Deactivation modes, coke

DEACTIVATING EFFECTS OF COKE MOLECULES MODES OF DEACTIVATION... 

In the MHC mode, the coke deposition on the catalysts is almost constant from the reactor top to the reactor bottom. This means that coke deactivation has the same significance for all catalyst types and is not such a dominant factor. Deactivation by metal deposition is dominant for each catalyst. Therefore demetallization catalysts with a high metal tolerance (higher metal absorption capacity) are essential for the catalyst life in the MHC mode. 

Coking, aging and regeneration of zeolites. Ill- Comparison of the deactivation modes of H mordenite, HZSM-5 and HY during n-heptane cracking, J. Catal. 106, 242-250. 

The reactor impregnated with nickel showed inferior performance again. Deactivation was observed, which was assumed to originate from coking, sintering, oxidation of the nickel or even losses of volatile nickel species. With increasing temperature, enhanced formation of by-products, namely methane and ethane, was observed in the reformate both under partial oxidation conditions and in the autothermal mode, which was attributed to thermal cracking. 

An important preliminary remark must be made. In contrast to non-zeolite catalysts the heavy secondary products responsible for zeolite deactivation are not always polyaromatics. Indeed the pores of certain zeolites are too narrow to allow the formation of potyaromatic compounds and even to accommodate them [1]. This has caused some confusion in the relevant literature, certain authors using the word coke only for the polyaromatic compounds and others for all the secondary products, polyaromatic and non polyaromatic retained in or on the zeolite. Although the designation of non polyaromatic and sometimes simple molecules as "coke may appear surprising it is this latter definition that will be used here. Indeed the non polyaromatic molecules contributing with the polyaromatic ones to deactivation and often through similar modes we considered preferable to use the same term (coke) for all the secondary products responsible for zeolite deactivation [8]. 

Deactivation of zeolites, like that of the other porous catalysts occurs in two ways, the first one in which at the maximum one active site per coke molecule is deactivated, the second in which several active sites are deactivated. The two modes of zeolite deactivation are shown in Figure 7. The effect of coke on the activity and on the pore volume accessible to the reactant is also indicated. 

The first mode of deactivation is clearly shown with HZSM5, At low coke content, Vr/Va is close to 1 4 coke molecules are needed to deactivate one acid site, This weak deactivating effect can be explained by a competition for adsorption on the acid sites between the reactant and the coke molecules which are too weakly basic to be "irreversibly adsorbed at the reaction temperature. However limitations in the rate of diffusion of the reactant can also be responsible for deactivation. The size and the basicity of the coke molecules increase with the coke content, which causes an increase in the deactivating effect of the coke molecules. Beyond a certain size of the coke molecules the channel intersection is completely inaccessible to the reactant and to the adsorbates and Vr/Va can be lower than T This first mode of deactivation occurs also with USHY. However the deactivating effect of coke molecules is initially very high because coke molecules are formed on the strongest (hence the most active) acid sites. 

With zeolite catalysts it is possible to determine the coke composition, essential for the understanding of the modes of coke formation, of deactivation and of coke oxidation. As the micropores cause an easy retention of organic molecules through condensation, electronic interactions or steric blockage, the formation of coke molecules begins within these micropores. Their size is therefore limited by the size of channels, of cavities or of channel intersections. However the growth of coke molecules trapped in the cavities or at the channel intersections close to the outer surface of the crystallites leads to bulky polyaromatic molecules which overflow onto this outer surface. 

In addition, oxidized V also catalyzes coke and H2 formation through dehydrogenation as described in the case of Ni. V is therefore a significant contributor to catalyst deactivation, Figure 5.6. In addition, the more oxidized the V, the more destructive the effect if, therefore, the regenerator is operated in the more efficient full burn mode (lower levels of CO emissions), the destruction of the zeolite is increased, Figure 5.7. 

In contrast to Fig. 12a, the spectrum of a coke from the same process shown in Fig. 12c, surprisingly, strongly resembles the signals of the well-defined species [Fe(H20)Cl5] (49), the simulated spectrum of which is also included Fig. 12d. Alternative structures would show quite different vibrational spectra. The strongest band, at 386cm is assigned to the Fe-OH2 torsional mode. The presence of this species indicates another cause of catalyst deactivation. This species was probably the result of traces of moisture in the HCl recycle gas stream, which can lead to dew point corrosion and hence to the formation of [Fe(H20)Cl5] species, which may dominate the whole IINS spectrum of this type of coke. 

The mode of coke deposition is closely related to the pore structure of the zeolite (5-8). Figure 1 shows how coke deposits on typical zeolites. In the case ofZSM-5, coke deposits at intersections of the straight and zigzag channels, and also on the outer surface of the crystal. Whereas, Y type zeolites and mordenites have supercages whose sizes are almost equal to the molecular sizes of aromatic compounds composed of a few benzene rings, and coke is easily formed in the supercages. These differences in the manner of the coke formation reflect on mode of the deactivation... 

Modes of Coking and Deactivation of Acid Zeolite Catalysts... 

Mass Spectrometry. Mass spectrometry was conducted on the coke concentrates using a VG instrument in which the probe was heated from ambient to 5mass range 50-600 were recorded every 5 s. Spectra were recorded in both electron impact (El) and chemical ionisation (Cl, with ammonia) modes. Field ionisation (FI) spectra of some of the deactivated catalysts from the n-hexadecane MAT runs were obtained at the Stanford Research Institute as described elsewhere (16). 

The wide-ranging effects of pore structure on coke toxicity lead us to define four modes of deactivation (Figure 8) instead of the two (site coverage and pore blockage) which are generally proposed. Thus, deactivation could be due to ... 

---