Viorel Sasca

Studies on the Non-Isothermal Decomposition of H3PMo12O40·xH2O and H4PVMo11O40·yH2O

This paper reports a comparative study of the non-isothermal decompositions of the heteropolyacids HPM and HPVM, with structures consisting of Keggin units (KUs). Non-isothermal analysis at low heating rates demonstrated the existence of 4 crystal hydrate species, depending on the temperature. The stability domains of the anhydrous forms of HPM and HPVM were found to be 150–380°C, respectively. Processing of the TG curves obtained at different heating rates by the Ozawa method revealed that the decomposition of anhydrous HPM takes place according to a unitary mechanism, whilst for anhydrous HPVM two mechanisms are observed. Thus, the first part of the constitution water is lost simultaneously with the departure of vanadium from the KU as VO2+, while the second part is lost at higher temperatures as in the case HPM.

THERMAL BEHAVIOUR OF THE HOMOPOLYNUCLEAR GLYOXYLATE COMPLEX COMBINATIONS WITH Cu(II) AND Cr(III)

Homopolynuclear complexes of Cu(II) respectively Cr(III) with the glyoxylate dianion, C2H2O42-, have been studied in non-isothermal regime in air and nitrogen. The results of the non-isothermal analysis performed for the synthesised complexes, Cu(C2H2O4)·0.5H2O, respectively [Cr2(OH)2(C2H2O4)2(OH2)4]·2H2O, correlated with the results of the IR and TG analysis of the compounds obtained by thermal treatment from the initial complexes and the results of the GLC and XR analysis have led to the establishment of the thermal decomposition mechanisms for the two studied complexes. The decomposition mechanisms confirm the stoichiometric and structural formulae proposed for the two synthesised homopolynuclear complexes.

Studies on the thermal decompositions of heteropolynuclear glyoxylates of Cr(III) and Cu(II)

Complexes of Cr(III):Cu(II) with the glyoxylate dianion as ligand were synthesized in the range of cation atomic ratios (0.01–8):1.0.The results of non-isothermal analysis of the synthesized compounds correlated with the results of IR and UV-VIS spectroscopy, and gas chromatography of the volatile products of the decomposition allowed the formulation of a mechanism for the decomposition of the complex with Cr(III):Cu(II)=2:1 and the assumption that the other complexes are mixtures of this with the homopolynuclear complexes of Cr(III) and Cu(II), depending on the ratio of the cations Cr(III):Cu(II).The thermal conversion of the complexes takes place at relatively low temperatures, with partial transformation of the ligand into oxalate and of the oxide mixture into CuCrO4.

Band-gap energy of heteropoly compounds containing Keggin polyanion-[PVxMo12-xO40]−(3+x) relates to counter-cations and temperature studied by UV-VIS diffuse reflectance spectroscopy

The band gap energy (absorption edge energies) of the pure H3[PMo12O40]·13H2O and H4[PVMo11O40]·13H2O, respectively, supported on SiO2 and SiC and some of its NH4+ and Cs+ salts were determined by different methods. The influence of the counter-cations and the temperature on band gap energy was studied. In this purpose, the diffuse reflectance spectra of above mentioned compounds were registered at different temperatures, and it were transposed in the curves of the Kubelka-Munk function vs. wavelength. The band gap energies were determined by processing of low field energy of the ligand-metal charge transfer band (O2− → Mo6+ and O2− → V5+) usually observed between 200 and 400 nm on these curves. In this aim, the Taucs relation was adapted for Kubelka-Munk function use and it was plotted for n = 1/2 (direct transition) and 2 (indirect transition) vs. wave energy (photon energy). The intersection of the curves’ tangent drawn to their point of inflection with horizontal axis gives the band gap energy. The o...

Thermal behavior of the polyoxometalates derived from H3PMo12O40 and H4PVMo11O40

The aim of this study is to investigate the influence of some monovalent counter-ions (NH4+, K+ and Cs+) on thermal behavior of polyoxometalates derived from H3PMo12O40 (HPM) and H4PVMo11O40 (HPVM) by replacing the protons. The IR and UV-VIS-DRS spectra of some acid and neutral NH4+, K+, Cs+ salts, which derived from HPM and HPVM, confirmed the preservation of Keggin units (KU) structure. The X-ray diffraction spectra clearly showed the presence of a cubic structure. The non-isothermal decomposition of studied polyoxometalates proceeds by a series of processes: the loss of crystallization water; the loss of O2 accompanying with a reduction of V5+→V4+ and Mo6+→Mo5+; the loss of constitution water started at 360°C for HPVM salts and 420°C for HPM salts; the decomposition of ammonium ion over 420°C with NH3, N2 and H2O elimination and simultaneous processes of reduction (V5+→ V4+ and Mo6+→ Mo5+ or Mo4+) associating with endothermic effects; reoxidation of Mo5+, Mo4+ and V4+with a strong exothermic effect; destruction of KU to the oxides: P2O5, MoO3 and V2O5 and the crystallization of MoO3.

