Vera Maria Martins Salim

Modeling and optimization of the combined carbon dioxide reforming and partial oxidation of natural gas

The optimization of the combined carbon dioxide reforming and partial methane oxidation over a 1% Pt/-Al2O3 catalyst was studied in order to produce synthesis gas with hydrogen/carbon monoxide ratio close to 1, for applications in metallurgical and polycarbonates processes and for production of oxygenated compounds and hydrocarbons. The study was performed with the help of experimental design and two mathematical modeling approaches: empirical and phenomenological. Empirical polynomial models were employed to analyze the effects of the process variables on the response factors and the final correlation coefficients obtained were above 95%. The phenomenological model was obtained from individual mass balances and the obtained correlation coefficients were above 95% for CH4 and N2, 90% for CO2 and H2O and near 70% for H2 and CO. The empirical modeling approach was found to be more efficient, simpler and led to better results than those obtained with the phenomenological model approach. Therefore, the empirical modeling was used for optimization of the process operation conditions. At an oxygen/methane ratio of 0.55 gmol/gmol and temperature of 950 ◦ C, optimized process conditions were obtained with complete methane conversion, maximum carbon monoxide selectivity of 43% and minimum hydrogen/carbon monoxide ratio of 1.3, in absence of water.

Stability and selectivity of bimetallic Cu-Co/SiO2 catalysts for cyclohexanol dehydrogenation

Abstract Bimetallic Cu–Co/SiO 2 catalysts with different metal loading (5:5, 15:15, 35:35 – Cu:Co) were prepared by deposition–precipitation method and evaluated by using the cyclohexanol dehydrogenation. XRD results show that only the 15%Cu–15%Co/SiO 2 oxide precursor exhibits mainly a phase containing Cu and Co with spinel-like structure, while other bimetallic precursors present CuO and Co 3 O 4 as distinct phases. The Rietveld method was used to quantify the different phases on the precursors and to determinate how much Cu there was in the spinel form and as CuO. After reduction and passivation, the 15%Cu–15%Co/SiO 2 catalyst showed a Cu–Co alloy formation and a different H 2 chemisorption capacity. The H 2 uptake was 2.5 greater than 35%Cu–35%Co/SiO 2 catalyst and the same order of magnitude for the monometallic 35%Co/SiO 2 . Catalytic results showed that the 15%Cu–15%Co/SiO 2 catalyst presented the highest stability and selectivity to cyclohexanone, with the lowest phenol production.

Quantitative XPS analysis of silica-supported Cu–Co oxides

Abstract Copper–cobalt oxides with Cu/Co=5:5, 15:15 and 35:35 bulk ratio have been prepared by deposition–precipitation method at constant pH from copper and cobalt nitrate solutions. Different oxides were obtained by decomposition of the precursors at 673 K for 7 h in air and analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). XRD data showed the formation of different oxide phases; for the bulk atomic ratio of 15Cu:15Co, a phase containing Cu and Co with spinel-like structure was observed, while the other bimetallic oxides presented CuO and Co3O4 as distinct phases. The XPS qualitative analysis has shown that all samples exhibited Cu2+ and Co3+ species at the surface. The Cu–Co spinel presented a displacement in Cu 2p binding energy value. A mathematical model was proposed from relative intensity ratios, which allowed the determination of the oxide particle thickness and the fraction of coverage at the support. This model described accurately the system and showed that cobalt improved the copper dispersion.

Preparation of high loading silica supported nickel catalyst: simultaneous analysis of the precipitation and aging steps

The precipitation and aging steps used for the preparation of high loading silica supported nickel catalysts were simultaneously analyzed with the aid of statistical design of experiments. Empirical models relating catalyst final properties and preparation variables were developed. It was found that the precipitation step determines the final catalyst nickel content, and that at low nickel concentrations high metallic area per gram of reduced nickel may be attained with the precipitated precursor without performing the aging step. When it is performed, the aging step has a strong influence on final catalyst properties, such as specific surface area, metallic area per gram of nickel and degree of reduction. Therefore, catalyst aging may be an effective method to control dispersion, reducibility and active phase accessibility. The importance of the aging step was shown to increase with the aging temperature, which is linked to the larger rates of silicate formation at high temperatures. A semi-empirical model was developed to describe the final catalyst specific surface area as a function of the rate of silicate formation. This model is able to describe changes of the catalyst surface area fairly well and allow the proper manipulation of preparation conditions in order to maximize the catalyst specific area, leading to maximum catalyst activities.

Ethyl oleate production by means of pervaporation-assisted esterification using heterogeneous catalysis

Pervaporation-assisted esterification of oleic acid and ethanol was investigated by means of heterogeneous acid catalysis with the aim of increasing the ethyl oleate yield. The experimental strategy comprised kinetic tests with Amberlyst 15 Wet (Rohm & Haas), the characterization of hydrophilic Pervap 1000 membrane (Sulzer) and the evaluation of the membrane-assisted reactor. Kinetic tests were carried out to study the effect of temperature, catalyst loading and ethanol/organic acid molar ratio for the esterification of oleic acid and ethanol. The ester yield and initial reaction rate were used as response. The hydrophilic poly(vinyl alcohol) membrane was able to remove water from the reaction medium and, hence, the ester yield was increased. The potential of coupling esterification and pervaporation was demonstrated, with a two-fold increase in the reaction yield of ethyl oleate.

