Effect of alkene co-feed on the MTO reactions over SAPO-34

The methanol-to-olefin (MTO) process occurs by the hydrocarbon pool mechanism and has auto-catalytic and rapid deactivation behavior. This work studied the effect of alkene co-feeds on the MTO reactions over a SAPO-34 zeolite and included the correlation of activity with characterization of the species formed inside the zeolite. Under the reaction conditions used, an alkene co-fed with methanol shortened the induction period by promoting the formation of the co-catalytic aromatic species. Propene and butene were more effective than ethene in shortening the induction period. Analysis of the species formed inside the zeolite indicated that deactivation of the catalyst was not due to coke formation but due to increased internal diffusion resistance. Although an alkene co-feed promoted the formation of hydrocarbon pool species, however, it also caused faster deactivation because the higher alkene concentration promoted an over-abundance of aromatics. To be a better industrial MTO catalyst, the diffusion length of the SAPO-34 zeolite should be small (<0.2 μm) and/or the acidic cage fraction should be low (<73%) to avoid severe internal diffusion resistance. Co-fed alkene will benefit a MTO or MTP process in the start-up period, especially in a fixed bed reactor.