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Chapter 22
Bioadvantaged Nylon from Renewable Muconic Acid: Synthesis, Characterization, and Properties Sanaz Abdolmohammadi,1,2 Nacú Hernández,1 Jean-Philippe Tessonnier,1,2 and Eric W. Cochran*,1 1Department
of Chemical and Biological Engineering, Ames, Iowa 50011, United States 2NSF Engineering Research Center for Biorenewable Chemicals (CBiRC), Ames, Iowa 50011, United States *E-mail:
[email protected].
We present a new route for converting glucose into a bioadvantaged monomer through a hybrid process combining biological and chemical conversions. This monomer gives access to a new family of unsaturated polyamides (bioadvantaged nylon 6,6) with superior performance compared to their petrochemical counterparts. Specifically, we demonstrate that this chemical enables the introduction of new functionalities, hereby facilitating the synthesis of nylon 6,6 with tailored functional properties.
Introduction The adverse effects of fossil carbon on the environment are driving the search for alternative and more sustainable feedstocks for the production of fuels and chemicals. In this regard, biomass represents a promising source of renewable carbon for the synthesis of a wide variety of monomers and polymers (1, 2). Hence, bio- and chemocatalytic routes are being developed to fractionate biomass into carbohydrates and lignin followed by their conversion into oxygen-rich platform molecules (1, 3–11). Dicarboxylic acids are a large class of oxygen-rich molecules that can be produced via biological catalysis and that have a wide variety of applications spanning plastics, coatings, and adhesives (12, 13). These broad applications © 2018 American Chemical Society
have attracted many researchers and encouraged them to explore new pathways for the production and utilization of biobased diacids (14–19). Hybrid processes combining biological and chemical conversions have emerged as an effective strategy for building-block diversification (7, 8). For this strategy, sugars are first converted to a platform intermediate through fermentation using metabolically engineered bacteria or yeast. The biologically produced intermediate is then further converted to the target diacid using a chemical or chemocatalytic step. However, challenges associated with separation and purification costs, low conversion rates, as well as catalyst deactivation were reported while trying to transform biomass into higher value compounds (8, 20). It is to mitigate these issues that electrochemistry was proposed as an alternative to conventional chemocatalytic conversions, in particular for hydrogenation reactions (20–22). Electrochemical hydrogenation (ECH) offers several key advantages for the conversion of biologically derived intermediates. In ECH, hydrogen is generated at the surface of the working electrode material (cathode) through the hydrogen evolution reaction (23–25). This approach suppresses the mass transfer limitations typical for three-phase reactions and replaces natural gas by water as a source of hydrogen (26–29). In addition, the Earth-abundant metals used as electrode materials are resistant to biogenic impurities, thus offering opportunities to convert the intermediate directly in the fermentation broth without any separation, which significantly drops the production costs (21). Muconic acid (MA), a biologically produced C6 diunsaturated dicarboxylic acid, has recently been identified as a bioprivileged molecule (30). The structure of this platform compound makes it a unique intermediate for the production of both drop-in monomer replacements and novel chemicals (Figure 1) (17, 22, 30–35).
Figure 1. Value-added products obtained from biobased muconic acid. (Reproduced with permission from ref. (22). Copyright 2016 American Chemical Society.) 356
Full hydrogenation of MA yields adipic acid (AA), the most prevalent dicarboxylic acid with a global market size over 2.7 million tons per year. AA is mainly used as a monomer in the synthesis of nylon 6,6 and as a plasticizer in the production of poly(vinyl chloride) (PVC) and poly(vinyl butyral) (PVB) (30, 36–38). In addition to conversion of MA to large-scale commodity chemicals, electrochemistry gives access to promising novel species, including the monounsaturated monomer trans-3-hexenedioic acid (t-3HDA) (20–22). The presence of a double bond advantageously positioned in the center of t-3HDA provides opportunities for insertion in the nylon backbone and subsequent functionalization, thereby introducing novel properties in a commodity polymer. So far, t-3HDA is not easily accessible from petrochemical building blocks, but it is obtained through ECH of MA with selectivity and yield of higher that 94% (22). Hence, biomass provides new opportunities for the synthesis of bioadvantaged polymers with superior functional properties compared to its parent polymer derived from petroleum.
