DOI: 10.1002/chem.200802097

Chemical and Structural Properties of Carbonaceous Products Obtained byHydrothermal Carbonization of Saccharides

Marta Sevilla* and Antonio B. Fuertes[a]

Introduction

When an aqueous solution/dispersion of a saccharide (e.g.,glucose, sucrose, starch, etc.) is heat-treated at a moderatetemperature in the 170–350 8C range (under pressure), acarbon-rich black solid is obtained as insoluble product.This process, which will be termed hydrothermal carboniza-tion, gives rise to other substances besides the solid residue.These include aqueous soluble products (furfural, hydroxy-methylfurfural, acids, and aldehydes) and gases (CO2, CH4,etc.).[1–4] In the present work our primary interest is the car-bonaceous solid product. The first research work on the hy-drothermal carbonization of saccharides was carried outduring the first decades of the 20th century with the aim ofunderstanding the mechanism of coal formation. Thus, in1913 Bergius and Specht subjected cellulose to hydrother-mal carbonization at temperatures in the 250–310 8C range,as a result of which they obtained a black residue with a O/C atomic ratio of 0.1–0.2 (O/C atomic ratio of cellulose:0.84).[5] Later, in 1932, Berl and Schmidt investigated the hy-

drothermal treatment of cellulose over a wider temperaturerange (200–350 8C).[6] In 1960, van Krevelen et al.[7] noticedthat the solid products derived from the hydrothermal treat-ment of the cellulose and glucose have the same composi-tion, which suggests that the hydrolysis products for bothsubstances are similar. In relation to this process van Kreve-len proposed an H/C versus O/C diagram to analyze thechemical transformations that take place during the hydro-thermal carbonization of these substances.[8]

Renewed interest in the hydrothermal carbonization ofsaccharides has recently been established. However, theACHTUNGTRENNUNGobjectives of these new investigations are completely differ-ent to those previously mentioned. Now the main purpose isto use this process as a way to produce carbonaceousACHTUNGTRENNUNGmaterials with specific properties (i.e. , shape, size, chemicalfunctionalities, etc.). In 2001, Wang et al. reported the syn-thesis of carbonaceous microspheres of a tunable size (inthe 0.25–5 mm range) through the hydrothermal carboniza-tion of sucrose at 190 8C.[9] Much attention has also been fo-cused on the hydrothermal carbonization of sugars in thepresence of inorganic salts, which gives rise to the formationof hybrid carbon/metal materials (C/Ag, C/Cu, C/Au, C/Pd,and C/Te), with complex nanoarchitectures.[10–15] In addition,the microspheres resulting from the hydrothermal carboni-zation have been employed as sacrificial templates for fabri-cating hollow spheres of inorganic compounds (Ga2O3, GaN,WO3, SnO2, etc.).[16–21] Recently, Yao et al.[22] investigatedthe mechanism of formation of carbonaceous microspheres

Abstract: A carbon-rich solid product,here denoted as hydrochar, has beensynthesized by the hydrothermal car-bonization of three different saccha-rides (glucose, sucrose, and starch) attemperatures ranging from 170 to240 8C. This material is made up of uni-form spherical micrometer-sized parti-cles that have a diameter in the 0.4–6 mm range, which can be modulatedby modifying the synthesis conditions

(i.e. , the concentration of the aqueoussaccharide solution, the temperature ofthe hydrothermal treatment, the reac-tion time, and type of saccharide). Theformation of the carbon-rich solidthrough the hydrothermal carboniza-

tion of saccharides is the consequenceof dehydration, condensation, or poly-merization and aromatization reactions.The microspheres thus obtained pos-sess, from a chemical point of view, acore–shell structure consisting of ahighly aromatic nucleus (hydrophobic)and a hydrophilic shell containing ahigh concentration of reactive oxygenfunctional groups (i.e., hydroxyl/phe-nolic, carbonyl, or carboxylic).

