Synthesis, Crystal Structure And Catalytic Oxidation Performance Of Polyoxomolybdate Decorated By N-heterocyclic Ligands

Recently, the design and synthesis of inorganic-organic hybrid materials basedon polyoxometalates have drawn considerable attention. It was found that thestructures and properties of polyoxometalates (POMs) could be effective regulationby using the advantages of organic molecules, and constructed polyoxometalatebased functional materials with unique structure and properties. These materialswere often showed the advantage both of POMs and organic ligands, and haveextensive research and development space in the fields of medicine, optics, magneticmaterials, and catalysis and so on. However, compared to the rapid devepment in thefield of synthesis and structure, inorganic-organic hybrid materials based onpolyoxometalstes application in epoxidation of olefin have reported hardly. Recently,by the self-assembly of organic ligands and POMs, and already shown some specialadvantages for the application as efficient heterogeneous catalysts in olefinepoxidation. Presently, it is still a meaning study to develop N-heterocyclic ligandsmodified POMs compounds with novel structure, and investigate theses catalyticperformance for olefin epoxidation. Furthermore, although N-heterocyclic ligandsmodified POMs materials exhibit good catalytic activity, catalytic performance ofthe kinds of materials (activity, selectivity and stability) needs to be further improved.Besides, some basic issues also need further research and discussion, such asrelationship catalytic performances of heterogeneous catalyst based POMssupramolecular compounds with its organic ligand types, bond modes and spacestructure.Therefore, in this paper, we have synthesized a series of polyoxo molybdatecompounds based on N-heterocyclic ligands/copper complexes under hydrothermalconditions. The form, structure and bond mode of these materials were investigated by various characterization methods. In addition, their catalytic performances areinvestigated in epoxidation of olefin with tert-butylhydroperoxide (t-BuOOH) as theoxidant. The relationship catalytic performances of these catalysts with its organicligand types, bond modes and space structure were discussed. The main researchcontents and results were summarized as follows:1. Supramolecular Assembly Based on γ-Octamolybdate and TriazoleDerivative: Crystal Structure and Catalytic Application in Olefin Epoxidation.A novel polyoxometalate-based network supramolecular compound,[H(atrz)]4[(atrz)2(Mo8O26)]·2H2O (1)(atrz=3-amino-1,2,4-triazole), has beensynthesized under traditional hydrothermal method, using atrz as organic ligand andammonium molybdate as molybdenum source. In the structure of catalyst1, atrzligand through one nitrogen atom of a triazole ring covalently connected with onemolybdenum atom of γ-octamolybdate. This is a novel connection mode betweenatrz ligands and octamolybdate. Two ligand molecules covalently attached tooctamolybdate at two positions of center symmetry, and construct a[(atrz)2(γ-Mo8O26)]4-unit, forming a2D layer via hydrogen bonds interactions eachunits. The adjacent2D layers further interactions construct3D networksupramolecular structure via π-π stacking interactions.The catalytic properties of catalyst1were investigated in epoxidation of olefin(cyclooctene, styrene and1-octene) with t-BuOOH as the oxidant. The resultsindicated that catalyst1exhibited good catalytic activity in cyclooctene epoxidation.The catalyst1showed high selectivity for styrene oxide in epoxidation of styrenewhen chloroform as reaction solvent. The further results indicated that the catalyst1aslo exhibits good catalytic performances about the relatively inert terminal alkeneof1-octene. Leaching experiment results showed that no leaching of active species,and is a truly heterogenenous catalysis. The catalytic activity of the catalyst noobvious loss after five reaction cycles under identical conditions. It was found thatthe various characterizations of the used one are well consistent with fresh catalyst,suggested basic structure of catalyst1keep well during reaction process. The XPS confirmed that the catalyst1contains six valent MoVIspecies as active sites, whichshould be major reason for high catalytic activity and selectivity in catalyst1. Inaddition, structure analyses have shown that the existence of multi-weak interactions,like hydrogen bonds and π-π interactions, in the supramolecular structure of catalyst1should play positive role in improving