Ind. Eng. Chem. Res. 2010, 49, 1247–1251
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Recovery of L-Lactic Acid from Poly(L-lactic acid) under Hydrothermal Conditions of Dilute Aqueous Sodium Hydroxide Solution Masaru Yagihashi and Toshitaka Funazukuri* Department of Applied Chemistry, Chuo UniVersity 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
Poly(L-lactic acid) (PLLA) pellets were degraded in a batch reactor under hydrothermal conditions of aqueous sodium hydroxide solution, from 343 to 453 K, compared to water in the absence of NaOH. Over an entire range of conversion, components solubilized from PLLA mainly consisted of L-lactic acid (LLA) in the presence of NaOH. For instance, PLLA was nearly completely converted into LLA at 433 K for 60 min or 453 K for 20 min with 0.6 M aqueous NaOH solution, where D-lactic acid was not observed. The degradation reaction with/without NaOH proceeded in the induction stage followed by the major degradation stage, as has been seen for polyesters reported in our previous studies. In the major degradation stage the overall reaction rate for PLLA was represented by 2/3-order reaction kinetics with respect to the amount of unreacted polymer, suggesting that the reaction occurred on the polymer surface. The NaOH concentrations above 0.6 M at 343 K hardly affected the reaction rates, but did affect the induction time remarkably. It can be considered that the degradation reaction is controlled by not chemical reaction but dissolution of products on the pellet surface. Introduction Huge amounts of plastic wastes have been discharged, and chemical recycling is required to establish a sustainable society. Biodegradable polymers such as poly(lactic acid) (PLA) have become the focus of attention as one of alternatives to conventional undegradable polymers. Even though the polymer is biodegradable, it takes a long time to be degraded, and an efficient degradation process is required for chemical recycling. Although stabilities of PLA and composites containing PLA have extensively been studied in various environments or by various media such as thermal, hydrolytic, enzymatic, bacteria, irradiation, photodegradation, etc., the studies to recover the monomer are limited. There are two routes studied via depolymerization and hydrolysis for chemical recycling of PLA. In the former, with/ without catalysts, PLA is thermally depolymerized and converted to lactide,1-5 which can be polymerized via ring-opening. In contrast, PLA can be hydrolytically converted to lactic acid with water under hydrothermal conditions.6-8 Mohd-Adnan et al.9 also studied the hydrolysis under high pressure steam. In both thermal degradation and hydrolysis relatively high temperatures are required to obtain practically high reaction rates, but the racemization reaction is inevitable.8,10 The reaction should be avoided to produce high quality polymers.10 To develop a chemical recycling process, high selectivity of monomer production and relatively fast reaction rates under mild conditions are essential. Recently, the authors demonstrated the effectiveness of conversion of various polyesters into monomers for poly(ethylene terephthalate)(PET), poly(ethylene naphthalate)(PEN), and polycarbonate(PC) under alkali hydrothermal conditions.11-14 Although the degradation of PLA in the presence of alkali was examined to study the stability of PLA or surface morphology at 37 °C,15-17 the recovery of lactic acid has not been made. To examine an effectiveness for alkali hydrothermal conversion of poly(L-lactic acid) (PLLA) to L-lactic acid (LLA), PLLA was hydrolytically converted into LLA under various hydrothermal * To whom correspondence should be addressed. E-mail: tfunazo@ kc.chuo-u.ac.jp.
