In a recent investigation of methanol to olefins (MTO) on H-ZSM-5, deactivation in a continuous stirred tank reactor (CSTR) was compared with a plug-flow reactor (PFR).1 At similar initial conversion levels, and comparable contact times, the rate of deactivation is remarkably slower in the CSTR than in the PFR. This indicates that the methanol concentration (that is low and equal to the CSTR exit concentration for the CSTR, but high and an averaged concentration from the initial to exit concentrations for the PFR), has a direct affect on deactivation. In a follow-up investigation of methanol chemistry in the initial stages of reaction, it was demonstrated that methanol dehydrogenation (to formaldehyde and hydrogen) and disproportionation (to formaldehyde and methane) occur, even in the absence of Brønsted acid sites, e.g. on silicalite-1.2 It is through these reactions, and the resulting formaldehyde concentration, that methanol affects deactivation. The Bhan group, working with the CHA topology, have contributed two relevant papers on the issue of direct affect of methanol partial pressure on deactivation.3,4 How formaldehyde, and diffusional limits on its reaction products, control deactivation will be discussed in the light of work in the CHA topology of HSSZ-13 (aluminosilicate CHA structure)3 and very recent work on the diffusional aspects of deactivation of HSAPO-34 (silicoaluminophosphate structure).

Deactivation of methanol to olefins revisited

2019 Fall National Meeting

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