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Enhancement of Lignin Production from Olive Tree Pruning Integrated in a Green Biorefinery Ana Toledano, Luis Serrano,* and Jalel Labidi Chemical Engineering Department, University of the Basque Country, Plaza Europa, 1, 20018, Donostia-San Sebastian, Spain ABSTRACT: The green biorefinery concept relies on economically feasible processing to achieve a complete utilization of most lignocellulosic biomass components using green technologies. Among the main component of lignocellulosic biomass, lignin is one of the most interesting components since its aromatic nature makes lignin unique. The study presented here was focused on the enhancement of the organosolv lignin produced from olive wood from the point of view of not only operational conditions but also taking into account the purity and contamination of the obtained lignin. The results showed that the amount of lignin recovered in the liquor was strongly influenced by the process temperature. The ethanol concentration presented the opposite behavior with lignin, reaching high yields at low concentrations. Then, the obtained lignins under optimized conditions were analyzed in order to verify their purity. Almost all the obtained lignins presented hemicellulose contamination but there were differences between the polymerization degree of hemicelluloses and the abundance of hemicelluloses.
1. INTRODUCTION Renewable plant based carbon sources are promising raw materials for energy, chemicals, and fuel production and may help to overcome the problem of decreasing fossil fuel reserves. Among lignocellulosic biomass, agricultural residues constitute an interesting feedstock for biorefineries due to their abundance, low cost, and worldwide availability. Olive tree cultivation is spread mainly through the Mediterranean countries, Spain being the main producer in the world. Olive tree pruning is a periodical culture operation by means of which less productive branches are cut off and trees are rejuvenated. This action generates an annual volume of lignocellulosic residues estimated at 3000 kg/ha,1 thus constituting a widely available and renewable resource. Just in Spain, the main olive oil producer in the world, some 7 106 t/y of olive tree pruning biomass are available. A typical pruning lot includes leaves, thin branches, and wood. To prevent propagation of vegetal diseases, these residues are commonly ground and abandoned in the fields or burnt with associated costs, environmental concerns, and, until date, no economical alternatives.2 An efficient fractionation performance requires selectivity in the separation of constitutive components, accessibility to each of them after fractionation, high yields, good quality of obtained products, and process economic viability.3 Lignocellulosic biomass fractionation can be achieved using traditional process such as the soda, Kraft, and sulphite processes. There are different viable alternatives to the established sulfate, soda, and sulphite technologies. Among these, organosolv pulping has attracted much interest in the last decades. Organosolv treatment also seems to be a viable future pulping alternative because of the relatively low capital investment required for a new mill, the absence of pollution problems,4 and the advantage of obtaining polyoses and lignin easily and largely unchanged for further highvalue utilization.5 However, in the 1990s the organosolv pulping mill experience failed because of capital/operational costs. The complete utilization of the major components of the lignocellulosic r 2011 American Chemical Society
biomass, i.e. lignin valorization, will help to overcome the economy of the organosolv pulping mill, turning the whole process into a biorefinery. In addition, as mentioned before, organosolv technology enabled the fractionation of the raw material into different products (cellulose, hemicellulose-derived sugars, and lignin) allowing the subsequent recovery of the solvents by distillation with high yields and low energy consumption3 converting organosolv treatment into a green technology process. Much research has been published about the utilization of the olive tree pruning residues by applying different treatments such as hydrothermal conditions or ethanol pulping in order to make profit from the holocellulosic fraction to produce paper or bioethanol,68 and steam explosion has been