Nanosheet MgO-Based CO2 Sorbent Promoted by Mixed-Alkali-Metal

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Nanosheet MgO-Based CO2 Sorbent Promoted by Mixed Alkali Metal Nitrate and Carbonate: Performance and Mechanism Lei Wang, Zhiming Zhou, Yuanwu Hu, Zhen-Min Cheng, and Xiangchen Fang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 2, 2017

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Industrial & Engineering Chemistry Research

Nanosheet MgO-Based CO2 Sorbent Promoted by Mixed Alkali Metal Nitrate and Carbonate: Performance and Mechanism Lei Wang,a Zhiming Zhou,*,a, Yuanwu Hu,a Zhenmin Cheng,a and Xiangchen Fang*,b

a

State Key Laboratory of Chemical Engineering, East China University of Science and Technology,

Shanghai 200237, China b

Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC, Fushun 113001, China

* Corresponding Author Phone: +86 21 6425 2230; Fax: +86 21 6425 3528 Email: [email protected] (Z. Zhou); [email protected] (X. Fang)

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ABSTRACT 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 owing to its thin-sheet structure. Among various alkali metal salt-promoted nanosheet MgO, the sorbents promoted with mixed alkali metal nitrate (LiNO3 and KNO3) and carbonate (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 gCO /gsorbent and MgO conversion 2

of 65% after 20 cycles (60 min of absorption in 40% CO2/60% N2 at 350 ºC, 15 min of regeneration in 100% N2 at 400 ºC). Based on the structure-activity relationship of the sorbents, a possible mechanism consisting of three stages was put forward 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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1. INTRODUCTION It is well known that CO2 as a greenhouse gas contributes greatly to the global warming, but unfortunately, the rapid consumption of fossil fuels in anthropogenic activities still release large amounts of CO2 into the atmosphere. In particular, around 42% of global CO2 emissions come from existing fossil fuel-fired electricity and heat production plants.1 Therefore, the development of effective methods to capture CO2 from the flue gas of power plants is an urgent concern for academic and industrial communities.2,3 Among different approaches investigated, sorption-based technologies account for the majority of research activities.4 The CO2 absorption by aqueous amine solutions is a well-established technique for CO2 capture, however, it still suffers from some problems, such as high cost, corrosive nature and thermal degradation of amine as well as loss of available amine from the scrubbing system and through byproduct formation.5 In this regard, solid sorbents have advantages such as wide operating temperature range, relatively low cost and ready availability in natural minerals.4,6 Different types of solid sorbents are used for CO2 capture from power plant flue gas, depending on the working temperature and the technology adopted (pre-combustion, post-combustion and oxy-combustion). At intermediate temperature (200~500 ˚C), which is the operating window for pre-combustion CO2 capture in integrated gasification combined cycle (IGCC) power plants, layered double hydroxides (LDHs) and MgO are among the most frequently-studied sorbents.7 Compared to LDH-based sorbents, MgO has a much higher theoretical CO2 capture capacity (1.1 gCO /gMgO ), but the slow absorption kinetics, low actual capacity and poor thermal stability during 2

cyclic operation limit its practical applications.8

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Recently, a novel concept based on molten salts to facilitate the CO2 absorption with MgO was put forward by Mayorga et al.,9 and further extended by Hamad et al.10 and Zhang et al.11 It was found that the molten nitrate was able to dissolve to some extent bulk MgO, whereby the high lattice energy constraints were overcome, which gave rise to the enhancement of CO2 absorption.12,13 In a 30-cycle CO2 absorption on a NaNO3-Na2CO3 promoted MgO-based sorbent, a stable CO2 capture capacity of 0.26 gCO /gsorbent was obtained.14 Harada et al.15 synthesized 2

(Li,Na,K)NO3 coated MgO particles with an average size of 100 nm by simple calcination of a mixed suspension of magnesium carbonate hydroxide hydrate and alkali metal nitrates in water, which showed a good stability over 40 cycles, with a capacity of about 0.30 gCO /gsorbent . In a 2

following study performed by the same group,16 LiNO3-(Na,K)NO2 coated MgO with an average primary particle size of 10 nm were prepared by a nonhydrolytic sol-gel reaction (for MgO) and a subsequent deposition of alkali metal salts by solvent evaporation-induced surface precipitation. Compared to (Li,Na,K)NO3 coated MgO, LiNO3-(Na,K)NO2 coated MgO exhibited faster and higher uptake of CO2, around 0.53 gCO /gsorbent at the 20th cycle; however, the preparation of 2

MgO is a little complex and costly, with the aid of two types of surfactants (oleic acid and oleyl amine) using magnesium acetylacetonate dihydrate as the magnesium precursor and 1,2-tetradecandiol as the solvent.16 Very recently, Vu et al.17 developed a one-step technique, an aerogel method with a supercritical drying, to prepare mesoporous MgO-based sorbents promoted by NaNO3 and Na2CO3. The best sorbent synthesized showed a capacity of 0.32 gCO /gsorbent at the 2

14th cycle, which was lower than that of LiNO3-(Na,K)NO2 coated MgO by Harada et al.16 Detailed information on the sorbent material, evaluation condition and CO2 capture performance is presented and discussed in subsection 3.6. The above findings indicate that the performance of 4


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MgO-based sorbents can be optimized by using small MgO particles and by adjusting the composition of alkali metal salts. The challenge is to prepare high-performance alkali metal salt-promoted MgO-based sorbents by a simple and inexpensive method. The mechanism of CO2 absorption on alkali metal salt-promoted MgO is still not fully understood and remains subject to controversial discussion in the literature. Zhang et al.13,14 thought that the reaction between CO2 and MgO took place at triple phase boundaries comprising gaseous CO2, molten salt and solid MgO, while a mechanism by which CO2 was dissolved into the molten salt followed by diffusion through the melt to the MgO surface for reaction, is incorrect. On the contrary, Harada et al.15,16 held that the CO2 absorption occurred by the initial dissolution of CO2 in the molten salt layer and the following reaction of CO2 with MgO partially dissolved in the molten salt. Therefore, it is necessary to clarify the CO2 absorption mechanism of this type of sorbent. To this end, a simple method was developed for preparing MgO-based sorbents promoted by mixed alkali metal nitrate and carbonate, and the composition of the sorbents was tuned by varying the molar ratio of nitrate to carbonate and the molar ratio of total salts to MgO. The structureactivity relationship of the sorbents was explored, based on which a reasonable mechanism was proposed for the CO2 absorption process. In addition, the effects of temperature and CO2 partial pressure on the CO2 uptake were investigated, and the cyclic stability of the sorbents was evaluated. Compared to previous investigations, the novelty of this study mainly lies in two aspects: (1) the preparation method is more simple and scalable, and more importantly, the resulting sorbent has excellent CO2 capture performance, even at a relatively low CO2 concentration of 40% (a typical concentration of the flue gas in the water-gas shift reactor of IGCC power plants); and (2) the proposed CO2 absorption mechanism is more comprehensive and cohesive, which resolves the 5



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controversy in the literature. The findings of this work will help design more effective MgO-based sorbents for CO2 capture.

