Decomposition Of Hydrotalcites Research Articles

A high-speed method to obtain Ni-Zn ferrite nanoparticles by microwave hydrotalcite decomposition for magnetic applications

Nanoparticles of Zn0.375Ni0.625Fe2O4 ferrite were successfully synthesized using an innovative, rapid, and efficient synthesis method based on microwave-assisted hydrotalcite decomposition, enabling nanoparticle crystallization in less than 15 minutes. A single composition was studied to better assess the impact of the synthesis method on structural, morphological, and magnetic properties. This new synthesis route was compared with the traditional ceramic method. The prepared hydrotalcite was analyzed by high-resolution transmission electron microscopy (HRTEM), identifying hydrotalcite diffraction planes consistent with selected area electron diffraction (SAED) and X-ray diffraction (XRD) patterns. The decomposition phases of the hydrotalcite were identified by simultaneous thermogravimetric and differential thermal analysis (DTA/TG). After hydrotalcite decomposition, the ferrite obtained was analyzed by XRD, confirming a single-phase cubic structure. Scanning electron microscopy (SEM) micrographs revealed nanoparticles less than 100 nm in size with the new method and larger than 2 µm with the traditional route. At a temperature of 5 K, the M-H graph obtained with a superconducting quantum interference device (SQUID) indicated that the spinel-type ferrite exhibited a smooth ferrimagnetic behavior, which was also corroborated by electron paramagnetic resonance (EPR). Magnetic saturation values of 105.73 emu/g were obtained in the synthesis microwave-assisted hydrotalcite decomposition and 51.49 emu/g in the traditional synthesis, representing an increase of over 100 %. The two synthesis methods were also compared using density functional theory (DFT) calculations, confirming the influence of morphology on magnetic properties, obtained thanks to different synthesis methods. These results indicate that the synthesized Ni-Zn spinel ferrite nanoparticles can be used in high-frequency applications, such as ceramic induction plates.

On the effect of yttrium promotion on Ni-layered double hydroxides-derived catalysts for hydrogenation of CO2 to methane

Ni-containing mixed oxides derived from layered double hydroxides with various amounts of yttrium were synthesized by a co-precipitation method at constant pH and then obtained by thermal decomposition. The characterization techniques of XRD, elemental analysis, low-temperature N2 sorption, H2-TPR, CO2-TPD, TGA and TPO were used on the studied catalysts. The catalytic activity of the catalysts was evaluated in the CO2 methanation reaction performed at atmospheric pressure. The obtained results confirmed the formation of nano-sized mixed oxides after the thermal decomposition of hydrotalcites. The introduction of yttrium to Ni/Mg/Al layered double hydroxides led to a stronger interaction between nickel species and the matrix support and decreased nickel particle size as compared to the yttrium-free catalyst. The modification with Y (0.4 and 2 wt%) had a positive effect on the catalytic performance in the moderate temperature region (250–300 °C), with CO2 conversion increasing from 16% for MO-0Y to 81% and 40% for MO-0.4Y and MO-2.0Y at 250 °C, respectively. The improved activity may be correlated with the increase of percentage of medium-strength basic sites, the stronger metal-support interaction, as well as decreased crystallite size of metallic nickel. High selectivity towards methane of 99% formation at 250 °C was registered for all the catalysts.

Influence of reduction temperature on Ni particle size and catalytic performance of Ni/Mg(Al)O catalyst for CO2 reforming of CH4

