Platinum on alumina

Hydrogenation of Acetylenes. Complete hydrogenation of acetylenes to the corresponding alkanes, which maybe requited to remove acetylenic species from a mixture, or as a part of a multistep synthesis, may be accompHshed using <5 wt % palladium or platinum on alumina in a nonreactive solvent under very mild conditions, ie, <100°C, <1 MPa (10 atm). Platinum is preferred in those cases where it is desired to avoid isomeri2ation of the intermediate olefin. Silver on alumina also can be used in this appHcation as can unsupported platinum metal. 

They represent an improvement over earlier platinum on alumina catalysts in their abiHty to resist coke fouling when operated at low pressures. Dehydrogenation and hydrogenation occur on the active metal sites isomerization takes place on the acidic alumina surface. 

Standard Test Methodfor Surface Area of Catalysts. (D3663—78) Standard Test Method for Hydrogen Chemisorption on Supported Platinum on Alumina Catalysts. (D3908-80) American Society for Testing and Materials (ASTM), Philadelphia, PA. 

Aryl or vinyljluoroformates, when passed in the vapor phase with helium over an alumina or platinum on alumina catalyst eliminate carbon dioxide to give, respect vely,/ZMOroare er or vinyl fluorides [id] (equation 26)... 

Solvents influence rate as well as selectivity. The effect on rate can be very great, and a number of factors contribute to it. In closely related solvents, the rate may be directly proportional to the solubility of hydrogen in the solvent, as was shown to be the case for the hydrogenation of cyclohexene over platinum-on-alumina in cyclohexane, methylcyclohexane, and octane 48). Solvents can compete for catalyst sites with the reacting substrates, change viscosity and surface tension (108), and alter hydrogen availability at the catalyst surface. 

Reactor material Silicon Pyrex Catalyst material Platinum on alumina... 

Barnett et al. [AIChE J., 7 (211), 1961] have studied the catalytic dehydrogenation of cyclohexane to benzene over a platinum-on-alumina catalyst. A 4 to 1 mole ratio of hydrogen to cyclohexane was used to minimize carbon formation on the catalyst. Studies were made in an isothermal, continuous flow reactor. The results of one run on 0.32 cm diameter catalyst pellets are given below. 

Iso-Kel [Isomerization-Kellogg] A fixed-bed, vapor-phase isomerization process for making high-octane gasoline from aliphatic petroleum fractions. The catalyst is platinum on alumina. Developed by MW Kellogg. 

Oxycat A catalytic oxidation process for removing combustible vapors from air and industrial exhaust gases. The catalyst is platinum on alumina, supported inside a porcelain tube. 

Sinclair-Baker A naphtha reforming process, catalyzed by platinum on alumina. See also reforming. 

In fact, there are only two heterogeneous catalysts that reliably give high enantioselectivities (e.s. s) (90% e.e. or above). These are Raney nickel (or Ni/Si02) system modified with tartaric acid (TA) or alanine for hydrogenation of /(-kctocstcrs [12-30], and platinum-on-charcoal or platinum-on-alumina modified with cinchona alkaloids for the hydrogenation of a-ketoesters [31-73],... 

Recent experiments confirmed the role of tin added to platinum on alumina (present mainly as Sn " ) as a regulator for surface hydrogen 75a). A very specific spillover mechanism is assumed between platinum and tin atoms. The hydrogen accessibility of Pt increases proportionally to the amount of added Sn. To this end, it is sufiicient that 3-6% of the tin atoms... 

Davis (94b) aromatized several Cg and C9 hydrocarbons with a quaternary carbon atom over chromia- and platinum-on-alumina catalysts. Here the reactions of 1,1-dimethylcyclohexane, and 2,2- and 3,3-dimethylhexanes will be compared (Table V). 1,1-Dimethylhexane suffered demethylation predominantly over chromia and alkaline platinum however, with less alkaline platinum, isomerization to xylenes occurred. 

Kazansky et al. (5) estimated the role of C5 cyclic intermediates in aromatization to be about 5% over platinum on carbon. Dautzenberg and Platteeuw found about 11% C5 cyclic pathway with nonacidic platinum on alumina (23). 2,2,4-Trimethylpentane is forced to produce aromatics via C5 cyclization because of its structure here the quaternary carbon atom facilitates ring enlargement (5, 23). 

With hydrocarbons higher than Cg, there are more reaction possibilities. The aromatization of l-[ C]- -heptane over nonacidic platinum on alumina permitted the following estimates of various ring closure pathways (128) ... 

In Figure 5 the conversion of 1-phenylethanol and the open circuit potential of alumina-supported catalysts are plotted as a function of reaction time. There is a striking difference between the curves of unpromoted (a, a ) and bismuth-promoted (c, c ) catalysts. When air is introduced to the reactor, the potential of the platinum-on-alumina catalyst quickly increases to the anodic direction and after one minute the catalyst potential is above -300 mV. One may conclude that there is practically no hydrogen on the platinum surface and after a short period an increasing fraction of platinum is covered by OH. The influence of bismuth promotion is a higher reaction rate (final conversion) and lower catalyst potential during reaction. 

One of the most extensively used addition reactions of cyclobutanes is hydrogenolysis.36 With regard to the mechanistic aspect, evidence has been provided that hydrogenolysis of cyclobutane is structure sensitive to the particle size of the platinum on alumina catalysts.37 Moreover, a kinetic study has also revealed that the mechanism for the hydrogenolysis of cyclobutanes is likely to be different from that for cyclopropanes.37... 

Platinum on alumina reforming catalysts are commonly used commercially in the form of cylindrical pellets about X i in. in size, since this is about the smallest size acceptable from the standpoint of avoiding excessive pressure drop. For fundamental studies in small-scale reactors, where pressure drop limitations are less severe, it may be preferable to use the catalyst in the form of small granules to minimize diffusional limitations. 

The foregoing paragraphs have described platinum on alumina catalysts in a very general way and have also attempted to give a brief discussion of the reason why platinum is so commonly used as a reforming catalyst. In the following sections, the physical and chemical properties of platinum-alumina catalysts will be discussed in more detail, since this information will be of considerable help in understanding the way in which these catalysts function. 

The high degree of dispersion of platinum on alumina has also been confirmed by the hydrogen chemisorption measurements of Keavney and Adler (Kl), and by the chemisorption studies of Gruber (G2). In addition, high dispersion of platinum on silica-alumina has been observed by Hughes and associates, using a carbon monoxide chemisorption technique (H10). 

McHenry and co-workers (Ml) have suggested that platinum on alumina catalysts, which are active for the dehydrocyclization of paraffins,... 

Sinfelt and associates (S6) for a 0.3% platinum on alumina catalyst. At these temperatures diffusional effects are much less important than at the usual reforming temperatures. Over the range of methylcyclohexane and hydrogen partial pressures investigated, 0.07 to 2.2 atm. and 1.1 to 4.1 atm., respectively, the reaction was found to be zero order with respect to hydrogen and nearly zero order with respect to methylcyclohexane (Table III). The kinetic data were found to obey a rate law of the form... 

A study of the kinetics of isomerization of n-pentane at 372°C. over a platinum on alumina catalyst (0.3% platinum) has been reported by Sinfelt et al. (S4). The rate measurements were made in a flow system at low conversion levels (4-18%). The n-pentane was passed over the catalyst in the presence of hydrogen at total pressures ranging from 7.7 to 27.7 atm. and at hydrogen to n-pentane ratios varying from 1.4 to 18. Over this range of conditions the rate was found to be independent of total pressure and to increase with increasing n-pentane to hydrogen ratio (Fig. 4). The rate data were correlated by an expression of the form... 

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