Isotopic tracer studies of methanol-to-olefins conversion over HSAPO-34: The role of the olefins-based catalytic cycle

Abstract

The concurrent propagation of the aromatics-based and olefins-based catalytic cycles at early stages of the methanol-to-olefins reaction over HSAPO-34 and the resulting consequences on light olefins selectivities are demonstrated with 13C3-propylene/12C2-dimethyl ether isotopic tracing studies at 623K and sub-complete dimethyl ether conversions. Transients in effluent product selectivities were rationalized by the maturation of the entrained hydrocarbon pool where catalyst turnover number is introduced as a compendious descriptor of hydrocarbon pool maturity and reaction progress. The distinct 13C-content of ethylene from other effluent products and its agreement with the 13C-content of entrained polymethylbenzenes indicate that ethylene is a product of aromatics-based dealkylation events while the match between methylation-predicted and experimentally observed 13C-contents for C5+ olefins establishes that they are products of olefins-based methylation events. Methanol-to-olefins conversion proceeds through a dual cycle mechanism proposed earlier for methanol conversion over other solid acid catalysts where the topology of HSAPO-34 specifically engenders the prevalence of the aromatics-based cycle at >∼200molCmolH+-1 catalyst turnovers.