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Solventless esterification of fatty acids with trimethylolpropane using sulfonated amorphous carbons derived from wood powder
Catalysis Communications 96 (2017) 32–36
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Solventless esterification of fatty acids with trimethylolpropane using sulfonated amorphous carbons derived from wood powder Dasom Mun a,b, Anh Thi Hoang Vo c, Bora Kim a, Yong-Gun Shul b, Jin Ku Cho a,c,⁎ a b c
Green Material and Process R&D Group, Korea Institute of Industrial Technology (KITECH), Cheonan, Chungnam 31056, Republic of Korea Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Republic of Korea Green Process and System Engineering Department, Korea University of Science and Technology (UST), Cheonan, Chungnam 31056, Republic of Korea
a r t i c l e
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Article history: Received 7 December 2016 Received in revised form 15 March 2017 Accepted 16 March 2017 Available online 18 March 2017 Keywords: Solventless esterification Fatty acids Polyols Sulfonated amorphous carbon Biobased Heterogeneous acid catalysts
a b s t r a c t Polyolesters synthesized by an esterification between polyols and fatty acids are value-added oleochemicals widely used as lubricants, cosmetics, and food additives among other applications. However, homogeneous acid catalysts are still preferred in their industrial production, despite their associated energy cost and the environmental issues that they present. In this paper, we describe lignocellulose-derived amorphous carbons with a high loading level of SO3H and their application to the synthesis of polyolesters as a biobased heterogeneous acid catalyst. These sulfonated amorphous carbons could be readily prepared via (i) heat treatment at 400°C for 1h and (ii) sulfonation with chlorosulfuric acid. XRD and BET analyses demonstrated that these carbonaceous materials were not crystalline but were amorphous structures with a low surface area. The attachment of SO3H groups was confirmed by FT-IR and XPS, and the loading level of SO3H was determined by CHNS elemental analysis. Chlorosulfuric acid gave a higher loading level of SO3H than other sulfating agents, such as conc. sulfuric acid and fuming sulfuric acid. The higher SO3H-loaded amorphous carbons exhibited a greater catalytic activity for esterification between trimethylolpropane and fatty acids under solventless conditions. Esterification of oleic acid derived from vegetable oil with trimethylolpropane using this catalyst afforded the desired biolubricant at over 93% yield in 3h. The sulfonated amorphous carbons could be reused three times without any significant loss of catalytic activity. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Polyolesters (POEs) are oleochemicals widely used as lubricants, cosmetics, and food additives among other applications, and they are made by esterification of fatty acids with polyols [1]. Esterification is performed in the presence of Brønsted acids, such as sulfuric acid, phosphoric acid, or p-toluenesulfonic acid. Although these homogeneous acid catalysts are preferred in industrial production due to their low cost and strong acidity, (i) they are difficult to separate from reaction mixtures and reuse; (ii) a large amount of acidic waste water is discharged, which causes environmental problems; and (iii) they often cause serious corrosion in process units [2]. To overcome the drawbacks of homogeneous acid catalysts, a variety of heterogeneous acid catalysts were explored. Cation exchange resins with high loading of sufficient sulfonic acid functionalities exhibit good performance in certain cases, but they are expensive and have low thermal stability [3]. Zeolitic solid acids including H-beta and H-ZSM-5 were attempted but the small pore sizes were unsuitable for an effective access of ⁎ Corresponding author at: Green Material and Process R&D Group, Korea Institute of Industrial Technology (KITECH), Cheonan, Chungnam 31056, Republic of Korea. E-mail address: [email protected] (J.K. Cho).
http://dx.doi.org/10.1016/j.catcom.2017.03.015 1566-7367/© 2017 Elsevier B.V. All rights reserved.
bulky substrates (fatty acids and polyols) [4]. Esterification of fatty acids using heteropolyacid (HPA) supported on clay K-10 [5] or mesoporous silica [6], an Fe-Zn double-metal cyanide complex [7], and ZrOCl2·8H2O [8] as catalysts have been reported, but the studies were confined to simple alcohol compounds used primarily to produce the fatty acid methyl esters (FAME) of biodiesel. Additionally, immobilized lipase has been applied to the esterification of fatty acids with long chain alcohols, but there are still economic issues [9]. Currently, amorphous carbons (AC) containing SO3H groups attract considerable attention as biobased heterogeneous acid catalysts for esterification due to their physical/chemical stability and high catalytic activity. These carbon materials are typically made from natural carbon sources, such as glucose [10] or cellulose [11], via incomplete carbonization followed by sulfonation. Incomplete carbonization results in AC composed of small polycyclic aromatic carbon sheets in a three-dimensional sp3bonded structure, and these AC can afford highly acidic catalysts by sulfonation because there are numerous reactive sites where SO3H groups may attach covalently, compared with crystalized carbon (CC) [12]. In this study, amorphous carbons containing SO3H (AC-SO3H) were prepared from lignocellulose, the most abundant biomass, and they were applied to the synthesis of polyolesters as biobased heterogeneous acid catalysts. AC-SO3H could be readily obtained by the thermal
D. Mun et al. / Catalysis Communications 96 (2017) 32–36
treatment of wood powder and then sulfonation with sulfonating agents such as conc. sulfuric acid (H2SO4), fuming sulfuric acid (SO3H2SO4), and chlorosulfonic acid (ClSO3H). AC-SO3H was characterized by XRD, FT-IR, XPS, BET, and elemental analysis. Esterification between trimethylolpropane (TMP), a polyol containing three symmetric alcohol moieties, and fatty acids, including 2-ethylhexanoic acid (2-EHA), isononanoic acid (INA), and oleic acid, was performed without solvent in the presence of as-synthesized AC-SO3H. The effects of the loading level of SO3H, the temperature, and the structure of the fatty acids on the synthesis of polyolesters were investigated. The reusability of ACSO3H was also tested. 2. Experimental procedures 2.1. Materials A wood powder (pine tree, 50–100μm in diameter) used as a raw material of the lignocellulose was supplied by G·biotech (Korea). The fatty acids 2-ethylhexanoic acid (2-EHA), isononanoic acid, and oleic acid, as well as trimethylolpropane (TMP), a polyol, were provided by Oh Sung Chemical Industry Co., Ltd. (Korea). All other chemicals, including conc. sulfuric acid (H2SO4, 95.0–98.0%), fuming sulfuric acid (SO3H2SO4, 28.0–32.0% free SO3), chlorosulfonic acid (ClSO3H, N98%), ptoluenesulfonic acid monohydrate (PTSA, 99%), and 1,4-dioxane (99.8%), were purchased from Sigma-Aldrich (USA) and directly used without further purification.
