Article pubs.acs.org/IECR
Manageable Conversion of Lignin to Phenolic Chemicals Using a Microwave Reactor in the Presence of Potassium Hydroxide Hyeung Geun Kim and YoonKook Park* Department of Biological and Chemical Engineering, Hongik Unversity, 2639 Sejong Road, Sejong, S. Korea, 339-701 S Supporting Information *
ABSTRACT: Sulfonated lignin was converted to phenol and phenolic compounds using a conventional batch or a microwave reactor. In the batch reactor, changing the medium from pure water to an aqueous solution of potassium hydroxide (KOH) dramatically increased the number of liquid products detected by gas chromatography. Similar liquid products were also obtained when a hydrogen peroxide solution was used. In addition to the reaction medium in the batch reactor, the severity factor also plays a critical role in determining the type and the number of liquid products detected. When a microwave reactor was used, lignin conversion resulted in several different liquid products. Of the liquid products, six phenolic compounds (guaiacol, vanillin, homovanillic acid, phenol, acetovanillone, and syringol) were quantitatively analyzed. The mass of the liquid products increased as the temperature increased. Altering the KOH concentration produced mixed results as the amounts of vanillin, acetovanillone, phenol, and syringol decreased, while that of guaiacol increased and homovanillic acid did not change. Thus, the phenolic compounds produced by lignin conversion can be adjusted by varying the operating conditions in a less energy intensive microwave reactor.
1. INTRODUCTION Because of its strong phenolic resin backbone structure, lignin has not been studied much as a source for chemical production compared to other lignocellulosic biomass primary component counterparts of cellulose and hemicellulose. Lignin, which consists of coniferyl alcohol, sinapyl alcohol, and coumaryl alcohol polymers, represents a major fraction of the lignocellulosic biomass (10−30 wt %). It is currently primarily used as a low-grade fuel to provide heat in the pulp and paper industry.1 However, Pandey and Kim2 recently described several new methods, consisting of variations of three primary processes: pyrolysis, oxidation, and combustion, that have been introduced to utilize lignin. Others have suggested the use of catalysts to convert lignin into valuable chemicals. For example, Ma et al.3 obtained phenol alkoxy species from lignin using a H-ZSM5 catalyst at 400−700 °C and depolymerized lignin in aqueous alkaline solutions using a continuous flow reactor to generate four fractions: gaseous, small organics, aromatic monomers, and oligomers.4 Recently, Mahmood et al.5 proposed a hydrolysis reaction to obtain depolymerized lignin from kraft lignin (KL). They demonstrated that higher temperatures favored reducedmolecular weight polyols, while a longer reaction time promoted dehydration reactions. However, the KL underwent severe reaction conditions to produce pyrocatechol and other chemicals. Rodrigues’ group6 has been heavily involved in research on new ways to convert lignin to vanillin and lignin-based polyurethane using both batch and continuous reactors. Another approach, reported to improve the value of lignin for production of catechol and 4-methylcatechol, is to vary the severity factor.7 Labidi’s group8 reported that the liquid product composition obtained from organosolv lignin depolymerization strongly depended on the applied catalyst. © XXXX American Chemical Society
Although a comprehensive blueprint of an integrated lignin conversion process has been presented, it remains unsuitable for commercial application because of its high energy consumption. Microwave reactors have been investigated as an alternative method for biomass conversion. Microwave heating reduces the heating time and boosts energy absorption, both of which improve the overall process efficiency.9 For example, Kim et al.10 demonstrated the successful use of microwave heating to enhance conversion of hemicelluloses to furfural, in the presence of maleric acid. The selective, high yield transformation of lignin to aromatics with reduced coke formation was described by Zakzeski et al.11 Ye et al.12 showed that by adjusting the reaction conditions for lignin depolymerization different chemical products may be produced. In this study, a less energy intensive microwave reactor was used to convert lignin to phenol and phenolic chemicals in the presence of potassium hydroxide (KOH) and the composition of the resulting phenolic chemicals was determined using quantitative analysis. The effect of an ultrasonic pretreatment step on the overall lignin conversion process was also investigated.
