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Open Access Publications from the University of California

A computational study of CH4storage in porous framework materials with metalated linkers: Connecting the atomistic character of CH4binding sites to usable capacity

  • Author(s): Tsivion, E
  • Mason, JA
  • Gonzalez, MI
  • Long, JR
  • Head-Gordon, M
  • et al.

© 2016 The Royal Society of Chemistry. To store natural gas (NG) inexpensively at adequate densities for use as a fuel in the transportation sector, new porous materials are being developed. This work uses computational methods to explore strategies for improving the usable methane storage capacity of adsorbents, including metal-organic frameworks (MOFs), that feature open-metal sites incorporated into their structure by postsynthetic modification. The adsorption of CH4on several open-metal sites is studied by calculating geometries and adsorption energies and analyzing the relevant interaction factors. Approximate site-specific adsorption isotherms are obtained, and the open-metal site contribution to the overall CH4usable capacity is evaluated. It is found that sufficient ionic character is required, as exemplified by the strong CH4affinities of 2,2′-bipyridine-CaCl2and Mg, Ca-catecholate. In addition, it is found that the capacity of a single metal site depends not only on its affinity but also on its geometry, where trigonal or "bent" low-coordinate exposed sites can accommodate three or four methane molecules, as exemplified by Ca-decorated nitrilotriacetic acid. The effect of residual solvent molecules at the open-metal site is also explored, with some positive conclusions. Not only can residual solvent stabilize the open-metal site, surprisingly, solvent molecules do not necessarily reduce CH4affinity, but can contribute to increased usable capacity by modifying adsorption interactions.

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