Ethylene and cyclohexene hydrogenation on acidic or basic erionite activated by double spillover of oxygen and hydrogen

Two types of erionite were submitted to a double spillover treatment: O₂ first then H₂ at 430 ° C. The first type was acidic erionite while the second one was basic erionite (Na, K-erionite). Blank tests were also performed using only O₂-H₂ thermal activation procedures. Ethylene was hydrogenated at 180 ° C only on basic erionite activated by spillover. The double O₂-H₂ thermal treatment was unable to convert either acidic or basic erionite into a ethylene hydrogenation catalyst. Cyclohexene was converted to cyclohexane exclusively on acidic erionite activated either by double spillover or thermally, the latter activation being less efficient than the former. It is the first time that hydrogenating sites have been created by spillover on a basic form of zeolite.     

Growth and Optical Properties of Quantized CdS Crystallites in TEOS Silica Xerogels

Quantized CdS crystallite-doped tetraethylorthosilicate (TEOS) silica xerogels are prepared by the sol-gel method. In this method, cadmium acetate [Cd(CH₃COO)₂2H₂O]-doped TEOS alcogel is formed by the hydrolysis and polycondensation of ethanolic TEOS in the presence of hydrochloric acid (HCl) and ammonium hydroxide (NH₄OH) catalysts and Cd(CH₃COO)₂.2H₂O. The CdS crystallites are formed in the alcogel by the reaction of Cd(CH₃COO)₂.2H₂O present in the gel and methanolic sodium sulfide (Na₂S), which is added over the alcogel. The effect of CdS/TEOS, EtOH/TEOS, S/Cd molar ratios, and temperature on the optical properties and CdS crystallite sizes in the xerogels are studied. A blue shift is observed in optical absorption spectra by decreasing the CdS/TEOS molar ratio from 2 × 10^–2 to 1 × 10^–4. It is observed that the crystallite size is increased from 1.6 to 3.4 nm by increasing the EtOH/TEOS molar ratio from 2 to 20, respectively, for a constant CdS/TEOS molar ratio of 5 × 10^–4. Emission spectra of xerogels are measured and found that the emission peak maxima shifted toward lower energies (higher wavelengths) by increasing the CdS/TEOS molar ratio in the xerogels. It is known from the X-ray diffraction (XRD) measurements of CdS-doped xerogels that the CdS crystallite structure in the xerogels is hexagonal wurtzite. The crystallite sizes were calculated from the XRD patterns and tight bonding calculations. There is a significant change in the color and size of CdS crystallite in the xerogels with a variation in temperature from 200 to 400°C.     

Effect of solvents and catalysts on monolithicity and physical properties of silica aerogels

The effect of various solvents and catalysts on the monolithicity and physical properties of silica aerogels is reported. The aerogels were prepared by hydrolysis and polycondensation of tetramethoxysilane, followed by hypercritical drying, using 6 solvents of different chain lengths, and 17 catalysts consisting of strong and weak acids, bases and their mixtures. It was found that solvents of longer chain lengths and strong basic catalysts resulted in semitransparent to opaque aerogels, whereas strong acids and their combinations with a weak basic catalyst produced transparent but cracked aerogels. While weak acids and their combinations with a weak base were found to produce shrunk and semitransparent (opaque for CH₃COOH + NH₄OH) aerogels, the best quality transparent, monolithic, low-density and refractive index, and large surface area aerogels were obtained for a combination of weak basic catalysts and solvents of shorter branching and chain lengths. The physical properties of the aerogels were studied by BET analysis, porosity, density, refractive index and optical transmission measurements.     

Effect of interparticle forces on the hydrodynamic behaviour of fluidized aerogels

Aerogel powders, particles of the group C according to the Geldart classification, are not particularly attractive as such for fluidized bed applications. The following study shows, however, that these fine solids form agglomerates above a minimum superficial velocity of 0.04 m/s and that the resulting new system fluidizes smoothly and homogeneously at room and at higher temperatures. A comprehensive model developed to represent the hydrodynamic behaviour of the system proposes that the agglomerate is a cluster of the original fixed bed materials and hence has the properties of the bulk matrix. Van der Waals forces on the elementary grains are used to predict the cluster size. This clustering process is finally analysed in the light of the classical fluidizability diagram.     

Mixed nickel-aluminium oxides catalysts for the nitroxidation of toluene by no into benzonitrile

This paper describes the catalytic properties (activities, selectivities and stabilities with time on stream) of nickel oxide-alumina aerogels and xerogels, towards the transformation of toluene by NO into benzonitrile at 440°C in a differential flow reactor. The stability of the catalysts depends upon the NiO content. When the ratio Ni/Al increases to unity, the catalysts (aerogel as well as xerogel) are very stable with time on stream and are active and selective like pure nickel oxide. The latter is indeed initially active and selective, but very unstable with time on stream. A redox model is proposed to explain the reaction mechanism.     

Catalytic properties of new Cu based catalysts containing Zr and/or V for methanol synthesis from a carbon dioxide and hydrogen mixture

Pure unsupported copper is a poor catalyst for CH₃OH synthesis from CO₂+H₂ when it is compared to Cu coprecipitated with Zr or Zr+V in which case its selectivity and yield in methanol are strongly enhanced. The two Zr and V components added to pure Cu are shown to be textural promoters towards zerovalent Cu which participate to the building of the active sites. Methanol formation on these catalysts is a structure insensitive reaction with respect to the metallic Cu dosed by N₂O surface decomposition.     

