Search

International audience; The catalytic oxidation of dichloromethane (DCM, 1000 ppm) in wet air was carried out in a fixed bed reactor over a range of Pt/γ-Al2O3 catalysts. The dichloromethane oxidation occurs following a bifunctional mechanism. DCM is disproportionated over γ-Al2O3 into a 1/1/3molar mixture of CO, CH3Cl, and HCl; the DCM transformation is followed by CH3Cl and CO oxidation into CO2. A mechanism of DCM disproportionation is proposed involving, as intermediates, formate species as well as alumina hydroxyl groups formed by water adsorption

International audience; The aim of this work was to evaluate the possible synergy in hybrid catalysts composed of the two types of solids and to study their performances in the catalytic combustion of isopropanol. Several catalysts, based on cerium oxide and zeolites, for the catalytic oxidation of volatile organic compounds (VOCs), especially isopropanol, were tested. This study was assessed by determining the catalysts activity, selectivity and deactivation at various temperatures after 5 h of reaction. An important part of the work was the preparation and characterization of the catalysts, in particular the study of oxygen storage and mobility. Hybrid catalysts exhibit a synergy effect in terms of oxygen storage capacity and oxygen isotopic exchange. However, even if a synergy effect is also observed in catalytic oxidation over some samples, the oxygen mobility does not ensure better results in isopropanol combustion. (C) 2011 Elsevier B.V. All rights reserved.

The catalytic combustion of 1-methylnaphthalene (1-MN), a PAH representative molecule, was carried out over USHY and 0.8%PtUSHY catalysts in presence of steam. These catalysts are able to transform low concentration of 1-MN into carbon dioxide at 300 °C over USHY catalyst and at 200 °C over PtUSHY, without by-products formation. The presence of platinum on the USHY increases the catalytic activity, hence the reaction rate. The influence of platinum content showed that 0.5% of platinum deposited on the zeolite was sufficient to oxidize 900 ppm of 1-MN at 300 °C. The influence of water content in the feed was investigated and several assumptions were put forth to explain our results. The carbon dioxide yield decreases in the 0–12% range of relative humidity (RH). When RH increases up to 12% the carbon dioxide yield increases. Over USHY catalyst and for low water content (RH 2 O dimer (neutral complex form) formation. Over PtUSHY catalysts, water adsorption over the PtO phase could be considered. For high water content (RH > 12%), water cluster formation via H 2 O dimer (ion-pair complex form) is proposed on both catalysts. This form of H 2 O dimer (ion-pair complex) could generate an acidity, which facilitates the formation of oxygenated compounds easily degradable into carbon dioxide.

Catalytic oxidation of isopropylbenzene (cumene, 210 ppm) in wet air was essentially investigated over basic NaX zeolite. Experiments were carried out in presence of water (11,000 ppm) and at a gas hourly space velocity (GHSV) of 18,000 h−1. Over NaX, the cumene showed a catalyst activation phenomenon during the reaction time. Indeed, after 175 min of reaction at 250 °C the conversion of cumene increases from 45% to 100% and the CO2 yield from 30% to 95% probably because of the formation of active species mainly localized at the cavities apertures of the basic zeolite. Peroxide or peroxy radicals were proposed to be these particular active species.

The catalytic oxidation of tetrachloroethylene (PCE), chosen as model for chlorinated hydrocarbons, was investigated over Pt/HY, Pt/NaY and Pt/NaX under humid conditions at a space velocity of 15,300 h −1 . PCE is selectively transformed into carbon dioxide (CO 2 ) and hydrochloric acid (HCl) over all catalysts studied in this work. 1.2%Pt/HY exhibits the highest activity with the complete conversion of PCE at 500 °C and undergoes no significant deactivation after 4 h of reaction. It was also highlighted in this work, that water vapour plays a preponderant role for PCE transformation and limits catalyst deactivation. On the other hand, platinum-exchanged sodic faujasites (Pt/NaFAU) are also active in PCE transformation but it appeared that 1%Pt/NaX underwent a high and rapid deactivation by the formation of sodium chloride (NaCl) clusters.

Catalytic oxidation of monochlorobenzene (667 ppm) in wet air was investigated over PtHFAU(5) catalysts, differing by their Pt content (from 0 to 1.1 wt.%), their Pt dispersion (for identical Pt content) and the electronic state of Pt. PtHFAU(5) catalysts show a higher activity compared to more conventional PtAl2O3 and PtSiO2 samples. For a given temperature, chlorobenzene oxidation over PtHFAU(5) is independent of the platinum particles size. On the other hand, a plateau in activity is reached from 0.4 to 0.6% Pt. Amount of polychlorinated benzenes (PhCl2+) were produced in the order PtAl2O3 > PtSiO2 > PtHFAU(5). The formation of these compounds, especially PhCl2, over PtHFAU(5) was function on the Pt content (PhCl2 isomers appear only from 0.6% PtHFAU), and of the electronic state of Pt. Thus PhCl2, were mainly found over reduced Pt0 particles certainly through a chlorination of platinum (formation of PtClx species). A reaction scheme for the PhCl2 formation was proposed.

