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Enhancing Van der Waals Interactions of Functionalized UiO‐66 with Non‐polar Adsorbates: The Unique Effect of para Hydroxyl Groups
Author(s) -
Tovar Trenton M.,
Iordanov Ivan,
Sava Gallis Dorina F.,
DeCoste Jared B.
Publication year - 2018
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201704779
Subject(s) - physisorption , van der waals force , adsorption , chemistry , density functional theory , binding energy , metal organic framework , polar effect , ring (chemistry) , molecule , computational chemistry , organic chemistry , physics , nuclear physics
UiO‐66 is a highly stable metal‐organic framework (MOF) that has garnered interest for many adsorption applications. For small, nonpolar adsorbates, physisorption is dominated by weak Van der Waals interactions limiting the adsorption capacity. A common strategy to enhance the adsorption properties of isoreticular MOFs, such as UiO‐66, is to add functional groups to the organic linker. Low and high pressure O 2 isotherms were measured on UiO‐66 MOFs functionalized with electron donating and withdrawing groups. It was found that the electron donating effects of ‐NH 2 , ‐OH, and ‐OCF 3 groups enhance the uptake of O 2 . Interestingly, a significant enhancement in both the binding energy and adsorption capacity of O 2 was observed for UiO‐66‐(OH) 2 ‐p, which has two ‐OH groups para from one another. Density functional theory (DFT) simulations were used to calculate the binding energy of oxygen to each MOF, which trended with the adsorption capacity and agreed well with the heats of adsorption calculated from the Toth model fit to multi‐temperature isotherms. DFT simulations also determined the highest energy binding site to be on top of the electron π‐cloud of the aromatic ring of the ligand, with a direct trend of the binding energy with low pressure adsorption capacity. Uniquely, DFT found that oxygen molecules adsorbed to UiO‐66‐(OH) 2 ‐p prefer to align parallel to the ‐OH groups on the aromatic ring. Similar effects for the electron donation of the functional groups were observed for the low pressure adsorption of N 2 , CH 4 , and CO 2 .