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

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. This journal is