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)]), adsorb ed 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 superposit ion of one solidlike and two liquidlike signals, whereas for iBA[1-d(2)], i t is a superposition of the solidlike and the liquidlike lines. Two liquidl ike 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-O H-Si groups, which are located inside the channels and at channel intersect ions of the zeolite channel system. Being adsorbed on Al-OH-Si groups, thes e two types of alcohol molecules differ in the effective amplitude of libra tion 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). Th e solidlike signal with the observed quadrupole splitting of 38 kHz is assi gned to the alcohol molecules located inside the zeolite channels. These al cohol 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) si milar to 19 degrees. Methyl groups of the alcohol molecules located at chan nel 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, w hereas 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 ch annel than at channel intersections.