Dynamic structures of iodine molecules accommodated in nanopores of zeolite crystals
by Hu Juanmei
Ph.D. Nano Science and Technology
xv, 109 p. : ill. (some col.) ; 30 cm
Zeolite crystals with uniform nanopores and large fraction of pore volume are promising materials for the rearrangement of incorporated species. In this thesis, AEL and AFI zeolite crystals are used as templates to guide the incorporated neutral iodine molecules....[ Read more ]
Zeolite crystals with uniform nanopores and large fraction of pore volume are promising materials for the rearrangement of incorporated species. In this thesis, AEL and AFI zeolite crystals are used as templates to guide the incorporated neutral iodine molecules.
The structures and dynamics of the iodine molecules inside the channels of AEL single crystals are studied by polarized Raman spectroscopy and molecular dynamics simulations. It shows that the iodine molecules inside the elliptical channels of AEL crystals are restricted to two orientations, either lying along the channel direction or standing along the major axes of the elliptical cross-sections of the channels. These configurations are attributed to both geometry and size match between the iodine molecules and the AEL channels, which not only have elliptical shapes but their size alternates with a period similar to the size of the iodine atoms. The two configurations of the iodine molecules are exchangeable by overcoming a rotational barrier. In addition, the structures of the iodine molecules are loading dependent. With the increase of iodine loading, the iodine molecules change from isolated single molecules to one-dimensional chains and ribbon sheet structures. In the ribbon sheets, the iodine molecules iodine molecules maintain their standing configurations from several picoseconds to tens of picoseconds with uniform distribution.
At low loading, the orientation preference of the iodine molecule is highly affected by the density of water molecules adsorbed inside the AEL channels. In well-hydrated AEL crystals, all the iodine molecules can self-assemble to stand along the major axes of the elliptical cross-sections of the channels; however, the iodine molecules rotate to lie along the channels after driven out the water molecules. This effect has been demonstrated with polarized Raman spectra by controlling the air pressure and temperature which are used to control the degree of hydration. The experimental results are well reproduced by MD simulations with different density of water molecules. It is the special confinement and the mildly hydrophilic property of the AEL channels that make the orientation of iodine molecules modulated. At last, we succeed to reversibly control the orientation of the iodine by a focused laser spot with different intensity.
AFI crystals, which have the same composition as AEL crystals but circular channels, are employed to study the effect of zeolite architectures on the structures and dynamics of the adsorbed iodine molecules. The polarized Raman spectra show that the iodine molecules in the AFI channels are almost orientation isotropic. MD simulations demonstrate that due to the circular shape of the channels and the reduced confinement, the iodine molecules are distributed close to the channel walls instead of the center of the channels and the distribution is not as regular as that in AEL channels. The vibrational and rotational analysis further validates that the iodine molecules inside AFI channels can rotate continuously and the special dynamic properties of the iodine molecules inside AEL channels disappear. The comparison of the iodine molecules inside AFI and AEL channels implies that the geometry correlation plays a key role in determining the properties of adsorbates in confined states.