Controllable Synthesis of Various CaCO3 Morphologies Based on a

May 13, 2016 - ... of 9.0–9.75 μm accumulated by small square particles (Figure 3D). ..... In other words, the sheaf-like CaCO3 microparticles were...
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Research Article pubs.acs.org/journal/ascecg

Controllable Synthesis of Various CaCO3 Morphologies Based on a CCUS Idea Feng Sha, Ning Zhu, Yijia Bai, Qiang Li, Bo Guo, Tianxiang Zhao, Fei Zhang, and Jianbin Zhang* College of Chemical Engineering, Inner Mongolia University of Technology, Hohhot 010051, China S Supporting Information *

ABSTRACT: A novel, green, and economic CaCO3 synthesis method using a CO2-storage material (CO2SM) was developed based on the strategy of carbon capture, utilization, and storage (CCUS). In this process, CO2SM was used as both a CO2 source and a crystal modifier to regulate and control the crystallization of CaCO3. 1,4-Butanediol (BDO) and/or 1,2-ethanediamine (EDA) from CO2SM played unexpected roles as a surfactant, structure-directing agent, and/or pH modulator. The effects of CO2SM concentrations, reaction temperatures, and reaction times were systematically investigated. Pure calcite (bundle-like and dumbbell-like) and pure vaterite (oblate spheroid and snowball-like) CaCO3 products were obtained. Moreover, the CO2SM was recycled to prepare same crystal phase CaCO3 upon the addition of an appropriate amount of Ca(OH)2 with 120 g·L−1 CO2SM concentration. KEYWORDS: Calcium carbonate, CO2-storage material, Morphology control, Cycle preparation, CO2 capture, Utilization and storage



INTRODUCTION Carbon dioxide (CO2) emission from fossil fuel combustion is a major anthropogenic factor in global warming and rising sea levels.1,2 Because of international efforts to decrease greenhouse gases, the reduction of CO2 has been extensively studied using electrochemical and photochemical reactions;3,4 however, the results obtained are still not satisfactory. Recently, CO2 capture and sequestration (CCS) has attracted great attention.5,6 It is a process that captures CO 2 by means of absorption, 7 adsorption,8 or membrane separation technologies,9,10 and then disposes CO2 in deep earth or oceans.11 Although CCS has helped to reduce continuing damage to the environment by CO2, it has significant disadvantages. The sequestered CO2 can leak from storage sites, and could then migrate to the surface and affect soil conditions, infiltrate groundwater, alter plant growth, and contribute to carbon waste.12,13 In contrast, CO2 capture, utilization and sequestration (CCUS) offers some distinct advantages because it not only consumes CO2 but also uses CO2 to afford an environmentally friendly C1 feedstock and to produce value-added chemicals.14,15 Nevertheless, the stability of CO2 and high activation barrier of various chemical transformations are some of the challenges that need to be overcome in order to develop CCUS methods. Conventional and well-developed amine absorption technology has been used in industrial scale CO2 capture, and conversion into various carbamate or carbamate esters;7 however, the drawbacks include high energy consumption, equipment corrosion, and solvent loss.16,17 In recent years, Jessop et al.18,19 put forward an innovative class of CO2 binding organic liquids (CO2BOLs), which consisted of an alcohol and an amidine (or guanidine) © XXXX American Chemical Society

superbase. Upon exposure to CO2, amidinium or guanidinium alkylcarbonate salts are formed with good reactivity, and the high absorption capacity of this material helps to reduce the amine loss. However, the CO2BOLs are unable to attract industrial interest because the amidine (or guanidine) superbase is quite expensive. Simultaneously, the sequestration of CO2 in mineral CaCO3 had also been extensively studied by biomimetic synthesis and CO2 bubbling method.20−27 These strategies have been especially attractive because CaCO3 has many biomedical and industrial applications.28,29 In biomimetic synthesis methods, various kinds of natural or synthetic polymers were used as templates to control nucleation and assembly of CaCO3 crystals.30−32 Biomimetic synthesis is easy to implement, it requires only a small quantity of additive (ppm or g/L) that does not alter the chemical properties of the CaCO3, and it displays a vast array of outcomes due to the sheer number of organic additives that exist. Liu et al.22,33 used biomimetic agents, such as surfactants, polyelectrolytes, double hydrophilic block copolymers, triblock copolymers, polysaccharides, and proteins, as various types of additives to achieve different CaCO3 particle morphologies, polymorphs and sizes. Yu et al.34,35 carried out studies on CaCO3 and obtained products with various morphologies by using different additives. In addition, various organics, such as a Langmuir monolayer,36 self-assembled monolayers,37 polysaccharides,38 and polyReceived: December 29, 2015 Revised: April 26, 2016

