Selective benzoylation of biphenyl to 4-phenylbenzophenone over zeolite H-beta

Abstract

Liquid phase benzoylation of biphenyl (BP) to 4-phenylbenzophenone (4-PBP) with benzoyl chloride (BOC) has been investigated at atmospheric pressure in the presence of various zeolite catalysts. Conventional homogeneous catalyst, AlCl3, is also included for comparison. Under identical reaction conditions, zeolite H-beta is considerably more active than the other zeolite catalysts, whereas AlCl3 shows higher PBP yield but less selectivity than the H-beta in the benzoylation of BP. The conversion of BP, rate of BP conversion (TOF: turnover frequency), selectivity to 4-PBP and 4-PBP/2-PBP ratio over H-beta after 6h of reaction time and at 443K are ca. 41.0%, 31.7 (10−5s−1mol−1Al), 97.4% and 37.4, respectively. For comparison, the conversion of BP, TOF, selectivity to 4-PBP and 4-PBP/2-PBP ratio over AlCl3 after 0.5h of reaction time, under identical reaction conditions, are estimated to be 48.6%, 72.1 (10−5s−1mol−1Al), 80% and 4.0, respectively. A higher amount of 2-PBP (20%) is achieved over non-shape-selective AlCl3 as compared to the zeolite H-beta (2.6%). Acidity and structure of the zeolites play an important role in the conversion of BP, rate of BP conversion and products distribution. Acidity of the zeolite catalysts is measured by the temperature-programmed desorption of ammonia. The effect of reaction time, catalyst concentration, reaction temperature and BOC/BP molar ratio on the catalyst performance is examined in order to optimise the conversion of BP and selectivity to 4-PBP. The conversion of BP using zeolite H-beta is increased significantly with the increases in reaction time, catalyst concentration, reaction temperature and BOC to BP molar ratio. The acetylation and propionylation of BP is also investigated over zeolite H-beta catalyst. In addition, the influence of various acylating agents (BOC, benzoic anhydride, propionyl chloride, propionic anhydride, acetyl chloride, acetic anhydride) is also seen on the conversion of BP and selectivity for the para-products. In the acylation reactions, the conversion of BP decreases in the following order: benzoylation>propionylation>acetylation. H-beta is recycled two times (fresh plus two cycles) and a decrease in BP conversion is observed after each cycle, which is related to the minor dealumination of zeolite catalyst by HCl (formed during reaction). The formation of acylated products of BP is explained by an electrophilic attack of acyl cation (RCO+; where R=C6H5 or CH3CH2 or CH3) on the BP ring, whose formation is facilitated by acid sites of the zeolite catalysts.