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Bioinspired Metal−Organic Framework Catalysts for Selective Methane Oxidation to Methanol
Summary:
The authors from the University of California–Berkeley, Lawrence Berkeley National Laboratory, Dong-A University, and King Abdulaziz City for Science and Technology developed bioinspired metal–organic framework (MOF) catalysts (MOF-808-His-Cu, MOF-808-Iza-Cu, MOF-808-Bzz-Cu) with high selectivity for methane oxidation, achieving high methanol productivity (up to 71.8 ± 23.4 μmol g⁻¹) in the application of selective methane oxidation to methanol.
Background:
1. To address the challenge of selective methane oxidation to methanol (due to strong C−H bonds in methane, bond dissociation energy = 104 kcal mol⁻¹, causing selectivity and activity issues), previous researchers developed synthetic catalysts like Cu-exchanged zeolites (limited active site diversity) and liquid-phase systems (overoxidation of methanol), yet these have problems such as stepwise reaction at different temperatures or byproduct formation.
2. The authors proposed using MOF-808 as a scaffold, via postsynthetic modification to install imidazole ligands and metalation with Cu(I), obtaining catalysts with bis(μ-oxo) dicopper active sites for high-selectivity isothermal methane oxidation.
Research Content:
1.Synthesis
The authors synthesized MOF-808 first (dissolving 1,3,5-benzenetricarboxylic acid and ZrOCl₂·8H₂O in DMF/formic acid, heating at 100 °C for 1 day, centrifuging and drying). Then, they performed postsynthetic ligand exchange (heating MOF-808 with saturated solutions of L-histidine/4-imidazoleacrylic acid/5-benzimidazolecarboxylic acid to get MOF-808-L) and metalation (reacting MOF-808-L with CuI in acetonitrile at room temperature for 3 days) to obtain the target catalysts.
2.Characterizations
1) BET and pore size: MOF-808-His-Cu (385 m²g⁻¹), MOF-808-Iza-Cu (580 m²g⁻¹), MOF-808-Bzz-Cu (580 m²g⁻¹); pore size distribution analyzed via DFT.
2) SEM: Uniform microcrystalline morphology; EDS showed uniform Zr, N, Cu distribution.
3) XAS: N K-edge (398.8/400.6 eV peaks shifted after metalation, confirming Cu-N coordination); Cu K-edge (peaks at 8979/8984/8989/8988 eV, indicating mixed Cu(I)/Cu(II)).
4) Resonance Raman: ~560/640 cm⁻¹ peaks (¹⁶O₂) shifted to ~545/630 cm⁻¹ (¹⁸O₂), confirming bis(μ-oxo) dicopper.
3.Application
Catalytic methane oxidation at 150 °C (He pretreatment → N₂O oxidation → CH₄ activation → steam desorption): MOF-808-Bzz-Cu had the highest methanol productivity (71.8 ± 23.4 μmol g⁻¹), with only methanol/water as products (no CO₂ below 150 °C).
4.Mechanism
Combined spectroscopy and DFT: Bis(μ-oxo) dicopper is the active site. Cu(I) oxidizes to Cu(II) with N₂O to form active species, which reduces to Cu(I) after methane activation; MOF framework stabilizes the active site to avoid ligand oxidation.
Outlook:
This research realizes bioinspired MOF catalysts for isothermal high-selectivity methane oxidation, providing a blueprint for designing bioinspired MOF catalysts for other catalytic reactions.
Bioinspired Metal−Organic Framework Catalysts for Selective Methane Oxidation to Methanol
Authors: Jayeon Baek, Bunyarat Rungtaweevoranit, Xiaokun Pei, Myeongkee Park, Sirine C. Fakra, Yi-Sheng Liu, Roc Matheu, Sultan A. Alshmimri, Saeed Alshehri, Christopher A. Trickett, Gabor A. Somorjai, Omar M. Yaghi
DOI: 10.1021/jacs.8b11525
Link: https://pubs.acs.org/doi/10.1021/jacs.8b11525
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