Hydrotalcite-derived Ni/Mg(Al)O is promising for CH4–CO2 reforming. However, the catalysts reported so far suffer from sever coking at low temperatures. In this work, we demonstrate that a significant improvement of coke-resistance of Ni/Mg(Al)O can be achieved by fine tuning the Ni particle size through adjusting the reduction condition of catalyst. Ni particles having average size within 4.0–7.1 nm are in situ generated by reducing the catalyst at a selected temperature within 923–1073 K. Controllability of Ni particle size is related to the formation of Mg(Ni,Al)O solid solution upon hydrotalcite decomposition. It is found that the catalyst reduced at 973 K exhibits high activity, stability, and coke-resistance even at reaction temperature as low as 773 K. In contrast, the catalyst reduced at 923 K has low activity and deactivates due to Ni oxidation, while those reduced at 1023 and 1073 K suffer from sintering and severe coking. STEM and O2-TPO reveal that coke deposition is directly proportional to the Ni particle size but becomes negligible at a size below 6.2 nm. It is evidenced that a critical size of about 6 nm is required to inhibit coking effectively. CO2 temperature-programmed surface reaction indicates that the deposited carbon on small Ni particles can be easily removed by the CO2 activated at the Ni–Mg(Al)O interfaces, accounting for the better resistance to coking.

Cu–Zn–Al mixed oxides as catalysts for the hydrogenolysis of glycerol to 1,2-propanediol

This study is aimed to compare Cu–Al–Zn catalysts prepared by different methods and their effectiveness in glycerol hydrogenolysis to 1,2-propanediol. The preparation methods were: co-precipitation and thermal decomposition of hydrotalcite, co-precipitation by KOH and thermal decomposition of the precipitate, sol–gel auto-combustion method, oxalate–gel co–precipitation and thermal decomposition of the precipitate and finally, mechanical mixing of Cu, Zn, Al oxides. It was found that the activity of the catalysts correlates well with the size of Cu crystallites. The smaller Cu crystallites, the higher the activity. However, also the texture of the catalyst, which also depends on the preparation method, has an impact on the catalyst activity. On the other hand, no correlation between activity and acidity, as well as internal surface area was found.

The influence of nickel content on the performance of hydrotalcite-derived catalysts in CO2 methanation reaction

A series of mixed oxides obtained by thermal decomposition of hydrotalcites containing different amounts of Ni and constant MII+/MIII+ molar ratio were characterized by XRD, XANES, XES, H2-TPR, CO2-TPD, elemental analysis and low temperature nitrogen sorption technique. The results confirm the formation of periclase-like structured materials after thermal treatment, with nickel present as NiO in octahedral coordination environment. As proven by H2-TPR with increasing Ni content the interaction between Ni and hydrotalcite matrix weakens, which has a positive influence on the catalytic performance. The catalysts containing different amounts of nickel (10.3, 16.2, 27.3, 36.8, 42.5 wt.%) showed at 300 °C a very good catalytic performance in carbon dioxide methanation, with CO2 conversion of 36 (Ni10.3)–82% (Ni42.5) and CH4 selectivity of 98–99%.

Mg/Al mixed oxides: Heterogeneous basic catalysts for the synthesis of salicylamide from urea and phenol

Abstract Several Mg/Al mixed oxides were prepared by thermal decomposition of hydrotalcites and used for the synthesis of salycilamide from urea and phenol. Their physicochemical properties were characterized by ICP, XRD, CO2-TPD and N2 adsorption/desorption techniques. Based on the results of catalytic testing and catalyst characterization, relationships between their catalytic performances and base properties were investigated. In addition, XRD characterization and re-usability testing indicated that these catalytic systems for salycilamide synthesis are heterogeneous in nature.

Mg and Al mixed oxides and the synthesis of n-butanol from ethanol

n-Butanol is used in a wide range of applications, especially for the production of paint, solvents and plasticizers. Recently, some companies have proposed the use of n-butanol as a fuel, which can be employed pure or blended with gasoline or diesel. At present, there is great interest in developing a one-step process that generates n-butanol from ethanol. Some works showed that Mg–Al mixed oxides are very promising catalysts for this matter. Analyzing the physical–chemical properties of these catalysts, this work aims at further studying this reaction. The Mg–Al mixed oxides were obtained by thermal decomposition of hydrotalcites employing two Mg/Al different ratios. Magnesium oxide (MgO) and alumina (Al 2O 3) were also prepared following the same synthetic route used for the Mg–Al mixed oxides. The synthesized samples were characterized by the following techniques: XRD, NMR, BET and XRF. The acid and basic properties of the samples were also analyzed employing probe molecules. The catalytic tests were performed using a fixed bed reactor at atmospheric pressure. It was verified that the Mg and Al mixed oxides are able to promote the synthesis of C4 compounds from ethanol. Adjacent acid and medium basic sites are needed in order to generate the intermediate compounds. These pairs of acid and medium basic sites are obtained by the insertion of Al in the MgO lattice or in consequence of the presence of Mg in the γ-Al 2O 3 lattice. It was observed that the higher the concentration of Mg is, the higher the hydrogenation capacity of the catalyst, since a greater selectivity to n-butanol was observed. Strong basic sites and a specific superficial atomic arrangement seem not to be essential for the synthesis of C4 from ethanol.

