Sol-gel hydrothermal synthesis of microstructured CaO-based adsorbents for CO2 capture

Article abstract of DOI:10.1039/c4ra14355h

In this study, microstructured CaO-based adsorbents were synthesized by a sol-gel hydrothermal method using calcium nitrate tetrahydrate, citric acid and sodium hydroxide as precursors. Experiments with different NaOH concentrations (2, 6 and 10 M) were carried out to investigate the effects on the morphologies and CO₂ adsorption activities of the synthesized adsorbents. X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM) results showed that different NaOH concentrations resulted in different crystal phases and morphologies. A novel three-dimensional (3D) hierarchical calcite (CaCO₃) hollow microspherical adsorbent composed of one-dimensional (1D) spike-shaped nanorods was obtained with 2 M NaOH. XRD analyses confirmed that the hierarchical CaCO₃ hollow microspheres were characteristic of the calcite phase. The FESEM image revealed that the microspheres were composed of 1D spike-shaped nanorods with an average length of 500 nm. The cross-sectional FESEM image showed that the microspheres had hollow structures with an average inner cavity of 2 μm and a shell thickness of approximately 0.5 μm. The CO₂ adsorption performance of the synthesized adsorbents was investigated using thermogravimetry-differential thermal analysis (TG-DTA) apparatus. The results indicated that the novel hierarchical calcite (CaCO₃) hollow microspherical adsorbent composed of one-dimensional (1D) spike-shaped nanorods possessed higher carbonation conversion of 45% after 15 cycles, which was about 22% higher than that of other adsorbents synthesized with 6 and 10 M NaOH concentration and limestone. This property could be attributed to the 3D hierarchical hollow microsphere structure, 1D spike-shaped nanorod structure, trimodal pore size distribution and large BET surface area (44.85 m² g^-1) of the novel adsorbent. This journal is

Full text of DOI:10.1039/c4ra14355h

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Sol–gel hydrothermal synthesis of microstructured
CaO-based adsorbents for CO2 capture†
Cite this: RSC Adv., 2015, 5, 6051
Nwe Ni Hlaing,abc Srimala Sreekantan, Radzali Othman,a Swee-Yong Pung,a
a
*
b
Hirofumi Hinode, Winarto Kurniawan,b Aye Aye Thant,c Abdul Rahman Mohamedd
*
and Chris Salimee
In this study, microstructured CaO-based adsorbents were synthesized by a sol–gel hydrothermal method
using calcium nitrate tetrahydrate, citric acid and sodium hydroxide as precursors. Experiments with
different NaOH concentrations (2, 6 and 10 M) were carried out to investigate the effects on the
morphologies and CO2 adsorption activities of the synthesized adsorbents. X-ray diffraction (XRD) and
field-emission scanning electron microscopy (FESEM) results showed that different NaOH
concentrations resulted in different crystal phases and morphologies. A novel three-dimensional (3D)
hierarchical calcite (CaCO3) hollow microspherical adsorbent composed of one-dimensional (1D) spike-
shaped nanorods was obtained with 2 M NaOH. XRD analyses confirmed that the hierarchical CaCO3
hollow microspheres were characteristic of the calcite phase. The FESEM image revealed that the
microspheres were composed of 1D spike-shaped nanorods with an average length of 500 nm. The
cross-sectional FESEM image showed that the microspheres had hollow structures with an average inner
cavity of 2 mm and a shell thickness of approximately 0.5 mm. The CO2 adsorption performance of the
synthesized adsorbents was investigated using thermogravimetry-differential thermal analysis (TG-DTA)
apparatus. The results indicated that the novel hierarchical calcite (CaCO3) hollow microspherical
adsorbent composed of one-dimensional (1D) spike-shaped nanorods possessed higher carbonation
conversion of 45% after 15 cycles, which was about 22% higher than that of other adsorbents
synthesized with 6 and 10 M NaOH concentration and limestone. This property could be attributed to
the 3D hierarchical hollow microsphere structure, 1D spike-shaped nanorod structure, trimodal pore size
distribution and large BET surface area (44.85 m2 gꢀ1) of the novel adsorbent.
Received 12th November 2014
Accepted 15th December 2014
DOI: 10.1039/c4ra14355h
www.rsc.org/advances
1. Introduction
In recent years, three-dimensional (3D) hierarchical hollow
microspheres composed of one-dimensional (1D) nano-
aSchool of Materials and Mineral Resources Engineering, Engineering Campus,
Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia. E-mail:
srimala@usm.my; nwenihlaing76@gmail.com; Fax: +60-604 5995215; Tel: +60-604
5941011
bDepartment of International Development Engineering, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro-ku, Tokyo, Japan. E-mail: hinode@ide.titech.ac.jp; Fax:
+81-3-5734-3245; Tel: +81-3-5734-3245
cDepartment of Physics, University of Yangon, 11041 Kamayut, Yangon, Myanmar
dLow Carbon Economy (LCE) Research Group, School of Chemical Engineering,
Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang,
Malaysia
structures have extensively attracted attention because of their
well-dened morphology, low density, large surface area and
high performance. These properties enable a wide range of
potential applications, such as in adsorption and separation,
energy conversion, electronics, optoelectronics, catalysis,
sensors and drugs delivery.1–3 Therefore, various types of
materials such as In2O3,4 SnO2,5 hydroxyapatite,6 V2O5,7
13
b-Ni(OH)2,8 ZnO,9 TiOSO4,10 MnO2,11 CuO,12 a-Fe2O3
CaCO3
and
14–19
hierarchical hollow microspheres have been
eEnvironmental Engineering, Surya University, Tangerang, 15810 Banten, Indonesia
† Electronic supplementary information (ESI) available: XRD pattern of
limestone, FTIR spectrum, EDX spectrum and TG-DTA curves of 3D hierarchical
CaCO3 hollow microspherical adsorbent, FESEM image of the commercial
limestone, FTIR spectra, EDX spectra, TG and DTA curves of Ca(OH)2 and
CaC2O4$H2O adsorbents, Multi cycles carbonation conversion curves of 3D
hierarchical calcite CaCO3 hollow microspherical adsorbent, FESEM images of
the adsorbents aer multiple cycles, BET surface areas and crystallite sizes of
synthesized adsorbents under different NaOH concentrations and TG data of
Ca(OH)2 and CaC2O4$H2O adsorbents. See DOI: 10.1039/c4ra14355h
prepared by different synthesis methods such as hydrothermal,
precipitation, solvothermal, mixing and gas-diffusion methods.
