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Synergistic Effect of EtOAc/H2O Bipahsic 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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02535 • Publication Date (Web): 25 Feb 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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ACS Sustainable Chemistry & Engineering

Synergistic Effect of EtOAc/H2O Bipahsic Solvent and Ru/C Catalyst for Cornstalk Hydrolysis Residue Depolymerization Wei Lv, [a, b] Zhan Si, [c] Zhipeng Tian,[a] Chenguang Wang,*[a] Qi Zhang, [a] Ying Xu, [a] Tiejun Wang,[a] Longlong Ma*[a,b] [a]

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) E-mail: [email protected]

[b]

School of environmental science and engineering, Tianjin University, No.92 Weijin Rd, Nankai District, Tianjin (China)

[c]

College of engineering, China Agricultural University, No.17 Stinghua east Rd, Haidian District, Beijing (China)

An EtOAc/H2O biphasic solvent-Ru/C coupling bio-refinery process was developed to selectively produce aromatics from cornstalk hydrolysis residue (CHR). In this process, CHR was depolymerized in ethyl acetate (EtOAc)/H2O biphasic solvent system over Ru/C catalyst, then produced aromatics and carbohydrates were instantly separated by EtOAc and H 2O. Most lignin in CHR was converted to aromatics and non-volatile fractions, accompany with partial degradation of cellulose. Optimized results showed that more than 42.7% of aromatics can be obtained under 260℃ for 5h without external hydrogen pressure. Monophasic and biphasic parallel experiment results show that the strong lignin dissolved ability of biphasic dissolution/separation system is helpful for aromatics production and separation, which promoted the CHR depolymerization over Ru/C. Furthermore, CHR, products, depolymeirzation residue solid (DRS) and catalysts were carefully characterized by GC-MS, FT-IR, GPC, HPLC-MS, NMR, XRD, XPS, SEM and 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 efficiently biomass degrade and product separate at mild condition without external hydrogen pressure.

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 source.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 are looking for more valuable ways to use or extract ligin in biomass over the past few decades. Lignin, the richest source ofaromatic polymer on the planet, is the most potential source for relevant aromatics, high value chemicals, 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 Conventional lignin conversion strategies for the well-defined products mainly through pyrolysis,11 gasification,12 acid/base depolymerization,13-14 oxidation15 and hydroprocessing degradation16-20 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, researches extended 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 of lignin in plant biomass can be effectively depolymerized into non-pyrolytic lignin bio-oil at 160-220 ℃ through isopropanol's hydrogen transfer reaction.27 Joseph Samec et al. designed a one-step process tandemed an organosolv hydrogenolysis and catalysis transfer reaction, which was an effective approach of converting pine wood and birch wood into 2-methoxy-4-(prop-1-enyl) phenol (23% yield) and 2,6dimethoxy-4-(prop-1-enyl)-phenol(49% yield) at 200℃.6 Nonetheless, most migrations from model system 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 products repolymerization and relatively low phenolic monomers yield.21, 28 In addition, if the feedstock is plant biomass, cellulose and hemisellulose will partially decompose to glucose, furanics and alcohols, the co-existence of these products with aromatics will affect further separation and upgrading to high-value 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' soluablity. Follow 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 were 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 weight and separate glucose, furans and alcohols from aromatics. With these advantages, lignin can be efficiently valorized. Here, we propose an integrated

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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 spectrometer (GC-MS) were employed to clarify the conversion process.

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Products 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 & catalyst were washed by EtOAc (10 mL per time, three times) and H2O (5 mL per time, three times), respectively, and filter was followed. The filtrate was mixed with liquid products (1) to liquid products (2). Solid fraction was dried at 105℃ until no obvious weight loss was detected. And liquid products (2) was separated into H2O phase (bottom) and EtOAc phase (upper). The bottom phase mainly contains saccharides, polyols, un-degraded lignin and water soluble oligomers, EtOAc phase comprised volatile products and non-volatile products.

