Ru-Catalyzed Hydrogenolysis of Lignin: Base-dependent Tunability of

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Ru-Catalyzed Hydrogenolysis of Lignin: Base-dependent Tunability of Monomeric Phenols and Mechanistic Study Helong Li, and Guoyong Song ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00556 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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ACS Catalysis

Ru-Catalyzed Hydrogenolysis of Lignin: Basedependent Tunability of Monomeric Phenols and Mechanistic Study Helong Li, Guoyong Song* Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing 100083, People’s Republic of China

ABSTRACT: Substantial attention has been given to depolymerization of lignin into monomeric phenols in recent years since lignin is a renewable and CO2-netural aromatic resource. Recent results indicated that the base can shift the selectivity from C3-frgmented phenols to C2frgmented phenols partially in transition metal-catalyzed lignin hydogenolysis, while reaction mechanisms have remained elusive. Using a series of dimeric, trimeric and polymeric β-O-4 lignin mimics, as well as their deuterated analogs, we now report an in-depth experimental study on the mechanism of Ru/C-catalyzed hydogenolysis lignin. Experimental evidences based on substrate probes, reactivity examination of possible intermediates and isotopic labeling experiments confirmed that the reported pathways, such as enol ether generated via α,βdehydration reaction or Cα carbonyl compounds generated via dehydrogenation or consecutive Cβ–O and Cγ–OH bonds hydrogenolysis, are irrelevant to current reactions. For C3-frgmented phenols with Ru/C catalyst under neutral condition, we deduced that the monolignol such as

1 Environment ACS Paragon Plus

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coniferyl alcohol is formed primarily through a concerted hydrogenolysis process, where Cα–O and Cβ–O bonds are ruptured synchronously. For C2-frgmented phenols generated by the combination of Ru/C and Cs2CO3, the reaction should start from quinone methide specie generated from the dehydration (or demethanolization) reaction between phenolic proton and Cα–OH (or Cα–OMe). The followed deprotonation of Cγ–OH and the coordination of oxygen with Ru results in a Ru specie, which undergoes Cβ–H, Cβ–O and Cβ–Cγ bonds cleavage to release 4-vinylphenol. In the case of Ru/C-catalyzed hydrogenolysis of an enzymatic mild acidolysis lignin (EMAL) derived from birch tree, the effects of some key parameters such as temperature, reaction time, as well as the type and dosage of base, were also examined in terms of monomer yields and selectivity. We found the formation of C2-phenols is a base-dependent process, which is in line with the proposed mechanism. Under optimized conditions, a high proportion of C2-phenols (44%) could be obtained with 26.6 wt% total monomers yield. KEYWORDS: lignin, hydrogenolysis, base-dependent, Ru/C, mechanism

1. INTRODUCTION Replacing fossil-based feedstocks with sustainable lignocellulosic biomass to produce chemicals and fuels is a key challenge facing humankind.1-4 Lignin is a main component of lignocellulosic biomass, which accounts for nearly 30% organic carbon on earth and is the few renewable sources of aromatic chemicals.5-7 In current biorefineries, lignin is treated as a waste product and the use of lignin is largely restricted to its caloric value through burning.5 Depolymerization of lignin into basic aromatic building blocks suitable for downstream processing such as phenols, in principle, is recognized as the most straightforward and economical method for lignin valorisation.7-14 Despite various progresses have been made in


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ACS Catalysis

valorisation of (hemi)cellulose from biomass,15-16 the catalytic conversion of lignin still remains one of the foremost challenges because of its natural complexity and high stability of lignin bonds. In the plant cell wall, lignin is biosynthesized through oxidative radical polymerization from primary monolignols, i.e. p-coumaryl alcohol (H), coniferyl alcohol (G), and sinapyl alcohol (S), among which β-O-4 units consisting a secondary benzylic alcohol (Cα–OH) and a primary aliphatic alcohol (Cγ–OH) are the most abundant connections (Figure 1).5-7,17 Reductive depolymerization of lignin, a converse process of lignin biosynthesis, is one of the most efficient methods to produce phenols from lignin, taking advantage of resulting products in high yields and in a narrow distribution form.10-13 Such processes usually use a heterogeneous catalyst based on transition metals, including Ru,18-23 Pd,18,22,24-31 Ni,32-38 W,38-39 Cu,40-41 Mo42-45 and bimetallic catalysts.24,26,38,44,46-47 Generally, the hydrogenolysis of isolated lignin, native lignin, as well as lignin model compounds aimed to cleave the weaken Cβ–O bonds in β-O-4 structure, thus affording phenolic products having a three-carbon end-chain (C3-fragmented) (Figure 1). Xu and Wang,32 Sels,19,28,35 Abu-Omar,24,26,34 Samec,37 Hensen31 and Román-Leshkov20,36 have reported that reductive catalytic fractionation (RCF) of native lignin in woody biomass afforded monomeric phenols bearing 4-propyl and/or 4-propanol substituents. Samec25 and we42 demonstrated that phenols with an unsaturated end-chain, such as 4-propenyl and 4-allyl group, could be generated from depolymerization of lignin with Pd/C or Mo catalysts, respectively. The organocatalyst, such as B(C6F5)3 also enabled the reductive depolymerization of lignin using hydrosilanes as reductants, which resulted in 4-propyl and 4-silyloxypropyl substituted aromatics.48 In addition to these systems for direct reductive cleavage, the two-step sequences


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involving oxidation at Cα position and Cβ–O cleavage, also furnished C3-fragmented phenols.4950

