Conformational properties of monosubstituted cyclohexane guest molecules constrained within zeolitic host materials. A solid-state NMR investigation

Abstract

The conformational properties of monosubstituted cyclohexane guest molecules (C₆H₁₁X with X = CH₃, OH, Cl, Br and I) included within microporous solid host materials (silicalite-I, H-ZSM-5, NH₄-mordenite and zeolite NH₄-Y) have been elucidated via high-resolution solid-state ¹³C NMR spectroscopy. For all of the inclusion compounds investigated, the fraction of monosubstituted cyclohexane molecules in the equatorial conformation is similar to that in solution, suggesting that these host materials do not impose any significant constraints upon the conformational properties of the monosubstituted cyclohexane guest molecules. For the monohalogenocyclohexane guest molecules (C₆H₁₁X with X = Cl, Br and I), this result is in marked contrast to the situation for the same guest molecules in the thiourea host structure, for which the conformational properties of the guest molecules are substantially different from those of the same molecules in solution. For cyclohexanol (C₆H₁₁OH) in H-ZSM-5, some amount of dicyclohexyl ether (C₆H₁₁OC₆H₁₁) is observed, and is analogous to the proposed production of dimethyl ether in the first stage of methanol-to-gasoline conversion on this zeolite. The comparatively low temperature (ambient temperature) at which this conversion from cyclohexanol to dicyclohexyl ether occurs is noteworthy. In addition to our high-resolution solid-state ¹³C NMR studies of these materials, ¹H MAS and ²⁷AI MAS NMR spectra have also been recorded, and are discussed.