A

DOI: 10.1021/acssuschemeng.5b01793 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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mers,39 have been used as templates to induce CaCO3 formation because organic matrixes are considered to function as the key factor in controlling the biomineralization process. In their work, additives were required and they were expensive. The great advantage of this study is that EDA and/or BDO from CO2SM in the reaction system played the unexpected role as an additive to regulate and control the crystallization of CaCO3. Compared with biomimetic synthesis, this CO2 bubbling method is simple and effective, and it uses a common industrial process for the production of CaCO3 particles. Li’s group40−42 reported an effective solid−liquid−gas carbonation system with ionic liquids or high pressure CO2 and obtained different products with various morphologies. Furthermore, Montes-Hernandez et al.43 investigated the growth of nanosized calcite through gas−solid carbonation of nanosized portlandite under anisobaric conditions. The polymorphisms could be achieved even in the presence of additives or under elevated pressures and moderate temperatures. This study demonstrated that different morphologies of CaCO3 could be obtained without additional additives and CO2. In our recent work, the CO2-storage material (CO2SM), which was confirmed as an alkylcarbonate salt, was simply and rapidly derived from the CO2 fixation with the system of BDO + EDA under mild conditions. And then, the CO2SM was used as both a CO2 source and a crystal modifier to regulate and control the crystallization of CaCO3 microparticles. In addition, the cycle use of CO2SM was important for the capture and utilization of CO2. The filtered solution was reused to absorb CO2, and it was repeatedly used to prepare the same crystalline phase CaCO3 microparticles upon the addition of Ca(OH)2 after the precipitation of CaCO3 microparticles at the higher CO2SM concentration of 120 g·L−1. As shown in Figure 1, the BDO + EDA system showed a significant CO2 capture capacity and exhibited promising potential as an efficient CO2 capture system.

Research Article

EXPERIMENTAL SECTION

Materials. The analytical grade EDA and BDO were purchased from Tianjin Reagent Company. Compressed CO2 (99.999 vol %) was purchased from the Standard Things Center (China). Ca(OH)2 was purchased from Sinopharm Chemical Reagent Co., Ltd. Doubly distilled water with a conductivity lower than 0.1 ms·cm−1 (25 °C) was used. All other reagents used were analytical grade. Preparation and Characterization of CaCO3 Microparticles. Preparation of Ca(OH)2 Saturated Suspensions. 10 g of Ca(OH)2 was placed into a 500 mL beaker containing 300 mL of doubly distilled water and stirred at room temperature for 30 min. And then, the suspensions was standing for 1 h. The supernatant upper layer was subjected to vacuum filtration to obtain Ca(OH)2 saturated suspensions. The lower layer of suspensions was added to 300 mL of doubly distilled water, and the above process was repeated. Formation of CO2SM. Pure compressed CO2 was continuously bubbled at 400 mL/min through a solution (approximately 40 g) of the system BDO + EDA (molar ratio = 1:1) for approximately 110 min to form solid CO2SM (Scheme 1 and Figure 2). In this process,

Scheme 1. Formation of the CO2SM from the Reaction of EDA + BDO with CO2

the system of BDO + EDA became turbid at t = 20 min and changed into solid powder at t = 110 min. The solid powder was thoroughly washed three times by ethanol. After washing, the solid powder was dried under vacuum at 60 °C for 3 h and stored at room temperature, which was named as CO2SM. Preparation of CaCO3 Microparticles. The crystallization of CaCO3 was carried out in the temperature range of 80 to 130 °C with 50 mL Ca(OH)2-saturated limpid suspensions and 0.03−6 g of CO2SM via hydrothermal reaction. Typically, 3 g of CO2SM was mixed with 50 mL of Ca(OH)2-saturated suspensions in 100 mL Teflon-lined stainless steel hydrothermal reactor. The reactor was sealed tightly and heated at 110 °C for 2 h. The as-obtained precipitate was separated with vacuum filtration and washed with double distilled water for three times. The as-synthesized CaCO3 was dried at 120 °C for 2 h. Characterization of CaCO3 Microparticles. The scanning electron microscopy (SEM, Quanta FEG 650, China) with an accelerating voltage of 20 kV and high magnification transmission electron microscopy (HR-TEM, JEM-2100, Japan) with an accelerating voltage of 200 kV of CaCO3 microparticles were obtained. Their X-ray diffraction (XRD) patterns were collected on a powder X-ray diffractometer (Siemens D/max-RB) with Cu Kα radiation at the scanning rate of 0.05°·S−1. X-ray photoelectron spectroscopy (XPS) data were obtained on a KRATOS Axis ultra X-ray photoelectron spectrometer with a monochromatized Al Kα X-ray (hν = 1486.6 eV) operated at 150 W. Fourier transform infrared spectroscopy (FTIR) was recorded on a Nexus 670 infrared spectrophotometer. 13C NMR spectra was recorded on a Bruker ARX-400 nuclear magnetic resonance spectrometer equipped with a 4 mm standard bore CP/ MAS probe head. Nitrogen sorption data were obtained on a Tristar II3020 automated gas adsorption analyzer. Isotherms were analyzed with the Barrett−Joyner−Halenda (BJH) theory to give the pore parameters, including Brunauer−Emmett−Teller (BET) surface areas, pore volume, and pore size distribution. Meanwhile, the CaCO3 product was characterized by SEM, HR-TEM, XRD, and FTIR, and the CO2SM product was characterized by XPS, XRD, FTIR, and 13C NMR. In addition, an elemental analyzer (Elementar Vario EL III, Germany) was used to measure the content of C, N and H in CaCO3 product. Thermogravimetry analysis (TGA, Entzsch-Sta 449) was employed to measure the weight percentage of the CO2SM and CaCO3 crystals. Furthermore, the relative percentage of each polymorph of CaCO3 was calculated from the Rietveld XRD whole pattern refinement by the

Figure 1. Morphology control of CaCO3 by using the CO2SM, which was synthesized by EDA + BDO absorbing CO2. In the processes, the filtered solution could not only repeatedly absorb CO2 but also recycle to produce the same crystal phase CaCO3 after bubbling CO2. This cycle with absorption of CO2 for preparing CaCO3 processes could potentially be optimized as a novel technique. B

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Figure 2. Reaction processes of EDA + BDO with CO2 at various time.

