E. G. Kovaleva

Spin probe study of acidity of inorganic materials

Acidic characteristics of various inorganic materials, such as silica gel, aluminum, and titanium(IV) oxides, original and modified with F− and SO42− ions, in the form of hydrogels and nanostructured powders doped by copper(II) ions, are studied. It is shown that pH values inside pores of the studied sorbents (pHintr) differ from the pH of external solutions by 0.5–1.1 units. In the pores of original γ-Al2O3, the pHintr values are higher, while, in pores of γ-Al2O3 samples modified with F− и SO42−, as well as nonmodified α-Al2O3, SiO2 gel, and TiO2 hydrogels, pHintr are lower compared to the pH of the external solution. It is found that pHintr decreases, as the content of Cu2+ ions in the TiO2 phase grows, and, beginning with a certain moment, it coincides with pH of an equilibrium solution. The pKa values of functional group dissociation in some materials are determined. It is shown that, using spin probes, that is, pH-sensitive nitroxyl radicals, the electrostatic potential of the surface of nanostructured TiO2 can be estimated.

Structure and characteristics of chitosan cobalt-containing hybrid systems, the catalysts of olefine oxidation

Cobalt-containing hybrid organo-inorganic materials based on the chitosan-SiO2, chitosan-Al2O3, and chitosan-cellulose systems were obtained. The surface structure and processes that occur during the formation of metal-containing materials, the catalytic properties of which were studied in the oxidation reactions of alkene, were investigated by EPR spectroscopy using a stable pH-sensitive nitroxyl radical, 4-dimethylamino-2-ethyl-5,5-dimethyl-2-(pyridin-4-yl)-2,5-dihydro-1H-imidazole-1-oxyl, as the adsorbed probe molecules.

Medium acidity and catalytic properties of composite materials based on silica and titania and powder cellulose in the presence of Cu2+ ions

Composite materials (CMs) were synthesized from silica and titania xerogels. The influence of the charged state of the CM surface on complexation with Cu2+ ions and the catalytic activity of the obtained samples in model oxidative dehydrogenation of trimethylhydroquinone with air oxygen was analyzed. Using powder cellulose in the synthesis of CMs increased the specific surface of inorganic dioxides and lowered the negative electric potential of the CM surface. This caused a redistribution of the ratio between different types of copper structures formed in the samples during the copper sorption and led to the catalytic activity of the CMs.

EPR study of copper(II) complexes in the matrix of titania gel modified with powder cellulose

The character of the copper(II) distribution over the surface of titania gel modified with powder cellulose has been studied by EPR. Three types of copper(II) compounds have been identified: mononuclear complexes, magnetic associates, and copper(II) compounds that do not give rise to an observable EPR spectrum at the used frequency and temperature. The modification of titania gel has been shown to have an effect on the relative contents of these compounds.

Investigation of the structure of chitosan hybrid systems by pH-sensitive nitroxyl radical

Hybrid organo-inorganic materials based on the chitosan-SiO2, chitosan-Al2O3, and chitosancellulose systems are prepared. The structure of the surface is studied by means of EPR spectroscopy using the stable pH-sensitive nitroxyl radical, 4-dimethylamino-2-ethyl-5,5-dimethyl-2-pyridine-4-yl-2,5-dihydro-1H-imidazole-1-oxyl as adsorbed probe molecules, and the processes that occur during the formation of hybrid materials are considered.

Effect of the surface charge on the complexing and catalytic properties of Cu2+-containing composite materials based on zirconia and powdered cellulose

Composite materials (CMs) based on zirconia xerogels and powdered cellulose (PC) were synthesized. The effect of the CM surface charge on their complexing properties with respect to Cu2+ ions and on the catalytic activity of the samples in model reactions of the decomposition of H2O2 and the oxidative dehydrogenation of trimethylhydroquinone with atmospheric oxygen was analyzed. It was experimentally proved that the use of PC for the synthesis of ZrO2-based CMs, as in the case of previously studied CMs based on SiO2 and TiO2, leads to an increase in the specific surface area Ssp and a decrease in the negative electric potential of their surface. The surface charge, sorption method, and the composition of the buffer solution used for sorption have a significant effect on the copper(II) content in the CM phase, the distribution pattern, and the composition of the sorbed copper(II) compounds and on the catalytic activity of the synthesized Cu2+-containing CMs.

