Journal of Materials Science & Technology  2020 , 38 (0): 183-188 https://doi.org/10.1016/j.jmst.2019.03.050

Research Article

A controllable soft-templating approach to synthesize mesoporous carbon microspheres derived from d-xylose via hydrothermal method

Jian Suab, Changqing Fangb*, Mannan Yangbc, Youliang Chengb, Zhen Wangb, Zhigang Huangd*, Caiyin Youa

aSchool of Materials Science and Engineering, Xi’an University of Technology, Xi’an, 710048, China
bFaculty of Printing, Packaging Engineering and Digital Media Technology, Xi’an University of Technology, Xi’an, 710048, China
cSchool of Mechanical and Precision Instrument Engineering, Xi’an University of Technology, Xi’an, 710048, China
dBeijing Key Laboratory of Quality Evaluation Technology for Hygiene and Safety of Plastics, Beijing Technology and Business University, Beijing, 100048, China

Corresponding authors:   ∗Corresponding authors.E-mail addresses: fcqxaut@163.com (C. Fang)huangzg@btbu.edu.cn (Z. Huang).

Received: 2018-12-19

Revised:  2019-02-25

Accepted:  2019-03-31

Online:  2020-02-01

Copyright:  2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

Highly dispersed carbon microspheres (CMSs) derived from D-xylose were successfully synthesized under hydrothermal conditions and followed by further carbonization, in which F127 was used as a soft template. As-synthesized products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), flourier transform infrared spectroscopy (FT-IR), thermal gravimetric (TG) and X-ray diffraction (XRD). The results showed that the morphology and structure of the CMSs prominently depended on the stirring speed during hydrothermal reaction. The resultant CMSs principally had non-porous structure without stirring and had a very smooth surface. When the stirring speed increased to 200 rpm, the synthesized mesoporous carbon microspheres at 220 °C for 24 h (CMSs-5) had a uniform size distribution of 1-1.4 μm and a specific surface area of 452 m2/g. Nevertheless, with further increasing to 400 rpm, as-fabricated carbon products were mostly amorphous with a low degree of sphericity. Results demonstrated that the diameter of the products decreased with the increase of stirring speed. Furthermore, the sphericity product yield of CMSs reduced with the increase of stirring speed. XRD result showed that all the obtained samples contained partial graphite phase. In addition, a formation mechanism was proposed that involved polymerization product as the precursors for microsphere formation. The controllable and green strategy may provide a great convenience to study properties and applications of carbon microspheres.

Keywords: d-xylose ; Mesoporous carbon microsphere ; Soft template ; Hydrothermal method ; Stirring

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Jian Su, Changqing Fang, Mannan Yang, Youliang Cheng, Zhen Wang, Zhigang Huang, Caiyin You. A controllable soft-templating approach to synthesize mesoporous carbon microspheres derived from d-xylose via hydrothermal method[J]. Journal of Materials Science & Technology, 2020, 38(0): 183-188 https://doi.org/10.1016/j.jmst.2019.03.050

1. Introduction

Owing to the superior electric conductivity, thermal conductivity, chemical stability and wide availability of raw materials, porous carbon materials have been applied in diverse fields [1]. It can be used as supports materials for catalytic and medicine, as sorbents for separation and gas storage, and as electrode materials for super capacitors, batteries and fuel cells etc [[2], [3], [4], [5]]. According to the pore diameters, porous carbon materials can be classified as microporous (pore size < 2 nm), mesoporous (2 nm < pore size < 50 nm), and macroporous (pore size > 50 nm) [6]. In order to improve the physical and chemical properties of porous carbon, great efforts have been made to seek more suitable precursors and controllable synthetic processes [7]. Available and inexpensive carbon precursor as well as facile, simple and environmental friend preparation method are still worth developing to produce mesoporous carbon mesosphere on a large scale.

Mesoporous carbon microspheres (MCMs) have attracted increasing attention due to the uniform and monodispersed morphology and controllable pore structure. Generally, a template was required to fabricate the MCMs with controllable morphology and porous structure. Commonly, there are two types of template-induced assembly strategies for fabrication of MCMs, namely soft template synthesis method and hard template synthesis method. Hard templates usually refer to the silica gel with pore structure, which serves as a mold for replication of pores, and no significant chemical interactions take place between templates and carbon precursors [8]. The carbon precursors can be infiltrated into the silica templates by sol-gel precipitation [9,10], chemical vapor deposition (CVD) [11,12] or chemical polymerization filling [13] etc. Obviously, the pore structures of these porous carbon materials are predetermined by the templates. As is known, hydrofluoric acid or strong base was needed to dissolve the silica hard templates, which makes the preparation process complicated, not environmental-friendly and time-consuming [14]. Therefore, more and more researchers explore the preparation process of mesoporous carbon microspheres by using soft templates, which generate the nanostructures through self-assembly formation between carbon precursors and specific organic molecules, such as Pluronic F127 [[15], [16], [17]] and P123 [[18], [19], [20]] (polyethylene oxide-polypropylene oxide-polyethylene oxide block copolymer) [21].

