Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions

Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions

CLAY-03459; No of Pages 10 Applied Clay Science xxx (2015) xxx–xxx Contents lists available at ScienceDirect Applied Clay Science journal homepage: ...

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CLAY-03459; No of Pages 10 Applied Clay Science xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions Lin Mei Wu a,b, Dong Shen Tong a, Chun Sheng Li b, Sheng Fu Ji c, Chun Xiang Lin d, Hui Min Yang e, Zhe Ke Zhong e, Chuan Yun Xu b, Wei Hua Yu a, Chun Hui Zhou a,f,⁎ a Research Group for Advanced Materials & Sustainable Catalysis (AMSC), State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China b Key Laboratory of Clay Minerals of Ministry of Land and Resources of The People's Republic of China, Zhejiang Institute of Geology and Mineral Resources, Hangzhou 310007, Zhejiang, China c State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029 China d Australian Institute of Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Queensland 4067, Australia e China National Bamboo Research Center, Hangzhou 310012, China f The Institute for Agriculture and the Environment, University of Southern Queensland, Toowoomba, Queensland 4350, Australia

a r t i c l e

i n f o

Article history: Received 4 March 2015 Received in revised form 21 May 2015 Accepted 10 June 2015 Available online xxxx Keywords: Clay minerals Montmorillonite Cellulose Hydrochar Nanocomposite Hydrothermal carbonization

a b s t r a c t Montmorillonite-hydrochar nanocomposite is of great significance in biofuel production, energy storage and conversion, catalysis, environmental protection, and biology and medicine. To understand how montmorillonite facilitates the conversion of cellulose into hydrochar, it is essential to accurately control the properties and structure of hydrochar and its nanocomposites for technological applications. In this work, hydrochar and montmorillonite-hydrochar nanocomposites were produced by hydrothermal carbonization of cellulose at 200 °C for 2–24 h in the absence and the presence of montmorillonite. The roles of montmorillonite in the formation of hydrochar were explored in detail. Results showed that montmorillonite acted as a catalyst, an adsorbent, and an inorganic template for hydrochar formation. The formation process of montmorillonite-hydrochar nanocomposite can be divided into two stages: first, the hydrolysis of cellulose yielding degraded liquid products (e.g. glucose, fructose, and organic acids); second, the formed liquid products were adsorbed by montmorillonite, which catalyzed condensation and aromatization to form hydrochar particles on the surface of montmorillonite. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Char produced by thermal carbonization of biomass has attracted much interest because of its uses and potentials in biofuel production, energy storage and conversion, catalysis, and environmental protection (Lehmann et al., 2006; Titirici and Antonietti, 2010). According to the process of the thermal conversion, the biomass-derived char can be roughly divided into two categories: biochar and hydrochar. Biochar refers to the char product formed by slow dry pyrolysis of biomass at a temperature between 180 °C and 450 °C and characterized by high aromaticity (Keiluweit et al., 2010). Hydrochar, also known as hydrothermal carbon, is a solid carbon-rich material produced from the hydrothermal carbonization of biomass (either isolated carbohydrates or crude plants) (Sevilla and Fuertes, 2009). Hydrochar is usually produced by the hydrothermal carbonization of biomass or biomassderived organic compounds such as saccharides (glucose, sucrose

⁎ Corresponding author at: Research Group for Advanced Materials & Sustainable Catalysis (AMSC), State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China. Tel./fax: +86 571 88320062. E-mail addresses: [email protected], [email protected] (C.H. Zhou).

or furfural, etc.) at temperatures in the 150 °C–350 °C range (under autogenous pressure or critical pressure) in the presence of water (Titirici et al., 2008; Sevilla and Fuertes, 2009). However, using a hydrothermal process to prepare hydrochar proves to have some advantages over using a pyrolysis process to obtain biochar. The hydrothermal process is usually conducted at relatively low temperatures (150 °C–350 °C) than dry pyrolysis processes. In addition, in a hydrothermal process, wet biomass can directly be used as the feedstock (wet animal manures, algae, etc.). Comparatively, a pyrolysis process is more energy-intensive both for heating the reactor and drying the feedstock (Xue et al., 2012). Hydrochar has been applied to water purification, energy storage (electrodes in Li ion batteries and carbon fuel cells), and CO2 sequestration (Hu et al., 2010; Titirici and Antonietti, 2010; Basso et al., 2013). Furthermore, hydrochar features a chemical structure comparable to fossil coals (Schuhmacher et al., 1960). Hydrochar is therefore regarded as an artificial storable biofuel; it can be combusted alone or with low-rank fossil coals (Libra et al., 2011). Converting biomass to hydrochar is an effective and economically feasible approach to sustainable energy production. In an attempt to obtain multifunctional and hierarchical-structured hydrochar materials with improved properties, similar hydrothermal carbonization have also recently been explored for the production of

http://dx.doi.org/10.1016/j.clay.2015.06.015 0169-1317/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: Wu, L.M., et al., Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.06.015

