Molecular Dynamics of iso-Butyl Alcohol Inside Zeolite H-ZSM-5 as Studied by Deuterium Solid-State NMR Spectroscopy

摘要

The molecular mobility of iso-butyl alcohol, selectively deuterated in the methylene group (iBA[1-d_2]) or in the methyl groups (iBA[3-d_6]), adsorbed on zeolite H-ZSM-5 was studied with ~2H NMR spectroscopy. At 115-293 K, the ~2H NMR line shape for the adsorbed iBA[3-d_6] represents a superposition of one solidlike and two liquidlike signals, whereas for iBA[1-d_2], it is a superposition of the solidlike and the liquidlike lines. Two liquidlike signals are assigned to the alcohol molecules isotropically reorienting with correlation time τ_R ～ 1 * 10~(-6) by jumping among Al-OH-Si groups, which are located inside the channels and at channel intersections of the zeolite channel system. Being adsorbed on Al-OH-Si groups, these two types of alcohol molecules differ in the effective amplitude of libration γ_0 (γ_0 is a libration cone semiangle) of the methyl groups, which is large for both adsorption sites (γ_0 ～ 52°for one of the types, and γ_0 ～ 72°for the other). The solidlike signal with the observed quadrupole splitting of 38 kHz is assigned to the alcohol molecules located inside the zeolite channels. These alcohol molecules reorient with a correlation time τ_R > 4.2 * 10~_(-6) s, and their methyl groups experience small librations with amplitude γ_0 ～ 19°. Methyl groups of the alcohol molecules located at channel intersections rotate about the CH_3-CH axis with correlation time τ_P = (1.4 - 2.6) * 10~(-11) s at 293 K and activation energy E_P = 11-12 kJ/mol, whereas those located inside the channel rotate with correlation time τ_j ～ 2 * 10~(-10) s at 293 K and activation energy E_j = 10.5 kJ/mol. the difference in the rotation rates in attributed to the influence of the walls of the zeolite channel on dynamics of one-axis methyl group rotation, which is expected to be more profound in the confined area of a narrow channel than at channel intersections.