Study Of The Novel Catalysts For Oxidative Dehydrogenation Of Propane And Ethylbenzene

The present dissertation, involving the synthesis and characterizations of novel catalysts for oxidative dehydrogenation (ODH) of propane and ethylbenzene (EB), consists of three parts. The first part (Chapter III) is the synthesis of mesostructured SiC materials with high surface area and the study for ODH of propane as catalytic support. The second part (Chapter IV) reports the synthesis of ceria-base catalysts and the catalysis study for ODH of ethylbenzene. The last part (Chapter V) discusses the catalysis study of ceria-supported nanogold catalysts for ODH of EB.1. Synthesis of SiC materials with high surface area and the catalysis study for ODH of propane as catalytic supportAs one of major building blocks for modern petrochemical industry, propylene is widely used for the production of diverse products, ranging from rubbers to plastics. Nowadays, propylene are mostly produced by steam pyrolysis of naphtha (as coproduct of ethylene), and fluid catalytic cracking (FCC) in oil refining. As the oil gets scarcer and more expensive, the two convenient processes could not meet the tremendous demand of propylene. Meanwhile, the new process of the synthesis of propylene via the direct dehydrogenation (DH) of propane still presents some disadvantages, such as the thermodynamical limitations, high temperature, and easy deactivation. Alternatively, based on the introduction of oxygen into the feedstock, the process of ODH of propane could decrease the reaction temperature and simultaneously, prevent the deactivation and thus has spurred the interest as a promising route. Up to now, the catalytic supports applied for ODH of propane are mainly traditional metal oxides such as SiO2, Al2O3. It is noteworthy that, in the heterogeneously exothermic reaction, the support with a low thermal conductivity cannot disperse the reaction heat well, issuing in partial hot spots on the surface of the catalyst bed, wherein the temperature is far beyond the average level. On the other hand, owing to its highly reactive nature, the propylene molecular in the vicinity of the hot spots is liable to consecutive oxidation, which leads to low selectivity of the target product. SiC material possesses highly thermal conductivity and excellent mechanical strength, which enables it wide application in the fields of catalysis, electronics, etc. Recently, Ledoux et al. has reported that the SiC-supported VPO catalyst demonstrated superior catalytic performance to SiO2-supported catalyst for the selective oxidation of butane into maleic acid, due to its high thermal conductivity. Moreover, SiC material has been extensively used as catalytic supports in the TWC catalytic system, partial oxidation of CH4, and other heterogeneously exothermal reactions.A wide range of methods have been reported for the synthesis of SiC materials and therein the sol-gel method presents the convenience and low cost and other several advantages, and therefore is widely used to fabricate the SiC materials. However, due to the low surface area (< 100 m2·g-1), the synthesized SiC material via sol-gel method is not suitable to load the vanadia catalyst for ODH of propane. In this paper, a given amount of alkyloxysilane is introduced into the precursor of the silica solution before the hydrolysis. The results of N2 adsorption-desorption indicated that the incorporation of alkyloxysilane can increase the surface area of SiC materials and the longer length of hydrophobic chain of alkyloxysilane was favorable for the higher surface area. Consequently, a series of mesostructuredβ-SiC with high surface area (151-350 m2·g-1) and tunable porosity (8-30 nm) were synthesized by adjusting the length of hydrophobic chain of alkyloxysilane (C1, C4, C6, and C8), which was confirmed by N2 adsorption-desorption, XRD, and HR-TEM techniques. Loaded with vanadia by wet impregnant method (NH4VO3-CH3OH), SiC was employed as catalyst support for ODH of propane to propylene (reaction temperature:600℃; C3H8/O2/N2=1/1/4; total gas velocity:30 mL·min-1; Catalyst weight:50 mg). The catalytic performances demonstrated the selectivity of propylene decreased with the increase of the loading amount of vanadia and the highest yield of propylene (20.2%) along with the selectivity of 62.1% was obtained when the loading amount was 1.5 wt%. However, as the counterparts, SiO2 and A12O3 gave low selectivity to propylene (51.0 and 47.8% respectively) under the identical reaction conditions. By employing two thermocouples, hot spot effect was tested and the results demonstrates the differential temperature for SiC-supported catalyst was 16℃, while the catalyst supported by SiO2 and A12O3 were 58 and 73℃respectively. The results above confirmed that, over the SiC-supported catalyst, high thermal conductivity could alleviate the hot spot effect and therefore prevent the deep oxidation of propylene. In addition, the results of XPS and H2-TPR characterizations revealed that the silica film on the surface of SiC material played a crucial role to disperse the vanadia species.2. Synthesis of ceria base catalysts and the catalysis study for ODH of ethyl benzeneStyrene (ST) is an important monomer extensively used in the chemical industry for the manufacture of plastic, resin, and rubber, and ranks the forth place in the highest demand alkenes, only following ethylene, propylene, and vinychorine. Nowadays, the annual capacity of the styrene synthesis is above 25 Mt over the world. Therein,90% of styrene is commercially produced by means of the direct DH of EB over potassium-iron oxide as a catalyst at high temperature (ca.650℃) in presence of a lot of steam. Due to its highly endothermic nature, this conventional route suffers from several disadvantages such as intensive energy consumption and rapid coking. Alternatively, the ODH of EB has attracted considerable recent attention since it can be operated at lower temperatures and the EB conversion would not be equilibrium limited. Currently, among