Enrique Lima

Structural and Thermochemical Chemisorption of CO2 on Li4+x(Si1–xAlx)O4 and Li4–x(Si1–xVx)O4 Solid Solutions

Different Li(4)SiO(4) solid solutions containing aluminum (Li(4+x)(Si(1-x)Al(x))O(4)) or vanadium (Li(4-x)(Si(1-x)V(x))O(4)) were prepared by solid state reactions. Samples were characterized by X-ray diffraction and solid state nuclear magnetic resonance. Then, samples were tested as CO(2) captors. Characterization results show that both, aluminum and vanadium ions, occupy silicon sites into the Li(4)SiO(4) lattice. Thus, the dissolution of aluminum is compensated by Li(1+) interstitials, while the dissolution of vanadium leads to lithium vacancies formation. Finally, the CO(2) capture evaluation shows that the aluminum presence into the Li(4)SiO(4) structure highly improves the CO(2) chemisorption, and on the contrary, vanadium addition inhibits it. The differences observed between the CO(2) chemisorption processes are mainly correlated to the different lithium secondary phases produced in each case and their corresponding diffusion properties.

Structural Analysis and CO2 Chemisorption Study on Nonstoichiometric Lithium Cuprates (Li2+xCuO2+x/2)

Lithium cuprate (Li(2)CuO(2)) was prepared by solid state reaction, using different quantities of lithium excess, which produced nonstoichiometric ceramics, Li(2+x)CuO(2+x/2). These ceramics were characterized by X-ray diffraction, transmission and scanning electron microscopies, solid state nuclear magnetic resonance, and atomic absorption. The results obtained showed that lithium excess is located mainly into the Li(2)CuO(2) interlayers forming nanoparticles of a different phase, perhaps lithium oxide. Additionally, the lithium excess produced morphological changed at a micrometric and nanometric levels. As lithium excess increased, the particle size increased as well and it formed some kind of filament-like structures. It was explained in terms sintering, due to the high mobility of lithium atoms. On the other hand, all these ceramics were tested as CO(2) captors, presenting encouraging properties through a chemisorption process. As expected, the CO(2) absorption increased as a function of total lithium contained into the ceramics. Finally, it was performed a kinetic analysis of the CO(2) absorption.

Thermokinetic study of the rehydration process of a calcined MgAl-layered double hydroxide.

The rehydration process of a calcined MgAl-layered double hydroxide (LDH) with a Mg/Al molar ratio of 3 was systematically analyzed at different temperatures and relative humidity. Qualitative and quantitative experiments were done. In the first set of samples, the temperature or the relative humidity was varied, fixing the second variable. Both adsorption and absorption phenomena were present; absorption process was associated to the LDH regeneration. Of course, in all cases the LDH regeneration was confirmed by other techniques such as TGA, solid state NMR, and SAXS. In the second set of experiments, a kinetic analysis was performed, the results allowed to obtain different activation enthalpies for the LDH regeneration as a function of the relative humidity. The activation enthalpies varied from 137.6 to 83.3 kJ/mol as a function of the relative humidity (50 and 80%, respectively). All these experiments showed that LDH regeneration is highly dependent on the temperature and relative humidity.

On the acid-base properties of microwave irradiated hydrotalcite-like compounds containing Zn2+ and Mn2+.

Microwave irradiated lamellar double hydroxides containing different divalent metals (Mn2+, Zn2+, or Mg2+) were prepared with Al3+ as the trivalent metal. Samples containing Mn2+ and Zn2+ were unstable at 400 degrees C, leading to formation of mixed oxides and spinel phases. Acid-base properties of the samples were characterized by nitromethane and CO2 adsorption followed by FTIR spectroscopy. Decomposition of adsorbed nitromethane leads to isocyanate species that acts as probe molecules of acid-base sites at the surface. These properties determine the ability of materials to retain CO2. Indeed, whereas Mn-O sites are able to interact directly with CO2 molecules, Mg-O and Zn-O are able to form carbonate species as a result of the CO2 sorption.

