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Review

Catalytic Production of Functional Monomers from Lysine and Their Application in High-Valued Polymers

by
Kangyu Liu
1,
Bingzhang Shao
1,
Bo Zheng
1,* and
Baoning Zong
2
1
National Engineering Research Center for Petroleum Refining Technology and Catalyst, Research Institute of Petroleum Processing Co., Ltd., SINOPEC, Beijing 100083, China
2
State Key Laboratory of Catalytic Material and Reaction Engineering, Research Institute of Petroleum Processing Co., Ltd., SINOPEC, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(1), 56; https://doi.org/10.3390/catal13010056
Submission received: 9 December 2022 / Revised: 20 December 2022 / Accepted: 21 December 2022 / Published: 27 December 2022
(This article belongs to the Special Issue Green Chemistry & Engineering towards Zero-Carbon Goals)
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Abstract

:
Lysine is a key raw material in the chemical industry owing to its sustainability, mature fermentation process and unique chemical structure, besides being an important nutritional supplement. Multiple commodities can be produced from lysine, which thus inspired various catalytic strategies for the production of these lysine-based chemicals and their downstream applications in functional polymer production. In this review, we present a fundamental and comprehensive study on the catalytic production process of several important lysine-based chemicals and their application in highly valued polymers. Specifically, we first focus on the synthesis process and some of the current industrial production methods of lysine-based chemicals, including ε-caprolactam, α-amino-ε-caprolactam and its derivatives, cadaverine, lysinol and pipecolic acid. Second, the applications and prospects of these lysine-based monomers in functional polymers are discussed such as derived poly (lysine), nylon-56, nylon-6 and its derivatives, which are all of growing interest in pharmaceuticals, human health, textile processes, fire control and electronic manufacturing. We finally conclude with the prospects of the development of both the design and synthesis of new lysine derivatives and the expansion of the as-synthesized lysine-based monomers in potential fields.

1. Introduction

Human civilization has come a long way with the progress of technology and the utilization of fossil resources. Since the middle of the 18th century, fossil resources, including coal, crude oil and natural gas, have brought prosperity and development to human society in succession [1,2,3]. Today, refinery processing with crude oil or coal continuously provides both bulk chemicals and value-added fine chemicals in million-ton scales, and it is expected that the fossil-based economy will still be flourishing through the 21st century. According to reports from the UN, “the world’s population is expected to increase to 9.7 billion in 2050 and could peak at nearly 11 billion around 2100” [4,5]. Such a rapid rise in population over the next few decades will exert a great impact on fossil fuel reserves. In addition, it should be noted that the increased energy consumption correlates with CO2 emissions, which causes climate change and a series of environmental problems [6,7,8]. These unavoidable subjects force humans to develop innovative solutions for the replacement of the finite fossil resources. Thus, the contribution of wind, hydropower, biomass and other explored sustainable strategies for overall energy demand has emerged [9,10].
In contrast to water, wind and solar [11], biomass is the only renewable resource that can be used as both energy and carbon sources, thus holding a crucial role in the sustainable production of carbon-based end products [12,13]. From the point of sustainability, biomass has a wide range of excellent merits, including reducing pollution from industry, diminishing hazardous chemicals production and complying with the carbon cycle of the global ecosystem [14,15,16]. More importantly, biomass is virtually inexhaustible with an estimated global production of 1.7 × 1011 tons per year [17,18]. Such tremendous reserves are sufficient to fulfill human demand as the starting material to produce various carbon-based commodities in the future. Among various biomass-derived materials, lysine (Lys or K) is considered to be one of the most promising chemicals and has been intensively studied in recent years [19].
As an essential amino acid of living creatures [20], lysine is responsible for the production of collagen and the absorption of calcium in living bodies [21,22]. On an industrial scale, lysine is commonly produced by fermenting sugars [23]. Thanks to the development of genetic engineering, various molecular techniques have been developed to improve the production of lysine [24,25], which thus drove to a leap forward in the development of its applications in food and pharmaceuticals [26].
Owing to its nutritional and commercial value, the market for lysine is growing steadily. According to the reports from Maximize Market Research (MMR), the lysine market is expected to grow at a rate of 7.5% CAGR from 2021 to 2027, reaching almost USD 13.05 billion in 2027 [27]. At present, the annual global productivity of lysine is ca. 386.1 million tons, only second to glutamic acid [28]. The major worldwide suppliers of lysine include Archer Daniels Midland (Chicago, IL, USA), Evonik Industries (Essen, Germany), Ajinomoto (Tokyo, Japan), etc. [29,30]. Over the past few years, China has become the world’s greatest lysine supplier owing to the boosted demand for food and dietary supplements, with a productivity of ca. 284.5 million tons in 2021. Such an enormous productivity covered ca. 73% of global supplements. Eppen, Changchun Dacheng and Meihua Bio are the major Chinese lysine suppliers.
The boosting establishment of lysine industries around the world has caused fierce market competition, which thus drove the price of lysine to decrease to ca. $1.5 per kilogram since 2013 [28,31]. Such an unprofitable market environment forces enterprisers and researchers to extend the application of lysine. Interestingly, the two amines and one carboxylic acid function groups on its backbone give lysine great potential to be a platform chemical to replace fossil resources for polymer synthesis. Currently, most monomers from the polyamide and nylon families are produced from fossil refineries. Replacing fossil-based monomers with lysine derivatives would greatly expand the lysine industry and help to develop novel utilizations for bio-based commodities (Figure 1).
Owing to the unique chemical structure and considerable global productivity of lysine, much more interest and attention have been paid to the production of lysine-based derivatives. In recent decades, massive important achievements have been achieved, and their synthetic process, downstream applications and commercial prospects have also been extensively investigated. Hence, a comprehensive and real-time review is needed.
This review aims to provide a thorough overview of catalytic synthesis strategies of industrially important lysine-based monomers and production routes for the high-valued polymer. This review consists of two main parts. The first part (Section 2) introduces the production strategies of functional monomers starting from lysine, and the other part (Section 3) summarizes the syntheses, properties and applications of polymers produced from the lysine-based monomers. Perceptions regarding how lysine-based commodities will develop, especially via green and sustainable strategies, are discussed throughout the review.

