Al Hydrotalcite-like Compounds Research Articles

Supported highly dispersed molybdenum nitride derived from molybdate-intercalated Mg‒Al hydrotalcite-like compounds as a catalyst for ammonia decomposition

Molybdate-intercalated magnesium–aluminum hydrotalcite-like compounds (HTlc) were synthesized by anion-exchange and used as precursor to prepare supported molybdenum nitride catalyst for ammonia decomposition. The as-synthesized precursors and the derived mixed metal oxides and nitride catalysts were characterized by ICP, XRD, Raman spectroscopy, XPS, H2-TPR, and HAADF-STEM-EDX. The characterizations indicated that molybdate was well intercalated into the HTlc interlayer as MoO42– and the Mo content was tunable by altering the intercalation degree of molybdate. Upon calcination, molybdena was dominantly presented as isolated tetrahedrally coordinated MoOx species. Upon nitridation with ammonia at 700 ºC, highly dispersed and composition-uniform molybdenum nitride (Mo2±xN) was formed. The Mo0.5Mg3Al nitride catalyst showed good activity for ammonia decomposition, providing nearly full conversion of ammonia at 625 ºC under a space velocity of 5,000 mL gcat–1 h–1. The optimal catalyst also exhibited high catalytic stability during 100 h operation at 625 ºC, and no obvious aggregation took place, attributable to the strong metal–support interaction between molybdenum nitride and Mg–Al mixed oxide. It is suggested that the hydrotalcite-derived Mg–Al mixed oxide supported highly dispersed Mo2±xN species was active and stable for ammonia decomposition.

Nickel‒cobalt bimetallic catalysts prepared from hydrotalcite-like compounds for dry reforming of methane

Ni–Co/Mg(Al)O alloy catalysts with different Co/Ni molar ratios have been prepared from Ni- and Co-substituted Mg–Al hydrotalcite-like compounds (HTlcs) as precursors and tested for dry reforming of methane. The XRD characterization shows that Ni–Co–Mg–Al HTlcs are decomposed by calcination into Mg(Ni,Co,Al)O solid solution, and by reduction finely dispersed alloy particles are formed. H2-TPR indicates a strong interaction between nickel/cobalt oxides and magnesia, and the presence of cobalt in Mg(Ni,Co,Al)O enhances the metal-support interaction. STEM-EDX analysis reveals that nickel and cobalt cations are homogeneously distributed in the HTlcs precursor and in the derived solid solution, and by reduction the resulting Ni–Co alloy particles are composition-uniform. The Ni–Co/Mg(Al)O alloy catalysts exhibit relatively high activity and stability at severe conditions, i.e., a medium temperature of 600 °C and a high space velocity of 120000 mL g−1 h−1. In comparison to monometallic Ni catalyst, Ni–Co alloying effectively inhibits methane decomposition and coke deposition, leading to a marked enhancement of catalytic stability. From CO2-TPD and TPSR, it is suggested that alloying Ni with Co favors the CO2 adsorption/activation and promotes the elimination of carbon species, thus improving the coke resistance. Furthermore, a high and stable activity with low coking is demonstrated at 750 °C. The hydrotalcite-derived Ni–Co/Mg(Al)O catalysts show better catalytic performance than many of the reported Ni–Co catalysts, which can be attributed to the formation of Ni–Co alloy with uniform composition, proper size, and strong metal-support interaction as well as the presence of basic Mg(Al)O as support.

Controlled preparation of Ni–Cu alloy catalyst via hydrotalcite-like precursor and its enhanced catalytic performance for methane decomposition

