Effect of Swelling Pretreatment on Properties of Cellulose-Based

May 16, 2019 - Hydrothermal carbonization (HTC) has been demonstrated as an effective method for preparing hydrochar to realize efficient utilization ...
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Cite This: ACS Sustainable Chem. Eng. 2019, 7, 10821−10829

Effect of Swelling Pretreatment on Properties of Cellulose-Based Hydrochar Jianglong Liu, Shen Zhang, Caidi Jin, Shuang E, Kuichuan Sheng, and Ximing Zhang* College of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou 310058, China

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ABSTRACT: Hydrothermal carbonization (HTC) has been demonstrated as an effective method for preparing hydrochar to realize efficient utilization of biomass resources. In this study, both trifluoroacetic acid (TFA) and phosphorous acid (PA) under low temperature were used as swelling solvents to disrupt the cellulose crystalline structure within 1 h with distinct physiochemical properties. After TFA and PA swelling pretreatment, by means of hydrothermal carbonization, two varieties of swollen cellulose-derived hydrochars were produced. The results showed that the TFA and PA swelling pretreatment significantly enhanced the dehydration and deoxidation of cellulose through an HTC process. The H/C value and O/C value of swelled cellulose are lower than nonswelled by more than 4% at 220 °C, which demonstrates that the dehydrogenation reaction is enhanced during the hydrothermal process of swollen cellulose. Particle size as well as microscopic arrangement of hydrochars were distinctly different due to swelling cellulose. The particle size of hydrothermal carbon microspheres increases by a swelling process at the same hydrothermal carbonazition temperature. TFA-HTC have an estimate diameter of 0.2 μm at 220 °C, which is equivalent to the size of CEL-HTC-280. This study illustrates the mechanism for hydrothermal carbonization of TFA and PA swollen cellulose and provides a novel route for directionally regulating hydrothermal carbon microspheres and developing carbon-based catalysts. KEYWORDS: Cellulose, Swelling pretreatment, Hydrothermal carbonization, Carbon microspheres morphology, Trifluoacetic acid, Phosphoric acid

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INTRODUCTION In recent years, hydrothermal carbonization (HTC) has attracted more and more attention in efficiently converting biomass resources to functionalized carbonaceous materials due to its economy and environmental friendliness characteristics.1,2 In the HTC process, cheap and renewable biomass has been used as a carbon source, and water has been employed as the reactive solvent. The carbonaceous materials were synthesized in a sealed steel container under mild conditions with the temperature below 375 °C (usually 150−280 °C).3,4 Due to the participation of subcritical water medium, the production of carbon materials has lots of inherent advantages, such as uniform size, regular morphology, physical and chemical stability, and abundant oxygen functionality groups on the surface of carbonaceous microspheres.5 These give hydrochar potential applications in several aspects, such as environmental remediation,6 catalyst synthesis,7 supercapacitor materials,8 and so on. It is well known, that cellulose accounted for 35−50% in the lignocellulosic biomass.9 In the raw material of biomass, two factors limit the effective transformation of cellulose. The natural network structure formed by lignin and hemicellulose tightly surrounding and entangling with cellulose impedes the cellulose conversion efficiency. On the other hand, crystallinity © 2019 American Chemical Society

possessed by cellulose itself is also an important factor that results in it being insoluble in most solvents.10,11 Previous studies have reported the hydrothermal carbonization mechanism of raw cellulose to prepare functionalized carbon materials.12,13 Study about cellulose carbonization will provide a theory reference to carbonization of cellulosic biomass.14,15 The aforementioned previous study16−18 used raw cellulose as a precursor of hydrochar, heretofore, researches focused on the effect of modified cellulose, especially, swollen cellulose on the property of hydrochar was rarely reported. Trifluoroacetic acid (TFA) and phosphorous acid (PA) can efficiently permeate into the interstices between the cellulose crystalline region, and the crystallinity of cellulose is significantly decreased under low temperature.19 When the cellulose was swelled by PA solution under low temperature, the crystal lattice structure drastically changes to a disorder one, as the hydrogen bonds interconnected in the cellulose chain will be disrupted, and amorphous cellulose formed simultaneously.20 In the case of TFA swelling, the molecules of swelling solvent penetrate into the cellulose crystal structure Received: March 23, 2019 Revised: April 28, 2019 Published: May 16, 2019 10821

DOI: 10.1021/acssuschemeng.9b01640 ACS Sustainable Chem. Eng. 2019, 7, 10821−10829