conditions in the presence of NaOH, and the rates were measured. The production rates of D-lactic acid were also determined. Experimental Section The hydrolytic degradation of PLLA was carried out mainly in a small bomb-type batch reactor, made of stainless-steel tubing, whose volume was 3.6 mL. The PLLA sample used was cylinder-shaped (average size: 2.5 × 2.7 × 3.2 mm), supplied by Sigma-Aldrich (Tokyo, Japan). Mainly 60 mg of PLLA pellets and a 2 mL of solvent (distilled water or a NaOH solution) were loaded in the reactor at room temperature and sealed. The reactor was immersed in a constant-temperature airoven (below 363 K), a silicon oil bath (from 433 to 453 K), or a molten salt bath (above 513 K). The reaction time was counted from the moment that the reactor had been immersed in the oven or the baths, whose temperatures were maintained within fluctuation (2 K. The heat-up periods in the three heating media were from one to a few minutes, but the values were negligible compared to the reaction times. When the prescribed time elapsed, the reactor was removed from the oven or bath and cooled by water. The reactor was opened, and then the product solution and residual solid were recovered. The inside reactor and residual solid were washed with distilled water sufficiently. The product solution and washing solution were filtered with a glass filter. Residual solid was weighed after drying in an oven. The filtrate was analyzed by HPLC with three kinds of columns: SH1821 and DE413 (both Shodex, Japan) for analyzing lactic acid and lactide, and CRX-853 (Shodex, Japan) for determining yields of D- and L-lactic acid. Contents of total organic carbon in the product solution and washing solution were measured by a TC analyzer (TC5000A, Shimadzu, Japan) after adjusting both the concentration and pH value. In this study TOC and product yields on carbon weight basis are defined as follows: g organic carbon in solution g carbon in initial PLLA sample g carbon in compound yield [%] ) 100 × g carbon in initial PLLA sample TOC [%] ) 100 ×
10.1021/ie9008925 2010 American Chemical Society Published on Web 12/16/2009
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Figure 2. Lactic acid yield vs TOC value with water and 0.6 M aqueous NaOH solution at various temperatures with water: (O) 433, (*) 453, (0) 513, (concentric square) 533, (4) 553, (À) 573, with NaOH, (2) 343, (b) 353, (9) 363, ([) 433, (1) 453 K.
Figure 1. Time changes of product yields, amounts of residual solid and total recovered, and TOC value with (a) water and (b) 0.6 M aqueous NaOH solution at 433 K: (9) total recovered, (2) residual solid, (O) TOC, ([) lactic acid, (0) lactide.
To examine the effects of external mass transfer resistance around PLLA pellets due to the use of the batch reactor without a stirrer, a semibatch reactor was employed to compare the rates to those in the batch reactor. The details on the semibatch reactor setup and the procedures were shown in our previous reports,13,14 and are briefly described as follows. The semibatch reactor was the same as the batch reactor, whose both ends were connected to the lines via frit disks with 2 µm pores instead of sealing with the Swagelok caps. PLLA pellets of 120 mg wrapped loosely with stainless-steel screen were placed in the reactor, and the solvent, distilled water or aqueous NaOH solution, was continuously supplied to the reactor at a constant flow rate of 3 mL/min (at room temperature). The pressure was maintained at 10 MPa by a back pressure regulator. Product solution that eluted from the back-pressure regulator was collected in every certain period, mainly 5 min. Results and Discussion Comparison between Aqueous NaOH Solution and Water. Figure 1 compares time changes of product yields, the amounts of residual solid, TOC, and total recovered (a) with water and (b) with 0.6 M aqueous sodium hydroxide solution at 433 K. With both solutions total recovered was higher than 90% for all reaction times. The amounts of residual solid on a carbon weight basis decreased with time, and correspondingly those of TOC increased. Note that the carbon contents of residual solid were assumed to be equal to those of PLLA. The yields of lactic acid with both solvents also increased with time. The yields of lactic acid with NaOH aq were almost equal to those of TOC, but those with water were lower than those of TOC. The difference may be caused by the presence of oligomers due to lower hydrolysis capability in the absent of NaOH. Lactide was slightly produced with water at lower temperatures and medium reaction times but was not produced with NaOH. Figure 2 shows lactic acid yield vs TOC value with water and 0.6 M aqueous NaOH solution at various temperatures and
Figure 3. Effects of PLLA loaded amount on yields of lactic acid ([) and lactide (0), the amount of residual solid (2), TOC value (O), and total recovered (9) with 0.6 M aqueous NaOH solution at 453 K for 15 min.
