Influences of pH value on the microstructure and phase transformation of aluminum hydroxide

The effects of pH value on the composition, structure, morphology, and phase transformation of aluminum hydroxides prepared by chemical precipitation were studied. Aluminum hydroxide precipitated at the pH values of 5 and 6 is amorphous and transforms to α-Al₂O₃ at 950 °C via the amorphous aluminum hydroxide → amorphous Al₂O₃ → α-Al₂O₃ transformation path. Aluminum hydroxide precipitated at pH = 7 is boehmite and transforms to α-Al₂O₃ at 950 °C via the γ-AlOOH → γ-Al₂O₃ → α-Al₂O₃ path. Aluminum hydroxide precipitated at pH values in the 8 to 11 range is bayerite and transforms to α-Al₂O₃ at 1000 °C via the α-Al(OH)₃ → γ-Al₂O₃ → ε-Al₂O₃ + θ-Al₂O₃ → α-Al₂O₃ path. Moreover, the pH value affects not only the morphology of aluminum hydroxide particles which changes from ultrafine floccules through 50 nm blowballs then to 150 nm irregular agglomerates with increasing pH value but also the microstructures of final decomposition products of aluminum hydroxides.     

Tuning product selectivity of CO2 hydrogenation by OH groups on Pt/γ-AlOOH and Pt/γ-Al2O3 catalysts

Herein, we explore how OH groups on Pt/γ-AlOOH and Pt/γ-Al₂O₃ catalysts affect CO₂ hydrogenation with H₂ at temperatures from 250°C to 400°C. OH groups are abundant on γ-AlOOH, but rare at Pt-(γ-AlOOH) interface which is the most favorable site for CO₂ conversion on Pt/γ-AlOOH. This makes CO₂ hydrogenation on Pt/γ-AlOOH form CO weakly bonding to γ-AlOOH, which prefers to desorption from Pt/γ-AlOOH rather than further conversion, thus enhancing CO production on Pt/γ-AlOOH. Different from Pt/γ-AlOOH, OH groups are abundant at Pt-(γ-Al₂O₃) interface which is the most favorable site for CO₂ conversion on Pt/γ-Al₂O₃. This promotes CO₂ hydrogenation on Pt/γ-Al₂O₃ to form CO strongly bonding to Pt, which prefers to further hydrogenation to CH₄, and thereby increases CH₄ selectivity on Pt/γ-Al₂O₃. Therefore, the OH groups at metal-support interface are crucial factor influencing product distribution, and must be considered seriously when fabricating catalysts.     

Beneficial effect of adding γ-AlOOH to the γ-Al2O3 washcoat of a PdO catalyst for methane oxidation

The beneficial effect of adding γ-AlOOH to the γ-Al₂O₃ washcoat of a ceramic cordierite (2MgO · 2Al₂O₃ · 5SiO₂) monolith, used to support a PdO catalyst, is reported for methane oxidation in the presence of water at low temperature (<500°C). The mini-monolith (400 cells per square inch (CPI), 1 cm diameter × 2.54 cm length; ~52 cells) was washcoated using a suspension of γ-Al₂O₃ plus boehmite (γ-AlOOH), followed by calcination and then deposition of Pd by wet impregnation. An optimum solid content of 25 wt% in the washcoat suspension was used to obtain a ~25 wt% washcoat on the monolith. The presence of γ-AlOOH enhanced the thermal and mechanical stability of the washcoat, provided that the γ-AlOOH content was <8 wt%. Temperature-programmed methane oxidation (TPO) showed that the addition of γ-AlOOH to the γ-Al₂O₃ washcoat decreased the catalyst activity. However, when H₂O (2 vol% and 5 vol%) was present in the feed gas, the γ-AlOOH improved the catalyst activity and stability. A γ-AlOOH content of ~5 wt% in the washcoat was determined to provide the highest catalyst activity and stability for CH₄ oxidation in the presence of water.     

