Al Sites Research Articles

First-principles lattice dynamics and thermodynamic properties of α-, θ-, κ- and γ-Al2O3 and solid state temperature-pressure phase diagram

γ-Al2O3 is a commonly observed high temperature alumina phase during Al-based alloy oxidation. Due to the partially ordered, defective nature of the spinel structure arising from the partial occupancy in the Al sites, first-principles prediction of the thermodynamic properties of γ-Al2O3 has been challenging. In this work, employing the first-principles quasiharmonic approach, we obtain the finite temperature thermodynamic properties of γ-Al2O3, including entropy, chemical potential, heat capacity, thermal expansion coefficient, and elastic constants and the results are compared with those calculated for the other three alumina phases, i.e., α-Al2O3, θ-Al2O3, and κ-Al2O3. The calculated lattice constants and the predicted relative phase stability under 0 K and 0 GPa, α>κ>θ>γ, are consistent with experimental values and existing calculations, respectively. Based on the results, we constructed the temperature-pressure phase diagram covering the four Al2O3 phases (α, κ, θ, γ), which could provide for experimentally synthesizing the different alumina phases.

Synthesis and characterization of (Al,Si)3(Zr,Ti)-D022/D023 intermetallics: Understanding the stability of silicon substitution

(Al,Si)3(Zr,Ti)-D022/D023 are phases that may form in aerospace and automotive aluminium alloys. The substitution of Zr/Ti in these solid solutions is widely reported in the literature; however, it remains relatively unexplored for Si. In this work, in situ precipitation of (Al,Si)3(Zr,Ti)-D022/D023 intermetallics was performed using Al-Si-Zr-Ti alloys. The precipitation, sedimentation and concentration of numerous intermetallic particles were accomplished by filtrating the residual molten aluminium using a temperature/pressure-controlled vessel adapted with a PoDFA filter. A combination of SEM, TEM, XRD and EMP analysis allowed the identification of (Al,Si)3(Zr,Ti)-D022/D023 intermetallics concentrated within α-FCC matrices of non-Si-doped (sample S2) and Si-doped (samples S4 and S6) alloys. EDS analysis confirmed that Zr and Ti substitute each other in the D022 and D023 phases, whereas Si substitutes in Al sites. Acceptance of Si inside the D023 phase was not expected according to FTlite (FactSage) and TCAL7 (Thermo-Calc) databases. Additionally, Si was found to enhance the formation of (Al,Si)3(Zr,Ti)-D022 intermetallics with high Zr-content, contrary to FactSage 7.3 predictions. TEM results showed intermetallic/FCC crystal coherency for samples S2 and S6, implying that these intermetallics acted as nucleation sites for the Al-phase due to their small lattice mismatch. Furthermore, Si site occupancy was calculated for both (Al,Si)3Ti-D022 and (Al,Si)3Zr-D023 phases via DFT, showing that sites 2b and 4e are the most favorable for Si occupation, respectively. Finally, a thermodynamic model is derived to describe Si substitution upon solidification. Experimental and numerical examinations indicate that Si substitution preferentially occurs in the D022 intermetallics compared to the D023 phase.

Tuning Main Group Element-based Metal-Organic Framework to Boost Electrocatalytic Nitrogen Reduction Under Ambient Conditions.

Main group element-based materials are emerging catalysts for ammonia (NH3 ) production via a sustainable electrochemical nitrogen reduction reaction (N2 RR) pathway under ambient conditions. However, their N2 RR performances are less explored due to the limited active behavior and unclear mechanism. Here, an aluminum-based defective metal-organic framework (MOF), aluminum-fumarate (Al-Fum), is investigated. As a proof of concept, the pristine Al-Fum MOF is synthesized by the solvothermal reaction process, and the defect engineering method namely solvent-assisted linker exchange, is applied to create the defective Al sites. The defective Al sites play an important role in ensuring the N2 RR activity for defective Al-Fum. It is found that only the defective Al-Fum enables stable and effective electrochemical N2 RR, in terms of the highest production rate of 53.9µg(NH3 ) h-1 mgcat -1 (in 0.4m K2 SO4 ) and the Faradaic efficiency of 73.8% (in 0.1m K2 SO4 ) at -0.15V vs reversible hydrogen electrode) under ambient conditions. Density functional theory calculations confirm that the N2 activation can be achieved on the defective Al sites. Such sites also allow the subsequent protonation process via the alternating associative mechanism. This defect characteristic gives the main group Al-based MOFs the ability to serve as promising electrocatalysts for N2 RR and other attractive applications.

