Cracking of hexanes

A different kind of shape selectivity is restricted transition state shape selectivity. It is related not to transport restrictions but instead to size restrictions of the catalyst pores, which hinder the fonnation of transition states that are too large to fit thus reactions proceeding tiirough smaller transition states are favoured. The catalytic activities for the cracking of hexanes to give smaller hydrocarbons, measured as first-order rate constants at 811 K and atmospheric pressure, were found to be the following for the reactions catalysed by crystallites of HZSM-5 14 n-... 

Scheme 5.1-69 The cracking of hexane in [EMiM]Ci/AiCi3 (X(AiCi3) added copper(ii) chioride.

Few authors considered the reactivity of hydroxyl groups at catalyti-cally interesting temperatures. In situ infrared spectroscopy showed that in the cumene cracking reaction the 3550 cm-1 hydroxyls in a HY sample are only affected above 325° C. The 3650 cm-1 hydroxyl decreased in intensity at 250° C (6). During the cracking of hexane on a similar sample the gradual deactivation of the catalyst is accompanied by the progressive... 

Location of coke produced during cracking of hexane and ortho-xylene on HV zeolites has been studied by 129-Xe NMR. Coupled with HRADS (high resolution adsorption) technique we obtained information about the distribution of coke during coking and decoking. We also showed that complete reoxidation under relatively mild conditions did not permit to restore the initial catalyst. 

Cracking of Hexanes Over Alkali MetaU-Exchanged Xeolites... 

Hydroisomerizadan and cracking of hexane (Ni-cx)-Al-S, impr. with Pd. An enhancement of catalytic activity when Si02 was replaced with A1 was observed. The presence of Ni in the T-O-T layer also increase the activity. 54... 

Cracking of //-hexane is a well-established test reaction for the activity and activity/time stability of a catalyst. Therefore we used //-hexane and //-butane for the temperature switched (300 K - 600 K) MAS NMR studies. 

Recently, Froment et al. [ref. 9], in their study of the deactivation of zeolite catalysts in the cracking of hexane observed a very different behavior for the main and the coking reactions. This behavior strongly depended on the structure of the zeolite Fiq 1 shows the results obtained with ZSM-48, which has a one-dimensional channel structure. 

Basic Catalysis. The catalytic properties of alkali zeolites free of acidic sites have been investigated for the cracking of hexanes (25, 26). At 500 C K-Y zeolite cracks easily n-hexane and its isomers resulting in product distributions markedly different from those obtained over acidic zeolites or even by thermal cracking (pyrolysis). Free radical-type mechanism predominates on the zeolite surface. The relative rates of H atom abstraction (bimolecular) and B-scission (unimolecular) are greatly affected by the zeolite matrix. Zeolites also concentrate hydrocarbon reactants within the crystal, which enhances the rate of bimolecular reaction step. Comparison with silicalite (Al-free ZSM-5 zeolite) and quartz chips has been done in order to characterize the zeolitic effect. Silicalite behaves as inert quartz chips with no effect on the rate of H-abstraction step,... 

The distribution of the products obtained from this reaction depends upon the reaction temperature (Fig. 5.2-2) and differs from other polyethylene recycling reactions in that aromatics and alkenes are not formed in significant concentrations. Another significant difference is that this ionic liquid reaction occurs at temperatures as low as 90 °C, whereas conventional catalytic reactions require much higher temperatures, typically 300-1000 °C [90j. A patent filed under the Secretary of State for Defence (UK) has reported a similar cracking reaction for lower molecular weight hydrocarbons in chloroaluminate(iii) ionic liquids [91]. An example is the cracking of hexane to products like propene and isobutene (Scheme 5.2-40). The reaction was... 

Johnson and coworkers investigated the cracking and isomerization of various alkanes such as nonane, tetradecane and 2-methylpentane in acidic pyridinium chloride-aluminum chloride ionic liquids. Similar product types to the cracking of hexane (above) were observed and after 15 days some polymerization of the cracked products had occurred [91]. A similar reaction occurs vyith fatty acids (such as stearic acid) or methyl stearate, which undergo isomerization, cracking, dimerization, and oligomerization reactions. This has been used to convert solid stearic acid into the more valuable liquid isostearic acid [92] (Scheme 5.2-41). The isomerization and dimerization of oleic acid and methyl oleate have also been found to occur in chloroaluminate(iii) ionic liquids [93]. 

Discussions of OH groups in the context of catalysis normally focus on their role as active centers in a number of reactions. The work by Haag et al. (94) constitutes a classic example the authors estahhshed a linear relationship between the concentration of aluminum in HZSM-5 (which imphes an equal concentration of bridging hydroxyls) and the activity for cracking of -hexane. It was concluded that aU protonic acid sites in the zeohte are characterized by the same turnover frequency. Many other correlations between catalytic properties of materials and the strength and/or density of their Bronsted acid sites are well estabHshed. We will not discuss this aspect in detail and recommend instead a number of recently pubhshed reviews (59,60,87). Two more points are worth mentioning. One point is that the cooperative action of Bronsted and Lewis acid sites has been demonstrated. The second is that, of course, OH groups must not necessarily be involved in a catalytic conversion in fact, they can even block the catalyt-icaUy active sites. 

Figure 3. Schematic diagram illustrating the elemental reactions involved in thermal cracking of hexane [Used by permission of Elsevier Seience, fromUngerer (1990) Organic Geochemistry, Vol. 16, Fig. 3, p. 5].

WORKED PROBLEM 11.9 What products do you expect from the thermal cracking of hexane ... 

In an elegant example Haag et aL [80,81] used the cracking of -hexane over H-ZSM-5 zeohtes with A1 concentrations ranging from 20 up to 50000 ppm (Si/Al = 10 to 100000) to show that all acid sites had the same TOF, by demonstrating a linear relationship between concentration of A1 (add site concentration) and the reaction rate (see Fig. 2). 

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