Surfactant assisted CaO-based sorbent synthesis and their application to high-temperature CO2 capture

Powder Technology 344 (2019) 208–221

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Surfactant assisted CaO-based sorbent synthesis and their application to high-temperature CO2 capture Panupong Jamrunroj a, Suwimol Wongsakulphasatch b,⁎, Atthaphon Maneedaeng c, Chin Kui Cheng d, Suttichai Assabumrungrat e a

Department of Chemical Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhon Pathom 73000, Thailand Center of Ecomaterials and Cleaner Technology, Department of Chemical Engineering, Faculty of Engineering, King Mongkut's University of Technology North Bangkok, Bangkok 10800, Thailand School of Chemical Engineering, Institute of Engineering, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand d Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Pahang 26300, Malaysia e Center of Excellence in Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand b c

a r t i c l e

i n f o

Article history: Received 26 October 2017 Received in revised form 17 November 2018 Accepted 3 December 2018 Available online 04 December 2018 Keywords: CaCO3 CaO-based sorbent CO2 sorption SDS Gemini surfactant

a b s t r a c t The concern of carbon dioxide (CO2) emissions, a main contribution of greenhouse gases, has been emerged as an important issue for environmental impact. Adsorption of CO2 by porous solid materials is proven to be one of efficient techniques for CO2 capture technologies. In the present work, attempted has been made to improve property of porous solid materials, CaO-based sorbent, applied for high-temperature CO2 capture. CaCO3 and CaO-based alumina was synthesized using precipitation technique with the addition of sulfonic single chain (SDS) and gemini (12-carbon hydrophobic chains and 3-carbon alkyl spacer, 12-3-12) surfactants for controlling/modifying physical properties. Our studies showed that the addition of anionic surfactants affected phase formation and polymorph of CaCO3, where stronger effect was observed with gemini surfactant. The synthetic CaCO3 was derived to form CaO and applied for capturing CO2 at 600 °C, 15% v/v CO2 (N2 balanced). The results showed that CaO synthesized with adding gemini surfactant offered higher CO2 sorption capacity than single chain surfactant. By incorporating calcium with alumina using co-precipitation technique, the addition of gemini surfactant showed a good impact on CO2 capture performance as an increase in CO2 sorption capacity was observed. However, sintering effect was still not yet be resolved with the addition of gemini surfactant as CO2 sorption capacity decreased upon multiple cycles of CO2 capture. © 2018 Elsevier B.V. All rights reserved.

1. Introduction CaO is used in many applications, for example, biomedical applications [1], catalyst for transesterification reaction [2], reactant of cement [3], additive in paper industry [4], or gas separation [5], etc. An interesting application of CaO is used to capture CO2 at high temperature, which is known as calcium looping. In calcium looping, CaO is reacted with CO2 to form CaCO3 (Eq. (1)). This process is exothermic reaction and called carbonation reaction. The reverse reaction (Eq. (2)) is endothermic reaction and named as calcination reaction [6]. Carbonation : CaOðsÞ þ CO2 ðgÞ⟷CaCO3 ðsÞ ΔH ¼ −178 kJ=mol

ð1Þ

Calcination : CaCO3 ðsÞ⟷CaOðsÞ þ CO2 ðgÞ ΔH ¼ þ178 kJ=mol

ð2Þ

CaO is widely used as CO2 sorbent as it is abundant in nature. However, CaO from natural resources such as natural lime, egg shell, ⁎ Corresponding author. E-mail address: [email protected] (S. Wongsakulphasatch).

https://doi.org/10.1016/j.powtec.2018.12.011 0032-5910/© 2018 Elsevier B.V. All rights reserved.

seashells, or snail shells have uncertain morphologies, which would be difficult to control their applications [1,7,8]. As a consequence, synthetic CaO becomes an attractive option for CO2 capture technology. Studies have shown that performances of CaO-based sorbent on calcium looping technology depend upon surface area, pore size, pore volume, and particle size [9–11]. One technique used to control such morphology and properties of CaO is the addition of additive or structure directing agent in the synthesis step. Additives that are used to enforce structure of CaCO3 or CaO include polymers, surfactants, or organic compounds, etc. [12–21]. Zhao et al. [17] studied an employment of surfactant to control morphologies of CaCO3. The results showed that the addition of SDS provided a belt-like morphology, the presence of CTAB provided plate-like and the presence of PVP provided network-like nanoparticles. Olivares-Marín et al. [5] precipitated CaCO3 from Na2CO3 and CaCl2 with the addition of surfactant TX-100 and commercial dish washing liquid detergent (DLD) of 1% v/v. The results showed the precipitated CaCO3 without additive formed rhombohedral calcite and needle aragonite. Addition of TX-100 showed similar morphology to that without additive whereas morphology of CaCO3 with the addition of DLD obtained spherical of vaterite. All CaCO3 sorbents were

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Scheme 1. Chemical structure of SDS and gemini surfactants.

