Isomerization of heptanes

In this paper, the results of the isomerization of hexane, heptane and octane over a Mo2C-oxygen-modified-catalyst, a Mo03-carbon-modified catalyst and a Pt//l-zeolite catalyst, at atmospheric pressure, are presented. Also, the results for a conventional Pt/Al203 catalyst are presented for the isomerization of hexane. Then, the effect of pressure on the isomerization of heptane and octane over the molybdenum catalysts and the Pt//l-zeolite catalyst is shown. Finally, the ability of the molybdenum catalysts to catalyse the isomerization reaction at high conversion with high selectivity even with hydrocarbons larger than hexane is demonstrated this is not possible over the Pt catalysts. The differences between the catalysts are discussed in terms of the reaction mechanisms. 

V. Isomerization of Heptanes and Higher Alkanes 1. Without Cracking Suppressors... 

The cyclopropanation of 1-trimethylsilyloxycyclohexene in the present procedure is accomplished by reaction with diiodomethane and diethylzinc in ethyl ether." This modification of the usual Simmons-Smith reaction in which diiodomethane and activated zinc are used has the advantage of being homogeneous and is often more effective for the cyclopropanation of olefins such as enol ethers which polymerize readily. However, in the case of trimethylsilyl enol ethers, the heterogeneous procedures with either zinc-copper couple or zinc-silver couple are also successful. Attempts by the checkers to carry out Part B in benzene or toluene at reflux instead of ethyl ether afforded the trimethylsilyl ether of 2-methylenecyclohexanol, evidently owing to zinc iodide-catalyzed isomerization of the initially formed cyclopropyl ether. The preparation of l-trimethylsilyloxybicyclo[4.1.0]heptane by cyclopropanation with diethylzinc and chloroiodomethane in the presence of oxygen has been reported. "... 

A non-acidic isomerization catalyst system has unexpectedly emerged from recent studies by French workers [4] in the area of Mo-oxycarbides. Although at an early stage of development, these new materials exhibit high selectivities for the isomerization of paraffins such as n-heptane. An alternative non-carbenium ion mechanistic route to achieve isomerization of higher alkanes could potentially overcome some of the limitations of conventional solid acid based catalyst systems. 

P. A. (1997) Isomerization and hydrocracking of heptane over bimetallic bifunctional PtPd/H-beta and PtPd/USY zeolite catalysts. . Catal,... 

Constitutional isomerism becomes more complex as the size of the hydrocarbon molecule is increased. For example, there are three constitutional isomers of pentane, C5H12. The number of constimtional isomers increases quite rapidly with an increasing number of carbon atoms. Thus, there are five constimtional isomers of hexane, CeH, nine isomers of heptane, C7H16, 75 isomers of decane, C10H22, and 366,319 isomers of eicosane, C20H42. You can begin to understand why it is possible to make so many different molecules based on carbon. 

The first reaction is the isomerization from a zero-octane molecule to an alkane with 100 octane the second is the dehydrocyclization of heptane to toluene with 120 octane, while the third is the rmdesired formation of coke. To reduce the rate of cracking and coke formation, the reactor is run with a high partial pressure of H2 that promotes the reverse reactions, especially the coke removal reaction. Modem catalytic reforming reactors operate at 500 to 550°C in typically a 20 1 mole excess of H2 at pressures of 20-50 atm. These reactions are fairly endothermic, and interstage heating between fixed-bed reactors or periodic withdrawal and heating of feed are used to maintain the desired temperatures as reaction proceeds. These reactors are sketched in Figure 2-16. 

From this beginning, an extensive study of the isomerization of n-heptane was made with platinum on silica-alumina catalysts. Figure 2 shows curves plotted from the data obtained illustrating the total isomer yield versus conversion and the temperatures that produced these conversions. The conversion-isomer yield curve follows closely the 45° theoretical yield line, goes through a maximum at about 65% isomer yield, and then drops sharply because of cracking. The temperature at which the maximum yield of isomers was obtained was about 660° F. 

The isomerization of paraffinic heptanes presents still more difficult problems, and no successful method has been found for suppressing the side reactions. 

In a series of experiments carried out by Pines and coworkers,202 the intrinsic acidity of alumina was neutralized before use to avoid ionic-type skeletal isomerization. Radiotracer studies showed that toluene formed from [l-l4C]-heptane over chromia-alumina contained only 18-32% labeling in the methyl group, that is, less than 50% required by 1,6 carbon-carbon closure.204 This indicates that direct 1,6 carbon-carbon closure is not the only path of cyclization of /(-heptane. 

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.

Fig. 6. Effect of hydrogen pressure on rate of isomerization of n-heptane over platinum-alumina catalyst (R3). The rate n is relative to the rate of isomerization at 471 °C., ph = 5.8 atm.

Heptane Isomerization—It is not expected that isomerization of a heptane fraction per se will be commercially feasible since straight run heptanes are a choice stock for catalytic reforming. The estimated equilibrium octane number (RON clear) for C7 paraffins at 98°F is only about 82 using Rossini s equilibrium data. [These data have been checked experimentally at 98.2°F by G. M. Kramer and A. Schriesheim (44). Good agreement was obtained except for the 2,2-DMP and 3,3-DMP which underwent side reactions.]... 

The effect of pressure on the isomerization of n-heptane and n-octane was determined over the Pt//l-zeolite, Mo2C-oxygen-modified and M0O3-carbon-modified catalysts. The weight hour space velocity (WHSV) was changed with the pressure to keep the conversion at a similar level, enabling the effect on the isomerization selectivity and the product distributions to be seen. Other conditions were kept constant. 

The results for the isomerization of n-heptane are presented in Table 20.5. Over all the catalysts the main products are again the 2-methyl (M2H) and 3-methylhexanes (M3H), with a significant contribution from the dimethylpentanes of 12-13% over the platinum and Mo2C-oxygen-modified catalysts, and 21% over the Mo03-carbon-modified catalyst. 3-Ethylpentane always contributes around 3% to the isomer distribution and almost no cyclic products are observed. Increasing the pressure over the... 

Figure 20.8 (a and b) show a comparison of the Pt// -zeolite catalyst which activated molybdenum oxycarbide catalysts, prepared from both Mo2C and Mo03, for the isomerization of n-heptane and n-octane at elevated pressure. For the platinum catalyst the chain length is important, as explained above, with the isomerization selectivity obtained for n-octane dropping more quickly with increasing conversion than that for n-heptane over this plantinum catalyst the isomerization of n-hexane can be... 

Figure 20.9 Pseudo-Arrhenius plots for the isomerization of n-heptane (6 bar, H2/n-C7 = 30) (a) Pt/jS-zeolite (b) MoO -carbon-modi ficd (c) Mo2C-oxygen-modified.

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