Role of acidity over rare earth metal ion-exchanged heteropoly tungstates during oxidation of alcohols

•Rare earth metal ion-exchanged heteropolytungstate shows high catalytic activity for oxidation of benzyl alcohol.•Good catalytic activity due to Brønsted-Lewis acid synergy of catalyst.•Acidic properties of rare earth exchanged heteropolytungstate catalysts probed by the 31P-TMPO MAS NMR approach.•Experimental variables during oxidation over Ce1/3H2PW12O40 catalyst optimized by RSM.

A series of Brønsted-Lewis acidic catalysts, denoted RE-TPA (MyH3‒3yPW12O40; M = La, Ce, Pr, Nd, Sm; y = 1/3, 2/3, 1), prepared by incorporating rare earth (RE) metal cations as Lewis centers onto the superacidic tungstophosphoric acid (TPA), were characterized by XRD, FT-IR, TGA-DTA and NMR. In particular, their acidic properties were studied by Pyridine-IR and solid-state 31P NMR of adsorbed phosphorous probe molecule. The high catalytic activity for the RE-TPAs was found to invoke Brønsted-Lewis acid synergy. Among various catalysts examined for oxidation of benzyl alcohol (BzOH) with hydrogen peroxide, the Ce1/3H2PW12O40 was utilized to optimize the reaction conditions while nearly full BzOH conversion, and excellent bezaldehyde yield (98.2%) and selectivity (98.4%) could reach by response surface methodology (RSM). Additional recyclability and kinetic studies revealed that the RE-TPA catalysts are robust, reusable, and highly efficient for alcohols oxidation reaction with an apparent order of 2.5 and an active energy of 37.3 kJ/mol.

Graphical abstractRare earth metal partly exchanged heteropolytungstate catalysts show superior activity and durability for the oxidation of benzyl alcohol. The synergy of Brønsted and Lewis acid sites of catalyst was responsible for high catalytic activity.Download high-res image (118KB)Download full-size image