Reactions hydroisomerization

The last vertical column of the eighth group of the Periodic Table of the Elements comprises the three metals nickel, palladium, and platinum, which are the catalysts most often used in various reactions of hydrogen, e.g. hydrogenation, hydrogenolysis, and hydroisomerization. The considerations which are of particular relevance to the catalytic activity of these metals are their surface interactions with hydrogen, the various states of its adatoms, and admolecules, eventually further influenced by the coadsorbed other reactant species. 

This has been demonstrated by a comparison of the cracking rates of small linear hydrocarbons in ZSM-5 [12] and also for reactions in different zeolites for the hydroisomerization of hexane [13]. Differences in catalytic conversion appear to be mainly due to differences in 9. 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

Hydroisomerization of n-hexadecane on Pt/HBEA bifunctional catalysts effect of the zeolite crystallites size on the reaction scheme. 

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 development of composite micro/mesoporous materials opens new perspectives for the improvement of zeolytic catalysts. These materials combine the advantages of both zeolites and mesoporous molecular sieves, in particular, strong acidity, high thermal and hydrothermal stability and improved diffusivity of bulky molecules due to reduction of the intracrystalline diffusion path length, resulting from creation of secondary mesoporous structure. It can be expected that the creation of secondary mesoporous structure in zeolitic crystals, on the one hand, will result in the improvement of the effectiveness factor in hydroisomerization process and, on the other hand, will lead to the decrease of the residence time of products and minimization of secondary reactions, such as cracking. This will result in an increase of both the conversion and the selectivity to isomerization products. 

Since ITQ-4/SSZ-42/MCM-58 have been prepared as aluminosilicates with Si/Al ratios of 20 to °°, which possess Brpnsted sites, there is potential for acid catalysis. Some preliminary accounts of catalytic cracking, hydrocracking, dewaxing, alkylation, hydroisomerization, and reforming reactions have been reported (47, 62-64). 

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. 

Of course, certain features of overall kinetics are inaccessible via a cluster model method, such as the influence of pore structure on reactivity. The cluster model method cannot integrate reaction rates with concepts such as shape selectivity, and an alternative method of probing overall kinetics is needed. This has recently been illustrated by a study of the kinetics of the hydroisomerization of hexane catalyzed by Pt-loaded acidic mordenite and ZSM-5 (211). The intrinsic acidities of the two catalysts were the same, and differences in catalyst performance were shown to be completely understood on the basis of differences in the heat of adsorption of hexene, an intermediate in the isomerization reaction. Heats of adsorption are strongly dependent on the zeolite pore diameter, as shown earlier in this review (Fig. 11). 

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... 

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... 

Figure 3. Influence of reaction temperature on hydroisomerization conversion of n-alkanes with different chain length (Fa = 12 10 3 mole hr1)...

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

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).

Hydrogenolysis has been concluded in a preceeding section to be mainly responsible for the formation of + Cm 1 and Cg + Cm 2 Branching in the fractions p = m-1 and p = m-2, then, may be due to hydroisomerization of either the feed or the cracked products according to the reaction sequences ... 

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