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Catalytic Depolymerization of Organosolv Lignin in a Novel Water/ Oil Emulsion Reactor: Lignin as the Self-Surfactant Zhenping Cai,† Yingwen Li,† Hongyan He,‡ Qiang Zeng,† Jinxing Long,*,† Lefu Wang,† and Xuehui Li*,† †

School of Chemistry and Chemical Engineering, Pulp & Paper Engineering State Key Laboratory of China, South China University of Technology, Guangzhou, 510640, China ‡ Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China S Supporting Information *

ABSTRACT: A novel and efficient water/oil emulsion reactor for organosolv lignin depolymerization is presented using an ionic liquid catalyst based on the self-surfactivity of lignin. The physical−chemical properties of the emulsion reactor and lignin were intensively studied using optical photo, dynamic light scattering, surface tension measurement, and hydrophile−lipophile balance value determination. The results show that the emulsion reactor demonstrates a more significant process intensification effect on lignin depolymerization, with more than 29.60 mg g−1 desired phenolic compounds obtained, which is about 3.3 times higher than that from a reactor without emulsification. Another advantage of this water/oil emulsion reactor is that both the organic solvent (n-butanol) and the ionic liquid catalyst can be recycled easily, as the depletion of lignin surfactant at the end of depolymerization can result in the phase partition and the enrichment of final products in the oil phase automatically.

1. INTRODUCTION Lignin, the most abundant natural aromatic polymer on the globe,1 is regarded as a promising raw material for the sustainable biofuel and aromatic chemical production.2,3 However, most of lignin in fact is directly burned as a low value fuel and only less than 2% of it is commercially used due to the difficulty in breaking its complicated and disordered chemical bond connections, resulting in significant energy and resource waste.4 Therefore, efficient technologies for lignin valorization are crucially important and imperative. Various processes with conventional heterogeneous or homogeneous catalysts were widely explored for this issue in recent years.5−15 For example, lignin could be efficiently depolymerized over the base catalyst of CuMgAlOx, by which more than 23% of phenolic monomer yield was obtained.11 Recent investigation also showed that an increased yield of volatile aromatic chemicals with a declination of the oligomer could be obtained under the synergic effect between a base catalyst and solvent.12,13 Furthermore, solid acid was used for the lignin hydrolysis, in which 60% organic solvent soluble products could be obtained.14 Moreover, the NiM (M = Ru, Rh, Pd and Au) bimetallic catalysts, which showed a significant advantage over the Ni catalyst and can be further promoted by base solution, were found to be efficient for lignin hydrogenolysis as well, in which more than 56% of lignin conversion could be obtained under the optimized condition.15,16 However, these heterogeneous catalysts are limited to contact with lignin and access the targeting bonds.7 A homogeneous process using proton acid and base was also utilized for the lignin depolymerization.17 Unfortunately, it generally suffers a labor-intensive posttreatment. Ionic liquids (ILs), which have advantages for both heterogeneous and homogeneous catalysts and show excellent lignin dissolution capability, have been used for the lignin valorization as well.18,19 However, it has been a great © XXXX American Chemical Society

challenge for ILs in a industrialization process due to difficulties in product separating and catalyst recycling. Emulsion, a mixture of water, oil, and surfactant, overcomes the solubility and incompatibility problems of the polar and nonpolar reactants.20 Hydrophilic and hydrophobic compounds can dissolve, respectively, in aqueous and oil regions of an emulsion and accumulate at the oil−water interface.21 Hence, this interface is highly reactive toward the two otherwise incompatible reagents and enhances the reaction rate and selectivity due to the large interface area.21 Lignin, containing both lipophilic and hydrophilic groups, is a kind of surfactant in nature and has been widely used as a precursor for the preparation of novel surfactants as well.22,23 Based on the knowledge of the excellent surfactant property of lignin, we propose here a novel water/oil (W/O) emulsion reactor for lignin depolymerization using lignin as a selfsurfactant, IL aqueous as catalyst, and n-butanol/n-hexane as oil phase in a batch reactor after careful screening (Scheme 1). During the reaction process, lignin could stabilize the emulsion and be degraded simultanously at the W/O interface. Namely, the hydrophilic α-O-4 and β-O-4 connecting points in lignin are thus led to be exposed to the oil−water interface, resulting in more significant contact with IL catalyst and substantial promotion of lignin depolymerization. On the other hand, this system provides superiority in the demulsification process, giving an automatic phase partition at the end of the reaction due to the degradation and condensation of lignin. Two distinct phases are observed after reaction, where the depolymerization products are dissolved in the upper phase of n-butanol dispersion while IL catalyst exists in the lower phase of water. Received: September 2, 2015 Revised: November 3, 2015 Accepted: November 6, 2015

A

DOI: 10.1021/acs.iecr.5b03247 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Proposed Emulsion Reactor and W/O Interfacea

products were determined acoording to the reported procedure.26 Generally, 0.2 g of bagasse lignin was dissolved in 20 mL of dioxane−benzene solution (v/v = 90:4). Then the mixture was submitted for HLB value determination using water titration (eq 1). The solubility of bagasse lignin was measured by a Shimadzu UV-2450 spectrophotometer with extra standard method. The calibration curve was obtained according to the dissolution of lignin in ethanol. HLB value = 57.91 log W − 58.55

(1)

2.3. Lignin Depolymerization and Product Separation. Scheme 2 shows the whole procedure of organosolv Scheme 2. Procedure for Lignin Depolymerization in the Emulsion Reactor and the Product Separation

a

(a) Emulsion reactor; (b) after reaction.

