CO ₂ utilization as a soft oxidant for the synthesis of styrene from ethylbenzene over Co ₃O ₄ supported on magnesium aluminate spinel: role of spinel activation temperature

Abstract

Magnesium aluminate spinel (MgAl ₂O ₄) supported Co ₃O ₄ catalysts are synthesized and tested for the oxidative dehydrogenation (ODH) of ethylbenzene using CO ₂ as a soft oxidant. The effect of spinel calcination temperature on the catalytic performance has been systematically investigated. With an increase in the activation temperature from 600 to 900 °C, the active presence of a single-phase MgAl ₂O ₄ spinel is observed. A catalyst series consisting of MgAl ₂O ₄ spinel with varying Co loadings (10–20 wt%) were prepared and systematically distinguished by ICP, XRD, BET, TPR, NH ₃-TPD, UV–Vis DRS, FT-IR, XPS, SEM, and TEM. Among the tested cobalt catalysts, 15Co/800MA sample derived by calcination of MgAl ₂O ₄ support at 800 °C exhibits the most excellent catalytic performance with the maximum ethylbenzene conversion (≥ 82%). Also, high yields of styrene (≥ 81%) could be consistently achieved on the same active catalyst. Further, the catalyst exhibited almost stable activity during 20 h time-on-stream with a slow decrease in the ethylbenzene conversion from 82 to 59%. However, the selectivity of styrene (98%) stayed almost constant during the reaction. Activation of the MgAl ₂O ₄ spinel at 800 °C facilitates a dramatic chemical homogeneity for the alignment of Co ₃O ₄ nanoparticles on the surface of the active catalyst. Moreover, the isolated Co ₃O ₄ clusters have a strong chemical/electronic interaction with the Mg ²⁺ and Al ³⁺ ions on the support perform a crucial role to achieve the maximum catalytic activity.