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gently stirred at 80°C in an oil bath for 3h and additionally at room temperature (25°C) for 3h. The resulting sulfonated amorphous carbonaceous materials were washed with hot distilled water (70°C) until pH paper was no longer changed to an acid-indicating color and then washed with 1,4-dioxane. Afterwards, to remove the loosely bonded materials, sulfonated amorphous carbonaceous materials were washed by Soxhlet extraction with 1,4-dioxane for 24h and dried in vacuum oven for one day. The AC-SO3H species obtained by treatment with conc. sulfuric acid, fuming sulfuric acid, and chlorosulfonic acid were denoted as AC-co-SO3H, AC-f-SO3H, AC-ch-SO3H, respectively. 2.3.2. Esterification between fatty acids and TMP Into a 250mL two-neck round-bottom flask were placed TMP (3.7mmol, 1equiv) and an excess of fatty acid (12.3mmol, 3.3equiv), and AC-SO3H (0.123mmol, 1mol% of fatty acid) was added as a catalyst. The reaction mixture was heated to the desired temperature and stirred at 300rpm under N2 flow for 24h. Volatile substances formed during the reaction were removed by N2. At intervals of 1h, a small amount of sample was taken out from the reaction mixture using a syringe and then it was diluted with dichloromethane (DCM) and submitted for GC–MS (GCMS-QP2010 Ultra, Shimadzu, Japan) with an Rxi-5ms column (50m in length). Helium was used as a carrier gas at 1mLmin−1of flow rate. Injector temperature was 150°C. Column temperature was programmed from 150°C to 330°C at a rate of 5°Cmin−1 and held isothermal for 14min. The split ratio, ion source temperature, mass scan range were 1:20, 300°C, 35–1000massunits, respectively. The yield of polyolester was calculated by the following equation.
2.2. Analysis AC-SO3H was analyzed by XRD (Powder X-ray diffraction, D8 ADVANCE with DAVINCI, BRUKER, Germany), FT-IR (Fourier transform infrared spectroscopy, Nicolet 6700, Termo Scientific system, USA) and XPS (X-ray photoelectron spectroscopy K-Alpha™+, Thermo Scientific, USA). The loading amounts of sulfur were determined by elemental analysis (Automatic Elemental Analyzer, FLASH 2000 Series, Thermo Scientific, USA), and the surface areas were measured by BET analysis (Brunauer-Emmett-Teller, ASAP2010, Micromeritics, USA), respectively. XRD was conducted at 2θ from 2.5° to 30°, at a scanning step size of 0.02° and a scan speed per step of 0.5s using Cu Kα radiation. FT-IR samples were prepared in a pellet form by mixing the catalyst sample with KBr, and spectra were recorded in ATR mode; 126 spectra were accumulated and averaged to improve the signal-to-noise ratio. XPS measurements were performed on a Thermo Scientific K-alpha instrument using monochromatized Al Kα radiation (hν=1486.6eV) and processed using Thermo Avantage software. The calculated spectra represented the transmittance. The specific surface area was determined on a BET surface analyzer using N2 as the adsorbent at liquid nitrogen temperature (77K) in the relative pressure (P/P0) range of 0–0.25. The powder samples were degassed in air over 12h at 100°C prior to analysis. 2.3. Procedure 2.3.1. Preparation of AC-SO3H Amorphous carbonaceous materials were prepared directly from wood powder. A wood powder was carbonized by heating at 400°C under N2 for 1h. Fifty grams of wood powder (WP) were placed into a rectangular shaped ceramic alumina crucible (10×10×5cm3), and the WP-containing crucible was placed in a chamber-type electric furnace. The furnace was heated to 400°C over 80min under flowing N2, and the temperature was maintained for 1h to afford approximately 15g of amorphous carbonaceous materials (weight yield: ~30%). For the attachment of SO3H groups onto the aromatic rings of the amorphous carbonaceous materials, sulfonation proceeded as follows: In the 500mL round bottom flask were placed 10g of amorphous carbonaceous materials, and 100mL of a sulfonating agent such as conc. sulfuric acid, fuming sulfuric acid, or chlorosulfuric acid were added. The mixtures were
Yield of polyolester ð%Þ ¼ Conversion of TMP Selectivity of polyolesters ðT−nFÞ where T is an initial letter of trimethylolpropane; F is an initial letter of fatty acid (E, I, O in the cases of 2-EHA, isononanoic acid, oleic acid, respectively); and n is the number of fatty acids coupled with a TMP. 3. Results and discussion 3.1. Characterization of AC-SO3H According to the previous report by Okamura et al., heat treatment above 450°C afforded large carbon sheets in a well-crystallized form, which indicated a lack of reactive sites for the attachment of SO3H groups on the carbon sheets [13]. Therefore, the carbonization process was conducted at a moderate temperature (under 450°C) in this study. Wood powders were changed to black carbonaceous materials after heat treatment under N2 at 400°C for 1h. In the XRD patterns of the carbonaceous materials, two broad peaks were observed at 10–30° and at 35–50°, corresponding to randomly oriented aromatic carbon sheets [14] (Fig. S1a). The results indicated that the carbonaceous materials formed an amorphous structure due to incomplete carbonization. The BET surface area, under 100m2g−1 of the carbonaceous materials, confirmed that they are not crystalline materials. Next, SO3H groups were attached to the aromatic rings of the amorphous carbon (AC) to impose acidic character. Three sulfonating agents, specifically sulfuric acid, fuming sulfuric acid, and chlorosulfonic acid, were examined for efficient sulfonation. Sulfonation of AC was analyzed by FT-IR and XPS. FT-IR spectra of the sulfonated amorphous carbon (AC-SO3H) contained bands at 1377 and 1040cm−1, which were assigned as O_S_O and SO− 3 stretching bands, respectively (Fig. S1b) [14]. A peak appearing at 168.8eV in the XPS spectrum corresponded to the S 2p binding energy of the SO3H groups [13] (Fig. S1c). The XRD spectra of the AC-SO3H were similar to that of AC. It was understood that no structural change occurred during sulfonation. The BET surface areas were also sustained after sulfonation, except for AC-ch-SO3H sulfonated with chlorosulfuric acid (286m2g−1). It could be explained that further carbonization occurred during treatment with chlorosulfuric acid, and the crystallinity
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Fig. 1. Comparison of AC-co-SO3H and AC-ch-SO3H in the esterification between polyol and fatty acid.