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various hydrochar nanocomposites, including metal-hydrochar (Xuan et al., 2007), silicon-hydrochar (Cakan, 2008), and mineral-hydrochar nanocomposites (Chen et al., 2011). For example, Ding et al. (2012) synthesized a nickel-hydrochar nanocomposite via the hydrothermal carbonization of carbohydrates (glucose, xylose, and fructose) in a nickelous acetate solution and investigated its electrochemical performance. The results showed that such a nickel-hydrochar nanocomposite effectively improved the specific capacity of pure hydrochar by 149%. Cakan (2008) obtained spherically shaped silicon-hydrochar nanocomposites via the hydrothermal carbonization of glucose in the presence of silicon nanoparticles. The resultant silicon-hydrochar nanocomposite displayed a highly stable reversible capacity (ca. 460 mA h g−1) at a current rate of 300 mA g−1 in the voltage range of 0.05 to 1.2 V. This capacity is remarkably higher than that of pure hydrothermal carbon under the same electrochemical conditions (ca. 160 mA h g−1). Of further interest, the researchers could demonstrate that mineral-hydrochar nanocomposites are capable of functional adsorption-desorption and biofuel-storage using earth-abundant resources. Mineral-hydrochar nanocomposites, for example, have been developed through the hydrothermal carbonization of natural biomass along with natural minerals, especially natural layered clay minerals. The clay minerals used include palygorskite/ attapulgite (Chen et al., 2011; X. Wu et al., 2014; Luo et al., 2015; R. Zhang et al., 2015; X. Zhang et al., 2015; Zhou et al., 2015), and montmorillonite (Mt) (Ai and Li, 2013; R. Zhang et al., 2015; X. Zhang et al., 2015). Compared to pure carbon materials, attapulgite-hydrochar nanocomposites exhibited higher adsorption capacities for Cr (VI) and Pb (II) ions, reaching up to 177.74 and 263.83 mg/g, respectively (Chen et al., 2011). Ai and Li (2013) produced Mt-hydrochar nanocomposites by hydrothermal process and evaluated their performances in the adsorptive removal of organic dyes (methylene blue) in aqueous solution (Ai and Li, 2013). Compared to the adsorption capacities of methylene blue onto various adsorbents (activated carbon, bentonite, iron terephthalate, titanate nanotubes, carbon–graphene hybrid, etc.), Mt-hydrochar nanocomposites exhibited a much higher capacity in the removal of methylene blue (194.2 mg g − 1) than other adsorbents. X. Wu et al. (2014) prepared palygorskite-hydrochar nanocomposites at 210°C–250 °C for 2–48 h and investigated the adsorption capacities of phenol. The palygorskite-hydrochar nanocomposite obtained at 250 °C in 48 h exhibited the highest adsorption of phenol (92 wt.%), which was significantly higher than that of raw palygorskite (18 wt.%). Among layered clay minerals, the use of naturally occurring Mt for hydrochar nanocomposite has peculiar advantages over other minerals. Mt is a 2:1 clay mineral composed of aluminosilicate layers with exchangeable, hydrated cations in the interlayer spaces. Thus, the twodimensional layer and the expandable interlayer space have all been used as an inorganic template to produce a high surface area of carbonous material (Sonobe et al., 1990; Bakandritsos et al., 2004; Jiang et al., 2010). In addition, each aluminosilicate layer consists of two Si–O tetrahedral sheets sandwiching an Al–OH octahedral sheet by means of –Si–O–Al– O–Si– bonds (Bergaya and Lagaly, 2013). Such naturally occurring Mt provided large surface areas and solid acidic sites for catalysis and adsorption (Aylmore et al., 1970; Zhou et al., 2012; Zhou and Keeling, 2013). Therefore, the addition of Mt into the hydrothermal system should play a catalytic role in the transformation of biomass into hydrochar and form a nanocomposite material with hydrophilic (mineral) and hydrophobic (hydrochar) phases. This allows the effective adsorption of both polar and nonpolar substances (Ai and Li, 2013). In addition, the evolution of biomass to hydrochar, namely, the mechanism of hydrochar formation, should differ considerably from the mechanism driving the hydrothermal carbonization of biomass (e.g. cellulose) (Sevilla and Fuertes, 2009; Funke and Ziegler, 2010). Therefore, our focus was to clarify the interactions between Mt and cellulose under hydrothermal conditions. The evolution of biomass to hydrochar, and the role of Mt in this evolution, was explored and the possible mechanism was proposed.

