Hydrocracking of n-hexane

Figure 9. Hydrocracking of n-hexane and different n-alkanes at low degrees of conversion. Distribution of the cracked products.

Figure 13.36 Dimethylbutane n-hexane selectivity ratios from hydrocracking of n-hexadecane over various framework types [75].

It is significant that the mixture yielded propane as the major product (Table III). As noted in our earlier paper on catalytic cracking (6), the predominance of C3 fragments in the cracked products and the absence of isobutane appeared to be a unique property of erionite. Our present data indicate that this is also true for hydrocracking over a dual function erionite. The only exception was that when n-pentane alone was hydro-cracked, equimolal quantities of ethane and propane were found. This shift in product distribution in the presence of n-hexane, a second crackable component, indicated that the reaction path within the intracrystalline space was complicated. 

H. Robson We were primarily interested in the hydrocracking of n-pentane and n-hexane to low molecular weight gases, principally propane. There were minor secondary effects such as isomerization. 

An interesting related study introduced the concept of inverse shape selectivity in molecular sieves [99]. Relative computed adsorption heats for n-hexane and 2,2-dimethylbutane in a series of zeolites with 1-dimensional channels were compared with corresponding experimental adsorption data and data for the relative selectivity to production of these two C6 isomers in hydrocracking of n-Ci6H34. A peak in the relationship between 2,2-dimethylbutane n-hexane selectivity and channel diameter at intermediate pore sizes indicated a channel size domain in which the branched isomer was... 

Paraffin isomerization over dual function catalysts based on zeolite Y and mordenite has been reviewedand a reaction mechanism was proposed in which olefin-paraffin equilibrium is established and carbonium ions are formed from both paraffins and olefins. The isomerization of n-hexane and hydrocrack-... 

Table VIII shows similar data on the conversion of n-hexane. Again, even at the favorable low pressure, the selectivity to benzene is relatively poor. As the temperature is increased, the selectivity to aromatization increases somewhat, but hydrocracking increases at an even greater rate. The data in Table VII show better conversion and selectivity for the aromatization of methylcyclopentane to benzene. It appears, therefore, that the rate-limiting step in hexane conversion is the conversion of hexane to methylcyclopentane. This could be explained by reference to...

On the other hand, R-exchanged zeolites have also been used in combination with a metal function, for carrying out the isomerization and hydrocracking of paraffins and cycloalkanes. This is the case for isomerization of n-hexane to isohexane and 2,2-dimethyl-butane (Rabo et al. 1961) the isomerization of n-undecane to mixed Ci 1 isomers at 275°C on Pt/Ce-Y zeolite (Weitkamp et al. 1985) the isomerization of c (7o-exo-tricyclo[5.2.1.02,6]-decane or exo-tricyclo[6.2.1.02,7]-undecane into adamantane or 1-methyladamantane, respectively, on R-Y at 150-270°C (Lau and Maier 1987) the isomerization of tetrahydrodicyclopentadiene into adamantane on Re-Y in a H2/HC1 atmosphere at 250°C (Honna et al. 1986) or the double bond relocation of 2-alkyl acrolein into fran.j-2-methyl-2-alkenals over Ce,B-ZSM-5 (Fisher et al. 1986). Recently, it has been reported that Ce-promoted Pd/ZSM-5 is an active and selective catalyst in the dehydroisomerization of a-limonene to / -cymene (Weyrich et al. 1997). 

Metal clusters in zeolites are catalysts for a number of reactions, including alkene hydrogenation and alkane hydrocracking. The former is an example of shape selective catalysis, whereby straight diain alkenes can enter the zeolite pores and react but branched alkenes cannot enter and so do not substantially react. The latter have been apidied commerdally. Pt dusters in the zeolites KL and BaKL are remarkably selective catalysts for the dehydroi dization of n-hexane to give benzene, and they are now applied commerdally. The origin of the selectivity is still not fully understood, but it may be primarily a consequence of the smallness of the Pt clusters, which consist of only about S or 6 atoms on average, as determined by EXAFS spectroscopy, H2 chemisorption, and electron microscopy. 

In addition, the effect of coke deposition on the acid function also supports these observations. The rate of n-pentane isomerization on catalysts coked in a commercial reactor, containing up to 14%C decreases linearly with coke content on the acid support (24). When coke content increases up to 6%, there is an increase in the yield of iso-hexanes, dnring the reforming of n-hexane. This is because, as previously mentioned, the iso-paraffins are intermediate compovmds, and as catalyst deactivates the concentration of intermediate compounds increases due to a decrease in the conversion to hydrocracking products. 

In view of the complicated reaction kinetics of multicomponent systems, it was not clear whether or not the diffusional effects would also affect the relative rate of conversion of feed molecules in a mixture. To answer this question we studied the hydrocracking of three multicomponent systems. The first was a C5-C8 mixture, a C5 360° C boiling range midcontinent reformate which contained 12.5 wt % n-paraffins including 4.2% n-pentane, 4.3% n-hexane, 2.9% n-heptane, l.l%n-octane, and <1% C9+ n-paraffins, with the remainder isoparaffins and aromatics. The reaction was carried out at 400 psig, 2 H2/HC, 2 LHSV, and 800°F. Secondly, a Cg-Cie mixture... 

A study is presented of the synthesis and properties of the novel synthetic zeolite omega. The synthesis variables and kinetics of formation are discussed, as well as the ion exchange, sorption, and thermal properties. By decomposition of imbibed tetra-methylammonium ions and exhaustive treatments of the zeolite with ammonium ions, a pure hydrogen form can be obtained which is a suitable substrate for the preparation of hydrocarbon conversion catalysts. Several catalysts were prepared and utilized to isomerize n-hexane, and to hydrocrack a heavy gas oil. 

Ideal hydrocracking of alkanes requires a minimum of seven carbon atoms. This may be derived from Figure 9 in which distributions of the cracked products are compared that were gained with n-hexane (left hand side) and three of its higher homo-... 

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 chain length of n-alkanes has a marked influence on reactivities for hydroisomerization, and especially for hydrocracking. To a very small extent a methane and ethane abstracting mechanism, probably hydrogenolysis as predicted in a basic work on bifunctional catalysis (14), is found to be superimposed when lower carbon number feeds (C, Cg, Cg) are used. n-Hexane is excluded from ideal hydrocracking. On the Pt/Ca-Y-zeolite catalyst it is cracked via a mechanism that is mainly hydrogeno-lytic. 

Figure 1. Effect of temperature on isohexanes/n-hexane hydrocracking of California gas oil...

Figure 4. Relationship between liquid yield and isohexanes/n-hexane at 590 15°F hydrocracking of California gfis oil...

Erionite has been synthesized at i00°-I50°C from a (Na,K) aluminosilicate gel with Si02/AUOs = 10. X-ray and electron diffraction results on the product show intergrowths of the related offretite structure, which is a large-pore zeolite. Adsorption capacity for n-hexane is consistent with the density but adsorption rates are far slower than for zeolite A. Adsorption rates for n-octane are even slower but still better than for natural erionite. Hydrocracking tests on a C /Cq naphtha show strong selectivity for converting normal paraffins to Cf gas, particularly propane. As temperature is increased, other components of the naphtha feed are cracked and selectivity decreases. 

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