Reaction Chemistry of W-Mn/SiO 2 Catalyst for the Oxidative Coupling of Methane

Transcription

1 Journal of Natural Gas Chemistry 12(2003)1 9 Reaction Chemistry of W-Mn/SiO 2 Catalyst for the Oxidative Coupling of Methane Shuben Li State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou , China [Manuscript received December 9, 2002] Abstract: Reaction chemistry of the OCM reaction on W-Mn/SiO 2 catalyst has been reviewed in this account. Initial activity and selectivity, stability in a long-term reaction, reaction at elevated pressures and a modelling test in a stainless-steel fluidized-bed reactor show that W-Mn/SiO 2 has promising performance for the development of an OCM process that directly produces ethylene from natural gas. A study on surface catalytic reaction kinetics and used catalyst structure characterization revealed a possible reason why C 2 and CO x selectivity changed during the long-term reaction. Further improvement of the catalyst composition and preparation method should be a future direction of study on OCM reaction over W- Mn/SiO 2 catalyst. Key words: W-Mn/SiO 2 catalyst, oxidative coupling of methane, elevated pressure, methane 1. Introduction Oxidative coupling of methane (OCM) is a promising catalytic process for direct natural gas conversion to ethylene, which is used as an important raw material in the production of petrochemicals and liquid fuels. 20 years ago Keller and Bhasin [1] published a pioneering work in this area, and since then, thousands of catalysts for the OCM reaction have been reported on in patents and publications throughout the world. Metal oxides (alkali earth metal oxides [2 4], rare earth metal oxides [5 7], heavy metal oxides [8] and transition metal oxides [9 11]) were found to be effective catalyst systems for this reaction. Alkali metal and halogen ions can be used as promoters but often result in decreased stability. Experiments were conducted using Al 2 O 3, SiO 2 and TiO 2 supports, but these were only useful for heavy and transition metal oxide catalysts. The advances in catalyst performance and reaction mechanisms, kinetic and engineering aspects, and economics and future directions have been reviewed by Lunsford[12], Mleczko and Bearns [13] and Bhasin and Campbell [14], respectively. Ten years ago, Fang et al. [15,16] discovered a new catalyst system for the OCM reaction, which included binary transition metal oxides promoted by alkali metal ions and supported on SiO 2 or TiO 2. The reaction chemistry of W-Mn/SiO 2 and related catalyst systems for the OCM process will be discussed based on catalytic performance and reaction kinetics using different reactors and operating conditions. The purpose of this paper is to introduce the future direction of catalyst research in the development of the OCM process. 2. Features of the OCM reaction Methane is the most stable molecule in the hydrocarbons. The catalytic activation of methane to directly produce higher hydrocarbons is a difficult process in thermodynamics. Using molecular oxygen as an oxidant to activate methane for production of oxy- Corresponding author. lzcpcn@ns.lzb.ac.cn

2 2 Shuben Li/ Journal of Natural Gas Chemistry Vol. 12 No genated compounds has to be done at high pressures. In the gas phase reaction, with or without a catalyst to convert methane to methanol, the selectivity is lower than 80% at 20% methane conversion, because deep oxidation occurs to form carbon oxides as byproducts. Until now, industrial production of higher hydrocarbons and methanol from methane had to use indirect conversion that combines two processes of methane reforming with FT synthesis or methanol synthesis. In the past two decades, the OCM reaction and the partial oxidation