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- Biomass-derived nanoporous carbons as electrocatalysts for oxygen reduction reactionPublication . Fernandes, Diana M; Mestre, Ana S.; Martins, Angela; Nunes, Nelson; Carvalho, A. P.; Freire, CristinaElectrocatalysts (ECs) for the oxygen reduction reaction (ORR) are crucial in fuel cells and for this reason developing cost-effective metal-free ECs with high electrocatalytic activity and high-volume production remains a huge challenge. Herein, we report the application as ORR electrocatalysts of a series of high grade nanoporous carbons, prepared by chemical activation of acid-chars obtained from the H2SO4 digestion and polycondensation (acid-mediated carbonization) of a biomass residue (Agave sisalana). All the nanoporous carbons presented good ORR electrocatalytic activities in alkaline medium. The AC(1) carbon exhibited the most promising ORR performance with E-onset = 0.84 vs. RHE, j(L, 0.26 V, 1600 rpm) = -3.12 mA cm(-2) and n(O2) = 3.6 electrons. The Tafel slopes of all carbons varied between 47 mV dec(-1) (AC(3)) and 250 mV dec(-1) (AC(1)). Furthermore, the carbons revealed superior tolerance to methanol when compared with commercial Pt/C and a competitive long-term electrochemical stability, with current retentions of 75-85 % after 20,000 s. The results obtained in this work suggest a promising method based on sustainable and economical biomass residues towards the development and engineering of novel value-added biomass-derived carbons as effective metal-free electrocatalysts for alkaline fuel cells.
- Zooming in with QSPR on Friedel-Crafts acylation reactions over modified BEA zeolitesPublication . Aleixo, Odrigo; Elvas Leitao, Ruben; Martins, Filomena; Carvalho, Ana; Brigas, Amadeu Fernandes; Nunes, Ricardo; Fernandes, Auguste; Rocha, Joao; Martins, Angela; Nunes, NelsonThe catalytic behaviour of hierarchical BEA zeolites with Si/Al ratio of 12.5 and 32 was studied in Friedel-Crafts acylation reactions using furan, anisole and pyrrole as substrates and acetic anhydride as acylating agent. Hierarchical BEA samples were submitted to alkaline and alkaline + acid treatments. Kinetic modelling using nonlinear regressions applied to a simplified Langmuir-Hinshelwood equation showed that the Si/Al ratio of the parent materials strongly influenced the catalytic behaviour. Catalytic results were correlated with physicochemical properties using a Quantitative Structure-Property Relationship (QSPR) methodology. This approach provided detailed information about the role of key properties on the catalytic behaviour, and pointed out which properties should be modified through direct synthesis and/or post-synthesis treatments to obtain materials with optimized catalytic performance.
- Using solvatochromic probes to investigate intermolecular interactions in 1,4-dioxane/methanol/acetonitrile solvent mixturesPublication . Nunes, Ricardo; Nunes, Nelson; Elvas Leitao, Ruben; Martins, FilomenaUV–Vis absorption spectra of five solvatochromic probes namely, betaine (30), 4-nitrophenol, 4-nitroanisole, 4-nitroaniline and N,N-dimethyl-4-nitroaniline, were obtained at 298.15 K in the binary systems 1,4-dioxane/methanol (MeOH) and 1,4-dioxane/acetonitrile (MeCN), and in the ternary system 1,4-dioxane/methanol/acetonitrile, in a total of 40 new solvent mixtures. Preferential solvation trends were assessed through the Bosch and Rosès formalism for the binary mixtures, and the extended model approach for the ternary mixture, thus allowing the setting up of a preferential solvation order for all entities present in solution, including solvent “complexes”. Synergistic behaviors were spotted in most binary mixtures involving 1,4-dioxane but not in the ternary system which is dominated by a combination of contributions from the three underlying binary mixtures. Kamlet-Taft parameters π*, α and β were also computed for all 40 systems and their variation with composition was thoroughly examined. The addition of small amounts of 1,4-dioxane to the mixtures was seen to cause a significant variation in π*, whereas on the other extreme, in dioxane rich mixtures, a large effect in α was observed, particularly for the system 1,4-dioxane/methanol.