the stability of the catalyst.2. Catalytic Epoxidation of Olefins over Supramolecular Compounds Based onMolybdenum Oxide Clusters and Copper ComplexesThe copper complexes modified molybdenum oxide cluster compounds,[Cu(bipy)]4[Mo15O47]·2H2O (2) and [Cu(bix)][(Cubix)(-Mo8O26)0.5](3) weresynthesized by hydrothermal method, using1,4-bis(imidazole-1-ylmethyl)benzene)(bix) or4,4′-bipyridine (bipy) as N-heterocyclic ligands. Single-crystal X-raydiffraction analysis reveals that compound2exhibits an open multitrack Cu-Ncoordination polymeric chain-modified molybdenum oxide3D supramolecularstructure. As for compound3, a main feature of this compound is that its3Dframework is penetrated by copper-organic polymeric chain, to form a3Dpolythreaded framework. The catalytic performances of two compounds wereinvestigated in epoxidation of cyclooctene, styrene and1-octene with t-BuOOH asthe oxidant. The catalytic results showed that the catalyst2and3exhibit highcatalytic activity in epoxidation of cyclooctene or1-octene, and both marked betterthan that (H2bix)[(Hbix)2(γ-Mo8O26)]2.H2O (4) of without Cu ions introduction. Thecatalytic performance of catalyst is solvent-dependent in epoxidation of styrene.When acetonitrile is chosen as solvent, the benzaldehyde was detected as majorproduct. Changing the solvent from acetonitrile to chloroform did not change theactivity order of the catalysts, but the selectivity to epoxide increased notably withthe decreasing of benzaldehyde selectivity. In addition, the leaching and recycleexperiments results that the catalyst2and3exbibit high stability and recyclicity.The catalyst2showed high catalytic performances in comparition with catalyst3could be attributed to two aspects: on the one hand, it was found that changing thetype of ligands could result in electropositivity of molybdenum species, thus considerably affecting their catalytic performance for the olefins epoxidation. TheXPS spectra showed electropositivity of catalyst2is higher than catalyst3, and infavor of obtain high activity catalyst in epoxidation of olefins; on the other hand,structure analysis showed that compared with the polythreaded structure of catalyst3,it seems that catalyst2have more open framework, which is more suitable to diffusethe reactants and to increase the opportunity what reactio n substrate contact with thecatalyst active sites, thus being helpful on improving the catalytic activity of thecatalyst2. Furthermore, the presence of different kinds of copper complexes incatalyst2and catalyst3might be also a key factor in influencing the catalyticproperties of the POM-based catalysts since the copper complex may also act as anew active site for the activation of t-BuOOH to form active intermediate, and showhigh efficiency catalyst. Besides, the presence of relatively strong coordinationbonds between the copper complexes and the molybdenum oxide clusters should bemajor reason for stable heterogeneous catalysts for the epoxidation of olefins.3. Cyclohexene Oxidation Performance Study of Polyoxometalates CompoundsDecorated by N-Heterocyclic Ligands/Copper ComplexesThe catalytic properties of catalyst1-4were also investigated in theepoxidation/oxidation of cychexene with t-BuOOH as the oxidant. The catalyticresults show that all the catalysts could serve as active and stable heterogeneouscatalyst for cyclohexene epoxidation. Among them, catalysts1and4exhibitrelatively high selectivity to cyclohexene epoxide, while cyclohexanol andcyclohexanone turn to major products in the case of2and3were used as catalysts.The influence of reaction parameters (temperature, solvent types etc.) on thecatalytic performance was studied for optimizing the reaction conditions to achivehigh yield for epoxide. The results of leaching and recyclable tests showed thatcatalyst1possesses high stability and recyclablility. These results suggest that thesupramolecular compounds based on molybdenum can act as truly heterogeneouscatalysts in epoxidation of cyclohexene under liquid-phase reaction conditions. Sixvalence MoVIspecies as active sites could be a major reason for high catalytic activity and selectivity (cyclohexene epoxide) in catalyst1and4. Besides, thecatalyst2and3showed high selectivity for cyclohexanol and cyclohexanone couldbe assigned to the fact that copper complexes could act as a sort of active sites makeoxidant produce radical species(C(CH3)OO.and C(CH3)O.), thus resulting in theformation of cyclohexanol and cyclohexanone and influence on product distributionof cyclohexene epoxidation.