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). 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 8.0wt% sulfuric acid solution treatment cornstalk chips (particle size of 20-40 mesh) at 180℃ for 2 hours at 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 ℃ oven to remove water). The component analysis showed that it contained 56.71% lignin, 27.55% cellulose, 2.06% moister, 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 2.0 g CHR, 1.0g 5 wt% Ru/C and 50 ml reaction medium (for example: 40mL EtOAc and 10mL distilled water ) were charged into a 100 ml stainless autoclave (316L stainless, made by Weihai Chemical Machinery Co., Ltd.) equipped with a megnetic stirring. After the air displacement by N2 for three times, the reactor was heated to 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. Ru/C catalyst reused：2.0 g CHR, 1.0 g 5 wt% Ru/C, 40 mL EtOAc and 10mL H2O, at 260℃ for 5h. Repetitive reaction condition: the solid (catalyst and DSR) from last run was used in next run reaction, and CHR was replenished to make the amount of feedstock to 2.0g (1.0g of 5 wt% Ru/C was dried in oven at 105℃ as the same condition used to dry the solid (catalyst and DSR), 0.5231g of Ru/C was obtain after drying), each run repetitive experiments was conducted in 40 mL EtOAc and 10mL H2O at 260℃ for 5h.

Scheme 1 Procedure for product separation

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 Translation Infrared Spectroscopy (FT-IR). They were measured on a Nicolet is 50 FT-IR spectrometer in the range of 4000–400 cm-1, using KBr pelleting method. Analysis and measurement of the products Detail product analysis could be found in 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 (30m ×0.25 mm ×0.25μm) and was identified according to the NIST MS library. The oven temperature was programmed as 40℃ hold 5 min, and then ramped up to 300℃ with 5.2℃/min and hold for another 4 min. The quantitative analysis of these chemicals was carried out on an Agilent 7890 GC with a FID using acetophenone as internal standard at the same capillary column and temperature program as the GC–MS analysis.


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The HPLC–MS was used to analyze 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 non-volatile 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 Eq. (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 Eq. (3) and (4). The yield of linear alcohols/esters, aromatics and non-aromatic 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 non-aromatic cyclic compounds and non-volatile fraction. Therefore, the yield of water soluble fraction and non-volatile fraction were calculated by the weight subtraction method (Eq. (8) and (9)).

[D6] DMSO (1.0 mL), and about 50mg non-volatile fraction was dissolved in 0.5 mL dimethylsulfoxide-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 4.089s; 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, 24038.4 Hz. For the HSQC analysis, the collecting and processing parameters were listed as follows: number of scans, 48; receiver gain, 203; acquisition time, 0.2129/0.0636s; relaxation delay, 2.0 s; pulse width, 10 s; spectrometer frequency, 400.15/100.61 MHz; and spectral width, 4807.8/20124.9 Hz. 34-35 The MestReNova software was used to process the data. Analysis of Ru/C catalyst Powder X-ray diffraction (XRD)：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 X-ray source and a delay-line detector. Spectra were obtained using the aluminium anode (Al Kα=1486.6 eV) operating at 150W. The scanning electron microscope (SEM) images were obtained on a Hitachi S-4800 instrument (10 kV). Ru leach 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 CHR(%) =[(Wa-Wr) /Wa]×100%

(1)

Yield of DRS (%) = (Wr /Wa)×100%

(2)

Conversion of lignin(%) = [(L0×Wa-L1×Wr) /(L0×Wa)]×100%

(3)

Conversion of cellulose(%)= [(C0×Wa-C1×Wr)/(C0×Wa)]×100%

(4)

Yield of linear alcohols/esters(%)= [Waa/(L0×Wa)]×100%

(5)

Yield of aromatics(%) = [Wac/(L0×Wa)]×100%

(6)

Yield of non-aromatic cyclics (%) = [Wnac/(L0×Wa)]×100%

(7)

Results and Discussion

Yield of non-volatile products (%) =[Wnvp/(L0×Wa)]×100%

(8)

Depolymerization CHR in mixture solvents

Yield of Water soluble fractions (%) = (Wwsp/Wa)×100%

(9)

In this work, CHR is used as feedstock in all experiments. CHR is a lignin rich feedstock after most cellulose and hemi-cellulose in cornstalk was removed in 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 degradation degree and product distribution. Thus, we firstly investigated the effect of different solvent systems.