used in order to assess the antioxidant capacity of the extractives of the olive tree pruning.9 However, none of these studies have been focused on the lignin fraction and its potential use even though dissolving as much lignin as possible in the liquid fraction favors the two main streams, the solid and the liquid, to be used in their most efficient way. Lignin is a phenolic polymer built up by oxidative coupling of three major C6C3 (phenylpropanoid) units, namely, syringyl alcohol (S), guaiacyl alcohol (G), and p-coumaryl alcohol (H), which form a randomized structure in a three-dimensional network inside the cell wall.10 However, the percentage or the presence/absence of these substructures varies depending on the orignin of the lignocellulosic biomass. This special structure makes lignin a unique compound in nature. However, it has traditionally been considered in the pulp and paper industries as a stream useful only for energy production missing their potential uses as high value added products such as a dispersant in cement gypsum blends,11,12 emulsifiers,13 and carbon fibers.14 Received: October 22, 2010 Accepted: April 13, 2011 Revised: April 12, 2011 Published: April 27, 2011 6573
dx.doi.org/10.1021/ie102142f | Ind. Eng. Chem. Res. 2011, 50, 6573–6579
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Table 1. Experimental Design for the Enhancement of Lignin Production From Olive Tree Pruning by Organosolv Treatmenta experiment
XT
1
0
2 3
Xt
XC
LP (%)
Table 2. Snedecor’s F Values for the Terms in the Obtained Equations 3 and 4 for the Lignin in Liquor (LL) and Lignin Content in Pulp (LP) terms with the
LL (g/L)
0
0
18.05
1
0
0
27.16
7.86
0
1
0
18.78
15.75
4
0
0
1
21.33
25.23
5 6
1 1
1 1
1 1
25.50 11.50
10.08 36.55 12.53
equation
18.62
7
1
1
1
24.90
8
1
1
1
29.28
0.08
9
0
1
1
19.62
35.77
10
1
0
1
28.99
9.01
11
1
1
0
27.82
6.02
12
1
1
1
14.75
25.66
13 14
1 1
1 1
1 1
27.78 18.97
5.92 20.76
15
1
1
1
11.39
35.09
16
0
1
1
25.78
14.53
17
1
0
1
18.48
23.69
18
1
1
0
23.45
32.35
19
1
0
1
11.17
33.69
20
0
1
1
24.93
8.65
21 22
1 0
1 0
0 1
26.63 24.43
7.69 10.67
23
0
1
0
20.82
21.73
24
1
0
0
10.01
29.33
25
1
0
1
29.00
4.21
26
0
1
1
14.88
28.68
27
1
1
0
14.30
28.46
a
XT temperature normalized; Xt time normalized; XC ethanol concentration normalized.
The main objective of this work was to revalorize lignin. With this purpose, the aim was to establish the best conditions for organosolv delignification of the olive tree pruning (only branches and wood) in particular the maximization of the lignin concentration in the liquid fraction. Mathematical equations were employed to establish the relationships between the studied factors. High lignin concentrated liquors were treated to precipitate the lignin in order to study the purity of each obtained batch and to achieve high lignin concentration liquors with good quality lignin.
2. MATERIALS AND METHODS 2.1. Olive Tree Pruning. The raw material was kindly supply by the independent producer Luis Angel Toledano and originated from an olive tree (Olea europea) cultivated in Navarra belonging to the variety called Arroniz. It was locally collected and then dried at room temperature. The olive tree prunings (branches and wood) were milled in a Retsch 2000 hammer mill to produce 46 cm chips free of small stones, dust, and soil. They were then characterized following the TAPPI standards: ash content 3.1% ( 0.4 (TAPPI T211 om-93), solubility in hot water 23.3% ( 1.3 (TAPPI 207 om-93), solubility in 1% NaOH 34.4% ( 1.9 (TAPPI T212 om-98), ethanolbenzene extractives 9.5% ( 0.4 (TAPPI T204 cm-97), acid-insoluble lignin 23.2% ( 0.7
Snedecor’s F
variable XT
LP LL
75.01
XC
11.43
XT
245.10
XC XtXC
75.83 4.64
Table 3. Snedecor’s F, R2, and R2-Adjusted Values Obtained from the Adjusted Model Equations 3 and 4 equation
Snedecor’s F
R2
R2-Adjusted
LP LL
54.75 101.79
0.82 0.93
0.80 0.92
