Study on the Acidity and the Catalytic Activity of the Halogenated Silica Gel

Abstract

The acidities and the catalytic activities of silica gels halogenated by treatment with the aqueous solutions of ammonium halides were investigated.
On the surfaces of these halogenated silica gels acid sites were created which were ascertained as Brönsted acid sites by an infrared spectroscopic method. Their total acidities increased with their halogen contents. Their distribution patterns of acid strength were characteristic to the kinds of halogen atoms. The activities for cumene cracking did not directly correlate with the total acidities but depended mainly on the amount of the acid sites stronger than pKa=-5.6 which varied in order of Cl>Br>I>>F.
OH groups on the silica surfaces were partially replaced with OCH₃ groups by treatment with CH₃OH at 260°C, and the total acidities and the catalytic activities for isomerization of 1-pentene were decreased linearly with increases of these OCH₃ groups.
From these results the mechanism of enhancement of the acidity in silica gel by halogenation was discussed.