H2O ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Synergistic Effect of EtOAc/H2O Biphasic Solvent and Ru/C Catalyst for Cornstalk Hydrolysis Residue Depolymerization Wei Lv,†,‡ Zhan Si,§ Zhipeng Tian,† Chenguang Wang,*,† Qi Zhang,† Ying Xu,† Tiejun Wang,† and Longlong Ma*,†,‡

Downloaded via KAOHSIUNG MEDICAL UNIV on June 30, 2018 at 17:42:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, CAS Key Laboratory of Renewable Energy, Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, No. 2 nengyuan Rd, Tianhe District, Guangzhou 510640, China ‡ School of environmental science and engineering, Tianjin University, No. 92 Weijin Rd, Nankai District, Tianjin 300072, China § College of engineering, China Agricultural University, No. 17 Stinghua east Rd, Haidian District, Beijing 100083, China S Supporting Information *

ABSTRACT: An EtOAc/H2O biphasic solvent−Ru/C coupling biorefinery process was developed to selectively produce aromatics from cornstalk hydrolysis residue (CHR). In this process, CHR was depolymerized in an ethyl acetate (EtOAc)/H2O biphasic solvent system over Ru/C catalyst, which produced aromatics and carbohydrates instantly separated by EtOAc and H2O. Most lignin in CHR was converted to aromatics and nonvolatile fractions, accompanied with partial degradation of cellulose. Optimized results showed that more than 42.7% of aromatics can be obtained under 260 °C for 5 h without external hydrogen pressure. Monophasic and biphasic parallel experimental results show that the strong lignin dissolving ability of the biphasic dissolution/separation system is helpful for aromatics production and separation, which promoted CHR depolymerization over Ru/C. Furthermore, CHR, products, depolymeirzation residue solid (DRS), and catalysts were carefully characterized by gas chromatography (GC-MS), Fourier transform infrared (FT-IR), gel permeation chromatrography (GPC), high-performance liquid chromatrography mass spectrometry (HPLC-MS), nuclear magnetic resonance (NMR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis. Results demonstrated that the biphasic solvent−Ru/C coupling process can significantly alleviate repolymerization reactions. Based on these analysis results, the catalytic process in biphasic EtOAc/H2O was discussed. This work demonstrates that such a green biphasic catalytic/separation coupling system highlights a promising route for efficient biomass degradation and product separation at mild conditions without external hydrogen pressure. KEYWORDS: Cornstalk hydrolysis residue, Biphasic solvent system, Hydrogenolysis, Depolymerization, Aromatics

■

INTRODUCTION With increasing global energy demands and environment concerns (CO2 emission and air pollution), renewable chemicals and fuels from biomass gain great interest in order to decrease the dependence on petroleum sources.1−3 Cellulose and hemicellulose can easily convert to renewable chemicals and fuels, such as, ethanol, soap, pulp, and alkanes.4,5 In comparison, lignin, which holds 15−40% dry weight in typical biomass plant cell walls, is not effectively used.4,6 Also its recalcitrant structure inhibits the conversion of lignin, and it is typically slated for combustion in most applications with a low heating value.7,8 Researchers have been looking for more valuable ways to use or extract ligin in biomass over the past few decades. Lignin, the richest source of aromatic polymer on the planet, is the best potential source for relevant aromatics, high value chemicals, and alternative fuels.5,9 However, the approach of effeciently producing aromatics are still hindered by lignin’s high-cross-linked structure and high oxygen content.10 Conven© 2017 American Chemical Society

tional lignin conversion strategies for well-defined products mainly through pyrolysis,11 gasification,12 acid/base depolymerization,13,14 oxidation, 15 and hydroprocessing degradation,16−20 etc. Among these strategies, hydroprocessing degradation is considered as a promising method for lignin conversion to aromatics.8,17−22 However, most reported hydroprocessing methods require extreme condictions like high temperature and hydrogen pressure.21 Recently, selective cleavage of specific linkages within model polymer were conducted over catalysts under mild conditions.7,23−26 Meanwhile, research works extending lignin model compounds to “real” technical lignin or biomass under mild conditions without external hydrogen were reported. For instance, Roberto Rinaldi et al. reported that most lignin in plant biomass can be effectively depolymerized into nonpyrolytic Received: October 28, 2016 Revised: February 23, 2017 Published: February 25, 2017 2981

DOI: 10.1021/acssuschemeng.6b02535 ACS Sustainable Chem. Eng. 2017, 5, 2981−2993

Research Article

ACS Sustainable Chemistry & Engineering lignin bio-oil at 160−220 °C through isopropanol’s hydrogen transfer reaction.27 Joseph Samec et al. designed a one-step process that tandemed an organosolv hydrogenolysis and catalysis transfer reaction, which was an effective approach for converting pine wood and birch wood into 2-methoxy-4-(prop1-enyl)-phenol (23% yield) and 2,6-dimethoxy-4-(prop-1enyl)-phenol (49% yield) at 200 °C.6 Nonetheless, most migrations from model systems to real lignin were subjected to some obstacles. For example, conversion processes always require high temperature which is thermodynamically desirable for secondary reactions,16 making these processes suffer problems like serious char formation, intermediate product repolymerization, and relatively low phenolic monomer yield.21,28 In addition, if the feedstock is plant biomass, cellulose and hemisellulose will partially decompose to glucose, furanics, and alcohols, the coexistence of these products with aromatics will affect further separation and upgrading to highvalue chemicals.27 The problems mentioned above cause low product selectivity and yield. In order to solve the problem, in situ separation is proposed as a promising way to increase the selectivity based on different products’ soluability. Following this strategy, researchers turned to biphasic reaction/separation coupling system for high conversion and selectivity.29−31 For example, Chunyan Shi et al.30 proposed a switchable ionic liquid/organic biphasic system for efficient preparation HMF from fructose. In their work, reactants and catalyst were mainly enriched in the IL phase and the produced HMF was extracted into the organic solvent phase with yield at 84%. The HDO of lignin-derived phenolic products in a biphasic solvent-Ru/CNT catalyst system was reported by Yao Fu’s group. In their biphasic system, eugenol was effectively converted to alkane with a higher conversion (>99%) and alkane selectivity (98%) than in the monophasic system (56.5% yield of alkanes in H2O).29 In Yanqin Wang’s recent work, the toluene/water biphasic system with added isopropanol was employed to transfer intermediates (furfural and isopropyl levulinate) to organic phase and efficiently conduct aldol condensation to produce C15 oxygenates.31 In our opinion, a green process for lignin (from biomass plant) conversion can effectively decrease intermediates’ molecular weights and separate glucose, furans, and alcohols from aromatics. With these advantages, lignin can be efficiently valorized. Here, we propose an integrated biphasic system in which lignin is efficiently converted to aromatics, since reaction intermediates were instantly transferred to organic phase which can effectively prevent the repolymerization reaction. Ethyl acetate (EtOAc) is widely used as solvent for lignin transformation and aromatics extraction.32 In this work, EtOAc was combined with H2O as the biphasic solvent system for reaction, separation, product transfer and target products enrichment. We present the degradation results of cornstalk hydrolysis residue (CHR) in a biphasic solvent−Ru/C coupling system. Then the parellel experiments of CHR depolymerization in monophasic system over Ru/C were carried out, as well as the separation and characterization of hydrogenolysis products (i.e., liquid (EtOAc phase and water-soluble phase)) and residue. The coupling effect of biphasic solvent and Ru/C was carefully discussed. Analytic methods such as gel permeation chromatography (GPC), Fourier transform infrared (FTIR), nuclear magnetic resonance spectroscopy (quantitative heteronuclear single quantum coherence spectra (HSQC) 2D HSQC NMR techniques), high performance liquid chromatography combined with mass spectrometer (HPLC-MS) and gas

chromatography−mass spectrometry (GC-MS) were employed to clarify the conversion process.

■

EXPERIMENTAL SECTION

Materials. Ethanol (99%), ethyl acetate (99%), acetic acid (99%), tetrahydrofuran (99%), (NH4)2CO3, and water were analytic grade and purchased from Tianjin Fu Yu Fine Chemical Co., Ltd. (Tianjin, China). A portion of 5 wt % Ru/C was provided from Aladdin (Shanghai, China) and used as received. The material was CHR (from Yingkou pilot, Liaoning China. CHR was obtained from the hydrolysate of 8.0 wt % sulfuric acid solution treatment cornstalk chips (particle size of 20−40 mesh) at 180 °C for 2 h in the 316L stainless steel hydrolysis reaction kettle. The CHR was collected and washed with distilled water to the filtrate at pH about 7.0, then transferred to 105 °C oven to remove water). The component analysis showed that it contained 56.71% lignin, 27.55% cellulose, 2.06% moisture, 13.33% extraction, and 0.35% ash. Elemental analysis demonstrated that it was composed of 57.68% C, 5.39% H, 0.26% N, 0.11% S, and 36.56% O. Typical Process for CHR Depolymerization. A 2.0 g portion of CHR, 1.0 g 5 wt % Ru/C, and 50 mL reaction medium (for example: 40 mL EtOAc and 10 mL distilled water) were charged into a 100 mL stainless autoclave (316L stainless, made by Weihai Chemical Machinery Co., Ltd.) equipped with magnetic stirring. After the air was displaced by N2 three times, the reactor was heated to the designed temperature for a certain time. When the reaction time had elapsed, the mixture was cooled to room temperature by the natural cooling-off process. The Ru/C catalyst was reused: 2.0 g CHR, 1.0 g 5 wt % Ru/C, 40 mL EtOAc, and 10 mL H2O, at 260 °C for 5 h. Repetitive reaction conditions: the solid (catalyst and DSR) from last run was used in the next reaction run, and CHR was replenished to make the amount of feedstock to 2.0 g (1.0 g of 5 wt % Ru/C was dried in oven at 105 °C as the same condition used to dry the solid (catalyst and DSR), 0.5231g of Ru/C was obtain after drying), each run repetitive experiment was conducted in 40 mL EtOAc and 10 mL H2O at 260 °C for 5 h. Product Separation. The separation procedure used for the degradation products was displayed in Scheme 1. Gaseous fractions were collected after the stainless autoclave cooled to room temperature. The other mixture products were neutralized with (NH4)2CO3 until the pH value of liquid to 7.0 through stirring and filtered to divide to liquid products 1 and solid fraction (including catalyst and depolymerization residue solid (DRS). And then, the DRS

