Preparation, Characterization And Catalytic Properties Of Phosphotungstic Ammonium Salts, Cesium Salts And Phosphotungstic Acid Supported On M41S Mesoporous Molecular Sieves

Heteropoly compounds have been widely studied in relation to their expressiveconductive, catalytic, antiviral, photochromic and many other properties. Amongheteropoly compounds,12-tungstophosphoric acid (HPW) having Keggin structure,characterized by strong acidity are classified into a ‘‘superacids’’. However, bulkheteropolyacids have poor thermal stability, low surface area and porosity, expensiveand difficult to recycle. Hence, there has been interest in replacing H+inthe surface with Cs+and NH4+or using supported HPAs, which resulted in thepreparation of a family of (NH4)xH3-xPW12O40, CsxH3-xPW12O40and HPW supportedcatalysts.Benzoic acid has been widely used in the fields of pharmaceutical, food,chemical, and so on. The traditional production of benzoic acid is basically usinginorganic acid catalysts, which involves obvious shortcomings, such as difficultseparation from the solution, corrosion of equipment, pollution. Sinceenvironmentally benign, economical, practical and efficient processes have becomeincreasingly important in the chemical engineering community, it is urgent to developan environmental-friendly technology under neutral, mild and practical conditions tosynthesize benzoic acid.In this thesis, a series of cesium salts and ammonium salts of tungstophosphoricacid, M41S mesoporous molecular sieves and their correspounding supportedHPW/M41S catalysts were prepared, respectively. The samples werecharacterizatized by various techniques and their catalytic performance for theoxidation of benzaldehyde and/or benzyl alcohol with H2O2was investigated. Theprimary work was as follows:(1) Bulk HPW was synthesized via an improved diethyl ether extractionmethod. A series of (NH4)xH3-xPW12O40and CsxH3-xPW12O40(x=0.5-3.0) weresynthesized by a precipitation method, respectively. M41S mesoporous molecularsieves including MCM-48, Al-MCM-48, MCM-41and Al-MCM-41were synthesized by the hydrothermal method or an open synthesis process. Then, these molecularsieves was used as supports to prepare the supported catalysts with different HPWloadings (5-45wt.%) by using an impregnation technique.(2) The as-synthesized samples were characterized by XRD, XRF, FT-IR, Pyr-IR,N2adsorption-desorption, DR UV-Vis, NH3-TPD, SEM, and TEM. The resultsshowed that the substitution of H+by NH4+or Cs+in HPW did not change the Kegginstructure. Furthermore, the particles became more uniformity and the texturalparameters increased with the increase of x in CsxH3-xPW12O40and(NH4)xH3-xPW12O40. Meanwhile, the HPW/M41S catalysts retained the mesoporousstructure of M41S molecular sieves and bulk HPW was highly dispersed into thesupports to some extent. On the other hand, the incorporation of HPW leaded theinteraction between HPW and Si-OH, which may destroy the long-range order ofM41S, and the influence became more serious with the increase of HPW loadings. Inaddition, the supported catalysts had both Lewis acid and Br nsted acid sites andincorporation of HPW increased the Br nsted acid sites with increase of the totalconcentrations of acid sites.(3) The optimal catalytic conditions (0.25g cat,80℃,8h) for benzaldehydeoxidation with30%H2O2was selected out over (NH4)2.5H0.5PW12O40catalyst in theabsence of any organic solvent, for the first time with salts of tungstophosphoric acidcatalyzed synthesis of benzoic acid. The yield of benzoic acid yield may rise up to75.93%under the conditions. Moreover, the catalytic performance of the as-preparedcatalysts for benzaldehyde and/or benzyl alcohol oxidation to benzoic acid wasinvestigated, respectively.(NH4)2.5H0.5PW12O40, Cs2.5H0.5PW12O40and the supportedcatalysts with30-40wt.%HPW loadings exhibited the best catalytic activity amongthe correspounding series.