Highly efficient and stable PEI@Al2O3 adsorbents derived from coal fly ash for biogas upgrading

• Polyethylenimine is impregnated on porous nano-Al 2 O 3 derived from coal fly ash. • The PEI@Al 2 O 3 adsorbent is efficient for biogas upgrading via adsorbing CO 2 . • The PEI@Al 2 O 3 adsorbent is extremely stable under realistic regeneration condition. • Nano-Al 2 O 3 support enhances cyclic stability via resisting urea linkages formation. • PEI impregnation process is not restricted by the pore size of nano-Al 2 O 3 support. Cyclic CO 2 adsorption via amine-functional adsorbents is widely accepted as an efficient route for biogas upgrading and CO 2 mitigation. However, the fast deactivation and insufficient CO 2 uptake are the major drawbacks of this technology. Herein, we developed a green and facile approach, including the recycling of porous nano-Al 2 O 3 support from coal fly ash (CFA) and the impregnation of PEI onto the nano-Al 2 O 3 support with large pore volume, to enhance both the CO 2 adsorption capacity and the cyclic stability of amine-functional adsorbents. The as-synthesized 50%PEI@Al 2 O 3 adsorbent possessed a CO 2 adsorption capacity of 136 mg g adsorbent −1 and showed superior cyclic stability with merely a negligible decay of <0.3% after 10 cycles. Even under the realistic regeneration conditions (at 165 °C in pure CO 2 ), it still maintained extremely stable adsorption capacity with a final CO 2 uptake of 111 mg g adsorbent −1 , markedly increased by 5.5-fold than that of traditional 40%PEI@SiO 2 adsorbent. It was concluded that the nano-Al 2 O 3 support was benefical for the resistance of urea linkages formation during the regeneration stage, which would inactivate the functional amidogen. This strategy significantly enhanced the cyclic CO 2 uptakes of amine-functional adsorbents through the recycling of CFA-derived nano-Al 2 O 3 support, and is thus a green technology for practical biogas upgrading via CO 2 adsorption.