Effective Capture of Carbon Dioxide Using Hydrated Sodium Carbonate Powders

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Article Effective Capture of Carbon Dioxide Using Hydrated Sodium Carbonate Powders

Yuanhao Cai 1, Weilin Wang 1,2,3, Liang Li 1, Zhaofeng Wang 2,3, Suying Wang 1 ID , Hao Ding 2,3, Zhengguo Zhang 1 ID , Luyi Sun 2,3,4,* ID and Weixing Wang 1,* ID

1 School of Chemistry and Chemical Engineering, China Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, South China University of Technology, Guangzhou 510640, Guangdong, China; [email protected] (Y.C.); [email protected] (W.W.); [email protected] (L.L.); [email protected] (S.W.); [email protected] (Z.Z.) 2 Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA; [email protected] (Z.W.); [email protected] (H.D.) 3 Department of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, CT 06269, USA 4 Department of Biomedical Engineering, University of Connecticut, Storrs, CT 06269, USA * Correspondence: [email protected] (L.S.); [email protected] (W.W.); Tel.: +86-0486-6895 (L.S.); +86-20-8711-3171 (W.W.)

Received: 27 December 2017; Accepted: 20 January 2018; Published: 24 January 2018

Abstract: The emission of CO2 has been considered a major cause of greenhouse effects and global warming. The current CO2 capture approaches have their own advantages and weaknesses. We found that free-ﬂowing hydrated sodium carbonate (Na2CO3) powders with 30 wt % water can achieve a very high CO2 sorption capacity of 282 mg/g within 60 min and fast CO2 uptake (90% saturation uptake within 16 min). The results suggest that the alkaline solution resulting from the dissolution of partial Na2CO3 can freely attach onto the hydrated Na2CO3 particles, which provides an excellent gas–liquid interface for CO2 capture, leading to signiﬁcantly enhanced CO2 sorption capacity and kinetics.

Keywords: hydrated sodium carbonate; CO2 capture; free-ﬂowing powders

1. Introduction

Emission of CO2 is identiﬁed as the main contributor to global climate change. Reducing the levels of CO2 in the atmosphere has become a pressing issue worldwide, and capturing and sequestrating CO2 as an option to decrease levels of CO2 has been widely explored [1–4]. A number of promising materials for CO2 capture were reported [5–10]. The best developed are probably aqueous amines [11,12], including monoethanolamine (MEA) [13,14] and diethanolamine (DEA) [15,16]. However, liquid amines have some serious disadvantages, including amine evaporation [17,18], corrosion to equipment [19], and high energy cost for regeneration [20,21]. A feasible way to reduce the corrosivity and the regeneration energy is to use supported amine adsorbents [22–26], but the raw materials are currently too expensive to be applied in large-scale industrial settings [27]. As an alternative to supported amine sorbents, alkali metal carbonates such as K2CO3 and Na2CO3 as solid sorbents have received wide attention with both high sorption capacity and low cost [28–32]. However, the main problem of using carbonates is their slow reaction kinetics [33–35]. Cooper and co-workers reported that dry K2CO3 solution (K2CO3 aqueous solution coated with hydrophobic silica powders) exhibited signiﬁcantly increased CO2 uptakes [36], but the recyclability of this sorbent was poor. It has been generally accepted that K2CO3 is superior to Na2CO3 in terms of both CO2 uptake capacity and kinetics [37–39]. However, using Na2CO3 will be more competitive for

Materials 2018, 11, 183; doi:10.3390/ma11020183 www.mdpi.com/journal/materials Materials 2018, 11, 183 2 of 10 large-scale industrial applications because of its lower cost, especially if one can dramatically promote the rate of the key reaction: Na2CO3 + H2O + CO2 2NaHCO3 (1)

One of the most common approaches to tackle this problem is to disperse Na2CO3 powders on solid supports [40,41], but such a strategy also reduces CO2 sorption capacity because the inclusion of the supports greatly decreases the amount of active components per unit mass [42]. In this report, we demonstrate that support-free hydrated sodium carbonate powders (HSCPs) prepared by simply mixing a certain amount of water and Na2CO3 powders exhibit effective CO2 capture. The alkaline solution resulting from the dissolution of partial Na2CO3 can freely attach into hydrated Na2CO3 particles, which provides an excellent gas–liquid interface for CO2 capture, leading to signiﬁcantly enhanced CO2 sorption capacity and kinetics. The elimination of supports not only reduces the overall cost of raw materials, but also increases the CO2 sorption capacity, both of which are critical for large-scale applications.

