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.

Presented in this paper is a deep investigation into the defect chemistry of UiO-66 when synthesized in the presence of monocarboxylic acid modulators under the most commonly employed conditions. We unequivocally demonstrate that missing cluster defects are the predominant defect and that their concentration (and thus the porosity and composition of the material) can be tuned to a remarkable extent by altering the concentration and/or acidity of the modulator. Finally, we attempt to rationalize these observations by speculating on the underlying solution chemistry.

No abstract

The air-free reaction between FeCl 2 and H 4 dobdc (dobdc 4À = 2,5-dioxido-1,4-benzenedicarboxylate) in a mixture of N,N-dimethylformamide (DMF) and methanol affords Fe 2 (dobdc) 3 4DMF, a metalÀorganic framework adopting the MOF-74 (or CPO-27) structure type. The desolvated form of this material displays a BrunauerÀEmmettÀTeller (BET) surface area of 1360 m 2 /g and features a hexagonal array of onedimensional channels lined with coordinatively unsaturated Fe II centers. Gas adsorption isotherms at 298 K indicate that Fe 2 (dobdc) binds O 2 preferentially over N 2 , with an irreversible capacity of 9.3 wt %, corresponding to the adsorption of one O 2 molecule per two iron centers. Remarkably, at 211 K, O 2 uptake is fully reversible and the capacity increases to 18.2 wt %, corresponding to the adsorption of one O 2 molecule per iron center. M€ ossbauer and infrared spectra are consistent with partial charge transfer from iron(II) to O 2 at low temperature and complete charge transfer to form iron(III) and O 2 2À at room temperature. The results of Rietveld analyses of powder neutron diffraction data (4 K) confirm this interpretation, revealing O 2 bound to iron in a symmetric sideon mode with d OÀO = 1.25(1) Å at low temperature and in a slipped side-on mode with d OÀO = 1.6(1) Å when oxidized at room temperature. Application of ideal adsorbed solution theory in simulating breakthrough curves shows Fe 2 (dobdc) to be a promising material for the separation of O 2 from air at temperatures well above those currently employed in industrial settings. ' INTRODUCTIONWith over 100 million tons produced annually, O 2 is one of the most widely used commodity chemicals in the world. 1 Its potential utility in processes associated with the reduction of carbon dioxide emissions from fossil fuel-burning power plants, however, means that the demand for pure O 2 could grow enormously. For implementation of precombustion CO 2 capture, pure O 2 is needed for the gasification of coal, which produces the feedstock for the waterÀgas shift reaction used to produce CO 2 and H 2 . 2 In addition, oxyfuel combustion is receiving considerable attention for its potential utility as an alternative to postcombustion CO 2 capture. Here, pure O 2 is diluted to 0.21 bar with CO 2 and fed into a power plant for fuel combustion. Since N 2 is absent from the resulting flue gas, the requirement for postcombustion separation of CO 2 from N 2 is eliminated. 3 The separation of O 2 from air is currently carried out on a large scale using an energy-intensive cryogenic distillation process. 4 Zeolites are also used for O 2 /N 2 separation, 5 both industrially and in portable medical devices; however, this process is inherently inefficient as the materials used adsorb N 2 over O 2 with poor selectivity. By employing materials that selectively adsorb