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ARTICLES Hydrothermal Syntheses of Colloidal Carbon Spheres from Cyclodextrins Yongsoon Shin,* Li-Qiong Wang, In-Tae Bae, Bruce W. Arey, and Gregory J. Exarhos Pacific Northwest National Laboratory, 902 Battelle BlVd, P.O. Box 999, MSIN K2-44, Richland, Washington 99354 ReceiVed: February 14, 2008; ReVised Manuscript ReceiVed: July 23, 2008
Colloidal carbon spheres have been prepared from aqueous R-, β-, and γ-cyclodextrin (CD) solutions in closed systems under hydrothermal conditions at 160 °C. Both liquid and solid-state 13C NMR spectra taken for samples at different reaction times have been used to monitor the dehydration and carbonization pathways. CD slowly hydrolyzes to glucose and forms 5-hydroxymethyl furfural (HMF) followed by carbonization into colloidal carbon spheres. The isolated carbon spheres are 70-150 nm in diameter, exhibit a core-shell structure, and are comprised of a condensed core (CdC) peppered with resident chemical functionalities including carboxylate and hydroxyl groups. Evidence from 13C solid-state NMR and FT-IR spectra reveal that the evolving carbon spheres show a gradual increase in the amount of aromatic carbon as a function of reaction time and that the carbon spheres generated from γ-CD contain significantly higher aromatic carbon than those derived from R- and β-CD. Introduction Synthesis of nanostructured materials with tunable size and shape has become a critical issue for specific applications in drug delivery,1 manipulation of light (photonic band gap crystals),2 and contaminant sequestration.3 Among them, porous carbon spheres have attracted much attention owing to tenability of particle size and shape and resident porosity that promotes diffusion of guest molecules through interconnected micropores.4 The chief obstacles in regard to high temperature processed (800-1000 °C) carbon spheres formed by polymerization and carbonization are aggregation of the carbon spheres5 and their relatively chemically inert surfaces, which require harsh chemical treatment for further modification.6 In particular, nanosized carbon precursors exhibit a very strong tendency toward aggregation during the carbonization process, which makes it difficult to prepare well-dispersed carbon nanospheres. A “green” synthetic approach has been developed that involves the transformation of sugars such as glucose and fructose into homogeneous carbon spheres by inter- or intramolecular dehydration and subsequent sequestering in aqueous solutions.7,8 The surface of carbon spheres prepared by this method contains hydrophilic functionalities including -OH and -CdO groups. Such stable colloidal surfaces have also been utilized as templates to grow hollow metal and metal oxides shell structures.9-11 Our previous work involved mechanistic studies designed to probe the dehydration dynamics of glucose and fructose in water using magnetic resonance and in situ light scattering approaches, and to understand at the molecular level the key processes associated with the evolution of carbon spheres from heated carbohydrate solutions under pressure.12 We found that when an aqueous fructose solution was heated in a closed system at * Corresponding author. Fax: (509) 375-2693. E-mail: yongsoon.shin@ pnl.gov.
temperatures between 120 and 140 °C, 5-hydroxymethyl-2furaldehyde (HMF)13-15 was formed by an intramolecular dehydration reaction. Microscopic carbon spheres spontaneously assembled through continued dehydration (polymerization). Subsequent loss of water by these assemblies led to further coalescence of microscopic spheres to larger spheres thereby generating a grain-like surface morphology with attendant interconnected porosity. For reactions involving glucose, it was difficult to detect HMF formation during the initial hydrothermal treatment even at a higher reaction temperature (180 °C), suggesting that carbon spheres were more likely formed via an intermolecular dehydration route followed by carbonization.12 Cyclodextrins (CD) are cyclic oligosaccharides of R-Dglucopyranose that are brewed by the cyclomaltodextrin glucanotransferase (E.C. 2.4.1.19; CGTase) catalyzed degradation of starch. The common CDs used in chromatography are the R-, β-, and γ-CDs which have been shown to contain 6 (cyclohexamylose), 7 (cycloheptamylose), and 8 (cyclooctamylose) glucose units, respectively. These cyclic, chiral, torus shaped macromolecules contain D(+)-glucose residues bonded through R-(1-4) glycosidic linkages. They are among the most popular phases for accomplishing chiral separations in HPLC. The cavities have different diameters dependent on the number of glucose units: 5.6 Å for R-CD, 7.0 Å for β-CD, and 8.8 Å for γ-CD. CD rings are amphiphilic with the wider rim displaying 2- and 3-OH groups and the narrower rim displaying 6-OH groups on its flexible arm. These hydrophilic groups appear on the outside of the hydrophobic molecular cavity that is lined with ether-like anomeric oxygen and hydrogen atoms. CDs have been used to increase the water solubility of hydrophilic compounds or minimize undesirable properties including odor and taste in certain food additives.16 The exceptional low water solubility (1.8%) of β-CD, compared to R- (13.0%) and γ-CDs (24.9%), is still puzzling.17 In this report, we present the synthesis of colloidal carbon spheres from
10.1021/jp801343y CCC: $40.75 2008 American Chemical Society Published on Web 08/26/2008
Hydrothermal Syntheses of Colloidal Carbon Spheres
Figure 1. standard.
