Loading...
12 results
Search Results
Now showing 1 - 10 of 12
- N2O-Free single-pot conversion of cyclohexane to adipic acid catalysed by an iron(II) scorpionate complexPublication . Da Costa Ribeiro, Ana Paula; Martins, Luisa; Pombeiro, ArmandoThe highly efficient eco-friendly synthesis of adipic acid (96% yield, 98% selectivity, TONs up to 1.8 × 103) directly from cyclohexane is achieved by using ozone and [FeCl2{κ3-HC(pz)3}] (pz = pyrazol-1-yl) in a solvent-, heating-, radiation- and N2O-free new protocol.
- Supported C-Scorpionate Vanadium(IV) Complexes as Reusable Catalysts for Xylene OxidationPublication . Wang, Jiawei; Martins, Luisa; Da Costa Ribeiro, Ana Paula; Carabineiro, Sonia Alexandra Correia; Figueiredo, José L.; Pombeiro, ArmandoC-Scorpionate vanadium(IV) [VOxCl3-x{k(3)-RC(pz)(3)}] [pz = pyrazol-1-yl; x=0, R = SO3 (1); x = 1, R= CH2OH (2) or CH2OSO2Me (3)] complexes supported on functionalized carbon nanotubes (CNTs) are the first V-scorpionate catalysts used so far for the neat oxidation of o-, m- or p-xylene, with TBHP (70% aqueous solution), to the corresponding toluic acids (main products), tolualdehydes and methylbenzyl alcohols. Remarkably, a p-toluic acid yield of 43% (73% selectivity, TON = 1.34 V 10(3)) was obtained with 2@CNT in a simple microwave-assisted mild oxidation procedure, using a very low catalyst charge (3.2 x 10(-2) mol% vs. substrate). Further, this occurred in the absence of any bromine source, what is significant towards the development of a greener and more sustainable process for oxidation of xylenes. Moreover, reuse of catalysts with preservation of their activity was found for up to six consecutive cycles. The effects of reaction parameters, such as reaction time, temperature, amount of catalyst or type of heating source, on the performance of the above catalytic systems are reported and discussed.
- Nickel (ii)-2-amino-4-alkoxy-1,3,5-triazapentadienate complexes as catalysts for heck and henry reactionsPublication . Da Costa Ribeiro, Ana Paula; Karabach, Yauhen; Martins, Luisa; Mahmoud, Abdallah G.; Guedes Da Silva, M. Fátima C.; Pombeiro, ArmandoNickel (II)-triazapentadienate complexes, [Ni(tap)(2)], are synthesized and found to act as catalysts in the Heck reaction of deactivated aryl halides. The use of room-temperature ionic liquids instead of organic solvents allows easy separation of the catalyst from the products and substrates. The synthesized complexes also catalyse the Henry reaction of benzaldehydes with nitroethane, at 25 degrees C, leading to the corresponding nitroalkanols with diastereoselectivity in favour of the anti isomer.
- Catalytic performance of Fe(II)-scorpionate complexes towards cyclohexane oxidation in organic, ionic liquid and/or supercritical CO2 media: a comparative studyPublication . Da Costa Ribeiro, Ana Paula; Martins, Luisa; Alegria, Elisabete; Matias, Inês A. S.; Duarte, Tiago A. G.; Pombeiro, ArmandoThe catalytic activity of the iron(II) C-scorpionate complexes [FeCl2{HC(pz)3}] 1 (pz = pyrazol-1-yl) and [FeCl2{HOCH2C(pz)3}] 2, and of their precursor FeCl2·2H2O 3, towards cyclohexane oxidation with tert-butyl hydroperoxide was evaluated and compared in different media: acetonitrile, ionic liquids (1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], and 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, [bmim][FAP]), supercritical carbon dioxide (scCO2), and scCO2/[bmim][X] (X = PF6 or FAP) mixtures. The use of such alternative solvents led to efficient and selective protocols for the oxidation of cyclohexane. Moreover, tuning the alcohol/ketone selectivity was possible by choosing the suitable solvent.
- Nickel (ii)-2-amino-4-alkoxy-1,3,5-triazapentadienate complexes as catalysts for heck and henry reactionsPublication . Da Costa Ribeiro, Ana Paula; Karabach, Yauhen Yu; Martins, Luisa; Mahmoud, Abdallah G.; Guedes Da Silva, M. Fátima C.; Pombeiro, ArmandoNickel (II)-triazapentadienate complexes, [Ni(tap)(2)], are synthesized and found to act as catalysts in the Heck reaction of deactivated aryl halides. The use of room-temperature ionic liquids instead of organic solvents allows easy separation of the catalyst from the products and substrates. The synthesized complexes also catalyse the Henry reaction of benzaldehydes with nitroethane, at 25 degrees C, leading to the corresponding nitroalkanols with diastereoselectivity in favour of the anti isomer.
