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Esterification of Levulinic Acid into n-Butyl Levulinate Catalyzed by Sulfonic Acid-Functionalized Lignin-Montmorillonite Complex

  • Corresponding author: Xianxiang Liu; 
  • Received Date: 2020-04-15
    Accepted Date: 2020-06-12
    Fund Project:

    the National Natural Science Foundation of China 21975070

    China Postdoctoral Science Foundation 2019M662787

    Hunan Provincial Natural Science Foundation of China 2018JJ3334

    the National Natural Science Foundation of China 21606082

    the National Natural Science Foundation of China 21776068

  • In this study, sulfonic acid-functionalized lignin-montmorillonite complex (LMT-SO3H) was prepared and employed as an efficient heterogeneous catalyst for the esterification of levulinic acid (LA) into n-butyl levulinate (BL). An intermediate pseudo-butyl levulinate (p-BL) was determined by distilled water treatment and nuclear magnetic resonance (NMR) analysis, and a possible mechanism for the esterification of LA is proposed. The effects of various process parameters were studied and the results showed that the LMT-SO3H catalyst had the excellent catalytic performance for esterification of the LA. Under optimum reaction conditions, the yield of BL was 99.3% and the conversion of LA was 99.8%. The LMT-SO3H catalyst exhibited strong acidic sites and high stability even after seven cycles of usage. Furthermore, esterification of the LA with various alcohols over the LMT-SO3H was further investigated.
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Esterification of Levulinic Acid into n-Butyl Levulinate Catalyzed by Sulfonic Acid-Functionalized Lignin-Montmorillonite Complex

    Corresponding author: Xianxiang Liu; 
  • National & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of Resources, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China
Fund Project:  the National Natural Science Foundation of China 21975070China Postdoctoral Science Foundation 2019M662787Hunan Provincial Natural Science Foundation of China 2018JJ3334the National Natural Science Foundation of China 21606082the National Natural Science Foundation of China 21776068

Abstract: In this study, sulfonic acid-functionalized lignin-montmorillonite complex (LMT-SO3H) was prepared and employed as an efficient heterogeneous catalyst for the esterification of levulinic acid (LA) into n-butyl levulinate (BL). An intermediate pseudo-butyl levulinate (p-BL) was determined by distilled water treatment and nuclear magnetic resonance (NMR) analysis, and a possible mechanism for the esterification of LA is proposed. The effects of various process parameters were studied and the results showed that the LMT-SO3H catalyst had the excellent catalytic performance for esterification of the LA. Under optimum reaction conditions, the yield of BL was 99.3% and the conversion of LA was 99.8%. The LMT-SO3H catalyst exhibited strong acidic sites and high stability even after seven cycles of usage. Furthermore, esterification of the LA with various alcohols over the LMT-SO3H was further investigated.

1.   Introduction
  • The conversion of renewable biomass resources into various valuable chemicals has been the focus of academic research to reduce the excessive dependence on nonrenewable fossil resources over the past few decades (Zhang et al., 2012). Levulinic acid (LA), one of the most important biomass-derived platform molecules, has received massive importance due to its broad substrate applicability to synthesize various value added chemicals such as angelica lactone, diphenolic acid, alkyl levulinates, succinic acid, 2-methyl tetrahydrofuran, 3-hydroxy propanoic acid and γ-valerolactone (Mukherjee et al., 2015; Pileidis and Titirici, 2016; Yan et al., 2017; Kang et al., 2018). Among them, alkyl levulinates are known to have widespread applications such as plasticizing agents, solvents, odorous substances, and fuel additives (Demolis et al., 2014; Marcel et al., 2019; Badgujar et al., 2020). It has been considered as a strong potential for substituting current chemicals produced from petro-chemical routes. Conventionally, alkyl levulinates were mainly obtained by direct esterification of the LA with alcohols in the presence of acid catalysts such as H2SO4, H3PO4 or HCl (Christensen et al., 2011). However, drawbacks associated with environmental problems necessitate a replacement of the acids by more suitable catalysts, which would be less corrosive and reusable such as heterogeneous catalyst (Pavlovic et al., 2019; Zhang et al., 2019).

