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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.
- 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.
- 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.
- Eco-friendly cyclohexane oxidation by a V-scorpionate complex immobilized at hierarchical MOR zeolitePublication . Ottaviani, Davide; Van-Dúnem, Vanmira; Carvalho, Ana; Martins, Angela; Martins, LuisaThe hydrotris(3,5-dimethylpyrazol-1-yl)borate dioxido-vanadium(V) complex [VO2{HB(3,5-Me(2)pz)(3)}] (V, pz= pyrazol-1-yl) immobilized at hierarchical MOR zeolite modified through surfactant mediated technology using NaOH as alkaline agent, V@MOR_NaOH, provides an efficient (TON up to 5.2x10(2)) catalyst for the peroxidative oxidation of cyclohexane with tert-butyl hydroperoxide to KA oil (52% yield), at room temperature, and is easily recovered and reused up to four consecutive catalytic cycles with no significant leaching of vanadium. The textural modifications performed on the zeolitic matrix produced an effective support for the anchorage of the vanadium complex allowing an improved catalytic performance and reusability. This work constitutes an unprecedented use of supported vanadium B-scorpionate complexes as catalysts for the oxidation of alkanes.
- Improved cyclohexane oxidation catalyzed by a heterogenized Iron (II) complex on hierarchical Y zeolite through surfactant mediated technologyPublication . Van-Dúnem, Vanmira; Carvalho, Ana; Martins, Luisa; Martins, AngelaThe hydrotris (pyrazol-1-yl) methane iron (II) complex [FeCl2 (Tpm)] [Tpm=HC(C3N2H3)3] was successfully immobilized, by the incipient wetness impregnation method, in parent and hierarchical Y zeolite modified through surfactant mediated methodology using different bases (NH4OH, NaOH or TPAOH). The catalytic performance and recyclability of supported Fe complex was evaluated in the oxidation of cyclohexane with hydrogen peroxide to cyclohexanol and cyclohexanone under mild conditions and two distinct behaviors were observed: using commercial Y and modified Y with NaOH the complex is anchored mainly at the outer surface of the zeolite supports, resulting in high lixiviation after the first catalytic cycle. When NH4OH and TPAOH are used during the zeolite treatments the particular textural parameters exhibited by these two supports are able to effectively retain the complex molecules, resulting in a high recyclability observed after three consecutive cycles, especially when NH4OH treated zeolite was used as support.