increased. However, this surface area was still considerably lower than that of activated carbon. The loading level of \\SO3H groups on ACSO3H after sulfonation was determined by elemental analysis and the amount of sulfur per AC-SO3H was calculated to mmolg−1. The loading levels of \\SO3H on AC-co-SO3H, AC-f-SO3H, and AC-ch-SO3H were 0.67mmolg−1, 1.29mmolg−1, and 1.57mmolg−1, respectively. The total Brønsted acidity of AC-ch-SO3H was measured by an acid-base titration using a 10mM NaOH aqueous solution to give 2.02mmolofH+g−1. This finding indicated that approximately 30% of the total Brønsted acidity would be from other acidic moieties, such as \\CO2H, rather than\\SO3H.
3.2. Effect of the SO3H loading level on the esterification between TMP and 2-EHA using AC-SO3H In the previous section, we found that the sulfonation of AC relied on sulfonating agents, and chlorosulfuric acid resulted in a higher loading level of SO3H than any of the other sulfonating agents. The higher loading level of SO3H means that the acid sites exist more densely on the support. To examine the effect of the SO3H loading level on the esterification between a polyol and a fatty acid, two catalysts (AC-co-SO3H and AC-ch-SO3H) with different loading level of SO3H (0.67mmolg−1 vs 1.57mmolg−1) were compared under identical conditions (Fig. 1). The polyol and fatty acid sources used were 3.7mmol of TMP (500mg) and 2-EHA (12.3mmol, 3.3equiv of TMP), respectively. TMP not containing a β-H was employed owing to its thermal and hydrolytic stability [15]. The amount of catalyst was determined as 1mol% of the fatty acid (0.123mmol of SO3H). In other words, 184mg of AC-co-SO3H and 78mg of AC-ch-SO3H were added to the reaction. The esterification
was carried out without a solvent at 180°C for 24h. Interestingly, ACch-SO3H with a high loading level of SO3H showed a much faster reaction rate, even though the total amount of SO3H is the same as that in AC-co-SO3H. The half-life (t1/2) of TMP in the reaction using AC-coSO3H was 31.8min, while the t1/2 of TMP was only 11.4min in the case of AC-ch-SO3H (Fig. 2). The yields of esterification products were also enhanced when AC-ch-SO3H was used. A tri-ester (T-3E) was obtained as the major product in 58.1% yield, and N95% of the mono-ester (T1E) was consumed after 7h (Fig. S2b). However, the reaction in the presence of AC-co-SO3H gave di-ester (T-2E) as the major product in 52.9% yield and a significant amount of the mono-ester (T-1E), over 10%, remained even after 24h (Fig. S2a). All results were summarized in Table 1. Moreover, a trace amount of unknown products (about 1– 2%) was found in GC–MS when using AC-ch-SO3H. It is due to the high activity of AC-ch-SO3H and the reason about formation of unknown products will be discussed in Section 0.
3.3. Effect of the temperature on the esterification between TMP and 2-EHA using AC-co-SO3H The effect of the reaction temperature on the esterification of TMP and 2-EHA in the presence of AC-co-SO3H was investigated. Prior to test, the activity of AC-co-SO3H was compared with that of PTSA as a reference. AC-co-SO3H showed a little bit higher activity than PTSA in both reaction rate (t1/2=30.6min for AC-co-SO3H vs 36.6min for PTSA) and yield of T-3E (36.5% for AC-co-SO3H and 35.4% for PTSA) (see Figs. S3 and S4). From the results, we can understand that AC-co-SO3H has a comparative activity to homogeneous acid catalysts. As expected, the esterification progressed more rapidly as the reaction temperature increased (180°C–220°C), and the t1/2 of TMP was shortened from 31.8min to 13.2min (Fig. S5). The yields of the esterification products were increased at 220°C. T-3E was obtained in 51.4% yield and T-1E almost disappeared after 24h (Fig. S6). However, a couple of unknown compounds began to form after 3h at 220°C (Table S2). From GC–MS, the molecular weights of unknown compounds were same as T-2E and T-3E, respectively. It indicates that unknown compounds are diastereomers of T-2E and T-3E because 2-EHA has a chiral center. Probably, these diastereomers seem to be more steric hindered than major products, T-2E and T-3E because the steric hindered diastereomers of T-2E Table 1 Characteristic data and the esterification results of AC-co-SO3H and AC-ch-SO3H. Catalyst
Surface Loading Amount t1/2 of TMP Product yields area (min) (%) level of SO3H used 2 −1 −1 (m g ) (mmolg ) (mg) T-1E T-2E T-3E
AC-co-SO3Ha 63 AC-ch-SO3Hb 286 Fig. 2. Comparison of the TMP (reactant) profiles according to the time of the esterification between TMP (3.7mmol) and 2-EHA (3.3equiv of TMP) using AC-co-SO3H and AC-chSO3H (1mol% of 2-EHA each) at 180°C.