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calcined to produce CaO at 950 °C for 4 h and then tested its performances on CO2 capture at condition of 15% v/v CO2 and 650 °C. The results revealed that conversion was 25.66% for CaO without additive, 23.84% for CaO-TX-100, and 18.30% for CaO-DLD. Lower capacity of CaO with the addition of DLD additive was found to be due to large agglomeration of particles. It is noted, however from this study, that capacity of all sorbents showed no correlation between adsorption capacities with surface area. Du et al. [18] synthesized aragonite CaCO3 using calcium dodecyl benzenesulfonate as an additive to control crystalline form in microreactor. The results showed whiskers of ca. 27 μm in length with 98–99.5% crystalline purity was obtained under preparation at room temperature. An interesting application of CaO for CO2 capture is in H2 production process, which is mostly operated at high temperature where partial sintering could occur. A technique that is used to prevent sintering is an incorporation of CaO with metal oxide. Type of metal, synthesis method, and the effect of amount of metal adding, etc., are found to

Fig. 1. XRD patterns of CaCO3 with/without the addition of a) SDS and b) gemini surfactants at different concentrations.

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affect CO2 sorption capacity [22–27]. Martavaltzi and Lemonidou [22] varied weight ratio of CaO (from Ca(CH3COO)2)/Al2O3 (from Al (NO3)3) of 65/35, 75/25, and 85/15 and tested for CO2 capture. The conversion of CaO-based at weight ratio of 65/35 showed the lowest conversion of 23%, at weight ratio of 75/25 provided 35% conversion, and at weight ratio of 85/15 offered 45% conversion, for carbonation at

690 °C and 15% v/v CO2. The increasing inert metal was found to have two opposite effects: CO2 sorption capacity was decreased due to a decrease amount of CaO active site, whereas high thermal stability of sorbent could be obtained upon multiple repeated cycles. Xu et al. [28] studied the effect of mixed percent of Al2O3 to CaO on CO2 capture performances by sol-gel synthesis. Percent of Al2O3 from aluminum

Fig. 2. SEM image of CaCO3 at various concentration of a) SDS and b) gemini surfactants.

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isopropoxide were varied from 10% to 30%wt. The resulted CaO-based contained Ca9Al6O18 and CaO. CO2 sorption was tested at carbonation temperature of 650 °C under 15% v/v CO2 (balanced N2). The results showed 10% and 20%wt of Al2O3 provided 80% conversion whereas 30%wt of Al2O3 support yielded 95% conversion. Florin et al. [25] incorporated CaO into calcium alumina complex (Ca12Al14O33) by coprecipitation of Ca(OH)2, Al(NO3)3 and CO2, the CaO-based sorbent consisted of 85%wt CaO and 15%wt Ca12Al14O33. Experimental results showed 50% conversion could be obtained. Broda et al. [29] prepared CaO/Ca12Al14O33 sorbent by sol-gel method and investigated the effect of CaO content on CO2 sorption performance. The results showed that 49% conversion could be obtained with the sorbent contained 90% CaO whereas 41% conversion was observed with 80% CaO at 650 °C in 20% v/v CO2 (Balanced N2). Liu et al. [30] synthesized CaO/Ca12Al14O33 with the use of sulfonated polystyrene as template. The sorbent of 85% wt CaO with 15%wt Ca12Al14O33 formed hollow spherical structure showed high conversion of 96%. Zhao et al. [31] synthesized CaO-based sorbent by sol-gel method, which consisted of Ca(NO3)2 as calcium precursor, Al(NO3)3 as aluminum precursor, and PEG (MW = 300 g/mol) as additive. The results of CO2 sorption exhibited approximately 82% conversion of CaO. In the present study, we attempted to improve properties of CaObased sorbent for high-temperature CO2 capture, where an interesting application is found in sorption-enhanced hydrogen production process. The effect of the addition of surfactant, anionic surfactant SDS and anionic gemini surfactant (12-3-12), as structure directing agent on properties of CaCO3 and CaO-based alumina sorbents, as well as the ability to adsorb CO2 are subjects of interest. Two synthesis methods, co-precipitation and wet-mixing, are of interest in the present

