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 structural integrity of the parent material.
Highlights
Porous metal−organic frameworks (MOFs) have attracted a great deal of interest because of their potential applications in gas adsorption and separation, catalysis, and drug delivery.[1−3] The assembly of MOF materials from various metal ions and organic linkers, usually via solvothermal reactions, allows the fine-tuning of their crystal structures and the incorporation of designed functional groups for specific applications
Significant enhancement of CO2 adsorption capacity by up to 49% was observed by doping with Fe(III), reflecting the increased structural integrity of the
In situ single-crystal X-ray diffraction studies of CO2-loaded materials revealed, on a molecular level, key details into the preferred binding sites within the pores of these materials that were in excellent agreement with the results of the in situ polarized IR spectroscopic study of CO2-loaded MFM300(Ga2)
Summary
Porous metal−organic frameworks (MOFs) have attracted a great deal of interest because of their potential applications in gas adsorption and separation, catalysis, and drug delivery.[1−3] The assembly of MOF materials from various metal ions and organic linkers, usually via solvothermal reactions, allows the fine-tuning of their crystal structures and the incorporation of designed functional groups for specific applications. The properties, size, and functionality of the cavity of these porous materials can be optimized by using different metal centers or organic ligands.[4,5] Amine (−NH2) groups can bind selectively to CO2 due to the formation of strong electrostatic interactions between the electronegative N center of the −NH2 group and the electropositive C center of the CO2 molecule.[6] For this reason, a number of amine-functionalized MOFs have been designed and synthesized to capture CO2 from flue gases.[7,8] the effect of the different metal ions in MOFs on CO2 adsorption properties has been rarely studied. MOFs are usually constructed from a single type of metal cation and organic linker, but there are increasing examples of MOFs which contain two different types of ligand linkers or metal cations with a homogeneous distribution, as found in solid solutions.[12−23] Kitagawa et al.[17] and Cheetham[18] et al have reported the syntheses and properties of some binary and ternary
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