Structural Study of D2 within the Trimodal Pore System of a Metal Organic Framework

D 2 loaded to saturation at 25 K into the porous coordination framework Cu 3 (1,3,5-benzenetricarboxylate) 2 was studied using neutron powder diffraction. A saturation loading equivalent to 6.1 wt % H 2 is reached at 25 K. D 2 is located at up to nine distinct sites with six stable at saturation and three “metastable” sites at doses intermediate to saturation. Filling of the trimodal pore system is complex, and a concentration dependency on the optimal D 2 arrangement is noted. A dynamic rearrangement of the D 2 in the pores to a cubic-closed packed equilibrium structure occurs at doses close to saturation, with 4 D 2 molecules arranged tetrahedrally in the smallest pore, 32 D 2 arranged in a truncated octahedron with capped hexagonal faces in the intermediate sized pore, and 48 D 2 arranged in a face-capped rhombic dodecahedron in the largest pore. The equilibrium structure of D 2 in the largest pore at doses close to and at saturation was found to be the same arrangement that is optimal for 48 circles on a sphere. The structural response of the framework to D 2 adsorption is dependent on the amount of D 2, where the host lattice expands with increasing amount of adsorbed D 2 at low and high D 2 doses, and contracts upon adsorption of intermediate amounts of D 2, commensurate with the known flexibility exhibited by this framework.