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Alkali Nitrates Molten Salt Modified Commercial MgO for Intermediate-Temperature CO2 Capture: Optimization of the Li/Na/K Ratio Yaqian Qiao,† Junya Wang,† Yu Zhang,† Wanlin Gao,† Takuya Harada,‡ Liang Huang,† T. Alan Hatton,‡ and Qiang Wang*,† †

College of Environmental Science and Engineering, Beijing Forestry University, 35 Qinghua East Road, Haidian District, Beijing 100083, P. R. China ‡ Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ABSTRACT: The promoting effect of alkali nitrates molten salt on the CO2 capture capacity of a commercial MgO was investigated in detail. In particular, the ratio of Li/Na/K nitrates and the loading of the molten salt mixture on the MgO particles were optimized, and the influence of calcination and adsorption temperatures was evaluated. The MgO sample doped with 10 mol % (Li0.3Na0.6K0.1)NO3 was demonstrated to possess the highest CO2 uptake (up to 16.8 mmol g−1), which is the highest value reported for MgO based adsorbents in the literature. The CO2 adsorption/ desorption cycling stability was studied using both temperature swing adsorption (TSA) and pressure swing adsorption (PSA). The morphology and structure of the optimized adsorbent, 10 mol % (Li0.3Na0.6K0.1)NO3· MgO, were characterized thoroughly using XRD, SEM, FTIR, and BET analyses. The thermal stability of doped alkali nitrates was investigated via temperature program desorption and XRD analysis, which indicated that the phase status of the molten salts is crucial for the marked improvement in CO2 capture capacity of MgO. can capture CO2 at higher temperatures (200−700 °C) and can be disposed of without undue environmental precautions.4,14−20 Among the various solid adsorbents studied, layered double hydroxide (LDH)-derived metal oxides18,21−24 and magnesium oxide (MgO) compounds 6,13 have been identified as particularly suited adsorbent materials for CO2 adsorption in the SEWGS and SEBR technologies, which operate in the temperature range of 200−400 °C.18 LDH materials have attracted intense attention because they have good thermal stability, relatively fast CO2 adsorption kinetics, and a moderate regeneration temperature.25 It has generally been accepted that the adsorption capacity of layered double oxides (LDOs) adsorbents derived from MgAlCO3-LDH is usually less than 0.8 mmol g−1. To enhance both the CO2 capacity and the adsorption/desorption kinetics of the original LDH, Li et al.25 successfully synthesized K2CO3 promoted MgAl-stearate-LDH with an increased CO2 adsorption capacity of up to 1.9 mmol g−1 at 300 °C. Until very recently, this value was by far the highest adsorption capacity reported in the literature. Lee et

1. INTRODUCTION It has been well accepted that the increased atmospheric concentration of carbon dioxide (CO2) is a leading contributor to global climate change.1−4 Thus, much effort is being expended currently on the search for new carbon-free energy conversion systems and the utilization of various alternative energy sources. To date, fossil fuels, which supply a majority of the world’s energy needs, are by far the largest source of CO2 emissions and a major contribution to greenhouse gas effects on the climate, making them obvious targets for the implementation of advanced CO2 capture and storage technologies.5,6 To solve this problem, worldwide efforts have been devoted to the development of new technologies/ processes for CO2 capture, utilization, and storage (CCUS)4,7 and suitable CO2 capture materials that could contribute to the establishment of advanced CCUS systems. Absorption by aqueous absorbent solvents is the most widely used technology for CO2 capture,1,8−11 but these systems are only limited to postcombustion CO2 capture because they only operate at relatively low temperatures.12,13 However, for precombustion CO2 capture that is required in the integrated gasification combined cycles (IGCC) related processes, e.g. sorption enhanced water gas shift (SEWGS) and sorption enhanced biomass reforming (SEBR) reactions, solid type adsorbents are preferred rather than liquid absorbents as they © XXXX American Chemical Society

