One-Pot Catalytic Transformation of Lignocellulosic Biomass into

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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One-Pot Catalytic Transformation of Lignocellulosic Biomass into Alkylcyclohexanes and Polyols Xiangcheng Li,† Tianye Guo,† Qineng Xia, Xiaohui Liu, and Yanqin Wang* Shanghai Key Laboratory of Functional Materials Chemistry, Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, China S Supporting Information *

ABSTRACT: The direct transformation of lignocellulosic biomass into valuable chemicals and fuels, which is recognized as a promising strategy for easing the present situation of energy depletion, has brought a hot-spot of research. Here we describe the one-pot catalytic conversion of cornstalk into liquid alkylcyclohexanes (from lignin fraction) and polyols (from cellulose and hemicellulose components) over Ru/C catalysts, which are prepared by oxidized commercial Ru/C and then reduced at different temperature. These Ru/C catalysts are well-characterized by the techniques of X-ray photoelectron spectroscopy, H2-TPR, and NH3-TPD correlated with the conversion of cornstalk and products distribution. It is found that the Ru/C catalyst reduced at 300 °C has the best performance for cornstalk conversion; the mole yield of liquid alkylcyclohexanes reached 97.2% (based on lignin monomers in cornstalk) with 52.7% polyols (24.5% sorbitol, 12.2% xylitol, and 16.0% C2−C4 polyols) at 200 °C. The detailed investigations reveal a synergistic effect between Ru and RuO2 species in the conversion of lignocellulosic biomass, where metallic Ru is responsible for hydrogenation and RuO2 species plays crucial roles in cleaving the interlinkages of lignin− carbohydrate (ester and ether linkages) efficiently and in promoting the retro-aldol condensation of carbohydrates. Furthermore, the Ru/C-300 catalyst can be reused for five runs and can be extended to several different raw biomass substrates, indicating that this Ru/C catalysts-based reaction system is promising for the total conversion of lignocellulosic biomass into liquid alkylcyclohexanes and polyols. KEYWORDS: Biomass, Alkylcyclohexanes, Polyols, Sorbitol, Xylitol, Ru/C

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INTRODUCTION Lignocellulosic biomass consists of lignin, cellulose, and hemicellulose and is a sustainable feedstock compared with fossil fuels.1−3 It can be transformed and upgraded into fuels and chemicals by sustainable biorefineries and has the potential to share responsibility for solving the situations of energy depletion.4−7 Due to the requirement of high efficiency in the full valorization of lignocellulosic biomass, great efforts and plenty of strategies have been initiated rapidly in recent years.8−21 Moreover, the direct catalytic conversion of raw lignocellulosic biomass into fuels and chemicals has attracted a lot of attention. In 2012, Zhang et al. developed a one-pot catalytic process to convert raw lignocellulosic biomass into phenols and alcohols over a Ni−W2C/AC catalyst.15 The synergistic effect existed in Ni−W2C/AC effectively converted the lignin component into © XXXX American Chemical Society

phenols and carbohydrate fractions into ethylene glycol and other diols. Ma et al. reported the production of gasoline alkanes (e.g., hexane, methylcyclopentane, pentane, and so on) and value-added chemicals (phenols with a wide distribution) from raw lignocellulosic biomass over layered LiTaMoO6 and Ru/C catalysts in aqueous phosphoric acid medium.17 Both these two catalytic processes are of high importance in the full utilization of lignocellulosic biomass, while the subsequent separation and purification of phenols derived from the lignin component may lead to energy consumption. More recently, a multifunctional Pt/NbOPO4 catalyst was developed to convert raw lignocellulosic biomass into liquid alkanes (e.g., hexane, Received: January 4, 2018 Revised: January 29, 2018

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DOI: 10.1021/acssuschemeng.8b00012 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Scheme 1. Schematic Representation of the Direct Catalytic Transformation of Lignocellulosic Biomass into Liquid Alkylcyclohexanes and Polyols over Partially Reduced Ru/C Catalysts

