Transition Alumina Research Articles

Application of 4D STEM and DPC Techniques to Study Surface Reconstruction of Transition Aluminas

Application of 4D STEM and DPC Techniques to Study Surface Reconstruction of Transition Aluminas

Enhancing propane direct dehydrogenation performances through temperature induced VOx dispersion and alumina support phase transformation

Vanadium oxide-based catalysts are promising candidates for propane direct dehydrogenation. A strategy was developed in the present study to enhance the intrinsic activity and coking-resistance of supported VOx catalysts at the same time. The annealing temperature and VOx surface density were identified as regulation parameters to tailor the dispersion of VOx and micro-environment of V-O-Al active sites. The surface acidic VOx was found capable of inducing local structure rearrangement of transition alumina and the transformation of amorphous δ- to more crystalline θ-alumina at relatively lower temperatures, contributing to lowering VOx-support interactions and enhancing VOx reducibility. The DFT calculations verify that the chemical states of V-O active sites could be tuned by the ligand Al3+ coordination micro-environment, and the octahedral Al3+ serving as ligands contributes to enhancing the reducibility and C–H bond rupture capability of V-O site. The annealing temperature was also found capable of dispersing VOx species over alumina surface with the optimal temperature dependent of specific VOx loadings. As a consequence, the intrinsic activity of 3.1 V/Al2O3-900 and the deactivation rate of 3.1 V/Al2O3-700 with dominantly isolated VOx is 1.4 times and 0.65 times that of 3.1 V/Al2O3-550, respectively.

“ω-Al2O3“ a microcrystalline ordered form of transition spinel alumina metastable up to 1200 ​°C

A new variety of alumina with micron-sized crystallization was obtained in mixture with other phases (a Si-doped transition alumina, α-Al2O3 and small amounts of mullite) by firing an Al2O3:SiO2 (12:1) gel at 1200 ​°C, beyond the domain of existence of the heretofore known transition aluminas. The so-called ω-Al2O3 transforms very slowly into α-Al2O3, even at such temperatures. According to X-ray diffraction /Rietveld analysis, the ordered orthorhombic (space group P21212) crystal structure corresponds to a 1 ​× ​1.5 ​× ​1 superstructure of the cubic spinel cell, with a 50 % deficiency in the occupancy of the octahedral sites in 1/3rd of the (010) layers. Though many arguments are in favor of a pure - or almost pure - form of alumina, no evidence can be given of the absence of silicon.

Effect of firing temperature on humidity self-regulation functionality in a ceramic tile composition

The effect of sintering temperature on the humidity regulating capacity of a ceramic composition based on the mineral gibbsite has been studied. The humidity self-regulation functionality is due to the presence of mesopores in the material.In this work, it has been found that the adsorption-desorption capacity decreases drastically with sintering temperature, preserving a certain regulating activity up to temperatures as high as 1200 °C. The variation of the regulating capacity with temperature is consistent with the transformation of gibbsite from transition alumina to alpha alumina, which leads to a sharp decrease of the mesopore volume which can be determined from mercury porosimetry and especially nitrogen gas adsorption. The adsorption-desorption curves fit well to a pseudo-second order kinetic model and it is found that the equilibrium moisture is proportional to the specific surface area while the sorption rate constant depends on permeability for water diffusion through the porous structure.

On the Nature of Extra-Framework Aluminum Species and Improved Catalytic Properties in Steamed Zeolites.

Steamed zeolites exhibit improved catalytic properties for hydrocarbon activation (alkane cracking and dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was suggested to be related to the increased strength of zeolitic Bronsted acid sites after dealumination. We now utilize state-of-the-art infrared spectroscopy measurements and prove that during steaming, aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano) facets of hydroxylated transition alumina surfaces. The Bronsted acidity of the zeolite does not increase and the total number of Bronsted acid sites decreases during steaming. O5Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al1O5 sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively with much lower barriers (at lower temperatures) than for protolytic C-H bond activation, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying mechanism behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on Al-O sites of aluminum oxide clusters confined in zeolitic pores. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites.