2. EXPERIMENTAL SECTION 2.1. Materials. LiNO3 (≥ 96.7%), KNO3 (≥ 99.0%), Na2CO3 (≥ 99.8%), K2CO3 (≥ 99.0%), MgCl2·6H2O (≥ 98.0%), NH3·H2O (25-28 wt%) and absolute methanol (≥ 99.5%) were purchased from Sinopharm Group Chemical Reagent Co. All chemicals were used as received. 2.2. Preparation of MgO. First, 55 mL of MgCl2 aqueous solution (0.4 mol/L) was added at a rate of 0.25 mL/min using a syringe pump to 100 mL of 2.5 wt% ammonia solution in a 3-neck flask at room temperature under strong agitation. Then, the precipitated Mg(OH)2 was collected by filtration and washed several times with deionized water. Finally, the precipitate was dried in an oven at 110 ˚C for 12 h, and the cake obtained was ground into powder, followed by calcination at 500 ˚C (with 5 ˚C/min heating from room temperature) for 5 h in static air. As a reference, a commercial MgO (≥ 99.9%), represented by MgO(c), was purchased from Aladdin Reagent Inc. 2.3. Preparation of MgO-based Sorbents. The sorbents were prepared by the method developed by Harada and Hatton,16 i.e., deposition of alkali metal salts on the surface of MgO by solvent evaporation-induced surface precipitation: predetermined amounts of MgO, alkali metal nitrates and carbonates were mixed in absolute methanol followed by evaporation of methanol to deposit the salts. It should be noted that all the sorbents prepared in this work had a fixed molar ratio of LiNO3: KNO3 of 0.44: 0.56, because the double nitrate salt with such a ratio had been reported to have a low melting point of around 137 ˚C.18 In addition, the molar ratio of Na2CO3 to K2CO3, if both carbonates were used, was fixed to be 0.5: 0.5, which was arbitrarily chosen 6


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because we were primarily interested in the effects of the amount of mixed salts with respect to that of MgO and the amount of nitrates with respect to that of carbonates. In a typical procedure, 5 mmol of MgO, 0.22 mmol of LiNO3, 0.28 mmol of KNO3, 0.125 mmol of Na2CO3 and 0.125 mmol of K2CO3 were first mixed in 80 mL of absolute methanol and then ultrasonicated for 2 h to obtain a white slurry. Next, the excess methanol was evaporated at 60 ˚C in a rotary evaporator, and the white powder obtained was dried in an oven at 110 ˚C for 12 h. The asprepared MgO-based sorbent has a molar ratio of (Li,K)NO3-(Na,K)2CO3 to MgO of 0.15: 1 and a molar ratio of (Li,K)NO3 to (Na,K)2CO3 of 2:1. Unless otherwise specified, the sorbents are denoted for simplicity by [(Li,K)x-(Na,K)]y/MgO, where the first term (Li,K) represents nitrate (LiNO3 and KNO3) with a fixed molar ratio of 0.44/0.56 (LiNO3/KNO3); the second term (Na,K) stands for carbonate (Na2CO3 and K2CO3) with a fixed molar ratio of 0.5/0.5 (Na2CO3/K2CO3); x represents the molar ratio of nitrate to carbonate; and y is the molar ratio of total salts (nitrate + carbonate) to MgO. For example, the sorbent mentioned above is named [(Li,K)2-(Na,K)]0.15/MgO. In some case where no carbonates are added, the sorbent is simply represented as [(Li,K)]y/MgO. 2.4. Characterization of Sorbents. The textural properties of the sorbent such as BET surface area, pore volume and pore diameter were measured by N2 physisorption at -196 ˚C (Micromeritics ASAP 2010). The crystalline structure of the sorbent was characterized by X-ray diffraction (XRD, Rigaku D/Max 2550) using Cu Kα radiation at room temperature from 10 to 80°. Thermal analysis was performed using a differential scanning calorimeter (DSC, TA Q20) from 50 to 400 ˚C with a heating ramp of 10 ˚C/min. The morphology of the sorbent was examined by field emission scanning electron microscopy (FESEM, Nova NanoSEM 450) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100). 7



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2.5. CO2 Uptake Test of Sorbents. The CO2 uptake of sorbent, expressed as mass of CO2 absorbed per unit mass of sorbent, was measured at ambient pressure using a thermogravimetric analyzer (TGA, WRT-3P, Shanghai Precision & Scientific Instrument Co., Ltd.). In each test, around 5 mg of fresh sample placed in a platinum basket was first preheated to 400 ˚C for 30 min under a flow of 100% N2 (50 mL/min) to remove the possible adsorbed H2O and CO2, and then multiple sorption-regeneration cycles were performed alternatively between 350 ˚C in 100% CO2 for 20 min and 400 ˚C in 100% N2 for 15 min. The gas flow rate was maintained at 50 mL/min and the heating/cooling rate was set at 25 ˚C/min.

3. RESULTS AND DISCUSSION 3.1. Effect of MgO Structure. Figure 1 compares the CO2 uptake of commercial MgO-derived sorbent to that of home-made MgO-derived sorbent. It is clear that the former has a CO2 capture capacity lower than the latter, which is mainly attributed to the difference in the structural features of the sorbents. As shown in Figure 2, both the commercial MgO (Figure 2(a)) and the corresponding sorbent ([(Li,K)2-(Na,K)]0.15/MgO(c), Figure 2(b)) are in granular form with an average diameter of around 200 nm, while the home-made MgO (Figures 2(c), 2(e) and 2(f)) and the associated [(Li,K)2-(Na,K)]0.15/MgO (Figures 2(d), 2(g) and 2(h)) generally display a sheet-like geometry with average thickness of about 30 and 40 nm, respectively. The thin-sheet structure of [(Li,K)2-(Na,K)]0.15/MgO allows for high MgO utilization efficiency due to decreased product layer diffusion, which will be discussed in subsection 3.3. [Figures 1-3] The asprepared nanosheet MgO exhibits an interconnected pore system (Figures 2(e) and 2(f)), 8


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whereas the alkali metal salt-promoted MgO-based sorbent has few visible pores (Figures 2(g) and 2(h)), suggesting the successful deposition of alkali metal salts on the nanosheet MgO. This result is confirmed by N2 physisorption. As shown in Figure 3, both commercial and home-made MgO samples display hysteresis loops, indicating the presence of mesopores. Once alkali metal salts are deposited on the surface of MgO, the gas volume adsorbed drops dramatically. As listed in Table 1, the commercial MgO, with a BET surface area of 55.68 m2/g and a pore volume of 0.235 cm3/g, shows reduced surface area and pore volume after deposition of alkali metal salts (i.e., [(Li,K)2-(Na,K)]0.15/MgO(c)), being 11.85 m2/g and 0.064 cm3/g, respectively. Likewise, the same trend is observed for home-made MgO and the corresponding [(Li,K)2-(Na,K)]0.15/MgO sorbent. Therefore, the difference in the micromorphology and the textural property between MgO and MgO-based sorbent clearly demonstrates the successful deposition of alkali metal salts on MgO. [Table 1] [Figure 4] 3.2. Effect of Different Alkali Metal Salts. Figure 4 shows the CO2 uptake of various MgO-based sorbents promoted with or without alkali metal salts. Note that the MgO contents of the four promoted sorbents are similar (Table 1). The pure MgO has a very low CO2 uptake of less than 0.01 gCO /gsorbent after 2 h of reaction with CO2, while the capacity of the alkali metal 2

salt-promoted sorbent is remarkably increased, with the total CO2 uptake of about 0.56, 0.68, 0.72 and 0.73 gCO /gsorbent for [(Li,K)]0.15/MgO, [(Li,K)2-K]0.15/MgO, [(Li,K)2-Na]0.15/MgO and 2

