Factors Affecting the Generation and Catalytic Activity of Extra-Framework Aluminum Lewis Acid Sites in Aluminum-Exchanged Zeolites

Aluminum ion exchange was employed to introduce Lewis acidity into zeolites BEA, mordenite (MOR), MFI, and FAU (Si/Al = 11–15) and thereby evaluate what factors affect the generation and activity of extra-framework Lewis acid sites (LAS) in zeolites. After the treatment, all zeolites retain their framework structure and porosity characteristics, as evidenced by diffraction and nitrogen physisorption. The increase in the total aluminum content in BEA and FAU was appreciable, whereas MOR and MFI showed very little uptake of aluminum. The increase in total aluminum content quantitatively follows the increase in total LAS content determined by Fourier transform infrared (FTIR) spectroscopy of adsorbed pyridine after dehydration and increases the concentration of octahedrally coordinated extra-framework aluminum after hydration, determined by ²⁷Al magic-angle spinning (MAS) and multiple-quantum magic-angle spinning (MQMAS) NMR spectroscopy. Likewise, the catalytic activity for Meerwein–Ponndorf–Verley reduction of 4-tert-butylcyclohexanone changed accordingly with no significant change in MOR and MFI and significant and maximum increase in BEA and FAU. The selectivity of zeolites toward cis respectively trans 4-tert-butylcyclohexanols was affected by the pore size and framework type of the zeolite and not by the number or structure (extra-framework/framework-associated aluminum) of LAS they contain. While the number of LAS in BEA and FAU significantly increased, their total Brønsted acid site (BAS) content remained constant, suggesting that the incorporated LAS are neutral moieties. The incorporation of extra-framework LAS by aluminum ion exchange and their catalytic activity depend on the zeolite framework type, pore size, and possibly on the aluminum location within the zeolite framework.