Composition-uniform Ni–Cu/Al2O3 alloy catalysts have been prepared from Ni–Cu–Al hydrotalcite-like compounds (HTlcs) and tested for methane decomposition at 650°C. The catalysts were characterized by XRD, XPS, H2 chemisorption, H2-TPR, STEM-EDX, SEM, TEM, and Raman. The characterizations reveal that calcination of Ni–Cu–Al HTlcs leads to Ni(Cu,Al)O oxide solid solution, both nickel and copper ions being homogeneously distributed in HTlcs as well as in Ni(Cu,Al)O, and upon reduction the well-mixed Cu2+/Ni2+ species are step-wise reduced to form composition-uniform Ni–Cu alloy with an average size of 9.5–10.4 nm. Alloying Ni with an appropriate amount of Cu remarkably enhances the catalytic life and carbon yield. The highest carbon yield of 132.9 g-C/g-cat is obtained at atomic ratio of Ni:Cu = 7:3, which is about 78 times that of the Ni/Al2O3 counterpart. Moreover, carbon morphology is changed from thin CNTs to thick fishbone-CNFs and platelet-CNFs depending on the copper content. Under the reaction atmosphere, Ni–Cu alloy is sintered to large particles by contact with methane. It is suggested that Ni–Cu alloying favors the formation of large alloy particles, which inhibits methane dissociation and enhances carbon bulk diffusion, thus facilitating the CNFs growth and leading to a significant increase of carbon yield.

Structure and surface properties of Ni–Al hydrotalcite-like compounds and their catalytic application in highly selective acetalization at room temperature

Oxygen vacancies improve the catalytic activity of acid sites, and their amount decreases due to the conversion of [Ni(OH)5NO3] into [Ni(OH)6] units.

Ni–Fe/Mg(Al)O alloy catalyst for carbon dioxide reforming of methane: Influence of reduction temperature and Ni–Fe alloying on coking

Various Ni–Fe/Mg(Al)O alloy catalysts were obtained by calcination of Ni–Fe–Mg–Al hydrotalcite-like compounds, followed by reduction at different temperatures (973–1173 K). The characterizations of XRD and STEM-EDX suggest that the resulting Ni–Fe alloy particles are composition-uniform and size-controllable. The alloy composition is little affected by the reduction temperature, whereas the particle size (5.8–8.2 nm) increases with the increase of reduction temperature. This property is ascribed to the homogeneous distribution of nickel and iron species during the catalyst preparation. All of the Ni–Fe/Mg(Al)O alloy catalysts show relatively high and stable activity for CH4–CO2 reforming during 25 h of investigation at 773–1073 K. Particularly, the 973 K-reduced catalyst exhibits higher coke-resistance due to its smaller particle size. Ea-CH4 and CH4-TPSR measurements indicate that Ni–Fe alloying inhibits CH4 dissociation. It is considered that during DRM CH4 is dissociated at the Ni sites and CO2 may be activated at the metal-support interface as well as the Fe sites. Ni–Fe alloying may inhibit CH4 dissociation and/or promote CO2 activation, thus contributing to the suppression of coke deposition.

Characterization and catalytic behavior of hydrotalcite-derived Ni–Al catalysts for methane decomposition

A series of Ni catalysts were prepared from Ni–Al hydrotalcite-like compounds (HTlcs) by varying the Ni/Al molar ratio (1–4) and calcination temperature (773–1173 K) of HTlcs. The catalysts were reduced with H2 at 1073 K and tested for CH4 decomposition at 773–923 K on a thermal gravimeter. Various techniques including N2 physical adsorption, XRD, H2-TPR, XPS, HAADF-STEM, TEM, and Raman were applied to characterize the catalysts and the as-produced carbon. The characterizations show that calcination of Ni–Al HTlcs leads to Ni(Al)O solid solution and minor NiO and/or NiAl2O4 spinel may be formed depending on the Ni/Al ratio and calcination temperature; upon reduction at 1073 K, most nickel species are reduced to metallic Ni. In CH4 decomposition, carbon yield shows a volcano-type dependence on the Ni content with the optimum Ni/Al ratio equal to 3. On the other hand, carbon yield is affected by the calcination temperature of the Ni3Al HTlcs to a small extent. Carbon yield is also significantly affected by the reaction temperature, which decreases remarkably with a rise of temperature to 923 K. TEM and Raman indicate that fish-bone carbon nanofibers are formed at 773–823 K, whereas multi-walled carbon nanotubes are formed at 873–923 K.

