Powder Technology 215-216 (2012) 54–58
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Powder Technology
journal homepage: www.elsevier.com/locate/powtec
Hydrothermal synthesis of hierarchical micron flower-like γ-AlOOH and γ-Al2O3
superstructures from oil shale ash
Guijuan Ji a, Mengmeng Li a, Guanghuan Li a, Guimei Gao a, Haifeng Zou a, Shucai Gan a,⁎, Xuechun Xu b
a
b
College of Chemistry, Jilin University, 6 Ximinzhu street Changchun 130026, P.R. China
College of Earth Sciences, Jilin University, 6 Ximinzhu street Changchun 130026, P.R. China
a r t i c l e
i n f o
Article history:
Received 7 July 2011
Received in revised form 31 August 2011
Accepted 1 September 2011
Available online 17 September 2011
Keywords:
Oil shale ash
Alumina
Surfactant
a b s t r a c t
The hierarchical micron flower-like γ-AlOOH and γ-Al2O3 superstructures were synthesized using oil shale ash
(OSA) as a new raw material. In order to prepare the better quality alumina with uniform morphology, the effects of ethanol and surfactant CTAB on the formation of alumina particles were investigated and the formation
mechanism of flower-like composite particles was also discussed. The structural and morphological properties
of the uncalcined and calcined powders were characterized by X-ray diffractometer (XRD), Scanning electron
microscope (SEM), Brunauer Emmett Teller (BET) and Fourier transform infrared (FT-IR). The results indicated
that uniform flower-like γ-AlOOH and γ-Al2O3 assembled by nanosheets which thickness is less than 100 nm
were prepared. After calcined at 600 °C, morphology of the flower-like superstructure was maintained perfectly.
The results obtained in this work prove that the oil shale ash can be used for production of hierarchical superstructures alumina and open a new way for comprehensive application of OSA.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
A by-product residue from oil shale processing is oil shale ash
(OSA), which is considered as a serious environmental pollutant
[1,2]. Therefore, many attempts were made to benefit from such material, especially in construction industries and chemical industries,
such as Na-Y, X and -P1 zeolites [3,4], adsorbent [5], tobermorites
[6,7], portland cement concrete [8], while the large portion is dumped
in landfills. Therefore, there is a need for a proper strategy for ash
handling, disposal and utilization. Previous experimental results indicated that the OSA consists of two parts: inorganic and organic components, whose major chemical compositions are silicon dioxide
(SiO2) and alumina (Al2O3) etc. So, an alternative manner is the
conversion of this ash into high-grade alumina product, which is considered an environmentally friendly product.
Alumina is widely used as adsorbents, catalysts supports, ceramics,
heat insulating materials, and reinforcements for composite materials
result of its unique physical and chemical characteristic [9,10]. In various transition alumina, γ-Al2O3 has been utilized as a carrier for catalyst
in petroleum refining and petrochemical industries due to its large
surface area, low cost, thermal stability, good mechanical strength and
volatile acidity [11-14]. Boehmite (γ-AlOOH) which is used as a precursor for γ-Al2O3 materials, can serve as catalyst supports, flame retardants and adsorbents [15-17]. Nowadays, there are many strategies to
synthesize micro/nanostructure boehmite, such as template method,
⁎ Corresponding author. Tel.: + 86 431 88502259.
E-mail address: [email protected] (S. Gan).
0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.powtec.2011.09.005
electrochemical method, hydrothermal method, sol–gel method, chemical vapor deposition method, pyrolysis method and seed precipitation
[18,19]. However, the hierarchical 3D micron flower-like γ-AlOOH and
γ-Al2O3 superstructures which were synthesized using oil shale ash as
raw material have not been reported.
In this study, we reported a facile hydrothermal route to synthesize
micron 3D flower-like γ-AlOOH and γ-Al2O3 assembled by nanosheets
which thickness is less than 100 nm in ethanol–water solution using oil
shale ash as raw material.
