Bioresource Technology 179 (2015) 84–90

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Efficient and product-controlled depolymerization of lignin oriented by metal chloride cooperated with Pd/C Riyang Shu a,b, Jinxing Long a, Zhengqiu Yuan a,b, Qi Zhang a,⇑, Tiejun Wang a, Chenguang Wang a, Longlong Ma a a b

Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China

h i g h l i g h t s  Lignin depolymerization was presented using metal chloride cooperated with Pd/C.  85.6% lignin liquefaction with 35.4% phenolic monomer yield was shown.  Highly controllable product distribution could be realized.  Both the acid catalytic and coordination catalytic mechanism were proposed.

a r t i c l e

i n f o

Article history: Received 23 October 2014 Received in revised form 4 December 2014 Accepted 5 December 2014 Available online 12 December 2014 Keywords: Lignin Product-controlled Guaiacols Phenols Phenolic monomer

a b s t r a c t An efficient lignin depolymerization process with highly controllable product distribution was presented using metal chloride (MClx) cooperated with Pd/C. The catalytic performances of MClx were investigated. The effect of reaction conditions on the lignin depolymerization and products distribution were also studied. Results showed that more than 35.4% yield of phenolic monomer including 7.8% phenols and 1.1% guaiacols could be obtained under optimized condition. And the product distribution can be efficiently controlled by the modification of the metal cation through different pathway of Lewis acid catalysis and coordination catalysis. Furthermore, the Pd/C catalyst showed an excellent recyclability, where no significant loss of the catalytic activity was exhibited after 3 runs. Moreover, the product control mechanism was proposed. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Lignin, which comprises up to 40% of the energy content of typical plant matter (Regalbuto, 2009), shows great potential on high value biochemical/bio-fuel aromatics production (Barta et al., 2010). However, lignin is also the most recalcitrant components of renewable biomass. Lignin structure mainly consists of three types of 4-propenyl-phenols units interlinked through C–O–C ether covalent bonds (Zakzeski et al., 2010), making it much difficult for conversion. Furthermore, intensive hydrogen bond in the lignin molecule is also a main challenge for lignin depolymerization (Kubo and Kadla, 2005). Therefore, most of lignin is only used as low quality fuel for biomass refinery. Successful conversion of lignin into high value products has not been achieved yet. Catalytic depolymerization of lignin while preserving its aromatic nature is a

⇑ Corresponding author. Tel./fax: +86 20 87057789. E-mail address: [email protected] (Q. Zhang). http://dx.doi.org/10.1016/j.biortech.2014.12.021 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

promising process to provide a valuable stream of material, which is currently acquired from petroleum sources exclusively. Reductive depolymerization has been considered as a promising way for lignin conversion to phenols. With hydrogenation or hydrogenolysis process, C–O–C linkages of lignin can be selectively cleaved into phenolic monomer and oligomer (Yan et al., 2008). For example, Zakzeski et al. had reported that lignin could be reduced and liquid-phase reformed to 17.6 wt% phenolic monomer over noble metal catalysts (Pt, Ru, Pd, and Rh supported on aluminum oxide) (Zakzeski et al., 2012). Pepper et al. also studied the effect of different catalysts (Raney Ni, Pd/C, Rh/C, Rh/Al2O3, Ru/C, Ru/ Al2O3) on softwood lignin (spruce wood) hydrogenation. A significant amount of the original lignin was converted into the monomeric products such as 4-propylguaiacol and dihydroconiferyl alcohol under mild conditions (3.4 MPa, 195 °C) (Pepper, 1969). However, most lignin depolymerization processes suffered obvious disadvantage of low conversion and phenolic monomer yield. Especially, the depolymerization products were uncontrollable (Barta et al., 2010).

