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Hydrolysis of cellulose by sulfonated magnetic reduced graphene oxide
Accepted Manuscript Hydrolysis of cellulose by sulfonated magnetic reduced graphene oxide Zhenzhen Yang, Renliang Huang, Wei Qi, Liping Tong, Rongxin Su, Zhimin He PII: DOI: Reference:
S1385-8947(15)00779-2 http://dx.doi.org/10.1016/j.cej.2015.05.091 CEJ 13732
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
21 March 2015 23 May 2015 26 May 2015
Please cite this article as: Z. Yang, R. Huang, W. Qi, L. Tong, R. Su, Z. He, Hydrolysis of cellulose by sulfonated magnetic reduced graphene oxide, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej. 2015.05.091
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Hydrolysis of cellulose by sulfonated magnetic reduced graphene oxide Zhenzhen Yang a, Renliang Huang b, Wei Qi a,c,d,*, Liping Tong a, Rongxin Su a,c,d , Zhimin He a
a
State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China.
b
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, P. R. China. c
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China.
d
Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin, 300072, China
* Corresponding author (Wei Qi). Tel: +86 22 27407799 Fax: +86 22 27407599 E-mail address: [email protected]
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Abstract Reduced graphene oxide functionalized with magnetic Fe3O4 nanoparticles and -PhSO3H groups (Fe3O4-RGO-SO3H) was reported for environmentally benign hydrolysis of cellulosic materials, which is an important process in integrated utilization of biomass resource. Except for -PhSO3H groups, -COOH and -OH groups derived from graphene oxide (GO) were also expected in the structure of Fe3O4-RGO-SO3H. Fe3O4-RGO-SO3H exhibited outstanding catalytic performance for the hydrolysis of cellulosic materials, due to its crumpling feature with clear layers and the coexistence of -COOH and -OH groups. The unique structure of Fe3O4-RGO-SO3H favors the accessibility of cellulose to the active sites of the material and facilitates the diffusion of the product molecules. -COOH groups on the surface of the material provide a large concentration of acidic functionality for hydrolyzing cellulose, while -OH groups readily absorb cellulose through forming strong hydrogen bonds between -OH groups and the oxygen atoms in β-1, 4 glycosidic bonds. Furthermore, Fe3O4-RGO-SO3H can also be easily separated from the reaction residue with an extra magnetic force and further used at least five times.
Keywords: Biomass, Cellulose, Hydrolysis, Glucose, magnetic solid acid, Fe3O4-RGO-SO3H
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1. Introduction The increasing energy crisis and environmental concerns have prompted the utilization of readily available lignocellulosic biomass resource as an alternative to the limited fossil resource. [1, 2] Lignocellulosic biomass is composed of lignin (15-20%), hemicellulose (25-35%) and cellulose (40-50%). [3] Lignin is a methoxylatedphenylpropane three-dimensional structure responsible for the structural rigidity of plants and that surrounds hemicellulose and cellulose. The hemicellulose polymer is formed by C5 and C6 sugar monomers. Finally, cellulose, the most abundant component of nonfood biomass, is a water-insoluble saccharide polymer composed of glucose monomers linked by β-1,4 glycosidic bonds. Among the three compositions of lignocellulosic biomass, cellulose has been widely studied as a potential material for production of advanced materials, biofuels and value-added chemicals. [4-7] Since glucose is a major platform for synthesis of various important chemicals such as 5-hydroxymethlfurfural (5-HMF), [8-10] levunilic acid [11, 12] and 2, 5-dimethylfuran, [13,14] hydrolysis of cellulose into glucose is of critical importance for chemical and biochemical industries based on sugars. Simultaneously, the process of cellulose hydrolysis offers great challenges to researchers due to the tight H-bond network in cellulose structure. Thus far, a great deal of effort has been devoted to the study of cellulose depolymerization and substantially appropriate cellulose hydrolysis schemes have been developed, including enzymedriven reactions, catalysis with mineral acid and ionic liquid promoted dissolution and hydrolysis of cellulose. Enzymatic degradation of cellulose has received considerable attention due to its high selectivity and moderate hydrolysis conditions. [15, 16] However, pretreatment process is necessary before enzymatic hydrolysis of cellulose, because the complex structure of cellulose makes it difficult for enzymes to access cellulose. Furthermore, the high cost and long residence
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time of cellulase are also worth mentioning. Mineral acid such as sulfuric acid seems to be a more economic catalyst for cellulose hydrolysis. [17, 18] However, several drawbacks such as severe reaction conditions, corrosion hazard, environmental pollution and difficult separation of catalysts and products must be taken into consideration. Ionic liquids, which can dissolve cellulose to form a homogeneous solution, have been widely used for the degradation of cellulose. [19, 20] Although a high efficiency of cellulose saccharification can be obtained in ionic liquid medium, product separation is another bottleneck for the degradation of cellulose in ionic liquids. Therefore, hydrolysis of cellulose catalyzed by solid acid in water medium seems more environmental sustainable and has received increasing attention. Up till now, numerous papers have been published on the transformation of cellulose with solid acids. As a type of strong solid acid, transition-metal oxides with different morphology including mesoporous Nb-W oxide and layered HNbMoO6 exhibited different catalytic performance for cellulose hydrolysis. [21, 22] Heteropoly acids, another type of solid acid, consisting of early transition metal-oxygen anion clusters, have been widely used for cellulose hydrolysis. For example, Shimizu et al. reported H3PW12O40 and H3SiW12O40 for the hydrolysis of ball-milled cellulose and cellubiose into glucose or sugars. [23] Ogasawara et al. reported negatively charged heteropoly acids including H3BW12O40, H3AlW12O40 and H3GaW12O40 for the hydrolysis of non-pretreated crystalline cellulose into saccharides under mild reaction conditions. [24] Among various types of solid acids for cellulose hydrolysis, carbon-based solid acids have superior catalytic activities. Especially those carbonaceous acids derived from cheap naturally occurring raw materials are better candidates for cellulose transformation. The first carbonaceous acid from sulfonated D-glucose/sucrose materials was reported by Hara. [25, 26] Then, BCSO3H derived from bamboo, cotton and starch, [27] and CSA prepared from hydrolyzed corncob
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residue were used for the hydrolysis of cellulosic materials. [28] Other solid acids, such as ionexchange resins, [29] polymer based acids [30] and H-formed zeolites, [31] have also been extensively investigated for cellulose degradation. Although these solid acids presented considerable catalytic activities for cellulose hydrolysis, a significant challenge remains for these catalytic systems when it is converted to non-pretreated cellulose or real biomass. Common solid acids cannot be efficiently separated from solid residue in biomass hydrolysis systems including non-reacted cellulose, lignin component in real biomass and by-product humins. Nowdays, magnetic materials have been widely used in various field due to the simply separation. [32-34] To overcome the problem aforementioned in heterogeneous catalyzed hydrolysis of cellulose, Lai et al. were the first to design a magnetic sulfonated mesoporous silica (Fe3O4-SBA-SO3H) and evaluate its catalytic performance for cellulose hydrolysis. [35, 36] Hydrolysis of avicel cellulose and amorphous cellulose with Fe3O4-SBA-SO3H gave 26% and 52% yields of glucose under corresponding reaction conditions, respectively. The magnetic solid acid can be easily separated with the solid residue by an external magnetic force after reaction and further used at least 5 times. This study made a fundamental contribution to the degradation of lignocellulose. Later, other magnetic solid acids such as CoFe2O4-embedded silica nanoparticles with -SO3H groups, Fe3O4@C-SO3H and PCM-SO3H were reported for cellulose hydrolysis in succession. [37-39] To provide more candidates for the hydrolysis of cellulosic materials, more high efficiency and stability magnetic solid acids need to be developed. Graphene, a promising carbon material, has excellent mechanical properties, a large surface area and a distinctive two-dimensional structure, providing an ideal platform to anchor functional groups and embed nanoparticles. [40, 41] For example, sulfonated graphene which was prepared by anchoring of –PhSO3H groups to the surface of reduced graphene has been used for the
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hydrolysis of ethyl acetate. [42] While reduced graphene oxide functionalized with Fe3O4 nanoparticles has been synthesized for arsenic removal. [43] Herein, we introduced -PhSO3H groups and Fe3O4 nanoparticles to the structure of reduced graphene oxide (RGO) to prepare a new magnetic solid acid. The magnetic solid acid (Fe3O4-RGO-SO3H) was synthesized by simultaneously precipitation Fe3+ and Fe2+ ions with ammonia solution and reducing graphene oxide with hydrazine hydrate, followed by direct anchoring of sulfonic acid-containing aryl radicals to the surface of Fe3O4-RGO through C-C covalent bonds. The as synthesized Fe3O4RGO-SO3H not only contained post-introduced Fe3O4 nanoparticles and -PhSO3H groups, COOH, -OH groups and the crumpling feature with clear layers derived from GO were also preserved. The -COOH groups contribute to the total acid density and the superior hydrothermal stability of Fe3O4-RGO-SO3H, the -OH groups readily adsorb cellulose and the crumpling feature facilitates the diffusion of the product molecules. In one word, the unique structure and the existence of -COOH and -OH groups make Fe3O4-RGO-SO3H an excellent catalyst for the hydrolysis of cellulosic materials.
