Thermoanalytical studies of natural potassium, sodium and ammonium alunites

Dynamic and controlled rate thermal analysis (CRTA) has been used to characterise alunites of formula [M(Al)₃(SO₄)₂(OH)₆] where M⁺ is the cations K⁺, Na⁺ or NH₄ ⁺. Thermal decomposition occurs in a series of steps: (a) dehydration, (b) well-defined dehydroxylation and (c) desulphation. CRTA offers a better resolution and a more detailed interpretation of water formation processes via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of water formation reveal the subtle nature of dehydration and dehydroxylation.     

Thermal decomposition of the vivianite arsenates—implications for soil remediation

The presence of arsenate compounds in soils and mineral dump leachates is common. One potential method for the removal of the arsenates from soils is through thermal treatment. High-resolution thermogravimetric analysis has been used to follow this thermal decomposition of selected vivianite arsenates. This decomposition occurs as a series of steps. The first two steps involve dehydration with 6 mol of water lost in the first step and two in the second. The third major weight loss step occurs in the 750-800 °C temperature range with de-arsenation. The application of infrared emission spectroscopy confirms the loss of water by around 250 °C and the loss of arsenic as arsenic pentoxide is observed by the loss of AsO stretching bands at around 826 cm⁻¹. Thermal activation of arsenic contaminated soils may provide a method of decontamination.     

Thermal decomposition of syngenite, K2Ca(SO4)2·H2O

The thermal decomposition of syngenite, K₂Ca(SO₄)₂·H₂O, formed during the treatment of liquid manure has been studied by thermal gravimetric analysis, differential scanning calorimetry, high temperature X-ray diffraction (XRD) and infrared emission spectroscopy (IES). Gypsum was found as a minor impurity resulting in a minor weight loss due to dehydration around 100 °C. The main endothermic dehydration and decomposition stage of syngenite to crystalline K₂Ca₂(SO₄)₃ and amorphous K₂SO₄ is observed around 200 °C. The reaction involves a solid-state re-crystallisation, while water and the K₂SO₄ diffuse out of the existing lattice. The additional weight loss steps around 250 and 350 °C are probably due to presence of larger syngenite particles, which exhibit slower decomposition due to the slower diffusion of water and K₂SO₄ out of the crystal lattice. A minor endothermic sulphate loss around 450 °C is not due to the decomposition of syngenite or its products or of the gypsum impurity. The origin of this sulphate is not clear.     

Thermal analysis of ammonium,mono-, di- and triethanolammonium alginates

Thermal behaviour of ammonium (NH₄alg), mono- (MEAalg), di- (DEAalg) and triethanolammonium (TEAalg) salts of alginic acid (Halg) was investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC). Salts were prepared by the direct reaction of alginic acid with the ammonium hydroxide and with the respective ethanolamines. After preparation the compounds were lyophilized during 24 h and characterized by FTIR spectroscopy and elemental analysis (C, H and N). Under air the compounds exhibited three successive thermal decomposition steps: dehydration, decomposition of the polymeric matrix and finally, burning of carbonaceous residue. Under nitrogen two steps (dehydration and decomposition) were observed. The stability order of this series of compounds was: TEAalg this series of compounds was: TEAalg<DEAalg<NH₄alg<Halg≈MEAalg. DSC curves between –50 and 150°C did not show any thermal events suggesting that after lyophilization probably non-freezing type water is present in the system.     

A thermogravimetric and infrared emission spectroscopic study of alunite

Thermogravimetric and differential thermogravimetric analysis has been used to characterize alunite of formula [K₂(Al³⁺)₆(SO₄)₄(OH)₁₂]. Thermal decomposition occurs in a series of steps (a) dehydration up to 225°C, (b) well defined dehydroxylation at 520°C and desulphation which takes place as a series of steps at 649, 685 and 744°C.The alunite minerals were further characterized by infrared emission spectroscopy (IES). Well defined hydroxyl stretching bands at around 3463 and 3449 cm^?1 are observed. At 550°C all intensity in these bands is lost in harmony with the thermal analysis results. OH stretching bands give calculated hydrogen bond distances of 2.90 and 2.84–7 Å. These hydrogen bond distances increase with increasing temperature. Characteristic (SO₄)^2? stretching modes are observed at 1029.5, 1086 and 1170 cm^?1. These bands shift to lower wavenumbers on thermal treatment. The intensity in these bands is lost by 550°C.     

