Calcined Mg-Al Research Articles

Removal of HCl from gases using modified calcined Mg-Al-CO3 hydrotalcite: Performance, mechanism, and adsorption kinetics

To achieve carbon neutrality, it is essential to remove HCl resulting from the extensive use of alternative fuel feedstock in thermochemical plants. In this work, the HCl removal performance of modified calcined Mg-Al hydrotalcite (Mg-Al-CO3) on gaseous stream was investigated. The layered structure of Mg-Al-CO3 intercalated with CO32– was successfully prepared using the coprecipitation method with a molar ratio of Mg/Al equal to 3.1. The optimal performance of Mg-Al-CO3 for HCl removal was observed at 300 °C, and the breakthrough chlorine capacity was 102.7 mg g−1. Among calcined Mg-Al samples, only Mg-Al-400 exhibited better HCl removal performance than Mg-Al-CO3. The specific surface area of Mg-Al-400 was 183.73 m2 g−1, which was 2.8 times greater than that of Mg-Al-CO3. The average HCl removal rate was 90.11 % within the first 120 min, which was 1.12 times higher than that of Mg-Al-CO3. Moreover, the breakthrough chlorine capacity of Mg-Al-400 was increased by 16.9 %, reaching 120.1 mg g−1. Two reaction paths were identified for Mg-Al-400, first Mg2+ reacts with Cl− to generate MgCl2, and then Cl− acted as a new interlayer anion of hydrotalcite. According to the results of adsorption kinetics, the pseudo-second-order and the Bangham model were found to fit well with the HCl removal process of Mg-Al-400.

Enhancing the phosphorus content of layered double hydroxide fertilizers by intercalating polymeric phosphate instead of orthophosphate: A feasibility study

HypothesisLayered double hydroxide (LDH) loaded with orthophosphate (PO4) are suggested as slow-release P fertilizers. However, PO4-LDHs have a low maximal P content, related to high charge HPO42−/PO43− anions occupying the anion exchange capacity (AEC) of LDHs. We postulate that the P content of LDHs can be enhanced by exchanging them with polymeric-P (i.e. trimetaphosphate, P3O9), which has a lower molar charge/P ratio than its monomer. ExperimentsAdsorption capacities were compared between PO4 and P3O9 for as-synthesized and calcined MgAl LDHs with Mg/Al ratio of 2, 3, or 4; the P-LDHs were characterized (XRD, FTIR). Dialysis and soil incubation experiments were performed with PO4−LDHs, P3O9−LDHs, and corresponding soluble fertilizers to compare their P release and P solubility (CaCl2 extract). FindingsThe P adsorption capacities were 1.25–1.60 fold larger for P3O9 compared to PO4, yet the high theoretical P contents with P3O9 were not achieved (incomplete loading, P3O9 depolymerization). P3O9-Mg3Al released polymeric-P whereas P3O9-Mg2Al released depolymerized PO4, and P release from P3O9-LDHs was slower than that of PO4-LDHs. With soil incubation, soluble P from P3O9-LDH was initially lower but later converged to that of PO4-LDH as result of continued hydrolysis, yet did not exceed that of the soluble P3O9 and PO4 fertilizers.

Different dye removal mechanisms between monodispersed and uniform hexagonal thin plate-like MgAl–CO32--LDH and its calcined product in efficient removal of Congo red from water

Monodispersed and uniform hexagonal thin plate-like magnesium-aluminum layered double hydroxide intercalated with CO32− (MgAl–CO32−-LDH) is synthesized by an ethanol–water mediated solvothermal method. The maximum adsorption capacities of the as-synthesized and calcined MgAl−CO32−−LDH are 129.9 and 143.27 mg g−1, respectively, which are higher than those of the other metal oxide nanostructures reported to date. More importantly, although N2 adsorption–desorption analysis indicates that the BET surface area changed from 24.74 m2 g−1 for the as-synthesized MgAl−CO32−−LDH to 165.07 m2 g−1 for the calcined MgAl−CO32−−LDH, the as-synthesized and calcined hexagonal thin plate-like MgAl−CO32−−LDH structures show similar adsorption capacity of Congo red (CR) from aqueous solution. The adsorption mechanism for the as-prepared LDH is anion exchange while adsorption mechanism for the calcined LDH is reconstruction. Our study opens a new insight for understanding and preparing high-performance adsorbents for organic dye removal.

Synthesis of Formic Acid from Monosaccharides Using Calcined Mg-Al Hydrotalcite as Reusable Catalyst in the Presence of Aqueous Hydrogen Peroxide

Formic acid (FA) can be synthesized from monosaccharides such as glucose, galactose, xylose, arabinose and lyxose by using both calcined Mg-Al hydrotalcite as a solid catalyst and aqueous H2O2 as an oxidant in ethanol solvent at 343 K for 5 h. For the glucose oxidation, the FA yield and H2O2 utilization efficiency reach 78% and 100%, respectively. The used hydrotalcite catalyst can be easily separated from the reaction mixture and is reusable at least twice.

