Studies On The Synthesis And Catalytic Application Of Novel Tungsten-based Nano-scale Materials In The Selective Oxidation Of Olefins

As we know,the oxidation of olefins is a very useful and important reaction in organic synthesis.Classic routes for the reaction employ chlorocarbon such as chloroform and 1,2-dichloroethane as solvents and peroxo acids as oxidants.These reaction processes fall short of green chemistry.Nowadays,the development of environmental friendly techniques is one of the prior goals of chemical research,which is especially true in the field of the oxidation of olefins where there is an urgent need to replace wasteful and toxic stoichiometric oxidants with "clean" oxygen donors,such as hydrogen peroxide and oxygen.It is known that oxidation of organic substrates with hydrogen peroxide is very attractive and has been long studied.Hydrogen peroxide as oxidant is riskless and non-polluting. In hydrogen peroxide system,the reaction is catalyzed by a number of transition metal compounds such as W,Mn,Mo,and Rh.Among the most efficient catalysts,tungsten containing compounds are cheap and do not decompose to H₂O₂.Thus,there is considerable interest in the synthesis and characterization of novel tungsten-containing catalysts.It is commonly accepted that 9-oxabicyclo[3.3.1]nonane-2,6-dioles(1) and 2-hydroxy-9-oxabicyclo[3.3.1]nonane-6-one(2) are the major starting material for the synthesis ofγ-butyrolactones.γ-Butyrolactone structure is a versatile building block in organic synthesis since countless compounds containing this function group show interesting biological activity.For example,5-hydroxy-γ-decalactone is a potent cytotoxic agent on different tumor cell lines.Therefore,it is of great significance to develop efficient and accessible approaches to afford these products(1 and 2).Classic routes for 1 and 2 employ peroxo acid or permanganate as oxidants.However,the performic acid,as well as peracetic acid and permanganate,is expensive chemicals and leads to great amounts of byproducts.Therefore,difficult separation of 1 from the products mixture is inevitable in these routes because of the low conversion and selectivity.Herein we report a green procedure for the O-hetero-cyclization of cycloocta-1,5-diene(COD) by catalytic oxidation with aqueous H₂O₂.The main purpose of this dissertation is to develop novel catalysts that is highly efficient,highly stable and can be reused for several times.In addition,different types of tungsten containing catalyst is investigated,as well as their catalytic activity for the selective oxidation of COD,and all the catalysts have been characterized with various analytical and spectroscopic techniques.1.Green Catalytic Process for the Selective Oxidation of cycloocta-1,5-diene over tungstic acid and phosphotungstic acidNovel one-step green process for the synthesis of compounds(1 and 2) from the selective oxidation of COD has been reported with hydrogen peroxide(H₂O₂) as oxidant,tert-butanol as solvent,and tungstic acid(H₂WO₄) or phosphotungstic acid (H₃PW₁₂O₄₀) as catalyst.It can be seen that the conversion of COD rises with the increase of molar ratios of H₂O₂ to COD.And the molar ratios of H₂O₂ to COD can dramatically affect the yield of the object products.Considering both good catalytic performance and H₂O₂ utility,the 3:1 ratio of the H₂O₂ to COD is needed.It was found that conversion of COD in a fixed reaction period ascends with increasing catalyst dosage,and the amounts of catalysts not only affect the rate of the reaction but also affect the distribution of 1 and 2.For the alcoholic solvents,tert-butanol is the best,the large alkyl of which hinders the formation of ethers.Some other water-soluble solvents, such as 1,4-dioxane,acetonitrile and tetrahydrofuran,having no trend to give ethers, were also tested,and the conversion of COD and selectivity of object products are also high.There is an optimum volume ratio of tert-butanol to COD at which a maximum amount of 1+2 is formed.Initially,with a decrease in the volume ratio from 20 to 5,the selectivity of object products increases from 88 to 98%.Among different heteropoly acids,phosphotungstic acid is the best one in conversion and selectivity of the reaction.It is also found that phosphotungstic acid was very suitable as catalyst for this reaction.The conversion of COD rises with the