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work as these methods are likely more relevant to facilitate large-scale industry productions [16]. 2. Experimental 2.1. Materials Calcium acetate monohydrate (Ca(CH3COO)2⋅H2O, 99%) was used as calcium precursor. Urea ((NH2)2CO, 99%) was used as carbonate source. Sodium dodecyl sulfate (SDS, 97%) purchased from Carlo Erba and gemini surfactants synthesized followed Wang et al. [32] were used as additives. All chemicals were used as received. 2.2. Preparation of gemini surfactant Gemini surfactant (Scheme 1) was synthesized followed Wang et al. [32] by firstly connecting hydrophobic part through a spacer followed by adding the hydrophilic part head groups. To synthesize tail group, a 20-mL of 2.5 M of 1,3-dibromopropane in ethanol solution and 50-mL of 3 M dodecylamine in ethanol solution were mixed in a 4-necked round bottom flask. The mixture solution was stirred under reflux at 78 °C for 48 h. The solvent was removed by evaporation at 78 °C. The wet residue was extracted by diethyl ether and filtrated 3 times. The solid was recrystallized 3 times by 1:1 mixture of petroleum ether and acetone. At this step, two tails of hydrophobic (dodecylamine) were connected by the spacer (1,3-dibromopropane). To connect the head groups, 1,3-propanesultone was reacted with the tail groups obtained from the first step (n,n-didodecylpropane-1,3-diamine) in methanol. The 20-mL solution of 2.5 M 1,3-propanesultone and 50-mL of 0.5 M

Fig. 3. TGA results of CaCO3 with a) SDS and b) gemini surfactants at different concentrations.

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n,n-didodecylpropane-1,3-diamine were stirred under reflux at 64 °C for 24 h. The solution was neutralized by 0.05 mol of Na2CO3 for 1 h, then the solvent was removed by heating at 64 °C. The solid was separated by filtration. Finally, the gemini surfactant was recrystallized by 1:3 of methanol and acetone mixture 3 times.

(carbonate precursor) solution was added into the mixture solution under vigorous stirring at 90 °C for 24 h. The obtained precipitate was filtered, washed with distilled water, and dried at 30 °C. The products obtained with the additive of surfactant were denoted as CaCO3-SDS xx mM, and CaCO3-GS xx mM, where xx stands for concentration of SDS or gemini surfactant.

2.3. Preparation of CaCO3 2.4. Preparation of CaO sorbent CaCO3 was prepared by dissolving 100 mL of 2.5 M of Ca(CH3COO)2 with a desired concentration of SDS (10, 20, and 40 mM) or gemini surfactant (0.045, 0.08, 0.12, 2, and 4 mM). Then, 2.5 M of CO(NH2)2

To produce CaO sorbent for CO2 capture, CaCO3 sample was calcined under air at 850 °C for 30 min.

Fig. 3 (continued).

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2.5. Preparation of CaO-based alumina sorbent CaO-based alumina sorbents both with/without the addition of gemini surfactant were incorporated with alumina by co-precipitation and wet-mixing techniques. The ratio of calcium to alumina for each sample was fixed at 70:30 by weight. Details of each preparation technique are summarized as follows: Co-precipitation method: CaO-based alumina sorbent was prepared by mixing 33.3 g of Ca(CH3COO)2 with 33.1 g of Al(NO3)3, then 16.6 g solution of (NH2)2CO was gradually added into the mixture solution under vigorous stirring at 90 °C for 24 h. The obtained precipitates were filtered, washed with distilled water, and dried at 30 °C. The powder was thereafter calcined at 850 °C for 2 h. The same procedure was carried out for the sample with the addition of gemini surfactant, except 0.35 g of gemini surfactant, which corresponds to 2 mM, was added into the mixture of 33 g of Ca(CH3COO)2 and 33.1 g of Al(NO3)3 before (NH2)2CO was added into the mixture. The products were denoted as Co-precipitation and Co-precipitation-GS 2 mM.

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Wet-mixing method: Ca(CH3COO)2 6 g and Al(NO3)3 3.76 g were mixed in DI-water and allowed to stir at 75 °C for 1 h. The solution was dried in an oven at 110 °C for 12 h. The obtained white powder was then calcined at 900 °C for 1.5 h. For CaO-based alumina sample with the addition of gemini surfactant, 0.35 g of gemini surfactant was added into the mixture of Ca(CH3COO)2 and Al(NO3)3, then the solution was stirred, dried, and calcined as the same procedure as the case of without adding surfactant as mentioned above. The products were denoted as Wet mixing and Wet mixing-GS 2 mM.

2.6. Characterization of synthetic sorbent Compositions and crystalline structure of either CaCO3 or CaO were analyzed by X-ray powder diffractometer (XRD, Rigakuminiflex II) with Cu Kα radiation. Measurements were carried out with the scanning step of 2° per minute and the 2θ ranged from 10° to 80°. Crystallite size of each sample was approximated by the Scherrer Equation from

a)

CaCO3

CaCO3 with SDS 10 mM and 20 mM

CaCO3 with SDS 40 mM

b)

CaCO3

CaCO3 with Gemini surfactant at 0.045 mM CaCO3

CaCO3 with Gemini surfactant at 0.08-2 mM

CaCO3

CaCO3 with Gemini surfactant at 4 mM mM Fig. 4. Proposed mechanism of adding surfactant on polymorph of CaCO3 a) SDS and b) gemini surfactant.

