Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Deep Eutectic Solvent Assisted Facile Synthesis of Lignin-Based Cryogel Kuan-Ting Lin,⊥,† Ruoshui Ma,⊥,†,‡ Peipei Wang,† Junna Xin,§ Jinwen Zhang,§ Michael P. Wolcott,§ and Xiao Zhang*,†,‡ †
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Voiland School of Chemical Engineering & Bioengineering Bioproducts, Science & Engineering Laboratory, Washington State University, 2710 Crimson Way, Richland, Washington 99354, United States ‡ Pacific Northwest National Laboratory, Richland, Washington 99354, United States § Composite Materials & Engineering Center, Washington State University, Pullman, Washington 99164, United States S Supporting Information *
ABSTRACT: In this study, we reported a method to prepare a lignin based cryogel by cross-linking deep eutectic solvent (DES) extracted lignin and formaldehyde as an alternative to replace resorcinol-formaldehyde (RF) based aerogel. The resulting lignin-formaldehyde cryogel has a highly porous structure (172.8 m2/g surface area) with a high dimension stability. We found that the hydroxyl group and carbonyl group of DES lignin provide reactive sites to cross-link with formaldehyde through electrophilic addition and aldol condensation reaction. We also found that using choline chloridelactic acid (ChCl-Lac) DES as a solvent during the lignin-formaldehyde cryogel formation is critical to prevent the shrinkage of the final cyrogel. The ensuing lignin-formaldehyde cryogel has promising properties such as high thermostability, low thermal conductivity, and good fire retardancy.
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INTRODUCTION Resorcinol-formaldehyde (RF) based aerogel was first demonstrated in 1989.1 Since then it has gained increasing interest because of its unique physical properties such as high specific surface area and low thermal conductivity.2−4 RFbased gels have found many potential applications as adsorbents, electrodes, energy storages, and thermal insulates.5−8 However, the cost of RF aerogel manufacturing has significantly limited its applications. Two major cost barriers are associated with the expensive precursors (i.e., resorcinol) and drying process to maintain the porosity structure. Resorcinol is an expensive chemical, and supercritical dry is an expensive process, which is difficult for large scale commercial production.9 Thus, identifying alternative precursors and economically viable drying methods can bring breakthrough toward expanding RF gel commercialization. Lignin is the largest renewable resource with an aromatic skeleton.10 Its chemical structure as well as monomeric constituents resembles many types of phenolic polymers (e.g., acrylonitrile butadiene styrene (ABS), rosin, etc.) and their precursors (e.g., polyurethane, epoxy resin, polyethylene terephthalate, etc.) respectively.11−14 There has been a significant amount of effort toward modifying lignin macromolecular structure/chemistry for material applications15−17 or depolymerizing lignin to low molecular weight and monomeric phenolic compounds for polymer synthesis.18−20 The phenylpropane units in lignin are connected through a variety of © XXXX American Chemical Society
chemical linkages as well as intricate electronic interactions,21,22 which makes lignin a compact and amorphous macromolecule. Lignin separated from the plant by various means and processes typically show a low surface area and porosity (Table S1). Recently, Jiang and Lubineau have demonstrated the preparation of a lignin-resorcinol-formaldehyde aerogel in alkaline solution, incorporating alkaline lignin into bacterial cellulose.23 The materials have shown promising application in energy storage. However, the protocol requires the presence of 70 wt % of bacterial cellulose gel as a support, and the aerogel can only form with a low lignin content up to 7.5 wt %. A few other studies also investigated the incorporation of lignin into polymer composites to facilitate the formation and support the pore structure formed by these composites.24−28 However, blending polymeric materials with lignin loses the feasibility of replacing petroleum precursors with lignin since the higher cost of polymer become the obstacle to prepare porous materials with reasonable price. The low surface area is a major shortcoming hinders the use of lignin macromolecules for many materials applications. Identifying ways to convert lignin to highly porous material can expand the potential for lignin Received: October 23, 2018 Revised: December 7, 2018
