Cécile Daniel

Heats of adsorption for seven gases in three metal - Organic frameworks: Systematic comparison of experiment and simulation

The heat of adsorption is an important parameter for gas separation and storage applications in porous materials such as metal-organic frameworks (MOFs). There are, however, few systematic studies available in the MOF literature. Many papers report results for only one MOF and often only for a single gas. In this work, systematic experimental measurements by TAP-2 are reported for the heats of adsorption of seven gases in three MOFs. The gases are Kr, Xe, N2, CO2, CH4, n-C4H10, and i-C4H10. The MOFs studied are IRMOF-1, IRMOF-3, and HKUST-1. The data set provides a valuable test for molecular simulation. The simulation results suggest that structural differences in HKUST-1 experimental samples may lead to differing heats of adsorption.

Absolute molecular sieve separation of ethylene/ethane mixtures with silver zeolite A.

Absolute ethylene/ethane separation is achieved by ethane exclusion on silver-exchanged zeolite A adsorbent. This molecular sieving type separation is attributed to the pore size of the adsorbent, which falls between ethylene and ethane kinetic diameters.

Structure–property relationships of water adsorption in metal–organic frameworks

A set of 15 metal–organic frameworks (MIL-53, MIL-68, MIL-125, UiO-66, ZIF) exhibiting different pore size, morphology, and surface chemistry is used to unravel the numerous behaviors of water adsorption at room temperature in this class of materials. Outstanding “S”-shaped (type V) adsorption isotherms are observed for MIL-68 type solids. We show that the underlying mechanism of water adsorption can be rationalized using a simple set of three parameters: the Henry constant (i.e. the slope of the adsorption pressure in the low pressure range), the pressure at which pore filling occurs, and the maximum water adsorption capacity. While the Henry constant and pore filling pressure mostly depend on the affinity of water for the surface chemistry and on pore size, respectively, these two parameters are correlated as they both reflect different aspects of the hydrophobicity–hydrophilicity of the material. For a given type of porous structure, the functionalization of the material by hydrophilic moieties such as hydrogen bonding groups (amine or aldehyde) systematically leads to an increase in the Henry constant concomitantly with a decrease in the pore filling pressure. As for the adsorption mechanism, we show that, for a given temperature, there is a critical diameter (Dc ∼ 20 A for water at room temperature) above which pore filling occurs through irreversible capillary condensation accompanied by capillary hysteresis loops. Below this critical diameter, pore filling is continuous and reversible unless the material exhibits some adsorption-induced flexibility.

Engineering the Environment of a Catalytic Metal–Organic Framework by Postsynthetic Hydrophobization

