Microenvironment-controlled Polyoxometalate Catalysts For Epoxidation Of Olefins And Oxidative Desulfurization

Sulfur in the fuel is converted into SOxin internal-combustion engines.SOxnotonly poisons the noble metal catalyst in the exhaust gas purification, but also causesair pollution, acid rain and mist. Ultra low sulfur diesel with10-15ppm sulfur hasbeen implemented during the past several years in Europe, Japan and USA. Chinesenational standard GB19147-2013prescribes sulfur content must be not more than50ppm.The more stringent standard for diesel will be issued in the future, when thesulfur content of gasoline and diesel will be less than10ppm. Hydrodesulfurizationprocess can effectively remove sulfides and disulfides, but it is not efficient fordibenzothiophene and its derivatives. Hydrodesulfurization process requires severeprocessing conditions to remove the refractory sulfur compounds in fuel, which leadsto escalating the cost of hydrogen and the capital cost for hydrotreating sulfur removal.Therefore, it is necessary to develop methods for production of ultra-low sulfur dieselunder mild conditions. Oxidation desulfurization with extraction or adsorption is avery promising approach. Hydrogen peroxide is cheap, non-polluting andnon-corrosive, which is an ideal oxidant. The development of oxidativedesulfurization process under mild conditions with hydrogen peroxide as oxidant forproduction of clean diesel is very important.Epoxide is an important chemical intermediate, which can be used to synthesizedrugs, pesticides, resins, plastics and other products. The traditional industry factureof epoxide is chlorohydrination process and organic peroxide process.Chlorohydrination process produces large quantities of poisonous wastewater.Organic peroxide process overcomes the disadvantages of chlorohydrination process,but it requires a large investment. Hydrogen peroxide process is a new technology forepoxide production, which produces only epoxide and water and eliminates thepollution from the source.Polyoxometalate is a variety of transition metal oxide clusters, which is anefficient redox catalyst. Polyoxometalate-surfactant complexes are combined with themesoporous silica and graphene oxide to form three microenvironment-controlledpolyoxometalate catalysts: micro reaction-controlled phase-transfer catalyst,polyoxometalate lyotropic liquid crystal catalyst, and amphiphilic polyoxometalate-graphene oxide with thermosensitive property. The composition, structure andcatalytic performance of these catalysts are described. These catalysts are applied tooxidative desulfurization and olefin epoxidation.A micro reaction-controlled phase-transfer catalyst is prepared using silica gel assupport. Using the method of ship in a bottle, surfactant precursor andpolyoxometalate precursor (N,N-dimethyl hexadecyl amine and PW12O403-) areencapsulated in silica, while the ends of silica channels is sealed with micellar-likeaggregates. With the action of H2O2, PW12O403-is converted into {PO4[WO(O2)2]4}3-and N,N-dimethyl hexadecyl amine is converted into C16H33(CH3)2NOH. A microreaction-controlled phase-transfer system is made in the channels of silica, whichpreventes the catalytic active species from losing. Dibenzothiophene is converted intodibenzothiophene sulfone with100%yield under the optimal reaction conditions(H2O2/S molar ratio is4:1, atmospheric pressure,70oC,2h). The catalyst showes highcatalytic activity, stability and recyclability.[C16H33(CH3)2NOH]3[PO4{WO(O2)2}2] is synthesized by one-pot method inwater with hydrogen peroxide as oxidant (N,N-dimethyl hexadecyl amine andPW12O403-as precursors). It is a polyoxometalate lyotropic liquid crystal catalyst. Thelyotropic liquid crystal property is determined by polarizing optical micrograph. The dissolution and precipitation of the catalyst is controlled by the polar in the system. Totake advantage of this property, a self-separating microemulsion system for olefinepoxidation is designed and implemented, which solve the problem of demulsification.The polar of substrate and product in the reaction system controls the emulsificationand demulsification, which facilitates the separation of the product. A three phase(polar phase, nonpolar phase and liquid crystal phase) balanced catalytic system foroxidative desulfurization of diesel is designed and implemented, which facilitates theseparation of product, by-product and catalyst. Cyclooctene is converted intocyclooctene oxide with>99%yield and>99%selectivity under the optimal reactionconditions (H2O2/cyclooctene molar ratio is1:1,60oC,1h). Dibenzothiophene isconverted into dibenzothiophene sulfone with>99%yield under the optimal reactionconditions (H2O2/S molar ratio is2:1,60oC,3h).Polyoxometalate-graphene oxide catalyst is synthesized by one-pot method inwater with hydrogen peroxide as oxidant (N,N-dimethyl hexadecyl amine andPW12O403-as precursors). Using hydrogen peroxide to peel off the modified grapheneoxide, the catalyst is prepared. It is a solid particle with one hydrophilic side and onehydrophobic side, while the catalyst has thermal property. The catalyst canencapsulate the hydrophobic substrate and transfer into the aqueous phase to emulsifythe catalytic system when heated. Therefore, the catalyst can catalyze the reaction inwater. Cyclooctene is converted into cyclooctene oxide in water with>94%yield and>94%selectivity under the optimal reaction conditions (H2O2/cyclooctene molar ratiois1:2,50oC,20h). Benzothiophene is converted into benzothiophene sulfone in waterwith>96%yield and>99%selectivity under the optimal reaction conditions (H2O2/Smolar ratio is4:1,50oC,20h). The thermal property of the catalyst facilitates theseparation and reuse of the catalyst.