Acidic characterization and activity of (NH4)xCs2.5−xH0.5PW12O40 catalysts in the esterification reaction of oleic acid with ethanol

Ammonium and cesium derivatives from H3PW12O40 (H3PW), namely (NH4)xCs2.5−xH0.5PW12O40 (x = 0.5, 1, 1.5, 2), were synthesized and structurally characterized by FT-Raman spectroscopy, and their thermal stability was evaluated by FTIR and TGA/DTA. The acidity was characterized by the adsorption/desorption of gaseous pyridine, by FTIR and TGA/DTA as well as by the reaction of oleic acid and ethanol. The stability of the mixed salts regarding the Keggin structure was much higher than the parent acid, with the onset decomposition around 520 °C. Nonetheless, calcination up to 300 °C is recommended for the integrity of the mixed salt. The FTIR of adsorbed pyridine displayed only Brønsted acidic sites, which was confirmed by TGA measurements of the formation of Py–H+⋯Py adducts. The best esterification result was for (NH4)2Cs0.5H0.5PW12O40 with TOF = 1.314 molEO mol−1 proton s−1 with a 1:6 (oleic acid:ethanol) molar ratio, at 80 °C and 10 wt% catalyst in relation to the acid.

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► The mixed salts are much more stable than the parent acid (H3PW12O40).
► The FTIR of adsorbed pyridine displayed only Brønsted acidic sites.
► TG measurements indicated the formation of Py–H+⋯Py adducts.
► The best catalyst for esterification of oleic acid with ethanol was (NH4)2Cs0.5H0.5PW12O40.
► Good reutilization and no leaching were observed for (NH4)2Cs0.5H0.5PW12O40 catalyst.