Methane Sorption in Nanoporous Metal-Organic Frameworks and First-Order Phase Transition of Confined Methane

Nanoporous metal-organic frameworks (MOFs) are promising materials for methane sorption and storage. Using neutron powder diffraction, we have directly determined the methane sorption sites in two prototypical MOF materials: zeolitic irnidazolate framework-8 (ZIF-8, Zn-6(N2C4H5)(12)) and metal-organic framework-5 (MOF-5, Zn4O13(C8H4)(3)). The primary methane adsorption sites are associated with the organic linkers in ZIF-8 and the metal oxide clusters in MOF-5. Methane molecules on these primary sites possess well-defined orientations, implying relatively strong binding with the framework. With higher methane loading, extra methane molecules populate the secondary sites and are confined in the framework. The confined methanes are orientationally disordered and stabilized by the intermolecular interactions. Below 100 K a maximum of similar to 18 and similar to 24 methane molecules per formula unit can be stored in ZIF-8 and MOF-5, corresponding to similar to 20 and similar to 50 wt % storage capacities, respectively. An unusual reversible methane-induced structural phase transition in MOF-host lattice is observed at similar to 60 K in both ZIF-8 and MOF-5 due to strong intermolecular interaction between confined methane molecules in the pores of the host lattice.