Activating and shaping MOFs

MOF activation and shaping on one page: solvent exchange followed by heating under vacuum, the three ways to empty the pores (vacuum, supercritical CO₂, freeze-drying), pore collapse as the solvent leaves, and the shaped forms (pellets, extrudates, granules, sol–gel and 3D-printed monoliths), what lies inside them and a dense sol–gel monolith. Every step and number is quoted from the cited papers.

1. Exchange, then vacuum: On heating, ZIF-4 first releases the framework-templating N,N-dimethylformamide (DMF), which does not make the framework collapse (Bennett 2015). Solvent exchange is fast: minutes rather than days suit many MOFs, and with a very low surface tension solvent such as n-hexane or perfluoropentane, MOFs that had failed to activate reach their predicted surface areas (Ma 2017). As-made ZIF-62 crystals, for example, were washed with DMF and dichloromethane and dried in a vacuum oven at 150 °C for 24 h (Hou 2019). 2. Three ways to empty pores: Monolithic and powdered MOFs were fully activated by heating under vacuum before their porosity was measured (Madden 2021). Processing four MOFs with liquid and supercritical CO₂ raised their gas-accessible surface area substantially, in some cases by up to 1200%; preliminary evidence points to inhibited mesopore collapse as the reason (Nelson 2009). Freeze-drying significantly increased the permanent porosity and hydrogen uptake of 4,4-connected MOFs (Ma 2009). 3. Pore collapse: Removing the synthesis solvent from the pores without damaging the framework is often a challenge; supercritical CO₂ activates the solvent-filled pores mildly and efficiently (Farha & Hupp 2010). MOFs that had failed to activate reached their predicted surface areas after exchange into a very low surface tension solvent such as n-hexane or perfluoropentane (Ma 2017). 4. Five shaped forms: Shaping MOF powders into bodies of the desired size, shape, density and mechanical stability keeps pressure drops low and volumetric capacity high in adsorption columns (Madden 2021). Shaped MIL-100(Fe) granules with a 5% polyvinyl butyral binder had less surface area, pore volume and water uptake than the powder, but more mechanical strength and faster adsorption (Chen 2024). 3D-printed (robocast) MOF-74(Ni) and UTSA-16(Co) monoliths held 80 and 85 wt% MOF and took up 79% and 87% of the CO₂ of the powders at 5000 ppm CO₂ and 25 °C (Thakkar 2017). 5. Inside the body: Extrudates made at low pressure have low bulk densities because of large voids; in powder pressing the crystals are prone to collapse under high pressure, leaving crumple zones of amorphous material: dense pellets with much less porosity (Madden 2021). High-pressure pelletisation or binders are conventionally used to shape MOF powders but have been shown to decrease the material’s efficacy significantly (Hou 2019). 6. A sol–gel monolith: Monolithic HKUST-1 has a bulk density of 1.06 g cm⁻³, the powder 0.5 g cm⁻³: in the sol–gel synthesis the crystallites pack closely (Madden 2021). A sol–gel monolithic MOF, packed and densified, reached a methane capacity of 259 cm³(STP) cm⁻³, the highest then reported for conformed-shape porous solids, and was at least 130% harder than conventional MOF counterparts (Tian 2018).

Starting materials: As-synthesised MOF crystals with solvent-filled pores; exchange solvents, supercritical CO₂; binders such as polyvinyl butyral. Product: Activated MOF powders, pellets, extrudates, granules and sol–gel or 3D-printed monoliths.

Bennett et al., “Hybrid glasses from strong and fragile metal-organic framework liquids”, Nat. Commun. 2015 (doi:10.1038/ncomms9079); Ma et al., “Rapid guest exchange and ultra-low surface tension solvents optimize metal-organic framework activation”, Angew. Chem. Int. Ed. 2017 (doi:10.1002/anie.201709187); Hou et al., “Metal-organic framework crystal-glass composites”, Nat. Commun. 2019 (doi:10.1038/s41467-019-10470-z); Madden et al., “Monolithic metal-organic frameworks for carbon dioxide separation”, Faraday Discuss. 2021 (doi:10.1039/d1fd00017a); Nelson et al., “Supercritical processing as a route to high internal surface areas and permanent microporosity in metal-organic framework materials”, J. Am. Chem. Soc. 2009 (doi:10.1021/ja808853q); Ma et al., “Freeze drying significantly increases permanent porosity and hydrogen uptake in 4,4-connected metal-organic frameworks”, Angew. Chem. Int. Ed. 2009 (doi:10.1002/anie.200904983); Farha & Hupp, “Rational design, synthesis, purification, and activation of metal-organic framework materials”, Acc. Chem. Res. 2010 (doi:10.1021/ar1000617); Chen et al., “Development of shaped MIL-100(Fe) granules for high-performing adsorption desalination: from formulation optimization to system test”, Chem Bio Eng. 2024 (doi:10.1021/cbe.4c00008); Thakkar et al., “3D-printed metal-organic framework monoliths for gas adsorption processes”, ACS Appl. Mater. Interfaces 2017 (doi:10.1021/acsami.7b11626); Tian et al., “A sol-gel monolithic metal-organic framework with enhanced methane uptake”, Nat. Mater. 2018 (doi:10.1038/nmat5050).

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