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pentane, cycloalkanes, and so on),18 where the synergistic effect between Pt, NbOx species, and acidic sites played critical roles in the direct hydrodeoxygenation of lignocellulose. In this system, although liquid alkanes are easily separated, keeping the oxygen-containing functional groups of cellulose and hemicellulose fractions is still of great significance. As is well-known, sorbitol and xylitol are versatile precursors to chemicals and fuels;4,22,23 meanwhile, the C2−C4 polyols (e.g., 1,2-butanediol, glycerol, 1,2-propanediol, and glycol) can be used as feedstocks for production of hydrogen23,24 and can stabilize the depolymerization of lignin via a acetal stabilization strategy.25,26 Thus, an alternative strategy for the one-pot conversion of lignocellulosic biomass into liquid alkylcyclohexanes and polyols is expected. In our recent study,27 the lignin extracted from birch can be highly selectively converted into C7−C9 cycloalkanes (29.3 wt % yield) merely over Ru/C catalyst in aqueous phase; it is likely that the combination of metallic Ru and H3O+ ions formed in situ from hot water (250 °C) is effective for depolymerization and hydrodeoxygenation of lignin. Moreover, Liu et al. investigated the high production of sorbitol and mannitol from cellulose over a Ru/C catalyst in aqueous phase, where cellulose hydrolysis to glucose catalyzed by in situ formed H3O+ ions in hot water and subsequent glucose hydrogenation by metallic Ru.28 Based on the preceding results, we propose that Ru/C would be an excellent catalyst for carbohydrates and lignin conversion in aqueous solution. Here, we report a partially reduced Ru/C catalyst, with RuO2 as Lewis acid sites for completely stripping the lignin fraction from cornstalk and metallic Ru acting as active sites for hydrogenation, and high yields of liquid alkylcyclohexanes and polyols (the majority products are sorbitol and xylitol) are achieved in aqueous phase (Scheme 1). The obtained products are easily separated for liquid alkylcyclohexanes suspended in the upper layer and polyols kept in the aqueous phase. It is found that the synergistic effect between Ru and RuO2 at Ru/C catalyst plays an important role on the whole catalytic process. In addition, the Ru/C catalyst can be recycled five times and extended to other raw biomass such as birch, beech, and pine sawdust successfully.

EXPERIMENTAL SECTION

Materials. The 5 wt % Ru/C catalyst and active carbon were purchased from Aladdin Chemical Co., Ltd. Sorbitol (98%), mannitol (98%), xylitol (99%), and arabitol (98%) were purchased from Adamans Reagent Co., Ltd. 1,2-Butanediol (97%) was purchased from Energy Chemical Co., Ltd. Glycerol, propylene glycol, and ethylene glycol were purchased from Shanghai Taitan Scientific Co., Ltd. Cornstalk, pine, beech, and birch samples were purchased from Shandong Province, China. The ball-milled lignocellulosic biomass samples were prepared by using a laboratory ball mill (QM-3SP04). To get the desired sample, about 2 g of lignocellulosic biomass was charged into the grinding cell and the ball milling was operated at a frequency of 50 Hz with 6 mm agate balls for 12 h. The lignin is obtained from birch according to the studies of Shao et al.14 Catalyst Preparation. The RuO/C catalyst was obtained by treating 5 wt % Ru/C catalyst in air at 100 °C for 12 h. Ru/C-150, Ru/ C-300, and Ru/C-400 catalysts are prepared by reducing RuO/C catalyst in a flow of 10% H2/Ar at 150, 300, and 400 °C for 2 h, respectively, and then purged with N2 for 2 h until to room temperature. The carbon (carbon support) was also treated as the blank sample at the corresponding temperature under H2. Characterization. Powder XRD patterns were recorded on a Bruker diffractometer (D8 Focus) by using Cu Kα (λ = 0.154056 nm) radiation. Nitrogen adsorption−desorption isotherms were measured at −96 °C on a Micromeritics ASAP 2020 M sorption analyzer. The Brunauer−Emmett−Teller (BET) method was used to calculate the specific surface area. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo ESCA LAB-250 spectrometer with monochromatic Al Kα radiation. Ammonia temperature-programmed desorption (NH3-TPD) was carried out in a PX200 apparatus (Tianjin Golden Eagle Technology Ltd. Corp.) with a thermal conductivity detector (TCD). The catalyst (50 mg) was charged into the quartz reactor, and the temperature was increased from room temperature to 500 °C at a rate of 10 °C min−1 under a flow of N2 (40 mL min−1), and then the temperature was decreased to 90 °C. Finally, NH3 was injected into the reactor at 90 °C under a flow of N2 (40 mL min−1). When the adsorption saturation was reached, followed by a flow of N2 for 1 h at 90 °C, then the temperature was increased from 90 to 500 °C at a rate of 10 °C min−1 and the amount of desorbed ammonia was detected by using a thermal conductivity detector (TCD) at 110 °C. The total acid sites were determined by the amounts of desorbed NH3 using HCl as absorbent. Besides, a blank measurement was conducted with active carbon B


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ACS Sustainable Chemistry & Engineering Table 1. Comparison of Cycloalkanes and Polyols’ Yields over Various Ru/C Catalystsa yield of polyols (mol %)c yield of alkylcyclohexanes (mol %)

