Gas-phase synthesis and characterization of CH₄-loaded hydroquinone clathrates

A CH₄-loaded hydroquinone (HQ) clathrate was synthesized via a gas-phase reaction using the a-form of crystalline HQ and CH₄ gas at 12 MPa and room temperature. Solid-state ¹³C cross-polarization/magic angle spinning (CP/MAS) NMR and Raman spectroscopic measurements confirm the incorporation of CH₄ molecules into the cages of the HQ clathrate framework. The chemical analysis indicates that about 69% of the cages are filled by CH₄ molecules, that is, 0.69 CH₄ per three HQ molecules. Rietveld refinement using synchrotron X-ray powder diffraction (XRD) data shows that the CH₄-loaded HQ clathrate adopts the /3-form of HQ clathrate in a hexagonal space group R3 with lattice paramet ers of a = 16.6191 Å and c = 5.5038 Å. Timeresolved synchrotron XRD and quadrupole mass spectroscopic measurements show that the CH₄-loaded HQ clathrate is stable up to 368 K and gradually transforms to the a-form by releasing the confined CH₄ gases between 368-378 K. Using solid-state ¹³C CP/MAS NMR, the reaction kinetics between the a-form HQ and CH₄ gas is qualitatively described in terms of the particle size of the crystalline HQ,