Normally, hydrothermal method is one of the most common way to prepare porous carbon spheres when soft templates were used [22], and usually follow with an activation step by KOH, NaOH or ZnCl2 [9,23]. Until now, various materials have been used as the carbon precursor, such as polydopamine [24], phenolic resin [7], fructose, xylose [15], glucose [25], poly(vinyl alcohol) [26], lignin [27]. As a biomass raw material, carbohydrates have become more and more popular as a carbon precursor owing to the superior water solubility and renewability. Although many efforts have been made, the preparation of excellent performance mesoporous carbon spheres using soft template is still challenging.

In this work, a 2 L high temperature and high pressure autoclave with mechanical stirring was used to prepare carbon spheres via hydrothermal process. D-xylose was applied as the carbon precursor, and Pluronic F127 was used as the only soft templates. The formation mechanism of the carbon microspheres and the influence of stirring speed on the morphologies and structures of carbon spheres were investigated in this paper.

2. Experimental

2.1. Materials

F127 (EO106PO70EO106, Pluronic®F-127, Sigma-Aldrich chemical Co., Ltd.), D-xylose (C5H10O5, 98 wt% Shanghai Macklin biochemical Co., Ltd.), deionized water (Produced by UPH series ultra-pure water machine, Ulupure), magnetic stirrer (IKA, Germany), ethanol absolute (C2H6O, 99.7 wt%, 0.789-0.791 g/mL, purchased from Tianli Chemical Tianjin, China) were used anf KBr (spectral purity) was supplied by Sinopharm Chemical Reagent Co., Ltd.

2.2. Preparation of CMSs

As shown in Fig. 1, CMSs were prepared using a 2 L high-temperature and high-pressure autoclave with mechanical stirring (CJF-2 L, DaLian TongDa). The mass ratio of xylose/F127 was 6:1. First, 10 g of F127 was dissolved in 500 ml of deionized water at 30 °C by magnetic stirring for 30 min. Then 60 g of xylose was added into the solution and stirred for another 10 min. The above aqueous solution was diluted to 1 L and transferred to the vitreous liner in autoclave. Following, the autoclave was set to design conditions (temperature, hold time and stirring speed). After the reaction was finished, the reactor was cooled down to room temperature naturally. Finally, the vitreous liner containing brown solution and precipitation was taken out. The precipitation (self-assembly gels) was separated by filtering and then washed with anhydrous ethanol and deionized water until the filtrate became colorless. The obtained gels were dried at 80 °C for 12 h. Last, the gels were carbonized in a tube furnace (KeJing 1400XL) under N2 atmosphere (200 ml/min). The heating process was as follows: the temperature increased to 350 °C from room temperature at a heating rate of 5 °C/min and held for 60 min, then increased to 800 °C at a heating rate of 10 °C/min and held for 2 h. After the tube furnace was cooled down to room temperature, the CMSs were obtained. The reaction conditions listed in Table 1 were designed through the orthogonal method using the IBM SPSS Statistics software.

Fig. 1.   Preparation process of CMSs and polymerization of precursors.

Table 1   The different reaction conditions, diameter and sphericity yield of preparing carbon microspheres (CMSs).

SamplesStirring speed
(rpm)
Temperature
(°C)
Reaction time
(h)
Diameter
(μm)
Yield of carbon spheres
(%)
CMSs-10180121.4-1.786
CMSs-20220181.2-1.598
CMSs-30260241.5-2.396
CMSs-4200180180.7-1.484
CMSs-5200220241.0-1.482
CMSs-6200260121.0-1.397
CMSs-7400180240.7-1.270
CMSs-840022012/0
CMSs-9400260181.0-2.650

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2.3. Characterization

The morphology of the CMSs was analyzed by field emission scanning electron microscopy (FE-SEM, SU8000 Hitachi) at 20 kV. An X-ray diffractometer (XRD) instrument (XRD-7000, SHIMADZU LIMITED, Japan) was used to analyze the structure of the CMSs. A JEM-3010 transmission electron microscope (TEM) with a Gatan894 CCD camera was performed to investigate the microstructures of CMSs-5, 9 at an accelerating voltage of 300 kV. Nitrogen adsorption and desorption isotherms were recorded using a JW-BK100A analyzer (JWGB). The pore size distribution was obtained using nitrogen desorption data via the Barrett-Joyner-Halenda (BJH) model and specific surface area was calculated by Brunauer-Emmett-Teller (BET) model. The flourier transform infrared (FT-IR) spectrum was recorded on a Shimadzu FTIR-8400S spectrometer using KBr pellets as the sample matrix in the frequency range of 1000‒4000 cm-1 and the number of scans was 32 times/s. The thermogravimetric (TG) curves were measured at standard atmospheric pressure air with N2 as the shielding gas using a NETZSCH TG209 F3 thermogravimetric analyzer from Germany.

