The catalytic activity of zeolites originates from the crystalline aluminosilicate structure, which yields Brønsted acid sites (BAS) and the defects, which give rise to Lewis acid sites (LAS). Bronsted acidity is well-defined as it stems from a hydroxyl group, which bridges the framework silicon and aluminum atoms of the zeolite. The structure of LAS, however, is much less defined. We introduce Lewis acidity into zeolite Y (Si/Al=30) by a facile ion-exchange of aluminum cations followed by calcination. X-ray diffraction and nitrogen physisorption suggest that the zeolitic framework is retained after these treatments. With an increase in the total extra-framework aluminum in the modified zeolites, there was a pronounced increase in the number of Lewis acid sites, as illustrated by FTIR spectroscopy of adsorbed pyridine and carbon monoxide, whereas no significant change in the number of Brønsted acid sites was observed. Meerwein–Ponndorf–Verley reduction of aldehydes and ketones is widely studied reaction for the exploration of Lewis acid zeolite catalysts because of highly selective nature of this reaction. The catalytic activity, i.e. the conversion and rate of reaction for the MPV reduction of 4-tert butyl cyclohexanone using isopropanol over zeolite catalysts, improved considerably after ion-exchange. A quantitative agreement was observed between the concentration of these introduced extra-framework aluminum species with the total Lewis acid content from pyridine-FTIR and with the initial catalytic rates. This illustrates that the newly introduced aluminum Lewis acidic species adopt an octahedral coordination under the conditions of NMR measurement and are responsible for enhanced Lewis acid catalytic activity of the aluminum exchanged zeolites. The preservation of Brønsted acid sites further endorses the charge neutrality of these extra-framework aluminum complexes.

Lewis acid activity of extra-framework aluminum introduced in zeolite Y by ion-exchange

ACS Spring 2022

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