Applied Clay Science最新文献

The utilization of efficient and environmentally friendly heterogeneous catalysts in the Baeyer-Villiger (BV) reaction gained significant importance in the field of green chemistry. In this study, the catalytic properties of magnesium-aluminium hydrotalcite in the BV oxidation of cyclic ketones using H₂O₂ were investigated. The research focused on the influence of two different synthetic routes on the clustering of aluminium centers and its impact on catalytic activity in the Baeyer-Villiger reaction. Surprisingly, the catalyst with a high degree of aluminium clustering exhibited superior performance compared to the ordered aluminium solid. Moreover, the most active catalyst displayed sustained activity even after three successive reaction cycles. This study presented the first report on the influence of trivalent metals in the BV reaction, thus opening up a new avenue for further exploration of such catalysts in the context of Baeyer-Villiger oxidation.

Aluminum oxide (Al₂O₃) compounds are extensively used in ionizing radiation dosimetry due to their high sensitivity when doped with carbon. However, their production is difficult and expensive, leading to much research on alternative ways to increase its sensitivity. This paper proposes a seed-mediated synthesis of α-Al₂O₃ by the combustion method with halloysite nanocrystals as seeds, which also have the ability to scavenge heavy metal ions. The dosimetric features were studied by radioluminescence (RL), thermoluminescence (TL), and optically stimulated luminescence (OSL). Scanning electron microscopy images and X-ray diffraction patterns pointed to the halloysite nanotubes (HNT) acting as heterogeneous nucleation seeds, and as adsorber centers for the chromium ions (Cr³⁺), as evidenced by the decreased crystallite size and Cr³⁺ RL emission. Decreased TL intensity upon increasing HNT content in addition to the RL data suggested that the Cr³⁺ ions strongly participate in the TL emission process as a luminescent center. Remarkable 6-fold enhanced OSL area intensity and 69-fold OSL initial intensity enhancement were registered from the samples seed-mediated by HNT, revealing that, by scavenging Cr³⁺, the HNT eliminated a luminescent center that competes with the OSL emission. Therefore, HNTs are promising nanomaterials to enhance the sensitivity of Al₂O₃ dosimeters with potential application in medical physics, where a decrease in the density of concurrent luminescent centers and an increase in OSL intensity were evidenced by the presence of HNT in Al₂O₃.

Alkali-activated materials (AAMs) have good resistance to chemical corrosion compared to silicate cements, but the effect of the pH of chloride-containing solutions on the resistance of AAMs to chloride diffusion is not clear. In this paper, the resistance of alkali-activated slag (AAS) with different metakaolin replacement rate to chloride diffusion in HCl/NaCl solution was investigated in terms of mechanical properties, hydration product composition and pore structure. The experimental results show that acidic solutions lead to alkali leaching, and dissolution of gel and hydrotalcite. The capillary pores that appeared in the AAS after the replacement rate of 10% metakaolin resulted in a decrease in the average pore size of the AAS and low depth of chloride diffusion. 30% metakaolin replacement led to a decrease in the diffusion resistance. In HCl solution, the high [AlO₄]⁵⁻ retention corroded layer was observed in the AAS with 30% metakaolin replacement and this layer slowed down the migration rate of [Cl⁻]. This study shows the promising application of alkali-activated slag/metakaolin in complex chloride solution environments.

Aiming to impart epoxy with a phosphorous-free super-efficient fire safety and multifunctions via a facile interface-manipulation protocol, we innovatively proposed a proof of concept of a two-in-one catalytic function via covalently inducing an interfacial supramolecular assembly of Salen-Fe complex on organic layered double hydroxide (LDH-DBS). Various characterizations confirmed the target LDH-DBS@Salen-Fe with a surface-located uniform and ultrathin deposition of Salen-Fe complex, which was conducive to a better nanodispersion in epoxy matrix. An exceptionally low loading of 2 wt% LDH-DBS@Salen-Fe (i.e., 0.6 % Salen-Fe) endowed epoxy with a UL-94 V-0 level and intensive fire protection with a suppressed peak heat release rate by 45.0 %. An insightful mechanism investigation demonstrated that the interface-located Salen-Fe rapidly catalyzed a charring reaction with an ultrafast formation of protective fire chars to resist an early-stage fire attack. Additionally, relative to EP/2LDH-DBS, a mere 0.6 % Salen-Fe increased the tensile, flexural and impact strength by 39.6 %, 31.5 % and 37.0 %, respectively based on the optimized interface compatibilization. Interestingly, an ultralow loading of Salen-Fe significantly contributed to a degradation recycling of epoxy under a mild condition with mass loss after 7 h treatment 392.8 % higher than its counterpart via catalytically promoting the generation of CHCOO∙ and HO∙ at the interface. In perspective, an interfacial supramolecular assembly of two-in-one catalysts exploits a novel route towards a phosphorous-free fire-safe and multifunctionally reinforced polymers.

