José A. F. Gamelas

Keggin-type polyoxotungstates as catalysts in the oxidation of cyclohexane by dilute aqueous hydrogen peroxide

Abstract Oxidation of cyclohexane by hydrogen peroxide in the presence of catalytic amounts of the Keggin-type heteropolytungstates [PW 11 O 39 ] 7− and [PW 11 M(L)O 39 ] (7− m )− , M m + =first row transition metal cation, L=H 2 O or CH 3 CN, was found to produce cyclohexanol, cyclohexanone and, in certain cases, cyclohexyl hydroperoxide. The presence of the latter was demonstrated by negative chemical ionization GC-MS. The reactions were carried out in acetonitrile, using tetra n -butylammonium salts of the catalysts and aqueous 30% hydrogen peroxide as oxidant. The polyanions [PW 11 O 39 ] 7− and [PW 11 Fe(H 2 O)O 39 ] 4− showed higher catalytic activity and different selectivity for the oxidation of cyclohexane than did the corresponding Cu-, Co-, Mn- and Ni-substituted complexes.

Alternatives for lignocellulosic pulp delignification using polyoxometalates and oxygen: a review

A review on the use of polyoxometalates (POMs) and oxygen, for the delignification of lignocellulosic pulps is presented. Two main processes using POMs for the delignification of pulp have been investigated: an anaerobic process in which the POMs have been used, in aqueous solutions, as stoichiometric reagents for the oxidative degradation of residual lignin, with their re-oxidation by oxygen occurring in a subsequently separate stage; and an aerobic process in which the POMs were designed to perform as catalysts in the oxygen delignification. In the aerobic approach, the lignin oxidation and the reactivation of the POM take place in the same stage of the process. The feasibility of the catalytic system was shown by pilot plant experiments. Following the green chemistry goals, these processes are environmentally friendly approaches and may allow a significant decrease of chlorine-based chemical consumption by the pulp-and-paper industry. The use of the POMs together with laccase for the oxygen delignification of lignocellulosic pulps will also be considered. Final comments regarding the practical application of the POMs are pointed out.

Synthesis, properties and photochromism of novel charge transfer compounds with Keggin anions and protonated 2,2′-biquinoline

Abstract Several novel compounds with protonated 2,2′-biquinoline (biqui) and Keggin polyoxoanions (α-isomers), having the general formula (Hbiqui) m [XM 12 O 40 ]· n solv, X=P ( m =3), Si ( m =4); M=Mo, W; n =0, 3; solv=H 2 O, N , N -dimethylformamide (dmf), were synthesized and characterized by analytical, spectroscopic and X-ray diffraction techniques. Electronic spectroscopy (visible/UV) indicated the presence of intermolecular charge transfer between the organic and inorganic moieties in the solid state. Evidence for the existence of intermolecular electronic interaction in solution was found for compounds with the [SiW 12 O 40 ] 4− anion, a fact quite uncommon for charge transfer compounds based on Keggin anions. A single crystal X-ray diffraction study could be performed on (Hbiqui) 4 [SiW 12 O 40 ]·3dmf crystals, but the refinements revealed the presence of highly disordered 2,2′-biquinoline molecules and α-[SiW 12 O 40 ] 4− anions. Photosensitivity to sunlight and to W-lamp visible light was assessed for all compounds. Photochromic properties were found for solids with [XMo 12 O 40 ] m − Keggin anions. Reduction of these anions upon irradiation was observed by diffuse reflectance and EPR spectroscopy. After a minimum of 5 h under sunlight, the extent of anion reduction followed the order (Hbiqui) 3 [PMo 12 O 40 ]·3dmf>(Hbiqui) 4 [SiMo 12 O 40 ]·3dmf>(Hbiqui) 3 [PMo 12 O 40 ]>(Hbiqui) 4 [SiMo 12 O 40 ]·4H 2 O. The Kurtz powder test was used to evaluate the second-order non-linear optical properties of the prepared compounds. (Hbiqui) 4 [SiW 12 O 40 ]·3H 2 O originated a second harmonic generation signal with intensity about 15% that of urea for 1064 nm radiation.

