Immobilization Of Phosphomolybdates On Imidazolium Functionalized Organic-inorganic Hybrid Materials For Catalytic Olefin Epoxidation

Epoxide compounds are useful intermediates and chemical raw materials inorganic synthesis, and widely used for petrochemicals, fine chemicals, polymers andelectronic industry. Epoxidation of olefins is the most important reactions toapproach epoxides. Polyoxometalates (POMs) are a class of early‐transition‐metaloxides, which contain a number of coordinated metal atoms at high oxidation state.They could serve as excellent multi-electron acceptors, possess a good transmissioncapacity of electrons and protons, and exhibit excellent oxidation resistance.Therefore, POMs have shown attractive catalytic performance in the selectiveoxidation of olefins. However, almost all sophisticated POMs or POM-basedepoxidation catalysts are homogenous in essence. Much recent effort has beenfocused on achieving the heterogenization of homogeneous POM catalysts in orderto overcome their drawbacks, separation and recycling problems. Differentpreparation strategies have been developed to obtain heterogeneous POM‐basedcatalyst systems. These include immobilizing POMs on porous support surfaces,incorporating POMs in silica or metal oxide matrices via sol‐gel techniques,encapsulating POMs within nanocages of metal‐organic frameworks, andself‐assembling POMs with organic compounds to form supramolecular structures.To obtain highly efficient and stable POM‐based heterogeneous catalysts, anappropriate interaction between POMs and the support (or host) is usually necessary.This may include covalent bonds, electrostatic binding, hydrogen bonds, and/orother interactions. The support should possess a sufficiently high surface area, a large pore size, and a suitable surface polarity, as these parameters influence thedistribution and accessibility of catalytically active species.Based on the above, in this thesis, the supported POM-based catalysts wereachieved by the electrostatic immobilization of different kinds ofphosphomolybdates on the imidazolium functionalized organic-inorganic hybridmaterials. Their catalytic performances were investigated for the liquid-phaseepoxidation of olefins. Combined with different characterizations and experimentalresults, the effect of structures and surface properties of the supports on the catalyticproperties of POM-based catalysts was studied, and the essence of interactionbetween POM units and supports was also discussed for clarifying the nature ofactive sites and understanding the catalytic reaction mechanism. The main resultsand conclusions are as follows:1. Immobilization of phosphomolybdic acid on imidazolium functionalizedmesoporous materials for catalytic epoxidation reactionsA series of phosphomolybdic acid-based hybrid materials were prepared by ionexchange method, which referred to immobilize phosphomolybdic acid (PMA) onthe imidazolium ionic liquid functionalized supports. These supports includedmesoporous silica materials (SBA-15), mesoporous aluminum phosphate (AlPxO, x=0.9,1.0,1.1) and mesoporous carbon (NC-2). All the resulting hybrid materialsexhibited relatively high catalytic activity and nearly100%selectivity to cycloocteneepoxide in the cyclooctene epoxidation with TBHP as the oxidant. Among them,PMA@IL/AlP1.0O and PMA@IL/AlP1.1O were more active and stable than the otherthree kinds of hybrid materials. Combined with different characterizations, therelatively high catalytic performance of these two catalysts should be mainlyattributed to the characteristics of the AlPxO (x=1.0,1.1) supports, such as suitablestructures, compositions and surface properties.In addition, the catalytic activity of these PMA-based catalysts also beeninvestigated in the epoxidation of a wide range of allylic alcohols(3-methyl-2-butene-1-ol, trans-2-butylethylene-1-ol and geraniol). It was found that PMA@IL/NC-2acted as an efficient catalyst under TBHP and methanol system.This could be ascribed to the presence of the oxygen groups on the surface of NC-2support, which have improved the accessibility between allylic alcohols and PMAactive sites in protic solvent (methanol).2. Immobilization of phosphomolybdic acid on imidazolium-basedmesoporous organosilicas for catalytic olefin epoxidationThe periodic mesoporous organosilicas containing different imidazoliumcations contents (PMO-ILs, a and b) were chosen as supports for the electrostaticimmobilization of12-phosphomolybdic acid (PMA). The resulting hybrid materials(PMA@PMO-ILs,2a and2b) were characterized by a variety of techniques. Theresults illustrated that PMA was successfully anchored on the surface and in thechannels of the PMO-ILs via electrostatic interaction and the structure of both thePMO-IL supports and the PMA were retained during the preparation processes.The catalytic properties of these materials (2a and2b) were investigated for theliquid-phase epoxidation of cyclooctene, and found that they were active with nearly100%selectivity to cyclooctene epoxide using TBHP as the oxidant. Furthermore,these hybrid catalysts could be reused several times without obvious loss of activityor selectivity under identical reaction conditions. Through comparative studies witha PMA-functionalized SBA-15material (2c), it suggested that the presence of theimidazolium cations in the framework of PMO-ILs should play key role instabilizing the catalytic active units against leaching during the reaction process. Therole of imidazolium cations could be further confirmed by studying anotherPMA-based catalyst (2d), which was obtained by anchoring PMA species on aminofunctionalized PMO-IL support. The catalytic activity and stability of2d decreasedslightly owning to the introduction of amino groups, which might prevent the PMAanions from interacting with the imidazolium cations in PMO-IL support. Based onthe above results, it can be conclude that an appropriate interaction between thePOM units and the supports should be established, and the supports should possess arationally high specific surface area and large pore size in order to fabricate highly efficient and stable POM-based heterogeneous catalysts.3. Immobilization of transition-metal-substituted phosphomolybdate onimidazolium-based mesoporous organosilica for catalytic olefin epoxidationThrough the electrostatic immobilization of transition-metal-substitutedphosphomolybdate (TMSP) onto PMO-IL (a), a series of suppored catalysts wereobtained. The resulting hybrid materials, including PCoMo11@PMO-IL (3a),PMnMo11@PMO-IL (4a), PNiMo11@PMO-IL (5), PCuMo11@PMO-IL (6a) werecharacterized by means of ICP-AES, N2adsorption-desorption, powder XRD, FT-IR,UV-Vis, and XPS. Characterizations indicated that TMSP were successfullyimmobilized on the surface and in the channels of PMO-IL and the structuralintegrity of PMO-IL and TMSP was remained throughout the preparation processes.The catalytic properties of these catalysts were evaluated for the epoxidation ofolefins using molecular oxygen as oxidant in combination with a co-reductantisobutyraldehyde. It was found that these functionalized materials showed goodactivity in the epoxidation of a wide range of olefins (including terminal ones).Under test conditions,3a exhibited the highest activity and selectivity to epoxides.In addition, the influence of different reaction parameters (such as temperature,time, the molar ratio of isobutyraldehyde to olefin, and oxidants) on the catalyticproperty of3a has also been studied. Based on the related publications andexperimental results, a possible reaction mechanism for the epoxidation of olefinswith molecular oxygen in the presence of isobutyraldehyde over these TMSPfunctionalized catalysts was proposed. The first stage is the generation of an acylradical through the reaction between the aldehyde and the transition metal site of theTMSP. The acyl radical in turn react with dioxygen to produce acyl peroxy radical,which have several pathways to achieve the selective oxidation of olefins. In short,the types and valence states of transition metal species in the supported catalystscould impact the oxidation rate and selectivity towards epoxides. The relatively high catalytic activity and selectivity of3a might be assigned to the presence of CoIIwithrelatively high electropositivity.