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.