Synthesis and Implementation of Aerogel for Thermal Insulation in Buildings and Atmospheric Restoration

Carbon dioxide (CO2) and methane (CH4), the two most abundant greenhouse gases, continue to rise every year in our atmosphere driving global climate changes despite the international efforts to minimize emissions of mainly CO2. Immediate action is required in developing solutions that significantly improve energy efficiency in buildings by installing thermal superinsulation, which minimizes the energy requirement for heating or cooling and with it the release of CO2. The conversion of the far more potent methane to a less harmful alternative would further enable atmospheric restoration. The class of materials that can bring a solution to these challenging but necessary goals is aerogel. Aerogels are not yet widely used mainly due to their high cost price, need for high amounts of non-aqueous solvent when ambient pressure dried or high pressures and temperatures when supercritically dried. Furthermore, their reliability remains questionable as commercially available aerogels can show relatively large deviations from their nominal properties. This dissertation therefore focused on novel synthesis, functionalization, and implementation protocols for aerogels having outstanding properties with low standard deviations while being compatible with mass production. This work is divided into three main parts: the synthesis of hydrophobic silica aerogel particles, the dispersion of hydrophobic silica aerogel in gypsum plaster slurries, and the synthesis and use of copper-doped polyimide aerogels to convert methane to CO2. A tremendous amount of research has already been performed on the synthesis of mainly silica aerogel and continues to go on in which the focus most often lies in making the production process less cost-intensive and safer. In this dissertation, three different silica aerogels synthesis protocols were developed, in which all silica gels were prepared from waterglass, an aqueous sodium silicate solution, as it is a cheap and relative harmless precursor. The aim was to obtain the aerogel in particle form, preferably powder. For each protocol, a different sol-gel step was developed for diluted waterglass solutions having a silica content ranging from 6 to 8 weight percentage, as well as a solution enabling a combined hydrophobization and solvent exchange of the gel. Each sol-gel was critical not only because it would determine the final properties of the aerogel, but special care had to be taken for the sodium ions, which had to be removed before gelation or washed out afterwards. All aerogels were characterized for density, porosity, pore volume, BET surface area, pore size distribution, and thermal conductivity. The pore structures were imaged with scanning electron microscopy. In the first protocol, the main objective was driven by repeatability such that aerogels would be obtained with properties, especially thermal conductivity, comparable to today's standards, but having insignificant deviations from the nominal values. A gel was obtained after partial neutralization of a diluted waterglass solution to a pH no less than 10.5, while the solvent exchange, washing out of sodium salts, and hydrophobization were achieved simultaneously with a hexane/trimethylchloro-silane/isopropyl alcohol solution with a trimethylchlorosilane/pore water ratio of only 0.11 and a hexane volume about 35 times smaller than typical processes in literature. During and after drying at ambient pressure, practically no shrinkage was observed for the final aerogel powder consisting of 8 wt% silica. Later samples were hydrophobized with hexamethyldisiloxane instead of trimethylchlorosilane to avoid the release of highly corrosive hydrogen chloride. Thermal conductivities in the range of 22 to 25 mW m-1 K-1 were measured with standard deviations no larger than 0.3, compared to a commercial available sample for which a thermal conductivity of 22 ± 1.4 mW m-1 K-1 was measured with the same equipment. A second protocol was then developed in which the main focus was to obtain aerogel powder with similar properties as in previous protocol but in a shorter period of time by eliminating the separate gelation and aging step in a co-precursor method. A slightly modified solution of hexane/trimethylchlorosilane/isopropyl alcohol was again used but the hydrogen chloride from trimethylchlorosilane was now used to induce gelation as well as to hydrophobize the growing solid network forming a gel. As larger amounts of hydrogen chloride were present in this synthesis protocol compared to the previous, the pore structure of the gel was different. A thermal conductivity less than 24 ± 0.9 mW m-1 K-1 was measured. Finally, a last synthesis protocol was designed with the aim to produce hydrophobic silica aerogel particles having outstanding properties. Therefore an ion exchange step was included to turn diluted waterglass solutions into silicic acid before