A simple reaction scheme based on the heterogeneous intercalation of pillaring ligands (HIPLs) provides a convenient method for systematically tuning pore size, pore functionality, and network flexibility in an extended series of pillared cyanonickelates (PICNICs), commonly referred to as Hofmann compounds. The versatility of the approach is demonstrated through the preparation of over 40 different PICNICs containing pillar ligands ranging from ∼4 to ∼15 Å in length and modified with a wide range of functional groups, including fluoro, aldehyde, alkylamine, alkyl, aryl, trifluoromethyl, ester, nitro, ether, and nonmetalated 4,4'-bipyrimidine. The HIPL method involves reaction of a suspension of preformed polymeric sheets of powdered anhydrous nickel cyanide with an appropriate pillar ligand in refluxing organic solvent, resulting in the conversion of the planar [Ni2(CN)4]n networks into polycrystalline three-dimensional porous frameworks containing the organic pillar ligand. Preliminary investigations indicate that the HIPL reaction is also amenable to forming Co(L)Ni(CN)4, Fe(L)Ni(CN)4, and Fe(L)Pd(CN)4 networks. The materials show variable adsorption behavior for CO2 depending on the pillar length and pillar functionalization. Several compounds show structurally flexible behavior during the adsorption and desorption of CO2. Interestingly, the newly discovered flexible compounds include two flexible Fe(L)Ni(CN)4 derivatives that are structurally related to previously reported porous spin-crossover compounds. The preparations of 20 pillar ligands based on ring-functionalized 4,4'-dipyridyls, 1,4-bis(4-pyridyl)benzenes, and N-(4-pyridyl)isonicotinamides are also described.
A detailed correlation is presented between the in situ Fourier transform-infrared (FT-IR) spectra of adsorbed CO2, CuBzPyz host bands, and CO2 adsorption sites using previously reported crystal structures of CO2-loaded CuBzPyz and CO2 adsorption isotherms. Through the analysis of both in situ attenuated total reflectance FT-IR spectra taken at several points on the high pressure isotherm and in situ transmission FT-IR spectra acquired at low pressures and cryogenic temperatures, we provide additional insight into the pore-filling mechanism of CO2 on the structurally dynamic CuBzPyz host. The FT-IR spectrum of adsorbed CO2 shows distinct ν2 and ν3 spectral features that can be attributed to known CO2 adsorption sites observed in the reported crystal structure of the CO2-saturated phase of CuBzPyz. The availability of detailed high quality CO2-loaded structural data for CuBzPyz makes this system a case study for associating infrared spectral features with CO2 adsorption sites and should prove valuable for future interpretations of CO2 host−guest and guest−guest interactions when X-ray quality structural data is unavailable.
Flexibility provides selectivity: The selective adsorption of CO2 from mixtures with N2, CH4, and N2O in a dynamic porous coordination polymer (see monomer structure) was evaluated by ATR‐FTIR spectroscopy, GC, and SANS. All three techniques indicate highly selective adsorption of CO2 from CO2/CH4 and CO2/N2 mixtures at 30 °C, with no selectivity observed for the CO2/N2O system.
Keywords: Metal-organic frameworks / Spin crossover / Density functional calculations / IR spectroscopy / Adsorption Variable-temperature in situ ATR-FTIR spectra are presented for the porous spin-crossover compounds [Fe(pyrazine)-Ni(CN) 4 ] and [Fe(pyrazine)Pt(CN) 4 ] under CO 2 pressures of up to 8 bar. Significant shifts in the ν 3 and ν 2 IR absorption bands of adsorbed CO 2 are observed as the host materials undergo transition between low-and high-spin states. Computational models used to determine the packing arrangement of CO 2 within the pore structures show a preferred orientation of one of the adsorbed CO 2 molecules with close O=C=O···H contacts with the pyrazine pillar ligands. The interaction is a consequence of the commensurate distance of [a]
This technical effort was performed in support of the National Energy Technology Laboratory's ongoing research in CO2 capture under the RDS contract DE-FE0004000. Reference in this work to any specific commercial product is to facilitate understanding and does not necessarily imply endorsement by the United States Department of Energy. Certain commercial materials and equipment are identified in this paper only to specify adequately the experimental procedure. In no case does such identification imply recommendation by NIST nor does it imply that the material or equipment identified is necessarily the best available for this purpose. This work utilized neutron scattering facilities supported in part by the National Science Foundation under Agreement No. DMR-0454672. The authors would like to acknowledge Juscelino Leao, Wendy Queen, and Craig Brown of the NIST Center for Neutron Research and Martin Green of NIST Ceramics Division for their valuable technical discussions and contributions.
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