Improving Performance of a Dense Membrane Reactor for Thermal

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Ind. Eng. Chem. Res. 2007, 46, 2000-2005

MATERIALS AND INTERFACES Improving Performance of a Dense Membrane Reactor for Thermal Decomposition of CO2 via Surface Modification Chun Zhang, Xianfeng Chang, Yiqun Fan, Wanqin Jin,* and Nanping Xu Membrane Science and Technology Research Center, Nanjing UniVersity of Technology, Nanjing 210009, People’s Republic of China

In this study, a SrCo0.4Fe0.5Zr0.1O3-δ (SCFZ) dense mixed-conducting membrane was applied to a membrane reactor for the thermal decomposition of carbon dioxide (TDCD) (CO2 T CO + 1/2O2). The SCFZ membrane broke after the membrane reactor ran for about 36 h, because the eroding gases, such as CO2 and CO, corroded the membrane material. To improve the stability of the membrane reactor, the surface modification of the SCFZ membrane was applied by coating a porous layer. After surface modification on the membrane surface, the porous layer can reduce effectively the corrosion of gases for the membrane material. The effect of coating a porous layer on the membrane surface exposed to the feed side (CO2) on improving the performance of the membrane was more remarkable than that on the membrane surface exposed to the permeate side. This phenomenon can be elucidated by the reaction pathway of TDCD in the membrane reactor. 1. Introduction The decomposition of carbon dioxide (CO2) into carbon monoxide (CO) and oxygen (2CO2 f 2CO + O2, ∆H0298 ) 552 kJ/mol) has been one of the targeted technologies for the utilization of CO2 and the mitigation of the “greenhouse effect”.1 However, this reaction is a highly endothermic reaction, takes place only at high temperature, and is limited by the thermodynamic equilibrium. This reaction is, therefore, not easy to realize in conventional fixed-bed reactors. To date, a few research groups have attempted to apply oxygen-permeable ceramic membrane reactors to realize this reaction.2-4 The use of membranes in chemical reactors is motivated principally by the equilibrium shift, leading to a higher conversion or selectivity. Recently, we proposed the thermal decomposition of carbon dioxide (TDCD) coupled with the partial oxidation of methane (POM) to syngas in the SrCo0.4Fe0.5Zr0.1O3-δ (SCFZ) membrane reactor.5 The conversion of CO2 was up to 11.1% at the temperature of 1123 K. However, because of the presence of TDCD and POM reactions respectively in either side of the SCFZ membrane, both surfaces of the membrane were exposed to the reducing atmospheres (one side is CO, the other side CO and H2); after the SCFZ membrane underwent a 33-h operation, severe degradation of the SCFZ material occurred, and finally the membrane cracked. One of the challenges for our current research is now focused on improving the stability of the membrane reactor by some effective ways. A feasible solution is to coat a porous mixedconducting oxide layer on the surface of the dense membrane. If a porous mixed-conducting oxide layer is coated on the membrane surface, the reaction may occur mainly on the inner surface of the porous layer, and the effective specific surface * To whom correspondence should be addressed. Tel.: +86-258358-7211. Fax: +86-25-8358-7211. E-mail: [email protected].

area for the exchange of oxygen is increased. Thus, on one hand, the oxygen flux can be enhanced by surface modification.6-9 For given external conditions, the enhancement of oxygen permeation through a mixed-conducting membrane is beneficial to the decomposition of CO2.5 On the other hand, the porous layer can reduce the corrosion of the atmosphere to the surface of the membrane and prolong the lifetime of the membrane. Therefore, this work continued our previous research5 on the reaction of TDCD in the SCFZ membrane reactor and studied the contribution of the surface modification to the performance of the SCFZ membrane reactor and the reaction pathway of TDCD in membrane reactor. 2. Experimental Section 2.1. Powder and Membrane Preparation. The perovskitetype SrCo0.4Fe0.5Zr0.1O3-δ (SCFZ) ceramic powders were synthesized by a solid-state reaction of appropriate amounts of SrCO3, Co2O3, Fe2O3 (the second Chemical Industry of Shanghai, purity of 99.9%), and monoclinic ZrO2 (Shengzhen Nanbo Structure Ceramics Co., Ltd.). The details of powder preparation conditions were outlined in our previous work.10 The diskshaped SCFZ membranes were prepared by isostatic pressing at a pressure of 200 MPa using an oil press. The green membranes were sintered in air at 1473 K for 5 h in a MoSi2 furnace to form dense membranes. The densities of the sintered membranes, which were determined by the Archimedes method, in all cases exceeded 90% of the theoretical one. The sintered membranes without a porous layer were polished to the thickness of 1.5 mm before being used in our experiments, and the coated membranes were obtained by coating the surface of the polished membrane with a porous layer of the SCFZ material and subsequently sintered at the temperature of 1313 K for 4 h. The thickness of the porous layer was controlled around 10 µm based on our previous work,9 and the porosity of the porous layer is

