Platinum/zeolite hydrogenation, selective

The noble metal component may be either palladium or platinum the effect of the concentration of both metals on methylpentane as well as on dimethylbutane selectivity in C6 hydroisomerization on lanthanum and ammonium Y-zeolite with Si/Al of 2.5 has been studied by M.A. Lanewala et al. (5). They found an optimum of metal content for that reaction between 0.1 and 0.4 wt.-%. The noble metal has several functions (i) to increase the isomerization activity of the zeolite (ii) to support the saturation of the coke precursors and hence prevent deactivation, as was shown by H.W. Kouvenhoven et al. (6) for platinum on hydrogen mordenite (iii) to support the hydrodesulfurization activity of the catalysts in sulfur containing feedstocks. 

The paper deals with some new data concerning the state of the metal after reduction and the catalytic functions of zeolite catalysts containing nickel and platinum. By using the molecular sieve selectivity in the hydrogenation of mesitylene it has been proved that metal (platinum) is contained in the volume of the zeolite crystal. The temperature dependence of the formation of nickel crystals was investigated. The aluminosilicate structure and the zeolite composition influence mainly the formation of the metal surface which determines the catalytic activity. In the hydrocracking of cumene and disproportionation of toluene a bifunctional action of catalysts has been established. Hydrogen retarded the reaction. 

In addition to performing acid/base catalysis, zeolite structures can serve as hosts for small metal particles. Transition metal ions, e.g., platinum, rhodium, can be ion exchanged into zeolites and then reduced to their zero valent state to yield zeolite encapsulated metal particles. Inside the zeolite structure, these particles can perform shape selective catalysis. Joh et al. (16) reported the shape selective hydrogenation of olefins by rhodium encapsulated in zeolite Y (specifically, cyclohexene and cyclododecene). Although both molecules can be hydrogenated by rhodium supported on nonmicroporous carbon, only cyclohexene can be hydrogenated by rhodium encapsulated in zeolite Y since cyclododecene is too large to adsorb into the pores of zeolite Y. 

Blackmond et al. compared the selectivities of ruthenium, platinum, and rhodium supported on NaY and KY zeolites with those supported on carbon, in the hydrogenation of cinnamaldehyde and 3-methylcrotonaldehyde in isopropyl alcohol at 100°C (for rhodium and ruthenium) or 70°C (for platinum) and 4 MPa H2.52 Good selectivities to unsaturated alcohols were obtained over zeolite-supported ruthenium and platinum with... 

Here, we also mention a composite system developed by Van der Puil et al. [90] in which an MFI (Silicalite-1) layer is completely covering a platinum-on-flat support catalyst. The zeolite layer is governing the acces to the platinum and this leads in the competitive hydrogenation of 1-heptene and 3,3-dimethyl-l-butene seeFigure 31 + 32) to highly selective conversion of 1-... 

Hasegawa Y, Kusakabe K, and Morooka S. Selective oxidation of carbon monoxide in hydrogen-rich mixtures by permeation through a platinum-loaded Y-type zeolite membrane. J Membr Sci 2001 190 1-8. 

A novel method for the preparation of metal containing small pore zeolites is described. The metal is introduced at elevated temperatures by solid state ion exchange. The zeolites obtained by the new method are highly shape selective. As an example, the competitive hydrogenation of an equimolar mixture of hexene-(l) and 2,4,4-trimethylpentene-(l) over various platinum, palladium and rhodium containing 8-membered ring zeolites was studied. 

Iridium clusters in zeolite KLTL, like the platinum clusters, consisting of 4 to 6 atoms on average, have also been prepared by hydrogen reduction of [Ir(NH3)5Cl]Cl2 in the pores at temperatures >300°C [26]. Even though the iridium clusters were as small as the selective platinum clusters in the same basic zeolite support, they were found to be unselective catalysts, being similar to other iridium catalysts for conversion of n-hexane and hydrogen principally into hydrogenolysis products. It is inferred that the combination of cluster size, electronic... 

A competitive test in which a mixture of methylpyruvate and t-butylpyruvate were hydrogenated with the platinum salen complex occluded in USY zeolite shows that only the methylpyruvate gives a conversion of 10%. The t-butylpyruvate gives only a conversion of less than 0.5 %. So it is possible to hydrogenate only one of the two substrates, highly probably because of the reactant selectivity of the zeolite. 

S. J. Miller (Chevron) published results from early work that highlighted the selectivity of the platinum form of SAPO-11 catalyst compared to a number of others. These others were amorphous silica-alumina, from which one would expect little or no selectivity, ZSM-5, HY, and Na-Beta zeolites. All the catalysts carried 1 wt. % platinum and the feed employed was n-octane. He found that at 30% conversion, only SAPO-11, the amorphous silica-alumina, and the HY catalysts exhibited better than 94% selectivity for feed isomerization to isooctanes. ZSM-5 and Na-Beta catalysts behaved poorly in this regard. Selectivity for dimethylhexanes was low. SAPO-11 also produced equal quantities of 2- and 3-methyl heptanes, whereas the other catalysts favored 3-methyl heptane, with a ratio close to that favored by thermodynamics. SAPO-11 also produced one of the lowest levels of doubly-branched hexanes (Table 10.1646) and the predominant ones formed were those separated by more than one carbon—only minor amounts of the less thermally stable (bond breaking here can produce tertiary carbonium ions) geminal-dimethyl (2,2 and 3,3-) ones were formed. Noble metal presence was a key to success since replacement of the hydrogenation metal platinum by pallodium did not alter the isomeri-zation selectivity much, but replacement by nickel led to very poor isomerization. 

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