2,3-Dimethylbutane, cracking

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 cracking of 2,3-dimethylbutane at 400° was found to be proportional to the weight of the catalyst and inversely proportional to the rate of flow of the carrier gas. The products were not propane and propylene as would be expected from simple scission (Fig. 6), but were 38.4 mole% C3H10, 24.8 mole% C3H8, 13.8% C2H4, 3.7% C2H6, 4.2% C4H10, and 1.3% and 0.8% of nonidentifiable products. The... 

Fio. 7. Effect of heat treatment of zeolite on the catalytic activities for cracking of 2,3-dimethylbutane. 

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. 

Fig. 5.1. Chromatograms of products of catalytic cracking (A) without reactor and (B) with reactor. Sorbent, 11% quinoline on refractory brick temperature, 25 C column length, 10.5 m. Peaks 1 = propane 2 = propylene 3 = isobutane 4 = n-butane 5 = isobutene 6 = butene-1 7 = rmns-butene-2 8 = cis-butene-2 9 = isopentane 10 = 3-methylbutene-l 11 = n-pentane 12 = pentene-1 13 = 2,2-dimethylbutene 14 = 2-methylbutene-l 15 = tnms-pentene-2 16 = cfsi)entene-2 17 = 2-methyl-butene-2 18 = 2,3-dimethylbutane 19 = 2-methylpentane 20 = 3-methylpentane 21 = 3-methylpen-tene-1 22 = 4-methylpentene-l 23 = c -4-methylpentene-2 24 = cyclopentane 25 = 2,3-dimethyl-butene-1 26 = fmns-4-methylpentene-2 27 = w-hexane 28 = cyclopentene 29 = 2-methylpentene-l 30 = hexene-1 31 = 2,4-dimethylpentane 32 = cis-hexene-3 33 = tnms-hexene-3 34 = 2-ethylbu-tene-1 35 = trans-hexene-2 36 = methylcyclopentane 37 = cis-methylpentene-2 38 = 2-methylpen-tene-2 39 = pisns-3-methylpentene-2 40 = methylcyclopentene-4 41 = 4-methylcyclopentene 42 = cw-3-methylpentene-2 43 = 2,3-dimethylpentane 44 = 2-methylheptane 45 = 2,3-dimethylbutene-2 46 = methylheptane 47 = cyclohexane 48 = C, olefin. Reprinted with permission from ref. 1.

At high-temperature conditions, the product distributions of typical decar-boxylative and aldol condensations vary with temperature, time on stream, and catalyst age. Several ketone isomers can be produced. With acetic acid, e.g., C5-C7 (e.g., methylhexanone, pentan-2-one, 3,3 -dimethylbutan-2-one) ketones and alkylphenols arise from acetone aldolization. An important cyclic product in low temperature acetone aldolization is isophorone (2-cyclohexen-l-one, 3,5,5 -trimethyl), formed by the aldol condensation of acetone with mesityl oxide, followed by 1,6-Michael addition. In reactions with acetic acid, we have observed 2-cyclohexen-l-one, 3,5-dimethyl, which is probably a cracking product of isophorone, and small amounts of isophorone itself. Cracking to produce... 

The main reaction products were 2-methylpentane and 3-methyl pentane followed by 2,3-dimethylbutane. The main cracking product was butane methane and ethane were not detected. 

Both the intrinsic rate constant and the effective diffusivity (KD) can be extracted from measurements of the reaction rate with different size fractions of the zeohte crystals. This approach has been demonstrated by Haag et al. [116] for cracking of n-hexane on HZSM5 and by Post et al. [117] for isomerization of 2,2-dimethylbutane over HZSM-5. It is worth commenting that in Haag s analysis the equilibrium constant (or distribution coefficient K) was omitted, leading to erroneously large apparent diffusivity values. 

Namba et al. studied the cracking of octane on H-ZSM-5 in the presence of other alkanes [27]. The reaction conditions were such that the conversion of octane obeyed first order kinetics and that the coverage of the active sites was low. The octane conversion was not affected by the presence of 3-methylheptane or 2,2,4-trimethylpentane in the feed. 3-Methylheptane is expected to diffuse rapidly through the ZSM-5 pores, while 2,2,4-trimethylpentane is excluded from the pores. Secondary shape-selectivity does not occur with these two molecules. However, the octane conversion dropped sharply with increasing partial pressures of 2,2-dimethylbutane in the feed. This strong inhibition caimot be the result of primaiy shape-selectivity, since the competing 2,2-dimethylbutane molecule should not be selectively adsorbed over octane. The explanation is that the slowly diffusing 2,2-dimethylbutane molecules retard the diffusion and, consequently, the conversion of the octane molecules. 

Table 2. Reactant and transition shape-selectivity of 3-methylpentane and 2,2-dimethylbutane versus hexane cracking on ZSM-5 at 811 K (data adapted from ref.29)...

Kinetics and Mechanism. The isoparaffins are intermediate compounds in the reforming reaction network, as shown in (Scheme 1) for n-heptane. At very low conversion, isomers are the main products. In the reaction of n-heptane on Pt/Al203, at zero conversion the isomers are the 52% in moles of the products, whereas at high conversion (95%), cracking products are the main products and the isomers yield is 3%, which shows that after being formed, the isomers are converted by successive steps (19). There is a maximum in the formation of i-hexanes both as a function of space velocity (19) and as a function of temperature (6,20). The equilibrium between ra-C6 and methylpentanes is rapidly established, but this is not true for the dimethylbutanes (8,11). This observation indicates that there is a very low kinetic constant for the transformation of single branched into doubly branched isomers (8). 

Figure 4.37 illustrates the sensitivity dependence of the molecular product distribution on zeolite micropore dimensions for the cracking of n-Cie. The product ratio of branched dimethylbutane (DMB) versus n-Ce is taken as a measure of the selectivity. A maximum in selectivity towards the bulky branched molecule is found for the intermediate pore-size zeolite AFI. This result is curious since one would have expected the wide-pore zeolite Fau to lead to the maximum yield for the more bulky molecule. The differences from expectation appear to be related again to the adsorption properties of hydrocarbons. 

Similarly, 2-methylpentane runs through the column faster than its linear isomer n-hexane, while 2,2-dimethylbutane, the isomer of hexane with the shortest linear chain, elutes even faster. Thus, mixtures of 2-methylbutane, n-pentane, 2,2-dimethylbutane, 2-methylpentane, and n-hexane can be easily separated with this new type of microporous MOF column. The potential applications of this mi-croporous MOF column in the efficient GC separation of natural gas and alkane mixtures are remarkable and foreseeable. The column could be used to identify the impurities in natural gas, and to monitor the amounts of mono- and multibranched alkanes formed in cracking reactions. 

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