[(Li,K)2-(Na,K)]0.15/MgO, respectively. This result is consistent with previous studies,13,15 indicating that the presence of alkali metal salts can enhance the CO2 uptake of MgO. It should be noted that the CO2 uptake of the salt-promoted sorbents is not directly proportional to the surface 9



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area (Table 1). The inset in Figure 4 shows around 2.3 min of induction time (the time to initiation of the increase in CO2 uptake) for [(Li,K)]0.15/MgO, which is almost zero for the sorbents promoted with both nitrate and carbonate, especially for [(Li,K)2-K]0.15/MgO and [(Li,K)2-(Na,K)]0.15/MgO. In addition, the CO2 uptake of MgO promoted with both nitrate and carbonate is much higher than that of MgO promoted only with nitrate. These observations clearly suggest the synergetic effect of carbonate and nitrate on the CO2 absorption of MgO, which will be explained in detail in the following discussion on the CO2 absorption mechanism. Similar results were reported by Zhang et al.14 and Vu et al.17 using (NaNO3-Na2CO3)-promoted MgO sorbents. However, the maximum total CO2 uptake of sorbent was 0.7114 and 0.5617 gCO /gsorbent , lower than 0.73 gCO /gsorbent of 2

2

[(Li,K)2-(Na,K)]0.15/MgO. 3.3. CO2 Absorption Mechanism. In order to clarify the mechanism of the enhanced CO2 absorption by the (nitrate-carbonate)-promoted MgO-based sorbent, the evolution of the crystalline structure of [(Li,K)2-(Na,K)]0.25/MgO with the absorption time was analyzed. A series of CO2 absorption experiments were carried out in a fixed-bed absorber (ID 6 mm) at 350 ˚C in 100% CO2. For each experiment, once the preset absorption time (0.5, 1, 2, 4, 20 or 120 min) was reached, the CO2 absorption process was stopped by switching to pure N2 together with fast cooling to room temperature to prevent sorbent regeneration, after which the sample was withdrawn and kept in N2 until analysis. [Figures 5 and 6] As shown in the time-evolution XRD curves (Figure 5), for the fresh sorbent (t = 0), diffraction peaks belonging to MgO (JCPDS 75-0447), Na2CO3 (JCPDS 37-0451), K2CO3 (JCPDS 49-1093), NaNO3 (JCPDS 36-1474), KNO3 (JCPDS 71-1558) and LiNaCO3 (JCPDS 34-1193) are observed, 10


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but no LiNO3 was detected. It appears that the ionic recombination occurs during the sorbent preparation because neither NaNO3 nor LiNaCO3 is added as precursor. With regards to the absence of LiNO3, two reasons can be put forward: first, a part of Li+ ions are consumed in the formation of LiNaCO3; and second, LiNO3 is highly dispersed as small crystallites and/or poor crystallites on MgO. Indeed, the LiNO3 phase is also undetectable in [(Li,K)]0.25/MgO for which the disturbance resulting from carbonates is excluded (Figure 6). After 0.5 min of CO2 absorption, the diffraction peaks assigned to MgCO3 (JCPDS 08-0479) emerge as expected and accordingly the MgO peaks are weakened, with the former becoming stronger and the latter weaker as the CO2 absorption time increases. In addition, the LiNaCO3 phase disappears, accompanied by the formation of Li2CO3 (2θ = 30.6˚, JCPDS 22-1141), implying that LiNaCO3 is transformed into Li2CO3 and Na2CO3 during CO2 absorption. A strange but interesting phenomenon is the presence of new peaks at 2θ = 26.6˚ and 27.4˚, which correspond to KNO2 (JCPDS 79-1985) and K2C2O6 (JCPDS 22-0807), respectively. Moreover, the peak intensity of K2C2O6 decreases with increasing the absorption time and becomes nearly zero after 4 min of CO2 absorption, while few changes are seen in KNO2. In the meantime, K2Mg(CO3)2 (2θ = 32.6˚, JCPDS 33-1495) and Na2Mg(CO3)2 (2θ = 34.4˚, JCPDS 24-1227) are detected. To the best of our knowledge, it is the first time that such a behavior is observed for the alkali metal salt-promoted MgO-based sorbent. [Figure 7] Based on the absorption curves (Figure 4) and the XRD patterns (Figure 5) as well as the literature13,15 we propose a possible mechanism for the CO2 absorption by [(Li,K)x-(Na,K)]y/MgO, as illustrated in Figure 7. At the first stage (0 < t ≤ 4 min), K2CO3 dissolved in the molten nitrates reacts with NO3- and CO2 to form K2C2O6 and NO2- (eq 1); K2C2O6 is unstable19 and further reacts 11



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with the dissolved MgO to form the double salt K2Mg(CO3)2 (eq 2). Meanwhile, the reaction between dissolved Na2CO3, MgO and CO2 yields Na2Mg(CO3)2 (eq 3), as reported by Zhang et al.14 and Vu et al.17 NO3- + K2CO3 + CO2 → NO2- + K2C2O6

(1)

K2C2O6 + MgO → K2Mg(CO3)2 + 0.5O2

(2)

Na2CO3 + MgO + CO2 → Na2Mg(CO3)2

(3)

[Figure 8] The partial dissolution of carbonates in molten salts have been found by some researchers,20 which is also verified here by DSC analysis as follows. The melting points of LiNO3, NaNO3, KNO3, Na2CO3, and K2CO3 are 255, 307, 334, 858 and 901 ˚C, respectively. Therefore, the endothermic peaks with temperature lower than 334 ˚C in the DSC profiles (Figure 8) are assigned to melting of nitrates, while those higher than 334 ˚C correspond to melting of carbonates. As regards the melting of nitrates, [(Li,K)2-Na]0.25/MgO has the lowest temperature (109 ˚C), whereas [(Li,K)2-K]0.25/MgO has the highest (137 ˚C). It has been reported that a binary nitrate (LiNO3:KNO3 = 0.44:0.56) has a melting point of about 137 ˚C,18 while for a ternary nitrate (LiNO3:NaNO3:KNO3 = 0.30:0.18:0.52) it is around 133 ˚C.15 It appears that the ternary nitrate has a lower melting point than the binary one. Considering that both [(Li,K)2-Na]0.25/MgO and [(Li,K)2-(Na,K)]0.25/MgO contain NaNO3 that is ultimately derived from ionic recombination during the sorbent preparation, it is reasonable that they have a lower melting point than [(Li,K)2-K]0.25/MgO. The peaks at 355 ˚C for [(Li,K)2-K]0.25/MgO, 346 ˚C for [(Li,K)2-Na]0.25/MgO and 352 ˚C for [(Li,K)2-(Na,K)]0.25/MgO are associated with melting of K2CO3, Na2CO3 and their mixture, respectively, indicating that a part of carbonate will be dissolved in the molten nitrate at 12


