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Propylene production via oxidative dehydrogenation of propane with CO2 over modified metal oxides catalysts

Antonakos Georgios

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Summary

Propylene (C3H6) is ranked among the most important products with extensive applications in the chemical industry, while its demand is continuously increasing.Propylene production through the oxidative dehydrogenation of propane (C3H8) with CO2 (Oxidative Dehydrogenation of propane-CO2, ODP-CO2) constitutes an efficient method that can meet the continuously increasing demand for propylene and simultaneously contribute to the utilization of CO₂, offering various benefits. In this process, CO2 (a) acts as a mild oxidant, limiting the over-oxidation of propane, (b) consumes the produced H₂ via the reverse water-gas shift reaction (RWGS), shifting the reaction equilibrium towards propylene production, and (c) contributes to the removal of carbon, that may be deposited on the catalyst surface under reaction conditions, via the reverse Boudouard reaction. In the present work, a series of promoted TiO₂ catalysts with alkali (Na, K, Cs, Rb, Li) and alkaline earth (Ca, Ba, Mg, Sr) metals, as well as composite x%Ga₂O₃-Al₂O₃ (x=0-100 wt.%) oxides, were prepared, characterized, and evaluated with respect to their activity for the ODP-CO2 reaction. Catalysts preparation was carried out using the wet impregnation method, while their characterization was employed using the BET, XRD, and CO₂-TPD techniques to determine their specific surface area, crystalline structure and surface basicity, respectively. The reactions network taking place under ODP-CO2 conditions was further studied by temperature-programmed surface reaction (TPSR) experiments using mass spectrometry. The results showed that promoting TiO₂ with alkali and alkaline earth metals leads to a significant increase in surface basicity and an improvement in propane conversion to propylene compared to the unpromoted TiO₂, in a manner dependent on the nature and content of the promoter. The catalysts 0.2%Cs-TiO₂ and 0.2%Ca-TiO₂, characterized by intermediate surface basicity, exhibited optimal performance. The selectivity towards CO increases significantly in the presence of alkali and alkaline earth metals, indicating that the reverse WGS and Boudouard reactions are enhanced. This increase is accompanied, in both cases, by a decrease in the selectivity towards the undesirable products ethylene (C₂H₄), methane (CH₄), and ethane (C₂H₆), which are formed via side reactions, and by a reduction in carbon deposition. The selectivity towards propylene decreases in the presence of alkali metals and remains practically unaffected by the presence of alkaline earth metals, making alkaline earth metals a more ideal promoter choice compared to alkali metals. Results obtained from the investigation of the effect of Ga₂O₃ content on the Al₂O₃ surface showed that both propane conversion and propylene yield are significantly influenced by the Ga₂O₃ content. A volcano-type correlation was found between the catalytic activity and basicity, according to which propane conversion and propylene yield presented optimal values for an intermediate value of surface basicity, which corresponds to the sample containing 30% Ga₂O₃. Specifically, propane conversion at 600 °C increases from 4% to 58% with an increase in Ga₂O₃ content from 0 to 30 wt.%, while the propylene yield increases from 1.5% to 39%, respectively, which is among the highest values reported so far in the literature. The 30%Ga₂O₃-Al₂O₃ catalyst is able not only to enhance the conversion of C₃H₈ to C₃H₆ but also to limit the undesired reactions of C₃H₈ hydrogenolysis and C₃H₈/C₃H₆ cracking, which are responsible for the formation of C₂H₄, CH₄, C₂H₆, and carbon deposition. Finally, a kinetic study was performed over the 10%Ga₂O₃-Al₂O₃ catalyst at three different temperatures (550, 600, and 650°C). The results showed that the propane conversion rate increases significantly with increasing the partial pressure of C₃H₈ and decreases slightly with increasing the partial pressure of CO2. These results can be used to determine the reaction orders of the CO2-ODP reaction and to develop a detailed kinetic model able to describe the mechanism of the reaction.

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