Quantitative Analyses of Lignin Hydrothermolysates from Subcritical

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Quantitative Analyses of Lignin Hydrothermolysates from Subcritical Water and Water−Ethanol Systems Weikun Jiang,† Gaojin Lyu,*,† Yu Liu,*,† Chao Wang,† Jiachuan Chen,† and Lucian A. Lucia†,‡ †

Key Lab of Pulp & Paper Science and Technology of the Ministry of Education, Qilu University of Technology, Jinan, Shandong 250353, People’s Republic of China ‡ Departments of Chemistry, Wood & Paper Science (Forest Biomaterials), The Laboratory of Soft Materials & Green Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States ABSTRACT: The present research relates a universally practical and feasible approach toward chemically deconstructing the macromolecular architecture of lignin, specifically alkali lignin (AL), for the production of a number of valuable side-stream aromatic nuclei byproducts. The hydrothermolysates obtained at different subcritical conditions were tentatively qualitatively identified by gas chromatography mass spectrometry (GC-MS) and subsequently quantified by gas chromatography (GC). The influence of temperature (220−340 °C), residence time (0−60 min), and ethanol volume-% in a water−ethanol cosolvent (0− 100%) on the conversion of AL and the yield of the side-stream (liquid) products were explored. The results show that the yield and identity of the phenolic and methoxy-benzene compounds were strongly correlated to the conversion temperature and the ethanol volume-%, whereas residence time in the autoclave had only a minor influence. The following individual chemicals (mg liquid side-stream/g lignin) and associated yields were determined from the optimal hydrothermal conditions (30 min, 310 °C, 25% ethanol): phenol (4.25 mg/g), 4-methylguaiacol (2.93 mg/g), 3,5-dimethoxyacetophenone (0.78 mg/g), 1,2,4trimethoxybenzene (2.47 mg/g), and 2,6-dihydroxy-4-methoxyacetophenone (2.47 mg/g). The highest yield of guaiacol (11.87 mg/g) and 2,6-dimethoxyphenol (12.17 mg/g) was obtained at a reaction temperature of 310 °C over 60 min in neat water.

1. INTRODUCTION Lignin is the most abundant noncellulosic-based biopolymer in nature. Although it has been industrially extracted from wood and associated lignocellulosics since the middle of the 19th century, its primary industrial role has been to provide thermal energy. It has not shown sufficient promise in chemical applications because of its complex structure, polydispersity, and random polymeric nature (it does not have a true monomeric building block). It can be primarily characterized as a C9-based (poly) propylphenolic structure that is putatively biosynthesized via a free radical (combinatorial) polymerization mechanism.1 The pulp industry is the main industry that extracts lignin from wood to liberate cellulose for making paper-based materials, while sending its hydrolysates/residues to (energy) recovery boilers so that they can be burned to recover energy. Annually, over 50 million tons of lignin worldwide is burned in the recovery boilers of pulp mills to provide up to 60% of the energy needs of mills.2 However, this latter trend clearly highlights the fact that there is still a lack of high-value applications for lignin, by which it can further enhance the mills’ capacity and improve the mill economy. In view of its abundance and the continuous depletion of petroleum as a primary chemical/material resource, the concept of using lignin as a precursor chemical substitute for petroleum byproducts has gained significant traction. Petroleum is a vitally important raw material for industrial manufacture because it contains a number of aromatic functionalities that can be distilled to give benzene, toluene, and xylenes, valuable commodity chemicals used for resin, plastic, and composites’ manufacture. If lignin, a macromolecular material possessing © XXXX American Chemical Society

many of these aromatic functionalities in its nuclei, is to supplant petroleum for similar starting materials, then an indepth understanding of its chemical hydrolysis is necessary. Structurally, lignin is a 3-D networked polymer that is biosynthesized in plants from three phenylpropane moieties by a radical combinatorial process. The precursors are specifically three aromatic alcohols (monolignols): p-coumaryl, coniferyl, and sinapyl alcohol.3 Phenolic functionalities and their associated properties allow lignin to offer numerous possibilities for providing higher value-added renewable products applications. A number of protocols have been suggested and adopted to convert lignin into useful petroleum-like byproducts that include the following: hydrothermal conversion, pyrolysis, enzymatic degradation, photocatalytic degradation, electrochemical degradation, ionic liquids degradation, and microwave irradiation oxidation. Recently, there has been growing interest in using subcritical water as an environmentally benign reaction medium for a number of organic transformations. It has attracted much attention because subcritical water is a liquid water reaction medium at temperatures between the atmospheric boiling point and the critical temperature (374 °C) of water. Synonymous terms are pressurized hot water (PHW), hot compressed water (HCW), near-critical water (NCW), or superheated water. Hydrothermal conversion typically requires the use of hot Received: March 28, 2014 Revised: June 4, 2014 Accepted: June 4, 2014

