Facile synthesis of dicamba ester over heterogeneous magnesium oxide and kinetic modelling

Abstract

Dicamba (3,6-dichloro-2-methoxybenzoic acid) is one of the widely used herbicides. Methyl 2-methoxy-3,6-dichloro benzoate (Dicamba ester) is a key intermediate which upon hydrolysis forms dicamba. For the first time esterification as well as etherification of 3,6-dichloro-2-hydroxybenzoic acid (3,6-dichlorosalicylic acid) was studied to achieve dicamba ester as a product using dimethyl carbonate (DMC) as methylating agent as well reaction solvent and MgO as catalyst. Reaction proceeds through the formation of two intermediates, namely, methyl 3,6-dichloro-2-hydroxybenzoate (3,6-Dichlorosalicylate) and dicamba. DMC has emerged as green reagent substituting other alternatives like methyl halide and dimethyl sulphate. Different heterogeneous base catalysts were synthesized and their activity and selectivity studied in the formation of dicamba ester. Calcinated hydrotalcite (CHT), calcinated hydrotalcite supported on hexagonal mesoporous silica (CHT-HMS), ZrO₂, Al₂O₃ and MgO were used for this reaction. Combustion synthesized MgO (CS-MgO) was found to be the best catalyst for this reaction. Activity and selectivity of CS-MgO catalyst were excellent. The catalyst was characterised completely with FTIR, DCS-TGA, SEM, TPD, BET and XRD. Reaction proceeds through formation of two intermediates, finally leading into formation of dicamba ester. Reaction mechanism and kinetics were studied to throw light on the exact path of product formation. The catalyst was found reusable with constant activity and selectivity. The overall process is simple and green using active, selective and reusable catalyst with nontoxic alkylating reagent.