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

•Water-involved methane-selective oxidation by dioxygen over Cu–CHA zeolite catalyst•Methanol space-time yield of 543 mmol/molCu/h achieved at methanol selectivity 91%•Both monomeric and dimeric Cu species can catalyze the methane-to-methanol conversion•Various undergo interconversion under employed reaction conditions Methane is an abundant fossil resource widely distributed throughout earth, direct functionalization methane a challenging topic in chemistry catalysis. Inspired natural monooxygenase that dioxygen, first-row transition-metal cations stabilized matrix have been developed for dream transformation. However, suffers from extremely low efficiency its mechanism hotly debated. Here, we report water-involved catalytic copper zeolites. A state-of-the-art 91%. The fast redox cycle Cu2+–Cu+–Cu2+ during identified related to high activity Cu–CHA. Both conversion with CuOOH as key intermediate. selective functionalization, which remains challenge catalysis hot controversy. Herein, At 573 K, 91% catalyst. Temperature-programmed surface reactions isotope labeling suggest water apparent oxygen hydrogen source hydroxyl methanol. Spectroscopic analyses reveal Cu2+-Cu+-Cu2+ oxidation, closely Density functional theory calculations both CuOH monomer dimer Cu–OOH intermediate, meanwhile, various sites may conditions. Methane, main component gas, earth.1McFarland E. Chemistry. Unconventional unconventional gas.Science. 2012; 338: 340-342Crossref PubMed Scopus (160) Google Scholar In energy-intensive industrial processes, first converted syngas via reforming or partial oxidation2Sun L. Wang Y. Guan N. Li activation utilization: current status future challenges.Energy Technol. 2020; 8: 1900628Crossref (21) then transformed fuel chemicals. By contrast, appears be more intriguing; however, it greatly challenged large C–H bond dissociation energy 435 kJ/mol. Several strategies, example, oxidative nonoxidative coupling3Keller G.E. Bhasin M.M. 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To establish promising transformation, continuous desired27Dinh Viewpoint Cu- Fe-exchanged zeolites.ACS 8306-8313Crossref (75) several requirements should satisfied, namely: use inexpensive oxidants like methanol, sustainable rate, (4) good catalyst stability operation. production presence dioxygen. With elaborate optimization conditions, (STY) simultaneously achieved, makes important step forward methane. clear picture depicted combination spectroscopic theoretical calculations, clarifies misunderstandings will stimulate further development reaction. Copper were prepared wet ion exchange (see Supplemental experimental procedures, Figures S1–S6; Table S1) applied different temperature was raised 473 523–723 K promote while concentration regulated (from impurities extra dioxygen) minimize byproduct carbon dioxide production. (Si/Al = 11.4; Cu/Al 0.23), significant contrast other zeolites, such Cu-MOR Cu-MFI, exhibited remarkable (Figures S7–S21). performance controlled multiple factors, including but not limited concentration, space velocity. Experimentally, STY 542 (195 ?mol/gcat/h) 91 % 2% 400 ppm (Figure 1A). This value order magnitude higher than those mentioned previous reports similar processes (Table S2). crucial trigger (vide infra) dioxygen/methane ratio (<1/2,000) maintaining toward desired time-on-stream behaviors investigated, amazingly stable 500-h operation 1B). Besides, very reproducibility errors <5% repeated tests. These data clearly demonstrate potential one-step oxidation. Since reaction, aqueous solution mass fraction 1.2% could obtained S22) valuable utilization. For insight pathway MTM, temperature-programmed (TPSR) performed. TPSR mode, started ?523 generating S23; detected dominating Cu-MFI; S24 S25). Dihydrogen also outlet amount lower lagged slightly behind absence water, dominant trace above 673 when produced total S26), indicating role MTM. 13CH4 isotopically labeled reagent, definitively confirmed S27). Deuterium experiments (D2O reagent Figure 2A CD4 2B) indicated predominantly came whereas dihydrogen apparently abstracted dissociated water. Subsequently, 18O-labeling performed oxidant H218O 2C), CH318OH observed together CH3OH due non-labeled system. It seems acted conversion. CO2 ruled out C18O2 CO18O come (major route, Equation 1) (minor S28; 2), respectively. Meanwhile, consumption >573 confirming participation 18O2 2D), mainly formation demonstrating reaction.CH3OH+H2O?CO2+3H2(Equation 2CH3OH+3O2?2CO2+4H2O(Equation 2) According results, participate Provided oxidized stoichiometric 1 (Equation 3). far below dependent system 3A), i.e., nH2/nCH3OH increased increasing 50 550 (high >85% all cases). 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(CH4–H2O), Cu(II)-OH (3,655 synchronously hydrolysis bare 6).Z2?Cu2++CH4?Z?[Cu?CH3]++Z?H+(Equation 5) Z2?Cu2++H2O?Z?[Cu?OH]++Z?H+(Equation 6) steady-state spectra sampled recorded. 5C, treatment isotope-labeled did bring about Cu–CHA, excluding gas-phase oxygen-isotope 3,605 3,595 3,570 respectively) groups 3,655 3,645 ?O-H/?18O-H ?1.003. hydroxyls 18O-exchange led 18O-carboxyl (band 2,105 ?C–O/?C–18O =1.024) accordance 13C-carboxyl 2,107 feeding (?C–O/?13C–O 1.023). Following particulate MMO (pMMO), dicopper proposed process. recently argued pMMO contains mononuclear centers.44Ross M.O. MacMillan Nisthal Lawton T.J. Olafson B.D. Mayo S.L. Hoffman Particulate centers.Science. 364: 566-570Crossref (104) there signs our even if they certain would excess Furthermore, series samples loadings specific compared. S33, site-specific yields similarly level 400–600 0.6–2.3 wt <300 loading 3.5 %. context, rational propose 5), catalytically although excluded. Finally, spin-polarized density (DFT) rationalize observations Data S1, energies spin states S3). 6, set Regarding evolution variation 6A, 6C, S34), (TS-A, 90 kJ/mol) subsequent adsorption CH3Cu[OH2]2 cation cage (M3). Dioxygen molecule demonstrated chemically adsorb center four-coordinated complex (M4). framework-bound methoxide (ZCH3) demethylation (TS-B) accompanied two-coordinated H2OCuO2 (M5). Methanol typical methylation (TS-C) adjacent H2O, leaving H atom H2OCuO2. barriers steps 101 kJ/mol, adsorbed found play facilitating Framework-bound (M7) intermediate CuOH. (TS-D, 99 produce regenerate center. served explicit steps. case CuOH-dimer 6B, 6D, S35), assumed follow aforementioned CuOH-monomer pathway, close proximity another If appropriate distance, binuclear (D2) formed. feasible breaking O–OH (TS-E, 82 immobilized HOCuOCuOH (D3). (TS-F) needed overcome barrier ?59 kJ/mol dimer. comparing kinetics pathways (TS-D versus TS-F), seemed activation. highlight Cu(II), CuOCu, CuOOCu interconverted S34–S39). originating provides alternative producing without 3B) reveals experiments, proved necessary chemisorption 6). origin reflected variations Bader charge, S40. some comes 6), 5C) 2C 2D). Water known prerequisite conversion, plays route hydrolyzing Cu–O bond, (CuOCu, CuOOCu) (CuOH). noted host allows free diffusion (M3) within separate cages aggregation across cages, inactive Cu0. ideal choice Cu-SAPO-34 exhibits S41). ratios S33 S42–S45 Note S1). roles explicated. parameters, kinetic labeling. unique established considering exchange.