Multiple Coordination Exchanges for Room-Temperature Activation of Open-Metal Sites in Metal–Organic Frameworks

Abstract

The activation of open coordination sites (OCSs) in metal-organic frameworks (MOFs), i.e., the removal of solvent molecules coordinated at the OCSs, is an essential step that is required prior to the use of MOFs in potential applications such as gas chemisorption, separation, and catalysis because OCSs often serve as key sites in these applications. Recently, we developed a "chemical activation" method involving dichloromethane (DCM) treatment at room temperature, which is considered to be a promising alternative to conventional thermal activation (TA), because it does not require the application of external thermal energy, thereby preserving the structural integrity of the MOFs. However, strongly coordinating solvents such as N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), and dimethyl sulfoxide (DMSO) are difficult to remove solely with the DCM treatment. In this report, we demonstrate a multiple coordination exchange (CE) process executed initially with acetonitrile (MeCN), methanol (MeOH), or ethanol (EtOH) and subsequently with DCM to achieve the complete activation of OCSs that possess strong extracoordination. Thus, this process can serve as an effective "chemical route" to activation at room temperature that does not require applying heat. To the best of our knowledge, no previous study has demonstrated the activation of OCSs using this multiple CE process, although MeOH and/or DCM has been popularly used in pretreatment steps prior to the TA process. Using MOF-74(Ni), we demonstrate that this multiple CE process can safely activate a thermally unstable MOF without inflicting structural damage. Furthermore, on the basis of in situ H nuclear magnetic resonance (H NMR) and Raman studies, we propose a plausible mechanism for the activation behavior of multiple CE.

Suspension Processing of Microporous Metal-Organic Frameworks: A Scalable Route to High-Quality Adsorbents

“…It should be noted that this is a preliminary cost analysis of the technique. Further analysis could be performed, which would be beneficial to the development of the technology in the future, such as taking into account alternative solvents such as hexane (Ma et al., 2017) or CH 2 Cl 2 (Bae et al., 2017) or even to account for possible recycling of solvent.…”

Drinking and Breathing: Solvent Coordination‐driven Plasticity of IRMOF‐9

“…This opens two possible scenarios, the first involving the MOF framework inability to adapt to the ongoing coordination change and the consequent phase transformation with an unavoidable crystallinity decrease. In the second one, however, some MOFs are capable to adapt their network geometry allowing modification of the SBUs coordination without losing the original connectivity . This structural versatility was described by Kitagawa et al.…”

Metal–Organic Framework Hybrid Materials and Their Applications

“…For numerous MOFs, solvent molecules in the solvation shell coordinating to terminal metal nodes can be removed and the resultant coordinatively unsaturated sites (CUSs) are exposed upon activation (i.e., desolvation by heating under vacuum) [27][28][29]. CUSs are available for covalent coordination with other materials, giving rise to MOF hybrids exhibiting additional and improved properties [30].…”

“…Remarkably, some of these hybrid catalysts obtained by ALD also exhibit high selectivity towards desired products, presumably due to the unique structures of the grafted metal/metal clusters and the confinement from the MOFs [34,40]. activation (i.e., desolvation by heating under vacuum) [27][28][29]. CUSs are available for covalent coordination with other materials, giving rise to MOF hybrids exhibiting additional and improved properties [30].…”