    Up to now, various heterogeneous catalysts such as zeolites (Maheria et al., 2013; Nandiwale and Bokade, 2015; Morawala et al., 2019; Morawala et al., 2020), heteropoly acids (Dharne and Bokade, 2011; Su et al., 2013), mesoporous molecular sieves (Najafi Chermahini et al., 2017), lipase (Zhou et al., 2018), magnetic organosilica nanoflower (Gao et al., 2017), WO x/ZrO2 (Ciptonugroho et al., 2016), HPA/C-Sil-1(Manikandan and Cheralathan, 2017), HClO4/SiO2 (Yang and Tang, 2019), acid ion exchange resins (Iborra et al., 2019), WO3-SBA-16 (Enumula et al., 2017), SnTUD-1 (Pachamuthu et al., 2019), Zr-MOF (Cirujano et al., 2015), sulfonated carbon (Yang et al., 2018), TNTs-SO3H (Zhou et al., 2019), sulfonic acid-functionalized organic polymer (Tian et al., 2020) and ionic liquid (Kalghatgi and Bhanage, 2019), have been utilized to produce alkyl levulinates from LA with n-butyl alcohol. Although these promising heterogeneous catalysts effectively catalyzed the conversion of LA to n-butyl levulinate (BL), the expensive cost, easy leaching of active sites and low catalytic activity retard their industrial applications. Accordingly, an environmentally friendly, reusable and efficient solid acid catalyst is urgently desired for the esterification of LA into BL.

    Montmorillonite (MMT), one of the abundant naturally occurring clays and composed of stacked, two-dimensional alu minosilicate layers, has a significant cation exchange ability, good adsorption capacity, flexible acidity, and expansible interlayer space (Cao et al., 2019). Thus, it can adsorb hydroxyl-containing substances such as lignin. In addition, montmorillonite has been used as effective environmentally benign heterogeneous catalysts and promising acidic supports for many chemical reactions (Varadwaj et al., 2016; Bonacci et al., 2019). For example, Xu et al. (2015) reported a highly efficient conversion of carbohydrates such as glucose to methyl levulinate in methanol with a series of sulfated montmorillonite. Under the optimal conditions, the conversion of glucose and fructose was up to 100%, and the methyl levulinate yields obtained from glucose and fructose were 48% and 65%, respectively.

    Papermaking black liquor is the main pollutant in the papermaking industry, causing water pollution. The lignin is the main component of papermaking black liquor and its recovery is expensive and difficult to be treated due to fine particle size of alkali lignin (< 500 nm) and suspending particles (< 50 nm), high alkalinity (pH = 13). Recently many researchers have reported about using kraft lignin to produce activated carbon (Gao et al., 2013), carbon-based nanocomposite anodes (Yi et al., 2017) and research on ammoniated lignin diesel oil (Sun et al., 2014). Thus, the utilization of lignin from papermaking black liquor has received extensive attention in recent years.

    Inspired by the present research situations, we synthesized an efficient and low-cost solid acid catalyst LMT-SO3H for the esterification of renewable LA with n-butyl alcohol (Fig. 1). The sulfonic acid-functionalized lignin-montmorillonite complex (LMT-SO3H) catalyst was prepared by carbonization and sulfonation method using papermaking black liquor as precursor and montmorillonite as support. Furthermore, the catalyst amount, reaction time, the molar ratios of the acid to the alcohol, substrates and water-carrying agents were found to have significant effects on this catalytic system. The LMT-SO3H catalyst was expected to be a good candidate for alkyl levulinates production from biomass-derived platforms.

    Figure 1.  Esterification of levulinic acid (LA) with n-butyl alcohol over LMT-SO3H catalyst

2.   Materials and Methods
  • Montmorillonite (cation exchange capacity = 0.9 mmol/g) was purchased from Zhejiang Fenghong New Material Co., Ltd. (Huzhou, China), papermaking black liquor (concentrated, lignin content of about 25%) was collected from Hongjiang Wanyuan Chemical Co., Ltd. (Huaihua, China). Sulfuric acid was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the chemicals were of analytical grade and used without further purification.

