Accessibility of the Active Sites

High surface areas are normally obtained by using porous materials, and the pore sizes may condition the accessibility of the reactants to the active sites, especially in the case of microporous materials such as activated carbons. Pore diffusion limitations become more important as the pore sizes decrease in addition, the smaller pores may be more easily blocked (e.g., by coke deposition). Therefore, deactivation and diffusion phenomena will in general affect more strongly the performance of microporous carbons. As a result, there has been a drive to develop mesoporous carbon catalysts (such as aerogels, xerogels, and templated carbons) for some applications, especially in the liquid phase. 

Nevertheless, the presence of small micropores (ultra-micropores, i.e., pores smaller than 0.7 nm) may be useful in some cases, due to their ability to retain reaction products in the liquid state such an example is discussed in Section 6.3.3. 

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Another possible explanation for the limitations of catalytic antibodies raised against TSA can be found in the different accessibility of the active site. In the case of natural enzymes, it is that their catalytic machinery and bound substrates are often buried. This feature isolates from the solvent the reactive functionalities that mediate chemical transformations. On the contrary, in antibody catalysis, the moieties of the bound haptens that mimic the TS are often positioned near the entrance of the antibody-combining site. This disparity in the overall architecture of natural enzymes and catalytic antibodies is undoubtedly a factor in the lower catalytic... 

Electron transfer from the chiral catecholamines to iron(III) in the FeTL or FeTD system proceeds stereoselectively only when the polypeptide matrices are predominantly in the a-helical conformation and the accessibility of the active sites is, at least partially, hindered. 

Two significant drawbacks of Nafion-H in catalytic applications are its very low surface area (0.02 m2 g ) and the hindered accessibility of the active sites (sulfonic acid groups) located inside the pockets of the polymeric backbone. Consequently, the specific activity of Nafion, namely, the number of reacting molecules per unit weight,... 

Friedel-Crafts alkylation of benzene,220 221 toluene,222para-xylene,220 and naphthalene223 with benzyl alcohols have been studied over Nafion-silica nano-composite catalysts, including the kinetics of alkylation.221,223 In most cases, 13% Nafion-silica showed the highest activity, testifying again to the much higher accessibility of the active sites. Complete conversion of para-xylene was found in the presence of triflic acid, and it was the only reaction when ether formation as side reaction did not occur. 

Again, the activity of Nafion-H in the above protection processes (transformation of alcohols to THP ethers672 and methoxymethyl ethers679) is lower, and consequently, longer reaction times and elevated temperatures are required to achieve yields comparable to those over Nafion nanocomposites. Obviously, this is due to the low specific surface area and low accessibility of the active sites of Nafion-H as compared with Nafion SAC-13. 

While comparing the pseudo-first order constant for the same polymers with the relative non-imprinted polymers ( MIP/ NIP),the rate increase dropped to 24 for carbonate and 11 for carbamate. The corresponding bulk polymers showed better results since the rate enhancement, due to the imprinted polymer, was 588 for the carbonate hydrolysis and 1,435 for the carbamate. However, when comparing the imprinted with the non-imprinted bulk, the ratio dropped to 10 for carbonate and 5.8 for carbamate, suggesting that the higher selectivity showed by the beads could be due to an enhanced accessibility of the active sites compared to the bulk. 

In the area of catalyst architecture, the effect of an enhanced accessibility of the active sites by the larger resid molecules, can significantly improve both conversion and bottoms cracking. 

Preparation of the apofiavoprotein (1) and its reconstitution with and N-enriched flavins and subsequent NMR analysis yields information about the 7t-electron density of the atoms of the isoalloxazine ring in the different redox states (47). Reconstitution with chemically modified ( artificial ) flavins can be of help in substrate structure-activity relationship studies (23) and provide information about the solvent accessibility of the active site (48). 

Although static and dynamic disorder had been detected for other enzyme substrate systems before [10-12], one could argue that its observation in this case is an artifact of the way, how the experiments were performed. The nonspecific immobilization procedure might result in static disorder. Enzymes with different orientations on the surface might possess a different lid mobility and accessibility of the active site and, as a result, show different activities. And the use of the highly unnatural substrate might be a possible source of dynamic disorder. An alternative detection scheme for this class of enzymes, which solves these shortcomings, will be presented in Sect. 25.3. 

A comparison between phosphinated polystyrene and phosphinated silica catalysts used in hydrogenation [8, 38] and hydrosilylation [72] reactions indicated, in either case, a higher reaction rate for the silica-supported analogs. This was expected and has been attributed to the better accessibility of the active sites on the silica surface. 

Increase the accessibility of the active sites, and thus the number of accessible sites per catalyst weight ... 

To overcome the disadvantage of the low surface area (< 0.02 m g ) of pure Nafion NR50 beads, researchers at DuPont developed Nafion-silica composites, in which small (20-60 nm) Nafion resin particles are embedded in a porous silica matrix [7]. The composites, available under the trade name SAC 13 (containing 13 % (w/w) Nafion) are prepared by a sol-gel technique. Because of the higher surface area and accessibility of the active sites the application of this material as a solid-acid catalyst has become attractive. 

Surface area and its accessibility are important both in catalysis and gas cleanup. Nano-structured micro-porous catalysts or catalyst supports offer intensified catalysis since they provide an enhanced surface area which is accessible to the reactants and products through a network of arterial channels feeding into the regions of catalytic activity. In non-structured catalysts, although the surface area might be large, as determined by gas adsorption, they are often not accessible as a result of surface fouling and the diffusion resistance can slow down the rates of reactions. Catalysts are either deposited as a thin film on a support or they are used as pellets. These two techniques have certain drawbacks in coated systems, catalyst adhesion can be non-uniform and weak while the accessibility of the active sites within the interior of the catalyst is hindered due to low porosity. 

While retentions of the moleailarly enlarged systems of up to 99.75% were observed, the formation of insoluble purple species occurred under continuous-flow conditions. Addition of [Bu4N]Br prevented catalyst precipitation but a fast decrease in the conversion was detected. After 45 cycles, the activity of the catalyst dropped to almost zero, while the retention of the catalyst under the applied conditions was 98.6% (Figure 3). The authors state that the main decrease in activity was due to formation of inactive Ni(III) species. Furthermore, the carbosdane support plays a pivotal role in the accessibility of the active sites surface congestion can lead to the formation of mixed-valence Ni(II)/Ni(III) complexes on the dendrimer periphery that compete for reactions with substrate radicals. 

Lower catalytic activity than homogeneous catalysts because of poor accessibility of the active sites for the substrate, steric effects of the matrix, incompatibility of solvent and polymer, deactivation of active centers. 

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