increase of molar ratios of H₂O₂ to COD.The conversion of COD rises with the increase of catalyst dosage.Tert-butanol is also a good solvent.The conversion of COD and selectivity of 1+2 rises with the decrease of volume ratios of solvent to COD in the volume ratio from 20 to 10,the good results can be got within the volume ratio of 10.The conversion of COD and selectivity of 1+2 rises with the increase of reaction temperature.The optimum temperature is 60℃considering the H₂O₂ utility.This process that doesn't need any phase-transfer catalysts meets the requirements of environmental protection with H₂O₂ as the oxidant and water as the only by-product.In addition,this method was highly efficient,which accords well with all of the requirements of green chemistry,resulting in its possibility in large-scale industrial production.2.Synthesis of W-containing mesocellular silica foam catalysts and their application in the O-heterocyclization of cycloocta-1,5-diene with aqueous H₂O₂In our previous work,tungstic acid was reported as an efficient homogeneous catalyst for the title reaction.However,the difficulties of separating and recovering the catalysts from the product mixture during the homogeneous process made such catalysts impractical for large-scale industrial production processes.The in situ synthesized method-derived 10 wt.%WO₃-MCF and the impregnated method-derived 10 wt.%WO₃/MCF catalysts both exhibit excellent performances for the target reaction(O-heterocyclization).The ultralarge mesopores of the catalysts are helpful for the transport of the large raw material and products during the reaction.The heterogeneous tungsten trioxide containing MCF catalyst synthesized via the in situ synthesized method shows higher WO₃ species dispersion compared to the one synthesized by the impregnation method as proved by XRD,TEM,Raman and UV-vis DRS results.According to the XRD result,the tungsten oxide on the catalyst synthesized by the impregnation method is more easily aggregated after the reaction than the one on the catalyst synthesized by the in situ synthesized method.TPR and XRD results show that the 10 wt.%WO₃-MCF catalyst shows stronger interaction between active tungsten oxide species and the MCF material than that of WO₃/MCF catalysts.The recycling experiment results indicate that the in situ synthesized method- derived catalyst shows far better stability than the impregnated one.Although there is detectable leaching of tungsten species from 10%WO₃-MCF based on ICP-AES analysis,the in situ method derived WO₃-MCF catalyst can still be reused for more than 4 times.3.High-activity,single-site mesoporous WO₃-MCF materials for the catalytic epoxidation of cycloocta-1,5-diene with aqueous hydrogen peroxideIn our previous work,WO₃-containing mesocellular silica foam catalysts is highly efficient in the O-heterocyclization of cycloocta-1,5-diene.However,there is detectable leaching of tungsten species from 10%WO₃-MCF based on ICP-AES analysis.So it is important to find a more stable catalyst.According to the characterizations,it can be seen that the tungsten species are presented as the isolated tetrahedral {WO₄} species,low-condensed polymeric tungsten oxide species, high-condensed polymeric tungsten oxide species and a little crystalline WO₃.It is hard to understand the role of the isolated tetrahedral {WO₄} species over the reaction. In the present study,in order to understand the role model of isolated tetrahedral {WO₄} species on mesocellular silica foam materials and how these properties influence the catalytic activity and selectivity,the isolated tetrahedral {WO₄} species doped mesocellular silica foam(MCF) materials are synthesized and systematically characterized by various analytical and spectroscopic techniques,including N₂ sorption,TEM,UV-vis DRS,UV-Raman and XPS.The AMA-treated WO₃-MCF catalysts exhibit good performance which has been attributed to the ultra-large mesopores of the catalysts for this reaction.The AMA-treated catalysts retain the special structure of the supports under the treating conditions.The most isolated tungsten atoms are well embedded in the supports when the catalysts were treated with AMA,which were proved by UV-Vis.DRS,XPS and UV-Raman experiments. The UV-Vis.DRS results also indicate the crystalline tungsten trioxide species were firstly removed,then the polymeric