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the width at half-height of the highest intensity at 2θ of 37.4° for CaO: d¼

0:89λ 180 °  B cosθ π

ð3Þ

where d is the mean crystallite diameter, λ is the X-ray wave length of CuKα (1.542 Ǻ), and B is the full width half maximum (FWHM) of the most intense ray of each phase. Composition of phase CaCO3 was calculated with the use of major intensity peak of each phase at 29.4° of calcite, 26.7° of vaterite, 45.6° of aragonite. Mixed-phase was calculated using the following equations [33,34]: For mixed-phase of calcite and aragonite: XA ¼

3:157I45:6 A

o

o

I29:4 þ 3:157I45:6 C A

o

;

X C ¼ 1−X A

ð4Þ

For mixed-phase of vaterite and calcite: o

XV ¼

7:691I26:7 V o

o

I 29:4 þ 7:691I26:7 C V

;

X C ¼ 1−X A

ð5Þ

Morphologies and surface texture of the materials were examined by scanning electron microscopy (SEM). To determine surface area, pore volume, pore diameter, and pore size distribution of the samples, N2 adsorption/desorption isotherms were examined using BEL Japan, INC Belsorp mini II. The samples were degassed at 200 °C for 3 h prior to conducting the measurement at -196 °C. From the results of N2 isotherms, the appearances of hysteresis loops of N2 isotherm at P/P0 ~ 1 was used to characterize the possible structure of pore of the samples (IUPAC). Specific surface area was estimated by applying the BET (Brunauer-Emmett-Teller) method at 0.05 b P/P0 b 0.30. Total pore volume was estimated from the amount of N2 adsorbed at relative pressure of 0.95. Pore diameter and pore size distribution of the sample was examined by the BJH (BarrettJoyner-Hallenda) method. 2.7. CO2 sorption performance test CO2 uptake capacity of the synthetic CaO was tested via packed-bed reactor. For each experiment, the sample of 0.8 g was placed in a quartz tube and heated from ambient temperature to 850 °C under N2 flow and hold for 30 min before taking measurement to refresh the material. CO2

sorption (carbonation reaction) was carried out at 600 °C under 15 mL/min gas flow containing 15% v/v CO2 (balanced N2). Cyclic stability of the synthetic material was investigated through CO2 sorption/desorption in fixed-bed reactor. CO2 desorption (calcination reaction) was conducted by heating the sorbent with the addition of steam at 850 °C at flow rate of 15 mL/min until CO2 was completely released from the sorbent. 3. Results and discussion 3.1. Synthesis of CaCO3 using surfactant as structure directing agent 3.1.1. Effect of addition of anionic surfactant on morphology of CaCO3 XRD patterns of CaCO3 synthesized with the use of different concentrations of SDS surfactant are shown in Fig. 1a). Addition of SDS at low concentrations of 10 and 20 mM does not affect phase formation; pure aragonite is obtained, which is similar to that observed with nonadded surfactant. However, when SDS concentration is increased to 40 mM, pure aragonite phase is transformed to a mixed-phase of calcite (24%) and aragonite (76%). In case of CaCO3 synthesized with the addition of gemini surfactant, XRD patterns (Fig. 1b)) show aragonite phase of CaCO3 when gemini surfactant is added up to 2 mM, then the mixedphase of calcite (3%) and vaterite (97%) is found when concentration of gemini surfactant is increased to 4 mM. The results of phase transformation observed with the addition of gemini surfactant indicates that gemini surfactant can enhance surface active of CaCO3. Morphology of CaCO3 observed by SEM images are shown in Fig. 2a). CaCO3 without adding surfactant shows morphology of aggregated particles whereas CaCO3 with adding SDS surfactant presents hexagonal structure with different particle sizes depending upon surfactant concentration used. CaCO3-SDS 10 mM and CaCO3-SDS 20 mM possess hexagonal rod-like structure with average particle length of 3–5 μm. The rod shape of CaCO3 becomes shorter than 1 μm when concentration of SDS is raised to 40 mM together with the aggregation of particles. SEM images of CaCO3 synthesized with the addition of gemini surfactant are shown in Fig. 2b). The results of using gemini surfactant as template at concentrations 0.045 mM, 0.08 mM, and 0.12 mM provide uniform needle-like structure. CaCO3-GS 2 mM shows large needle-like with width 1 μm and length 5 μm, whereas 4 mM concentration of gemini surfactant offers aggregation of small particles. Larger particle size observed with the addition of SDS when compared with gemini surfactant could be due to the difference of micellar structures. Since the critical micelle concentration of SDS and GS are 7.9 and 0.045 mM, respectively,

Table 1 Polymorphs of CaCO3 prepared from different calcium and carbonate sources and different surfactants. Precursors

Additive

Concentration of additive

Polymorph

Morphology

Refs.

