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Kinetic Trapping of D2 in MIL-53(AL) Observed using Neutron Scattering



Rachel Ann Pollock, Jae-Hyuk Her, Craig Brown, Yun Liu, Anne Dailly


We have studied gas adsorption effects on the structure of a metal organic framework, MIL-53(Al), a material well known due to the controllable framework 'breathing¿ phenomenon. Neutron powder diffraction between 4 K and 77 K and up to 4.5 bar pressure D2 confirms that a structural phase transition is responsible for the observed H2/D2 isotherm hysteresis at 77 K. We find two crystallographically distinct D2 adsorption sites in MIL-53(Al) when the pores are fully opened, similar to those reported for D2 in MIL-53(Cr), but in contrast with the previously published cases of CO2 and H2O. Upon desorption of D2 at 77 K, we find strong evidence for the existence of D2 molecules kinetically trapped in the center of the closed pore of MIL-53(Al). This 'molecular clamp¿ appears to be functional until approximately equal) 120 K, where the D2 eventually desorbs under dynamic vacuum. Hydrogen diffusion constants calculated using quasielastic neutron scattering data collected along the isotherm are also consistent with H2 being trapped in the closed pore structure. Diffraction experiments performed with N2 and He gases under similar conditions show the range of structural response from immediate pore opening at low N2 pressures (< 1 bar) to no observable effect at 10 bar He.
Journal of Physical Chemistry C


MIL-53, neutron powder diffraction, hydrogen storage, quasielastic neutron scattering, metal-organic framework


Pollock, R. , Her, J. , Brown, C. , Liu, Y. and Dailly, A. (2014), Kinetic Trapping of D<sub>2</sub> in MIL-53(AL) Observed using Neutron Scattering, Journal of Physical Chemistry C, [online], (Accessed April 21, 2024)
Created July 13, 2014, Updated October 12, 2021