Phosphotungstic acid supported on acid-leached porous kaolin for friedel-crafts acylation of anisole

Abstract

Porous clay heterostructures (PCH) was derived from natural kaolin through intercalation with cationic potato starch as the template. Leaching of PCH was performed in concentrated acid solutions consisting of HCl and H2SO4. Phosphotungstic acid (HPW) supported on PCH and modified PCH were synthesized by wet impregnation method. The resulting PCH showed remarkable increase in surface area starting from 15 m2g–1 for the parent kaolin to maximium value of 725 m2g–1 for PCH. Acidity studies by pyridine adsorption and FTIR spectra showed that both natural kaolin and PCH possessed strong Lewis acid sites. In contrast, the surface acidity of HPW supported on PCH was significantly enhanced and comprising mainly Brönsted acid sites. The correlation between the Brönsted to Lewis acid ratios (B/L) and either conversion or selectivity of the catalysts has been studied in Friedel-Crafts acylation of anisole. The PCH/30HPW catalyst with the highest number of Brönsted acid sites showed excellent catalytic activity giving 86% conversion of anisole and high selectivity of 95% toward p-methoxypropiophenone.