times. Lactic acid yields with water at 433 and 453 K were lower than TOC values. The yield was nearly proportional to the TOC value over an entire range of TOC values except for those with water at 433 and 453 K. Lactic acid yields with NaOH were almost equal to TOC values at all temperatures. According to HPLC chromatograms and the material balances shown in Figure 1, in the presence of NaOH, PLLA was fully converted into monomer, lactic acid, and oligomers hardly remained in product solutions. Effect of PLLA Loaded Amount. Figure 3 shows the effects of PLLC loaded amounts on product yields, and the amounts of TOC and residual solid at 453 K for 15 min with 0.6 M aqueous NaOH solution. The yields and TOC values decreased with increasing the loaded amounts, and those of residual solid increased. All these values were significantly influenced by the loaded amount. This also supports that the reaction was significantly affected by dissolution of products. Hydrolysis Rate. In depolymerization of polyesters such as poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), and polycarbonate (PC) under hydrothermal conditions in the presence/absence alkali, we found that the reaction rates were represented by 2/3-order reaction kinetics with respect to unreacted amounts of polymer, suggesting that the reaction took place on the surface of polymer pellets.11-14 In the present case the shapes of pellets also remained, and the sizes were reduced until the reaction was nearly completed. Thus, by assuming that pellets are spherical, the reaction rate can be expressed by eq 1:11-14 dY ) k(1 - Y)2/3 dt
(1)
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Figure 4. Plot of (1 - Y)1/3 - 1 vs reaction time with water at 433 (O) and 453 K (*), and 0.6 M aqueous NaOH solution at 343 (2), 353 (b), and 363 K (9).
Figure 5. Plot of (1 - Y)1/3 - 1 vs reaction time with aqueous NaOH solution at various concentrations and 343 K: 0.6 (4), 1.0 (2), 1.3 (À), 1.6 (5), 2.0 M (3).
where Y is the conversion, and k is the overall rate constant. By integrating eq 1 from the conversion Yi ≈ 0 at an induction time ti to Y at t, k (2) (1 - Y)1/3 - 1 ) - (t - ti) 3 Figure 4 plots (1 - Y)1/3 - 1 vs reaction time with both solvents at various temperatures, where the conversion Y is based on lactic acid yield. As had been seen for depolymerization of PET, PEN, and PC under hydrothermal conditions,11-14 the plots in all cases were represented by straight lines over wide ranges of conversion. The induction times in which almost no reaction apparently proceeded were observed. Since eq 2 was found to be valid, the reaction can be considered to proceed on the polymer surface, similar to other polymers. In fact Tsuji and Ikada16 observed that hydrolysis of PLLA film in 0.01 M NaOH aq at 37 °C proceeded mainly via the surface erosion mechanism. Figure 5 also plots (1 - Y)1/3 - 1 vs reaction time with aqueous NaOH solution at various concentrations from 0.6 to 2.0 M and 343 K. Similar to Figure 4, the plots were expressed with straight lines, but the slopes were almost the same. Figure 6 depicted the reaction rate constants and the induction times obtained from the slopes and the intercepts, respectively, shown in Figure 5. Note that the induction time was the intercept at Y ) 0 of the straight line at each NaOH concentration. The rate constants were almost irrespective of NaOH concentration. The hydrolytic degradation reaction of PLLC proceeded, but the rates were not affected by alkali concentration. The induction times decreased with increasing NaOH concentration up to 1.3 M, and were almost unchanged at higher concentrations. It can be
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Figure 6. Effects of NaOH concentration on reaction rate constant k (2) and induction time ti (0) at 343 K.
Figure 7. Plot of (1 - Y)1/3 - 1 vs reaction time for lactic acid yields with water at various temperatures, obtained by Saeki et al.:7 453 (0), 473 (9), 503 (double square), 533 K(!), 573 K (open split square).
considered that during the induction times PLLC was degraded into oligomers, whose molecular weights were higher than those of compounds soluble in the alkali solution. Thus, the reaction rates could be controlled by not chemical reaction but dissolution of products. Hydrolysis of PLLA with Water by Saeki et al. Saeki et al.7 measured lactic acid yields obtained from hydrolysis of PLLA with water at various hydrothermal conditions in a batch reactor, similar to ours, but they did not determine the reaction rates. The validity of eq 2 was examined with the yield data of Saeki et al., as shown in Figure 7. The plots were also represented by straight lines over a wide range of temperature, and eq 2 was valid up to conversions higher than 90%. Figure 8 shows the rate constants and the induction time against 1000/ T, obtained from plots in Figure 7. At temperatures lower than 533 K (1000/T > 1.87), the rates were represented by a straight line, and the values were almost unchanged at higher temperatures. On the other hands, the induction times were also expressed with a straight line at all temperatures. Thus, the rate constant in eq 1 and the induction times with water are
[ -E RT ]
kwater [min-1] ) 1.041 × 105 exp
with E ) 54.59 kJ · mol-1
[ ]
Eti with Eti ) 50.93 kJ · mol-1 RT Since the rate data were obtained in the batch reactors without a stirrer employed by Saeki et al.7 and in the present study, the rate data measured in the semibatch reactor were also compared to those in the batch reactors. Figure 9 compares (1 - Y)1/3 1 values on the basis of TOC yield obtained in the semibatch ti [min] ) 4.565 × 10-5 exp
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Figure 10. Time changes of D/L ratio with water at various temperatures, 533 (4), 543 (9), 553 (O), 573 K ([), obtained in the present study. Figure 8. Rate constant k (9) and induction time ti (4) vs 1000/T, obtained from Figure 7 by Saeki et al.7 at various temperatures.
