Monomolecular mechanism

Industrial metal-zeolite catalysts undergo a bifunctional, monomolecular mechanism [1-5, 7]. Carbenium ions are the critical reaction intermediates to complete chain reactions. In the zeolite channels, carbenium ions likely exist as an absorbed alkoxyl species, rather than as free-moving charged ions [8], Figure 14.2 illustrates the accepted reaction mechanism, using hexanes as an example. 

With increasing reaction severity, the concentrations of the individual isomers approach their equilibrium values. The monomolecular route is the most effective for achieving high yields of PX, which is typically the most desirable for petrochemical applications. The schematic above shows the stepwise interconversion of OX to MX and MX to PX, which is consistent with a 1,2-methyl shift route. However, the results of kinetics studies provide some indications in favor of a reaction step that directly converts OX to PX [62]. It is not clear what the form of the reaction intermediate for this transformation is. Some in situ time-resolved spectroscopic methods have been used to look at how modification of zeoMtes like MFl affects the monomolecular mechanism by constraining the diffusion of MX [63]. 

A new study with n-hexane, however, suggests a monomolecular mechanism with the participation of Lewis acid-Lewis base site pairs generating the carboca-tion. The role of Pt is to activate hydrogen which spills over to form hydride species for the desorption of the isocarbocation.311... 

Two extreme possibilities can be considered in ligand substitution. In the SnI (nucleophilic, monomolecular) mechanism, first the ligand, X", to be substituted dissociates in a slow reaction ... 

The usually considered monomolecular mechanism of substitution implies that one-electron reduction activates a substrate sufficiently so that it could dissociate with no further assistance from a nucleophile. The next steps of the reaction consist of transformations of the resultant radical. However, in substrates having sp3 carbon as a reaction center, the influence of the leaving group has been fixed (Russell Mudryk 1982a, 1982b). This led to the formulation of the SRN2 bimolecular mechanism of radical-nucleophilic substitution. In this mechanism, the initial products of single-electron transfer are combined to form the... 

In studies of catalytic reactions, linear (monomolecular) mechanisms are observed in the following two cases. 

Various models are used in the literature to account for the kinetics of the excitons involved in optical processes. In the simplest cases, the signal evolution n(t) can be reproduced by considering either a single exponential or multiexponential time dependences. This model is well suited for solutions or solids in which monomolecular mechanisms happen alone. Since in most transient experiments the temporal response is a convolution of a Gaussian-shaped pulse and of the intrinsic kinetics, the rate of change with time of the excited-state population decaying exponentially is given by... 

The skeletal isomerization of butane to isobutane is a typical reaction catalyzed by superacidity. Early in the history of this work, S04/Fe203, S04/Ti02, and S04/Zr02, were termed superacids owing to their ability to isomerize butane at room temperature or below [32, 37, 39] The formation of isobutane from butane, however, does not necessarily require superacidic strength. A bimolecular reaction pathway based on the intermediacy of butane is energetically lower than a monomolecular mechanism [129-133]. The monomolecular and bimolecular mechanisms are shown in Schemes 17.1 and 17.2, respectively, using pentane as a model. 

The processes are the monomolecular reaction through a protonated cyclopropane produced by the abstraction of H" over Lewis acid sites and the bimolecular mechanism where an olefin takes part in the reaction. The olefin is produced over Bronsted acid sites, in the case of butane in the monomolecular mechanism, isobutane is formed through protonated methylcyclopropane with an activation energy of 8.4kcalmoT followed by the formation of the primary isobutyl cation with high energy [134]. 

Cyclohexane is known to be isomerized to methylcyclopentane when catalyzed by strong acids. In fact, the SO jT rOi catalyst converts cyclohexane into methylcyclopentane and methylcyclopentane into cyclohexane [119, 142, 143]. The reactions proceed by the monomolecular mechanism via the intermediacy of secondary and tertiary carbenium ions followed by protonated cyclopropanes. 

I he recent literature related to selective skeletal isomerization of -butenes catalyzed by medium-pore zeolites and Me-aluminophosphates is reviewed. In the presence of medium-pore molecular sieve catalysts, o-butenes are selectively transformed into isobutylene via a monomolecular mechanism. This is an example of restricted transition state shape selectivity, whereby the space available around the acidic site is restricted, constraining the reaction to proceed mainly through a monomolecular mechanism. Coking of (he ciitalysl that leads to poisoning of (he acidic sites located on the external surfaces and to a decrease in the space around the acidic sites located in the micropores renders the catalyst more selective. 