10.1021/ie060913n CCC: $37.00 © 2007 American Chemical Society Published on Web 03/02/2007

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Figure 1. Illustrations of the reactor module: (a) the non-coated membrane, (b) the coated membrane with a porous layer on the surface exposed to the feed side, and (c) the coated membrane with a porous layer on the surface exposed permeate side. The solid bars and checkered bars stand for the dense membrane and the porous layer, respectively.

about 35% which was analyzed by the homemade software. The surface area of the calcined SCFZ powder was determined by nitrogen adsorption using the BET method. The surface areas per unit volume of the porous layer were about 7000 cm-1 estimated from the weight and the surface area of the powder. 2.2. Membrane Reactor Configurations and Experimental Procedures. The reactor module for membrane reaction experiments is illustrated in Figure 1a, which is similar to those previously reported by our laboratory.9 A prepared membrane disc was mounted on a quartz tube (6 mm i.d., 12 mm o.d.) using a gold ring seal, and the gold rings were the same dimensions as the quartz tubes, which left an effective area of about 0.283 cm2 for oxygen permeation. A gas mixture of CO2 and He was introduced into the upper chamber, while the sweep gas (a gas mixture of CH4 and Ar or single Ar) was introduced into the lower chamber. Both upper and lower chambers were maintained at the atmospheric pressure. The effluent streams were analyzed by two on-line gas chromatographs (model Shimadzu GC-8A, Japan). A 2 m 5A molecular sieve column was used for the separation of O2, CH4, and CO, and a 1 m TDX-01 column was used for the separation of CO2. The unreacted feed gases and the products flowed through the loop of the on-line analysis. In this work, CO2 and CO which were produced from the TDCD were in the same chamber. Therefore, the carbon balance could be checked by determining the concentrations of CO2 and CO at the same time. On the basis of the carbon balance, we calculated the conversion of CO2, which is defined as follows:

XCO2 )

FCO2,inlet - FCO2,outlet FCO2,inlet

(1)

The oxygen flux through the dense SCFZ membrane could be calculated by the mass balance on the basis of the components of CO, H2, CH4, CO2, O2, and H2O in the exit stream from the upper chamber.5 3. Results and Discussion 3.1. Performance of the SCFZ Membrane Reactor. In our previous work,5 we proposed coupling the TDCD with POM to syngas in the SCFZ membrane reactor, in which methane reacted with oxygen that permeated through the membrane from

Figure 2. Temperature dependence of CO2 conversion and oxygen flux in the membrane reactor (feed conditions: in the upper chamber 6 cm3 (STP) min-1 CO2 and 24 cm3 (STP) min-1 He; in the lower chamber 1 cm3 (STP) min-1 CH4 and 19 cm3 (STP) min-1 Ar).

the CO2 decomposition over supported transition metal catalysts. After the SCFZ membrane underwent a 33-h operation in the reactive atmosphere, severe degradation of the SCFZ material occurred, and finally the membrane cracked. To deeply study the influence of reactive atmosphere on membrane stability, in this work, we first investigated the performance of non-coated SCFZ membrane for TDCD without packing catalyst for POM on the surface exposed to CH4 (permeate side). The details of the experimental conditions were outlined in our previous work.5 The oxygen flux of the membrane and the conversion of CO2 were related to operating temperature. As shown in Figure 2, the conversion of CO2 and oxygen flux increase with increasing temperature. At 1173 K, the conversion of CO2 in the SCFZ membrane reactor reached 7.8%. The corresponding Arrhenius plots of the membrane and the decomposition of CO2 are shown in Figure 3. These straight lines in Figure 3 have approximately the same slope, giving the apparent activation energy of 110 ( 3 kJ/mol between 1023 and 1223 K. It was implied that the permeation rate of oxygen affected the conversion of CO2 strongly. Subsequently, we observed the stability of the membrane at high temperature. Figure 4 shows the stability of the SCFZ membrane reactor at 1173 K. It was found that the membrane could be operated for only about 36 h. Figure 5 shows the

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Figure 3. Arrhenius plot of the oxygen permeation rate of the membrane and the CO2 conversion.

