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

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(-)MIDLINE HORIZONTAL ELLIPSIS(CH3)(3)O+, 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)(3)O+, which could be regarded as a mobile methyl cation carrier ((CH3)(2)O-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(-)MIDLINE HORIZONTAL ELLIPSIS(CH3)(3)O+ and Z-CH3 rather than through the well-accepted concerted mechanism via CH3OH and CH3OCH3 regardless of temperature. Z(-)MIDLINE HORIZONTAL ELLIPSIS(CH3)(3)O+ 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(-)MIDLINE HORIZONTAL ELLIPSIS(CH3)(3)O+ 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.

Automated summary

A study of ethene methylation over H-ZSM-5 using a multiscale approach revealed the importance of methanol dehydration products as methyl donors, rapid formation of propene, and the essential role of a mobile methyl cation carrier in the process.