Mullite Formation of Colloidal Matrix Hybrid Aluminosilicate Gel

A hybrid aluminosilicate gel composed of a relatively small amount of single-phase gel in a diphasic matrix forms when a diphasic gel of AlOOH and SiO₂ is heated at 700°C for 7 h. The formation of mullite from this hybrid gel is a sequential conversion process, but not of two entirely independent processes of single-phase gel and diphasic gel. The high commencement temperature (>1250°C) of mullite formation from this hybrid gel indicates that the dominant component (diphasic component) strongly affects or controls the mullite transformation behavior of the hybrid gel. The apparent activation energy for mullitization from this hybrid gel is 846 kJ/mol, which is somewhat lower than that of pure diphasic gel but much higher than that of pure single-phase gel.     

Use of noncontact dilatometry for the assessment of the sintering kinetics during mullitization of three kaolinitic clays from Cameroon

Noncontact dilatometry, compared to differential scanning calorimetry (DSC), was used together with scanning electron microscopy and densification behavior studies to investigate the parameters that govern the kinetics of transformation of kaolin to mullite during sintering. Three kaolinitc clays from Cameroon, with different SiO₂/Al₂O₃ molar ratio, were examined. The temperatures of mullite nucleation were 973, 979, and 984 °C at 5 °C/min heating rate, respectively, for values of SiO₂/Al₂O₃ molar ratio equal to 4.22, 2.22, and 2.08. At 20 °C/min heating rate, the temperatures are shifted to higher values, 992, 997, and 1,001 °C. The mullitization phenomenon, which includes a first step of nucleation and a second one of crystal growth, presented activation energy in the range of 650–730 kJ/mol, depending on the nature of the sample investigated. These values, obtained by noncontact dilatometer measurements, were comparable to those obtained by means of DSC and are in agreement with literature values. The difference in sintering kinetics for the three kaolinitic clays could explain the different morphologies obtained for the mullite grains.     

Effect of nickel and cobalt ions on low temperature synthesis of mullite by sol–gel technique

Synthesis of mullite has been achieved at a low temperature of 600 °C by sol–gel technique in presence of nickel and cobalt ions. Samples were characterized by DTA, XRD, FESEM and FTIR spectroscopy. Mullite formation was found to depend on the concentration of the ions to a certain extent. Highly crystalline spherical mullite particles of dimension 35 nm were obtained at 0.02 M nickel or cobalt concentration.     

Kinetics of cordierite crystallization from diphasic gels

Diphasic cordierite gels were prepared from colloidal silica, aluminum and magnesium nitrates and citric acid. The mechanism of xerogel decomposition was studied by infrared spectroscopy (FT-IR) and thermal gravimetric analysis (TGA). The thermal decomposition of the xerogel forms a solid mixture of MgO, Al₂O₃ and SiO₂ at around 250 °C. Cordierite crystallization was studied by X-ray diffraction (XRD) and differential thermal analysis (DTA). Xerogels were initially thermally treated, and this sample crystallized to μ-cordierite at 850 °C, at 900 °C α-cordierite crystallizes and at 1150 °C α-cordierite is the major phase and μ-cordierite is totally consumed. The apparent activation energy for cordierite crystallization process was determined based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory, Ligero methods and the Arrhenius law for dependence of activation energy with temperature. The apparent activation energy was (466.8 ± 34.3) kJ/mol, the exponent of Avrami was (1.9 ± 0.2) and the frequency factor was (1.55 × 10²⁰) s⁻¹. The Avrami value indicates a nucleation controlled process, which can be a consequence of the high xerogel homogeneity, a consequence of the early and simultaneous formation of the MgO, Al₂O₃ and SiO₂ mixture.     

Effects of mechanical activation on the non-isothermal kinetics of mullite formation from kaolinite

The non-isothermal kinetics of mullite formation from both non-activated and mechanically activated kaolinite have been studied by differential thermal analysis (DTA). Kaolinite was mechanically activated in a planetary mill, while amorphization in the structure was studied by X-ray diffraction analysis. It was established that the mechanical activation especially affected the loss of structural water. The activation energies depending on the conversion for mullite formation have been calculated from the DTA curves by using the non-isothermal method of Coats and Redfern at heating rates of 5, 10, 15, and 20 °C min⁻¹. The mechanical activation and amorphization of the kaolinite brings to the formation of mullite at a lower heating temperature.     

