lactic acid production by alkaline hydrothermal treatment of corn cobs
TRANSCRIPT
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Chemical Engineering Journal 181– 182 (2012) 655– 660
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
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actic acid production by alkaline hydrothermal treatment of corn cobs
ristina Sánchez, Itziar Egüés, Araceli García, Rodrigo Llano-Ponte, Jalel Labidi ∗
hemical and Environmental Engineering Department, University of the Basque Country, Pza Europa, 1, 20018, Donostia-San Sebastián, Spain
r t i c l e i n f o
rticle history:eceived 18 October 2011eceived in revised form 6 December 2011
a b s t r a c t
An experimental study was carried out for the corn cobs thermal conversion to obtain the maximumcontent in lactic acid. For this purpose, under the same conditions (275 ◦C and 30 min) different con-centrations of Ca(OH)2 as alkaline catalyst were used (from 0.32 M to 1 M). The maximum content of
ccepted 9 December 2011
eywords:orn cobshermal conversionatalyst
lactic acid (6.72 ± 0.31 g/L) was obtained with 0.7 M of Ca(OH)2. With this catalyst concentration, differ-ent reaction conditions were used (250, 275 and 300 ◦C and 15, 30 and 45 min). The optimal conditions toproduce the highest yield of lactic acid from corn cobs in alkaline conditions were determined at 300 ◦Cand 30 min, achieving 44.76 ± 2.59% respect to the total cellulose and hemicellulose contained in theinitial corn cobs (7.38 ± 0.43 g/L of lactic acid).
© 2011 Elsevier B.V. All rights reserved.
actic acid. Introduction
Lignocellulosic materials are characterized to be abundant andnexhaustible, renewable, with low pollution character and withow price. Due to these properties, lignocellulosic materials repre-ent an important source to obtain polymeric materials, energy and
wide variety of chemical compounds with high added value [1–3].mong lignocellulosic materials, corn cobs are classified as abun-ant agricultural wastes with high content in hemicelluloses andellulose (about 30–40 wt.% hemicelluloses and 33–41 wt.% cellu-ose of the dry material). Owing to their composition, corn cobsresent a great potential for producing a lot of added-value chem-
cals [4].The hemicellulosic fraction is made up of amorphous het-
ropolysaccharides containing different structural units (xylose,lucose, arabinose, mannose, galactose or rhamnose), which cane substituted with phenolic, uronic or acetyl groups [5]. Foremicelluloses recovery, xylooligosaccharides can be produced
rom xylan-rich lignocellulosic biomass by different hydrothermalrocesses. These obtained xylooligosaccharides can be used for syn-hesis of biopolymers such as lactic acid [6]. Cellulose, the mainomponent of biomass, has become an attractive substrate becausef its conversion to useful chemical products, as well as for itotential for the production of bioethanol. The cellulose moleculesre a linear homopolymers composed of a repeating unit of glu-
ose linked by (1 → 4)-�-glycosidic bonds. The associated celluloseolecules form subunits called microfibrils, which are grouped∗ Corresponding author. Tel.: +34 943017178; fax: +34 943017140.E-mail address: [email protected] (J. Labidi).
385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.12.033
forming fibrils with crystalline and amorphous structures parts[7–9].
Lactic acid (2-hydroxypropionic acid) is an organic acid bear-ing a hydroxyl group an acid function, and has particularly gainedinterest for use in producing biodegradable lactic acid poly-mers, solvents, metal pickling and food additives [10,11]. Dueto the recent increase in environmental concerns, the techni-cal applications of lactic acid include use as a preservative infood, pharmaceuticals and cosmetics, and in the production ofpolylactic acid (PLA). PLA is a biodegradable polyester used inhealth-demanded new materials such as medical sutures and clipsfor wound closure, controlled drug or in artificial prostheses thatmay be an environment friendly alternative to plastics derivedfrom petrochemical materials [12–15]. Lactic acid also has poten-tial applications as controlled release systems for pesticides anddrugs [16].
Lactic acid can be manufacture by chemical synthesis or by fer-mentation of different carbohydrates such as starch, glucose orxylose. Presently, most lactic acid is produced by fermentation ofglucose, but a minor quantity is produced by chemical synthe-sis employing highly toxic and expensive feedstock [17,18]. Thefermentation is a complex and sensitive process, especially therecovery steps, because it has a limited production capacity andit takes 2–8 days to complete the reaction. The pH and tempera-ture also must be carefully monitored, and the yields to lactic acidare 85–95% based on fermentable sugars [19].
