Can the Rate of a Catalytic Turnover Be Altered by Ligands in the Absence of Direct Binding Interactions?

Second sphere coordination effects ubiquitous in enzymatic catalysis occur through direct interactions, either covalent or non-covalent, with reaction intermediates and transition states. We present herein evidence of indirect second sphere coordination effects in which ligation of water/alkanols far removed from the primary coordination sphere of the active site nevertheless alter energetic landscapes within catalytic redox cycles in the absence of direct physicochemical interactions with surface species mediating catalytic turnovers. Density functional theory, in situ X-ray absorption and infrared spectroscopy, and a wide array of steady-state and transient CO oxidation rate data suggest that the presence of peripheral water renders oxidation half-cycles within two-electron redox cycles over µ3-oxo-bridged trimers in MIL-100(M) more kinetically demanding. Communication between ligated water and the active site appears to occur through the Fe-O-Fe backbone, as inferred from spin density variations on the central µ3-oxygen 'junction'. Evidence is provided for the generality of these second sphere effects in that they influence different types of redox half-cycles or metals, and can be amplified or attenuated through choice of coordinating ligand. Specifically in the case of MIL-100(M) materials, the Cr isostructure can be made to kinetically mimic the Fe variant by disproportionately hindering oxidation half-cycles relative to the reduction half-cycles. Kinetic and spectroscopic inferences presented here significantly expand both the conceptual definition of second sphere effects as well as the palette of synthetic levers available for tuning catalytic redox performance through chemical ligation.

Bifunctional Gas Diffusion Electrode Enables In Situ Separation and Conversion of CO2 to Ethylene from Dilute Stream.

The requirement of concentrated carbon dioxide (CO2 ) feedstock significantly limits the economic feasibility of electrochemical CO2 reduction (eCO2 R) which often involves multiple intermediate processes, including CO2 capture, energy-intensive regeneration, compression, and transportation. Herein, a bifunctional gas diffusion electrode (BGDE) for separation and eCO2 R from a low-concentration CO2 stream is reported. The BGDE is demonstrated for the selective production of ethylene (C2 H4 ) by combining high-density-polyethylene-derived porous carbon (HPC) as a physisorbent with polycrystalline copper as a conversion catalyst. The BGDE shows substantial tolerance to 10 vol% CO2 exhibiting a Faradaic efficiency of ≈45% toward C2 H4 at a current density of 80 mA cm-2 , outperforming previous reports that utilized such partial pressure (PCO2 = 0.1 atm and above) and unaltered polycrystalline copper. Molecular dynamics simulation and mixed gas permeability assessment reveal that such selective performance is ensured by high CO2 uptake of the microporous HPC as well as continuous desorption owing to the molecular diffusion and concentration gradient created by the binary flow of CO2 and nitrogen (CO2 |N2 ) within the sorbent boundary. Based on detailed techno-economic analysis, it is concluded that this in situ process can be economically compelling by precluding the C2 H4 production cost associated with the energy-intensive intermediate steps of the conventional decoupled process.

Spectroscopic and reactive characterization of mixed-metal Fe-Cr trimer nodes in metal-organic framework MIL-100.

Metal-organic framework MIL-100 is synthesized featuring heterometallic Fe and Cr M3O nodes where mixing of the metals within the nodes is evidenced using a combination of in situ IR spectroscopy, NO titrations, and CO oxidation kinetics. Reactivity data indicate distinctive properties of the bimetallic nodes.

Technological Options for Direct Air Capture: A Comparative Process Engineering Review.

The direct capture of CO2 from ambient air presents a means of decelerating the growth of global atmospheric CO2 concentrations. Considerations relating to process engineering are the focus of this review and have received significantly less attention than those relating to the design of materials for direct air capture (DAC). We summarize minimum thermodynamic energy requirements, second law efficiencies, major unit operations and associated energy requirements, capital and operating expenses, and potential alternative process designs. We also highlight process designs applied toward more concentrated sources of CO2 that, if extended to lower concentrations, could help move DAC units closer to more economical continuous operation. Addressing shortcomings highlighted here could aid in the design of improved DAC processes that overcome trade-offs between capture performance and DAC cost.

Plastic Waste Product Captures Carbon Dioxide in Nanometer Pores.

Plastic waste (PW) and increasing atmospheric carbon dioxide (CO2) levels are among the top environmental concerns presently facing humankind. With an ambitious 2050 zero-CO2 emissions goal, there is a demand for economical CO2 capture routes. Here we show that the thermal treatment of PW in the presence of potassium acetate yields an effective carbon sorbent with pores width of 0.7-1.4 nm for CO2 capture. The PW to carbon sorbent process works with single or mixed streams of polyolefin plastics. The CO2 capacity of the sorbent at 25 °C is 17.0 ± 1.1 wt % (3.80 ± 0.25 mmol g-1) at 1 bar and 5.0 ± 0.6 wt % (1.13 ± 0.13 mmol g-1) at 0.15 bar, and it regenerates upon reaching 75 ± 5 °C. The CO2 capture cost from flue gas via this technology is estimated to be <$21 ton-1 CO2, much lower than competing CO2 capture technologies. Hence, this PW-derived carbon material should find utility in the capture of CO2 from point sources of high CO2 emissions while providing a use for otherwise deleterious PW.