Oxidative condensation and chemiluminescence of 5-amino-2,3-dihydro-1,4-phtalazinedione

The oxidative condensation of (5-amino-2,3-dihydro-1,4-phtalazinedione) luminol was carried out under their oxidation by (NH₄)₂S₂O₈ and KIO₃ in the mixed water-organic (namely dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) or N-methyl-2-pyrrolidinone (MPD)) solvent under the volume ratio 1:9. The structure of the products was studied by IR and Raman spectroscopy, elemental analysis and the derivatographic method. It was determined that oxidation by KIO₃ (E = 1.085 V) occurs on the amide nitrogen atoms, while in the case of (NH₄)₂S₂O₈ (E = 2.05 V) it occurs on the amino-group. The structure and thermal stability of the obtained products is determined by the nature of the oxidant. The active decomposition of luminol begins at t_d = 334.5 °C, while for the specimens of the polyluminols, which were synthesized in the water-DMSO and water-MPD mixtures, t_d equals 356.7 and 409.1 °C respectively. The worst thermal stability has products of luminol oxidation by KIO₃ (t_d = 282.5 °C). The mechanism of the luminol oxidative polymerization by (NH₄)₂S₂O₈ and KIO₃ has been proposed.     

Thermoanalytical studies of silver and lead jarosites and their solid solutions

Dynamic and controlled rate thermal analysis has been used to characterise synthesised jarosites of formula [M(Fe)₃(SO₄)₂(OH)₆] where M is Pb, Ag or Pb–Ag mixtures. Thermal decomposition occurs in a series of steps. (a) dehydration, (b) well defined dehydroxylation and (c) desulphation. CRTA offers a better resolution and a more detailed interpretation of water formation processes via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of water formation reveal the subtle nature of dehydration and dehydroxylation. CRTA offers a better resolution and a more detailed interpretation of the decomposition processes via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of non-isothermal nature reveal separation of the dehydroxylation steps, since in these cases a higher energy (higher temperature) is needed to drive out gaseous decomposition products through a decreasing space at a constant, pre-set rate.     

Thermogravimetric analysis of organoclays intercalated with the surfactant octadecyltrimethylammonium bromide

Summary High resolution thermogravimetric analysis has been used to study the thermal decomposition of montmorillonite modified with octadecyltrimethylammonium bromide. Thermal decomposition occurs in 4 steps.The first step of mass loss is observed from ambient to 100°C temperature range and is attributed to dehydration of adsorbed water. The second step of mass loss occurs between 87.9 to 135.5°C temperature range and is also attributed to dehydration of water hydrating metal cations such as Na⁺. The third mass loss occurs between 179.0 and 384.5°C; it is assigned to the loss of surfactant. The fourth step is ascribed to the loss of OH units due to dehydroxylation of the montmorillonite and takes place between 556.0 and 636.3°C temperature range. These TG steps are related to the arrangement of the surfactant molecules intercalating the montmorillonite. Changes in the basal spacing of the clay with surfactant are followed by X-ray diffraction. Thermal analysis provides an indication of the stability of the organo-clay.     

Thermal decomposition of hydrotalcite with chromate, molybdate or sulphate in the interlayer

The thermal decomposition of hydrotalcites with chromate, molybdate and sulphate in the interlayer has been studied using thermogravimetric analysis coupled to a mass spectrometer measuring the gas evolution. X-ray diffraction shows the hydrotalcites have a d(0 0 3) spacing of 7.98 Å with very small differences in the d-spacing between the three hydrotalcites. XRD was also used to determine the products of the thermal decomposition. For the sulphate-hydrotalcite decomposition the products were MgO and a spinel MgAl₂O₄, for the chromate interlayered hydrotalcite MgO, Cr₂O₃ and spinel. For the molybdate interlayered hydrotalcite the products were MgO, spinel and MgMoO₄. EDX analyses enabled the formula of the hydrotalcites to be determined. Two processes are observed in the thermal decomposition namely dehydration and dehydroxylation and for the case of the sulphate interlayered hydrotalcite, a third process is the loss of sulphate. Both the dehydration and dehydroxylation take place in three steps each for each of the hydrotalcites.     

