LAUSR

High-pressure (700 MPa or ~100 000 psi) compaction of dehydroxylated and hydroxylated UiO-66 for H2 storage applications is reported. The dehydroxylation reaction was found to occur between 150 - 300 oC. The H2 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 H2 uptake capacity of dehydroxylated UiO-66 pellets reduces by 66% from 3.8 wt% to 1.3 wt%, while for hydroxylated UiO-66 the pellets show only a 9% reduction in capacity from 4.6 wt% to 4.2 wt%. This implies the H2 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 H2 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 oC. These compacted samples exhibit XRD patterns indicative of an amorphous material, low porosity (surface area reduces from between 700 and 1300 m2/g to ca. 200 m2/g and pore volume from between 0.4 and 0.6 cm3/g to 0.1 and 0.15 cm3/g) and very low hydrogen uptake (0.7 - 0.9 wt% at 77 K and 100 bar). The observed activation temperature-induced dynamic behaviour of UiO-66 is unusual for MOFs and has previously only been reported in computational studies. After compaction at 700 MPa, the structural properties and H2 uptake of hydroxylated UiO-66 remain relatively unchanged, but are extremely compromised upon compaction of dehydroxylated UiO-66. Therefore, UiO-66 responds in a dynamic manner to changes in activation temperature within the range in which it has hitherto been considered stable.