Tandem c-cl cleavage and oxidation over Pt/ZSM-5 catalysts for CH2Cl2 deep oxidation: Acidity – Redox synergy, rate – Limiting step, and catalyst stability

A Pt/ZSM-5-18 catalyst with a Si/Al molar ratio of 18 in the support was highly active for the oxidation of CH2Cl2, exhibiting high selectivity towards CO2 formation. The excellent performance can be attributed to its abundant surface acidity, which facilitates the C-Cl cleavage of CH2Cl2, as well as the strong oxidation ability of the Pt species that promotes the sequential oxidation of intermediates such as CH3Cl and CO. Quantitative analysis of the turnover frequencies (TOFs) further revealed that the C-Cl cleavage of CH2Cl2 is the rate-limiting step, while the oxidation of CH3Cl and CO on the Pt surface is more facile, with TOFs of 10-fold and 50-fold higher than that of the C-Cl cleavage, respectively. However, the rate-limiting step in CH3Cl oxidation is related to the oxidation step, suggesting a reactant-dependent kinetic relevance. The partial deactivation of the catalyst can be attributed to either coke deposition under dry conditions or competitive adsorption of water under wet conditions. Nonetheless, the catalyst’s stability is hardly influenced by Pt aggregation through the migration of framework Al sites in the support.

Controlling Metal-Oxide Reducibility for Efficient C-H Bond Activation in Hydrocarbons.

Knowing the structure of catalytically active species/phases and providing methods for their purposeful generation are two prerequisites for the design of catalysts with desired performance. Herein, we introduce a simple method for precise preparation of supported/bulk catalysts. It utilizes the ability of metal oxides to dissolve and to simultaneously precipitate during their treatment in an aqueous ammonia solution. Applying this method for a conventional VOx -Al2 O3 catalyst, the concentration of coordinatively unsaturated Al sites was tuned simply by changing the pH value of the solution. These sites affect the strength of V-O-Al bonds of isolated VOx species and thus the reducibility of the latter. This method is also applicable for controlling the reducibility of bulk catalysts as demonstrated for a CeO2 -ZrO2 -Al2 O3 system. The application potential of the developed catalysts was confirmed in the oxidative dehydrogenation of ethylbenzene to styrene with CO2 and in the non-oxidative propane dehydrogenation to propene. Our approach is extendable to the preparation of any metal oxide catalysts dissolvable in an ammonia solution.

Oxidative Activation of Small Aluminum Nanoclusters with Boron Atom Substitution prior to Completing the Endohedral B@Al12- Superatom.

Elemental substitution and doping validate the optimization of chemical and physical properties of functional materials, and the composition ratio of the substituting atoms generally determines their properties by changing their geometric and electronic structures. For atomically precise nanoclusters (NCs) consisting of countable atom aggregates, the composition can be controlled accurately to provide an ideal model to study the heteroatom substitution effects. Since aluminum (Al) and boron (B) both belong to group 13 in the periodic table, the effect of B atom substitution on Aln NCs can be investigated while maintaining the total number of valence electrons in AlnBm NCs. In this study, oxidative reactivities of small Al NCs with B atom substitution are studied for AlnBm NCs (m = 1, n = 6-14 and m = 2, n = 11) supported on organic surfaces by using X-ray photoelectron spectroscopy and oxygen molecule (O2) exposure measurements. Before completing the endohedral B@Al12- superatomic NC, one B atom substitution in Al NCs (AlnB) enhances oxidative reactivities 3-20 times compared to those of Aln+1, particularly for n ≤ 11. When one Al atom of Al12B is further substituted by a B atom to form Al11B2, the reactivity drastically increases (6.6 × 102 times), showing that the B atom substitution makes the NC chemically active or inactive geometrically depending on the exohedral or endohedral site for the B atom in the Al NC. In addition, density functional theory calculations show that the electronegative B atom contributes to forming a locally positive Al site to facilitate O2 adsorption except in Al12B, in which the B atom is geometrically shielded by the surface of the Al12 cage in B@Al12.