For the whole life cycle of the textile industry, the disposal of printing and dyeing wastewater and the reuse of spent fibers were two of the main environmental problems. In this work, activated carbon fiber/layered double oxides (ACF/LDO) were prepared by the pyrolysis of spent cotton fiber/layered double hydroxides (LDH) composite. Compared to 2D structure LDH, 3D hierarchical ACF/LDO was formed by introducing activated carbon nanofibers as a skeleton. ACF/LDO had excellent adsorption properties for organic dye and the maximum adsorption capacity of acid red 27 (AR27) exceeded 800 mg/g. Due to the memory effect of LDH, the interconversion of ACF/LDO to ACF/LDH was observed during the adsorption and regeneration process. Meanwhile, the shrinkage and expansion of adsorbent occurred, which was similar to that of a sponge absorbing water. During the adsorption process, the average pore diameter of the adsorbent increased as ACF/LDO was converted to ACF/LDH, which increased the diffusion of organic dye inside the pores. On the contrary, the specific surface area and pore structure were restored during the regeneration process. Therefore, this composite displayed excellent adsorption capacity and repeatability, with no significant decrease was observed in adsorption capacity even after five consecutive cycles. The adsorption mechanism revealed that the removal of AR27 was dominated by electrostatic attraction and π-π interactions. The experiment provided a new strategy of treating wastewater with waste fiber.

Alkali leaching is an effective desilication method for improving the alumina-silica mass ratio (A/S) of bauxite. The paper aims to study the kinetics of kaolinite dissolution and hydrosodalite precipitation during alkali leaching of kaolinite-rich diasporic bauxite. Alkali leaching of bauxite in NaOH solutions was studied at Na₂O concentrations of 200–260 g/L, temperatures of 90–105 °C, and times of 30–150 min. The leached bauxites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) techniques. The rate equations describing kaolinite dissolution and hydrosodalite precipitation were derived by fitting the Avrami model. The results show that kaolinite with finer particle sizes was preferentially dissolved during alkali leaching, providing a material basis for the precipitation of hydrosodalite. Hydrosodalite was heterogeneously nucleated on the dissolved edges of some kaolinite and grew in both one- and two-dimensions, presenting acicular and lamellar morphology. The dissolution rate of kaolinite and the crystallization mechanism of hydrosodalite were primarily influenced by Na₂O concentration and temperature. Both kaolinite dissolution and hydrosodalite precipitation were controlled by chemical reactions with activation energies of 70.152 (± 1.429) kJ/mol and 289.089 (± 2.063) kJ/mol, respectively, and the orders of reaction with respect to Na₂O of 0.733 (± 0.070) and 7.165 (± 0.047), respectively. The kinetic equations describing kaolinite dissolution, hydrosodalite precipitation and even the leaching of SiO₂ were eventually modeled with Na₂O concentration, temperature and time as variables.

Due to the constraints associated with diffusion and mixing time, traditional kinetic and thermodynamic approaches were inadequate for probing the true mechanism of interaction between chromate and Layered Double Hydroxide (LDH). To circumvent these limitations, colloidal suspensions of Mg/Al-NO₃ LDH, characterized by a positively charged surface (approximately +50 mV) in ultrapure water and a mean average diameter of 140 nm, allowing the formation of stable suspensions for days, were swiftly mixed with Cr(VI) suspensions at both pH = 4 and 9 using a stopped flow technique. This rapid mixing, accomplished in <5 milliseconds, enabled the examination of the initial stages of interaction between the toxic anion and the host compound. Two distinct steps in the adsorption process were identified: a very fast step (completed in <5 ms), representing up to 80% of the measured variation, and a slower step lasting up to 100 s. The fast step assumed to be driven by electrostatic interaction (ζ ∼ +50 mV) with the surface, and sites close to the surface are easily accessible to the chromate anions. The slower step corresponded to a diffusion process close or inside the particles. Chromate extraction efficiency was investigated through ultrafiltration tests, varying the LDH and chromate amounts, indicating that 2 nitrate ions are exchanged for 1 chromate, regardless of the pH considered, and a total exchange can be fulfilled with 0.1 g L⁻¹ of LDH within the explored concentration range.

