Coking leads to the deactivation of solid acid catalyst. This phenomenon is a ubiquitous problem in the modern petrochemical and energy transformation industries. Here, we show a method based on microwave cavity perturbation analysis for an effective examination of both the amount and the chemical composition of cokes formed over acid zeolite catalysts. The employed microwave cavity can rapidly and non-intrusively measure the catalytically coked zeolites with sample full body penetration. The overall coke amount is reflected by the obtained dielectric loss (ε″) value, where different coke compositions lead to dramatically different absorption efficiencies (ε″/cokes’ wt%). The deeper-dehydrogenated coke compounds (e.g., polyaromatics) lead to an apparently higher ε″/wt% value thus can be effectively separated from lightly coked compounds. The measurement is based on the nature of coke formation during catalytic reactions, from saturated status (e.g., aliphatic) to graphitized status (e.g., polyaromatics), with more delocalized electrons obtained for enhanced Maxwell–Wagner polarization.

Methanol-to-hydrocarbon (MTH) process over acidic zeolite catalysts has been widely utilised to yield many types of hydrocarbons, some of which are eventually converted into the highly dehydrogenated (graphitized) carbonaceous species (cokes). The coking process can be divided into two parallel pathways based on the accepted hydrocarbon pool theory. From extensive investigations, it is reasonable to conclude that inner zeollite cavity/channel reactions at acidic sites generate cokes. However, coke formation and accumulation over the zeolite external surfaces play a major role in reaction deactivation as they contribute a great portion to the total coke amount. Herein we have reviewed previous literatures and included some recent works from KOPRC in understanding the nature and mechanism of coke formation, particularly during an H-ZSM-5 catalysed MTH reaction. We specially conclude that rapid aromatics formation at the zeolite crystalite edges is the main reason for later stage coke accumulation on the zeolite external surfaces. Accordingly, the catalyst deactivation is in a great certain to arise at those edge areas due to having the earliest contact with the incoming methanol reactant. The final coke structure is therefore built up with layers of poly-aromatics, as the potential sp 2 carbons leading to pre-graphite structure. We have proposed a coke formation model particularly for the acidic catalyst, which we believe will be of assistance in understanding-and hence minimising-the coke formation mechanisms.

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