Microwave-assisted catalytic esterification of α-glucoisosaccharino-1,4-lactone with tall oil fatty acids
© Kumar and Alén. 2016
Received: 22 February 2016
Accepted: 19 April 2016
Published: 27 April 2016
Carbohydrates-rich materials are partly degraded during alkaline kraft pulping into a complex mixture of aliphatic carboxylic acids consisting of α-glucoisosaccharinic acid as one of the main acids. On the other hand, crude tall oil, containing significant amounts fatty acids, is obtained as a by-product from kraft pulping. One interesting novel approach is to utilize chemically both these side-streams by producing renewable surfactants from aliphatic carboxylic acids and fatty acids via esterification.
In this paper, lauric (C12:0) and oleic (C18:1) acids as well as a mixture of tall oil fatty acids were esterified with α-glucoisosaccharino-1,4-lactone in a microwave-assisted batch reactor using a respective molar ratio of 3:1 in the presence of p-toluenesulfonic acid as a catalyst to obtain a mixture of the corresponding mono- and diesters (nonionic surfactants) at 70–100 °C for 6–24 h. The total yields varied in the range of 40–62 % and the mass ratio of monoesters to diesters (0.4–2.7) was greatly dependent on the fatty acid feedstock.
It seems that relatively high yields of mono- and diesters of typical fatty acids can be obtained when they are esterified with one of the most significant lactones (α-glucoisosaccharino-1,4-lactone) in black liquor from kraft pulping. These esters are potential surfactants based on renewable feedstocks.
KeywordsGlucoisosaccharinic acid Lauric acid Oleic acid Tall oil fatty acids Esterification Batch reactor Kraft pulping Microwave
During kraft pulping, about half of the initial wood material is dissolved in the cooking liquor (“black liquor”, BL) . BL typically contains degraded lignin, aliphatic carboxylic acids, and inorganic compounds, as well as small amounts of extractives and hemicellulose residues. In the kraft pulping industry, the tall oil soap, mainly containing sodium salts of fatty and resin acids, is removed by skimming from the BL during its multiple-effect evaporation before burning in a recovery furnace to generate energy and recover the inorganic cooking chemicals .
Conventionally, crude tall oil (CTO) is obtained from the tall oil soap by adding sulfuric acid to liberate free fatty and resin acids . The CTO is then further purified and fractionated by vacuum distillation to obtain several commercial fractions including tall oil fatty acids (TOFAs) (20–40 %), tall oil rosin (TOR) (25–35 %), light oil (10–15 %), and pitch residue (20–30 %) [2–4]. The average yield of CTO is in the range of 30–50 kg/ton of pulp. In general, oleic (C18:1) and linoleic (C18:2) acids are the most significant fatty acid components in TOFAs. Recently, TOFAs are used as such or as various derivatives in many commercial applications including, for example, the manufacture of paints, surfactants, inks, and fuel additives [5–8].
The main fraction of aliphatic carboxylic acids in BL contains volatile acids (formic and acetic acids) together with non-volatile hydroxy carboxylic acids [1, 9]. The hydroxy acids, originating from feedstock carbohydrates by alkali-catalyzed degradation reactions, can be further divided into low-molar-mass (LMM) hydroxy acids with 2–4 carbon atoms (mainly glycolic, lactic, and 2-hydroxybutanoic acids) and high-molar-mass (HMM) hydroxy acids with 5 or 6 carbon atoms (mainly 3,4-dideoxy-pentonic, 3-deoxy-pentonic, xyloisosaccharinic, and α- and β-glucoisosaccharinic (α- and β-GISA, respectively) acids [1, 10].
Since the aliphatic carboxylic acids are formed in considerable amounts in the pulping industry, their partial recovery has gradually become an interesting alternative to using them as fuel . On the other hand, carboxylic acids possess a low heating value in comparison to that of lignin. Therefore, their recovery from BL reduces the heat content of BL less than expected [11–13]. It has also been studied potential process alternatives for separating sodium-free aliphatic acids [1, 14, 15]. Aliphatic carboxylic acids are used in numerous applications as single components or as more or less purified mixtures. Especially, the LMM acids are commercially utilized for many purposes. However, the chemical utilization of the HMM acids is still challenging and it has been studied only to some extent.
The esters of carboxylic acids with carbohydrates (“sugar fatty acid esters”) and their ethoxylated derivatives belong to a family of synthetic nonionic surface active agents (“surfactants”) . Almost one-fourth of the total industrial production of surfactants account from nonionic surfactants, which are widely used in washing up liquids and personnel care products  and as emulsifiers in food, cosmetics, and medicinal products [18–20]. The free HMM hydroxy acids exist as intramolecular cyclic esters (i.e., lactones) . These molecules can also be directly esterified with long chain fatty acids in the presence of an acid catalyst to obtain nonionic surfactants. In addition, the HMM acid lactones contain multiple hydroxyl groups that practically makes possible, besides the formation of monoesters, the preparation of diesters.
Chemicals and esterification tests
α-GISAL (3-deoxy-2-C-hydroxymethyl-d-erythro-pentono-1,4-lactone) was prepared from lactose  and was gas chromatographically pure (m.p. 94–95 °C) . Lauric acid (~98 %, Fluka Chemical Corporation, Seeltze, Germany), oleic acid (≥99 %, Sigma Aldrich, Inc., St. Louis, MO, USA), p-toluenesulfonic acid (pTSA) monohydrate (~98 %, Sigma Aldrich), and the fraction of TOFAs (Forchem Oy, Finland) were commercial products and used without further purification. All the other chemicals and solvents were of analytical grade.