- Zeolites and related materials as catalyst supports for hydrocarbon oxidation reactionsPublication . Martins, Angela; Nunes, Nelson; Carvalho, Ana P.; Martins, LuisaCatalytic oxidation is a key technology for the conversion of petroleum-based feedstocks into useful chemicals (e.g., adipic acid, caprolactam, glycols, acrylates, and vinyl acetate) since this chemical transformation is always involved in synthesis processes. Millions of tons of these compounds are annually produced worldwide and find applications in all areas of chemical industries, ranging from pharmaceutical to large-scale commodities. The traditional industrial methods to produce large amounts of those compounds involve over-stoichiometric quantities of toxic inorganic reactants and homogeneous catalysts that operate at high temperature, originating large amounts of effluents, often leading to expensive downstream processes, along with nonrecovery of valuable catalysts that are loss within the reactant effluent. Due to the increasingly stringent environmental legislation nowadays, there is considerable pressure to replace these antiquate technologies, focusing on heterogeneous catalysts that can operate under mild reactions conditions, easily recovered, and reused. Parallelly, recent advances in the synthesis and characterization of metal complexes and metal clusters on support surfaces have brought new insights to catalysis and highlight ways to systematic catalysts design. This review aims to provide a comprehensive bibliographic examination over the last 10 years on the development of heterogeneous catalysts, i.e., organometallic complexes or metal clusters immobilized in distinct inorganic supports such as zeolites, hierarchical zeolites, silicas, and clays. The methodologies used to prepare and/or modify the supports are critically reviewed, as well as the methods used for the immobilization of the active species. The applications of the heterogenized catalysts are presented, and some case-studies are discussed in detail.
- Kinetic study of Friedel-Crafts acylation reactions over hierarchical MCM-22 zeolitesPublication . Aleixo, Rodrigo; Elvas Leitao, Ruben; Martins, Filomena; Carvalho, Ana; Brigas, Amadeu; Martins, Angela; Nunes, NelsonFriedel-Crafts acylation was studied under mild conditions using hierarchical MCM-22 zeolites prepared by desilication and by desilication + acid treatment, using furan, pyrrole and anisole as substrates, and acetic anhydride as acylating agent. Enhanced catalysis was observed for furan and anisole, especially at short reaction times. Kinetic results modelling using non-linear regressions applied to a simplified Langmuir-Hinshelwood equation showed that desilication treatment followed by acid treatment improved kinetics (higher k and TOF). A QSPR methodology using nine substrate and zeolite descriptors was applied to model kinetics and adsorption. For both processes, the best QSPR model equations lead to the same descriptors, namely, Bronsted acidity, as zeolite's feature, and both van der Waals volume and Dimroth-Reichardt E-T(N) parameter (related to dipolarity and Lewis acidity) as substrates' characteristics. Normalization of descriptors allowed quantification of each descriptor's relative importance leading to a better understanding of the catalytic and adsorption processes.
- Probing substrate/catalyst effects using QSPR analysis on friedel-crafts acylation reactions over hierarchical BEA zeolitesPublication . Elvas Leitao, Ruben; Martins, Filomena; Borbinha, Leonor; Marranita, Catarina; Martins, Angela; Nunes, NelsonAttempts to optimize heterogeneous catalysis often lack quantitative comparative analysis. The use of kinetic modelling leads to rate (k) and relative sorption equilibrium constants (K), which can be further rationalized using Quantitative Structure-Property Relationships (QSPR) based on Multiple Linear Regressions (MLR). Friedel-Crafts acylation using commercial and hierarchical BEA zeolites as heterogeneous catalysts, acetic anhydride as the acylating agent, and a set of seven substrates with different sizes and chemical functionalities were herein studied. Catalytic results were correlated with the physicochemical properties of substrates and catalysts. From this analysis, a robust set of equations was obtained allowing inferences about the dominant factors governing the processes. Not entirely surprising, the rate and sorption equilibrium constants were found to be explained in part by common factors but of opposite signs: higher and stronger adsorption forces increase reaction rates, but they also make the zeolite active sites less accessible to new reactant molecules. The most relevant parameters are related to the substrates' molecular size, which can be associated with different reaction steps, namely accessibility to micropores, diffusion capacity, and polarizability of molecules. The relatively large set of substrates used here reinforces previous findings and brings further insights into the factors that hamper/speed up Friedel-Crafts reactions in heterogeneous media.