Combining Metabolic Engineering and Electrochemistry Recently, we reported a unique approach of utilizing an engineered strain of Saccharomyces cerevisiae yeast for the conversion of sugar into MA, with the highest MA titer reported in yeasts (21). Furthermore, this biologically derived MA was converted into the bioadvantaged monomer t-3HDA with 94% yield through the electrochemically hydrogenated pathway in the presence of biogenic impurities without any separation step and at low pH (21). Our successful ECH of MA to t-3HDA with optimized condition is illustrated in Figure 2. As a summary, the reaction was carried out in a three-electrode jacketed cell at room temperature and atmospheric pressure. The total reaction time was 1 h and the progress of the reaction was monitored by withdrawing 0.5 mL samples at 5, 15, 30, and 60 min for high performance liquid chromatography (HPLC) and 1H nuclear magnetic resonance (NMR) analysis. The fermentation broth was electrochemically hydrogenated by potentiostatic control with an applied potential of -1.5 V vs Ag/AgCl on a 10 cm2 lead rod. Lead was chosen as an electrode material (working electrode) due to its Earth abundance, low cost, and resistance to biogenic impurities. Figure 2a shows a 95% conversion of MA and 81% selectivity of t-3HDA within 1 h, as well as no evidence of catalyst deactivation within the 5 successive runs. Figure 2b shows the yield under the optimized reaction conditions using the fermentation broth as a substrate. According to the results, slightly higher MA conversion 96 ± 2% with 98 ± 4% selectivity toward t-3HDA is reported in acidic media with a pH of 2.00.
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Figure 2. (a) ECH of MA to t-3HDA with five successive runs. (b) ECH of MA in acidic fermentation broth with pH of 2.0. (Reproduced with permission from ref. (21). Copyright 2016 John Wiley and Sons.)
In addition, with the basis of the current yield and MA titer, our early stage technoeconomic analysis revealed that the cost for production of t-3HDA could be at approximately $2.00 Kg-1. Hence, the low production cost of t-3HDA will be one of the key parameters of hybrid metabolic engineering and ECH . Figure 3 shows the process flow diagram for the production of t-3HDA used in the technoeconomic analysis (TEA) (20).
Figure 3. Anticipated overall process flow diagram for the production of t-3HDA through fermentation and ECH . (Reproduced with permission from ref. (20). Copyright 2016 American Chemical Society.) 358
To investigate more on the effect of bioadvantaged monomers as the backbone of polymers, t-3HDA replaced AA as the monomer used in the synthesis of unsaturated polyamides, such as nylon 6,6.
Production of Polyamides from Sugar Hence, a bioadvantaged nylon (BAN) was prepared via a polycondensation reaction between t-3HDA, AA, and hexamethylenediamine (HMDA). AA and t3HDA with the molar ratio of x:(1-x), respectively, were both dissolved separately in methanol (CH3OH), and the resulting solution was mixed with a 1:1 molar ratio with HMDA dissolved in CH3OH. Then the reactants were heated in a roundbottom flask at 60 ˚C. The precipitated salt was filtered, washed with CH3OH and left to dry in a fume hood. To complete the polycondensation, the resulting salt was mixed with deionized (DI) water and heated up to 250–270 ˚C under an N2 purge, and then cooled to room temperature (21). The differences in the color and physical properties of bioadvantaged nylon compared to nylon 6,6 are shown in Figure 4. BAN samples are named based on the molar ratio of t-3HDA. Increasing the amount of t-3HDA changed the color of nylon 6,6 from translucent to clear for BAN100.
Figure 4. Nylon blends with various molar ratios of t-3HDA.
