Validating the Mechanism Underlying the Slacking of the Gate-Opening Behavior in Flexible Metal-Organic Frameworks Arising from the Application of External Force

Flexible metal-organic frameworks (MOFs) are innovativeadsorbents expected to revolutionize conventional separation systemsas they exhibit stepwise adsorption arising from structural transitions,commonly known as gate opening. However, because MOFsare typically obtained in powder form, they require shaping for industrialapplications. In our previous study, we reported that the stepwiseuptake observed in the CO2 gate opening of ELM-11 ([Cu(BF4)(2)(4,4 & PRIME;-bipyridine)(2)]) became less distinct when molded with polymer binders andfound that this slacking phenomenon could be caused by the polymerbinder inhibiting the structural change of the ELM-11 particles. Inthis study, we aimed to fully validate and generalize the mechanismbehind the slacking of gate adsorption from both theoretical and experimentalperspectives. First, we conducted grand canonical molecular dynamicssimulations for a simplified MOF model to directly calculate freeenergy profiles of the particle to validate the slacking theory withoutany assumptions. The results confirmed the fundamental assumptionmade in our previous study that the deformation of the flexible motifswithin the MOF particles occurs sequentially, which is a key factorcontributing to the slacking phenomenon. The second part of the studyfocused on the relationship between the volume expansion ratio ofMOFs and the degree of slacking. The relationship predicted by thetheory was experimentally validated by comparing ELM-11, which exhibits30% volume expansion, to another MOF with a mutually interpenetratingjungle-gym structure, which exhibits 10% volume expansion. These findingsstrengthened and generalized the understanding of the mechanism underlyingthe slacking of gate adsorption induced upon the application of externalforce, which could guide the fabrication of molded MOFs while maintaininga high adsorption efficiency for various industrial applications.

Automated summary

Flexible metal-organic frameworks (MOFs) exhibit stepwise adsorption and have the potential to revolutionize conventional separation systems. However, MOFs in powder form need to be shaped for industrial applications. This study aimed to validate the mechanism behind the slacking of gate adsorption from both theoretical and experimental perspectives. Molecular dynamics simulations confirmed the sequential deformation of flexible motifs within MOF particles as a key factor contributing to the slacking phenomenon. Experimental validation showed the relationship between the volume expansion ratio of MOFs and the degree of slacking. These findings enhance the understanding of the slacking mechanism and guide the fabrication of molded MOFs with high adsorption efficiency.