Keywords: carbohydrates · carbon ·hydrothermal synthesis · micro-spheres · saccharides

[a] Dr. M. Sevilla, Dr. A. B. FuertesDepartamento de Qu�mica de MaterialesInstituto Nacional del Carb�n (CSIC)P.O. Box 73, 33080 Oviedo (Spain)Fax: (+34) 985-29-76-62E-mail : martasev@incar.csic.es

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in the course of the hydrothermal treatment of glucose andfructose at low temperatures (120–160 8C). They concludedthat during hydrothermal treatment, glucose loses waterfirst (T=160 8C) through an intermolecular condensation re-action and that subsequently an aromatization (carboniza-tion) process occurs.

Most of the works recently published in this area havebeen mainly focused on the synthesis of carbonaceous prod-ucts and hybrid carbon/inorganic materials. Surprisinglylittle attention has been paid to the chemical and structuralproperties of such synthesized solid products. Accordingly,the main goal of the present work is to provide a full under-standing of the chemical properties and structural character-istics of carbonaceous microspheres obtained by hydrother-mal carbonization of saccharides. For this purpose we sub-jected different saccharides (glucose, sucrose, and starch) tohydrothermal carbonization over a wide range of operation-al conditions (temperature, time of reaction, and concentra-tion). Such synthesized carbonaceous materials were charac-terized by means of different experimental techniques: scan-ning electron microscopy (SEM), transmission electron mi-croscopy (TEM), X-ray photoelectron spectroscopy (XPS),infrared spectroscopy, Raman spectroscopy, nitrogen physi-sorption, and elemental C/H/O chemical analysis.

Experimental Section

Synthesis of the materials : The selected starting materials were a-d-Glu-cose (96 %, Aldrich), d-(+ )-sucrose (Rectapur, Prolabo), and potatostarch (Sigma-Aldrich). First, the saccharide was dissolved (glucose andsucrose) or dispersed (starch) in water (50 mL) until a concentration of0.1–1 mol L�1 was reached (see Table 1 for details). The mixture was thenplaced in a Teflon-lined autoclave and kept at a temperature in the 170–240 8C range for a period of time between 0.5 and 15 h. The solid prod-

ucts were separated by centrifugation, washed with distilled water andacetone, and finally dried at 100 8C for 2 h. The codes used to identify thesamples are listed in Table 1.

Characterization methods : SEM images were obtained by using a ZeissDSM 942 microscope. To determine the size distribution of the synthe-sized microspheres, the diameter of around one hundred particles (as vi-sualized by SEM) was measured. TEM images were taken using a JEOL(JEM-2000 EX II) microscope operating at 160 kV. Fourier transform dif-fuse reflectance infrared spectra of the materials in powder form were re-corded using a Nicolet Magna-IR 560 spectrometer fitted with a diffusereflection attachment. The Raman spectra were recorded by using aHoriva (LabRam HR-800) spectrometer. The source of radiation was alaser operating at a wavelength of 514 nm and a power of 25 mW. Thecurve fitting was performed with the combination of Gaussian–Lorentzian line shapes that gave the minimum fitting error. X-ray photo-electron spectroscopy (XPS) was carried out by means of a Specs spec-trometer, using MgKa (1253.6 eV) radiation emitted from a double anodeat 50 W. Binding energies for the high-resolution spectra were calibratedby setting C 1s at 284.6 eV. Adsorption measurements were performedusing a Micromeritics ASAP 2020 volumetric adsorption system. The ele-mental analysis (C, H, and O) was performed using a LECO CHN-932microanalyzer.

Results and Discussion

Structural characteristics of the hydrothermally carbonizedmaterials : The hydrothermal reaction of glucose, sucrose,and starch in an autoclave at temperatures above 170 8Cgenerates a solid residue, which we denote as hydrochar, aname that reflects both the nature of the product and thesynthesis procedure used. The hydrochar material is madeup of particles with a spherical morphology that have a di-ameter in the 0.4–6.0 mm range, as evidenced by the SEMimages shown in Figures 1 and 2. These figures also includehistograms of the diameter distribution of the microspheres,which clearly show they have a high degree of uniformity.The mean diameter and standard deviations of the hydro-char microspheres synthesized under a variety of operation-al conditions are listed in Table 1. We observed that for tem-peratures in the 230–240 8C range and short reaction times(0.5–1 h), the microspheres fuse, thereby giving rise to parti-cles that have a peanut shape (Figure 1d). The N2 adsorptionmeasurements reveal that these microspheres have a poorporosity, the BET surface areas being less than 3 m2 g�1.