Thus, efficient product separation and catalyst recycling can be simultaneously achieved through a one-step separatory filtration procedure, overcoming the intrinsic problems about the separation of catalyst in a traditional reactor or the demulsification process applied for most emulsion reactors. Finally, n-butanol is also recyclable after distillation, which significantly promotes the economic performance of the lignin depolymerization process.

2. EXPERIMENTAL SECTION 2.1. Materials. 1,4-Butane sultone and tetrahydrofuran (THF; HPLC grade) were purchased from Acros (Belgium). N-Alkylimidazole was purchased from Lanzhou Greenchem ILS, LICP, CAS (Lanzhou, China) and used as received. Other reagents (analytical grade) were supplied by Guanghua Chemical Factory Co. Ltd. (Shantou, Guangdong, China). Acid ILs were synthesized according to the reported literature24 and characterized by 1H nuclear magnetic resonance (NMR), 13 C NMR (Bruker AV-400 spectrometer, where D2O was used as solvent), and Fourier transform infrared spectroscopy (FTIR; a Bruker Tensor 27 FT-IR spectrophotometer with KBr disks in the range 400−4000 cm−1). The results are shown in the Supporting Information. Organosolv lignin was obtained according to our previous study.25 FT-IR and 1H NMR characterizations show that the purity of lignin is more than 97%. There was also no sugar detected by high performance liquid chromatography (HPLC) when lignin was hydrolyzed with the catalysis of 5 wt % H2SO4 at 423 K for 60 min. 2.2. Emulsion Characterization. The size distribution of the solvent with bagasse lignin was evaluated with dynamic light scattering (Zetasizer Nano, Malvern). Before measurement, 3.0 mmol of butyl-3-(butyl-4-sulfonate) imidazolium hydrogen sulfate (BSbimHSO4), 0.5 g of bagasse lignin, 2.0 mL of nhexane, 20 mL of water, and 20 mL of n-butanol were added into a 100 mL flask and vigorously stirred at 303 K for 2 h. Three parallel measurements were performed for each sample at 298 K. The images of emulsion were obtained by a microscope (OLYMPUS BX53). The surface tension of the nbutanol solution with different lignin and depolymerization product concentrations was measured on an automatic surface tensiometer (Data Physics oca40) at 298 K. The hydrophile− lipophile balance (HLB) values of lignin and depolymerization

lignin depolymerization in a emulsion reactor and the product separation. In general, 0.5 g of lignin, 20 mL of water, 20 mL of n-butanol, 2 mL of n-hexane, and 3.0 mmol of acidic IL were charged into a 100 mL stainless autoclave (316 L stainless, Tongda Chemical Machinery Co., Ltd. Liaoning, China). The reactor was purged with nitrogen three times, and then it was heated from room temperature to the designated temperature and maintained for the desired time. When the reaction was finished, the reactor was moved from the heating furnace and cooled in air to room temperature. After removal of products, the reactor was washed with n-butanol three times (10 mL × 3). Triplicate experiments were conducted for the reducing of the error. The solid residue was filtered and washed by n-butanol and ethanol, respectively. Then it was dried at 333 K under vacuum until no significant weight loss was detected. The filtrate was separated and the n-butanol phase was washed by water (10 mL × 3) to remove the IL catalyst residual. The water phase was collected. After removal of excess water by reduced pressure distillation, the soltuion containing IL catalyst was reused for the next reaction directly. The n-butanol phase was diluted to a fixed volume of 100 mL. Then it was sperated to two sections, one for qualitative and quantitative analysis, and another for FT-IR and molecular weight distribution analysis after the removal of solvent (n-butanol) under reduced pressure. 2.4. Product Analysis. The degree of lignin depolymerization was measured by weight comparison of the solid residue and the feed lignin according to eq 2. W − WS degree of liquefaction (%) = F ·100 WF (2) B

DOI: 10.1021/acs.iecr.5b03247 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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interface appears when the mixture is heated at 523 K for 30 min (Figure 1f). It can be ascribed to the depolymerization of the lignin and the decrease of the surfactivity of lignin. After reaction, lignin cracking products (phenolic monomer and oligomer) were extracted by the n-butanol (oil phase), so a black solution is obtained (upper phase), whereas the water phase enriched the IL catalyst which can be used directly for the next run. Thus, an efficient and enhanced process for lignin depolymerization and IL recycling could be achieved (Figure 1f). We also examined the role of the co-oil reagent n-hexane. As shown in Figure 1g,h, n-hexane, the conventionally used oil in the emulsion process,27 is mainly responsible for the improved stability of the emulsion system through the increase of the lipophilicity; for example, a slightly demulsified solution was demonstrated after 24 h in the absence of n-hexane (Figure 1h). 3.2. Particle Size Distribution and Optical Image of the Emulsion Reactor. To gain more insight into the emulsion behavior, dynamic light scattering was used to investigate the size distribution of the emulsion (Figure 2).