Their applications in various elds are summarized in Table 1.
Among these synthesis methods, hard and so templates
approaches are favourable for the formation of hollow
spheres.1–19 However, hollow spheres prepared using the hard
template route usually possess disadvantages related to high
cost and tedious synthetic procedures such as rinsing and
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Table 1 Summary of different types of hierarchical hollow microspheres and their applications
Samples
Templates/additives/solvents
Method
Application
References
4
In2O3
SnO2
Ethylenediamine
Sulfonated polystyrene hollow spheres
Hydrothermal Gas sensors
Hydrothermal Anode materials for lithium-ion
batteries
5
Hydroxyapatite Polyaspartic acid
Hydrothermal Water treatment
6
7
V2O5
Polyvinylpyrrolidone and ethylene glycol
Mixing
Cathode material in lithium-ion
batteries
b-Ni(OH)2
ZnO
TiOSO4
MnO2
CuO
Urea
Hydrothermal
Solvothermal
Solvothermal
Mixing
—
—
8
9
10
11
Pluronic P123 and hexamethylenetetramine
Glycerol and ethanol
Ethanol
—
Photocatalytic
Waste-water treatment
Hydrothermal Anode materials for lithium ion 12
batteries
a-Fe2O3
Glycerol-quasiemulsion microdroplets
Hydrothermal Anode materials for lithium-ion 13
batteries
CaCO3
CaCO3
CaCO3
CaCO3
CaCO3
Polyvinylpyrrolidone and sodium dodecyl sulfonate
Carboxyl-terminated hyperbranched polyglycerol
Low methoxy pectin
Precipitation
Gas diffusion
Precipitation
Mixing
—
—
—
14
15
16
17
18
Soluble starch
Anticancer drug carrier
—
Poly(ethylene oxide)-block-poly(methacrylic acid) and sodium
dodecylsulfate
—
CaCO3
Polyoxyet hylene sorbitan monooleate
Interfacial
reaction
Filler and coating pigment
19
calcination or chemical etching to remove the templates; which biodegradability, and pH-sensitive properties of CaCO3 hollow
not only destroys the core–shell structures, but is also time and microspheres. Moreover, Enomae and Tsujino19 fabricated
energy-consuming. In addition, the size and morphology of the CaCO3 hollow spheres with polyoxyethylene sorbitan mono-
nal hollow spheres mainly depend on the templates (e.g., SiO2 oleate using the interfacial reaction method. They proposed
carbon spheres, polymers).20,21 Using the so template (e.g., CaCO3 hollow spheres to be used as a ller and paper coating
micelles, ionic liquid) route is relatively easier, however, large pigment because hollow structure scatters more light, resulting
quantities of surfactants or functionalized organic acids are to high brightness and opacity. According to literatures, the
required. Moreover, the shape, shell thickness, and diameter hierarchical CaCO3 hollow microspheres were mostly prepared
distribution of the resulting hollow particles are difficult to using templates or additives, which lead to complicated
control because of the deformability of the so templates.1,21,22 synthesis procedures. These procedures are time and energy
In other words, both methods possess some disadvantages. consuming, require toxic raw materials, produce pollutants
Therefore, it is highly desirable to develop a new approach from the removal of the templates and are costly. Therefore, an
without templates for the preparation of 3D hierarchical hollow economical and environmental-friendly template-free synthesis
microspheres assembled by 1D nanostructure materials.
method must be developed to fabricate CaCO3 hollow spheres
Calcium carbonate is one of the most abundant natural from the scientic research and practical application point of
minerals found in different polymorphs: three anhydrous view.25,26 To the best of our knowledge, no studies have been
crystalline polymorphs (calcite, aragonite, and vaterite) and reported on the synthesis of 3D hierarchical calcite CaCO3
three metastable forms (amorphous calcium carbonate (ACC), hollow microspherical adsorbent composed of one-
crystalline hexahydrate and crystalline monohydrate).23,24 Hier- dimensional (1D) spike-shaped nanorods for CO2 capture
archical CaCO3 hollow microspheres are attracting signicant using the sol–gel hydrothermal method. Furthermore, most of
interest because of their excellent properties such as low these literatures focused on the hierarchical CaCO3 hollow
density, high surface areas, and their potential application in microspheres as smart carriers for anticancer drugs, ller, and
industry (e.g., ller and coating pigment)19 and health care (e.g., coating pigment, but not so much on CO2 capture.