Carbon–Al2O3–Ag composite molecular sieve membranes for gas separation

Phenolic resins loaded with two different inorganic fillers (boehmite (γ-AlO(OH)) and silver (Ag)) were used to prepare composite carbon membranes. Polymer solutions containing γ-AlO(OH) and AgNO₃ were prepared and the effect of Ag on the transport properties of the composite membrane was evaluated. The polymer solutions were coated on α-Al₂O₃ tubes and carbonized in a single dipping-drying-pyrolysis step. After pyrolysis at 550 °C, γ-AlO(OH) yielded γ-Al₂O₃, and Ag agglomerated forming spherical nanoparticles of 30 nm in diameter. Ag loading enhanced the carbon membrane performance for several gas pairs of interest, especially for C₃H₆/C₃H₈ separation, where the C₃H₆/C₃H₈ permselectivity increased from a maximum of 15 to 38.     

Synthesis of plate-like α-Al2O3 single-crystal particles in NaCl–KCl flux using Al(OH)3 powders as starting materials

Plate-like α-Al₂O₃ single-crystal particles were successfully synthesized in NaCl–KCl flux using Al(OH)₃ powders as starting materials, and the influence of pre-calcining of Al(OH)₃ powders on the phase formation and morphology of α-Al₂O₃ powders was focused. When Al(OH)₃ powders are used as starting materials, the synthesized product at 900 °C is mainly composed of α-Al₂O₃ and κ-Al₂O₃, and most synthesized particles show alveolate morphology. At 1100 °C, single-phase α-Al₂O₃ powders are developed, in which there are many aggregations of intensively bound plate-like particles. In contrast, using porous amorphous Al₂O₃ powders obtained by pre-calcining Al(OH)₃ powders at 550 °C for 3 h as the starting material, plate-like α-Al₂O₃ single-crystal particles can be well developed above 900 °C. The reason of the influence of pre-calcining of Al(OH)₃ powders on the phase formation and morphology of α-Al₂O₃ powders is also discussed in the paper.     

Lyothermal synthesis of nanocrystalline BaSnO3 powders

The synthesis of BaSnO₃ powders has been investigated at lyothermal conditions (temperature of 250 °C; t = 6 h), starting from SnO₂·xH₂O and Ba(OH)₂ and methanol, ethanol, isopropanol and acetone as solvents. Among them isopropanol was found to be the most suitable medium for preparing BaSnO₃. By addition of the modifier Genapol X-080 during the processing, the BET specific surface area of the end-powder was increased by a factor of 10. The as-prepared powder consisted of BaSn(OH)₆. The thermal behavior, the crystallization behavior and the structure evolution of the powder during heating treatment have been studied with the TG–DTA–MS, XRD and FTIR. The weight loss of the as-prepared powder of about 12 wt% heated up to 1200 °C is mainly attributed to the dehydration around 260 °C which leads to the structure rearrangement and the building of the [SnO₆] octahedra. At this temperature BaSn(OH)₆ converts to an amorphous phase, from which BaSnO₃ nucleates and grows with increasing temperature. The obtained BaSnO₃ powders had a BET specific surface area of 16.56 m²/g and a primary crystallite size of 49 nm.     

Effect of carbon nanotubes on sintering behavior of alumina prepared by sol–gel method

Alumina (Al2O3)/carbon nanotube (CNT) (99/1 by weight) composite was prepared by mixing CNT dispersion with AlCl₃-based gel, followed by high temperature sintering at a temperature up to 1150 °C in argon. Composite alumina precursor showed phase transition order from amorphous to γ-Al₂O₃ after sintered at 900 °C for 2 h, partially to θ-Al₂O₃ after sintered at 1000 °C for 2 h, and then partially to α-Al₂O₃ after sintered at 1150 °C for 2 h. By comparison, control alumina precursor directly transformed from amorphous to α-Al₂O₃ after sintered at a relatively low temperature of 600 °C for 2 h. Composite alumina showed porous structure with pore diameter ranging from 100 nm to 2 µm, whereas control alumina was relatively pore-free. The elevated alumina-crystal phase transition temperatures and the formation of porous structure were ascribed to the presence of CNTs in alumina precursor. The composite alumina sintered at 900 °C for 2 h containing only γ-Al₂O₃ had a BET surface area of 138 m²/g, which was significantly higher than that of control alumina sintered at 1150 °C for 2 h containing only α-Al₂O₃, ~15 m²/g.     

