Confinement mechanisms, zeolite

The formation of conducting polymers within the confines of zeolite channels has been shown to depend on several factors, including the nature of the cation, reaction pressure and the presence of acid sites. From a detailed study of pyrrole polymerisation it is possible to postulate the following reaction mechanism. Interaction of pyrrole with a Cu site initially gave a radical monomer species, (which presumably became associated with AIO4 sites in the framework) and a Cu site. Further creation of radical polymers leads to a polymerisation reaction in which aromatic polymer chains were formed. As the polymer chain lengthens the energy levels for the n system are lowered and thus oxidation... 

The restriction for a nucleophile to penetrate and react with the confined cation-radical sometimes leads to unexpected results. Comparing the reactions of thianthrene cation-radicals, Ran-gappa and Shine (2006) refer to the zeolite situation. When thianthrene is absorbed by zeolites, either by thermal evaporation or from solution, thianthrene cation-radical is formed. The adsorbed cation-radical is stable in zeolite for a very long time. If isooctane (2,2,4-trimethylpentane) was used as a solvent, tert-butylthianthrene was formed in high yield. The authors noted it is apparent that the solvent underwent rupture, but the mechanism of the reaction remains unsolved. ... 

For molecular mean free paths much less than the mean free diameters of the intercrystalline void space in the zeolite bed, Din, is controlled by the same mechanism as in the gas phase, with a self-diffusion coefficient Dg. Due to the steric confinement, Dimer is reduced with respect to Dg by a tortuosity factor, Tb, with values typically of the order of 2-3. The mean free path can be estimated through the relation... 

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. 

For the acid catalysed conversion of hydrocarbons, the reaction mechanisms in absence of sterical hinderance are rather well understood, so that molecular shape-selective effects exerted by constrained environments can be isolated [8,9]. Shape-selective catalysis is also possible when other than acid functions are confined to the intracrystalline void volumes of zeolite crystals, e.g. metal [10,11], bifunctional [12] and basic functions [13]. Nowadays, catalysis on zeolites with organic substrates containing heteroatoms receives much attention. Molecular shape-selectivity seems to be superimposed on electronic factors determining the selectivities [14,15]. 

Wei and Srivastava [8] reported diffusion of polymers to be several orders of magnitude faster than through the zeolite chaimels (of comparable size) and also report the translocation time through the nanotube scales as N, where N is the number of monomers in a polymer. Gao et al. [9] calculated that a single-stranded DNA will spontaneously insert into the nanotubes from water solutions provided that the nanotube is big enough, attributing the mechanism to van der Waals attraction. Ye H and Hummer studied electrophoretic transport of nucleic acids through 1.5-nm carbon nanotubes. Their simulation showed that without electric field, RNA would remain trapped in the hydrophobic pores. Sorin and Pande [10] recently also demonstrated that confinement inside a nanotube denatures protein helices. 

The size, location, and structure of platinum clusters in H-mordenite were modeled by molecular mechanics energy minimization and molecular dynamics simulation techniques [96G1]. It was suggested that the relative stability of monoatomic platinum sites in aluminosilicate mordenites is related to the specific aluminum insertion in T sites of the framework structure. The structural features of the platinum cluster confined to the 12-ring main channel are almost independent of the total Pt content and strongly dependent upon the surrounding zeolite structural field. 

Both the framework stmcture and the acid/exchange sites of a zeolite support play important roles in the SCR reaction [4, 16]. The openings of the framework structure allow gases such as NOx, NH3, and O2, to enter into the interconnected pores of the zeolite support. The intra-crystalhne pores generate a large surface area for low concentrations of NOx and NH3 to condense on the surface and provide spatial confinement for these molecules to react. The acid sites are directly involved in the SCR reaction. They adsorb NH3 forming ions, which is beheved to be a key step in the SCR reaction mechanism. The exchange sites anchor Cu cations so that Cu ions are atomically dispersed inside the matrix of the zeolite. 

Unusual oxidation states of mononuclear metal complexes can be realized when the complexes are confined in the pores of zeolites or silicoaluminophosphates. The matrix mechanically prevents the thermodynamically favored and kinetically facile disproportionation to higher and lower oxidation states. Using this concept of site isolation, mononuclear [Au"(en)2] complexes (en = 1,2-ethylenediamine)... 

Solvation behavior can be effectively predicted using electronic structure methods coupled with solvation methods, for example, the combination of continuum solvation methods such as COSMO with DFT as implemented in DMoF of Accelrys Materials Studio. An attractive alternative is statistical-mechanical 3D-RISM-KH molecular theory of solvation that predicts, from the first principles, the solvation structure and thermodynamics of solvated macromolecules with full molecular detail at the level of molecular simulation. In particular, this is illustrated here on the adsorption of bitumen fragments on zeolite nanoparticles. Furthermore, we have shown that the self-consistent field combinations of the KS-DFT and the OFE method with 3D-RISM-KH can predict electronic and solvation structure, and properties of various macromolecules in solution in a wide range of solvent composition and thermodynamic conditions. This includes the electronic structure, geometry optimization, reaction modeling with transition states, spectroscopic properties, adsorption strength and arrangement, supramolecular self-assembly,"and other effects for macromolecular systems in pure solvents, solvent mixtures, electrolyte solutions, " ionic liquids, and simple and complex solvents confined in nanoporous materials. Currently, the self-consistent field KS-DFT/3D-RISM-KH multiscale method is available only in the ADF software. 

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