From a practical perspective, it is worthwhile that lignin can be depolymerized into different monomeric phenols in a tunable and selective fashion. In light of multifunctional groups and tunable reactivity of β-O-4 structures, the production of different aromatic compounds through C–C bond cleavage of lignin model compounds has been realized.51-55 However, the catalytic version involving C–C bond cleavage for actual lignin biopolymer is still rare. Hartwig and coworkers described that the both 4-propyl (C3-fragmented) and 4-ethyl (C2-fragmented) phenols were generated in Pd/C-catalyzed fragmentation of dioxanesolv pine wood lignin.29 Mechanistic study by using a series of dimeric lignin model compounds suggested that the formation of 4-ethylphenols involved dehydrogenation of Cα–OH

to form Cα=O,

dehydrogenative decarbonylation (C–C bond cleavage) and hydrogenolysis of Cβ–O bond. Luterbacher and coworkers reported that the isolated lignins with protection group on α,γ-diol, could be converted into phenols with ethyl end-chain,21-22 where 4-ethyl substituted phenols were probably generated from Ru/C-catalyzed dehydroxymethylation of monomeric 4-propanol substituted phenols.22 OMe

OH MeO

 HO

O

 O 4 

O

OMe

O

OH

OH

metal catalyst NaOH C-O and C-C cleavage

O HO

R

HO

OMe

HO

MeO

MeO

OMe C3-frgmented phenols

OMe

Lignin

OMe

O

C-O cleavage

O

HO

MeO HO

metal catalyst

O

-O-4

O O

ref 30, 47

MeO HO OMe C2-frgmented phenols

Figure 1. Transition metal-catalyzed lignin hydrogenolysis leading to C3- and C2-fragmented phenols.


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ACS Catalysis

Alkaline regimes, such as NaOH, Ca(OH)2 and ammonia, are known for promotion delignification, as well as lignin fragmentation, dissolution, and/or repolymerization in biomass pretreatment processes.56 Recently, Sels30 and Yan47 found that the combination of a transition metal catalyst and NaOH enabled the generation of 4-ethyl substituted phenols partially from lignin hydogenolysis, together with observation of an obvious decreasing of total monomers (Figure 1).30,47 Under such a case, a quinone methide and a vinyl ether (such as 8) were hypothesized as the possible intermediates for 4-ethyl phenols.30 Despite monomeric phenols distribution has been regulated by alkaline regime, many questions remain unsolved. We initiated our studies with the following questions: (1) What are the possible pathways leading to C3- and C2-fragmented phenols in transition metal-catalyzed lignin hydogenolysis? (2) Can we shift the selectivity from C3-fragmented phenols to C2-fragmented products with desirable total monomers yields? Herein, we report an in-depth experimental investigation on the reaction mechanism of Ru/C-catalyzed hydrogenolysis of lignin by using a series of dimeric, trimeric and polymeric model compounds, as well as their deuterated analogs. We found that the formation of C2-fragmented phenols in Ru/C-catalyzed depolymerization of enzymatic mild acidolysis lignin (EMAL) is a base-dependent process, and a high proportion of 4-ethyl substituted phenols could be generated under optimized conditions from catalytic hydrogenolysis of EMAL with a desirable total monomers yield. On the basis of the reactivity model compounds and related intermediates, the plausible pathways towards C3- and C2-fragmented phenols were also depicted in the paper.


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2. RESULTS AND DISCUSSION The reactivity of β-O-4 model compound and derivatives. It is well known that both alkaline catalysis56 and reductive metal catalysis8-9 can been used to delignificate of lignocellulosic

biomass

and

produce

well-defined

aromatic

compounds

via

lignin

depolymerization. To identify the roles of the base and transition metal in such a process, a phenolic dimeric lignin model compound 1 having a β-O-4 structure was firstly examined under various conditions (scheme 1). On the treatment of 1 with Ru/C (10 wt%) in MeOH under H2 (20 atm) and 160 oC for 3 h, a complete conversion of 1 was observed, which gave coproducts derived from the cleavage of Cβ–O bond and subsequent hydrogenation, 4-propyl guaiacol 2 (35%), 4-propanol guaiacol 3 (33%) and guaiacol 4 (90%), as was consistent with a prior report (Scheme 1a).57 In a comparable condition, but with no H2 present, the reaction proceeded to Cβ– O bond cleavage and dehydroxylation of Cγ–OH to give 4-propenyl guaiacol 5 and guaiacol 4 in 55% and 60% yield, respectively (Scheme 1b). To our surprise, a control experiment in the absence of Ru/C under N2 also led to the formation of coniferyl methyl ether 6 and 4 in relative low yields (26% and 19%, respectively) via Cβ–O bond cleavage (Scheme 1c). The addition of radical

scavengers,

such

as

2,2,6,6-tetramethyl-1-piperidinyloxy

(TEMPO)

or

1,1-

diphenylethylene (2 to 5 equiv), did not influence this Ru/C-free reaction, which may rule against a radical process. Raising reaction temperature or prolonging reaction time caused significant decrease of the yields of 6 and 4, with observation of insoluble polymers. In this reaction mixture, formaldehyde (HCHO) was also detected in ca. 12% yield, in contrast with Rucatalyzed reaction (Scheme 1a), where no HCHO was detected. Performing the reaction in CD3OD led to the generation of CD2O by analysis of GC-MS (for details, see SI). It appears, therefore, that the decomposition of 1 is a redox transformation, where MeOH acted as a


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ACS Catalysis

hydrogen donor, verifying the hypothesis proposed by Sels.35 The above results indicated that Ru/C can accelerate the C–O bond cleavage in β-O-4 moiety, as well as can avoid the recondensation of resulted monomers and etherification reaction of Cγ–OH. We then introduced Cs2CO3 (10 mol%) into the Ru/C-catalyzed system, from which a C2fragmented product, 4-ethyl guaiacol 7 (56%), together with guaiacol 4 (85%) were generated. No C3-fragmented product was detected (Scheme 1d). Such a process should involve Cβ–Cγ and Cβ–O bonds cleavage, from which formaldehyde was supposed to release under basic condition.30 An insoluble polymer, generated from recondensation reaction, was also observed, which is consistent with the phenomenon of catalytic reductive fractionation of lignocellulose by using Pd/C and NaOH.30 In the absence of Ru/C, the reaction of 1 with Cs2CO3 in MeOH afforded vinyl ether 8 in 90% yield (Scheme 1e), with loss of the terminal hydroxymethyl group (HCHO). The generation of 8 rather than 7 implied that Ru/C takes part in Cβ–O bond cleavage step under basic condition. Previous report hypothesized that vinyl ether 8 is a possible intermediate for 7.30,58 To test this, we treated 8 with Ru/C (10 wt%) under H2 with or without Cs2CO3 (Scheme 1f). Both of them gave 2-methoxy-4-(2-(2-methoxyphenoxy)ethyl)phenol as a hydrogenated product in high yields, wherein trace 4-ethyl guaiacol 7 (5%) was detected even under harsh reaction conditions (260 oC, 3 h) (see SI). These results implied that vinyl ether 8 may be not enroute to the formation of 4-ethyl guaiacol 7 and the use of Ru/C can avoid the pitfall of 8.


