Giant tunable barocaloric effects using guest-molecule adsorption in a porous metal-organic framework

Metal-organic frameworks (MOFs) that exhibit first-order structural phase transitions with significant volume changes are promising candidates for barocaloric cooling and heating applications. Here we report giant barocaloric effects in a zeolitic-imidazolate framework [ZIF-4(Zn)] that can be reversibly driven over a wide range of temperatures with pressure, near a starting temperature that is highly tunable using gas adsorption. For nitrogen-loaded frameworks of ZIF-4(Zn), we report isothermal entropy changes |∆S_it| > 38 J K^-1 kg^-1 and adiabatic temperature changes |∆T_ad| > 10 K driven reversibly by |∆p| = |p - p_atm| ~ |p| ~ 1 kbar (p_atm ~ 1 bar) near T₀ ~ 190 K. For desolvated frameworks of ZIF-4(Zn), we estimate larger barocaloric effects up to |∆S_it| ~ 210 J K^-1 kg^-1 and |∆T_ad| ~ 60 K near T₀ ~ 137 K driven using the same pressure change. Due to the dynamic behaviour of this porous MOF, a combination of quasi-direct, indirect and direct methods was used to determine the caloric performance. This combination of methods underlines the cooling and heating potential of porous MOFs, while highlighting the necessity of quantifying the presence of any inactive mass. Our findings should establish a benchmark for MOFs as environmentally benign solid-state refrigerants that can be tuned using guest molecules as a caloric vector.