Browse

You are looking at 81 - 90 of 392 items for :

  • Chemistry and Chemical Engineering x
  • Architecture and Architectonics x
  • All content x
Clear All

Abstract

Asymmetric electro-epoxidation of olefins had been achieved with sodium hypochlorite (NaClO) as an oxidant, which was generated by electrolysis in two-phase systems of aqueous sodium chloride (NaCl) and ionic liquids (1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4)). The electrolysis conditions by different current densities (0.8, 0.9, 1.0, 1.1 and 1.3 mA/cm2) and pH values (8, 9, 10, 11, 12 and 13) were optimized and 1.1 mA/cm2 and pH 11 were selected. The proposed reaction mechanism is also discussed. The performance of new catalytic systems in four kinds of reaction media in the presence or absence of ammonium acetate (NH4OAc) as a cocatalyst was investigated systematically. Compared to the chemical epoxidation systems, the enantiomeric excess (ee) values and yields for the epoxidation of styrene, α-methylstyrene and indene were acceptable in the electrocatalytic epoxidation systems.

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors: Luis Ernesto Sandoval-Díaz, Ericsson David Coy-Barrera, and Carlos Alexander Trujillo

Abstract

The catalytic effect of fluoride on the polymerization of sodium silicate and phosphoric acid solutions at pH 7.15 was studied through a simple and inexpensive method: the observation of the meniscus of the resulting sol in a test tube. Reaction was found to be second-order regarding silicate concentration as reported previously. Novel results show that the reaction also follows a pseudo-second-order kinetics with respect to fluoride. A rate law for the catalyzed polymerization of the form r = K v[Si(OH)4][HF]2 was established. A mechanism for fluoride action on silica polymerization, where silicon increases its coordination number to five, is proposed.

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, M. Scurtu, C. Hornoiu, M. Caldararu, C. Munteanu, G. Postole, N. I. Ionescu, T. Yuzhakova, and A. Redey

Abstract

The surface dynamics of SnO2/γ-Al2O3 catalysts with different tin dioxide loadings (3 and 20 wt%) prepared by the impregnation method were investigated by using in situ electrical/dielectric measurements. The samples were characterized by BET, inductively coupled plasma atomic emission spectroscopy and X-ray diffraction methods. The propylene oxidation reaction, in the temperature range of 30–400 °C, was used to identify the catalytic activity of SnO2 based catalysts. Results are discussed on the bases of the electrical changes accompanying variation of the active phase content. The conductance/capacitance of supported samples evidenced the reciprocal influence of the support and of the guest oxide.

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, Monica Dan, Maria Mihet, Alexandru R. Biris, Petru Marginean, Valer Almasan, George Borodi, Fumiya Watanabe, Alexandru S. Biris, and Mihaela D. Lazar

Abstract

The effect of Ag (1 wt%) and Au (1 wt%) on the catalytic properties of Ni/Al2O3 (7 wt% Ni) for methane steam reforming (MSR) was studied in parallel with the effect of CeO2 (6 wt%) and La2O3 (6 wt%) addition. The addition of 1 wt% Ag to the alumina supported nickel catalyst drastically decreased its catalytic properties at temperatures lower than 600 °C, due to the blockage of metal catalytic centers by silver deposition. The addition of Au and CeO2 (La2O3) to the nickel catalyst improved the methane conversion, CO2 selectivity and hydrogen production at low reaction temperatures (t < 600 °C). At 700 °C under our working conditions, the additives have no important effect in hydrogen production by MSR. The best hydrogen production at low temperatures was obtained for Ni–Au/Al2O3, due to the higher CO2 selectivity, cumulated with slightly higher methane conversion in comparison with Ni/CeO2–Al2O3. At high temperature, Ni/CeO2–Al2O3 is stable for 48 h on stream. Ni–Au/Al2O3 and Ni–Ag/Al2O3 are mainly deactivated due to the temperature effect on Au and Ag nanoparticles and less through coke formation. On Ni/Al2O3 and Ni/La2O3–Al2O3, crystalline, graphitic carbon was deposited after 48 h of reaction leading to catalyst partial deactivation. On the Ni/CeO2–Al2O3 surface, a porous amorphous form of deposited carbon was found, which does not decrease its catalytic activity after 48 h of reaction.

Restricted access

Abstract

Horseradish peroxidase immobilization inside molecular sieves with pores hexagonally structured as such or functionalized with copper ions is presented in comparison with the bidimensional immobilization onto aerosil A380 particles. Immobilization was performed under mild conditions that do not lead to enzyme denaturation. Guest–host interactions were pursued spectrophotometrically and by thermal analysis measurements. FTIR spectroscopy showed no major secondary structural change for the entrapped enzyme despite some influence of the support functionalization that can be supposed. The activity of the immobilized enzyme was tested in the oxidation of Alizarin Red S (ARS) with hydrogen peroxide. It was found that the dye oxidation is catalyzed by the enzyme in homogeneous reaction but the rate becomes much slower in heterogeneous catalytic processes with prepared catalysts. It seems that horseradish peroxidase immobilization in simple porous silica or alumino-silica post synthesis blocks the enzyme activity. In addition, copper ions make a stable complex with ARS hindering its oxidation as well.

