Activation Energy Of Dehydration Research Articles

Effect of Salt Type on Hygroscopicity of a New Cephalosporin S-3578

Effect of salt type on hygroscopicity was evaluated using S-3578 salts. The hydration behavior of a sulfate and a nitrate salt of S-3578 were evaluated by powder X-ray diffraction (PXRD), simultaneous measurement of PXRD-differential scanning calorimetry (DSC), moisture sorption analysis, simultaneous measurement of thermogravimetric/differential thermal analyses, and solid state 13C-nuclear magnetic resonance (C-NMR). The sulfate salt incorporated two types of lattice water to form a monohydrate or a trihydrate. Additional water could also be absorbed as channel water to expand the lattice structure. The activation energy for dehydration was very high, probably due to steric hindrance in the lattice structure. The nitrate salt incorporated only one water molecule per compound as the lattice water. The additional water was absorbed as channel water as observed for the sulfate salt. X-ray diffractograms showed little dependence on the salt type under the ambient condition. The hydration number was likely to be affected by the size of the counter acids. The hygroscopicity of S-3578 salts was significantly altered by the salt type. The difference in the amount of the lattice water could be explained in terms of the difference in the molecular size of the counter acids.

Dehydration of isopropyl alcohol used as an indicator of the type and strength of catalyst acid centres

The subject of our studies was determination of the kind and strength of the catalyst acid centres on isopropyl alcohol conversion. The investigations were carried out for two groups of catalysts: typical Lewis acids γ-Al 2O 3 and ZrO 2 and Brønsted acids: H 3PW 12O 40 and H 3PMo 12O 40. Considerable differences between Lewis and Brønsted acids in the conversion of isopropyl alcohol were observed. The influence of Brønsted acid centres was studied for a group of catalysts with different strength: mixtures of two different heteropolyacids. It was observed that the increasing strength of Brønsted acids centres leads to higher catalytic activity but it does not significantly affect the activation energy of dehydration to propene.

Dehydration kinetics of calcium aluminate cement hydrate under non-isothermal conditions

Calcium aluminate cement (containing 75% Al 2O 3), which finds extensive high temperature applications, was synthesized by sintering a mixture of calcined alumina and synthetic calcium carbonate. The cement was hydrated and its kinetics of thermal dehydration was studied under non-isothermal conditions by thermo-gravimetry using the integral approximation model. It was observed that on heating the cement hydrate followed multistage dehydration with different reaction orders at different stages. The activation energies of dehydration for these different stages were compared.

Characterization of dehydration behavior of untreated and pulverized creatine monohydrate powders

Creatine, which is well known as an important substance for muscular activity, is synthesized from amino acids such as glycine, arginine and ornithine in liver and kidney. It then accumulates in skeletal muscle as creatine phosphoric acid. The aim of this study was to understand the dehydration behavior of untreated and pulverized creatine monohydrate at various temperatures. The removal of crystal water was investigated by using differential scanning calorimetry (DSC), X-ray powder diffraction and scanning electron microscopy (SEM). The X-ray diffraction pattern of untreated and pulverized creatine monohydrate agreed with reported data for creatine monohydrate. However, the diffraction peaks of the (1 0 0), (2 0 0) and (3 0 0) planes of pulverized creatine monohydrate were much stronger than those of untreated creatine monohydrate. On the other hand, the diffraction peaks of the (0 1 2) and (0 1 3) planes of untreated creatine monohydrate were much stronger than those of pulverized creatine monohydrate. The dehydration of untreated and pulverized creatine monohydrate was investigated at various storage temperatures, and the results indicated that untreated and pulverized creatine monohydrate were transformed into the anhydrate at more than 30 °C. After dehydration, the particles of untreated and pulverized creatine anhydrate had many cracks. The dehydration kinetics of untreated and pulverized creatine monohydrate were analyzed by the Hancock–Sharp equation on the basis of the isothermal DSC data. The dehydrations of untreated and pulverized creatine monohydrate both followed a zero-order mechanism (Polany–Winger equation). However, the transition rate constant, calculated from the slope of the straight line, was about 2.2–7.7 times higher for pulverized creatine monohydrate than for untreated creatine monohydrate. The Arrhenius plots (natural logarithm of the dehydration rate constant versus the reciprocal of absolute temperature) of the isothermal DSC data for untreated and pulverized creatine monohydrate were linear. The activation energies of dehydration in the 40–60 °C range for untreated and pulverized creatine monohydrate were 15.02 and 10.1 kJ/mol, respectively. Dehydration of untreated creatine monohydrate had a pronounced effect on the particle size of the powder. Compared with pulverized creatine monohydrate, the particle size of untreated creatine monohydrate was significantly decreased by dehydration.

