Liquid phase benzoylation of diphenyl oxide (DPO) to 4-(benzoyl)-diphenyl oxide (p-isomer) with benzoyl chloride (BC) has been investigated with 12-tungstophosphoric acid supported on zirconia (TPA/ZrO2) as the catalyst. Among the catalysts, 15 wt.% TPA/ZrO2 calcined at 750degreesC is found to be more active than others in the benzoylation reaction. The acid strengths of different TPA loaded catalysts are estimated by TPD of NH3 and among these 15 wt. %TPA/ZrO2 calcined at 750degreesC has the highest acidity and hence more active in the reaction. The optimization of reaction conditions of benzoylation of DPO by BC is performed with 15 wt. %TPA/ZrO2 calcined at 750 degreesC (higher acidity) by varying catalyst concentration, 1-5 (wt.% of reaction mixture), temperature, 100-140degreesC and DPO:BC molar ratio 1-15. Under the optimized reaction conditions, conversion of benzoyl chloride (39.3%) gave high selectivity to 4-(benzoyl) diphenyl oxide (97.4%) and remaining 2-(benzoyl) diphenyl oxide in 3 It. The reaction is found to be heterogeneously catalyzed and no contribution from homogeneous (leached) TPA into reaction medium. (C) 2004 Elsevier B.V. All rights reserved.