anticancer drug carrier).17 Zhao and Wang14 synthesized hollow
To capture CO2 from power plants or industrial processes,
CaCO3 microspheres by the precipitation method employing several kinds of adsorbents and absorbents (such as amines,
polyvinylpyrrolidone with sodium dodecyl sulfonate as zeolite, activated carbon, metal–organic framework and
template for the controlled growth of hollow spheres. Butler calcium oxide) have been investigated.27 Currently, absorption
et al.16 reported that low methoxy pectin played a key role in the mainly uses aqueous amine compounds (e.g., monoethanol-
formation of hollow shells of CaCO3. In 2008, Wei et al.17 amine (MEA) and diethanolamine (DEA)) is a commercialized
reported the formation of hierarchical CaCO3 hollow micro- technology for CO2 separation due to their selectivity for acidic
spheres using a soluble starch for anticancer drugs carriers. gases and fast reactivity.28 However, this technique has some of
This application could be attributed to the biocompatibility, major drawbacks such as high solvent regeneration cost, low
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CO2 loading capacity and high equipment corrosion rate.29 In
recent years, calcium oxide based adsorbents are one of the
most promising adsorbents to capture CO2 due to their high
adsorption capacity (0.786 g-CO2/g-adsorbent), wide availability
in natural minerals (e.g. limestone and dolomite) and reversible
carbonation–calcination reaction.27 However, the main disad-
vantage of using CaO adsorbents is the rapid decay in adsorbent
performance during multiple cycles because of sintering during
every calcination step.30 In general, the carbonation conversion
of the CaO adsorbent derived from limestone is about 80% for
the initial cycle and sharply drops to about 25% aer 10
cycles.31,32 Therefore, thermal precalcination,33 modication34
and steam hydration35 of limestones and synthetic CaO-based
adsorbents36–39 have been proposed to improve the CO2
adsorption performance.
Herein, we rstly report a 3D hierarchical calcite CaCO3
hollow microspherical adsorbent composed of one-
dimensional (1D) spike-shaped nanorods for CO2 capture
synthesized by a sol–gel hydrothermal method. The effects of
NaOH concentration on the structural properties as well as CO2
adsorption performance of synthesized adsorbents were
studied in detail. The experimental results indicated that the 3D
hierarchical calcite CaCO3 hollow microspherical adsorbent
composed of one-dimensional (1D) spike-shaped nanorods
could be obtained at 2 M NaOH concentration using hydro-
thermal reaction. This novel structure exhibited higher
carbonation conversion of 45% aer 15 cycles, which was about
22% higher than that of other adsorbents synthesized with 6
and 10 M NaOH concentration and limestone.
2.2 Characterization
The crystal structures of the as-synthesized adsorbents were
characterized by X-ray diffraction (XRD) using a Rigaku X-ray
diffractometer with Cu Ka1 radiation (l ¼ 0.154056 nm); the
working current and voltage were 20 mA and 40 kV, respectively.
The surface morphology of the adsorbents was observed under
a S-4700 (Hitachi) eld-emission scanning electron microscope
(FESEM). The resulting CaCO3 hollow spheres were crosscut
using an argon ion beam (cross section polisher (CP) SM-09010,
JEOL) and the cross sectional FESEM images were obtained
using a S-4700 (Hitachi, FESEM). The samples were coated with
a thin layer of platinum to avoid sample charge up before
FESEM analysis. The presence of the functional groups (CO3
2ꢀ
,
OHꢀ) was determined using a JASCO FT/IR-6100 FV Fourier-
transform infrared spectroscopy (FTIR) over the region of 400–
4000 cmꢀ1 with the KBr pellet technique having spectral reso-
lution of 4 cmꢀ1. A thermogravimetry-differential thermal
analysis (TG-DTA) was carried out using a Rigaku TG-DTA. The
adsorbents were placed in a platinum crucible and heated at the
rate of 10 ꢁC minꢀ1 up to 900 ꢁC, under N2 purge at a ow rate of
40 ml minꢀ1. Al2O3 was used as reference material. The pore
structure parameters of the adsorbents were determined from
nitrogen adsorption–desorption isotherms using the Quan-
tachrome (Autosorb-1-MP) instrument and the Brunauer–
Emmett–Teller (BET) method was used for surface area
calculation.
2.3 CO2 carbonation–calcination performances
To measure the CO2 adsorption, a Rigaku thermogravimetry-
differential thermal analysis (TG-DTA) apparatus with Ther-
moplus
2
soware was used. Carbonation–calcination
processes were performed at 800 ꢁC. A small amount of the
adsorbent (ꢂ6.5 mg) was placed in a platinum crucible and was
heated ꢀfr1om room temperature to 800 ꢁC at a heating rate of 10
ꢁC min under 100% N2 gas ow and atmospheric pressure,
and then the temperature was held for 6 min to achieve
complete decomposition. Subsequently, carbonation perfor-
mance was carried out for 30 min under 100% CO2 gas ow.
