Zeolites confinement effect

The reaction used to test these solid catalysts was the aziridination of styrene with AT-tosyliminophenyliodinane (Phi = NTos) (Scheme 10). In most cases, enantioselectivities were low or moderate (up to 60% ee). The loss of enantioselectivity on changing from ligand 11a to ligand 12 was attributed to the fact that ligand 12 is too big for the copper complex to be accommodated into the zeolite supercages. Further studies carried out with hgands 11a and 11b [62] demonstrated that the reaction is more enantioselective with the supported catalyst (82% ee with 11a and 77% ee with 11b) than in solution (54% ee with 11a and 31% ee with 11b). This trend supports the confinement effect of the zeolite structure on the stereoselectivity of the reaction. 

Shape selectivity and orbital confinement effects are direct results of the physical dimensions of the available space in microscopic vessels and are independent of the chemical composition of nano-vessels. However, the chemical composition in many cases cannot be ignored because in contrast to traditional solution chemistry where reactions occur primarily in a dynamic solvent cage, the majority of reactions in nano-vessels occur in close proximity to a rigid surface of the container (vessel) and can be influenced by the chemical and physical properties of the vessel walls. Consequently, we begin this review with a brief examination of both the shape (structure) and chemical compositions of a unique set of nano-vessels, the zeolites, and then we will move on to examine how the outcome of photochemical reactions can be influenced and controlled in these nanospace environments. 

The majority of intrazeolite photooxygenations have been conducted in NaY,84 85 however, one study in the pentasil zeolite ZSM-5 demonstrates that steric confinement effects can play important roles.84 A comparison of the reactions of a series of tri-substituted alkenes in isooctane, NaY, and in ZSM-5 is given in Fig. 21. The reactions... 

Marquez, F., Marti, V., Palomares, E., Garcia, H. and Adam, W. (2002). Observation of azo chromophore fluorescence and phosphorescence emissions from DBH by applying exclusively the orbital confinement effect in siliceous zeolites devoid of charge-balancing cations. J. Am. Chem. Soc. 124, 7264-7265... 

The regioselectivity for the hydrogen atom abstraction from each of two geminal methyl groups (twin or twix) in trisubstituted alkenes such as 34, 35, 39 and 122 (Table 17) was studied by specific deuterium labeling. Independent studies revealed that the cis effect selectivity found in solution no longer operates within the zeolite. As seen in Table 19, for the case of 124-127, the twin methyl group reactivity increases up to 14 times (see substrate 126) by zeolite confinement. 

On HZSM5 both hydroxyacetophenones are formed by trans-acylation. Disproportionation (reaction e) probably does not exist because of steric contraints. Moreover since ortho-hydroxyacetophenone does not react with phenylacetate (probably for the same reason) to give ortho-acetoxyacetophenone, reaction g cannot take place. On the other hand, the formation of products resulting from the oligomerization of ketene (dehydroacetic acid, 6-methyl 4-acetoxy 2-pyrone, reaction h) is favoured presumably because of the confinement effect in the zeolite. These compounds are supposed to be to a large extent responsible for the deactivation of HZSM5. 

Our conclusions support the ideas of Derouane et al. (16) who, by analogy with enzymes, proposed that confinement effects must be important for catalysis with zeolites. 

Competition between reactant, solvent and product molecules for adsorption within the zeolite micropores is demonstrated directly (adsorption experiments) and indirectly (effect of the framework Si/Al ratio on the activity, kinetic studies) to occur during Fine Chemical synthesis over molecular sieve catalysts. This competition, which is specific for molecular sieves (because of confinement effects within their micropores), adds up to the competition which exists over any catalyst for the chemisorption of reactant, solvent and product molecules on the active sites. Both types of competition could affect significantly the activity, stability and selectivity of the zeolite catalysts. Although the relative contributions of these two types of competition cannot be estimated, the large change in the activity of the acidic sites (TOF) with the zeolite polarity seems to indicate that the competition for adsorption within the zeolite micropores often plays the major role. 

