Article pubs.acs.org/IECR
Synthesis of Lactide from Alkyl Lactate via a Prepolymer Route Pravin P. Upare,†,‡ Young Kyu Hwang,†,‡ Jong-San Chang,†,‡ and Dong Won Hwang*,† †
Biorefinery Research Group, Korea Research Institute of Chemical Technology (KRICT), Sinseoungno 19, Yuseong, Daejeon 305-600, Korea ‡ School of Science, University of Science and Technology (UST), 113 Gwahangno, Yuseong, Daejeon 305-333, Korea ABSTRACT: Poly(lactic acid) is a biodegradable polymer that has enormous potential for use as a replacement for some petroleum-based materials. However, the properties of this polymer depend strongly on the quality of the lactide monomer from which it is often synthesized. In this work, lactide was synthesized from alkyl lactate via a prepolymer route, and the reaction kinetics was compared with lactide synthesis from lactic acid. A number of different parameters were investigated in order to obtain the highest possible yield of lactide, as well as to achieve L-isomer selectivity. Among the various acid catalysts tested, Sn(Oct)2 was found to be the most effective for L-lactide selectivity as well as for producing a high oligomer yield from the alkyl lactate. The effect of the alkyl group length of the starting materials was investigated with the highest lactide yield being obtained from ethyl lactate, over methyl and butyl lactate. The deoligomerization reaction was also studied in detail. It was found that an oligomer with a molecular weight of 600−800 g/mol gave the highest lactide yield. The optimum depolymerization temperature for the oligomer with a molecular weight of 1274 g/mol was shown to be 180 °C, as above this temperature, the reaction rate of oligomerization for heavy residue was much faster than that of deoligomerization for crude lactide, which resulted in a lower lactide yield.
1. INTRODUCTION Owing to the rapid depletion of fossil fuel resources and plastic disposal problems, much attention has been paid to the production of biomass-based materials and biodegradable polymers. Polylactic acid (PLA) is an aliphatic polyester whose properties are similar to those of some petroleumbased polymers. It is renewable and biodegradable, making it a promising alternative to polymers such as PET for uses including fibers, coatings, and many other plastic materials.1,2 When PLA is exposed to heat and water, it breaks down into low molecular weight oligomers and is eventually degraded to CO2 by microorganisms in the environment.3 Although PLA can be prepared by direct condensation of lactic acid,4,5 the preferred method for producing the polymer with a molecular weight greater than 100 000 is the ringopening polymerization of lactide, a six-membered dimeric cyclic ester of lactic acid, which was commercialized by Natureworks in 2001.6 Purac, a Netherlands-based specialty chemicals producer, is in the process of building a 75 000 tonne/year lactide plant in Rayong, Thailand.7 In this method, lactic acid is first polymerized to form a lactic acid oligomer with a molecular weight less than 3000. It is then depolymerized to lactide by a backbiting mechanism using a Sn-based catalyst. The reaction rate, molecular weight, and optical purity of PLA are highly influenced by the chemical and optical purity of lactide. Hence, the production of lactide is regarded as one of the most crucial processes in the synthesis of PLA from lactic acid. Lactic acid is often produced by the fermentation of glucose. However, high levels of contamination with impurities such as residual sugar, colorings, and other organic acids affect the final lactide yield and optical selectivity.8 To overcome this drawback, the impure lactic acid needs to be purified before the lactide formation reaction is carried out. However, it is © 2012 American Chemical Society