2. Materials and methods
2.1. Raw materials and reagents
The oil shale and oil shale ash in the experiment was taken from oil
retorting factory of Heilongjiang province. The mineral compositions of
oil shale are 49% Quartz, 34% Kaolinite, 5% Alkali feldspar, 4% Plagioclase
and 8% gypsum. The chemical compositions of the oil shale (OS) and oil
shale ash (OSA) are given in Table 1, which indicate that oil shale ash
contains mainly around 28% alumina, 60% silica. All chemicals were
analytical grade reagents supplied by Beijing Chemical Reagent Research Institute.
2.2. Experimental procedure
2.2.1. Leaching of aluminium
OSA was heated at 700 °C for 3 h to remove residual organic
compounds, and then leached by sulfuric acid solution. After elapsing
the reaction time, the slurry was filtered and the pH value of the filtrate
G. Ji et al. / Powder Technology 215-216 (2012) 54–58
55
Table 1
Chemical component of oil shale and oil shale ash.
Samples SiO2
OS
OSA
TiO2 Fe2O3 Al2O3 P2O5 MnO CaO MgO K2O
13.38 0.79 1.22
60.35 0.78 5.50
5.66 0.03 0.03
27.50 0.12 0.14
0.06 0.23
3.57 0.29
0.16
1.05
Na2O LOI
0.17
0.72
58.57
0.00
was adjusted to 5 by adding sodium carbonate. The obtained turbid
solution was filtrated so that calcium, magnesium, potassium ions
were removed then transferred the precipitation to a 500 mL pyrex
beaker. Sodium hydroxide solution was added to the precipitation
until the pH value equaled 13. Then, the mixture was filtered to remove
ferric hydroxide and obtain alkaline sodium aluminate solution.
2.2.2. Synthesis of hierarchical micron flower-like alumina
Firstly, the appropriately alkaline sodium aluminate solution
produced by 2.2.1 was diluted to 0.6 M and 0.8 M. Then 0, 0.025,
0.05 M CTAB solution (in ethanol) was subsequently dropwise added
into the aforementioned diluted solution respectively. After 24 h stirring, the resulting milky mixture was transferred into a Teflon-lined
stainless steel autoclave heating at 160 °C for 12 h. After cooling to
room temperature, the as-prepared products were collected and
washed by centrifugation with distilled water and ethanol several
times, then vacuum-dried at 60 °C for 12 h. The as-obtained products
were calcined at 600 °C for 4 h.
2.3. Method of characterization
Fig. 2. FTIR spectra of flower-like γ-AlOOH (a) and γ-Al2O3 (b) obtained by calcining at
600 °C.
3. Results and discussion
3.1. Properties of 3D flower-like composite particles
Fig. 1 shows the XRD patterns of samples prepared before (a) and after
(b) calcining at 600 °C respectively. Before calcination, the diffraction patterns of the products can be indexed with a crystalline γ-AlOOH, which is
in good agreement with the literature (JCPDS card no. 21–1307) (Fig. 1
(a)). Fig. 1b can be indexed to the pattern of γ-Al2O3 according to JCPDS
card (no. 10–425), which indicate γ-AlOOH transform to γ-Al2O3 after
The chemical compositions of the OSA were determined by X-ray
fluorescence (XRF) analysis (PW1404/10. Philips, Holland). X-ray
diffraction (XRD) analysis for powders was carried out by D/Max-IIIC
(Rigaku, Japan) with Cu Kα radiation to identify the crystallite phases.
Morphological analysis was performed by an S-4800 field emission scanning electron microscope (FE-SEM, Hitachi, Japan) with an acceleration
voltage of 3 kV. The N2 adsorption measurement was examined by a
surface analyzer (ASAP 2010, USA) and the BET surface area, the pore
volumes were calculated. The pore size distributions were estimated
by the Barrette–Joynere–Halenda (BJH) method. Infrared spectra of
the samples were investigated on a FT-IR (Nicolet Impact 410, USA)
by KBr disk method. The fourier transform infrared (FT-IR) spectroscopy
(Iraffinity-1, Shimadzu, Japan) was used to confirm the surface chemical
structure of the products in the wave number range of 400–4000 cm− 1.
