Continuous catalytic depolymerisation and conversion of industrial kraft lignin into low-molecular-weight aromatics

The commercial kraft lignin, Indulin AT, which is a pine softwood lignin that is precipitated from black liquor of linerboard-grade pulp, was obtained as dry powder from MeadWestvaco Corporation (Charleston Heights, SC) [26]. In this work, Indulin AT was used as the starting raw material in all experiments. This lignin stream has a moisture content of about 4.5 wt% and ash content of 3.5 wt%. The sample was analysed for its elemental composition at BELAB AB in Norrköping, Sweden; and full proximate, ultimate and compositional analyses for the lignin sample are reported in Table 1. The higher heating value (HHV) of the lignin sample was calculated based on the composition of main elements through an applied formula [27] with more than 90% predictions accuracy and in the range of ± 5% error (i.e. HHV (MJ/kg) = − 1.3675 + 0.3137 C + 0.7009 H + 0.0318 O*). The elemental compositions are in weight percentage, and O* is the sum of the contents of oxygen and other elements in the organic matter, including sulphur and nitrogen.

The feed was prepared prior to each experimental run and was composed typically of 5 wt% lignin substrate, 5 wt% NaOH and 90 wt% deionised (DI) water. Vacuum filtration for the feed was performed to ensure that no precipitation could occur in the pump and to prevent clogging problems within the pressure valve further after reaction. Sodium hydroxide, hydrochloric acid (37%) and all other chemicals were purchased from Sigma-Aldrich Sweden AB.

The base-catalysed lignin depolymerisation experiments were performed in a new-tailored continuous flow reactor (CFR), and a schematic diagram of the designed setup is presented in Fig. 2.

The system is composed of a Gilson 307 HPLC pump (Middleton, WI, USA), a Salamander tubular CFR reactor (Cambridge Reactor Design Ltd., Cottenham, UK) with a heated volume of 18 and 50 mL total system volume (ensured in safe reaction zone, i.e. max P × V = 8.15 bar L < 20), and a specially assembled pressure control valve from nickel-molybdenum-chromium superalloy (Hastelloy C276; UNS N10276). The same material used in manufacturing the valve—Hastelloy C276—was also used for the reactor tubes to replace normal stainless steel tubes, offering excellent corrosion resistance in such severely basic environment. A filter was installed after the cooler in order to protect the pressure control valve from any char depositions if any. Before starting each experimental run, the setup was heated up and pressurised to the targeted operating conditions, keeping a continuous flow of DI water within the whole system. The feed was then continuously pumped to the reactor at volumetric flowrates of 10, 5 and 2.5 mL/min, respectively. These mixtures flowing through the reactor system typically correspond to residence times in the heating zone of about 1, 2 and 4 min, respectively. The experimental parameters and conditions are summarised in Table 2.

The experiments were conducted at different screening temperatures ranging from 170 to 250 °C, with 10 °C intervals, at pressures around 130 bar and retention times span of 1–4 min. For the reaction parameters, the temperature was set using a Eurotherm temperature controller (Eurotherm, Ashburn, Virginia, USA) and the pressure was adjusted through a backpressure regulator (Bronkhorst High-Tech B.V., Ruurlo, Netherlands). After exiting the reaction zone and expanding the product mixture to atmospheric pressure, the liquid products were continuously collected separately during the different operating conditions for workup and analysis purposes. The collected product samples were also divided into two cuts for storage stability testing, where one moiety was stored at room temperature and the other at 4 °C.