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DOI: 10.1021/acs.macromol.8b02279 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules application. So far, there has been very few successful attempts to prepare porous gel materials primarily based lignin.29−31 It is conceivable that maximizing the cross-linking between lignin and formaldehyde provides an effective way to support lignin porosity or surface area. Attempts have been made by attaching phenolic compounds to lignin side chain to afford new active sites with formaldehyde.16,28,32 While this approach did improve cross-links in the mixture, it added a significant cost to the process because the use of the phenols and additional process steps. Of course, this approach defies the purpose of eliminating phenols from petrochemical feedstock. This finding, however, provided an insight that, if lignin side chains have sufficient reactive groups such as free hydroxyl groups, the lignin itself can have a high reactivity toward formaldehyde. Recently, we have demonstrated that applying deep eutectic solvent (DES) to extract woody biomass can yield lignin product, deep eutectic solvent extracted lignin (DESL), with a high purity, low condensation, and high functional group and phenolic group content.33 The selective cleavage of ether linkages between phenylpropane units resulted in a significant increase in aliphatic and phenolic hydroxyl groups. On the basis of the C9 formulation, the DESL contains 1.99 aliphatic−OH and phenolic−OH group per C9 unit,34 which is comparable to two hydroxyl groups per aromatic ring of resorcinol. In addition to hydroxyl group, carbonyl group can also react with formaldehyde through condensation with formaldehyde to form C−C linkage. These unique features of DESL make it a suitable candidate for directly replacing resorcinol for RF aerogel preparation. It also opens an opportunity to apply low cost drying method for gel formation. In this study, we first reported a lignin based cryogel with up to 100% resorcinol substitution by DES extracted lignin in common alkaline solution. Then we applied DES as reacting solvent to solve the shrinking problem during freeze-drying process. The ensuing cryogels have the comparable properties to typical aerogel with low shrinkage and high porosity. The mechanism behind the transforming compact lignin molecule to this high surface area material is also discussed. The potential applications of these cryogels as adsorbent and thermal barriers were demonstrated.
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Table 1. Composition of RF, LXRF, RF-DES, and LXRFDES Cryogelsc sample
lignin (g)
resorcinol (g)
formaldehyde (g)
lignin content in L+R (%)
RFa L20RFa L50RFa L75RFa L100Fa RF-DESb L20RF-DESb L50RF-DESb L75RF-DESb L100F-DESb
0 0.04 0.1 0.15 0.2 0 0.04 0.1 0.15 0.2
0.2 0.16 0.1 0.05 0 0.2 0.16 0.1 0.05 0
0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
0 20 50 75 100 0 20 50 75 100
a
Synthesized in alkaline aqueous solution. bSynthesized in DES. cRF and LXRF are gels synthesized in alkaline solution, and RF-DES and LXRF-DES are gels synthesized in DES system. X stands for percentage of resorcinol replaced by DESL.
environment (130 Pa). High voltage was set at 20 kV (30 kV for raw material) and the magnification was set from 50−5000×. Fourier Transform Infrared Spectroscopy (FTIR). The FTIR spectra were collected by preparing LXRF-KBr, LXRF-DES-KBr pellets. A blank freshly prepared KBr pellet was used as standard to collect the background of the test. The IR spectrum was collected at a range of wavenumber from 4000 to 800 cm−1 with are solution of 4 cm−1 for 64 cycles. Molecular Length Simulation. The simulation of DES was calculated by MM2 energy minimization from Chem 3D, PerkinElmer. N2-Physisorption. The cryogels were degassed at 80 °C under N2 atmosphere for 20 h before testing. The BET surface areas and pore size distributions were obtained by physisorption in a Micromeritics ASAP 2020 unit. The pore volumes were determined using the Horvath−Kawazoe method.35 Micromeritics analysis software provided the method. Thermal Gravity Analysis (TGA). The TGA was performed on SDT Q600 TGA instrument (TA Instruments). Each sample was scanned from 30 to 400 °C at a heating rate of 10 °C/min under a nitrogen atmosphere. Thermal Conductivity. The thermal conductivity of cryogels was collected by 2500 S Thermal Property System.
