Within zeolites

Encapsulation of [Co(bpy)3]2+ within zeolite frameworks has also been shown to have a remarkable influence on the electronic spin state of the complex.240 Distortions imparted on the tris-chelate complex by the confines of the zeolite supercage are found to be responsible for stabilizing the unusual low-spin electronic ground state.241,242 The [Co(bpy)3]3+/2+ couple has been measured for the encapsulated complex and it has been found that the complexes remain within the zeolite and do not exchange with the bulk solution.243 Electrochemistry of [Co(bpy)3]3+/2+ immobilized within a sol-gel has also been studied.244... 

Common to all encapsulation methods is the provision for the passage of reagents and products through or past the walls of the compartment. In zeolites and mesoporous materials, this is enabled by their open porous structure. It is not surprising, then, that porous silica has been used as a material for encapsulation processes, which has already been seen in LbL methods [43], Moreover, ship-in-a-bottle approaches have been well documented, whereby the encapsulation of individual molecules, molecular clusters, and small metal particles is achieved within zeolites [67]. There is a wealth of literature on the immobilization of catalysts on silica or other inorganic materials [68-72], but this is beyond the scope of this chapter. However, these methods potentially provide another method to avoid a situation where one catalyst interferes with another, or to allow the use of a catalyst in a system limited by the reaction conditions. For example, the increased stability of a catalyst may allow a reaction to run at a desired higher temperature, or allow for the use of an otherwise insoluble catalyst [73]. 

Alvaro, M., Ferrer, B., Fornes, V., Garcia, H. and Scaiano, J.C. (2002). Bipyridinium macroring encapsulated within zeolite Y supercages. Preparation and intrazeolitic photochemistry of a common electron acceptor component of rotaxanes and catenanes. J. Phys. Chem. B 106, 6815-6820... 

Corma, A., Fornes, V., Garcia, H., Miranda, M.A., Primo, J. and Sabater, M.-J. (1994). Photoinduced electron transfer within zeolite cavities eiv-Stilbcnc isomerization photosensitized by 2,4,6-triphenylpyrylium cation imprisoned inside zeolite Y. J. Am. Chem. Soc. 116, 2276-2280... 

Shailaja, J., Sivaguru, J., Robbins, R.J., Ramamurthy, V., Sunoj, R.B. and Chandrasekhar, J. (2000). Singlet oxygen mediated oxidation of olefins within zeolites selectivity and complexities. Tetrahedron 56, 6927-6943... 

Kupfer, R., Poliks, M.D. and Brinker, U.H. (1994). Carbenes in constrained systems. 2. First carbene reactions within zeolites-solid state photolysis of adamantane-2-spiro-3 -diazirine. J. Am. Chem. Soc. 116, 7393-7398... 

Li, X. and Ramamurthy, Y. (1996). Electron transfer reactions within zeolites photooxidations of stilbenes. Tetrahedron Lett. 37, 5235-5238... 

Robbins, R.J. and Ramamurthy, V. (1997). Generation and reactivity of singlet oxygen within zeolites remarkable control of hydroperoxidation of alkenes. J. Chem. Soc. Chem. Commun. 1071-1072... 

Yoon, K.B. (1993). Electron- and charge-transfer reactions within zeolites. Chem. Rev. 93, 321-339... 

The resultant cycloalkenyl carbenium ions, especially the cyclopentenyl cations, are very stable (103,104) and can even be observed as free cations in zeolites 105,106). These ions can oligomerize further and, within zeolites, irreversibly block the acidic hydroxyl groups. With liquid acids, the oligomers will dilute the acid and thus lower its acid strength. 

Figure 29. Two modes of interaction of CHf within zeolite structure, leading to H-H exchange (left) and dehydrogenation (right). 

Supramolecular concepts involved in the size- and shape-selective aspects of the channels and cavities of zeolites are used to control the selectivity of reactions of species produced by photoexcitation of molecules encapsulated within zeolites. The photochemistry of ketones in zeolites has been extensively studied. Photoexcitation of ketones adsorbed on zeolites at room temperature produces radical species by the Norrish type 1 reaction. A geminate (born together) radical pair is initially produced by photolysis of the ketone, and the control of the reaction products of such radicals is determined by the initial supramolecular structure... 

Recently, the PER of phenyl acetate and phenyl benzoate has been studied within zeolites of the faujasite and pentasil families, which are structurally very different. [277,278] It was found that for phenyl acetatet in faujasites X and Y, the predominating product is the ortho isomer, whereas in the pentasils ZSM-5 and ZSM-11, the preferred product is the para isomer. These differences have been explained taking into account the different sizes and shapes of the cavities and channels of the two families. The faujasites are made up of channels of 8 A diameter, leading to supercages of 13 A diameter, where the reaction is expected to occur, whereas the pentasils are made up of narrow channels of 6 A diameter. In the X and Y faujasites, the ortho position would be favored due to the restricted mobility, whereas in the channels of ZSM-5 and ZSM-11, there would be a shape limitation because the ortho isomer does not fit but the para isomer does (Table 15). With phenyl benzoate inside X or Y zeolites, the same... 

Chapter 6, by Clennan, on oxygenation in zeolites critically summarizes the results on oxidation of organic molecules within zeolites by singlet oxygen and superoxide anion. This chapter complements Chapter 6, Volume 5 by Va-senko and Frei. In addition to Chapter 6, Chapter 7 in this volume and Chapters 3, 4, and 5 in Volume 5 are exceptional resources in the area of excited-state behavior of organic molecules within zeolites. 

In Chapter 7, Maas et al., summarize their elegant work on the photophysics of dye molecules included within zeolites. This chapter illustrates how a marriage between photophysics and materials science could lead to useful chemistry. 

A similar reservation applies to the need to remove ligand precursors or byproducts from the support. This is a particular issue in relation to intrazeolite com-plexation reactions in which the corresponding homogeneous synthesis is less than quantitative. Another difficulty relates to estabUshing the efficacy of ligand synthesis within a zeoUte. Thus, for example, although several reports of the syntheses of metal porphyrin complexes within zeolites have appeared, the ligands are, in fact, domed porphodimethenes (diameter -14 A), or protoporphyrins [138,... 

A variant on the ship-in-a-bottle approaches described above is to react a preformed catalyst with the zeolite framework. This is analogous to procedures developed for immobilizing organometallic and coordination compounds on surfaces, but occurs within zeolite pores. These compounds that have reacted with the external surface must be removed, along with those adsorbed on the surface, to produce a true single-site catalyst. Two examples, illustrating different synthetic approaches, are discussed below. 

TABLE 24. Regioselectivity and diastereoselectivity in the photooxygenation of chiral alkenes within zeolite Na-Y and in solution (values in parentheses)... 

The study of sorption of guests within zeolite hosts is complementary to the study of diffusion in zeolites. Having discussed the pathways and trajectories of molecules through micropores, we now consider the favored sorption locations, conformations of sorbates, and sorption energetics. Indeed, so close are the two subjects that they are frequently considered within the same paper. 

As was the case for diffusion calculations, tremendous advances have been made recently in the simulation of the sorption locations, energetics, and conformations of adsorbates within zeolites. As far as the prediction... 

In this review, we focus on cluster models of Br0nsted acid sites, bridging hydroxyl groups that result from the incorporation of trivalent aluminum atoms into the siliceous framework during synthesis. These sites are by no means the only active sites within zeolites, but they are among the best characterized. 

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