Scott M. Auerbach, Lucy M. Bull, Neil J. Henson, Horia I. Metiu,
and Anthony K. Cheetham,
``Behavior of benzene in Na-X and
Na-Y zeolites: A comparative study by 2H
NMR and molecular mechanics,''
J. Phys. Chem. 100, 5923-5930 (1996).
Abstract
We have performed solid state 2H NMR spin-lattice
relaxation measurements on perdeuterated
benzene in the zeolites
Na-X and Na-Y over the temperature range 155-350 K.
The resulting NMR correlation times
exhibit Arrhenius temperature dependence, with the
following Arrhenius parameters:
tau0(Y) = (9.6 +/- 0.4)x10-13 s,
Ea(Y) = (23.5 +/- 0.9) kJ mol-1,
tau0(X) = (8.8 +/- 2.3)x10-12 s,
Ea(X) = (14.0 +/- 0.6) kJ mol-1.
This significant enhancement of benzene mobility
from Na-Y to Na-X
is reproduced with atomistic molecular
mechanics simulations, yielding
Ea(Y) = 35 kJ mol-1 and
Ea(X) = 15 kJ mol-1, in quite reasonable agreement
with the NMR measurements.
We propose a simple length scale model which accounts
for both the mobility enhancement and its order of
magnitude. We find that the enhancement from Na-Y to Na-X
is due to attractive interactions from SII sites
influencing adjacent SII sites, making
mobility more energetically favorable in Na-X.
Based on the relative magnitudes
of our calculated hopping activation
energies, we predict that while
intracage and intercage benzene mobilities are comparable in Na-X,
intracage mobility is much more rapid than intercage migration
for benzene in Na-Y.
Prof SM Auerbach
28 August 2000