Following an established method [28], the molecular weight distributions of Indulin AT lignin as well as depolymerised lignin samples were determined and screened using a SEC system. The system used for the size screening studies was a Waters 600E chromatography system (Waters, Milford, MA, USA) equipped with a Waters 2414 refractive index (RI) detector, a Waters 486 UV absorbance detector and an analytical column packed with 30 cm of Superdex 30 and 30 cm of Superdex 200 (GE Healthcare, Uppsala, Sweden). The column was operated at ambient temperature and eluted with 125 mM NaOH solution (analytical grade) as the mobile phase at a flowrate of 1.0 mL/min. Calibration was performed using polyethylene glycol (PEG) standards ranging from 400 to 35,000 g/mol in the eluent (Merck Schuchardt OHG, Hohenbrunn, Germany). The samples were dissolved/diluted at concentrations of about 1 mg/mL in the eluent, and the solutions were further filtered using a 0.2-μm filter (Schleicher and Schuell, Dassel, Germany) in order to remove any suspended matter; and 500 μL from the filtered solution was lastly injected into the SEC system. Due to the comparison with PEG standards, both the molecular weight and the molecular number should be considered relative. For the storage stability tests, the feed mixture and the depolymerised lignin product samples (240 and 250 °C) were stored at different temperatures for periods of 1, 2 and 3 months before finally being analysed using the SEC system.

For detailed analysis of substrate and produced products, a workup procedure was developed for better characterisation results where separation tasks were involved. Basically, the product reaction mixture was first acidified with 6 M HCl to pH 1, which resulted in the precipitation of insoluble fragments. The supernatant—generated after acidification—was saved after centrifugal separation and extracted three times with ethyl acetate as extraction solvent. The ethyl acetate extract was then transferred quantitatively to a glass tube and evaporated under a gentle N₂ flow, where residue A (low-molecular-weight degradation products) was obtained. Residue A was finally stored for further SFC-MS analysis and subjection to a set of 2D ¹H–¹³C HSQC NMR experiments. The acid-insoluble lignin product (residue B) originated after acidification was washed more than three times with DI water, freeze-dried for 24 h and stored for other analysis set.

For the SFC-DAD/MS analysis, residue A was first dissolved in 1 mL of methanol. The solution was then stored in a freezer at − 80 °C before being analysed. An ultra-high-performance supercritical fluid chromatography coupled with quadrupole-time-of-flight mass spectrometry method for the separation/analysis of major lignin-derived phenolic compounds was adopted in this sense, following a newly developed procedure [29]. Chromatographic separation and detection of the sample was performed with a Waters UltraPerformance Convergence Chromatography System (Milford, MA, USA) coupled to a Waters Xevo G2 Q-TOF mass spectrometer with electrospray ionisation (ESI) and atmospheric pressure chemical ionisation (APCI) (Waters, MS Technologies, Manchester, UK). The column employed was Waters Torus DIOL (1.7 μm, 3 × 100 mm) for target compounds separation. The mobile phase was composed of mainly CO₂ gas and methanol as co-solvent. The column temperature was set at 50 °C, and the pressure of the backpressure regulator was adjusted at 130 bar. A binary gradient elution program was employed with supercritical CO₂ and methanol as solvents. The mobile phase gradient started with 100:0 (CO₂/CH₃OH vol%) and ramped up to 91.5–8.5% of CO₂ solvent in 2.5 min, then ramped up to 75–25% of CO₂ solvent in 3 min and hold for 1 min and lastly decreased to the starting composition after 0.5 min. Ammonia (5 mM) in methanol was used as a makeup solvent, 0.2 mL/min. The data acquisition was done in a negative ion ESI mode. The data acquisition range was m/z 150–800. The capillary voltage was 3 kV, the extraction cone voltage was 4 V and the sampling cone voltage was 20 V. The source temperature was 120 °C, and the desolvation temperature was 600 °C with the desolvation gas flowrate of 1200 L/h. The cone gas flowrate was 40 L/h.

