Preparation of Ultrafine Carbon Spheres by Controlled Polymerization

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Preparation of Ultrafine Carbon Spheres by Controlled Polymerization of Furfuryl Alcohol in Microdroplets Minhua Ju,† Changfeng Zeng,‡ Chongqing Wang,† and Lixiong Zhang*,† †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, No 5 Xin Mofan Rd., Nanjing 210009, P. R. China ‡ College of Mechanic and Power Engineering, Nanjing University of Technology, No 5 Xin Mofan Rd., Nanjing 210009, P. R. China ABSTRACT: Carbon microspheres with mean particle sizes of 0.7−1.2 μm were prepared by polymerization of furfuryl alcohol (FA) in H2SO4 solution droplets without any surfactant, followed by pyrolysis. FA was introduced by diffusion from FAcontaining oil phase into the droplets formed in an interdigital micromixer. The oil to aqueous phase flow rate ratio and the FA and H2SO4 concentrations should be kept at medium values. A decrease in the total flow rate ratio, oil to aqueous phase flow rate ratio, or FA concentration results in small-sized carbon microspheres, and a decrease in the FA or H2SO4 concentration leads to formation of the carbon microspheres exhibiting rough surface. The carbon microspheres show good dispersity in ethanol and microporosity. Their particle sizes could be further reduced to 150 nm by using polyvinyl pyrrolidone, and their mesoporosity can easily be endowed by adding silica nanoparticles in the droplets.

1. INTRODUCTION Carbon microspheres have attracted a lot of attention because of their wide applications in many fields, such as adsorbents, catalyst supports, electrodes for lithium ion batteries, and so on.1−5 There are a couple of methods for preparing carbon microspheres, including hydrothermal carbonization of carbohydrate sources such as glucose,6−8 sucrose,9 fructose,10 cellulose,11 and cyclodextrin;12 chemical vapor deposition of hydrocarbons such as ethylene,13 toluene,14 mesitylene,15 and deoiled asphalt16 on substrates; ultrasonic spray pyrolysis;17 and pyrolysis of carbon-rich polymer microspheres formed by emulsion, templating method, self-assembly, and Stö ber method.18−25 The organic precursors for formation of polymeric microspheres are abundant, and resorcinol-formaldehyde,18,19,23 melamineformaldehyde,25 and furfuryl alcohol (FA)24 are commonly used. Among them, FA, which can be easily polymerized to poly(furfuryl alcohol) (PFA) by acidic catalysts, is known to be a high-carbon-yield carbon source, and is widely used in the preparation of carbon membranes26 and bulk carbon.27,28 However, there have been limited reports involving preparation of carbon microspheres from FA. The reason is believed to be that the formation of both the spheres and PFA matrix takes place almost simultaneously as the polymerization process of FA in a batch reactor is difficult to be well controlled. To avoid formation of the PFA matrix, careful control of the polymerization is needed and surfactants acting as the template to assist the formation of spheres are used.24,29 Consequently, a two-step emulsion polymerization of FA to synthesize dispersive colloidal microporous carbon spheres was developed.24 In the first step, the slow polymerization of FA was carried out in an aqueous solution containing FA, the surfactant (F127), ethanol, and a small amount of the HCl catalyst, leading to preferential formation of PFA nuclei by prepolymerization of FA. In the second step, a high concentration of H2SO4 was added into the above PFA solution to accelerate growth of spherical PFA from PFA nuclei © 2014 American Chemical Society

accompanied by solvent evaporation. Obviously, complicated preparation steps and surfactants have to be applied. Thus, the preparation of carbon microspheres from FA by simply acidic catalysts remains a challenge. Recently, we have developed a microfluidic technique to prepare hollow carbon microspheres through the carbonization of hollow PFA microspheres.30 The hollow PFA microspheres were formed in a T−type microfluidic device through interfacial polymerization of FA in continuous oil phase on the outer surface of the aqueous H2SO4 microdroplets. The average particle size (100−300 μm) of these PFA microspheres can be easily controlled by adjusting the flow rate ratios of two phases. Close observation reveals that these microspheres are constructed by incalculable aggregated small microspheres with particle sizes of 0.5−5 μm. The reason for their aggregation is because of the existence of a PFA matrix, which comes from the excess polymerization of FA.31 During the preparation, the flow rate ratios of oil to water phases used ranged from 100−300, suggesting that the amount of FA monomer surrounding the H2SO4 microdroplets was almost constant. Under such a circumstance, the PFA matrix is preferentially formed, and dispersed PFA microspheres are difficult to be produced. It is reasonable to postulate that formation of a PFA matrix can be avoided when the content of FA around the microdroplets continuously decreases during the polymerization process. Therefore, in this paper, we first examine the effect of the oil to aqueous phase flow rate ratio on the formation of PFA microspheres so that we can prepare dispersed spheres with particle sizes of around 1 μm using FA and H2SO4 in a microfluidic device. We also examine the effect of other preparation parameters, such as the acid concentration, Received: Revised: Accepted: Published: 3084

