L. Calvo

Pd-Al pillared clays as catalysts for the hydrodechlorination of 4-chlorophenol in aqueous phase.

Catalysts based on pillared clays with Pd-Al were synthesized from a commercial bentonite and tested for catalytic hydrodechlorination (HDC) using 4-chlorophenol (4-CPhOH) as target compound and formic acid as hydrogen source. Stable Pd-Al pillared clays, with a strong fixation of the active phase to the solid support were obtained since no Pd was detected in the reaction media. The incorporation of Pd to the pillared clay structure yielded catalysts with high activity in the reaction studied reaching a complete removal of the 4-CPhOH under mild conditions of temperature (50-70 degrees C). Phenol was not the only reaction product formed, since a more hydrogenated product such as cyclohexanone was detected in the effluent, which indicates additional hydrogenation of phenol. The influence of the method of introduction of Pd in the pillared clay (ion-exchange or impregnation) and Pd concentration in the catalytic activity were studied as well as other important operating variables such as reaction temperature, catalyst concentration, 4-CPhOH initial concentration and formic acid to 4-CPhOH molar ratio. The catalysts prepared suffered deactivation after three consecutive runs, probably due to carboneous deposits formation since no appreciable Pd leaching was observed.

Hydrodechlorination of 4-chlorophenol in water with formic acid using a Pd/activated carbon catalyst

This work reports on the feasibility of hydrodechlorination as a treatment technique for chlorophenols-bearing wastewaters using formic acid as a hydrogen source. 4-Chlorophenol (4-CPhOH) has been used as target compound and the experiments were carried out in batch and continuous mode with a commercial activated carbon-supported Pd (0.5 wt.%) catalyst. The variables studied in the batch runs were HCOOH/4-CPhOH molar ratio (10-1000), temperature (25-75 degrees C) and catalyst concentration (250-1000 mg/L). The continuous experiments were performed in a fixed bed reactor where aqueous solutions of formic acid and 4-CPhOH with molar ratios between 50 and 100 were continuously fed to the reactor, at different space-time values in the range of 10.7-42.8 kg(cat)h/mol. Reaction temperatures from 35 to 100 degrees C were tested and the pressure was fixed at 2.5bar. Conversion values above 99% for 4-CPhOH were obtained in batch experiments, but using a HCOOH/4-CPhOH molar ratio as high as 500. Moreover, most of the phenol produced was adsorbed on the catalyst. Continuous runs were performed to evaluate the efficiency of the catalyst under lower HCOOH/4-CPhOH ratios and to explore the possibility of converting phenol to more hydrogenated products. The results indicated that the HCOOH/4-CPhOH molar ratios needed were an order of magnitude lower than those required in batch runs to achieve conversions of 4-CPhOH close to 95%. Besides, phenol was not the only reaction product formed, since a more hydrogenated product such as cyclohexanone was detected in the effluent, which indicates additional hydrogenation of phenol in contrast to the behaviour observed in batch experiments. A loss of activity was observed in the continuous runs after 20-30 h on stream.

Identification of by-products and toxicity assessment in aqueous-phase hydrodechlorination of diuron with palladium on activated carbon catalysts

The hydrodechlorination (HDC) of diuron in aqueous phase with hydrogen using two different activated carbon-supported Pd catalysts was studied. A commercial activated carbon and one prepared by chemical activation of grape seeds with phosphoric acid (GS) were evaluated as supports, being the catalysts tested in a wide range of temperature (30-100 °C) and space-time (78-311 kgcat h mol(-1)). Diuron conversion was above 70% under all the conditions tested. The Pd catalyst supported on GS showed the highest activity in terms of diuron conversion within the temperature range studied, allowing nearly complete conversion above 50 °C. However, a gradual loss of activity with time was observed for this catalyst. A complete route of hydrogenation of diuron was elucidated. Two reaction routes one leading to fenuron and another to aniline were identified. As the temperature and space-time were increased, the formation of fenuron (via monuron) was found to be favored. The toxicity of the reaction products was evaluated, being the route to fenuron and monuron, the one giving rise to a significant decrease of ecotoxicity.

Improved synthesis and hydrothermal stability of Pt/C catalysts based on size-controlled nanoparticles

A novel method for the preparation of stable Pt/C catalysts with size-controlled nanoparticles has been developed. The method is based on in situ synthesis of the nanoparticles (reduction with NaBH4 in the presence of a support and PVP). Compared to the conventional ex situ route (colloidal synthesis followed by impregnation), this in situ route yields smaller nanoparticles (2.5–3.9 nm) of narrower size distribution. The catalysts prepared by the in situ synthesis showed a higher stability in water at 80 °C, indicating a stronger interaction between the support and the metallic phase. Hydrothermal stability tests were also conducted under conditions equivalent to those of aqueous phase reforming (200 °C, 17 bar and water and diluted acetic acid). Hydrothermal treatment proved to be an excellent method to improve the resistance to leaching of the catalysts. Metal loss was negligible while PVP was almost completely removed from the catalyst; hence, most of the porosity was recovered and the dispersion measured by CO chemisorption increased from 5 to 34–75%. Water at 200 °C was more effective than diluted acetic acid for the removal of PVP. TEM images confirmed that the Pt nanoparticles did not undergo significant changes either in size or morphology upon the hydrothermal treatment, and XPS analysis showed a homogeneous distribution of Pt nanoparticles within the catalyst granules.

