Letter pubs.acs.org/journal/ascecg
Lignin as Renewable and Superior Asphalt Binder Modifier Shangxian Xie,†,‡,§,∇ Qiang Li,†,‡,§,∇ Pravat Karki,∥ Fujie Zhou,*,⊥ and Joshua S. Yuan*,†,‡,§ †
Synthetic and Systems Biology Innovation Hub, Texas A&M University, College Station, Texas 77843, United States Department of Plant Pathology and Microbiology, Texas A&M University, College Station, Texas 77843, United States § Institute for Plant Genomics and Biotechnology, Texas A&M University, College Station, Texas 77843, United States ∥ Texas A&M Transportation Institute, Texas A&M University, College Station, Texas 77843, United States ⊥ College of Transportation Engineering, Tongji University, Shanghai 201804, China Downloaded via UNIV OF THE SUNSHINE COAST on June 25, 2018 at 19:22:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: The utilization of lignin for fungible products remains a major challenge for biofuel, pulp and paper industries. We hereby demonstrated the potential of lignin to be used as the asphalt binder modifier, and addressed the challenges in producing high-performance asphalt binder modifiers from lignin. We first demonstrated that Kraft lignin could improve the high temperature performance of asphalt binder, yet compromise the low temperature performance. To address the challenge, we developed both enzyme-mediator-based biological processing and formic acid-based chemical processing to derive lignin fractions to improve the high temperature performance of asphalt binder without compromising its low temperature performance. Moreover, the soluble fraction of biologically processed lignin could improve both high temperature and low temperature performance of asphalt binder, which enabled lignin to serve as a modifier with unique features. We also carried out a thorough characterization of different lignin fractions, and revealed the potential mechanisms for lignin to improve the asphalt binder performance. Overall, the study opened the new avenues for lignin to serve as an exceptional modifier and renewable substitute to improve both high and low temperature performance of asphalt binder. The novel application also transformed lignin waste into a valuable byproduct with market size compatible to biorefinery, pulp and paper industries. KEYWORDS: Asphalt binder modifier, Lignin, Temperature performance
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INTRODUCTION
biorefinery, it needs to be compatible with the scale of the industries like biorefinery, pulp and paper industry. We hereby demonstrated that lignin can be fractionated and modified to enable the utilization as effective modifiers to improve asphalt binder performance. More than 90% of US roads are paved with asphalt mixes. The annual production of asphalt mixes for highway pavement in United States is 360 million tons. Eighteen million tons of asphalt binders with good performance at both high and low temperatures are needed annually for maintaining a safe and smooth highway system.5 Asphalt binder is a mixed petroleum-derived material composed of asphaltenes, resins, saturates and aromatics, where the more polar components including asphaltenes and resins render the asphalt binder modulus and high temperature properties, and the less polar components including saturates and aromatics promote asphalt flexibility and low temperature properties.6−9 A superior binder should have a higher performance grade (the higher, the better) to prevent melting and associated pavement distress (such as rutting) at high temperature. Meanwhile, it should also have a lower perform-
Lignin utilization for fungible products remains a major challenge for lignocellulosic biorefinery, pulp and paper industries.1−3 Even though lignin is the second most abundant biopolymer on earth, very little of it has been transformed into value-added bioproducts.2 More than 50 million tons of lignin are generated from pulp and paper industry annually, whereas only 2% of this waste lignin has been utilized for bioproducts.4 Likewise, essentially all biomass conversion platforms result in the formation of a major lignin-containing waste stream, which needs to be upgraded into fungible products.2 The utilization of this excess lignin as feedstock for renewable products offers a significant opportunity to enhance the operational efficiency, reduce the cost, minimize carbon emissions and maximize sustainability of lignocellulosic biorefinery. Despite the imminent needs, bioproduct development from lignin is highly challenging due to its recalcitrance nature.1−3 The technologies pursued by the industry included bioconversion, thermoconversion, specialty chemicals and materials from lignin, whereas each of the technologies has its limitations. These limitations include the low titer for bioconversion, corrosive products from thermoconversion, small market size and low cost-effectiveness for specialty chemicals. In particular, for bioproduct to enable © 2017 American Chemical Society
Received: December 15, 2016 Revised: February 23, 2017 Published: March 20, 2017 2817
DOI: 10.1021/acssuschemeng.6b03064 ACS Sustainable Chem. Eng. 2017, 5, 2817−2823
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Figure 1. Limitation of raw Kraft lignin as asphalt binder modifier, and the strategies to process lignin toward effective asphalt binder modifier.
