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

Abstract

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.

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

“…Ashtekar et al 143 showed with FT-Raman that hexane has two distinct adsorption sites. Single-crystal XRD data 188 confirmed a periodic ordering of the n-hexane molecules in the channels of MFI. However, the experimental data of Floquet et al 189 indicate a very different mechanism; n-heptane has two filling modes.…”

One‐Pot Synthesis of Zeolitic Strong Solid Bases: A Family of Alkaline‐Earth Metal‐Containing Silicalite‐1

“…Thes pectral assignment of thesep eaks to crystallographically inequivalent Si atoms can be found in previous studies. [46,47] Twoa dditional peaks emerge at À98 and À102 ppm for all the 29 Si MAS NMR spectra of the MgS-1 series.W ith the increase of Mg content,t he intensity of the former peaks continuouslyi ncreases, whereas the latter peaks increasea tf irst, before reaching am aximum for the MgS-1(30) sample, and then decrease ( Figure 4). By contrast, the 29 Si NMR spectrum for MgO/S-1 is similart ot hat of S-1, that is, no peak at À98 or À102 ppm is observed.…”

Towards a molecular understanding of shape selectivity

“…Typical examples of the accuracy that can now be achieved for computing thermodynamic and transport data are shown in Box 2 Figure a and 12 and entropic separation 47 , that were only recently confirmed by experiments 48,49 .…”