Two-Step, Catalytic C–C Bond Oxidative Cleavage Process Converts

Letter pubs.acs.org/acscatalysis

Two-Step, Catalytic C−C Bond Oxidative Cleavage Process Converts Lignin Models and Extracts to Aromatic Acids Min Wang,† Jianmin Lu,† Xiaochen Zhang,† Lihua Li,† Hongji Li,†,‡ Nengchao Luo,†,‡ and Feng Wang*,† †

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: We herein report a two-step strategy for oxidative cleavage of lignin C−C bond to aromatic acids and phenols with molecular oxygen as oxidant. In the first step, lignin β-O-4 alcohol was oxidized to β-O-4 ketone over a VOSO4/TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl)] catalyst. In the second step, the C−C bond of β-O-4 linkages was selectively cleaved to acids and phenols by oxidation over a Cu/1,10-phenanthroline catalyst. Computational investigations suggested a copper-oxo-bridged dimer was the catalytically active site for hydrogen-abstraction from Cβ−H bond, which was the rate-determining step for the C−C bond cleavage. KEYWORDS: lignin, biomass, oxidation, copper, C−C cleavage Scheme 1. Two-Step Strategy for β-O-4 Cleavagea

T

he depolymerization of lignin will provide a renewable way of producing substituted aromatics for our society.1 The valorization of this complex natural polymer must selectively break its major linkages down. The β-O-4 bond accounts for ca. 50% of all the lignin linkages.1c Targeting its cleavage, researchers have developed solvolysis,2 pyrolysis,3 reduction,4 and oxidation methods, 5 as well as their combinations.6 The most-studied approach is the catalytic reduction of the Cβ−O bond.7 We recently reported that birch wood could be converted to phenols via C−O cleavage by a tandem alcoholysis-hydrogenolysis strategy.8 While most efforts have involved cleavage of the Cβ−O bond, we focused on the Cα−Cβ bond in this study. The selective cleavage of C−C bond is a challenge because of its nonpolarity and robustness. Although some works have reported the Cα−Cβ cleavage of β-O-4 models without Cγ− OH,9 the oxidative cleavage of the lignin Cα−Cβ bond, especially in the presence of Cγ−OH, to afford acid is still unsuccessful.10 Toste’s group11 observed an unusual C−O bond cleavage to afford an enone product using vanadium oxocomplex catalysts under aerobic conditions. Hanson and Baker12 studied the oxidative cleavage of C−O and C−C bond in methoxyl-substituted arylglycerol β-aryl ether (β-O-4 linkage model with Cγ−OH) with vanadium catalysts, and β-O4 ketone was the dominant product or reaction intermediate. The oxidation of β-O-4 alcohol to β-O-4 ketone lowers the Cβ−O bond energy from 247.9 to 161.1 kJ mol−1 but increases the Cα−Cβ bond energy from 264.3 to 294.2 kJ mol−1, making the Cα−Cβ bond harder to be converted (Scheme 1).11,12 Bypassing the β-O-4 ketone intermediates, the Cα−Cβ bond © 2016 American Chemical Society

a

The bond energy was obtained from density functional theory (DFT) calculation using 2l (eq 2) as model.

was cleaved using stoichiometric CuCl/TEMPO (43% yield, 100 °C, 40 h, in pyridine),13 or using catalytic amount of copper(I) trifluoromethanesulfonate with 1 equiv of TEMPO and 10 equiv of 2,6-lutidine (52% yield, 100 °C, 40 h),14 or under photoirradiation (70% yield).15 However, the major products of these studies were aldehydes. Substituted aromatic acids are very useful chemical intermediates, and currently, they are produced by hydrocarbon oxidation.16 Thus, developing a sustainable production route for acids from biomass is particularly interesting. Recently our group and others found that the copper catalysts could oxidize Received: July 21, 2016 Revised: August 8, 2016 Published: August 10, 2016 6086

DOI: 10.1021/acscatal.6b02049 ACS Catal. 2016, 6, 6086−6090

Letter

ACS Catalysis C−C bond of ketones to acids and its derivatives.17 We thus investigated the possibility of oxidizing β-O-4 ketone to acids. Herein we report a two-step strategy for converting lignin model compounds to aromatic acids and phenols (Scheme 1). In the first step, the oxidation of lignin β-O-4 alcohol to β-O-4 ketone was realized over a VOSO4/TEMPO catalyst. In the second step, a copper/1,10-phenanthroline complex was highly active and selective for oxygenation of Cβ−H bond to a hydroxyl ketone-like intermediate. This oxidation lowers the Cα−Cβ bond energy by ca. 100 kJ mol−1 and facilitates its cleavage. A wide range of methoxyl-substituted β-O-4 ketones were converted to aromatic acids with 80−95% yields. The βO-4 ketone with Cγ−OH was also successfully oxidized to acid. We commenced our study with catalytic oxidation of lignin β-O-4 alcohol to β-O-4 ketones (Table 1). Previous studies on Table 1. Oxidation of Lignin β-O-4 Alcohol Modelsa

