Metal Function of the Catalyst

A metal, a metal oxide, a metal sulfide, or a combination of these compounds may supply the metal function of the catalyst. The key requirement for the metal function is that it must activate hydrogen and catalyze dehydrogenation and hydrogenation reactions. In addition, metal-catalyzed hydrogenolysis (carbon-carbon breaking) is undesirable because the distribution of the hydrogenolysis products is less desirable relative to hydrocracking. 

Because the Group VIA and Group VIIIA metals are most conveniently prepared as oxides, a sulfiding step is necessary. That will be discussed in Section 7 (Catalyst Loading and Activation). 

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Parera et al. showed that coke formation results mainly from the polymerization of cyclopentadiene (CPde). This diene is produced by dehydrogenation of cyclopentane (CP) on the metallic function of the catalyst. Then, it is possible to assume that the difference in coke formation from n-pentane and n-hexane on the different catalysts could be due to the different selectivities to CP and MCP and the different dehydrogenation capacities to transform them into CPde and MCPde, respectively. 

When properly optimized, the metal function of the catalysts essentially maintains hexane-hexene equilibrium. The reaction energy scheme deduced" for hexene protonation and isomerisation is shown in figure 2. 

The metal function of the catalyst is poison by sulfur, as discussed later. The elimination of sulfur from the naphthas is carried out in the hydrodesulfurization vmit, in which the sulfur compounds are hydrogenolyzed, producing H2S, which is separated. This unit contains a catalyst, usually Co-Mo, Ni-Mo, or Ni-W supported on alumina. This catalyst also eliminates the nitrogen as NH3 and retains the arsenic and metals present in the feed. 

Kinetics. The dehydrogenation reaction is catalyzed by the metal function of the catalyst. It is a very fast reaction approaching equilibrium in commercial operation. 

Figure 4.61 shows the location of the catalyst library and also lists the available catalyst types. A catalyst type essentially contains tuning or calibration factors responsible for light gas distribution, small adjustments to product bulk properties (RON, MON, etc.) and distribution of coke produced by the metal function of the catalyst The catalyst library contains catalyst from a variety of manufacturers and sources. If the exact catalyst is not available, we recommend using a similar match. It is possible to tune away variations in the tuning factors due to catalyst type, but this may produce an overcalibrated model with unrealistic yield predictions. For this model, we will use the af 3.csv catalyst... 

Figure 5.5 shows the relationship between the acid and metal functions of the catalyst and particular classes of reactions. The acidic function of the catalyst promotes the isomerization reactions, namely, reactions that convert paraffins into napthenes and isoparaffins. Iso-paraffins are important contributors to high-octane number. The metal function promotes the dehydrogenation reactions, where the napthenes are dehydrogenated into aromatics. The metal function is also a significant source of coke (or polyaromatic compound) that adsorbs to the catalyst surface. In addition, the olefins are hydrogenated producing paraffins for further reaction. 

Another significant feature is that the coke generation is rigorously modeled and included in the deactivation and adsorption factor F for each reaction. The deactivation factor is a function of reactor pressure, adsorbed hydrocarbons, coke on catalyst and acid/metal function of the catalyst This feature allows us to calibrate the model to a variety of operating conditions and catalyst behavior. In this work, we model a CCR with a hydrotreated feed therefore, we do not include any significant changes in catalyst activity due to changes in add of the catalyst... 

The active site on the surface of selective propylene ammoxidation catalyst contains three critical functionalities associated with the specific metal components of the catalyst (37—39) an a-H abstraction component such as Sb ", or Te" " an olefin chemisorption and oxygen or nitrogen insertion component such as Mo " or and a redox couple such as Fe " /Fe " or Ce " /Ce" " to enhance transfer of lattice oxygen between the bulk and surface... 

Oxidation and chlorination of the catalyst are then performed to ensure complete carbon removal, restore the catalyst chloride to its proper level, and maintain full platinum dispersion on the catalyst surface. Typically, the catalyst is oxidized in sufficient oxygen at about 510°C for a period of six hours or more. Sufficient chloride is added, usually as an organic chloride, to restore the chloride content and acid function of the catalyst and to provide redispersion of any platinum agglomeration that may have occurred. The catalyst is then reduced to return the metal components to their active form. This reduction is accompHshed by using a flow of electrolytic hydrogen or recycle gas from another Platforming unit at 400 to 480°C for a period of one to two hours. 

Metal halides like zinc chloride are used as Lewis-acid catalysts. Other Lewis-acids or protic acids, as well as transition metals, have found application also. The major function of the catalyst seems to be the acceleration of the second step—the formation of the new carbon-carbon bond. 

