Catalysts, supported

Supported catalysts represent the largest group of heterogeneous catalysts and are of major economic importance, especially in refinery technology and the chemical industry. Supported catalysts are heterogeneous catalysts in which small amounts of catalytically active materials, especially metals, are applied to the surface of porous, mostly inert solids - the so-called supports. The supports can have special forms such as pellets, rings, extrudates, and granules. 

Typical catalyst supports are porous solids such as aluminum oxides, silca gel, MgO, Ti02, Zr02, aluminosilicates, zeolites, activated carbon, and ceramics. Table 5-31 lists widely used catalyst supports. 

What are the reasons for the predominant use of supported catalysts in industry  

Aluminosilicates up to 180 cracking reactions, dehydrations, isomerizations, ammoxidation 

Silica Si02 200-1000 polymerization, hydrogenation, oxidation, NO c reduction (SCR process) 

In many cases of catalysis by metals the actual catalyst particles are supported. This capitalizes on the observation that only a small proportion of the metal atoms in the surface are catalytically active further a larger surface area is achieved for a given quantity of metal if it is dispersed on a high surface carrier. This also means that the catalyst is made in such a way that the support (frequently a metal oxide, though a wide variety of other matrices have been used) comprises the bulk of the material. This also has an obvious economic benefit since the actual (expensive) catalyst is diluted by a much cheaper material. Some support materials are inert - they play no role in the catalyses. 

However frequently the support material does have a very important function. This is particularly so when the support acts as a (Bronsted) acid or a base. An example is in the catalytic cracking, alkylation, and isomerization of hydrocarbons (Section 5.2.6) The role of the transition metal is in oxidation or hydrogen transfer reactions while the support, for example acidic oxides such as aluminosilicates, act to protonate, rearrange and dehydrate organic species. 

Catalysts are manufactured by various methods (such as precipitation, extrusion and spray drying) in the form of cylinders, rings, multi-lobed extru-dates and other shapes. They range in size from a few millimetres to several centimetres small spheres are used in fluidized bed reactors. Active phases can be dispersed on the pre-shaped support by several methods such as by impregnation of a solution of the active components. Alternatively the catalysts can be made by the extrusion of mixtures of solid components the support, active phase, and binder. For some reactions that are diffusion limited, the catalyt-ically active species are not uniformly distributed instead they are deposited on the outer shell of the catalyst particle (egg-shell catalysts), since those inside the particle cannot be involved in the reaction. 

It is obvious that the surface of the catalyst can be enhanced when supporting of the catalysts has been achieved successfully. Therefore, there are a lot of efforts to use this supporting strategy to increase the catalytic activity, some of which has been discussed below. 

Hydrotreatment processes, including hydrodesulfurization, are carried out in practice on supported catalysts, mostly on alumina or silica supports, on 

Sulfidation by H2 S, represented in Refe. 26, 40-44 is a widely used method for determining the sulfur uptake of catalysts, based on the radioactivity balance in between the introduced H2 S/H2 and the outflow-, or line-out gasesl l  

Smob is determined in the pulse-flow system from the radioactivity of fraction containing the reversibly taken up and the desorbed, by flowing gas (e g., by H2 or N2) of I3 radioactivity collected between injections of two hydrogen sulfide pulses and calculated by the expression  

In case of the circulation system, the vessel is evacuated for the removal of S]p0v Introduction of the H2 S/H2 mixture of initial radioactivity is repeated, and Srev is calculated from the radioactivity loss (41) by expression (3), applying the radioactivity, I2, at line out. The St value in this run does not contain Sirr, as the catalyst was sulfided in Run 1. No Sgxo is measured either, as the gas phase S specific radioactivity is equivalent to that of sulfur in the Scat- 

Expression (4) is also applicable for calculation of actual sulfur exchange (aSexc) between S-labelled catalyst and non-labelled (non-radioactive) H2S circulated over the catalyst, until constant radioactivity (I3) is reached. 

Further aspects of catalyst preformation and catalyst aging are covered in the context of supported catalysts in Sects. 2.1.7 and 3.2. 

The use of supported Nd catalyst systems is described for the polymerization of BD and IP in solution, gas phase and slurry. In this section the use of supported catalyst is reviewed for solution processes, only. The use of supported catalysts for gas-phase polymerization (Sect. 3.2) and for slurry polymerization (Sect. 3.1) is addressed in the cited sections. 

As early as in 1985, supported catalysts were described for the use in a solution process [399]. Bergbreiter et al. used catalyst supports on the basis of divinylbenzene-styrene-copolymers as well as on polyethylene. These authors found that the use of supported catalysts has no influence on the stereospecificity of diene polymerization. 

