Experimental Demonstration of Dynamic Temperature-Dependent Behavior of UiO-66 Metal-Organic Framework: Compaction of Hydroxylated and Dehydroxylated Forms of UiO-66 for High-Pressure Hydrogen Storage

High-pressure (700 MPa or similar to 100 000 psi) compaction of dehydroxylated and hydroxylated UiO-66 for H-2 storage applications is reported. The dehydroxylation reaction was found to occur between 150 and 300 degrees C. The H-2 uptake capacity of powdered hydroxylated UiO-66 reaches 4.6 wt % at 77 K and 100 bar, which is 21% higher than that of dehydroxylated UiO-66 (3.8 wt %). On compaction, the H-2 uptake capacity of dehydroxylated UiO-66 pellets reduces by 66% from 3.8 to 1.3 wt %, while for hydroxylated UiO-66 the pellets show only a 9% reduction in capacity from 4.6 to 4.2 wt %. This implies that the H-2 uptake capacity of compacted hydroxylated UiO-66 is at least three times higher than that of dehydroxylated UiO-66, and therefore, hydroxylated UiO-66 is more promising for hydrogen storage applications. The H-2 uptake capacity is closely related to compaction-induced changes in the porosity of UiO-66. The effect of compaction is greatest in partially dehydroxylated UiO-66 samples that are thermally treated at 200 and 290 degrees C. These compacted samples exhibit XRD patterns indicative of an amorphous material, low porosity (surface area reduces from between 700 and 1300 m(2)/g to ca. 200 m(2)/g and pore volume from between 0.4 and 0.6 cm(3)/g to 0.1 and 0.15 cm(3)/g), and very low hydrogen uptake (0.7-0.9 wt % at 77 K and 100 bar). The observed activation-temperature-induced dynamic behavior of UiO-66 is unusual for metal-organic frameworks (MOFs) and has previously only been reported in computational studies. After compaction at 700 MPa, the structural properties and H-2 uptake of hydroxylated UiO-66 remain relatively unchanged but are extremely compromised upon compaction of dehydroxylated UiO-66. Therefor; UiO-66 responds in a dynamic manner to changes in activation temperature within the range in which it has hitherto been considered stable.