2. Experiment 2.1. Preparation of Mt-hydrochar nanocomposite Microcrystalline cellulose (MCC) was purchased from Yi Xing Shen De Li Synthetic Leather Material Co., Ltd., China and was used without further treatment. The cellulose content was above 99% (wt.%), and the particle size of MCC was 48 μm. Naturally occurring Mt from a bentonite was used. The chemical composition of bentonite was determined by using an X-ray fluorescence spectrometer (XRF, ARL ADVANT’X, IntelliPower™ 4200, ThermoFisher) and contained 3.54% Na2O, 4.63% MgO, 15.28% Al2O3, 71.38% SiO2, 0.02% P2O5, 1.32%, K2O, 2.30% CaO, 0.03% TiO2, 0.07% MnO2, and 1.35% Fe2O3. Mt dispersion was prepared by adding 0.6 g of Mt powder to 10.0 g of distilled water under vigorous magnetic stirring for 30 min. 1.0 g of MCC was then added to the Mt dispersion and the mixture was stirred at room temperature for 30 min. The resultant mixture was transferred into a 30 mL Teflon-lined stainless steel autoclave and hydrothermally treated at 200 °C under autogenic pressure. A series of reactions was conducted at 2, 4, 8, 16, 20, and 24 h. Thereafter, the autoclave was cooled to room temperature naturally. Following the addition of 30 mL of distilled water into the autoclave, all products were taken out and the solid was separated by centrifuge. The as-prepared Mt-hydrochar nanocomposite (solid fraction) were collected and dried at 70 °C for 24 h. The Mt-hydrochar nanocomposite samples using different reaction times for preparation were labelled as 2 h-MC, 4 h-MC, 8 h-MC, 16 h-MC, 20 h-MC, and 24 h-MC. The liquid fraction was filtered through quantitative filter paper (pore size 10 μm; Hangzhou Special Paper Industry, Zhejiang, China) to remove the remaining floating solids that could not be separated during centrifugation. Finally, the filtrate was transferred into a 100-mL volumetric flask and diluted to a required volume with distilled water for the analysis of liquid products. In each control experiment, either 1 g of MCC alone or 0.6 g of Mt alone was added into 10.0 g of distilled water under vigorous magnetic stirring for 30 min. The mixture was transferred into a 30 mL Teflon-lined stainless steel autoclave and hydrothermally treated at 200 °C under static autogenic pressure. The reaction time was 2, 4, 8, 16, 20, and 24 h. When the autoclave cooled down to room temperature, the solid products were collected by centrifuge. The samples from the treatment of MCC alone were labelled as 2 hC, 4 h-C, 8 h-C, 16 h-C, 20 h-C, and 24 h-C; those from the treatment of Mt alone were labelled as 2 h-M, 4 h-M, 8 h-M, 16 h-M, 20 h-M, and 24 h-M. 2.2. Quantitative analysis of liquid products The amounts of reducing sugar (RS), furfural and its substitutes (FF), and organic acid (Acd) were measured immediately following the reaction. The 3,5-dinitrosalicylic acid (DNS) method was employed as a standard protocol to determine the amount of RS (Miller, 1959). Furfural and its substitutes were determined using a colorimetric method (Dinsmore and Nagy, 1974). The amount of organic acid was determined by titration of the aqueous solution with an aqueous 0.1 mol/L NaOH solution. 2.3. Removal of Mt from Mt-hydrochar nanocomposite To isolate and analyze the fraction of hydrochar in an Mt-hydrochar nanocomposite, a mixed aqueous HCl/HF solution was used to dissolve Mt. In a typical procedure, a 1.0 g sample was treated with 20 mL of a mixture of 6 mol/L HCl and 40% (wt.%) HF at a 1:2 volumetric ratio. Following the reaction was carried out at room temperature for 24 h, the sample was repeatedly washed with distilled water until pH 7 of the filtrate was reached. The solid Mt-free sample was then dried at 70 °C for 24 h. Similar procedures were used to prepare a series of

Please cite this article as: Wu, L.M., et al., Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.06.015

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samples treated for different times and labelled as HF-2hMC, HF-4hMC, HF-8hMC, HF-16hMC, HF-20hMC, and HF-24hMC. 2.4. Characterization Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 6700 Fourier transform spectrometer. The FTIR spectra of samples were measured by collecting 124 scans at a resolution of 2 cm− 1 in the wave number ranging from 4000 to 400 cm− 1 . The X-ray

(a)

3

diffraction (XRD) patterns were collected on a PNAlytical X’Pert PRO diffractometer with Cu Ka radiation (λ = 1.54056 Å). The elemental analyses were conducted on an elementary analyzer (Vario MICRO Cube, Germany). The oxygen content (percentage by weight) was calculated according to Oxygen% = 100% − Carbon% − Hydrogen%. Scanning electron microscopy (SEM) analysis was conducted on a Hitachi S-4700 (II) electron microscope. The liquid products were analyzed on an ultraviolet-visible (UV-Vis) spectrophotometer (UV2550, Shimadzu Co., Japan).

(b)

(c)

(d)

(e)

Fig. 1. FTIR spectra of samples: (a) hydrochar samples, (b) Mt-hydrochar nanocomposites, (c) Mt-free samples obtained by HCl/HF-treated hydrochar-Mt composites, (d) FTIR absorbance spectra in the 900–700 cm−1 region of hydrochar samples, and (e) FTIR absorbance spectra in the 900–700 cm−1 region of Mt-free samples.

Please cite this article as: Wu, L.M., et al., Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.06.015