The immobilization of Nd tetramethyl aluminate complexes on porous silica supports (MCM-48) was reported by Fischbach et al. [406,407]. In this study activation was accomplished by DEAC. The polymerization of IP in hexane yielded IR with high cis- 1,4-contents ( 99%). 

Tris-allyl-neodymium Nd(//3-C3I Ishdioxane which performs as a single site catalyst in solution polymerization was heterogenized on various silica supports which differed in specific surface area and pore volume. The catalyst was activated by MAO. In the solution polymerization the best of the supported catalysts was 100 times more active (determined by the rate constant) than the respective unsupported catalyst [408]. 

More detailed discussions of the optimum choice of quaternary ammonium salt for phase-transfer catalysis are available elsewhere [1, 5-8]. 

In the following chapters, published procedures are given for a wide range of phase-transfer catalysed reactions. Particular catalysts, reaction conditions, and work-up procedures are recommended. In general, these are thought to be the optimum conditions but, where there are viable alternatives, they are indicated. 

M Halpern, in Phase-Transfer Catalysis. Fundamentals, Applications and Industrial Perspectives. Chapman Hall, New York, 1994. 

M Halpern, in Phase Transfer Catalysis in Organic and Polymer Synthesis. Mechanism and Synthesis (ed. ME Halper), ACS Symposium Series 659, Am. Chem. Soc., Washington, 1997, 

The removal of highly lipophilic catalysts from the reaction product poses problems, which can be obviated by binding the ammonium Catalyst to a solid support. As well as being easily removed by nitration and recycled, such catalysts also have potential application in continuous flow phase-transfer catalytic processes. 

In an overwhelming majority of cases, the transition metal atom is bound to the carrier surface by an oxygen bridge 

The Solvay catalysts are activated by aluminium organometals, particularly Et2AlCl, (isoBu)3 A1 or (isoBu)2 A1H [267a]. 

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Supported catalysts, in which the active catalytic material is... 

Fig. XVIII-27. Specific rates of CO oxidation on single crystal and supported catalysts as a function of temperature. (From Ref 308. Reprinted with permission from American Chemical Society, copyright 1988.)...

Increasing the surface-to-bulk ratio of the sample to be studied. This is easily done in the case of highly porous materials, and has been exploited for the characterization of supported catalysts, zeolites, sol-gels and porous silicon, to mention a few. 

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

Because x-rays are particularly penetrating, they are very usefiil in probing solids, but are not as well suited for the analysis of surfaces. X-ray diffraction (XRD) methods are nevertheless used routinely in the characterization of powders and of supported catalysts to extract infomration about the degree of crystallinity and the nature and crystallographic phases of oxides, nitrides and carbides [, ]. Particle size and dispersion data are often acquired with XRD as well. 

In the case of metal particles distributed on a support material (e.g. supported catalysts), XPS yields infomiation on the dispersion. A higher metal/support intensity ratio (at the same metal content) indicates a better dispersion [3]. 

MgCl2-Supported Catalysts. Examination of polymerizations with TiCl catalysts has estabUshed that only a small percentage of titanium located on lateral faces, edges, and along crystal defects is active (52) (see Titanium and titanium alloys). This led to the recognition that much of the catalyst mass acted only as a support, promoting considerable activity aimed at finding a support for active titanium that would not be detrimental to polymer properties. 

Third-generation high yield supported catalysts are also used in processes in which Hquid monomer is polymerized in continuous stirred tank reactors. The Hypol process (Mitsui Petrochemical), utilizes the same supported catalyst technology as the Spheripol process (133). Rexene has converted the hquid monomer process to the newer high yield catalysts. Shell uses its high yield (SHAC) catalysts to produce homopolymers and random copolymers in the Lippshac process (130). 

Supported aqueous phase (SAP) catalysts (16) employ an aqueous film of TPPTS or similar ligand, deposited on a soHd support, eg, controlled pore glass. Whereas these supported catalysts overcome some of the principal limitations experienced using heterogeneous catalysts, including rhodium leaching and rapid catalyst deactivation, SAP catalysts have not found commercial appHcation as of this writing. 

Toluene reacts with carbon monoxide and butene-1 under pressure in the presence of hydrogen fluoride and boron trifluoride to give 4-methyl-j iYbutyrophenone which is reduced to the carbinol and dehydrated to the olefin. The latter is cycHzed and dehydrogenated over a special alumina-supported catalyst to give pure 2,6- dim ethyl n aph th a1 en e, free from isomers. It is also possible to isomerize various dim ethyl n aph th a1 en es to the... 

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