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Herein, a novel and efficient lignin hydrogenolysis process was proposed using MClx cooperated with Pd/C. In which, the hydrogen bond and other chemical bonds such as b-O-4 and 4-O-5, could be weakened by the strong electronegativity group Cl and cleaved more easily under the catalysis of Pd/C and the acid center of MClx. Furthermore, the phenolic products could be modified by the change of the metal cation based on the various depolymerization mechanisms. 2. Methods 2.1. Materials Alkali lignin was purchased from Sigma–Aldrich China (Shanghai, China). Elemental analysis showed that it is composed of 47.7% C, 4.9% H, 24.4% O, 0.1% N, 3.9% S and 19.0% ash. Methanol (99%), ethanol (99%), n-propanol (99%), isopropanol (99%), n-butanol (99%), NiCl2, LiCl, BaCl2, CuCl2, ZnCl2, AlCl3, MgCl2, FeCl2, FeCl3, CrCl3, Cr(NO3)3, Cr3C2, HCl and H2SO4 were analytical grade and purchased from Tianjin Fu Yu Fine Chemical Co., Ltd. (Tianjin China). 5 wt% Pd/C was provided by Aladdin Reagent Co., Ltd. (Shanghai, China). 2.2. Typical process for lignin depolymerization 0.5 g alkali lignin, 0.5 mmol Lewis acid, 0.1 g 5 wt% Pd/C and 40 mL CH3OH were charged into a 100 mL stainless autoclave (316L stainless, Weihai Chemical Machinery Co., Ltd., Shandong, China) equipped with a mechanical agitation. After air displaced by H2 for three times, the reactor was heated to 260 °C (rate of 4 °C min1) for 5 h. When the reaction was finished, the mixture was cooled down to room temperature during 30 min using flowing water. 2.3. Products separation and analysis The product mixture was first filtered. Solid fraction (include catalyst and residual solid) was washed three times with methanol and deionized water respectively. Then it was dried at 80 °C until a constant weight. The filtrate was collected together with the methanol which was used for solid fraction washing. And then, it was diluted by CH3OH to a given volume for the qualitative and quantitative analysis. Gas fraction was indentified and measured on an Agilent 6890 gas chromatogram (GC) with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The results showed that the gaseous products from this process were less than 1 wt% of the feed lignin. Therefore, the yield of gas products was negligible and not taken into account in the mass balance. Qualitative analysis of the lignin-derived phenolic compounds was carried out on a gas chromatography/mass spectrometry (GC–MS) equipped with a HP-INNOWAX column (30 m  0.25 m m  0.25 lm). The oven temperature was programmed as 60 °C hold 2 min, and then ramped up to 260 °C with 10 °C min1 and hold for another 10 min. The injector was kept at 280 °C in spit mode (5:1) with helium as the carrier gas. Quantitative analysis of the volatile products was determined by SHIMADZU GC 2014C with a FID and a HP-INNOWAX column. The oven temperature program was the same as the GC–MS analysis. Acetophenone was used as internal standard chemical. The yield of liquid product was represented by the degree of lignin liquefaction, which was calculated through the lignin weight loss ratio (Eq. (1)). The yields of phenols, guaiacols and other phenolic monomer were evaluated according to the following equations (Eqs. (2)–(4)) based on the GC results. The main products from this process were divided into gaseous fraction, volatile

products (including phenolic monomer and other volatile products), nonvolatile products and residual solid. As shown above, the weight of the gaseous fraction was negligible. Therefore, the yield of nonvolatile products was measured by the mass balance as shown in Eq. (6):

DL ð%Þ ¼ Degree of lignin liquefaction ¼ ðW F  W R Þ=W F  100% ð1Þ Y M ð%Þ ¼ Yield of the phenolic monomer ¼ W M =W F  100% ð2Þ Y P ð%Þ ¼ Yield of phenols ¼ W P =W F  100%

ð3Þ

Y G ð%Þ ¼ Yield of guaiacols ¼ W G =W F  100%

ð4Þ

Y Others ð%Þ ¼ Yield of other phenolic monomer ¼ Y M  Y P  Y G ð5Þ Y Nonvolatile ¼ Yield of the nonvolatile products ¼ ðW F  W R  W V Þ=W F  100%

ð6Þ

WF: the weight of feed lignin; WR: the weight of residual solid; WM: the weight of the phenolic monomer; WP: the weight of phenols; WG: the weight of guaiacols; Wv: the weight of volatile products (including phenolic monomer and other volatile chemicals) which was measured by GC. 2.4. Characterization of the fresh and recovered Pd/C catalyst The Pd metal leaching of the catalyst in the CH3OH was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES, OPTIMA 8000). Thermogravimetric analysis (TGA) was couducted on NETZSCH STA449C instrument under nitrogen flow, ramping up to 700 °C at 10 °C/min. Scanning electron microscope (SEM) images of the fresh and recovered Pd/C were obtained on a Hitach S-4800 instrument (10 kV). Fourier transform infrared spectroscopy (FT-IR) analysis was conducted on a Nicolet is 50 FTIR spectrometer using KBr pelleting method. 3. Results and discussion 3.1. Catalytic hydrogenolysis of lignin Generally, the lignin depolymerization product is complex (Long et al., 2015). Table S1 also showed that the products distribution of this lignin hydrogenolysis is complicated. Phenols, guaiacols and syringols could be observed clearly. Compared with others, the yield of syringols was significantly low. FT-IR spectrum of this alkali lignin (Fig. S1) showed that the peak strength of guaiacyl unit (1267 cm1, G-lignin) was much stronger than that of the syringyl unit (1326 cm1, S-lignin). And the previous works (Long et al., 2015, 2014a) also demonstrated that S-lignin is much more recalcitrant than the G-lignin. Hence few of syringols were detected in the volatile products. And thus, the content of syringols was not discussed individually in the following biochemical yield measurement, whereas it was included in yield of other phenolic monomer as shown in the Eq. (5). It should be noted that no chloride contained compound was detected in all runs, indicating that the dissolved metal chloride did not react with the feedstock. Table 1 summarized the degree of lignin liquefaction and the volatile product distribution over different catalysts. It could be seen that no more than 49.8% of lignin could be liquefied with 5.6% yield of total phenolic monomer in the absence of any catalyst (Table 1, entry 1). The addition of Pd/C facilitated the lignin depo-

