Catalysis

Aromatic aldehydes as tuneable and ppm level potent promoters for zeolite catalysed methanol dehydration to DME

Authors

Abstract

Dimethyl ether (DME) is a valuable chemical intermediate and renewable fuel that can be made, via methanol, from many sources of carbon, including carbon dioxide and biomass. Benzaldehyde and its derivatives have been found to be promoters for zeolite catalysed methanol dehydration to DME at low temperature (110 to 150 oC). For the 3-dimensional medium pore zeolite H-ZSM-5 (MFI) the promotion is readily reversible and the potency of the promoter can be tuned by varying the substituent on the aromatic ring of the aldehyde. The most potent promoters are active at concentrations as low as 1 ppm relative to methanol. High throughput experimentation (HTE) is used to screen and rank potential promoters and catalysts and to collect high quality kinetic data for the most promising candidates discovered. The catalytic data and in-situ FT-IR-MS experiments combined with molecular modelling studies indicate a mechanism involving competitive adsorption of the aldehyde promoter on a Brønsted acid (BA) site, followed by reaction with methanol to give a hemi-acetal intermediate. Loss of water from the hemi-acetal intermediate generates a transient and highly reactive methyl oxonium species, [ArC(H)(=O-Me)]+, which then directly reacts with methanol via a SN2 mechanism to give DME and regenerate the aldehyde promoter and BA site. The methyl oxonium species is stabilized by electron-donating groups on the aromatic ring and the solvent like effect of the zeolite pore walls. Molecular descriptors were calculated by molecular modelling for the 22 aromatic aldehyde promoters tested. Multivariate linear regression analysis was used to build an interpretable model for aldehyde promotional activity in H-ZSM-5 and in another 3-dimensional medium pore zeolite, H-ZSM-11 (MEL).

Version notes

File and header name change to ChemRxiv due to typo. Superscript changes to R2 in 2 places, subscript change to second O in NBOO. Some minor changes to bold text (references in some places).

Content

Thumbnail image of Aromatic aldehydes as promoters for zeolite catalysed methanol dehydration to DME 23rd January 2023 ChemRxiv version.pdf

Supplementary material

Thumbnail image of ESI Aromatic aldehydes as promoters for zeolite catalysed methanol dehydration to DME 21st Jan 2023 ChemRxix version.pdf
Electronic Supporting Information : Aromatic aldehydes as tuneable and ppm potent promoters for zeolite catalysed methanol dehydration to DME
Electronic Supplementary Information (ESI) available: Experimental and other additional details for reagents, catalyst testing, catalyst characterisation, IR band assignments, in-situ NMR studies, molecular modelling and MLR analysis plus additional data, figures and associated commentary.