Intermediates and Dominating Reaction Mechanism During the Early Period of the Methanol-to-Olefin Conversion on SAPO-41

摘要

The formation and evolution of initial reaction intermediates as well as the reaction mechanism during the early period of the methanol conversion on the silicoaluminophosphate SAPO-41 with one-dimensional and 10-numbered ring pore system was elucidated. According to in situ UVvis spectroscopy, the formation and nature of intermediates formed on the catalysts in the methanol conversion process were monitored. The intermediates remaining on the catalysts after quenching the methanol conversion were determined by ex situ UVvis, 1H MAS NMR, and C-13 MAS NMR spectroscopy, and the reactivity of these species was investigated by adsorption of ammonia and subsequent solid-state NMR spectroscopy. The above-mentioned spectroscopic studies gave a detailed mechanistic insight into the induction period of the methanol conversion on SAPO-41. Monoenylic carbenium ions, being the dominating species during the initial period of the methanol conversion, were rapidly formed and gradually transferred to dienylic carbenium ions, benzene-based carbenium ions, and trienylic carbenium ions, in addition to three-ring compounds and dienes with different chain lengths. On the basis of these spectroscopic observations and the catalytic results, the olefin-based reaction cycle is disclosed to be the dominating reaction mechanism in the initial period of the methanol conversion on SAPO-41.