Hydrocarbon activation elementary reaction steps

The activation energy for proton transfer can be viewed as a lattice oxygen Lewis-base and proton Br0nsted-acid synergetic event [3]. One generally finds that activation energies of proton-activated reactions arc rather high between 100 and 200 kJ/mol for proton-activated elementary reaction steps in hydrocarbon conversion catalysis. ITiis is the main reason for the relatively low TOP per proton ( 102 s ) for this type of reaction. Similarly to enzymes [31], the weak van der Waals-type interaction determines the size- and shape-dependent behavior. 

The elementary reaction steps of the hydrocarbons considered in this section are summarized in Fig. 8. Tlie occurrence of monomolecular reactions with linear hydrocarbons that produce hydrogen and alkane fragments was first demonstrated by Haag and Dessau [94], For convenience, the zeolite lattice to which the proton is attached is not explicitly shown in the scheme. However, it will become clear later that proton activation cannot be understood properly without explicitly taking into account the interaction of the carbonium and carbenium ion intermediates with the negatively charged zeolite wall. 

Because of their great importance in chemical industry, much effort has been devoted to the study of hydrocarbon oxidation, and a large data base of rate coefficients and activation energies of common elementary reaction steps has been compiled [50,51,60,69-71]. 

All of this is, of course, speculative at present and will remain so until more details become available about the wall active species, the elementary reaction steps, and the physical transformations occurring within a pyrolysis reactor. However, precise control of reactor treatment times and conditions still provides excellent conditions for investigating the effect of such treatments during pyrolysis reactions. As research workers continue to identify the effects and reactions of both the homogeneous and heterogeneous pyrolysis of hydrocarbons, and as these reactions mechanisms become quantitative then such efforts will lead to better control of conversion, product yields and in carbon laydown in commercial tubes. 

The approach to hydrocarbon cracking taken by the Froment school is to model the actual elementary steps of radicals at the various molecular configurations [38]. These are relatively few initiation hydrogen abstraction from a primary, secondary, or tertiary carbon and radical decomposition by scission of a carbon-carbon bond in /3-position to the unpaired electron. Boolean relation matrices are used to reflect the structures of the hydrocarbon reactants by indicating the existence and location of all their carbon-carbon bonds. Computer software generates reaction networks on the basis of known rate coefficients and activation energies at the various positions. Froment states the number of components in naphtha cracking as around 200, that of radicals as 40, and that of elementary radical steps... 

The hydrocarbon catalytic cracking is also a chain reaction. It involves adsorbed carbonium and carbenium ions as active intermediates. Three elementary steps can describe the mechanism initiation, propagation and termination [6]. The catalytic cracking under supercritical conditions is relatively unknown. Nevertheless, Dardas et al. [7] studied the n-heptane cracking with a commercial acid catalyst. They observed a diminution of the catalyst deactivation (by coking) compared to the one obtained under sub-critical conditions. This result is explained by the extraction of the coke precursors by the supercritical hydrocarbon. 

Carbonaceous species on metal surfaces can be formed as a result of interaction of metals with carbon monoxide or hydrocarbons. In the FTS, where CO and H2 are converted to various hydrocarbons, it is generally accepted that an elementary step in the reaction is the dissociation of CO to form surface carbidic carbon and oxygen.1 The latter is removed from the surface through the formation of gaseous H20 and C02 (mostly in the case of Fe catalysts). The surface carbon, if it remains in its carbidic form, is an intermediate in the FTS and can be hydrogenated to form hydrocarbons. However, the surface carbidic carbon may also be converted to other less reactive forms of carbon, which may build up over time and influence the activity of the catalyst.15... 

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. 

Another important zeolite-catalyzed chemical reaction is the decomposition of NO. Cu-exchanged zeolites, especially Cu-ZSM-5, have been shown to catalyze the decomposition of NO in the presence of hydrocarbons and excess oxygen. The increasing awareness of the detrimental effects of automobile exhaust has prompted several theoretical studies on the active site and reaction mechanism. ° Cu-ZSM-5 was described using an empirical force field and energy minimization to locate the copper ions in ZSM-5. Isolated copper atoms and copper clusters were found in the micropores, mostly associated with framework aluminium species. A cluster of two copper ions bridged via an OH species not part of the zeolite framework ( extra-framework ) was proposed as the active site. Quantum mechanical cluster calculations were carried out to study the elementary steps in the NO decomposition. A single T-site model was used to represent the zeolite framework. 

Until fairly recent times, most of the reported homogeneous HDS-related reactions were limited to stoichiometric model transformations in which thiophenes and related molecules could be activated by coordination to the metal centers so as to undergo C-S bond scission and eventually hydrogenolysis to the corresponding free thiol, or in some cases even complete desulfurization to yield hydrocarbons. This has represented a remarkable contribution to the understanding of many of the elementary steps involved in complex heterogeneous HDS schemes, but it had obvious limitations, and it offered little promise in terms of developing catalytic systems of any mechanistic or practical importance. 

Most of the homogeneous Ziegler-type catalysts have been preferentially investigated in order to understand the elementary steps of the polymerization, which is simplified in soluble systems. Bis(cyclopentadienyl)titanium(IV) compounds, which are soluble in aromatic hydrocarbons could be used together with aluminum alkyls to give Ziegler-catalysts. As to the kinetics of polymerization and to side reactions of the catalyst components, this system is probably the best understood. It has not been used in a technical process because of the low activity and short life of systems that contain chloride (see Table 1). 

It appears that a significant amount of energy is required for CH activation, the primary elementary step of the hydrocarbon conversion reaction. If one chemisorbs methane at low temperature on a transition metal surface, it desorbs before reaction can occur. Aliphatic hydrocarbons can dissociate from a preadsorbed state, if they contain enough carbon atoms in their chain to induce a high heat of adsorption whereby CH dissociation can take place at a rate large compared to the desorption rate. So far this has only been found for surfaces containing highly active metal atoms, such as the reconstructed Ir (110) surface or stepped surfaces l. As we will see, both electronic and steric effects may play a role. 

The activation of CO is one of the critical elementary steps that controls Fischer-Tropsch synthesis in the production of higher hydrocarbons from synthesis gas. It is well established that the Fischer-Tropsch reaction proceeds by activating CO to form surface carbon and oxygen ]. The surface carbon subsequently hydrogenates to form various CH intermediates which can react further with hydrogen, couple with other CHa, fragments and ultimately desorb as different hydrocarbon products. 

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