Transition metal salts Nb₂O₅ (99.9%), CoCl₂⋅6H₂O (98%), CuSO₄ (99%), MnN₂O₆⋅4H₂O (97%), TiO₂ (99.8%), CrKO₈S₂⋅12H₂O (98%), and NiCl₂⋅XH₂O (99.9%) were obtained from Sigma Aldrich (St. Louis, MO, USA). The IL ([C₂C₁Im][OAc]), alkaline (Kraft) lignin, 2-methyltetrahydrofuran (MeTHF, 99%), H₂O₂ (30%), guaiacol (98%), syringol (98%), vanillin (99%), acetovanillone (98%), and homovanillic acid (98%) were also procured from Sigma Aldrich (St. Louis, MO, USA).

Lignin oxidation reactions were performed in batch reactors. Prior to the reaction, lignin (10 wt%) was treated in [C₂C₁Im][OAc] for 3 h at 140°C by following a typical IL pretreatment condition shown elsewhere (Li et al., 2010); subsequently the IL–lignin mixtures were diluted by adding water at a mass ratio of 1:1 to a 2 mL total volume. To the above mixture, a predetermined amount of catalyst and H₂O₂ were added and the reaction was conducted at different temperatures (74–126°C) for 3 h with constant mixing by a magnetic stir bar at 200 rpm. The catalyst and hydrogen peroxide (commercially available 30% H₂O₂ solution) loadings were based on the total weight of IL and lignin. After reaction, residual lignin was precipitated by addition of 2 mL of water followed by centrifugation at 4,000 rpm for 15 min to separate solids from liquid stream. The liquid stream was subjected to a liquid–liquid extraction using MeTHF for three times. Finally, the MeTHF was evaporated to leave thick oil in a vacuum oven at 30°C for 24 h. The oil was re-dissolved in 750 µL of ethyl acetate and analyzed by GC/MS for monomeric compounds.

The solid stream (residual lignin) was washed by 5 mL warm water for four times and subsequently oven dried for 24 h at 70°C. Lignin conversion was calculated using Eq. 1 (1)Conversion (%)=weight of initial lignin−weight of precipitated ligninweight of initial lignin×100

Identification and quantification of the monomeric products from the depolymerization reaction were performed by Agilent 7890B GC coupled 5977B MS with an HP-5ms (60 m × 0.32 mm) capillary column. The temperature program started at 50°C and increased to 120°C at 10°C min⁻¹ with a holding time of 5 min; then it was raised to 280°C at 10°C min⁻¹ with a holding time of 8 min and to 300°C at 10°C min⁻¹ with holding time of 2 min (Prado et al., 2016a). Helium was used as a carrier gas at a flow rate of 1.2 mL min⁻¹. Calibration curves were created using commercially available pure compounds: guaiacol, syringol, vanillin, acetovanillone, and homovanillic acid (Sigma Aldrich, St. Louis, MO, USA).

In a series of preliminary screening experiments, various transition metal catalysts including Nb₂O₅, CoCl₂⋅6H₂O, CuSO₄, MnN₂O₆⋅4H₂O, TiO₂, CrKO₈S₂⋅12H₂O, and NiCl₂⋅XH₂O were tested in batch reactors. Catalyst and H₂O₂ were both loaded at 5 wt% loading (IL + lignin); the screening experiments were conducted at 120°C and a reaction time of 3 h (Prado et al., 2016a). Identification and determination of product formation and concentration were performed according to the batch experiments mentioned in the Section “Lignin Depolymerization, Extraction, and Analytical Method.”

An orthogonal central composite design (CCD) (Table 1) was used to study the effect of H₂O₂ concentration, catalyst loading, and temperature on product concentration for selected catalysts. Levels for independent variables were selected to be, 1–5%, 1–5%, and 80–120°C for H₂O₂ concentration, catalyst loading, and temperature, respectively.

A CCD consists of 2^p factorial runs with 2p axial runs and P_c center runs. In this study, the independent variables were H₂O₂ concentration (X₁), catalyst loading (X₂), and temperature (X₃). For each independent variable, a 2³ full factorial CCD for the three variables consisting of 8 factorial points, 6 axial points, and 3 replicates at the center points were employed, which accounted for a total of 17 experiments.

The center points were used to determine the experimental error and reproducibility of the data. Low and high levels of the independent variable were coded as −1 and +1. Axial points were located at (±α, 0, 0), (0, ±α, 0), and (0, 0, ±α) where α is the distance of the axial point. In this study, α value was fixed at 1.287. The experimental sequence was randomized to minimize the effects of uncontrolled errors. The response variable for this study was product concentration (Y₁). Response was used to develop an empirical model corresponding to product concentration using a second-degree polynomial equation as given in Eq. 2: (2)Y^=β0+∑ inβixi+∑iin βiixi2+∑i=1n−1∑j=i+1nβijxixj where Y^ is the response, β₀ is the intercept, β_i the linear coefficients, β_ii the quadratic coefficients, βij the interaction coefficients, and x_ix_j are the coded values.