(TAPPI T222 om-98), and holocellulose15 66.8% ( 1.2, R-cellulose16 58.4% ( 0.5, and hemicelluloses 8.4% ( 0.3. 2.2. Organosolv Treatment. The raw material was treated with an ethanolwater mixture using different ethanol concentrations, reaction times, and temperature. The solid:liquid ratio used was 1:6 and held constant for all experiments. The reactions were carried out in a 1500 mL pressure stainless steel reactor (Parr 4836) equipped with a heating mantle, mechanical stirrer, and manometer. Temperature was controlled with the reactor controller and time with a chronometer. The time needed to reach the desired temperature was around 15 min for all experiments. When the reaction was stopped, the reactor was cooled to room temperature as soon as possible. The solid fraction was separated by gravity filtration from the liquid fraction. The solid fraction was dried and then analyzed in order to determine the lignin content (Klason lignin) following the TAPPI standard T-222. The liquid fraction was treated to precipitate the lignin present, and its concentration was determined; two volumes of acidified water (pH around 2) were added to a known volume of liquid fraction in order to precipitate the lignin. The precipitated lignin was then separated by centrifugation (4000 rpm, 20 min). The lignin concentration was determined gravimetrically. 2.3. Experimental Design. The applied model uses a series of points (experiments) around a central one (central experiment) and several additional points (additional experiments) to estimate the first- and second-order interaction terms of a polynomial. This design meets the general requirement that every parameter in the mathematical model can be estimated from a fairly small number of experiments.17 Experimental data were fitted to the following second-order polynomial: Y ¼ a0 þ
n X i¼1
bi xni þ
n X i¼1
ci xni 2 þ
n X
dij xni xnj
ði < jÞ
i ¼ 1;j ¼ 1
ð1Þ where _ XX xn ¼ 2 X max X min 6574
ð2Þ
dx.doi.org/10.1021/ie102142f |Ind. Eng. Chem. Res. 2011, 50, 6573–6579
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Figure 1. Variation of lignin concentration (g/L) in the liquid fraction (LL) with the temperature (normalized) and the ethanol concentration (normalized) at the central time point.
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Figure 3. Variation of lignin concentration (g/L) in the liquid fraction (LL) with the reaction time (normalized) and the ethanol concentration (normalized) at the central temperature point.
Figure 2. Variation of lignin concentration (g/L) in the liquid fraction (LL) with the temperature (normalized) and the reaction time (normalized) at the central ethanol concentration point.
Y denotes dependent variables (lignin content in solid fraction and lignin concentration in liquid fraction), n denotes the number of independent variables, Xn, independent variables, and a0, bi, ci, and dij are constants. The Xn were normalized from 1 to þ1 using eq 2 in order to facilitate direct comparison of the coefficients and visualization of the effects of the individual independent variables on the response variable. This also results in more accurate estimates of the regression coefficients as it reduces inter-relationships between linear and quadratic terms.16 In eq 2, Xn is the normalized value of temperature (XT), time (Xt), and ethanol concentration (XC); _ X is the experimental value of the variable concerned; X is the middle point of the variation range value for the variable in question; and Xmax and Xmin are the maximum and minimum values of such a variable. Experimental results were subjected to regression analysis using the STATGRAPHIC software.17 Nonsignificant variables were eliminated one at a time using the stepwise method to discard those terms with a Snedecor’s F-value smaller than four.19 The normalized values of independent variables, for the 27 experiments of the experimental design, are shown in Table 1. 2.4. Lignin Characterization. Obtained lignins were characterized by attenuated total reflection infrared spectroscopy (ATR-IR) and thermogravimetric analysis (TGA).
Figure 4. Variation of lignin content (%) in pulp (LP) with the temperature (normalized) and the reaction time (normalized) at the central ethanol concentration point.
Attenuated total reflection infrared (ATR-IR) spectroscopy was by direct transmittance in a single-reflection ATR System (ATR top plate fixed to an optical beam condensing unit with ZnSe lens) with an MKII Golden Gate SPECAC instrument. The region between 4000 and 600 cm1 with a resolution of 4 cm1 and 20 scans was recorded. Lignin samples were dried before the analysis. TGA was carried out using a Mettler Toledo TGA/SDTA RSI analyzer. Samples of ∼5 mg were heated from 25 °C up to 800 °C at a rate of 10 °C/min. A constant nitrogen flow was used, and an inert atmosphere during the pyrolysis allowed the extraction of the gaseous and condensable products that could cause secondary interactions.