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the absorption temperature of 350 ˚C. In addition, all the temperatures are much lower than the melting point of individual carbonate without addition of nitrate, implying that nitrate can facilitate the dissolution of carbonate. These dissolved carbonates are closely associated with the enhanced absorption ability of [(Li,K)x-(Na,K)]y/MgO in that: (1) it has been reported that the dissolved Na2CO3 can facilitate the CO2 absorption through the formation of Na2Mg(CO3)2,14,17 which is in line with the observed decreased induction time and increased CO2 uptake of [(Li,K)2-Na]0.15/MgO compared to [(Li,K)]0.15/MgO (Figure 4); and (2) in the case of K2CO3 as a promoter, the immediate formation of the intermediate compound K2C2O6 accelerates the CO2 dissolution in the molten salt on the one hand, and on the other hand one of the products, alkali metal nitrite, possesses a higher concentration of oxide ions (O2-) in the molten state than nitrate and thus provides more CO32-.16,21 Both factors are favorable for improving the CO2 absorption ability, which well explains the zero induction time found for [(Li,K)2-K]0.15/MgO and [(Li,K)2-(Na,K)]0.15/MgO. Harada and Hatton16 also showed that the nitrite/nitrate-coated MgO sorbent had faster and higher CO2 uptake than the nitrate-coated MgO. For [(Li,K)2-Na]0.15/MgO, K2C2O6 and KNO2 are not detected in the timeevolution XRD patterns (not shown here), and the induction time is therefore present, being about 0.5 min (inset in Figure 4). MgO + CO2 → MgCO3

(4)

Besides the above reactions, the reaction between MgO and CO2 in the molten salt (eq 4), which essentially occurs between Mg2+ derived from the dissolved MgO and CO32- from the dissolved CO2 and O2-, can take place at the first stage, but its contribution to the CO2 uptake at this stage is less than those of other reactions because a threshold CO2 concentration in the molten salt is 13



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required for achieving a respectable reaction rate of MgO with CO2.15,17 The above analysis demonstrates that the addition of alkali metal carbonate can shorten the time needed for reaching the threshold, which increases in the order of [(Li,K)2-K]0.15/MgO < [(Li,K)2-(Na,K)]0.15/MgO < [(Li,K)2-Na]0.15/MgO < [(Li,K)]0.15/MgO. At the second stage (4 < t ≤ 20 min), the CO2 uptake of sorbent is accelerated, which is mainly ascribed to the MgO-CO2 reaction. The sorbents promoted with both carbonate and nitrate, i.e., [(Li,K)2-(Na,K)]0.15/MgO, [(Li,K)2-K]0.15/MgO and [(Li,K)2-Na]0.15/MgO, have a CO2 absorption rate much faster than [(Li,K)]0.15/MgO promoted only with nitrate, which is reasonable because the former can provide additional CO32- due to the dissolution of carbonates in molten salts as mentioned above, and thus enhance the driving force for CO2 absorption. At the end of this stage, [(Li,K)2-(Na,K)]0.15/MgO rather than [(Li,K)2-K]0.15/MgO has the highest CO2 uptake (about 0.69 gCO /gsorbent ), which is probably because the former could dissolve more MgO in the molten salt 2

due to its lower melting point (Figure 8). On the other hand, although [(Li,K)2-Na]0.15/MgO has the lowest melting point (Figure 8) and thus probably the highest concentration of dissolved MgO, its relatively lower CO2 concentration in the molten salt compared to that of [(Li,K)2-(Na,K)]0.15/MgO due to the promotion of K2CO3, limits the conversion of MgO. At the third stage (t > 20 min), the absorption rate is greatly decreased and this stage can be called the product layer diffusion-controlled stage, just as for the CaO-CO2 reaction.22,23 This is because with the continuous nucleation and growth of the MgCO3 crystals as well as the growth of the MgCO3 product layer, when the product layer thickness exceeds a critical value, the apparent absorption rate is controlled by the slow counter-current diffusion of inward CO32- and outward O2through the MgCO3 layer. 14


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It should be noted that the transition time from the first stage to the second one or from the second to the third varies from each other, depending on the composition of sorbent. For example, for [(Li,K)2-K]0.15/MgO, [(Li,K)2-Na]0.15/MgO and [(Li,K)2-(Na,K)]0.15/MgO, the transition times from the first stage to the second one are about 2, 4 and 4 min, respectively. As regards the transition time from the second stage to the third one, it follows the order [(Li,K)]0.15/MgO > [(Li,K)2-Na]0.15/MgO > [(Li,K)2-(Na,K)]0.15/MgO > [(Li,K)2-K]0.15/MgO. The above mechanism can explain the experimental phenomenon found in this work, but further investigations are still needed, e.g., the relationship of the CO2 or CO32- concentration in the molten salt to the property of the alkali metal nitrate and carbonate. In addition, for all (or most) of the alkali metal salt-promoted MgO-based sorbents, is there a common critical product layer thickness marking the onset of the slow product layer diffusion-controlled stage? In the case of CaO-based sorbents, the common critical product layer thickness is around 50 nm.24 If the particle size of MgO is close to or even smaller than the critical product layer thickness, a high MgO utilization efficiency could be reasonably expected because the effect of the rather slow product layer diffusion will be greatly reduced or probably non-existent. This explains the improved CO2 uptake of nanosheet [(Li,K)2-(Na,K)]0.15/MgO (30-40 nm) compared to the granular counterpart (ca. 200 nm), as shown in Figure 1, and the MgO conversion (in terms of MgO rather than the whole sorbent) of nanosheet [(Li,K)2-(Na,K)]0.15/MgO is as high as about 91%. 3.4. Effects of Nitrate/Carbonate and Total Salts/MgO Molar Ratios. Figure 9 shows the effect of the nitrate/carbonate molar ratio (x = 0.5~3) on the CO2 uptake of (nitrate-carbonate)promoted MgO-based sorbents, where the molar ratio of total salts (nitrate + carbonate) to MgO is fixed at 0.15. All sorbents display a three-stage CO2 absorption curve. Moreover, there exists an 15



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optimal x at 2, either above or below which would cause a decrease in the total CO2 uptake. This is probably because increasing x means, on the one hand, an increase in the amount of nitrate, which allows for more dissolution of MgO in the molten nitrate and thus benefits the CO2 absorption, but on the other hand, a decrease in the amount of carbonate, which reduces the CO32- concentration in the molten salt and consequently adversely affects the CO2 absorption. The tradeoff between these two conflicting factors results in an optimal nitrate/carbonate molar ratio. The inset in Figure 9 indicates that at the first stage of CO2 absorption, the sorbent with higher content of carbonate has higher absorption rate, which confirms the CO2 absorption mechanism proposed above. [Figures 9-11] The cyclic stability of these sorbents is presented in Figure 10. Despite the fact that all sorbents are subject to decay in CO2 capture capacity with cycling, [(Li,K)2-(Na,K)]0.15/MgO has the best stability, whose total CO2 uptake remains almost unchanged after 15 cycles. The main reason for the deteriorated CO2 capture performance with cycling is the formation of large agglomerates originated from fusion and coalescence of neighboring grains (Figure 11). The one-cycle-used [(Li,K)2-(Na,K)]0.15/MgO displays a granular morphology (Figure 11(a)) different from the original sheet-like shape (Figure 4(d)), and an agglomeration of small grains emerges (Figure 11(b)), though the sheet-like structure is still preserved in some regions (Figure 11(c)). As the number of cycles increases, the size of the agglomerate becomes larger (Figure 11(d) for the 5-cycle-used sorbent and Figure 11(e) for the 30-cycle-used), which is indeed so large that HRTEM beam cannot pass through it. Another possible reason for the decreased cyclic activity is the redistribution of molten salt in the sorbent. As shown in Figures 11(e) and 11(f), the 30-cycle-used sample exhibits some smooth surfaces, which are probably owing to the molten salt that migrates to the outer 16