  • LMT-SO3H: the lignin-montmorillonite complex (LMT) was prepared by flocculating precipitation of lignin in black liquor from the paper industry with MMT. In a typical procedure, 5.0 g MMT was mixed with 25 mL distilled water and 20 g black liquor, stirring for 24 h, then 1 mol/L sulfuric acid was added into the mixture to adjust to pH 6–7, and stirring for 24 h again. Finally, the mixture was milled for three hours, followed by rotary evaporation to a moisture content of about 50%, dried at 120 ℃ for 12 h to obtain the LMT. The above LMT sample is placed in a tube furnace at 325 ℃ under N2 protection for charring two hours to obtain carbonized LMT (LMT-C). Sulfated LMT was prepared as follows: 1.0 g LMT-C was added to 8 mL of a solution of sulfuric acid at 150 ℃ with constant stirring for four hours. After the reaction, the product was filtered and washed repeatedly with deionized water until the pH was neutral, and then dried at 80 ℃ for 12 h. The obtained powder is denoted as LMT-SO3H.

  • Thermogravimetric (TG-DTG, Germany) curves were recorded in air flow on a Netzsch Model STA 409PC instrument with a heating rate of 20 ℃/min from the room temperature to 750 ℃ using α-Al2O3 as the standard material. The Fourier transform infrared spectrum (FT-IR) of the samples was collected by the KBr pellet technique on a Nicolet 370 infrared spectrophotometer in the range of 4000–500 cm–1.

  • The catalytic esterification of the LA was carried out in a 25 mL two-neck flask equipped with a water knockout trap, reflux condenser and magnetic stirrer. In a typical reaction, LA (15 mmol) was mixed with n-butanol (21 mmol), cyclohexane (10 mL) as the water-taking agent. After the target catalyst was added in the above solution, the mixture was heated at 120 ℃ under stirring for four hours. After cooling the reactor to room temperature, 0.1 mL of the reaction mixture was withdrawn and then diluted with acetonitrile to 5 mL. The diluted suspension was centrifuged, and the clear solution was analyzed by gas chromatography (Agilent Technologies 6890N) with a crosslinked HP-5 capillary column (30 m × 0.25 mm × 0.50 μm), which was equipped with a flame ionization detector (FID). Operating conditions were as follows: the flow rate of the N2 carrier gas was 40 mL/min, the injection port temperature was 250 ℃, the oven temperature was 180 ℃, and the detector temperature was 250 ℃. The peaks were identified by comparison of the retention time of the unknown compounds with those of standard compounds and quantified based on the internal standard method.

3.   Results and Discussion
  • The functional groups of these samples were identified by FT-IR analysis, and the corresponding FT-IR spectra were depicted in Fig. 2. In the case of the MMT, the peak around 3600–3235 cm–1 is associated with O—H stretching vibration and the peak at 1480 cm–1 is related to the symmetric stretching vibration of Si—OH (Shirini et al., 2011). For LMT-C, a new vibration bands C=O (1610 cm–1) emerged and the intensity of —OH peak around 3600–3235 cm–1 was amplified, due to the covalent attachment of lignin to the MMT. After sulfonation, the existence of sulfonic —SO3H group in the samples was confirmed by the presence of stretching modes at 1150 cm–1, which were considered as the evidence for S=O asymmetric and symmetric stretching modes (Kolvari et al., 2015).

    Figure 2.  The FT-IR spectra of montmorillonite (MMT), carbonized lignin-montmorillonite complex (LMT-C) and sulfonic acid-functionalized lignin-montmorillonite complex (LMT-SO3H)

    The TG/DTG was performed for the characterization of LMT-SO3H in comparison with LMT-C, and the results are shown in Fig. 3. The initial weight loss of 6.2% at about 110 ℃ corresponding to the loss of the physically adsorbed water was observed for LMT-C (Fig. 3a). However, a great additional weight loss appeared at 200 ℃–600 ℃ with an endothermic DTG signal at 250 ℃, which can be attributed to the dehydroxylation of montmorillonite (Shirini et al., 2011; Varadwaj et al., 2016). Compared with the observation of the LMT-C, the DTG curve pattern of LMT-SO3H (Fig. 3b) exhibits a strong endothermic DTG signal occurred at 450 ℃, which could be attributed to the decomposition of SO3H group, and agrees with the reported literature (Zhou et al., 2019). The TG/DTG results show that the LMT-SO3H catalyst has great thermal stability and applicability in organic reaction with temperatures below 220 ℃.

