Metal organic frameworks (MOF) are coordination polymers with high surface area varying between 1000 -10000 m²/g ¹. Like ionic solids, MOFs are composed by units with positive and negative charge. In this case, rigid organic linkers with more than a coordinating group (normally a carboxylate) take anions’ position and metal-oxide cluster replace the cations. The chemical and physical properties and the size and connectivity of the pores of MOFs vary with the characteristics of the linker and/or the metal-oxide cluster used, thus presenting a rich and varied chemistry and several potential applications. Unlike other porous materials, MOFs are crystalline and all distances are well defined with pores of the same size.

One of the most studied MOFs is MOF-199², which can be described as a periodic arrangement of Cu₂C₄O₈, known as paddlewheels. These secondary building units (SBU) are spaced by the rigid linker trimesic acid (BTC). The arrangement has crystalline order and can be described as a tridimensional periodic array of identical copper paddlewheels linked by BTC.

MOF-199 has special interest in methane storage because its gas storage capacity is close to the goal defined by the USA Department of Energy (DoE). One way to achieve and overcome this goal is to create uncoordinated metal sites and improve the activation procedure, as these processes dominate the metal-gas interaction in MOFs.

In this work we follow the structural changes in MOF-199 during the activation process by X-ray diffraction (XRD) and the changes in the local environment of the uncoordinated metal sites by Electron Paramagnetic Resonance (EPR).