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- Heterogenisation of a c-scorpionate feII complex on carbon materials for cyclohexane oxidation with hydrogen peroxidePublication . Martins, Luisa; Almeida, M. Peixoto de; Carabineiro, S. A. C.; Figueiredo, J. L.; Pombeiro, ArmandoThe hydrotris(pyrazol-1-yl)methane iron(II) complex [FeCl2{h3-HC(pz)3}] (pz=pyrazol-1-yl) (1) was immobilized on three diferente carbon materials (activated carbon, carbon xerogel and multi-walled carbon nanotubes) with three different surface treatments (original, treated with nitric acid, and treated with nitric acid followed by sodium hydroxide) to produce active, selective and recyclable catalysts. The heterogenisation process was more efficient for carbon nanotubes treated with nitric acid and sodium hydroxide. An outstanding improved catalytic performance of complex 1 upon heterogenisation on carbon nanotubes treated with nitric acid and sodium hydroxide (turnover numbers up to 5.6_103 and overall yield of 21 %), relative to the homogeneous system, was achieved for the single-pot peroxidative oxidation of cyclohexane to the cyclohexanone and cyclohexanol mixture. The heterogenised systems allowed their easy recovery and reuse, at least for five consecutive cycles, maintaining 96% of the initial activity and concomitante rather high selectivity to cyclohexanol and cyclohexanone.
- Pyrazole or tris(pyrazolyl)ethanol oxo-vanadium(IV) complexes as homogeneous or supported catalysts for oxidation of cyclohexane under mild conditionsPublication . Silva, Telma F. S.; Mac Leod, Tatiana C. O.; Martins, Luisa; Guedes Da Silva, M. Fátima C.; Schiavon, Marco A.; Pombeiro, ArmandoThe oxovanadium(IV) complexes [VO(acac)(2)(Hpz)].HC(pz)(3) 1.HC(pz)(3) (acac= acetylacetonate, Hpz = pyrazole, pz = pyrazoly1) and [VOCl2{HOCH2C(pz)(3)}] 2 were obtained from reaction of [VO(acac)(2)] with hydrotris(1-pyrazolyl)methane or of VCl(3)with 2,2,2-tris(1-pyrazolyl)ethanol. The compounds were characterized by elemental analysis, IR, Far-IR and EPR spectroscopies, FAB or ESI mass-spectrometry and, for 1, by single crystal X-ray diffraction analysis. 1 and 2 exhibit catalytic activity for the oxidation of cyclohexane to the cyclohexanol and cyclohexanone mixture in homogeneous system (TONS up to 1100) under mild conditions (NCMe, 24h, room temperature) using benzoyl peroxide (BPO), tert-butyl hydroperoxide (TBHP), m-chloroperoxybenzoic acid (mCPBA), hydrogen peroxide or the urea-hydrogen peroxide adduct (UHP) as oxidants. 1 and 2 were also immobilized on a polydimethylsiloxane membrane (1-PDMS or 2-PDMS) and the systems acted as supported catalysts for the cyclohexane oxidation using the above oxidants (TONs up to 620). The best results were obtained with mCPBA or BP0 as oxidant. The effects of various parameters, such as the amount of catalyst, nitric acid, reaction time, type of oxidant and oxidant-to-catalyst molar ratio, were investigated, for both homogeneous and supported systems. (C) 2012 Elsevier B.V. All rights reserved.
- Highly active and selective supported rhenium catalysts for aerobic oxidation of n-Hexane and n-HeptanePublication . Mishra, Gopal; Alegria, Elisabete; Pombeiro, Armando; Martins, LuisaA series of derivative C-scorpionate rhenium complexes, i.e., [ReCl2{NNC(O)C6H5} (Hpz)(PPh3)2] (A) (where Hpz is pyrazole), [ReCl2{NNC(O)C6H5}(Hpz)2(PPh3)] (B), [ReClF {NNC(O)C6H5}(Hpz)2(PPh3)] (C), and their precursor [ReOCl3(PPh3)2] (D), immobilized on 3-aminopropyl-functionalized silica have been prepared and used for neat O2 oxidation of n-hexane and n-heptane mainly to the corresponding alcohols and, in lower amounts, ketones. The supported catalyst C, with fluoro- and diazenido-ligands, exhibits the highest activity for both alkanes (overall turnover numbers (TONs) up to 3.8 _ 103 and 2.5 _ 103, for n-hexane and n-heptane, respectively) and can be reused in consecutive catalytic cycles. Improved conversion was observed after addition of hetero-carboxylate co-catalysts. A free-radical-based mechanism is proposed to explain the product formation.
