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534 Chinese Science Bulletin Vol. 47 No. 7 April 2002

Partial oxidation of methaneto syngas in tubular oxygen-permeable reactorWANG Haihui, CONG You & YANG Weishen

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,Chinese Academy of Sciences, Dalian 116023, ChinaCorrespondence should be addressed to Yang Weishen (e-mail: yangws@dicp.ac.cn)

Abstract A dense Ba0.5Sr0.5Co0.8Fe0.2O3−−−−δ membrane tubewas prepared by the extruding method. Furthermore, amembrane reactor with this tubular membrane was success-fully applied to partial oxidation of methane (POM) reaction,in which the separation of oxygen from air and the partialoxidation of methane are integrated in one process. At 875�,94% of methane conversion, 98% of CO selectivity, 95% ofH2 selectivity, and as high as 8.8 mL/(min�cm2) of oxygenflux were obtained. In POM reaction condition, the mem-brane tube shows a very good stability.

Keywords: membrane reactor, oxygen separation, POM, mixed-conducting membrane, syngas, perovskite.

Syngas, a mixture of H2 and CO, is used as feedstock for many important industrial processes[1], such asmethanol synthesis and Fischer-Tropsch (F-T) processes.Up to now, steam reforming of methane (SRM) is thedominant process for producing syngas[3] (CH4 + H2O �

CO + 3H2, ∆H(25�) = 206.16 kJ/mol). Due to its strongendothermic property, the typical SRM reaction usually isoperated at high temperature (850�900�), which re-quires more investments in plant construction and moreenergy supply in process operation. Moreover, the syngasproduced through this process has a higher H2/CO ratio(> 3), which is unsuitable for methanol synthesis or F-Tsynthesis. In recent years, a new process, i.e. partial oxi-dation of methane (POM) to syngas (CH4 + 0.5O2 � CO+ 2H2, ∆H(25�) = −36.67 kJ/mol), is extensively deve-loped in order to better meet the needs of methanol syn-thesis or F-T synthesis. Due to its weak exothermic prop-erty, this reaction has some advantages over SRM process.For example, POM reaction can be carried out automati-cally once it starts, and the reaction rate is 1�2 orders ofmagnitude faster than the SRM reaction. Furthermore, theratio of CO/H2 is 1/2, which is a perfect stoichiometry formethanol synthesis or F-T synthesis. However, methanolsynthesis or F-T synthesis cannot tolerate the presence ofnitrogen, so pure oxygen is required in POM process.Traditionally, pure oxygen is obtained from air throughcryogenic process or pressure swing adsorption process.The utilization of these oxygen separation processes again

increase the investment and operation cost of POM pro-cess, which makes POM process difficult to be economi-cally commercialized in industry. Recently, Balachandranet al.[2] developed a membrane reactor for POM reaction,in which a mixed-conducting oxygen-permeable mem-brane was used to simultaneously separate pure oxygenfrom air in the reaction conditions. In this way, the separa-tion of pure oxygen and POM reaction were unified in oneprocess, in which the low-priced air can be directly usedas the oxygen source for POM reaction. The developedmembrane reactor not only simplified the POM process,but also reduced the operation cost. Therefore, it is apromising process for producing syngas for methanolsynthesis or F-T synthesis more economically.

Tsai et al.[3] studied the direct conversion of methaneto syngas in a disk-shaped oxygen-permeable La0.2Ba0.8-Co0.2Fe0.8O3−δ membrane reactor at 1123 K. They foundthat packing a 5% Ni/Al2O3 catalyst directly on the reac-tion-side surface of the membrane resulted in a fivefoldincrease in O2 permeation and fourfold increase in CH4

conversion, compared with the blank reactor. Recently, wehave developed a novel mixed-conducting oxygen-per-meable Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane[4], which show-ed extremely high oxygen permeability and promisingstability. We have also studied POM to syngas in the disk-shaped Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor withLiLaNiO/γ-Al2O3 catalyst[5, 6]. At 850�, CH4 conversionis greater than 88%, CO selectivity is greater than 97%and oxygen permeation is about 7.8 mL/(min�cm2). Atpresent, most investigations are based upon disk-shapedmembrane reactor. One of the disadvantages in using adisk-shaped membrane is the lower ratio of the reactionarea to the total reactor volume, which results in a loweroverall conversion of methane. Therefore, it is necessaryto increase the ratio of the reaction area to the total reactorvolume. This problem can be partially solved by applyingtubular membranes with small diameters. However, thereare few laboratories using tubular membrane becausethere are some technical difficulties in the fabrication oftubular membrane and the sealing of membrane with re-actor body at high temperature. Furthermore, the mem-brane tubes are easy to be broken in reducing atmospheredue to stress. Balachandran et al.[7] investigated POM in atubular La0.2Sr0.8Co0.2Fe0.8O3−δ membrane reactor. Theyfound that the membrane tube had been broken into seve-ral pieces after methane was introduced to the inside ofthe membrane tube for a few minutes. They attributed thisphenomenon to the stress induced by the expansion of theinside surface of the membrane tube exposed to methane,because there is a higher oxygen vacancy concentration inthe reducing atmosphere. Pei et al.[8] studied POM to syn-