Physicochemical Processes in Pyrolysis and Combustion of Fireproof Polyacrylonitrile and Viscose Fibres

The characteristics of thermooxidative degradation of fibres in the presence of flame retardants, manifested by predominance of cross-linking processes which favors formation of CR over degradation processes, are established. Modification with flame retardants decreases the yield and toxicity of combustible volatile products, the rate of thermolysis, the activation energy of dehydration (for VF) and cyclization (for PAN fibres), and the exothermic effects and reverse heat flow to the polymer. The fibres obtained have OI of 32 vol. % and are resistant to wet treatments with a fireproofing effect.

The parallel dehydrative and dehydrogenative catalytic action of γ-Al 2O 3 pure and doped by MgO : Kinetics, selectivity, time dependence of catalytic behaviour, mechanisms and interpretations

The catalytic test reaction of HCOOH decomposition on pure γ-Al 2O 3 and doped (+ MgO (1% w/w)) was studied at temperatures 275–400 °C. A double dehydrative and dehydrogenative, instead the thought single dehydrative, behaviour of γ-Al 2O 3 was disclosed. The dehydration activity is higher for pure alumina and decreases with operation time while the dehydrogenation activity is higher for doped and increases with time at the higher but decreases at the lower temperatures showing an unusual temperature dependent activation and ageing. Doping and increase of temperature reduce dehydration selectivity. It decreases with operation time at higher but increases at lower temperatures. The Arrhenius kinetic parameters of reactions differ strongly. Activation energy of dehydration seems unaffected and pre-exponential factor slightly affected by catalyst kind and operation time. Parameters concerning dehydrogenation are affected slightly by catalyst kind and strongly by operation time. Based on an atomic scale structure analysis and results, two dehydration and one dehydrogenation coexisting mechanisms stemming from identical initial step of dissociative adsorption of HCOOH as H + and HCOO − were suggested interpreting the results. The observed behaviour is due to changes of surface atomic spacing by doping Mg 2+ and of mean distances of adsorptive pair sites with operation time caused by the second mechanism.

Chain dynamics and stability of the poly(3-methacryloxypropyltrimethoxysilane-co-vinylimidazole)

The chain dynamics and the apparent activation energy of dehydration and decomposition of the ester bond for poly(3-methacryloxypropyl-trimethoxysilane-co-vinylimidazole) copolymer (PMPS–PVI), were examined by high-resolution solid-state 29Si and 13C nuclear magnetic resonance spectroscopy and thermogravimetric analysis (TGA). The 29Si–1H cross-polarization process revealed that the 3-methacryloxypropyltrimethoxysilane (MPS) components species had similar CP efficiency. The spin–lattice relaxation time in rotating frame (T1ρH) of the copolymers indicated that the molecular motion of the imidazole ring was independent of the MPS content. The apparent activation energy of dehydration under nitrogen was dependent on the MPS content, while that of decomposition ester bond was independent of the MPS content.

