Metal and acid sites

Catalyst stability with time on stream is an important characteristic. Acidic catalysts can be deactivated by basic poisons such as nitrogen. Carbonaceous species can build up on both metal and acid sites. These are the two prevalent mechanisms for catalyst deactivation. Other ways that a catalyst can be damaged, such as a temperature excursion, may be more likely to occur during the initial start up or during coke burning regenerations. Regeneration is discussed in the next section. 

The elements of range in value from 0 to 1 and are the ratio of the reformer kinetic constants at time on stream t to the values at start of cycle. At any time on stream t, the deactivation rate constant matrix K(a) is determined by modifying the start-of-cycle K with a. From the catalytic chemistry, it is known that each reaction class—dehydrogenation, isomerization, ring closure, and cracking—takes place on a different combination of metal and acid sites (see Section II). As the catalyst ages, the catalytic sites deactivate at... 

The C6 component reaction sequence involves several elementary steps with adsorbed intermediates on both the metal and acid sites. An example is the reaction of methylcyclopentane to cyclohexane ... 

Thus, the number of ring isomerization sites available for reaction is the total number of active sites minus those sites blocked by the formation of coke. Since the metal and acid sites can be used for reactions other than isomerization, intermediates from these other reactants adsorbed on the site also affect the ring isomerization reaction by reducing the number of active sites for the ring isomerization reaction. Since isomerization involves both metal and acid sites in a unique way, all intermediates from other reactions that utilize metal and acid sites in this manner have the potential to block these active sites. This is illustrated in Fig. 13. 

Dual-function catalysts possessing both metallic and acidic sites bring about more complex transformations. Carbocationic cyclization and isomerization as well as reactions characteristic of metals occurring in parallel or in subsequent steps offer new reaction pathways. Alternative reactions may result in the formation of the same products in various multistep pathways. Mechanical mixtures of acidic supports (silica-alumina) and platinum gave results similar to those of platinum supported on acidic alumina.214,215 This indicates that proximity of the active sites is not a requirement for bifunctional catalysis, that is, that the two different functions seem to operate independently. 

Commercial reforming catalysts have both metal and acid sites. Both could contribute to cyclization. If there are four or more carbon atoms in the side chain of a mono-alkylaromatic or ortho-substituted dialkylaromatic hydrocarbon, cyclization can yield either five- or six-membered rings. This multiplicity of reaction pathways helps to clarify the roles of the metal and acid components in dehydrocyclization and other reactions. 

Heterogeneous catalysts for hydrocarbon conversion may require metal sites for hydrogenation-dehydrogenation and acidic sites for isomerisation-cyclisation and these reactions may be more or less susceptible to the effect of carbonaceous overlayers depending on the size of ensembles of surface atoms necessary for the reaction. In reality we must expect species to be transferred and spilled-over between the various types of sites and if this transfer is sufficiently fast then it may affect the overall rate and selectivity observed. If there is spillover of a carbonaceous species [4] then there may be a common coke precursor for the carbonaceous overlayer on the two types of site. Nevertheless, the rate of deactivation of a metal site or an acidic site in isolation may be very different from the situation in which both types of site are present at a microscopic level on the same catalyst surface. The rate at which metal and acid sites deactivate with carbonaceous material may of course not be identical. Indeed metal sites may promote the re-oxidation of a carbonaceous species in TFO at a lower temperature than the acid sites would allow on their own and this may allow differentiation of the carbonaceous species held on the two types of site. 

Study of Deactivation of Metal and Acid Sites in Hydrocarbon Conversion... 

Thus it is clear that both metal and acid sites can be poisoned and deactivated rather quickly in hydrocarbon conversion reactions and at very mild experimental conditions. 

Quantitative data on the relation of Pt loading, concentration of acid sites, and activity for -heptane conversion have been published by Guisnet et al. for HY, HMor, and HZSM-5 40). Their findings are illustrated in Fig. 22. Using p,/ a as a parameter for the relative concentrations of metal and acid sites, respectively, it appears that the minimum pt/ A values required for maintaining catalytic stability are different for different zeolites. 

Hydrogenolysis may occur on the metal. Other secondary reactions of monobranched isomers result in multibranched isomers or cracking products. The relative concentrations of metal and acid sites are of importance for the selectivities to primary and secondary isomerization and cracking i40). 

Since metallic and acidic sites are both created during reduction of NiSMM, it is very difficult to measure exclusively the influence of the metal function on the bifunctional activity of the catalyst. 

Assumption 4. The possible steps of physisorption, hydrogenation/dehydrogen-ation, shift between the metallic and acid sites, and protonation/deprotonation, are in equilibrium. 

Isomerization reactions of alkanes and cycloalkanes occur very readily on bifunctional catalysts containing both metal and acidic sites (3,4), the latter being associated with the carrier employed for the metal. This mode of reaction is very important for the catalysts used in commercial reforming, which will be discussed in Chapter 5. In such bifunctional catalysts, the metal and acidic sites catalyze different steps in the reaction sequence. 

The mode of transport of olefin intermediates between metal and acidic sites must be considered in this type of reaction scheme. In the isomerization of alkanes or dehydroisomerization of alkylcyclopentanes, a reaction sequence involving transport of olefin intermediates from metal to acidic... 

Investigations of the isomerization of alkanes in recent years have provided evidence that the reaction can occur on certain metals, notably platinum, in the absence of a separate acidic component in the catalyst (20-22). While it has been shown that a purely metal-catalyzed isomerization process can occur, the findings do not challenge the commonly accepted mode of action of bifunctional reforming catalysts in which separate metal and acidic sites participate in the reaction. The available data at conditions commonly employed with commercial reforming catalysts indicate that a purely metal-catalyzed process does not contribute appreciably to the overall isomerization reaction on a bifunctional catalyst. 

Methylcyclopentane (MCP) is a convenient probe molecule for interrogating the metal and acid sites of a bifunctional catalysts. For instance, metal clusters are formed in the cavities of zeolite Y by ion exchange, followed by calcination and reduction with hydrogen. Protons which act as Bronsted acid sites are formed during reduction of the metal ions. A monofunctional catalyst can be obtained by neutralizing these protons with NH3 or by secondary exchange with Na ions. With this acid-free form of such catalysts the ring-... 

Silicon poisons both metalic and acid sites of Pt/Al203... 

The criterion of evaluated commercial catalyst takes in account a large rage of characterization. On of the most important is the microactivity catal34ic test, that presents good sensibility to detect metal, acid sites and coke deactivation. The great number of reactionals routes makes the specific characterization more difficult. In this way cyclohexane presents ease improvement and can characterize simultaneously metallic and acid sites. 

In the context of alkanes, hydrogenolysis is the breaking of C—C bonds by the action of hydrogen, leading to alkanes of lower molar mass. It is not a reaction that is deliberately practised on a large scale, but it is a parasitic reaction that occurs in parallel with other useful reactions of alkanes to be considered in the next chapter. In order to learn how to avoid or minimise it, it becomes necessary to find out as much as possible about it, and this is most easily done with molecules containing only two to four carbon atoms. Alkanes can also be cracked by an acid-catalysed reaction on solid acids or acidic supports, but in this chapter we are solely concerned with reactions that proceed on purely metallic sites the cooperation of metallic and acidic sites in petroleum reforming will be briefly considered in Chapter 14. 

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