Tungsten-phosphate supported on titania-silica as oxidative-acidic bifunctional catalyst for diol synthesis

Abstract

Diols are important as raw materials and intermediates for different processes in a wide variety of chemical industries. The preparation of diols is commonly carried out in two steps involving the epoxidation of olefin and continued by hydrolysis of the epoxides; the reactions are carried out using different catalysts in two separate chambers. Therefore, development of a bifunctional catalyst for production of diol in a single step reaction is highly desirable. In this research, a new oxidative-acidic bifunctional catalyst of tungsten-phosphate supported on TiO2-SiO2 has been synthesized. TiO2- SiO2 support was prepared via sol-gel method, followed by impregnation of WO3 and PO43-. The amounts of WO3 and PO43- in the materials were varied in order to investigate the role of tungsten oxide and phosphoric acid in generating oxidative and acidity active sites. X-ray diffraction analysis showed the TiO2-SiO2 support remained amorphous even after loading of WO3 and PO43-, indicating that WO3 and PO43- werehighly dispersed on the surface of SiO2, while TiO2 was fused into the SiO2 framework. The relatively higher tungsten loading seemed to increase the amount of octahedrally coordinated Ti species which acted as oxidative active site. Although titania-silica impregnated with 5 wt% tungsten possessed high surface area (473 m2/g), the addition of both WO3 and PO43- resulted in a dramatic decrease in the surface area (15 m2/g). FTIR results of tungsten-phosphate supported titania-silica implied the attachment of the phosphate to the hydroxyl groups. Increment in acidity of TiO2-SiO2 after the modification with WO3 and PO43- was confirmed by TPD-NH3 analysis. Pyridine adsorption analysis monitored by FTIR spectroscopy showed the 10WO3/TiO2-SiO2 had the highest amount of Lewis and Brønsted acid sites. However, the Brønsted acid sites formation within 0.2 M PO43-/10WO3/TiO2-SiO2 might be obstructed by the competition between WO3 and PO43- to react with TiO2-SiO2. The catalytic activities of TiO2-SiO2 and xW/TiO2-SiO2, (x = 1, 5, 10, 15 wt%) were evaluated through 1,2- octanediol formation in the conversion of 1-octene to 1,2-epoxyoctane using aqueous H2O2 as oxidant. Only samples with more than 5 wt% tungsten loading were found to act as oxidative-acidic bifunctional catalysts to produce 1,2-epoxyoctane and 1,2- octanediol. In the series of bifunctional catalysts, 10W/TiO2-SiO2 showed the best performance with the highest formation of 1,2-epoxyoctane (754 µmol) and 1,2- octanediol (51 µmol). Further modification with H3PO4 demonstrated that 0.2 M PO4 3-/10WO3/TiO2-SiO2 was the best bifunctional oxidative-acidic catalyst which produced 679 µmol 1,2-epoxyoctane and 436 µmol 1,2-octanediol from 1-octene after 24 h. As a conclusion, more active sites were generated via the tungsten-phosphate modification, thus accelerating the production of epoxide for diol formation in the one step reaction. The resulting bifunctional oxidative-acidic catalysts are potentially useful for the chemical production industry.