Liquid phase bromination of phenol: II. Over heteropoly acid (HPA)-impregnated zirconium phosphate (ZrP)

Heteropoly acid (HPA)-impregnated zirconium phosphate (ZrP) was prepared by an incipient wetness impregnation method by varying the wt.% of HPAs like phosphotungstic acid (PWA), phosphomolybdic acid (PMoA), silicotungstic acid (SiWA) and silicomolybdic acid (SiMoA). All the samples were characterized by X-ray diffraction studies, FTIR, determination of acid sites and TG–DTA. The catalytic activities of all the samples were evaluated by carrying out the liquid phase bromination of phenol using KBr and hydrogen peroxide. The reaction products were analysed by GC through a capillary column. From the product analysis it was found that PWA/ZrP shows the highest conversion. Even so SiWA/ZrP heated at 110 °C shows the highest para-selectivity (in the range of 32–41%), higher than any other HPA/ZrP. The selectivity increases with an increase in the activation temperature up to 400 °C and thereafter it remains constant. The reaction proceeds through the formation of Br+ (bromonium ion) which attacks the phenol ring, forming different brominated products.