2. Materials and methods 2.1 Materials Natural graphite powder, cellobiose, avicel cellulose, 1-methylimidazole, C4H9Cl, Amberlyst15 and Amberlite-IRC76 were purchased from Aladdin Reagent Co., Ltd. 30 wt% H2O2, FeCl3·6H2O, FeCl2·4H2O, CH3CH2OH, NH4OH were purchased from Tianjin Guangfu Chemical Reagent Co. Ltd. 80% hydrazine hydrate, NaNO3, KMnO4, NaNO2, starch, sucrose, xylose, glucose, sulfanilic acid were purchased from Tianjin Kemiou Chemical Reagent Co. Ltd. H-Beta (25) and HZSM-5 were purchased from The Ctalyst Plant of Nankai University, γ-Al2O3
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was purchased from Nanjing Tianxing New Material Co. Ltd. Corn cob was obtained from a local farm. 2.2 Preparation of water-soluble GO Natural graphite powder was oxidized to graphene oxide according to Hummers method. [44] First, graphite powder (2 g) was added to 50 mL of 98% H2SO4 and 2 g NaNO3 solution in an ice bath. KMnO4 (6 g) was slowly added with vigorous stirring, avoiding a sudden increase in temperature. The mixture was stirred at 35 oC until it became pasty brownish and then diluted with water, followed by adding 10 mL of 30 wt% H2O2. The obtained brilliant yellow mixture was centrifuged and washed with 5% HCl several times, then by a lot of water. Finally the resulted brown powder solid was dried at room temperature under vacuum condition. 2.3 Preparation of Fe3O4-RGO In a typical procedure, 0.7 g GO was first dispersed in 450 mL water under ultrasonic for 60 min, and 0.2027g FeCl3·6H2O and 0.0792 g FeCl2·4H2O (2:1 molar ratio) were dissolved in 25 ml water. The solution of Fe3+/Fe2+ salts was added to the GO solution slowly with constant stirring, then the pH of the solution was adjusted to 9~10 by 30% ammonia solution. The threeneck ground flask was transferred to an oil bath with a temperature of 80 oC and 6 ml 80% hydrazine hydrate was added slowly. After being stirring for 4 h, the resulted black solution was cooled to RT. The black solid was centrifuged and washed with water for several times, finally Fe3O4-RGO was obtained after drying at 60 oC under vacuum condition. Fe3O4 nanoparticles were synthesized by precipitation Fe3+ and Fe2+ ions with ammonia solution at pH=10. 2.4 Preparation of Fe3O4-RGO-SO3H
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Fe3O4-RGO-SO3H was obtained by reacting aryl diazonium salt of sulfanilic acid with Fe3O4RGO. Firstly, 4-Benzenediazoniumsulfonate was synthesized by diazotization of sulfanilic acid as follows, 5.2 g sulfanilic acid was dispered in 1 M HCl aqueous solution by vigorous stirring in an ice bath, and the temperature was controlled at 3-5 oC. After 10% excess 1 M NaNO2 solution (33 ml) was added dropwise, a clear solution was obtained. The solution was stirred for another 1 h, and the synthesized white precipitate was filtered and washed with cool water. 4-Benzenediazoniumsulfonate obtained was moved to a three-necked ground flask with ethanol (40 mL) and a small amount of water. Then, Fe3O4-RGO was added, and the temperature was controlled at 50 oC. After stirring for 1.5 h, the obtained Fe3O4-RGO-SO3H was intensively washed with deionized water until the pH was about 6 (the pH value of deionized water we used is about 6) to insure the absence of free acid in the structure of Fe3O4-RGO-SO3H and then dried overnight at 80 oC. (Fig. 1). 2.5 Characterization High resolution transmission electron microscopy (HRTEM) was performed on a JEM-2100 microscope (JEOL Ltd., Akishima, Japan) operating at an acceleration voltage of 200 kV. Scanning electron microscopy (SEM) was performed on an S-4800 field emission scanning electron microscope (Hitachi Hightechnologies Co., Japan) at the acceleration voltage of 3 kV. All the samples were sputter-coated with platinum using an E1045 Pt-coater (Hitachi Hightechnologies Co., Japan) before SEM observation. Energy dispersive X-ray spectroscopy (EDS) analysis was used to identify the elemental composition and distribution of Fe3O4-RGO-SO3H. The density of -SO3H was estimated based on the sulfur content which was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, Therom Jarell-Ash Corp, USA). Fourier transform infrared spectroscopy (FTIR) analysis was performed using a
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spectrophotometer (BIO-Rad; Japan), which was with a spectral resolution of 4 cm-1 in the wave number range of 500-4000 cm-1. X-ray diffraction (XRD) analysis was performed on D/max 2500 (Rigaku; Japan). The electronic binding energy of Fe3O4-RGO-SO3H was measured using X-ray photoelectron spectroscopy (Quantum 2000, USA), which was conducted with a PHI Quantum 1600 Scanning ESCA Microprobe equipped with Al Kα radiation as an excitation source. 2.6 Catalytic test 30 mg cellulose and 30 mg catalyst were weighed into a pressure tube, and then 3 mL deionized H2O was added. The tube was heated at 150 °C, and zero time was taken when the temperature of the reactor reached 150 °C. After reaction, the catalyst was removed by an extra magnetic force and 1 mL of the residue mixture was taken out by an injection syringe and then filtered with a 0.45 µm syringe filter prior to analysis by High-performance liquid chromatography (HPLC). The separated Fe3O4-RGO-SO3H was washed with water several times under ultrasonic for further catalyzed reaction. The yields of water soluble organic compounds were calculated as follows. Glucose yield (%) = (moles of glucose in water soluble products)/(mol of C6H12O5 in charged cellulose)×100 Degradation products yield (%) = (moles of degradation products in water soluble products)/(mol of C6H12O5 in charged cellulose)×100 Here, 30 mg cellulose is approximately equal to 0.18 mmol of anhydroglucose, which was calculated based on considering the repeating anhydroglucose units in cellulose. In this study, the initial substrate concentration was calculated in terms of anhydroglucose, rather than in the form of the cellulose polymer, and the product yield was expressed as molar yield of glucose. 9
3. Results and discussion 3.1 Characterization of Fe3O4-RGO-SO3H The SEM image of Fe3O4-RGO-SO3H shows the presence of magnetite and the crumpled sheets of RGO can be seen throughout the morphology. In addition, a layered structure is observed at the edges of the agglomerates (Fig. 2a). This unique structure of Fe3O4-RGO-SO3H has potential advantages in acid-catalyzed hydrolysis reaction. On one hand, the reactants can easily access the active sites on both sides of two-dimensional graphene sheets. On the other hand, the crumpling feature facilitates the diffusion of the product molecules. [42] The TEM image of Fe3O4-RGO-SO3H shows magnetite nanoparticles well dispersed in the RGO matrix, and the nanoparticles are entrapped inside the RGO sheets rather than simply mixed up or blended with RGO. Furthermore, the lattice fringes of the magnetite nanoparticles are found surrounding of the RGO matrix, and the lattice spacing is 0.21 nm which corresponding to the indexes (400, 040 and 004) reflections (Fig. 2b). The powder XRD pattern and the XPS spectra of Fe3O4-RGO-SO3H further proved the chemical properties of the magnetite nanoparticles. In the powder XRD pattern of Fe3O4-RGOSO3H (Fig. 3), the main peaks at 2θ = 18.5o (111), 30.4o (220), 35.32o (311), 43.2o (400), 54.4o (422), 57.46o (511) and 62.56o (440) show the characteristics of Fe3O4 (JCPDS No. 75-0033), and the broad peak at 23.9 o which is for RGO. The wide scan XPS spectrum of Fe3O4-RGOSO3H (Fig. 4) shows the photoelectron lines at a binding energy of about 168, 232, 285, 532, 712, and 725 eV attributed to S 2p, S 2s, C 1s, O 1s, Fe 2p3/2, and Fe 2p1/2, respectively. In the spectrum of Fe 2p which is inserted in the wide scan XPS spectrum of Fe3O4-RGO-SO3H, the peaks Fe 2p1/2 and Fe 2p3/2 are located at 725 and 712 eV, not at 710 and 724 eV which are for
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Fe2O3. Besides, there was no satellite peak at about 719 eV, characteristic of Fe2O3, which is indicative of the formation of the Fe3O4 phase in the material.[43] The EDS mapping images of Fe3O4-RGO-SO3H show homogeneous distribution of iron and sulfur elements in the entire range (Fig. 5). And the element mass percents of carbon, oxygen, sulfur and iron in the solid acid were 66.42, 23.92, 3.51 and 6.15 wt%, respectively, which were obtained from quantitative analysis. Thus, considering the O/S atom ratio (13.6) and the O/Fe atom ratio (13.6), residual epoxide, hydroxyl and carboxyl groups derived from GO are also expected in the composition of Fe3O4-RGO-SO3H. The density of -SO3H was estimated to be 0.95 mmol/g based on the sulfur content which was analyzed by ICP. The total -SO3H + -COOH and -SO3H + -COOH + -OH contents were estimated from the exchange of Na+ in aqueous NaCl and NaOH solutions respectively, [45] to obtain the proportions of each functional group, and the data was summarized in Table 1. The functionalization of RGO with magnetic Fe3 O4 nanoparticles and -PhSO3H groups was further confirmed by the FTIR (Fig. 6). The very broad intense peak at 3431 cm-1 is seen for the O-H stretching frequency. The FTIR spectrum of RGO shows C=O (1708 cm-1), aromatic C=C (1600 cm-1), carboxyl C=O (1352 cm-1), epoxy C-O-C (1209 cm-1), and alkoxy C-O (1013 cm-1) stretching vibrations. [43] In the FTIR spectrum of RGO-SO3H, the presence of -PhSO3H is confirmed by the peaks at 1033 cm-1, 1118 cm-1, 1164 cm-1 (two S-O and one S-phenyl vibrations), and the peak at 1003 cm-1 shows the characteristic vibration of a para-disubstituted phenyl group. Furthermore, the intensive of the peak at 1598 cm-1 also indicates the presence of phenyl groups. In the FTIR spectrum of Fe3O4-RGO-SO3H, the newly appeared peak at 623 cm1
was a characteristic peak corresponding to the stretching vibration of Fe-O as compared to
RGO and RGO-SO3H.