The surface properties of cellulose and lignocellulosic materials assessed by inverse gas chromatography: a review

The physicochemical surface properties of cellulose and lignocellulosic materials are of major importance in the context of the production of composites, in papermaking, and textile area. These properties can be evaluated by using inverse gas chromatography (IGC), a particularly suitable technique for the characterization of the surface properties of fibrous materials and powders. At infinite dilution conditions of appropriate gas probes, IGC may provide important parameters including the dispersive component of the surface energy of the material under analysis, thermodynamic data on the adsorption of specific probes, and Lewis acid–base interaction parameters between the matrix and the filler of composite materials. This paper critically reviews the most relevant results available in the literature concerning the characterization of cellulose and lignocellulosic materials using IGC. Emphasis will be put into the cellulose and nanocellulose surface properties, changes in the surface properties of cellulose and lignocellulosic materials after chemical and physical modifications, and in the compatibility of cellulose-based materials with polymeric matrices. The surface properties of non-woody fibers will also be considered. Before discussing the results available in the literature, the theoretical background and the main approaches used for the calculation of parameters accessed by IGC will be given. It is expected that this review can contribute to a better knowledge of the physicochemical surface properties of cellulosics.

Nanostructured bacterial cellulose-poly(4-styrene sulfonic acid) composite membranes with high storage modulus and protonic conductivity.

The present study reports the development of a new generation of bio-based nanocomposite proton exchange membranes based on bacterial cellulose (BC) and poly(4-styrene sulfonic acid) (PSSA), produced by in situ free radical polymerization of sodium 4-styrenesulfonate using poly(ethylene glycol) diacrylate (PEGDA) as cross-linker, followed by conversion of the ensuing polymer into the acidic form. The BC nanofibrilar network endows the composite membranes with excellent mechanical properties at least up to 140 °C, a temperature where either pure PSSA or Nafion are soft, as shown by dynamic mechanical analysis. The large concentration of sulfonic acid groups in PSSA is responsible for the high ionic exchange capacity of the composite membranes, reaching 2.25 mmol g(-1) for a composite with 83 wt % PSSA/PEGDA. The through-plane protonic conductivity of the best membrane is in excess of 0.1 S cm(-1) at 94 °C and 98% relative humidity (RH), decreasing to 0.042 S cm(-1) at 60% RH. These values are comparable or even higher than those of ionomers such as Nafion or polyelectrolytes such as PSSA. This combination of electric and viscoelastic properties with low cost underlines the potential of these nanocomposites as a bio-based alternative to other polymer membranes for application in fuel cells, redox flow batteries, or other devices requiring functional proton conducting elements, such as sensors and actuators.

On the morphology of cellulose nanofibrils obtained by TEMPO-mediated oxidation and mechanical treatment.

The morphological properties of cellulose nanofibrils obtained from eucalyptus pulp fibres were assessed. Two samples were produced with the same chemical treatment (NaClO/NaBr/TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) oxidation), but distinct mechanical treatment intensities during homogenization. It was shown that the nanofibrils production yield increases with the mechanical energy. The effect of mechanical treatment on the yield was confirmed by laser profilometry of air-dried nanocellulose films. However, no significant differences were detected regarding the nanofibrils width as measured by atomic force microscopy (AFM) of air-dried films. On the other hand, differences in size were found either by laser diffraction spectroscopy or by dynamic light scattering (DLS) of the cellulose nanofibrils suspensions as a consequence of the differences in the length distribution of both samples. The nanofibrils length of the more nanofibrillated sample was calculated based on the width measured by AFM and the hydrodynamic diameter obtained by DLS. A length value of ca. 600 nm was estimated. The DLS hydrodynamic diameter, as an equivalent spherical diameter, was used to estimate the nanofibrils length assuming a cylinder with the same volume and with the diameter (width) assessed by AFM. A simple method is thus proposed to evaluate the cellulose nanofibrils length combining microscopy and light scattering methods.

Novel charge transfer supramolecular assemblies with Keggin anions and 2-amino-5-nitropyridine

Several novel compounds with the non-linear optical chromophore 2-amino-5-nitropyridine (2A5NP) and Keggin polyoxoanions (alpha-isomers), having the general formula (2A5NP)(m)H(n)[XM12O40].xH2O, M = Mo, W, were synthesised. Compounds were obtained with X = P, n = 3, m = 3 and 4 and X = Si, n = m = 4 (x = 2-6). Thus, for each of the anions [PMo12O40]3- and [PW12O40]3- two different compounds were obtained, with the same anion and organic counterpart but with a different stoichiometric ratio. These presented different charge transfer properties and thermal stability. All compounds were characterised by spectroscopic and analytical techniques. The single crystal X-ray diffraction structure of (2A5NP)4H3[PMo12O40].2.5H2O.0.5C2H5OH showed that the water solvent molecules and the organic chromophores are assembled via infinite one-dimensional chains of hydrogen bonds with formation of open channels, which accommodate [PMo12O40]3- and ethanol solvent molecules.