2. Catalytic Production of Chemicals from Lysine

2.1. ε-Caprolactam

ε-Caprolactam (CPL) is an important monomer used for the fabrication of nylon-6 polymers, which is the most commonly used raw material for synthesizing fibers and resin [32,33]. The global production of CPL is anticipated to surpass 10 million tons per year in 2023. Specifically, Asian regions holds a major share in global CPL output due to the commissioning of new and expansion of existing facilities and will be expected to grow at ca. 4% per year in the next few years [34]. The fossil-based CPL production process has gone through a long period of development and evolution (Figure 2). In 1919, the IG Farben company took the lead in developing the CPL synthesis process, and the first industry for CPL production was established in 1943. Phenol was used as the starting material, from which cyclohexanone was produced and then converted to the corresponding oxime with hydroxylamine. In this process, hydoxylamine was produced by the Raschig method [35]. BASF replaced the Raschig method with the NO reduction method to produce hydroxylamine in 1956 [36]. Since the 1970s, several new technologies including the boric acid (BA) and hydroxylamine phosphate (HPO) methods invented by Mitsubishi Chemical, Du Pont and Royal DSM N.V. have been applied, which thus drove a sharp increase in global CPL productivity. In the 1990s, SINOPEC developed a series of sustainable technologies for CPL production, including a green oxidation process for cyclohexanone synthesis from benzene, gas-phase Beckmann rearrangement of cyclohexanone oxime and CPL purification with amorphous Ni-based alloy catalysts (Figure 3) [37,38]. These synthesis strategies greatly expand CPL productivity and supported China to dominate worldwide CPL production. Currently, 90% of CPL is produced via the aforementioned cyclohexanone route. However, the unavoidable complicated processes and fossil-based raw materials bring CPL an uncompetitive prospect. An alternative route for CPL synthesis with renewability, eco-friendliness and economic viability is highly desirable. To this extent, lysine is an ideal raw material with low-cost and green separation processes.
J. Frost first developed a route for the production of CPL from lysine [39]. In this process, lysine first underwent cyclization forming α-amino-ε-caprolactam (α-ACL) at ca. 190 °C using 1,2-propanediol as the solvent, and then deamination was subsequently carried out using a stoichiometric amount of KOH and hydroxylamine-O-sulfonic acid at ca. −5 °C resulting in 99% molar yields of CPL. In order to explore a more sustainable process, Frost converted lysine monohydrochloride to CPL with a Pt-S/C catalyst [40]. A 15% molar yield of CPL was obtained in a one-step process under H2 and H2S atmospheres. H. Kim et al. invented an advanced two-step strategy to produce CPL [41]. Lysine monohydrochloride was converted to 2-aminocaprolactam by using NaOH and Al2O3 at 117 °C in 1-butanol, followed by using hydroxylamine-O-sulfonic acid to break the C-N bond of the α-amine group at room temperature. The optimum yield of CPL reached as high as 87%. Analogously, P. Preishuber-Pfluegl et al. found that 2-aminocaprolactam could be formed in the mixture of lysine, NaOH, ethylene glycol and water at pH 10 at 190 °C, and CPL could be subsequently obtained in 87% yields [42]. According to these studies, the efficient conversion of lysine to CPL lies in selectively breaking the C-N bond at the α-amine group under a mild condition. To improve the reactivity of the C-N bond at the α-amine group, M. Mochizuki developed a three-step strategy [43]. Lysine was transformed into 6-amino-2-chlorohexanoic acid with hydrochloric acid and nitrous acid. The chloride group could be easily removed by using a Palladium catalyst under a H2 atmosphere, and the obtained 6-aminohexanoic acid reversed into the cyclic type (CPL) in tetraethylene glycol at 185 °C.
To develop a green and simple manner for CPL synthesis from lysine, T. Zhang et al. report the one-pot catalytic conversion of lysine to CPL by using an Ir/HBeta zeolite catalyst [44]. Over this multifunctional catalyst, acid sites catalyzed the cyclization of lysine and then synergistically broke the C-N bond of the α-amine group at hydrogenation sites, thus giving an ideal CPL yield (58%). Recently, K. Berger et al. developed a versatile deamination method for amines [45]. They used Cesium carbonate and propanoic acid as the catalysts to convert lysine monohydrochloride to CPL in acetonitrile with a 58% molar yield.

