K. Mark Thomas

High Capacity Hydrogen Adsorption in Cu(II) Tetracarboxylate Framework Materials: The Role of Pore Size, Ligand Functionalization, and Exposed Metal Sites

A series of isostructural metal-organic framework polymers of composition [Cu2(L)(H2O)2] (L= tetracarboxylate ligands), denoted NOTT-nnn, has been synthesized and characterized. Single crystal X-ray structures confirm the complexes to contain binuclear Cu(II) paddlewheel nodes each bridged by four carboxylate centers to give a NbO-type network of 64.82 topology. These complexes are activated by solvent exchange with acetone coupled to heating cycles under vacuum to afford the desolvated porous materials NOTT-100 to NOTT-109. These incorporate a vacant coordination site at each Cu(II) center and have large pore volumes that contribute to the observed high H2 adsorption. Indeed, NOTT-103 at 77 K and 60 bar shows a very high total H2 adsorption of 77.8 mg g(-)- equivalent to 7.78 wt% [wt% = (weight of adsorbed H2)/(weight of host material)] or 7.22 wt% [wt% = 100(weight of adsorbed H2)/(weight of host material + weight of adsorbed H2)]. Neutron powder diffraction studies on NOTT-101 reveal three adsorption sites for this material: at the exposed Cu(II) coordination site, at the pocket formed by three {Cu2} paddle wheels, and at the cusp of three phenyl rings. Systematic virial analysis of the H2 isotherms suggests that the H2 binding energies at these sites are very similar and the differences are smaller than 1.0 kJ mol-1, although the adsorption enthalpies for H2 at the exposed Cu(II) site are significantly affected by pore metrics. Introducing methyl groups or using kinked ligands to create smaller pores can enhance the isosteric heat of adsorption and improve H2 adsorption. However, although increasing the overlap of potential energy fields of pore walls increases the heat of H2 adsorption at low pressure, it may be detrimental to the overall adsorption capacity by reducing the pore volume.

A partially interpenetrated metal–organic framework for selective hysteretic sorption of carbon dioxide

The selective capture of carbon dioxide in porous materials has potential for the storage and purification of fuel and flue gases. However, adsorption capacities under dynamic conditions are often insufficient for practical applications, and strategies to enhance CO(2)-host selectivity are required. The unique partially interpenetrated metal-organic framework NOTT-202 represents a new class of dynamic material that undergoes pronounced framework phase transition on desolvation. We report temperature-dependent adsorption/desorption hysteresis in desolvated NOTT-202a that responds selectively to CO(2). The CO(2) isotherm shows three steps in the adsorption profile at 195 K, and stepwise filling of pores generated within the observed partially interpenetrated structure has been modelled by grand canonical Monte Carlo simulations. Adsorption of N(2), CH(4), O(2), Ar and H(2) exhibits reversible isotherms without hysteresis under the same conditions, and this allows capture of gases at high pressure, but selectively leaves CO(2) trapped in the nanopores at low pressure.

Separation of rare gases and chiral molecules by selective binding in porous organic cages

The separation of molecules with similar size and shape is an important technological challenge. For example, rare gases can pose either an economic opportunity or an environmental hazard and there is a need to separate these spherical molecules selectively at low concentrations in air. Likewise, chiral molecules are important building blocks for pharmaceuticals, but chiral enantiomers, by definition, have identical size and shape, and their separation can be challenging. Here we show that a porous organic cage molecule has unprecedented performance in the solid state for the separation of rare gases, such as krypton and xenon. The selectivity arises from a precise size match between the rare gas and the organic cage cavity, as predicted by molecular simulations. Breakthrough experiments demonstrate real practical potential for the separation of krypton, xenon and radon from air at concentrations of only a few parts per million. We also demonstrate selective binding of chiral organic molecules such as 1-phenylethanol, suggesting applications in enantioselective separation.

The release of nitrogen oxides during char combustion

The release of nitrogen oxides during coal combustion has a major environmental impact. In low-NOx burners and fluidized bed combustors the char nitrogen is the major contributor to nitrogen oxide emissions. In this review the release of nitrogen oxides during the combustion of chars is considered in relation to coal and char structural characteristics. The changes in the nitrogen functionality during the conversion of coal nitrogen to char nitrogen are discussed in detail. The results available in the literature relating nitrogen oxide emissions to coal and char properties are discussed in terms of the mechanisms for production of nitrogen oxides and their reduction in the pores or on the surface of the char.

Enhancement of H2 adsorption in Li+-exchanged co-ordination framework materials

H(2) adsorption in (Me(2)NH(2))[In(L)] is enhanced by exchange of Me(2)NH(2)(+) for Li(+) cations; the Li(+)-exchanged material displays a lower isosteric heat for H(2) adsorption than the parent material, indicating that the increase in H(2) capacity is due to an increase in the accessible pore volume on cation exchange, while the lower adsorption enthalpy is consistent with increased pore size.