Research Article

ACS Sustainable Chemistry & Engineering

temperatures of 220, 240, 260, and 280 °C were 22 , 32, 50, and 79 bar, respectively. After the reactor cooled down to room temperature, the hydrochar in the mixture of liquid solution and solid product was collected through filtration and dried at 105 °C for 4 h for further analysis. Hydrochars prepared by TFA-CEL and PA-CEL were labeled as “TFA-HTC-X” and “PA-HTC-X”, respectively. Hydrothermal carbonization with raw cellulose was carried out under the same conditions as the control group, and the obtained hydrochar was labeled as “CEL-HTC-X”, where X represented the temperature of hydrothermal carbonization. Characterizations. Surface Areas and Porosity. Surface areas and pore size characteristics of hydrochars were determined by nitrogen adsorption−desorption isotherm measurements at −196 °C using a static nitrogen adsorption apparatus (JW-BK, JWGB SCI &TECH, China). The surface area was calculated by the Brunauer− Emmett−Teller (BET) method, and pore volume and pore size parameters were calculated by the Barrett−Joyner−Halendar (BJH) method. Elemental Analysis. The composition of carbon, hydrogen, and nitrogen in hydrochars were determined using an Elementar (Vario Micro, Elenemtar Analysensysteme GmbH, Germany), while the content of oxygen was calculated by difference. Samples were held in tin cups, and the setting combustion temperature was 950 °C. The combustion gases were separated by an adsorption analytical column with helium as the mobile phase. Scanning Electron Microscope (SEM). The morphology of the hydrothermal carbon was analyzed by SEM (SU8010, Hitachi, Japan) with an acceleration voltage of 3.0 kV. The sample subjected sputtering with gold before the test was conducted. Fourier Transform Infrared (FTIR) Spectra. FTIR spectra of every sample were obtained utilizing an FTIR spectrometer (Nicolet 6700, Thermo Fisher Nicolet Corporation, USA). The spectra information on samples was acquired in the wavenumber range from 4000−400 cm−1, and the scan time was 32. The hydrochars and bromatum kalium were mixed with a ratio of 1:100 and ground in an agate mortar. Then, the mixtures were pressed into thin slices under a hydraulic press. Thermogravimetric (TG). TG analysis was performed using a TG analyzer (TG209F3, Netzsch, Germany) to measure the thermal stability of the sample. A temperature-programmed test was conducted with a heating rate of 10 K/min from 40 to 900 °C under an N2 atmosphere with a flow rate of 10 mL/min.

with a cyclic dimer formed at around 0 °C and decrease its crystallinity. TFA has a low boiling point that will make it efficiently recycled, and PA can be easily recycled by means of centrifugation and filtration.21 The two swelling agents provide a potential effective and economical way to break the intermolecular hydrogen bond and reduce the crystallinity of cellulose. Most studies have reported that the cellulose HTC mechanism mainly includes two pathways.12,17,22 The first proposed mechanism is cellulose hydrolyze into individual glucose units and then following the same reaction route as the hydrothermal treatment of glucose; while the second reaction pathway is similar to the pyrolysis process, where the hydrochar was attributed to complex reactions of intramolecular condensation, dehydration, and decarboxylation. We presume that the swelling treatment on cellulose may affect the two different carbonization pathways significantly in the hydrothermal process. In this study, TFA and PA were selected as the swelling solvents to pretreat the cellulose and change its crystal structural at 0 °C. Then, based on hydrothermal carbonization technology, swelling cellulose was converted into high-value functionalized carbon materials. This study reports the transformation mechanism of swollen cellulose on the HTC process and comparison of the property of different pretreatedcellulose derived hydrochars. In the end, a new route for efficient utilization of the lignocellulosics will be proposed.

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MATERIALS AND METHODS

Sample Preparation. α-Cellulose with a particle size of 5 μm was purchased form Aladdin Chemistry Co., Ltd. (China). Trifluoroacetic acid (TFA, ≥ 99%), phosphoric acid (PA, ≥ 85%), and ethanol (≥99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China), and all were analytical grade. Deionized water was purchased form Macklin Biochemical Co., Ltd. (China). Cellulose Swelling Pretreatment. TFA Swelling of Cellulose. The progress of TFA swelling cellulose was according to Zhang’s method.19 Nine grams of cellulose and 90 mL of TFA (0 °C) solution were mixed in a 250 mL serum bottle. After that, the mixture was kept in and ice-cold water mixture, and the swelling progress took about 60 min. After that, 120 mL of water was added to the serum bottle with stirring. By using a vacuum pump, the cellulose was washed with 60 mL (20 mL each time, 3 times in a row) of ethanol to remove residual TFA and washed to neutral with deionized water (0 °C). The resultant cellulose was freeze-dried using a vacuum freeze-dryer (SCIENTZ-10N, Ningbo Xinzhi, China). The TFA swollen cellulose was collected and named as “TFA-CEL”. PA Swelling of Cellulose. Cellulose swelled by PA was conducted according to prior work.23,24 Three grams of cellulose was added to a 250 mL serum flask, and 9 mL of deionized water was added for wetting the cellulose powder. Then, 60 mL of ice-cold PA was added to the mixture with vigorous stirring. About 5 min later, the mixture was changed to transparent gel and was moved to ice-cold water for 60 min. Then, 100 mL of deionized water (0 °C) was added with violent agitation, and the sample turned to a white cloudy-like precipitate. The mixture was filtrated and freeze-dried. The PA swollen cellulose was collected and named “PA-CEL”. Synthesis Hydrochars. The hydrothermal carbonization progresses were carried out using a Micro Reactor (YZPR - 25, Yanzheng, China) with a 25 mL volume. One gram of swelled cellulose and 10 mL of deionized water were added into the reactor and heated up to the setting temperature. Temperatures of 220, 240, 260, and 280 °C were chosen as the desired temperatures, and the mixture was held at this temperature for 4 h with a stirring speed of 200 r/min. Three parallel tests were conducted for each group of samples. The selfgenerated pressures by hydrothermal carbonization cellulose with the

