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 #tau#_R approx 1 * 10~(-6) s 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 #gamma#_0 (#gamma#_0 is a libration cone semiangle) of the methyl groups, which is large for both adsorption sites (#gamma#_0 approx 52 deg for the types, and #gamma#_0 approx 72 deg for the other). The solidilike 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 #tau#_R > 4.2 * 10~(-6) s, and their methyl groups experience small librations with amplitude #gamma#_0 approx 19 deg. Methyl groups of the alcohol molecules located at channel intersections rotate about the CH_3-CH axis with correlation time #tau#_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 #tau#_j approx 2 * 10~(-10) s at 293 K and activation energy E_j = 10.5 kJ/mol. The difference in the rotation rates is attributed to the influence of hte 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.