William F. Schneider

Equimolar CO2 Absorption by Anion-Functionalized Ionic Liquids

Amino acid ionic liquid trihexyl(tetradecyl)phosphonium methioninate [P(66614)][Met] and prolinate [P(66614)][Pro] absorb CO(2) in nearly 1:1 stoichiometry, surpassing by up to a factor of 2 the CO(2) capture efficiency of previously reported ionic liquid and aqueous amine absorbants for CO(2). Room temperature isotherms are obtained by barometric measurements in an accurately calibrated stirred cell, and the product identity is confirmed using in situ IR. Density functional theory (DFT) calculations support the 1:1 reaction stoichiometry and predict reaction enthalpies in good agreement with calorimetric measurements and isotherms.

Critical review of Pd-based catalytic treatment of priority contaminants in water

Catalytic reduction of water contaminants using palladium (Pd)-based catalysts and hydrogen gas as a reductant has been extensively studied at the bench-scale, but due to technical challenges it has only been limitedly applied at the field-scale. To motivate research that can overcome these technical challenges, this review critically analyzes the published research in the area of Pd-based catalytic reduction of priority drinking water contaminants (i.e., halogenated organics, oxyanions, and nitrosamines), and identifies key research areas that should be addressed. Specifically, the review summarizes the state of knowledge related to (1) proposed reaction pathways for important classes of contaminants, (2) rates of contaminant reduction with different catalyst formulations, (3) long-term sustainability of catalyst activity with respect to natural water foulants and regeneration strategies, and (4) technology applications. Critical barriers hindering implementation of the technology are related to catalyst activity (for some contaminants), stability, fouling, and regeneration. New developments overcoming these limitations will be needed for more extensive field-scale application of this technology.

Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites

The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic reduction (SCR) of NOx with NH3 are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Löwensteins rule. Further, exchanged Cu(II) ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as Cu(II)OH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordination environment as a function of environmental conditions including low-temperature (473 K) SCR catalysis and are rationalized through first-principles thermodynamics and ab initio molecular dynamics. Experiment and theory together reveal that the Cu sites respond sensitively to exposure conditions, and in particular that Cu species are solvated and mobilized by NH3 under SCR conditions. While Cu sites are spectroscopically and chemically distinct away from these conditions, they exhibit similar turnover rates, apparent activation energies and apparent reaction orders at the SCR conditions, even on zeolite frameworks other than SSZ13.

Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu‐SSZ‐13

Operando X-ray absorption experiments and density functional theory (DFT) calculations are reported that elucidate the role of copper redox chemistry in the selective catalytic reduction (SCR) of NO over Cu-exchanged SSZ-13. Catalysts prepared to contain only isolated, exchanged Cu(II) ions evidence both Cu(II) and Cu(I) ions under standard SCR conditions at 473 K. Reactant cutoff experiments show that NO and NH3 together are necessary for Cu(II) reduction to Cu(I). DFT calculations show that NO-assisted NH3 dissociation is both energetically favorable and accounts for the observed Cu(II) reduction. The calculations predict in situ generation of Brønsted sites proximal to Cu(I) upon reduction, which we quantify in separate titration experiments. Both NO and O2 are necessary for oxidation of Cu(I) to Cu(II), which DFT suggests to occur by a NO2 intermediate. Reaction of Cu-bound NO2 with proximal NH4(+) completes the catalytic cycle. N2 is produced in both reduction and oxidation half-cycles.

Chemically Tunable Ionic Liquids with Aprotic Heterocyclic Anion (AHA) for CO2 Capture

Ionic liquids (ILs) with aprotic heterocyclic anions, or AHAs, can bind CO2 with reaction enthalpies that are suitable for gas separations and without suffering large viscosity increases. In the present work, we have synthesized ILs bearing an alkyl-phosphonium cation with indazolide, imidazolide, pyrrolide, pyrazolide and triazolide-based anions that span a wide range of predicted reaction enthalpies with CO2. Each AHA-based IL was characterized by NMR spectroscopy and their physical properties (viscosity, glass transition, and thermal decomposition temperature) determined. In addition, the influence of substituent groups on the reaction enthalpy was investigated by measuring the CO2 solubility in each IL at pressures between 0 and 1 bar at 22 °C using a volumetric method. The isotherm-derived enthalpies range between -37 and -54 kJ mol(-1) of CO2, and these values are in good agreement with computed enthalpies of gas-phase IL-CO2 reaction products from molecular electronic structure calculations. The AHA ILs show no substantial increase in viscosity when fully saturated with CO2 at 1 bar. Phase splitting and compositional analysis of one of the IL/H2O and IL/H2O/CO2 systems conclude that protonation of the 2-cyanopyrrolide anion is improbable, and this result was confirmed by the equimolar CO2 absorption in the presence of water. Taking advantage of the tunable binding energy and absence of viscosity increase after the reaction with CO2, AHA ILs are promising candidates for efficient and environmental-friendly absorbents in postcombustion CO2 capture.

