Acidity of Substituted MCM‐41‐Type Mesoporous Silicates Probed by Ammonia.
1997
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 Bronsted 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 Bronsted sites) and at ca. 1620 and
1300 cm
-1
(ammonia located at weak and strong Lewis sites,
respectively). The vibration bands of bridging Bronsted 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 Bronsted sites falls in the same order. The
strength of medium-to-strong Bronsted 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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