Abstract
This study demonstrates that bio-based products of lignin depolymerisation can potentially replace common antioxidants and UV absorbers in the plastics and cosmetics industries. The kraft lignin Indulin AT was used to obtain low-MW lignin and monomers & oligomers rich in phenolic hydroxyl groups. Both thermo-oxidative stability and antioxidant activity significantly improved upon lignin depolymerisation. The results from oxidation induction time differential scanning calorimetry (OIT-DSC) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) measurements show that the lignin based depolymerisation products are comparable with commercial antioxidants like pentaerythritol-tetrakis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate) (Irganox 1010). UV/vis spectroscopy was investigated to confirm absorption of the depolymerisation products in the UVA and UVB range (280–400 nm).
1 Introduction
Interest in finding replacements for fossil-fuel-based products has been growing in recent years, as these are derived from non-renewable resources and are associated with global warming. Lignin is an especially promising raw material because of its abundance in nature: It makes up between 12 and 30% of the biomass found in plants. Every year more than 50 million tons of lignin accumulate as side product in cellulose production. Kraft lignin in particular is a by-product of the most commonly used pulping technique and therefore readily available (Duval et al. 2016; Hansen et al. 2016; Rinaldi et al. 2016).
Although numerous valorisation techniques that transform lignin into other chemicals have emerged, it is used mainly for energy recovery. Only a small fraction of lignin is used for other purposes, for instance, as an additive in adhesives based on phenol-formaldehyde resins. The reason for that is the complex composition of lignin: It is a phenolic polymer consisting of phenylpropane (C9) units with an irregular structure that comprises various functional groups, including methoxy, carbonyl and carboxyl groups, as well as phenolic and aliphatic hydroxyl groups (Aufischer et al. 2021; Laurichesse and Avérous 2014; Ponnusamy et al. 2019).
Numerous studies have considered the depolymerisation of lignin necessary for its conversion to value-added substances. Various methods have been published, including pyrolysis and treatment with water, organic solvents, ionic liquids or supercritical fluids (Agarwal et al. 2018; Ahlbom et al. 2022). During lignin depolymerisation, ether and carbon-carbon linkages are cleaved between the C9 units of the macromolecules. Among hydrothermal techniques, base-catalysed lignin depolymerisation is well established especially, using sodium hydroxide. Its catalytic performance is based on the cleavage of the β-O-4 ether bond which is the most common linkage found in lignin (Erdocia et al. 2014; Toledano et al. 2014).
Previous studies have already demonstrated the antioxidant and UV-absorbing behaviour of lignin. Its antioxidant activity results from scavenging free radicals formed by an oxidation process. The phenolic hydroxyl groups in lignin react with free radicals and trap them within the sterically hindered phenol structure (Alzagameem et al. 2018; Chen et al. 2018; Domenek et al. 2013; Gordobil et al. 2018). UV absorption in lignin is based mainly on benzene rings and ketone structures within the macromolecule. Carbon–carbon and carbon–oxygen double bonds act as chromophores, while hydroxyl and ether groups play important roles as auxochromes (Falkehag et al. 1966; You and Xu 2016).
Safe use of lignin in cosmetic products has been confirmed, and it has been shown that lignin is non-toxic to human skin. Industrially produced lignin also does not exhibit any cytotoxic effects (Ugartondo et al. 2008). Lee et al. successfully used various lignins to prepare sun-screens and checked for UV-absorbing characteristics (Lee et al. 2019).
Lignin has been successfully incorporated into polymers such as polyethylene (PE), polypropylene (PP) and polylactic acid (PLA) in previous studies. However, low molecular weight and polydispersity of the lignin are crucial to homogeneous distribution within the polymer matrix (Pouteau et al. 2003; Ten and Vermerris 2015). In this context, depolymerisation could allow customization of lignin-based products to each type of polymer.
In this work, the effect of depolymerisation of kraft lignin on its antioxidant and UV-absorbing properties was investigated.
