Water Cluster Confinement and Methane Adsorption in the Hydrophobic Cavities of a Fluorinated Metal-Organic Framework
Water Cluster Confinement and Methane Adsorption in the Hydrophobic Cavities of a Fluorinated Metal-Organic Framework
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DOI：
10.1021/ja400754p
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发表时间：
2013-08-28
影响因子：
15
通讯作者：
Chabal, Yves J.
中科院分区：
文献类型：
--
作者：
Nijem, Nour;Canepa, Pieremanuele;Chabal, Yves J.
通过原位振动光谱、吸附等温线和第一性原理DFT计算（使用dW-DF），研究了疏水多孔金属有机骨架内水簇形成和甲烷吸附。具体来说，研究了氟化金属有机骨架（FMOF-1）疏水腔中H2O簇的形成和稳定性。尽管水的等温线显示没有可测量的吸收（见Yang等人）。j。化学。Soc. 2011, 133, 18094)，水内部模式的大偶极子使得利用红外光谱在固体框架表面附近的孔隙中检测低水浓度成为可能。结果表明，即使在低压条件下（100 mTorr ~ 3 mTorr），水分子也优先占据大空腔，其中氢键和壁疏水性促进了水团簇的形成。我们确定了在低于3托的压力下形成的五聚体水团簇和超过该压力的更大的团簇。根据DFT计算结果和红外吸收数据证实，水与壁面的结合能可以忽略不计。因此，分子间氢键占主导地位，随着团簇大小的增加，水团的稳定性增强。受宿主扰动可忽略不计的水团的形成可以与对大气中低浓度的较大水团的实验环境研究进行定量比较。研究了水团簇的稳定性作为压力降低和甲烷气体存在的函数。活化FMOF-1的甲烷吸附等温线在288k和31bar条件下为67v (STP)/V ~ 133v (STP)/V，在173k和5bar条件下为133v (STP)/V，在288k ~ 318k高温条件下吸附热约为14kj /mol。总体而言，实验和计算数据表明，在工作温度和湿度条件下，FMOF-1孔隙对甲烷气体的吸收率高于水蒸气，易于解吸，骨架稳定性高。
Water cluster formation and methane adsorption within a hydrophobic porous metal organic framework is studied by in situ vibrational spectroscopy, adsorption isotherms, and first-principle DFT calculations (using dW-DF). Specifically, the formation and stability of H2O clusters in the hydrophobic cavities of a fluorinated metal organic framework (FMOF-1) is examined. Although the isotherms of water show no measurable uptake (see Yang et al. J. Am. Chem. Soc. 2011, 133, 18094), the large dipole of the water internal modes makes it possible to detect low water concentrations using IR spectroscopy in pores in the vicinity of the surface of the solid framework. The results indicate that, even in the low pressure regime (100 mTorr to 3 Torr), water molecules preferentially occupy the large cavities, in which hydrogen bonding and wall hydrophobicity foster water cluster formation. We identify the formation of pentameric water clusters at pressures lower than 3 Torr and larger clusters beyond that pressure. The binding energy of the water species to the walls is negligible, as suggested by DFT computational findings and corroborated by IR absorption data. Consequently, intermolecular hydrogen bonding dominates, enhancing water cluster stability as the size of the cluster increases. The formation of water clusters with negligible perturbation from the host may allow a quantitative comparison with experimental environmental studies on larger clusters that are in low concentrations in the atmosphere. The stability of the water clusters was studied as a function of pressure reduction and in the presence of methane gas. Methane adsorption isotherms for activated FMOF-1 attained volumetric adsorption capacities ranging from 67 V(STP)/V at 288 K and 31 bar to 133 V(STP)/V at 173 K and 5 bar, with an isosteric heat of adsorption of ca. 14 kJ/mol in the high temperature range (288-318 K). Overall, the experimental and computational data suggest high preferential uptake for methane gas relative to water vapor within FMOF-1 pores with ease of desorption and high framework stability under operative temperature and moisture conditions.