A High-Pressure GasâSolid Carbonation Route to Produce Vaterite

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A High-Pressure Gas-Solid Carbonation Route to Produce Vaterite Benoit Rugabirwa, David Murindababisha, Hongtao Wang, and Jun Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01314 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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Crystal Growth & Design

Cover page A High-Pressure Gas-Solid Carbonation Route to Produce Vaterite Benoit Rugabirwa, David Murindababisha, Hongtao Wang, Jun Li* Department of Chemical and Biochemical Engineering, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Xiamen University, Xiamen 361005, China ABSTRACT: A novel synthetic strategy based on high pressure gas-solid reaction using urea as an additive to produce vaterite CaCO3 crystals was reported. Results showed that at about 150 oC and 12 MPa the reaction attained complete carbonation with predominant spherical vaterite crystals via self-sustained reaction. The mechanism study showed that, high-pressure CO2 acted as an inhibitor for urea decomposition besides as a reactant; the urea entrapped Ca(OH)2 via its melt to stabilize vaterite formation by amine functional group and initiated the carbonation process via tiny water triggered from its slight decomposition.

Corresponding author Tel./Fax: (+86)-592 2183055. Contact e-mail: [email protected]

ACS Paragon Plus Environment

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A High-Pressure Gas-Solid Carbonation Route to Produce Vaterite Benoit Rugabirwa, David Murindababisha, Hongtao Wang, Jun Li* Department of Chemical and Biochemical Engineering, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Xiamen University, Xiamen 361005, China ABSTRACT A novel synthetic strategy based on high pressure gas-solid reaction using urea as an additive to produce value-added CaCO3 material was reported. Results showed that at about 150 oC and 12 MPa the reaction attained complete carbonation with predominant spherical vaterite crystals via self-sustained reaction. The influences of working conditions including pressure, temperature, Ca(OH)2/urea mass ratio and the reaction time on the mineralization process were studied. The as-prepared CaCO3 powder exhibited vaterite content as high as 94.2% and the surface area up to 32.9 m2/g. The mechanism study showed that, the high-pressure CO2 acted as an inhibitor for urea decomposition at high temperature besides as a reactant; the urea entrapped Ca(OH)2 via its melt to stabilize vaterite formation by amine functional group and initiated the carbonation process via tiny water triggered by its slight decomposition. The separation to obtain the fine product was simple and allowed high urea recovery (for example 93.6%). The method has both merits for the high CO2 utilization and production of value-added CaCO3 without discharging inorganic salts; thus, would be an environment-friendly and potential route for industrial application.

Corresponding author Tel./Fax: (+86)-592 2183055. Contact e-mail: [email protected]


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INTRODUCTION A continuous rise of carbon dioxide (CO2) in the atmosphere is a worldwide environmental concern for its impact on the global warming.1 A larger emission of CO2 is associated to industrialization and human activities with the utilization of fossil fuels.2 CO2 mitigation strategies based on CO2 capture and storage (CCS) and CO2 capture and utilization (CCU) have been promising techniques for reducing greenhouse gas emissions.3 Mineral carbonation is one of the accounted technologies to store CO2 permanently4 but also to produce value-added materials. For instance, calcium carbonate (CaCO3) is one of the solid mineral carbonates that falls into calcium looping for CO2 capture5 and have proven industrial interest with potential applications6 in improving existing material and advanced material design. As such, CaCO3 is an important material for sustainable cementation,6, 7 energy storage8 and as filler9 in a broad range (biomedical, plastic industry,..) of engineering. Moreover, CaCO3 is known for successfully being employed in environmental remedial strategies.10 This way, tailoring CaCO3 crystal properties to meet certain desired characteristics of the material has been a focus of the emerging engineering processes that would be leading the forthcoming industrial community for green manufacturing. In the designing process of

advanced CaCO3 materials, polymorphism

selectivity, particle size,11 surface area, morphology and purity12 are characteristics of choices among others that adhere to a given application. CaCO3 occurs as three crystalline polymorphs: calcite, aragonite and vaterite with thermodynamic stability decreasing in respective order.9 Qualitatively the least stable vaterite demonstrated beneficial properties for industrial applications which include high specific surface area, high dispersion, high solubility and low specific gravity.13 The higher surface area characteristic of vaterite enhanced effective ionexchange, making vaterite powder a potential candidate for removal of heavy metal ions.14


