Graduate Researcher, Virginia Polytechnic Institute and State University
Metal-organic frameworks with new photochemical properties upon incorporation of ruthenium complex
Metal-organic frameworks (MOFs) are crystalline formations of organic ligand struts connected by metal-oxide knots. They have been shown to be extremely porous, extremely stable, and extremely tunable. Swapping out struts, knots, pore size, and molecules within pores has made for novel useful properties such as gas storage, photocatalysis, conductivity, and even solar applications.
My research focuses on the photochemical aspect, furthering the tunability of MOFs by utilizing a standard ruthenium metal complex as a substitute for some of the struts of the crystal formation. Struts (or linkers) have been shown in the past, though simply organic ligands, to induce photoactive charge-separation by donating electrons to the metal knots (or nodes). This was already a long-lived excited state which, upon re-emission, could induce photocatalysis in reactions such as proton reduction. However, with a metal complex substituting the organic ligand for a more photoactive compound, catalytic and solar applications are expected to expand vastly.
Abstract: Aimed to developing renewable energy based processes, researchers are paying increasing interest to light induced organic transformations. Metal-organic frameworks (MOFs), a class of micro-mesoporous hybrid materials, are recently emerging as a new type of photoactive materials for organic syntheses due to their unique structural characteristics. In this review, we summarized the recent applications of MOFs as photocatalysts for light induced organic transformations, including (1) oxidation of alcohols, amines, alkene, alkanes and sulfides; (2) hydroxylation of aromatic compounds like benzene; (3) activation of the C-H bonds to construct new C-C or C-X bonds; (4) atom-transfer radical polymerization (ATRP). This review started with general background information of using MOFs for photocatalysis, followed by a description of light induced organic transformations promoted by photoactive inorganic nodes and photocatalytic active ligands in MOFs, respectively. Thereafter, the use of MOFs as multifunctional catalysts for light induced organic transformations via an efficient merge of the metal/ligand/guest based catalysis where the photocatalytic activity of MOFs plays a key role are discussed. Finally, the limitations, challenges and the future perspective of the application of MOFs for light induced organic transformations were also addressed. The objective of this review is to serve as a starting point for other researchers to get into this largely unexplored field. It is also our goal to stimulate intensive research in this field for rational designing of MOF materials to overcome their current limitations in photocatalysis, which can lead to more creative visible-light-induced organic transformations.
Pub.: 16 May '17, Pinned: 05 Jul '17
Abstract: The preparation of transparent porous materials can offer a different access towards the study of molecules under solid confined space. Metal-organic frameworks represent a unique opportunity due to their tunable pore size, however aromatic linkers present strong absorption and reduce the transparency. Herein, we report the first example of a MOF with bicyclic organic dicarboxylic linkers and its use as a solid solvent.
Pub.: 24 May '17, Pinned: 05 Jul '17
Abstract: The removal of nitrogen-containing compounds (NCCs) from fossil fuels prior to combustion is currently of particular importance, and so we investigated an adsorptive method using metal organic frameworks (MOFs) for the removal of indole (IND) and quinoline (QUI), which are two of the main NCCs present in fossil fuels. We herein employed an amino (-NH2)-functionalized MIL-125 (MIL-125-NH2) MOF, which was further modified by protonation (P-MIL-125-NH2). These modified MOFs exhibited extraordinary performance in the adsorption of both IND (as representative neutral NCC) and QUI (as representative basic NCC). These MOFs were one of the most efficient adsorbents for the removal of NCCs. For example, P-MIL-125-NH2 showed the highest adsorption capacity for QUI among ever reported adsorbent. The improved adsorption of IND was explained by H-bonding and cation-π interactions for MIL-125-NH2 and P-MIL-125-NH2, respectively, while the mechanisms for QUI were H-bonding and acid-base interactions, respectively. This is a rare phenomenon for a single material (especially not with very high porosity) to exhibit such remarkable performances in the adsorption of both basic QUI and neutral IND. The adsorption results obtained using regenerated MIL-125-NH2 and P-MIL-125-NH2 also showed that these materials can be used several times without any severe degradation.