Table 1. Summary of the Synthesis Conditions for As-Prepared CaCO3 Products CO2SM (g/L)

pH

temp. (°C)

CaCO3-1 CaCO3-2 CaCO3-3 CaCO3-4 CaCO3-5 CaCO3-6

0.6 1 1.6 3 6 10

12.06 11.82 11.20 9.86 9.41 9.15

110 110 110 110 110 110

2 2 2 2 2 2

CaCO3-7

20

8.89

110

2

CaCO3-8 CaCO3-9 CaCO3-10 CaCO3-11 CaCO3-12 CaCO3-A CaCO3-B CaCO3-C CaCO3-D CaCO3-E CaCO3-F CaCO3-G CaCO3-H CaCO3-I CaCO3-J CaCO3-K CaCO3-L CaCO3-M CaCO3-N

40 60 80 100 120 1 1 1 1 1 1 3 3 3 3 3 3 60 60

8.69 8.59 8.58 8.56 8.55 11.82 11.82 11.82 11.82 11.82 11.82 9.86 9.86 9.86 9.86 9.86 9.86 8.59 8.59

110 110 110 110 110 80 90 100 110 120 130 80 90 100 110 120 130 80 90

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

sample no.

time (h)

polymorphs

sample no.

100 wt % calcite 100 wt % calcite 100 wt % calcite 100 wt % calcite 100 wt % calcite 37.8(3) wt % calcite 62.2(3) wt % aragonite 49.5(8) wt % calcite 44.4(5) wt % vaterite 100 wt % vaterite 100 wt % vaterite 100 wt % vaterite 100 wt % vaterite 100 wt % vaterite 100 wt % calcite 100 wt % calcite 100 wt % calcite 100 wt % calcite 100 wt % calcite 100 wt % calcite 100 wt % calcite 100 wt % calcite 100 wt % calcite 100 wt % calcite 100 wt % calcite 100 wt % calcite 100 wt % vaterite 100 wt % vaterite

CaCO3-O CaCO3-P CaCO3-Q CaCO3-R CaCO3-S CaCO3-T CaCO3-W CaCO3-V CaCO3-U CaCO3-X CaCO3-(1) CaCO3-(2) CaCO3-(3) CaCO3-(4) CaCO3-(5) CaCO3-(6) CaCO3-(7) CaCO3-(8) CaCO3-(9) CaCO3-(10) CaCO3-(11) CaCO3-(12) CaCO3-(13) CaCO3-(14) CaCO3-(15) CaCO3-(16) CaCO3-(17) CaCO3-(18) CaCO3-(19) CaCO3-(20)

Gass-Expgui software. The peaks of 110 plane (IV110, 2θ = 43.90°), 221 plane (IA221, 2θ = 46.06°), and 104 plane (IC104, 2θ = 29.40°) represented vaterite, aragonite, and calcite, respectively. The Rietveld XRD fitting results of Sample 5 (CaCO3-5), Sample 6 (CaCO3-6), and Sample 7 (CaCO3-7) were given in Figure S1, and other samples’ data are listed in Table 1.

CO2SM (g/L) 60 60 60 60 120 120 120 120 120 120 1 1 1 1 1 3 3 3 3 3 60 60 60 60 60 120 120 120 120 120

pH

temp. (°C)

time (h)

8.59 8.59 8.59 8.59 8.55 8.55 8.55 8.55 8.55 8.55 11.82 11.82 11.82 11.82 11.82 9.86 9.86 9.86 9.86 9.86 8.59 8.59 8.59 8.59 8.59 8.55 8.55 8.55 8.55 8.55

100 110 120 130 80 90 100 110 120 130 110 110 110 110 110 110 110 110 110 110 110 110 110 110 110 110 110 110 110 110

2 2 2 2 2 2 2 2 2 2 0.5 1.0 2.0 3.0 5.0 0.5 1.0 2.0 3.0 5.0 0.5 1.0 2.0 3.0 5.0 0.5 1.0 2.0 3.0 5.0

polymorphs 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt wt

% % % % % % % % % % % % % % % % % % % % % % % % % % % % % %

vaterite vaterite vaterite vaterite vaterite vaterite vaterite vaterite vaterite vaterite calcite calcite calcite calcite calcite calcite calcite calcite calcite calcite vaterite vaterite vaterite vaterite vaterite vaterite vaterite vaterite vaterite vaterite

certified the CO2SM had a structure with 17.50°, 21.48°, 22.40°, 22.60°, 29.24°, and 35.42° (JCPDS card no. 34-1993), which was similar to ethlenediamine carbamate (−NH− COO−) characteristic diffraction peaks. FTIR spectra (Figure S3(b)) suggested the CO2SM was an alkylacrbonate salt.49−51 The peaks at 3307 and 2189 cm−1 were attributed to N−H and −NH3+ groups, which suggested the CO2SM was a primary amine salt.52,53 Two intense absorption peaks at 1574 and 1483 cm−1 were associated with −CO2− groups.54 Specially, the peak at 1373 cm−1 denoted carbonate (CO32−) rather than bicarbonate because the typical peaks of bicarbonate appeared at 1360 and 835 cm−1. Moreover, 13C NMR spectra (Figure S4f) also demonstrated the CO2SM was alkycarbonate salt rather than bicarbonate because bicarbonate was often found at about 160 ppm in 13C NMR.55−61 In addition, TGA-DSC results (Figure S5) indicated the CO2SM could release CO2 between 50 and 112 °C. Observably, the decomposition of



