Dehydration Mechanism Research Articles

Thermal Dehydration of Magnesium Acetate Tetrahydrate: Formation and in Situ Crystallization of Anhydrous Glass

The kinetics and mechanism of the thermal dehydration of magnesium acetate tetrahydrate were investigated as a typical example of the glass formation process via the thermal decomposition of solids. Formation of an intermediate fluid phase was identified as the characteristic phenomenon responsible for the formation of anhydrous glass. Thermal dehydration from the surface fluid layer regulates the zero-order-like rate behavior of the mass-loss process with an apparent activation energy E(a) ≈ 70-80 kJ mol(-1). Because of variations in the mechanism of release of the water vapor with changes in the reaction temperature range, the mass-loss behavior is largely dependent on the particle size of the sample and heating conditions. The formation of hollow anhydrous glass is the novel finding of the present study. The mechanism of formation is discussed in terms of complementary interpretations of the morphological changes and kinetic behavior of the thermal dehydration. On further heating, the as-produced anhydrous glass exhibits a glass transition phenomenon at approximately 470 K with an E(a) ≈ 550-560 kJ mol(-1), and subsequently crystallizes via the three-dimensional growth of nuclei controlled by diffusion. The crystallization process is characterized by an E(a) ≈ 280 kJ mol(-1) and an enthalpy change ΔH = -13.3 kJ mol(-1), resulting in the formation of smaller, rounded particles of crystalline anhydrate.

Effect of Anions on the Cloud Point Temperature of Aqueous Poly(2-ethyl-2-oxazoline) Solutions

Poly(2-alkyl-2-oxazoline)s have recently gained attention in especially biological applications due to their lower critical solution temperature being close to the body temperature and their biocompatibility. The understanding of how cloud point temperature (T(c)) depends on the salt concentration and the molecular mechanisms responsible for such behavior are important to tune T(c) as desired by the applications. In this paper, we report the effect of a series of sodium salts on T(c) of aqueous poly(2-ethyl-2-oxazoline) (PEOX) solutions by dynamic light scattering. PEOX samples having four different molecular weights were investigated, and the results were compared with those of poly(N-isopropylacrylamide) (PNIPAM), the mostly investigated and used thermoresponsive polymer. Kosmotropic anions decreased T(c) linearly while chaotropic anions increased T(c) nonlinearly with salt concentration. The contributions of different mechanisms to T(c) change have been discussed. Our results indicate that the dominant mechanism is the dehydration of PEOX for divalent kosmotropic anions (CO(3)(2-), SO(4)(2-), S(2)O(3)(2-)) and direct binding for chaotropic anions (NO(3)(-), I(-), ClO(4)(-), SCN(-)). For the remaining monovalent kosmotropic anions (H(2)PO(4)(-), F(-), Cl(-), Br(-)), a combination of dehydration and surface tension mechanisms was in effect. The additional contribution of the surface tension mechanism for the monovalent kosmotropic anions was inferred for different molecular weight PEOX samples and also for PNIPAM. With PEOX molecular weight decreasing from 500,000 to 5000 g/mol, T(c) decreased less with salt concentration which was attributed to the contribution of the surface tension mechanism. For PEOX samples, the decrease of T(c) with kosmotropic anion concentration was faster compared to PNIPAM due to differences in their chemical structure. Our results show that the molecular mechanisms of interactions between PEOX chains and specific anions can simply be inferred from determination of T(c) by a common technique-dynamic light scattering.

Propan-2-ol dehydration on H-ZSM-5 and H-Y zeolite: a DFT study

The catalytic dehydration of propan-2-ol over H-Y and H-ZMS-5 aluminated zeolite models, mimicking both internal cavities and external surfaces, was studied by DFT calculations to investigate the reaction mechanism. After the adsorption of propan-2-ol on the zeolite, the dehydration mechanism starts with alcohol protonation, occurring by one acidic –OH group of the zeolite fragment, followed by a concerted β-elimination to give propene. The catalytic activity is affected by the size of the zeolite cavity, which is larger in the H-Y than in the H-ZMS-5 zeolite. The adsorption energy of the reagent, as an example, decreases in the order: H-Y cavity ≃ H-ZMS-5 surface > H-ZMS-5 cavity, pointing that the adsorption process should preferentially occur either on open surface or inside larger cavity. More interestingly, confinement effects play a twofold role in driving the reaction pathway, resulting in two different effects on the reaction outcomes. The thermodynamic stability, evaluated by the standard free energy difference of the products (water and propene) with respect to the reactant (propan-2-ol), would indeed suggest that the reaction more smoothly could occur for the systems: H-ZMS-5 surface > non-catalyzed > H-Y cavity > H-ZMS-5 cavity. The activation standard free energy of the process conversely decreases in the order: non-catalyzed > H-ZMS-5 surface > H-ZMS-5 cavity > H-Y cavity, suggesting that the reaction is faster inside zeolite cavities. Experimental and computational results are in agreement, giving confidence on the atomistic-level insights provided.

