Utilization of a CO₂ Storage Material: Shape-Controlled Preparation of CaCO₃ Microspheres

Abstract

Homogeneous CaCO₃ microspheres were controllably synthesized via a simple hydrothermal method without using extraneous additives by Ca(OH)₂ saturated solution reacted with CO₂ storage material (CO₂SM), which was obtained by absorbing CO₂ gas in a system containing equimolar ethylenediamine (EDA) and polyethylene glycol 400 (PEG). Specifically, homogeneous pure vaterite spheres were obtained by using 160 g/L CO₂SM solution at 100 °C and a reaction time of two hours. It is noteworthy that the EDA and PEG, which are released from CO₂SM, have dual functions as a CO₂ absorber and crystal regulator in the process of controlling the crystallization of CaCO₃ crystals.

1. Introduction

Climate change, one of the most disturbing global environmental issues, primarily relates to continuously increasing CO₂ emission from burning fossil fuels [1,2,3]. Thus, it is necessary to develop CO₂ capture and utilization (CCU) technology to reduce CO₂ emission. Currently, there are four CO₂ separation technologies, consisting of pre-combustion decarburization, chemical chain combustion decarburization, oxygen-rich combustion decarburization, and post-combustion decarburization [4,5]. As an important CO₂ abatement technology, the post-combustion decarbonization captures CO₂ from the flue gas after combustion and reduces CO₂ in flue gases by about 80% [6,7]. Therefore, it has broad application prospects. In recent years, carbon capture and storage (CCS) and CCU processes have been developed to control CO₂ emission and resource recovery [8,9,10,11,12]. In particular, the preparation of CaCO₃ via the chelation of CO₂ has been studied extensively because CaCO₃, as one of the cheaper biological materials, has important applications in various fields [13,14,15,16,17,18,19,20]. There are four polymorphic phases that have been prepared as follows: calcite, aragonite, vaterite, and amorphous CaCO₃ (ACC) [21,22]. Calcite and aragonite CaCO₃ crystals are the most stable structures of thermodynamics, and vaterite would rapidly transform to aragonite and (or) calcite because it is the least thermodynamically stable [23]. Among them, the pure vaterite phase CaCO₃ crystals is studied most widely because it has a larger specific surface and porous nature that could improve mechanical properties when they are used as filler [24,25,26]. More importantly, it is challenging for the synthesis of stable vaterite when CO₂ is used as a direct raw chemical because the process is more dependent on the calcium source.

CaCO₃ powders are prepared by carbonization in industry. The morphology of CaCO₃ powders prepared by carbonization is disordered and the particle size is not uniform [27,28]. Recently, Mari et al. [13,19] captured CO₂ in aqueous solutions of methyldiethanolamine (MDEA) and 2-amino-2-methyl-1-propanol (AMP). At the same time, soluble calcium salts were added to the system, and the CO₂ captured by amine in the system was further converted by the reaction with Ca²⁺ in the system to form CaCO₃ precipitation. However, CaCO₃ is easy to agglomerate, its morphology is disordered, and the size of particle is larger. Therefore, the preparation of CaCO₃ with fixed morphology, monodisperse, small particle size, and single crystal phase has become a hot spot in the research of CaCO₃ mineralization.

In the work, the polyethylene glycol 400 (PEG400) was added into ethylenediamine (EDA) to enhance the ability of the system absorbing CO₂, which formed CO₂ storage material (CO₂SM) (Figure 1). After that, the as-prepared CO₂SM was used to investigate the crystallization of CaCO₃ crystals via a simple hydrothermal reaction. Whereas EDA provided a high pH environment, polyethylene glycol (PEG) induced the formation of vaterite CaCO₃ crystals. Furthermore, the optimal synthesis conditions were identified by exploring the concentration of CO₂SM solution, reaction temperature, and reaction time. Specially, the stable pure vaterite phase CaCO₃ crystals in nature also could be prepared under experiment condition. It is anticipated that the result of this is significant for the indirect utilization of CO₂. The process integrates CO₂ capture and resource utilization, effectively utilizes CO₂, and generates value-added CaCO₃.