A set of ¹H and 2D ¹H–¹³C HSQC NMR experiments for lignin and depolymerised products at 25 °C were acquired on Agilent VNMRS NMR spectrometers (Santa Clara, CA, USA) operating at 500 and 600 MHz, both equipped with a 5-mm HCN probe with triple-axis gradients. Typically, 50 mg of sample was dissolved in 0.5 mL of deuterated dimethyl sulfoxide-d ₆ (DMSO-d ₆ ) after stirring. Chemical shifts were referenced to the residual solvent signal at 2.5 and 39.5 ppm for ¹H and ¹³C, respectively. The ¹H spectra were acquired using a 90° pulse angle, an acquisition time of 2 s, a relaxation delay of 2 s, 32 scans for the lignin sample and 4 scans for depolymerised products. The HSQC experiments were acquired using the Agilent pulse sequence gHSQC, a relaxation delay of 2 s, an acquisition time of 150 ms and 256 complex data points in the indirect dimension but were otherwise not acquired identically for the two samples. For the sample of depolymerised lignin residue, an edited experiment was used, with spectral widths of 13.4 and 170 ppm for the ¹H and ¹³C dimensions, respectively, and 64 transients. The total experimental time was 20 h. For the lignin sample, the experiment was run without editing, in order to maximise the signal intensities, with spectral widths of 12 and 170 ppm for the ¹H and ¹³C dimensions, respectively, and 80 transients. The total experimental time was 25 h. The spectra were processed using VnmrJ software (v4.2).

Size exclusion chromatography (SEC) was performed to assess changes in the distribution of molecular mass of lignin samples before and after depolymerisation at temperatures ranging from 170 to 250 °C and, initially, with the flowrate of 10 mL/min. Figure 3a, b shows the corresponding UV absorbance chromatograms and refractive index elution profiles of different lignin samples, respectively. A flowrate of 10 mL/min was employed, corresponding to residence time of approximately 1 min in the reactor. As depicted in Fig. 3a, the initial molecular weight distribution profile of Indulin AT lignin generally distributed in a distinct wide region from 1 to 100 kDa with a peak around 4 kDa, illustrating that the main parts of Indulin AT are heavy compounds (heavier than 1 kDa). In addition, a small peak curve was also shown at approximately 0.9 kDa, representing a fraction of low-molecular-weight oligomers. The molecular weight distribution of the products gradually shifts towards lower molecular weight components with the increasing depolymerisation temperature (i.e. from 170 to 250 °C). The heaviest components with a molecular weight higher than 10 kDa almost disappeared when the temperature increased to 250 °C. It was noted that the bimodal curve distribution around 200–400 Da, which represents basically monomers and dimers, appeared when the temperature was above 220 °C. The higher depolymerisation temperature yielded in turn substantially more monomers and dimers. Here, the lowest molecular mass distribution and the clearest peak at 200–400 Da were observed in the highest temperature (250 °C). The results at this juncture suggest that a more complete depolymerisation of lignin takes place with higher temperatures for such operation.

On the other hand, as shown in Fig. 3b, all the lignin samples had refractive index (RI) peaks at 40–70 Da, which comes from the sodium hydroxide. This illustrates that the hydrothermal depolymerisation reaction at such conditions is a sodium hydroxide consuming process. Such phenomenon was found to be comparable with other similar studies [25, 30], indicating/confirming that a high consumption of base is to be expected upon depolymerising lignin under alkaline conditions. Also during operation, the pH was lowered by two orders of magnitude confirming the consumption of alkali. A wide peak at about 1–10 kDa was also observed in the RI profiles (Fig. 3b). The peak weakened slightly with the increasing depolymerisation temperature, indicating the decrease of the molecular weight of Indulin AT lignin after depolymerisation.

Towards a better understanding of the reaction mechanism/behaviour and the CFR system performance, another screening attempt was performed at the same temperature range (170–250 °C) but with a lower feed flowrate of 5 mL/min, corresponding to a residence time of approximately 2 min. The UV absorbance chromatograms and the refractive index elution profiles for the lignin and depolymerised lignin samples at the aforementioned operating conditions are depicted as Fig. 4a, b, respectively.

