Hongjie Cui,Qiming Zhang,Yuanwu Hu,Chong Peng,Xiangchen Fang,Zhenmin Cheng,Vladimir V. Galvita,Zhiming Zhou
As a potential candidate for precombustion CO2 capture at intermediate temperatures (200–400 °C), MgO-based sorbents usually suffer from low kinetics and poor cyclic stability. Herein, a general and facile approach is proposed for the fabrication of high-performance MgO-based sorbents via incorporation of CaCO3 into MgO followed by deposition of a mixed alkali metal salt (AMS). The AMS-promoted MgO–CaCO3 sorbents are capable of adsorbing CO2 at an ultrafast rate, high capacity, and good stability. The CO2 uptake of sorbent can reach as high as above 0.5 gCO2 gsorbent–1 after only 5 min of sorption at 350 °C, accounting for vast majority of the total uptake. In addition, the sorbents are very stable even under severe but more realistic conditions (desorption in CO2 at 500 °C), where the CO2 uptake of the best sorbent is stabilized at 0.58 gCO2 gsorbent–1 in 20 consecutive cycles. The excellent CO2 capture performance of the sorbent is mainly due to the promoting effect of molten AMS, the rapid formation of CaMg(CO3)2, and the plate-like structure of sorbent. The exceptional ultrafast rate and the good stability of the AMS-promoted MgO–CaCO3 sorbents promise high potential for practical applications, such as precombustion CO2 capture from integrated gasification combined cycle plants and sorption-enhanced water gas shift process.
Molten salt-doped MgO adsorbent is considered one of the most promising CO 2 adsorbents in the field. In this work, MgO-based adsorbents were prepared by one-step calcination using MgCl 2 ·6 H 2 O as magnesium source. The CO 2 adsorption performance of MgO-based adsorbents was investigated via different methods. Results showed that the maximum CO 2 adsorption capacity of MgO doped by LiNO 3 -NaNO 3 -KNO 3 was 57.1% at the CO 2 concentration of 80% and 350 ℃, and the MgO-based adsorbents showed good regeneration. The nanosheet structure of the MgO-based adsorbents decreased with the increase in the number of cycles, whereas the crystal structures of MgO and alkali metal nitrates did not change because of multiple decarbonization. DFT computation revealed selective adsorption of CO 2 on different crystal faces of MgO. The (200) crystal face of molten salt-doped MgO did not have CO 2 trap ability. In addition, the doped nitrate did not directly participate in the reaction but reduced the adsorption energy of MgO carbonation. The adsorption energies of the MgO (220) and (222) crystal faces after doping with nitrate were reduced to − 2.07 and − 3.26 eV, respectively. The overall energy level of adsorption decreased as the number of resonance peaks and the stability of the structure increased. This study explains why MgO currently fails to reach the theoretical adsorption capacity and reveals the underlying mechanism of molten salts.
Lei Wang,Zhiming Zhou,Yuanwu Hu,Zhenmin Cheng,Xiangchen Fang
A series of nanosheet MgO-based sorbents promoted by mixed-alkali-metal nitrate and carbonate were prepared by a simple precipitation–deposition method and applied to CO2 capture. The structural properties of these sorbents were characterized by various techniques, and their CO2 capture performance was evaluated using a thermogravimetric analyzer. Compared to the sorbent derived from commercial MgO, the nanosheet MgO-based sorbent had faster and higher CO2 uptake, because of its thin-sheet structure. Among various alkali-metal-salt-promoted nanosheet MgO, the sorbents promoted with mixed-alkali-metal nitrates (LiNO3 and KNO3) and carbonates (Na2CO3 and K2CO3) exhibited a high total CO2 uptake at a respectable rate. In particular, the sorbent with a MgO content of 73 wt % and a nitrate/carbonate molar ratio of 2 possessed the highest total CO2 uptake and the best cyclic stability, with a total uptake of 0.52 gCO2/gsorbent and MgO conversion of 65% after 20 cycles (60 min of absorption in 40% CO2/60% N2 at 350 °C, 15 min of regeneration in 100% N2 at 400 °C). Based on the structure–activity relationship of the sorbents, a possible mechanism consisting of three stages was presented for the CO2 absorption process, in which the dissolution of CO2 and carbonation in the molten nitrate played an important role.
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