Pub.: 02 Jun '17, Pinned: 05 Jul '17
Abstract: Two-dimensional metal-organic frameworks represent a family of materials with attractive chemical and structural properties, which are usually prepared in the form of bulk powders. Here we show a generic approach to fabricate ultrathin nanosheet array of metal-organic frameworks on different substrates through a dissolution-crystallization mechanism. These materials exhibit intriguing properties for electrocatalysis including highly exposed active molecular metal sites owning to ultra-small thickness of nanosheets, improved electrical conductivity and a combination of hierarchical porosity. We fabricate a nickel-iron-based metal-organic framework array, which demonstrates superior electrocatalytic performance towards oxygen evolution reaction with a small overpotential of 240 mV at 10 mA cm(-2), and robust operation for 20,000 s with no detectable activity decay. Remarkably, the turnover frequency of the electrode is 3.8 s(-1) at an overpotential of 400 mV. We further demonstrate the promise of these electrodes for other important catalytic reactions including hydrogen evolution reaction and overall water splitting.
Pub.: 06 Jun '17, Pinned: 05 Jul '17
Abstract: We employed metal-organic framework (MOF) supports to modulate the electronic states of loaded Pt nanoparticles (NPs) in their composite catalysts (Pt/MOFs). Pt NPs were homogenously deposited on four MOFs characterized with different electronic states (Zn-MOF-74, Mg-MOF-74, HKUST-1, and UiO-66-NH2). Theoretical and experimental studies demonstrated that a charge-transfer interaction between Pt NPs and MOFs is a critical factor for controlling the catalytic activity of Pt NPs supported on MOFs.
Pub.: 07 Jun '17, Pinned: 05 Jul '17
Abstract: Identifying the metal ions that optimize charge transport and charge density in metal-organic frameworks is critical for systematic improvements in the electrical conductivity in these materials. In this work, we measure the electrical conductivity and activation energy for twenty different MOFs pertaining to four distinct structural families: M2(DOBDC)(DMF)2 (M = Mg(2+), Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+)); H4DOBDC = 2,5-dihydroxybenzene-1,4-dicarboxylic acid; DMF = N,N-dimethylformamide), M2(DSBDC)(DMF)2 (M = Mn(2+), Fe(2+); H4DSBDC = 2,5-disulfhydrylbenzene-1,4-dicarboxylic acid), M2Cl2(BTDD)(DMF)2 (M = Mn(2+), Fe(2+), Co(2+), Ni(2+); H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i]dibenzo[1,4]dioxin), and M(1,2,3-triazolate)2 (M = Mg(2+), Mn(2+), Fe(2+), Co(2+), Cu(2+), Zn(2+), Cd(2+)). This comprehensive study allows us to single-out iron as the metal ion that leads to the best electrical properties. The iron-based MOFs exhibit at least five orders of magnitude higher electrical conductivity and significantly smaller charge activation energies across all different MOF families studied here and stand out materials made from all other metal ions considered here. We attribute the unique electrical properties of iron-based MOFs to the high-energy valence electrons of Fe(2+) and the Fe(3+/2+) mixed valency. These results reveal that incorporating Fe(2+) in the charge transport pathways of MOFs and introducing mixed valency are valuable strategies for improving electrical conductivity in this important class of porous materials.
Pub.: 16 Jun '17, Pinned: 05 Jul '17
Abstract: A series of Ru(II)L2L′ (L = 2,2′-bipyridyl, L′ = 2,2′-bipyridine-5,5′-dicarboxylic acid), RuDCBPY, -containing zirconium(IV) coordination polymer thin films have been prepared as sensitizing materials for solar cell applications. These metal–organic framework (MOF) sensitized solar cells, MOFSCs, each are shown to generate photocurrent in response to simulated 1 sun illumination. Emission lifetime measurements indicate the excited state quenching of RuDCBPY at the MOF–TiO2 interface is extremely efficient (>90%), presumably due to electron injection into TiO2. A mechanism is proposed in which RuDCBPY-centers photo-excited within the MOF-bulk undergo isotropic energy migration up to 25 nm from the point of origin. This work represents the first example in which a MOFSC is directly compared to the constituent dye adsorbed on TiO2 (DSC). Importantly, the MOFSCs outperformed their RuDCBPY–TiO2 DSC counterpart under the conditions used here and, thus, are solidified as promising solar cell platforms.
Pub.: 16 Oct '15, Pinned: 05 Jul '17