Fig. 1. XRD patterns of the flower-like γ-AlOOH (a) and γ-Al2O3 obtained by calcining
at 600 °C (b).
Fig. 3. N2 adsorption–desorption isotherm (a) and pore size distribution of the flower-like
γ-Al2O3 (b).
56
G. Ji et al. / Powder Technology 215-216 (2012) 54–58
being calcined. No other phases of alumina XRD peaks are observed in
Fig. 1a and b, which revealed only γ-AlOOH and γ-Al2O3 were prepared.
In addition, there is no obvious change in the XRD patterns of other samples when concentration of CTAB and alkaline sodium aluminate range
from 0 to 0.05 M, 0.6 to 0.8 M respectively.
The IR spectrum of calcined and uncalcined flower-like products is
shown in Fig. 2. In Fig. 2a, the intensive bands at 3304 and 3085 cm− 1 belong to the νas (Al)O–H and νs (Al)O–H stretching vibrations. The bands
located in 1065 cm− 1 and 1162 cm− 1 are attributed to the δ s Al–O–H
and δas Al–O–H modes, respectively. The three bands at 736, 610, and
482 cm− 1 are consistent with the AlO6 octahedra vibration mode. The
shoulder at 1639 cm− 1 is the feature of the bending mode of absorbed
water. The two weak bands at 2092 and 1970 cm− 1 are the combination
bands [20]. In Fig. 2b, the intensive band at 3458 cm− 1 and weak band at
1638 cm− 1 are attributed to the stretching vibrations of OH groups in the
hydroxide structure and adsorbed water in physically, and the bands at
789 and 574 cm− 1 correspond to the vibration mode of AlO6 octahedra.
The absorption bands mentioned above are perfectly consistent with
the IR spectrum of γ-AlOOH and γ- Al2O3 reported in previous literature
[21].
The N2 adsorption–desorption isotherms and pore size distributions (PSD) were obtained to investigate the pore structure and distribution of the pores in the flower-like γ-Al2O3. The nitrogen
adsorption–desorption isotherm are shown in Fig. 3a. The isotherm
obtained for the sample is of type IV which is the characteristic of
the mesoporous materials. Fig. 3b depicts that the PSD of the products
is unimodal with the peak pore diameters at 34 nm. This implies that
the as-made alumina consisted of interpenetrating meso- (pores
below 50 nm) and macrostructures (pores above 50 nm) [22].
3.2. Effect of the concentration of reactant on the morphology of composite
particles
In order to investigate the effect of reactant concentration on the
morphology of composite particles, a series of samples using different
concentrations of CTAB and NaAlO2 were prepared. It is found that
CTAB and NaAlO2 concentrations have a remarkable effect on the
morphology as shown in Fig. 4. When NaAlO2 was 0.6 M and CTAB
concentration changes from 0 to 0.05 M, the SEM images of assynthesized samples were shown in Fig.4(a–c). It is clearly indicated
that leaf-like samples with thickness less than 100 nm, length and
width 5.0–7.0 μm, 2.0–3.5 μm were synthesized when no CTAB was
added. With increasing dosage of CTAB, the morphology of products
was uneven due to self-assembly of leaf-like alumina. The SEM images of as-synthesized samples when increasing NaAlO2 to 0.8 M
and CTAB concentration varies from 0 to 0.05 M, were shown in
Fig. 4. SEM images of as-synthesized samples (a–c) CNaAlO2 = 0.6 M, CCTAB = 0,0.025, 0.05 M; (b–f) CNaAlO2 = 0.8 M, CCTAB = 0,0.025, 0.05 M.
G. Ji et al. / Powder Technology 215-216 (2012) 54–58
57
bonding between CTAB and hydroxyl groups of γ-AlOOH layers effectively reduces the inclination of interlayer stacking through hydrogen
bonds [24]. Besides, under the hydrothermal conditions, the CTAB
and the NaAlO2 may form CTAB–AlO−
2 ion pairs by electrostatic interaction. The assembly of CTAB–AlO−
2 ion pairs also contributes to form
an ordered flower-like structure when the pH value of solution
reaches 13 and the concentration of NaAlO2 increase to 0.8 M [25].