RESULTS AND DISCUSSION Characterizations of CO2SM. The CO2SM was characterized by the XPS, XRD, FTIR, and 13C NMR technologies. XPS spectra of CO2SM showed that the CO2SM contained the groups of NH3+ and CO32−. The peaks at 288.80 and 284.80 eV (Figure S2A,B) were due to CO32− groups.44,45 Two peaks at 532.05 and 533.25 eV were due to O 1s (Figure S2C) in carbonate-like structures.46,47 The peak at 399.45 eV referred to N 1s (Figure S2D), and peak at 284.80 eV (Figure S2B) showed NH3+ existed.48 Furthermore, XRD (Figure S3a) C

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Figure 3. SEM images of CaCO3 with different morphologies obtained with various CO2SM concentrations (g/L): (A) 0.6, (B) 1.0, (C) 1.6, (D) 3.0, (E) 6.0, (F) 10.0, (G) 20.0, (H) 40.0, (I) 60.0, (J) 80.0, (K) 100.0, and (L) 120.0. Reaction conditions: 110 °C, 2 h, and 50 mL saturated pellucid Ca(OH)2 suspensions.

Figure 4. (a) XRD patterns and (b) FTIR spectra of CaCO3 obtained in the presence of varying CO2SM concentrations (g/L): (A) 0.6, (B) 1.0, (C) 1.6, (D) 3.0, (E) 6.0, (F) 10.0, (G) 20.0, (H) 40.0, (I) 60.0, (J) 80.0, (K) 100.0, and (L) 120.0. Reaction conditions: 110 °C, 2 h, and 50 mL saturated pellucid Ca(OH)2 suspensions.

CO2SM was accelerated at about 103 °C and completely decomposed at about 112 °C. Effects of CO2SM Concentration. A series of CaCO3 microparticles were synthesized under different conditions, including different concentrations of CO2SM, reaction temperatures, and times. The results were listed in Table 1. To investigate the effect of concentration of CO2SM on the morphology of the products, the experiments for the crystallization of CaCO3 with varying CO2SM concentrations were carried out at 110 °C for 2 h. The results are shown in Figure 3.

The CaCO3 microparticles showed different morphologies at different CO2SM concentrations, such as sheaf-like (Figure 3A−C), dumbbell-like (Figure 3D−E), inflorescence-like (Figure 3F), spheroidicity-like (Figure 3H−K), and microspheres assembled by many particles (Figure 3L), which were all in good agreement with previous reports.62,63 Especially, four specific uniform CaCO3 morphologies were obtained at 1, 3, 60, and 120 g/L CO2SM concentrations, respectively. When the CO2SM concentration was 1 g/L, the products appeared as uniform and regular bundle-like structures with lengths in the range of 8.75−10.0 μm, and diameters in the range of 1.75−3.5 μm (Figure 3B). A dramatic shape change occurred when the D

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Figure 5. SEM images of CaCO3 obtained in varying reaction temperature (°C): (A) 80, (B) 90, (C) 100, (D) 110, (E) 120, and (F) 130 at 1.0 g/L CO2SM concentration for 2 h with 50 mL saturated pellucid Ca(OH)2 suspensions; (G) 80, (H) 90, (I) 100, (J) 110, (K) 120, and (L) 130 at 3.0 g/ L CO2SM concentration for 2 h with 50 mL saturated pellucid Ca(OH)2 suspensions.

Furthermore, two new peaks appeared at 744 cm−1 in sample G and 854 cm−1 in sample F, which indicated that the two samples were in transition from calcite to vaterite.66 Effects of Reaction Temperature. Effect of Reaction Temperature in the Lower Concentration of CO2SM (1.0 and 3.0 g·L−1). As shown in Figure 5, the morphology and size of the products were affected by reaction temperature at lower CO2SM concentrations. When the CO2SM concentration was 1.0 g/L, excessively high or low temperatures resulted in both sheaf-like, inflorescence-like particles and irregular aggregates (Figure 5A,F). In contrast, relatively uniform sheaf-like microparticles were formed when the temperature changed from 90 to 120 °C (Figure 5B−E), and the largest particle sizes gradually decreased from 11.32 × 10.37 μm to 7.92 × 6.04 μm upon increasing the reaction temperature. When the CO2SM concentration was increased to 3.0 g/L, the amount of halfdumbbells, which were hollow and made up of square particles, gradually increased, and the crystals had a tendency to change into a sphere-shape with increasing reaction temperature (Figure 5G−L). XRD patterns of the prepared products (Figure S6a,c) showed that the polymorph formed the most stable calcite at experimental temperatures. The result was further demonstrated by the FTIR spectra (Figure S6b,d), in which all samples had two characteristic peaks at 875 and 711 cm−1 indicating that pure calcite was formed.67 According to the results shown in Figures 5 and S6, the polymorph of as-prepared CaCO3 microparticles did not change with varying reaction temperatures at lower CO2SM concentrations. This was because the initial pH was higher, which made the system reliant on thermodynamic control.68 It