September 11, 2013 December 29, 2013 February 7, 2014 February 7, 2014 dx.doi.org/10.1021/ie4029939 | Ind. Eng. Chem. Res. 2014, 53, 3084−3090

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Figure 1. (a) Experimental setup for synthesis of PFA spheres, (b) interdigital micromixer in a metal frame, (c) the glass chip, and (d) the structure illustration of microchannels in the interdigital micromixer.

Table 1. Preparation Parameters and the Morphologies and Particle Sizes of the Resultant Carbon Products flow rate (mL/h) sample

oil phase

water phase

FA content (wt %)

H2SO4 content (mol/L)

spherical morphology

1 2 3 4 5 6 7 8 9 10a

5 10 10 10 5 5 5 7.5 2.5 5

5 10 10 10 5 5 5 2.5 7.5 5

5 5 2 12 5 5 5 5 5 5

8 8 8 8 5 6 10 8 8 8

yes yes yes yes, aggregated yes, aggregated fair yes, aggregated yes yes yes

surface roughness smooth bumpy golf-like

mean particle size (μm) 0.9 0.8 0.7

concave−convex smooth smooth smooth

1.2 0.9 0.16

a

The experiment was conducted by adding 2 wt % PVP in the aqueous solution with the other conditions being the same as those for preparing sample 1.

(750 mm height × 20 mm ID). The interdigital micromixer consisted of a lead frame and a glass chip which has 30 identical parallel inlet geometries (50 μm width, 150 μm height) and a rectangular mixing chamber (3 mm width, 150 μm height), as shown in Figure 1b−d. Both the inlet and outlet of the micromixer were connected with poly(tetrafluoroethylene) (PTFE) tubes (1.0 mm ID), with the length of the one connected to the outlet of 3.0 cm. The glass sedimentation column installed underneath the outlet of the PTFE tube was assembled with a condenser and a single-neck round-bottom flask and filled with FAME. In a typical synthesis of the PFA spheres, a continuous oil phase containing 5 wt % of FA, 5 wt % of Span-80 and 90 wt % of FAME and a dispersed phase containing 8 M H2SO4 aqueous solution were pumped into the interdigital micromixer through two syringe pumps. The aqueous solution substreams were first constricted and then decayed into microdroplets surrounded by the oil phase in the mixing chamber.33 The microdroplets flowed with the oil phase in the mixing chamber, while FA in the oil phase would diffuse

the FA content, and the polymerization temperature to investigate the formation of PFA microspheres vs PFA matrix. Carbonization of these PFA spheres results in the formation of ultrafine carbon spheres. Further decrease in the size of the carbon microspheres to nanometer range was explored.

2. EXPERIMENTAL SECTION 2.1. Materials. FA used as carbon source, sulfuric acid (H2SO4, 98 wt %) used as catalyst for polymerization of FA, polyvinyl pyrrolidone (PVP, K35), and Span 80 were purchased from Shanghai Chemical Reagent Corporation (Shanghai, China) with chemically purity. Fatty acid methyl ester (FAME) was produced from cottonseed oil in a microreactor by transesterification with methanol at the catalysis of KOH.32 2.2. Preparation of the Carbon Microspheres. The ultrafine carbon microspheres were prepared by carbonization of the PFA microspheres synthesized in the setup shown in Figure 1. The setup mainly consisted of an interdigital micromixer (mikroglass) and a glass sedimentation column 3085

