Deactivation of Zeolite Catalysts by Coke

Abstract

Zeolite catalysts are widely used in conversion processes of hydrocarbons, because of their high activity and shape selectivity. The conversion rate and the shape selectivity are closely related to intracrystalline diffusion rate and acidic properties of the zeolite catalyst. Carbonaceous materials called coke deposit on the catalyst during reactions, leading to changes in catalytic properties (diffusivity and acidic properties) as well as the reaction performance. This presentation reviews the mechanisms of the deactivation of zeolite catalysts caused by coke deposition from the following points of view. (1) Characterization of coke. Examination of the location of coke formation. (2) Relationship between the changes in acidic properties, and the acid strength and the acid amount. Prediction of the reduction of diffusion rate taking account of pore structure and molecular size. (3) Development of models for predicting the change in the activity and the shape selectivity.Zeolites are solid acid catalysts which are widely used in hydrocarbon processing, such as naphtha cracking, isomerization, dispropornation and alkylation. During reactions carbonaceous materials called coke deposit on the zeolite and reduces its activity and selectivity. Coke deposited not only covers the acid sites of the catalyst, but also blocks the pores, and restrain reactants from reaching the acid sites, leading to the decrease in the apparent reaction rate (1, 2). Here, we will mainly deal with the intracrystalline diffusivity of zeolites, and will discuss the relationship between it and the change in catalyst selectivity. THE PORE STRUCTURES OF ZEOLITES AND THE MODES OF COKE DEPOSITIONZeolites are composed of crystals of alumino silicate with the size of 0.1-5 micron. The pores of HY type zeolites consist of super cages and windows (0.74 nm). ZSM-

Improvement in the catalytic properties of ZSM-5 zeolite nanoparticles via mechanochemical and chemical modifications

“…6c) due to selective removal of Al species (working as Brønsted or Lewis acid sites) on the external surface of the ZSM-5 particles. In paraffin cracking over ZSM-5 zeolite catalysts, it is well-known that the external acid sites give rise to deactivation due to coke formation on these sites, [33][34][35][36] although large 12R micropores of zeolite beta catalysts during catalytic ethanol transformation would induce the formation of coke precursors, such as alkylbenzenes, alkylnaphthalenes and alkylpyrenes. 37 Since Lewis acid sites tend to cause the dehydrogenation of hydrocarbons and related reactions leading to the formation of coke species mainly consisting of polyaromatic compounds, selective leaching of external Lewis acid sites in the ZSM-5 catalyst could lead to diminution of coke formation to give a long-lived catalyst.…”

Performance of ZSM-5 Catalyst in the Dimethyl Ether to Olefins Process

“…Such a harsh environment can often structurally damage the zeolite by dealumination, structural alteration, sintering of supported metals, etc. (Guisnet, 1995;Masuda, 1995).…”

“…20 hrs)·pores. Theref~re a great deal of the existing coke should be located on the catalyst surface thereby resulting in the high stability of ZSM-5 in comparison to other zeolites(Masuda, 1995). (b) Perhaps the only evidence of deactivation, aside from the deposition of coke in the catalyst pores, is the change in concentration of the different hydrocarbon I , product species.…”