Figure 4. Stability of the membrane reactor. (Feed conditions: in the upper chamber 6 cm3 (STP) min-1 CO2 and 24 cm3 (STP) min-1 He; in the lower chamber 1 cm3 (STP) min-1 CH4 and 19 cm3 (STP) min-1 Ar; T ) 1173 K).

scanning electron microscopy (SEM) photographs of the fresh and used membranes. It can be seen from the Figure 5a that the particles are closely connected to each other with clear grain boundaries. Figure 5b,c shows the surface of the membrane exposed to CO2 and CH4 after 36 h of the experiment, respectively. In contrast to Figure 5a, grain boundary is not visible on the membrane surfaces after the experiment. It appears that there is a porous layer on the surface. The porous layer could have caused the decomposition of the SCFA in a strongly reducing atmosphere. In our previous study,11 the SCFZ membrane could be operated continuously for over 180 h in the oxygen permeation

experiment using the air as the oxygen source. It is implied that the stability of a mixed-conducting membrane is not only influenced by temperature and oxygen partial pressure but also by atmospheres on both sides of the membrane. Material degradation can occur either by reduction (phase decomposition) under reducing atmosphere or by phase segregation under the oxygen partial pressure gradient.12,13 The results of X-ray diffraction (XRD) analysis of the membrane after the membrane reaction are shown in Figure 6. As can be seen from the figure, the fresh sample is of a cubic perovskite structure with a trace of the SrZrO3 phase. However, most of the characteristic peaks of the perovskite phase of the used membrane weaken obviously. The surface exposed to CO2 (feed side) of the used membrane contains a little amount of SrCO3, SrZrO3, and oxides (including Fe2O3, Co2O3) besides the perovskite phase, as shown by the curve B of Figure 6. In this study, the origin of SrCO3 was attributed to the interaction of CO2 with the material of membrane. The presence of CO2 could erode the membrane strongly.14 The membrane surface to the permeate side contained only the perovskite phase and a small amount of unknown phases which may be assigned to the partial decomposition of the perovskite phase, as shown by the curve C of Figure 6. As a result of the presence of CO, CO2, and CH4, respectively, on either side of the SCFZ membrane, both surfaces of the membrane were exposed to the corrosive atmospheres; besides, a larger oxygen partial pressure gradient existed. Thus, the SCFZ membrane cracked after it underwent a 36-h operation in a reactive atmosphere. 3.2. Performance of SCFZ Membrane Reactor after Surface Modification. As mentioned above, both surfaces of the membrane were exposed to the reducing atmospheres (one side is CO, the other side is CH4). Moreover, the CO2 could react with the membrane material to form carbonate and erode the membrane seriously. In light of practical application of an oxygen-permeable membrane, contacting the membrane directly with the eroding gases should be avoided. Therefore, we expected to improve the performance of the membrane by coating a porous layer of the mixed-conducting oxide on the surface of the membrane. In this work, the material of the porous layer was the same as the membrane material so that the match of thermal expansion between the membrane and the porous layer could be realized for the good interfacial adhesion. The arrangement for studying the contribution of the porous layer to the performance of the membrane was illustrated in Figure 1b,c (Figure 1b denoted for Case I and Figure 1c for Case II). First, we investigated Case I and conducted the measurements for the performance of the membrane reactor in the temperature range of 1073-1223 K. The flow rates of the feed and the sweep gases were the same as the former experiment of the non-coated membrane. The performance of the membrane reactor after

Figure 5. SEM photographs of (a) the surface of the fresh membrane, (b) the surface of the used membrane exposed to CO2, and (c) the surface of the used membrane exposed to CH4.

