Practical Use of Microwave for Activation of Metal-Organic Frameworks under Ambient Condition

Abstract

Metal−organic frameworks (MOFs) are an interesting class of porous materials that are self- assembled by coordination bonding between metal node and organic ligand. Open coordination site (OCS) in MOFs, which is coordinatively unsaturated sites, has proven to play an important role in various applications of MOFs, such as molecule separation, energy storage, electronic conduction, ionic conduction, catalysis, and sensing. To utilize the OCS for such applications, activation process, which is a process to remove coordinated molecules at OCSs, is essentially required. To date, thermal activation (TA), an activation process normally conducted by applying thermal energy higher than the bond dissociation energy of solvent molecules in its extent, has been the most popularly used method. However, TA process needs to maintain high vacuum (> 103 torr) for approximately 20 hours even at high temperature. Thus, this process requires extra-equipment such as schlenk line connected to a vacuum pump and electric heater in order to make the conditions. Meanwhile, microwave accelerates the rotational motion of molecules. In the aspect of thermodynamics, the excited molecule in its rotational motion is promptly relaxed, and subsequently, the relaxation energy is converted to thermal energy. We hypothesized that solvents molecules residing in the nanospace of MOF can be excited in rotational motion by absorbing microwave, and the subsequent rotational relaxation can emit nanoscale thermal energy. By utilizing this thermal energy, we further hypothesized that both coordinated solvent molecules and pore-filled solvent molecules can be removed. In this presentation, we demonstrate that applying microwave is an efficient method for the activation of OCS-containing MOFs. More precisely, we observed that MOFs such as HKUST-1 and MOF-74 series can be completely activated under ambient conditions.