Activation Energy Of Dehydration Research Articles

Preparation and Dehydration Kinetics of Complex Sulfadiazine Calcium Hydrate with Both Channel-Type and Coordinated Water

A new hydrate of sulfadiazine calcium (Hydrate I) was discovered, and the crystal structure was determined using single crystal X-ray diffraction. Both channel-type water (9.23 wt %) and calcium-ion coordinated water (11.87 wt %) existed in the unit cell. The thermal stability and dehydration of Hydrate I were investigated by thermal gravimetric analysis, hot stage microscopy, powder X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. A two-step dehydration process was detected from Hydrate I to anhydrous phase (AP) with the intermediate of the less-hydrated form (Hydrate II). The dehydration kinetics of Hydrate I with both channel-type and coordinated water was studied using model fitting method and model free method in isothermal mode. The dehydration activation energy was derived via the Friedman method. Further, the first-step dehydration of Hydrate I was determined to be the 2D phase boundary reaction mechanism, and the second-step dehydration was found to be...

Effect of pulverization on dehydration behavior of crystals of GK-128, a serotonin3 receptor antagonist.

The effect of pulverization on the dehydration behavior of 2-[(2-methylimidazol-1-yl)methyl]benzo[i]thiochromen-1-one monohydrochloride hemihydrate (GK-128), a newly developed serotonin3 receptor antagonist, was studied using powder X-ray diffraction analysis, thermogravimetry (TG), and differential scanning calorimetry (DSC). The crystalline forms of GK-128 obtained by pulverizing with a jet mill and an agale mortar were each confirmed to be the same as that of intact GK-128, since the powder X-ray diffraction patterns of the two prepared samples were identical with that of intact GK-128. However, after pulverization by jet mill, the dehydration temperature of GK-128 was markedly lowered and the DSC endothermic peak due to dehydration disappeared. The activation energy for dehydration, calculated by the Ozawa method using TG data, decreased with decreasing particle size and/or crystallinity of GK-128 crystals, e.g., the activation energies for dehydration of intact and jet-milled GK-128 were 128.1 and 75.9 kj/mol, respectively. The results of comparison of powder X-ray diffraction patterns of pulverized GK-128 and intact GK-128 and of determination of the crystal structure of GK-128 suggested that water molecules could be removed easily along the c-axis of GK-128 crystals.

The physical and chemical properties of the polyvinylalcohol/polyvinylpyrrolidone/hydroxyapatite composite hydrogel

A hydrogel of polyvinylalcohol (PVA)/polyvinylpyrrolidone (PVP)/hydroxyapatite (HA) was prepared by a repeated freezing and thawing technique. The effect of HA on the hydrogel was evaluated by comparing the physical and chemical properties of PVA/PVP/HA and PVA/PVP hydrogels. By using theoretical models, the information about the swelling kinetics and the dehydration kinetics have been obtained. From the analysis of structure, mechanical properties, and molecular interaction, the application of PVA/PVP/HA hydrogel as a biomaterial has been evaluated.Relative to PVA/PVP, the PVA/PVP/HA hydrogel is of denser network structure, lower water content, larger storage modulus, and higher dehydration activation energy. These results reveal that, as HA fills in the hydrogel, the molecular interaction is enhanced, the free space of network is compressed, and the diffusion activation energy of water is increased. In spite of its water content being decreased, it is still in the range of meeting the requirement of bio-application. When the hydrogel is subjected to external forces, the matrix will transfer the load to the HA powder, thus enhance the strength of the hydrogel. For application in bio-materials, HA will still have osteoinductivity because its crystalline structure is not interrupted in PVA/PVP/HA hydrogel environment.

The kinetics of non-isothermal decomposition of the Z/Al(OH)3 mixtures (Z = ZnO or Zn4CO3(OH)6·H2O)

The kinetics of non-isothermal decomposition of ZnO/Al(OH)3 and Zn4CO3(OH)6·H2O/Al(OH)3 has been studied both for the starting mixtures and for mixtures after co-grinding. The process has been controlled by X-ray diffraction, scanning electron microscopy, thermogravimetry and differential scanning calorimetry under air atmosphere (the heating rates were 5, 10 and 15 °C min−1). The activation energy was calculated using the Friedman analysis and Ozawa–Flynn–Wall analysis. It was shown that both methods gave similar results. It was found that the dehydration activation energy of gibbsite to boehmite in the mixture with ZnO gradually is decreased from 150–170 to 100 kJ mol−1. At the final stage (conversion degree was more than 0.9), the dehydration process converted from kinetic mode into the diffusion mode (the activation energy was 50–70 kJ mol−1). It was established that the activation energy of Zn4CO3(OH)6·H2O/Al(OH)3 decomposition at a conversion degree <0.5 (decomposition of basic zinc carbonate to ZnO) has values 130–170 kJ mol−1. For a conversion degree 0.5–0.9 (dehydration of gibbsite to boehmite), the activation energy amounted to 90–130 kJ mol−1. At the final stage of decomposition (degree of conversion above 0.9), the process occurred in the diffusion mode in which the activation energy amounted to 40–70 kJ mol−1. It was revealed that after co-grinding of the ZnO/Al(OH)3 composition, the dehydration process of activation energy has decreased by 10–20 % compared with the original mixture. After the co-grinding of Zn4CO3(OH)6·H2O/Al(OH)3 mixture, the activation energy has increased by 10–15 %. These facts had been explained by changes in the size and defectiveness of the solid-phase particles.

