Abstract

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 sp2 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.

Highlights

  • 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 carbonaceous species

  • 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

  • We will address the recent progresses on the study of coke formation at different zeolite positions, for the case of an acidic ZSM-5 catalyst system employed in the methanol to hydrocarbon process [15,16,17,18,19]

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Summary

ORIGINAL ARTICLE

This article is published with open access at Springerlink.com

Coke accumulation originates at regions near zeolite edges
Coke accumulation in large pores over the zeolite surfaces
Conclusion
Full Text
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