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3. Results and discussion 3.1. The effect of Mt on the chemical structure of hydrochar The FTIR spectra of raw MCC and MCC-derived hydrochar obtained at different reaction times (2 h, 4 h, 8 h, 16 h, 20 h, and 24 h) are illustrated in Fig. 1a. Characteristic absorption of MCC displayed absorbance bands at 3400–3300 cm − 1 (O–H stretching), 2900 cm− 1 (C–H stretching), 1430 cm− 1 (CH2 deformation), 1370–1315 cm− 1 (C–H deformation and C–OH deformation), 1160–896 cm− 1 (skeletal vibration involving C–O stretching, glycosidic linkages between sugar units), and 668 cm− 1 (O–H out-of-plane bending in C–OH alcoholic groups) (Sakanishi et al., 1999). The FTIR spectra of the sample (2 h-C) is similar to that of the raw MCC, indicating that no obvious chemical transformation occurs when the MCC is hydrothermally treated at 200 °C for less than 2 h (Fig. 1a). However, with the increased reaction time (≥4 h), the obtained hydrochars exhibited different FTIR spectra compared with the sample 2 h-C (Fig. 1a). Characteristic absorption bands of MCC disappeared in the FTIR spectra of the samples (4 h-C to 24 h-C) and some new absorption bands (e.g. bands at 1700 cm−1 and 1600 cm−1) appeared. These changes relate to different events occurring in hydrothermal carbonization processes. Our three main observations are discussed in detail below: (1) There is a remarkable decrease in the intensity of the bands at 1160–896 cm− 1 (associated to C–O stretching of glycosidic linkages between sugar units) and the broad band at 3400–3300 cm−1 (associated to O–H stretching vibrations in hydroxyl or carboxyl groups). The decrease suggests that hydrolysis and dehydration occurred during the hydrothermal carbonization of cellulose (Fig. 1a). (2) The presence of aromatic rings is also evidenced by the band at 1600 cm−1 attributed to C = C vibrations, and the bands in the 900–750 cm− 1 region assigned to aromatic C–H out-of-plane bending vibrations. This reveals that the polymerization and aromatization processes took place during hydrothermal carbonization (Fig. 1a). (3) The aliphatic structures, ether structures, and acid groups formed in the samples (4 h-C to 24 h-C) could be deduced from the bands at 3000–2815 cm−1 (aliphatic C − H), 1700 cm−1 (C = O vibrations corresponding to carbonyl, quinone, ester or carboxyl), and 1000–1460 cm−1 (C–O stretching vibrations in hydroxyl, ester, or ether) (Fig. 1a). All these results suggest that, with an increase in reaction time, the solid carbon-rich hydrochar is produced through the hydrothermal carbonization of cellulose. However, compared with the spectra of hydrochar samples (Fig. 1a), there are obvious differences in the FTIR spectra of Mt-hydrochar nanocomposite (Fig. 1b). The resulting Mt-hydrochar nanocomposite appeared to contain unchanged Mt (Fig. 1b), as reflected by its FTIR spectrum with the typical bands of Mt at 1113, 1035, 797, 530, and 464 cm− 1. These bands were attributed to vibrations from out-ofplane stretching of Si–O, in-plane stretching of Si–O, quartz, Si–O–Al bending (octahedral Al), and Si–O–Si bending, respectively (Fig. 1b) (Wu et al., 2014a). In addition to the typical bands of Mt, the bands at 3000–2815 cm−1 (aliphatic C − H), 1700 cm−1 (C = O vibrations corresponding to carbonyl, quinone, ester or carboxyl), and 1600 cm− 1 (C = C stretching vibrations in aromatic rings) are also visible (Fig. 1b). However, the intensities of these bands in Mt-hydrochar nanocomposite decreased compared to those in the spectra of the hydrochar samples (4 h-C to 24 h-C). The bands at 1000–1460 cm−1 (C–O stretching vibrations in hydroxyl, ester, or ether) and 900 to 750 cm−1 (aromatic C–H out-ofplane bending vibrations) are not visible in the FTIR spectra of the Mthydrochar nanocomposite (Fig. 1b). One possible explanation is that the Mt-hydrochar interaction occurred in the hydrothermal condition, or that the hydrochar particles were wrapped by Mt. Thus, the

fraction of Mt in Mt-hydrochar nanocomposite should be removed using an HCl/HF solution to dissolve Mt so that the hydrochar can be released from the mineral matrix. However, it deserves noting that the HCl/HF solution had no significant influence on the structure of hydrochar (Wu et al., 2014a, 2014b). Following treatment in a HCl/HF solution, the characteristic absorption bands of Mt at 1113, 1035, 797, 530, and 464 cm−1 disappeared (Fig. 1c). Also, the spectrum of the sample HF-2hMC was similar to that of the raw MCC, indicating that no obvious chemical transformation occurred when the MCC was hydrothermally treated at 200 °C for 2 h in the presence of Mt (Fig. 1c). In addition, the absorbance bands at 1000–1460 cm−1, attributed to the C–O stretching vibrations of esters, appeared (Fig. 1c). The C–O vibrations are barely recognizable in the samples of Mt-hydrochar nanocomposite due to the strong absorbance bands from the Si–O vibrations of Mt. After the removal of Mt, the intensities of absorbance bands at 1700 cm−1 increased (Fig. 1c) compared to their intensities in Fig. 1b. One possible explanation for the absorption intensity increase at 1700 cm−1 (C = O for carbonyl or carboxyl) was that the hydrochar formed were tightly bonded with Mt rather than a physical mixture of hydrochar and Mt. That is, an interaction occurred between the carboxyl group of hydrochar and hydroxyl group of Mt as per the following sequential reactions (Eqs. (1) and (2)) (Sposito, 1984). Mt  OH þ Hþ ⇆Mt  OHþ 2