Chemical Heat Storage with LiOH/LiOH·H&lt;sub&gt;2&lt;/sub&gt;O Reaction for Low-Temperature Heat below 373 K

There is a great demand on promotion of heat utilization below 373 K to establish highly-efficient energy system, because such heat is enormously unused and discharged from every process. Towards this demand, we have focused on chemical heat storage due to its high heat storage density. In this study, the promising inorganic hydrates were investigated for low-temperature heat storage with the differential scanning calorimetry. Consequently, it is found that lithium hydroxide monohydrate dehydrates at 337 K with endothermic heat of 1,440 kJ/kg-LiOH・H2O. Due to its high storage density and the simplicity of dehydration reaction, LiOH/LiOH・H2O reaction was chose as the most promising reaction for chemical heat storage below 373 K. From the chemical equilibrium calculation, this reaction system is found to be more suitable for chemical heat storage than chemical heat pump. Fundamental study of dehydration behavior of LiOH・H2O was also performed with a thermogravimetric analyzer, and the apparent activation energy of dehydration of LiOH・H2O was determined to be 51.7 kJ/mol in the conversion ranges of 0.4-0.7.

Effect of Experimental and Sample Factors on Dehydration Kinetics of Mildronate Dihydrate: Mechanism of Dehydration and Determination of Kinetic Parameters

The dehydration kinetics of mildronate dihydrate [3-(1,1,1-trimethylhydrazin-1-ium-2-yl)propionate dihydrate] was analyzed in isothermal and nonisothermal modes. The particle size, sample preparation and storage, sample weight, nitrogen flow rate, relative humidity, and sample history were varied in order to evaluate the effect of these factors and to more accurately interpret the data obtained from such analysis. It was determined that comparable kinetic parameters can be obtained in both isothermal and nonisothermal mode. However, dehydration activation energy values obtained in nonisothermal mode showed variation with conversion degree because of different rate-limiting step energy at higher temperature. Moreover, carrying out experiments in this mode required consideration of additional experimental complications. Our study of the different sample and experimental factor effect revealed information about changes of the dehydration rate-limiting step energy, variable contribution from different rate limiting steps, as well as clarified the dehydration mechanism. Procedures for convenient and fast determination of dehydration kinetic parameters were offered.

Thermal behavior and properties of chitosan fibers enhanced polysaccharide hydrogels

Abstract Composite hydrogel based on chitosan fibers (CSFs), high- and low-acyl gellan gum (HLG) was fabricated. The strong interactions between CSFs and HLG were confirmed by Fourier transform infrared (FTIR) spectra, X-ray diffraction (XRD) and thermogravimetric (TG) analysis. Differential scanning calorimetry (DSC) study shows that the dehydration activation energy decreases as the content of CSFs increases. The rheological measurement showed that the storage modulus of 2.0CSF–HLG (where 2.0 represents the mass ratio between CSF and HLG) hydrogels is 48 kPa at regular frequency of 1 rad/s, which is 1.8 times higher than that of HLG hydrogel. In addition, the SEM images demonstrated that 2.0CSF–HLG hydrogel exhibits three dimensional architectures with HLG attaching to CSFs. A weak swelling capacity of CSF–HLG hydrogels was observed, indicating fibroid CSFs would hinder the movement of HLG chain segments in solution.

Physico-chemical characteristics of gamma irradiation crosslinked poly(vinyl alcohol)/magnetite ferrogel composite

Magnetic field sensitive gels, ferrogels are new promising class of hydrogels. The coupling of hydrogels and magnetic particles has potential application in soft actuators such as artificial muscles or for hyperthermia application. Here a composite of magnetite particles (Fe3O4) and poly(vinyl alcohol) (PVA) hydrogel is developed using gamma irradiation as a crosslinking agent. PVA and Fe3O4 were chosen because of their well-established biocompatibility, radiation crosslinking potential (PVA), mild magnetic properties and the Curie point near desired heating temperature range (Fe3O4). Physico-chemical characteristics of these systems show the effect of the presence of particles in the process of radiation-induced crosslinking. Obtained PVA/Fe3O4 ferrogel composite has greater swelling capacity, activation energy of dehydration and dehydration rate compared to PVA hydrogel, crosslinked under the same conditions.