    Figure 3.  The thermogravimetric (TG-DTG) curves of (a) LMT-C and (b) LMT-SO3H

  • Firstly, the catalytic performance of different acid catalysts for the synthesis of the BL was investigated (Table 1). The results reveal that oxalic acid as the active acid type shows the lowest catalytic activity for the conversion of LA, while the LMT-SO3H affords the highest catalytic activity as compared with other acid catalysts. Interestingly, by using A- and R- TiO2NTs-SO3H catalysts, the LA conversion values of 73.4% and 74.0% were obtained, respectively, whereas the yield of BL production was below 20%, probably due to the production of various by-products.

    Entry Catalyst Conversion (%) BL selectivity (%) BL yield (%)
    1 LMT-SO3H 99.8 98.7 98.5
    2 Pine-SO3H 98.1 97.5 95.6
    3 A-TiO2NTs-SO3H 73.4 17.6 12.9
    4 R-TiO2NTs-SO3H 74.0 13.1 9.7
    5 HOOC-COOH 48.5 85.5 41.5
    6 H3PO4 70.5 96.7 68.2
    7 ZnCl2 51.3 78.5 40.3
    8 CuSO4 89.3 83.0 74.1
    Notes: reaction conditions are 15 mmol LA, 21 mmol BuOH, 10 wt% catalyst amount, 10 mL cyclohexane, and reflux reaction at 120 ℃ for four hours. BL, n-butyl levulinate.

    Table 1.  Catalytic performance of different catalysts in esterification of levulinic acid (LA)

  • Figure 4 shows the effect of reaction time on the esterification of LA in the presence of LMT-SO3H. The conversion of LA rose from 40.7% to 99.8% with increasing reaction time from 0.5 h to 4.0 h, the selectivity of BL was increased from 63.9% to 98.7%. In addition, an intermediate p-BL during the reaction process was identified by GC-MS techniques. When the reaction time increased from 0.5 h to 4.0 h, the selectivity of p-BL significantly decreased from 36.1% to 1.3%. It indicated that the intermediates p-BL that produced during the reaction gradually transformed into BL with increasing reaction time and the path of LA to BL was: LA→p-BL→BL under the catalysis of LMT-SO3H. When increasing reaction time from 2 h to 4 h, there was an insignificant change in the selectivity of BL and p-BL, therefore the conversion process of p-BL to BL was the reaction rate-determining step. Based on the results above, we proposed a possible reaction pathway for LA to be converted to BL over the LMT-SO3H.

    Figure 4.  Effect of reaction time on LA esterification

  • Since the esterification reaction is reversible, the choice of water-carrying agent is more important. In this paper, the yield of esterification of LA was studied in the presence of different water-carrying agents. As shown in Table 2, when using benzene as the water-carrying agent, the yield of the product is significantly lower than that of toluene and cyclohexane. Furthermore, considering that benzene is more toxic and the reflux temperature of cyclohexane is lower, cyclohexane is a suitable water-carrying agent for the reaction.

    Entry Water-carrying agent T (℃) t (h) Yield (%)
    1    Benzene 80 4 74.0
    2    Toluene 110 4 96.3
    3    Cyclohexane 80 4 98.6
    Notes: reaction conditions are 15 mmol LA, 21 mmol BuOH, 10 wt% catalyst amount and 10 mL water-carrying agent.