ance grade (the lower, the better) to reduce cracking potential at low temperature. As a renewable aromatic polymer, lignin consists of monoligonol precursors of ρ-hydroxycinnamyl alcohols including ρ-coummaryl alcohol, coniferyl alcohol and sinapyl alcohol, which are further connected with different types of interunitery covalent linkages.10,11 Considering the structural similarity to the fossil fuel-based asphalt binder, lignin could serve as a renewable substitute and potentially modifier for asphalt binder. Even though previous studies have explored the possible antioxidant activity of lignin,12 very limited amount of lignin is needed for the antioxidant function and the application does not create a market size-compatible utilization of lignin. Furthermore, it is not clear how adding lignin into asphalt binders will impact the most important characteristics of asphalt binder: permanent deformation (or rutting) resistance at high temperature and cracking resistance at low temperature.12,13 The impact on the high and low temperature performance will determine the capacity of lignin to serve as modifier of asphalt binder. Asphalt binder modifier could enhance the properties of asphalt binder to confer better high or low temperature performances of millions of miles of pavements. Few studies have explored how lignin can serve as asphalt binder modifier.
The high heterogeneity and complexity of lignin make it difficult to predict how the lignin addition will change the high and low temperature performance of asphalt binder. Neither do we understand how to develop lignin products to enhance high temperature performance without compromising the low temperature performance of asphalt binder, and vice versa. In this study, we have first established that lignin can modify asphalt binder performance and has the potential to serve as asphalt binder modifier. More importantly, we have developed various biological and chemical processes to fractionate lignin into fractions with various molecular weights and chemical properties, which can in turn serve as elite modifiers to significantly improve both high temperature and low temperature performance. The study further reveals the potential mechanisms for processed lignin to serve as effective asphalt binder modifier, and opens new avenues for lignin-based valueadded products and renewable substitute for road materials.
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RESULTS AND DISCUSSION
Lignin Can Change the Performance of Asphalt Binder. We first evaluated how raw Kraft lignin could impact asphalt binder performance. The study suggested that the addition of raw Kraft lignin at different concentrations (5− 2818
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Figure 2. Performance of laccase-HBT processed lignin fractions as asphalt binder modifiers. (A) The high temperature performance grade of asphalt binder with different percentage of various lignin fractions. (B) The low temperature performance grade of asphalt binder with different percentage of various lignin fractions. (C) The GPC analysis of the different processed lignin fractions. (D) The 13C NMR analysis of the different processed lignin fractions. KL, Kraft lignin without processing; KL-L/H-Insol, the insoluble fraction of the Kraft lignin after laccase-HBT processing; KL-L/H-Sol, the soluble fraction of the Kraft lignin after laccase-HBT processing.