The phenolic monomers were identified by gas chromatography−mass spectrometer (GC−MS; Agilent 5977A, GC−MS apparatus with a capillary column, HP-5 MS 5% phenyl methyl silox; 30 m × 250 μm × 0.25 μm) based on an Agilent MS library. The initial oven temperature was 323 K (hold for 3 min), and then it was programmed to 553 K at the rate of 8 K min−1 and held for another 5 min. The quantitative analysis of these volatile products was conducted on an Aglient 7890B gas chromatograph with a flame ionization detector (GC-FID). The same chromatography column and temperature program as the GC−MS analysis was used. The content of the compound was measured by extra standard method with the commercial compound (purity >99%) as the standard (eq 3). yield of phenolic monomer (mg g −1) =

WP WF

(3)

In eqs 2 and 3, WF is the weight of feed lignin (g), WS is the weight of solid residue (g), and WP is the weight of phenolic monomer, which was determined by GC-FID (mg). The molecular weight distributions of original lignins, depolymerization products, and solid residues were measured by gel permeation chromatography (GPC) analysis on an Agilent 1260 HPLC apparatus with a refractive index detector (RID). THF was used as the eluent with a flow rate of 1 mL min−1. Polystyrene was used as the standard compound for the molecular weight calibration curve. FT-IR spectra of bagasse lignin, depolymerization products, and solid residues were obtained in KBr disks by use of a Bruker Tensor 27 FT-IR spectrophotometer in the range 400−4000 cm−1.

3. RESULTS AND DISCUSSION 3.1. Self-Surfactivity of Lignin. Figure 1 shows the selfsurfactivity of lignin in this novel emulsion reactor. A distinct

Figure 2. Particle size distribution of the mixtures after stirring for 2 h: (a) 20 mL of [email protected] g of [email protected] mmol of BSbimHSO4; (b) addition of 20 mL of H2O to sample a; (c) addition of 2 mL of nhexane to sample b.

Figure 2a shows that the lignin particle in n-butanol accords well with normal distribution with an average size of 1108 nm. This apparent average particle size is substantially increased when an emulsion is formed. For example, the average size increases to 1579 nm when water is added (Figure 2b), and further addition of n-hexane results in an abnormal size distribution (from 3091 to 5560 nm). This particle size change could be ascribed to the formation of emulsion (Figure 1d) as much more lignin is gathered in the spherical wall of emulsion droplets (which can be observed by the dynamic optical images). Optical photographs (Figure 3) clearly illustrate the emulsion formation and the increase of the apparent particle size. Figure 3a demonstrates that the dissolution of lignin in nbutanol generates a homogeneous solution and the suspension of some large lignin particles. The addition of water resulting in the formation of primary emulsion system deduces the increase of the particle size (Figure 3b). As shown in Figure 3c, the image of this system showed obvious emulsion behavior where the distinct interface of oil and water could be observed. It is considered that further addition of n-hexane favors formation of

Figure 1. Images of (a) mixture of BSbimHSO4 (3.0 mmol)@water (20 mL)@n-hexane (2 mL)@n-butanol (20 mL), (b) vigorous stirring for 2 h of (a), (c) added 0.50 g of bagasse lignin to (b), (d) vigorous stirring for 2 h of (c), (e) standing for 24 h of (d), (f) after treatment of (e) at 523 K for 30 min, (g) without n-hexane in (d), and (h) standing for 24 h of (g).

water−oil interface can be observed when the IL solution is mixed with n-butanol and n-hexane (Figure 1a). After fast stirring for 2 h, no obvious change of this solution is observed in both the appearance and the volume of each phase (Figure 1b). When lignin is added, the upper oil phase becomes brown due to the dissolution of lignin (Figure 1c). However, a homogeneous solution is formed from visual observation after stirring (Figure 1d), which is stable even after standing for 24 h (Figure 1e). This clearly illustrates that the self-surfactivity of the lignin is sufficient for the formation of a stable emulsion system containing IL aqueous, n-butanol, and n-hexane. Interestingly, the demulsification occurs and a new two-phase C

DOI: 10.1021/acs.iecr.5b03247 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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butanol is sharply decreased with the increase of lignin dosage when the lignin concentration is less than 1.0 wt %. However, with the continuous increase of the lignin amount, the change of the surface tension is negligible. This suggests that the emulsion is formed when 1.0 wt % lignin is presented, namely, the critical micelle concentration (cmc) is 1.0 wt % lignin, which can also be observed from Figure S1 (the isotherms of surface tension against log X, where X is the concentration of lignin or the depolymerization product). After depolymerization, the self-surfactivity of lignin decreased, resulting in automatic phase separation (Figure 1). Therefore, an obvious difference is shown for the surface tension (Figure 4 and Figure S1). Compared to the solvent with original lignin, a higher surface tension is demonstrated for this fraction. Furthermore, the change of surface tension is insubstantial with the increase of the aromatic product concentration, because it does not tend to form emulsion (Figure 1f). As reported previously,28 the stability of the W/O emulsion significantly depends on the HLB value of the surfactant, where a larger HLB value generally indicates the weakening of the emulsion stability. In this reactor, the lignin is gradually degraded to phenolic products, which has more hydrophilic groups than the original lignin. The HLB value of the lignin thereby increases from 13.1 to 16.5 after reaction (Table S1), which further confirms the demulsification by the lignin depolymerization (Figure 1f). 3.4. Enhancement Effect of the Emulsion Reactor on the Lignin Dissolution. As shown in Table 1, emulsion has an obvious promotion effect on the lignin solubility. The organosolv lignin is undissolvable in water/n-hexane. By contrast, its solubility in n-butanol/n-hexane is determined as 19.0 g L−1. With the addition of IL, a slight increase of the lignin solubility is observed. However, when water is added into the emulsion, a significantly higher solubility of 37.3 g L−1 can be obtained. According to the reported literature,29 emulsion favors the immiscibility-lignin dissolution. Therefore, the increase of lignin solubility can be ascribed to the enhancement effect of the emulsion. This improvement of lignin solubility could also be confirmed by the optical image (Figure 3c) where obviously lignin gathered in the interface between oil and water