The diameter of the hydrochar microspheres can bemodulated by modifying the operational conditions. Actual-ly, we observed that, independently of the type of saccha-ride, increasing the reaction temperature, the concentrationof the reaction mixture, or the reaction time leads to an in-crease in the mean diameter of the microparticles (seeTable 1). We also observed that under similar operationalconditions, the diameter of the microspheres changes as afunction of the type of saccharide used. Thus, the mean di-ameter follows the tendency: glucose< starch< sucrose. Thisvariation is related to the number of decomposed speciesgenerated from the different saccharides during the hydro-thermal treatment, which is obviously greater in the case ofstarch and sucrose due to their polysaccharide and disac-charide nature, respectively.

Table 1. Physical properties of hydrochar materials resulting from the hy-drothermal treatment of different saccharides.

Saccharide Samplecode

c[mol L�1]

T[8C]

t[h]

Sphere diameter[mm][a]

Yield[%][b]

glucose HC-G1 0.50 170 4.5 0.40 (�0.06) 1.5HC-G2 180 0.44 (�0.09) 5.1HC-G3 190 1.2 (�0.3) 9.4HC-G4 210 1.2 (�0.3) 28HC-G5 230 1.4 (�0.4) 36HC-G6 0.50 170 15.0 1.0 (�0.3) 6.0HC-G7 180 1.9 (�0.8) 15HC-G8 1.00 190 4.5 1.4 (�0.3) 26HC-G9 1.00 230 1.0 0.92 (�0.08) 31HC-G11 240 1.9 (�0.2) 43HC-G10 1.00 240 0.5 1.0 (�0.1) 37

starch HC-A1 0.50 180 4.5 3.6 (�0.9) 7.1HC-A2 0.25 180 1.3 (�0.2) 5.1HC-A3 0.25 200 1.7 (�0.5) 25HC-A4 0.10 200 0.40 (�0.07) 15

sucrose HC-S1 0.50 190 4.5 6.0 (�2.1) 23

[a] Mean spherule diameter size. Standard deviation is indicated in pa-renthesis. [b] Yield is defined as: g hydrochar per 100 g saccharide.

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Chemical properties of the hydrochar samples : The elemen-tal chemical composition (C, O, and H) of the saccharidesand different hydrochar samples are listed in Table 2. It canbe seen that the carbon content increases from 40–44 % inthe saccharides to approximately 64–66 % in the hydrocharsamples. At the same time there is a reduction in theoxygen and hydrogen contents. These variations becomegreater as the reaction temperature increases, which is con-sistent with a carbonization process. These changes were an-

Figure 1. SEM microphotographs and size histograms of the hydrocharmicrospheres obtained by hydrothermal carbonization of glucose. a) Glu-cose (0.5 m) hydrothermally carbonized at 170 8C for 4.5 h (HC-G1).b) Glucose (0.5 m) hydrothermally carbonized at 230 8C for 4.5 h (HC-G5). c) Glucose (0.5 m) hydrothermally carbonized at 170 8C for 15 h(HC-G6). d) Glucose (1 m) hydrothermally carbonized at 240 8C for 0.5 h(HC-G10).

Figure 2. SEM microphotographs and size histograms of the hydrocharmicrospheres obtained by hydrothermal treatment of starch and sucrose.a) Starch (0.25 m) hydrothermally treated at 180 8C for 4.5 h (HC-A2).b) Starch (0.25 m) hydrothermally treated at 200 8C for 4.5 h (HC-A3).c) Starch (0.10 m) hydrothermally treated at 200 8C for 4.5 h (HC-A4).d) Sucrose (0.5 m) hydrothermally treated at 190 8C for 4.5 h (HC-S1).

