Tristan Lescouet

The Origin of the Activity of Amine‐Functionalized Metal–Organic Frameworks in the Catalytic Synthesis of Cyclic Carbonates from Epoxide and CO2

The conversion of carbon dioxide into bulk chemicals at lower energy cost is a scientific and technological challenge. 2] Acid–base-pair catalysts are interesting for such applications because they can promote concerted reactions. The adsorption of CO2 occurs on the basic sites to form activated species; then, the epoxide coordinates onto the neighboring acidic site and ring-opening occurs by nucleophilic attack of the activated species. One example of rationally designed acid–base catalysts is amine-functionalized mesoporous Ti(Al)-SBA-15. Ratnasamy and co-workers reported a “volcanic plot” of the reaction rate as a function of amine basicity, in which secondary amines showed the optimum reactivity. They suggested that CO2 is too-weakly activated on primary amines, whereas it is too-strongly adsorbed onto tertiary amines. Hence, moderate CO2-adsorption onto amine-functionalized solid acids appears to provide good candidates as catalyst for this reaction. It is generally acknowledged that metal–organic frameworks (MOFs) are appropriate materials for designing single-site acid– base catalysts. In a spectroscopic study on MOFs, Gascon et al. showed the functionalization of MOF-5 by an amino substituent (2-amino-1,4-benzenedicarboxylate), also known as IRMOF-3. The amine group acted as an electron donor (Lewis base) on CO2. [7] This “amino effect” on CO2-adsorption has been experimentally observed on various MOFs and was later confirmed by ab initio calculations. The concept of concerted reactions on acid–base MOFs has been reported by Baiker and co-workers in the synthesis of propylene carbonate with amine-containing mixed-linker MIL-53 (co-catalyzed by tetraalkylammonium halides). A turnover frequency (TOF) of 400 h 1 was measured under solvent-free conditions. Another significant example is the activity of amine-functionalized UiO66 in the cross-aldol reaction reported by De Vos and co-workers. Hence, we anticipated that the use of MOFs that contain acid–base pairs, such as a Brønsted acid MOF that is functionalized with NH2, could lead to potential catalyst candidates for the synthesis of carbonate from CO2. Herein, we elucidate the role of the NH2-functionalization of MIL-68(In)-NH2 [14] as a catalyst for the synthesis of styrene carbonate from styrene oxide and CO2. Surprisingly, we show by using ab initio calculations and spectroscopic investigations that the modification of the electronic structure of the inorganic component by ligand-substitution has a much-larger impact on the activation of CO2 than the amine substituents. MIL-68(In) and MIL-68(In)-NH2 were prepared by precipitation reactions of indium nitrate and terephthalic acid or aminoterephthalic acid in DMF. X-ray diffraction, surface areas, DRIFT analysis, and the H NMR spectra were in agreement with previous reports (see the Supporting Information). These results confirmed that the MOFs were empty of any organic solvent or occluded reactants. MIL-68(In) and MIL-68(In)-NH2 were tested in the synthesis of carbonate (Scheme 1). This evaluation was performed in a glass vial at 150 8C under CO2 pressure (8 bar).

Homogeneity of flexible metal–organic frameworks containing mixed linkers

Very sophisticated porous materials known as multivariate functional MOFs (also known as MixMOFs) can be designed using a synthesis method that starts from solutions composed of two or more different linkers. For this procedure to be successful, one must have access to techniques that characterize the homogeneity of MOF crystallites containing two different linkers. This is of particular relevance for MOFs made of 2-aminobenzene-1,4-dicarboxylate (abdc), which are excellent platforms for the introduction of additional functions by post-modification. In this paper, we show that adsorption/desorption isotherms and thermodiffraction studies on flexible structures can indirectly characterize the homogeneity of MOFs made from a mixture of linkers. Breathing pressures and temperatures for a series of MIL-53(Al) functionalized with amino tags, i.e. Al(OH)(bdc)1−n(abdc)n, were measured as a function of the amino content. The linear relationship between the CO2 breathing pressure and the amine content in the MIL-53(Al) structure clearly illustrates the homogeneity of the crystallite composition; in other words, the crystallites have the same abdc : bdc ratio. On the other hand, the functionalization of MIL-53(Al) with low amine content (10% abdc) results in a profound modification of the breathing properties triggered by the temperature. Much higher temperatures are required for full conversion of the np (narrow pore) to the lp (large pore) phase. We also suggest an interplay between coexisting np and lp microcrystalline domains that may “smooth” the breathing properties at the macroscopic level.

Hierarchical Zeolitic Imidazolate Framework-8 Catalyst for Monoglyceride Synthesis