Scheme 1. Procedure for Product Separation

2982


Research Article

ACS Sustainable Chemistry & Engineering and catalyst were washed by EtOAc (10 mL per time, three times) and H2O (5 mL per time, three times), respectively, and filtering followed. The filtrate was mixed with liquid products 1 and liquid products 2. The solid fraction was dried at 105 °C until no obvious weight loss was detected. And, liquid products 2 was separated into an H2O phase (bottom) and an EtOAc phase (upper). The bottom phase mainly contains saccharides, polyols, undegraded lignin, and water-soluble oligomers, the EtOAc phase comprised volatile products and nonvolatile products. Determine the Component of CHR and DSR. The component of CHR and DSR were tested according to NREL K-lignin analysis (NREL LAP Determination of structural carbohydrates and lignin in biomass).33 Raw CHR and DRS were characterized by Fourier transform infrared spectroscopy (FT-IR). They were measured on a Nicolet iS 50 FT-IR spectrometer in the range of 4000−400 cm−1, using the KBr pelleting method. Analysis and Measurement of the Products. Detailed product analysis can be found in the Supporting Information. The gas chromatography mass spectrometer (GC-MS) analysis of the volatile products was conducted on Agilent 7890A-5975C equipped with a Pxi-17Sil MS Cap. column (30 m × 0.25 mm × 0.25 μm) and was identified according to the NIST MS library. The oven temperature was programmed as 40 °C hold 5 min and, then, ramped up to 300 °C at 5.2 °C/min and held for another 4 min. The quantitative analysis of these chemicals was carried out on an Agilent 7890 GC with a flame ionization detector (FID) using acetophenone as the internal standard at the same capillary column and temperature program as the GC-MS analysis. HPLC-MS was used to analyze the water-soluble fraction on a quadrupole-time chromatography−mass spectrometry (LC-MS, Agilent, USA) equipped with HiP Sampler, Binary Pump and triplequadrupole mass spectrometer (TOF/Q-TOF). The molecular weights of nonvolatile fractions derived from the reactions in different solvents, under variable temperature and the reaction time were determined by gel permeation chromatography (GPC; Algient 1260 HPLC) with a differential refraction detector (RID). The average molecular weight of the sample was measured according to the external standard method with narrow polystyrene as the standard compound. CHR conversion and DRS yield were calculated by the weight comparison between the recovered and the original material as shown in eqs 1 and 2. The conversion of lignin and cellulose were measured by the weight comparison between the recovered and the original lignin and cellulose as shown in eqs 3 and 4. The yield of linear alcohols/esters, aromatics and nonaromatic cyclic compounds was measured according to eqs 5, 6, and 7, respectively based on the GC results. According to above separation procedure, the liquid products were separated into water phase and EtOAc phase. EtOAc phase comprises of linear alcohols/esters, aromatics and nonaromatic cyclic compounds, and nonvolatile fraction. Therefore, the yield of the water-soluble fraction and nonvolatile fraction were calculated by the weight subtraction method (eqs 8 and 9).

conversion of CHR (%) = [(Wa − Wr)/ Wa] × 100%

(1)

yield of DRS (%) = (Wr /Wa) × 100%

(2)

yield of water‐soluble fractions (%) = (Wwsp/ Wa) × 100%

Wa the weight of CHR feedstock; Wr the weight of DRS; Waa the weight of linear alcohols/esters; Wac the weight of volatile aromatic compounds; Wnac the weight of nonaromatic cyclic compounds; Wnvp the weight of EtOAc phase after removed all solvents and volatile substances; Wwsp the weight of water-soluble phase after removed water and volatile substances; L0 the content of lignin in CHR feedstock (L0 = 56.71%); L1 the content of lignin in DRS; C0 the content of cellulose in CHR feedstock (C0 = 27.55%); C1 the content of cellulose in hydrolysis residual solid. The contents of linear alcohols/esters, nonaromatic cyclic compounds, and volatile aromatic compounds were measured by GC with the same capillary column (Pxi-17Sil MS Cap., 30 m × 0.25 mm × 0.25 μm)) and temperature program as used in the GC-MS analysis. Acetophenone was employed as the internal standard compound. The weights of others and the water-soluble phase were both obtained by weight after the solvents and volatile substances were removed. In addition, L0, L1, C0, and C1 were determined following standard NREL procedures.33 1 H, 13C, and 1H−13C HSQC NMR Analysis of Nonvolatile Fraction, CHR, and DRS. To investigate the depolymerization performance and the evolution of structure on the CHR and DRS, NMR spectra of CHR, DRS, and nonvolatile fractions were measured using a Bruker AvanceIII 400 MHz spectrometer. Both CHR (100 mg) and DRS (100 mg) were dissolved in [D6] DMSO (1.0 mL), and about 50 mg nonvolatile fraction was dissolved in 0.5 mL dimethyl sulfoxide-d6 ([D6] DMSO). For the 1H NMR analysis, the collecting and processing parameters were set as follows: number of scans was 16; 32 receiver gain; acquisition time was 4.089 s; relaxation delay 1.0 s; pulse width was 10.0 s; 400.15 MHz spectrometer frequency. For the 13C NMR analysis, the collecting and processing were listed as following parameters: number of scans, 1024; receiver gain, 203; acquisition time, 1.363 s; relaxation delay, 2.0 s; pulse width, 10.0 s; spectrometer frequency, 100.61 MHz; and spectral width, 24 038.4 Hz. For the HSQC analysis, the collecting and processing parameters were listed as follows: number of scans, 48; receiver gain, 203; acquisition time, 0.2129/0.0636 s; relaxation delay, 2.0 s; pulse width, 10 s; spectrometer frequency, 400.15/100.61 MHz; and spectral width, 4807.8/20 124.9 Hz.34,35 The MestReNova software was used to process the data. Analysis of Ru/C Catalyst. Powder X-ray diffraction (XRD): the Bruker Endeavor D4 with Cu Kα radiation (40 kV and 30 mA) was used to analyze the fresh and recovered Ru/C catalyst. They were recorded with 0.0167° steps over the 5 to 80° angular range. X-ray photoelectron spectrometer (XPS): XPS spectra were performed using a Thermo ESCALAB 250XI equipped with a monochromatic Al Kα Xray source and a delay-line detector. Spectra were obtained using the aluminum anode (Al Kα = 1486.6 eV) operating at 150 W. The scanning electron microscope (SEM) images were obtained on a Hitachi S-4800 instrument (10 kV). Ru leaching from catalyst used in this study was determined by measuring the concentration of dissolved components in the collected EtOAc and water phase samples, using ICP-AES (Agilent ICP-725ES, Agilent Technologies, USA), respectively.

■

conversion of lignin (%) = [(L0Wa − L1Wr)/(L0Wa)] × 100%

RESULTS AND DISCUSSION Depolymerization of CHR in Mixture Solvents. In this work, CHR is used as a feedstock in all experiments. CHR is a lignin rich feedstock left after most cellulose and hemicellulose in cornstalk is removed in the hydrolysis process. Degradation of CHR in biphasic and monophasic systems were investigated. Results show that lignin can be degraded in all the solvent systems, more than 41.7% lignin was converted and total aromatics yield was higher than 12.8% without external hydrogen in different solvents which were also widely used in cellulose pretreatment and lignin valorization.23,27,36 Nevertheless, different solvent systems dramatically affected CHR

(3)

conversion of cellulose (%) = [(C0Wa − C1Wr)/(C0Wa)] × 100% (4)

yield of linear alcohols/esters (%) = [Waa/(L0Wa)] × 100% (5)

yield of aromatics (%) = [Wac/(L0Wa)] × 100%

(6)

yield of nonaromatic cyclics (%) = [Wnac/(L0Wa)] × 100%

(7)

(9)

yield of nonvolatile products (%) = [Wnvp/(L0Wa)] × 100% (8) 2983


Research Article

ACS Sustainable Chemistry & Engineering Table 1. Effect of Monophasic and Biphasic Reaction Mediaa liquid weight yield (wt %) EtOAc phased

entry

solvent (v/v/v)

1 2

EtOAc/H2O (8:2) ethanol + H2O (8:2) acetic acid + H2O (8:2) acetic acid + ethanol + H2O (4:4:1) EtOAc/H2O (6:4) EtOAc/H2O (4:6)