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Figure 6. XRD patterns of (a) the fresh membrane, (b) the surface of the used membrane exposed to CO2, and (c) the surface of the used membrane exposed to CH4.

Figure 7. Temperature dependence of CO2 conversion and oxygen flux of the coated membrane reactor (feed conditions: in the upper chamber 6 cm3 (STP) min-1 CO2 and 24 cm3 (STP) min-1 He; in the lower chamber 1 cm3 (STP) min-1 CH4 and 19 cm3 (STP) min-1 Ar).

coating a porous layer is shown in Figure 7. It can be seen that the oxygen flux and the conversion of CO2 were enhanced to 0.78 mL (STP)/(cm2 min) and 9.2% by coating a porous layer on the CO2 side at 1173 K, respectively. The long-time stability of the coated membrane is shown in Figure 8. The reaction of TDCD in the SCFZ membrane reactor was operated stably for about 120 h at 1173 K, which was nearly three times longer than the non-coated membrane. Subsequently, we observed the performance of the membrane of Case II using the identical operation condition with Case I. The conversion of CO2 and the oxygen flux for are given in Figure 7. The oxygen flux and the conversion of CO2 were enhanced to 0.66 mL (STP)/(cm2 min) and 8.2% by coating a porous layer on CH4 side at 1173 K, respectively, and the membrane could be operated continuously for about 68 h, which is nearly one time longer than the non-coated membrane, as show in Figure 8. From the Figure 7 and Figure 8, we find that the performance of the membrane can be enhanced by coating a porous layer on either side of the membrane, which is mainly attributed to the surface modification. However, the contribution of the porous layer coated on the surface exposed to CO2 (high oxygen partial pressure) for enhancing the performance of the membrane is larger than that on the surface exposed CH4 (low oxygen partial

Figure 8. Stability of the coated membrane reactor (feed conditions: in the upper chamber 6 cm3 (STP) min-1 CO2 and 24 cm3 (STP) min-1 He; in the lower chamber 1 cm3 (STP) min-1 CH4 and 19 cm3 (STP) min-1 Ar; T ) 1173 K).

pressure). This is not in agreement with the results of previous literature.9,15-20 In general, when only one side of the membrane is coated, a larger enhancement is observed when the low oxygen partial pressure side of the membrane is modified. Therefore, it is necessary to investigate the reaction pathway of TDCD in a mixed-conducting membrane reactor for elucidating the phenomenon. 3.3. Reaction Pathways of TDCD in the SCFZ Membrane Reactor. There are likely two kinds of pathways for the TDCD in a dense membrane reactor. One is that the reaction of TDCD occurs mainly in the bulk of gas phase. The oxygen produced from the reaction of TDCD migrates selectively through the dense membrane, and the thermodynamic equilibrium is shifted, which leads to a higher conversation of CO2. The other is that CO2 decomposes mainly on the surface of the membrane, and the oxygen from the decomposition of CO2 combines with the oxygen vacancies of the membrane to form lattice oxygen which transfers through the membrane because of the presence of a driving force. In this study, an experiment was conducted using single Ar of 20 cm3 (STP) min-1 as the sweep gas to investigate the possible pathways of TDCD at the temperature of 1073 K. Meanwhile, a gas mixture of CO2 of 6 cm2 (STP) min-1 and He of 24 cm3 (STP) min-1 was fed into the upper chamber. Because Ar could not supply enough low oxygen partial pressure in the permeate side, we observed that the highest conversion of CO2 obtained was only 0.3% and the oxygen flux was about 0.023 mL (STP)/(cm2 min). On the basis of the data of the conversion of CO2 and the flow rate of the inlet gas, we could calculate that about 0.009 mL (STP) of oxygen was formed in 1 min by the reaction of TDCD. Meanwhile, we knew that about 0.0065 mL (STP) of oxygen transported through the membrane in 1 min on the basis of the data of oxygen flux. By calculating, we deduced that over 70% oxygen produced from the reaction of TDCD transported through the membrane. The oxygen partial pressures of gas phase in the lower chamber and the upper chamber are about 3.25 × 10-4 atm and 8.33 × 10-5 atm, respectively. That is to say, the oxygen chemical potential of the gas phase in the upper chamber was lower than that of the lower chamber. For a better understanding of the change process of oxygen chemical potential in the membrane reactor, the schematic diagram of the gradient in oxygen chemical potential in Figure

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Figure 9. Schematic diagrams of the gradient in oxygen chemical potential in membrane reactors (a) in the process of oxygen permeation used air as oxygen source and (b) in the process of TDCD membrane reaction (VO¨ ) oxygen ion vacancy, h• ) free-electron hole, and µ0 ) chemical potential).