Investigations of electronic, elastic, and optical properties of (Ag, Cd)-doped LaAlO3: a computational insight

In this study, we used density functional theory to examine how the wide band gap optoelectronic properties of pure, silver-, and cadmium-doped LaAlO3 perovskites changed. Structural, electrical, elastic, and mechanical properties were calculated using generalized gradient approximation. Silver and cadmium incorporated at Al site demonstrated a decreasing trend in the band gap. Both impurities showed the reduction in band gap from 2.98 to 0.468 eV and from 2.98 to 2.0238 eV. The density of states was examined for pristine and doped structures to understand behavioral change of LaAlO3. It was observed that p states in upper valance band and d states in lower conduction band were contributing toward the reduction of band gap. Elastic properties were computed. Elastic parameters were used to calculate Born’s stability and it is predicted that the material is stable mechanically for both pristine and doped forms. Mechanical properties were also predicted and brittle nature was found for both pristine and doped materials. Optical responses such as dielectric function, absorption, and refractive index were also predicted. The impurity inclusion in pristine structure not only reduces the bad gap but also alters the optical behavior. Absorption edge shifted toward lower energy shows a clear redshift, whereas refractive index also reduces from 2.4 to 1.9, making the material more transparent. The absorption spectrum as well as electronic band gap makes this material more useful for solar cell application as well as photocatalytic applications.

Role of imperfect coordination of amorphous metal oxide in its chemical reaction with epoxy resin in metal − polymer hybrid material

The chemical bonding between laser − induced metal oxide and polymer highly determines the interfacial bonding performance of metal − polymer hybrid materials. To achieve reliable interfacial chemical bonding, it is crucial to gain a molecular/atomic level understanding of the chemical reaction mechanism between the metal oxide and epoxy resin (main components of epoxy polymer). Previous research has demonstrated that laser ablation induces the formation of amorphous aluminium oxide on aluminium alloy surfaces. Our solid − state nuclear magnetic resonance (ssNMR) investigation reveals that this amorphous aluminium oxide is consisted mainly of 4 − fold and 5 − fold coordinated Al sites. Through an innovative combination of ssNMR and density functional theory calculations, this aluminium oxide was confirmed to form Al − O − C bonds with epoxy resin, preferentially at imperfectly coordinated Al sites due to active donor − acceptor interactions. It is clarified that imperfect coordination of amorphous aluminium oxide plays a more critical role than its amorphous nature in forming stable adsorption structure with epoxy resin. Furthermore, compared to on crystalline surface, 5 − fold coordinated Al site on amorphous surface exhibits higher reactivity, and the origin of activity is revealed through distortion/interaction analysis. This study provides a research guidance for further regulation and optimization of interfacial reactivity and bonding characteristics of metal − polymer hybrid materials.

Nature of GaOx Shells Grown on Silica by Atomic Layer Deposition.

