IR spectroscopic studies of the acidic properties of the mesoporous molecular sieve MCM-41

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Abstract
The concentration, acid strength and thermal stability of the Brønsted- and Lewis-acid sites in the proton form of the mesoporous molecular sieve MCM-41 were monitored by IR spectroscopy as function of the Al content. The results were compared with those for zeolite H+-ZSM-5 with the same elemental composition. The spectra of hydroxy groups in purely siliceous [Si]-MCM-41 and H+-[Si, Al]-MCM-41 contain a single band at 3740 cm–1, assigned to the Si—OH groups, and are independent of the Al content. Sorption of ammonia or pyridine generates bands from the ammonium and pyridinium ions. The concentrations of both Brønsted- and Lewis-acid sites in H+-[Si, Al]-MCM-41 are lower than the concentration of four-coordinate aluminium. The Brønsted sites in H+-MCM-41 are more prone to dehydroxylation than those in zeolite H+-ZSM-5, and the effect increases with the Al content. The acid strength of the Brønsted sites in H+-MCM-41 is independent of the Al content and much lower than in zeolite H+-ZSM-5.

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