Figure 9. Sensitivity analyses on methane conversions by changing H2/ H2O selectivity.

Conclusions A composite catalytic membrane reactor in which metal catalyst was deposited in the porous alumina substrates was successfully obtained by the counterdiffusion CVD method. The silica layer for H2 selective permeation was stable for 82 h during steam permeation tests at 773 K. Rh was superior to Ni by catalytic activity stability at 773 K under S/C ) 2. A membrane reactor test was conducted at 773 K under S/C ) 2, and methane conversion was increased with 64.5% from 31.4% (equilibrium) by extracting H2 through the silica layer at W/F ) 0.41 g-cat. h mol-1. The methane conversions for the membrane reactor were almost the same as the 100 cells simulation, indicating that the experimental conditions were under simulation assumptions (plug flow, isothermal, etc.). The high methane conversions obtained by the membrane reactor were due to a high H2/H2O permeance ratio through the silica layer. The H2/H2O permeance ratio was over 35 by the sensitivity analyses of the membrane reactor. Acknowledgment This work has been supported by NEDO as a part of the R&D Project on Highly Efficient Ceramic Membranes for HighTemperature Separation of Hydrogen promoted by METI, Japan. This work has been partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Young Scientists (A), Grant 17686065, 2005. Literature Cited (1) Kikuchi, E. Palladium/ceramic membranes for selective hydrogen permeation and their application to membrane reactor. Catal. Today 1995, 25, 333. (2) Tsuru, T.; Tsuge, T.; Kubota, S.; Yoshida, K.; Yoshioka, T.; Asaeda M. Catalytic membrane reactor for methane steam reforming using porous silica membranes. Sep. Sci. Technol. 2001, 36, 3721.

(3) Kurungot, S.; Yamaguchi T.; Nakao, S. Rh/γ-Al2O3 catalytic layer integrated with sol-gel synthesized microporous silica membrane for compact membrane reactor applications. Catal. Lett. 2003, 86, 273. (4) Kurungot, S.; Yamaguchi, T. Stability improvement of Rh/γ-Al2O3 catalyst layer by ceria doping for steam reforming in an integrated catalytic membrane reactor system. Catal. Lett. 2004, 92, 181. (5) Tsuru, T.; Yamaguchi, K.; Yoshioka, T.; Asaeda, M. Methane steam reforming by microporous catalytic membrane reactors. AIChE J. 2005, 50, 2794. (6) Okubo, T.; Inoue, H. Introduction of specific gas selectivity to porous glass membranes by treatment with teraethyoxysilane. J. Membr. Sci. 1989, 42, 109. (7) Ha, H.-Y.; Nam, S.-W.; Hong, S.-A.; Lee, W.-K. Chemical vapor deposition of hydrogen-permselective silica films on porous glass supports from tetraethyorthosilicate. J. Membr. Sci. 1993, 85, 279. (8) Morooka, S.; Yan, S.-C.; Kusakabe, K.; Akiyama, Y. Formation of hydrogen-permselective SiO2 membrane in macropores of R-alumina support tube by thermal decomposition of TEOS. J. Membr. Sci. 1995, 101, 89. (9) Prabhu, A. K.; Oyama, S. T. High hydrogen selective ceramic membranes: application to the transformation of greenhouse gases. J. Membr. Sci. 2000, 176, 233. (10) Nomura, M.; Kasahara, S.; Nakao, S. Silica membrane reactor for the thermochemical iodine-sulfur process to produce hydrogen. Ind. Eng. Chem. Res. 2004, 43, 5874. (11) Gavalas, G. R.; Megiris, C. E.; Nam, S.-W. Deposition of H2-permselective SiO2 films. Chem. Eng. Sci. 1989, 44, 1829. (12) Megiris, C. E.; Glezer, J. H. E. Synthesis of H2-permselective membranes by modified chemical vapor deposition. Microstructure and permselectivity of SiO2/C/Vycor membranes. Ind. Eng. Chem. Res. 1992, 31, 1293. (13) Nakao, S.; Suzuki, T.; Sugawara, T.; Tsuru, T.; Kimura, S. Preparation of microporous membranes by TEOS/O3 CVD in the opposing reactants geometry. Microporous Mesoporous Mater. 2000, 37, 145. (14) Yamaguchi, T.; Ying, X.; Tokimasa, Y.; Nair, B. N.; Sugawara, T.; Nakao, S. Reaction control of tetraethyl orthosilicate (TEOS)/O3 and tetramethyl orthosilicate (TMOS)/O3 counter diffusion chemical vapor deposition for preparation of molecular-sieve membranes. Phys. Chem. Chem. Phys. 2000, 2, 4465. (15) Xomeritakis, G.; Han, J.; Lin, Y.-S. Evolution of pore size distribution and average pore size of porous ceramic membranes during modification by counter-diffusion chemical vapor deposition. J. Membr. Sci. 1997, 124, 27. (16) Nomura, M.; Yamaguchi, T.; Nakao, S. Silicalite membranes modified by counterdiffusion CVD technique. Ind. Eng. Chem. Res. 1997, 36, 4217. (17) Nomura, M.; Ono, K.; Gopalakrishnan, S.; Sugawara, T.; Nakao, S. Preparation of a stable silica membrane using a counter diffusion chemical vapor deposition method. J. Membr. Sci. 2005, 251, 151. (18) Nomura, M.; Aida, H.; Gopalakrishnan, S.; Sugawara, T.; Nakao, S.; Yamazaki, S.; Inada, T.; Iwamoto, Y. Steam stability of a silica membrane prepared by a counter diffusion chemical vapor deposition. Desalination, in press. (19) Xu, J.; Froment, G. F. Methane steam reforming, methanation and water-gas shift: I. Intrinsic kinetics. AIChE J. 1989, 35, 88. (20) Zahir, Md. H.; Sato, K.; Iwamoto Y. Development of hydrothermally stable sol-gel derived La2O3-doped Ga2O3-Al2O3 composite mesoporous membrane. J. Membr. Sci. 2005, 247, 95.

ReceiVed for reView December 1, 2005 ReVised manuscript receiVed February 16, 2006 Accepted March 1, 2006 IE051345Z