Highly selective chlorination of 1,2-dichlorobenzene to 1,2,4-trichlorobenzene using a combination of zeolite K-L and ClCH2COOH Original Research Article

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-TCB1,2,3-TCB=6.2) is compared with that of conventional catalyst, FeCl3 (1,2,4-TCB1,2,3-TCB=2.1). The selectivity for 1,2,4-TCB (1,2,4-TCB1,2,3-TCB=15) is found to markedly increase when a combination of a co-catalyst (ClCH2COOH) and zeolite K-L is used. The IR spectra of K-L impregnated with ClCH2COOH show the transformation of some ClCH2COOH molecules into ZeolOH..OCCH2Cl; 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 FeCl3 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 ClCH2COOH/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 ClCH2COOH/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-TCB1,2,3-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 (Cl2) is first polarised (Clδ+ xh Clδ−) by the zeolite catalyst. Thus the generated electrophile (Cl+) attacks the ring of 1,2-DCB resulting in the formation of trichlorobenzenes.