One-Step Hydrothermal Synthesis of Carbonaceous Spheres from

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One-step hydrothermal synthesis of carbonaceous spheres from glucose with aluminium chloride catalyst and its adsorption characteristic for U(VI) Huaming Cai, Xiaoyan Lin, Linyuan Tian, and Xuegang Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02540 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016

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Industrial & Engineering Chemistry Research

One-step hydrothermal synthesis of carbonaceous spheres from glucose with aluminium chloride catalyst and its adsorption characteristic for U (VI) Huaming Caia,b, Xiaoyan Lin a,b,*, Linyuan Tian a,b, Xuegang Luo b

a

School of Materials Science and Engineering, Southwest University of Science and

Technology, Mianyang 621010, Sichuan, China b

Engineering Research Center of Biomass Materials, Ministry of Education,

Mianyang 621010, Sichuan, China

Postal address: 59 Qinglong Road, Mianyang, 621010, Sichuan, China. Email: [email protected] Phone number: +86 08166089372

Abstract Hydrothermal carbonization (HTC) of carbohydrates has been widely used for the synthesis of carbon materials. Most of the chemical transformations (e.g., fragmentation, dehydration) of carbohydrates in the HTC synthesis of carbon rich microspheres

need

relatively

high

temperatures.

However,

the

superficial

functionalities (e.g., reactive oxygen groups) of the microspheres are susceptible to the synthesis temperature. It is reported seldom to synthesize the HTC microspheres with highly abundant reactive oxygen groups at low temperatures. Herein, HTC of glucose was proposed to synthesize the carbon materials using AlCl3 as catalyst at 120-150 oC in this study. The chemical properties and structural characteristics of HTC microspheres were analyzed using FTIR, XPS,

13

C solution NMR and SEM,

respectively. The results showed that the HTC microspheres were successfully

ACS Paragon Plus Environment


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synthesized with near single spherical shapes and smooth surfaces, and the size of diameter in the range of 0.5-5.5µm. There were cross-linked furanic structure and abundant reactive oxygen groups on the surface of HTC microspheres. The 5-hydroxymethylfural (HMF) was detected by 13C solution NMR, which played a key role in the HTC synthesis of carbon rich microspheres. The HTC microspheres exhibit the maximum adsorption capacity of 163mg/g for U (VI). Keywords: Hydrothermal carbonization; glucose; catalyst; adsorption; uranium

1. Introduction As a branch of carbon materials, HTC has been emerged as early as 1913.1 The HTC microspheres from the biomass exhibit significant potential in applications such as adsorption/separation,2-4 catalysis,5 energy storage/conversion,6 drug delivery7 due to their controllable size and shape, and the templates for preparation of hollow spheres.8 In recent years, HTC microspheres were prepared from widespread saccharides or other biomass by hydrothermal process at different temperatures,9-13 most of them were formed without any addition of surfactants and catalysts. There were not surfactants and catalysts in the synthesis process of HTC microspheres from the fructose aqueous solution under hydrothermal treatment in closed vessel,14 the 5-hydroxymethylfural (HMF) was formed through initial intramolecular dehydration of fructose at 120-140°C firstly, and then the microscopic nonpolar carbon-containing spheres were formed, the carbonaceous spheres with grain-like surface morphology were finally obtained by subsequent dehydration and assemblies. However, the hydrothermal carbonization reaction of the saccharides usually needed a higher temperature than 180 oC,10,15 as for other biomass such as cellulose, needed the temperature reaching to 220 oC during the production process of carbon materials.16 Sun and Li17 investigated the carbonaceous spheres were formed from glucose aqueous solution by hydrothermal treatment at 160-180 oC through a significant reaction of aromatization. Supposing addition of some surfactants or catalysts to the reaction system, are the experimental conditions of production of carbonaceous spheres changed? Some researches proved that is true. The study by Wang et al.18


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demonstrated that the temperature of production of carbonaceous spheres was 150 oC when the ammonia, which was good catalyst for the synthesis of carbonaceous materials, existed in the precursor solution. The synthesis temperature of 130 oC was reported recently when the thiourea was used as the sulfur source and catalyst for formation of carbon-coated CsS nanoparticles.19 The increase in the temperature of hydrothermal carbonization of the saccharides will lead to a decrease in active oxygen groups of the hydrochar,20 which goes against the application of HTC microspheres for adsorption and chemical modification.21,22 So it is more significant and urgent to reduce the reaction temperature of production of HTC microspheres. Moreover, the synthesis of HTC microshperes at low temperature can effectively cut down the costs of the production of HTC microspheres and obtain highly abundant active oxygen groups simultaneously. As mentioned above, the HMF is obtained by intramolecular dehydration of fructose at 130 oC, it roles as a bridge between saccharide and HTC microsphere, so it is important that the HMF is obtained firstly in the forming process of HTC microspheres when glucose is used as the carbon source at low temperature. Most of the formation reactions of HTC microspheres in which the HMF is generated from saccharides or other biomass have superior yields of HTC microspheres, when liquid23 or solid acids24,25 and bases26 were used as catalysts at mild temperature. Furthermore, one of them indicated that the mild temperature and the catalyst were employed to minimise competing degradation reactions of fructose to produce levulinic acid or other resultants, so that the yield of HMF was improved finally,24 which resulted in large formation of HTC microsphere. As an effective lewis acid catalyst, AlCl3 was more environmentally friendly than other halide catalysts (eg., Cr (II) or Cr (III)-chlorides), and widely used to catalyze the formation of HMF, which was the important intermediate in the process of HTC synthesis of carbon rich microspheres, from carbohydrate in water rather than in expensive ionic liquid reported in the literature,27,28 moreover, a relatively high yield of HMF can be obtained from carbohydrate using AlCl3 catalyzer. Therefore, the HTC microspheres are synthesized from glucose aqueous solution by hydrothermal treatment in