0.67 1.57
184 78
31.8 11.4
10.4 0.7
52.9 39.3
36.5 58.1
a Reaction conditions: TMP (3.7mmol), 2-EHA (12.2mmol, 3.3equiv of TMP), AC-co-SO3H (0.12mmol, 1mol% of 2-EHA), N2 flow, 180°C, 24h. b Reaction conditions: TMP (3.7mmol), 2-EHA (12.2mmol, 3.3equiv of TMP), AC-ch-SO3H (0.12mmol, 1mol% of 2-EHA), N2 flow, 180°C, 24h.
D. Mun et al. / Catalysis Communications 96 (2017) 32–36
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Fig. 3. Reaction scheme for the esterification of TMP with fatty acids and the structures of the fatty acids.
and T-3E requiring higher activation energy can be produced only at high temperature. 3.4. Effect of the chemical structure of the fatty acids on the esterification with TMP using AC-ch-SO3H Polyolesters coupled by multiple numbers of fatty acids are highly bulky compounds. Therefore, the chemical structures of fatty acids can affect their reactivity during esterification. In this study, the reactivity of three fatty acids, including 2-EHA, INA, and oleic acid, in an esterification with TMP was evaluated in the presence of AC-ch-SO3H (Fig. 3). All the reactions were carried out under solventless conditions. 2-EHA and INA are synthetic fatty acids. 2-EHA has a C8 alkyl chain and the ethyl branch is adjacent to the carboxylic acid moiety. INA has a C9 alkyl chain and the isopropyl group is located at the end of the alkyl chain. Meanwhile, oleic acid is a fatty acid derived from vegetable oil. Oleic acid has a C18 linear alkyl chain and one double bond that is located at the middle of the chain. Therefore, the alkyl chain of oleic acid produces a lesser steric factor than those of 2-EHA and INA. Particularly, polyolesters made with oleic acid are known as biolubricants and they have several significant advantages: non-toxicity, biodegradability, carbon-neutrality, high temperature stability, and no need for various additives such as anti-oxidants, pour point depressants, emulsion stabilizers, and detergents [16–21]. Oleic acid showed the highest reactivity even
Fig. 4. Yields of the products from the esterification of TMP with 2-EHA, INA, and oleic acid. Reaction conditions: TMP (3.7mmol), fatty acid (3.3equiv of TMP), AC-ch-SO3H (1mol% of fatty acid), N2 flow, 24h for T-nE and T-nI and 3h for T-nO.
though its chain length is the longest. The tri-ester of oleic acid (T-3O) can be obtained in a 93% yield after 3h (Fig. 4). Following reaction, the catalyst (AC-ch-SO3H) was filtered out and then the filtrate was passed through Celite layer to afford the products. The reason why the reactivity of oleic acid is much higher than the other fatty acids (2-EHA and INA) can be understood due to the less steric hindered alkyl chain of oleic acid. Although the alkyl chain length of oleic acid is nearly twice longer than that of 2-EHA and INA, the bulkiness of oleic acid is relatively low because the alkyl chain of oleic acid is linear without any branch and cis-form double bond in the middle of chain prevents free rotation of the chain. At a prolonged reaction time, it was found that a small portion of T-3O (about 3%) was hydrolyzed and converted back to T-2O (see Table S3 and Fig. S7a for details). 2-EHA and INA showed a lower reactivity than oleic acid due to the bulkiness caused by the branching and the flexibility of their alkyl chains. The reactivity of 2-EHA was lower than that of INA to give T3E in only 40.6% yield after 24h. Meanwhile, T-3I was obtained in 59.3% yield. It is believed that branch of 2-EHA has negative effect on the esterification compared with that of INA because ethyl group is bigger sized branch than methyl branch. In addition, ethyl group of 2-EHA is much closer to carboxylic acid that is the reactive site of esterification than methyl of INA (Fig. 4).
Fig. 5. Reusability test of AC-ch-SO3H in the esterification of TMP with 2-EHA. Reaction conditions: TMP (3.7mmol), 2-EHA (3.3equiv of TMP), AC-ch-SO3H (1mol% of 2-EHA), N2 flow, 24h.
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3.5. Reusability test of AC-ch-SO3H in the esterification between TMP and 2EHA The reusability of AC-ch-SO3H was evaluated as a biobased heterogeneous acid catalyst in the esterification between a polyol and a fatty acid. TMP and 2-EHA were used as substrates, and the reaction was performed in the presence of 1mol% of the catalyst without solvent at 180°C for 8h. After each run, AC-ch-SO3H was filtered and washed with 1,4dixoane. Any further regeneration of the catalyst was not conducted. AC-ch-SO3H can be reused 3 times without any significant loss of activity (Fig. 5). This result was confirmed by FT-IR analysis of the fresh and the reused AC-ch-SO3H. The peaks corresponding to\\SO3 and O_S_O were retained after the reaction (Fig. S5). 4. Conclusions Lignocellulose-derived amorphous carbons with a high loading level of SO3H were readily prepared via (i) heat treatment at 400°C for 1h and (ii) sulfonation with chlorosulfuric acid. The as-synthesized sulfonated amorphous carbons were successfully applied to synthesis of polyolesters as a biobased heterogeneous acid catalyst. From the results of the XRD and BET analyses, it was observed that these carbonaceous materials were not crystalline, but rather were amorphous structures with low surface areas. The attachment of SO3H groups was confirmed by FT-IR and XPS, and the loading level of SO3H was determined by CHNS elemental analysis. The higher SO3H-loaded amorphous carbons showed a greater catalytic activity for esterification between a polyol and a fatty acid under solventless conditions. The esterification of oleic acid derived from vegetable oil with trimethylolpropane using this catalyst afforded the desired biolubricant in over 93% yield after 3h. The sulfonated amorphous carbons could be reused three times without significant loss of activity. Acknowledgments We would like to acknowledge the financial support from the R&D Convergence Program of NST (National Research Council of Science
and Technology) (CAP-11-04-KIST) of the Republic of Korea and KITECH (Korea Institute of Industrial Technology) and from the Internal Research Program (PEO17250) of KITECH. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2017.03.015.
