Reaction Ensemble Monte Carlo Simulation of Xylene Isomerization in Bulk Phases and under Confinement.

The original reaction move for the reaction ensemble Monte Carlo (RxMC) method is adapted 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 ΔGrxn ideal as an input. We compare three methods for computing ΔGrxn ideal : using tabulated enthalpies and entropies of formation, using the harmonic oscillator and rigid rotor approximations and using QM/MM alchemical transformation combined with multistate Bennett acceptance ratio. 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 the zeolite 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.