2. MATERIALS AND METHODS 2.1. Materials. Lignin (low sulfonate content, Mw ∼ 10 000, CAS no. 8068-05-1), potassium hydroxide (CAS no. 306568; 99.99%), guaiacol (CAS no. 90-05-1; >98%), vanillin (CAS no. 121-33-5; 99%), homovanillic acid (CAS no. 306-08-1; 99.0%), phenol (CAS no. 108-95-2l; ∼99%), acetovanillone (CAS no. 498-02-2; ≥98%), 4-methylcatechol (CAS no. 452-86-8; Received: March 5, 2013 Revised: June 27, 2013 Accepted: July 6, 2013
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≥95%), toluene (CAS no. 108-88-3; ≥99.9%), and syringol (CAS no. 91-10-1; 99%) were all purchased from SigmaAldrich Co. (St. Louis, USA). Ethyl acetate (CAS no. 141-78-6; 99.9%) and hydrogen peroxide (CAS no. 7722-84-1; ∼30%) were purchased from Samjun Chem. Co. (Seoul, S. Korea). All chemicals were used without further purification. 2.2. Experimental Methods. Approximately 10 g of lignin and 140 g of deionized water were loaded into a high-pressure batch reactor (with a volume of ∼500 cm3) made of SS 316 (Wonil industry Co., Namyangjoo, S. Korea). To investigate the effect of KOH on the conversion of lignin under near critical conditions, a 0.1 M of KOH solution was used instead of deionized water. The influence of an oxygen source on lignin conversion under near critical conditions was also examined by replacing some water with 20 g of 30 wt % H2O2 solution. The microwave reactor used in this study was an Initiator Exp EU (Biotage, Shanghai, China) that allows the reaction temperature to be varied with a fixed output of 400 W at 2.45 GHz. The range of temperature applied in this study was 125− 175 °C. A temperature of 175 °C was chosen as the highest reaction temperature because the maximum holding pressure of the microwave reactor is 20 bar. The way of reading the temperature of vial surface in the microwave reactor system is infrared sensing. We have periodically (once a year) calibrated the temperature reading with the aid of an engineer from the vendor. In order to avoid a hotspot problem, we added a magnetic stirrer bar inside the vial before the reaction gets started. A fixed amount of pure lignin (∼ 0.3 g) was dissolved in either deionized water or a known solution of KOH (∼3 g) in a 20 mL vial and placed in the microwave reactor. In experiments where an ultrasonicator (Model No. VCX-750, Sonic Inc., Newtown, CT) was used to examine the effect of ultrasonic pretreatment on the lignin conversion reaction, the pretreatment was applied prior to the lignin aqueous solution being placed in the microwave reactor. 2.3. Analytical Methods. When the reaction was complete, the liquid containing the phenolic organic products was filtered with filter paper (Whatman, cat. no. 1820-055) to remove any residual fine particles, and then, the products were extracted with ethyl acetate in a separating funnel. There are many possible solvents used for extraction. Unfortunately, the choice of acetone and tetrahydrofuran was not valid because those two solvents are soluble in water. In terms of high volatility and extractability, we used ethyl acetate as a solvent in extraction. Further methylene chloride has been used as a solvent for extraction. However, it led us to obtain much less phenolic compounds that we intended to produce. Once the organic products were analyzed with the gas chromatography/mass spectrometer (GC/MC), gas chromatography (GC) was used to quantitatively analyze the liquid products. The instruments used for these analyses were a GCMS (HP 6890) equipped with a mass selective detector (MSD 5937) and DB-5 column (0.32 mm × 30 m × 0.25 μm; Agilient) and a GC (GC-2014; Shimadzu) equipped with a flame ionization detector and DB-5 column. Both the GC-MS and GC were operated in a programming temperature mode. The initial column temperature of 50 °C was held for 2 min before linearly increasing to 280 °C at a rate of 10 °C per min, and the column was then held at the upper temperature for 2 min. The detector was maintained at 280 °C and the injector port at 290 °C.
