Studies On The Synthesis Of Novel Tungsten-based No No-mate Rials And Its Catalytic Application In The Green Selective Oxidation Reactions

Hydrogen peroxide (H2O2) is one kind of environmentally friendly oxides for its product is only water which is non-toxic, therefore, using H2O2as the oxidant accords well with the demand of "green chemistry". In the H2O2system, the reaction is catalyzed by a number of transition metal compounds such as W, Mo and Mn. Among these efficient catalysts, tungsten containing compounds are cheap and do not decompose H2O2. Thus, there is considerable interest in the studies on the synthesis and characterization of novel tungsten-containing catalyst used in H2O2systemLactones and their derivatives are one kind of important compounds. Due to their high boiling point, solubility, conductivity and stability, the lactones are widely used as solvent, extraction agent and also can be used for the synthesis of a variety of polymers. The synthesis of lactones in industry has been attracted more and more attention in recent years. Phthalide and its derivatives are one kind of important lactones, which are commonly used in the manufacturing of dyes, pharmaceuticals, bactericides and other useful products. Phthalide is traditionally synthesized by reaction of phthalic anhydride with zinc and hydrochloric acid; reaction of phthalimide and sodium hydroxide; or hydrogenation of phthalic anhydride. During these processes, serious environmental pollution is generated, low yield of phthalide is obtained, or a high reaction temperature is required, which needs a great deal of energy.In this dissertation, we use tungsten-based nano-materials as the catalyst, aqueous H2O2as the oxidant to transform1,2-benzenedimethanol to phathalide. The purpose of this dissertation is to develop novel catalysts that is green and highly efficient. The preparation conditions of the catalysts and the composition of the catalysts were optimized. All the catalysts were characterized with various analytical and spectroscopic techniques and the influence of the morphologies, compositions and structure of the support on the catalytic activities has been studied intensively.1. Green catalytic process for the selective oxidation of1,2-benzenedimethanol to phthalide over tungstic acidA new, economic and efficient route for the production of phthalide by catalytic oxidationof1,2-benze nedimethanol with aqueous H2O2has been developed using tungstic acid as a catalyst under mild conditions. When the molar ratio of H2O2to1,2-benzenedimethanol is not higher than2.2:1, the conversion of substrate and the yield of main product (phthalide) increase with the increase of the molar ratio. When the molar ratio is higher than2.2:1, the yield of phthalide decrease with the increase of the molar ratio. Therefore, the optimized molar ratio is2.2:1. The influence of the reaction temperature, the dosage of the catalyst and the solvent were also investigated and the reaction condition was optimized as:reaction temperature353K, molar ratio of catalyst to substrate1.1%, solvent/-BuOH or water. Bearing in mind that phthalide is soluble in hot water and insoluble in cold water, a novel route to obtain phthalide with high purity was developed. Using water as the solvent, the catalyst can be easily removed when the excess H2O2is decomposed at363K after the reaction The product was separated out when the filtrate was cooled to room temperature and the separated phthalide was obtained. Melting point test,1H NMR spectroscopy and the XRD pattern of the product confirmed that the structure of the powder was pure phthalide.2. Synthesis of nano-scale tungsten oxides by hydrothermal method and their application in the selective oxidation of1,2-benzenedimethanolIn our previous work, tungstic acid was reported as an efficient homogenous catalysts for the title reaction. However, the difficulties of separating and recovering the catalysts restrict its application in large-scale industrial production processes. We then synthesized nano-scale tungsten oxides by hydrothermal method and investigated the influence of the preparation condition on their morphologies, and microstructure on their catalytic performances in the title reaction. TEM images showed that the nano-scale tungsten oxides are ribbonlike. With the prolonging of the hydrothermal time, the tungsten oxide ribbons became longer and longer. Too long hydrothermal time will lead to the rupture of the tungsten oxides. With the prolonging of the thermal treatment time, the BET surface area of the tungsten oxides increased. XRD results showed that the majority crystalline phase of the tungsten oxides is h-WO3. When the hydrothermal time is6h, the tungsten oxides contain WO3·0.5H2O and WO3·0.33H2O, and if prolonging the time to12h, the amount of WO3·0.33H2O increased. Further prolonging the time will lead to the disappearance of the WO3·0.5H2O and WO3·0.33H2O. It is found that the tungsten species was first transformed to WO3·0.5H2O and then WO3·0.33H2O, and finally converted to h-WO3. UV-vis DRS and Raman results confirmed that the as-obtained tungsten oxides are h-WO3. In the catalytic performance of the nano-scale tungsten oxides synthesize by hydrothermal method, it