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Improved mechanical properties of vaterite doped-membranes and plastics had excellent performance in separation since vaterite could be successfully incorporated into the compositematerial matrix associated to high dispersion and solubility bearing properties.15 Smaller particle size of vaterite assumed to correlate to the low specific gravity rendered vaterite crystal systems suitable for improved polymers toughness.16 Microsized and nanosized vaterite as well as vaterite-composite materials have been used in advanced technologies and biomedical area; it was demonstrated that vaterite could efficiently enhance drugs loading and release useful in nanomedicine technology.11 Incorporated magnetite (Fe3O4) ascribed vaterite-Fe3O4 composite with useful magnetic properties for advanced biosensor materials.17 CO2 bubbling through solutions and solution reaction of calcium salts with carbonate compounds3, 18 are commonly reported practical approaches for synthesizing CaCO3 materials. These techniques render easy manufacturing process with regard to production time and controlling material’s property; however, would be limited to the potential environmental risk because separation process to obtain fine product would definitely discharge sewage, harmful to the surrounding environment; thus, limiting their applicability. Consequently, high pressure or high temperature carbonation approaches were investigated in recent years. Ibrahim et al.19 reported the carbonation of Ca(OH)2 in CO2 expended ethanol-water solution. In this system, chameleonic phase transformation (normal to abnormal) between the CaCO3 polymorphs was observed. Solid-gas carbonation utilizing high pressure has never been reported in the mineralization of vaterite CaCO3 particularly in the absence of aqueous media. Solid-gas mineralization of CO2 in the absence20 and/or presence of relative humidity21 have been reported for low carbonation efficiency. Nevertheless, the carbonation outcomes resulted into calcite formation. Montes-Hernandez et al.20 investigated carbonation of nanosized portlandite catalyzed


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by water activity; they reported high efficiency of CO2 mineralization to calcite (>95%) for 24 h reaction time. This reaction model suggested that the carbonation would be achieved into a threestep process starting from the initial transformation phase of portlandite grains to calcite (25-40 wt %), a fast CO2 mineralization followed by a slow CO2 mineralization. Ibrahim et al.22 suggested the rapid solid-gas carbonation of Ca(OH)2 under high pressure CO2 with assistance of an effective solid state ionic liquid, only pure calcite was produced by this way. Lopéz-Periago et al.23 evidenced that supercritical gas-liquid-solid and wet gas-solid systems could attain high yield of carbonation process and efficiently influenced the morphology and size of calcite particles formed. Similar findings were also described by Gu et al.24 who successfully achieved high conversions (>98%) in favor of calcite formation using supercritical CO2 contained water. Materic and Smedley25 proposed a mechanism of direct carbonation of Ca(OH)2 at high temperatures (>500.0 oC) suggesting that a dehydration process occurred during the thermal decomposition of superheated Ca(OH)2 followed with a subsequent carbonation of the dehydrated CaO according to Equations (1) and (2). Ca(OH)2 → CaO +H2O

(1)

CaO + CO2 → CaCO3

(2)

Technically, stabilization and formation of the metastable vaterite phase require facilitating factors such as additives. Various additives including amino acids,26 urea,6 biopolymers27 and polypeptide28 have been successfully employed in the crystallization of vaterite in solutions. Literature reports demonstrated the influence of urea in the precipitation of the metastable CaCO3 phases; thus aragonite29 and vaterite30 were precipitated in water solvent. However, these systems employed different calcium resources (CaCl2, NaCO3, NaHCO3, K2CO3 or Ca(CH3COO)2) as inputs which would result in side-products accumulation and increased cost of