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into the microdroplets, with subsequent occurrence of polymerization of FA to PFA microspheres. Then the mixture in the mixing chamber was dropped into the sedimentation column that was heated by circulating hot water to 60 °C. The PFA microspheres sedimented at the bottom of the flask and were then transferred into a polypropylene beaker by solid− liquid separation using a separating funnel. They were heated at 100 °C for 6 h, allowing the water to evaporate to obtain a concentrated H2SO4 solution before rinsing and centrifuging with ethanol and deionized water for three cycles. This is essential for the preparation of carbon microspheres because the concentrated H2SO4 solution serves as a good dehydration agent, which is effective for eliminating the surface reactivity of PFA microspheres, thus avoiding the aggregation during the carbonization process.24,34 Finally, the as-synthesized PFA microspheres were transferred in a quartz boat and were carbonized in a tube furnace in N2 with a N2 flow rate of 12 mL/min at 550 °C for 5 h with a heat rate of 1 °C/min. Low carbonization temperature was chosen as we would like to prepare metal nanoparticles/carbon composite materials later to avoid growth of metal particles at higher temperatures.35 The microspheres were mechanically stable after carbonization. Table 1 lists the preparation conditions and the morphologies and particle sizes of the products. 2.3. Characterizations. Scanning electron microscope (SEM, Philips Quanta 200 and Hitachi−S4800) analyses were used to observe the morphology of microspheres. N 2 adsorption/desorption measurements were performed at 77 K on BELSORP II instrument to probe the pore texture. The sample was outgassed at 200 °C for 6 h before the measurement. The total pore volume (VT) was evaluated at a relative pressure of about 0.99 and the Brunauer-EmmettTeller (BET) surface area (SBET) was calculated from the adsorption branches in the relative pressure range of 0.05−0.25. The micropore size was calculated by the Horvath−Kawazoe method. Photon correlation spectroscopy (PCS) with a Zetasizer 3000 HSa equipment (Malvern Instruments Ltd. U.K.) at room temperature (25 °C) was used to determine the mean particle size and particle size distribution (PSD) of carbon microspheres that were dispersed in ethanol. The wavelength of the internal laser was 633 nm and the selected measurement scattering angle was 90°.

Figure 2. SEM pictures of carbon microspheres prepared from various flow rates and precursor concentrations: (a and b) flow rates of both oil and water phases are 5 mL/h, and the concentrations of FA and H2SO4 are 5 wt % and 8 mol/L, respectively (sample 1); (c and d) flow rates of both oil and water phases are 10 mL/h, and the concentrations of FA and H2SO4 are 5 wt % and 8 mol/L, respectively (sample 2). (e) Particle size distribution of sample 2. The inset in panel e shows the carbon spheres (sample 1 or sample 2) dispersed in ethanol solution for 30 min.

μm for sample 1 and in the range of 0.7−1.1 μm centered at 0.8 μm for sample 2. Close observation of the two samples finds small difference in their surface roughness, with a smoother surface for sample 1 and a bumpy surface for sample 2. These results suggest that increase in the total flow rate produces carbon microspheres with narrower PSD, and smaller particle sizes, but rougher surface. They can be uniformly dispersed in ethanol (inset in Figure 2e) and are stable for at least 0.5 h. 3.2. Effect of the Preparation Parameters. 3.2.1. FA Concentration. We first examined the effect of FA concentration by using 2 and 12 wt % FA and keeping the H2SO4 concentration at 8 mol/L and equal oil and aqueous phase flow rates of 10 mL/h. The obtained carbon microspheres (samples 3 and 4, Table 1) show different surface morphologies, particle sizes, and PSDs, as shown in their SEM pictures (Figure 3a−c) and PSD curve for sample 3 (Figure 3d). Sample 3 exhibits a golf ball like wrinkled surface, with particle sizes ranging from 0.5 to 1.0 μm, centered at 0.7 μm. On the other hand, sample 4 shows agglutinating spherical particles as large as 4.1 μm attached by some particles as small as less than 100 nm, indicating heterogeneous growth of the PFA particles at high FA concentrations. The surfaces of the particles are smooth. Compared with the surface roughness of sample 2, that of sample 3 is much deeper and bumpier. We thus can conclude that, with an increase in the FA concentration, the resulting carbon microspheres change from a golf ball like wrinkle

