Toluene oxidation

Abstract: Nanosized rod-like, wire-like, and tubular α-MnO(2) and flower-like spherical Mn(2)O(3) have been prepared via the hydrothermal method and the CCl(4) solution method, respectively. The physicochemical properties of the materials were characterized using numerous analytical techniques. The catalytic activities of the catalysts were evaluated for toluene oxidation. It is shown that α-MnO(2) nanorods, nanowires, and nanotubes with a surface area of 45-83 m(2)/g were tetragonal in crystal structure, whereas flower-like spherical Mn(2)O(3) with a surface area of 162 m(2)/g was of cubic crystal structure. There were the presence of surface Mn ions in multiple oxidation states (e.g., Mn(3+), Mn(4+), or even Mn(2+)) and the formation of surface oxygen vacancies. The oxygen adspecies concentration and low-temperature reducibility decreased in the order of rod-like α-MnO(2) > tube-like α-MnO(2) > flower-like Mn(2)O(3) > wire-like α-MnO(2), in good agreement with the sequence of the catalytic performance of these samples. The best-performing rod-like α-MnO(2) catalyst could effectively catalyze the total oxidation of toluene at lower temperatures (T(50%) = 210 °C and T(90%) = 225 °C at space velocity = 20,000 mL/(g h)). It is concluded that the excellent catalytic performance of α-MnO(2) nanorods might be associated with the high oxygen adspecies concentration and good low-temperature reducibility. We are sure that such one-dimensional well-defined morphological manganese oxides are promising materials for the catalytic elimination of air pollutants.

Abstract: The dissimilatory Fe(III) reducer, GS-15, is the first microorganism known to couple the oxidation of aromatic compounds to the reduction of Fe(III) and the first example of a pure culture of any kind known to anaerobically oxidize an aromatic hydrocarbon, toluene. In this study, the metabolism of toluene, phenol, and p-cresol by GS-15 was investigated in more detail. GS-15 grew in an anaerobic medium with toluene as the sole electron donor and Fe(III) oxide as the electron acceptor. Growth coincided with Fe(III) reduction. [ring-14C]toluene was oxidized to 14CO2, and the stoichiometry of 14CO2 production and Fe(III) reduction indicated that GS-15 completely oxidized toluene to carbon dioxide with Fe(III) as the electron acceptor. Magnetite was the primary iron end product during toluene oxidation. Phenol and p-cresol were also completely oxidized to carbon dioxide with Fe(III) as the sole electron acceptor, and GS-15 could obtain energy to support growth by oxidizing either of these compounds as the sole electron donor. p-Hydroxybenzoate was a transitory extracellular intermediate of phenol and p-cresol metabolism but not of toluene metabolism. GS-15 oxidized potential aromatic intermediates in the oxidation of toluene (benzylalcohol and benzaldehyde) and p-cresol (p-hydroxybenzylalcohol and p-hydroxybenzaldehyde). The metabolism described here provides a model for how aromatic hydrocarbons and phenols may be oxidized with the reduction of Fe(III) in contaminated aquifers and petroleum-containing sediments. Images

Abstract: . The Master Chemical Mechanism has been updated from MCMv3 to MCMv3.1 in order to take into account recent improvements in the understanding of aromatic photo-oxidation. Newly available kinetic and product data from the literature have been incorporated into the mechanism. In particular, the degradation mechanisms for hydroxyarenes have been revised following the observation of high yields of ring-retained products, and product studies of aromatic oxidation under relatively low NOx conditions have provided new information on the branching ratios to first generation products. Experiments have been carried out at the European Photoreactor (EUPHORE) to investigate key subsets of the toluene system. These results have been used to test our understanding of toluene oxidation, and, where possible, refine the degradation mechanisms. The evaluation of MCMv3 and MCMv3.1 using data on benzene, toluene, p-xylene and 1,3,5-trimethylbenzene photosmog systems is described in a companion paper, and significant model shortcomings are identified. Ideas for additional modifications to the mechanisms, and for future experiments to further our knowledge of the details of aromatic photo-oxidation are discussed.

Abstract: Thermal and hydrothermal treatments have been applied to an amorphous TiO2 precursor for obtaining nanosized TiO2 particles (P11t and P11h, respectively) of different photocatalytic properties. The activity of these catalysts has been tested by performing the toluene oxidation in gas phase in a continuous photoreactor. A Fourier transform infrared (FTIR) investigation of the catalysts under conditions prevailing during the test photoreaction has also been carried out. The photoreactivity results showed that CO2 was the main oxidation product and benzaldehyde a stable intermediate. Anatase P11t photoactivity was similar to that observed for commercial photocatalysts, while anatase P11h presented a marked improvement. The FTIR study on these samples indicate that P11h has a higher number of hydrogen-bonded hydroxyl groups that are more stable under RT outgassing than P11t. The higher photoactivity of P11h is attributed to the participation of these hydrogen-bonded hydroxyls in the toluene conversion to CO2. FTIR spectra also suggest that benzaldehyde, the minor oxidation product, originates from toluene adsorbed on isolated hydroxyls. Benzaldehyde is more strongly adsorbed on P11h than on P11t; the presence of water vapor in the reacting mixture, however, facilitates its desorption and/or photooxidation.

Abstract: Photocatalyzed degradation of trace level trichloroethylene (TCE) and toluene in air were carried out over near-UV-illuminated titanium dioxide (anatase) powder in a flow reactor using a residence time of about 5–6 ms. Concentration ranges for TCE and toluene were 0–800 mg/m3. TCE photooxidation was very rapid under our experimental conditions, and ∼100% conversion was achieved for TCE concentration examined up to 753 mg/m3as a single air contaminant. Initial photodegradation rates for toluene in humidified air were fitted by a Langmuir–Hinshelwood rate form. Toluene photooxidation sole contaminant rates were of the same order of magnitude as reported previously form-xylene and acetone. No significant intermediates were detected by GC/FID during TCE or toluene photooxidation reactions. Humidification had significant influence: Toluene photooxidation rate increases with the water concentration up to about 1650 mg/m320% relative humidity) and decreases thereafter. The presence of sufficient TCE (225–753 mg/m3) promoted the toluene photooxidation reaction rate to achieve 90–100% toluene conversion in 5.6 ms. When feed toluene levels measured below ∼90 mg/m3. Higher toluene feed levels “quenched” this TCE promotion effect and also depressed TCE conversion very strongly, but the toluene conversion fell just to the toluene-only levels observed in single contaminant experiments. Photooxidation kinetics of TCE and toluene mixtures in air are thus shown to exhibit strong promotion and inhibition behavior vs that expected from the single-species kinetic degradation data. A previously suggested chlorine radical oxidation of TCE is modified to rationalize the TCE enhancement of the toluene photooxidation rate and the corresponding toluene “quench” of TCE destruction. Time-dependent catalyst activation (by TCE) and deactivation (by toluene or toluene oxidation products and, eventually, even by TCE products) were observed. Carboxylate formation and carboxylic acid accumulation postulated by previous investigators could be a major cause of such catalyst deactivation.