Grand Canonical Monte Carlo (GCMC) simulation
All computation simulations were performed using Sorption module in
Materials Studio 7.0 package (Accelrys Inc., USA). The adsorption of
1-hexene on different ion-exchanged zeolite X structures were calculated
based on the Grand Canonical Monte Carlo (GCMC) and configurational bias
methods with periodic boundary conditions.36
The parent FAU framework topology was obtained from the Materials Studio
database. According to the compositions of synthesized samples
determined by EDS analyses (see Table S1), the Al atoms were randomly
replaced by Si atoms, automatically following the Lowenstein’s Al-O-Al
avoidance rule.37 In addition, the corresponding
extra-framework ion distribution was conducted by using the Locate Task
in Sorption module,38,39 which can avoid the choice of
ion amount at a specific position caused by the exchange of ion
species.40 The structural models of ion-exchanged
zeolite X samples were enabled by replacing n Na+cations with n/x Mx+ ones in unit cell based on the
found compositions. Moreover, the extra-framework ions incorporated in
the zeolite structures can be Na+,
Mx+, or mixed cations. The Cartesian and fractional
atomic positions were fixed during the simulation to restrict the
adsorption in a specific region.
The 1-hexene module was built by a united-atom description, in which
each CH3 (sp3), CH2(sp3), CH (sp2), and
CH2 (sp2) group was treated as a
single interaction center with effective potential
parameters.41 Table S2 lists the Lennard-Jones 12-6
potential parameters of 1-hexene groups and partial charges of the
zeolite X framework system we used.41,42 The
Lennard-Jones potential parameters for zeolite X was obtained using the
COMPASS force field,43 which has a wide application in
prediction the covalent molecules adsorption on FAU
zeolite.44,45 During simulation, all bond lengths were
considered to be rigid, while bond angles were allowed to bend.
Interactions among different sites were computed based on the standard
Lorentz-Berthelot combining rule, and all interactions were cut off at a
radius of 12.5 Å and a cubic spline truncation of 1 Å width. The Ewald
summation method was used to handle the electrostatic interactions
between guest and host atoms with accuracy of 0.0001 kcal/mol. The guest
– host potential energy and the density field of the guest molecules
were sampled with 25 points between two evaluations of the field data on
a three-dimensional grid of 0.25 Å spacing.
GCMC simulation was employed to calculate the adsorption capacity and
van der Waals interaction energy. The temperature (T ), volume
(V ) and chemical potential (μ ) of the system were fixed,
while the system energy (E ), pressure (P ) and total number
of molecules (N ) varied. For each adsorption calculation point,
the computational process was equilibrated during 5,000,000 steps and
followed by 5,000,000 production steps for data collection. In order to
validate the experimental results, the predicated simulation data were
converted from absolute values to excess adsorption properties.