Light-driven flow synthesis of acetic acid from methane with chemical looping

Methane conversion to value-added chemicals under mild conditions is a promising strategy for utilizing CH4 and mitigating greenhouse effects. Partial oxidation of CH4 at low temperature (<200 °C) can generate valuable oxygenates (e.g., CH3OH, HCHO, HCOOH, CH3COOH) with reduced energy input and emissions compared to conventional high-temperature gas-phase processes. Acetic acid (CH3COOH) is an important industrial feedstock typically produced from CH4 via multistep routes (syngas to methanol to acetic acid), incurring high resource use and safety concerns. Recent thermocatalytic oxidative carbonylation approaches can produce CH3COOH from CH4 but require additional oxidants (e.g., O2, H2SO4) and/or CO, leading to side reactions (e.g., HCOOH, CO2) and limited selectivity. Photocatalysis offers a green alternative, in which OH radicals from water oxidation substitute for added oxidants. Metal-decorated semiconductor photocatalysts can synergistically enhance CH4 activation via improved charge separation and intermediate adsorption; Pd-based systems are known to produce C1 oxygenates with OH radicals facilitating methyl generation and Pd sites stabilizing methyl intermediates. However, forming carbonyl intermediates and achieving controlled methyl–carbonyl coupling to synthesize CH3COOH remain challenging. This study addresses these challenges by integrating Pd and PdO active sites on a WO3 support, enabling CH4 activation, in situ carbonyl formation from PdO via a Mars–van Krevelen pathway, and subsequent methyl–carbonyl coupling to form acetyl intermediates that hydrolyze to CH3COOH. A photochemical flow reactor with arc-shaped channels is designed to promote continuous cascade reactions and enhance selectivity and rate.

Prior strategies for converting methane to acetic acid often rely on oxidative carbonylation requiring added oxidants (O2, H2SO4) and/or CO, which limits applicability due to safety and selectivity issues. Photocatalytic systems have shown effectiveness in CH4 activation using metal–semiconductor composites, where OH radicals from water oxidation serve as oxidants and metals such as Pd stabilize methyl intermediates for further reactions. Pd-based photocatalysts have produced C1 oxygenates (CH3OH, CH3OOH, HCHO) but not efficiently CH3COOH due to difficulties in generating carbonyl intermediates and controlling methyl–carbonyl coupling. Thermocatalysis has demonstrated that carbonyl groups generated in situ from CH4 oxidation can couple with adsorbed methyl to form acetic acid, suggesting that if carbonyls can be formed photochemically from CH4, added CO is unnecessary. These insights motivate designing heterostructured catalysts with sites for CH4 activation and carbonyl generation and reactor configurations that enhance coupling of key intermediates.

Catalyst synthesis: WO3 nanosheets were prepared via hydrothermal synthesis (Na2WO4·2H2O and citric acid in water; acidified with HCl; 120 °C, 24 h), washed, dried, and calcined in air at 400 °C for 2 h. Pd/WO3 nanocomposites were obtained by depositing Pd (from PdCl2 reduced with NaBH4) onto WO3. PdO/Pd-WO3-x (x=1–5) nanocomposites were prepared by calcining Pd/WO3 in air at different temperatures to tune PdO content: 120 °C (x=1), 200 °C (x=2), 260 °C (x=3) for 5 h at 1 °C min−1; 350 °C (x=4) for 3 h and 450 °C (x=5) for 2 h at 2 °C min−1. Pd loading was kept constant (verified by ICP-OES). An 18O-labeled sample (Pd18O/Pd-WO3-2) was prepared by annealing Pd/WO3 in 18O2 atmosphere. Characterization: TEM, HRTEM, XRD, HAADF-STEM and EELS were used to examine morphology and heterostructures; Pd NPs decorated with PdO were observed with Pd (111) and PdO (101) lattice spacings. XPS characterized surface states. In situ DRIFTS monitored surface vibrational species during light-driven CH4 conversion. In situ near-ambient-pressure XPS (NAP-XPS) tracked surface carbon and oxygen species under reaction conditions. EPR detected methyl radicals. H2-TPR quantified PdO content/consumption. ICP-OES measured Pd content. Gas products were analyzed by GC (TCD, FID) and GC–MS for isotopic tracing. Photochemical CH4 conversion (batch): 10 mg catalyst dispersed in 10 mL water in a 30 mL quartz reactor; CH4 atmosphere at 0.1 MPa; room temperature; illuminated with a 300 W Xe lamp (200 mW cm−2). Products were analyzed by GC; time-dependent measurements assessed performance decay and regeneration. Oxygen addition tests assessed selectivity. Regeneration: post-reaction heating in air to replenish lattice oxygen in PdO and WO3; cyclic reaction–regeneration tests performed. Flow reactor: A custom photochemical flow device with arc-shaped flow channels was fabricated. CH4 and H2O were premixed to form monodispersed gas bubbles and pumped through the reactor to establish gas–liquid–solid three-phase contact. The reactor facilitated downstream migration/capture of methyl species by adsorbed *CO on PdO/Pd-WO3 layers, enabling continuous methyl–carbonyl coupling. Performance was compared between flow and conventional batch devices, and cyclic regeneration was demonstrated.

