Design of Amine-Containing Nanoporous Materials for Postcombustion CO₂ Capture from Engineering Perspectives

ConspectusCarbon dioxide (CO₂) capture and storage (CCS) is a means to enable the continued use of fossil fuels in the short term. In particular, postcombustion CO₂ capture has attracted considerable attention because it can be retrofitted into existing power plants and industrial plants. Among various CO₂ capture technologies, the absorption of CO₂ using aqueous amines has been industrially employed for decades. However, such amine scrubbing technologies have inherent limitations of environmental and health concerns due to volatile amine loss, corrosion, and high energy demands for regeneration. To overcome these limitations, CO₂ adsorption using solid adsorbents has emerged as a promising alternative due to its noncorrosiveness and low energy demand. Various amine-containing adsorbents have been synthesized and investigated for postcombustion CO₂ capture. These materials are prepared by physically impregnating low-vapor-pressure amine polymers or by chemically grafting amines onto nanoporous materials. A wide variety of amine guests and nanoporous hosts (e.g., SiO₂, Al₂O₃, zeolites, MOFs, and polymers) have been combined to develop advanced CO₂ adsorbents.The design of CO₂ adsorbents is a multifaceted puzzle that must ultimately consider integration with large-scale CO₂ capture processes. Various engineering aspects need to be carefully considered. Unfortunately, a significant proportion of previous studies has primarily focused on the use of novel materials for improving the CO₂ adsorption capacity. In this Account, we describe key challenges and solutions to develop energy-efficient and stable amine-containing adsorbents for postcombustion CO₂ capture via temperature swing adsorption (TSA). We found that a high CO₂ working capacity, often overemphasized in the literature, does not necessarily guarantee a low energy demand for CO₂ capture. Suppressing coadsorption of H₂O during the CO₂ adsorption in humid flue gas is also a significant factor. Amine-containing adsorbents can be degraded through various pathways, including hydrothermal degradation of nanoporous hosts and chemical degradation of amine guests via urea formation and oxidation. To inhibit such degradation pathways, it is extremely important to properly design the nanoporous structures of the hosts and the molecular structures of the amine guests. By combining macroporous silica hosts, poly(ethylenimine) (PEI) functionalized with various alkyl epoxides, and phosphate-based oxidative stabilizers, we could synthesize adsorbents exhibiting low energy demands for CO₂ capture and unprecedentedly high thermochemical stability under TSA conditions. The macroporous silica host synthesized by assembling fumed silica particles via spray-drying exhibited high hydrothermal stability and enabled uniform distribution of bulky amine polymers within its pores. The functionalization of PEI with alkyl epoxides converted its primary amines into hindered secondary amines, leading to a significant reduction in energy demand for TSA cycles and a remarkable improvement in long-term stabilities. The oxidative stability of amines could be drastically improved by adding phosphate metal-binding reagents, which can poison ppm-level metal impurities that catalyze amine oxidation. The present discussions will provide important insights into designing practical adsorbents for CO₂ capture from engineering perspectives.