The laccase-mediator system has not been used to fractionate commercial lignin toward different applications. However, from a fundamental scientific perspective, the laccase-mediator system has potential to enhance the electron transfer in redox reactions for better linkage cleavage during depolymerization processes, and thus has potential to be used for efficient fractionation of lignin toward broad usage. In particular, electron mediator could both facilitate more rapid electron transfer and allow better penetration of redox reactions into lignin molecular structure, both of which will result in better linkage cleavage, broader modification of functional group, and fractionation of lignin into portions with altered molecular weights. We have established an efficient laccase-mediator system using 1-hydroxybenzotriazole (HBT) as mediator to fractionate lignin for asphalt binder modifier usage. The laccase-HBT system could fragmentize and solubilize over 35% of Kraft lignin as a water-soluble fraction. Both the water-soluble and insoluble lignin fractions were investigated for their performance as asphalt binder modifiers. The asphalt binder blended with different lignin fractions and concentrations were characterized for both high and low temperature performance. The analysis showed that the addition of both soluble and insoluble lignin fractions could significantly improve the high temperature performance of asphalt binder, in a way similar to the raw Kraft lignin (Figure 2A). However, unlike the raw Kraft lignin, the addition of water-soluble lignin fraction could slight improve the low temperature performance of the asphalt binder when added at 10 to 20% (Figure 2B). The water-insoluble lignin fraction also had little effects on the low temperature performance when added 5%−10% as modifier, whereas it reduced the low temperature performance of asphalt binder
20%) could improve the high temperature performance (rutting resistance) of the asphalt binder, indicating that the lignin modified binder can stand hotter summer temperatures without rutting problem. The result opens the opportunity for lignin to serve as renewable asphalt binder modifiers (Figure 1). Despite the potential, the addition of raw Kraft lignin led to significantly compromised low temperature cracking property of the asphalt binder, when more than 5% of the lignin was added into the asphalt binder. Specifically, the low temperature performance was increased by 7 °C when 20% Kraft lignin was added into asphalt binder. In other words, the asphalt binder with raw Kraft lignin might crack at −17 °C vs at −24 °C without lignin. The results indicated the improvement of high temperature performance came with a cost of compromised low temperature performance of asphalt binder. Therefore, raw Kraft lignin would not be a suitable modifier for asphalt binder. Even though the direct use of raw Kraft lignin as asphalt binder modifier is challenging, it is possible to process lignin to tailor the molecular weight and functional groups through different biological and chemical fractionation. 14,15 We hypothesized that different lignin functional groups and molecular weight could impact the high and low temperature performance of asphalt binders. On the basis of this hypothesis, we developed two different types of lignin processing and investigated their effects on the performance of asphalt binder as shown in Figure 1. Development of Biological Process To Improve Lignin Property as Asphalt Binder Modifier. We first developed a new biological process to fractionate lignin into portions with different molecular weights and functional groups. In particular, the processing exploited the laccase-mediator system that was previously used in delignification in pulp and paper industries. 2819
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Figure 3. Performance of formic acid/Fenton-processed lignin. (A) The high temperature performance grade of asphalt binder with the addition of different percentage of various lignin fractions. (B) The low temperature performance grade of asphalt binder when adding various lignin fractions at different concentrations. (C) The GPC analysis of the different lignin fractions after chemical processing. (D) The 13C analysis of the different lignin fractions after chemical processing. KL, Kraft lignin without processing; KL-FA-Insol, the insoluble fraction of the Kraft lignin after formic acid/ Fenton processing; KL-FA-Sol, the soluble fraction of the Kraft lignin after formic acid/Fenton processing.