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separation. For example, vaterite mesocrystals with a hexagonal prism structure were achieved in the presence of sodium citrate and sodium dodecyl benzenesulfonate by using CO2 gas (generated from ammonium carbonate) and CaCl2.31 However, Beuvier et al.,32 reported the preparation of hollow vaterite CaCO3 microspheres by using supercritical CO2 with a Ca(OH)2 aqueous solution which necessitated NaCl, glycine and template directing agent hyaluronic acid. Expectedly, from the accomplishments mentioned above, although direct carbonation of Ca(OH)2 (the best calcium resource which only introduces OH- group) to calcite CaCO3 has been widely studied either in solution or in high pressure or high temperature CO2, it is currently not successful for producing vaterite or aragonite CaCO3 except the work of Beuvier et al.32 (it still needs several additives in aqueous solution); the successful researches of gas-solid reaction of Ca(OH)2 with CO2 at high pressure would provide a platform for preparing vaterite or aragonite CaCO3 to avoid the solution carbonation which is susceptible to environmental and economical limitations; and possible dissociation and transformation of vaterite or aragonite to calcite.33 Thus, we have been motivated by exploring a new route in a free-solvent system using urea as an additive to stabilize vaterite formation through high pressure CO2 carbonation of Ca(OH)2 with risk-free to sustainability of the growing engineering demand. The reaction conditions and carbonation mechanism were examined to develop a fast and green route for crystallization of vaterite crystals, which revealed that the CO2 acted as an inhibitor for urea decomposition at high temperature besides as a reactant; the urea entrapped Ca(OH)2 via its melt to stabilize vaterite formation by amine functional group and initiated the carbonation process via tiny water triggered by its slight decomposition. This technique in its unique feature needs to be understood in the sense that it is a benign process using CO2 with no side-products or direct discharge into the environment while applied to production of high value material with recyclability concern.


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EXPERIMENTAL SECTION Materials The analytical grade Ca(OH)2 (≥95.0%), urea (≥99.9%) and ethanol (99.9%) were purchased from Sinophram Chemical Reagent Co., Ltd (China). XRD analysis of Ca(OH)2 indicated that it contained about 1.1% CaCO3 calcite and other impurities. CO2 (≥99.9%) was purchased from Linde Gas Co., Ltd (China). Preparation of vaterite particles From a cheap and readily available commercial Ca(OH)2, vaterite crystals have been produced via the experimental setup (Figure 1) by using high pressure CO2 at various conditions (Table 1). The reaction pressure P was controlled with a back-pressure regulator (in a CO2 feeding system, not shown) for the CO2 reservoir at  0.05MPa accuracy. The reaction temperature was controlled with a water thermostat at  0.5 oC.

Figure 1 Representation of the experimental setup for gas-solid reaction.

In a typical experiment, an appropriate mass ratio (R=1:1-1:6) of Ca(OH)2 to additive urea was manually grinded to about average particle size of less than 8 µm (supplementary Figure S1) and sufficiently mixed in a mortar before loading into the reactor (a 150 ml stainless steel high pressure vessel). The reactor was then sealed tightly, subjected to a vacuum pump and heated to


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reach the melting temperature (~133 oC, ambient pressure) or less than the melting temperature using an oil bath. At thermal equilibrium, CO2 from the CO2 reservoir was charged to the reactor, through valve v-1, the dryer (P2O5 inside to remove tiny water in CO2), v-2, and finally a coil preheater. Accordingly, the reaction temperature (T= 90.0-180.0 oC) was further adjusted and stabilized. The pressure of the reaction system was regulated (P= 6.0-15.0 MPa) by the valves and the CO2 reservoir and displayed by a pressure gauge. Upon the reaction completion (reaction time t=30-150 min), CO2 is released through valves v-3. The additive-free product could be obtained by ethanol washing and drying at 80 oC or direct calcination at 200 oC. Characterization of products For quantitative analysis of as-obtained CaCO3, powder X-ray diffractometer (XRD, Rigaku Ultima IV, Japan) with Cu Kα radiation was used for phase detection. The patterns were collected in the 2θ range 5-75o at scanning speed of 0.05 o/s. Rietveld refinement method was used to analyze the compositional diffraction patterns of the as-prepared polymorphs.19,

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Fourier transform infrared spectroscopy (FT-IR) was implemented on Nicolet 330 (Thermo Electron Corporation) with the KBr pellet method. All spectra were recorded in the 4000-400cm1

wavenumber region at room temperature and manipulated using OMNICTM software. The

morphological structures of the powders were examined by using scanning electron microscopy (SEM, ZEISS Sigma, JEOL JFC-1600 instrument). The pore volume, Brunauer-Emmet-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) pore size of the powders were determined by the nitrogen adsorption-desorption measurement (ASAP 2020 Micrometrics, USA). 870 KF Titrino Plus was enabled to determine the water content of the samples. Differential Scanning Calorimetry (DSC 204 HP Phoenix®, Netzsch) was enabled to measure the melting and decomposition temperature of urea with high pressure CO2 and without CO2.