Fatty acid esters of α-GISAL were directly prepared using a microwave-assisted batch reactor (10 mL). The lauric acid (75.5 mg, 0.37 mmol) was added to the pTSA catalyst (4.5 mg, 0.026 mmol) and α-GISAL (20 mg, 0.12 mmol) was then added in four portions (5 mg) at an interval of 20 min. The resulting suspension was esterified at 100 °C under magnetic stirring for 6 h. The other esterifications were made in the same way; a fatty acid component of oleic acid (103.4 mg, 0.36 mmol) and TOFAs (110 mg, ~0.38 mmol) were separately esterified in the presence of pTSA with α-GISAL, at 70 °C for 24 h. In each case, the crude reaction product was collected for further analyses. Two replicates were made for each condition. The ester products could be recovered by dissolution of the reaction mixture into toluene (unreacted α-GISAL was removed by decanting) followed by washing the liquid phase in aqueous KHCO3 solution (1 % in H2O) to precipitate the unreacted fatty acids as an emulsion. Finally, the ester products were obtained by evaporation of the toluene phase to dryness.
The quantitative analysis of TOFAs was performed by GC/FID according to Lappi and Alén . The relative mass-based response factors used between the GC peak areas derived from internal standards (ISs), heneicosanoic acid and betulinol, and those of compounds were equal to 1. α-GISAL and the product esters were also determined by GC/FID; in each case, the dried sample was dissolved in toluene (1:100 v/v) and then per(trimethylsilyl)ated with a mixture of 0.5 mL N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) containing 1 % of trimethylchlorosilane (TMCS) in 1 mL pyridine prior to chromatographic analysis. Xylitol was used as an IS. The identification of the peaks was conducted by a gas chromatograph equipped with a mass selective detector (GC/MSD). The results were analyzed with the Enhanced ChemStation G1701CA (version C.00.00, Agilent Technologies, Palo Alto, CA, USA) software and the mass spectra were compared to those in the Wiley7n database. The relative mass-based response factors in relation to that of the IS (1.0) were determined by the separately-purified mono- and dilaurate of α-GISA using GC/FID under the same conditions as those in GC/MSD; they were 3.5 and 3.7, respectively. These values were also used for the quantitative determination of other fatty acid mono- and diesters.
HRMS data were obtained in the positive mode using a MicroMass LCT-TOF Premier Spectrometer, Micromass UK Ltd. The results were analyzed using the Mass Lynx software. The accurate sample masses were measured in relation to IS [for monolaurate aspartame, for dilaurate and dioleate erythromycin, and for monooleate 2-(4-hydroxyphenylazo)benzoic acid]. The 1H NMR spectra were recorded in CDCl3 on a Bruker Avance 500 or 300 spectrometer (Bruker Corporation, Billerica, MA, USA). The chemical shifts were reported in ppm relative to the residual CHCl3 (δ 7.26).
IR spectra were recorded with a Tensor27 FT-IR spectrometer (Bruker GmbH, Germany). The spectra were taken as an average of 32 scans in the wave number between 400 and 4000 cm−1 with a resolution of 4 cm−1.
The formation of esters was also followed by thin-layer chromatography (TLC). The mobile phase consisted of hexane and ethyl acetate in the ratio of 50:50 (v/v). The Rf values of lauric acid esters were: 0.74 for a diester and 0.19 and 0.35 for monoesters when esterification took place at the group C2 and C4, respectively.
Results and discussion
Chemical composition of the faction of TOFAs (% of the total compounds)
Yields (%) of the esterification of α-GISAL with fatty acids
The high resolution mass spectrometric (HRMS) (ESI+) data are shown in Additional file 1: Figure S1: m/z calculated for [C18H32O6Na]+ 367.2091 (found 367.2094), which corresponded to monolaurate and m/z calculated for [C30H54O7Na]+ 549.3762 (found 549.3778) representing dilaurate. Similarly, the m/z data on mono- and dioleate were, respectively: 449.2874 (calculated for [C24H42O6Na]+) and 449.2853 (found) and 713.5327 (calculated for [C42H74O7Na]+) and 713.5313 (found).
The IR spectra of the α-GISAL and the lauric esters of α-GISAL are shown in Additional file 1: Figure S2. The strong bands were observed in the bands of 3321, 1776, and 1026 cm−1 for α-GISAL and in the bands of 3460, 1773, 1742, 1710, and 1182 cm−1 for lauric esters of α-GISAL.
In kraft pulping, significant amounts of carbohydrates-derived hydroxy acids and extractives-derived fatty acids are formed as by-products and they can be partly recovered. In general, the mutual esterification of these organic fractions to manufacture value-added nonionic surfactants seems possible.
This investigation performed with simple model substances clearly indicates that relatively high yields of mono- and diesters can by produced under optimized conditions in a microwave-assisted reactor. This new approach may offer a potential integrated biorefinery concept for utilizing more effectively wood feedstock materials with the simultaneous increase in profitability and expansion of the product portfolio of modern kraft mills.
crude tall oil
tall oil fatty acids
HK performed the experiments and analyses. RA made preliminary plan. Both authors read and approved the final manuscript.
The authors gratefully acknowledge the financial support of the Foundation for Research of Natural Resources in Finland (Suomen Luonnonvarain Tutkimussäätiö).
The authors declare that they have no competing interests.
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