Oxidation Of Low-carbon Alkane Dehydrogenation Of Vanadium-based Oxide Catalytic CO ₂

The oxidative dehydrogenation of light alkanes (C2-C4) with CO2(CO2-ODLA) is very important on the aspects of the effective utilization of carbon resources and environmental protection. Moreover, deep understanding on CO2-ODLA is also theoretically beneficial to the catalyst design for oxidative reactions. Thus, CO2-ODLA has long been extensively inverstigated and significant progresses have been made in the aspects of reaction mechanism and suitable catalytic systems. However, lacking of a highly efficient catalyst is still a bottleneck for the further development of the titled reaction.In our previous work, the idea by using the redox of Ce3+-Ce4+to stabilize V5+is applied to the catalyst design for the oxidative dehydrogenation of ethylbenzene (CO2-ODEB), and a highly active and stable catalyst V2O5/Ce0.6Zr0.4O2-Al2O3) is obtained. Based on the primary results of CO2-ODLA over V2O5, CeO2, ZrO2, and Al2O3, V2O5/Ce0.6Zr0.402-Al203is optimized in the aspects of composition, preparation method, and the addition of alkaline promoters for the oxidative dehydrogenation of propane and iso-butane with CO2under the condition of P=1atm, T=600～650℃, flow rate of reactants of30mL/min, and CO2to alkane molar ratio of5. The textural, structural and acidic properties of the catalysts are evaluated by XRD, CO2-TPD and N2adsorption at low temperatures. The amount and properties of the carbonaceous deposits over the used catalysts are inverstigated by TG/DTA and O2-TPD techniques. Based on the results of the reaction and the catalyst characterization, the role of CO2in the oxidative dehydrogenation of iso-butane and the reaction mechanism are discussed from the in-situ FT-IR and temperature programmed surface reaction/mass spectroscopy (TPSR-MS). The experimental and main conclusions are summarized as follows.(1) By using the sol-gel technique, V2O5, CeO2, ZrO2, Al2O3and Ceo.6Zro.402-Al203were prepared, and were evaluated for the oxidative dehydrogenation of iso-butane with CO2(CO2-ODB). Results indicate that the activity of all the oxides was low, and V2O5showed the highest conversion and selectivity to iso-butane.(2) After loading a centain amount of V2O5, the supported catalysts showed increased activity for CO2-ODB. In the cases of Ceo.6Zro.402-Al203as a support, the conversion of iso-butane was clearly affected by its composition, preparation method, and the loading of V2O5while a similar selectivity of iso-butane (about90%) was achieved for all the catalysts. Among the V2O5-supported catalysts evaluated,6wt.%V2O5/Al2O3showed the best performance for CO2-ODB.(3) A series of x wt.%V2O5-Ceo.6Zro.4O2(7wt.%)-Al2O3was prepared by a modified sol-gel and hydrothermal methods, respectively, and was applied for CO2-ODB. Results indicate that the stability of the mixed oxides for CO2-ODB was better than that of the V2O5-supported catalysts although similarly high selectivity of iso-butane (about90%) was obtained for the two types of catalysts. In the case of the sol-gel x wt.%V2O5-Ceo.6Zro.4O2(7wt.%)-Al2O3, the activity for CO2-ODB was increased with increasing V2O5content. However, the stability of the catalysts with a higher V2O5content was decreased. When the hydrothermal x wt.%V2O5-Ceo.6Zro.4O2(7wt.%)-Al2O3was concerned, the activity for CO2-ODB was increased first and then decreased with increasing V2O5content, and9wt%V205-Ceo.6Zro.402(7wt.%)-Al2O3showed the best performance.(4) Based on the results of XRD, CO2-TPD, and N2adsorption at low temperatures, the results of CO2-ODB over different catalysts are explained as the dispersion of vanadium species and the acidic properties of the catalysts. The propeties of the carbonaceous species deposited over the used catalysts were evaluated by TG/DTA and TPO-MS techniques. As indicated from the TG results, a large amount of carbon (10～22wt.%) was deposited over all the catalysts. Moreover, by correlating the peak temperature of TPO-MS with the stability of the catalysts for CO2-ODB, it is concluded that the stability of the catalyst was decreased when the carbonaceous deposit was rich in carbon.(5) The TPSR-MS and In-situ FTIR were applied for probing the role of CO2and the reaction mechanism of CO2-ODB. It is conclusive that the presence of CO2significantly decreases the temperature required for the dehydrogenation of iso-butane, and increases the activity of the catalyst and the selectivity of iso-butane.(6) The oxidative dehydrogenation of propane with CO2(CO2-ODP) was studied over V2O5, CeO2, ZrO2, Al2O3, and the V2O5-supported catalysts, respectively, under the conditions of P=1atm,T=650℃, flow rate of reactants of30mL/min, and CO2to alkane molar ratio of5.All the catalysts showed fast deactivation although their initial activity for CO2-ODP was relatively high. In the mean time, the selectivity of propene was low (about60%) over all the catalysts, which is clearly different from those of the CO2-ODB. As clearly indicated from the comparative results for the dehydrogenation of propane in the presence of CO2or N2, the obvious promotional effect of CO2was unambiguously manifested.