Jeffrey A. Reimer

Cooperative insertion of CO2 in diamine-appended metal-organic frameworks

The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as ‘phase-change’ adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg2+ within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.

Metal-Organic Frameworks with Precisely Designed Interior for Carbon Dioxide Capture in the Presence of Water

The selective capture of carbon dioxide in the presence of water is an outstanding challenge. Here, we show that the interior of IRMOF-74-III can be covalently functionalized with primary amine (IRMOF-74-III-CH2NH2) and used for the selective capture of CO2 in 65% relative humidity. This study encompasses the synthesis, structural characterization, gas adsorption, and CO2 capture properties of variously functionalized IRMOF-74-III compounds (IRMOF-74-III-CH3, -NH2, -CH2NHBoc, -CH2NMeBoc, -CH2NH2, and -CH2NHMe). Cross-polarization magic angle spinning (13)C NMR spectra showed that CO2 binds chemically to IRMOF-74-III-CH2NH2 and -CH2NHMe to make carbamic species. Carbon dioxide isotherms and breakthrough experiments show that IRMOF-74-III-CH2NH2 is especially efficient at taking up CO2 (3.2 mmol of CO2 per gram at 800 Torr) and, more significantly, removing CO2 from wet nitrogen gas streams with breakthrough time of 610 ± 10 s g(-1) and full preservation of the IRMOF structure.

Mapping of Functional Groups in Metal-Organic Frameworks

Mapping Molecular Linkers In metal-organic framework compounds, inorganic centers (metal atoms or clusters) are linked by bidentate organic groups. Normally, the same group is used throughout the structure, but recently, synthesis with linkers bearing different functional groups has produced well-defined materials. Kong et al. (p. 882, published online 25 July) combined solid-state nuclear magnetic resonance and molecular simulations to map the distributions of linkers in these materials as random, well-mixed, or clustered. Solid-state nuclear magnetic resonance and simulations map the distribution of linking groups in metal-organic frameworks. We determined the heterogeneous mesoscale spatial apportionment of functional groups in a series of multivariate metal-organic frameworks (MTV-MOF-5) containing BDC (1,4-benzenedicarboxylate) linkers with different functional groups—B (BDC-NH2), E (BDC-NO2), F [(BDC-(CH3)2], H [BDC-(OC3H5)2], and I [BDC-(OC7H7)2]—using solid-state nuclear magnetic resonance measurements combined with molecular simulations. Our analysis reveals that these methods discern between random (EF), alternating (EI and EHI), and various cluster (BF) forms of functional group apportionments. This combined synthetic, characterization, and computational approach predicts the adsorptive properties of crystalline MTV-MOF systems. This methodology, developed in the context of ordered frameworks, is a first step in resolving the more general problem of spatial disorder in other ordered materials, including mesoporous materials, functionalized polymers, and defect distributions within crystalline solids.

Catalysis over molybdenum carbides and nitrides. II: Studies of CO hydrogenation and C2H6 hydrogenolysis

Abstract The hydrogenation of CO and the hydrogenolysis of C2H6 were studied over the hcp and fcc phases of MO2C, and the fcc phase of Mo2N. The CO hydrogenation activity and selectivity of the fcc phases of Mo2C and Mo2N are identical. The activity of the hcp phase of Mo2C is half that of the other two catalysts but the olefin selectivity is higher. Elemental analysis reveals that the catalysts contain a substantial amount of oxygen following extended use for CO hydrogenation. The hydrogenolysis activity of both Mo2C phases increases markedly with decreasing content of oxygen in the catalyst. Consistent with the known structure sensitivity of C2H6 hydrogenolysis, the activity of the hcp phase of Mo2C is 200-fold higher than that of the fcc phase. This difference in activity is attributed to differences in the structure of the principal planes exposed by each phase of the carbide.

CO2 Dynamics in a Metal–Organic Framework with Open Metal Sites

Metal-organic frameworks (MOFs) with open metal sites are promising candidates for CO(2) capture from dry flue gas. We applied in situ(13)C NMR spectroscopy to investigate CO(2) adsorbed in Mg(2)(dobdc) (H(4)dobdc = 2,5-dihydroxyterephthalic acid; Mg-MOF-74, CPO-27-Mg), a key MOF in which exposed Mg(2+) cation sites give rise to exceptional CO(2) capture properties. Analysis of the resulting spectra reveals details of the binding and CO(2) rotational motion within the material. The dynamics of the motional processes are evaluated via analysis of the NMR line shapes and relaxation times observed between 12 and 400 K. These results form stringent and quantifiable metrics for computer simulations that seek to screen and improve the design of new MOFs for CO(2) capture.