3 4 5 6

conversion of feedstockb (wt %)

residual solidc (wt %)

lignin conversion (wt %)

cellulose conversion (wt %)

linear alcohols/ esters

volatile aromatics

nonaromatic cyclics

nonvolatiles

watersoluble products

60.6 47.5

39.0 52.1

72.7 51.0

10.4 27.0

8.8 16.0

23.8 16.2

4.6 4.2

38.7 26.7

5.1 19.2

50.5

49.0

53.1

45.9

8.3

12.8

17.6

21.9

26.6

52.5

46.8

41.7

62.3

10.9

14.3

11.2

24.7

41.5

48.8 42.7

50.4 55.5

56.1 43.4

37.4 42.0

7.1 8.6

19.7 12.5

12.6 18.2

28.9 23.2

20.0 22.5

General condition: 2.0 g CHR (40 mesh), 1.0 g 5 wt % Ru/C, 50.0 mL solvent mixture, 180 °C, 3 h. bThe feedstock conversion was determined via the weight comparison of initial and recovered residue (catalyst was not included). cBased on the weight of CHR feedstock. dThe yields of linear alcohols/esters, volatile aromatics, nonaromatic cyclics, and nonvolatile fractions were measured by an internal standard method using acetophenone as a standard compound, and the yield of residual solid was determined by the weight comparison of it and the feedstock. The others yield obtained by the weight comparison of that solvent was moved and the feedstock. Water-soluble fractions yield also obtained by the weight comparison of that water was moved and the feedstock.

a

degradation degree and product distribution. Thus, we first investigated the effect of different solvent systems. Effect of the Solvent. As shown in Table 1, monophasic and biphasic systems show different depolymerization behavior. GC-MS analysis also displays that biphasic systems dramatically affected the product distribution as shown in Figure 3. Entry 1 (EtOAc/H2O) shows a higher lignin conversion and aromatics yield than monophasic cases (entry 2−4, Table 1), as shown in Table 1 and Figure S1 (GC-MS spectrogram peak area). In contrast, when conducting the reaction in monophasic systems, higher cellulose conversion, nonaromatic cyclic products, and water-soluble products yield were obtained (entry 2−4, Table 1). Therefore, to obtain high aromatics yield, a biphasic solvent system should be a better candidate. Since all the reactions were conducted in same conditions except phases, we came to the conclusion that all the differences were related to the property of biphasic and monophasic solvents.29,37,38 As we know, EtOAc hydrolyzing is a reverse reaction of ethanol and acetic acid’s esterification. This means that ethanol, acetic acid, EtOAc, and H2O could coexist in the EtOAc/H2O solvent system. All these components can affect Ru/C catalytic hydrogenolysis reaction. In order to investigate the effect of solvents, a model solvent system with ethanol + acetic acid + H2O = 4:4:1 (v/v/v) was used; CHR was degraded in this monophasic system. However, the depolymerization performance in the EtOAc/H2O (8:2, v/v) case is quite different from the case of ethanol + acetic acid + H2O (4:4:1, v/v/v), especially in the conversion of lignin and cellulose, also the product distribution. For instance, 23.8% of aromatics yield (EtOAc phase products) and 72.7% of lignin conversion were obtained by a biphasic system (entry 1 in Table 1), while only 14.3% of aromatics yield and 41.7% of lignin conversion were given in the ethanol + acetic acid + H2O case, accompanying 62.3% cellulose conversion (entry 4 in Table 1). This result means even EtOAc hydrolyzation products ethanol and acetic acid can affect the depolymerization process, and the effect of EtOAc as a single phase is more important for lignin conversion. This conclusion is also supported by FT-IR results of CHR structure evolution (Figure 1C, Table S7). For example, from the spectra of b and f, the characteristic

Figure 1. FT-IR spectra of CHR and DRS. (a) CHR material. (b) EtOAc/H2O (8:2, v/v). (c) EtOAc/H2O (6:4, v/v). (d) EtOAc/H2O (4:6, v/v). (e) Ethanol + H2O (8:2, v/v). (f) Acetic acid + ethanol + H2O (4:4:1, v/v/v). (g) Acetic acid + H2O (8:2, v/v).

absorption of cellulose at 1371, 1162, 1115, 1058, and 896 cm−1 39 in f is much weaker than in b, while benzene structure infrared absorbent (λ = 1606, 1513, 1463, 1323, and 834 cm−1)40,41 showed a reverse trend. Although the variations of cellulose and benzene structure were found in all DRS of b, e, f, and g, cellulose and lignin content still shows dramatic differences. The benzene structure’s absorption peak in b is weaker than peaks in f, b, 2984


Research Article

ACS Sustainable Chemistry & Engineering

Table 2. Relative Semiquantification by 1H NMR

and g, while the adsorption peak of cellulose in spectra b is stronger than that in f. It clearly shows that lignin content is enriched and cellulose is dramatically desreased in the monophasic H2O + acetic acid + ethanol system. These differences demonstrate that the monophasic system promotes cellulose conversion, biphasic solvent EtOAc/H2O (8:2, v/v) system facilitates lignin decomposition and results in high aromatics concentration, which are consistent with the conversion results in Table 1 Meanwhile, another significant difference of CHR depolymerization in different solvents is shown by 1H NMR and 13C NMR spectra of nonvolatile fractions (Figure 2) and the yields

% integration

groups aliphatic H aromatic H phenolic OH CH−CO, CH− O, Cal−OH

chemical shift (ppm)

EtOAc/ H2O

ethanol/ H2O

acetic acid/ H2O

EtOAc/ ethanol/ H2O

0.6−2.3 6.0−7.7 7.8−9.6 nd

43.38 15.36 10.90 30.36

27.06 12.90 9.00 51.04

46.05 16.15 19.36 18.44

56.66 9.82 10.19 23.33

water-soluble products in biphasic EtOAc/H2O (8:2, v/v) case was observed. We came to the conclusion that biphasic solvent, EtOAc hydrolysis rate, soluble capacity, and extraction ability of aromatics (as well as lignin) control the concentration of aromatics and water enriches the cellulose decomposition products. So the coupling effect can adjust product distribution, resulting in the differences of conversion, product yield, and selectivity. In order to further clarify the different degradation performances between monophasic and biphasic solvent systems, we compared the total-ion chromatogram (TIC) of the volatile fraction from a monophasic (ethanol + H2O, acetic acid + H2O) and a biphasic system (EtOAc/H2O). From Figure 3, we can find more species in a and the peak intensity is also much stronger than that in b−d (equal volume of solvents were used during the reaction and the post reaction treatment, including same volume of liquid product was took to analyze by GC-MS). This result also shows biphasic EtOAc/H2O system gets higher volatile fraction yield than monophasic system. Interestingly, more long-chain fatty acid ethyl esters and higher yields of aromatics and nonvolatile fractions were obtained if ethanol is present, these corresponding compounds are shown in Tables S1, S2, and S4. Especially, the highest yield of linear alcohols/esters (16.01%) was observed in the ethanol + H2O case (Table 1). These linear alcohols/esters are longchain fatty acid ethyl esters and fatty acids, which mainly from grease in CHR.45,46 Fingerprint correlation signals of aliphatic chain (at δH = 0−5.0 ppm and δC = 0−90 ppm) also demonstrate abundant aliphatics in ethanol + H2O and EtOAc/ H2O cases (both spectra a and b in Figure 2A and B, Figure S1, and Tables 2 and 3). Meanwhile, more alkylated aromatics and esterification products were observed in solvent systems contain ethanol, particularly in the EtOAc/H2O case, as Tables S1, S2, and S4 displayed. Combining the weak fingerprint signal of phenolic hydroxyl group in spectrum of a and b in Figure 2A (EtOAc/H2O, ethanol + H2O case), it can be concluded that ethanol is likely to be the key for lower productivity of phenolic hydroxyl group during CHR depolymerization. As reported, ethanol used in our mixture solvents may function as a “capping agent” of active phenolics’s alkylation, esterification, and suppressing aldol condensation.47,48 On the other hand, the Mw of volatile fractions are 1154 and 1550 g·mol−1 in the ethanol + H2O and acetic acid + H2O cases, respectively (Table 4). They are higher than in the EtOAc/H2O case (965 g·mol−1). It is shown that the biphasic system is more effective in depolymerization reaction or/and suppression repolymerization than in the monophasic system. Similarly, based on the result of c and d TIC, we also found that a small amount of volatile fractions were detected after 35 min (Figure 3d). These may ascribe to vivid fragments that are prone to convert to larger molecular other than volatile substances or phenolic hydroxyl groups (HO−Caromatic, δH =

Figure 2. (A) 1H NMR, (B) 13C NMR spectrum of nonvolatile fractions from catalytic degrade hydrogenolysis of cornstalk residue over Ru/C at 180 °C in different mixture solvents. (a) EtOAc/H2O (8:2, v/v). (b) Ethanol + H2O (8:2, v/v). (c) Acetic acid + H2O (8:2, v/v). (d) Acetic acid + ethanol + H2O (4:4:1, v/v/v).

in Table 1. The fingerprint signals of H−Caromatic (δH = 5.5−7.5 ppm) and aromatic C (δC = 100−155 ppm) in the spectra of Figure 2A-a and B-a are much stronger than other spectra b, c, and d.42−44 The fingerprint correlation signals of H−Caromatic and aromatic C relative semiquantification by 1H NMR and 13C NMR spectrum turned out to be similar, as displayed in Tables 2 and 3. These results clearly illustrate that much more aromatic fragments exist in nonvolatile fractions after the reaction in biphasic system. Additionally, as shown in Table 1, a large number of products were concentrated in water (monophasic system, entry 4), while only a little amount of 2985


Research Article

ACS Sustainable Chemistry & Engineering Table 3. Relative Semiquantification by 13C NMR % integration groups

chemical shift (ppm)