The interaction effects of dehydration function on catalytic performance and properties of hybrid catalysts upon LPDME process

Three bi-functional catalysts have been prepared by physical mixing of a commercial methanol synthesis catalyst (CuO–ZnO–Al₂O₃) with three different methanol dehydration catalysts including: H-MFI90, γ-Al₂O₃ and H-Mordenite in order to investigate the role of interaction effects of dehydration component on characteristic properties and performance of these admixed catalysts. The bi-functional catalysts have been characterized by XRD, N₂ adsorption, H₂-TPR, NH₃-TPD and XRF techniques and tested in a mixed slurry bed reactor at the same operating conditions (T = 240 °C, P = 50 bar, H₂/CO = 2, SV = 1100 ml g-cat^− 1 h^− 1) for 60 h time on stream. Among the examined bi-functional catalysts, the physical mixture of KMT + HMFI-90, which had lower reducing peak temperature (T = 200 °C), higher S_Cu (39.1 m² g-cat^− 1) and Cu Dispersion (11.6%), showed higher X_CO (84 mol%), yield of DME (Y_DME = 55.5 mol%), DME selectivity (Select_DME = 66.7 mol%) and also good stability over 60 h time on stream as compared to the other catalysts. This could be assigned, from NH₃-TPD results, to more middle strength acidic sites of H-MFI90 zeolite (SiO₂/Al₂O₃ = 90, total acid site density = 476 µmol/g-cat) which inhibits detrimental interactions with methanol synthesis catalyst and deep dehydration of methanol.     

Hydrodearomatization of petroleum fuel fractions on silica supported Ni-W sulphide with increased stacking number of the WS2 phase

Ni-W catalysts supported on commercial γ-alumina and silica displayed similar activity in dibenzothiophene hydrodesulfurization (DBT HDS), while the activity of the Ni-W/SiO₂ catalyst in toluene hydrogenation (HYD) was 6 times higher compared with Ni-W/Al₂O₃. The dearomatization performance of Ni-W/SiO₂ catalyst was tested over a wide range of operation conditions with naphtha and middle distillates. 90% saturation of aromatics in FCC naphtha (340 °C, LHSV of 1 h⁻¹) and 50% in Light cyclic oil (LCO) (360 °C, LHSV of 1 h⁻¹) was achieved at 5.4 MPa. In a two stage process with the same Ni-W/SiO₂ and intermediate separation of hydrogen sulphide 90% saturation of aromatics in LCO was achieved at 320 °C and total LHSV of 0.5 h⁻¹. At equal conditions, Ni-W/Al₂O₃ catalyst yielded 1.5-4 times lower total aromatics saturation.     

Influence of sintering temperature and holding time on the densification, phase transformation, microstructure and properties of hot pressing WC-40 vol.%Al2O3 composites

WC-40 vol.%Al₂O₃ composites were prepared by high energy ball milling followed by hot pressing. The tungsten carbide (WC) and commercial alumina (Al₂O₃) powders composed of amorphous Al₂O₃, boehmite (AlOOH) and χ-Al₂O₃ were used as the starting materials. The phase transformation during sintering, the influence of sintering temperature and holding time on the densification, microstructure, Vickers hardness and fracture toughness and the toughening effects of WC-40 vol.%Al₂O₃ composites were investigated. The results showed that the amorphous Al₂O₃, AlOOH and χ-Al₂O₃ were transformed to α-Al₂O₃ completely during the sintering process. With the increasing sintering temperature and holding time, the relative density increased and both the Vickers hardness and fracture toughness increased initially to the maximum values and then decreased. When the as milled powders were hot pressed at 1540 °C for 90 min, a relative density of 97.98% and a maximum hardness of 18.65 GPa with an excellent fracture toughness of 10.43 MPa m^1/2 of WC-40 vol.%Al₂O₃ composites were obtained.     