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environmentally benign solvents with AlCl3 Lewis acid as catalyst at 120-150 oC in this paper. A series of experimental factors such as the concentration of catalyst, reaction temperature and time were researched, and the chemical properties and structural characteristics of HTC microspheres were characterized by Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), 13

C solution nuclear magnetic resonance (13C NMR) and scanning electron

microscopy (SEM). The synthesis mechanism of HTC microspheres from glucose at low temperature is analyzed. Finally, the adsorption characteristic of HTC microspheres for U (VI) is evaluated.

2．．Experimental Section 2.1 Materials and method 2.1.1 Materials Glucose (Sigma-Aldrich) was used as the precursor for hydrothermal treatment without further purification. The hydrated AlCl3 was purchased from Sigma-Aldrich and used without further purification. Deionized water and hydrated AlCl3 were used as the aqueous phase and catalyst for all reactions, respectively.

2.1.2 Synthesis of the hydrothermal carbon Glucose of 1.58g and AlCl3 of 0.1-0.6g were dissolved in 25ml and 5ml of deionized water to form 5 wt% of glucose aqueous solution with 10-50 mol% of AlCl3 catalyst. Then a 50 ml Teflon-lined stainless steel autoclave was 60% filled with this solution, and it was placed into a pre-heated oven at a temperature in the 120-150 oC range, which is the temperature range of batch production of HMF in the presence of AlCl3 catalyzer,27 for various periods of time between 8 and 20 h. After cooled down to room temperature, the solid products were separated by centrifugation, and washed with deionized water and ethanol until the filtrate was colorless. Finally the products were dried in a vacuum oven overnight at 60 oC. The codes used to identify the samples were listed in Table 1, and these samples were also denoted as HTC-x-y, where x stands for the dose of AlCl3 catalyzer (mol %) and y for the


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treatment temperature of hydrothermal process (oC), respectively.

2.2 Characterization The morphology of HTC microsphere was characterized by scanning electron microscopy (SEM, Zeiss EVO 18 special edition) and FEG field emission scanning electron microscope (FESEM, Zeiss Ultra 55). Fourier transform infrared (FT-IR) spectroscopy was used to analyze the molecular structure of KBr-diluted samples using a Perkin–Elmer spectrum Nicolet-5700 with resolution of 0.4 cm-1. For NMR analysis, 0.5ml of D2O was added to 0.2ml of the untreated glucose solution, and 0.2ml of D2O was added to 0.5ml of the other sample solution.

13

C NMR spectra of

sample solutions were recorded using a Bruker Avance 600 MHz (14.1T) spectrometer and the tests were acquired using inverse gated decoupling to minimise the nuclear Overhauser effect.29 The element composition and distribution of HTC microspheres were analyzed by Energy Dispersive Spectroscopy on the SEM (EDS, Oxford

IE450X-Max80),

X-ray

photoelectron

spectroscopy

(XPS,

Thermo

SCIENTIFIC ESCALAB 250) and elemental C/H/O chemical analysis (EA, Vario EL CUBE). Nitrogen adsorption-desorption measurement was used to analysis the mesoporous pore size on a JW-BK112 mesoporous pore size analyzer. The thermogravimetric analysis (TGA) of microspheres was carried out by the thermogravimetric analyzer (United States TA Corporation) in nitrogen atmosphere at the heating rate of 20 oC/min and the temperature varying from 35 to 800 oC.

2.3 Evaluation of U (VI) adsorption of HTC microspheres The HTC microspheres, coded as HTC-20-130, were adopted for adsorbing U (VI) in aqueous solutions. This sample was synthesized at the catalyst dose of AlCl3 of 20 (mol %) and the hydrothermal treatment temperature of 130 oC, a further calcination for the microspheres in a muffle furnace at 300 oC in air for 5 h was implemented in order to enhance the amount of carboxylic groups on outer surface of pristine HTC microspheres.21 In adsorption experiment, 10 mg of HTC-20-130 was dispersed in 20 ml of different concentration U (VI) solutions (20-140mg/L), and then the mixture



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was stirred at 25 °C and different pH value for different times. After adsorption experiment, the mixed solutions were filtered, and the concentrations of uranyl ions were determined by U-3900 ultraviolet spectrophotometer before and after adsorption experiments. The adsorption equilibrium amount (qe, mg/g) of U (VI) was calculated by the equation, expressed as: qe=(C0-Ce)V/m, where qe, C0 and Ce are the adsorption capacity of U (VI) (mg/g), initial and equilibrium concentration (mg/L), respectively; V and m are the volume of testing solution used for adsorption (L) and the weight of adsorbent (g), respectively.