Ca(CH3COO)2 + urea

Sodium dodecyl sulfate (SDS)

Ca(CH3COO)2 + urea

– Sodium dodecyl sulfate (SDS) Hexadecyltrimethylammonium bromide (CTAB) – Hexadecyltrimethylammonium bromide (CTAB) Sodium dodecyl sulfate (SDS) Sodium dodecylbenzenesulfonate (SDBS) Sodium dodecyl sulfonate (DDS) Sodium dodecyl sulfate (SDS)

Ca(CH3COO)2 + urea

Gemini (12-3-12)

Aragonite Vaterite Calcite and Vaterite Calcite Calcite Calcite n/a n/a Calcite Vaterite Calcite Aragonite Aragonite Calcite and Aragonite Aragonite Aragonite Aragonite Aragonite Calcite and Vaterite

Rod-like Flower-like Tube, Rhombohedral, Hexagonal Rhombohedral Cubic with rough surface Rhombohedral Rhombohedral Rhombohedral Monodispersed hollow-sphere particles Spherical particles Rhombohedral with smooth surface Hexagonal rod-like Hexagonal rod-like Hexagonal rod-like Needle-like Needle-like Needle-like Needle-like Flake-like

[7]

Ca(OH2) + CO2

0.5 mM 1 mM 2.5 mM – 2 g/L 2 g/L – 1 mM 5 mM 5 mM 5 mM 10 mM 20 mM 40 mM 0.045 mM 0.08 mM 0.12 mM 2 mM 4 mM

CaCl2 + Na2CO3 CaCl2 + Na2CO3

[43]

[44] [45]

This work

This work

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thus, at the same surfactant concentration above CMC, gemini surfactant will self-assembly to rod-like or lamellar structure faster than SDS, which would limit the confine space for crystal growth of CaCO3, while SDS remains in the monomeric surfactant molecules [35]. Our results showed that the addition of anionic surfactants during the synthesis of CaCO3 at high temperature affect phase and polymorph of CaCO3. This observation is similar to that presented by Chen et al. [7] of which the addition of SDS affects polymorphs of CaCO3 at elevated temperature. The role of anionic surfactant as structure directing agent on the formation of CaCO3 could be explained as a result of electrostatic interaction between anionic head groups of surfactant and Ca2+ and between Ca2+ and CO32−. In the aqueous solution of Ca(CH3COO)2 and CO (NH2)2 containing surfactant system, Ca2+ and CO32− formed solid CaCO3 through the liquid-solid-solution (LSS) strategy [36,37]. The presence of anionic head group SO3 electrostatically interact with Ca2+, resulting in higher concentration of Ca2+ around the surfactants when compared to the concentration in bulk solution. Because Ca2+ could, at

215

the same time, attractively interact with CO32−, this would induce the crystallization process to occur at the interface of surfactant aggregates, leading to lowering of surface energy and inducing the transformation to a more stable phase. Previous studies have been reported that the role of surfactant in the solution phase could inhibit crystal growth of their crystallizing species because of the formation of energy barrier by surfactant at solid-liquid interfacial [38–40]. Since the strong interfacial adsorption ability of the surfactant, the growth rate of nanoparticle could be reduced and the absorbed atoms at the active growth site could be inhibited [41]. In addition, it was reported that the diffusion of growth species from the solution phase to the active surface of the growing particles could also be hindered by coverage surfactant molecules [42]. The behavior could be used to explain for the case of adding SDS, the facet of CaCO3 crystals would form in the vicinity of SDS molecules, which would reduce nucleation energy of calcite to form. To prove this hypothesis, decompositions of CaCO3 synthesized with the use of different SDS concentrations were analyzed by TGA. As seen in Fig. 3a), the

Fig. 5. XRD patterns of CaO derived from synthetic CaCO3 using a) SDS and b) gemini surfactants.

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Fig. 6. SEM images of CaO derived from CaCO3 with adding a) SDS and b) gemini surfactants at different concentrations.

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Table 2 Textural properties of CaO sorbents. Sample

Surface area (m2/g)

Pore volume (cm3/g)

Pore size diameter (nm)

Crystal size

CaO CaO-SDS 10 mM CaO-SDS 20 mM CaO-SDS 40 mM CaO-GS 0.045 mM CaO-GS 0.08 mM CaO-GS 0.12 mM CaO-GS 2 mM CaO-GS 4 mM

9.8 7.6 9.4 1.7 12.1 14.9 14.6 16.3 12.9

0.063 0.026 0.036 0.004 0.138 0.084 0.063 0.059 0.097

27.5 14.1 15.0 9.9 45.6 22.6 17.4 14.5 30.2

44.3 45.0 44.0 40.4 37.8 37.9 37.7 37.6 41.1

0.70

Capacity (gCO2/gCaO)

0.60 0.50 0.40 0.30 0.22 0.20

0.25

0.29

0.27 0.22

0.24

0.25 0.15

0.15

0.10 0.00

Fig. 7. Capacity of CaO with the addition of surfactant.

