Direct Photocatalytic Synthesis of Acetic Acid from Methane and CO at Ambient Temperature Using Water as Oxidant

• Type: Journal Article
• Author: Dong, Chunyang; Marinova, Maya; Tayeb, Karima Ben; Safonova, Olga V.; Zhou, Yong; Hu, Di; Chernyak, Sergei; Corda, Massimo; Zaffran, Jérémie; Khodakov, Andrei Y.; Ordomsky, Vitaly V.
• Journal: Journal of the American Chemical Society
  • Volume: 145
  • Issue: 2
  • Pages: 1185-1193
• Year: 2023
• DOI: 10.1021/jacs.2c10840

Abstract

Direct functionalization of methane selectively to value-added chemicals is still one of the main challenges in modern science. Acetic acid is an important industrial chemical produced nowadays by expensive and environmentally unfriendly carbonylation of methanol using homogeneous catalysts. Here, we report a new photocatalytic reaction route to synthesize acetic acid from CH₄ and CO at room temperature using water as the sole external oxygen source. The optimized photocatalyst consists of a TiO₂ support and ammonium phosphotungstic polyoxometalate (NPW) clusters anchored with isolated Pt single atoms (Pt₁). It enables a stable synthesis of 5.7 mmol·L^–1 acetic acid solution in 60 h with the selectivity over 90% and 66% to acetic acid on liquid-phase and carbon basis, respectively, with the production of 99 mol of acetic acid per mol of Pt. Combined isotopic and in situ spectroscopy investigation suggests that synthesis of acetic acid proceeds via a photocatalytic oxidative carbonylation of methane over the Pt₁ sites, with the methane activation facilitated by water-derived hydroxyl radicals.

Files and Links

• Url: https://doi.org/10.1021/jacs.2c10840
• Local Library: Zotero

Detail

Key points:

• Photocatalytic reaction at room temperature, water as sole external oxygen source

• TiO₂ support, Pt single atom achored by NPW (ammonium phosphotungstic polyoxometalate)

• 5.7 mmol/L acetic acid in 60 h (sel>90% in liquid phase, 66% in carbon basis), TON of 99

• Isotopic and in situ spectroscopy investigation

• Photocatalytic oxidative carbonylation over Pt, methane activation facilitated by water-derived hydroxyl radicals

Learning points:

• IR with temperature-programmed desorption confirmed atomically dispersion of Pt

• The investigation of mechanism is impresive (especially EPR and XANES)

• The acidity drives the Pt/NPW aggregates’ disassembly over the basic TiO₂ surface.

• The variation of oxidation state of W species is interesting

• The synergy of Pt/NPW and TiO₂ is surprising

• Mars van Krevelen type mechanism

Role of Water

Sushkevish et al.: soft oxidant and intermediate stabilizer in anaerobic oxidation of methane for methanol

Liu et al.: O-provider and site-blocker in methane oxidaiton to methanol

Photocatalytic Synthesis

4 h (2 bar of CO, 8 bar of CH₄), different catalysts

Pt/TiO₂: a part of the oxygen species for CO and/or methane oxidation to CO₂ comes from the catalysts

The production of AcOH was observed only for (M/
NPW)/TiO₂ (M = Pt, Pd, Rh, Co, and Ru) and NPW/TiO₂

No oxygen was detected during the CH₄ photocatalytic conversion.

(Pt/NPW)/TiO₂ irradiated with visible light: AcOH synthesis over (Pt/NPW)/TiO₂ is essentially driven by photoinduced charge carriers from TiO₂

lower hydrogen production for other catalysts can be attributed to the partial reduction of NPW and metal oxide species with H₂, which do not contain Pt, Pd, and Rh.

12 h, NPW/TiO₂ VS (Pt/NPW)/TiO₂

catalysts changed color to dark blue: formation of partially reduced NPW

Pt species are capable of utilizing H₂O as the external oxidant for regeneration of NPW oxygen species and simultaneously produce H₂.

Ratio of CO/CH₄

Acetaldehyde: derived from the intermediates

CO₂ and H₂ were produced by WGS reaction

Best condition: 1 bar of CO, 10 bar of CH₄, 15 h

Composition of (Pt/NPW)/TiO₂

Varying the content of Pt/NPW in the catalyst and Pt content in NPW

Best composition: 0.2 wt % Pt loading and the weight ratio between Pt/NPW and TiO₂ at 3:10

Catalyst Stability

CO adsorption over Pt sites during the reaction preserves the AcOH from rapid decomposition (Figure S10)

Catalyst Characterization

a. XRD patterns, b. HAADF-STEM and EDS-mapping of Pt/NPW, c. HAADF-STEM of (Pt/NPW)/TiO₂, d. EDS-mapping of (Pt/NPW)/TiO₂, e. Atomic-resolved HAADF-STEM of (Pt/NPW)/TiO₂, f. IR spectra of temperature-programmed CO desorption over (Pt/NPW)/TiO₂

Atomic dispersion of primary Pt species over the catalyst was confirmed by IR of TPD.

The acidity drives the Pt/NPW aggregates’ disassembly over the basic TiO₂ surface.

Mechanism

a. In situ W L₃-edge XANES spectra of (Pt/NPW)/TiO₂(3:20) b. In situ IR spectra of (Pt/NPW)/TiO₂(3:5) c. In situ EPR spectra with DMPO radical-trapping agent

Gradual reduction of W species was observed.

IR absorption bad: NH₄⁺ at 1420, absorbed H2O at 1620, *CH₃COO at 1486 and 1542, absorbed CO on cationic Pt between 2159 and 2100, CO on reduced Pt at 2074

Pt/NPW clusters promote the charge separation efficiency of TiO₂ by accepting photoexcited electrons (Figure S19)

·OH played fundamental roles in promoting methane activation

The methyl groups are derived from CH₄, while acyl groups of AcOH are produced from CO.

CO₂ is principally produced from CO by the photocatalytic WGS reaction (Figure S18)

Proposed Mechanism:

Path Ⅲ is more favored (Mars van Krevelen type mechanism)