Cu–Mn and Co–Mn catalysts synthesized from hydrotalcites and their use in the oxidation of VOCs

Mixed oxides of the Cu/Mn/Mg/Al and Co/Mn/Mg/Al type were obtained by means of the thermal decomposition of hydrotalcites, with manganese as the main active phase and with variations in their content by means of the addition of Cu or Co (molar ratio Cu/Mn or Co/Mn between 0.05 and 0.5). The effect of a second active phase on the material was evaluated in the total oxidation of three volatile organic compounds, VOCs (toluene, ethanol and butanol). The catalysts were characterized by means of X-ray fluorescence (XRF), X-ray diffraction (XRD), temperature-programmed reduction with hydrogen (TPR-H 2) and physisorption of nitrogen at 77 K. All the solids were active in the oxidation reaction of the VOCs, where the scale of difficulty to oxidize the different organic compounds evaluated was: butanol < ethanol < toluene. The use of mixed active phases optimized the physicochemical and catalytic properties compared with the system derived exclusively from manganese. The Co–Mn catalysts being the most active; this behavior is associated to the generation of amorphous phases and redox cycles as a consequence of the cooperative effect among the metals.

Thermal decomposition, gas phase hydration and liquid phase reconstruction in the system Mg/Al hydrotalcite/mixed oxide: A comparative study

The thermal decomposition of a synthetic Mg/Al hydrotalcite and the subsequent hydration of the calcined products in the presence of water vapor at 30 °C were studied by in situ XRD. A comparative study between the gas phase hydration and reconstruction of calcined hydrotalcite in Na 2CO 3 solution at 25 and 70 °C was investigated. TG coupled to a mass spectrometer was used to study the thermal behaviour of synthetic hydrotalcite. SEM images were taken to show the layering and size of crystals. Thermal decomposition of hydrotalcite led to the formation of a dehydrated intermediate phase at 200 °C, followed by crystalline MgO formation at 450 °C. Hydration of calcined hydrotalcite in H 2O/N 2 resulted in the formation of broad patterns of the meixnerite phase after 2 h. This phase retained its structure even after 52 h exposure to water vapor. Reconstruction of the calcined products in carbonated water at 25 and 70 °C resulted in intensified patterns of the hydrotalcite phase. Due to the different structural and compositional properties of the layered products obtained by different chemical reactions of the mixed oxide phase a clarification of nomenclature for all phase transforming steps is proposed.

Thermal characterization of hydrotalcite used in the transesterification of soybean oil

Hydrotalcite was synthesised by co-precipitation method, calcined and characterized by XRD, BET, IR and TG/DTA/DTG analyses and tested as solid base catalyst in the transesterification of soybean oil with methanol, achieving a methyl ester content of 99.5%. The thermal decomposition of hydrotalcite calcined occurred in four mass loss steps at 28, 105, 203 and 400 °C. The hydrotalcite was recovered and through a simple evaluation by TG/DTA/DTG techniques it was found that at 500 °C is the temperature, where the organic matter should be eliminated from the catalyst. This study shows the importance of thermal analysis in the evaluation of the recovery temperature of hydrotalcite.