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Table 1 The effect of Lewis acid on lignin depolymerization and product distribution. Entry

Lewis acid

DL (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

–a –b LiCl NaCl KCl BaCl2 CuCl2 FeCl2 NiCl2 ZnCl2 MgCl2 FeCl3 AlCl3 CrCl3 Cr3C2 Cr(NO3)3 H2SO4 HCl CrCl3c

49.8 58.5 72.0 63.5 63.4 64.0 66.0 72.0 74.0 76.0 82.7 84.0 91.4 80.7 78.8 89.7 88.6 90.8 67.9

Yield of the phenolic monomer (%)

YOther

YPhenols

YGuaiacols

YOthers

Total

0.3 1.5 2.4 2.6 1.7 0.8 2.1 4.1 0.7 2.2 4.1 1.0 5.3 7.3 0.2 3.8 0.7 0.9 7.3

3.9 2.6 3.3 4.7 4.9 4.7 3.5 2.3 5.8 6.9 3.1 5.1 0.9 2.8 3.6 5.2 6.4 5.8 2.5

1.4 2.7 5.4 0.9 2.2 1.3 3.6 2.1 3.8 3.9 12.4 0.6 19.8 18.4 4.1 7.6 3.6 4.5 14.2

5.6 6.8 11.1 8.2 8.8 6.8 9.2 8.5 10.3 13.0 19.6 6.7 26.0 28.5 7.9 16.6 10.7 11.2 24.0

1.4 1.0 0.6 0.8 0.7 0.2 0.5 0.3 0.2 3.4 3.4 0.4 3.2 4.0 2.2 1.3 0.7 0.9 3.2

volatile

(%)

Ynonvolatile (%)

42.8 50.7 60.3 54.5 53.9 57.0 56.3 63.2 63.5 59.6 59.7 76.9 62.2 48.2 68.7 71.8 77.2 78.7 40.7

DL: the degree of lignin liquefaction, YPhenols: the yield of phenols, YGuaiacols: the yield of guaiacols, YOthers: the yield of other phenolic monomer excluded phenols and guaiacols; YOthervolatile: the yield of other volatile chemicals exclude phenolic monomer; YNonvolatile: the yield of nonvolatile products. Condition: 0.5 g alkali lignin, 0.5 mmol Lewis acid, 0.1 g 5 wt% Pd/C, 40 mL methanol, 4 MPa H2, 260 °C, 5 h. a Without both Pd/C and Lewis acid. b Without Lewis acid. c Without Pd/C.