3. RESULTS AND DISCUSSION 3.1. Preparation of the Carbon Microspheres. In our previous work to prepare the hollow PFA microspheres in a Ttype microfluidic device, the flow rate ratio of the oil phase to aqueous phase is over 100.30 Thus, the concentration of FA surrounding the microdroplets could be considered to be constant. In this preparation, we first used a flow rate ratio of the oil to water phases of 1 so that the concentration of FA surrounding the microdroplets gradually decreases with occurrence of the polymerization. However, it is difficult to form stable microdroplets at such a low flow rate ratio in the traditional T-type micromixer. Therefore, an interdigital micromixer was applied to produce the aqueous microdroplets with total flow rates of 10 and 20 mL/h (Table 1). Figure 2 shows the SEM pictures of the resulting PFA microspheres and corresponding carbon microspheres (samples 1 and 2). Both samples show regular spherical morphology and decreased particle sizes after carbonization. Their corresponding PSD curves also shown in Figure 2 exhibit that these carbon microspheres are in the range of 0.8 to 1.2 μm centered at 0.9 3086

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broad particle size distributions are formed (samples 5 and 7, Figure 4a,c). They stick to each other, forming a monolithic structure. At a H2SO4 concentration of 6 mol/L, nonsticky PFA microspheres can be obtained, with a PSD in the range of 1.1 to 3.7 μm and centered at 2.1 μm (sample 6, Figure 4b,d). The surface is quite rough, with many concave-convexes. In addition, the sample is not as round as sample 1. Thus, the H2SO4 concentration has significant influence on successfully preparation of the microspheres and the suitable H2SO4 concentrations are in the range of 6−8 mol/L. 3.2.3. Flow Rate Ratio. The effect of the oil to aqueous phase flow rate ratio was examined by keeping a constant total flow rate of 10 mL/h and choosing flow rate ratios of 3:1 and 1:3, with the FA content of 5 wt % and the H 2 SO 4 concentration of 8 mol/L. Figure 5 shows SEM pictures of the carbon samples (samples 8 and 9). They both show spherical morphology and smooth surface without adhesion. Figure 5c shows the PSD curves for samples 8 and 9. It can be seen that the particle size is ranging from 0.7 to 2.1 μm centered at 1.2 μm for sample 8 and from 0.6 to 1.0 μm centered at 0.9 μm for sample 9. The PSD of sample 8 is broader than that of sample 9. Figure 5d shows the relationship between particle size and flow rate ratio, which reveals that the particle size increases with the increase in the oil to aqueous phase flow rate ratio. 3.3. Discussion. Carbon microspheres with mean particle sizes of 0.6−1.2 μm were prepared by phase-transfer of FA into aqueous microdroplets and subsequent polymerization without using any surfactant and needing multistep, followed by pyrolysis. Diffusion of FA from the oil phase to the aqueous microdroplet is controlled by the oil to aqueous phase flow rate ratio and the FA concentration, while polymerization of FA is mainly determined by the FA and H2SO4 concentrations. Results presented in sections 3.1 and 3.2 indicate that dispersed microspheres are successfully prepared in the medium-range of the oil to aqueous phase flow rate ratio and the FA and H2SO4 concentrations, which means that massive formation of the PFA matrix can be avoided at these conditions. To better understand the formation process, we collected one droplet at the exit of the PTFE tube connected to the interdigital micromixer on a glass slide in the course of preparation for sample 3 and examined it by SEM after drying (Figure 6). We found that PFA microspheres are formed in a very short time (less than 7 s). They are consisted of many small particles with nonuniform spherical morphology (Figure 6a). After treating this sample in the FAME column, it becomes more round in shape (Figure 6b). Thus, the formation process of PFA microspheres in the microdroplets can be reasonably assumed to follow the following three steps: (1) diffusion of FA from the oil phase into the aqueous phase, (2) generation and precipitation of hydrophobic reactive PFA primary particles through polymerization of FA monomers at the catalysis of H2SO4, and (3) their immediate aggregation to form microspheres. At high FA concentrations, much more FA monomer diffuses into the microdroplets, leading to formation of more primary particles and larger microspheres. On the other hand, more FA monomer in the oil phase may produce a high FA concentration, thus resulting in a disordered reaction system that forms various sized particles or even a PFA matrix. Consequently, the PFA microspheres exhibit a broad particle size distribution and are aggregated (sample 4). At low FA concentrations, less primary particles are formed, which gather together to form smaller microspheres (sample 3). The H2SO4