Crystal structure of low magnesium-content alite: Application to Rietveld quantitative phase analysis

Five members of tricalcium silicate solid solution, Ca_{3 − x − y}Mg_xAl_y(Si_1 − yAl_y)O₅, have been prepared. T_1, T₃ and M₃ forms have been identified by X-ray powder diffraction, but pure M₁ form was not stabilized. The crystal structure of a sample nominally Ca_2.96Mg_0.03Al_0.01(Si_0.99Al_0.01)O₅ has been studied by a joint Rietveld refinement using strictly monochromatic laboratory X-ray and neutron powder diffraction data, with soft constraints of interatomic distances. The crystal structure of this alite is a T₃ form with a triclinic cell, space group P 1?1­, of dimensions a = 11.6389(2) Å, b = 14.1716(3) Å, c = 13.6434(3) Å, α = 104.982(2)°, β = 94.622(1)°, γ = 90.107(2)° and V/Z = 120.346(6) ų. Laboratory and commercial clinkers were studied by synchrotron X-ray powder diffraction and the Rietveld method. The reported T₃ structure for alite fits properly a variety of laboratory Portland clinkers with low magnesium contents. The alite refined volume(s) (V/Z) is useful to predict the magnesium oxide content of a clinker and the alite-type. Thus, a refined V/Z value between 121.0 and 120.3 Å³ should contain up to ~ 1.0 wt.% of MgO, being T₃ type. If refined C₃S V/Z is smaller than ~ 119.8 Å³ the clinker may contain more than ~ 2.1 wt.% of MgO with alite as M₃. For intermediate magnesium (and sulfur) contents, alites phase coexistence may be detected by using strictly monochromatic laboratory or synchrotron X-ray powder diffraction. However, the application of these results to commercial materials has to be taken cautiously due to the influence of other foreign ions in volume and alite-type.     

Methanol dehydration to dimethyl ether over highly porous xerogel alumina catalyst: Flow rate effect

Template-free sol-gel synthesis in the absence of an acid catalyst resulted in mesoporous nanocrystalline γ-alumina, meso-γ-Al₂O₃, possessing high surface areas, 400-460 m²/g, and high porosity, 1.4-1.9 cm³/g. The prepared alumina was characterized by powder XRD, SEM, and N₂ adsorption for BET surface area and porosity measurements. FTIR spectroscopy was employed to study the catalytic activity of meso-γ-Al₂O₃ and commercial γ-alumina, com-γ-Al₂O₃, in the dehydration reaction of methanol to dimethyl ether, DME. The prepared meso-γ-Al₂O₃ showed higher catalytic activity than the commercial catalyst with a conversion around 86% and DME selectivity around 99%. The products' selectivity showed a significant dependence on the flow rate of the feed gas stream. As the flow rate increased, the selectivity to DME increased on the account of the minor products, CO₂ and CH₄. However, as the flow rate decreased, more CO₂ formed and the DME selectivity decreased.     

Structure and properties of highly stereoregular isotactic poly(methyl methacrylate) and syndiotactic poly(methyl methacrylate) blends treated with supercritical CO2

The structure and properties of highly stereoregular isotactic poly(methyl methacrylate) (it-PMMA) and syndiotactic poly(methyl methacrylate) (st-PMMA) blends with crystalline stereocomplex formed by supercritical CO₂ treatment at temperatures ranging from 35 to 130 °C were investigated by means of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and dynamic mechanical analysis (DMA) measurements. The melting temperature, T_m, and the heat of fusion, ΔH_m, had maximum values at about 200 °C and 25 J/g, respectively. The degree of crystallinity evaluated by WAXD ranged in value from 32 to 38%. The fringed-micellar stereocomplex crystallites were formed in case of treatment temperatures below 90 °C, and the orderliness perpendicular to the helix axis of the fringed-micellar crystallites was considered to be increased with increasing treatment temperature. In case of treatment temperature of 130 °C, the fringed-micellar crystallites and the lamellar crystallites with high orderliness parallel to the helix axis coupled with the perpendicular orderliness were formed, and the respective double endothermic peaks, T_m¹ and T_m³, were observed in DSC due to the melting of the two kinds of stereocomplex crystallites. The it-PMMA/st-PMMA blends containing the fringed-micellar crystallites maintained high values of storage modulus, E′, up to higher temperature compared with the amorphous blends. The E′ of the blend treated with CO₂ at 130 °C decreased twice at temperatures corresponding to T_m¹ and T_m³.     

