Protolytic cracking

Fig. 15 A proposed reaction network of direct ring opening of decalin reaction over acidic zeolites. PC Protolytic cracking HeT Hydride transfer HT Hydrogen transfer I Isomerization P /i-scission DS Desorption TA Transalkylation. Adapted from ref. 47.

The formation of methane and ethane can be explained by the protolytic cracking of an alkane (1 ) (specially branched alkanes) on very strong Brdnsted sites. However, it would be difficult to... 

Investigation of n-butane conversion over H-forms of the ferrierite and theta-1 zeolites demonstrated that the isobutene selectivities were similar (and low) for these catalysts. The maximum selectivities (7-8 %) were obtained at low n-butane conversions (5-10 %) and decreased with increasing conversion of n-butane due to olefin interconversion and aromatisation reactions. Isobutene was in equilibrium with the other butene isomers due to the high isomerisation activity of the parent zeolites. The maximum selectivity to butenes, which was observed at low conversions, was around 20 %. This value reflects a moderate contribution of the dehydrogenation steps in n-butane transformation over H-forms of the ferrierite and theta-1 zeolites and indicates an important role of the n-butane protolytic cracking steps over these two catalysts. 

More recently, it has been proposed (78, 79) that on zeolite catalysts the reaction can also start by protonation of a C-C bond by the acid site of the zeolite forming a pentacoordinated carbonium ion transition state. This can then eliminate H2 or a short alkane molecule leaving an adsorbed carbenium ion on the zeolite (protolytic cracking), as shown below ... 

In summary, it could be said that, in the case of n-alkanes, the first molecules of reactant are cracked by protolytic cracking. Then, once a smaller carbenium ion is left on the catalyst surface, cracking can continue by one of the following routes ... 

The results on tables 9 and 10 show that increasing the size of the cluster and/or the basis set does not change appreciably either the structure of the TS s or the activation energies obtained at a lower level of calculation (3T cluster and DZP basis set). Irrespective of the substrate and level of calculation employed, the results show that the protolytic cracking involves the attack of the zeolitic proton to a... 

As for the dehydrogenation reaction, IRC calculations [67] indicate that the protolytic cracking of linear and branched alkanes follows different mechanisms. For ethane and propane the products of the reaction are methane and the proper alkoxide. For isobutane, as one follows the reaction path towards the products, the t-butyl cation decomposes into propene and a proton which restores the acid site of the zeolite. 

The protolytic cracking involves the attack of the zeolitic proton to a carbon atom of the alkane molecule and the simultaneous rupture of one its adjacent C-C bond. The carbon atom being attacked and the C-C bond being broken will be preferentially those which produce the most stable carbenium ion. As for the dehydrogenation reaction, the protolytic cracking of linear and branched alkanes also follow different mechanisms, the latter ones producing olefins instead of alkoxides. 

Abstract The ab initio pseudopotential plane wave DPT simulation of the structure and properties of zeolite active sites and elementary catalytic reactions are discussed through the example of the protonation of water and the first step in the protolytic cracking mechanism of saturated hydrocarbons. 

In a similar way, hydride transfer reactions in alkane/alkene transformations depend in a nonlinear fashion upon the varying concentration of acid sites. Post et al. [50] showed elegantly that the rates of these bimolecular reactions depend upon the square of the concentration of the acid sites, while the rates of the monomolecular reactions (protolytic cracking [51]) were linearly dependent on the proton concentration. This suggests that similar effects can also be expected in more complex organic transformations, where less thoroughly developed structure-activity relations exist. 

Fig. 11. Corrections to the calculated activation energies for the cluster acid strength. Activation energies for protolytic cracking of ethane, protolytic dehydrogentation of ethane, methane-methoxy hybrid transfer, and methane deprotonation energies are computed at the MP2/6-31++G //HF/6-31G level with the ZPE corrections [17,113].

Figure 10 suggests the following order of protolytic cracking in n-hexane at 400°c (Table 4), over the CSY2 zeolite. 

El Tanany et al. (ref. 22) have investigated the n-heptane hydroconversion at 428 K and 1013 mbar on H-erionite. According to Haag et al. (ref. 26) and their results the formation of C3/C4 species should take place via a pentacoordinated carbonium ion by protolytic cracking. In a following chain process higher hydrocarbons may result. 

S. Kotrel, H. Knozinger, B. C. Gates, The Haag-Dessau mechanism of protolytic cracking of alkanes, Micropor. Mesopor. Mater, 2000, 35-36, 11-20. 

A non-stationary kinetics approach for the determination of the kinetic parameters of the protolytic cracking of methylcyclohexane. 

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