The most important process to make hydrogen is based on steam reforming of natural gas according to an overall reaction CH 4 + 2H 2 O!4H 2 + CO 2 , where after reformation of the natural gas to a CO/H 2 mixture (syngas) a water-gas shift reaction results in a predominantly CO 2 /H 2 mixture. [1,2] Steam reforming potentially offers one technological path to extend the trend of decarbonization of the primary fossil fuel by taking advantage of the low 1:4 carbon-to-hydrogen ratio of methane compared to coal (~8:4) and oil (~2:4), provided that subsequent separation of the CO 2 /H 2 mixture and sequestration of CO 2 is cost-effective and feasible. We present here a guest-free hydroquinone (HQ) clathrate, prepared by gas-phase synthesis, which reveals unique selectivities towards CO 2 /CH 4 and CO 2 /H 2 mixtures. A dynamical pore-widening process allows CO 2 to be adsorbed with a selectivity of 29:1 from a CO 2 /CH 4 (50:50 v/v) mixture and with a selectivity of 60:1 reversibly stored at 7 MPa and 298 K in the presence of a CO 2 /H 2 (50:50 v/v) mixture. This first example of a flexible hydrogen-bonded organic framework (HOF) that can reversibly and selectively absorb and store CO 2 opens up a host of applications.The

A CH(4)-loaded hydroquinone (HQ) clathrate was synthesized via a gas-phase reaction using the alpha-form of crystalline HQ and CH(4) gas at 12 MPa and room temperature. Solid-state (13)C cross-polarization/magic angle spinning (CP/MAS) NMR and Raman spectroscopic measurements confirm the incorporation of CH(4) molecules into the cages of the HQ clathrate framework. The chemical analysis indicates that about 69% of the cages are filled by CH(4) molecules, that is, 0.69 CH(4) per three HQ molecules. Rietveld refinement using synchrotron X-ray powder diffraction (XRD) data shows that the CH(4)-loaded HQ clathrate adopts the beta-form of HQ clathrate in a hexagonal space group R3 with lattice parameters of a = 16.6191 A and c = 5.5038 A. Time-resolved synchrotron XRD and quadrupole mass spectroscopic measurements show that the CH(4)-loaded HQ clathrate is stable up to 368 K and gradually transforms to the alpha-form by releasing the confined CH(4) gases between 368-378 K. Using solid-state (13)C CP/MAS NMR, the reaction kinetics between the alpha-form HQ and CH(4) gas is qualitatively described in terms of the particle size of the crystalline HQ.

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