Compared with the flowrate of 10 mL/min, a more thorough depletion of components over 10 kDa was observed from the temperature of 170 °C (Fig. 4a). Another stage of reduction happened when the temperature increased up to 190 °C. Accordingly, the peaks at around 400 Da and 1 kDa just get more and more distinct and higher above 190 °C. This represents a substantial increase in the production of monomers, dimers and other low-molecular-weight oligomers with the depolymerisation of Indulin AT kraft lignin. However, compared with 240 °C, an increase of the peak value at about 3 kDa was observed in the temperature of 250 °C. For the experiments performed at 5 mL/min flowrate, char was experienced/formed at the temperature of 250 °C. The flowrate suddenly dropped, and the reactor tubing was plugged upon raising the temperature and feed flowrate, hindering any flow of feed solution throughout the CFR system. Therefore, temperature of 250 °C is not recommended for the continuous depolymerisation of Indulin AT lignin at a residence time of 2 min.

Figure 4b clearly shows the consumption of NaOH (40–70 kDa), i.e. lower refractive index exhibited, with the increased depolymerisation temperature. The results from the experiments at 240 and 250 °C displayed RI peaks at approximately 0.5–0.8 kDa which indicates the production of low-molecular-weight components tentatively at such conditions (Fig. 4b). In addition, the RI peaks at 1–10 kDa obviously decreased with the increasing depolymerisation temperature (170–250 °C), which indicates that more cleavage of the chemical bonds in the lignin molecules occurred under this operating conditions.

Final set of depolymerisation experiments was performed at a flowrate of 2.5 mL/min in the temperature range from 170 to 240 °C. At 2.5 mL/min feed flowrate, the residence time in the reactor was estimated to be 4 min. Under this condition, the heavy components with molecular weight higher than 10 kDa were totally depolymerised at different temperatures from 170 to 240 °C (Fig. 5a). There is also a remarkable growth in the amount of components with molecular mass between 1 and 5 kDa. A trend confirmed that UV absorbance and RI response increase with increased temperature was observed. However, no clear trend was experienced upon increasing the residence time, indicating that repolymerisation could be an issue at τ of 4 min (Fig. 5) in comparison to τ of 2 min (Fig. 4). More distinct peaks at 0.2–1 kDa were exhibited in UV absorbance and refractive index, which illustrates the production of more monomers, dimers and/or other oligomers (Fig. 5a, b).

To gain more comparative insights between different attained results, Fig. 6 was plotted showing different SEC profiles recorded for the three residence times tested (1, 2 and 4 min) at 230 and 240 °C (sweet depolymerisation spots). Comparing the changes encountered at different residence times under the same depolymerisation temperature, it can be deduced that longer residence time yields more low-molecular-mass oligomers, which can be demonstrated from Fig. 6. However, with 4 min of reaction time, the repolymerisation of Indulin AT became prominent. These phenomena can be visually seen in the comparative plot, where the curves for the experiments performed at temperatures of 230 and 240 °C exhibited a slight shift to the right when the residence increased from 2 to 4 min. The results here corresponded well with the changing of molecular mass distribution of lignin during autohydrolysis process [19]. Based on these results, for further investigation, we suggest 2 min residence time and 240 °C reaction temperature as the optimum operating conditions for Indulin AT base-catalysed depolymerisation in a continuous-mode processing.

The storage stability of lignin oils and lignin-derived bio-oils is a critical factor that would significantly influence the bio-oil applications—from an industrial point of view. In this context, the term “storage stability” may refer to the ability of the bio-oil to retain its initial chemical and physical properties in a relatively high temperature environment, i.e. thermal stability, or under atmospheric oxidation (oxidative stability) storage conditions [31]. The stability of lignin and lignin-derived bio-oils can be tested through storage time and/or storage temperature, and here, in the present study, we investigate both the long-term storage of lignin bio-oils and the effect of storage temperature on lignin and lignin oils’ properties. Figure 7 presents the molecular weight distribution variations of Indulin AT lignin starting substrate (the mixture of 5% lignin with 5% NaOH and 90% distilled water) at room temperature and different storage times in order to assess the initial substrate stability. As can be seen, the lignin basic feed mixture appears to be very stable with time at normal storage conditions. This can be attributed to that there were no significant variations in size (molecular mass) encountered during this long-term storage.