concentration of CO2SM increased to 3 g/L, which produced well-defined monodisperse dumbbell-like crystals with a length of 9.0−9.75 μm accumulated by small square particles (Figure 3D). Remarkably, two half-dumbbells (marked by an arrow in Figure 3D) with an average length of approximately 4.25 μm, which was broken from a whole crystal, were hollow. Furthermore, increasing the concentration of CO2SM to 60 g/L, a novel oblate spheroid consisting of sheet-like structures with mussy granules were obtained, and the diameters were in the range of 3.0 to 3.75 μm (Figure 3I). As the CO2SM concentration increased to 120 g/L, snowball-like microspheres assembled by many particles (Figure 3L) with a size of 5.0− 6.25 μm were easily acquired. XRD patterns and FTIR spectra of products were obtained and the results are shown in Figure 4. In general, XRD peak intensities of calcite decreased whereas those of vaterite increased with increasing CO2SM concentrations (Figure 4a and Table S1). All products were shown to be calcite when the concentration was less than or equal to 6 g/ L, and they were vaterite when the concentration was greater than or equal to 40 g/L. When the CO2SM concentration was 10 g/L, as-prepared CaCO3 contained calcite and aragonite. When the CO2SM concentration was 20 g/L, as-obtained CaCO3 products contained calcite and vaterite. Furthermore, the relative percentage of each polymorph of CaCO3 was calculated from the Rietveld XRD whole pattern refinement technique by the Gass-Expgui software. And the Rietveld XRD fitting results are listed in Table 1. From Figure 4b, the absorption peaks at 875 and 711 cm−1 in samples A−E were characteristics of pure calcite,64 and the peaks at 874 and 744 cm−1 were characteristic of pure vaterite65 (samples H−L). E

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Figure 6. SEM images of CaCO3 obtained in varying reaction temperature (°C): (A) 80, (B) 90, (C) 100, (D) 110, (E) 120, and (F) 130 at 60.0 g/ L CO2SM concentration for 2 h with 50 mL saturated pellucid Ca(OH)2 suspensions; (G) 80, (H) 90, (I) 100, (J) 110, (K) 120, and (L) 130 at 120.0 g/L CO2SM concentration for 2 h with 50 mL saturated pellucid Ca(OH)2 suspensions.

exceeded 110 °C. The morphology was irregular microspheres, which was made up of two cake-like spheres embedded together, and the maximum diameter (3.77 or 3.21 μm) was less than the maximum diameter of samples G−J. XRD patterns (Figure S7a,c) showed that the polymorph was vaterite via (110) planes68,70 without change in the reaction temperatures. Furthermore, the vibrational bands at 875 and 746 cm−1 in FTIR spectra (Figure S7b,d) were characteristic of vaterite.72,73 Figures 6 and S7 indicated that the reaction temperature significantly affected the morphology and size of the products at higher CO2SM concentrations. This was related to the speed of CO2 released by the CO2SM. According to the TGA result (Figure S5), the CO2SM slowly released CO2 when the reaction temperature was lower than 103 °C, and the CO2SM released CO2 instantly when reaction temperature was higher. Thus, excess CO2SM and low temperature could not only lead to greater supersaturation but also to free CO2 being converted into CO32− in the reaction system. Then, Ca2+ would be surrounded by abundant CO32− to form small micelles because there would be not enough Ca2+.74,75 Then, CO32− on the outside surface of small micelles could continue to attract Ca2+ via electrostatic interactions to form bigger micelles.76 This process was repeated until all Ca2+ ions were consumed in the presence of EDA and BDO (samples G−J). In contrast, higher reaction temperatures caused the entire system retain little CO32− because CO2 was released rapidly and supersaturation was achieved.72 The initiating micelles were not complete and they finally formed samples K and L. Specially, the size of products was smaller at 120 and 130 °C, which resulted from lots of EDA/BDO blocking movement among different ions.

is well-known that nucleation occurs in a supersaturated ionic solution, and the supersaturation is inversely proportional to temperature.69,70 This reduced the conversion of CO2 into CO32− with increasing temperatures, and it led to a low concentration of CO32− that was surrounded by Ca2+ in the presence of EDA and BDO. In other words, the gradually decreasing solubility of CO2 (g) in the solution with the increasing reaction temperature resulted in the formation of smaller particles and made the size of the as-prepared CaCO3 microparticles smaller. At the same time, Colfen71 had reported uniform dumbbell-like particles at pH 9.5, which was in good agreement with results obtained in this work. Effect of Reaction Temperature in the Higher Concentration of CO2SM (60.0 and 120.0 g·L−1). From Figure 6, the effect of reaction temperature on the products was relatively large. Although the morphologies of products were more complex, they were mainly the oblate spheroid at 80 °C (Figure 6A). Following, the morphologies of the products were uniform spheres with increasing reaction temperatures (Figure 6C−F), but some rod-shaped products also formed at 90 °C (Figure 6B). At the same time, the largest diameter of spheres in each sample first decreased then increased with increasing reaction temperatures. The diameter was the smallest at 110 °C, which may have been a result of supersaturation.69,70 When the concentration of CO2SM was 120.0 g/L and temperature changed from 80 to 110 °C (Figure 6G−J), it was found that obtained products were uniform defective snowball-like microspheres, which were composed of many particles, and the maximum diameter (5.28 μm) was nearly uniform except for sample J (6.25 μm). However, the morphology and size of the product changed greatly (Figure 6K,L) when temperature F

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Figure 7. SEM images of CaCO3 obtained in varying reaction time (h): (A) 0.5, (B) 1.0, (C) 2.0, (D) 3.0, and (E) 5.0 at 1.0 g/L CO2SM concentration for 2 h with 50 mL saturated pellucid Ca(OH)2 suspensions; (F) 0.5, (G) 1.0, (H) 2.0, (I) 3.0, and (J) 5.0 at 3.0 g/L CO2SM concentration for 2 h with 50 mL saturated pellucid Ca(OH)2 suspensions.

