Cracking alkane

MCM-22, with a larger pore volume than ZSM-5, revealed behavior intermediate between what was observed for large- and medium-pore zeolites (126). Unverricht et al. (141) also examined MCM-22 at 353 and 393 K, it was found to produce mainly cracked products and dimethylhexanes and to deactivate rapidly. MCM-36 gained considerable interest that is evidenced by the patent literature (171-174). MCM-36 is a pillared zeolite based on the structure of MCM-22. Ideally, it should contain mesopores between layers of MCM-22 crystallites. This structure was found to be much more active and stable than MCM-22 (175). Alkane cracking experiments with zeolites having various pore dimensions evidenced the preference of monomolecular over sterically more demanding bimolecular pathways, such as hydride transfer, in small- and medium-pore zeolites (146). 

Mechanism for protonation of alkenes was previously discussed in Section 13.5.1. In general, protonation of alkenes is an exothermic process. Protonation of alkanes was discussed in Section 13.5.2. There wiU be further discussion on this step in Section 13.8.4 within the context of alkane cracking mechanisms. The formation of a penta-coordinated carbonium ion from alkane protonation is typically an endothermic process, the reverse being true for deprotonation. 

Beta scission of a carbenium ion is an elementary step that is inihated by the weakening of the bond beta to the positive charge, leading to a smaller carbenium ion and an alkene. This elementary step is further discussed in Sections 13.8.1, 13.8.3.1 and 13.8.4 within the context of alkene skeletal isomerization, isobutane-2-butene alkylation and alkane cracking, respectively. 

Table 13.8 Monomolecular and bimolecular kinetics of alkane cracking [92],...

Table 13.9 Monomolecular kinetics for alkane cracking when surface reaction controls the rate.

Van Bokhoven, J.A., Williams, B.A., Ji, W Koningsberger, D.C., Rung, H.H., and Miller, J.T. (2004) Observation of a compensation relation for monomole-cular alkane cracking by zeolites the dominant role of reactant sorption. 

Do these results also suggest that five-coordinate carbonium ions are not essential to explain alkane cracking The evidence is mixed. Kazansky and van Santen (132) reported low-level calculations and found a metastable carbonium ion (CH3-H-CH1) formed from ethane and a zeolite Brpnsted site, but this species was so high in energy that it did not appear to be thermally accessible. More extensive work by van Santen (133) shows, however, that the transition states leading from this species do not relate to ethane cracking Blaszkowski, Nascimento, and van Santen (134) found other transition states for ethane cracking (Fig. 26) that are similar to carbenium ions albeit with stabilization from the lattice. 

In the cases of mesoporous silica, AMS and quartz chip, the 0.5 C3/C4 ratio being close to unity means that two reactions proceed with almost equal probability to each other. This is in accordance with the classical radical mechanism of alkane cracking supposing that the energy required to form tertiary radical is not so different from that required for secondary radical and that both radicals are cracked by P-scission mechanism shown below [13]. Thus, the results shown in Fig. 4 strongly suggest that isohexane is cracked via the radical mechanism on the mesoporous silica catalysts, or, in other words, MCM-41, both with and without aluminum impurity, and FSM-16 exhibit radical type catalytic function. 

Number of moles of cracked products with carbon number i [moles per 100 moles n-alkane cracked J... 

In the case of alkane cracking, dispersion forces between the alkane molecules and the siliceous walls of the zeolites and perhaps other nanoporous crystalline and ordered materials are possibly the most important interactions for stabilizing adsorption in the cavities, since the proton affinity of alkanes is low [104] and the electrostatic interactions between the alkane and the adsorbent are negligible [97],... 

Fig. 2.7 Reaction pathway for alkane cracking on zeolite catalysts...

Alkane Cracking and Isomerization on Solid Acid Catalysts... 

Section 5.2 aromatic Friedel-Crafts-type alkylations C. Perego and P. Ingallina, Catal. Today 2002, 73, 3 A. Corma, Chem. Rev. 1995, 95, 559 alkane cracking and isomerization Y. Ono, Catal. Today 2003,81, 3 A. Feller and J. A. Lercher, Adv. Catal. 2004, 48, 229 isoparaffin-olefin alkylation A. Corma and A. Martinez, Catal. Rev. Sci. Eng. 1993, 35, 483 acid-base catalysis with metal oxides K. Tanabe and W. F. Floelderich, Appl. Catal. A Gen. 1999, 181, 399. 

The zeolites have been first used as catalyst in the 1960s for alkane cracking reactions in petroleum industry.They replaced favorably previously employed alumina based catalysts because of their better therm and mechanical stability. Moreover, they showed higher selectivity. The selectivity finds its source in the zeolite micropore structure with different... 

The apparent first-order rate constant of n-alkane cracking over ZSM-5 at 380°C. [Reprinted from J. Wei, Adsorption and Cracking of N-Alkanes over ZSM-5 Negative Activation Energy of Reaction, Chem. Eng. Sci., 51 (1996) 2995, with permission from Elsevier Science.]... 

The temperature of zeolite samples containing various adsorbed molecules was switched from room temperature to 500-600 K within 30-40 seconds by means of a laser beam. Catalytic n-alkane cracking and H-D exchange with deuterated cyclohexane were monitored by IH MAS NMR in time steps of down to one second. A two-dimensional representation of the chemical shift and the chemical reaction of the species will be given, allowing a good characterization of reaction steps. At low temperature a weak proton transfer without chemical reaction can be observed, whereas at 430 K and 530 K the proton transfer is accompanied, respectively, by an isomerization or a decomposition to methane and coke. In addition to the effect of high temperature, the laser radiation itself can force the conversion of alkanes to methane and coke. 

Alkane cracking, alkane-alkene alkylation, alkane disproportionation Alkane isomerization Alkene partial oxidation Oxidation of CO, SOj, and hydrocarbons... 

The anomalous decrease in the activation energies of n-alkanes cracking after Cie the C- sixteen effect[31]", could be explained in terms of the contribution from the internal vibrations of zeolites whose frequencies are in resonance with the skeleton vibrations of hydrocarbons at isocatal)4 ic temperature and has been described in detail elsewhere[32]. 

In two papers by Walsh and Rollman [14-C]labelled hydrocarbons were used to study the origin of carbonaceous deposits on zeolites. With feeds composed of an aliphatic + an aromatic hydrocarbon, the initial reaction involved in the formation of coke was the alkylation of aromatics by the olefmic fragments of alkane cracking. Since ZSM-5 and mordenite have the same framework A1 content, it was possible to compare directly the coke yields of these zeolites. Under the same experimental conditions it was found that C deposition on mordenite was almost two orders of magnitude greater than on ZSM-5. The differences were explained in terms of pore size. In the smaller ZSM-5 pore, the alkylaromatics, once formed were prevented from reacting further to produce coke, because of the spacial constraints. 

Haag proposed an explanation of hydrogen, methane and ethane formation during the alkane cracking over H-ZSM-5 and H-ZSM-11 catalysts by the reactions typical of superacids ... 

RTC is characteristic for the high reaction rate. In the case of relatively light oil, kinetics of alkane cracking can be described by a simple first-order reaction ... 

Among the other mieroporous solid acids, SAPOs and MAPOs tend to possess acidities that are weaker than those found for aluminosilicates. Not all substituted aluminophosphates are weak acids, however. Magnesium-substituted AIPO4-36, for example, gives a strong performance in acid catalysed alkane cracking. " ... 

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