CATION AND LOADING EFFECTS ON MOTION OF C-6 HYDROCARBON GUESTS IN L-ZEOLITES

The motions of the perdeuterated C-6 hydrocarbons benzene, hexane and cyclohexane have been examined in the pores of zeolite L by use of H-2 NMR techniques. The pore dimensions of this particular zeolite framew ork type are such that typically only one or two of these C-6 molecule s can reside in any given pore, providing an opportunity to study the motion of a given molecule in the presence of the constraining walls o f the zeolite, cationic species which coexist in the pore, and at most one other molecule. For the base case, chosen to be K-L zeolite (i.e. exchangeable cations are potassium) with one molecule per unit cell, a study of the H-2 NMR of the perdeuterated guest molecules shows no e vidence of isotropic motion on the time scale of the H-2 NMR experimen t (about 1 mus), presumably as a result of the constraining influence of the pore walls. Internal molecular degrees of freedom are observed, even at relatively low temperatures (about 150 K): C6 spinning for be nzene, C3 spinning for cyclohexane, and rotation about the carbon-carb on bond axes for hexane. In addition, transitions between conformation s of the cyclohexane molecule become sufficiently rapid to influence t he H-2 NMR spectrum for temperatures above 275 K. Libration and site h opping (for benzene) are also observed at higher temperatures. Higher guest densities (two molecules per unit cell) introduce correlation ef fects in the guest motions, and varying the non-framework cations (Li, K+, Cs+) also influences the dynamics of the molecules.