It should be emphasized that is the actual, promoter modified, work function of the catalyst surface and not that of a clean metal surface for which we reserve the symbol o- It should also be clarified that the kinetic constant kR is also expected to vary with . Since, however, we have no rules on how it varies with we will attempt here to rationalize some classical promotional kinetics treating it as a constant. What is amazing is that this procedure works, which indicates that the promoter action effect on kD andkA, together with the 1-0P term, is dominant. 

In this equation, Y is the catalyst performance, the variables X and ni are normalized variables representing the reaction conditions and catalyst s metal weight loadings, respectively. The model coefficients C, a , and (3 , are functions of the catalyst composition, as shown in Eqns (6) and (7), where m.j refers to the nominal weight loading of Pt, Ba, or Fe. The equation for (3 takes the same form as Eqns (6) and (7). 

These reactions have been known for almost 100 years and they have been extensively studied. The reactions are catalyzed by the corresponding ammonium salt in each case, although other protonated amines function as catalysts. It appears that the function of the catalyst is to supply ff+, which helps to force an end of the ethylenediamine molecule away from the metal. 

The variety of functions of the catalyst is pronounced, in particular, in the technological catalytic oxidation of -paraffins to aliphatic acids [5]. This technology consists of several stages among which the central place is occupied by oxidation. It is conducted at 380 420 K in a series of reactors, with a mixture of salts of aliphatic acids of K+ and Mn2+ or Na+ and Mn2+ as the catalyst. The alkaline metal salt stabilizes (makes it more soluble and stable) the manganese salt [152]. Studies have revealed the multifunctional role of the catalyst (manganese ions) (Mn) [152-154]. 

The surface area of the catalyst as well as the pore size distribution can easily be measured, and the zeolite and matrix surface areas of the catalyst can be determined by the t-plot method. The different FCC yields can then be plotted as a function of the ZSA/MSA ratio, zeolite surface area or matrix surface area, and valuable information can be obtained [9], The original recommendation was that a residue catalyst should have a large active matrix surface area and a moderate zeolite surface area [10,11]. This recommendation should be compared with the corresponding recommendation for a VGO catalyst a VGO catalyst should have a low-matrix surface area in order to improve the coke selectivity and allow efficient stripping of the carbons from the catalyst [12], Besides precracking the large molecules in the feed, the matrix also must maintain the metal resistance of the catalyst. 

A more detailed interpretation of the chemistry of catalytic cracking was based on studies with pure hydrocarbons.121-123 A simplified summary put forward by Heinemann and coworkers123 (Fig. 2.1) shows how Cg open-chain and cyclic alkanes are transformed to benzene by the action of both the hydrogenating (metal) and acidic (halogenated alumina) functions of the catalyst. 

The acid function of the catalyst is supplied by the support. Among the supports mentioned in the literature are silica-alumina, silica-zirconia, silica-magnesia, alumina-boria, silica-titania, acid-treated clays, acidic metal phosphates, alumina, and other such solid acids. The acidic properties of these amorphous catalysts can be further activated by the addition of small proportions of acidic halides such as HF, BF3, SiFit, and the like (3.). Zeolites such as the faujasites and mordenites are also important supports for hydrocracking catalysts (2). 

In absence of a catalyst, simple olefins are essentially fixed in their bonding configurations reaction paths to interconversions through molecular collisions, fusions, and disassociations are apparently closed because of orbital symmetry restrictions, as proposed by Hoffman and Woodward 8°). Mango 8 has postulated that in the presence of certain transition metal catalysts, these orbital symmetry restraints are lifted, allowing bonds to flow freely and molecular systems to interchange. Thus, the conservation of molecular orbital symmetry is a key function of the catalyst. 

Halogen exchange at less activated sites requires a Lewis acid catalyst and an important part of the function of the catalyst, usually a metal fluoride or a chromium species, is to assist the removal of halogen as halide ion. Therefore, these reactions could be considered to involve carbocationic intermediates (Figure 2.4). 

Test Procedures. The tests were performed with 35 g of catalyst in the reactor, and the catalyst was oxychlorinated, reduced and sulfided in the reactor prior to testing. Oxychlorination was carried out in order to ensure uniform chloride content as well as a highly dispersed metal function on the catalyst. Oxychlorination was carried out at 500°C in an air stream containing H2O and HCl at a given ratio, before "rejuvenation" of the metal function in dry air. The catalyst was then reduced in Hj... 

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