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R OH a)

Ru/C (10 wt%) + H2 (20 atm), MeOH HO OMe 160 oC, 3 h 2, R = H, 35% con: 95% 3, R = OH, 33%

OMe 4, 90%

OH b)

Ru/C (10 wt%) MeOH, 160 oC, 3 h con: 94%

HO

O 

HO

OMe

4, 60% OMe

4

 OH

OMe

OMe 5, 55%

MeO HO

+

c)

MeOH 160 oC, 3 h con: 68%

OH

1

d)

e)

+

HO OMe 6, 26%

+ HCHO

Ru/C (10 wt%) Cs2CO3 (10 %)

OH

H2 (20 atm), MeOH HO 160 oC, 3 h OMe con: 96% 7, 56%

Cs2CO3 (10%) MeOH, 160 oC, 3 h HO con: 98%

OMe 4, 19%

+ OMe 4, 85%

O

X

Ru/C w/ or w/o OMe Cs2CO3 MeOH H2 f)

OMe 8, 90% (E : Z = 60:40)

Scheme 1. The reactivity of phenolic β-O-4 lignin model compound 1

It has been reported that enol ether such as 9, an α,β-dehydrated product from β-O-4 structure, is an important intermediate in lignin decomposition reactions either under alkaline59-60 and acidic conditions61-62 or with transition metal catalysts.21,63 This compound can be generated from dehydration reaction of 1 in ionic liquids;59-60 however, the reactivity studies of enol ether are rare probably owning to its unstable nature. To synthesize the enol ether, we changed the solvent from MeOH to MeCN, in which Cs2CO3 proved to be a good catalyst for dehydration 1 into 9 with 75% NMR yield and 34% isolated yield at 100 oC (Scheme 2). Other bases, such as NaOH, K2CO3 and CsOAc, failed to catalyze the α,β-dehydration of 1. Compound 9 is sensitive to moisture, and polymerized into an undissolved solid in CDCl3 in 2 days. We performed a series of experiments by using 9 as a substrate. The treatment of 9 with Ru/C in MeOH at 160 oC gave a complicated mixture containing polymeric product. Further analysis of the soluble


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ACS Catalysis

fraction confirmed that in addition to 2 (8%), 3 (15%), 5 (6%), and 4 (50%), Hibbert ketones 10, the proposed intermediates in Ru/C-catalyzed hydrogenolysis of β-O-4 moiety,21 and compound 11 generated via hydrogenation of 9,29,63 were also produced (Scheme 2). In the absence of Ru/C, independent experiments of 9 with Cs2CO3 or the combination Ru/C and Cs2CO3 led to an insoluble polymer with no detectable vinyl ether 8 or monomeric phenols. These observations agreed to the conclusion drawn by Marks that 9 is most likely responsible for the dimer polymerization.63 The reactivity studies hinted that 9 may be not the intermediate for C2 and C3 phenols in Ru/C catalytic system.

OH

OH Ru/C (10 wt%) H2 (20 atm), MeOH 160 oC, 3 h con: 96%

MeO

+

+

R 2, R = H, 8% 3, R = OH, 10%

MeO

1

Cs2CO3 (10%) MeCN, 100 oC, 12 h con: 75% HO

5, 6%

R O 10a, R = H, 9% 10b, R = OH, 10% OMe

OH

O +

OH

OMe

OH

MeO

MeO

MeO

O + HO

4, 65%

9, 34% Cs2CO3 (10%) w/ or w/o Ru/C (10 wt%)

OH OMe

11, 28%

polymeric products

H2 (20 atm), MeOH 160 oC, 3 h

Scheme 2. The synthesis and reactivity of enol ether 9

The influence of hydroxyl group. In case MeOH was used as a solvent, Ni-catalyzed hydrogenolysis of lignin35 as well as model compound 164 can undergo a nucleophilic substitution at Cα position with MeOH to form an α-methoxylated intermediate, which showed higher reactivity than its precursor in β-O-4 cleavage.64-65 To assess whether the α-OH group influence the activity and selectivity, we treated model compound 12 under above reaction conditions (Scheme 3). The reactions of 12 occurred in a fashion akin to those of 1, that is,


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forming 2 (38%) and 3 (33%) with Ru/C, affording 7 (51%) exclusively with Ru/C and Cs2CO3, and giving vinyl ether 8 (90%) with Cs2CO3, respectively (Scheme 3a). The completely compatible reactivity and selectivity suggested that 12 is a possible intermediate towards C3- and C2-frgmented phenols. To estimate the influence of phenolic group, a nonphenolic dimeric β-O4 mimic 13 was subjected to the reactions (Scheme 3b). With Ru/C catalyst, hydrogenolysis of 13 proceeded to Cβ–O cleavage to afford veratrole derivatives 14 as a C3-fragmented product, being in line with previous report.21 Unlike compound 1, neither C2 end-chained product nor vinyl ether specie was formed when Cs2CO3 was introduced. Obviously, the desert of phenolic group disables the regulation ability of Cs2CO3 in these reactions. The formation of 15 under Ru/C-free condition testified again that Cα methoxylation may be the onset of all transformations. Based on these results, it was concluded that 1) Ru/C is essential for the rupture of nonphenolic dimeric β-O-4 unit, and 2) Cs2CO3-participated reactions, such as the formation of enol ether 9, vinyl ether 866 and 4-ethyl guaiacol 7, may start from the phenolic group. The critical role of phenolic group is consistent with the pioneering results in the context of alkaline pulping processes, where a quinone methide can be formed from phenolic units when a suitable leaving group such as OH and OMe is present in the α-position.56,60,67