Figure 3. SEM pictures of carbon microspheres prepared from various flow rates and precursor concentrations: (a and b) flow rates of both oil and water phases are 10 mL/h, and the concentrations of FA and H2SO4 are 2 wt % and 8 mol/L, respectively (sample 3); (c) flow rates of both oil and water phases are 10 mL/h, and the concentrations of FA and H2SO4 are 12 wt % and 8 mol/L, respectively (sample 4). (d) Particle size distribution of sample 3.

surface to a less rough surface and smooth surface, with their PSDs and mean particle sizes becoming broader and narrower. 3.2.2. H2SO4 Concentration. Figure 4 shows SEM pictures of carbon products prepared at H2SO4 concentrations of 5, 6, and 10 mol/L, with the FA content of 5 wt %, the oil to aqueous phase flow rate ratio of 1:1 and a total flow rate of 10 mL/h. At H2SO4 concentrations of 5 and 10 mol/L, many small particles with nonuniform spherical morphology and very

Figure 4. SEM pictures of carbon microspheres prepared from various concentrations of H2SO4 of (a) 5 mol/L (sample 5), (b) 6 mol/L (sample 6), and (c) 10 mol/L (sample 7). The other preparation parameters are same. Both the flow rate of both oil and water phases are 5 mL/h, and the concentration of FA is 5 wt %. (d) PSD curve of the carbon microspheres sample 6. 3087

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Figure 5. SEM pictures of carbon microspheres prepared from different flow rate ratios of oil to water phases: (a) 3:1, i.e., flow rates of oil and water phases are 7.5 and 2.5 mol/h, respectively (sample 8); (b) 1:3, i.e., flow rates of oil and water phases are 2.5 and 7.5 mol/h, respectively (sample 9). In both syntheses, the concentrations of FA and H2SO4 are 5 wt % and 8 mol/L, respectively. (c) PSD curves of these two samples; (d) the mean particle size of the carbon microspheres as a function of flow rate ratios of two phases.

The carbon microspheres prepared at a high total flow rate, low FA concentration, or low H2SO4 concentration (samples 2, 3, and 6) exhibit rough surfaces. This may be ascribed to the production of a small quantity of primary particles, which loosely pack together and finally transform into microspheres with concave−convex surfaces during curing at a high temperature, as illustrated in the top row of Figure 7. On the

Figure 6. SEM pictures of the fresh PFA microspheres (a) and after heat treatment (b).

concentrations mainly determine the polymerization rate and the activity of fresh PFA.29 At high concentrations, a large amount of primary particles are formed because of fast polymerization rates and the activity of fresh PFA is eliminated because of the dehydration effect of the acid,24,29 thus leading to formation of both PFA matrix and many small particles that cannot aggregate to large spheres (sample 7). At low concentrations, the high activity of fresh PFA makes them interact with each other, leading to mainly PFA matrix (sample 5). Only at medium values of the H2SO4 concentrations are PFA microspheres readily formed. The variation of the oil to aqueous phase flow rate ratio affects the droplet size and the FA diffusion rate, thus varying the FA concentration and its gradient in the droplet. At a high oil to aqueous solution flow rate ratio, small-sized aqueous droplets are formed, suggesting large contact surface areas of the droplets with the oil phase. This results in a high FA concentration in the aqueous droplets, thus forming PFA microspheres with large particle sizes and broad PSD (sample 8). The increase in the total flow rate reduces the contact time between the oil phase and the aqueous droplets and thus reduces the FA amount in microdroplets, resulting in formation of the PFA microspheres with small particle size (sample 2).

Figure 7. Schematic illustration of the formation process of carbon microspheres with bumpy and smooth surface.

contrary, formation of the carbon microspheres exhibiting smooth surfaces at a low total flow rate, high FA concentration, or high H2SO4 concentration may result from production of enough primary particles that densely pack together at first, thus forming smooth surfaces after curing (bottom row in Figure 7). To further decrease the particle size of the carbon microspheres, we tried to add 2 wt % PVP in the aqueous solution with the other condition same as that for preparing sample 1. The resultant carbon products (sample 10, Table 1) also exhibit spherical morphologies and smooth surfaces, similar to sample 1, as shown in the SEM image (Figure 8a). However, this sample is much smaller and narrower than 3088

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area, mesopore size, total pore volume and mesoporosity are 1443 m2 g−1, 6.2 nm, 2.43 cm3 g−1, and 94.5%, respectively. Thus, the method developed in this paper also provides a simple route to prepare mesoporous carbon microspheres with ultrafine particle sizes.