A

dx.doi.org/10.1021/ie5011178 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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reactor equipped with an intelligent digital temperature control system. The maximum operation temperature and pressure of the reactor were 400 °C and 30 MPa, respectively. In each run, 1 g lignin and 80 mL water or water−ethanol solution were loaded into the reactor. The reactor was then purged with nitrogen three times to remove any residual air. The reactor was heated to a desired temperature (220, 250, 280, 310, or 340 °C) at a heating rate of 10 °C/min, a process that required 20−35 min in total, after which the reaction temperature was held for the indicated reaction times of 0, 15, 30, 45, and 60 min. The corresponding pressures at 220, 250, 280, 310, and 340 °C were 0.9, 1.9, 3.9, 7.0, and 13.8 MPa, respectively. The ethanol volume-% in water−ethanol cosolvent (0%−100%) experiments were conducted at 310 °C and 30 min. After the reaction, the reactor was first cooled down to 180 °C by simple fan (ventilation) cooling and quickly quenched to room temperature by placing it into a water bath. The liquid and solid product fractions were collected by washing the reactor vessel with ethyl acetate. Each run of the hydrothermal liquefaction experiments was repeated three times. 2.3. Separation of the Products of Lignin Hydrothermolysis. The focus of the current study was the quantitative analyses of lignin hydrothermolysates, which allowed the gas yield to be ignored because the experiments were run at relatively low temperatures during liquefaction. When the reactor was cooled to room temperature, the liquid contents in the reactor were completely rinsed with ethyl acetate. The mixture of liquid contents was filtered with a preweighed G3 funnel, and filtered with a polytetrafluoroethylene (PTFE) membrane. The obtained solid residue (SR) was oven-dried at 105 °C overnight before weighing. The hydrothermal conversion of alkali lignin was calculated from the following equation:

compressed water (subcritical water) as the reaction medium. The technique is invaluable for the conversion of biomass/ waste streams into innocuous byproducts (remediation) or for production into high-value products. What makes the technique so appealing from an energetic perspective is that the evaporation of water and its associated high-energy consumption is avoided by operating at high pressures, while the temperatures and pressures are still lower than found in supercritical water media (most operational temperatures for subcritical reactions tend to be ∼200 °C). In general, the application of hydrothermal conversion (subcritical water) has been utilized in a number of fields.4−10 For example, a survey of the literature has uncovered several research efforts to obtain bio-oil products or phenolic chemicals, employ lignin in sub/supercritical fluids such as water,11−13 methanol,14,15 water−ethanol cosolvents,16 and ethanol.17 Cheng and co-workers demonstrated that a cosolvent of 50/50 (v/v) water−ethanol is much more reactive than either pure ethanol or water alone, leading to a much higher biomass conversion. These methods can partly change the chemical structure of alkali lignin, or improve the yield of phenolic compounds to a certain extent, but there has been a scarcity of data conveying detailed quantitative analyses of the resultant phenolic compounds as a function of ethanol fractions in the reaction medium. Such a determination would greatly facilitate an understanding of the chemical efficiency of the hydrolysis reaction and how it could be more properly utilized. The current work therefore provides the first detailed quantitative data of the phenols and methoxy-benzenes produced from the hydrothermal conversion of alkali lignin in subcritical water and water−ethanol media.

