New insights into the breathing phenomenon in ZIF-4

Abstract

Structural changes in ZIFs upon adsorption remain a paradigm due to the sensitivity of the adsorption mechanism to the nature of the organic ligands and gas probe molecules. Synchrotron X-ray diffraction under operando conditions clearly demonstrates for the first time that ZIF-4 exhibits a structural reorientation from a narrow-pore (np) to a new expanded-pore (ep) structure upon N₂ adsorption, while it does not do so for CO₂ adsorption. The existence of an expanded-pore structure of ZIF-4 has also been predicted by molecular simulations. In simulations the expanded structure was stabilized by entropy at high temperatures and by strong adsorption of N₂ at low temperatures. These results are in perfect agreement with manometric adsorption measurements for N₂ at 77 K that show the threshold pressure for breathing at ∼30 kPa. Inelastic neutron scattering (INS) measurements show that CO₂ is also able to promote structural changes but, in this specific case, only at cryogenic temperatures (5 K).