Dehydration, hydration behavior, and structural analysis of fenoprofen calcium

Fenoprofen calcium (FC) is a nonsteroidal, anti-inflammatory, analgesic, and antipyretic agent. The dehydration behavior of FC dihydrate and the rehydration of the dried FC were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and powder X-ray diffractometry (PXRD). The stoichiometry, the crystal packing arrangement, and water environments in FC dihydrate were determined using single-crystal X-ray diffraction (XRD) analysis. The Arrhenius plot (natural logarithm of the dehydration rate constant versus the reciprocal of absolute temperature) for FC dihydrate from isothermal TGA is not linear. The activation energy of dehydration was 309 kJ/mol in the 50–60°C range and 123 kJ/mol in the 60–80°C range. The difference in activation energy can be explained from the crystal structure data where one water molecule is sandwiched between repeating polar carboxylate groups and the other water is in a slightly less polar region of the crystal. Single-crystal XRD analysis also indicated each calcium ion is coordinated to six oxygens. Two coordinating oxygens are provided by two water molecules and the other four oxygens are provided by the carboxylate group of four separate fenoprofen anions. Each fenoprofen anion, which can provide two oxygens for coordination, is associated with two different calcium ions. Hot-stage PXRD suggested that only a loss of 1 mole of water per mole of FC dihydrate (forming a monohydrate) was required to convert the material to a partially crystalline state. The monohydrate is not completely disordered as evidenced by a strong diffraction peak as well as some weaker peaks in the PXRD pattern. The rehydration of the anhydrous form of FC follows a solution-mediated transformation, prior to crystallizing as the dihydrate. © 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:845–859, 2001

Investigation of solid-state reactions using variable temperature X-ray powder diffractrometry. I. Aspartame hemihydrate.

The object of this study was to demonstrate the applicability of variable temperature X-ray powder diffractometry (XRD) to investigate solid-state reactions using aspartame as a model compound. Aspartame exists as a hemihydrate (ASH) under ambient conditions and converts to aspartame anhydrate (ASA) at approximately 130 degrees C. ASA on further heating to approximately 180 degrees C undergoes decomposition (intramolecular cyclization) to form a diketopiperazine derivative (DKP). The dehydration as well as the decomposition kinetics were studied isothermally at several temperatures. The unique feature of this technique is that it permits simultaneous quantification of the reactant as well as the product. While the dehydration of ASH appeared to follow first-order kinetics, the cyclization of ASA was a nucleation controlled process. The rate constants were obtained at various temperatures, which permitted the calculation of the activation energies of dehydration and cyclization from the Arrhenius plots. The activation energy of dehydration was also calculated according to the method described by Ng (Aust. J. Chem., 28:1169-1178, 1975) and the two values were in good agreement. The study demonstrates that XRD is an excellent complement to thermal analysis and provides direct information about the solid-states of various reaction phases.

Studies of dehydration process for isostructural series of lanthanide(III) 2,6-dihydroxybenzoates

Dehydration kinetics of lanthanide 2,6-dihydroxybenzoates has been studied by thermogravimetry using the model-fitting method of calculation. Several kinetic models gavea good representation of the measured thermogravimetric (TG) curves but the calculation method allowed to propose the most probable dehydration mechanism. The activation energies of dehydration for isostructural 2,6-dihydroxybenzoates of lanthanides are similar for a given kinetic model. A change in the heating rate did not alter the dehydration mechanism. A linear correlation between the activation energy and the heating rate was observed for the systems studied.

TGS CRYSTAL GROWTH POLYMORPHISM AND SURFACE DIFFUSION GROWTH MECHANISM

Four faces of TGS crystal were chosen to study the growth kinetics. BCF spiral growth mechanism for the surface diffusion model was analyzed using the kinetic data of the four faces. We found it is more important that the growth units enter the surface layer to grow after overcoming the dehydration activation energy, than the process of surface diffusion. Two simple crystal forms of TGS spontaneousnucleation crystals were also observed.

Kinetics of dehydration of Ca-montmorillonite

Isothermal thermogravimetric experiments have been carried out to determine the reaction kinetics of the dehydration processes in fuller's earth, a natural Ca-montmorillonite. Dehydration in swelling clays is a complex reaction, and analysis of the thermogravimetric data using empirical rate equations and time-transformation analysis reveals that the nature of the rate controlling mechanism is dependent upon both the temperature regime of the sample as well as the extent of reaction. For fuller's earth, we find that the dehydration kinetics are dominated by a nucleation and growth mechanism at low temperatures and fractions transformed (stage I), but above 90 °C the last stages of the reaction are diffusion controlled (stage II). The activation energy for dehydration during stage I is around 35 kJ · mol−1, whereas the removal of water during stage II requires an activation energy of around 50 kJ · mol−1. These two stages of dehydration are associated with primary collapse of the interlayer (stage I) and movement of water that is hydrated to cations within the interlayer (stage II).

