Understanding the Early Stages of the Methanol-to-Olefin Conversion on H‑SAPO-34

Abstract

Little is known on the early stages of the methanol-to-olefin (MTO) conversion over H-SAPO-34, before the steady-state with highly active polymethyl­benzenium cations as most important intermediates is reached. In this work, the formation and evolution of carbenium ions during the early stages of the MTO conversion on a H-SAPO-34 model catalyst were clarified via ¹H MAS NMR and ¹³C MAS NMR. Several initial species (i.e., three-ring compounds, dienes, polymethyl­cyclopentenyl, and polymethyl­cyclohexenyl cations) were, for the first time, directly verified during the MTO conversion. Their detailed evolution network was established from theoretical calculations. On the basis of these results, an olefin-based catalytic cycle is proposed to be the primary reaction pathway during the early stages of the MTO reaction over H-SAPO-34. After that, an aromatic-based cycle may be involved in the MTO conversion for long times on stream