Aer the 30 min carbonation process, 100% N2 gas was owed
for 6 min to calcine the carbonated adsorbent. To investigate
the CO2 adsorption capacity and cyclic stability of the adsor-
bents, the carbonation–calcination performance was repeated
for multiple cycles. The constant ow rate for CO2 and N2 gas
was 40 ml minꢀ1. The CO2 adsorption capacity and CO2
conversion were calculated using the following equations;
2. Materials and methods
2.1 Sol–gel hydrothermal synthesis
All reagents used were of analytical grade and were used without
further purication. First, dry gel was prepared by the sol–gel
method. An equal molar ratio (1 : 1) of calcium nitrate tetra-
hydrate (Ca(NO3)2$4H2O, R&M chemical) and citric acid mon-
ohydrate (C6H8O7$H2O, Merck) was dissolved in 100 ml distilled
water. The solution was evaporated by heating at 80 ꢁC on a hot
plate with vigorous stirring for 5 h. As water evaporated, the
solution became viscous and nally formed a very viscous pale
yellow gel. The obtained viscous gel was then dried overnight in
an oven at 140 C.40,41 Aer which, 2 g of dry gel was dissolved
ꢁ
into a 2 M NaOH solution and the mixture was stirred for 30
min. Then the stirred mixture was placed in a Teon-lined
stainless-steel autoclave. Hydrothermal reaction was con-
WN ꢀ WI
CO2 adsorption capacity; C ¼
(1)
ꢁ
WI
ducted at 170 C for 16 h. Aer the hydrothermal reaction, the
precipitates were centrifuged, washed several times with
distilled water and ethanol, and then dried in an oven at 70 ꢁC
for 24 h to obtain the nal adsorbent. To investigate the effects
WN ꢀ WI MCaO
CO2 Conversion; XN ð%Þ ¼
ꢃ
ꢃ 100 (2)
MCO
2
WI
of NaOH concentration on the structural properties and CO2 where C is the CO2 adsorption capacity, XN is the carbonation
adsorption capacities, experiments with 6 and 10 M NaOH conversion, WN is the weight (%) of the carbonated adsorbents
concentration were carried out under similar conditions. For aer N cycle(s), WI is the initial weight (%) of the calcined
further comparison, commercial limestone (CaCO3) from adsorbents, MCaO and MCO are molar masses of CaO and CO2,
2
Malaysia was used in this study.
respectively.
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3. Results and discussion
3.1 Structure and morphology
Fig. 1(a) shows the X-ray diffraction (XRD) pattern of hierar-
chical CaCO3 hollow microspherical adsorbent with a rhom-
bohedral phase calcite (JCPDS-05-0586) with R3ꢀc(167) space
group.42 No diffraction peaks related to other impurities was
detected. The peak at 2q ¼ 29.40ꢁ, assigned to the (104) plane
showed the strongest diffraction which could be assumed that
CaCO3 grows mainly along the (104) plane, which was in
agreement with the XRD pattern of limestone, common CaCO3
(Fig. S1 (ESI†)). Obviously, highly crystallized CaCO3 crystals
could be observed under mild hydrothermal condition without
calcination at high temperature. The crystallite size calculated
using Scherrer equation (D ¼ 0.9l/b cos q) was 76.75 nm.
Fig. S2 (ESI†) shows the FTIR spectrum of the hierarchical
calcite CaCO3 hollow microspheres. The characteristic bands of
calcite CaCO3 at 711, 874 and 1423 cmꢀ1 could be observed in
Fig. S2 (ESI†).43 The broad band observed at 3456 cmꢀ1 corre-
sponded to the stretching vibration mode of O–H bond and due
to the absorbed water on the surface of the 1D spike-shaped
nanorods. The strong and wide band at 1423 cmꢀ1 attributed
to the asymmetric stretching vibration of the C–O bond while
the sharp bands at 874 and 711 cmꢀ1 corresponded to the in-
Fig. 2 FESEM images of microstructured CaO-based adsorbents: (a)
microsphere (b) cross sectional and (c) higher magnification images of
3D hierarchical CaCO3 hollow microspherical adsorbent composed of
1D spike-shaped nanorods, (d) hexagonal and rectangular-shaped
Ca(OH)2 and (e) plate-shaped CaC2O4$H2O adsorbents.
atomic percentage (at%) of the elements in the CaCO3 hollow
spheres was determined using EDX analysis. The EDX spectrum
(Fig. S3 (ESI†)) proved that the composition of the hierarchical
hollow microsphere adsorbent was pure CaCO3. The pure
CaCO3 hollow spheres were composed of 19.96 at% Ca, 27.71
at% C, and 52.33 at% C. The Pt peaks in the EDX spectrum was
due to the plated platinum. The FESEM image of the commer-
cial limestone (CaCO3) was shown in Fig. S4 (ESI†).
Fig. 3 shows the trimodal pore size distribution of hierar-
chical calcite CaCO3 hollow microspherical adsorbent. The rst
pore size peak was 1.70 nm, which corresponded to the micro-
pores, while pore sizes of 3.07 and 5.65 nm corresponded to
meso-pores. The N2-adsorption–desorption isotherm of the
hierarchical CaCO3 hollow spheres (inset gure) was classied
as type II isotherm (no plateau at high P/Po) according to the
International Union of Pure and Applied Chemistry (IUPAC)
classication.44–46 The BET surface area of the CaCO3 hollow
2ꢀ
plane and out-of-plane bending modes of CO3 , respectively.42
Fig. 2(a) shows the FESEM image of hierarchical calcite
CaCO3 hollow microsphere with an average diameter of 3.3 mm
composed of an array of spike-shaped CaCO3 nanorods. The
nanorods have grown radially on the surface of the sphere.
Fig. 2(b) shows the cross sectional FESEM image of the CaCO3
hollow sphere with an average inner cavity of 2 mm. The inner
part of hollow structures was found to comprise particles with
several nanometers. The shell thickness of the CaCO3 hollow
sphere was about 0.5 mm. Additional detail high magnication
structural features are shown in Fig. 2(c). The spike-shaped
CaCO3 nanorods had a root size of 110–150 nm and a tip size
of 50–70 nm with an average length of 500 nm. The average
Fig. 3 Pore size distribution and N2-adsorption–desorption isotherm
(inset) of 3D hierarchical CaCO3 hollow microspherical adsorbent
composed of 1D spike-shaped nanorods.
Fig. 1 XRD patterns of microstructured CaO-based adsorbents.