3. RESULTS AND DISCUSSION 3.1. Organosolv Experimental Design. Two dependent variables were considered to follow the delignification process, lignin content (%) in pulp (LP) and lignin concentration (g/L) in the liquid fraction (LL). As mentioned above, three independent variables were varied during the delignification process: time (t) 6575
dx.doi.org/10.1021/ie102142f |Ind. Eng. Chem. Res. 2011, 50, 6573–6579
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studied by Luis Jimenez et al.7 The results of the 27 experiments are shown in Table 1. Applying the STATGRAPHICS Centurion software to the data in Table 1, the following equations that predict the behavior of the dependent variables studied for the organosolv delignification process of olive tree pruning were obtained: LP ¼ 21:1 6:28X T þ 2:45111X C
ð3Þ
LL ¼ 18:8374 þ 11:2322X T 6:24778X C þ 1:89333X t X C ð4Þ
Figure 5. Variation of lignin content (%) in pulp (LP) with the temperature (normalized) and the ethanol concentration (normalized) at the central reaction time point.
Figure 6. Variation of lignin content (%) in pulp (LP) with the reaction time (normalized) and the ethanol concentration (normalized) at the central temperature point.
Tables 2 and 3 show the statistical values (Snedecor’s F, R2, and R2-adjusted) for the different terms in the eqs 3 and 4. From the statistical values obtained for both eqs 3 and 4, it is found that the proposed model fitted the experimental values of lignin concentration in liquid fraction (LL) well since the calculated values for the dependent varaiable (LL) reproduces the experimental results with errors of 7%. However, for the lignin content in pulp (LP), the model did not fit as wells as for LL. In this case, the calculated values for LP obtained from eq 3 reproduced the experiment results with errors higher than 15%. This is in accordance with other author’s works8,20 where the error in lignin content measured as Klason lignin was around 15% and always higher than other parameters that can be measured in the solid fraction such as holocellulose and Rcellulose. Experimentally, the highest lignin concentration in the liquid fraction (LL), 36.55 g/L, was obtained at the highest temperature (normalized value 1), the shortest reaction time (normalized value 1), and lowest ethanol concentration (normalized value 1). From Figures 13, eq 4, and data in Table 1, it can be observed that the lignin concentration in the liquid fraction is highly influenced by the conditions used in the delignification process. The experimental values for LL varied from nearly 0 to 36.55 g/L with the temperature being the most important factor that determined the yield of dissolved lignin in the liquid fraction. As the temperature used in the organosolv treatment was raised, the lignin concentration in the liquid fraction increased while increasing the ethanol concentration resulted in the opposite behavior. The presence of water in the reaction mixture seemed to be important because the lower the concentration of ethanol, the higher the lignin concentration in the liquid fraction. The reaction time was not a variable that affected the LL much;
Figure 7. ATR-IR spectra of the analyzed lignins. 6576
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Table 4. Thermogravimetric Analyses Data of the Organosolv Obtained Lignins experiment 4
6
9
12
14
15
17
18
19
23
24
temperature (°C) 75.1
6.0
139.4 192.7
4.2 4.86
359.1
45.34
70.4
4.4
134.4
2.8
196.6
5.3
361.3
46.7
61.8
1.9
204.1 353.1
7.7 50.0
73.8
3.2
133.2
0.6
363.0
57.1
70.0
3.8
240.6
7.7
362.5
48.5
73.9 116.9
3.3 0.5
136.4
0.3
236.4
5.7
363.0
49.3
72.2
4.5
259.0
10.6
362.5
47.4
75.5 150.22
4.1 0.8
244.9
6.7
364.1
48.1
75.5
4.1
146.2
1.1
231.4
8.3
314.3
9.4
364.1 71.7
36.6 5.3
188.2
7.8
363.0
49.2
77.2
4.7
210.6
5.5
364.6 26
27
weight loss (%)
residue (%) 39.6
40.8
40.4
39.1
40.0
40.9
37.5
40.3
40.5
37.7
39.8
50
68.8
5.4
222.6 359.1
11.8 45.7
72.4
4.8
225.3
7.7
364.1
48.6
37.1
38.9
however, it had a synergetic effect with the ethanol concentration. To maximize the lignin concentration dissolved in the liquid fraction, the organosolv conditions to be used are high temperature and low ethanol concentration. For the present study, it was desirable to minimize the lignin content in pulp (LP) meaning that the lignin present in the raw