Effects of SiO2 Addition on Phase Transition and Microstructure Evolution of Alumina Fibers Prepared Using the Sol-Gel Method

In order to optimize the phase composition and grain size of alumina fibers, 0-3 wt% SiO2 were induced to prepare alumina fibers using sol-gel method with the reaction of aluminium powder, formic acid, acetic acid, nitric acid and SiO2 sol. Effects of low SiO2 content on the phase transition and microstructure of alumina fibers were investigated. X-ray diffraction analysis and differential scanning calorimetry results showed that the addition of silica delayed the formation of α-Al2O3, and the phase transformation path of alumina was changed from amorphous alumina → γ-Al2O3 → α-Al2O3 to amorphous alumina → γ-Al2O3 → θ-Al2O3 → α-Al2O3. Moreover, scanning electron microscopy and transmission electron microscopy results confirmed that the addition of SiO2 reduced the grain size and was beneficial for the dense structure. Fine-grained dense alumina fibers composed of α-Al2O3 phase and transitional alumina phase were obtained by adding 1 wt% and 3 wt% silica after sintering at 1200°C for 1 h.

Crystal‐chemical investigation of the “cubic Al–Si spinel” issued from the thermal transformation of kaolinite and halloysite

AbstractSince its discovery in 1959 as a transient nanocrystalline product of the thermal transformation of aluminum‐rich clays, the so‐called Al–Si spinel has been the object of numerous studies aimed at determining its composition. In this work, samples obtained from fired kaolinite‐ and halloysite‐containing clays were analyzed using X‐ray diffraction, leading to conclude in a single cubic (space group Fd‐3m) form, with an unvarying size of crystallites of 5 nm and a constant cell volume, typical of a transition alumina form. Trial‐and‐error Rietveld fitting procedure revealed a crystal structure consisting in an oxygen close packing and a disordered cation sublattice that differs from the known forms of spinel alumina. Rietveld quantitative phase analysis based on this model was successfully implemented on a series of fired halloysite clays, giving a first quantitative insight on the transformations in the 980–1260°C range.

Pozzolanic and hydraulic activity of bauxite for binder production

Bauxite is mainly used to produce aluminium and calcium aluminate cements. Both applications carry high environmental impacts. However, the carbon emissions generated from mining bauxite (3–5 kg CO2e/t) are negligible when compared those generated for aluminium or cement production (620–700 kg CO2e/t for CEM II). Hence, an alternative use for bauxite would lower environmental impacts worldwide. This paper measures the pozzolanic and hydraulic capacity of raw and thermally activated bauxite with a view to use it, as a binder, in construction.The results indicate that the bauxite is pozzolanic and, when fired over 900 °C it becomes slightly hydraulic. An amorphous reactive phase begins to appear at 300 °C, but the lime binding potential is enhanced at 550 °C because most of the aluminium is in amorphous form. Some active phases (except for ϒ- Al2O3) could not be detected with XRD due to their amorphousness. However, their pozzolanicity was evidenced with direct and indirect measurements.The bauxite's pozzolanic and mechanical indices are high, comparable or superior to other pozzolanic and cementing materials. Gibbsite, boehmite and kaolinite are the main phases in the bauxite. Their layered atomic structures with high specific surface areas (SSA) and active surface hydroxyls that enhance adsorption, and hence nucleation, precipitation and dissolution, provide a high pozzolanic activity. The high kaolinite content, and the presence of the highly-reactive, kaolinite-polymorph nacrite, enhance reactivity, and the presence of sulphates also contributes to the initial activity.The SSA of the bauxite particles, is superior to CEM II and other pozzolanic and cementing materials, and heating, even at low temperature (300 °C), drastically increases the SSA of the particles (by∼60%). The methods that determine initial pozzolanic activity rate the 550 °C bauxite as the most active, while the mechanical methods rate the 700–800 °C bauxites as the most reactive. This is because the high SSA of the 550 °C bauxite enhances lime combination in the earlier stages of the pozzolanic reaction, while the 700–800 °C bauxite has the highest amorphous/active alumina content which control late pozzolanic activity. As evidenced with XRD, at 700–800 °C, boehmite has disappeared, transformed into amorphous transition aluminas, and kaolinite has completely turned into reactive metakaolin.

Advances in the understanding of mesoporous transition aluminas: Unveiling the correlation between morphology and thermostability

The thermostabilities of mesoporous γ-Al2O3 samples with similar initial surface areas and average pore radii, but significantly different pore volumes, were studied in detail. The results show that γ-Al2O3 with highest initial pore volume converts to α-Al2O3 at a significantly higher temperature, along with larger residual surface area and pore volumes after aging. These observations are directly related to the morphologies of the individual particles and the resulting aggregate structures, which were studied by automated crystallite orientation mapping (ACOM-TEM). Large particles with well-developed plate-like morphologies preferentially form dense stacks by assembly via the {110} main basal planes (pseudo-cubic lattice). This arrangement favors sintering and the formation of α-Al2O3 at comparatively low temperatures. Aluminas with higher proportions of the lateral {100} and {111} facets tend to form aggregate structures with higher porosities and lower interparticle contact areas, thus explaining their superior thermostabilities. These findings highlight the importance of the aggregate structure for the purposeful development of alumina-supported catalysts.