Catalysts were screened based on the total product concentration and conversion. Results obtained from the screening experiments show that out of the seven catalysts used CoCl₂ and Nb₂O₅ gave the highest product concentration and conversion compared to the other catalysts (Figure 1). Subsequently, optimization of batch experiments was performed using these two catalysts. Cobalt (Co)-based catalysts have been used for oxidation of lignin model compounds and alcohols in 1-ethyl-3-methylimidazolium diethylphosphate ([C₂C₁Im][DEP]) (Zakzeski et al., 2010b, 2011). Co species facilitate the oxidation reaction by readily catalyzing the disproportionation of H₂O₂ to form O₂ and H₂O (Pokutsa et al., 2009). In addition, several studies reported niobium oxide as a selective and effective oxidation catalyst for a wide range of oxidation reactions (Wachs et al., 2000). Apart from acting as a strong Lewis acid, niobium oxide also has the capability to stabilize oxidation cation species, which could assist in the electrophilic attack of H₂O₂ on lignin (Fielicke et al., 2003; Nakajima et al., 2011).

Concentrations (mg L⁻¹) of five products: guaiacol, syringol, vanillin, acetovanillone, and homovanillic acid were determined by GC/MS for all the reaction conditions based on the central composite design. Results illustrate that the total product concentrations for CoCl₂ catalyst were in the range of 545.3–1,200.5 mg L⁻¹; whereas for Nb₂O₅ catalyst the total product concentrations fell into 340.7–1,325.6 mg L⁻¹ range (Table 2). In addition, obtained data were fitted to a polynomial model to optimize the reaction parameters for the formation of products from lignin using a desirability approach (JMP 12, SAS Institute, Inc., Cary, NC, USA). The suggested optimum levels for CoCl₂ catalyst were temperature of 120°C, catalyst loading of 1%, and H₂O₂ concentration of 1%, corresponding to a predicted total product concentration of 1,079.5 mg L⁻¹ at 95% confidence interval (989.0, 1,169.9). For Nb₂O₅ catalyst, the optimum reaction parameters were temperature of 120°C, catalyst loading of 4.45%, and H₂O₂ concentration of 3.08%, corresponding to a predicted total product concentration of 1,262.6 mg L⁻¹ at 95% confidence interval (1,182.8, 1,342.3). The quadratic regression models for the CoCl₂ and Nb₂O₅ catalysts were shown below: (3)Y^CoCl2=718.91−37.43 X1−92.79 X2+155.90X3+14.71 X12+12.57 X22+111.19 X32+31.01 X1X2+36.17 X1X3+58.88 X2X3 (4)Y^Nb2O5=735.42−37.09 X1+104.98 X2+195.64 X3−33.49 X12−84.95 X22+285.65 X32−6.65 X1X2+45.10 X1X3+19.73 X2X3 where, X₁, X₂, and X₃ are H₂O₂ concentration, catalyst loading, and temperature, respectively. The linear effects were represented by the coefficients of one factor (X₁, X₂, X₃), while the coefficients of the second-order term (X12,X22,X32) and two-factor term (X₁X₂, X₁X₃, X₂X₃) represent the quadratic effects and interaction effects, respectively.

Products obtained from the catalytic oxidation process were identified and quantified using GC/MS. Figure 2 shows a GC chromatography representing the mixture of aromatic products derived from lignin after catalytic oxidation in aqueous IL. Several lignin derived monomers of commercial interest such as guaiacol, syringol, vanillin, acetovanillone, and homovanillic acid were determined quantitatively for both catalysts. Guaiacol was the major oxidation product followed by homovanillic acid, acetovanillone, vanillin, and syringol. For CoCl₂ catalyst, the concentration of guaiacol was in a range of 265.0–536.9 mg L⁻¹ whereas for Nb₂O₅ catalyst the concentration was in a range of 235.4–649.4 mg L⁻¹. The concentrations of homovanillic acid were in the range of 25.5–314.3 and 45.1–361.7 mg L⁻¹, for CoCl₂ and Nb₂O₅ catalysts, respectively. Other products such as syringol, vanillin, and acetovanillone were also present at small proportions. The alkaline lignin used in this study was derived from softwood, which contains primarily G-lignin and a small quantity of S-lignin in the original feedstock (Sigma-Aldrich, 2016). The fact that products obtained from oxidation reactions were mainly G-lignin derived compounds corroborates the G-lignin rich source. Similar profile of compounds were reported in a previous study using [C₂C₁Im][OAc] IL for the pretreatment of switchgrass, eucalyptus, and Kraft lignin (Varanasi et al., 2013). In another study, Stärk et al. (2010) reported similar product profiles from the catalytic oxidation of organosolv beech wood lignin with guaiacol as the main product probably due to the cleavage of β-aryl ether bond.

