Synthesis Of Polyoxometalates-based Carbon Nitride Catalysts And Their Ultra-deep Desulfurization Performance

Although the development of new energy sources is in full swing,fossil fuels still dominate in transportation and industrial production.The combustion of fuels contributes to the emission of sulfur oxides and thus creates hazards,which has been widely concerned in the world.The cleanup of energy is a major direction of energy development,and how to effectively remove sulfide from fuels to meet the strict national regulations for fuel emissions（<10 ppm）is an urgent issue.Extractive catalytic oxidation desulfurization（ECODS）has received a lot of attention due to its ability to achieve ultra-low levels of sulfur in fuels under mild conditions（reaction temperature<100°C at atmospheric pressure）.The development of high-performance catalysts is the keyway to realize this technology.In this paper,we synthesize catalysts based on carbon nitride and two types of polyoxometalates,Keggin and Linqvist,and perform extractive catalytic oxidation desulfurization experiments,and the main experimental results are as follows:1.The graphitic phase carbon nitride（g-C₃N₄）was prepared by thermal polymerization with melamine as a precursor,and then the loaded catalyst PMo₆W₆/g-C₃N₄ was generated by thermal polymerization.By this method,the polyoxometalate PMo₆W₆ was loaded onto the carrier g-C₃N₄,which can reduce the loss of active site PMo₆W₆ and facilitate the recycling of catalyst PMo₆W₆/g-C₃N₄.The results of desulfurization experiments showed that the optimum loading of PMo₆W₆ in the catalyst was 50%,and 0.015 g 50.0%PMo₆W₆/g-C₃N₄ could completely remove dibenzothiophene（DBT）under the optimal reaction conditions of T=70℃,t=20 min and O/S=8.In addition,the catalyst showed good performance for benzothiophene（BT）and 4,6-dimethyldibenzothiophene（4,6-DMDBT）also showed good degradation.2.The catalyst PW₁₂/g-C₃N₄ was synthesized in situ by thermal polymerization method using PW₁₂,which has excellent thermal stability among Keggin-type polyoxometalate,and melamine,and the synthesis process was simple and green and environmentally friendly.The PW₁₂/g-C₃N₄ was used in the ECODS system,and the optimum loading of 0.20 g was obtained by adjusting the loading of PW₁₂.Complete desulfurization was achieved in 20 min under the reaction conditions of the catalyst PW₁₂/g-C₃N₄-0.20 using 0.015 g,the reaction temperature of 60℃and O/S=4.The catalyst also showed some degradation effect on BT and 4,6-DMDBT.The stability of the catalyst was evaluated by cyclic experiments and the mechanism of desulfurization was speculated.3.Interfacial effect is one of the important factors affecting the performance of extractive catalytic oxidative desulfurization.The long carbon chain modification and immobilization were combined to synthesize the catalysts（DTA）₂[Mo₆O₁₉]/g-C₃N₄、（CTA）₂[Mo₆O₁₉]/g-C₃N₄、（OTA）₂[Mo₆O₁₉]/g-C₃N₄ to enhance their dispersion performance in oil and aqueous phases and stability.The successful synthesis of these three catalysts was demonstrated by characterization such as FT-IR and PXRD.The results of desulfurization experiments showed that the catalyst（CTA）₂[Mo₆O₁₉]/g-C₃N₄ had better catalytic ability for the hard-to-degrade aromatic sulfides in fuel,and DBT could be completely removed at 60℃for 15 min,and the catalyst was also effective for BT,4,6-DMDBT under this condition.The results of the catalyst cycling experiments showed that（CTA）₂[Mo₆O₁₉]/g-C₃N₄ has excellent stability and can still remove DBT completely in 12 cycles and 90%of DBT can be removed in the 14th cycle.