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December 12, 2016 January 7, 2017 January 23, 2017 January 23, 2017 DOI: 10.1021/acs.iecr.6b04793 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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and coated with gold for ∼120 s under an argon atmosphere. The surface area of each adsorbent was measured by Brunauer−Emmett−Teller (BET, Builder SSA-7000). Fourier transform infrared spectrometer (FT-IR) analyses were performed using a Bruker Vertex 70 spectrophotometer to examine the molecular speciation of adsorbed CO2. 2.3. Evaluation of CO2 Adsorption Capacity. The CO2 adsorption/desorption on (Li−Na−K)NO3 modified MgO derivatives was measured using a thermogravimetric analyzer (TGA, Q50 TA Instrument). The flow rates of N2 and CO2 were controlled by a mass flow controller (MFC). Samples were first calcined at 450 °C for 4 h in air before adsorption experiments were performed. For each run, a sample of about 15 mg was loaded into the sample container. To avoid errors caused by preadsorbed species (atmospheric CO2, water, and other impurities), samples were first pretreated at 450 °C for 60 min under a flow of high purity N2 (20 mL min−1). The temperature was then lowered to the desired adsorption temperature, the gas was switched from N2 to CO2 with a constant flow of high purity CO2 (1 atm, 40 mL min−1), and the CO2 adsorption uptake was measured for 4 h. The CO2 adsorption/desorption cycling tests were also evaluated using TGA. The operating temperatures for both temperature swing adsorption (TSA) and pressure swing adsorption (PSA) were determined from initial trial tests at various temperatures. For TSA, an adsorption temperature of 300 or 350 °C in the presence of pure CO2 and a desorption temperature of 350 or 400 °C in the presence of pure N2 were selected for the experiments. For PSA, the adsorption and desorption temperatures were kept the same while just simply switching the atmosphere between pure CO2 and pure N2. The time duration for both adsorption and desorption steps was 30 min. The thermal stability of each alkali metal-coated MgO particle was examined by temperature-programmed desorption (TPD) with pure Ar. The temperature of the furnace was increased from room temperature to 600 °C at a ramp rate of 2 °C min−1. The slightly desorbed gas was swept off under a flow rate of 100% Ar at a rate of 100 mL min−1. The concentration of the outlet desorbed gas was recorded on a quadrupole mass spectrometer (QGA, Hidden, UK).

al.26 have now reported an NaNO3 promoted LDH with high Mg/Al ratio and achieved a high CO2 capture capacity of up to 9.3 mmol g−1 at 240 °C. Among the many metal oxide-based sorbents available,14−20 MgO is considered to be a suitable adsorbent for CO2 capture at intermediate temperatures.27−29 However, pure MgO has a moderate CO2 adsorption capacity and poor thermal stability during regeneration. Gregg et al.30 used MgO to adsorb CO2, for instance, and obtained a capacity of only 0.4 mmol g−1, but several selective and reversible MgObased materials with significantly improved CO2 adsorption performance have since been reported. For example, MgObased sorbents promoted with K2CO3 showed relatively high CO2 adsorption capacity of 1.9 mmol g−1 at a CO2 partial pressure of 100 kPa.12 Vu et al.14 systematically investigated KNO3 coated MgO composites and obtained a high CO2 adsorption capacity of 3.2 mmol g−1, higher than the capacity of other MgO composites promoted by alkali metal salts (K2CO3, KOH, NaNO3, Na2CO3, Na2HPO4, LiNO3, and Li2CO3). In a recent breakthrough, Harada and Hatton13,31 developed (Li−Na−K)NO3 and LiNO3-(Na−K)NO2 coated MgO particles with a record CO2 capture capacity of 15.7 mmol g−1.31 MgO particles and colloidal nanoclusters prepared with magnesium carbonate hydroxide hydrate and colloidal nanoclusters by a novel nonhydrolytic sol−gel reaction were used in the synthesis of nitrate-coated composites with a nonoptimized ratio of [Li]:[Na]:[K] fixed at 0.3:0.18:0.52. In this contribution, we optimized the ratio of Li/Na/K in the coating of a commercially available MgO used in the preparation of the composites. The influence of molten salt loading, calcination temperature, and adsorption temperature was investigated systematically. The cycling performance of this alkali nitrate molten salt-modified commercial MgO as a CO2 adsorbent was evaluated under both temperature swing adsorption (TSA) and pressure swing adsorption (PSA) conditions. Finally, the optimized alkali nitrates molten salt-modified commercial MgO adsorbent was characterized in-depth.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Alkali metal nitrates-modified MgO adsorbents were prepared by first forming a mixed suspension of light MgO (98.5% in purity, Sinopharm Chemical Reagent) and three kinds of alkali metal nitrates (Sinopharm Chemical Reagent) in deionized water. To obtain the final product, the aqueous slurry was dried and then calcined at a high temperature. Typically, 2 g of light MgO (corresponding to ∼0.05 mol Mg) and 0.005 mol alkali metal nitrates (the ratio of Li/Na/K can be varied) were mixed in 20 mL of water and stirred vigorously at room temperature until the mixed solution became slurry. The obtained aqueous slurry was dried in the oven at 120 °C for overnight. The dried powders were ground and then calcined at temperatures ranging from 300 to 600 °C (450 °C was the standard condition) for 4 h in air. The desired temperatures of the furnace were increased at a rate of 3 °C min−1 during all cases. 2.2. Sample Characterization. XRD patterns were recorded on a Shimadzu XRD-7100 instrument in reflection mode with Cu Kα radiation to study the phase compositions and crystallographic structures of the sample. The accelerating voltage was set at 40 kV with 30 mA current (λ = 1.542 Å) at 5° s−1 from 10 to 80° with a scan rate of 5° min−1 and a step size of 0.02°. The morphologies of the samples were characterized using scanning electron microscopy (SEM, S-3400N II). Before observation, samples placed directly on the stub were sputtered

