Immobilization Of Tungsten/Molybdenum Compounds And Their Catalytic Oxidation Desulfurization Performance For Fuel Oil

The market demand for energy liquid fuels is increasing with the rapid economic growth,the low-quality fossil fuels containing more impurities(such as S,N,etc.)are developed and used,in which the combustion of many sulfur-containing compounds would emit SO_Xcompounds,causing a series of environmental problems.Oxidative desulfurization(ODS)has emerged as a promising method for deep desulfurization of liquid fuels due to its mild reaction conditions and high desulfurization rate.Usually,polyoxometalates(POMs)with high active are used as outstanding catalysts.However,POMs as homogeneous catalysts have suffered from some disadvantages,including high solubility in polar solvents,difficult separation and recycling,and so on.To overcome the above defects,in this study,the loaded POMs with high dispersion and strong bond were prepared by different methods,such as,covalent binding method,electrostatic binding method,etc.,so as to improve the catalytic activity and reusability of POMs.Then,the supported POMs catalysts were annealed at high temperature to regulate the structure of POMs,and enhance the active of H₂O₂.Finally,the shortcomings of existing catalysts would be overcome,for example,the poor catalytic activity and difficult removal for sulfur compounds with large concentration in fuel oil,poor stability,and so on.The main research contents are as follows:(1)Functional polymer-modified silica gel were used as carrier.Firstly,amino-modified Dawson type plyoxometalate K₁₀[?-P₂W₁₇O₆₁]·20H₂O(NH₂-P₂W₁₇)was covalently bonded to PGMA/Si O₂ to prepare heterogeneous P₂W₁₇-PGMA/Si O₂ catalyst.The prepared catalyst with Na HCO₃ as basic compound had the best catalytic oxidation active on THT,and the THT conversion can reach 100%within 60 min,which may be due to the following reasons:1)high redox activity of P₂W₁₇ cluster;2)the full swelling and stretch of polymer chains in organic solvents;3)hydrophobic effect of the polymer chain is conducive to the adsorption of THT on the catalyst,providing a microenvironment for the oxidation of THT.Also,the hydrophobic polymer makes the tetrahydrothiophene oxide(THTO)excluded easily from the catalyst in time.All of these resulted in the outstanding catalytic performance of P₂W₁₇-PGMA/Si O₂-3.(2)Graphene oxide(GO)with three-dimensional network structure(3D GO)was used as a carrier,and the mono-vacant Dawson polyoxotungstate(EPO-P₂W₁₇)modified with epoxy group was covalently immobilized on 3D GO for preparing a heterogeneous catalyst P₂W₁₇/3D GO.The removal results of dibenzothiophene(DBT)and tetrahydrothiophene(THT)in fuel oil by P₂W₁₇ before and after loading on on 3D GO and 2D GO were compared and analyzed.The results showed that P₂W₁₇/3D GO exhibited the enhanced catalytic performance because of the strong adsorption between thiophene compounds and 3D GO through?-?and S-?interactions and the acid-base interactions,and the microenvironmental effect provided by 3D GO with reticular holes.Finally,the removal efficiency of DBT followed:P₂W₁₇/3D GO>P₂W₁₇/2D GO>P₂W₁₇.Complete oxidative removals of DBT(500 ppm)and THT(5000 ppm)by P₂W₁₇/3D GO were achieved for 120 and 30 min at 60?,respectively.In addition,the catalyst showed good reusability,and its catalytic activity remains almost unchanged after 5 cycles.(3)In order to further simplify the catalyst recovery process,using chitosan modified Fe₃O₄(Fe₃O₄@CS)as a carrier,Keggin-and Dawson type polyoxomethoate were immobilized on magnetic chitosan microsphere by electrostatic interaction,respectively,obtaining five heterogeneous catalysts Fe₃O₄@CS@POMs.The experimental results showed that Fe₃O₄@CS@PMo₁₂ showed the best desulfurization performance,and the removal rate of DBT(500 ppm)reached 100%for 60 min under the optimum conditions,i.e.60?and 0.1 g of Fe₃O₄@CS@PMo₁₂.The high desulfurization efficiency was achieved in a short time,which was attributed to the strong catalytic oxidation active of PMo₁₂ and the microenvironmental effect provided by Fe₃O₄@CS,and thus promoting the oxidation of DBT.In addition,the as-prepared catalyst could be easily separated and recycled in the external magnetic field,and it still had a good reusability.