extremely difficult to recover lactic acid from the fermentation broth by distillation, given the fact that lactic acid is nonvolatile and is easily oligomerized at temperatures above 150 °C.9 The most efficient and economically viable method to obtain polymer-grade lactic acid is the esterification of impure lactic acid with alcohol to produce alkyl lactate, which is then separated easily by distillation.10−12 The distilled alkyl lactate is then run through a reactive distillation column filled with ionexchange resin, where it is hydrolyzed into pure lactic acid.13 Finally, the purified aqueous lactic acid is concentrated to enhance the oligomerization rate, which is described in Scheme 1. The costs involved in this process are still relatively high, and thus, an alternative method is currently being sought. One such method involves synthesizing lactide directly from alkyl lactate without converting it into lactic acid by the hydrolysis process. A substantial amount of energy and a large and complex distillation column are required for the hydrolysis procedure. Although the use of a prepolymer route for synthesizing lactide from methyl or ethyl lactate using organic or inorganic acid catalysts is the subject of a number of patents,14,15 a detailed study on this process has not yet been reported. In this study, we have investigated the effects of a number of variables on the oligomerization of alkyl lactate, such as the catalyst type, alkyl group length, and reaction conditions. We have also assessed how the lactide yield and selectivity are affected by the temperature and oligomer molecular weight in the deoligomerization reaction. In addition, we have evaluated the reaction kinetics of oligomerization and compared the properties of the Received: Revised: Accepted: Published: 4837
November 22, 2011 March 6, 2012 March 9, 2012 March 20, 2012 dx.doi.org/10.1021/ie202714n | Ind. Eng. Chem. Res. 2012, 51, 4837−4842
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Scheme 1. Synthesis of Lactide by the Prepolymer Route: Ethyl Lactate versus Lactic Acid
Table 1. Effect of Catalyst Type on Oligomer Yield and L-Lactide Selectivity catalyst feed a
lactic acid ethyl lactateb ethyl lactate ethyl lactate ethyl lactate ethyl lactate
oligomerization
deoligomerization
type
Mw (g/mol)
amount (mmol)
oligomer yield (%)c
Mw (g/mol)
crude lactide yield (%)
SnO SnO SnCl2 Sn(Oct)2 BuSnO2H H2SO4
135 135 190 405 209 98
0.37 3.70 2.63 1.20 2.27 5.00
78 0 0 81 73 76
1394
53 0 0 46 31 29
2346 2772 2859
d
. e
L-lactide sel. (%)
L-lactide produced (mmol)
TONf
98
260 0 0 191 108 116
708 0 0 160 47 23
98 82 94
a Oligomerization: lactic acid (90%) 50 g, 180 °C, 720 Torr for 6 h and 10 Torr for 5 h, N2 flow rate 20 mL/min. Deoligomerization: oligomer 10 g, SnO 0.05 g, 210 °C, 10 Torr for 5 h. bOligomerization: ethyl lactate 50 g, catalyst 0.5 g, 160 °C, 720 Torr for 6 h and 10 Torr for 5 h, N2 flow rate 20 mL/min. Deoligomerization: oligomer 10 g, 180 °C, 10 Torr for 5 h. cOligomer yield (%) = Oligomer obtained (g)/theoretical amount of oligomer (30.5 g for ethyl lactate and 36.0 g for 90% lactic acid). dCrude lactide yield (%) = 100 − Oligomer disappeared (g)/oligomer charged (10 g). eLLactide sel. (%) = L-Lactide (g)/crude lactide (g). fTON (turnover number) = L-Lactide produced/moles of catalyst used.
unreacted alkyl lactate and alcohol could be distilled off completely from the produced oligomer. The oligomerization reaction was also carried out for lactic acid (88%) at 180 °C and 720 Torr under nitrogen atmosphere for 6 h; it was then carried out for another 5 h at 10 Torr. 2.2.2. Lactide Synthesis from Oligomer. The oligomer synthesized from alkyl lactate was transferred to a three-necked 50-mL round-bottom flask. The flask was equipped with a magnetic stirrer, a temperature controller, and a distillation column at 100 °C, which was connected to a collector in an ice bath. The reaction was carried out under nitrogen atmosphere at 180 °C and 10 Torr for 5 h. The oligomer from lactic acid was depolymerized at 210 °C for 5 h after adding 0.5 wt % SnO catalyst. 2.3. Analysis. Concentrations of the volatile compounds (methanol, ethanol, butanol, methyl lactate, ethyl lactate, butyl lactate) and lactide were analyzed using a gas chromatograph (DONAM DS6200) equipped with flame ionization detector (FID) and a Cyclosil B column (30 m × 0.320 mm). The column temperature was increased from 70 to 250 °C, with a ramping rate of 15 °C/min. The injector and FID temperatures were maintained at 270 and 280 °C, respectively. For quantification of the meso- and L-lactide from the GC spectra, the calibration factor of L-lactide was used, because L-lactide is commercially available. The molecular weight of the produced
final lactide produced from alkyl lactate with that produced from lactic acid.
