Rational Design of Phosphotungstate Hybrids with Dual Active Sites for Efficient Catalytic Oxidation

Phosphotungstate organic–inorganic hybrid materials [CH 2 COOHmim]H 2 PW 12 O 40 , [(CH 2 ) 3 COOHmim]H 2 PW 12 O 40 and [Bmim]H 2 PW 12 O 40 with dual catalytic oxidation active centers, including carboxylic acid group and phosphotungstate anion, were synthesized from bisulfate acidic ionic liquid in a one-step method, and well characterized by XRD, 1 H NMR, 13 C NMR, 31 P NMR, FT-IR, and UV–vis. Then, these obtained hybrids were employed as catalysts for the selective oxidation of N-methylmorpholine (NMM) into N-methylmorpholine-N-oxide (NMMO) using H 2 O 2 as an oxidant. As expected, the –COOH functionalized hybrids showed much better activity than the conventional hybrid [Bmim]H 2 PW 12 O 40 , and even much better than the mixture of corresponding ionic liquid and H 3 PW 12 O 40 , suggesting the existence of dual active species and synergistic effect between the cation and phosphotungstate in hybrids. The best catalyst [(CH 2 ) 3 COOHmim]H 2 PW 12 O 40 gave an NMM conversion of 98.7% and selectivity to NMMO of 98.8%. The structural characterization by 31 P NMR, UV–vis, Raman and 13 C NMR analysis of the catalyst before and after oxidation demonstrated the existence of the catalytic active species PW 12 O 40 3− and peroxy carboxylic acid. Finally, a possible catalytic mechanism has been proposed. Graphical Abstract