Figure 9. Comparison of (1 - Y)1/3 - 1 values with water (0,9) and 0.6 M-NaOH (O,b) at 433 K in the semibatch and batch reactor. Open and solid keys designate the semibatch and batch reactor, respectively.
and batch reactor with two solvents, 0.6 M NaOH and water, at 433 K. Since the reactor type did not affect the values with each solvent, the external mass transfer resistance may be insignificant with water and 0.6 M aqueous NaOH solution. The slopes with water and NaOH were almost the same while the plots scattered somewhat, and the induction times were remarkably different. Thus, the dissolution of products may be significant. Note that the fluid residence time in the semibatch reactor is estimated to be less than 30 s,18 and the soluble components can be expected to be instantly swept out of the reactor. Racemization. In chemical recycling of poly(lactic acid) racemization is unfavorable, as shown by Tsukegi et al.10 When the monomer produced is recovered, D- and L-form should be separated. Figure 10 shows time changes of the yield ratio of D-lactic acid to L-form with water at temperatures from 533 to 573 K in the present study. When the ratio is unity, the yield of L-form is equal to that of D-form. As seen in Figure 10 the ratio was proportional to time at each temperature, and the ratio was only a function of temperature. Tsukegi et al.10 studied racemization of L,L-lactide at temperatures from 453 to 573 K, and determined the rates of the transformation of L,L-lactide. They found that the conversion to meso-lactide increased with increasing temperature and time. Faisal et al.8 also observed racemization of LA subjected to water at 250-300 °C. Similar to these studies, the racemization was also observed in hydrolysis of poly(L-lactic acid) with water above 533 K in the present study and was significantly affected by temperature. Note that no D-lactic acid was observed in hydrolysis of PLLA in the presence of NaOH at temperatures up to 453 K. The ratio was
Figure 11. Arrhenius plot for rate constant kD/L with water obtained from Figure 10.
expressed well by the zero order reaction kinetics. The ratio seemed to become unity infinitely. Figure 11 shows an Arrhenius plot for rate constant kD/L for the formation of D-lactic acid, defined by eq 3, obtained from the slope in Figure 10. d (D/L) ) kD/L dt
(3)
where D/L is the yield ratio of D-form to L-form. The preexponential factor of 9.164 × 1011 min-1 and the activation energy of 145.9 kJ mol-1 of the rate constant kD/Lwere obtained. Conclusions Poly(L-lactic acid) was subjected to water in the presence/ absence of sodium hydroxide under hydrothermal conditions in a batch reactor. In the presence of NaOH PLLA was almost fully converted into L-lactic acid, for instance for 20 min at 453 K in 0.6 M aqueous NaOH solution. The reaction proceeded in the first induction period, where almost no product was eluted, followed by the major degradation stage. The induction times with water were also correlated with temperature. In the major stage with/without NaOH the reaction was represented by the 2 /3 order reaction kinetics over a nearly entire range of conversion, suggesting that the reaction proceeded on surfaces of solid polymer. The rate constants with NaOH were almost independent of NaOH concentration at 433 K, and the rates at this temperature were almost the same in both the semibatch and batch reactor with 0.6 M aqueous NaOH solution and water. Thus, the reaction could be controlled by dissolution of products from polymer surfaces. The production rates of D-lactic acid
