Jean-Pierre Bellat

Formaldehyde: catalytic oxidation as a promising soft way of elimination.

Compared to other molecules such as benzene, toluene, xylene, and chlorinated compounds, the catalytic oxidation of formaldehyde has been studied rarely. However, standards for the emission level of this pollutant will become more restrictive because of its extreme toxicity even at very low concentrations in air. As a consequence, the development of a highly efficient process for its selective elimination is needed. Complete catalytic oxidation of formaldehyde into CO2 and H2 O using noble-metal-based catalysts is a promising method to convert this pollutant at room temperature, making this process energetically attractive from an industrial point of view. However, the development of a less expensive active phase is required for a large-scale industrial development. Nanomaterials based on oxides of manganese are described as the most promising catalysts. The objective of this Minireview is to present promising recent studies on the removal of formaldehyde through heterogeneous catalysis to stimulate future research in this topic.

Molecular simulation studies of water physisorption in zeolites.

We report a series of Grand Canonical Monte Carlo simulations of water adsorption in NaY and NaX faujasite, as well as in silicalite-1. Computed adsorption isotherms and heats of adsorption were in good agreement with the available experiments. The existence of cyclic water hexamers in NaX located in the 12-ring windows, recently disclosed by neutron diffraction experiments (Hunger et al., J. Phys. Chem. B, 2006, 110, 342-353) was reproduced in our simulations. Interestingly enough, such cyclic hexamer clusters were also observed in the case of NaY, in which no stabilizing cation is present in the 12-ring window. We also report cation redistribution upon water adsorption for sodium faujasite with varying cation contents (Si ratio Al ratio in the range 1.53-3). A simple and transferable forcefield was used, that enabled to reproduce the different aspects of water physisorption in stable zeolites. The high pressure water condensation in hydrophobic silicalite-1 was reproduced without any parameter readjustment. The method and forcefield used here should be useful for engineering oriented applications such as the prediction of multi-component mixture adsorptive separations in various stable zeolites. It allows to address the issue of the effect of the small amounts of water that are almost inevitably present in zeolite-based separation processes.

MCM-41 silica monoliths with independent control of meso- and macroporosity

Centimetre sized macroporous silica monoliths consisting of MCM-41 have been prepared by a two-step procedure allowing an independent control of the meso- and macro-porosity. In the first step a monolith with a macroporosity tailored between 2 and 20 μm is prepared under acidic medium by a phase separation, named spinodal decomposition, leading to a bicontinuous structure of a silica/polymer phase and a water phase. The monolith is then reacted in an alkaline solution of cetyltrimethyl ammonium to transform the silica skeleton into MCM-41 under conditions which preserve the original morphology and macroporosity of the material. The combination of spinodal decomposition and pseudomorphic transformation proves very efficient to precisely tune the textural characteristics of macroscopic objects.

Adsorption of water vapor on X and Y zeolites exchanged with barium

Abstract The adsorption of water vapor on BaY and BaX zeolites is studied by thermogravimetry and calorimetry under isothermal or isobaric conditions. The faujasite zeolite exhibits a stronger adsorption affinity for water molecules with barium than with sodium or potassium as exchangeable cations. The adsorption–desorption isotherm at 150°C shows a hysteresis loop attributed to the migration of compensating cations during their interaction with water molecules. Calorimetric measurements and the Dubinin–Radushkevich’s model both show that the adsorption of water occurs via a three-step mechanism: adsorption on compensating cations, formation of a monolayer on the walls and multilayer adsorption in the cavities. Water probes the complete microporosity of the zeolite. After several isobaric adsorption–desorption cycles, the structure of BaX undergoes severe damages (complete loss of the microporosity and amorphization of the solid) whereas that of BaY remains intact.

Adsorption equilibria of water vapor on cork.

We report here for the first time a complete thermodynamic study of water vapor adsorption on crude cork powder and plate. Adsorption-desorption isotherms were accurately measured by thermogravimetry at 283, 298, and 313 K in a large range of relative pressure. Adsorption enthalpies were determined by calorimetry as a function of loading. Adsorption-desorption isotherms exhibit a hysteresis due to the swelling of the material. The influence of the presence of lenticels on the adsorption properties of cork is found to be negligible. A detailed analysis and interpretation of adsorption data allow proposal of an adsorption mechanism in two steps. (i) First, water adsorbs on hydrophilic sites constituted by hydroxyl and methoxyl groups. (ii) Then water adsorption continues by clusters formation around the hydrophilic sites.

