Tsivion Ehud, Mason Jarad A, Gonzalez Miguel I, Long Jeffrey R, Head-Gordon Martin
Materials Sciences Division , Lawrence Berkeley National Laboratory , Berkeley , California 94720 , USA.
Department of Chemistry , University of California , Berkeley , California 94720 , USA . Email:
Chem Sci. 2016 Jul 1;7(7):4503-4518. doi: 10.1039/c6sc00529b. Epub 2016 Mar 29.
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 CH on 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 CH usable capacity is evaluated. It is found that sufficient ionic character is required, as exemplified by the strong CH affinities of 2,2'-bipyridine-CaCl and 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 CH affinity, but can contribute to increased usable capacity by modifying adsorption interactions.
为了以足够的密度廉价地储存天然气（NG）以用作运输部门的燃料，正在开发新型多孔材料。这项工作使用计算方法来探索提高吸附剂（包括金属有机框架（MOF））可用甲烷储存容量的策略，这些吸附剂通过后合成修饰在其结构中引入了开放金属位点。通过计算几何结构和吸附能并分析相关相互作用因素，研究了CH在几个开放金属位点上的吸附。获得了近似的位点特异性吸附等温线，并评估了开放金属位点对整体CH可用容量的贡献。发现需要足够的离子特性，例如2,2'-联吡啶-CaCl和Mg，Ca-儿茶酚酸盐对CH的强亲和力所证明的。此外，发现单个金属位点的容量不仅取决于其亲和力，还取决于其几何形状，其中三角形或“弯曲”的低配位暴露位点可以容纳三个或四个甲烷分子，例如Ca修饰的次氮基三乙酸所证明的。还探索了开放金属位点处残留溶剂分子的影响，并得出了一些积极的结论。残留溶剂不仅可以稳定开放金属位点，令人惊讶的是，溶剂分子不一定会降低CH亲和力，而是可以通过改变吸附相互作用来提高可用容量。
