Zeolite nanocavities

Kojima, M., Nakajoh, M., Matsubara, C. and Hashimoto, S. (2002). Photooxygenation of aromatic alkenes in zeolite nanocavities. J. Chem, Soc. Perkin Trans. 2, 1894-1901... 

Finally, it is increasingly clear that molecules confined in the zeolitic nanocavities see their electronic properties modified. It has been shown for instance (19) that the dipole moment of acetonitrile increases significantly upon its introduction in the side pockets of MOR compared to the linear channels of the same zeolite. The guest molecule is made more basic and is easily protonated in such a confined environment. Zeolites also act as solid solvents and the anionic framework acts as the conjugate base of the proton thereby stabilizing some charged intermediates along concerted catalytic pathways. 

Bohs, V., Broyer, M., Barbagha, A., Busco, C., Foddanu, G.M., and Ughengo, P. Van der Waals interactions on acidic centers in zeolites nanocavities a calorimetric and computer modelling study. J. Mol. Catal A Chem. 2003, 204-205, 561-569. 

CO is a soft Lewis base which is easily polarized by the electrostatic field generated by the extra-framework alkaline-metal cations located in the MFl zeolite nanocavities. As a consequence, it is reversibly taken up by the surface when put in contact with the activated zeolite [23],... 

CO specpure from either Matheson or Praxair was used as a molecular probe in order to assess the Lewis acidic properties of coordinatively unsaturated (cus) cations either located in the dehydrated zeolite nanocavities as charge-balancing cations, or exposed at the dehydrated surface of oxidic materials. CO is capable of interacting with the cus cations leading to the formation of adducts of different stability according to the chemical nature of the cation. Weak electrostatic adducts are formed on alkaline metal cations, u-coordinated species of intermediate stability on non d/d metal cations, whereas high-stability carbonyl-like species originated by a a-coordination -l-7r-back donation of d electrons are formed on d block metal cations. 

In all cases, the q versus Hads curves are typical of heterogeneous surfaces. In H-BEA several interactions, of different strength, take place on sites of different nature located either within the zeolite nanocavities or at the external surface. 

All interactions taking place simultaneously contributed to the calorimetrically measured heat, and consequently it is hard to single out the energetics of the individual contributions to the interaction. At the Brpnsted Si(OH)+Al sites H2O molecules are either strongly H-bonded or protonated, whereas at the Lewis acidic sites, i.e. the cus framework Al (III) cations, H2O molecules are oxygen-down coordinated [25]. H2O molecules interacted also via H-bonding with Si-OH nests located within the zeolite nanocavities and, more weakly, with (isolated) Si-OH species exposed at the external surface. Aspeciflc interactions generated by confinement [82-84] also contributed to the overall measured heat. 

V. Bohs,M. Broyer, A. Barbaglia, C. Busco,G.M. Foddanu, P. Ugliengo, VanderWaalsinteractions on acidic centres localized in zeolites nanocavities a calorimetric and computer modeling study. J. Mol. Catal. aChem. 204, 561-569 (2003). doi 10.1016/S1381-1169(03)00339-X... 

V. Bolis, A. BarbagUa, M. Broyer, C. Busco, B. Civalleri, P. Ugliengo, Entrapping molecules in zeolites nanocavities a thermodynamic and ab-initio study. Orig. Life Evol. Biosph. 34(1-2), 69-77 (2004). doi 10.1023/B ORIG.0000009829.11244.dl... 

It is nearly twenty five years since the application of Quantum Chemistry to the investigation of the catalytic properties of zeolites, yet it is still in its infancy with regard to the many basic questions relating to chemical changes in cavities of molecular dimensions. The recent emphasis on studies of the chemistry in nanostructures and reactions in nanocavities, is indicative of the importance of this field and its relation to biological systems in which chemical changes occur within the firework of macromolecules. 

Silicalite is a microporous crystalline silica, i.e., an aluminium-free zeolite, belonging to the MFI-type structure and being the Si/Al oo limit of the ZSM-5 zeohte. The nature, population, and acidic properties of the hy-droxylated species (hydroxyl nests) present in the nanocavities of variously prepared defective silicahtes have been characterized by adsorption of NH3 at room temperature, monitored through the combined use of microcalorimetry and IR spectroscopy [196]. It was foimd that a perfect sihcahte sample eidiibits a very low activity towards NH3, confirming the almost complete absence of defects. The energetics of the interaction indicated that the (mild) acidity of silanols increases as far as the extension of the silanol patches increases. 

Femtosecond studies of caged molecules in nanocavities provide direct information on the relationship between time and space domains of molecular relaxation [3]. Therefore, simple and complex molecular systems have been studied using CD s proteins, micelles, pores and zeolites as nanohosts, demonstrating... 

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