The hydrothermal processing of lignocelullosic wastes is oneof the most prominent methods for converting biomass in lactic
acid, because under a high temperature and high pressure, waterbehaves as a reaction medium having unusual properties, and actsas an effective alkaline catalyst [20,21]. Some studies showed thatlactic acid may be formed by this process without the addition of
6 eering Journal 181– 182 (2012) 655– 660
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Table 1Experiments conditions for Ca(OH)2 as catalyst.
Experiment T (◦C) t (min)
1 250 152 250 303 250 454 275 155 275 306 275 45
the total content in cellulose and hemicelluloses in the initial rawmaterial. Using Ca(OH)2 0.32 M with the same conditions (275 ◦Cand 30 min), the minimum content in lactic acid (4.09 ± 0.12 g/L)was obtained, corresponding to yield of 24.82 ± 0.75%.
Table 2Composition (%) of corn cobs according to other authors.
Component Reference
56 C. Sánchez et al. / Chemical Engin
ny catalyst through a base catalytic effect of high-temperatureater [22], but the addition of an alkali under hydrothermal con-itions can facilitate the lactic acid formation and increase theeaction yield [23].
The main purpose of this study was to determine the best reac-ion conditions to produce lactic acid by thermal conversion of cornobs. Different reaction conditions have been proposed varying thelkaline catalyst content, the temperature and reaction time. Toromote the lactic acid formation, most of reported studies useds alkaline catalyst NaOH or Ca(OH)2 with a concentration rang-ng from 0.32 M to 2.5 M [22–24]. In this work both catalysts haveeen used under the same conditions (0.32 M, 275 ◦C and 30 min)o determine the most suitable. Ca(OH)2 was chosen as the best.
The catalyst concentration has been varied to determine theptimal concentration to be used in the experimental study. Usinghe optimum catalyst concentration determined previously, dif-erent alkaline thermal treatments have been carried out. In thexperimental study, the temperature (250, 275 and 300 ◦C) and theime of reaction (15, 30 and 45 min) have been varied to determineheir effects on the reaction yield.
. Materials and methods
.1. Raw material characterization
The corn cobs used as raw material in this investigation wereindly supplied by an independent farmer from Pontevedra (Spain).he raw material with constant moisture, were grounded in a millnd then sieved, to obtain 0.4 mm as the size fraction according withhe standards that will be used. Milled corn cobs were chemicallyharacterized using TAPPI standards in terms of moisture (TAPPI264 cm-97), ash (TAPPI T211 om-93), extracts (TAPPI T204 cm-97),lason lignin (TAPPI T222 om-98), �-hollocellulose [25], cellulose
26], hemicellulose and others.
.2. Alkaline hydrothermal process
By using the alkaline hydrothermal treatments, cellulose andemicellulose are mainly converted into glucose, which discom-osed to other products including aldehydes or ketones, fromhich lactic acid or other organic acids are produced (such as
ormic, acetic or acrylic acid) [27].The reaction was carried out in a Mini Compact Reactor of
00 mL (Parr 5500) equipped with a Reactor Controller 4848 thatupports a maximum pressure of 200 bar and a temperature rangef −10/350◦ C. In each experiment 1.23 g of finely milled corn cobsith 50 mL of dissolution were used. To carry out the treatments,ifferent reaction conditions were used; firstly, the selected cata-
yst Ca(OH)2 concentration was varied (0.32, 0.5, 0.6, 0.7, 0.8 and M) under the same conditions of temperature and reaction time275 ◦C and 30 min). After obtaining the optimal catalyst concen-ration to produce the higher concentration in lactic acid, differentxperiments were carried out varying the temperature and reac-ion time to improve the lactic acid yield. In total, as it can be seenn Table 1, 9 experiments were performed using different condi-ions: at temperature of 250, 275 and 300 ◦C and reaction time of5, 30 and 45 min.
.3. Quantification of lactic acid and other organic acids
To determine the lactic acid content, the hydrolysated sug-rs and the rest of formed compounds in liquors after alkaline
ydrolysis processes, a High Performance Liquid ChromatographyHPLC) Jasco LC-Net II/ADC was used, equipped with a refractivendex detector, photodiode array detector and Rezex ROA-Organiccid H+ (8%) column. As mobile phase, 0.005 N H2SO4 dissolution7 300 158 300 309 300 45
were prepared with 100% deionised and degassed HPLC water. Asinjection conditions 40 ◦C, 0.35 mL/min flow and 40 �L as injec-tion volume were used. For the calibration curve, a standardizedsolution of lactic acid was used. To determine the rest of liquorscompounds, high purity glucose, xylose, arabinose, xylitol, formicacid, acetic acid, ethanol, and acrylic acid were used.
