Molecular dynamics of iso-butyl alcohol inside zeolite H-ZSM-5 as studied by deuterium solid-state NMR spectroscopy

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 H-2 NMR spectroscopy. At 115-293 K, the H-2 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) similar to 1 x 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) similar to 52 degrees for one of the types, and gamma(0) similar to 72 degrees 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 tau(R) > 4.2 x 10-6 s, and their methyl groups experience small librations with amplitude gamma(0) similar to 19 degrees. Methyl groups of the alcohol molecules located at channel intersections rotate about the CH3-CH axis with correlation time tau(p) (1.4-2.6) x 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) similar to 2 x 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 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.