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microspheres was 44.85 m2 gꢀ1, which was higher than that of and the width range from 0.6 to 1 mm were obtained at 6 M
the commercial limestone (1.38 m2 gꢀ1) and others CaCO3 NaOH. Fig. 2(e) shows the morphology of the adsorbent
adsorbents reported in literature such as CaCO3 nanopod (10.40 obtained at 10 M NaOH. The microplates adsorbent with the
m2 gꢀ1),37 commercial microsized CaCO3 (12.40 m2 gꢀ1) and length of 2–2.5 mm and the width range from 0.7–1.2 mm was
nanosized CaCO3 (17.00 m2 gꢀ1).47 High surface area has been formed under this condition. Fig. S7 (ESI†) shows the EDX
reported to enhance the CO2 adsorption capacity and cyclic spectra of the Ca(OH)2 and CaC2O4$H2O adsorbents. EDX
stability of the adsorbent.48 The larger BET surface area (44.85 results identied that the composition of the adsorbents was
m2 gꢀ1) together with the micro-pore and meso-pore size Ca, C and O. For Ca(OH)2 adsorbent Fig. S7(a) (ESI†), the C peak
distributions suggested that as-synthesized calcite CaCO3 at 0.27 keV could be attributed to the content of CaCO3 and
hollow spheres would exhibit good CO2 adsorption capacity.
Thermal analysis was carried out to investigate the thermal
CaC2O4$H2O.
The TG-DTA curves of the Ca(OH)2 and CaC2O4$H2O adsor-
decomposition temperature of the CaCO3 adsorbent. The TG bents synthesized with 6 and 10 M NaOH are shown in Fig. S8
and DTA curves of the CaCO3 hollow microspheres are shown in (ESI†). Weight losses in the TG curves occurred in three stages
Fig. S5 (ESI†). The wide endothermic peak at 779 ꢁC, which (Fig. S8(a) and (b) (ESI†)). However, different adsorbents
coincided with the TG weight loss in the temperature range of (Ca(OH)2 and CaC2O4$H2O) resulted to different weight losses.
ꢁ
715–795 C, represented the thermal decomposition of CaCO3 This might be due to the different amounts of H2O, CO, and CO2
into CaO. Based on this data, the calcination temperature of the in the adsorbents. The rst stage of weight loss (100–160 ꢁC) in
hierarchical calcite CaCO3 hollow spheres for multiple cycles Fig. S8(a) (ESI†) represented the evolution of water molecule
were xed at 800 ꢁC. Results will be discussed in the later from the small amount of CaC2O4$H2O phase present in the
section.
adsorbent. The second weight loss in the temperature range
(370–440 ꢁC) represented the dehydroxylation of Ca(OH)2.52,53
The nal weight loss (560–700 ꢁC) was related to the thermal
decomposition of the content of CaCO3 in the Ca(OH)2 adsor-
3.2 Effect of NaOH concentration
The NaOH concentration was increased from 2 M to 6 and 10 M bent. In Fig. S8(b) (ESI†), the rst stage of signicant weight loss
to investigate the effects of NaOH concentration on the struc- (100–190 ꢁC) corresponded to the removal of water molecule
tural properties and CO2 adsorption performances. As dis- from CaC2O4$H2O (CaC2O4$H2O to CaC2O4). The second stage
ꢁ
cussed previously, using 2 M NaOH resulted in CaCO3. With (390–510 C) was related to the evolution of carbon monoxide
increased concentration to 6 M NaOH, Ca(OH)2 (calcium (CaC2O4 to CaCO3) and the nal weight loss (770–830 ꢁC) could
hydroxide, JCPDS-78-0315) with
a hexagonal phase was be attributed to the liberation of CO2 (CaCO3 to CaO), respec-
observed (Fig. 1(b)). In addition, the presence of minor peaks, tively.54 The TG weight losses (%) of synthesized adsorbents aer
which belong to the CaCO3 and CaC2O4$H2O were found. When listed in Table S2 (ESI†). The experimental weight losses were
the concentration of NaOH was further increased to 10 M, a lower than the theoretical value for the adsorbents synthesized
monoclinic phase CaC2O4$H2O (whewellite, JCPDS-20-0231) at 6 and 10 M, because of the presence of different types of
was obtained (Fig. 1(c)). Some minor peaks at 2q values of impurity phases in the adsorbents.
18.23ꢁ, 28.84ꢁ, and 34.23ꢁ displayed the coexistence of Ca(OH)2
In Fig. S8(c) (ESI†), the sharp endothermic peak at 423 ꢁC
with CaC2O4$H2O. Crystallite sizes and BET surface areas of the represented the decomposition of Ca(OH)2. Water vapour and
ꢁ
synthesized adsorbents are summarized in Table S1 (ESI†).
FTIR spectra were recorded to exhibit the chemical groups of
each adsorbent with different NaOH concentrations. The char-
acteristic band of Ca(OH)2 could be seen at 3643 cmꢀ1 which
represented the O–H stretching mode (Fig. S6(a) (ESI†)).49
Moreover, the bands at 781, 1318, and 1623 cmꢀ1 as well as 874,
1428 cmꢀ1, proved that CaC2O4$H2O and CaCO3 coexist,
respectively. In Fig. S6 (ESI†), the sharp band at 1318 cmꢀ1 and
the strong band at 1618 cmꢀ1 attributed to symmetric metal-
carboxylate and antisymmetric carbonyl stretching C]O
modes, which corresponded to the characteristic bands of
CaC2O4$H2O.50,51 In Fig. S6(b) (ESI†), the weak band at 3642
cmꢀ1 signied the existence of Ca(OH)2. In summary, the FTIR
spectra are in agreement with the XRD patterns.
carbon dioxide evolved at 140 and 680 C, which was from the
The morphology and size of the adsorbents were examined
using FESEM. Fig. 2(d) and (e) show the representative FESEM
images of the synthesized adsorbents with 6 and 10 M NaOH.