2.4. Characterization of precipitated solid fraction
2.4.1. Fourier transform infrared spectroscopy (FTIR)The remaining solid fraction was filtered and washed, and
then characterized by FTIR. For this purpose, a PerkinElmer 16PCinstrument by direct transmittance with an MKII Golden Gate SPEA-CAC accessory was used. Spectra were recorded over 20 scanswith a 4 cm−1 as resolution in the range between 4000 cm−1 and600 cm−1. By the FTIR spectroscopy, the most characteristic bandsof initial raw material were compared with the obtained solidresidues after hydrothermal conversion.
3. Results and discussion
3.1. Composition of corn cobs
The main components compositions (% on an oven-dry weightbasis) resulted after corn cobs characterization, were 0.95 ± 0.03ash, 2.43 ± 0.03 extracts, 23.13 ± 3.40 lignin, 36.75 ± 0.54 cellulose,29.98 ± 3.6 hemicelluloses and others 6.76 ± 1.52.
Table 2 presents obtained corn cobs compositions reportedresults by other authors.
From these results, it can be concluded that corn cobs are a suit-able feedstock for lactic acid production due to their high contentin cellulose and hemicellulose.
3.2. Election of catalyst concentration
In order to determine the optimal catalyst concentration pre-liminary experiments were carried out. According to Fig. 1, themaximum content in lactic acid (6.73 ± 0.31 g/L) at 275 ◦C and30 min was obtained using Ca(OH)2 0.7 M as catalyst. The achievedyield at these conditions was 40.82 ± 1.88% of lactic acid, respect to
[4] [5] [28]
% Hemicellulose 39.0 31.1 33.7–41.2% Cellulose 34.3 34.3 30.0–41.7% Lignin 14.4 18.8 4.5–15.9
C. Sánchez et al. / Chemical Engineering
3.5
4.5
5.5
6.5
7.5
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Lact
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Fig. 1. Lactic acid concentration versus catalyst concentration.
Taking into account these results, 0.7 M Ca(OH)2 was selectedo perform the experimental study.
.3. Experimental study
.3.1. Quantification of lactic acidUsing 0.7 M Ca(OH)2 as alkaline catalyst, the same solid:liquid
atio (1:40) and modifying the temperature and time of reac-ion a set of experiments were carried out. Fig. 2 represents theesulting lactic acid concentrations for the used experimental con-itions.
The highest concentration of lactic acid (7.38 ± 0.32 g/L) wasbtained with 300 ◦C and 30 min as reaction condition, followedy 300 ◦C and 15 min (7.36 ± 0.20 g/L of lactic acid). At theseonditions, a 44.76 ± 2.59 and 44.67 ± 1.23% yield respectivelyere achieved respect to the total cellulose and hemicelluloses
ontained in the initial raw material. The lowest concentra-ion resulted from the experiment carried out under 300 ◦C and5 min, reaching only 3.62 ± 0.12 g/L of lactic acid (21.97 ± 0.71%ield). Yan et al. [23,24] studied the hydrothermal conversionf the carbohydrates including glucose, cellulose and starchchieving a high lactic acid yield of 27% from glucose usingaOH 2.5 M and 20% yield using Ca(OH)2 0.32 M at 300 ◦Cnd 60 s as reaction conditions. By fermentation, Walton et al.15] achieved a 72% yield of lactic acid from hemicellulosextracts.
It can be concluded that the lactic acid concentration increasesith the increase of temperature and reaction time. Neverthe-
ess, using higher reaction time (45 min) at 300 ◦C decreased
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
250 ºC 15 min
250 ºC 30 min
250 ºC 45 min
275 ºC 15 min
275 ºC 30 min
275 ºC 45 min
300 ºC 15 min
300 ºC 30 min
300 ºC 45 min
lact
ic a
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(g/L
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Reaction conditions
Fig. 2. Lactic acid concentrations at different reaction conditions.
Journal 181– 182 (2012) 655– 660 657
dramatically the lactic acid concentration, that could be explainedby the degradation of lactic acid.
3.3.2. Quantification of other compoundsDuring the alkaline thermal treatments of corn cobs at high tem-
peratures, some byproducts (as acetic acid, formic acid, xylitol. . .)were generated due to the fractionation of hemicelluloses, par-tial degradation or because of depolimerization [29–31]. Theconcentrations of most significant compounds generated were rep-resented in Fig. 3.
