Controlled Formation of Hierarchical Metal–Organic Frameworks Using CO₂-Expanded Solvent Systems

Abstract

It is shown that a crystalline metal–organic framework (HKUST-1) can be rapidly synthesized from a DMSO/MeOH solution with greatly reduced amounts of organic solvents using a supercritical CO2 (scCO2) solvent expansion technique. The precursor solution is stable for months under ambient conditions, and CO2-driven MOF (metal–organic framework) crystallization is achieved under mild conditions (40 °C, 40–100 bar) with excellent reproducibility. As the degree of liquid-phase expansion drives MOF nucleation and growth, the crystallite size and overall yield can be tuned by adjusting the CO2 pressure. Furthermore, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), and gas sorption analyses showed that, in the presence of scCO2, HKUST-1 crystallites with a hierarchical pore structure are generated through a postcrystallization etching process. These findings demonstrate that scCO2 is a time- and material-efficient route to MOF synthesis with a high level of control over the crystallization process for accessing tailored material properties.

Using Supercritical CO2 in the Preparation of Metal-Organic Frameworks: Investigating Effects on Crystallisation

“…The results of these syntheses are further discussed below. The use of DMSO in BTC to form hydrogen-bonded complexes as reported in the HKUST-1 synthesis [47] was extended to the Zn-BTC. However, this method did not seem to be successful for this MOF, with no precipitation occurring from the stock solution after adding up to ten volume equivalents of MeOH and introducing scCO2 into the system after 3 days.…”

“…Using scCO 2 during the synthesis has been shown to be useful for introducing additional porosity to traditionally microporous or mesoporous MOFs [2,3,47,48]. Unlike other methods such as templating or gelation, using scCO 2 can preclude the use of additional purification or activation steps to obtain hierarchical structures without pore collapse.…”

“…The latter synthesis can be related to the etching mechanism, which was further studied using different acidic agents [36,37,51]. In addition, use of scCO 2 as an anti-solvent (i.e., a solvent in which the crystals are less soluble) to trigger nucleation from a MOF stock solution [47] was shown to remove the need for the 50-times excess of methanol antisolvent used in the original syntheses [52]. Here, we report our investigations into the use of scCO 2 in different MOF syntheses to provide information which could improve the understanding and utility of such approaches.…”

“…The scCO 2 synthetic method which was successfully employed for HKUST-1 [39,47] and Znbenzene-1,3,5-tricarboxylate (BTC) (in DMF) [38] has here been extended to the Zn-MOF-74, UiO-66 and Zn-BTC (in DMSO) MOF systems, [Cu 24 (OH-mBDC) 24 ] n MOPs, as well as to two new flexible MOF materials {[Zn 2 (L 1 )(DPE)]•4H 2 O} n and {[Zn 3 (L 1 ) 3 (4,4 -azopy)]•7.5H 2 O} n . The first four MOF/MOP materials were examined to see the effect of scCO 2 on the structure and yield of the products.…”

“…[Cu 24 (OH-mBDC) 24 ] n (consisting of metal-organic polyhedral nanocages) was self-assembled from the coordination between copper ions and the angular bifunctional ligand benzene-1,3-dicarboxylate and N,N'-dimethylacetamide, forming a discrete structure [54]. Zn-MOF-74 was also synthesised with scCO 2 in the same manner as reported in the literature [47], in order to investigate the behaviour of CO 2 towards different solvent systems. UiO-66 was chosen as an example of a well-known MOF whereby low-temperature syntheses (40 • C) affords gel-like UiO-66 with many defect sites and low crystallinity.…”