3. RESULTS AND DISCUSSION 3.1. Evaluation of CO2 Adsorption Capacity. 3.1.1. The Influence of Different Types of Alkali Metal Nitrates on CO2 Capture. The influence of alkali metal nitrates on the CO2 adsorption capacity of the prepared samples was examined by performing CO2 (99.995%) adsorption experiments on a Q50 TGA analyzer. A sample weight of ca. 15 mg was loaded into a platinum sample pan in the TGA unit, and the CO2 adsorption performance was evaluated.32 The initial activation of the samples was carried out at 450 °C for 1 h in nitrogen atmosphere. The adsorption and desorption runs were conducted using high purity CO2 gas and N2 flow, respectively. Both the gases, CO2 and N2, were controlled using an automatic valve to ensure continuous adsorption and desorption profiles. Figure 1 summarizes the CO2 capture capacity of MgO particles modified with various types of alkali metal nitrates in 100% CO 2 at atmospheric pressure (1 bar). The CO2 adsorption capacity of pure (nonmodified) MgO particles and MgO particles loaded with 10 mol % alkali metal nitrates (0.1 RNO3·MgO; R = Li, Na, K, (Na−K), (Li−Na), (Li−K), and (Li−Na−K)) is shown in Figure 1(a). All samples were B

DOI: 10.1021/acs.iecr.6b04793 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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MgO particles modified with (Li−Na−K)NO3 quickly reached adsorption equilibrium and had the highest capacity (12.3 mmol g−1) of all the adsorbents shown; its uptake quickly exceeded 11.2 mmol g−1 within 30 min and reached 12.3 mmol g−1 after 4 h exposure. These results indicated that the CO2 uptake was enhanced by the introduction of alkali nitrate molten salts to the coatings, as reported recently by Harada and Hatton.13,31 The influence of the loading of alkali nitrates is shown in Figure 1(b). The results demonstrated that the CO2 adsorption capacity was improved dramatically by the nitrate salt coatings. When the loading of alkali nitrates was low (2 mol %), the CO2 adsorption capacity was only slightly increased to 0.5 mmol g−1. However, with an increase in alkali nitrate loading to 5 or 10 mol %, the CO2 adsorption capacity increased significantly, with capacities of 11.0 and 12.3 mmol g−1, respectively. Further increases in the loading to 15 mol % did not lead to additional CO2 adsorption capacity; rather, the final CO2 uptake decreased slightly to 11.9 mmol g−1. Thus, 10 mol % was selected as the best alkali nitrates loading for the subsequent studies. Figure 1(c) shows the CO2 uptake of 10 mol % (Li0.3Na0.18K0.52)NO3·MgO at different adsorption temperatures, ranging from 200 to 400 °C. First, the CO2 uptake increased with an increase in adsorption temperature from 200 to 300 °C, to 3.7, 8.4, and 12.3 mmol g−1 at 200, 250, and 300 °C, respectively. During the early stages of the adsorption process, up to 120 min, the highest adsorption rate and highest capture capacity both occurred at 300 °C, with the uptake quickly reaching 11.7 mmol g−1 in 80 min and leveling off at 11.3 mmol g−1 after 120 min. The adsorption capacity at 325 °C at times beyond 120 min reached 13.1 mmol g−1, which is even higher than that at 300 °C. The uptake decreased monotonically with increasing temperature higher than 350 °C. At 400 °C, the CO2 uptake was very poor, with a very low capacity of 0.1 mmol g−1. 