2-Methylpentane, cracking

For 2-methylpentane cracking the assumed reaction mechanism was based on that proposed by Zhao et al [8], Reactions are initiated by adsorption of 2-methylpentane feed on to Bronsted acid sites. Subsequent protolysis produces a carbenium ion on the surface of the catalyst (an adsorbed olefin) and a smaller gas phase paraffin molecule. Propagation reactions can then occur by hydride transfer between carbenium ions on the surface of the catalyst and gas phase feed molecules. Zhao experimentally determined certain bimolecular reactions to be more significant than others, the most significant were implemented in our model. Reactions are terminated by desorption of carbenium ions to yield a gas phase olefin molecule... 

NHs adsorption microcalorimetry has been used to characterize the acid sites of a H-USY zeolite and another USY sample in which the strong Lewis acid sites were poisoned with ammonia. Poisoning of the Lewis acid sites did not affect the rate of deactivation, the cracking activity, or the distribution of cracked products during 2-methylpentane cracking. Thus, strong Lewis acid sites do not seem to play any important role in cracking reactions [148]. 

In order to differentiate the available reaction space, Frillette et al. [122] introduced the constraint index (Cl), which is determined from the ratio between the relative rates of w-hexane and 3-methylpentane cracking ... 

Hydrogenolysis of 2-methylpentane, hexane, and methylcyclopentane has been also studied on tungsten carbide, WC, a highly absorptive catalyst, at 150-350 °C in a flow reactor [80], These reforming reactions were mainly cracking reactions leading to lower molar mass hydrocarbons. At the highest temperature (350 °C) all the carbon-carbon bonds were broken, and only methane was formed. At lower temperatures (150-200 °C) product molecules contained several carbon atoms. 

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

Adequate acid characteristics of solids are needed since the selectivity to tri-methylpentanes (TMP), and especially the 2,2,4-isomer, strongly depends on the nature and strength of the acid sites of the catalyst. Thus, excessively strong acidity will drive the reaction toward cracking reactions, while an acidity that is too low will mostly produce oligomerization of the olefins (Figure 13.5). " ... 

R.M. (1981) Introduction of constraint index as a diagnostic test for shape selectivity using cracking rate constants for n-hexane and 3-methylpentane. 

Zeolite A (Ca) shows reactant selectivity. The straight-chain /j hexane can pass through the windows and undergo reaction but the branched-chain 3-methylpentane is excluded. The selective cracking of straight-chain hydrocarbons in the presence... 

Higher hydrocarbon molecules allow study of the unique cracking pattern of metals. These studies are usually carried out at low conversion to observe only primary hydrogenolysis. Nickel exhibits high selectivity to cleave terminal C—C bonds leading to demethylation that is, it cleaves only bonds that involve at least one primary carbon atom. For example, in the transformation of n-hexane, only methane and n-pentane are formed (180°C, Ni-on-silica catalyst, 0.3% conversion), whereas 2-methylpentane and 3-methylpentane yield methane, n-pentane, and isopentane.260 In the transformation of 2-methylpentane, the n-pentane isopentane ratio is close to 2, which corresponds to the statistical value. Under more forcing conditions, successive demethylations lead eventually to methane as the only product. 

The 13C-labelling experiments allowed one to determine the relative contributions of cyclic and of bond-shift mechanisms in the isomerization and cracking reactions of 2-methylpentane and hydrogenolysis of methylcyclopentane over Pt TiC>2 catalysts prepared by different methods566. 

Cracking and disproportionation in the reaction of hexane could be suppressed by the addition of cycloalkanes (cyclohexane, methylcyclopentane, cyclopentane).101 Furthermore, 3-methylpentane and methylcyclopentane also reduced the induction period. These data indicate that reactions are initiated by an oxidative formation of alkene intermediates. These maybe transformed into alkenyl cations, which undergo cracking and disproportionation. When there is intensive contact between the phases ensuring effective hydride transfer, protonated alkenes give isomerization products. 