Figure 3. Optical images of the mixture solvents with lignin after stirring for 2 h: (a) 20 mL of [email protected] g of [email protected] mmol of BSbimHSO4; (b) addition of 20 mL of H2O to sample a; (c) addition of 2 mL of n-hexane to sample b.

uniform emulsion droplets with continuous and perfect spherical walls; this also improves the stability of the emulsion. 3.3. Surface Tension and HLB Value. We also measured the surface tension of the solvents with various lignin addition dosages and the HLB value of lignin and its depolymerization product. As shown in Figure 4, the surface tension of the n-

Figure 4. Isotherms of surface tension of n-butanol plotted against concentration of bagasse lignin and depolymerization products.

Table 1. Solubility of Bagasse Lignin in the Emulsion Reactora

a

Conditions: IL dosage 3.0 mmol, n-butanol 20 mL, n-hexane 2 mL, 20 mL of H2O, 303 K, and 6 h. D

DOI: 10.1021/acs.iecr.5b03247 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 2. Depolymerization of Bagasse Lignin in the W/O Emulsion Reactora

a

Conditions: 0.5 g of bagasse lignin, 20 mL of n-butanol, 20 mL of H2O, 2 mL of n-hexane, 3.0 mmol of IL catalysts, 523 K, and 30 min. bDLDP: degree of lignin depolymerization; c−, not added or not detected.

Scheme 3. Lignin Depolymerization in Emulsion Reactor

competition process between depolymerization and repolymerization occurred during the lignin conversion, resulting in phenolic monomer and solid product, respectively. Therefore, both the weight of solid residue (degree of depolymerization, eq 2) and the volatile chemicals were measured. GC−MS analysis demonstrates that the depolymerization in this emulsion reactor is highly selective with comparison to the most current lignin depolymerization processes.30−32 Phenol, 4ethylphenol, 4-ethyl-2-methoxyphenol, and 2,6-dimethoxylphenol are found to be the most abundant components of the volatile product, indicating that all of the structural units of lignin (hydroxyphenyl (H), guaiacyl (G), and syringyl (S)) are degraded. Compared to the lignin raw material, the components of solid residue (features monitored by FT-IR spectra, Figure S3) show no significant differences. This indicates that the solid

can be observed. However, it should be noticed that the lignin solubility is negligibly influenced by the IL species (Table 1), suggesting that IL catalyst is not a key factor for emulsion formation. Generally, first lignin with small particle size is dissolved, and then the larger one. Therefore, with the increase of the lignin solubility, the particle size of the solvent is increased, showing good agreement with the above result of dynamic light scattering (Figure 2). 3.5. Process Intensification Effect of Emulsion on Lignin Depolymerization. Table 2 summarizes the depolymerization of bagasse lignin and the yield of four desired phenolic monomers in the emulsion reactor (Scheme 3). GC− MS analysis shows that almost no phenolic product existed in the water-soluble fraction (Figure S2). Therefore, the yield of the phenolic monomer in this fraction is not taken into the account. According to our previous work,12,13,19 a simultaneous E

DOI: 10.1021/acs.iecr.5b03247 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. Effect of process parameters on lignin depolymerization: (a) n-butanol dosage; (b) n-hexane dosage; (c) BSbimHSO4 dosage; (d) reaction temperature; (e) reaction time; (f) IL recyclability. 1, Phenol; 2, 4-ethylphenol; 3, 4-ethyl-2-methoxyphenol; 4, 2,6-dimethoxyphenol.

accelerating effect of the emulsion33 is considered to be responsible for this observation. As shown in Figure 1, n-hexane is beneficial for the emulsion stability. Therefore, a slight increase of depolymerization and decrease of repolymerization are shown (Table 2, entries 2−6). Table 2 also demonstrates that acid IL is crucial for lignin degradation in this emulsion, where a significant increase of the phenolic monomer yield is shown in the presence of IL catalyst (Table 2, entries 6−9). Generally, the acid strength of the IL catalyst is positive for its catalytic activity on the biomass depolymerization.19,34 Interestingly, the effect of IL is varied in the emulsion reactor. For example, the performance of lignin depolymerization is not

residue is mainly composed of the repolymerization products rather than char, the conventional byproduct during lignin conversion.4 As shown in Table 2, the lignin depolymerization hardly occurred in the absence of catalyst and without the formation of the emulsion reactor (Table 2, entry 1). IL catalyst favors the lignin depolymerization;19 therefore, both the degree of depolymerization and the yield of the volatile product are enhanced (Table 2, entry 2). However, when the lignin is depolymerized in the emulsion reactor, the yield of the desired volatile phenolic monomer sharply increases to 21.04 mg g−1 with 88.8% depolymerization degree (Table 2, entry 4). The F

DOI: 10.1021/acs.iecr.5b03247 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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the reaction temperature has a significant effect on the lignin depolymerization. Increased temperature results in the increase of depolymerization performance when it is less than 523 K. The slight declination of the desired phenolic monomers at elevated temperature could be ascribed to the repolymerization of the unsaturated chemicals according to the previous work.19 This repolymerization is more obvious at the prolonged reaction time when the reaction is carried out at 523 K. For example, 90.1% of lignin is liquefied in 45 min. However, it decreases to 85.6% at 60 min (Figure 5e). The recyclability of the IL catalyst was also examined. As discussed above (Figure 1), an automatic phase separation occurred due to the demulsification caused by the lignin degradation and temperature-induced phase separation. At the end of reaction, the depolymerization products are dissolved in the upper oil layer and the IL catalyst exists in the water layer. Hence, the lower layer of water solution containing IL catalyst can be reused after a very simple filtration process for the removal of solid residue and distillation of the excess water. In addition, the results shown in the Figure 5f demonstrate that the IL catalyst shows good reusability in this emulsion reactor, where no significant loss of the catalytic activity is observed even after five recycle runs. 3.6. Analysis of the Lignin Depolymerization Product. As discussed above (Table 2), GC−MS is used to identify the volatile products generated by the lignin depolymerization process (Table S2). To give a more clear understanding of the lignin depolymerization, GPC analysis is further applied to this investigation by monitoring the molecular weight change. It can be seen that the molecular weight distributions of the depolymerization product without catalyst and water are the most similar to that of raw lignin material (Figure 6a,b), where only a slight decrease of the molecular weight and increase of the dispersity are observed. This indicates insignificant lignin depolymerization without the emulsion reactor, and explains well the small yield of the desired products as listed in Table 2.

significantly affected by the acidic strength of IL catalysts (Table 2, entries 6−9), where IL with longer side chains results in weaker acidity.34 For example, 29.60 mg g−1 of the identified phenolic monomer with more than 89.1% depolymerization degree is achieved by the weakest acidic IL, BSbimHSO4, in this work (Table 2, entry 6). By contrast, the strongest acid IL, BSmimHSO4, only gives 19.22 mg g−1 of the phenolic monomers and 86.1% depolymerization degree (Table 2, entry 9). This indicates that the accelerated effect of the emulfication is more significant than that of IL acid strength. According to the reported literature,35 the hydrophilicity of IL decreases with the side-chain extension. In this emulsion reactor, the polar IL is dissolved in water. With the increase of the alkyl side chain of the cation, it has a tendency to accumulate at the interface, resulting in the acceleration of the reaction rate and the increase of selectivity. Furthermore, the results shown in Table 2 demonstrate that the lignin depolymerization in the emulsion is highly selective, where more than 50% of the phenolic monomers are phenol and 4ethylphenol. This suggests that H lignin is more flexible for degradation in the emulsion reactor, partially due to the excellent water reducing property of this fraction.22,23 The process intensification effect of the emulsion reactor is further investigated by the lignin depolymerization in the conventional alcohol solvents such as ethanol and methanol. As desired, the depolymerization of the lignin is less significant, where only 73.7 and 63.0% of it is liquefied in ethanol and methanol, respectively. The analysis of volatile products using GC−MS (Table S2) demonstrates that the lignin depolymerization processes in these solvents are complicated, where more than 15 kinds of the products could be detected. However, the yield of typical products, e.g., is far lower than that in the emulsion of the n-butanol, water, and n-hexane mixture (Table 2, entries 10 and 11). The low carbon alcohols such as ethanol and methanol are dissolvable in water, so the emulsion reactor cannot be formed. Therefore, a normal lignin depolymerization in the solvent of alcohol/water as shown in previous work2,3,19 occurred, resulting in serious char formation and less product selectivity. We also investigated the effect of the reaction conditions on the competition process of lignin depolymerization and repolymerization. It can be seen from Figure 5a that this lignin depolymerization process is substantially affected by the nbutanol/water ratio. The presence of a much higher or lower volume of n-butanol results in difficulty in the emulsion formation and, thus, leads to a low degree of depolymerization (Figure 5a). As discussed above, n-hexane is crucial for emulsion stability (Figure 3); therefore, the lignin depolymerization is improved at the appropriate n-hexane dosage (here, it may be 2.0 mL). However, n-hexane also increases the lipophilicity, which thickens the oil/water interface and hampers the efficient contact of lignin and IL catalyst. Thus, both the lignin depolymerization and the yield of desired phenolic monomer are decreased (Figure 5b) with excessive nhexane. The effect of catalyst dosage was also investigated. It can be seen from Figure 5c that a slight increase of the degree of depolymerization and phenolic monomer yield is demonstrated with the increase of catalyst dosage (Figure 5c). However, it should be noted that the influence of the IL catalyst dosage is less significant, compared with that of the oil/water ratio. It further confirms that the acceleration effect of the emulsion is the main reason for the promotion of lignin depolymerization (Table 2). Figure 5d also demonstrates that

Figure 6. GPC curves of (a) original bagasse lignin and depolymerization products from (b) lignin reacted in n-butanol only, (c) without water and n-hexane, (d) without n-hexane, (e) BSbimHSO4, (f) BSpimHSO4, (g) BSeimHSO4, and (h) BSmimHSO4. G

DOI: 10.1021/acs.iecr.5b03247 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research In the presence of IL catalyst and n-butanol, the depolymerization product has a weight-average molecular weight (Mw) and a number-average molecular weight (Mn) of 735 and 268 g mol−1, respectively (Figure 6c, Table 3, entry 3). These values Table 3. Molecular Weight of Bagasse Lignin and Depolymerization Products molecular weighta(g mol−1) entry

sampleb

Mw

Mn

Dc

1 2 3 4 5 6 7 8

a b c d e f g h

1395 1191 735 822 768 993 934 1054

528 395 268 343 300 392 397 436

2.64 3.01 2.74 2.46 2.56 2.53 2.35 2.42

a

Molecular mass of bagasse lignin and depolymerization products. Sample a, original bagasse lignin and depolymerization products from sample b, lignin reacted in n-butanol only; c, without water and hexane; d, without hexane; e, BSbimHSO4, f, BSpimHSO4; g, BSeimHSO4; and h, BSmimHSO4. cD = Mw/Mn. b

Figure 7. FT-IR spectra of (a) original bagasse lignin and depolymerization products from (b) lignin reacted in n-butanol only, (c) without water and n-hexane, (d) without n-hexane, (e) BSbimHSO4, (f) BSpimHSO4, (g) BSeimHSO4, and (h) BSmimHSO4.

are significantly lower than those obtained from the process without catalyst of 1191 and 395 g mol−1 (Table 3, entry 2) due to the degradation of lignin in the emulsion reactor. By comparison of the GPC curves in Figure 6, intensive degradation is observed based on the generated lower molecular weight products (both Mw and Mn) when n-hexane and catalysts are added. It suggests better reaction performance of the emulsion reactor. Figure 6e−h shows the molecular weight distributions of the lignin depolymerization products over various IL catalysts. By increasing the number of carbons in alkyl substituents, the molecular weight of the depolymerization product becomes lower, suggesting a more thorough degradation and according well with the results listed in Table 2. We also examined the molecular weight distribution of the solid residue. As shown in Figure S4 and Table S3, a significant increase of the average molecular weight (Mw and Mn) is demonstrated in comparison with the original lignin. This suggests the simultaneous condensation of the lignin in the emulsion. However, it should be noticed that the solid residue is soluble in THF, which indicates that the solid residue consists of the repolymerization products rather than char. This conclusion is in line with the results of the above FT-IR spectra (Figure S3). To elucidate the structure evolution of degraded lignin, FTIR spectra of the original lignin and the depolymerization products from various conditions were recorded (Figure 7, Table S4). In the FT-IR spectra, the absorptions at 3422 and 2934 cm−1 are ascribed to O−H stretching and C−H stretching, respectively.36 Aromatic skeletal vibrations, a characteristic absorption of lignin, are assigned at 1609, 1511, 1422, and 834 cm−1. As shown in Figure 7, the difference of these features between the original bagasse lignin and the low molecular weight aromatic chemicals is insignificant, suggesting the reservation of the aromatic structure of lignin in the depolymerization product. However, the band at 1115 cm−1 assigned to the characteristic absorption of H lignin36 was substantially weakened when the lignin was depolymerized in the emulsion reactor. This indicates the efficient degradation of H lignin, resulting in more products of phenol and 4-

ethylphenol (Table 2). The stretch of C−O for primary alcohol at 1037 cm−137 is intensified with the lignin depolymerization, implying the formation of C−O bond in this process. 3.7. Catalytic Depolymerization of the Herbaceous Lignin in the Emulsion Reactor. The depolymerization of other grass lignins, such as tapioca lignin, rice straw lignin, corn stalk lignin, and corncob lignin, was also investigated in the emulsion reactor under the optimized conditions. The results listed in Table 4 demonstrate that all grass lignins can be efficiently degraded in this emulsion reactor with more than 89.1% degree of depolymerization. However, the lignin species has a substantial effect on its depolymerization. For example, corn stalk and corncob lignin show higher yields of the desired phenolic chemical (42.06 and 47.52 mg g−1, respectively), whereas tapioca lignin is found to be the most recalcitrant in this process. We consider that the inner structure of the lignin from various species is probably responsible for the different depolymerization performances. The profound reason for this difference needs further investigation. Furthermore, Table 4 illustrates that the product from the degradation of the H lignin (phenol and 4-ethylphenol) is the most abundant component of the volatile product in each run (for example, more than 82.2% selectivity for corncob lignin), which further confirms that H lignin is the most flexible fraction for depolymerization in the emulsion. GPC results (Figure 8) indicate that the depolymerization product shows a significant decrease of the molecular weight for each lignin. Especially, as for tapioca, corn stalk, and corncob lignin, two obvious shoulder peaks are shown, which could be ascribed to the phenolic oligomer and dimer based on the average molecular weight of this fraction and that of the structural unit of lignin. It suggests the high efficiency of the emulsion reactor for lignin depolymerization.

4. CONCLUSION Summarily, a novel and effective W/O emulsion reactor for lignin depolymerization is explored using lignin as selfsurfactant for the first time. This emulsion reactor shows a H

DOI: 10.1021/acs.iecr.5b03247 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 4. Depolymerization of Various Lignins in This Emulsion Reactora

a Conditions: 0.5 g of lignin, 20 mL of n-butanol, 20 mL of H2O, 2 mL of n-hexane, 3.0 mmol of BSbimHSO4, 523 K, and 30 min bDLDP: degree of lignin depolymerization.

Figure 8. GPC analysis of depolymerization products from various lignin species.

substantial process intensification effect on lignin depolymerization, where lignin can be depolymerized at the interface of the water/n-butanol phase. Under the optimized conditions, more than 89.1% of lignin could be converted and the yield of four desired phenolic monomers from bagasse lignin could reach up to 29.60 mg g−1 (which was far higher (3.3 times) than those without emulsion) with 53.9% 4-ethylphenol selectivity. Furthermore, this emulsion reactor is also efficient for other typical grass lignins. Moreover, this emulsion reactor shows a novel characteristic for the separation of IL catalyst and the products as the demulsification process occurs at the end of the depolymerization: the IL catalyst enriched in the water phase can be reused directly. Thus, the results and process in this study have a great potential for the industrial utilization of lignin and IL due to its substantial process intensification effect, simple technology, ready product separation, and easy catalyst reusability.

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molecular weight of solid residue; Table S4, assignment of FT-IR spectra (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 0086 20 8711 4707. Fax: 0086 20 8711 4707 (X. Li). *E-mail: [email protected]. Tel.: 0086 20 8711 4707. Fax: 0086 20 8711 4707 (J. Long). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the Natural Science Foundation of China (No. 21336002, 21276094, 51306191, and 21406230) and the Natural Science Foundation of Guangdong Province, China (No. 2015A030311048).

ASSOCIATED CONTENT

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03247. Results of 1H NMR, 13C NMR, and FT-IR analyses of ILs; Figure S1, isotherms of surface tension against log X; Figure S2, GC−MS analysis of n-butanol and water volatile products; Figures S3 and S4, FT-IR and GPC results of solid residue, respectively; Table S1, HLB value of bagasse lignin and depolymerized products; Table S2, GC−MS results of volatile products; Table S3, average

REFERENCES

(1) Heitner, C.; Dimmel, D. R.; Schmidt, J. A. Lignin and Lignans: Advances in Chemistry; CRC Press: Boca Raton, FL, 2010. (2) Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski, T. J.; Tuskan, G. A.; Wyman, C. E. Lignin valorization: improving lignin processing in the biorefinery. Science 2014, 344, 1246843. (3) Zhang, X. H.; Zhang, Q.; Wang, T. J.; Ma, L. L.; Yu, Y. X.; Chen, L. G. Hydrodeoxygenation of lignin-derived phenolic compounds to

I

DOI: 10.1021/acs.iecr.5b03247 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research hydrocarbons over Ni/SiO2-ZrO2 catalysts. Bioresour. Technol. 2013, 134, 73. (4) Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen, B. M. The catalytic valorization of lignin for the production of renewable chemicals. Chem. Rev. 2010, 110, 3552. (5) Bouxin, F. P.; McVeigh, A.; Tran, F.; Westwood, N. J.; Jarvis, M. C.; Jackson, S. D. Catalytic depolymerisation of isolated lignins to fine chemicals using a Pt/alumina catalyst: part 1- impact of the lignin structure. Green Chem. 2015, 17, 1235. (6) Strassberger, Z.; Prinsen, P.; Klis, F. v. d.; Es, D. S. v.; Tanase, S.; Rothenberg, G. Lignin solubilisation and gentle fractionation in liquid ammonia. Green Chem. 2015, 17, 325. (7) Hanson, S. K.; Wu, R. L.; Silks, L. A. P. C-C or C-O bond cleavage in a phenolic lignin model compound: selectivity depends on vanadium catalyst. Angew. Chem., Int. Ed. 2012, 51, 3410. (8) Liu, S. W.; Zhang, C.; Li, L.; Yu, S. T.; Xie, C. X.; Liu, F. S.; Song, Z. Q. Application of dissociation extraction in oxidation dedradation reaction of lignin. Ind. Eng. Chem. Res. 2014, 53, 19370. (9) Gao, D. N.; Schweitzer, C.; Hwang, H. T.; Varma, A. Conversion of guaiacol on noble metal catalysts: reaction performance and deactivation studies. Ind. Eng. Chem. Res. 2014, 53, 18658. (10) Zhang, X. H.; Wang, T. J.; Ma, L. L.; Zhang, Q.; Jiang, T. Hydrotreatment of bio-oil over Ni-based catalyst. Bioresour. Technol. 2013, 127, 306. (11) Huang, X.; Korányi, T. I.; Boot, M. D.; Hensen, E. J. M. Catalytic depolymerization of lignin in supercritical ethanol. ChemSusChem 2014, 7, 2276. (12) Long, J. X.; Xu, Y.; Wang, T. J.; Yuan, Z. Q.; Shu, R. Y.; Zhang, Q.; Ma, L. L. Efficient base-catalyzed decomposition and in situ hydrogenolysis process for lignin depolymerization and char elimination. Appl. Energy 2015, 141, 70. (13) Long, J. X.; Zhang, Q.; Wang, T. J.; Zhang, X. H.; Xu, Y.; Ma, L. L. An efficient and economical process for lignin depolymerization in biomass-derived solvent tetrahydrofuran. Bioresour. Technol. 2014, 154, 10. (14) Deepa, A. K.; Dhepe, P. L. Lignin depolymerization into aromatic monomers over solid acid catalysts. ACS Catal. 2015, 5, 365. (15) Zhang, J. G.; Teo, J.; Chen, X.; Asakura, H.; Tanaka, T.; Teramura, K.; Yan, N. A series of NiM (M= Ru, Rh, and Pd) bimetallic catalysts for effective lignin hydrogenolysis in water. ACS Catal. 2014, 4, 1574. (16) Konnerth, H.; Zhang, J. G.; Ma, D.; Prechtl, M. H. G.; Yan, N. Base promoted hydrogenolysis of lignin model compounds and organosolv lignin over metal catalysts in water. Chem. Eng. Sci. 2015, 123, 155. (17) Zhang, X.; Tu, M. B.; Paice, M. G. Routes to potential bioproducts from lignocellulosic biomass lignin and hemicelluloses. BioEnergy Res. 2011, 4, 246. (18) Stärk, K.; Taccardi, N.; Bösmann, A.; Wasserscheid, P. Oxidative depolymerization of lignin in ionic liquids. ChemSusChem 2010, 3, 719. (19) Long, J. X.; Lou, W. Y.; Wang, L. F.; Yin, B. L.; Li, X. H. [C4H8SO3Hmim]HSO4 as an efficient catalyst for direct liquefaction of bagasse lignin: decomposition properties of the inner structural units. Chem. Eng. Sci. 2015, 122, 24. (20) Walstra, P. Principles of emulsion formation. Chem. Eng. Sci. 1993, 48, 333. (21) Huang, J. P.; Yang, H. Q. A pH-switched Pickering emulsion catalytic system: high reaction efficiency and facile catalyst recycling. Chem. Commun. 2015, 51, 7333. (22) Lou, H. M.; Lai, H. R.; Wang, M. X.; Pang, Y. X.; Yang, D. J.; Qiu, X. Q.; Wang, B.; Zhang, H. B. Preparation of lignin-based superplasticizer by graft sulfonation and investigation of the dispersive performance and mechanism in a cementitious system. Ind. Eng. Chem. Res. 2013, 52, 16101. (23) Pang, Y. X.; Gao, W.; Lou, H. M.; Zhou, M. S.; Qiu, X. Q. Influence of modified lignosulfonate GCL4−1 with different molecular weight on the stability of dimethomorph water based suspension. Colloids Surf., A 2014, 441, 664.

(24) Cole, A. C.; Jensen, J. L.; Ntai, I.; Tran, K. L. T.; Weaver, K. J.; Forbes, D. C.; Davis, J. H. Novel Brønsted acidic ionic liquids and their use as dual solvent-catalysts. J. Am. Chem. Soc. 2002, 124, 5962. (25) Long, J. X.; Li, X. H.; Guo, B.; Wang, L. F.; Zhang, N. Catalytic delignification of sugarcane bagasse in the presence of acidic ionic liquids. Catal. Today 2013, 200, 99. (26) Greenwald, H. L.; Brown, G. L.; Fineman, M. N. Determination of the hydrophile-lipophile character of surface active agents and oils by a water titration. Anal. Chem. 1956, 28, 1693. (27) Luwang, M. N.; Ningthoujam, R. S.; Srivastava, S. K.; Vatsa, R. K. Preparation of white light emitting YVO4: Ln3+ and silica-coated YVO4: Ln3+ (Ln3+ = Eu3+, Dy3+, Tm3+) nanoparticles by CTAB/nbutanol/hexane/water microemulsion route: energy transfer and site symmetry studies. J. Mater. Chem. 2011, 21, 5326. (28) Shinoda, K.; Saito, H. The stability of O/W type emulsions as functions of temperature and the HLB of emulsifiers: the emulsification by PIT-method. J. Colloid Interface Sci. 1969, 30, 258. (29) Liu, H. F.; Zhang, Z. M.; Yang, H. Q.; Cheng, F. Q.; Du, Z. P. Recycling nanoparticle catalysts without separation based on a Pickering emulsion/organic biphasic system. ChemSusChem 2014, 7, 1888. (30) Lavoie, J. M.; Baré, W.; Bilodeau, M. Depolymerization of steam-treated lignin for the production of green chemicals. Bioresour. Technol. 2011, 102, 4917. (31) Gosselink, R. J. A.; Teunissen, W.; van Dam, J. E. G.; de Jong, E.; Gellerstedt, G.; Scott, E. L.; Sanders, J. P. M. Lignin depolymerisation in supercritical carbon dioxide/acetone/water fluid for the production of aromatic chemicals. Bioresour. Technol. 2012, 106, 173. (32) Jongerius, A. L.; Bruijnincx, P. C. A.; Weckhuysen, B. M. Liquidphase reforming and hydrodeoxygenation as a two-step route to aromatics from lignin. Green Chem. 2013, 15, 3049. (33) Crossley, S.; Faria, J.; Shen, M.; Resasco, D. E. Solid nanoparticles that catalyze biofuel upgrade reactions at the water/oil interface. Science 2010, 327, 68. (34) Long, J. X.; Guo, B.; Teng, J. J.; Yu, Y. H.; Wang, L. F.; Li, X. H. SO3H-functionalized ionic liquid: efficient catalyst for bagasse liquefaction. Bioresour. Technol. 2011, 102, 10114. (35) Zhang, G. P.; Zhou, H. H.; Hu, J. Q.; Liu, M.; Kuang, Y. F. Pd nanoparticles catalyzed ligand-free Heck reaction in ionic liquid microemulsion. Green Chem. 2009, 11, 1428. (36) Xu, F.; Jiang, J. X.; Sun, R. C.; Tang, J. N.; Sun, J. X.; Su, Y. Q. Fractional isolation and structural characterization of mild ball-milled lignin in high yield and purity from Eucommia ulmoides Oliv. Wood Sci. Technol. 2008, 42, 211. (37) Yang, Q.; Wu, S. B.; Lou, R.; LV, G. J. Structural characterization of lignin from wheat straw. Wood Sci. Technol. 2011, 45, 419.

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