Research Article pubs.acs.org/journal/ascecg
Visible-Light Induced Thiol−Ene Reaction on Natural Lignin Hailing Liu and Hoyong Chung* Department of Chemical and Biomedical Engineering, Florida State University, 2525 Pottsdamer Street, Building A, Suite A131, Tallahassee, Florida 32310, United States S Supporting Information *
ABSTRACT: The current use of lignin as a raw material is very limited and focused only on cheap and poorly defined nonfunctional materials mainly due to challenges in synthetic modification of lignin. Herein, we report a new low energy and environmentally friendly lignin modification method induced by visible blue light. The key modification reaction is a photoredox catalyzed thiol−ene reaction. The lignin was modified to possess alkenes for the thiol− ene reaction. Three photochemical reagentsRu(bpy)3Cl2, Eosin Y, and 2,2-dimethoxy-2-phenylacetophenonewere tested to determine the best thiol−ene modification method. The thiol−ene reaction between lignin−alkene and 1-decanethiol revealed that Ru(bpy)3Cl2 was the most efficient, resulting in conversions of 97% with 2.5 mol % catalyst loading. The Ru(bpy)3Cl2 was further investigated with diverse thiol compounds. All tested thiol−ene reactions showed excellent efficiencies, with conversions of 93−97% under low-energy 3W blue LED light. In particular, thiol terminal poly(ethylene glycol) also displayed 94% conversion after 80 min of irradiation. The developed photoredox catalyzed thiol−ene modification of lignin was very conveniently controlled by simply turning the light source on and off. Excellent conversion, 95%, of lignin thiol−ene modification was achieved even by natural sunlight after 4 h of irradiation. KEYWORDS: Lignin, Photoredox reaction, Thiol−ene reaction, Lignin polymer modification
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INTRODUCTION As a sustainable raw material, lignin is the second most abundant plant-based biopolymer after cellulose.1−3 Biologically, lignin is mostly found in plant cell walls, where it is an important structural component of plants due to its physical strength and durability. Other functions of lignin include the formation of water-conducting vascular networks via hydrophobic interactions with the vascular channels and to protect plants from external microorganisms and insects. Chemically, lignin demonstrates a random polymeric network that is composed of phenylpropane groups. Three monomeric units include coumaryl alcohol, coniferyl alcohol and sinapyl alcohol as shown in Figure 1a. Those three monomeric units undergo a biosynthetic process to form the polymeric structure of lignin. The biosynthetic polymerization yields three types of segments within lignin: p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S). Different plant sources yield diverse lignin varieties by altering the concentrations of H, G, and S.4 Another feature of lignin is its complex three-dimensional structure with various types of functionalities and covalent links as a biomass.5,6 For example, lignin’s important C−O linkages are β-O-4, α-O-4, and 4-O-5, and C−C links are β-5, 5-5, β-1, and β-β linkages (Figure 1b).2,7 Other common functional groups in lignin include methoxyl, phenolic hydroxyl, aliphatic hydroxyl, and other carbonyl groups.8 Chemical characterization of lignin, lignin derivatives, and other lignin-based materials is no trivial task due to its networked structure, diverse chemical linkages, various functional groups, difficult isolation, and poor solubility in organic solvents. This is further complicated by the many © 2017 American Chemical Society
different types of lignin available depending on the plant source and processing method. Due to lignin’s chemically inert nature and structural complexity, modification is necessary for advanced applications. By and large, there are two actively studied lignin modification fields: catalytic cleavage and polymeric modification. The catalytic cleavage of lignin is important because lignin contains a large amount of aromatic groups that can be used as substitutes for petroleum-based aromatic fine chemicals. While research into the catalytic cleavage of lignin has seen progress recently, much of that research utilizes model compounds of lignin.9−12 Polymeric modification of lignin can produce practically useful advanced materials. Polymeric modification methods integrate lignin and polymer either by covalent bonds or physical mixing (blending) with a heat and pressure.13,14 Comparatively, covalent linkage of lignin and polymer provides better reliability and controllability than physical mixing. Most importantly, polymeric modification of lignin compounds provide more specific advanced properties.15−17 For example, advanced mechanical properties and self-healing properties were recently improved in a lignin-graf t-poly(5-acetylaminopentyl acrylate) material.18 Controlled hydrophobicity was observed from rosin−lignin composites.19 Lignin-graf t-poly(methyl methacrylate-co-butyl acrylate) exhibits high UV absorption capability.20 Among the examples of lignin’s Received: June 23, 2017 Revised: July 21, 2017 Published: August 10, 2017 9160
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Figure 1. (a) Monomers of lignin biopolymer. (b) Chemical structure of natural lignin.
