MgO composites for intermediate

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Study on MNO3/NO2 (M = Li, Na, and K)/MgO Composites for Intermediate-Temperature CO2 Capture Wanlin Gao,† Tuantuan Zhou,† Yanshan Gao,† Qiang Wang,*,† and Weiran Lin*,‡ †

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College of Environmental Science and Engineering, Beijing Forestry University, 35 Qinghua East Road, Haidian District, Beijing 100083, People’s Republic of China ‡ Beijing Research Institute of Chemical Industry, SINOPEC, Beijing 100013, People’s Republic of China S Supporting Information *

ABSTRACT: Molten salt [MNO3/NO2 (M = Li, Na, and K)]-promoted MgO composite-type CO2 adsorbents with high capacity and cycling stability are fabricated by controlled hydrolysis along with an incipient impregnation approach. The remarkable results of molten salts on the CO2 capture performance of MgO adsorbents and the CO2 capture capacities with different loadings and calcination and adsorption temperatures were comprehensively investigated. Notably, optimized NaNO3promoted MgO exhibited the highest CO2 capture capacity of 17.0 mmol g−1 at 350 °C and maintained excellent regenerability through multiple cycles. The initial CO2 uptakes of MNO3/NO2 (M = Li, Na, and K)/MgO composites follow the order of NaNO3 (17.0 mmol g−1) > LiNO3 (14.1 mmol g−1) > NaNO2 (12.5 mmol g−1) > KNO2 (9.3 mmol g−1) > KNO3 (5.0 mmol g−1). Furthermore, NaNO3 and KNO3 with good chemical stability are the effective constituents for the NaNO3 system and KNO3/KNO2-promoted system, respectively, and preserved their original form after the interaction with CO2. The NaNO2 system consists of NaNO3 and Na2CO3 to form double carbonates Na2Mg(CO3)2 after CO2 adsorption. The morphology and structure of the MgO skeleton and the introduction of promoters are essential factors for intermediate-temperature CO2 capture. tion.26,27 Molten salts were found to be able to inhibit the generation of a harsh, CO2-impermeable, monodentate carbonate layer on the MgO surfaces, allowing for the high rate of CO2 uptake with the promotion of rapid generation of carbonate ions.24,25 Molten salts serve as a liquid channel to promote the adsorption of CO2 and facilitate the ion transfer to the reactant, thereby boosting the CO2 capture rate.28−30 This extraordinary enhancement on the CO2 uptake of a MgObased adsorbent is exceedingly attractive for CO2 capture and sequestration systems, even though limited properties of commercial MgO currently restrict its wider applications. It is therefore highly desirable to develop alternative, novel MgObased CO2 materials promoted by molten salts,18,31 because the morphology and textural structure are crucial to the overall performance of MgO. Our previous studies reported MgO nanosheets fabricated via a facile hydrothermal strategy, exhibiting extraordinarily high uptake for the CO2 capture.16,32 The layered structure of synthesized MgO allows for consideration of both platelet size and thickness and, therefore, generates abundant surface defects as oxygen vacancies. In addition, the high specific surface area of MgO nanosheets ensures the active sites accessible to CO2 without traditional diffusion limitations.33 Inspired by this, MgO nanosheets coated with molten salts

1. INTRODUCTION Molten salts, typically described as liquid electrolytes, which consist entirely of ions displaying an ionic−covalent crystalline structure,1,2 have been widely used in electrochemistry technologies as a result of their excellent chemical and thermal stabilities.3 With the advantages of good electrical conductivity and high ionic mobility,4−6 molten salts have been explored as green solvents for chemical processes and reactions,7 electrochemical solvents for metal/semiconductor electrodeposition,8,9 and media for physical and analytical chemistry10 and even in the biology and biomimetic processes.11−13 Motivated by mitigating greenhouse gas emissions and serious on-going global climate changes,14 regenerable CO2 adsorbents with high capacity are desirable for the development of technical advances to pause the increasing atmospheric CO2 concentrations.15−17 Magnesium oxide (MgO)-based intermediate-temperature adsorbents possess the advantage of high theoretical capacity (24.8 mmol g−1)18 but are hugely limited as a result of their poor regenerability and slow reaction kinetics.19 In the past few decades, tremendous efforts have been devoted to developing MgObased adsorbents with good adsorption performance as well as excellent physical properties.20−23 In 2015, Harada and coworkers successfully fabricated molten alkali metal nitratedoped highly regenerable MgO-based CO2 adsorbents with superior capture capacity above 10.2 mmol g−1 under ambient pressure at 300 °C, creating huge potential for CO2 capture and sequestration.24,25 Molten salts were reported to have the ability to partly dissolve bulk MgO and overcome the high lattice energy constraints, leading to the improvement of CO2 adsorp© XXXX American Chemical Society

Special Issue: Carbon Dioxide Capture and Utilization - Closing the Carbon Cycle Received: August 8, 2018 Revised: August 31, 2018 Published: September 7, 2018 A

DOI: 10.1021/acs.energyfuels.8b02749 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

promoters and the loss of pores confirmed the existence of molten salts introduced by the IWI approach. Figure 1a displays the CO2 uptake for molten salt-promoted MgO-based adsorbents with different loading contents exposed

(alkali metal nitrites/nitrates) were synthesized using an incipient impregnation approach. These nanosheets exhibit extraordinarily high capture capacities for the intermediate adsorption of CO2. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) provide evidence for phase transition and morphology alteration through multiple adsorption and desorption processes. This work provides a basic understanding of the promotion rule of molten salts on the CO2 uptake of MgO-based adsorbents and highlights that the molten salt-promoted MgO CO2 adsorbent is a promising platform for CO2 capture and sequestration.

