Blanca Casal

Adsorption of monovalent organic cations on sepiolite; experimental results and model calculations

Adsorption of neutral organic molecules and the monovalent organic cations methylene blue (MB) and crystal violet (CV) to sepiolite was determined experimentally and investigated by an adsorption model. The largest amounts of MB and CV adsorbed were about 4-fold of the cation exchange capacity (CEC) of sepiolite. Consequently, it was proposed that most of the above described adsorption was to neutral sites of the clay. The adsorption model considered combines the Gouy-Chapman solution and specific binding in a closed system. The model was extended by allowing cation adsorption to neutral sites of the clay, in addition to adsorption to negatively charged sites and adsorption to neutral complexes formed from 1 cation adsorbed to a negative surface site. The amount of available neutral sites was determined from the adsorption of the neutral molecule Triton-X 100 (TX100). The model could adequately simulate the adsorption of the neutral molecules TX100 and crown ether 15-crown-5 (15C5) as well as the organic cations. Due to aggregation of MB molecules in solution, their adsorption was somewhat less than that of CV at the larger added concentrations. A consideration of the molecular dimensions of TX100, MB and CV suggested that their adsorption was mostly to external sites of the clay and that their entry to the sepiolite channels was largely excluded. This interpretation is supported by infrared spectroscopy (IR) measurements, which show large perturbations of the peak corresponding to vibrations of external Si-OH groups of the clay and confirm complete occupancy of external sites by MB and CV.

Fe-containing pillared clays as catalysts for phenol hydroxylation

An alternative method for the preparation of mixed Fe–Al-pillared clays (Fe–Al-PILCs) derived from two natural smectites, Wyoming SWy-1 and Tunisia-Gafsa VI (an iron-rich sample) based on the use of a mixture of FeCl3 and chlorhydrol is described. The effect of the pH of the pillaring solution and the Al/Fe ratio are studied in order to assess their influence in the characteristics of the resulting solids: specific surface area, porosity and thermal stability. The behaviour of these solids as catalysts has been checked in the hydroxylation of phenol, under conventional heating or microwave irradiation, this last one allowing better yields and shorter reaction times. The presence of redox centers in the layers or in galleries of the materials, together with a Bronsted acid environment in the galleries of the PILCs, (mixed Fe–Al-PILCs and Al-PILCs derived from the Tunisian clay) induces the hydroxylation of phenol reaching conversions close to 70% under microwave irradiation, even at low reaction time (5 min). These values are comparable or even greater than conversions obtained from other catalysts used for these reactions (i.e. modified MCM).

Microwave decomposition of a chlorinated pesticide (Lindane) supported on modified sepiolites

Inorganic porous materials based on modified sepiolites have been studied as supports for controlled degradation of Lindane, a chlorinated pesticide. In dry media conditions under microwave (MW) irradiation, sepiolite can dehydrohalogenate Lindane to pentachlorocyclohexene (PCCH). Trichlorobenzene (TCB) and PCCH are obtained with NaOH-modified sepiolite and, when nickel oxide is present in the solid, CO2 is also found as a product of the reaction, in different proportions depending on the surface characteristics.

Sepiolite-based materials for the photo- and thermal-stabilization of pesticides

The present work is related to the use of sepiolite for the stabilization of certain photo or thermolabile herbicides by their adsorption on sepiolite-modified materials that act as organo-inorganic supports of the pesticides. The study demonstrates that formulations based on sepiolite containing a cationic dye (thioflavine-T (TFT)) are very effective in the stabilization of a photolabile herbicide (trifluralin (TF)). A modification of the hydrophilic character of the sepiolite surface by adsorption of cationic surfactants enhances the adsorption on the mineral substrate of non-polar pesticides, such as the herbicides alachlor (ACH) or metolachlor (MCH), thus contributing to the decrease of their losses by volatilization.

Ionic conductivity in layer silicates controlled by intercalation of macrocyclic and polymeric oxyethylene compounds

Abstract This contribution concerns intercalation materials based on the formation of intracrystalline polymer—salt complexes obtained by insertion of poly(ethylene oxide) (PEO) and crown-ether compounds in a layer silicate (montmorillonite), containing Na + exchangeable cations in their interlayer space. Polyoxyethylene compounds (crown ethers and PEO) are able to associate interlayer cations modifying dramatically the ionic conductivity of the natural silicate. The new organo–inorganic materials exhibit a two-dimensional structure able to induce an anisotropic character in their electrical properties.

