Investigating parameters affecting the production of alumina gel by sol-gel method

Master thesis

in the field of chemical engineering

Abstract

Investigation of parameters affecting the production of alumina gel by the sol-gel method

The sol-gel method is one of the common methods that is widely used to produce various nanoparticles. can be In this method, a solution of initiators is mixed with water in the presence of a catalyst, and hydrolysis and polymerization reactions take place. In the continuation of the polymerization process, cell particles form a three-dimensional network called gel. Among the parameters affecting the formation time of alumina gel are molar ratio of water to alkoxide, hydrolysis rate, type and volume of catalyst used, initial concentration of precursor and hydrolysis and drying temperature. After preparing the gel, the liquid phase (solvent) is separated by two methods of evaporative and supercritical drying. The resulting material is called xerogel and aerogel, respectively. Due to the surface tension of the liquid, a pressure difference occurs in the walls of the gel cavities, which can destroy a large part of the volume of the gel cavities. Supercritical drying prevents this from happening. The strength, high stability, porous structure and morphology of aerogels have made them widely used in recent years. Among the applications of aerogels in the construction of thermal insulation, are types of absorbers and catalyst bases. Among hundreds of catalyst bases, alumina bases are of great interest due to their heat resistance, chemical stability, and high porosity. In the current research, the synthesis of alumina gel as a nanocatalyst base was investigated with the help of hydrolysis of aluminum trisec butoxide at two temperatures of 60 and 32 degrees Celsius, with different concentrations of precursor and water and two different types of acid as catalysts, in different amounts. Gel formation speed, hydrolysis and polymerization speed and amount of synthesized gel were measured and analyzed. The results showed that adding 0.2 milliliters of nitric acid per 1 milliliter of aluminum trisec butoxide in a solution of 16% by weight of aluminum trisec butoxide and the molar ratio of water to precursor is equal to 2, at a temperature of 60 degrees Celsius, the amount of gel and the formation time of alumina gel is optimal, and the maximum amount of gel is obtained in the shortest possible time.  Then the synthesized gel was dried under optimal conditions at ambient temperature and pressure and supercritical carbon dioxide conditions, and the results of scanning electron microscope analysis were compared.

Key words: alumina gel, effective parameters, gelation point, supercritical, supercritical, airgel

1-1- sol-gel

1-1-1- The history of the sol-gel process

For the first time in 1845, Abelman [3] accidentally observed that silicon tetrachloride (SiCl4), which was left in a container, first hydrolyzed and then turned into a gel. For about a century, the sol-gel method was only of interest to chemists, but in 1930, Schott Company used the sol-gel method to cover industrial glass with oxide layers using metal compounds as precursors. In 1950, extensive studies on the synthesis of ceramics and glass structures using this method began. Roy and his colleagues [4] proposed a method for the synthesis of ceramic oxide compounds using the sol-gel process, which the final product was more homogeneous than other methods.

Inorganic gels from multi-aqueous salts have been studied for a long time. Graham showed that the water inside the silica gel can be replaced by organic solvents, which confirmed the theory of the porous structure of the solid gel network. While other theories considered the gel structure as a coagulated sol whose particles are surrounded by a layer of water or an emulsin. In the 1930s, the network and porous structure of gel was widely accepted, which was the result of Hard's efforts.. Synthesis of airgel by drying the gel using supercritical fluid by Kiesler[5] in 1932 was also a confirmation of the network structure of the gel. At the same time, the production of homogeneous powder from gel and sol for study in the field of phase balance received a lot of attention. It is worth mentioning that with this method, many inorganic oxides such as TiO2, SiO2, ZrO2 were synthesized (Brinker and Schere, 1990). To produce a product by sol-gel method, it is necessary to provide the prerequisites and necessary conditions for the reactions.

         Preparation of a homogeneous solution[6]: It is necessary to prepare a homogeneous solution including the solvent and precursors that are supposed to form the final product during the process. For this purpose, first the solvent (water, alcohol, organic solvents or a ratio of them) and the precursor are mixed in a container to obtain a homogeneous solution. Sometimes it is necessary to use the combination of two solvents with certain ratios so that the precursors are completely dissolved in that solution and a homogeneous solution is obtained. For example, some organic metal precursors must first be dissolved in an organic solvent soluble in water and then the resulting solution dissolved in water. But in cases where the desired precursor is a metal salt, it can be directly dissolved in water and there is no need for an organic solvent. Alkoxide precursors are mostly used, but it can proceed in another way other than the alkoxide route, the sol-gel method, which is called the colloidal route (Niederberger and Pinna, 2009). The hydrolysis reaction [7] is the basis of this step. Basically, the word hydrolysis is formed from the combination of two words hydro[8] and lysis[9] and its meaning is decomposition by water. The term decomposition refers to any case where a complex state is transformed into a simpler state. In chemistry, sometimes water can break a molecule and turn it into simpler molecules. In short, a group of reactions in which molecular water is broken (due to a chemical reaction) and converted into simpler molecules is called hydrolysis (Niederberger and Pinna, 2009).

Hydrolysis reaction is not very complicated. As can be seen in Figure 1-2, to start this process, a little water is added to the reaction medium, which is in the case that a homogeneous solution is prepared in a solvent without water. The presence of water will cause the hydrolysis reaction to take place on the precursor and somehow activate it so that the metal oxide particles come together and form fine and solid particles that are dispersed in the solvent. Such a combination of sol is referred to as an intermediate product of the process.

Sol is taken from the English word solution[10] which means solution, and therefore it is a colloidal solution of various reactants, including precursors, solvent, acid or base catalysts and other required additives, which are supposed to turn into gel in the continuation of the reaction during the stages of hydrolysis and condensation. Tubes are slightly different from real solutions. In the real solution, the solute is uniformly dispersed in the form of atoms, molecules or ions in the solution and the size of the particles does not exceed one nanometer. But if the size of the particles is larger than one hundred nanometers, they gradually settle (suspension mixture). If the size of the particles varies between 1 and about 100 nm, they usually remain dispersed throughout the mixture, and such mixtures are called colloids. Cells contain very small particles (100 nm) dispersed in the solvent phase and actually form a solution or, more correctly, a colloidal mixture. Therefore, tuberculosis is a mixture of solids dispersed in a liquid, which, due to the smallness of the solid particles, is able to remain stable for a very long time (months) and does not settle. The most important thing to say about a good and acceptable tuber is that the resulting tuber should be prepared in such a way that it can be stable for months and not precipitate. In other words, the size of the particles should be so small that the Brownian motion [11] of the particles overcome the gravitational force of the earth, do not settle and remain homogeneous for a long time. If the resulting tuber has such characteristics, we can hope to produce a homogeneous, pure and high yield product (Hench and West, 2009).