of methane (POM), as the new selective oxidation technologies, have been attractive processes for methane conversion to directly produce higher hydrocarbons [1] and synthesis gas [17,18] by improving indirect process (Figure 1). Figure 1. Scheme of the OCM and related reactions of methane conversion to ethylene The OCM and POM reaction equations are given below. CO 2 is the co-byproduct of the OCM and POM reactions because the OCM and POM reactions use the same reactants (methane and molecular oxygen) even though the catalysts and reaction conditions of the two are different. In other words, the OCM and POM reactions face the same problem (enhancing the selectivity of valuable products) as the other selective oxidation of hydrocarbon reactions. (1) OCM reaction to C 2 hydrocarbons 4CH 4 + 3/2O 2 C 2 H 6 + C 2 H 4 + 3H 2 O (1) (2) POM reaction to Methanol CH 4 + 1/2O 2 CH 3 OH (2) of the POM reaction to methanol and syngas (Equations 2,3), because the activation energy of C 2 hydrocarbon formation is higher than that of CO x formation (Equations 4,5), and higher temperatures are favorable for the C 2 hydrocarbon selectivity. Increasing the system pressure causes the catalytic activity and selectivity to decrease during the OCM reaction, and this phenomenon is similar to the selective oxidation of other hydrocarbons. The space velocity of methane (GHSV) in the OCM reaction is much higher than that in the POM reaction to methanol but less than that in the POM reaction to syngas. The conversion of methane can be increased by increasing the GHSV, but selectivity to C 2 hydrocarbons will be decreased, especially after a long time on stream. This feature is quite different than in other selective oxidation of hydrocarbon reactions. The significant homogeneous gas phase reactions, which occur at high temperatures during the OCM reaction, are a serious problem for the enhancement of C 2 selectivity. Using redox mode instead of co-feeding mode can improve selectivity of C 2 hydrocarbons, but single pass conversion of methane is very low in redox mode. From the features of the OCM reaction mentioned above, it can be seen that the high temperature and significant homogeneous gas phase reactions should seriously influence the enhancement of C 2 hydrocarbon selectivity at high methane conversion. The sum of CH 4 conversion and C 2 selectivity reaching 100% was considered desired performance of the best catalyst discovered for the OCM reaction during the past two decades. Unfortunately, only one catalyst system with supported two transition metal oxides promoted by alkali metal ions has been proven to attain this level. The alkali earth metal oxides and/or rare earth metal oxides without halogen ion doping have not achieved this goal yet. In this paper, we will mainly discuss composite sodium tungstate and manganese oxide supported on silica (W-Mn/SiO 2 ) systems. (3) POM reaction to syngas CH 4 + 1/2O 2 CO + 2H 2 (3) 3. Catalyst screening and development (4) Deep oxidation to CO x CH 4 + 2O 2 CO 2 + 2H 2 O (4) CH 4 + 3/2O 2 CO + 2H 2 O (5) The temperature of the OCM reaction to C 2 hydrocarbons (Equation 1) must be higher than that Initial research that screened various metal oxides to catalyze the OCM reaction was reported by Keller and Bhasin [1] in They tested 25 α- Al 2 O 3 supported metal oxides, and the oxides of Mn, Cd, Sn, Sb, Tl, Pb and Bi were active for the OCM reaction. However, using 5% manganese on α-al 2 O 3 as a catalyst produced a C 2 selectivity of only 45%