Restricted access

Abstract

Ti-SBA-15, one of the latest titanium silicalite catalysts, has been prepared according to the literature by the direct hydrothermal synthesis using Pluronic 123 as structure-directing agent. The characterization of the catalyst was performed by means of the following methods: XRD, IR, UV–Vis, X-ray microanalysis and SEM. The catalytic properties of the Ti-SBA-15 catalyst have been tested in the epoxidation of allyl alcohol, methallyl alcohol, crotyl alcohol and 1-butene-3-ol with hydrogen peroxide. The process has been described by the following main functions: the selectivity to epoxide compound in relation to allylic compound consumed and the conversion of allylic compound.

Open access

Abstract

Ta2O5 powder was heat treated in flowing NH3 at varying temperature for 2 h. When heat treated at 680–720 °C, nitrogen was doped to Ta2O5, but showed only a slight improvement of photocatalysis of rhodamine B dye in solution under visible light irradiation. From the heat treatment temperature of 740 °C, seeds of TaON and Ta3N5 formed newly and grew with the temperature. The optical absorption edge at 410–510 nm by TaON and absorption shoulder at 535 nm by Ta3N5 were observed. These specimens with N-doped Ta2O5, TaON, and Ta3N5 demonstrated notably improved photocatalytic performance, and the photocatalysis was facilitated by surface photosensitization.

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors: Cristina Benincá, Patrício Peralta-Zamora, Ronaldo Cardoso Camargo, Célia Regina Granhen Tavares, Everton Fernando Zanoelo, and Luciana Igarashi-Mafra

Abstract

A 22 factorial design of experiments complemented with a central point assayed in triplicate was proposed to investigate the influence of initial Fenton's reagent (H2O2 and Fe2+) concentration on the maximum oxidation of the Ponceau 4R dye in aqueous solution. The reactions took place in batch reactors without (Fenton process) and with (photo-Fenton process) UV irradiation under well-stirred conditions at approximately 25 °C. Based on these preliminary results, the kinetics of color degradation, removal of total organic carbon and consumption of reactants was experimentally investigated for 7200 s at optimal initial concentration of H2O2 and Fe2+. The results show that the Fenton and photo-Fenton process caused approximately 95% of decoloration after only 300 s of reaction and complete color degradation at 7200 s. At this final time of reaction, TOC removals of 97% and approximately 100% were observed when applying Fenton and photo-Fenton conditions, respectively. A toxicity assay revealed that A. salina was not sensitive to the synthetic effluent treated by the photo-Fenton process and diluted with aqueous nutritive solution at 25, 12.5, 10 and 5%. A suggested simplified mechanism involving three elementary reactions was able to reproduce the experimental kinetic results of Fe2+, H2O2 consumption, Fe3+ formation and azo dye degradation. On the whole, the results obtained at the investigated conditions show that the photo-Fenton and Fenton processes are effective processes to achieve TOC reduction and color degradation of Ponceau 4R dye typically found in liquid effluents from the Brazilian confectionary industries.

Restricted access

Abstract

In this work, we describe the synthesis and characterization of a new water soluble [RhCl(CO)(PySO3Na)2], complex (1); where [PySO3Na] is the sodium salt of p-sulfonated pyridine ligand. Complex (1) was obtained in methanol, reacting the dried sodium salt of p-sulfonated pyridine ligand with [Rh(Cl)(CO)2]2 complex under inert atmosphere. Complex (1) was characterized by FT-IR and 1H NMR techniques. Complex (1) was active in the catalytic hydroformylation of 1-hexene and real naphtha in a biphasic (water/toluene) system. The effects of reaction time, temperature and syngas pressure were studied with 1-hexene as a substrate. Formation of metallic particles was tested with a mercury drop trial, giving negative results. Reuse of the aqueous phase for several times under the same conditions showed little catalytic activity loss. Complex (1) was active for the hydroformylation reaction of 1-hexene, in the presence of thiophene compounds, showing better performance in comparison with water soluble rhodium analogues.

Restricted access

Abstract

The effect of the introduction of niobium carbide was studied on the dehydro-aromatization of methane over a molybdenum containing HMCM-22 zeolite at 973 K. The introduction of niobium decreased Br⊘nsted acidity, affecting catalyst behavior on the dehydro-aromatization of methane. Niobium carbide containing zeolite showed lower activity, lower coke deposition and predominant formation of naphthalene, in the expenses of benzene, the major product observed for Mo containing zeolite. Besides carbidic carbon, two other types of coke were observed: one associated to molybdenum and the other associated to Br⊘nsted acid sites located both on the catalyst surface and inside zeolite pores.

Restricted access