EtOAc/H2O

ethanol/H2O

acetic acid/H2O

EtOAc/ethanol/H2O

Caliphatic (DMSO not included) methoxy CH3−O Caliphatic−O (including O−CH3) Caromatic CO DMSO

10−90 (not include 42.4−38.4) 55 50−90 100−155 165−180 42.4−38.4

11.54 1.32 0.67 7.37 0.23 83.80

9.85 0.79 3.31 3.64 0.60 87.95

2.78 0.68 1.48 0.64 0.86 96.93

30.85 0.62 12.68 2.46 0.31 67.86

respectively. The high cyclic and aqueous phase product yields were obtained when acetic acid existed in monophasic systems, as entries 3 and 4 shown in Table 1 and Figure S1. In the EtOAc/H2O biphasic system, part of cyclic products attributed to esters of furans derivative. These esters and poyols (hexose and pentose) were transferred from water phase to organic phase after the esterification between acetic acid and polyols. Nonvolatile fractions show strong H−Caromatic (1H NMR, in Figure 2A-a) and Caromatic (13C NMR, in Figure 2B-a) fingerprint signals. High yield of aromatics and nonvolatiles show in entry 1 of Table 1 was due to hydrogenolysis, stabilization, and a separation coupling effect in the biphasic EtOAc/H2O system. It can be concluded that the selectivity of depolymerization products are affected by both EtOAc hydrolysis rate and separation ability of the EtOAc/H2O system. Effect of EtOAc/H2O Volume Ratio. To better understand the influence of EtOAc/H2O volume ratio on the product selectivity, further studies were carried out under different EtOAc/H2O ratios. Table 1 (entries 1, 5, and 6 in the present of Ru/C) presents the CHR conversion and product distribution with different EtOAc/H2O ratios. It could be seen that the conversion and product distribution were influenced by EtOAc/H2O volume ratio in the biphasic system. For example, the reaction carried out in the 8:2 (v/v, entry 1) case afforded 72.7% conversion of lignin, 23.8% (/38.7%) yield of aromatics (nonvolatile products), and only 5.1% yield of water phase products. When EtOAc/H2O volume ratio decreased from 8:2 to 6:4 and 4:6, much lower conversion of feedstock (to 48.8% and 42.7%) and lignin (to 56.1% and 43.4%) were given. Volatile aromatics yield also decreased as well as nonvolatiles yield, while the conversion of cellulose (37.4% and 42.0%) and the yield of aqueous fractions (12.6% and 18.2%) increased. These results correspond to DRS results shown in FT-IR spectra (Figure 1B). From the intensity of all spectra in Figure 1B, the characterized absorbance peaks changed after hydrogenolysis, indicating that the evolution of the CHR structure occurred. The cellulose absorbance of DRS b−d were similar to that of CHR (lignin, 1606, 1513, 1463, 1323, 834 cm−1; cellulose, 1371, 1162, 115, 1058, 896 cm−1),39−41 while the increased absorbance indicated the increase of cellulose content in DRS. Cellulose characterized absorbance was also increased with the EtOAc/H2O volume ratio increase, while benzene structure in lignin exhibited an opposite trend, those suggested higher relative content of cellulose and lower content of lignin were obtianed in DRS from a higher EtOAc/H2O ratio process. These results demonstrated that the adjusting of EtOAc/H2O ratio will adjust hydrolytic rate and extracting medium volume, these will change CHR delignification and product migration, leading to concentrated aromatic in the EtOAc phase23 and transfer of polyhydric alcohols to the water phase.

Figure 3. Total-ion chromatogram (TIC) of the volatile fraction of EtOAc phase from the reaction in biphasic and monophasic systems.

Table 4. Average Molecular Weight of Depolymerization Products from Various Conditionsa condition solvents

reaction temperature (°C)

reaction time (h)

EtOAc/H2O ethanol + H2O acetic acid + H2O aceticacid + ethanol + H2O 160 180 220 260 0.5 3 5 8

Mnb

Mwb

Mzb

Db

726 821 926 851

965 1154 1550 1317

1346 1520 2725 2520

1.33 1.41 1.67 1.55

897 726 684 585 846 726 653 702

1470 965 822 806 1245 965 821 904

2266 1346 1232 1074 2007 1346 1206 1011

1.64 1.33 1.20 1.38 1.47 1.33 1.26 1.29

a

Conditions: 2.0 g CHR feedstock (40 mesh), 1.0 g 5 wt % Ru/C, 50.0 mL mixture solvents (l), 180 °C, 0.5−8 h. bMn number-average molecular weight; Mw weight-average molecular weight; Mz Z-average molecular weight; D dispersion degree

7.8−9.6 ppm, considered as one of the main functional groups led to larger molecular formation or repolymerization reactions34). The 1H NMR spectrum of c (Figure 2A) is the strongest fingerprint signals in all the spectra; this result can explain the highest Mw (Table 4). The high reactivity of acetic acid lignin gave rise to generate nonvolatile large molecular compounds during the preparation of acetic acid pulp for paper.49−52 In this work, acetic acid effectively degraded cellulose with 45.9% and 62.3% conversion in the cases of entries 3 and 4, 2986


Research Article


Effect of Reaction Temperature. The yield of aromatic compounds increased remarkably with the elevation of reaction temperature. At higher temperature, for example at 260 °C, 88.7% CHR can be converted and 42.7% aromatic compound yield was obtained. With temperature increasing, DRS reduced and volatile aromatics increased, a higher nonvolatile fraction yield was also received that would be oligomers or lignin fragments. However, increasing the reaction temperature led to increase of the cyclic fractions, which indicates that carbohydrate might be favorable to decomposition at higher temperature. This finding supports the conclusion made in previous reports.39 Moreover, we investigated the temperature influence on delignification by GPC analyzing nonvolatile fractions. The results listed in Table 4 clearly demonstrate that the Mw was decreased gradually (160−260 °C) with the increase of reaction temperature. It was reported that more lignin and cellulose molecular fragments can be degraded when the temperature was raised.22 In the studied temperature range, the higher temperature employed, the higher activity for hydrolysis and alkylation reaction to produce much more phenolic derivatives. Products Analysis. Volatile Fractions in the EtOAc Phase. No gaseous product was detected after the stainless autoclave was cooled to room temperature in all experiments except when the temperature reached 220 and 260 °C. The main components were H2, CO, CO2, CH4, as shown in Table S5, deriving from decarbonylation reaction, decarboxylation reaction, acetic acid dehydrogenation, and removed methoxy groups on lignin fragments. The liquid products were divided into the EtOAc phase and water phase first and then qualitatively and quantitatively analyzed. The EtOAc phase was analyzed using GC-MS. Tables S1−S4 list the structure of components in the organic phase derived from CHR catalytic degradation reaction in different solvent systems (at 180 °C for 3 h over Ru/C catalyst, reaction conditions listed in the Supporting Information). As compound structures show, the main products are aromatics, nonaromatic cyclic products, and linear products. Most aromatic products are phenolic derivatives, alkylated and esterified aromatics, and part of benzoquinones such as 2,5-di-tert-butyl-1,4-benzoquinone. Alkylated aromatics were mainly alkylated aromatics with different alkyl groups, esterified aromatics with ethyl or butyl ester functional groups, and a small amount of alkylated benzylic alcohols. Substituted aromatics were also observed from lignin degradation in alcohol solvents (particularly ethanol and methanol); similar products were reported by different groups.34,54,55 A wide range of linear products were also found. They were generated from ethanol and acetic acid’s Guerbet reaction, alkylation and esterification reactions. The linear products are mainly long chain fatty acid/esters and short chain alcohols/ esters, which are the result of esterification and aldol reactions occurring in the presence of ethanol and acetic acid. These products are coming from alcoholysis lignin with ethanol (or methanol).34 The long-chain linear products were produced from the grease and cellulose depolymerization.56,57 The grease comes from the extraction part of CHR, which was also proved in the side-chain region of 1H−13C HSQC NMR spectrum of CHR (Figure S4-a), nonvolatile fractions (Figure 5a), and DRS (Figure 6a). Apart from these linear and aromatic products, a wide range of nonaromatic cyclic products (mainly HMF, hexose, pentose, 1,4-anhydro-D-mannitol or/and D-sorbitol) and condensation

Effect of Reaction Time. In order to obtain high aromatic yield, the effect of reaction time and temperature were examined carefully (Figure 4a and b). Results show that both reaction time and temperature affect the target compound yield.

Figure 4. Effects of time (a) and temperature (b) on the CHR material decomposition. Conditions: (a) CHR 2.0 g; 5 wt % Ru/C 1.0 g; solvent EtOAc 40 mL; H2O 10 mL; 180 °C; 1−8 h. (b) CHR 2.0 g; 5 wt % Ru/C 1.0 g; solvent EtOAc 40 mL; H2O 10 mL; 160−260 °C; 3 h.