9 was assumed. Figure 9a describes the gradient of the oxygen chemical potential in the process of oxygen permeation using the air as the oxygen source.21 The oxygen chemical potential of the gas phase in the feed side is higher than that of the permeate side and decreases gradually from the bulk of the feed side to the permeate side. In the process of TDCD, however, the oxygen chemical potential in the upper chamber is lower than that of the lower chamber, and the oxygen chemical potential of the membrane surface exposed to CO2 is higher than that of the other sites of the system. The possible gradient of the oxygen chemical potential can be described as shown in Figure 9b. If the reaction of TDCD happened in the bulk of the gas phase, the oxygen chemical potential in the upper chamber would be higher than those of lower chamber and the surface of the membrane in the feed side. According to the experiment, however, we found that the oxygen chemical potential of gas phase in the upper chamber was lower than that in the lower chamber. Therefore, we thought that the reaction of TDCD occurred mainly on the surface of the membrane instead of in the bulk of gas phase. Both CO2 and CO that formed from the TDCD would corrode the membrane seriously (just as the analysis of the XRD pattern of the membrane after reactions in Figure 6 shows) and lead to cracks of the SCFZ membrane. After surface modification on the feed side, the reaction of TDCD occurs mainly on the inner surface of the porous layer instead of the membrane surface. The porous layer could reduce effectively the corrosion of gases for the membrane material. Thus, the stability of the membrane can be enhanced remarkably. Furthermore, the reaction of TDCD takes place mainly on the inner surface of porous layer instead of the membrane surface after coating a porous layer on feed side. It is obvious that the effective surface area increases and the exchange rate of oxygen can be enhanced with the increasing of effective surface area. Therefore, the performance of the membrane could be increased more remarkably by coating a porous layer on the feed side than on the permeate side. 4. Conclusions The contribution of surface modification to the performance of the SCFZ mixed-conducting membrane for TDCD was investigated in detail. It was found that the performance of the membrane reactor could be improved by coating a porous layer on the surface of the dense membrane. The reaction of TDCD was found to occurr mainly on the surface of the membrane instead of in the bulk of the gas phase, and the effect of coating a porous layer on the feed side was more remarkable than that on the permeate side in increasing the performance of the membrane. At 1173 K, the lifespan of the modified membrane was nearly three times longer than that of the non-coated membrane.

