The effect of methyl functionalization on microporous metal-organic frameworks' capacity and binding energy for carbon dioxide adsorption

Hui Liu, Yonggang Zhao, Zhijuan Zhang, Nour Nijem, Yves J. Chabal, Heping Zeng, Jing Li

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Abstract

The design, synthesis, and structural characterization of two new microporous metal-organic framework (MMOF) structures is reported; Zn(BDC)(DMBPY) 0.5·(DMF) 0.5(H 2O) 0.5 (1; H 2 BDC = 1,4-benzenedicarboxylic acid; DMBPY=2,2′-dimethyl-4,4′-bipyridine) and Zn(NDC)(DMBPY) 0.5·(DMF) 2 (2; H 2NDC = 2,6-naphthalenedicarboxylic acid, DMF=N,N,-dimethylformamide), which are obtained by functionalizing a pillar ligand with methyl groups. Both compounds are 3D porous structures of the Zn 2(L) 2(P) type and are made of a paddle-wheel Zn 2(COO) 4 secondary building unit (SBU), with the dicarboxylate and DMBPY as linker (L) and pillar (P) ligands, respectively. Comparisons are made to the parent structures Zn(BDC)(BPY) 0.5·(DMF) 0.5(H 2O) 0.5 (3; BPY = 4,4′-bipyridine) and Zn(NDC)(BPY) 0.5·(DMF) 1.575 (4) to analyze and understand the effect of methyl functionalization. CO 2-adsorption studies indicate substantially enhanced isosteric heats of CO 2 adsorption (Q st) for both compounds, as a result of adding methyl groups to the BPY ligand. The CO 2 uptake capacity, however, is affected by two opposing and competing factors: the enhancement due to increased MMOF-CO 2 interactions (higher Q st values) and detraction due to the surface area and pore-volume reduction. For 1′ (the guest-free form of 1), the positive effect dominates, which leads to a significantly higher uptake of CO 2 than that of its parent structure 3′ (the guest-free form of 3). In 2′ (the guest-free form of 2), however, the negative effect rules, which results in a slightly lower CO 2 uptake with respect to 4′ (the guest-free form of 4). All four compounds exhibit a relatively high separation capability for carbon dioxide over other small gases, including CH 4, N 2, and O 2. The separation ratios of CO 2 to O 2 and N 2 (at 298 K and 1 atm) are 39.8 and 23.5 for compound 1′, 57.7 and 40.2 for 2′, 25.7 and 29.5 for 3′, 89.7, and 20.3 for 4′, respectively. IR and Raman spectroscopic characterization of CO 2 interactions with 1′ and 2′ provides indirect support of the importance of the methyl groups in the interaction of CO 2 within these systems.

Original languageEnglish
Pages (from-to)4754-4762
Number of pages9
JournalAdvanced Functional Materials
Volume21
Issue number24
DOIs
Publication statusPublished - Dec 20 2011

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Keywords

  • carbon dioxide capture
  • gas adsorption
  • gas separation
  • ligand functionalization
  • microporous metal organic frameworks

ASJC Scopus subject areas

  • Chemistry(all)
  • Materials Science(all)
  • Condensed Matter Physics

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