2. EXPERIMENTAL SECTION 2.1. Lignin Isolation, Purification, and Characterization. A poplar sulfate pulping black liquor, obtained from the Key Laboratory of Pulp and Paper Science & Technology of the Ministry of Education, was used to isolate alkali lignin. The chemical composition of poplar, the pulping method, and the chemical properties of the black liquor have been reported in previous efforts.18 The methods for the isolation and purification of lignin in the black liquor were based on a previously documented method.19 Briefly, the method involved filtering black liquor to remove the solid impurities of which 100 mL was diluted to 1000 mL. Diethylenetriaminepentaacetic acid (DTPA, 0.6 g) was added to the diluted liquor and mixed for 1−2 h. The lignin was precipitated from the black liquor by adding acidified water (pH = 2) made from H2SO4. It was then frozen at −10 °C overnight (∼12 h), and centrifuged at 3000 rpm for 15 min. Precipitated lignin was separated and washed with acidified water (pH = 2) three times. The sample was freeze-dried to obtain alkali lignin, which was vacuum-dried overnight at room temperature before use. Elemental analyses of the lignin and impurities were performed on a Vario Micro analyzer (EL III, Germany). C, H, N, and S contents were measured and the O content was calculated by difference. FT-IR spectroscopy of the alkali lignin and the solid residues were recorded on a SHIMADZU FT-IR spectrophotometer (IRPrestige-21, Japan) using a KBr pellet containing ∼1% sample. The scan was conducted over the range of 4000 to 600 cm−1. 2.2. Hydrothermal Conversion. Hydrothermal conversions of lignin were performed in a 200 mL stainless autoclave

conversion (wt%) = 100 wt% − m(SR)/ m(AL) × 100%

where m(SR) is the mass of over dried solid residue (g), and m(AL) is the mass of the alkali lignin used for the reaction (g). 2.4. Analysis of the Products. Qualitative examination of the liquid products was performed on a GC−MS (Shimadzu QP 2010, Shimadzu, Japan) equipped with a SHRXI-5 MS capillary column (30 m × 0.25 mm × 0.25 μm). Helium was used as the carrier gas at a flow rate of 1 mL/min. The GC oven temperature ramps for the work were as follows: begin heating at 50 °C that was held for 2 min, which was then raised at a rate of 10 °C/min to 160 °C; this was held for 2 min; raised to 250 °C at a rate of 10 °C/min; and this final temperature was held for 2 min. The injector temperature was 250 °C with a split ratio of 50:1. The mass selective detector was operated in electron impact (EI) ionization mode. The data were analyzed using a NIST library. From the results of the qualitative analysis, the main phenols and methoxy-benzenes compositions displaying a relatively high content in the liquid products was identified. The standard compounds of these major compositions were purchased from Sigma−Aldrich and further used as external standards to allow for accurate quantitative analyses. The major liquid constituents were subsequently quantified by external standard method in a GC (Varian CP3900, America) equipped with a flame ionization detector (FID) and a DB-1 capillary column (30 m × 0.25 mm × 0.25 μm). The analytical conditions for the phenolic constituents were as follows: an injection temperature of 250 °C, detector B



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temperature of 270 °C, and a column temperature of 50−250 °C (programmed temperature; held at 50 °C for 2 min, raised to 250 °C at 5 °C/min) in a helium stream at a flow rate of 1.1 mL/min. Each liquid sample was injected into a GC at least in duplicate until acceptable peak reproducibility was achieved.

3. RESULTS AND DISCUSSION 3.1. Lignin Characterization. 3.1.1. Proximate and Ultimate Analysis. Proximate and ultimate analysis results of the lignin samples are listed in Table 1. The moisture content Table 1. Proximate and Ultimate Analysis of the Original Alkali Lignin proximate analysis (wt %, d.b.)a

ultimate analysis (wt %, d.a.f)b

VMc

FCd

ash

moisture

C

H

Oe

N

S

84.7

12.3

2.9

1.6

55.9

3.6

36.7

0.56

3.2

Figure 2. Relative concentration FT-IR spectra of the control lignin and the solid residues (test cases) after different reaction times. Curve a: control or raw alkali lignin; curve b: the solid residue after 0 min, 310 °C, pure water; curve c: the solid residue after 30 min, 310 °C, pure water; and curve d: the solid residue after 60 min, 310 °C, pure water.

a

On a dry basis. bOn a dry and ash free basis. cVM: volatile matter. FC: fixed carbon (VM and FC was determined by thermogravimetric analysis (TGA) in N2 at 10 °C/min to 900 °C). eBy difference. d

of the lignin was 1.6 wt %, with an ash content of 2.9 wt %. Elemental analysis demonstrated that the lignin contained the following atoms and relative percentages: carbon (55.9%), hydrogen (3.6%), oxygen (36.7%), nitrogen (0.56%), and sulfur (3.2%). The extremely high content of sulfur is likely indicative of the effect of the acid (sulfuric acid) precipitation. 3.1.2. FTIR Spectrum of Raw Alkali Lignin and the Solid Residues. Figures 1−3 demonstrate the FT-IR spectrum of the

Figure 3. FT-IR spectra of raw alkali lignin and the solid residues under different ethanol volume-%s. Curve a: raw alkali lignin; curve b: the solid residues after 30 min, 310 °C, pure water; curve c: the solid residues after 30 min, 310 °C, ethanol 50%; and curve d: the solid residues after 30 min, 310 °C, ethanol 100%.

