Carboxylic acid-modified polysilsesquioxane aerogels for the selective and reversible complexation of heavy metals and organic molecules

Organofunctional porous methyltrimethoxysilane (MTMS)-based aerogels are attractive for various adsorption purposes due to the combination of their unique properties such as low densities and high specific surface areas with tunable and accessible functional groups that can coordinate to, e.g., heavy metals and/or organic dye molecules in polar and non-polar solutions. Furthermore, the MTMS backbone gives these aerogels mechanical strength, the ability to be dried under ambient conditions and ensures their non-degradability in aqueous media and recyclability. Herein, we report the preparation of carboxylic acid-modified polysilsesquioxane aerogels via a simple and straightforward acid-base catalyzed sol-gel approach by using MTMS and the novel and stable 5-(trimethoxysilyl)pentanoic acid. In this surfactant assisted co-condensation approach, all parameters (concentration, pH, and temperature) have been carefully designed to yield porous (porosities between 82% and 53% and specific surface areas between 345 m2.g−1 and 36 m2.g−1), light (bulk densities between 1.38 g.cm−3 and 1.16 g.cm−3), and hydrophobic aerogels with accessible and reactive functional carboxylic acid groups (-COOH) (accessible surface loading up to 0.19 mmol.g−1) for the adsorption of heavy metals ions (Zn2+ and Cu2+) and cationic dyes (methylene blue and rhodamine B). The maximum adsorption capacities obtained from Langmuir isotherms were 154 mg.g−1, 106 mg.g−1, 111 mg.g−1, and 78 mg.g−1 for RhB, MB, Zn2+, and Cu2+, respectively. An increasing content of carboxylic acid groups influences the morphology, specific surface area and adsorption behavior of the synthesized aerogels. Optimized functionalized aerogels can be dried ambiently and show high and reversible adsorption abilities of 87% over several cycles towards cationic dyes in aqueous media. Moreover, these carboxylic acid-modified aerogels demonstrate excellent adsorption selectivity by adsorbing only positively charged molecules from mixed dye solutions, making them ideal candidates for diverse adsorption processes in polar and non-polar solutions.