Dehydration kinetics of sodium perborate tetrahydrate to monohydrate in a fluidized-bed drier

Dehydration kinetics of sodium perborate tetrahydrate to monohydrate is investigated at four different temperatures between 314.7 and 331 K in a controlled fluidized-bed drier. During the dehydration, no thermal decomposition of sodium perborate to sodium metaborate is observed. First-order kinetic behavior in this conversion is proved and the activation energy for dehydration is calculated as 80.18 kJ/mol. Effect of temperature on the bulk density of the product is given and compared with theoretical calculations. Change in the bulk density is attributed to two different competitive effects, one is shrinking and the other is puffing. Bulk density of the product can be adjusted by controlling these two parameters.

Thermoanalytical study of the dehydration of cyclodextrin inclusion complexes of clofibric acid

Hydrated inclusion complexes of the hosts β-CD (CD=cyclodextrin), γ-CD and permethylated β-CD with the guest clofibric acid were analysed by TG and DSC methods to characterise their dehydration behaviours. Activation energies for dehydration of the β- and γ-CD clofibric acid complexes, determined by isothermal thermogravimetry, are significantly lower (∼20-25%) than those for the corresponding uncomplexed hydrated CDs. These data can be reconciled with X-ray structural data which show that H2O molecules in the complexes occupy different crystal sites from those occupied in the parent CDs.

Effects of environmental factors on the dehydration of diclofenac HEP dihydrate and theophylline monohydrate

The effects of temperature and humidity on the kinetics of dehydration of diclofenac N-(2-hydroxyethyl)pyrollidine dihydrate (DHEPH) were examined. Dehydration threshold temperature ( T d) was shown to be unsuitable for such investigations. Nine solid state reaction models were applied to the dehydration data. In the temperature range 34–40°C, a diffusion model gave the best fit to the data, dehydration rate varied significantly and activation energies of dehydration ( E aD) were constant (average 240.7 kJ/mol) with respect to change in relative humidity. In the temperature range 42–48°C, best fit to the data was given by a phase boundary model, dehydration rate did not vary with relative humidity change and activation energies of dehydration were constant (average 140.2 kJ/mol). Choice of solid state reaction model within the diffusion or phase boundary types did not significantly affect the derived activation energy values. Previously reported differences in E aD values for theophylline monohydrate (TH) may be explained in terms of the different experimental temperature ranges used. The use of activation energy of dehydration values in the study of factors affecting dehydration and in comparing hydrate physical stability, requires careful consideration of sample factors, relative humidity and temperature range of experimentation.

A Study on the Thermal Properties of Dibasic Calcium Hydrogen Phosphate and Monobasic Calcium Phosphate.

Dibasic calcium hydrogen phosphate (CaHPO 4 . 2H 2 O) and monobasic calcium phosphate (Ca(H 2 PO 4 ) 2 ° H 2 O) are usually mixed with calcium carbonate or phosphoric acid as the raw material to prepare calcium phosphate glass by melting. In this work, the dehydration mechanisms and the activation energies of dehydration of CaHPO 4 . 2H 2 O and Ca(H 2 PO 4 ) 2 . H 2 O are investigated using nonisothermal TA technique and X-ray powder diffraction patterns.

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[f]thiochromen-1-one monohydro-chloride hemihydrate (GK-128), a newly developed serotonin3 receptor antagonist, was studied using powder X-ray diffraction analysis, thermo-gravimetry (TG), and differential scanning calorimetry (DSC). The crystalline forms of GK-128 obtained by pulverizing with a jet mill and an agate 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.

