Xuebo Zhao

High gas storage capacities and stepwise adsorption in a UiO type metal–organic framework incorporating Lewis basic bipyridyl sites

A UiO type MOF with Lewis basic bipyridyl sites was synthesized and structurally characterized. After being activated by Soxhlet-extraction, this MOF exhibits high storage capacities for H2, CH4 and CO2, and shows unusual stepwise adsorption for liquid CO2 and solvents, indicating a sequential filling mechanism on different adsorption sites.

High oxygen reduction activity on a metal–organic framework derived carbon combined with high degree of graphitization and pyridinic-N dopants

N-doped porous carbons have been considered as one of the most promising earth-abundant catalysts for the oxygen reduction reaction (ORR) owing to their high activity and excellent stability. Among various types of N-containing groups, pyridinic-N has been identified as the most effective catalytic sites for the ORR, as demonstrated by a recent study. However, the fabrication of porous carbons with a high density of exposed pyridinic-N sites has been rarely reported. In this work, the ORR catalytic properties of a series of pyridinic-N doped porous carbon (PNPC) which was derived by carbonization of a pyridyl-ligand based MOF were investigated. At different carbonization temperatures, this series of carbon exhibits different pyridinic-N contents and different degrees of carbonization. The ORR studies show that the graphitization degree of carbon has a significant impact on ORR catalytic activity besides N-groups. Electrochemical impedance spectroscopy (EIS) reveals that the electron transfer resistance in the ORR decreases significantly with the higher degree of graphitization, which gives rise to a higher ORR activity for these PNPCs. The synergistic effect of the high density of pyridinic-N sites and decreased electron impedance results in remarkably improved ORR activity which is comparable with that of the commercial Pt/C (10 wt%) catalyst. The result of this work could provide some guidance for designing or synthesizing highly efficient ORR catalysts.

Oxygen reduction in the nanocage of metal-organic frameworks with an electron transfer mediator

A highly porous metal–organic framework (MOF) containing copper metal centres and nanocages was modified onto a glassy carbon electrode as a noble-metal-free electrocatalyst for oxygen reduction reaction. The nanocages in the metal–organic framework were fully activated by the solvent-exchange method. Although both the as-prepared MOF and activated MOF samples showed electrochemical activity of Cu2+/Cu+ redox pairs by cyclic voltammetric studies, only the activated MOF samples could catalyze an oxygen reduction reaction. In order to avoid detachment of the activated sample from the glassy carbon electrodes surface owing to a low-effective electron-transfer pathway during electrochemical scanning in aqueous solution, reduced graphene oxide (RGO) was immobilized onto a glassy carbon electrode surface as a binder and electron transfer mediator under MOF active layer. The MOF layer on RGO immobilized glassy carbon electrode can catalyze the oxygen reduction reaction through a 2–4 electrons reduction pathway. Furthermore, the occurrence potential of ORR versus Ag/AgCl by MOF catalyst shifted to the positive near 100 mV in comparison with other MOF catalysts.

Enhanced Uptake and Selectivity of CO2 Adsorption in a Hydrostable Metal-Organic Frameworks via Incorporating Methylol and Methyl Groups

A new methylol and methyl functionalized metal-organic frameworks (MOFs) QI-Cu has been designed and synthesized. As a variant of NOTT-101, this material exhibits excellent CO2 uptake capacities at ambient temperature and pressure, as well as high CH4 uptake capacities. The CO2 uptake for QI-Cu is high, up to 4.56 mmol g(-1) at 1 bar and 293 K, which is top-ranked among MOFs for CO2 adsorption and significantly larger than the nonfunctionalized NOTT-101 of 3.93 mmol g(-1). The enhanced isosteric heat values of CO2 and CH4 adsorption were also obtained for this linker functionalized MOFs. From the single-component adsorption isotherms, multicomponent adsorption was predicted using the ideal adsorbed solution theory (IAST). QI-Cu shows an improvement in adsorptive selectivity of CO2 over CH4 and N2 below 1 bar. The incorporation of methylol and methyl groups also greatly improves the hydrostability of the whole framework.

