The thermally induced decarboxylation mechanism of a mixed-oxidation state carboxylate-based iron metal–organic framework

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Summary:
The authors from Texas A&M University developed a decarboxylation-activated iron carboxylate MOF, PCN-250, which exhibits a mixed-valence Fe(II)Fe(III)₂ cluster, achieving enhanced CH₄ and CO₂ uptake with robust thermal cyclability.

Background:
1. To create open-metal sites for gas storage, prior work thermally activated carboxylate MOFs, yet the underlying redox mechanism and long-term stability remained unclear.
2. The authors herein propose a thermally induced decarboxylation mechanism that selectively reduces Fe(III)₃ to Fe(II)Fe(III)₂ while generating 37 Å mesopores.

Research Content:
1. Synthesis: PCN-250 (MIL-127) was solvothermally synthesized from FeCl₃ and H₄ABTC, then washed 25 days to introduce mesopores.
2. Characterizations:
   1) BET surface area rose from 1560 to 1609 m² g⁻¹ after 10 cycles at 240 °C; pore-size distribution confirmed 37 Å mesopores.
   2) SEM/TEM images revealed no particle aggregation or loss of crystallinity after cycling.
   3) ⁵⁷Fe Mössbauer, TGA-MS and GC-MS evidenced CO₂ release and decarboxylated ligand fragments, corroborating the Fe(III)→Fe(II) reduction.
3. Application: At 195 K and 1 bar, CH₄ uptake increased from 138.1 to 187.4 cm³ g⁻¹ (STP); CO₂ uptake and heats of adsorption also improved.
4. Mechanism: Decarboxylation above 200 °C removes carboxylate ligands, reduces one Fe(III) to Fe(II), and creates open sites plus mesopores; the process is reversible under oxidizing atmospheres.

Outlook:
This study establishes decarboxylation as a controllable route to generate stable defective MOFs with superior gas-sorption performance, offering broad implications for other carboxylate frameworks.

The thermally induced decarboxylation mechanism of a mixed-oxidation state carboxylate-based iron metal–organic framework
Authors: Hannah F. Drake, Gregory S. Day, Shaik Waseem Vali, Zhifeng Xiao, Sayan Banerjee, Jialuo Li, Elizabeth A. Joseph, Jason E. Kuszynski, Zachary T. Perry, Angelo Kirchon, Osman K. Ozdemir, Paul A. Lindahl, Hong-Cai Zhou
DOI: 10.1039/c9cc04555d
Link: https://pubs.rsc.org/en/content/articlelanding/2019/cc/c9cc04555d

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