C4−C2 polyols

b

entry

catalyst

substrate

1

2

3

total

C6

C5

C4

C3-G

C3-P

C2

total

total

1 2 3d 4 5 6 7 8 9

carbon Ru/C-400 Ru/C-400 RuO/C Ru/C-150 Ru/C-300 Ru/C-300 Ru/C-300 Ru/C-300

cornstalk cornstalk cornstalk cornstalk cornstalk cornstalk birch beech pine

− 0.8 13.6 9.0 10.9 14.9 20.4 19.7 17.5

− 4.1 45.2 34.2 48.3 66.7 46.5 52.5 55.2

− 1.6 11.7 9.6 13.1 15.6 29.1 26.2 23.8

− 6.5 70.5 52.8 72.3 97.2 96.0 98.4 96.5

− 12.6 8.5 2.6 7.2 24.5 18.6 17.6 20.6

− 5.1 3.4 2.9 4.0 12.2 10.8 13.3 7.5

− 0.7 1.2 1.3 2.8 2.5 2.3 3.0 2.6

− 0.8 1.6 7.6 8.1 5.2 4.8 4.7 3.8

− 1.0 1.2 3.2 4.2 3.4 3.7 3.7 3.2

− 1.8 2.3 9.7 8.3 4.9 4.3 4.1 5.1

− 4.3 6.3 21.8 23.4 16.0 15.1 15.5 14.7

− 22.0 18.2 27.3 34.6 52.7 44.5 46.4 42.8

Reaction condition: 0.1 g of substrate, 0.1 g of catalyst, 10 mL of H2O, 3 MPa H2, 200 °C, 8 h. bProducts 1, 2, and 3 represent methylcyclohexane, ethylcyclohexane, and propylcyclohexane, respectively. cProducts: C6, sorbitol; C5, xylitol; C4, 1,2-butanediol; C3-G, glycerol; C3-P, 1,2-PG; C2, EG. d With 2 mg (TfO)3Yb (3.7 μmol). The mannitol and arabitol can be detected, and their yields are less than 1%. a

the formation of humins. The quantification of products was performed using an external standard method. The yields of polyols are calculated based on the moles of hexose and pentose units in lignocellulosic biomass. The cellulose content in cornstalk, birch, beech, and pine are 38.0, 56.1, 41.0, and 43.2% (by weight), with hemicellulose content in cornstalk, birch, beech, and pine being 15.1, 19.3, 20.7, and 10.3% (by weight), respectively; The compositions of lignocellulosic biomass were adapted from ref 15 according to the procedures of the Van Soest method. The yields of polyols (sorb, sorbitol; mann, mannitol; xyl, xylitol; arab., arabitol; 1,2-BG, 1,2-butanediol; gly, glycerol; PG, propylene glycol; EG, ethylene glycol) were calculated as (hexU, hexose units; pentU, pentose units; LigB, lignocellulosic biomass)

prepared at the corresponding temperature under H2; the NH3-TPD curves of the catalysts were obtained after the subtraction of the blank measurement. The temperature-programmed reduction (H2-TPR) of Ru/C catalysts was carried out in a gas mixture of 5 vol % hydrogen in nitrogen at a flow rate of 40 cm3 min−1 and a heating rate of 10 °C min−1 from 25 to 400 °C, and H2 uptake was measured using a TCD. Reaction Procedure and Product Analysis. The direct catalytic transformation of lignocellulosic biomass, the conversion of lignin, and the conversion of glucose and xylose were conducted in a 50 mL Teflon-lined stainless-steel autoclave (Anhui Kemi Machinery Technology Co. Ltd.). In a typical run, cornstalk (0.10 g), 5 wt % Ru/C-300 catalyst (0.20 g), and H2O (14 mL) were charged in the reactor, which was then sealed, purged three times with H2, and charged to an initial pressure of 2.0 MPa with H2. The reactor was then slowly heated to 200 °C with a magnetic stirring speed of 700 rpm and held at this temperature for 10 h. After the reaction, the reactor was cooled quickly. Then the organic products were extracted using ethyl acetate, and catalyst was separated from the aqueous phase containing polyols by centrifugation. The “used” catalyst was washed with ethyl acetate and water, dried under vacuum (50 °C, 2 h), and directly recharged for the next run. The amounts of alkylcyclohexanes were analyzed by gas chromatography (GC) and GC−mass spectroscopy (GC-MS) on an Agilent 7890B gas chromatograph with flame ionization detector (FID) and an Agilent 7890A GC-MS instrument, both equipped with HP-5 capillary columns. Tridecane was added as an internal standard. The mole yields of cycloalkanes are calculated based on the mole of lignin monomers in lignocellulosic biomass (LigB) and organic lignin determined by the nitrobenzene oxidation method (NBO) from ref 20. The corresponding mole of lignin monomers in cornstalk, birch, beech, and pine are 0.48, 0.82, 0.87, and 0.64 mmol/g, respectively. The mole yield of C7−C9 cycloalkanes (MY(C7−C9)) was defined as follows:

sorb (mann) yield =

xyl (arab.) yield =

(mol of xyl (arab.) produced) × 5 (mol of hexU in LigB) × 6 + (mol of pentU in LigB) × 5