Table 2. Chemical elemental analysis of hydrochar materials resultingfrom the hydrothermal treatment of different saccharides.

Sample C [wt %] H [wt %] O [wt %] O/C[a] H/C[a]

glucose 40.00 6.67 53.33 1.000 2.000HC-G1 64.91 4.20 30.89 0.357 0.776HC-G4 66.29 4.15 29.56 0.334 0.752starch 44.44 6.17 49.38 0.830 1.670HC-A2 64.16 4.10 31.74 0.371 0.768HC-A3 65.85 3.99 30.16 0.343 0.727sucrose 42.11 6.43 51.46 0.920 1.830HC-S1 65.02 4.21 30.77 0.355 0.777

[a] Atomic ratio.

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alyzed by means of a van Krevelen diagram (see Figure 3).[8]

This graph offers the advantage that the elemental reactionsthat occur during carbonization can be represented by

straight lines that describe the dehydration, decarboxylation,and demethanation processes. The evolution from the sac-charides to the hydrochar samples follows the diagonal line,which suggests that dehydration reactions prevail during hy-drothermal carbonization. In this process possibly ether, an-hydride, and lactone bonds are formed.[8] It should also benoted that the location of the hydrochar samples in the H/Cversus O/C diagram is far away from that of the coal, whichhas a lower O/C ratio as a consequence of decarboxylationand demethanation reactions that take place during naturalcoalification.[23]

The percentage of carbon fixed in the hydrochar materialscan be calculated from the comparison of the chemical com-position of the starting carbohydrate and that of the finalcarbon material (see Table 2). Thus, depending on the op-erational conditions (i.e., temperature, reaction time, con-centration of the aqueous saccharide solution, and type ofsaccharide), between 2.4 % (HC-G1) and 46.4 % (HC-G4)

of the carbon contained in the saccharide is retained in thehydrochar. The parameter that influences to a higher extentthe carbon fixation in the hydrochar is the reaction tempera-ture, as it is the parameter with a major influence on theproduct yield (see Table 1).

The changes in the chemical characteristics of the saccha-rides that take place during the hydrothermal carbonizationhave been investigated by different spectroscopic techniques(i.e., infrared, Raman, and X-ray photoelectron spectroscop-ic analysis). The IR spectra corresponding to several repre-sentative hydrochar samples are shown in Figure 4a. Inde-pendently of the type of saccharide and operational condi-tions used, the spectra contain the same IR bands, which in-dicates that they have a similar chemical nature. The bandsat 1710 and 1620 cm�1 (together with the band at 1513 cm�1)can be attributed to C=O (carbonyl, quinone, ester, or car-boxyl) and C=C vibrations respectively, whereas the bandsin the 1000–1450 cm�1 region correspond to C�O (hydroxyl,ester, or ether) stretching and O�H bending vibra-tions.[10, 24,25] The bands at 875–750 cm�1 are assigned to aro-matic C�H out-of-plane bending vibrations,[26] whereas thebands at approximately 2900 and 3000–3700 cm�1 corre-spond to stretching vibrations of aliphatic C�H and O�H(hydroxyl or carboxyl), respectively.[27,25] A comparativeanalysis of the FTIR spectra of the hydrochars (Figure 4a)and those of the saccharides (Figure 4b) suggests that dehy-dration and aromatization processes take place during thehydrothermal carbonization, which confirms the results de-duced from the van Krevelen diagram. Thus, the intensitiesof the bands corresponding to the hydroxyl or carboxylgroups (3000–3700 and 1000–1450 cm�1) in the hydrocharsare weaker than those of the corresponding saccharide,thereby disclosing dehydration reactions. New vibrationbands at 1710 cm�1, corresponding to C=O groups, and 1620and 1513 cm�1, corresponding to C=C groups, appear in thehydrochar material. The appearance of the bands at 1620and 1513 cm�1 reveals the aromatization of the samples. Anincrease in the temperature of the hydrothermal carboniza-tion of glucose is accompanied by a diminution in the inten-sities of the band at 1710 cm�1 (C=O) and the wide band atapproximately 3000–3700 cm�1 (O�H) (see Figure 4a), dueto oxygen removal. At the same time, both the aromatic hy-drogen and aromatic carbon (C=C) content increase, asACHTUNGTRENNUNGevidenced by the increase in the intensity of the bands at875–750 cm�1 and 1620 cm�1, respectively. These data revealan increase in the aromatization of the hydrochar as the re-action temperature rises, which is the normal tendency for acarbonization process.[25]

