Raymond Le Van Mao

Application of concept of relative photonic efficiencies and surface characterization of a new titania photocatalyst designed for environmental remediation

Abstract The relative photonic efficiency ζr for the TiO2 photocatalized transformation and mineralization of phenol (standard process and product) has been determined for a new industrially available TiO2 specimen. Under otherwise identical conditions of catalyst loading, pH, phenol concentration and temperature (ambient) the Hombikat UV-100 specimen was approximately four times less efficient than the Degussa P-25 analog (standard photocatalyst). The new titania material was also characterized by X-ray powder diffraction and by nitrogen adsorption-desorption isotherms at liquid nitrogen temperature. The sample is porous with mesopores of about 5.6 nm (diameter) and the Brunauer-Emmett-Teller surface area is 189 m2 g−1. X-ray powder patterns confirm the anatase structure of the UV-100 sample.

The bioethanol-to-ethylene (B.E.T.E.) process

Abstract Ethanol present at low concentration in a fermentation broth which also contains some leftover glucose as well as other fermentation residues, is totally converted into ethylene over a solid superacidic catalyst. Trifluoromethanesulfonic acid bound to the silica rich surface of the acidic ZSM-5 zeolite is thermally stable up to 300°C and can catalyse the reaction with nearly complete conversion at a temperature as low as 170°C. Below 200°C, the catalyst is extremely stable even in the presence of the high amount of water found in the feed. A rapid economical evaluation shows that ethylene produced by the B.E.T.E. (bioethanol-to-ethylene) process has a production cost 10% to 40% higher than petroleum ethylene. However, a more promising figure can be envisaged with a better integrated process working with cheaper raw materials.

Conversion of n-butane into BTX aromatics on new hybrid catalysts

Abstract Hybrid catalysts prepared by contacting zinc or gallium oxide containing particles with the ZSM-5 zeolite crystallites or by coating these latter with zinc or gallium species, exhibit aromatizing activity which is comparable to that of the zeolite catalysts prepared using the conventional impregnation technique. Involvement of the hydrogen back spillover mechanism has been suggested.

Selective removal of silicon from zeolite frameworks using sodium carbonate

The mild treatment of various zeolites (X, Y and ZSM-5) with sodium carbonate in aqueous solutions has been investigated. The removal of silicon from highly siliceous zeolites led to a significant decrease of the Si : Al ratio, mostly with the ZSM-5 and Y zeolites. However, the original structure, surface area and size of the micropores were essentially preserved, although healing processes did occur after silicon removal from the framework. All the aluminium atoms remained in the tetrahedral configuration during the treatment, which, therefore, provides a novel means for significantly increasing the ion–exchange capacity or the density of protonic sites of the zeolites studied.

Long distance hydrogen back-spillover (LD-HBS) phenomena in the aromatization of light alkanes

Abstract Hybrid catalysts consisting of ZSM-5 zeolite and a ZnO based co-catalyst intimately mixed within a bentonite matrix, have been studied in the conversion of propane and n-butane into aromatics and particularly into benzene, toluene, xylenes (BTX aromatics). It has been observed that the ZnO based co-catalyst is not actively involved in any step of the aromatization reaction of alkanes. Therefore, the main role of the metal oxide co-catalyst is to help removing the hydrogen species released by the zeolite acid sites through a process which is called long-distance hydrogen back-spillover.

Hybrid catalyst containing a microporous zeolite and a mesoporous cocatalyst forming a pore continuum for a better desorption of reaction products

Abstract This paper describes a general procedure for preparing and testing hybrid catalysts which comprise two components: microporous and mesoporous particles. The microporous component is a ZSM-5 zeolite in the protonic form, eventually having some active metallic or metal oxide species incorporated into its pore network. The mesoporous component is a (reducible or irreducible) oxide, which may be used as a support for some active metallic or metal oxide species. The particles of microporous zeolite are embedded in the larger particles of mesoporous material using the technique of extrusion with a binder. Conversion and product selectivities in several reactions including the aromatization of n -butane and the upgrading of the propane steam-cracking products are reported. The beneficial effect of the hybrid system when compared with the parent zeolite alone is attributed to the formation of a micropore–mesopore continuum within the hybrid catalyst system. Such a “funnel-shaped” pore configuration decreases the effects of the energy barrier at the pore openings of the zeolite particles which otherwise slows down the desorption of the products of the reaction over the zeolite active sites. In addition, the metal oxide species supported on the cocatalyst surface occupies a favorable position which allows it to intervene on both the inward-diffusing reactant molecules and the outward-diffusing product or intermediate molecules.

Hybrid catalysts containing zeolite ZSM-5 and supported gallium oxide in the aromatization of n-butane

Abstract It is shown that in the aromatization of n -butane, hybrid catalysts containing irreducible oxides (silica and alumina) as co-catalysts provide aromatic yields which are almost twice that obtained with “pure” zeolite ZSM-5 zeolite. The interpretation of these results is based on the hydrogen back spillover concept. The addition of Ga 2 O 3 to these co-catalysts enhances further the aromatizing performance of the hybrid catalysts. It is believed that such an additional improvement is due to the gallium species located in the interface between the two types of particles, and the Ga containing regions of the zeolite crystallites near the pore openings. This contamination results from the transfer of some partially reduced gallium oxide from the co-catalyst surface to the zeolite pores. In such a case, it is probable that the reaction is governed by the conventional bifunctional catalysis mechanism. These gallium oxide sites appear more active than the normal ones because of their location near the zeolite pore openings which enables them to react with both inward-and outward-diffusing molecules.

pH of the sodium carbonate solution used for the desilication of zeolite materials

Sodium hydroxide is added to the sodium carbonate solution used for the selective removal of silicon from silica-rich zeolites. This adjusts the initial pH of the suspension for a more efficient desilication process. As a general rule, the higher the Si : Al ratio of the zeolite, the lower is the pH of the desilication medium.

Modified zeolites for the removal of calcium and magnesium from hard water

The cation-exchange capacity (cec) and the pore size of zeolite materials are the two key factors which influence the removal of calcium and magnesium in aqueous solution. Na–X and Na–Y zeolites have been modified by selective extraction of Si from the zeolite framework using sodium carbonate. At ambient temperature, the modified Na–X zeolite was more efficient than the Na–A commercially used in detergency, for both Ca and Mg ions.

Kinetic study of n-butane aromatization on ZSM-5 and gallium bearing ZSM-5 catalysts

Abstract The apparent activation energies and pre-exponential factors for n-butane conversion over “pure” ZSM-5 zeolite and two gallium bearing ZSM-5 catalysts have been determined. Although these three catalysts give the same value for the activation energy (100 kJ/mol), the values of the pre-exponential factor exhibit a sequence which is coincident to that of the aromatization performance: parent zeolite