Direct Synthesis of C ₆ –C ₈ Light Aromatics via CO ₂ Hydrogenation over K/FeCo@Z5-Si Tandem Catalysts

The synthesis of valuable aromatics via CO 2 hydrogenation is a sustainable and promising technique which could substitute the carbon intensive fossil route based on naphtha catalytic reforming. However, the selective production of C 6 –C 8 light aromatics, especially for the para -xylene (PX) is still challenging. Herein, aiming at optimizing the selectivity toward C 6 –C 8 light aromatics, we prepared a series of K/FeM@Z5 and K/FeM@Z5-Si tandem catalysts, which are composed of K/FeM (M = Co, Zn, and Mn) combined with parent or SiO 2 -coated HZSM-5. It is found that the introduction of Co facilitates the formation of stable χ-(Co x Fe 1– x ) 5 C 2 alloy carbides, which facilitates the synthesis of olefin intermediates, and the K/FeCo@Z5 tandem catalyst shows a high aromatics selectivity of around 42%, among which C 6 –C 8 aromatics account for approximately 60%. However, among the formed xylenes, the proportion of PX in xylenes is only 27.8%. In contrast, that of the K/FeCo@Z5-Si-2 catalyst is determined to be 64.0%, which is more than doubled after silica coating to passivate the external acid sites of the parent ZSM-5, indicating a success in suppressing the transformation of PX, although a relatively worse catalytic stability is observed. Moreover, a possible reaction pathway for CO 2 hydrogenation over the K/FeCo@Z5-Si tandem catalysts was proposed to understand its reaction mechanism toward the formation of aromatics. This work could provide useful insights into the synthesis of C 6 -C 8 light aromatics over the tandem catalytic system.