Things go better with coke: the beneficial role of carbonaceous deposits in heterogeneous catalysis

Abstract

Carbonaceous deposits on heterogeneous catalysts are traditionally associated with catalyst deactivation. However, they can play a beneficial role in many catalytic processes, e.g. dehydrogenation, hydrogenation, alkylation, isomerisation, Fischer–Tropsch, MTO etc. This review highlights the role and mechanism by which coke deposits can enhance catalytic performance.

Coke formation and deactivation during catalytic reforming of biomass and waste pyrolysis products: A review

“…The range of reactions affected by this phenomenon includes, among others, oxidative dehydrogenation, isomerization, hydrogenation, Fischer-Tropsch synthesis and reforming, for which coke deposition is the main cause of catalytic activity loss [55,203].…”

Application of Inelastic Neutron Scattering to the Methanol-to-Gasoline Reaction Over a ZSM-5 Catalyst

“…The past 30 years have seen numerous investigations of the reaction pathways and mechanisms by which methanol is converted to hydrocarbons over acid zeotype catalysts, as reviewed recently for example in references [1][2][3][4]. Three different components of the reaction pathway can be distinguished: (1) the initial reaction steps in which methanol reacts with acid sites in the zeolite or SAPO catalysts; (2) the formation of hydrocarbon products during steady-state working conditions; and (3) the catalyst deactivation through so-called coke formation.…”

Structural changes in FeOx/γ-Al2O3 catalysts during ethylbenzene dehydrogenation

“…Iron carbides have previously been reported in similar systems such as FischerTropsch synthesis, 36 where they have been proposed to act as the catalytically active phase. 4,7,10 The mechanism of formation of carbides has been proposed by Ding et al to involve reduction of haematite to magnetite in the surface and bulk regions of the catalyst under a CO atmosphere, and the subsequent transformation of the magnetite surface into iron carbides. 8 Elsewhere, the formation of CO in the direct dehydrogenation of ethylbenzene over CrO x /Al 2 O 3 has been demonstrated and its role in catalyst performance discussed.…”

“…3 The structural changes that catalysts undergo during reaction can, however, also lead to the formation of catalytically-active phases. 4,6,7 The formation of different oxides and carbides is well known for iron catalysts, particularly in Fischer-Tropsch synthesis, 7,8 during which iron-based materials undergo complex phase changes, 9 with the identification of the active phase still disputed. 10 Carbide phases formed in situ also play a catalytic role in, e.g.…”

“…10 Carbide phases formed in situ also play a catalytic role in, e.g. dehydrogenation and isomerisation over molybdenum oxycarbides 7 and dehydrogenation over PdC x species. 6 The dehydrogenation of ethylbenzene is a further example of a reaction where Fe based catalysts undergo structural and phase changes as a function of time on stream.…”