Structures and mechanisms of the dehydration of benzaldoxime over Fe-ZSM-5 zeolites: a DFT study

With considerable academic and industrial attention being focused on green chemical processes, environmentally friendly nanomaterials such as zeolites have been systemically modified and tuned to meet such requirements in various industrial chemical reactions. The dehydration of benzaldoxime over Fe-ZSM-5 zeolite has thus been systematically investigated by means of the ONIOM(M06:UFF) scheme. Three different forms of iron-oxo species are used for the active center of Fe-ZSM-5: (a) [FeO]+, (b) [FeO2]+, and (c) [Fe(OH)2]+. Both the oxidative and polar elimination mechanisms for the dehydration process were investigated. For the oxidative dehydration mechanism, this route begins with the oxygen-end benzaldoxime adsorption complex. The energy barrier is calculated to be 24.5, 31.4, and 33.6 kcal/mol for [FeO]+Z−, [FeO2]+Z−, and [Fe(OH)2]+Z−, respectively. These are significantly lower than the energy barrier found for the polar elimination one. A comparison of the energetic profiles suggest that the dehydration process would take place through the oxidative dehydration mechanism in which the [FeO]+Z− is the most reactive form of mononuclear Fe-ZSM-5 zeolite. Over the [FeO]+Z−, an α-oxygen has the function of migrating the hydrogen of the benzaldoxime and the hydroxyl group of substrate that consecutively leads to bonding with the metal center to produce the [Fe(OH)2]+Z− and the benzonitrile product.

Mechanism of Dehydration–Hydration Processes of Lisinopril Dihydrate Investigated by ab Initio Powder X-ray Diffraction Analysis

Lisinopril is extensively used as an ethical anti-hypertensive drug in its dihydrate crystalline phase, where this phase undergoes dehydration to the metastable monohydrate and anhydrous phases upon being heated. The mechanistic aspects of this two-step dehydration process are successfully established from their crystal structures, determined for the first time by ab initio powder X-ray diffraction (PXRD) analysis. Furthermore, the hydration process of the anhydrous phase was investigated by humidity-controlled PXRD and dynamic vapor sorption. Although the dehydration process of the dihydrate phase proceeded in two steps via the metastable monohydrate phase, the hydration process from the anhydrous phase to the dihydrate phase proceeded in a single step. The mechanisms of these different pathways for the dehydration and hydration processes were also established from the crystal structures.

Interfacial oxidation–dehydration induced formation of porous SnO2 hollow nanospheres and their gas sensing properties

Abstract Uniform porous SnO 2 hollow nanospheres with average diameters of about 100–200 nm have been reproducibly synthesized via a facile template- and surfactant-free hydrothermal method, using hydrogen peroxide 30% and stannous sulfate as precursors. The morphology, composition and structure of the resultant products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy and nitrogen adsorption–desorption technique. Experimental results demonstrated that the formation of these porous SnO 2 nanostructures is ascribed to an interfacial oxidation–dehydration mechanism. H 2 O 2 usage has an important effect on both the morphology and purity of the final products. The gas sensing properties of the as-prepared porous SnO 2 hollow nanospheres were investigated. By comparative gas sensing tests, the porous SnO 2 hollow nanospheres exhibited superior gas sensing performances over commercial SnO 2 nanopowders toward some typical volatile organic compounds (VOCs), implying their promising applications in gas sensors.

Inactivation Mechanism of Glycerol Dehydration by Diol Dehydratase from Combined Quantum Mechanical/Molecular Mechanical Calculations

Inactivation of diol dehydratase during the glycerol dehydration reaction is studied on the basis of quantum mechanical/molecular mechanical calculations. Glycerol is not a chiral compound but contains a prochiral carbon atom. Once it is bound to the active site, the enzyme adopts two binding conformations. One is predominantly responsible for the product-forming reaction (G(R) conformation), and the other primarily contributes to inactivation (G(S) conformation). Reactant radical is converted into a product and byproduct in the product-forming reaction and inactivation, respectively. The OH group migrates from C2 to C1 in the product-forming reaction, whereas the transfer of a hydrogen from the 3-OH group of glycerol to C1 takes place during the inactivation. The activation barrier of the hydrogen transfer does not depend on the substrate-binding conformation. On the other hand, the activation barrier of OH group migration is sensitive to conformation and is 4.5 kcal/mol lower in the G(R) conformation than in the G(S) conformation. In the OH group migration, Glu170 plays a critical role in stabilizing the reactant radical in the G(S) conformation. Moreover, the hydrogen bonding interaction between Ser301 and the 3-OH group of glycerol lowers the activation barrier in G(R)-TS2. As a result, the difference in energy between the hydrogen transfer and the OH group migration is reduced in the G(S) conformation, which shows that the inactivation is favored in the G(S) conformation.

