Structural Investigation of Silicalite-I Loaded with n-Hexane by X-ray Diffraction, 29Si MAS NMR, and Molecular Modeling

The room temperature (298 K) structure of zeolite Silicalite-I loaded with approximately eight n-hexane molecules per unit cell was solved from twinned single-crystal X-ray diffraction (XRD) data in the monoclinic space group P121/n1 with a = 19.8247(2) Å, b = 20.1292(2) Å, c = 13. 4510(2) Å, and β = 90.29(8)°. At this temperature, the guest molecules are dynamically disordered and distributed throughout the entire channel system. The structure determined from a Rietveld refinement of room-temperature powder XRD data, which is not affected by the twinning, confirmed this. A twinned crystal refinement was also carried out for data collected at 180 K (P121/n1, a = 19.9310(2) Å, b = 20.1730(3) Å, c = 13.4191(3) Å, β = 90.20(5)°). At 180 K, the sorption sites of the n-hexane molecules are well-defined within the channel system, being located only in the straight and sinusoidal channels, leaving the intersections unoccupied. This ordering is commensurate with the framework structure of Silicalite-I. 29Si HPDEC MAS NMR shows that the loading of n-hexane induces a phase transition to an orthorhombic space group (most likely Pnma) only above 340 K. Force field simulations confirm that the absorption of n-hexane molecules occurs only inside the straight and sinusoidal channels and leads to an energetically minimized host−guest structure. By optimizing the van der Waals interactions between the n-hexane molecules and the silica host framework, the nonbonding energy is minimized, leading to a general minimization of the total potential energy, and the energetically most favorable structure is obtained.