lymerization, but the promotion effect of phenolic monomer production was insignificant. However, when lignin was treated with MClx cooperated with Pd/C, the degree of liquefaction significantly increased (Table 1, entries 3–14). For example, when degraded in CH3OH with CrCl3 and Pd/C, more than 80.7% lignin was liquefied, giving more than 28.5% yield of total phenolic monomer (Table 1, entry 14). This phenolic monomer yield was higher than many previous studies (Long et al., 2014b; Yoshikawa et al., 2013; Zhang et al., 2014). The ether bonds in lignin molecule, such as b-O-4 and 4-O-5, were more flexible to be corrupted when an acidic catalyst was presented (Deepa and Dhepe, 2014), therefore, higher liquefaction degree and phenolic monomer yield were obtained in comparison with the non-catalysis process. In parallel, the active energy of these ether bonds was also significantly decreased with the help of atomic H on Pd surface (Zhao et al., 2011), which further promoted the lignin depolymerization, resulting in high lignin liquefaction degree and phenolic monomer yield. Moreover, it is well known that Cl is a high electronegativity element, and Cl was widely used in biomass dissolution and conversion as an excellent hydrogen bonding acceptor and nucleophilic reagent (Long et al., 2011a; Swatloski et al., 2002). For example, Swatloski found that cellulose can be efficiently dissolved in ionic liquid [bmim]Cl with more than 10% solubility (Swatloski et al., 2002). During the dissolution, the strong inter and extra hydrogen bond in cellulose molecules was broken, resulting in more flexible for conversion to reducing sugar and platform chemicals such as HMF and LA. And in most cellulose conversion process with [bmim]Cl, the C–O bond in cellulose molecules was easier to be broken than that with traditional solvent. Lignin was also composed of C–O bond. And the hydrogen bond was widely existed in lignin molecules as well. Therefore, it is considered that the Cl in Lewis acid can be an efficient hydrogen bond acceptor for lignin and a polarization reagent for C–O bonding in this process. And higher degree of lignin liquefaction and phenolic monomer yield were obtained in the presence of MClx cooperated with Pd/C. Metal cation also has a significant impact on lignin depolymerization. The results listed in Table 1 showed that the degree of lig-

nin liquefaction increased gradually with the increase of metal cation valence. The Lewis acid strength of metal cation is considered to be responsible for it. The higher valence metal cations could produce more acidic centers and increase the acidic strength of the catalyst (Gary et al., 2014), which promoted the lignin hydrogenolysis by enhancing the degree of lignin liquefaction. It explains well with the fact that the degree of lignin liquefaction with AlCl3 was higher than that of MgCl2 and LiCl. Furthermore, the larger amount of Cl in the AlCl3 molecule is also considered to contribute to the high degree of lignin liquefaction. However, it should be noted that both the degree of lignin liquefaction and the yield of phenolic monomer in presence of LiCl were higher than that with most of bivalence metal chlorides such as BaCl2, CuCl2, FeCl2 and NiCl2 (Table 1, entries 3, 6–9). The fact that Li+ has almost the same ionic radius as the H+ (0.155 nm for Li and 0.159 nm for H) resulting in the similar catalytic performance of LiCl and HCl (Zhdanow, 1965) is responsible for it. It is noteworthy that the product distribution of phenolic monomer was also significantly influenced by metal chloride. As shown in Fig. S1, the lignin used in this study was mainly composed of guaiacyl unit (G/S = 2.59, determinate by the infrared absorption strength comparison of 1267 cm1 (G lignin) and 1326 cm1 (S-lignin) (Ibarra et al., 2005; Long et al., 2011b)). Hence, most of lignin depolymerization processes listed in Table 1 showed that the main volatile products were guaiacol and its derivates. However, when lignin was decomposed over CrCl3 cooperated with Pd/C, phenols were found to be the most abundant species. In which, 7.8% yield of phenols was obtained with 2.8% yield of guaiacols. Generally, the breakage of the low activation energy C–O–C bonds in G-lignin molecule, such as b-O-4 and 4-O-5, was occurred at acid centers on the catalyst surface, which directly gave guaiacol and its derivates. Therefore, the selectivity of guaiacols was higher than that of phenols with most of MClx catalysts (Table 1). However, it was reported that CrCl3 could act as both Lewis acid and coordinated catalyst when a C–O bond abundant chemical was presented (Zhao et al., 2007). Therefore, both guaiacols and phenols were shown in the product distribution with the coordinated catalyst of CrCl3 and Pd/C.

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Cl is a high electronegativity element and widely used in biomass dissolution and conversion as an excellent hydrogen bonding acceptor and nucleophilic reagent (Long et al., 2011a; Swatloski et al., 2002). Therefore, the effect of this anion was further investigated. Table 1 showed that more than 28.6% yield of phenolic monomer including 7.3% phenols could be obtained with CrCl3. However, a significant decrease of phenolic monomer and dramatic change of product distribution were exhibited when Cl was replaced by carboanion or NO 3 (Table 1, entries 15, 16). It indicated that the synergic effect between Cl and Cr3+ is mainly contributed to the high phenolic monomer and phenols yield. The catalytic activities of traditional proton acids such as H2SO4 and HCl were also investigated. More than 88.6% degree of lignin liquefaction could be achieved. However, less than 11.2% of phenolic monomer yield was shown, and guaiacols were found to be the most abundant. Furthermore, the fact that HCl exhibited better catalytic activity than H2SO4 on both the degree of lignin liquefaction and the phenolic monomer yield further confirmed that Cl plays a crucial role in the lignin depolymerization. Therefore, Table 1 demonstrated clearly that lignin can be efficiently depolymerized into phenolic monomer under the catalysis of MClx cooperated with Pd/C. The depolymerization performance of lignin and the product distribution were significantly depended on the instinct of metal cations. Among all metal cations investigated, Cr3+ was found to be the most efficient (28.5% phenolic monomer yield, which is far higher than the reported level), due to both acid catalytic (which gave guaiacols) and coordination catalytic depolyemrization (which gave phenols). 3.2. The effect of solvents The interaction between alcohol solvent molecules and polar groups of lignin can produce a solvent effect of lignin depolymerization (Wang and Rinaldi, 2012). Methanol, ethanol, propanol and butanol are excellent Lewis basic solvents (Marcus, 1993). Furthermore, the excellent solubility of phenolic monomer and oligomer in alcohols is helpful to suppress the repolymerization. Therefore, the alcohols showed a significant solvent effect on the lignin hydrogenolysis. For example, 66.2–80.7% degree of lignin liquefaction with 7.3–28.5% yield of phenolic monomer was obtained in the presence of alcohols (Table S2). However, no more than 62.5% degree of lignin liquefaction and 6.5% yield of phenolic monomer were shown in H2O and tetrahydrofuran. Table S2 also showed that both the degree of lignin liquefaction and the yield of phenolic monomer decreased with the increase of carbon chain in alcohols. The fact that the Lewis basic strength decreases with the chain length (Wang and Rinaldi, 2012) is considered to be the main cause.

87

Fig. 1. Effect of atmosphere on the lignin depolymerization. Condition: lignin 0.5 g, CrCl3 0.5 mmol, Pd/C 0.1 g, 260 °C, 5 h, 2 MPa pressure.

For example, the degree of lignin liquefaction was gradually increased with the rise of H2 pressure, and 80.7% of lignin was converted into phenolic monomer with 28.5% yield when 4.0 MPa H2 was inflated. However, when the hydrogen initial pressure sequentially increased (5.0 MPa), the degree of lignin liquefaction and the yield of total phenolic monomer dropped sharply. The product distribution (Table S3) demonstrated that the yield of phenolic monomer decreased significantly, whereas, other volatile products yield was increased. GC–MS results (Table S4) showed that this fraction was mainly composed of the aliphatic compound, which was generally originated from the hydrogenation of the phenolic monomer (Zhang et al., 2014). Therefore, the fact that some of the phenolic monomer produced in this process was saturated to give more other volatile products was responsible for the decrease of phenolic monomer yield. It is well known that the volatility of the compounds could be suppressed by the physical effect of increasing gas pressure (Guell et al., 1993). Hence, the repolymerization of the unsaturated phenolic monomer was also occurred at higher H2 pressure, resulting in more nonvolatile products (Table S3). 3.4. Effect of reaction temperature and time The effects of reaction temperature and time were shown in Fig. 3. It could be found that this lignin hydrogenation process is highly temperature dependent. Both the lignin liquefaction degree

3.3. The effects of hydrogen Lignin depolymerization process with MClx and Pd/C catalyst was a hydrogen consuming process, so the effect of hydrogen was also investigated. Results shown in Fig. 1 demonstrated that H2 had an obvious advantage on the degree of the lignin depolymerization and the yield of the phenolic monomer than N2 and air. For example, 73.6% of lignin liquefaction with 23.6% of phenolic monomer could be obtained under H2 atmosphere. However, in the presence of air, no more than 67.4% of lignin was liquefied with 10.3% yield of the phenolic monomer. It should be noted that 69.8% degree of lignin liquefaction and 20.2% yield of phenolic monomer were shown when N2 was used, because of the in situ production of hydrogen from CH3OH over the Pd/C catalyst (Xu et al., 2015). The initial H2 pressure also has a significant effect on both the lignin liquefaction and product distribution adjustment (Fig. 2).

Fig. 2. Effect of hydrogen pressure on the lignin depolymerization. Condition: lignin 0.5 g, CrCl3 0.5 mmol, Pd/C 0.1 g, 260 °C, 5 h.

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and the phenolic monomer yield were sharply increased with the elevated reaction temperature. No more than 47% of lignin could be liquefied with less than 7.5% yield of phenolic monomer at 240 °C. However, when the temperature was increased, the degree of lignin liquefaction was sharply increased to 80.7% at 260 °C, and then kept steady. Fig. 3a also demonstrated that the yield of phenols was significantly increased with the rise of temperature, indicating that the coordination catalysis was more temperature sensitive than the acid catalysis. Certainly, the conversion of guaiacols to phenols at higher temperature (Lin et al., 2011; Zhang et al., 2013) was also attributed to the phenols increase and guaiacols decrease from 260 °C to 300 °C. In addition, the slight decrease of the phenolic monomer yield at 300 °C could be attributed to the repolymerization based on the previous studies (Long et al., 2015, 2014b). Compared to temperature, the effect of reaction time is insignificant (Fig. 3b). The slight declination of the lignin liquefaction degree and the phenolic monomer yield also should be attributed to the repolymerization of phenolic products as shown in the previous study (Long et al., 2015, 2014b).

The effects of both Lewis acid and Pd/C dosages had also been carefully examined (Fig. 4). Generally, MClx acts as the acid center, which is responsible for the breakage of the low activity energy

chemical bonds in lignin molecule such as b-O-4 and 4-O-5. Therefore, both the degree of lignin liquefaction and the yield of phenolic monomer were sharply increased when CrCl3 was increased from 0.25 (75.3% degree of lignin liquefaction with 10.7% yield of phenolic monomer) to 0.50 mmol (80.7% degree of lignin liquefaction with 28.5% yield of phenolic monomer). However, the repolymerization of the phenolic dimer and oligomer could also be promoted efficiently over acid catalyst (Zakzeski and Weckhuysen, 2011). Hence, the degree of lignin liquefaction was gradually declined when the catalyst dosage was more than 0.5 mmol (Fig. 4a). Fig 4b exhibited that the degree of lignin liquefaction was gradually increased with the increase of the Pd/C dosage. And 92.8% of lignin was liquefied when 0.3 g Pd/C was used. The yield of phenolic monomer showed similar tendency when Pd/C was less than 0.2 g. However, when the Pd/C dosage reached 0.3 g, the phenolic monomer yield was sharply decreased to 30.7%. Product distribution (Table S5) showed that both the other volatile products (exclude phenolic monomer) and the nonvolatile production have an increase. GC–MS analysis of the volatile products showed an obvious increase on the aliphatic chemicals (Table S6). Hence, the decrease of phenolic monomer could be attributed to the hydrodeoxygenation of the products. The results shown in Fig. S2 also demonstrated that the Pd/C had a good recyclability in this hydrogenolysis process. After three runs, only a slight decrease of the phenolic monomer yield was observed. ICP-AES analysis showed no Pd leaching in the solvent.

Fig. 3. Effect of (a) reaction temperature and (b) time on the lignin depolymerization and yield of phenolic monomer. Conditions: (a) lignin 0.5 g, CrCl3 0.5 mmol, Pd/ C 0.1 g, H2 4 MPa, 5 h. (b) lignin 0.5 g, CrCl3 0.5 mmol, Pd/C 0.1 g, H2 4 MPa, 280 °C.

Fig. 4. Effect of (a) CrCl3 and (b) Pd/C dosage on the lignin depolymerization and yield of phenolic monomer. Conditions: (a) lignin 0.5 g, Pd/C 0.1 g, H2 4 MPa, 280 °C, 5 h. (b) lignin 0.5 g, CrCl3 1 mmol, H2 4 MPa, 280 °C, 5 h.

3.5. Effect of Lewis acid and Pd/C dosages

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TG analysis (Fig. S3) of the fresh and recovered catalysts also showed no obvious change on the thermal degradation property of the Pd/C catalyst. However, the onset decomposition temperature of recovered catalyst was slightly decreased and less weight loss of that was also shown at 700 °C. SEM images of the fresh and recovered Pd/C (Fig. S4) showed an obvious tar formation. FT-IR spectrum (Fig. S5) of the recovered catalyst also exhibited a characteristic infrared absorption of lignin, where the characteristic absorptions of the aromatic hydroxyl (3434 cm1), methoxyl (2952 cm1) and benzene ring (1621, 1442, 1377, 1031, 869, 811 cm1) could be observed clearly (Long et al., 2014b). Therefore, the slight decrease of the catalytic activity of the Pd/C catalyst cannot be attribute to the Pd leaching, whereas, the slight tar formation on the catalyst surface was considered to be responsible for that. 3.6. Analysis of the volatile products As discussed above, the main components of volatile product are significant for lignin utilization. They can be directly converted to high quality gasoline through HDO. The lignin hydrogenolysis volatile product was therefore further investigated. Qualitative analysis by GC–MS was showed in Fig. S6 and Table S1. It could be clearly seen that the components were rather complex and the product distribution was remarkably different when ZnCl2 and CrCl3 were used as Lewis acid. For ZnCl2 cooperated with Pd/ C, the guaiacols were the major product, whereas the phenols were found to be the most abundant with CrCl3 and Pd/C. It indicated that the catalytic mechanism of ZnCl2 and CrCl3 was different. Quantitative analysis of major components contained in the phenolic monomer was carried out by GC-FID according to the added internal standard, and the results were displayed in detail in Table 2. For ZnCl2 as Lewis acid, the guaiacols were major product and the yield was up to 6.94%, including 2.95% 2-methoxy-4-propyl-phenol and 1.54% 2-methoxy-phenol. When CrCl3 was used, 7.30% yield of phenol and its derivatives was achieved. It should be noticed that the volatile aromatic chemicals from lignin hydrogenolysis were mainly composed of multiple methyl chemicals with

C8–C13. Furthermore, carboxylic acid, which is the general product from lignin thermal degradation and the main challenge for bio-oil utilization (Kaewpengkrow et al., 2014) was not detected. Therefore, the volatile products obtained from this system have a great potential for excellent and high octane value alternatives for the unsustainable gasoline. 3.7. Proposed catalytic mechanism As discussed above, both the acid catalysis which results in guaiacols, and coordination catalysis which results in phenols were existed in presence of CrCl3 cooperated with Pd/C. therefore, both the Lewis acid catalytic depolymerization and the coordination catalytic mechanism were proposed based on the detailed analysis of the lignin, the phenolic monomer and other volatile products with various catalysts (Fig. S7). It can be seen from Fig. S7a that the Lewis acid catalytic process is in good agreement with the proton catalysis. The low activity energy b-O-4 bond of the lignin molecule is cracked on the acid center of Lewis acid catalyst. Under the synergic effect of Pd/C and the weaken effect of Cl, a series of products with the native structural units were given (guaiacol and its derivatives). This mechanism accords well with most acid catalytic lignin depolymerization processes (Jia et al., 2010; Long et al., 2015; Yan et al., 2008). It was also reported that CrCl3 was an excellent catalyst for cellulose conversion to 5-hydroxymethyl furfural (HMF) (Bali et al., 2012; Tan et al., 2011). Mechanism investigation demonstrated that Cr cations could coordinate with the electron abundant element oxygen to give a complex, which further degraded to the final products HMF (Choudhary et al., 2013; Zhao et al., 2007). Based on knowledge of the above processes and the results of this study, a possible reaction pathway for the phenols generation was proposed. In this pathway (Fig. S7b), the Cr cation was first conjuncted with O atom and the electron-abundant benzene ring of lignin molecule to form a stable complex (1). Simultaneously, the high electronegativity Cl was also attached to the lignin molecule, resulting in the weakening of C–O bond. And then, under the polarization effect of Cl and the synergic effect of Pd/C, which had cap-

Table 2 The main components of the lignin-derived phenolic monomer.a

a

Component

Yieldb (%) ZnCl2

CrCl3

Phenols 1,4-Benzenediol, 2,3,5-trimethylPhenol, 2,6-dimethyl30 ,50 -Dihydroxyacetophenone Phenol, 2,4,6-trimethyl2,5-Dimethylhydroquinone Phenol, 2,3,6-trimethylPhenol, m-tert-butylPhenol, 4-(methoxymethyl)-2,6-dimethylPhenol, 2,3,5,6-tetramethylPhenol, 2,3,4,6-tetramethylPhenol, 3,4,5-trimethylPhenol, 2-(1,1-dimethylethyl)-5-methyl3-Methyl-4-isopropylphenol Phenol, 2-(1,1-dimethyl-2-propenyl)-3,6-dimethylGuaiacols Phenol, 2-methoxyPhenol, 4-methoxy-3-methylPhenol,2-Methoxy-5-methyl Ethanone, 1-(4-hydroxy-3-methoxyphenyl)Phenol, 2-methoxy-4-propyl3-tert-Butyl-4-hydroxyanisole

2.20 – – 0.12 0.28 0.14 0.12 0.22 – 0.15 0.23 0.17 0.35 0.30 – 6.94 1.54 0.26 0.85 1.34 2.95 —

7.30 0.1 0.15 0.14 0.46 0.30 0.50 0.54 0.37 0.91 0.68 0.39 0.28 1.88 0.24 2.82 0.18 0.18 0.14 0.64 0.14 1.54

Component

Yieldb (%) ZnCl2

CrCl3

Others Benzenemethanol, 4-(1,1-dimethylethyl)-.alpha.-methyl-

0.36

0.53

Benzene, 1-methoxy-4-methyl-2-(1-methylethyl)Benzene, 1-(1,1-dimethylethyl)-2-methoxy-4-methylEthanone, 1-(2,4,5-triethylphenyl)Propanal, 2-(4-ethoxyphenyl)-2-methyl4,7-Dimethoxy-2-methyl-1H-indene 2,3,3,4,7-Pentamethyl-2,3-dihydro-benzofuran Benzofuran, 2,3-dihydro-2,2,5,6-tetramethyl1,3-Benzenedicarboxylic acid,dimethyl ester

0.23 – – – 1.17 0.11 0.20 –

4.02 0.55 0.11 0.34 0.93 0.29 1.40 0.15

Condition: 0.5 g alkali lignin, 0.5 mmol Lewis acid, 0.1 g 5wt% Pd/C, 40 mL methanol, 4 MPa H2, 260 °C, 5 h. Measured by GC 2014C, where acetophenone was used as internal standard chemical. Components listed are those represented by more than 0.1% of the yield determined by GC 2014C. b

90

R. Shu et al. / Bioresource Technology 179 (2015) 84–90

tured hydrogen to promote the hydrogenolysis (Parsell et al., 2013), the cleavage of –OCH3 and b-O-4 bond occurred at the same time. With the crack of ether bonds, an oligomer (4) and a simple chemical (5) were formed. Afterwards, a series of typical processes for the intermediate (4) were occurred, such as dehydration, hydrogenation, hydrolysis and concerted reaction (Zhao et al., 2011), which would vary the product. Eventually, diverse phenols were obtained in the following processes. Therefore, the efficient regulating of the catalytic mechanism by changing metal chlorides could efficiently control the product distribution. 4. Conclusions The alkali lignin hydrogenolysis could be realized by the synergistic effect of Pd/C and CrCl3 at 280 °C for 5 h with 85.6% lignin liquefaction and 35.4% phenolic monomer. And the products could be regulated efficiently through the modification of the metal cation of MClx due to the acid catalysis and coordination catalysis mechanism. The acid catalytic procedure is preferred to produce guaiacols, whereas the coordination catalytic mechanism is more likely to result in phenols. Hence this efficient lignin depolymerization process will be a beneficial reference for the future utilization of this sustainable aromatic polymer. Acknowledgments The authors gratefully acknowledge the financial support of Natural Science Foundation of China (Nos. 51306191, 51476178), and the National Key Technology R&D Program (NO. 2014BAD02B01). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.201 4.12.021. References Bali, S., Tofanelli, M.A., Ernst, R.D., Eyring, E.M., 2012. Chromium(III) catalysts in ionic liquids for the conversion of glucose to 5-(hydroxymethyl)furfural (HMF): insight into metal catalyst: ionic liquid mediated conversion of cellulosic biomass to biofuels and chemicals. Biomass Bioenergy 42, 224–227. Barta, K., Matson, T.D., Fettig, M.L., Scott, S.L., Iretskii, A.V., Ford, P.C., 2010. Catalytic disassembly of an organosolv lignin via hydrogen transfer from supercritical methanol. Green Chem. 12, 1640–1647. Choudhary, V., Mushrif, S.H., Ho, C., Anderko, A., Nikolakis, V., Marinkovic, N.S., Frenkel, A.I., Sandler, S.I., Vlachos, D.G., 2013. Insights into the interplay of Lewis and Bronsted acid catalysts in glucose and fructose conversion to 5(Hydroxymethyl)furfural and levulinic acid in aqueous media. J. Am. Chem. Soc. 135, 3997–4006. Deepa, A.K., Dhepe, P.L., 2014. Solid acid catalyzed depolymerization of lignin into value added aromatic monomers. RSC Adv. 4, 12625–12629. Gary, L.M., Paul, J.F., Donald, A.T., 2014. Inorganic Chemistry, fifth ed. Pearson Education, New York. Guell, A.J., Li, C.Z., Herod, A.A., Stokes, B.J., Hancock, P., Kandiyot, R., 1993. Effect of H2-pressure on the structures of bio-oils from the mild hydropyrolysis of biomass. Biomass Bioenergy 5, 155–171. Ibarra, D., del Rio, J.C., Gutierrez, A., Rodriguez, I.M., Romero, J., Martinez, M.J., Martinez, A.T., 2005. Chemical characterization of residual lignins from eucalypt paper pulps. J. Anal. Appl. Pyrol. 74, 116–122.

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