HIGHLY SELECTIVE CHLORINATION OF 1,2-DICHLOROBENZENE TO 1,2,4-TRICHLOROBENZENE USING A COMBINATION OF ZEOLITE K-L AND CLCH2COOH

The catalytic liquid phase chlorination of 1,2-dichlorobenzene (1,2-DC B) 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 t o 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 catal yst, FeCl3 (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 combin ation of a co-catalyst (ClCH2COOH) and zeolite K-L is used. The IR spe ctra of K-L impregnated with ClCH2COOH show the transformation of some ClCH2COOH molecules into [GRAPHICS] by reaction with Ki-ions of the z eolite which creates steric hindrance in the zeolite channels and enha nces the selectivity for 1,2,4-TCB. The rate of 1,2-DCB conversion cat alysed by zeolite K-L at 428 K is about 49.1 mmol g(-1)h(-1). For comp arison, 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 mm ol 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 importa nt parameters for controlling catalyst activity. The rate of 1,2-DCB c onversion and product yields increase with the increase in reaction te mperature (up to 418 K) and catalyst concentration, whereas both decre ase monotonically with the increase in ClCH2COOH/catalyst ratio. HCl t reated 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 act ivity resulting from a minor dealumination and Kt-exchange of zeolite K-L with HCl (produced in the reaction). A reaction path is proposed i n which chlorine gas (Cl-2) is first polarised (Cldelta+---Cldelta-) b y the zeolite catalyst. Thus the generated electrophile (Cl+) attacks the ring of 1,2-DCB resulting in the formation of trichlorobenzenes. ( C) 1997 Elsevier Science B.V.