Preparation, Characterization And Catalytic Performance Of Exposed Keggin - Type Polysaccharide Catalysts For Transition Metal Doping

Heteropoly acids (HPAs) are well-defined transition metal oxygen clusters with composition and structural diversity, and they can be controlled at the molecular level to match the different reaction system and catalytic conditions. HPAs have been widely used as the catalysts in both oxidative and acid-catalyzed reactions as homogeneous and heterogeneous catalysts for the oxidation of different organic molecules because of their strong acid and the rich redox chemistry. The catalytic activity of HPAs can be improved by partial substitution of redox metal ions in Keggin structure. However, HPAs still suffer from some shortcomings, such as the difficulties to separation from the reaction solution and repeated utilization.In this paper, a series of new doped hybrid catalytic materials were prepared by means of doped transition metal (such as, Cu, Fe, Mn, Co, Ni and V) in the monovalent Keggin type silicotungstic acid, K8[SiWuO39]·14H2O (SiWu). The structural integrity and physicochemical properties of these catalytic materials were characterized by ICP, IR, XRD, TG,SEM and XPS.The catalytic activities of these samples were evaluated by the reactions of selective oxidation of alcohols into the benzaldehyde and desulfurize simulation gasoline with 30% aqueous H2O2 as oxidant. The silicotungstic acids doped with transition metal are more reactive than the corresponding undoped compounds. With Cui.oFei.oSiWll as heterogeneous catalyst to selective oxidation of benzyl alcohol into benzaldehyde, it shows excellent activity with a benzyl alcohol conversion of 98.0% and benzaldehyde selectivity of 97.3%. The orthogonal test shows that the optimized conditions is as follows:0.02 g catalyst,5 mL toluene as solvent,4 mmol alcohol, V(H2O2)/V(benzyl alcohol)= 5:1, and 80℃ for only 1 h. We also found that the samples of Cu1.0SiW11O39, Cu0.75Fe0.75SiW11O39, and Cu1.0Fe1.0SiW11O39 could desulfurize the Sulfur Content from 500 ppm to 10 ppm. The orthogonal test reveals that the optimized conditions is as follows:5 mL Simulation gasoline, 0.01 g catalyst,2 mL H2O2 acetonitrile as extracting agent,2 mL H2O2,80℃ for 80 min.The recyclability of the catalysts was carried out by separating from the aqueous phase, centrifugating, washing and drying at 80℃ for 6 h, then, the catalyst was reused in the next reaction. After reused more than five times, its catalytic efficiency was not decreased significantly, meanwhile, the fresh catalysts and the recycled catalysts were characterized with FT-IR spectra, we found that structure of recycled catalysts were maintained the Keggin structure.In conclusion, a series of new doped hybrid catalytic materials were prepared by introducing the transition metal to modulate the structure or doped transition metal in the monovalent Keggin type silicotungstic acid. These insoluble catalytic materials present excellent catalytic activities for the reactions of selective oxidation of alcohols into the benzaldehyde and desulfurize simulation gasoline with 30% aqueous H2O2 as oxidant, more important, they are very stable and recyclable in aqueous solution.