Evolution of Methanol Molecules within Pyridine-Modified Mordenite Unveiled by Solid-State NMR Spectroscopy

Pyridine modified Mordenite (Py-MOR) has been successfully applied in multiple methanol-related catalytic reactions, e.g., carbonylation and methanol-to-hydrocarbon conversions, etc.; however, the fundamental insights into the methanol molecules within its interconnected micropores remain elusive. Herein, we comprehensively studied the adsorption and evolution of methanol on Py-MOR under thermal treatment by employing solid-state NMR spectroscopy. An interesting evolution route of methanol (8-MR) -> dimethyl ether -> methanol (12-MR) is observed when increasing the reaction temperature up to 573 K, while no carbon-carbon bond can be formed in the 8-MR and 12-MR channels, possibly due to the confinement effect on methanol either from the narrow 8-MR pores or pyridine partially occupied 12-MR channels. More importantly, the vigorous competitive adsorption on the Bronsted acid site for methanol versus pyridine is experimentally verified by 1H MAS NMR and two-dimensional hetero- and homonuclear correlation NMR spectra. The pyridine-H+ bond of the pyridinium ions in the 8-MR side pockets easily breaks upon methanol adsorption, even at room temperature, while the strongly bonded pyridine-H+ moiety in the 12-MR channel breaks only at elevated temperatures up to 523-573 K, due to the competitive adsorption of methanol. We further show that the CO carbonylation reaction occurs for methanol residing in 8-MR pores of Py-MOR, while the methanol within the 12-MR is "locked" by the pyridine molecules. These fundamental findings are critical for a more comprehensive understanding of the methanol-related reactions on Py-MOR.