ð1Þ

 Mt  OHþ 2 þ OOC  hydrochar⇆Mt  OOC  hydrochar þ H2 O

ð2Þ

Moreover, following HCl/HF acid treatment, the absorbance bands at 900–700 cm−1, attributed to the aromatic C–H out-of-plane bending vibrations, appeared. Fig. 1d and e display the evolution of aromatic hydrogen in the 900–700 cm− 1 zone. The number of adjacent aromatic hydrogens per ring influences and decides the wave numbers of characteristic absorption according to the proposition by Bellamy (Bellamy, 1958). 6H led to an absorption peak at 688 cm− 1, 5H at 710–690 cm − 1 , 4H at 770–735 cm − 1, 3H at 810–750 cm− 1, 2H at 860–800 cm− 1, and 1H at 860 to 900 cm− 1. Regarding the hydrochar samples, three out-of-plane C–H deformation bands were observed in the 900–700 cm− 1 (Fig. 1d). These bands are assigned to aromatic structures with isolated aromatic hydrogens (1H, 886–873 cm−1), two adjacent hydrogens per ring (2H, 810 cm−1) and four adjacent aromatic hydrogens (4H, 749 cm− 1) (Yen et al., 1984). There was no obvious change for the three out-of-plane C–H deformation bands with the increased reaction time (Fig. 1d). However, the sample HF-4hMC exhibited only two types of out-of-plane C–H deformation bands at 873 cm−1 (1H) and 810 cm−1 (2H) (Fig. 1e). With an increase in reaction time, the out-of-plane C–H deformation band at 756 cm−1 (4H) appeared in the FTIR spectra of samples (HF-8hMC and HF-16hMC) (Fig. 1e). Interestingly, with a further increase in reaction time to 24 h, the outof-plane C–H deformation band at 756 cm−1 disappeared. In its place, the out-of-plane C–H deformation bands at 842 cm−1 (2H), 775 cm−1 (3H), and 746 cm−1 (4H) appeared. It is noteworthy that the number of adjacent hydrogens per ring provides an estimate of the degree of aromatic substitution (Ibarra et al., 1996). The aromatic substitution increased with the decrease of adjacent hydrogens per aromatic ring. Thus, it can be inferred that in the presence of Mt, the generated hydrochar exhibited a chemical structure with a lowly substitution aromatic ring with an increase in reaction time. 3.2. The effect of Mt on the evolution of liquid products Cellulose carbonized via hydrothermal treatment not only yields hydrochar (solid product), but also yields liquid products such as reducing sugar (RS), furfural, and its substitutes (FF) and organic acid (Acd) (Xiao et al., 2012). RS, FF, and Acd are typical products of cellulose decomposition under hydrothermal conditions and their quantities can reflect the extent of the hydrothermal reaction of MCC (Sevilla and

Please cite this article as: Wu, L.M., et al., Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.06.015

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(b) RS-C RS-MC FF-C FF-MC

1.0 0.8 0.6 0.4

Acd-C Acd-MC

0.2 0.0 2

4

6

8

10 12 14 16 18 20 22 24 26

Time (h)

C MC C (%): 60.05 H (%): 4.61 O (%): 35.34

1.2

0.8

C (%): 67.07 H (%): 4.77 O (%): 28.16

C (%): 64.02 H (%): 4.48 O (%): 31.51

0.4

0.0 0

C (%): 39.82 H (%): 5.60 O (%): 54.18

1.6 The obtained solid product (g)

Product distribution (mmol/g MCC)

(a) 1.2

5

C (%): 75.03 H (%): 4.63 O (%): 20.34

0

2

4

6

8 10 12 14 16 18 20 22 24 26

Time (h)

Fig. 2. The evolution of (a) liquid products and (b) solid products. C, hydrothermal carbonization of MCC alone; MC, hydrothermal carbonization of MCC in the presence of Mt.

Fuertes, 2009). In addition, when considering the hydrothermal carbonization of MCC alone, hydrolysis, and dehydration of MCC could mainly take place during the reaction period of 0–2 h, as verified by the present experiments. For example, Fig. 2a reveals that the obtained reducing sugar (RS), furfural and its substitutes (FF) reached the maximum after 2 h of reaction time (Fig. 2a; RS-C and FF-C). Meanwhile, as the hydrolysis of MCC proceeded, the weight of raw MCC decreased sharply (Fig. 2b, C). The formed RS and FF derived from the hydrolysis of relatively reactive amorphous regions of MCC and the crystalline regions of MCC were little changed at this stage. There was evidence that the FTIR spectrum of the sample 2 h-C was similar to that of the raw MCC as mentioned above. To determine the relative crystallinity, the crystallinity index (CrI) of cellulose was calculated according to the Segal method: CrI (%) = [(I200 − Iam)/I200] × 100%, where I200 is the intensity value of the (200) reflection at 22.6° and Iam is the intensity value of the reflection of the amorphous phases (2θ = 18.6°) (Segal et al., 1959). The CrI can provide information on the relative amount of crystalline and non-crystalline (or amorphous) components in a cellulosic material. The sample 2 h-C exhibited an XRD pattern similar to that of MCC, indicating that their microcrystalline structure nearly remained intact (Fig. 3a). Nevertheless, the calculated Segal CrI of sample 2 h-C increased to 79.18 compared to raw MCC (CrI = 69.11), indicating that the relatively reactive amorphous regions of MCC could decompose. However, with an increase in reaction time to 20 h, the typical reflections of MCC in the XRD patterns of the samples (20 h-C) disappeared and the Segal CrI sharply decreased to 2.54 (Fig. 3a). These changes indicated that crystalline MCC completely decomposed after hydrothermal treatment at 200 °C for more than 4 h. These phenomena are in agreement with the determinations of the FTIR spectra of the samples obtained at corresponding times. Meanwhile, when reaction time increased to 4 h, the yields of RS and FF decreased sharply (Fig. 2a, RS-C and FF-C). A possible explanation was the decomposition of FF to organic acid (Sevilla and Fuertes, 2009); thus, the amount of organic acid (Acd) reached a maximum at 4 h (Fig. 2a, Acd-C). However, the contents of organic acid (Acd) decreased with a further increase in reaction time to 24 h. This may be caused by the consumption of the autogenetic organic acid for hydrolysis and dehydration of cellulose and the degradation products of cellulose (Fig. 2a, Acd-C) (Funke and Ziegler, 2010). In regards to the hydrothermal carbonization of MCC in the presence of Mt, a large number of RS and FF formed due to the hydrolysis of the reactive amorphous regions of MCC; the maximums of these two liquid products were obtained after 2 h (Fig. 2a, RS-MC and FF-MC). The crystalline regions of MCC nearly remained intact at this stage, as reflected by the XRD pattern of the sample 2 h-MC, which exhibits characteristic

