Green synthesis of methyl salicylate using novel sulfated iron oxide–zirconia catalyst

Abstract

Esters of salicylic acid are in demand for the synthesis of drugs, food preservatives, pharmaceuticals, perfumes and solvents. A novel sulfated Fe2O3–ZrO2 catalyst with different iron loadings was prepared by the combustion technique and used in the preparation of methyl salicylate from salicylic acid and dimethyl carbonate. This process is a viable and safe substitute to other esterification processes, involving harmful alkylating agents and chemicals. The activity of sulfated Fe2O3–ZrO2 was evaluated for the esterification of salicylic acid with dimethyl carbonate. No solvent was used and hence the quality of the product was excellent. The product methyl salicylate is extensively used in the food and pharma industries. Catalysts with different loadings of iron on zirconia (5, 10, 15, 20 wt.%) were synthesized, and their activities evaluated vis-a-vis ZrO2 and sulfated zirconia. The catalysts were characterized using SEM, XRD, FTIR, TGA, NH3-TPD and BET surface area analysis. The activity of the catalysts increases after the loading of iron on zirconia and 10 wt.% sulfated Fe2O3–ZrO2 gave the highest acidity and activity for the synthesis of methyl salicylate. The effect of various kinetic parameters on the rate of esterification of salicylic acid was studied. The optimum conditions for 99% conversion of salicylic acid with 100% selectivity was 120 °C after 150 min at a 1:10 molar ratio of salicylic acid to dimethyl carbonate and catalyst loading of 0.03 g/cm3. The reaction mechanism and kinetics were studied. The activation energy for this reaction was found to be 13.82 kcal/mol.