Previous efforts to enhance gas adsorption in metal organic frameworks (MOFs) have focused on increasing the gas binding energy within each pore. In contrast, we show here that exposure of ethylenediamine (EDA) after gas loading forms a monolayer cap that can effectively trap weakly adsorbed molecules (CO, CO2, SO2, C2H4, NO) or prevent their penetration, because it only deposits on the external surfaces of the MOF microcrystals. Combining in-situ infrared spectroscopy, X-ray photoelectron spectroscopy with gas cluster sputtering, low energy ion spectroscopy, and ab initio calculations, we demonstrate that the EDA layer is less than 1nm thick and arranges in a manner that increases the CO diffusion barrier by ~24 times. We further show that water molecules can traverse the same EDA barrier without hindrance (i.e. barrierless diffusion), in sharp contrast to CO, CO2, SO2 C2H4, and can fully displace these small molecules out of the MOF altogether. This unexpected selectivity, as accounted for by ab initio modeling, is attributed to the ability of water to H-bond to the terminal amine network, thus opening a space for water molecules to pass through. These findings not only provide a unique means for trapping gases inside MOF at atmospheric conditions without having to synthetically increase their binding energy within the MOFs, but also suggest a new concept to develop future selective membranes for adsorption in porous materials.

Trapping gases in metal organic frameworks with a selective surface molecular barrier layer

253rd National Meeting (2017)

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