The decomposition kinetics of mechanically activated alunite ore in air atmosphere by thermogravimetry

The thermal decompositions of both non-activated and mechanically activated alunite ore have been studied by thermogravimetry (TG). The ore was activated mechanically in an attritor for 15 min and amorphisation in the structure was studied by X-ray diffraction analysis. It can be verified that alunite decomposes in two steps, which are dehydration and desulphation. It was also established that the mechanical activation affected especially on the temperature range of dehydration reaction. The activation energies of dehydration and desulphation reactions have been calculated from the thermogravimetric data at heating rates of 5, 10, 15 and 20 K min⁻¹ involving isoconversional methods of Ozawa and Kissenger-Akahira-Sunose (KAS).     

Thermal decomposition of natural and synthetic plumbojarosites: Importance in ‘archeochemistry’

Plumbojarosite and argentoplumbojarosite were sources of lead and silver in ancient and medieval times. The understanding of the chemistry of the thermal decomposition of these minerals is of vital importance in ‘archeochemistry’. The thermal decomposition of plumbojarosite was studied using a combination of thermogravimetric analysis coupled to a mass spectrometer. Three mass loss steps are observed at 376, 420 and 502 °C. These are attributed to dehydroxylation, loss of sulphate occurs at 599 °C, and loss of oxygen and formation of lead occurs at 844 and 953 °C. The temperatures of the thermal decomposition of the natural jarosite were found to be less than that for the synthetic jarosite. This is attributed to a depression of freezing point effect induced by impurities in the natural jarosite. Raman spectroscopy was used to study the structure of plumbojarosite. Plumbojarosites are characterised by stretching bands at 1176, 1108, 1019 and 1003 cm⁻¹ and bending modes at 623 and 582 cm⁻¹. Changes in the molecular structure during thermal decomposition were followed by infrared emission spectroscopy. The technique shows the loss of intensity in the hydroxyl stretching region attributed to dehydroxylation. Loss of sulphate only occurs after dehydroxylation. Lead is formed at higher temperatures through oxygen evolution.     

The synthesis of transition metal nitrides via thermolysis of metal-ammine complexes, Part I: Chromium nitride

The thermal decomposition of a CrN precursor, hexaammine chromium(II) chloride, in ammonia has been investigated via a combination of thermogravimetric analysis, differential thermal analysis, Fourier transform infrared spectroscopy, and X-ray diffraction. Upon heating, [Cr(NH₃)₆]Cl₂ sequentially loses ammonia ligands, ultimately forming CrCl₂·NH₃ at ∼400 °C. When heat-treated to 500 °C in ammonia, this compound ammonolyzes to form nanocrystalline CrN.     

Mixed oxides obtained from Co and Mn containing layered double hydroxides: Preparation, characterization, and catalytic properties

Co-Mn-Al layered double hydroxides (LDHs) with various Co:Mn:Al molar ratios (4:2:0, 4:1.5:0.5, 4:1:1, 4:0.5:1.5, and 4:0:2) were prepared and characterized. Magnesium containing LDHs Co-Mg-Mn (2:2:2), Co-Mg-Mn-Al (2:2:1:1), and Co-Mg-Al (2:2:2) were also studied. Thermal decomposition of prepared LDHs and formation of related mixed oxides were studied using high-temperature X-ray powder diffraction and thermal analysis. The thermal decomposition of Mg-free LDHs starts by their partial dehydration accompanied by shrinkage of the lattice parameter c from ca. 0.76 to 0.66 nm. The dehydration temperature of the Co-Mn-Al LDHs decreases with increasing Mn content from 180 °C in Co-Al sample to 120 °C in sample with Co:Mn:Al molar ratio of 4:1.5:0.5. A subsequent step is a complete decomposition of the layered structure to nanocrystalline spinel, the complete dehydration, and finally decarbonation of the mixed oxide phase. Spinel-type oxides were the primary crystallization products. Mg-containing primary spinels had practically empty tetrahedral cationic sites. A dramatic increase of the spinel cell size upon heating and analysis by Raman spectroscopy revealed a segregation of Co-rich spinel in Co-Mn and Co-Mn-Al specimens. In calcination products obtained at 500 °C, the spinel mean coherence length was 5-10 nm, and the total content of the X-ray diffraction crystalline portion was 50-90%. These calcination products were tested as catalysts in the total oxidation of ethanol and decomposition of N₂O. The catalytic activity in ethanol combustion was enhanced by increasing (Co+Mn) content while an optimum content of reducible components was necessary for high activity in N₂O decomposition, where the highest conversions were found for calcined Co-Mn-Al sample with Co:Mn:Al molar ratio of 4:1:1.     

