Methanol to hydrocarbons reaction over Hβ zeolites studied by high resolution solid-state NMR spectroscopy: Carbenium ions formation and reaction mechanism

Abstract

Carbenium ions as active intermediates play very important roles in methanol conversion to hydrocarbons (MTH) reactions, while their observation and structure confirmation is of great difficulty. In this contribution, solid-state magic-angle-spinning (MAS) NMR spectroscopy has been applied in the investigation of methanol conversions over Hβ zeolite. By attempting the batch-like and continuous-flow reactions over the catalysts with varied acid site densities and under the condition of different reaction temperatures and atmosphere pressures, an optimized way for direct observation of carbenium ions has been developed. For the first time, two types of carbenium ions, including methylcyclopentenyl (MCP+) cations and heptamethylbenzenium (heptaMB+) cation, have been successfully captured by 13C MAS NMR during methanol conversion over Hβ zeolite. For the batch-like reactions, methanol conversion and carbenium ions observation have been promoted by using the Hβ catalysts with more Brönsted acid sites and performing the reaction under relatively high atmosphere pressure. The competitive adsorption from reactants and products on the catalyst surface may cause the deprotonation of the carbenium ions. In the continuous-flow reactions, both the heptaMB+ and MCP+ cations are formed and more easily observed compared with batch-like reactions due to the improved methanol conversion and the reduced competitive adsorption on the Brönsted acid sites. Based on the carbenium ions identification, a catalytic cycle is proposed to explain the generation of isobutene, a predominantly generated product on Hβ zeolite, during methanol conversion.