Immobilization Of The Polyoxometalate And Its Oxidation Desulfurization Performance

Polyoxometalates?POMs?are the kind of oxygen cluster compounds,which are consisted of transition metals of Mo,W and V etc.and has the advantages of oxidation-reduction properties,easy to modify the structure.Therefore,they have showed great potential application in the fields of catalysis,magnetic materials and nano device surface,etc.However,when POMs are used as catalyst in homogeneous catalysts,they show obvious difficulty in recovery,small specific surface area of solid POMs(<10 m²·g^-1)and lacking of surface active sites.As a result,how to effectively solve the above problems is very important for development of POMs on the realization of practical application.Considering the above shortcomings,the purpose of this study is to develop immobilized POM-based catalysts,and to solve the existing problem of application of POM functional materials,such as small specific surface area,separation recycling and reuse,etc.The research contents are as follows.Using mesoporous metal-organic framework MIL-101 as support,with Weakley(LaW₁₀O₃₆)and Keggin type heteropoly acid as the guest and active substances,through post-synthetic impregnation and“ship-in-bottle”?in situ assembly of guest?method,three kinds of POMs-based host-guest composites were prepared.LaW₁₀@MIL-101 and PMoV@MIL-101were successfully prepared by impregnation method,because the guest molecules LaW₁₀O₃₆and H₄PMo₁₁VO₄₀ have poor stability.In order to obtain more POMs based catalysts with high dispersion and high load,using phosphotungstic acid?PTA?as guest molecules,the PTA@MIL-101 composite was successfully prepared by in situ assembly of guest method.The composites were characterized by XRD,FI-TR,³¹P NMR,BET,XPS,TEM,SEM/Mapping and etc.The results show that due to the small dynamic diameter and unique“cylindrical”molecular shape of Weakley structure,achieved“single molecule”dispersion in both large and small mesocage of MIL-101.Composite PMoV@MIL-101 was prepared by impregnation method,the guest molecules exist only in large cage of MIL-101,but not into the small cage.The PTA@MIL-101materials prepared by“ship-in-bottle”exhibit high dispersion and high loading,but when the loading of PTA is more than 36%,PTA molecules were growthed and agglomerated in the surface of MIL-101 nanocrystals.The catalytic oxidation coupled with extraction desulfurization performances of the synthesized three composites are studied in model diesel containing 500 ppm DBT.Using H₂O₂as the oxidant,acetonitrile as extractant,the POMs@MIL-101 showed excellent catalytic performance for the removal of refractory sulfur-containing compounds in model diesel under mild reaction conditions.The DBT conversion could reach as high as 97%with the catalyst mass 125 mg,reaction time 120 min and O/S=30.Moreover,the POM@MIL-101 could be easily recovered and reused several times without leaching or loss of activity.The results showed that the structure of composite LaW₁₀@MIL-101 still maintained after using more than three times.