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RESULTS AND DISCUSSION Surface Areas and Pore Size Characteristics. The obvious overlap of nitrogen adsorption isotherms and desorption isotherms of hydrochars is shown in Figure S1. The results showed that the hydrochars present isotherm type III. The amount of nitrogen adsorption at the low relative pressure zones is small, and the adsorption amount has a significant increase with the relative pressure increasing. The specific surface area of CEL-HTC and TFA-HTC reaches the maximum value under a hydrothermal condition at 260 °C, while the optimum value of the specific surface area for PAHTC occurs at the hydrothermal temperature of 280 °C. Total pore volume of three hydrothermal carbons also showed the same trend as the specific surface area at elevated temperatures. This may attribute to the microstructure changes when the cellulose was pretreated by different swelling agents. The TFA swelling produces more pore structures on the surface of cellulose and destroys partial hydrogen bonds between cellulose molecules.19 However, to a certain degree, the crystalline structure is still partially retained in the cellulose structure. PA swelling generates a more violent disruption of hydrogen bonds for cellulose.23 Furthermore, as shown in Table 1, the average pore diameter distributes in 4−5 nm, and there is no clear 10822

DOI: 10.1021/acssuschemeng.9b01640 ACS Sustainable Chem. Eng. 2019, 7, 10821−10829

Research Article

ACS Sustainable Chemistry & Engineering

qualitatively determine the reaction type in the hydrothermal carbonization of cellulose at different temperatures,25 the Van Krevelen diagram is plotted in Figure 1.

Table 1. Product Yields and Physical Properties of the Hydrochar at Different Temperatures textural properties material CEL CEL-TFA CEL-PA CEL-HTC

TFA-HTC

PA-HTC

T (°C)

P (Bar)

SBET (m2g−1)

VTotal (cm3g−1)a

Da (nm)b

22 32 50 69 22 32 50 69 22 32 50 69

6.690 16.815 14.130 47.935 42.181 118.263 62.730 67.182 87.819 203.456 85.186 77.397 51.903 80.673 196.804

0.011 0.025 0.027 0.083 0.066 0.164 0.085 0.100 0.121 0.266 0.116 0.091 0.087 0.108 0.263

4.834 4.754 5.273 4.921 4.672 4.420 4.660 4.619 4.594 4.506 4.663 4.627 4.790 4.426 4.582

220 240 260 280 220 240 260 280 220 240 260 280

a

Total pore volume. bAverage pore diameter.

Figure 1. Van Krevelen diagrams about CEL-HTC, TFA-HTC, and PA-HTC.

distinction among three cellulose-derived hydrochars. To the total pore volume of the three types of hydrochars, the total pore volume of TFA-HTC generally increased compared to the CEL-HTC at different temperatures. Under the hydrothermal condition of 220 and 240 °C, the total pore volumes of PA-HTC were slightly greater than the CEL-HTC, but it did not appear obvious under the condition of 260 and 280 °C. By analyzing the pore size distribution curve, it was found that the TFA-HTC pore volume under different pore size distributions was significantly larger than that of CEL-HTC. From nitrogen adsorption−desorption isotherms and pore size distributions of CEL-HTC, TFA-HTC, and PA-HTC at different temperatures, after swelling treatment, the BET and pore size distribution of solid products produced by hydrothermal cellulose showed a significant difference. Elemental Composition of Hydrochars. The elemental compositions of raw materials and hydrochars were analyzed, and atomic ratios of oxygen to carbon (O/C) and hydrogen to carbon (H/C) are calculated as shown in Table 2. In order to

The three different types of HTC hydrochar contain low O/ C and H/C atomic ratios when compared with raw materials, and both O/C and H/C atomic ratios decrease with increasing the reaction temperature. The direction vector shows that the dehydration and decarboxylation reactions are the main pathway. The atomic ratios of H/C for three different types of hydrochar are distributed in 0.75−1. According to the research by Visser et al.,26 the structure of hydrothermal carbon is dominated by aromatic rings. As the hydrothermal temperature increases, the values of O/C and H/C decrease as the content of the carbon element increases due to the dehydration and carboxyl reaction being enhanced with the increase of temperature.11 Compared to the CEL-HTC, the content of C in TFA-HTC does not significantly improve, but it increases with different degrees for PA-HTC at different temperatures. The H/C value and O/C value of TFA-HTC and PA-HTC are lower than CEL-HTC.

