Monomolecular Cracking Mechanism

In later work by Haag and Dessau product selectivity data were provided for n-butane cracking at 426-523 °C over HZSM-5 with Si/Ah = 70 [90]. The selectivity results at 496 °C and 1-10 kPa for n-butane were extrapolated to zero percent conversion in Table 13.6 to be able to identify the primary products and to assess the decomposition pattern of the n-butyl carbonium ion. Similar selectivities to methane and propylene implied, as expected, that the decomposition of the car- 

As part of the same study selectivity data were provided at 10-100 kPa partial pressures of n-butane at 0-17% conversion over HZSM-5 [90]. With increase in pressure and conversion secondary reactions started to occur. These results are also summarized in Table 13.6. The lowered selectivity to hydrogen, methane and ethane was attributed to increasingly less favorable conditions for monomolecular cracking. The dramatic increase in selectivity to propane which was absent at zero conversion, along with decrease in propylene was considered as signature for bimolecular cracking. More specifically, it was suggested that hydride transfer 

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The duality of cracking mechanisms is summarized in Fig. 5, where RH paraffin feed, R -C=C = olefinic product, Kq = equilibrium constant of olefin chemisorption. Free Bronsted acid sites HZ interact directly with the paraffin feed by protonation, producing monomolecular cracking. When the acid sites are covered with adsorbed olefins to form... 

The selective reaction occurs through a monomolecular mechanism, and the bimolecular (afkylation/cracking) mechanism is unselective. Both reactions are catalyzed by Brpnsted acids. 

Krannila, H., Haag, W.O., and Gates, B.C. (1992) Monomolecular and bimolecular mechanisms of paraffin cracking n-butane cracking catalyzed by HZSM-5./. Catal, 135, 115-124. 

Cracking of small saturated hydrocarbons, catalyzed by zeolites, can proceed via two mechanisms, both involving carbocations the bimolecular chain reaction, which involves carbenium ions that are further transformed by / -scission, and the unimolecular protolytic mechanism, involving alkanium ions that are formed by the direct protonation of the alkane by the Br0nsted acid OH groups of the catalyst. This latter mechanism, originally proposed by Haag and Dassau, is the predominant one at about 800 K in medium-pore zeolites, like HZSM-5, which favor monomolecular reactions. While rela-... 

The mechanism of skeletal isomerization of n-butenes may be rationalized in terms of the steps presented previously the key reaction intermediate is the 5-butyl cation. The predominent structure of the adsorbed intermediate was recently considered to be an alkoxy 50), which cither adds to one butene molecule and cracks into C3, C4, or C5 fragments (the bimolccular mechanism) or rearranges into isobutylene (the monomolecular mechanism) via a primary carbenium ion. 

Pore size may also affect the reaction order. Cracking of small (i.e., less than C ) paraffins over amorphous acid catalysts and large-pore zeolites may proceed either by a bimolecular or by a monomolecular mechanism. In medium- and small-pore zeolites the space is insufficient to form bulky bimolecular transition states. This makes a monomolecular path more likely. Low reactant partial pressure, low acid site density, and high temperatures (above 450-500 C) also favor the monomolecular mechanism. According to Haag and Dessau [24] and Kranilla, Haag, and Gates [25], the transition state of the monomolecular reaction involves a penta-coordinated carbonium ion. 

A detailed examination of the reaction products of 2MP on both USHY catalysts indicates the modes of reaction are 1) isomerization, monomolecular and dimerization-cracking on pristine sites and 2) biinolecular chain processes between an adsorbed product carbocations and a gas phase reactant molecule. Figure 1 presents the experimental average conversion and the corresponding predicted conversion, with respect to time on stream, along lines of constant catalyst to reactant ratio. The sigmoidal behaviour exhibited at low conversions and short times on stream is consistent with the presence of the second type of reaction, one mediated by chain processes. Such behaviour contrasts sharply with that previously reported for linear paraffins on USHY (9) where 1 was observed to increase monotonically with reaction time. A kinetic model has been proposed (10) which accounts for all the mechanisms active in this system. The model assumes that the surface reaction is rate controlling in all cases and is ... 

Light alkane conversion over HZSM-5 zeolite occurs usually by a protolytic monomolecular mechanism. In the present study we will analyse a set of experimental results obtained for the transformation of light alkanes over HZSM-5, at various temperatures (350 C - 500 C), and compare these results with quantum chemical calculations for these transformations over model acid sites. It was concluded that similar transition states were formed for the cracking of C-C bonds in different alkanes, always with relatively high activation energies. 

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