3. Results and Discussion 3.1 Morphology and microstructure of HTC microsphere Study of Sun and Li17 indicated that the HTC microspheres with larger sizes were obtained at the constant concentration and temperature when the reaction time of hydrothermal processes reached to 10 h. The reaction times of hydrothermal processes were designed to be more than 8 h in this study. The surface morphology and size distributions of the various products were demonstrated by SEM images (Figure 1) and the histograms (Figure 2). The mean diameter, standard deviations and yields of HTC microspheres synthesized at a variety of operational conditions were listed in Table 1. We can see from Figure 1, there were single spherical shapes of HTC at the temperature less than 130 oC (see in Figure 1a and 1b). However, an increased number of microspheres bonded together to form a “peanut shape” when the reaction temperatures rose to 150 oC,20 see in Figure 1c. All the microspheres exhibited smooth surfaces in the higher magnification SEM images in Figure 1a, 1f and 1g, except those exhibited in Figure 1d. The microspheres with sharkskin-like surface were obtained when the dosage of AlCl3 catalyst was 10 mol % at the constant time and temperature, shown in the higher magnification of SEM image in Figure 1d. In addition, we can see from Figure 2 and Table 1, most of the size distribution of HTC products were 0.5-5.5µm, it was obvious of the change in the size of HTC microspheres obtained at different reaction temperatures. The size distribution and the mean diameter of the microspheres were dependent on reaction temperature of the synthesis process, which


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were respectively 0.5-4µm, 1-5.5µm and 2-5.5µm for the reaction temperature of 120 o

C, 130 oC, and 150 oC (Figure 2a-c ) , and 2µm, 3.2µm and 4.1µm (Table 1a-c). The

lesser yield of microspheres, which was 0.5% exhibited in Table 1(a), indicated that the reaction temperature was required for above 120 oC, in order to obtain a higher yield of HTC microspheres. The size distribution and the mean diameter of the microspheres obtained at the reaction time of 8h, 12h and 16h were 0.5-5.5µm, 2.7µm, 1-5.5µm, 3.32µm and 1.5-5.5µm, 4.75µm, respectively (Figure 2g-i and Table 1g-i). The results revealed that the size distribution and the mean diameter of HTC microspheres were also the dependence of the reaction time in synthesis process. The dose of AlCl3 catalyzer had an effect on the yield of microspheres, shown in Table 1d-f. The high yield of 16.4% of HTC microsphere was obtained at the following conditions: 5wt% of glucose used as raw material with 20 mol% of AlCl3 catalyzer adding in aqueous solution, reacting at 130 oC for 20 h, shown in Figure 2e and Table 1e. The N2 adsorption–desorption isotherm of the HTC-20-130 microspheres is presented in the Figure 3 and the pore structure parameters of the microspheres are listed in the Table 2. As we can see from Figure 3 and Table 2, the BET surface area, BJH desorption cumulative volume and BJH desorption average pore width of the microspheres are respectively 4.02 m2/g, 0.34 cm3/g and 4.20 nm, which indicates the HTC microspheres have a poor porosity.

3.2 Chemical structure of HTC microsphere The changes in the chemical characteristics of three representative HTC samples which obtained at different reaction temperatures were investigated by FTIR. All the spectra almost exhibited similar IR bands which indicated that the HTC microspheres had similar chemical nature (shown in Figure 4). The band at 1700 cm-1 was attributed to C=O (carbonyl, ester, or carboxyl) vibrations,13 and the bands at 1625 and 1513 cm-1 were attributed to C=C vibrations.12,15 The band of lactone group appeared at 1390 cm-1, whereas the C-O (hydroxyl, ester, or ether) stretching and O-H bending vibrations bands appeared in the 1000-1450 cm-1 region.20,21 Meanwhile, the band at 3430 cm−1 corresponded to hydroxyl groups, and 2850, 2925 cm−1



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corresponded to C–H stretching vibrations respectively.3 Some characteristic bands in the 875-750 cm-1 region were assigned to aromatic C-H out-of-plane banding vibrations.30 In particular, the band at 798 cm-1 was attributed to furanic out-of-plane C–H deformation, and 1023 cm-1 was attributed to characteristic furan.12 Compared Figure 4a with Figure 4b and c, when the reaction temperature rose from 120 oC to 150 oC, the bands intensity at 798 and 1023 cm-1 decreased, which indicated that the ratio of furan existed in the HTC microsphere decreased. The lesser furan structure existed, the lesser oxygen-containing groups remained in the HTC microsphere. The results reveal that high reaction temperature will lead to reduce the quantity of oxygen-containing groups in HTC microspheres. The aforementioned functional groups presented in HTC microspheres were further confirmed by a XPS analysis, as shown in Figure 5, and the XPS data were listed

in

were chosen

Table for

3.

The two typical samples of

XPS

analysis

because they

HTC-20-130 and HTC-20-150 were

synthesized

with

lesser doses of AlCl3 adding and higher yields in the process. In Figure 5, the C 1s and O1s spectrum of two samples were similar, the peak at 284.6 eV was attributed to carbon group (C=C, CHx, C-C), 286.0 eV to hydroxyl, alcohol or ether groups (-C-OR), and 288.4 eV to carboxylic groups, esters, or lactones (-COOR). As oxygen-containing groups in two samples according to O 1s spectrum showed in Figure 5b and d, the peak at 531.7 eV was attributed to O=C groups, and 533.0 eV to -O-C- and -COOR groups.20,21 The atomic ratios of O/C of two samples were respectively 0.261 and 0.225 in Table 3, which indicated that the quantity of oxygen-containing groups decreased when the reaction temperature rose from 130 oC to 150 oC. These results agreed well with those conclusions obtained from the FTIR spectra. Meanwhile, the higher ratio of C=C groups existed in HTC-20-150 microspheres indicated that they possess more aromatic structure than HTC-20-130 microspheres, which is one of the probable cause for the “peanut shape” structure of HTC-20-150 microsphere. The C=C groups existed in the sample was mainly attributed to both HMF and keto-enol tautomerism,18 and the C=O groups was attributed to both HMF and acids.