In Situ Synthesis Strategy for Hierarchically Porous Ni2P Polyhedrons from MOFs Templates with Enhanced Electrochemical Properties for Hydrogen Evolution

The development of highly active and stable noble metal-free electrocatalysts of hydrogen evolution reaction (HER) under both acidic and basic conditions for renewable-energy conversion techniques is of great significance. Herein, a practical in situ synthesis strategy for a three-dimensional Ni2P polyhedron with a hierarchically porous structure was presented, which was efficiently obtained from a nickel centered metal-organic frameworks (MOF-74-Ni) by direct low-temperature phosphorization. The as-prepared Ni2P polyhedron showed a high BET surface area (175.0 m2·g-1), hierarchically porous property, and outstanding metal dispersion, which well inherited the morphology and porosity of its MOF precursor. Compared with Ni2P particles obtained from a nonporous precursor, the as-prepared Ni2P polyhedron used as electrocatalyst exhibited excellent electrocatalytic performance toward the HER, with a low overpotential of 158 mV to produce the cathodic current density of 10 mA cm-2. A small Tafel slope of 73 mV per decade is obtained for Ni2P polyhedron, which revealed a Volmer-Heyrovsky mechanism during the HER. In addition, benefiting from the structural stability, the porous Ni2P polyhedron used as a electrocatalyst showed satisfactory long-term durability for the HER in acidic media.

Second ligands-assisted structural variation of entangled coordination polymers with polycatenated or polythreaded features

The self-assembly of transition metal salts with dicarboxylate ligands and second ligands afford a series of entangled coordination frameworks based on different metal clusters, namely, [Ni2(oba)2(bpy)2(H2O)2]·(DMF)(H2O)2 (1), [Ni2(oba)2(bpy)2(sa)(H2O)2]·(H2O)4 (2), [Cu5(oba)4(bpy)5(pca)2(H2O)4]·(H2O)9 (3), [Co3(oba)2(bpy)2(dbca)2]·(H2O)6 (4), [Co3(oba)2(dbca)2(H2O)] (5) and [Co3(oba)2(bph)2(SO4)]·(H2O)4 (6) [oba = 4,4′-oxydibenzoic acid; bpy = 4,4′-bipyridine; dbca = [1,1′-biphenyl]-2,2′-dicarboxylic acid; sa = 2-hydroxybenzoic acid; pca = picolinic acid; bph = (1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine]. Their structures were determined by single-crystal X-ray diffraction analysis and further characterized by elemental analysis, IR spectra and TG analyses. The auxiliary ligands play important roles in the formation of various entangled networks. 1 shows a box-like structure and is interlocked in a parallel fashion. Compound 2 represents the first example of a 3D polycatenated framework with different size arms. 3 displays 2D → 3D entangled polymers with polycatenated and polythreaded features based on 4,4-sql layers, which are formed by short/long alternate pillars. 4 shows an interesting framework of 2D → 3D polythreading feature with a Co3 cluster occupying the corner of the grid. However, 5 displays 3D polycatenation with a 1D zigzag metal chain. An unusual [Co3SO4] unit, which is linked by four bph ligands and four oba ligands, exhibiting an anionic three-dimensional (3D) framework is observed in 6. Compounds 4–6 display antiferromagnetic properties.