2.2. α-Amino-ε-Caprolactam (α-ACL) and Its Derivatives

As a rising valuable bio-based platform compound, α-ACL has exhibited varied applications in producing polymer units, additives and therapeutical drugs (Table 1). Because of the modifiability of the α-amino group, α-ACL can be easily derived and utilized in various fields. Compared with the industrial process and appreciable worldwide productivity of CPL, α-ACL and its derivatives perform poorly in industrial production but have a reliable manufacturing potential.
Several strategies have been explored for α-ACL production. H. Matsumoto et al. transformed lysine to α-ACL in a methanol/H2O mixture at 250 °C, using NaOH as the hazardous catalyst [46]. T. Phi et al. [47] and I. Jerman et al. [48] developed a similar route for α-ACL synthesis with basic chemicals (NaOH and pyridine) as catalysts. Considering the recoverability and contamination of the homogeneous basic catalysts, H. Matsumoto et al. improved the process without using NaOH, yielding 76% α-ACL [49]. In addition, hexamethyldisilazane and titanium isopropoxide with neutral pH properties were also applied in α-ACL production, but only yielded 15% and 27% α-ACL, respectively [50,51].
Due to the diversity in chemical structures, properties and application fields, it is a real challenge to compose a comprehensive review of α-ACL derivatives. Generally, α-ACL derivatives are designed and used as functional monomers, additives and therapeutical drugs. Herein, we mainly focus on the production and utilization of several representative α-ACL derivatives in these three fields.
Table
Table 1. Derivation and utilization of α-ACL.
Table 1. Derivation and utilization of α-ACL.
EntryRaw
Material		Reaction
Condition		Chemical StructureApplication FieldsRef.
1Lysine hydrochlorideHCHO, H2, Pd/C, room temp.Catalysts 13 00056 i001Functional polyamide[52]
22,5-Hexanedione, AcOH, NaOHCatalysts 13 00056 i002[53]
3Benzyl chloride, refluxCatalysts 13 00056 i003[53]
4α-ACLPerfluorobutyl chloride, ice bathCatalysts 13 00056 i004[54]
5α-ACLAcryloyl chlorideCatalysts 13 00056 i005Functional polyethylene[55]
6Lysine hydrochloride1,4-Diethoxybutane, t-BuOK, acetyleneCatalysts 13 00056 i006[56]
7α-ACL2,3,4,5,6-Pentafluorophenyl (2R)-2-[[formyl(phenylmethoxy)amino]methyl]hexanoate, H2, Pd/C, room temp.Catalysts 13 00056 i007Antibacterial agent[57]
8α-ACLDiacyl chlorideCatalysts 13 00056 i008Polymer chain extender[58]
9α-ACLDi-tert-butyl decarbonate, room temp.Catalysts 13 00056 i009Coagulation inhibitor[59]
10α-ACLCarboxyl derivative, ice bathCatalysts 13 00056 i010Treating visceral dyskinesia[60]
11α-ACLSulfone derivative, 3eq HClCatalysts 13 00056 i011HIV integrase inhibitor[61]
Owing to the lactam group, α-ACL and its derivatives possess a reliable potential in polyamide synthesis. Y. Tao et al. adopted a series of α-amino protecting groups [methyl (Table 1, Entry 1), 2,5-dimethylpyrrole (Table 1, Entry 2), aryl (Table 1, Entry 3), Boc, Cbz, etc.] [62] to produce α-ACL derivatives. These derivatives can be utilized to produce high-value functionalized polymers (See detailed discussion in Section 3.2 and Section 3.3). An ingenious design was developed by E. Tarkin-Tas who modified the α-amino with a vinyl group (Table 1, Entry 5). Such a novel derivative was polymerized via vinyl groups and forced the lactam group pendant onto the polymer backbone [55]. Tao et al. derived the monomer bearing pendant 2,5-dimethylpyrrole-protected amino group with the vinyl group to yield a sustainable N-vinylcaprolactam (Table 1, Entry 6). The polymerization was attributed to the vinyl group but not the ROP of the amide group in the as-synthesized N-vinylcaprolactam derivatives. The polymer performed pH-regulated thermos-responsiveness with the cleavage of pendant 2,5-dimethylpyrrole protecting groups [56]. Different from the common polyolefins, this obtained functional polyethylene displayed both high glass transition point (Tg) values and unexpectedly large sub-Tg transitions that are believed to be due to the relaxation of hydrogen bonding of pendant ACL units, which thus allows the pendant group to rotate flexibly. P. Beckett et al. found that compounds with a specific N-formyl hydroxylamine function can be used as antibacterial agents. Particularly, α-ACL grafted with a branched amide structure (Table 1, Entry 7) possesses such a N-formyl hydroxylamine function, which has excellent antibacterial activity and provides a viable application for this compound in treating bacterial contamination [57]. W. Ji et al. developed a specific α-ACL dimer linked with two amide groups and an alkane chain (Table 1, Entry 8). Due to the symmetrical structure with multi-amide groups, the designed compound can be utilized in increasing the intrinsic viscosity, length of the polymer chains and branch degree of polyamides and polyesters [58]. As the α-amino group can be modified with ester, ketone, sulfone or other complicated functional groups (Table 1, Entry 9-11), the produced α-ACL derivatives could serve as therapeutical drugs for various diseases [59,60,61]. Different from the above studies, for these sorts of α-ACL derivatives, we still cannot put forward an exact conclusion of the relationship between the modified groups on the α-amino group and the therapeutic indication of the corresponding α-ACL derivatives due to the complex pharmacological mechanism for every drug.