Catalytic oxidation of dichloromethane (DCM, 1000 ppm) in wet air was investigated over PtHFAU catalysts differing by their Pt content (from 0 to 0.87 wt.%), the Si/Al ratio being constant and equal to 5. For a given temperature, the rate of DCM transformation is independent of the platinum particles size and of the platinum content. On the other hand, DCM oxidation into CO2 occurs through a bifunctional mechanism: DCM was firstly hydrolysed into HCl and formaldehyde on the Bronsted acid sites of the support. Afterwards, formaldehyde was oxidised into CO2 and H2O on the platinum sites. A reaction scheme was proposed to explain the complete oxidation of DCM over PtHFAU catalysts.

The catalytic oxidation of dichloromethane (DCM, 1000 ppm) in wet air was carried out in a fixed bed reactor over NaY, pure, exchanged with 0.5 wt% Pt or added with 0.4 wt% PtSiO 2 . At low temperatures (⩽250 °C) DCM is selectively hydrolyzed into 2/1 molar mixture of HCl and formaldehyde ; at higher temperatures with Pt catalysts, DCM hydrolysis is followed by formaldehyde oxidation into CO 2 and water, which suggests a bifunctional scheme. DCM transformation was investigated over pure NaY, the zeolite samples being characterized after reaction by adsorption of nitrogen, of pyridine followed by IR spectroscopy, and of DCM followed by microcalorimetry. During the first 15 min of reaction, a significant decrease in conversion could be observed as well as large changes in the NaY properties: disappearance of the strongest sites for DCM adsorption, creation of acidic bridging OH groups, etc., suggesting a stoichiometric reaction of DCM with NaY. Afterward, the process became catalytic with no more change in DCM conversion and in the physicochemical properties of the zeolite. A mechanism of DCM hydrolysis is proposed involving as intermediates chloromethoxy, hydroxymethoxy, and NaCl species as well as the OH groups created on the NaY zeolite.

A high-throughput vapor-phase catalytic experimentation unit with 15 parallel microreactors and fast, on-line GC product analysis was built. The 15 open tubular microreactors were mounted in a single furnace and fed with a common gas stream. Decane hydroisomerization experiments were conducted on 35 mg quantities of reference platinum-loaded ultrastable Y zeolite particles at 200 °C, 4.5 bar, using a hydrogen/hydrocarbon molar ratio of 375/1 and contact times of 100–800 kg s/mol. The microreactor outlets were sampled sequentially. GC analysis of the C1C10 alkane product mixtures was achieved in 3.2 min with a multi-capillary column. The actual gas flow rates and contact times in the individual microreactors were derived from the GC FID signals after dilution of the reaction product stream with a calibrated hydrogen gas flow. The maximum temperature difference among the microreactors, measured in an empty reactor unit was ca. 6 °C at 200 °C. The conversion data were in excellent agreement with a kinetic model established for a conventional reactor. Fitting of the kinetic data obtained in the individual microreactors to the model confirmed the small temperature variations depending on the location of each microreactor in the catalyst furnace.

The competitive transformation of cyclopentanone and of two thiols (cyclopentanethiol and cyclohexanethiol) was carried out on a sulfided CoMo/Al2O3 catalyst at 220°C under atmospheric pressure in the presence of H2/H2S (90/10 vol.%). The results made it possible to explain the good selectivity in cyclopentanethiol obtained in the thioreduction of cyclopentanone. This is the consequence of the inhibition due to a competition to adsorption, of the thiol desulfurization into the corresponding olefin. The kinetic order with respect to cyclopentanone of the transformation of cyclohexanethiol into cyclohexene was found equal to −0.6.

Two series of sulfided Ni or Co promoted Mo/alumina catalysts, having different Ni or Co loadings, were characterized by their activities for the transformation of cyclopentanone into cyclopentanethiol (flow reactor, 220°C, atmospheric pressure) and for the hydrodesulfurization of dibenzothiophene (flow reactor, 340°C, 3 MPa hydrogen pressure). The addition of the promoter increased significantly the activity of the Mo/alumina catalyst for both reactions, up to a maximum obtained with the catalysts having a (promoter)/(promoter+Mo) molar ratio equal to 0.3–0.4. This increase in activity was due in part to an increase in the hydrogenating properties of the Mo/alumina catalyst. However, an additional modification of the catalyst (basic and nucleophilic properties) must be considered to account for the spectacular effect of the promoter on the rate of the dibenzothiophene direct desulfurization reaction.