(376bi) Study of Functional Groups of Ligands in Cu2+ MOFs in the Efficiency and Selectivity of Gas Adsorption

Metal Organic Frameworks (MOFs), are coordination compounds with crystalline structure formed by two sections organic ligands, comprising carboxylic acids, imidazoles, and other molecules with active heteroatoms, and metal compounds (ions or clusters). MOFs form reticular structures with big cavities and large surface area.

MOFs are extensively used for gas adsorption and separation [1-4], which are directly related to the pore size of the material and the affinity of the host molecules with the metals or the ligands of the MOF.

In this work we synthesize and analyze a family of copper derived MOFs as adsorbents of gases such as NH₃ and SO₂. The objective is to relate the chemical structure with the efficiency and selectivity of the adsorption process. MOFs are synthesized by mixing the ligands (terephthalic acid, phthalic acid and succinic acid) with the copper source (Cu(NO₃)₂⸱6H₂O) in a molar ratio of 1:4 in dimethyl formamide (DMF). The mixtures were homogenized and left at 80°C for 48 hours. The solid formed is filtered and washed with DI water and let dry at 120°C for 3 hours. MOFs are then characterized by solubility tests, melting point determination, FT-IR and X-ray diffraction.

For the adsorption tests, filters with 0.1 g of MOF were built, and two different tests were carried on. For the first test, NH₃ detection tubes are connected to a source of NH₃ (a solution of NH₄OH), the adsorption efficiency is determined in one step by correlating the concentration of gas detected by the color tubes in the system with and without filter. MOFs were compared to active carbon. MOFs showed better efficiency than carbon; terephthalic acid MOF adsorbed twice than the succinic acid MOF.

In a second test, a solution containing 3% of NH₄OH v/v is heated. The liberated gas is separated in two currents, one of them is connected to the MOF filter and then both currents are bubbled in corresponding vapor traps containing a solution of 0.1 M H₂SO₄. After 10 minutes, the solution in the vapor trap is titrated with NaOH 0.01 M previously validated with a reference. Terephthalic acid MOF registered a 64.7% of efficiency of NH₃ while succinic acid MOF registered 35.2%. Phthalic acid MOF showed similar efficiency than terephthalic acid MOF.

Test was repeated for SO₂. For generating SO₂ the reaction of NaSO₃ and H₂SO₄ was carried instead of the heating of the solution and using water in the vapor traps. Terephthalic acid MOFs showed bigger efficiency than succinic acid MOF but lower than activated carbon. A specific efficiency could not be registered because degradation of the MOF was observed due to the acidity of the gas.

Aromatic ligands in MOFs have a better efficiency in adsorbing basic and acid gases, however the stability of the material gets compromised in strong acid environments. The work in our laboratory continues in the synthesis of new MOF ligands and the design of systems for gas separations.

[1] Han, S.; Huang, Y.; Watanabe, T.; Nair, S.; Walton, K.; Sholl, D.S.; Meredith, C. Microporous and Mesoporous Materials 2013, 173, 86-91 “MOF stability and gas adsorption as a function of exposure to water, humid air, SO₂ and NO₂”

[2] Jacoby, M. C&EN Chicago, Science and Technology 2014, September, 26-28 “New sorbents for greener cooling”

[3] Zhao, Z.; Ma, X.; Kasik, A.; Li, Z.; Lin, Y.S. Ind. Eng. Chem. Res. 2013, 52, 1102-1108 “Gas Separation Properties of Metal Organic Framework (MOF-5) Membranes”

[4] Hartlieb, K.J.; Holcroft, J.M.; Moghadam, P.Z.; Vermeulen, N.A.; Algaradah, M.M.; Nassar, M.S.; Botros, Y.Y.; Snurr, R.Q.; Stoddart, F. J. Am. Chem. Soc. 2016, 138(7), 2292-2301 “CD-MOF: A versatile separation method”