Synthesis of arsenic sorbent by the reaction of magnesium hydroxide with aqueous iron(III) chloride solution

The reaction of magnesium hydroxide with a concentrated aqueous solution of iron(III) chloride yields a mixture of magnesium–iron layered double hydroxide and iron oxide–hydroxide in the akaganeite form. The content of these phases depends on the Mg/Fe atomic ratio in the starting reactant mixture. Iron oxide–hydroxide is the major reaction product at the Mg/Fe atomic ratio in the interval 1.5–1.75, and layered magnesium–iron layered double hydroxide, at Mg/Fe = 3–4. The ability of the synthesized products to take up As(III) from aqueous solutions was studied. These sorbents allow the arsenic concentration to be decreased from 3–5 mg L^–1 to values below MPC (0.01 mg L^–1).     

Reactivity of Mg–Al hydrotalcites in solid and delaminated forms in ammonium carbonate solutions

Treatment of Mg–Al hydrotalcites (LDHs, layered double hydroxides) in aqueous (NH₄)₂CO₃ at 298 K leads to composites of dawsonite, hydrotalcite, and magnesium ammonium carbonate. The mechanism and kinetics of this transformation, ultimately determining the relative amounts of these components in the composite, depend on the treatment time (from 1 h to 9 days), the Mg/Al ratio in the hydrotalcite (2-4), and on the starting layered double hydroxide (solid or delaminated form). The materials at various stages of the treatment were characterized by inductive coupled plasma-optical emission spectroscopy, X-ray diffraction, transmission electron microscopy, infrared spectroscopy, thermogravimetry, and nitrogen adsorption at 77 K. The progressive transformation of hydrotalcite towards crystalline dawsonite and magnesium ammonium carbonate phases follows a dissolution–precipitation mechanism. A gradual decrease of the Mg/Al ratio in the resulting solids was observed in time due to magnesium leaching in the reacting medium. Dawsonite–hydrotalcite composite formation is favored at high aluminum contents in the starting hydrotalcite, while the formation of magnesium ammonium carbonate is favored at high Mg/Al ratios. The synthetic strategy comprising hydrotalcite delamination in formamide prior to aqueous (NH₄)₂CO₃ treatment is more reactive towards composite formation than starting from the bulk solid hydrotalcite.     

Thermal and X-ray diffraction studies of the solid–solid interactions in CuO–MoO3/MgO system

The solid–solid interactions in pure and MoO₃-doped CuO/MgO system were investigated using TG, DTA and XRD. The composition of pure mixed solids were 0.1CuO/MgO, 0.2CuO/MgO and 0.3CuO/MgO and the concentrations of MoO₃ were 2.5 and 5 mol%. These solids were prepared by wet impregnation of finely powdered basic magnesium carbonate with solutions containing calculated amounts of copper nitrate and ammonium molybdate followed by heating at 400–1000°C. The results revealed that ammonium molybdate doping of the system investigated enhanced the thermal decomposition of copper nitrate and magnesium hydroxide which decomposed at temperatures lower than those observed in case of the undoped mixed solids by 70 and 100°C, respectively. A portion of CuO present dissolved in the lattice of MgO forming CuO–MgO solid solution with subsequent limited increase in its lattice parameter. The other portion interacted readily with a portion of MoO₃ at temperatures starting from 400°C yielding CuMoO₄ which remained stable up to 1000°C. The other portion of MoO₃ interacted with MgO producing MgMoO₄ at temperatures starting from 400°C and remained also stable at 1000°C. The diffraction peaks of Cu₂MgO₃ phase were detected in the diffractograms of pure and MoO₃-doped 0.3CuO/MgO precalcined at 1000°C. The formation of this phase was accompanied by an endothermic peak at 930°C.     