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The components of the typical sample (HTC-20-130) also was analyzed by energy dispersive X-ray (EDX), see Table 4. Compared with the resluts of XPS in Table 3, a little Al and Cl were detected by EDX measure, which indicated that a potential concentration of cationic species [Al(OH)(H2O)5]2+ (see in Figure 6) embedded inside the carbon matrix of HTC microspheres. As the study of Baccile et al.31 and Zhang et al.29 shown, there were the five sets of main peaks, which were indexed to five main species of glucose, fructose, HMF, levulinic acid and formic acid, existed in the NMR spectra (Figure 6). The chemical structures of the five species with different carbons identified numerically were schematically shown in the spectra, the peaks in the spectra were marked the same as the codes of carbons. The two sets of peaks identified in the dashed box correspond to glucose and fructose, and one identified by single arrows was fructose. Only fructose and glucose were detected in the vestigial solution when the duration of hydrothermal treatment of glucose was 3h. Both of the concentrations of fructose and glucose decreased rapidly along with increase of the duration of hydrothermal treatment from 3h to 9h, while those of other species of HMF, levulinic acid and formic acid increased at the same time. The results indicated obviously that a large number of glucose were converted to fructose just within a short reaction time at 130 oC. However, the fructose was so unstable to be converted to HMF and other acids along with the increase of the reaction time, finally the converted HMF from the fructose went to form HTC microsphere. The results confirmed the existence of furanic rings in the HTC microspheres, which were in good agreement with the FTIR data outlined above. Comparing Figure 5a, b with Figure 5e, f, it is obvious that the peak position of the C 1s and O 1s spectra of the HTC-20-130 and calcined HTC-20-130 samples are consistent, which indicates that the type of the groups on/in the HTC-20-130 microspheres before and after calcination does not change. Nevertheless, the ratio of the groups such as C=C and -COOR increases respectively from 62.5 to 74.2% and 6 to 10% (see in Table 5), suggesting that the amount of carboxylic groups on the surface of calcined HTC-20-130 microspheres increases compared with those of



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HTC-20-130 microspheres. The results reveal that a further calcination for the microspheres in a muffle furnace will enhance the amount of C=C and carboxylic groups in the HTC microspheres or on their surface.21,32 The carboxylic groups on the surface of the HTC microspheres are important active sites for U (VI) adsorption, the further calcined HTC microspheres with abundant carboxylic groups on the surface can enhance their adsorption capacity towards uranium.33

3.3 Mechanism of formation of HTC microsphere Carbonaceous spheres were produced from glucose using hydrothermal treatment have been widely reported in recent years. Almost all of the products were synthesized at relatively high reaction temperature (>180 oC). The results of this study demonstrate that HTC microspheres have the similar surface topography and size distribution, but yet more oxygen-containing functional group compared with other carbonaceous spheres which obtained from glucose at a temperature range of 160-180 o

C. The formation mechanism of HTC microspheres was exhibited in Figure 7, which

was similar to some suggested in the literature.29,31,32,34 The cationic species [Al(OH)(H2O)5]2+

formed

from

AlCl3

hydrolysis

promoted

formation

of

fructofuranose, H+, the other production of AlCl3 hydrolysis, led to assist synthesis of HMF subsequently (shown in Figure 7a and 7b).27 The results from 13C NMR spectra confirmed that there was HMF in the vestigial glucose solutions. The CHO, OH and H terminals on the furanic ring of HMF were potentially most active to participate directly in the polycondensation reactions of HMF, and the H2O was released from polycondensation reaction of HMF monomers. The abundant HMF in the vestigial glucose solutions subsequently established a cross-linked furanic structure by a series of polymerization–polycondensation reactions. The evident C=O groups were existed in the cross-links between furanic rings and end functional groups.31 Levulinic acid, another species which was existed in the vestigial glucose solution, acted as a connecting function to connect the cross-linked furanic structure, led to formation of the polyfuranic clusters. When the concentration of polyfuranic clusters reached to the critical supersaturation point, a burst nucleation process took place successfully. Then


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the nuclei uniformly and isotropically grew by continually reacting on the surface of the nuclei until the HTC microspheres formed finally. As a result of these reactions, the superficial functional groups of the microspheres mainly consist of the active oxygen-containing groups from HMF and hydroxyl and carboxyl from levulinic acid. The inference of the formation mechanism of HTC microspheres above was powerfully supported by the results obtained from FTIR analysis, XPS and EDX. The nucleationgrowth mechanism and formation of HTC microspheres reasonably conforms to the classical LaMer model.17,35,36

3.4 Thermal analysis of HTC microsphere The TGA curves of HTC-20-130 samples before and after calcination are shown in Figure 8. The small weight loss for the two samples (HTC-20-130 and calcined HTC-20-130) at the range of temperature from 30 to 270 oC can be attributed mainly to loss of residual water within the microspheres, and the further weight loss for the two samples when the temperature rise to 800 oC mainly comes from the thermal decomposition of the HTC microspheres. As we can see, the temperature (370 oC) of the calcined HTC-20-130 starting to decompose is relatively higher than that (270 oC) of the HTC-20-130, and the calcined HTC-20-130 shows a smaller weight loss (59.87%) when the temperature rise to 800 oC than the HTC-20-130 sample (62.78%), which indicates that the calcined HTC-20-130 microspheres are more suitable than the HTC-20-130 microspheres.