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3.2 Catalytic study The catalytic performance of Fe3O4-RGO-SO3H was evaluated for the catalyzed hydrolysis of cellulosic materials, Fe3O4, Fe3O4-RGO and RGO-SO3H were also used as catalysts for comparison. Cellobiose, an intermediate product of cellulose hydrolysis, was selected as the model compound for hydrolysis reaction first. As shown in Table 2, no product has been detected when cellobiose was heated in water at 150 oC for 3 h without any catalyst (Entry 1) or in the presence of pure Fe3O4 (Entry 2). Fe3O4-RGO, which excludes –PhSO3H groups, exhibited relatively low efficiency for the hydrolysis of cellobiose, only gave 21% molar yield of glucose (Entry 3). However, in the presence of RGO-SO3H or Fe3O4-RGO-SO3H (total acid densities were 1.67 and 1.56 mmol/g), the molar yields of glucose were up to 96% and 94%, and the conversions of cellobiose were 98% and 97% respectively (Entry 4, 6). In addition, glucose yield increased obviously with the amount of acid in Fe3O4-RGO-SO3H (Entry 5, 6). Cellulose has a low reaction activity for decomposition in water due to its stubborn structure. Amorphous cellulose, which has a significantly lower crystallinity than cellulose, was prepared by dissolving avicel cellulose in ionic liquid (1-butyl-3-methylimidazolium chloride) at 80 oC and then precipitating it from water (the detailed method was shown in Supplementary data). The XRD patterns of avicel cellulose and amorphous cellulose were shown in the Supplementary data (Fig. S1). By using amorphous cellulose as substrate, the molar yield of glucose was 31% under the same reaction conditions as that for cellobiose hydrolysis due to the poor solubility of cellulose in water (Entry 7). Whereas, glucose yield was increased to 52% by prolonging the reaction time from 3 h to 5 h (Entry 8). A longer reaction time led to a loss in glucose yield (46%) because of the formation of levulinic acid, formic acid and 5-HMF (Entry 9). Furfural (FF), which is the dehydration product of xylose, was detected as another by-product
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in the reaction, but negligible (less than 1%), may be due to the small amount of xylan in the cellulose we used. When avicel cellulose was hydrolyzed, the molar yield of glucose decreased to 28% (Entry 10), which is close to the content of the amorphous region in avicel cellulose (degree of crystallinity is about 75%). However, this interesting finding does not mean that Fe3O4-RGO-SO3H can only decompose the amorphous proportion of cellulose. By prolonging reaction time to 12 h, levulinic acid, which is the main product of acid-catalyzed dehydration of glucose, became the major product with yield of 44%. The total yields of glucose and levulinic acid were up to 56%, implying that the crystalline proportion of cellulose was also partially decomposed by Fe3O4-RGO-SO3H (Entry 11). Fe3O4-RGO-SO3H has also been evaluated for hydrolysis of another two common polycaccharides, starch and sucrose, under the reaction conditions shown in Table 2, the molar yields of glucose were 93% and 95% respectively (Entry 12, 13). Moreover, a typical biomass corn cob was employed as a feed to determinate the capability of Fe3O4-RGO-SO3H for hydrolysis of lignincellulosic biomass. The molar yield of total reducing sugars was 40% based on the contents of cellulose (45 wt%) and hemicellulose (35 wt%) in corn cob, 5-HMF, levulinic acid and FF were detected as by-products (Entry 14). To verify the origin of these by–products, Fe3O4-RGO-SO3H was then used as catalyst for dehydration of glucose and xylose, two kinds of monosaccharides (Supplementary data, Table S1). As shown in Table S1, Fe3O4-RGO-SO3H exhibited moderate catalytic activity for glucose/xylose dehydration, 5-HMF/FF and levulinic acid were detected as products. These are only preliminary results, the systematic study of carbohydrate decomposition by Fe3O4-RGO-SO3H are under way. To further investigate the superior catalytic performance of Fe3O4-RGO-SO3H, another several commercially available solid acids have also been tested for cellulose hydrolysis for
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comparation. The experimental results and the acid densities of these solid acids were listed in Table 3. Among the solid acids we used, only Amerlyst-15, an ionic-exchange resin with -SO3H groups, gave a moderate glucose yield of 12%. Other catalysts, such as HZSM-5, H-Beta and γAl2O3, exhibited lower activities for the hydrolysis of avicel cellulose. Then dynamics experiments were performed for cellulose hydrolysis catalyzed by Fe3O4-RGO-SO3H, Amberlyst 15 and 0.0156 mol/L H2SO4 (the concentration of -SO3H groups bonded to 30 mg Fe3O4-RGOSO3H
in 3 mL water was 0.0156 mol/L). The dependence of the yield of glucose and
degradation products (5-HMF and levulinic acid ) on reaction time were shown in Fig. 7. The results indicate that the catalytic activity of Fe3 O4-RGO-SO3H is much higher than that of Amberlyst 15 and 0.0156 mol/L H2SO4. For further quantitative comparation of the activity of Fe3O4-RGO-SO3H, Amberlyst 15 and 0.0156 mol/L H2SO4, a kinetic model of two consecutive firt-order pseoduhomogeneous reactions was proposed for cellulose hydrolysis. [46,47] This model assumes that cellulose can be treated as a homogeneous reactant when water is present in excess. As follows: Cellulose → glucose → Degradation products
(1)
In accordance with eqn (1), the glucose-production rate is
= −
(2)
which, integrated, lead to =
[exp− − exp− ]
(3)
The values of the kinetic contants k1 and k2 were obtained by the simplex method, and the values are presented in Table 4. As expected, the results once again proved that Fe3O4-RGOSO3H exhibited much higher activity than Amberlyst 15 and 0.0156 mol/L H2SO4 for cellulose hydrolysis. 14
The large differences in catalytic properties among these solid acid catalysts cannot be adequately explained as being due solely to the density and strength of acid sites on their surface. Acid-catalyzed hydrolysis of cellulose into glucose involves two stages: H+ attacks to hydrogen and β-1, 4 glycosidic bonds in avicel cellulose to form water-soluble β-1, 4 glucan, followed by the hydrolysis of β-1, 4 glucan to yield glucose. Therefore, efficient conversion of cellulose into glucose using a solid acid requires a strong interaction between the solid acid and β-1, 4 glycosidic bonds. Because the acid sites of solid acids cannot approach the surface of cellulose without such an interaction, which is different from homogeneous acid. To further investigate the interaction between the solid acids and β-1, 4 glycosidic bonds in β1, 4 glucan, adsorption experiments were performed using cellobiose as a substrate. (the detailed experiment method was shown in Supplementary data) As shown in Table S2 (Supplementary data), the commercially available solid acids such as Amberlyst 15, H-Beta, HZSM-5 and γ-Al2O3 did not adsorb cellobiose in water even after stirring for one day. However, Fe3O4-RGO-SO3H can efficiently adsorb cellobiose, and the adsorption reached a plateau of 20% of the cellobiose in only 3 h. The results were further confirmed by soaking cellulose filter papers in solid acid-water suspensions, only Fe3O4-RGO-SO3H particles attached to the cellulose (Supplementary data, Fig. S2). From the results, the inability of Amberlyst 15 to adsorb β-1,4 glucan demonstrates that SO3H groups are not adsorption sites in Fe3O4-RGO-SO3H (Table S2, Entry 3). Furthermore, Amberlite-IRC76, a polystyrene-based resin contains high density of -COOH groups (4.0 mmol/g), exhibited much lower ability than Fe3O4-RGO-SO3H to adsorb β-1,4 glucan (Table S2, Entry 7). Therefore, -COOH groups are not the dominant sites responsible for the adsorption of β-1,4 glucan to Fe3O4-RGO-SO3H. Taking into account these results, the neutral -OH groups
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were considered as active sites for adsorption β-1,4 glucan. In addition, it is worth noting that Fe3O4-RGO-SO3H cannot adsorb glucose as a monomer of cellobiose at all (Table S2, Entry 1). This suggests that the glycosidic bonds in β-1,4 glucan, not -OH groups, participate in the adsorption on Fe3O4-RGO-SO3H. In one word, -OH groups in Fe3O4-RGO-SO3H can be linked to the oxygen atoms in β-1, 4 glycosidic bonds by strong hydrogen bonds, and thereby the substrate can be adsorbed onto the surface of Fe3O4-RGO-SO3H for the following reaction. The excellent adsorption ability as well as the crumpling feature with clear layers which was aforementioned of Fe3O4-RGO-SO3H is responsible for the high catalytic activity for cellulose hydrolysis. In the heterogeneous acid-catalyzed hydrolysis of cellulosic materials, full conversion is hardly achieved, so the separation of catalyst from the reaction residue is another bottleneck in this field. Magnetic solid acid Fe3O4-RGO-SO3H can easily solve the problem, because it can be recovered from the resulted mixture by an extra magnetic force as shown in Fig. 8. Furthermore, Fe3O4-RGO-SO3H can be used again directly after washing and drying, and no obvious deactivation was found after being used 5 times (Fig. 8). The possibility for loss of active sites of Fe3O4-RGO-SO3H during the hydrolysis reaction was also tested. The reaction was stopped after 2 h, and Fe3O4-RGO-SO3H was filtered. The filtrate was reacted again for another 3 h under the same reaction conditions. The result shows glucose yield does not change after Fe3O4-RGOSO3H was removed (Fig. S3). In addition, the pH values of the reaction solution before and after the catalysis reaction were detected, as shown in Table S3. The pH value had no change after pure water was heated at 150 oC for 5 h (Table S3, Entry 1). But when cellulose was heated at 150 oC for 5 h, a little decrease in pH value, may be due to the formation of little soluble organic acid (Table S3, Entry 2). A little greater decrease in pH value after Fe3O4-RGO-SO3H was
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heated at 150 oC for 5 h, indicates a little loss of -SO3H in the reaction (less than 1%), may be due to the self-hydrolysis of -PhSO3H (Ar-SO3H + H2O ↔ Ar-H + H2SO4) (Table S3, Entry 3). [25] The much greater decrease in pH value before and after cellulose hydrolysis reaction may be due to the formation of levulinic acid and formic acid (Table S4, Entry 4). Moreover, the proportions of each functional group in Fe3O4-RGO-SO3H almost remain constant after being used 5 times, which further confirms the stability of the catalyst (Table 1, Entry 4). The aforementioned results show the considerable hydrothermal stability of Fe3O4-RGOSO3H. However, it is notable that -SO3H bonded to aromatic hydrocarbons is generally subject to leaching in the presence of water, as observed for Amberlyst 15, as a result of the equilibrium between sulfoaromatic compounds and aromatic hydrocarbons. [47] It has been reported that the aromatic carbon -SO3H bonds in sulfoaromatic compounds bearing electron-withdrawing functional groups are more stable as a result of the increased electron density between carbon and sulfur atoms afforded by the electron-withdrawing functional groups. [48] So it is expected that electron-withdrawing -COOH groups in Fe3 O4-RGO-SO3H increase the electron density between carbon and sulfur atoms. This may be the origin of the stability of Fe3O4-RGO-SO3H in reactions involving excess water.
