Photocatalytic esterification of acetic acid with methanol over metal-exchanged phosphotungstate

Abstract

In this work, a series of metal-exchanged phosphotungstate salts, Fe-TPA, Ni-TPA, Cu-TPA, and Cr-TPA, were tested for esterification of acetic acid to methyl acetate under UV irradiation. The results of UV-DRS analysis confirmed the preservation of the Keggin structure, and the presence of metals in the structure, as well as the redshifts compared with P25 in the UV absorbance spectrum. All M−TPA photocatalysts exhibit significantly enhanced activity with respect to P25 for the esterification of acetic acid under UV irradiation, except for Cr-TPA. Among them, Fe-TPA salt was the best catalyst with the highest conversion. The excellent photocatalytic activity can be explained by the nano-size, good oxygen-to-transition metal charge transfer, and high acid concentration in the esterification reaction. Besides, the lower photoluminescence intensity of Fe-TPA suggests a lower recombination rate of electron-hole pairs, indicating efficient charge separation and a higher potential for catalytic reactions. Fe-TPA also absorbs light at slightly longer wavelengths, indicating modifications in the electronic structure that can enhance its ability to capture and utilize light energy for photocatalytic reactions. The optimum reaction conditions for the esterification reaction were found to be 25 °C, a methanol/acetic acid molar ratio of 15/1, and the presence of 5 wt% catalyst loading for 240 min. Experimental data agree with pseudo-first-order reaction kinetics with 44.9 kJ/mol of activation energy. The iron phosphotungstate salts are a promising photocatalyst with sustainable acidity during the reaction, high conversion at low temperature, easy synthesizability, inexpensiveness, and an environmentally friendly nature.