Article pubs.acs.org/IECR
Kinetic Study on the Selective Production of 2-(Hydroxybenzyl)-4methylphenol from Organosolv Lignin in a Mixture of Supercritical Water and p-Cresol Seiichi Takami, Kazuhide Okuda,† Xin Man, Mitsuo Umetsu, Satoshi Ohara,‡ and Tadafumi Adschiri* Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *
ABSTRACT: We have realized chemical recovery of a phenolic compound, 2-(hydroxy-benzyl)-4-methyl-phenol (BMP), from organosolv lignin without forming char in a mixture of supercritical water and p-cresol (Okuda, K.; et al. J. Phys.: Condens. Matter 2004, 16, S1325). In this paper, we evaluated the reaction rate constants of the depolymerization process of organosolv lignin using Monte Carlo simulation. We also evaluated the formation and decomposition rate of BMP based on the proposed reaction scheme. The obtained reaction rate constants gave insight into the mechanism why the mixture of supercritical water and p-cresol suppressed the formation of char. They also suggested the reaction path for the formation of BMP. In addition, these reaction rate constants enable the design and optimization of the chemical recovery processes to realize the highest yield.
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INTRODUCTION Using biomass as a source of chemicals and/or fuels decreases the consumption of fossil fuels and, therefore, has attracted recent interests.1−8 Chemical conversion of wood biomass is one strategy to realize carbon-neutral production of useful chemicals. Wood biomass comprises approximately 40−55 wt % cellulose, 25−40 wt % hemicellulose, and 20−35 wt % lignin.8 Lignin, a complex heteropolymer of p-coumaryl, coniferyl, and sinapyl alcohols, is of interest as a natural source of aromatic compounds.9−12 The annual production of wood in 2007 was 3.6 × 109 m3,13 which contained about 8 × 108 tons of lignin. In comparison, the annual production of benzene, toluene, xylene, and phenol in the world was 3.9, 1.7, 2.6, and 0.7 × 107 tons in 2006, respectively.14 Therefore, wood is a possible source of aromatic compounds. Many attempts have been made to depolymerize lignin into lighter chemicals.8,15−19 However, most of them resulted in the formation of char, i.e., solid products formed by uncontrolled polymerization. We studied chemical conversion of lignin in a mixture of supercritical water and p-cresol and found that organosolv lignin (the soluble component of lignin in organic solvents) with a mean molecular weight (MW) of 2.1 × 103 was depolymerized successfully without forming char.20 It produced a single chemical species, 2-(hydroxybenzyl)-4-methylphenol (BMP, Figure 1), with MW = 214 as the major product; the yield approached 75%. On the basis of the chemical structure of BMP and the yield of BMP as a function of reaction time, we proposed that the conversion of organosolv lignin into BMP involved hydrolysis (depolymerization) by supercritical water to form reactive fragmented intermediate species. In the presence of p-cresol, the intermediate species was efficiently stabilized by the reaction with p-cresol to form BMP (Scheme 1). We considered the possible structures for the fragmented species and found that guaiacylglycerol-β-guaiacyl ether (GGGE),21 where two guaiacyl units are connected by a glyceol unit, is the key to selective formation of BMP in a mixture of supercritical water © 2012 American Chemical Society
Figure 1. (a) Prominent structures in softwood lignin,2 (b) the recovered phenolic compound, and (c) the model compound for organosolv lignin as the minimum unit of lignin.
and p-cresol (Figure 1). We also confirmed that the yield of BMP from GGGE was much larger than that from the mixture of glycerol and guaiacol,21 indicating that the chemical structure of GGGE was important to produce BMP. In the present paper, we report the results of a kinetic study on the chemical conversion of organosolv lignin and GGGE to Received: Revised: Accepted: Published: 4804
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Scheme 1. Proposed Conversion Pathway of Organosolv Lignin into BMP in a Water−p-Cresol Mixture at 400 °C
Figure 2. Course of depolymerization and the resulting change in the MW distribution.
products for a given value of k1. Monte Carlo simulation was performed assuming that (1) dissociation between two adjacent monomer units occurs at a rate of k1 (s−1) independent of the position in the polymer structure; (2) dissociation produces two polymers with lower molecular weight, while conserving the total number of monomer units; (3) the dissociated bond is randomly selected from all bonds in the starting polymer; and (4) the structure of the starting polymer is linear. The detailed information about the simulation is shown in the Supporting Information. The molecular weight distribution of organosolv lignin was used as the initial condition for the simulation. To evaluate k1, we performed the Monte Carlo simulation using different k1 values and fitted the experimental results at a reaction time of 15 min, which has the least effect from the decomposition of produced fragments (labeled with k3 in Scheme 1), with the simulated molecular weight distribution. Figure 3 shows the result for depolymerization at 350 °C. We fitted the experimental result focusing on the MW distribution
determine the reaction rate constants for depolymerization (k1), formation (k2), and decomposition (k3) (Scheme 1). The results suggest the reasons why BMP was selectively produced while suppressing the formation of char. The procedures used to determine reaction rate constants should be applicable to other biomass conversion processes to maximize chemical yields from biomass.
