Disclosing the Complex Structure of UiO-66 Metal Organic Framework: A Synergic Combination of Experiment and Theory

Abstract

Through a combined use of experimental and theoretical approaches such as XRPD, EXAFS, IR, and UV−vis spectroscopies and ab initio periodic DFT calculations, we report a detailed characterization of structural, vibrational, and electronic properties of UiO-66 (Zr-BDC MOF) in its hydroxylated and dehydroxylated forms. The stability of the materials with respect to the most common solvents, acids, and bases is determined by combining XRPD and TGA/MS techniques. The structures of the two forms of UiO-66 are refined through an interactive XRPD/EXAFS approach and validated by ab initio calculations. Experimental and calculated IR spectra are reported and compared to enlighten the nature of vibrational modes upon dehydroxylation and to show the complete reversibility of the dehydration/hydration phenomenon. Experimental and calculated band gaps are also reported and compared. In this work, we show the necessity to combine, in a synergic way, different experimental techniques and periodic ab initio approaches to disclose and fully understand the nature of complex novel materials such as UiO-66 on structural, vibrational, and electronic grounds. The correct structure refinement could not be possible using one of these three approaches alone, in particular, XRPD data were unable to detect an important distortion of the Zr6O6 units of the dehydrated material that was, however, foreseen in the ab initio calculations and measured in the EXAFS spectra.

Zirconium-containing metal organic frameworks as solid acid catalysts for the esterification of free fatty acids: Synthesis of biodiesel and other compounds of interest

“…These results indicate that linker defects are present in both compounds, as already concluded by others in previous studies [26]. A detailed analysis of the linker deficiency is given in the Supporting Information.…”

Conversion of levulinic acid into chemicals: Synthesis of biomass derived levulinate esters over Zr-containing MOFs

“…Similarly, the Lewis acid properties of UiO-type materials arise (at least partially) from the formation of crystalline defects associated to linker deficiencies, as already established in previous works by the groups of Lamberti (Valenzano et al, 2011) and De Vos (Vermoortele et al, 2012). Indeed, the analysis of the thermogravimetric (TGA) curves of these Zr-MOFs indicates that the amount of organic linker (either terephthalate or aminoterephthalate for UiO-66 and UiO-66-NH 2 , respectively) with respect to the inorganic residue (ZrO 2 in both cases) is systematically lower that the expected from the ideal stoichiometry of the MOF, the exact fraction depending on the particular synthesis conditions of the sample.…”

“…S1 in Supporting Information), with linker deficiencies ranging from 2.5 to 13.2% for UiO-66, and between 2.1 and 8% for UiO-66-NH 2 , as determined from the corresponding TGA curves following the procedure described by (Valenzano et al, 2011 Table 1). …”

“…However, in spite of lacking coordination vacancies in their ideal crystalline structure, Zr-containing UiO-66 type MOFs (Cavka et al, 2008) also display the typical reactivity expected for a Lewis acid catalyst (Cirujano et al, 2014;Vermoortele et al, 2011;Vermoortele et al, 2012). According to recent characterization studies (Valenzano et al, 2011), the catalytic activity of these Zr-MOFs would arise from the occurrence of a (thermal) dehydroxylation of the [Zr 6 O 4 (OH) 4 ] 12+ clusters to [Zr 6 O 6 ] 12+ exposing µ 3 -vacancies, together with the systematic formation of crystalline defects associated to linker deficiencies. The crystalline structure of UiO-66 is shown in Figure 1, together with the type of Zr 4+ coordination vacancies that are created when a linker molecule is missing.…”

Stability and Hydrolyzation of Metal Organic Frameworks with Paddle-Wheel SBUs upon Hydration

“…The sensitivity of the carboxylate mode to hydration and dehydration was mentioned in a previous study of UiO-66 metal organic frameworks. 74 The structure is maintained stable after 3-cycle absorption and desorption of water at the pressure up to 8 Torr (See Fig. S12).…”