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C NMR spectra for aqueous R-CD solutions quenched during the hydrothermal reaction. An asterisk (/) indicates CH3OD internal
aqueous CD solutions in an autoclave. 13C NMR and FT-IR have been used to investigate the mechanistic pathway that converts CDs to colloidal carbon spheres. Electron imaging studies (SEM and TEM) are used to characterize surface morphologies and dimensions of carbon spheres isolated from these aqueous reaction mixtures. Experimental Methods A 2.0 g sample of each CD was dissolved in 20 mL of water in separate vessels (at this point solubility of the CD is not important). The solution mixtures were transferred to 50 mL sealed chambers and heated to 160 °C without agitation for 16 h. After cooling to room temperature, solutions were centrifuged at 10 000 rpm for 12 min (eppendorf centrifuge) and dried under vacuum overnight to isolate the colloidal carbon. 13C NMR spectra were collected by using two Chemagenetics spectrometers (500 MHz for solution and 300 MHz for solid). Timedependent measurements were obtained on samples prepared by the same hydrothermal procedure (in an autoclave at 160 °C) for specific reaction times (2, 4, 8, 12, 16 h). The reactions were quenched by slow cooling to room temperature. For the 13C solution NMR spectra, CH OD (or D O) was used as an 3 2 internal standard, and for the 13C solid-state cross-polarization (CP) magic angle spinning (MAS) NMR spectra, samples were loaded into 5.0 mm zirconia PENCIL rotors and spun at 3-4 kHz. All spectra were recorded at ambient temperature. Single pulse excitation (SPE) was used for quantitative analysis. The spectra were quantified by summing signals for aromatic and aliphatic carbons after baseline correction. Uncertainties in area measurements were e5.0%. Dried carbon spheres were characterized by using Field Emission Scanning Electron Microscopy (FE-SEM) JEOL-JSEM 633F and Transmission Electron Microscopy (TEM) JEOL JEM 2010 operating at 200 kV. Fourier-Transform Infrared (FT-IR) spectra of the resulting carbon sphere solids were recorded on a Nexus 670 FT-IR
spectrophotometer. KBr was used to prepare clear and transparent sample disks. The spectra were acquired at 4 cm-1 resolution, and 64 scans were averaged to reduce noise. Three spectra were run on each sample. To quantify the amounts of CdC and CsC groups of the isolated carbon sphere products, the integrated band areas of absorbances at 1650-1590 and 1270-1190 cm-1 were computed by measuring the lengths of lines drawn from the baseline to the maximum of each band. Linearly corrected baselines were drawn from 2000 to 900 cm-1. The standard deviation between replicate FT-IR analyses was (2.8%. Results and Discussion In our experiment, it was required to have 10-15% of aqueous CD solution to isolate a sufficient amount of colloidal carbon spheres following hydrothermal treatment at 160 °C for 16 h. The high temperature treatment and internal pressure in these closed systems helped to overcome the low solubility of β-CD. Time-dependent carbonization was followed for the aqueous R-CD solution (10%). After 6 h, R-CD solution showed a light brown color and a pH of 2.7, which suggests the formation levulinic acid (4-oxopentanoic acid) from the glucose unit of R-CD.18 In fact, levulinic acid is generated by rehydration of HMF and further catalyzes the dehydration of glucose or fructose. High-resolution 13C NMR spectra of aqueous R-CD solutions are used to probe the reaction pathway (hydrolysis and dehydration) of R-CD (Figure 1). R-CD (61.5, 74.8, 75.1, 77.1, 83.2, and 105.2 ppm) is stable at the reaction temperature for at least 2 h. Glucose is generated by the hydrolysis of R-CD over 4 h. After a reaction time of 8 h, the 13C NMR spectrum confirmed the existence of HMF (58.0, 112.8, 128.3, 152.9, 163.2, and 181.9 ppm) and complete hydrolysis of R-CD to glucose (61.9, 72.4, 73.0, 74.8, 75.4, 77.8, 93.0, and 97.9 ppm).19 The longer reaction time (16 h) produces collectable carbon spheres from centrifugation at 10 000 rpm and the supernatant
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Figure 2. SEM images of carbon spheres prepared from different CD solutions at 160 °C for 16 h: (a) R-CD, (b) β-CD, and (c) γ-CD.