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Solventless esterification of fatty acids with trimethylolpropane using sulfonated amorphous carbons derived from wood powder Dasom Mun a,b, Anh Thi Hoang Vo c, Bora Kim a, Yong-Gun Shul b, Jin Ku Cho a,c,⁎ a b c
Green Material and Process R&D Group, Korea Institute of Industrial Technology (KITECH), Cheonan, Chungnam 31056, Republic of Korea Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Republic of Korea Green Process and System Engineering Department, Korea University of Science and Technology (UST), Cheonan, Chungnam 31056, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 December 2016 Received in revised form 15 March 2017 Accepted 16 March 2017 Available online 18 March 2017 Keywords: Solventless esterification Fatty acids Polyols Sulfonated amorphous carbon Biobased Heterogeneous acid catalysts
a b s t r a c t Polyolesters synthesized by an esterification between polyols and fatty acids are value-added oleochemicals widely used as lubricants, cosmetics, and food additives among other applications. However, homogeneous acid catalysts are still preferred in their industrial production, despite their associated energy cost and the environmental issues that they present. In this paper, we describe lignocellulose-derived amorphous carbons with a high loading level of SO3H and their application to the synthesis of polyolesters as a biobased heterogeneous acid catalyst. These sulfonated amorphous carbons could be readily prepared via (i) heat treatment at 400°C for 1h and (ii) sulfonation with chlorosulfuric acid. XRD and BET analyses demonstrated that these carbonaceous materials were not crystalline but were amorphous structures with a low surface area. The attachment of SO3H groups was confirmed by FT-IR and XPS, and the loading level of SO3H was determined by CHNS elemental analysis. Chlorosulfuric acid gave a higher loading level of SO3H than other sulfating agents, such as conc. sulfuric acid and fuming sulfuric acid. The higher SO3H-loaded amorphous carbons exhibited a greater catalytic activity for esterification between trimethylolpropane and fatty acids under solventless conditions. Esterification of oleic acid derived from vegetable oil with trimethylolpropane using this catalyst afforded the desired biolubricant at over 93% yield in 3h. The sulfonated amorphous carbons could be reused three times without any significant loss of catalytic activity. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Polyolesters (POEs) are oleochemicals widely used as lubricants, cosmetics, and food additives among other applications, and they are made by esterification of fatty acids with polyols [1]. Esterification is performed in the presence of Brønsted acids, such as sulfuric acid, phosphoric acid, or p-toluenesulfonic acid. Although these homogeneous acid catalysts are preferred in industrial production due to their low cost and strong acidity, (i) they are difficult to separate from reaction mixtures and reuse; (ii) a large amount of acidic waste water is discharged, which causes environmental problems; and (iii) they often cause serious corrosion in process units [2]. To overcome the drawbacks of homogeneous acid catalysts, a variety of heterogeneous acid catalysts were explored. Cation exchange resins with high loading of sufficient sulfonic acid functionalities exhibit good performance in certain cases, but they are expensive and have low thermal stability [3]. Zeolitic solid acids including H-beta and H-ZSM-5 were attempted but the small pore sizes were unsuitable for an effective access of ⁎ Corresponding author at: Green Material and Process R&D Group, Korea Institute of Industrial Technology (KITECH), Cheonan, Chungnam 31056, Republic of Korea. E-mail address: [email protected] (J.K. Cho).
http://dx.doi.org/10.1016/j.catcom.2017.03.015 1566-7367/© 2017 Elsevier B.V. All rights reserved.
bulky substrates (fatty acids and polyols) [4]. Esterification of fatty acids using heteropolyacid (HPA) supported on clay K-10 [5] or mesoporous silica [6], an Fe-Zn double-metal cyanide complex [7], and ZrOCl2·8H2O [8] as catalysts have been reported, but the studies were confined to simple alcohol compounds used primarily to produce the fatty acid methyl esters (FAME) of biodiesel. Additionally, immobilized lipase has been applied to the esterification of fatty acids with long chain alcohols, but there are still economic issues [9]. Currently, amorphous carbons (AC) containing SO3H groups attract considerable attention as biobased heterogeneous acid catalysts for esterification due to their physical/chemical stability and high catalytic activity. These carbon materials are typically made from natural carbon sources, such as glucose [10] or cellulose [11], via incomplete carbonization followed by sulfonation. Incomplete carbonization results in AC composed of small polycyclic aromatic carbon sheets in a three-dimensional sp3bonded structure, and these AC can afford highly acidic catalysts by sulfonation because there are numerous reactive sites where SO3H groups may attach covalently, compared with crystalized carbon (CC) [12]. In this study, amorphous carbons containing SO3H (AC-SO3H) were prepared from lignocellulose, the most abundant biomass, and they were applied to the synthesis of polyolesters as biobased heterogeneous acid catalysts. AC-SO3H could be readily obtained by the thermal
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treatment of wood powder and then sulfonation with sulfonating agents such as conc. sulfuric acid (H2SO4), fuming sulfuric acid (SO3H2SO4), and chlorosulfonic acid (ClSO3H). AC-SO3H was characterized by XRD, FT-IR, XPS, BET, and elemental analysis. Esterification between trimethylolpropane (TMP), a polyol containing three symmetric alcohol moieties, and fatty acids, including 2-ethylhexanoic acid (2-EHA), isononanoic acid (INA), and oleic acid, was performed without solvent in the presence of as-synthesized AC-SO3H. The effects of the loading level of SO3H, the temperature, and the structure of the fatty acids on the synthesis of polyolesters were investigated. The reusability of ACSO3H was also tested. 2. Experimental procedures 2.1. Materials A wood powder (pine tree, 50–100μm in diameter) used as a raw material of the lignocellulose was supplied by G·biotech (Korea). The fatty acids 2-ethylhexanoic acid (2-EHA), isononanoic acid, and oleic acid, as well as trimethylolpropane (TMP), a polyol, were provided by Oh Sung Chemical Industry Co., Ltd. (Korea). All other chemicals, including conc. sulfuric acid (H2SO4, 95.0–98.0%), fuming sulfuric acid (SO3H2SO4, 28.0–32.0% free SO3), chlorosulfonic acid (ClSO3H, N98%), ptoluenesulfonic acid monohydrate (PTSA, 99%), and 1,4-dioxane (99.8%), were purchased from Sigma-Aldrich (USA) and directly used without further purification.
