Toward Sustainable Adsorbents for CO₂ Capture: Reuse of Silica from Spent Silica-Polyethylenimine

Silica-polyethylenimine (silica-PEI) and, more recently, silica-alkoxylated polyethylenimine (APEI) adsorbents are promising materials for CO₂ capture due to their high adsorption capacity and selectivity. The ability to reuse silica supports recovered from spent adsorbents can enhance overall material efficiency and facilitate the development of regeneration strategies given our earlier work demonstrating that spent PEI generates pyrazines and other chemicals during pyrolysis. In this study, spent silica-APEI adsorbents were subjected to four different treatments to recover silica supports and to investigate how thermal regeneration influences pore structure and subsequent reimpregnation performance. Single-stage and two-stage pyrolysis at 500-600 °C resulted in moderate mesopore contraction (13%) and total pore volume reduction (10%) compared to the initial silica, largely independent of the specific pyrolysis route applied. The recovered silicas were successfully reimpregnated with fresh PEI, exhibiting an optimal loading of 45 wt %, slightly lower than that of the original silica (47 wt %). At optimal PEI loadings, the regenerated silica-PEI displayed an approximately 10% reduction in CO₂ adsorption capacity relative to fresh silica-based adsorbents. These results demonstrate that silica supports largely retain functional compatibility with PEI after high-temperature pyrolysis, laying the foundation for further studies with multiple cycles to assess the overall cost and life cycle benefits.