Product formation during a catalytic oxidation process is partially dependent on the basicity or acidity of the reaction system. Formation of vanillin from Kraft lignin by different oxidants such as H₂O₂, O₂, and nitrobenzene in NaOH medium has been reported (Xiang and Lee, 2000; Rodrigues Pinto et al., 2010; Pandey and Kim, 2011). It was reported that the IL, [C₂C₁im][OAc] possesses dual basic and acidic characteristics as a function of temperature (Varanasi et al., 2012). The hydrogen bond basicity (β) decreased by 1.4% with an increase in temperature from 120 to 160°C (Varanasi et al., 2012). In an aqueous IL system, the hydrogen bond basicity (β value) can be correlated with the disruption of the inter- and intramolecular hydrogen bondings in cellulose, hemicellulose, and lignin (Shi et al., 2014). It is likely that the G-lignin monomeric products obtained in this study were phenolic moieties of G-lignin caused by losing phenolic proton under alkaline conditions at low reaction temperatures (Varanasi et al., 2012). These results lead to the hypothesis that the temperature-dependent dual basic and acidic characteristics of an IL could serve as a design basis to tune the aqueous IL system for better lignin solubility and product selectivity (Park and Kazlauskas, 2003; Shi et al., 2014).

Temperature and catalyst loading were the significant factors for both catalysts. Guaiacol, syringol, vanillin, acetovanillone, and homovanillic acid were the main compounds of the total quantified products (Figure 6). Yields of the total quantified products are detailed in Table S2 in Supplementary Material. Nb₂O₅ catalyst led to higher yield of products than that of CoCl₂ catalyst. Yield (%) with respect to the loaded lignin ranged from 0.26 to 0.99 for Nb₂O₅ catalyst and from 0.41 to 0.90 for CoCl₂ catalyst, respectively, under the tested conditions. The product yields obtained from this study were comparable with results from previous studies using vanadium-based POM in presence of IL 1-butylimidazolium hydrogen sulfate and H₂O₂ with a product (vanillin, syringaldehyde, and guaiacol) yield of 0.055% (De Gregorio et al., 2016; Prado et al., 2016a). Low product yield in this study may be due to repolymerization of depolymerized lignin and/or over-oxidization products to other intermediates which were not detected in the GC/MS. Based on the insight gained from this study, a few possible approaches to improve product yield could be (a) a better control of the reaction time and temperature of the oxidation reaction; (b) selection of more selective catalysts; and (c) in situ product recovery such as using a biphasic system or via continuous product separation.

Complexity of lignin structure hindered the understanding of the reaction mechanisms of the substrate under catalytic oxidation. Lignin model compounds (monomeric and oligomeric) have been investigated to understand the cleavage mechanism and parameters effecting the depolymerization product and yield (Xu et al., 2014). Depolymerization of organosolv lignin in ILs [C₂C₁Im][OTf] and [C₂C₁Im][Cl] under reductive conditions using Lewis and Brønsted acid catalysts showed low conversion despite the high yields obtained on lignin model compounds (Behling et al., 2016). On the other hand, catalytic oxidation of Kraft lignin, soda lignin, Alcell/organosolv lignin in ILs [C₂C₁Im][DEP], [C₂C₁Im][MeSO₄], [C₂C₁Im][CF₃SO₃], and [C₂C₁Im][EtSO₄] involving various metal oxides such as Ni, Co, V, Cu, Fe, and Mn coupled with oxidants such as O₂ and H₂O₂ were effective on lignin conversion, however, did not lead to high product yield (Chatel and Rogers, 2013; Xu et al., 2014). Zakzeski et al. (2010b) studied the oxidation of organosolv lignin and soda lignin with 1-ethyl-3-methylimidazilium diethylphosphate, [C₂C₁Im][DEP] IL in presence of transition metal catalysts; however, the study failed to detect any monomeric products via GC/MS. In another study, oxidation of lignin model compounds and lignin was investigated using dispersed metal nanoparticle catalyst in [C₄C₁Im][MeSO₄] or [C₄C₁Im][PF₆] (Zhu et al., 2012). It was postulated that there was insufficient disruption of lignin linkages to yield monomeric products or over-oxidation of monomeric product in the latter case.

Oxidation of lignin is a complex process, involving various kinds of reaction pathways. On the basis of previous literature (Zakzeski et al., 2011) and results obtained for the current study, we proposed a plausible reaction sequence for lignin oxidation in aqueous [C₂C₁Im][OAc]. Anion [OAc] from the IL coordinates to the Co or Nb, forming Co[OAc] or Nb[OAc] species. We hypothesize that at 120°C IL behaves as an alkaline medium and under these conditions hydrogen peroxide dissociates to perhydroxyl anion (HOO^–), an extremely strong nucleophile (Xiang and Lee, 2000; Pokutsa et al., 2009). Perhydroxyl anion (HOO^–) combines with Co[OAc] or Nb[OAc] species to cleave the aryl ether bonds of the phenolic moieties of the lignin to form lignin monomers (Varanasi et al., 2012).