https://doi.org/10.1016/j.ccst.2021.100011
“Metal oxides including SiO2, MgO, MgO/Al2O3, CaO, ZrO2, TiO2, TiO(OH)2, FeOOH and SnO2 have been utilized as supports (Cho et al., 2019; Cho et al., 2018; Cho et al., 2016; Lee et al., 2006a; Lee et al., 2014b; Li et al., 2011; Li et al., 2013; Tuwati et al., 2013; Zhang et al., 2011; Zhao et al., 2009b). Unlike the inert supports such as carbon and mesoporous Al2O3 that function to provide surface area and pore channels to anchor and stabilize K2CO3, most of the metal oxides supports also act as active promoters that afford extra alkaline sites or strengthen the interaction between K2CO3 and supports. K2CO3/SiO2 generally exhibited poor CO2 adsorption capacity, since the reinforced interaction between K2CO3 and SiO2 induced the formation of the undesired components (K2Si2O5 and K2Si4O9). A careful optimization of calcination temperature and molar ratio of K2CO3 to SiO2 was required to achieve high CO2 uptakes (Cho et al., 2019). Alkaline supports of MgO and MgO/Al2O3 could afford abundant basic active sites for enhanced CO2 capture capacities (2.45-2.70 mmol CO2/g), due to the formation of new structures of KAl(CO3)2(OH)2, K2Mg(CO3)2, K2Mg(CO3)2•4H2O, and MgCO3. Yet, the formation of these products had increased the required regeneration temperature from 120-200°C to 480°C (Li et al., 2011; Li et al., 2013). The incorporation of CaO would promote the initial carbonation reactivity, as evidenced by the high CO2 uptake of 7.89 mmol CO2/g for K2CO3/CaO in the first cycle. This was because the active CaO had been involved in the carbonation process under a moisture condition. Nonetheless, its CO2 capture capacity declined remarkably to 0.85 mmol CO2/g within 3 cycles, and this was ascribed to the fact that K2CO3 had reacted with CaO to form K2Ca(CO3)2, which could not be regenerated at lower temperatures (Cho et al., 2018). TiO2 had also been adopted as a suitable support for potassium-based adsorbents, while its role in carbonation remained controversial. One view insisted that K2CO3/TiO2 would deactivate in repeated cycles, because of the formed inactive K-Ti alloys (K2Ti2O5 and K2Ti6O13) during regeneration (Lee et al., 2014b). Another opinion was that TiO2 support played positive roles in carbonation, because it had provided more active sites and increased OH coverage by promoting H2O dissociation (Qin et al., 2018; Zhao, W. et al., 2017). The latter view enlightened the use of TiO(OH)2 and FeOOH as supports or catalysts to enhance OH coverage for improved carbonation performance and reduced energy consumption for regeneration (Tuwati et al., 2013; Zhang et al., 2011). Unfortunately, the TiO(OH)2 and FeOOH supported K2CO3 adsorbents exhibited obvious loss-in-capacity within the initial few cycles. ZrO2 and SnO2 supported K2CO3 adsorbents showed excellent thermal stability and stabilized CO2 capture performance in repeated cycles, because no inactive alloy species were formed even after calcination at higher temperatures (Cho et al., 2016; Lee et al., 2014b). ZrO2 and SnO2 were therefore suggested as promising alternatives to TiO2 and γ-Al2O3 for synthesizing supported K2CO3 adsorbents. In addition, silicate-based supports such as Al6Si2O13, CaSiO3 and ZrSiO4 had also been employed to load K2CO3 and the supported adsorbents presented relatively stabilized CO2 uptakes over repeated cycles (Cho et al., 2018).”