3.1.2. The Influence of the Li/Na/K Ratio on CO2 Capture. Figure 1(a) shows that MgO particles modified with double alkali metal salts also exhibited good adsorption capacity, especially those modified with (Li−Na)NO3 (12.4 mmol g−1). From a comparison of the performance of (Na−K)NO3, (Li− Na)NO3, and (Li−K)NO3 modified MgO particles, it is clear that the CO2 adsorption capacities of (Li−Na)NO3 and (Na− K)NO3 modified MgO samples were significantly higher than that of the (Li−K)NO3 modified MgO sample. In addition, single NaNO3 modified MgO also led to a CO2 uptake as high as 9.8 mmol g−1. Therefore, we speculated that among the above three alkali metals, NaNO3 played an important role in the promotion of CO2 adsorption. The influence of NaNO3 on CO2 capture capacity was first investigated, as shown in Figure 2(a). Here we fixed the total (Li−Na−K)NO3 loading at 10 mol % and adsorption temperature at 325 °C and then changed the proportion of NaNO3 while keeping the same proportions of LiNO3 and KNO3. The CO2 capture capacity increased from 13.1 to 15.0 mmol g−1 with an increase in the NaNO3 proportion from 0.18 to 0.6 and then declined to 14.5 and 9.5 mmol g−1 when the proportion of NaNO3 was further increased to 0.8 and 0.9, respectively. Thereafter, we fixed the total (Li−Na−K)NO3 loading at 10 mol %, the adsorption temperature at 325 °C, and the proportion of NaNO3 at 0.6 and changed the relative proportions of LiNO3 and KNO3, as shown in Figure 2 (b). The maximum CO2 adsorption capacity of 15.2 mmol g−1 was achieved when the ratio of nitrate composition [Li]:[Na]:[K] = 0.3:0.6:0.1. That is, the optimized

Figure 1. (a) CO2 adsorption at 300 °C for pure MgO particles and 10 mol % alkali metal nitrates modified MgO particles (10 mol % (LixNayKz)NO3·MgO, where the proportions of binary mixtures of the nitrates [Na]:[K], [Li]:[Na], and [Li]:[K] were 0.5:0.5 and the proportions of ternary [Li]:[Na]:[K] mixtures of the nitrates were 0.30:0.18:0.52); (b) the influence of alkali nitrates loading on the CO2 uptake of (Li0.3Na0.18K0.52)NO3·MgO tested at 300 °C; (c) the influence of adsorption temperature on the CO2 uptake of 10 mol % (Li0.3Na0.18 K0.52)NO3·MgO.

adsorbed at 300 °C with a constant flow of high purity CO2 (40 mL min−1), and the adsorption uptake was measured for 4 h. It is clear that all MgO particles modified with alkali metal nitrates other than KNO3 and LiNO3 showed a much higher uptake of CO2 than did the nonmodified MgO particles, for which the saturation point of CO2 adsorption capacity was less than 0.02 mmol g−1 attained within about 5 min. Among all the samples, C

DOI: 10.1021/acs.iecr.6b04793 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. (a) The influence of the NaNO3 amount on the CO2 capture capacity of 10 mol % (LixNayKz)NO3·MgO; (b) the influence of loadings of LiNO3 and KNO3 on the CO2 capture capacity 10 mol % (LixNayKz)NO3·MgO.