polymeric modification,21 one of the most extensively used reaction is click chemistry.18,22,23 Click chemistry refers to reactions that are high yielding, possess an extended scope, are experimentally simple using easily removable solvents, and create only byproducts that can be easily removed without complex purification methods.24 Of the most noteworthy of click reactions are the thiol−ene (hydrothiolation of olefins, Figure 2) and thiol−yne
reagents. Herein, we have decided to focus our efforts on photochemically driven thiol−ene reactions. There are generally two methods for photochemically driven thiol−ene reactions, by photoinitiator and photoredox catalyst. Typically, organic photoinitiators such as 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, 2,2-dimethoxy-2-phenylacetophenone (DMPA), benzophenone, and 2-hydroxy-4′-(2hydroxyethoxy)-2-methylpropiophenone generate radicals through either homolytic cleavage of covalent bonds (type I) or abstraction of hydrogen, forming a thiyl radical (type II).31 The activation of the initiator is typically performed by UV irradiation. Herein, the radical initiators can be used in substoichiometric amounts; however, it does not function as a catalyst. A mechanism of the thiol−ene reaction, which is initiated by conventional radical initiators, is presented in Figure 2a. Photoredox catalysis undergoes photoinduced singleelectron-transfer processes by photoexcitation via the absorption of photons. The photoexcited state (metal to ligand charge transfer (MLCT) state) needs to be stable for a relatively long time (1000 ns) to prevent rapid deactivation quenching pathways. For the photoredox thiol−ene reaction, the excited state undergoes a reductive quenching cycle. In this pathway, the single electron photooxidation of a thiol produces a thiol radical cation (RSH+·) (Figure 2b). The deprotonation of thiol radical cation generates an electrophilic thiyl radical (RS·). The thiyl radical adds to the alkene via an anti-Markovnikov route yielding the alkyl radical. Next, the alkyl radical abstracts hydrogen atom from the unreacted thiol compound (RSH) resulting a hydrothiolated species (RS−CH2−CH2−R′) with an equivalent thiyl radical (RS·).32−35 In particular, photoredox catalysis acts as a sustainable chemistry method because it can be activated by visible light. The efficiency of photoredox catalysis can be tuned by ligand modification and the use of additives.32,35,36 Actual examples of photoredox catalysts, photoinitiators, and optimized reaction conditions are discussed in this report for modification of natural polymer, lignin. The photochemical thiol−ene reaction for natural lignin modification will be crucial from the standpoint of practical application. The reaction is activated by visible blue light, an inexpensive and renewable clean energy source, which will make the lignin modification process environmentally
Figure 2. (a) Mechanism of radical mediated thiol−ene click reaction using a radical initiator. (b) Commonly accepted mechanism of the photoredox catalyzed thiol−ene reaction.
reactions.25−28 The thiol−ene click reaction is particularly important because the reaction manipulates carbon−sulfur bonds that are frequently found in natural and pharmaceutical products.29 Also, the thiol−ene reaction is an important fundamental research theme in chemistry due to its antiMarkovnikov fashioned products.30 The thiol−ene click reaction occurs with base-catalyzed electron-deficient alkenes or by a radically initiated reaction with unactivated olefins. The radical initiation takes place by UV irradiation or thermolysis of 9161
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Scheme 1. (a) Lignin Modification to Prepare Lignin−Alkene, (b) Thiol−Ene Reaction of Lignin and 1-Decanethiol,a (c) Terminal Functionalization of Poly(ethylene glycol) (PEG) to Synthesize PEG−Thiol, (d) Thiol−Ene Reaction of Lignin− Alkene and PEG−Thiol
a
Other thiol compounds were also tested as shown in Table 1.