Hydroxylammonium fluorometalates: Synthesis and characterisation of a new fluorocuprate and fluorocobaltate

The first layered hydroxylammonium fluorometalates, (NH₃OH)₂CuF₄ and (NH₃OH)₂CoF₄, were prepared by the reaction of solid NH₃OHF and the aqueous solution of copper or cobalt in HF. Both compounds crystallize in monoclinic, P2₁/c, unit cell with parameters: a = 7.9617(2) Å, b = 5.9527(2) Å, c = 5.8060(2) Å, β = 95.226(2)° for (NH₃OH)₂CuF₄ and a = 8.1764(3) Å, b = 5.8571(2) Å, c = 5.6662(2) Å, β = 94.675(3)° for (NH₃OH)₂CoF₄, respectively. Magnetic susceptibility was measured between 2 K and 300 K giving the effective Bohr magneton number of 2.1 for Cu and 5.2 BM for Co. At low temperatures both complexes undergo a transition to magnetically ordered phase. The thermal decomposition of both compounds was studied by TG, DSC and X-ray powder diffraction. The thermal decomposition of (NH₃OH)₂CuF₄ is a complex process, yielding NH₄CuF₃ as an intermediate product and impure Cu₂O as the final residue, while (NH₃OH)₂CoF₄ decomposes in two steps, obtaining CoF₂ after the first step and CoO as the final product.     

Thermal behavior of aluminum powder and potassium perchlorate mixtures by DTA and TG

In this work the thermal decomposition characteristics of micron sized aluminum powder + potassium perchlorate pyrotechnic systems were studied with thermal analytical techniques. The results show that the reactivity of aluminum powder in air increases as the particle size decreases. Pure aluminum with 5 μm particle size has a fusion temperature about 647 °C, but this temperature for 18 μm powder is 660 °C. Pure potassium perchlorate has an endothermic peak at 300 °C corresponding to a rhombic-cubic transition, a fusion temperature around 590 °C and decomposes at 592 °C. DTA curves for Al₅/KClO₄ (30:70) mixture show a maximum peak temperature for thermal decomposition at 400 °C. Increasing the particle size of aluminum powder increases the ignition temperature of the mixture. The oxidation temperature increased by enhance in the aluminum content of the mixture.     

Reaction kinetics of the microwave enhanced digestion of zircon with ammonium acid fluoride

Zircon is notorious for its chemical inertness. Extreme processing conditions such as alkaline fusion (NaOH at 600 °C or Na₂CO₃ at 1200 °C) are used to extract the zirconium values from the mineral. In this study zircon was treated with ammonium acid fluoride (NH₄F·1.5HF) by means of microwave assisted digestion. Reaction times ranged from 15 to 260 min at temperatures between 120 °C and 240 °C. Successive microwave digestion steps, interrupted by an aqueous wash procedure, resulted in a >99% conversion of zircon to the water soluble intermediates (NH₄)₃ZrF₇ and (NH₄)₂SiF₆. Arrhenius rate laws are derived for both reaction control (progressively shrinking particle) and diffusion control by the product layer. Both models show reasonably good agreement with the experimental data. The derived diffusion coefficient corresponds to a solid-liquid case.     

