Advances in the study of coke formation over zeolite catalysts in the methanol-to-hydrocarbon process

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 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.

Dual catalyst system for selective vinyl chloride production via ethene oxychlorination

“…The selectivity to coke decreased with a rising Si : Al ratio as expected, since acidic sites in the zeolite and aromatics formation on its external surface are known to generate coke. 44 The fact that strong acidity is necessary for EDC dehydrochlorination, while it seems to promote CO x and coke formation, excludes the as-prepared zeolite based catalysts from successful application in ethene oxychlorination to VCM. One remaining strategy to tune the Table 2.…”

Overview of Current and Future Perspectives of Saudi Arabian Natural Clinoptilolite Zeolite: A Case Review

“…Specifically, exploration and the accrued knowledge of dehydration catalytic reaction at specific temperatures will later serve to inform on zeolite acidic or basic active sites during reaction. A review of the advantages and the practical implementations of employing such characterization techniques using the various kinds of zeolites is given in the literature [21,[92][93][94][95][96][97][98][99][100]. Moreover, it is possible to retard coke formation [101] by utilizing local eight-ring clinoptilolite zeolite as acid catalyst.…”

“…Table 2 presents summary of conditions and parameters for the methyl butynol catalytic test reaction on natural materials [99,102]. A good understanding of the operating parameters like reaction temperatures, reaction mechanisms, the conversion, selectivity, and the activation energy and more results about iele modulus are crucial factors in deciding on diffusion limitation and subsequently, will lead to a greater knowledge of zeolite coke formation during reaction [100,101]. In addition, the implementation of the characterization method, the gas phase test reaction, methyl butynol (MBOH) as suggested in this article, will also lead to a better comprehension of the behaviour of active sites in their three existing forms: acidic, basic, and amphoteric reaction pathways.…”

“…Additionally, using local zeolite in production will encourage greater interest in its research in academic institutions in Saudi Arabia together with the collaborative projects, which may potentially be established with institutes worldwide. Such future research should lead to the discovery of novel, beneficial methodologies and procedures for zeolite modification such as those suggested in this article [38,100,101,103]. e end goal is to gain a more detailed understanding and information about the structure, active sites, and chemical properties of local modified zeolites.…”

“…In addition, the identification of the active acidic centers by temperature-programmed desorption of ammonia is required. (iv) Based on previous studies [87,100,101,103,122,128], local investigation of natural clinoptilolite by acid or base treatment is required in order to gain a complete understanding of its properties and in discovering ways to exploit them and use the local zeolite in a variety of industrial applications such as in coke reduction. Performance of kinetics and thermodynamics investigations will be necessary to examine the diffusion limitation for the new formed nano local zeolite.…”