Figure 3. Bright-field TEM images of carbon spheres prepared from different CD solutions at 160 °C for 16 h: (a) R-CD, (b) β-CD, and (c) γ-CD. The inset in panel b indicates the core-shell structure of a carbon sphere.
solution contains mixtures of glucose, HMF, and levulinic acid. For glucose dehydration, HMF was observed in 13C NMR spectra after a 2 h reaction time (see the Supporting Information, S1). Continuous generation of HMF and small amounts of levulinic acid (28.6, 30.3, 38.6, and 178.4 ppm) were observed after a 12 h reaction time. Figure 2 shows FESEM images of the final products isolated from the CD reaction mixtures prepared at 160 °C and 4 atm of pressure for 16 h followed by centrifugation at 10 000 rpm. They show homogeneous carbon spheres with a very narrow size distribution (100-200 nm in diameter). Specially, carbon spheres prepared from γ-CD solution showed a monodisperse size distribution with an average dimension of 100 nm. In our previous report, we observed that higher reaction temperatures produced more homogeneous sample sizes than found at lower temperatures; the carbon sphere diameter is heavily influenced by reaction time and precursor concentration as well as the reaction temperature.11 In this study, about 10 wt % of CD in water is necessary in order to collect carbon colloids at 160 °C for 16 h. A lower CD concentration made particle collection difficult upon centrifugation (10 000 rpm) after thermal treatment, and more concentrated CD solutions or longer reaction times produced highly aggregated carbon spheres. Detailed TEM images of colloidal carbon spheres are shown in Figure 3. The isolated carbon spheres exhibit a narrow size distribution (70-150 nm in diameter) and the carbon spheres prepared from β- and γ-CD showed a core-shell structure comprised of a relatively dense hydrophobic core and a hydrophilic shell (Figure 3b,c). The carbon surface appears smooth, and results from continuous intermolecular dehydration of glucose followed by carbonization. Our previous paper suggested that an aqueous glucose solution forms colloidal carbon spheres with smooth carbon surfaces via a dominant intermolecular dehydration route
followed by carbonization, while an aqueous fructose solution initially forms HMF by intramolecular dehydration followed by polymerization of microscopic carbon-containing spheres that then assemble to larger spheres, thereby generating a grainlike surface morphology.11 13C NMR peak intensities and areas are usually not directly proportional to the number of carbon nuclei giving rise to the signal because of variations in spin-lattice relaxation times and nuclear Overhauser enhancements.20 For quantitative analysis, attempts have been made to suppress such variations through use of spin-relaxation reagents and 1H decoupling routines.21 Quantitative results by 13C NMR should be applicable to many complex mixtures as well as to composition analysis of a compound.22 13C solid-state NMR spectra for carbon spheres prepared from all CD solutions exhibit several distinct peaks in the aliphatic (0-90 ppm), aromatic (91-170 ppm), and carboxyl (171-220 ppm) regions (Figure 4). The relative ratio of peak resonances can be used to examine product compositions.23 The relative ratios of peak areas for aromatic carbon to aliphatic carbon for carbon sphere products are 1.40 (R-CD), 1.40 (β-CD), and 2.80 (γ-CD), respectively (Table 1). Interestingly, carbon spheres prepared from γ-CD solution contain twice as much aromatic carbon as those prepared from R- and β-CD solutions. A dramatic increase of the ratio in γ-CD may result from rapid hydrolysis (due to its higher solubility in water) to glucose followed by dehydration over the same reaction time (16 h). FT-IR spectra of carbon spheres show that they contain two prominent features assigned to the broad O-H absorption in the region from 3700 to 2800 cm-1 and the carbon stretching absorption at 1707 cm-1 (Figure 5). In addition, the presence of aliphatic hydrocarbon (-CsH) (2965 and 2930 cm-1), CdC (1605 cm-1), C-C (1230 cm-1), and C-O groups (1030 cm-1)
Hydrothermal Syntheses of Colloidal Carbon Spheres
J. Phys. Chem. C, Vol. 112, No. 37, 2008 14239 SCHEME 1: The Hydrolysis, Dehydration, and Carbonization Process of Cyclodextrins under Hydrothermal Condition
Figure 4. 13C solid-state NMR spectra of carbon spheres prepared from different CD solutions at 160 °C for 16 h: (a) R-CD, (b) β-CD, and (c) γ-CD.
