Eric M. Gaigneaux

Exploring, Tuning, and Exploiting the Basicity of Hydrotalcites for Applications in Heterogeneous Catalysis.

Basic catalysis! The basic properties of hydrotalcites (see picture) make them attractive for numerous catalytic applications. Probing the basicity of the catalysts is crucial to understand the base-catalysed processes and to optimise the catalyst preparation. Various parameters can be employed to tune the basic properties of hydrotalcite-based catalysts towards the basicity demanded by each target chemical reaction.Hydrotalcites offer unique basic properties that make them very attractive for catalytic applications. It is of primary interest to make use of accurate tools for probing the basicity of hydrotalcite-based catalysts for the purpose of 1) fundamental understanding of base-catalysed processes with hydrotalcites and 2) optimisation of the catalytic performance achieved in reactions of industrial interest. Techniques based on probe molecules, titration techniques and test reactions along with physicochemical characterisation are overviewed in the first part of this review. The aim is to provide the tools for understanding how series of parameters involved in the preparation of hydrotalcite-based catalytic materials can be employed to control and adapt the basic properties of the catalyst towards the basicity demanded by each target chemical reaction. An overview of recent and significant achievements in that perspective is presented in the second part of the paper.

Tuning the acid/metal balance of carbon nanofiber-supported nickel catalysts for hydrolytic hydrogenation of cellulose.

Carbon nanofibers (CNFs) are a class of graphitic support materials with considerable potential for catalytic conversion of biomass. Earlier, we demonstrated the hydrolytic hydrogenation of cellulose over reshaped nickel particles attached at the tip of CNFs. The aim of this follow-up study was to find a relationship between the acid/metal balance of the Ni/CNFs and their performance in the catalytic conversion of cellulose. After oxidation and incipient wetness impregnation with Ni, the Ni/CNFs were characterized by various analytical methods. To prepare a selective Ni/CNF catalyst, the influences of the nature of oxidation agent, Ni activation, and Ni loading were investigated. Under the applied reaction conditions, the best result, that is, 76 % yield in hexitols with 69 % sorbitol selectivity at 93 % conversion of cellulose, was obtained on a 7.5 wt % Ni/CNF catalyst prepared by chemical vapor deposition of CH(4) on a Ni/γ-Al(2)O(3) catalyst, followed by oxidation in HNO(3) (twice for 1 h at 383 K), incipient wetness impregnation, and reduction at 773 K under H(2). This preparation method leads to a properly balanced Ni/CNF catalyst in terms of Ni dispersion and hydrogenation capacity on the one hand, and the number of acidic surface-oxygen groups responsible for the acid-catalyzed hydrolysis on the other.

Theoretical condition for transparency in mesoporous layered optical media: application to switching of hygrochromic coatings

Mesoporous Bragg stacks are able to change color upon infiltration or displacement of liquid compounds inside their porous structure. Reversible switching from transparency to coloration offers additional functionality. Based on Bruggemans effective medium theory, we derive a transparency master equation, which is valid for bilayers of arbitrary host materials and pore-filling compounds. The transparency condition fixes pore volume fractions such that the effective refractive index is homogenized through the bilayer, hence, through arbitrary layered optical media built from this bilayer. This general concept is applied to the case of switching of hygrochromic coatings made of mesoporous mixed oxide Bragg stacks.

One-pot aerosol route to MoO3-SiO2-Al2O3 catalysts with ordered super microporosity and high olefin metathesis activity

Aerosol processing coupled with surfactant-templated sol–gel synthesis is used to produce MoO3-SiO2-Al2O3 catalysts. By quenching the sol–gel kinetics by fast drying of the aerosol, molecular-scale dispersion of each component is achieved. The structuring agent generates an organized porosity at the nanoscale. The catalysts have a high specific surface area and outstanding olefin metathesis activity.

Operando resonance Raman spectroscopic characterisation of the oxidation state of palladium in Pd/γ-Al2O3 catalysts during the combustion of methane

Resonance Raman spectroscopy is used in order to investigate the behaviour of Pd/γ-alumina (2 wt.%) in the catalytic combustion of methane. Simultaneous spectroscopic and catalytic measurements allow an operando spectroscopic investigation of the catalyst under operational conditions. Temperature cycles were performed by alternately heating and cooling the system under the reaction conditions after various in situ gaseous pre-treatments of the catalyst. Our results show that neither the Raman features nor the conversion of methane are influenced by initial thermal activation treatment under reducing or oxidizing atmospheres. Pure metallic palladium was found to be inactive for the catalytic combustion of methane. Under the reaction conditions, the Pd/γ-Al2O3 catalyst is always in an oxide PdO form with its surface in an intermediate state between surface lacunary PdO and crystalline PdO species.

Further on the mechanism of the synergy between MoO3 and gamma-Sb2O4 in the selective oxidation of isobutene to methacrolein. Restructuration of MoO3 via spillover oxygen.