The effect of reaction time was significant during the catalytic hydrogenolysis of lignin in EtOAc/H2O (8:2, v/v) at 180 °C over the Ru/C catalyst (Figure 4a). When the reaction time was only kept for 0.5 h after the temperature raised to 180 °C, a significant amount of DSR (49.2 wt %) was obtained, and the yield of aromatic fraction was only 8.5%. The high DSR content and low yield of aromatics are associated with the factor reported by X. Huang et al.34,53 They claimed that the high content of DRS was ascribed to the recondensation reactions which is the predominant pathway at the beginning of the cooking process in delignification. The aromatics yield enhanced and no char was found in all of these cases with the reaction time increasing at 180 °C. Nevertheless, the reaction time is extended, a large amount of nonvolatile products and water-soluble phase products were obtained. Correspondingly, the GPC analysis of nonvolatile fraction shows that Mw continuous decrease with increasing reaction time, whereas the Mw increased with cooking duration for 8 h (Table 4). This trend of Mw variation is similar to the results obtained in the hydrogen transfer depolymerization plant biomass in 2-PrOH/H2O (7:3, v/v) at 180 °C for 3 h (and 8 h) using Raney Ni catalyst.27 Therefore, the recondensation reactions took place at the beginning of delignification process with high-Mw, then the depolymerization resulted a low DRS yield, a low Mw, and an high yield of aromatics and nonvolatile fractions (demonstrated as the reaction time for 3 and 5 h). When the reaction time extended to 8 h, high-Mw lignin fragments were produced. This Mw change may first be due to the decomposed fragments migrating to the organic phase then repolymerization was prevented. However, when time goes on, the solubility and extraction ability reached their maximum values because of the limited EtOAc and water volume in biphasic solvents system, so unsegregated lignin fragments would be prone to reunion to increase Mw. 2987


Research Article


Figure 6. Side-chain region (a) and the aromatic region (b) of 1H−13C HSQC NMR spectrum of DRS derived from catalytic CHR over Ru/ C at 180 °C in the EtOAc/H2O biphasic solvent system. Figure 5. Side-chain region and aromatic region of 1H−13C HSQC NMR spectrum of nonvolatile fractions derived from the reaction at 180 °C for 3 h with the Ru/C catalyst in a biphasic solvent system (EtOAc/H2O, 8:2, v/v).

breaked by demethoxylation from the raw lignin and then were eliminated in the precence of methanol or/and ethanol in the beginning of reactions.48,58,59 In addition, some reports implied that formaldehyde can be directly obtained from the γ-carbon of the alkyl side-chain in lignin during hydrolysis and pyrolysis.60 Formaldehyde and methanol from the methoxy groups in lignin degradation were eliminated by ethanol and acetic acid in EtOAc/H2O biphasic system during the depolymerization. Therefore, biphasic systems are helpful to alleviate repolymerization for the stabilization of aromatics and the utilization of formaldehyde. Nonvolatile Fractions in the EtOAc Phase. To obtain a more comprehensive structural characterization, the nonvolatile products were subjected to 2D HSQC NMR analysis. The assignment of the main cross-signals is listed in Table S6.39,43,44,61 The spectra of the nonvolatile products and the major substructures of lignin are presented in Figure 5. Crosssignals from syringyl (S2,6, S′2,6), guaiacyl (G5, G6), and hydroxyphenyl (H2,6) lignin units can be clearly observed in the

products were obtained. Furans, anhydrosugar, sorbitol, and polyhydric alcohols generally originated from the hydrolysis, dehydration and hydrogenolysis the unit of hexose (pentose, 1,4-anhydro-D-mannitol or/and D-sorbitol) derived from cellulose. Furthermore, the aldol reactions occurred between polyols (glucose, galactose and sorbitol) and formaldehyde (formed during lignin depolymerization process) to the condensation products (such as 2, 4:3, 5-dimethylene-l-iditol and 1,3:2,4-dimethylene-d-epirhamnito) (Table S1). To a certain extent, it could be concluded that eliminating formaldehyde by aldol reactions with polyols is a helpful way to alleviate repolymerization. Because formaldehyde is one of the main reactants of undesired repolymerization reactions that gives rise to low aromatic yield and char formation.48 As documented by X. Huang et al., methoxy groups were easily 2988


Research Article


aliphatics derived from grease, strong signals of δC/δH at 70.5− 75.0/3.0−3.8 ppm are associated with C−H in β-D-xylopyranoside.39,43,62 These results are agree with the data of material component analysis and FT-IR results (Figure 2), which indicated that the DRS and CHR material contains cellulose and extractives. But correlation signals in Figure 6a such as X4 and X 3 disappeared and β- D -xylopyranoside decreased compared with that in Figure S4-a, due to the CHR degradation. Additionally, a large amount of interunit linkages β−β′ (B) and β−5 (C), the correlations in side-chain region at δC/δH 53.8/3.07, 71.3/3.84−4.2, and 62.8/3.74 ppm (Figure S4-b), were converted, and only a few were found in DRS (Figure 6). In particular, the disappearance of the A, C, and H linkage signals are prominent, suggesting that the cleavage of the A and C linkage were the most common occurrences when lignin was dissociated from CHR into liquid fractions. Strong signals correspond to FA, T6, PB2,6, and PCE2,6 units in the aromatic region in Figure 4b, while weak signals of those units were found in the aromatic region in Figure 6b. It is suggested that the strong signals corresponding to A, T6, PB2,6, and PCE2,6 units mostly were degraded (or/and dissolved) and migrated to the EtOAc phase as nonvolatiles during reaction (Figure 4b). Meanwhile, it is also worth noting that many new peaks assigned to ethyl groups of alkylated products and esters appear in the side-chain region (Figure 6a). Compared to the side-chain region in Figure S4-a, the signals of ethyl groups of alkylated and esters groups products significantly increase. Those can be conclude that ethanol and acetic acid interacted with the groups on DRS to attach ethyl and ester groups on the residue fragment. An important observation is the formation of ethyl ester cross peaks in the oxygenated side-chain region. A strong cross signal assigned to ethoxy and ethyl groups bonded to aromatic rings and linear rings appears, and new cross peaks corresponding to ethyl groups bonded to aromatic rings are clearly seen in the aliphatic side-chain region. Those results imply that esterification and alkylation (alkylation on phenolic OH groups and on the aromatic ring) took place on DRS. Ethylation reaction occurred during reaction in the ethanol solvent. Ring-ethylated products may be formed by rearrangement reactions55,63 or by removal of methoxy groups via hydrogenolysis followed by ring-methylation with the product methanol.54,64 The recycle experiments were performed to investigate the stability of Ru/C catalyst. Although the gaseous volume percentages of H2, CO, CH4, CO2, O2, and N2 in every run slightly changed with repeating use (Table S8), the conversion of CHR and the yield of fractions in liquid were sharply decreased, especially, volatile aromatics and nonaromatic cyclic fractions (Table S9). Those undesired repeat reaction results mainly ascribed to an growing amount of feedstock (DSR and replenished CHR) were converted to C sphere under 260 °C (Figure S7). The active sites of Ru/C catalyst and pore channels were covered by C sphere and tar (almost no Ru mental leaked in liquid fractions (Table S10), SEM (Figure S7)) and the skeleton of C support was damaged, leading to catalyst deactivation and weakening the ability for CHR depolymerization. Those are also confirmed by the characterizations of reused Ru/C catalyst by XRD, XPS, and ICP-AES analysis, as shown in Figure S5−S6 and Table S10. Discussion. Main Reactions in EtOAc/H2O Biphasic System. The components and their yields shown in Tables S1−S4 and 1 and Figure S1 and the fragments detected by 2D HSQC (Figures 5 and 6) and analyzed using GPC (Table 4)

aromatic region of the spectrum (Figure 5b). Significant signals of ferulate (FA6), p-hydroxyphenol (H), and p-hydroxybenzoate (PB2,6), p-coumarate (PCE3,5, PCE2,6), pyridine, and 2,6dimethyl-4-tert-butylphenol units are also clearly observed; these units are the typical structures of cornstalk.21 The interunit linkages (β−O−4′, β−5, β−β′) were not detected except the weak signal of the β−β′ linkage (Figure 5), while in the CHR side-chain region (Figure S4), these three main interunit linkages (β−O−4′, β−5, β−β′) can be observed. These mean that most of the interunit linkages were destroyed. Also, β-D-xylopyranoside and X4 signals in the side-chain region of Figure S4 can not be detected by HSQC NMR in Figure 5a.61 It is believed that the cellulose fragment of β-Dxylopyranoside and X4 cannot be collected by EtOAc. Clearly, the strong correlations in the side-chain region at δC/δH 30.0/ 1.31 ppm is attributed to −CH2 in aliphatics, provided the possibility that part of the grease was extracted or degraded from CHR to EtOAc phase. Additionally, it is worth noting that an abundance of ethyl signal as well as some methyl groups of alkylated products and esters appear in the side-chain region (Figure 5a), indicating that the acetic acid and ethanol solvents were successfully attached onto the depolymerized fragments by alkylation and esterification reactions. A small amount of lignin intermediates and β-D-xylopyranoside were present in the nonvolatile fraction, as shown in Figure 5b. Those indicate that some kind of (poly-)hexose and pentose sugars or cellulose fragments were also transferred in the EtOAc phase, while most polyols were migrated to the water phase. Water-Soluble Products. HPLC-MS was used to analysis molecular weight and main compounds in water phase. The water-soluble products had the molecular weight distribution from 141 to 1034, and several main compounds were C8H16O7, C16H32O2, C18H36O2, C23H32O2, and C6H14O7. And the largest volume of compounds is 3,4,5,6-tetrahydroxy-2-(1-hydroxyethoxy)-hexanol (C8H16O7, peak 1 in Figure S2) further detected by HPLC-MS-MS (Figure S3). Therefore, the HPLCMS results indicated that the water-soluble fractions are mainly cellulose degradation products, a small amount of long chain aliphatic ester and acid, and lignin and cellulose fragments from the incomplete decomposition of lignin and cellulose. DRS Analysis. To examine the lignin degradation degree, the physical−chemical properties of CHR and the residual solids were compared by FT-IR and 2D HSQC NMR analysis (Figures 2, S4, and 6). From the result, the decomposition of lignin was significantly improved with the synergic effect between catalyst and biphasic solvents (Table 1). The 2D HSQC NMR characterizations of the CHR and its DRS in EtOAc/H2O (8:2, v/v) at 180 °C for 3 h over the Ru/ C catalyst are shown in Figures S4 and 6. Just a few cornstalk typical units, FA, PCA, PCE, and PB were detected in the HSQC NMR spectrum of cornstalk,21 which may due to the structural changes after cornstalk hydrolysis leading to the low solubility of CHR in DMSO. However, the HSQC spectra of DRS in Figure 6b demonstrated that S′, G′, G, T A, T6, PB2,6, and PCE2,6 were the main unit structures in the aromatic-chain region, and the signals of A and C units are weak or even disappear (Figure 6a), demonstrating more typical lignin units in DRS can be dissolved by DMSO after hydrogenolysis treatment for the lignin substructure depolymerized in EtOAc/ H2O over Ru/C. From the side-chain region of Figures 6a and S4-a, they both have several kinds of typical functional groups from grease and cellulose. For example, the fingerprint signal of δC/δH assignments at 30.1/1.32 ppm is regarded as C−H in 2989


