Preparation Of Metal-organic Frameworks Confining Heteropoly Acid Composites And Their Oxidative Desulfurization Activities

In recent years,under the continuous implementation of the national environmental protection policy for clear water and green mountains,the pressure of environmental protection is gradually increasing.China has a clear regulation on the content of sulfur compounds in fuel oil.Looking around the world,China and most countries have clearly required that the content of sulfur compounds in fuel oil should not be higher than 10ppm,and the future may require no sulfur emissions.Therefore,the ultra-deep desulfurization with low cost and high efficiency has become a global research hotspot.Generally speaking,hydrodesulfurization is often used in traditional industrial desulfurization work.In fact,hydrodesulfurization technology in general for thiophene sulfur compounds,such as dibenzothiophene(DBT),4,6-dimethyldibenzothiophene(4,6-DMDBT),are difficult to be removed.Fortunately,oxidation desulfurization technology(ODS)can efficiently remove such thiophene compounds,and the consumption cost is low,the reaction conditions are relatively mild,can be carried out under room temperature and atmospheric pressure conditions,thus ODS is a green desulfurization technology with great development potential.In this paper,four kinds of highly efficient phosphoric molybdenum blue@defect Zr-MOF composite ODS catalysts were designed from the two perspectives of electronic effect and steric hindrance effect:PMA/Ui O-66?[Bmim]₃PMo₁₂O₄₀/Ui O-66?PMA/Thiourea/Ui O-66 and[Bmim]₃PMo₁₂O₄₀/Thiourea/Ui O-66.In the preparation process,the addition of Thiourea made more Mo⁵⁺and electron-bound oxygen vacancies generated in the composite catalyst,which improved the catalytic activity of oxidative desulfurization.In addition,due to the affinity between the high electron density thiophene ring and the electrophilic imidazole ring,the adsorption of4,6-DMDBT on the active center Mo atom is promoted,and the 1-butyl-3-methylimidazole(Bmim)phosphoric-molybdenum blue in Ui O-66 shows higher oxidation desulfurization activity than that of the phosphoric-molybdenum blue.The great chemical and structural tunability makes metal-organic frameworks(MOFs)to be used as functional supports for catalytic nanoparticles in a wide range of applications.However,a lack of understanding the structure-activity relationship for nanoparticle-encapsulated MOFs as efficient heterogeneous catalysts limits their usage.Herein,phosphomolybdic acid(PMA)is utilized as a precursor to disperse Mo nano species throughout the determined V-MOF structure.By varying the ratio of PMA to V-MOF,tetrahedrally coordinated MoO₄^2-confined in the V-MOF structure or octahedrally coordinated MoO₃ phase dispersed on the surface of V-MOF can be tuned to be formed,thus influencing the oxidative desulfurization(ODS)activity.The difference in the confining effects of V-MOF on resulted MoO₄^2-or MoO₃ active species are clearly clarified including spatial confining effect,electronic confining effect and kinetic confining effect by various characterizations.Oxygen vacancies and V⁴⁺with abundant electrons are found to be formed in the PMA/V-MOF composites,which are beneficial to ODS activity.Further,according to DFT calculations,tetrahedrally coordinated MoO₄^2-species confined in the V-MOF with V⁴⁺state is confirmed to be the most active among the Mo-O-V species.The clear structure-activity relationship is thus critical for designing new composite materials that can integrate the advantages of encapsulated nanoparticles and MOFs.