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), the reversible adsorption-driven entropy changes under 0.4 MPa of applied pressure were calculated to be ΔS = 186.2 J K^-1 kg^-1 and 151.8 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.