Desarrollo de nuevos aerogeles de aplicación industrial mediante la técnica de liofilización

Energy conservation is an important issue to get a human sustainable development. An appreciable part of total energy consumption of the world belongs to the building industry. Thus, one of the most important goals in the construction of future buildings, is the reduction of the energy consumption through their life cycle, from construction to demolition. In consequence, it is crucial to promote the energy efficiency of buildings. One way to get this objective is by means of thermal insulation to reduce heat loss and decrease the energy demands of heating and cooling system. Furthermore, insulation materials results in an increase in both energy savings and buildings lifetime, conserving natural resources and decreasing of pollutants emissions. As a consequence of the thermal insulation materials advantages, many industries are interesting in the production of insulation materials with enhanced characteristics, such as improvements in sound insulation properties. Noise is recognised as a serious health hazard and its exposure may cause numerous physiological and psychological effects. Thus, the development of thermal insulation materials with sound absorption properties could be the solution of both, excessive energy consumption and health risks derived from the noise exposure.
\nOn the other hand, the continuous energy demand all over the world has resulted in the extraction, refining, production, transportation, storage and use of large amounts of oil. Millions of oil tons are spilled each year accidentally into the environment causing important health, environmental, safety and economical problems. There are different methods to remediate the oil spills but one of the most attractive techniques as a consequence of its low cost and efficiency is the use of sorbent materials. Thus, the development of sorbent materials could be the solution of both water and soil pollution.
\nOther global problem that is a health risk to humans and impact aquatic life, is the presence of pharmaceutical compounds in the aquatic environment. Furthermore, wastewater treatment plants cannot completely remove them and the necessity for their removal has intensified. There are different techniques for the removal of pharmaceutical compounds from the aquatic environment, but the use of biocatalysts, such as enzymes, is an environmentally friendly technique and possess lower energy consumption and moderate operational conditions in comparison with the other treatments techniques. The only problem when using enzymes is that an efficient method for its immobilization is required to facilitate the recovery and reusability. Among the different enzyme immobilization methods, the covalent attachment between the enzyme and thesupport is the most commonly used method as it prevents enzyme leaching and improves enzyme stabilization. Thus, the development of efficient support materials to immobilize the enzyme could be the solution to carry out the pharmaceutical compounds degradation.
\nTherefore, the purpose of this work is trying to find new solutions and improvements in several fields: the energy efficiency in buildings and the removal of pollutants from soil and water through the development of different types of aerogels.
\nChapter 1, 2 and 3 correspond to the general introduction, the objectives and motivation of the work and the methodology, respectively. On the other hand, in Chapter 4, 5, 6, 7 and 8 the results obtained were discussed. They are summarized below:
\nIn Chapter 4 carbon nanofibres-reinforced polymer aerogels were successfully synthesized by means of a freeze-drying process. The influence of the operation conditions of freeze-drying process (freezing time, vacuum pressure and freeze-drying temperature during secondary drying step) on aerogels physical properties was evaluated. The porosity of the aerogels increased when the freezing time, vacuum pressure and freeze-drying temperature during secondary drying step increased. In addition, the thermal conductivity of carbon nanofibres-reinforced polymer aerogels varied as a function of the porosity, in the range 0.037-0.060 W/m•K, corroborating their possible use as thermal insulation material.
\nIn Chapter 5 the development of three different types of aerogels based on poly (vinyl alcohol ) (PVA), using freeze-drying method, to be used as building insulation materials, was carried out. Firstly, carbon nanofibres-reinforced polymer aerogels were analysed. The effect of synthesis conditions (amount of carbon nanofibres (CNF) and use of different solvents) on the properties of the aerogels was studied. Thermal conductivity and surface area decreased when the CNF amount decreased. On the other hand, the presence of CNF improved the mechanical properties of the aerogels. Otherwise, the use of tert-butanol as the solvent reaction, allowed to decrease the sublimation time and, consequently, reduced costs.
