Nanoencapsulation

Catalyst nanoencapsulation is an excellent fit to the concepts of green chemistry [2] in the area of process intensification - enabling incompatible catalysts to function in the same reactor, thereby achieving what otherwise simply cannot be done. 

Although the scope of this chapter is limited to catalyst nanoencapsulation for the purpose of process intensification, we take a broad view of the definition of nanoencapsulation. The capsule or catalyst, or both, may be on the nanoscale. Additionally, the various methods of nanoencapsulation may be of the order of up to a few microns. 

In this stoichiometric forerunner, the use of a polymeric support demonstrated the concept of using an immobilization method to prevent reagents from reacting with each other in an undesired manner, permitting a reaction to occur that is not normally possible. By analogy, there are possibilities where various immobilization methods (in this case, we are interested in nanoencapsulation methods) are used to enable two incompatible catalysts to work concomitantly in an otherwise impossible reaction. 

Cascade Reactions with Incompatible Catalysts and Nanoencapsulation... 

The use of an enzyme in a cascade using nanoencapsulation has also been demonstrated [23]. In this case, the dynamic kinetic resolution (DKR) of secondary alcohols was achieved with an acidic zeolite and an incompatible enzyme, Candida antarctica lipase B (CALB) (Scheme 5.8). 

To the best of the authors knowledge, these examples comprise most of the literature that fits the criteria of cascade reactions with incompatible catalysts achieved with nanoencapsulation. However, there are many more examples where cascade reactions have been achieved with incompatible catalysts, but without the use... 

One of the drawbacks with such a macroscopic system is the increased time for the diffusion of molecules relative to that in nanoscale systems. Molecules will clearly take longer to pass through thick barriers and to diffuse great distances than in the nanoscale regime. Therefore, the nanoencapsulation of such systems is desirable as it potentially reduces these limitations very significantly. Our attention now proceeds to various potential methods for nanoencapsulation. 

Similar structures using a silica-in-carbon core-shell structure have also been synthesized [76], which afford new possibilities for nanoencapsulation. Carbon can be removed by calcination, leaving silica, and (if the positions of silica and carbon are reversed) a carbon shell can be created using NH4OH solution to dissolve the silica. The formation of silica microparticles using silanol-functionalized polystyrene latexes proceeds along similar lines (Scheme 5.16) [77]. 

Although the systems described here have not been used for nanoencapsulated cascade reactions, or of course, for mutually incompatible catalysts, they offer an attractive possibility for the extension of this field, especially given the availability of a wide range of protein-based nanometer-sized cages, such as chaperonins, DNA binding proteins, and the extensive class of viruses [107]. 

The use of multiple otherwise incompatible catalysts allows multistep reactions to proceed in one reaction vessel, providing many potential benefits. In this chapter, literature examples of nanoencapsulation for the purpose of process intensification have been discussed comprehensively. Current efforts in the literature are mostly concentrated in the areas of LbL template-based nanoencapsulation and sol-gel immobilization. Other cascade reactions (without the use of nanoencapsulation) that allow the use of incompatible catalysts were also examined and showcased as potential targets for nanoencapsulation approaches. Finally, different methods for nanoencapsulation were investigated, thereby suggesting potential ways forward for cascade reactions that use incompatible catalysts, solvent systems, or simply incompatible reaction conditions. 

Nonetheless, a wide variety of potential methods are available to achieve the goal of nanoencapsulation for the purpose of facilitating the use of two or more incompatible catalysts in cascade reactions. The many multistep reactions that are of importance in the fine chemicals industry are prime targets for the application of the principles of nanoencapsulation and, therefore, of green chemistry. 

Pinto Reis, C., Neufeld, R.J., Ribeiro, A.J., Veiga, F. and Nanoencapsulation., I. (2006) Methods for preparation of drug loaded polymeric nanopartides. Nanomedicine Nanotechnology, Biology, and Medicine, 2, 8—21. 

Krol S, Del Guerra S, Gmpillo M et al (2006) Multilayer nanoencapsulation. New approach for immune protection of human pancreatic islets. Nano Lett 6 1933-1939... 

Ng V, PI G, Ss S-C et al (2007) Nanoencapsulation of stem cells within polyelectrolyte multilayer shells. Macromol Biosci 7 877-882... 

D. Trau, W. Yang, M. Seydack, F. Caruso, N.T. Yu, and R. Renneberg, Nanoencapsulated microcrystalline particles for superamplified biochemical assays. Anal. Chem. 74, 5480-5486 (2002). 

Lubbers DW, Opitz N, Speiser PP, Bisson HJ (1977) Nanoencapsulated fluorescence indicator molecules measuring pH and p02 down to submicroscopical regions on the basis of the optode principle. Z Naturforsch 32 133-134... 

FIGURE 8.16 Representation of a biomolecule nanoencapsulated in the silica network. (Reprinted from M. Kato et al., J. Sep. ScL, 28 1893 (2005). With permission. Copyright Wiley-VCH 2005.)... 

Fig. 14.1 Nanobiocatalysis publications in ISl Web of Science by year (1995-2009). Research articles, proceedings and reviews, Search in title, abstract and keywords for enzyme or biocatalysts and nanostructured or nanosized or nanoscale or nanoporous or nanoencapsulation or nanoparticle or nanotube or nanocluster or nanofibre or nanotechnolog or nanocolloid or nanocapsule or nanoreactor or nanoliposome, Search hke in excluding biosens or sensor or electrode or detect or electrochem or chemiluminescence or assay in title, abstract and keywords...

Ong KK, Dong H et al. (2003) Nanoencapsulation of OPAA with mesoporous materials for chemical agent decontamination in organic solvents. DoD Joint service scientific conference on chem bio defense research. Hunt Valley (MD)... 

Macro-coating is used mainly to stabilise fragrances or transform them from liquid to free-flowing solid powder. Microencapsulation or nanoencapsulation is the process of enclosing a substance inside a miniature capsule. These capsules are referred to as microcapsules or nanocapsules. The substance inside the capsule can be a gas, liquid or solid. The capsule wall can consist of various materials, such a wax, plastic or biopolymers like proteins or polysaccharides. 

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