Esterification of salicylic acid with methanol/dimethyl carbonate over anion-modified metal oxides

Abstract

The esterification of salicylic acid to methyl salicylate over solid acids such as zirconia, alumina and silica and their sulphate, phosphate and borate modified forms, has been studied for the first time in an autoclave under autogenous conditions using methanol and dimethyl carbonate to compare their methylating ability. Metal oxides such as zirconia, alumina and silica as well as their sulphate, phosphate and borate modified forms are effective catalysts for the conversion of salicylic acid to methyl salicylate. Superacid, sulphated zirconia, is the most suitable catalyst due to high yield, selectivity, reusability and the simple method used for the preparation of the catalyst. The percentage yield of methyl salicylate was found to be higher when methanol was used as the methylating agent over these catalysts rather than dimethyl carbonate. Hence, methanol appears to be a better methylating agent compared to dimethyl carbonate in terms of yield as well as atom economy. The influence of reaction temperature, amount of catalyst, duration of reaction and molar ratio of salicylic acid: methanol/dimethyl carbonate has been studied over sulphated zirconia.