Thermal decomposition of hydrotalcite with hexacyanoferrate(II) and hexacyanoferrate(III) anions in the interlayer

The mechanism for the decomposition of hydrotalcite remains unsolved. Controlled rate thermal analysis enables this decomposition pathway to be explored. The thermal decomposition of hydrotalcites with hexacyanoferrate(II) and hexacyanoferrate(III) in the interlayer has been studied using controlled rate thermal analysis technology. X-ray diffraction shows the hydrotalcites have a d(003) spacing of 10.9 and 11.1 Å which compares with a d-spacing of 7.9 and 7.98 Å for the hydrotalcite with carbonate or sulphate in the interlayer. Calculations show dehydration with a total loss of 7 moles of water proving the formula of hexacyanoferrate(II) intercalated hydrotalcite is Mg6Al2(OH)16[Fe(CN)6]0.5·7H2O and 9.0 moles for the hexacyanoferrate(III) intercalated hydrotalcite with the formula of Mg6Al2(OH)16[Fe(CN)6]0.66·9H2O. CRTA technology indicates the partial collapse of the dehydrated mineral. Dehydroxylation combined with CN unit loss occurs in two isothermal stages at 377 and 390°C for the hexacyanoferrate(III) and in a single isothermal process at 374°C for the hexacyanoferrate(III) hydrotalcite.

Thermal decomposition of hydrotalcites with variable cationic ratios

Thermal analysis complimented with evolved gas mass spectrometry has been applied to hydrotalcites containing carbonate prepared by coprecipitation and with varying divalent/trivalent cation ratios. The resulting materials were characterised by XRD, and TG/DTG to determine the stability of the hydrotalcites synthesised. Hydrotalcites of formula Mg4(Fe,Al)2(OH)12(CO3)·4H2O, Mg6(Fe,Al)2(OH)16(CO3)·5H2O, and Mg8(Fe,Al)2(OH)20(CO3)·8H2O were formed by intercalation with the carbonate anion as a function of the divalent/trivalent cationic ratio. XRD showed slight variations in the d-spacing between the hydrotalcites. The thermal decomposition of carbonate hydrotalcites consists of two decomposition steps between 300 and 400°C, attributed to the simultaneous dehydroxylation and decarbonation of the hydrotalcite lattice. Water loss ascribed to dehydroxylation occurs in two decomposition steps, where the first step is due to the partial dehydroxylation of the lattice, while the second step is due to the loss of water interacting with the interlayer anions. Dehydroxylation results in the collapse of the hydrotalcite structure to that of its corresponding metal oxides and spinels, including MgO, MgAl2O4, and MgFeAlO4.

Thermal decomposition of hydrotalcite with molybdate and vanadate anions in the interlayer

Hydrotalcites containing carbonate, vanadate and molybdate were prepared by coprecipitation. The resulting materials were characterized by XRD, and TG/DTA to determine the stability of the hydrotalcites synthesized. The thermal decomposition of carbonate hydrotalcites consist of two decomposition steps between 300 and 400°C, attributed to the simultaneous dehydroxylation and decarbonation of the hydrotalcite lattice. Water loss ascribed to dehydroxylation occurs in two decomposition steps, where the first step is due to the partial dehydroxylation of the lattice, while the second step is due to the loss of water interacting with the interlayer anions. Dehydroxylation results in the collapse of the hydrotalcite structure to that of its corresponding metal oxides, including MgO, Al2O3, MgAl2O4, NaMg4(VO4)3 and Na2Mg4(MoO4)5. The presence of oxy-anions proved to be beneficial in the stability of the hydrotalcite structure, shown by the delay in dehydroxylation of oxy-anion containing hydrotalcites compared to the carbonate hydrotalcite. This is due to the substantial amount of hydroxyl groups involved in a network of hydrogen bonds involving the intercalated anions. Therefore, the stability of the hydrotalcite structure appears to be dependent on the type of anion present in the interlayer. The order of thermal stability for the synthesized hydrotalcites in this study is Syn-HT-V>Syn-HT-Mo> Syn-HT-CO3-V>Syn-HT-CO3-Mo>Syn-HT-CO3. Carbonate containing hydrotalcites prove to be less stable than oxy-anion only hydrotalcites.