Synthesis of submicronic α-alumina from local aluminum slags

In this study, a high valued product submicronic ?-alumina was successfully extracted from aluminum slags generated by the local aluminum industry. The extraction technique was based on the leaching of slags by H2SO4 followed by precipitation. The coarser aluminum-rich fractions of the slags were used in this study instead of the finer oxide-rich fractions that were commonly used in previous studies. The precipitation of the leached slags by NH4OH was controlled by a zetameter in order to determine the optimal precipitation pH. Then, the obtained gel showing the higher precipitation rate and the finer particle size was calcined at 1200 ?C and characterized by the XRF, XRD, FTIR, SEM, EDS, and laser granulometry. Even without any pretreatment of slags, the XRF analysis revealed that a high purity and high extraction efficiency of 99.2% and 93.75% respectively could be achieved just at a leaching acid concentration of 15%. The XRD spectrum shows that the produced alumina was pure a-corundum, which was confirmed by the FTIR spectrum showing only the Al-O bonds. The laser granulometry showed that the recovered powder exhibited a wide particle size distribution, which was between 50 nm and 20 ?m, while the average particle size (d50) was about 400 nm. The SEM observations revealed that the grains were in the form of submicronic whiskers. The above characteristics allowed the obtained alumina powder in this study to be used in the usual applications of alumina such as refractory, ceramic fibers, abrasive, etc. The obtained powders may also assume applications as a thermally stable substitute for the commonly used transition alumina powders, which needs further investigations in future studies.

Phosphate Adsorption on Amorphous Aluminum Hydroxide-Yielded Transition Aluminas: Insights on Fundamental Chemistry and Making Engineering Adsorbents

Phosphate Adsorption on Amorphous Aluminum Hydroxide-Yielded Transition Aluminas: Insights on Fundamental Chemistry and Making Engineering Adsorbents

A comparative study on the thermal stability, textural, and structural properties of mesostructured γ-Al2O3 granules in the presence of La, Sn, and B additives

Mesoporous γ-alumina is widely used as catalyst support in various catalytic reactions of industrial interest. However, due to the instability of γ-alumina at elevated temperatures, many efforts have been reported to inhibit the α-alumina phase transition through doping with suitable metalloids, as well as transition, post-transition, or rare-earth elements. In the present study, undoped and La-, Sn-, and B-doped alumina granules were synthesized via sol-gel/oil drop method with the aim to clarify the role of the additives and their content on the porous structure as well as on the chemical, structural, and microstructural behavior of γ-alumina. XRD and DTA/TG results demonstrated that thermal stability of transition aluminas increases more than 100 °C by 3 wt% lanthanum and tin doping; however, boron doping seems to have only negligible effect on the thermal stability. On the other hand, based on nitrogen adsorption-desorption analysis, tin and boron-doped aluminas showed a higher surface area at 750 °C (between 214.74 m2/g to 245.97 m2/g) but higher loss in the surface area after calcination at 1200 °C (between 25.45 m2/g to 8.57 m2/g). On the contrary, the 3 wt% La-doped alumina sample, with a relatively high surface area at 750 °C (227.17 m2/g), exhibited the highest surface area after calcination at 1200 °C (53.07 m2/g). 27Al MAS NMR and HRTEM studies indicated that the presence of 3 wt% La in alumina structure leads to thermal and mesoporous structure stability up to 1200 °C by inhibiting oxygen lattice restructuring. These results provide a comparative perspective of La, B, and Sn additives' behavior in γ-alumina.