Thermal analysis of hydrated calcium aluminates

Differential scanning calorimeter (DSC) has been used to study the dehydration characteristics of hydrated calcium aluminates such as CA, CA2 and C12A7 where C and A stand for CaO and Al2O3 respectively. Dehydration of CAH10 and C2AH8 (whereH=H2O) occur ∼ at 160–180°C and 200–280°C respectively. These two phases are unstable and ultimately get transformed to AH3 and C3AH6. Dehydration of AH3 and C3AH6 occur between 290 and 350°C and overlap at lower scanning rate. The activation energy for dehydration of the stable AH3 and C2AH6 phases has been found to be 107.16 and 35.58 kJ mol−1 respectively. The compressive strength of the hydrated calcium aluminates has been determined. The result shows that in the case of CA, almost 90% of ultimate strength has been attained in 1 day whereas in CA2, ultimate strength has been attained in 14 days and in C12A7 in 1 day. DSC results have been correlated with the rate of strength developments.

Dehydration of theophylline monohydrate powder — effects of particle size and sample weight

From isothermal thermogravimetric analysis, the dehydration mechanism and activation energy of dehydration of theophylline monohydrate powder were found to be dependent on both the particle size and sample weight. With a large sample weight (i.e., 17.0 mg), the dehydration process was best described by the Avrami-Erofeev equation n = 1 4 ) for both the unfractionated samples (needles of length ≤ 3 mm) and the < 500 μm sieve fraction. In contrast, dehydration for the < 150 μm sieve fraction was consistent with a two-dimensional phase boundary mechanism. However, with a smaller sample weight (i.e. 6.0 mg), dehydration in both the unfractionated samples and the < 150 μm fraction conformed to the same mechanism, the Avrami-Erofeev model n = 1 4 ). The activation energy of dehydration, E a , estimated from these mechanisms was 70–90 kJ mol −1, depending on the mechanism of dehydration, sample weight and particle size. These analyses suggest that both the mechanism and the activation energy of dehydration of a hydrate could be significantly influenced by sample pre-history such as particle size, sample weight, crystal defects and surface characteristics. Therefore, meaningful comparisons of kinetic parameters should relate data acquired under similar experimental conditions.

The solid state dehydration ofdlithium potassium tartrate monohydrate is complete in two rate processes II. The nucleation and growth second reaction and dehydration mechanism

A kinetic and mechanistic study has been undertaken of the nucleation and growth reaction that is the second of the two consecutive rate processes that occur during the dehydration ofdlithium potassium tartrate monohydrate. Electron microscopic examinations of the cleaved surfaces of partly reacted crystals show the development of three-dimensional nuclei that are composed of small crystals of the anhydrous product and above 450 K there is evidence of intranuclear melting. Consistent with this model, the second reaction obeys the Avrami-Erofe’ev equation {[ — ln (1 — α)]1/2=kt}. Overall rates of the dehydrations of single crystals and of crushed powder samples were closely similar. The activation energy for dehydration was 150-160 kJ mol-1for both first (reported in part I, preceding paper) and second reactions and for both single crystal and crushed powder reactants. The addition of product crystallites to the reactant reduced sharply, or eliminated, the induction period to the nucleation and growth process. From consideration of the kinetic characteristics, together with the textural changes observed microscopically, we conclude that the following mechanism very satisfactorily accounts for our results. The first reaction proceeds to the dehydration of all crystal surfaces, representing water losses from a layerca. 10 µm thickness. This deceleratory process occurs initially in a structure resembling that of the reactant but later the increasing water site vacancy concentration results in increasing reactant disorder and possibly includes fusion of the outer layer. When the first reaction water evolution has slowed, recrystallization to the structure of the anhydrous product occurs at a limited number of sites to generate germ nuclei that effectively act as seed crystals for nucleus growth. During the second reaction the reactant—product contact interface is identified as a zone of diffusive water loss, similar to that described for the first reaction. Here, however, the product crystallites promote reorganization of dehydrated material, thereby opening channels for water escape and continually exposing new hydrate surfaces at which dehydration continues. This product recrystallization enables advance of the nucleus interface to be maintained, so that rates of both first and second reactions are subject to control by diffusive loss of water from an active boundary of the reactant. Product reorganization removes the inhibiting character of accumulated product layer by introducing escape channels for water loss so that interface advance continues and, although spasmodic, this migrates forward at a constant average linear rate. The work is of interest because kinetic measurements have been obtained for both of the consecutive rate processes that contribute to the overall reaction. The controls of both are shown to be closely similar. The reaction model proposed here provides insight into the structure of the dehydration interface and the mechanism of water release.