Strength of Brønsted acid sites in boralites

Abstract

H-Boralites contain OH groups vibrating at ca. 3700–3720 cm^–1(Si—OH⋯B groups, Brønsted acid sites), 3450 cm^–1(Si—OH⋯O) and 3670 cm^–1(B—OH). The last two kinds of hydroxy group are fragments of Si—OH⋯[graphic omitted]—B units. The amounts of both 3450 cm^–1(Si—OH⋯O) and 3670 cm^–1(B—OH) groups decrease with activation temperature. This is due to the dehydroxylation Si—OH⋯[graphic omitted]—B → Si—O—B + H₂O. On the other hand, the concentration of Si—OH⋯B that are Brønsted acid sites is practically independent of activation temperature. The increase of activation temperature increases, however, the acid strength of Si—OH⋯B. This is evidenced by IR studies of OH groups interacting with benzene and n-hexane, as well as by IR studies of pyridine thermodesorption. The increase of the acid strength of Si—OH⋯B at high activation temperatures is explained by the loss (by dehydroxylation) of Si—OH⋯O and B—OH groups situated in the near vicinity of Si—OH⋯B.