1, 2‐BG yield =

(moles of 1, 2‐BG produced) × 4 (mol of hexU in LigB) × 6 + (mol of pentU in LigB) × 5

gly (PG) yield =

(mol of gly (PG) produced) × 3 (mol of hexU in LigB) × 6 + (mol of pentU in LigB) × 5

EG yield =

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MY(C7 − C9) =

(mol of sorb (mann) produced) × 6 (mol of hexU in LigB) × 6 + (mol of pentU in LigB) × 5

(mol of EG produced) × 2 (mol of hexU in LigB) × 6 + (mol of pentU in LigB) × 5

RESULTS AND DISCUSSION Study on the Conversion of Raw Lignocellulosic Biomass. As expected, no hydrogenation products but trace amounts of glucose (7.2%) and xylose (3.7%) are detected over the pure carbon (carbon-support) catalyst (Table 1, entry 1). Based on the previous studies,27,28 Ru/C catalyst solely containing metallic Ru is capable of converting lignin and cellulose into liquid alkylcyclohexanes and sorbitol, respectively. The Ru/C-400 catalyst which is almost completely reduced at 400 °C, is used for the conversion of cornstalk, a largely produced agricultural waste in China. Unexpectedly, quite low yield of alkylcyclohexanes (6.5%) is obtained at 200 °C, due to the incomplete stripping of lignin from cornstalk (Table 2,

mol of C7 −C9 cycloalkanes produced (μmol/(g of LigB)) mol of C7 −C9 hydrocarbons by NBO method (μmol/(g of LigB))

The conversion of lignin and the yield to its liquid products were calculated in the same way. The analysis of polyols, glucose, and xylose were carried out by means of an HPLC apparatus (Agilent 1200 Series) equipped with a Shodex SUGAR SC1011 column (8 mm × 300 mm) with a refractive index detector (Agilent G1362A) by using highly pure water as a mobile phase at a flow rate of 0.8 mL min−1 at 80 °C. An autosampler (Agilent G1329A) was used to enhance the repeatability of analysis. Polyols were the only products and detected in the aqueous solution after reaction, while the byproducts were humins as many reported befor in the literature;29,30 therefore the carbon imbalance belongs to C


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ACS Sustainable Chemistry & Engineering Table 2. Characterization of Ru/C Catalysts entry

catalyst

BET surface area (m2 g−1)

pore size (nm)

molar ratio of Ru/RuO2a

reduction degree (%)

RuO2 (μmol/g)

total acid sitesb (μmol/g)

1 2 3 4 5 6 7

RuO/C Ru/C-150 Ru/C-300 Ru/C-400 Ru/C-300−5th-used pristine-Ru/C carbon

795.3 841.2 804.3 877.5 796.3 826.4 896.3

3.2 3.2 3.2 3.2 3.2 3.2 −

0 0.76 2.9 8.2 3.4 7.5 −

0 43.2 74.4 89.1 77.3 88.2 −

375.9 213.5 96.2 40.9 72.0 44.2 −

158.6 130.9 108.6 69.2 95.2 73.5 56.8

a

Molar ratio of different Ru species calculated from the result of pseudo in XPS. bDetermined from ammonia TPD.

bonds in the sugar intermediates, thus leading to the formation of C4−C2 polyols instead of sorbitiol and xylitol. These findings show that RuO2 species as Lewis acid play an important role in the cleavage of lignin−carbohydrate interlinkages of cornstalk and the retro-aldol reaction, demonstrating the efficient production of alkylcyclohexanes and polyols over Ru/C-300 catalyst. We also attempted the direct conversion of other raw lignocellulosic biomass (i.e., birch, beech, and pine) over Ru/C300 catalyst (Table 1). Interestingly, the lignin, cellulose, and hemicellulose fractions of the above lignocellulosic biomass are found efficiently converted, the molar yields of alkylcyclohexanes and polyols from birch, beech, and pine are recorded as 96.0/44.5%, 98.4/46.4% and 96.5/42.8%, respectively (Table 1, entries 7−9). Significantly, the yields of alkylcyclohexanes (96.0−98.4%) in terms of lignin monomers in lignocellulosic biomass are comparable to the near theoretical yield of guaiacyl and syringyl monomers obtained by formaldehyde stabilization method,20 implying the high efficiency of this Ru/C-300 catalyst-based system in aqueous phase for the full valorization of the raw lignocellulosic biomass. Characterizations of Various Ru/C Catalysts. The XRD patterns of various Ru/C catalysts are shown in Figure 1. Two