CoFe2O4@Methylcellulose/AC as a New,Green, and Eco-friendly Nano-magnetic adsorbent for removal of Reactive Red 198 from aqueous solution

Methylcellulose (MC) is the most common commercial cellulose ether and the most attractive biopolymer due to its cheap cost of biodegradability, biocompatibility, hydrophilicity, and lack of toxicity. In this study, CoFe₂O₄@MC/activated carbon (AC) was synthesized as a unique magnetic nano-adsorbent in the presence of MC biopolymer for Reactive Red 198 (RR198) dye removal. The nano-magnetic adsorbent was characterized by FESEM (Field emission scanning electron microscopy), EDS (Energy-dispersive X-ray spectroscopy), Mapping, Linescan, BET (Brunauer–Emmett–Teller), FTIR (Fourier Transform Infrared Spectroscopy), XRD (X-Ray Diffraction), and VSM (Vibrating-Sample magnetometer). For simple separation by external magnetic fields, the Ms value was 57.91 emu/g. According to XRD analysis, the nano-adsorbent maintains its crystal structure, with an average crystal size of 11 nm. The maximum removal efficiencies of RR198 for synthetic and real wastewater samples under optimal conditions (an initial concentration of 10 mg/L, pH 3, contact time of 10 min, nanocomposite dose of 1.5 g/L, and a temperature of 25 °C) were 92.2% and 78%, respectively. The adsorption experiments were fitted well with the Freundlich isotherm (R² = 0.989) and pseudo-second-order kinetic (R² = 0.995). The values of entropy changes (ΔS = 35.087 kJ/mol.k), enthalpy changes (ΔH = -9.862 kJ/mol), and negative Gibbs free energy changes (ΔG) showed that the adsorption process was exothermic. Finally, the reusability findings showed that after six recovery cycles, the efficiency decreased slightly (90.1%). In the end, it can be concluded that the prepared CoFe₂O₄@Methylcellulose/AC can be used as an efficient adsorbent for the removal of RR198 from an aqueous solution.     

Atenolol sequestration using activated carbon derived from gasified Glyricidia sepium

Activated carbon (AC) derived from gasified Glyricidia sepium woodchip (GGSWAC) was prepared using KOH and CO₂ activation via microwave radiation technique to remove atenolol (ATN) from aqueous solution. The surface area (S_BET) and total pore volume (TPV) of GGSWAC were 483.07 m²/g and 0.255 cm³, respectively. The n-BET model fits well with the isothermal data indicating a multilayer adsorption with the saturation capacity of 121, 143 and 163 mg/g at 30, 45 and 60 °C, respectively. The kinetic study showed that ATN adsorption followed Avrami model equation (R² ≅ 0.99). Based on the thermodynamic parameters, the adsorption of ATN onto GGSWAC was endothermic (ΔH_S = 234.17 kJ/mol) in the first layer of adsorption and exothermic in the subsequent layer (ΔH_L = −165.62 kJ/mol). The ATN adsorption was controlled by both diffusion and chemisorption. In continuous operation, the Thomas (R² = 0.9822) and Yoon–Nelson (R² = 0.9817) models successfully predicted the ATN adsorption.     

Adsorption of tetracycline using CuCoFe2O4@Chitosan as a new and green magnetic nanohybrid adsorbent from aqueous solutions: Isotherm,kinetic and thermodynamic study

Utilizing a new microwave-assisted method, CuCoFe₂O₄@Chitosan (Ch) was synthesized as a very strong, magnetically separable nano-adsorbent. The magnetic nanohybrid adsorbent was characterized by FESEM (Field emission scanning electron microscopy), EDS (energy dispersive X-ray), Mapping & Linescan, BET (Brunauer-Emmett-Teller), FTIR (Fourier-transform infrared spectroscopy), XRD (X-ray diffraction analysis), TGA (Thermogravimetric analysis), and VSM (Vibrating Sample Magnetometer) techniques. Then, the adsorption process of Tetracycline (TC) was investigated. The highest percentage of pollutant adsorption on the synthetic and real samples was recorded at an initial concentration of 5 mg/L, pH 3.5, contact time of 20 min, the dose of 0.4 g/L, and temperature of 25 °C, 93.07 %, and 67%, respectively. The TC adsorption process via the synthesized magnetic nanocomposite was consistent with the Freundlich isotherm model (R² = 0.992) and pseudo-second-order kinetic (K₂ = 0.267). The outcomes of thermodynamic analyses, which included entropy changes (ΔS = 10.122 J/mol.k), enthalpy changes (ΔH = ?1.975 kJ/mol), and the Gibbs negative free energy (ΔG = ?4.992 kJ/mol), revealed that the adsorption process was spontaneous, favorable, and exothermic. The good magnetic properties allow easy separation after the adsorption operation. Finally, the efficiency of the nano-adsorbent in the removal process was 82.16% after four adsorption–desorption cycles. Some advantages of this research are a fast and green method for synthesis of adsorbent, fast kinetic, and magnetic properties to easy separation.     

