Coke formation in the oxidative dehydrogenation of ethylbenzene to styrene by TEOM

Abstract

A packed bed microbalance reactor setup (TEOM-GC) is used to investigate the formation of coke as a function of time-on-stream on γ-Al₂O₃ and 3P/SiO₂ catalyst samples under different conditions for the ODH reaction of ethylbenzene to styrene. All samples show a linear correlation of the styrene selectivity and yield with the initial coverage of coke. The CO_X production increases with the coverage of coke. On the 3 wt% P/SiO₂ sample, the initial coke build-up is slow and the coke deposition rate increases with time. On alumina-based catalyst samples, a fast initial coke build-up takes place, decreasing with time-on-stream, but the amount of coke does not stabilize. A higher O₂ : EB feed ratio results in more coke, and a higher temperature results in less coke. This coking behaviour of Al₂O₃ can be described by existing “monolayer–multilayer” models. Further, the coverage of coke on the catalyst varies with the position in the bed. For maximal styrene selectivity, the optimal coverage of coke should be sufficient to convert all O₂, but as low as possible to prevent selectivity loss by CO_X production. This is in favour of high temperature and low O₂ : EB feed ratios. The optimal coke coverage depends in a complex way on all the parameters: temperature, the O₂ : EB feed ratio, reactant concentrations, and the type of starting material.