Dynamic multinuclear sites formed by mobilized copper ions in NOx selective catalytic reduction

X-ray vision spies copper on the move Copper ions in zeolites help remove noxious nitrogen oxides from diesel exhaust by catalyzing their reaction with ammonia and oxygen. Paolucci et al. found that these copper ions may move about during the reaction (see the Perspective by Janssens and Vennestrom). Zeolite catalysts generally fix metals in place while the reacting partners flow in and out of their cagelike structures. In this case, though, x-ray absorption spectroscopy suggested that the ammonia was mobilizing the copper ions to pair up as they activated oxygen during the catalytic cycle. Science, this issue p. 898; see also p. 866 Copper ions can move about and pair up in a zeolite framework as they catalyze nitric oxide removal from diesel exhaust. Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia (NH3), but the low-temperature rate dependence on copper (Cu) volumetric density is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calculations to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O2)–mediated CuI→CuII redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the volume that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.

DFT-Based Coverage-Dependent Model of Pt-Catalyzed NO Oxidation

A coverage‐dependent, mean‐field microkinetic model of catalytic NO oxidation, NO+0.5 O2⇌NO2, at a Pt(111) surface has been developed, based on large supercell density functional theory (DFT) calculations. DFT is used to determine the overall energetics and activation energies of candidate reaction steps as a function of surface coverage. Surface coverage is found to have a significant but non‐uniform effect on the energetics, pathways, and activation energies of reaction steps involving formation or cleavage of ONO and OO bonds, and inclusion of this coverage dependence is essential for obtaining a qualitatively correct representation of the catalysis. Correlations are used to express all reaction parameters in terms of a single coverage variable θ and steady‐state solutions to the resultant mean‐field models are obtained in the method of DeDonder relations. At conditions representative of NO oxidation catalysis, the surface coverage is predicted to be 0.25≤θ<0.4 ML and to be controlled by equilibrium between gas‐phase NO and NO2 and chemisorbed O. O2 dissociative adsorption (O2(g)→2O*) is rate limiting in the model. The DFT‐based mean‐field model captures many features of the experimentally observed catalysis, and its short‐comings point the way toward more robust models of coverage‐dependent kinetics.

Theoretical analysis of oxygen-bridged Cu pairs in Cu-exchanged zeolites

O- and O2-bridged Cu pairs in zeolitic environments are examined using density functional theory with cluster models. Both types of oxocation ([CuOCu]2+ and [CuO2Cu]2+) are found to be highly stable for conditions likely to exist in Cu-ZSM-5. A variety of geometric isomers with different electronic states and preferred Cu–Cu distances are described. Possible implications for “autoreduction”, NO decomposition, and other reactions involving Cu pairs in ZSM-5 are explored.

Computational Comparison of the Reactions of Substituted Amines with CO2

Substituted amines are a popular choice as molecules to selectively react with and separate CO(2) from gas mixtures. Such separations are of particular interest, for example, for CO(2) separations for carbon capture and sequestration. It is desirable to tune amine-CO(2) reaction energies to suit a particular separation. Herein, we use DFT-B3LYP simulations to characterize the products and energetics of reactions of CO(2) with a range of substituted amines, considering both 1:1 and 2:1 amine/CO(2) reaction stoichiometries. The results show that by adjusting both the nature and the placement of functional groups, it is possible to tune reaction energies over a substantial range. Decomposition of the 2:1 reaction into separate carbamate and ammonium formation steps shows that the Brønsted basicity and Lewis basicity towards CO(2) are largely uncorrelated and provide an independent means of tuning the overall reaction energies.