According to Fig. 3, formic acid and acetic acid were the mostabundant final degradation compounds (between 60% and 83% ofthe total quantified degradation products) generated from the glu-cose and xylose oxidation. Formic acid concentration increasedwith the temperature and reaction time due to the sugars degra-dation up to 300 ◦C and 30 min (1.37 ± 0.14 g/L formic acid), butat 300 ◦C and 45 min, this final degradation by-product decreased(0.49 ± 0.4 g/L). Acetic acid, produced by the depolymerization orby the hemicelluloses decomposition into their monomeric sugars,increased always with the temperature and reaction time. Aceticacid concentration reached 1.18 ± 0.19 g/L and 1.79 ± 0.10 g/L forreaction conditions of 250 ◦C at 15 min and 300 ◦C at 45 min, respec-tively.
In smaller quantities ethanol, acrylic acid and xylitol were pro-duced owing to the catalytic hydrogenation of xylose. When thetemperature and reaction time increases, ethanol and acrylic acidcontent were increased, but at 300 ◦C and 45 min, the ethanoldegradation began. As results the ethanol maximum content of0.47 ± 0.02 g/L and a minimum content of 0.047 ± 0.004 g/L, wereachieved under 300 ◦C and 30 min; 250 ◦C and 15 min respectively.Acrylic acid highest concentration was achieved under 300 ◦Cand 45 min (0.32 ± 0.03 g/L), while the lowest concentration wasobtained at 250 ◦C and 15 min (0.066 ± 0.006 g/L). Finally, as finaldegradation by-product, xylitol also was produced, but when thetime and reaction temperature increase, the xylitol concentrationwas decreased. At 250 ◦C and 15 min the xylitol concentration wasthe highest (0.16 ±0.01 g/L), whereas at 300 ◦C and 15 min, xylitoldisappeared almost completely.
In Table 3 the yields of all final products are presented, respectto the total cellulose and hemicelluloses in the initial biomass. Thehighest and lowest yields of each by-product were highlighted.
Based on Table 3, high yields of lactic acid as the main com-ponent formed in all alkaline thermal treatments was observed.Considerable yields of final degradation products have beenobserved, especially acetic acid and formic acid. In general, atmore severe reaction conditions, higher by-products concentra-tions were produced due to the sugars degradation occurring attemperature above 200 ◦C.
Analyzing hemicellulosic sugars in resulted liquors after alka-line thermal treatments, mainly the presence of arabinose wasobserved. By contrast, glucose and xylose have been detected invery small amounts. This is due to the fact that glucose and xyloseare the main monomeric precursors of hemicellulosic degrada-tion products (lactic acid, formic acid. . .). The main hemicellulosicmonomers concentrations were presented in Fig. 4.
The hemicellulosic sugars tend to decrease their concentrationswith the increase of reaction temperature and time. It was observedthat arabinose takes longer to degrade than xylose and glucose.Unlike glucose and xylose, arabinose presents a considerable con-centration at 250 ◦C (0.21 ± 0.02 g/L of arabinose). The xylose andglucose monomers were detected in very small amounts and theirquantities also decrease when the reaction temperature and time
increase (from 0.006 g/L to 0.03 g/L of xylose and from 0.01 g/L to0.05 g/L of glucose). With these results, it can be conclude that thexylose and glucose are the main sugars which are degraded before,producing different final degradation products.
658 C. Sánchez et al. / Chemical Engineering Journal 181– 182 (2012) 655– 660
Fig. 3. Generated by-products concentrations.
Table 3Yields of produced organic acids from corn cobs alkali hydrothermal treatment using Ca(OH)2 0.7 M as catalyst.
Hydrothermal conditions Yields (%)
T (◦C) Time (min) Lactic acid Formic acid Acetic acid Acrylic acid Ethanol
250 15 28.51 ± 2.12 6.48 ± 0.54 7.14 ± 1.15 0.40 ± 0.03 0.29 ± 0.0230 30.81 ± 0.29 6.57 ± 0.64 7.48 ± 0.21 0.43 ± 0.02 0.33 ± 0.0645 31.43 ± 2.32 6.72 ± 0.71 7.26 ± 0.46 0.44 ± 0.05 0.36 ± 0.24
275 15 37.00 ± 0.31 6.40 ± 0.02 7.80 ± 0.22 0.61 ± 0.04 0.27 ± 0.0230 40.96 ± 0.14 7.30 ± 0.32 9.34 ± 0.42 0.75 ± 0.03 0.59 ± 0.1245 41.05 ± 0.48 6.73 ± 0.18 8.29 ± 0.30 0.73 ± 0.21 0.62 ± 0.19
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300 15 44.67 ± 1.23 7.56 ±30 44.76 ± 2.59 8.32 ±45 21.97 ± 0.72 2.97 ±
.4. Precipitated solid fraction characterization
.4.1. Fourier transform infrared spectroscopyThe FTIR absorption spectra for milled corn cobs and most
haracteristic dry wastes were represented in Fig. 5 between000 cm−1 and 600 cm−1. The differences between the wastes ofhe experiments in which highest and lowest concentration of lac-ic acid were obtained, were compared with the initial raw material
pectra.Fig. 6 shows an enlargement of part with lower wavenumbers.The three solid spectra present two characteristic bands at
300 cm−1 and 2915 cm−1 which correspond with the aromatic and
Fig. 4. Hemicellulosic sugars in alkali
10.34 ± 0.48 1.31 ± 0.36 1.04 ± 0.1510.76 ± 0.82 1.39 ± 0.41 2.87 ± 0.1110.85 ± 0.61 1.79 ± 0.18 2.19 ± 0.08
aliphatic stretching of the OH groups and with the C H stretchingvibration of CH2 and CH3 groups respectively [32].