2 Materials and methods
2.1 Materials
The kraft lignin Indulin AT was purchased from MeadWestvaco, sodium hydroxide (NaOH) from ACROS ORGANICS; ethyl acetate, tetrahydrofuran (THF), sodium sulfate, toluene, ethanol (absolute) and squalane from Carl Roth; hydrochloric acid (HCl) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) from VWR; vanillin, 2,6-Di-tert-butyl-4-methylphenol (BHT), 2-tert-butyl-4-methoxyphenol & 3-tert-butyl-4-methoxyphenol (BHA), dimethyl sulfoxide (DMSO), 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-[(hexyl)oxy]-phenol (DPTHP), Folin & Ciocalteu’s phenol reagent and 5-chloro-2-hydroxybenzophenone (CHBP) from Sigma-Aldrich; 2-hydroxy-4-methoxybenzophenone (HMBP) and oxanilide from Alfa Aesar and pentaerythritol-tetrakis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate) (Irganox 1010) from Ciba Specialty Chemicals.
2.2 Lignin depolymerisation
Lignin was depolymerised in a 0.5 L stainless steel reactor (Parr 4560). Sodium hydroxide was used as a homogeneous depolymerisation catalyst. For each experiment, 20 g of lignin was dissolved in a solution of 180 g 5 wt % NaOH and heated to 250 °C within 44 min. After 30 min at 250 °C and 37 bar, the reactor was cooled down to room temperature within 15 min. The reaction mixture was stirred at 300 rpm the entire time. After lignin depolymerisation the resulting product mixture was acidified with HCl (37 wt%) to a pH of 2 to precipitate low-MW lignin. The solids (low-MW lignin and coke) were separated from the mixture by a fritted glass filter (porosity no. 3). The filtrate was extracted three times with ethyl acetate. The solvent of the combined organic phases was removed via rotary evaporation followed by lyophilisation to obtain a fraction containing monomers & oligomers. The solvent of the aqueous phase was removed in an analogous manner to obtain a water-soluble residue. The precipitated low-MW lignin and coke were dispersed in THF. Subsequently the dissolved low-MW lignin was separated from the coke by a fritted glass filter (porosity no. 3). The solvent of the filtrate was removed via rotary evaporation and lyophilisation to obtain the low-MW lignin fraction. The filter cake was desiccated in a drying cabinet to obtain the coke fraction. The yields were determined gravimetrically, elating the weight of each product fraction to the initial weight of lignin.
2.3 Characterisation of lignin and its depolymerisation products
Lignin and its depolymerisation products (low-MW lignin and monomers & oligomers) were analysed by gel permeation chromatography (GPC) using a Thermo Scientific DIONEX ICS-5000 + including a PSS MCX analytical 100A + 1000A + 100,000A column (8 mm × 300 mm, Thermo Fischer). 0.1 mol L−1 NaOH was used as the eluent at a temperature of 30 °C and a flow rate of 0.5 mL min−1. Detection was performed with a UV detector at 280 nm. Polystyrene sulfonate standards with a molar mass of 891–976,000 g mol−1 and vanillin were used for calibration. Prior to injection, all samples were dissolved in 0.1 M NaOH and clarified with a 0.45 µm syringe filter.
The monomers & oligomers were dissolved in ethyl acetate and analysed by gas chromatography mass spectroscopy (GC-MS) using a Shimadzu QP 2020 equipped with an HP SM5 capillary column (60 m × 0.25 mm × 0.25 µm) and helium as the carrier gas. The oven temperature started at 50 °C, was raised to 120 °C, held for 5 min, raised to 280 °C, held for 8 min and finally raised to 300 °C. In each step, the rate heating rate was 10 °C min−1. Electron impact ionization was applied.
The numbers of phenolic hydroxyl groups in lignin, low-MW lignin and monomers & oligomers were determined with Folin & Ciocalteu’s phenol reagent as described by de Sousa et al. (2001).
2.4 Antioxidant activity and UV absorption of lignin and its depolymerisation products
Oxidation induction time differential scanning calorimetry (OIT-DSC) was used to characterise lignin, low-MW lignin, monomers & oligomers and the reference substances BHT, BHA and Irganox 1010. A stock solution of each substance was prepared by dissolving it in EtAc at a concentration of 30.0 g L−1. 100 µL of each stock solution was added to 3.00 g squalane and homogenised in an ultrasonic bath. Pristine squalane and each squalane spiked with stock solution were transferred to a Tzero pan and analysed with a DSC Q20. The sample size was 20 mg, and each test was carried out in triplicate. During measurement the sample was heated from 25 to 190 °C at a rate of 10 °C min−1 under nitrogen flow (50 mL min−1). After 5 min at 190 °C, the gas flow was switched to oxygen (50 mL min−1). Heat and oxygen flow were applied for 100 min.
