Deactivation of catalysts, by coking,

In spite of the intense effort carried out in the past, the deactivation of catalysts by coke deposition, and its subsequent regeneration, still poses one of the most important problems in industrial catalytic processes. 

Beeckman JW, Froment GF. Deactivation of catalysts by coke formation in the presence of internal diffusion limitation. Industrial Engineering Chemistry Fundamentals 198i 21(3) 243-250. 

We can also distinguish a few lines of approach regarding catalyst poisons which deactivate the catalyst by coke formation, that is by blocking of pores and the catalyst active sites,... 

In this paper we contribute to clarifying the role of coke in the activation/deactivation of FER for olefin conversions with emphasis on butene isomerization. To this end we establish the nature of the coke deposits by adsorption/desorption measurements. Furthermore, the isotopic scrambling for butene isomerization is determined in support of the Guisnet mechanism. Finally, slow deactivation of FER by coke is studied as this limits the ultimate lifetime of the catalysts. Details of the coke deposits in spent catalysts are revealed using electron microscopy and X-ray excited Photo-electron Spectroscopy. 

Deactivation of catalyst by physically covering the active sites or by physical blocking of the pores in the catalyst by coke. 

RFCC (resid fluid catalytic cracking) is one of the processes for the conversion of heavy oils in modem refineries. The problem with vacuum residue as FCC feedstock is quick deactivation of catalysts by the coking of asphaltene fractions and the deposition of metals involved in metallorganic polycyclic compounds. Therefore, developing novel zeolites to achieve metal tolerance has long been a goal of catalyst researchers... 

Let us now use the sequence of elementary steps to explain the activity loss for some of the catalysts The combination of hydrogen chemisorption and catalytic measurements indicate that blocking of Pt by coke rather than sintering causes the severe deactivation observed in the case of Pt/y-AljOj The loss in hydrogen chemisorption capacity of the catalysts after use (Table 2) is attributed mainly to carbon formed by methane decomposition on Pt and impeding further access. Since this coke on Pt is a reactive intermediate, Pt/Zr02 continues to maintain its stable activity with time on stream. 

In this way, the conjunct polymers serve as a reservoir of hydride ions. Under some conditions, the polymers are a source of hydride ions, but they accept these ions under other conditions. Substantial amounts of the saturated products are supposedly formed via this route with sulfuric acid. In zeolites, species similar to conjunct polymers also form. The heavy hydrocarbon molecules, which deactivate the catalyst by pore blocking or by site blocking, are generally termed soft coke or low-temperature coke , because of the absence of aromatic species. 

Bifunctional catalysis is one of the most important routes to green (more economical and more environmentally friendly) processes. Indeed, the deactivation of bifunctional catalysts by coking is much slower than that of monofunctional catalysts and their selectivity generally higher (e.g., hydrocracking compared to... 

The catalyst remained active and did not start to deactivate for about 50 h on stream without any regeneration. After that, the catalyst activity started to drop slowly but steadily.The deactivation could be caused either by sintering of Pt nanoparticles or by coke deposited on the Pt atoms. If the catalyst deactivation was caused by coking, regeneration may reestablish catalyst activity. However, regeneration in air did not improve catalyst activity, which suggests that the cause of deactivation was not coke. 

The deactivation of catalysts used in hydrotreating and hydroconversion of heavy petroleum feedstocks is associated with coking and metals deposition. Deactivation by metals has been thoroughly studied [1], but, little is known about deactivation by carbonaceous deposits. The initial decline in activity has often been attributed to this coke formation [2, 3, 4], However, in a recent study [51 it has been shown that coke deactivation can account for more than 50% of the deactivation in resid upgrading. 

The deactivation of cracking catalysts by coking with vacuum gas oils (VGO) is studied in relation to the chemical deactivation due to site coverage, and with the increase of diffusional limitations. These two phenomena are taken into account by a simple deactivation function versus catalyst coke content. The parameters of this function arc discussed in relation to feedstock analysis and change of effective diffiisivity with catalyst coke content. 

The deactivation of catalysts concerns the decrease in concentration of active sites on the catalyst Nj. This should not be confused with the reversible inhibition of the active sites by competitive adsorption, which is treated above. The deactivation can have various causes, such as sintering, irreversible adsorption and fouling (for example coking or metal depositions in petrochemical conversions). It is generally attempted to express the deactivation in a time-dependent expression in order to be able to predict the catalyst s life time. An important reason for deactivation in industry is coking, which may arise from a side path of the main catalytic reaction or from a precursor that adsorbs strongly on the active sites, but which cannot be related to a measurable gas phase concentration. For example for the reaction A B the site balance contains also the concentration of blocked sites C. A deactivation function is now defined by cq 24, which is used in the rate expression. 

Corma el al. (126) found that PtNaY was an active but rather unstable catalyst for methylcyclohexane dehydrogenation to toluene. These workers studied both the dehydrogenation and the catalyst decay kinetics. It was concluded that the reaction occurs via a series of consecutive partial dehydrogenation steps, the first of which was rate determining. Further, catalyst deactivation was caused by coke deposition from partially unsaturated precursor molecules. 

Conversion and coke formation during catalytic ethene oligomerization catalyzed by HZSM-5 have been investigated in the TEOM and in a conventional gravimetric microbalance under similar conditions (2). The results show that the TEOM is a powerful tool for determination of the kinetics of deactivation of catalysts, with a design that makes determination of the true space velocity (or space time) easy. The TEOM combines the advantages of the conventional microbalance with those of a fixed-bed reactor, and the same criteria can be used to check for plug flow and differential operation. 

MASUDA HASHIMOTO Deactivation of Zeolite Catalysts by Coke... 

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