Abstract
Four kinds of hydroxyls are present in H-boralite: 3450 cm-1 (Si-OH...O), 3670 cm-1 (B-OH), 3700-3720 cm-1 (Si-OH...B) and 3740 cm-1 (Si-OH). Only 3700-3720 cm-1 (Si-OH...B) hydroxyls are Bronsted acid sites. Their concentration, as well as the concentration of Lewis acid sites, was found to be independent of the activation temperature (in the range 473-873 K). On the other hand, both 3450 and 3670 cm-1 hydroxyls are very prone to dehydroxylation which starts at as low temperature as 473 K. This is due to a reaction: Si-OH + O(H)-B → SiOB + H2O. It has been found that the stretching frequency of Si-OH···B groups (Bronsted acid sites) depends on the activation temperature (3700 cm-1 at 573 K and 3720 cm-1 at 773 K) suggesting a variation of the properties of Bronsted sites with temperature. We explain it by the loss of Si-OHO and B-OH hydroxyls situated in close vicinity of Si-OH...B ones.
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