3.5 Adsorption characteristic of the HTC microspheres for U (VI) The effect of solution pH on the adsorption capability of HTC microspheres toward uranium is given in Figure 9. The reason why the pH values was selected within 4.5, is that the soluble form of uranyl ions in the solution will convert into insoluble when the pH of the solution is more than 4.5 according to the species distribution of uranium in solution.37 It is obviously exhibited that the adsorption process of HTC microspheres for U (VI) is clearly depend on the pH value of the solution. The amount of uranium adsorbed onto HTC microspheres sharply increased



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from pH 1.5 to 3.5, and then became steady when pH was beyond 3.5. Finally, the maximum adsorption capacity of 164 mg/g was obtained at pH 4.5. The functional groups on the surface of HTC microspheres were protonated at low pH, leading to form the positive charged surface and then the electrostatic repulsion between the positive charge on the surface of the adsorbent and the uranyl ions takes place, which lead to poor adsorption capability of the HTC microspheres for U (VI). Another reason was the competition between H+ and uranyl ions for the limited active sites on the surface of the HTC microspheres at low pH.37 The effect of contact time on the adsorption of U (VI) onto the HTC microspheres is presented in Figure 10a. The contact time was investigated in the range of 3-24 h, and the adsorption characteristics can be clearly observed from Figure 10a. The amount of U (VI) adsorbed onto the HTC microspheres increased rapidly during the first 9 h, and then slightly altered to reach the maximum value of 163 mg/g at the contact time of 22 h. The result reveals that the adsorption equilibrium is established at about 22 h and the maximum capacity of the HTC microspheres for U (VI) is about 163 mg/g. The effect of initial U (VI) concentrations on the U (VI) adsorption was studied with the concentration ranging from 20 mg/L to 140 mg/L, as shown in Figure 10b. As we can see, the adsorption capacity increases rapidly with an increase in the initial U (VI) concentration of 20–80 mg/L, but rises slowly beyond 80 mg/L. The increase of adsorption capacity is attributed to a more intensity interaction between U (VI) and HTC microspheres as the initial U (VI) concentration increases, the slowly rising in adsorption capacity is due to that the active sites onto the surfaces of HTC microspheres tend to be saturated.

3.6 Comparison with other carbon rich materials Compared to the reported carbon rich materials, there are mainly several improvements in the morphologies and properties of the microspheres for advanced applications shown in the following: 1) the microspheres synthesized in the manuscript appear near single spherical shapes and smooth surfaces, rather than serious adhesion of the spheres;9,21 2) the microspheres have a relatively large


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diameter which is conducive to application and recycling of the materials in adsorption compared to the nano-size carbon rich materials; 3) there are a larger quantity of oxygen-containing groups on the outer surfaces of the microspheres which are important active sites for U (VI) adsorption. The adsorption capacity of the calcined HTC microspheres is compared against that of other carbon rich materials and listed in Table 6.33,38,39 The comparison in the adsorption capacity of HTC microspheres with other carbon rich materials reveals that the microspheres have higher adsorption capacity than other carbon rich materials, suggesting that the microspheres have good potential towards uranium removal.

4. Conclusions HTC microspheres are synthesized by hydrothermal treatment of aqueous glucose solutions at low temperatures of 120-150 oC when AlCl3 is used as the catalyzer. The synthesis conditions of concentration of AlCl3 in the aqueous solution, reaction temperature and time predominantly influence surface morphology, size distributions, the yield and the quantity of superficial oxygen-containing groups of the microspheres. A high yield of HTC microspheres with size of diameter in the 0.5-5.5µm range, near single spherical shapes and smooth surfaces, especially abundant reactive oxygen groups on the outer surfaces were obtained at the optimal experimental conditions: 130 oC, 20h of reaction time, and 20 mol % of AlCl3. The carbonization process of glucose at low temperature and the addition of AlCl3 catalyzer to form HTC microspheres, we speculate based on the results of

13

C NMR, EDX, XPS and FTIR,

involves three stages, (i) conversion of glucose into fructose by isomerization, then into HMF by partial dehydration under the attendance of [Al(OH)(H2O)5]2+ and H+, (ii) formation of the cross-linked furanic structure by release of water and then formation of the polyfuranic clusters by Levulinic acid acted as a connecting function, further formation of nuclei, (iii) the nuclei uniformly and isotropically grew by continually reacting on the surface of the nuclei until the HTC microspheres formed finally. The maximum adsorption capacity of the HTC microspheres for U (VI) was 163mg/g at pH of 4.5 and 25 oC for 22 h. The HTC microspheres with abundant



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superficial active oxygen groups and the low-cost property due to the low synthesis temperature are especially suitable for applied in the field of adsorbent.