a range of catalysts reported for DH of EB, nanocarbon materials like carbon nanotubes and carbon nanofibers exhibit an excellent catalytic performance with a yield of ST of ca.50%. Nevertheless, the industrial application of the carbon-based catalysts has been till now prevented by their fine powder nature and intrinsically low resistance to combustion. As the most widely used oxide in the family of rare earth, Ceria (CeO2) is a key redox component in the catalyst formulations for many industrially important reactions, such as TWC catalysts for automobile exhaust treatment, CO2 activation, CO oxidation, and low temperature water-gas shift reaction. The success of ceria in various applications is largely attributed to its superior oxygen storage capacity, viz. releasing and storing the oxygen lattice species under the lean-O2 and rich-02 conditions respectively. In the reactions based on the oxidative dehydrogenation, high 02-supplying rate are favorable for the high catalytic performance and therefore ceria-based catalysts have been received tremendous attention in the ODH reactions.In the present work, a series of mesostructured ceria with high surface area has been prepared via template-assisted precipitation method. The results of N2 adsorption-desorption revealed that the surface area of ceria samples decrease with the increment of the calcination temperature, accompanied by the wider peak in the corresponding XRD patterns. In the reaction of ODH of EB to ST (reaction temperature:450℃; EB/O2/N2=0.5/0.25/20; 20.75 mL·mim-1; Catalyst weight:50 mg), the CeO2-500 sample with the highest surface area displayed the highest catalytic performance with the EB convention of 30% and ST selectivity of 82%. By analysis of the H2-TPR profiles, it is found the CeO2-500 sample presented a highly intense reduction peak at 200-450℃compared with other samples with low surface areas, which revealed that high surface are would favor the mobility of lattice oxygen species and consequently enhance the catalytic activity. As to the results above,10 wt% of the second metal species as dopant (Al3+, Sn4+, Zr4+, Mn4+, and Ni3+) were introduced into the ceria samples. The XRD patterns indicated the second metal species have been incorporated into the ceria lattice and leads to the lattice defeats. As catalysts for ODH of EB, the introduction of other metal ions haves changed the catalytic activity of the pristine ceria and the order of the catalytic activity was as follows:Ce0.90Ni0.10Ox> Ce0.90Mn0.10Ox> Ce0.90Sn0.10Ox> CeO2> Ce0.90Zr0.10Ox> Ce0.90Al0.10Ox (reaction temperature:500℃; EB/O2/N2=0.5/0.25/20; 20.75 mL·min-1; Catalyst weight:50 mg). To further investigate the effect of the amount of Ni on the catalytic performance, a series of Ni-doped ceria catalysts were studied for ODH of EB and the highest yield of ST (ca.50%) was obtained over Ce0.90Ni0.10Ox catalyst. The XRD patterns revealed that the Ce0.90Ni0.10Ox sample presented a highest lattice defeat, compared with other Ni-doped ceria catalysts, which is correlated with the corresponding reduction peak with the lowest temperature at ca.284℃in H2-TPR profiles, indicating the Ce0.90Ni0.10Ox possessed the considerable lattice oxygen mobility rate. Moreover, the H2-total oxygen storage experiments proved that the Ce0.90Ni0.10Ox catalyst possessed the highest oxygen storage capacity at 500℃, which could explained the highest catalytic performances in ODH of EB.3. Catalysis study of ceria supported nanogold catalysts for ODH of ethylbenzeneGold has been originally labeled as a chemically inert metal and long disregarded for catalytic applications. Compared with other noble metals such as Pt and Ru, the catalytic potential of gold has not been received considerable attention. Since Haruta discovered a remarkable activity of supported gold nanoparticles (NPs) in CO oxidation, catalysis by gold has become one of the most intensively studied topics in chemistry. Currently, gold could become a highly active and selective catalyst in many reactions including oxidation, hydrogenation, selective isomerization, and one-pot multistep reactions. Of particular interest to the current chemical community is the Au-catalyzed selective oxidation, which is believed to be essential for the development of new alternative and greener routes toward sustainability. Recently, several liquid phase aerobic oxidations, including the selective oxidation of alcohols, aldehydes, and amines have been developed. Despite significant research effort, there have been few studies on the selective oxidation of less reactive hydrocarbons in the gas phase over the supported Au nanoparticles probably due to the fact that both the reactants and products are prone to undesired combustion in the presence of molecular oxygen at high temperatures. In the context, the application of Au NPs into the ODH of EB in gas phase not only serves as a major supplement to the field of selective oxidative over Au catalyst in gas phase, but also presents a practical significance to develop a novel catalytic system for ODH of EB.A range of noble catalysts including Pt/CeO2, Ru/CeO2, Au/Al2O3, Au/Mn2O3, and Au/CeO2 were prepared via different methods and investigated for ODH of EB. Compared with Au/CeO2 catalysts, the Pt and Ru based catalyst, as well as other metal oxides supported Au catalyst demonstrated low selectivity to ST, due to the direct and deep combustion. Moreover, according the results of XRD patterns and HR-TEM images of the Au/CeO2 catalysts prepared by different methods, NaOH-DP, rather than urea-DP and wet impregnant methods, could assure a good dispersion of Au nanoparticles over ceria at the same loading amount of Au. In the ODH of EB, the existence of Au NPs could improve the catalytic activity of the pristine ceria and highest yield of 60.9% was obtained over 6.0 wt%Au/CeO2 catalyst. The results of H2-TPR profiles and H2-TOSC experiments revealed that the 6.0 wt%Au/CeO2 presented the highest OSC and oxygen mobility, which proved the highest catalytic performances.