3 Reviews / Journal of Natural Gas Chemistry Vol. 12 No at 11%CH 4 conversion. Five years later, Sofranko and co-workers [9] screened 18 metal oxides that had been used in different selective oxidation of hydrocarbon processes including transition metal oxides (e.g. Mn 2 O 3 /Mn 3 O 4, MoO 3, WO 3 and V 2 O 5 ) for the OCM reaction in a redox system. Only manganese and tin oxides were stable without volatility problems during the 6 12 redox reaction cycles at A number of identified manganese oxides supported on SiO 2 that the manganese-silica system promoted by sodium ions is most attractive for the OCM reaction [10]. The best result (22% methane conversion and 61%C 2 and 15.4%C 3+ selectivity) could be obtained on 15%Mn-5%Na 4 P 2 O 7 /SiO 2 catalyst at 850, 860 h 1 methane GHSV and CH 4 : air=1 : 1 in a redox reaction system [11]. The co-feeding mode resulted in decreasing the selectivity to 59%C 2 and 7%C 3+. In order to maintain the same methane conversion (22%), the reaction temperature had to be raised to 900. Based on the previous research mentioned above and our experience with the selective oxidation of other hydrocarbons, adding a second active component to form binary composite transition metal oxides should improve the catalytic behavior of the manganese-silica system for activation of methane and molecular oxygen at different reaction sites in co-feeding mode. After identifying many different transition metals, the catalyst W-Mn/SiO 2 was found, which has the potential to be used for developing the OCM process at co-feeding mode and a redox cycle system in a fixed bed and fluidized bed reactor. The best catalyst (composed of 1.9%Mn-5%Na 2 WO 4 /SiO 2 ) demonstrated 37.7% methane conversion and 66.9%C 2 selectivity with a higher ratio of C 2 H 4 to C 2 H 6 at 800, 48,000 h 1 methane GHSV and CH 4 : O 2 : N 2 =3 : 1 : 2.5 in cofeeding mode OCM reaction. The repeatability of this catalyst system is perfect, and similar results have been obtained in other laboratories [19 22] at different conditions. The initial activity and selectivity of W-Mn/SiO 2 and comparative catalyst systems with methane conversion >20% and C 2 selectivity >65% are listed in Table 1. More interesting results with 80%C 2 selectivity at 20% and 33% methane conversion were achieved by Lunsford and the group of Abetini and Lambert, respectively. Manganese on silica without sodium promotion showed very low methane conversion and C 2 selectivity. High selectivity at high methane conversion could not be achieved for sodium tungstate on silica [23,24]. Adding sodium chloride to a 1.9%Mn-5%Na 2 WO 4 /SiO 2 catalyst could enhance C 2 selectivity to 82.7% with 26% methane conversion, but stability would decrease[25]. Table 1. Initial activity and selectivity of W-Mn/SiO 2 catalyst systems Temperature CH 4 Cofeeding Catalyst ( ) GHSV (h 1 ) ratio (V/V) X(CH 4 )/% S(C 2+ )/% S(CO x)/% Ref 15%Mn 5%Na 4 P 2 O 7 /SiO ,800 CH 4 :air=1: (0) 34 [11] 20 68(3,500 h) %NaMnO 4 /MgO 800 2,800 CH 4 :air=1: [10] 925 9,600 CH 4 :air=1: % Na 2 WO 4 /SiO ,000 CH 4 :O 2 :N 2 =3:1: %Mn/SiO ,000 CH 4 :O 2 :N 2 =3:1: [15,16] 1.9%Mn 5%Na 2 WO 4 /SiO ,000 CH 4 :O 2 :N 2 =3:1: %Mn 5%Na 2 WO 4 /SiO ,000 CH 4 :O 2 =7.4: [19] 2%Mn 5%Na 2 WO 4 /MgO 800 7,000 CH 4 :O 2 =7.4: (40 h) 19 2%Mn/5%Na 2 WO 4 /SiO CH 4 :O 2 :He=4.5:1: [20] 10% Na 2 WO 4 /SiO ,000 CH 4 :air=1: [23] 800 6,000 CH 4 :air=1: %Rb 2 WO 4 /SiO ,000 CH 4 :O 2 :He=4.5:1: [24] 5%Na 2 WO 4 /SiO ,000 CH 4 :O 2 :He=4.5:1: %Mn/5%Na 2 WO 4 /SiO ,320 CH 4 :O 2 =7.5: <20 [21] 5%Na 2 WO 4 /SiO ,320 CH 4 :O 2 =7.4: <26