2. Experimental Section

2.1. Materials

The purity of EDA was 98%, which was provided by Yongsheng Fine Chemical Co., Ltd., Tianjin, China. The solid of Ca(OH)₂ was provided by Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. PEG 400 with a molecular weight range of 380–420 g/mol, was analytical grade and was provided by the Beijing Reagent Company Co., Ltd., Beijing, China. The purity of compressed CO₂ was 99.999%, which was provided by the China Standard Things Center, Beijing, China. CO₂SM was obtained by capturing via the equimolar EDA + PEG400 system according to the previous study.

2.2. Synthetic Procedures

In line with a typical CaCO₃ preparing process, 1 g CO₂SM was added into a stainless steel reactor (the specifications of Teflon-lined is 100 mL), which was filled with 50 mL saturated Ca(OH)₂ solution. The reaction was controlled at 100 °C for 2 h, and then was cooled down until it reached room temperature. The mixture of precipitate and solution was separated by vacuum filtration, and it was washed at least three times by aqua distillate. The collected filter cake was dried at 120 °C for more than 4 h under vacuum.

2.3. Characterization

The morphology of CaCO₃ precipitates were observed by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (Quanta FEG 650, Beijing, China), with an accelerating voltage of 20 kV. The crystal lattice spacing was observed by high resolution transmission electron microscope (HR-TEM; JEM-2100, Beijing, China), and the accelerating voltage was 200 kV. The X-ray diffractometer (XRD) was made in Germany from Siemens D/max-RB at 2θ = 5–80°, and the scanning rate was 0.05°/s (the mass of a sample was about 0.5 g), which used a powder with Cu Kα (λ = 1.5418 Å) radiation. The Fourier transform infrared spectroscopy (FTIR) technology was made in the USA by Nexus 670, and the CO₂SMs dispersed in KBr powder, with a resolution of 1 cm⁻¹ from (4000 to 400) cm⁻¹. The thermogravimetric analysis (TGA) technology was made in Germany by Entzsch-Sta 449 and the samples were met with the heating rate of 5 °C min⁻¹ (the mass of a sample was about 0.3–0.5 mg). The automatic surface area (BET method) and porosity analyzer (BJH model) was made in the USA by TristarII 3020 to obtain nitrogen adsorption-desorption data (the mass of a sample was about 2.0 g). The BET method and BJH model are depended on the Equations (1) and (2):

P/[V(P₀ − P)] = 1/(V_m × C) + [(_C − 1)/(Vm × C)] × (P/Po)

(1)

where P is nitrogen partial pressure, P₀ is adsorption temperature, nitrogen saturated vapor pressure, V is the actual adsorption amount of nitrogen on the sample surface, V_m is nitrogen monolayer saturated adsorption amount, C is constants related to the adsorption capacity of the sample

$$\ln \frac{P}{P_0}=\frac{2\gamma V_m}{rRT}$$

(2)

where P is the actual vapor pressure, P₀ is the saturated vapor pressure, γ is the surface tension, V_m is the molar volume of the liquid, R is the general gas constant, r is the radius of the droplet, T is the temperature.

The X-ray photoelectron spectroscopy (XPS) technology was made in the USA by ESCLAB-250Xi with a monochromatized Al Kα X-ray (hν = 1486.6 eV) operating at 150 W from (200 to 600) eV.