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from PLLA were significantly affected by temperature, and represented by the zero order reaction kinetics. Literature Cited (1) Noda, M.; Okuyama, H. Thermal catalytic depolymerization of poly(L-lactic acid) oligomer into LL-lactide: Effects of Al, Ti, Zn, and Zr compounds as catalysts. Chem. Pharm. Bull. 1999, 47, 467. (2) Tsuji, H.; Fukui, I.; Daimon, H.; Fujie, K. Poly(L-lactide) XI. Lactide formation by thermal depolymerization of poly(L-lactide) in a closed system. Polym. Deg. Stab. 2003, 81, 501. (3) Nishida, H.; Fan, Y.; Mori, T.; Oyagi, N.; Shirai, Y.; Endo, T. Feedstock recycling of flame-resisting poly(lactic acid)/aluminum hydroxide composite to L,L-lactide. Ind. Eng. Chem. Res. 2005, 44, 1433. (4) Omura, M.; Tsukegi, T.; Shirai, Y.; Nishida, H.; Endo, T. Thermal degradation behavior of poly(lactic acid) in a blend with polyethylene. Ind. Eng. Chem. Res. 2006, 45, 2949. (5) Motoyama, T.; Tsukegi, T.; Shirai, Y.; Nishida, H.; Endo, T. Effects of MgO catalyst on depolymerization of poly-L-lactic acid to L,L-lactide. Polym. Deg. Stab. 2007, 92, 1350. (6) Tsuji, H.; Daimon, H.; Fujie, K. A new strategy for recycling and preparation of poly(L-lactic acid): Hydrolysis in the melt. Biomacromolecules 2003, 4, 835. (7) Saeki, T.; Tsukegi, T.; Tsuji, H.; Daimon, H.; Fujie, K. Depolymerization of poly(L-lactic acid) under hydrothermal conditions. Kobunshi Ronbunshu 2004, 61, 561. (8) Faisal, M.; Saeki, T.; Daimon, H.; Fujie, K. Racemization of lactic acid under hydrothermal conditions. Asian J. Chem. 2006, 18, 248. (9) Mohd-Adnan, A. F.; Nishida, H.; Shirai, Y. Evaluation of kinetics parameters for poly(L-lactic acid) hydrolysis under high-pressure steam. Polym. Deg. Stab. 2008, 93, 1053. (10) Tsukegi, T.; Motoyama, T.; Shirai, Y.; Nishida, H.; Endo, T. Racemization behavior of L,L-lactide during heating. Polym. Deg. Stab 2007, 92, 552.
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(11) Zenda, K.; Funazukuri, T. Hydrothermal depolymerization of poly(ethylene terephthalate) in the presence of ammonia. Joint 8th International Symposium on Hydrothermal Reactions and 7th International Conference on Solvo-Thermal Reactions, August, 2006, Sendai, Japan. (12) Zenda, K.; Hatakeyama, K.; Arai, R.; Funazukuri, T. Hydrothermal depolymerization of poly(ethylene terephthalate), poly(ethylene naphthalate), and polycarbonate with aqueous ammonia solutions. Proceedings of SuperGreen 2007, The 5th International Conference on Supercritical Fluids, November 2007, Seoul, Korea. (13) Zenda, K.; Funazukuri, T. Depolymerization of poly(terephthalate) in dilute aqueous ammonia solution under hydrothermal conditions. J. Chem. Technol. Biotechnol. 2008, 83, 1381. (14) Arai, R.; Zenda, K.; Hatakeyama, K.; Yui, K.; Funazukuri, T. Reaction kinetics of hydrothermal depolymerization of poly(ethylene naphthalate), poly(ethylene terephthalate), and polycarbonate with aqueous ammonia solution. Chem. Eng. Sci. 2010, 65, 36. (15) Cam, D.; Hyon, S. H.; Ikada, Y. Degradation of high molecular weight poly(L-lactide) in alkaline medium. Biomaterials 1995, 16, 833. (16) Tsuji, H.; Ikada, Y. Properties and morphology of poly(L-lactide). II. Hydrolysis in alkaline solution. J. Polym. Sci., Part A 1998, 36, 59. (17) Yuan, X.; Mak, A. F. T.; Yao, K. Surface degradation of poly(Llactic acid) fibres in a concentrated alkaline solution. Polym. Deg. Stab. 2003, 79, 45. (18) Miyazawa, T.; Funazukuri, T. Hydrothermal production of mono(galacturonic acid) and the oligomers from poly(galacturonic acid) with water under pressures. Ind. Eng. Chem. Res. 2004, 43, 2310.
ReceiVed for reView May 31, 2009 ReVised manuscript receiVed November 24, 2009 Accepted November 29, 2009 IE9008925