Adsorption of gaseous p-xylene and m-xylene on NaY, KY, and BaY zeolites: Part 1. Adsorption equilibria of pure xylenes

Abstract Isothermal and isobaric adsorptions of p -xylene and m -xylene on NaY, KY, and BaY are studied by thermogravimetry between 25°C and 400°C, and 10 Pa and 1014 Pa. At low filling BaY shows a stronger affinity for xylenes than the other zeolites. At high temperature and low pressure its adsorption capacity is greater than those of NaY and KY. These two zeolites have a similar behavior and adsorb more m -xylene than p -xylene. At low temperature and high pressure KY adsorbs p -xylene like NaY and m -xylene like BaY, and, unlike NaY, the adsorption capacities of KY and BaY are higher for p -xylene than for m -xylene. The substitution of K + or Ba 2+ for Na + decreases the adsorption capacity of α-cages.

Mesoporous Silica‐Confined Manganese Oxide Nanoparticles as Highly Efficient Catalysts for the Low‐Temperature Elimination of Formaldehyde

Manganese(IV) oxide was synthesized through crystallization in confined space by using the SBA‐15 silica support. The evolution of textural, morphological, structural, and redox properties of the manganese phase in the composites has been studied in terms of different parameters such as impregnation route, manganese loading, and activation temperature. High performances of nanoscaled manganese(IV) oxide for low‐temperature formaldehyde oxidation have been obtained. The optimization of preparation parameters enabled the complete conversion of formaldehyde at temperatures as low as 130 °C, which is comparable to the activity of the reference platinum catalyst that demonstrates complete conversion at similar temperatures.

Adsorption of gaseous p-xylene and m-xylene on NaY, KY, and BaY zeolites. Part 2: Modeling. Enthalpies and entropies of adsorption

Abstract The adsorption isotherms of xylenes are described by Henrys law and Langmuirs model on NaY and KY and the Dubinin-Radushkevich model on BaY. The isoteric enthalpies of adsorption show on each zeolite that the adsorbate-adsorbent interactions are nearly the same with both isomers. They are stronger on BaY than on NaY and KY. The adsorbate-adsorbate interactions are stronger with m-xylene than with p-xylene and especially outstanding on KY. The entropies of adsorption show that the mobility of the adsorbate is lower with m-xylene than with p-xylene. The isosteric free enthalpies of adsorption are related to the acidity of each zeolite. BaY has a stronger acidity and a greater ability to adsorb xylenes than NaY and KY.

Differential calorimetric enthalpies of adsorption ofp-xylene andm-xylene on y faujasites at 25°C

AbstractThe differential enthalpies of adsorption ofp-xylene andm-xylene on NaY, KY and BaY zeolites were measured by isothermal calorimetry coupled with isothermal volumetry at 25‡C. Whatever the zeolite, the enthalpies of adsorption ofp-xylene andm-xylene at low filling were of the same order of magnitude. They did not show significantly the effect of the dipolar moment ofm-xylene. Their absolute values varied in the sequence Q0(pX/NaY)<Q0(pX/KY) =Q0(mX/KY)<Q0(pX/BaY)<Q0(mX/NaY) =Q0(mX/BaY)During the adsorption of the third molecule of xylene per α-cage, BaY zeolite exhibited specific behaviour: the differential enthalpies of adsorption decreased with the filling of the α-cages in such a way that they became lower than those of the other two zeolites. Some arguments concerning the structures of the zeolite and xylene molecules can explain such behaviour. Whatever the zeolite, the adsorption capacity of the α-cages was 3.5 molec.α−1. For relative pressures ranging from 0.2 to 0.5, an additional adsorption of about 0.1 molec.α−1 occurred on the external surface.

Capture of formaldehyde by adsorption on nanoporous materials

The aim of this work is to assess the capability of a series of nanoporous materials to capture gaseous formaldehyde by adsorption in order to develop air treatment process and gas detection in workspaces or housings. Adsorption-desorption isotherms have been accurately measured at room temperature by TGA under very low pressure (p<2 hPa) on various adsorbents, such as zeolites, mesoporous silica (SBA15), activated carbon (AC NORIT RB3) and metal organic framework (MOF, Ga-MIL-53), exhibiting a wide range of pore sizes and surface properties. Results reveal that the NaX, NaY and CuX faujasite (FAU) zeolites are materials which show strong adsorption capacity and high affinity toward formaldehyde. In addition, these materials can be completely regenerated by heating at 200°C under vacuum. These cationic zeolites are therefore promising candidates as adsorbents for the design of air depollution process or gas sensing applications.