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gently stirred at 80°C in an oil bath for 3h and additionally at room temperature (25°C) for 3h. The resulting sulfonated amorphous carbonaceous materials were washed with hot distilled water (70°C) until pH paper was no longer changed to an acid-indicating color and then washed with 1,4-dioxane. Afterwards, to remove the loosely bonded materials, sulfonated amorphous carbonaceous materials were washed by Soxhlet extraction with 1,4-dioxane for 24h and dried in vacuum oven for one day. The AC-SO3H species obtained by treatment with conc. sulfuric acid, fuming sulfuric acid, and chlorosulfonic acid were denoted as AC-co-SO3H, AC-f-SO3H, AC-ch-SO3H, respectively. 2.3.2. Esterification between fatty acids and TMP Into a 250mL two-neck round-bottom flask were placed TMP (3.7mmol, 1equiv) and an excess of fatty acid (12.3mmol, 3.3equiv), and AC-SO3H (0.123mmol, 1mol% of fatty acid) was added as a catalyst. The reaction mixture was heated to the desired temperature and stirred at 300rpm under N2 flow for 24h. Volatile substances formed during the reaction were removed by N2. At intervals of 1h, a small amount of sample was taken out from the reaction mixture using a syringe and then it was diluted with dichloromethane (DCM) and submitted for GC–MS (GCMS-QP2010 Ultra, Shimadzu, Japan) with an Rxi-5ms column (50m in length). Helium was used as a carrier gas at 1mLmin−1of flow rate. Injector temperature was 150°C. Column temperature was programmed from 150°C to 330°C at a rate of 5°Cmin−1 and held isothermal for 14min. The split ratio, ion source temperature, mass scan range were 1:20, 300°C, 35–1000massunits, respectively. The yield of polyolester was calculated by the following equation.
2.2. Analysis AC-SO3H was analyzed by XRD (Powder X-ray diffraction, D8 ADVANCE with DAVINCI, BRUKER, Germany), FT-IR (Fourier transform infrared spectroscopy, Nicolet 6700, Termo Scientific system, USA) and XPS (X-ray photoelectron spectroscopy K-Alpha™+, Thermo Scientific, USA). The loading amounts of sulfur were determined by elemental analysis (Automatic Elemental Analyzer, FLASH 2000 Series, Thermo Scientific, USA), and the surface areas were measured by BET analysis (Brunauer-Emmett-Teller, ASAP2010, Micromeritics, USA), respectively. XRD was conducted at 2θ from 2.5° to 30°, at a scanning step size of 0.02° and a scan speed per step of 0.5s using Cu Kα radiation. FT-IR samples were prepared in a pellet form by mixing the catalyst sample with KBr, and spectra were recorded in ATR mode; 126 spectra were accumulated and averaged to improve the signal-to-noise ratio. XPS measurements were performed on a Thermo Scientific K-alpha instrument using monochromatized Al Kα radiation (hν=1486.6eV) and processed using Thermo Avantage software. The calculated spectra represented the transmittance. The specific surface area was determined on a BET surface analyzer using N2 as the adsorbent at liquid nitrogen temperature (77K) in the relative pressure (P/P0) range of 0–0.25. The powder samples were degassed in air over 12h at 100°C prior to analysis. 2.3. Procedure 2.3.1. Preparation of AC-SO3H Amorphous carbonaceous materials were prepared directly from wood powder. A wood powder was carbonized by heating at 400°C under N2 for 1h. Fifty grams of wood powder (WP) were placed into a rectangular shaped ceramic alumina crucible (10×10×5cm3), and the WP-containing crucible was placed in a chamber-type electric furnace. The furnace was heated to 400°C over 80min under flowing N2, and the temperature was maintained for 1h to afford approximately 15g of amorphous carbonaceous materials (weight yield: ~30%). For the attachment of SO3H groups onto the aromatic rings of the amorphous carbonaceous materials, sulfonation proceeded as follows: In the 500mL round bottom flask were placed 10g of amorphous carbonaceous materials, and 100mL of a sulfonating agent such as conc. sulfuric acid, fuming sulfuric acid, or chlorosulfuric acid were added. The mixtures were
Yield of polyolester ð%Þ ¼ Conversion of TMP Selectivity of polyolesters ðT−nFÞ where T is an initial letter of trimethylolpropane; F is an initial letter of fatty acid (E, I, O in the cases of 2-EHA, isononanoic acid, oleic acid, respectively); and n is the number of fatty acids coupled with a TMP. 3. Results and discussion 3.1. Characterization of AC-SO3H According to the previous report by Okamura et al., heat treatment above 450°C afforded large carbon sheets in a well-crystallized form, which indicated a lack of reactive sites for the attachment of SO3H groups on the carbon sheets [13]. Therefore, the carbonization process was conducted at a moderate temperature (under 450°C) in this study. Wood powders were changed to black carbonaceous materials after heat treatment under N2 at 400°C for 1h. In the XRD patterns of the carbonaceous materials, two broad peaks were observed at 10–30° and at 35–50°, corresponding to randomly oriented aromatic carbon sheets [14] (Fig. S1a). The results indicated that the carbonaceous materials formed an amorphous structure due to incomplete carbonization. The BET surface area, under 100m2g−1 of the carbonaceous materials, confirmed that they are not crystalline materials. Next, SO3H groups were attached to the aromatic rings of the amorphous carbon (AC) to impose acidic character. Three sulfonating agents, specifically sulfuric acid, fuming sulfuric acid, and chlorosulfonic acid, were examined for efficient sulfonation. Sulfonation of AC was analyzed by FT-IR and XPS. FT-IR spectra of the sulfonated amorphous carbon (AC-SO3H) contained bands at 1377 and 1040cm−1, which were assigned as O_S_O and SO− 3 stretching bands, respectively (Fig. S1b) [14]. A peak appearing at 168.8eV in the XPS spectrum corresponded to the S 2p binding energy of the SO3H groups [13] (Fig. S1c). The XRD spectra of the AC-SO3H were similar to that of AC. It was understood that no structural change occurred during sulfonation. The BET surface areas were also sustained after sulfonation, except for AC-ch-SO3H sulfonated with chlorosulfuric acid (286m2g−1). It could be explained that further carbonization occurred during treatment with chlorosulfuric acid, and the crystallinity
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Fig. 1. Comparison of AC-co-SO3H and AC-ch-SO3H in the esterification between polyol and fatty acid.
