• The nature of surface hydroxyl groups (Al–OH vs. Si–OH) dictates distinct deactivation pathways for supported PEI adsorbents, with Al–OH promoting oxidative degradation and Si–OH promoting urea formation.
• PEG modification effectively suppresses urea formation on Si–OH supports, but does not prevent oxidation on Al–OH supports, highlighting the need for tailored support design.
• The optimized 40PEI-20PEG-SBA-15 adsorbent exhibits exceptional stability with 2.45 mmol g−1 CO2 capacity over 1000 cycles and negligible loss after one month in simulated flue gas.
• This work provides mechanistic insights essential for designing ultra-stable solid amine adsorbents for practical post-combustion CO2 capture.