Nafion pockets

To explain how solid acids such as Nafion-H or HZSM-5 can show remarkable catalytic activity in hydrocarbon transformations, the nature of activation at the acidie sites of such solid acids must be eon-sidered. Nafion-H contains acidic -SO3H groups in clustered pockets. In the acidic zeolite H-ZSM-5 the active Bronsted and Tewis acid sites are in close proximity (—2.5 A). 

The highest level, at structural scales >10 nm, is that over which long-range transport takes place and diffusion depends on the degree of connectivity of the water pockets, which involves the concept of percolation. The observed decrease in water permeation with decreasing water volume fraction is more pronounced in sulfonated poly(ether ketone) than in Nafion, owing to differences in the state of percolation. Proton conductivity decreases in the same order, as well. 

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,... 

As in the case of the ZSM-5 zeolite sample [160], in substrate in Nafion-DCA in CH2C12, due to the isolation of the sensitizer in solution from the substrate in Nafion, only the products derived from the energy-transfer pathway were detected. The sensitized photo-oxidation of DPB gave 6 as the unique product (Fig. 18), whereas that of TS yielded 10 and 13 (Fig. 19). The oxidation products of TS with 102 in this mode are the same as in solution sensitized by HA or tetraphenylporphyrin, but different from that in ZSM-5 zeolite, where benzaldehyde 1 is the unique product. This observation suggests that the constrained space in the water-containing pocket of Nafion membranes can accommodate the molecules of 10 and 13. In contrast, irradiation of the water-swollen... 

FIGURE 34.25 SEM-XRF dot-map of Na in a cross-section of (a) Nafion 117 and (b) lonac MC 3470. It may be noted that the charged sites are more uniformly distributed in the homogeneous membrane Nafion 117, while the heterogeneous membrane lonac MC 3470 has nonconducting pockets shown by dark space. 

The fifth type of enzyme immobilization is encapsulation in an already formed polymer. An example of this is to utilize alcoholic solutions to swell the micellar pockets of hydrophobically modified Nafion and then to add enzyme and dehydrate the polymer. This encapsulates the enzyme within the polymer and provides a structural entrapment without covalent binding. This technique can tailor the polymer to provide added stabilization to the enzyme, but these polymers typically retard transport of the substrate/product in and out of the film. 

Petersen, M. K., Wang, R, Blake, N. P., Metiu, H., and Voth, G. A. 2005. Excess proton solvation and delocalization in a hydrophilic pocket of the proton conducting polymer membrane Nafion. 109(9), 3727-3730. 

Alternatively, methods of enzyme encapsulation can provide a means to stabilize proteins in a protective environment by either trapping the protein, wiring the protein to a polymer backbone, or specifically depositing enzymes within micellar pockets [29-32]. Enzymes immobilized within the pores of hydrophobically modified micellar polymers such as Nafion and chitosan, for example, have been shown to effectively stabilize enzymes at electrode surfaces and promote operation lifetimes of more than 2 years [29]. 

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