Hydroxy groups in ferrisilicates studied by IR spectroscopy

Abstract

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—AI 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—AI in HZSM-5. This is explained in terms of the lower electronegativity of Fe than that of AI. 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.