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RESULTS AND DISCUSSION A control RF gel was first prepared by reaction of formaldehyde and resorcinol aromatic ring in alkaline solution following the standard protocol.36 The chemistry of RF formation is shown in Scheme S1. The presence of free hydroxyl group and the vacant ortho- or para- sites on the aromatic ring is a prerequisite to this reaction. Lignin monomer units, such as p-hydroxyphenyl and guaiacyl, possess the similar ortho reactive site with resorcinol, which can crosslink with formaldehyde (Scheme 1). While the presence of 5−
EXPERIMENTAL METHOD
General Materials and Methods. All chemicals and solvents were reagent grade from Fisher Scientific. The alkaline aqueous solution was 0.25 M sodium hydroxide solution with 0.05 wt % sodium carbonate. The preparation of choline chloride and lactic acid deep eutectic solvent, and DES extracted lignin were followed our previous procedure.33 Preparation of LXRF and LXRF-DES Cryogels. Lignin, resorcinol (98% purity), and formaldehyde (37 wt % in water) were added into an 8 mL vial, dissolved in 4 mL of either alkaline solution or choline chloride-lactic acid DES, and then incubated for 48 h in a 90 °C oven. The weight ratios of lignin and resorcinol were different, which are listed in Table 1. After the wet gel was formed, water and ethanol were added to wash the gel and DES or alkaline was removed. The water/ethanol remained with gel was then exchanged with t-butanol. After solvent was exchanged, the wet gels were frozen under −80 °C, then placed in a freeze-dryer under −49 °C and 0.2 mbar to remove the t-butanol, and dry cryogels were obtained. LXRF and LXRF-DES Cryogels Characterization. Scanning Electronic Microscope (SEM). The SEM was performed on a FEI 200F SEM system with large field detector and a low vacuum
Scheme 1. Reactive Site for Lignin Monomer Polymerization
B
DOI: 10.1021/acs.macromol.8b02279 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules 5 linkage and methoxy groups reduce formaldehyde attachment on the aromatic, the presence of aliphatic hydroxyl group on DESL side chain creates new binding sites to formaldehyde. This also offers a new scaffolding structure. Lignin-Resorcinol-Formaldehyde (LXRF) Preparation by DES Lignin. Lignin based LXRF gels were prepared with 20% (L20RF), 50% (L50RF), 75% (L75RF), and 100% (L100F) substitution of resorcinol by DESL (X stands for percentage replace of resorcinol by DESL). It was found that stable wet gels were formed at all these substitution ratios. At 100% substitution, the final gel appeared to be identical to the control with 100% resorcinol. After complete gelation, the ensuing LXRF all showed high elasticity. As we know, this is the first cryogel with 100% substitution of resorcinol by lignin. Fourier transform infrared spectroscopy (FTIR) was used to determine the cross-linking between DES lignin and formaldehyde. An additional peak appeared at 1078.0 cm−1 representing the C−O−C bending of benzyl ether,30 which confirmed the cross-linkage of polymerization between lignin and formaldehyde (Figure 5). However, an increasing in gelation time was observed along with the increase in lignin substitution ratio. L20RF, L50RF, and L75RF can form within 48 h. However, it took 72 h for L100F to form the final stable gel. In addition to DESL, a number of other biorefinery lignin samples were tested at the same condition with DESL with 75% lignin substitution (detail is described in Supporting Information). The result shows that only DESL can form the gel after reacting for 48 h. Using a modified condition with increasing reaction time to 72 h, corn stover and wheat straw lignin-based gels can be formed. The key structural characterizations of these representative biorefinery lignins were provided in Table S2. Compared to these biorefinery lignins, wheat straw lignin, corn stover lignin, and DES extracted lignin had lower molecular weight. In addition, DES extracted lignin had the highest phenolic-hydroxyl content (0.49 per aromatic ring). The high hydroxyl content made DES extracted lignin could be polymerized in shorter time. As a result, DESL