Zn Al Hydrotalcite-like Compounds Research Articles

Boosting COS catalytic hydrolysis performance over Zn-Al oxide derived from ZnAl hydrotalcite-like compound modified via the dopant of rare earth metals and the replacement of precipitation base

In this work, a series of Zn-Al oxides calcined from ZnAl hydrotalcite-like catalysts were prepared using the co-precipitation method and then applied to remove COS. The effects of the content of Sm and the species of rare earth metal on the catalytic activities of COS hydrolysis are investigated. Experimental results show that the prepared catalyst doped with 20 wt% Sm performs better catalytically. Based on the optimization of Sm dopant, the effect of the precipitation base in the ZnAl hydrotalcite-like compounds on the catalytic activities of the catalysts is further investigated. The results show that the prepared catalyst using KOH and K2CO3 as the precipitation base exhibits the best catalytic performance. The characterization results show that the doping of rare earth metals can weaken the crystallinity and the introduction of an appropriate amount of Sm ions can significantly enhance the proportion of adsorbed oxygen species. Moreover, the results show that the synergistic effect of the doping of Sm and the replacement of precipitating alkali contribute to the enhancement of the number of basic sites, especially weak and medium basic sites. Furthermore, combined with in-situ DRIFTS, the catalytic mechanism of COS hydrolysis on ZnAl-20Sm-Na MMO is proposed.

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.

ZnAl-Zr hydrotalcite-like compounds activated at low temperature as solid base catalyst for the transesterification of vegetable oils

ZnAl hydrotalcite-like compounds were modified through the incorporation of Zr+4 using molar ratios of Zr/Al(X)=0.0, 0.10 and 0.25. The synthesis of these materials was performed by the co-precipitation method and their physicochemical properties were characterized by XRD, UV-Vis-DRS, N2 physisorption, TGA, and finally, the basic properties were analyzed by titration with benzoic acid and adsorption of methanol analyzed by FT-IR. It was found that 200ºC is the most appropriate activation temperature, since the catalytic activity of ZnAl-Zr(X) materials on the transesterification reaction was enhanced, obtaining FAME yields ranging from 68% to 82%. This high catalytic activity at low activation temperature (200ºC) was attributed to the presence of a mixture of the active phases: hydrotalcite, zincite, and hydrozincite; as well as the formation of Brönsted and Lewis basic sites when the hydrotalcite-type materials are dehydrated. In addition, it was possible to calculate the apparent reaction rate constants by means of a simplified lumped first-order reaction model. The reaction rate constant for the ZnAl-Zr(0.25) catalyst was 12.4 x10-3 L/gcat.min which is 1.5 times greater than that of the catalyst without Zr.

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.

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.

Ab Initio Simulation of Changes in Geometry, Electronic Structure, and Gibbs Free Energy Caused by Dehydration of Hydrotalcites Containing Cl−and CO32−Counteranions

This ab initio study was performed to better understand the correlation between intercalated water molecules and layered double hydroxides (LDH), as well as the changes that occur by the dehydration process of Zn-Al hydrotalcite-like compounds containing Cl⁻ and CO₃²⁻ counterions. We have verified that the strong interaction among intercalated water molecules, cointercalated anions, and OH groups from hydroxyl layers is reflected in the thermal stability of these compounds. The Zn(2/3)Al(1/3)(OH)₂Cl(1/3)·2/3H₂O hydrotalcite loses all the intercalated water molecules around 125 °C, while the Zn(2/3)Al(1/3)(OH)₂(CO₃)(1/6)·4/6H₂O compound dehydrates at about 175 °C. These values are in good agreement with experimental data. The interlayer interactions were discussed on the basis of electron density difference analyses. Our calculation shows that the electron density in the interlayer region decreases during the dehydration process, inducing the migration of the Cl⁻ anion and the displacement of the hydroxyl layer from adjacent layers. Changes in these compound structures occur to recover part of the hydrogen bonds broken due to the removal of water molecules. It was observed that the chloride ion had initially a lower Löwdin charge (Cl(-0.43)), which has increased its absolute value (Cl(-0.58)) after the water molecules removal, while the charges on carbonate ions remain invariant, leading to the conclusion that the Cl⁻ anion can be more influenced by the amount of water molecules in the interlayer space than the CO₃²⁻ anion in hydrotalcite-like compounds.

On the study of KF/Zn(Al)O catalyst for biodiesel production from vegetable oil

A KF/Zn(Al)O catalyst was prepared by doping KF on calcined Zn–Al hydrotalcite-like compounds and tested for transesterification of vegetable oil with methanol to biodiesel. The obtained catalyst was characterized using techniques of nitrogen physisorption, Hammett indicator, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Its catalytic activity was compared with that of Zn–Al hydrotalcite-like compounds, Zn(Al)O, KF, KF/γ-Al2O3 and KF/ZnO. It was found that the prepared KF/Zn(Al)O catalyst showed the highest activity. This catalyst was particularly effective since, for a methanol to oil molar ratio of 6:1, an addition of 3wt.% KF/Zn(Al)O catalysts, at 65°C reaction temperatures, the biodiesel yield exceeded 95% at a reaction time of 3h. Its high activity was ascribed to the formation of new phases KF and KOH and effect of Zn(Al)O support.