Catalytic Hydrogenation of CO2 to Formic Acid with Silica‐Tethered Iridium Catalysts

The combustion of fossil fuels as the primary source of chemical energy has led to increasing quantities of CO2 in the environment. To combat the negative impact of these releases and therefore to make the use of fossil fuels cleaner, research efforts have focused on the capture, sequestration, and transformation of CO2. [3] With sufficient input of chemical energy, CO2 can be transformed into useful products (e.g. , chemicals, fuels, and polymers). 4] Without a catalyst, these transformations are too slow to be of practical value and thus require sufficiently active, durable, and selective catalysts. A direct and promising approach to generating liquid products from CO2 is the catalytic hydrogenation of CO2 to formic acid (FA). [5] Typical unsupported, homogeneous transition-metal catalysts capable of catalyzing this reaction include Ru, Rh, Pd, Ir, and Pt in the form of halides (M Cl) or hydrides (M H). 6] There are few reports of heterogeneous catalysts for this reaction. Organic–inorganic hybrid catalysts are a class of tethered heterogeneous catalysts designed to retain the selectivity of homogeneous catalysts while being immobilized on heterogeneous supports to allow for easy separation. However, very few reports have focused on the use of tethered heterogeneous catalysts for the reduction of CO2 to liquids. Baiker and co-workers reported a co-condensation method to incorporate a transition-metal complex based on Ru, Ir, Pt, or Pd within a silica framework. As reported, Ru–phosphine hybrid catalysts showed the best activities for the synthesis of N,N-diethylformamide from CO2, H2, and diethylamine. Yu et al. reported aminosilica-tethered Ru complexes with a turnover frequency (TOF) of 1482 h 1 for CO2 hydrogenation to FA when PPh3 was added under supercritical CO2 conditions (80 8C, 18 MPa of total pressure). 10] In this study, a new silica-tethered iridium catalyst Ir-PN/SBA-15 was synthesized for the hydrogenation of CO2 to FA (Scheme 1). To the best of our knowledge, we report the first use of this organic–inorganic hybrid silica-tethered bidentate Ir complex for the hydrogenation of CO2 to FA. The precatalyst was synthesized through a multistep grafting approach by using an iminophosphine ligand tethered to mesoporous SBA-15. Once activated by H2, these new materials exhibited relatively high catalytic activities with a turnover number (TON) of 2.8 10 in 20 h (60 8C, total pressure of 4.0 MPa). The alkoxysilane-containing bidentate iminophosphine ligand o-Ph2PC6H4CH = N(CH2)3Si(OMe)3 (3) was synthesized through the Schiff base reaction of o-(diphenylphosphino)benzaldehyde [Ph2P(o-C6H4CHO)] (1) with 3-(aminopropyl)trimethoxysilane [NH2(CH2)3Si(OMe)3] (2) in anhydrous toluene (see the Supporting Information for a detailed procedure). The structure of the ligand was confirmed by P NMR, C NMR, and H NMR spectroscopy (Figures S1–S3, Supporting Information). The mesoporous silica support, SBA-15, was synthesized in a manner similar to that previously reported. N2 physisorption results showed a type IV isotherm, a BET surface area of 950 m g , and a Barrett–Joyner–Halenda (BJH) pore size of 6.2 nm (Table S1 and Figure S4, Supporting Information). Iminophosphine ligand 3 was subsequently grafted onto SBA-15 to afford PN/SBA-15 (3 a), and this species underwent metalation with IrCl3 hydrate in refluxing anhydrous ethanol to afford Ir-PN/SBA-15 (3 b) (Figure 1, see the Supporting Information for a detailed procedure). For comparison, monodentate phosphine precatalysts 4 b and 5 b and primary amine precatalyst 2 b were also synthesized (Figure 1). The unsupported analogue Ir(Cl3)Ph2PC6H4CH = N(CH2)2CH3 (denoted as Ir-PNPr ) was also synthesized for comparison to the hybrid materials (see the Supporting Information). The tethered hybrid materials were characterized by a battery of techniques including FTIR, UV/Vis, thermogravimetric analysis (TGA), N2 physisorption, inductively coupled plasma optical emission spectrometry (ICP-OES), and X-ray photoelectron spectroscopy (XPS) (see the Supporting Information). The FTIR spectra of unsupported PNPr and Ir-PNPr showed the Schiff base (CH=NR) double-bond adsorption at 1639 cm , as well as phenyl ring vibrations at 3061, 1587, 1435, 743, and 694 cm . Similar peaks also appeared in the FTIR spectra of [a] Dr. Z. Xu, N. D. McNamara, G. T. Neumann, Prof. W. F. Schneider, Prof. J. C. Hicks Department of Chemical and Biomolecular Engineering University of Notre Dame 182 Fitzpatrick Hall, Notre Dame, IN 46556 (USA) Fax: (+ 1) 574-631-8366 E-mail : jhicks3@nd.edu Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201200839. Scheme 1. Structure of the tethered bidentate Ir catalyst used in the hydrogenation of CO2.