The percent molar content of calcite, vaterite, and aragonite in the precipitate was calculated by Equations (3) to (5) [29]:

$$X_A=\frac{{3.157\times I}_A^{221}}{I_C^{104}+{3.157\times I}_A^{221}+{7.691\times I}_V^{110}},$$

(3)

$$X_C=\frac{I_C^{104}{\times X}_A}{{3.175\times I}_A^{221}},$$

(4)

$${\mathrm{X}}_{\mathrm{V}}=\frac{\left(I_C^{104}+{3.157\times I}_A^{221}+{7.691\times I}_V^{110}\right)\times {3.175\times I}_A^{221}}{\left(I_C^{104}+{3.157\times I}_A^{221}+{7.691\times I}_V^{110}\right)\times {3.175\times I}_A^{221}}-\frac{{3.157\times I}_A^{221}\times {3.175\times I}_A^{221}-\left(I_C^{104}+{3.157\times I}_A^{221}+{7.691\times I}_V^{110}\right){\times I}_C^{104}{\times X}_A}{\left(I_C^{104}+{3.157\times I}_A^{221}+{7.691\times I}_V^{110}\right)\times {3.175\times I}_A^{221}}.$$

(5)

The percent of molar content of only containing vaterite and calcite in the precipitate is calculated by Equations (6) and (7):

$$\frac{I_C^{104}}{I_V^{110}}=7.691\frac{X_C}{X_V},$$

(6)

$${\mathrm{X}}_{\mathrm{V}}=\frac{\left(I_C^{104}+{3.157\times I}_A^{221}+{7.691\times I}_V^{110}\right)\times {3.175\times I}_A^{221}}{\left(I_C^{104}+{3.157\times I}_A^{221}+{7.691\times I}_V^{110}\right)\times {3.175\times I}_A^{221}}-\frac{\left(I_C^{104}+{3.157\times I}_A^{221}+{7.691\times I}_V^{110}\right){\times I}_C^{104}{\times X}_A}{\left(I_C^{104}+{3.157\times I}_A^{221}+{7.691\times I}_V^{110}\right)\times {3.175\times I}_A^{221}},$$

(7)

where X_A, X_C, and X_V are the percent molar content of aragonite, calcite, and vaterite, respectively. The lattice planes of 221 (I²²¹A), 110 (I¹¹⁰V), and 104 (I¹⁰⁴C) are due to aragonite, vaterite, and calcite.

3. Results and Discussion

Conventional protocols for regulating CaCO₃ polymorph mainly include variation of solvents [30], temperatures [31], and organic or inorganic additives [32]. Thus, the effect of three major factors were examined in the CaCO₃ crystallization process, such as CO₂SM concentration, temperature, and time.

3.1. Influences of CO₂SM Concentration

Various CO₂SM concentrations were examined to identify the influences of CO₂SM concentration on the preparation of CaCO₃ crystals, and the SEM photos of CaCO₃ crystals obtained at the CO₂SM concentration range of 20 to 160 g/L at 100 °C for 2 h are shown in Figure 2.

Table 1. The reaction conditions and polymorph composites of as-obtained CaCO₃ crystals under different CO₂SM concentrations at 100 °C for 2 h.

Sample a	Reaction Conditions b(CO2SM g/L; pH)	Polymorph Composition c (%)
		Calcite	Aragonite	Vaterite
A	20; 9.33	20.60	0	79.40
B	60; 8.75	0	0	100
C	95; 8.57	0	0	100
D	120; 8.53	0	0	100
E	160; 8.49	0	0	100

^a The Ca(OH)₂ saturated liquid was 50 mL. ^b The unit of g/L was calculated by CO₂SM dispersed in Ca(OH)₂ saturated liquid. ^c Results were calculated by the X-ray diffractometer (XRD) patterns.

illustrates the preparation conditions and crystal composition of CaCO₃ crystals prepared with different CO₂SM concentrations, which indicated that the CaCO₃ crystals exhibited spherical-like shapes with an approximate particle diameter of 5–7 μm, at the same time, there was no obvious change in CaCO₃ crystal size with the increase of CO₂SM concentration. As listed in Table 1, the solution pH decreased gradually from 9.33 to 8.49 with increasing CO₂SM concentration. These results unambiguously revealed that the morphology of CaCO₃ crystals varied significantly with the increasing CO₂SM concentrations.

