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News > Mixed-Metal Strategy on Metal−Organic Frameworks (MOFs) for Functionalities Expansion: Co Substitution Induces Aerobic Oxidation of Cyclohexene over Inactive Ni-MOF-74
Mixed-Metal Strategy on Metal−Organic Frameworks (MOFs) for Functionalities Expansion: Co Substitution Induces Aerobic Oxidation of Cyclohexene over Inactive Ni-MOF-74
Summary:
The authors from Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University developed mixed-metal Co/Ni-MOF-74 with solid-solution-like structure and adjustable Co content, achieving excellent catalytic activity and selectivity in the application of aerobic oxidation of cyclohexene field.
Background:
To address the problem that many MOFs lack specific functionalities (e.g., inert Ni-MOF-74 for cyclohexene oxidation) and mixed-metal effect on solid-solution-like MOFs in heterogeneous catalysis is largely unexplored, previous researchers conducted work on mixed-metal MOFs (e.g., Ti-substituted NH₂-Uio-66(Zr) for improved CO₂ adsorption, Ti-containing NH₂-Uio-66(Zr/Ti) for enhanced photocatalysis), achieving success in some aspects, yet there are problems and deficiencies such as limited studies on heterogeneous catalysis application and unclear mixed-metal effect mechanism.
The authors in this study proposed an innovative post-synthetic metal exchange method to substitute Ni²⁺ in Ni-MOF-74 with Co²⁺, obtained Co/Ni-MOF-74 that enabled inert Ni-MOF-74 to show cyclohexene oxidation activity and exhibited superior performance to pure Co-MOF-74 with similar Co content.
Research Content:
1. Synthesis
The authors synthesized Ni-MOF-74 by mixing H₄DOBDC and Ni(NO₃)₂·6H₂O in DMF/EtOH/H₂O (1:1:1) solution, stirring at room temperature, heating at 120 °C for 24 h in a Teflon liner, centrifuging, washing with DMF and methanol, and vacuum-treating at 180 °C for 5 h. Co-MOF-74 was synthesized via the same procedure but replacing Ni(NO₃)₂·6H₂O with Co(NO₃)₂·6H₂O. Co/Ni-MOF-74 was prepared by dispersing Ni-MOF-74 in DMF solution of Co(NO₃)₂·6H₂O, stirring for 30 min, sealing and incubating at 80 °C for different times, then centrifuging, washing, Soxhlet-extracting with deionized water overnight and drying.
2. Characterizations
BET and pore size distribution: Ni-MOF-74 had a Langmuir specific surface area of ~860 m²g⁻¹; Co/Ni-MOF-74-4 (incubated for 4 days) showed a Type I N₂ isotherm and a higher Langmuir specific surface area of 1085 m²g⁻¹, with no amorphous impurities in pores.
XRD tests: Ni-MOF-74, Co/Ni-MOF-74-4 and Co-MOF-74 had similar XRD patterns, indicating Co/Ni-MOF-74 retained Ni-MOF-74 framework; the 2θ peak at ~6.8° of Ni-MOF-74 shifted to lower value in Co/Ni-MOF-74-4. Fresh and used Co/Ni-MOF-74-4 had similar XRD patterns, showing good stability.
Other tests:
ICP-MS: Co content in Co/Ni-MOF-74 increased with incubation time, reaching ~61% after 4 days and saturating; corresponding Ni was detected in solution, confirming Co-Ni metathesis.
EXAFS: Ni in Ni-MOF-74 was 6-coordinated by O with Ni-O bond length of 2.04 Å; Co in Co/Ni-MOF-74-4 was also 6-coordinated by O with Co-O bond length of 2.04 Å, and no other Co species peaks were observed, confirming successful Co substitution.
UV-vis DRS: Not detailed in results, but mentioned as a characterization method.
3. Application
The material was tested in aerobic oxidation of cyclohexene (20 mg catalyst, 2 mL cyclohexene, O₂ balloon, 80 °C). Results:
Ni-MOF-74 was inactive, with ~37.2% conversion (similar to catalyst-free system, 37.0%), main product D (cyclohexene hydroperoxide) yield 18.8%.
Co/Ni-MOF-74-1 (4.7% Co): Conversion increased to 39.2%, D yield dropped to 6.6%, B (2-cyclohexen-1-ol) and C (2-cyclohexen-1-one) yields rose to 9.9% and 20.8%.
Co/Ni-MOF-74-4 (61% Co): Conversion reached 54.7%, D yield only 0.6%, B and C yields 20.7% and 30.4%, total selectivity to B and C 93.4%. After removing Co/Ni-MOF-74-4, reaction conversion only increased slightly to 55.7% in 5 h, showing heterogeneous catalysis. It had no obvious activity loss after 3 cycles.
Co/Ni-MOF-74-12 (60% Co) vs pure Co-MOF-74 (similar Co content): Co/Ni-MOF-74-12 had higher conversion (54.2% vs 50.0%) and higher total selectivity to B and C (93.2% vs 83.4%).
4. Mechanism
Analysis of experiment results: Catalytic activity correlated with Co content; Co substitution facilitated conversion of D to B and C; Co in Co/Ni-MOF-74 was at more substrate-accessible positions than in pure Co-MOF-74, leading to better performance.
Mechanism reasoning: O₂ binds to Co²⁺ in Co/Ni-MOF-74 to form Co-O₂ adduct, which reacts with cyclohexene to form Co³⁺-OOH and cyclohexenyl radicals. Co-O bond fission in Co³⁺-OOH releases HOO•, which forms D or A (cyclohexene oxide) with cyclohexenyl radicals. D is further converted to B and C via Haber-Weiss process involving Co²⁺/Co³⁺ cycling. Allylic oxidation (forming D, then B/C) is easier than epoxidation (forming A), so B/C are main products.
Outlook:
This research successfully prepared Co/Ni-MOF-74 via post-synthetic metal exchange, endowing inert Ni-MOF-74 with cyclohexene oxidation activity. It provides a facile method to develop solid-solution-like MOFs for heterogeneous catalysis, highlights the great potential of mixed-metal strategy in preparing MOFs with specific functionalities, and enriches the understanding of mixed-metal effect in MOFs' catalytic performance, laying a foundation for further design and application of functional mixed-metal MOFs.
Mixed-Metal Strategy on Metal−Organic Frameworks (MOFs) for Functionalities Expansion: Co Substitution Induces Aerobic Oxidation of Cyclohexene over Inactive Ni-MOF-74
Authors: Dengrong Sun, Fangxiang Sun, Xiaoyu Deng, and Zhaohui Li*
DOI: 10.1021/acs.inorgchem.5b01278
Link: https://pubs.acs.org/doi/10.1021/acs.inorgchem.5b01278
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