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.

Research on Additives Assisted Catalytic Cyclo-Dehydration of 1, 4-Butanediol to Tetrahydrofuran in Near-Critical Water

We herein report the cyclo-dehydration of 1,4-butanediol (BD) assisted with additives to form tetrahydrofuran (THF) in near-critical water (NCW).Three additives including Fe2(SO4)3,ZnSO4 and NaHSO4 were screened. Effects of various experimental parameters such as temperature (260-340°C), time (60-180min), reactant/water ratio (r/w, 1:10-1:40) and pressure (15-25MPa) on the yield of THF were examined. Without the presence of additives, the results showed that increasing temperature favored the cyclo-dehydration of BD to form THF. The maximum yield (52.61 wt. %) was obtained at temperature of 340°C and time of 180 min. With the presence of additives, at temperature range of 260~300°C and time range of 60~130min, all the additives selected can promote the cyclo-dehydration of BD to yield THF. However, with further increasing temperature and time, the additives would suppress the formation of THF. The catalytic activity toward the production of THF in the order of Fe2(SO4)3> ZnSO4> NaHSO4. The maximum yield of THF assisted with Fe2(SO4)3 could reach as high as 59.85 wt. % at 320°C, 120 min. Large reactant/water ratio would not benefit the yield of THF because of the dilution of additive. Increasing the pressure will slightly increase the yield of THF. Base on experimental results, a possible reaction mechanism and pathway of dehydration of BD was proposed in NCW.

An ONIOM investigation of reaction mechanisms of propylene glycol dehydration over H-ZSM-5 and H-MOR catalysts

The reaction mechanisms for dehydration of propylene glycol (1,2-propanediol) to propanone and propanal over the H-ZSM-5 and H-MOR catalysts have been theoretically studied using ONIOM(MO:MO) approach. The H-ZSM-5 and H-MOR catalysts modeled as 52T and 68T clusters were employed in the ONIOM(B3LYP/6-31+G(d):AM1) method. For the reaction either over the H-ZSM-5 or H-MOR catalysts, four steps for the propylene glycol conversion to propanone and three steps for the conversion to propanal were found. Thermodynamic properties, rate and equilibrium constants of these reactions are reported.

Mechanisms of ethylene glycol carbonylation with carbon dioxide

The direct synthesis of cyclic carbonate from polyol and CO2 is very attractive from both the environmental and carbon utilization points of view. In this work, the direct carbonylation of ethylene glycol and CO2 to ethylene carbonate without catalyst was investigated theoretically by quantum mechanical calculations. Two mechanisms, hydroxyl dehydration and carbonyl substitution, were proposed. Four reaction pathways and the corresponding transition states were identified and the corresponding energy profiles were calculated. All four reaction pathways contain the intramolecular cyclic process, which is the rate-determining step, and the energy barriers of the carbonyl substitution mechanism are lower than that of the hydroxyl dehydration mechanism. Solvent effects calculation indicates that the electrophilic attack of CO2 on the 1-hydroxyl group of ethylene glycol can be promoted by a polar solvent, but for other steps solvent effects are very limited.

Process-induced phase transformation of carbamazepine dihydrate to its polymorphic anhydrates

Phase transformation of carbamazepine dihydrate during low pressure and atmospheric dehydration was investigated for a temperature range between 20 °C and 50 °C. The dihydrate was prepared by crystallisation from a mixture of ethanol and water, prior to dehydration studies at vacuum pressure (10− 3 to 102 Torr), 5% to 60% relative humidity (RH) and 5% to 90% solvent partial pressure (P/P0). During vacuum pressure dehydration, the dihydrate collapsed to the amorphous form undergoing quick liberation of water of hydration, before converting to a mixture of a metastable anhydrous C-monoclinic polymorph (major component) and a P-monoclinic polymorph (minor component) (Fig. 1). Humidified dehydration (≥ 30 °C) is shown to result in nucleation to a metastable triclinic polymorph, while organic solvent vapour-mediated phase transformation causes reversion to the most stable P-monoclinic anhydrate. The role of water and organic solvent during the dehydration process is investigated and discussed (in context with/with respect to) the molecular mobility and relaxation period required to initiate the nucleation of the more stable anhydrates. It is concluded that the dehydration mechanism itself as well as the metastability of the polymorphic anhydrate product eventually formed are determined by the accessibility of solvent vapour during the dehydration process.