Gallia-based shells with a thickness varying from a submonolayer to ca. 2.5 nm were prepared by atomic layer deposition (ALD) using trimethylgallium, ozone, and partially dehydroxylated silica, followed by calcination at 500 °C. Insight into the atomic-scale structure of these shells was obtained by high-field 71Ga solid-state nuclear magnetic resonance (NMR) experiments and the modeling of X-ray differential pair distribution function data, complemented by Ga K-edge X-ray absorption spectroscopy and 29Si dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) studies. When applying one ALD cycle, the grown submonolayer contains mostly tetracoordinate Ga sites with Si atoms in the second coordination sphere ([4]Ga(Si)) and, according to 15N DNP SENS using pyridine as the probe molecule, both strong Lewis acid sites (LAS) and strong Brønsted acid sites (BAS), consistent with the formation of gallosilicate Ga-O-Si and Ga-μ2-OH-Si species. The shells obtained using five and ten ALD cycles display characteristics of amorphous gallia (GaOx), i.e., an increased relative fraction of pentacoordinate sites ([5]Ga(Ga)), the presence of mild LAS, and a decreased relative abundance of strong BAS. The prepared Ga1-, Ga5-, and Ga10-SiO2-500 materials catalyze the dehydrogenation of isobutane to isobutene, and their catalytic performance correlates with the relative abundance and strength of LAS and BAS, viz., Ga1-SiO2-500, a material with a higher relative fraction of strong LAS, is more active and stable compared to Ga5- and Ga10-SiO2-500. In contrast, related ALD-derived Al1-, Al5-, and Al10-SiO2-500 materials do not catalyze the dehydrogenation of isobutane and this correlates with the lack of strong LAS in these materials that instead feature abundant strong BAS formed via the atomic-scale mixing of Al sites with silica, leading to Al-μ2-OH-Si sites. Our results suggest that [4]Ga(Si) sites provide strong Lewis acidity and drive the dehydrogenation activity, while the appearance of [5]Ga(Ga) sites with mild Lewis activity is associated with catalyst deactivation through coking. Overall, the atomic-level insights into the structure of the GaOx-based materials prepared in this work provide a guide to design active Ga-based catalysts by a rational tailoring of Lewis and Brønsted acidity (nature, strength, and abundance).

Tailoring the structure, mechanical, electronic, and thermodynamic properties of Al8Mn5 by doping Ti atom with different atomic site configurations

To solve the problem of the influence of Ti particles on Al8Mn5 phases is difficult to observe from the experiments, the effects of Al8Mn5 doped with Ti atom at different Wyckoff sites on its thermodynamic properties, mechanical properties, and electronic structural characteristics were systemically studied by first-principles calculations in this work. It was found that the doping of Ti can enhance the thermodynamic, dynamic, and mechanical stability, change the elastic anisotropy in different directions, and decrease the ductility of Al8Mn5. The calculated mechanical results showed that the bulk modulus decreased, while the Young's modulus (E), hardness (HV), and shear modulus improved for the Ti-doped Al8Mn5 phases. The Al–Ti5 phase in Ti doping Al sites and the Mn–Ti3 phase in Ti doping Mn sites have the largest E with the values of 234.59 and 233.25 GPa, respectively. Additionally, the Al–Ti5 and the Mn–Ti3 phases have the highest HV of 16.24 and 16.06 GPa in Chen's model, and 14.31 and 14.48 GPa in Tian's model, respectively. The electronic structures and chemical bonding characteristic information elaborated that the improvement in mechanical properties is due to the improvement in Al–Mn and Al–Al bond strength.

Effect of Tetrahedrally Coordinated Al on the Surface Acidity of Mg-Al Binary Mixed Oxides.

Metal oxides (MOs) having Mg and Al with Mg/Al ratios of 1, 2, 3, and 4 were synthesized via calcination of the layered double hydroxides (LDH). The X-ray diffraction analysis revealed that all the MO consisted of periclase (MgO) crystallite with comparable crystallinity regardless of the metal ratio. According to the 27Al magic-angle spinning nuclear magnetic resonance, the phase transformation from LDH to MO upon calcination facilitated the evolution of the Al3+ ions with unsaturated coordination at the surface of MO. The specific surface area values of MOs were not significantly different from each other, ranging between 100 and 200 m2/g, suggesting that the metal ratio did not strongly influence the porous structure of MO. The temperature-dependent desorption of ammonia demonstrated that the Lewis acidity of the Al-rich MOs was the largest with an Mg/Al ratio of 1, attributed to the efficient exposure of the surface-active site Al3+-O2- pairs. The acidity of heterogenous Al-rich MOs significantly increased with the exposed tetrahedral Al site on the surface and dramatically diminished when the molar ratio (Mg/Al) was over two.

