Journal of Physical Chemistry B, Vol.107, No.31, 7647-7653, 2003
An FT-Raman spectroscopic study of the conformational properties of chlorocyclohexane in zeolites
The conformational behavior of chlorocyclohexane (ClCH) adsorbed inside several representative zeolites such as completely siliceous ZSM-5, Na-Y, and siliceous Y (Si-Y) has been investigated by FT-Raman spectroscopy. The results have clearly shown that the conformational and dynamic properties of ClCH depend on the zeolitic framework structure, the presence of charge-balancing cations, as well as the Si/Al ratio. Compared to pure liquid, the population of equatorial conformation increases upon adsorption into ZSM-5. Apparently, the ZSM-5 framework stabilizes the equatorial conformation (with a larger molecular volume) via maximizing the van der Waals interactions between the host and guest. Within the ZSM-5 framework, the axial and equatorial conformers are still in dynamic equilibrium. It continues to shift toward the equatorial configuration upon lowering the temperature, and at 153 K the ClCH in ZSM-5 adopts the equatorial conformation exclusively. For the ClCH/Na-Y system, the situation is remarkably different. Upon incorporation of ClCH into Na-Y, the conformation of ClCH is immediately frozen as a result of very strong interaction with Na+ ions. The sorbate-cation interactions also lock the ClCH molecules into different orientations which vary from cage to cage, yielding a static disorder. However, if ClCH molecules are adsorbed in a Si-Y where the cation-sorbate interactions are lacking, the dynamic equilibrium between axial and equatorial conformation can be resumed. The relative population of equatorial conformer in Si-Y is higher than that in ZSM-5. The slightly larger equatorial concentration is attributed to the orderly packing of ClCH inside the supercage of Si-Y.