4 4 Shuben Li/ Journal of Natural Gas Chemistry Vol. 12 No Stability testing Stability tests for manganese-silica catalysts were first reported by Sofranko and co-workers [11] in They found that 15%Mn, 5%Na 4 P 2 O 7 /SiO 2 catalyst longevity could be achieved to 3,500 h at , 4,800 h 1 methane GHSV and CH 4 : air=1 : 1. The initial activity was 22% methane conversion with 65%C 2+ selectivity, and the last analysis showed 20% methane conversion and 68%C 2+ selectivity. Lin et al. [26] were encouraged by this promising achievement to investigate the stability of the 1.9%Mn- 5%Na 2 WO 4 /SiO 2 catalyst for the OCM reaction. The experiments were carried out in 0.2 ml and 30 ml fixed-bed reactor under different reaction conditions. A continuous run in the 0.2 ml reactor for 1,000 h ( , 8,000 15,000 h 1 methane GHSV and CH 4 : O 2 : N 2 = : 1 : ) demonstrated that the C 2 yield was maintained at 16% 17%, C 2 selectivity was 60% 65% and the C 2 H 4 to C 2 H 6 ratio was 2. This result could be repeated in the 30 ml reactor running for 500 h at , 5,000 5,500 h 1 methane GHSV and CH 4 : O 2 : H 2 O= : 1 : In this case, the C 2 yield was about 17%, and the C 2 selectivity remained higher than 70%. The best C 2 selectivity reached 78% 80% during the initial seven days of the continuous reaction (Figures 2,3). Figure 2. Stability test of the W-Mn/SiO 2 catalyst for a 1,000 h OCM reaction in a micro-quartz reactor at , 8,000 15,000 h 1 CH 4 GHSV, CH 4 : O 2 : N 2 = : 1 : [26] (1) S(C 2 ), (2) Y (C 2 ), (3) Y (C = 2 ), (4) Y (CO 2), (5) Y (CO) Figure 3. Stability test of the W-Mn/SiO 2 catalyst for a 500 h OCM reaction in a 30 ml quartz reactor at , 5,000 5,500 h 1 CH 4 GHSV, CH 4 : O 2 : N 2 = : 1 : [26] (1) S(C 2 ), (2) Y (C 2 ), (3) Y (CO), (4) Y (CO 2 )

5 Reviews / Journal of Natural Gas Chemistry Vol. 12 No Structural changes in the catalyst used for longterm reactions were characterized using XRD and FT- IR methods. The XRD patterns and FT-IR spectra show that the crystal state of support SiO 2 changed from α-cristobalite into α-tridymite and α-sio 2. The active components (Na 2 WO 4 and Mn 2 O 3 ) have partially been volatile at high temperature and reaction atmosphere with steam (Figures 4,5). The change in catalyst structure from 500 to 1,000 h did not significantly influence catalytic activity, but C 2 selectivity slightly decreased and CO selectivity increased. The interesting result is that the sum of C 2 and CO selectivity remained at 93%, while CO 2 yield decreased from 5% to 1.5% at 510 h on stream. These phenomena were not found in similar experiments for La 2 O 3 - BaCO 3 and MgO-BaCO 3 catalysts (Table 2). Table 2. Results of stability testing on the W-Mn/SiO 2 catalyst Catalyst X(CH 4 ) Y (C 2 H 4 ) Y (C 2 H 6 ) Y (CO) Y (CO 2 ) Y (C 2 ) Y (C 2 +CO) S(C 2 ) S(C 2 +CO) % % % % % % % % % W-Mn/SiO 2 (510 h)(940 ) La 2 O 3 /BaCO 3 (520 h)(915 ) MgO/BaCO 3 (516 h)(990 ) Reaction conditions: CH 4 : O 2 : H 2 O=5.8 : 1 : 5.25, CH 4 GHSV=5,000 h 1 5. Reaction kinetics and mechanisms The kinetics of the OCM reaction over W- Mn/SiO 2 catalysts were studied at different conditions by Wu and Li [27]. Based on previous work on catalyst performance [15,16] and structure characterization [28 30], the experimental data on reaction kinetics can be explained by a Rideal-redox mechanism which includes four elementary reactions: (1) O 2 activation on the catalyst surface to form surface lattice oxygen Figure 4. XRD patterns of the W-Mn/SiO 2 catalyst structure change after a 1,000 h stability test in a micro-quartz reactor [26] (1) fresh catalyst, (2) used catalyst S cat + O 2 2O S (6) (2) Methyl radical formation on the surface CH 4 + O S CH 3 +OH S (7) (3) Methyl radical coupling to C 2 hydrocarbons in the gas phase 2CH 3 C 2 H 6 C 2 H 4 (8) (4) CO and CO 2 formation on the surface CH 3 +XO S CO x (9) Figure 5. XRD patterns of W-Mn/SiO 2 catalyst structure change after a 500 h test in a 30 ml fixed-bed quartz reactor [26] (1) fresh catalyst, (2) used catalyst The overall reaction rate of methane conversion depends on methyl radical formation, which is a slow step of the reactions. Apparent activation energy of the overall reaction for methane conversion is kj/mol (58.1 kcal/mol). Lunsford et al. [31] recently reported a value of 64 kcal/mol for the same catalyst, which is higher than ours because of different