entry 1; ca. 40 wt % residue left, its detailed composition is shown in Supporting Information Table S1). The low polyols yield (22.0%), especially for xylitol (5.1%), and mass amounts of glucans and xylans that existed in the residue also confirm that the lignin−carbohydrate interlinkages are rather stable under the reaction condition. H3O+ ions formed in situ from hot water (250 °C) acts as weak instead of strong Brønsted acid and is not efficient for the stripping of lignin fragments from the whole biomass.31 It is reported that Lewis acid (TfO)3Yb is excellent for breaking the lignin−carbohydrate interlinkages,32,33 This is further confirmed by adding (TfO)3Yb to Ru/C-400 (Table 1, entry 3), where the alkylcyclohexanes yield increases from 6.5 to 70.5% (based on lignin monomer). However, the yield of polyols is still lower, only 18.2%; this might be due to the strong Lewis acidity of (TfO)3Yb, and it is reported that strong Lewis acid would lead to more humins formation from the hexose.34 Since Lewis acid is needed for the efficient cleavage of lignin−carbohydrate interlinkages, commercial Ru/C catalyst was first oxidized and then reduced at different temperatures to get bifunctional catalyst with suitable Ru and RuO2 species. In this case, RuO2 can act as Lewis acid sites,35 which can be verified by glucose isomerization as probe reaction (Table S2), and the metallic Ru acts as active sites for hydrogenation, leading to the bifunctional catalyst having excellent performance for cornstalk transformation. Catalytic studies are carried out over RuO/C, Ru/C-150, and Ru/C-300 catalysts. Over RuO/C catalyst, the yield of alkylcyclohexanes and polyols reached 52.8 and 27.3% (Table 1, entry 4). This is unexpected because this catalyst was oxidized at air and RuO2 would be its main component. The activity may come from the gradual reduction of RuO2 during reaction, and the generated metallic Ru species act as active the component for hydrogenation. With the increase of the reduction temperature from 150 to 300 °C, both alkylcyclohexanes and polyols yield increases. Surprisingly, as high as a 97.2% yield of alkylcyclohexanes is obtained with a 52.7% yield of polyols (contains 24.5% sorbitol, 12.2% xylitol, and 16.0% C4− C2 polyols) over Ru/C-300 catalyst, implying its suitable ratio of Ru/RuO2 species for the depolymerization and hydrogenation of cornstalk. Significantly, the yields of sorbitol (24.5%) and xylitol (12.2%) are comparable to that (17.5% sorbitol and 18.5% xylitol) from cornstalk catalyzed by using zirconium phosphate (ZrP) combined with commercial Ru/ C.36 Moreover, low yields of sorbitiol and xylitol (2.6 and 2.9%, respectively) are obtained over unreduced RuO/C catalyst, and the yields gradually increase with the reduction degree. This result is consistent with the studies of Liu et al.,37,38 where the presence of Lewis acid promotes the cleavage of the C−C

Figure 1. XRD patterns of various Ru/C materials.

broad diffraction peaks can be seen at 20−50°, corresponding to amorphous carbon, but no obvious characteristic diffraction peaks of RuO2 and Ru are detected, indicating Ru species are well-dispersed on carbon. The surface areas of Ru/C catalysts are in the range of 795.3−877.5 m2 g−1 (Table 2) from N2 sorption, and pore size remains the same, at ca. 3.2 nm. The histogram of the Ru particle size distribution shows that most of the Ru particles are distributed in the range 3.2−3.6 nm (Figure 2). The average Ru particle sizes in Ru/C-untreated, D


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Figure 2. TEM images of various Ru/C materials.

