Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 157–160
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In situ preparation of biochar coated silica material from rice husk Yujiao Li, Xiaofeng Wang, Yanchao Zhu, Lili Wang, Zichen Wang ∗ College of Chemistry, Jilin University, Changchun 130012, China
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Article history: Received 19 August 2011 Received in revised form 20 October 2011 Accepted 7 December 2011 Available online 16 December 2011 Keywords: Composite material Rice husk utilization Carbon coated silica Biochar
a b s t r a c t An alternative approach to the utilization of lignocellulosic biomass is reported in this paper. It was the ﬁrst time that rice husk was used as carbon source to synthesize carbon coated silica particles. Rice husk was hydrolyzed and mixed with silica powder, then biochar coated silica particles were simply in situ synthesized. The coating morphology was similar to those who used pure chemicals or glucides as carbon source, while the mild reacting conditions could keep some organic functional groups on the particle surface and make the modiﬁcation easier. Acid could be recycled to form a semi-closed reaction system. The composite was designed as one of the bases for the preparation of activated carbon shell. Thus, this paper may provide a low-cost and simple method to prepare functional materials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The utilization of lignocellulosic biomass to prepare carbon and carbon composite materials is an important focus of science and technology [1,2]. Biomass is a renewable carbon source; it could reduce the consumption of fossil fuels. Discarded biomass would release large amount of carbon dioxide due to the decomposition of microorganisms. So it is essential to comprehensively utilize biomass considering both environmental and resource factors. Rice husk (RH) is one of the main kinds of agricultural by-products. About 80 million tons rice husk , which consists of about 50% polysaccharide and 15% silica, is left behind every year. If these elements can be made good use to, residue problems would be solved, and at the same time, great proﬁts could be gained. In recent years, synthesis of carbon coated silica materials has obtained new progresses. Carbon nanotubes are utilized as the coating layer in some works. For example, silica gel particles coated with a monolayer of pristine single-walled carbon nanotubes are useful for a wide range of materials, such as the stationary phase of liquid chromatography and catalyst supporting materials . An ultrathin graphene coat is able to endow silica particles with hydrophobicity pore nature and electrical conductivity [5,6]. Carbon nanoshell–silica core and carbon nanoshells, which will be useful in electronic applications, are prepared by chemical vapor deposition . Accordingly, a unique carbon coated silica nanowire hybrid structure was directly synthesized by a chemical vapor deposition method using ethanol as the precursor . Besides, pure
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(Z. Wang). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.023
saccharides, such as sucrose, are also widely used carbon sources [9,10]. Rice husk based micro- and mesoporous carbon/silica composites are synthesized using beet sugar as a binder, by means of CO2 gas activation . To the best of our knowledge, in the synthesis of carbon coated silica particles, biomass has not been utilized as carbon source. In our previous work, sphere-like hydrochar with outstanding properties has already been synthesized under the catalysis of sulphuric acid . Similarly, in this paper, biochar coated silica core–shell particles were directly in situ prepared using rice husk as the carbon source. Because the raw material is cheap and easily accessible, such method is supposed to be more approachable for industrialization. 2. Materials and methods 2.1. Materials Rice husk was washed and dried under 80 ◦ C before hydrolyzing. Sulphuric acid (95–98 wt%), tetraethoxysilane (TEOS; 28.0 wt%), absolute ethanol (99.7 wt%), ammonia hydroxide (25–28 wt%) and sodium hydroxide (96.0 wt%) were purchased from Beijing Chemical Works. All chemicals were of analytical grade and were used without further puriﬁcation. Distilled water was used through all the processes. 2.2. Preparations 2.2.1. Synthesis of SiO2 The monodisperse silica spheres were prepared by Stöber’s method [9,11,12]. In a typical case, at room temperature, 25 mL of
Y. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 157–160 Table 1 Coating experiment conditions (T1 : prepolymerization temperature; t1 : prepolymerization time; T2 : carbonization temperature; t2 : carbonization time).
Filtrate : SSA
SSA concentration Composite
14.0 mg/mL 18.0 mg/mL 19.0 mg/mL
Silica:SSA 1 g:55 mL 1 g:90 mL 1 g:100 mL 1 g:200 mL 1 g:600 mL
55 ◦ C 60 ◦ C 62.5 ◦ C 65 ◦ C
0h 1h 2h 3h 4h
90 ◦ C 92 ◦ C 95 ◦ C
0h 1h 2h 3h 4h
2.2.2. Synthesis of biochar coated silica particles Fig. 1 shows the schematic drawing of synthesis route. RH was hydrolyzed by 72% (wt%) sulphuric acid (1 g RH:10 mL H2 SO4 ) to prepare solution contains saccharide and acid (SSA), which was based on our previous work . The saccharide concentration of the obtained ﬁltrate was about 25.2 mg/mL. SSA was diluted to make the saccharide concentration 18.0 mg/mL, then ultrasonic mix with silica. Under mechanical stirring and water-bath heating, the suspension was prepolymerized and then carbonized in a 250 mL three-necked ﬂask. After being cooled to room temperature, the suspension was ﬁltered and the residue was the product, while the acid ﬁltrate could be recycled. When silica prepared from RH residue after hydrolization was utilized, a semi-closed reaction system was formed, the schematic drawing of the semi-closed synthesis system of biochar coated silica is shown in Fig. 1. 2.2.3. Characterization The morphology of biochar coated silica core–shell particles were examined using transmission electron microscopy (TEM; Hitachi H-800) at an acceleration voltage of 200 kV. FT-IR spectra were measured by a JIR-5500 (JEOL) spectrophotometer at room temperature using KBr tablets. Saccharides content was tested by vis spectrophotometer (Jinghua 723PC vis spectrophotometer) using 3,5-dinitrosalicylic acid (DNS) as the color reagent . 