Geometric control catalysis

By using different templates, different positions on the steroid are attacked. This is a case of enzyme-like geometric control of the site of functionalization rather than the usual control by reactivity seen in chemical reactions. These template-directed halogenation reactions show both catalysis and specificity and are indeed biomimetic (350). 

Control of emissions of CO, VOC, and NOj, is high on the agenda. Heterogeneous catalysis plays a key role and in most cases structured reactors, in particular monoliths, outperform packed beds because of (i) low pressure drop, (ii) flexibility in design for fast reactions, that is, thin catalytic layers with large geometric surface area are optimal, and (iii) attrition resistance [17]. For power plants the large flow... 

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

Succinate esters serve as examples of derivatives that exhibit less than optimal pH-hydrolysis rate behavior owing to their increased reactivity in water as a result of intramolecular catalysis of hydrolysis by the terminal carboxylic acid functionality (Anderson and Taphouse, 1981 Anderson etal., 1984 Damen etal., 2000). Since intramolecular catalytic effects are quite sensitive to geometric factors and distances separating the interactive groups (Anderson and Conradi, 1987), intramolecular catalysis by a terminal ionizable group should be easily controlled by varying the alkyl chain length. 

Pt-alumina is a profoundly subtle and possibly unique system for bifunctional reforming catalysis. The Pt crystallite sizes which gamma alumina can stabilise contain high proportions of sites upon which selective catalysis can take place in addition, the acidity of gamma alumina can be promoted and controlled for selective carbenium chemistry in-situ. Further enhancement of Pt selectivity has been achieved by the addition of a second element, notably Re, Ir and Sn. Both ensemble and electronic models have been put forward to explain the effects, and have been vigorously discussed in the open literature. Geometric effects tend to be preferred for Pt-Re and Pt-Sn, but inevitably in such multivariate systems, there is still room for debate. The commercial success of these catalyst systems depends on their ability... 

As examples of aluminosilicate materials, it is difficult to draw a clear demarcation between zeolites and other ionic materials. Zeolites are unusual in that the relevant catalysis appears to occur within the well-defined cage and channel structure of the material. Their catalytic activity appears primarily to arise because of their Bronsted acidity, while their control over selectivity arises primarily from geometrical considerations of which molecules (and transition states) can be accommodated within the structure. 

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