Acknowledgements This work was supported by National key scientific projects for decommissioning of nuclear facilities and radioactive waste management (14zg6101), Engineering Research Center for Biomass Materials, Ministry of Education, China (14tdsc02) and Postgraduate Innovation Fund Project by Southwest University of Science and Technology (14ycx015). Thanks for the technology support of Engineering Research Center of Biomass Materials, Ministry of Education, Southwest University of Science and Technology.


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of

tetracycline. Carbon 2014, 77, 627. 3 Chen, Z.; Ma, L.; Li, S.; Geng, J.; Song, Q.; Liu, J.; Li, S. Simple approach to carboxyl-rich materials through low-temperature heat treatment of hydrothermal carbon in air. Appl. Surf. Sci. 2011, 257, 8686. 4 Demir-Cakan, R.; Baccile, N.; Antonietti, M.; Titirici, M. M. Carboxylate-rich carbonaceous materials via one-step hydrothermal carbonization of glucose in the presence of acrylic acid. Chem. Mater. 2009, 21, 484. 5 Roldán, L.; Pires, E.; Fraile, J. M.; García-Bordejé, E. Impact of sulfonated hydrothermal carbon texture and surface chemistry on its catalytic performance in esterification reaction. Catal. Today 2015, 249, 153. 6 Zhang, X.; Ran, F.; Fan, H.; Tan, Y.; Zhao, L.; Li, X.; Kang, L. A dandelion-like carbon microsphere/MnO2 nanosheets composite for supercapacitors. J. Energy Chem. 2014, 23, 82. 7 Guo, S. R.; Gong, J. Y.; Jiang, P.; Wu, M.; Lu, Y.; Yu, S. H. Biocompatible, luminescent silver@ phenol formaldehyde resin core/shell nanospheres: Large-scale synthesis and application for In vivo bioimaging. Adv. Funct. Mater. 2008, 18, 872. 8 Abdelaal, H. M. Facile hydrothermal fabrication of nano-oxide hollow spheres using monosaccharides as sacrificial templates. ChemistryOpen 2015, 4, 72. 9 Titirici, M. M.; Antonietti, M.; Baccile N. Hydrothermal carbon from biomass: a comparison of the local structure from poly-to monosaccharides and pentoses/hexoses. Green Chem. 2008, 10, 1204. 10 Wang, Q.; Li, H.; Chen, L.; Huang, X. Monodispersed hard carbon spherules with uniform nanopores. Carbon 2001, 39, 2211. 11 Wang, Q.; Li, H.; Chen, L.; Huang, X. Novel spherical microporous carbon as anode material for Li-ion batteries. Solid State Ionics 2002, 152, 43.



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12 Ryu, J.; Suh, Y. W.; Suh, D. J.; Ahn, D. J. Hydrothermal preparation of carbon microspheres from mono-saccharides and phenolic compounds. Carbon 2010, 48, 1990. 13 Aydıncak, K.; Yumak, T.; Sınağ, A.; Esen, B. Synthesis and characterization of carbonaceous materials from saccharides (glucose and lactose) and two waste biomasses by hydrothermal carbonization. Ind. Eng. Chem. Res. 2012, 51, 9145. 14 Yao, C.; Shin, Y.; Wang, L. Q.; Windisch, C. F.; Samuels, W. D.; Arey, B. W.; Exarhos, G. J. Hydrothermal dehydration of aqueous fructose solutions in a closed system. J. Phys. Chem. C 2007, 111, 15141. 15 Sakaki, T.; Shibata, M.; Miki, T.; Hirosue, H.; Hayashi, N. Reaction model of cellulose decomposition in near-critical water and fermentation of products. Bioresource Technol. 1996, 58, 197. 16 Sevilla, M.; Fuertes, A. B. The production of carbon materials by hydrothermal carbonization of cellulose. Carbon 2009, 47, 2281. 17 Sun, X.; Li, Y. Colloidal carbon spheres and their core/shell structures with noble-metal nanoparticles. Angew. Chem. Int. Edit. 2004, 43, 597. 18

Wang,

X.;

Liu,

J.;

Xu,

W.

One-step

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preparation

of

amino-functionalized carbon spheres at low temperature and their enhanced adsorption performance towards Cr (VI) for water purification. Colloid. Surface. A 2012, 415, 288. 19 Zou, S.; Fu, Z.; Xiang, C.; Wu, W.; Tang, S.; Liu, Y.; Yin, D. Mild, one-step hydrothermal synthesis of carbon-coated CdS nanoparticles with improved photocatalytic activity and stability. Chinese J. Catal. 2015, 36, 1077. 20 Sevilla, M.; Fuertes, A. B. Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides. Chem-Eur. J. 2009, 15, 4195. 21 Song, Q.; Ma, L.; Liu, J.; Bai, C.; Geng, J.; Wang, H.; Li, S. Preparation and adsorption performance of 5-azacytosine-functionalized hydrothermal carbon for selective solid-phase extraction of uranium. J. colloidinterf. sci. 2012, 386, 291. 22 Wang, F.; Zhao, J.; Zhu, M.; Yu, J.; Hu, Y. S.; Liu, H. Selective