Investigations on dehydration processes of trisodium citrate hydrates

The dehydration processes of trisodium citrate (Na3C6H5O7) hydrates were investigated using thermogravimetry (TG), differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD). It was found that the temperature of dehydration of trisodium citrate dihydrate was at 430.99 K. For trisodium citrate pentahydrate, there is a two-step dehydration process and the endothermal peaks appear at 337.23 K and 433.83 K, respectively. During the first step of dehydration process, the structure of trisodium citrate pentahydrate changed into the structure of trisodium citrate dihydrate. In addition, the kinetics of dehydration for trisoidum citrate hydrates was also investigated using TG data. According to the activation energies of dehydration calculated by Ozawa equation, it was found that the dehydration mechanisms of the two hydrates were different.

Studies on the dehydration kinetics and rehydration of YAG precursor powder in the hydroxyhydrogel form

YAG precursor powder was prepared in the hydroxyhydrogel form. Dehydration kinetic study and rehydration experiment was carried out to know the behavior of water molecules and hydroxide bonds present in the hydroxyhydrogel network structure with temperature. Rate constants and activation energies for dehydration and dehydroxylation were evaluated by static thermogravimetry. Percent rehydration was determined at different heat treatment temperatures. The results obtained were explained, correlated to establish the thermal stability of hydroxyhydrogel network structure and finally supported by the FTIR analysis.

Thermal analysis of hydrogen titanate nanotubes prepared by potassium dititanate with water vapour treatment

Hydrogen titanate nanotubes were prepared by K2Ti2O5 with water vapour treatment. TG/DTG techniques were used to investigate their water content and dehydration kinetics. It was shown that chemical formula for hydrogen titanate nanotubes is TiO2·0.58H2O which is close to H2Ti2O5. The activation energy of dehydration from hydrogen titanate nanotubes is about 60 kJ/mol according the calculation of Friedman and Flynn–Wall–Ozawa methods. With multivariate non-linear regression calculation, it was found that dehydration of hydrogen titanate nanotubes is a two-step consecutive reaction. The first step is a reaction of nth order with autocatalysis. The second step is a reaction of nth order.

Thermal and spectral features of nickel compounds synthesized within the pores of photonic crystals based on SiO2

Nanoparticles of NiC2O4 · 2H2O were obtained within the interspheric voids of synthetic opal polycrystalline samples based on SiO2. The thermal decomposition of NiC2O4 · 2H2O inclusions within the pores of crystallites was studied by means of thermal gravimetry and differential scanning calorimetry under static conditions in atmospheres of He and air. The efficient activation energy of dehydration (180 ± 10 kJ mol−1) and dehydration enthalpy of nickel oxalate crystalline hydrate (69.9 kJ mol−1) within the pores of photonic crystals based on SiO2 were calculated. Pyrolysis of the nickel oxalate led to the formation of NiO nanoparticles within the voids and on the external surface of the photonic crystallites. The behavior of polycrystalline samples of the photonic crystals in a medium of immersion liquids was studied. It was shown that the partial population of the interspheric pores of photonic crystals with NiC2O4 · 2H2O, NiO, and Ni that diffuse visible light do not lead to a total loss of light conductivity in the immersion medium. The diffuse reflection spectra of a sample of photonic crystals populated with NiC2O4 · 2H2O and NiO phases were recorded. A sample of photonic crystals with inclusions of metallic nickel particles exhibiting magnetic properties was obtained by treating nickel oxide-containing synthetic opal based on SiO2 with molecular hydrogen.

Kinetics of Hydrogenolysis of Glycerol to Propylene Glycol over Cu-ZnO-Al 2O 3 Catalysts

A series of Cu-ZnO-Al 2O 3 catalysts with various metal compositions of Cu/Zn/Al were prepared by the co-precipitation method, and screened for glycerol hydrogenolysis to propylene glycol. The catalyst with a Cu/Zn/Al molar ratio of 1 : 1 : 0.5 exhibited the best performance for glycerol hydrogenolysis, and thus selected for kinetic investigation. Under elimination of external and internal diffusion limitation, kinetic experiments were performed in an isothermal fixed-bed reactor at a hydrogen pressure range of 3.0-5.0 MPa and a temperature range of 493-513 K. Based on a dehydration-hydrogenation two-step hydrogenolysis mechanism, a two-site Langmuir-Hinshelwood kinetic model taking into account competitive adsorption of glycerol, acetol and propylene glycol was proposed and successfully fitted to the experimental data. The average relative errors between observed and predicted outlet concentrations of glycerol and propylene glycol were 6.3% and 7.6%, respectively. The kinetic and adsorption parameters were estimated by using the fourth-order Runge-Kutta method together with the Rosenbrock algorithm. The activation energies for dehydration and hydrogenation reactions were 86.56 and 57.80 kJ·mol −1, respectively.