(4)The loaded Fe₃O₄@CS@POMs catalysts aforementioned were calcined under air atmosphere to modulate the structure of transition metal compounds and the carrier,thus to improve the catalytic efficiency.The effect of high temperature calcination on oxidation desulfurization of catalyst was studied.The experimental results showed that?-Fe₂O₃@C@P₂W₁₈-400 showed the best desulfurization performance.The desulfurization rate of DBT(4000 ppm)reached 100%for 15 min at 70?with 0.03 g of?-Fe₂O₃@C@P₂W₁₈-400 and the H₂O₂/S molar ratio of 2.After the 400?calcination,the oxygen defectives with the strong catalytic activity were found in the structure of the Dawson-type P₂W₁₈ crystals,therefore,the desulfurization efficiency of?-Fe₂O₃@C@P₂W₁₈-400 was significantly enhanced.Furthermore,magnetic porous carbon spheres produced by air etching provide the good channel to facilitate the contact between substrate and catalytic active center.Therefore,comparing with uncalcined Fe₃O₄@CS@P₂W₁₈,?-Fe₂O₃@C@P₂W₁₈-400 significantly shortened the time for complete desulfurization,reduced catalyst dosage,and broadened the suitable concentration range of the model oil.And that,after calcination at 400?,the magnetic properties of the catalyst still remained,so the catalyst could be separated quickly and effectively under the external magnetic field,and the catalytic activity was almost unchanged after 6 recycles.The desulfurization mechanism was explored by radical scavenging experiments,and it was found that the hydroxyl radicals(·OH)and peroxypoic acid intermediates synergistically accelerate the oxidization of DBT into DBTO₂.(5)The ultrafine transition metal carbides(W₂C or Mo₂C)and transition metal elemental(W)loaded on carriers have been firstly developed as oxidation desulfurization catalysts to realize the rapid and deep desulfurization of high S concentration.Using PW₁₂-PEI/GO,PMo₁₂-PEI/GO and Si W₁₂-PEI/GO ternary nanocomposites as precursors,they were annealed at the high temperature under N₂ atmosphere,so as to obtain graphene-loaded transition metal carbides or elemental transition metal heterogeneous catalysts.The results showed that the different active metal center of precursors would result in the different structures of active centers of products annealed at different temperatures,and then the different desulfurization performance.And that,the desulfurization performance became better with the increase of annealing temperature.All of three catalysts obtained by annealing at 1000?could make the model fuel oil with 4000 ppm removed completely within only 10 min,and the kinetic constant could be as high as 0.58 min^-1.The excellent ODS performances of these catalysts can be mainly attributed to the following three aspects:1)Ultrafine W₂C,Mo₂C or metal W elemental as active species are highly dispersed on the carrier.2)The strong adsorption of the catalyst towards DBT through the?-?and S-?interactions between the NC/r GO and the benzene rings on the DBT as well as the acid-base interactions between DBT with a basic inherence and graphene at the edge of which a small number of acidic functional groups(such as carboxyl,lactone,and phenol)is,thus promoting the ODS reaction.3)DFT calculation depictes that the electron-donating N-r GO can increase the dispersion of W₂C and reduce its leaching via the strong interfacial electrostatic interaction caused by the electron transfer from N-r GO to W₂C,and the increased electronegativity of W₂C could accelerate the decomposition of H₂O₂ into·OH radicals,so as to improve the catalytic activity and stability.Radical experiments and density functional theory calculations indicate that the ODS reaction proceeds mainly via a·OH radical mechanism.