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adsorption-deposition of gold nanoparticles onto monodispersed hydrothermal carbon spherules: a reduction–deposition coupled mechanism. J. Mater. Chem. A 2015, 3, 1666. 23 Pagan-Torres, Y. J.; Wang, T.; Gallo, J. M. R.; Shanks, B. H.; Dumesic, J. A. Production of 5-hydroxymethylfurfural from glucose using a combination of Lewis and Brønsted acid catalysts in water in a biphasic reactor with an alkylphenol solvent. ACS Catal. 2012, 2, 930. 24 Osatiashtiani, A.; Lee, A. F.; Brown, D. R.; Melero, J. A.; Morales, G.; Wilson, K. Bifunctional SO4/ZrO2 catalysts for 5-hydroxymethylfufural (5-HMF) production from glucose. Catal. Sci. Technol. 2014, 4, 333. 25 Ordomsky, V. V.; Van der Schaaf, J.; Schouten, J. C.; Nijhuis, T. A. The effect of solvent addition on fructose dehydration to 5-hydroxymethylfurfural in biphasic system over zeolites. J. Catal. 2012, 287, 68. 26 Ohara, M.; Takagaki, A.; Nishimura, S.; Ebitani,

K. Syntheses of

5-hydroxymethylfurfural and levoglucosan by selective dehydration of glucose using solid acid and base catalysts. Appl. Catal. A 2010, 383, 149. 27 De, S.; Dutta, S.; Saha, B. Microwave assisted conversion of carbohydrates and biopolymers to 5-hydroxymethylfurfural with aluminium chloride catalyst in water. Green Chem. 2011, 13, 2859. 28 Wu, X.; Fu, J.; Lu, X. Hydrothermal decomposition of glucose and fructose with inorganic and organic potassium salts. Bioresource Technol. 2012, 119, 48. 29 Zhang, M.; Yang, H., Liu, Y.; Sun, X., Zhang, D.; Xue, D. Hydrophobic precipitation of carbonaceous spheres from fructose by a hydrothermal process. Carbon. 2012, 50, 2155. 30 Lua, A. C.; Yang, T. Effect of activation temperature on the textural and chemical properties of potassium hydroxide activated carbon prepared from pistachio-nut shell. J. Colloid Interf. Sci. 2004, 274, 594. 31 Baccile, N.; Laurent, G.; Babonneau, F.; Fayon, F.; Titirici, M. M.; Antonietti, M. Structural characterization of hydrothermal carbon spheres by advanced solid-state MAS 13C NMR investigations. J. Phys. Chem. C 2009, 113, 9644.



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32 Bertarione, S.; Bonino, F.; Cesano, F.; Damin, A.; Scarano, D.; Zecchina, A. Furfuryl alcohol polymerization in HY confined spaces: Reaction mechanism and structure of carbocationic intermediates. J. Phys. Chem. B. 2008, 112, 2580. 33 Liu, Y. H.; Wang, Y. Q.; Zhang, Z. B.; Cao, X. H.; Nie, W. B.; Li, Q.; Hua, R. Removal of uranium from aqueous solution by a low cost and high-efficient adsorbent. Appl. Surf. Sci. 2013, 273, 68. 34 Falco, C.; Baccile, N.; Titirici, M. M. Morphological and structural differences between glucose, cellulose and lignocellulosic biomass derived hydrothermal carbons. Green Chem. 2011, 13, 3273. 35 Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Monodispersed colloidal spheres: old materials with new applications. Adv. Mater. 2000, 12, 693. 36 Mer, V. K. L. Nucleation in Phase Transitions. Ind. Eng. Chem. Res. 1952, 44, 1270. 37 Bai, C.; Zhang, M.; Li, B.; Tian, Y.; Zhang, S.; Zhao, X.; Li, S. Three novel triazine-based materials with different O/S/N set of donor atoms: One-step preparation and comparison of their capability in selective separation of uranium. J. Hazard. Mater. 2015 300, 368. 38 Zhang, Z. B.; Liu, Y. H.; Cao, X. H.; Liang, P. Sorption study of uranium on carbon spheres hydrothermal synthesized with glucose from aqueous solution. J. Radioanal. Nucl. Chem. 2013, 295, 1775. 39 Zhang, Z. B.; Cao, X. H.; Liang, P.; Liu, Y. H. Adsorption of uranium from aqueous solution using biochar produced by hydrothermal carbonization. J. Radioanal. Nucl. Chem. 2013, 295, 1201.


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Figure 1. SEM images of (a) HTC-20-120, (b) HTC-20-130, (c) HTC-20-150, (d) HTC-10-130, (e) HTC-20-130, (f) HTC-50-130, produced of 5 wt% glucose for 20 h reaction time; (g-i) for HTC-20-130, produced of 5 wt% glucose for 8, 12 and 16 h reaction time, respectively.



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Figure 2. The size distribution histograms of (a) HTC-20-120, (b) HTC-20-130, (c) HTC-20-150, (d) HTC-10-130, (e) HTC-20-130, (f) HTC-50-130, produced of 5 wt% glucose for 20 h reaction time; (g-i) for HTC-20-130, produced of 5 wt% glucose for 8, 12 and 16 h reaction time, respectively.


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Figure 3. N2 adsorption–desorption isotherms of HTC-20-130 microspheres at 77 K.

