Elucidation of the structural transformation of MIL-53(Al) upon D2 responsive 2nd Breathing

MIL-53(Al), a well-known flexible MOF, exhibits a dynamic 'breathing' phenomenon, transitioning from narrow pore (np) to large pore (lp) states. Understanding the structural flexibility of MIL-53(Al) holds the key for advancements, making it ideal for precise gas separation, including hydrogen isotopes. This proposal seeks beamtime to investigate unresolved questions concerning MIL-53(Al)'s isotope-specific second 'breathing' transitions induced by D2 adsorption below 25 K. To date, neutron powder diffraction (NPD) studies have been hampered by high noise levels. Leveraging the superior flux of the WISH beamline, we aim for precise identification of D2 adsorption sites and their site-specific binding energies as functions of pressure. Concurrently, we will examine how crystal size influences the kinetics of these transitions. We can readily prepare pure-phase MIL-53(Al) with high crystallinity over 1 g of the material for gas loading experiments. To complete these experiments, it will require approximately three days for in situ NPD measurements. We believe that NPD, benefiting from the high flux of WISH, is the optimal tool for this purpose, allowing us to identify hydrogen isotope adsorption sites within MIL-53(Al).