Research Article


catalyzed by Ru/C catalyst with acid facilitation are important in stabilizing the reactive sites of products and allevating repolymerization. Role of Biphasic Solvent System. From the results of volatile fractions summarized in Table S1−S3 analyzed by GCMS, it is found that chemicals derived from EtOAc/H2O (8:2,v/v) case is similar to that of in ethanol + H2O and acetic acid + H2O cases. We can further confirm that similar roles of ethanol and acetic acid exhibit in biphasic solvent system or/ and in monophasic ethanol/H2O and acetic acid/H2O systems. Although solvent changes, the depolymerization process still follows similar pathways. The extraction and transfer effects of biphasic system increased the lignin conversion and aromatic yield (no added H2, H+ proton comes from ethanol, acetic acid, and water in a hydrothermal environment). Thus, the important roles of EtOAc/H2O are summarized: First, EtOAc is converted to ethanol and acetic acid serving as hydrogen source and H+ proton to facilitate the lignin hydrolysis and hydrogenolysis. Second, ethanol from EtOAc hydrolysis acts as a scavenger of formaldehyde formed by removal of methoxy groups from the lignin, thereby suppressing formaldehyde envolved repolymerization reactions. Third, ethanol and acetic acid act as the stabilizers for aromatics’ active groups by alkylation, acylation, and esterification reactions. Four, another role of EtOAc/H2O are separating agents. EtOAc as an extracting agent concentrates aromatics, cyclic products, linear alcohols/esters, nonvolatile aromatics, and lignin fragments to restrain repolymerization. H2 O collects glucose, HMF, anhydro-sorbitol, mannitol/anhydro-mannitol, and the associated deviants derived from cellulose decomposition. Ethanol lowers the repolymerization rate of phenolic products, which may eliminate the char in the system. As a result, Ru/C catalyst combined with biphasic EtOAc/H2O system not noly promotes lignin degradation, alkylation, acylation, and esterification reaction but also benefits of the EtOAc and H2O phase fraction follow-up unitization.

demonstrate that CHR was degraded in biphasic solvent EtOAc/H2O over Ru/C catalyst. In this process, part of EtOAc is hydrolyzed to ethanol and acetic acid. Ethanol, acetic acid, and H2O are not only solvents but also reactants participating in the reaction. They are considered to be the hydrogen-donor solvent in the absence of external hydrogen resources.37,38,65 The products derived from ethanol and acetic acid are mainly C4−C8 alcohols and esters generated by Guerbet and esterification reactions.66 Ru/C catalyst possess well-known transfer hydrogenolysis properties in hydrogen involved reactions and bears higher intrinsic activity and, hence, is widely used in direct hydrogenolysis of raw and pretreated lignin.21,57 Chang et al. reported that Ru/C exhibited outstanding hydrogenolysis activities for cornstalk lignin hydrogenolysis to 4-ethylphenolics (4-EP) with high selectivity.68 A. Fukuoka et al.67 claimed that Ru/C catalyst is active for converting cellulose using 2-propanol or H2 as sources of hydrogen atom. In this work, the combination of Ru/C and acid solvent facilitated a hydrogen resource (solvent dehydrogenation, the release of H+ proton of ethanol, and H2 release from acetic acid reduction) for transfer hydrogenolysis and hydrolysis reactions. Those resulted in providing the acid environment for both solvolytic processes to break α−O−4 lignin linkages and β−O−4 linkages27,69 and promote hydrolysis of cellulose releasing C5 and C6 sugars.70 After lignin degrading to lignin fragments with formaldehyde as byproduct,5,48,71 aromatics and nonvolatile compounds were also obtained. The alkylation took place between the lignin fragments and ethanol to stabilize aromatic contained ethyl group substituents on their rings (Table S1). Esterification reaction occurred between acetic acid (or ethanol) and aliphatic acids/alcohols (or/and aromatic acid/alcohol, DRS fragment) to stabilize ethoxycarbonyl substituents. And a small amount aromatics with active species (such as phenolic hydroxyl groups) were subjected to acylation reaction with acetic acid. In addition, the reactive side-chains of lignin fragments were also protected by forming ring-ethylated products.55,63 To a certain extent, such reactions alleviated the repolymerization of lignin fragments into high-molecular weight products. Cellulose in CHR is degraded by acid hydrolysis and hydrogenolysis over Ru/C with hexose and pentose and their anhydrosugars as products, which are further hydrogenlyzed and dehydrated to sorbitol, mannose, HMF, epirhamnitol, linear alcohols, 4-oxopentanoic acid, and so on.72 The esterification reaction also happens to form esters in the presence of ethanol, acetic acid, and polyols during the hydrolysis and hydrogenolysis. Furthermore, sorbitol (glucose, galactose, hexose, and mannose) and formaldehyde (formed during the lignin depolymerization process) in the water phase were converted to 2,4:3,5-di-O-methylene-D-epirhamnitol and 1,3:2,4- di-O-methylene-D-epirhamnitol (Table S1) by aldol reaction and then were transferred to the EtOAc phase. Due to the extraction effect of EtOAc and H2O, a small amount of hexose, pentose, and anhydrosugar were also extracted by the EtOAc phase. With all these results discussed, we can firmly conclude that the degradation for CHR strongly affected by hydrolysis, transfer hydrogenolysis, aldol, Guerbet, esterification, and alkylation reactions and the separation-transfer coupling abilities of the biphasic system. Because these reactions, solvent separation, and transfer abilities alleviated lignin repolymerization and cellulose fragments for a higher aromatic yield. Among these reactions, the alkylation, aldol, and esterification reactions

■

CONCLUSION In summary, we have exhibited a one-pot decomposition process for producing aromatic biorefinery feeds that can effectively depolymerize CHR and separate products in a green biphasic EtOAc/H2O system over Ru/C catalyst. The use of EtOAc/H2O biphasic solvent system was found to be more effective than a monophasic system (ethanol + H2O, acetic acid + H2O, and acetic acid + ethanol + H2O). EtOAc and H2O played the roles of separation compounds by significantly different polarities and alleviation fragment repolymerization. EtOAc/H2O can not only act as a separating agent but also the hydrogen-donor solvent, and it can carry out the alkylation and esterification reactions with the substance contained hydroxyl, carboxylic acid, and aromatic rings groups to stabilize the highly active intermediates, with alkyl groups substituted on the phenolic hydroxyl group and on the aromatic ring, essentially inhibiting repolymerization reactions. When the fractions separation and repolymerization were alleviated, high aromatic yield can be obtained without char formation and most fractions from cellulose degradation were migrated and collected into the H2O phase. This green biphasic solvent− Ru/C catalyst/separation coupling system shows excellent hydrogenolysis depolymerization and separation abilities, yielding a large amount of aromatic fractions and enriching polyols and associated derivatives in mild condition without external hydrogen resource. The aromatics with oxygen2990


