Enhancement of CO₂ Adsorption and Catalytic Properties by Fe-Doping of [Ga₂(OH)₂(L)] (H₄L = Biphenyl-3,3′,5,5′-tetracarboxylic Acid), MFM-300(Ga₂)

Abstract

Metal–organic frameworks (MOFs) are usually synthesized using a single type of metal ion, and MOFs containing mixtures of different metal ions are of great interest and represent a methodology to enhance and tune materials properties. We report the synthesis of [Ga2(OH)2(L)] (H4L = biphenyl-3,3′,5,5′-tetracarboxylic acid), designated as MFM-300(Ga2), (MFM = Manchester Framework Material replacing NOTT designation), by solvothermal reaction of Ga(NO3)3 and H4L in a mixture of DMF, THF, and water containing HCl for 3 days. MFM-300(Ga2) crystallizes in the tetragonal space group I4122, a = b = 15.0174(7) Å and c = 11.9111(11) Å and is isostructural with the Al(III) analogue MFM-300(Al2) with pores decorated with −OH groups bridging Ga(III) centers. The isostructural Fe-doped material [Ga1.87Fe0.13(OH)2(L)], MFM-300(Ga1.87Fe0.13), can be prepared under similar conditions to MFM-300(Ga2) via reaction of a homogeneous mixture of Fe(NO3)3 and Ga(NO3)3 with biphenyl-3,3′,5,5′-tetracarboxylic acid. An Fe(III)-based material [Fe3O1.5(OH)(HL)(L)0.5(H2O)3.5], MFM-310(Fe), was synthesized with Fe(NO3)3 and the same ligand via hydrothermal methods. [MFM-310(Fe)] crystallizes in the orthorhombic space group Pmn21 with a = 10.560(4) Å, b = 19.451(8) Å, and c = 11.773(5) Å and incorporates μ3-oxo-centered trinuclear iron cluster nodes connected by ligands to give a 3D nonporous framework that has a different structure to the MFM-300 series. Thus, Fe-doping can be used to monitor the effects of the heteroatom center within a parent Ga(III) framework without the requirement of synthesizing the isostructural Fe(III) analogue [Fe2(OH)2(L)], MFM-300(Fe2), which we have thus far been unable to prepare. Fe-doping of MFM-300(Ga2) affords positive effects on gas adsorption capacities, particularly for CO2 adsorption, whereby MFM-300(Ga1.87Fe0.13) shows a 49% enhancement of CO2 adsorption capacity in comparison to the homometallic parent material. We thus report herein the highest CO2 uptake (2.86 mmol g–1 at 273 K at 1 bar) for a Ga-based MOF. The single-crystal X-ray structures of MFM-300(Ga2)-solv, MFM-300(Ga2), MFM-300(Ga2)·2.35CO2, MFM-300(Ga1.87Fe0.13)-solv, MFM-300(Ga1.87Fe0.13), and MFM-300(Ga1.87Fe0.13)·2.0CO2 have been determined. Most notably, in situ single-crystal diffraction studies of gas-loaded materials have revealed that Fe-doping has a significant impact on the molecular details for CO2 binding in the pore, with the bridging M–OH hydroxyl groups being preferred binding sites for CO2 within these framework materials. In situ synchrotron IR spectroscopic measurements on CO2 binding with respect to the −OH groups in the pore are consistent with the above structural analyses. In addition, we found that, compared to MFM-300(Ga2), Fe-doped MFM-300(Ga1.87Fe0.13) shows improved catalytic properties for the ring-opening reaction of styrene oxide, but similar activity for the room-temperature acetylation of benzaldehyde by methanol. The role of Fe-doping in these systems is discussed as a mechanism for enhancing porosity and the s...

Modulating supramolecular binding of carbon dioxide in a redox-active porous metal-organic framework

“…MFM-300(V III ) crystallizes in the space group I 4 1 22 and is isostructural to MFM-300(M) (M=Al, Ga, In, see Supplementary Table 1 for single crystal data) 26 27 28 . The V III centre shows octahedral coordination defined by four carboxylate oxygen atoms from the ligand [V–O=2.014(2) and 2.030(2) Å] and two oxygen atoms from two hydroxyl groups [V–O=1.948(2) Å] ( Fig.…”

Understanding the adsorption process in ZIF-8 using high pressure crystallography and computational modelling

“…1 ) 4 . Whilst reports using techniques such as IR 5 , 6 , Raman 7 and solid-state NMR spectroscopy 8 have inferred structural changes on the binding modes of gases within MOFs, more direct evidence of structural changes can be obtained using in-situ crystallography. Such experiments usually involve a gas capillary cell, where MOFs can be exposed to varying pressures of gas.…”

Selective Adsorption of Sulfur Dioxide in a Robust Metal–Organic Framework Material

“…The complex MFM‐300(In) (MFM = Manchester Framework Material replacing the previous NOTT (Nottingham) designation) [In 2 (OH) 2 (L)] (H 4 L = biphenyl‐3,3′,5,5′‐tetracarboxylic acid) was synthesized by solvothermal reaction of In(NO 3 ) 3 and H 4 L in DMF ( N , N ‐dimethylformamide) at 90 °C . MFM‐300(In) is isostructural to MFM‐300(Al)[12a] and MFM‐300(Ga), and comprises chains of corner sharing [InO 4 (OH) 2 ] octahedra linked by mutually cis ‐μ 2 ‐OH groups, and further bridged by tetracarboxylate L 4− ligands. This arrangement generates a highly porous material, defined by one‐dimensional pore channels bounded by In‐OH‐In groups and ligand phenyl rings in a “wine rack” array.…”