Malonate complexes at γ-alumina surface determined by a multi-technique characterization approach and a surface complex model

γ-Al2O3 is a transition alumina oxide used in many different applications. Its surface chemistry seems to be a key factor in the preparation of heterogeneous catalyst where γ-Al2O3 is used as a support. The modification and description of its surface are necessary to understand the role of the support on the catalyst preparation and on its performance. In this work, the modification of the surface is performed with a carboxylic diacid, malonic acid, which specifically adsorbs at the γ-Al2O3 surface and modifies the surface chemistry (surface sites density and global surface charge). The impact of the impregnation pH, malonic acid content, and drying are studied. By using a multi-technique approach, both the solid/liquid interface and the dried surface were characterized. The infrared spectroscopic information coupled with zetametry measurements indicate the formation of an inner sphere complex adsorbed via both side-on ester linkage and bridging bidentate mode involving the two carboxylic groups. A surface complexation model was built to support the experimental results and obtain complementary information. It underlines the different reactivity of the γ-Al2O3 facets and of the surface sites responsible for the adsorption. The whole (100) facet is involved in the adsorption while only the µ1-AlVI-OH and µ1-AlIV-OH sites of the (110) facet participate in the malonic acid adsorption. The model also confirms the consumption of 3–4 surface hydroxyls per adsorbed molecule, in accordance with ester linkage and bridging bidentate structure. This molecular level description of the surface chemistry is of interest in different environmental fields such as wastewater treatment and pollutant removal. In the catalysis field, the detailed surface chemistry will be very useful to understand the impact of this modified support on the catalytic performance.

Photoluminescence characterizations in phase transition alumina with boehmite nanostructures

Photoluminescence characterizations in phase transition alumina with boehmite nanostructures

Effect of transitional aluminas on Portland cement hydration, phase assemblage and the correlation to ASR preventing effectiveness

This study reveals the impact of transitional aluminas on the hydration kinetics, phase assemblage of Portland cement, and ASR preventing effectiveness. Transitional aluminas were obtained by annealing reactive alumina mainly consisting of γ-Al2O3 at various temperatures. Their impacts on hydration kinetics at early age and phase assemblage were examined on a Portland cement blended with 7 wt% transitional aluminas. The increase in annealing temperature reduces the reactivity of transitional aluminas, manifesting into a decreasing dissolution rate in stimulated pore solution. The rapid dissolution of transitional aluminas annealed below 600 °C prolonged the induction period and inhibited the dissolution of C3S. Their continuous dissolution contributes to the formation of monosulfate, (SO4-OH) AFm, and katoite. As a result, the incorporation of these transitional aluminas prevents ASR effectively. In contrast, transitional aluminas annealed above 600 °C present as fillers due to their decreased reactivity, hence promoting the hydration process and exerting little effect on phase assemblage.

Pozzolanic and Cementing Activity of Raw and Pyro-Processed Saudi Arabian Red Mud (RM) Waste

This paper investigates the composition, properties and reactivity of a red mud waste generated in Saudi Arabia with a view to find alternative materials to replace construction binders of high environmental impact. The phase transformation triggered by the sintering of the RM up to 1000°C is determined with thermal and X-Ray Diffraction analyses. Reactivity is investigated with chemical and physical methods including the Chapelle test, setting times, mechanical index and microscopy. The RM is clearly pozzolanic, and its activity is mainly due to the reaction of feldespathoids and the formation of zeolitic and feldspathoid-based hydrates. The positive effects of the thermal treatment are seen below 750°C, and include the loss of water in the zeolite/feldespathoids, and the destruction of the crystal structures of the clay minerals inherited form the parent bauxite. The negative effects of the thermal treatment are evidenced over 750°C, with a decrease in specific surface area, devitrification and crystal formation, whereby the active transition aluminas and the fedespathoids/zeolites (mainly cancrinite) transform into nepheline, tricalcium aluminate (C3A) and gehlenite. Despite the occurrence of nepheline, C3A and gehlenite in the RM sintered at 1000°C, the formation of pozzolanic hydrates that cause setting and strength development are greater at lower temperature. The optimum thermal treatment that enhances pozzolanic activity lies at c.400°C, as evidenced by the highest lime combination, the greatest mechanical index and the fastest set. The RM consists of gibbsite and boehmite, inherited from the bauxite, and cancrinite, chantalite and sodalite formed during the Bayer process. Feldespathoids have formed, instead of zeolites, due to the available silica and the high alkali content of the RM. The quick lime -CaO -, added twice during the refining process, has transformed the original goethite into hematite, and produced cancrinite. The Saudi RM has high SiO2 and high alkalinity, and an abundant specific surface area available for reaction. The chloride and carbon contents are low, and no environmental toxicity is inferred from its chemistry.

Processing and Thermal Conductivity of Bulk Nanocrystalline Aluminum Nitride.