Figure 3. (a) 1H NMR of unmodified natural lignin (top) and lignin−alkene (bottom) in DMSO-d6. (b) FT-IR of unmodified natural lignin (red, bottom) and lignin−alkene (black, top).
friendly.37 The visible light source is readily available, requiring no special equipment. Thus, application for industrial purposes would be relatively simple. Moreover, more versatile modification methods can be achieved thanks to the temporal and spatial control offered by visible light irradiation.38 Above all, the use of photochemical thiol−ene reactions for the modification of natural lignin will derive various subsequent research and applications because of the cost-effectiveness and environmentally benign nature of both sustainable base materials and the modification method.
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purchased from TCI America (Product number L0045, softwood lignin, kraft lignin). These alkene groups were used for radical thiol−ene reactions as shown in Scheme 1a and b. The functionalization of hydroxyl groups on lignin was accomplished by carbodiimide-mediated esterification using 4pentenoic acid in the presence of 4-(dimethylamino)pyridine and N,N′-dicyclohexylcarbodiimide. The introduction of alkene moieties onto natural lignin was characterized spectroscopically by 1H NMR and FT-IR as shown in Figure 3. In the 1H NMR, the appearance of terminal alkene protons (a in the presented chemical structure in Figure 3a) at 5.79 ppm and b at 4.97 ppm confirm the successful integration of alkene moieties on lignin. According to the chemical structure in Figure 3a, the aliphatic chains (signals near 1.70 and 1.20 ppm) next to the formed ester group on the lignin also appeared post modification. The signal of aliphatic chains are resulting from the esterification of aliphatic alcohols at lignin. As shown in Figure 1, natural lignin contains large amount of aliphatic alcohols (−CH2−OH) in the
EXPERIMENTAL SECTION
Full experimental procedures of synthesis and characterization data are available in the Supporting Information (SI).
RESULTS AND DISCUSSION Synthesis of Lignin−Alkene. Lignin was chemically functionalized to introduce alkene groups. Lignin was 9162
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Figure 4. Chemical structures of examined visible-light photochemical reagents.
Figure 5. (a) Efficiency study of lignin−alkene and 1-decanethiol thiol−ene reaction under different photochemical reagents: DMPA [the DMPA experiment was performed under UV irradiation, keeping all other conditions identical to the other photoredox catalysts], Ru(bpy)3Cl2, and Eosin Y. (b) Ru(bpy)3Cl2 catalyst loading effect on lignin−alkene and 1-decanethiol thiol−ene reaction.
chemical structure. The low resolution in the 1H NMR spectrum is the result of several known factors: (1) the wide distribution of molecular weight of natural lignin molecules, (2) the randomly distributed functionalities of both phenolic and aliphatic hydroxyl groups on lignin, and (3) unreacted hydroxyl groups on lignin without ester linkage formation. Similar resolution issues with spectrometric characterization of lignin have been reported elsewhere.15,22,39 Another spectroscopic characterization, FT-IR was carried out to clarify the results of the introduction of alkene to natural lignin as shown in Figure 3b. The increase of carbonyl group stretching absorption at ester linkage frequency (1750 cm−1) between lignin and alkene after esterification of lignin to incorporate alkene was clearly observed. The characteristic hydroxyl group stretching vibration (3350 cm−1) from unmodified natural lignin also decreased significantly after incorporation of alkene to lignin due to the consumption of hydroxyl groups during esterification. The prepared lignin−alkene was purified three times by precipitation in cold hexane in order to make sure the complete removal of unreacted 4-pentenoic acid. Comparing the Efficiency of Photochemical Agents for the Thiol−Ene Reaction of Lignin. The alkene-modified lignin was then used in radical thiol−ene reactions. Unlike conventional addition mechanisms, the radical thiol−ene reaction follows anti-Markovnikov orientation.40 Thus, the thiol−ene reaction is an excellent modification method to functionalize terminal groups of biopolymers such as lignin. In addition, recently developed low-energy consuming photoredox catalyzed thiol−ene reactions can be an ideal chemical modification method for natural polymer lignin with a low environmental impact.32,33,35 In this work, three types of photochemical reagents were tested to determine the best
