Dehydrogenation methylcyclohexanes

The formed methylcyclohexane carbocation eliminates a proton, yielding 3-methylcyclohexene. 3-Methylcyclohexene can either dehydrogenate over the platinum surface or form a new carbocation by losing H over the acid catalyst surface. This step is fast, because an allylic car-bonium ion is formed. Losing a proton on a Lewis base site produces methyl cyclohexadiene. This sequence of carbocation formation, followed by loss of a proton, continues till the final formation of toluene. 

Naphthalene itself is solid at ambient temperatures (m.p. 80.5°C) but is dissolved easily in aromatic compounds such as toluene (refer Table 13.1) [10,12], so that the oily mixture can be handled as a "naphthalene oil." The naphthalene oil is catalytically hydrogenated to decalin and methylcyclohexane simultaneously. Decalin and methylcyclohexane are converted into hydrogen and naphthalene oil again by dehydrogenation catalysis. From the handling viewpoint, the naphthalene oil may be deemed as a preferential and practical material for hydrogen storage and transportation. 

Catalytic Dehydrogenation of Decalin and Methylcyclohexane over Carbon-Supported Platinum-Based Nanoparticles under Superheated Liquid-Film Conditions... 

As shown in Table 13.1, toluene is a candidate compound to form the naphthalene oil. To utilize the reaction pair of methylcyclohexane dehydrogenation/toluene hydrogenation as an additive component, it is, thus, necessary to generate hydrogen efficiently from methylcyclohexane under mild reaction conditions. 

Heat flow from any external thermo-source into the dehydrogenation reactor should take the role of affording the endothermic reaction heat and the evaporation heat of both reactant and product in addition to the apparent heat for raising their temperatures from the ambient up to the external heating one. Under assumptions of the sufficient amounts of active catalyst and the adequate feed rates of organic chemical hydride, the minimum required heat is obtained as shown in the example of methylcyclohexane at 285°C on the basis of 100% conversion of methylcyclohexane to toluene and hydrogen (Table 13.5). 

Re2Pt(CO)n Dehydroxylated AI2O3 Adsorption from solution and thermal treatment under H2 Re4Pt2 entities, model catalyst for methylcyclohexane dehydrogenation [57, 60]... 

In support of the conclusion based on silver, series of 0.2, 0.5, 1.0, 2.0, and 5.0 % w/w of platinum, iridium, and Pt-Ir bimetallic catalysts were prepared on alumina by the HTAD process. XRD analysis of these materials showed no reflections for the metals or their oxides. These data suggest that compositions of this type may be generally useful for the preparation of metal supported oxidation catalysts where dispersion and dispersion maintenance is important. That the metal component is accessible for catalysis was demonstrated by the observation that they were all facile dehydrogenation catalysts for methylcyclohexane, without hydrogenolysis. It is speculated that the aerosol technique may permit the direct, general synthesis of bimetallic, alloy catalysts not otherwise possible to synthesize. This is due to the fact that the precursors are ideal solutions and the synthesis time is around 3 seconds in the heated zone. 

A fourth type of petroleum isomerization, which was commercialized on a small scale, involves the rearrangement of naphthenes. In the manufacture of toluene by dehydrogenation of methylcyclohexane, the toluene yield can be increased by isomerizing to methylcyclohexane the dimethylcyclopentanes also present in the naphtha feed. This type of isomerization is also of interest in connection with the manufacture of benzene from petroleum sources. 

Naphthene Isomerization. In addition to the paraffin isomerization processes, naphthene isomerization also proved useful during the war in connection with the manufacture of toluene. In the Shell dehydrogenation process for the manufacture of toluene, good yields depend upon increasing the methylcyclohexane content of the feed by isomerization of dimethylcyclopentanes. This process was employed commercially at one refinery in the Midwest and one on the Pacific Coast. 

A study of the kinetics of methylcyclohexane dehydrogenation to toluene in the temperature range 315 to 372°C. has been reported by... 

Summary op Rate Data for Methylcyclohexane Dehydrogenation over Pt-AljOa" (S6)... 

Here M and T represent methylcyclohexane and toluene in the gas phase, and Ttt represents adsorbed toluene. The first step in the above reaction sequence represents the adsorption of methylcyclohexane with subsequent reaction to form toluene, while the second step is the desorption of toluene from the surface. Very likely the first step represents a series of steps involving partially dehydrogenated hydrocarbon molecules or radicals. However, at steady-state conditions the rates of the intermediate steps would all be equal, and the kinetic analysis is, therefore, not complicated by this factor. To account for the near zero-order behavior of the reaction, it was suggested that the active catalyst sites were heavily covered with... 

Thus it appears that an interpretation not involving adsorption equilibria is reasonable in accounting for the observed kinetics of dehydrogenation of methylcyclohexane to toluene. However, some additional information, such as data on the heat of adsorption of toluene on supported platinum, would be desirable in establishing the correctness of this interpretation. 

Figure 13.20 Methylcyclohexane conversion to toluene as a function of reactor temperature in a membrane and a nonmembrane reactor [45]. Reprinted with permission from J.K. Ali and D.W.T. Rippin, Comparing Mono and Bimetallic Noble Metal Catalysts in a Catalytic Membrane Reactor for Methyl-cyclohexane Dehydrogenation, Ind. Eng. Chem. Res. 34, 722. Copyright 1995, American Chemical Society and American Pharmaceutical Association...

Shuikin (370) passed methyl and dimethyl cyclohexanes over nickel at 330-350°. In addition to the usual demethylation and dehydrogenation reactions, he found evidence of methyl transfer methylcyclohexane gave some p-xylene, while dimethylcyclohexane gave some trimethylbenzene. Platinum at these temperatures did not cause this methyl transfer. Plate and O. A. Golovina (306) reported that appreciable demethylation of 2,2,4-trimethylpentane took place over molybdena-alumina at 150-250°C. and was accompanied by the formation of small amounts of aromatics. 

Corma el al. (126) found that PtNaY was an active but rather unstable catalyst for methylcyclohexane dehydrogenation to toluene. These workers studied both the dehydrogenation and the catalyst decay kinetics. It was concluded that the reaction occurs via a series of consecutive partial dehydrogenation steps, the first of which was rate determining. Further, catalyst deactivation was caused by coke deposition from partially unsaturated precursor molecules. 

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