SO42-/TiO2-SiO2 mixed oxide catalyst 2. Effect of the fluoride ion and calcination temperature on esterification of acetic acid

A series of fluoride-modified SO42-/TiO2-SiO2 (4 wt.% SO42-) catalysts have been prepared by an incipient wetness impregnation technique and compared for their catalytic activity/selectivity in esterification of acetic acid a nd cumene conversion reactions. The characterisation of the catalysts was p erformed 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 sulp hate content by titrimetric method. TiO2-SiO2 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, whe reas fluoride ion decreases it. The specific surface area of a particular w t.% promoted fluoride ion (2. wt.%) enhances with increase in calcination t emperature up to 723 K and thereafter decreases on further heating up to 92 3 K. Surface hydroxyl groups measurement and IR result shows that both the internal LI weakly H-bonded hydroxyl groups and free hydroxyl groups are pr esent on the surface of the mixed oxides. All types of OH groups are reduce d in number by anion treatments (both SO42-, F-, SO42- with F-) as well as on calcination. However. all catalysts retain some hydroxyl groups even aft er anion treatment. which increases the strength of the remaining acid site s, as observed from Hammett indicator method as well as cumene conversion r eaction. The simultaneous presence of SO42- and F- exert?, II stronger effe ct than the presence of only one of these modifiers. The dispersed F- stabi lises the SO42- ions that are co-adsorbed on TiO2-SiO2 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. (C) 2001 Elsevier Science B.V. All rights reserved.