The XRD patterns of CaCO₃ crystals are shown in Figure 3. The main peaks of sample A (20 g/L) at 29.2° and 24.7° in the XRD patterns were assigned to the (104) face of calcite and the (110) face of vaterite, respectively [33], indicating the phase structure of the sample A was a mixture of calcite and vaterite, of which the CO₂SM concentration was 20 g/L. With the increasing CO₂SM concentration from 60 g/L to 160 g/L (samples B–E), the phase structures were pure vaterite because their characteristic peaks at 24.7° corresponded to the (110) face of vaterite, which is consistent with our previous work. The change trend above suggests that a high concentration of CO₂SM favors the formation of vaterite because of the low pH value of the medium [34,35,36,37], which probably favors the formation of the most thermodynamically stable polymorph—vaterite. In addition, the polymorph composition of CaCO₃ crystals are listed in Table 1.

In general, there are four modes for the characteristic vibrations of the C–O bond of CaCO₃ crystals in FTIR spectra, which are the symmetric stretching mode, the out-of-plane bending mode, the doubly asymmetric stretching mode, and the doubly in-plane bending mode [38,39,40]. As shown in Figure 3b, two characteristic peaks at 875 and 711 cm⁻¹ were observed, which was due to the υ₄ mode of calcite in the sample A. Meanwhile, the characteristic peak at 745 cm⁻¹ was also observed, which corresponds to the υ₄ mode of vaterite. These results indicated that the crystal phase of sample A was a mixing crystal phase of calcite and vaterite. When the CO₂SM concentration was higher than 20 g/L, the characteristic peak at 711 cm⁻¹ disappeared, indicating that the samples of B to E were pure vaterite phase. The analysis results of FTIR spectra and XRD spectra are unanimous.

3.2. Influent of Reaction Temperature

Preparation conditions and crystal composition of CaCO₃ crystals at different reaction temperatures are listed in

Table 2. The reaction conditions and polymorph composites of as-obtained CaCO₃ crystals under different temperatures in 160 g L⁻¹ CO₂SM solution for 2 h.

Samples a	Preparation Conditions b (°C)	Polymorph Compost c (%)
		Calcite	Aragonite	Vaterite
A	80	0	0	100
B	90	0	0	100
C	100	0	0	100
D	110	0	0	100
E	120	0	0	100

^a The Ca(OH)₂ saturated liquid was 50 mL. ^b The unit of g/L was calculated by CO₂SM dispersed in Ca(OH)₂ saturated liquid. ^c Results were calculated by the XRD patterns.

. It is known that the reaction temperature hardly influenced the morphology of CaCO₃ in Figure 4. In contrast, with increasing reaction temperature, the size of as-obtained CaCO₃ crystals increased firstly, and subsequently decreased. In the crystallization process, supersaturation was the driving force, as the supersaturation decreased with the rise of temperature and the nucleation rate slowed down, which made the growth rate slower and helped to increase the size of the crystals. The solubility of CO₂ in solution further decreased, and the content of CO₃²⁻ decreased with the further increase of temperature, which led to the formation of smaller particles and the smaller size of CaCO₃ crystals [15].

The XRD patterns of CaCO₃ crystals were shown in Figure 5, which were obtained with the increasing temperature from 80 °C to 120 °C with 160 g/L CO₂SM for 2 h. As shown in Figure 5a, all samples were pure vaterite crystals because the main peak at 24.7° could be assigned to the 110 phase of vaterite. Furthermore, the polymorph composition of CaCO₃ crystals is listed in Table 2. The FTIR spectra in Figure 5 show that all the samples displayed three vibration bands at 1083, 875, and 745 cm⁻¹, which confirmed the presence of vaterite. These results suggest that there was no influence of reaction temperature on the phase.

