Study On Oxidative Dehydrogenation Of Isob-Utane To Isobutene Over V-Based MSU-1 Catalysts

The oxidative dehydrogenation of light alkanes into valuable alkenes has become a challenging research area in heterogeneous catalysis. As one of the major greenhouse gases, carbon dioxide can also act as a mild oxidant. Utilization of carbon dioxide in the oxidative dehydrogenation of isobutane reactions can retard greatly coke deposition on the catalysts surface and enhance isobutane conversion and isobutene selectivity. The processes of catalytic conversion of isobutane to isobutene by oxidative dehydrogenation are of increasing importance because of growing demand for isobutene. Thus, The research and development of the effective catalysts for oxidative dehydrogenation of isobutane to isobutene with carbon dioxide plays an important role in catalysis science field.In this paper, the supports, the surface active components and the reacting conditions of the catalysts were studied in dehydrogenation of isobutane to isobutene with CO₂. A new catalyst VO_x/MSU-1 used in dehydrogenation alkanes to the corresponding alkenes was developed and it had not yet been reported in reaction of dehydrogenation of isobutane to isobutene with CO₂. The main contents and results were summarized as follows:1) The effect of support (MSU-1,γ-Al₂O₃, Cact, MgO or TiO₂) on catalytic activity over Cr-based catalysts was investigated for the dehydrogenation of isobutane with CO₂. The catalytic activity order was CrO_x/MSU-1> CrO_x/Al₂O₃> CrO_x/C_act>CrO_x/MgO>CrO_x/TiO₂. The samples were characterized by N₂ adsorption-desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It was found that the nature of support has significant effect on the catalytic performance. The N₂ adsorption-desorption and XRD results show that the order of specific surface area is the same as catalytic activity. Larger surface area resulted in better dispersion of Cr and higher activity. The XPS results reveal Cr⁶⁺ is one kinds of the active center. The results of NH₃-TPD indicate that catalytic activity of catalysts is proportional to the amount of weak-acid adsorption sites. As a result, a 59.17% conversion of isobutane and 39.51% yield of the isobutene were achieved on the CrO_x/MSU-1 catalyst, and MSU-1 was better catalyst in investigative catalysts.2) In this study, the Cr,V,Ga,Fe,Ag and Ni catalysts was prepared by impregnation of MSU-1 with the corresponding nitrate solution and was tested for dehydrogenation of isobutane to isobutene with the mild oxidant CO₂. The results showed that VO_x/MSU-1 catalyst exhibited highest conversion and selectivity, The conversion of isobutane was 54.83% and the selectivity of isobutene was 40.06%. The VO_x/MSU-1 samples were characterized by X-ray diffraction (XRD), N₂ adsorption-desorption and temperature-programmed reduction (TPR). The XRD results show the VO_x active species can be highly dispersed on the surface of MSU-1. The N₂ adsorption- desorption results indicate that MSU-1 has a larger surface area and uniform pore size distribution. Furthermore, the catalysts preserve the 3D wormlike pore structure after impregnation. The TPR results suggested that the VO_x active species was easier reduced than the V₂O₅. Because the initial temperature of the reduction peak was lower when the low loading VO_x active species was supported. The high surface area and the highly dispersed were the main reason of the high catalyst activity.3) According to the characteristics of the catalyst and the existing process, Linde technology was fit for dehydrogenation of isobutane to isobutene on VO_x/MSU-1 with CO₂. The fixed bed reactor of the Linde technology was imitated in the laboratory. The temperature, the space velocity and the ratio of CO₂/i-C₄H₁₀ has played a very important role in the dehydrogenation of isobutane with CO₂. The optimum process was determined: reaction temperature 853 K, space velocity 7200 mL/(h·g), the ratio of CO₂/i-C₄H₁₀= 3/1, the best conversion of isobutane reached 38.4% and selectivity of isobutene reached 83.2%.