Supported K2CO3 for CO2 capture

“Supported potassium-based adsorbents have been synthesized to improve the dispersion of K2CO3 for higher K2CO3 utilization efficiency and enhanced carbonation reactivity. Whereas, the involved supports with tunable acid and/or basic sites and varied structural properties might alter the carbonation pathways. Al2O3 supports with varied surface properties have been extensively employed to stabilize K2CO3 for CO2 capture. In most of the open documents, a new byproduct of KAl(CO3)(OH)2 could be observed in the carbonation process of K2CO3/γ-Al2O3, and the formed byproduct required a higher temperature for regeneration (Cho et al., 2018). Lee et al. further pointed out that the carbonation pathways depended on the alumina phases employed in the Al2O3 supported K2CO3 adsorbents, and the alumina phases heavily depended on calcination temperature. The formation of the inactive byproduct of KAl(CO3)(OH)2 was primarily associated with the γ-Al2O3 phase. During the preparation of Al2O3 support, high-temperature calcination (1200°C) would yield α-Al2O3 phase, which could alleviate the formation of KAl(CO3)(OH)2. A moderate calcination temperature (600°C) could benefit the formation of δ-Al2O3 phase, which could effectively improve the regeneration performance of the K2CO3/δ-Al2O3 adsorbent (Lee et al., 2014a). SiO2 has also been widely employed as support for potassium-based adsorbents. Cho et al. reported that K2CO3 would interact with the SiO2 support to form inactive components of K2Si2O5 and K2Si4O9 (Cho et al., 2019Cho et al., 2018). Sanna et al. confirmed that K2CO3 could react with SiO2 in the fly ash support during high-temperature calcination to form K2SiO3. Unlike the inactive K2Si2O5 and K2Si4O9 reported by Cho et al., the formed K2SiO3 phase was highly active for high-temperature CO2 capture (Sanna and Maroto-Valer, 2016).

As illustrated above, TiO2 also plays a significant role in altering the carbonation and regeneration pathways of K2CO3/TiO2 and KHCO3/TiO2/Al2O3 Eqs. 7(9) (Qin et al., 2018Zhao, W. et al., 2017). Alternatively, nanoporous TiO(OH)2 had been employed to support K2CO3, and the TiO(OH)2 support contributed to high initial CO2 uptakes by the formation of TiOCO3 (Tuwati et al., 2013). MgO is a promising support or promoter for potassium-based adsorbents. MgO captures CO2 at an intermediate temperature (200°C) to form MgCO3. When moisture is present, MgO can even combine with CO2 at a lower temperature (60°C) (Kumar and Saxena, 2014). The carbonation pathways of MgO promoted potassium-based adsorbents are more complicated. For K2CO3/MgO adsorbent, both the active K2CO3 and MgO would interact with CO2 and H2O to form byproducts of K2Mg(CO3)2 and K2Mg(CO3)2•4H2O (Byun et al., 2016Gao, W. et al., 2017Lee et al., 2006aLee and Kim, 2007). The K2CO3/MgO/Al2O3 adsorbent also showed complicated carbonation pathways, since the ternary oxides had been involved in the carbonation process to yield intermediates and products such as K2Mg(CO3)2, K2Mg(CO3)2•4H2O, KAl(CO3)2(OH)2, Mg4Al2(OH)12CO3•3H2O, Mg2Al2(CO3)4(OH)2•15H2O, and KHCO3•MgCO3•4H2O (Li et al., 2011Li et al., 2013). Similarly, both K2CO3 and CaO in the K2CO3/CaO adsorbent interacted with CO2 to form the K2Ca(CO3)2 byproduct (Cho et al., 2018). Besides, some alkaline solid wastes with considerable basic sites have also been valorized as supports for potassium-based adsorbents. K2CO3 had been loaded on naturally occurring potassium feldspar mineral, and the KAlSi3O8 and NaAlSi3O8 in the mineral support contributed to enhanced carbonation in a way similar to CO2 mineralization (Guo et al., 2015a).”

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