opened reaction. The potential ring-opening reactions could also be supported by the reduction in aromatic region at 160− 102 ppm in Figure S1. The results thus indicated that the soluble fraction of laccase-HBT processed lignin was a mixture of lignin-derived small molecule aromatic compounds, and nonaromatic compounds derived from lignin benzene ring opening reaction. The structural features well correlated with the performance of lignin as asphalt binder. Asphaltenes are generally considered as a highly polar aromatic material with the highest molecular weight in asphalt,8,16 and lignin is polyphenylpropanoid macromolecule with aromatic monomers.2,14 The similarity in molecular structures makes it possible for lignin to interact with asphaltene in asphalt binder. The results suggested that lignin could act as a cross-linker to modify asphalt binder performances. First, lignin as a branched aromatic macromolecule could cross-link with polar asphaltenes through dipolar−dipolar intermolecular forces to create a closely interacted macromolecular structure. Such cross-linking could generate macromolecular structure in a similar way as in plant cell wall. The cross-linked macromolecular structure may allow the asphalt to be more stable at high temperature, and thus prevent the melting (or rutting) at high temperature. However, such macromolecular structure could also lead to less molecular flexibility and thus higher viscosity of asphalt, which in turn could make asphalt binder more brittle and prone to cracking at low temperature. An increase in the molecular weight of insoluble lignin fraction (Figure 2C) would increase the asphalt binder stability at high temperature (Figure 2A) due to the enhanced dipolar−dipolar intramolecular forces between lignin and asphalt. On the other hand, the soluble lignin derived from
when added at 20% (Figure 2B). The results highlighted that the laccase-HBT fractionated lignin could serve as a good asphalt binder modifier to enhance the high temperature performance of asphalt binder without compromising the low temperature performance. The detailed structural analysis of fractionated lignin was subsequently carried out to elucidate the potential mechanisms for altering asphalt binder performance by lignin. The molecular weight and structural characteristics for the biologically fractionated lignin were characterized using gel permeation chromatography (GPC) and 13C-nuclear magnetic resonance (13C NMR), respectively. The GPC analysis revealed that the molecular weight of insoluble lignin fraction was increased 1.6 times as compared to that of the raw Kraft lignin, whereas the soluble fraction had significantly decreased molecular weight (Figure 2 C). NMR analysis revealed the changes of functional groups in fractionated lignin, which could contribute to the performance of lignin as asphalt binder modifier. The amount of aliphatic hydroxyl (172−168.6 ppm), phenolic hydroxyl (168.6−166 ppm) and aliphatic carboxylic (173−171 ppm) functional groups in the insoluble fraction were slightly increased as compared to that of the raw Kraft lignin (Figure 2 D), indicating that biological fractionation has led to lignin degradation. However, the aforementioned aliphatic hydroxyl group, phenolic hydroxyl group, and methoxyl group (57−54 ppm) in the soluble fraction had 13-, 4.3- and 2.5-fold increases, respectively (Figure 2 D), suggesting that lignin had been depolymerized and/or decyclized. In particular, the significant increase in aliphatic region for soluble fraction of laccase-HBT processed lignin indicated that the benzene ring of lignin had undergone ring2820
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aliphatic hydroxyl group, yet almost no phenolic hydroxyl groups were detected (Figure 3D). Meanwhile, aliphatic hydroxyl group in formic acid/water-soluble fraction was similar to that of Kraft lignin, whereas the content of phenolic hydroxyl groups was increased by 3 times (Figure 3D). The differences in functional groups between the two fractions could be that most of the phenolic hydroxyl groups were removed in the insoluble fraction, whereas the benzene ring was slight degraded in the soluble portion, leading to an increase in the phenolic hydroxyl group per aromatic ring. The functional groups, in particular, the Ar−COOH group might have a significant impact on the asphalt binder performance. As a strong polar and more oxidized group, the Ar−COOH group could have prevented the better cross-linking of lignin with asphaltenes and thus reduced the melting temperature at high temperature. For the insoluble fraction, the lignin fraction with much higher molecular weight would enhance the dipolar− dipolar intermolecular forces between lignin and asphaltene to form an interacted macromolecular structure as aforementioned. The Ar−COOH group might interfere with the formation of macromolecular structure, yet the combinatory effects still led to the enhancement of high temperature performance. However, for the soluble fraction, the effects of Ar−COOH group could be more predominant considering the low molecular weight of lignin molecules. The increase of concentrations for soluble lignin fraction led to a decrease in high temperature performance of asphalt binder. Processed Lignin Improved Aging Resistance, Mechanical and Rheological properties. To evaluate further the potential of fractionated lignin as renewable asphalt binder modifier, we investigated the aging resistance, mechanical and rheological properties of the asphalt binder when mixed with fractionated lignin. One of the key considerations of the asphalt binder performance is the aging resistance. To study the effects of lignin on binder aging resistance, we compared the G*/sin δ ratio of aged to unaged among the