material has been dissolved in the liquid fraction and, so, can more easily be isolated and utilized for further applications. The statistical program STATGRAPHICS Centurion gave the following optimized organosolv conditions to minimize the lignin content in pulp (LP): highest temperature (normalized value 1), lowest reaction time (normalized value 1), and lowest ethanol concentration (normalized value 1), i.e., these conditions would be the same as with the other objective of this study: enhancement of lignin concentration in the liquid fraction. Nevertheless, the experimental data did not agree with the aforementioned conditions. The lowest LP data was 10.01% obtained at high temperature (normalized value 1), medium reaction time (normalized value 0), and medium ethanol concentration (normalized value 0). Such a variation could be explained by the error associated to the LP eq 3. Others authors studies presented similar error in their equations to predict the lignin content in pulp behavior.8,20,21 In our opinion, this could be justified due to the complex structure of the plant cell wall and its heterogeneity. These two factors affect the way the reaction mixture acts and make this variable (lignin content in pulp) difficult to predict with an error less than 10%. Also, repolymerization and lignin redeposition on the fiber surface22 are other phenomena that could explain the obtained results. From eq 1 and Figures 46, it is evident that the temperature used to extract lignin is the most important factor. The higher the temperature, the less lignin content in pulp. As in the case of LL, employing a low ethanol concentration led to low lignin content in pulp. The reaction time, as it can be noted in Figure 4 and 6, did not influence the percentage of lignin content in pulp since the surface of the graphics plot remains constant in the time axis. 3.2. Lignin Characterization. The enhancement of the lignin production also involves production of good quality lignin. From the 27 experiments performed those which yielded liquors with a high lignin concentration were treated to precipitate the lignin in order to establish its physicochemical properties. With this purpose, the lignin concentration average of the 27 experiments was calculated, and those experiments (experiment numbers: 4, 6, 9, 12, 14, 15, 17, 18, 19, 23, 24, 26 and 27) that produced a liquid fraction with a lignin concentration equal or higher than the average were considered for further characterization. 3.2.1. Attenuated Total Reflection Infrared (ATR-IR) Spectroscopy. The ATR-IR spectra of the obtained lignin from the optimized experimental conditions are shown in Figure 7. Around 3345 cm1, a vibration can be observed caused by the stretching of the O—H group. All spectra present bands between 2950 and 2830 cm1 that correspond to the vibration of C—H bond in methyl and methylene groups. The asymmetric deformation of this bond also produces a band at around 1460 cm1. The vibration at around 1710 cm1 is associated to the CdO bond stretching in unconjugated ketones, carbonyl, and ester groups while the vibration at around 1660 cm1 is related to the CdO bond stretching in conjugated p-substituted aryl ketones. Three typical vibrations are present in aromatic compound such as lignin; these bands appear around 1595, 1515, and 1425 cm1. The bands most significant in the lignin spectra are those that correspond to its main substructures—guaiacylpropane (G), syringylpropane (S), and 4-hydroxyphenylpropane (H)—such as that at around 1330 cm1 related to the breathing of the syringyl ring with CO stretching and at around 1270 cm1 (shoulder) and 1220 cm1 associated to the breathing of the guaiacyl ring with CO stretching. Around 1150 cm1, it appears a vibration that is caused by the deformation of the 6577
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Figure 8. Thermogravimetric analysis of two lignin samples with different degradation profile: (a) experiment 19, (b) experiment 24.