The DPPH method was used to analyse lignin, low-MW lignin, monomers & oligomers and the reference substances BHT, BHA and Irganox 1010. DMSO was used as a solvent to prepare stock solutions of each sample at concentrations of 2, 1, 0.5, 0.25 and 0.125 g L−1. A 50.0 mg L−1 DPPH solution in ethanol was prepared. 0.1 mL stock solution was added to 3.9 mL DPPH solution. A control solution was prepared by adding 0.1 mL DMSO to 3.9 mL DPPH solution. After an incubation time of 90 min, the absorbance was measured at 517 nm by a Thermo Scientific Multiskan GO spectrometer. The inhibition was calculated according to:
where A0 and AS are the absorbances of the control solution and the sample solution, respectively. The antioxidant activity index (AAI) was calculated according to:
where IC50 is the sample concentration in mg L−1 that provides 50% inhibition.
UV/vis spectroscopy was performed with a Thermo Scientific Multiskan GO spectrometer to characterise lignin, low-MW lignin and the monomers & oligomers obtained. These were then compared to the reference UV absorbers 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-[(hexyl)oxy]-phenol, 5-chloro-2-hydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone and oxanilide. For analysis the samples were dissolved in THF at a concentration of 20.0 mg L−1 and transferred to a quartz cuvette. Each sample was scanned at wavelengths from 200 to 800 nm.
3 Results and discussion
3.1 Products of lignin depolymerisation
The gravimetric yields of the product fractions from lignin depolymerisation are listed in Table 1. Low-MW lignin and monomers & oligomers were the product fractions of interest in this study and are shown in Figure 1.
Product fraction yields from lignin depolymerisation.
| Low-MW lignin | Monomers & oligomers | Water-soluble residue | Coke | |
|---|---|---|---|---|
| Yield/% | 82.9 | 9.0 | 3.7 | 0.5 |
Products of interest from lignin depolymerisation; low-MW lignin (left) and monomers & oligomers (right).
Low-MW lignin and monomers & oligomers were the main products of lignin depolymerisation and made up 92% of the starting weight of lignin. The low yield of coke is in accordance with data from literature (Toledano et al. 2014) and was the reason for choosing a lignin depolymerisation method that involved sodium hydroxide under hydrothermal conditions.
3.2 Molecular weight distribution
The molecular weight distributions of lignin (Indulin AT) and its depolymerisation products (low-MW lignin and monomers & oligomers) are illustrated in Figure 2. The corresponding values – number-average molecular weight (Mn), weight-average molecular weight (Mw) and polydispersity index (PDI) – are shown in Table 2.
Molecular weight distributions of lignin and its depolymerisation products.
Characteristic values of lignin and its depolymerisation products from GPC data.
| Mn/g mol−1 | Mw/g mol−1 | PDI | |
|---|---|---|---|
| Indulin AT | 1240 ± 21 | 4950 ± 10 | 3.99 ± 0.06 |
| Low-MW lignin | 1102 ± 14 | 2102 ± 14 | 1.91 ± 0.02 |
| Monomers & oligomers | 328 ± 3 | 525 ± 2 | 1.60 ± 0.01 |
In comparison to lignin the depolymerisation products had a lower molecular weight: The Mw values of low-MW lignin and monomers & oligomers were 58 and 89% lower, respectively. This was due to cleavage of covalent bonds within the lignin structure during depolymerisation (Toledano et al. 2014). Additionally, low-MW lignin and monomers & oligomers had a sharper molecular weight distribution than lignin. By comparison of the molecular weight distributions of lignin and low-MW lignin, it was obvious that in the low molecular weight region the lignin and low-MW lignin were about the same. In contrast, the high molecular fraction of the lignin could not be found for MW lignin. Obviously, the high molecular constituents of the lignin reacted predominantly during depolymerisation. GPC results of the monomers & oligomers indicated that the product fraction contained monomeric, dimeric and trimeric compounds consisting of phenylpropane (C9) units. These findings match those from MALDI-TOF mass spectroscopy performed by Erdocia et al. (2014).
