Acidity of substituted MCM-41-type mesoporous silicates probed by ammonia

Abstract

The acid properties of Al-, Ga- and Fe-substituted mesoporous silicates have been investigated using ammonia as a probe molecule. Substitution leads to the formation of Brønsted and Lewis acid sites of different type and strength. The IR spectra of ammonia-loaded samples show bending vibration bands near 1450 cm^-1 (ammonium ions at Brønsted sites) and at ca. 1620 and 1300 cm^-1 (ammonia located at weak and strong Lewis sites, respectively). The vibration bands of bridging Brønsted acid hydroxy groups appear at 3605 (Al) and 3591 cm^-1 (Ga). Initial heats of ammonia chemisorption decrease in the order: 185 (Al), 162 (Ga) and 144 (Fe) kJ mol^-1. The degree of isomorphous substitution of framework silicon by trivalent atoms decreases in the order AlGa>Fe. The ratio of strong to medium-strong Brønsted sites falls in the same order. The strength of medium-to-strong Brønsted sites of Al-substituted mesoporous silicate is similar to the strength of corresponding sites in HY. Desorption of ammonia from Lewis sites indicates the existence of a broad acid-site distribution ranging in strength from weak to strong. In contrast, Fe-substituted material contains only Lewis sites of weak acid strength. The results indicate that isomorphous substitution is a useful tool to tailor the acidic properties of mesoporous molecular sieves.

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