alkali nitrates modified MgO adsorbent was determined to be 10 mol % (Li0.3Na0.6K0.1)NO3·MgO in this work. 3.1.3. The Influence of Adsorption Conditions. With the optimized CO2 adsorbent (Li0.3Na0.6K0.1)NO3·MgO, the influence of calcination temperature, adsorption temperature, and loading of (Li0.3Na0.6K0.1)NO3 was further evaluated. Figure 3(a) shows the effect of calcination temperature on the CO2 uptake of 10 mol % (Li0.3Na0.6K0.1)NO3·MgO. The CO2 capture capacity first increased with the increase in calcination temperature from 300 to 450 °C and then started to decrease when the calcination temperatures were too high (500 and 600 °C). In particular, at 600 °C the final CO2 capture capacity became very poor, at only 0.7 mmol g−1 even after 4 h. Figure 3(b) shows the effect of adsorption temperature on the CO2 uptake of the optimized 10 mol % (Li0.3Na0.6K0.1)NO3· MgO adsorbent. The maximum CO2 capture capacity (16.8 mmol g−1) was obtained with an adsorption temperature fixed at 300 °C. However, according to Figure 1(b), when the proportion of [Li]:[Na]:[K] fixed at 0.3:0.18:0.52, its highest adsorption capacity occurred at 325 °C. This result indicates that the optimal adsorption temperature changes with the changing ratio of alkali metal nitrates on the particles. When the proportion of [Li]:[Na]:[K] was fixed at 0.3:0.6:0.1, both the highest uptake rate and the maximum CO2 capture capacity occurred at 300 °C. Figure 3(c) shows the effect of the alkali nitrates loading on the CO2 uptake of (Li0.3Na0.6K0.1)NO3·MgO. A similar trend

Figure 3. (a) The effect of calcination temperature on the CO2 uptake capacity of 10 mol % (Li0.3Na0.6K0.1)NO3·MgO (adsorption at 325 °C); (b) the effect of adsorption temperature on the CO2 uptake of 10 mol % (Li0.3Na0.6K0.1)NO3·MgO (calcined at 450 °C); and (c) the effect of alkali nitrites loading on the CO2 uptake of (Li0.3Na0.6K0.1)NO3·MgO (calcined at 450 °C and adsorbed at 300 °C).

was observed with the adsorbent (Li0.3Na0.18K0.52)NO3·MgO, and the optimal alkali nitrates loading for (Li0.3Na0.6K0.1)NO3· MgO was also 10 mol %. Both the highest CO2 uptake and the most rapid rate occurred with 10 mol % alkali nitrates loading. In contrast to the quite low adsorption capacity of 0.02 mmol g−1 observed for the nonmodified MgO, the adsorption capacity of 10 mol % (Li0.3Na0.6K0.1)NO3·MgO exceeded 16.8 mmol g−1 under the same reaction conditions. These results indicate that by fixing the calcination temperature at 450 °C, and the adsorption temperature at 300 °C, the 10 mol % (Li0.3Na0.6K0.1)NO3·MgO particles can exhibit a CO2 uptake as high as 16.8 mmol g−1, which is by far the highest value reported in the literature to date. D

DOI: 10.1021/acs.iecr.6b04793 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 3.1.4. Cycling Stability Tests. In addition to the CO2 capture capacity, the continuous adsorption/desorption cycling stability of the synthetic adsorbent was also evaluated using typical TSA and PSA procedures, as shown in Figure 4. For TSA, the adsorption was performed at 300 °C in a flow of high purity CO2 for 30 min, and the desorption was performed at a higher temperature (350 or 400 °C) in a flow of high purity N2 for 30 min. The adsorption−desorption cycles were repeated for 20 times. Independent of whether the desorption was performed at 350 or 400 °C, the CO2 uptake capacity initially decreased