    Table 2.  Effect of water-carrying agent on yield of BL at reflux temperature

  • The effect of cyclohexane amount was investigated ranging from 2 mL to 10 mL, and the results are shown in Fig. 5. Under the same amount of water-carrying agent reaction conditions, the yield of BL gradually increases with the reaction time. Further increasing reaction time from 3 h to 4 h, the yield of BL has unchanged, which should be attributable that the conversion of LA achieves chemical balance. The BL yield rises with the increase of water-carrying agent in the same reaction time. When the amount of cyclohexane is 8 mL, the yield of BL is up to 99.3%. However, further increasing the amount of cyclohexane, the BL yield remains basically unchanged. Taking the cost and the efficiency into consideration, the optimal condition is 8 mL cyclohexane.

    Figure 5.  Effect of cyclohexane amount on LA esterification

  • Then, the catalyst amount was optimized and the results are presented in Fig. 6. As can be observed, the yield of BL increases from 61.1% to 99.3% as the amount of catalyst increases from 2 wt% to 10 wt%. At higher catalyst concentration, due to the availability of more active sites, more reactants were transformed to give higher conversion. However, improving the catalyst amount to 12 wt%, the yield of BL decreased to 98.0%. The inverse effect of the further increase in catalyst amount on the yield of BL is probably due to the increase of the available active sites of the catalyst which facilitates the reverse reaction (Chermahini and Nazeri, 2017). Therefore, the optimum catalyst amount of 10 wt% was selected for further experiments.

    Figure 6.  Effect of catalyst amount on LA esterification

  • The results obtained from the study of the effect of molar ratio (LA: n-butyl alcohol) on the esterification of LA are depicted in Fig. 7. The yield of BL was found to be increased from 90.2% to 99.3% with an increase in the molar ratio from 1꞉1.0 to 1꞉1.4, respectively. However, the noteworthy decrease in the yield of BL is observed when the molar ratio beyond 1꞉1.4. The decrease in the yield for the molar ratio 1꞉1.6 to 1꞉1.8, may be attributed to the fact that the excessive amount of alcohol is accumulated on the surface of the catalyst and blocks the catalytic active sites (Das and Parida, 2007). Hence, the optimum LA to n-butanol molar ratio of 1꞉1.4 was selected for further experiments.

    Figure 7.  Effect of molar ratio (acid꞉alcohol) on LA esterification

  • To develop the application range of the catalyst, the esterification reaction of LA was investigated when different alcohols were used as substrates. As shown in Table 3, most alcohols trans formed to afford the corresponding esters in excellent yields. The yield of BL decreased with the increase of steric hindr ance, and the following order of substrate reactivity was obse rved: primary alcohol > secondary alcohol > tertiary alcohol (Table 3, entries 1, 2, 4, 5 and 8), which is in agreement with the general rules of esterification reaction. Interestingly, when using primary alcohol as a substrate, the yield of BL is generally high (Table 3, entries 1, 3, 5, 6 and 7). In connection with the possible reaction mechanism proposed below, this phenomenon may be due to that primary alcohols are more likely to attack carbocations than other alcohols.

    Entry Alcohol Yield (%)
    1 n-propanol 98.2
    2 isopropanol 58.5
    3 n-butanol 99.3
    4 isobutanol 61.5
    5 n-pentanol 98.6
    6 n-hexanol 98.2
    7 n-heptyl alcohol 98.5
    8 tert-butanol 9.5
    Notes: reaction conditions are 15 mmol LA, 21 mmol alcohol, 10 wt% catalyst amount, 8 mL cyclohexane, and reflux reaction at 120 ℃ for 4 h.

    Table 3.  Effect of different alcohols on LA esterification

  • The recycle experiment was carried out to investigate the reu sability of the catalyst under the optimum reaction conditions. After the reaction, the reaction solution was removed by centrifugation to recover the catalyst, washed by cyclohexane three times and dried at 60 ℃ before the usage for the next time. As shown in Fig. 8, the recycled catalyst could be reused for at least five times without observing a decrease in yield, which shows that the catalyst maintains high activity and good stability. After the sixth cycle, the conversion of LA decreased from 93.7% to 84.7%, which may be due to the partial —SO3H groups on the catalyst surface falling off.

    Figure 8.  Stability of LMT-SO3H on esterification of LA

  • According to above-obtained results, the mechanism proposed for the conversion of LA to BL is shown in Fig. 9. It is similar that recently reported by Ciptonugroho et al. (2016) and Enumula et al. (2017).