Magnetic alginate-layered double hydroxide composites for phosphate removal

The objective of this study was to investigate phosphate removal using magnetic alginate-layered double hydroxide (LDH) composites. The magnetic composites were prepared by entrapping synthetic magnetic iron oxide and calcined Mg-Al LDH in polymer matrix (alginate). Results showed that the magnetic composites (2% magnetic iron oxide and 6% calcined Mg-Al LDH) were effective in the removal of phosphate with the sorption capacity of 5.0±0.1 mgP/g under given experimental conditions (adsorbent dose=0.05 g in 30 ml solution; initial phosphate concentration=10 mgP/l; reaction time=24 h). Both magnetic iron oxide and calcined Mg-Al LDH have the ability to adsorb phosphate, with the latter having much higher sorption capacity. In the magnetic composites, calcined Mg-Al LDH functions as a phosphate adsorbent while magnetic iron oxide provides both magnetic and sorption properties. Results also demonstrated that phosphate sorption to the magnetic composites reached equilibrium at 24 h. The maximum phosphate sorption capacity was determined to be 39.1 mgP/g. In addition, phosphate removal was not sensitive to initial solution pH between 4.1 and 10.2. Only 9% of the phosphate sorption capacity was reduced as the solution pH increased from 4.1 to 10.2. This study demonstrated that magnetic alginate-LDH composites could be used for phosphate removal in combination with magnetic separation.

Reconstruction of calcined MgAl- and NiMgAl-layered double hydroxides during glycerol dehydration and their recycling characteristics

MgAl- and NiMgAl-layered double hydroxides (LDHs) were prepared by the co-precipitation method. On calcination MgAl and NiMgAl mixed oxides were formed. Crystalline MgO and NiO could be identified, while aluminium ions were well-dispersed on these oxides. The mixed oxides were effective in the transformation of glycerol to currently more valuable products like acetol (hydroxyacetone) and acrolein. Acetol was found to be the main product and some part was further dehydrated to acrolein. During the reaction, full reconstruction of the LDH structure occurred. The used catalysts could be reactivated by repeated calcination.

Co-production of high quality NH 4SCN and sulfur slow release agent from industrial effluent using calcined MgAl–hydrotalcite

Thiosulfate (S 2O 3 2−) is the main impurity in ammonium thiocyanate (NH 4SCN) extracted from the effluent from industries such as electroplating or mining. It significantly lowers the quality of NH 4SCN. Here we report a method for selective removal of S 2O 3 2− from aqueous solutions containing thiocyanate (SCN −) using the material produced by calcination of MgAl–CO 3 hydrotalcites also known as layered double hydroxides (LDHs). The calcined materials, which we denote CLDHs, are well known scavengers for anionic contaminants. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) showed that CLDH exhibits selectivity in adsorption between S 2O 3 2− and SCN − anions, with preferential adsorption of S 2O 3 2− anions resulting in S 2O 3 2−-intercalated MgAl–LDHs. Selective removal of S 2O 3 2− from an aqueous mixed solution lowered the ratio of S 2O 3 2−:SCN − from 1:1 to approximately 1:7, which meets the requirements for production of high quality NH 4SCN after recrystallization, or for direct reuse in the industrial process which produced the original effluent. At the same time, the by-product, S 2O 3 2−-intercalated MgAl–LDH showed satisfactory S 2O 3 2−-release behavior in a simulated solution, showing that is a potential slow-release sulfur fertilizer. Furthermore, its alkaline nature allows it to act as an effective acid soil conditioner. In this way, waste effluent posing an environmental hazard can be converted into two valuable products by a simple and economical process.

Environmentally Benign Route for the Synthesis of Polycarbonate Diols (PCDLs)−Calcined MgAl Hydrotalcites as Heterogeneous Catalysts

Polycarbonate diols (PCDLs) are known as important intermediates for the manufacture of novel polyurethanes. In this work, PCDLs were prepared via the reaction of hexane-1,6-diol (HD) and dimethyl carbonate (DMC) using calcined MgAl hydrotalcites (HTc) as solid base. The effects of preparative parameters of the catalysts and reaction conditions on the transesterification reaction were investigated. X-ray diffraction (XRD), thermogravimetric analysis−differential scanning calorimetry (TG-DSC), CO2 temperature-programmed desorption (CO2−TPD), and Brunauer−Emmett−Teller (BET) techniques were used to characterize the HTc. The experimental results showed that the pore structure of the catalysts played a crucial role for the synthesis of PCDLs with high efficiency.