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ACS Catalysis

Ru/C (10 wt%)

a)

H2 (20 atm), MeOH 160 oC, 3 h

OH

MeO

R

HO

+ OMe

2, R = H, 38% 3, R = OH, 33%

4, 83%

MeO MeO

O OH

Ru/C (10 wt%) Cs2CO3 (10%) H2 (20 atm), MeOH 160 oC, 3 h

HO

OMe

OH

MeO +

OMe

HO 7, 51%

4, 80%

12 OMe O Cs2CO3 (10 %) MeOH, 160 oC, 3 h

HO OMe

b)

Ru/C (10 wt%) H2 (20 atm), MeOH 240 oC, 3 h

O OH

MeO

OMe

R

OMe

14a, R=H 20% 14b, R=OH 15%

MeO

H2 (20 atm), MeOH 240 oC, 3 h

MeO

4, 38%

OH + OMe

14c, 38%

4, 41% OMe

OMe w/ or w/o Cs2CO3 (10 %) MeOH, 200 oC, 3 h

+

MeO

Ru/C (10 wt%) Cs2CO3 (10%)

13

OH

MeO

MeO HO

8, 90% E : Z = 60:40

O OH

MeO OMe

15, 85%

Scheme 3. The influences of hydroxyl group on β-O-4 model compounds

A more complex trimetric β-O-4 model compound 16 bearing two β-O-4 units was treated with Ru/C, which led to a clean Cβ–O cleavage either under neutral condition or with catalytic amount of Cs2CO3 (Scheme 4). The selectivity was different. In the absence of a base, monomeric phenols such as 2 (38%) and 3 (17%), as well as o-ethoxylanisole derivatives such as 17a (41%) and 17b (15%), were generated as C3-fragmented products (Scheme 4a). In the case of the combination of Ru/C and Cs2CO3, o-ethoxylanisole moiety was all transformed into C3 products, such as 17a (14%), 17b, (15%) and 17c (56%), while both C2-fragmented phenol 7 (38%) and C3-fragmented phenol 2 (12%) from intermediary unit of 16 were detected (Scheme 4b). It was inferred that phenolic group in intermediary unit would be exposed after the first Cβ– O bond cleavage, thus paving the route to C2 product under basic condition. To assess the possibility that 4-ethyl guaiacol 7 was produced from direct C–C bond cleavage of C3


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monomeric phenols, a series of monophenols involving 2, 3, 5 and sinapyl alcohol were tested over Ru/C and Cs2CO3 at 220 oC (see SI). As results, no 4-ethyl guaiacol 7 was observed in all cases, indicating that the generation of C2-fragmented phenol starts from the β-O-4 structure under current conditions.22 OEt

OH

HO

MeO a) Ru/C (10 wt%) H2 (20 atm) MeOH,160 oC, 12 h

+

MeO

OH + MeO

HO

MeO

O EtO

4, 88 %

OH OH

MeO

R R 17a, R = H, 41% 2, R = H, 38% 3, R = OH, 17% 17b, R = OH, 15%

O

MeO

OH

16

OH

MeO

MeO

OH MeO

+

+

b) Ru/C (10 wt%) Cs2CO3 (10 mol%) H2 (20 atm) MeOH,160 oC, 12 h

7, 38%

2, 12%

4, 90 %

R EtO

+

EtO

MeO

MeO

17a, R = H, 14% 17b, R = OH, 15%

17c, 56%

Scheme 4. Ru/C-catalyzed decomposition of trimetric β-O-4 model compound

Deuterated Experiments. To elucidate plausible pathways towards C3- and C2-fragmented phenols under current conditions, a series of deuterated experiments were conducted (Scheme 5). Performing the Ru/C-catalyzed decomposition of 1 in CD3OD under N2 resulted in 5-D1 incorporated one deuterium at γ position, wherein no H/D exchange was detected at α and β positions, suggesting that dehydroxylation is a hydrogenolysis process (Scheme 5a). The experiments by using an α,β-deuterated model compound 1-D2 were also conducted. Similarly to 1, the hydrogenolysis of 1-D2 catalyzed by Ru/C under H2 underwent C–O bond cleavage and subsequent hydrogenation to furnish C3-alkylated phenols 2-D2 (18%) and 3-D2 (65%), as well as guaiacol 4 (79%). Evidences from 1H NMR and mass analysis of isolated 2-D2 and 3-D2


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ACS Catalysis

confirmed that deuterium still preserved well at α and β positions (Scheme 5b, for details, see SI). In the case of reaction carried out under N2, 4-propenyl guaiacol 5-D2 was generated in 48% yield, without the loss of deuterium (Scheme 5c). Under catalyst-free conditions, the reactivity of 1-D2 was same to 1, thus affording 6-D2 (25%) bearing two deuterium (Scheme 5d). Previous results have suggested some possible pathways leading to C3-alkylated phenols in late transition metal catalysis, including 1) dehydrogenation at Cα position to form a ketone intermediate, followed by hydrogenolysis Cβ–O cleavage, hydrogenation and dehydration;41,68 2) the generation of enol ether such as 9 via α,β-dehydration (or demethanolization),60 followed by either hydrogenation of C=C bond and hydrogenolysis Cβ–O bond62 or hydrolysis of Cβ–O (to form Hibbert ketones) and hydrogenolysis;21,61,63 3) the formation of α,β-OH-propyl substituted monophenols from alkyl-alkyl ether or glycosidic bond cleavage, followed by dehydration and tautomerization;69 and 4) consecutive Cβ–O and Cγ–OH bonds cleavage via hydrogenolysis, followed by Cα–OH and Cβ–H dehydration, and subsequent hydrogenation (for details, see SI).32 Obviously, these mechanisms does not account for current reactions, because deuterium would be lost partially there. The homolysis of β-O-4 units, which starts from the free phenolic OH group and undergoes a quinone radical specie,37 seems to be compatible with experiments from deuterated compounds. To assess whether the cleavage of β-O-4 unit by Ru/C occur by homolytic pathway, we conducted experiment of a nonphenolic deuterated compound 13-D2 (Scheme 5f). As a result, α,β-deuterated aromatic compounds 14a-D2 and 14b-D2 were generated, being consistent with the observation of 1-D2 experiments. This result indicated the homolysis is not the necessary pathway for retaining α and β deuterium in Ru/C-catalyzed cleavage of β-O-4 structure.