The unique properties of zeolites and other micro- or mesoporous solids that may favour their application to fine chemical synthesis are (1) the compatibility between the size and shape of their channels or cavities with the size of the reactants and/or products (generally referred to as molecular shape selectivity) that may direct the reaction away from the thermodynamically favoured route (2) the occurrence of confinement effects increasing the concentration of reactants near the catalytic sites and (3) the ability to tune their catalytic properties (acidic, basic, or other) via various treatments as described in this Volume. 

This concept of zeolites as enzyme mimics was used by Derouane and Vanderveken (59) to explain the selective aromatization of n-hexane on Pt/LTL catalysts confinement effects combined with the unique pore structure of LTL zeolite would be responsible for the fast and selective conversion of n-hexane to benzene. 

A zero order with respect to phenol was found, which can be related to the strong physical adsorption of phenol into the zeolite micropores (confinement effect). This explains the pronounced increase in phenol conversion with the methanol/phenol ratio. This strong retention of phenols and phenolic products within the zeolite micropores is responsible for a large part of the high apparent ratio of secondary reactions and especially of coking , i.e. of formation of heavy products which remain trapped in the zeolite micropores. This fast coking of zeolites is responsible for their rapid deactivation by pore blockage. 

When people consider confinement effects, they consider mainly an increase in the encounter probability inside a single pore and therefore, expect an acceleration of the reaction. Such in-pore acceleration has been quantified by Tachiya and co-workers for diffusion-limited reactions through the so-called confinement factor [see Eq. (11.58) in Ref. 40]. From this treatment, confinement effects are expected to disappear when the reaction radius is less than one tenth of the confinement radius. Considering the reaction radii of radiolytic species, no acceleration by confinement should be expected for pore diameter larger than 10 nm. For smaller pore size, acceleration of the recombination reactions within spurs would be critical in the determination of radiolytic yields in the nanosecond time range. However, the existence of such an acceleration of radiolytic reactions has not been suggested in the nanosecond pulse radiolysis of zeolites and has still to be assessed using picosecond pulse radiolysis. 

The zeolite Y-supported, heterogeneous catalyst (54-CuHY) was also used by Hutchings et al. [61] for the enantioselective carbonyl- and imino-ene reactions. In a carbonyl-ene reaction of methylene cyclopentane with ethyl glyoxylate, the heterogeneous catalyst 54-CuHY exhibited superior enantioselectivity (93% ee) compared to the homogeneous catalyst 54-Cu(OTf)2 (57% ee) (Scheme 2.30), due to the confinement effect of the zeolite pores. 

Since all of the CdS clusters reside in the sodalite cages of the zeolite Y framework, the larger supercages of the structure are still available for absorption of substrate molecules - in this case olefins for photo- oxidation via electron transfer. Colloidal CdS in free solution has been used for such oxidations previously(19) and in a competitive oxidation of styrene and 1,1-diphenylethylene we find that unconfined bulk CdS will effect oxidation in a ratio of 1 2 for these two olefins (irradiation at 365nm). In the zeolite confined system we find however that the ratio becomes 1 1 ie a slight shift in selectivity toward the smaller substrate as may be expected on the basis of size/diffusion effects. From the viewpoint of the enzyme mimic, we have here a system... 

Another way to look at zeolites is to look at them as microscopic catalytic reactors. Selectivity control can be achieved either by shape selectivity (23) or by a confinement effect... 

The interactions of a reagent with a zeolite are governed by the shape-selective properties and the confinement effects previously mentioned. The cavities and pores of a zeolite can be considered as microreactors, containing stabilized active sites, which can be designed to have particular properties. This is comparable to the active site of an enzyme, where the catalytic activity depends on the three-dimensional arrangement of the functional groups on the side-chains of the amino acids of the protein. Several of these zeolite mimics of enzyme action have been developed, and three are worthy of mention here. 

Last but not least, the versatility of zeolites is demonstrated by exchanging the acidic proton with deuteron which enables investigation of interesting mechanisms related to catalysis and by exchanging the proton with transition metal cations, such as Cu(I), and opens new areas of enviromnentaUy friendly organic chemistry. For these reasons, we are including in this chapter acidic-zeolite catalysed reactions from our own work which can be best understood as examples of confinement effects superelectrophilic, Cu(I) catalysed Click chemistry, and specific H/D exchange reactions. 

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