Figure 5a shows the Raman spectra for three representa-tive hydrochar samples. The signal obtained in the 1000–1900 cm�1 range is typical of carbonized materials.[28,29] Itconsists of two broad overlapping bands at around 1360 and1587 cm�1. The band at 1587 cm�1, known as the G band, in-volves the in-plane bond-stretching motion of pairs of C sp2

atoms both in aromatic and olefinic molecules.[30] The bandat 1360 cm�1, known as the D band, is assigned to ring-breathing vibrations in benzene or condensed benzene rings

Figure 3. H/C versus O/C van Krevelen diagram of different saccharidesand the hydrochar products resulting from hydrothermal carbonization.For comparison purposes, other carbonaceous materials are also repre-sented.

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in amorphous (partially hydrogenated) carbon films.[31]

Therefore these data reveal the existence of small aromaticclusters in the hydrochar samples, which agrees well withthe aromatization of the materials observed by IR spectros-copy. The deconvolution of these two overlapping bands(see Figure 5b) gives rise to three additional peaks. Thepeak at 1460 cm�1 can be attributed to semicircle ringstretching or carbon atoms in single aromatic rings or fusedaromatic rings.[32] The peak at 1242 cm�1 is assigned to aryl–

Figure 4. a) FTIR spectra of the hydrochar samples obtained by hydro-thermal treatment of saccharides. (s= stretching vibration, d=deforma-tion or bending vibration). b) FTIR spectra of the saccharides.

Figure 5. a) Raman spectra of three representative samples of hydrocharand b) deconvolution of the bands in the 1000–1900 cm�1 range for theHC-S1 sample.

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alkyl ether whereas the peak at 1506 cm�1 is due to amor-phous carbon structures.[33] These spectra also exhibit pro-nounced bands in the 2400–3600 cm�1 range. These peakscan be assigned to sp3 C�Hx stretching modes, the main con-tributions being from the sp3 C�H2 and sp3 C�H groups(2800–3000 cm�1), and the 2G peak (�3170 cm�1).[34]

X-ray photoelectron spectroscopy (XPS) was used tocharacterize the oxygen functionalities present on the outerlayer (shell) of the hydrochar microspheres. As a represen-tative example, Figure 6 shows the C 1s and O 1s core-levelspectra of a starch-based hydrochar (HC-A3), and the peak-fitting of the C 1s and O 1s envelopes. For the C 1s spec-trum represented in Figure 6a, four signals at 284.6, 285.7,287.2, and 289.0 eV were identified. They are attributed, re-spectively, to carbon group (C=C, CHx, C�C), hydroxyl

groups or ethers (�C�OR), carbonyl or quinone groups(>C=O), and carboxylic groups, esters, or lactones(�COOR).[35–37] The presence of these oxygenated groupswas confirmed by the O 1s spectrum (Figure 6b), in whichtwo signals were identified at 531.7 and 533.0 eV.[35] The firstsignal corresponds to O=C groups, whereas the peak at533.0 eV is mainly attributed to �O�C� groups, although acertain contribution from �COOR is probably also present,as evidenced by the C 1s envelope. These results agree wellwith those obtained from the FTIR and Raman measure-ments, which also reveal the presence of both aromatic andaliphatic carbon, as well as abundant oxygenated functionalgroups. It should also be pointed out that the percentages ofoxygen in the core and in the shell are analogous, as can beinferred from a comparison of the O/C atomic ratios deter-mined by elemental analysis (see Table 1) and those calcu-lated by XPS (0.387, 0.353, and 0.344 for the HC-G3, HC-A3, and HC-S1 samples, respectively). However, it is rea-sonable to imagine that there is a notable difference in thenature of the oxygen functionalities present in the core andin the shell of the hydrochar microspheres. Indeed, theoxygen in the core is probably in the form of stable groups(i.e., ether, quinone, pyrone, etc.), which were identified bymeans of the FTIR measurements, whereas the oxygen func-tionalities present on the shell may consist of more reactive/hydrophilic groups (i.e., hydroxyl, carbonyl, carboxylic,ester, etc.), as evidenced by XPS analysis.