Figure 1 shows that the relative concentrations of solid residues were influenced by the different reaction temperatures. The spectra of solid residues showed attenuating peaks relative to the alkali lignin control. As the temperature increased (spectra a → d), these samples showed a stepwise drop in the intensity of the aromatic OH bands. Clearly, the solid residue after 30 min, 340 °C, and pure water displays nearly no bands. This result implicates the decomposition of the aromatic OH groups as a function of subcritical water reaction temperature. Figure 2 shows the spectra of solid residues under different reaction times. At all reaction times, it was observed that the alkali lignin degraded. Even at a reaction time of 0 min, significant changes took place as compared to raw lignin. Finally, Figure 3 shows the spectra of raw alkali lignin and accompanying reacted samples under different ethanol volume%s (all at 310 °C). It was very apparent that when the ethanol volume-% was 50%, the test samples relative to the control degraded significantly. Indeed, the efficiency of the degradation appeared to be much greater than what is observed for the same conditions, but at 100% (neat) water (Figure 2). The result

Figure 1. FT-IR spectra of raw alkali lignin and the solid residues after different reaction temperatures. Curve a: raw alkali lignin; curve b: the solid residue after 30 min, 220 °C, pure water; curve c: the solid residue after 30 min, 280 °C, pure water; and curve d: the solid residue after 30 min, 340 °C, pure water.

alkali lignin and the solid residues after different conditions, illustrating the typical IR bands that may be observed. The bands in the 3600−3200 cm−1 region are assigned to the stretching frequencies of aromatic OH groups, whereas in the 3000−2800 cm−1 range they are assigned to the stretching frequencies of aromatic and aliphatic CH groups, while in the 1600−1510 cm−1 range, they are assigned to the stretching frequencies of aromatic CC groups. Signals in the region 1400−1300 cm−1 range and 900−700 cm−1 (fingerprint region) were assigned to bending vibrations of saturated aromatic hydrocarbons and aromatic CH groups.20,21 C



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suggested that the addition of ethanol highly encouraged the oxidative chemistry. 3.2. Hydrothermal Conversion. 3.2.1. Effects of Reaction Temperature. The effects of reaction temperature on the conversion of alkali lignin (neat water for 30 min) are displayed in Figure 4a. A lower reaction temperature produced a lower

Table 2. Identification of Compounds Obtained from Alkali Lignin Hydrothermolysis (30 Min, 340°C, and Neat Water)

Figure 4. Conversion of alkali lignin under different reaction conditions (Curve a: 30 min and neat water, reaction temperatures: 220, 250, 280, 310, and 340 °C, respectively; curve b: at 310 °C and neat water, reaction times: 0, 15, 30, 45, and 60 min, respectively; and curve c: at 310 °C and 30 min, ethanol volume-% in water−ethanol cosolvent: 100%, 75%, 50%, 25%, and 0%, respectively).

yield, likely because depolymerization reactions are endothermic, indicating that they are thermodynamically favorable at a higher temperature. When the temperature increased from 220 to 310 °C, the conversion continued to increase from 82.49% to 95.91% as the temperature increased. Again, it was evident that at 50% ethanol volume-%, the conversion (degradation) is the highest among all of the conditions employed. 3.2.2. Effects of Reaction Time. The experiments in neat water were carried out at 310 °C for reaction times ranging from 0 to 60 min; the conversions are displayed in Figure 4b. The conversions were not significantly influenced by reaction time, a result that is similar to what has been found previously.16,22 Conversions of all the runs were nearly 94.00%. When the reaction time was 30 min, the highest conversion was 95.91%; at extended reaction times, the conversion dropped to 92.90%, findings that indicate that extension of the reaction time does not appear to favor the reaction. 3.2.3. Effects of Ethanol Volume-% in Water−Ethanol Cosolvent. Figure 4c showed the effects of ethanol volume-% in water−ethanol cosolvent on the hydrothermal conversion. All the reactions were performed at 310 °C for 30 min. When the ethanol concentration was 50%, the conversion (98.39%) was the highest. However, in subsequent quantitative experiments, the yield of phenolic compositions identified by GC was low. This suggested that in all likelihood, the reaction generated volatiles. 3.3. Qualitative Analysis. Table 2 shows the results of the qualitative examination of the identified compounds obtained from the hydrothermal decomposition of alkali lignin (30 min, 340 °C, and neat water) using GC−MS. The main product fractions of alkali lignin hydrothermolysis were phenols and methoxy-benzenes compositions. The quantitatively dominant substances (based on peak area percentage) were phenol,