the direct catalytic transformation of lignocellulosic biomass into alkylcyclohexanes and polyols, mainly due to its suitable molar ratio of Ru/RuO2 (2.9). Further analyses are carried out by H2-TPR and NH3-TPD (Figure 4). The H2-TPR profiles of all Ru/C samples show a peak at 80−160 °C, and the H2 consumption during the reduction increases in the following order: Ru/C-400 < Ru/C300 < Ru/C-150 < RuO/C (Figure 4A), indicating the increase of RuO2 species. These results are further confirmed by NH3TPD method, where the total amount of acid sites determined by using NH3-TPD increases in the following order: Ru/C-400 < Ru/C-300 < Ru/C-150 < RuO/C (Table 2 and Figure 4B). The main acids are Lewis acidity from RuO2. The amounts of RuO2 species decrease with increasing reduction temperature, and a suitable molar ratio of Ru/RuO2 is needed for the direct catalytic transformation of lignocellulosic biomass into liquid alkylcyclohexanes and polyols, which is discussed in the next section. Study on the Conversion of Glucose and Xylose. The Ru/C catalysts with various reduction degrees are examined in the glucose and xylose conversion; the results are shown in Table 3. Glucose can be obtained from the hydrolysis of cellulose; then three subsequent conversion pathways coexist: (i) direct hydrogenation to sorbitol; (ii) retro-aldol condensation to erythrose and glycoladehyde, followed by conversion into 1,2-butanediol (1,2-BG) and ethylene glycol (EG); (iii) isomerization to fructose, which is then cracked into glycerosone with glyceraldehyde as intermediate, final generating 1,2-propylene glycol (1,2-PG) and glycerol (Scheme 2A).15,23,37 The proposed reaction pathway of converting xylose to polyols is similar to that of glucose conversion and shown in Scheme 2B; xylitol is obtained through the hydrogenation of xylose over metallic Ru, while RuO2 species promotes the yields of C3−C2 products via the retro-aldol condensation forming glycolaldehyde and glyceraldehyde. High glucose conversions (89.1−99%) are obtained over these Ru/C catalysts, while RuO/C catalyst shows higher catalytic activities (full conversions of glucose), implying RuO2

RuO/C, Ru/C-150, Ru/C-300, and Ru/C-400 catalysts were 3.2, 3.4, 3.4, 3.5, and 3.6 nm, respectively, and increased gradually as reduction temperature increased. In order to investigate the valence state of Ru in Ru/C catalysts, the X-ray photoelectron spectroscopy (XPS) analysis was performed. The XPS spectra of Ru 3d5/2 are shown in Figure 3. The binding energy at 280.2 eV is the characteristic of

Figure 3. Ru 3d5/2 XPS spectra of various Ru/C catalysts.

Ru0 species (metallic Ru). The shift in the peak for Ru 3d5/2 to higher value (281.2 eV) can be attributed to Ru4+ species, which most likely exists in the form of RuO2.35,39,40 Meanwhile, the molar ratio of Ru/RuO2 species is calculated and summarized in Table 2. Prior to reduction, the RuO/C sample only contains RuO2 species. After H2 reduction at 150 °C for 2 h, RuO2 species are partially reduced to metallic Ru with a molar ratio of Ru/RuO2 being 0.76, the corresponding reduction degree is 43.2%. Further increasing the reduction temperature to 300 and 400 °C, RuO2 species gradually decrease with the molar ratio of Ru/RuO2 increasing to 2.9 and 8.2, respectively, leading to the reduction degrees of Ru/C-300 and Ru/C-400 samples reaching 74.4 and 89.1%, respectively. Moreover, Ru/C-300 catalyst shows the best performance for E


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Figure 4. H2-TPR (A) and NH3-TPD (B) profiles of various Ru/C catalysts.

Table 3. Comparison Yields of Polyols over Various Ru/C Catalystsa selectivity to polyols (mol %)b C4−C2 polyols entry

catalyst

substrate

conversion (%)

C6

C5

C4

C3-G

C3-P

C2

total

total

1 2 3 4 5 6 7 8

RuO/C Ru/C-150 Ru/C-300 Ru/C-400 RuO/C Ru/C-150 Ru/C-300 Ru/C-400

glucose glucose glucose glucose xylose xylose xylose xylose

>99 94.8 92.7 89.7 >99 92.8 90.4 89.6

2.6 8.9 30.3 39.2 − − − −

4.1 8.1 8.3 7.7 4.9 10.9 30.5 38.5

1.9 3.3 4.2 1.8 1.6 0.9 1.1 0.9

7.2 5.4 4.0 2.0 11.1 15.0 8.3 2.6

4.5 3.7 4.5 3.2 3.2 5.0 5.5 1.7

12.4 10.9 8.2 4.3 12.9 9.1 7.5 2.4

26.0 23.3 20.9 11.3 28.8 30.0 21.4 7.6

32.7 40.2 58.9 58.3 33.6 39.7 51.9 46.1

Reaction condition: 0.1 g of substrate, 0.1 g of catalyst, 10 mL of H2O, 3 MPa H2, 180 °C, 2 h. bProducts: C6, sorbitol; C5, xylitol; C4, 1,2butanediol; C3-G, glycerol; C3-P, 1,2-PG; C2, EG. The mannitol and arabitol can be detected, and their selectivitives are less than 2%. a

