Selective Para Chlorination of o-Xylene over Zeolite Catalysts

Abstract

The liquid phase chlorination of diphenylmethane (DPM) to 4,4′-dichlorodiphenylmethane (4,4′-DCDPM) is investigated at 333 K, under atmospheric pressure over a number of zeolite catalysts using sulfuryl chloride (SO2Cl2) as the chlorinating agent. The results obtained are compared with those over the conventional Lewis acid catalyst, AlCl3 as well as without any catalyst. Zeolite K-L is found to be highly active and selective catalyst for the conversion of DPM to 4,4′-DCDPM. The conversion of DPM, rate of DPM conversion and the selectivity (4,4′-DCDPM/2,4′-DCDPM isomer ratio) over zeolite K-L after 1 h of reaction time are found to be 96.8 wt.%, 19.1 mmol g−1 h−1 and 7.4, respectively. The influence of solvent, catalyst concentration, reaction temperature, DPM/SO2Cl2 molar ratio, recycle of zeolite K-L, etc. are also examined. 1,2-Dichloroethane is the best solvent and gives the highest selectivity for 4,4′-DCDPM (4,4′-DCDPM/2,4′-DCDPM isomer ratio=9.7) with zeolite K-L at 353 K after 1 h of reaction time. The formation of 4,4′-DCDPM is favoured by increase in catalyst concentration, reaction temperature and higher concentration of SO2Cl2 (lower DPM/SO2Cl2 molar ratio). In all these cases, the yield of 4,4′-DCDPM increases with a decrease in the yield of 4-CDPM which suggests that the formation of 4,4′-DCDPM takes place by the consecutive reaction of 4-CDPM. Higher SiO2/Al2O3 ratio (obtained by HCl treatment) of zeolite K-L decreases the conversion of DPM. A noticeable decrease in the activity and selectivity of zeolite K-L is observed on recycling, probably due to reduced crystallinity as well as extraction of small amounts of Al+3 and K+ ions by the HCl, generated in the reaction. Mechanistically, SO2Cl2 is first decomposed into SO2 and Cl2 the latter being polarized by the zeolite catalyst to an electrophile (Cl+) which then attacks the DPM and subsequently produce the monochlorodiphenylmethane (MCDPM). The MCDPM further is attacked by the electrophile (Cl+) and result in the formation of DCDPM.