base asphalt binder and modified binders with different lignin fractions. A smaller G*/ sin δ ratio means better aging resistance. The results showed that G*/sin δ ratio of the modified asphalt binders with the biologically processed lignin fractions and the chemically processed insoluble lignin fraction were significantly lower than that of the base binder (Figure S2). Thus, these modified lignin fractions significantly improved the aging resistance property of asphalt binder. However, the asphalt binder modified with chemically processed soluble lignin fraction has lower aging resistance. This observation suggested that the method used for lignin fractionation would have significant impact on the aging resistance. The results also indicated that three out of four fractions from processed lignin could improve aging resistance of asphalt binder. In combination with temperature performance (Figures 2A,B, 3A,B), the results highlighted that these fractions could serve as quality asphalt binder modifiers. Another important consideration for asphalt binder performance is the mechanical and rheological property.18,19 The shear modulus (or stiffness) of binders doped with different percentage of lignin were measured from the frequency sweep tests. The results showed that stiffness of either base asphalt binder or lignin modified asphalt binder increased with an increase in loading frequency, a typical response of viscoelastic material (Figure S3). The results also demonstrated that, at any given loading frequency, binder stiffness increases with an increase in the percentage of lignin (Figure S3). Our
laccase-mediator processing with lower molecular weight led to less extent of enhancement for high temperature performance (KL-L/H-Sol in Figure 2A). Second, the differences in low temperature performance could be explained by the various functional groups in different lignin fractions. The improvement of asphalt binder’s low temperature performance by adding soluble lignin fraction could be due to the increase in hydroxyl groups in lignin structure. The soluble lignin with lower molecular weight (Figure 2C) had much more hydroxyl groups (Figure 2D), which could form intermolecular hydrogen bonds between this lignin and asphalt. The increased hydrogen bonding could enhance the flexibility of asphalt binder, and thus improve the asphalt binder’s low temperature performance. The aliphatic and phenolic moieties could also increase the saturate and aromatic components of asphalt binder, and thus enhance the flexibility of asphalt binder. The significantly improved hydroxyl groups in soluble lignin fraction could balance off the effects on cross-linkage of lignin with asphaltene, and thus overall prevents the cracking of asphalt at a lower temperature. Overall, lignin could cross-link with asphaltenes through dipolar−dipolar interactions to improve the high temperature performance of asphalt binder. In addition, the increased hydroxyl groups in biological processed lignin could also form intermolecular hydrogen bond to prevent the cracking of asphalt at a lower temperature. Development of Chemical Processes To Improve Lignin Property as Asphalt Binder Modifier. Besides the biological lignin processing, we further developed a chemical process, where formic acid and Fenton reagent (iron ions and H2O2) were used to derive both soluble and insoluble lignin fractions. The combination of formic acid, iron ions and H2O2 might synergize the formic acid and Fenton reactions to achieve maximized fractionization of Kraft lignin into formic acidsoluble and -insoluble fractions. These fractions were then evaluated for their capacity to serve as renewable asphalt binder modifiers. The results showed that the addition of the insoluble lignin fraction at 5% to 20% could significantly improve the high temperature performance of asphalt binder (Figure 3A). Meanwhile, the addition of the insoluble lignin had no significant impact on the low temperature properties (Figure 3B). The results highlighted the insoluble fraction from the chemical fractionation could serve as a quality asphalt binder modifier. However, the soluble fraction had a drastic different effect on asphalt performance as compared to all other type of lignin fractions. Basically, the high temperature performance was significantly reduced when >5% of soluble lignin fraction from chemical fractionation was added (Figure 3A), whereas the addition of 20% of the fraction could improve the low temperature performance of the asphalt binder. The distinct performance as asphalt binder modifier for chemically processed lignin fractions could be due to the unique pattern of functional groups. GPC and 13C NMR analyses were carried out to understand the potential mechanisms for asphalt binder performance when adding different lignin fractions. Besides the molecular weight considerations, one of the key features is that carboxylate peaks were detected as C4 in Ar−COOH (163−161 ppm) for both insoluble and soluble fractions out of chemical fractionation (Figure S1). These carboxylate moieties could be derived from the oxidation of aliphatic hydroxyl group by formic acid.17 In addition, the semiquantitative 13C NMR showed that the insoluble fraction had a 2-fold increase in 2821
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lack. The study thus not only provided a new approach to utilize an industrial waste for valued products but also enabled asphalt binders with unique features.
study also indicated that, under low lignin dosage, the binder modified with biologically processed insoluble fraction was stiffer than the binder modified with raw Kraft lignin and the soluble fraction (Figure S4), which is consistent with our aforementioned results on high and low temperature properties.
