Selective Removal of Nitrosamines from a Model Amine Carbon-Capture Waterwash Using Low-Cost Activated-Carbon Sorbents.

Nitrosamines generated in the amine solvent loop of postcombustion carbon capture systems are potent carcinogens, and their emission could pose a serious threat to the environment or human health. Nitrosamine emission control strategies are critical for the success of amine-based carbon capture as the technology approaches industrial-scale deployment. Waterwash systems have been used to control volatile and aerosol emissions, including nitrosamines, from carbon-capture plants, but it is still necessary to remove or destroy nitrosamines in the circulating waterwash to prevent their subsequent emission into the environment. In this study, a cost-effective method for selectively removing nitrosamines from the absorber waterwash effluent with activated-carbon sorbents was developed to reduce the environmental impact associated with amine-based carbon capture. The results show that the commercial activated-carbon sorbents tested have a high capacity and selectivity for nitrosamines over the parent solvent amines, with capacities up to 190 mg/g carbon, under simulated amine waterwash conditions. To further reduce costs, an aerobic thermal sorbent regeneration step was also examined due to the low thermal stability of nitrosamines. To model the effect of oxidation on the sorbent performance, thermal- and acid-oxidized sorbents were also prepared from the commercial sorbents and analyzed. The chemical and physical properties of nitrosamines, the parent amine, and the influence of the physical properties of the carbon sorbents on nitrosamine adsorption was examined. Key sorbent properties included the sorbent hydrophilicity and hydrophobicity, surface pKa of the sorbent, and chemical structure of the parent amine and nitrosamine.

Spray-Coated Multiwalled Carbon Nanotube Composite Electrodes for Thermal Energy Scavenging Electrochemical Cells.

Spray-coated multiwalled carbon nanotube/poly(vinylidene fluoride) (MWCNT/PVDF) composite electrodes, scCNTs, with varying CNT compositions (2 to 70 wt %) are presented for use in a simple thermal energy-scavenging cell (thermocell) based on the ferro/ferricyanide redox couple. Their utility for direct thermal-to-electrical energy conversion is explored at various temperature differentials and cell orientations. Performance is compared to that of buckypaper, a 100% CNT sheet material used as a benchmark electrode in thermocell research. The 30 to 70 wt % scCNT composites give the highest power output by electrode area-seven times greater than buckypaper at ΔT = 50 °C. CNT utilization is drastically enhanced in our electrodes, reaching 1 W gCNT(-1) compared to 0.036 W gCNT(-1) for buckypaper. Superior performance of our spray-coated electrodes is attributed to both wettability with better use of a large portion of electrochemically active CNTs and minimization of ohmic and thermal contact resistances. Even composites with as low as 2 wt % CNTs are still competitive with prior art. The MWCNT/PVDF composites developed herein are inexpensive, scalable, and serve a general need for CNT electrode optimization in next-generation devices.

Carbonic anhydrase mimics for enhanced CO2 absorption in an amine-based capture solvent.

Two new small-molecule enzyme mimics of carbonic anhydrase were prepared and characterized. These complexes contain the salen-like ligand bis(hydroxyphenyl)phenanthroline. This ligand is similar to the salen-type ligands previously incorporated into carbonic anhydrase mimics but contains no hydrolyzable imine groups and therefore serves as a promising ligand scaffold for the synthesis of a more robust CO2 hydration catalyst. These homogeneous catalysts were investigated for CO2 hydration in concentrated primary amine solutions through which a dilute CO2 (14%) fluid stream was flowed and showed exceptional activity for increased CO2 absorption rates.

Harnessing redox-active ligands for low-barrier radical addition at oxorhenium complexes.

