STRENGTH OF BRONSTED ACID SITES IN BORALITES

H-Boralites contain OH groups vibrating at ca. 3700-3720 cm-1 (Si-OH ... B grouPs, Bronsted acid sites), 3450 cm-1 (Si-OH ... O) and 3670 cm-1 (B-OH). The last two kinds of hydroxy group are fragments of [GRAPHICS] 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 [GRAPHICS] On the other hand, the concentration of Si-OH ... B that are Bronsted 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 ... 0 and B-OH groups situated in the near vicinity of Si-OH ... B.