Para-selective chlorination of benzyl chloride to 4-chlorobenzyl chloride over zeolite catalysts

Abstract

The liquid-phase chlorination of benzyl chloride (BC) has been investigated in the presence of a series of zeolite catalysts at 353 K under atmospheric pressure. A comparative study reveals that of these catalysts zeolite H(26.1) K–L exhibits the highest rate of BC conversion (98.4 mmol g −1 h −1) and zeolite K–L the highest selectivity of 4-chlorobenzyl chloride (4-ClBC/2-ClBC=3.7). In the absence of any catalyst or with zeolites K–X and K–Y mainly the side chain chlorination of BC to α,α-dichlorotoluene is observed. The effects of reaction time, solvents, catalyst concentration and reaction temperature are also examined. A combination of solvent and the reaction temperature not only affect the rate of BC conversion but also enhance markedly the isomer ratio of 4-ClBC/2-ClBC. 1,2-Dichloroethane is the best solvent and the isomer ratio of 4-ClBC/2-ClBC and the rate of BC conversion over zeolite K–L are enhanced from 2.21 to 7.21 and 13.2 to 51.9 mmol g −1 h −1 (in the presence of solvent), respectively, when the reaction temperature is raised from 313 to 353 K. Further, the use of 1,2-dichloroethane as solvent in the reaction decreases the formation of side chain and consecutive products significantly. As the reaction time and amount of the catalyst in the reaction mixture are increased, an increase in the conversion of benzyl chloride is noticed. The zeolite K–L is recycled three times with a decline in catalytic activity. The probable mechanism involves the formation of an electrophile (Cl +) from Cl 2 gas over zeolite catalyst which reacts with BC to form the ring-chlorinated products.