Monomolecular reaction 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. 

An oriented cycle of the length more than two is not solvable. For each number of vertices one can calculate the set of all maximal solvable mechanisms. For example, for five components there are two maximal solvable mechanisms of monomolecular reactions ... 

In conclusion, extensive research has revealed that the Lewis and Brpnsted acid sites on the promoted sulfated zirconia catalysts are not necessarily stronger acids than the corresponding sites in zeolites, but sulfated zirconia circumvents the energetically unfavorable monomolecular reaction path by following a bimolecular mechanism. The question of superacidity of sulfated zirconia, however, is still debated.312... 

If all the elementary reactions are monomolecular, i.e. can be written as Ax —> Aj, it is more convenient to represent reaction mechanisms in a different way, namely nodes correspond to substances, edges are elementary reactions, and edge directions are the directions of reaction processes. As usual, this graph is simpler than the bipartite graph. For example, for the system of three isomers Al A2 and A3 we obtain... 

Strictly speaking, mechanisms for heterogeneous catalytic reactions can never be monomolecular. Thus they always include adsorption steps in which the initial substances are a minimum of two in number, i.e. gas and catalyst. But if one considers conversions of only surface compounds (at a constant gas-phase composition), a catalytic reaction mechanism can also be treated as monomolecular. It is these mechanisms that Temkin designates as linear (see Chap. 2). 

It was established that the reaction of the methyl radical with the silanone group occurs via two steps first it adds to the silicon atom to form the oxy radical, and then the radical is isomerized via a shift of the H atom to oxygen (reaction 2 in Table 7.11). The estimated activation energy of this monomolecular reaction was 16kcal/mol. This suggests that the addition of other alkyl radicals (reactions 3 and 4) to the silanone groups also occurs via steps. The mechanism of the addition of an H(D) atom to the silanone group remains unclear. 

The atomic mechanism responsible for monomolecular reactions, including thermal decompositions, was first discussed by Polanyi and Wigner.26 Their model assumes that decomposition occurs when, due to energy fluctuations in the bonds of the molecule, the bond strength is exceeded or more precisely, the bond energy resides in harmonic vibrations and that decomposition occurs when their amplitude is exceeded. The resulting expression for the first-order Polanyi-Wigner rate constant is... 

Tertiary alkyl chlorides are easily dehydrochlorinated by base (via the E2, or bimolecular elimination reaction mechanism), but the environment of the degrading resin is not basic. Loss of hydrogen chloride to yield an olefin can occur principally by the El, or monomolecular elimination reaction. This is a slow reaction because, in the rate-determining step, the C—Cl bond is broken to form two separated oppositely charged particles. The reaction rate is not assisted by the acid present. 

As many organic compounds may transform simultaneously through mono molecular (intramolecular) and bimolecular (intermolecular) processes, it is easy to understand that the shape and size of the space available near the active sites often determine the selectivity of their transformation. Indeed the transition state of a bimolecular reaction is always bulkier than that of a monomolecular reaction, hence the first type of reaction will be much more sensitive to steric constraints than the second one. This explains the key role played by the pore structure of zeolites on the selectivity of many reactions. A typical example is the selective isomerization of xylenes over HMFI the intermediates leading to disproportionation, the main secondary reaction over non-spatioselective catalysts, cannot be accommodated at its channel intersections (32). Furthermore, if a reaction can occur through mono and bimolecular mechanisms, the significance of the bimolecular path will decrease with the size of the space available near the active sites (41). 

A general mechanism for heterogeneously catalyzed monomolecular reactions is... 

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 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]. 

The isomerization of open-chain alkanes with more than six carbon atoms gives isobutane as the main product, together with disproportionated materials, even though the reaction proceeds by the monomolecular pathway [144]. On the other hand, for cyclic alkanes the monomolecular process with preservation of the cyclic structure seems to be the most probable, judging from the results for cyclohexane. The absence of isobutane in the products indicates that the reaction path does not involve open-chain intermediate species. Therefore, it is of interest to try cycloalkanes larger than cyclohexane for clarification of the reaction mechanism along with the catalytic action of S04/Zr02. 

It has been shown (Fig. 9) that for most of the molecular sieve catalysts the selectivity for isobutylene improves with TOS. Furthermore, it has been demonstrated with ferrierite that the change with TOS from the nonselective form (the noncoked state) to the selective form (the coked state) is accompanied by a change in the reaction mechanism from bimolecular to monomolecular. Here, the physicochemical properties of the molecular sieves and their variation with TOS are described and analyzed in terms of the correlations between the state of the molecular sieves and their activities and selectivities for n-butenc isomerization. 

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. 

Even though this appears to be a monomolecular reaction, it is not, as it proceeds through the following mechanism ... 

Let us have a look at the thermodynamic approach [91], The reaction rate for a monomolecular dissociation mechanism depends in transition state theory on the partial pressure of the metal oxide PmcKgy If we incorporate the loss of atoms we... 

Most feeds contain some olefin as an impurity moreover many sulfated zirconia catalysts contain traces of iron or other transition metal ions that are able to dehydrogenate hutane. In the presence of such sites, the olefin concentration is limited by thermodynamics, i.e a high pressure of H2 leads to a low olefin concentration. That aspect of the reaction mechanism has been proven in independent experiments. The isomerization rate over sulfated zirconia was dramatically lowered by H2. This effect is most pronounced when a small amount of platinum is deposited on the catalyst, so that thermodynamic equilibrium between butane, hydrogen and butene was established. In this way it was found that the isomerization reaction has a reaction order of +1.3 in -butane, hut -1.2 in hydrogen [40, 41]. The byproducts, propane and pentane, are additional evidence that a Cg intermediate is formed in this process. As expected, this kinetics is typical for butane isomerization only in contrast pentane isomerization is mainly a monomolecular process, because for this molecule the protonated cyclopropane ring can be opened without forming a primary carbenium ion [42]. 

---