The current interest in porous crystalline metal-organic frameworks (MOFs) [1] is largely due to their wide range of compositions and structure types with low framework densities, their tunability, and the possibility of accessible, coordinatively unsaturated metal sites (CUS). The existence of CUS can strongly modify interactions with gases [2] or liquid adsorbates, [3] and is thus of particular importance gas storage and separations.The redox properties of transition-metal-substituted zeolites and mesoporous materials have been extensively studied and used for selective catalysis in liquid-phase oxidation, [4] removal of nitrogen oxides, [5] and photocatalytic reactions.[6]These features are very rare for MOFs containing 3d metals, in particular with respect to the reducibility of the framework metal ions. [7] For this reason, we examine herein both the conditions of generation of iron CUS with mixed valence Fe II

The present study illustrates the importance of the oxidation state of iron within the mesoporous iron trimesate [{Fe(3)O(H(2)O)(2)F(0.81)(OH)(0.19)}{C(6)H(3)(CO(2))(3)}(2)] denoted MIL-100(Fe) (MIL= Material from Institut Lavoisier) during adsorption of molecules that can interact with the accessible metal sites through π-back donation. Adsorption of CO has been first followed by FTIR spectroscopy to quantify the Lewis acid sites in the dehydrated Fe(III) sample, outgassed at 150 °C, and on the partially reduced Fe(II/III), outgassed at 250 °C. The exposure of MIL-100(Fe) to CO(2), propane, propene and propyne has then been studied by FTIR spectroscopy and microcalorimetry. It appears that π-back donating molecules are strongly adsorbed on reduced iron(II) sites despite the weaker Lewis acidity of cus Fe(2+) sites compared to that of Fe(3+) ones, as shown by pyridine adsorption.

The current interest in porous crystalline metal-organic frameworks (MOFs) [1] is largely due to their wide range of compositions and structure types with low framework densities, their tunability, and the possibility of accessible, coordinatively unsaturated metal sites (CUS). The existence of CUS can strongly modify interactions with gases [2] or liquid adsorbates, [3] and is thus of particular importance gas storage and separations.The redox properties of transition-metal-substituted zeolites and mesoporous materials have been extensively studied and used for selective catalysis in liquid-phase oxidation, [4] removal of nitrogen oxides, [5] and photocatalytic reactions.[6]These features are very rare for MOFs containing 3d metals, in particular with respect to the reducibility of the framework metal ions. [7] For this reason, we examine herein both the conditions of generation of iron CUS with mixed valence Fe II

Infrared spectra of MIL-100(Al) have been recorded after evacuation from room temperature up to 623 K. In addition to adsorbed water molecules characterized by specific (ν+δ)H2O combination bands at about 5300 cm–1, spectra analysis shows the presence of impurities like carboxylic acid and nitrates resulting from the synthesis step, explaining the low amount of Al–OH groups detected. The Lewis acidity has been characterized by CO [ν(CO) at 2183 cm–1], pyridine [ν8a band estimated at 1618 cm–1], and CD3CN [ν(CN) at 2326 cm–1] adsorption on the activated sample. The acidity is strong as revealed by the ν(CN) wavenumber. Interestingly, CO gives rise to an interaction weaker than that expected from pyridine and CD3CN results. Quantitative results relative to the number of Al3+ 5c sites are in full agreement with those reported elsewhere from 27Al NMR experiments. The Brønsted acidity mainly results from the presence of coordinated water species in the nonfully dehydrated sample and not from the structural Al–OH groups.

An alternative activation method was developed to stabilize the Metal Organic Framework (MOF) MIL-47(V) material in the VIII oxidation state. This solid and the oxidized forms were investigated by in situ infrared and Raman spectroscopies, X-ray diffraction (XRD), and Complex Impedance Spectroscopy (CIS). Unlike MIL-47(VIV), MIL-47(VIII) is a flexible structure which presents μ2-hydroxyl groups acting as preferential adsorption sites for H2O or CO2. The modulation of the oxidation state of the metal center of this porous material leads to new intermediate porous solids with mixed oxidation states VIII/VIV. In these materials, the VIII and VIV centers seem to occur in close vicinity. However, the presence of VIV centers inhibits the flexibility to a large extent.

International audienceThe adsorption of the acid gas H2S has been explored in both MIL-47(V) and MIL-53(Cr) porous metal organic frameworks (MOFs) by combining infrared measurements and molecular simulations. It is shown that while the MIL-47(V) structure remains rigid upon H2S adsorption up to a pressure of 1.8 MPa, the MIL-53(Cr) solid initially present in the large pore form (LP) switches to its narrow pore version (NP) at very low pressure before undergoing a second structural transition from the NP to the LP versions at higher pressure. Such structural transitions further explain the different shape of the adsorption isotherms for both MILs. A further step consists of providing some insights into the microscopic arrangements of the adsorbate molecules within the pores of the MILs. At the initial stage of adsorption, the H2S molecules mainly form hydrogen bonded species, either as hydrogen donor (in MIL-47 V) or hydrogen-acceptor (in MIL-53Cr) with the mu(2)-O and mu(2)-OH groups, respectively, present at the MOF surfaces. At higher pressure (1.8 MPa), the adsorbates are preferentially arranged within the channel in order to form dimers with a high orientational disorder. Both experimental and simulated adsorption enthalpies for H2S decrease in the following sequence: MIL-53(Cr) NP > MIL-47(V) > MIL-53(Cr) LP. The conclusions drawn from this work are then discussed considering the use of such materials for the CH4/H2S separation by means of Pressure Swing Adsorption

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