6 6 Shuben Li/ Journal of Natural Gas Chemistry Vol. 12 No conditions used in their experiment. The apparent activation energies for C 2 H 6 and CO x formation are 305 and 128 kj/mol, respectively. The former reaction activation energy is much higher than the later value. This is an important reason for OCM reaction, which has to be conducted at very high temperatures, because increasing the reaction temperature is favorable for enhancing C 2 selectivity. Similar to the Mars and van Krevelen redox mechanism, the rates of C 2 and CO x formation can be expressed as follows: ln(r C2 /R COx ) = ln(k C2 /k COx ) xlnp O2 (10) Unfortunately, the OCM reaction is very complicated and is not a typical Mars and van Krevelen mechanism redox reaction [32]. The bulk lattice oxygen does not participate in selective oxidation, and a gas phase homogeneous reaction is significant for deep oxidation of CH 4 and C 2 hydrocarbons at high temperature (Figure 6). This model can only be used for the catalytic surface reaction. Figure 6. Scheme of the OCM reaction over the surface of the W-Mn/SiO 2 catalyst and gas phase 6. Reaction at elevated pressures The OCM reaction operated at elevated pressures will be beneficial to commercial applications, but only a few papers have been reported on the OCM catalyst, which is favorable to selective oxidation at higher pressures. Pinabian-Calier et al. [33] studied an Sr- La catalyst system at a pressure of bar, 860 and the CH 4 to O 2 ratio of 10, and the results showed that both CH 4 conversion and C 2+ selectivity decreased with increasing pressure. The C 2+ yield fell from 9.9% to 3.4%, and even GHSV increased by the same factor as the pressure. Lunsford and coworkers [19] first reported a result from W-Mn/SiO 2 catalysts operated at elevated pressures. They found that using 0.1 g catalyst, 250 ml/(min g) GHSV, at 0.5 MPa, 750 and CH 4 /O 2 =10, both CH 4 conversion and C 2+ selectivity only slightly decreased with increasing pressure. Using 1 ml catalyst, 550 ml/min flow rate, at 800, a methane conversion of 13% 14% with a C 2+ selectivity of 80% could be maintained at a pressure of 0.5 MPa during 60 hours of continuous reaction. Chou et al. [34 36] has investigated OCM reaction performance over 3 different kinds of W-Mn/SiO 2 catalysts at MPa pressure and using a stainless steel micro-reactor at various CH 4 GHSV and CH 4 to O 2 ratios. Increasing the CH 4 GHSV and decreasing the CH 4 to O 2 ratio caused the CH 4 conversion to increase while the C 2 selectivity decreased. It was also interesting that C 2 selectivity decreased with increasing C 3 and C 4 selectivity, while the C 2+ yield remained constant at about 13% 14%. Using a CeO 2 or SnO 2 doped catalyst at 0.6 MPa caused the C 3 and C 4 selectivity to remarkably increase (as shown in Table 3). Table 3. Best results of the OCM reaction at elevated pressures Catalyst p Temperature CH 4 n(ch 4 ) X(CH 4 ) S(C 2+ ) S(C 2 ) S(C 3 ) S(C 4 ) Y (C 2+ ) MPa ( ) GHSV (h 1 ) n(o 2 ) % % % % % % 2%Mn-5%Na 2 WO 4 /SiO %CeO 2-2%Mn-5%Na 2 WO 4 /SiO %SnO 2-2%Mn-5%Na 2 WO 4 /SiO Modeling test in fluidized bed reactor Wang et al. [37] conducted an OCM modeling experiment with W-Mn/SiO 2 catalysts in a stainless steel fluidized bed reactor with 200 ml (130 g) catalyst loading and 600 g/h steam as a diluent. During the 450 h period of continuous reaction without recharging the catalyst, the C 2 selectivity and yield were stabilized at over 70% and 17%, respectively. The best results were at 800, 7,000 h 1 CH 4 GHSV and 11.8%O 2 content, C 2 selectivity and yield were 82.6% and 17.9%, respectively; at 875, 15.1%O 2 content, C 2 selectivity and yield were 75.7% and 19.4%, respectively. Using XRD, ICP-AES and BET methods to characterize the change in catalyst structure after a 450 h reaction produced similar results to the stabil-