4. Conclusions Fe3O4-RGO-SO3H was successfully synthesized based on reduced graphene oxide (RGO), containing Fe3O4 nanoparticles and benzene sulfonic acid groups which were directly anchored onto the surface of RGO through C-C covalent bonds. The unique structure of Fe3O4-RGO-SO3H and its high dispersion in water promote the accessibility of cellulose to the active sites. In addition, the coexistence of -OH groups facilitated the adsorption of cellulose to the surface of the material by forming strong hydrogen bonds with the oxygen atoms in β-1, 4 glycosidic bonds, 17
and thus enhanced the catalytic activities of Fe3O4-RGO-SO3H for cellulose hydrolysis. Furthermore, Fe3O4-RGO-SO3H exhibited superior stability in the reaction of cellulose hydrolysis, may be due to the existence of electron-withdrawing -COOH groups, which increase the electron density between carbon and sulfur atoms.
Acknowledgments This work was supported by the Natural Science Foundation of China (Nos. 51173128, 31071509), the 863 Program of China (Nos. 2012AA06A303, 2013AA102204), the Ministry of Science and Technology of China (No. 2012YQ090194), the Beiyang Young Scholar of Tianjin University (2012) and the Program of Introducing Talents of Discipline to Universities of China (No. B06006).
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Figure Captions 22
Fig. 1. Preparation of magnetic solid acid Fe3O4-RGO-SO3H.
Fig. 2. (a) SEM spectrum of of Fe3O4-RGO- SO3H, and (b) Low and high magnification TEM images of Fe3O4-RGO- SO3H.
Fig. 3. X-ray dirrrication (XRD) patterns of GO and Fe3O4-RGO-SO3H.
Fig. 4. X-ray photoelectron spectroscopy (XPS) spectra: wide scan and Fe 2p spectra.
Fig. 5. The SEM image of Fe3O4-RGO-SO3H (a), and corresponding quantitative EDS element mapping of S (b) and Fe (c). (d) The EDS spectrum and element analysis of Fe3O4-RGO-SO3H.
Fig. 6. FT-IR spectrums of the Fe3O4-RGO- SO3H, RGO-SO3H and RGO.
Fig. 7. Kinetic study for the hydrolysis of cellulose by different solid acids. Reaction conditions: 30 mg cellulose, 30 mg solid catalysts, 3 mL H2O, at 150 oC. (black line) Fe3O4-RGO- SO3H, (blue line) Amberlyst 15, (red line) 0.0156 mol/L H2SO4, (■) glucose yield (●) degradation products yield.
Fig. 8. (a) and (b) Separation of Fe3O4-RGO-SO3H from the reaction residue. (c) Hydrolysis of cellulose to glucose by reused Fe3O4-RGO-SO3H. Reaction conditions: 30 mg cellulose, 30 mg Fe3O4-RGO-SO3H, 3 mL H2O, at 150 oC for 5 h.
23
Fig. 1. Preparation of magnetic solid acid Fe3O4-RGO-SO3H.
24
Fig. 2. (a) SEM spectrum of of Fe3O4-RGO- SO3H, and (b) Low and high magnification TEM images of Fe3O4-RGO- SO3H.
25
40
(440)
(422)
(333)
(311) (400)
(220) RGO 002
(111)
Intensity (a.u.)
20
60
80
2 theta (degree) Fig. 3. X-ray dirrrication (XRD) pattern of Fe3O4-RGO-SO3H.
26
720
600
500
400
300
710
700
200
Fe 3s Fe 3p
S 2p
Binding Energy (eV)
S 2s
C 1s
730
700
Fe 2p3/2
B
O 1s
Fe 2p 1/2 Fe 2p 3/2
Intensity (a.u.)
Fe 2p1/2
100
0
Bonding Energy (eV) Fig. 4. X-ray photoelectron spectroscopy (XPS) spectra: wide scan and Fe 2p spectra.
27
Fig. 5. The SEM image of Fe3O4-RGO-SO3H (a), and corresponding quantitative EDS element mapping of S (b) and Fe (c). (d) The EDS spectrum and element analysis of Fe3O4-RGO-SO3H.
28
Transmittance (%)
Fe3O4-RGO-SO3H RGO-SO3H
Fe-O
RGO C-H
C=O C=C
-OH
4000
3500
O=C-O
3000
2500
2000
C-H
SO3 O=S=O
1500
1000
500
Wavenumbers (cm-1) Fig. 6. FT-IR spectrums of the Fe3O4-RGO- SO3H, RGO-SO3H and RGO.
29
50
Molar Yield (%)
40 30 20 10 0 0
2
4
6
8
10
12
14
Time (h) Fig. 7. Kinetic study for the hydrolysis of cellulose by different solid acids. Reaction conditions: 30 mg cellulose, 30 mg solid catalysts, 3 mL H2O, at 150 oC. (black line) Fe3O4-RGO- SO3H, (blue line) Amberlyst 15, (red line) 0.0156 mol/L H2SO4, (■) glucose yield (●) degradation products yield.
30
c
Glucose Levulinic acid
Molar yield (%)
30
20
10
0 1
2
3
4
5
Cycle Fig. 8. (a) and (b) Separation of Fe3O4-RGO-SO3H from the reaction residue. (c) Hydrolysis of cellulose to glucose by reused Fe3O4-RGO-SO3H. Reaction conditions: 30 mg cellulose, 30 mg Fe3O4-RGO-SO3H, 3 mL H2O, at 150 oC for 5 h.
31
Table 1 Concentration determination of different functional groups in functionalized RGO. -COOH groups (conc. in mmol/g) 0.60
-PhSO3H groups (conc. in mmol/g)
Fe3O4-RGO
-OH groups (conc. in mmol/g) 0.26
RGO-SO3H
0.26
0.61
1.06
Fe3O4-RGO- SO3H
0.26
0.61
0.95
a
0.24
0.6
0.93
Catalyst
Fe3O4-RGO- SO3H a
Fe3O4-RGO- SO3H has been reused 5 times.
32
Table 2 Hydrolysis of carbohydrates to glucose.a
Entry
Substrate
Catalyst
S content (mmol/g)b
1 2 3 4 5 6
Cellobiose Cellobiose Cellobiose Cellobiose Cellobiose Cellobiose Amorphous cellulose Amorphous cellulose Amorphous cellulose Cellulose Cellulose Starch Sucrose Corn cob
Fe3O4 Fe3O4-RGO RGO-SO3H Fe3O4-RGO- SO3H Fe3O4-RGO- SO3H
1.06 0.63 0.95
Total acid density (mmol/g)c 0.61 1.67 1.23 1.56
Fe3O4-RGO- SO3H
0.95
Fe3O4-RGO- SO3H
7 8 9 10 11 12 13 14 a
Reaction time (h)
Yield (mol%)
3 3 3 3 3 3
n.d. n.d. 21 96 (98)d 61 (66)d 94 (98)d
1.56
3
31
0.95
1.56
5
52
Fe3O4-RGO- SO3H
0.95
1.56
6
46
Fe3O4-RGO- SO3H Fe3O4-RGO- SO3H Fe3O4-RGO- SO3H Fe3O4-RGO- SO3H Fe3O4-RGO- SO3H
0.95 0.95 0.95 0.95 0.95
1.56 1.56 1.56 1.56 1.56
5 12 3 3 5
28 12 (44)e 93 95 40f
Reaction conditions: 30 mg carbohydrates, 30 mg Fe3O4-RGO- SO3H, 3 mL H2O, 150 oC.
Based on sulfur content which was analyzed by ICP.
c
b
Acid density values was determined by
acid-base titration. d The figures in parentheses were the conversion of cellobiose. e The figure in parenthese was the yield of levulinic acid. f The yield of total reducing sugars.
33
Table 3 Hydrolysis of cellulose to glucose by various solid acids.a
a
Entry
Catalyst
1 2 3 4 5
Fe3O4-RGO- SO3H Amberlyst-15 HZSM-5 (45) H-Beta (15) γ-Al2O3
Total acid density (mmol/g)b 1.56 1.9 0.3 0.98 0.05
Yield (mol%) Glucose Degradation products 28 8 15 4 3 nd 5 nd 2 nd
Reaction conditions: 30 mg cellulose, 30 mg solid acid, 3 mL H2O, 150 oC, 5 h. b Acid density
values was determined by acid-base titration.
34
Table 4 Experimental values of the kinetic contants k1 and k2. Catalyst Fe3O4-RGO- SO3H Amberlyst-15 0.0156 mol/L H2SO4
k1 ×103 (h-1) 144.32 124.89 14.8
k2 ×103 (h-1) 295.43 197.72 45.9
35
Magnetic solid acid based on reduced graphene oxide (Fe3O4-RGO-SO3H) was synthesized, which exhibited outstanding catalytic performance for cellulose hydrolysis.
36
·Novel magnetic solid acid (Fe3O4-RGO-SO3H) was successfully synthesized. ·Fe3O4-RGO-SO3H exhibited outstanding catalytic performance for cellulose hydrolysis. ·The ability to adsorb cellulose of Fe3O4-RGO-SO3H endow its high catalytic performance. ·Fe3O4-RGO-SO3H can be easily separated from the reaction residue.