no.

RT (min)

compound

peak area (%)

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

8.251 10.317 10.917 11.258 13.442 14.025 14.117 15.825 16.367 16.958 17.533 18.217 18.375 18.625 19.067 19.417 19.642 19.717 20.017 20.133 20.525 20.800 21.100 21.475 21.925 22.342 22.442 22.591 22.865 25.188 25.309 25.373 25.434 26.400

phenol 3-methylphenol 4-methylphenol guaiacol 4-ethyphenol naphthalene 4-methylguaiacol 3-methoxycatechol 4-ethylguaiacol 2-methyl-naphthalene 3-methoxy-5-methylphenol 2,6-dimethoxyphenol 2-methoxy-4-(2-propenyl)-phenol 4-propylguaiacol 3,5-dimethoxyacetophenone vanillin 2-methoxy-4-(1-propenyl)-phenol 1,5-dimethyl-naphthalene 1,3-dimethyl-naphthalene 1,8-dimethyl-naphthalene 1,2,4-trimethoxybenzene 2-methoxy-4-propylphenol 4-ethenyl-1,2-dimethoxybenzene 4-hydroxy-3-methoxyacetophenone 1,2-dimethoxy-4-n-propylbenzene 1,2,3-trimethoxy-5-methylbenzene 4-hydroxy-3-methoxy-benzeneacetic acid flopropione p-nitrobenzaldehyde 4-hydroxy-3,5-dimethoxy-benzaldehyde 3-methoxy-2-naphthalenol 2,6-dimethoxy-4-(2-propenyl)-phenol 2,6-dihydroxy-4-methoxyacetophenone brevifolin

5.48 0.19 0.28 20.05 0.64 0.20 5.25 2.08 4.84 1.84 0.63 31.21 0.25 1.07 3.59 1.84 0.27 0.49 0.43 0.28 4.21 3.97 0.76 1.91 0.47 3.49 1.52 0.25 0.27 0.51 0.32 0.44 0.16 0.81

guaiacol, 4-methylguaiacol, 3-methoxycatechol, 4-ethylguaiacol, 2,6-dimethoxyphenol, 4-propylguaiacol, 3,5-dimethoxyacetophenone, vanillin, and so forth. Wherein, 2,6-dimethoxyphenol and guaiacol were the two most abundant compounds that accounted for 31.2% and 20.1% of the total peak area, respectively. Precise quantification of the selected major compounds will be a discussed later in this section. 3.4. Quantitative Analysis. 3.4.1. Product Yields under Different Reaction Temperatures. Table 3 shows the results of the qualitative examination of the main biophenols from lignin hydrothermal conversion at different temperatures. Phenol, guaiacol, 4-methylguaiacol, 4-ethylguaiacol, and 2,6-dimethoxyphenol were the main products, and Figure 5 shows the influence of subcritical water temperature on their yield. As the temperature increased, phenol yield showed no obvious differences, while the best yield (2.65 mm/g) was obtained at 310 °C, and then decreased slightly with temperature increasing to 340 °C. From 250 to 310 °C, the yield of guaiacol increased steeply (from 1.08 mg/g to 10.57 mg/g) as it did for 2,6dimethoxyphenol (from 0.65 mg/g to 9.20 mg/g), and then slowly ticked up. 4-Methylguaiacol showed the highest yield (2.52 mg/g) at a temperature of 340 °C and a retention time of 30 min. Hydrolysis and cleavage of the ether bond and the C C bond, demethoxylation, alkyation, and condensation were the D



