Catalyst deactivation reversible poisoning

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

The importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

This reversibility of the poisoning is an important parameter. Two of the catalysts exhibiting inhibited activity shown in Fig. 8 were also tested following a water wash to determine the permanence of the deactivation. In the test results shown in Fig. 14, it is apparent that the catalyst deactivation noted in an initial test can be reversed by the water wash, and the catalyst activity can be returned to a level at or near that of unused catalyst. The effect was demonstrated for both ammonium carbonate and ammonium hydroxide. 

This value was verified in a continuous laboratory reactor used to study the catalyst deactivation in long time kinetic runs[2]. On the basis of experimental observations, we recognized that the palladium catalyst is subjected to both reversible and irreversible poisoning. Water beiing responsible for reversible poisoning of the catalyst. Thus, we suggested, the following mechanism ... 

Sulfur oxides (S02 and S03) present in flue gases from upstream combustion operations adsorb onto the catalyst surface and in many cases form inactive metal sulfates. It is the presence of sulfur compounds in petroleum-based fuels that prevent the super-sensitive base metal catalysts (i.e., Cu, Ni, Co, etc.) from being used as the primary catalytic components for many environmental applications. Precious metals are inhibited by sulfur and lose some activity but usually reach a lower but steady state activity. Furthermore the precious metals are reversibly poisoned by sulfur compounds and can be regenerated simply by removing the poison from the gas stream. Heavy metals such as Pb, Hg, As, etc. alloy with precious metals and permanently deactivate them. Basic compounds such as NH3 can deactivate an acidic catalyst such as a zeolite by adsorbing and neutralizing the acid sites. 

FTIR model experiments were performed to reveal the nature of catalyst deactivation in C02. The spectrum taken at 15 bar in a C02/H2 mixture is shown in Fig. 1. The bands at 2060 and 1870 cm 1 indicate considerable coverage of Pt by linearly and bridge-bonded CO [12], formed by the reduction of C02 on Pt (reverse water gas shift reaction). The three characteristic bands at 1660, 1440 and 1235 cm 1 are attributed to C02 adsorption on A1203, likely as carbonate species [13, 14], It is well known [15] that CO is a strong poison for the hydrogenation of carbonyl compounds on Pt, but can improve the selectivity of the acetylene — olefin type transformations. Based on the above FTIR experiments it cannot be excluded that there are other strongly adsorbed species on Pt formed in small amounts. It is possible that the reduction of C02 provides also -COOH and triply bonded COH, as proposed earlier [16]. 

In binary catalysts two types of propagation centers can be kinetically identified stereospecific CJ and non stereospecific C. The aluminum alkyl causes the formation of such centers by means of irreversible alkylation reactions of the corresponding S and SA sites. Moreover, it brings about the reversible deactivation of the propagation species, which is preferential for the non-stereospecific centers. The external base, in equilibrium and competition with the organoaluminum, would reversibly poison the non-stereospecific centers and, to a much lower degree, also the stereospecific centers. In the ternary catalysts a further stereospecific center, would be present. This center is most likely, but not necessarily, donor associated. In this case the aluminum alkyl, besides deactivating the various active centers to different... 

It is well known that metal catalysts are poisoned by compounds of Group VB and VIB elements (ref. 90). The precise effect of a given poison however, may vary from system to system. In addition, catalyst deactivation may also result from the adsorption of a product of the reaction onto the active surface. The poisoning effect may be reversible, and in some cases catalytic activity can be restored by eliminating the source of poison. On the other hand, when poisoning occurs by an irreversible process, regeneration of the catalyst may not be possible, and so it may have to be discarded. 

Reversible/Irreversible Poisoning- Further definition, however, is needed to clarify the related concepts of reversible and irreversible deactivation, and poisons versus simple competitive adsoiption. Deactivation, which may involve the loss of the catalyst s conversion ability, activity or selectivity, over time is often irreversible. If irreversible, continued in situ operation of the catalyst in the absence of the deactivating agent does not restore the catalyst to its original activity. 

Steam reforming catalysts are poisoned by sulfur, arsenic, chlorine, phosphorus, copper and lead. Poisoning results in catalyst deactivation however, sulfur poisoning is often reversible. Reactivation can be achieved by removing sulfur from the feed and steaming the catalyst. Arsenic is a permanent poison therefore, feed should contain no more than 50 ppm of arsenic to prevent permanent catalyst deactivation by arsenic poisoning 13]. 

If the activity of the catalyst is slowly modified by chemisorption of materials that are not easily removed, the deactivation process is termed poisoning. It is usually caused by preferential adsorption of small quantities of impurities (poisons) present in the feedstream. Adsorption of extremely small amounts of the poison (a small fraction of a monolayer) is often sufficient to cause very large losses in catalytic activity. The bonds linking the catalyst and poison are often abnormally strong and highly specific. Consequently, the process is often irreversible. If the process is reversible, a change in the temperature or the composition of the gas to which it is exposed may be sufficient to restore catalyst... 

The intentional design of model systems can be envisioned, as for instance binary or multiple assemblies (clusters) of active components and poisons, for the examination of their activity in chemisorption, or specific reactions. The results can then be compared with respective clusters containing the active species only. Perhaps, such model systems will be amenable to computational methods capable of predicting their chemisorptive behavior and their surface reactivity. Such approaches are now employed for the design of improved multicomponent catalysts and can, obviously, be used to study the reverse effect, i.e., the mutual deactivation of the cluster components. 

Moffat and Clark 84> found that a Langmuir-Hinshelwood model applied to a heterogeneous surface can be used to describe both the general kinetics and the rate-temperature maxima reported by Banks and Bailey (Fig. 2) for olefin disproportionation on cobalt molybdate-alumina catalyst. They conclude that the rate-temperature maximum was caused by the reversible deactivation of sites superimposed on the irreversible poisoning of sites. 

Controlled burning of carbon does not regenerate all catalysts. Catalysts can be deactivated by particle growth, compound formation, tramp metal deposition, crystal-phase changes, and adsorption of catalyst poisons that cannot be reversed by thermal oxidative treatment. 

Catalysts can be regenerated, that is, the performance of deactivated catalysts can be improved by regeneration. That is, when the catalyst activity and/or selectivity is reduced during operation, a particular treatment allows the proper activity and/or selectivity of the catalysts to be restored [7], Deactivation is produced by inhibition, fouling, or sintering, and all of these can be reversed, by the removal of poisons, or fouling agents, like coke, or by the re-dispersion of the active species [8], The reproducibility of a catalyst is related to the consistency of its properties in different sets of production lots [7],... 

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