Platinum-silica catalysts conditions

Subsequent to the discovery of skeletal rearrangement reactions on plati-num/charcoal catalysts, the reality of platinum-only catalysis for reactions of this sort was reinforced with the observation of the isomerization of C4 and C5 aliphatic hydrocarbons over thick continuous evaporated platinum films (68,108, 24). As we have seen from the discussion of film structure in previous sections, films of this sort offer negligible access of gas to the substrate beneath. Furthermore, these reactions were often carried out under conditions where no glass, other than that covered by platinum film, was heated to reaction temperature that is, there was essentially no surface other than platinum available at reaction temperature. Studies have also been carried out (109, 110) using platinum/silica catalysts in which the silica is catalytically inert, and the reaction is undoubted confined to the platinum surface. 

Fig. 2. Isomerization of n-heptane over mixtures of particles of silica-alumina and particles of inert-supported platinum (W5). The dashed lines represent conversions over platinum-impregnated silica-alumina. Conditions of runs 25 atm., Hj/nC = 4/1, space velocity = 0.7 g. nC7 per hour per gram of catalyst.

The catalytic activation of CO2 and its reaction with C2H4 and H2O was studied over several silica-supported platinum-tin catalysts under different reaction conditions. The lactic acid production is related to the content of the PtSn alloy in the catalyst. 

Polyaromatics react under hydrocracking conditions to undergo partial or complete saturation of the aromatic rings, together with isomerization and cracking of intermediate and perhydro products. Phenanthrene, for example, can lead to tetralin and methyl cyclohexane as the principal products. With a platinum/ silica-alumina catalyst,67 the major products are isomerized perhydrophenan-threnes, which includes some adamantanes and cracked products, the total number of products exceeding 100. 

Several years ago, one of the authors found that nickel, platinum, and some other hydrogenating agents, when deposited on fresh synthetic silica-alumina cracking catalyst, made a new catalyst that would isomerize paraffin and naphthene hydrocarbons in the presence of hydrogen at elevated pressures and nominal temperatures. Table I shows some early typical results calculated from mass spectrometer analyses of the products obtained by passing methyl cyclopentane, cyclohexane, and n-hexane over a catalyst composed of 5% nickel in silica-alumina at the indicated reaction conditions. Isomerization of a number of other hydrocarbons has also been studied and reported elsewhere (2). 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

However, in heterogeneous catalysis, metals are usually deposited on nonconducting supports such as alumina or silica. For such conditions electrochemical techniques cannot be used and the potential of the metallic particles is controlled by means of a supplementary redox system [8, 33]. Each particle behaves like a microelectrode and assumes the reversible equilibrium potential of the supplementary redox system in use. For example, with a platinum catalyst deposited on silica in an aqueous solution and in the presence of hydrogen, each particle of platinum takes the reversible potential of the equilibrium 2H+ + 2e H2, given by Nemst s law as... 

Among the transition metals, Pd, Pt, Ir, and Rh in various forms have been reported active in methanol synthesis (32-34), as noted in Section II. Palladium, platinum, and iridium metals were supported on silica, and it has recently been suggested that palladium is present in its valence state Pd(II) which is the active form of the catalyst (70). Under the synthesis conditions the Pd(II) ions could not survive the highly reducing atmosphere of the CO/H2 synthesis gas, and so this valence state would have to be induced by the presence of silicon dioxide. Should this be a general case, silica would not act merely as an inert support, and the silica-supported transition metals would have to be considered binary catalysts whose active state is formed by a support-metal interaction. ... 

In the gas-phase, benzene shows a single line,77 78 and can yield useful information regarding the diffusion/transport properties. Benzene trapped within pores in glasses and silica gels too yields results, about pore size and adsorbed versus liquid-phase conditions.79 Chemisorption on alumina-supported platinum catalysts leads to disclosure as to how and where the benzene molecules are located, via FT NMR.80... 

In addition to palladium, the catalysts used commercially always contain alkali salts, preferably potassium acetate. Additional activators include gold, cadmium, platinum, rhodium, barium, while supports such as silica, alumina, aluminosilicates or carbon are used. The catalysts remain in operation for several years but undergo deactivation. The drop in activity is due to a gradual sintering of the palladium particles which causes the catalytically active area to decrease progressively. Under reaction conditions potassium acetate is slowly lost from the catalyst and must continuously be replaced. 

P. S. Nix and S. Lucki, at our laboratories, have demonstrated (unpublished) the ability of separate platinum and acidic catalysts, as a mixed composite, to perform the skeletal transition from ethyl-benzene to xylenes under hydrogenative conditions (pn, = 11.8 atm., Peb = 1.2 atm. 427 C, r = 3.3 sec.) with 40% conversion to xylenes. Yet at the same temperature, but at atmospheric pressure where production of cyclo-olefin intermediates is not favored, they obtained no measurable conversion even with platinum directly impregnated on the silica-alumina. 

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