is found that the h-WO3was better than m-WO3and appropriate amount of WO3·0.5H2O and WO3·0.33H2O was beneficial for the catalytic performance of the tungsten oxides.3. Synthesis of WO3/W compounds and their application in the selective oxidation of1,2-benzenedimethanol and cycloocta-1,5-dieneIn our previous work, due to the high dispersion of the catalysts in the reaction system and the very small size of the catalyst, the collection of the catalyst after reaction is difficult. Metallic tungsten powder has high density and could quickly deposit after reaction. Therefore, we use tungsten and WO3/W synthesized by oxidation of tungsten with nitric acid as the catalysts in the selective oxidationof1,2-benzenediemethanol. In addition,9-oxabicyclo[3.3.1]nonane-2,6-dioles and2-hydroxy-9-oxabicyclo[3.3.1]nonane-6-one are the major starting material for the synthesis of lactones. They could be synthesized from selective oxidation of1,5-COD. Thus, we also use WO3/W as the catalyst in the selective oxidation of1,5-COD and investigate the influence of the preparation conditions on the catalytic performances. It is found that tungsten powder could easily deposit after the reaction. The WO3/W composite showed good catalytic performance in the selective oxidation of1,2-benzenedimethanol and1,5-COD. XRD and SEM results showed that the tungsten was gradually oxidized to WO3·H2O. Prolonging the oxidation time with nitric acid at348K is beneficial to the synthesis of rod like tungsten oxides, and prolonging the oxidation time with nitric acid at353K is beneficial to the synthesis of nano-sheets of tungsten oxides. At the same oxidation time, rod like tungsten oxides showed better catalytic performances than the nano-sheet tungsten oxides. In addition, the well-crystallized tungsten oxides showed better catalytic activities than the poorly-crystallized ones. When the WO3/W is treated in Ar atmosphere at elevated temperature, the morphologies disappeared and m-W03was formed, leading to the deactivationof the catalysts. 4. Synthesis of WO3/ZrO2catalysts and their application in the selective oxidation of1,2-benzenedimethanolIn our previous work, although the WO3/W can quickly deposit after the reaction, there is still leaching of tungsten species from WO3/W catalysts. So it is important to synthesize a more stable catalyst. We then prepared supported tungsten oxide using ZrO2as the support to study their catalytic application in the title reaction.A series of tungsten oxide supported on commercial ZrO2was synthesized via a traditional impregnation method using ammonium tungstate (AM), phosphotungstic acid hydrate (HPW) and tungstic acid-oxalic acid complex (OA) as the tungsten precursors. The supported catalysts were characterized by XRD, UV-vis DRS, Raman and XPS. It was found that the tungsten precursor and the calcination temperature were crucial to the dispersion and the nature of the tungsten species on ZrO2-The catalytic performances of the catalysts were investigated in the selective oxidation of1,2-benzenedimethanol to phthalide with H2O2. WO3/ZrO2(OA) shows the best catalytic performances because the tungsten species were highly dispersed on the catalyst and there was polymeric WO6units in it. WO3/ZrO2(HPW) showed the second best catalytic performance because there was polymeric WO6units in it while a small amount of crystallized WO3appeared on the catalyst. We also found that the optimum calcinations temperature of the catalyst is823K. When the temperature is too high, the tungsten species on the catalyst would aggregated and lead to the deactivation of the catalyst. ICP result of the reaction solution showed that there is almost no leaching of tungsten species during the reaction.5. Synthesis of WO3/SnO2catalysts and their application in the selective oxidation of1,2-benzenedimethanolAlthough there is no leaching of tungsten species when using WO3/ZrO2as the catalyst, the reusability of the catalyst is not good. Thus, we choose SnO2as the support to synthesize WO3/SnO2catalysts due to the strong interaction between SnO2and WO3.It is found that the calcinations temperature of the catalyst and the support is essential to the structure and the dispersion of the tungsten species on the WO3/SnO2and the catalytic performances in the selective oxidation of1,2-benzenedimethanol The optimum calcinations temperature of the catalyst is823K. BET results showed that the BET surface areas of SnO2decreased with the increase of the calcinations temperature. After addition of tungsten species, the BET surface area decreased slightly. XRD results showed that the addition of tungsten species suppressed the crystallization of SnO2and SnO2could also suppress the crystallization of WO3. When the calcinations temperature of the catalyst is too high, tungsten species begin to aggregate as crystallized WO3, leading to the deactivation of the catalyst. Raman and XPS results showed that when the support is not calcinated, there is too much W6+entered the lattice of SnO2, resulting in the deactivation of the catalyst. The optimized temperature is1023K. When the calcinations temperature of the support is too high, the tungsten species begin to aggregate and lead to the deactivation of the catalysts.