Low-Temperature, Ambient Pressure Oxidation of Methane to Methanol Over Every Tri-Iron Node in a Metal-Organic Framework Material.

In contrast with metal-modified zeolites, metal-organic framework materials (MOFs) provide a platform that may be significantly more amenable to creating catalysts in which every metal site is endowed with the same coordination environment, and hence, catalytic function. Using MIL-100(Fe) as a prototype, we present the first example of a synthetic heterogeneous catalyst comprised exclusively of active tri-iron moieties participating in the low-temperature oxidation of methane to methanol; in contrast with prior reports on iron-MOFs, we report the near-exclusive formation of methanol at low temperatures and sub-ambient methane pressures, and evidence its effectuation solely by Fe2+ sites. The study captures the utility of exploring classes of materials endowed with a high level of definition in structure and catalytic function for the purposes of overcoming persistent scientific and technological challenges in the field of synthetic heterogeneous catalysis.

Synthesis of NiO Crystals Exposing Stable High-Index Facets.

Metal oxides exposing high-index facets are potentially impactful in catalysis and adsorption processes owing to under-coordinated ions and polarities that alter their interfacial properties compared to low-index facets. Here, we report molten-salt syntheses of NiO particles exposing a variety of crystal facets. We show that for a given anion (nitrate or chloride), the alkali cation has a notable impact on the formation of crystals exposing {311}, {611}, {100}, and {111} faces. Based on a parametric analysis of synthesis conditions, we postulate that the crystallization mechanism is governed by the formation of growth units consisting of NiII complexes whose coordination numbers are determined by temperature and the selection of anion (associated to the coordination sphere) and alkali cation (associated with the outer coordination sphere). Notably, our findings reveal that high-index facets are particularly favored in chloride media and are stable under prolonged periods of catalysis and steaming.

Quantification of Open-Metal Sites in Metal-Organic Frameworks Using Irreversible Water Adsorption.

Metal-organic frameworks (MOFs) have been the focus of extensive research over the past couple of decades owing to their utility in enhancing performance in a range of applications including but not limited to gas separations, heterogeneous catalysis, and sensing. A rigorous understanding of the role of open-metal sites in molecular processes pertinent to these applications is first and foremost reliant on an accurate measure of the quantity of metal atoms that are coordinatively unsaturated under a given set of experimental conditions. Existing methods for quantifying open-metal sites exhibit drawbacks originating from unselective adsorption, use of high pressures and/or low temperatures, or the handling of potentially hazardous reagents. Here we investigate for the first time the use of room-temperature water adsorption isotherms for the quantification of MOF open-metal site density. We report that the quantity of water adsorbed irreversibly at room temperature on MIL-100 represents the open-metal site density under a given set of activation conditions. We use for this purpose a hydroxyl-containing version of MIL-100(Cr) that enables us to track (using in situ Fourier transform infrared spectroscopy) both dehydration and dehydroxylation events leading to open-metal site creation, providing evidence for site counts measured using irreversible water adsorption. Crucially, this approach circumvents the need for assumptions relating to the identity of open-metal sites and the degree of adsorbate saturation, while also obviating risks associated with the use of hazardous reagents. Given the near-universal presence of water as a labile ligand in the first coordination sphere of possible MOF open-metal sites, we envision that the protocols presented here could represent an approach to counting open-metal sites that is broadly applicable within (and maybe even beyond) the field of MOF research.

Improving HSAPO-34 Methanol-to-Olefin Turnover Capacity by Seeding the Hydrocarbon Pool.

Seeding the hydrocarbon pool before exposure to methanol ensures the presence of active olefinic and aromatic chain carriers in the HSAPO-34 cavity before the first methanol-to-olefin turnover. The primordial hydrocarbon pool enables the introduction, at low turnover numbers, of chain propagation steps that compete with methanol transfer dehydrogenation-mediated chain termination steps, thereby increasing the fraction of converted methanol used for productive turnovers during methanol-to-olefin catalysis over HSAPO-34. Seeding the hydrocarbon pool results, concurrently, in higher light-olefin yields and lower rates of carbon loss. The increasing relative preponderance of methanol transfer dehydrogenation steps with increasing methanol pressure renders seeding more effective at higher methanol pressures. Under the conditions used in this study, seeding appears to accelerate the buildup of the hydrocarbon pool without significantly altering its composition. The results reported here outline a strategy for mitigating the deleterious effects of methanol transfer dehydrogenation reactions while reemphasizing their primacy in effecting catalyst deactivation during methanol-to-olefins conversion.

Important roles of enthalpic and entropic contributions to CO2 capture from simulated flue gas and ambient air using mesoporous silica grafted amines.