XRD patterns and FTIR spectra (Figure S8) indicated that the polymorph of the products obtained at lower CO2SM concentrations at 110 °C with 50 mL saturated pellucid Ca(OH)2 suspensions was calcite at all reaction times. These results showed that reaction time had a small influence on morphology and polymorph of the as-prepared CaCO3 microparticles at lower CO2SM concentration at 110 °C with 50 mL saturated pellucid Ca(OH)2 suspensions. This might have been caused by the fast nucleation rate and the system of thermodynamic control.68,77 In other words, the CaCO3 microparticles nucleation process had completed before 0.5 h, and then, small particles began to grow with extending reaction times. Effect of Reaction Time in the Higher Concentration of CO2SM (60.0 and 120.0 g·L−1). As shown in Figure 8, although the size of the products showed an increasing trend, the morphology did not change with the increasing reaction times at 60.0 g/L CO2SM concentration at 110 °C (Figure 8A−E). With the increasing CO2SM concentrations up to 120.0 g/L, all the products were defective snowball-like microspheres, which were made up particles with diameters of 6.75−7.17 μm that did not change upon increasing reaction times (Figure 8F−J). XRD patterns and FTIR spectra (Figure S9) showed that the polymorph of products were all vaterite, which resulted from both kinetic control68,77 and reaction temperatures.77−79 These data showed that reaction times had only a small effect on the morphology and polymorph of the products at higher CO2SM concentrations at 110 °C with 50 mL of saturated pellucid Ca(OH)2 suspensions. This observation could be explained by the fast nucleation rate of products. Therefore, extending the reaction time would only be helpful to

The polymorphs of the products were vaterite at all reaction temperatures. This might have been caused by the kinetics of the crystallization processes67,77 and the reaction temperatures, which promoted the formation of vaterite.78,79 In addition, the arrangement of Ca2+ was affected only by temperature, which also favored crystallization of the thermodynamically stable vaterite.74,80 Effect of Reaction Time. Effect of Reaction Time in the Lower Concentration of CO2SM (1.0 and 3.0 g·L−1). Figure 7 shows SEM images of the products synthesized at lower CO2SM concentrations of 1.0 and 3.0 g/L, and reaction temperature of 110 °C with 50 mL saturated pellucid Ca(OH)2 suspensions for 0.5, 1.0, 2.0, 3.0, and 5.0 h intervals. According to SEM images, sheaf-like products were formed at 0.5 h when the CO2SM concentration was 1.0 g/L (Figure 7A). As the reaction time was extended to 1.0, 2.0, 3.0 and 5.0 h, the morphologies of the products were similar to the sheaf-like products (Figure 7B−E); however, the length and width of products first decreased from 7.92 × 10.37 μm to 7.0 × 3.5 μm and then increased to 7.92 × 7.36 μm upon extending the reaction time. When the CO2SM concentration increased to 3.0 g/L, whole dumbbell-like products were only obtained at a reaction time of 2 h (Figure 7H). At other reaction times, although there were no complete dumbbell-like products, hollow half-dumbbell products were formed (marked by an arrow in Figure 7F−J). Furthermore, the largest size of particles on the surface of the products gradually increased from 1.88 × 1.88 μm to 5.47 × 3.41 μm with varying reaction times when the CO2SM concentration was 3.0 g/L at 110 °C (Figure 7F− J). G

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Figure 8. SEM images of CaCO3 obtained in varying reaction time (h): (A) 0.5, (B) 1.0, (C) 2.0, (D) 3.0 and (E) 5.0 at 60.0 g/L CO2SM concentration for 2 h with 50 mL saturated pellucid Ca(OH)2 suspensions; (F) 0.5, (G) 1.0, (H) 2.0, (I) 3.0, and (J) 5.0 at 120.0 g/L CO2SM concentration for 2 h with 50 mL saturated pellucid Ca(OH)2 suspensions.

Figure 9. HR-TEM of as-obtained products. (A) bundle-like sample obtained in 1.0 g/L CO2SM concentration, 110 °C, and 3 h; (B) dumbbell-like sample obtained in 3.0 g/L CO2SM concentration, 110 °C, and 2 h; (C) sheet-like sphere sample obtained in 60.0 g/L CO2SM concentration, 110 °C, and 2 h; and (D) microspheres sample obtained in 120.0 g/L CO2SM concentration, 110 °C, and 3 h.