Temperature-Dependent, High-Resolution Magic-Angle-Spinning (HRMAS) NMR Studies of Poly(N-isopropylacrylamide-co-acrylic Acid)

High-resolution magic-angle-spinning NMR spectroscopy is used to investigate the phase transition of poly(N-isopropylacrylamide-co-acrylic acid) hydrogels crosslinked with N,N’-methylene bisacrylamide, hereafter poly(NIPAAm-co-AAc). Van't Hoff ΔH and ΔS for polymer dehydration are derived from temperature-dependent NMR spectra, and the thermodynamic data strongly support a four-stage dehydration mechanism for pure poly(NIPAAm). Acrylic acid stabilizes the hydration sphere around the polymer chains. Reduced amounts of water released during the phase transition translates into smaller values for ΔH and ΔS. Enhanced rehydration kinetics for poly(NIPAAm-co-AAc) is attributed to water remaining in the samples at elevated temperatures, which may produce facile diffusion pathways and enable faster rehydration kinetics than poly(NIPAAm).

Sodium ibuprofen dihydrate and anhydrous

(R,S)-(±)-ibuprofen sodium salt (racemate) dihydrate (SID) was dehydrated and the physicochemical properties of SID and the anhydrous forms (SIA) were compared by different analytical techniques (scanning electron microscopy, helium pychnometry, differential scanning calorimetry, X-ray powder diffractometry). The dehydration of SID, followed by thermogravimetry in isothermal conditions, allowed to calculate the activation energy of the dehydration process and to predict the mechanism of dehydration. Dehydration occurred in one step and the activation energy was rather low, indicating the ease of water removal from the crystal. The mechanism of dehydration followed a three dimensional diffusion (Jander equation). Similarly to the dehydration, the hydration process was followed under isothermal conditions by exposing the anhydrous powder at 64% RH at different temperatures. The mechanism of hydration was governed by a two dimensional diffusion and the energy associated to the process was very low, indicating the ease of crystal hydration. The driving force for the hydration is higher than that for the dehydration. From a thermodynamic point of view this fact may explain why the hydrated form is more stable than the anhydrous one. Solubilities, determined at different temperatures in water and in phosphate buffer (pH 6.8), showed that SID is more soluble in water than SIA for temperatures higher than approximately 283 K. On the contrary, in phosphate buffer SIA is always more soluble than SID in the temperature range considered for the experiments. Drug release reflects the solubility in water and phosphate buffer previously reported.

Gastric lactobezoar - a rare disorder?

Gastric lactobezoar, a pathological conglomeration of milk and mucus in the stomach of milk-fed infants often causing gastric outlet obstruction, is a rarely reported disorder (96 cases since its first description in 1959). While most patients were described 1975-1985 only 26 children have been published since 1986. Clinically, gastric lactobezoars frequently manifest as acute abdomen with abdominal distension (61.0% of 96 patients), vomiting (54.2%), diarrhea (21.9%), and/or a palpable abdominal mass (19.8%). Respiratory (23.0%) and cardiocirculatory (16.7%) symptoms are not uncommon. The pathogenesis of lactobezoar formation is multifactorial: exogenous influences such as high casein content (54.2%), medium chain triglycerides (54.2%) or enhanced caloric density (65.6%) of infant milk as well as endogenous factors including immature gastrointestinal functions (66.0%), dehydration (27.5%) and many other mechanisms have been suggested. Diagnosis is easy if the potential presence of a gastric lactobezoar is thought of, and is based on a history of inappropriate milk feeding, signs of acute abdomen and characteristic features of diagnostic imaging. Previously, plain and/or air-, clear fluid- or opaque contrast medium radiography techniques were used to demonstrate a mass free-floating in the lumen of the stomach. This feature differentiates a gastric lactobezoar from intussusception or an abdominal neoplasm. Currently, abdominal ultrasound, showing highly echogenic intrabezoaric air trapping, is the diagnostic method of choice. However, identifying a gastric lactobezoar requires an investigator experienced in gastrointestinal problems of infancy as can be appreciated from the results of our review which show that in not even a single patient gastric lactobezoar was initially considered as a possible differential diagnosis. Furthermore, in over 30% of plain radiographs reported, diagnosis was initially missed although a lactobezoar was clearly demonstrable on repeat evaluation of the same X-ray films. Enhanced diagnostic sensitivity would be most rewarding since management consisting of cessation of oral feedings combined with administration of intravenous fluids and gastric lavage is easy and resolves over 85% of gastric lactobezoars. In conclusion, gastric lactobezoar is a disorder of unknown prevalence and is nowadays very rarely published, possibly because of inadequate diagnostic sensitivity and/or not yet identified but beneficial modifications of patient management.

