MOF synthesis: solvothermal, room temperature, modulated

The classic routes to metal–organic frameworks on one page: solvothermal MOF-5 in a Teflon-lined vessel, UiO-66 step by step, hydrothermal HKUST-1 from molecules to the framework, ZIF-8 mixed at room temperature in methanol, how the Hmim/Zn ratio sets its crystals, and modulators that compete with the linker. Every step and number is quoted from the cited papers.

1. Solvothermal MOF-5: Solvothermal reactions run in closed vessels under autogenous pressure above the solvent’s boiling point, usually at 50–260 °C for hours or days; the cooling rate affects crystal growth (Raptopoulou 2021). MOF-5: zinc acetate (4.24 g) and H₂BDC (1.26 g) stirred in DMF for 2 h, heated in a Teflon-lined sealed vessel in an oven at 125 °C for 24 h, cooled gradually, centrifuged (7000 rpm, 15 min), washed with DMF and dried under vacuum (Kumar & Masram 2021). Heating a formamide solvent releases a base by thermal decomposition, which starts the self-assembly (Forgan 2020); DMF breaks down into carbon monoxide, dimethylamine and formate (Wu 2021). MOF-5 is built from octahedral Zn–O–C clusters and benzene links (Eddaoudi 2002). 2. UiO-66, step by step: ZrCl₄ (0.233 g, 1.0 mmol) and H₂BDC (0.166 g, 1.0 mmol) in 20 mL DMF in a 50 mL PTFE liner, dissolved with ultrasound, heated at 120 °C for 24 h and washed three times with DMF and ethanol (Liu 2023). UiO-66 is Zr₆O₄(OH)₄(BDC)₆ (Drenchev 2020); its Zr₆ building brick gives frameworks that decompose above 500 °C and stay crystalline after 10 tons/cm² of pressure (Cavka 2008). The high hygroscopic character of ZrCl₄ can hurt reproducibility; the less acidic ZrOCl₂·8H₂O seemed a suitable alternative, and adding HCl or water sped up the reaction (Ragon 2014). 3. HKUST-1, hydrothermal: [Cu₃(TMA)₂(H₂O)₃]ₙ (TMA = benzene-1,3,5-tricarboxylate) formed in 80% yield; its Cu₂(O₂CR)₄ units make 1 nm channels and about 40% accessible porosity, the aqua ligands can be replaced by pyridines, and it is stable up to 240 °C (Chui 1999). HKUST-1 is conventionally made hydrothermally; microwave heating gives better yields in a much shorter time (Raptopoulou 2021). ZIFs, by contrast, form by copolymerizing Zn(II) or Co(II) with imidazolate links; ZIF-8 is permanently porous (Langmuir surface area 1,810 m²/g), stable up to 550 °C and resists boiling alkaline water and organic solvents (Park 2006). 4. ZIF-8 at room temperature: Zn(NO₃)₂·6H₂O (1.484 g) in 50 mL methanol and 2-methylimidazole (3.278 g) in 50 mL methanol; the second solution poured slowly into the first, stirred for 60 min at 25 ± 3 °C, centrifuged, washed with methanol three times and dried in air (Zhang 2018). In water, ZIF-8 formed at room temperature in several minutes as nanocrystals of about 85 nm (Pan 2011); with a trialkylamine, ZIF-67 and ZIF-8 formed in water in 10 min (Gross 2012). HKUST-1 was made at room temperature from H₃BTC and NaHCO₃ in water and Cu(NO₃)₂·3H₂O in ethanol, stirred for 12 h; yield 89.6% (Cortés-Súarez 2019). 5. The Hmim/Zn ratio: At room temperature in methanol, an Hmim/Zn ratio of 1 gave cubes (0.4 and 0.8 µm) together with Zn(OH)(NO₃)(H₂O) and other phases; higher ratios gave truncated rhombic dodecahedra or rhombic dodecahedra. The particles shrank from 1 µm to 0.1 µm as the ratio rose from 4 to 16; a ratio of 8 gave the best crystallinity, yield (51.09%), BET area (1268 m²/g) and micropore volume (0.62 cm³/g), and excess Hmim hindered growth and made the particles aggregate (Zhang 2018). The shapes follow the competition between the {100} and {110} faces of ZIF-8 (Kar 2023). 6. Modulated synthesis: Coordination modulation adds monotopic ligands that mimic the binding group of the linker; modulator and linker are thought to compete for the metal, slowing crystallization and giving kinetic control over self-assembly (Forgan 2020). Modulating the coordination equilibria regulates the rate of framework extension and crystal growth (Tsuruoka 2009). With more benzoic or acetic acid, Zr-bdc (UiO-66) changes from intergrown to individual crystals whose size can be tuned (Schaate 2011). Acetic acid and the synthesis time tune the missing linkers of UiO-66, about 10% in one sample (Wu 2013); in UiO-66, the lower the modulator’s pKa, the more defects it induced (Forgan 2020).