Notably, different NaOH concentrations led to the formation of
different morphologies. As shown in Fig. 2(d), hexagonal-
shaped microstructures with an average size of 1.6 mm and
Fig. 4 A schematic diagram of the morphology and phase evolution of
rectangular-shaped microstructures with the length of 1–1.3 mm the synthesized adsorbents as a function of NaOH concentration.
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content of CaC2O4$H2O and CaCO3, respectively. In Fig. S8(d) observed that the maximum weight gain of the CaCO3 adsor-
(ESI†), a sharp exothermic peak at 494 ꢁC indicated that the bent increased with increasing the carbonation temperature. At
ꢁ
oxidation of carbon and carbon monoxide were released from 800 C, the carbonation reaction proceeded quickly.
the CaC2O4. The two endothermic peaks at 176 and 817 ꢁC
represented the evolution of water molecule from CaC2O4$H2O carried out at the same temperature (800 C) in order to avoid
In this study, the carbonation and calcination reactions were
ꢁ
and the thermal decomposition of CaCO3, respectively.
the repetitive heating and cooling of the adsorbents between
The morphology and phase evolution of the synthesized cyclic operations which in turn resulted to reduce the effect of
adsorbents as a function of NaOH concentration based on the the thermal stress on the adsorbents.48,56 Sun et al.57 and Lysikov
aforementioned results are presented in Fig. 4. Firstly, in the et al.38 reported that the carbonation processes carried out at
sol–gel process, the citrate anion (C6H5O7)3ꢀ is presumed to long carbonation time (up to 30 min) exhibited substantially
combine with the calcium ion Ca2+ to form calcium–citric acid higher residual conversions than the processes performed at
chelate complexes during the gelation of sol. Aerwards, the dry short carbonation time (less than 10 min). In contrast, the effect
gel was dissolved by adding different concentrations of NaOH of calcination time (3–60 min) was only visible for rst few
solution. In this process, some of the OHꢀ ions in the solution cycles and it became modest when the number of cycles was
might neutralize the H+ ions derived from the citric acid, while increased.32,38 In this experiment, 30 min was selected to
other OHꢀ ions might react with the Ca2+ citric acid chelate perform carbonation reaction to attain substantially higher
complexes. During the hydrothermal reaction, decomposition residual conversions and the calcination time was xed for
of citric acid would take place and subsequently produce 6 min to reduce sintering of the adsorbents for multiple cycles.
2ꢀ
carbonate ions CO3 in the aqueous solution. Therefore, at a
low concentration of NaOH (2 M), three different ionic species
(Ca2+, OHꢀ and CO32ꢀ) were available as reactants. At these
conditions, Ca2+ citric acid chelate complexes would preferably
2ꢀ
react with CO3
to form CaCO3 because the free energy
formation of CaCO3 are ꢀ1129.1, which is lower than that of
2ꢀ
calcium hydroxide (ꢀ897.5). Once the entire CO3 ions were
consumed for the formation of CaCO3, the rest of the Ca2+ in the
citric acid chelate complexes would react with OHꢀ to form
Ca(OH)2. This may represent the results obtained at 6 M which
showed that the as-prepared particles were dominantly
Ca(OH)2.
As the number of nanoparticles increased, the total surface
energy of the nanoparticles in the solution increased accord-
ingly, and the Ca(OH)2 nanoparticles aggregated to form the
hexagonal and rectangular shaped microstructures. When the
concentration of NaOH was increased to 10 M, high excess of
OHꢀ ions would react with Ca2+ citric acid chelate complexes in
the solution to form the CaC2O4$H2O nanoparticles. The
CaC2O4$H2O nanoparticles would then grow into the irregular
nanoplates in the solution through oriented aggregation.55 The
irregular nanoplates coalesced with each other through side-by-
side means to enlarge the planar area thus forming microplates.
Fig. 5 Effect of temperature on the carbonation of 3D hierarchical
CaCO3 hollow microspherical adsorbent composed of 1D spike-sha-
ped nanorods.
3.3 CO2 adsorption analysis
To investigate the effect of temperature on the carbonation of
CaCO3 hollow microspheres adsorbent, a set of experiment was
conducted at temperature ranging from 600 to 800 ꢁC (Fig. 5). It
could be seen that the curves evolved into a plateau stage within
6 min aer the carbonation started and the adsorbent achieved
over 80% weight gain. The results showed that the CaCO3
hollow microspheres possessed high carbonation rate, which
offered a great chance for this adsorbent in practical applica-
tion. The fast carbonation could be attributed to the larger BET
surface area, micro-pore and meso-pore size distributions of the
CaCO3 hollow microspherical adsorbent. Aer 30 min carbon-
ation, the maximum weight gain of the adsorbent at 600, 700
Fig. 6 Weight change associated with 29 carbonation–calcination
cycles of 3D hierarchical CaCO3 hollow microspherical adsorbent
and 800 ꢁC were 84%, 86% and 89%, respectively. It was composed of 1D spike-shaped nanorods.