In corn cobs spectrum a change can be observed at 1625 cm−1,which is referred to cellulose. The next absorption bands at 1600,1505 and 1425 cm−1 approximately are related to the aromaticskeletal vibrations of lignin. On the other hand, the absorptionsat 1425, 1319, 1248, and 1157 cm−1 are associated with hemi-celluloses. At 1035 cm−1 appears the aromatic C H deformation
as a complex vibration associated with the C O, C C stretch-ing vibration and the glycosidic linkage (C O C) contributions inhemicelluloses indicates a typical absorbance for xylans in the iso-lated hemicelluloses. The band recorded at 897 cm−1 is referred tone thermal hydrolysate liquors.
C. Sánchez et al. / Chemical Engineering
4000 300 0 200 0 100 04000 300 0 200 0 100 0
814
1505
3642
897
1030
1248
13191425
1600
1625
3300
Tran
smita
nce(
%)
Wavenumber (cm-1)
2915
Corn cob s
250 ºC 1 5 min
300 ºC 30 min
Fig. 5. IR spectra of initial raw material and wastes obtained at 300 ◦C 30 min andat 250 ◦C 15 min.
2000 100 0
711
871
1430
11571248
897
1035
1425
1505
1319
1600
Tran
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nce(
%)
Wavenumber (cm-1)
Corn cobs
250 ºC 15 min
300 ºC 30 min
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ig. 6. IR spectra of initial raw material and wastes obtained at 300 ◦C 30 min andt 250 ◦C 15 min with enlarged scale.
he domain of �-(1 → 4)-glycosidic bonds between the xylose unitsn the hemicelluloses [33,34].
The solid wastes spectra gives a signal at 3642 cm−1 that isharacteristic of the O H stretching vibration in Ca(OH)2 used asatalyst. Finally, the spectra changes between 1430 cm−1, as wells absorptions at 871 and 711 cm−1 are referred to CaCO3 rapidormation due to the presence of catalyst [35].
After an exhaustive analysis of Fig. 6, it can be said that corn cobsresented all the main compounds of lignocellulosic materials (cel-
ulose, lignin and hemicelluloses). However, the obtained wastes at00 ◦C 30 min and 250 ◦C 15 min are mainly composed by the useda(OH)2 as catalyst, formed CaCO3, and some hemicelluloses thatere present in the solid fraction. The most of the biomass com-ounds have been converted into other compounds due to the higheaction temperature.
. Conclusions
The experimental results showed that Ca(OH)2 was the best cat-lyst among the screened once to convert corn cobs in lactic acidsing a hydrothermal reaction under the studied conditions.
At 300 ◦C 15 min and 300 ◦C 30 min using Ca(OH)2 0.7 M as
atalyst, acceptable results were obtained (a yield of 44.67 ± 1.23nd 44.76 ± 2.59% of lactic acid respectively), but a considerableoncentration of final degradation products were also obtained.better yields of lactic acid could be obtained by using other
[
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Journal 181– 182 (2012) 655– 660 659
catalyst concentrations under others reaction conditions, but surelymore final degradation products could be produced. The studiedexperimental conditions have generated a high production of lac-tic acid. Comparing our results with other similar works, the yieldof lactic acid has been improved and the reaction time is reducedconsiderably compared to the production of lactic acid by enzymesor microorganisms.
Acknowledgements
Authors would like to thank the Department of Education, Uni-versities and Research, and the Department of Agriculture, Fishingand Food of the Basque Government (pre-doctoral scholarship pro-grams of young researchers training) for supporting financially thisresearch work.
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