Structural Transformation Studies Of I2 And H₂O Confined In The One-dimensional （1D） Channels Of Aluminophosphate Molecular Sieves

Porous materials with one-dimensional(1D) nanochannels are ideal models to study 1D nanoconfinement systems. The atoms/molecules confined in 1D nanochannels exhibit many new structures and properties, which are different from those observed in the bulk materials. High pressure provides an extreme condition, which can change the distance between atoms and molecules and construct unusual new structures and new phenomena. Combining high pressure technology with nanoconfinement effect and studying on the structures and dynamics of atoms/molecules in nanoconfinement environment are not only important for deep understanding the host-guest interaction in nanoconfinement systems, but also significant for exploring the new structures and properties in confinement environment.Both the representative diatomic I2 and ubiquitous H2 O molecules exhibit rich structural transformation under high pressure or in nanoconfinement. High pressure studies on confined I2 and H2 O are new domains of science and the structural transformations are still unclear. Based on these, we carried out the following studies on confined I2 and H2 O in the channels of aluminophosphate molecular sieve AEL.1. In situ high pressure study has been carried out on I@AEL samples. Upon compression(0~6GPa), the contraction of zeolite framework under high pressure induces the individual iodine molecules in the channels to transform from direction perpendicular to the channel axis to parallel to the channel axis, consequently leading a population increase of iodine chains. Higher pressures distort the zeolite framework seviously and make the iodine chains broken. Polarized Raman measurements and theoretical simulations offer a clear picture of the transformation process of confined iodine molecules in nanochannels upon compression. These results demonstrate that high pressure is an effective method to control iodine molecules and weave long iodine chains and it will provide useful information for the pressure induced structural transformation studies of other diatomic molecules in nanoconfinement environment and provide experimental evidences and theoretical guidances for preparing new 1D nanomaterials.2. In situ low temperature Raman spectra studies have been carried out on I@AEL and I@AFI. Upon cooling down, the individual iodine molecules confined in 1D channels transform from direction perpendicular to the channel axis to parallel to channel axis and leading a population increase of iodine chains. Upon heating up to room temperature, the orientation of confined iodine molecules recover. These suggest that the orientation of iodine molecules can be controlled reversibly by simple cooling and heating process. We thus provide a new way to modulate the orientation of iodine molecules in nanochannels, which may have implications in the development of low-temperature-sensitive nanoscale devices.3. We have performed an infrared spectroscopy study on the hydrogen bonded structures of water in the channels of AlPO4-11(AEL) at ambient condition. It has been found that the confined water molecules exist in the state of water molecules coordinated with framework Al sites, ice-like structure, liquid-like water and low water polymers. The appearance of liquid-like structures is related to the strong confinement in the one-dimensional(1D) elliptical channels offered by the AEL crystals. The unique coordination between water molecules and framework Al sites is responsible for the presence of ice-like structures in the channels above the melting point of ice. We find that both the geometry of the channels and the interaction between the water molecules and the framework are responsible for the structures of confined water. Higher relative humidity of the air does not change the hydrogen bonded structures of confined water, but speeds up water absorption from the air. Our results provide useful information on the hydrogen bonded structures, the vibrational properties of water and the interactions between water molecules and host media in the study of confined water.4. High pressure studies have been carried out on H2O@AEL to explore the structural transformations of confined water. Pressure induced the framework of AEL contract and the confined water transform from ice-like structure to water molecules coordinated with framework Al sites, liquid-like water and low water polymers. It is the first time that we found this unique structural transformation of confined water and it has never been reported in previous papers. When Ar is used as pressure transmitting medium(PTM), the penetrating Ar atoms disrupt the hydrogen bonded structures of confined water, consequently facilitate structural transformation and weaken the supporting effect of confined water to the framework. At the same time, the existence of Ar atoms make the distance between hydrogen atoms of water and oxygen atoms of the framework decrease and attractive interaction between water molecules and the framework increase. This is responsible for the higher compressibility of the AEL framework when Ar is used as PTM. Our results are of great importance for deep understanding on interaction between guest molecules and host media in confinement systems and expanding the potential applications of porous materials.