Wa: the weight of CHR feedstock; Wr: the weight of DRS. W aa: the weight of linear alcohols/esters. Wac: the weight of volatile aromatic compounds. Wnac: the weight of non-aromatic 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 content of linear alcohols/esters, nonaromatic cyclic compounds and volatile aromatic compounds were measured by GC with the same capillary column (Pxi-17Sil MS Cap., 30m ×0.25 mm ×0.25μm)) and temperature program as the GC-MS analysis. Acetophenone was employed as internal standard compound. The weight of others and water soluble phase were both obtained by weight after the solvents and volatile substances 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 non-volatile 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 by using a Bruker AvanceШ 400 MHz spectrometer. Both CHR (100 mg) and DRS (100 mg) were dissolved in

Effect of the solvent As shown in Table 1, monophasic and biphasic system show different depolymerization behavior. GC-MS analysis also displays that biphasic system 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, non-aromatic cyclic products and water soluble products yield were obtained (entry 2~4，Table 1). Therefore, to obtain high aromatics yield, 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



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Table 1 The effect of monophasic and biphasic reaction medium[a] Liquid weight yield (wt%) Solvent Entry

1 2 3 4 5 6

(v/v/v)

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)

Conversion of feedstock [b] (wt %)

60.6 47.5 50.5 52.5 48.8 42.7

Residual

Lignin

Cellulose

solid [ c]

conversion

conversion

(wt%)

(wt%)

(wt%)

39.0 52.1 49.0 46.8 50.4 55.5

72.7 51.0 53.1 41.7 56.1 43.4

10.4 27.0 45.9 62.3 37.4 42.0

EtOAc phase [d]

Water soluble products

Linear alcohols/esters

Volatile aromatics

Non-aromatic cyclics

non-volatiles

8.8 16.0 8.3 10.9 7.1 8.6

23.8 16.2 12.8 14.3 19.7 12.5

4.6 4.2 17.6 11.2 12.6 18.2

38.7 26.7 21.9 24.7 28.9 23.2

5.1 19.2 26.6 41.5 20.0 22.5

[a] General Condition: 2.0 g CHR (40 mesh ), 1.0 g 5 wt% Ru/C, 50.0 ml solvent mixture, 180℃, 3 h. [b] The feedstock conversion was determined via the weight comparison of initial and recovered residue (catalyst was not included). [c] Based on the weight of CHR feedstock. [d] The yields of linear alcohols/esters, volatile aromatics, non-aromatic cyclics and non-volatile fractions were measured by internal standard method using acetophenone as 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.


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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. It means, 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 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 biphasic system (entry 1 in Table 1), while only 14.3% of aromatics yield and 41.7% of lignin conversion were given in ethanol + acetic acid + H2O case, accompanying with 62.3% cellulose conversion (entry 4 in Table 1). This result means even EtOAc hydrolyze product ethonal and acetic acid can affect the depolymerization process, 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 characterize 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 an reverse trend.

than peaks in (f), (b) and (g), while the adsorption peak of cellulose in spectra (b) is stronger than 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

Figure 2 (A) 1H NMR, (B)13C NMR spectrum of non-volatile fractions from catalytic degrade hydrogenolysis of cornstalk residue over Ru/C at 180℃ 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).

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)

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 different. Benzene structure’s absorption peak in (b) is weaker

Meanwhile, another significant difference of CHR depolymerization in different solvent is shown by 1H NMR and 13C NMR spectrum of nonvolatile fractions (Figure 2) and the yields in Table 1. The fingerprint signals of H-Caromatic (δH= 5.5-7.5 ppm) and aromatic C (δC= 100-155ppm) in spectrum of Figure 2A-a and Figure 2B-a are much stronger than other spectra (b), (c) and (d).42-44 The fingerprint correlation signals of H-Caromatic and aromatic C relative semi-quantification by 1H NMR and 13C NMR spectrum turned out to be similar, as displayed in Table 2 and Table 3. These results clearly illustrate that much more aromatic fragments exist in non-volatile 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 water soluble products in biphasic EtOAc/H2O（8:2, v/v）case was observed. We came