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Table 3. Identities of the Phenols and Methoxy-Benzenes Obtained from Lignin Hydrothermal Conversions under Different Reaction Temperatures (30 Min and Neat Water) mg/g of dry alkali lignin retention time (min)

compound

220 °C

250 °C

280 °C

310 °C

340 °C

8.27 11.28 14.13 15.85 16.38 18.22 18.64 19.38 19.44 20.54 21.52 22.26

phenol guaiacol 4-methylguaiacol 3-methoxycatechol 4-ethylguaiacol 2,6-dimethoxyphenol 4-propylguaiacol 3,5-dimethoxyacetophenone vanillin 1,2,4-trimethoxybenzene 4-hydroxy-3-methoxyacetophenone 2,6-dihydroxy-4-methoxyacetophenone

1.14

2.01 1.08

2.48 5.83 0.62

2.65 10.57 1.87

0.76 5.97

1.56 9.20 0.59

2.02 11.82 2.52 1.10 1.62 9.68 0.39 0.58

2.15

1.80

0.65 0.45

0.43 0.07 0.49 0.21

0.99

conditions of relatively low temperature. With a temperature increase, the demethoxylation and alkylation of lignin-derived phenolics compounds were enhanced to yield various additional alkyl phenols at high temperature. 3.4.2. Product Yields from Different Retention Times. Table 4 shows the results of the qualitative examination of the main phenolic and methoxy-benzene compositions from lignin hydrothermal conversions at different retention times. Figure 6 shows the influence of subcritical water time on the yield of phenol, guaiacol, 4-methylguaiacol, 4-ethylguaiacol, and 2,6dimethoxyphenol. In general, most of the compounds yields increased with increasing time. At first, they were easily observed to increase, and then they increased slowly as the reaction time was extended to 30 min. With an increasing retention time, phenol, 4-methylguaiacol, and 4-ethylguaiacol demonstrated no obvious changes over further heat treatment times. The optimal yields of phenol (2.77 mg/g), 4methylguaiacol (1.87 mg/g) were obtained at times of approximately 45 and 30 min, respectively. Any further increases in reaction times lead to secondary, undesirable reactions. In addition, the highest yields of guaiacol (11.87 mg/ g) and 2,6-dimethoxyphenol (12.17 mg/g) were obtained at a temperature of 310 °C and retention time of 60 min. 3.4.3. Product Yields under Different Ethanol Volume-%s in Water−Ethanol Cosolvents. Table 5 shows the results of a qualitative examination of the main phenols and methoxy-

Figure 5. Main phenols and methoxy-benzenes compositions measured as a function of reaction temperature (30 min, and neat water).

general reactions that occurred during the hydrothermal liquefaction of lignin to produce the phenolic monomers. More specifically, the monomers were obtained by hydrolyses of the ether bond and aliphatic CC bond under the mild

Table 4. Identities of the Phenols and Methoxy-Benzenes from Lignin Hydrothermal Conversions at Different Retention Times (310°C and Neat Water) mg/g of dry alkali lignin retention time (min)

compound

0 min

15 min

30 min

45 min

60 min

8.27 11.28 14.13 16.38 15.85 18.22 18.64 19.38 20.54 21.52 22.26 22.35

phenol guaiacol 4-methylguaiacol 4-ethylguaiacol 3-methoxycatechol 2,6-dimethoxyphenol 4-propylguaiacol 3,5-dimethoxyacetophenone 1,2,4-trimethoxybenzene 4-hydroxy-3-methoxyacetophenone 4-ethyl-2,6-dimethoxyphenone 2,6-dihydroxy-4-methoxyacetophenone

1.62 5.08 0.49

1.66 6.66 0.96 0.79

2.65 10.57 1.87 1.56

4.25

6.78

9.20

2.77 10.91 1.77 1.50 0.59 11.81 0.37

2.45 11.87 1.86 1.68 0.68 12.17 0.37

1.38 0.87 0.68

1.35 0.24 1.22 1.15

0.59 2.15 0.36 0.72 E

0.99



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Figure 6. Main phenols and methoxy-benzenes compositions changes as a function of reaction time (310 °C, and pure water).

