(373x) Feasibility Study of Membrane Reactor for Reverse Water Gas Shift By ZSM-5 Zeolite Membrane

Reverse water gas shift (RWGS) is one of the promised techniques for carbon utilization. To increase the level of conversion of RWGS, higher reaction temperature is required because of the large endotherm of this reaction. When water is selectively removed from reaction system by using membrane, high conversion would be obtained at lower temperature. In this study, we developed a membrane reactor for RWGS using ZSM-5 zeolite as water-selective membrane and investigated the effect of dehydration on improvement of the level of conversion.

ZSM-5 membrane was synthesized on the outer surface of porous tubular a-alumina support by a secondary growth method. The outer surface of support was seeded by a dip-coating method using a slurry of ZSM-5 seed crystals. A seeded support was hydrothermal treated at 453 K for 12 h. After hydrothermal treatment, the membrane was washed with boiling water and dried at 383 K for 12 h, and then we finally obtained a ZSM-5 membrane.

RWGS was taken place using the mixture of CO₂/H₂ (= 1/3, 24 mL(STP) min^-1) as feed gas with 3.2 g of Cu/Zn/g-Al₂O₃ (CuO/ZnO/Al₂O₃ = 40/40/20wt%). as a catalyst prepared by polymerizable complex method. Reaction temperature was changed in the range of 513 – 610 K. The membrane was placed in reactor by graphite o-rings and catalyst located around membrane. For comparison, RWGS without ZSM-5 membrane was also carried out using non-porous glass tube instead of membrane.

Here, we compared the yield of CO in RWGS with and without ZSM-5 membrane. Unsurprisingly, the yield of CO without membrane were almost the same as equilibrium conversion. In contrast, CO yield in membrane reactor exceeded that without membrane and thermodynamic equilibrium; i.e. the yield of CO with and without membrane at 586 K were 28.9 and 25.1%. About 20% of water generated in reaction system was removed by ZSM-5 membrane.

By selective removal of water, equilibrium conversion was shifted according to Le Chatelier’s law, and then CO yield improved over thermodynamic equilibrium constraint. In addition, it is expected that CO yield further increase with increasing the removal ratio of water from reaction system.