increased. However, this surface area was still considerably lower than that of activated carbon. The loading level of \\SO3H groups on ACSO3H after sulfonation was determined by elemental analysis and the amount of sulfur per AC-SO3H was calculated to mmolg−1. The loading levels of \\SO3H on AC-co-SO3H, AC-f-SO3H, and AC-ch-SO3H were 0.67mmolg−1, 1.29mmolg−1, and 1.57mmolg−1, respectively. The total Brønsted acidity of AC-ch-SO3H was measured by an acid-base titration using a 10mM NaOH aqueous solution to give 2.02mmolofH+g−1. This finding indicated that approximately 30% of the total Brønsted acidity would be from other acidic moieties, such as \\CO2H, rather than\\SO3H.
3.2. Effect of the SO3H loading level on the esterification between TMP and 2-EHA using AC-SO3H In the previous section, we found that the sulfonation of AC relied on sulfonating agents, and chlorosulfuric acid resulted in a higher loading level of SO3H than any of the other sulfonating agents. The higher loading level of SO3H means that the acid sites exist more densely on the support. To examine the effect of the SO3H loading level on the esterification between a polyol and a fatty acid, two catalysts (AC-co-SO3H and AC-ch-SO3H) with different loading level of SO3H (0.67mmolg−1 vs 1.57mmolg−1) were compared under identical conditions (Fig. 1). The polyol and fatty acid sources used were 3.7mmol of TMP (500mg) and 2-EHA (12.3mmol, 3.3equiv of TMP), respectively. TMP not containing a β-H was employed owing to its thermal and hydrolytic stability [15]. The amount of catalyst was determined as 1mol% of the fatty acid (0.123mmol of SO3H). In other words, 184mg of AC-co-SO3H and 78mg of AC-ch-SO3H were added to the reaction. The esterification
was carried out without a solvent at 180°C for 24h. Interestingly, ACch-SO3H with a high loading level of SO3H showed a much faster reaction rate, even though the total amount of SO3H is the same as that in AC-co-SO3H. The half-life (t1/2) of TMP in the reaction using AC-coSO3H was 31.8min, while the t1/2 of TMP was only 11.4min in the case of AC-ch-SO3H (Fig. 2). The yields of esterification products were also enhanced when AC-ch-SO3H was used. A tri-ester (T-3E) was obtained as the major product in 58.1% yield, and N95% of the mono-ester (T1E) was consumed after 7h (Fig. S2b). However, the reaction in the presence of AC-co-SO3H gave di-ester (T-2E) as the major product in 52.9% yield and a significant amount of the mono-ester (T-1E), over 10%, remained even after 24h (Fig. S2a). All results were summarized in Table 1. Moreover, a trace amount of unknown products (about 1– 2%) was found in GC–MS when using AC-ch-SO3H. It is due to the high activity of AC-ch-SO3H and the reason about formation of unknown products will be discussed in Section 0.
3.3. Effect of the temperature on the esterification between TMP and 2-EHA using AC-co-SO3H The effect of the reaction temperature on the esterification of TMP and 2-EHA in the presence of AC-co-SO3H was investigated. Prior to test, the activity of AC-co-SO3H was compared with that of PTSA as a reference. AC-co-SO3H showed a little bit higher activity than PTSA in both reaction rate (t1/2=30.6min for AC-co-SO3H vs 36.6min for PTSA) and yield of T-3E (36.5% for AC-co-SO3H and 35.4% for PTSA) (see Figs. S3 and S4). From the results, we can understand that AC-co-SO3H has a comparative activity to homogeneous acid catalysts. As expected, the esterification progressed more rapidly as the reaction temperature increased (180°C–220°C), and the t1/2 of TMP was shortened from 31.8min to 13.2min (Fig. S5). The yields of the esterification products were increased at 220°C. T-3E was obtained in 51.4% yield and T-1E almost disappeared after 24h (Fig. S6). However, a couple of unknown compounds began to form after 3h at 220°C (Table S2). From GC–MS, the molecular weights of unknown compounds were same as T-2E and T-3E, respectively. It indicates that unknown compounds are diastereomers of T-2E and T-3E because 2-EHA has a chiral center. Probably, these diastereomers seem to be more steric hindered than major products, T-2E and T-3E because the steric hindered diastereomers of T-2E Table 1 Characteristic data and the esterification results of AC-co-SO3H and AC-ch-SO3H. Catalyst
Surface Loading Amount t1/2 of TMP Product yields area (min) (%) level of SO3H used 2 −1 −1 (m g ) (mmolg ) (mg) T-1E T-2E T-3E
AC-co-SO3Ha 63 AC-ch-SO3Hb 286 Fig. 2. Comparison of the TMP (reactant) profiles according to the time of the esterification between TMP (3.7mmol) and 2-EHA (3.3equiv of TMP) using AC-co-SO3H and AC-chSO3H (1mol% of 2-EHA each) at 180°C.