The addition of an [X](+) electrophile to the five-coordinate oxorhenium(V) anion [Re(V)(O)(ap(Ph))(2)](-) {[ap(Ph)](2-) = 2,4-di-tert-butyl-6-(phenylamido)phenolate} gives new products containing Re-X bonds. The Re-X bond-forming reaction is analogous to oxo transfer to [Re(V)(O)(ap(Ph))(2)](-) in that both are 2e(-) redox processes, but the electronic structures of the products are different. Whereas oxo addition to [Re(V)(O)(ap(Ph))(2)](-) yields a closed-shell [Re(VII)(O)(2)(ap(Ph))(2)](-) product of 2e(-) metal oxidation, [Cl](+) addition gives a diradical Re(VI)(O)(ap(Ph))(isq(Ph))Cl product ([isq(Ph)](•-) = 2,4-di-tert-butyl-6-(phenylimino)semiquinonate) with 1e(-) in a Re d orbital and 1e(-) on a redox-active ligand. The differences in electronic structure are ascribed to differences in the π basicity of [O](2-) and Cl(-) ligands. The observation of ligand radicals in Re(VI)(O)(ap(Ph))(isq(Ph))X provides experimental support for the capacity of redox-active ligands to deliver electrons in other bond-forming reactions at [Re(V)(O)(ap(Ph))(2)](-), including radical additions of O(2) or TEMPO(•) to make Re-O bonds. Attempts to prepare the electron-transfer series monomers between Re(VI)(O)(ap(Ph))(isq(Ph))X and [Re(V)(O)(ap(Ph))(2)](-) yielded a symmetric bis(µ-oxo)dirhenium complex. Formation of this dimer suggested that Re(VI)(O)(ap(Ph))(isq(Ph))Cl may be a source of an oxyl metal fragment. The ability of Re(VI)(O)(ap(Ph))(isq(Ph))Cl to undergo radical coupling at oxo was revealed in its reaction with Ph(3)C(•), which affords Ph(3)COH and deoxygenated metal products. This reactivity is surprising because Re(VI)(O)(ap(Ph))(isq(Ph))Cl is not a strong outer-sphere oxidant or oxo-transfer reagent. We postulate that the unique ability of Re(VI)(O)(ap(Ph))(isq(Ph))Cl to effect oxo transfer to Ph(3)C(•) arises from symmetry-allowed mixing of a populated Re≡O π bond with a ligand-centered [isq(Ph)](•-) ligand radical, which gives oxyl radical character to the oxo ligand. This allows the closed-shell oxo ligand to undergo a net 2e(-) oxo-transfer reaction to Ph(3)C(•) via kinetically facile redox-active ligand-mediated radical steps. Harnessing intraligand charge transfer for radical reactions at closed-shell oxo ligands is a new strategy to exploit redox-active ligands for small-molecule activation and functionalization. The implications for the design of new oxidants that utilize low-barrier radical steps for selective multielectron transformations are discussed.

Redox-active ligands facilitate bimetallic O2 homolysis at five-coordinate oxorhenium(V) centers.

Five-coordinate oxorhenium(V) anions with redox-active catecholate and amidophenolate ligands are shown to effect clean bimetallic cleavage of O(2) to give dioxorhenium(VII) products. A structural homologue with redox-inert oxalate ligands does not react with O(2). Redox-active ligands lower the kinetic barrier to bimetallic O(2) homolysis at five-coordinate oxorhenium(V) by facilitating formation and stabilization of intermediate O(2) adducts. O(2) activation occurs by two sequential Re-O bond forming reactions, which generate mononuclear eta(1)-superoxo species, and then binuclear trans-mu-1,2-peroxo-bridged complexes. Formation of both Re-O bonds requires trapping of a triplet radical dioxygen species by a cis-[Re(V)(O)(cat)(2)](-) anion. In each reaction the dioxygen fragment is reduced by 1e(-), so generation of each new Re-O bond requires that an oxometal fragment is oxidized by 1e(-). Complexes containing a redox-active ligand access a lower energy reaction pathway for the 1e(-) Re-O bond forming reaction because the metal fragment can be oxidized without a change in formal rhenium oxidation state. It is also likely that redox-active ligands facilitate O(2) homolysis by lowering the barrier to the formally spin-forbidden reactions of triplet dioxygen with the closed shell oxorhenium(V) anions. By orthogonalizing 1e(-) and 2e(-) redox at oxorhenium(V), the redox-active ligand allows high-valent rhenium to utilize a mechanism for O(2) activation that is atypical of oxorhenium(V) but more typical for oxygenase enzymes and models based on 3d transition metal ions: O(2) cleavage occurs by a net 2e(-) process through a series of 1e(-) steps. The implications for design of new multielectron catalysts for oxygenase-type O(2) activation, as well as the microscopic reverse reaction, O-O bond formation from coupling of two M=O fragments for catalytic water oxidation, are discussed.

Deoxygenation of nitroxyl radicals by oxorhenium(V) complexes with redox-active ligands.

Five-coordinate oxorhenium(V) anions with redox-active catecholate ligands deoxygenate stable nitroxyl radicals, including TEMPO(*), to afford dioxorhenium(VII) complexes and aminyl radical-derived products. A structural homologue with redox-inert oxalate ligands does not react with TEMPO(*). Redox-active ligands are proposed to lower the kinetic barrier to TEMPO(*) deoxygenation by giving access to 1e(-) redox steps that are crucial for the formation and stabilization of intermediate species.