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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, 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 totalion chromatogram (TIC) of the volatile fraction from monophasic system (ethanol+H2O, acetic acid+H2O) and 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), (c) and (d) (equal volume of solvents were used during the reaction and the post reaction treatment, including same volume of liquid product was took to analyse 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 yield of aromatics and non-volatile fractions were obtained if ethanol is present, these corresponding compounds are showed in Table S1, S2 and S4. Especially, the highest yield of linear alcohols/esters (16.01%) was observed in ethanol+H2O case (Table 1). These linear alcohols/esters are long-chain 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 Figure 2B, Figure S1, Table 2 and Table 3). Meanwhile, more alkylated aromatics and esterification products were observed in solvent systems contain ethanol, particularly in EtOAc/H2O case, as Table 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, esterificationand supressing aldol condensation.47-48

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shown that biphasic system is more effective in depolymerization reaction or/and suppression repolymerization than in 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 3-d). These may ascribe to vivid fragments that are prone to convert to larger molecular other than volatile substance or phenolic hydroxyl group (HO-Caromatic, δH=7.8~9.6 ppm, considered as one of the main functional groups led to larger molecular formation or repolymerization reactions 34 ). The 1H NMR spectrum of (c) (Figure 2A) is the strongest fingerprint signals in all the spectra, this results can explain the highest Mw (Table 4). The high reactivity of acetic acid ligningave rise to generate non-volatile 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 entry 3 and 4 cases, respectively. The high cyclic and aqueous phase products yield were obtained when acetic acid existed in monophasic systems, as entry 3 and 4 shown in Table 1 and Figure S1. In 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. Non-volatile fractions show strong HCaromatic (1H NMR, in Figure 2A-a) and Caromatic (13C NMR, in Figure 2B-a) fingerprint signals. High yield of aromatics and non-volatiles show in entry 1 of Table 1 was due to hydrogenolysis, stabilization and separation coupling effect in the biphasic EtOAc/H2O system. Which can be concluded that the selectivity of depolymerization products are affected by both EtOAc hydrolysis rate and separation ability of EtOAc/H2O system. Table 2 Relative semi-quantification by 1H NMR % Integration Groups Aliphatic H Aromatic H Phenolic OH CH-CO, CH-O, Cal-OH

Chemical shift (ppm)

EtOAc/ H2O

Ethanol/ H2O

Acetic acid/H2O

0.6-2.3 6.0-7.7 7.8-9.6

43.38 15.36 10.90

27.06 12.90 9.00

46.05 16.15 19.36

EtOAc/ ethanol/ H2O 56.66 9.82 10.19

n.d.

30.36

51.04

18.44

23.33

Effect of EtOAc/H2O volume ratio

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

On the other hand, the Mw of volatile fractions are 1154 g  mol-1 and 1550 g · mol-1 in ethanol+H2O and acetic acid+H2O case, respectively (Table 4). They are higher than in EtOAc/H2O case (965 g.mol-1). It is

To better understand the influence of EtOAc/H2O volume ratio on the product selectivity, further studies were carried out under different EtOAc/ H2O ratio. Table 1 (entry 1, 5~6 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 products distribution were influenced by EtOAc/H2O volume ratio in the biphasic system. For example, the reaction carried out in 8:2 (v/v, entry 1) case afforded 72.7% conversion of lignin, 23.8 % (/38.7 %) yield of aromatics (non-volatile 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 non-volatiles 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 show in FT-IR spectra (Figure 1B). From the intensity of all spectra in Figure 1B, the characterized absorbance peaks changed after hydrogenolysis, indicating the evolution of the CHR structure occurred. The cellulose absorbance of DRS (b), (c) and (d) were similar to that of


Table 3 Relative semi-quantification by C NMR

Caliphatic (DMSO not included) Methoxy CH3-O Caliphatic-O (including O-CH3) Caromatic C=O DMSO

Chemical

130

160

100

120

(a)

90

140

110 80

100

50

60

40

50 40

10-90 (not include 42.4-38.4)

11.54

9.85

2.78

30.85

55

1.32

0.79

0.68

0.62

50-90

0.67

3.31

1.48

12.68

100-155 165-180 42.4-38.4

7.37 0.23 83.80

3.64 0.60 87.95

0.64 0.86 96.93

2.46 0.31 67.86

70

100

60

90 80

50

70 40

60 50

30

Conversion (wt %)

60

70

30

40 20

30 20

10

10

EtOAc/ H2O

80

110 Yield (%)

70

80

20

shift (ppm)

90

120

90

EtOAc/ ethanol/ H2O

(b)

130

20

% Integration Acetic Ethanol/ acid/ H2O H2O

100

150

30

13

Groups

system, so unsegregated lignin fragments would be prone to reunion to increase Mw.

