Alkane Hydroisomerization

This A cracking was shown to be 20 times faster than the rearrangement of type B involved in n-alkane hydroisomerization. 

Bifunctional zeolite catalysts are used in various commercial processes light alkane hydroisomerization (chapter 7), hydrocracking (chapter 6),hydrodewaxing (chapter 8), light alkane aromatization and hydroisomerization of the C8 aromatic cut (chapter 9). The hydrogenation/dehydrogenation components included in zeolite catalysts can be very different and located in different positions ... 

LP mordenite doped with platinum is the catalyst used in the Shell Hysomer process for alkane hydroisomerization this can be linked to the Isosiv separation process whereby -/n-alkanes are separated on LTA zeolite. This link is the basis of the joint Shell/UOP Total Isomerization Process (TIP). Similar catalysts have been employed to hydrocrack (i.e. dewax) diesel fuels. There have also been reports of commercial use of mordenite to catalyze methanol amination with ammonia, to produce dimethylamine. 

From all the above results, it is inferred that the formation of aromatic compounds from parasite reactions in alkane hydroisomerization play an important role for generation coke precursors, which, during the coke formation, continued their polymerization in the acid sites of the Pd/HM catalyst to be eventually converted into polyaromatic or pseudo-graphitic structures. 

One notes in Table 1.2 a uniform increase in the adsorption energies of the alkanes when the microspore size decreases (compare 12-ring-channel zeohte MOR with 10-ring-channel TON). However, at the temperature of hydroisomerization the equilibrium constant for adsorption is less in the narrow-pore zeohte than in the wide-pore system. This difference is due to the more limited mobility of the hydrocarbon in the narrow-pore material. This can be used to compute Eq. (1.22b) with the result that the overall hydroisomerization rate in the narrow-pore material is lower than that in the wide-pore material. This entropy-difference-dominated effect is reflected in a substantially decreased hydrocarbon concentration in the narrow-pore material. 

The hydroisomerization of heavy linear alkanes is of a great interest in petroleum industry. Indeed, the transformation of long chain n-alkanes into branched alkanes allows to improve the low temperature performances of diesel or lubricating oils [1-3]. On bifunctional Pt-exchanged zeolite catalysts, n-CK, transformed into monobranched isomers, multibranched isomers and cracking products [4], The HBEA zeolite based catalyst was more selective for isomerization than those containing MCM-22 or HZSM-5 zeolites [4], This was explained on one hand by a rapid diffusion of the reaction intermediates inside the large HBEA channels, and on the other hand by the very small crystallites size of this zeolite (0.02 pm). 

The hydroisomerization of linear alkanes nowadays is among the most demanded technologies for transformation of naphtha into high octane gasoline. However, while the processes for hydroisomerization of C4 and C5 - C6 cuts are well established (PENEX, ISOTEX, TIP, HYSOMER, ISOFIN, SKIP, PAR-ISOM), there is no suitable technology for the conversion of longer alkanes (C7 - C8 cuts). 

Paracrystallinity, Cu/ZnO/AIjOj, 31 295 2"-Paracyclophanes, 32 453 Paracyclophanes, macrocycles, 29 206-208 Paraffin, see also Alkanes alkylation, 10 165, 27 98 carbon selectivity, bed residence time effects, 39 249-250 cracking, 39 283 cyclization, 28 295 rates, 28 301, 302 double cyclization, 28 312-314 in exhaust gases, 24 66, 67 hydrogenolysis, 30 43-44 hydroisomerization, 39 183 oxidation, 32 118-121 solubility enhanced hydrogeolysis, 39 285 Parahydrogen conversion rate correlations, 27 48-50... 

Soualah, A., Lemberton, J.L., Pinard, L., Chater, M., Magnoux, P., and Moljord, K. (2008) Hydroisomerization of long-chain n-alkanes on bifunctional Pt/zeolite catalysts effect of the zeolite strucmre on the product selectivity and on the reaction mechanism. Appl. Catal. A., 336, 23-28. 

Hollo, A., Hancsok,)., and Kallo, D. (2002) Kinetics of hydroisomerization of Cs-Cy alkanes and their mixtures over platinum containing mordenite. Appl. Catal. A., 229, 93-102. 

The Influence of Chain Length in Hydrocracking and Hydroisomerization of w-Alkanes... 

It was found that hydroisomerization and hydrocracking of n-dodecane over the Pt/Ca-Y-zeolite require low reaction temperatures, a typical value being 275 °C (9). This temperature was chosen in the present work to investigate the influence of chain length on the reactivity of the n-alkanes. In Figure 1 the degree of overall conversion has been plotted versus the superficial... 

Figure 1. Hydroisomerization and hydrocracking of n-alkanes with different chain length (T — 275°C)...

In Figure 5 the generally accepted reaction path (14) for hydroisomerization of n-alkanes has been represented along with different possibilities for the cracking step. The n-alkane molecules are adsorbed at a dehydrogenation/hydrogenation site where n-alkenes are formed. After desorption and diffusion to an acidic site chemisorption yields secondary carbenium ions that rearrange... 

According to Figure 5 a series of elementary reactions are involved in hydroisomerization and hydrocracking of n-alkanes. 

Literature on hydroisomerization of long chain alkanes > is very limited (2, 9,15,16) due to both analytical difficulties and the fact that hydrocracking predominates unless the bifunctional... 

Figure 5. Reaction scheme for hydroisomerization of n-alkanes on bifunctional catalysts and possible modes of cleavage...

According to the reaction scheme shown in Figure 5 both hydroisomerization and hydrocracking of the n-alkanes (except n-hexane) proceed via branched alkyl carbenium ions. In the range of medium degrees of conversion (40 % <,X <, 90 %) both reactions may be investigated simultaneously. A relationship between the products of both types of reaction will be discussed in the present section. 

The values defined in this manner do not represent any probability for rupture of definite carbon-carbon bonds in the feed molecule. This term is meaningless if rearrangement of the carbon skeleton precedes the cracking step. Rather, the values indicate the probability of an n-alkane for being hydrocracked according to the overall cracking reaction in question. These probabilities are useful for a comparison with the relative concentrations of the products formed by hydroisomerization (cf. Table IV). 

The mechanistic background for such a comparison is illustrated in Figure 10 which represents in more detail the pathway of hydroisomerization and hydrocracking of two n-alkanes. Branched carbenium ions are formed via n-alkenes and linear carbenium ions. Then, either desorption or (3 -scission may occur in parallel reactions. Desorption (followed by hydrogenation) of a given carbenium ion yields an iso-alkane with the same carbon skeleton. f3 - scission, on the other hand, yields fragments of definite carbon numbers ( (3 -scissions which would yield or C2... 

Table IV. Hydroisomerization and Hydrocracking of -Alkanes. Comparison Between Relative Concentrations of Iso-alkanes and Probabilities of Overall Cracking Reactions (Fa = 12 10 3mole h 1).

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