Tuning the structure of metal phosphonates using uncoordinating methyl group: syntheses, structures and properties of a series of metal diphosphonates

Eight new transition metal diphosphonates, namely, [Mn(H2L)(H2O)2][(H2O)2] (1), [Co(H2O)6][H2L][(H2O)2] (2), [Co(H2O)6][(H3L)2][(H2O)2] (3), [Ni(H2O)6][H2L][(H2O)2] (4), [Cu(H2L)][(H2O)2] (5), [Zn(H2O)6][(H3L)2][(H2O)2] (6), [Zn(H2L)(H2O)2] (7) and [Cd(H2L)(H2O)2] (8), have been synthesized hydrothermally from 2,5-dimethyl-1,4-phenylenediphosphonic acid (H4L) and thoroughly characterized using EA, IR, TGA, powder and single-crystal XRD, luminescence and magnetism methods. The single-crystal X-ray diffraction measurements indicate that the diphosphonate ligands adopt three kinds of coordination modes which have a huge influence on the structure formation. Compounds 1 and 8 are isostructural and feature 3D framework structures which are constructed by the connectivity of MO6 (M = Mn and Cd) octahedrons and tetradentate organic linkers. Compounds 2 and 4, 3 and 6 are also isostructural, but have isolated mononuclear structures. Compound 5 exhibits 2D layered structure, in which four-connected copper(II) centers are bridged by tetradentate diphosphonate ligands. Compound 7 has a 1D infinite chain structure which is constructed from ZnO4 tetrahedrons and bidentate bridging diphosphonate ligands. It is also found that compounds 6–8 display interesting luminescent properties, whereas 1 shows an antiferromagnetic property.

An rht type metal-organic framework based on small cubicuboctahedron supermolecular building blocks and its gas adsorption properties

A metal–organic framework NPC-5 was synthesized via reaction of a methyl-functionalized ligand 2,4,6-trimethyl benzene-1,3,5-triyl-isophthalate (TMBTI) with Co(NO3)2·6(H2O) under solvothermal conditions. The steric hindrance induced by the methyl groups on the central phenyl ring led to a non-planar configuration of the ligand and further resulted in a small cubicuboctahedron SBB sustained (3, 24)-connected rht network, which comprised three types of cages and exhibited high porosity. Experimental results showed that despite the use of different synthetic methods the same structure was obtained. Gas sorption study of this MOF revealed high CO2 and CH4 uptake capacities and relatively low adsorption enthalpies.

Kinetic molecular sieving, thermodynamic and structural aspects of gas/vapor sorption on metal organic framework [Ni1.5(4,4′-bipyridine)1.5(H3L)(H2O)3][H2O]7 where H6L = 2,4,6-trimethylbenzene-1,3,5-triyl tris(methylene)triphosphonic acid

A metal organic framework [Ni1.5(4,4′-bipy)1.5(H3L)(H2O)3]·[H2O]7 where H6L = 2,4,6-trimethylbenzene-1,3,5-triyl tris(methylene)triphosphonic acid and 4,4′-bipy = 4,4′-bipyridine has been prepared. The structures of [Ni1.5(4,4′-bipy)1.5(H3L)(H2O)3]·[H2O]7 and the desolvated form [Ni1.5(4,4′-bipy)1.5(H3L)(H2O)3] have been determined by single crystal X-ray diffraction and the framework structures are virtually identical with the former having disordered water molecules in the pores. The framework structure comprises of two-dimensional Ni1.5(H3L) layers and 4,4′-bipy linkers acting as pillars with an unusual framework topology of a (3, 3, 6) net that can be denoted as: {4·62}2{63}2{68·85·102}. The framework has one-dimensional channels decorated with acidic O–H groups with irregular shape varying from narrow windows (cross section: 4.2 × 4.2 A) to pore cavities (diameter: ∼12 A). Thermogravimetric studies showed that both coordinated and lattice water molecules adsorbed in pores were removed in ultra-high vacuum to give [Ni1.5(4,4′-bipy)1.5(H3L)]. The water vapor adsorption isotherm for [Ni1.5(4,4′-bipy)1.5(H3L)] showed that 3 coordinated and ∼7 pore lattice water molecules were adsorbed and the framework structure was reformed. The desorption isotherm showed that the lattice water was easily desorbed in vacuum at 20 °C to form [Ni1.5(4,4′-bipy)1.5(H3L)(H2O)3]. The ethanol adsorption isotherms for [Ni1.5(4,4′-bipy)1.5(H3L)] for temperature range 20–50 °C were markedly hysteretic. The stoichiometry was [Ni1.5(4,4′-bipy)1.5(H3L)]·[1.11C2H5OH] at p/p0 = 0.97 and 20 °C gave a total pore volume approximately half that of [Ni1.5(4,4′-bipy)1.5(H3L)(H2O)3]. The desorption isotherms show that ethanol is strongly retained with decreasing pressure indicating a stable framework structure. The kinetic profiles for oxygen, nitrogen, carbon dioxide, and water and ethanol vapors, can be described by Fickian, combined barrier resistance/diffusion (CBRD), and stretched exponential models for both adsorption and desorption. Gas adsorption studies for [Ni1.5(4,4′-bipy)1.5(H3L)] reveal kinetic molecular sieving occurs with very high kinetic selectivity for O2/N2 at 0 °C. Carbon dioxide adsorption has intermediate rates of adsorption between oxygen and nitrogen. The isosteric enthalpy for CO2 adsorption at zero surface coverage was 30.7 ± 2.4 kJ mol−1. The corresponding activation energy for diffusion of CO2 into the framework was ∼48 kJ mol−1. Narrow constrictions in the porous structure of [Ni1.5(4,4′-bipy)1.5(H3L)] give rise to kinetic molecular sieving effects and do not allow adsorption of molecules such as methane, which has a larger cross-section. The selectivity for CO2/CH4 was very high (x1000) at 30 °C. The adsorption results are discussed in terms of diffusion, thermodynamics and surface interactions in pores.