The 240 and 250 °C (1 min of residence time) depolymerised lignin samples were first selected for the stability research and further compared with the ones obtained at 2 min reaction time, referring that the best depolymerisation results were experienced at these particular temperatures for such residence times. The molecular weight distribution of the first selected two depolymerised lignin samples which were stored at room temperature and refrigerated at 4 °C, respectively, were compared during 0–3 months of storage times (Fig. 8). In the condition of 4 °C, the molecular weight distribution of lignin samples was rarely changed after even 3 months’ storage (Fig. 8b). This means that samples stored at 4 °C of storage condition were rather stable. However, it was clear that the peaks of the depolymerised lignin samples which were stored at room temperature moved to some extent at different storage periods (Fig. 8a). One explanation can be that the depolymerised components might have been repolymerised during such storage periods. Bearing in mind the capability of produced phenolic structures to react with available reactive sites in water during lignin conversion [32], cross-linking between the active sites and generated phenolic moieties in large fragments could take place, i.e. lignin could give in turn higher-molecular-weight fragments. Similar results can be observed in the other two depolymerised lignin samples at 240 and 250 °C with the residence time of 2 min (Fig. 9). It performed/revealed almost perfect stability in the 4 °C refrigerator, while on the other end, some changes at room temperature were exhibited. The results here suggest that it is better/recommended to store the depolymerised lignin samples at lower temperatures for enhanced stability behaviour.

The lignin starting material as well as the monomeric-rich fraction extracted from the depolymerised lignin products were qualitatively characterised using the newly introduced SFC-MS technique. Two samples from the depolymerised lignin streams produced where chosen beside the Indulin AT sample, residue A which entails the recommended/optimum operating conditions among all the runs performed (240 °C; 2 min) and the run performed at 230 °C and 1 min for the sake of drawing a comparative discussion. Figure 10 presents the typical base peak intensity chromatograms for the three investigated samples, and Table 3 reports the most abundant chemical compounds identified along with the MS/MS fragmentation data.

Seven major compounds were identified in the tested samples/fractions: guaiacol, vanillin, acetovanillone, p-hydroxyacetophenone, p-hydroxybenzaldehyde, vanillic acid and p-hydroxybenzoic acid, where the latter three compounds appeared in the starting Indulin kraft lignin sample. It was revealed from the analysis that guaiacol and vanillin are the main products generated from the current depolymerisation approach with higher concentrations/intensities for sample A over sample X, confirming/supporting the results acquired from the SEC attempts.

For the interesting sample A, it can be noted that the compounds exhibiting a retention time higher than 3 min were not identified due to the reason of being at low concentrations. These compounds may comprise vanillic acid and other distinct mono-aromatic moieties with aldehyde, acidic and methoxy functionalities. The decrease/disappearance of these particular compounds in the depolymerised lignin product obtained is due to the increasing severity of the reaction. This finding agrees well with results reported by Beauchet and co-workers [17]. In addition, the high selectivity towards guaiacol formation was attained for the higher reaction time (2 min). It can be suggested that the increase in the residence/reaction time (by lowering the feed pump flowrate) assisted in surging the guaiacol production which may have been generated not only from lignin itself but also from the extra depolymerisation of intermediate oligomers. The increased reaction severity also helped—to a great extent—in cleaving the acidic and aldehyde functions which were originally present in the initial starting lignin material. The depolymerised lignin products generated in the present study, i.e. guaiacol and vanillin, are known to have the potential of being key renewable aromatic building blocks for sustainable chemical industry [4, 33].

The integrated ¹H NMR spectra obtained for Indulin Kraft starting material and depolymerised lignin samples are shown in Fig. 11. As can be seen, the increased intensities of proton chemical shifts (sharper peaks) of depolymerised lignin sample in comparison to the original lignin material suggest an increase in the degree of depolymerisation and higher formation of low-molecular-weight products.

The measurements on the produced depolymerised residue A show that the aliphatic compounds are rather present, but in very low concentrations. However, the main ¹H NMR peaks were assigned to typical aromatic protons (signals 6.0–8.0 ppm), besides the methoxyl and aldehyde groups, confirming the presence of small molecular entities within the depolymerised sample and supporting the findings of both SEC and SFC-MS.