Moreover, oblate spheroid (C) and snowball-like microsphere (D) could also be obtained when reaction temperature was at 110 °C, and the CO2SM concentration was 60.0 g/L for 2 h and 120.0 g/L for 3 h. To understand the structure of CaCO3 microparticles and to study their crystalline phase transformation and composition, the properties of the above-

microparticle growth, but it would not affect the morphology and polymorph of the as-prepared CaCO3 microparticles. Properties of CaCO3 Microparticles. The bundle-like (A) and dumbbell-like (B) calcite CaCO3 were obtained with a 1.0 g/L CO2SM concentration at 110 °C for 3 h and 3.0 g/L CO2SM concentration at 110 °C for 2 h, respectively. H

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ACS Sustainable Chemistry & Engineering described four kinds of CaCO3 microparticles were investigated. HR-TEM. As shown in Figure 9A, the lattice spacing of 2.48 Å corresponded to the (110) plane of calcite.80 On the other hand, from Figure 9B, the (110) plane with a lattice spacing of 2.5 Å was found, which could be assigned to crystal lattice of the calcite.81 In addition, the (112) plane with a lattice spacing of 3.2 Å and the (114) plane with a lattice spacing of 2.6 Å are found in Figure 9C, which were consistent with crystal lattice of the vaterite.82 And as shown in Figure 9D, the lattice spacing of 3.2 Å corresponded to the (114) plane of vaterite.81 TGA-DSC. The thermal behavior of the four kinds of CaCO3 samples was studied by TGA in the temperature range of 20− 1000 °C (Figure S10). When the temperature was changed from 20 °C to approximately 550 °C, the main weight reduction was due to the loss of the adsorbed organics on the surface of CaCO3, which could be demonstrated by elemental analysis and FTIR spectroscopy (Figure S11). Elemental analysis determined a nitrogen content of 0.349%, 0.372%, 0.428%, and 0.644% in the as-prepared bundle-like, dumbbelllike, oblate spheroid, and snowball-like CaCO3 microspheres, respectively. In addition, nonsymmetrical and symmetrical stretching vibrations of C−H (−CH2−)83 of EDA and/or BDO at 2931 and 2873 cm−1 (Figure S10) indicated that the asprepared products contained CaCO3 crystals and organics molecules EDA and/or BDO. An obvious mass loss with the maximum weight loss peak occurred between 550 and 780 °C. This was mainly caused by the thermal decomposition of CaCO3 (CaCO3 → CaO + CO2 ↑).84 At the same time, it was found that the thermal decomposition of CaCO3 was fastest at about 760 °C via DSC. Dumbbell-like products mainly formed numerous small blocks, and thus it contained little organics upon drying before TGA-DSC. As a result, a small amount of organics and thermal decomposition was found in the TGADSC analysis (Figure S10b). N2 Adsorption−Desorption Isotherm. N2 adsorption− desorption was carried out, and the isotherms are plotted in Figure S12. The bundle-like CaCO3 had a 7.27 m2/g specific surface area and the average pore size was mainly distributed at 7.8 nm (Figure S12A). On the contrary, dumbbell-like CaCO3 exhibited a small specific surface area of 6.34 m2/g and the average pore size was mainly in the range of 4−7 nm (Figure S12B). Two sphere-like CaCO3 products possessed similar specific surface area of 21.69 m2/g (Figure S12C), and 21.98 m2/g (Figure S12D), respectively. In addition, the average pore size of the sheet-like sphere products was distributed at 5 nm, and the snowball-like microspheres products had an average pore size of 3−5 nm. Possible Crystallization Process of CaCO3 Microparticles. In this study, the CO2SM concentration played a crucial role in regulating and controlling the morphologies of the CaCO3 microparticles (Figure 10). In this process, CO2 from the CO2SM was converted into CO32−, which reacted with Ca2+ to form CaCO3 aggregates, and the remaining EDA and BDO served as dispersant, localized nucleation center, agglomerant, structure-directing agent, or pH regulating agent,80,85,86 which arranged aggregates to assemble into different morphological CaCO3 microparticles. When the CO2SM concentration was lower (1.0 or 3.0 g/L), the concentration of CO2 released from the CO2SM was lower, which led to lower CO32− concentrations and lower pressures in the equipment. In contrast, the remaining EDA and BDO could sequester Ca2+ due to the high local concentration of

Figure 10. Schematic illustration of different structure of CaCO3 obtained under different CO2SM concentrations at 110 °C for 2 h: (A) 1.0, (B) 3.0, (C) 60.0, and (D) 100 g/L.

Ca2+.87 This not only reduced the collision frequency among different ions but also resulted in the formation of small CaCO3 aggregates. When the concentration of CO2SM was 1.0 g/L, these aggregates were further transformed into the multi- or Ybranched structures as rod-like subunits via specific hydrogen bonding interactions and favorable adsorption interactions.88 Through parallel aggregation, they aggregated into highly monodisperse sheaf shapes87,89 under the action of EDA (and/ or BDO) dispersant, localized nucleation centers, or structuredirecting agents (Figure 10A). In other words, the sheaf-like CaCO3 microparticles were formed by mesoscale assembly of hierarchical rods.90 Increasing the concentration to 3.0 g/L, these small aggregated particles were first formed in the early stages of dumbbell-like formation, and then grew and transformed into irregular hexahedron subunits,91 which were completely driven by crystal facet selective adsorption and similar orientation attachment modes.92,93 Ammonium ions adhered to the surfaces of subunits, which led to a lowering of surface energy and inhibition the growth of surfaces in this direction.94 This contributed to the upward oriented growth that simultaneously and nearly symmetrically occurred at both ends of the irregular hexahedron subunits, which ultimately resulted in the formation of dumbbell-like CaCO3 microparticles (Figure 10B). These results clearly demonstrate a multistep aggregation mechanism, which should be consistent with particle aggregation along electric field lines.95 In addition, the higher pH values also significantly promoted the formation of sheaf-like and dumbbell-like particles when the CO2SM concentration was lower.71 By comparison, the amounts of CO32−, EDA, and BDO in the reaction system largely increased when the CO2SM concentration was higher (60.0 and 120.0 g/L), which resulted in greater collision frequencies among different ions and formation of more aggregates. Specifically, the lone-pair electrons of nitrogen atoms in EDA and oxygen atoms in BDO have very important effects on the formation of CaCO3 aggregates because of the strong electrostatic interactions I

