Improved performance of non-thermal plasma reactor during decomposition of trichloroethylene: Optimization of the reactor geometry and introduction of catalytic electrode

The decomposition of trichloroethylene ITCE) by non-thermal plasma was investigated in a dielectric barrier discharge (DBD) reactor with a copper rod inner electrode and compared with a plasma-catalytic reactor. The particularity of the plasma-catalytic reactor is the inner electrode made of sintered metal fibers (SMF) coated by transition metal oxides. In order to optimize the geometry of the plasma reactor, the efficiency of TCE removal was compared for different discharge gap lengths in the range of 1-5 mm. Shorter gap lengths (1-3 mm) appear to be more advantageous with respect to TCE conversion. In this case TCE conversion varies between 67% and 100% for input energy densities in the range of 80-480 J/l, while for the 5 turn discharge gap the conversion was lower (53-97%) for similar values of the input energy. As a result of TICE oxidation carbon monoxide and carbon dioxide were detected in the effluent gas. Their selectivity was rather low, in the range 14-24% for CO2 and 11-23% for CO, and was not influenced by the gap length. Several other chlorinated organic compounds were detected as reaction products. When using MnOx/SMF catalysts as the inner electrode of the DBD reactor, the TCE conversion was significantly enhanced, reaching similar to 95% at 150 J/l input energy. The selectivity to CO2 showed a major increase as compared to the case without catalysts, reaching 58% for input energies above 550 J/l. (C) 2007 Elsevier B.V. All rights reserved

Kinetic study of the selective hydrogenation of styrene over a Pd egg-shell composite catalyst

This is a study on the kinetics of the liquid-phase hydrogenation of styrene to ethylbenzene over a catalyst of palladium supported on an inorganic–organic composite. This support has a better mechanical resistance than other commercial supports, e.g. alumina, and yields catalysts with egg-shell structure and a very thin active Pd layer. Catalytic tests were carried out in a batch reactor by varying temperature, total pressure and styrene initial concentration between 353–393 K, 10–30 bar, and 0.26–0.60 mol L−1. Kinetic models were developed on the assumptions of dissociative hydrogen chemisorption and non-negligible adsorption of hydrogen and styrene. Final chemical reaction expressions useful for reactor design were obtained. The models that best fitted the experimental data were those ones that considered the surface reaction as the limiting step. In this sense, a two-step Horiuti–Polanyi working mechanism with half hydrogenation intermediates gave the best fit of the experimental data. The heats of adsorption of styrene and ethylbenzene were also estimated.The authors are gratefully indebted to CONICET, ANPCyT and Universidad Nacional del Litoral for financially sponsoring this research work

Role of nitrogen on the acid-base properties of zirconophosphate (ZrPON) oxynitride catalysts

Activation of amorphous zirconium phosphate powder by high temperature treatment under NH3 allows to prepare series of zirconium phosphate oxynitrides. The adjustment of the N/O ratio allows the tuning of the acid-base properties of these high surface area and amorphous catalysts. It is evidenced that the substitution of oxygen by nitrogen induces a decrease of the number and strength of acid site while the basic character of the surface increases. By increasing the nitrogen content in the ZrPON powders, the strong acidic centres of ZrPO precursor are modified and a "soft" surface is obtained with good equilibrium between acid and basic sites of medium strength. In ZrPON with high N content, the basicity clearly dominates as numerous strong basic centres are identified. The basicity enhancement is directly connected to the presence of nitrogen atoms N3- anions and hydrogenated nitrogen species in the zirconophosphate structure. It is then proposed that in addition to nitride N present at the surface and in the bulk, some oxygen species may be also responsible of the most strongest basic sites. (C) 1999 Elsevier Science B.V. All rights reserved

Acylation of alcohols and activated aromatic compounds on silica embedded-triflate catalysts

A series of triflate derivatives (La(OTf)(3), tert-butyl-dimethyl-silyltrifluoromethane-sulfonate (BDMST) and triflic acid (HOTf)) embedded in a silica matrix were Used as heterogeneous catalysts for the acylation of alcohols and activated aromatic Compounds. Acylation of saturated alcohols and cyclohexanol resulted in O-acylated products. The acylation of the aromatic compounds was dependent on temperature and substituents. At low temperatures, for phenols and naphtols, acylation occurred mainly to O-acylated. Under these conditions, the presence of catalysts only enhanced the reaction rate. The increase of the temperature above 150 degrees C also led to C-acylation. For anisole and 1-methoxynaphtalene, acylation occurred at the aromatic ring even at low temperatures. (c) 2005 Elsevier B.V. All rights reserved