Study of the decomposition of aqueous citratoperoxo-Ti(IV)-gel precursors for titania by means of TGA-MS and FTIR

An aqueous solution-gel route is developed for the preparation of TiO₂. In this report, we study an aqueous citratoperoxo-Ti(IV)-precursor at pH 2.0, which is compatible with polyvinyl alcohol (PVA) and therefore can be applied for the preparation of a thick mesoporous TiO₂ film.With regard to deposition of films, it is important to gain insight in the behaviour of the precursor during thermal treatment. Therefore, the thermal decomposition mechanism of a citratoperoxo-Ti(IV)-gel and a PVA modified citratoperoxo-Ti(IV)-gel is studied. Weight losses and evolved gasses are characterized by TGA-MS (5 °C min⁻¹), while gel structure and changes in the solid upon heating are studied by means of FTIR. For both gels, decomposition in dry air can be divided into five regions. After drying of the samples in the first region (∼100 °C), decomposition of the organic matter not coordinated to the metal ions occurs (∼200 °C). The third region (∼310 °C) involves the decomposition of citrato ligands. Finally, the residual organic matter is combusted in the last two regions. Only in dry air it is possible to fully remove the organic matrix in both gels at temperatures below 600 °C.It is also proven that the citratoperoxo-Ti(IV)-complexes, seen at pH 7.0, exist in the precursor gel already at pH 2.0.     

Thermal reactions of guaiacol and syringol as lignin model aromatic nuclei

Thermal reactions of guaiacol (2-methoxyphenol) and syringol (2,6-dimethoxyphenol) were compared in a closed ampoule reactor (N₂/400-600 °C/40-600 s) to obtain information on the thermal reactivities of lignin aromatic nuclei, guaiacyl and syringyl types. For both compounds, the O-CH₃ bond homolysis, which was observed at >400 °C, initiated their decomposition. This homolysis was followed by several temperature-dependent reactions; radical-induced rearrangement to convert the aromatic OCH₃ to aromatic CH₃ structures and condensation into high molecular weight (MW) products were the next steps (≈400 °C); then, coke formation became extensive (≈450 °C); effective gas formation required higher temperature such as >550-600 °C. The syringol- and guaiacol-characteristic GC/MS-detectable low MW products were explained with the above mentioned reactions. As for the difference between guaiacol and syringol, coke and gas (especially CH₄ and CO₂) formation was more extensive in syringol. This effective coking can be explained by the influence of the additional OCH₃ group in syringol, which doubles the opportunity for coke formation. This, in turn, reduces the yields of GC/MS-detectable low MW products. Demethoxylation to form guaiacol was also observed in syringol, even though the reactivity was not so high. These reactions are discussed at the molecular level.     

Dehydration of synthetic and natural vivianite

Thermal analyses of synthetic and natural vivianite (Fe²⁺)₃(PO₄)₂·8H₂O) were determined using a high-resolution thermal analyser coupled to a mass spectrometer.Five dehydration weight loss steps were observed for the natural vivianite at 105, 138, 203, 272 and 437 °C. The first weight loss step involves the reaction (Fe²⁺)₃(PO₄)₂·8H₂O→(Fe²⁺)₃(PO₄)₂·3H₂O+5H₂O. The TGA/MS for the synthetic vivianite gave similar results to that of the natural sample. Mass spectrometry shows that water is lost up to 450 °C and after this temperature oxygen is lost. Changes in the structure of vivianite were followed using infrared emission spectroscopy. A model is proposed for the dehydration of vivianite.     

Thermal dehydration of lithium metaborate dihydrate and phase transitions of anhydrous product

Reaction steps and mechanisms of the thermal dehydration of lithium metaborate dihydrate were investigated by means of thermoanalytical measurements, high temperature powder X-ray diffractometry, FT-IR spectroscopy, and microscopic observations. The first half of thermal dehydration was characterized by the melting of the sample producing viscous surface layer, the formation of bubbles on the particle surfaces, and the sudden mass-loss taking place by an opportunity of cracking and/or bursting of the bubble surface layer. The second half of the dehydration with a long-tailed mass-loss process in a wide temperature region was divided further into three distinguished reaction steps by the measurements of controlled rate thermal analysis. During the course of the thermal dehydration, four different poorly crystalline phases of intermediate hydrates were observed, in addition to an amorphous phase produced by an isothermal annealing. Just after completing the thermal dehydration, an exothermic DTA peak of the crystallization of β-LiBO₂ was appeared at around 750 K. The phase transition from β-LiBO₂ to α-LiBO₂ was observed in the temperature range of 800-900 K, which subsequently melted by indicating a sharp endothermic DTA peak with the onset temperature at 1101.4 ± 0.6 K.