Figure 1. Partial 2D HSQC NMR spectra of organosolv Alcell lignin (a) before and (b) after oxidation.

a 0.5 mmol β-O-4 alcohol, 0.1 mmol VOSO4 (20%), 0.1 mmol TEMPO (20%), 0.4 MPa O2, 2 mL MeCN, 100 °C, 12 h.

benzyl hydroxyl groups to ketones have been widely documented.6b,c,18 Based on our work on alcohol oxidation,19 we found VOSO4/TEMPO to be a selective catalyst for converting β-O-4 alcohols to ketones. Model compounds with different methoxyl substituents were oxidized to the ketones (2a−2f) with 80−98% yields. Only a minor amount of Cα−Cβ/ Cβ-O cleavage occurred, and the selectivity is different compared to the vanadium oxo-complex catalysts of Hanson and Toste.9c,11a,12a,13 Phenolic lignin model was also successfully oxidized to the corresponding ketone (2g) with 82% yield. The oxidation of lignin models with Cγ−OH (1k) gave a mixture of 2k and 2h. 2h may be formed via a retro-aldol reaction of 2k (eq 1). In addition, when decreasing the catalyst

Figure 2. Catalyst screen. 0.2 mmol 2a, 0.04 mmol Cu, 0.04 mmol ligand, 2 mL MeOH, 0.4 MPa O2, 80 °C, 2 h. The major product for CuO, CuCl2, Cu(NO3)2, and CuSO4 was the α-keto ester (phenyl 2oxo-2-phenylacetate).

Cu(OAc)2 showed the highest activity with 40% conversion. The conversion was further increased when nitrogen-containing ligands were added with Cu(OAc)2. 1,10-Phenanthroline (L4) and 2,2′-bipyridine (L5) were among the best two ligands for the oxidation, with 93% and 92% conversion, respectively. However, Cu(OAc)2/L4 catalyst was inactive toward direct oxidation of β-O-4 alcohols to acids. We next investigated the Cu(OAc)2/L4 catalyst in the oxidation of β-O-4 ketones with CH3O groups. With CH3O groups either on the acetophenone part or on the phenoxyl part, β-O-4 ketones were converted to acids with 85−99% yields (Table 2). In some cases, higher reaction temperatures

amount to 5 mol %, nearly the same yield of ketone (2a) can also be obtained with longer reaction time (24 h). The catalyst was also tested on an organosolv Alcell lignin, which was rich in syringl units and consisted of a high proportion of β-O-4 linkages.6b,18c Based on the Aα integrals of the 2D HSQC NMR spectra (Figure 1), approximately 36% of β-O-4 was oxidized.20 The β-O-4 ketone (2a) oxidation was then conducted over several Cu(I), Cu(II) salts, and Cu-oxide catalysts (Figure 2). 6087

DOI: 10.1021/acscatal.6b02049 ACS Catal. 2016, 6, 6086−6090

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ACS Catalysis Table 2. Oxidation of Lignin β-O-4 Ketone Modelsa

Figure 3. (a) X-ray structure of Cu(OAc)2/L4 complex and (b) its differential charge density (DCD), which was defined as the difference in the electron density with and without L4 ligand. The blue and yellow regions in (b) indicate a charge increase and decrease, respectively. (c) Electron paramagnetic resonance (EPR) spectra (at 77 K) and (d) cyclic voltammetry (CV) of Cu(OAc)2 and Cu(OAc)2/ L4. 0.2 mmol β-O-4 ketone models, 0.04 mmol Cu(OAc)2, 0.04 mmol L4, 2 mL MeOH, 0.4 MPa O2, 80 °C, 3 h. a

g⊥ signals of Cu(II) appeared. After coordination with L4, the g⊥ peak apparently shifted to high magnetic field along with five-line hyperfine splittings (Figure 3c), which was caused by the two coordinated nitrogen atoms. CV characterization showed the oxidation potential of copper greatly decreased from 0.5 V vs SCE to −0.04 V vs SCE after coordinating with L4 (Figure 3d). Bader charge analysis from DFT calculation indicated 0.31 electrons were transferred from L4 ligand to copper acetate. The DCD result revealed an anisotropic distribution of copper 3d electrons, with a decrease in the four-coordinated direction and increase in the unoccupied perpendicular direction (Figure 3b). The increase of the electron density in the unoccupied site was beneficial for the adsorption and activation of oxygen. This may account for the enhanced catalytic performance of Cu(OAc)2/L4 complex. We then explored the oxygen activation by Cu(OAc)2/L4. In the enzyme, oxygen is activated to copper-oxo-bridged oxygen dimers, which is active in H-abstraction.22 We calculated the reaction energy for the oxygen activation by Cu(OAc)2/L4 (Figure 4), and we found the copper-oxo-bridged dimer was

