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Amine-functionalized adsorbents have become a hot topic in research on CO 2 separation from biogas. However, developing superior adsorbents with high adsorption capacity and stable cyclic stability under a CO 2 regeneration atmosphere remains difficult. In this study, azeotropic distillation, a green, recyclable method, was used to expand the pores of the nano-Al 2 O 3 support. The pore volume of pore-expanded nano-Al 2 O 3 reached 1.62 cm3 g−1, and the derived amine-functionalized nano-Al 2 O 3 adsorbent ('60%PEI@AD-Al 2 O 3 ') possessed a superior CO 2 uptake of 199.4 mg·g−1 adsorbent with rapid adsorption kinetics. '60%PEI@AD-Al 2 O 3 ' showed favourable cyclic stability under CO 2 regeneration atmosphere with a final CO 2 uptake of 133.8 mg·g−1 adsorbent after 50 cycles, which possessed a competitive CO 2 uptake compared with the reported amine-functionalized adsorbents with anti-urea stability. This superior performance is attributed to the large pore volume of the pore-expanded nano-Al 2 O 3 support and the anti-urea stability for '60%PEI@AD-Al 2 O 3 ', which does not require active amine modification and avoids diluting the active amine content. Furthermore, the effect of water vapour in biogas on adsorption performance was studied. Water vapour could significantly promote CO 2 adsorption at low temperatures (<90 °C), and it was accompanied by strong water vapour adsorption, which would potentially increase the regeneration energy consumption. Additionally, water vapour could effectively inhibit the formation of urea compounds, further improving the cyclic stability. Because of the facile, scalable preparation method, the synthesised amine-functionalized nano-Al 2 O 3 adsorbent has broad application prospects for CO 2 separation. [Display omitted] • Nano-Al 2 O 3 was pore-expanded via n-butanol solvent to a pore volume of 1.62 cm3 g−1. • Polyethylenimine was impregnated on nano-Al 2 O 3 for CO 2 separation from biogas. • '60%PEI@AD-Al 2 O 3 ' adsorbent shows superior CO 2 uptake of 199.4 mg·g−1 adsorbent. • '60%PEI@AD-Al 2 O 3 ' adsorbent possesses favourable cyclic and anti-urea stability. • Water vapour can inhibit the urea formation and improve the cyclic stability. [ABSTRACT FROM AUTHOR]