Preparation Of Sodium-tungstate-based Temperature-sensitive Hydrogel And Catalytic Oxidation Of Alcohol

Tungsten-based catalyst systems are widely used in a variety of catalytic reactions due to their high efficiency and high selectivity.Sodium tungstate?Na₂WO₄·2H₂O?as a typical polyacid salt has attracted much attention in the oxidation of alcohols and epoxidation of olefins because of its low cost and availability.Due to its special three-dimensional network structure and controllable hydrophilic-hydrophobic microenvironment,thermosensitive hydrogels can usually serve as a carrier for catalytically active centers,improving the mass transfer of reactants/substrates in the water-organic two phases.In addition,the recovery and reuse of the catalyst can be achieved by changing the hydrophilicity and hydrophobicity of the thermosensitive hydrogel.In this paper,a porous thermosensitive sodium tungstate composite hydrogel was successfully synthesized by one step.The structure and properties of the thermosensitive hydrogel were systematically studied.Besides,it was used as a catalyst for the catalytic oxidation of benzyl alcohol and its derivatives,and the catalytic properties and influencing factors of the hydrogel were discussed.The paper includes the following sections:1.Athermosensitivecompositehydrogel P[NIPAM-co-methacryloxyethyltrimethyl ammonium chloride?DMC?]/Na₂WO₄ was prepared by copolymerization of NIPAM and cationic monomer DMC in the aqueous solution of Na₂WO₄.Na₂WO₄ interacted with quaternary ammonium groups of DMC by electrostatic interaction producing the crosslinking sites,and could also act as a porogen precusor to form a porous structure in situ.The structure and composition of the hydrogel were determined by FTIR,EDS,TG and ESEM.The resultant hydrogel showed a fast swelling rate and good salt tolerance.The hydrogel reached the swelling equilibrium within 10 min.Moreover,the swelling ratio of the hydrogels increased with the increase of the polarity of the solvent.In the water,the swelling ratio decreased with the increasing of temperature,but remained at a high level even at 80^oC due to porous structure.The swelling ratio increased instead in low polar solvent,while it became negligible in non-polar solvent with the increasing of temperature.2.The P?NIPAM-co-DMC?/Na₂WO₄ hydrogel was used as catalyst to catalyze the oxidation of benzyl alcohol and its derivatives with H₂O₂ as oxidizer.The influences of different solvents,H₂O₂ concentration,reaction temperature,reaction time,heating mode and reaction substrate on the catalytic performance were investigated.The hydrogel changes before and after the catalytic reaction were analyzed by FTIR and TG.In the absence of solvents,when the mass of N₅D₁W₁₇ gel was 0.05 g,n?H₂O₂?:n?benzyl alcohol?was 1:1.1,the reaction was carried out at20^oC for 1 h,and then gradually increased to 80^oC for 6 h,the conversion of benzyl alcohol was 41.5%,while selectivity to benzaldehyde was 90.4%.When ACN was used as a solvent,the catalytic effect of the hydrogel on the benzyl alcohol derivative could be improved,and when the benzene ring was para-substituted by connected to the electron-attracting substituent,the conversion of the corresponding alcohol was higher than that of the electron-withdrawing group.However,due to the large influence of the reaction temperature on the loading of tungstic acid in the catalyzed hydrogel,the effect of H₂O₂ concentration was relatively small.Therefore,when the temperature was high,there observed large loss of Na₂WO₄ which led to poor reusability.