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Figure 11. (a) FTIR spectra and (b) XRD pattern of cycle preparation CaCO3 microparticles with five cycles. A−E represent the experimental number.

between Ca2+ and EDA (and/or BDO).85 In other words, EDA (and/or BDO) can combine with Ca2+ (and/or CO32−) and easily adsorb onto the surface of CaCO3 aggregates. When the concentration of CO2SM was increased to 60.0 g/L, CO2, EDA, and BDO were released from the decomposing CO2SM, and then EDA, BDO, Ca2+, and CO32− aggregated in an aqueous solution of EDA and BDO to form CaCO3 aggregates.96−100 With this reaction, the ammonium ions adsorbed on aggregates reduced the surface energy of the aggregates,95 which would be conducive to the aggregates by promoting nucleation, crystallization, and other self-assembly processes to produce aggregated flake-structure subunits. With further mineralization, these flake-structure subunits were further assembled and eventually became oblate spheroids with rough surfaces under the adherence of EDA and BDO (Figure 10C). When the CO2SM concentration was further increased to 120 g/L, EDA (and/or BDO) interacted with Ca2+ through electrostatic interaction, and CO2 was converted into CO32− by OH−, which promoted the synthesis of more CaCO3 aggregates in a chemical environment that contained a certain amount of Ca2+ and a large amount of ammonium ions, CO32−, EDA, and BDO. These aggregates can be further assembled into strip subunits, which have smaller surface energy, under the action of excess ammonium ion in the solution. Then, these strip subunits rapidly snowballed as defective spheres with the help of EDA and BDO (Figure 10D). The formation process of snowball-like spherical superstructures was similar to that of snowball-like cobalt oxide superstructures observed by Jones et al.101 At the same time, the reaction system had a lower pH when the CO2SM concentration was higher and promoted the formation of sphere-shaped CaCO3 microparticles,71 which was in good agreement with this work. Cyclic Preparation of CaCO3 Microparticles. After the snow-like products were prepared using 120.0 g/L of CO2SM, the filtrate was recycled to make more CaCO3 microparticles at 110 °C within 2 h. The filtrate containing EDA and BDO could be reused to absorb CO2, which was released from the steel cylinder. After the absorption of CO2, an appropriate amount of Ca(OH)2 was added into the solution to prepare CaCO3 microparticles by a hydrothermal reaction. After this process, the materials were reused again. As a result, the same polymorph CaCO3 microparticles were smoothly produced at 110 °C within 2 h after five-successive absorption−preparation cycles. Each sample was characterized by FTIR spectroscopy and XRD patterns (Figure 11), which

showed that all CaCO3 microparticles had the same mixed crystal phase of vaterite. Thus, the filtered solution without the CaCO3 precipitates could be used repeatedly to absorb CO2 and to produce the same crystal phase CaCO3 microparticles after bubbling with CO2.



CONCLUSION In this study, a novel, green, and economical CaCO3 synthesis method was developed based on an innovative CCUS approach, in which CO2SM served as both the CO2 source and the crystal modifier. It is noteworthy that CO2 was bubbled into the system EDA−BDO at mild conditions to regenerate the solid CO2SM, which was confirmed as a kind of alkylcarbonate salts. Subsequently, the influences of CO2SM concentration, reaction temperature, and reaction time were systematically investigated, and the resulting products were fully characterized. Pure calcite (bundle-like and dumbbell-like) and pure vaterite (oblate spheroid and snowball-like) CaCO3 products were obtained. In addition, the CO2SM solution could be recycled to prepare the same crystal phase CaCO3 with the addition of an appropriate amount of Ca(OH)2 at 120 g·L−1 CO2SM, which is important for the capture and utilization of CO2. Thus, this new CCUS approach may open up a wide range of potential applications in environmental protection.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01793. Rietveld XRD whole pattern refinement results, XPS spectra of CO2SM, XRD pattern and (b) FTIR spectrum of CO2SM, 13C-NMR spectra of EDA + BDO system absorbed CO2 and formed solid CO2SM, TGA curve for CO2SM, XRD patterns and FTIR spectra of CaCO3 obtained in varying reaction temperature, XRD patterns and FTIR spectra of CaCO3 obtained in varying reaction time, TGA curves of as-prepared CaCO3 samples with different preparation conditions, FTIR spectra of asprepared CaCO 3 particles under three optimum conditions, nitrogen adsorption−desorption isotherm and pore size distribution of as-prepared CaCO3 samples with different preparation conditions, and peak intensity values (PDF). J

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AUTHOR INFORMATION

Corresponding Author

*J. Zhang. Tel.: +86-471-6575722. Fax: +86-471-6575722. Email: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21166017), Program for New Century Excellent Talents in University (NCET-12-1017), the Program for Grassland Excellent Talents of Inner Mongolia Autonomous Region, the Inner Mongolia Science and Technology Key Projects, and training plan of academic backbone in youth of Inner Mongolia University of Technology.



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DOI: 10.1021/acssuschemeng.5b01793 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.5b01793 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX