Kinetic Trapping of D₂ in MIL-53(Al) Observed Using Neutron Scattering

We have studied gas adsorption effects on the structure of a metal–organic framework, MIL-53(Al), a material well-known because of the controllable framework “breathing” phenomenon. Neutron powder diffraction between 4 and 77 K and up to 4.5 bar pressure D 2 confirms that a structural phase transition is responsible for the observed H 2 /D 2 isotherm hysteresis at 77 K. We find two crystallographically distinct D 2 adsorption sites in MIL-53(Al) when the pores are fully opened, similar to those reported for D 2 in MIL-53(Cr), but in contrast with the previously published cases of CO 2 and H 2 O. Upon desorption of D 2 at 77 K, we find strong evidence for the existence of D 2 molecules kinetically trapped in the center of the closed pore of MIL-53(Al). This “molecular clamp” appears to be functional until ≈120 K, the temperature at which the D 2 eventually desorbs under dynamic vacuum. Hydrogen diffusion constants calculated using quasielastic neutron scattering data collected along the isotherm are also consistent with H 2 being trapped in the closed pore structure. Diffraction experiments performed with N 2 and He gases under similar conditions show the range of structural response from immediate pore opening at low N 2 pressures (<1 bar) to no observable effect at 10 bar He.