2. EXPERIMENTAL SECTION 2.1. Materials. Tin oxide (SnO, 99%, Aldrich), tin chloride (SnCl2, 99%, Aldrich), tin octoate (Sn(Oct)2, 97%, Alfa Aesar), n-butyltin hydroxide oxide (BuSnO2H, 95%, Alfa Aesar), and sulfuric acid (98%, Samchun Chemical) were used as the catalysts, without further treatment. Lactic acid (89%, Aldrich), methyl lactate (99%, Alfa aesar), ethyl lactate (99%, Alfa aesar), butyl lactate (98%, Aldrich), and L-lactide (99.9%, Purac) were used as feedstocks, without further treatment. Triethylene glycol dimethyl ether (TEGDME 99.9% Alfa aesar) was used as a standard compound for the quantitative analysis of each component in the gas chromatography analysis. 2.2.1. Methods. Oligomer Synthesis. A mixture of 50 g of alkyl lactate and 0.5 g of catalyst was oligomerized in a 250mL three-necked round-bottom flask equipped with a magnetic stirrer, a temperature controller, and a distillation condenser. The reaction was carried out at 150−180 °C and 720 Torr under nitrogen atmosphere (20 mL/min) for 6−10 h, depending on the parameters being investigated. During the reaction, alcohol was removed continuously from the reactor. After this time, the pressure was decreased to 10 Torr, following which the reaction continued for 2−5 h so that the 4838
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notable that BuSnO2H showed much lower L-lactide selectivity (82%) than Sn(Oct)2 (98%), which was attributed to the formation of meso-lactide (8%) and some low molecular weight oligomers (10%), not detected by gas chromatography. The “coordination−insertion” mechanism of Sn(Oct)2, which has been well established for ring-opening polymerization of cyclic esters, may also be applicable to the oligomerization of ethyl lactate.18−20 Sulfuric acid (H2SO4), as an example of Bronsted acid, was also tested for comparison with the Sn-based Lewis acids. H2SO4 produced a lower oligomer yield with a higher molecular weight than Sn(Oct)2. It also produced a much lower crude lactide yield and L-lactide selectivity than Sn(Oct)2. It is possible that the lower optical purities of Llactide when using BuSnO2H and H2SO4 might be due to the hydroxyl functional group of the catalysts, because the optical purity of L-lactide decreases with the concentration of ionic impurities.7 In addition, the turnover number of Sn(Oct)2 was found to be around 3.4 and 7.0 times higher than that of BuSnO2H and H2SO4, respectively. Although Sn(Oct)2 was found to be the most active catalyst system for synthesizing lactide from ethyl lactate, it has a critical limitation. Sn(Oct)2 is highly hygroscopic and was easily hydrolyzed when the ethyl lactate feed contained water impurities, thus eliminating its activity for oligomerization. It is therefore essential that all water should be removed from the ethyl lactate feed and the reaction system during oligomerization. 3.3. Oligomerization Reaction Kinetics. The kinetics of the oligomerization of ethyl lactate using Sn(Oct)2 was investigated as this was the most efficient catalyst. In a closed reaction system equipped with a total reflux condenser, the ethyl lactate concentration decreased rapidly up to 2 h, while lactoyl ethyl lactate, the dimer of ethyl lactate, was produced together with ethanol (Figure 1). After this time, the concentration of all species remained almost constant. In addition, oligomer with a molecular weight higher than 500 g/ mol was not observed, which indicates that the dimerization of ethyl lactate over an Sn(Oct)2 catalyst is reversible as is represented in Scheme 2. These results show that it is essential
oligomer was analyzed using a gel permeation chromatograph equipped with a refractive index detector (RID-10A, Shimadzu) and Styragel HR1 column (7.8 × 300 mm, Waters) using THF as the mobile phase with a flow rate of 1 mL/min. Polystyrenes of known molecular weights, i.e., 580, 780, 1200, 1650, 2340, 3950, 6180, and 10200 (Shodex standard, SL-105), were used as calibration standards and universal callibration curve (K = 1.67 × 10−4, a = 0.692 in the [η] = KMa Mark−Houwink equation)[2].
