Selective oxidation of benzyl alcohol catalyzed by (TEAH)nH3-nPW12O40 and its reaction mechanism

Several triethylamine (TEA) salts of phosphotungstic acid were synthesized by an acid-base reaction using a Keggin-type phosphotungstic acid and TEA, and used to catalyze the oxidation of benzyl alcohol to benzaldehyde in water with 30% aqueous hydrogen peroxide as oxidant. (TEAH)nH3-nPW12O40 (n = 1, 2, 3) were excellent catalysts for the reaction and could be isolated and recycled. With (TEAH)H2PW12O40, the conversion of benzyl alcohol and selectivity to benzaldehyde were as high as 99.6% and 100%, respectively, under optimized reaction conditions. The catalysts and their transformation and distribution during the reaction were investigated by IR, 31P NMR and analysis of the reaction system, and the reaction mechanism was deduced. In this water-oil biphasic reaction, (PW12O40)3– was first oxidized and degraded into small (PO4(WO(O2)2)4)3– and free tungsten species that were soluble in the aqueous phase upon reaction with H2O2. Then (PO4(WO(O2)2)4)3–, as the actual oxidant, oxidized benzyl alcohol soluble in the aqueous phase to benzaldehyde, and was converted into a SAR (species after reaction) after losing its active oxygen. The catalytic cycle was completed by the polymerization of the SAR with the free tungsten species back to larger catalyst precursor (PW12O40)3–, which was soluble in the oil phase.

Graphical Abstract(TEAH)nH3-nPW12O40 exhibited excellent catalytic performance for the selective oxidation of benzyl alcohol with hydrogen peroxide as oxidant. The reaction mechanism was investigated and described in detail.Figure optionsDownload as PowerPoint slide