2.3. Cadaverine and Lysinol

Cadaverine (1,5-diaminopentane, Figure 4a) is a potential feedstock both in industry and agriculture [63]. As a diamine with a C5 backbone, it has been used to develop high-valued polyamides and polyurethanes [64,65]. Initially, cadaverine was produced via fermentation of sugars and mineral salts using engineered Escherichia coli [66,67]. These rough strategies only obtained undesired product yields, which thus created a challenge in using cadaverine in industrial-scaled production. The production of cadaverine through the decarboxylation of lysine with lysine decarboxylases was viable with a molar yield of up to 99.9% but offered an expensive production process and complicated recovery of cadaverine [68,69,70]. Hence, the industrial-scale process requires an economical, reusable and stable strategy for the decarboxylation of lysine, which provides a great opportunity for the development of the chemo-catalytic heterogeneous process.
Several approaches have been further explored for cadaverine production. J. Verduyckt et al. reported the decarboxylation of lysine over the commercial Ru/C with a 32% yield of cadaverine [71]. Based on this attainment, Y. Huang et al. developed a series of Ru-based catalysts for the decarboxylation of lysine to cadaverine [72,73]. In particular, zeolite-supported RuO2-catalysts demonstrated a greatly enhanced yield (54%) of cadaverine. The active surface oxygen vacancies in the catalysts facilitated the adsorption of intermediates, and the under-coordinated Ru-O bonds in the ruthenium oxide catalysts were regarded as active surface sites for H2 dissociation. These two factors synergistically benefited the hydrogenation–decarboxylation process of lysine [74]. S. Xie et al. put forward an alternative route of the conversion of lysine to diamines and cyclic amines using a Ru/C catalyst with phosphoric acid (H3PO4) as the cocatalyst [75]. Under the optimum condition, 100% conversion of lysine and 94% yield of amines, including 51% selectivity to diamines, were achieved. In the presence of H2, the hydrogenation of lysine to lysinol is the direct step for the subsequent deoxygenation process. During the catalysis process, H3PO4 protonated the amino groups and helped to minimize the deamination of lysine. The hydroxymethyl group of lysinol (2,6-diamino-1-hexanol) exhibited a reversible dehydrogenation–hydrogenation process on the surface of Ru species, and the obtained aldehyde intermediate acted as the key precursor of cadaverine. Combined with corresponding characterization results, decarbonylation of the aldehydes and the transformation of CO to CH4 on the surface of Ru were considered as the critical steps for the formation of the target product during the catalytic process. As H3PO4 is corrosive and cannot be recycled, it is important to develop a suitable solid acid for the expansion of such a potential catalytic strategy.
Recently, lysinol (Figure 4b) has attracted certain interest due to its similarity with cadaverine and other major classes of petrochemical-derived diamines. K. Moloy et al. used Ru/C catalysts to transform lysine to lysinol under modest conditions (100–150 °C, 48–70 bar, pH 1.5–2) with 100.0% lysine conversion and >90% selectivity. Based on the reported literature of the catalytic hydrogenation of α-amino acids to the corresponding 1,2-amino alcohols [76,77,78], the authors speculated that under low pH conditions (pH < 3), α-amino acids are largely present as the cation, which thus supported the adjacent ammonium cation to render the carboxylic acid functionality more reactive towards reduction. D. Vos et al. further exploited a general methodology that hydrogenating amino acids to bio-based amino alcohols by using Rh-MoOx/SiO2 catalysts. H3PO4 was chosen as the cocatalyst to guarantee a low pH condition throughout the catalytic process. The optimized selectivity of lysinol reached 87% with 96% conversion of lysine at 80 °C and 70 bar H2 [79]. Such an ideal product yield shows the great potential of this synthesis strategy for the valorization of protein-rich biomass waste. Furthermore, owing to the urgent demand for the fabrication of polymers from bio-based monomers and the structural similarity with diamines synthesized from fuel refineries, cadaverine and lysinol are both expected to exhibit great usage in the near future.