7 Reviews / Journal of Natural Gas Chemistry Vol. 12 No ity testing experiments. A new crystal phase of SiO 2 support (α-tridymite and α-sio 2 ) was observed, but the catalyst was seriously worn during the reaction in the fluidized bed reactor, resulting in a loss of active components and the surface area decreasing from 2.31 m 2 /g to 0.14 m 2 /g. ICP-AES analysis showed that W, Na, Mn content lost was 96.7%, 57.5% and 17.7%, respectively. Most W and Na loss was due to Na 2 WO 4 enriched on the surface of catalyst and more Mn distributed in the bulk (Table 4). By means of XAFS and XPS, Kou and Zhang et al. [38] found that the dispersed active species of tetrahedral WO 4 and octahedral MnO 6 disappeared, while dispersed tetrahedral MnO 4 and Mn 2+ species appeared after the catalyst was used for 450 h in fluidized bed reactor (Figure 7). The MnO 4 may be a new active site (instead of WO 4 ) that maintains catalyst activity after undergoing a long term reaction. Possible reasons for the decrease in the C 2 selectivity and the increase in CO selectivity include a change in the catalyst structure on the SiO 2 crystal phase, a loss of active components, a decrease in surface area and a change in the active site. Nevertheless, the sum of C 2 and CO selectivity (i.e. efficiency of carbon utilization) could reach higher than 90%. It should also be emphasized that industrial operation, with the catalyst recharged to maintain constant catalyst loading during the reaction, will significantly improve the reaction results. After modeling experiment finished, discharged catalyst decreased to 72 ml (42.4 g), almost 64% catalyst was worn and lost, the catalyst mechanical lifetime was about 700 h, catalyst cost was 2 kg/t of C 2 H 4 production. These data obtained from a modeling test could be used for an economic assessment of developing a commercial fluidized bed OCM process. Figure 7. XPS spectra of Mn 2p region changed after a 450 h reaction in a 200 ml stainless steel fluidized bed reactor [38] (1) fresh catalyst, (2) fresh catalyst at 620, (3) catalyst used for 450 h at 620 Table 4. Near-surface compositions (mol%) of catalyst components [38] Catalyst Mn 2p Na 1s W 4f Si 2p O 1s SiO 2 MO x C 1s Fresh catalyst in situ at 620 for fresh catalyst h catalyst a 450 h catalyst in situ at 620 for 450 h catalyst a: 2.7% C 1s at ev was not included Lunsford and Pak [39] investigated thermal effects during the OCM reaction over W-Mn/SiO 2 and W- Mn/MgO catalysts. They found that the temperature rose about 150 in the catalyst bed. During an extended duration on stream the W-Mn/MgO catalyst deactivated quickly, but the W-Mn/SiO 2 catalyst was stable until after 97 hours of reaction time. A decrease in surface area during the reaction was observed for both catalysts, but the loss of Mn content was discovered only from the near surface of the MgO supported catalyst. Lunsford and Rosynek et al. [40 42] suggested two recycling systems for ethylene and ethane produced by the OCM reaction over W-Mn/SiO 2 to subsequently convert them to C 4+ aliphatics and aromatics, which can be used as gasoline components. The recycling yields were 60% 70% of the initial feed methane, and these two modeling processes would be helpful to develop a new natural gas conversion technology that would produce a fuel and method of fuel transport from remote regions.

