The Synergic Effect of h-MoO3, α-MoO3, and ß-MoO3 Phase Mixture as a Solid Catalyst to Obtain Methyl Oleate.

Extensive research in the last few decades has conclusively demonstrated the significant influence of experimental conditions, surfactants, and synthesis methods on semiconductors' properties in technological applications. Therefore, in this study, the synthesis of molybdenum oxide (MoO3) was reported by the addition of 2.5 (MoO3_2.5), 5 (MoO3_5), 7.5 (MoO3_7.5), and 10 mL (MoO3_10) of nitric acid, obtaining the respective concentrations of 0.6, 1.10, 1.6, and 0.6 mol L-1. In this study, all samples were synthesized by the hydrothermal method at 160 °C for 6 h. The materials obtained were structurally characterized by X-ray diffraction (XRD) and structural Rietveld refinement, Raman spectroscopy, and infrared spectroscopy (FTIR), confirming the presence of all crystallographic planes and bands associated with active modes for the pure hexagonal phase (h-MoO3) when the solution's concentration was 0.6 mol L-1 of nitric acid. For concentrations of 1.10, 1.60, and 2.10 mol L-1, the presence of crystallographic planes and active modes associated with the formation of mixtures of molybdenum oxide polymorphs was confirmed, in this case, the orthorhombic, monoclinic, and hexagonal phases. X-ray photoelectron spectroscopy reveals the occurrence of the states Mo4+, Mo5+, and Mo6+, which confirm the predominance of the acid Lewis sites, corroborating the analysis by adsorption of pyridine followed by characterization by infrared spectroscopy. The images collected by scanning electron microscopy confirmed the information presented in the structural characterization, where microcrystals with hexagonal morphology were obtained for the MoO3_2.5 sample. In contrast, the MoO3_5, MoO3_7.5, and MoO3_10 samples exhibited hexagonal and rod-shaped microcrystals, where the latter morphology is characteristic of the orthorhombic phase. The catalytic tests carried out in the conversion of oleic acid into methyl oleate, using the synthesized samples as a heterogeneous catalyst, resulted in conversion percentages of 52.5, 58.6, 69.1, and 97.2% applying the samples MoO3_2.5, MoO3_5, MoO3_7.5, and MoO3_10, respectively. The optimization of the catalytic tests with the MoO3_10 sample revealed that the conversion of oleic acid into methyl oleate is a thermodynamically favorable process, with a variation in the Gibbs free energy between -67.3 kJ mol-1 and 83.4 kJ mol-1 as also, the energy value of activation of 24.6 kJ mol-1, for the temperature range from 80 to 140 °C, that is, from 353.15 to 413.15 K, respectively. Meanwhile, the catalyst reuse tests resulted in percentages greater than 85%, even after the ninth catalytic cycle. Therefore, the expressive catalytic performance of the mixture of h-MoO3 and α-MoO3 (MoO3_10) phases is confirmed, associated with the synergistic effect, mainly due to the increase in the surface area and available Lewis sites of these phases.

Calcium ferrites for phosphate adsorption and recovery from wastewater.

In this study, calcium ferrites with different Ca : Fe atomic ratios (1 : 1, 1 : 2, 1 : 3 and 2 : 1) were prepared from Ca and Fe nitrates treated at 300, 700 and 900 °C and evaluated for phosphate adsorption and recovery from wastewater. TG, XRD, Mössbauer spectroscopy, SEM, VSM magnetic measurements, and BET analyses showed the formation of two different calcium ferrite phases, i.e., CaFe2O4 and Ca2Fe2O5 at 700 and 900 °C. The adsorption results indicated that the formation of calcium ferrite structure is critical for phosphate adsorption/recovery. Evaluation of the pH, initial phosphate concentration, contact time, coexisting ions and desorption conditions showed remarkable adsorption capacities of 62-75 mg g-1 for CaFe1:2-700 and 28-43 mg g-1 for CaFe1:2-900. The phosphate adsorption on the Ca ferrite surfaces is so strong that the recovery/desorption showed limited efficiencies, e.g., 15-39%.

Micromesoporous Activated Carbons as Catalysts for the Efficient Oxidation of Aqueous Sulfide.

