Bifunctional H2WO4/TS-1 catalysts for direct conversion of cyclohexane to adipic acid: Active sites and reaction steps

Adipic acid production may account for 10% of the annual increase in atmospheric nitrous oxide level. Here we developed a hollow H2WO4/TS-1 bifunctional catalyst with excellent catalytic activity for the direct transformation of cyclohexane to adipic acid with 30% hydrogen peroxide by a non-HNO3 route. XRD diffraction, N-2 physisorption, TEM, NH3-TPD and Raman, XPS, Infrared, and UV-visible spectroscopies were used to characterize catalysts to increase understanding of the structure of active species on the hollow TS-1 support. The enhanced catalytic activity in comparison of reference catalysts (H2WO4. H2WO4/Silicate-1 and TS-1) was explained in terms of their bifunctional catalytic sites and perfect hollow morphology with large intraparticle voids. Experimental results on the product distribution indicated that the tetrahedral Ti and surface W sites played important roles in the synergistic effect: tetrahedral Ti worked in oxidation of cyclohexane to intermediates (cyclohexanone and cyclohexanol) and surface W worked in further oxidation of late intermediates to adipic acid. Theoretical calculation results on catalytic activity of the Ti-O-O-H and W-O-O-H active sites in the H2WO4/TS-1 for different reaction steps validate the experimental results. The catalytic activity was also correlated to the strong interaction between W sites and HTS-1 surface induced increase in acidity of the materials. Increasing calcination temperature led to structural evolution of supported active species and subsequent activity change. A cyclohexane conversion of 31% with 78% adipic acid selectivity was achieved over this catalyst calcinated at 500 degrees C. Based on the catalytic and characterization results, possible reaction pathways were thus proposed to explain high adipic acid yield over hollow H2WO4/TS-1 catalysts. (C) 2016 Elsevier B.V. All rights reserved.