Table 2. Yield and Elemental Composition of Hydrochar at Different Temperatures elemental analysis sample CEL CEL-TFA CEL-PA CEL-HTC

TFA-HTC

PA-HTC

T (°C)

220 240 260 280 220 240 260 280 220 240 260 280

C (wt %) 42.01 41.61 40.3 67.43 69.14 71.63 72.91 68.00 69.32 71.68 72.85 68.31 69.92 71.72 73.73

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.01 0.04 0.19 0.12 0.00 0.05 0.00 0.07 0.03 0.00 0.06 0.03 0.00 0.01 0.10

H (wt %) 6.48 6.36 6.44 4.76 4.69 4.70 4.73 4.64 4.53 4.57 4.65 4.57 4.41 4.60 4.55

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.02 0.05 0.02 0.01 0.01 0.01 0.01 0.06 0.01 0.00 0.00 0.05 0.03 0.02 0.02

atomic ratio O (wt %)

O/C (at.)

H/C (at.)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.92 0.93 0.99 0.31 0.28 0.25 0.23 0.30 0.28 0.25 0.23 0.30 0.27 0.25 0.22

1.86 1.85 1.92 0.85 0.81 0.79 0.78 0.81 0.78 0.77 0.77 0.80 0.76 0.77 0.74

51.46 52.00 53.25 27.73 26.05 23.53 22.22 27.34 26.12 23.71 22.48 27.09 25.57 23.60 21.61

0.07 0.03 0.22 0.14 0.01 0.04 0.00 0.13 0.04 0.02 0.06 0.05 0.04 0.01 0.13

10823

yield (%)

21.25 25.90 21.55 20.95 32.43 28.16 27.35 26.56 31.20 32.93 31.35 29.57

± ± ± ± ± ± ± ± ± ± ± ±

0.25 2.5 0.75 1.15 1.96 1.09 1.15 0.82 1.06 3.06 0.54 2.70

carbon efficiency (%)

34.11 42.63 37.34 36.26 52.96 48.14 47.11 45.52 52.29 57.17 56.57 51.14

± ± ± ± ± ± ± ± ± ± ± ±

0.46 4.11 0.71 2.00 3.83 0.69 0.09 0.38 1..08 3.30 0.97 3.27

DOI: 10.1021/acssuschemeng.9b01640 ACS Sustainable Chem. Eng. 2019, 7, 10821−10829

Research Article

ACS Sustainable Chemistry & Engineering All of these indicate that a swelling pretreatment would significantly enhance the dehydration and deoxidation of cellulose. Especially, at 220 °C, the H/C value of the HTC hydrochars prepared by swelled cellulose below the value of CEL-HTC is more than 4−5%, while the O/C value only decreases less than 0.01. This demonstrates that the dehydrogenation reaction is enhanced more obviously than the deoxidation reaction during the hydrothermal process of swollen cellulose. The changes of hydrothermal carbon yield and carbon efficiency in the hydrothermal process were analyzed. It was found that the hydrothermal carbon yield and fixed carbon efficiency of CEL-HTC experienced a process of increasing first and then decreasing from 220 to 280 °C. The maximum value was reached at temperature of 240 °C. The solid product yield is consistent with the results of Sevilla et al.,13 which noted that the carbonization temperature of 250 °C results in excellent performance of yield. The yield indexes of PA-HTC follow relevant trends. The yield of hydrochar and fix carbon yield reach 32.93 and 57.17%, respectively, at the hydrothermal temperature of 240 °C. However, the solid material yield and carbon efficiency of TFA-HTC decrease as temperature increases. Both the yield of solid and fixed carbon of PAHTC are higher than that of TFA-HTC at the temperature range of 240−280 °C. Functional Groups on Surface of Hydrochars. The evolving chemical structure of cellulose during the swelling and hydrothermal progress was evaluated through one-dimensional spectroscopy (Figure 3). Compared with untreated cellulose, the functional groups of cellulose are well retained after the TFA swelling treatment, and no new functional group appeared. The absorbance peaks at 3600−3200 cm−1 and 3000−2800 cm−1 attribute to valence vibration of O−H and C−H, respectively.27 The absorbance peak of 1800−900 cm−1 associates with the characteristic functional group on the glucose pyran ring.28 For cellulose swelled by PA, the signal intensities at the typical functional groups O−H and C−H are significantly weakened. Besides, a new absorption peak appears at 2850 cm−1, caused by the aliphatic C−H stretching vibration, which is marked by circles in the enlarged view of Figure 2c. The spectral characteristics of the band 3000−2800 cm−1 according to the aliphatic C−H stretching vibration were further analyzed for PA-CEL. The symmetry and dissymmetry vibration of C−H on the methyl group was located on the absorption peaks at 2872 and 2962 cm−1, respectively. On the methylene group, an absorption peak of the symmetry vibration of C−H located at 2853 cm−1 and absorption peak of C−H asymmetric vibration appear at 2926 cm−1. Generally, the asymmetric peaks have a more intensive absorbance.29 The overlap of absorption peaks appears in the case of the resolution not being high enough, and the absorption peak displacement occurs due to the connection of different chemical structures. After the swelling progress, PA-CEL translates more thoroughly from the crystalline structure to the amorphous structure than TFA-CEL because of the disruption of the intermolecular hydrogen bonds. The symmetric vibration of C−H on the methylene group of the glucopyranose ring results in a new absorption peak at 2850 cm−1.30,31 There is a disappearing absorption peak at 1431 cm−1 in the FTIR of PA-CEL, which is attributed to the H− C−H bond symmetrical bending vibration. The difference indicated that the vibration form of methylene on the pyran

