Titration of H2SO4/SiO2-mounted catalysts in cyclohexane with pyridine based on a new acidity function considering the effect of physisorption sites

Abstract

A spectroscopic and visual titration of H₂SO₄/SiO₂-mounted catalysts in cyclohexane with pyridine has been carried out by applying a new acidity function S′_A in which the strength and amounts of acid and physisorption sites are taken into consideration. The S′_A function is defined by S′_A= pβ_I+ pK_{P, I}+ log([PI]/[AI]) for a surface on which a kind of acid site and a kind of physisorption site exist, where pβ_I is the basic strength of a Hammett indicator and K_{P, I} represents the equilibrium constant of the indicator for physisorption; [PI] and [AI] are the amounts of the indicator adsorbed on physisorption and acid sites, respectively. The physical properties of silica gel used here as the carrier were estimated based on the adsorption isotherm of pyridine and several indicators. It was revealed through simulation of the base titration system based on the S′_A function that the strength and amounts of physisorption sites influence the estimate of acid strength distribution by the titration method, and that existence of strong physisorption sites often makes the titration impossible. The results of the improved spectroscopic titration showed that acid sites equal to the amount of sulphuric acid exist on the mounted catalyst. The strength of these sites was estimated by analysing the obtained S′_A curve and was expressed in terms of the equilibrium constant for pyridine adsorption. At the same time, the relative strength of several indicators on the catalyst was determined. From the results of the improved visual titration, it was revealed that acid sites that correspond to the amount of sulphuric acid exist on each of the three kinds of mounted catalyst. It was pointed out that the physical meaning of the strength and amount of acid sites obtained by the conventional method is very obscure.