TABLE 1: The Integration Ratios for CdC vs. C-C of Carbon Spheres Isolated from Hydrothermally Treated r-, β-, and γ-CD in FT-IR and 13C SS NMR Spectra 13
R-CD β-CD γ-CD
C SS NMRa 1.40 1.40 2.80
FT-IRb 1.87 1.90 3.80
a CdC (91-170 ppm) vs. C-C (0-90 ppm). b CdC (1590-1650 cm-1) vs. C-C (1190-1270 cm-1).
peak ratios for CdC vs CsC for carbon spheres isolated from the aqueous R-CD solution have also been investigated (see the Supporting Information, S2). The carbon spheres could not be collected at treatment times less than 8 h. The carbon product showed an aromatic/aliphatic ratio of 0.90 after 8 h of treatment and 0.95 after 12 h. However, after 16 h the product showed a ratio of 1.87 due to the dramatic decrease in aliphatic carbon. Conclusions From the results described above, we conclude that cyclodextrin hydrolyzes into glucose followed by dehydration into HMF and transformation into porous carbon spheres under hydrothermal conditions (Scheme 1). High-resolution liquidstate 13C NMR spectroscopy showed progressive structural changes for carbon spheres prepared at different reaction times. At the beginning of the hydrothermal treatment, the slow hydrolysis step, which requires 4-8 h at 160 °C, leads to smaller microscopic carbon spheres (70-150 nm) than that (g300 nm) seen from glucose. 13C SS CP/MAS NMR and FT-IR spectroscopy show that the highly water-soluble γ-CD forms uniform colloidal carbon spheres containing a higher ratio of aromatic carbon than those prepared from R- and β-CD solutions. This work provides insight for preparing highly porous carbon spheres from networked sugars in a simple hydrothermal treatment. These microscopic carbon sphere materials that are easily prepared and that contain resident chemical functionality are very attractive for applications involving sequestration of water contaminants and as catalyst support materials.
Figure 5. FT-IR spectra of carbon sphere samples prepared from different CD solutions: (a) R-CD, (b) β-CD, and (c) γ-CD. The arrows indicate CsC and CdC bands.
Acknowledgment. This work supported by the Office of Basic Energy Sciences, Division of Materials Science and Engineering Physics, U.S. Department of Energy, under contract DE-AC06-76RL0 1830 with the Battelle Memorial Institute.
suggests that these carbon spheres contain an aromatic core and resident functionalities in the shell. The C-C peak is shifted from 1154 cm-1 (pure R-CD) to 1230 cm-1 in the carbon sphere product.24 The relative intensities of CdC (1650-1590 cm-1) and CsC bands (1270-1190 cm-1) for each of the isolated carbon spheres were calculated.25,26 The final carbon products prepared from aqueous R-, β-, and γ-CD solutions showed intensity ratios of 1.87, 1.90, and 3.80 (CdC/CsC), respectively. This result suggests that highly soluble γ-CD hydrolyzes to glucose followed by carbonizing into sphere form. This result is in agreement with that of previous 13C solid-state NMR spectra of carbon sphere products.12 The time-dependent relative
Supporting Information Available: Time-dependent 13C NMR spectra of aqueous glucose solution of aqueous R-CD solution, and time-dependent FT-IR spectra of carbon sphere products isolated from aqueous R-CD solution. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhu, H.; McShane, M. J. J. Am. Chem. Soc. 2005, 127, 13448– 13449. (2) Kubo, S.; Gu, Z.-Z.; Takahashi, K.; Fujishima, A.; Segawa, H.; Sato, O. J. Am. Chem. Soc. 2004, 126, 8314–8319. (3) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923–926.
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Shin et al. (17) Sabadini, E.; Cosgrovea, T.; do Carmo Egı´dio, F. Carbohydr. Res. 2006, 341, 270–274. (18) Kuster, B. F. M.; van der Baan, H. S. Carbohydr. Res. 1977, 54, 165–176. (19) Bagno, A.; Rastrelli, F.; Saielli, G. J. Org. Chem. 2007, 72, 7373– 7381. (20) Levy, G. C.; Lichter, R. L.; Nelson, G. L. Carbon-13 NMR Spectroscopy; Wiley: New York, 1980; pp 36-44. (21) Shoolery, J. N. Prog. Nucl. Magn. Reson. Spectrosc. 1977, 11, 79– 93. (22) Islas, C. A.; Suelves, I.; Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Fuel 2003, 82, 1813–1823. (23) Chang, J. H.; Wang, L.-Q.; Shin, Y.; Jeong, B.; Birnbaum, J. C.; Exarhos, G. J. AdV. Mater. 2002, 14, 378–382. (24) Available at http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_ top.cgi. (25) Parrish, J. R. Anal. Chem. 1975, 47, 1999–2003. (26) Celi, L.; Schnitzer, M.; Ne´gre, M. Soil Sci. 1997, 162, 189–197.
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