Reaction diffusion template effects

The propagation and termination rate constants have been determined for many systems (5,12,43-46). In most cases, the propagation rate constant is lower than that for a reaction carried out in the same conditions but without a template. The termination rate constant in the template polymerization is much lower in comparison with the blank reaction. Retarded termination may be caused by retarded segmental diffusion of the template-boimd radicals. This leads to the template increasing the overall rate of polymerization. Much experimental data from the literature confirm this, and it is sometimes called the kinetic template effect. 

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

For the synthesis of enzyme mimics, control experiments are of central importance, especially if the catalytic effects are relatively small and if the precise nature of the catalyst is difficult to establish. This proved to be of special relevance for the imprinting of silica. Among the problems encountered were the lack of reproducibility, side reactions due to impurities, diffusion limited reaction rates and difficulties to attach and to remove the template molecules [39-41]. 

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