3.3. Influents of Reaction Time

Preparation conditions and crystal composition of CaCO₃ crystals prepared with different reaction time are listed in

Table 3. The reaction conditions and polymorph composites of as-obtained CaCO₃ crystals with different reaction times at 100 °C and in 160 g L⁻¹ CO₂SM solution.

Samples a	Preparation Conditions b (min)	Polymorph Compost c (%)
		Calcite	Aragonite	Vaterite
A	60	0	0	100
B	120	0	0	100
C	240	0	0	100
D	360	0	0	100
E	480	0	0	100

^a The Ca(OH)₂ saturated liquid was 50 mL. ^b The unit of g/L was calculated by CO₂SM dispersed in Ca(OH)₂ saturated liquid. ^c Results were calculated by the XRD patterns.

. All the samples were spherical-like shapes, regardless of the reaction time in Figure 6. When the reaction time was extended, the microsphere of CaCO₃ gradually grew into defective spheres. As a result, the size of microspheres gradually decreased and became densely agglomerated. The reason may be that the nucleation rate of crystals is faster and the system is controlled by thermodynamics. In other words, the nucleation process of CaCO₃ particles was completed within 120 min, and then small particles began to agglomerate over time [15].

The XRD patterns of CaCO₃ are shown in Figure 7, which were obtained at the reaction time ranging from 1 to 8 h and the reaction temperature of 100 °C with 160 g/L CO₂SM. As shown in Figure 7a, all the samples were pure vaterite crystals because their main peaks at 24.7° correspond to the 110 phase of vaterite. The polymorph composition of CaCO₃ crystals are listed in Table 3. The FTIR spectra in Figure 7b showed that all the samples displayed three vibration bands at 1083, 875, and 745 cm⁻¹, which confirmed the presence of vaterite.

It was suggested that the reaction time could affect the size of CaCO₃, which may have been because of the fast nucleation rate of CaCO₃, but it hardly impacted the morphology and crystal phase of CaCO₃ crystals.

3.4. Continuous Preparation of CaCO₃ Microspheres

The system consisting of equimolar EDA + PEG reacted with CO₂ to yield CO₂SM, which was added into Ca(OH)₂ solution to prepare CaCO₃ precipitate via the hydrothermal process. Then, the mixture of precipitate and solution was separated via vacuum filtration to obtain CaCO₃ powder and filtrate. Subsequently, CO₂ was bubbled into filtrate before the Ca(OH)₂ solution was added. The mixing solution was reacted at 100 °C and 160 g L⁻¹ CO₂SM for 2 h. The process was recirculated five times. The characteristic peaks of products in FTIR were found at 877 and 745 cm⁻¹, as shown in Figure 8, which were ascribed to the CO₃²⁻ group of the vaterite phase, indicating that the uniform crystalline phase of CaCO₃ crystals could still be obtained after filtration.

3.5. Possible Preapared Mechanism of as-Obtained CaCO₃ Microspheres

In this process (Figure 9), EDA would play pH-adjusting agent roles and PEG would play co-solvent roles in the growing process of CaCO₃ crystals [35,36,37,41]. The CO₂, which was released from the CO₂SM, transformed to CO₃²⁻ via the zwitterion mechanism, and at the same time, the CO₂SM produced RNH₃⁺ and HCO₃⁻ at a higher pH (pH > 8) and subsequently abstracted H⁺ by OH⁻ to produce CO₃²⁻. On the other hand, the lone-pair electrons of O in -OH and N in NH₃ showed the strong electrostatic interactions between PEG and Ca ions in the mixing solution [35,42,43,44,45]. Therefore, the formation mechanism of CaCO₃ crystals was as follows:

RNH₂ + CO₂ + H₂O → RNH₃⁺ + HCO₃⁻

RNH₂ + H₂O → RNH₃⁺ + OH⁻

2RNH₃⁺ + CaX₂ → 2RNH₂ + Ca²⁺ + 2HX(X= Cl⁻, NO₃⁻, Br⁻, CH₃COO⁻)

PEG + Ca²⁺ → PEG-Ca²⁺

2PEG-Ca²⁺ + 2HCO₃⁻ + 2OH⁻ → 2CaCO₃ + 2H₂O + 2PEG

The normal polymorph phase transformation of CaCO₃ is vaterite to calcite. The reverse phase transformation was observed in the systems as a result of the electrostatic interactions between surface layers containing two orientations of the CO₃²⁻ groups, and the calcium-rich surface promotes the crystallization of calcite.

