Reactive Atmosphere Synthesis of Polymer‐Derived Si–O–C–N Aerogels and Their Cr Adsorption from Aqueous Solutions

Polymer derived ceramic (PDCs) aerogels belonging to the Si–O–C–N system are synthesized by crosslinking a preceramic polymer in a diluted solution followed by supercritical or atmospheric drying and pyrolysis in inert (N2) or reactive (NH3/CO2) atmosphere. Accordingly, aerogels with hierarchical porosity ranging from some microns to few nanometers together with high specific surface area in the range 30–400 m2 g−1 have been obtained. Moreover, their surface contains a broad range of moieties (Si–OH, Si–NH, C=O, etc.) that can interact and bind metal ions thanks to electrostatic interactions. This combination of hierarchical porosity, high SSA, and broad range of chemical functionalities makes these PDCs aerogels interesting candidates for water purification. In this work, SiOC and SiCN aerogels have been tested as adsorbents for Cr(III)/(VI) ions from aqueous solutions with promising results for the SiOC aerogel pyrolyzed in N2 and the SiCN treated in NH3. Correlations and similarities among the Cr(VI)/(III) adsorption capacity with the main features of the porous substrates (SSA, N2 TPV, amount of free C, bulk density, isoelectric point, main IR peaks (Si–OH, OH, NH, C=O, C=C, Si–O, C–N, Si–N) have been investigated by applying the Principal Component Analysis (PCA). Polymer derived SiOC and SiCN aerogels are produced, starting from preceramic aerogels, through a pyrolysis process with reactive atmospheres (Ar, CO2, NH3) and are characterized for the adsorption of Cr(VI/III) from aqueous solutions. Results show that SiOC and SiCN aerogels pyrolyzed in inert N2 and reactive NH3 flow, respectively, display a good adsorption capacity for Cr(VI) (30 and 20 mg g−g, respectively, after 1 h contact time).