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surface of the sorbent during cyclic operation. A similar result about the molten salt redistribution was reported by Lee et al.25 in the study of (KNO3-K2CO3)-promoted MgO-based sorbents. [Figure 12] Figure 12 describes the dependence of CO2 uptake of sorbents on the molar ratio of total salts to MgO (y = 0.05~0.75), where the nitrate/carbonate molar ratio is fixed at 2. The sorbent with a lower salt content or a higher MgO content shows, as anticipated, a higher total CO2 uptake, except for [(Li,K)2-(Na,K)]0.05/MgO. Although [(Li,K)2-(Na,K)]0.05/MgO has the largest amount of MgO (89 wt%), the highest surface area (37.39 m2/g) and pore volume (0.199 cm3/g) among these sorbents (Table 1), its salt content (11 wt%) is probably too low to markedly facilitate the CO2 absorption in the molten salt as compared to other sorbents, and as a result its MgO utilization is low. As shown in the inset in Figure 12, the MgO conversion of [(Li,K)2-(Na,K)]0.05/MgO at either 20 or 120 min of absorption, is the lowest among all the sorbents, while [(Li,K)2-(Na,K)]0.15/MgO with 73 wt% of MgO content has the highest conversion. An interesting phenomenon is observed from the inset: although the surface areas of some sorbents are very small, e.g., [(Li,K)2-(Na,K)]0.65/MgO and [(Li,K)2-(Na,K)]0.75/MgO with nearly zero of surface area, implying complete coverage of the MgO surface with alkali metal salts, the MgO conversion is far beyond zero. This result is different from that reported by Zhang et al.13 who noted that the capacity of the sorbent with a much smaller surface area had an extremely low MgO conversion (almost zero), based on which a mechanism for CO2 absorption at the triple phase boundaries was proposed. Therefore, the mechanism suggested by Zhang et al.13 seems to have some limitations. The corresponding XRD patterns of the sorbents are given in Figure 13. With increasing y, the peak intensity of alkali metal salts increases as expected, and correspondingly, the MgO peak 17



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decreases gradually. In addition, like [(Li,K)2-(Na,K)]0.25/MgO discussed above, the LiNO3 phase is not found in all other [(Li,K)2-(Na,K)]y/MgO. The stability of several [(Li,K)2-(Na,K)]y/MgO sorbents is shown in Figure 14. It appears that [(Li,K)2-(Na,K)]0.05/MgO instead of [(Li,K)2-(Na,K)]0.15/MgO has the best stability, whose total CO2 uptake remains constant after the first cycle. Unfortunately, the low uptake (around 0.05 gCO /gsorbent ) limits its application. 2

[Figures 13 and 14] 3.5. Effects of Temperature and CO2 Partial Pressure. Figure 15(A) shows the CO2 uptake profiles of [(Li,K)2-(Na,K)]0.15/MgO at different temperatures. During the initial stage of the CO2 absorption process (inset), the lower the temperature, the higher the rate, which is mostly due to the higher CO2 concentration in the molten salt at a lower temperature.26 With the progress of CO2 absorption, the disadvantage of slow kinetics at low temperatures is more prominent, which finally gives rise to a low CO2 uptake after 120 min of absorption for the low temperature operation. Figure 15(B) shows the CO2 uptake profiles of [(Li,K)2-(Na,K)]0.15/MgO at different CO2 partial pressures or bulk concentrations. A higher CO2 concentration is no doubt favorable for CO2 absorption, resulting in higher rate and uptake. Note that the CO2 uptake at 20% of bulk CO2 concentration is very low, which is mainly caused by the slow rate of CO2 absorption derived from the small driving force (the difference between the bulk and equilibrium CO2 partial pressure). However, once the concentration is increased to 30-40%, a marked increase is observed, with a CO2 uptake of about 0.66 gCO /gsorbent for 30% CO2 and 0.70 gCO /gsorbent for 40% CO2 after 120 2

2

min of absorption. This result is particularly attractive, as the concentration of CO2 exiting the water-gas shift reactor in an IGCC power plant is around 40%.27,28 [Figure 15] 18


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In order to evaluate the potential of [(Li,K)2-(Na,K)]0.15/MgO for the pre-combustion CO2 capture, 20 consecutive absorption-regeneration cycles are performed using 40% CO2 (balance N2) as the absorption gas. As displayed in Figure 16, after 12 cycles of operation, the total CO2 uptake remains unchanged, being about 0.52 gCO /gsorbent , demonstrating the good stability of the sorbent. 2

In addition, the MgO conversion is maintained at around 65% (inset in Figure 16), indicating a relatively high utilization of MgO. [Figure 16] [Table 2] 3.6. Comparison with Other Sorbents. The cyclic CO2 capture performance of [(Li,K)2-(Na,K)]0.15/MgO is compared with other alkali metal salt-promoted MgO-based sorbents in the literature. As summarized in Table 2, [(Li,K)2-(Na,K)]0.15/MgO has excellent CO2 capture performance comparable to that of [LiNO3-(Na,K)NO2)]0.2/MgO,16 the best MgO-based sorbent ever reported in the literature. However, [LiNO3-(Na,K)NO2)]0.2/MgO tested under 100% CO2 has a total uptake of 0.53 gCO /gsorbent at the 20th cycle, which can be obtained on 2

[(Li,K)2-(Na,K)]0.15/MgO under 40% CO2 at the same cycle. In this respect, our sorbent is superior to [LiNO3-(Na,K)NO2)]0.2/MgO.

4. CONCLUSIONS A simple “precipitation-deposition” two-step method was developed to prepare alkali metal salt-promoted, nanosheet MgO-based CO2 sorbents with an average thickness of about 30-40 nm, which showed much higher CO2 uptake at 350 ˚C compared to either a nanosheet MgO without alkali metal salt or a commercial MgO with alkali metal salt. The improved CO2 capture capacity 19



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of the novel sorbents was mainly attributed to the nanosheet structure and the mixed alkali metal nitrate and carbonate: the former was able to reduce the adverse effect of the slow product layer diffusion-controlled stage, and the latter can promote the dissolution of CO2, MgO and carbonate in the molten salt, which in turn accelerate the CO2 absorption. A reasonable mechanism based on the formation of K2C2O6, nitrite, K2Mg(CO3)2, Na2Mg(CO3)2 and MgCO3 was put forward to describe the CO2 absorption process on the MgO-based sorbent promoted by mixed alkali metal nitrate (LiNO3 and KNO3) and carbonate (Na2CO3 and K2CO3). In addition, the absorption process mainly occurred in the molten salt, especially prior to the product layer diffusion-controlled stage. An increase in the absorption temperature was found to decrease the CO2 absorption rate at the initial stage, but increase the final CO2 uptake except for 375 ˚C. By increasing the CO2 concentration from 20% to 100%, both the absorption rate and the CO2 uptake were increased. Among all the MgO-based sorbents prepared in this study, the best one ([(Li,K)2-(Na,K)]0.15/MgO) exhibited high CO2 uptake and good stability during cyclic operation, with a stable uptake of 0.52 gCO /gsorbent 2

and MgO conversion of 65% after 20 consecutive cycles (40% CO2 at 350 ºC for absorption, 100% N2 at 400 ºC for regeneration).