bond CH in guaiacyl substructures while the same linkage, but in syringyl substructures appears at around 1120 cm1. The vibration at around 1030 cm1 is due to the deformation or the aromatic CH linkages in guaiacyl substructures and can also be related to the deformation of the bond CO in primary alcohols. Finally, at around 835 cm1, there is a vibration caused by the CH linkage in positions 2 and 6 of the aromatic ring in syringyl substructures and in all positions of the aromatic ring in H substructures. This is in good agreement with previous studies23,24 where different types of lignins were studied. On the basis of the ATRIR spectra, lignin is the major component of the precipitated obtained from the liquid fraction. 3.2.2. Thermogravimetric Analysis. Selected samples of lignin were also subjected to thermogravimetric analysis in order to study their thermal behavior. The obtained results are presented in Table 4. Figure 8 shows the thermogravimetric analysis of two lignin samples with different degradation profile. All samples showed a weight loss around 70 °C that was associated with the moisture present in the lignin samples. Significant differences between lignins obtained from different experiments were observed in regards to their degradation profile. Samples 4, 6, 12, 15, 18, and 19 presented weight losses below 185 °C that can be attributed to hemicellulose degradation products. The aggressive conditions used for the lignin extraction may affect the hemicellulose structure causing degradation into small components such as acetic acid, furfural, hydroxymethylfurfural, and ferulic acid. These components could be dissolved in the liquid fraction during the delignification process, and in the lignin isolation stage, they may be coprecipitated with the lignin. Between 185 and 260 °C, another weight loss was observed that can be related to the presence of hemicelluloses25,26 signifying the contamination of lignin samples. The percentage of hemicellulose contamination varies between the experiments because of the different conditions employed to extract the lignin from the raw material. Lignin degradation happened slowly in a wide range of temperatures with a maximal mass loss rate between 300 and 400 °C, this fact being associated with the complex structure of lignin with phenolic hydroxyl, carbonyl groups, and benzylic hydroxyl, which are connected by straight links.27 All lignin samples presented a high percentage of final residue due to lignin aromatic polycondesations.
4. CONCLUSIONS A process was developed for the optimization of lignin production from olive tree pruning by organosolv treatment.
Optimization of lignin extraction from the raw material also ensures an almost lignin-free solid fraction rich in cellulose. The maximization of the lignin concentration in the liquid fraction entailed using high temperatures and low ethanol concentration. These conditions also guarantee minimization of lignin content in the remaining pulp (solid fraction). Reaction time was of less importance for the delignification process in the studied cases. The precipitated lignin from the highest concentrated liquid fraction was characterized in order to select a procedure to produce lignin in a large amount and good quality. Fourier transform infrared (FTIR) results revealed that the precipitate was mainly lignin and that the lignin obtained from olive tree pruning is H:G:S type. From the thermogravimetric analysis data the experimental conditions employed to extract lignin also extracted other components from the raw material such as hemicelluloses or other degradation products in different proportion depending on the severity of the treatment. A compromise between the yield of lignin concentration in the liquid fraction and the composition of the lignin has to be achieved.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: þ34-943017178. Fax: þ34943017125.
’ ACKNOWLEDGMENT The authors would like to thank the Spanish Ministry of Science and Innovation (CTQ2010-19844-C02-02) and the University of the Basque Country (EHU 09/01). ’ REFERENCES (1) Sanchez, S.; Moya, A. J.; Moya, M.; Romero, I.; Torrero, R.; Bravo, V.; San Miguel, M. P. Aprovechamiento del Residuo de Poda del Olivar. Ing. Quim. 2002, 34, 194–202. (2) Romero, I.; Ruiz, E.; Castro, E.; Moya, M. Acid Hydrolysis of Olive Tree Biomass. Chem. Eng. Res. Des. 2010, 88, 633–64. (3) Gonzalez Alriols, M.; García, A.; Llano-ponte, R.; Labidi, J. Combined Organosolv and Ultrafiltration Lignocellulosic Biorefinery Process. Chem. Eng. J. 2010, 157, 113–120. (4) Sanchez, R.; Rodríguez, A.; García, J. C.; Rosal, A.; Jimenez, L. Exploitation of Hemicellulose, Cellulose and Lignin from Hesperaloe Funifera. Bioresour. Technol. 2011, 102, 1308–1315. (5) Fengel, D., Wegener, G., Eds. Wood: Chemistry, Ultrastructure, Reactions; Walter de Gruyter: Berlin and New York, 1989. 6578
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Industrial & Engineering Chemistry Research
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dx.doi.org/10.1021/ie102142f |Ind. Eng. Chem. Res. 2011, 50, 6573–6579