3.3 GC-MS qualification and quantification
Figure 3 shows the GC spectrum of the monomers & oligomers obtained from lignin depolymerisation. The 10 main monomer compounds including retention time and content in weight percent for each compound are listed in Table 3.
GC spectrum of the monomers obtained from lignin depolymerisation.
Retention time and content for each monomer compound obtained from lignin depolymerisation.
| Monomer compound | Retention time/min | Content/% |
|---|---|---|
| Guaiacol | 11.3 | 9.47 |
| Vanillin | 18.9 | 2.19 |
| Apocynin | 20.4 | 1.35 |
| Catechol | 14.2 | 1.33 |
| Ferulic acid | 17.2 | 0.65 |
| 4-Ethylcatechol | 18.5 | 0.55 |
| 3-Methoxycatechol | 16.0 | 0.46 |
| Phenol | 8.6 | 0.43 |
| Phloretic acid | 22.4 | 0.41 |
| 4-Methylcatechol | 16.5 | 0.39 |
The main components found in the monomers & oligomers were guaiacol, vanillin, apocynin, catechol and ferulic acid and accounted for 15% of the entire fraction. The remaining compounds identified were various phenolic substances that made up less than 1% of the product fraction. These results are in accordance with those of Erdocia et al. (2014).
3.4 Phenolic hydroxyl group content
The concentrations of phenolic hydroxyl groups according to the Folin-Ciocalteu method in lignin (Indulin AT) and of its depolymerisation products (low-MW lignin and monomers & oligomers) are listed in Table 4.
Phenolic hydroxyl group content in lignin and its depolymerisation products.
| Indulin AT | Low-MW lignin | Monomers & oligomers | |
|---|---|---|---|
| Phenolic hydroxyl groups/mmol g−1 | 3.16 ± 0.04 | 4.38 ± 0.09 | 8.65 ± 0.06 |
The phenolic hydroxyl group content was 39% higher in the low-MW lignin and 174% higher in the monomers & oligomers compared to Iignin, which was mostly due to cleavage of β-O-4 ether bonds in the lignin structure resulting in phenolic hydroxyl groups during lignin depolymerisation (Toledano et al. 2014).
3.5 Oxidation induction time (OIT)
OIT was used as a measure of antioxidant activity in terms of thermal oxidation. During DSC measurement an antioxidant sample embedded in squalane was heated under nitrogen flow. After a specified time, the gas flow was switched to oxygen. OIT is defined as the time interval between switching to oxygen and the first steep increase of the DSC curve, which indicates the onset of oxidation (Schmid and Affolter 2003). The DSC curves and corresponding OIT values of pristine squalane and of squalane containing reference substances (BHT, BHA and Irganox 1010), lignin (Indulin AT) or its depolymerisation products (low-MW lignin and monomers & oligomers) are shown in Figure 4.
DSC curves (left) and oxidation induction times (OIT) of squalane and of squalane containing reference substances, lignin or its depolymerisation products.
Among the reference substances, BHT and BHA showed little antioxidant activity (OIT < 2 min) compared to Irganox 1010 (OIT = 25 min). Lignin had a short OIT (2 min) similar to that of BHT. The depolymerisation products, in contrast, showed higher antioxidant activity – between that of Irganox 1010 and that of BHT – with an OIT of 12 min for the low-MW lignin and 18 min for the monomers & oligomers.
The higher antioxidant activity of the monomers & oligomers compared to low-MW lignin can be attributed to a higher phenolic hydroxyl group content. A correlation between thermo-oxidative antioxidant activity and phenolic hydroxyl groups has been reported by Kabir et al. (2019).
Among the reference substances the molecular weight of the antioxidant is crucial for a high OIT value. BHT and BHA have a low molecular weight (<250 g mol−1) and exhibited a short OIT, while Irganox 1010 with a higher molecular weight (1178 g mol−1) showed a longer OIT. These results correspond well to those of other studies which reported that antioxidant activity drops significantly if the molecular weight decreases below 300–400 g mol−1. This effect is attributed to a rapid volatilization of low-molecular-weight molecules (Tocháček 2004). Compared to BHT and BHA, Irganox 1010 is a high molecular weight phenol with four 3,5-di-t-butyl-4-hydroxy-phenyl units (Andreucetti et al. 1998).