Preparation of high loading silica-supported nickel catalyst: analysis of the reduction step

The reduction step used for preparation of high loading silica-supported nickel (Ni/SiO 2) catalysts was analyzed with the help of statistical experimental design. Two catalyst precursors prepared by deposition–precipitation (DP) method and presenting significantly different metal–support interactions were studied. Empirical models were developed for the degree of reduction and for the metallic area of the Ni/SiO2 catalysts as functions of the reduction variables (final reduction temperature, heating rate, flow rate and hydrogen concentration of the reducing mixture) for each precursor. For the precursor with low nickel–support interaction, high degrees of reduction were attained regardless of the reducing conditions used; however, operation conditions exerted a significant influence upon the final catalyst metallic area. For the precursor with high nickel–support interaction, both the degree of reduction and the final catalyst metallic area were strongly affected by the reducing conditions. The reducing conditions were then optimized for both catalyst precursors in order to maximize the final catalyst metallic area. Simulation results were validated experimentally and indicated that optimum reducing conditions may depend significantly on the nature of the catalyst precursor. Finally, a mixed model was built by the linear combination of the two original models, allowing the successful optimization of the reducing conditions for a precursor with intermediate nickel–support interaction. These results suggest that reducing conditions may be tuned at plant site as a function of the metal–support interaction of catalyst precursors, with the aid of mathematical models built for model catalyst precursors.

Analysis of experimental errors in catalytic tests for production of synthesis gas

The proper determination of experimental errors in catalytic processes may be very important because experimental errors can exert a major impact upon the analysis of experimental results. For this reason, the influence of temperature upon the experimental errors observed during the combined carbon dioxide reforming and partial oxidation of methane over Pt/-Al2O3 is studied here. It is shown that fluctuations of output stream compositions may decrease more than one order of magnitude as reactor temperature increases in the range from 600 to 1100 ◦ C during catalytic tests. Additionally, it is shown that the covariance matrix of composition measurements is not diagonal, as usually assumed, and may change very significantly with the experimental conditions. Therefore, experimental errors should not be regarded as constant and covariance matrices should not be assumed to be diagonal a priori for kinetic model building and parameter estimation. It is also shown that the covariance matrix may contain significant amount of information about the reaction mechanism, which can be used for model building and interpretation of kinetic experiments. Particularly, it is shown that the actual experimental error may be much smaller than usually obtained when covariance terms are neglected and that fluctuation of catalyst activity may concentrate most of the experimental fluctuations observed experimentally.

Interfacial behaviour of bovine testis hyaluronidase

The interfacial properties of bovine testicular hyaluronidase were investigated by demonstrating the association of hyaluronidase activity with membranes prepared from bovine testis. Protein adsorption to the air/water interface was investigated using surface pressure-area isotherms. In whichever way the interfacial films were obtained (protein injection or deposition), the hyaluronidase exhibited a significant affinity for the air/water interface. The isotherm obtained 180 min after protein injection into a pH 5.3 subphase was similar to the isotherm obtained after spreading the same amount of protein onto the same subphase, indicating that bovine testicular hyaluronidase molecules adopted a similar arrangement and/or conformation at the interface. Increasing the subphase pH from 5.3 to 8 resulted in changes of the protein isotherms. These modifications, which could correspond to the small pH-induced conformational changes observed by Fourier-transform IR spectroscopy, were discussed in relation to the pH influence on the hyaluronidase activity. Adding hyaluronic acid, the enzyme substrate, to the subphase tested the stability of the interfacial properties of hyaluronidase. The presence of hyaluronic acid in the subphase did not modify the protein adsorption and allowed substrate binding to a preformed film of hyaluronidase at pH 5.3, the optimal pH for the enzyme activity. Such effects of hyaluronic acid were not observed when the subphase was constituted of pure water, a medium where the enzyme activity was negligible. These influences of hyaluronic acid were discussed in relation to the modelled structure of bovine testis hyaluronidase where a hydrophobic region was proposed to be opposite of the catalytic site.

Quantitative XPS Analysis of Bimetallic Cu–Co Catalysts

The surfaces of bimetallic Cu-Co/SiO 2 catalyst with 1 Cu:2 Co atomic ratio has been studied focusing on the determination of bimetallic phase thickness and its distribution using a mathematical formalism from X-ray photoelectron spectroscopy (XPS) intensities. X-ray diffraction analysis indicated a Cu-Co alloy formation and XPS qualitative analysis showed that Cu 0 and Co 0 species were present on the surface. Atomic ratios of Cu/Si, Co/Si and Co/Cu from XPS and atomic absorption (A.A.) analyses suggested homogeneous particle formation with a slight Cu surface enrichment in the catalyst. A mathematical model was proposed from relative intensity ratios, which was able to determine the metallic particle thickness and its coverage fraction at the support. Considering that these values were comparable to those for the particle size obtained from transmission electron microscopy (TEM) measurements, it may be said that the model describes the evaluated system accurately.

Hyaluronidase binds differently DPPC, DPPS or GlcNAc-bearing glycolipid biomimetic monolayers

Bovine testis hyaluronidase (btHyal) had been shown to have direct effects on cancer cells and to be a useful adjuvant in several medicines. Furthermore this enzyme had been found to be membrane-associated. Thus, in this work, the interactions between btHyal and membranes were analyzed by using lipid monolayers at the air-water interface as a biomimetic membrane system. This allowed us to define the btHyal interactions with two residues of hyaluronic acid (a btHyal substrate), GlcNAc and carboxylic group, which are present in cholesteryl-triethoxy-N-acetylglucosamine (Chol-E3-GlcNAc) and in DPPS, respectively. btHyal bound preferentially Chol-E3-GlcNAc monolayers and showed a decreasing affinity for Chol-E3-GlcNAc-DPPC monolayers containing decreasing amount of glycolipid, suggesting a crucial role of the glycolipid GlcNAc. Furthermore the significant btHyal binding to DPPS was not affected by the presence of free GlcNAc in the subphase. These results and the absence of significant binding of btHyal to pure DPPC monolayer suggest that the protein interacts with the lipid monolayer by mimicking the enzyme-substrate interactions or by electrostatic interactions.