ACKNOWLEDGMENTS

Financial support from the National Natural Science Foundation of China (21276076), the Program for New Century Excellent Talents in University (NCET-13-0801), the Fundamental Research Funds for the Central Universities (222201718003) and the “111” Project (B08021) is gratefully acknowledged.

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REFERENCES (1) International Energy Agency. CO2 emissions from fuel combustion: highlights; Organisation for Economic Co-operation and Development/International Energy Agency (OECD/IEA): Paris, 2015. (2) Spigarelli, B. P.; Kawatra, S. K. Opportunities and challenges in carbon dioxide capture. J. CO2 Utilization 2013, 1, 69-87. (3) Abanades, J. C.; Arias, B.; Lyngfelt, A.; Mattisson, T.; Wiley, D. E.; Li, H.; Ho, M. T.; Mangano, E.; Brandani, S. Emerging CO2 capture systems. Int. J. Greenhouse Gas Control 2015, 40, 126-166. (4) Wang, Q.; Luo, J.; Zhong, Z.; Borgna, A. CO2 capture by solid adsorbents and their applications: current status and new trends. Energy Environ. Sci. 2011, 4, 42-55. (5) Dumée, L.; Scholes, C.; Stevens, G.; Kentish, S. Purification of aqueous amine solvents used in post combustion CO2 capture: a review. Int. J. Greenhouse Gas Control 2012, 10, 443-455. (6) Kierzkowska, A. M.; Pacciani, R.; Müller, C. R. CaO-based CO2 sorbents: from fundamentals to the development of new, highly effective materials. ChemSusChem. 2013, 6, 1130-1148. (7) Wang, J.; Huang, L.; Yang, R.; Zhang, Z.; Wu, J.; Gao, Y.; Wang, Q.; O'Hare, D.; Zhong, Z. Recent advances in solid sorbents for CO2 capture and new development trends. Energy Environ. Sci. 2014, 7, 3478-3518. (8) Wang, S.; Yan, S.; Ma, X.; Gong, J. Recent advances in capture of carbon dioxide using alkali-metal-based oxides. Energy Environ. Sci. 2011, 4, 3805-3819. (9) Mayorga, S. G.; Weigel, S. J.; Gaffney, T. R.; Brzozowski, J. R. Carbon dioxide adsorbents

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containing magnesium oxide suitable for use at high temperatures. U.S. Pat. 2001/6280503 B1, 2001. (10) Hamad, E. Z.; Al-sadat, W. I.; Coleman, L.; Shen, J. P.; Gupta, R. Mixed salt CO2 sorbent, process for making and uses thereof. U.S. Pat. 2013/0195742 A1, 2013. (11) Zhang, K.; King, D. L.; Li, X. S.; Li, L.; Rohatgi, A.; Singh, P. System, sorbent, and processes for capture and release of CO2. U.S. Pat. 2013/0287663 A1, 2013. (12) Zhang, K.; Li, X. S.; Duan, Y.; King, D. L.; Singh, P.; Li, L. Roles of double salt formation and NaNO3 in Na2CO3-promoted MgO absorbent for intermediate temperature CO2 removal. Int. J. Greenhouse Gas Control 2013, 12, 351-358. (13) Zhang, K.; Li, X. S.; Li, W.Z.; Rohatgi, A.; Duan, Y.; Singh, P.; Li, L.; King, D. L. Phase transfer-catalyzed fast CO2 absorption by MgO-based absorbents with high cycling capacity. Adv. Mater. Interfaces 2014, 1, 1400030. (14) Zhang, K.; Li, X. S.; Chen, H.; Singh, P.; King, D. L. Molten salt promoting effect in double salt CO2 absorbents. J. Phys. Chem. C. 2016, 120, 1089-1096. (15) Harada, T.; Simeon, F.; Hamad, E. Z.; Hatton, T. A. Alkali metal nitrate-promoted high-capacity MgO adsorbents for regenerable CO2 capture at moderate temperatures. Chem. Mater. 2015, 27, 1943-1949. (16) Harada, T.; Hatton, T. A. Colloidal nanoclusters of MgO coated with alkali metal nitrates/ nitrites for rapid, high capacity CO2 capture at moderate temperature. Chem. Mater. 2015, 27, 8153-8161. (17) Vu, A.T.; Ho, K.; Jin, S.; Lee, C. H. Double sodium salt-promoted mesoporous MgO sorbent with high CO2 sorption capacity at intermediate temperatures under dry and wet conditions. 22


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Chem. Eng. J. 2016, 291, 161-173. (18) Zhang, X.; Xu, K.; Gao, Y. The phase diagram of LiNO3-KNO3. Thermochimica Acta 2002, 385, 81-84. (19) Dinnebier, R. E.; Vensky, S.; Stephens, P. W.; Jansen, M. Crystal structure of K2[C2O6]-first proof of existence and constitution of a peroxodicarbonate ion. Angew. Chem. Int. Ed. 2002, 41, 1922-1924. (20) Temple, R. B.; Lockyer, C. J. Solubility data for Na2CO3 and K2CO3 dissolved in molten NaNO3-KNO3 eutectic. Aust. J. Chem. 1979, 32, 1849-1850. (21) Kust, R. N.; Burke, J. D. Thermal decomposition in alkali metal nitrate melts. Inorg. Nucl. Chem. Lett. 1970, 6, 333-335. (22) Bhatia, S. K.; Perlmutter, D. D. Effect of the product layer on the kinetics of the CO2-lime reaction. AIChE J. 1983, 29, 79-86. (23) Zhou, Z.; Xu, P.; Xie, M.; Cheng, Z.; Yuan, W. Modeling of the carbonation kinetics of a synthetic CaO-based sorbent. Chem. Eng. Sci. 2013, 95, 283-290. (24) Alvarez, D.; Abanades, J. C. Determination of the critical product layer thickness in the reaction of CaO with CO2. Ind. Eng. Chem. Res. 2005, 44, 5608-5615. (25) Lee, C. H.; Kwon, H. J.; Lee, H. C.; Kwon, S.; Jeon, S. G.; Lee, K. B. Effect of pH-controlled synthesis on the physical properties and intermediate-temperature CO2 sorption behaviors of K-Mg double salt-based sorbents. Chem. Eng. J. 2016, 294, 439-446. (26) Novozhilov, A. L.; Bamburov, V. G.; Fedotova N. N. Solubility of carbon dioxide in molten alkali-metal nitrates. Russ. J. Inorg. Chem. 2007, 52, 1679-1681. (27) Olajire, A. A. CO2 capture and separation technologies for end-of-pipe applications ‒ a review. 23



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Energy 2010, 35, 2610-2628. (28) Bhatta, L. K. G.; Subramanyam, S.; Chengala, M. D.; Olivera, S.; Venkatesh, K. Progress in hydrotalcite like compounds and metal-based oxides for CO2 capture: a review. J. Clean. Prod. 2015, 103, 171-196. (29) Xiao, G.; Singh, R.; Chaffee, A.; Webley, P. Advanced adsorbents based on MgO and K2CO3 for capture of CO2 at elevated temperatures. Int. J. Greenhouse Gas Control 2011, 5, 634-639. (30) Liu, M.; Vogt, C.; Chaffee, A. L.; Chang, S. L. Y. Nanoscale structural investigation of Cs2CO3-doped MgO sorbent for CO2 capture at moderate temperature. J. Phys. Chem. C. 2013, 117, 17514-17520. (31) Vu, A. T.; Park, Y.; Jeon, P. R.; Lee, C. H. Mesoporous MgO sorbent promoted with KNO3 for CO2 capture at intermediate temperatures. Chem. Eng. J. 2014, 258, 254-264. (32) Lee, C. H.; Mun, S.; Lee, K. B. Characteristics of Na-Mg double salt for high-temperature CO2 sorption. Chem. Eng. J. 2014, 258, 367-373.