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RESULTS AND DISCUSSION Effects of various reaction conditions Generally, gas-solid carbonation reactions were reported to achieve incomplete conversion.34 To the best of our knowledge, synthesis of vaterite particularly using pressurized CO2 in free-solvent system had not yet been reported. In the current study we showed the possibility to fabricate spherical vaterite particles using a solvent-free system to transform Ca(OH)2 into value-added CaCO3 material by high pressure CO2 at various reaction conditions as shown in Table 1. Note that the temperatures were selected according to the melting temperature about 132 oC of urea at ambient pressure. Table 1 Effects of various parameters on the conversion of Ca(OH)2 Sample No.

R

P

T

t

Calcite

Vaterite

Ca(OH)2

(Ca(OH)2/urea)

(MPa)

(oC)

(min)

(%)

(%)

(%)

1

1:1

12.0

150.0

120

20.9

79.1

0.0

2

1:2

12.0

150.0

120

12.8

87.2

0.0

3

1:3

12.0

150.0

120

9.5

90.5

0.0

4

1:4

12.0

150.0

120

8.0

92.0

0.0

5

1:6

12.0

150.0

120

5.8

94.2

0.0

6

1:3

6.0

150.0

120

48.8

51.2

0.0

7

1:3

9.0

150.0

120

44.8

55.2

0.0

8

1:3

12.0

150.0

120

9.5

90.5

0.0

9

1:3

15.0

150.0

120

9.8

90.2

0.0

10

1:3

12.0

90.0

120

35.9

7.2

56.9

11

1:3

12.0

120.0

120

41.3

58.7

0.0

12

1:3

12.0

150.0

120

9.5

90.5

0.0

13

1:3

12.0

160.0

120

9.9

90.1

0.0

14

1:3

12.0

180.0

120

55.0

45.0

0.0


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15

1:3

12.0

150.0

30

19.4

42.2

38.0

16

1:3

12.0

150.0

60

15.5

78.6

6.4

17

1:3

12.0

150.0

90

10.8

89.2

0.0

18

1:3

12.0

150.0

120

9.5

90.5

0.0

19

1:3

12.0

150.0

150

12.4

87.6

0.0

20

1:3

12.0

150.0

1440

38.8

61.2

0.0

Considering the influence of mass ratio R (Ca(OH)2/urea), the analysis of diffraction patterns by Rietveld refinement showed that the conversion of Ca(OH)2 attained 100.0% for all investigated R which evidenced the complete carbonation. Increased R from 1:1 to 1:6 positively influenced the vaterite yield; for instance, at the mass ratios of 1:3 and 1:6 while keeping other parameters (temperature T, pressure P and time t) constant, the vaterite contents were 90.5% and 94.2%, respectively. However, increasing R above 1:3 to 1:6 did not significantly influence the high yield of vaterite (it is only about 4% difference; yet could not be 100% due to the calcite formation in the open atmosphere), indicating enough urea to contact Ca(OH)2. Thus, R=1:3 could be chosen from practical and economical consideration for avoid more urea decomposition (see the later study). As Figure 2A shows, it emerged that the characteristic peaks of calcite at 2θ angles of 29.37 and 35.96 decreased gradually with the increasing of R which resulted in high vaterite content, indicating that the composition of CaCO3 crystals was a mixture of calcite and predominant vaterite phase. For further characterization of these samples, FT-IR measurement was performed and recorded in the 4000-400 cm-1 wavenumber region; all displayed four characteristic peaks of vaterite at 744.62, 876.79, 1087.50 and 1436.87 cm-1, and only one short peak at 713.31 cm-1 showing the presence and decreasing trace amounts of calcite as the mass ratio increased (Figure 2B). However, an additional absorption band at 2499.15 cm-1 could be


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observed in the spectrum which is likely assigned to the presence of carbamate formation between adsorbed CO2 and amine functional group of urea.35 Samples prepared at different R were further demonstrated by SEM analysis (Figure 2a-d). It was observed that all samples formed spherical vaterite particles and trace amounts of calcite crystals scattered around and/or on spherical vaterite particles. SEM images of products obtained by direct calcination or through ethanol washing showed differences in textural property of the crystals (Figure S2): the sample from ethanol washing exhibited smooth surface while the calcined vaterite particles had rough surface with pores which could be generated from decomposing urea during the calcination process. The specific areas, average pore sizes and pore volumes of some typical vaterite powder samples obtained at R=1:3 and 1:4 were showed in Table 2. Obviously, the vaterite obtained by ethanol washing (sample No.4) showed the BET surface area of 32.9 m2/g; slightly higher compared to the product obtained when calcination was used during product post-treatment.