WO₃ species.The tungsten percent comes to a fixed value for the catalysts of different WO₃ loadings as confirmed by ICP method. Different preparation methods lead to different final residual tungsten percent,but only the in situ method can lead to the perfect single-site {WO₄} tetrahedral species. It is also demonstrated from the recycling experiment that the AMA-treated WO₃-MCF catalyst shows excellent stability.4.Preparation and catalytic behavior of highly active and stable amiuopropyl-immobilized phosphotungstie acid on mesoeellular silica foam for the O-heterocyclization of cycloocta-1,5-diene with aqueous H₂O₂Although it is demonstrated from the recycling experiment that the AMA-treated WO₃-MCF catalyst shows excellent stability,the main product is COD epoxide. Hence,there is a strong driving force to find a highly active catalyst without any leaching of tungsten species so that no heavy metal containing waste water was generated.The heteropoly phosphotungstic acid,H₃PW₁₂O₄₀,has been successfully immobilized on the surface of mesoporous MCF,SBA-15 and MCM-41 by means of chemical bonding to aminosilane groups.Characterization results from N₂ sorption indicate that the surface area decreased after grafting amine to silica.The aminopropyl functional groups were successfully grafted on the MCF silica from ¹³C and ²⁹Si MAS NMR results.The strong interaction between the NH₂ groups in the silanes moieties and HPW molecules can be convinced from FT-IR and XPS results.³¹p MAS-NMR indicated that the HPW can keep its Keggin structure after the immobilization.The HPW-NH₂-MCF is highly efficient in the title reaction with a COD conversion up to 100%and(1+2) selectivity up to 98%.Comparison of 16%HPW-NH₂-MCF,16% HPW-NH₂-SBA-15 and 16%HPW-NH₂-MCM-41 reveals that 16%HPW-NH₂-MCF is the most effective one.This finding uncovers that the ultra large pores and the unique three dimensional cell-window of MCF are more favorable for the title reaction than the SBA-15 and MCM-41 counterparts owning only two dimensional pore structures.The HPW-NH₂-MCF could be used for more than six times without any significant loss of activity and leaching of tungsten species in the reaction mixture. The good stability can be attributed to the strong interaction between the -NH₂ groups in the silanes moieties and HPW molecules.4.The study of the selective oxidation of other olefins over the immobilized catalysts and the low cost exploring of the catalystsThe HPW-NH₂-MCF is highly efficient in the selective oxidation of cyclopentene(CPE) to glutaraldehyde(GA) with a CPE conversion up to 100%and GA selectivity up to 71.8%.CPE conversion keeps up to 99%and GA selectivity keeps up to 60%,when the 24%HPW-NH₂-MCF was reused for eight times. Heterogeneous experiment indicates that the 24%HPW-NH₂-MCF catalyst is actually a heterogeneous one.The HPW-NH₂-SBA-15 is highly efficient in the selective oxidation of dicyclopentadiene(DCPD) to dicyclopentadiene dioxide with a DCPD conversion up to 100%and dicyclopentadiene dioxide selectivity up to 97%.DCPD conversion keeps up to 100%and Dicyclopentadiene dioxide selectivity keeps up to 96%,when the HPW-NH₂-SBA-15 was reused for six times.The recycling experiment results indicate that the immobilized catalyst shows much stability than the impregnated one.Heterogeneous experiment indicates that the 16% HPW-NH₂-SBA-15 catalyst is actually a heterogeneous one.The HPW-NH₂-SiO₂ is highly efficient in the O-heterocyclization of COD to 1 and 2 with a COD conversion up to 100%and(1+2) selectivity up to 95.2%.Heterogeneous experiment indicates that the 16%HPW-NH₂-SiO₂ catalyst is actually a heterogeneous one.The recycling experiment results indicate that the immobilized catalyst shows far better stability than the impregnated one.The HPW-NH₂-SiO₂ could be used for more than seven times without any significant loss of activity and leaching of tungsten species in the reaction mixture.The HPW-NH₂-SiO₂(commercial) is highly efficient in the O-heterocyclization of COD to 1 and 2 with a COD conversion up to 100%and(1+2) selectivity up to 95.7%.The HPW-NH₂-SiO₂(commercial) could be used for more than six times without any significant loss of activity and leaching of tungsten species in the reaction mixture.Heterogeneous experiment indicates that the 16% HPW-NH₂-SiO₂ catalyst is actually a heterogeneous one.