10-mM CaCO3-SDS and the 20-mM CaCO3-SDS show one step decomposition of CaCO3 at approximately 600-800 °C, indicating the release of CO2 as ca. 44% weight loss of CO2 is observed. The 40-mM CaCO3-SDS exhibits weight loss at 220-450 °C and 630-740 °C, indicating the release of SDS surfactant and CO2 from CaCO3, respectively. The presence of two decomposition temperature implies that surfactant can adsorb on surface of CaCO3. In case of adding gemini surfactant, needle-like morphology observed with addition of low concentration of gemini surfactant at 0.045 mM might be due to the adsorption of gemini surfactant on CaCO3 surface, similar to that observed with the case of adding SDS. This hypothesis could be confirmed by TGA result shown in Fig. 3b), where two steps of decomposition, which are the weight loss of gemini surfactant at 343–428 °C and of CO2 from CaCO3 at 571–702 °C, are observed. However, for CaCO3-GS 0.08 mM, 0.12 mM, 2 mM, and 4 mM, only one step of decomposition of 44% weight loss is found at 600–800 °C, which indicates the release of CO2 molecules. This observation might be due to the result of self-interaction among gemini surfactant molecules, leading to lowering of electrostatic interaction between gemini surfactant and CaCO3 surface (Fig. 4). At 4 mM of gemini surfactant, morphology of ellipsoid small particles is observed, which could be due to the control by complex ellipsoid micelle structure. Proposed mechanism of

Fig. 8. XRD patterns of CaO-based alumina sorbent synthesized from different methods.

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Fig. 9. SEM images of CaO-based alumina sorbents synthesized from a) wet mixing, b) wet mixing with gemini surfactant 2 mM, c) co-precipitation, d) co-precipitation with gemini surfactant 2 mM.

the formation of CaCO3 with the presence of SDS and gemini surfactants are shown in Fig. 4. In Table 1 is summarized polymorphs of CaCO3 obtained from different sources of calcium and carbonate and different surfactants added. As seen, a variety of polymorphs and morphologies of CaCO3 could be obtained depend upon calcium and carbonate precursors as well as type and concentration of surfactant.

3.1.2. CO2 sorption tests In this section, synthetic CaCO3 was calcined to form CaO for an application to high-temperature CO2 capture. Fig. 5 shows XRD patterns of CaO obtained from calcination of CaCO3 synthesized with the use of SDS surfactant and gemini surfactant. The pattern exhibits major peaks at 2θ of 32.3, 37.4, 53.8, 64.3, 67.6°, which represent CaO, for all samples, indicating complete decomposition of CaCO3 to CaO could be obtained at temperature of 800 °C. Morphology of CaO derived from CaCO3 synthesized with the use of SDS demonstrated by SEM photographs are shown in Fig. 6. CaO-SDS 10 mM has oval-like morphology with average size of 2–5 μm. Increasing SDS concentration leads particles to aggregate as dense packing with larger particle size are observed with CaO-SDS 20 mM and CaO-SDS 40 mM. In case of adding gemini surfactant, low concentration of gemini addition, CaO-GS 0.045 mM, shows aggregation of rod-like particles and the particles become more agglomerated when gemini concentration is increased. The aggregation observed with higher surfactant concentration could be attributed to heat induce particles to collapse due to greater void space resulted from the addition of surfactant [46].

Crystal size of CaO sorbents calculated by Scherrer equation at the highest peak of CaO of 37.4° are shown in Table 2. CaO synthesized with adding SDS surfactant has similar crystal size to CaO synthesized without adding surfactant, whereas gemini surfactant offers smaller crystal size of CaO when compared to those without using surfactant and adding SDS. Textural properties such as surface area, pore volume, and pore diameter of CaO synthesized with the addition of surfactant at different concentrations are shown in Table 2. The BET surface area of CaO synthesized with the use of SDS surfactant is lower than that without surfactant whereas those synthesized with adding gemini surfactant possess higher BET surface area than others. The observed results could be due to the possession of greater void space, resulted from the addition of gemini surfactant. Fig. 7 shows CO2 sorption capacity adsorbed by different CaO sorbents derived from different CaCO3. CaO-SDS 10 mM and CaO-SDS 20 mM exhibit capacity of 0.25 and 0.27 gCO2/gCaO, respectively, the value of which is closed to CO2 sorption capacity of CaO without surfactant. In contrast, CO2 sorption capacity of CaO-SDS 40 mM has the lowest sorption capacity of 0.18 gCO2/gCaO. This result could be due to the sorbent has low surface area (1.7 m2/g) and dense particles as shown in Table 2 and Fig. 6, respectively. CaO sorbents with adding gemini

Table 3 Textural properties of CaO-based alumina sorbents. Sample

Surface area (m2/g)

Pore volume (cm3/g)

Pore size diameter (nm)

Wet mixing Co-precipitation Wet mixing-GS 2 mM Co-precipitation-GS 2 mM

8.8 2.2 4.3 17

0.040 0.033 0.108 0.0035

18.1 58.6 30.7 8.42

Fig. 10. CO2 sorption capacity of CaO-based alumina sorbents.