Based on the results obtained by the elemental chemicalanalysis and by means of the spectroscopic techniques(FTIR, Raman, and XPS spectroscopy), we propose a chem-ical model for the hydrochar microspheres that reflects thechemical differences existing between the core and the shellof these microparticles. This model is depicted in Figure 7.

Mechanism of formation of hydrochar microspheres : Thereis a large number of works related to the reactions thatoccur when saccharides are treated under sub- or supercriti-cal water conditions at temperatures in the 150–350 8Crange.[4,38–41] Based on this vast information, it is possible toreconstruct the mechanism of formation of hydrochar micro-spheres by hydrothermal treatment of saccharides. Thus, inthe first step, under the experimental conditions of the hy-drothermal treatment, the disaccharides (sucrose) and poly-saccharides (starch) undergo hydrolysis, thereby giving riseto the corresponding monosaccharides (i.e. , glucose in thecase of starch, and glucose and fructose in the case of su-crose).[42] In the case of starch, oligosaccharides such as mal-tose are also formed and the monosaccharide fructose isproduced by isomerization of glucose.[43,44] The hydrolysis re-actions are possible due to the relatively high values of theionic product of H+ and OH� under hydrothermal condi-tions.[45, 46] On the other hand, the decomposition of themonosaccharides leads to the formation of organic acids(e.g., acetic, lactic, propenoic, levulinic, and formicacids),[7,44,40] which rapidly decrease the pH to pH�3. Thehydronium ions generated from these acids are in further re-action stages the catalyst for the degradation of the oligosac-

Figure 6. a) C 1s and b) O 1s core-level spectra of the HC-A3 hydrocharsample.

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M. Sevilla and A. B. Fuertes

charides generated in the case of the starch.[46] These oligo-saccharides generated from starch then decompose into thecorresponding monosaccharides, which then suffer dehydra-tion and fragmentation (i.e. , ring-opening and C�C bond-breaking) processes, thereby giving rise to different solubleproducts, such as furfural-like compounds (5-hydroxyme-thylfurfural, furfural, 5-methylfurfural), the hyACHTUNGTRENNUNGdroxy-ACHTUNGTRENNUNGmethylfurfural-related 1,2,4-benzenetriol, acids, and alde-hydes (acetaldehyde, acetonylacetone). As an example, glu-cose suffers fragmentation to dihydroxyacetone, glyceralde-hyde and erythrose, and dehydration to 1,6-anhydroglu-cose.[4] The further decomposition of the furfural-likecompounds generates acids/aldehydes and phenols.[4,39, 47–50]

Then, the monomers (glucose and fructose) and/or their de-composition products undergo polymerization or condensa-tion reactions, leading to the formation of soluble poly-mers.[50] These polymerization or condensation reactionsmay be induced by intermolecular dehydration [Eq. (1)] oraldol condensation [Eq. (2)]. Thus, the reactions may pro-ceed in the following manner:

in which R1, R2 =alkyl, vinyl, cycloalkyl, or aryl groups. Atthe same time, the aromatization of the polymers takesplace. A mechanism that can account for the appearance ofC=O groups (see FTIR spectra in Figure 4) is the dehydra-tion of water from the equatorial hydroxyl groups in themonomers.[51] On the other hand, the formation of the C=C

Figure 7. a) TEM image of a starch-based hydrochar microsphere and b) schematic illustration of the hydrophobic/hydrophilic core–shell chemical struc-ture of the hydrochar microspheres resulting from the hydrothermal carbon-ization of saccharides.

