The aim of this work is to highlight the importance of controlling the residual water vapour pressure above the sample as
well as the rate of the thermal decomposition during the thermal dehydration of cerium cyclotriphosphate trihydrate CeP3O9·3H2O.
For this reason, the dehydration of the titled compound was followed by both techniques: the constant rate thermal analysis
at PH2O = 5 hPa and the conventional TG-DTA in air.
It has been shown that the pathway of the thermal dehydration depends strongly on the nature of atmosphere above the sample.
However, in air atmosphere CeP3O9·3H2O decomposes in two well defined steps to give first an amorphous, phase in the temperature range 440–632 K, then the cerium
polyphosphate Ce(PO3)3 crystallizing in orthorhombic system (C2221) at T>632 K. Whereas decomposition carried out at 5 hPa water vapour pressure, also occurring in two steps, leads first to a crystallized
intermediate monohydrate at 259<T<343 K and second to a crystallized anhydrous cerium polyphosphate, at 343<T<791 K, with a structure different from those of all lanthanide polyphosphate known actually and particularly from that of
Ce(PO3)3 obtained in air.
The activation energy corresponding to the dehydration of the initial phosphate was also measured experimentally by means
of two CRTA curves and was found equal to 81±5 kJ mol−1.
Authors:Y. Erten, A. Güneş-Yerkesikli, A. Çetin, and F. Çakιcιoǧlu-Özkan
In this study, NaX synthetic zeolite was modified by following the conventional cation exchange method at 70°C. 82, 81, 79
and 48% of sodium were exchanged with Li+, K+, Ca2+ and Ce3+, respectively. Thermal analysis data obtained by TG/DSC was used to evaluate the dehydration behavior of the zeolites. The
strongest interaction with water and the highest dehydration enthalpy (ΔH) value were found for Li-exchanged form and compared with the other forms. The temperature required for complete dehydration
increased with decreasing cation size (cation size: K+>Ce3+>Ca2+>Na+>Li+). CO2 adsorption at 5 and 25°C was also studied and the virial model equation was used to analyze the experimental data to calculate
the Henry’s law constant, Ko and isosteric heat of adsorption at zero loading Qst. Ko values decreased with increasing temperature and the highest Qst was obtained for K rich zeolite. It was observed that both
dehydration and CO2 adsorption properties are related to cation introduced into zeolite structure.
The catalytic properties of a novel MFI-type zeolite with different SiO2/Al2O3 ratio in the dehydration of glucose to levulinic acid (LA) were investigated in this work. The results demonstrate the strength
of acidic sites and the mesoporosity of the zeolites have significant effects on LA formation.
The effect of the water vapor pressure on the thermal dehydration of manganese(II) formate dihydrate was studied by means
of isothermal gravimetry under various water vapor pressure, ranging from 4.6 to 24.4 torr.
The kinetics of dehydration was described by a two-dimensional phase-boundary model,R2. The rate of dehydration decreased with increasing atmospheric water vapor pressure, but the Smith-Topley phenomenon was
not observed for the present dehydration. The activation energy and the frequency factor for the dehydration were 110–170
kJ·mol−1 and 1010–1016 cm·s−1, respectively. These values increased with increasing water vapor pressure, and were much larger than those reported for
the dehydration in vacuum.
Reliable kinetic information for thermal analysis kinetic triplets can be determined by the comparative method: (1) An iterative
procedure or the KAS method had been established to obtain the reliable value of activation energy Ea of a reaction. (2) A combined method including Coats-Redfern integral equation and Achar differential equation was put forward
to confirm the most probable mechanism of the reaction and calculate the pre-exponential factor A. By applying the comparative
method above, the thermal analysis kinetic triplets of the dehydration of CaC2O4H2O were determined, which apparent activation energy: 813 kJ mol-1, pre-exponential factor: 4.51106-1.78108 s-1, the most probable mechanism function: f(α)=1 or g(α)=α, which the kinetic equation of dehydration is dα/dt=Ae-Ea/RT.
An increase of the specific surface area of solid phases is often desirable e.g. for the bioavailability of pharmaceuticals or in chemical processes. Such an increase can a.o. be achieved by suspending crystalline substances in a solvent that induces phase transformations. Hence, the original substance has to be in a metastable state in the solvent. If the stable phase after transformation has in addition a very low solubility in the solvent, a dendritic growth is forced to occur because of the high local supersaturations during the phase change. This dendritic growth of the stable phase in term leads to needle- or whisker-like crystals, which have the desired larger specific surface area in comparison to the initial crystalline substance.In order to investigate this phenomenon several hydrates of salts were chosen, which undergo phase transformations to their anhydrates accompanied by a corresponding loss of crystal water when suspended in excess in lower alcohols. Consequently, anhydrous forms were created by dehydrating these hydrates. The transformation rate or in this case the dehydration level can thus be indirectly measured by Karl-Fischer titration. The thermodynamic background of the dehydration phenomena can be clarified by solubility studies of the hydrates and anhydrates in water/alcohol-mixtures.
Authors:K. Mouaïne, P. Becker, and C. Carabatos-Nédelec
Differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) of lithium formate monohydrate (LiHCOOH2O) were performed in the temperature range 300–700 K. The DSC/TG measurements show that the dehydration process to anhydrous
lithium formate (LiHCOO) is complex and occurs in two stages. The data are correlated to the structure and to the arrangement
of the molecules in the crystal, including the hydrogen-bonding. Infrared transmittance and Raman spectra of this crystal
are reported and commented on.
Extraction coefficients for all lanthanides have been determined in two systems: 0.2M TBP-3M NaNCS, and 3.6M TBP-0.2M NaNCS. The data have been used for the calculation of relative changes in thermodynamic functions accompanying the investigated extraction process. The compensation of enthalpy and entropy changes is found as a result of dehydration of the lanthanide aquaions.
The analysis of dehydration of the complexes, [La(C8H8NO3)3.2H2O] and [Yb(C8H8NO3)3.3H2O] for the evaluation of kinetic parameters (Z, E &δS*) and mechanism of dehydration by non-isothermal methods are reported. The complexes decompose in three well defined steps involving random nucleation mechanism. First two steps involving the dehydration and the third step the loss of the ligand moiety. The intermediates formed during decomposition were found to be unstable for carrying out any significant studies.
The derivatographic method was used for the quantitative determination of water content in selected flour and fat products.
Weight losses and thermal effects taking place in the examined products under the influence of heat were read from the obtained
derivatograms. Percentage water contents were calculated from TG curves within the range from 5.6% (rice cereal) to 16.0%
(potato flour), based on the diagrams recorded within the temperature range 20–1000C, narrower temperature ranges of dehydration
and decomposition processes were determined. In addition, the number of stages of thermal decomposition of the examined products
was determined and appropriate kinetic parameters were calculated.
The values of the activation energy (Ea) of dehydration, frequency factor (A) and reaction order (n) were calculated from TG, DTG andT curves within the temperature range from 20 to 90C. For potato flour the following values were obtained:Ea=27.825 kJ/mol,A=1.05105 1/s andn=2.9710−3, for the remaining samples under investigation the activation energyEa is several dozen kJ/mol and the reaction ordern is very low. The obtained data show that the dehydration process under study is mainly associated with evaporation of water
adsorbed on products and to less extent with chemical processes.