    Figure 9.  Proposed mechanism for conversion of LA into BL

    Firstly, the enol form of LA can undergo intramolecular lactonization to afford α-angelica lactone, which can undergo nucleophilic addition of an alcohol to give as a product. Sub sequently, the protonation of p-BL facilitates another nucleo philic addition and the intermediate I was formed. Then, the intermediate I underwent ring opening under the action of the LMT-SO3H catalyst to produce hemiacetal material (inter mediate II). After that, BL was generated by the elimination reaction of alcohols.

  • To highlight the superiority of synthesized LMT-SO3H, some previous catalytic results are presented in Table 4. Comparing with these reported catalysts employed for the synthesis of BL, the present LMT-SO3H catalyst exhibits high activity without using an excessive amount of alcohols and the reaction time is the shortest. More importantly, LMT-SO3H has a good sta bility which can be repeatedly used for more than nine times. Besides, the BL yield of 99.3% indicates that LMT-SO3H can be superior to other materials. In summary, the present catalyst shows the obvious advantages of low cost, easy separation and satisfactory catalytic activity.

    Entry Catalyst T (℃) t (h) Mole ratio (LA꞉BuOH) Yield (%) Catalyst reuse References
    1 H-BEA 120 4 1꞉7 82.2 5 times (Maheria et al., 2013)
    2 H3PW12O40/ZrO2-Si(PhSi)-1.0 120 3 1꞉7 82.8 3 times (Su et al., 2013)
    3 H-ZSM-5 120 5 1꞉6 98.0 6 times (Nandiwale and Bokade, 2015)
    4 HPA/K10 120 4 1꞉6 97.0 3 times (Dharne and Bokade, 2011)
    5 WO3-SBA-16 250 10 1꞉7 94.0 10 times (Enumula et al., 2017)
    6 CALB@nanoflowers 50 24 N/A 85.5 10 times (Gao et al., 2017)
    7 Al-MCM41 120 8 1꞉5 90.0 Several runs (Najafi Chermahini et al., 2017)
    8 ZSM-5 120 5 1꞉10 86.0 4 times (Morawala et al., 2019)
    9 Sulfonated carbon 100 4 1꞉5 90.5 5 times (Yang et al., 2018)
    10 NER@3DOM/m-OS 40 12 1꞉10 74.6 9 times (Zhou et al., 2018)
    11 Ten sulfonic resins 115–155 4–6 1꞉3 72.3–82.4 N/A (Iborra et al., 2019)
    12 WO x/ZrO2 120 2 1꞉2 66.0 N/A (Ciptonugroho et al., 2016)
    13 Zr-MOF 120 5 1꞉6 99.0 3 times (Cirujano et al., 2015)
    14 TNTs-SO3H 120 8 1꞉11.2 82.7 5 times (Zhou et al., 2019)
    15 HPA/C-Sil-1 100 5 1꞉5 92.0 4 times (Manikandan and Cheralathan, 2017)
    16 LMT-SO3H 120 4 1꞉1.4 99.3 9 times This work

    Table 4.  Summary of catalytic results obtained for synthesis of BL from LA over different catalysts

4.   Conclusions
  • In summary, sulfonic acid-functionalized lignin-montmorillonite complex as an efficient and low-cost solid acid catalyst was successfully prepared from papermaking black liquor and it exhibited good catalytic performance for esterification of biomass- derived LA with n-butyl alcohol. Under optimized reaction conditions, the LMT-SO3H catalyst exhibited the highest yield (99.3%) to BL. Furthermore, the catalyst was used for nine consecutive cycles, showing a high activity. The yield of alkyl levulinates (n-propanol, n-butanol, n-pentanol, n-hexanol, n-heptyl alcohol) was more than 98.0%, which designated the excellent performance of the LMT-SO3H catalyst in the esterification of LA.

Conflict of Interest
  • There are no conflicts to declare.

Acknowledgements
  • The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 21606082, 21776068 and 21975070), Hunan Provincial Natural Science Foundation of China (No. 2018JJ3334), and China Postdoctoral Science Foundation (No. 2019M662787).

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