Defective hierarchical porous copper-based metal-organic frameworks synthesised via facile acid etching strategy

Abstract

Introducing hierarchical pore structure to microporous materials such as metal-organic frameworks (MOFs) can be beneficial for reactions where the rate of reaction is limited by low rates of diffusion or high pressure drop. This advantageous pore structure can be obtained by defect formation, mostly via post-synthetic acid etching, which has been studied extensively on water-stable MOFs. Here we show that a water-unstable HKUST-1 MOF can also be modified in a corresponding manner by using phosphoric acid as a size-selective etching agent and a mixture of dimethyl sulfoxide and methanol as a dilute solvent. Interestingly, we demonstrate that the etching process which is time- and acidity- dependent, can result in formation of defective HKUST-1 with extra interconnected hexagonal macropores without compromising on the bulk crystallinity. These findings suggest an intelligent scalable synthetic method for formation of hierarchical porosity in MOFs that are prone to hydrolysis, for improved molecular accessibility and diffusion for catalysis.

Etching and Polymerization Reactions of Alkoxythiophenes in HKUST-1: Choosing Between Filled and Core-Shell MOF/Polymer Composite Structures

“…(Figure 4) In each composite for which significant reaction/modification (based on IR, BET, and EDS measurements) occurred, significant changes in the intensity are seen in the very low diffraction angle MOF peaks. The modified materials show a sharp increase in the ratio of the (002) peak to the (111) peak, based on literature peak assignments, 19,24 consistent with our observation of the etching-and-recrystallization phenomenon seen in Figure 3. Smaller monomers EDOT and 3MOT result in a near-complete disappearance of the (111) peak, while this peak is still seen with significant intensity in the bulkier hexyloxy monomer 3HOT.…”

“…Both species have the capacity to etch the lattice by protonation of linker ends and introduction of Cu(I) defects respectively. 19,20 The partially degraded HKUST-1 is recrystallized by heating, resulting in full conversion to the more stable 21 hexagonal form with a largerproportion of the (111) face. The barrier to annealing of HKUST-1 is likely reduced by the presence of defects introduced during the monomer/MOF reaction, which accounts for the structural change during heating.…”

Etching and Polymerization Reactions of Alkoxythiophenes in HKUST-1: Choosing Between Filled and Core-Shell Composite Structures

“…18 SEM of the intermediate material, (Figure 3, top) in which monomer was loaded but not yet heat-treated, suggests an explanation for this conversion. Visibly etched octahedral crystals can be seen in the micrographs of the 19,20 The partially degraded HKUST-1 is recrystallized by heating, resulting in full conversion to the less stable 21 orthorhombic form with a smaller proportion of the {111} face. The barrier to annealing of HKUST-1 is likely reduced by the presence of defects introduced during the monomer/MOF reaction, which accounts for the structural change during heating.…”

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

“…SEM analysis of these samples revealed that the typical morphology of normal Zn-MOF-74 is octahedral with smooth faces, while Zn-MOF-74 in scCO2 shows a similar morphology but with very rough faces (Figure 2b), which might be due to an acidic etching effect of the CO2 at 75 bar and 40 °Ϲ for 72 h. Effect of the acid etching on surface roughness have been previously investigated on amorphous silica and ceramic [68,69]. In addition, etching of MOFs in aqueous acidic solutions such as a mixture of hydroquinone and MeOH (up to 180 °Ϲ for 72 h) or a mixture of phosphoric acid, MeOH and DMSO (at 40 °Ϲ for up to 10 days) has been shown to result in remarkable effects such as interconnected geometrical macropores and etched hole features [37,51,70]. 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, by changing to hydrogen-bonded complexes (using dimethyl sulfoxide (DMSO) instead of dimethylformamide (DMF) to form a precursor solution in HKUST preparation), Doan et al reported that the formation of the MOF occurred before additional macropores were introduced by scCO 2 . 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].…”