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Table 1. Textural Properties of MgO and Alkali Metal Salt-Promoted MgO-Based Sorbents Sorbent

MgO content

BET surface 2

Pore volumec 3

Average pore

(wt%)

area (m /g)

(cm /g)

diameterc (nm)

MgO(c)a

100

55.68

0.235

19.2

MgOb

100

139.37

0.287

7.4

[(Li,K)]0.15/MgO

75

10.10

0.039

44.4

[(Li,K)2-Na]0.15/MgO

74

15.26

0.091

28.5

[(Li,K)2-K]0.15/MgO

72

19.02

0.125

26.7

[(Li,K)2-(Na,K)]0.05/MgO

89

37.39

0.199

25.2

[(Li,K)2-(Na,K)]0.15/MgO

73

14.97

0.120

35.9

[(Li,K)2-(Na,K)]0.25/MgO

62

12.19

0.093

38.8

[(Li,K)2-(Na,K)]0.35/MgO

54

4.05

0.026

70.1

[(Li,K)2-(Na,K)]0.45/MgO

47

3.32

0.022

85.6

[(Li,K)2-(Na,K)]0.55/MgO

42

2.51

0.014

102.5

[(Li,K)2-(Na,K)]0.65/MgO

38

0.62

0.011

144.8

[(Li,K)2-(Na,K)]0.75/MgO

35

0.45

0.008

157.9

73

11.85

0.064

31.3

[(Li,K)2-(Na,K)]0.15/MgO(c) a

b

c

Purchased from Aladdin Reagent Inc; Prepared in this study; Determined from the adsorption branch by the

BJH method.

25



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Table 2. Comparison of Different Alkali Metal Salt-Promoted MgO-Based Sorbents in Cyclic Operation MgO

Sorbent

content

of cycles

Total CO2 uptake at last cycle

Ref

(wt%) 80

330/60/100% CO2

385/30/100% N2

9

0.26

13

77

360/90/100% CO2

400/60/100% N2

30

0.26

14

75

300/60/100% CO2

350/30/100% N2

40

0.30

15

[LiNO3-(Na,K)NO2)]0.2/MgOb

72

340/60/100% CO2

450/30/100% N2

20

0.53

16

[(NaNO3)2-Na2CO3]0.3/MgO

59

325/60/100% CO2

450/10/100% N2

14

0.32

17

[(KNO3)0.18-K2CO3]1.3/MgO

19

350/30/100% CO2

500/60/100% N2

10

0.04

25

[K2CO3]0.95/MgO

23

375/20/100% CO2

375/30/100% N2

17

0.07

29

[Cs2CO3]0.18/MgO

41

370/12/50% CO2(Ar)

370/24/100% Ar

25

0.02

30

[KNO3]0.2/MgO

66

325/20/100% CO2

450/30/100% N2

12

0.08

31

[(NaNO3)2.39-Na2CO3]0.61/MgO

40

325/180/100% CO2

500/100/100% N2

7

0.15

32

[(Li,K)2-(Na,K)]0.15/MgOc

73

350/20/100% CO2

400/15/100% N2

30

0.40

this work

c

73

350/60/40% CO2(N2)

400/15/100% N2

20

0.52

this work

[(NaNO3)1.36-Na2CO3]0.13/MgO [(Li,Na,K)NO3]0.15/MgO

a

[(Li,K)2-(Na,K)]0.15/MgO

Regeneration

Number

Absorption

[NaNO3]0.12/MgO

a

Temperature (˚C) / time (min) / atmosphere

(gCO /gsorbent) 2

LiNO3:NaNO3:KNO3 (molar ratio) = 0.30:0.18:0.52; b LiNO3:NaNO2:KNO2 (molar ratio) = 0.30:0.18:0.52; c LiNO3:KNO3 (molar ratio) =

0.44:0.56, Na2CO3:K2CO3 (molar ratio) = 0.5:0.5.

26


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Figure Captions Figure 1. CO2 uptake of [(Li,K)2-(Na,K)]0.15/MgO and [(Li,K)2-(Na,K)]0.15/MgO(c).(absorption: 350 ˚C, 1 atm, 100% CO2) Figure 2. FESEM (a-d) and HRTEM (e-h) images of MgO and alkali metal salt-promoted MgO: (a) commercial MgO(c); (b) [(Li,K)2-(Na,K)]0.15/MgO(c); (c,e,f) home-made MgO; (d,g,h) [(Li,K)2-(Na,K)]0.15/MgO. Figure 3. N2 adsorption-desorption isotherms of commercial MgO(c), home-made MgO and the corresponding [(Li,K)2-(Na,K)]0.15/MgO(c) and [(Li,K)2-(Na,K)]0.15/MgO sorbents. Figure 4. CO2 uptake of MgO, [(Li,K)]0.15/MgO, [(Li,K)2-Na]0.15/MgO, [(Li,K)2-K]0.15/MgO and [(Li,K)2-(Na,K)]0.15/MgO (absorption: 350 ˚C, 1 atm, 100% CO2). Figure 5. Variation of the XRD pattern of [(Li,K)2-(Na,K)]0.25/MgO with the CO2 absorption time (absorption: 350 ˚C, 1 atm, 100% CO2). Figure 6. XRD pattern of [(Li,K)]0.25/MgO. Figure 7. Proposed mechanism of CO2 absorption on the MgO-based sorbent promoted by mixed alkali metal nitrate and carbonate.

Figure

8.

DSC

profiles

of

[(Li,K)2-K]0.25/MgO,


and

[(Li,K)2-(Na,K)]0.25/MgO. Figure 9. CO2 uptake of various [(Li,K)x-(Na,K)]0.15/MgO (x = 0.5~3) (absorption: 350 ˚C, 1 atm, 100% CO2).