The monomolecular mechanism, which involves the formation of a primary carbenium ion, is highly energetically unfavorable. However, such a mechanism would result theoretically in 100% selectivity for the conversion of n-butenes into isobutylene. 

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. 

The monomolecular mechanism would transform n-butenes into isobutylene via the formation of a secondary carbenium ion as foUows ... 

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

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. 

The increase in isobutene selectivity with time-on-stream is a particular property of the ferrierite. This zeolite has two types of active sites the external sites (on the external surface of the zeolite crystallites) which are non-selective for skeletal isomerization and the internal sites (inside the zeolite pores) which are selective for this reaction (9). The changes observed on the selectivity have been associated with modifications of pore shapes ough coke deposition that favor reactions involving small molecules, such as -butene to isobutene isomerization (8). More recently, it has been reported that a bimolecular mechanism takes place at the non-selective acid sites, while a monomolecular mechanism occurs on the selective sites (10), the coke deposition being necessary in order to poison, block, and modify the non-shape selective acid sites. 

The theoretical scheme took into account the reaction of chain propagation limitation by transfer to the Al-organic compound, monomer, and hydrogen, as well as the chain termination following the monomolecular mechanism (spontaneous termination or the immurement of active center). 

If such a monomolecular mechanism is accepted a possible explanation for the low pre-exponential factors observed is, that in the initial state (the formate ion) there are internal degrees of freedom—e.g., rotation with the axis perpendicular to the surface, and bending vibrations— which are lost in the transition to the activated state. This can be schematically represented as follows ... 

Isobutene and tran -2-butene were the main reaction products. Neither C3-C5 nor C2-C6 by-products were observed among the reaction products, as in the case of smaU-pore zeolites, suggesting that the reaction followed a monomolecular mechanism and that there was no dimerization, probably due to an effect of the carbon aerogel porosity. 

The reactivity and specific behavior of free radicals produced during initiator s thermal decomposition strongly depend on the type of the radicals formed, which is determined by the nature of peroxide (28,37). Table 10.2 lists primary and secondary radicals formed during the decomposition of an initiator, while Table 10.3 gives data on the activity of certain types of free radicals in abstraction reactions of hydrogen atoms from carbon (33). Primary radicals are formed directly at breakdown of an initiator molecule secondary radicals result from transformations of primary radicals by a monomolecular mechanism. 

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. 

At the same time, after the UV-illumination was stopped in the field of process autoacceleration, simultaneously with the bimolecular chain termination chemical mechanism, the monomolecular mechanism is also observed, the presence of which is explained by the authors [11, 12] as a Stacking (or trapping) of the radicals in the polymeric matrix, resulting in the loss of their diffusion mobility in such a ca.se these radicals can not continue the kinetic chain. At conversions higher than the autoacceleration state (20-80%) chain termination is realized only in accordance with the monomolecular chemical mechanism, which from the authors point of view [11, 12] leads to the... 

The dehydrochlorination reaction continues to be studied - and a variety of mechanisms has been proposed to account for the process. These include a radical-chain mechanism in which the termination step depends upon the stage of the reaction, a monomolecular mechanism involving an activated complex, and a three-step mechanism, beginning with the random formation of a single c s-carbon—carbon double bond, followed by elimination of hydrogen chloride via a six-membered transition state and isomerization of the polyene so formed. ... 

Monomolecular Mechanism This, the simplest, mechanism is based on the assumptions that we have a simple A-to-B reaction and that the reacting species A is adsorbed on the surface according to the Langmuir isotherm, which leads to the rate equation ... 

Xylene isomerization is a test reaction which is claimed to require moderately strong Bronsted acid sites to proceed. One reason for this is the very good stabilization of the formed carbenium ions over the benzene ring. The reaction proceeds via a benzenium ion and the rate-Umiting step in this reaction is the intramolecular methyl transfer. Besides the monomolecular mechanism, xylene isomerization can also proceed via a bimolecular reaction pathway as outlined by Morin et al. [ 172]. They determined the contributions of both pathways and determined the contribution of the monomolecular reaction, which they propose to compare activity and acidity in zeolites. These findings emphasize that the reaction pathway should be known in order to properly estimate acidity. Especially for large pore zeolites, this may be a problem. 

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