reflections of MCC at 14.9°, 16.4°, and 22.6° assigned to the (1–10), (110), and (200) reflections (Fig. 3c). However, with an increase in reaction time to ≥2 h, the crystalline regions of MCC decomposed, as illustrated Fig. 3c. The characteristic reflections of MCC disappeared in the XRD spectra of all Mt-hydrochar samples (4 h-MC, 8 h-MC, 16 h-MC, and 20 h-MC). Moreover, following a further increase in the reaction time to 4 h, the yields of RS and FF decreased sharply (Fig. 2a, RS-MC, and FF-MC), whereas the yield of organic acid (Acd) reached a maximum (Fig. 2a, Acd-MC). The autogenetic organic acid is partly consumed for the hydrolysis and dehydration of cellulose and degradation products of cellulose (e.g. RS and FF), but it can also serve as a proton source. The proton can partly replace the original cations (mainly, Na+ and Ca2+), thereby leading to the formation of proton-exchanged Mt (H-Mt) (Eqs. (3) and (4)). The H-Mt has enhanced Brönsted acidity (Zhao et al., 2013) and thus it could serve as catalyst to promote reactions, for example, hydrolysis of MCC, in hydrothermal conditions (Tong et al., 2013). Ca2þ  Mt þ 2Hþ →2Hþ  Mt þ Ca2þ :

ð3Þ

Naþ  Mt þ Hþ →Hþ  Mt þ Naþ :

ð4Þ

However, the amounts of RS and FF obtained from the hydrothermal carbonization of MCC in the presence of Mt (Fig. 2a, RS-MC and FF-MC) were lower than that from the hydrothermal carbonization of MCC alone (Fig. 2a, RS-C and FF-C), a possible explanation was that the RS and FF formed in aqueous solution were adsorbed by Mt, which usually has good capability for adsorption of organic compounds (Greenland, 1956). Specifically, the organic species formed could be adsorbed on the surface, at the edge of platelets and/or in the channel (Fig. 5b) (Wu et al., 2014a). The adsorption of the liquid products formed into interlayer spaces could cause the change of the d001 spacing (interlayer distance) of Mt; the d001 value increased with the increase of the amount of adsorbed liquid products located in the interlayer spaces of Mt (Yu et al., 2014). All XRD patterns clearly showed the 001 reflection of all samples at around 7.5° (2θ) with the d001 at 1.26 nm (Fig. 3c). However, compared with the control experiments of Mt alone, the specific 001 reflection of the Mt in Mt-hydrochar nanocomposite shifted towards the lower 2θ angle regions, resulting from an increase in the d001 value (Fig. 3c). For example, following 2 h of reaction time, the obtained sample 2 h-MC showed the (001) reflection of the Mt around 6.61° (2θ) with the d001 value at 1.33 nm. With a further increase in reaction time to 4 h, the obtained sample 4 h-MC showed the (001) reflection of Mt around 4.69° (2θ) with the d001 value at 1.88 nm (Fig. 3c). Within 0–2 h, the d001 values increased from 1.26 nm (sample 2 h-M)

Please cite this article as: Wu, L.M., et al., Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.06.015

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(a)

(b)

(200) (110) (1-10)

2h-C

CrI(%) =79.18

4h-C

CrI(%) =2.54

8h-C

5

10

15

20

7.14 CrI(%) =69.11

Intensity (a.µ .)

Intensity (a.µ .)

Raw MCC

CrI(%) =2.96

d001=1.26 nm

2h-M

d001=1.26 nm

4h-M

d001=1.26 nm

8h-M

16h-C

CrI(%) =2.54

d001=1.26 nm

16h-M

20h-C

CrI(%) =2.54

d001=1.26 nm

20h-M

25 30 35 40 2θ (degree)

(c)

45

50

55

60

10

20

30

40

50

60

70

80

2θ (degree)

(001)

(200)

(110) 6.61

(1-10)

Intensity (a.µ .)

2h-MC

d001=1.33 nm

4h-MC

d001=1.88 nm

8h-MC

d001=1.88, 1.33 nm

16h-MC

d001=1.33 nm

20h-MC

d001=1.33 nm

4.69

5

10

15

20

25

30

35

40

45

50

55

60

2θ (degree) Fig. 3. XRD patterns of (a) hydrochars, (b) samples of hydrothermal treatment of Mt alone, and (c) Mt-hydrochar nanocomposites.

to 1.33 nm (sample 2 h-MC). Thus, it is likely the result from swelled Mt that had already adsorbed liquid products (e.g. RS and FF) from the hydrolysis of amorphous regions of MCC (Fig. 3b and c). Similarly, within 2–4 h, the d001 values further increased from 1.33 nm (sample 2 h-MC) to 1.88 nm (sample 4 h-MC) (Fig. 3c). Nevertheless, in this sequence, the hydrolysis of crystalline regions of MCC produced liquid products (e.g. RS and FF), which could then be adsorbed by Mt. However, as seen from Fig. 3c, the d001 reflections of Mt in Mthydrochar nanocomposite obtained after 8 h (sample 8 h-MC) split into two reflections at 4.69° (2θ) with the d001 value at 1.88 nm and at 6.61° (2θ) with the d001 value at 1.33 nm. Moreover, with a further increase in the reaction time of N 8 h, the (001) reflections at 4.69° (2θ) disappeared and the (001) reflection of Mt in the samples (16 h-MC and 20 h-MC) shifted to 6.61° (2θ) with the d001 at 1.33 nm (Fig. 3c). The gradual decrease of the value of d001 to 1.33 nm probably indicates that with the increase of the reaction time, the adsorbed organic species (e.g. RS and FF) began to generate solid carbon species through polymerization and aromatization reactions on the mineral surface. These polymerization and aromatization reactions led to an increase in the carbon content of the solid residue compared to raw MCC. Elemental composition of raw MCC was C 39.82 wt.%, H 5.6 wt.%, and O 54.18 wt.% (Fig. 2b). Compared to raw MCC, the content of carbon in hydrochars increased up to 64.2 wt.% and 75.03 wt.% for the 16 h-C and 24 h-C samples, respectively (Fig. 2b). The carbon in Mthydrochar nanocomposite increased up to 60.05 wt.% and 67.07 wt.%