Scheme 2. Possible Reaction Pathways from Glucose (A) and Xylose (B) to Polyols

including 26.0 and 23.3% C4−C2 polyols, respectively, with trace amounts of sorbitol (Table 3, entries 1 and 2). The diols yields (18.8% and 17.9%, respectively) over RuO/C and Ru/C150 catalysts are much lower than the results (68.6%) obtained from cellulose over 4%Ni−30%W2C/AC catalyst,15 implying tungsten sites are more favorable for retro-aldol condensation than that of ruthenium sites. With the increase of the molar ratio of Ru/RuO2 species from 0 to 2.9, the sorbitol selectivity

species promotes glucose conversion via the retro-aldol condensation reaction.41,42 In the distribution of the products, the polyols are quite different from one another, which could be due to the different molar ratios of Ru/RuO2 species on these catalysts, where a competition exists in hydrogenation and retro-aldol condensation over Ru and RuO 2 species, respectively. The total polyols selectivity from glucose on RuO/C and Ru/C-150 catalysts are only 32.7 and 40.2%, F


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ACS Sustainable Chemistry & Engineering Table 4. Main Product Yields from Direct Hydrodeoxygenation of Lignin over Various Ru/C Catalystsa

a Reaction condition: 0.1 g lignin, 0.1 g catalyst, 14 mL H2O, 3 MPa H2, 230 °C, 20 h. bThe corresponding mole of lignin monomers in lignin is 2.74 mmol/g. cThe mass yields of cycloalkanes are calculated based on the mass of lignin. dThe phenolic monomers contain the following:

e

The yields of others are calculated based on the mass of lignin, and the products contain the following:

Scheme 3. Catalytic Process of One-Pot Conversion of Lignocellulosic Biomass Liquid Alkylcyclohexanes and Polyols

accordance with the above results in which the retro-aldol condensation catalyzed by RuO2 species promotes the xylose conversion. The Ru/RuO2 ratio also has a similar effect on the product selectivity in the xylose conversion. Clearly, the Ru/C400 catalyst with the highest Ru/RuO2 ratio of 8.2 favored the efficient production of xylitol (38.5%) from the direct hydrogenation of xylose (Table 3, entry 8), while the C2−C4 polyols tend to be the dominant products as the Ru/RuO2 ratio decreases to 0.76 (Table 3, entries 5−8). Study on the Conversion of Lignin. Based on the experimental results above, it is clear that the synergistic effect of Ru and RuO2 species at Ru/C catalysts plays a very important role in the products distribution from glucose and xylose. In this part, Ru/C catalysts with different reduction degrees are tested for lignin conversion under the same conditions (Table 4) as were used for cornstalk; the mole yields of C7−C9 cycloalkanes are high over theses four catalysts, varying from 76.9 to 92.7%. For example, over the Ru/C-400 catalyst, the mole yield of C7−C9 cycloalkanes is 92.7% (total

increases to 30.3%, much higher than that obtained over RuO/ C and Ru/C-150 catalysts. Meanwhile, the selectivity to the cracking products, e.g., C2−C4 polyols, decreases from 26.0 to 20.9%, and the maximum polyols yield (58.9%) is obtained over Ru/C-300 catalyst (Table 3, entries 1−3). Moreover, further increasing the molar ratio of Ru/RuO2 species (8.2) leads to a concurrent decline in the C2−C4 polyols selectivity. Ru/C-400 catalyst affords the superior direct hydrogenation activity, and the sorbitol selectivity increases to as high as 39.2% (Table 3, entry 4); the corresponding products distribution is similar to the result with 2 wt % Ru/C catalyst reported by Liu and co-workers.28 These changes in polyols selectivity reflect the effect of the molar ratio of Ru/RuO2 species on the synergistic effect of Ru and RuO2 species, and this effect is further examined in the hydrogenation of xylose (the key intermediate in the hemicellulose conversion to polyols). The xylose conversions are 89.6 and 90.4% on Ru/C-400 and Ru/C-300 catalysts after 2 h at 180 °C, which increase to 99% on RuO/C catalyst, in G


Research Article

ACS Sustainable Chemistry & Engineering Table 5. Recycling Tests of Ru/C-300 Catalyst for Cornstalk Conversion in Aqueous Phase yield of polyols (mol %)c C4−C2 polyols

b

yield of alkylcyclohexanes (mol %) cycle

1

2

3

total

C6

C5

C4

C3-G

C3-P

C2

total

total

1 2 3 4 5

14.9 14.5 12.8 12.6 11.8

66.7 66.8 65.7 64.5 64.2

15.6 15.4 14.9 14.5 14.8

97.2 96.7 93.4 91.6 90.8

24.5 24.8 24.6 25.7 26.4

12.2 11.2 12.1 11.3 10.6

2.5 2.4 2.3 2.3 2.2

5.2 4.9 5.1 4.8 4.8

3.4 3.6 3.2 3.3 2.9

4.9 4.7 4.5 4.8 4.5

16.0 15.6 15.1 15.2 14.4

52.7 51.6 51.8 52.2 51.4

Reaction condition: 0.1 g of cornstalk, 0.1 g of catalyst, 10 mL of H2O, 3 MPa H2, 200 °C, 8 h. bProducts 1, 2, and 3 represent methylcyclohexane, ethylcyclohexane, and propylcyclohexane, respectively. cProducts: C6, sorbitol; C5, xylitol; C4, 1,2-butanediol; C3-G, glycerol; C3-P, 1,2-PG; C2, EG. The mannitol and arabitol can be detected, and their yields are less than 1%. a