- A PdO/Pd-WO3 heterointerface enables direct, light-driven synthesis of acetic acid (CH3COOH) from methane (CH4) and water without added oxidants or CO. - Achieved selectivity and rates: in the flow reactor, 91.6% selectivity to CH3COOH with a mass-normalized production rate of 90.7 µmol g−1 h−1; normalized to Pd loading, 1.5 mmol gpd−1 h−1. Initial CH4 conversion rate reached 181.5 µmol g−1 h−1 (first 3 h) before decay in batch. - Mechanistic evidence: in situ DRIFTS detected adsorbed *CO at ~2060 cm−1 and C=O vibrations only in presence of PdO; NAP-XPS revealed growth of surface CH (285.5 eV), C–O (286.1 eV), and COO (289.1 eV) species, and O 1s features for hydroxyl, C–O, adsorbed H2O, and CO upon illumination. - Isotope tracing (18O): GC–MS showed CH3CO18O and 18OOH, indicating the carbonyl oxygen originates from PdO lattice oxygen (Mars–van Krevelen mechanism). - Quantitative correlation: H2-TPR showed PdO content closely matches CH3COOH yield at low PdO loadings (x=1–2), confirming PdO lattice oxygen consumption during acetic acid formation; excessive PdO reduces metallic Pd and hampers charge separation, lowering performance. - Interface requirement: CH3COOH was not produced by simply mixing Pd/WO3 with PdO; the Pd–PdO interface quality and structure are critical for C–C coupling and acetic acid formation. - Reactant effects: OH radicals from water oxidation activate CH4; adding O2 promotes CO2 rather than liquid products; negligible H2 detected, indicating WO3 lattice oxygen participates in water formation but with minor loss (~1.28%). - Stability: In batch, performance decays after ~3 h due to PdO and WO3 lattice oxygen depletion; activity is recovered by air regeneration. Durable performance maintained over five reaction–regeneration cycles (5 h each).

The study addresses the long-standing challenge of synthesizing acetic acid directly from methane under mild, oxidant-free photocatalytic conditions by engineering a PdO/Pd heterointerface on WO3. The Pd sites enable CH4 activation and stabilization of methyl intermediates via OH radicals derived from photocatalytic water oxidation, while PdO supplies lattice oxygen to generate carbonyl (*CO) species via a Mars–van Krevelen pathway. The subsequent methyl–carbonyl coupling at the Pd–PdO interface forms acetyl intermediates that hydrolyze to CH3COOH. In situ spectroscopies (DRIFTS, NAP-XPS) and isotope labeling unambiguously identify *CO formation and confirm the PdO lattice as the oxygen source for the carbonyl, linking PdO consumption to acetic acid yield. The arc-shaped photochemical flow reactor enhances continuous contact among intermediates, facilitating cascade reactions and maximizing utilization of PdO and methyl species, thereby boosting selectivity and rates compared with conventional batch configuration. These findings validate heterointerface design and reactor engineering as effective levers for controlling intermediate formation and coupling, enabling selective C2 oxygenate synthesis from methane without added oxidants or CO.

This work demonstrates a direct, light-driven route to acetic acid from methane and water by integrating Pd and PdO active sites on WO3 to control carbonyl formation and methyl–carbonyl coupling. Mechanistic studies establish that carbonyl oxygen originates from PdO lattice oxygen, motivating a conversion–regeneration strategy to sustain performance. A custom photochemical flow reactor induces cascade reactions, achieving 91.6% selectivity and a production rate of 1.5 mmol gpd−1 h−1 toward acetic acid under mild conditions. The results underscore the importance of rational heterostructure engineering and reactor design for selective methane valorization. Future work could focus on optimizing Pd–PdO interface structure and stability, minimizing oxygen depletion, extending continuous operation without regeneration, scaling flow architectures, and generalizing this strategy to other C2 oxygenates and methane upgrading pathways.

- Catalyst deactivation arises from consumption of PdO lattice oxygen during operation, causing performance decay after ~3 h in batch; activity requires air regeneration to replenish PdO and WO3 lattice oxygen. - WO3 undergoes partial reduction under illumination, with minor but measurable lattice oxygen loss (~1.28%), contributing to decay. - Excessive PdO decreases metallic Pd content and impairs charge separation, lowering activity; thus, there is a narrow optimal PdO/Pd balance. - Adding O2 shifts selectivity toward CO2, undermining liquid oxygenate formation; process relies on water-derived OH radicals and must avoid external oxidants. - In gas–solid phase, methyl self-coupling can form C2H6, competing with desired methyl–carbonyl coupling; reactor design is needed to manage intermediates. - Continuous long-term stability beyond five reaction–regeneration cycles and catalyst robustness under scale-up remain to be fully validated.