has special structural features, which allow it to readily replace resorcinol. The ability to form stable wet gel is the first step for porous material preparation. In the subsequent processes, solvent in the gel will be removed before gel can be processed for applications.37 As a high porous material, the RF gel is susceptible to dimensional shrinking during drying. Therefore, the selection of drying method is crucial to maintain the pore structure and surface area of final gel products. Supercritical drying with CO2 is a standard method to produce aerogel with minimal shrinkage during drying process. However, large-scale drying presents an economic challenge.38,39 Freeze-drying is a common method to prepare cryogel, which typically suffered with a significant dimensional shrinkage, often over 60%, due to stronger capillary pressure encountered during solvent exchange and removal.24,31,40 Producing a cryogel with similar shrinkage and porosity to aerogel is a significant challenge for lignin gel preparation. However, a breakthrough will bring out an enormous economic benefit. After freeze-dried, up to 85% of volume shrinkage was observed in LXRF (Figure 1a). To understand the cause for the shrinkage, scanning electron microscopy was used to investigate the morphology of LXRF after freeze-drying (Figure 1b,c). The dried LXRF showed a uniformed compact structure, and lignin-RF resin appeared in nanospheric particles
Figure 1. (a) LXRF gel before and after freeze-drying. Microscopic structure of L75RF under SEM (b) at 5 μm and (c) at 2 μm.
with approximately 50 nm in diameter. These particles were attached together after drying, which left little porous structure. The porosity, average pore size, and the surface area of LXRF were determined using isothermal nitrogen adsorption/ desorption. The LXRF only had a surface area less than 4 m2/g. Prior to freeze-drying, the wet gels were subjected to solvent exchange in the following sequence: water, ethanol, and tertbutanol. This solvent exchange process was applied to minimize the expanding volume of water after freezing and reduce shrinkage during the drying stage. It was noticed that a significant shrinkage (∼20%) occurred during water exchange with ethanol. There was little shrinkage during ethanol exchange with tert-butanol. However, the shrinkage of after drying off the tert-butanol was the largest (∼75%). On the basis of these observations, it is likely that the capillary pressure from sublimating tert-butanol from the pores is a main force responsible for the shrinkage. A direct measurement of the pore volume and size during the solvent exchange was difficult. However, it can be surmised that pores formed during gel formation in water have small radius (R). Water has a relatively large surface tension (r), The capillary force (ΔP) is expressed as ΔP = 2r cos θ/R based on Young−Laplace equation equals.41,42 Thus, a large surface tension and small radius led to large capillary force, which explains why a significant shrinkage was first observed with ethanol−water exchange, as surface tension of ethanol is 22.7 mN m−1, which is much lower than water. The surface tensions between ethanol and tert-butanol (20.7 mN m−1) were comparable; therefore, little shrinkage occurred during ethanol-tert-butanol exchange. Because of small pore radius initially formed in gel, the last step drying, an exchange of tert-butanol and air, produced a large capillary force leading to significant shrinkage. A common problem encountered in all previous research is the significant shrinkage during drying of lignin-based gel (Figure 2).24,31 In this study, we proposed and test the use of DES solvent during lignin gel preparation based on a hypothesis that both the large radius of DES molecule and its compatibility with lignin instigate the formation of stable pore and thus reduce shrinkage.43,44 Deep Eutectic Solvent Assist Preparation of LXRFDES with Low Shrinkage. As shown in Figure 3, DES is constructed by hydrogen bond between lactic acid and choline chloride. The simulated length of DES is 10.387 Å, which is seven-times larger than a water molecule. DES is orderly arranged by intermolecular hydrogen bond, but DES can maintain a larger bulk volume than water. In addition, DES is dense and high viscosity solvent, possessing low free volume.45−47 The low free volume of DES helps maintain its C
