Carbonation pathways of potassium based sorbent for carbon capture

https://doi.org/10.1016/j.ccst.2021.100011

“Unravelling carbonation pathways is particularly important for potassium-based adsorbents, because it offers insights into identifying the rate-limiting step, and this further provides significant guidance for the rational design of robust adsorbents and process optimizations. The overall reaction pathway of the carbonation process is listed as Eq. (1). A consensus has been reached that the carbonation process consists of hydration and carbonation reactions. However, there remains controversial conclusions on the detailed carbonation pathways since water vapor is involved in the carbonation process. Besides, the carbonation process also involves several intermediates, and the understandings upon the role of these intermediates remain controversial. Hayashi et al. thought that K₂CO₃ reacted with water vapor to form the intermediate of K₂CO₃•1.5H₂O under a moist condition, and K₂CO₃•1.5H₂O further captured CO₂ to form KHCO₃ Eqs. 2 and (3) (Hayashi et al., 1998; Shigemoto et al., 2006). The involved hydration and carbonation reactions were in a sequential relationship.”

“Lee et al. confirmed that K₂CO₃ could be hydrated to form K₂CO₃•1.5H₂O, and the role of the K₂CO₃•1.5H₂O intermediate had been clarified. A lower temperature and a higher H₂O concentration favored K₂CO₃•1.5H₂O formation, which would contribute to facilitating the subsequent carbonation reaction Eqs. 2 and (3) Lee et al., 2006a). Besides, a new active intermediate of K₄H₂(CO₃)₃•1.5H₂O was formed upon excessive water vapor pretreatment, and the formed K₄H₂(CO₃)₃•1.5H₂O intermediate further captured CO₂ to produce KHCO₃ (Eqs. 4 and (5) (Lee et al., 2011a; Lee et al., 2006b). However, the definite role of the K₄H₂(CO₃)₃•1.5H₂O intermediate (promoting or inhibiting) in the carbonation process had not been clearly clarified.”

“Zhao et al. illustrated that both parallel and sequential reactions occurred in the carbonation process. The hydration reactions occurred concurrently to form K₂CO₃•1.5H₂O and K₄H₂(CO₃)₃•1.5H₂O Eqs. 2 and (4), and the two intermediates were then converted into KHCO₃ through the parallel carbonation reactions Eqs. 3 and (5). The hydration and carbonation reactions were in a sequential relationship. The roles of K₂CO₃•1.5H₂O and K₄H₂(CO₃)₃•1.5H₂O intermediates had been identified as that the former showed slow carbonation rate and the latter contributed to fast carbonation Zhao et al., 2009c; Zhao et al., 2009d). Recently, Luo et al. divided the bicarbonate formation of K₂CO₃ into three sequential steps: the formation of K₂CO₃•1.5H₂O from K₂CO₃ (Eq. 2), the subsequent formation of K₄H₂(CO₃)₃•1.5H₂O from K₂CO₃•1.5H₂O (Eq. 6), and the slow formation of KHCO₃ from K₄H₂(CO₃)₃•1.5H₂O (Eq. 5) (Chioyama et al., 2015; Luo et al., 2015). More recently, Mahinpey’s group experimentally verified the reaction pathways of solid K₂CO₃ during CO₂ capture. The carbonation process of K₂CO₃ consisted of the competing reversible parallel carbonation and hydration reactions (Eqs. 1 and (2). Despite that the K₂CO₃•1.5H₂O intermediate could be formed through the hydration reaction, it could not be directly converted to KHCO₃ via CO₂ occlusion (Fig. 3) (Gomez et al., 2016; Jayakumar et al., 2016, 2017).”

“Fig. 3. Carbonation pathways of solid K₂CO₃ adsorbent. Reproduced with permission from (Gomez et al., 2016). Copyright 2016 American Chemical Society.”