Chemistry on fluorinated surfaces. Brønsted and lewis acid sites

Abstract

The fluorination of high surface area metal oxides is an important step in the production of heterogeneous catalysts for halogen exchange, e.g. fluorinated γ-alumina or silica. Two situations have been encountered experimentally when the defect spinel γ-alumina is fluorinated at room temperature using a variety of volatile inorganic fluorides : (i) formation of new Brønsted acid surface sites, or (ii) formation of new Brønsted and Lewis sites. Models for both types of behaviour have been developed from an idealised γ-alumina surface by combining the results obtained from several experimental methods. I.r. spectroscopy and [ l8F]-labelling were used to follow the course and extent of the gas-solid reactions, to obtain kinetic data and to determine the lability of surface fluoride. Adsorption-desorption phenomena were studied using [ 14C]- and [ 35S]-labelled molecules and reactions involving but-1-ene or CH 3CCl 3 were used to probe Brønsted and Lewis acid character respectively. Polymerization of a C 2 chloroolefin in the presence of Lewis acid promoted, fluorinated γ-alumina leads to the formation of an organic layer on the surface. The nature of these surfaces, which are also catalytically active, will be discussed in the light of 1H, 13C and 27Al solid state MAS n.m.r. measurements. Support of this work from the SERC and I.C.I. plc, is gratefully acknowledged.