Role of interface on electrical conductivity of carbon nanotube/alumina nanocomposite

Interface of multiwalled carbon nanotube (MWCNT)/alumina (Al₂O₃) nanocomposites have been studied using TEM. At low sintering temperature (T_sin=1500 °C), a 3–5 nm thick amorphous interface region was noticed. Nanocomposite sintered at 1700 °C possessed a well-defined graphene layer coating on matrix grains as the interface between CNT and Al₂O₃. A mechanism of such layered interface formation has been proposed. No traceable chemical reaction product was observed at the interface even after sintering at 1700 °C. It was noticed that while DC electrical conductivity (σ_DC) of 1500 °C sintered 2.4 vol% MWCNT/Al₂O₃ nanocomposite was only~0.02 S/m, it raised to ~21 S/m when sintering was done at 1700 °C. Such 10³ times increase in σ_DC of present nanocomposite at a constant CNT loading was not only resulted from the exceptionally high electron mobility of CNT but the well-crystallized graphene interface on insulating type Al₂O₃ grains also significantly contributed in the overall increase of electrical performance of the nanocomposite, especially, when sintering was done at 1700 °C.     

Synthesis of γ-Al2O3 nanowires through a boehmite precursor route

By regulating the pH values of the reaction solution, the boehmite (γ-AlOOH) nanowires and nanoflakes were successfully synthesized with a simple hydrothermal route using anhydrous AlCl₃, NaOH and NH₃·H₂O as starting materials. Crystalline γ-Al₂O₃ nanowires with diameter of 10–30 nm and length of several hundreds of nanometer have been prepared by thermal decomposition of γ-AlOOH precursor. X-ray diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAED), and high resolution transmission electron microscope (HRTEM) were used to characterize morphology and structure feathers of the synthesized samples. The pH values of the solution play important roles in the formation of γ-AlOOH nanowires. After calcinated at 500 °C for 2 h, the obtained γ-Al₂O₃ with a linear structure is similar to the γ-AlOOH precursor.     

Performance of modified H-ZSM-5 zeolite for dehydration of methanol to dimethyl ether

The conversion of methanol to dimethyl ether was carried out over various commercial zeolites and modified H-ZSM-5 catalysts to evaluate their catalytic performance. A series of commercially available zeolite samples were used for vapor-phase dehydration of methanol to DME. Catalyst screening tests were performed in a fixed-bed reactor under the same operating conditions (T = 300 °С, P = 16 barg, WHSV = 3.8 h⁻¹). It was found that all the H-form zeolite catalysts in this study were active and selective for DME synthesis. According to the experimental results MDHC-1 catalyst exhibited the highest activity in dehydration of methanol.After finding the most active catalyst, the H-MFI90 zeolite was modified with Na content varying from 0 to 120 mol%, via wet-impregnation method to further improve its selectivity. All of catalysts were characterized by BET, XRD, NH₃-TPD, ICP, TGA, SEM, FT-IR and TPH techniques. It was found that these materials affected activity of MDHC-1 zeolite by changing its acidity. Ultimately, among all the catalysts studied, Na₁₀₀-modified H-MFI90 zeolite exhibited optimum activity, selectivity and stability at methanol dehydration reaction.     

Enhanced performance of a macroporous ceramic support for nanofiltration by using α-Al2O3 with narrow size distribution

Enhanced performance of a macroporous disk alumina support was fabricated through colloidal filtration route, by using α-Al₂O₃ powder with an average particle size of 1.1 μm. The support, sintered at 1250 °C, showed relative high permeances towards water (101 L h⁻¹ m⁻² bar⁻¹) and nitrogen (∼2×10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹), with an average surface roughness of ∼175 nm and a high mechanical strength of 61.1 MPa. Titania supported γ-Al₂O₃ mesoporous layers were deposited onto this promising disk α-Al₂O₃ support through dip-coating. The disk membrane A1100/TiO₂/γ-Al₂O₃, with pore size of ca. 4.4 nm, showed a pure water flux as high as 4.5 L m⁻² h⁻¹ bar⁻¹, which is four times higher than that of γ-Al₂O₃ membrane reported in literature. This mesoporous membrane showed relative high retention rate (∼80%) towards di-valent cations like Ca²⁺, Mg²⁺, but not for the mono-valent cation (Na⁺).     

