Trimethyloxonium ion – a zeolite confined mobile and efficient methyl carrier at low temperatures: a DFT study coupled with microkinetic analysis

Abstract

The reaction network of ethene methylation over H-ZSM-5, including methanol dehydration, ethene methylation, and C₃H₇⁺ conversion, is investigated by employing a multiscale approach combining DFT calculations and microkinetic modeling. The methanol dehydration process produces a mixture of CH₃OH, CH₃OCH₃, Z–CH₃, and Z⁻⋯(CH₃)₃O⁺, which would serve as a methyl donor for methylation. C₃H₇⁺ conversion occurs quickly to yield propene. The results demonstrate that the zeolite ionic environment (Z⁻) stabilized (CH₃)₃O⁺, which could be regarded as a mobile methyl cation carrier ((CH₃)₂O–CH₃⁺) and is an indispensable methylation species under low temperature due to its flexible mobility and the availability of three methyl groups. The propene formation process does mostly occur by means of the stepwise mechanism via Z⁻⋯(CH₃)₃O⁺ and Z–CH₃ rather than through the well-accepted concerted mechanism via CH₃OH and CH₃OCH₃ regardless of temperature. Z⁻⋯(CH₃)₃O⁺ dominates at low temperatures, while Z–CH₃ is prevalent at high temperatures; interestingly, when increasing the CH₃OCH₃ partial pressure in the feed, the methylation through the Z⁻⋯(CH₃)₃O⁺ has a larger contribution at higher temperatures. The traditionally assumed rate law for the zeroth and first order with respect to CH₃OH/CH₃OCH₃ and ethene, respectively, prevails under the conditions that the partial pressure of CH₃OH/CH₃OCH₃ is much larger than that of ethene.