37
S1385-8947(15)00779-2 http://dx.doi.org/10.1016/j.cej.2015.05.091 CEJ 13732
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
21 March 2015 23 May 2015 26 May 2015
Please cite this article as: Z. Yang, R. Huang, W. Qi, L. Tong, R. Su, Z. He, Hydrolysis of cellulose by sulfonated magnetic reduced graphene oxide, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej. 2015.05.091
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hydrolysis of cellulose by sulfonated magnetic reduced graphene oxide Zhenzhen Yang a, Renliang Huang b, Wei Qi a,c,d,*, Liping Tong a, Rongxin Su a,c,d , Zhimin He a
a
State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China.
b
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, P. R. China. c
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China.
d
Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin, 300072, China
* Corresponding author (Wei Qi). Tel: +86 22 27407799 Fax: +86 22 27407599 E-mail address: [email protected]
1
Abstract Reduced graphene oxide functionalized with magnetic Fe3O4 nanoparticles and -PhSO3H groups (Fe3O4-RGO-SO3H) was reported for environmentally benign hydrolysis of cellulosic materials, which is an important process in integrated utilization of biomass resource. Except for -PhSO3H groups, -COOH and -OH groups derived from graphene oxide (GO) were also expected in the structure of Fe3O4-RGO-SO3H. Fe3O4-RGO-SO3H exhibited outstanding catalytic performance for the hydrolysis of cellulosic materials, due to its crumpling feature with clear layers and the coexistence of -COOH and -OH groups. The unique structure of Fe3O4-RGO-SO3H favors the accessibility of cellulose to the active sites of the material and facilitates the diffusion of the product molecules. -COOH groups on the surface of the material provide a large concentration of acidic functionality for hydrolyzing cellulose, while -OH groups readily absorb cellulose through forming strong hydrogen bonds between -OH groups and the oxygen atoms in β-1, 4 glycosidic bonds. Furthermore, Fe3O4-RGO-SO3H can also be easily separated from the reaction residue with an extra magnetic force and further used at least five times.
Keywords: Biomass, Cellulose, Hydrolysis, Glucose, magnetic solid acid, Fe3O4-RGO-SO3H
2
1. Introduction The increasing energy crisis and environmental concerns have prompted the utilization of readily available lignocellulosic biomass resource as an alternative to the limited fossil resource. [1, 2] Lignocellulosic biomass is composed of lignin (15-20%), hemicellulose (25-35%) and cellulose (40-50%). [3] Lignin is a methoxylatedphenylpropane three-dimensional structure responsible for the structural rigidity of plants and that surrounds hemicellulose and cellulose. The hemicellulose polymer is formed by C5 and C6 sugar monomers. Finally, cellulose, the most abundant component of nonfood biomass, is a water-insoluble saccharide polymer composed of glucose monomers linked by β-1,4 glycosidic bonds. Among the three compositions of lignocellulosic biomass, cellulose has been widely studied as a potential material for production of advanced materials, biofuels and value-added chemicals. [4-7] Since glucose is a major platform for synthesis of various important chemicals such as 5-hydroxymethlfurfural (5-HMF), [8-10] levunilic acid [11, 12] and 2, 5-dimethylfuran, [13,14] hydrolysis of cellulose into glucose is of critical importance for chemical and biochemical industries based on sugars. Simultaneously, the process of cellulose hydrolysis offers great challenges to researchers due to the tight H-bond network in cellulose structure. Thus far, a great deal of effort has been devoted to the study of cellulose depolymerization and substantially appropriate cellulose hydrolysis schemes have been developed, including enzymedriven reactions, catalysis with mineral acid and ionic liquid promoted dissolution and hydrolysis of cellulose. Enzymatic degradation of cellulose has received considerable attention due to its high selectivity and moderate hydrolysis conditions. [15, 16] However, pretreatment process is necessary before enzymatic hydrolysis of cellulose, because the complex structure of cellulose makes it difficult for enzymes to access cellulose. Furthermore, the high cost and long residence
3
time of cellulase are also worth mentioning. Mineral acid such as sulfuric acid seems to be a more economic catalyst for cellulose hydrolysis. [17, 18] However, several drawbacks such as severe reaction conditions, corrosion hazard, environmental pollution and difficult separation of catalysts and products must be taken into consideration. Ionic liquids, which can dissolve cellulose to form a homogeneous solution, have been widely used for the degradation of cellulose. [19, 20] Although a high efficiency of cellulose saccharification can be obtained in ionic liquid medium, product separation is another bottleneck for the degradation of cellulose in ionic liquids. Therefore, hydrolysis of cellulose catalyzed by solid acid in water medium seems more environmental sustainable and has received increasing attention. Up till now, numerous papers have been published on the transformation of cellulose with solid acids. As a type of strong solid acid, transition-metal oxides with different morphology including mesoporous Nb-W oxide and layered HNbMoO6 exhibited different catalytic performance for cellulose hydrolysis. [21, 22] Heteropoly acids, another type of solid acid, consisting of early transition metal-oxygen anion clusters, have been widely used for cellulose hydrolysis. For example, Shimizu et al. reported H3PW12O40 and H3SiW12O40 for the hydrolysis of ball-milled cellulose and cellubiose into glucose or sugars. [23] Ogasawara et al. reported negatively charged heteropoly acids including H3BW12O40, H3AlW12O40 and H3GaW12O40 for the hydrolysis of non-pretreated crystalline cellulose into saccharides under mild reaction conditions. [24] Among various types of solid acids for cellulose hydrolysis, carbon-based solid acids have superior catalytic activities. Especially those carbonaceous acids derived from cheap naturally occurring raw materials are better candidates for cellulose transformation. The first carbonaceous acid from sulfonated D-glucose/sucrose materials was reported by Hara. [25, 26] Then, BCSO3H derived from bamboo, cotton and starch, [27] and CSA prepared from hydrolyzed corncob
4
residue were used for the hydrolysis of cellulosic materials. [28] Other solid acids, such as ionexchange resins, [29] polymer based acids [30] and H-formed zeolites, [31] have also been extensively investigated for cellulose degradation. Although these solid acids presented considerable catalytic activities for cellulose hydrolysis, a significant challenge remains for these catalytic systems when it is converted to non-pretreated cellulose or real biomass. Common solid acids cannot be efficiently separated from solid residue in biomass hydrolysis systems including non-reacted cellulose, lignin component in real biomass and by-product humins. Nowdays, magnetic materials have been widely used in various field due to the simply separation. [32-34] To overcome the problem aforementioned in heterogeneous catalyzed hydrolysis of cellulose, Lai et al. were the first to design a magnetic sulfonated mesoporous silica (Fe3O4-SBA-SO3H) and evaluate its catalytic performance for cellulose hydrolysis. [35, 36] Hydrolysis of avicel cellulose and amorphous cellulose with Fe3O4-SBA-SO3H gave 26% and 52% yields of glucose under corresponding reaction conditions, respectively. The magnetic solid acid can be easily separated with the solid residue by an external magnetic force after reaction and further used at least 5 times. This study made a fundamental contribution to the degradation of lignocellulose. Later, other magnetic solid acids such as CoFe2O4-embedded silica nanoparticles with -SO3H groups, Fe3O4@C-SO3H and PCM-SO3H were reported for cellulose hydrolysis in succession. [37-39] To provide more candidates for the hydrolysis of cellulosic materials, more high efficiency and stability magnetic solid acids need to be developed. Graphene, a promising carbon material, has excellent mechanical properties, a large surface area and a distinctive two-dimensional structure, providing an ideal platform to anchor functional groups and embed nanoparticles. [40, 41] For example, sulfonated graphene which was prepared by anchoring of –PhSO3H groups to the surface of reduced graphene has been used for the
5
hydrolysis of ethyl acetate. [42] While reduced graphene oxide functionalized with Fe3O4 nanoparticles has been synthesized for arsenic removal. [43] Herein, we introduced -PhSO3H groups and Fe3O4 nanoparticles to the structure of reduced graphene oxide (RGO) to prepare a new magnetic solid acid. The magnetic solid acid (Fe3O4-RGO-SO3H) was synthesized by simultaneously precipitation Fe3+ and Fe2+ ions with ammonia solution and reducing graphene oxide with hydrazine hydrate, followed by direct anchoring of sulfonic acid-containing aryl radicals to the surface of Fe3O4-RGO through C-C covalent bonds. The as synthesized Fe3O4RGO-SO3H not only contained post-introduced Fe3O4 nanoparticles and -PhSO3H groups, COOH, -OH groups and the crumpling feature with clear layers derived from GO were also preserved. The -COOH groups contribute to the total acid density and the superior hydrothermal stability of Fe3O4-RGO-SO3H, the -OH groups readily adsorb cellulose and the crumpling feature facilitates the diffusion of the product molecules. In one word, the unique structure and the existence of -COOH and -OH groups make Fe3O4-RGO-SO3H an excellent catalyst for the hydrolysis of cellulosic materials.