Effect of CeO2 addition on densification and microstructure of Al2O3-YSZ composites

Alumina (Al₂O₃) and alumina-yttria stabilized zirconia (YSZ) composites containing 3 and 5 mass% ceria (CeO₂) were prepared by spark plasma sintering (SPS) at temperatures of 1350-1400 °C for 300 s under a pressure of 40 MPa. Densification, microstructure and mechanical properties of the Al₂O₃ based composites were investigated. Fully dense composites with a relative density of approximately 99% were obtained. The grain growth of alumina was inhibited significantly by the addition of 10 vol% zirconia, and formation of elongated CeAl₁₁O₁₈ grains was observed in the ceria containing composites sintered at 1400 °C. Al₂O₃-YSZ composites without CeO₂ had higher hardness than monolithic Al₂O₃ sintered body and the hardness of Al₂O₃-YSZ composites decreased from 20.3 GPa to 18.5 GPa when the content of ZrO₂ increased from 10 to 30 vol%. The fracture toughness of Al₂O₃ increased from 2.8 MPa m^1/2 to 5.6 MPa m^1/2 with the addition of 10 vol% YSZ, and further addition resulted in higher fracture toughness values. The highest value of fracture toughness, 6.2 MPa m^1/2, was achieved with the addition of 30 vol% YSZ.     

Anomalous reduction in surface area during mechanical activation of boehmite synthesized by thermal decomposition of gibbsite

Mechanical activation of boehmite (γ-AlOOH), synthesized by thermal decomposition of gibbsite, has been carried out in a planetary mill up to 240 min. After an initial decrease in particle size up to 15 min, the particle size shows an increase with further milling; the median size (d₅₀) has increased from 1.8 to 5 μm during 15 to 240 min of milling. Quite unexpectedly, the BET specific surface area of the sample (N₂ adsorption method) decreases continuously from 264 m²/g to 67 m²/g with milling. A detailed analysis of N₂ adsorption/desorption isotherms has indicated that the decrease in surface area is associated with: (a) change in narrow slit like pores with microporosity to slit shaped pores originating from loose aggregate of platelet type particles; and (b) shift of maxima in pore size distribution plot at ~ 2 nm and ~ 4 nm to dominantly ~ 23 nm size pores. Scanning electron microscopy (SEM) studies have revealed that during milling, initial breakage is followed by agglomeration/fusion of particles with consequent loss in porosity. Amorphisation, decrease in microcrystallite dimension (MCD) and increase in microstrain (ε) are indicated from a detailed analysis of X-ray powder diffraction patterns and Fourier Transform Infrared (FTIR) spectra. Reactivity of samples, expressed in terms of increase in dissolution in alkali (in 8 M NaOH at 90 °C) and decrease in boehmite to γ-Al₂O₃ transformation temperature, increases with milling time. The nature of correlations between reactivity and physico-chemical changes during milling has been analyzed and discussed.     

Titanium-doped alumina for catalytic dehydration of methanol to dimethyl ether at relatively low temperatures

Mesoporous Al-Ti oxide composites with molar %Ti of 3, 5, 10, and 20 as well as pure γ-alumina were prepared using a template-free sol-gel method in the absence of a catalyst. The prepared composites were characterized by powder XRD, FTIR spectroscopy and N₂ adsorption for BET surface area and porosity measurements. The composites and the pure alumina possessed relatively high surface areas, 350-410 m²/g, and high porosities after calcination at 500 °C. FTIR spectroscopy was employed to study the products of the catalytic dehydration of methanol to dimethyl ether, DME, over the prepared catalysts at reaction temperatures between 180 and 300 °C. Compared with pure γ-alumina, the Ti-modified alumina with %Ti < 10 showed higher catalytic activity in the methanol dehydration and better selectivity to DME. Composites with %Ti of 3 and 5 showed the highest activity at relatively lower temperatures than the other catalysts where they showed their highest activity at 190 and 200 °C, respectively. The activity of all studied catalysts slightly decreased as the temperature was raised to 300 °C and dropped considerably when the temperature was decreased to 180 °C. However, the activity of Al-Ti-3 dropped only slightly at both temperatures. The selectivity to DME was dependent on the reaction temperature where 100% DME selectivity was obtained at temperatures ?220 °C and as the temperature was raised to 300 °C, some CH₄ and CO₂ formed on the account of DME.