Continuous Partial Oxidation of Methane to Methanol Catalyzed by Diffusion-Paired Copper Dimers in Copper-Exchanged Zeolites

Journal of the American Chemical Society, Journal Year: 2019, Volume and Issue: 141(29), P. 11641 - 11650

Published: June 21, 2019

Copper-exchanged zeolites can continuously and selectively catalyze the partial oxidation of methane to methanol using only oxygen water at low temperatures, but genesis nature active sites are currently unknown. Herein, we demonstrate that this reaction is catalyzed by a [Cu–O–Cu]2+ motif forms via hypothesized proton-aided diffusion hydrated Cu ions within cages SSZ-13 zeolites. While various configurations may be present for oxidation, dimeric primary site selective oxidation. Mechanistically, CH4 activation proceeds rate-determining C–H scission form surface-bound C1 intermediate either desorbed as in presence H2O/H+ or completely oxidized CO2 gas-phase O2. High selectivity obtained with (i) high pressures (ii) maximizing dimer formation Al content loadings.

Language: Английский

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Structure and Reactivity of Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-Ion-Exchanged Chabazite Catalyst for NH₃-Mediated Selective Catalytic Reduction

Journal of the American Chemical Society, Journal Year: 2020, Volume and Issue: 142(37), P. 15884 - 15896

Published: Aug. 24, 2020

The NH3-mediated selective catalytic reduction (NH3-SCR) of NOx over Cu-ion-exchanged chabazite (Cu-CHA) catalysts is the basis technology for abatement from diesel vehicles. A crucial step in this reaction activation oxygen. Under conditions low-temperature NH3-SCR, oxygen only reacts with CuI ions, which are present as mobile diamine complexes [CuI(NH3)2]+. To determine structure and reactivity species formed by oxidation these at 200 °C, we have followed reaction, using a Cu-CHA catalyst Si/Al ratio 15 2.6 wt% Cu, X-ray absorption spectroscopies (XANES EXAFS) diffuse reflectance UV-Vis spectroscopy, support DFT calculations advanced EXAFS wavelet transform analysis. results provide unprecedented direct evidence formation [Cu2(NH3)4O2]2+ complex side-on μ-η2,η2-peroxo diamino dicopper(II) structure, accounting 80–90% total Cu content. These completely reduced to [CuI(NH3)2]+ °C mixture NO NH3. Some N2 well, suggests role dimeric NH3-SCR reaction. NH3 leads partial without any N2. an almost complete CuI, under This indicates that proceeds via NO.

Language: Английский

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In Situ UV–Vis–NIR Absorption Spectroscopy and Catalysis

Chemical Reviews, Journal Year: 2024, Volume and Issue: 124(5), P. 2352 - 2418

Published: Feb. 26, 2024

This review highlights

Language: Английский

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Competition between Mononuclear and Binuclear Copper Sites across Different Zeolite Topologies

JACS Au, Journal Year: 2024, Volume and Issue: 4(1), P. 197 - 215

Published: Jan. 4, 2024

A key challenge for metal-exchanged zeolites is the determination of metal cation speciation and nuclearity under synthesis reaction conditions. Copper-exchanged zeolites, which are widely used in automotive emissions control potential catalysts partial methane oxidation, have particular evidenced a wide variety Cu structures that observed to change with exposure conditions, zeolite composition, topology. Here, we develop predictive models CHA, MOR, BEA, AFX, FER topologies using interatomic potentials, quantum chemical calculations, Monte Carlo simulations interrogate this vast configurational compositional space. Model predictions rationalize experimentally differences between Cu-zeolites wide-body literature, including populations, structural variations, methanol per yields. Our results show both topological features commonly Al-siting biases MOR increase population binuclear sites, explaining small mononuclear sites these materials relative other such as CHA BEA. Finally, machine learning classification model determine preference form or at different Al configurations 200 international database. reveal several extreme ends vs spectrum, highlighting synthetic options realization strong preferences.

Language: Английский

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Solvation and Mobilization of Copper Active Sites in Zeolites by Ammonia: Consequences for the Catalytic Reduction of Nitrogen Oxides

Accounts of Chemical Research, Journal Year: 2020, Volume and Issue: 53(9), P. 1881 - 1892

Published: Aug. 11, 2020

ConspectusCopper-exchanged chabazite (Cu-CHA) zeolites are catalysts used in diesel emissions control for the abatement of nitrogen oxides (NOx) via selective catalytic reduction (SCR) reactions with ammonia as reductant. The discovery these materials early 2010s enabled a step-change improvement aftertreatment technology. Key advantages Cu-CHA over prior include their effectiveness at lower temperatures characteristic exhaust, durability under high-temperature hydrothermal conditions, and resistance to poisoning from residual hydrocarbons present exhaust. Fundamental catalysis research has since uncovered mechanistic kinetic features that underpin ability selectively reduce NOx strongly oxidizing conditions achieve improved conversion relative other zeolite frameworks, particularly low exhaust instead reductants.One critical feature is NH3 solvation exchanged Cu ions (<523 K) create cationic Cu-amine coordination complexes ionically tethered anionic Al framework sites. This ionic tethering confers regulated mobility facilitates interconversion between mononuclear binuclear complexes, which necessary propagate SCR through Cu2+/Cu+ redox cycle during turnover. dynamic mechanism, wherein single dual metal sites interconvert mediate different half-reactions cycle, combines canonically associated homogeneous heterogeneous reaction mechanisms.In this Account, we describe how unified experimental theoretical interrogation operando provided quantitative evidence ion its role mechanism. approach relied on new synthetic methods prepare model varied active-site structures spatial densities order verify models behavior family diverse composition, computational approaches capture structure dynamics representative catalysis. Ex situ revealed depends synthesis, influence substitution patterns, statistical electronic can enumerate site populations known distribution. recognition unifies seemingly disparate spectroscopic observations inferences regarding responses external conditions. rates depend density composition regimes where Cu+ oxidation O2 becomes rate-limiting, occurs fuel-rich Transient experiments, ab initio molecular simulations, relate sensitivities constraints imposed by CHA NH3-solvated ions, regulate pore volume accessible pair complete cycle. highlights key characteristics enable superior performance low-temperature conditions.This work illustrates power precise material, simultaneous wide range strategies tailored those reveal microscopic detail complex widely practiced In doing so, it thus potentially more general phenomenon reactant active mobilization catalyzed zeolites.

