(218f) Reaction Ensemble Monte Carlo Simulations of Xylene Isomerization Under Confinement

Xylene exists as three isomers: meta- (m-), ortho- (o-), and para- (p-). m-Xylene is the most abundant isomer in bulk fluids at
equilibrium, but p-xylene is a valuable feedstock in the production of terephthalic acid. p-Xylene can be
selectively produced using ZSM-5, a zeolite with narrow channels that prevent
m- and o-xylene from escaping interior pores. Using reaction ensemble Monte
Carlo (RxMC), we demonstrate that confining a xylene
fluid to the interior of a carbon nanotube shifts the equilibrium composition
as a function of nanotube diameter.

We adapt the original RxMC
reaction move to align both the position and orientation of inserted product
molecules and deleted reactant molecules. The accuracy and efficiency of this move
is demonstrated for xylene isomerization in vapor, liquid, and supercritical
phases. Classical RxMC requires the ideal gas free
energy of reaction as an input. We compare three methods for computing this
free energy: using tabulated enthalpies and entropies of formation, using the
harmonic oscillator and rigid rotor approximation, and using QM/MM alchemical
transformation combined with the multistate Bennett acceptance ratio (MBAR). We
find that the tabulated free energies of reaction give the best agreement with
experimental equilibrium compositions in bulk fluids. RxMC
simulations in a carbon nanotube with an inner diameter of approximately 6 Å
show that p-xylene becomes the dominant isomer under confinement, an effect
consistent with the production of p-xylene in ZSM-5. We also show that o-xylene
becomes the dominant isomer in nanotubes with an inner diameter of 7-8 Å.
We find that both m- and p-xylene exhibit a loss of rotational entropy in
nanotubes of this diameter, effectively allowing o-xylene to fit into cavities
inaccessible to the other isomers.