- Tuning cyclohexane oxidation: combination of microwave irradiation and ionic liquid with the c-scorpionate [FeCl2(Tpm)] catalystPublication . Da Costa Ribeiro, Ana Paula; Martins, Luisa; Kuznetsov, Maxim L.; Pombeiro, ArmandoFor the first time, microwave (MW) irradiation was successfully applied to peroxidative cyclohexane oxidation catalyzed by a C-scorpionate complex, [FeCl2(Tpm)] (1; Tpm = hydrotris(pyrazol-1-yl)methane), providing a highly selective and fast eco-friendly procedure to produce a KA (cyclohexanol + cyclohexanone) oil mixture (up to ca. 100% selectivity) with 28% yield in 1 h at 50 degrees C in MeCN. Water, organics, and ionic liquids (ILs, [bmim][N(CN)(2)] and [brnim][BF4]) were used as solvents, and their interactions with 1 were determined by DFT calculations. The possibility to recycle and reuse 1 (up to nine consecutive cycles) without loss of activity is also observed for the IL [bmim] [BF4]. Moreover, higher catalytic activities under additive-free conditions are obtained with the ILs in comparison to those with the other solvents. Tuning the alcohol/ketone selectivity is also possible by choosing the appropriate IL solvent.
- New trendy magnetic C-scorpionate iron catalyst and its performance towards cyclohexane oxidationPublication . Da Costa Ribeiro, Ana Paula; Matias, Inês; Alegria, Elisabete; Ferraria, Ana Maria; Rego, Ana; Pombeiro, Armando; Martins, LuisaFor the first time, a magnetic C-scorpionate catalyst was prepared from the iron(II) complex [FeCl2{_3-HC(pz)3}] (pz = pyrazol-1-yl) and ferrite, using the sustainable mechanochemical synthetic procedure. Its catalytic activity for the cyclohexane oxidation with tert-butyl hydroperoxide (TBHP) was evaluated in different conditions, namely under microwave irradiation and under the effect of an external magnetic field. The use of such magnetic conditions significantly shifted the catalyst alcohol/ketone selectivity, thus revealing a promising, easy new protocol for tuning selectivity in important catalytic processes.
- Catalytic oxidation of cyclohexane with hydrogen peroxide and a tetracopper(II) complex in an ionic liquidPublication . Da Costa Ribeiro, Ana Paula; Martins, Luisa; HAZRA, SUSANTA; Pombeiro, ArmandoThe catalytic peroxidative oxidation (with H2O2) of cyclohexane in an ionic liquid (IL) using the tetracopper(II) complex [(CuL)2(μ4-O,O′,O′′,O′′′-CDC)]2·2H2O [HL = 2-(2-pyridylmethyleneamino)benzenesulfonic acid, CDC = cyclohexane-1,4-dicarboxylate] as a catalyst is reported. Significant improvements on the catalytic performance, in terms of product yield (up to 36%), TON (up to 529), reaction time, selectivity towards cyclohexanone and easy recycling (negligible loss in activity after three consecutive runs), are observed using 1-butyl-3-methylimidazolium hexafluorophosphate as the chosen IL instead of a molecular organic solvent including the commonly used acetonitrile. The catalytic behaviors in the IL and in different molecular solvents are discussed.
- Liquid phase oxidation of xylenes catalyzed by the tripodal C-scorpionate iron(II) complex [FeCl2{κ3-HC(pz)(3)}]Publication . Mendes, Marta; Da Costa Ribeiro, Ana Paula; Alegria, Elisabete; Martins, Luisa; Pombeiro, ArmandoA simple and mild (room temperature) process for oxidation of o-, m- or p-xylene to the corresponding methylbenzyl alcohols, tolualdehydes and toluic acids, using H2O2 (30% aqueous solution) and the iron(II) C-scorpionate [FeCl2{κ3-HC(pz)3}] (pz = pyrazol-1-yl) catalyst is presented. Remarkably, after 5 min of reaction an overall oxygenates yield of 22% (TOF = 1.3 × 102 h−1) was obtained. The effects of reaction parameters, such as reaction time, temperature, amount of catalyst, type and amount of oxidant are reported and discussed.
- Highly selective cyclohexane oxidation catalyzed by ferrocene in ionic liquid mediumPublication . Martins, Luisa; Da Costa Ribeiro, Ana Paula; Pombeiro, ArmandoBackground: Although economically very important in view of the significance of the oxidized products for the manufacture of adipic acid and caprolactam (precursors to polyamides widely used in several industries), the current cyclohexane oxidation industrial process leads to very low conversions (ca. 4%) to assure a reasonable selectivity (ca. 85%). Therefore, there is an urgent need to develop sustainable selective cyclohexane oxidation chemistry, which is the main objective of this work. Methods: The partial oxidation of cyclohexane by aqueous tert-butyl hydroperoxide (TBHP, 70% aq. sol.) in phosphonium ionic liquid medium [P6.6.6.14][DCA] and in the presence of catalytic amounts of ferrocene was investigated. The reaction proceeded during 2 h at room temperature. Results: It was found that TBHP, under the above conditions, oxidizeD cyclohexane with high selectivity (> 98%), yielding mainly cyclohexanone (up to 16%, based on the substrate) with total TOFs up to 1 x 104 h-1. Conclusion: The combination of a commercial iron-complex catalyst (ferrocene) and well-adjusted unconventional reaction conditions led to a highly selective, fast and reusable catalytic system for the mild oxidation of cyclohexane. Moreover, the found [Fe(C5H5)2]/[P6.6.6.14][DCA] catalytic system can be of significance to produce polyamide 6.