Gel-based morphological design of zirconium metal-organic frameworks

Abstract

The ability of metal-organic frameworks (MOFs) to gelate under specific synthetic conditions opens up new opportunities in the preparation and shaping of hierarchically porous MOF monoliths, which could be directly implemented for catalytic and adsorptive applications. In this work, we present the first examples of xero- or aerogel monoliths consisting solely of nanoparticles of several prototypical Zr⁴⁺-based MOFs: UiO-66-X (X = H, NH₂, NO₂, (OH)₂), UiO-67, MOF-801, MOF-808 and NU-1000. High reactant and water concentrations during synthesis were observed to induce the formation of gels, which were converted to monolithic materials by drying in air or supercritical CO₂. Electron microscopy, combined with N₂ physisorption experiments, was used to show that irregular nanoparticle packing leads to pure MOF monoliths with hierarchical pore systems, featuring both intraparticle micropores and interparticle mesopores. Finally, UiO-66 gels were shaped into monolithic spheres of 600 μm diameter using an oil-drop method, creating promising candidates for packed-bed catalytic or adsorptive applications, where hierarchical pore systems can greatly mitigate mass transfer limitations.

Fig. 1. Gels and monolithic particles of UiO-66. (a) ‘Non-flowing’ gels of UiO-66 synthesized from ZrOCl₂·8H₂O and H₂bdc (Table S1, entry 10). (b and c) Optically transparent monolithic xerogel particles obtained by crushing ‘non-flowing’ gels dried in air at 200 °C. Prior to drying, the gels were washed and solvent-exchanged with ethanol. Scale bar = 100 μm (see also Fig. S1†).

Fig. 2. X-ray diffraction patterns of UiO-66 gels and monoliths. (a) Simulated diffraction pattern of UiO-66. (b) UiO-66 prepared as microcrystalline powder. (c) UiO-66 prepared as ethanol-exchanged gel (Table S1, entry 10). (d) Air-dried xerogel monolith prepared from the gel in (c). (e) Aerogel monolith obtained after supercritical CO₂ extraction of the gel in (c).

Fig. 3. Electron microscopy of monolithic UiO-66 xerogels. (a) Scanning electron microscopy (SEM) image (scale bar = 25 μm) of a xerogel particle. (b and c) TEM images (b, scale bar = 1 μm; c, scale bar = 50 nm) of xerogel particles, which consist of irregularly packed MOF nanoparticles with interparticle voids. (d) ADF-STEM image of the xerogel in (b), illustrating the crystalline nature of the nanoparticles. Bright contrast corresponds to areas of high density (scale bar = 10 nm). The inset shows the Fourier transform of the nanoparticle circled in white, consistent with the face-centered cubic lattice of UiO-66 viewed along the [100] direction. (e) ADF-STEM image of the sample in (c), showing an individual UiO-66 nanoparticle oriented along [110]. Each bright spot corresponds to a single column of [Zr₆O₄(OH)₄(R–COO)₁₂] clusters (scale bar = 10 nm). The Fourier transform of this nanoparticle (inset) features reflections that can be indexed as the (111) and (200) reflections of UiO-66, corresponding to d-spacings of 12.1 Å and 10.5 Å, respectively. (f) SAED pattern of the aggregate particle in (b). The diffraction ring corresponds to a d-spacing of ∼11.5 Å, consistent with UiO-66's (111) reflection (12.0 Å).

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.

Fig. 5. Monolithic UiO-66 xerogel spheres. (a) Optical image of a single sphere (scale bar = 150 μm). (b) SEM micrograph of a single sphere (scale bar = 150 μm). (c) SEM micrographs of a cross-section of the interior sphere architecture (scale bar = 3 μm). (d) Close-up of (c), highlighting the nanoparticulate structure and mesoporosity (scale-bar = 0.5 μm). (e) X-ray diffraction pattern of UiO-66 monolithic spheres. (f) Nitrogen physisorption isotherm (77 K) of monolithic UiO-66 spheres (full symbols = adsorption branch; open symbols = desorption branch).

Fig. 6. Schematic overview of gel formation. (a) Synthesis in dilute conditions, with a limited amount of water leads to microcrystalline particles. (b & c) High reactant and water concentration stimulates formation of the [Zr₆O₄(OH)₄]¹²⁺ clusters and nucleation of the Zr-MOF (yellow stars). This leads to gel-like viscous colloidal suspensions of Zr-MOF nanoparticles, in which further crystal growth is hampered. (b) At intermediate nanoparticle concentrations, ‘flowing’ gels can be observed, (c) high nanoparticle concentrations lead to viscoelastic ‘non-flowing’ gels with a network of weakly aggregated particles throughout the entire solvent volume. (d & e) Tuning the nanoparticle concentration interconverts the system between these two states. (f) Solvent removal from the ‘non-flowing’ gels yields monolithic mesoporous xerogels or aerogels, consisting of randomly packed nanoparticles.