As known by crystal growth kinetics, the growth rate of CaCO₃ crystals on one special direction might slow down due to the fact that the crystal adsorbs PEG and/or EDA on the same surface, which might lead to an isotropic growth of the crystal [46,47]. Thus, it is known that the concentration of PEG and EDA, which is directly related from CO₂SM, was of great significance in the CaCO₃ crystallization processes. The higher the CO₂SM concentration, the higher the EDA and PEG concentrations, which leads to more chances for collisions between different ions and the further formation of spherical particles.

3.6. Properties of CaCO₃ Microspheres

To further know the properties of the CaCO₃ microspheres, including the crystalline phase transformation and compositions, we further systematically investigated the spherical-like CaCO₃ micro-particles.

3.6.1. HR-TEM

Firstly, the lattice structure and the edge structure of CaCO₃ microspheres were analyzed by HR-TEM. As shown in Figure 10, there were the (112) plane (0.322 nm) and (114) plane (0.274 nm), which were attributed to the lattice of vaterite [48].

3.6.2. TGA-DSC

TGA refers to the relationship between the mass of the sample to be measured and the temperature change under programmed temperature control. DSC refers to the relationship between the power difference (e.g. in the form of heat) input to the sample and the reference material and the temperature under programmed temperature control.

The spherical CaCO₃ microspheres were also analyzed by using TGA-DSC, and the result is shown in Figure 11. At the first stage of weight loss from the origin temperature at 592 °C, the weight loss rate of CaCO₃ microspheres was 4.83%, which is attributed to the volatilization of organic matters contained on the surface of CaCO₃ microspheres. At 790 °C, a very strong exothermic peak was observed in the DSC curve, which was caused by the thermal decomposition of CaCO₃ via the following reaction: CaCO₃ → CaO + CO₂↑ [49].

3.6.3. Surface Area and Pore Diameter

The N₂ adsorption–desorption curve and pore size distribution curve were also studied to evaluate the property of CaCO₃ microspheres. A specific surface area of CaCO₃ microspheres arrived at 152.04 m²/g. Their average pore diameter and pore volume were determined to be 3.862 nm and 0.245 cc/g, respectively.

3.6.4. XPS and EDX

EDX refers to the determination of elements in samples according to the different characteristic X-ray wavelengths of different elements.

The microspheres of CaCO₃ were also analyzed by XPS and EDX, which are shown in Figure 12 and Figure 13. From the EDX and XPS spectra, as-prepared CaCO₃ microspheres contained C, O, and Ca elements, but other elements were not observed, partially because of the low content of EDA and PEG in CaCO₃ microspheres.

4. Conclusions

CO₂SM, which was obtained by absorbing CO₂ via the EDA + PEG binary system solution, was mixed with Ca(OH)₂ to prepare CaCO₃ microspheres via hydrothermal reactions. The suitable preparation conditions were confirmed as follows: a CO₂SM concentration of 160 g/L, a reaction temperature of 100 °C, and a reaction time of 2 h. The as-prepared CaCO₃ crystals were homogeneous pure vaterite phase and the properties of CaCO₃ microspheres was confirmed. In addition, the uniform crystalline phase of CaCO₃ crystals could still be obtained after five times of cyclic preparation by filtrate. The as-obtained CaCO₃ by this method has high purity and uniform particle size, which may have a good application prospect in the paper industry.