A compilation and critical evaluation of the literature data on thermal dehydration of hydrated salts

The article provides a detailed compilation of literature data on the thermal dehydration of simple (formed from single cation and anion) hydrated salts consisting of a wide variety of cations and anions or ligands. An attempt has been made to correlate the temperature and apparent activation energy of dehydration with either charge density (z/r) of cation or stability constant (log K) of metal-ligand complex. It is observed that the correlations are complex as they are dependent on the nature of both cation and anion. It is suggested that precise and well-documented data of the dehydration of crystalline hydrated salts are necessary to arrive at meaningful correlation with adequate theoretical basis.

Thermal Decomposition and Dehydration Kinetics of Tetra(piperidinium) Octamolybdate Tetrahydrate in Air

AbstractThermal decomposition of tetra(piperidinium) octamolybdate tetrahydrate, [C5H10NH2]4[Mo8O26]·4H2O, was investigated in air by means of TG‐DTG/DTA, DSC, TG‐IR and SEM. TG‐DTG/DTA curves showed that the decomposition proceeded through three well‐defined steps with DTA peaks closely corresponding to mass loss obtained. Kinetics analysis of its dehydration step was performed under non‐isothermal conditions. The dehydration activation energy was calculated through Friedman and Flynn‐Wall‐Ozawa (FWO) methods, and the best‐fit dehydration kinetic model function was estimated through the multiple linear regression method. The activation energy for the dehydration step of [C5H10NH2]4[Mo8O26]·4H2O was 139.7 kJ/mol. The solid particles became smaller accompanied by the thermal decomposition of the title compound.

Swelling of the Zwitterionic Copolymer Networks and Dehydration of their Hydrogels

AbstractSummary: The polyelectrolyte and antipolyelectrolyte effects on the degree of swelling (Q) and dehydration activation energy (Ea) of homo‐ and copolymer single and double networks, consisting of monomer units with polyelectrolyte and antipolyelectrolyte association abilities are determined and compared. An original correlation between Q and Ea is outlined and explained.

Physicochemical Characteristics of Lanthanum Hydroxide in Hydroxyhydrogel and Conventional Gel Form

Lanthanum hydroxide was prepared by wet interaction through precipitation as well as flash polycondensation, using lanthanum nitrate and ammonium hydroxide. The prepared materials were subjected to various physicochemical characterization. Rate constants and activation energies for dehydration, dehydroxylation and decarboxylation of synthesized materials were determined by dynamic and static thermogravimetry. Rate constants and activation energies were calculated with structural transformations, which were supported by XRD and FTIR analyses. The results showed that the physicochemical properties of the materials differ considerably with respect to the process parameters adopted during synthesis.

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).

Influence of H2on the Gas-Phase Decomposition of Formic Acid: A Theoretical Study

Gas-phase decomposition of formic acid results in final products CO + H2O and CO2 + H2. Experimentally, the CO/CO2 ratio tends to be large, in contradiction with mechanism studies, which show almost equal activation energies for dehydration and decarboxylation. In this work, the influence of H2 on the decomposition mechanism of HCOOH was explored using ab initio calculations at the CCSD(T)/6-311++G**//MP2/6-311++G** level. It was found that, in the presence of H2, the reaction channels leading to CO + H2O are more than those leading to CO2 + H2. With competitive energy, H2 addition to HCOOH can reduce the latter into HCHO, which then dissociates into CO + H2 catalyzed by H2O. Compared to trans-HCOOH, cis-HCOOH and cis-C(OH)2, conformers required for decarboxylation, are less populated due to interactions with H2.

Study on the Kinetics of Non-isothermal Dehydration of Alkaline Earth Metal Selenites

The dehydration kinetics of crystallohydrates of beryllium, magnesium and calcium selenites were studied under non-isothermal conditions. The values of the activation energy of dehydration, the pre-exponential factor in Arrhenius equation and the change of entropy for the formation of the activated complex were calculated using the calculation procedure of Coats and Redfern. Thermal stability and activation energy of dehydration of the crystallohydrates were found to increase by the same order. The relationships observed were interpreted using Klopman’s generalized perturbation theory of chemical reactivity. The same theory was applied to explain the differences in the IR spectra of the selenite crystallohydrates studied.