(A)

v

(B)

c: calcite v: vaterite

v

(c)

v

v

c v

c v

(a) 713 745

(b)

(b)

(c)

876

10

20

30 40 50 2theta (degrees)

60

70

2499

(a)

v

1088

v

Transmittance

Intensity (a.u)

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


1425

500 1000 1500 2000 2500 3000 3500 4000 Wavenumber (cm-1)


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Figure 2 (A) XRD patterns (B) FT-IR spectra and SEM pictures of samples after calcination produced at different R: (a) R=1:1; (b) R=1:2; (c) R=1:3; (d) R=1:4 Table 2 BET surface area, pore volume and pore size of the as-synthesized vaterite particles Sample No.

Post-treatment

Surface area

Pore volume

(m2/g)

(cm3/g)

Pore size (nm)

4

Ethanol washing

32.9

0.07

7.5

3

Ethanol washing

30.3

0.10

12.7

4

Calcination

25.8

0.14

22.7

3

Calcination

18.5

0.13

26.3

Pressure has been reported to have effect on CO2 uptake capacity and then influences the overall carbonation process.36 As shown in Table 1, mineralization of CO2 was complete in the studied range of pressure, but lower pressure (6.0 and 9.0 MPa) promoted the formation of calcite while higher pressure in the range of 12–15 MPa favored the formation of vaterite with high content of about 90 %. High pressure of CO2 could enhance the dissolution of CO2 in molten urea due to the obvious decrease in melting temperature at high pressure37 (the melting temperature at ambient pressure is 133.1oC and that at 5.5 MPa is 129.1 oC, supplementary Figure S3), which raised the contact of urea with Ca(OH)2, therefore increased the formation of vaterite. The influence of temperature as shown in Table 1 has significant effect on the solid-gas reaction. We investigated five different reaction temperatures (90.0, 120.0, 150.0, 160.0, and 180.0 oC).


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Results showed that lower temperature (90.0 oC) resulted into incomplete conversion (sample No.10; very tiny water about 0.05 % in the dry CO2 can still be detected for the carbonation occurring). The corresponding product at lower temperature is a mixture of portlandite (56.9%), calcite (35.9%) along with vaterite (7.2%) suggesting not enough contact of Ca(OH)2 with urea which was under melting temperature. High vaterite yield (94.2%) was obtained with temperature around 150-160 oC, but formation of calcite at higher temperature 180.0 oC could be anticipated; at 180.0 oC the thermolysis of urea was accelerated to result in failure to stabilize vaterite.38 As could be detected from the FT-IR spectra analysis of the sample prepared at 180.0 oC

(Figure S4), the peak arising at 3428 cm-1 is assigned to melamine which is a urea

decomposing product.39 On the other hand; as Table 1 shows, the conversion of Ca(OH)2 was time-dependent: the unreacted Ca(OH)2 decreases from 38.0% at 30 min to 0% after 90 min; the content of vaterite increased from 42.2% at 30 min to about 90 % after 90 min (Figure S5 for the detailed SEM images). From the time-dependent reaction for a typical sample (No.4) we can observe clearly the development of vaterite morphology formation changing from primitive structure of vaterite nuclei and immature crystal just after 5 min reaction time to mature vaterite crystals with clear boundary after 60 min (Figure S6). The increasing of vaterite with the reaction time is because of the increased conversion of Ca(OH)2 by slow diffusion of CO2 within the bulk and water effect on the dissociation of reacting species to complete the carbonation. However, further increase of the reaction time made the obvious transformation of vaterite to calcite from 120 min to 1440 min, revealing the instability of vaterite at high temperature. Table 1 indicated that to some extent the production of calcite along vaterite is inevitable. This can be ascribed to the initial calcite content present in Ca(OH)2 attributed to contact of Ca(OH)2