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EXPERIMENTAL SECTION
Organosolv lignin was purchased from Aldrich and GGGE from Tokyo Kasei Kogyo. The average molecular weight of organosolv lignin was 2.1 × 103. Tetrahydrofuran (THF, HPLC grade) and p-cresol (99.0%) were purchased from Wako Pure Chemical Industries. The chemicals were used without further purification. Reactions were performed in a pressure reactor (5.0 cm3, SUS316). The reactor was loaded with either organosolv lignin or GGGE (0.10 g) and a mixture of distilled water (1.8 g) and p-cresol (2.5 g). The reactor was not purged with inert gas and was heated in an electrical furnace to 350, 380, 400, and 420 °C. The time required to reach the desired temperature was about 3 min. After the desired reaction time, the reaction was terminated by cooling the reactor in a water bath. The product was recovered by rinsing the reactor with THF (50 mL). Under the experimental conditions described in this paper, all products were soluble in THF. The products in THF were analyzed by gel permeation chromatography (GPC, GPC-900, JASCO) and gas chromatography equipped with a mass spectroscope (GC-MS, Saturn 2000, Varian). Detailed procedure and the used columns were described elsewhere.20,21 The amount of BMP was evaluated using the GC-MS areas of BMP and naphthalene, which was added in the THF soluble products and used as an internal standard for quantitative analysis.
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RESULTS AND DISCUSSION First, we determined the depolymerization rate of organosolv lignin (k1). We have studied the conversion of organosolv lignin into BMP at 400 °C as a function of reaction time and measured the MW distribution of the products by GPC.20 In this study, we performed similar experiments at 350, 380, and 420 °C. The rate of the change in the MW distribution closely relates to the depolymerization rate of the polymeric reactants (Figure 2). However, we could not evaluate the reaction rate constants by the conventional method because this reaction results in a decrease in the MW instead of a change in the concentration of the reactants and products. Therefore, we performed kinetic Monte Carlo simulation to predict the MW distribution of the
Figure 3. MW distribution of organosolv lignin and products at a reaction time of 15 min at 350 °C obtained by GPC measurements. This figure also shows the simulation results with different k1 values. The thick dotted line, which corresponds to k1 = 1.9 × 10−4 s−1, best fits the experimental data.
between 1 × 103 and 1 × 104 because this range was sensitive to the dissociation reaction. The simulation result with k1 = 1.9 × 10−4 s−1 best fitted the experimental data at 350 °C. We performed similar procedures to evaluate k1 for the experimental results obtained at 380, 400, and 420 °C. The Arrhenius plot of the reaction rate constants for depolymerization is shown in Figure 4 (open circles). The activation energy 4805
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r3 = k3[BMP]
(2)
On the basis of eqs 1 and 2, the BMP concentration can be expressed as [BMP] 1 = [fragment]0 1 − (k3/k2[p‐cresol]) × (e−k3t − e−k2[p‐cresol]t )
(3)
for the case where p-cresol is present in excess. To fit the experimental results with eq 3, the initial concentration of the fragmented species, [fragment]0, must be known. GGGE has two guaiacol units and a glycerol unit. Thus, as a side reaction, it may decompose in the presence of water to produce guaiacol and glycerol, which do not contribute to the formation of BMP.21 To determine the contribution of GGGE to the formation of BMP, we measured the concentration of the products from both the main and side reactions and evaluated that the initial concentration of the fragmented species can be approximated by
Figure 4. Arrhenius plot of the depolymerization reaction of organosolv lignin in water−p-cresol mixture (open circles). Solid circles indicate k2[p-cresol].
was estimated to be 79 kJ mol−1. McDermott et al. summarized the kinetic rate constants for the thermal decomposition reaction of lignin model compounds in an inert gas.22,23 Reported values ranged from 52.3 to 99.9 kJ/mol, and therefore the obtained k1 values were close to these values. We then determined the reaction rate constants, k2 and k3, for the formation and decomposition of BMP, respectively. To study the kinetic rate constants, the initial concentrations of the reactants must be known. However, the fragmented intermediate species are gradually produced by the depolymerization of organosolv lignin in this process. Thus, we decided to use GGGE, which was shown to be a minimal model structure of organosolv lignin, to study the formation of BMP21 and the rate constants for the reaction. We have investigated the yield of BMP from GGGE as a function of reaction time in the presence of supercritical water and p-cresol. We performed similar experiments at 350, 380, and 420 °C. Experimental results are shown in Figure 5.