Structural and textural evolution during folding of layers of layered double hydroxides.

Layers of a layered double hydroxide, containing aluminum 4-fold coordinated, were partially folded in order to obtain a fibrous hydrotalcite-like compound. The hydrotalcite layers, in the presence of an anionic surfactant (sodium dodecyl sulfate) after hydrothermal treatment for 2 weeks, acquire a mesoporous-like arrangement. The transformation was monitored by techniques sensitive to structural and textural properties. Results suggest that brucite-like layers can be joined throughout unsaturated coordinated aluminum, that is, tetrahedral aluminum which links through hydrogen bonds to form aluminum octahedrally coordinated. The fractal dimension parameter was very sensitive to evolution from layered to fibrous hydrotalcites.

Capturing natural chromophores on natural and synthetic aluminosilicates

Several materials such as zeolites, clays, alumina, and hydrotalcite were tested as adsorbents of carmine cromophores from hibiscus Sabdariffa L. tropical flower. Majority of materials were unable to trap and stabilize the chromophore, some of them were not coloured and by the other ones the chromophore was chemically degraded, a change of colour was observed. The best adsorbents were natural mordenite zeolite from Oaxaca, Mexico and montmorillonite from Wyoming, USA. These two materials sequester the colour of flower and maintain this colour for several months and they resist even acid attacks. From characterization of pigments the interaction anthocyanine-adsorbent is proposed.

Removal of CO2 from CH4 and CO2 capture in the presence of H2O vapour in NOTT-401

From a binary equimolar gas-mixture of CO2 and CH4, NOTT-401 exhibits CO2 separation from CH4. By kinetic uptake experiments, NOTT-401 shows a maximum of 1.47 wt% CO2 capture at 30 °C and a significant 7-fold increase (∼9.90 wt%) in CO2 capture under 40% relative humidity.

Fluorinated Hydrotalcites: The Addition of Highly Electronegative Species in Layered Double Hydroxides To Tune Basicity

Hydrotalcite-like compounds were synthesized by coprecipitation using a constant-pH method. Aluminum was introduced in the form of an octahedral aluminum fluorine complex during the synthesis. Controlling the pH during the synthesis ensured that aluminum fluorine did not decompose to tetrahedral species but remained in the octahedral (AlF(6))(3-) form to be incorporated into brucite-like sheets. The physicochemical, thermal, and spectroscopic characterizations showed significant modifications by fluorine introduction regarding structural, textural, and adsorption properties. The memory effect of fluorinated hydrotalcites differed from the memory effect commonly observed in fluorine-free hydrotalcites. Nitromethane adsorption showed that the basicity of hydrotalcite was greatly modified by the fluorine loading.

CO2 capture in the presence of water vapour in MIL-53(Al)

MIL-53(Al) shows a CO2 capture of 3.5 wt% by kinetic uptake experiments, under anhydrous conditions at 30 °C. When this material is exposed to water vapour (20% RH and 30 °C), there is a considerable 1.5-fold increase in the CO2 capture up to 5.2 wt%.

CO2 Capture at Low Temperatures (30–80 °C) and in the Presence of Water Vapor over a Thermally Activated Mg–Al Layered Double Hydroxide

The carbonation process of a calcined Mg-Al layered double hydroxide (LDH) was systematically analyzed at low temperatures, varying the relative humidity. Qualitative and quantitative experiments were performed. In a first set of experiments, the relative humidity was varied while maintaining a constant temperature. Characterization of the rehydrated products by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and solid-state NMR revealed that the samples did not recover the LDH structure; instead hydrated MgCO(3) was produced. The results were compared with similar experiments performed on magnesium oxide for comparison purposes. Then, in the second set of experiments, a kinetic analysis was performed. The results showed that the highest CO(2) capture was obtained at 50 °C and 70% of relative humidity, with a CO(2) absorption capacity of 2.13 mmol/g.