Mechanism for hydrotalcite decomposition: A controlled rate thermal analysis study

The mechanism for the decomposition of hydrotalcite remains unsolved. Controlled rate thermal analysis enables this decomposition pathway to be explored. Hydrotalcites containing carbonate, vanadate and molybdate were prepared by coprecipitation. The resulting materials were characterised by XRD, simultaneous TG–DTG–DTA and controlled rate thermal analysis (CRTA) to determine the stability and thermal decomposition pathway of the synthesised hydrotalcites. For the carbonate intercalated hydrotalcite dehydration takes place in three steps two of which are quasi-isothermal and one non-isothermal. Dehydroxylation and decarbonation occur separately over the 235–330 and 330–370 °C temperature range. A second non-isothermal decarbonation step is observed in the 371–541 °C range. In comparison the mixed carbonate–vanadate and carbonate–molybdate hydrotalcites show two dehydration steps and the dehydroxylation and decarbonation occur simultaneously. The observation of three dehydration steps is used to support the model of water molecules in three structurally distinct environments in the hydrotalcite interlayer. CRTA technology provides a mechanism for the decomposition of hydrotalcites.

Memory Effect of Activated Mg–Al Hydrotalcite: In Situ XRD Studies during Decomposition and Gas‐Phase Reconstruction

The thermal decomposition of Mg-Al hydrotalcite and the subsequent reconstruction of the decomposed products in the presence of water vapor (2 vol. % H(2)O in N(2)) have been investigated by in situ XRD. Thermographic analysis and temperature-programmed desorption MS results complemented the diffraction data. Valuable mechanistic and kinetic insights into these processes, which are of prime importance for optimal activation of this type of material for catalytic applications, were obtained. Hydrotalcite decomposition to the mixed oxide proceeds via formation at 423-473 K of an intermediate phase, consisting of a highly disordered, dehydrated, layered structure. The latter evolves by removal of interlayer water on heating, causing a shrinking of the interlayer space (it is up to 45 % smaller than in the as-synthesized hydrotalcite). Above 623 K, Mg(Al)O(x) oxide with the periclase structure is formed. Reversion of the intermediate dehydrated structure to hydrotalcite upon contact with water vapor is complete and very fast at room temperature. Recovery of hydrotalcite from the oxide calcined at 723 K is two orders of magnitude slower than rehydration of the intermediate layered structure and one order of magnitude slower than the typically practiced liquid-phase reconstruction. In contrast to the decomposition, the reconstruction mechanism does not involve an intermediate phase. The gas-phase rehydration and reconstruction was interrupted above 303 K. This is attributed to the poor wetting of the surface of the decomposed materials induced by hampered H(2)O adsorption above room temperature at the water vapor pressure applied. The Avrami-Erofe'ev model describes the reconstruction kinetics well.

Thermal decomposition of hydrotalcitewith hexacyanoferrate(II) and hexacyanoferrate(III) anions in the interlayer

The mechanism for the decomposition of hydrotalcite remains unsolved. Controlled rate thermal analysis enables this decomposition pathway to be explored. The thermal decomposition of hydrotalcites with hexacyanoferrate(II) and hexacyanoferrate(III) in the interlayer has been studied using controlled rate thermal analysis technology. X-ray diffraction shows the hydrotalcites have a d(003) spacing of 10.9 and 11.1 A which compares with a d-spacing of 7.9 and 7.98 A for the hydrotalcite with carbonate or sulphate in the interlayer. Calculations show dehydration with a total loss of 7 moles of water proving the formula of hexacyanoferrate(II) intercalated hydrotalcite is Mg6Al2(OH)16[Fe(CN)6]0.5·7H2O and 9.0 moles for the hexacyanoferrate(III) intercalated hydrotalcite with the formula of Mg6Al2(OH)16[Fe(CN)6]0.66·9H2O. CRTA technology indicates the partial collapse of the dehydrated mineral. Dehydroxylation combined with CN unit loss occurs in two isothermal stages at 377 and 390°C for the hexacyanoferrate(III) and in a single isothermal process at 374°C for the hexacyanoferrate(III) hydrotalcite.