DOI: 10.1021/acs.macromol.8b02279 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
drying. It is very exciting to find that the ensuing LXRF-DES cryogels did maintain a good dimensional stability with a low degree of shrinkage, below than 30%, after freeze-drying (Figure 4a). There is little shrinkage detected during solvent exchange process. The overall shrinkage was comparable to the aerogel that was obtained from supercritical drying. The ensuing cryogel was examined using scanning electron microscopy image analysis (Figure 4b,c). The LXRF-DES prepared in DES as a media showed a highly porous structure with predominantly meso and macro pores. This result confirms that DES with a large molecular radius compared to most of solvent can facilitate the formation of large pores. The predominant meso (2−50 nm) and macro (larger than 50 nm) pores remained after drying further suggested that DES can form the pores with much larger size than the dimension of a single solvent molecule (shown in BET data, Table 2). This is probably due to the low free volume and high viscosity of DES’s characteristics. The lower free volume of the DES also likely reduces the contact angle (cos θ) with solid surface that help maintain the pore size during solvent exchange; therefore, little shrinkage was observed during DES and ethanol exchange. Compared to the LXRF, DES can help form large pore size with a lower interfacial surface tension. These characteristics significantly reduced capillary pressure subjected to the pore structures during subsequent solvent exchange and drying. This is the first time a lignin-based cryogel was demonstrated to have a high porous and stable pore structure by freeze-drying process. The use of DES for preparation of lignin based cryogels presents a novel approach to feasibly conquer the shrinkage problem without blending any support or using expensive drying process. Lignin Cryogel Characterization. Fourier transform infrared (FT-IR) spectroscopy was applied to analyze the functional groups presented in final LXRF and LXRF-DES. Figure 5 presented the FT-IR spectra of L75RF and L75RFDES with DESL as a reference sample. Most of the reactions during the gelation are expected to occur on C5 of lignin aromatic nucleus or with the lignin propanyl side-chains. The region of 1900 to 1000 cm−1 provides the key structural information on C−O stretch bond on side-chain and C=C on aromatic rings. Spectra of two cryogel samples showed a similar increase in intensity at 1654 cm−1, which is due to C=C vibration. This suggested resorcinol is participated in the gelation reaction. The aromatic ring structure was not significantly affected during the gelation as the absorbance at
Figure 2. Degree of shrinkage of different lignin gel materials, from left to right is kraft lignin freeze-dried cryogel, ionic liquid lignin freeze-dried cryogel, L75RF, and L75RF-DES.
Figure 3. Schematic pore size comparison between DES and water.
structure dimension during reaction. DES has been evaluated as both solvent-template system as well as gel precursor.48 A recent research has shown that resorcinol-formaldehyde based carbon monoliths and carbon nanotube formed in ethylene glycol-choline chloride DES solvent has a higher dimensional stability.49,50 We then prepared cryogels, LXRF-DES, in ChCl-Lac DES with 0%, 25%, 50%, and 75% substitution of resorcinol by DES lignin. After gel formation, the DES was exchanged consecutively with ethanol and tert-butanol prior to freezing
Figure 4. (a) L75RF-DES before and after freeze-drying. Microscopic structure of L75RF-DES under SEM (b) at 5 μm and (c) at 2 μm. D
DOI: 10.1021/acs.macromol.8b02279 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. Surface Area and Specific Pore Volume of LXRF-DES sample
SSABETa (m2/g)
SSAmicrob (m2/g)
SSAexternal (m2/g)
Vmicroc (cm3/g)
Vmesoc (cm3/g)
Vtotalc (cm3/g)
mean pore size (nm)
RF-DES L20RF-DES L50RF-DES L75RF-DES
230.5 161.9 172.8 76.6
3.01
