HYDROXY-GROUPS IN FERRISILICATES STUDIED BY IR SPECTROSCOPY

Bridging Si-OH-Fe groups, as well as three kinds of silanol Si-OH group (3724, 3730 and 3740 cm-1) have been found in both Na-ferrisilicate and H-ferrisilicate. Studies of 2,2,4-trimethylpentane sorption revealed the presence of only Si-OH (3740 cm-1 vibration) on the external faces of the crystals. Si-OH groups vibrating at 3724 and 3730 cm-1 were localized inside the channels. It was also shown that the 3724 and 3730 cm-1 hydroxy groups were formed during the decomposition of the template. Bridging Si-OH-Fe hydroxy groups in H-ferrisilicates were found to be more sensitive to dehydroxylation than Si-OH-Al in zeolites. Benzene sorption experiments demonstrated that the Si-OH-Fe groups are heterogeneous; a few kinds of hydroxy groups with different acid strengths exist. The acid strength of all of these Si-OH-Fe groups was found to be lower than that of the corresponding Si-OH-Al in HZSM-5. This is explained in terms of the lower electronegativity of Fe than that of Al. In contrast to HZSM-5 zeolite, partial dehydroxylation does not change the distribution of the acid strengths of the Si-OH-Fe groups in H-ferrisilicate.