3. RESULTS AND DISCUSSION 3.1. Formation of Lactide from Lactic Acid. The oligomer yields and molecular weights obtained from the oligomerization reactions of ethyl lactate and lactic acid as well as the lactide yield and selectivity of the L-isomer in the deoligomerization reactions are summarized in Table 1. The oligomer yield was calculated from the ratio of the amount of oligomer obtained after the reaction to the theoretical yield, which was determined using the assumption that all water and ethanol molecules were removed by condensation from lactic acid and ethyl lactate, respectively. When lactic acid was oligomerized with continuous removal of water, the oligomer yield was 78% and the molecular weight was found to be 1394 g/mol. In this study, lactic acid was first condensed to oligomer at 180 °C and 720 Torr for 6 h and then the pressure was decreased to 10 Torr in order to increase the oligomerization reaction rate. During the reaction, it is likely that some of the unreacted lactic acid monomer, dimer, and trimer was vaporized as the boiling point of lactic acid is 122 °C at 12 Torr and 217 °C at 760 Torr.16 This is thought to be the main reason for a lower than theoretical oligomer yield. It should be noted that lactic acid was self-oligomerized in the absence of an acid catalyst because it is known that the condensation reaction between the carboxyl and alcohol groups could proceed with ease.17 The formed oligomer was then decomposed in the presence of 0.5 wt % SnO at 210 °C for 5 h. The crude lactide yield obtained was 53%, and the L-lactide selectivity was found to be 98%. The loss in yield was likely due to the formation of heavy residues with molecular weights of 5000−10000 g/mol, which would have remained in the reactor after the deoligomerization reaction. The loss in the L-lactide selectivity can be attributed to the formation of lactic acid monomer and meso-lactide. 3.2. Formation of Lactide from Alkyl Lactate. When ethyl lactate was oligomerized at 160 °C in the absence of a catalyst, the reaction rate was extremely low compared to that of lactic acid. For this reason, various Sn-based materials were tested as Lewis acid catalysts to facilitate the oligomerization reaction of ethyl lactate. The most effective catalysts tested were Sn(Oct)2 and BuSnO2H, which were found to afford oligomer yields of 81% and 73%, respectively, whereas SnO and SnCl2 did not produce any oligomer. These results indicate that the level of reactivity in the oligomerization of ethyl lactate is not proportional to the acid strength of the catalyst. The species with the highest acid strength (SnCl2) showed no activity, whereas it was a species with only moderate acid strength (Sn(Oct)2) that showed the highest activity. For the deoligomerization reaction, Sn(Oct)2 showed a much higher selectivity for L-lactide as well as a higher crude lactide yield than BuSnO2H. The slightly lower lactide yield from BuSnO2H than that from Sn(Oct)2 might be ascribed to the higher molecular weight of the oligomer (2772 versus 2346 g/ mol), which will be discussed further in Section 3.4. It is
Figure 1. Concentration profile of reactant and products during oligomerization of ethyl lactate in a closed reaction system. Oligomerization: ethyl lactate 50 g, Sn(Oct)2 0.5 g, 160 °C, 760 Torr for 9 h, N2 flow rate 20 mL/min. 4839
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Scheme 2. Reversible Reaction of Ethyl Lactate Dimerization
were selected as 150, 160, and 185 °C, respectively, owing to their boiling points. Both the oligomerization rate and the molecular weight of the oligomer from butyl lactate were much lower than those from methyl lactate and ethyl lactate, even though the reaction temperature for butyl lactate was higher. The lower level of condensation of butyl lactate over Sn(Oct)2 compared to methyl and ethyl lactate may be attributed to the steric hindrance caused by the bulky alkyl group adjacent to the lactic acid. In addition, the rate of the removal of butanol produced by condensation of butyl lactate is much slower than that of methanol and ethanol owing to its much higher boiling point (methanol, 65 °C; ethanol, 78 °C; and n-butanol, 118 °C). The crude lactide yield and the L-lactide selectivity produced from deoligomerization of the butyl lactate oligomer were also much lower than those obtained from the methyl and ethyl lactate oligomers. This lower lactide yield was ascribed mainly to the incomplete removal of butyl lactate and the light oligomer (Mw < 500 g/mol), which could not be transformed efficiently to lactide, from the main oligomer (Mw ≈ 1478). The produced