This paper concerns the synergetic effects between a (010) oriented MoO3 and α-Sb2O4 in the oxidation of isobutene to methacrolein at 420°C. The catalysts were prepared by mechanical mixture of the pure oxides prepared separately. Important increases were observed both for the conversion of isobutene and for the selectivity to methacrolein, suggesting that new selective sites have been created. The characterization of the samples before and after the reaction by SEM and XRD shows an important reconstruction of the (010) face of MoO3 into steps exposing (100) faces, when the reaction is carried out in the presence of α-Sb2O4. It is known from literature that the (100) face of MoO3 performs selectively partial oxidation, while the (010) face is not selective. On the other hand, α-Sb2O4 has shown to produce spillover oxygen, which flows onto the MoO3 surface. It is, therefore, concluded that the increase of selectivity is intimately connected to the reconstruction under the action of spillover oxygen. Spillover oxygen would favour the “selective coordination” of Mo atoms or groups of atoms, typical of (100), at the expense of the “non-selective one”, typical of (010). This result corresponds to the theoretical prediction of the Remote Control theory.

Adsorption capacity of methylene blue, an organic pollutant, by montmorillonite clay

The isotherms and kinetics of the adsorption of a cationic dye in aqueous solution, methylene blue, on a local Algerian montmorillonite clay mineral (raw, sodium and thermally activated at 300 and 500°C) were determined experimentally. Various parameters influencing the adsorption were optimized, mainly solid–liquid contact time, mass of adsorbent, initial concentration of dye, pH of the solution and temperature. Results showed that the adsorption kinetics were fast: 30 min for the raw clay mineral, and 20 min for sodium clay mineral (SC) and thermally activated at 300°C, whereas with the clay mineral calcined at 500°C, the equilibrium was reached after 150 min only. The maximum adsorption capacity was reached at pH 6.6. Results deducted from the adsorption isotherms also showed that the retention follows the Langmuir model. In addition, it was found that the kinetics were in the order of 2 (K = 2.457 × 106 g/mg.h) for sodium clay and were limited by an intra-particle diffusion. SC was found to be a better adsorbent to remove methylene blue from industrial wastewater.

Supramolecular Organization in Organic–Inorganic Heterogeneous Hybrid Catalysts Formed from Polyoxometalate and Poly(ampholyte) Polymer

Hybridization of polyoxometalates (POMs) via the formation of an organic-inorganic association constitutes a new route to develop a heterogeneous POM catalyst with tunable functionality imparted through supramolecular assembly. Herein, we report on strategies to obtain tunable well-defined supramolecular architectures of an organic-inorganic heterogeneous hybrid catalyst formed by the association of a hydrophobically substituted polyampholyte copolymer (poly N, N-diallyl-N-hexylamine-alt-maleic acid) and phosphotungstic acid (H3PW12O40) POMs. The self-assembling property of the initial polyampholyte copolymer matrix is modulated by controlling the pH of the hybridization solution. When deposited on a mica surface, isolated, long and extended polymer chains are formed under basic conditions (pH 7.9), while globular or coiled structures are formed under acidic conditions (pH 2). The supramolecular assembly of the POM-polymer hybrid is found to be directed by the type and quantities of charges present on the polyampholyte copolymer, which themselves depend on the pH conditions. The hypothesis is that the Keggin type [PW12O40](3-) anions, which have a size of ~1 nm, electrostatically bind to the positive charge sites of the polymer backbone. The hybrid material stabilized at pH 5.3 consists of POM-decorated polymer chains. Statistical analysis of distances between pairs of POM entities show narrow density distributions, suggesting that POM entities are attached to the polymer chains with a high level of order. Conversely, under acidic conditions (pH 2), the hybrid shows the formation of a core-shell type of structure. The strategies reported here, to tune the supramolecular assembly of organic-inorganic hybrid materials, are highly valuable for the design and a more rational utilization of POM heterogeneous catalysts in several chemical transformations.

Toluene oxidation in a plasma-catalytic system

Oxidative removal of toluene in a dielectric barrier discharge reactor combined with manganese catalysts downstream was investigated. Toluene input concentration was varied in the range of 415-2227 ppm. The discharge was operated in pulsed mode, with short pulses of 23-35 kV peak voltage. At 7 W average power, toluene conversion was 60%-70%, independent on the toluene input concentration and on the total gas flow rate in the range of 110-330 SCCM (SCCM denotes cubic centimeter per minute at STP). Toluene total oxidation was favored at high residence time of the gas in the discharge zone and low toluene concentration, when the main reaction product was CO2 with selectivities of 80%-85%. The addition of the catalysts led to a 15%-20% increase in toluene conversion with respect to the values obtained in the plasma, due to oxidation with ozone on the catalyst surface. (c) 2006 American Institute of Physics.

A New Bio‐Inspired Route to Metal‐Nanoparticle‐Based Heterogeneous Catalysts

Onion-type multilamellar vesicles are made of concentric bilayers of organic surfactant and are mainly known for their potential applications in biotechnology. They can be used as microreactors for the spontaneous and controlled production of metal nanoparticles. This process does not require any thermal treatment and, hence, it is also attractive for material sciences such as heterogeneous catalysis. In this paper, silver-nanoparticle-based catalysts are prepared by transferring onion-grown silver nanoparticles onto inorganic supports. The resulting materials are active in the total oxidation of benzene, attesting that this novel bio-inspired concept is promising in inorganic catalysis.