Hysteresis Kinetics of Propene Oxidation Characterized by a Ag-Re Supported Membrane Reactor

摘要

The partial oxidation of propene (PP) to propylene oxide over a Ag-Re catalyst immobilized in a porous ceramic tube membrane reactor was studied by a continuous forced flow system under a differential reactor condition. The steady state rate equations for the production of propylene oxide (PO) and carbon dioxide were separately proposed by two different reaction pathways as follows. r_(po)=kKppKoPppPo_2/{(KppPpp+l)(KoPo_2+l)} r_(CO2) = kKppKoPppPo_2/(l+KppPpp+KoPo_2)~2 In the CO_2 -pathway, a characteristic hysteresis was observed according to the propene concentration increase (C-up) or decrease (C-down) operation each of which was separately characterized by own kinetic parameters. An identical stable reaction intermediate (In) which was commonly formed on the surface in the courses of C-up and C-down operations at a given reaction steady state was quantitatively evaluated by the transient response method using two different reactions, In+O_2 →CO_2 and In + H_2 →C_3H_8 ,monitoring the amounts of CO_2 (q_(CO2)) and propane (q_H) produced. The value of q_(CO2)/q_H gave about three without depending on the concentration of PP and C-up or C-down operation indicating the intermediate to be a structure of C-C-C bond. The surface coverage of In for the C-down was about six times higher than the C-up operation at most. The kinetic modification due to surface silver rearrangement induced by the strong adsorption of In was possibly proposed for the explanation of the hysteresis behavior. The selectivity to PO (S) was drastically varied as S=13 ～ 55 and S=5 ～ 12 % depending on the C-up and C-down operations respectively. The total gas flow rate effectively contributed to the enhancement of S as S=18 ～ 41 % with increasing the flow rate, indicating an advantage of the membrane reactor (which was characterized by a convection flow in membrane pores) rather than conventional packed bed reactors (which were characterized by molecular diffusion in the pores of catalyst particles).