L-lactide needs to be separated rapidly from the reaction system, as it is highly unstable at temperatures above 150 °C, especially when exposed to impurities, and can undergo racemization into meso-lactide.21 In the deoligomerization reaction using the butyl lactate oligomer, the residual butyl lactate and light oligomer may have induced the transformation of L-lactide to meso-lactide and lactoyl lactic acid, which then resulted in a lower L-lactide selectivity. Among the different lactate starting materials tested, ethyl lactate showed the best results for the oligomerization reaction. This monomer produced an oligomer of a higher molecular weight in a shorter time, although the higher reaction temperature is likely to be a contributing factor to this. The yield obtained from the oligomerization of ethyl lactate was also dependent on the amount of the Sn(Oct)2 catalyst (Figure 3). The oligomer yield increased with the catalyst concentration from 0.1 to 1 wt %, at which point no further increases occurred. The molecular weight increased with the catalyst concentration up to 5 wt %, with a value of approximately 1300 g/mol at 1 wt %. Thus, 1 wt % of Sn(Oct)2 was considered to be an adequate amount of catalyst for the oligomerization of ethyl lactate. 3.5. Optimization of Deoligomerization Reaction. The properties of the oligomer have a significant influence on the
to rapidly remove the ethanol produced by condensation of ethyl lactate in order for the lactoyl ethyl lactate to be oligomerized further. The oligomerization kinetics of ethyl lactate over 1 wt % Sn(Oct)2 was also investigated in an open reaction system equipped with a partial condenser, where only ethyl lactate was refluxed and the ethanol produced was removed from the reaction system continuously. In this system, the ethyl lactate concentration decreased continuously with the reaction time and conversion of ethyl lactate reached 100% after 9 h, as shown in Figure 2. It is notable that the concentration of lactoyl ethyl lactate increased up to 3 h and then decreased with the
Figure 2. Concentration profile of reactant and products during oligomerization of ethyl lactate in an open reaction system. Oligomerization: ethyl lactate 50 g, Sn(Oct)2 0.5 g, 160 °C, 720 Torr for 6 h and 10 Torr for 5 h, N2 flow rate 20 mL/min.
reaction time. The oligomer yield also increased with the reaction time, reaching 100% after 11 h, whereas the ethanol concentration in the reactor was almost negligible during the entire reaction. 3.4. Effect of Alkyl Chain Length. The effect of the length of the alkyl group on the Sn(Oct)2-catalyzed oligomerization of the alkyl lactate is shown in Table 2. The oligomerization temperatures for methyl lactate, ethyl lactate, and butyl lactate
Table 2. Effect of Alkyl Group Length on Oligomerization and Deoligomerizationa oligomerization
deoligomerization
feed
temp (°C)
time (h)
oligomer yield (%)
Mw (g/mol)
temp (°C)
time (h)
crude lactide yield (%)
L- lactide sel. (%)
methyl lactate ethyl lactate butyl lactate
150 160 185
13 11 19
65 81 71
1918 2346 1478
180 180 210
5 5 5
49 46 24
98 98 64
a
Oligomerization: alkyl lactate 50 g, Sn(Oct)2 0.5 g, 720 Torr for 6 h and 10 Torr for 2 h, N2 flow rate 20 mL/min. Deoligomerization: oligomer 10 g, 180 °C, 10 Torr for 5 h. 4840
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The crude lactide yield was also affected by the deoligomerization reaction temperature (Figure 5). In this study, the prepolymer with a molecular weight of 1274 g/mol was used since the oligomer yield from ethyl lactate was below
Figure 3. Effect of catalyst concentration on oligomerization of ethyl lactate. Oligomerization: ethyl lactate 50 g, Sn(Oct)2, 160 °C, 720 Torr for 6 h and 10 Torr for 2 h, N2 flow rate 20 mL/min.
lactide yield and L-selectivity obtained from the deoligomerization reaction. To investigate this further, oligomers of varying molecular weights were prepared by altering the reaction time while keeping the catalyst concentration (1 wt % Sn(Oct)2) and temperature (160 °C) constant. The crude lactide yield decreased linearly with the molecular weight of the oligomer used, whereas the L-lactide selectivity was not different much for all oligomers (Figure 4). This result may be attributed to the lower mobility of the oligomer with a higher molecular weight. These species can more easily transform into oligomer heavier
Figure 5. Effect of deoligomerization temperature. Oligomerization: ethyl lactate 50 g, Sn(Oct)2 0.5 g, 160 °C, 720 Torr for 6 h and 10 Torr for 2 h, N2 flow rate 20 mL/min (Mw 1274 g/mol). Deoligomerization: oligomer 3 g, 10 Torr for 5 h.