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with air. A further careful experiment in glove box confirmed 99.0% vaterite was achieved at R=1:4, 12.0 MPa and 150.0 oC except about 1.0% calcite present in the raw portlandite. Regeneration of urea To recover the additive urea from the end product, sample was washed with ethanol. After filtration, the filtrate was simply treated with evaporation at 80 oC to recycle the urea up to approximately 93.6 wt% of the original feeding mass. The recovered urea was further confirmed by its XRD pattern and FT-IR spectrum (supplementary Figure S7) which displayed strong matching patterns. Eichelbaum et al.,40 demonstrated that about 10% of the original mass of urea was decomposed at about 150 oC (the sample was treated for a short time at high heating rate to minimize the decomposition). However, as tested by the urea mass loss, the decomposition of pure urea in CO2 was about 3% of the original mass at 12.0 MPa and 150.0 oC for 2 h, against about 30% at ambient pressure and 150.0 oC for 2 h. In other words, it can be said that besides as the reactant, high pressure CO2 used in the current system also acted as an inhibitor for the thermolysis of urea at relatively high temperature, altering the decomposition of urea to shift to higher temperature and resulting in high recovery of the additive urea. Proposed reaction mechanism Figure 3 represents the suggested mechanism for this high-pressure gas-solid carbonation system with urea to crystalize vaterite CaCO3. Accordingly, when heating to melting point of urea (~133 oC), the molten urea enhances the entrap of Ca(OH)2 particles (Figure S8). After the melting temperature, a small amount of urea decomposes to ammonium cynate (NH4+NCO-); dehydration of the latter species releases water molecules and other species41 (water content of 0.29% was detected after about 1h for pure urea at 150.0 oC and 12.0 MPa in dried CO2). The


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produced water resulted into the formation of Ca2+ and bicarbonate ion HCO3- upon pressurized CO2 and the urea-entrapped Ca(OH)2 following further formation of complex between Ca2+ and amine functional group of urea.42 It was reported that HCO3- could be easily trapped by amine group forming intermediate carbamate,42 providing nucleation site for vaterite particles. Following this, HCO3- would supply CO32- which reacted with Ca2+ to precipitate CaCO342, 43 with moisture being released; the so-produced water in the system would sustain the advancement of carbonation process by enhancing further dissolution of residual solid Ca(OH)2 until the carbonation process was complete; suggesting that this configuration involving gassolid carbonation in the presence of urea was self-sustained reaction. Note that Hernandez et al. reported CO2 mineralization of portlandite catalyzed by water activity to calcite (>95%) for 24h reaction time.20 Therefore, both the additive urea and high-pressure CO2 played key roles in this system: the urea entrapped the portlandite to stabilize vaterite formation through its amine functional group and trigged the formation of tiny water via its decomposition to initiate the carbonation reaction; the CO2 acted as the carbonate source for vaterite product and assured the recovery of urea through inhibiting effect for the urea decomposition.


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Figure 3 Representation model of high-pressure gas-solid carbonation of Ca(OH)2 mechanism with urea as the additive.

CONCLUSION In summary, we demonstrated a novel solvent-free high-pressure solid-gas carbonation technique for CO2 mineralization with Ca(OH)2 for vaterite crystallization using urea as the additive. The technique in the presence of urea demonstrated a high efficiency, relatively fast and green route to completely transform Ca(OH)2 into spherical vaterite as the predominant phase. Moreover, the post-treatment of the product after reaction ensured a simple regeneration of the additive urea with high recovery. Compared to the normal solution way where Ca(OH)2 is not applicable for producing vaterite by using urea as the additive, the current system refrains from generating inorganic salts as byproducts, which simplified the separation process. Consequently, the proposed solvent-free solid-gas carbonation of Ca(OH)2 using high pressure CO2 would be a potential green route for industrial production of CaCO3 polymorphs. ASSOCIATED CONTENT Supporting Information. SEM images of raw materials, SEM images showing textural property of crystals, DSC graph and FT-IR spectra, SEM images of as-prepared CaCO3 powder at different reaction duration and particle evolution morphology. XRD patterns and IR spectra of recycled additive. The following files are available free of charge. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (J.L)