Effects of inert gas dilution of silane on plasma‐deposited a‐Si:H films

Electrical, optical, and structural characterization of hydrogenated amorphous silicon films plasma‐deposited from mixtures of SiH4 with different inert‐gas diluents reveals substantial differences in a number of properties. A general trend of increasing defect density with atomic weight of the inert gas is observed. Of specific interest to device applications is the observation that high deposition rates can be achieved concurrently with low defect densities when helium is used as a deluent.

Understanding CO2 Dynamics in Metal–Organic Frameworks with Open Metal Sites

Hopping along: Metal-organic frameworks such as Mg-MOF-74 possess open metal sites that interact strongly with CO2. Molecular simulations reveal detailed CO2 dynamics (hops between metal sites and localized fluctuations), which can be used to accurately explain the experimentally measured 13C NMR chemical shift anisotropy pattern. Copyright

Inhomogeneous carbon bonding in hydrogenated amorphous carbon films

Hard‐carbon films prepared by the rf‐plasma decomposition of acetylene have been investigated by high‐resolution 13C nuclear magnetic resonance spectroscopy, x‐ray photoelectron spectroscopy (XPS), and the H(15Nα,γ)C nuclear resonant reaction. It was found that the ratio of sp2:sp3 bound carbon was 1.6, and that virtually all sp3 carbon atoms are, in fact, bound to one or more hydrogen atoms. Bulk layers contain about 40% hydrogen; however, results of the measurements of the hydrogen concentration, as well as those of XPS, confirm that the composition and properties of these carbon films are a strong function of their distance from the initial growth interface, and are spatially varying over the first 40 nm.

Catalysis over molybdenum carbides and nitrides: I. Catalyst characterization

The hcp and fcc phases of Mo2C and the fcc phase of Mo2N have been prepared and characterized. Mo2C (hcp) was produced by carburization of metallic Mo. The resulting material is polycrystalline, has a BET surface area of 10–30 m2/g, and pores 30 A in diameter. Mo2N (fcc) is obtained by NH3 reduction of MoO3. The nitride has a high degree of crystallinity, a BET surface area of about 180 m2/g, and pores about 17 A in diameter. The fcc phase of Mo2C is produced by exposing Mo2N to a CH4H2 mixture. This results in a substitution of C for N atoms in the lattice without major changes in the BET area or pore diameter. The preparation of both Mo2C (hcp) and Mo2C (fcc) is accompanied by the deposition of free carbon. Selective removal of the free carbon without concurrent removal of lattice carbon cannot be achieved. Air exposure of the carbides does reduce the inventory of free carbon, but at the expense of introducing oxygen into the carbide lattice. The dissolved oxygen cannot be removed by H2 reduction without removal of lattice carbon. It is possible, though, to remove oxygen from the near-surface region of the carbide particles by pretreating the air-exposed carbide powder in a CH4H2 mixture at 623 K. Use of the carbide and nitride catalysts for CO hydrogenation and C2H6 hydrogenolysis causes further changes in the catalyst composition. CO hydrogenation can deposit free carbon and introduces oxygen into the lattice of the catalyst particles. C2H6 hydrogenolysis is very efficient in removing oxygen dissolved in the catalyst lattice.

An in situ infrared study of NO reduction by C3H8 over Fe‐ZSM‐5

The interactions of NO, O2 and NO2 with Fe‐ZSM‐5, as well as the reduction of NO by C3H8 in the presence of O2, have been investigated using in situ infrared spectroscopy. The sample of Fe‐ZSM‐5 (Fe/Al =0.56) was prepared by solid‐state ion exchange. NO adsorption in the absence of O2 produces only mono‐ and dinitrosyl species associated with Fe2+ cations. Adsorbed NO2/NO3 species are formed via the reaction of adsorbed O2 with gas‐phase NO or by the adsorption of gas‐phase NO2. The reduction of NO in the presence of O2 begins with the reaction of gas‐phase C3H8 with adsorbed NO2/NO3 species to form a nitrogen‐containing polymeric species. A reaction pathway is proposed for the catalyzed reduction of NO by C3H8 in the presence of O2.