2.4. Pipecolic Acid

Pipecolic acid is an important non-proteinogenic amino acid and acts as an intermediate of many pharmaceutical and biological compounds (e.g., the immunosuppressive agents FK506, the anesthetic analogue bupivacaine and ropivacaine, etc.) [80,81,82,83,84]. Currently, the market price of pipecolic acid reaches up to USD 15,000–30,000 per ton due to the lack of an efficient and economic industrial-scaled production strategy. The developed synthesis strategies include biosynthetic and chemosynthetic strategies. The key part of biosynthetic strategies is the production and upgrade of the structure and catalytic reactivity of lysine cyclodeaminase (LCD, Figure 5) [85]. In the presence of LCD, lysine cyclodeamination can be implemented via the condensation of the α-NH2 group and ε-NH2 group of lysine [86]. G. Tsotsou et al. further found that the addition of Fe (II) and glycerol into the biocatalytic system could accelerate the reactivity of LCD [87]. Owing to the delicate structure of cyclodeaminase and the complicated process of fermentation and purification [88,89], the biosynthetic strategies have limited potential in the large-scale production of pipecolic acid.
To date, several pathways have been developed for the production of pipecolic acid, including enantioselective reduction, diastereoselective synthesis, stereoselective transformation and condensation−deamination processes from various reactants [90,91,92]. Among these methods, deamination of lysine to pipecolic acid is considered as the most reliable and sustainable pathway by virtue of the easy access to raw materials and simple reaction procedure. Typically, catalysts holding TiO2 species performed better than other catalytic systems [93]. S. Nishimoto et al. first discovered that TiO2 (anatase crystal phase) supported with Pt nanoparticles could convert lysine to pipecolic acid under photoirradiation at ambient temperatures. The authors attributed the brilliant reactivity to the larger surface area with fewer defects and the higher dispersion of Pt nanoparticles acting as an efficient reduction site for a Schiff base intermediate during the catalytic process [94]. B. Ohtani et al. elucidated the redox-combined mechanism of photocatalytic conversion of lysine into pipecolic acid via rigorous isotope measurements [95]. The detailed influence of the reaction environment on the selectivity of chiral synthesis of pipecolic acid has been found by S. Chuang et al. It was demonstrated that the chiral selectivity toward lysine was found to be governed by the stereo-structure of the adsorbed lysine and the crystal structure of TiO2 [96].

3. Polymers Synthesis from Lysine-Based Monomers

3.1. Poly (α-Lysine)

Poly (α-lysine) (α-PL) consists of lysine residues linked through their α-carboxyl groups and α-amino groups (Figure 6a). Such a specific structure can only be fabricated via chemo-synthetic routes, typically by using dicyclohexyl carbodiimide and 18-crown-6-ether in chloroform as an activating agent for the polycondensation of lysine [97,98]. Due to its polycationic property (similar with poly (ε-lysine), see detailed discussion in Section 3.2), α-PL has been extensively studied as an interferon inducer, antitumor agent and gene transfer agent due to its cationic nature [99,100,101]. For example, α-PL was found to improve drug transport by reducing drug resistance and increase the efficiency of organelle and liposomes fusion in hematocytes [102]. Although α-PL is toxic to humans, its application in therapeutics and pharmaceutics has been developed on the basis of copolymerization with other residues [103]. Y. Yang et al. developed a novel delivery system for cancer treatment which was self-assembled to form a nanoparticle with a negatively charged pH-sensitive α-PL-copolymer [poly (ethylene glycol)-b-poly (L-lysine)-graft-cyclohexene-1,2-dicarboxylic anhydride] and α-PL-copolymer [polypeptide guanidinium-functionalized poly (α-lysine), PLL-Gua] via electrostatic interactions. This neutral nanoparticle killed cancer cells in a dose- and time-dependent manner by inducing cell death via apoptosis [104]. B. Ernst et al. found that mannose-functionalized α-PL can efficiently inhibit the attachment of viral glycoproteins of cells with picomolar affinity, which brought a promising perspective for this functionalized polymer in the development of antibiotic treatments [105].

