J. Phys. Chem. C 2007, 111, 15141-15145
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ARTICLES Hydrothermal Dehydration of Aqueous Fructose Solutions in a Closed System Chunhua Yao,†,‡ Yongsoon Shin,*,† Li-Qiong Wang,† Charles F. Windisch, Jr.,† William D. Samuels,† Bruce W. Arey,† Chongmin Wang,† William M. Risen, Jr.,‡ and Gregory J. Exarhos*,† Pacific Northwest National Laboratory, 902 Battelle BlVd, P.O. Box 999, MS K2-44, Richland, Washington 99354, and Department of Chemistry, Brown UniVersity, ProVidence, Rhode Island 02912 ReceiVed: May 30, 2007; In Final Form: August 21, 2007
Aqueous monosaccharide solutions including glucose or fructose have been hydrothermally treated in a closed system to form colloidal carbon spheres. In-situ Raman and 13C NMR have been used to quantify the intramolecular dehydration moiety, HMF, as an intermediate. An aqueous glucose solution forms a carbon sphere via an intermolecular dehydration route without forming HMF during initial hydrothermal treatment and followed by carbonization at 170-180 °C. However, an aqueous fructose solution initially forms HMF by intramolecular dehydration at 120-140 °C. Upon subsequent polymerization, microscopic carbon-containing spheres assemble to larger spheres, thereby generating a grain-like surface morphology. The carbon sphere contains a dense hydrophobic carbon core and a hydrophilic shell.
1. Introduction The synthesis of materials with targeted size and shape has attracted much attention. Specifically, the ability to synthesize fixed diameter colloidal spheres has opened the door to a variety of applications involving drug delivery,1,2 or manipulation of light (photonic band gap crystals).3 In particular, colloidal carbon spheres are of great interest because the diffusion of guest species through the micropores can be significantly manipulated by changing their particle sizes and shapes.4 Surface modification is a key to realizing many of these applications as the prepared surface is often inert.5 There have been only a few reports regarding colloidal carbon spheres.6 The main concern is the aggregation of carbon nanospheres. Nanosized polymer particles exhibit a strong tendency toward aggregation during carbonization, which makes it difficult to prepare well-dispersed carbon nanospheres.7 The remarkable transformation of carbohydrate molecules including sugars to form homogeneous carbon spheres readily occurs by a dehydration mechanism and subsequent nanoscale sequestering in aqueous solutions when heated at 160-180 °C in a pressurized vessel.6b,8 Under such conditions, these molecules actually dehydrate even though they are dissolved in water. The synthetic “green” approach involves none of the toxic organic solvents, initiators, or surfactants that are commonly used for the preparation of polymer micro- or nanospheres. The surface of colloidal sphere products is hydrophilic and a distribution of -OH and -CdO groups, which makes surface modification unnecessary. Size-tunable metal and metal oxides with uniform shell thickness have also been prepared by using the carbon spheres as templates.9 * Corresponding author. Y.S.: Phone, (509) 375-2693; fax, (509) 3752186; e-mail,
[email protected]. G.J.E.: Phone, (509) 376-4125; fax, (509) 376-5106; e-mail,
[email protected]. † Pacific Northwest National Laboratory. ‡ Brown University.
However, the detailed dehydration mechanism during the colloidal carbon formation of glucose and fructose remains unknown. The goal of this present work is to invoke magnetic resonance and in-situ light scattering approaches to probe the dehydration dynamics, and understand at the molecular level the key processes associated with the evolution of carbon spheres from heated carbohydrate solutions under pressure. This highly regular and porous form of carbon can be isolated with sphere diameters in the range from a hundred nanometers to a few micrometers depending upon the processing conditions. For reactions involving glucose, it was difficult to detect 5-hydroxymethyl-2furaldehyde (HMF)10 formation during initial hydrothermal treatment at 160 °C)
Dehydration of Aqueous Fructose Solutions and pressure 6-7 atm to transform into porous carbon sphere dispersions, while fructose dehydrates in water under 3-4 atm at somewhat lower temperature (120 °C) due to the presence of a more reactive furanose unit in contrast to glucose, where a pyranose group is present. We speculate that glucose loses water first through an intermolecular condensation reaction as a result of its stable pyranose structure when heated under pressure.6 However, fructose initially forms HMF through an intramolecular dehydration process followed by subsequent water loss to form carbon. Further intermolecular dehydration then generates surface roughness (raspberry structure) during carbon sphere formation. Ease of preparation, chemical robustness, and resident hierarchical porosity are attractive for applications ranging from sequestration of contaminants in air or waste streams, as catalyst supports for petrochemical production, and as electrically conducting phases in energy conversion devices including fuel cells. Acknowledgment. This work was 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. Supporting Information Available: TEM image, FT-IR, and Raman spectra of carbon spheres. 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, 1344813449.
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