3. Results and discussion 3.1. The optimal coating conditions The coating conditions investigated were the concentration of SSA, the ratio of silica mass to SSA volume, prepolymerization temperature and time, carbonization temperature and time (Table 1). The optimal coating conditions were investigated through single-factor experiments. Fig. 2 shows the effects of time and temperature on the coating morphology. In order to obtain coated particles, the minimum concentration of SSA was 18.0 mg/mL. The ratio of silica to SSA affected the thickness of the coating layer. Commonly, as the dosage of SSA increased, the coating layer became thicker and smoother. While if the ratio was larger than 1 g:90 mL, silica spheres were incompletely covered, and biochar tended to form spheres independently on the surfaces of silica. Typical
3.2. FT-IR characterizations Generally speaking, the biochar coated silica curve was similar to the biochar curve rather than the silica curve. In all the three kind of curves (Fig. 4), there were peaks around 1100 cm−1 , which implied the C–O (hydroxyl, ester, or ether) stretching. Near 3000 cm−1 in the biochar curve located the band that corresponded to stretching vibrations of aliphatic C–H. Different from both silica and biochar, the coated curves had peaks at 1707 cm−1 , which
ammonia hydroxide and 250 mL of absolute ethanol was mixed in a 500 mL three-necked glass ﬂask. Under mechanical stirring, 20 mL of TEOS was slowly added dropwise. After 6 h, monodisperse silica spheres 450 nm in diameter were obtained. The solution was then centrifugalized, dried and pulverised for further experiments. During the investigation of optimal coating conditions (see Section 3.1) and the acid recycling experiments (see Section 3.3), this kind of silica was used, in order to simplify the discussions. Silica could also be prepared from hydrolyzed rice husk residue, by calcining, or NaOH dissolving.
morphologies of the partly coated and uncoated particles are shown in Fig. 3a and b, respectively. Under optimal coating conditions (18.0 mg/mL SSA, 1 g silica: 100 mL SSA, prepolymerize at 60 ◦ C for 2 h, carbonize at 95 ◦ C for 2 h), the coating morphology was preferable (Fig. 3c), while there was a little aggregate.
Prepolymerization time Carbonization time
Reaction time (h)
Fig. 1. The schematic drawing of the semi-closed synthesis system of biochar coated silica.
Prepolymerisation temperature (ºC) Fig. 2. Effects of (a) time and (b) prepolymerization temperature on the coating morphology.
Y. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 157–160
Fig. 3. TEM images of (a) partly coated particles, (b) uncoated particles and (c) biochar coated silica particles under optimal coating conditions (c inset, silica particles prepared by sol–gel procedure from TEOS).
mechanism. The activation of carbon and dissolvation of silica can be carried out simultaneously, according to our previous work . To avoid collapse of the shell structure during activation is one of the most signiﬁcant works.
3.3. The recycling of acid
The ﬁltrate of SSA after reaction was still with high acid concentration. If it was poured away directly, negative effects would be brought to soil and groundwater. Also, it is a waste of chemicals. Taking environmental and economical factors into consideration, the acid was recycled. Acid concentration was measured by chemical titration, and saccharide concentration was tested by vis spectrophotometer using DNS as the color reagent. Fig. 5 shows the acid and saccharide concentration changes during recycling. Basically, the concentration of acid and the convert ratio of saccharide were declined as the recycle time increased. The decline of acid concentration was quite small, and the use of recycling acid did not have obvious affections to the coating morphology. The coated product was designed as the ultimate base for the preparation of activated carbon shell, which is our following experimental plan. Before that, carbonized polymer coated silica and pure saccharide synthesized biochar coated silica spheres might be used as the bases in order to investigate the feasibility and the formation
Saccharide concentration (mg/mL)
attributed to C O vibrations. According to the results, the coated material combined some functional groups of silica and biochar. At the same time, the product itself had unique organic groups. So the coated product might be more suitable to prepare polymernanocomposite or load metal ions than absolute silica and carbon.
Before reaction After reaction
Acid concentration (wt %)
Before reaction After reaction
Recycling time Fig. 4. FT-IR spectrum of (a) silica, (b) biochar and (c and d) biochar coated silica particles.
Fig. 5. The concentration changes of (a) saccharide and (b) acid during recycling.
Y. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 157–160
4. Conclusions Biochar coated silica material which was designed as the base for the preparation of activated carbon shell can be simply in situ synthesized. Rice husk and other lignocellulosic biomass had not been utilized as carbon source in the preparation of carbon coated silica particles before. The optimal coating conditions: 18.0 mg/mL SSA, 1 g silica:100 mL SSA, prepolymerize at 60 ◦ C for 2 h, carbonize at 95 ◦ C for 2 h, were obtained through single-factor experiments. The functional groups on the particle surface made the modiﬁcation easy. Acid could be recycled to form a semi-closed reaction system. Acknowledgements This work was supported by Key Project of the National Eleventh Five-Year Research Program of China (2008BAE66B00) and Scientiﬁc and Technological Planning Project of Jilin Province (200,75,009). References  L. Wang, Y. Guo, Y. Zhu, Y. Li, Y. Qu, C. Rong, X. Ma, Z. Wang, A new route for preparation of hydrochars from rice husk, Bioresour. Technol. 101 (2010) 9807–9810.  S. Kumagai, H. Ishizawa, Y. Aoki, Y. Toida, Molded micro- and mesoporous carbon/silica composite from rice husk and beet sugar, Chem. Eng. J. 156 (2010) 270–277.
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