chemical reagent to use with lignin. The tested photochemical reagents are tris(bipyridine)ruthenium(II) chloride (Ru(bpy)3Cl2), Eosin Y, and DMPA (Figure 4). The Ru catalyst, Ru(bpy)3Cl2 is a representative transition metal photoredox catalyst. Eosin Y represents a metal-free organic photoredox catalyst. DMPA is a commonly used organic photoinitiator that generates radicals under UV irradiation followed by fast αcleavage.41 DMPA was tested in a Luzchem UV reactor, keeping all other conditions identical to the other photoredox catalysts. Although it has not been tested in our work, Fadeyi et al. reported a visible-light driven thiol−ene reaction with bismuth oxide catalyst in the presence of additive, BrCCl3.42 All three reagents showed successful thiol−ene reactions, but the reaction efficiency varied depending on the reagent. The summary of thiol−ene reactions between lignin−alkene and 1decanethiol in the presence of diverse photochemical reagents are demonstrated in Figure 5. Reagent loading amount (5 mol %), temperature (room temperature, 23 °C), and solvent (DMF) were kept the same over different photochemical reagents to compare the reagent efficiency only. In order to reach 80% conversion, the Ru(bpy)3Cl2 took the shortest time, 40 min (Figure 5a). DMPA and Eosin Y required 100 and 130 min, respectively. Hence, the Ru(bpy)3Cl2 photoredox catalyst showed the best efficiency on lignin−alkene modification with 1-decanethiol, and the efficient behavior of Ru(bpy)3Cl2 compares well with other reported works using synthetic polymers.43 After identifying Ru(bpy)3Cl2 as the principle catalyst for further investigation, catalyst loading was varied to perform an optimal thiol−ene reaction. As shown in Figure 5b, Ru(bpy)3Cl2 loading of 1, 2.5, and 5 mol %, with p-toluidine additive (20 mol equivalent to Ru(bpy)3Cl2) were used to catalyze the reaction between 9163
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Figure 6. (a) 1H NMR spectra of the alkene functional group changes in DMSO-d6 during lignin−alkene and 1-decanethiol thiol−ene reaction. (b) 1 H NMR spectra of unmodified natural lignin (top), lignin−alkene (middle), and decane-functionalized lignin (bottom) in DMSO-d6.
Table 1. Thiol−Ene Reaction of Lignin−Alkene with a Variety of Thiol Compounds in the Presence of 2.5% Ru(bpy)3Cl2 and 0.5 equiv p-Toluidine
a
Mn of PEG: 550 g/mol.
lignin−alkene and 1-decanethiol. Hereafter, p-toluidine was consistently used for all Ru(bpy)3Cl2 catalyzed thiol−ene reactions in this report because p-toluidine is known to serve as a redox mediator resulting in the expedition of radical thiol− ene reactions.32 The general trend of catalyst loading revealed that higher catalyst loading yielded a faster thiol−ene reaction. The thiol−ene reaction was complete after 40 min with 5% Ru(bpy)3Cl2 and 60 min using 2.5%. Catalyst loading of 1% resulted in a relatively slow reaction as shown in Figure 5b. Although 5% catalyst loading demonstrated the fastest rate, the cost of the catalyst and inconsequential increase in reaction rate led us to choose 2.5% as the optimal catalyst loading moving forward. Synthesis and Characterization of Model Thiol−Ene Reactions with Lignin−Alkene. The performed thiol−ene reaction was monitored by the alkene protons of lignin via 1H NMR spectroscopy. During the reaction process, the alkene
groups in lignin were observed to deplete over time as shown in Figure 6a. The presented example is a thiol−ene reaction between lignin−alkene and 1-decanethiol (5 equiv) in the presence of 2.5% Ru(bpy)3Cl2 and p-toluidine (see the SI for details). As shown in the Figure 6a, the alkene peaks at 5.79 and 4.97 ppm were depleted rather slowly at the beginning, before depleting more quickly. The same trend was also observed in the kinetics studies in Figures 5a and b. The slow initiation of this reaction is presumably due to the steric hindrance of the functional groups in the highly networked lignin structure. Prior studies suggest the rate-limiting step of the radical thiol− ene addition involves slow hydrogen atom transfer from thiol to the intermediate carbon-centered radical in a chain transfer step (Figure 2b).44,45 In our case, the sterically hindered complex structure of lignin prolonged the time before hydrogen atom transfer from thiol to the carbon-centered radical from lignin at the beginning of the reaction. This would be the one of the 9164