4. CONCLUSION Ultrafine carbon microspheres with mean particle sizes of 0.7− 1.2 μm were successfully prepared in an interdigital micromixer. Only FA and H2SO4 that act as the carbon source and the polymerization catalyst, respectively, were used for the preparation, without needing any surfactant and complex multistep. The oil to aqueous phase flow rate ratio, the FA concentration, and the H2SO4 concentration all play an important role in successful formation of the microsphere and determine the particle size and morphology of the resulting carbon microspheres. A decrease in the total flow rate ratio, the oil to aqueous phase flow rate ratio, and the FA concentration results in formation of small microspheres, while a decrease in the FA concentration and the H2SO4 concentration leads to formation of rough microspheres. The mean particle sizes of the carbon microspheres could be adjustable in the range of 0.6−1.2 μm and could be further reduced to 150 nm by using PVP as the surfactant. These carbon microspheres possess microporosity with a mean pore size of ca. 0.6 nm, and their mesoporosity can easily be endowed by simple addition of silica nanoparticles in the preparation process. This work may provide a novel route to prepare dispersed metal nanoparticles/ carbon composite materials with uniform metal content and distribution, thanks to the in situ encapsulation skill of the microfluidic technique.

Figure 8. (a) SEM image and (b) PSD curve of carbon microspheres (sample 10) with 2.0 wt % PVP in water phase. The flow rates of both oil and water phases are 5 mL/h, and the concentration of FA and H2SO4 are 5 wt % and 8 mol/L, respectvely.

sample 1, in terms of the particle size and PSD, exhibiting a mean particle size of 159 nm and a PSD ranging from 93 to 237 nm (Figure 8b). The addition of PVP greatly reduces the particle size, due to decrease in the surface tension of solution.19 Carbon materials obtained from FA are known to be microporous.29 To explore the porous property of the carbon microspheres, we measured the N2 adsorption−desorption isotherms of samples 1 and 2 that show a type I isotherm (Figure 9a), indicating typical micropores of the microspheres.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Figure 9. (a) N2 adsorption−desorption isotherms for the carbon spheres with microporosity (samples 1 and 2). The synthesis conditions for sample 1: The flow rates of both oil and water phases are 5 mL/h, and the concentrations of FA and H2SO4 are 5 wt % and 8 mol/L. respectvely. The synthesis conditions for sample 2: The flow rates of both oil and water phases are 10 mL/h, and the concentrations of FA and H2SO4 are 5 wt % and 8 mol/L. respectvely. (b) N2 adsorption−desorption isotherms for the carbon spheres with mesoporosity prepared by adding 0.8 wt % of silica nanoparticles in the water phase. The other synthesis conditions are similar to those for sample 1.

Notes

The authors declare no competing financial interest. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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ACKNOWLEDGMENTS This work is supported by Natural Science Key Project of the Jiangsu Higher Education Institutions (12KJA530002), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Research and Innovation Program for College Postgraduates of Jiangsu Province (No. CXZZ11_0356).

They exhibit surface areas of about 510 m2 g−1, pore volumes of 0.25 cm3 g−1, and a mean pore diameter of 0.6 nm. The surface areas and pore volumes are larger than those of the carbon spheres synthesized by a two-step polymerization of FA in the presence of a surfactant.24 We could easily prepare the carbon microspheres with abundant mesoporosity by adding silica nanoparticles in the aqueous phase during preparation of the PFA microspheres, followed by treatment of the corresponding carbon microspheres in a HF solution to remove the silica template. Figure 9b shows the N2 adsorption−desorption isotherm of the carbon microspheres prepared by adding 0.8 wt % of silica nanoparticles with a mean particle size of ca.14 nm under the same condition as that for preparing sample 1. The resulting carbon microspheres exhibit a type IV isotherm and an H3 hysteresis loop, suggesting existence of mesopores. The surface

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dx.doi.org/10.1021/ie4029939 | Ind. Eng. Chem. Res. 2014, 53, 3084−3090