F.M. Labajos

Effect of hydrothermal and thermal treatments on the physicochemical properties of Mg-Al hydrotalcite-like materials

The synthesis and characterization of hydrotalcite-like materials is described. As the time to which the samples have been submitted to a hydrothermal treatment is prolonged, a change in the Mg/Al ratio is observed, together with a more ordered structure of the species existing in the interlayer space. Calcination at increasing temperatures leads to decomposition of this compound, with the final formation of MgO and MgAl2O4. Changes taking place during these processes have been related to the decrease observed in the specific surface area of the samples upon prolonging the hydrothermal treatment, as well as its development and further decrease upon thermal treatment.

The effect of iron on the crystalline phases formed upon thermal decomposition of Mg-Al-Fe hydrotalcites

Layered double hydroxides (LDH) containing Mg II , Fe III , and Al III in the brucite-like layers and interlayer carbonate (with a constant M II /M III ratio but varying Al III /Fe III ratios) have been prepared and characterised by X-ray diffraction, thermal analysis, FT-IR and UV-VIS/diffuse reflectance spectroscopies, temperature-programmed reduction and specific surface area assessment through low temperature adsorption of N 2 . An Mg,Al-LDH, but with intercalated hexacyanoferrate(III), has been also prepared and characterised, in which simultaneous formation of the carbonate analogue did not occur. Thermal decomposition in air at 450 and 750°C leads to MgO and poorly crystallised MgFe 2 O 4 spinel (crystallinity increasing with the iron content), while for the hexacyano-containing sample, crystallization only is observed after calcination at 900°C. This different behaviour has been related to the initial location of the iron ions.

Microwave-assisted homogeneous precipitation of hydrotalcites by urea hydrolysis.

The use of urea as a precipitating agent in the synthesis of Ni-Al and Zn-Al layered double hydroxides having a hydrotalcite-like structure via a microwave-hydrothermal method is reported. For comparison purposes, the samples were also prepared by a conventional hydrothermal method. Ni-Al compounds with the hydrotalcite-like structure were obtained in shorter periods of time by the microwave method than by the conventional method, whereas when zinc cations were involved, no successful synthesis was achieved regardless of the method used. In order to find the best synthesis conditions for the Ni-Al solids, samples were submitted to microwave-hydrothermal treatment at different temperatures for increasing periods of time, and the structural, thermal, and textural properties of the synthesized materials were evaluated. All of the solids were fully characterized by chemical elemental analysis, powder X-ray diffraction (PXRD), FT-IR spectroscopy, and transmission and scanning electron microscopy as well as by N 2 adsorption/desorption at -196 degrees C for assessment of specific surface area and porosity. The PXRD patterns showed that the layered structure appeared after merely 10 min when the synthesis was carried out at 125 degrees C; however, the FT-IR spectra showed the presence of some cyanate groups that were formed during urea hydrolysis and were quite difficult to remove completely. When the conventional hydrothermal treatment was used, longer periods of time were required in order to develop the hydrotalcite-like structure, but increasing the aging time improved the crystallinity of the compounds and yielded large particles.

Acetylene hydrogenation on Ni–Al–Cr oxide catalysts: the role of added Zn

Abstract Ni-containing catalysts for selective acetylene hydrogenation to ethylene have been prepared by controlled calcination of hydrotalcite-like precursors. In addition to Ni and Al, Cr and Zn were added to improve the catalytic performance. The hydrotalcite-like precursors and the calcined catalysts have been characterized by powder X-ray diffraction, FT-IR and Vis–UV/Diffuse Reflectance spectroscopies, temperature-programmed reduction, and specific surface area assessment by nitrogen adsorption at 77 K. Despite addition of Zn hinders coke formation, the activity to gaseous products decreases as the Ni content is increased. An increase in coke concentration increases activity and selectivity to ethylene, specially in those samples with not too high Ni contents. The highest selectivity to ethylene is achieved for Zn/Ni≈4 (molar ratio).