The non-isothermal kinetics of mullite formation in boehmite–zircon mixtures

In this work, we studied the kinetics of mullite formation in different composites under non-isothermal conditions using DTA. Different composites based of mullite, alumina, zircon and zirconia were prepared by reaction sintering of boehmite (as alumina source) and zircon. Several mixtures were used while varying the percentage of the boehmite from 30 to 70 mass% with a step of 10. Five compositions marked as B30, B40, B50, B60 and B70 corresponding to boehmite–zircon ratios (mass%) of 30/70, 40/60, 50/50, 60/40 and 70/30 were fabricated and studied. The DTA conducted at heating rates of 10, 20 and 30 K min^?1 showed an endothermic peak in all composites at about 1,603 K associated with mullite formation. The activation energies measured from non-isothermal treatments for five compositions (30, 40, 50, 60 and 70 mass% of boehmite) were 1,029, 1,085, 1,262, 1,508 and 1,321 kJ mol^?1, respectively. The n values (Avrami parameter) for all compositions are larger than 2.5, the mullite crystallization of these composites is followed by three-dimensional growth.     

Melting behaviors,crystallization kinetics,and spherulitic morphologies of poly(butylene succinate) and its copolyester modified with rosin maleopimaric acid anhydride

This article investigated the melting behaviors, crystallization kinetics, and spherulitic morphologies of poly(butylene succinate) (PBS) and its copolyester (PBSR) modified with rosin maleopimaric acid anhydride, using wide‐angle X‐ray diffraction, differential scanning calorimeter (DSC), and polarized optical microscope. Subsequent DSC scans of isothermally crystallized PBS and PBSR exhibited two melting endotherms, respectively, which was due to the melt‐recrystallization process occurring during the DSC scans. The equilibrium melting point of PBSR (125.9 °C) was lower than that of PBS (139 °C). The commonly used Avrami equation was used to describe the isothermal crystallization kinetics. For nonisothermal crystallization studies, the model combining Avrami equation and Ozawa equation was employed. The result showed a consistent trend in the crystallization process. The crystallization rate was decreased, the perfection of crystals was decreased, the recrystallization was reduced, and the spherulitic morphologies were changed when the huge hydrogenated phenanthrene ring was added into the chain of PBS. The activation energy (ΔE) for the isothermal crystallization process determined by Arrhenius method was 255.9 kJ/mol for PBS and 345.7 kJ/mol for PBSR. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 900–913, 2006     

Theoretical study on the isomerization and tautomerism in barbituric acid

Isomerization and tautomerism of 16 isomers of barbituric acid (BA) were studied at the MP2 and B3LYP levels of theory. Activation energies (E _a), imaginary frequencies (υ), and Gibbs free energies (ΔG ^#) of the amine-imine and keto-enol tautomerisms and O–H internal rotations were calculated. The activation energies of amine-imine tautomerisms were in the range of 110–200 kJ/mol and for keto-enol tautomerisms were larger than 200 kJ/mol. The calculated activation energies of internal O–H rotations were smaller than 60 kJ/mol. Effect of micro-hydration on the transition state structures and activation energies of the tautomerisms were also investigated. Water molecule catalyzed the tautomerisms and decreased the activation energies of both the amine-imine and keto-enol tautomerisms about 100–120 kJ/mol.     

On the preparation of nanosized Al2(WO4)3 by a precipitation method

Nanosized aluminum tungstate, Al₂(WO₄)₃, is prepared by a precipitation reaction between Na₂WO₄ and Al(NO₃)₃. The structure of the precipitated composition is determined by powder XRD analysis, IR and ²⁷Al MAS NMR spectroscopy. The thermal properties are examined by DSC, DTA and TG analyses combined with gas evolved analysis. Particle sizes and morphology are examined by TEM analysis. Precipitation reaction leads to the formation of an amorphous composition, which consists of dimer and trimer aluminum hydroxide species and WO₄^2? groups. Finely dispersed particles with dimensions of about 25 nm are formed. The precipitated composition is decomposed to amorphous Al₂(WO₄)₃ immediately after H₂O release. At 630 °C, amorphous Al₂(WO₄)₃ crystallizes in an orthorhombic modification of Al₂(WO₄)₃, the enthalpy of crystallization being 58 kJ/mol. The nanosized particles remain intact after the crystallization of amorphous Al₂(WO₄)₃. A significant particle growth take places when nanosized Al₂(WO₄)₃ is heated from 600 to 800 °C.     