Figure 2. FTIR spectra of CEL-HTC, TFA-HTC, and PA-HTC. (a) FTIR spectra of cellulose and its derived hydrochar; (b) FTIR spectra of TFA-swelled cellulose and its derived hydrochar; (c) FTIR spectra of PA-swelled cellulose and its derived hydrochar. 10824

DOI: 10.1021/acssuschemeng.9b01640 ACS Sustainable Chem. Eng. 2019, 7, 10821−10829

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ACS Sustainable Chemistry & Engineering

Figure 3. TG and DTG curves for pyrolysis CEL-HTC (a), TFA-HTC (b), and PA-HTC (c)

spectrograms, the three strong auto peaks at 3400, 2920, and 2851 cm−1 occur at the bands of 3700−2700 cm−1. There is a significant difference in intensity of the three auto peaks for different hydrochars. Combined with the one-dimensional spectral analyses, the susceptibility of the C−H stretching vibration on the ring form pyran changes as the hydrothermal temperature increases in the hydrothermal process of CELHTC, TFA-HTC, and PA-HTC. The positive cross peaks (3400 and 2820 cm−1) observed in the synchronous spectrum indicate that the stretching vibrations of O−H and C−H are the same in the hydrothermal process of cellulose and present a strong synergy. The intensity of the auto peaks and cross peaks was further analyzed.37 It was found that CEL-HTC and PA-HTC showed a strong auto peak at 3400 cm−1, while TFA-HTC appeared at 2920 and 2851 cm−1. The major positive cross peaks (3400 and 2920 cm−1) are observed in Figure S2a,b,c. C−H and O− H have a strong synergistic effect in the hydrothermal process of CEL-HTC, and this effect is weakened through swelling treatment. The asynchronous correlation spectral in bands 3700−2700 cm−1 is analyzed to obtain the variation order of the spectral signal. In the case of positive correlation, the spectral change of V1 precedes V2, while the spectral intensity of V1 changes before V2 in a negative correlation. CEL-HTC showed positive correlation peaks at 2920 and 3400 cm−1 and 2851 and 3400 cm−1, indicating that the change of the O−H stretching vibration was later than C−H because of the external disturbance of the hydrothermal temperature. The same positive correlation peaks appear in TFA-HTC, but the intensity of the peak was significantly enhanced. For PA-HTC, only a positive correlation peak at 2920 and 3400 cm−1 occurs. Combined with the one-dimensional spectrum of swelled cellulose, we can conclude that the methylene (−CH2) structure of PA-CEL was exposed by a swelling process. When the swollen cellulose suffers hydrothermal carbonization, the methylene groups on the surface easily have a dehydration reaction that results in the intensity decreasing in the absorption peak of C−H. The synchronous correlation spectral in the region of 1800− 900 cm−1 is shown in Figure S2h,i,j. In the region of 1800− 1500 cm−1, the strong auto peaks at 1701 and 1616 cm−1, which cross each other, indicated that CO and CC are highly correlated in the hydrothermal process. While the auto peaks appearing at 1440, 1383, and 1207 cm−1 in the bands range of 1500−1100 cm−1 correspond to the spectrum of C O and O−H on the aromatic ring. The intensities of the auto peaks for different hydrochars in this region show a significant difference, which indicates that the functional group character-