The relative reactivity of hexane and 3-methylpentane (about 1000) in the isomerization mode was shown to be the same as found for isomerization in HF-SbF5.102 In the cracking mode, however, the ratio is about 10, resulting from the dramatic acceleration of the reaction of hexane compared to that of 3-methylpentane. Further characteristics of the cracking mode are a large excess of branched isomers in the C4—C5 fractions, the absence of unsaturated cracking products, and formation of... 

As a model for cracking of alkanes, the reaction of 2-methylpentane (16, 2MeP) over SbF5-intercalated graphite has been studied in a flow system, with the hydrocarbon being diluted in a hydrogen stream.104,105 A careful study of the product... 

Figure 5.13. Distribution of cracking products (mass%) of 2-methylpentane (16) versus time on stream at 20°C.104...

This result can only be explained by the /3-scission of the trivalent 4-methylpent-2-yl ion 30 as the initial step in the cracking process. Based on this and on the product distribution versus time profile, a general scheme for the isomerization and cracking process of the methylpentanes has been proposed103,104 (Scheme 5.16). 

Relative hydrogen transfer activity can be determined using an HTI test (11), where the index is a measure of the degree of saturation in the reaction product. The test determines the product ratio of 3-methylpentenes to 3-methylpentane derived from a 1-hexene feed. While the branched products come mainly from oligomerization followed by cracking, the results should be relevant here as well. The higher the index, the lower the relative hydrogen transfer activity. 

We wish to report a study of the cracking of n-hexane, 2-methyl-pentane, 3-methylpentane, and 2,3-dimethylbutane over K-exchanged Y, NaY, and Na,K-exchanged L zeolites at 500° and 1 atm at low conversion levels (LHSV — 0.3), as well as thermal cracking in a quartz wool-packed... 

The ZSM-5 family of zeolites show further interesting shape-selective effects. Both normal and methyl-substituted paraffins have access to interior sites, so both hexane and 3-methylpentane are cracked by ZSM-5, but steric constraints cause hexane to be cracked faster than 3-methylpentane. Further shape selectivity was found between 3-methylpentane and 2,3-dimethylbutane. No window effect with paraffin chain length was found with ZSM-5. In the conversion of methanol to hydrocarbons over ZSM-5 catalysts, the distribution 94,152,195 of aromatic products ends at Cio- The distribution of tetramethylbenzenes is not far from equilibrium, but has excess 1,2,4,5-tetramethylbenzene. Measurements of diffusion coefficients of alkyl benzenes show rapid decrease, by orders of magnitude, as ring substitution increases. 

On the other hand, the selective dehydrocyclization, which does not allow the formation of secondary-primary C-C bonds, must involve only two methylic carbon atoms in the 1 and 5 positions. Although the reverse reaction (selective hydrogenolysis of methylcyclopentane) could be observed on platinum catalysts of low dispersion at 220°C (86), the selective dehydrocyclization of methylpentanes on these catalysts is detectable only at higher temperatures (280°-300°C), where it competes with another process, ascribed to Mechanism C (33). Fortunately, it was found recently that iridium supported on AI2O3 or SiOj selectively catalyzes at 150°C the cyclic type interconversion of 2-methyl- and 3-methylpentanes (88). n-Hexane under the same conditions yields only cracked products (702) (Scheme 52). Similarly,... 

There are three possible types of shape and size selectivity effects, as shown in Fig. 4.2S. First, the reactant molecules may be too large to enter the cavities. Comparison of Tables 4.11 and 4.13 shows that all of the molecules access faujasite structures. Only molecules larger than penta-methyl benzene are excluded. Early examples of shape and size selectivity were almost completely limited to small openings and normal versus branched paraffins. For example, n hexane was selectively cracked in the presence of 3-methylpentane over zeolite A catalysts. Other, more subtle, effects may occur. The diffusivity of frart5 butane-2 is 200 times larger than that of cis-butene-2 in zeolite CaA. By adding Pt to CaA, selective hydroge nation of fr[Pg.79]

---