Fluorine and related complexes in α-Al2O3

Using first-principles calculations based on hybrid-density-functional theory, we examine the energetics and electronic structure of fluorine in α-Al2O3. The F atom can be incorporated as an interstitial (Fi) or substitutional impurity on the oxygen site (FO); the latter tends to be lower in energy, particularly under Al-rich conditions. Fluorine on the oxygen site acts as a donor, but for Fermi-level positions high in the bandgap, a negatively charged DX configuration is lower in energy. Fluorine substituting on the Al site is not energetically stable. We also examine complexes between F and hydrogen or carbon, which can easily be unintentionally incorporated during growth or processing. Our calculated defect levels, combined with band alignments, allow us to assess the impact on Al2O3/semiconductor heterostructures. We find that F can passivate oxygen-vacancy related traps in the Al2O3 dielectric. Complex formation with H or C is either ineffective or could even be detrimental.

A novel Al-Si-Ni-Fe near-eutectic alloy for elevated temperature applications

A novel near-eutectic Al-15.0Si-4.1Ni-1.9Fe (wt%) alloy with a ternary eutectic reaction of Liquid→α-Al+Si+(Al,Si)5(Fe,Ni) was investigated. Eutectic Si and (Al,Si)5(Fe,Ni) phases exhibit short nanoscale fibrous morphologies with volume fractions of 14.3 ± 1.6% and 15.1 ± 1.9%, respectively. The (Al,Si)5(Fe,Ni) phase has a tetragonal Al2.7FeSi2.3-type crystal structure with excellent thermal stability, which contributes to high mechanical properties at room and elevated temperatures. First-principles density-functional theory (DFT) calculations reveal its chemical composition of tetragonal (Al4.75Si0.25)(Fe0.5Ni0.5) having Si solution at the Al sites with two Fe neighbours contributes to the lowest solution energy. The newly developed alloy has superior mechanical properties at room and elevated temperatures compared with other typical heat-resistant aluminium alloys, which has great potential for industrial applications.

Influence of Ga Substitution on the Local Structure and Luminescent Properties of Eu-Doped CaYAlO4 Phosphors

Understanding the local environment of luminescent centers in phosphors serves as a blueprint for designing the luminescent properties of phosphors. Chemical substitution is a general strategy for engineering the local structure around luminescent center ions. In this study, we systematically investigate the luminescent properties of Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) phosphors and the local structure of the Eu ions. The Ga substitution at the Al sites leads to a significant enhancement in the electric dipole transition of Eu3+ (5D0 → 7F2). The Judd–Ofelt analysis reveals that Eu3+ ions are substituted for Ca/Y, and the Ga substitution increases the asymmetricity of the local structure around the Eu ions because of the different ionic radii and electronegativities of Al and Ga. In addition, Eu2+ emission is missing regardless of the Ga substitution and post-hydrogen treatments. The present work provides deeper insight into the role of chemical substitution in oxide phosphors.

CO2 Cycloaddition under Ambient Conditions over Cu-Fe Bimetallic Metal-Organic Frameworks.

Carbon dioxide cycloaddition into fine chemicals is prospective technology to solve energy crisis and environmental issues. However, high temperature and pressure are usually required in the conventional cycloaddition reactions of CO2 with epoxides. Moreover, metal active sites play a vital role in the CO2 cycloaddition, but it is still unclear. Herein, we select the isostructural MOF-919-Cu-Fe and MOF-919-Cu-Al as models to promote the performance and clarify the effects of metal type on the CO2 cycloaddition. The MOF-919-Cu-Fe with exposed Fe and Cu Lewis acid sites reaches the CO2 cycloaddition with over 99.9% conversion and over 99.9% selectivity at room temperature and a 1 bar CO2 atmosphere, 3.0- and 52.6-fold higher than those of the MOF-919-Cu-Al with Al and Cu sites (33.8%) and the 1H-pyrazole-4-carboxylic acid, Fe, and Cu mixed system (1.9%), respectively. The proposed mechanism demonstrated that the exposed Fe3+ sites facilitate the ring opening of epoxide and CO2 activation to boost the CO2 cycloaddition reaction. This work provides a new insight to tune the catalytic sites of MOFs to achieve high performance for CO2 fixation.

