Synthesis of 2D and 3D MOFs with tuneable Lewis acidity from preformed 1D hybrid sub-domains

Abstract

Novel MOF-type materials with different morphologies based on assembled 1D organic-inorganic sub-domains were prepared using specific monodentate benzylcarboxylate spacers with functional substituents in the para-position as structure modulating agents. The combination of electron-withdrawing or electron-donating functions in the organic spacers with suitable solvothermal synthesis conditions allowed modulating the structuration level (2D or 3D), vacancies, physico-chemical properties and Lewis acidity strength of the metal-organic structures. Furthermore, bimetallic (Al/Fe) MOF-type materials were synthesized by a one-pot direct process without modification of the structural framework. The activity of these hybrid materials as Lewis acid catalysts was evaluated to prepare cyanohydrins as precursors for the synthesis of biologically active compounds, and for aerobic oxidation of thiols to disulfides. The catalytic results showed that the derived MOFs exhibited modulatable Lewis acid capacities which are a function of the morphology, functionality of monodentate substituents present in the networks and a cooperative effect between metallic nodes of different nature.

Scheme 1. Synthesis routes to obtain Al-MOF-type materials with different morphologies, based on associated 1D organic–inorganic nanoribbons, depending on functional organic spacers used during the solvothermal process. At the bottom of the illustration, details of 1D sub-domains are shown with a (organic spacer number/Al) ratio of 1 for clarity (in blue, chain based on associated aluminium octahedra following a Kagomé conformation; in grey, ethylbenzoate monodentate spacers; in red, oxygen atoms from carboxylate groups).

Fig. 1. XRD patterns of the derived Al-metal–organic materials: (a) 3D standard MIL-53(Al), (b) Al-ITQ-Br, (c) Al-ITQ-NO₂, (d) Al/Fe-ITQ-NO₂, (e) L-MOF-EB and (f) L-MOF-AB.

Fig. 2. FESEM images of 3D Al-MOFs: (a) and (b) Al-ITQ-Br, (c) and (d) Al-ITQ-NO₂ and (e) and (f) Al/Fe-ITQ-NO₂. Scale bars correspond to 1 μm for (a), (c), (e) and (f) micrographs, and 200 nm for (b) and (d) micrographs.

Fig. 3. EDS mapping of metallic elements in 3D Al/Fe-ITQ-NO₂.

Fig. 4. TEM images of 2D Al-MOFs: (a) and (b) L-MOF-EB and (c) and (d) L-MOF-AB. Scale bars correspond to 5 μm, 200 nm, 1 μm and 100 nm for (a), (b), (c) and (d) micrographs, respectively.

Fig. 5. ¹³C MAS NMR spectra: (a) Al-ITQ-Br, (b) Al-ITQ-NO₂, (c) Al/Fe-ITQ-NO₂, (d) L-MOF-EB and (e) L-MOF-AB.

Fig. 6. ²⁷Al MAS NMR spectra: (a) Al-ITQ-Br, (b) Al-ITQ-NO₂, (c) Al/Fe-ITQ-NO₂, (d) L-MOF-EB and (e) L-MOF-AB.

Fig. 7. IR spectra: (a) Al-ITQ-Br, (b) Al-ITQ-NO₂, (c) Al/Fe-ITQ-NO₂, (d) L-MOF-EB and (e) L-MOF-AB. On the left, infrared spectra in the hydroxyl stretching region. On the right, infrared spectra in the framework region.

Fig. 8. CO₂ isotherms at 0 °C up to 1 bar of Al and Al/Fe-metal–organic materials.

Scheme 2. Cyanosilylation of benzaldehyde.

Fig. 9. Kinetics for cyanohydrins production using Al-ITQ-NO₂, Al/Fe-ITQ-NO₂, Al-ITQ-Br, L-MOF-AB, L-MOF-EB, MIL-53(Al) and MIL-53(Al)–NO₂ as solid catalysts (0.5 mol% Al). Blank test was performed, obtaining a yield of 24% at 5 h.

Fig. 10. Kinetics for cyanosilylation of aromatic aldehydes catalyzed by Al-ITQ-NO₂ (0.5 mol% Al).

Scheme 3. Oxidation of thiophenol to diphenyldisulfide.

Fig. 11. Kinetics for diphenyldisulfide production using Al/Fe-ITQ-NO₂, Al-ITQ-NO₂, MIL-53(Al) and MIL-53(Al)–NO₂ as solid catalysts (10 mol% Fe). Blank test was performed, obtaining a yield of 3% at 10 h.