with an increase in cycle numbers, but the decline became smaller after 15 cycles, with a final CO2 uptake of 3.2 mmol g−1. In the PSA process, both the adsorption and desorption temperatures were set at 350 °C, and the time for each adsorption and desorption step was 30 min. It can be seen that although the initial CO2 uptake was not that high, the CO2 uptake during the 20 cycles was very stable, with a reversible capacity of ca. 3.2 mmol g−1. This value is very similar to that obtained in the TSA process. For practical reaction-based CO2 separation processes, the industrial standard is monoethanol amine (MEA), which has a CO2 capture capacity of ca. 1.36 mmol g−1.33−35 The performance of our synthesized adsorbent 10% mol (Li0.3Na0.6K0.1)NO3·MgO has a much higher reversible CO2 capture capacity (3.2 mmol g−1) than the industrial standard, suggesting that this novel CO2 adsorbent is very promising in the sorption-enhanced hydrogen production processes.34,35 3.2. Characterization of the Optimized 10 mol % (Li0.3Na0.6K0.1)NO3·MgO Adsorbent. To clarify the details of the reactions occurring during CO2 adsorption and desorption processes, neat MgO and 10 mol % (Li0.3Na0.6K0.1)NO3·MgO were thoroughly characterized using XRD, SEM, FTIR, and BET analyses. The structural evolution of the adsorbent 10 mol % (Li0.3Na0.6K0.1)NO3·MgO after being calcined at 450 °C for 4 h, and sequentially adsorbing CO2 at 300 °C for 4 h, and releasing CO2 at 450 °C with N2 flow for 1 h was examined using XRD analyses, as shown in Figure 5(a). The XRD pattern of pure MgO is also provided for comparison. It is well-known that pure MgO easily absorbs vapor or CO2 in air to form magnesium hydroxide (Mg(OH)2) or magnesium carbonate (MgCO3). Thus, the MgO samples were first calcined at 450 °C for 4 h before the XRD analysis. The characteristic peaks of MgO (JCPDS 45-0946) at 2θ = 36.94°, 42.89°, 62.41°, 74.57°, and 78.60° are clearly observed, which reveals that no impurities, such as Mg(OH)2 or MgCO3, were incorporated in MgO. Then, 10 mol % (Li0.3Na0.6K0.1)NO3·MgO samples were prepared by the mixing of pure MgO and three kinds of alkali metal nitrates. Before calcination, the synthetic product was magnesium hydroxide (JCPDS 44-1482) mixed with alkali metal nitrates. Following calcination at 450 °C for 4 h, the magnesium hydroxide lost water to revert to MgO (JCPDS 450946). After reaction with CO2, well-crystallized magnesium carbonate (JCPDS 08-0479) was generated; the carbonate decomposed on subsequent complete regeneration of the adsorbent.36 The morphological changes of 10 mol % (Li0.3Na0.6K0.1)NO3· MgO during the thermal treatment and the CO2 adsorption/ desorption processes were also examined using SEM analyses. Figure 5(b) indicates that pure MgO was comprised of small spherical particles with an average size of 140 nm.37 During the preparation of alkali nitrates modified MgO, after contact with nitrate aqueous solution the MgO transformed into a rosettelike conformation consisting of a composite of magnesium hydroxide and nitrates (Figure 5(c)). After calcination at 450 °C for 4 h in air, magnesium hydroxide lost water and transformed back into MgO. It was clear that the (Li0.3Na0.6K0.1)NO3·MgO sample was composed of small spherical particles with an average size of 140 nm (Figure 5(d)), which is similar to that of the pure MgO. After adsorbing CO2 at 300 °C for 4 h, the grains were partly coalesced and expanded to form larger particles with size twice as large as that before adsorption (Figure 5(e)).

Figure 4. CO2 adsorption and desorption cycling tests with 10% mol (Li0.3Na0.6K0.1)NO3·MgO adsorbent: (a) TSA with adsorption at 300 °C and desorption at 350 °C, (b) TSA with adsorption at 300 °C and desorption at 400 °C, and (c) PSA with adsorption and desorption both at 350 °C. E

DOI: 10.1021/acs.iecr.6b04793 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. (a) XRD patterns of pure MgO and 10 mol % (Li0.3Na0.6K0.1)NO3·MgO following calcination in air at 450 °C for 4 h, adsorption of CO2 at 300 °C in 100% CO2 for 4 h, and subsequent regeneration at 450 °C in 100% N2 for 1 h. SEM images of (b) pure MgO, (c) fresh 10 mol % (Li0.3Na0.6K0.1)NO3·MgO, (d) 10 mol % (Li0.3Na0.6K0.1)NO3·MgO calcined at 450 °C in air for 4 h, and (e) 10 mol % (Li0.3Na0.6K0.1)NO3·MgO after adsorbing CO2 at 300 °C in 100% CO2 for 4 h.

the carbonate ion (CO32−). Shoulders at 1500 and 1355 cm−1 for unidentate carbonates were also enhanced in this spectrum.38−41 These results indicated that the large amounts of carbonate ions were generated after reaction with CO2. After the regeneration of the particles, peaks of the carbonate ions disappeared, and the peaks from the nitrate ions and atmospheric CO2 appeared again. The introduction of nitrate salts led to a marked enhancement in the CO2 uptake of the MgO particles. The BET specific surface areas were measured under four conditions: fresh material, before the CO2 adsorption, after the CO2 adsorption, and after the regeneration. The samples were outgassed under vacuum for 4 h at room temperature before the introduction of nitrogen at 77 K. The sample specific surface area was calculated with the BET equation, and the pore volume distribution was calculated by the Barrett−Joyner− Halenda (BJH) method (Table 1). Fresh material showed a

