Progress on CO₂-induced inactivation of solid amine adsorbents

Climate warming caused by massive CO2 emission is a global problem facing mankind. The carbon capture and storage (CCS) technology can substantially reduce CO2 emissions from heavy industries, such as power, steel, cement and chemical industries. By eliminating billions of tonnes of CO2 from fossil fuels, CCS can help limit the increase in global temperatures to under 2 degrees C by the end of the century. The CCS technology has rapidly developed to include several technologies, such as liquid ammonia absorption, physical adsorption, membrane separation and solid amine adsorption. Solid amine adsorption, in particular, can be used to capture CO2 from flue gas and air, and separate it from biogas or syngas because of its various advantages, such as high CO2 selectivity, low energy consumption during regeneration, the ability to process large amounts of CO2, and the non-corrosive equipment used. By designing the pore structure of the support material, a solid amine adsorbent can adsorb more than 5 mmol/g of CO2 at a time. However, to understand its commercial potential, the adsorbent should be stable over hundreds of adsorption-desorption cycles to ensure a longer lifetime and lower CO2 capture cost. The cyclic stability of adsorbents is seriously hindered by CO2-induced inactivation, which occurs during their regeneration. To obtain highly pure CO2, adsorbents are regenerated under a CO2 atmosphere instead of an inert atmosphere, but the urea groups formed when CO2 reacts with amine molecules are too stable to release CO2, hampering the reusability of the adsorbent. In this paper, we focus on the challenges posed by CO2-induced inactivation of solid amine adsorbents as well as discuss the inactivation process, its critical factor and its mechanism. We also analyse the current preventive strategies employed to tackle this problem. CO2-induced inactivation widely occurs in different types of solid amine adsorbents, with an inactivation rate of more than 70% in 10 cycles. The regeneration temperature is a critical factor in this process. When the regeneration temperature exceeds 120 degrees C, the solid amine adsorbents form several urea groups, resulting in prolific CO2-induced inactivation. Mechanistically, the carbamic acid intermediate formed when a primary amine reacts with CO2 gets easily dehydrated to generate isocyanate, which can subsequently form urea group by combining with another primary or secondary amine. The dehydration of carbamic acid to form isocyanate, thus, plays a key role in the formation of urea groups. The formation of urea can be effectively inhibited by decreasing the amount of primary amines in the adsorbent, reducing the regeneration temperature, and increasing the water vapour content in the regeneration atmosphere. Accordingly, three strategies to inhibit urea formation were proposed: Organic amine modification (primary amines conversion into secondary amines), water vapour repression (inhibition of dehydration) and the use of support materials cross-linking organic amines (change the primary amine properties). Nevertheless, the challenges associated with CO2-induced inactivation, including high regeneration temperatures and the non-compatibility of CO2 adsorption capacity with cyclic stability, still plague solid amine adsorbents. Finally, potential solutions that can facilitate the industrial application of this process have been discussed.