Study of ground state magnetic structure of L10-type MnAl via spin-spiral density wave

We investigated the ground state magnetic structure of L10-type MnAl with the density functional theory via the spin-spiral density wave state. Our results indicate that stoichiometric perfectly ordered MnAl prefers an antiferromagnetic state, while the ferromagnetic state is metastable. When a chemical disorder is introduced with the degree of order S = 0.9, the spiral period becomes longer than that of the antiferromagnetic structure, and below S = 0.8, the system adopts the ferromagnetic state. Furthermore, doping of Mn atoms into the Al sites above 5 at% led to the ferromagnetic state. We confirmed that chemical or compositional disorder induces ferromagnetic transitions in MnAl alloys.

Silicon Incorporation Reduces the Reactivity of Short-range Ordered Aluminosilicates Toward Organic Acids

The structure and composition of short-range ordered aluminosilicates (SROAS) may control their affinity for organic acids with potential effects on soil organic matter stabilization. Adsorption mechanisms of model organic acids were studied to resolve the effect of Si incorporation. Adsorption of oxalic, salicylic, and octanoic acid on Al-rich (Al:Si = 3.7) and Si-rich (Al:Si = 1.4) SROAS was quantified by analyses of dissolved organic carbon using catalytic high-temperature combustion. The initial pH of 5 and 6.5 increased to 6.3–8.2 during adsorption of oxalic and salicylic acid, demonstrating hydroxyl release by ligand exchange. Minor changes in pH indicated weak interactions of octanoic acid with both SROAS. Adsorbates were characterized by Fourier-transform infrared spectroscopy. Asymmetric stretching of carboxylate groups at 1720 and 1700 cm–1, and symmetric stretching at 1430 cm–1 evinced the formation of chelate complexes for oxalic acid. An absorption band centered at 1545 cm–1 indicated partial inner-sphere binding of salicylic acid on both SROAS. Silicon-rich SROAS adsorbed 80–90% less than Al-rich SROAS, suggesting that adsorption of oxalic and salicylic acid was controlled by surface aluminol groups. Fast kinetics of oxalate adsorption on Al sites was studied by a conductivity-based stopped-flow technique. Ligand exchange proceeded at a rate constant of 3.5 s–1 (25°C), similar to solute Al complexation, with an activation energy of up to 34.1 kJ mol–1. A slow process with a rate constant of 0.13 s–1 (25°C) was attributed to diffusion of oxalate at the surface or into SROAS particles. As supported by structural characterization of Si-rich SROAS, the much lower susceptibility of Si-rich SROAS to ligand exchange relates to Al speciation. The formation of tetrahedral Al precludes its complexation by carboxyl groups.

Surface solid frustrated Lewis pair constructed on alumina for selective hydrogenation reaction