It can be interpreted as well from the spectra that the methoxyl content, in particular, was dramatically decreased after the hydrothermal reaction, and this phenomenon agrees well with the reported literature on the hydrothermal treatment of lignin [34]. In addition, the large concentrations of hydroxyl and phenolic groups/protons in residue A can explain the increased hydrophilicity of produced aromatic compounds.

2D NMR spectroscopy was run on the depolymerised lignin residue A extracted from the run performed at T = 240 °C and τ = 2 min which is considered to be optimum and compared with the untreated Indulin Kraft lignin sample. The measured spectra were interpreted qualitatively and divided into an aliphatic region (δ_C/δ_H 8.0–52.5/0.5–4.5 ppm), an inter-unit/oxygenated aliphatic region (δ_C/δ_H 52.5–90.0/2.8–5.7 ppm) and an aromatic region (δ_C/δ_H 105–155/6.0–9.2 ppm), as presented in Fig. 12 reporting different chemical shifts acquired. Tentative assignments of ¹³C–¹H correlation signals/cross-peaks within the 2D NMR spectra were also identified for both lignin samples following recent literature reports [35, 36].

In general, guaiacol rings form about 95% of the major aromatic composition of lignin from softwood origin. Differences in the cross-peak patterns were detected for the two samples in the aliphatic 2D NMR region (Fig. 12; left compartment). The most characteristic cross-peaks of lignin macromolecule are found in the region of CH₂ groups (black rings), where they correspond to the primary aliphatic network present in lignin G–CH₂–CH₂–CH(R)–OH/R located at the α and β positions, bounded to the guaiacol ring. Most of the cross-peaks in the depolymerised fractions were found to be connected to an aromatic ring in the α-position, which suggests a shortening in the aliphatic chains within the bio-oil—residue A—in comparison to the Indulin AT substrate.

The inter-unit aliphatic region (also termed as oxygenated aliphatic region), shown in intermediate compartment of Fig. 12, is the region where carbon atoms bonded to alcohol, ether and Ar-methoxyl groups resonate. The known aliphatic inter-unit linkages β-O-4 (β-ether), β-β (pinoresinol), β-β (secoisolariciresinol) and β-5 (phenylcoumaran) were found for the Indulin AT lignin basic structure. However, the β-1 (diphenyl ethane), α-O-4 and dibenzodioxocine (5–5-α, β-O-4) structures were not found/detected, as they were probably hydrolysed during the kraft pulping process. From the visual inspection of the 2D NMR spectra for this region, it can be revealed that all the inter-unit/C–O linkages from Indulin AT lignin have disappeared post pretreatment reaction. This entails that the aliphatic C–O bonds have been cleaved under the tested reaction conditions (240 °C and 2 min residence time), i.e. in the known lignin inter-unit structures β-O-4, β-β, β-1 (weak signal) and β-5. This is indeed an important finding as it subsequently proves that all of the lignin starting material was reacted effectively under basic conditions in our CFR system. It also demonstrates that the amount of oxygen connected to the aliphatic carbons in depolymerised fraction is lowered. To note, the content of atomic oxygen comes almost exclusively from oxygen-containing groups (–OH and –OMe/R) connected to the aromatic ring, like phenols, guaiacols and other aromatic ethers.

Looking into the aromatic region, right compartment of Fig. 12, it was found that the main CH cross-peaks/signals of Indulin AT lignin correspond to the guaiacol ring (G) and the recognised substitution pattern of softwood kraft lignin, i.e. G₂, G₅ and G₆. It can further be seen that depolymerised lignin—residue A—exhibited a pattern of cross-peaks different than that of Indulin AT lignin. One of the principal reasons for this is that all monomeric/dimeric low-molecular-weight aromatics derived/produced ended up in this fraction. It was also confirmed from the SFC-MS analysis that phenol and phenol derivatives, e.g. guaiacol, dominate this residue/fraction, complementing such results.