Encapsulated Polyoxometalate In Metal-Organic Frameworks And Its Oxidative Desulfurization Performance

SO2in the atmosphere result in serious pollution which damages the natural environment and human activities, so it is an urgent and important task to reduce the sulfur content in diesel fuel for the continuous stringent environmental requirements. Hydrodesulfurization (HDS) technology is difficult to meet the demand for the production of ultra-low sulfur diesel, however, the new oxidative desulfurization (ODS) of diesel fuel is considered as the most potential industrialized ultra-deep desulfurization technology in the near future because of its mild reaction conditions, less investment in equipment, high removal rate for refractory substituted dibenzothipohene which is difficult to remove using HDS technology. The addition of the catalyst can improve the oxdative desulfurization rate of diesel fuel. Polyoxometalates (POMs) as a novel oxidative desulfurization catalyst has attracted considerable attention because of the easily adjustable structure of POMs, the behavior of the pseudo-liquid phase, its multi-function property, and so on. But soluble POMs directly as oxidative desulfurization catalyst are difficult to separate and recycle, and the insoluble solid POMs have very small specific surface area (<10m2·g-1), which limit the catalytic activity of solid catalyst. Howerver, the supported POMs catalysts can solve the above problems, so far the supporters, such as SiO2, TiO2, ion exchange resins, activated carbon and molecular sieves, and so on are used as the supporters of POMs, but they are faced with the low loading, uneven dispersion, the leakage of catalysts, the poisoning of active sites. In order to overcome the above problems, it is urgent to find a new support for preparing the new supported POMs catalysts. The new porous metal-organic frameworks (MOFs) material, owning a stable crystalline porous structure, the larger surface area, adjustable pore diameter, modified surface of pore channel, is an ideal carrier to prepare the supported POMs catalyst. In this thesis, MIL-101@POMs composite material, combining the catalytic oxidation activity of POMs with mesoporous of MIL-101, is prepared and applied to oxidative desulfurization reaction.POMs@MIL-101composite materials are successfully prepared by the impregnation, situ assembly of host method and situ assembly of guest method, which are characterized by XRD, FI-TR,31P solid state NMR, BET, SEM/EDS and TEM. The results show that the specific surface area and pore volume of the POMs@MIL-101composite material prepared by impregnation decrease with the increase of phosphotungstic acid (PTA), PTA keep the complete Keggin structure and are evenly distributed in the MIL-101with the serious problem of plugging pore, which is only encapsulated in the large cage of MIL-101; PTA in the POMs@MIL-101composite materials obtained in situ assembly of host keep the Keggin structure and are successfully encapsulated in the large and middle cages of MIL-101, which is very evenly distributed in the MIL-101; PTA in the POMs@MIL-101composite materials obtained in situ assembly of guest keep the complete Keggin structure and successfully grow to MIL-101in situ, but PTA are unevenly dispersed in MIL-101, when the loading is more than36%, PTA begin to aggregate and grow up in the composites materials, the specific surface area and pore volume begin to decline with the increase of PTA obtained in situ, and PTA is focused on encapsulating in the middle cage of MIL-101.H3PW12O40@MIL-101composites materials obtained in siut assembly of host is used as oxidative desulfurization catalyst to evaluate the oxidation desulfurization activity of the model oil and real oil. The results indicate that DBT conversion rate reaches up to99%when the amount of surfactant is30mg and O/S reaches up to100; DBT removal rates gradually increase with the increase of the oxidation reaction time and reaction temperature, which is very advantageous for the reaction; The sequence for oxidation activity of different sulfides is DBT>4,6-DMDBT>BT; The H3PW12040@MIL-101composite materials have superior oxidative desulfurization performance. The performance of H3PW12O40@MIL-101composite materials for the thiophenes sulfide with methyl steric hindrance in hydrogenated diesel is investigated, the results show that the H3PW12O40@MIL-101catalysts combinated with MIL-101as the adsorbent have high oxidative desulfurization activity for thiophenic sulfur compounds in hydrogenated diesel, the sulfide in oil after oxidative desulfurization can be reduced from193ppm to33ppm.This paper discusses the oxidation desulfurization mechanism of H3PW12O40@MIL-101composites materials obtained in siut assembly of host for thiophenic sulfur compounds, which is combined with the various factors and the sulfide oxidation kinetics. The apparent reaction order of BT, DBT,4,6-DMDBT is first-order reaction, and the order of apparent activation energy of three macromolecular sulfide is BT>4,6-DMDBT>DBT, which is consisted with the rule of oxidation activity of different sulfide; PTA in the cage of MIL-101is first oxidized to{PO4[WO(O2)2]4}3-by hydrogen peroxide, which is confirmed by FT-IR, and the mechanism of oxidative desulfurization in the new oil in water micelles system is described.The H3PW12O40@MIL-101composite materials exhibit high selective adsorption capacity for DBTO2, which is mainly ascribed to the π-π stacking interactions between the sulfur in DBTO2with the positive charge and terephthalate bridges of metal-organic framework MIL-101. Therefore, H3PW12O40@MIL-101composite material not only exhibits high oxidative activity but also removes oxidized sulfone products by selective adsorption on catalyst simultaneously, which is a kind of oxidation-adsorption synergistic catalysts. The influence of the different situ assembly and the type of carrier for the adsorption of DBTO2are also investigated in detail. The results show that the composite material obtained via situ assembly of host has the highest selective adsorption capacity for DBTO2, the adsorption capacity for DBTO2is about74.8mg S-g-1Catalyst, and the relative adsorption capacity is96.8%.