Interlayer adsorption of ammonia and pyridine in V2O5 xerogel

Intracrystalline adsorption of ammonia and pyridine in V2O5·1.5 H2O xerogel, takes place spontaneously with protonation of the bases. Although partial Vv→ VIV transformation in the host lattice has been detected by electron spin resonance spectroscopy, infrared studies show that the intercalation process is due mainly to a proton-transfer reaction involving interlayer water molecules, which denotes the Bronsted-acidic character of the xerogel. Considerations of the symmetry and orientation of the interlamellar species are also reported.

Intracrystalline alkylation of benzoate ions into layered double hydroxides

Several layered double hydroxides (LDHs) containing metals of different nature intercalated with benzoate anions have been synthesised in order to study the O-alkylation reaction of the interlayered species with alkyl halide in dry media conditions. In all the cases, the expected ester was obtained with good yield (>60%). The presence of anionic benzoate in the structure was prefered to obtain high selectivity and yield, in comparison with the use of sodium benzoate supported as a salt. The use of MW irradiation to promote these reactions has permitted the reaction time to be reduced considerably. The influence of water present in the LDH on the ester formation has also been investigated. Characterisation of the LDH support before and after reaction is also presented, showing the influence of the nature of the cations located in the octahedral sheets. The support can be regenerated, with no effect on the reactivity.

Intercalation mechanism of nitrogenated bases into V2O5 xerogel

It has been shown by i.r. and XPS techniques that protonation of nitrogenated bases (ammonia, pyridine and pyrazine) is the major process that occurs on adsorption of these compounds in the intracrystalline environment of V2O5· 1.5 H2O xerogel. At room temperature the intercalation process takes place mainly by a proton-transfer reaction involving protons inherent to the solid and interlayer water molecules. Thermal treatment of the intercalated compounds gives rise to coordination of guest species to vanadium ions included in the host lattice, and VV/–VIV transformations are found to different extents depending on the nature of the intercalated molecules. The protons of the solid and the interlayer water appear to be the origin of the protonic conductivity on the V2O5· 1.5 H2O xerogel, as shown by the impedance spectra. The V5+/V4+ couples formed in the intercalation process produce the electronic conductivity detected when the bases are intercalated in the layered solid.

Proton conductivity in Al-montmorillonite pillared clays

Abstract Pillared clays (PILCs) are materials derived from smectite clay minerals (2:1 phyllosilicates) in which an interlayer formation of metal oxides, acting as pillars, produces a permanent separation between the silicate layers. It is assumed that protons are generated during the thermal treatment necessary for the pillar oxide formation, as it is the case for Al-montmorillonite PILCs. We have applied the electrochemical impedance technique with the aim of studying the proton conductivity of these pillared materials. Enhancement of electrical conductivity has been observed upon adsorption of oxyethylene compounds (crown-ethers and PEO) into the PILC galleries.

MVO5(M = Nb, Ta) mixed oxides: sol–gel synthesis, structural and thermal characterization and electrochemical Li+ insertion

Our studies on the preparation, via the sol–gel techniques, of MVO5(M = Nb, Ta) oxides have been rewarded here with an increased understanding of the synthesis process. The sol–gel method is at the moment the only procedure for producing NbVO5 as a pure phase. We present results concerning the influences of different parameters on the nature of the final products in the V–Nb–O system. Compounds NbVO5 and TaVO5 are isostructural with orthorhombic unit cells. The structural parameters have been refined using the Rietveld method with space group Pnma. Thermal analyses of the precursor xerogels show that the thermal effects depend on the nature of the M cations. The crystallization and decomposition temperatures are lower for NbVO5 than for TaVO5. The electrochemical insertion of lithium ions into the new MVO5(M = Nb, Ta) mixed oxides has also been investigated. For both materials, this process is complex and consists of three main steps. The maximum insertion degrees (x in LixMVO5) achieved after the reduction process are x= 1.95 for NbVO5 and x= 1.57 for TaVO5. The galvanostatic cycling studies show the good rechargeability of the Li+/e– insertion/deinsertion process in the MVO5 oxides.