Starting materials: Zinc acetate or nitrate, ZrCl₄ and copper(II) salts; terephthalic acid (H₂BDC), trimesic acid (H₃BTC) and 2-methylimidazole; DMF, methanol or water; monocarboxylic acid modulators. Product: MOF-5, UiO-66, HKUST-1 and ZIF-8 crystals.

Raptopoulou, “Metal-Organic Frameworks: Synthetic Methods and Potential Applications”, Materials 2021 (doi:10.3390/ma14020310); Kumar & Masram, “Sustainable Synthesis of MOF-5@GO Nanocomposites for Efficient Removal of Rhodamine B from Water”, ACS Omega 2021 (doi:10.1021/acsomega.1c00143); Forgan, “Modulated self-assembly of metal-organic frameworks”, Chem. Sci. 2020 (doi:10.1039/d0sc01356k); Wu et al., “Facile and Rapid Room-Temperature Electrosynthesis and Controlled Surface Growth of Fe-MIL-101 and Fe-MIL-101-NH₂”, ACS Cent. Sci. 2021 (doi:10.1021/acscentsci.1c00686); Eddaoudi et al., “Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage”, Science 2002 (doi:10.1126/science.1067208); Liu, Lyu & Bai, “One-Step Synthesis of Al-Doped UiO-66 Nanoparticle for Enhanced Removal of Organic Dyes from Wastewater”, Molecules 2023 (doi:10.3390/molecules28052182); Drenchev et al., “In situ FTIR Spectroscopy as a Tool for Investigation of Gas/Solid Interaction: Water-Enhanced CO₂ Adsorption in UiO-66 Metal-Organic Framework”, J. Vis. Exp. 2020 (doi:10.3791/60285); Cavka et al., “A new zirconium inorganic building brick forming metal organic frameworks with exceptional stability”, J. Am. Chem. Soc. 2008 (doi:10.1021/ja8057953); Ragon et al., “In situ energy-dispersive X-ray diffraction for the synthesis optimization and scale-up of the porous zirconium terephthalate UiO-66”, Inorg. Chem. 2014 (doi:10.1021/ic402514n); Chui et al., “A chemically functionalizable nanoporous material”, Science 1999 (doi:10.1126/science.283.5405.1148); Park et al., “Exceptional chemical and thermal stability of zeolitic imidazolate frameworks”, Proc. Natl. Acad. Sci. USA 2006 (doi:10.1073/pnas.0602439103); Zhang et al., “Influence of the 2-methylimidazole/zinc nitrate hexahydrate molar ratio on the synthesis of zeolitic imidazolate framework-8 crystals at room temperature”, Sci. Rep. 2018 (doi:10.1038/s41598-018-28015-7); Pan et al., “Rapid synthesis of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals in an aqueous system”, Chem. Commun. 2011 (doi:10.1039/c0cc05002d); Gross, Sherman & Vajo, “Aqueous room temperature synthesis of cobalt and zinc sodalite zeolitic imidizolate frameworks”, Dalton Trans. 2012 (doi:10.1039/c2dt30174a); Cortés-Súarez et al., “Synthesis and Characterization of an SWCNT@HKUST-1 Composite: Enhancing the CO₂ Adsorption Properties of HKUST-1”, ACS Omega 2019 (doi:10.1021/acsomega.9b00330); Kar et al., “Guiding Metal Organic Framework Morphology via Monolayer Artificial Defect-Induced Preferential Facet Selection”, JACS Au 2023 (doi:10.1021/jacsau.2c00692); Tsuruoka et al., “Nanoporous nanorods fabricated by coordination modulation and oriented attachment growth”, Angew. Chem. Int. Ed. 2009 (doi:10.1002/anie.200901177); Schaate et al., “Modulated synthesis of Zr-based metal-organic frameworks: from nano to single crystals”, Chem. Eur. J. 2011 (doi:10.1002/chem.201003211); Wu et al., “Unusual and highly tunable missing-linker defects in zirconium metal-organic framework UiO-66 and their important effects on gas adsorption”, J. Am. Chem. Soc. 2013 (doi:10.1021/ja404514r); Mottillo & Friščić, “Advances in Solid-State Transformations of Coordination Bonds: From the Ball Mill to the Aging Chamber”, Molecules 2017 (doi:10.3390/molecules22010144); Carné-Sánchez et al., “Protecting metal-organic framework crystals from hydrolytic degradation by spray-dry encapsulating them into polystyrene microspheres”, Adv. Mater. 2015 (doi:10.1002/adma.201403827).

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