Research Article


(9) Himmel, M. E.; Ding, S. Y.; Johnson, D. K.; Adney, W. S.; Nimlos, M. R.; Brady, J. W.; Foust, T. D. Biomass recalcitrance: Engineering plants and enzymes for biofuels production. Science 2007, 315 (5813), 804−807. (10) Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski, T. J.; Tuskan, G. A.; Wyman, C. E. Lignin Valorization: Improving Lignin Processing in the Biorefinery. Science 2014, 344 (6185), 709−710. (11) Neumann, G. T.; Hicks, J. C. Novel Hierarchical CeriumIncorporated MFI Zeolite Catalysts for the Catalytic Fast Pyrolysis of Lignocellulosic Biomass. ACS Catal. 2012, 2 (4), 642−646. (12) Lin, Y. C.; Huber, G. W. The critical role of heterogeneous catalysis in lignocellulosic biomass conversion. Energy Environ. Sci. 2009, 2 (1), 68−80. (13) Deuss, P. J.; Scott, M.; Tran, F.; Westwood, N. J.; de Vries, J. G.; Barta, K. Aromatic Monomers by in Situ Conversion of Reactive Intermediates in the Acid-Catalyzed Depolymerization of Lignin. J. Am. Chem. Soc. 2015, 137 (23), 7456−7467. (14) Roberts, V.; Fendt, S.; Lemonidou, A. A.; Li, X. B.; Lercher, J. A. Influence of alkali carbonates on benzyl phenyl ether cleavage pathways in superheated water. Appl. Catal., B 2010, 95 (1−2), 71−77. (15) Herrmann, W. A.; Kuhn, F. E.; Romao, C. C. Multiple-Bonds between Main-Group Elements and Transition-Metals 0.148. Alkynylrhenium(Vii) Oxides - Synthesis, Properties and Decomposition Reactions. J. Organomet. Chem. 1995, 495 (1−2), 209−213. (16) Schutyser, W.; Van den Bosch, S.; Dijkmans, J.; Turner, S.; Meledina, M.; Van Tendeloo, G.; Debecker, D. P.; Sels, B. F. Selective nickel-catalyzed conversion of model and lignin-derived phenolic compounds to cyclohexanone-based polymer building blocks. ChemSusChem 2015, 8 (10), 1805−1818. (17) Van den Bosch, S.; Schutyser, W.; Koelewijn, S. F.; Renders, T.; Courtin, C. M.; Sels, B. F. Tuning the lignin oil OH-content with Ru and Pd catalysts during lignin hydrogenolysis on birch wood. Chem. Commun. (Cambridge, U. K.) 2015, 51 (67), 13158−13161. (18) He, J.; Zhao, C.; Lercher, J. A. Ni-catalyzed cleavage of aryl ethers in the aqueous phase. J. Am. Chem. Soc. 2012, 134 (51), 20768− 20775. (19) Vispute, T. P.; Zhang, H. Y.; Sanna, A.; Xiao, R.; Huber, G. W. Renewable Chemical Commodity Feedstocks from Integrated Catalytic Processing of Pyrolysis Oils. Science 2010, 330 (6008), 1222−1227. (20) Yan, N.; Zhao, C.; Dyson, P. J.; Wang, C.; Liu, L. T.; Kou, Y. Selective Degradation of Wood Lignin over Noble-Metal Catalysts in a Two-Step Process. ChemSusChem 2008, 1 (7), 626−629. (21) Li, C.; Zhao, X.; Wang, A.; Huber, G. W.; Zhang, T. Catalytic Transformation of Lignin for the Production of Chemicals and Fuels. Chem. Rev. 2015, 115 (21), 11559−11624. (22) Long, J.; Zhang, Q.; Wang, T.; Zhang, X.; Xu, Y.; Ma, L. An efficient and economical process for lignin depolymerization in biomass-derived solvent tetrahydrofuran. Bioresour. Technol. 2014, 154, 10−17. (23) Galkin, M. V.; Dahlstrand, C.; Samec, J. S. Mild and Robust Redox-Neutral Pd/C-Catalyzed Lignol beta-O-4′ Bond Cleavage Through a Low-Energy-Barrier Pathway. ChemSusChem 2015, 8, 2140. (24) Hanson, S. K.; Wu, R. L.; Silks, L. A. C-C or C-O Bond Cleavage in a Phenolic Lignin Model Compound: Selectivity Depends on Vanadium Catalyst. Angew. Chem., Int. Ed. 2012, 51 (14), 3410− 3413. (25) Galkin, M. V.; Samec, J. S. Selective route to 2-propenyl aryls directly from wood by a tandem organosolv and palladium-catalysed transfer hydrogenolysis. ChemSusChem 2014, 7 (8), 2154−2158. (26) Zhang, G. Q.; Scott, B. L.; Hanson, S. K. Mild and Homogeneous Cobalt-Catalyzed Hydrogenation of CC, CO, and CN Bonds. Angew. Chem., Int. Ed. 2012, 51 (48), 12102− 12106. (27) Ferrini, P.; Rinaldi, R. Catalytic Biorefining of Plant Biomass to Non-Pyrolytic Lignin Bio-Oil and Carbohydrates through Hydrogen Transfer Reactions. Angew. Chem., Int. Ed. 2014, 53 (33), 8634−8639.

containing functional groups are the main products. The compounds in EtOAc phase and H2O phase both can be further converted to a variety of useful chemicals, respectively.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02535. Characterization of catalysts, ICP-AES, SEM, XPS, XRD, and HPLC-MS analysis of aqueous phase products, and GC-MS of EtOAc phase analysis (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail address: [email protected]. Tel.: +86-2037029721. Fax: +86-20-87057673. *E-mail address: [email protected]. Tel.: +86-20-87057673. Fax: +86-20-87057673. ORCID

Wei Lv: 0000-0003-0567-8665 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by NSFC (Natural Science Foundation of China) project (Nos. 51606205, 51476175), the Chinese Academy of Sciences “one hundred talented plan” (No. y507y51001), and The National Natural Science Foundation of China (No. 51536009). Prof. Bert Sels is thanked for fruitful discussions.

■

REFERENCES

(1) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Liquid-phase catalytic processing of biomass-derived oxygenated hydrocarbons to fuels and chemicals. Angew. Chem., Int. Ed. 2007, 46 (38), 7164−7183. (2) Roman-Leshkov, Y.; Barrett, C. J.; Liu, Z. Y.; Dumesic, J. A. Production of dimethylfuran for liquid fuels from biomass-derived carbohydrates. Nature 2007, 447 (7147), 982−U5. (3) Somerville, C.; Youngs, H.; Taylor, C.; Davis, S. C.; Long, S. P. Feedstocks for Lignocellulosic Biofuels. Science 2010, 329 (5993), 790−792. (4) Hanson, S. K.; Baker, R. T.; Gordon, J. C.; Scott, B. L.; Sutton, A. D.; Thorn, D. L. Aerobic Oxidation of Pinacol by Vanadium(V) Dipicolinate Complexes: Evidence for Reduction to Vanadium(III). J. Am. Chem. Soc. 2009, 131 (2), 428−429. (5) Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen, B. M. The Catalytic Valorization of Lignin for the Production of Renewable Chemicals. Chem. Rev. 2010, 110 (6), 3552−3599. (6) Parsell, T.; Yohe, S.; Degenstein, J.; Jarrell, T.; Klein, I.; Gencer, E.; Hewetson, B.; Hurt, M.; Kim, J. I.; Choudhari, H.; Saha, B.; Meilan, R.; Mosier, N.; Ribeiro, F.; Delgass, W. N.; Chapple, C.; Kenttamaa, H. I.; Agrawal, R.; Abu-Omar, M. M. A synergistic biorefinery based on catalytic conversion of lignin prior to cellulose starting from lignocellulosic biomass. Green Chem. 2015, 17 (3), 1492−1499. (7) Galkin, M. V.; Sawadjoon, S.; Rohde, V.; Dawange, M.; Samec, J. S. M. Mild Heterogeneous Palladium-Catalyzed Cleavage of beta-O-4 ′-Ether Linkages of Lignin Model Compounds and Native Lignin in Air. ChemCatChem 2014, 6 (1), 179−184. (8) Long, J. X.; Xu, Y.; Wang, T. J.; Yuan, Z. Q.; Shu, R. Y.; Zhang, Q.; Ma, L. Efficient base-catalyzed decomposition and in situ hydrogenolysis process for lignin depolymerization and char elimination. Appl. Energy 2015, 141, 70−79. 2991