Mechanically Stimulated Thermal Synthesis of Lithium Pentaaluminate from Lithium Carbonate and Aluminum Hydroxide

It is shown that the phase composition of lithium aluminates formed when aluminum hydroxide in the form of gibbsite interacts with lithium carbonate in their mixture with Al: Li atomic ratio of 5: 1 depends on the duration of a preliminary mechanical activation of the mixture and on the temperature of the subsequent thermal treatment. A thermal treatment of the starting mixture at temperatures exceeding 800°C yields LiAl₅O₈ with a substantial admixture of α- and γ-LiAlO₂. Raising the duration of the mechanical activation to 5 min and more makes it possible to obtain highly dispersed single-phase LiAl₅O₈ with a specific surface area larger than 20 m² g^–1     

Synthesis of microtubes with a surface of "house of cards" structure via needlelike particles and control of their pore size

The conditions for synthesizing microtubes with a surface of "house of cards" structure via needlelike particles were examined in detail. Magnesium carbonate trihydrate was formed as a metastable phase in the reaction process using magnesium hydroxide and carbon dioxide as starting materials. Subsequently, in the formation of basic magnesium carbonate from magnesium carbonate trihydrate, microtubes with a surface of house of cards structure were obtained via needlelike particles of magnesium carbonate trihydrate under certain conditions where the temperature and added amount of sodium hydroxide were properly controlled. The pore size of the microtubes could be controlled within a range of 0.5-6 microm by adjusting the condition of needlelike particle formation. In addition, the sustainability of naphthalene release from the microtube was found to be about 6 times higher than that from naphthalene crystal.     

Influence of Mg surface layer for induction period of Grignard reagent formation

Factors that affect the induction period of Grignard reagent formation, which involves heterogeneous reaction between magnesium metal (Mg) and an alkyl halide in ether solvent, has been clarified to achieve safer and more efficient operation in chemical processes. The influence of the Mg surface, especially the effects of carbonate, hydroxide, and oxide layers on the induction period were investigated by measuring the exothermic behavior of Grignard reagent formation by a differential reaction calorimeter. Mg powder was kept in water bubbled with CO₂ or N₂ gas to form a coating on the Mg surface. The calorimetry results for the reaction indicated that both treatments increased the induction period. Thermogravimetric analysis-mass spectrometry was conducted to identify the chemical species and quantify the amount of surface material on the Mg particles. It was found that basic magnesium carbonate and magnesium hydroxide were formed on Mg exposed to CO₂ and N₂, respectively. Subsequent heating the carbonate- or hydroxide-coated Mg at 500 °C caused a MgO layer to form on the surface, which was found to dramatically reduce the induction period.     

Hydration of medium reactive industrial magnesium oxide with magnesium acetate

Medium reactive magnesium oxide reacts incompletely with available water to form magnesium hydroxide. To enhance the hydration of medium reactive magnesium oxide, the effect of magnesium acetate as hydrating agent was studied. The extent to which different parameters (concentration of magnesium acetate, solution temperature and solid to liquid ratio of MgO to magnesium acetate) influence the hydration rate of a medium reactive industrial sample of magnesium oxide were evaluated. The degree of rehydration measured as percentage Mg(OH)₂being formed, increases from approximately 56% using 0.5 M magnesium acetate solutions at 25°C to 64% at 50°C, to more than 70% at 70°C. The major part of rehydration of the medium reactive MgO sample occurs within the first few minutes of the reaction for all three temperatures studied. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of the base catalytic activity of MgO prepared by thermal decomposition of hydroxide,basic carbonate,and oxalate under atmospheric conditions

Research on Chemical Intermediates - To obtain highly active solid base catalyst of magnesium oxide (MgO) under atmospheric conditions, hydroxide (Mg(OH)2), basic carbonate...     

Preparation and Characterization of MgO Thin Films by a Novel Sol-Gel Method

MgO thin films have been prepared on Si substrates by a novel and simple sol-gel method using magnesium nitrate and collodion as starting material. Solutions consisting of magnesium nitrate in a mixture of collodion and ethanol were spin-coated onto silicon substrates. It was found that collodion was a necessary component to form stable sols and the crystallization and structures were clearly dependent on the amount of the collodion and the annealing temperature. The MgO thin films with good crystallization were obtained after annealing at 800°C. Meanwhile, the microstructure of the MgO films was examined by transmission electron microscopy and atomic force microscopy.     

