Fig. 4. Hierarchical porosity in UiO-66 monoliths. (a) Electron tomographic reconstruction of a single mesoporous monolithic xerogel particle (approx. 100 nm wide). Matter is represented in red. (b) Slice through the 3D reconstruction in (a). Bright contrast corresponds to matter, revealing intraparticle mesoporous voids. (c) Nitrogen physisorption isotherms (77 K) for a microcrystalline UiO-66 powder (black circles) and xerogel (red diamonds) and aerogel (blue triangles) monoliths (full symbols = adsorption branch; open symbols = desorption branch). The hysteretic desorption above p/p 0 = 0.8 is attributed to capillary condensation in the mesopores. The inset shows a logarithmic representation of the adsorption branch at low p/p 0. The two-step profile corresponds to the uptake of N2 in the smaller tetrahedral (6 Å) and larger octahedral cages (8 Å), respectively.