2. Experimental

2.1. Preparation of HSCPs

Na2CO3 (99.8%) was purchased from Tianjin Qilun Chemical Technology Co. Ltd., Tianjin, China. Na2CO3·H2O (99%) was purchased from Aladdin Co. Ltd., Shanghai, China. MEA (99%) was purchased from Jiangsu Yonghua Chemical Technology Co. Ltd., Changshu, China. CO2 (99.9%) was supplied by Zhuozheng Gas Co. Ltd., Guangzhou, China. All the chemicals were used as received without further puriﬁcation. A series of HSCPs with different Na2CO3 contents were prepared by thoroughly mixing an appropriate amount of Na2CO3 and deionized water at room temperature.

2.2. Characterization X-ray diffraction (XRD) patterns of the samples were recorded using a Bruker D8 diffractometer (Bruker, Karlsruhe, Germany) with Bragg–Brentano θ−2θ geometry (20 kV and 5 mA), using a graphite monochromator with Cu Kα radiation. To measure the CO2 capture capacity of the HSCP samples, 5.0 g HSCP was charged into a 50 mL container, which was exposed to CO2 using a balloon containing a sufﬁcient amount of CO2 gas (ca. 5 L with a pressure of ca. 1.05 bar). The amount of CO2 captured by each HSCP sample was measured using a balance. A mufﬂe furnace (Luoyang BSK Electronic Materials Co. Ltd., Luoyang, China) was used to regenerate the sorbents at 250 ◦C for 1 h, which was mixed with water to reform HSCPs.

3. Results and Discussion

Figure1a shows the CO 2 uptake kinetic curves using various HSCPs (labelled as HSCP-X, where X ◦ is the mass percentage of Na2CO3 in the mixture) as a sorbent at 30 C. It was found that HSCP-10 to HSCP-60 had a very low CO2 sorption capacity (<32 mg/g of HSCP). The CO2 uptake capacity rapidly rose to 156 mg/g when the mass fraction of Na2CO3 was increased to 65 wt %, i.e., HSCP-65, but it still suffered from low sorption kinetics. Further increasing mass fraction of Na2CO3 led to another signiﬁcant increase in term of both sorption capacity and kinetics. At the optimum concentration of 70 wt % (i.e., HSCP-70), the CO2 uptake capacity reached 282 mg/g within 60 min, and the t90 (the time to achieve 90% of this capacity) was only 16 min. This capacity is much higher than that of other Na2CO3-based CO2 sorbents reported in the literature, which varies between 32 and 140 mg/g [43,44]. Although HSCP-75 achieved the highest capacity (286 mg/g), its CO2 sorption rate was relatively slow and t90 was about 45 min. Materials 2018, 11, x FOR PEER REVIEW 3 of 9 Materials 2018, 11, 183 3 of 10

◦ FigureFigure 1.1. ((aa)) COCO22 sorptionsorption kineticskinetics ofof variousvarious HSCPsHSCPs atat 3030 °C;C; (b(b)) conversionconversion ratioratio ofof Na Na2CO2CO33 inin variousvarious HSCPsHSCPs andand differentdifferent reactionreaction time. time.