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A plausible pathway for C3-frgmented phenols was proposed. The fragmentation of β-O-4 structure gives monolignol such as coniferyl alcohol, which undergoes catalytic hydrogenation at C=C double bond and/or alcohol hydrogenolysis at γ-position to result in 2 and 3. In the case of Ru/C-free condition, etherification of coniferyl alcohol with MeOH forms compound 6. Taking into consideration of the primary formation of monolignol containing a C=C bond, together with the well-preservation of α- and β-D, we deduced that the cleavages of Cα–OH (or Cα–OMe) and Cβ–O bonds in β-O-4 structure occur synchronously via a concerted hydrogenolysis mechanism, either with Ru/C catalyst or under catalyst-free condition (for 1). To access whether the hydrogenolysis of Cα–OH bond by Ru/C influence the Cα proton, we conducted the experiments by using α-deuterated 4-hydroxy-3-methoxy-α-methylbenzyl alcohol 18-D and an incomplete βO-4 mimic 19-D as substrates (Scheme 5g). In both cases, 4-ethyl guaiacol 7 was generated, with retaining deuterium at α-position well. Therefore, the existed α-deuterium contradicted with the Pd/C-catalyzed redox neutral transfer hydrogenolysis mechanism (see SI).70 Unlike the reaction of compound 1, the experiment of 1-D2 catalyzed by the combination Ru/C and Cs2CO3 resulted in vinyl ether 8-D2 (72%) instead of 4-ethyl guaiacol 7-D1 (20%) as the major product (Scheme 5e). It was evidenced that α and β deuterium still remained in 8-D2, while β-D has been lost in 7-D1 by comparing their 1H NMR spectra (see SI). These results confirmed again that vinyl ether 8 is not an intermediate for 7 and the formation of 7 and 8 should be competitive. The variant chemoselectivity suggested that Ru and Cs2CO3-participated deprotonation (Cβ–H cleavage) should be involved in the rate-determining step for 7, and Cβ–D bond cleavage would be suppressed due to the isotope effect. Taking into account the existence of α-D in 7-D1, the pathway proposed in Pd/C-catalyzed hydrogenolysis of lignin model compounds leading to C2-frgmented aromatics that involved dehydrogenation of secondary


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ACS Catalysis

alcohol, decarbonylation of primary alcohol and Cβ–O cleavage,29 cannot be a viable pathway for current reaction. Therefore, quinone methide should be a proper intermediate for 7 (vide supra).

a)

H

OMe

OH O HO

1

OMe

MeO

Ru/C (10 wt%) CD3OD, 160 oC, 4 h

OH

H D

+ 4

HO

41%

5-D1, 35% D

D MeO + HO

b) Ru/C (10 wt%) MeO H2 (20 atm), MeOH HO 160 oC, 3 h

D

OH + 4

D 3-D2, 65%

2-D2, 18%

79%

D

c) Ru/C (10 wt%) MeO

MeO

o

MeOH, 160 C, 3 h

DO

HO

D

4

5-D2, 48%

OH

52%

D

1-D2 OMe

HO

+

D

HO

d)

OMe

MeO

MeOH,160 oC, 3 h

HO

+

30%

6-D2, 25%

D

D

(10 wt%) MeO e) Ru/C Cs CO (10 %) 2

4

D

OMe

MeO

O

+

3

H2 (20 atm), MeOH HO 7-D1, 20% 160 oC, 3 h

D

HO

8-D2, 72%

OH OMe

f)

4, 21%

D OH

D

O

Ru/C (10 wt%)

D OH

MeO OMe

13-D2

D

MeO

OMe

D

MeO

H2 (20 atm) MeOH, 240 oC, 4 h

OH

MeO +

D

MeO

14a-D2, 19% +

14b-D2, 21%

HO 4, 45%

MeO D

OH

g)

Ru/C (10 wt%) D HO OMe 18-D

H2 (20 atm) MeOH, 160 oC, 3 h

HO OMe 7-D1, 88% D

OH O D HO

MeO OMe

19-D

Ru/C (10 wt%) H2 (20 atm) MeOH, 160 oC, 3 h

+ HO HO OMe 7-D1, 58%

MeO 4, 45%

Scheme 5. Deuterated experiments of model compounds

Hydrogenolysis of β-O-4 synthetic lignin polymer. Having shown that the protocols for Ru/C-catalyzed Cβ–O bonds cleavage of lignin model compounds leading to C2 and C3-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

fragmented phenols, we investigated cleavage of a synthetic lignin polymer composed exclusively of the β-O-4 substructure, prepared through polycondensation and subsequent reduction (Figure 2).71 The identification and quantification of monomers were assessed by GCMS and GC, by comparison with authentic samples. On the treatment of Ru/C (10 wt%) in MeOH at 160 oC and 20 atm H2, the guaiacyl-type β-O-4 polymer was depolymerized into an oily product, in which 4-propyl guaiacol 2 (28 wt%) and 4-propanol guaiacol 3 (28 wt%) were identified as primarily products (Figure 2a).24 When the combination of Ru/C (10 wt%) and Cs2CO3 (20 mol% based on β-O-4 unit) was used, in addition to C3 products such as 2 (19 wt%) and 3 (9 wt%), C2 product 7 was also obtained in 26 wt% yield simultaneously (Figure 2b). The molar ratio of C2- and C3-frgmented phenols was identified as 1.1, lower than that from trimetric β-O-4 model compound (intermediary unit, 3.2). 2D HSQC (heteronuclear singlequantum coherence) spectra of resulted lignin oils were collected and compared in Figures 2b. In both cases, the signals of Aα, Aβ and Aγ corresponding to benzylic alcohols and secondary alkyl protons of β-O-4 linkages, almost completely disappeared, as evidenced by comparing HSQC NMR spectra of the oily products with synthetic lignin sample (see SI). The correlation signals of δC/δH = 37.3/2.44, 24.7/1.54 and 14.0/0.88 ppm from propyl group of 2, and signals of δC/δH = 35.0/1.68, 31.6/2.50 and 60.6/3.40 ppm from propanol group of 3, could be observed in both conditions. Particularly, in the case of Cs2CO3-participated hydrogenolysis reaction, the expected C–H correlations corresponding to ethyl group of 7 were found at δC/δH = 28.1/2.49 and 16.3/1.14 ppm (labelled in red) (Figure 2b, right), in agreement with the GC/MS structural analysis.