Aluminum Nitride Oxidation by Simultaneous TG and DTA

This work is a study, by simultaneous thermogravimetry (TG) and differential thermal analysis (DTA), of the oxidation of a water resistant aluminum nitride powder which has a special protective coating, and an uncoated AlN powder which has become partially hydrated during its use. The activation energy for oxidation is estimated by the Kissinger and isoconversional methods. In the former method, the temperatures of the oxidation peaks were obtained from DTA and DTG curves. The activation energies for oxidation of the water resistant AlN, obtained by the Kissinger method, are 357±10 kJ mol^–1, 392±12 kJ mol^–1 using respectively DTG and DTA data. For the uncoated AlN, the values are 243±7 and 257±8 kJ mol^–1, respectively. By the isoconversional method, the average values obtained for coated and uncoated samples are, respectively, 323±10 and 224±7 kJ mol^–1. Therefore, the special coating, which protects the aluminum nitride from humidity action, also provides a higher resistance to oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

exo-Selective inverse-electron-demand hetero Diels–Alder reactions of norbornene with 5-benzylidine-2-arylimino-3-aryl-thiazolidine-4-thiones at room temperature

2-Arylimino-3-aryl-thiazolidine-4-thiones were synthesized from the corresponding thiazolidine-4-ones using Lawesson's reagent (LR) and converted into 5-benzylidine-2-arylimino-3-aryl-thiazolidine-4-thiones by reaction with benzaldehyde, which were then used as heterodienes in the inverse-electron-demand hetero Diels–Alder cycloadditions with norbornene as a dienophile at 25 ^°C. The reactions with norbornene were found to proceed with 100% exo-selectivity as determined by NMR experiments. The hetero Diels–Alder reactions with axially chiral heterodienes with ΔG^#>116 kJ/mol showed kinetic atroposelectivities up to 11:1. However, the products were found to equilibrate, as revealed by the 97.1 kJ/mol barrier to hindered rotation of the most sterically hindered product, to produce 2:1 diastereoselectivities after the 24 h reaction time.     

Thermal analysis applied in the crystallization study of SrSnO3

SrSnO₃ was synthesized by the polymeric precursor method with elimination of carbon in oxygen atmosphere at 250 °C for 24 h. The powder precursors were characterized by TG/DTA and high temperature X-ray diffraction (HTXRD). After calcination at 500, 600 and 700 °C for 2 h, samples were evaluated by X-ray diffraction (XRD), infrared spectroscopy (IR) and Rietveld refinement of the XRD patterns for samples calcined at 900, 1,000 and 1,100 °C. During thermal treatment of the powder precursor ester combustion was followed by carbonate decomposition and perovskite crystallization. No phase transition was observed as usually presented in literature for SrSnO₃ that had only a rearrangement of SnO₆ polyhedra.     

New synthesis of mullite. Structural evolution study by 17O, 27Al and 29Si MAS NMR spectroscopy

Mullite has been prepared from a new combination of precursors. An aluminum alkoxide, aluminium isopropoxide, and silicon tetrachloride, are hydrolysed in tetrahydrofuran solution by ¹⁷O enriched water. The resulting powder is chemically homogeneous, crystallizing into mullite at 980°C. The structural evolution has been studied by DTA, TGA, XRD and ¹⁷O, ²⁷Al and ²⁹Si MAS NMR spectroscopy.     

Growth of Nd-doped YAG powder by sol spray process

Yttrium aluminum garnet (YAG) and neodymium-doped yttrium aluminum garnet (Nd-YAG) nano-crystalline powders were successfully grown using cost effective sol spray process without the addition of any chelating agent or organic templates. Thermal decomposition behavior was studied by thermogravimetry (TG) and differential thermal analysis (DTA). Results revealed that crystallization of YAG started around 920 °C. The shrinkage/expansion behavior of synthesized powder was examined by dilatometer and revealing that sintering kinetics of these materials can be related to the evaporation of binder and formation of crystalline phases. Nano-crystallinity and garnet structure of YAG and Nd-YAG specimens were analyzed by Raman, fourier transform infra red (FTIR), and X-ray diffraction (XRD) techniques. XRD patterns were indexed using Rietveld refinement method. Smaller lattice parameter and a small change in atomic position of oxygen were found in Nd-YAG when compared with YAG structure. Scanning electron microscope (SEM) results indicated that particle size of Nd-YAG was <150 nm. The morphology of Nd-YAG nanosized powder was rounded in shape.     