2. MATERIALS AND METHODS 2.1. Materials. Lithium nitrate (LiNO3, 99.6%), sodium dodecyl sulfate (SDS), sodium nitrate (NaNO3, 99%), magnesium carbonate hydroxide hydrate [4MgCO3·Mg(OH)2·4H2O, 99%], sodium nitrite (NaNO2, 99%), magnesium nitrate hexahydrate (Mg(NO3)2·6H2O, 99%), potassium nitrate (KNO3, 99%), light magnesium oxide (MgO, 98.5%), potassium nitrite (KNO2, 99%), and urea were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used without further purification. Demineralized water was used in all of the experimental processes. 2.2. Synthesis of Samples. The magnesium precursor nanosheets with the formula of 4MgCO3·Mg(OH)2·4H2O were synthesized by a hydrothermal method, and more details can be referred to our previous work.32 A total of 0.4 g of SDS, 12 g of urea, and 10.24 g of Mg(NO3)2·6H2O were dissolved with 200 mL of deionized water. The mixture was heated at 120 °C in an autoclave for 12 h. The resulting solid was collected by filtration and dried at 80 °C. The molten salt-coated MgO adsorbents were synthesized via an incipient wetness impregnation (IWI) approach. The necessary mass of molten salts was dissolved with deionized (DI) water. The solution was added dropwise to the as-prepared magnesium precursor powders until it appeared wet, dried at 70 °C overnight, and calcined at 450 °C for 5 h in a muffle furnace in air. 2.3. Characterizations and CO2 Adsorption Tests. XRD spectra were collected using a Shimadzu XRD-7000 diffractometer. Sample morphologies were characterized using SU8010 SEM. Brunauer−Emmett−Teller (BET) surface areas were determined using a Builder SSA-7000 apparatus at 77 K. A thermogravimetric analyzer Q50 TA Instrument was used for CO2 capture experiments. The adsorbent sample (10 mg) was spread on the platinum container. Prior to the measurement, the sample was evacuated at 450 °C under 100% dry N2 to eliminate any absorbed species (atmospheric moisture, CO2, etc.). Then, CO2 (99.999%) was introduced to the sample at a desired set temperature for 4 h.

Figure 1. (a) Loading content and (b) calcination temperature dependence of the CO2 uptake after 4 h of reaction of CO2 with molten salt-promoted MgO adsorbents with different molten salt compositions.

3. RESULTS AND DISCUSSION Molten salt-promoted MgO-based nanosheets have been prepared by controlled hydrolysis along with an IWI approach. The XRD patterns of the molten salt-promoted MgO-based samples with different loading contents (5−40 mol %) and calcination temperatures (400−550 °C) are shown in Figures S1−S5 of the Supporting Information. Both the MgO skeleton with a cubic structure [Joint Committee on Powder Diffraction Standards (JCPDS) number 45-0946] and molten salts as the promoter in the samples were detected in the composites. The intensity of the characteristic peaks of molten salts grew stronger by increasing the loading content. Texture parameters and CO2 adsorption capacities of molten salt-promoted MgO adsorbents with different loadings and calcination temperatures are organized in Table S1 of the Supporting Information. The significant reduction in the specific surface area of the adsorbents suggested covering and occupying the mesoporous pores by molten salts.28,32 The crystals of

to 100% CO2 at 300 °C and ambient pressure. In the presence of molten salts, a significance increase in the CO2 capture capacity is observed for the MgO adsorbents coated with molten salts. The highest uptake, 14.2 mmol g−1, was obtained with 10 mol % NaNO3 with CO2 after 4 h of contact time. Evidently, more LiNO3 loading content of 35 mol % could promote MgO with a similar CO2 uptake of 14.1 mmol g−1. The optimal loading content turned out to be 25 mol % for both of the nitrite promoters, and the CO2 capture capacity increased to 12.5 mmol g−1 with the NaNO2 promoter and 9.3 mmol g−1 with the KNO2 promoter. The uptake in the KNO3promoted MgO adsorbent was very poor and possessed the highest uptake of 1.2 mmol g−1, with a loading content of 10 mol %. The difference between the optimal loading content and the promotion effect arises primarily from the essential contrasts among the molten salts. A higher amount of loading B


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Energy & Fuels Table 1. Texture Parameters and CO2 Adsorption Capacities of Optimized Molten Salt-Promoted MgO Adsorbents molten salt

loading (mol %)

NaNO3 NaNO2 LiNO3 KNO2 KNO3

10 25 35 25 10

calcination (°C and h) 450 450 450 450 500

and and and and and

BET specific surface area (m2 g−1)

pore volume (cm3 g−1)

average pore size (nm)

CO2 uptake at 300 °C (mmol g−1)

28.9 24.6 13.2 10.0 21.9

0.3 0.2 0.1 0.1 0.4

21.4 18.3 19.1 21.2 32.0

14.2 12.0 14.1 9.3 4.0

5 5 5 5 5

more CO2 than the sodium nitrite sample, but on the potassium case, it was exactly the contrary. It is reported that alkali metal nitrites have the ability to accommodate higher concentrations of oxygen ions, facilitating the fast generation of CO32− and, thus, enabling fast nucleation and growth of MgCO3.39,40 In this case, the sodium nitrite sample reacted with the magnesium precursor to form Na2CO3 with the solid state during the carbonation process, resulting in partial losses of the available molten nitrate promoters. MgO possesses an intermediate operational adsorption temperature range (200−400 °C) and a low energy cost for regeneration at ∼450 °C. The enhancement of the CO2 capture capacity by MgO was facilitated by molten salts at temperatures above the melting point (253−337 °C; Table 2) through the dramatic increment of CO2 solubility on the phase transition from the solid to liquid state of the molten salts.25 Therefore, the optimized molten salt-promoted MgO adsorbents were subsequently examined over a temperature range of 275−375 °C, as shown in Figure 2. Table 3

may result in agglomeration of the active oxide phase and, hence, lower the specific surface area of active components and lower activity.34 Figure 1b presents the calcination condition dependence of the CO2 capture capacity after 4 h of adsorption. The highest uptakes tended to have occurred at a calcination of around 450 °C for the molten salt-promoted MgO-based nanosheets other than KNO3 (at 500 °C). Table 1 lists the CO2 adsorption capacities and texture parameters of optimized molten saltpromoted MgO adsorbents. The CO2 uptakes became higher with the increase in the calcination temperature. The thermal degradation of the magnesium precursor and the premelting and surface melting of the molten salts take place through the calcination process and offer an excellent blend of the MgO particle and molten salt promoter. However, irreversible reduction occurred with the continued increasing calcination temperature above 500 °C as a result of the particle coarsening of the aggregated crystal.35 Table 2 summarizes the thermal Table 2. Summary of the Thermal Dissociation and Stability of the Molten Salts molten salt NaNO3 NaNO2 LiNO3 KNO2 KNO3

melting point (°C) 28

308 27128 25337 33738 33429

decomposition temperature (°C)36 600 330 350 410 530

dissociation and stability of the molten salts. LiNO3, as the least stable alkali metal nitrate, which is expected from the high polarizing power of the cation, slowly decomposes at 350 °C and becomes faster with the increment of the temperature.36,37 KNO2 starts to decompose evidently at 410 °C, while KNO3 remains stable in the temperature range of 400−600 °C. KNO3 melts at ∼334 °C to the liquid state without decomposition, which remains stable at least to 530 °C in air.37 NaNO2 is unstable above 330 °C, while NaNO3 melts to the liquid state without decomposition, which remains stable at least to 500 °C in air and starts to slowly decompose at 600 °C.38 It is noteworthy that the characteristic peaks of LiNO3 (JCPDS number 08-0466) and NaNO3 (JCPDS number 36-1474) transformed into Li2CO3 (JCPDS number 22-1141) and Na2CO3 (JCPDS number 37-0451) as a result of their poor thermal stability and, thus, exhibited irreversible decay in CO2 adsorption capacities, as shown in Figures S1b, S2b, and S3b and Table S1 of the Supporting Information. On the contrary, potassium nitrate/nitrite displayed high heat resistance and maintained good CO2 uptake as a result of their high melting point and thermal stability, as shown in Figures S4b and S5b and Table S1 of the Supporting Information.29 These results showed that an appropriate calcination temperature was favorable for the property preservation and activation of the molten salt promoter. The sodium nitrate sample adsorbed

Figure 2. Adsorption temperature dependence of the CO2 uptake after 4 h of reaction of CO2 with molten salt-promoted MgO adsorbents with different molten salt compositions compared to the non-coated MgO adsorbent.

summarizes CO2 adsorption capacities of molten salt-coated MgO adsorbents at different adsorption temperatures. The CO2 uptake of the non-coated MgO adsorbent is simultaneously pretended for comparison. The neat MgO adsorbents showed limited CO2 uptake of less than 0.7 mmol g−1 over the temperature range. With the introduction of molten salt promoters, the CO2 capture performance was dramatically increased over 10 times of its original value. For MgO promoted by NaNO3, the CO2 uptake increased from 9.4 to 17.0 mmol g−1 when the temperature increased from 275 to 350 °C. The highest uptakes were recorded at 300 °C for MgO promoted by NaNO2, KNO2, and LiNO3, reaching 12.0, 9.3, and 14.1 mmol g−1, respectively. For MgO coated with KNO3, C


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Energy & Fuels

Table 3. CO2 Adsorption Capacities of Molten Salt-Promoted MgO Adsorbents Compared to the Non-coated MgO Adsorbent at Different Adsorption Temperatures CO2 uptake (mmol g−1) molten salt

loading content (mol %)

NaNO3 NaNO2 LiNO3 KNO2 KNO3 non-coated

10 25 35 25 10

calcination condition (°C and h) 450 450 450 450 500 450

and and and and and and

5 5 5 5 5 5

the uptake rose from 2.7 to 5.0 mmol g−1 when the temperature increased from 275 to 325 °C. Adsorbed CO2 started to desorb when the temperature was too high.41 Further increment in the adsorption temperature exhibited suppressed CO2 capture capacity, and a severe decrease was discovered at 375 °C, indicating nearly no CO2 sorption. Besides, relatively low CO2 capture obtained at 375 °C was largely dominated by the chemical thermal equilibrium, in which the decarbonation of MgCO3 began to take place at high temperatures above 373 °C.42,43 The CO2 capture capacity is strongly affected by the adsorption condition. The overall performance of the MgO adsorbent is dictated by the balance of several effects, including fast reaction kinetics, the CO2 solubility within the molten salt, and the enthalpy changes of MgCO3 formation, and therefore, an optimal capture temperature exists with the interaction between CO2 and the molten salt-coated MgO adsorbents.28 The molten salt promotion system can take up CO2 over a higher temperature range than MgO only, whereas the reaction of MgO with CO2 becomes unfavorable at temperatures above 400 °C in terms of the thermodynamic data.35 In general, the CO2 uptake by MgO promoted with molten salts could be sequentially followed as NaNO3 (17.0 mmol g−1) > LiNO3 (14.1 mmol g−1) > NaNO2 (12.5 mmol g−1) > KNO2 (9.3 mmol g−1) > KNO3 (5.0 mmol g−1). The chemical speciation and crystal phases were examined by XRD to clarify the details of the interaction with CO2, as exhibited in Figures 3 and 4. The crystal structures of NaNO3 (JCPDS number 36-1474, rhombohedral) and KNO3 (JCPDS number 32-0824, rhombohedral) were maintained. The XRD spectra of NaNO2-promoted MgO adsorbents exhibited

275 °C

300 °C

325 °C

350 °C

375 °C

9.4 9.5 12.5 7.4 2.7 0.6

14.1 12.0 14.1 9.3 4.0 0.6

16.2 4.8 6.4 4.6 5.0 0.7

17.0 2.3 0.2 1.5 1.2 0.4

0.6 1.2 0.1 0.4 0.1 0.4

Figure 4. XRD patterns of the optimized molten salt-promoted MgO adsorbents after 4 h of reaction of CO2.

characteristic peaks of NaNO3 (JCPDS number 36-1474, rhombohedral) and Na2CO3 (JCPDS number 37-0451, monoclinic), indicating that the available composition of the promoter turned out to be sodium nitrate through the calcination process, as shown in Figure 1a and panels a and b of Figure S2 of the Supporting Information. To explore the existence of Na2CO3, we coated NaNO2 on two different magnesium precursors: light MgO and magnesium carbonate hydroxide hydrate. XRD results showed that the characteristic peaks of NaNO2 completely transformed into NaNO3 when coated on light MgO material (Figure S6a of the Supporting Information). The reaction of NaNO2 and O2 to establish equilibrium with NaNO3 can be written as NaNO2 + O2 → NaNO3

(1)

Notably, the coexistence of NaNO3 and Na2CO3 was discovered when coated on the magnesium carbonate hydroxide hydrate (Figure S6b of the Supporting Information). Therefore, various components of precursors led to different reaction processes to form Na2CO3, which can be written as 4MgCO3 · Mg(OH)2 · 4H 2O + NaNO2 + O2 → MgO + NaNO3 + Na 2CO3

(2)

Similarly, XRD results of KNO2-coated magnesium precursors showed that the characteristic peaks of KNO2 completely transformed into KNO3 when coated on either light MgO material or magnesium carbonate hydroxide hydrate without the existence of potassium carbonate (Figure S7 of the Supporting Information). The reaction of KNO2 and O2 to establish equilibrium with KNO3 can be written as

Figure 3. XRD patterns of the optimized molten salt-promoted MgO adsorbents before 4 h of reaction of CO2. D


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Figure 5. SEM images of (a) NaNO3-coated magnesium precursors, (b) fresh NaNO3-promoted MgO adsorbents, (c) NaNO3-promoted MgO adsorbents after 4 h of reaction of CO2, and (d) NaNO3-promoted MgO adsorbents after 20 cycles of CO2 adsorption and desorption.

KNO2 + O2 → KNO3

nation with molten salt. During the thermal decomposition process, NaNO3 tends to melt and cover the surface of MgO to form smaller particles. The surface of NaNO3-promoted MgO adsorbents turned more knobby, and fine grains aggregated to produce a rougher surface underwent reaction with CO2 for 4 h (Figure 5c). The pores became much larger after regeneration at the 20th cycle, suggesting the irreversible decay of the mesoporous structures, accompanied by the gradual loss of the active molten salt promotor. Therefore, the mitigation of the destruction of the MgO skeleton is a major determinant to the material deactivation over multiple cycles. SEM images of the molten salt-coated magnesium precursor, fresh molten salt-promoted MgO adsorbents, molten saltpromoted MgO adsorbents after adsorption, and molten saltpromoted MgO adsorbents after 20 cycles of CO2 adsorption and desorption followed the same rule and were displayed in Figures S8−S11 of the Supporting Information. The regenerability of molten salt-promoted MgO adsorbents was examined by repeating 20 cycles of CO2 adsorption (in 100% CO2 for 30 min at 300 °C) and desorption (in 100% N2 for 30 min at 450 °C). The adsorbents were hardly regenerated and experienced an extended period to complete a regeneration at low temperatures below 400 °C. Besides, significant degradation of the textural structure of composites took place at temperatures above 500 °C. Therefore, the regeneration process was executed at 450 °C. In both cases, other than LiNO3, the CO2 uptake declined gradually in the first several cycles and then eventually reached a constant value without further degradation. The uptake decreased from 3.1 mmol g−1 for the first cycle to 2.3 mmol g−1 for the 20th cycle for the NaNO3-promoted MgO adsorbent and from 2.9 to 2.1 mmol g−1 for the NaNO2-promoted MgO adsorbent. The regenerability of the NaNO2-promoted system showed better thermal stability compared to the NaNO3-promoted system, which may be attributed to the generation of double salt Na2Mg(CO3)2. The uptake declined from 1.7 mmol g−1 for the first cycle to 1.1 mmol g−1 for the 20th cycle for the KNO2promoted MgO adsorbent and from 1.7 to 0.8 mmol g−1 for the NaNO2-promoted MgO adsorbent. The regenerability of

(3)

During the carbonation process, nitrate−nitrite transformed to the melting state without decomposition to generate any NOx production. Proper thermal dissociation and excellent thermal stability are key factors influencing the chemical differences produced during CO2 adsorption when using molten salts. XRD results also demonstrated that Na2Mg(CO3)2 was generated during the reaction process as a result of the exposure of MgO and Na2CO3 to CO2 via the reaction as follows:30 MgO + Na 2CO3 + CO2 → Na 2Mg(CO3)2

(4)

The adsorption mechanism was chemisorption through the formation of Na2Mg(CO3)2 and MgCO3 under pure dry CO2. It is observed that the double carbonates appear thermodynamically more stable, with an enthalpy lower by about 24 kJ mol−1, than MgCO3, with a Gibbs free energy of formation lower by about 35 kJ mol−1. Consequently, the driving force for Na2Mg(CO3)2 formation is larger than MgCO3 formation at equivalent conditions.44 Similarly, the XRD spectra of KNO2-promoted MgO adsorbents exhibited characteristic peaks of KNO3 (JCPDS number 32-0824, rhombohedral), and LiNO3-promoted MgO adsorbents exhibited characteristic peaks of LiNO3 (JCPDS number 08-0466, rhombohedral) and Li2CO3 (JCPDS number 22-1141, monoclinic) as a result of their poor chemical stability. Figure 4 presents XRD spectra of molten salt-promoted MgO adsorbents after 4 h of interaction with CO2 and indicated that well-crystallized magnesium carbonate (MgCO3 , JCPDS number 08-0479) with a rhombohedral structure was generated after the adsorption of CO2. Figure 5 presents the SEM images of the NaNO3-coated magnesium precursor (Figure 5a), fresh NaNO3-promoted MgO adsorbents (Figure 5b), NaNO3-promoted MgO adsorbents after adsorption (Figure 5c), and NaNO3promoted MgO adsorbents after 20 cycles of CO2 adsorption and desorption (Figure 5d). The NaNO3-coated magnesium precursor preserved its original sheet-like form after impregE


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Energy & Fuels the KNO 3 /KNO 2 -promoted system showed a similar tendency, which may be correlated with the presence of active ingredient KNO3 coated on the MgO skeleton. Gradual deterioration over repeated cycles could be attributed to a sintering effect induced by phase transition.45,46 Notable, the LiNO3-promoted MgO adsorbent showed a slight increase from 1.6 mmol g−1 at the first cycle to 3.7 mmol g−1 for the ninth cycle and gradually dropped to 0.8 mmol g−1 for the 20th cycle, which may result from the crystal and textural agglomeration over phase transitional cyclic operation, as shown in Figure S9 of the Supporting Information.28,30 To understand the cause of the CO2 adsorption deterioration over several repeated cycles, we subsequently performed XRD results of the optimized molten salt-promoted MgO adsorbents after 20 cycles of CO2 adsorption and desorption, as shown in Figure 7a. Both the sodium nitrate/nitrite and

Figure 7. XRD patterns of the (a) optimized molten salt-promoted MgO adsorbents after 20 cycles of CO2 adsorption and desorption and (b) 35 mol % LiNO3-promoted MgO adsorbent undergoing different cycles (cycle numbers = 0, 1, 5, 10, 15, and 20, respectively) of CO2 adsorption and desorption.

gradually disappeared with increasing cycles, and the increasingly stronger characteristic peaks of Li2CO3 indicated the loss of the LiNO3 promoter with relatively poor thermal stability, which thus gave rise to an irreversible deterioration of the CO2 uptake over repeated cycles (Figure 7b). To summarize, molten salts with simultaneously superior thermal stabilities and appropriate melting points are favorable to attain high CO2 uptake and excellent regenerability for MgO-based adsorbents. The selection of the molten salt promoter is essential to the CO2 capture enhancement. Besides, the crystal and textural agglomeration over phase transitional cyclic operation explored via SEM images suggested that the stability of the MgO skeleton structure is of vital importance for the regenerability of MgO-based CO2 adsorbents as well. To address future applicability for commercial CO2 capture, the CO2 adsorption/desorption cycling stability of 25 mol % NaNO2-doped MgO was then examined at different CO2 concentrations (20 or 50% CO2, respectively), as shown in Figure 6b. The CO2 uptake displays an excellent regenerability of around 2 mmol g−1 when exposed to a 50% CO2 atmosphere during the adsorption period. Although the CO2 uptake of the MgO-based adsorbent met a gradual decline of 0.3 mmol g−1 for the final several cycles when the CO2 concentration decreased to 20%, the adsorption capacity appeared considerably stable, with a reversible capacity of over 1.3 mmol g−1. Generally, the CO2 adsorption performance

Figure 6. (a) Uptake in molten salt-promoted MgO adsorbents over 20 cycles of CO2 adsorption (at 300 °C in 100% CO2 for 30 min) and desorption (at 450 °C in 100% N2 for 30 min) and (b) uptake in 25 mol % NaNO2-promoted MgO adsorbents over 20 cycles of CO2 adsorption (at 300 °C in diluted CO2 for 30 min) and desorption (at 450 °C in 100% N2 for 30 min).

potassium nitrate/nitrite systems preserved their original characteristic peaks compared to the fresh samples exhibited in Figure 3 which are in accordance with their relatively stable regenerability. The excellent thermal stability of the molten salt promoter as sodium nitrate/nitrite and potassium nitrate/ nitrite makes it possible to preserve their original character without further decomposition under harsh repeated conditions. On the contrary, the characteristic peaks of LiNO3 F


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Energy & Fuels exhibited a relatively stable manner against the reduced CO2 concentration, demonstrating great prospect of application in the intermediate CO2 capture and utilization.

4. CONCLUSION We have performed a detailed study of molten salt [MNO3/ NO2 (M = Li, Na, and K)]-promoted MgO as regenerable high-capacity CO2 adsorbents. The property of the MgO skeleton with its porous structure was found to be an essential property to yield a high-performance adsorbent. The CO2 adsorption performances of the synthesized MgO adsorbents with different promoter loadings and calcination and adsorption temperatures were systematically compared to provide a basic understanding of the promotion effect of molten salts on the CO2 uptake of MgO. It is noteworthy that optimized NaNO3-promoted MgO displayed the highest CO2 capture capacity of 17.0 mmol g−1 at 300 °C for 4 h and maintained excellent regenerability after 20 cycles of CO2 capture and adsorbent regeneration. The sintering-induced deterioration of the CO2 uptake of the adsorbents over multiple operation cycles indicated that it is essential to mitigate the deterioration of the microstructure of the MgO skeleton. The CO2 uptake promoted by MNO3/NO2 (M = Li, Na, and K)/MgO composites was in the order of NaNO3 (17.0 mmol g−1) > LiNO3 (14.1 mmol g−1) > NaNO2 (12.5 mmol g−1) > KNO2 (9.3 mmol g−1) > KNO3 (5.0 mmol g−1). Furthermore, NaNO3 and KNO3 with good chemical stability are the effective constituents for the NaNO3 system and KNO3/KNO2-promoted system, respectively, and preserved their original form after the interaction with CO2. The NaNO2 system composed of NaNO3 and Na2CO3 formed double carbonate Na2Mg(CO3)2 after CO2 adsorption. Detailed electron microscopy-based analyses confirm that the morphology and structure of the MgO skeleton and the introduction of promoters are important components for the intermediatetemperature CO2 capture. Given their unique structural features, outstanding properties, and promising applications, MNO3/NO2 (M = Li, Na, and K)/MgO composites enable enormous potential for the intermediate-temperature CO2 capture.

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carbonate hydroxide hydrate samples before and after calcination (Figure S6), XRD patterns of KNO2-coated light MgO samples before and after calcination and KNO2-coated magnesium carbonate hydroxide hydrate samples before and after calcination (Figure S7), SEM images of NaNO 2-coated magnesium precursors, NaNO2-promoted MgO adsorbents, NaNO2-promoted MgO adsorbents after 4 h of reaction of CO2, and NaNO2-promoted MgO adsorbents after 20 cycles of CO2 adsorption and desorption (Figure S8), SEM images of LiNO3-coated magnesium precursors, LiNO3promoted MgO adsorbents, LiNO3-promoted MgO adsorbents after 4 h of reaction of CO2, and LiNO3promoted MgO adsorbents after 20 cycles of CO2 adsorption and desorption (Figure S9), SEM images of KNO2-coated magnesium precursors, KNO2-promoted MgO adsorbents, KNO2-promoted MgO adsorbents after 4 h of reaction of CO2, and KNO2-promoted MgO adsorbents after 20 cycles of CO2 adsorption and desorption (Figure S10), and SEM images of KNO3coated magnesium precursors, KNO3-promoted MgO adsorbents, KNO3-promoted MgO adsorbents after 4 h of reaction of CO2 , and KNO 3-promoted MgO adsorbents after 20 cycles of CO2 adsorption and desorption (Figure S11) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] and/or qiangwang@bjfu. edu.cn. *Telephone: 86-13810270882. E-mail: linwr.bjhy@sinopec. com. 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 National Natural Science Foundation of China (51622801 and 51572029), the Beijing Excellent Young Scholar (2015000026833ZK11), and the Beijing Natural Science Foundation (2184114).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b02749. Texture parameters and CO2 adsorption capacities of molten salt-promoted MgO adsorbents with different loadings and calcination temperatures (Table S1), XRD patterns of NaNO3-promoted MgO adsorbents with different loadings and calcination temperatures (Figure S1), XRD patterns of NaNO2-promoted MgO adsorbents with different loadings and calcination temperatures (Figure S2), XRD patterns of LiNO3-promoted MgO adsorbents with different loadings and calcination temperatures (Figure S3), XRD patterns of KNO2promoted MgO adsorbents with different loadings and calcination temperatures (Figure S4), XRD patterns of KNO3-promoted MgO adsorbents with different loadings and calcination temperatures (Figure S5), XRD patterns of NaNO2-coated light MgO samples before and after calcination and NaNO2-coated magnesium

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REFERENCES

(1) Dupont, J.; de Souza, R. F.; Suarez, P. A. Ionic Liquid (Molten Salt) Phase Organometallic Catalysis. Chem. Rev. 2002, 102 (10), 3667−3692. (2) Wasserscheid, P.; Keim, W. Ionic LiquidsNew “Solutions” for Transition Metal Catalysis. Angew. Chem., Int. Ed. 2000, 39 (21), 3772−3789. (3) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-liquid Materials for the Electrochemical Challenges of the Future. Nat. Mater. 2009, 8 (8), 621−629. (4) Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Dialkylimidazolium Chloroaluminate Melts: A New Class of Roomtemperature Ionic Liquids for Electrochemistry, Spectroscopy and Synthesis. Inorg. Chem. 1982, 21 (3), 1263−1264. (5) Hussey, C. L. Room Temperature Haloaluminate Ionic Liquids. Novel Solvents for Transition Metal Solution Chemistry. Pure Appl. Chem. 1988, 60 (12), 1763−1772. (6) Dieter, M. K.; Dymek, C. J., Jr.; Heimer, N. E.; Rovang, J. W.; Wilkes, J. S. Ionic structure and interactions in 1-methyl-3-

G


Article

Energy & Fuels

MgO-based Absorbents with High Cycling Capacity. Adv. Mater. Interfaces 2014, 1 (3), 1400030. (27) 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. (28) Zhao, X.; Ji, G.; Liu, W.; He, X.; Anthony, E. J.; Zhao, M. Mesoporous MgO Promoted with NaNO3/NaNO2 for Rapid andHigh-Capacity CO2 Capture at Moderate Temperatures. Chem. Eng. J. 2018, 332, 216−226. (29) 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. (30) Vu, A.-T.; Ho, K.; Jin, S.; Lee, C.-H. Double Sodium Saltpromoted Mesoporous MgO Sorbent with high CO2 Sorption Capacity at Intermediate Temperatures under Dry and Wet Conditions. Chem. Eng. J. 2016, 291, 161−173. (31) Qiao, Y.; Wang, J.; Zhang, Y.; Gao, W.; Harada, T.; Huang, L.; Hatton, T. A.; Wang, Q. Alkali Nitrates Molten Salt Modified Commercial MgO for Intermediate-Temperature CO2 Capture: Optimization of the Li/Na/K Ratio. Ind. Eng. Chem. Res. 2017, 56 (6), 1509−1517. (32) Gao, W.; Zhou, T.; Wang, Q. Controlled Synthesis of MgO with Diverse Basic Sites and its CO2 Capture Mechanism under Different Adsorption Conditions. Chem. Eng. J. 2018, 336, 710−720. (33) Zhao, Y.; Chen, G.; Bian, T.; Zhou, C.; Waterhouse, G. I.; Wu, L. Z.; Tung, C. H.; Smith, L. J.; O’Hare, D.; Zhang, T. Defect-rich Ultrathin ZnAl-layered Double Hydroxide Nanosheets for Efficient Photoreduction of CO2 to CO with Water. Adv. Mater. 2015, 27 (47), 7824−7831. (34) Yamaguchi, T.; Zhu, J. H.; Wang, Y.; Komatsu, M. A.; Ookawa, M. Supported K-salts as a New Solid Base Catalyst. Chem. Lett. 1997, 26 (10), 989−990. (35) 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 (2), 1089−1096. (36) Stern, K. H. High Temperature Properties and Decomposition of Inorganic Salts Part 3. Nitrates and Nitrites. J. Phys. Chem. Ref. Data 1972, 1 (3), 747−772. (37) Bordyushkova, E. A.; Prostsenko, P. I.; Venerovskaya, L. N. Kinetics of Thermal Dissociation and Stability of Melts and AlkaliMetal Nitrates. J. Appl. Chem. 1967, 40, 1386−1390. (38) Freeman, E. S. The Kinetics of the Thermal Decomposition of Sodium Nitrate and of the Reaction between Sodium Nitrite and Oxygen. J. Phys. Chem. 1956, 60 (11), 1487−1493. (39) Kust, R. N.; Duke, F. R. A Study of the Nitrate Ion Dissociation in Fused Nitrates. J. Am. Chem. Soc. 1963, 85, 3338−3340. (40) Kust, R. N.; Burke, J. D. Thermal Decomposition in Alkali Metal Nitrate Melts. Inorg. Nucl. Chem. Lett. 1970, 6, 333−335. (41) Zheng, Q.; Huang, L.; Zhang, Y.; Wang, J.; Zhao, C.-Z.; Zhang, Q.; Zheng, W.; Cao, D.; O’Hare, D.; Wang, Q. Unexpected Highly Reversible Topotactic CO2 Sorption/Desorption Capacity for Potassium Dititanate. J. Mater. Chem. A 2016, 4 (33), 12889−12896. (42) Centnerszwer, M.; Bružs, B. Zerfallsgeschwindigkeit fester Stoffe. I. Z. Phys. Chem. 1925, 115U (1), 365−376. (43) Centnerszwer, M.; Bružs, B. Die stufenweise Dissoziation des Magnesiumkarbonats. Z. Phys. Chem. 1924, 114 (1), 237−250. (44) 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. Int. J. Greenhouse Gas Control 2013, 12 (1), 351−358. (45) Sánchez-Camacho, P.; Gómez-García, J. F.; Pfeiffer, H. Thermokinetic and Conductivity Analyzes of the High CO2, Chemisorption on Li5AlO4, and Alkaline Carbonate Impregnated Li5AlO4, Samples: Effects Produced by the Use of CO2 Partial Pressures and Oxygen Addition. J. Energy Chem. 2017, 26 (5), 919− 926. (46) Olavarría, P.; Vera, E.; Lima, E. J.; Pfeiffer, H. Synthesis and Evaluation as CO2 Chemisorbent of the Li5(Al1‑xFex)O4 Solid

ethylimidazolium chloride−aluminum chloride molten salts. J. Am. Chem. Soc. 1988, 110 (9), 2722−2726. (7) Winther-Jensen, B.; Winther-Jensen, O.; Forsyth, M.; MacFarlane, D. R. High Rates of Oxygen Reduction over a Vapor Phase-polymerized PEDOT Electrode. Science 2008, 321 (5889), 671−674. (8) Abbott, A. P.; McKenzie, K. J. Application of Ionic Liquids to the Electrodeposition of Metals. Phys. Chem. Chem. Phys. 2006, 8 (37), 4265−4279. (9) Liu, Q. X.; El Abedin, S. Z.; Endres, F. Electroplating of Mild Steel by Aluminium in a First Generation Ionic Liquid: A Green Alternative to Commercial Al-Plating in Organic Solvents. Surf. Coat. Technol. 2006, 201 (3−4), 1352−1356. (10) Fuller, J.; Breda, A. C.; Carlin, R. T. Ionic Liquid-Polymer Gel Electrolytes. J. Electrochem. Soc. 1997, 144 (4), L67−L70. (11) Plechkova, N. V.; Seddon, K. R. Applications of Ionic Liquids in the Chemical Industry. Chem. Soc. Rev. 2008, 37 (1), 123−150. (12) Endres, F.; El Abedin, S. Z. Electrodeposition of Stable and Narrowly Dispersed Germanium Nanoclusters from An Ionic Liquid. Chem. Commun. 2002, 892−893. (13) Wasserscheid, P.; Bosmann, A.; Bolm, C. Synthesis and Properties of Ionic Liquids Derived From the ’Chiral Pool’. Chem. Commun. 2002, 200−201. (14) Chen, C.; Zhang, S.; Row, K. H.; Ahn, W.-S. Amine-silica Composites for CO2 Capture: A Short Review. J. Energy Chem. 2017, 26 (5), 868−880. (15) 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 (1), 42−55. (16) Gao, W.; Zhou, T.; Louis, B.; Wang, Q. Hydrothermal Fabrication of High Specific Surface Area Mesoporous MgO with Excellent CO2 Adsorption Potential at Intermediate Temperatures. Catalysts 2017, 7 (4), 116. (17) Su, X.; Yang, X.; Zhao, B.; Huang, Y. Designing of Highly Selective and High-temperature Endurable RWGS Heterogeneous Catalysts: Recent Advances and the Future Directions. J. Energy Chem. 2017, 26 (5), 854−867. (18) Gao, W.; Zhou, T.; Gao, Y.; Louis, B.; O’Hare, D.; Wang, Q. Molten salts-modified MgO-based Adsorbents for Intermediatetemperature CO2 Capture: A Review. J. Energy Chem. 2017, 26 (5), 830−838. (19) Han, S. J.; Bang, Y.; Kwon, H. J.; Lee, H. C.; Hiremath, V.; Song, I. K.; Seo, J. G. Elevated Temperature CO2 Capture on Nanostructured MgO−Al2O3 Aerogel: Effect of Mg/Al Molar Ratio. Chem. Eng. J. 2014, 242, 357−363. (20) Hiremath, V.; Shavi, R.; Seo, J. G. Controlled Oxidation State of Ti in MgO-TiO2 Composite for CO2 Capture. Chem. Eng. J. 2017, 308, 177−183. (21) Hanif, A.; Dasgupta, S.; Nanoti, A. High Temperature CO2 Adsorption by Mesoporous Silica Supported Magnesium Aluminum Mixed Oxide. Chem. Eng. J. 2015, 280, 703−710. (22) Li, L.; Wen, X.; Fu, X.; Wang, F.; Zhao, N.; Xiao, F.; Wei, W.; Sun, Y. MgO/Al2O3 Sorbent for CO2 Capture. Energy Fuels 2010, 24 (10), 5773−5780. (23) Kim, T. K.; Lee, K. J.; Yuh, J.; Kwak, S. K.; Moon, H. R. Multicore MgO NPs@C Core−Shell Nanospheres for Selective CO2 Capture under Mild Conditions. New J. Chem. 2014, 38 (4), 1606− 1610. (24) 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 (23), 8153−8161. (25) 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 (6), 1943−1949. (26) 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 H


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