Research Article

ACS Sustainable Chemistry & Engineering (28) Azadi, P.; Inderwildi, O. R.; Farnood, R.; King, D. A. Liquid fuels, hydrogen and chemicals from lignin: A critical review. Renewable Sustainable Energy Rev. 2013, 21, 506−523. (29) Chen, M. Y.; Huang, Y. B.; Pang, H.; Liu, X. X.; Fu, Y. Hydrodeoxygenation of lignin-derived phenols into alkanes over carbon nanotube supported Ru catalysts in biphasic systems. Green Chem. 2015, 17 (3), 1710−1717. (30) Shi, C.; Xin, J.; Liu, X.; Lu, X.; Zhang, S. Using Sub/ Supercritical CO2as “Phase Separation Switch” for the Efficient Production of 5-Hydroxymethylfurfural from Fructose in an Ionic Liquid/Organic Biphasic System. ACS Sustainable Chem. Eng. 2016, 4 (2), 557−563. (31) Li, C.; Ding, D.; Xia, Q.; Liu, X.; Wang, Y. Conversion of raw lignocellulosic biomass into branched long-chain alkanes through three tandem steps. ChemSusChem 2016, 9 (13), 1712−1718. (32) Jahan, M. S.; Chowdhury, D. A. N.; Islam, M. K.; Moeiz, S. M. I. Characterization of lignin isolated from some nonwood available in Bangladesh. Bioresour. Technol. 2007, 98 (2), 465−469. (33) Sluiter, A.; Hames, B.; Ruiz, R.; Scarlata, C.; Sluiter, J.; Templeton, D.; Crocker, D. Determination of structural carbohydrates and lignin in biomass. http://www.nrel.gov/biomass/analytical_ procedures.html (accessed Nov. 2015). (34) Huang, X.; Koranyi, T. I.; Boot, M. D.; Hensen, E. J. Catalytic depolymerization of lignin in supercritical ethanol. ChemSusChem 2014, 7 (8), 2276−2288. (35) Zhang, X. Q.; Chen, M. J.; Wang, H. H.; Liu, C. F.; Zhang, A. P.; Sun, R. C. Characterization of Xylan-graft-Polycaprolactone Copolymers Prepared in Ionic Liquid. Ind. Eng. Chem. Res. 2015, 54 (24), 6282−6290. (36) de Clippel, F.; Dusselier, M.; Van Rompaey, R.; Vanelderen, P.; Dijkmans, J.; Makshina, E.; Giebeler, L.; Oswald, S.; Baron, G. V.; Denayer, J. F.; Pescarmona, P. P.; Jacobs, P. A.; Sels, B. F. Fast and selective sugar conversion to alkyl lactate and lactic acid with bifunctional carbon-silica catalysts. J. Am. Chem. Soc. 2012, 134 (24), 10089−100101. (37) Shuai, L.; Luterbacher, J. Organic Solvent Effects in Biomass Conversion Reactions. ChemSusChem 2016, 9 (2), 133−155. (38) Marcus, Y. The Properties of Organic Liquids That Are Relevant to Their Use as Solvating Solvents. Chem. Soc. Rev. 1993, 22 (6), 409− 416. (39) Hu, L.; Luo, Y.; Cai, B.; Li, J.; Tong, D.; Hu, C. The degradation of the lignin in Phyllostachys heterocycla cv. pubescens in an ethanol solvothermal system. Green Chem. 2014, 16 (6), 3107−3116. (40) Long, J.; Xu, Y.; Wang, T.; Yuan, Z.; Shu, R.; Zhang, Q.; Ma, L. Efficient base-catalyzed decomposition and in situ hydrogenolysis process for lignin depolymerization and char elimination. Appl. Energy 2015, 141, 70−79. (41) Shu, R.; Long, J.; Yuan, Z.; Zhang, Q.; Wang, T.; Wang, C.; Ma, L. Efficient and product-controlled depolymerization of lignin oriented by metal chloride cooperated with Pd/C. Bioresour. Technol. 2015, 179, 84−90. (42) Crestini, C.; Sermanni, G. G.; Argyropoulos, D. S. Structural modifications induced during biodegradation of wheat lignin by Lentinula edodes. Bioorg. Med. Chem. 1998, 6 (7), 967−973. (43) Wen, J.-L.; Sun, S.-L.; Xue, B.-L.; Sun, R.-C. Recent Advances in Characterization of Lignin Polymer by Solution-State Nuclear Magnetic Resonance (NMR) Methodology. Materials 2013, 6 (1), 359−391. (44) Joffres, B.; Lorentz, C.; Vidalie, M.; Laurenti, D.; Quoineaud, A. A.; Charon, N.; Daudin, A.; Quignard, A.; Geantet, C. Catalytic hydroconversion of a wheat straw soda lignin: Characterization of the products and the lignin residue. Appl. Catal., B 2014, 145, 167−176. (45) Demirbas, A. Biodiesel production from vegetable oils via catalytic and non-catalytic supercritical methanol transesterification methods. Prog. Energy Combust. Sci. 2005, 31 (5−6), 466−487. (46) Madras, G.; Kolluru, C.; Kumar, R. Synthesis of biodiesel in supercritical fluids. Fuel 2004, 83 (14−15), 2029−2033. (47) Rinaldi, R.; Jastrzebski, R.; Clough, M. T.; Ralph, J.; Kennema, M.; Bruijnincx, P. C.; Weckhuysen, B. M. Paving the Way for Lignin

Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis. Angew. Chem., Int. Ed. 2016, 55 (29), 8164−8215. (48) Huang, X.; Korányi, T. I.; Boot, M. D.; Hensen, E. J. M. Ethanol as capping agent and formaldehyde scavenger for efficient depolymerization of lignin to aromatics. Green Chem. 2015, 17 (11), 4941− 4950. (49) Pan, X. J.; Sano, Y. Fractionation of wheat straw by atmospheric acetic acid process. Bioresour. Technol. 2005, 96 (11), 1256−1263. (50) Nimz, H. H.; Casten, R. Chemical-Processing of Lignocellulosics. Holz Roh Werkst 1986, 44 (6), 207−212. (51) Sano, Y.; Nakamura, M.; Shimamoto, S. Pulping of Wood at Atmospheric-Pressure 0.2. Pulping of Birch Wood with Aqueous Acetic-Acid Containing a Small Amount of Sulfuric-Acid. Mokuzai Gakkaishi 1990, 36 (3), 207−211. (52) Sano, Y.; Maeda, H.; Sakashita, Y. Pulping of Wood at Atmospheric-Pressure 0.1. Pulping of Hardwoods with Aqueous Acetic-Acid Containing a Small Amount of Organic Sulfonic-Acid. Mokuzai Gakkaishi 1989, 35 (11), 991−995. (53) Xu, W. Y.; Miller, S. J.; Agrawal, P. K.; Jones, C. W. Depolymerization and Hydrodeoxygenation of Switchgrass Lignin with Formic Acid. ChemSusChem 2012, 5 (4), 667−675. (54) Bui, V. N.; Laurenti, D.; Delichere, P.; Geantet, C. Hydrodeoxygenation of guaiacol Part II: Support effect for CoMoS catalysts on HDO activity and selectivity. Appl. Catal., B 2011, 101 (3−4), 246−255. (55) Zhu, X. L.; Lobban, L. L.; Mallinson, R. G.; Resasco, D. E. Bifunctional transalkylation and hydrodeoxygenation of anisole over a Pt/HBeta catalyst. J. Catal. 2011, 281 (1), 21−29. (56) Alonso, D. M.; Wettstein, S. G.; Dumesic, J. A. Bimetallic catalysts for upgrading of biomass to fuels and chemicals. Chem. Soc. Rev. 2012, 41 (24), 8075−8098. (57) Liu, Y.; Chen, L.; Wang, T.; Zhang, Q.; Wang, C.; Yan, J.; Ma, L. One-Pot Catalytic Conversion of Raw Lignocellulosic Biomass into Gasoline Alkanes and Chemicals over LiTaMoO6and Ru/C in Aqueous Phosphoric Acid. ACS Sustainable Chem. Eng. 2015, 3 (8), 1745−1755. (58) Onwudili, J. A.; Williams, P. T. Catalytic depolymerization of alkali lignin in subcritical water: influence of formic acid and Pd/C catalyst on the yields of liquid monomeric aromatic products. Green Chem. 2014, 16 (11), 4740−4748. (59) Zakzeski, J.; Weckhuysen, B. M. Lignin Solubilization and Aqueous Phase Reforming for the Production of Aromatic Chemicals and Hydrogen. ChemSusChem 2011, 4 (3), 369−378. (60) Pandey, M. P.; Kim, C. S. Lignin Depolymerization and Conversion: A Review of Thermochemical Methods. Chem. Eng. Technol. 2011, 34 (1), 29−41. (61) Yuan, T. Q.; Sun, S. N.; Xu, F.; Sun, R. C. Isolation and PhysicoChemical Characterization of Lignins from Ultrasound Irradiated FastGrowing Poplar Wood. Bioresources 2011, 6 (1), 414−433. (62) Rahimi, A.; Azarpira, A.; Kim, H.; Ralph, J.; Stahl, S. S. Chemoselective metal-free aerobic alcohol oxidation in lignin. J. Am. Chem. Soc. 2013, 135 (17), 6415−6418. (63) Marczewski, M.; Bodibo, J. P.; Perot, G.; Guisnet, M. Alkylation of Aromatics 0.1. Reaction Network of the Alkylation of Phenol by Methanol on Ushy Zeolite. J. Mol. Catal. 1989, 50 (2), 211−218. (64) Centeno, A.; Laurent, E.; Delmon, B. Influence of the Support of Como Sulfide Catalysts and of the Addition of Potassium and Platinum on the Catalytic Performances for the Hydrodeoxygenation of Carbonyl, Carboxyl, and Guaiacol-Type Molecules. J. Catal. 1995, 154 (2), 288−298. (65) Zhao, X. B.; Morikawa, Y.; Qi, F.; Zeng, J.; Liu, D. H. A novel kinetic model for polysaccharide dissolution during atmospheric acetic acid pretreatment of sugarcane bagasse. Bioresour. Technol. 2014, 151, 128−136. (66) Huang, X.; Atay, C.; Korányi, T. I.; Boot, M. D.; Hensen, E. J. M. Role of Cu−Mg−Al Mixed Oxide Catalysts in Lignin Depolymerization in Supercritical Ethanol. ACS Catal. 2015, 5 (12), 7359−7370. 2992


Research Article

ACS Sustainable Chemistry & Engineering (67) Kobayashi, H.; Matsuhashi, H.; Komanoya, T.; Hara, K.; Fukuoka, A. Transfer hydrogenation of cellulose to sugar alcohols over supported ruthenium catalysts. Chem. Commun. 2011, 47 (8), 2366− 2368. (68) Ye, Y. Y.; Zhang, Y.; Fan, J.; Chang, J. Selective production of 4ethylphenolics from lignin via mild hydrogenolysis. Bioresour. Technol. 2012, 118, 648−651. (69) Santos, R. B.; Hart, P. W.; Jameel, H.; Chang, H. M. Wood Based Lignin Reactions Important to the Biorefinery and Pulp and Paper Industries. BioResources 2012, 8 (1), 1456−1477. (70) Otieno, D. O.; Ahring, B. K. The potential for oligosaccharide production from the hemicellulose fraction of biomasses through pretreatment processes: xylooligosaccharides (XOS), arabinooligosaccharides (AOS), and mannooligosaccharides (MOS). Carbohydr. Res. 2012, 360, 84−92. (71) Onwudili, J. A.; Williams, P. T. Catalytic depolymerization of alkali lignin in subcritical water: influence of formic acid and Pd/C catalyst on the yields of liquid monomeric aromatic products. Green Chem. 2014, 16 (11), 4740−4748. (72) Hausoul, P. J.; Negahdar, L.; Schute, K.; Palkovits, R. Unravelling the Ru-Catalyzed Hydrogenolysis of Biomass-Based Polyols under Neutral and Acidic Conditions. ChemSusChem 2015, 8 (19), 3323−3330.

2993


H2O ... - ACS Publications

Recommend Documents