3.2. Poly (ε-Lysine)

Poly (ε-lysine) (ε-PL) comprises lysine units linked with amide groups formed from their α-carboxyl groups and ε-amino groups, with α-amino groups as pendants on the polymer backbone (Figure 6b). Different from α-PL, ε-PL is nontoxic [106]. Natural ε-PL was accidentally discovered in the culture filtrate of actinomycetes by Shima and his coworker Sakai in 1977 [62]. Following the identification of the constitutional unit, the antimicrobial and antiphage activity of ε-PL were discovered [107,108]. These valuable properties prompted microbiologists to optimize the culture conditions of ε-PL-producing strains, screen new ε-PL-producing strains and develop efficient fermentation processes [109,110,111,112].
The ε-PL obtained from the biosynthetic route usually consists of 25–35 lysine residues, and it has good solubility in water with 7.6 pKa owing to its polar functional groups (i.e., amide and α-amino group) [103]. It was found that the α-amino group with a positive charge played a key role in sterilization. As many bacteria possess a negative charge on the cell membrane [113], the positively charged polymer impelled the collapse of material, energy and information transfer between bacteria cells through electrostatic interactions, which eventually led to apoptosis of the bacteria [107,114]. Intriguingly, the antibacterial activity of ε-PL has a strong correlation with its molecular weight. ε-PL with a molecular weight between 3600–4300 Da exhibited the strongest antibacterial activity. As the molecular weight decreased below 1300 Da, the antibacterial activity was not detected [62]. Owing to its specific antibacterial property, ε-PL has been extensively used in food processing, cosmetic additives, antibacterial fibers, pesticides and pharmaceuticals [115,116,117,118,119,120,121].
Currently, ε-PL is mainly produced via microbial fermentation on an industrial scale, typically using Streptomyces strains and yielding the most expected 3 kDa polymers [122,123]. Similar with other polymers of amino acids, various mechanisms have been proposed for ε-PL biosynthesis [124,125]. It is generally believed that ε-PL synthesis is terminated by aliphatic polyols via esterification. M. Nishikawa et al. also suggested the ε-PL elongation occurs via the incorporation of lysine monomers into the carboxyl terminus of the polymer [126,127]. Although the bioprocess is mature and economical enough for large-scaled production, the dendric (branched) structures obtained from mutant strains and varied molecular weight seem to be a nonnegligible limitation, which results in the composition and properties of the synthesized ε-PL being difficult to regulate [128].
Compared with biosynthesis strategies, chemosynthesis is considered as a reliable strategy for ε-PL with controllable molecular weight and certain chemical structures. Initially, ε-PL was obtained by applying 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide as an activating agent for lysine in an aqueous medium [98]. As there are structural similarity between ε-PL and nylon-6 (See detailed discussion in Section 3.4), ring-opening polymerization (ROP) may be an ideal route for ε-PL synthesis [129,130,131]. Y. Tao et al. developed a series of ingenious ε-PL synthesis approaches. As linear ε-PL cannot be prepared via direct polymerization of α-ACL, a protective group for the α-amino group is necessary. Thus, several protecting groups such as Boc, Cbz and phthaloyl (Pht) were adopted to acylate α-ACL [53,62]. These classical protecting groups would lead to either undesired proton abstractions from the α-amide or easily be attacked by anionic species, resulting in double anionic propagation species or branched polymer chains during polymerization. On the basis of these studies, an uncommon amino protecting group, 2,5-dimethylpyrrole (Table 1, Entry 2) was developed to synthesize the desired linear ε-PL. The 2,5-dimethylpyrrole used to protect α-ACL was synthesized by a p-toluenesulfonic acid (TsOH)-catalyzed condensation reaction between the amino group of α-ACL and 2,5-hexadiione [53]. The as-synthesized ROP products were further reacted with hydroxylamine/triethylamine in refluxed THF to deprotect 2,5-dimethylpyrrole amino groups, and the expected ε-PL would be obtained.

3.3. Functionalized Poly (ε-Lysine)

Inspired by both the advantageous and undesired properties of ε-PL, many efforts have been made to derive ε-PL to feature varied properties for suitable applications. R. Rosal et al. incorporated poly (acrylic acid)/poly (vinyl alcohol) electrospun nanofibers with ε-PL. Such a dressing resulted in two orders of magnitude lower bacterial colonization than non-functionalized nanofibers after 14 days of incubation [118]. Y. Tao et al. developed several derivatives of α-ACL pendants with benzyl-protected hydroxyl, allyloxy and oligo-ethylene glycol groups. Different from the previous studies [53,62], these pendant moieties are stable and cannot be removed. They suggested that these as-synthesized ε-PLs had great potential for biomedical applications owing to their variable glass transition temperatures, minimal cytotoxicity and high solubility in water [132]. Recently, Y. Tao et al. developed an organocatalyzed ROP method for dimethyl-protected cyclic lysine (Table 1, Entry 1) that avoids subsequent deprotection of the pendant to prepare cationic poly (ε-lysine) mimics with quaternary ammonium groups. Such a functionalized ε-PL displayed potent antimicrobial activities and good biocompatibility [52,133]. K. Matsumura et al. fabricated a hydrogel composed of aldehyde-functionalized dextran and succinic anhydride-treated ε-PL. As a self-degradable adhesive, the applications in hemostatic agents, sealants, wound dressings for endoscopes and anti-adhesion materials have also been investigated owing to its unique backbone and multiple derived functionalities [134,135]. W. Xue et al. developed an injectable and high-efficiency gene delivery hydrogel with arginine-functionalized ε-PL and α-cyclodextrin via host-guest interactions [136]. Similarly, V. Kokol et al. fabricated a type of functional macromolecule with gelatine-conjugated ε-PL. The macromolecules had great antibacterial activity owing to the intrinsic properties (as mentioned in Section 3.2) of ε-PL and did not induce cytotoxicity in osteoblasts [137].

