Alkali metal modification of CaO

https://doi.org/10.1039/D0CP04410E

“Recent work has explored the effect of the addition of the double salt (Li-K)₂CO₃ to CaO on its CO₂ uptake.²¹ After 23 carbonation/calcination cycles a three times higher CO₂ uptake was observed when compared to pure CaO.²¹ It was speculated that the addition of (Li-K)₂CO₃ prevents the formation of a CaCO₃ layer that would otherwise impose a high diffusion resistance for CO₂, thus providing a direct access for CO₂ to unreacted CaO. ”

“Commercial CaCO₃ (Fischer Chemicals, CAS: 471-34-1, analytical reagent grade) and Na₂CO₃ (Acros Organics, CAS: 497-19-8, 99.5% purity) were used for the synthesis of the CO₂ sorbents (Fig. S1, ESI†). To achieve a homogeneous distribution of the sodium salt modifier, a wet ball-milling technique following a reported method was employed.¹⁴ Specifically, 7 g of the precursor powders, mixed with 10 mL of deionized water, were ball-milled in a 45 mL Si₃N₄ jar containing 5 mm Si₃N₄ balls. The ball-milling time was 48 h at 500 rpm. After ball milling, the suspension obtained was dried in an oven overnight at 100 °C. The following nomenclature is used for the samples: Ca/xNa, whereby x refers to the quantity of Na₂CO₃ (wt%). The following values for x were used: 1, 3, 6, 10 and 20 wt%. Pure ball-milled CaCO₃ was used as the reference material for the CO₂ uptake experiments (referred to as Ca/0Na).

The reference Na₂Ca(CO₃)₂ material was synthesized from a stoichiometric mixture of Na₂CO₃ and CaCO₃ at 800 °C in a CO₂ atmosphere for 2 h.”

“Fig. 1 (a) XRD patterns of CaO-based sorbents as prepared and calcined and Na₂CO₃ and CaCO₃ references; (b) cyclic CO₂ capture performance of ball-milled Ca/xNa (calcination at 900 °C in CO₂-rich atmosphere; carbonation at 650 °C in 20 vol% CO₂).”

“The cyclic CO₂ uptake performance of the ball-mill derived Ca/xNa materials (Fig. 1b) was determined under gas atmospheres and temperatures close to that prevailing in the envisioned large-scale CaL process, i.e. the calcination step was performed at 900 °C in a CO₂-rich atmosphere; and the carbonation at 650 °C in 20 vol% CO₂ in N₂. Ca/0Na (i.e. the pure ball-milled calcite) exhibited a relatively high initial CO₂ uptake (0.65 g_CO2 g_sorbent⁻¹), however, its CO₂ uptake gradually decreased with cycle number and reached 0.29 g_CO2 g_sorbent⁻¹ after 10 carbonation/calcination cycles. The decay in the CO₂ capacity of unsupported CaO is due to the sintering-induced loss in pore volume owing to the low T_T of CaCO₃ of 533 °C.⁷ We observed that the addition of Na₂CO₃ to CaO (Ca/xNa, with x = 1, 3, 6 10 and 20) led to a dramatic drop in the CO₂ uptake of the sorbents (Fig. 1b) when compared to pure CaO (Ca/0Na). For instance, the maximum CO₂ uptake of Ca/1Na in the 1st cycle was only 0.21 g_CO2 g_sorbent⁻¹, a value that is ca. three times lower compared to Ca/0Na (0.65 g_CO2 g_sorbent⁻¹). After the 10th cycle, Ca/1Na showed a CO₂ uptake of 0.05 g_CO2 g_sorbent⁻¹, a value that is ca. six times lower compared to Ca/0Na. Increasing the Na₂CO₃ content deteriorated further the CO₂ uptake of the sorbents; for example, Ca/20Na showed CO₂ uptakes of 0.14 g_CO2 g_sorbent⁻¹ and 0.04 g_CO2 g_sorbent⁻¹ in the 1st and 10th cycle, respectively.”

“Additionally, temperature-programed carbonation/calcination experiments (TPC, 150 cm³ min⁻¹, 20 vol% CO₂ in N₂, 50 to 1000 °C, 10 °C min⁻¹) of Ca/0Na, Ca/1Na and Ca/20Na as well as the reference Na₂CO₃ were performed in a TGA (Fig. S7, ESI†). As expected, pure Na₂CO₃ showed no weight increase over the entire temperature range probed. At temperatures above 850 °C, i.e. temperatures exceeding the melting temperature of Na₂CO₃ (melting temperature, T_M = 851 °C) a gradual weight loss was detected. This weigh loss was due to the slow decomposition of Na₂CO₃ into Na₂O and CO₂, in agreement with previous observations.²⁸ Turning to Ca/0Na, the TPC experiment showed a broad peak of weight gain with a maximum at ca. 570 °C due to the carbonation of CaO. The weight increase was followed by a rapid weight loss due to the decomposition of CaCO₃ starting at 800 °C. Interestingly, the addition of Na₂CO₃ to CaO changed dramatically the sorbent’s TPC curve profile. The Na₂CO₃-modified sorbents showed a lower CO₂ uptake at intermediate temperatures (T < 700 °C), while the onset temperature for carbonation was not affected by the addition of Na₂CO₃ (Fig. S7, ESI†). The broad shoulder at intermediate temperatures was followed first by a narrow, sharp peak at T ≈ 750 °C, and secondly by a weight loss corresponding to the decomposition of CaCO₃. Increasing the Na₂CO₃ content shifted the maximum of the carbonation peak towards higher temperatures, i.e. 770 °C for Ca/1Na and 785 °C for Ca/20Na (Fig. S7, ESI†). In contrast to pure Na₂CO₃, the Na₂CO₃-modified sorbents showed no weight loss at high temperatures, indicating the absence of Na₂CO₃ decomposition. We hypothesize that this was due to the formation of a stable Ca–Na mixed phase in CO₂-containing atmospheres (vide infra).”