Unimolecular cracking

Uncomplicated model reactions should be chosen in which the rate-determining step is an acid-catalyzed transformation. Reactants should therefore be selected that do not undergo extensive side reactions and that undergo kinetically simple reactions such as unimolecular cracking or isomerization. 

Formation of products in paraffin cracking reactions over acidic zeolites can proceed via both unimolecular and bimolecular pathways [4], Based on the analysis of the kinetic rate equations it was suggested that the intrinsic acidity shows better correlation with the intrinsic rate constant (kinl) of the unimolecular hexane cracking than with the apparent rate constant (kapp= k K, where K is the constant of adsorption equilibrium). In... 

As an example of a unimolecular decomposition reaction, we study the monomolecular catalytic cracking reaction of //-paraffins in high-silica acid zeolites or other crystalline or ordered acid porous materials, in this section [97-102],... 

With the model developed, we describe alkane adsorption in nanoporous crystalline and ordered acid catalysts with the intention of calculating three mathematical equations, one each for the geometry of the pore system, for describing the adsorption enthalpy and its relation with the activation energy, and for the monomolecular cracking of n-paraffins [97], This methodology can also be applied to other unimolecular reactions. 

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-... 

Thermal cracking of organic substances is an important reaction in the petroleum industry and has been extensively studied for over seventy years. At least for simple alkanes, the decay is first order in good approximation and therefore was long believed to occur in a single, unimolecular step [21]. However, in the 1930s, Rice and coworkers [22-24] established the presence of free radicals under the conditions of the reaction by means of the Paneth mirror technique [25,26], This observation led Rice and Herzfeld to propose a chain mechanism [22,27,28], Extensive later studies proved the essential features of their mechanism to be correct not only for hydrocarbons, but also for many other types of organic substances. 

Rates of thermal cracking are first-order in good approximation for propane, butane and still higher hydrocarbons [21], This is remarkable because chain mechanisms with initiation by break-up of a reactant normally result in reaction orders of one half or one-and-a-half, depending on which radical is consumed by termination. First-order behavior can result from "mixed" termination, which, however, can in most cases be ruled out as dominant mechanism (see Section 9.3). A more probable explanation is a combination of effects that key hydrocarbon radicals participate in several steps of different molecularities, that some steps are reversible, and that some unimolecular ones require collision partners. 

Basic Catalysis. The catalytic properties of alkali zeolites free of acidic sites have been investigated for the cracking of hexanes (25, 26). At 500 C K-Y zeolite cracks easily n-hexane and its isomers resulting in product distributions markedly different from those obtained over acidic zeolites or even by thermal cracking (pyrolysis). Free radical-type mechanism predominates on the zeolite surface. The relative rates of H atom abstraction (bimolecular) and B-scission (unimolecular) are greatly affected by the zeolite matrix. Zeolites also concentrate hydrocarbon reactants within the crystal, which enhances the rate of bimolecular reaction step. Comparison with silicalite (Al-free ZSM-5 zeolite) and quartz chips has been done in order to characterize the zeolitic effect. Silicalite behaves as inert quartz chips with no effect on the rate of H-abstraction step,... 

More generally, a ratio of alkene to alkane is determined by the relative production rate of each. During free-radical cracking, pyrolysis of alkanes yields either alkenes by unimolecularly decomposing free radicals, alkanes by free radicals abstracting a hydrogen from another source, or both. For example, ethene can be formed by decomposition of primary radicals (including ethyl),... 

The reactions taking place during hydropyrolysis are probably dominated by free radical chemistry. Catalytic effects from reactor components or trace minerals found in the feedstocks cannot, however, be discounted. Cracking reactions are probably initiated through thermally induced unimolecular bond scission. 

An alumina addition to silica produces proton-active catalysts for cracking purposes. The selective adsorption of gases with proton affinity can be used to measure the surface area covered with protons 34). The aluminum ions seem to form a unimolecular layer on the surface of the silica 36). 

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