[fragment] = 0.6[GGGE]0
(4)
under the current experimental conditions using the delplot method, which was proposed by Klein.24 On the basis of eqs 3 and 4, we fitted the experimental results to evaluate k2 and k3 (Figure 5). The sensitivity analysis indicated that the obtained results had 10% or less error (see the Supporting Information). Figure 6 shows Arrhenius plots of k2 and k3. The activation energies of the formation and decomposition of BMP were
Figure 6. Arrhenius plot of the (a) formation and (b) decomposition of BMP. Closed circles represent the rate constants for the alkylation of phenol by 2-propanol reported by Sato et al.8.
estimated as 81 and 43 kJ mol−1, respectively. With regard to the liquefaction of GGGE, Lin et al. discussed the reaction of GGGE in phenol under acid catalysis at 150 °C.25,26 They proposed that a hydroxyl group at the α-position of GGGE was attacked by protons to form a benzyl cation that reacted with phenol to produce phenolated products. Although their reaction conditions differed from ours, we suppose that the formation of BMP involves dehydroxylation at the α-position of glycerol in the GGGE unit (1), followed by the addition of p-cresol. The products (3) had possibly undergone α−β cleavage as Lin et al. proposed25,26 and deformylation to produce BMP (5) (Scheme 2). To verify this scheme, we compared our results with the kinetic rate constants for the reaction of phenol with 2-propanol in supercritical water. Figure 6a compares our results with those reported by Sato et al.27 They reported that the activation energy for the reaction was 129 kJ mol−1. These results support the proposed reaction scheme, where p-cresol
Figure 5. Concentration of BMP produced from GGGE in a mixture of water and p-cresol (symbols). This figure also shows the curves fitted to determine k2 and k3 (curves).
The experimental results showed that the BMP concentration increased in the beginning and then decreased, suggesting that BMP was produced as an intermediate species of the consecutive reactions (Scheme 1). The reaction rates for the formation and decomposition of BMP are expressed as r2 = k2[p‐cresol][fragment]
(1) 4806
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on the yield of BMP from organosolv lignin. The yield was evaluated on the basis of the mass balance of benzene rings using the following equation,
Scheme 2. Proposed Reaction Pathway for BMP Formation from Organosolv Lignin without Char Formation
yield (%C) = moles of produced BMP moles of phenyl group in the phynylpropane structure of organosolv lignin
(5)
Figure 7 indicates that the simulation results well-reproduced the experimental results, indicating the validity of our approach.
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SUMMARY In summary, we performed a kinetic study on the chemical recovery processes from organosolv lignin. The results indicated that the faster reaction between p-cresol and fragmented intermediate species enabled the chemical conversion of organosolv lignin without forming char. The activation energy for the formation of BMP suggested that the rate-limiting process in BMP formation from intermediate species was the reaction of the α-carbon of glycerol in GGGE with p-cresol. These results suggest a way to design and optimize the chemical recovery processes of aromatic compounds from lignin.
reacted with the α-position of dehydroxylated GGGE (2), followed by α−β cleavage and deformylation. The proposed reaction path involves dehydroxylation at the α-position, followed by the reaction with p-cresol. Saisu et al. have discussed the reaction pathway of lignin in supercritical water.5 They suggested that lignin decomposed via hydration and dealkylation to form low-MW fragments. These fragments having reactive functional groups produced higher MW products through a cross-linking reaction. In our case, the dehydroxylated species 2 is possibly reactive. However, in the presence of p-cresol, the capping of the carbonium ion by pcresol might prevent unfavorable polymerization. In Figure 4, the reaction rate constants k1 and k2[p-cresol] that correspond to the depolymerization of lignin and the capping of the intermediate species, respectively, are compared. The rate constant for the capping reaction is ∼4−5 times faster than the depolymerization rate of organosolv lignin. This comparison supports the above analysis. The fragmented intermediate species 2, which was produced by depolymerization of organosolv lignin, was quickly stabilized by p-cresol. This quick capping possibly suppressed the formation of char by prohibiting polymerization of reactive intermediate species. On the basis of the proposed reaction scheme and the determined reaction rate constants, we predicted the yield of BMP from organosolv lignin. By combining Monte Carlo simulation and eqs 1 and 2, we calculated the yield of BMP as a function of reaction time and temperature. The results are shown in Figure 7. Figure 7 also shows the experimental results
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ASSOCIATED CONTENT
S Supporting Information *
Text discussing detailed information about the kinetic Monte Carlo simulation and the sensitivity analysis and figures showing a flow chart of the kinetic Monte Carlo simulation for lignin depolymerization and simulated fitting curves with changed k2 and k3 values. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Addresses †
Research Laboratory for Hydrothermal Chemistry, Kochi University, 2-5-1 Akebono, Kochi 780-8520, Japan. ‡ Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki 567-0047, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by KAKENHI, Grant-inAid for Scientific Research (B) 14350413.
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REFERENCES
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Figure 7. Concentration of BMP produced from organosolv lignin as a function of reaction time. Symbols show the experimental results. The lines show the simulation results using k1, k2, and k3 that were evaluated as discussed previously. 4807
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