It was found that too high a concentration of Na2CO3 in HSCP would actually lower the CO2 It was found that too high a concentration of Na2CO3 in HSCP would actually lower the CO2 uptake capacity. When the concentration of Na2CO3 in HSCP reached 80 and 85 wt %, the CO2 uptake capacity. When the concentration of Na2CO3 in HSCP reached 80 and 85 wt %, the CO2 uptake capacity decreased to 124 and 46 mg/g, respectively. Theoretically, the CO2 sorption capacity uptake capacity decreased to 124 and 46 mg/g, respectively. Theoretically, the CO2 sorption capacity is is directly related to the amount of Na2CO3 in HSCPs when the content of water is more than 14.5 wt directly related to the amount of Na2CO3 in HSCPs when the content of water is more than 14.5 wt % % according to Equation (1). Thus, the conversion ratio of Na2CO3 is a good indicator of the CO2 according to Equation (1). Thus, the conversion ratio of Na2CO3 is a good indicator of the CO2 sorption sorption behaviour. As shown in Figure 1b, with an increasing mass fraction of Na2CO3, the behaviour. As shown in Figure1b, with an increasing mass fraction of Na 2CO3, the conversion conversion ratio of Na2CO3 decreased initially, then dramatically increased to a maximum value ratio of Na2CO3 decreased initially, then dramatically increased to a maximum value close to 100% beforeclose to declining 100% before again. declining After 60 again. min of After reaction, 60 min HSCP-70 of reaction, exhibited HSCP-70 the highest exhibited conversion the highest rate conversion rate (97.1%), which suggested that most of Na2CO3 was consumed. The HSCPs with a (97.1%), which suggested that most of Na2CO3 was consumed. The HSCPs with a low mass fraction low mass fraction of Na2CO3, such as HSCP-10, also showed a high conversion ratio, in which of Na2CO3, such as HSCP-10, also showed a high conversion ratio, in which Na2CO3 dissolved in Na2CO3 dissolved in water to form a solution, due to its high degree of hydrolysis [45]. However, water to form a solution, due to its high degree of hydrolysis [45]. However, their corresponding CO2 their corresponding CO2 uptake capacity is low because of the limited amount of Na2CO3 presented uptake capacity is low because of the limited amount of Na2CO3 presented (Figure1a). The other two (Figure 1a). The other two sets of data in Figure 1b represent the conversion ratios of Na2CO3 after sets of data in Figure1b represent the conversion ratios of Na 2CO3 after ﬁve and 15 min of reaction. ForfiveHSCP-70, and 15 min its of conversion reaction. For ratio HSCP-70, increased its rapidlyconversion from ratio 5 to increased 15 min,but rapidly changed fromlittle 5 to 15 from min, 15 but to changed little from 15 to 60 min, which suggested that most of Na2CO3 was consumed within 15 60 min, which suggested that most of Na2CO3 was consumed within 15 min, thus showing a high reactionmin, thus rate. showing a high reaction rate.

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Overall, the above results show that the concentration ofof NaNa22CO3 inin HSCPs HSCPs has has a a great great influence inﬂuence on CO2 capture, which which can can be explained by the fact that the morphology of HSCPs varies from aqueous solution and slurry,slurry, toto powderspowders withwith anan increasing increasing Na Na2CO2CO33 concentration.concentration. AtAt lowlow NaNa22COCO33 concentrations, thethe HSCPsHSCPs existexist asas an an aqueous aqueous solution solution or or slurry slurry as as shown shown in in Figure Figure1b 1b (inset), (inset), which which is isnot not ideal ideal for for CO CO2 capture2 capture because because of of the the low low gas–liquid gas–liquid contact contact surface surface area. area. However, However, HSCP-70 HSCP-70 is is a asample sample of of free-ﬂowing free-flowing powders powders (Figure (Figure2) with 2) with a much a much higher higher gas–liquid gas–liquid contact contact surface surface area. This area. is whyThis is it haswhy a it rapid has a reaction rapid reaction rate and rate a high and COa high2 uptake CO2 uptake capacity. capacity.

Figure 2. Free-flowingFree-ﬂowing HSCP-70 from a glass funnel.

