Highly selective chlorination of 1,2-dichlorobenzene to 1,2,4-trichlorobenzene using a combination of zeolite K-L and ClCH 2COOH

Abstract

The catalytic liquid phase chlorination of 1,2-dichlorobenzene (1,2-DCB) is investigated over a number of zeolite catalysts. Both K-L and K-beta are effective catalysts for the selective conversion of 1,2-DCB to 1,2,4-trichlorobenzene (1,2,4-TCB). The performance of zeolite K-L ( 1,2,4-TCB 1,2,3-TCB =6.2 ) is compared with that of conventional catalyst, FeCl 3 ( 1,2,4- TCB 1,2,3-TCB =2.1 ). The selectivity for 1,2,4-TCB ( 1,2,4- TCB 1,2,3-TCB =15 ) is found to markedly increase when a combination of a co-catalyst (ClCH 2COOH) and zeolite K-L is used. The IR spectra of K-L impregnated with ClCH 2COOH show the transformation of some ClCH 2COOH molecules into ZeolOH..OCCH 2Cl; by reaction with K +-ions of the zeolite which creates steric hindrance in the zeolite channels and enhances the selectivity for 1,2,4-TCB. The rate of 1,2-DCB conversion catalysed by zeolite K-L at 428 K is about 49.1 mmol g −1h −1. For comparison, the rates of 1,2-DCB conversion catalysed by zeolite H·K-L and FeCl 3 under identical conditions are estimated to be 63.3 and 61.7 mmol g −1 h −1, respectively. Acidic H·K-L is found to be more active but poorly selective compared to basic K-L. The reaction temperature, catalyst concentration and ratio of ClCH 2COOH/catalyst are the important parameters for controlling catalyst activity. The rate of 1,2-DCB conversion and product yields increase with the increase in reaction temperature (up to 418 K) and catalyst concentration, whereas both decrease monotonically with the increase in ClCH 2COOH/catalyst ratio. HCl treated zeolite K-L shows a lower activity than the untreated K-L. The zeolite K-L is recycled four times without the loss of 1,2,4- TCB 1,2,3-TCB isomer ratio but with a decline in trichlorobenzene catalytic activity resulting from a minor dealumination and K +-exchange of zeolite K-L with HCl (produced in the reaction). A reaction path is proposed in which chlorine gas (Cl 2) is first polarised (Cl δ+ xh Cl δ−) by the zeolite catalyst. Thus the generated electrophile (Cl +) attacks the ring of 1,2-DCB resulting in the formation of trichlorobenzenes.