KOH activation of a mesophase pitch produces very efficient carbons for the removal of sulfide in aqueous solution, increasing the sulfur oxidation rate with the degree of activation of the carbon. These carbons are characterized by their graphitic structures, with domains of sizes of around 20 nm, and a moderate concentration of surface oxygen groups (0.2-0.5 mmol·g-1) dominating the basic groups. Because the activation leads first to a strong development of the micropores and later to a development of the mesopores, the surface area values are always high, reaching values of as high as 3250 m2·g-1 in the most activated carbon, with a volume of mesopores of as high as 44% of the total pore volume. In the presence of this carbon, the sulfide oxidation rate is 100 times higher than that found for a commercial activated carbon, the results indicating that the porosity of the carbon, especially mesoporosity, plays a role more important than the structure or the chemical nature of the carbon in the kinetics of sulfide oxidation to different polysulfides.

Tuning the surface properties of biochar by thermal treatment.

In this work, the effect of controlled thermal treatment to tune biochar surface properties such as area/porosity, functionalities and reactivity was investigated. TG-MS, CHN, Raman, IR, BET, Zeta and SEM analyses suggested that thermal treatment led to the decomposition of an organic complex/amorphous phase to produce micropores based on graphene nanostructures and a strong increase on surface area from 3m2g-1 for biochar to 30, 408 and 590m2g-1, at 400, 600 and 800°C, respectively. The treatment also led to a gradual decrease on oxygen content from 27 to 14wt% at 800°C due to decomposition of surface functionalities changing surface properties such as zeta potential, adsorption of anionic and cationic species and an increase on the activity for sulfide oxidation which is discussed in terms of increase in surface area and the presence of surface redox quinone groups.

Use of iron and bio-oil wastes to produce highly dispersed Fe/C composites for the photo-Fenton reaction.

This work describes the synthesis, characterization, and application of an active heterogeneous photo-Fenton system obtained from two different wastes, i.e., laterite (an iron mining waste) and the acid aqueous fraction (AAF) from bio-oil production. AAF with high acidity (ca. 3 molH+ L-1) and organic concentration (25 wt.%) obtained from biomass flash pyrolysis was used for the efficient extraction of Fe3+ from laterite waste. After extraction, the mixture Fe3+/AAF was dried and treated at different temperatures, i.e., 500, 650, and 800 °C, to obtain Fe/C reactive composites. Mössbauer, XRD, TG, elemental analyses, and SEM/EDS showed the presence of highly disperse Fe oxide nanoparticles at 500 and 650 °C and Fe0 particles in the material obtained at 800 °C with carbon contents varying from 74 to 80 %. The three composites were tested as heterogeneous catalysts in the photo-Fenton reaction for the oxidation of the model dye contaminant methylene blue, showing high activities at neutral pH.

Controlled formation of reactive Fe particles dispersed in a carbon matrix active for the oxidation of aqueous contaminants with H2O2.

In this work, reactive iron nanoparticles dispersed in a carbon matrix were produced by the controlled thermal decomposition of Fe(3+) ions in sucrose. During the sucrose decomposition, the Fe(3+) ions are reduced to form iron nanometric cores dispersed in a porous carbonaceous matrix. The materials were prepared with iron contents of 1, 4, and 8 wt.% and heated at 400, 600, and 800 °C. Analyses by X-ray diffraction, Mössbauer spectroscopy, magnetization measurements, Raman spectroscopy, termogravimetric analyses, BET surface area, scanning, and transmission electron microscopy showed that at 400 °C, the materials are composed essentially of Fe3O4 particles, while treatments at higher temperatures, i.e., 600 and 800 °C, produced phases such as Fe(0) and Fe3C. The composites were tested for the oxidation of methylene blue with H2O2 by a Fenton-type reaction and also H2O2 decomposition, showing better performance for the material containing 8 % of iron heated at 400 and 600 °C. These results are discussed in terms of Fe(2+) surface species in the Fe3O4 nanoparticles active for the Fenton reaction.

Application of Fenton's reagent to regenerate activated carbon saturated with organochloro compounds.

In this work Fenton's reagent was used to treat activated carbon saturated with organochloro compounds for the oxidation of the adsorbed contaminants and the regeneration of the carbon adsorbent. Activated carbon containing adsorbed chlorinated model substrates such as chlorobenzene, tetrachloroethylene, chloroform or 1,2-dichloropropane was treated with Fenton's reagent at room temperature resulting in a rapid consumption of the organochloro compounds. Thermogravimetric, infrared, BET surface area and MIMS adsorption studies showed that Fenton's treatment has no significant effect on the physico-chemical properties of the activated carbon. The used carbon adsorbents can be efficiently regenerated and recycled with no loss of its adsorption capacity even after five consecutive treatments with Fenton's reagent.