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 10. Variation of total CO2 uptake of [(Li,K)x-(Na,K)]0.15/MgO (x = 0.5~3) during 30 cycles. (absorption: 350 ˚C, 1 atm, 100% CO2, 20 min; regeneration: 400 ˚C, 1 atm, 100% N2, 15 min) Figure 11. FESEM (a,d-f) and HRTEM (b,c) images of used [(Li,K)2-(Na,K)]0.15/MgO: (a-c) 1-cycle used; (d) 5-cycle used; (e,f) 30-cycle used. (absorption: 350 ˚C, 1 atm, 100% CO2, 20 min; regeneration: 400 ˚C, 1 atm, 100% N2, 15 min) Figure 12. CO2 uptake of various [(Li,K)2-(Na,K)]y/MgO (y = 0.05~0.75) (absorption: 350 ˚C, 1 atm, 100% CO2). The inset shows the variation of MgO conversion and BET surface area of sorbent with the mass percentage of total salts (nitrate + carbonate) in the sorbent. Figure 13. XRD patterns of [(Li,K)2-(Na,K)]y/MgO (y = 0.05~0.75). Figure 14. Comparison of the total CO2 uptake of various [(Li,K)2-(Na,K)]y/MgO (y = 0.05~0.35). (absorption: 350 ˚C, 1 atm, 100% CO2, 20 min; regeneration: 400 ˚C, 1 atm, 100% N2, 15 min) Figure 15. Variation of CO2 uptake of [(Li,K)2-(Na,K)]0.15/MgO with (A) temperature and (B) bulk CO2 concentration. Figure 16. CO2 uptake of [(Li,K)2-(Na,K)]0.15/MgO during 20 cycles (absorption: 350 ˚C, 1 atm, 40% CO2/60% N2, 60 min; regeneration: 400 ˚C, 1 atm, 100% N2, 15 min). The inset shows the variation of MgO conversion with the number of cycles.

28


Page 28 of 45

Page 29 of 45

Figure 1

0.8 [(Li,K)2-(Na,K)]0.15/MgO

0.7

2

CO2 uptake / gCO /gsorbent

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[(Li,K)2-(Na,K)]0.15/MgO(c)

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

20

40

60 80 Time / min

29


100

120


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2

30


Page 30 of 45

Page 31 of 45

Figure 3

250 MgO

200

3

Volume adsorbed / cm /g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MgO(c) [(Li,K)2-(Na,K)]0.15/MgO [(Li,K)2-(Na,K)]0.15/MgO(c)

150 100 50 0 0.0

0.2

0.4

0.6

0.8

Relative pressure

31


1.0


Figure 4

0.8

[(Li,K)2-(Na,K)]0.15/MgO

0.7

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 45

0.6

[(Li,K)2-K]0.15/MgO

0.5


0.4

[(Li,K)]0.15/MgO 0.05 0.04

0.3

0.03 0.02

0.2

0.01

0.1

0.00

MgO

0

1

2

3

4

5

0.0 0

20

40

60 80 Time / min

32


100

120

Page 33 of 45

Figure 5

1-MgO; 2-MgCO3; 3-Na2CO3; 4-K2CO3; 5-Li2CO3;

2,12

6-LiNaCO3; 7-NaNO3; 8-KNO3; 9-KNO2; 10-K2C2O6; 11-Na2Mg(CO3)2; 12-K2Mg(CO3)2 1,2

8 2 2 8 9 7,8 3,8 8 5 11 8 2 8 8

8

2 2

2 21,2 2 2

22

120 min

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 min

4 min 2 min 11

1 min

1,2

10 2,12 9 5 21 2

2

2

1,2

0.5 min

1

8 6

10

20

8 8

7,8 3,8 1 8 48 8 8 8 8

30

1

40 50 60 2θ / degree

1

70

33


1

0 min

80

90


Figure 6

1

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 45

1-MgO; 2-KNO3

2

2 2 2

10

20

1

2

2 2 2 1 2 2 2 2 2 2

30

40

1

50

2

2

60

70

2θ / degree

34


1

80

Page 35 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7

CO2 [Na+…CO32-…Na+] Carbonates

NO3- +

Na2Mg(CO3)2

[K+…CO32-…K+]

[Mg2+…O2-]

NO2- [K+…C2O62-…K+]

K2Mg(CO3)2

MgCO3

Molten nitrates

-Mg-O-Mg-O-Mg-O-Mg-O …

…O

1st stage (0 < t ≤ 2 min) CO2 CO32-

Carbonates MgCO3

O2Mg2+

NO3NO2Molten nitrates

-Mg-O-Mg-O-Mg-O-Mg…

…O

2nd stage (2 < t ≤ 20 min) CO2

Carbonates

O2-

CO32-

NO3-

NO2Molten nitrates MgCO3

-Mg-O-Mg-O-Mg-O-Mg…

…O

3rd stage (t > 20 min)

35



Figure 8

o

355 C o

313 C

o

137 C

Heat flow / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 45

o

346 C

[(Li,K)2-K]0.25/MgO

o

o

109 C

328 C

o

213 C

o

162 C

o

352 C


o

324 C

o

133 C

o

304 C

[(Li,K)2-(Na,K)]0.25/MgO

50

100

150

200

250

300 o

Temperature / C

36


350

400

Page 37 of 45

Figure 9

0.8

[(Li,K)x-(Na,K)]0.15/MgO

0.7

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

0.6

1

2 x = 0.5

0.5 0.4

0.10 0.08

0.3

0.06 0.04

0.2

0.02

0.1

0.00

0

1

2

3

4

5

0.0 0

20

40

60

80

Time / min

37


100

120


Figure 10

0.8 [(Li,K)x-(Na,K)]0.15/MgO

0.7

x= 0.5 x=1 x=2 x=3

0.6

2

Total CO2 uptake / gCO /gsorbent

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 45

0.5 0.4 0.3 0.2 0.1 0.0 0

5

10 15 20 Number of cycles

38


25

30

Page 39 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 11

39



Figure 12

0.8 [(Li,k)2-(Na,K)]y/MgO y = 0.15

0.7 0.6

0.25

0.5

0.35

2


0.45

0.4

0.55 0.65

0.3 0.2

0.05

0.1 0.0 0

20

40

120 min

80 20 min

60

30 20

40

10

20 0

40

2

100

0

0 20 40 60 80 Total salts / wt%

60 80 Time / min

40


100

Surface area / m /g

0.75 MgO conversion / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 45

120

Page 41 of 45

Figure 13

1-MgO; 2-KNO3; 3-Na2CO3; 4-K2CO3;

[(Li,K)2-(Na,K)]y/MgO 2 2 5 2

1 6,2 3,2 42 12 22 2 2

5-LiNaCO3; 6-NaNO3 1

1

1

y=0.75 0.65 0.55

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.45 0.35 0.25

0.15

0.05

10

20

30

40 50 60 2θ / degree

70

41


80

90


Figure 14

0.8

2

Total CO2 uptake / gCO /gsorbent

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 45

[(Li,K)2-(Na,K)]y/MgO

0.7 0.6 0.5

y = 0.15

0.4 y = 0.25

0.3 0.2

y = 0.35

0.1

y = 0.05

0.0

0

5

10

15

20

Number of cycles

42


25

30

Page 43 of 45

Figure 15

0.8

A (0.1 MPa, 100% CO2) o

350 C o 325 C

0.6

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

375 C

o

300 C o

0.4

275 C

0.3 0.2

0.2

0.1 0.0 0

0.0

2

4

6

8

o

B (350 C)

0.6

80% 60% 40%

0.4

30% CO2

0.2 20%

0.0 0

20

40

60 80 Time / min

43


100

120


Figure 16

MgO Conversion

0.8 0.7

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.6

100 (%)

80 60 40

0

2

4

6

8

10 12 14 16 18 20

Number of cycles

0.5 0.4 0.3 0.2 0.1 0.0 0

300

600 900 1200 1500 1800 Time / min

44


Page 44 of 45

Page 45 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic

45


Nanosheet MgO-Based CO2 Sorbent Promoted by Mixed-Alkali-Metal

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