Figure 7. Concentration changes of the main phenols and methoxybenzenes compositions with respect to ethanol volume-% in a water− ethanol cosolvent (30 min, and 310 °C).

benzene from lignin hydrothermal conversions in different ethanol concentrations. The concentration changes of the phenol, guaiacol, 4-methylguaiacol, 4-ethylguaiacol, 2,6-dimethoxyphenol are shown in Figure 7. When the ethanol concentration is low, the yields of phenols such as phenol, guaiacol, 4-methylguaiacol, 2,6-dimethoxyphenol were higher. The results indicate that the ethanol concentration plays a significant effect on the production of phenols; in general, ethanol plays an inhibitory role on the phenols’ yield. It is suggested that the hydrogen-donor capability of ethanol can lead to the stabilization of the free radicals generated from lignin depolymerization.23 This appeared to be confirmed by increasing the alcohol concentration: when ethanol concentration was 25% (30 min, 310 °C), the highest yields of phenol, 4-methylguaiacol, 4-propylguaiacol, 3,5-dimethoxyacetophenone, 1,2,4-trimethoxybenzene, 2,6-dihydroxy-4-methoxyacetophenone were obtained and are shown as follows: 4.25, 2.93, 0.95, 0.78, 2.47, and 2.47 mg/g, respectively.

ethanol volume-%, whereas the residence time had only a slight influence. An optimal conversion of alkali lignin to liquid effluent was achieved at 310 °C, 30 min, and at an ethanol concentration of 50%. The yields of the phenolic and methoxy-benzene compounds were higher in 25% ethanol than that of pure water. It was found that the reaction time had a positive impact on the generation of phenols, while further extension of the reaction time did not significantly increase phenol yield. Higher reaction temperatures or lower contents of ethanol appeared to improve the byproduct yields.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 531 89631168. Fax: +86 531 89631630. E-mail: [email protected]. *Tel.: +86 531 89631168. Fax: +86 531 89631630. E-mail: [email protected].

4. CONCLUSIONS During lignin hydrothermal conversion in water/water−ethanol solvent systems, the reaction conditions were found to exert different effects on the overall yield and composition of the byproducts. The hydrothermal conversion of alkali lignin was markedly dependent upon the reaction temperature and the

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Science Foundation of China (Grant Nos. 31270626,

Table 5. Identities of the Phenols and Methoxy-benzenes from Lignin Hydrothermal Conversions under Different Ethanol Volume-%s in a Water−Ethanol Cosolvents (30 Min and 310 °C) mg/g of dry alkali lignin retention time (min)

compound

8.27 11.28 14.13 16.38 18.22 18.64 19.38 20.54 22.26 22.35

phenol guaiacol 4-methylguaiacol 4-ethylguaiacol 2,6-dimethoxyphenol 4-propylguaiacol 3,5-dimethoxyacetophenone 1,2,4-trimethoxybenzene 4-ethyl-2,6-dimethoxyphenone 2,6-dihydroxy-4-methoxyacetophenone

100%

75%

50%

25%

0%

2.58 1.23 0.60 1.30

2.47 3.81 1.97 1.20 1.55

3.12 6.43 2.68 3.30 4.13

4.25 10.5 2.93 3.26 10.15 0.95 0.78 2.47

2.65 10.57 1.87 1.56 9.20

2.47

0.99

0.12 1.23 1.00

F

0.59 2.15



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(20) Derkacheva, O.; Sukhov, D. In Investigation of Lignins by FTIR Spectroscopy; Macromolecular symposia, Wiley Online Library: 2008; pp 61. (21) Toledano, A.; García, A.; Mondragon, I.; Labidi, J. Lignin separation and fractionation by ultrafiltration. Sep. Purif. Technol. 2010, 71, 38. (22) Meier, D.; Ante, R.; Faix, O. Catalytic hydropyrolysis of lignin: Influence of reaction conditions on the formation and composition of liquid products. Bioresour. Technol. 1992, 40, 171. (23) Yuan, X.; Li, H.; Zeng, G.; Tong, J.; Xie, W. Sub-and supercritical liquefaction of rice straw in the presence of ethanol− water and 2-propanol−water mixture. Energy 2007, 32, 2081.

31170547, 31270627, and 31370580), the Science Foundation of Shandong Province (Grant Nos. Y2007B13, ZR2010CM65, and ZR2011CM011), and the Outstanding Young Scientist Fund of Shandong province (BS2013NJ014).

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DEDICATION The manuscript is dedicated to Prof. John Ralph (Wisconsin) on the occasion of his ACS Cellulose & Renewable Materials Division Anselm Payen Award (Dallas, 2014).

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Quantitative Analyses of Lignin Hydrothermolysates from Subcritical

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