Language: Английский

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Experimental and Theoretical Assessments of Aluminum Proximity in MFI Zeolites and Its Alteration by Organic and Inorganic Structure-Directing Agents

Chemistry of Materials, Journal Year: 2020, Volume and Issue: 32(21), P. 9277 - 9298

Published: Oct. 20, 2020

The molecular structure and cationic charge density of organic inorganic structure-directing agents (SDAs) influence the siting arrangement Al substituted in zeolite frameworks. Yet, developing robust synthesis–structure relations for MFI zeolites is difficult because complexities inherent to its low-symmetry framework (12 unique tetrahedral sites), which generates a large combinatorial space Al–Al site pairs exhaustively model by functional theory (DFT) quantify experiment. Here, we develop an experimental protocol reproducibly Co2+-titratable with saturation uptakes validated corroborating spectroscopic cation balance data. Using tetrapropylammonium (TPA+) as sole SDA, were crystallized varying contents (Si/Al = 37–185; 0.52–2.52 per unit cell) within composition range consistent mismatch occlusion one TPA+ channel intersection fractions paired (0.0–0.34) that increased bulk content. DFT calculations performed using 96 T-site cell containing either isolated (all configurations) or various (1773 out 13 680 total configurations), balanced two TPA+, respectively, reveal dominant electrostatic interactions between N anionic lattice on energies. Together Co2+ exchange energies at pairs, predicts cations confined adjacent intersections can form many pair ensembles are Co2+-titratable, rationalizing considerable presence sites samples only TPA+. use Na+ co-SDAs synthesis gel, while Na+/TPA+ ratio (0–5) constant SDA/Al ((TPA+ + Na+)/Al 30), similar content ≈ 50) but (0.12–0.44). Separate crystallization experiments charge-neutral SDAs, pentaerythritol mixture 1,4-diazabicyclo[2.2.2]octane methylamine, together compensate Al, lower (<0.14). Among SDA co-SDA, number formed generally co-occluded zeolite, relation resembles our prior observations CHA zeolites. combined theoretical approach used here provides microscopic define MFI, be adapted do so other topologies. These findings highlight how such models exercised quantitatively characterize materials synthetic strategies predictably vary their arrangements catalytic adsorption properties turn, shown essentially exploiting mixtures SDAs during hydrothermal crystallization.

Language: Английский

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Water-involved methane-selective catalytic oxidation by dioxygen over copper zeolites

Chem, Journal Year: 2021, Volume and Issue: 7(6), P. 1557 - 1568

Published: March 20, 2021

Language: Английский

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Insights into the Speciation of Cu in the Cu-H-Mordenite Catalyst for the Oxidation of Methane to Methanol

ACS Catalysis, Journal Year: 2019, Volume and Issue: 9(6), P. 5308 - 5319

Published: April 26, 2019

The proton form of Cu-exchanged mordenite (Cu-H-MOR) was prepared via ion-exchange, and the nature active Cu species in cyclic oxidation CH4 to CH3OH investigated by high-pressure reactivity testing, X-ray absorption spectroscopy (XAS), H2 temperature-programmed reduction (TPR). Increasing pressure from 1 35 bar reaction time 4 20 h increased product yield 0.30 0.42 mol (mol Cu)−1, suggesting that at lower pressures shorter times, activation is not complete site for formation likely contains fewer than three atoms. Linear combination fitting K-edge near edge spectra showed 83% freshly Cu-H-MOR can be autoreduced He 723 K. Analysis extended fine structure after O2 K resulted an oxygen coordination number 2.9. normalized redox-active fraction 0.50. All these findings are consistent with a dicopper site. same nonreducible observed autoreduction TPR H2, redox-inert inactive toward oxidation.

Language: Английский

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Identifying Cu-oxo species in Cu-zeolites by XAS: A theoretical survey by DFT-assisted XANES simulation and EXAFS wavelet transform

Catalysis Today, Journal Year: 2019, Volume and Issue: 345, P. 125 - 135

Published: Sept. 27, 2019

Language: Английский

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Highly-Dispersed Zinc Species on Zeolites for the Continuous and Selective Dehydrogenation of Ethane with CO₂ as a Soft Oxidant

ACS Catalysis, Journal Year: 2021, Volume and Issue: 11(5), P. 2819 - 2830

Published: Feb. 16, 2021

We report herein the preparation, characterization, and catalytic performance of a series heterogeneous catalysts featuring highly dispersed zinc sites on zeolitic SSZ-13 ZSM-5 frameworks. The materials are evaluated in CO2-assisted oxidative ethane dehydrogenation, very important reaction for synthesis platform chemicals. In particular, we find that Zn2.92/SSZ-13 exhibits high reactivity conversion C2H6 CO2 ethene selectivity. line with experimental results, show selective character catalyst is due to characteristic compositional structure support its topology can effectively confine molecules. An in-depth molecular analysis via operando studies DFT calculations shows rate-limiting step second C–H bond dissociation give C2H4. addition reduces energy barrier this step, favoring desorption C2H4 while limiting byproduct formation. Overall, work demonstrates breakthrough potential made species zeolites relevant transformations.

Language: Английский

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