2. Materials and methods 2.1 Materials Natural graphite powder, cellobiose, avicel cellulose, 1-methylimidazole, C4H9Cl, Amberlyst15 and Amberlite-IRC76 were purchased from Aladdin Reagent Co., Ltd. 30 wt% H2O2, FeCl3·6H2O, FeCl2·4H2O, CH3CH2OH, NH4OH were purchased from Tianjin Guangfu Chemical Reagent Co. Ltd. 80% hydrazine hydrate, NaNO3, KMnO4, NaNO2, starch, sucrose, xylose, glucose, sulfanilic acid were purchased from Tianjin Kemiou Chemical Reagent Co. Ltd. H-Beta (25) and HZSM-5 were purchased from The Ctalyst Plant of Nankai University, γ-Al2O3
6
was purchased from Nanjing Tianxing New Material Co. Ltd. Corn cob was obtained from a local farm. 2.2 Preparation of water-soluble GO Natural graphite powder was oxidized to graphene oxide according to Hummers method. [44] First, graphite powder (2 g) was added to 50 mL of 98% H2SO4 and 2 g NaNO3 solution in an ice bath. KMnO4 (6 g) was slowly added with vigorous stirring, avoiding a sudden increase in temperature. The mixture was stirred at 35 oC until it became pasty brownish and then diluted with water, followed by adding 10 mL of 30 wt% H2O2. The obtained brilliant yellow mixture was centrifuged and washed with 5% HCl several times, then by a lot of water. Finally the resulted brown powder solid was dried at room temperature under vacuum condition. 2.3 Preparation of Fe3O4-RGO In a typical procedure, 0.7 g GO was first dispersed in 450 mL water under ultrasonic for 60 min, and 0.2027g FeCl3·6H2O and 0.0792 g FeCl2·4H2O (2:1 molar ratio) were dissolved in 25 ml water. The solution of Fe3+/Fe2+ salts was added to the GO solution slowly with constant stirring, then the pH of the solution was adjusted to 9~10 by 30% ammonia solution. The threeneck ground flask was transferred to an oil bath with a temperature of 80 oC and 6 ml 80% hydrazine hydrate was added slowly. After being stirring for 4 h, the resulted black solution was cooled to RT. The black solid was centrifuged and washed with water for several times, finally Fe3O4-RGO was obtained after drying at 60 oC under vacuum condition. Fe3O4 nanoparticles were synthesized by precipitation Fe3+ and Fe2+ ions with ammonia solution at pH=10. 2.4 Preparation of Fe3O4-RGO-SO3H
7
Fe3O4-RGO-SO3H was obtained by reacting aryl diazonium salt of sulfanilic acid with Fe3O4RGO. Firstly, 4-Benzenediazoniumsulfonate was synthesized by diazotization of sulfanilic acid as follows, 5.2 g sulfanilic acid was dispered in 1 M HCl aqueous solution by vigorous stirring in an ice bath, and the temperature was controlled at 3-5 oC. After 10% excess 1 M NaNO2 solution (33 ml) was added dropwise, a clear solution was obtained. The solution was stirred for another 1 h, and the synthesized white precipitate was filtered and washed with cool water. 4-Benzenediazoniumsulfonate obtained was moved to a three-necked ground flask with ethanol (40 mL) and a small amount of water. Then, Fe3O4-RGO was added, and the temperature was controlled at 50 oC. After stirring for 1.5 h, the obtained Fe3O4-RGO-SO3H was intensively washed with deionized water until the pH was about 6 (the pH value of deionized water we used is about 6) to insure the absence of free acid in the structure of Fe3O4-RGO-SO3H and then dried overnight at 80 oC. (Fig. 1). 2.5 Characterization High resolution transmission electron microscopy (HRTEM) was performed on a JEM-2100 microscope (JEOL Ltd., Akishima, Japan) operating at an acceleration voltage of 200 kV. Scanning electron microscopy (SEM) was performed on an S-4800 field emission scanning electron microscope (Hitachi Hightechnologies Co., Japan) at the acceleration voltage of 3 kV. All the samples were sputter-coated with platinum using an E1045 Pt-coater (Hitachi Hightechnologies Co., Japan) before SEM observation. Energy dispersive X-ray spectroscopy (EDS) analysis was used to identify the elemental composition and distribution of Fe3O4-RGO-SO3H. The density of -SO3H was estimated based on the sulfur content which was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, Therom Jarell-Ash Corp, USA). Fourier transform infrared spectroscopy (FTIR) analysis was performed using a
8
spectrophotometer (BIO-Rad; Japan), which was with a spectral resolution of 4 cm-1 in the wave number range of 500-4000 cm-1. X-ray diffraction (XRD) analysis was performed on D/max 2500 (Rigaku; Japan). The electronic binding energy of Fe3O4-RGO-SO3H was measured using X-ray photoelectron spectroscopy (Quantum 2000, USA), which was conducted with a PHI Quantum 1600 Scanning ESCA Microprobe equipped with Al Kα radiation as an excitation source. 2.6 Catalytic test 30 mg cellulose and 30 mg catalyst were weighed into a pressure tube, and then 3 mL deionized H2O was added. The tube was heated at 150 °C, and zero time was taken when the temperature of the reactor reached 150 °C. After reaction, the catalyst was removed by an extra magnetic force and 1 mL of the residue mixture was taken out by an injection syringe and then filtered with a 0.45 µm syringe filter prior to analysis by High-performance liquid chromatography (HPLC). The separated Fe3O4-RGO-SO3H was washed with water several times under ultrasonic for further catalyzed reaction. The yields of water soluble organic compounds were calculated as follows. Glucose yield (%) = (moles of glucose in water soluble products)/(mol of C6H12O5 in charged cellulose)×100 Degradation products yield (%) = (moles of degradation products in water soluble products)/(mol of C6H12O5 in charged cellulose)×100 Here, 30 mg cellulose is approximately equal to 0.18 mmol of anhydroglucose, which was calculated based on considering the repeating anhydroglucose units in cellulose. In this study, the initial substrate concentration was calculated in terms of anhydroglucose, rather than in the form of the cellulose polymer, and the product yield was expressed as molar yield of glucose. 9
3. Results and discussion 3.1 Characterization of Fe3O4-RGO-SO3H The SEM image of Fe3O4-RGO-SO3H shows the presence of magnetite and the crumpled sheets of RGO can be seen throughout the morphology. In addition, a layered structure is observed at the edges of the agglomerates (Fig. 2a). This unique structure of Fe3O4-RGO-SO3H has potential advantages in acid-catalyzed hydrolysis reaction. On one hand, the reactants can easily access the active sites on both sides of two-dimensional graphene sheets. On the other hand, the crumpling feature facilitates the diffusion of the product molecules. [42] The TEM image of Fe3O4-RGO-SO3H shows magnetite nanoparticles well dispersed in the RGO matrix, and the nanoparticles are entrapped inside the RGO sheets rather than simply mixed up or blended with RGO. Furthermore, the lattice fringes of the magnetite nanoparticles are found surrounding of the RGO matrix, and the lattice spacing is 0.21 nm which corresponding to the indexes (400, 040 and 004) reflections (Fig. 2b). The powder XRD pattern and the XPS spectra of Fe3O4-RGO-SO3H further proved the chemical properties of the magnetite nanoparticles. In the powder XRD pattern of Fe3O4-RGOSO3H (Fig. 3), the main peaks at 2θ = 18.5o (111), 30.4o (220), 35.32o (311), 43.2o (400), 54.4o (422), 57.46o (511) and 62.56o (440) show the characteristics of Fe3O4 (JCPDS No. 75-0033), and the broad peak at 23.9 o which is for RGO. The wide scan XPS spectrum of Fe3O4-RGOSO3H (Fig. 4) shows the photoelectron lines at a binding energy of about 168, 232, 285, 532, 712, and 725 eV attributed to S 2p, S 2s, C 1s, O 1s, Fe 2p3/2, and Fe 2p1/2, respectively. In the spectrum of Fe 2p which is inserted in the wide scan XPS spectrum of Fe3O4-RGO-SO3H, the peaks Fe 2p1/2 and Fe 2p3/2 are located at 725 and 712 eV, not at 710 and 724 eV which are for
10
Fe2O3. Besides, there was no satellite peak at about 719 eV, characteristic of Fe2O3, which is indicative of the formation of the Fe3O4 phase in the material.[43] The EDS mapping images of Fe3O4-RGO-SO3H show homogeneous distribution of iron and sulfur elements in the entire range (Fig. 5). And the element mass percents of carbon, oxygen, sulfur and iron in the solid acid were 66.42, 23.92, 3.51 and 6.15 wt%, respectively, which were obtained from quantitative analysis. Thus, considering the O/S atom ratio (13.6) and the O/Fe atom ratio (13.6), residual epoxide, hydroxyl and carboxyl groups derived from GO are also expected in the composition of Fe3O4-RGO-SO3H. The density of -SO3H was estimated to be 0.95 mmol/g based on the sulfur content which was analyzed by ICP. The total -SO3H + -COOH and -SO3H + -COOH + -OH contents were estimated from the exchange of Na+ in aqueous NaCl and NaOH solutions respectively, [45] to obtain the proportions of each functional group, and the data was summarized in Table 1. The functionalization of RGO with magnetic Fe3 O4 nanoparticles and -PhSO3H groups was further confirmed by the FTIR (Fig. 6). The very broad intense peak at 3431 cm-1 is seen for the O-H stretching frequency. The FTIR spectrum of RGO shows C=O (1708 cm-1), aromatic C=C (1600 cm-1), carboxyl C=O (1352 cm-1), epoxy C-O-C (1209 cm-1), and alkoxy C-O (1013 cm-1) stretching vibrations. [43] In the FTIR spectrum of RGO-SO3H, the presence of -PhSO3H is confirmed by the peaks at 1033 cm-1, 1118 cm-1, 1164 cm-1 (two S-O and one S-phenyl vibrations), and the peak at 1003 cm-1 shows the characteristic vibration of a para-disubstituted phenyl group. Furthermore, the intensive of the peak at 1598 cm-1 also indicates the presence of phenyl groups. In the FTIR spectrum of Fe3O4-RGO-SO3H, the newly appeared peak at 623 cm1
was a characteristic peak corresponding to the stretching vibration of Fe-O as compared to
RGO and RGO-SO3H.
