1 Introduction

In a biorefinery concept, the complex biopolymers in biomass are deconstructed into small organics, which are then synthetically upgraded into specialty chemicals [1]. The chemical-catalytic value additions of biomass are particularly interesting since they are fast, selective, biomass agnostic, and work under relatively mild conditions [2]. The development of efficient, robust, inexpensive, and environment-friendly catalysts is of paramount importance in the chemical-catalytic pathway [3,4,5]. In this regard, the use of ionic liquids as the catalyst and alternative reaction media has become mainstream in the ambit of synthetic organic chemistry [6,7,8,9,10]. Ionic liquids have been utilized as selective and environment-friendly catalysts as well as a greener reaction media in sustainable synthesis, production of biofuels, and renewable chemicals [11, 12]. For example, protic ionic liquids have been used as acid catalysts for the hydrolysis and dehydration of biomass-derived carbohydrates into renewable chemical platforms such as 5-(hydroxymethyl)furfural (HMF), levulinic acid (LA), and furfural (FUR) [13]. Ionic liquids have also been employed in the downstream synthetic upgrading of the abovementioned biorenewable chemical intermediates [14,15,16]. However, one of the major obstacles associated with the scalability of the processes using ionic liquids is their limited availability and high cost [17]. Triethylammonium hydrogen sulfate (TEAHS), prepared by reacting triethylamine with sulfuric acid in equimolar quantities, has received considerable attention over the past decade as an inexpensive Brønsted ionic liquid catalyst [18]. TEAHS has been shown to catalyze the esterification reaction between alkyl carboxylic acid and alkyl alcohols of different chain lengths [19]. TEAHS has also been applied as a catalyst for the preparation of coumarins and various biologically important heterocycles [20,21,22]. TEAHS has served as an ionic liquid medium for the pretreatment of lignocellulosic biomass, and for the production of biodiesel from palm oil [23,24,25,26]. However, the application of TEAHS for the synthetic upgrading of biomass-derived chemical building blocks is virtually missing in the literature. In this work, we report TEAHS as an inexpensive, efficient, and recyclable catalyst for the esterification of various biomass-derived chemical intermediates into products of commercial significance. The alkyl esters of LA are of immense interest as renewable fuel oxygenate, green solvent, and chemical intermediate for further value addition [27, 28]. Hereby, we show TEAHS as an inexpensive but efficient catalyst for the preparation of methyl- to butyl levulinate from biomass-derived LA (Scheme 1). The alkyl esters of fatty acids have commercial applications as biodiesel and surface-active agents, among others [29, 30]. The acid-catalyzed esterification of free fatty acids (FAA) is of significant interest lately for the production of biodiesel from FAA-rich inexpensive feedstock [31, 32]. We report the preparation of alkyl stearates by the esterification of stearic acid with monohydric alkyl alcohols using TEAHS as the catalyst as well as the reaction medium. Alkyl 2-furoates are also important compounds with potential applications as novel oxygenate, green solvent, and as a constituent in various personal care products [33]. Alkyl 2-furoates have been produced in decent yields from biomass-derived 2-furoic acid (2-FA) and alkyl alcohol using TEAHS as the catalyst. The diesters of isosorbide, a glucose-derived diol, have potential applications as renewable plasticizers, surfactants, and monomers for polymeric applications [34]. The diacetates and dipropionates of isosorbide have been produced in good isolated yields using TEAHS as the catalyst. Since the small-chain (C1–C4) monohydric alkyl alcohols and carboxylic acids (C2–C3) are also sourced from biomass, the esters discussed here are biorenewable to their entirety.

Scheme 1
scheme 1

[Et3NH][HSO4]-catalyzed esterification of various biomass-derived renewable chemicals

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2 Experimental procedure

2.1 Synthesis of ethyl stearate

Stearic acid (1.002 g, 3.51 mmol), ethanol (5 mL), and TEAHS (0.2 g, 1.004 mmol, 28.60 mol%) were charged in a 75 mL high-pressure glass vessel fitted with a Teflon screw top and magnetically stirred for 4 h in a pre-heated oil bath at 120 °C. After the reaction, the reactor was cooled down to room temperature. Excess ethanol was evaporated under reduced pressure in a rotary evaporator. The reaction mixture was then diluted with petroleum ether (20 mL), and the ionic liquid was separated by phase separation. The petroleum ether layer was then dried by anhydrous Na2SO4 and evaporated under reduced pressure. The crude product was chromatographed (silica gel, petroleum ether) to get ethyl stearate 10 (1.05 g, 96%) as a colorless oil. 1H-NMR (400 MHz, CDCl3, δ ppm): 4.15 (q, 2H, J = 7.1 Hz), 2.30 (t, 2H, J = 7.6 Hz), 1.63 (m, 2H, J = 7.6 Hz), 1.27 (m, 31H), 0.90 (t, 3H, J = 7.1 Hz). 13C-NMR (100 MHz, CDCl3, δ ppm): 173.9, 60.1, 34.4, 31.9, 29.69, 29.65, 29.5, 29.4, 29.3, 29.2, 29.1, 25.0, 22.6, 14.2, 14.1. FTIR (ATR, cm−1): 2922, 2853, 1738, 1176. The same synthetic procedure was followed for the other three stearate esters.

