Metal-exchanged phosphotungstate nanoparticles with improved acidity as the catalyst for esterification of glycerol with acetic acid

A series of metal-exchanged phosphotungstate salts, Fe-TPA, Cr-TPA, Cu-TPA, and Ni-TPA were synthesized by sonication and tested for glycerol esterification with acetic acid to obtain biofuel additives. A systematic, chemical, structural, and morphological characterization and determination of acidity were used by various analytical techniques. XRD and FT-IR studies confirmed the Keggin structure of all metal exchanged phosphotungstate salts with no impurities coming from metal nitrate salts. The incorporation of metal cations in phosphotungstic acid leads to decreased crystallite size as seen from XRD diffractograms. N-2 adsorption-desorption analysis showed that the Type II isotherms were obtained for all metal exchanged phophostungstate salts, characteristic of non-porous or macroporous materials. SEM images of metal exchanged phosphotungstate salts displayed well-shaped crystalline particles. SEM-EDX analysis showed that the calculated tungsten metal ratios are in good agreement with the theoretical ratios of those for all the metal exchanged salts, verified by XRF results. The exchange of protons by the metal cations improved the thermal stability observed in the TGA analysis. The acidity strengths, from NH3-TPD analysis, were found in the following order; TPA > Fe-TPA > Cr-TPA > Cu-TPA. Additionally, pyridine-FTIR gives that the exchanging of transition metal into the phosphotungstic acid decreased the Bronsted acidity while increasing the Lewis acidity and this mostly affects the efficiency of the esterification reaction. The activity of metal exchanged salts was in the given order; Fe-TPA > Cr-TPA > Cu-TPA > Ni-TPA. Among these Fe-TPA salt, having a good ratio of Bronsted and Lewis acidity, have the selectivity for di- and triacetin (68 and 10%), with the highest conversion (96%). The iron phosphotungstate salts are a promising catalyst with sustainable Bronsted acidity during the reaction, high desired product selectivity at low temperature, easy synthesizability, inexpensive, and environmentally friendly.