Conversion (wt %)

CHR (lignin, 1606, 1513, 1463, 1323, 834cm-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 concentrate aromatic in EtOAc phase 23 and transfer polyhydric alcohols to water phase.

10

10

0 0

1

2

3

4

5

Reaction time ( h)

6

7

8

9

0 140

0

0 160

180

200 220 240 Temperature (℃ )

260

linear alcohols/esters non-volatile products volatile aromatics water soluble phase products non-aromatic cyclic products HRC conversion lignin conversion cellulose conversion

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 10mL:

180℃; 1-8 h (b) CHR: 2.0 g;

5 wt% Ru/C: 1.0 g; solvent: EtOAc 40 mL; H2O 10mL:

160-260℃; 3 h. Table 4 Average molecular weight of depolymerization products

from various

conditions a

Effect of reaction time

Solvents

The effect of reaction time was significant during the catalytic hydrogenolysis of lignin in EtOAc/H2O (8:2, v/v) at 180℃ over the Ru/C catalyst (Figure 4(a)). When the reaction time was only kept for 0.5 h after the temperature raised to 180℃, 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.M. 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℃. Nevertheless, the reaction time is extended, a large amount of non-volatile products and water soluble phase products were obtained. Correspondingly, the GPC analysis of non-volatile fraction shows that Mw continuous decrease with increasing reaction time, whereas the Mw increased with cooking duration for 8h (Table 4). This trend of M w variation are similar with the results obtained in the hydrogen transfer depolymerization plant biomass in 2PrOH/H2O (7:3, v/v) at 180℃ 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 non-volatile fractions (demonstrated as the reaction time for 3h and 5h). When the reaction time extended to 8h, high-Mw lignin fragments were produced. This Mw change may first due to the decomposed fragments migrate to organic phase then repolymerization were prevented. However, with the time goes on, the solubility and extraction ability reached at their maximum values because of the limited EtOAc and water volume in biphasic solvents

Mnb

Condition

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

Mwb

Mzb

Db

EtOAc/H2O

726

965

1346

1.33

Ethanol+H2O

821

1154

1520

1.41

Acetic acid+H2O

926

1550

2725

1.67

851

1317

2520

1.55

Aceticacid+ Ethanol +H2O Reaction

160 Ru3p 1/2 4+

(487.1 180 eV, Ru )

temperature (℃)

a

220

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Yield (%)

Page 7 of 15

260 0.5Ru3p1/2

Ru3p (463.3eV, 897 2266 RuCl3)1.64 Ru / C fresh 1470 3/2 (464.3 eV, RuOx/Ru) Ru / C run 1 726 965 1346 1.33 Ru / C run 2 Ru / C run 3 822 684 1232 1.20 Ru3p3/2 Ru / C run 4 0 585 806 1074 (462.1eV, Ru )1.38

846

1245

2007

1.47

Reaction

3

726

965

1346

1.33

time (h)

5

653

821

1206

1.26

8

702

904

1011

1.29

(484.2eV, Ru0)

Conditions: 2.0 g CHR feedstock (40 mesh ), 1.0 g 5 wt% Ru/C, 50.0 mL mixture

solvents (l), 180℃, 0.5-8 h. b 495 490 485 weight; 480 M 475 470average 465 molecular 460 455 Mn: number average molecular weight;450 Mz: Zw: weight Binding Energy (eV) average molecular weight; D: dispersion degree