Characterization of a passivation layer comprising MgO?SiO2 and ZrO2

Thin films with magnesium oxide (MgO) and silicon oxide (SiO₂) compounds mixed at various mixture ratios were deposited on flexible polyether sulfone (PES) substrates by an e‐beam evaporator to investigate their potential for transparent barrier applications. In this study, as the MgO fraction increased, thin films comprising MgO and SiO₂ compounds became more amorphous, and their surface morphologies became smoother and denser. In addition, zirconium oxide (ZrO₂) was added to the above‐mentioned compound mixtures, and the properties of the compound mixture comprising Mg? Si? Zr? O were then measured. ZrO₂ made the thin mixture films more amorphous, and made the surface morphology denser and more uniform. Whole thin films of 250 ± 30 nm in thickness were formed, and their water vapor transmission rates (WVTRs) decreased rapidly. The best WVTR was obtained by depositing thin films of Mg? Si? Zr? O compound among the whole thin films. The WVTRs of the PES substrate in the bare state decreased from 47 to 0.8 g m^?2 day^?1. This Mg? Si? Zr? O compound was deposited on polyethylene terephtalate (PET) substrates again to confirm the availability of the compound mixture. Thin films on the PET substrates decreased the WVTRs remarkably from 2.96 to 0.01 g m^?2 day^?1. These results were similar to those of thin films on PES substrates. As the thin mixture films became more amorphous and surface morphology denser and more uniform, the WVTRs decreased. Therefore, the thin mixture films became more suitable for flexible organic light emitting displays (OLEDs) as transparent passivation layers against moisture in air. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis of glycerol carbonate from glycerol and dimethyl carbonate catalyzed by K2CO3/MgO

A series of M/MgO (M?=?CaO, KNO₃, KOH, K₂CO₃) catalysts were prepared by a dry impregnation method and used for synthesis of glycerol carbonate from glycerol and dimethyl carbonate. It was found that K₂CO₃/MgO was the most efficient catalyst, with a glycerol carbonate yield of approximately 99% under the conditions: DMC/glycerol molar ratio 2.5:1, catalyst/raw material weight ratio 1%, reaction time 2?h, and reaction temperature 80?°C. FTIR, BET, TEM, and XRD were used for characterization of the catalyst and showed that the active sites seemed to be K₂O formed on the K₂CO₃/MgO catalyst. Finally, a recycling experiment showed that the catalyst was relatively stable and could be reused up to four times, at least, by regeneration.     

XPS study of the effect of aluminium on the atmospheric corrosion of the AZ31 magnesium alloy

The surface characteristics and corrosion behaviour of the AZ31 magnesium alloy exposed to a high relative humidity (RH) atmosphere were investigated. During the first 15 days of humidity test at 98% RH and 50 °C, a significant increase of magnesium carbonate and a decrease of magnesium oxide were detected on the surface film by XPS; after this stage, increased exposure times did not produce substantial changes on the relative amounts of these compounds. The surface film of commercially pure magnesium, also examined for comparison purposes, revealed more magnesium carbonate and less magnesium oxide compared with the AZ31 alloy. Unlike the AZ31 alloy, the surface of pure Mg disclosed almost complete substitution of MgO by magnesium carbonate after 30 days of exposure time. Mass gain values of tested specimens and scanning electron microscope characterisation of corroded surfaces indicated lower corrosion susceptibility of the AZ31 alloy compared with the commercially pure Mg, suggesting superior chemical stability of the oxide/hydroxide film formed over the magnesium–aluminium alloy surface. XPS and energy dispersive X‐ray (EDX) analyses did not revealed any substantial enrichment of aluminium in the corrosion products film on the AZ31 alloy after 30 days of testing. Copyright © 2008 John Wiley & Sons, Ltd.     

