Conformational isomerism tunes barocaloric potential of MIL-53(Ga)-fum frameworks

Abstract

With the negative implications of current refrigerants on the environment, there is a necessity for greener alternatives. Solid-state caloric materials exhibit large entropy and temperature changes that avoid liquid-vapour transitions of current refrigerants. Through guest adsorption, flexible metal-organic frameworks (MOFs) exhibit large entropy changes under small applied pressures, making them promising candidates. Though effective, flexible MOFs lack long-term mechanical stability owing to the mechanical stress induced during operation. Herein, we highlight and categorise the previously unexplored conformational isomerism of the MIL-53(X)-fum frameworks, which originates from the relative orientations of the fumarate linkers, and explore the effect on structural flexibility and barocaloric potential. Using single crystal X-ray diffraction (SCXRD) and ab initio molecular dynamics (AIMD), we reveal the third and final possible conformational isomer, MIL-53(X)-fum(c), to complement the two previously reported (MIL-53(X)-fum(a) and MIL-53(X)-fum(b)), and investigate the effects of the isomerism on the framework properties. We uncover the isomerically unique mechanical stability of MIL-53(Ga)-fum(c) through simulated potential energy surfaces and find that the vacated open phase has exceptional thermal stability of up to 650 K. Through grand canonical Monte Carlo (GCMC) simulations, the reversible adsorption-driven entropy changes under 0.4 MPa of applied pressure were calculated to be ΔS = 210(30) J K⁻¹ kg⁻¹ and 150(30) J K⁻¹ kg⁻¹ for the adsorption of H₂O and CO₂, respectively. The results of our studies suggest that MIL-53(Ga)-fum(c) can exhibit a caloric response that is competitive with current commercial refrigerants. Furthermore, we outline the significance of exploring conformational isomerism in MOFs to increase the space in which materials discovery can occur.