The design of solid frustrated Lewis pairs (FLPs) in heterogeneous systems is crucial to achieve the desired catalytic behavior in vital heterogeneous processes such as hydrogenation reactions. Herein, we report a synthesis approach for FLPs via calcination of AlOOH under different atmospheres (air and N2 atmospheres), in which unsaturated Al sites near OH vacancies and surface O atoms act as Lewis acidic sites and Lewis basic sites, respectively. A study combining 1H nuclear magnetic resonance (NMR), 27Al NMR, and electron paramagnetic resonance (EPR) demonstrates the presence of different types of OH vacancies, which results in spatial hindrance in generating various FLP sites. Notably, the selectivity of the hydrogenation of unsaturated aldehydes can be switched depending on the catalyst: the Al2O3–air catalyst with AlⅣ–O FLP sites exhibits over 90% selectivity to unsaturated alcohols via CO hydrogenation, while the Al2O3–N2 catalyst with AlⅤ–O FLP sites shows a unique selectivity toward saturated alcohols via hydrogenation of CC and CO bonds with a yield above 90%. Structure–property relationship studies based on in situ Fourier transform infrared spectroscopy (FT-IR) and density functional theory (DFT) calculations reveal that the different adsorption modes, i.e., vertical adsorption configuration at the AlⅣ–O FLP sites and flat adsorption configuration at the AlⅤ–O FLP sites, are responsible for the different selectivity in the hydrogenation reactions. This in-depth study on the structure–selectivity relationship provides new insights for the development of selective hydrogenation reactions heterogeneously catalyzed by solid FLPs.

Cation migration in siliceous CHA probed with DFT simulation and its catalytic implication

The migration or diffusion of cations in siliceous CHA with an approximate Si/Al ratio of 11 has been investigated to explore the impact of their location on catalytic performance, using DFT simulation. The migration of protons within the CHA cage and the resulting total energy were calculated to determine the activation energy for this migration. As the distance between the Al site and the nearby framework oxygen decreased, the activation energy for site-to-site migration decreased as well. Eventually, the proton was stabilized near the Al site, where the activation energy for reverse migration reached its maximum at 1.60 eV. The migration of Na cations and Cu cations was also examined. The activation energy for Na cations was only 0.49 eV, which can be attributed to multiple coordination and explains the facile migration. On the other hand, Cu+2 migration had an activation energy of 1.56 eV, while Cu+1 migration had an activation energy of 1.06 eV. Consequently, under the given reaction conditions, the facile migration of the catalytic component Cu+1 can be expected. Thus, CHA containing Na cations exhibits improved low-temperature activity in NH3 SCR, despite the potential for thermal degradation at high temperatures.

Pressure leaching of aluminum from kaolin by HCl: Experimental and DFT study

Kaolin is an important industrial mineral which has many industrial applications. It is also considered as an important source of aluminum; however, great technical challenges are required to be overcome to extract Al from kaolin. In this work, the extraction of aluminum from kaolin using HCl, as a leaching agent, was studied. Kaolin samples were collected from Wadi Al-Hafira, south of Jordan; these were crushed and ground to a particle size of 100% <75 μm. The ground samples were characterized in terms of chemical and mineral composition using X-ray fluorescence (XRF) and X-ray diffraction analysis (XRD) techniques. Kaolin was found to contain kaolinite, muscovite, and quartz as main mineral phases, in addition to anatase and hematite as trace mineral phases, while the chemical analysis revealed that kaolin contains about 36.1% Al2O3, 54% SiO2, 2.5% CaO, 1.96% Fe2O3 and 1.2% TiO2 as major oxides. The minor oxides were K2O, Na2O, and SO3. A microwave digestion vessel was used to provide a suitable environment for high pressure and temperature leaching. The effect of several leaching parameters was studied in this work including, leaching time (30 to 120 min), leaching temperature (80 to 200 °C), and molarity of leaching reagent (1, 2, 3 and 4 M). Additionally, two sources of HCl were used, including analytical grade HCl and HCl collected from the potassium sulphate industry. The extraction of four main elements were monitored during the leaching process employing ICP-OES multi element analysis technique, namely, aluminum, iron, titanium and silicon. The findings of this work suggest that the recovery of these elements increases with the increase of temperature and leaching time. The recovery of silicon decreased with the increase of acid molarity. Complete extraction of Al from kaolinite was obtained at temperatures above 180 °C and leaching time of 120 min, while Al present in muscovite was not amenable to leaching. However, traces of H2SO4 and chloride salts in waste HCl, from potassium sulphate industry, provided complete destruction of muscovite as evident from the XRD analysis of leach residues. Lower energy requirements (computed by DFT simulation) for chlorination of Al sites in reference to Si sites explains a higher recovery percentage of the former.