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

Glutaraldehyde (GA) is a very important fine chemical and intermediate product, which has been extensively used for purposes of disinfection and sterilization, the tanning process of leather, environmental protection, water treatment or oil field and so on. Currently, the commercial production of GA is mainly based on a multi-step process using expensive propenal and vinyl ethyl ether as starting materials, which however results in extremely high cost of GA. Thus, the potential applications of GA in wider fields are greatly compromised. An alternative way to produce GA is from the selective oxidation of cyclopentene (CPE) with environmentally benign aqueous H₂O₂ as the oxidant and tungstic acid as the catalyst, since a great quantity of CPE could be easily obtained by the selective hydrogenation of cyclopentadiene, a main by-product of the C-5 fraction in the petrochemical or coking industry. Although the high GA yield was obtained, their application in industrial processes seems impractical since the separation of these homogeneous catalysts from the reaction products is very difficult. One of the most promising approaches is to design the heterogeneous W-containing catalyst. Up to now, several heterogeneous W-containing catalysts (WO₃/SiO₂, WO₃/TiO₂-SiO₂, W-MCM-41 and W-SBA-15) have been synthesized for the aqueous hydrogen peroxide system by the conventional incipient wetness impregnation method or the novel in situ synthesis method. Although these heterogeneous approaches can solve the problem of catalysts separation, the yield of GA and the reusability of catalysts are not satisfied enough; moreover, the leaching of tungsten species could not be neglected and the intrinsic state of tungsten species in the catalysts is not clear yet, thus it is essential to solve the above problems. Therefore, the main purpose of this dissertation is to develop some novel tungsten doped mesoporous catalysts, in which the dispersion of the active species has been improved and the interaction between the active species and the supports has been strengthened or new reaction system is developed to improve the GA yield. In addition, the influence of different tungsten sources is investigated on the intrinsic state of tungstenspecies, as well as their catalytic activity for the selective oxidation of CPE, and all the catalysts have been characterized with various analytical and spectroscopic techniques.1. Synthesis of W-doped mesoporous zeolites and their application in the selective oxidation of CPE to GA by aqueous hydrogen peroxideA series of catalysts, such as W-HMS, W-MCM-48 and W-MCF, have been synthesized by in situ method with sodium tungstate and TEOS as tungsten and silicon resources, respectively. The optimum tungsten contents have been obtained through the activity test. The W-HMS catalyst with Si/W molar ratio of 30 shows excellent catalytic activity and selectivity. The conversion of CPE and the yield of GA are 100% and 76.3% after 24 h reaction, which are higher than those over 20wt.% W-MCM-48 catalyst on which the CPE conversion is 85.2% and the GA yield reaches 66.9%, respectively. For the W-MCF catalyst, the complete CPE conversion and 80.8% GA yield are obtained after 16 h reaction when the WO₃ content is 10 wt.%. The different catalytic activities among the three kinds of catalysts may be caused by their different texture and the different dispersion of tungsten species on the catalysts. The W-HMS possesses a much thicker framework wall, smaller domain size with short channels and wormhole structures, and larger textural mesoporosity; these properties provide better transport channels for reactants to access the active centers and better diffusion channels for products to move out. The W-MCF with well-defined ultra-large mesopores possesses high surface areas and porosities of ca. 80% and consists of uniform spherical cells interconnected with uniformly sized windows featuring a continuous 3D mesopore system, which entail more agitated flow and increase the interaction possibility between reactants and catalytic active centers. For the W-MCM-48 catalyst, although it has typical properties of mesoporous zeolite, the difficult synthesis method may cause the low dispersion of tungsten species and low catalytic activity.The catalysts are characterized by various modern techniques including BET, XRD, SEM, TEM, FT-IR and UV-Vis. DRS. BET results indicate that the surface area, pore volume of the catalysts decrease with the introduction of tungstic species while the pore diameter increases. Their special morphologies and mesoporous structures can be retained at low WO₃ contents and no peaks corresponding to crystalline WO₃are observed by XRD and Laser Raman, thus the tungsten species are highly dispersed in the catalysts. UV-Vis. DRS and H₂-TPR investigations confirm that the catalysts with low WO₃ contents mainly contain isolated [WO₄] tetrahedral tungsten species or monomeric or low condensed oligomeric oxide species, which have strong interaction with the silica-based matrix and can not be easily reduced. NH₃-TPD and Py-FT-IR adsorption experiments suggest that the strong Bronsted and Lewis acid sites are formed upon incorporation of tungsten in the mesoporous zeolite framework and the strong Bronsted acid sites of the catalysts are beneficial to their good catalytic