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

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 [Ga-2(OH)(2)(L)] (H4L = biphenyl-3,3',5,5'-tetracarboxylic acid), designated as MFM-300(Ga-2), (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(Ga-2) crystallizes in the tetragonal space group I4(1)22, a = b = 15.0174(7) angstrom and c = 11.9111(11) angstrom and is isostructural with the Al(III) analogue MFM-300(Al-2) 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(Ga-2) 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 Pmn2(1) with a = 10.560(4) angstrom, b = 19.451(8) angstrom, and c = 11.773(5) angstrom and incorporates mu(3)-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 [Fe-2(OH)(2)(L)], MFM-300(Fe-2), which we have thus far been unable to prepare. Fe-doping of MFM-300(Ga-2) 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(Ga-2)-solv, MFM-300(Ga-2), MFM-300(Ga-2)center dot 2.35CO(2), MFM-300(Ga1.87Fe0.13)-solv, MFM-300(Ga1.87Fe0.13), and MFM-300(Ga1.87Fe0.13)center dot 2.0CO(2) 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(Ga-2), 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.