A TG analysis of the effect of calcination conditions on the properties of reactive magnesia

The reactivity of MgO obtained from calcination of magnesium carbonate at different temperatures has been investigated by means of hydration in a constant relative humidity environment at 40°C for periods up to 24 days. Natural magnesite and AR grade basic MgCO₃ calcined in the range of 500–1000°C was characterised in terms of surface area, crystallite size, morphology, and hydration rate. It was found that the hydration rate is dependent on the surface area and crystallite size where temperature was the main variable affecting them. The most reactive MgO was produced at the lowest calcination temperature with the highest surface area and the smallest crystallite size. The basic MgO specimens showed higher degree of hydration compared to the natural MgO specimens due to the smaller surface area and larger crystallite size. The low MgO content of the starting natural magnesite is also attributable to the lower reactivity. This preliminary study serves as a mean to investigate potential utilisation of reactive MgO as a supplementary cementitious material in eco-friendly cements.     

Critical temperatures and pressures of reacting mixture in synthesis of dimethyl carbonate with methanol and carbon dioxide

Critical temperatures and pressures of nominal reacting mixture in synthesis of dimethyl carbonate （DMC） from methanol and carbon dioxide （quaternary mixture of carbon dioxide ＋ methanol ＋ water ＋ DMC） were measured using a high-pressure view cell. The results suggested that the critical properties of the reacting mixture depended on the reaction extent as well as its initial composition （initial ratio of carbon dioxide to methanol）. Such information is essential for determining the reaction conditions when one intends to carry out the synthesis of DMC with CO2 and methanol under supercritical conditions.     

MgO/CaO-loaded porous carbons for carbon dioxide capture

Nanoporous carbons loaded with both MgO and CaO were prepared by a simple heating of mixtures consisting of poly(ethylene terephthalate) and natural dolomite. Preparations were carried out at temperatures ranging from 850 to 1,000 °C that ensured complete thermal decomposition of the dolomite contained in the mixtures to the oxides. An influence of the PET/dolomite weight ratio and temperature of the preparation process on the porosity of the obtained composite products and on CaO and MgO crystallite sizes are discussed using the results of nitrogen adsorption/desorption at 77 K and X-ray diffraction analyses, respectively. Performances of the hybrid materials as sorbents for carbon dioxide were examined using thermogravimetric analyses. Finally, possibility of regeneration of the spent sorbent materials together with a side—effect accompanying this process are discussed on the basis of thermogravimetric measurements. As found, a part of CO₂ captured by the hybrid sorbents gets adsorbed weakly and another portion is fixed strongly. During thermal regeneration, the strongly fixed CO₂ reacts with carbon material. In this way small fraction of a sorbent is lost.     

The Mechanism Study on the Catalytic Synthesis of 2,6‐Dimethylphenol from KA‐Oil and Methanol over Magnesium Oxide Catalysts

Catalytic synthesis of 2,6‐dimethylphenol from KA‐oil (a mixture of cyclohexanol and cyclohexanone) and methanol was achieved by using magnesium oxide‐supported chromium oxide catalysts in one step. At higher conversion (> 90%), dimethylphenol was formed in high yield (>60 %). The activity of Cr/MgO catalysts depended on the concentration of chromium. The yield of 2,6‐dimethylphenol was also affected by the composition of the ratio of cyclohexanol to cyclohexanone in KA‐oil. Cyclohexanol and cyclohexanone reacting with methanol under the same conditions indicated that pure cyclohexanol or cyclohexanone is less reactive than their mixture, KA‐oil. The adsorption properties of cyclohexanol and cyclohexanone on the surface of Cr/MgO determined by FT‐IR spectroscopy suggest that cyclohexanone is easily reduced to cyclohexanol by the hydrogen which formed in the reaction, and then further reacted with methanol to form 2,6‐dimethyphenol.     

