Studies On The Synthesis Of Novel Tungsten-based Nanomaterials And Its Catalytic Application In The Synthesis Of Adipic Acid

Studies on the synthesis of novel tungsten-based nanomaterials and its catalytic application in the synthesis of adipic acidAdipic acid (AA) was known to be a versatile building block for an array of processes in the chemical, pharmaceutical and food industries, with a projected global market size of more than 6 billion pounds by 2017. Its primary use was as a precursor for the synthesis of the polyamide Nylon-6,6. Additionally, AA was widely used in the production of polyester and polyurethane resins, in the production of polyvinyl chloride as a plasticizer, and as an approved additive in cosmetics, lubricants, adhesives and insecticides. Notably, approximately 75 years after DuPont’s development of the first commercial AA process, long-standing interest in the improvement of AA synthesis strategy continues to inspire the catalytic community to explore new processes and resources.The current industrial process for the production of AA relied on the catalytic oxidation of a mixture of cyclohexanol and cyclohexanone (KA oil) with HNO3 as the oxidant. However, a major drawback of the nitric acid oxidation reaction was the stoichiometric reduction of HNO3 to NOx and the greenhouse gas nitrous oxide (N2O). The oxidation process of cyclohexene to AA with aqueous hydrogen peroxide as oxidant met the requirements of green production, because H2O2 as the oxidant was riskless and environment-friendly. However, the currently reported catalyst was mainly NaWO4·2H2O with organic solvents phase-transfer agents or organic ligand was added, which could not meet the requirements of green process, at the same time, the difficulties of separating and purifying the product and recovering the catalyst during this process made such catalysts impractical for large-scale industrial production process. Additionally, tungsten-containing silica or other nanomaterial has been also studied in the synthesis of AA, but these processes still had a lot of drawbacks, such as low conversion or poor selectivity and recyclability. The bio-catalytic process for the production of AA relied on the oxidation of D-glucose instead of using the petroleum product (non-renewable resources).This process was regarded as green route, but too expensive, which could not be used for large-scale industrial production..Therefore, the main purpose of this dissertation is to develop the novel and green route for the synthesis of AA with aqueous H2O2 as oxidant, at the same time, develop a novel catalyst with highly efficient, stable and recyclable performance in order to gain the economic benefit.1 Green catalytic process for adipic acid synthesis with W-based catalystA novel and green route for the synthesis of adipic acid (AA)from catalytic oxidation of cyclohexene oxide with aqueous H2O2 as the oxidant has been developped by using the tungstic acid (H2WO4) or phosphotungstic acid (HPW) as catalyst. The effect of reaction conditions on the yield of adipic acid was studied. The catalytic performance of the catalyst-supported with HPW as tungsten precursors was also explored initially.In the oxidation process of cyclohexene to adipic acid, compared with H2WO4 catalyst, HPW exhibited distinct advantage with better catalytic performance in terms of amount of catalyst and reaction time. The effect of substrate on the yield of adipic acid was also studied. It was found that high yield of adipic acid was obtained from the oxidation of cyclohexene oxide with low concentration of hydrogen peroxide. The optimal reaction conditions were:338 K(2.5 h),363 K(11.5 h), n(W):n(substrate)= 2.5%, n(H2O2):n(substrate)= 3.3,30 wt.% aqueous hydrogen peroxide as oxidant. Under above reaction conditions, the obtained AA yield was 95.5%. The catalytic performance of supported catalyst by using HMS as support and HPW as tungsten precursors was explored initially, it could be seen that the yield of AA was raised with the increasing loading amount of WO3. When the loading amount was 40%, the yield of AA reached 88.6%. However, high loading amount led to the weak interaction between the active species and support, so we studied the catalytic activity of the catalyst with the loading amount less than 40%. This heterogeneous catalytic process without any phase-transfer catalyst and organic solvent met the requirements of green chemistry, the simple separation between catalyst and the reaction system, as well as the easy purification of products.2. Effect of support on the structural evolution of W03-supported catalysts and its catalytic performance in the synthesis of adipic acidA series of tungsten-based catalysts were synthesized via a traditional impregnation method using SBA-15, HMS and SnO2 as the support. The supported catalysts were characterized by XRD, TEM (HRTEM), UV-vis DRS, Raman, XPS and FT-IR. It was found that the support was crucial to the dispersion and the morphology of the tungsten species on the catalyst. In this study,the catalytic performance of catalysts with different support were investigated in the synthesis of adipic acid from the selective oxidation of cyclohexane oxide. The excellent catalytic performance of the catalyst was obtained overWO3/SnO2, followed by WO3/HMS and WO3/SBA-15. The XRD results indicated that the crystalline degree of tungsten species of WO3/SnO2 catalyst was low and the particle size of WO3 was small. HRTEM and XPS results implied the high dispersion of tungsten species on the SnO2 support.The UV-vis DRS spectra demonstrated the existence of [WO4] and low-polymeric tungsten species. These characterizations could explain the excellent catalytic activity of WO3/SnO2 catalyst.3. Synthesis of WO3/SnO2 catalysts and their application in the oxidation of cyclohexene oxide to adipic acid with aqueous H2O2In our previous work, we found that W-Sn catalyst exhibited an excellent catalytic performance. Therefore, in this work, a series of WO3/SnO2 catalyst with different W loading amount and the calcination temperature of catalyst were studied. When the loading of WO3 was less than 20%, it was found that the particle size of WO3 was small and W species were highly dispersed on the catalyst. With the increasing loading amount of WO3, the W species began to aggregate which led to the formation of crystallized WO3 and the increase of particle size. However, the difference of catalytic activity of catalysts with different WO3 loading amount was not obvious, which maybe related to the catalytic mechanism of tungsten species in reaction system with hydrogen peroxide as oxidant. Calcination temperature of the catalyst played an essential role in the structure and activity of catalyst. When the catalyst was calcined below 923 K, the crystallization of WO3 was weak, and the tungsten species were mainly amorphous. With the temperature increased, part of WO3 was agglomerated to form crystalline WO3. Although the initial catalytic activity of the catalyst calcined at different temperature was similar, the stability of the catalyst varied greatly. When the catalyst calcined at 923 K, the catalyst could be used six times and the yield of adipic acid remained 80%or more which indicated that the proper calcination temperature of catalyst was essential to the structure and the dispersion of tungsten species on the WO3/SnO2 catalyst.