Estudio de la influencia de la composición química y el proceso de síntesis en las propiedades fisicoquímicas de hidróxidos dobles laminares y sus derivados
Authors:   , Prince Flores Julia Elena 
Published: March 2021

This thesis presents a series of investigations related to the family of materials known as layered double hydroxides (LDHs), carried out with the intention of understanding and fine-tuning their physicochemical properties. As explained in Chapter I, LDHs are a broad family of materials with a common structure and a great diversity of possible chemical compositions, summarized by the general formula: [M2+ (1-x) M3+ x (OH)2] Anx/ oxide solid solutions, which usually possess large specific surface areas and synergic interactions between components. It is possible to introduce in this structure diverse metal cations such as Mg2+, Zn2+, Ga3+, Al3+, among others. Therefore, LDHs and their calcination products may show, for example, acid-base and/or semiconducting properties. Syntheses of materials were carried out by three different methods, each of them with their particular advantages. The coprecipitation of metal salts is quite simple for the laboratory scale. The second method is the oxide dispersion, which was designed for industrial production of LDHs, as it is both economic and environment-friendly. Thirdly, there is the sol-gel method, which affords the possibility of preparing LDH thin films. Chapter III presents the studies carried out on LDHs, and mixed oxides derived thereof, with semiconducting properties. Results show that it is possible to obtain solids with band gap energies close to that of titania, one of the most widely studied semiconductors. Furthermore, it is possible to fine-tune the band gap energy of mixed oxides by the variation of the chemical composition of the parent LDH. For instance, cations such as Zn2+ or Ga3+ may be introduced in the layers, or oxides such as CeO2 may be deposited over the solid. Mg-Zn-Al mixed oxides are capable both of adsorbing and photodegrading acidic contaminants; the example of 2,4-diclorophenoxiacetic acid is presented. Also, it is possible to degrade aqueous contaminants such as phenol and chlorophenols using as photocatalysts mixed oxides derived from multimetallic LDHs. Degradation efficiency is by far superior to that of titania Degussa P25, taken as reference. These solids have several advantages to other semiconductors presented in literature reports. For example, the solid solutions obtained by LDH calcination may be considered as a doped metal oxide, where the dopants occupy fixed positions in the structure and therefore their preparation is easily reproduced. Contrarily, other doping methods require the strictest control over experimental conditions and their reproducibility may be complicated. Furthermore, this system works under natural conditions; it does not require acidification or addition of oxidants or sacrificial agents. The use of LDHs and their mixed oxides as semiconductors for photocatalytic applications is still a largely unexplored area with great potential. Another application of LDHs, particularly those of Mg and Al, is as basic catalysts. Calcined solids possess Lewis basicity, while rehydration of these mixed oxides generates Brönsted basic sites. Chapter IV presents an innovation to the rehydration method, carried out in this case at T = 80 oC, instead of room temperature as usual. This reduces the activation time from an average of 12 h to only 0.5 h. The relationship between basic strength and the Mg/Al molar ratio (represented by x in the general formula) was studied by adsorption of deuterated chloroform followed by infrared spectroscopy. Results show that basic strength is decreased as the Mg/Al molar ratio increases. Moreover, it was found that initial rehydration creates the sites with highest basic strength. Further rehydration produces sites with lower basic strength, up to the point where excess water becomes a poison to active sites. The conclusions derived from CDCl3 adsorption correspond well to the catalytic activity observed for the cyanoethylation of methanol and 2-propanol, according to the proposed reaction mechanism. Thus, basic strength may be fine-tuned for the specific needs of a catalytic reaction. Finally, different methodologies for the intercalation of anions in LDHs synthesized by sol-gel were studied (Chapter V). It is possible to intercalate a variety of anions and even a zwitter-ionic surfactant. By direct synthesis, it is preferable to add the intercalation molecule at the beginning of the synthetic procedure, allowing formation of the LDH structure around the anion. For a large molecule with low affinity to the positively charged layers, such as the zwitter-ionic surfactant, a certain ageing time must be given, to allow the separation of the layers in order to fit the surfactant. For anion-exchange reactions, the LDHs must have orderly stacked layers to permit appropriate diffusion of anions to and from the interlayer region. Additionally, multimetallic LDH thin films were successfully deposited over ITO (indium tin oxide) coated glass slides. Films were found to be uniform and well-adhered.