8 8 Shuben Li/ Journal of Natural Gas Chemistry Vol. 12 No Summary and future direction Systematic study on OCM reaction chemistry revealed that the W-Mn/SiO 2 catalyst has promising performance in a fixed and fluidized bed reactor. The sum of CH 4 conversion and C 2 selectivity can attain levels higher than 100% during the initial stage of the reaction. Between 500 and 1,000 hours, the reaction catalytic activity is stable, but C 2 selectivity slowly decreases with CO selectivity increase and CO 2 decrease. However, the sum of C 2 and CO selectivity remains over 90%. Characterization of the catalyst used during the long-term reaction showed that the SiO 2 support crystal phase changed from α-cristobalite to α-tridymite and α-sio 2, the active components lost from the catalyst surface at high temperatures, which could explain the change in C 2 and CO x selectivity during the later period of the reaction. The kinetics of the OCM reaction on the W-Mn/SiO 2 catalyst surface can be described by the Mars and van Crevelen redox mechanism. Activation energy of the overall reaction for methane conversion is kj/mol, and activation energy of C 2 and CO x formation was 305 kj/mol and 128 kj/mol, respectively. Enhancing the reaction temperature could increase C 2 selectivity. The W-Mn/SiO 2 catalyst can be operated at elevated pressures, high methane GHSV and lower CH 4 to O 2 ratios, resulting in a methane conversion increase and a C 2 selectivity decrease, but the C 2+ selectivity could be held constant. A modeling experiment in a fluidized bed reactor with 200 ml catalyst loading reproduced the reaction result obtained in a 30 ml fixed bed reactor, but the catalyst was seriously worn during the long reaction period without any new catalyst recharged into the reactor, and the mechanistic lifetime was about 700 h. Future directions of the study include improving the catalyst to stabilize active components during extended times on stream. Possible methods to achieve this goal include changing the preparation method and finding a new active component to replace W and/or Na. Acknowledgements The author expresses his gratitude to his coworkers (J. Lin, Z. Jiang, X. Wang, Y. Kuo, J. Gu, D. Yang, J. Niu, Y. Liu, J. Zhang, X. Fang, J. Wu, Y. Chu, S. Ji, Y. Zhang, B. Zhang, L. Chou, G. Wang and Y. Cai) for their contributions to the research progress on this project. The State Planning Commission, Ministry of Science and Technology and National Natural Science Foundation provided the financial support. The modeling experiment was conducted at Chengdu Institute of Organic Chemistry, and the reactor was designed by Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences. Curriculum vitae Shuben Li, graduated from the Department of Chemical Engineering of Tianjin University in As a scholar, he visited the Institute of Physical Chemistry, Swiss Federal Institute of Technology in He has been a Professor of Chemistry since 1986 and a Ph.D. supervisor of Physical Chemistry since He was Director of the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences from 1986 to 1999 as well as Director of the State Key Laboratory for Oxo Synthesis and Selective Oxidation from 1992 to His research interests include oxidation catalysis including heterogeneous catalysis, photocatalysis and enzyme catalysis. He has published