Stela Grozdanova

This study presents a novel approach for synthesizing porous silica materials using various biomass-derived carboxylic acids as non-surfactant, eco-friendly porogens. Different carboxylic acids were selected, and their influence on the properties of the final materials was systematically investigated. The silica synthesis was performed using only the intrinsic acidity of carboxylic acids without pre-hydrolysis of the silica precursor - tetraethyl orthosilicate (TEOS). Citric and tartaric acids had a favorable effect on the formation of mesoporous silica, whereas the oxalic, ascorbic, maleic, and mandelic acids led to the formation of microporous silica. The optimal synthesis compositions and parameters were thoroughly investigated. A mesoporous silica with a uniform pore size was prepared using tartaric acid, and the pore size was controlled by the drying temperature. Template removal via water extraction yielded silica materials with superior textural properties compared to conventional high-temperature calcination, while allowing the recovery and reuse of organic acids. Our results confirm that using carboxylic acids can be efficiently and economically applied for the controlled synthesis of mesoporous silica suitable for different applications, such as adsorption, drug delivery, and catalysis.

p-toluenesulfonic acid (pTSA) was used for the synthesis of porous silica xerogels while applying different synthesis conditions. Key parameters included acid concentration, drying temperature and the method of acid removal. The resulting organic–inorganic composites were investigated by nitrogen physisorption, X-ray powder diffraction (XRD), solid-state NMR and thermal analysis. The results demonstrated that both the drying temperature and quantity of the pTSA significantly influenced the pore structure of the xerogels. The utilization of such strong acids like pTSA yielded high surface area and pore volume, as well as narrow pore size distribution. Environmentally friendly template removal by solvent extraction produced materials with superior textural properties compared to traditional calcination, enabling the recovery and reuse of pTSA with over 95% efficiency. A selected mesoporous silica xerogel was modified by a simple two-step post-synthesis procedure with 1-(2-Hydroxyethyl) piperazine (HEP). High CO₂ adsorption capacity was determined for the HEP-modified material in dynamic conditions. The isosteric heat of adsorption revealed the stronger interaction between functional groups and CO₂ molecules. Total CO₂ desorption could be achieved at 60 °C. Leaching of the silica functional groups could not be detected even after four consecutive adsorption cycles. These findings provide valuable insights into the sustainable synthesis of tunable piperazine-modified mesoporous silica xerogels with potential applications in CO₂ capture.