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Fig. 6 shows the proles of 29 consecutive carbonation– rst ve cycles and thereaer became sluggish. The former
calcination cycles of the calcite CaCO3 hollow microspherical condition was due to the loss of pore volume of the adsorbent
adsorbent in percentage weight. The weight gain dropped from whereas the latter condition was caused by the sintering effect.60
89% of rst cycle to 71% aer 29 cycles. The detail of the On the other hand, the increase in grain size61 and the collapse
carbonation–calcination prole for 30 and 6 min could be of morphology during multiple cycles also affected the degra-
described by eqn (3) to (5), respectively. When the as- dation of the adsorbent62 and would be discussed by FESEM
synthesized CaCO3 adsorbent was heated from room tempera- images.
ture to 800 ꢁC under 100% N2 gas, the thermal decomposition of
Fig. 7 shows the selected carbonation conversion curves (1st
CaCO3 adsorbent was observed, as expressed in eqn (3). Then, and 15th cycle) of the synthesized CaCO3, Ca(OH)2, CaC2O4$H2O
ꢁ
the temperature was maintained at 800 C for 6 min to ensure adsorbents and commercial limestone. For the initial stage
complete decomposition of CaCO3. Aer that, the reaction (chemical controlled reaction), the conversion of the hollow
atmosphere was switched to a 100% CO2, the carbonation sphere CaCO3 adsorbent completed about 67% while Ca(OH)2,
reaction of CaO took place (eqn (4)) to form the carbonated CaC2O4$H2O and limestone attained about 60%, 55% and 65%,
CaCO3. This step was followed by the calcination reaction (eqn respectively. The CaCO3, CaC2O4$H2O adsorbents and lime-
(5)) whereby the carbonated CaCO3 converted to CaO under stone completed the initial stage within 2 min, however the
100% N2 gas.
Ca(OH)2 adsorbent completed around 3 min. Aer this stage,
the carbonation conversion increased relatively slow with time.
Notably, the increase in conversion of CaCO3, CaC2O4$H2O
adsorbents and limestone were about 12% in the second stage
(diffusion controlled reaction), however, the conversion for
Ca(OH)2 adsorbent increased to about 30%. Aer 30 min
carbonation, the carbonation conversion achieved 79%, 94%,
68% and 77% for CaCO3, Ca(OH)2, CaC2O4$H2O adsorbents and
limestone, respectively.
800 ꢁ
C
CaCO3 ðas-synthesizedÞ þ N2
CaO þ CO2 þ N2
(3)
(4)
(5)
ƒƒƒ!
800 ꢁ
C
CaO þ CO2
CaCO3 ðcarbonatedÞ
ƒƒƒ!
800 ꢁ
C
CaCO3 ðcarbonatedÞ þ N2
CaO þ CO2 þ N2
ƒƒƒ!
The carbonation conversion of the CaCO3 adsorbent for
multiple cycles is shown in Fig. S9 (ESI†). Notably, the
maximum conversion of 79% was achieved aer 30 min
carbonation in the 1st cycle compared with 38% following the
29th cycle. The carbonation reaction appeared in the two-stages.
The initial reaction (chemical controlled reaction) stage was
completed within the rst 2 min due to the rapid surface
reaction between CO2 and CaO to form the CaCO3 product layer
covering the CaO core.58 The higher conversion in the rst cycle
was due to the extent of conversion attained during the chem-
ical controlled reaction stage. In the second stage (diffusion
controlled reaction, 2–30 min), CO2 diffused through a layer of
The difference in carbonation conversion between the
adsorbents might be caused by the variance in the nucleation
rate of CaCO3.63 The conversion of the adsorbents in the rst
cycle was ranked as follows: Ca(OH)2 > CaCO3 > limestone >
CaC2O4$H2O. However, aer the 15th cycle, the ranking of the
conversion changed to be CaCO3 > CaC2O4$H2O > limestone >
Ca(OH)2. The decaying carbonation conversion of CaCO3,
Ca(OH)2, CaC2O4$H2O adsorbents and limestone aer multiple
cycles were attributed to the growth of crystallite of the CaO
particle due to the sintering accompanied by the calcination
process. The growth of crystallite results to a decrease in reac-
tive surface area, which consequently decreased the overall
reaction rate.64 In addition, the different carbonation conver-
sions of the CaCO3, Ca(OH)2, CaC2O4$H2O adsorbents and
limestone could be due to the different morphologies and BET
surface areas of the adsorbents. Lu et al.65 and Gupta and Fan36
assumed that the differences in reactivity appeared due to the
differences in the adsorbent morphology and not because of a
reection of chemistry of the gas-solid reaction that took place
on the CaO surface, as all of the CaO adsorbents which obtained
from different precursors showed similar crystal structure.
In this study, the mass of the adsorbents tested in multi
cycles TG experiments was not sufficiently high to perform the
physisorption analysis. Through the observation with SEM, Lu
et at. found that pore volumes and surface areas of the adsor-
bents were based on their different morphologies, which nally
determined their carbonation performances.48 Hence, the
FESEM analysis was used to investigate the sintering effect on
the morphologies of the adsorbents aer multiple carbonation–
calcination cycles. As shown in Fig. S10(a) (ESI†), the spike-
shaped CaCO3 nanorods merged into interconnected
networks aer 1 cycle, however, the hollow structure still
occurred. As shown in Fig. S10(b) (ESI†), the hollow structure
nascent CaCO3 to react with the unconverted CaO core.59
A
sharp decay in the carbonation conversion occurred during the
Fig. 7 The 1st and 15th carbonation conversion curves of CaCO3,
Ca(OH)2, CaC2O4$H2O adsorbents and limestone.