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surfactant concentrations of 0.045 mM, 0.08 mM, and 0.12 mM show sorption capacity of 0.22, 0.24, and 0.25 gCO2/gCaO, respectively. Maximum sorption capacity of 0.29 gCO2/gCaO is observed with 2 mM of adding gemini surfactant, whereas lower CO2 sorption capacity of 0.15 gCO2/gCaO is obtained when concentration is further increased to 4 mM. The highest CO2 sorption capacity observed with CaO-GS 2 mM could be due to the highest BET surface area of 16.3 m2/g and small crystal size, which are believed to be the main contribution to high CO2 sorption capacity. Increasing gemini concentration to 4 mM provides lower CO2 sorption capacity due to particles of CaO aggregated as revealed by SEM image shown in Fig. 6b). 3.2. CaO-based alumina for CO2 capture 3.2.1. Synthesis of CaO-based alumina In general, CaO applied for CO2 capture process is usually incorporated with metal oxide in order to enhance CO2 sorption capacity and ability to reuse. In the present work, an impact of adding gemini surfactant on CO2 sorption capacity using CaO incorporated with alumina was investigated as we envisioned that gemini surfactants provide a greater surface area of CaO than SDS. Two preparation techniques, coprecipitation at high temperature and wet-mixing at room temperature, were chosen to investigate the effect for the purpose of simply operation at large-scale production. Fig. 8 shows XRD patterns of sorbents synthesized from different techniques, all sorbents show major peaks of CaO at 2θ of 32.2, 37.4, 53.9, 64.2, and 67.4° and major peaks of Ca12Al14O33 at 2θ of 18.0, 29.8, 33.2, 36.4, 40.9, 46.4, 54.9, and 57.2°. The formation of calcium alumina complex occurs due to the diffusion of CaO into Al2O3 and the complex substance of Ca12Al14O33 is formed from the reaction of CaO with Al2O3 at high temperature (N800 °C) [23]. Morphologies of CaO-based alumina sorbents observed from SEM images are shown in Fig. 9. Aggregation of particles and rough surface are found with the sorbents synthesized by wet-mixing technique, whereas, slit on surface is observed with co-precipitation. Adding gemini surfactant shows denser packing particles with greater void space when compared with non-added surfactant. This result could be due to the adsorption of surfactant on CaCO3, leaving some space after heat treatment. Table 3 presents textural properties including surface area, pore volume, and pore size diameter of CaO-based alumina synthesized from wet-mixing and co-precipitation techniques. For CaO-based sorbents synthesized by wet-mixing technique, adding gemini surfactant provides lower surface area with higher pore volume and pore size diameter than non-added surfactant. On the other hand, adding gemini surfactant results in an increase of high surface area than that without

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surfactant. This observation could be attributed to 1) the interaction between gemini surfactant and CaO surface, resulted in structural arrangement of CaO-based alumina molecules and 2) self-aggregation between gemini molecules, which would leave void space after heat treatment. 3.2.2. CO2 sorption tests Fig. 10 shows CO2 sorption capacity of sorbents synthesized from wet-mixing and co-precipitation. Adding gemini surfactant provides a negative impact on CO2 sorption performance when CaO/Ca12Al14O33 is synthesized by wet-mixing technique as a slight reduction of sorption capacity is obtained. This result indicates non-effect of adding anionic surfactant at room temperature as structure directing agent [45]. In contrast, a positive impact is observed with CaO-based alumina sorbent synthesized from co-precipitation. CO2 sorption capacity increases from 0.24 to 0.31 gCO2/gsorbent when gemini surfactant is added into the system. This result could be because gemini surfactant adsorbed on the surface of Ca2+ and Al3+, inducing the formation of slit on the surface, which could be a reason of enhancing CO2 sorption capacity due to an increase of accessible active surface area. A pronounced effect observed with adding gemini surfactant when the material is synthesized at elevated temperature is in consistent with that reported by Wei et al. [45], where the effect of adding anionic surfactant is pronounced when CaCO3 is synthesized at elevated temperature. Amount of CO2 sorption capacity obtained by our synthetic CaO/Ca12Al14O33 synthesized by co-precipitation method is compared with other techniques as shown in Table 4. As seen, our results showed that gemini surfactant offers an advantage on enhancing CO2 sorption capacity; comparative amount of CO2 sorption can be obtained when compared with other synthesis methods, i.e. sol mixing [22], wet mixing [23], sol-gel [29], with lower amount of CaO content. For the application to calcium looping technology, cyclic stability of the synthetic CaO-based alumina sorbent has been tested. From the above results, Co-precipitation-GS 2 mM provides the highest CO 2 sorption capacity among sorbents investigated in the present work; it is therefore chosen to test for CO2 sorption/desorption upon multiple cycles run. Fig. 11 shows the results of CO2 sorption capacity gradually decreases from 0.31 to 0.10 gCO2/gsorbent from cycle 1 to cycle 5. This could be the result of sintering effect; particles of the sorbent possess large void space could aggregate to form dense packing particles during heat treatment. This hypothesis is confirmed by SEM images as shown in the inset of Fig. 11; particles of the sorbent after 5 cycles run become denser when compared with the fresh sorbent. Our results suggest that although applying surfactant for the purpose of enhancing high surface active