Heterogeneous catalysis is of paramount importance in many areas of the chemical and energy industries. Often, however, reactions can be hindered or reaction rates limited by poisoning effects originating from moisture in the air or from the water formed during the organic transformation. Water can be adsorbed and block catalytic sites, leading to their deactivation. This drawback has motivated the design and engineering of catalytic materials with hydrophobic features, such as the hydrophobic outer shell of enzymes, to prevent water-induced catalyst poisoning. Metal–organic frameworks (MOFs) represent an extensive class of porous organic–inorganic crystalline materials. Due to their calibrated pore size, they are regarded as new shapeselective catalysts analogous to zeolites. MOFs have already been reported to catalyze a broad range of organic transformations involving their Lewis acid nodes as well as their Brønsted acid–base properties. 13] Many reports have dealt with carbon–carbon bond formation catalyzed by unmodified MOFs through reactions such as Suzuki–Miyaura cross-coupling, the Mukaiyama aldol reaction, alkylation 17] or polymerization. 19] To obtain more sophisticated MOF catalysts, many research groups have examined the functionalization of these materials. The post-synthetic modification (PSM) of MOFs, an appealing route toward functionalized frameworks, involves chemical modification of the solid after formation of the crystalline structure, assuming that the primitive MOFs employed are sufficiently porous and robust. Post-synthetic modification can provide a wide range of isotopological structures from a single MOF by treating it as a substrate with a variety of organic reagents. The insertion of pendant groups onto or into the MOF makes it possible to add chemical functionality while retaining the MOF’s overall framework. Cohen and co-workers extensively studied the covalent organic PSM of a variety of amino-functionalized MOFs. This strategy was used by his group and others to generate metal complexes on MOFs, creating a new class of Lewis acid catalysts. 23, 30] Furthermore, Cohen extended his study to the hydrophobization of amino-containing MOFs through amide coupling, in order to increase their moisture resistance. In a parallel work, Yaghi et al. also reported the functionalization of porous materials and especially the reactivity of ZIF-90 against an amine or a reducing agent. The effect of the functionalization of a MOF by hydrophobic agents on its catalytic activity has, however, not been reported to date. In contrast to previous studies, our methodology is based on the modification of the environment of the catalytic centers and not on the insertion of new sites onto the MOF structure. We therefore studied the ability of modified zeolitic imidazolate framework (ZIF) materials to accelerate the rate of a reaction involving water formation. We chose the Knoevenagel condensation, which is a crossed aldol condensation of a carbonyl compound with an active methylene compound leading to C=C bond formation. This reaction is widely applied in the synthesis of fine chemicals and is classically catalyzed by bases in solution. This reaction can also be catalyzed by solid bases, such as metal oxides. The water that is produced in the course of the reaction usually competes with substrates for adsorption, however, thereby acting as a poison. Porous solids such as modified SBA-1, zeolites, 37] MIL-101 (TOF= 328 h ), 39] IRMOF-3 (TOF = 180 h ) or MIL-53, and others have already been employed as active heterogeneous catalysts for the Knoevenagel reaction. We report herein the fine tuning of hydrophobic properties of a MOF by post-synthetic modification to optimize its catalytic properties. To our knowledge, this is the first study showing that the engineering of the hydrophobic/hydrophilic environment can enhance the catalytic activity of a MOF by an order of magnitude. Our work focused on a porous substituted imidazolate material (SIM-1, formulated C10H10N4O2Zn) discovered by our group and belonging to the class of ZIFs. SIM-1 is a robust material, isostructural to ZIF-8 and consisting of ZnN4 tetrahedra linked by carboxylimidazolates. The aldehyde moiety present on the structure walls allows organic modification in the solid state, such as imine synthesis by condensation with primary amines to give the corresponding imino-functionalized SIM-2. This SIM-1 functionalization by imine condensation proceeds under mild conditions. In a typical experiment, a sample of desorbed SIM-1 (50 mg) was suspended in anhydrous methanol (5 mL) and the desired amine (1 mmol) was added under stirring. The suspension was allowed to react at room temperature for 24 h. After reaction, the solid was centrifuged and washed three times with ethanol and then dried under vacuum, providing the corresponding SIM-2 as a crystalline offwhite powder. In this manner, SIM-1 was treated with the primary amines dodecylamine to give SIM-2(C12) (Scheme 1). The C12 aliphatic chains present at the surface of the material create a hydrophobic shell surrounding the framework. Powder XRD analysis of the SIM-2(C12) sample showed a slight loss of crystallinity despite retention of the initial structure. Notably, the porosity of the SIM materials was maintained [a] Dr. J. Canivet, Dr. S. Aguado, C. Daniel, Dr. D. Farrusseng Universit Lyon 1, IRCELYON, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR CNRS 5256 Avenue Albert Einstein 2, 69626 Villeurbanne (France) Fax: (+ 33) 4-72-44-54-36 E-mail : david.farrusseng@ircelyon.univ-lyon1.fr Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201000386.

Direct Volumetric Measurement of Gas Oversolubility in Nanoliquids: Beyond Henry’s Law

The properties of condensed matter are strongly affected by confinement and size effects at the nanoscale. Herein, we measured by microvolumetry the increased solubility of H(2) in a series of solvents (CHCl(3), CCl(4), n-hexane, ethanol, and water) when confined in the cavities of mesoporous solids (gamma-alumina, silica, and MCM-41). Gas/liquid solubilities are enhanced by up to 15 times over the corresponding bulk values for nanoliquid sizes smaller than 15 nm as long as gas/liquid interfaces are mesoconfined in a porous network. Although Henrys law constant apparently no longer applies under these confinement, the concentration of dissolved H(2) still increases linearly with increasing pressure in the range 1-5 bar. We discuss the role and main implications of surface excess concentrations at mesoconfined gas/liquid interfaces in enhancing gas solubility.