Adsorption–desorption and catalytic properties of SBA-15 supported cesium salts of 12-molybdophosphoric acid for the dehydration of ethanol

Cesium salts of molybdophosphoric acid with varying amounts of the cations Cs1H2PMo12O40 and Cs3PMo12O40 have been prepared as pure or supported on SBA-15. The adsorption of ethanol and its temperature programmed desorption–TPD using thermogravimetry were studied for SBA-15 supported and unsupported Cs salts. The evolved gases during the adsorption–desorption of ethanol on CsHPM/SBA-15 composites were identified by online mass spectrometry coupled with thermogravimetry. The main products observed in the evolved gases during the desorption of ethanol were C2H4, C3H6 and unreacted ethanol. The amount of acidic sites of the composites was estimated by ammonia adsorption–desorption. The dehydration of ethanol was used to test the catalytic properties of the CsHPM samples incorporated on the silica matrix. SBA15—supported catalysts, appeared to be more active than bulk catalysts in vapor phase dehydration and dehydrogenation of ethanol reactions. The effect of the support leads to higher values of dehydration rates of ethanol as well as higher values of ethylene and acetaldehyde formation rates.

Thermal and spectroscopic analysis of Co(II)–Fe(III) polyglyoxylate obtained through the reaction of ethylene glycol with metal nitrates

The synthesis and thermal and spectroscopic studies of a new CoII–FeIII heteropolynuclear coordination compound are presented. The in situ oxidation product of ethylene glycol plays the role of ligand. Under specific working conditions, the reaction of ethylene glycol with FeIII and CoII nitrates in dilute acid solutions occurs with the oxidation of the former to glyoxylic acid, coordinated to the CoII and FeIII cations as glyoxylate anion (C2H2O42−), with simultaneous isolation of the heteropolynuclear coordination compound. In order to separate and identify the ligand, the synthesized coordination compound, having the composition formula Co4Fe10(L)9(OH)20(H2O)32·14H2O, where L is the glyoxylate anion, has been treated with R–H cationite (Purolite C-100). After the retention of the metal cations, the resulting glyoxylic acid was confirmed by measuring its physical constants, by specific reactions and through spectroscopic methods. The synthesized coordination compound was characterized by physical–chemical analysis, electronic spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD) and thermal analysis. Cobalt ferrite impurified with ferric oxide was obtained following the thermal decomposition of CoII–FeIII polyhydroxoglyoxylate. The oxides obtained through thermolysis were studied by FTIR, XRD, scanning electron microscopy (SEM) and elemental analysis.

Mixed-ligand complexes of iron(II), iron(III), copper(II), and cobalt(II) with pyrazolonic and 2,2′-bipyridine ligands

New mixed-ligand complexes, [M2(BAMP)(bipy)2][MCl4]2, M=Co+2(1), Cu+2(2), [M2(TAMEN)(bipy)2][MCl4]2, M=Fe+2(3), Co2+(4), and [Fe2(TAMEN)(bipy)2][FeCl6]2 (5), where BAMP and TAMEN stand for the Mannich bases N,N′-bis(antipyryl-4-methylene)-piperazine and N,N′-tetra(antipyryl-4-methylene)-1,2-ethane-diamine, respectively, have been obtained and characterized by elemental analyses, conductometric and magnetic susceptibility measurements at room temperature, mass spectrometry, UV-Vis, infrared, and mass spectroscopy, and 1H NMR spectra for the ligands.

The effect of hydration/dehydration on the AC electrical conductivity of H4PVMo11O40 and some of its cesium salts used as catalysts

Changes in AC electrical conductance G of vanadium substituted 12-molybdophosphoric acid and of its (mono)-acid and neutral cesium salts were studied. Depending on the environment, the electrical conductivity is a combination of protonic and electronic conduction, the protonic one dominating at low temperature.

Mononuclear Cu(II) complexes of novel salicylidene Schiff bases: synthesis and mesogenic properties

Two new Schiff base ligands 1 and 2 (where 1 = 4-(2-hydroxybenzilidenamino)-phenyl-4-(decyloxy)-2-(pent-4-enyloxy)benzoate, 2 = 4-(4-(decyloxy)-2-hydroxybenziliden amino)-phenyl-4-(decyloxy)-2-(pent-4-enyloxy)benzoate) and their copper (Cu)(II) complexes have been synthesised and characterised. The derivatives were fully characterised structurally, and their mesomorphic behaviour was investigated by polarised optical microscopyand differential scanning calorimetry. The structure of Cu(II) complex having 1 as ligand (3) was determined by X-ray diffraction. The Schiff base ligands exhibit enantiotropic nematic phases, the Cu(II) complex 4 shows monotropic nematic phase behaviour, while compound 3 does not show mesomorphism.