Preparation Of Transition Metal Substituted Phosphomolybdic Acid Encapsulated In The UiO-66 Framework For Catalytic Properties In Olefins Epoxidation

Epoxides are important chemical intermediates with wide range of applications in organic synthesis,polymer and pharmaceutical intermediates.Catalytic epoxidation of olefins is the main way for producing epoxides.So far,a variety of homogeneous catalysts,including various transition metal complexes,polyoxometalates and their derivatives,have been reported to be highly active for the epoxidation of olefins.However,these homogeneous catalytic processes face serious problems in product separation and catalyst recycling.For fitting the demand of eco-friendly chemistry,increasing attention has been drawn to studying and developing efficient heterogeneous catalysts.By immobilizing various polyoxometalates(POMs)on different solid supports,a variety of supported POM catalysts have been obtained.However,most of these supported POM catalysts still have some drawbacks,such as low loading of POM ingredients,unsatisfied epoxide selectivity,and easily loss of active POM species by leaching during the catalytic reaction term.Hence,it is still a highly attractive research subject to design and to prepare highly active and stable supported POM catalysts for the application in some industrially important olefin epoxidation reactions.Recently,the appearance of thermally and chemically stable Zr-based MOFs(like UiO-66 and UiO-67)has brought significant opportunity for designing more active and stable POM@Zr-MOFs hybrid heterogeneous catalysts.In general,POMs and metal?substituted POMs could be easily incorporated into MOFs via simple impregnation method.However,the supported POM catalysts prepared by this way usually suffer from low catalytic activity owing to the relatively low POM loading and nonuniform distribution of POM clusters.Moreover,leaching of POM commonly occurs during the liquid?phase catalytic process mainly due to the instability of the large POM clusters on the solid support surfaces.In this thesis,we mainly focus on the synthesis of a series of hybrid composites POM@UiO-66 based host-guest chemistry for the catalytic application in olefin epoxidation reaction.A variety of PMo₁₁M@UiO-66(M=Fe,Co,Ni,Cu)composites were synthesized by direct solvothermal synthesis method.The resultant POM@UiO-66 hybrids were characterized by various techniques,and their catalytic performance was investigated for the epoxidation of olefins with tert?butyl hydroperoxide as the oxidant,as well as for the aerobic epoxidation of styrene with molecular O₂ or air as oxidant,and aliphatic aldehydes as co-reductants.In addition,we also tried to clarify the nature of the catalytically active sites,and to reveal the epoxidation mechanism over the UiO-66 encapsulated POM catalysts by combining a variety of characterization results and the comparative experiments on free radical scavenging activities.The main research results are as follows:1.Hybrid composites of phosphomolybdic acid@UiO?66(PMo₁₂@UiO?66)and Co?substituted phosphomolybdic acid@UiO?66(PMo₁₁Co@UiO?66)were synthesized using the direct solvothermal method.A variety of characterization results demonstrated that phosphomolybdic acid(PMo₁₂)or Co?substituted phosphomolybdate acid(PMo₁₁Co)clusters are uniformly dispersed in the cages of Zr?based metal?organic UiO?66 frameworks.The catalytic properties of these hybrid composites were investigated by the epoxidation of olefins with tert?butyl hydroperoxide(TBHP)as the oxidant.Compared to PMo₁₂@UiO?66 and the reference catalyst prepared by impregnation method,PMo₁₁Co@UiO?66 showed much higher catalytic activity and stability,which could be simply recovered by filtration and reused for at least ten runs without significant loss of catalytic activity.The excellent catalytic activity and stability of the hybrid composite PMo₁₁Co@UiO?66 are mainly attributed to the confinement of the highly active PMo₁₁Co units within the smaller cages of UiO?66,to the suitable surface polarity of the hybrid composite for facilitating the access of reagents and solvent,and to the strong interface?interactions between the polyoxometalate and the UiO?66 framework.Additional experiments demonstrated that changing the types of solvents or oxidants could considerably influence the catalytic activity and stability of the catalysts.For instance,using acetonitrile or ethanol may lead to obvious decrease in the catalytic activity of the catalyst,while its stability turns to lower when using H₂O₂(aq)as the oxidant.Moreover,it was found that PMo₁₁Co@UiO?66 can also efficiently convert cyclic olefins like limonenes to the corresponding epoxides,and its selectivity to 1,2?limonene oxide could increase considerably from 44%to 91%when a small amount of radical inhibitor such as hydroquinone was added into reaction system.These results provide clear evidence that the path of producing Mo–OOR intermediate is responsible for the formation of epoxide,while the free radical intermediate route is in favor of the formation of allylic oxidation products.2.A series of transition metal-substituted phosphomolybdic acid,such as Fe,Co,Ni or Cu-monosubstituted POM,were encapsulated in UiO-66 frameworks(denoted as PMo₁₁M@UiO-66)by direct solvothermal method,and their catalytic properties were investigated for the epoxidation of styrene with molecular oxygen(O₂)as oxidant and pivalaldehyde(PIA)aldehydes as co-reductant.All the hybrid catalysts showed relatively high catalytic activity and epoxide selectivity.Among them,PMo₁₁Co@UiO-66 exhibited the highest catalytic activity,which could achieve an 80%conversion of styrene and 65%selectivity of STO after 2 h reaction at very mild conditions(313 K,ambient pressure).A 99%conversion of styrene could be reached when extending the reaction time to 3 h,which is among the most active and selective catalysts for the epoxidation of styrene with O₂/aldehyde.In addition,the hybrid composite catalyst can also efficiently catalyze the aerobic epoxidation of a variety of styrene derivatives with O₂/PIA.Their catalytic properties varied somewhat with the change of the substituted groups in types and positions.The substituted-transition metal atoms like Co in PMo₁₁M@UiO-66 should be the main active sites for the aerobic epoxidation of styrene with O₂/PIA,which can efficiently converting styrene to the corresponding epoxide through the activation of the in-situ generated acylperoxy radical intermediate.3.By using a cheap and easily-storable reagent of Zr OCl₂·8H₂O as Zr source,hybrid composites denoted as PMo₁₂@UiO-66-II and PMo₁₁Co@UiO-66-II were hydrothermally synthesized,which have shown almost identical structure and properties to that of derived from Zr Cl₄.By using air as oxygen source,which is much cheaper and easily available than molecular O₂,the catalytic properties of the hybrid composites were investigated for the epoxidation of styrene with PIA as co-reductant.Compared with molecular O₂ as the oxidant,the reaction rate of styrene epoxidation turned to slower when using air as the oxidant.The conversion of styrene reached 95%after 8 hours of reaction.Significantly,the STO selectivity increased considerably,up to 83%under the similar operation conditions,much higher than that of O₂/PIA system(65%).Moreover,PMo₁₁Co@UiO-66-II could also efficiently convert cyclohexene and1-octene to the corresponding epoxides by using air as the oxidant and PIA as the co-reducing agent.On the basis of the related literatures and some other comparative experiments,it could be revealed that reducing the oxygen concentration to a certain level may significantly reduce the concentration of acylperoxy radicals,thereby reducing the possibility of deep oxidation,thus leading to the significant improvement of the epoxide selectivity over the hybrid catalyst.