Synthesis, Crystal Structure And Catalytic Performance Of Phosphotungstic Heteropoly Acid Decorated By N-Heterocylic Ligand And Transition Metal

Polyoxometalates(POMs) have broad applications and prospects in many fieldsdue to their advantages: definite structures, controllable sizes, acid and redoxproperties， low corrosivity, low toxicity and low temperature highly activeproperties. Especially in the field of catalysis, POMs show excellent catalyticperformance, which can be used as acid-base catalyst, oxidation-reduction catalyst orbi-functional catalyst in both homogeneous and heterogeneous systems. In recentyears, introducing abudant organic ligands or their transition metal complexes intoPOMs, not only enrich the structures of POMs, but also regulate their structure,electricity, acidity, redox properties and electricity. This kind of organic-inorganichybrid materials have show their advantages in the fields of catalysis, magneticmaterials, pharmaceutical chemistry and so on, and obtained widespread attention.Recently, some papers about organic-inorganic materials based on POMs have beenreported, and these materials have shown some special advantages for theapplication as efficient heterogeneous catalysts in olefin oxidation.In this context, there will be huge space of extensively study and development, inpreparing N-heterocyclic ligands modified POMs compounds with novel structure,and using them as catalysts in olefin oxidation reactions. In addition, although thesecatalysts perform good catalytic activity, their stability and selectivity are also worthto study.Based on above conditions, we have synthesized three organic-inorganic hybridcompounds based on phosphotungstic heteropoly acid, N-heterocyclic ligands andtransition metal complexes under hydrothermal conditions. The compositions,structures and coordination modes of these materials are characterized by variouscharacterization methods. The catalytic properties of the three materials are investigated in the olefin oxidation reactions. The main research contents and resultswere summarized as follows:1、 Synthesis, crystal structure and catalytic properties study ofphosphotungstic heteropoly acid decorated by N-Heterocylic ligands andtransition metalA novel polyoxometalate-based network supramolecular compound,[Cu6(3atrz)6][PW12O40]2· H2O (1)(atrz=3-amino-1,2,4-triazole), has beensynthesized under traditional hydrothermal method, using organic ligand atrz,Keggin type H3PW12O40and transition metal Cu. The guest anion is a normal α-Keggin structure composed of12corner-or edge-sharing WO6octahedra with thecentral phosphorus ordered and coordi-nated to four oxygen atoms in a tetrahedralfashion. The host metal-organic cations and guest [PW12O40]3-ions formthree-dimensional supramolecular network through weak N-H O hydrogen bonds.When the guest polyanions are removed, the Cu-trz coordination fragments present1-D honeycomb tubular channels along the c axis.In the early experiment, we prepared two catalysts with bbi(bbi=1,1’-(1,4-butanediyl)bis(imidazolium), Keggin type H3PW12O40and Cu or Ag,denoted as bbi-Cu-PW12O40(catalyst2) and bbi-Ag-PW12O40(catalyst3) respectively.Although crystal structures are not obtained, related characterization methods showthe heteropoly tungstutes, transition metal clusters and the ligand are contained inthe structures.2、Olefins oxidation performance study of three catalyst materialsThe catalytic properties of the three catalysts were investigated in oxidation ofolefin (cyclooctene and styrene) with TBHP as the oxidant and chloroform as thesolvent. Catalyst1exhibits the best catalytic activity both in cyclooctene epoxidationand styrene oxidation reaction. Catalyst2exhibits the highest selectivity of styreneoxide in epoxidation of styrene, and catalysts3exhibits lower activity and selectivityin oxidation reactions.We optimize the reaction conditions for catalyst1with good catalytic activity.These conditions include reaction temperature, solvent types, and amount of solvent,oxidant and catalyst. Recycle experiments are tested with the same method everytime. The result shows that conversion of styrene remains basically the same after three times, slightly elevated in the fourth recycle reaction appears, indicating thatcatalyst1is stable relatively. IR and XRD characterization for fresh catalyst and theforth cycle catalyst show that the structure of catalyst1keeps relative stability andalmost no change. These results reveal that catalyst1has better recyclability inoxidation of styrene.