mass yield of 29.3 wt %) with 3.5 wt % yield of dimers; no phenolic monomers are detected (Table 4, entry 4). Similar results were obtained for the Ru/C-300 catalyst (Table 4, entry 3). Over Ru/C-150 and RuO/C catalysts, small amounts of phenolic monomers are detected (4.1 and 10.6%, respectively), implying that the lack of metallic Ru results in a decrease of hydrodeoxygenation ability. All of the above results indicate that Ru/C catalysts with different reduction degrees are efficient for the conversion of lignin, glucose, and xylose. But Ru/C-400 catalyst shows a poor reaction activity for converting cornstalk to alkylcyclohexanes and polyols due to the incomplete stripping of lignin from cornstalk, indicating the importance of RuO2 as Lewis acid in depolymerization of lignocellulosic biomass, well in accord with the previous reports.31−33 Moreover, this one-pot catalytic process enables a full valorization of the raw lignocellulosic biomass into liquid alkylcyclohexanes and polyols (Scheme 3), and the stability of catalysts during cycling has also been investigated, which will be described below. Study on the Recycling Potential of the Catalyst. Investigation of the reusability of Ru/C-300 catalyst for five successive batch reactions has been conducted, and the results are illustrated in Table 5. After each reaction, the aqueous phase containing polyols is separated from catalyst by centrifugation. The “used” catalyst is washed with ethyl acetate and water, then dried under vacuum (50 °C, 2 h), and directly recharged for the next run (Scheme 3). Even after reuse for five times, there is only a slight decline in the yields of alkylcyclohexanes and polyols (Table 5); it might be due to a little increase of the reduction degree of the Ru/C-300 catalyst (Table 3, increasing from 74.4 to 77.3%), and the loss of Ru after recycling (decreased from 4.5 to 3.6 wt %), but the average Ru particle size in Ru/C-300 has no obvious increase (Figure 2). Nevertheless, this Ru/C-300 catalyst-based catalytic system shows the reusability potential for the full valorization of lignocellulosic biomass.

into polyols through retro-aldol condensation. Furthermore, the Ru/C-300 catalyst-based catalytic system displays a good cycling stability on converting cornstalk into liquid alkylcyclohexanes and polyols and provides a highly industrially promising pathway for the direct production of valuable platform chemicals from renewable biomass.

CONCLUSIONS In conclusion, we have presented a simple and highly efficient approach for the direct transformation of lignocellulosic biomass into liquid alkylcyclohexanes and polyols over Ru/C300 catalyst in an aqueous phase. The partially reduced Ru/C300 catalyst contains both Ru and RuO2 species with a Ru/ RuO2 ratio of 2.9, in which RuO2 species acts as Lewis acid sites and Ru works for hydrogenation. RuO2 species not only promotes the cleavage of lignin−carbohydrate interlinkages in depolymerization of lignocellulosic biomass but also plays the critical role in converting cellulose and hemicellulose fractions

(1) Zhou, C. H.; Xia, X.; Lin, C. X.; Tong, D. S.; Beltramini, J. Catalytic conversion of lignocellulosic biomass to fine chemicals and fuels. Chem. Soc. Rev. 2011, 40, 5588−5617. (2) Sheldon, R. A. Green and sustainable manufacture of chemicals from biomass: state of the art. Green Chem. 2014, 16, 950−963. (3) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J., Jr.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. The path forward for biofuels and biomaterials. Science 2006, 311, 484−489. (4) Luterbacher, J. S.; Martin Alonso, D.; Dumesic, J. A. Targeted chemical upgrading of lignocellulosic biomass to platform molecules. Green Chem. 2014, 16, 4816−4838.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00012. Components of the derived residue and glucose isomerization reaction catalyzed by RuO/C catalyst (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: (+86)-21-64253824. E-mail: wangyanqin@ecust. edu.cn. ORCID

Qineng Xia: 0000-0001-5935-2455 Yanqin Wang: 0000-0002-5636-0617 Author Contributions †

X. Li and T. Guo contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the NSFC of China (Grant Nos. 91545103 and 21403065) and the Science and Technology Commission of Shanghai Municipality (Grant No. 10dz2220500).

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