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disappeared aer 29 cycles and a noticeable increase in grain were the typical of the sintering effect, led to the lower surface
size could be observed. The increase in CaO grain size with area,61,67,68 which in turn resulted to a decrease in carbonation
number of cycles could be affected the CO2 adsorption perfor- conversion of the CaCO3 adsorbent.
mance for subsequent cycles.61 The FESEM images in Fig. S10
Fig. S11 (ESI†) shows FESEM images of Ca(OH)2, CaC2O4$H2O
(ESI†) indicated a change in pore structure. Aer 1 cycle, the and limestone aer 15 cycles. As seen in FESEM images, there
adsorbent exhibited small pores (40–70 nm), whereas aer 29 were two different types of textures on the surface of the Ca(OH)2
cycles, small pores almost disappeared and larger pores (ꢂ700 and CaC2O4$H2O adsorbents, while the limestone showed cracks
nm) were observed. The increase in grains size, change in pore in the CaO crystalline structure as reported in literature.69 As
structure, the appearance of agglomerates and smooth surface shown in Fig. S11(a) and (b) (ESI†), one texture was a large
number of smaller CaO grains appeared to agglomerate together.
The other texture was a compact solid with smooth surface,
which covered the agglomerated CaO grains in some area. These
two textures might be decreased the CO2 reaction rate because
there were very few pores available for gas-solid reaction and CO2
had to penetrate the agglomerated grain during carbonation
reaction. The FESEM results demonstrated that Ca(OH)2 and
CaC2O4$H2O adsorbents experienced serious sintering effect,
which led to obviously reduce the surface area and CO2 adsorp-
tion performances.70
The cyclic carbonation conversions of the synthesized
adsorbents under different NaOH concentrations are presented
in Fig. 8. For comparison purposes, the solid line calculated
from a semi-empirical model that was proposed for the decay of
the capture capacity by Abanades and Alvarez69 was attached
(eqn (6)),
XN ¼ fNm(1 ꢀ fw) + fw
(6)
Fig. 8 Cyclic carbonation conversions of CaCO3, Ca(OH)2, CaC2O4$H2O
adsorbents and limestone.
Table 2 Summary of the carbonation conversions of CaCO3 adsorbents and limestone
Temperature (ꢁC)
Time (min)
Adsorbents
Morphology
Carbonation Calcination Carbonation Calcination Cycles (n) Conversion (%) References
CaCO3
(synthesized)
CaCO3
(synthesized)
CaCO3
(synthesized)
CaCO3
(synthesized)
CaCO3
(commercial)
CaCO3
(commercial)
Havelock
(limestone)
Cadomin
(limestone)
Hollow microsphere
800
800
800
700
750
700
850
850
850
30
30
45
20
20
20
10
10
6
29
29
18
29
29
29
29
29
ꢂ38
ꢂ30
ꢂ31
ꢂ30
ꢂ30
ꢂ30
ꢂ20
ꢂ20
This study
Spherical shaped mixed with cubic 800
shaped
Porous sphere shaped
7.5
38
39
37
37
66
32
32
650
750
600
650
650
650
30
Nanopod
20
20
20
10
10
—
Nanoparticle
—
—
Blanca (limestone) —
650
650
650
850
850
850
10
10
10
10
10
10
29
29
29
ꢂ20
ꢂ20
<20
32
32
32
Piasek (limestone)
Gotland
—
—
(limestone)
La Blanca
(limestone)
Strassburg
(limestone)
—
—
650
850
850
850
20
9
20
8
29
29
<20
<20
32
57
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where XN is the maximum carbonation conversion achieved
aer N cycles, fm ¼ 0.77 and fw ¼ 0.17. Aer 15 cycles, the
carbonation conversion of CaCO3 hollow microspheres was
45%, which was about 22% higher than the semi-empirical
model, CaC2O4$H2O, Ca(OH)2 adsorbents and limestone. In
comparison, the residual conversion ꢂ38% for CaCO3 hollow
microspheres aer 29 cycles was higher than those of spherical
shaped mixed with cubic shaped CaCO3 adsorbent, CaCO3
nanopod, commercial CaCO3 adsorbents and natural limestone
reported in literatures as summarized in Table 2.32,37–39,57,66 This
nding means that the hierarchical CaCO3 hollow micro-
spheres composed of spike-shaped nanorod adsorbent
possessed better anti-sintering performances than others CaO-
based adsorbents. This may be attributed to the novel 3D
structure, which has larger surface area and the trimodal pore
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We successfully synthesized the novel microstructured hierar-
chical calcite CaCO3 hollow spherical adsorbent composed of
spike-shaped nanorods by the sol–gel hydrothermal with
varying NaOH concentrations. This is a new proposed approach
to prepare 3D hierarchical calcite CaCO3 hollow microspheres
without surfactants. The hierarchical CaCO3 hollow micro-
spheres produced at 2 M NaOH exhibited the best CO2
adsorption performances, which was about 22% higher than
that of Ca(OH)2 and CaC2O4$H2O adsorbents synthesized with 6
and 10 M NaOH. This is attributed to the 3D hierarchical hollow
microsphere structure, 1D spike-shaped nanorod structure,
large BET surface area and trimodal pore size distribution. The
enhanced performance of the novel morphology indicated that
it is a promising adsorbent for CO2 adsorption and a potential
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The authors thank the AUN/SEED Net Project under grant no.
304/6050219, RU grant no. 814154, Japan International Coop-
eration Agency (JICA) and Long Term Research Grant (LRGS)
(203/PKT/6723001) from the Ministry of Higher Education
(MOHE) Malaysia for their nancial support. The authors thank
Mr Jon Koki, Technical Department, Center for Advanced
Materials Analysis, Tokyo Institute of Technology, for FESEM
analysis. The authors also greatly acknowledge the Universiti
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