ring in cellulose changes due to functional group exposure by PA swelling treatment.32 The HTC hydrochar prepared by three different cellulose materials has the same spectral characteristics as observed from the FTIR spectrum information. This indicates that the hydrothermal solid product has similar chemical structure functional groups on the surface. The 3600−3200 cm−1 peaks are the stretching vibrational band of O−H on the hydroxyl (−OH) and carboxyl (−COOH),13 and the absorption peak at 1612 cm−1 is associated with the stretching vibration band of the CC bond on the aromatic ring.33 The aromatic C−H out-of-plane bending vibration occurs at 875−750 cm−1, which indicates that the dehydration and aromatization reactions occur during hydrothermal carbonization. This is consistent with the results obtained from elemental analysis. Compared with the spectral of cellulose, CEL-HTC, TFA-HTC, and PAHTC not only have an absorption peak at the 1612 cm−1 wavenumber, but also emerge a new absorption band at 1701 cm−1, which is assigned to the stretching vibration of CO. The absorption bands at the 1000−1500 cm−1 region correspond to the symmetric stretching vibration of C−O and bending vibration of OH in a hydroxyl, ester, or ether group.34 There is a remarkable change in the intensity of the bands at 2920 and 2851 cm−1 of the hydrochar. TFA-HTC changed significantly with an increase in reaction temperature. Two narrow peaks assigned to the C−H stretching vibration occur at the temperatures of 240 and 260 °C, which can reflect the branch chain index of hydrochar and indicate that there are a large number of aliphatic chain branches or alkyl groups on the surface of TFA-HTC. In order to further examine the functional group changes of HTC hydrochars prepared by different swelled celluloses, twodimensional correction spectroscopy35 was analyzed, as shown in Figure S2, where red indicates a positive correlation, and blue indicates a negative correlation. The FTIR spectral information was interpreted through two-dimensional correlation spectral processing, and the spectral resolution was also improved. Meanwhile, by analyzing the synchronous cross peak and asynchronous cross peak, the change order of different functional group vibrations with the specific external disturbance will be acquired. Spectral information on HTC hydrochar at 240 °C in V1 and V2 on the left and top side of correlation map is used as the reference spectrum. The accuracy level is set to 20 for fully reflecting the real existence relevant peaks.36 The fine spectral information on different hydrochars caused by the overlap of the spectrum and the hydrothermal temperature disturbance was given by twodimensional correction analysis. As shown in synchronous 10825

DOI: 10.1021/acssuschemeng.9b01640 ACS Sustainable Chem. Eng. 2019, 7, 10821−10829

Research Article

ACS Sustainable Chemistry & Engineering

By analyzing the intensity of the DTG peak, it was found, that the maximum pyrolysis rate of CEL-HTC was higher than TFA-HTC and PA-HTC at different temperatures. This indicates that CEL-HTC has a more violent reaction during the pyrolysis process. Combined with the results of elemental analysis, the swelling treatment makes cellulose undergo a more intense dehydrogenation and deoxygenation in the hydrothermal process compared with cellulose raw materials. Therefore, a more stable chemical structure in hydrochar was generated. As a result, the weight loss of hydrothermal carbons obtained by the swelled cellulose is decreased in the pyrolysis progress. The weight loss rate after TGA of TFA-HTC and PA-HTC is lower than that of CEL-HTC, which also proves that carbonization in the HTC process was enhanced via swelling cellulose. The results also implied that the weight loss rate of hydrothermal carbon obtained at high temperature is lower than that at a low temperature. Morphology of Swelled Cellulose Hydrochars. The surface morphology of cellulose before and after swelling was evaluated under SEM. Figure 4a,b,c shows that untreated cellulose has a regular rod shape. Due to the intrusion of the TFA cyclic dimer, the surface morphology of the cellulose was irregular, and the particle size was significantly reduced after TFA swelling compared to untreated materials. After the PA treatment, the cellulose morphological surface changes generated an irregular loose-layered structure with numbers of micropores on the surface. Figure 4 shows scanning microscopy micrographs of hydrothermal carbon at temperatures of 220 and 280 °C. It can be observed, that the swelling pretreatment cellulose affects both particle size as well as microscopic arrangement of hydrochar. CEL-HTC and TFAHTC formed at different hydrothermal temperatures exhibit an uniform spherical particle with a homogeneous average size, while the PA swelling led the carbonaceous particle of PAHTC to have different sizes. In addition, the particle of PA-HTC showed irregular shapes, as shown in Figure 4f,i. Under hydrothermal conditions of 220 °C, the hydrothermal carbon microspheres of CELHTC have a particle size of about 0.1 μm, while TFA-HTC have an estimate diameter of 0.2 μm at the same temperature, which is equivalent to the hydrothermal carbon microsphere diameter size of CEL-HTC-280. When the hydrothermal reaction temperature reached 280 °C, the diameter of the TFA-HTC microspheres is about 0.5 μm, and some hydrothermal carbon microspheres with larger particle sizes were present in TFA-HTC and PA-HTC. The particle size of hydrothermal carbon microspheres increases with the increase of temperature.

