Cyclopentane, deuterium exchange

Figure 6. Initial rates vs Pt contents of the Pt/TiC>2 specimens for liquid methanol (A) or 1-propanol. (B) dehydrogenation at 298 K cyclopentane-deuterium exchange in gas phase at 263 K (C) oxygen isotope heteroexchange at 298 K over non-preoxidized (D) or preoxidized (E) samples.

Interpretation of the optimum metal content for these reactions. As already mentioned an optimum Pt content was found for dehydrogenation of liquid alcohols and cyclopentane-deuterium exchange in gas phase. Also, with Pt/Ti02 samples which had not been preoxidized and which were accordingly non-stoichiometric according to conductivity measurements, the same optimum content was found for the initial rate of OIE, whereas this rate decreased as a function of Pt content for preoxidized samples (44). 

As described in Chapter 3, the reasons given to explain this dilution effect of copper on nickel were that the hydrogenolysis reaction required a group or ensemble of nickel atoms on the catalyst surface and the presence of copper prevented the formation of the appropriately sized ensembles and the reaction was inhibited. On the other hand, the reactions involving C-H bond breaking take place on single atom sites so the surface dilution by copper has no effect until the surface is almost completely covered by the copper. The rate increase observed in the deuterium exchange on cyclopentane (Fig. 12.7b), however, is not easily rationalized by this surface ensemble effect alone. 

Photocatalytic cyclopentane-deuterium isotopic exchange (CDIE)... 

Indeed, experience shows that with deuterium exchange of cyclohexane and cyclopentane, the distribution curve with Pt, Pd, Ni, and Co has a maximum, whereas with Fe, Mo, W, Si, and Ge it has no maximum. Cyclobutane does not give a maximum either, not only with the last group of metals mentioned, but not with Ni, Pd, and Pt as well (for examples see Fig. 19c). We explain this by the absence of structural... 

That the adsorption of the cyclopentane ring seems to proceed mainly flatly in deuterium exchange on films has been stated above (see Section I,D). Of considerable interest are the investigations on asymmetric catalysis initiated by Schwab et al. (273). In their work, one of the optical isomers reacted a little faster than the other in a racemic mixture. Terent yev and Klabunovskii (274, 273) carried out the catalytic asymmetric synthesis from optically inactive substances. The reactions were accomplished on metals deposited on dextro- and levorotatory quartz. Organic optically active carriers and admixtures give a still greater effect. On this problem see Klabunovskii (276). At the present time still more active catalysts for the reaction of asymmetric hydrogenation and polymerization have been revealed (277-279). 

Copper-nickel alloy films similarly deposited at high substrate temperatures and annealed in either hydrogen or deuterium were used to study the hydrogenation of buta-1,3-diene (119) and the exchange of cyclopentane with deuterium (120). Rates of buta-1,3-diene hydrogenation as a function of alloy composition resemble the pattern for butene-1 hy-... 

F. C. Gault, J. J. Rooney, and C. Kemball, Catalytic exchange with deuterium of polymethyl-cyclopentanes on metal films Evidence for tt-bonded intermediates, J. Catal. 1, 225 (1962). 

That products of intermediate oxidation level can be detected in the photocatalytic reactions of hydrocarbons and fossil fuels is also consistent with a surface bound radical intermediate . Photocatalytic isotope exchange between cyclopentane and deuterium on bifunctional platinum/titanium dioxide catalysts indicates the importance of weakly adsorbed pentane at oxide sites. The platinum serves to attract free electrons, decreasing the efficiency of electron-hole recombination, and to regenerate the surface oxide after exchange. Much better control of the exchange is afforded with photoelectrochemical than thermal catalysis > ) As before, hydrocarbon oxidations can also be conducted at the gas-solid interface... 

In a series of papers, studies have been reported of the exchange of deuterium in, and the deuteration of, benzene (707,40), the exchange of deuterium in cyclopentane (62), and the reactivity of hexane (60) and methylcyclo-pentane (700) over nickel and Ni-Cu alloys. 

Figure 5. Number of deuterium atoms exchanged per cyclopentane molecule over a 5 wt "l Pt/Ti02 sample dotted lines, after illuminating for 15 min at 263 K,solid lines after 2,5 min in the dark at 343 K.

The selectivity of the H/D exchange of cyclopentane (CP) gives direct information about the different adsorption modes during the reaction. During the H/D exchange of CP, the hydrogen-atoms of cyclopentane are replaced by deuterium atoms. Typically, the H/D... 

Fig. 12.7. Reaction of cyclopentane with deuterium at 150°C over a) a nickel catalyst containing no copper and b) a nickel catalyst containing 5% copper. Open circles represent the exchange reaction (C-H bond breaking) closed circles the hydrogenolysis (C-C bond breaking). (Redrawn using data from Ref. 142.)...

G. Isotopic Exchange between Deuterium and Cyclopentane and Hexane... 

To assess any differences in isotopic exchange of a kanes on microcrystalline and amorphous catalysts, we examined isotopic exchange between deuterium and two saturated hydrocarbons, cyclopentane and hexane. The range of reaction temperatures for cyclopentane was 175-275° and the molar ratio of deuterium to hydrocarbon was 5.6. Catalysts were activated at 400° first in helium and then, after runs on the amorphous catalyst, the catalyst was reactivated in hydrogen to make it microcrystalline. Two series of such runs were made. At 250°, reaction rates on the amorphous catalysts were as follows run 285, 0.40 mmoles per hour per gram of Cr203 run 296, 0.36. On the micro-crystalline catalyst, the exchange rates were, for run 288, 2.0 run 299, 1.1. These rates were computed by... 

The photocatalytic isotopic exchange between cyclopentane and deuterium (CDIE) was carried out in a static fused silica photoreactor described in ref (23),... 

Few studies have been made of the hydrogenation characteristics of these classes of molecule. The deuteration of cyclopentadiene 52) occurred very rapidly at — 34° at the surface of an iron film cyclopen-tene was probably produced as an intermediate but full deuteration to cyclopentane was the only measurable process. A film sintered for 16 min at 200° was active for diene exchange at 90° and yielded product containing one deuterium atom, but it was not active for the addition reaction. 

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