Dehydrogenations with platinum

The reaction of 1,3,5-cycloheptatriene with CsHsCrCla in the presence of iso-C3H7MgBr under UV irradiation, followed by treatment with methanol, yields the brown, paramagnetic (/Xeff=1.63 B.M.), air-sensitive complex, (l,3,5-cycloheptatriene)(cyclopentadienyl)chrom-ium (19) (207). The complex is readily dehydrogenated with platinum to C7H7Cr(C5H5) or oxidized with oxygen in acetone-water to yield the... 

This overview is organized into several major sections. The first is a description of the cluster source, reactor, and the general mechanisms used to describe the reaction kinetics that will be studied. The next two sections describe the relatively simple reactions of hydrogen, nitrogen, methane, carbon monoxide, and oxygen reactions with a variety of metal clusters, followed by the more complicated dehydrogenation reactions of hydrocarbons with platinum clusters. The last section develops a model to rationalize the observed chemical behavior and describes several predictions that can be made from the model. 

Bursian et al. (66a) suggested metallic platinum sites for dehydrogenation and Pt " sites for ring closure. They studied the effect of several elements added to platinum-on-silica catalyst on the aromatizing activity of n-hexane. Benzene yield increased parallel to the amount of soluble platinum (66b) at the same time, the crystallinity of platinum decreased in the presence of additives promoting aromatization. These are elements (e.g., Ce, Sc, Zr) which do not form an intermetallic compound with platinum (66c). 

Alkanes appear to react with platinum(IV) in an identical manner to benzene (34, 84) chloromethane and chloroethane can be detected as the reaction products from methane and ethane, respectively. When propane, butane, or hexane is the reactant, the terminal chloro isomers predominate over the internal isomers. This was interpreted to mean that primary C—H bonds are the most reactive (34), but a more detailed study has shown that this conclusion does not necessarily follow from the experimental results (84). When cyclohexane is the reactant, dehydrogenation (or chlorination and then dehydrohalogenation) occurs to give benzene as one of the reaction products (29, 34, 84). 

With higher hydrocarbons, the spectra depend upon the structure of the olefin [70], With platinum—silica catalysts, linear chain olefins tend to form dehydrogenated surface residues more readily than branched chain olefins, which give predominantly saturated adsorbed alkyl species. 

A2-Pyrazolines are converted into pyrazoles by oxidation with bromine or Pb(OAc)4 and they can also be dehydrogenated with sulfur. 3,5-Diphenylpyrazoline (419) on heating with platinum disproportionates to the pyrazole (420) and the pyrazolidine (421) (66AHC(6)347). 

The paraffins dehydrogenation on platinum-alumina catalysts proceeds with constant rate up to some time-on-stream after which a slow deactivation of the catalysts takes place Since relative changes of the catalyst activity ( characterized by reaction rate) are proportional to relative amounts of the deposited coke it can suppose that coke formation is the main reason of deactivation. Deactivation can be related with an attainment of a threshold in coke concentration (Co) on catalysts. The threshold amounts are 1.8 wt.% for A-I, 6,8% and 2.2% for A-II and A-IXI catalysts respectively. The isobutane dehydrogenation in non-stationary region (C > Co) is described by the following kinetic equation ... 

Fig. 5 Reactions in platinum and acid sites in light paraffin dehydrogenation with unmodified catalyst.

Dehydrogenation of the cyclohexanol/n clohexanone mixture to phenol. The alcohol forms with the phenol an azeotrope with a high phenol content (for example, 75 molar per cent at 12 kPa absolute). In order to employ distillation as a means of purification, this means that the reactor output must have a phenol content higher than that of the azeotrope. In practice, conversion takes place in the vapor phase with platinum base... 

Rapid deactivation occurs when n-dodecane is dehydrogenated over platinum-alumina without any diluent or with an inert diluent such as nitrogen. The rate of deactivation is decreased greatly when hydrogen is used as a diluent. However, even with hydrogen dilution, a slow deactivation (accompanied by carbonization of the catalyst) occurs. Eventually it is necessary to regenerate the catalyst by combustion of the coke deposits and reactivation in hydrogen. 

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