Effects of amino trimethylene phosphonic acid on structure and properties of Cu–Zn–Al hydrotalcite-derived oxides for catalytic synthesis of iso-butanol and ethanol from synthesis gas

Ethanol and iso-butanol have received considerable interest to be used in automobiles as additives or as potential substitutes for gasoline. The Cu–Zn–Al hydrotalcite-derived oxides (CuZnAl-HTDO) can be used to catalyze synthesis of higher alcohols from syngas, and the complexing agents such as amino trimethylene phosphonic acid (ATMP) were used as novel modifiers to improve the catalytic activities of CuZnAl-HTDO. However, there is some lack of knowledge about the intrinsic relation between structure of the ATMP-modified CuZnAl-HTDO and its catalytic activities. Therefore, characterization techniques including FT–IR, XRD, N2 adsorption–desorption, H2–TPR, CO–TPD, CO2–TPD and SEM were carried out to investigate the composition, crystal structure, textural properties, reducibility, alkalinity, CO adsorption behavior and morphology of the ATMP-modified CuZnAl-HTDO and its precursors. The complexation reaction between Cu2+ ions and ATMP leads to expansion of crystal structure for the precursors (Cu–Zn–Al hydrotalcite-like compounds), and thus the amount of strained and defective lattices in CuZnAl-HTDO increases after calcination of the precursors. The lattice strain and defects improve hydrogen reduction of CuO in CuZnAl-HTDO and enhance CO adsorption and dissociation on Cu active sites. In addition, a network structure formed by Cu(II)-ATMP chelate delays desorption of alcohols from Cu active sites and thus promotes CO insertion reaction. These structural changes caused by ATMP promote catalytic synthesis of iso-butanol and ethanol from syngas.

The Heterogeneous Aminohydroxylation Reaction Using Hydrotalcite-Like Catalysts Containing Osmium

The aminohydroxylation reaction of olefins is a key organic transformation reaction, typically carried out homogeneously with toxic and expensive osmium (Os) catalysts. Therefore, heterogenisation of this reaction can unlock its industrial potential by allowing reusability of the catalyst. Os–Zn–Al hydrotalcite-like compounds (HTlcs), as potential heterogeneous aminohydroxylation catalysts, were synthesised by the co-precipitation method and characterised by several techniques. Reaction parameters (i.e., solvent system, reaction temperature, and catalyst structure) were optimized with cyclohexene, styrene, and hexene as substrates. The different classes of olefins (aliphatic, aromatic, and functionalised) that were tested gave >99% conversion and high selectivity (>97%) to the corresponding β-amino alcohol. The catalyst HTlc structure had a significant effect on the reaction time and yield of the β-amino alcohols. Under the same testing conditions, a heat treated catalyst (non-HTlc) showed a shorter reaction time, but drop in the yield of β-amino alcohols and rise in diol formation was observed. Leaching tests showed that 2.9% and 3.4% of Os (inactive) leached from the catalyst to the reaction solution when MeCN/water (1:1 v/v) and t-BuOH/water (1:1 v/v), respectively, were used as the solvent system. Recycling studies showed that the catalyst can be reused at least thrice, with no significant difference in the yield of the β-amino-alcohol.

Dehydrogenation Catalysts for Synthesis of O-Phenylphenol via Cu/Ni/Mg/Al Hydrotalcite-Like Compounds as Precursors

A series of copper containing catalysts were prepared by calcination of Cu/Ni/Mg/Al hydrotalcite-like precursors, using the coprecipitation method. The materials were characterized and show a well-crystallized layered structure of hydrotalcite with smaller Cu0 particles. We also studied their catalytic performance for conversion of 2-(1-cyclohexenyl) cyclohexanone into o-phenylphenol. The catalysts containing Ni showed higher catalytic activity; the optimum stability occurred when the Ni2+:(Ni2+ + Cu2+) atomic ratio was 0.4. The combination of Cu and Ni can greatly improve the stability and activities of the catalyst.