To understand the occurrence of natural gas hydrates in seabed sediments, it is crucial to examine the mechanisms of methane (CH₄) hydrate formation in sodium montmorillonite (Na-Mt) systems in the presence of amino acid. Accordingly, this study employed kinetics experiments and molecular dynamics simulations to investigate CH₄ hydrate nucleation and growth in an Na-Mt system containing alanine (Ala), leucine (Leu), and phenylalanine (Phe), respectively. Kinetics and Raman experiments showed that, compared with Ala, Leu and Phe enhanced hydrogen bonding between water molecules surrounding Na-Mt. This enhancement was due to the long carbon chain of Leu and the phenyl ring of Phe and facilitated CH₄ hydrate formation. Moreover, in the Na-Mt system, Ala reduced CH₄ consumption, whereas Leu and Phe increased CH₄ consumption. Molecular dynamics simulations revealed that the strength of electrostatic interactions between the negatively charged Na-Mt surface and the functional groups of amino acids affected the distribution of amino acids, thereby altering CH₄ aggregation and CH₄ hydrate nucleation processes. The strong interaction between Na-Mt and Ala significantly disrupted interfacial interactions between Na-Mt and water molecules. In contrast, the weaker interactions between Na-Mt and Leu and Phe, respectively, meant that these amino acids affected CH₄ hydrate nucleation in the bulk-like solution by influencing the arrangement of water molecules. These findings indicate that interfacial interactions between Na-Mt and amino acids play a crucial role in CH₄ hydrate formation. Overall, this study generated insights into the formation kinetics and nucleation properties of CH₄ hydrates in clay mineral–amino acid complexes that may increase understanding about the occurrence of natural gas hydrates in marine sediments.

Clay pellet mixtures are generally compressed to improve their engineering performance. Deepening the comprehension of the mechanical properties of these mixtures in the complete compression process facilitates the benefit to the engineering design and their utilization. In this study, the effects of soil grain size distribution, water content and dry density on the mechanical properties and microstructure of Teguline clay pellet mixtures during a continuous oedometric compression process are explored. Three types of soil pellet mixtures, including mixture A (grain size ≤5 mm), mixture B (≤ 0.4 mm) and mixture C (2–5 mm), were prepared with different water contents of 7%, 8% and 12% respectively. Subsequently, continuous oedometeric compression was undertaken to explore their mechanical behaviours of the soil pellet mixtures. After that, the microstropic structures of the compacted pellet mixtures were investigated using mercury intrusion porosimetry (MIP). The results indicated that mixture A has a minimal initial packing density of pellet mixtures, while mixture C has a maximum one at the initial compression stage. After completion of compression, the compression curves of the pellet mixtures tended to converge uniformity at a semilogarithm coordinate as the vertical stress increased. All of the compression curves presented a concave shape at the plastic compression stage, which is significantly influenced by grain size distribution and water content. In contrast, the elastic compression and rebound behaviours are little affected by the grain size distribution and water content. As far as the microstructure is concerned, compacted samples prepared by mixture A or C presented a unimodal pore structure, while those by mixture B showcased a bimodal pore structure. In comparison with the unimodal pore distribution of the undisturbed stiff clay, the compacted samples displayed a pseudo-unimodal pore distribution because the inter-aggregate pores still existed. A double tangent method was proposed to determine the delimiting pore diameter of the pseudo-unimodal pore distribution curves and found that the delimiting pore diameter decreased with the increase of dry density and water content. Moreover, the inflexion point for the pore diameter of compacted samples prepared by coarse soil was larger than that of fine soil. Combining this work with previous research, it was found that the high compression of coarse soil easily causes the pseudo-unimodal shape, which is also impacted by water content and particle properties. This work could help deepen the understanding of the mechanical characteristics and microstructure of the stiff clay pellet mixtures during continuous oedometric compression.