istics of hydrothermal carbon formed by different swelling pretreatments have obvious differences in the degree of aromatization. There is a strong center of the auto correction peaks for CEL-HTC, with a middle center of the auto correction peaks for TFA-HTC, and a minor center of the auto correction peaks for PA-HTC. The strong cross peaks at 1616 and 1440 cm−1, 1616 and 1383 cm−1, and 1616 and 1290 cm−1 are observed in the asynchronous spectral for CEL-HTC, TFAHTC, and PA-HTC. Both CEL-HTC and TFA-HTC also have cross peaks at 1701 and 1440 cm−1, 1701 and 1383 cm−1, and 1701 and 1290 cm−1, but PA-HTC did not appear to have cross peaks at this position. It was indicated that the CO and CC of CEL-HTC and TFA-HTC have the same change tendency with aromatic functional groups during the hydrothermal process. In addition, it can be seen from the asynchronous spectral, that CEL-HTC and TFA-HTC have a large number of centered cross peaks, while PA-HTC only appears to have weak positive correlation peaks and negative correlation peaks, which indicates that temperature has a greater influence in the region of 1800−900 cm−1 for CELHTC and TFA-HTC than PA-HTC. PA-HTC is less sensitive to a change of the hydrothermal temperature because there was a decrease in the absorption of groups after the cellulose was swelled by PA. TG Analysis. TG and DTG curves for pyrolysis of CELHTC, TFA-HTC, and PA-HTC are shown in Figure 3, and the pyrolysis characteristics of hydrochars are listed in Table 3. As Table 3. Pyrolysis Characteristics of Hydrochars sample

T (°C)

Ti (°C)a

Tpeak (°C)b

αpeak (%)c

Vpeakd

W900 (%)e

CEL-HTC

220 240 260 280 220 240 260 280 220 240 260 280

328.4 346.7 349.9 363.4 333.3 341.6 332.7 352.3 315.5 343.3 339.7 353.5

422.8 429.8 452.3 453.3 430.8 433.0 442.8 461.3 423.8 436.3 452.7 473.3

20.85 18.01 17.72 16.34 15.42 13.39 15.33 13.98 16.61 19.51 18.12 16.26

1.73 1.81 1.51 1.44 1.55 1.67 1.43 1.37 1.39 1.71 1.41 1.25

50.77 50.51 46.96 40.96 46.53 44.91 43.21 40.56 55.71 49.00 43.78 43.40

TFA-HTC

PA-HTC

a

Ti, the temperature corresponds to the initial pyrolysis. bTpeak, the temperature corresponds to the peak valley of TGA curve. cαpeak, the conversion rate at the Tpeak. dVpeak, the pyrolysis rate at peak of DTG. e W900, weight loss rate at 900 °C.

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DISCUSSION The thermodynamics in the hydrothermal carbonization process is analyzed. With the increase of the hydrothermal temperature and the self-generated pressure by they hydrothermal carbonization process, the characteristics (solubility, reactivity, and a certain acidity) of water as a solvent medium occur corresponding changes. In addition, the change of the hydrothermal environment of cellulose affects the mechanism in the hydrothermal carbon formation process, which leads to a difference in the particle size of hydrothermal carbon microspheres at four gradient temperatures. TFA swelling is capable of reducing the hydrothermal temperature required for the reaction. The uniform particle size and the serious agglomeration phenomenon for PA-HTC indicate that changes

depicted in the TG curves, the thermal decomposition behavior of the three different hydrochars showed the same trend. It mainly goes through three stages, namely, the preheating stage, weight loss stage, and carbonization stage.38 Weight loss also occurs at the progress of slow reduction; rapid reduction slows weight loss. The volatiles in hydrothermal carbon begin to separate out from Ti. For Ti to Tpeak, the rates of mass loss for the three hydrochars at different temperatures are significantly increased. The DTG peak position of CELHTC, TFA-HTC, and PA-HTC shifted to a high temperature with the increase of the hydrothermal temperature, indicating that the thermal stability improves with the increase of hydrothermal temperature, and it required a higher temperature for separating out volatiles. 10826

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Figure 4. SEMs of hydrochars and swelled cellulose.

undergoes reactions of the pyrolysis-like progress, characterizing with cellulose intramolecular condensation, dehydration, and decarboxylation. This reaction mechanism is occupied mainly in the cellulose hydrothermal conversion progress that results in a large amount of aromatic functional groups on the hydrothermal carbon structure. Under TFA and PA swelling, the microstructure and physicochemical properties of cellulose were changed. Zhang et al.19 analyzed the cellulose structure after swelling of TFA and PA by NMR, and pointed out that, in the natural state, cellulose is mainly composed of crystalline and para-crystalline structures. Besides, there is a partial amorphous structure which included solvent accessible and inaccessible fiber surfaces consisting within a crystalline region. After TFA and PA swelling treatment, the crystal structure of cellulose is destroyed, and the intermolecular hydrogen bonds are destroyed or rearranged, leading the crystal structure to be converted into an amorphous one. However, the substructure of cellulose swelled by TFA and PA showed obvious differences, since they differently impacted the swelling agents. Due to PA resulting in a more thorough swelling change in cellulose substructure than TFA, there are plenty of solvent accessible fiber surfaces that can sufficiently make contact with solvent water. This will accelerate the depolymerization of the cellulose molecular chain, resulting in a particle hydrothermal microsphere of PA-HTC with a larger size at the same temperature, as shown in Figure 4f. Compared with PA-CEL, after TFA swelling treatment, cellulose is not completely converted into an amorphous state, and each para-crystalline and crystalline region is still retained at about 15%. At the hydrothermal progress, the amorphous cellulose unit is hydrolyzed first, and the crystal structure of cellulose acts as a skeleton to support the nucleation of hydrothermal carbon. This also explains that the particles of TFA-CEL arrange in a spatial structure in the hydrothermal process. CEL-HTC, TFA-HTC, and PA-HTC are significantly different in uniformity and dispersion, as revealed in Figure 4. The crystalline structure occupies a dominant percentage in untreated cellulose. As a result, in the progress of the cellulose hydrothermal conversion process, the cellulosic substrate

