Sono-dispersion of Cr over nanostructured LaAPSO-34 used in CO2 assisted dehydrogenation of ethane: Effects of Si/Al ratio and La incorporation

Abstract

A series of SAPO-34 supported chromium oxide catalysts varying in the Si content and presence/absence of La3+ ions were prepared by sonochemically impregnation method and tested in the dehydrogenation of ethane with CO2. SAPO-34 supports with Si/Al ratios of 0.1, 0.2, 0.3 and 0.4 were synthesized via hydrothermal method. The samples were characterized by XRD, BET, FESEM, TEM, ICP, FTIR, EDX and TPD-NH3 techniques. The variation of silicon content indicates that the relative crystallinity, surface area and effective acidity of samples increased with the Si/Al ratio up to 0.2. However, a decreasing trend was observed with further increase of the silicon content. This can be justified by the appearance of irregular particles. It was found that there is the optimum silicon content in the SAPO-34(0.2) support for the best catalytic activity, attributable to higher relative crystallinity, surface area and appropriate surface acidity. The incorporation of La3+ into the SAPO-34(0.2) framework generated in LaAPSO-34 sample was proved by XRD and TEM techniques. The La incorporation not only increases the surface area but also neutralizes mostly strong acid sites, resulted in higher dispersion of Cr species and more effective surface acidity/basicity as evidenced by EDX and TPD-NH3 analysis, respectively. This reflects in higher content of redox Cr species stabilized in comparison with parent SAPO-34(0.2), which alongside homogenous and fine Cr nanoparticles account for the superior catalytic performance and stability of Cr/LaAPSO-34(0.2). It was found that this catalyst effectively dehydrogenated ethane to ethylene in the presence of CO2 at 700 °C even after 10 h on-stream operation, giving 42.3% ethylene yield. Following the obtained results, a possible reaction mechanism was also proposed.