Design Of Hybrid Catalysts Based On Polyoxometalate And Their Application For Biodiesel Production

Eco-friendly and commercially visble catalyst systems have been a focus of much recent research. Polyoxometalates (POMs) have attracted significant attention because of their high acidity and favourable redox behaviour, which make them suitable for applications in size- and shape-selective catalysis. However, POMs exhibit low surface area, low pore volume, and low thermal stability, which limit their utility in many catalytic reactions. Thus, the dispersion of POMs on porous solid supports with high surface area, large pore diameter, and high specific pore volume is seen as a critical means of improving their properties and obtaining better performance in many potential heterogeneous catalytic applications.In this dissertation, the micro- and mesoporous composite catalysts based on polyoxometalates have been prepared, which were further modified with organic silanes (methyltrimethoxysilane, phenyltrimethoxysilane, 1,2-bis-(triethoxysilyl)ethane or 1,4-bis-(triethoxysilyl)beneze). The catalytic performance of the resulting materials was evaluated the biodiesel production as a model reaction,1.In this paper, varying amounts of H3PW12O40 have been supported on tantalum pentoxide (Ta2O5) via hydrolysis of tantalum pentachloride (TaCl5) in the presence of H3PW12O40 using the sol-gel method for acid catalytic reaction. All obtained amorphous materials have been characterized by ICP-AES, FT-IR spectra, Raman scattering spectroscopy, nitrogen adsorption/desorption analysis and FESEM, in order to characterize the structure integrity of the Keggin unit in as-prepared composites as well as the morphology and surface textural properties of the composites. The results indicated that the primary Keggin structure of starting H3PW12O40 in as-prepared composites remain intact after formation of the composites. They have microporosity, large BET surface area and uniform pore size. Catalytic activity of the catalysts has been evaluated by esterification of acetic acid with ethanol as a probe reaction with different catalytic reaction parameters such as H3PW12O40 loading, reaction time, catalyst dose, molar ratio of the reactants, etc. The highest conversion of ethanol is about 87.4 %, and turnover frequency is 2.4×103. The catalyst is recyclable without significant loss of activity.2.A series of mesoporous polyoxometalate-tantalum pentoxide composite catalysts, H3PW12O40/Ta2O5, with H3PW12O40 loading from 3.6 to 20.1% was prepared by a one-step sol-gel-hydrothermal route in the presence of a triblock copolymer surfactant. Bulk and surface sensitive probe testing results indicated that the primary Keggin structure remained intact after formation of the composite, and strong interaction between the Keggin unit and Ta2O5 framework existed in the composite. Additionally, the composite exhibited larger and well-distributed three-dimensionally interconnected pores (3.9~5.0 nm), larger BET surface area (106.0~126.9 m2 g-1), high porosity (0.44~1.37 cm3 g-1), and homogeneous dispersion of the Keggin unit throughout the composite. As an environmentally friendly solid acid catalyst, the catalytic performance of the H3PW12O40/Ta2O5 was evaluated in the esterification of lauric acid and myristic acid, the transesterification of tripalmitin as well as the direct use of soybean oil for biodiesel production. Regardless of the presence of free fatty acids, the H3PW12O40/Ta2O5 composite showed high reactivity and selectivity towards simultaneous esterification and transesterification under mild conditions. The catalyst can be recovered, reactivated and reused several times.3.Mesoporous Ta2O5 materials functionalized with both alkyl group and a Keggin-type heteropoly acid, Ta2O5/SiO2?[H3PW12O40/R] (R = Me or Ph), was prepared by a single step sol-gel co-condensation method followed by a hydrothermal treatment in the presence of a triblock copolymer surfactant. The catalytic performance of the resulting multifunctionalized organic-inorganic hybrid materials was evaluated by a direct use of soybean oil for biodiesel production in the presence of 20 wt% myristic acid under atmosphere refluxing, and the influences of the catalyst preparation approaches, functional component loadings, and molar ratios of oil to methanol on the catalytic activity of the Ta2O5/SiO2?[H3PW12O40/R] were studied. In addition, the recyclability of the hybrid materials was evaluated via four catalytic runs. Finally, the network structures of the hybrid materials and the functions of the incorporated alkyl groups on the catalytic activity of the materials were put forward. The experimental results indicated that the co-condensed multifunctionalized catalyst exhibiting considerably high catalytic activity compared with the alkyl-free catalyst as well as the grafted ones,because ( Ta–OH2)n+(H2PW12O40)n? species, main acid sites of the catalysts with enhanced Br?nsted acidity compared to H3PW12O40 or Ta2O5 for catalyzing esterification or transesterification, were formed at the surface of the product via Ta–O–W covalent bonds.4.Organic-inorganic hybrid catalysts by co-hydrolysis and condensation reactions of TaCl5 and bridged organosilica precursors of the type (EtO)3Si-R-Si(OEt)3 instead of (MeO)3Si-R in the presence of POM. The organic units in this case are incorporated in the three-dimensional network structure of the silica and tantalum matrix through two covalent bonds and thus distributed totally homogeneously in the pore walls. The phase structure and surface textural property of the composites were characterized by several characterization techniques including FT-IR, XPS, TEM and N2 porosimetry. The catalytic activity of the hybrid catalysts was evaluated by a direct use of soybean oil for biodiesel production. For comparison, H3PW12O40/Ta2O5 and Ta2O5/SiO2?[H3PW12O40/R] were also tested.