Preparation and photophysical characterisation of Zn–Al layered double hydroxides intercalated by anionic pyrene derivatives

Zn–Al hydrotalcite-like compounds intercalated by 1,3,6,8-pyrenetetrasulfonate (PTS), 1-pyrenesulfonate (PS) and 1-pyrenecarboxylate (PC) anions were synthesised by an ion-exchange procedure. The materials were characterised by powder X-ray diffraction at different temperatures, thermogravimetric analysis, FTIR, 13C{1H} CP/MAS NMR and photoluminescence techniques. In the fully hydrated states, the interlayer distances are 13.7 A for Zn–Al–PTS, 18.9 A for Zn–Al–PS, and 24.8 A for Zn–Al–PC. These can be ascribed to a monolayer arrangement for intercalated PTS anions and bilayer arrangements for the 1-pyrenyl derivatives. The samples exhibit different thermal decomposition pathways, and in the case of Zn–Al–PTS the removal of physisorbed and interlayer water leads to a change in the orientation of the organic anion with respect to the hydroxide layers. The structural transformation is fully reversible upon hydration. The photophysical characterisation of the bulk materials was based on the determination of their emission and fluorescence excitation spectra, and the fluorescence lifetimes. From the steady-state (monomer and “excimer-like” bands) and time-resolved (triple exponential decays) data, evidence for the presence of a structure with similar characteristics to pyrene dimer together with monomer (by comparison with the emission of dilute solutions of PS and PC), pre-associated and (possibly) dynamic excimer species could be presented.

Effect of Permanent Charge on the Intrinsic Ionization Constant of Zn-Al Hydrotalcite-like Compounds

Effect of Permanent Charge on the Intrinsic Ionization Constant of Zn-Al Hydrotalcite-like Compounds

Space-resolved fluorescence properties of phenolphthalein-hydrotalcite nanocompositesPresented at the LANMAT 2001 Conference on the Interaction of Laser Radiation with Matter at Nanoscopic Scales: From Single Molecule Spectroscopy to Materials Processing, Venice, 3–6 October, 2001.

Phenolphthalein has been included into Zn–Al hydrotalcite-like compounds by taking advantage of the “memory effect” of hydrotalcites. In particular [Zn0.65Al0.35(OH)2] (CO3)0.1750.5H2O was calcined at 500 °C and the obtained mixture of Zn and Al oxides was dispersed in an aqueous alcohol solution containing 7 × 10−2 mol dm−3 phenolphthalein. The reconstruction of the layered hydrotalcite structure in the presence of the dye yielded formation of the composites in which phenolphthalein is intercalated in the interlayer region and/or adsorbed on the layer surface. Two samples were studied, the first, obtained at 25 °C, contained 0.17 mmol g−1 of phenolphthalein, mainly adsorbed on the surface, the second, obtained at 60 °C, contained 0.3 mmol g−1, in part intercalated. The samples were characterized by their X-ray diffraction patterns, specific surface areas and scanning electron micrographs. The photophysical characterisation of the bulk samples was based on the determination of their reflectance absorption and fluorescence spectra, the fluorescence lifetimes and fluorescence anisotropy. The results indicated different properties of the lowest singlet excited state of the adsorbed and intercalated dye. The space-resolved fluorescence images and fluorescence spectra obtained by confocal fluorescence spectroscopy of the two samples gave valuable information on the dye distribution and on the nature of the interactions between the dye and the inorganic matrix.

Preparation and Characterization of Al-Rich Zn-Al Hydrotalcite-Like Compounds

AbstractHydrotalcite-like compounds, described by the formula [Zn1-xAlx(OH)2][(CO3)x/2 · nH2O], were prepared by coprecipitation methods at 80°C and characterized by bulk chemical analysis, X-ray powder diffraction (XRD), nuclear magnetic resonance (NMR), and scanning-transmission electron microscopy (STEM). An x value of 0.33 was previously assumed to be an upper limit, but recently, Al-rich hydrotalcite-like compounds have been prepared with x as large as 0.44 by hydrothermal synthesis. In the Zn-Al system, Al-rich hydrotalcite was synthesized at normal pressure by coprecipitation. Zn-Al hydrotalcite-like compounds were obtained in the range of x = 0.3 to 0.4. An Al-rich hydrotalcite-like compound with x = 0.44 was formed in mixtures containing large amounts of a poorly crystalline Zn-Al phase. A continuous contraction of the hydrotalcite-like structure occurred as x increased, both the a and c lattice parameters decreasing for x values as large as 0.44. This study illustrates the advantages of using quantitative analytical electron microscopy with high spatial resolution to complement conventional (and bulk) characterization techniques for correlating structural and compositional characteristics of finely divided materials.