0.67 1.57
184 78
31.8 11.4
10.4 0.7
52.9 39.3
36.5 58.1
a Reaction conditions: TMP (3.7mmol), 2-EHA (12.2mmol, 3.3equiv of TMP), AC-co-SO3H (0.12mmol, 1mol% of 2-EHA), N2 flow, 180°C, 24h. b Reaction conditions: TMP (3.7mmol), 2-EHA (12.2mmol, 3.3equiv of TMP), AC-ch-SO3H (0.12mmol, 1mol% of 2-EHA), N2 flow, 180°C, 24h.
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Fig. 3. Reaction scheme for the esterification of TMP with fatty acids and the structures of the fatty acids.
and T-3E requiring higher activation energy can be produced only at high temperature. 3.4. Effect of the chemical structure of the fatty acids on the esterification with TMP using AC-ch-SO3H Polyolesters coupled by multiple numbers of fatty acids are highly bulky compounds. Therefore, the chemical structures of fatty acids can affect their reactivity during esterification. In this study, the reactivity of three fatty acids, including 2-EHA, INA, and oleic acid, in an esterification with TMP was evaluated in the presence of AC-ch-SO3H (Fig. 3). All the reactions were carried out under solventless conditions. 2-EHA and INA are synthetic fatty acids. 2-EHA has a C8 alkyl chain and the ethyl branch is adjacent to the carboxylic acid moiety. INA has a C9 alkyl chain and the isopropyl group is located at the end of the alkyl chain. Meanwhile, oleic acid is a fatty acid derived from vegetable oil. Oleic acid has a C18 linear alkyl chain and one double bond that is located at the middle of the chain. Therefore, the alkyl chain of oleic acid produces a lesser steric factor than those of 2-EHA and INA. Particularly, polyolesters made with oleic acid are known as biolubricants and they have several significant advantages: non-toxicity, biodegradability, carbon-neutrality, high temperature stability, and no need for various additives such as anti-oxidants, pour point depressants, emulsion stabilizers, and detergents [16–21]. Oleic acid showed the highest reactivity even
Fig. 4. Yields of the products from the esterification of TMP with 2-EHA, INA, and oleic acid. Reaction conditions: TMP (3.7mmol), fatty acid (3.3equiv of TMP), AC-ch-SO3H (1mol% of fatty acid), N2 flow, 24h for T-nE and T-nI and 3h for T-nO.
though its chain length is the longest. The tri-ester of oleic acid (T-3O) can be obtained in a 93% yield after 3h (Fig. 4). Following reaction, the catalyst (AC-ch-SO3H) was filtered out and then the filtrate was passed through Celite layer to afford the products. The reason why the reactivity of oleic acid is much higher than the other fatty acids (2-EHA and INA) can be understood due to the less steric hindered alkyl chain of oleic acid. Although the alkyl chain length of oleic acid is nearly twice longer than that of 2-EHA and INA, the bulkiness of oleic acid is relatively low because the alkyl chain of oleic acid is linear without any branch and cis-form double bond in the middle of chain prevents free rotation of the chain. At a prolonged reaction time, it was found that a small portion of T-3O (about 3%) was hydrolyzed and converted back to T-2O (see Table S3 and Fig. S7a for details). 2-EHA and INA showed a lower reactivity than oleic acid due to the bulkiness caused by the branching and the flexibility of their alkyl chains. The reactivity of 2-EHA was lower than that of INA to give T3E in only 40.6% yield after 24h. Meanwhile, T-3I was obtained in 59.3% yield. It is believed that branch of 2-EHA has negative effect on the esterification compared with that of INA because ethyl group is bigger sized branch than methyl branch. In addition, ethyl group of 2-EHA is much closer to carboxylic acid that is the reactive site of esterification than methyl of INA (Fig. 4).
Fig. 5. Reusability test of AC-ch-SO3H in the esterification of TMP with 2-EHA. Reaction conditions: TMP (3.7mmol), 2-EHA (3.3equiv of TMP), AC-ch-SO3H (1mol% of 2-EHA), N2 flow, 24h.
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3.5. Reusability test of AC-ch-SO3H in the esterification between TMP and 2EHA The reusability of AC-ch-SO3H was evaluated as a biobased heterogeneous acid catalyst in the esterification between a polyol and a fatty acid. TMP and 2-EHA were used as substrates, and the reaction was performed in the presence of 1mol% of the catalyst without solvent at 180°C for 8h. After each run, AC-ch-SO3H was filtered and washed with 1,4dixoane. Any further regeneration of the catalyst was not conducted. AC-ch-SO3H can be reused 3 times without any significant loss of activity (Fig. 5). This result was confirmed by FT-IR analysis of the fresh and the reused AC-ch-SO3H. The peaks corresponding to\\SO3 and O_S_O were retained after the reaction (Fig. S5). 4. Conclusions Lignocellulose-derived amorphous carbons with a high loading level of SO3H were readily prepared via (i) heat treatment at 400°C for 1h and (ii) sulfonation with chlorosulfuric acid. The as-synthesized sulfonated amorphous carbons were successfully applied to synthesis of polyolesters as a biobased heterogeneous acid catalyst. From the results of the XRD and BET analyses, it was observed that these carbonaceous materials were not crystalline, but rather were amorphous structures with low surface areas. The attachment of SO3H groups was confirmed by FT-IR and XPS, and the loading level of SO3H was determined by CHNS elemental analysis. The higher SO3H-loaded amorphous carbons showed a greater catalytic activity for esterification between a polyol and a fatty acid under solventless conditions. The esterification of oleic acid derived from vegetable oil with trimethylolpropane using this catalyst afforded the desired biolubricant in over 93% yield after 3h. The sulfonated amorphous carbons could be reused three times without significant loss of activity. Acknowledgments We would like to acknowledge the financial support from the R&D Convergence Program of NST (National Research Council of Science
and Technology) (CAP-11-04-KIST) of the Republic of Korea and KITECH (Korea Institute of Industrial Technology) and from the Internal Research Program (PEO17250) of KITECH. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2017.03.015.
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