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Chinese Science Bulletin Vol. 47 No. 7 April 2002 535

Fig. 1. The module of the tubular membrane reactor.

gas in a tubular SrCo0.8Fe0.2O3− δ membrane reactor, andthey observed two types of fractures occurring on theSrCo0.8Fe0.2O3−δ membrane tube. The first type occurredshortly after the reaction started (�1 h) and the mem-brane tube in the hot reaction zone was often broken intomany small pieces. The second type often occurred daysafter the reaction, and a large crack was formed parallel tothe axis of the tube. They also found the first fracture wasthe consequence of oxygen gradient in the membranefrom the reaction side to the air side, which caused a littlemismatch inside the membrane, leading to the fracture.The second type of fracture was the result of a chemicaldecomposition. The present note will report POM to syn-gas in a tubular Ba0.5Sr0.5Co0.8Fe0.2O3− δ membrane reactor.

1 Experimental

LiLaNiO/γ-Al2O3 catalyst with 10% (weight percent)Ni loading was prepared by the impregnating method[9].The tubular Ba0.5Sr0.5Co0.8Fe0.2O3− δ membrane was fabri-cated by the extrusion method. First, the Ba0.5Sr0.5Co0.8-Fe0.2O3− δ powder was prepared by a combined EDTA-citric acid complex method[4]. After grinding, the powderwas sifted to the size below 200 mesh, and mixed withseveral additives such as solvent, dispersant, binder andplasticizer to make a formulation (slip) with enough plas-ticity to be easily formed into tube while retaining satis-factory strength in the green state, then forced through adie under pressure to produce a hollow tube. The extrudedtube was sintered at 1100�1200� in stagnant air. Thesintered tubular membrane had an outer diameter of about8 mm and an inner diameter of about 5 mm, and with alength up to 30 cm.

The configuration of the tubular membrane reactor isshown in fig. 1. Ceramic powder was used to seal the twoends of the membrane tube to the quartz tubes. The effec-tive area of the membrane tube exposed to methane wasabout 2.0 cm2, and 150 mg LiLaNiO/γ-Al2O3 catalyst waspacked inside of the membrane tube. Air was introducedoutside of the membrane tube at the flow rate of300 mL/min, and a mixture of 80%CH4 + 20%He wasintroduced to the inside of the membrane tube at the flow

rate of 40 mL/min. The reaction temperature was set at875�. The products were condensed by a cool trap andwere further dehydrated by a column of Mg(ClO4)2, be-fore they were introduced to a GC (HP6890 with twoautomatic valves). A porapakQ column was used to sepa-rate CH4 and CO2, and a 13X column was used to separateH2, O2, N2 and CO. Concentrations of the products werecalculated by external stand method.

2 Result and discussion

The crystal phase of the tubular Ba0.5Sr0.5Co0.8-Fe0.2O3− δ membrane was determined by XRD at roomtemperature. As shown in fig. 2, the tubular Ba0.5Sr0.5-Co0.8Fe0.2O3−δ membrane has a cubic perovskite structure.In our previous studies, we have found that this materialshowed high phase stability. The membrane not onlycould keep its structure stable for a long time in helium,but also could be operated stably in POM reaction for500 h. The fresh tubular Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranewas analyzed by SEM and the results are shown in fig. 3.It can be seen from fig. 3(a) for the fresh membrane thatthe ceramic grains with clear grain boundaries are hexa-gon with size from 10 µm to 30 µm. Fig. 3(b) shows thatthere are some sphere-shaped pores with diameters of 0.01�3 µm exist in the cross-section of the membrane, whichmay be caused by the grain growth or decomposition oforganic additives during the sintering of the membranetube. The nitrogen permeation measurements indicated

Fig. 2. XRD of Ba0.5Sr0.5Co0.8Fe0.2O3− δ tubular membrane.

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536 Chinese Science Bulletin Vol. 47 No. 7 April 2002

Fig. 3. The SEM micrographs of the fresh Ba0.5Sr0.5Co0.8Fe0.2O3−δ tubular membrane. (a) Membrane surface; (b) cross-section of membrane.

that there was no nitrogen permeated through the mem-brane, which confirmed that these small sphere-shapedpores were closed pores and no open pores existed. Therelative density of the sintered membrane tube determinedby the Archimedes method using ethanol was higher than90%, which showed that the membrane tube was dense.

In order to seal the membrane tube to the quartztubes as shown in fig. 1, the membrane reactor was heatedto 1040� slowly and kept at 1040� for 10 min to makeceramic powder solidified, then the temperature was de-creased to 875�. Pure helium (60 mL/min) was intro-duced to check the leakage of the reactor and to measurethe oxygen flux of the tubular membrane. At 875�, theoxygen permeation flux is 1.1 mL/(min�cm2), which issimilar to the disk-shaped membrane in the same condi-tions. So the tubular Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranewas used to construct a membrane reactor for POM reac-tion.