2.2 Synthesis of ethyl levulinate

Levulinic acid (1.002 g, 8.62 mmol), ethanol (5 mL), and TEAHS (0.5 g, 2.510 mmol, 29.11 mol%) were charged in a 75 mL glass pressure vessel fitted with a Teflon screw top. The reactor was sealed and placed in a pre-heated (120 °C) oil bath for 3 h under continuous magnetic stirring. The reactor was cooled down to room temperature, and excess ethanol was evaporated in a rotary evaporator under reduced pressure. Chloroform (10 mL) was added to the mixture, and the ionic liquid was phase separated. The chloroform layer was dried over anhydrous Na2SO4 and evaporated under reduced pressure. The crude product was chromatographed (silica gel, chloroform) to get ethyl levulinate 6 (0.999 g, 80%) as light-yellow oil. 1H-NMR (CDCl3, 400 MHz) δ (ppm): 4.09 (q, 2H, J = 7.2 Hz), 2.71 (t, 2H, J = 6.4 Hz), 2.52 (t, 2H, J = 6.4 Hz), 2.15 (s, 3H), 1.21 (t, 3H, J = 7.2 Hz);13C-NMR (CDCl3, 100 MHz) δ (ppm): 202.4, 172.0, 59.6, 36.9, 28.8, 27.0, 13.1; FTIR (ATR, cm−1): 2982, 2931, 1716, 1155. The same synthetic procedure was applied for the other three levulinate esters.

2.3 Synthesis of 2-furoic acid

2-Furoic acid was synthesized from furfural by a slight modification of a literature procedure [35]. Furfural (20 g) was placed in a 250 mL conical flask and cooled in an ice-water bath while stirring magnetically. When the temperature reached < 10 °C, 16.5 g of 33.3% sodium hydroxide solution was added slowly over 30 min via a pipette so that the temperature in the flask not exceeding 20 °C. After that, the reaction was allowed to warm up to room temperature and stirred for 1 h. The reaction mixture was then diluted with water (80 mL) and the solution was extracted with diethyl ether (6 × 20 mL) to separate furfuryl alcohol. The aqueous layer containing the sodium salt of 2-furoic acid was cooled down in an ice-water bath and made acidic by adding 40% sulfuric acid slowly. The acidified solution was extracted with chloroform (3 × 20 mL). The chloroform layers were combined, dried over anhydrous Na2SO4, and evaporated in a rotary evaporator under reduced pressure to get crude 2-furoic acid as a yellow solid. The product was purified by triturating with boiling petroleum ether and 2-furoic acid was obtained in around 60% yield of the theoretical as a white crystalline solid.

2.4 Synthesis of ethyl 2-furoate

2-Furoic acid (0.502 g, 4.47 mmol), ethanol (5 mL), TEAHS (0.5 g, 2.510 mmol, 56.15 mol%), and Amberlyst-15 (0.1 g) were charged into a 75 mL glass pressure vessel fitted with a Teflon screw top. The reaction mixture was placed in a pre-heated (120 °C) oil bath and magnetically stirred continuously for 5 h. After completion of the reaction, the reactor was cooled down to room temperature and excess ethanol was evaporated. Chloroform (10 mL) was added to the reaction mixture, and TEAHS was phase separated. The chloroform layer was dried over anhydrous Na2SO4 and evaporated in a rotary evaporator under reduced pressure. The crude product was chromatographed (silica gel, chloroform) to get ethyl 2-furoate 2 (0.440 g, 70%) as a clear liquid. 1H-NMR (300 MHz, CDCl3, δ ppm): 7.48 (q, 1H, J = 0.9 Hz), 7.09 (q, 1H, J = 0.9 Hz and 2.7 Hz), 6.41 (q, 1H, J = 2.7 Hz), 4.27 (q, 2H, J = 7.2 Hz), 1.28 (t, 3H, J = 7.2 Hz); 13C-NMR (75 MHz, CDCl3, δ ppm): 158.6, 146.1, 144.8, 117.6, 111.7, 60.8, 14.2; FTIR (ATR, cm−1): 3142, 3126, 2983, 2874, 1714, 1292, 1008. The same synthetic procedure was adopted for the synthesis of the other three alkyl furoates.