Of the primary products defined as phenolic chemicals, more than 80% of the MS data obtained from the organic phase matched the spectra included in the GC-MS library. After the primary products had been identified, a known sample of each product was individually added to the organic layer for product verification. The retention times were compared as the area of each primary product correspondingly increased, and it was confirmed that the main products were guaiacol, vanillin, homovanillic acid, acetovanillone, phenol, and syringol. After the reaction, an internal standard of toluene was added to the ethyl acetate solution for quantitative GC analysis.
3. RESULTS AND DISCUSSION 3.1. Effect of Reactor Type on the Lignin Conversion Reaction. Table 1 shows the primary products obtained from a Table 1. Experimental Conditions and Results for the Batch Reactor exp. no.
KOH [M] H2O2 [wt %]
reaction temp [°C]
reaction time [min]
log Ro
B-1 B-2
0 0.1 M
300 300
60 60
7.7 7.7
B-3
5 wt %
300
60
7.7
B-4 B-5
5 wt % 5 wt %
300 350
120 60
8.0 9.2
a
primary liquid product nda catechol, 3methylcatechol, 4methylcatechol 4-methylcatechol, phenol, o-cresol, 2,5xylenol phenol, guaiacol catechol,4methylcatechol, 4ethylcatechol, guaiacol
Not detected.
lignin conversion reaction in a batch reactor. The lignin conversion reaction was carried out in the presence of neat water (B-1), and no liquid products were detected. When a 0.1 M KOH solution was used instead of water, with a reaction time of less than 1 h at 300 °C (B-2), catechol, 3-methyl catechol, and 4-methyl catechol were identified as the primary products. Under the same reaction time and temperature with a 5 wt % H2O2 solution (B-3), o-cresol, 4-methyl catechol, phenol, and 2,5-xylenol were identified as the primary products. An additional hour of reaction time (B-4) produced no significant changes in terms of the primary products detected. However, when the reaction temperature was increased to 350 °C (B-5), the major products were catechol, 4-methyl catechol, 4-ethyl catechol, and guaiacol; thus, changing the reaction temperature had a considerable effect on the liquid product distribution of phenolic compounds. Toledano et al.8 previously studied the effect of the severity factor, which was also investigated in this research. The severity factor, Ro, is an effective reaction condition that combines reaction temperature and time into a single factor and is defined as follows: ⎛ Texp − 100 ⎞ exp⎜ ⎟ dt ⎝ 14.75 ⎠
(1)
⎡ ⎛ Texp − 100 ⎞⎤ log R o = log⎢t ⎜ ⎟⎥ ⎣ ⎝ 14.75 ⎠⎦
(2)
Ro =
∫0
t
When a severity factor of 7.7 was applied, the reaction media (See B-1, B-2, and B-3) played a critical role in determining the B
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of valuable chemicals.14 Hence, in this study, the reaction mass was pretreated using an ultrasonicator prior to the microwave reaction. In order to quantify the effect of this pretreatment, the resulting masses of the identified liquid products from the microwave reactor were compared with those obtained without pretreatment. Unfortunately, no significant effect was observed with the ultrasonicator pretreatment, partially because the lignin used in this study had already undergone pretreatment when it was separated from the biomass. As expected, the masses of the identified liquid products increased as the reaction time increased from 1 (M-20) to 4 h (M-21). For example, the masses of guaiacol and acetovanillone increased from 4.72 to 10.01 μg and from 0.20 to 0.44 μg, respectively. Therefore, it seems likely that this lignin conversion reaction is kinetically controlled for periods up to 4 h. Although the maximum yields obtained for phenolic chemicals in this study were limited, the results are still of considerable interest. This technique is a far less energy intensive process than the traditional process, and it can be utilized to convert lignin into phenol and phenolic compounds. It also has the potential for further optimization as part of an integrated multistep process.
