Coke deposit structure effect

J. Wood, L. F. Gladden 2003, (Effect of coke deposition upon pore structure and self-diffusion in deactivated industrial hydroprocessing catalysts), Appl.Cat. A General, 249, 241. 

A natural extension of the investigations of single-pellet micro-imaging is to explore the effect of pore structure on the spatial distribution of coke deposition... 

The effect of coke deposition on pore structure and molecular diffusion within supported metal catalysts has also been investigated (73). 

In processing VGO it has been argued that the heavy poly-aromatic structures characterised by the Ramsbottom Carbon Residue (RCR, Table 1) can be considered as coke precursors [8]. An increase of the boiling point of those structures via condensation reactions or dehydrogenation reactions is responsible for coke deposition onto the catalyst. In order to increase our level of understanding of these processes we consider first the effects of catalyst parameters on the coke formation. 

The total acidity deterioration and the acidity strength distribution of a catalyst prepared from a H-ZSM-5 zeolite has been studied in the MTG process carried out in catalytic chamber and in an isothermal fixed bed integral reactor. The acidity deterioration has been related to coke deposition. The evolution of the acidic structure and of coke deposition has been analysed in situ, by diffuse reflectance FTIR in a catalytic chamber. The effect of operating conditions (time on stream and temperature) on acidity deterioration, coke deposition and coke nature has been studied from experiments in a fixed integral reactor. The technique for studying acidity yields a reproducible measurement of total acidity and acidity strength distribution of the catalyst deactivated by coke. The NH3 adsorption-desorption is measured by combination of scanning differential calorimetry and the FTIR analysis of the products desorbed. 

A systematic series of synthetic, characterization and butene isomerization catalysis studies of ferrierite and ferrierite-like materials such as ZSM-22,59 ZSM-23,60 and ZSM-35,6 was undertaken to study optimization of isobutylene product. Coke deposits in the pores of these materials play a key role in isobutylene formation as does the overall acidity and structure of the pore system. Such shape selective effects have been probed with TPD methods. 

Coke deactivation on Pt/Al203 catalysts have been studied intensively in the literature. Previous works have focused on the kinetics of catalyst deactivation [7] the influence of additives on coke formation [8] the coke deposition on different morphologic surfaces [9] the structure [10] and chemical composition of coke [11]. Deactivation by coke deposition on niobia supported catalysts, or even on other reducible supports which promote SMSI effect has not been studied. 

The Thiele modulus for the mesoporous structure of the eatalyst, ( ) i, was calculated using the following parameters particle size, Rp = 0.0137 cm mean pore radius, rpore.ave = 20 10 cm catalyst porosity, e = 0.52 catalyst density, pg = 1210 g 1. N2 adsorption-desorption isotherms were used for measurement. The calculated value of effective diffusivity coefficient in the mesoporous structure of the catalyst is Dg = 9.71 10 2 cm min . This value is not affeeted by coke deposition. 

Secondly, it can be expected that some coke will be formed even on those catalysts in which the ensemble size is controlled. Coke deposits have, eventually, been observed on doped nickel [17], albeit with a structure different from deposits formed on nickel. As a result, it could be argued that ensemble size control should best be effected using a second component that favours gasification of coke or coke forming intermediates. Metals such as Pt or Ir are known to catalyse coke gasification [18] and are obvious candidates. 

The activation energies for coke gasification on the three substrates shown in Figure 4 were 33 Kcal/mole for the three alumina-based materials and 54 Kcal/mole, for the silica-alumina catalysts. The increased activity and lower activation energy for the coke deposited on the aluminas (compared to that on the silica-aluminas) cannot be due to a direct catalytic effect of alumina on the gasification reaction, but rather to an indirect effect of the alumina that controls the nature and structure (surface area and structural disorder) of the coke during its deposition. 

The trend in oil processing towards more severe operating conditions and a heavier crude diet in order to enhance the upgrading of the barrel is bringing about more extensive coke formation, leading to catalyst deactivation and, ultimately, to reactor blockage. Characterisation of coke deposits may provide us with clues about catalyst characteristics essential in limiting coke formation under these extreme conditions. Furthermore, the development of deactivation models will be more effective when information not only on the quanti ty of coke hut also on the structure of the deposits is available (refs. 1—3). 

C.A.C., and Schmal, M. (2012) Structural investigation of LaCoOa and LaCoCuOs perovskite-type oxides and the effect of Cu on coke deposition in the partial oxidation of methane. Appl Catal. B Enviroit,... 

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