11
3.2 Catalytic study The catalytic performance of Fe3O4-RGO-SO3H was evaluated for the catalyzed hydrolysis of cellulosic materials, Fe3O4, Fe3O4-RGO and RGO-SO3H were also used as catalysts for comparison. Cellobiose, an intermediate product of cellulose hydrolysis, was selected as the model compound for hydrolysis reaction first. As shown in Table 2, no product has been detected when cellobiose was heated in water at 150 oC for 3 h without any catalyst (Entry 1) or in the presence of pure Fe3O4 (Entry 2). Fe3O4-RGO, which excludes –PhSO3H groups, exhibited relatively low efficiency for the hydrolysis of cellobiose, only gave 21% molar yield of glucose (Entry 3). However, in the presence of RGO-SO3H or Fe3O4-RGO-SO3H (total acid densities were 1.67 and 1.56 mmol/g), the molar yields of glucose were up to 96% and 94%, and the conversions of cellobiose were 98% and 97% respectively (Entry 4, 6). In addition, glucose yield increased obviously with the amount of acid in Fe3O4-RGO-SO3H (Entry 5, 6). Cellulose has a low reaction activity for decomposition in water due to its stubborn structure. Amorphous cellulose, which has a significantly lower crystallinity than cellulose, was prepared by dissolving avicel cellulose in ionic liquid (1-butyl-3-methylimidazolium chloride) at 80 oC and then precipitating it from water (the detailed method was shown in Supplementary data). The XRD patterns of avicel cellulose and amorphous cellulose were shown in the Supplementary data (Fig. S1). By using amorphous cellulose as substrate, the molar yield of glucose was 31% under the same reaction conditions as that for cellobiose hydrolysis due to the poor solubility of cellulose in water (Entry 7). Whereas, glucose yield was increased to 52% by prolonging the reaction time from 3 h to 5 h (Entry 8). A longer reaction time led to a loss in glucose yield (46%) because of the formation of levulinic acid, formic acid and 5-HMF (Entry 9). Furfural (FF), which is the dehydration product of xylose, was detected as another by-product
12
in the reaction, but negligible (less than 1%), may be due to the small amount of xylan in the cellulose we used. When avicel cellulose was hydrolyzed, the molar yield of glucose decreased to 28% (Entry 10), which is close to the content of the amorphous region in avicel cellulose (degree of crystallinity is about 75%). However, this interesting finding does not mean that Fe3O4-RGO-SO3H can only decompose the amorphous proportion of cellulose. By prolonging reaction time to 12 h, levulinic acid, which is the main product of acid-catalyzed dehydration of glucose, became the major product with yield of 44%. The total yields of glucose and levulinic acid were up to 56%, implying that the crystalline proportion of cellulose was also partially decomposed by Fe3O4-RGO-SO3H (Entry 11). Fe3O4-RGO-SO3H has also been evaluated for hydrolysis of another two common polycaccharides, starch and sucrose, under the reaction conditions shown in Table 2, the molar yields of glucose were 93% and 95% respectively (Entry 12, 13). Moreover, a typical biomass corn cob was employed as a feed to determinate the capability of Fe3O4-RGO-SO3H for hydrolysis of lignincellulosic biomass. The molar yield of total reducing sugars was 40% based on the contents of cellulose (45 wt%) and hemicellulose (35 wt%) in corn cob, 5-HMF, levulinic acid and FF were detected as by-products (Entry 14). To verify the origin of these by–products, Fe3O4-RGO-SO3H was then used as catalyst for dehydration of glucose and xylose, two kinds of monosaccharides (Supplementary data, Table S1). As shown in Table S1, Fe3O4-RGO-SO3H exhibited moderate catalytic activity for glucose/xylose dehydration, 5-HMF/FF and levulinic acid were detected as products. These are only preliminary results, the systematic study of carbohydrate decomposition by Fe3O4-RGO-SO3H are under way. To further investigate the superior catalytic performance of Fe3O4-RGO-SO3H, another several commercially available solid acids have also been tested for cellulose hydrolysis for
13
comparation. The experimental results and the acid densities of these solid acids were listed in Table 3. Among the solid acids we used, only Amerlyst-15, an ionic-exchange resin with -SO3H groups, gave a moderate glucose yield of 12%. Other catalysts, such as HZSM-5, H-Beta and γAl2O3, exhibited lower activities for the hydrolysis of avicel cellulose. Then dynamics experiments were performed for cellulose hydrolysis catalyzed by Fe3O4-RGO-SO3H, Amberlyst 15 and 0.0156 mol/L H2SO4 (the concentration of -SO3H groups bonded to 30 mg Fe3O4-RGOSO3H
in 3 mL water was 0.0156 mol/L). The dependence of the yield of glucose and
degradation products (5-HMF and levulinic acid ) on reaction time were shown in Fig. 7. The results indicate that the catalytic activity of Fe3 O4-RGO-SO3H is much higher than that of Amberlyst 15 and 0.0156 mol/L H2SO4. For further quantitative comparation of the activity of Fe3O4-RGO-SO3H, Amberlyst 15 and 0.0156 mol/L H2SO4, a kinetic model of two consecutive firt-order pseoduhomogeneous reactions was proposed for cellulose hydrolysis. [46,47] This model assumes that cellulose can be treated as a homogeneous reactant when water is present in excess. As follows: Cellulose → glucose → Degradation products
(1)
In accordance with eqn (1), the glucose-production rate is
= −
(2)
which, integrated, lead to =
[exp− − exp− ]
(3)
The values of the kinetic contants k1 and k2 were obtained by the simplex method, and the values are presented in Table 4. As expected, the results once again proved that Fe3O4-RGOSO3H exhibited much higher activity than Amberlyst 15 and 0.0156 mol/L H2SO4 for cellulose hydrolysis. 14
The large differences in catalytic properties among these solid acid catalysts cannot be adequately explained as being due solely to the density and strength of acid sites on their surface. Acid-catalyzed hydrolysis of cellulose into glucose involves two stages: H+ attacks to hydrogen and β-1, 4 glycosidic bonds in avicel cellulose to form water-soluble β-1, 4 glucan, followed by the hydrolysis of β-1, 4 glucan to yield glucose. Therefore, efficient conversion of cellulose into glucose using a solid acid requires a strong interaction between the solid acid and β-1, 4 glycosidic bonds. Because the acid sites of solid acids cannot approach the surface of cellulose without such an interaction, which is different from homogeneous acid. To further investigate the interaction between the solid acids and β-1, 4 glycosidic bonds in β1, 4 glucan, adsorption experiments were performed using cellobiose as a substrate. (the detailed experiment method was shown in Supplementary data) As shown in Table S2 (Supplementary data), the commercially available solid acids such as Amberlyst 15, H-Beta, HZSM-5 and γ-Al2O3 did not adsorb cellobiose in water even after stirring for one day. However, Fe3O4-RGO-SO3H can efficiently adsorb cellobiose, and the adsorption reached a plateau of 20% of the cellobiose in only 3 h. The results were further confirmed by soaking cellulose filter papers in solid acid-water suspensions, only Fe3O4-RGO-SO3H particles attached to the cellulose (Supplementary data, Fig. S2). From the results, the inability of Amberlyst 15 to adsorb β-1,4 glucan demonstrates that SO3H groups are not adsorption sites in Fe3O4-RGO-SO3H (Table S2, Entry 3). Furthermore, Amberlite-IRC76, a polystyrene-based resin contains high density of -COOH groups (4.0 mmol/g), exhibited much lower ability than Fe3O4-RGO-SO3H to adsorb β-1,4 glucan (Table S2, Entry 7). Therefore, -COOH groups are not the dominant sites responsible for the adsorption of β-1,4 glucan to Fe3O4-RGO-SO3H. Taking into account these results, the neutral -OH groups
15
were considered as active sites for adsorption β-1,4 glucan. In addition, it is worth noting that Fe3O4-RGO-SO3H cannot adsorb glucose as a monomer of cellobiose at all (Table S2, Entry 1). This suggests that the glycosidic bonds in β-1,4 glucan, not -OH groups, participate in the adsorption on Fe3O4-RGO-SO3H. In one word, -OH groups in Fe3O4-RGO-SO3H can be linked to the oxygen atoms in β-1, 4 glycosidic bonds by strong hydrogen bonds, and thereby the substrate can be adsorbed onto the surface of Fe3O4-RGO-SO3H for the following reaction. The excellent adsorption ability as well as the crumpling feature with clear layers which was aforementioned of Fe3O4-RGO-SO3H is responsible for the high catalytic activity for cellulose hydrolysis. In the heterogeneous acid-catalyzed hydrolysis of cellulosic materials, full conversion is hardly achieved, so the separation of catalyst from the reaction residue is another bottleneck in this field. Magnetic solid acid Fe3O4-RGO-SO3H can easily solve the problem, because it can be recovered from the resulted mixture by an extra magnetic force as shown in Fig. 8. Furthermore, Fe3O4-RGO-SO3H can be used again directly after washing and drying, and no obvious deactivation was found after being used 5 times (Fig. 8). The possibility for loss of active sites of Fe3O4-RGO-SO3H during the hydrolysis reaction was also tested. The reaction was stopped after 2 h, and Fe3O4-RGO-SO3H was filtered. The filtrate was reacted again for another 3 h under the same reaction conditions. The result shows glucose yield does not change after Fe3O4-RGOSO3H was removed (Fig. S3). In addition, the pH values of the reaction solution before and after the catalysis reaction were detected, as shown in Table S3. The pH value had no change after pure water was heated at 150 oC for 5 h (Table S3, Entry 1). But when cellulose was heated at 150 oC for 5 h, a little decrease in pH value, may be due to the formation of little soluble organic acid (Table S3, Entry 2). A little greater decrease in pH value after Fe3O4-RGO-SO3H was
16
heated at 150 oC for 5 h, indicates a little loss of -SO3H in the reaction (less than 1%), may be due to the self-hydrolysis of -PhSO3H (Ar-SO3H + H2O ↔ Ar-H + H2SO4) (Table S3, Entry 3). [25] The much greater decrease in pH value before and after cellulose hydrolysis reaction may be due to the formation of levulinic acid and formic acid (Table S4, Entry 4). Moreover, the proportions of each functional group in Fe3O4-RGO-SO3H almost remain constant after being used 5 times, which further confirms the stability of the catalyst (Table 1, Entry 4). The aforementioned results show the considerable hydrothermal stability of Fe3O4-RGOSO3H. However, it is notable that -SO3H bonded to aromatic hydrocarbons is generally subject to leaching in the presence of water, as observed for Amberlyst 15, as a result of the equilibrium between sulfoaromatic compounds and aromatic hydrocarbons. [47] It has been reported that the aromatic carbon -SO3H bonds in sulfoaromatic compounds bearing electron-withdrawing functional groups are more stable as a result of the increased electron density between carbon and sulfur atoms afforded by the electron-withdrawing functional groups. [48] So it is expected that electron-withdrawing -COOH groups in Fe3 O4-RGO-SO3H increase the electron density between carbon and sulfur atoms. This may be the origin of the stability of Fe3O4-RGO-SO3H in reactions involving excess water.