Effect of reaction temperature The yield of aromatic compounds increased remarkably with the elevation of reaction temperature. At higher temperature, for example at 260℃, 88.7% CHR can be converted and 42.7% aromatic compounds yield was obtained. With temperature increasing, DRS reduced and volatile aromatics increased, a higher non-volatile fraction yield was also received that would be oligomers or lignin fragments. However, increase the reaction temperature led to the 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 report.39 Moreover, we investigated the temperature influence on



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

delignification by GPC analyzing non-volatile fractions. The results listed in Table 4 clearly demonstrate that the Mw was decreased gradually (160~260℃) 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 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℃. 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 EtOAc phase and water phase first and then qualitatively and quantitatively analyzed. EtOAc phase was analyzed by using GC-MS. Table S1-S4 list the structure of components in the organic phase derived from CHR catalytic degradation reaction in different solvent systems (at 180℃ for 3 h over Ru/C catalyst, reaction conditions listed in the Supporting Information). As compound structures shown, the main products are aromatics, nonaromatic cyclic products and linear products. Most of 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 alcohols solvent (particularly ethanol, 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 occurrence in the present 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), non-volatile fractions (Figure 5-a) and DRS (Figure 6-a). Apart from these linear and aromatic products, a wide range of nonaromatic cyclic products (mainly HMF, hexose, pentose, 1,4-anhydro-DMannitol or/and D-sorbitol) and condensation 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, 5Dimethylene-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

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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 breaked by demethoxylation from the raw lignin and then were eliminated in the precence of methanol or/and ethanol in the beginning of reactions.5, 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 system are helpful to alleviate repolymerization for the stabilization of aromatics and the utilization of formaldehyde. Non-volatile fractions in 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 non-volatile products and the major substructures of lignin are presented in Figure 5. Cross-signals from syringyl (S2,6, S’2,6), guaiacyl (G5, G6), and hydroxyphenyl (H2,6) lignin units can be clearly observed in the aromatic region of the spectrum (Figure 5-b). Significant signals of ferulate (FA6), p-Hydroxyphenol (H), and p-hydroxybenzoate (PB2,6), pcoumarate (PCE3,5, PCE2,6), pyridine, and 2,6-dimethyl-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 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 side-chain region of Figure S4 can not be detected by HSQC NMR in Figure 5-a.61 It is believed that the cellulose fragment of β-D-xylopyranoside and X4 cannot be collected by EtOAc. Clearly, the strong correlations in side-chain region at δC/δH 30.0/1.31 ppm is attributed to -CH2 in aliphatics, provided the possiblity 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 a little bit methyl groups of alkylated products and esters appear in the side-chain region (Figure 5-a), indicated 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 5-b. Those indicate some kind of (poly-) hexose and pentose sugars or cellulose fragments were also transferred in EtOAc phase, while most polyols were migrated to 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-hydroxy-ethoxy)hexanal (C8H16O7, peak 1 in Figure S2) further detected by HPLC-MS-MS (Fig.S3). Therefore, the HPLC–MS results indicated that the water soluble fractions are mainly cellulose degradation products, a small amount of long chain aliphatic ester and acid, lignin and cellulose fragments from the incomplete decomposition of lignin and cellulose.


Page 9 of 15

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may due to the structures change after of cornstalk hydrolysis and lead to low solubility of CHR in DMSO. However, the HSQC spectra of DRS in Figure 6-b demonstrated that S’, G’, G, T A, T6, PB2,6, and PCE2,6 were the main unit structures in aromatic-chain region, and the signals of A and C units are weak or even disappear (Figure 6-a), 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 Figure 6-a and Figure 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 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 6-a such as X4, X3 disappeared and β-D-xylopyranoside decreased compared with that in Figure S4-a, due to the CHR degradation. Additionally, a large amount of interunits 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 particularly, the disappearance of the A, C and H linkage signals are prominent, suggested that the cleavage of the A and C linkage were the most common occurrence when lignin were dissociated from CHR into liquid fractions.

