posted on 2024-01-02, 06:05authored bySyeda
R. Batool, Vitaly L. Sushkevich, Jeroen A. van Bokhoven
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.