4. Conclusions Fe3O4-RGO-SO3H was successfully synthesized based on reduced graphene oxide (RGO), containing Fe3O4 nanoparticles and benzene sulfonic acid groups which were directly anchored onto the surface of RGO through C-C covalent bonds. The unique structure of Fe3O4-RGO-SO3H and its high dispersion in water promote the accessibility of cellulose to the active sites. In addition, the coexistence of -OH groups facilitated the adsorption of cellulose to the surface of the material by forming strong hydrogen bonds with the oxygen atoms in β-1, 4 glycosidic bonds, 17
and thus enhanced the catalytic activities of Fe3O4-RGO-SO3H for cellulose hydrolysis. Furthermore, Fe3O4-RGO-SO3H exhibited superior stability in the reaction of cellulose hydrolysis, may be due to the existence of electron-withdrawing -COOH groups, which increase the electron density between carbon and sulfur atoms.
Acknowledgments This work was supported by the Natural Science Foundation of China (Nos. 51173128, 31071509), the 863 Program of China (Nos. 2012AA06A303, 2013AA102204), the Ministry of Science and Technology of China (No. 2012YQ090194), the Beiyang Young Scholar of Tianjin University (2012) and the Program of Introducing Talents of Discipline to Universities of China (No. B06006).
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Figure Captions 22
Fig. 1. Preparation of magnetic solid acid Fe3O4-RGO-SO3H.
Fig. 2. (a) SEM spectrum of of Fe3O4-RGO- SO3H, and (b) Low and high magnification TEM images of Fe3O4-RGO- SO3H.
Fig. 3. X-ray dirrrication (XRD) patterns of GO and Fe3O4-RGO-SO3H.
Fig. 4. X-ray photoelectron spectroscopy (XPS) spectra: wide scan and Fe 2p spectra.
Fig. 5. The SEM image of Fe3O4-RGO-SO3H (a), and corresponding quantitative EDS element mapping of S (b) and Fe (c). (d) The EDS spectrum and element analysis of Fe3O4-RGO-SO3H.
Fig. 6. FT-IR spectrums of the Fe3O4-RGO- SO3H, RGO-SO3H and RGO.
Fig. 7. Kinetic study for the hydrolysis of cellulose by different solid acids. Reaction conditions: 30 mg cellulose, 30 mg solid catalysts, 3 mL H2O, at 150 oC. (black line) Fe3O4-RGO- SO3H, (blue line) Amberlyst 15, (red line) 0.0156 mol/L H2SO4, (■) glucose yield (●) degradation products yield.
Fig. 8. (a) and (b) Separation of Fe3O4-RGO-SO3H from the reaction residue. (c) Hydrolysis of cellulose to glucose by reused Fe3O4-RGO-SO3H. Reaction conditions: 30 mg cellulose, 30 mg Fe3O4-RGO-SO3H, 3 mL H2O, at 150 oC for 5 h.
23
Fig. 1. Preparation of magnetic solid acid Fe3O4-RGO-SO3H.
24
Fig. 2. (a) SEM spectrum of of Fe3O4-RGO- SO3H, and (b) Low and high magnification TEM images of Fe3O4-RGO- SO3H.
25
40
(440)
(422)
(333)
(311) (400)
(220) RGO 002
(111)
Intensity (a.u.)
20
60
80
2 theta (degree) Fig. 3. X-ray dirrrication (XRD) pattern of Fe3O4-RGO-SO3H.
26
720
600
500
400
300
710
700
200
Fe 3s Fe 3p
S 2p
Binding Energy (eV)
S 2s
C 1s
730
700
Fe 2p3/2
B
O 1s
Fe 2p 1/2 Fe 2p 3/2
Intensity (a.u.)
Fe 2p1/2
100
0
Bonding Energy (eV) Fig. 4. X-ray photoelectron spectroscopy (XPS) spectra: wide scan and Fe 2p spectra.
27
Fig. 5. The SEM image of Fe3O4-RGO-SO3H (a), and corresponding quantitative EDS element mapping of S (b) and Fe (c). (d) The EDS spectrum and element analysis of Fe3O4-RGO-SO3H.
28
Transmittance (%)
Fe3O4-RGO-SO3H RGO-SO3H
Fe-O
RGO C-H
C=O C=C
-OH
4000
3500
O=C-O
3000
2500
2000
C-H
SO3 O=S=O
1500
1000
500
Wavenumbers (cm-1) Fig. 6. FT-IR spectrums of the Fe3O4-RGO- SO3H, RGO-SO3H and RGO.
29
50
Molar Yield (%)
40 30 20 10 0 0
2
4
6
8
10
12
14
Time (h) Fig. 7. Kinetic study for the hydrolysis of cellulose by different solid acids. Reaction conditions: 30 mg cellulose, 30 mg solid catalysts, 3 mL H2O, at 150 oC. (black line) Fe3O4-RGO- SO3H, (blue line) Amberlyst 15, (red line) 0.0156 mol/L H2SO4, (■) glucose yield (●) degradation products yield.
30
c
Glucose Levulinic acid
Molar yield (%)
30
20
10
0 1
2
3
4
5
Cycle Fig. 8. (a) and (b) Separation of Fe3O4-RGO-SO3H from the reaction residue. (c) Hydrolysis of cellulose to glucose by reused Fe3O4-RGO-SO3H. Reaction conditions: 30 mg cellulose, 30 mg Fe3O4-RGO-SO3H, 3 mL H2O, at 150 oC for 5 h.
31
Table 1 Concentration determination of different functional groups in functionalized RGO. -COOH groups (conc. in mmol/g) 0.60
-PhSO3H groups (conc. in mmol/g)
Fe3O4-RGO
-OH groups (conc. in mmol/g) 0.26
RGO-SO3H
0.26
0.61
1.06
Fe3O4-RGO- SO3H
0.26
0.61
0.95
a
0.24
0.6
0.93
Catalyst
Fe3O4-RGO- SO3H a
Fe3O4-RGO- SO3H has been reused 5 times.
32
Table 2 Hydrolysis of carbohydrates to glucose.a
Entry
Substrate
Catalyst
S content (mmol/g)b
1 2 3 4 5 6
Cellobiose Cellobiose Cellobiose Cellobiose Cellobiose Cellobiose Amorphous cellulose Amorphous cellulose Amorphous cellulose Cellulose Cellulose Starch Sucrose Corn cob
Fe3O4 Fe3O4-RGO RGO-SO3H Fe3O4-RGO- SO3H Fe3O4-RGO- SO3H
1.06 0.63 0.95
Total acid density (mmol/g)c 0.61 1.67 1.23 1.56
Fe3O4-RGO- SO3H
0.95
Fe3O4-RGO- SO3H
7 8 9 10 11 12 13 14 a
Reaction time (h)
Yield (mol%)
3 3 3 3 3 3
n.d. n.d. 21 96 (98)d 61 (66)d 94 (98)d
1.56
3
31
0.95
1.56
5
52
Fe3O4-RGO- SO3H
0.95
1.56
6
46
Fe3O4-RGO- SO3H Fe3O4-RGO- SO3H Fe3O4-RGO- SO3H Fe3O4-RGO- SO3H Fe3O4-RGO- SO3H
0.95 0.95 0.95 0.95 0.95
1.56 1.56 1.56 1.56 1.56
5 12 3 3 5
28 12 (44)e 93 95 40f
Reaction conditions: 30 mg carbohydrates, 30 mg Fe3O4-RGO- SO3H, 3 mL H2O, 150 oC.
Based on sulfur content which was analyzed by ICP.
c
b
Acid density values was determined by
acid-base titration. d The figures in parentheses were the conversion of cellobiose. e The figure in parenthese was the yield of levulinic acid. f The yield of total reducing sugars.
33
Table 3 Hydrolysis of cellulose to glucose by various solid acids.a
a
Entry
Catalyst
1 2 3 4 5
Fe3O4-RGO- SO3H Amberlyst-15 HZSM-5 (45) H-Beta (15) γ-Al2O3
Total acid density (mmol/g)b 1.56 1.9 0.3 0.98 0.05
Yield (mol%) Glucose Degradation products 28 8 15 4 3 nd 5 nd 2 nd
Reaction conditions: 30 mg cellulose, 30 mg solid acid, 3 mL H2O, 150 oC, 5 h. b Acid density
values was determined by acid-base titration.
34
Table 4 Experimental values of the kinetic contants k1 and k2. Catalyst Fe3O4-RGO- SO3H Amberlyst-15 0.0156 mol/L H2SO4
k1 ×103 (h-1) 144.32 124.89 14.8
k2 ×103 (h-1) 295.43 197.72 45.9
35
Magnetic solid acid based on reduced graphene oxide (Fe3O4-RGO-SO3H) was synthesized, which exhibited outstanding catalytic performance for cellulose hydrolysis.
36
·Novel magnetic solid acid (Fe3O4-RGO-SO3H) was successfully synthesized. ·Fe3O4-RGO-SO3H exhibited outstanding catalytic performance for cellulose hydrolysis. ·The ability to adsorb cellulose of Fe3O4-RGO-SO3H endow its high catalytic performance. ·Fe3O4-RGO-SO3H can be easily separated from the reaction residue.
37