Hydrocarbon activation quantum-chemical calculations

In zeolites, this barrier is even higher. As discussed in Section II.B, the lower acid strength and the interaction between the zeolitic oxygen atoms and the hydrocarbon fragments lead to the formation of alkoxides rather than carbenium ions. Thus, extra energy is needed to transform these esters into carbonium ionlike transition states. Quantum-chemical calculations of hydride transfer between C2-C4 adsorbed alkenes and free alkanes on clusters representing zeolitic acid sites led to activation energies of approximately 200 kJ/mol for isobutane/tert-butoxide (29), 230-305 kJ/mol for propane/sec-propoxide, and 240 kJ/mol for isobutane/tert-butoxide (32), 130-150 kJ/mol for ethane/ethene (63), 95-105 kJ/mol for propane/propene, 88-109 kJ/mol for isobutane/isobutylene, and... 

The discussion of reactivity focused on the activation of hydrocarbons by zeolitic protons. The deprotonation energy of a proton is weakly dependent on the zeolite crystallographic position but may be strongly zeolite composition dependent, especially at high concentrations of three valent cations (Al, Ga) in the zeolite framework. Nonetheless, the deprotonation energy is a local property of the OH bond, which can be estimated using quantum-chemical calculations by extrapolation from properly terminated cluster calculations. 

Hydrocarbon autoxidation takes place via a complex set of radical reactions, some of which were only recently identified. One of the mechanistic difficulties is that the reactions can only be indirectly investigated by monitoring the evolution of stable products. The input of quantum-chemical calculations, in combination with theoretical kinetics, turned out to be a crucial tool to construct a generic mechanism. One of the new insights is the importance of the copropagation of the primary hydroperoxide product. A solvent-cage reaction, activated by the exother-micity of this secondary step, leads to the formation of the desired alcohol and... 

The activation of C-H bonds for different hydrocarbons can occur both at Zn + and ZnOZn + sites. We will first discuss hydrocarbon activation by Zn +. The results presented here are based on quantum-chemical cluster calculations. The reaction energies involved in the overall catalytic cycle for the activation of ethane over a Zn + cation and a ZnOZn " " oxycation adsorbed on a representative cluster chosen to model the ZSM-5 adsorption site are compared in Fig. 4.22. 

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. 

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