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OFF stage of the light source at each interval. At the beginning of reaction without light, the reaction did not occur at all. As the light was turned on, the reaction started rapidly. This experiment confirmed that thiol−ene reaction is conveniently controlled by light. In addition, the reaction only occurs during periods of irradiation because no alkene consumption was observed during OFF periods. With this light control technique, lignin modification can be precisely controlled without special facilities and/or treatments. The precise light control will enable the photochemically mediated lignin modification for advanced applications, including flat surface modification that could be the foundation of photolithography and nano/micro patterning.48−51 Graft Copolymerization of Lignin−Alkene and PEG. An efficient modification of natural lignin with synthetic or natural polymeric materials is an important issue for the development of modern materials for a broad range of practical applications.52,53 Due to the importance of polymer integration to lignin, further investigation of the photoredox-mediated thiol−ene reaction on lignin was performed with poly(ethylene glycol) (PEG, Mn 550 g/mol, TCI America P2184, note that degree of polymerization of the PEG550 is 11.8. Thus, the PEG can be also addressed as an oligomer.). PEG was covalently integrated onto lignin−alkene easily by the conditions as described in Scheme 1d. In other words, lignin-graf t-PEG was prepared via graft onto methods. Prior to the thiol−ene reaction, PEG was functionalized with 3-mercaptopropionic acid to have a thiol functional group at the polymer terminus (Scheme 1c, Figure 8a). The thiol−ene reaction of lignin− alkene and PEG−thiol occurred highly efficiently. The thiol− ene reaction was monitored by disappearance of alkene protons at 5.79 and 4.97 ppm in 1H NMR (Figure 8a). The structure of the produced lignin-graf t-PEG was verified by 1H NMR as shown in Figure 8a. The actual photos of products are shown in Figure 8c. The products are apparently distinctive on their visual appearances. While the lignin−alkene is dark brown solid powder, PEG−thiol is a transparent viscous liquid. The produced lignin-graf t-PEG has an intermediate appearance between these two polymers. The resulting lignin-graf t-PEG is a brown pastelike soft solid. The ON−OFF study was also performed to the reaction of lignin−alkene and PEG−thiol as presented in Figure 8d. The results show that the thiol−ene polymer modification of lignin was instantly controllable by the presence of light sources in the same way as was discovered for the reaction of small molecules. Sunlight Induced Thiol−Ene Reaction on Lignin. Visible-light photoredox catalysis has gained high interest as an important tool in polymer chemistry because of its low energy, high efficiency, and lack of special reaction facilities.35,54 Furthermore, visible-light photoredox catalysts can be activated by truly natural and environmentally friendly light from the sun. To verify our hypothesis of using sunlight as a source of irradiation, the lignin−alkene and thiol compounds (1decanethiol and PEG−thiol) were reacted on a windowsill on a typical Florida afternoon. The thiol−ene reaction conditions were used as described in Scheme 1b and d, except that the light source here is sunlight. The reaction was exposed to sunlight on fifth floor south-facing window in the Chemical Science Laboratory building of Florida State University (Figure 8b). The irradiation time was from 1:00 to 5:00 pm on January 5, 2017. After 4 h of sunlight irritation, the lignin−alkene and 1decanethiol attained alkene conversion of 97%, and the lignin− alkene and PEG−thiol showed conversions of 95%. The results