Fluorinated Mg–Al Hydrotalcites Derived Basic Catalysts for Transesterification of Glycerol with Dimethyl Carbonate

A series of fluorinated Mg–Al hydrotalcite-like (HTl) compounds were prepared by introduction of different amount of (AlF6)3− into HTl sheets via co-precipitation method. The fluorine-modified Mg–Al mixed oxides (CHT-F) with mesoporous structure were then acquired by thermal decomposition of as-prepared HTl precursors. The influence of fluorine modifier on the physicochemical properties of CHT-F samples was studied in detail and the results demonstrated that their structure and basicity were strongly related to the fluorine content. Simultaneously, the CHT-F samples were test for the glycerol carbonate (GC) production via transesterification between glycerol and dimethyl carbonate without organic solvent. For these catalysts, the role of basic sites was examined based on reactant conversion and product selectivities in overall reaction as well as in side reaction identified in this work. The catalytic results showed that the activity of CHT-F catalysts depended on the total number of surface basic sites. However, further study demonstrated that the undesired product was catalyzed by strong basic sites, resulting in decrease of GC selectivity. Thus, the introduction of appropriate amount of (AlF6)3− into the HTl structure can promote the production of GC, and the best catalytic performance was obtained over the catalyst with F:Al = 1.0. Besides, various parameters including reaction time, reaction temperature and catalyst amount were investigated to optimize the reaction conditions. Furthermore, these CHT-F catalysts possessed high stability on the basis of reusability test and catalyst characterization.

Supported Ni catalysts prepared from hydrotalcite-like compounds for the production of hydrogen by ammonia decomposition

Ni catalysts with high surface areas were prepared from Mg–Al hydrotalcite-like compounds containing Ni with different Mg-to-Al ratios for the production of hydrogen via ammonia decomposition. At atomic ratios of Mg-to-Al from 3:1 to 6:1, Ni reduction was greater than 90%, and the amounts of Ni0 exposed were kept at more than 200 μmol g−1. With the increase in Mg-to-Al ratio, turnover frequency was increased due to an increase in the basicity of the support. Of the catalysts examined, Ni_MgAl(6:1) exhibited the highest NH3 conversion, which was attributed to its relatively high basicity and large amount of Ni0 exposed.

Synthesis and characterisation of surfactant enhanced Mg–Al hydrotalcite-like compounds as potential 2-chlorophenol scavengers

Magnesium aluminium hydroxycarbonate hydrotalcites (denoted as MgAl–CO3-HTs) with different Mg/Al molar ratios (4, 3 and 2) were synthesised by the co-precipitation method under low supersaturation conditions, and then treated with sodium dodecylsulfate (SDS) and sodium dodecylbenzenesulfonate (SDBS) surfactants to produce the nanocomposites — organo-hydrotalcites; dodecylsulfate-hydrotalcites(DS-HTs) and dodecylbenzenesulfonate-hydrotalcites (DBS-HTs) through calcination-reconstruction method. Powder X-ray diffraction (PXRD) and infrared spectroscopy analysis of intercalated samples showed that dodecylsulfate and dodecylbenzenesulfonate guests were successfully intercalated into the parent hydrotalcites, with the PXRD revealing that the species could assume varying configurations within the interlayer gallery regions of this clay based materials, displaying monolayer and bilayer orientations.The uptake ability of the resulting DS-hydrotalcites and DBS-hydrotalcites for the adsorption of phenolic compounds from aqueous solution was investigated. The results showed that, the adsorption process can be described by pseudo-second order kinetics, while the capacity of 2-chlorophenol uptake is dependent on the MgII:AlIII ratio within the interlamellar of the organo-hydrotalcite, the anion present (with DBS modified samples showing higher adsorption capacities) and the pH at which adsorption was carried out.

CO2 capture by Mg–Al and Zn–Al hydrotalcite-like compounds

Hydrotalcite-like compounds (HTC) are distinguished by their properties for CO2 capture, like high surface area, basic sites, thermal stability and good adsorption/desorption efficiency. Mg–Al e Zn–Al HTCs with Al3+ molar ratios x = 0.20, 0.28 and 0.33 were synthesized by coprecipitation, and subsequently calcined at 400 °C. For both HTCs, X-ray diffraction patterns have attested the formation of mixed oxides through calcination. The amount of basic sites, measured by temperature-programmed desorption of CO2, decreases as x increases. The CO2 adsorption was performed in a thermogravimetric balance using an adsorption temperature of 50 °C. Mg–Al and Zn–Al samples with x = 0.33 molar composition presented the highest CO2 adsorption, 0.91 and 0.21 mmol g−1, respectively. The Langmuir isotherm fitted well to the experimental data. It was also found that increasing the number of adsorption/desorption cycles the CO2 adsorption decreases, which is associated with the irreversible chemisorption.

