During the recent decade, progress was made in the understanding of the mechanism of oxidative dehydrogenation of light alkanes in the presence of O2 [1, 2], while data on the mechanism of paraffin dehydrogenation in the CO2 presence are still missing. Some details of mechanism for propane oxidative dehydrogenation in CO2 presence over chromium-oxide catalysts were presented in [3]. This work is devoted to the mechanism of isobutane oxidative dehydrogenation over chromium-oxide catalysts in the presence of CO2. It is based on the results of transient response and adsorptiondesorption studies. The resistance of various samples of chromium-oxide catalysts to deactivation in this process was studied and reversible-reactivation catalysts were also chosen in [3]. Unstationary phenomena were studied in the flow step of small volume at atmospheric pressure. The step contained 3 independent lines and was connected with mass-spectrometer. Relaxation curves, describing a transition of the system to a new steady state, were obtained by a jump change of the corresponding concentrations. The following mass numbers were measured: 2 (hydrogen), 15 (methane), 18 (water), 28 (carbon monoxide), 43 (isobutane), 44 (carbon dioxide), 56 (isobutene). Only one substance was measured during the experiment. CrOx/SiO2 (prepared by impregnation methods) and CrOx/MCM-41 (prepared by direct hydrothermal synthesis) were used for the investigation. The catalysts contained 1.0 % (wt.) Cr. The partial pressures of isobutane and CO2 are changed in the limits 0.125-0.33 and 0.30-0.75 atm., respectively. The temperature was 600°C. Some adsorption-desorption tests of the reaction components and such experiments as (He+C4H8)/(He+CO2) in straight and reverse directions have been also done at 200oC. The shapes of the relaxation curves, obtained in the following responses He/(CO2+i-C4H10), (He+CO2)/i-C4H10+CO2) and (He+i-C4H10)/(CO2+i-C4H10) indicate that paraffins and carbon dioxide enter the reaction in the adsorbed state and adsorbed on the same sites of the catalyst surface (a slope line means a change of the reaction conditions). The experiments with different paraffins and carbon dioxide concentrations confirmed these conclusions. It is shown that CO2 didn’t participate in the formation of cracking products and they are formed from isobutane. The experiments on CO2 and i-C4H8 adsorption/desorption and on replacement of i-C4H8 by CO2 and replacement of CO2 by i-C4H8 showed that i-C4H8 is tied with the surface of CrOx/MCM-41 more strongly than with the surface of CrOx/SiO2 but CO2 is tied with CrOx/SiO2 surface more tightly than with the surface of CrOx/MCM-41. The share of weakly and strongly forms of adsorbed i-C4H8 and CO2 was evaluated for these catalysts. Mechanism isobutane dehydrogenation is the same over studied catalysts, but the adsorption abilities of the reaction components and surface covering of chromium-oxide catalysts, prepared by various methods, are different. The step-scheme of olefins and cracking products formation is proposed. The comparison of obtained results was carried out with the data of propane oxidative dehydrogenation in CO2 presence over analogous catalysts. References:
Mechanism of isobutene dehydrogenation in the presence of carbon dioxide over chromium-oxide catalysts
BOTAVINA, Maria
2009-01-01
Abstract
During the recent decade, progress was made in the understanding of the mechanism of oxidative dehydrogenation of light alkanes in the presence of O2 [1, 2], while data on the mechanism of paraffin dehydrogenation in the CO2 presence are still missing. Some details of mechanism for propane oxidative dehydrogenation in CO2 presence over chromium-oxide catalysts were presented in [3]. This work is devoted to the mechanism of isobutane oxidative dehydrogenation over chromium-oxide catalysts in the presence of CO2. It is based on the results of transient response and adsorptiondesorption studies. The resistance of various samples of chromium-oxide catalysts to deactivation in this process was studied and reversible-reactivation catalysts were also chosen in [3]. Unstationary phenomena were studied in the flow step of small volume at atmospheric pressure. The step contained 3 independent lines and was connected with mass-spectrometer. Relaxation curves, describing a transition of the system to a new steady state, were obtained by a jump change of the corresponding concentrations. The following mass numbers were measured: 2 (hydrogen), 15 (methane), 18 (water), 28 (carbon monoxide), 43 (isobutane), 44 (carbon dioxide), 56 (isobutene). Only one substance was measured during the experiment. CrOx/SiO2 (prepared by impregnation methods) and CrOx/MCM-41 (prepared by direct hydrothermal synthesis) were used for the investigation. The catalysts contained 1.0 % (wt.) Cr. The partial pressures of isobutane and CO2 are changed in the limits 0.125-0.33 and 0.30-0.75 atm., respectively. The temperature was 600°C. Some adsorption-desorption tests of the reaction components and such experiments as (He+C4H8)/(He+CO2) in straight and reverse directions have been also done at 200oC. The shapes of the relaxation curves, obtained in the following responses He/(CO2+i-C4H10), (He+CO2)/i-C4H10+CO2) and (He+i-C4H10)/(CO2+i-C4H10) indicate that paraffins and carbon dioxide enter the reaction in the adsorbed state and adsorbed on the same sites of the catalyst surface (a slope line means a change of the reaction conditions). The experiments with different paraffins and carbon dioxide concentrations confirmed these conclusions. It is shown that CO2 didn’t participate in the formation of cracking products and they are formed from isobutane. The experiments on CO2 and i-C4H8 adsorption/desorption and on replacement of i-C4H8 by CO2 and replacement of CO2 by i-C4H8 showed that i-C4H8 is tied with the surface of CrOx/MCM-41 more strongly than with the surface of CrOx/SiO2 but CO2 is tied with CrOx/SiO2 surface more tightly than with the surface of CrOx/MCM-41. The share of weakly and strongly forms of adsorbed i-C4H8 and CO2 was evaluated for these catalysts. Mechanism isobutane dehydrogenation is the same over studied catalysts, but the adsorption abilities of the reaction components and surface covering of chromium-oxide catalysts, prepared by various methods, are different. The step-scheme of olefins and cracking products formation is proposed. The comparison of obtained results was carried out with the data of propane oxidative dehydrogenation in CO2 presence over analogous catalysts. References:
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