Investigation of the structure variation of metal diphosphonates with the changing of N-donor auxiliary ligands and their properties

Nine metal diphosphonates, [Co(H2L)(pyz)(H2O)][(H2O)0.3] (1), [Ni(H2L)(pyz)(H2O)2] (2), [Ni(H2L)(2,2′-bipy)2][(H2O)2] (3), [Ni(H2L)(4,4′-bipy)(H2O)][(H2O)2] (4), [Ni(H2L)(dpe)(H2O)2][(H2O)2] (5), [Cu(H2L)(pyz)(H2O)2] (6), [Cu(H2L)(4,4′-bipy)][(H2O)2] (7), [Zn(H2L)(2,2′-bipy)(H2O)2] (8) and [Cd(H2L)(pyz)(H2O)2] (9), have been synthesized from a diphosphonate ligand 2,5-dimethyl-1,4-phenylenediphosphonic acid (H4L) and four N-donor auxiliary ligands (pyz = pyrazine, 2,2′-bipy = 2,2′-bipyridine, 4,4′-bipy = 4,4′-bipyridine, dpe = 1,2-di(4-pyridyl)ethylene). In compound 1, pyrazine molecules behave as pillars which connect the Co(H2L) layers into a 3D network structure. Compounds 2, 6 and 9 are isostructural and also show 3D framework structures, in which metal centers are linked by the bidentate diphosphonate ligands into 1D infinite chains and are connected by the pyrazine linkers. In compounds 3 and 8, the diphosphonate ligands, showing bidentate or tetradentate coordination modes, bridge the respective metal ions (Ni2+ and Zn2+) into a 1D infinite chain or 2D layer structure, respectively, in which the 2,2′-bipy ligands chelate to the central metal ions and complete the coordination spheres. In compound 4, the 4,4′-bipy molecules also behave as pillars between the 2D layers, which are constructed from tridentate bridging diphosphonate ligands and six-coordinated Ni2+ ions. Compounds 5 and 7 have similar square grid layered structures which are constructed from bridging bidentate diphosphonate ligands and 4,4′-bipy or dpe linkers. Photophysical measurements indicate that compounds 8 and 9 display ligand centered emissions. Magnetic studies reveal that dominant antiferromagnetic interactions are propagated in compounds 1–3 between the magnetic centers.