3.4. Nylon-6 and Its Derivatives

Nylon-6 was first discovered by P. Schlack in 1938 [138]. It possesses a similar backbone with nylon-66 but has an easier fabrication procedure both for monomer synthesis and polymer production. Generally, industries used water to catalyze ring-opening polymerization of CPL at high temperatures (240–280 °C) owing to the lack of the controllability of molecular weight and polymerization reproducibility when using a strong base catalyst under anhydrous conditions [139,140,141]. In the 1960s, the improvements in anionic ring-opening polymerization (AROP) offered higher rates of polymerization and low residual monomer content [142,143,144,145]. Compared with other thermoplastic materials (i.e., polyethylene, polypropylene, Polymethyl methacrylate, etc.), nylon-6 possessed greater strength, toughness, abrasion resistance and thermal resistance, which thus brought a flourishing nylon-6 market [146,147]. Since they have a similar backbone as ε-PL, the production strategies and application of functional nylon polymers had a promising future.
T. Emrick et al. fabricated a derived CPL pendant allyl group at α-methylene of the CPL backbone. The functionalized nylon-6 was prepared by anionic ring-opening copolymerization of pure CPL with the allyl-group-derived CPL. It was believed that the pendent allyl groups provided a pathway for nylon-6 in the application of cross-linked polyamide films and gels [148]. For pure nylon-6, the service life and applicability are limited because of its sensitivity to UV radiation [149]. D. Tunc et al. eliminated such a disadvantage and developed a novel photo-reactive functional nylon material modified with cinnamoyl chloride. This functionalized nylon-6 copolymer was synthesized from α-cinnamoylamido-ε-caprolactam (functionalized α-ACL) and CPL, exhibiting both excellent photo-reactivity and thermo-reactivity upon UV light irradiation or heating (Figure 7a) [150]. Comparing with the aforementioned report, the modification of the α-amino group on α-ACL is more efficient than the direct functionalization of CPL, which facilitates the production of functional nylon polymers. Such a specific pendant structure facilitated the polymers applied in electronic manufacturing. Y. Tao et al. fabricated nylon copolymers using dimethyl-protected cyclic lysine (α-ACL) and CPL on the basis of their previous study, which afforded good physical and mechanical properties and strong antimicrobial activities (Figure 7b) [52,151]. According to the quaternary ammonium group pendants on the backbone, we surmise a similar sterilization mechanism as ε-PL with this copolymer. S. Carlotti et al. produce a specific aliphatic polyamide bearing fluorinated groups, perfluorobutyryl-substituted α-ACL and CPL by AROP. The fluorinated monomer was obtained by the reaction between the α-amino group of α-ACL (Table 1, Entry 4) and perfluorobutyryl chloride. The obtained copolymer demonstrated better thermal stability and higher hydrophobic surface character than pure nylon-6 owing to the pendant aliphatic fluorinated groups, which exhibited great potential in self-cleaning materials, food preservation and house decorating materials (Figure 7c) [54]. These representative reports can be summarized in a unified strategy that the production of nylon copolymers can be implemented by copolymerizing the derived α-ACL and CPL. Such a miraculous strategy offers a reliable prospect for the design and production of target functional nylon materials.
Intrinsically flame-retardant nylon-6 is another sort of important functional nylon polymer and has been exploited in recent years. M. Buchmeiser et al. and P. Ji et al. prepared flame-retarded polyamide 6 via the direct co-condensation of CPL with two different organophosphorus compounds (Figure 8a,b) in a typical melt-polymerization process (Figure 8d,e) [152,153]. The multifilaments of the copolymers were prepared by melt spinning and exhibited tensile strengths up to 40 cN/tex and tenacities up to 0.5 GPa. Moreover, the as-synthesized copolymers exhibit high limiting oxygen index values around 35%. I. Jerman upgraded the polymer synthesis strategy via fabricating a new flame-retardant monomer using α-ACL as the starting reactant (Figure 8c). The as-synthesized flame-retardant nylon-6 copolymer was chemically bonded with 9,10-dihydro-9,10-oxa-10-phosphaphenanthrene-10-oxide as a pendant group via the -NH- group of α-ACL (Figure 8f) [154]. By virtue of these research achievements, these flame-retardant nylon copolymers have great potential in applications in fire control, home decoration and furniture manufacturing.

3.5. Nylon-56

Currently, nylon-6 and nylon-66 dominate the global nylon industry [155]. For nylon-66, the manufacturing process of traditional petroleum-based raw materials refers to toxic chemicals such as adiponitrile and has caused severe environmental pollution, which forced the evolution of both raw materials and production processes [156]. Bio-based nylon composed of biological monomers is considered an ideal substitute by virtue of their sustainability, nontoxicity and biodegradability. Nylon-56 is one of the typical types of bio-based nylon materials polymerized from bio-synthesized cadaverine (produced from lysine, see detailed discussion in Section 2.3) and adipic acid (Figure 9). With such a specific backbone, nylon-56 possesses desirable properties such as high melting points and low water absorption and has the advantages of good abrasion resistance and biocompatibility, which thus facilitate this copolymer’s applications in textiles, drug delivery, food packaging, and water treatment [157,158,159]. Many global companies [Ajinomoto (Japan), Toray (Japan) and Eppen (China)] have put forward strategic layouts for commercial nylon-56 production.