Page 16 of 38

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ACS Catalysis

Ru/C (10 wt%)

CH2OH H

O

H C

MeO

OH

C H

HO

H2 (20 atm) MeOH,160 oC, 12 h

MeO

R

2, R = H, 28 wt% 3, R = OH, 28 wt%

H n Ru/C (10 wt%) Cs2CO3 (20 mol%)

3200 g mol-1

+ HO

HO

H2 (20 atm) MeOH,160 oC, 12 h

MeO

MeO

R

2, R = H, 19 wt% 3, R = OH, 9 wt%

a) Ru/C

7, 26 wt%

2 3

7

Ru/C and Cs2CO3

IS

4

10

12

14 16 18 Retention time (min)

2

b) Ru/C

20

22

2

Ru/C and Cs CO 2

20

2

2 7

3

30

3 3

2

ppm

7

3

3

2

DMSO

DMSO 7

40 7

50 ArOMe

ArOMe

4.0

3.5

60

3

3

3.0

2.5

2.0

1.5

1.0

4.0

3.5

3.0

2.5

2.0

1.5

1.0

ppm

Figure 2. Catalytic hydrogenolysis of synthetic lignin polymer. (a) Peak identification of the monomers via GC-MS; (b) HSQC NMR spectra of oily products obtained with Ru/C (left) and with the combination of Ru/C and Cs2CO3 (right).

Catalytic hydrogenolysis of EMAL. For the degradation studies involving an actual plantderived substrate, a lignin sample containing β-O-4 (62%), β-5 (2%) and β-β (9%) linkages with molecular weight as 11530 g mol-1 was isolated from birch tree through the combination treatment of enzymatic hydrolysis and mild acidolysis extraction (EMAL).42 Initially, depolymerization reactions were performed in MeOH at 220 oC and 30 atm H2 for 4 h in the


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

presence of Ru/C (10 wt%). Upon completion, the resulting mixture was filtrated, evaporated to dryness and analysed by GC-MS, GPC and HSQC NMR, respectively. In the case of base-free condition, the EMAL underwent complete dissociation of β-O-4, β-5, and β-β structures via C–O bonds cleavage as determined by 2D HSQC NMR,19 to give an oily product with a decreased average molecular weight (610 g mol-1). Both syringyl and guaiacyl-derived monophenols, having a C3 end-chain (propyl, propanol and 3-methoxypropyl), were obtained in total 32 wt%, with an S/G ratio of 1.4 (Table 1, entry 1). The addition of Cs2CO3 (10 wt%) resulted in an incomplete cleavage of C–O bonds in β-O-4 and β-β linkages, which was evidenced by analysis of its HSQC spectrum (SI). Analysis of the resulting oily product by GPC showed that the average molecular weight Mw decreased to 520 g mol-1 (entry 2). The reaction of EMAL by the combination of Ru/C and Cs2CO3 produced 22.4 wt% combined yields of monomeric phenols, which, in addition to retained ether linkages, demonstrated that base hampers lignin depolymerization.30 In this oily residual, besides C3 phenolic monomers mentioned above, C2fragmented phenols such as 4-ethyl guaiacol (7) and 4-ethyl syringol (7S) were identified, directly analogous to those observed in the model studies. The molar ratio of C2 to C3 products was estimated as 0.38, being lower than that from guaiacyl-type β-O-4 polymer (1.1). Further analysis indicated that C2/C3 from guaiacyl-derived phenols (0.48) was higher than that from syringyl phenols, probably because the high reactivity of syringyl units makes the formation of C3 products more competitive in the lignin hydrogenolysis. Increasing the Ru/C dosage in the presence of Cs2CO3 led to continuing improvement of monomers yields, where more C3 phenols were generated via depolymerization of the oligomers (entries 3 and 4).


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ACS Catalysis

A range of bases was screened in the reductive depolymerization of birch lignin in the presence of Ru/C (Table 1). Analogous to the case for Cs2CO3, the hydrogenolysis reactions in the presence of KOH, NaOtBu, CsOAc and K2CO3 proceeded to give C3- and C2-fragmented phenols in 15.9-20.3 wt% combined yields, with the ratios of C2/C3 ranging from 0.33 to 0.42. High ratios of syringyl-derived monophenols (S/G = 2.5 to 5.3) were observed, probably because of a severe recondensation of guaiacyl-derived compounds. It should be noted that the addition of a base also led to the formation of oligomers as a black precipitate simultaneously. This observation, together with the low monomer yield and high S/G ratios, are consistent with basecatalyzed repolymerization processes.56,67

Table 1. Catalytic depolymerization of birch EMAL with combination of Ru/C and basea

Birch EMAL

Ru/C (10 wt%) base (0.015 mmol) MeOH, H2 (30 atm) 220 oC, 4 h

MeO

MeO

R + HO

HO MeO

R = H, OH C3 phenols

Entry

Ru/C (wt%)

Base

Oily product (wt%)

Monomerb (wt%)

