An FT-Raman spectroscopic study of the conformational behavior of trans-1,4-dichlorocyclohexane adsorbed in zeolites

We have examined the conformational properties of trans-1,4-dichlorocyclohexane (DCC) adsorbed in several zeolites, including completely siliceous ZSM-5, Na-Y and siliceous Y, by using FT-Raman spectroscopy. The results indicate that the conformational behavior of DCC is dependent on the zeolite framework structure, Si/Al ratio, and the charge-balancing cations. The conformational behavior of DCC in ZSM-5 differs from that in nonpolar solvents such as carbon tetrachloride in that the population of diequatorial (ee) conformer is significantly increased relative to the diaxial (aa) form upon adsorption. The ee and aa conformers are still in dynamic equilibrium as in solution, and lowering the temperature shifts the equilibrium further toward the ee configuration. The situation for DCC incorporated inside Na-Y is markedly different. Upon adsorption, the conformation of DCC is frozen by interactions with the extraframework sodium ions in the supercages of Na-Y. These interactions also lock the guest molecules into various orientations varying from cage to cage, resulting in static disorder. The dynamic conformation equilibrium can be reestablished inside the supercages of zeolite Y by removing the Na+ ions by using type Y zeolites with a much higher Si content.