50% due to evaporation of unreacted ethyl lactate for a molecular weight less than 1000 g/mol. The crude lactide yield was found to be the highest at 180 °C, with the content of heavy residue increasing with temperature. This result indicates that at reaction temperatures above 180 °C, the rate of the oligomerization reaction forming heavy residue was much faster than that of the deoligomerization reaction affording the lactide. For this reason, in this reaction system, 180 °C was considered to be the optimum deoligomerization temperature for the oligomer with a molecular weight of 1274 g/mol, where the crude lactide yield was about 81%. Finally, we compared the crude lactide yield between ethyl lactate and lactic acid for the prepolymer with similar molecular weight. The crude lactide yield from the lactic acid oligomer with molecular weight of 1374 g/mol was about 80% when 3 g of oligomer was depolymerized at 210 °C and 10 Torr, while the yield was 53% at 10 g of oligomer (Table 1). Thus, it is thought that the crude lactide yield from ethyl lactate would be almost the same with lactic acid at the optimized reaction conditions.
4. CONCLUSION In this study, the synthesis of lactide from alkyl lactate was investigated in detail. The highest lactide yield was obtained using ethyl lactate owing to its moderate vapor pressure and low steric hindrance in oligomerization. Among the various catalysts assessed, Sn(Oct)2 was found to be the most effective in both the oligomerization and the deoligomerization reactions. The higher catalytic activity of Sn(Oct)2 was thought to be related to the coordination−insertion mechanism between its 2-ethylhexanoate ligands and the alcohol of alkyl lactate. At the optimized reaction conditions (ethyl lactate, 1 wt % Sn(Oct)2, oligomer molecular weight of 600 g/mol), the Llactide yield reached 82%. The results show the feasibility of using alkyl lactate for the synthesis of high-quality lactide.
Figure 4. Effect of oligomer molecular weight on deoligomerization. Oligomerization: ethyl lactate 50 g, Sn(Oct)2 0.5 g, 160 °C, 720 Torr for 2 h to 6 h and 10 Torr for 2 h, N2 flow rate 20 mL/min. Deoligomerization: oligomer 3 g, 180 °C, 10 Torr for 5 h.
than 5000 g/mol rather than undergo the desired deoligomerization to lactide. The prepolymer with a molecular weight less than 600 g/mol showed a little lower L-lactide selectivity with higher lactoyl ethyl lactate selectivity than other ones, which might be insufficient cross-linking of ethyl lactate. The results indicate that 600−800 g/mol is the optimum oligomer molecular weight for maximizing the crude lactide yield and L-lactide selectivity. 4841
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(18) Trimalle, T.; Moller, M.; Gurny, R. Synthesis and ring-opening polymerization of new monoalkyl-substituted lactides. J. Polym. Sci., Part A 2004, 42, 4379. (19) Chisholm, M. Concerning the ring-opening polymerization of lactide and cyclic esters by coordination metal catalysts. Pure Appl. Chem. 2010, 82, 1647. (20) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; White, A. J. P.; Williams, D. J. A well defined tin (II) initiator for the living polymerisation of lactide. Chem. Commun. 2001, 283. (21) Tsukegi, T.; Motoyama, T.; Shirai, Y.; Nishida, H.; Endo, T. Racemization behavior of L,L-lactide during heating. Polym. Degrad. Stab. 2007, 92, 552.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +82-42-860-7674. Fax: +82-42-861-4245. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the R&D Program of MKE/KEIT [10031795, Conversion of C3 Platform Chemicals from Biomass] and by Institutional Research Program of KRICT [KK-1201-A0].
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REFERENCES
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