(2i and 2j, 100 °C) were needed to achieve 100% cleavage of the β-O-4 ketone. Lignin model with Cγ−OH (2l) was also converted with 100% conversion and 92% yield for acid (eq 2).

The Cγ−OH made the Cα−Cβ bond more resistant to oxidation, and higher reaction temperature was thus required (100 °C for 2i vs 150 °C for 2l). Phenolic β-O-4 lignin model (2g) majorly sustained Cα−Cβ bond cleavage. In addition, when decreasing the catalyst amount to 5 mol %, β-O-4 ketone (2a) was also effectively cleaved with longer reaction time (6 h). Note that the phenols with methoxyl groups were obtained in low yields. Attempts to perform the β-O-4 alcohol cleavage in a one-pot, two-step procedure without switching solvents were unsuccessful. The detection of gas products showed CO2 was generated (Figure S1). A minor amount of benzoylformic acid (3a) was detected (Figure 2), but using it as the reactant gave no benzoic acid (eq 3), indicating that it was not the reaction intermediate. Phenol formate was detected in the initial stage but disappeared after reaction, suggesting that the Cα−Cβ in 2a was first cleaved to phenol formate, followed by Cβ−O cleavage to phenol. A kinetic isotopic experiment (eq 4) showed a large value of kH/ kD (3.1), indicating the Cβ−H oxidation could be the ratedetermining step for the Cα−Cβ cleavage. We obtained the single crystal of the Cu(OAc)2/L4 complex. The X-ray diffraction showed copper and L4 formed a chelating structure (Figure 3a).21 In EPR spectra, well-resolved g// and

Figure 4. DFT calculation of the reaction energy for the molecular oxygen activation by Cu(OAc)2/L4 and H abstraction by copper− oxygen center. 6088

DOI: 10.1021/acscatal.6b02049 ACS Catal. 2016, 6, 6086−6090

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ACS Catalysis Notes

more favorable than the copper-superoxide monomer (−3.78 eV vs −1.16 eV). The ligands, such as L3, L7, and L8, showed poor catalytic results, which were due to the steric hindrance for forming the copper-oxo-bridged dimer.23 The H-abstraction by the copper−oxygen monomer and dimer was further calculated by DFT (Figure 4). The results showed that copper-oxo-bridged dimer was more active than the monomer with a larger reaction energy (−2.18 eV for dimer vs −0.42 eV for monomer). We come to a tentative reaction mechanism (Scheme 2). First, the oxidation of Cα−OH alcohol to a ketone activates the

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (projects 21422308, 21273231, 21403216) and the Dalian Excellent Youth Foundation (2014J11JH126).

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Scheme 2. Proposed Reaction Mechanism

Cβ−H. The Cu(OAc)2/L4 reacted with oxygen to form copper-oxo-bridged dimer. DFT calculation shows the most favorable way for Cβ-H bond oxidation is that the H is abstracted by one oxygen in the copper−oxygen center, and the remaining part is bound to the other oxygen via C−O bond formation. The H-abstraction step is exothermic with −2.18 eV. The activation of the Cα−Cβ bond in the form of hydroxyl ketone structure-like intermediate significantly decreases its bond energy from 307.7 kJ mol−1 to 205.5 kJ mol−1, which enables the Cα−Cβ bond to be easily broken,17 resulting in the formation of benzoic acid and phenol formate. Further transfer of hydroxyl group generates benzoic acid, restoring the initial copper complex. Finally, oxidative decarboxylation of phenol formate generates a phenol and CO2. In summary, we reported a two-step strategy for lignin C−C bond conversion via first, β-O-4 alcohol oxidation to ketone over the VOSO4/TEMPO catalyst, and second ketone oxidation over Cu(OAc)2/1,10-phenanthroline catalyst to acids and phenols.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b02049. Detailed experimental procedures, the synthesis of lignin models, NMR spectra, DFT calculation details, and the atomic coordinates (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 6089

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