Preparation Of Polyoxometalate-based Heterogeneous Solid Catalyst And Its Catalytic Effect On Oil Transesterification

This study investigated a class of polyoxometalate based（POMs）solid catalysts supported on metal organic frameworks（MOFs）materials.Heteropolyacids functionalized with metal cations,metal oxometalate clusters{Mo₁₃₂},and imidazole based ionic liquids were loaded onto mesoporous supports ZIF-8,multi-level porous supports SOM-ZIF-8,and SOM-Cu-BTC,respectively,to prepare Sn PW@ZIF-8,{Mo₁₃₂}@SOM-ZIF-8 and HPA/IL@SOM-Cu-BTC,three new efficient solid acid catalysts for oil transesterification.The chemical bond composition,element types,crystal structure,morphological characteristics and particle size,properties of porous structure,acid strength and acid type,hydrophobic property and thermal stability of the prepared catalytic materials were analyzed by characterization and testing methods.Using soybean oil and methanol as raw materials,the optimal conditions of transesterification reaction of the catalyst were investigated through single factor experiments,and the water and acid resistance and stability of the prepared solid acid catalyst were investigated.The Sn PW@ZIF-8 catalyst first prepared metal tin doped heteropoly acids through proton exchange reaction.Sn PW was bound to the carrier through coordination with the imidazole group in ZIF-8 through the W=O bond at the end of phosphotungstic acid,achieving the loading of active centers.The SEM and TEM test results indicate that the loading of Sn PW metal oxide salt still maintains the structure of ZIF-8 carrier.The N₂adsorption desorption test results show that the loaded catalyst still has a large specific surface area of 258.27 m²·g^-1.Py IR test results showed that the catalyst had Lewis acid site and Br（?）nsted acid site,and their synergistic effect could significantly improve the transesterification activity of the catalyst.Under optimized conditions of reaction temperature 140℃,methanol to oil molar ratio 35:1,and catalyst loading of 6 wt%,92.8%oil conversion rate and 96.4%FFA conversion rate were obtained.This solid catalyst has good tolerance to FFA and moisture in oils and fats,making it more suitable for catalyzing low-grade acidic oils.Moreover,the catalyst has good reusability,and even after 5 reaction cycles,the oil conversion rate remains above 84%.A 3D ordered macroporous SOM-ZIF-8 support was prepared using a template method.The polyoxometalate cluster{Mo₁₃₂}was grafted onto the surface and pores of SOM-ZIF-8through coordination,forming a multi-level porous{Mo₁₃₂}@SOM-ZIF-8 solid acid catalyst with efficient mass transfer.Compared with the catalyst without macropore,the catalyst showed higher catalytic activity,highlighting the high efficiency of mass transfer of macropore structure in catalytic transesterification.The oil conversion rate of 61.6%can be achieved when the reaction time is 2h,indicating that the hierarchical porous structure of the catalyst can improve the transesterification rate of oil macromolecules on the catalyst surface.SEM and TEM tests show that the catalyst has uniform channels and independent single crystal structure.The regular channel structure reduces the diffusion resistance of oil macromolecules on the catalyst surface,and the independent single crystal structure keeps the catalyst structural stability to a certain extent.The N₂adsorption desorption test results show that the loaded catalyst still has a large specific surface area of 736.26 m²·g^-1.The catalyst has good stability through reuse experiment,leakage experiment and leaching experiment,and the catalyst can be reused for 6 cycles in transesterification,and still has81.9%soybean oil conversion rate.HPA/IL@SOM-Cu-BTC The catalyst first constructs an ordered macroporous SOM-Cu-BTC carrier based on the metal organic skeleton Cu-BTC through template method.Then,based on the unique hierarchical porous structure of the multi-level porous metal organic skeleton material,the heteropolyacid ionic liquid is encapsulated into the pore structure of the skeleton material using encapsulation method,overcoming the disadvantage of heteropolyacid leakage in the reaction system and making the heteropolyacid ionic liquid uniformly dispersed on the surface of the carrier,It provides more active site for the transesterification of oils and fats,which is conducive to the efficient conduct of transesterification.And mesoporous HPMo/IL@Cu-BTC Compared with the catalyst,the catalyst has higher catalytic activity for transesterification reaction,which reflects the structural advantage of multi-stage pore structure in transesterification reaction system.Under the optimal conditions of transesterification,the catalyst can achieve 93.3%soybean oil conversion,and can be reused for 5 times without significant reduction in conversion.