(100e) Cu-BTC, Polar Or Apolar MOF?

Cu-BTC, polar or apolar MOF?

Tom R.C. Van Assche^a, Tim Duerinck^a, Juan José Guttiérez-Sevillano^b, Sofia Calero^b, Joeri F.M. Denayer^a

^a Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium
^b Department of Physical, Chemical and Natural Systems, University Pablo de Olivade, Ctra. Utera Km. 1, 41013 Seville, Spain

Copper-benzene-1,3,5-tricarboxylate, also known under the name Cu-BTC or HKUST-1 [1], is one of the most studied metal-organic frameworks (MOF). The material was first reported by Chui et al. in 1999 [1] and has since then received considerable attention in the fields of adsorption, gas separation, hydrogen storage, catalysis, etc. Despite the fact that the material is well studied and understood in relation to its structure and adsorption of mainly apolar hydrocarbons and light gasses, not much is known about the adsorption of more polar adsorbates. Among the polar adsorbates, water has received the most attention [2]. However, since the Cu-BTC structure has questionable water stability, it is difficult to compare adsorption data of water with other molecules. To further complicate the matter, the adsorption properties of the material tend to vary significantly (BET surface area of 692-1635 m²/g) between studies.       
Using commercially available Cu-BTC and strict storage and handling procedures, minimizing water contact, reproducible vapor isotherms were measured on the Cu-BTC framework, with very high saturation capacities for various polar and apolar adsorbates.  Additionally, the negative effect of water exposure was measured. With exception of water, pore volumes exceeding 0.66 cc/g were found for small alcohols, acetonitrile, acetone, THF, N,N-DMF and other molecules. Surprisingly, small and polar adsorbates give rise to a distinct two-step adsorption isotherm (Type IV). The isotherm step shifts to higher loading and lower relative vapor pressure for increasingly apolar adsorbates.           
Using Monte Carlo simulation, the two-step adsorption isotherm can be explained by the difference in the various Cu-BTC cages. Originally, the Cu-BTC material was ascribed two types of cages; small octahedral pockets (T1) and large central cages (L). However, a further distinction can be made for the large cages; those with copper unsaturated sites exposed (L3), and thus who do not have these copper sites (L2). The copper sites result in a more polar behavior of the corresponding cages. The combined behavior of the cages is proposed to result in a two-step adsorption isotherm for polar molecules, as the L2 cages will yield a S-shaped isotherm, while the L3 cages display more Langmuirian behavior.
The adsorptive behavior of the various cages is also reflected in the isosteric heat of adsorption and the Fickian and Maxwell-Stefan diffusion coefficients, as studied in this work for methanol. Here, the diffusion coefficients of polar molecules such as alcohols are significantly smaller than for apolar molecules (alkanes).       
The dual character of Cu-BTC, polar and apolar, is expected to reoccur for other MOF’s. Understanding the adsorption of various adsorbates in the well-known Cu-BTC structure is beneficial to understand the adsorptive properties of other, both hydrophilic or hydrophobic MOF. For example, hydrophobic MOF without free metal sites, like ZIF-8, show a very similar adsorption behavior as compared to the Cu-BTC apolar cages.

Acknowledgements:
T.R.C. Van Assche is grateful to the Agency for Innovation by Science and Technology (IWT) Flanders for financial support.

[1] Chui, S.S.Y.; Lo, S.M.F.; Charmant, J.P.H.; Orpen, A.G.; Williams, I.D. A Chemically Functionalizable Nanoporous Material [Cu₃(TMA)₂(H2O)₃]_n. Science 1993, 283, 1148-1150.
[2] Küsgens, P.; Rose, M.; Senkovska, I.; Föde, H.; Henschel, A.; Siegle, S.; Kaskel, S. Characterization of metal-organic frameworks by water adsorption. Microporous Mesoporous Mater. 2009, 120, 325-330