In Situ Monitoring and Mechanism of the Mechanochemical Formation of a Microporous MOF-74 Framework

Abstract

Mechanochemistry provides a rapid, efficient route to metal−organic framework Zn-MOF-74 directly from a metal oxide and without bulk solvent. In situ synchrotron X-ray diffraction monitoring of the reaction course reveals two new phases and an unusual stepwise process in which a close-packed intermediate reacts to form the open framework. The reaction can be performed on a gram scale to yield a highly porous material after activation.M etal−organic frameworks (MOFs) 1 are advanced materials with applications ranging from storage and separation of fuel gases, 2 CO 2 sequestration, 3 and degradation of nerve agents 4 to fuel cells, 5 catalysis, 6 drug delivery 7 and light harvesting. 8 Commercialization of MOFs has highlighted unique synthetic challenges, 9 often involving solvothermal conditions and soluble reagents which, while common in a laboratory, are intractable in large-scale manufacturing due to issues of cost, toxicity, and explosive (nitrates) or corrosive (chlorides) nature. 9,10 It was recently demonstrated that liquid-catalyzed mechanochemistry 11 (e.g., liquid-assisted grinding, LAG) permits facile, room-temperature transformation of safer metal oxide, carbonate, or hydroxide reactants into MOFs, resulting in cleaner, more atom-efficient processes that avoid external bases and production of mineral acids or their salts as byproducts. 12,13 Indeed, MOFs can now be manufactured mechanochemically on a large scale by extrusion. 14 However, scope of mechanochemistry for making currently relevant MOFs remains modest, limited to HKUST-1 and ZIF-8. 15 We now describe the development and mechanistic investigation of a mechanochemical milling approach to Zn-MOF-74, 16 a member of the popular M-MOF-74 (CPO-27) family of materials, 17−21 from stoichiometric ZnO and 2,5-dihydroxyterephthalic acid (H 4 dhta) (Figure 1). By using the very recently introduced technique for real-time in situ X-ray powder diffraction (XRPD) monitoring, 22,23 we reveal a previously not seen mechanism of mechanochemical MOF synthesis, where the formation of a low-density metal−organic structure proceeds via a close-packed reaction intermediate.Without included guests, Zn-MOF-74 has the composition Zn 2 (H 2 O) 2 (dhta), consisting of Zn 2+ coordinated by H 4 dhta anions and water. We attempted the synthesis of Zn-MOF-74 on 1.1 mmol scale (∼400 mg, see SI) by milling ZnO and H 4 dhta in 2:1 stoichiometric ratio, using 250 μL of water as the grinding liquid. 24 The liquid-to-solid ratio (η) 25 of 0.625 μL/mg was selected based on our previous experience in LAG mechanosynthesis of open MOFs. 13a,15a In situ experiments were done at the European Synchrotron Radiation Facility (ESRF) beamline ID15B using X-rays of 0.142 Å wavelength and also at a new measurement site at the Deutsches Elektronen-Synchroton (DESY) beamline P02.1, which provided improved signal-tonoise ratio and higher resolution data by using 0.207 Å radiation. 22,23 Milling was conducted using a modified Retsch mill operating at 30 Hz, in a 14 mL poly(methy...

Template‐Directed Approach Towards the Realization of Ordered Heterogeneity in Bimetallic Metal–Organic Frameworks

“…[8b] Thepostsynthetic approach enabled the random incorporation of metal ions that cannot be included through the conventional one-pot approach. [9c] Despite these promising results,achieving atomic-level ordering in multi-metal MOFs has been an enormous challenge.I mportantly,t he control over the arrangement of different metals could lead to the development of materials with tunable magnetic,e lectrical, and catalytic properties as well as interactions with guest molecules,t hus imparting complex functions to the resulting MOFs.O ne possible approach to synthesize ordered multimetal MOFs is atemplate-directed approach, which involves aone-dimensional (1D) metal-organic polymer incorporating well-defined binding sites as as tructural template for the inclusion of asecondary metal and subsequent conversion of this complex into the corresponding three-dimensional (3D) MOF structure.R ecently,J ulien and co-workers [10] have reported on the mechanochemical synthesis of as inglemetal MOF-74, in which they identified the 1D metal-organic polymer as an intermediate phase in the formation of 3D MOF-74, thus demonstrating the possibility of using 1D polymers as starting materials for the MOF synthesis.Herein, we report ( Figure 1) on the preparation of well-ordered bimetallic MOF-74 using a1 Dm etal-organic polymer incorporating 2,5-dihydroxy-1,6-benzenedicarboxylate and zinc ions as as tructural template and starting material. The presence of well-defined binding pockets within the polymer Figure 1.…”

An in situ investigation of the water-induced phase transformation of UTSA-74 to MOF-74(Zn)

“…including liquid assisted grinding 21 and water-based syntheses relying on stoichiometric amounts of NaOH to deprotonate the H4dobdc linkers. 22,23 However, most syntheses are based on the originally reported solvothermal synthesis, 8 where H4dobc and Zn(NO3)2·6H2O are dissolved in N,N-dimethylformamide (DMF) together with a small amount of water, presumably for solubility reasons, and allowed to react at 105 °C.…”

Efficient Solvent‐Free Carbon Dioxide Fixation Reactions with Epoxides Under Mild Conditions by Mixed‐Ligand Zinc(II) Metal–Organic Frameworks

“…Solvothermal and other solution‐based methods are the mainstream routes to achieve MOFs, and little attention has been paid to nonconventional synthetic methods. Mechanochemical (milling or grinding) reactions are a rapid synthetic alternative for the bulk preparation of MOFs with advantages such as a low energy requirement, low time consumption, minimal solubility issues, and environmental friendliness . To expand the scope of ecofriendly bulk synthesis, both MOFs were synthesized simply by the manual grinding of the precursor ligands with the metal salts in an appropriate stoichiometry.…”