In addition, CTAB availably manipulated the crystals growth and crystallites aggregation in an ordered and oriented way, which is indispensable in the formation of flower-like products. Subsequently,
calcination of the γ-AlOOH can result in topochemical transformation
to γ-Al2O3 with the retained flower-like morphologies due to the
transformation of boehmite into γ-Al2O3 is pseudomorphic, according
to the dehydration reaction of
2AlOOH→Al2 O3 þ H2 O
ð2Þ
Moreover, absolute ethanol also plays a key role in the formation of
the as-synthesized product. In order to avoid the ultra-fast hydrolysis
rate of aluminum precursor which always lead to irregular morphologies of products, ethanol/water mixed solvent was adopted. The
addition of ethanol could effectively inhibite the crystal growth, thus
leading to a better control over the reaction [24,26,27].
4. Conclusions
Fig. 5. SEM images of flower-like γ-Al2O3 obtained by calcining at 600 °C.
Fig. 4(d–f). Fig. 4(d–f) explicitly revealed that microscale flower-like
alumina which was constituted by leaf-like alumina was gradually
obtained with increase of CTAB concentration. Finally, uniform microscale flower-like alumina with an average diameter of 4.5–6.5 μm
were synthesized when CTAB concentration is 0.05 M. In addition, it
is shown in Fig. 5 that the morphologies of leaf-like and flower-like
alumina were maintained completely after calcining while the size
of leaf-like nanosheets and hierarchical 3D micron flower-like superstructures shrank to some extent.
3.3. The formation mechanism of flower-like composite particles
Acknowledgements
The formation of γ-AlOOH can be described by the following reaction
−
AlO2 þ H2 O→AlOOH þ OH
−
In this study, hierarchical 3D micron flower-like γ-AlOOH and γ-Al2O3
superstructures were successfully synthesized using OSA as raw material.
The results show that the surfactant CTAB can effectively manipulated the
crystals growth and crystallites aggregation in an ordered and oriented
way, which is beneficial to preparation γ-AlOOH and γ-Al2O3 powder
with uniform shape. The ethanol/water mixed solvent effectively can
slow down the ultra-fast hydrolysis rate of aluminum precursor and the
crystal growth, thus leading to a better control over the reaction. The
experiment provides a simple and effective approach for preparing hierarchical micron alumina superstructures with mesoporous and microporous structures. What's more, it opens a new way for comprehensive
utilization of OSA.
ð1Þ
The formation of 3D flower-like superstructures includes two
major procedures which are the primary self-assembly of boehmite
nanocrystals into leaf-like nanosheets and the secondary assembly
of the as-formed nanosheets into 3D flower-like superstructures. At
the beginning of the reaction, the boehmite nanocrystals which
further grew into nanosheets via oriented attachment began to generate. Boehmite (γ-AlOOH) has a lamellar tructure (orthorhombic
symmetry) with plenty of surface-located OH − groups and the layers
are stacked along [010] direction. The basic layer is composed of two
sheets of edge sharing AlO6 octahedra, and the sheets are fused again
via common edges of the octahedron. As is known to all that the
formation of smaller crystallites is kinetically favored at the initial
reaction stage, while larger crystallites are thermodynamically
favored. Therefore, the layered boehmite possesses a growth direction which needs the lowest growth energy, and the primary boehmite crystallites start to self-aggregate and grow into nanosheets
through dissolution and recrystallization (Ostwald ripening) [23].
Meanwhile, the self-assembly of boehmite nansheets into flowerlike superstructures occur because of OH groups on the surface of
nanosheets with the assistance of CTAB. Since the large number of
OH groups on the surface, γ-AlOOH is apt to interact with CTAB. The
This present work was financially supported by the key technology and equipment of efficient utilization of oil shale resources, No:
OSR-05, and the National Science and Technology major projects,
No: 2008ZX05018.
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