Direct alcoholysis of furfuryl alcohol to alkyl levulinates using nano-crystalline polyoxometalate-hexamethylphosphoramide salt as a defined solid Brønsted acid catalyst

A nano-crystalline cubic-type, organo-polyoxometalate salts prepared in a single step via quaternization of phosphotungstic acid (PWA) and hexamethylphosphoramide (HMPA) with different rations PWA-HMPA (1:1 to 1:3). The catalysts preserved their solid forms without hydrolysis of HMPA as verified through FT-IR and XRD analyses. A cubic nano-crystalline structure of the catalyst PWA-HMPA (1:3) was revealed by HR-TEM and FE-SEM and defined with moderate and strong Brønsted acid sites (1.19 mmols/g) remarkably provided 99 % conversion of FAL with 98 % selectivity for isopropyl levulinate (IPL) in one hour at 145 °C using isopropanol (IPA) as solvent. Two distinct reaction pathways over PWA-HMPA (1:3) observed with linear alcohols via 1,2 addition/tri-alkoxy intermediate showed lower AKLs selectivity while favouring high selectivity in IPA through 1,4 addition/furan ring opening. PWA-HMPA (1:3) exhibited notable stability over six recycling runs without leaching HMPA or PWA components, indicating its environmental compatibility.