Multistep mineral fouling growth on a cation-exchange membrane ruled by gradual sieving effects of magnesium and carbonate ions and its delay by pulsed modes of electrodialysis

The aim of this study was to reveal the mechanisms ruling a fouling growth on both sides of a CMX-SB cation-exchange membrane (CEM), run after run during three consecutive electrodialysis (ED) treatments. A model solution containing a high magnesium/calcium ratio (2/5) was demineralized under two different pulsed electric field (PEF) on-duty ratios and dc current. The results showed a series of mechanisms ruling a multilayer mineral fouling growth and its delay by PEFs. The nature of the fouling layer, during a first run, depended on the diluate pH-value evolutions and the ion migration rates through the membrane. A subsequent multilayer fouling growth during consecutive treatments was ruled by the already formed mineral layers, where gradual sieving effects inverted the migration rates and led to a multistep crystal growth. Calcium carbonate grew on the diluate side of CEM, starting from its amorphous phase to then crystallize in a coexisting presence of aragonite and calcite. Amorphous magnesium hydroxide appeared on CEM apparently through fouling dehydration ruled by the mineral layers themselves and by overlimiting current regimes. A delayed fouling growth was observed for PEF ratio 0.3. A long pause lapse during pulse modes was demonstrated as an important parameter for fouling mitigation.     

Thermogravimetric study on the decomposition of hydromagnesite 4 MgCO3 · Mg(OH)2 · 4 H2O

Isothermal and non-isothermal decomposition of hydromagnesite 4 MgCO₃ · Mg(OH)₂ · 4 H₂O was studied thermogravimetrically. Decarbonation was strongly influenced by the partial pressure of carbon dioxide. Decarbonation in an argon atmosphere proceeded via an amorphous lower carbonate to MgO. Decarbonation in a carbon dioxide atmosphere was interrupted at ～460–480°C. This interruption was explained by the formation of a metastable intermediate and the subsequent crystallization of MgCO₃, both from the amorphous lower carbonate. This explanation was supported by DTA and power X-ray diffraction analysis of the quenched specimens.     

Local Structure of Nanoscopic Magnesium Hydroxide ­Fluorides Studied by Natural Abundance 25Mg Solid State NMR Spectroscopy

Along with X‐ray diffraction measurements, ²⁵Mg solid state NMR experiments were performed in natural abundance at 9.4 T on crystalline and mechanically milled samples of MgO, Mg(OH)₂, MgF₂, and magnesium hydroxide fluorides Mg(OH)_xF_2–x prepared on mechanochemical and sol‐gel syntheses routes. In addition to single pulse and spin‐echo sequences, both static ¹H‐²⁵Mg CP and ¹⁹F‐²⁵Mg CP measurements allowed the registration of ²⁵Mg spectra in attractive short measurement times. Although an assignment of different magnesium species in magnesium hydroxide fluorides is only hardly possible, position and line shapes of ²⁵Mg spectra of magnesium hydroxide fluorides reflect chemically reliable trends. All samples studied here show a sixfold fluorine, oxygen, or mixed fluorine / oxygen coordination of magnesium, also in highly disordered samples.     

Structural and thermal interpretation of the synergy and interactions between the fire retardants magnesium hydroxide and zinc borate

The mechanism of cooperative action of commercial fire retardants is interpreted as resulting from specific chemical reaction and phase changes. This investigation focuses on the thermally initiated interactions between two forms of commercially available fire retardant compounds. The fire performance of a polyolefin with a metal hydroxide fire retardant, magnesium hydroxide, can significantly reduce the heat release rate through absorption of heat during conversion to its metal oxide. Formation of water, followed by vaporisation, decreases heat and dilutes volatiles from polymer degradation. The second form of fire retardant compounds are zinc borates (2ZnO·3B₂O₃·3H₂O and 4ZnO·B₂O₃·H₂O), that undergo dehydration with increasing temperature. Differential thermal analysis and wide-angle X-ray spectroscopy indicated that various structural changes occurred during heating. Endothermic transitions were observed for all components, while zinc borate (2ZnO·3B₂O₃·3H₂O) showed an exothermic crystallisation transition at relatively high temperature. The exotherm was modified by the development of a new crystalline phase, magnesium orthoborate (3MgO·B₂O₃) that formed on reaction with magnesium oxide (MgO) at temperatures greater than 500 °C. Formation of crystalline zinc oxide (ZnO) was also detected. From zinc borate (4ZnO·B₂O₃·H₂O), ZnO was primarily formed. No new crystalline phases were observed in the presence of MgO over the temperature range investigated.