Steam activation of Mg–Al hydrotalcite. Influence on the properties of the derived mixed oxides

The influence of steam activation of Mg–Al hydrotalcite on the properties of the resulting mixed oxides has been investigated and compared with the typically performed thermal decomposition under dry conditions. The as-synthesized hydrotalcite was treated at 723 K and 1 bar in nitrogen flow containing steam partial pressures in the range of 0–800 mbar. Thermal analysis indicated that the hydrotalcite decomposition was not affected by the presence of steam in the feed gas. The average crystallite size and morphology of the dry and steam-activated materials, as determined by XRD and TEM, respectively, were very similar too. The porous properties of the resulting oxides present significant differences. The BET surface area decreased ca. 20% when small amounts of steam were present as compared to the dry-decomposed product, but was not further reduced upon increasing the steam partial pressure. This has been attributed to the disappearance of small mesopores (<4 nm) due to the hydrolysis of Al–O–Al bonds from adjacent layers and platelets.

Hydrotalcite-Derived Mg/Al Mixed Oxides as Support Materials for the Friedel–Crafts Catalyst ZnCl2

Abstract ZnCl2 supported on Mg/Al mixed oxides obtained through the thermal decomposition of hydrotalcite showed high catalytic activity in the benzylation of benzene with benzyl chloride at room temperature. The surface properties of hydrotalcite-derived mixed oxides suggested that they increased the catalytic activities of ZnCl2 owing to their high mesoporosity, with pore sizes in the vicinity of 10 nm.

Thermal decomposition of hydrotalcite with chromate, molybdate or sulphate in the interlayer

The thermal decomposition of hydrotalcites with chromate, molybdate and sulphate in the interlayer has been studied using thermogravimetric analysis coupled to a mass spectrometer measuring the gas evolution. X-ray diffraction shows the hydrotalcites have a d(0 0 3) spacing of 7.98 Å with very small differences in the d-spacing between the three hydrotalcites. XRD was also used to determine the products of the thermal decomposition. For the sulphate-hydrotalcite decomposition the products were MgO and a spinel MgAl 2O 4, for the chromate interlayered hydrotalcite MgO, Cr 2O 3 and spinel. For the molybdate interlayered hydrotalcite the products were MgO, spinel and MgMoO 4. EDX analyses enabled the formula of the hydrotalcites to be determined. Two processes are observed in the thermal decomposition namely dehydration and dehydroxylation and for the case of the sulphate interlayered hydrotalcite, a third process is the loss of sulphate. Both the dehydration and dehydroxylation take place in three steps each for each of the hydrotalcites.

Thermal evolution of Mg-Al-CO3 hydrotalcites

AbstractThe thermal decomposition of hydrotalcite (HT), with chemical composition Mg1-xAlx(OH)2(CO3)x/2.(1-3x/2)H2O, (0.20 &lt; x ≤ 0.33), is a complex sequence of dehydration, dehydroxylation and decarbonization and leads to the formation of a series of metaphases: HT-D (dehydrated HT), HT-B (partially dehydroxylated HT) and MO (mixed oxides with periclase-like structure). The evolution of water and CO2 in natural and synthetic hydrotalcites (a Mg/Al ratio between 2:1and 3.7:1), heated to 800°C, was investigated by differential thermal analysis, thermogravimetry and evolved gas analysis. At least six endothermic and two exothermic effects were established by computer-aided resolving of the curves. The formation of each HT metaphase was related to the release of a discrete number of water molecules depending on the Al content in the samples and each appeared as a corresponding endothermic peak in the DTA curves. The exothermic processes associated with the crystallization of HT-B and MO metaphases were specified by decomposition of DTA curves. The evolution of CO2 during the thermal decomposition of the carbonate groups was found to be different for the samples studied. The preservation of CO3 even at high temperatures was established for synthetic samples with a high Al content. The release of volatile H2O and CO2 (which comprise ~40% of the sample mass) provokes fine cracking both along and across the layers.