4. Conclusions and Outlooks

Lysine exhibits great potential in the expansion from nourishment to the chemical industry. The key to the success of the industrial-scale transformation of lysine to chemical commodities is the competitive process cost compared with the traditional fuel refineries and corresponding industry chains. Thus, the development of efficient catalytic processes for these chemical commodities is necessary and urgent. Additionally, the requirement of developing highly efficient, safe, low-waste and low-toxicity processes brings us stern challenges but a promising future for the substitution of fossil resources with biomass.
Similar with several bio-based platform molecules such as glucose, 5-hydroxymethylfurfural and glycerol, the unique chemical structure of lysine exhibits great potential in the transformation towards various value-added chemicals. Even if the literature contains many academic publications and industrial patents for the synthesis of lysine-based chemicals, the published catalytic processes still have limited applications in large-scale production due to the complicated catalytic procedures, expensive catalysts and low product yields. Thus, efficient and sustainable production strategies need to be exploited. As the expansion of both lysine and lysine-based polymers towards other new fields (such as aviation, agriculture, automobile manufacturing, etc.) relies on the design of new lysine derivatives, both the modification and addition of specific functional groups would endow lysine or lysine-based polymers with new properties and drive innovation.
The production of lysine has just ushered in their prosperity at the end of the last century. Studies are needed to optimize the fermentation process and decrease lysine’s production costs, which perform completely differently than petroleum refining and their mature production processes. In addition, according to the aforementioned overview, the downstream processing of lysine and its derivatives is just at the beginning of its history, and there is an urgent need for the synthesis technologies to be improved. For the grand vision of industrial applications of lysine-based commodities, timely establishment of corresponding pilot technology as well as industrial chains also need to be arranged by the government and enterprises. With great efforts in the optimization of both the biomanufacturing of lysine and constant upgrading for the catalytic processes of functional derivatives, we believe that the industrial chain of lysine will be greatly expanded in various new fields and produce a fierce competition and allow us to touch the commercially produced lysine-based materials in the near future.

Author Contributions

Conceptualization, K.L.; investigation, K.L. and B.S.; writing—original draft preparation, K.L.; writing—review and editing, K.L. and B.Z. (Bo Zheng); supervision, B.Z. (Baoning Zong). All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

We thank Xin He for his valuable suggestions and contribution in manuscript editing.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Fossil strategy and biological fermentation strategy for the production of monomers and the subsequent functional polymers.
Figure 1. Fossil strategy and biological fermentation strategy for the production of monomers and the subsequent functional polymers.
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Figure 2. Overview of the development and evolution of CPL.
Figure 2. Overview of the development and evolution of CPL.
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Figure 3. The novel synthesis process of fossil-based CPL developed by SINOPEC.
Figure 3. The novel synthesis process of fossil-based CPL developed by SINOPEC.
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Figure 4. Chemical structure and catalytic processes of (a) cadaverine and (b) lysinol.
Figure 4. Chemical structure and catalytic processes of (a) cadaverine and (b) lysinol.
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Figure 5. Production of pipecolic acid from lysine.
Figure 5. Production of pipecolic acid from lysine.
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Figure 6. Chemical structures of (a) α-PL and (b) ε-PL.
Figure 6. Chemical structures of (a) α-PL and (b) ε-PL.
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Figure 7. Production strategies of the functional nylon materials in ref. [54,150,151].
Figure 7. Production strategies of the functional nylon materials in ref. [54,150,151].
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Figure 8. Chemical structures of flame-retardant monomers and polymers of ref. [152,154].
Figure 8. Chemical structures of flame-retardant monomers and polymers of ref. [152,154].
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Figure 9. Production process of bio-based nylon-56.
Figure 9. Production process of bio-based nylon-56.
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Liu, K.; Shao, B.; Zheng, B.; Zong, B. Catalytic Production of Functional Monomers from Lysine and Their Application in High-Valued Polymers. Catalysts 2023, 13, 56. https://doi.org/10.3390/catal13010056

AMA Style

Liu K, Shao B, Zheng B, Zong B. Catalytic Production of Functional Monomers from Lysine and Their Application in High-Valued Polymers. Catalysts. 2023; 13(1):56. https://doi.org/10.3390/catal13010056

Chicago/Turabian Style

Liu, Kangyu, Bingzhang Shao, Bo Zheng, and Baoning Zong. 2023. "Catalytic Production of Functional Monomers from Lysine and Their Application in High-Valued Polymers" Catalysts 13, no. 1: 56. https://doi.org/10.3390/catal13010056

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