Article pubs.acs.org/IECR
Application of Dissociation Extraction in Oxidation Degradation Reaction of Lignin Shiwei Liu,*,† Cong Zhang,† Lu Li,† Shitao Yu,*,† Congxia Xie,‡ Fusheng Liu,† and Zhanqian Song† †
College of Chemical Engineering and ‡College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, No. 53 Zhengzhou Road, Qingdao 266042, China ABSTRACT: A novel approach was developed in order to use lignin as a renewable resource for the production of high addedvalue aromatic aldehydes. The mixed phosphate solution was used as a reusable medium, and the desired products were separated by the method of the dissociation extraction during the reaction, which prevented the products aromatic aldehydes from oxidizing and increased their yields. The conversion of lignin reached 100%, and the total yield of aromatic aldehydes (vanillin, syringaldehyde and 4-hydroxybenzaldehyde) was 25.8%. Compared with the traditional solvent NaOH aqueous solution, this process gave some advantages to the reaction including less corrosive and environmental problems, easy product separation and purification, and recyclability of solvent.
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INTRODUCTION The valorization of bioresources is a major socioeconomic issue. Some important developments have been made, for example, the fermentation of sugars into ethanol, or the vegetable oil transesterification.1−4 However, new challenges are proposed in order to use resources that do not compete with food. Among these alternative bioresources, lignin is a major component with cellulose and hemicellulose of lignocellulosic plant and represents about 10−35% of the total biomass.4 Unfortunately, there are very few applications dealing with the transformation of lignin (biological or chemical conversion) except in the paper production industry, where it is burned to produce energy.5 Lignin can be converted into several functionalized hydrocarbons due to its primary structure, which can be assimilated to a biopolymer containing mostly polyaromatics and oxygenated functional groups through hydrogenolysis, hydrolysis, and oxidation.6−13 The obtained hydrocarbons are often used as platform molecules or monomers after the separation. Actually, the difficulty to convert lignin is complex, such as variability of its composition and structure, which depend on the separation process and nature of biomass, and low or no solubility in a usual solvent.14−16 Therefore, it is key to find a new solvent with a strong dissolving ability for converting crude lignin. Usually, the sodium hydroxide aqueous solution was used in this process, and the catalytic wet air oxidation process using air or oxygen in this solution has being indicated to be one of the most promising processes to convert lignin into high value-added aromatic aldehydes, such as 4-hydroxybenzaldehyde, vanillin, and syringaldehyde, which are widely used in flavoring as chemical intermediates for pharmaceutical drugs and agricultural pesticides.17−19 However, this process has some shortcomings including a complicated technique, nonrecyclability of catalyst, and serious environmental pollution. Especially, the selectivities of the products aromatic aldehydes are poor because they cannot be separated from the reaction mixture during the process of reaction, which leads to their overoxidation and form carboxylic acidic derivatives, such as vanillic acid, and syringic acid.20 The dissolution of lignin is critically © 2014 American Chemical Society
important for its efficient valorization but remains a challenge due to the particular property of the lignin structure that resists its chemical or enzymatic degradation.14 However, when lignin is dissolved or swollen in the solvent, the connecting points of the phenylpropane building units are exposed out and then the reagents are easy in touch with them, as a result the degradation carries out smoothly.14 Therefore, to find a novel approach used a new solvent that can efficiently promote the lignin degradation and simultaneously separate the products aromatic aldehydes is also a prerequisite for the oxidative degradation. Dissociation extraction is a two phase separation technique that exploits the difference in dissociation constants and distribution coefficients of organic acids/bases.21 When a neutralizing agent in stoichiometric deficient quantity is used, the competition between the components would be established, which leads to an enrichment of the weaker component in the raffinate phase while the extract phase is enriched by the stronger component.22 Dissociation extraction has been successfully used in the extraction and separation of chemical substances from coal−tar oil, polyphnols, substituted phenols, amines, and alkaloids.23 The mixed phosphate solvent is often used in the dissociation extraction of the substituted phenols, and the basic principle of this process is showed in Scheme 1. The phenolic hydroxyl group can react with Na2[HPO4] and Na3[PO4] to form water-soluble sodium phenolate and dissolves in the aqueous solution. When an organic extractant with a better solubility for the phenolic compounds is added, Scheme 1. Dissociation Extraction Principle of the Mixed Phosphate and Phenol
Received: Revised: Accepted: Published: 19370
September 6, 2014 November 4, 2014 November 12, 2014 November 12, 2014 dx.doi.org/10.1021/ie5035418 | Ind. Eng. Chem. Res. 2014, 53, 19370−19374
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its catheter (to prevent O2 touching with the extraction phase, N2 pressure was determined according to Claperon−Clausius equation),25 and then O2 was added through its catheter linking the distributor until its pressure balanced with N2 pressure and reached 3.0 MPa. The O2 pressure in the reactor was kept at 3.0 MPa by continuous flushing of O2 as a supplement because of its consumption during the reaction. During the reaction process, once the desired products aromatic aldehydes were produced, they were extracted simultaneously into the organic phase because of their better solubility in the organic extractant phase than in the aqueous phase, which decreased their residence time in aqueous phase and avoided them oxidation. When the reaction was finished, the upper extraction phase contenting of 4-hydroxybenzaldehyde (PHB), vanillin (VAN), and syringaldehyde (SYR) was obtained by liquid−liquid phase separation, and analyzed by the high performance liquid chromatography (HP-LC). Their concentrations were confirmed by the external standard method. The aqueous phase was processed by the following method to obtain the unreacted lignin. 30 wt % phosphoric acid was added to neutralize the aqueous phase until its pH value was 5.0, and then the precipitated lignin was filtrated, washed, and dried to determine its conversion.26 The filtration was distilled under vacuum to remove the water and obtain the solid of NaH2PO4, which contained the catalyst CuSO4 and was reused in the recycle experimental. Lignin conversion and the products yields were
the reversible reaction balance is broken and moves to the opposite direction, and then the oil-soluble phenolic compound is enriched in the extract phase. As a result, the separation of the phenolic compounds from the water is realized. In the lignin oxidative degradation, the desired products aromatic aldehydes are the substituted phenolic compounds. So, it would be feasible to separate them from the reaction mixture by the method of dissociation extraction. To the best of our knowledge, no study about the lignin oxidative degradation using dissociation extraction to separate the desired products has been published. Therefore, we first reported the lignin oxidative degradation to produce aromatic aldehydes coupled with their separation by the dissociation extraction in the presence of mixed phosphate.
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MATERIALS AND METHODS Chemicals and Instruments. Organosolv lignin was purchased from Sigma-Aldrich. Alkali lignin and sodium ligninsulfonate were purchased from Luohe Huadong Lignin Co. Ltd. Disodium hydrogen phosphate and other chemicals were on analytical grade and used without further purification. Qualitative analysis of the product sample was recorded by a Shimadzu LC-MS 8040 equipped with a column of an Agilent C18 (1.8 μm, 2.1 mm × 100 mm) and an UV detector set at 280 nm. The mobile phase was a mixture of ammonium acetate (5 mmol/L) and acetonitrile (100%). Quantitative analysis of the product sample was recorded by an Agilent HP-LC1100 equipped with a column of Dikma ODSC18 (5.0 μm, 4.6 mm × 250 mm), and an Agilent 1200 detector set at 280 nm. The mobile phase was a mixture of acetonitrile (8.5%), deionized water (90%), and acetic acid (1.5%).24 Oxidation Degradation Reaction of Lignin. The catalytic oxidation degradation process of lignin was carried out in a 100 mL high pressure reactor equipped with an oxygen distributor, a stirrer, and a thermometer (see Figure 1). The
calculated as follows Xlignin (%) = Yproduct (%) =
Fproduct,out Flignin,in
Flignin,in − Flignin,out Flignin,in
·100%,
·100%, Fi,in and Fi,out are the weight of
the i species for lignin and product at the inlet and outlet of the reactor, respectively. All the experiments were repeated for four times to confirm the results effectiveness. To select an appropriate solvent to extract products aromatic aldehydes from the water phase, some solvents with the good solubility to products aromatic aldehydes, such as acetone, methylisobutylketone, toluene, benzyl chloride, and trichloroethane, were investigated at room temperature. The results showed that trichloroethane, toluene, and benzyl chloride decreased the lignin solubility in mixed phosphate solution and made lignin separate from the water phase. Acetone and methylisobutylketone had good performances to build the coupled process of reaction-separation. However, acetone is easy to lose in the separation and reuse process, because its saturated vapor pressure is higher than that of methylisobutylketone under atmospheric pressure. Therefore, methylisobutylketone was selected and used in the coupled process of reaction-separation. Otherwise, it has been a general problem for the product identification in lignin oxidation, and the value products are the single phenolic compounds. So, only three aromatic aldehydes’ quantitative analyses are given in the paper, but by comparison of the results of the base peak chromatograms and UV detection at 280 nm recorded by liquid chromatography mass spectrometry (LC-MS), it was found that the products were composed of the single phenolic compounds and benzene ring opened products. The single phenolic compounds were mainly hydroxybenzoic acid, vanillic acid, 4hydroxybenzaldehyde, syringic acid, 4-hydroxybenzoate ethyl ketone, vanillin, syringaldehyde, and so on. The benzene ring opened products were difficult to determine. The reaction phase (white) contains water, Na2HPO4, Na3PO4, lignin, and catalyst CuSO4 and was represented in the bottom half of the batch reactor. The extraction phase
Figure 1. Batch process for productions of aromatic aldehydes from lignin with simulated countercurrent extraction.
homogeneous mixture of 0.35 mol/L Na2HPO4·2H2O 15 g, 0.05 mol/L Na3PO4·12H2O 15 g, lignin 1.5 g, catalyst CuSO4 0.02 g, and organic extractant methylisobutylketone 30 g were introduced into the reactor. The reactor was sealed and exchanged with 0.2 MPa N2 for three times. After that, the heating program was started under a slight N2 pressure. When the reaction mixture in the reactor reached the desired temperature, 2.36 MPa N2 was added into the reactor through 19371
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the aromatic aldehydes must be produced in ionized form. When the NaOH aqueous solution is used in the reaction, the pH of the aqueous phase (pH > 14) is too high to maintain the products in ionized form, which makes it impossible to extract them or phenol form from the aqueous solution to an organic solvent. Compared with NaOH aqueous solution, the pH of the alkaline of phosphate aqueous (pH ≈ 12) is lower; the phenolate and phenol exist in reversible equilibrium in the aqueous phase (see Scheme 1), which makes the degradation of lignin and the extraction of products aromatic aldehydes simultaneously happen in the two-phase system. Once lignin is oxidized to form aromatic aldehydes, they are extracted by the extractant methylisobutylketone and separated from the aqueous phase, which avoids their over oxidation reaction and increases their yields. Among all catalysts that were investigated, CuSO4 and ZrCl4 showed better catalytic performances for the oxidative degradation of lignin than the others (Entries 2−5). This is because Cu2+ and Zr4+ are good oxygen entrainers and have a good charge-transfer capacities,27 which promotes the oxidative degradation. When no catalyst was used in the reaction, the oxidation slight occurred. The conversion of lignin was 32.1%, and the total yield of the aromatic aldehydes was 5.6% (Entry 6). It was suggested that the mixed phosphate aqueous also had some catalytic performance for the oxidation of lignin. When sodium ligninsulfonate was used as the raw material, the result was not satisfied (Entry 7). This may be due to that the sodium sulfonate of sodium ligninsulfonate was not conducive to the oxidative degradation of lignin. But when alkali lignin was used, the result was better than that of organosolv lignin (Entry 8). This may be due to that alkali lignin degrades more serious than organosolv lignin in their preparation process, and alkali lignin with small molecular weight is easy to oxidative degradation. Effects of Reaction Conditions on the Reaction Results. Table 2 shows the effects of reaction conditions on the oxidative degradation reaction. It was obviously seen that the yields of products became higher with increasing of catalyst CuSO4 dosage from 0.01 to 0.02 g. However, when CuSO4 dosage was more than 0.02 g (Entry 3), the yields of products obviously decreased, which indicated that the excessive amount of catalyst promoted the products aromatic aldehydes over oxidation to form carboxylic acid or other oxidation byproducts. O2 pressure was very important for the oxidative degradation reaction. When O2 pressure was 1.0 MPa, the conversion of lignin was less than 90% under the given conditions (Entry 4).
(gray) contained methylisobutylketone and was represented in the top half of the batch reactor. Oxidant O2 through its distributor (total volume 6.0 mL, including pipeline) passed directly into the reaction layer.
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RESULTS AND DISCUSSION Effects of Different Catalysts on the Reaction Results. The results of the effects of catalysts on lignin oxidative degradation reaction are shown in Table 1. It was seen that, Table 1. Effects of Catalysts on Lignin Oxidative Degradation Reactiona Yproduct (%) entry
catalyst
Xlignin (%)
VAN
SYR
PHB
YTAP (%)
1b 2 3 4 5 6 7c 8d
CuSO4 CuSO4 ZrCl4 FeCl3 CoCl2
100 100 98.5 92.6 87.6 32.1 56.0 100
10.2 13.6 12.8 12.3 11.9 3.8 5.1 14.2
5.6 7.5 7.1 7.0 6.4 1.4 2.2 7.4
3.5 4.7 4.3 4.2 3.9 0.4 0.6 5.0
12.6 25.8 24.2 23.5 22.2 5.6 7.9 26.6
CuSO4 CuSO4
0.35 mol/L Na2HPO4·2H2O 15 g, 0.05 mol/L Na3PO4·12H2O 15 g, organosolv lignin 1.5 g, extractant 30 g, catalyst 0.02 g, O2 3.0 MPa, T = 175 °C, t = 1.5 h. bThe solvent was 30 g 0.4 mol/L NaOH aqueous solution, and the other conditions were the same as those in footnote a. cSodium ligninsulfonate 1.5 g, and the other conditions were the same in those footnote a. dAlkali lignin 1.5 g, and the other conditions were the same as those in footnote a. VAN, vanillin; SYR, syringaldehyde; PHB, 4-hydroxybenzaldehyde. YTAP: The total yield of the aromatic aldehydes. a
when NaOH aqueous solution was used as the reaction medium, the conversion of lignin was 100%, and the total yield of the aromatic aldehydes was 12.6% (Entry 1). Compared with traditional solvent−NaOH aqueous solution, the mixed phosphate aqueous as a new solvent in the lignin oxidative degradation reaction obtained a better result, lignin conversion reached 100% and the total yield of the aromatic aldehydes was up to 25.8% (Entry 2). The good reaction result maybe explained by the characteristics of the dissociation extraction.22 A strong alkaline medium is needed for the lignin oxidation due to the high pKa of the phenolic groups. Ionization is the first step of the reaction, which is the necessary condition for the oxidative degradation of the lignin. So, as the reaction proceeds,
Table 2. Effects of Reaction Conditions on the Reaction Resultsa Yproduct (%)
a
entry
catalyst (g)
P (MPa)
T (°C)
t (h)
Xlignin (%)
VAN
SYR
PHB
YTAP (%)
1 2 3 4 5 6 7 8 9 10
0.01 0.02 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02
3.0 3.0 3.0 1.0 2.0 4.0 3.0 3.0 3.0 3.0
175 175 175 175 175 175 160 190 175 175
1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.0 2.0
92.1 100 100 87.3 98.0 100 86.4 100 86.2 100
12.3 13.6 10.2 10.9 13.1 10.2 11.4 10.6 10.7 9.5
6.8 7.5 5.6 6.2 7.2 5.4 6.5 5.5 6.5 5.0
4.3 4.7 4.5 4.0 4.5 4.2 4.0 4.3 3.9 4.1
23.4 25.8 20.3 21.1 24.8 19.8 21.9 20.4 21.1 18.6
0.35 mol/L Na2HPO4·2H2O 15 g and 0.05 mol/L Na3PO4·12H2O 15 g. Catalyst, CuSO4; extractant, methylisobutylketone 30 g. 19372
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Figure 2. Proposed mechanism of lignin oxidation in phosphate solution.
When the O2 pressure was more than 3.0 MPa, the conversion of lignin reached 100% (Entry 6), but the yields of products obviously decreased. This result may be due to that the higher O2 pressure is, the stronger oxidative ability of the active oxygen is, which leads to aromatic aldehydes oxidizing and decreasing their yields. The effects of reaction temperature and time on the oxidative degradation reaction are also shown in Table 2 (Entries 2 and 6−9). It was found that both were very important for the degradation reaction. With increases of reaction temperature from 160 to 190 °C, the conversion of lignin significantly increased. However, when the reaction temperature reached 190 °C (Entry 8), the yields of products decreased. With increases of reaction time (Entries 2, 9 and 10), similar reaction results were obtained. Therefore, it was indicated that a higher reaction temperature and longer reaction time improved the depth oxidation of aromatic aldehydes, which made the yields of products decrease obviously. Otherwise, under harsh reaction conditions such as increasing reaction temperature and O2 pressure, or prolonging the reaction time, the results showed that the yields of vanillin and syringaldehyde decreased even more sharply than that of 4hydroxybenzaldehyde, which indicated that vanillin and syringaldehyde were more easily oxidated than 4-hydroxybenzaldehyde. This is because of the electronic effect.28 Compared with 4-hydroxybenzaldehyde, the methoxy of vanillin or syringaldehyde, as an electron donor, increases the electron cloud density of the aldehyde, which makes its oxidation easier occur. Based on the above results, the optimum conditions were obtained: 0.35 mol/L Na2HPO4·2H2O 15 g, 0.05 mol/L Na3PO4·12H2O 15 g, lignin 1.5 g, CuSO4 0.02 g, O2 3.0 MPa, reaction temperature 175 °C, and reaction time 1.5 h. Under these conditions, the conversion of lignin reached 100%, and the total yield of products was more than 25%.
By combining the above results and the previous literature report,10 the proposed mechanism of oxidation of lignin catalyzed by CuSO4 in the phosphate solution is depicted in Figure 2. Cu[H2PO4][OH] was formed from the reaction of CuSO4, Na2HPO4, and H2O, and then OH− of the Cu[H2PO4][OH] reacted with lignin to produce phenolate anion and Cu[HPO4]+. The carbon carbon double bond of phenolate anion was oxidized by Cu[HPO4]+ to form quinione methide radical. Meanwhile, Cu[HPO4]+ got an electron to form CuHPO4. After that, the quinone methide hydroperoxide and Cu[HPO 4]+ were produced by the reaction with O2 , respectively. And then the aldehyde ion was prepared by rearrangement and breaking of chemical bonds of quinone methide hydroperoxide. Reusability of the Mixed Phosphate Solution. To investigate the possibility of the mixed phosphate solution, the obtained aqueous phase during oxidation degradation of lignin was extracted three times with methylisobutylketone and then directly used in the next recycling experiment. The reusability of the mixed phosphate solution including catalyst CuSO4 is shown in Table 3. It was seen that the mixed phosphate solution could be reused four times without an obvious Table 3. Reusability of Solvent/Catalyst System Yproduct (%)
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no. of times
Xlignin (%)
VAN
SYR
PHB
YTAP (%)
1 2 3 4 5 6
100 100 100 100 92.6 100
13.7 13.9 13.7 13.4 12.5 13.8
7.6 7.8 7.6 7.5 7.1 8.0
4.9 5.0 4.6 4.5 4.2 4.9
26.2 26.7 25.9 25.4 23.8 26.7
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decrease in its performance. However, when it was reused five times, the total yield of the products was 23.8%. At the same time, lignin did not completely dissolve in the mixed phosphate solution. The above result and phenomenon maybe due to that the alkali is consumed in the process of the lignin oxidation, the alkaline dosage and strength of the mixed phosphate solution decrease with increasing of its reused times, as a result lignin does not dissolve in the phosphate solution used for five times. Therefore, some NaOH (0.2 wt % of the reused phosphate solution) was added to the phosphate solution to increase its alkaline strength, and the experiment result is shown in Entry 6. The conversion of lignin and the total yield of the products were similar to that of the new phosphate solution. It was said that the mixed phosphate solution containing the catalyst CuSO4 was of good reusable performance in the reaction.
bimetallic catalysts for effective lignin hydrogenolysis in water. ACS Catal. 2014, 4, 1574. (7) Zhang, J. G.; Asakura, H.; van Rijn, J.; Yang, J.; Duchesne, P.; Zhang, B.; Chen, X.; Zhang, P.; Saeys, M.; Yan, N. Highly efficient, NiAu-catalyzed hydrogenolysis of lignin into phenolic chemicals. Green Chem. 2014, 16, 2432. (8) Janesko, B. G. Acid-catalyzed hydrolysis of lignin β-O-4 linkages in ionic liquid solvents: A computational mechanistic study. Phys. Chem. Chem. Phys. 2014, 16, 5423. (9) Stark, K.; Taccardi, N.; Bosmann, A.; Wasserscheid, P. Oxidative depolymerization of lignin in ionic liquids. ChemSusChem 2010, 3, 719. (10) Zhang, J. H.; Deng, H. B.; Lin, L. Wet aerobic oxidation of lignin into aromatic aldehydes catalyzed by a perovskite-type oxide: LaFe1−xCuxO3 (x = 0, 0.1, 0.2). Molecules 2009, 14, 2747. (11) Nguyen, J. D.; Matsuura, B. S.; Stephenson, C. R. J. A photochemical strategy for lignin degradation at room temperature. J. Am. Chem. Soc. 2014, 136, 1218. (12) Biannic, B.; Bozell, J. J. Efficient cobalt-catalyzed oxidative conversion of lignin models to benzoquinones. Org. Lett. 2013, 15, 2730. (13) Abdel-Hamid, A. M.; Solbiati, J. O.; Cann, I. K. Insights into lignin degradation and its potential industrial applications. Adv. Appl. Microbiol. 2013, 82, 1. (14) 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. (15) Brown1, M. E.; Chang, M. C. Y. Exploring bacterial lignin degradation. Curr. Opin. Chem. Biol. 2014, 19, 1. (16) Busse, N.; Wagner, D.; Kraume, M.; Czermak, P. Reaction kinetics of versatile peroxidase for the degradation of lignin compounds. Am. J. Biochem. Biotechnol. 2014, 9, 365. (17) Sales, F. G.; Abreu, C. A. M.; Pereira, J. A. F. R. Catalytic wet-air oxidation of lignin in a three-phase reactor with aromatic aldehyde production. Braz. J. Chem. Eng. 2004, 21, 211. (18) Pinto, P. C. R.; Borges da Silva, E. A.; Rodrigues, A. E. Insights into oxidative conversion of lignin to high-added-value phenolic aldehydes. Ind. Eng. Chem. Res. 2011, 50, 741. (19) Araujo, J. D. P.; Grande, C. A.; Rodrigues, A. E. Vanillin production from lignin oxidation in a batch reactor. Chem. Eng. Res. Des. 2010, 88, 1024. (20) Longo, M. A.; Sanroman, M. A. Production of food aroma compounds: Microbial and enzymatic methodologies. Food Technol. Biotechnol. 2006, 44, 335. (21) Gaikar, V. G.; Sharma, M. M. Dissociation extraction: Prediction of separation factor and selection of solvent. Solvent Extr. Ion Exch. 1985, 3, 679. (22) Wadekar, V. V.; Sharma, M. M. Separation of close boiling substituted phenols by dissociation extraction. J. Chem. Technol. Biotechnol. 1981, 31, 279. (23) Ahmed, A. S.; Akhtar, M.; Hamid, A. Dissociation extraction process for the separation of isomeric organic compounds. Pak. J. Sci. Ind. Res. 2003, 46, 344. (24) Lobbes, J. M.; Fitznar, H. P.; Kattner, G. High-performance liquid chromatography of liginin-derived phenols in environmental samples with diode array detection. Anal. Chem. 1999, 71, 3008. (25) Koutsoyiannis, D. Clausius−Clapeyron equation and saturation vapour pressure: Simple theory reconciled with practice. Eur. J. Phys. 2012, 33, 295. (26) Zabkova, M.; Borges da Silva, E. A.; Rodrigues, A. E. Recovery of vanillin from lignin/vanillin mixture by using tubular ceramic ultrafiltration membranes. J. Membr. Sci. 2007, 301, 221. (27) Partenheimer, W. The aerobic oxidative cleavage of lignin to produce hydroxy aromatic benzaldehydes and carboxylic acids via metal/bromide catalysts in acetic acid/water mixtures. Adv. Syn. Catal. 2009, 351, 456. (28) Sun, C. J.; Wang, X. J.; Chen, T. Principle and Application of Organic Oxidation Reaction; Chemical Industry Press: Beijing, 2013.
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CONCLUSIONS A coupled process of reaction-separation composed the mixed phosphate and methylisobutylketone exhibited good performance in the oxidative degradation reaction of lignin. Compared with the traditional solvent NaOH aqueous solution, the coupled process avoided the depth oxidation of the products and improved significantly the lignin conversion and the yield of aromatic aldehydes. Otherwise, the mixture of product and the mixed phosphate phase was easily separated, and the mixed phosphate containing the catalyst CuSO 4 was a good reusability. Hence, a clean and environmentally friendly strategy for overall utilization of lignin and preparation of aromatic aldehyde is developed.
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AUTHOR INFORMATION
Corresponding Authors
*S. Liu. E-mail:
[email protected]. *S. Yu. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (973 Program) (2014CB460610), the Natural Science Foundation of China (31370570 and 31100430), and the Open Foundation from State Key Laboratory of Pulp and Paper Engineering of South China University of Technology (201120). The authors are grateful for the financial support.
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REFERENCES
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