Structure and Catalytic Properties of a Cu2+-Containing Hydrogel of Zirconium Dioxide

The distribution of Cu2+ ions in ZrO2 and sulfated ZrO2 hydrogel phases was studied by EPR spectroscopy and voltammetry. The formation of the following three types of copper structures was observed: mononuclear Cu2+ complexes (A), magnetic associates (B), and Cu2+ compounds (C) that gave no EPR signals under the conditions used in the spectroscopic measurements. The specific catalytic activity of various Cu2+ compounds in the liquid-phase reaction of 2,3,5-trimethyl-1,4-hydroquinone oxidation was determined. The copper complexes C were found to exhibit the highest catalytic activity.

Characterization of chitin and its complexes extracted from natural raw sources

It is known that the main source of chitin and chitosan are shells of shrimp and other sea crustaceans. Alternative row sources of chitin, chitosan and its complexes are the lowest plants – mushrooms and insects. Also industrial wastes, especially from brewing of beer and manufacture of wine and ethanol can be used for extracting chitosan-containing products. The present research is aimed to the extraction of chitin and its complexes from alternative row sources, such as insects (cockroaches Pariplaneta Americana linnaeus and bees Apis mellifera lineaus), mushrooms (Amanita phalloides and Lactarius subdulius), waste banana wine (Kovibar and Urwibutso Inc.) and beer products (Bralirwa Inc., traditional sorghum) characteristic for Rwanda and their characterization using FTIR spectroscopy and elementary analysis. In chitin and its complexes extraction from all used raw sources, conditions for deproteinization were: 8% NaOH at 95 °C for 1 h and demineralization involved treatment with 6.7 % HCl at room temper...

Sorption of boric acid by polymers containing dihydroxypropylamine fragments

Products of polymer-analogous transformations of chitosan, aminopolystyrene, and polyallylamine containing dihydroxypropylamine fragments were used as sorbents of boric acid. A dependence of their sorption properties on the structure of the main polymeric chain and its functionalization degree was found.

Interfacial Electrostatic Properties of Hydrated Mesoporous and Nanostructured Alumina Powders by Spin Labeling EPR

Acid-base equilibria and interfacial electrostatic properties of hydrated mesoporous and nanostructured alumina powders are determining factors for the use of these materials in heterogeneous catalysis and as a sorption media for filtration and chromatographic applications including life sciences. Here spin probe electron paramagnetic resonance spectroscopy of pH-sensitive nitroxides was employed to evaluate the surface charge and interfacial acid-base equilibria at the pore surface of mesoporous powders of α-Al2O3, γ-Al2O3, Al2O3 × nH2O, and basic γ-Al2O3 and nanostructured Al2O3 in the form of pristine materials and modified with aluminum-tri-sec-butoxide, hydroxyaluminum glycerate, and several phospholipids. A new pH-sensitive nitroxide probe, 4-dimethylamino-5,5-dimethyl-2-(4-(chloromethyl)phenyl)-2-ethyl-2,5-dihydro-1H-imidazol-1-oxyl hydrochloride semihydrate (nitroxide R1), has been synthesized and characterized. It was found that conditions of preparation of alumina powders exert strikingly large effects on the apparent pKa of nitroxides measured from electron paramagnetic resonance titration curves. Specifically, while the electron paramagnetic resonance titrations curves for the nitroxide R1 in mesoporous powders prepared from basic γ-Al2O3 and Al2O3 × nH2O were shifted by ΔpKa≈ +0.6 and up to ≈ +1.2 pH units respectively, the shift for γ-Al2O3 was found to be much higher: ΔpKa = +3.5. Assuming approximately the same ∆pH = 0.5–1.0 arising from a difference in the hydrogen ion activity between the bulk solution phase and that in a confined pore volume, the samples were ranked in the following order of descending magnitude of the effective surface electrostatic potential Ψ: mesoporous γ-Al2O3 > Al2O3 × nH2O > basic γ-Al2O3 > α-Al2O3. Conditions of the Al2O3 synthesis as well as the surface modification procedures were found to have profound effects on the interfacial electrostatic properties of hydrated samples that are likely related to the nature and concentration of the active sites on the alumina surfaces.