Presituated “coke”-determined mechanistic route for ethene formation in the methanol-to-olefins process on SAPO-34 catalyst

Abstract

Control of product selectivity is a central yet challenging issue in catalysis chemistry, both in academia and industry. The ever-increasing market demand for ethene, together with relatively low ethene selectivity, necessitates the upgrading of the current methanol-to-olefins (MTO) industry to enhance ethene selectivity. We report here an operable strategy, 1-butene “precoking” technology, by which ethene selectivity has been unprecedentedly enhanced while the catalyst lifespan has been conserved. We also made effects to provide more insight into the long-standing controversies regarding the mechanistic origin of the increase in ethene selectivity. With the aid of a 12C-/13C-methanol switch experiment, MD calculations, and FTIR analysis, it was demonstrated that ethene increase is neither simply—or solely—caused by the configurational diffusion effect, nor due to the transition-state shape selectivity or methylnaphthalene species, but rather to the spatial siting of the presituated “coke”. The relatively evenly distributed presituated coke extends the reaction zone toward the near-core of the SAPO-34 crystal, elongating the diffusion trajectories of molecules and allowing more acid sites to be utilized. The diffusion-hindered higher olefinic intermediates tend to evolve to active aromatic species, which, in turn, lead substantially to ethene. This mechanistic understanding could be also applicable to explaining ethene increase with time on stream (TOS) on parent SAPO-34.