Identifying pathways to metal-organic framework collapse during solvent activation with molecular simulations

19 June 2023, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Metal organic framework (MOF) materials are a vast family of nanoporous solids with potential applications ranging from drug delivery to environmental remediation. Application of MOFs in these scenarios is hindered, however, by difficulties in MOF ‘activation’ after initial synthesis – removal of the synthesis solvent from the pores to make the pore space accessible – leading to framework collapse if improperly performed. While experimental studies have correlated collapse to specific solvent properties and conditions, the mechanism of activation-collapse is currently unknown. Developing this understanding would enable researchers to create better activation protocols for MOFs, accelerating discovery and process intensification. To achieve this goal, we simulated solvent removal using grand-canonical Monte Carlo and free energy perturbation methods. By framing activation as a fluid desorption problem, we investigated activation processes in the IRMOF family of MOFs for different solvents. We identified two pathways for solvent activation – the solvent either desorbs uniformly from each individual pore or forms coexisting phases during desorption. These mesophases in turn lead to large capillary stresses within the framework, corroborating experimental hypotheses for the cause of activation-collapse. Finally, we found that the activation energy of solvent removal increased with pore size and connectivity due to the increased stability of solvent mesophases, matching experimental findings. Using these simulations, it is possible to screen MOF activation procedures, enabling rapid identification of ideal solvents and conditions and thus enabling faster development of MOFs for practical applications.

Keywords

MOFs
Activation collapse
Transition matrix Monte Carlo

Supplementary materials

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Additional technical details of this work: - Complete table of solvents used and their forcefield information. - Validation of TMMC effectiveness cf. unbiased GCMC. - Validation of TMMC simulation of solvent bulk surface tensions. - Geometric breakdown of instantaneous MOF-solvent interactions during activation. - Comparison of fluid-fluid vs. fluid-MOF interaction strengths.
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