QM/MM study of the reactivity of zeolite bound methoxy and carbene groups
The conversion of methanol-to-hydrocarbons (MTH) is known to occur via an autocatalytic process in zeolites, where framework-bound methoxy species play a pivotal role, especially during catalyst induction. Recent NMR and FT-IR experimental studies suggest that methoxylated zeolites are able to produ...
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Published in | Physical chemistry chemical physics : PCCP Vol. 23; no. 32; pp. 17634 - 17644 |
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Main Authors | , , |
Format | Journal Article |
Language | English |
Published |
England
Royal Society of Chemistry
28.08.2021
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Subjects | |
Online Access | Get full text |
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Summary: | The conversion of methanol-to-hydrocarbons (MTH) is known to occur
via
an autocatalytic process in zeolites, where framework-bound methoxy species play a pivotal role, especially during catalyst induction. Recent NMR and FT-IR experimental studies suggest that methoxylated zeolites are able to produce hydrocarbons by a mechanism involving carbene migration and association. In order to understand these observations, we have performed QM/MM computational investigations on a range of reaction mechanisms for the reaction of zeolite bound methoxy and carbene groups, which are proposed to initiate hydrocarbon formation in the MTH process. Our simulations demonstrate that it is kinetically unfavourable for methyl species to form on the framework away from the zeolite acid site, and both kinetically and thermodynamically unfavourable for methyl groups to migrate through the framework and aggregate around an acid site. Formation of carbene moieties was considered as an alternative pathway to the formation of C-C bonds; however, the reaction energy for conversion of a methyl to a carbene is unfavourable. Metadynamics simulations help confirm further that methyl species at the framework acid sites would be more reactive towards formed C
2+
species, rather than inter-framework migration, and that the role of carbenes in the formation of the first C-C bond will be
via
a concerted type of mechanism rather than stepwise.
The conversion of methanol-to-hydrocarbons (MTH) is known to occur
via
an autocatalytic process in zeolites, where framework-bound methoxy species convert to different types of hydrocarbons, but the exact reaction routes leading to the first C-C bond remain elusive. |
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Bibliography: | 10.1039/d1cp02535j Illustrations are provided of the general model of the GM and MM relaxed and fixed regions (Fig. S1), and a schematic of the CV applied in MTD simulations (Fig. S2). Also provided in tabular form is geometric data for the H-ZSM-5 unit cell after NPT simulations (Table S1) and geometric and electronic observables of methanol (Table S2), methoxy (Tables S3 and S4) and carbene models (Tables S5 and S6). Electronic Supplementary information (ESI) available at DOI ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
ISSN: | 1463-9076 1463-9084 |
DOI: | 10.1039/d1cp02535j |