types and the number of liquid products generated. In the absence of a catalyst (B-1), there were no detectable liquid products. However, when either KOH or H2O2 was used, several liquid products such as 4-methylcatechol were detected. When the severity was increased to 9.2 (B-5) from 7.7, guaiacol was produced along with the other liquid products, as Labidi’s group previously reported.7 To fully investigate the effect of the reactor type on lignin conversion, it was necessary to keep all the reaction parameters the same and change only the reactor type (i.e., the conventional batch reactor or microwave reactor). Unfortunately, because the maximum pressure for the microwave reactor in this study was limited to ∼20 bar, it was not possible to perform a corresponding set of experiments for direct comparison. As previously mentioned, the use of a microwave reactor provides numerous advantages, including rapid heat-up times and efficient energy absorption, to yield results that are very close, if not identical, to those possible using conventional heating methods.9 The experimental conditions used for the microwave reactor and the resulting liquid products identified by GC are listed in Table S1 (Supporting Information). A total of six identified compounds were quantitatively analyzed. Interestingly, the six compounds produced using the microwave reactor and the batch reactor were entirely different. The difference in the obtained liquid products may be due to the different reaction pathways, which may depend on the reactor type used for lignin conversion. 3.2. Effect of Temperature on Lignin Conversion Reactions in the Microwave Reactor. The mass of all six identified chemicals increased as the reaction temperature increased from 125 to 175 °C during a 60 min reaction time. This concentration enhancement is likely due to the fact that more energy was transferred directly to lignin, which considerably increased the quantity of oligomers formed from the phenolic resin polymer, resulting in higher concentrations of the identified products. For example, the mass of guaiacol produced, using a 1 M KOH solution and 1 h reaction time, increased from 0.29 μg at 125 °C to 4.85 μg at 175 °C (for M14 and M-16, respectively). Similar experimental results have been observed for styrene butadiene rubber under supercritical water oxidation.13 3.3. Effect of KOH Concentration on the Lignin Conversion Reaction in the Microwave Reactor. Interestingly, varying the concentration of KOH yielded mixed results in terms of the product yields obtained via microwave reaction. Specifically, the mass of guaiacol increased from 2.58 to 4.72 μg (for M-4 and M-20, respectively) as the KOH concentration increased from 0 to 2 M. However, the mass of vanillin obtained with 2 M KOH decreased to 0.17 from 2.32 μg in the absence of KOH. Similar decreases were observed for three other identified products: acetovanillone, phenol, and syringol. The effect of KOH concentration on the amount of homovanillic acid obtained via lignin conversion reaction was insignificant, falling within a range of 1.09−1.67 μg (for M-4 and M-20, respectively). In short, the role of KOH in the lignin conversion reaction is not fully understood. However, it is clearly possible to boost vanillin yield, for example, by carrying out the reaction with little to no KOH. 3.4. Effect of Ultrasonicator Pretreatment and Longer Reaction Times on the Lignin Conversion Reaction in the Microwave Reactor. Ultrasonic pretreatment for the biomass conversion reaction is known to boost the production
4. CONCLUSIONS Using a conventional batch reactor, catechol, 4-methylcatechol, and other phenolic compounds were the primary liquid products obtained from the conversion of sulfonated lignin under near critical water conditions in the presence of either KOH or H2O2. Using a microwave reactor, however, yielded different liquid products: guaiacol, vanillin, homovanillic acid, acetovanillone, phenol, and syringol. The difference in the liquid product mix obtained from batch and microwave reactors may be attributed to different reaction pathways in the two reactor types. In general, increasing the reaction temperature from 125 to 175 °C increased product yields. The effect of the KOH concentration on the masses of different products varied considerably. In the case of guaiacol, as increase in KOH concentration increased the mass of guaiacol obtained. In contrast, the masses of vanillin, acetovanillone, phenol, and syringol products decreased as the KOH concentration increased.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental conditions and liquid product concentration determined by GC for the microwave reactor. 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]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF2012-S1A2A1-A01029203). C
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