3.6 Antioxidant activity according to DPPH
Using DPPH allows the antioxidant activity of samples to be determined. It is an indirect method in which DPPH radicals are inhibited by antioxidants, and the resulting change in colour is detected spectrophotometrically (Kedare and Singh 2011). The inhibition curves indicating antioxidant activity of reference substances (BHT, BHA and Irganox 1010), lignin (Indulin AT) and its depolymerisation products (low-MW lignin and monomers & oligomers) are shown in Figure 5 (left). The corresponding AAI values are plotted in Figure 5 (right).
Antioxidant activities (left) and antioxidant activity indices (AAI, right) of reference substances and lignin and its depolymerisation products.
The AAI number is used as a measure of antioxidant activity to compare samples, since it is independent of both sample concentration and DPPH concentration (Scherer and Godoy 2009). Compared to the reference substances, lignin exhibited poor antioxidant activity with an AAI of 2.1. In contrast, the depolymerisation products showed activities that are comparable to those of the reference substances. The low-MW lignin had an AAI (3.8) similar to that of Irganox 1010 (3.9), and the AAI of the monomers & oligomers (4.9) was between that of Irganox 1010 (3.9) and that of BHA (7.6). Depolymerisation of lignin significantly increased the antioxidant activity of the resulting low-MW lignin and monomers & oligomers. This might be due to the formation of additional phenolic hydroxyl groups during lignin depolymerisation, as other studies have found that there is a relationship between phenolic hydroxyl groups and antioxidant activity in various lignin samples (Alzagameem et al. 2018).
3.7 UV absorption
UV spectra of common UV absorbers (DPTHP, CHBP, HMBP and oxanilide) and lignin (Indulin AT) and its depolymerisation products (low-MW lignin and monomers & oligomers) are shown in Figure 6.
UV spectra of common UV absorbers and lignin and its depolymerisation products.
Compared to lignin and its depolymerisation products, the common UV absorbers showed absorption bands of higher intensity. However, the lignin-based products exhibited broader and more uniform UV absorption. The common UV absorbers did not cover the entire UVA and UVB range (280–400 nm). Absorption decreased rapidly and approached zero close to the visible range. This was the case for oxanilide at approximately 340 nm, for HMBP at 375 nm, and for DPTHP at 390 nm. CHBP absorbed wavelengths up to 400 nm, but the intensities in the ranges 290–310 nm and 380–400 nm were very low. In comparison, the low-MW lignin and monomers & oligomers had absorption bands of higher intensity in the UVB range (280–315 nm). In the UVA range (315–400 nm) the low-MW lignin and lignin showed higher absorption than the monomers & oligomers. Absorption peaks at around 280 nm in lignin are basically attributed to phenolic groups (Falkehag et al. 1966; You and Xu 2016). Since depolymerisation of lignin leads to products richer in phenolic hydroxyl groups, this might be the reason for the higher absorption of low-MW lignin and monomers & oligomers in the UVB range. Compared to commercial UV absorbers, the UV/vis absorption of lignin and its depolymerisation products extends to the visual range. This might be desirable from a technical perspective but could be undesirable from a visual perspective for some applications. For example, in sunscreens the darker colour of the product might be disadvantageous.
4 Conclusions
In this study the kraft-lignin Indulin AT was depolymerised to obtain products with improved antioxidant and UV-absorbing properties. The low-MW lignin and monomers & oligomers obtained showed a reduction in molecular weight and polydispersity. At the same time the phenolic hydroxyl group content was increased by depolymerisation. The thermo-oxidative stability of the depolymerisation products was between that of BHT and that of Irganox 1010. According to the DPPH method the antioxidant activity of the low-MW lignin is similar to that of Irganox 1010, while that of the monomers & oligomers is even better. Compared to common UV absorbers, the depolymerisation products show a less intense but continuous UV-absorption across the UVA and UVB range. The products obtained from lignin have great potential as bio-based materials to replace common antioxidants and UV absorbers both in plastics and cosmetics industry.
Funding source: Österreichische Forschungsförderungsgesellschaft
Award Identifier / Grant number: Wood K plus Comet Funding Period 2019-2022
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Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
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Research funding: This work was funded by the Austrian Research Promotion Agency (FFG): Wood K plus Comet Funding Period 2019–2022.
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Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
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