Polymorphs and a Hydrate of Furosemide–Nicotinamide 1:1 Cocrystal

Five anhydrous polymorphs (forms I–V) and one hydrate of furosemide–nicotinamide 1:1 cocrystal were discovered, and their solid-state properties were characterized using X-ray powder diffraction and differential scanning calorimetry. The crystal structures of forms I–IV were determined from the X-ray powder diffraction data and showed the structural differences between forms, which are mainly attributable to molecular conformations and supramolecular synthons. The slurry conversion experiments revealed that the order of thermodynamic stability of the polymorphs at 25 °C is I > III > II > V > IV. Dynamic vapor sorption analysis and X-ray single-crystal structure determination of the hydrate were conducted to study the dehydration mechanism. We observed structural similarities between the hydrate and its dehydrate, form IV, such as lattice parameters (except the a-axis length), synthons between furosemide and nicotinamide molecules, and the molecular conformation of furosemide; after dehydration, however, the a-axis contracted and nicotinamide molecules were displaced, along with the pyridine ring twisting.

Effect of Particle Size and Morphology on the Dehydration Mechanism of a Non-Stoichiometric Hydrate

A hydrated form of 7-methoxy-1-methyl-5-(4-(trifluoromethyl)phenyl)-[1,2,4]triazolo[4,3-a]quinolin-4-amine (designated Form B) exhibits moisture sorption behavior that is very strongly affected by particle size and morphology. When studied pre- and post-micronization, the simple rate of dehydration at ambient temperature is faster by >2 orders of magnitude after micronization. Complementary techniques were employed to understand this behavior including environmental X-ray powder diffractometry (XRPD), gravimetric vapor sorption (GVS), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), hot-stage microscopy (HSM), single-crystal X-ray diffractometry (SCXRD), and solid-state nuclear magnetic resonance (SSNMR). Solid-state kinetics analysis of thermal data revealed that dehydration of the nonmicronized material follows a two-step consecutive reaction with the first step being a diffusion limited reaction and the second step being a first order reaction, whereas the micronized material ...

Thermal properties and microstructure of gypsum board and its dehydration products: A theoretical and experimental investigation

SUMMARYThis article addresses the thermal properties and the microstructure of gypsum boards produced from β‐hemihydrate by studying its dehydration process.Dehydration reaction of gypsum boards was investigated from both micro and macro levels, employing thermogravimetric analysis and a ventilated oven. A new dehydration mechanism of gypsum was proposed. The thermal properties of gypsum board such as enthalpy of reaction, specific heat capacity, and thermal conductivity were studied by differential scanning calorimetry and a heat transfer analyzer, respectively. The effect of void fraction and dehydration on thermal physical properties of gypsum board was investigated. The microstructure of the gypsum board at high temperature was studied with experiments and modeling. A model was proposed to describe the microstructure of a dehydrated system at high temperature, and experiments indicate its validity. Furthermore, the mechanical properties of gypsum at high temperature were investigated, and the effect of water content was discussed as well. Copyright © 2011 John Wiley & Sons, Ltd.

Dehydration of xylose over sulfated tin oxide catalyst: Influences of the preparation conditions on the structural properties and catalytic performance

Various sulfated metal oxides were tested as solid acid catalyst for the dehydration of xylose to furfural under milder conditions. On the basis of our findings that sulfated tin oxide exhibited the highest catalytic activity, the effects of the content of SO 4 2− group and the calcination temperature on the structural properties and catalytic performance were investigated, and the reusability of the sulfated tin oxide catalyst was evaluated. The acid property on the sulfated tin oxide catalyst was characterized by in situ FT-IR observations of the CO-adsorbed sample. Finally, the reaction mechanism of dehydration of xylose over the SO 4 2−/SnO 2 catalyst was considered.

Effect of fluorine additions on the stability of Mg3Si4O10(OH)2: Computer simulation

The computer simulation of the effect of dopant F atoms on the stability of Mg3Si4O10(OH)2 has been carried out using the density functional theory and pseudopotential method. It has been shown that the mechanism of dehydration is implemented by removing first hydrogen, and then (from the same OH positions) oxygen atoms. Incorporation of doped F atoms into the lattice leads to an increase in the O and H binding energies.