As comparison, we firstly performed POM reactionin the tubular Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactorwith no packed catalyst. The results showed that a largeamount of un-reacted CH4 and un-reacted O2 were de-tected in the outlet and a small amount of C2H6, C2H4,CO2, CO, H2 and H2O were also detected in the outlet. Inthis case, oxygen permeation flux through the Ba0.5Sr0.5-Co0.8Fe0.2O3−δ membrane tube calculated based on theoxygen balance was 1.35 mL/(min�cm2). It is almost thesame as that of non-reactive air/He experiment. The rea-son is that because of the presence of a large amount ofun-reacted O2 in the reaction chamber, the oxygen partialpressure difference between both sides of the membrane isin the same range as that with helium as the purge gas.

Compared with the POM reaction in the membranereactor without catalyst, we got different results when150 mg LiLaNiO/γ-Al2O3 catalyst was packed in the tu-

bular Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor. It wasfound that large amounts of CO and H2 were detected inthe outlet, and small amounts of CO2, H2O and un-reactedCH4 were detected, while no C2H4, C2H6 and O2 weredetected. No O2 detected in the outlet indicated that all O2

permeated through the membrane was consumed by CH4.So, the oxygen permeation flux increased greatly in thiscase. Fig. 4 shows CH4 conversion, CO selectivity, H2

selectivity and oxygen permeation fluxes for POM reac-tion in the tubular Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane re-actor with LiLaNiO/γ-Al2O3 catalyst. During 30 h opera-tion, CH4 was higher than 94%, CO selectivity was higherthan 98%, H2 selectivity was higher than 95% and oxygenpermeation fluxes were about 8.8 mL/(min�cm2).

Fig. 4. CH4 conversion, CO selectivity, H2 selectivity and O2 permea-tion flux in the membrane tube reactor at 875� in POM reaction.

Balachandran et al.[7] investigated POM in the tubu-lar La0.2Sr0.8Co0.2Fe0.8O3−δ membrane reactor. They foundthat the membrane tube broke into several pieces aftermethane was introduced inside of the membrane tube for afew minutes. Pei et al.[8] studied POM to syngas inthetubular SrCo0.8Fe0.2O3−δ membrane reactor, and they

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Chinese Science Bulletin Vol. 47 No. 7 April 2002 537

Fig. 5. The SEM micrographs of the Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane tube after used for POM reaction. (a) The surface exposed to methane; (b) thecross-section near the inside surface of the membrane tube; (c) the surface exposed to air.

observed two types of fractures occurring on the tubularSrCo0.8Fe0.2O3−δ membrane reactor. Compared with theirresults, there were no break and fracture formed in ourcase during 30 h operation. By proper substitution ofstrontium ion in SrCo0.8Fe0.2O3−δ with barium ion with alarger radium can increase the tolerate factor (near 1), thusgreatly improving the phase stability of the material.

The used membrane tube was analyzed by SEM andthe results are shown in fig. 5. Fig. 5(a) shows that thesurface exposed to methane became rough and porous.This was caused by the catalyst incorporated into mem-brane and reacted with the membrane to produce the po-rous materials at high temperature. The micrograph of thecross-section near the inside surface of the membrane tubein fig. 5(b) shows that the thickness of the porous layer isabout 15 µm. Compared with that of the membrane tube(about 1.5 mm), the porous layer can be neglected. So, themembrane tube still kept its integrity. Fig. 5(c) shows theSEM micrograph of the surface of the used membranetube exposed to air, which is the same as that of the freshmembrane. Therefore, we think that the Ba0.5Sr0.5Co0.8-Fe0.2O3−δ membrane tube can be operated steadily in thePOM reaction conditions.

In a dense oxygen-permeable membrane reactor,oxygen diffuses from air through the membrane and reactswith methane at the other side. So the permeation flux hasgreat effect on the reaction. However, the oxygen permea-tion flux of membrane depends on the oxygen pressuregradient between two sides of the membrane. It was foundthat the oxygen permeation flux in the POM conditionincreased greatly compared with that in no catalyst condi-tion or that in non-reactive air/He (with pure He as sweepgas) condition. In the presence of LiLaNiO/γ-Al2O3 cata-lyst in the membrane reactor, the permeated O2 throughthe membrane reacted rapidly with CH4, so the O2 partialpressure in the reaction side was greatly reduced. As re-sults, the oxygen flux increased significantly by a factor

of 8 compared with helium. For our membrane, the highO2 permeation flux was maintained at the same level dur-ing one week’s operation in the reducing atmosphere.

3 Conclusions

The dense Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane tubehas been fabricated by the extrution method, which wassuccessfully applied to POM reaction. High CH4 conver-sion and high CO/H2 selectivity were obtained. The mem-brane tube reactor can be operated steadily.

Acknowledgements This work was supported by the National HighTechnology Research and Development Program (Grant No. 715-006-0122) and the “973” Project of the Ministry of Science and Technology,China (Grant No. G1999022401).

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(Received November 14, 2001)