Figure 6(a) shows the variations in the FT-IR spectra of 10 mol % (Li 0.3 Na 0.6 K 0.1 )NO 3 ·MgO before and after CO 2 adsorption and the subsequent regeneration of the adsorbent. The broad absorption band at 1384 cm−1 observed before CO2 adsorption can be attributed to the nitrate ion (NO3−, doubly degenerated N−O stretch), and the shoulders and weak peaks can be attributed to various types of carbonates. The absorption bands at 1437 and 1355 cm−1 correspond to the asymmetric stretch of carbonate ions and symmetric stretch of unidentate carbonates, respectively. The peaks at 1630 and 1045 cm−1 were ascribed to the asymmetric and symmetric stretch of bidentate carbonates. The existence of these shoulders and weak peaks for various carbonate species was thought to be due to the adsorption of atmospheric CO2 on the surface of MgO.13,31 After CO2 adsorption, three peaks at 1437, 886, and 749 cm−1 were observed, which were assigned to asymmetric stretch, out of plane bending, and in plane bending modes of F

DOI: 10.1021/acs.iecr.6b04793 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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performance of this type of adsorbent dropped so obviously during the first several cycles before leveling off. To further explore whether the enhancement of CO2 uptake was related to the calcination temperature, structural changes under different calcination conditions were examined. Figure 6(b) shows the intensity of the characteristic peak of alkali metal nitrates at 2θ ≈ 30°, 32°, and 39°, which decreased as the calcination temperature exceeded 450 °C, and disappeared when the temperature was above 600 °C. The results indicated that the change of XRD patterns matched well with the CO2 capture performance of the corresponding samples in Figure 3(a). The highest CO2 capture capacity was obtained when the calcination temperature was 450 °C, and the capacity became much lower when the calcination temperature reached 600 °C. One possible reason for the disappearance of nitrates was that the crystalline phase of the composite had been completely transformed into an amorphous phase, and it is also possible that some decomposition of the alkali metal nitrates occurred. For further study, the thermal stability of the doped alkali nitrates molten salts was evaluated using TPD analysis, as shown in Figure 7(a). It is clear that the decomposition of LiNO3, NaNO3, and KNO3 begins from 270, 450, and 490 °C, respectively. The results indicate that LiNO3 and NaNO3 are partly decomposed when MgO particles coated with these nitrates are calcined at 450 °C but not for the case of the coating with KNO3. It is widely recognized that the nitrate salts (NO3−) are decomposed first into the nitrites (NO2−) with the generation of oxygen and that the nitrites are further decomposed into the metal and nitrogen oxides at higher temperatures.42 The calcination at 450 °C of MgO coated with ternary nitrates (Li−Na−K)NO3, including LiNO3 and NaNO3 as the components, may result in the formation of a mixture of the nitrates and nitrites. It has been reported that the coating with nitrites (NO2−) is more favorable for the acceleration of the uptake of CO2 than coatings with nitrates (NO3−), which is most likely because of the difference in the concentration of O2− to facilitate the formation of the carbonates in the coating layers. Here, the CO2 uptake by (Li0.3Na0.6K0.1)NO3·MgO shows the highest capacity on calcination at 450 °C, as shown in Figure 3(a) which could possibly be because calcination at this temperature maximized the composition of the nitrites in the salts. The specific influence of alkali metal nitrates was attributed to the variations in the CO2 solubility within these nitrates and their melting temperatures. When the adsorption temperature exceeds the nitrates melting point, the CO2 solubility dramatically increases during the phase transition from the solid to the liquid state, i.e., the molten phase of the doped alkali nitrates seems favorable for promoting the CO2 uptake. The optimum adsorption temperature was found to be 300 °C, and the uptake capacity decreased when the temperature was set higher, probably owing to the lowering of the equilibrium

Figure 6. (a) FT-IR spectra of 10 mol % (Li0.3Na0.6K0.1)NO3·MgO before CO2 adsorption, after CO2 adsorption at 300 °C for 4 h, and after CO2 desorption at 450 °C in 100% N2 for 1 h. (b) XRD patterns of calcined 10 mol % (Li0.3Na0.6K0.1)NO3·MgO at different temperatures.