performance.All the three kinds of catalysts show excellent stability in the title reaction, especially for the W-HMS. No detectable leaching of tungsten species into the product mixture could be found by ICP analyses (< 1.0 ppm). The used catalysts are characterized by XRD and Laser Raman. The results show that the crystalline WO3 appear and the active centers are covered by the organic contaminants, which might lead to the decrease of the activity. However, the high dispersion of WO3 and the outstanding catalytic activity can be recovered by simple calcination treatment.From the above studies, it can be concluded that the catalytic performance of the W-doped catalysts are strongly affected by the dispersion, the intrinsic state of tungsten species and the surface acidity. Therefore, the present work also studies the effect of the tungsten precursor on the dispersion, the state, and surface acidity of the tungsten oxide supported on SBA-15, including the tungstic complexes formed by tungstic acid and oxalic acid, the peroxo-tungstates formed by tungstic acid and hydrogen peroxide and the ammonium paratungstate. The results show that the highest dispersion and strongest surface acidity are obtained for the catalyst prepared from the tungstic complexes formed by tungstic acid and oxalic acid (WO₃/SBA-15(OA)). UV-Vis. DRS and H₂-TPR investigations confirm that the catalyst mainly contains isolated [WO₄] tetrahedral tungsten species or monomeric or low condensed oligomeric oxide species, which have strong interaction with the supports and can not be easily reduced. XPS results show that two types of tungsten oxide with oxidation states of 6+ and 5+ are present and the highly dispersed tungsten oxide may lead to the lower Si/W molar ratio. While the properties of the catalyst prepared from the ammonium paratungstate are much worse than those of WO₃/SBA-15(OA) catalyst. Hence, it is not surprised to find that the WO₃/SBA-15(OA) catalyst shows 100% CPE conversion and 84.5% GA yield.2. The study of the selective oxidation of CPE in the anhydrous H₂O₂/TBP systemThe mainly byproduct in the selective oxidation of CPE is cyclopentene-1,2-diol (>15%) from the reaction of the intermediate (CPE oxide) with H₂O according to the former reaction mechanism studies in our lab. Thus, it appears that decreasing water content in the reaction mixture may be beneficial for enhancing the GA yield. Thus, the anhydrous reaction system is developed with the non-aqueous hydrogen peroxide as the oxidant, tributyl phosphate (TBP) as the solvent and W-SBA-15 as the catalyst. The 20 wt.% W-doped SBA-15 catalyst shows 100% CPE conversion and 91.1% GA yield under the conditions with the CPE/TBP volume ratio of 7.5 and the CPE:H₂O₂:WO₃ molar ratio of 100:210:4, which is higher than those supported catalysts (WO₃/SBA-15, WO₃/MCM-41 and WO₃/SiO₂) and much higher (>12%) than those previously reported in aqueous system. The advantages compared to the aqueous system may be as follows: firstly, the extremely high yield of GA decreases the cost of the raw material remarkably; secondly, the yield of the main byproduct, cyclopentane-1,2-diol, is only 4.3%, which can be easily separated from the mixture by distillation; thirdly, GA with high purity (Medical Grade) can be easily obtained through decompressed rectification, due to the big difference of boiling point between GA and TBP(¹00°C).The W-doped SBA-15 catalyst is synthesized by in situ method under strong acid conditions with sodium tungstate as precursor. XRD patterns, FT-IR spectra and UV-Vis. DRS spectra all prove that tungsten has been incorporated into the SBA-15 framework uniformly at low WO₃ contents. The as-synthesized material shows typical structure of SBA-15. WO₃ species are highly dispersed into the lattice of the bulk and might be imbedded separately, which could be served as the active centers for the selective oxidation of CPE to GA. The FT-IR-pyridine adsorption has confirmed the presence of strong Bronsted acid sites and Lewis acid sites upon incorporating tungsten oxide species into the SBA-15 materials, which are beneficial to the catalytic performance.3. Preparation of mesoporous titania nanotubes and its application in the selective oxidation of CPE to GATitanium dioxide (TiO₂) is an important industrial material as a main component of paint, pigment, cosmetics and as a support for catalyst or photo-catalyst because of its high dielectric constant and refractive index. Owing to the various properties of mesoporous materials, it is becoming of great interest to synthesize mesoporous titania nanotube with TiO₂ nanoparticles as precursor. In this work, TiO₂ nanotube is prepared by using the alkali hydrothermal treatment of commercial P25 TiO₂ nanoparticles. Studies on the nanotube formation process indicate that the nanotube is formed during the alkali hydrothermal process, rather than washing process. XRD and BET results indicate that the as-synthesized TiO₂ nanotube shows only anatase phase with 209 m²/g BET surface area and 15.0 nm diameters calculated by using the BJH equation from the desorption branch of the isotherm. The WO₃/TiO₂ nanotube catalyst is prepared using incipient wetness impregnation method, which shows highly catalytic performance for the selective oxidation of CPE at 20 wt.% WO₃ content and the conversion of CPE as well as the GA yield reaches up to 97.9% and 67.9%, respectively.