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)

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• 作者：Cristina P. Krap ; Ruth Newby ; Amarajothi Dhakshinamoorthy ; Hermenegildo García ; Izabela Cebula ; Timothy L. Easun ; Mathew Savage ; Jennifer E. Eyley ; Shan Gao ; Alexander J. Blake ; William Lewis ; Peter H. Beton ; Mark R. Warren ; David R. Allan ; Mark D. Frogley ; Chiu C. Tang ; Gianfelice Cinque ; Sihai Yang ; Martin Schr?der
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：February 1, 2016
• 年：2016
• 卷：55
• 期：3
• 页码：1076-1088
• 全文大小：728K
• ISSN：1520-510X

文摘

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) ? and c = 11.9111(11) ? 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 [Ga_1.87Fe_0.13(OH)₂(L)], MFM-300(Ga_1.87Fe_0.13), 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 [Fe₃O_1.5(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) ?, b = 19.451(8) ?, and c = 11.773(5) ? 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(Ga_1.87Fe_0.13) 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(Ga_1.87Fe_0.13)-solv, MFM-300(Ga_1.87Fe_0.13), and MFM-300(Ga_1.87Fe_0.13)·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(Ga_1.87Fe_0.13) 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.