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (21476186). REFERENCES (1) Sha, F.; Zhu, N.; Bai, Y.; Li, Q.; Guo, B.; Zhao, T.; Zhang, F.; Zhang, J., Controllable Synthesis of Various CaCO3 Morphologies Based on a CCUS Idea. ACS Sustainable Chem. Eng. 2016, 43032-3044. (2) Chu, D. H.; Vinoba, M.; Bhagiyalakshmi, M.; Hyun Baek, II; Nam, S. C.; Yoon, Y.; Kim, S. H.; Jeong, S. K., CO2 mineralization into different polymorphs of CaCO3 using an aqueousCO2 system. RSC Adv. 2013, 321722-21729. (3) Boyjoo, Y.; Pareek, V. K.; Liu, J., Synthesis of micro and nano-sized calcium carbonate particles and their applications. J. Mater. Chem. A 2014, 214270-14288. (4) Fagerlund, J.; Teir, S.; Nduagu, E.; Zevenhoven, R., Carbonation of magnesium silicate mineral using a pressurised gas/solid process. Energy Procedia 2009, 14907-4914. (5) Farmer, T. C.; Schiebel, S. K.; Chmelka, B. F.; Doherty, M. F., Polymorph Selection by Continuous Precipitation. Cryst. Growth Des. 2018, 184306-4319. (6) Nawarathna, T. H. K.; Nakashima, K.; Fujita, M.; Takatsu, M.; Kawasaki, S., Effects of Cationic Polypeptide on CaCO3 Crystallization and Sand Solidification by Microbial-Induced Carbonate Precipitation. ACS Sustainable Chem. Eng. 2018, 610315-10322. (7) Vance, K.; Falzone, G.; Pignatelli, I.; Bauchy, M.; Balonis, M.; Sant, G., Direct Carbonation of Ca(OH)2 Using Liquid and Supercritical CO2: Implications for Carbon-Neutral Cementation. Ind. Eng. Chem. Res. 2015, 548908-8918. (8) Sarrion, B.; Sanchez-Jimenez, P. E.; Perejon, A.; Perez-Maqueda, L. A.; Valverde, J. M., Pressure Effect on the Multicycle Activity of Natural Carbonates and a Ca/Zr Composite for Energy Storage of Concentrated Solar Power. ACS Sustainable Chem. Eng. 2018, 67849-7858. (9) Chang, R.; Choi, D.; Kim, M. H.; Park, Y., Tuning Crystal Polymorphisms and Structural Investigation of Precipitated Calcium Carbonates for CO2 Mineralization. ACS Sustainable Chem. Eng. 2017, 51659-1667. (10) Zhou, Y.; Liao, C.; Shih, K., Combined Fe2O3 and CaCO3 Additives To Enhance the Immobilization of Pb in Cathode Ray Tube Funnel Glass. ACS Sustainable Chem. Eng. 2018, 63669-3675. (11) Liu, L.; Zhang, X.; Liu, X.; Liu, J.; Lu, G.; Kaplan, D. L.; Zhu, H.; Lu, Q., Biomineralization of Stable and Monodisperse Vaterite Microspheres Using Silk Nanoparticles. ACS Appl. Mater. Interfaces 2015, 71735-1745. (12) Udrea, I.; Capat, C.; Olaru, E. A.; Isopescu, R.; Mihai, M.; Mateescu, C. D.; Bradu, C., Vaterite Synthesis via Gas–Liquid Route under Controlled pH Conditions. Ind. Eng. Chem. Res. 2012, 518185-8193.


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For Table of Contents Use Only A High-Pressure Gas-Solid Carbonation Route to Produce Vaterite Benoit Rugabirwa, David Murindababisha, Hongtao Wang, Jun Li* Department of Chemical and Biochemical Engineering, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Xiamen University, Xiamen 361005, China

Synopsis Vaterite crystals are stabilized by urea additive via its melt. Calcium Ca(OH)2 is entrapped within the mass of molten urea which initiated the mineralization of CO2 triggered by a tiny water formation to complete the carbonation reaction in dry condition without leaving behind any inorganic salts.


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A High-Pressure GasâSolid Carbonation Route to Produce Vaterite

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A High-Pressure GasâSolid Carbonation Route to Produce Vaterite