In order to better understand the mechanism of CO2 sorption by HSCPs, the XRD patterns In order to better understand the mechanism of CO2 sorption by HSCPs, the XRD patterns (Figure 3) of various Na2CO3-based compounds were collected, including the reaction products of (Figure3) of various Na CO -based compounds were collected, including the reaction products HSCP-70 after 0, 5, 15, and2 603 min of sorption reaction at 30 °C. The XRD pattern of HSCP-70 was of HSCP-70 after 0, 5, 15, and 60 min of sorption reaction at 30 ◦C. The XRD pattern of HSCP-70 very close to the standard pattern of Na2CO3·H2O, which contains only 14.5 wt % water. This was very close to the standard pattern of Na2CO3·H2O, which contains only 14.5 wt % water. indicates that HSCP-70 contains extra water. As such, we also studied the CO2 sorption by pure This indicates that HSCP-70 contains extra water. As such, we also studied the CO2 sorption by Na2CO3·H2O, but it exhibited a low CO2 sorption capacity and rate (Figure 4). This suggests that the pure Na2CO3·H2O, but it exhibited a low CO2 sorption capacity and rate (Figure4). This suggests extra water contained in the sorbent plays a significant role in CO2 sorption. It indicates that the that the extra water contained in the sorbent plays a signiﬁcant role in CO2 sorption. It indicates reaction proceeds most rapidly and effectively when Na2CO3, H2O, and CO2 are present that the reaction proceeds most rapidly and effectively when Na CO ,H O, and CO are present simultaneously. Based on the above results, we propose that the extra2 water3 2 on the surface2 of HSCPs simultaneously. Based on the above results, we propose that the extra water on the surface of HSCPs helps to form a basic alkaline aqueous environment. When CO2 diffuses to the surface of HSCPs, it helps to form a basic alkaline aqueous environment. When CO diffuses to the surface of HSCPs, reacts with the basic aqueous media. Since the reaction is exothermic,2 the generated heat triggers the it reacts with the basic aqueous media. Since the reaction is exothermic, the generated heat triggers decomposition of sodium carbonate hydrates, meanwhile releasing water to drive the reaction to the decomposition of sodium carbonate hydrates, meanwhile releasing water to drive the reaction proceed continuously. In addition, along the reaction of HSCP-70 and CO2, we also found that the to proceed continuously. In addition, along the reaction of HSCP-70 and CO2, we also found that characteristic peaks of Na2CO3 disappeared gradually, then intermediate structures, such as the characteristic peaks of Na2CO3 disappeared gradually, then intermediate structures, such as Na3H(CO3)2·2H2O (i.e., Na2CO3·NaHCO3·2H2O) and Na2CO3·3NaHCO3, appeared after reacting for Na3H(CO3)2·2H2O (i.e., Na2CO3·NaHCO3·2H2O) and Na2CO3·3NaHCO3, appeared after reacting five and 15 min, respectively. Eventually, virtually pure NaHCO3 formed after 60 min of reaction, for ﬁve and 15 min, respectively. Eventually, virtually pure NaHCO formed after 60 min of reaction, which is expected. 3 which is expected.

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Figure 3. XRD patterns of various Na2CO3 based compounds and the reaction products of HSCP-70 Figure 3. XRD patterns of various Na2CO3 based compounds and the reaction products of HSCP-70 after 0, 5, 15, and 60 min of CO2 sorption reaction at 30 ◦°C. after 0, 5, 15, and 60 min of CO2 sorption reaction at 30 C.

The amine-based CO2 capture system is a proven technology that is already commercialized. To The amine-based CO capture system is a proven technology that is already commercialized. prevent excessive corrosion,2 typically 30 wt % MEA aqueous solution is used [11]. As shown in To prevent excessive corrosion, typically 30 wt % MEA aqueous solution is used [11]. As shown in Figure 4, a 30 wt % MEA aqueous solution showed similar CO2 uptake kinetics initially, but its Figure4, a 30 wt % MEA aqueous solution showed similar CO uptake kinetics initially, but its overall overall sorption capacity was relatively low (111 mg/g versus2 282 mg/g for HSCP-70). We also sorption capacity was relatively low (111 mg/g versus 282 mg/g for HSCP-70). We also studied studied the CO2 sorption capacity of pure water as a control, whose CO2 uptake capacity was ca. 0.7 the CO sorption capacity of pure water as a control, whose CO uptake capacity was ca. 0.7 mg/g mg/g (Figure2 4). 2 (Figure4).

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2 FigureFigure 4.4. COCO 2sorptionsorption kinetics kinetics of ofHSCP-70, HSCP-70, pure pure water, water, 30 wt 30 wt% MEA % MEA aqueous aqueous solution, solution, and Figure 4. CO2 sorption kinetics of HSCP-70, pure water, 30 wt % MEA aqueous solution, and ◦ andNa2CO Na23·HCO2O3· Hat2 30O at°C. 30 C. Na2CO3·H2O at 30 °C.

We also studied the CO2 uptake kinetics at different temperatures and the recyclability of WeWe also also studied studied the the CO 2COuptake2 uptake kinetics kinetics at different at different temperatures temperatures and the and recyclability the recyclability of HSCP-70. of HSCP-70. The suitable temperature range for CO2 capture was determined to◦ be 30–50 °C (Figure 5). TheHSCP-70. suitable The temperature suitable temperature range for CO range2 capture for CO was2 capture determined was determined to be 30–50 to Cbe(Figure 30–50 °C5). (Figure A higher 5). A higher temperature will cause excessive evaporation of water in HSCP-70, and a lower temperatureA higher temperature will cause excessive will cause evaporation excessive of waterevaporation in HSCP-70, of water and a lowerin HSCP-70, temperature and will a causelower temperature will cause the formation of Na2CO3·7H2O (as shown in Figure 6), both of which lead to a thetemperature formation will of Nacause2CO the3·7H formation2O (as shown of Na2 inCO Figure3·7H2O6 ),(as both shown of which in Figure lead 6), to both a lower of which CO 2 leaduptake to a lower CO2 uptake of HSCP-70. HSCP-70 also exhibited excellent recyclability with little deterioration oflower HSCP-70. CO2 uptake HSCP-70 of HSCP-70. also exhibited HSCP-70 excellent also exhibited recyclability excellent with recyclability little deterioration with little in CO deterioration2 sorption in CO2 sorption capacity and reaction rate after recycling (Figure 7). capacityin CO2 sorption and reaction capacity rate and after reaction recycling rate (Figure after recycling7). (Figure 7). 300 300 250 250 200 200 10 °C 10 °C 150 20 °C 150 3020 °C 4030 °C 100 40 °C

uptake (mg/g) 100 50 °C 2

uptake (mg/g) 50 °C 2 50 CO 50 CO 0 0 0 1020304050607080 0 1020304050607080 Time (min) Time (min)

Figure 5. CO2 sorption kinetics of HSCP-70 at different temperatures. FigureFigure 5.5.CO CO22 sorption kinetics of HSCP-70 atat differentdifferent temperatures.temperatures.

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FigureFigureFigure 6.6. 6. XRD XRD patterns patterns ofof of HSCP-70HSCP-70 HSCP-70 atat at 1010 10 andand and 3030 30 ◦°C. °C.C.

300300

250 250

200200

150

150 cycle 1

cycle 1

uptake(mg/g) cycle 2 2 uptake(mg/g) cycle 2 2 100 100 cyclecycle 33 CO CO 5050

00 102030405060 102030405060 TimeTime (min)(min)

◦ FigureFigure 7. 7. Recycling Recycling performance performance of of HSCP-70 HSCP-70 after after regenerationregeneration regeneration atat at 250250 250 °C. °C.C.

4.4. ConclusionsConclusions InInIn summary, summary,summary, we wewe have havehave demonstrated demonstrateddemonstrated that thatthat support-free support-freesupport-free HSCPs HSCPsHSCPs can cancan be be usedbe usedused as effective asas effectiveeffective sorbents sorbentssorbents for for CO2 capture with a high capacity (282 mg/g) and fast sorption rate (90% saturation uptake within COfor 2COcapture2 capture with with a high a high capacity capacity (282 (282 mg/g) mg/g) and and fast fast sorption sorption rate rate (90% (90%saturation saturation uptakeuptake withinwithin 16 min). The elimination of support and the low cost of Na2CO3 make this technology more 1616 min).min). The The elimination elimination of supportof support and theand low the cost low of Nacost2CO of3 makeNa2CO this3 make technology this technology more competitive more forcompetitivecompetitive large-scale forfor applications. large-scalelarge-scale applications.Inapplications. addition, based InIn additionaddition on the, reaction, basedbased onon principles, thethe reactionreaction HSCPs principles,principles, should HSCPs alsoHSCPs have shouldshould high also have high potential in capturing other acid gases, including SOx, NOx, H2S, and Cl2. potentialalso have inhigh capturing potential other in capturing acid gases, other including acid gases, SOx, NOx,including H2S, SOx, and ClNOx,2. H2S, and Cl2.

Acknowledgments:Acknowledgments:Acknowledgments: This ThisThis research research research is is sponsored issponsored sponsored by by the the by National National the National Natural Natural Natural Scienc Sciencee ScienceFoundation Foundation Foundation of of China China (21176093 of(21176093 China (21176093 and 21676107). L.S. acknowledges the partial support by the Air Force Office of Scientific Research andand 21676107).21676107). L.S.L.S. acknowledgesacknowledges thethe partialpartial supportsupport byby ththee AirAir ForceForce OfficeOffice ofof ScientificScientific ResearchResearch (Grant(Grant (GrantFA9550-12-1-0159). FA9550-12-1-0159). The authors The authors acknowledge acknowledge Pinzhen Pinzhen Lin, Haoxin Lin, Huang, Haoxin and Huang, Zhuyan and Tan Zhuyan for their Tan assistance for their assistanceFA9550-12-1-0159). for this research. The authors acknowledge Pinzhen Lin, Haoxin Huang, and Zhuyan Tan for their assistance forfor this this research. research. Author Contributions: Weixing Wang and Luyi Sun conceived and designed the experiments. Yuanhao Cai, LiangAuthorAuthor Li, Contributions:Contributions: Weilin Wang, Weixing SuyingWeixing Wang, WangWang andandand HaoLuyiLuyi Ding SunSun conceived performedconceived andand the dede experimentssignedsigned thethe experiments. and/orexperiments. analyzed YuanhaoYuanhao the data. Cai,Cai, ZhaofengLiangLiang Li,Li, Wang,WeilinWeilin Zhengguo Wang,Wang, SuyingSuying Zhang, Wang,Wang, and Hao andand Ding HaoHao discussed DingDing performedperformed the experiments thethe experimentsexperiments and data. and/orand/or Weixing analyzedanalyzed Wang, Luyi thethe data.data. Sun, Yuanhao Cai, Weilin Wang, Zhaofeng Wang, and Hao Ding wrote the manuscript. All authors discussed the ZhaofengZhaofeng Wang,Wang, ZhengguoZhengguo Zhang,Zhang, anandd HaoHao DingDing discusseddiscussed thethe experimentsexperiments andand data.data. WeixingWeixing Wang,Wang, LuyiLuyi results and commented on the manuscript. Sun,Sun, YuanhaoYuanhao Cai,Cai, WeilinWeilin Wang,Wang, ZhZhaofengaofeng Wang,Wang, andand HaoHao DingDing wrotewrote ththee manuscript.manuscript. AllAll authorsauthors discusseddiscussed Conflictsthethe results results of and and Interest: commented commentedThe authors on on the thedeclare manuscript. manuscript. no conflict of interest.

ConflictsConflicts of of Interest: Interest: The The authors authors declare declare no no conflict conflict of of interest. interest.

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