Use of hydrotalcites as catalytic precursors of multimetallic mixed oxides. Application in the hydrogenation of acetylene

Abstract The effect of the Ni/Zn molar ratio on the activity, selectivity, and coke formation of NiO·ZnO·Al2O3 catalysts (modified with Fe3+ or Cr3+) during acetylene hydrogenation has been studied. Coke formation is decreased significantly in the presence of ZnO, and a similar effect is also found when the catalysts are doped with Cr3+ instead of with Fe3+. An optimum Ni/Zn ratio for activity, selectivity and coke formation performance has been found. The existence of this maximum implies the necessity of adding ZnO to the support in order to modulate the catalytic properties of Ni. Furthermore, if the Ni concentration is increased, the conversion, selectivity, and yield to ethylene not only fails to increase, but actually decreases, while coke formation simultaneously increases. The existence of the above-mentioned optimum is the consequence of a minimum concentration of the hydrogenolytic (naked) metallic sites, the majority being hydrogenating metallic sites covered by a monolayer of ethylidines. A kinetic model of coke growth is proposed assuming the existence of two types of coke associated with the hydrogenolitic and hydrogenating sites respectively.

Preparation and thermal stability of manganese-containing hydrotalcite, [Mg0.75MnII0.04MnIII0.21(OH)2](CO3)0.11·nH2O

A magnesium–manganese hydroxycarbonate with the hydrotalcite structure has been synthesized by coprecipitation from aqueous solutions of Mg2+ and Mn2+. According to temperature-programmed reduction data, 84% of the manganese is oxidized to Mn3+ during synthesis. Fourier-transform infrared spectroscopy, powder X-ray diffraction and transmission electron microscopy data support the hydrotalcite structure. Thermal decomposition in air at 390 °C leads to an amorphous material with a simultaneous increase of the specific surface area. Further calcination at higher temperatures (670 and 1000 °C) leads to crystallization of MgO and Mg2MnO4 spinel and to sintering. Reduction of the spinel takes place in two separate steps, probably through formation of intermediate Mn3O4 and finally MnO.

Microwaves and layered double hydroxides: A smooth understanding*

Microwave-hydrothermal treatment (MWHT), a modification of conventional hydrothermal treatment, has been used during post-treatment of different layered double hydroxides (LDHs). In some cases, microwaves (MWs) have been used simultaneously with urea hydrolysis or for reconstruction of the LDH structure. The main advantages of replacing the conventional furnaces by MW ovens are a noticeable reduction in the time required to complete the process to obtain well-crystallized materials, and modification of their particle size distribution and textural and thermal properties. MW radiation leads to an increase in the rate of urea hydrolysis and consequently to fast precipitation of LDHs. Finally, the memory effect of Ni,Al-based LDHs is also improved.

A FT-IR and V-UV spectroscopic study of nickel-containing hydrotalcite-like compounds, [Ni1-xAlx(OH)2](CO3)x/2.nH2O

Abstract Hydrotalcite-like compounds containing Ni(II) and Al(III) cations and with different Ni/Al ratios in the brucite layers have been prepared and studied using FT-IR and V-UV/DR spectroscopies. It has been found that the local environment of the Ni(II) cations is the same in all cases, occupying octahedral holes, but the orientation of the interlayer carbonate anions changes with the Ni/Al ratio.

Thermal evolution of V(III)-containing layered double hydroxides

Abstract A study is reported on the thermal evolution of Mg, V layered double hydroxides with interlayer carbonate anions and different Mg/V molar ratios (from 1 to 4). Decomposition in O2 occurs at lower temperature than that in N2; the stability of the layered structure decreases as the V content increases. Oxidation of layer V3+ cations occurs in the same temperature range as dehydroxylation and decarbonation. The nature of the crystalline phases formed upon calcination at high temperatures strongly depends on the Mg/V molar ratio in the initial solid.

XAS study of transition metal ions in hydrotalcite-like compounds

Abstract EXAFS and XANES studies have been performed on hydrotalcite-like materials containing vanadium species in the layers, as V 3+ cations, or in the interlayer space as vanadate anions. It has been found that in the former sample V 3+ isomorphically substitutes Mg 2+ in the octahedral sites, while in the second case anions in the interlayer are as decavanadate clusters. Amorphous decomposition products obtained after calcination of the V 3+ hydrotalcite contain V 5+ cations as isolated VO a 3− units.