Table 4 Comparison of CO2 sorption capacity of CaO-based sorbents obtained by different synthesis methods. Method (Ref.)

Precursors

Supported precursors

Inert supports

%CaO

Carbonation

Reactor

gCO2/gsorbent

Sol mixing [22]

Al(NO3)3

Ca12Al14O33

75

650 °C, 15%CO2 (N2), 30 min

TGA

Al(NO3)3

80

650 °C, 15%CO2 (N2), 30 min

TGA

AlCl3 Al(CH3COO)3 Al(NO3)3

Ca9Al6O18 Ca9Al6O18 Absence Ca9Al6O18 Absence Ca9Al6O18 Ca12Al14O33 Absence Ca12Al14O33

75

650 °C, 15%CO2, 30 min

TGA

Co-precipitation [25] Sol-gel [29]

Ca(OH)2 Ca(CH3COO)2 Ca(HCOO)2 Ca(CH3COO)2 C6H10CaO4 Ca3(C6H5O7)2 C6H10CaO6 C12H22CaO14 C6H10CaO6 C6H10CaO6 Ca(CH3COO)2 C12H22CaO14 Ca(HCOO)2 C6H10CaO6 Ca(OH)2 and CO2 Ca(OH)2

Al(NO3)3 C9H21O3Al

Ca12Al14O33 Ca12Al14O33

650 °C, 15%CO2 (He), 10 min 650 °C, 20%CO2, 10 min

TGA TGA

Co-precipitation [This work]

Ca(CH3COO)2

Al(NO3)3

Ca12Al14O33

85 90 80 70

600 °C, 15%CO2 (N2), 15 mL/min

Fixed-bed

0.12 0.21 0.41 0.62 0.24 0.58 0.49 0.53 0.36 0.31 0.47 0.43 0.42 0.48 0.33 0.35 0.26 0.31

Wet mixing [23]

Sol mixing [24]

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Fig. 11. CO2 sorption capacity of CaO/Ca12Al14O33 synthesized by co-precipitation technique with the addition of gemini surfactant 2 mM operated for 5 consecutive cycles.

area could offer high CO2 sorption capacity; however, void space results from the addition of surfactant could in turn lead the aggregation to occur. 4. Conclusions Anionic surfactants, SDS and gemini (12–3-12) surfactants, have shown to have influence on phase formation and morphology of CaCO3 when they were synthesized by co-precipitation technique at elevated temperature. Aragonite phase was transformed into mixedphase of aragonite and calcite when concentration of SDS was increased to 40 mM, whereas the transformation of aragonite to mixed-phase of vaterite and calcite was found when concentration of gemini surfactant was increased to 4 mM. Morphology of synthetic CaCO3, hexagonal rodlike structure was obtained with the addition of SDS, whereas needlelike structure was observed with the addition of gemini surfactant. CaO derived from CaCO3 synthesized with the addition of gemini surfactant offered little higher CO2 sorption capacity that those synthesized with the addition of SDS. However, by incorporating CaO with alumina to form CaO/Ca12Al14O33 via co-precipitation technique, addition of gemini surfactant offered positive impact on increasing CO2 sorption capacity but negative effect on thermal stability upon multiple sorption/ desorption cycles was observed. Acknowledgements The authors would like to acknowledge the Ratchadapisek Sompoch Endowment Fund, Chulalongkorn University (CU-59-003-IC) and King Mongkut's University of Technology North Bangkok (KMUTNBKNOW-61-029) for research fund. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.powtec.2018.12.011. References [1] Ç.M. Oral, B. Ercan, Influence of pH on morphology, size and polymorph of room temperature synthesized calcium carbonate particles, Powder Technol. 339 (2018) 781–788. [2] Y. Tang, J. Xu, J. Zhang, Y. Lu, Biodiesel production from vegetable oil by using modified CaO as solid basic catalysts, J. Clean. Prod. 42 (2013) 198–203. [3] Y. Katsuyama, A. Yamasaki, A. Iizuka, M. Fujii, K. Kumagai, Y. Yanagisawa, Development of a process for producing high-purity calcium carbonate (CaCO3) from waste cement using pressurized CO2, Environ. Prog. Sustain. 24 (2005) 162–170.

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