Evolution of carbon deposits during ethylene aromatization over Zn/ZSM-5

Abstract

Zn/ZSM-5 catalysts exhibit remarkable catalytic activity in the ethylene aromatization process. Nonetheless, they frequently suffer from rapid deactivation owing to coke deposition. This study investigated the coking process of Zn/ZSM-5 catalyst during the ethylene aromatization, as well as the evolution of carbon deposits and their impact on the catalyst's structure and surface properties. The results showed that, despite ethylene conversion remaining at 100 % during the initial high-activity stable phase (0–24 h), even small amounts of carbon deposition resulted in significant decrease in microporosity and strong acid sites of the catalyst. Methyl-substituted benzene was identified as the primary deposit on the catalyst in the initial reaction stage, as confirmed by infrared (IR) spectroscopy, ultraviolet-Raman (UV-Raman), thermogravimetry-mass spectrometer (TG-MS), gas chromatography-mass spectrometry (GC–MS), and electron paramagnetic resonance (EPR), which can act as an active hydrocarbon pool species for ethylene oligomerization. Thus, the catalyst was able to maintain high ethylene conversion activity despite the loss of some acid sites. However, during the decline in activity period (24–72 h), the composition and location of carbon deposits underwent significant changes. A large number of polycyclic aromatic hydrocarbons formed on the catalyst surface, which resulted in the weakening of the interaction between Zn species and the zeolite, ultimately leading to the transformation into large grain ZnO and decrease in aromatics selectivity. Additionally, the bulky carbon deposits covered the pores of the zeolite, leading consequently to a decrease in ethylene conversion and ultimately catalyst deactivation.