SO 42−/TiO 2-SiO 2 mixed oxide catalyst : 2. Effect of the fluoride ion and calcination temperature on esterification of acetic acid

Abstract

A series of fluoride-modified SO 4 2−/TiO 2-SiO 2 (4 wt.% SO 4 2−) catalysts have been prepared by an incipient wetness impregnation technique and compared for their catalytic activity/selectivity in esterification of acetic acid and cumene conversion reactions. The characterisation of the catalysts was performed using X-ray powder diffraction (XRD), infrared spectroscopy (IR), thermal analysis (TG-DTA), nitrogen adsorption–desorption methods, surface acid strength by Hammett indicator method, surface hydroxyl groups and sulphate content by titrimetric method. TiO 2-SiO 2 mixed oxides are amorphous up to 1073 K, whereas crystallisation starts with sulphation and fluorination at a lower temperature, 923 K. Sulphate ion enhances the surface area, whereas fluoride ion decreases it. The specific surface area of a particular wt.% promoted fluoride ion (2 wt.%) enhances with increase in calcination temperature up to 723 K and thereafter decreases on further heating up to 923 K. Surface hydroxyl groups measurement and IR result shows that both the internal weakly H-bonded hydroxyl groups and free hydroxyl groups are present on the surface of the mixed oxides. All types of OH groups are reduced in number by anion treatments (both SO 4 2−, F −, SO 4 2− with F −) as well as on calcination. However, all catalysts retain some hydroxyl groups even after anion treatment, which increases the strength of the remaining acid sites, as observed from Hammett indicator method as well as cumene conversion reaction. The simultaneous presence of SO 4 2− and F − exerts a stronger effect than the presence of only one of these modifiers. The dispersed F − stabilises the SO 4 2− ions that are co-adsorbed on TiO 2-SiO 2 mixed oxides and the increase of surface sulphate concentration as well as the strength of the acid sites accounts for the higher rate of esterification of acetic acid.