A green synthesis of PEI@nano-SiO2 adsorbent from coal fly ash: selective and efficient CO2 adsorption from biogas

Abstract

As an alternative renewable energy, upgraded biogas by separating CH₄ and CO₂ can be used to replace natural gas in automobiles and power grids. Adsorption using solid amine adsorbents is a promising technology for CO₂ separation, yet the high cost and complex preparation of support materials have seriously hindered its industrial application. In this study, we developed a green and economical method to prepare nano-SiO₂ supports and silica-based solid amine adsorbents; i.e., coal fly ash (CFA) derived nano-SiO₂ supports, produced using CO₂-assisted precipitation technology without adding templates or pore-expanding agents, were impregnated with polyethylenimine (PEI) to synthesize PEI@nano-SiO₂ adsorbents for biogas upgrading via adsorbing CO₂. The as-synthesized “40%-PEI@SiO₂-6%” adsorbent possessed an initial CO₂ uptake of 131 mg g⁻¹ under simulated biogas flow, and showed high adsorption selectivity with a CH₄ uptake of <1.0 mg g⁻¹. “40%-PEI@SiO₂-6%” also exhibited excellent stability under an Ar regeneration atmosphere with only 2.4% decay after 50 cycles; even under a severe regeneration atmosphere of CO₂, it still showed competitively high cycling stability. The larger pore volume and the higher hydroxyl density were beneficial for enhancing the CO₂ diffusion and PEI loading, and thus improved the CO₂ adsorption capacity. The higher hydroxyl density could also promote the conversion of –NH₂ to –NH–, which could resist the deactivation caused by the formation of urea compounds, and thus improved the cycling stability of “40%-PEI@SiO₂-6%”. Therefore, the use of efficient PEI@nano-SiO₂ adsorbents derived from CFA appears to be a promising strategy for biogas upgrading.