for the HF-16hMC and HF-24hMC samples (Fig. 2b). Furthermore, in the absence of Mt, the yield of solid product decreased to a minimum after 2 h of reaction time due to the hydrolysis of MCC (Fig. 2b, C series). However, with an increase in reaction time to 4 h, the yield of solid product increased. When the reaction time was increased further, the yields were almost unchanged. The increase of solid product may be explained by the liquid-soluble polymers derived from polymerization of reactive hydrolysis products (e.g. furfural-like compounds), which accumulated and reached the critical supersaturation point in the aqueous solution. The carbon solid then formed and gradually grew by condensation polymerization and aromatization (Sevilla and Fuertes, 2009; Funke and Ziegler, 2010). However, in the presence of Mt, the yield of solid products (Mt-hydrochar nanocomposite) decreased to a minimum after 4 h of reaction time, with a further increase in the reaction time not altering the yield of solid products (Fig. 2b, MC series). The increase of solid yield was not seen in the presence of Mt. A possible explanation was that the probability of the condensation reactions of Mt surface-adsorbed intermediate compounds (e.g. RS and FF) was much higher than those in solution, because entropy change (ΔS) of the reaction on the Mt surface is much smaller than that of the dissolved compounds in the aqueous solution. But the enthalpy change (ΔH) was essentially the same regardless of whether the reactants were dissolved or adsorbed to the Mt surface. As such, the free-energy change (ΔG = ΔH – T · ΔS) of the reaction on the Mt surface was more negative (Wächtershäuser, 1988). Although the condensation reactions may be energetically favorable on the Mt surface, the rates of reaction may be

Please cite this article as: Wu, L.M., et al., Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.06.015

L.M. Wu et al. / Applied Clay Science xxx (2015) xxx–xxx

much slower due to less mobilization and mutual contact of adsorbed molecules (Collins et al., 1995). However, because the chances of collision of molecules in the solution were more much higher, a burst of homogeneous nucleation and a higher growth rate could occur in the solution (Sevilla and Fuertes, 2009). Thus, compared with the condensation polymerization and aromatization of the intermediate compounds in the solution, longer times were necessary for the slower reactions of adsorbed species on the Mt surface.

Raw MCC

C-2h

100 µm

100 µm

2 µm C-2h

HF-2hMC

7

3.3. The effect of Mt on the morphological structure of hydrochar Following the 2-h reaction time and the removal of amorphous regions of MCC by the hydrolysis reaction, the rod-like raw MCC form (Fig. 4, sample raw MCC) disappeared. In its place, a block-like crystalline cellulose is visible (Fig. 4, sample 2 h-C). With an increase in reaction time to 24 h, carbon microspheres formed (Fig. 4, sample 24 h-C). This result is in agreement with previous studies, which also found

Raw Mt

C-24h

C-24h

2 µm HF-24hMC

2 µm

100 µm

2 µm

2 µm

Fig. 4. SEM images of raw materials, hydrochars, and Mt-free samples.

Please cite this article as: Wu, L.M., et al., Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.06.015

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L.M. Wu et al. / Applied Clay Science xxx (2015) xxx–xxx

that carbon microspheres resulted from the polymerization of products from MCC hydrolysis, such as glucose and furfural and its substitutes (Sevilla and Fuertes, 2009; Yu and Wu, 2009). However, more interestingly, here the morphological structure of hydrochar appeared to be related to the use of Mt. After removing the fraction of Mt in the Mt-hydrochar nanocomposite, the relative pure organic fraction (hydrochar) can be obtained. It can be reasonably expected that the SEM imaging of raw Mt showed the presence of large multi-layered flakes (Fig. 4, sample raw Mt). The Mt-free sample (HF-2hMC) also showed a flake-like structure. However, with an increase in reaction time to 24 h, the larger flakes were barely observed in the sample of HF-24hMC. However, the SEM image of sample HF-24hMC revealed porous structures (Fig. 4, sample HF-24hMC). Moreover, cylindrical white colloids were obtained in the control experiments of Mt alone after completion of the hydrothermal reaction (Fig. 5c, sample 16 h-Mt). In contrast, the obtained Mt-hydrochar nanocomposite were in the form of a black cylinder (Fig. 5c, sample 16 h-MC) and the hydrochar obtained by the hydrothermal carbonization of MCC alone existed in the form of black dispersion (Fig. 5c, sample 16 h-C). Therefore, the evolution of cellulose to hydrochars in the presence of Mt indicated that Mt might play a template role. The Mt presented various templates resulting from diverse Mt particle associations. When a dispersion of plate-like Mt particles flocculates, three different modes of particle association may occur: face-to-face (FF), edge-to-face (EF), and edge-to-edge (EE) (Fig. 5a) (Van Olphen, 1964). FF association leads to thicker and larger flakes, and EF and EE associations lead to porous structures (Luckham and Rossi, 1999). Thus, during hydrothermal carbonization, different morphological structures of hydrochar can be obtained from the Mt particle associations. The pH of the medium is an important factor related to Mt particle associations (Luckham and Rossi, 1999). The surface charge of Mt layers on the basal planes is the permanent negative charge caused by the isomorphic substitutions, while the edge charge is variable (Van Olphen, 1964). Decreasing the pH led to an increase of the number of positive charges by the protonation reaction of Al-OH sites at edges (AlOH + H+ → Al-OH2 +). This resulted in an increasing number of EE and EF associations between the positive charge at the edges and