Diuron Multilayer Adsorption on Activated Carbon from CO2 Activation of Grape Seeds

Granular activated carbons were obtained from grape seeds by pyrolysis at 600°C and subsequent physical activation with CO2 (750–900°C, 1–3 h, 25–74% burn-off). The carbon and ash content increased during the activation, reaching values of 79.0% and 11.4%, respectively. Essentially microporous materials with BET surface areas between 380 and 714 m2/g were obtained. The performance of the activated carbon in the adsorption of diuron in aqueous phase was studied within the 15–45°C temperature range. Equilibrium data showed that the maximum uptake increased with temperature from 120 to 470 µmol/g, also evidencing some dependence of the adsorption mechanism on temperature. Data were fitted to five isotherm models [Langmuir, Freundlich, Dubinin–Radushkevich, BET, and GAB (Guggenheim, Anderson, and de Boer)]. Kinetic data were analyzed using first- and second-order rate equations and intraparticle diffusion model. The second-order rate constant values obtained (2.8–13.5 × 10−3 g/µmol min) showed that the hollow core morphology of the material favors the adsorption kinetics.

Photolysis of clethodim herbicide and a formulation in aquatic environments: Fate and ecotoxicity assessment of photoproducts by QSAR models

The photochemical fate of the herbicide clethodim in natural waters was investigated under simulated and natural sunlight radiation. This herbicide exhibited a rapid degradation rate in simulated aquatic environment with half-lives ranged from 27.9min to 4.6h. The commercial formulation of clethodim showed a faster degradation with half-lives from 19.3min to 1.4h. It has also been demonstrated that the photolytic behavior of clethodim was affected by the water composition and the radiation intensity. Nine major photoproducts were identified and their distribution was dependent on the experimental conditions. Photodegraded solutions of clethodim were shown to be more toxic to the bacteria Vibrio fischeri than the herbicide itself, reaching the maximum toxicity when the herbicide is completely degraded. QSAR analysis of the fate, ecotoxicological and physicochemical endpoints of the degradation products provided positive alerts for several identified by-products. Environmental fate and transport estimates showed that all photoproducts, unlike the active substance, are potential leachers. Moreover, predicted vapor pressures suggested that dermal contact and ingestion are the most probable exposure routes for workers and general population to both clethodim and its photoproducts. These results highlight the importance of the degradation products in attaining a complete knowledge of the fate and behavior of an herbicide in the environment. To our knowledge, this is the first study to report a detailed QSAR study on clethodim photoproducts under environmental conditions. These results provide a very valuable information that will guide further experimental studies leading to a better pesticide risk assessment.

Photodegradation behaviour of sethoxydim and its comercial formulation Poast® under environmentally-relevant conditions in aqueous media. Study of photoproducts and their toxicity

Photolysis is an important route for the abiotic degradation of many pesticides. However, the knowledge of the photolytic behaviour of these compounds and their commercial formulations under environmentally-relevant conditions are limited. The present study investigated the importance of photochemical processes on the persistence and fate of the herbicide sethoxydim and its commercial formulation Poast® in aqueous media. Moreover, the effect of important natural water substances (nitrate, calcium, and ferric ions) on the photolysis of the herbicide was also studied. The results showed that additives existing in the commercial formulation Poast® accelerated the rate of photolysis of sethoxydim by a factor of 3. On the contrary, the presence of nitrate and calcium ions had no effect on the photodegradation rate while ferric ions resulted in an important decrease in the half-life of sethoxydim possibly due to the formation of a complex. Different transformation products were identified in the course of sethoxydim irradiation and the effect of experimental conditions on their concentrations was investigated. Finally, Microtox® test revealed that aqueous solutions of sethoxydim photoproducts increased the toxicity to the bacteria Vibrio fischeri.

Enhancement of the activity of Pd/C catalysts in aqueous phase hydrodechlorination through doping of carbon supports

The enhancement of the activity of Pd catalysts for aqueous phase hydrodechlorination (HDC) was studied using N-doped tailored supports. N-Doped (1–1.7 wt% N) and non-doped mesoporous carbon materials with equivalent pore structures were prepared via templating with MSU-F silica. At 30 °C, doping of the support resulted in a higher catalytic activity (26 vs. 49 mmol g−1 Pd min−1 for non-doped and doped supports, respectively) in the removal of 4-chlorophenol from water. However, the activity of the catalyst with non-doped carbon supports increased more with reaction temperature, even being the most active at 70 °C (131 vs. 102 mmol g−1 Pd min−1 for non-doped and doped supports, respectively). The activation energy of the process was found to decrease from 32 to 16 kJ mol−1 due to nitrogen doping of the support. Nitrogen doping of the carbon support is an interesting strategy to prepare catalysts with high HDC activity under mild temperature conditions, whereas non-doped supports are more convenient for the intensification of the process by increasing the reaction temperature.