\nThe second part was based on the development of nanoclay-based PVA aerogels by means of freeze-drying process, at pilot plant scale, to be used as thermal insulation materials with fire-proof properties. Furthermore, the influence of nanoclay/PVA mass ratio on the morphological, thermal and mechanical properties of the aerogels was studied as well as their fire retardant properties. The nanoclay/PVA mass ratio increases, allowed to produce compact structures with smaller pore size. On the other hand, the nanoclay incorporation improved the mechanical properties and the thermal stability of the aerogels. The synthesised materials had good heat insulation performance (0.029-0.052 W/m• K) and exhibited appropriate fire-proof properties.
\nFinally, the last part of this chapter was focused on the development of PVA/nanoclay/ graphene oxide aerogels by means of freeze-drying process, at pilot plant scale, to be used as thermal insulation materials with enhanced sound absorption properties. The influence of the thermally reduced graphene oxide (trGO) addition on the physical, thermal and acoustic properties of the aerogels was evaluated. The trGO incorporation lead to a decrease of the average pore diameter and improved the thermal stability of the final product. Moreover, the presence of trGO into the aerogel enhanced the sound absorption coefficient of the aerogels. PVA/nanoclay/graphene oxide aerogels presented exceptional properties as thermal insulator (values of the thermal conductivity from 0.0255 to 0.0289 W/m•K) with enhanced sound absorption properties.
\nIn Chapter 6, the development of hydroxyethyl cellulose/alumina-based aerogels by means of an environmentally friendly freeze-drying process was carried out. In the first part of the chapter, the influence of the hydroxyethyl cellulose (HEC) to aluminum tri-sec butoxide (ASB) solution mass ratio on the morphological, mechanical and thermal properties of the synthesized aerogels were evaluated. The HEC incorporation resulted in compact structures with smaller pore size. Furthermore, the incorporation of HEC into the ASB matrix enhanced the mechanical properties of the aerogels. In addition, all synthesized aerogels showed low thermal conductivities. Once aerogels were characterized, they were evaluated as sorbent materials for oil spill in the second part of the chapter. The influence of the HEC/ASB solution mass ratio on the oil sorption capability of the aerogels was evaluated and the aerogel synthesised using 10 wt. % (HEC/ASB solution mass ratio) was selected as the optimal, due to its remarkable characteristics and its good sorbent properties. This aerogel allowed to obtain an oil retention percentage of 5.5 times higher than its dry weight. Finally, reusability test results showed that the washing with acetone improved the sorption capability to be reusable as sorbent materials.
\nIn Chapter 7, linear and crosslinked polyimide aerogels (PI aerogels) were synthesized by means of a simple and eco-friendly process. Water-soluble poly (amic acid) (PAA) solution was produced and thermal imidization was employed. In the first part of the chapter, the influence of the solid content of the PAA solution and the crosslinking addition on the properties of the PI aerogels, were evaluated. An increase in solid content in linear polyimide aerogels resulted in denser lamellar structures with smaller pore size. Furthermore, polyimide aerogels presents very high thermal stability up to 550 ºC. On the other hand, the crosslinking addition allowed to improve the mechanical properties of the PI aerogels. In addition, the synthesized linear polyimide aerogels were evaluated as sorbent material for oil spill, resulting in a retained oil amount of 14 times higher than its dry weight. Moreover, linear polyimide aerogels were analysed as possible acoustic and thermal insulation materials. Results showed very good values of sound absorption coefficient (maximum value of 0.91 at high frequencies) and low thermal conductivities (0.038-0.045 W/m•K). 
\nThe second part of the chapter was carried out in the Institut National de la Recherche Scientifique (INRS) in Quebec, (Canada). It was based on the evaluation of linear PI aerogels as support materials for enzyme immobilization to carry out the carbamazepine (CBZ) removal from the wastewater. Thus, laccase was immobilized on modified polyimide aerogels by means of covalent bonding. The immobilized laccase on polyimide aerogels was carried out successfully and improved the enzyme activity (for all the pH and temperature range studied) in comparison with the free enzyme. The storage stability was improved by the immobilization on this support material. Immobilized laccase on polyimide aerogels for CBZ degradation exhibited 76% and 74% removal in spiked water and secondary effluent, respectively. Furthermore, after 7 use cycles the CBZ removal efficiency remained higher (50% and 65% for spiked water and secondary effluent, respectively).
\nIn Chapte

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