Cracking of n-hexane

The activity advantage of zeolite catalysts over amorphous silica-alumina has well been documented, Weisz and his associates [1] reported that faujasite Y zeolite showed 10 to 10 times greater activity for the cracking of n-hexane than silica-alumina. Wang and Lunsford et al. [2] also noted that acidic Y zeolites were active for the disproportionation of toluene while silica-alumina was inactive. The activity difference between zeolite and silica-alumina has been attributed to their acidic properties. It is, however, difficult to explain the superactivity of zeolite relative to silica-alumina on the basis of acidity, since the number of acid sites of Y-type zeolite is only about 10 times larger than that of silica-alumina. To account for it, Wang et al. [2] proposed that the microporous structure of zeolite enhanced the concentration of reactant molecules at the acid sites. The purpose of the present work is to show that such a microporous effect is valid for pillared clay catalysts. 

The present paper is an extension of previous work (8) in the following ways. (1) Samples were examined in which residual sodium, but not aluminum, was removed by exchange with ammonium nitrate followed by calcination. (2) The cracking of n-hexane was studied in a pulsed microreactor. (3) The relative effective diffusivity was estimated by a gas chromatographic technique involving broadening of a N2 pulse in a He carrier. 

The effect of coke on the rate of 2-Me-pentane formation, a primary product of the catalytic cracking of n-hexane, will be dealt with in detail to illustrate the methodology. 

Industrial samples of HY zeolites partially dealuminated (HYD) then acid washed (HYDW) were studied. Coke was produced during cracking of n-hexane (4.7 % (HYDW4.7H) and 8.6% (HYDW8.6H) of coke w/ w) or ortho-xylene (8.7% (HYDW8.7X) of coke w/w) at 673K, Partial or total oxidation was carried out in a... 

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... 

In contrast, cracking of n-hexane over amorphous silica-alumina (Aerocat) gave modestly increased activity and a typical carbonium ion" product mix containing more hydrogen, less methane and C2 s, more propane, and isomerized higher paraffins (isobutane). 

Various reactions have been studied on mixed rare earth and the La and Ce forms. These include ethylation of benzene 18), propylation of toluene 14), o-xylene isomerization 21), butane cracking 14), cracking of n-hexane, n-heptane, and ethylbenzene (8), and isomerization and disproportionation of 1-methy 1-2-ethylbenzene (7). Other reactions are summarized by Venuto and Landis 18). In several reports, an optimum calcination temperature for best catalytic performance has been demonstrated (7, 8, 14, 18, 21). 

The effect of hydrogen spillovo on the cracking of n-hexane on Pt/H-erionite was investigated. Changes in selectivities were found in dependence on the nature of the carrier gas and the presence of metal. An extended model of the bifunctional catalysis involving spilt-over hydrogen is proposed. 

Figure 3. Cracking of n-hexane on Pt/H-erionite in a flow reactor after 5 min time on stream in dependence on relative hydrogen pressure.

In the present work FAU, EMO and EMT zeohtes steamed at 450-800 C were characterised employing XRD, NMR and FTIR techniques. Cracking of n-hexane was carried out over the three series of catalysts in order to evaluate the effects of structure and aridity on catalytic performance. 

The catalytic cracking of n-hexane was earned out in a continuous flow reaction system over 0.5 g of electrically heated Ni/Al203 catalyst at 733K and 1 atm. The total flow rate of hexane and nitrogen was controlled at 60 ml per minute. I he amount of carbon deposited on the used catalysts was determined by chemical analysis. The eilluent gas was analyzed by gas chromatography (HP 5890 A) using a flame ionization detector. 

So far these processes have been modeled in terms of lumps. In catalytic cracking the 3-lump -and the 10-lump model [Nace et al, 1971 Jacob et al, 1976] are still widely used although the lumps are based on boiling ranges rather than on chemical nature. These models contain in general only one deactivation function of an empirical nature for the reactions of the various lumps, b their study of the catalytic cracking of n-hexane on a US-Y-zeolite in an electrobalance with recycle Beimaert et al, [1994] derived an empirical deactivation function of the type (2) for the various reactions, but with different a-values, as illustrated in Table 2 for the isomerizations. 

Figure 2. Molecular reaction scheme for the catalytic cracking of n-hexane.

The only way out is to express the transformation of the molecules in terms of the elementary steps of carbenium-ion chemistry protonation, H- and Me-shifts, protonated cyclo-propane-type isomerization, P-scission. .. [Froment, 1996]. In Fig. 3 the cracking of n-hexane is represented in terms of the single event approach introduced by Froment and co-workers [Baltanas et al 1989 Vynckier and Froment, 1991 Froment, 1991 Svoboda et al, 1995]. 

Typical test reactions often used for the characterization of zeolites are the cracking of n-hexane - and disproportionation of ethylbenzene. The catalytic activity of a zeolite is determined by the concentration of protons and the acid strength. 

In this paper, the cracking of n-hexane, n-dodecane and n-hexadecane on ZSM-5 zeolites at about atmosphere and temperatures of 260-400°C were studied. The results showed that both mono-molecular cracking and bimolecular reaction (disproportionation) for n-hexane cracking took place. A network for initial reactions was proposed, and the apparent kinetic parameters of the reactions were estimated. An examination for the factors affecting the product destribu-tion of n-hexadecane indicated that hydrogen transfer on the surface of HZSM-5 zeolites plays an important role in cracking reaction. 

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. 

A simple example is the cracking of alkanes by zeolites with a low density of acid groups. H-ZSM-5 can be regarded as an ideal solution of acidic groups, since they are too far apart to influence one another. At low A1 contents (up to 4 % AI2O3) there is a direct relationship between the catalytic activity in the cracking of n-hexane and... 

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