Improvement of a Bulk Optical Basicity Table for Oxidic Systems

Abstract

Optical basicity (Λ), defined by J. A. Duffy, can be an interesting parameter for the characterization of acid–base properties and the choice of solids used, for example, in glass working or heterogeneous mild oxidation catalysis. In this last field, the Lewis acid–base property of a catalyst is relevant. The purpose of the present work is to complete and improve an optical basicity table of ionic–covalent oxides and oxysalts, despite the lack of data for these solids. Values of optical basicity for most covalent transition metal oxides are indeed often missing or wrongly assessed. Furthermore, catalysts in mild oxidation of hydrocarbons mainly contain transition metal oxides with covalent bonds. To overcome these difficulties, attempts were made to correlate ICP (the ionic–covalent parameter defined by J. Portier) and Λ of simple oxides. Four straight lines are observed depending on the electron configuration of the cations (s–p,d1–d9,d10,d10s2). On the other hand, for highly covalent oxides (d0electron configuration), Λ values were assumed based only on experimental logical considerations, thus giving a fifth correlation line. These results allow the calculation of Λ values for any mixed oxide, depending on the valence and coordination numbers of each ion, and will help with the prediction of the Lewis acid–base and hence, the catalytic properties of the solids concerned.