2.5 Synthesis of isosorbide-2,5-diacetate

Isosorbide (1.002 g, 6.85 mmol), glacial acetic acid (5 mL), and TEAHS (1.0 g, 5.020 mmol, 73.28 mol%) were charged into a 75 mL ACE glass pressure reactor fitted with a Teflon screw top with a magnetic stirrer bar. The reactor is placed in the pre-heated oil bath at 120 °C for 4 h with continuous stirring. After the reaction, the reactor was cooled down to room temperature and opened. Excess acetic acid was evaporated in a rotary evaporator under reduced pressure. The reaction mixture was extracted with chloroform (10 mL), and TEAHS was phase separated. The chloroform layer was dried over anhydrous Na2SO4 and evaporated under reduced pressure. The crude product was chromatographed (silica gel, chloroform) to get isosorbide diacetate 13 (0.790 g, 50%) as a clear oil. 1H-NMR (300 MHz, CDCl3, δ ppm): 5.14 (m, 2H), 4.80 (t, 1H), 4.46 (d, 1H), 3.95 (m, 3H), 3.76 (m, 1H), 2.10 (s, 3H), 2.05 (s, 3H); 13C-NMR (75 MHz, CDCl3, δ ppm): 170.4, 170.1, 85.8, 80.6, 78.0, 73.9, 73.3, 70, 20.8, 20.5; FTIR (ATR, cm−1): 2980, 2875, 1742, 1718, 1089. The same synthetic procedure was followed for the synthesis of isosorbide-2,5-dipropionate.

3 Results and discussion

Initially, the preparation of alkyl 2-furoates was undertaken by the esterification of 2-FA with C1–C4 alkyl alcohols using TEAHS as the acid catalyst (Scheme 2).

Scheme 2
scheme 2

Preparation of alkyl 2-furoates from 2-FA

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An attempt to esterify 2-FA with excess ethanol in the absence of an acid catalyst did not produce alkyl 2-furoates in perceptible yields even after heating the reaction mixture at 120 °C for an extended time. However, when TEAHS (20 wt% of 2-FA) was used as the catalyst, ethyl 2-furoate 2 was obtained in a 58% isolated yield within 5 h at 120 °C (Table 1, entry 2). The mass balance was essentially the unreacted 2-FA. Extended reaction time or increased reaction temperature did not improve the yield of 2, appreciably. In all cases, attempt was made to attain the equilibrium and maximize the conversion of the starting carboxylic acid. Lower temperatures required a long duration for the reaction to establish equilibrium and make the process economically less attractive. The use of lesser amounts of TEAHS significantly lowered the yield of 2. We reasoned that the concomitant use of a sulfonic acid-based co-catalyst could help to establish the equilibrium faster due to the availability of superior Brønsted acidic sites and potentially afford better yields of the ester products. Amberlyst-15 (A-15) was selected as a co-catalyst since it is robust, heterogeneous, and available in bulk. As anticipated, the use of A-15 as a co-catalyst noticeably improved the isolated yield of 2 (Table 1, entry 3). The texture and heterogeneity of the A-15 catalyst were visibly unchanged post reaction. The reaction was then extended to the preparation of other alkyl 2-furoates. Methyl 2-furoate 1 was obtained in 64% isolated yield when 2-FA was reacted with excess anhydrous methanol in the presence of the combination of TEAHS and A-15. Propyl 2-furoate 3 and butyl 2-furoate 4 were obtained in 72% and 74% yields, respectively. The slightly better yields of the esters with higher alcohol may be explained by their capability of forming a low-boiling azeotrope with water, formed as a byproduct.

Table 1 The preparation of alkyl 2-furoates from 2-FA
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Thereafter, the esterification of biomass-derived LA was attempted using TEAHS as the catalyst as well as reaction medium (Scheme 3).