Production of C2 and C3 polyols from d-sorbitol over hydrotalcite-like compounds mediated bi-functional Ni–Mg–Al–Ox catalysts

Production of C2 and C3 polyols from d-sorbitol over Ni/MgxAlyOx+1.5y catalysts with different Mg/Al molar ratio were performed under mild conditions. It was found that this reaction proceeded effectively over these bi-functional catalysts prepared via thermal decomposition and reduction of Ni–Mg–Al hydrotalcite-like compounds. The detected conversion of d-sorbitol over Ni/Mg1.29Al0.06O1.38 reached 80.7% at 453K and further increased to 97.0% at 473K. Characterizations indicate that the performance of these catalysts depended mainly on its surface basicity.

Synthesized of macroporous composite electrode by activated carbon fiber and Mg–Ca–Al (NO3) hydrotalcite-like compounds to remove bromate

Abstract In this study, a novel composite electrode was been modified by using activated carbon fiber (ACF) and Mg–Ca–Al (NO3) hydrotalcite-like compounds by electrophoretic deposition (EPD) method. The resulting electrode was characterized by SEM, BET and cyclic voltammetry (CV). The results indicated the specific surface area of the composite electrode (MACF electrode) is twice as the basic one (ACF electrode) and the MACF electrode revealed a higher capacity of BrO3− ions. The adsorption experiments results showed that the adsorption capacity of electrode increased with the increase of electrical potential and initial bromate concentration. Furthermore, the bromate adsorption of the two electrodes was suppressed by the presence of coexisting anions, wherein the inhibition is ordered as follows: SO42− > CO32− > Cl−. What is more, the modeled results indicated that the adsorption process of bromate by the electrode derived from physical sorption and controlled by the intra-particle diffusion.

Phosphoric acid intercalated Mg–Al hydrotalcite-like compounds for catalytic carboxylation reaction of methanol in a continuous system

Mg–Al-hydrotalcite-like catalysts (HTlcs) with different amount of P/Mg molar ratio were prepared via co-precipitation method and calcined at 450°C. The synthesized catalysts were tested in the direct-carboxylation reaction of methanol in gas phase in a continuous system such as chemical route for CO2 valorization. Activities around 2% with total selectivity towards the dimethyl carbonate (DMC) was obtained at moderate temperatures (150°C) with both Mg/Al mixed oxides (HTO) and phosphated Mg/Al mixed oxides (HTc-9c). Even though the conversion increased until 16% at higher temperatures (200°C), the selectivity with both catalysts (HTO and HTc-9c) decreased due to the decomposition of DMC to dimethyl ether (DME). Nevertheless, the catalyst with P showed less DMC decomposition and a higher selectivity towards the desired product. This is explained by the presence of orthophosphate species bonded to the Al3+ metals of the HTs (OP(OAl)3) which give to the catalyst a higher structural stability and specific acid properties.In addition, the solids were characterized in-depth by XRD, ICP, NH3-TPD, FTIR, Raman, 27Al and 31P MAS NMR spectroscopy and SEM.

Study on preparation of layered double hydroxides (LDHs) and properties of EPDM/LDHs composites

The aim of this work was to study preparation and characterisation of layered double hydroxides (LDHs) and their effects on the mechanical and flame-retardant properties of ethylene propylene diene (EPDM) polymer. A series of Mg–Al, Ni–Al, and Cu–Al hydrotalcite-like compounds were prepared. The surface morphology, interlayer space, interlamellar structure, and thermal properties of these LDHs were investigated. Results showed that a best thermal stability could be obtained with Cu–Al–LDHs as the most effective components. EPDM/Cu–Al–LDHs composites were prepared by conventional compounding with EPDM and Cu–Al type of LDHs. The cure characteristics, tensile strength, wear resistant, and flame-retardant properties were investigated. The best properties were observed for 10 phr of Cu–Al–LDHs filled composite, which resulted in no obvious changes of tensile strength, increased thermal stability, 45% decrease in abrasion loss, and 53% increase in vertical burning time, respectively, compared to that of pure EPDM matrix.