Testing catalytic activity of ruthenium(III) acetylacetonate in the presence of trialkylphosphite or trialkylphosphine in hydrogen generation from the hydrolysis of sodium borohydride

Catalytic activity of Ru(acac)₃ in the presence of different phosphorus compounds (P(OMe)₃, P(OPh)₃, PPh₃ and dppe) was investigated for the first time in the hydrolysis of NaBH₄. Phosphorus compound, usually known as poison in catalysis, is involved in the formation of a species which has higher catalytic activity in comparison with Ru(acac)₃ alone. Varying the phosphorus compound affects the catalytic activity and lifetime of the catalyst as well as the kinetics and the activation parameters of the hydrolysis of NaBH₄. For all of the phosphorus compounds, the hydrogen generation was found to be zero-order with respect to the substrate concentration and first-order regarding the catalyst concentration. The catalyst system with P(OMe)₃ shows the highest catalytic activity and provides the largest total turnover number (TTON = 20,700 over 72 h) in the hydrolysis of NaBH₄. The highest activation energy and enthalpy values were obtained for the catalyst with dppe (E_a = 59 ± 2, ΔH^# = 60 ± 2 kJ/mol) while the lowest values were found for the catalyst system with PPh₃ (E_a = 46 ± 2, ΔH^# = 43 ± 1 kJ/mol).     

Some studies on the phase formation and kinetics in TiO<Subscript>2</Subscript> containing lithium aluminum silicate glasses nucleated by P<Subscript>2</Subscript>O<Subscript>5</Subscript>

Lithium aluminum silicate (LAS) glasses of compositions (wt%) 10.6Li₂O–71.7SiO₂–7.1Al₂O₃–4.9K₂O–3.2B₂O₃–1.25P₂O₅–1.25TiO₂ were prepared by the melt quench technique. Crystallization kinetics was investigated by the method of Kissinger and Augis–Bennett using differential thermal analysis (DTA). Based on the DTA data, glass ceramics were prepared by single-, two-, and three-step heat treatment schedules. The interdependence of different phases formed, microstructure, thermal expansion coefficient (TEC) and microhardness (MH) was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermo-mechanical analysis (TMA), and microhardness (MH) measurements. Crystallization kinetics revealed that Li₂SiO₃ is the kinetically favored phase with activation energy of 91.10 kJ/mol. An Avrami exponent of n = 3.33 indicated the dominance of bulk crystallization. Based upon the formation of phases, it was observed that the two-stage heat treatment results in highest TEC glass ceramics. The single-step heat treatment yielded glass ceramics with the highest MH.     

Removal of cibacron blue 3G-A (CB) dye from aqueous solution using chemo-physically activated biochar from oil palm empty fruit bunch fiber

A novel biodegradable adsorbent called pyrolysed empty fruit bunch fibres (PEF) was prepared by chemo-physical activation of empty fruit bunch fibres (EFB) biochar for removal of cibacron blue 3G-A (CB) dye from aqueous solution. PEF was characterized using FTIR, SEM-EDX, XRD and BET techniques. The N₂ adsorption-desorption isotherms indicated PEF’s surface area to be 362.84 m²g⁻¹ and XRD attributed amorphous nature to PEF. After adsorption process, PEF has smoother surface morphology, increase in carbon by weight and shift in functional groups. The established adsorption optimum conditions were pH 10, 45 min contact time and 0.10 g/100 mL adsorbent dosage with 99.05% CB dye removal capacity at 343 K and initial dye concentration 100 mg/L. Desorption ratio >90% after seventh cycle of adsorption-desorption experiments confirmed high reusability (regeneration) of PEF. Pseudo second order kinetic and Freundlich were better fitted with kinetic and isotherm model respectively, while mechanism of adsorption was controlled by film diffusion (external mass transfer). Thermodynamic studied revealed ΔG, ΔS and ΔH to be −3.12 MJ/mol K, 9.11 kJ/mol K, 6.83 kJ/mol respectively at 343 K. The negative value of ΔG, positive values of ΔS and ΔH indicated spontaneity, feasibility and endothermic nature of CB dye adsorption from aqueous solution onto PEF.