in the chemical structure of the cellulose after PA swelling treatment have a significant affect in the physicochemical properties of the hydrothermal products. Cellulose, a linear polymer, possesses an extended, firm, rodlike structure with the aid of the flat bond conformation of the glucose residue. The chains are closely linked together to form a high tensile strength structure because of the intermolecular hydrogen bonding.39 The diagram of the evolution mechanism in the HTC of cellulose and swelled cellulose is showed in Figure 5. When cellulose is treated under hydrothermal

Figure 5. Evolution mechanism in the HTC of cellulose and swelled cellulose.

carbonization, the fibrous network disrupted partly, leading to nano/micro-sized fragments being formed. In order to minimize the contact of the interface with the hydrothermal solution environment, the cellulose fragment unit collapsed into a bulk with a spherical shape. Cellulose exposed to hydrothermal liquids and cellulose in the encapsulation spheres of fragments have different hydrothermal carbonization mechanisms. Part of the cellulose in full contact with the homogeneous aqueous phase is hydrolyzed into glucose and undergoes the same progress of glucose-derived hydrothermal carbon formation.40 While the cellulose inside of the bulk 10827

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mainly undergoes the pyrolytic-like progress to form a hydrothermal carbon. The homogeneous aqueous phase preferably leads to the uniformity and dispersion of the hydrothermal carbon microspheres. PA-CEL is exposed to a large amount of solvent accessible fibril surfaces due to the cellulose structure converting from crystal to amorphous. PACEL is easy to hydrolyze and undergoes the same progress of the glucose hydrothermal carbon formation. Some microspheres with a diameter of 2.5 μm appear in Figure 4i, also demonstrating this fact. However, for dominating pyrolytic-like progress to form a hydrothermal carbon, the solvent accessible fibril surfaces show that PA-CEL particles tend to be easy to contact with surrounding water and difficult to form a spherical bulk. These cause the hydrothermal carbon to have an irregular shape. However, both the crystalline and amorphous structure in TFA-CEL bring about the two hydrothermal reaction processes occuring simultaneously at a certain ratio, as show in Figure 4e,h of SEM. The arrangement of TFA-HTC is partially ordered, and there are large microspheres with a partial diameter of 2 μm in SEM. So we can conclude, that the transformation of the cellulose crystal structure, stimulated by PA and TFA swelling agents, is the main reason to make a difference in the physical and chemical properties of hydrochars.

AUTHOR INFORMATION

Corresponding Author

*Tel: +86 88982142; E-mail: [email protected]. ORCID

Ximing Zhang: 0000-0002-4889-8791 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Zhejiang Province (LQ19B060010) and China National Key R&D Program (2016YFD0800804).

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CONCLUSIONS Due to the TFA and PA swelling pretreatment, physical properties, surface function groups, as well as microscopic morphology of the resultant cellulose of HTC show distinct differences. CEL-HTC formed at different hydrothermal temperatures exhibited an uniform spherical particle with homogeneous average size, the spherical particles arranged orderly, and its size increases with the change of temperature. TFA-HTC formed at different hydrothermal temperatures also exhibited an uniform spherical particle with a homogeneous average size, while the particles arrange in a spatial structure. The most difference is the branch chain index increasing on TFA-HTC at hydrothermal temperatures of 240 and 260 °C. As for the cellulose-swelled PA, it led to the carbonaceous particle of PA-HTC having different sizes. The swelling treatment makes cellulose undergo a more intense dehydrogenation and deoxygenation in the hydrothermal process compared with cellulose raw materials, which results from forming a more stable chemical structure in hydrochar. This study also shows how the swelling pretreatment affected the HTC mechanism of cellulose. Overall in this work, a novel way to synthesize functional carbon materials will be opened up for efficient utilization of the lignocellulosics.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01640. Nitrogen adsorption−desorption isotherms and pore size distributions of CEL-HTC, TFA-HTC, and PAHTC at different temperatures and two-dimensional correction spectroscopy of synchronous and asynchronous spectrograms of hydrochars (PDF) 10828

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