Figure 5 The side-chain region and aromatic region of 1H-13C HSQC NMR spectrum of non-volatile fractions derived from the reaction at 180℃ for 3 h with the Ru/C catalyst in biphasic solvent system (EtOAc/H2O, 8:2, v/v)

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 (Figure 2, Figure S4 and Figure 6). From the result, the decomposition of lignin was significantly improved with the synergic effect between catalyst and biphasic solvents (Table1). The 2D HSQC NMR characterization of the CHR and its DRS in EtOAc/H2O (8:2, v/v) at 180℃ for 3 h over the Ru/C catalyst are showed in Figure S4 and Figure 6. Just a few cornstalk typical units, FA, PCA, PCE and PB were detected in the HSQC NMR spectrum of cornstalk, 21 which



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6 The 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℃ in the EtOAc/H2O biphasic solvent system.

The strong signals corresponding to FA, T6, PB2,6, and PCE2,6 units in aromatic region in Figure 4-b, while weak signals of those units were found in the aromatic region in Figure 6-b. 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 EtOAc phase as non-volatiles during reaction (Figure 4-b). 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 6-a). 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 signals assigned to ethoxy and ethyl groups bonded to aromatic rings and linear rings appear, 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. Ringethylated products may be formed by rearrangement reactions 55, 63 or by removal of methoxy groups via hydrogenolysis followed by ringmethylation 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 non-aromatic 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℃ (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 Table S1-S4, Table 1 and Figure S1 and the fragments detected by 2D HSQC (Figure 5, 6) and analyzed using GPC (Table 4) 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

Page 10 of 15

participated 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, are widely used in direct hydrogenolysis of raw and pretreated lignin.21, 57 Such as, Chang. J et al reported that Ru/C exhibited outstanding hydrogenolysis activities for corn stalk lignin hydrogenolysis to 4-ethylphenolics (4-EP) with high selectivity.67 Atsushi Fukuoka et al.68 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 to afford 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, 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 non-volatile compounds were also obtained. The alkylation took place between the lignin fragments and ethanol to stabilize aromatics 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 group) 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 At 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 present of ethanol, acetic acid, polyols during the hydrolysis and hydrogenolysis. Furthermore, sorbitol (glucose, galactose, hexose and mannose) and formaldehyde (formed during lignin depolymerization process) in water phase were converted to 2,4:3,5-Di-O-methylene-D- epirhamnitol and 1,3:2,4- Di-Omethylene-D-epirhamnito (Table S1) by aldol reaction and then were transferred to EtOAc phase. Due to the extraction effect of EtOAc and H2O, a small amount of hexose, pentose and anhydrosugar were also extracted by 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 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 catalyzed by Ru/C catalyst with acid facilitation are important in stabilizing the reactive sites of products and allevating repolymerization. The role of biphasic solvent system


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From the results of volatile fractions summarized in Table S1-S3 analyzed by GC-MS, 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 hydrothermal environment). Thus, the important roles of EtOAc/H 2O 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. H 2O collects glucose, HMF, anhydro-sorbitol, mannitol/anhydro-mannitol and the associated deviants derived from cellulose decomposition. Ethanol lowers the repolymerization rate of phenolic products, this 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 fractions follow-up unitization.

Associated content

Conclusion

Reference

In summary, we have exhibited 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 monophasic system (ethanol+H2O, acetic acid+H2O and acetic acid+ethanol+H2O). EtOAc and H2O played the roles of separation compounds by significant different polarities and alleviation fragments repolymerization. EtOAc/H2O not only act as a separating agent, but also the hydrogen-donor solvent and 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. With the fractions separation and repolymerization were alleviated, high aromatics yield can be obtained without char formation and most fractions from cellulose degradation were migrated and collected into 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 oxygen-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.

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 Supporting Information Characterization of catalysts, mental of ICP-AES, SEM, XPS, XRD and HPLC-MS analysis of aqueous phase products and GC-MS of EtOAc phase analysis.

Author Information Corresponding Author E-mail address: [email protected]. Tel.: +86-20-37029721; Fax: +86-20-87057673 Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by NSFC (Natural Science Foundation of China) project (51606205, 51476175). Chinese Academy of Sciences “one hundred talented plan”().

Keywords:

Cornstalk Hydrolysis Residue (CHR), Biphasic Solvent System, Hydrogenolysis, Depolymerization, Aromatics

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Proposed Reaction Network of Catalytic Hydrogenolysis Depolymerization of CHR in Biphasic EtOAc/H2O system over Ru/C Catalysts


C Catalyst

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