Adsorption dynamics of gases and vapors on the nanoporous metal organic framework material Ni2(4,4'-bipyridine)3(NO3)4: guest modification of host sorption behavior

This study combines measurements of the thermodynamics and kinetics of guest sorption with powder X-ray diffraction measurements of the nanoporous metal organic framework adsorbent (host) at different adsorptive (guest) loadings. The adsorption characteristics of nitrogen, argon, carbon dioxide, nitrous oxide and ethanol and methanol vapors on Ni2(4,4'-bipyridine)3(NO3)4 were studied over a range of temperatures as a function of pressure. Isotherm steps were observed for both carbon dioxide and nitrous oxide adsorption at approximately 10-20% of the total pore volume and at approximately 70% of total pore volume for methanol adsorption. The adsorption kinetics obey a linear driving force (LDF) mass transfer model for adsorption at low surface coverage. At high surface coverage, both methanol and ethanol adsorption follow a combined barrier resistance/diffusion model. The rates of adsorption in the region of both the carbon dioxide and methanol isotherm steps were significantly slower than those observed either before or after the step. X-ray diffraction studies at various methanol loadings showed that the host structure disordered initially but underwent a structural change in the region of the isotherm step. These isotherm steps are ascribed to discrete structural changes in the host adsorbent that are induced by adsorption on different sites. Isotherm steps were not observed for ethanol adsorption, which followed a Langmuir isotherm. Previous X-ray crystallography studies have shown that all the sites are equivalent for ethanol adsorption on Ni2(4,4'-bipyridine)3(NO3)4, with the host structure undergoing a scissoring motion and the space group remaining unchanged during adsorption. The activation energies and preexponential factors for methanol and ethanol adsorption were calculated for each pressure increment at which the linear driving force model was obeyed. There was a good correlation between activation energy and ln(preexponential factor), indicating a compensation effect. The results are discussed in terms of reversible adsorbate/adsorbent (guest/host) structural changes and interactions and the adsorption mechanism. The paper contains the first evidence of specific interactions between guests and functional groups leading to structural change in flexible porous coordination polymer frameworks.     

Flexibility in metal-organic framework materials: Impact on sorption properties

Recent years have seen the development of a new class of porous coordination polymers known collectively as metal organic framework materials (MOFs). This review outlines recent progress in understanding how adsorption characteristics of these systems differ from rigid classical sorbents such as activated carbon and zeolites. Gas/vapor adsorption studies for characterization of the porous structures of MOF materials are reviewed and differences in adsorption characteristics based on detailed measurement of equilibrium and dynamical sorption behavior, compared with previous generations of sorbents, are highlighted. The role of framework flexibility and specific structural features, such as windows and pore cavities, within the MOF porous structures are discussed in relation to adsorption mechanisms.     

Structural and Magnetic Chemistry of La2Sr2BMnO8 (B=Mg, Zn)

Polycrystalline samples of the layered perovskites La₂Sr₂MgMnO₈ and La₂Sr₂ZnMnO₈ have been studied by X-ray and neutron powder diffraction, electron diffraction and magnetometry. X-ray and neutron powder diffraction indicate that the average structure is that of K₂NiF₄, with disordering of Mn and (Zn, Mg) cations over the octahedral sites. Electron diffraction data indicate that cation ordering is present over these sites in the xy planes, with the xy ordered planes being stacked in a disordered manner along z. No long-range magnetic ordering is observed in the temperature range 5≤T (K)≤300.     

Generation of a solid Brønsted acid site in a chiral framework

Protonation of chiral porous materials introduces a Br?nsted acid centre, the structure of which is unique to the heterogeneous phase requiring pore wall confinement for stable isolation.     

Isolation of two-dimensional 2:1 cation-ordered perovskite units by anion vacancy ordering in Ba6Na2Nb2P2O17

A new six-layer perovskite-related structure Ba 6Na 2Nb 2M 2O 17 (M = P, V), which consists of cubic (c) BaO 3 layers and oxygen-deficient pseudocubic (c') BaO 2 layers stacked in the sequence c'ccccc, is presented. In Ba 6Na 2Nb 2M 2O 17, two-dimensional slabs of the well-known 2:1 octahedral cation-ordered perovskite motif are isolated between layers of tetrahedral units formed by anion vacancy ordering: two consecutive NbO 6 octahedral layers are sandwiched by two single NaO 6 octahedral layers, which, in turn, connect with two isolated MO 4 tetrahedral layers. Both oxides are derived from the 2:1 ordered perovskite structure (e.g., Ba 3ZnTa 2O 9) by ordered removal of O atoms in every sixth BaO 3 layer. Both materials exhibit a relative permittivity of approximately 20-23, Q x f 0 values of approximately 7800-10600 GHz, and negative temperature coefficients of the resonant frequency of approximately -23 to -7 ppm/ degrees C.     

Fluorescent or not? Size-dependent fluorescence switching for polymer-stabilized gold clusters in the 1.1-1.7 nm size range

The synthesis of fluorescent water-soluble gold nanoparticles by the reduction of a gold salt in the presence of a designed polymer ligand is described, the size and fluorescence of the particles being controlled by the polymer to gold ratio; the most fluorescent nanomaterial has a 3% quantum yield, a 1.1 nm gold core and a 6.9 nm hydrodynamic radius.