gelation in order to obtain homogeneous mesoporous gels. Again a combined hydrophobization and solvent exchange was a major step within the synthesis process, this time acquired by two variants of a hexamethyldisiloxane/ethanol solution to which either trimethylchlorosilane or nitric acid was added to obtain aerogel in powder or monolithic form, respectively. In case of trimethylchlorosilane, this silylating agent had three more functions next to hydrophobizing the gels due to its release of hydrogen chloride: catalyzing hexamethyldisiloxane, pulverizing the gels, and finally recovering the ion exchange resin. Aerogels were then obtained having a density of around 0.09 g cm-3, porosity higher than 90%, BET surface area of 900 m2 g-1, and a thermal conductivity of 16 mW m-1 K-1. The addition of hydrophobic aerogel to traditional building materials offers many advantages, namely the possibility to construct thinner insulation panels with improved fire safety, protection against moist, and reduction in mass. A second major challenge of this dissertation was therefore to disperse hydrophobic silica aerogel particles in water-based slurries, more specifically gypsum plaster. Aerogel P300 particles from Cabot Corporation were closely packed and cemented together with plaster in a layer within the resulting plasterboard. These aerogel-rich layers were effective in eliminating thermal bridges especially when the layer thickness was less than 25% of the total composite board thickness. As such, for plaster composite boards which contained overall only 15 vol% of aerogel, a reduction of 63% in thermal conductivity could be achieved compared to a pure plasterboard. In case of an aerogel content higher than 60 vol% within the aerogel-plaster layer itself, (3-amino-propyl)triethoxysilane was required in order to obtain stable dispersions. The thickness of the aerogel-containing plaster layer and the aerogel content were varied, while aerogel had been used in granulate as well as in powder form after crushing the former. The influence of the addition of hydrophobic aerogel particles and additives on the setting of the gypsum plaster slurries was monitored and investigated. X-ray computed tomography was used to characterize the aerogel dispersion in the boards. The last focus of this dissertation was on the removal of the greenhouse gas methane, which due to its much higher global warming potential compared to CO2 and its relative short atmospheric lifetime would result in a larger and more immediate effect on climate mitigation. Polyimide aerogels were synthesized based on a recent process developed by NASA, which involved the mixing of two diamines, i.e. 4,4'-oxydianiline and 2,2'-dimethylbenzidine, combined with 3,3',4,4'-biphenyltetracarboxylic dianhydride. The resulting oligomers were then cross-linked with triisocyanate Desmodur N3300A. However, before chemical imidization of the resulting polyamic acid, soluble copper salts were incorporated, such that during the sol-gel copper compounds would be homogeneously dispersed and deposited or impregnated into the resulting polyimide gel, similar to a protocol designed by Aspen Aerogels. The gels were then solvent exchanged and dried by supercritical CO2. Because of the high specific surface area and many functional groups of polyimide aerogels, these were considered in this work as potentially more effective catalysts compared to the typical copper-exchanged zeolites. Conversion of methane to CO2 rather than traditionally to methanol was opted as it offers several advantages pointed out by recent literature. Major advantages are found in the operation of the catalyst as the conversion of methane can be made continuous more easily given that CO2 is a gas and can therefore simply leave the catalyst. Methanol, on the other hand, has to be extracted from the actives sites, which in addition often results in damage of the latter while requiring energy as well as a collection system for the liquid methanol. The reduction in radiative forcing due to the removal of methane was proven to be still tremendously beneficial for the environment and climate, even with the additional release of CO2 from this catalytic process. Next to making the catalytic process continuous, it was also made less energy intensive by using a temperature of only 100°C throughout the whole process compared to temperatures typically higher than 400°C, while not applying extra pressure on the gas streams. The polyimide aerogels then had conversion yields in the range of those in recent literature. As this dissertation comprised the synthesis of both inorganic silica and organic polyimide aerogels for which an ambient pressure drying and supercritical drying technique, two significantly different approaches, had been used, respectively, in-depth understanding and practical experience in synthesizing aerogels had been established, while novel ideas for optimization were successfully implemented as well.status: publishe