negative charge on the basal surfaces (Luckham and Rossi, 1999). As mentioned, a large quantity of organic acids formed in the course of the hydrothermal treatment of MCC (Fig. 2a). Thus, under acidic conditions, positive charges at the edge of the Mt particle can be enhanced. Therefore, EE and/or EF associations are promoted by the attraction of opposite charges on the two surfaces (Van Olphen, 1964). Less organic acids were generated within 2 h (Fig. 2). In such a case, the number of positive charges on the edges were not large enough. Therefore, FFtype associations, which lead to thicker and larger flakes, are predominant. That is the possible reason why the SEM image of the sample (HF-2hMC) revealed the presence of large flakes, which were also observed in the SEM image of raw Mt (Fig. 4). However, these large flakes were scarce in the HF-24hMC sample (Fig. 4). Rather, the SEM image of the HF-24hMC sample showed the presence of pores between the agglomerates of carbon particles (Fig. 4). With increased reaction times, the quantity of organic acid increased (Fig. 2a). As a result, the number of positive charges at the edges also increased due to the protonation reaction of Al-OH sites at the edge. Therefore, EF and/or EE-type associations, which led to the porous structures, became predominant. In such cases, hydrothermal carbonization of cellulose in the presence of Mt led to porous hydrochar (Fig. 4). To summarize, with reference to chemical transformations and product evolutions, and the hydrochar morphological structure formed during the hydrothermal carbonization of cellulose alone and those in the presence of Mt, the evolution mechanism of the formation of Mt-hydrochar nanocomposite was illustrated in Fig. 6. According to Sevilla and Fuertes (2009), in the first step, when a cellulose aqueous dispersion was hydrothermally treated, both the amorphous and crystalline cellulose hydrolyzed. At this stage, the hydrolysis of cellulose produced glucose, which subsequently isomerized to form fructose. Next, the decomposition of furfural and 5-hydroxymethylfurfural from fructose and glucose produced organic acids. During this stage, two things occurred: (1) the formed degradation products (glucose, fructose, furfural, etc.) of cellulose adsorbed on the surface of Mt, which assisted subsequent reactions such as polymerization and aromatization to generate carbon particles; and (2) the organic acid generated reacted with Mt, leading to the existence of proton-exchanged Mt in

Fig. 5. Modes of Mt particle associations (a), adsorbed sites in Mt (b), and pictures of samples (c).

Please cite this article as: Wu, L.M., et al., Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.06.015

L.M. Wu et al. / Applied Clay Science xxx (2015) xxx–xxx

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Fig. 6. The evolution of cellulose and Mt in the reaction of hydrothermal carbonization for Mt-hydrochar nanocomposites formation.

the system. This proton-exchanged Mt served as a catalyst that could promote reactions in the hydrothermal condition. Moreover, with the increased quantities of organic acids, the modes of Mt particle association could gradually change from face-to-face (FF) to edge-to-face (EF) and/or edge-to-edge (EE). The polymerization or condensation and aromatization reactions followed. Through polymerization and aromatization of the adsorbed organic species (e.g. glucose, fructose, and furfural), Mt-hydrochar nanocomposites were finally generated (Fig. 6). Furthermore, the morphological structures of hydrochar in Mt-hydrochar nanocomposites were different due to the alteration of the Mt particle associations (templates) during hydrothermal carbonization.

4. Conclusion Hydrochar and Mt-hydrochar nanocomposite were successfully obtained via the hydrothermal carbonization of cellulose in the absence and presence of Mt at 200 °C for 2–24 h. In the absence of Mt, hydrothermal carbonization of cellulose resulted in hydrochar consisting mainly of aggregates of carbon microspheres. In contrast, in the presence of Mt, the evolution of cellulose took place through Mt-templated and Mt-catalyzed carbonization. Hence, the resulting hydrochar appeared to have thin and large flakes or porous structures. The morphology and the structure of hydrochar were tunable by changing the Mt particle associations during hydrothermal carbonization. In addition to the template, Mt also acted as a catalyst and an adsorbent. In the course of hydrochar formation, cellulose under hydrothermal treatment yielded liquid (e.g. reducing sugar and furfural) and solid products (hydrochar). Hydrothermal carbonization of cellulose produces organic acid, which then served as a source that yielded proton-exchanged montmorillonite into the system. The proton-exchanged Mt can then act as a solid acid catalyst to promote reactions in the hydrothermal condition and hydrochar formation. Mt also played a role in adsorbing the formed degradation liquid products (e.g. reducing sugar and furfural) on the surface, at the edge of platelets or/and in the channel. The adsorption aided the condensation and aromatization of cellulose-degraded products to form carbon particles. These findings not only provide new insights into the Mt-facilitated formation

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Please cite this article as: Wu, L.M., et al., Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.06.015