Porous crystals are strategic materials with industrial applications within petrochemistry, catalysis, gas storage, and selective separation. Their unique properties are based on the molecular-scale porous character. However, a principal limitation of zeolites and similar oxide-based materials is the relatively small size of the pores, typically in the range of medium-sized molecules, limiting their use in pharmaceutical and fine chemical applications. Metal organic frameworks (MOFs) provided a breakthrough in this respect. New MOFs appear at a high and an increasing pace, but the appearances of new, stable inorganic building bricks are rare. Here we present a new zirconium-based inorganic building brick that allows the synthesis of very high surface area MOFs with unprecedented stability. The high stability is based on the combination of strong Zr-O bonds and the ability of the inner Zr6-cluster to rearrange reversibly upon removal or addition of mu3-OH groups, without any changes in the connecting carboxylates. The weak thermal, chemical, and mechanical stability of most MOFs is probably the most important property that limits their use in large scale industrial applications. The Zr-MOFs presented in this work have the toughness needed for industrial applications; decomposition temperature above 500 degrees C and resistance to most chemicals, and they remain crystalline even after exposure to 10 tons/cm2 of external pressure.

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.

XRD, UV-Vis, EXAFS, XANES, and Raman techniques have been used to study the removal of water molecules coordinated to the Cu(II) framework atoms of the novel HKUST-1 metal-organic framework. The dehydration process preserves the crystalline nature of the material, just causing a reduction of the cell volume due to the shrinking of the [Cu 2 C 4 O 8 ] cage. The removal of adsorbed H 2 O molecule makes the framework Cu(II) sites available for interaction with other probe molecules. In situ IR spectroscopy has evidenced the formation at liquid nitrogen temperature of labile Cu(II)‚‚‚CO adducts characterized by a ν(C-O) ) 2178 cm -1 and at 15 K of Cu(II)‚‚‚H 2 adducts characterized by a ν(H-H) ) 4100 cm -1 . To the best of our knowledge, we have observed for the first time a clear signal of Cu(II) carbonyl and dihydrogen complexes formed inside a crystalline microporous hosting matrix. The sinking of the oxygens of the carboxyl units, undergone by the Cu(II) framework ions in the dehydration process, is responsible for the rather low coordinative unsaturation of Cu(II). The important shielding effect of the four oxygen framework atoms is testified by the low polarization factor of the Cu(II) site probed by both CO and H 2 molecules.