3. Results and discussion

3.1. Morphologies of obtained CMSs

In all cases, carbon microspheres were successfully prepared from D-xylose by the hydrothermal reaction under different reaction conditions except CMSs-8, 9. The SEM images in Fig. 2 show the morphologies of CMSs prepared under different conditions. The diameter size distribution and the sphericity product yield of as prepared carbon spheres are listed in Table 1. The dates were roughly measured and calculated according to the scale bar and occupied area in the SEM images, respectively. Results show that the diameter of carbon microspheres was decreased with the increase of stirring speed. In addition, when the stirring speed was increased, the yield of carbon spheres showed an obvious decrease. In short, highly dispersed smooth surface carbon spheres (CMSs-1, CMSs-2, CMSs-3) with uniform size distribution and very few pores were obtained without stirring. When the stirring speed increased to 400 rpm, amorphous porous carbon materials (CMSs-7, CMSs-8, CMSs-9) was obtained instead of spherical carbon spheres. Surprisingly, when the stirring speed was 200 rpm, mesoporous carbon microspheres (CMSs-5) with uniform size distribution of 1-1.4 μm were successfully synthesized at 220 °C for 24 h. The pore size distribution and N2 adsorption desorption isotherm of CMSs-5 are shown in Fig. 3. The average pore size was 2.593 nm and specific surface area was 452 m2/g. The pores were mainly mesoporous according to the type of the hysteresis loop. In addition, the detailed view of CMSs-5 in SEM and TEM images is also shown in Fig. 3. The remaining samples were not further characterized by TEM and nitrogen adsorption and desorption test since no sphericity products with obvious porous structure were found in SEM images. However, the porous structure of CMSs-5 was not clearly reflected in the TEM image because of the relatively big size of the carbon spheres.

Fig. 2.   SEM images of CMSs prepared under different conditions. The CMSs-1, 2, 3 were prepared from xylose without stirring; the CMSs-4, 5, 6 with 200 rpm stirring; the CMSs-7, 8, 9 with 400 rpm string, the insets of CMSs 1, 2, 5, 8 are detailed view.

Fig. 3.   The N2 adsorption desorption isotherms, pore size distribution curve, detailed view of CMSs-5 in SEM and TEM images.

As is generally known, the influence of reaction temperature and time on morphology of prepared carbon materials has been reported in many literatures. However, there is no obvious regularity effect of reaction temperature (180 °C, 220 °C and 260 °C) and time (12 h, 18 h and 24 h) on the morphology of prepared carbon materials in this work. It seems that the stirring speed has a strong effect on the morphology of CMSs. Without stirring, dehydrated polymerization product of fructose was tightly assembled with F127 and wrapped the F127 micelle, which leads to the nonporous carbon spheres. However, the self-assembly process between xylose and F127 was disorganized entirely under the high stirring speed, so extensive amorphous porous carbon was generated instead of carbon spheres. Under a moderated stirring speed, the self-assembly process between xylose and F127 was disturb in a certain extent by the high-speed flow of aqueous solutions, which resulted in a loose spherical composite, and after carbonization porous carbon spheres were obtained. Therefore, an appropriate stirring speed was beneficial to the formation of porous carbon spheres with narrow size distribution and uniform pores size.