- Insights into the thermochemistry of paracetamol in ternary mixtures of eutectic solventsPublication . Lopes, Andreia; Nunes, Nelson; Elvas Leitao, Ruben; Martins, FilomenaSolution enthalpies of paracetamol, ΔsolH, in choline chloride–ethylene glycol–water mixtures were evaluated at 298.15 K. Solvatochromic UV–vis shifts of fve diferent probes (4-nitroaniline, 4-nitroanisole, 4-nitrophenol, N,N-dimethyl-4-nitroani line and 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate) were also assessed for the same mixtures at the same temperature in order to compute the corresponding Kamlet–Taft parameters, α, β, and π*. Quantitative structure–property relationships (QSPR) between ΔsolH and the solvatochromic parameters were established to investigate the solution pro cesses under consideration. Due to the small variability in the solvent parameters, a further set of eight pure solvents were considered. The behavior of paracetamol in eutectic mixtures is shown to be not signifcantly diferent from its behaviour in the pure solvents. The solvents’ basicity measured by β, is the most relevant factor modeling the solution process of this solute in the total set of solvents.
- Study of the ternary mixture of methanol/formamide/acetonitrile via solvatochromic probesPublication . Nunes, Nelson; Elvas Leitao, Ruben; Martins, FilomenaFollowing previous studies, the ternary mixture of methanol/formamide/acetonitrile (MeOH/Formamide/MeCN) was studied using the UV-Vis absorption spectra at 298.15 K with a set of five probes, 4-nitroaniline, 4-nitroanisole, 4-nitrophenol, N,N-dimethyl-4-nitroaniline and 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (Reichardt betaine dye), for a total of 22 mole ternary fractions. In addition, nine mole fractions of the underling binary mixtures, MeOH/Formamide and Formamide/MeCN were also tested. Spectroscopic results were used to model the preferential solvation order for each probe in the mixtures. The Kamlet–Taft solvatochromic solvent parameters, α, β, and π*, were also computed through the use of the solvatochromic shifts of the five probe indicators. Moreover, discrepancies in the spectroscopic behavior of 4-nitrophenol in formamide-rich mixtures were observed and analyzed.
- Volumetric and refractive index study of the ternary mixture methanol/formamide/acetonitrile at 298.15 KPublication . Martins, Filomena; Elvas Leitao, Ruben; Nunes, NelsonThe ternary mixture methanol (MeOH)/formamide (Form)/acetonitrile (MeCN) was studied by means of refractive index and density measurements. Data for a total of 22 ternary mole fractions, and 19 binary mole fractions for each formamide containing mixture, as well as values from the previously studied methanol/acetonitrile mixture were obtained and evaluated. Excess properties were computed and modeled by polynomial equations, namely Redlich-Kister for binary fractions and Cibulka for ternary mixtures. Partial molar volumes were likewise calculated and together with previously published values allowed the interpretation of structural effects associated with molecular interactions.
- Friedel-crafts acylation reaction over hierarchical Y zeolite modified through surfactant mediated technologyPublication . Martins, Angela; Neves, Vera; Moutinho, João; Nunes, Nelson; Carvalho, AnaFriedel-Crafts acylation reaction was studied under mild conditions using hierarchical HY zeolite samples prepared through surfactant mediated technology, in the presence of NH4OH, using CTAB or DTAB as surfactants and changing the duration of treatment from 6 to 48 h. The materials were characterized by powder X-ray diffraction, low temperature N2 adsorption isotherms, SEM microscopy and pyridine adsorption followed by FTIR. The catalytic behaviour was studied in the acylation of furan by acetic anhydride. The catalytic results reflect the role of the duration of the treatment as well as the surfactant molecule used. As the time of treatment increases, the enlargement of the pores leads to an increase of the rate constant and turnover frequency (TOF), except for sample modified during longer time, 48 h, due to the occurrence of secondary reactions that produce larger products or reaction intermediates that become trapped inside the pores. The role of the surfactant molecule is also relevant since the sample modified in the presence of the larger surfactant molecule, CTAB, a significant increase in product yield and rate constant is obtained when compared with the sample prepared in the same conditions using DTAB. However, in that case, by prolonging the treatment a substantial decrease in the same parameters occurs due to the occurrence of deactivation phenomena, pointing out that the optimized modification of porosity needs to be customized according to the needs of each catalytic system.