The measurement of isosteric heats of adsorption of silica supported amine materials in the low pressure range (0-0.1 bar) is critical for understanding the interactions between CO2 and amine sites at low coverage and hence to the development of efficient amine adsorbents for CO2 capture from flue gas and ambient air. Heats of adsorption for an array of silica-supported amine materials are experimentally measured at low coverage using a Calvet calorimeter equipped with a customized dosing manifold. In a series of 3-aminopropyl-functionalized silica materials, higher amine densities resulted in higher isosteric heats of adsorption, clearly showing that the density/proximity of amine sites can influence the amine efficiency of adsorbents. In a series of materials with fixed amine loading but different amine types, strongly basic primary and secondary amine materials are shown to have essentially identical heats of adsorption near 90 kJ/mol. However, the adsorption uptakes vary substantially as a function of CO2 partial pressure for different primary and secondary amines, demonstrating that entropic contributions to adsorption may play a key role in adsorption at secondary amine sites, making adsorption at these sites less efficient at the low coverages that are important to the direct capture of CO2 from ambient air. Thus, while primary amines are confirmed to be the most effective amine types for CO2 capture from ambient air, this is not due to enhanced enthalpic contributions associated with primary amines over secondary amines, but may be due to unfavorable entropic factors associated with organization of the second alkyl chain on the secondary amine during CO2 adsorption. Given this hypothesis, favorable entropic factors may be the main reason primary amine based adsorbents are more effective under air capture conditions.

Enhanced CO2 adsorption over polymeric amines supported on heteroatom-incorporated SBA-15 silica: impact of heteroatom type and loading on sorbent structure and adsorption performance.

Silica supported amine materials are promising compositions that can be used to effectively remove CO(2) from large stationary sources, such as flue gas generated from coal-fired power plants (ca. 10 % CO(2)) and potentially from ambient air (ca. 400 ppm CO(2)). The CO(2) adsorption characteristics of prototypical poly(ethyleneimine)-silica composite adsorbents can be significantly enhanced by altering the acid/base properties of the silica support by heteroatom incorporation into the silica matrix. In this study, an array of poly(ethyleneimine)-impregnated mesoporous silica SBA-15 materials containing heteroatoms (Al, Ti, Zr, and Ce) in their silica matrices are prepared and examined in adsorption experiments under conditions simulating flue gas (10 % CO(2) in Ar) and ambient air (400 ppm CO(2) in Ar) to assess the effects of heteroatom incorporation on the CO(2) adsorption properties. The structure of the composite adsorbents, including local information concerning the state of the incorporated heteroatoms and the overall surface properties of the silicate supports, are investigated in detail to draw a relationship between the adsorbent structure and CO(2) adsorption/desorption performance. The CO(2) adsorption/desorption kinetics are assessed by thermogravimetric analysis and in situ FT-IR measurements. These combined results, coupled with data on adsorbent regenerability, demonstrate a stabilizing effect of the heteroatoms on the poly(ethyleneimine), enhancing adsorbent capacity, adsorption kinetics, regenerability, and stability of the supported aminopolymers over continued cycling. It is suggested that the CO(2) adsorption performance of silica-aminopolymer composites may be further enhanced in the future by more precisely tuning the acid/base properties of the support.

Dramatic enhancement of CO2 uptake by poly(ethyleneimine) using zirconosilicate supports.

The CO(2) adsorption characteristics of prototypical poly(ethyleneimine)/silica composite adsorbents can be drastically enhanced by altering the acid/base properties of the oxide support via incorporation of Zr into the silica support. Introduction of an optimal amount of Zr resulted in a significant improvement in the CO(2) capacity and amine efficiency under dilute (simulated flue gas) and ultradilute (simulated ambient air) conditions. Adsorption experiments combined with detailed characterization by thermogravimetric analysis, temperature-programmed desorption, and in situ FT-IR spectroscopy clearly demonstrate a stabilizing effect of amphoteric Zr sites that enhances the adsorbent capacity, regenerability, and stability over continued recycling. It is suggested that the important role of the surface properties of the oxide support in these polymer/oxide composite adsorbents has been largely overlooked and that the properties may be even further enhanced in the future by tuning the acid/base properties of the support.

Structural changes of silica mesocellular foam supported amine-functionalized CO2 adsorbents upon exposure to steam.

Three classes of amine-functionalized mesocellular foam (MCF) materials are prepared and evaluated as CO(2) adsorbents. The stability of the adsorbents under steam/air and steam/nitrogen conditions is investigated using a Parr autoclave reactor to simulate, in an accelerated manner, the exposure that such adsorbents will see under steam stripping regeneration conditions at various temperatures. The CO(2) capacity and organic content of all adsorbents decrease after steam treatment under both steam/air and steam/nitrogen conditions, primarily due to structural collapse of the MCF framework, but with additional contributions likely associated with amine degradation during treatment under harsh conditions. Treatment with steam/air is found to have stronger effect on the CO(2) capacity of the adsorbents compared to steam/nitrogen.