Study On Oxidative Dehydrogenation Of Butene To Butene By Vanadium - Based Oxides

n-Butane is an important chemical of light alkanes. Butenes and butadiene, as products for oxidative dehydrogenation (ODH) of n-butane, are useful intermediates for producing synthetic rubbers, plastics, and a number of industrial important chemicals. The ODH of n-butane is taken as a probe reaction to investigate the activation and selective conversion of C-H. It is important in both theoretical studies and practical applications for the effective utilization of light alkanes and environmental protection. However, alkenes are easily oxidized into carbon dioxide and carbon monoxide, resulting in the decrease in selectivity to C4 alkenes.How to design the catalysts with high activity and high selectivity to C4 alkenes plays a critical role in both theoretical studies and industrial applications and is still a challenging work in the domain of ODH of n-butane.Based on the results reported literature, the titanosilicates and alkaline-earth metals modified SBA-15 mesoporous materials were designed and synthesized in the present thesis. The synthesized supports were used as vanadia based catalysts for the ODH of n-butane. The supports and catalysts are characterized by powder X-ray diffraction (XRD), N2 physical adsorption, transmission electron microscope (TEM), infrared spectroscopic analysis (FT-IR), hydrogen temperature-programmed reduction (H2-TPR), temperature-programmed desorption of O2 (O2-TPD), and X-ray photoelectron spectra (XPS). Meanwhile, the reaction results over catalysts for the ODH of n-butane were quantitatively correlated with the characterization results, and the mechanistic relationships between performance over catalysts for the ODH of n-butane and the factors of redox ability, especially the key role of lattice oxygen in activation of n-butane, were proposed. The main results are summarized as follows:(1) Titanosilicates with different Ti/Si ratios were synthesized by a sol-gel method and the catalytic performance over V-containing titanosilicates for the ODH of n-butane was investigated. Results indicated that titanium was successfully incorporated into silica framework. Moreover, titanium oxide assembled and formed crystalline TiO2 on the surface of pore wall. The partial structural collapse of the SBA-15 ordered mesoporous after incorporation of TiO2 was observed. However, the mesoporous structure of titanosilicates was still retained.(2) To obtain better catalytic performance over V-containing titanosilicate catalysts, synthetic method for titanosilicates, hydro-thermal synthesis conditions, and both reaction composition and reaction temperature in catalytic test were studied.(3) The reaction performance over catalysts for the ODH of n-butane was investigated under the conditions of atmospheric pressure, T=460℃, n-butane/oxygen/nitrogen with a molar ratio of 1/2/7, GHSV=48,000 L/(kg·h). V-containing titanosilicates with lower vanadia content for the ODH of n-butane at lower temperature showed a high catalytic activity. The 1V-Ti-SBA-15(5) catalyst reached the best TOF∑C4 and STY∑C4 value, which are 3.8 μmol/(V·s) and 0.90 kg/(kgcat·h), respectively. Moreover, 1V-Ti-SBA-15(5) catalyst showed a good stability. The conversion of n-butane changed slowly from 25.2% to 20.4% and selectivity to C4 alkenes increased from 25.7% to 33.4%, then decreased from 33.4% to 29.3%. Characterization results indicate that lattice oxygen exists in surface of the V-containing titanosilicate catalysts, enhances the reducibility of vanadia-based catalysts with increase in TiO2 loadings, and plays a critical role in activation of n-butane.(4) Alkaline-earth metals modified the SBA-15 with different Si/Z (Z=Mg, Ca, and Ba) ratios was synthesized by an impregnation method. Results indicate that the partial structural collapse of the SBA-15 ordered mesoporous after incorporation of alkaline-earth metals. Different decreasing levels in the BET specific surface areas and total pore volumes happen. The reducibility of vanadia-based catalysts reduces after incorporation of alkaline-earth metals.(5) Compared with the 2.9V-SBA-15 catalyst, alkaline-earth metals modified VOx-SBA-15 catalysts exhibit low reaction activity, attributing to the partial structural collapse of catalysts and decrease in the reducibility. The partial structural collapse of catalysts is not beneficial to substance transfer and diffusion, and the strong interaction between alkaline-earth metals and vanadium oxide reduces the activity of vanadium oxide over the surface of catalysts, causing catalyst activity to decrease.