An Investigation of the Dynamic and Conformational Properties of Alkylcyclohexanes Absorbed in Siliceous Zeolite ZSM-5

Abstract

The dynamic and conformational behavior of three related alkylcyclohexane molecules, cyclohexane (CH), methylcyclohexane (MCH), and trans-1,4-dimethylcyclohexane (DMCH), loaded within completely siliceous zeolite ZSM-5 has been investigated by variable temperature FT-Raman spectroscopy. Our results indicate that at room temperature CH molecules inside the ZSM-5 framework undergo significant reorientational motions around different axes as well as ring inversion and that the reorientational motions have ceased when the temperature is lowered to 153 K. For MCH and DMCH, in contrast, the major molecular motions are removed upon initial loading into the ZSM-5 framework. At low temperatures, the symmetry of CH inside ZSM-5 clearly deviates from the perfect D3d symmetry of the free molecule. At room temperature, the conformational equilibrium of MCH guest molecules still exists in the ZSM-5 host, but at low temperatures, MCH adopts the equatorial conformation exclusively. Interestingly, for DMCH absorption into ZSM-5 results in an increase in the population of the high-energy, small-volume diaxial conformer. The low-temperature Raman spectra of the ZSM-5 framework have also shown that for CH/ZSM-5 and MCH/ZSM-5 complexes, the zeolite framework is not rigid and undergoes significant thermal motion at room temperature. For DMCH/ZSM-5, the zeolitic framework also undergoes thermal motion, but there is a larger distribution of Si−O−Si angles.