Results and Discussion Thermal characterization was performed using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). DSC of the polymer powder was conducted using a DSC Q2000 (TA Instruments) in aluminum hematic pans by three consecutive heating and cooling cycles between 25 and 300 °C, at a heating-cooling rate of 10 °C/min under a 50 mL/min N2 flow. TGA measurement of all samples was carried out using a NETZSCH model STA 449 F1 Jupiter thermogravimetric analyzer, on 3–5 mg samples placed in alumina crucibles. The samples were heated from room temperature to 700 ˚C with a heating rate of 10 ˚C/min. Nitrogen with a flow rate of 20 mL/min was used to maintain an inert atmosphere. 359
Thermal properties of nylon 6,6 and BAN samples are shown in Figure 5. DSC results in Figure 5a illustrate that the existence of the unsaturated double bond decreases the melting point for bioadvantaged nylon. Commercial nylon 6,6 exhibited a melting temperature (Tm) of 253 ˚C, however, for BAN 50 the Tm decreased to 172 ˚C. This phenomenon is attributed to the incorporation of t-3HDA into the polymer structure disrupting the crystalline structure of polymer, resulting in the reduction of the Tm (39–41). The decrease of about 81 ˚C in the melting point of BAN 50 makes it suitable for applications that require lower processing temperatures. Above 50% of t-3HDA loading, the DSC trace of BAN 100 shows the existence of only a glass transition temperature (Tg) at 59 ˚C and no visible Tm, indicating an amorphous structure of the biobased polyamide. Unfortunately, DSC is not sensitive enough for detection of glass transition temperatures of polyamide; therefore, further investigation has been done using Dynamic Mechanical Analyzer (DMA) studies. Figure 5b shows the thermal decomposition of nylon 6,6 and BAN samples. Similar to saturated nylon 6,6, the unsaturated and bioadvantaged nylon has the same decomposition temperature range between 320–500 ˚C (42). With the addition of unsaturated bonds in the structure of nylon, the thermal stability of the bioadvantaged nylon determined at 50% weight loss of sample (Td50) slightly shifts to a higher temperature. The value of Td50 for nylon 6,6 varies from 431 ˚C to 447˚C for BAN 50 and 452 ˚C for BAN 100. The higher value of Td50 for BAN samples shows that the addition of t-3HDA has no negative effect on the stability of the polyamide structure (41). The higher stability temperature makes bioadvantaged nylon a good candidate for melt processing.
Figure 5. (a) DSC comparison of nylon 6,6 and BAN samples from 25 ºC to 300 ˚C. (b) TGA plot of comparison of nylon 6,6 and BAN samples from 40 ºC to 700 ˚C. 360
The mechanical behavior of the samples was determined using the DMA mode of a TA ARES-G2 instrument. Two unsaturated BAN samples, one semicrystalline (BAN 50) and one amorphous (BAN 100) were compared with semicrystalline nylon 6,6. The storage modulus (E’) and the α-relaxation temperature (Tα) are summarized in Table 1. Nylon 6,6 shows the highest storage modulus compared to BAN 50 and BAN 100 which is due to the higher degree of crystallization of nylon 6,6 compared to amorphous BAN 100 (43). Incorporation of t-3HDA in the BAN structure has an effect on the chain mobility of the polymer, resulting in a decrease in the α-relaxation temperature when increasing t-3HDA content.
Table 1. Storage modulus @ 30 ˚C and 1 Hz and α-relaxation temperature calculated from peak maxima in Tan δ Sample
Storage modulus (MPa)
α-relaxation temperature (°C)
Nylon 6,6
2.45 × 103 ± 515
62.43 ± 2.02
103
34.02 ± 0.23
BAN 50
1.24 ×
BAN 100
5.41 ± 3.13
23.03 ± 1.64
The crystalline structures of BAN samples and nylon 6,6 were studied by wide-angle X-ray scattering (WAXS) using a XENOCS Xeuss 2.0 SWAXS system with an X-ray wavelength of λ = 1.542 Å from Cu Κα radiation. Figure 6 shows WAXS data comparing nylon 6,6 and BAN samples at room temperature. The diffractogram of BAN 50 is very similar to that of nylon 6,6 consisting of both an amorphous and a crystalline part. The two characteristic peaks of nylon 6,6 are at a q of 1.42 and 1.64 (Å-1) approximately, corresponding to (100) and (010)/(110) doublet, respectively. These characteristic peaks correspond to intrasheet and intersheet scattering and represent the α-phase of the triclinic nylon 6,6 (44–46). Upon the addition of t-3HDA, the intensity of the (010/110) peak decreases, which translates into a lower crystallinity for BAN 50 compared to nylon 6,6. Further addition of unsaturated diacid completely diminishes the (010/110) peak, indicating a change in crystallinity from semicrystaline nylon 6,6 and BAN 50 to amorphous BAN 100. DSC data corroborate these results, as above 50% t-3HDA loading the DSC curve only shows a glass transition, in agreement with an amorphous structure.
