Methane storage mechanism in the metal-organic framework Cu3(btc)2: An in situ neutron diffraction study

The adsorption of deutero-methane (CD4) in Cu3(btc)2 (HKUST-1) was investigated at 77 K using high-resolution neutron powder diffraction. Rietveld refinement of the neutron data revealed a sequential filling of the rigid framework at distinct preferred adsorption sites, and showed the importance of open metal sites even for non-polar molecules such as methane. Four main adsorption sites were identified, located inside the small and two larger pores of the framework. The shorter distances between the CD4 center and the pore wall atoms are covering a range from 3.07 to 3.547 Å. The maximum occupation of 170 CD4 molecules per unit cell, estimated from the refined occupancy of the adsorption sites, is close to the value estimated from volumetric adsorption isotherms at 77 K (176 molecules per cell). Molecular simulation gave further insight into the adsorption mechanism.