Catalytic cycle ensembles

Figure 13.2 Proposed catalytic cycle for the selective delignification of wood (lignocellulose) fibres with an equilibrated ensemble of POMs. (From Weinstock, I. A. et al., Nature, 414, 191,2001.)...

This proline-catalysed aldol proceeds via initial formation of an enamine with the donor component. This enamine then reacts with the re-face of the acceptor through a closed-transition state, as depicted in Figure 7.4, to give the anti-product as the major diastereomer. The proline also activates the acceptor by hydrogen bonding within this ensemble and thus plays a bifunctional role in the reaction. The resulting iminium ion is then hydrolysed to close the catalytic cycle. 

To describe the catalytic reaction, the catalyst must be included in the catalytic cycle as a participating species. The simplest way to do so is to consider a solid catalyst as an ensemble of single active sites ( ). The transformation from to A2 can be presented as a sequence of elementary steps ... 

This chapter proceeds with a general discussion of the overall catalytic cycle and Sabatier s principle in order to illustrate the comparison of relative kinetic and thermodynamic steps in the overall cycle. This is followed by a fundamental discussion of the intrinsic surface chemistry and the application of transition state theory to the description of the surface reactivity. We discuss the important problem of the pressure and material gap in relating intrinsic rates with overall catalytic behavior and then describe the influence of the tatic reaction environment including promoters, cluster size, support, defects, ensemble, coadsorption and stereochemistry. Lastly, we discuss the transient changes to the surface structure as well as intermediates and their influence on catalytic performance. 

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