Biogas Upgrading via Cyclic CO₂ Adsorption: Application of Highly Regenerable PEI@nano-Al₂O₃ Adsorbents with Anti-Urea Properties

Abstract

Solid amine adsorbents are among the most promising CO 2 adsorption technologies for biogas upgrading due to their high selectivity toward CO 2 , 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 2 atmosphere. Herein, we demonstrated a facile and efficient synthesis route, involving the synthesis of nano-Al 2 O 3 support derived from coal fly ash with a CO 2 flow as the precipitant and the preparation of polyethylenimine (PEI)-impregnated Al 2 O 3 -supported adsorbent. The optimal 55%PEI@2%Al 2 O 3 adsorbent showed a high CO 2 uptake of 139 mg•g −1 owing to the superior pore structure of synthesized nano-Al 2 O 3 support and exhibited stable cyclic stability with a mere 0.29% decay per cycle even under the realistic regenerated CO 2 atmosphere. The stabilizing mechanism of PEI@nano-Al 2 O 3 adsorbent was systematically demonstrated, namely, the cross-linking reaction between the amidogen of a PEI molecule and nano-Al 2 O 3 support, owing to the abundant Lewis acid sites of nano-Al 2 O 3 . 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-Al 2 O 3 adsorbents by markedly inhibiting the formation of urea compounds. Therefore, this facile and efficient strategy for PEI@nano-Al 2 O 3 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 2 separation processes.

Insights into the Critical Role of Abundant-Porosity Supports in Polyethylenimine Functionalization as Efficient and Stable CO2 Adsorbents

“…Similar phenomena could be observed based on several previous investigations. ,,,,,,,− We summarized the maximum CO 2 adsorption capacities and the corresponding optimal adsorption temperatures of the reported PEI-functionalized adsorbents all with around 50% loadings in these studies, as presented in Figure f. Thereinto, various adsorbents were divided into PEI/SiO 2 , PEI/Al 2 O 3 , PEI/AC, and PEI/resin, according to the selected porous supports.…”

“…As for 50%PEI/HP20 and 50%PEI/XAD4, the long-term operation of adsorption–regeneration cycles was significantly more stable, and the working capacities for CO 2 adsorption in each cycle gradually varied from 3.59 to 3.30 mmol/g and from 2.98 to 2.62 mmol/g with the losses of 8.08 and 12.09%, respectively. Despite the decrease, the stability of 50%PEI/HP20 and 50%PEI/XAD4 was still similar to or even better than that of many PEI-impregnated adsorbents by CO 2 purging regeneration. , Nevertheless, the attenuations of working capacities using 50%PEI/Al 2 O 3 and 50%PEI/AC were more apparent and reached 24.68 and 66.04%, respectively, after 30 consecutive adsorption–regeneration cycles. After all, the high-temperature CO 2 purging flow would promote amine leaching of the functionalized Al 2 O 3 and AC with poor thermal stability, which could be observed from the decaying adsorption–regeneration curves.…”

“…Notably, this finding was generally overlooked in previous studies. Most investigations concluded that amine types and temperatures were the main factors affecting the resistance to CO 2 -induced degradation, but ignored the critical role of porous support types. , Overall, the selection of appropriate supports for PEI functionalization could effectively inhibit the degradation due to the CO 2 purging regeneration approach, and the resistance was also demonstrated in some reported non–SiO 2 -based adsorbents. , …”

“…Despite the decrease, the stability of 50%PEI/HP20 and 50%PEI/XAD4 was still similar to or even better than that of many PEI-impregnated adsorbents by CO 2 purging regeneration. 44,56 Nevertheless, the attenuations of working capacities using 50%PEI/Al 2 O 3 and 50%PEI/AC were more apparent and reached 24.68 and 66.04%, respectively, after 30 consecutive adsorption−regeneration cycles. After all, the high-temperature CO 2 purging flow would promote amine leaching of the functionalized Al 2 O 3 and AC with poor thermal stability, which could be observed from the decaying adsorption−regeneration curves.…”

“…62,66 Overall, the selection of appropriate supports for PEI functionalization could effectively inhibit the degradation due to the CO 2 purging regeneration approach, and the resistance was also demonstrated in some reported non−SiO 2 -based adsorbents. 25,56 ■ CONCLUSIONS…”