Producing bulk AlN with grain sizes in the nano regime and measuring its thermal conductivity is an important milestone in the development of materials for high energy optical applications. We present the synthesis and subsequent densification of nano-AlN powder to produce bulk nanocrystalline AlN. The nanopowder is synthesized by converting transition alumina (δ-Al2O3) with <40 nm grain size to AlN using a carbon free reduction/nitridation process. We consolidated the nano-AlN powder using current activated pressure assisted densification (CAPAD) and achieved a relative density of 98% at 1300 °C with average grain size, ~125 nm. By contrast, high quality commercially available AlN powder yields densities ~75% under the same CAPAD conditions. We used the 3-ω method to measure the thermal conductivity, κ of two nanocrystalline samples, 91% dense, = 110 nm and 99% dense, = 220 nm, respectively. The dense sample with 220 nm grains has a measured κ = 43 W/(m·K) at room temperature, which is relatively high for a nanocrystalline ceramic, but still low compared to single crystal and large grain sized polycrystalline AlN which can exceed 300 W/(m·K). The reduction in κ in both samples is understood as a combination of grain boundary scattering and porosity effects. We believe that these are finest reported in bulk dense AlN and is the first report of thermal conductivity for AlN with ≤220 nm grain size. The obtained κ values are higher than the vast majority of conventional optical materials, demonstrating the advantage of AlN for high-energy optical applications.

High purity alumina synthesised from iron rich clay through a novel and selective hybrid ammonium alum process

The demand for high purity alumina (HPA) has been steadily increasing due to its use in advanced electronics and battery applications. This paper presents an innovative synthesis pathway, which is one of a series of pathways investigated to obtain HPA from a local Australian clay source with minimal clay processing. The novelty in this work was centred around impurity control since the feed clay had a high proportion of iron phases (19%). Ammonium aluminium sulfate (NH4Al(SO4)2·12H2O) was used as an intermediate to both selectively recover aluminium from sulfuric acid leachates and as a transition to finer refinement of aluminium in a chloride system through crystallisation of aluminium chloride (AlCl3·6H2O). The aluminium chloride was decomposed at 800 °C and 1200 °C to yield alumina with a purity of >99.99% Al2O3. The HPA product was characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and was found to be comprised of transitional alumina phases including κ and χ-Al2O3 after calcination at 800 °C and α-Al2O3 after calcination at 1200 °C. The purity of the HPA was assessed using an acid digestion method followed by analysis with ICP-OES and ICP-MS.

A sequential Box-Behnken Design (BBD) and Response Surface Methodology (RSM) to optimize SAPO-34 synthesis from kaolin waste

Kaolin waste from an industry in the Amazon region was selected as an economic source of Si and Al to optimize the synthesis of SAPO-34 molecular sieve. A sequential BBD, RSM and global desirability function (D) was used. The factors studied for optimization were the SiO2/Al2O3 molar ratio, the concentration of the structure-directing agent (SDA; morpholine) and the crystallization time. The responses evaluated in the synthesized SAPO-34 were the relative crystallinity (%), the enthalpy of formation of transition alumina (J.g−1) and the mass loss (%). The synthesis as a function of time and its products post-DSC and calcined at 1000 °C was performed to relate the enthalpy of formation of the transition alumina with the SAPO-34 formation. Prediction models and response surfaces were corroborated by the level values optimized by the desirability function. The ideal values based on the data used in this study included a crystallization time of 20 h, a xSiO2 value of 1.3 and a ySDA value of 1.6. The experimental validation based on the ideal values was within the established confidence level (±95%). The formation of the transition alumina at 985 °C influences the enthalpy of exothermic transformation identified at this temperature. Low values of this enthalpy are related to the effective formation of SAPO-34.

Low Temperature Transitional Aluminas: Structure Specifics and Related X-ray Diffraction Features

Despite the fact that metastable aluminum oxides are actively used in industry, there is a discrepancy in the literature regarding their crystal structure. All this leads to difficulties in data interpretation and, as a consequence, classification problems. This work is aimed at solving these tasks. The main features of powder X-ray diffraction of typical samples of three Al2O3 polymorphs (γ-, χ-, η-) are analyzed. Specifics and fundamental differences in X-ray scattering and their relationship with the structural organization at the nanostructure level are clearly shown. The work demonstrates the possibilities of analyzing experimental powder X-ray diffraction data using a modern approach based on the Debye Scattering Equation for studying the organization of such complex systems.