Metal–organic frameworks (MOFs) are heterogeneous porous catalysts with unprecedented catalytic functionality via coordinatively unsaturated metal nodes, provoked defects, organic functions of linker molecules, and intrinsic chirality. These materials are amenable to catalytic modification by means of encapsulation of nanoparticles, polyoxometalates, and porphyrins. MOFs have been demonstrated as active heterogeneous catalysts for various organic reactions, including acid–base and redox reactions, such as esterification, transesterification, epoxidation, alcoholysis, Knoevenagel condensation, 6] among others. The introduction of a secondary pore system, demonstrated to be advantageous in zeolite catalysis, is still in its infancy in the world of MOFs and might further lift the potential of MOF materials in catalysis. A very challenging area of heterogeneous catalysis is the synthesis of monoglycerides. Monoglycerides are produced according to two major routes: 1) direct esterification of fatty acid with glycerol and 2) transesterification of triglycerides with glycerol. Monoglycerides are popular natural emulsifiers in food and beverages, personal care products, and pharmaceuticals. According to a recent prediction, the global emulsifier market is primed to reach volume sales of 2.6 million metric tons by the year 2017. Industrial monoglyceride production is currently performed at high temperature (493–523 K) using a basic homogeneous catalyst with limited monoglyceride selectivity, owing to formation of diand triglyceride side products and soap. The high reaction temperature bears the risk of deterioration of taste, aroma, and color of the product. Developing a heterogeneous catalytic process at lower temperature for selective monoglyceride production is a major scientific challenge. We discovered nanoparticles of zeolitic imidazolate framework-8 (ZIF-8) transformed into hierarchical material through reaction with fatty acid to be a promising truly heterogeneous catalyst for monoglyceride synthesis. ZIFs possess excellent thermal stability up to 693 K and a tunable pore architecture. The ZIF-8 structure resembles sodalite with 1.16 nm wide cavities connected through 0.34 nm wide windows formed by four-ring and six-ring ZnN4 clusters. ZIF-8 is a promising material for gas separation, as well as for catalytic applications. ZIF-8 exhibits a unique pressure-induced change of pore size and amorphization. It has attractive tribological properties, and guest molecule trapping behavior. Immobilization of catalytically active nanoparticles, such as Pt, Ni, Au, and Co3O4, is a recent avenue in ZIF catalysis. ZIF-8 originally was synthesized via the solvothermal route from inorganic zinc compound and 2-methylimidazole in dimethylformamide (DMF) solvent and temperatures of 358–423 K. Alternative synthesis methods are precipitation from methanol or water solution at room temperature, steam-assisted synthesis, mechanochemical, and ultrasound treatment. Here we report another facile synthesis of ZIF-8 nanomaterial with large specific surface area and micropore volume. Nanosized ZIF-8 was prepared from a synthesis solution 2.2 times more concentrated than usual (see the Supporting Information for Experimental Details). The reproducibility was excellent. Phase purity was confirmed by XRD (Figure 1 a). The XRD pattern can be indexed with cubic unit cell parameter a = 1.710(5) nm, in agreement with literature. IR spectra further confirmed sample purity (Figure S1). SEM revealed a particle size of about 150 nm qualifying as nanomaterial (Figure 1 b). TGA in nitrogen (Figure S2) revealed this ZIF-8 nanopowder is stable till 723 K. The microporous nature of ZIF-8 is apparent from the Type 1 nitrogen adsorption isotherm (Figure 1 c). The BET and Langmuir specific surface area and micropore volume are 1388 m g , 2110 m g 1 and 0.78 cm g , respectively. These are high values for a nanosized ZIF-8 sample given that there were no chemical additives nor excess solvent used in the synthesis. [a] Dr. L. H. Wee, Prof. J. A. Martens Centre for Surface Science and Catalysis KU Leuven Kasteelpark Arenberg 23, Heverlee, B3001 (Belgium) Fax: (+ 32) 163-21998 E-mail : likhong.wee@biw.kuleuven.be johan.martens@biw.kuleuven.be [b] T. Lescouet, Dr. D. Farrusseng Institut de Recherche sur la Catalyse et l’Environnement de Lyon (IRCELYON), University Lyon 1, CNRS 2 Av. Albert Einstein, Villeurbanne, 69626 (France) [c] J. Ethiraj, Dr. F. Bonino, Prof. S. Bordiga NIS Centre of Excellence and INSTM Reference Centre Department of Chemistry, University of Turin Via Quarello 15, Turin, 10135 (Italy) [d] Dr. R. Vidruk, Prof. M. Herskowitz Blechner Center for Industrial Catalysis and Process Development Ben-Gurion University of the Negev Beer-Sheva (Israel) [e] Dr. E. Garrier, Dr. D. Packet Novance Rue les rives de l’Oise, Venette, B.P. 20609 Compiegne, Cedex, 60206 (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201300581.

An alternative pathway for the synthesis of isocyanato- and urea-functionalised metal–organic frameworks

We have developed a generic two-step post-functionalisation technique for transforming amino-functionalised MOFs into their isocyanate analogues. The first part of the synthetic pathway consists in the conversion of the amino moieties into azido groups. Next, the thermal activation of these azido groups leads to nitrene species that can react with carbon monoxide to yield the desired products. As a proof of concept, this method was applied to the highly stable Al-MIL-53-NH2 and to the acid-sensitive In-MIL-68-NH2. The resulting nitrene species were highly reactive, with side reactions dominating initially. This issue was overcome through the use of a mixed-linker strategy applied during the MOF synthesis that decreased the nitrene radical density within the pore, thereby permitting In-MIL-68-NH2 to be converted into its isocyanate analogue with 100% selectivity. To illustrate the potential of this method for grafting a wide library of potentially active organic groups inside MOFs, amines were condensed onto isocyanato MOFs to form urea analogues.

Soft synthesis of isocyanate-functionalised metal–organic frameworks

We have developed an original synthetic pathway for the conversion of a MIL-68(In)-NH(2) metal-organic framework into its corresponding isocyanate (-NCO) derivative. This two-step soft post-modification technique leads to highly porous isostructural materials.