Synthesis of hydrolytically stable metal-organic frameworks by control of inner surface hydrophobicity
Final Report Abstract
A post-synthetic functionalization approach was used to tailor the hydrophobicity of DUT-67 (DUT - Dresden University of Technology) as well as create catalytically active sites. DUT-67 is a MOF consisting of eight-connected Zr6O6-clusters and 2,5-thiophenedicarboxylate as ligand. Using the post-synthetic exchange of the modulator by fluorinated monocarboxylates, rational functionalization of the inner surface was possible. In DUT-67 only one out of three available pore types could be functionalized because of the peculiarity of the framework topology. Functionalization of the inner pore surface by fluorinated monocarboxylates leads not only to the change of hydrophobicity reflected by changes in the water contact angle, but also causes a reversal in the pore filling sequence. These changes in water adsorption mechanism are detected using water vapour adsorption isotherms analysis, and in parallel pore size distribution analysis, performed on the model crystal structures. In the case of DUT-67, the stability of the framework during water exposure could be significantly improved by integrating fluorinated acids, rendering this material as a potential candidate for adsorption-based heat exchange applications and solar cooling. We believe our approach can be widely applied to other Zr-based MOFs with reduced cluster connectivity. The same post-synthetic modification approach, in acidic treatment with HCl or H2SO4, changes in the coordination environment of the metal cluster could be achieved. Modulator molecules were removed from the framework and corresponding anions of the inorganic acids are incorporated for charge balancing. Thereby, it was possible to tune the relevant properties of the MOF - the hydrophilicity as well as the acidic strength. These modifications have a huge impact on the catalytic activity. While pristine DUT-67 has relative small TOFs in the esterification of levulinic acid with ethanol (6 h^-1 for Zr based – and 2 h^-1 for Hf-based DUT-67), DUT-67_HCl shows an ~18-fold increase of TOFs (Zr-based) and even a 50-fold increase for the Hf-based MOF. The sulphate-bearing DUT-67 is slightly less active in comparison to DUT-67_HCl, likely due to steric factors. Recycling tests showed a poisoning of the catalysts, but catalytic activity could be completely regained by a subsequent washing procedure, involving a diluted HCl solution. The full recovery illustrates the importance of absence of molecules, like modulators or other metal coordinating molecules, which block catalytically active centres. The Zr-MOF DUT-67 also was demonstrated to be an ideal platform for the exchange of chiral monocarboxylic acids at the Zr-nodes, for example by soaking in L-proline solution. The catalytic activity of DUT-67-pro was confirmed in the asymmetric Michael reaction of cyclohexanone to transβ-nitrostyrene to form the enantiomers of 2-(2-nitro-1-phenylethyl) cyclohexanone, with an excellent yield (up to 96%) and enantioselectivity (approximately 38% ee) comparable to that of the homogeneous catalyst L-proline. In addition, DUT-67-pro can be separated easily from the products and reused many times without substantial degradation in catalytic activity. Chiral DUT-67-pro is a promising catalyst in asymmetric Michael addition reaction of unmodified ketones as cyclohexanone to trans-β-nitrostyrene. In conclusion, the post-synthetic modification procedure can be applied to the majority of 8- or 6- connected Zr-MOFs. These porous systems can be precisely tuned for a wide spectrum of applications, including adsorption, catalysis, and sensing.
Publications
- (2018) Chiral Functionalization of a Zirconium Metal-Organic Framework (DUT-67) as a Heterogeneous Catalyst in Asymmetric Michael Addition Reaction. Inorganic chemistry 57 (3) 1483–1489
Nguyen, Khoa D.; Kutzscher, Christel; Drache, Franziska; Senkovska, Irena; Kaskel, Stefan
(See online at https://doi.org/10.1021/acs.inorgchem.7b02854) - Gas Storage in a Partially Fluorinated Highly Stable Three-Dimensional Porous Metal–Organic Framework. Inorg. Chem. 2013, 52(13), 7358-7366
Santra, A.; Kaskel, S.; Bharadwaj, P.K.
(See online at https://doi.org/10.1021/ic302645r) - Postsynthetic Inner-Surface Functionalization of the Highly Stable Zirconium-Based Metal–Organic Framework DUT-67. Inorg. Chem. 2016, 55 (15), 7206-7213
Drache, F.; Bon, V.; Senkovska, I.; Marschelke, C.; Synytska, A.; Kaskel, S.
(See online at https://doi.org/10.1021/acs.inorgchem.6b00829) - Vapochromic Luminescence of a Zirconium-Based Metal-Organic Framework for Sensing Applications. Eur. J. Inorg. Chem. 2016, (27), 4483-4489
Drache, F.; Bon, V.; Senkovska, I.; Adam, M.; Eychmüller, A.; Kaskel, S.
(See online at https://doi.org/10.1002/ejic.201600261) - The modulator driven polymorphism of Zr(IV) based metal-organic frameworks. Philos. Trans. R. Soc., A 2017, 375 (2084)
Drache, F.; Bon, V.; Senkovska, I.; Getzschmann, J.; Kaskel, S.
(See online at https://doi.org/10.1098/rsta.2016.0027)