Kinetics of adsorption and diffusional characteristics of carbon molecular sieves

Commercially available carbon molecular sieve (CMS) materials, from a variety of sources, and molecular sieving carbons produced from the carbonization of coal have been studied. These materials have been assessed in terms of their kinetics of adsorption of oxygen and nitrogen. The diffusion kinetics of gases into the carbons have been analysed using Fickian and phenomenological models. The Arrhenius activation energies and pre-exponential factors for gas adsorption in the two types of material have been compared. The adsorption kinetic results are discussed in relation to structural models for carbon molecular sieves where the kinetic selectivity for gases has been introduced by carbon deposition in the case of commercial CMS and by the variation of carbonization conditions in the case of the coal-based carbons produced in the laboratory.

NOx release and reactivity of chars from a wide range of coals during combustion

NOx release from a wide range of chars prepared in an entrained-flow reactor from coals and vitrinite concentrates of different rank and geographic origin was studied. Temperature-programmed combustion of the chars using thermogravimetric analysis and mass spectrometry showed that the fractional conversion of char-N to NO increased with increasing rank of the original coal at first and then reached a plateau above a vitrinite reflectance of ~1.5%. The fractional conversion of char-N to N2 was roughly constant throughout the suite of chars. The HCN level was significantly lower for the chars than for the parent coals. The HCN is believed to be a devolatilization product in advance of oxidation to N2, NO and N2O. The conversion of char-N to NO was correlated with char reactivity. Normalization by surface area halved the range of reactivity values, but the relation between char-N conversion to NO and reactivity was still apparent. The results suggest that as well as the surface area, the intrinsic reactivity of the char is important in determining the reduction of NO on the carbon during combustion.

Detection of reactive intermediate nitrogen and sulfur species in the combustion of carbons that are models for coal chars

Abstract The release of nitrogen and sulfur during the combustion of coal chars is a major environmental problem. Model coal chars, prepared by the high-pressure carbonisation of polynuclear aromatic precursors to 873 K and subsequent calcination at atmospheric pressure to 1273 K, were studied using temperature-programmed combustion (TPC) in 20% O 2 He . These carbons, which are derived from pure organic precursors with well-defined nitrogen and sulfur functionality, are highly reproducible and are virtually free of catalytic effects due to the absence of metallic species. The reactions were conducted using a thermogravimetric analyser coupled to a quadrupole mass spectrometer (TG-MS). The evolved gases were analysed throughout the course of the combustion. Sampling of the gases directly above the sample by means of a heated capillary line allowed the detection of reactive species including HCN, C2N2, and OCS. Sampling the gases at the exhaust of the TGA allowed the estimation of the gas composition at nearequilibrium conditions. In this case, the reactive species were no longer detected and gas-phase reactions, such as the conversion of HCN and C2N2 to NO and the conversion of CO to CO2, were apparently occurring in the gas phase. The results are discussed in terms of the nature of the surface nitrogen and sulfur species present during combustion.

Dimensionality transformation through paddlewheel reconfiguration in a flexible and porous Zn-based metal-organic framework.

The reaction between Zn and a pyrene-based ligand decorated with benzoate fragments (H(4)TBAPy) yields a 2D layered porous network with the metal coordination based on a paddlewheel motif. Upon desolvation, the structure undergoes a significant and reversible structural adjustment with a corresponding reduction in crystallinity. The combination of computationally assisted structure determination and experimental data analysis of the desolvated phase revealed a structural change in the metal coordination geometry from square-pyramidal to tetrahedral. Simulations of desolvation showed that the local distortion of the ligand geometry followed by the rotation and displacement of the pyrene core permits the breakup of the metal-paddlewheel motifs and the formation of 1D Zn-O chains that cross-link adjacent layers, resulting in a dimensionality change from the 2D layered structure to a 3D structure. Constrained Rietveld refinement of the powder X-ray diffraction pattern of the desolvated phase and the use of other analytical techniques such as porosity measurements, (13)C CP MAS NMR spectroscopy, and fluorescence spectroscopy strongly supported the observed structural transformation. The 3D network is stable up to 425 °C and is permanently porous to CO(2) with an apparent BET surface area of 523(8) m(2)/g (p/p° = 0.02-0.22). Because of the hydrophobic nature, size, and shape of the pores of the 3D framework, the adsorption behavior of the structure toward p-xylene and m-xylene was studied, and the results indicated that the shape of the isotherm and the kinetics of the adsorption process are determined mainly by the shape of the xylene isomers, with each xylene isomer interacting with the host framework in a different manner.

Irreversible network transformation in a dynamic porous host catalyzed by sulfur dioxide.

Porous NOTT-202a shows exceptionally high uptake of SO2, 13.6 mmol g(-1) (87.0 wt %) at 268 K and 1.0 bar, representing the highest value reported to date for a framework material. NOTT-202a undergoes a distinct irreversible framework phase transition upon SO2 uptake at 268-283 K to give NOTT-202b which has enhanced stability due to the formation of strong π···π interactions between interpenetrated networks.