Fabrication of porous microspheres and network arrays of Zn–Al hydrotalcite-like compounds on Al substrate via facile hydrothermal method

The porous microspheres and network arrays of Zn–Al hydrotalcite-like compounds were synthesized on Al substrate using sodium oxalate via a facile one-step hydrothermal approach at low temperature (70°C). The Zn–Al hydrotalcite-like microspheres assembled by numerous interlaced curved nanoplates with thickness of about 50nm were generated when the concentration of sodium oxalate is 0.21M. The XRD pattern indicates that the product was of good quality in terms of phase purity and crystallinity. The morphology of Zn–Al hydrotalcite-like compounds and the thickness of nanoplates varied with the concentration of sodium oxalate, which should be attributed to the different nucleation density and growth rate. When the concentration of Zn2+ and sodium oxalate was proportionally reduced the low nucleation density and growth caused the formation of porous network arrays. The reaction temperature directly affected the diffusion rates of ions and thus the density of the nucleation and growth is responsible for the morphology change when the reaction temperature is varied. Additionally, the Zn–Al hydrotalcite-like microspheres transformed into porous network arrays when the sodium oxalate was substituted by the equivalent sodium acetate, which should be attributed to the low alkalinity of sodium acetate. The density and thickness of nanoplates composed of the porous network arrays can be effectively tailored by adjusting the concentration of sodium acetate. On the basis of experimental results, the growth mechanism was proposed and discussed.

Low temperature hydrolysis of carbonyl sulfide using Zn–Al hydrotalcite-derived catalysts

A series of Zn–Al hydrotalcite-like compounds (HTLCs) were synthesized by co-precipitation method. The mixed oxides derived from HTLCs were tested for the catalytic hydrolysis of carbonyl sulfide (COS) at relatively low temperatures of 50°C. These catalysts were characterized by X-ray diffraction (XRD) and Thermogravimetry/Derivative Thermogravimetry Analysis (TG-DTA) and the results can help us to understand the effect of preparation conditions. The results showed that the Zn–Al hydrotalcite-like compounds were efficient precursor in preparing active and stable catalysts for hydrolysis of COS. The catalyst performance was strongly related to the synthesis pH and calcination temperature. The optimum condition was pH of 10, and calcination temperature of 400°C. Although water was indispensable for hydrolysis of COS, the excess water would hinder the adsorption of COS. In general, COS was hydrolyzed to H2S, which is depending on the chemistry of the derived oxides. The end-products of hydrolysis were simple substance S and sulfate. The presence of oxygen will accelerate the formation of the final product.

Synthesis, characterization, and SO2 removal capacity of MnMgAlFe mixed oxides derived from hydrotalcite-like compounds

Fe (III), Mn (II)-substituted Mg–Al hydrotalcite-like compounds were prepared by co-precipitation method and MnMgAlFe mixed oxides derived from hydrotalcite-like compounds (calcination at 1023K for 4h) were tested for SO2 removal under the conditions similar to those of FCC units. XRD, IR, TG/DTA and N2 adsorption analysis were performed to investigate the physicochemical and textural properties of the samples. The results showed that all samples exhibited good dispersion of metal oxides in the matrix. The SO2 adsorption–reduction tests showed that on the basis of MgAl adsorbent, addition of a certain amount manganese and iron could enhance the the SO2 removal capacity and adsorption rates of catalysts in FCC units and the sample that contains 5% Fe and 10% Mn showed the best effect. Further analysis suggested that Fe and Mn could cooperatively promote the oxidation of SO2 to SO3 that is easier to be chemisorbed by catalysts and Fe simultaneously played a role as reducing promoter which is essential for reusability of the catalysts.