1

10

none

78

32

2

10

Cs2CO3

69

3

20

Cs2CO3

4

30

5

MeO C2 phenols

Mw

S/Gc

C2/C3c

610

1.4

0

22.4

520

2.5

0.38

71

23.7

480

1.9

0.32

Cs2CO3

74

26.7

450

1.4

0.24

10

KOH

43

15.9

730

3.7

0.42

6

10

NaOtBu

66

18.8

940

2.8

0.33

7

10

CsOAc

60

20.3

720

3.9

0.41

8

10

K2CO3

45

19.4

760

5.3

0.35

aReaction

(g

mol-1)

conditions: EMAL (50 mg), Ru/C (10-30 wt%), base (0.015 mmol), MeOH (10 mL), H2 (30 atm), 220 °C, 4 h. bDetermined by comparison of lignin degradation products with authentic samples on GC-MS. cMolar ratios.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 38

To test the possibility of base modulated the activity and selectivity, a set of Ru/C-catalyzed reductive depolymerization of birch EMAL was therefore performed at 220 oC in 4 h, varying the dosage of Cs2CO3 from 10 wt% to 100 wt%. The data are presented in Figure 3. The yield of total monomeric phenols and the average molecular weight of lignin oil steadily declined with the increasing of the dose of Cs2CO3 (see also SI). These observations, together with appearance of more generation of oligomers (precipitate), confirmed again that base intensifies the repolymerization reactions. In the selectivity profile, the ratios of C2/C3 is linear with the amount of Cs2CO3 between the 10 wt% to 60 wt% range, and became less dependent over 80 wt% amount of Cs2CO3. Clearly, the base can accelerate the generation of C2 products, thus modulating the selectivity of Ru/C-catalyzed hydrogenolysis of EMAL. Finally, in the presence of 100 wt% Cs2CO3, the reductive depolymerization of birch EMAL gave a 16.2 wt% yield of total monomers, where the molar ratio of C2/C3 products was determined as 0.68. Since the formation of C2 and C3 phenols is competitive reactions (vide supra), we proposed that the high concentration of base may facilitate the formation of quinone methide specie, the deprotonation of Cγ–OH, as well as Cβ–H cleavage.72


Page 21 of 38

0.7 30

25

R2 = 0.9917

0.6

20 total monomers

15

0.5 10

C3 phenols

5

C2 phenols

C2/C3 (molar ratio)

monomer yield (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

0.4 0 0

10

20

30

40

50

60

70

80

90

100

Cs2CO3 dosage (wt%)

Figure 3. Influence of Cs2CO3 dose to monomers yield and selectivity in hydrogenolysis of EMAL. Reaction conditions: EMAL (50 mg), Ru/C (5 mg), MeOH (10 mL), H2 (30 atm), 220 °C, 4 h. Above results suggested the finding of proper balance between shifting selectivity towards C2fragmented phenols and avoiding recondensation is a key issue to achieve high monomers yield with high ratio of C2 products in base-participated hydrogenolysis of lignin. As β-O-4 structures still remained under the conditions of 220 oC and 4 h (vide supra), we carried out the experiments either by prolonging reaction time or improving the reaction temperature. Considering that large dose of Cs2CO3 would lead to irreversible recondensation severely, 10 wt% of Cs2CO3 was employed in all reactions. Reaction time-course data, obtained at 220 oC and 30 atm H2 with Ru/C (10 wt%), presented in Figure 4. Prolonging reaction time to 8 h led to a higher total monomer yield (27.4 wt%), compared to those in 2 h (17.4 wt%) and 4 h (22.3 wt%), under which the C2/C3 molar ratio was increased to 0.40. Performing the reaction for 12 h caused a lower monomer yields (22.9 wt%), probably because of a severe recondensation under


ACS Catalysis

harsh conditions. Under such condition, a high molar ratio of C2/C3 was determined as 0.48, implying the formation of C2 phenols is a thermal favoured process. 0.7

0.5

20 0.4

C2/C3 (molar ratio)

0.6

25

monomer yield (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

0.3 15 2

4

6

8

10

12

Time (h)

Figure 4. Reaction time-course data for hydrogenolysis EMAL with Ru/C and Cs2CO3. Reaction conditions: EMAL (50 mg), Ru/C (5 mg), Cs2CO3 (5 mg), MeOH (10 mL), H2 (30 atm), 220 °C.

Varied temperatures were also screened for hydrogenolysis of EMAL with Ru/C and Cs2CO3 during 4 h, which revealed that both the yield of phenolic monomers and the selectivity to C2 fragmented phenols were improved with increasing of the reaction temperature systematically (Figure 5). At 260 oC, the combined monomer yield reached to 26.6 wt%, wherein a high molar ratio of C2/C3 was detected as 0.8. The detailed products distribution is represented in SI. Under such a case, the S/G ratio of monomeric phenols (3.0) was slight higher than the S/G monomer composition in the birch EMAL (2.0). The HSQC spectra of resulted lignin oil illustrated a nearly complete conversion of β-O-4 linkages to monomers alkyl chains. The cross peaks of δC/δH = 28.6/2.51 and 16.3/1.15 ppm (labeled in red) ascribed to ethyl chain in 4-ethyl guaiacol (7) and 4-ethyl syringol (7S), and the C–H cross signals for propyl and propanol groups in C3fragmented could also be found in the spectra (labeled in blue) (Figure 6b). 22 Environment ACS Paragon Plus

Page 23 of 38

25

0.8

0.6

15

10 0.4

C2/C3 (molar ratio)

20 monomer yield (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

5

0.2

0 160

180

200

220

240

260

Temperature (C)

Figure 5. Effect of reaction temperature. Reaction conditions: EMAL (50 mg), Ru/C (5 mg), Cs2CO3 (5 mg), MeOH (10 mL), H2 (30 atm), 4 h.

a)

2(S)

2

3(S)

IS

Ru/C

3 7(S)

Ru/C-Cs2CO3

7

20

25

30

35

40

45

Retention time (min)

b)



OH

 







7(S)





2(S)

ppm 20

2(S) OMe OH 2(S)

OMe OH 3(S)

OMe

7(S)

30

3(S)

OH

2(S)

7(s)

3(S) 40 50

C

ArOMe

60

3(S)

70 80

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

ppm


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

HO HO 





HO 

 O 4 OMe

5



4 1 1

A -Aryl ether (-O-4)

B Phenylcoumaran (-5)

O 1



O



 O

C Resinol ()

Figure 6. Catalytic hydrogenolysis of birch EMAL. (a) Peak identification of the monomers via GC-MS; (b) HSQC NMR spectra of oily products obtained with the combination of Ru/C and Cs2CO3 at 260 oC for 4 h.