Effect of pH,Aging, and Drying on the Size of CdS Nanocrystallites,Monolithicity, and Transparency of Nanocrystalline CdS-Doped Silica Xerogels

CdS semiconductor nanocrystals are prepared in SiO₂ matrix xerogels by a sol–gel method using ethanolic tetraethylorthosilicate [TEOS, Si(OC₂H₅)₄], catalyst (HCl or NH₄OH), cadmium acetate [Cd (CH₃COO)₂ 2 H₂O], and thiourea [SC (NH₂)₂] and drying the alcogels at room temperature and heating up to 250°C. The effect of alcosol pH on the CdS crystallite size in silica matrix, color, monolithicity, and transparency of the xerogels is studied by varying the pH from 1 to 10, and maintaining the CdS/SiO₂ molar ratio constant at 0.0025. It was found that the color of samples varied from light yellow to orange and transparency decreased from 85 to 15% with the variation of alcosol pH from 1 to 10, respectively. The cracked sample were found for pH < 3, opaque, powdery samples for the pH > 8, and transparent monolithic yellow-colored samples in the pH range 3–7. The quantum size effect was observed in the optical absorption spectra of the samples. The threshold peak decreased from 520 to 250 nm, energy increased from 2.4 to 4.9 eV, and the CdS crystallite size decreased from 6 to 1 nm with the variation of pH of the alcosol from 10 to 1, respectively. In the case of the effect of aging, a minimum of 5 days aging is required for monolithic samples and the CdS crystallite to increase in size with and remain constant, even when the aging period is increased for 10 days. Monolithic samples were obtained with drying and heating rates of <0.4 wt.% loss/h and 30–50°C/h range and cracked samples with >0.4 wt.% loss/h and >50°C/h drying and heating rates, respectively. The density of the CdS crystallites doped-silica xerogels decreased from 1.57 to 1.30 gm/cm³ with an increase in pH from 1 to 10, respectively. From XRD spectra, the CdS crystallite structure was found to be a hexagonal wurtzite structure. The intensity of the peaks increased and the breadth decreased with the variation of pH of the alcosol from 1 to 10 because of an increase in CdS crystallite size.     

Surface characterization of palladium-copper bimetallic catalysts by FTIR spectroscopy and test reactions

Different bi-metallic palladium-copper catalysts have been prepared according to different sequences of impregnation of a commercial alumina. CO adsorption followed by FTIR spectroscopy was performed in order to identify the nature of the superficial sites. Two test reactions were also used to characterize the nature of the active surface site: gas phase toluene hydrogenation was selected in order to show the hydrogenating activity of metallic palladium, whereas the reaction of decomposition of ethanol, which exhibits 100% selectivity in the formation of ethanal on pure copper catalysts, was also studied on this series of catalysts. The results obtained by both methods are complementary. By comparison with the behaviour of pure palladium or copper based catalysts, they allow to propose hypotheses on the surface composition: separated phases, alloys or a phase covered by the other one.     

Epidural scar tissue formation after spinal surgery: an experimental study

The bone-resorptive cytokines interleukin 1 (IL-1) and tumor necrosis factor (TNF) have been implicated in the pathogenesis of many chronic inflammatory diseases, including pulpitis and apical periodontitis.To further elucidate their role in these disorders, we have identified cells that express IL-1 alpha and TNF alpha in infected pulps and in developing rat periapical lesions after surgical pulp exposure. As detected by immunohistochemistry, IL-1 alpha- and TNF alpha-positive cells were present as early as 2 days after pulp exposure in both the pulp and periapical region. The numbers of cytokine-expressing cells increased up to day 4 in the pulp and up to day 30 in the periapex. In contrast, cells expressing IL-1 beta and TNF beta, the homologous forms of these mediators, were not found in pulp or periapical lesions during this period. Cells expressing IL-1 alpha and TNF alpha were identified primarily as macrophages and fibroblasts, with occasional staining of polymorphonuclear leukocytes. Osteoblasts and osteoclasts were also positive, whereas lymphocytes were negative. In general, cytokine-expressing cells were located proximal to abscesses and the root apex. These findings demonstrate that cells that express bone-resorptive cytokines IL-1 alpha and TNF alpha are present immediately after pulp exposure in this model, which supports the hypothesis that these mediators play a key role in pulpal and periapical pathogenesis, including the concomitant bone destruction. They also indicate that both resident connective tissue cells as well as infiltrating cells express bone-resorptive cytokines in response to infection in these lesions.     

Structure and texture of alumina aerogel monoliths made by complexation with ethyl acetoacetate

Alumina gels can be made by spontaneous gelation of alkoxides in organic solvents, their hydrolysis rate can be controlled by chelating agents and the wet gels obtained can be transformed to aerogels by supercritical drying. The present study reports on such alumina aerogels made from aluminum tri-sec-butoxide in organic solvent and complexed with ethylacetoacetate at high temperature (260°C) in methanol. The aerogels were studied by X-ray diffraction, nitrogen adsorption, infra-red spectroscopy and ²⁷Al nuclear magnetic resonance. The results show that both the chemical complexation and drying methods can significantly affect the texture and structure of these gels.