Estimation of acidic and physical properties of solid acids based on adsorption of basic molecules in non-polar solvents

Abstract

The adsorption equilibrium among a solid acid having both acidic and physically adsorptive sites, a base and a Hammett indicator have been investigated theoretically to derive a new surface acidity function S_A and a physical adsorptivity function S_P. A system composed of silica–alumina as a solid acid, pyridine as a base and each Hammett indicator have been experimentally treated for application of S_A and S_P. S_A and S_P are defined by S_A=–log (Σ^m_{i= 1}α_i[A_i]) where α_i and [A_i] are the strength and amount of the ith group of vacant acid sites, respectively, and S_P=–log (Σⁿ_{j= 1}µ_j and [P_j]), where µ_j and [P_j] are the strength and amount of the jth group of vacant physical adsorptive sites, respectively. Silica–alumina dispersed in benzene or benzene–cyclohexane was neutralized to various degrees with pyridine in the presence of one of five Hammett indicators and the solid and liquid phases were subjected to u.v.–visible spectrophotometric measurements. A data analysis based on S_A has revealed that the silica–alumina contains 0.11 m mol g^–1 acid sites of –log α=–3.9 ± 0.6 as referenced to –logβ= 1.5, assigned as a standard for 4-(phenylazo)diphenylamine, a Hammett indicator, where β is the strength of a base in a similar sense to α. Relative strengths have been evaluated for the five Hammett indicators as basic and physical adsorbates to the silica-alumina. An experimental criterion has been set up for the application of S_A to the determination of the strength–amount distribution for a solid acid.