Polymorphism in tetra-butylammonium salts of Keggin-type polyoxotungstates

Abstract Some tetra-n-butylammonium salts of Keggin-type polyoxotungstates, namely α-[PW11O39]7−, α-[PW11M(H2O)O39]5−, MII=Co, Ni, α-[PW11CuO39]5−, α-[PW11Fe(H2O)O39]4− and α-[PW11Fe(OH)O39]5−, with the general formula [(C4H9)4N]4Hx[anion]·nH2O, were studied by powder X-ray diffraction. The salts α-[(C4H9)4N]4H3[PW11O39] and α-[(C4H9)4N]4H[PW11CuO39] were obtained in two crystalline phases, a behavior that has been scarcely documented in compounds with Keggin polyoxometalates. One of the phases corresponds to a body-centered cubic unit cell (Im 3 symmetry) and the other has a body-centered tetragonal unit cell (I41 or I4122 symmetry). For the compounds with Cu, the cell parameters of the cubic and tetragonal structures were a=17.675(2) and a=18.775(2), c=14.646(2) A, respectively. The cubic phase was obtained by precipitation from aqueous solution. The tetragonal phase was isolated by crystallization from acetonitrile. The remaining compounds were found to crystallize with one of these two crystal structures. α-[(C4H9)4N]4Hx[PW11M(H2O)O39]·nH2O, with M=Co, Ni, x=1, and M=Fe, x=0, n=1–2, were found to crystallize with the body-centered cubic structure. The method of synthesis used to prepare α-[(C4H9)4N]4H[PW11Fe(OH)O39] afforded the tetragonal phase. The conversion of the cubic into the tetragonal phase and the stability of each one are discussed.

New polyoxometalate-functionalized cellulosic fibre/silica hybrids for environmental applications

Cellulosic fibre/silica hybrid materials functionalized with Keggin-type polyoxometalates ([PV2Mo10O40]5−, [PVMo11O40]4−, ([PMo12O40]3− or [PW12O40]3−) were prepared by a sol–gel method at room temperature. The novel materials are composed of ca. 56 wt% of polysaccharides, ca. 37 wt% of propylamine-modified silica, 2 wt% of polyoxometalate, and 5% of hydration water. The silica network of these hybrids exhibits relatively high degree of condensation being distributed mainly on the surface of the cellulosic fibres as a dense film. The functionalization of silica with polyoxometalates via electrostatic interactions with protonated propylamino groups of modified silica was unambiguously confirmed. Despite their high silica content cellulose/silica hybrids retained basic cellulosic pulp properties—supramolecular and fibrous structure, porosity, relatively low density, etc. The novel bio-based material functionalized with 2% of [PVMo11O40]4− shows particularly high activity towards the oxidation of volatile organic compounds (VOCs) present in urban air thus anticipating future environmental applications.

Oxidation of phenols employing polyoxometalates as biomimetic models of the activity of phenoloxidase enzymes

A kinetic study of the oxidation of substituted phenols with either vanadium(V) polyoxotungstate, [α-SiVVW11O40]5− (viz. SiW11V), or manganese(III) polyoxotungstate, [α-SiMnIIIW11(H2O)O39]5− (viz. SiW11Mn), has been carried out. Because the redox potentials of the polyoxometalate SiW11V and SiW11Mn compounds, viz. POMs, are 0.67 and 0.76 V/NHE, respectively, and those of the phenolic substrates are in the range of 0.4–0.9 V/NHE in water, the resulting oxidation reactions are exoergonic or slightly endoergonic. The reactivity of oxidation of substituted phenols by SiW11V, at 50 °C in buffered (pH = 4) water solution, has been found to correlate with the σ+ parameter of the substituents, yielding an Okamoto–Brown ρ value (i.e., −3.1) consonant with the electronic requirements of a rate-determining electron-transfer route. The negligible value of the solvent kinetic isotope effect (kH/kD = 1.06) obtained for the oxidation of p-MeO-phenol with SiW11V in H2O vs. D2O solution was also in favour of a rate-determining one-electron abstraction from the substrate, followed by fast deprotonation of the intervening radical cation. Additional support to an outer-sphere oxidation with SiW11V was provided by a satisfactory correlation of the reactivity vs. redox potential of the phenols, and by a Marcus analysis of the experimental oxidation data. A sizeable value for the reorganisation energy (λ) was accordingly obtained. The slightly stronger oxidant SiW11Mn resulted to be more reactive than SiW11V by ca. two powers of magnitude. Our scrutiny of the reactive behaviour of the two POMs towards phenols may provide a model of interpretation for the phenoloxidase activity of the laccase enzymes, a class of multicopper oxidases endowed with redox potentials well comparable with those of the two POMs.