major reasons that many lignin-based reactions are well-known to require longer reaction times compared to other reagents.18,22 Despite the initial delay, the alkene was continuously consumed over time until the reaction is complete. The 1H NMR of decane-functionalized lignin is shown in Figure 6b. Importantly, the disappearance of alkene and appearance of alkyl group confirmed the formation of the desired product; and peaks on the 1H NMR were assigned with the specified chemical structures of the products in Figures 3, 6, and 8. Scope of Thiols Participating in the Thiol−Ene Reaction. In order to demonstrate that the lignin-based thiol−ene reaction is general and not limited to 1decanethiol,46 we set out to examine the versatility of the thiol−ene reaction with seven thiol substrates (Table 1). Most thiols reached 90% conversion or higher in 3 h. Primary thiols such as 1-decanethiol, methyl thiolglycolate, 1-thiolglycerol, and thioacetic acid reacted efficiently in nearly quantitative yields. Among the tested primary thiol substrates, a ketone containing thioacetic acid required a longer reaction time yet still produced thiol−ene adducts in high yield after 3 h. The secondary thiol of cyclohexanethiol reacted in short time, 60 min and attained 81% of yield. The tertiary thiol, the tert-butyl mercaptan took longer reaction time than primary and secondary thiols, but it still resulted in 95% conversion after 3 h. This trend is likely the result of increased radical stability in the secondary radical, which is then overcome by steric limitations in the tertiary radical. Temporal and Spatial Control of the Thiol−Ene Reaction. Blue light is an essential factor in the photocatalyzed thiol−ene reaction. According to a widely accepted mechanism (Figure 2b), the light exposure generates a strong MLCT excited state on the ruthenium polypryridyl complex to produce electrophilic thiyl radical cations.35,47 Hence, elimination of the light source will instantly stop the entire thiol−ene reaction mechanism, this is the OFF state. On the other hand, as soon as the light is turned on, the thiol−ene reaction immediately proceeds by MLCT of the ruthenium complex, this is the ON state. To confirm the necessity of light and subsequent temporal control of the thiol−ene reaction, a visible light ON−OFF test was carried out on lignin and 1-decanethiol reaction. A repeat cycle of 20 min of a light OFF period followed by 20 min of irradiation was performed on the reaction (Figure 7). An aliquot of the reaction was analyzed by 1 H NMR for alkene conversion prior to changing the ON−
Figure 7. “ON−OFF” study of thiol−ene reaction with lignin−alkene and 1-decanethiol. 9165
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Figure 8. (a) 1H NMR of lignin-graft-PEG in DMSO-d6. (b) Image of natural sunlight induced photoredox catalyzed thiol−ene reactions: lignin− alkene and 1-decanethiol (left); lignin−alkene and PEG−thiol (right). (c) Images of lignin−alkene; lignin-graft-PEG; PEG−thiol (from left to right). (d) ON−OFF study of thiol−ene reaction with lignin−alkene and PEG−thiol under 3 W blue LED light.
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confirm that sunlight can be used as a light source to activate thiol−ene reaction on lignin.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02065. Experimental procedures and characterization of all compounds (PDF)
CONCLUSIONS
In conclusion, we have shown that lignin can be efficiently modified via radical thiol−ene reactions using low energy visible-light or sunlight. This is the first example of successful modification of the natural biopolymer, lignin, using these two light sources. Three different photochemical reagents, Ru(bpy)3Cl2, Eosin Y, and DMPA, were tested to find the best performing catalyst under visible-light and UV irradiation. The selected Ru(bpy)3Cl2 was further studied to examine the efficiency of the thiol−ene reaction with various thiol substrates resulting high conversion of alkene between 81% and 97%. The successful substrate scope was extended to a polymer, PEG, which can open a new avenue of material applications. The developed thiol−ene reaction of lignin instant ON−OFF feature that enables convenient control of reaction. Finally, successful thiol−ene reactions of lignin and PEG−thiol or 1decanethiol were demonstrated under natural sunshine. The developed new chemistry, visible-light thiol−ene reaction of lignin, offers numerous opportunities as an energy conserving and ecofriendly method in the natural polymer lignin modification field, which we are currently investigating further.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hoyong Chung: 0000-0003-0370-230X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Florida State University Energy and Materials Hiring initiative and FSU Department of Chemical and Biomedical Engineering. We would like to acknowledge Dr. Rimantas Slegeris and Dr. Brian Ondrusek for helpful discussions. We thank Dr. Banghao Chen for the support of NMR facilities.
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