Scheme 3
scheme 3

Esterification of levulinic acid (LA) using TEAHS as an acid catalyst

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Although levulinate esters formed even under the reflux condition, superior yields of the same were achieved by conducting the reactions in a sealed glass vessel. The result may be explained by the higher reaction temperature achievable in the closed system and no evaporative loss of the alcohol reagent. Initially, the reaction was carried out by using only triethylamine and H2SO4 as a catalyst. The use of triethylamine alone formed the salt of levulinic acid and did not form any ester. However, when conc. H2SO4 was used as a catalyst, ethyl levulinate 6 was isolated in 75% yield under the same reaction conditions employed for TEAHS (1 g LA, 5 mL ethanol, 120 °C, 3 h, H2SO4 (0.245 g, 29 mol%)). Methyl levulinate 5 was obtained in an 82% yield using excess anhydrous methanol and 50 wt% of TEAHS as catalyst and reaction medium (Table 2, entry 1). Ethyl levulinate 6 was obtained in an 80% yield within 3 h at 120 °C using excess of absolute ethanol. Propyl levulinate 7 and butyl levulinate 8 were isolated in 83% and 84% yields, respectively, under identical conditions (Table 2, entry 3 and 4).

Table 2 The preparation of alkyl levulinates from LA
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The gram-scale preparation of alkyl stearates was attempted from stearic acid using excess alcohol reagent and 20 wt% of TEAHS as catalyst (Scheme 4).

Scheme 4
scheme 4

Esterification of stearic acid using the TEAHS catalyst

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The reactions were performed in a 75 mL glass pressure vessel fitted with a Teflon screw top. Ethyl stearate 10 was obtained in 96% isolated yield within 3 h at 120 °C. After the reaction, the excess alcohol was distilled off under reduced pressure. Alkyl stearate was conveniently separated from the TEAHS catalyst by extracting with petroleum ether. The catalyst was then dried at 110 °C for 4 h under vacuum before resubmitting to the next cycle. Longer duration or higher reaction temperature had negligible effect on the isolated yield of 10 (Table 3, entry 3). The use of more TEAHS lowered the reaction time but did not improve the yield appreciably. However, a lower quantity of TEAHS decreased the yield of 10 due to the lower conversion of stearic acid (Table 3, entry 2). The use of only slight excess of the alcohol reagents also provided lesser yields of alkyl stearates. Methanol, propanol, and butanol provided similar yields of their corresponding stearates (9, 11, and 12) under identical reaction conditions. The use of recycled TEAHS provided only a slightly diminished yield of 10 (Table 3, entry 7) (supplementary information).

Table 3 Esterification of stearic acid with monohydric alkyl (C1–C4) alcohols
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The recyclability and stability studies of TEAHS were then undertaken for the synthesis of 10. After each reaction, the excess ethanol was evaporated in a rotary evaporator under reduced pressure and 10 was extracted using light petroleum ether. The ionic liquid was then dried at 110 °C under vacuum for 4 h before subjecting it for the consecutive reaction. Figure 1 shows the results of the recyclability studies of TEAHS. The ionic liquid catalyst was recycled for four cycles and only marginal decrease in the yield of 10 was observed. The chemical identity and stability of the recycled TEAHS were confirmed by 1H-NMR and TGA (supplementary information). The data showed that no chemical decomposition occurred to TEAHS during the reaction. It was interesting to note that when the recycled TEAHS was subjected to the next catalytic cycle without undergoing the drying process, the yield of 10 was significantly lowered. This result can be justified by the fact that TEAHS not only acts as a Brønsted acid catalyst for the esterification reaction but also absorbs the water byproduct formed during the esterification reaction.

Fig. 1
figure 1

Recyclability study of TEAHS during the preparation of 10

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Finally, the preparation of isosorbide 2,5-alkanoates was attempted by the TEAHS-catalyzed esterification of glucose-derived isosorbide using excess carboxylic acids (Scheme 5).

Scheme 5
scheme 5

TEAHS-catalyzed preparation of isosorbide-2,5-dialkanoates from isosorbide

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Isosorbide-2,5-diacetate 13 was prepared in 50% isolated yields by reacting isosorbide with excess glacial acetic acid in the presence of the TEAHS catalyst. The mass balance was unreacted isosorbide and monoacetate of isosorbide. Isosorbide-2,5-dipropionate 14 was isolated in 56% yield under identical conditions (Table 4, entry 2).

Table 4 Preparation of isosorbide-2,5-dialkanoates from isosorbide
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The TEAHS catalyst was recovered and characterized by 1H-NMR and TGA to confirm its chemical integrity. No observable decomposition product was observed in the recycled TEAHS.

4 Conclusions

High-yielding, gram-scale esterification of various biomass-derived chemical intermediates was carried out using [Et3NH][HSO4] as an inexpensive, efficient ionic liquid catalyst. Both aromatic and aliphatic esters of commercial interests were synthesized in good isolated yields. The catalyst was conveniently recovered from the reaction media and reused.