Preparation, characterisation and catalytic behaviour of cobalt-niobia catalysts

Cobalt-niobia catalysts were prepared using the colloidal sol-gel technique. Niobium chloride or niobia oxide were used as precursor. The differences between the procedures used are due to the methods of preparation of the colloidal suspensions and gelification. The catalysts were characterised using adsorption and desorption curves of Kr and N-2 at 77 K, H-2-Chemisorption, XRD, FT-IR, XPS and electron microscopy investigations. Preparation of these catalysts without experimental precautions led to a very inhomogeneous structural and textural material. In contrast, the colloidal sol-gel technique controls both the structure of the niobia oxide and the tailoring of cobalt. A strong metal support interaction effect (SMSI) was present irrespective of the sample preparation variant. Although the rate of butane hydrogenolysis was low for all catalysts, a correlation between TOF and the catalyst crystallite size was found. Selectivity to methane, ethane, propane or to isomerization also depends on the catalyst crystallite size. (C) 1998 Elsevier Science B.V. All rights reserved

TG and DTA investigation of ZrO2-SO42- catalysts exposed to hexane, methylcyclopentane and cyclohexane

ZrO2-SO42- catalysts with different sulfur contents were analysed with thermal methods coupled with mass spectrometry after exposure to mixtures of hexane, methylcyclopentane, and cyclohexane in argon. The reaction of the hydrocarbons led to carbonaceous deposits, but an important part of hydrocarbon remained chemisorbed as well. Heating these samples in He atmosphere provoked the decomposition of these deposits with evolution of CO2 and CO, and also of SO2 and SO. At the same time, COS was evidenced in the reaction products. The release of these molecules occurred below the activation temperature of the catalysts. The behavior of the catalysts depended both on reactant molecule and sulfur content. The analyses clearly evidenced the oxidation ability of ZrO2-SO42- catalysts

Alkylation of hydroquinone with tert-butanol over silica-immobilized triflate derivatives

Silica immobilized La(OTf)(3), tertbutyldimethylsilyl-triflate and triflic acid proved to be very efficient catalysts for the hydroquinone alkylation with tert-butanol. In solvent free conditons, the conversion was higher than 95% with the dialkylated product (2,5-DTBHQ) formed in high yields. When a polar solvent (1,4-dioxane) was used, the selectivity towards the monoalkylated product (2-TBHQ) increased up to 90%

Transformation of C6 hydrocarbons over sulfated zirconia catalysts

The conversion of the C6 saturated hydrocarbons hexane, methylcyclopentane and cyclohexane was studied in a flow microreactor in the temperature range 120-240 degrees C. Sulfated zirconia with different sulfate contents were used as catalysts. Catalysts were characterized before and after reactant exposure using Raman spectroscopy and XPS. The obtained results confirm data on the reaction mechanism already reported in the literature. The first step is the hydrogen abstraction which could occur to a more or less advanced extent as a function of the sulfate density species. This process is the main cause of the catalyst reactivity but also of the catalyst deactivation. More advanced dehydrogenated species lead to polymerized species which remain strongly chemisorbed on the catalyst surface. XPS data also indicated that during these redox processes a part of the sulfur remained on the catalysts surface. Higher sulfate densities lead to the cyclization of hexane. (C) 1999 Elsevier Science B.V. All rights reserved

Reaction of hexane, cyclohexane, and methylcyclopentane over gallium-, indium-, and thallium-promoted sulfated zirconia catalysts

Sulfate-zirconia catalysts were prepared via the colloidal-sol-gel method using ZrOCl2 as precursor. After precipitation and washing, the solids were peptized with a 1 : 1 mixture of CH3COOH and H2SO4. The promoters were introduced as gallium, indium, or thallium nitrates into the solution containing the zirconium sols. After a gel formation period, the samples were dried and calcined. The catalysts were characterized by chemical analysis, nitrogen sorption isotherms, FTIR analysis of adsorbed pyridine and deuterated acetonitrile, XRD, laser Raman and FTIR spectroscopy, and XPS. The catalytic tests were performed in a fixed-bed quartz microreactor using as reactant molecules hexane, cyclohexane, and methylcyclopentane. Characterization of the samples by the different techniques allowed us to propose that the main factors controlling the observed properties are the high sulfur content and the promoter effect. Low surface areas of the catalysts were related to the high sulfur contents. Sulfate species in different coordinations were indentified : isolated sulfate groups, polynuclear sulfate species, and sulfuric acid. Lewis acid sites as well as the polynuclear sulfate species determined the acidity of these solids. The presence of promoters contributed not only to the increase of the redox behavior of the catalysts, but also to a decrease of the population of polynuclear sulfates, in the order Tl > In > Ga. In the reaction conditions adopted, on unpromoted catalysts and (especially) Ga-promoted catalysts, dehydroizomerization and ring isomerization were the principal reaction pathways for hexane and methylcyclopentane, and for cyclohexane, respectively. Tl was found to promote ring cleavage. These data are consistent with previous studies pointing to the dehydrogenation activity of Ga embedded in zeolite matrices. (C) 1998 Academic Press