3.2. Chemical structure of the gels

According to previous researches, F127 can be dissolved in deionized water and assembled with the d-xylose dehydration carbonized product to fabricate a gel composite during the hydrothermal treatment process, which contains a lot of hydroxyl. As shown in Fig. 1, d-xylose firstly lost water through intermolecular condensation reaction and turn into furfural [22,28]. Then furfural polymerized into benzene ring structure and fabricated into gel with F127. Finally, the oxygen and protium of self-assembly gel were removed by high temperature thermal treatment under nitrogen atmosphere. The functional groups of the resulting gels were characterized by FT-IR, and Fig. 4 shows the FT-IR spectra. The spectra show several characteristic peaks at 3570‒3050 cm-1, 2960‒2888 cm-1, 1700 cm-1, 1608 cm-1, 1434 cm-1, 1286 cm-1, 1022 cm-1 and 756 cm-1, respectively [23]. The peaks at 3570‒3050 cm -1 were due to the O—H stretching of d-xylose and its dehydration product, peaks at 2960‒2888 cm-1 may attribute to the—CH3, —CH2 asymmetrical stretching of F127. Peaks at 1700 cm-1 means that C═O group were formed in the process of hydrothermal carbonization, which may be a carboxyl group or a carbonyl group generated during dehydration of the hydroxyl group in d-xylose. The peaks at 1610 cm-1 indicated C═C stretching of aromatic and furan rings due to the dehydration condensation of carbon precursors (d-xylose). Peaks at 1286 cm-1 and 1022 cm-1 were due to the C—O—C symmetric stretching of aromatic esters or lactone. The single substitution of aromatic peaks at 756 cm-1 was found in the FT-IR spectrum, which were formed from d-xylose during the hydrothermal carbonization.

Fig. 4.   FT-IR spectra of the gels.

3.3. Carbonization behavior of the gels

The TG curves of F127, d-xylose and the self-assembly gels were shown in Fig. 5. As shown in Fig. 5, the main weight loss temperature ranges of d-xylose and F127 were about 185-350 °C and 350-427 °C, respectively, and F127 was decomposed completely with no residue. The weight loss of prepared gels mainly occurred between about 185-350 °C and 350-600 °C with different rates, respectively. The weight loss between 185-350 °C of gels was probably due to the unreacted d-xylose and moisture, which was in accordance with the TG curve of d-xylose and FT-IR result. The more obvious weight loss between 350-600 °C was mainly because of the decomposition of F127 (350-427 °C) and both of further polycondensation and the removal of oxygen-containing functional groups of gels (427-600 °C), which was also supported by the TG curve of F127 and FT-IR result. Besides, the weight loss rate of as-prepared gels was slowly under 350 °C, which indicated that the gels were relatively stable below 350 °C. In addition, the residual mass of gels after heat treatment were about 52.1-61.4 wt% except CMSs-7. Fig. 5 shows that CMSs-7 lost greater weight than other gels, probably because of the incomplete polymerization reaction of d-xylose in relatively low temperature (180 °C) and high stirring speed (400 rpm).

Fig. 5.   TG curves of F127, D-xylose and the self-assembly gels (10 °C/min to 800 °C).

3.4. Structural analysis of obtained MCMs

Fig. 6 shows the XRD patterns of all obtained carbon microspheres. Obviously, there are two relatively broad peaks around 24° and 43° in the pattern of each sample. The diffraction peak around the angle (2θ) of 23°‒25° indexed to (002) plane diffractions of graphitic carbon [29,30], while the board peak indicated a relatively low graphitization degree [24]. The diffraction peaks situated at about 43° were attributed to the interlayer reflection of (100) plane of hexagonal graphite [31,32]. In short, the results show that all the obtained CMSs were partial graphitized.

Fig. 6.   XRD patterns of the CMSs.

4. Conclusion

In summary, porous carbon spheres CMSs-1-CMSs-9 derived from d-xylose with different yields of sphericity have been prepared through hydrothermal method with stirring. Among these samples, the CMSs-5 (200 rpm, 220 °C and 24 h) with a uniform size distribution between 1-1.4 μm and specific surface area of 452 m2/g, had uniform holes, which are shown in SEM images and BET results. Mechanical stirring was innovatively used during hydrothermal reaction processes in this work. Surprisingly, the results show that the stirring speed has an obvious effect on the morphology and structure of carbonized products. The obtained carbon microspheres had principally non-porous structure and very smooth surface without stirring. When the stirring speed increased to 200 rpm, mesoporous carbon microspheres (CMSs-5) were obtained. Nevertheless, the obtained porous carbon materials were mostly amorphous under 400 rpm stirring, and had a low degree of sphericity. Furthermore, with the rise of stirring speed, the size of carbon microspheres was reduced, and the yields of carbon microspheres showed an obvious decrease. Moreover, the prepared carbon materials contained a part of graphite phase. In brief, the controllable and green strategy in this work may provide a great convenience to study properties and applications of carbon microspheres. In order to further improve the physical and chemical properties, more research will be carried out for precise control of morphology, size and pore size of carbon microspheres in our following work.

Acknowledgements

This work was supported financially by the Outstanding Youth Science Fund of Shaanxi Province (No. 2018JC-028), the fund of Beijing Key Laboratory of Quality Evaluation Technology for Hygiene and Safety of Plastics, Beijing Technology and Business University (No. 51772243), the Innovation Team Plan of Shaanxi Province (No. 2017KCT-17) and the National Natural Science Foundation of China (No. 51772243).


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