more than 300 papers in domestic and international journals. References [1] Keller G E, Bhasin M M. J Catal, 1982, 73: 9 [2] Driscoll D J, Marbir W, Wang J X, Lunsford J H. J Am Chem Soc, 1985, 107: 58 [3] Ito T, Wang J X, Lin C H, Lunsford J H. J Am Chem Soc, 1985, 107: 5062 [4] Campbell K D, Lunsford J H. J Phys Chem, 1988, 92: 5792 [5] Otsuka K, Jinno K, Morikawa A. Chem Lett, 1985, 160: 499 [6] Lin C-H, Cambell K D, Wang J-X, Lunsford J H. J Phys Chem, 1986, 90: 534 [7] Tong Y, Rosynek M P, Lunsford J H. J Phys Chem, 1989, 93: 2896 [8] Hinsen W, Bytyn W, Baerns M. Proc Int Congr Catal, 8th 1984, 1984, 3, 581 [9] Sofranko J A, Leonard J J, Jones C A. J Catal, 1987, 103: 302 [10] Jones C A, Leonard J J, Sofranko J A. J Catal, 1987, 103: 311 [11] Sofranllo J A, Leonard J J, Jones C A, Gaffney A M, Withers H P. Catal Today, 1988, 3: 127 [12] Lunsford J H. Angew Chem, Int Ed Engl, 1995, 34: 970 [13] Mleczko L, Baerns M. Fuel Process Technol, 1995, 42: 217

9 Reviews / Journal of Natural Gas Chemistry Vol. 12 No [14] Bhasin M M, Campbell K D. In: Bhasin M M, Slocum D W eds. Oxidative Coupling of Methane A Progress Report Methane and alkane Conversion Chemistry Edited, Plenum Press, New York, [15] Fang X-P, Li S-B, Lin J-Z, Chu Y-L. J Mol Catal (China), 1992, 6: 255 [16] Fang X-P, Li S-B, Lin J-Z, Gu J-F, Yang D-X. J Mol Catal (China), 1992, 6: 427 [17] Vennon P D F, Green M L, Cheetham A K. Catal Lett, 1990, 6: 181 [18] Aschcrofe A T, Cheethan A K, Green M L H, Vernon P D F. Nature, 1991, 352: 225 [19] Wang D-J, Rosynek M P, Lunsford J H. J Catal, 1995, 155: 390 [20] Palermo A, Vazquez J P H, Lee A F, Tikhov M S, Lambert R M. J Catal, 1998, 177: 159 [21] Malekzadeh A, Khodadadi A, Abedini M, Amini M, Bahramian A, Dalai A K. Catal Comm, 2001, 2: 241 [22] Ji S-S, Xiao T-C, Li S- B, Xu C-Z, Hou R-L, Coleman K S, Green M L H. Appl Catal A: General, 2002, 225: 271 [23] Yan Q J, Wang Y, Jin Y-S, Chen Y. Catal Lett, 1992, 13: 221 [24] Palermo A, Vazquez J P H, Lambert R M. Catal Lett, 2000, 68: 91 [25] Wu J-G, Li S-B. J Mol Catal (China), 1994, 8: 131 [26] Lin J-Z, Gu J-F, Yang D-X, Zhang C-W, Yang Y-L, Chu Y-L, Li S-B. Petrochem Technol (China), 1995, 24(5): 293 [27] Wu J-G, Li S-B. Chinese J Catal, 1995, 16: 376 [28] Jiang Z-C, Yu C-J, Fang X-P, Li S-B, Wang H-L. J Phys Chem, 1993, 97: [29] Wu J-G, Li S-B, Niu J-Z, Fang X-P. Appl Catal A, 1995, 124: 9 [30] Wu J-G, Li S-B. J Phys Chem, 1995, 99: 4566 [31] Pak S, Qiu P, Lunsford J H. J Catal, 1998, 179: 222 [32] Li S-B. Chinese J Chem, 2001, 19: 16 [33] Pinabiau-Carlier M, Hadid A Ben, Cameron C J. Stud Surf Sci Catal, 1991, 61: 183 [34] Chou L-J, Cai Y-C, Zhang B, Niu J-Z, Ji S-F, Li S-B. React Kinet Catal Lett, 2002, 76: 311 [35] Chou L-J, Cai Y-C, Zhang B, Niu J-Z, Ji S-F, Li S-B. Chem Comm, 2002, 996 [36] Chou L-J, Cai Y-C, Zhang B, Niu J-Z, Ji S-F, Li S-B. Appl Catal, 2003, 238: 185 [37] Wang X-L, Zhang J-N, Yang D-X, Zhang C-W, Lin J-Z, Li S-B. Petrochem Technol (China), 1997, 26: 361 [38] Kou Y, Zhang B, Niu J-Z, Li S-B, Wang H-L, Tanaka T, Yoshida S. J Catal, 1998, 173: 399 [39] Pak S, Lunsford J H. Appl Catal A, 1998, 168: 131 [40] Qiu P, Lunsford J H, Rosynek M P. Catal Lett, 1997, 48: 11 [41] Lunsford J H, Cordi E M, Qiu P, Rosynek M P. Stud Surf Sci Catal, 1998, 119: 227 [42] Pak S, Rades T, Rosynek M P, Lunsford J H. Catal Lett, 2000, 66: 1