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linkages can result from keto–enol tautomerism of the dehy-drated species [Eq. (3)] containing

or via intramolecular dehydration:[51]

Finally, the formation of aromatic clusters can also be pro-duced by the condensation (by intermolecular dehydration)of aromatized molecules generated in the decomposition/de-hydration of the monosaccharides. As an example, seeEquation (5):

It is reasonable to assume that the formation of the hy-drochar microspheres takes place according to a nucleation-growth mechanism following the LaMer model,[52,53] as Sunand Li[10] proposed in relation to the hydrothermal treat-ment of glucose. Thus, when the concentration of aromaticclusters in the aqueous solution reaches the critical super ACHTUNGTRENNUNGsat-ACHTUNGTRENNUNGuration point, a burst nucleation process takes place. Thenuclei so formed grow by diffusion to their surface of thechemical species present in the solution. These species aresuperficially linked to the microspheres through the reactiveoxygen functionalities (hydroxyl, carbonyl, carboxylic, etc.)present in both the outer surface of the microspheres and inthe reactive species. As a result of these reactions, stableoxygen groups such as ether or pyrone, which are found inthe core of the resulting carbonaceous microspheres, areformed. Once the growth process stops, the superficial func-tionalities of the hydrochar microspheres will mainly consistof reactive oxygen groups. At the end of the reaction, twotypes of products can be found in the reaction medium:1) an insoluble residue consisting of carbonaceous sphericalmicroparticles with a core–shell chemical structure (hydro-char microspheres) and 2) aqueous soluble organic com-pounds (i.e. , furfural-like compounds, acid, and aldehydes).As the concentration of the aqueous saccharide solution, thereaction temperature, or the reaction time rise, the processesof aromatization and polymerization will be favored and, inconsequence, the diameter of the microspheres and the hy-drochar yields increase, as clearly shown in Table 1.

Conclusion

The treatment of saccharides (glucose, sucrose, and starch)under hydrothermal conditions (in an aqueous medium at>170 8C) leads to the formation of a solid carbonaceous res-idue (hydrochar) made up of uniform micrometer-sizedspheres (0.4–6.0 mm). The diameter of these microspherescan be modulated by modifying the synthesis conditions.Thus, the diameter widens with an increase in reaction tem-perature, saccharide concentration, or reaction time. The hy-drochar microspheres obtained from these saccharides con-tain a large number of oxygen functional groups, as inferredby spectroscopic techniques (FTIR spectroscopy and XPS).Although oxygen is uniformly distributed along the micro-spheres, the nature of the oxygen-functional groups differsbetween the core and the shell of the particles. Thus, the mi-crospheres have a core–shell chemical structure consistingof a highly aromatic nucleus (hydrophobic) with oxygen-forming stable groups (i.e. , ether, quinone, pyrone, etc.) anda hydrophilic , which contains a high density of reactive/hy-drophilic oxygen functional groups (hydroxyl, phenolic, car-bonyl, carboxylic, ester, etc.). The hydrochar microsphereshave two important properties that make them suitable foruse in the fields of catalysis, drug delivery, or enzyme immo-bilization. Firstly, they possess a high concentration of sur-face oxygen groups, which suggests that they may be easilylinked to other substances with complementary functionali-ties useful for the fabrication of functional nanocomposites.Secondly, they can be synthesized with uniform diametersless than 500 nm, which are compatible with in vivo applica-tions.

Acknowledgements

The financial support for this research work provided by the SpanishMCyT (MAT2008-00407) is gratefully acknowledged. M.S. acknowledgesthe assistance of the Spanish MCyT for the award of a FPU grant.

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Received: October 10, 2008Revised: January 22, 2009

Published online: February 26, 2009

Chem. Eur. J. 2009, 15, 4195 – 4203 � 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 4203

FULL PAPERHydrophilic Carbonaceous Microspheres