(177c) Research in Novel Solid Sorbents for Direct Air Capture and Carbon Capture.

Between 2023 and 2024, for the first time and due to El Niño phenomenon, global temperatures were in a ten-month record where temperatures surpassed the 1.5 °C threshold. If global warmth drop after the current meteorological effects from El Niño, this increment will be seen as an abnormal outlier from the trending global increment. Yet, if the temperature remain high or rapidly accelerates, this implicates that we are in uncharted territory. Numerous mathematical models stated that this boundary will be exceeded -in the absence of El Niño effects-, at the beginning of the next decade. In order to transition towards a sustainable future, it will be necessary the implement of renewable sources of energy as well as the implement of policies, however, this is not enough. The Intergovernmental Panel for Climate Change (IPCC) stated very clearly that if we overshoot the 1.5 °C limit, Carbon Dioxide Removals (CDRs), such as Direct Air Capture (DAC) would be necessary to reduce the global concentrations of CO₂.

DAC emerged during a conference in 1999 by Professor Klaus Lackner from the Arizona State University, where he stated the necessity of reduce the CO₂ concentration in the atmosphere. Yet, it was not until the recent years, where the first DAC plant was built. DAC and Carbon Capture are different processes; besides, they share many similarities: both activities aim to capture the CO₂ and safely store it. Carbon Capture focus on extract the CO₂ from point sources, as for example, industries that are finding difficulties to decarbonise such as the cement industry. On the other hand, DAC is an emerging industry with the objective of removing the excess of CO₂ in the atmosphere. For the moment, DAC remains more expensive than Carbon Capture, however, DAC industry is in its infancy, and in the last two decades there have been breakthrough developments which have awaken the interest from various stakeholders. Therefore, DAC is a key technology in the near future, but research is necessary to reduce the associated costs. For the moment, there are two main methods of DAC: by using solvents or solid sorbents. These last ones have been an attractive option to explore due to the low operational costs, yet the key element for scale-up this industry is the research in novel sorbents with a great affinity for CO₂.

The most promising solid sorbents used for CO₂ capture are silica, zeolites, alumina, amine sorbents and Metal Organic Frameworks (MOFs). MOFs are crystalline elements with an extraordinary large surface area, ranging from 1000 to 10000 m²/g, and with pore sizes between 3 to 20 Å. They are synthesised by metal organic clusters and organic ligands which results in different types of MOFs with unique properties that could be produced by only changing the organic ligands. Depending on the MOF, this material have a great selectivity to retain certain gases. One of the main disadvantages of MOFs is that are expensive materials, yet they are becoming commercially viable in recent years. On the other hand, amines have a great affinity for CO₂ and do not require large temperatures to regenerate the sorbent. One of the drawbacks of DAC is the presence of moisture where the water molecule occupies the spaces of the sorbent. Yet, amines, can work in the presence of high levels of humidity, which makes this material an interesting option to explore for DAC purposes. Another great advantage of the amines is its low manufacturing cost.

Our study aims to analyse different materials, focusing on the preparation of a mesoporous silica (SBA-15) with two types of amines: Polyethylenimine (PEI) and Tetraethylenepentamine (TEPA). SBA-15 is a highly porous material with low selectivity for CO₂. After the amine modification, this material have a high affinity for CO₂. Yet, the load of amine to the SBA-15 can be counterproductive. This is due to the fact that the amine blocks the active sites of the porous material and reduces the CO₂ capture capacity. Besides, there is a serious degree of toxicity and corrosiveness due to the organic components. Therefore, it is vital to study the optimum load of amine in order to optimise the efficiency of the adsorbent. The CO₂ adsorption capacity of these adsorbents was analysed using a Pressure Swing Adsorption (PSA). Other materials explored out of the box are for example the use of biochar which in contrast with the mesoporous silica, the costs and environmental impacts of manufacturing biochar are further down.

Thus, there is an emergency to rapidly scale-up this new industry, however, capturing CO₂ from the atmosphere is challenging in contrast to capture it from point sources: the concentration in the air is 0.04%. Operating conditions such as temperature, pressure and humidity, can affect the performance of the different materials This paper aims to help in the progress towards sustainable solid sorbents that will help future researchers and scale this emerging industry.