Ultrasonic Preparation of Metal-Organic Frameworks (MOFs)

• Metal-Organic Frameworks are compounds formed from metal ions and organic molecules so that a one-, two, or three-dimensional hybrid material is created. These hybrid structures can be porous or non-porous and offer manifold functionalities.
• The sonochemical synthesis of MOFs is a promising technique as the metal-organic crystals are produced very efficient and environmental-friendly.
• The ultrasonic production of MOFs can be linearly scaled-up from the preparation of small samples in the lab to full commercial production.

Metal-Organic Frameworks

Crystalline metal-organic frameworks (MOFs) fall within the category of high potential porous materials, which can be used in gas storage, adsorption/ separation, catalysis, as adsorbents, in magnetism, sensor design, and drug delivery. MOFs are typically formed by self-assembly where secondary building units (SBUs) get connected with organic spacers (ligands) to create complex networks. The organic spacers or the metallic SBUs can be modified in order to control the porosity of the MOF, which is crucial regarding its functionalities and its utility for particular applications.

Sonochemical Synthesis of MOFs

Ultrasonic irradiation and the thereby generated cavitation are well known for its unique effects on chemical reactions, known as sonochemistry. The violent implosion of cavitation bubbles generates localized hot spots with exceedingly high transient temperatures (5000 K), pressures (1800 atm), and cooling rates (10¹⁰Ks^-1) as well as shock waves and resulting liquid jets. At these cavitational hot spots, the crystal nucleation and growth, e.g. by Ostwald ripening, is induced and promoted. However, the particle size is limited since those hot spots are characterized by extreme cooling rates meaning that the temperature of the reaction medium falls within milliseconds.
Ultrasound is known to synthesize MOFs rapidly under mild process conditions, such as solvent-free, at room temperature and under ambient pressure. Studies have shown that MOFs can be produced cost-effectively at high yield via sonochemical route. Finally, the sonochemical synthesis of MOFs is a green, environmentally friendly method.

Preparation of MOF-5

In the study of Wang et al (2011), Zn₄O[1,4-benzenedicarboxylate]₃ was synthesized via sonochemical route. 1.36g H₂BDC and 4.84g Zn(NO₃)₂·6H₂O were inilially dissolved in 160mL DMF. Then 6.43g TEA was added to the mixture under ultrasonic irradiation. After 2 h the colorless precipitate was collected by filtration and washed by DMF. The solid was dried at 90°C in vacuum and then stored in a vacuum desiccator.

Preparation of microporous MOF Cu₃(BTC)₂

Li et al. (2009) report the efficient ultrasonic synthesis of three-dimensional (3-D) metal–organic framework (MOF) with 3-D channels, such as Cu₃(BTC)₂ (HKUST-1, BTC = benzene-1,3,5-tricarboxylate). The reaction of cupric acetate and H₃BTC in a mixed solution of DMF/EtOH/H₂O (3:1:2, v/v) under ultrasonic irradiation at ambient temperature and atmospheric pressure for short reaction times (5–60 min) gave Cu₃(BTC)₂ in high yield (62.6–85.1%). These Cu₃(BTC)₂ nano-crystals have dimensions of a size range of 10–200 nm, which are much smaller than those synthesized using conventional solvothermal method. There were no significant differences in physicochemical properties, e.g. BET surface area, pore volume, and hydrogen storage capacity, between Cu₃(BTC)₂ nano-crystals prepared using ultrasonic method and the microcrystals obtained by using improved solvothermal method. Compared with traditional synthetic techniques, such as solvent diffusion technique, hydrothermal and solvothermal methods, the ultrasonic method for the construction of porous MOFs was found to be highly efficient and more environmentally friendly.

Preparation of a One-Dimensional Mg(II) MOF

Tahmasian et al. (2013) report an efficient, low cost, and environmental-friendly route to produce a 3D supramolecular metal-organic framework (MOF) based on MgII, {[Mg(HIDC)(H₂O)₂]⋅1.5H₂O}_n (H3L = 4,5-imidazole-dicarboxylic acid) by using an ultrasonically-assisted route.
Nanostructured {[Mg(HIDC)(H₂O)₂]⋅1.5H₂O}_n was synthesized via the following sonochemical route. To prepare nanosized {[Mg(HIDC)(H2O)2]⋅1.5H₂O}n (1), 20 mL of an solution of the ligand H₃IDC (0.05M) and potassium hydroxide (0.1 M) was positioned a high-density ultrasonic probe with a maximum power output of 305 W. Into this solution 20 mL of an aqueous solution of magnesium nitrate (0.05M) was added dropwise. The obtained precipitates were filtered off, washed with water andethanol, and air-dried (m.p.> 300ºC. (Found: C, 24.84; H, 3.22; N, 11.67%.). IR (cm^-1) selected bands: 3383 (w), 3190 (w), 1607 (br), 1500 (m), 1390 (s), 1242 (m), 820 (m), 652 (m)).
For studying the effect of concentration of initial reagents on size and morphology of nanostructured compound, the above processes were done under the following concentration condition of initial reagents: [HL2−] = [Mg2+] = 0.025 M.

Sono-Synthesis of fluorescent microporous MOFs

Qiu et al. (2008) found a sonochemical route for the rapid synthesis of fluorescent microporous MOF, Zn₃(BTC)₂⋅12H₂O (1) and selective sensing of organoamines using nanocrystals of 1. The results reveal that ultrasonic synthesis is a simple, efficient, low cost, and environmentally friendly approach to nanoscale MOFs.
MOF 1 was synthesized using the ultrasonic method at an ambient temperature and atmospheric pressure for different reaction times of 5, 10, 30, and 90 min, respectively. A control experiment was also carried out to synthesize compound 1 using the hydrothermal method, and the structures were confirmed by IR, elemental analysis and Rietveld analysis of the powder X-ray diffraction (XRD) patterns using WinPLOTR and Fullprof¹³. Surprisingly, the reaction of zinc acetate dihydrate with benzen-1,3,5-tricarboxylic acid (H₃BTC) in 20% of ethanol in water (v/v) under ultrasonic irradiation at ambient temperature and pressure for 5 min gave 1 in a remarkably high yield (75.3%, based on H₃BTC). Also, the yield of 1 increased gradually from 78.2% to 85.3% with increasing the reaction time from 10 to 90 min. This result suggests that rapid synthesis of MOF can be realized in a significantly high yield using sonication. Compared with hydrothermal synthesis of the same compound MOF 1, which is carried out at 140°C at high pressure for 24 h, ultrasonic synthesis is found to be a highly efficient method with high yield and low cost.
Since no product was obtained by mixing zinc acetate with H3BTC in the same reaction medium at ambient temperature and pressure in the absence of ultrasound, it can be concluded that sonication plays an important role during the formation of MOF 1.
 

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