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Figure 6. WAXS patterns of nylon 6,6 and BAN samples at room temperature.
Figure 7 shows transmission electron microscopy (TEM) images of nylon samples with less than 50% of t-3HDA loading (nylon 6,6), as well as more than 50 % of t-3HDA loading (BAN 60). The TEM grids were prepared by dispersing the samples in 1,4 butanediol, and placing a droplet of the nylon dispersion on the TEM grid followed by drying in a vacuum oven at 60 ˚C. In Figure 7a there is an obvious existence of crystalline structures in nylon 6,6 as shown by sheet-like formations which are randomly dispersed in the structure (47). However, increasing t-3HDA to approximately 60% formed aggregation in the structure of BAN 60 without any sheet-like thin film dissociation (see Figure 7b). In addition, Figure 7c shows there is some orientation in the electron diffraction pattern of nylon 6,6 samples, which can be attributed to the semicrystalline structure of nylon 6,6; however, in Figure 7d existence of the circular halo ring is corresponding to the amorphous structure. These apparent differences in the structure of nylon samples is supported by the WAXS studies presented above.
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Figure 7. (a,b) TEM images of nylon 6,6 and BAN 60, respectively. (c,d) Selected area electron diffraction (SAED) patterns of nylon 6,6 and BAN 60, respectively.
Applications Nylon 6,6 is selected because this polyamide has the capability of being synthesized from biobased-derived monomers. Nylon 6,6 is a versatile polyamide produced from polycondensation of AA and hexamethylenediamine. It is used for various applications (such as automotive parts, electronics, food packaging, as well as high-strength fiber) or at elevated temperatures (42, 48, 49). Despite all the advantages of nylon (e.g., resistance to high temperatures and chemicals, processing flexibility, and toughness), there are some drawbacks, such as moisture absorption and poor surface wettability (50). The alkene bond in the structure of t-3HDA is amenable to a variety of functionalization strategies; thus, adding t-3HDA to nylon 6,6 forms a novel bioadvantaged nylon. 363
There is a great interest in replacing petroleum-based polymers by green alternatives. This study presents a value opportunity for bioadvantaged polymers derived from biological monomers such as MA. The existence of an unsaturated bond in the bioadvantaged nylon can enable its functionalization through sulfur vulcanization, or crosslinking, or allow the attachment of different chemical groups via thiol-ene chemistry. These pendant groups can enhance polymer properties, such as hydrophobicity, flame resistance, and antistaticity (20, 39). Figure 8 displays potential applications of t-3HDA in the functionalization of unsaturated polyamides.
Figure 8. Applications of t-3HDA that enhance properties of polyamides. (Reproduced with permission from ref. (20). Copyright 2016 American Chemical Society.)
Conclusion Overall, this study demonstrates the synthesis of a bioderived monomer and its incorporation into a novel bioadvantaged polyamide. In addition, this work provides insight into ECH reactions of other biorenewable compounds. Moreover, it involves determination of key conditions and parameters in both selective ECH and polymerization processes (e.g., choice of catalyst, concentration of the media, pH, and temperature) to synthesize bioadvantaged polymers with enhanced properties.
Acknowledgments This material is based upon work supported in part by the National Science Foundation grants EEC-0813570, CBET-1512126, IIP-1701000, and NSF-DMR 1626315. Research at the Ames Laboratory was supported by the U.S. Department of Energy-Laboratory Royalty Revenue (DE-AC02-07CH11358). 364
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