Biogas Upgrading via Cyclic CO Adsorption: Application of Highly Regenerable PEI@nano-AlO Adsorbents with Anti-Urea Properties.

Solid amine adsorbents are among the most promising CO adsorption technologies for biogas upgrading due to their high selectivity toward CO, low energy consumption, and easy regeneration. However, in most cases, these adsorbents undergo severe chemical inactivation due to urea formation when regenerated under a realistic CO atmosphere. Herein, we demonstrated a facile and efficient synthesis route, involving the synthesis of nano-AlO support derived from coal fly ash with a CO flow as the precipitant and the preparation of polyethylenimine (PEI)-impregnated AlO-supported adsorbent. The optimal 55%PEI@2%AlO adsorbent showed a high CO uptake of 139 mg·g owing to the superior pore structure of synthesized nano-AlO support and exhibited stable cyclic stability with a mere 0.29% decay per cycle even under the realistic regenerated CO atmosphere. The stabilizing mechanism of PEI@nano-AlO adsorbent was systematically demonstrated, namely, the cross-linking reaction between the amidogen of a PEI molecule and nano-AlO support, owing to the abundant Lewis acid sites of nano-AlO. This cross-linking process promoted the conversion of primary amines into secondary amines in the PEI molecule and thus significantly enhanced the cyclic stability of PEI@nano-AlO adsorbents by markedly inhibiting the formation of urea compounds. Therefore, this facile and efficient strategy for PEI@nano-AlO adsorbents with anti-urea properties, which can avoid active amine content dilution from PEI chemical modification, is promising for practical biogas upgrading and various CO separation processes.