Acknowledgment This work is sponsored by the National Basic Research Program of China (No. 2003CB615702), National Natural Science Foundation of China (NNSFC, Nos. 20576051, 20436030, and 20306010), Scientific Research Foundation for the Returned Overseas China Scholars of MOE (2004527) and the Key Laboratory of Material-oriented Chemical Engineering of Jiangsu Province and MOE. Literature Cited (1) Shin, H. C.; Choi, S. C. Mechanism of M ferrites (M ) Cu and Ni) in the CO2 decomposition reaction. Chem. Mater. 2001, 13, 1238. (2) Nigara, Y.; Cales, B. Production of CO by Direct Thermal Splitting of CO2 at High Temperature. Bull. Chem. Soc. Jpn. 1986, 59, 1997. (3) Itoh, N.; Sanchez, M. A.; Xu, W. C.; Haraya, K.; Hongo, M. Application of a membrane reactor system to thermal decomposition of CO2. J. Membr. Sci. 1993, 77, 245. (4) Fan, Y. Q.; Ren, J. Y.; Onstot, W.; Pasale, J.; Tsotsis, T. T. Reactor and technical feasibility aspects of a CO2 decomposition-based power generation cycle, utilizing a high-temperature membrane reactor. Ind. Eng. Chem. Res. 2003, 42, 2618. (5) Jin, W. Q.; Zhang, C.; Zhang, P.; Fan, Y. Q.; Xu, N. P. Thermal decomposition of carbon dioxide coupled with POM in a membrane reactor. AIChE J. 2006, 52, 2545. (6) Diethelm, S.; Herle, J. V. Oxygen transport through dense La0.6Sr0.4Fe0.8Co0.2O3-δ perovskite-type permeation membranes. J. Eur. Ceram. Soc. 2004, 24, 1319. (7) Steele, B. C. H. Interfacial reactions associated with ceramic ion transport membranes. Solid State Ionics 1995, 75, 157. (8) Lee, K. S.; Lee, S.; Kim, J. W.; Woo, S. K. Enhancement of oxygen permeation by La0.7Sr0.3CoO3 coating in La0.7Sr0.3Ga0.6Fe0.4O3-δ membrane. Desalination 2002, 147, 439. (9) Chang, X. F.; Zhang, C.; Wu, Z. T.; Jin, W. Q.; Xu, N. P. Contribution of the Surface Reactions to the Overall Oxygen Permeation of the Mixed Conducting Membranes. Ind. Eng. Chem. Res. 2006, 45, 2824. (10) Gu, X. H.; Jin, W. Q.; Chen, C. L.; Xu, N. P.; Shi, J.; Ma, Y. H. YSZ-SrCo0.4Fe0.6O3-δ membranes for the partial oxidation of methane to syngas. AIChE J. 2002, 48, 2051. (11) Yang, L.; Tan, L.; Gu, X. H.; Jin, W. Q.; Zhang, L. X.; Xu, N. P. A new series of Sr(Co, Fe, Zr)O3-δ perovskite-type membrane materials for oxygen permeation. Ind. Eng. Chem. Res. 2003, 42, 229. (12) Xu, S. J.; Thomson, W. J. Stability of La0.6Sr0.4Co0.2Fe0.8O3-δ Perovskite Membranes in Reducing and Nonreducing Environments. Ind. Eng. Chem. Res. 1998, 37, 1290. (13) Elshof, J. E. T.; Bouwmeester, H. J. M.; Verweij, H. Oxidative coupling of methane in a mixed-conducting perovskite membrane reactor. Appl. Catal. A 1995, 130, 195. (14) Yi, J. X.; Feng, S. J.; Zuo, Y. B.; Liu, W.; Chen, C. S. Oxygen permeability and stability of Sr0.95Co0.8Fe0.2O3-δ in a CO2 and H2O containing atmosphere. Chem. Mater. 2005, 17, 5856. (15) Steele, B. C. H. Interfacial reactions associated with ceramic ion transport membranes. Solid State Ionics 1995, 75, 157. (16) Lee, K. S.; Lee, S.; Kim, J. W.; Woo, S. K. Enhancement of oxygen permeation by La0.7Sr0.3CoO3 coating in La0.7Sr0.3Ga0.6Fe0.4O3-δ membrane. Desalination 2002, 147, 439.

Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 2005 (17) Lee, T. H.; Yang, Y. L.; Jacobson, A. J.; Abeles, B.; Milner, S. Oxygen permeation in SrCo0.8Fe0.2O3-δ membranes with porous electrodes. Solid State Ionics 1997, 100, 87. (18) Kharton, V. V.; Kovalevsky, A. V.; Yaremchenko, A. A.; Figueiredo, F. M.; Naumovich, E. N.; Shaulo, A. L.; Marques, F. M. B. Surface modification of La0.3Sr0.7CoO3 ceramic membranes. J. Membr. Sci. 2002, 195, 277. (19) Lee, S.; Lee, K. S.; Woo, S. K.; Kim, J. W.; Ishihara, T.; Kim, D. K. Oxygen-permeating property of LaSrBFeO3 (B)Co, Ga) perovskite membrane surface-modified by LaSrCoO3. Solid State Ionics 2003, 158, 287.

(20) Teraoka, Y.; Honbe, Y.; Ishii, J.; Furukawa, H.; Moriguchi, I. Catalytic effects in oxygen permeation through mixed-conductive LSCF perovskite membranes. Solid State Ionics 2002, 152-153, 681. (21) Bouwmeester, H. J. M. Dense ceramic membrane for methane conversion. Catal. Today 2003, 82, 141.

ReceiVed for reView July 14, 2006 ReVised manuscript receiVed January 5, 2007 Accepted February 5, 2007 IE060913N