The role of RWGS in the dehydrogenation of ethylbenzene to styrene in CO2

The addition of CO2 to the dehydrogenation of ethylbenzene (EB) to styrene over promoted and un-promoted vanadia, iron, and chromia catalysts on alumina improves the yield of styrene, in comparison with the use of N2 as diluent. Depending on the catalyst, EB conversion increases 5–10%, while selectivity does not change significantly. The potassium promoted vanadium catalyst shows the highest CO2 conversion for dehydrogenation with the largest increase in EB conversion. The activity of these catalysts in the reverse water–gas-shift (RWGS) reaction, in the presence and absence of the dehydrogenation, is very different. Vanadium catalysts exhibit the lowest CO2 RWGS activity. The presence of coke on the catalyst suppresses the RWGS reaction over chromium and iron catalysts, but improves the activity of the vanadium catalyst. This is attributed to hydrogen spill-over from the coke to the metal catalyst. A two-step mechanism with a hydrogen spill-over is proposed for the EB dehydrogenation in CO2. Step-response experiments show that the redox mechanism for the RWGS and the two-step pathway for dehydrogenation in CO2 are the dominant routes. A slight RWGS activity of the bare alumina support is attributed to an associative mechanism.

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► Styrene yield over iron, vanadia and chromia catalysts is improved in the presence of CO2.
► Coke improves the RWGS activity of the vanadia catalysts.
► Hydrogen spill-over from coke to the catalyst, in a two-step mechanism, is proposed for dehydrogenation in CO2.
► RWGS takes place via redox and associative mechanism, the redox mechanism is dominant.