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ASSOCIATED CONTENT
S Supporting Information *
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03064. Methods and materials; 13C NMR spectra of lignin; G*/ sin δ ratios of aged and unaged samples for unmodified base binder and modified binders doped with 10% different lignin fractions; mechanical and rheological properties of base asphalt binder and asphalt binders modified with different dosage of Kraft lignin; mechanical and rheological properties of asphalt binders modified with 3% different lignin; assignments and quantification of functional groups in 13C NMR spectra (PDF)
CONCLUSION Overall, the results highlighted the potential of modified lignin to serve as renewable asphalt binder modifier. First, we have established that raw Kraft lignin can improve high temperature performance of asphalt binder, yet decrease the low temperature performance. The results highlighted the potential and limitation of lignin as a renewable asphalt binder modifier, and indicated the necessity of modifying lignin for the application. Second, we have developed two different methods to process lignin to produce quality asphalt binder modifiers. Even though lignin has been proposed to serve as asphalt antioxidant before, previous studies fall short in evaluating lignin as an asphalt binder modifier to enhance both high temperature and low temperature performance. We hereby established the concept that lignin can be properly processed to derived fractions to serve asphalt binder modifier to improve asphalt binder performance under different temperatures. In particular, two lignin processing strategies were developed, a biological process based on enzyme-mediator system, and a chemical process using formic acid, iron and H2O2. Both the soluble fraction of biologically processed lignin and the insoluble fraction of the chemically processed lignin can serve as quality asphalt binder modifier to enhance high temperature performance of asphalt mixtures without compromising the low temperature performance. Moreover, the soluble fraction of biologically processed lignin might even improve both high and low temperature performance of asphalt binder, offering unique features as modifier. Third, we have established that some lignin fractions not only improved temperature performance but also enhanced aging resistance of asphalt binder. In particular, three out of four fractions of processed lignin could increase aging resistance, including the soluble fraction of biologically processed lignin. The fraction thus can serve as unique asphalt binder modifier for improving aging resistance along with the high and low temperature performance. Fourth, the study established potential mechanisms regarding how molecular weight and functional groups could impact the performance of asphalt binder. Extensive studies need to be carried out to verify further the hypothesis established in this study. Nevertheless, the results could provide practical guidance on how to fractionate lignin toward asphalt binder modifiers to suite different applications. Lignin is a major waste from biorefinery and paper-making industry. The study not only provides fundamental understanding on how to produce quality modifier from lignin to improve asphalt binder performance but also enables the biological and chemical modification of industrial lignin for value-added material to benefit the entire biorefinery supply chain. Thus, the utilization of lignin as asphalt binder modifier renders practical solutions for both road pavement and biorefinery industry. The additional stream will enable a multistream biorefinery with better sustainability and costeffectiveness. In particular, the soluble fraction of biologically processed lignin can actually improve both high temperature and low temperature performance of asphalt binder, which is a unique feature that most of petroleum-based asphalt binders
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AUTHOR INFORMATION
Corresponding Authors
*J. S. Yuan. E-mail:
[email protected]; Phone: +1-979-8453016. *F. Zhou. E-mail:
[email protected]. ORCID
Joshua S. Yuan: 0000-0002-3837-4214 Author Contributions ∇
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was supported by the U.S. DOE (Department of Energy) EERE (Energy Efficiency and Renewable Energy) BETO (Bioenergy Technology Office) (grant No. DEEE0006112 and DE-EE0007104) to J.S.Y. The research was also supported by Texas A&M Agrilife Research’s biofuel initiative to J.S.Y.
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