higher BET specific surface area (65.76 m2 g−1) and a larger pore volume (0.438 cm−3 g−1). After calcination, the BET specific surface area and the pore volume decreased significantly to 18.62 m2 g−1 and 0.276 cm−3 g−1, respectively. It was reasonable to assume that the alkali metal nitrates were in the molten state during calcination under a high temperature and thus would lead to pore clogging. After the adsorption, CO2 reacted with MgO which caused a further decrease (0.123 cm−3 g−1) in pore volume. However, the BET specific surface area and pore volume of MgO further decreased after being regenerated at 450 in N2. In general, our results suggested that during the regeneration process, the nitrates return to a molten state with some pore blocking resulting from the calcination step. These data suggest well explains why the cycling

Table 1. BET Specific Surface Areas and Pore Volumes of Different Phases of Neat MgO and 10 mol % (Li0.3Na0.6K0.1)NO3· MgO samples MgO 10 mol % (Li0.3Na0.6K0.1)NO3·MgO

fresh material before adsorption after adsorption after regeneration G

specific surface area (m2 g−1)

BJH pore volume (cm−3 g−1)

20.0 65.8 18.6 21.0 18.0

0.103 0.438 0.276 0.123 0.109 DOI: 10.1021/acs.iecr.6b04793 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 8. Comparison of the CO2 uptake between neat MgO and 10 mol % (Li0.3Na0.6K0.1)NO3·MgO adsorbents.

optimal Li/Na/K ratio and the molten salts loading was determined. The sample with 10 mol % (Li0.3Na0.6K0.1)NO3· MgO presented the highest CO2 adsorption capacity of 16.8 mmol g−1 following calcination at a temperature of 450 °C with an adsorption temperature of 300 °C, which is by far the highest uptake for the MgO based CO2 adsorbents. We also demonstrated that the optimal adsorption temperature is dependent on the Li/Na/K ratio of the doped molten salts. The cycling performance of 10 mol % (Li0.3Na0.6K0.1)NO3· MgO was investigated under both PSA and TSA protocols, both of which showed a stable reversible CO2 capture capacity of ca. 3.2 mmol g−1 over 20 cycles. The structure and morphology of the best CO 2 adsorbent, 10 mol % (Li0.3Na0.6K0.1)NO3·MgO, was characterized using XRD, SEM, FTIR, and BET analyses. The thermal stability study and XRD analysis suggested that the molten phase of the doped alkali nitrates plays an important role in promoting the uptake of CO2 in the intermediate temperature ranges of interest to power producers exploiting SEWGS and SEBR technologies.

Figure 7. (a) The evolution of NO as a function of decomposition temperature for 10 mol % (Li0.3Na0.6K0.1)NO3·MgO, 10 mol % LiNO3· MgO, 10 mol % NaNO3·MgO, and 10 mol % KNO3·MgO. (b) Thermal stability of adsorbed CO2 on 10 mol % (Li0.3Na0.6K0.1)NO3· MgO.

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conversion ratio of MgO into MgCO3 under CO2 at these elevated temperatures. Figure 7(b) shows the variation in the concentration of CO2 desorbed from preadsorbed 10 mol % (Li0.3Na0.6K0.1)NO3·MgO with increasing temperature under a nitrogen flow. Here, the adsorption of CO2 was performed at 325 °C for 1 h in pure CO2. It is clear that the desorption rate attained a maximum at 450 °C. These results indicate that the alkali-metal nitrates coated MgO particles investigated in this study can be regenerated readily by use of various sources of waste heat in the intermediate temperature range. In all, we have revealed that the ratio of Li/Na/K greatly influences the promoting effect of the molten salt for MgO. With the optimized Li/Na/K ratio, we demonstrated that by doping with only 10 mol % (Li0.3Na0.6K0.1)NO3 molten salt, the CO2 capacity of a commercial MgO sample can be significantly improved from ca. 0.01 to 16.8 mmol g−1 (as compared in Figure 8), which is the highest value reported for MgO based adsorbents in the literature.

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-13699130626. E-mail: [email protected], [email protected]. ORCID

Qiang Wang: 0000-0003-2719-2762 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (2016ZCQ03), the National Natural Science Foundation of China (51622801, 51572029, and 51308045), and Beijing Excellent Young Scholar (2015000026833ZK11).

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4. CONCLUSIONS The CO2 capture performance of commercial MgO particles coated with alkali nitrates molten salts was studied in detail. The influence of the types of alkali nitrates (LiNO3, NaNO3, and KNO3) and their combinations were evaluated, and the H

DOI: 10.1021/acs.iecr.6b04793 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.6b04793 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX