Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10111-10115
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Tetrabutylammonium Hydroxide 30-Hydrate as Novel Reaction Medium for Lignin Conversion Kohei Yamamoto,† Takashi Hosoya,† Koichi Yoshioka,† Hisashi Miyafuji,*,† Hiroyuki Ohno,‡ and Tatsuhiko Yamada§
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†
Graduate School of Life and Environmental Sciences, Kyoto Prefectural University, 1-5 Hangi-cho, Shimogamo, Sakyo-ku, Kyoto 606-8522, Japan ‡ Graduate School of Engineering Department, Tokyo University of Agriculture and Technology, 2-24 Nakacho, Koganei, Tokyo 184-8588, Japan § Forestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaragi 305-8687, Japan S Supporting Information *
ABSTRACT: The efficient conversion of lignin, a major component of lignocellulosics, to monomeric aromatics is a hot topic in biorefining. In this study, we developed a method for selective degradation of several types of lignin [milled wood lignin, sodium lignosulfonate, soda lignin, and Japanese cedar (Cryptomeria japonica) wood flour] in Bu4NOH·30H2O (mp 27−30 °C). Degradation at 120 °C for 43−70 h gave low-molecular-weight (MW) compounds such as vanillin, vanillic acid, acetoguiacone, and p-hydroxybenzaldehyde in considerable yields. The total yields were 16.3, 6.5, 6.7, and 22.5 wt % from milled wood lignin, sodium lignosulfonate, soda lignin, and wood flour, respectively. Similar degradation in aqueous NaOH solution with the same OH− concentration (1.25 mol/L) as that in molten Bu4NOH·30H2O gave much lower yields of these products. This suggests that in addition to the effect of Bu4NOH·30H2O as a strong alkali, the Bu4N+ cation increased the selectivity of lignin degradation for low-MW products. Degradation under N2 gave significantly lower yields of the low-MW products, even in Bu4NOH·30H2O, suggesting that aerobic oxidation is involved in the formation of low-MW compounds and oxidation is affected by the presence of the Bu4N+ cation. KEYWORDS: Vanillin, Degradation, Bu4NOH, Oxidation, Lignocellulosic biomass
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INTRODUCTION Vanillin (4-hydroxy-3-methoxybenzaldehyde) is widely used in industry. In addition to its traditional use as a flavoring agent, it is an important source of chemical products, especially those for medical applications.1 It is also used as a monomeric unit in various biobased polymeric materials.2 Current vanillin production in the chemical industry involves formylation of guaiacol derived from fossil resources.3 However, lignin, a major component of lignocellulosics, is also converted to vanillin in many biomass conversion processes such as alkaline oxidation, pyrolytic liquefaction, hydrothermal conversion, and ionic-liquid treatments.4−13 Vanillin production from lignin, which is a renewable resource, is strongly desirable in environmental terms. Vanillic acid (4-hydroxy-3-methoxybenzoic acid), which is an oxidized form of vanillin, is frequently formed along with vanillin in biomass conversion processes and is a potential chemical source for polymer synthesis.1,2 Vanillin was formerly produced mostly from lignosulfonate in wastes from sulfate pulp mills via alkaline degradation combined with air oxidation.14 This lignin-based process gave vanillin in significant yields (∼8.0%)15,16 considering the highly complicated chemical nature of the wastes, although this yield is not as high as those achieved currently using fossil-based processes. The SO3H moiety at the α-position of the © 2017 American Chemical Society
monomeric unit plays a key role in achieving these high yields of vanillin.15,16 These yields were sufficiently high to make this biomass-based vanillin production more competitive than the process based on eugenol, which was an alternative vanillin production method until the 1980s. However, lignin-based vanillin production produces wastes containing harmful sulfides, sulfoxides, and sulfones.3 Because of this drawback and the introduction of the above guaiacolbased process in the 1990s, most industries switched to petroleum-based production. The harsh reaction conditions, e.g., a strong alkali, high temperature (∼200 °C), and compressed air, required for vanillin production from lignosulfonate were another reason for this feedstock switch. Around 85% of global vanillin supplies are produced from fossil resources, with only 15% from lignosulfonate.17 The oxidation of lignin by nitrobenzene under alkaline conditions is another potential method for vanillin production from lignin. This oxidation gives vanillin in substantial yields, e.g., ∼30% from softwood milled wood lignin,18,19 and this method has been predominantly used in lignin analysis since its Received: June 26, 2017 Revised: October 5, 2017 Published: October 9, 2017 10111
DOI: 10.1021/acssuschemeng.7b02106 ACS Sustainable Chem. Eng. 2017, 5, 10111−10115
Research Article
ACS Sustainable Chemistry & Engineering
in a 10 mL stainless-steel bomb. The bomb was heated at 170 °C for 2.5 h in an oil bath. The bomb was then cooled with cold water to quench the reaction. The solution was worked up using the procedure described above and analyzed using HPLC under the same analytical conditions. The peaks of low-MW products were not overlapped with that of nitrobenzene (see Figures S1 and S2 in the Supporting Information for the chromatogram).
discovery. However, the use of oxidation for industrial production of vanillin is not feasible because of the harmful nature of nitrobenzene and its reduced products.2 Another drawback is that the nitrobenzene dosage must be greater than the molar equivalent of the monomeric units in lignin. The above considerations suggest that if the yields of ligninbased vanillin can be improved, chemical industries will again use lignin as a vanillin source, with the drawbacks being compensated for by improved yields. In this study, we investigated the production of low-molecular-weight (MW) compounds, especially vanillin and vanillic acid, from various lignin samples via degradation in Bu4NOH·30H2O. Alkaline conditions are known to promote lignin degradation. Our idea was to use this information to develop a method for the production of industrially useful compounds from lignincontaining materials. Our screening of various alkaline media identified Bu4NOH·30H2O as the most effective medium for chemical conversion of lignin. We found that the selectivity of lignin degradation for low-MW products was substantially higher in Bu4NOH·30H2O than in a simple alkali, namely NaOH.
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RESULTS AND DISCUSSION Degradation of Lignin Samples. We first degraded milled wood lignin from Japanese cedar (Cryptomeria japonica) in Bu4NOH·30H2O at 120 °C under air. Bu4NOH·30H2O is completely molten at 120 °C and exists as an aqueous Bu4NOH solution containing 1.25 mol/L of Bu4N+ and OH−. A typical chromatogram obtained by HPLC analysis of the sample is shown in Figure 1. The chromatogram has four major
EXPERIMENTAL SECTION
Materials. Bu4NOH·30H2O (≥98%) and sodium lignosulfonate were purchased from the Sigma-Aldrich Co. and Tokyokasei Co., respectively. Milled wood lignin was prepared according to the method reported in the literature20 from Japanese cedar (Cryptomeria japonica) wood. This method is frequently used as a standard procedure for milled wood lignin preparation. Soda lignin was obtained from the Forestry and Forest Products Research Institute, Japan. The particle size of the Japanese cedar wood flour was 90−180 μm. Degradation of Lignin Samples and Analysis of Reaction Mixture. Bu4NOH·30H2O (2.0 g) and the lignin sample (14 mg) were put in a 10 mL glass tube, and the tube was tightly sealed. We used a small amount of lignin sample because the major aim of this study was to determine the effectiveness of Bu4NOH·30H2O as the reaction medium. In most cases, the experiments were performed under air, without any replacement of the atmosphere in the reactor with a specific gas. In the experiments that were performed under N2, the tube was flushed with N2 and immediately sealed. The tube was then heated at 80−140 °C in an oil bath with stirring. At a certain reaction time, the tube was cooled with cold water, and then 1,5dihydroxy-1,2,3,4- tetrahydronaphthalene/ethanol solution (2.0 g/L, 800 μL) was added as an internal standard. A sample (100 μL) of the resulting reaction mixture was taken and added to acetonitrile (900 μL) containing 1.5% acetic acid. The solution was filtered and used directly for high-performance liquid chromatography (HPLC) analysis. HPLC was performed using an HPLC system (Shimadzu Ltd., Kyoto, Japan) equipped with a pump (LC-10AD), column oven (CTO-10A), and ultraviolet−visible detector (SPD-10A) set at 280 nm. The analytical conditions were as follows: Cadenza CD-C18 column; flow rate, 0.8 mL/min; gradient, 1.5% acetic acid aqueous/ acetonitrile eluent (90/10 → 45/55 0−30 min; 45/55 → 0/100 30− 35 min; 0/100 35−40 min; 0/100 → 90/10 40−45 min; 90/10 45−60 min); and column temperature, 30 °C. HPLC analysis only provides the product retention times; therefore, verification of the product identities was needed. We therefore performed HPLC analysis of the same product mixture under different analytical conditions to doublecheck the identifications. In this case, the reaction mixture (20 μL) was taken and added to a water/methanol (9/1, v/v) solution (180 μL) containing sulfuric acid (2.0 μL). The solution was filtered and used directly for HPLC analysis. The analytical conditions were as follows, with other conditions the same as above: flow rate, 1.0 mL/min; gradient, water/methanol eluent (90/10 0−40 min; 90/10 → 0/100 40−100 min; 0/100 100−110 min); and column temperature, 40 °C. Alkaline Nitrobenzene (AN) Oxidation. NaOH (2 mol/L, 7.0 mL), nitrobenzene (0.4 mL), and the lignin sample (50 mg) were put
Figure 1. HPLC chromatogram of reaction mixture obtained after Bu4NOH·30H2O degradation of milled wood lignin at 120 °C for 70 h under air; the aqueous AcOH/CH3CN eluent was used for HPLC analysis (see the Experimental Section for details). I.S. denotes internal standard.
peaks, at 13, 14.5, 16.5, and 18.2 min, which were identified as vanillic acid, p-hydroxybenzaldehyde, vanillin, and acetoguaiacone [1-(4-hydroxy-3-methoxyphenyl)ethanone], respectively. We double-checked the identifications by analyzing the same sample using different HPLC conditions (see Figure S3 in the Supporting Information). The main products were then quantified at several reaction times. Figure 2A shows that the yields of vanillin, vanillic acid, acetoguaiacone, and p-hydroxybenzaldehyde increased with increasing degradation time and reached 16.3% at 43 h. To optimize the reaction conditions, we investigated the yields of vanillin, the major product, at other reaction temperatures, i.e., 80 and 140 °C, and for a series of reaction times. The results
Figure 2. Changes with time in yields of vanillin (⧫), vanillic acid (■), acetoguaiacone (▲), and p-hydroxybenzaldehyde (×), and their total yields (∗) during degradation of milled wood lignin at 120 °C in Bu4NOH·30H2O under air (A), in aqueous NaOH under air (B), and in Bu4NOH·30H2O under N2 (C). 10112
DOI: 10.1021/acssuschemeng.7b02106 ACS Sustainable Chem. Eng. 2017, 5, 10111−10115
Research Article
ACS Sustainable Chemistry & Engineering indicate that the vanillin yield obtained by degradation at 120 °C was higher than those obtained at other degradation temperatures (see Table S1 in the Supporting Information for details). NMR spectroscopic analysis of Bu4NOH·30H2O heated for 72 h at 120 °C showed that Bu4NOH·30H2O was chemically stable under the conditions used (the NMR spectrum is shown in Figure S4 in the Supporting Information). Similar degradation experiments were then performed in aqueous NaOH solution instead of Bu4NOH·30H2O. The OH− concentration in the NaOH solution was the same as that (1.25 mol/L) in liquid Bu4NOH·30H2O. The low-MW products vanillin, vanillic acid, acetoguaiacone, and phydroxybenzaldehyde were produced in aqueous NaOH, but their yields were lower than those obtained in Bu4NOH· 30H2O, particularly at reaction times longer than 43 h (Figure 2A,B). This suggests that the degradations in aqueous NaOH and Bu4NOH·30H2O differ significantly, and degradation in Bu4NOH·30H2O gives greater selectivity for low-MW products. This difference is probably a result of the change in the countercation of OH− from Na+ to Bu4N+. At shorter reaction times such as 10 h, the differences between the yields in the two reaction media were not significant (Figure 2A,B). This suggests that Bu4N+ can open up a specific reaction pathway for vanillin formation at longer reaction times. The degradation of milled wood lignin in Bu4NOH·30H2O was performed after replacement of the gas in the reactor with N2. Figure 2C shows that replacement of air by N2 significantly suppressed the formation of low-MW products. This suggests that lignin degradation to low-MW products in Bu4NOH· 30H2O requires O2. The formation of small amounts of the low-MW products even after air replacement by N2 is explained by the presence of residual O2, because the procedure for N2 replacement did not ensure complete exclusion of O2 from the system (see the Experimental Section). Our next paper will report that degradation under O2 greatly increases the reaction rate and further improves the yields of low-MW compounds. We also performed degradation of several other lignin samples, i.e., sodium lignosulfonate, soda lignin, and Japanese cedar wood flour. Figure 3 summarizes the yields of the lowMW products from the lignin samples. The maximum total yields in Bu4NOH·30H2O were 6.5−22.5% depending on the sample type, where the yield from wood flour is based on the lignin content (the total amount of Klason lignin and acidsoluble lignin) of the wood (34.3 wt %). These yields were all higher than those obtained in aqueous NaOH under the same conditions. The results for sodium lignosulfonate were exceptional; the low-MW products were obtained in 4.8% yield in 43 h, even in aqueous NaOH. Considering the large differences between the yields obtained using Bu4NOH·30H2O and aqueous NaOH for the other lignin samples (Figures 2 and 3), the 4.8 wt % yield from sodium lignosulfonate in aqueous NaOH is similar to the maximum yield of low-MW compounds achieved in Bu4NOH·30H2O for 72 h (6.5%). This relatively good performance for sodium lignosulfonate in NaOH may be one of the reasons why sodium lignosulfonate is usually used for vanillin production. We now compare the yields of low-MW products obtained in our degradation in Bu4NOH·30H2O with those obtained using AN oxidation, which is currently the most selective method for lignin conversion.18,19 The data in Table 1 show that the total yields of the low-MW products achieved in Bu4NOH·30H2O reached significant percentages of those obtained by AN oxidation for all types of lignin. In the case of wood flour, the
Figure 3. Changes with time in yields of vanillin (⧫), vanillic acid (■), acetoguaiacone (▲), and p-hydroxybenzaldehyde (×), and their total yields (∗) during degradation of soda lignin (A), sodium lignosulfonate (B), and wood flour (C) in Bu4NOH·30H2O (A-1, B1, and C-1) and aqueous NaOH (A-2, B-2, and C-2) at 120 °C. The yield from wood flour is based on the lignin content (the total amount of Klason lignin and acid-soluble lignin) of the wood (34.3 wt %).
yield of low-MW products (22.5 wt %) in our degradation in Bu4NOH·30H2O reached 76.0% of that in AN oxidation (29.6 wt %). Considering that AN oxidation is not suitable for industrial vanillin production (see the Introduction) and substantial improvements in the selectivity are achieved by using Bu4NOH·30H2O instead of NaOH (see above), our degradation in Bu4NOH·30H2O has potential as a novel method for industrial vanillin production. Our next paper will report that addition of solid NaOH to Bu4NOH·30H2O further improves the yields of low-MW products, especially vanillin, and their total yield becomes comparable to that achieved using AN oxidation. It is also noted that the lignin-based total yield from the wood (22.5 wt %) is higher than that from milled wood lignin (16.3 wt %). This is an interesting observation, but the underling mechanisms have not been clear at the moment. Lignin degradation under alkaline conditions has been extensively studied in terms of efficient pulp production.21−28 Several studies using β-ether-type lignin model compounds have suggested that, as shown in Scheme 1, the phenolic end units of lignin polymers are degraded via a quinone methide intermediate QM to give an enol ether EE; the nonphenolic middle units of lignin polymers undergo β-ether cleavage via formation of an oxirane intermediate Ox.29 The two reactions shown in Scheme 1 do not require O2; therefore, it is unlikely that they are directly involved in the formation of low-MW products, including vanillin. However, the products EE and Ox 10113
DOI: 10.1021/acssuschemeng.7b02106 ACS Sustainable Chem. Eng. 2017, 5, 10111−10115
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ACS Sustainable Chemistry & Engineering
Table 1. Yields (wt %) of Vanillin, Vanillic Acid, Acetoguaiacone (AG), and p-Hydroxybenzaldehyde (pHBA) from Lignin Samples Degraded in Bu4NOH·30H2O at 120 °C and Their Yields from Alkaline Nitrobenzene (AN) Oxidation yield (wt %) reaction timea (h) Bu4NOH·30H2O AN oxidation
43 2.5
Bu4NOH·30H2O AN oxidation
70 2.5
Bu4NOH·30H2O AN oxidation
70 2.5
Bu4NOH·30H2O AN oxidation
43 2.5
vanillin
vanillic acid
Milled Wood Lignin 11.7 4.1 25.7 1.9 Sodium Lignosulfonate 4.6 1.2 7.1 1.3 Soda Lignin 4.2 1.9 7.3 2.2 Japanese Cedar Wood Flourc 15.4 ± 1.2 3.9 ± 0.6 27.2 1.2
pHBA
AG
total
0.5 0.1
0.1 0.8
16.3 (57.2)b 28.5
0.6 0.5
0.1 0.3
6.5 (70.7) 9.2
0.5 0.6
0.1 0.3
6.7 (64.4) 10.4
0.5 ± 0.2 1.2
22.5 ± 1.4 (76.0) 29.6
2.6 ± 1.8 Trace
a
The yields are those at the reaction time at which the total yield of the low-MW products was maximum. For AN oxidation, the reaction time was fixed at 2.5 h, according to the standard procedure for AN oxidation. bThe numbers in parentheses show the percentages of the AN oxidation yields achieved in our degradation in Bu4NOH·30H2O. cThe yield from wood flour is based on the lignin content (the total amount of Klason lignin and acid-soluble lignin) in Japanese cedar (34.3 wt %).
suggesting that aerobic oxidation is involved in the formation of these products. One of the advantages of the present Bu4NOH·30H2O method is that low-MW compounds, mainly vanillin, are produced in significant yields not only from sodium lignosulfonate, but also from other lignin-containing materials, including wood flour; therefore, the emissions of harmful sulfur-containing byproducts generated in the current vanillin production process can be avoided. However, there are several issues to be addressed to enable industrial applications of the present method. For example, the long reaction time (∼72 h) needed for the production of low-MW compounds has to be significantly shortened. The dependences of the yields of the low-MW products on the initial concentration of the lignin sample and the scale of the reaction also need to be investigated.
Scheme 1. Degradation of Phenolic End and Non-Phenolic Middle Units of Lignin under Alkaline Conditions12
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02106. HPLC chromatograms, vanillin yield (wt %) from soda lignin at various temperatures in Bu4NOH·30H2O, yield values used for Figures 2 and 3, and NMR spectrum of Bu4NOH·30H2O heated at 120 °C for 72 h (PDF)
in Scheme 1 can be further oxidized to low-MW compounds. Furthermore, the monomeric C6−C3 units in lignin polymers could react directly with O2 to give low-MW products. We hypothesize that the Bu4N+ cation affects such oxidation steps and enhances the formation of low-MW products, but more detailed investigation is necessary to fully elucidate the mechanisms of the formation of low-MW products and the roles of the Bu4N+ cation.
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CONCLUSIONS The degradation of lignin samples in Bu4NOH·30H2O at 120 °C for 43−70 h gave low-MW products, mainly vanillin and vanillic acid, in total yields of 6.5−22.5%, depending on the type of lignin sample. A comparison of the yields with those obtained in aqueous NaOH indicated that the formation of low-MW products was significantly enhanced by the presence of the Bu4N+ cation. This high selectivity for low-MW products was confirmed by the fact that the yields of low-MW products, especially that of vanillin, achieved in our degradation in Bu4NOH·30H2O reached 57.2−76.0% of those obtained by AN oxidation. The introduction of N2 into the reaction system greatly decreased the yields of low-MW products, strongly
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kohei Yamamoto: 0000-0001-8384-218X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS NMR analysis was performed at the Division of Forest and Biomaterials Science, Graduate School of Agriculture, Kyoto University, Japan. This work was supported by the Technologies for Creating Next-Generation Agriculture, 10114
DOI: 10.1021/acssuschemeng.7b02106 ACS Sustainable Chem. Eng. 2017, 5, 10111−10115
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ACS Sustainable Chemistry & Engineering
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Forestry, and Fisheries under the Cross-Ministerial Strategic Innovation Promotion Program (SIP) administered by the Council for Science, Technology, and Innovation (CSTI), Japan, and a Grant-in-Aid for Young Scientists (B) (No. 17K18008) from the Japan Society for the Promotion of Science. We thank Helen McPherson, PhD, from Edanz Group for editing a draft of this manuscript.
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