Figure 4. FT-IR spectra of (a) HTC-20-120, (b) HTC-20-130, (c) HTC-20-150, produced of 5 wt% glucose for 20 h reaction time.



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Figure 5. XPS spectra of (a) C 1s and (b) O 1s for HTC-20-130, (c) C 1s and (d) O 1s for HTC-20-150, and (e) C 1s and (f) O 1s for calcined HTC-20-130, produced of 5 wt% glucose for 20 h reaction time.


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Figure 6.

13

C NMR spectra of the vestigial glucose solutions obtained by

hydrothermal treatment of 5 wt% glucose with 20 mol% AlCl3 in aqueous solution for different duration at 130℃.



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Figure 7. Schematic illustration of the formation mechanism and chemical structure of the carbonaceous spheres resulted from glucose aqueous solution by using AlCl3 catalyzer under hydrothermal treatment.

Figure 8. TGA curves of the HTC-20-130 samples before (A) and after (B) calcination at a heating rate of 10oC min-1, under N2 flow.


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Figure 9. Effect of pH on the adsorption of U (VI) (c0 = 140 mg/L, t = 24 h, V = 20 mL, T = 293.15 K, and w = 10 mg).

Figure 10. Effect of contact time (a) and initial U (VI) concentration (b) on the adsorption of U (VI) onto the HTC microspheres. (a: c0 = 140 mg/L; b: t=24 h, pH = 4.5, V = 20 mL, T = 298.15 K, and w = 10 mg)



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Table 1. Mean diameters and yield of HTC microspheres synthesized at different conditions Entry

Sample

t[h]

Sphere diameter[µm]i

Yield[%]ii

a

HTC-20-120

20

2 (±0.67)

0.5

b

HTC-20-130

20

3.2(±1.1)

16.4

c

HTC-20-150

20

4.1(±0.85)

16

d

HTC-10-130

20

3.9(±0.97)

9.4

e

HTC-20-130

20

3.8 (±0.88)

16.4

f

HTC-50-130

20

3.17(±0.95)

17.5

g

HTC-20-130

8

2.7(±0.98)

6.6

h

HTC-20-130

12

3.32(±0.85)

15.3

i

HTC-20-130

16

4.75(±0.85)

16.2

*The HTC microspheres resulted from the hydrothermal treatment of 5 wt% glucose. i The mean diameter of microspheres and the standard deviation in parenthesis. ii Yield is defined as: g HTC microspheres per 100 g glucose.

Table 2. Pore structure parameters of the HTC-20-130 microspheres . Sa (m2/g)

Sample

HTC-20-130 4.02 a

BET surface area.

b

BJH desorption cumulative volume.

c

BJH desorption average pore width.

Vb (cm3/g)

Pore sizec (nm)

0.27

4.23

Table 3. The components of microspheres obtained by XPS analysis Sample

C [at.%]

O [at.%]

O/C a

HTC-20-130

79.3

20.7

0.261

HTC-20-150

81.6

18.4

0.225

*HTC-20-130 and HTC-20-150 microspheres resulted from the hydrothermal treatment of 5 wt% glucose for 20 h reaction time. a Atomic ratio.


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Table 4. The components of the HTC-20-130 microspheres obtained by EDX Sample

C[wt%]

H[wt%]

O[wt%]

Al[wt%]

Cl[wt%]

O/C a

HTC-20-130

75.74

4.95

18.83

0.44

0.04

0.187

*HTC-20-130 microspheres resulted from the hydrothermal treatment of 5 wt% glucose for 20 h reaction time. a Atomic ratio.

Table 5. Element contents of HTC-20-130 and calcined HTC-20-130 samples analyzed by XPS Sample

C [at.%]

O [at.%]

O/Cb

C=C [%]c

-C-O [%]d -COOR [%]e

HTC-20-130

79.3

20.7

0.261

62.5

31.5

6

HTC-20-130a

80.3

19.7

0.245

74.2

15.8

10

a

Calcined HTC-20-130 samples,

b

Atomic ratio,

c,d,e

The ratios of -C=C, -C-O,

-COOR on the surface of HTC-20-130 and calcined HTC-20-130 samples are calculated based on the C1s fitting data of XPS analysis.

Table 6. Comparison of the reported carbon rich materials for the adsorption of uranium (VI). Adsorbent

pH

Capacity (mg/g)

C0 (mg g-1)

reference

Carbon spheres (CSs)

6

57.53

50

[38]

Hydrothermal carbon (HTC)

6

55.90

50

[33]

Biochar

6

55.91

50

[39]

Calcined hydrothermal carbon

4.5

96.72

50

This work

Calcined hydrothermal carbon

4.5

162.21

140

This work



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Table of Contents Graphic

In order to improve the morphologies and properties of the carbon rich microspheres for advanced applications, we have investigated the effect of AlCl3 catalyzer on the HTC synthesis of carbon rich microspheres from aqueous glucose solutions at low temperature (eg., 120 oC). The 5-hydroxymethylfural (HMF) plays a key role in the formation of carbon rich microspheres, and the carbon rich microspheres formed at the existence of AlCl3 catalyzer and low temperature have near single spherical shapes, smooth surfaces and a larger quantity of oxygen-containing groups on their outer surfaces.


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75×58mm (1200×1200 DPI)



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One-Step Hydrothermal Synthesis of Carbonaceous Spheres from

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