- Trinuclear Cu-II structural isomers coordination, magnetism, electrochemistry and catalytic activity towards the oxidation of alkanesPublication . Sutradhar, Manas; Martins, Luisa; Guedes Da Silva, M. Fátima C.; Mahmudov, Kamran; Liu, Cai-Ming; Pombeiro, ArmandoThe reaction of the Schiff base (3,5-di-tert-butyl-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H3L) with copper(II) nitrate, acetate or metaborate has led to the isomeric complexes [Cu-3(L)(2)(MeOH)(4)] (1), [Cu-3(L)(2)(MeOH)(2)]2MeOH (2) and [Cu-3(L)(2)(MeOH)(4)] (3), respectively, in which the ligand L exhibits dianionic (HL2-, in 1) or trianionic (L3-, in 2 and 3) pentadentate 1O,O,N:2N,O chelation modes. Complexes 1-3 were characterized by elemental analysis, IR spectroscopy, single-crystal X-ray crystallography, electrochemical methods and variable-temperature magnetic susceptibility measurements, which indicated that the intratrimer antiferromagnetic coupling is strong in the three complexes and that there exists very weak ferromagnetic intermolecular interactions in 1 but weak antiferromagnetic intermolecular interactions in both 2 and 3. Electrochemical experiments showed that in complexes 1-3 the Cu-II ions can be reduced, in distinct steps, to Cu-I and Cu-0. All the complexes act as efficient catalyst precursors under mild conditions for the peroxidative oxidation of cyclohexane to cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone, leading to overall yields (based on the alkane) of up to 31% (TON = 1.55x10(3)) after 6 h in the presence of pyrazinecarboxylic acid.
- 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.
- Trits(Pyrazol-1-YL)methane metal complexes for catalytic mild oxidative functionalizations of alkanes, alkanes and ketonesPublication . Martins, Luisa; Pombeiro, ArmandoThis work concerns recent advances (since 2005) in the oxidative functionalization of alkanes, alkenes and ketones, under mild conditions, catalyzed by homoscorpionate tris(pyrazol-1-yl)methane metal complexes. The main types of such homogeneous or supported catalysts are classified, and the critical analysis of the most efficient catalytic systems in the different reactions is presented. These reactions include the mild oxidation of alkanes (typically cyclohexane as a model substrate) with hydrogen peroxide (into alkyl hydroperoxides, alcohols, and ketones), the hydrocarboxylation of gaseous alkanes (with carbon monoxide and potassium peroxodisulfate) into the corresponding Cn+1 carboxylic acids, as well as the epoxidation of alkenes and the Baeyer-Villiger oxidation of linear and cyclic ketones with hydrogen peroxide into the corresponding esters and lactones. Effects of various reaction parameters are highlighted and the preferable requirements for a prospective homogeneous or supported C-scorpionate-M-based catalyst in oxidative transformations of those substrates are identified. (C) 2014 Elsevier B.V. All rights reserved.
- p-xylene oxidation to terephthalic acid: new trendsPublication . Lapa, Hugo; Martins, LuisaLarge-scale terephthalic acid production from the oxidation of p-xylene is an especially important process in the polyester industry, as it is mainly used in polyethylene terephthalate (PET) manufacturing, a polymer that is widely used in fibers, films, and plastic products. This review presents and discusses catalytic advances and new trends in terephthalic acid production (since 2014), innovations in terephthalic acid purification processes, and simulations of reactors and reaction mechanisms.
- V(IV), Fe(II), Ni(II) and Cu(II) complexes bearing 2,2,2-tris(pyrazol-1-yl)ethyl methanesulfonate: application as catalysts for the cyclooctane oxidationPublication . Silva, Telma F. S.; Rocha, Bruno G. M.; Guedes Da Silva, M. Fátima C.; Martins, Luisa; Pombeiro, ArmandoWater-soluble compounds [VOCl2{CH3SO2OCH2C(pz)(3)}] (pz = pyrazol-1-yl) 1, [FeO2{CH3SO2OCH2C(pz)(3)}] 2, [NiO2{CH3SO2OCH2C(pz)(3)}] 3 and [Cu{CH3SO2OCH2C(pz)(3)}(2)](OTf)(2) 4 were obtained by reactions between the corresponding metal salts and 2,2,2-tris(pyrazol-1-yl)ethyl methanesulfonate, CH3SO2OCH2C(pz)(3). They were isolated as air-stable solids and fully characterized by IR, FTIR, NMR (for 2), EPR (for 1), ESI-MS(+/-), elemental analysis and (for 4) single-crystal X-ray diffraction. In all, half- (1-3) or full-sandwich (4), compounds the C-scorpionate ligand shows the N,N,N-coordination mode. 3 and 4 appear to provide the first examples of a Ni(II) and a full-sandwich Cu(II) compound respectively, bearing that scorpionate ligand. Compound 3 is the first Ni(II) tris(pyrazol-1-yl)methane type complex to be applied as catalyst for the oxidation of alkanes. Compounds 1-4 exhibit catalytic activity for the peroxidative (with aq. H2O2) oxidation, in water/acetonitrile medium and under mild homogeneous conditions, of cyclooctane to the corresponding alcohol and ketone (yields up to ca. 27%). The effect of the presence of additives, such as nitric acid or pyridine, was studied.
- Enhancing alkane oxidation using Co-doped SnO2 nanoparticles as catalystsPublication . Silva, Telma F.S.; Silvestre, A. J.; Rocha, Bruno G. M.; Nunes, Manuel R.; Monteiro, Olinda; Martins, LuisaA novel eco-friendly KA oil synthesis at room temperature (up to 25% yield) via solvent-free cyclohexane oxidation using Sn1 − xCoxO2 − δ (x = 0, 0.01 or 0.05) nanoparticles as catalyst (TON up to 2 × 103) is here reported. These nanoparticles are the first SnO2-based material able to catalyze the oxidation of alkanes. The most active nanocatalyst was the Sn0.95Co0.05O2 − δ, allowing an easy recovery and reuse, at least for five consecutive cycles, maintaining high selectivity concomitant with 92% of its initial activity.
- 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.
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