Mechanism perspective. Having these results in hand, the plausible pathway for Ru/Ccatalyzed hydrogenolysis of lignin leading to C3 and C2-fragmented monophenols was rationalized in Scheme 6. The reaction starts from a nucleophilic attack of MeOH on the benzylic position of β-O-4 units of lignin A to give α-OMe β-O-4 structures such as B. The Cβ–O and Cα– O bonds are cleaved synchronously through a concerted process, thus gives monolignols such as coniferyl alcohol and sinapyl alcohol, which undergoes hydrogenation and dehydroxylation to give 4-propyl and 4-propanol phenols as C3 products. The dehydrated product, such as enol ether 9, is most likely responsible for the repolymerization, and the use of Ru/C may suppress the formation of 9. The Cβ–O bond cleavage causes the exposing of the phenolic hydroxyl group in C. Under basic condition, C converts quinone methide moiety D via demethanolization reaction of phenolic hydroxyl and Cα-OMe, which competes with Cβ–O bond cleavage. Deprotonation of Cγ–OH in D gives negative oxygen specie E, which should be a base-dependent process. In the absence of Ru catalyst, E can be converted to a vinyl ether F by loss of a formaldehyde molecule, where Cα–H and Cβ–H do not participate the reaction. In the presence of Ru catalyst, the coordination of oxygen with Ru takes place quickly to give G. The process from G to 4vinylphenol should comprise the cleavage of Cβ–O and Cβ–Cγ in terms of overall transformation.


Page 24 of 38

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ACS Catalysis

Similar to the formation of vinyl ether F, HCHO is released through Cβ–Cγ bond cleavage under basic condition.73 Deuterated experiment proved that Ru center is essential for the Cβ-H cleavage (vide supra), which probably proceeds via a nucleophilic C–H activation mechanism, where strong base can accelerate the C–H cleavage.72 This also implied that the Ru centers are proximal to Cβ-O bond, thus facilitating the Cβ-O bond cleavage to give 4-vinylphenol. Finally, 4-ethyl substituted phenols are obtained as C2 fragmented product by Ru/C-catalyzed hydrogenation of 4-vinylphenols. In these processes, the concentration of base may accelerate the generation of C2 products at the steps involving the formation of quinone methide, deprotonation of Cγ–OH as well as Cβ–H bond cleavage.

OMe

OH  MeO

HO

O  

OMe

OH

 O 

O

OH

Lignin

OMe

Lignin

O

MeOH

 OH

MeO

HO

-H2O



Lignin OH

MeO

OMe

HO

C-O bond cleavage

OMe



O +

 OMe

A

MeOH

B

OMe

OMe

OLignin

HCHO

OMe OH

Ru/C

OLignin

C

OLignin

base

Ru/C

C-O bond cleavage

hydrogenation and dehydroxylation

-MeOH HO

O

OMe

E

[Ru]O Lignin

O

H+



OMe

F

OMe

OMe

O(Lignin)

OH

Lignin

  O

[Ru]

OMe

-HCHO

HO

D

O

O

MeO

R u/ C

OMe

-H+

OMe OMe

O

base

O

OH

OH

Lignin

Lignin

OMe O proposed

G

C-O, C-H and C-C bonds cleavage

OMe OH

R u/ C

OMe OH

Scheme 6. Proposed reaction mechanisms for the catalysis of lignin by Ru/C and Cs2CO3


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3. CONCLUSIONS The mechanism of alkaline regime shifting the selectivity from C3-frgmented phenols to C2frgmented phenols in Ru/C-catalyzed hydrogenolysis of lignin was systematically studied by means of various experiments using a series of dimeric, trimeric and polymeric β-O-4 lignin model compounds, as well as their deuterated analogs. Some key possible intermediates were isolated and tested in perspective of activity and selectivity. Substantiated by experimental evidences, the reported mechanisms of transition metal-catalyzed hydrogenolysis of lignin, such as enol ether pathways or Cα carbonyl compound pathways or consecutive Cβ–O and Cγ–OH bonds cleavage pathways, are not applicable for C3-frgmented phenols by Ru/C catalyst. It was deduced that Ru/C-catalyzed cleavage of β-O-4 moiety proceeds through a concerted hydrogenolysis process, where Cα–O and Cβ–O bonds are ruptured synchronously. For the case of C2-frgmented phenols generated with the combination of Ru/C and Cs2CO3, the vinyl ether can be ruled out from the mechanistic steps. The reaction is proposed starting from the generation of quinone methide specie via base-catalyzed the dehydration (or demethanolization) reaction. The using of Ru/C can supress the formation of vinyl ether. The followed deprotonation of Cγ–OH and Cβ–H cleavage are also base-dependent steps. In catalytic hydrogenolysis of an EMAL from birch tree with Ru/C and Cs2CO3, the formation of C2-phenols is a base-dependent process, which is according to the proposed mechanism. The type of bases, reaction temperature and time has significant influences on both the activity and selectivity of lignin hydrogenolysis. Under optimized conditions, a high proportion of C2phenols (44%) could be produced with a desirable total monomers yield (26.6 wt%). This work links studies of lignin mimics and actual plant-derived lignin, which provides useful information in the design of novel catalytic systems for lignin depolymerization.


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ACS Catalysis

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: Details of experimental procedures, GPC and GC chromatograms and NMR spectra, tabulated monomer yields (PDF)

AUTHOR INFORMATION Corresponding Author * [email protected] ORCID Guoyong Song: 0000-0002-9221-4078

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT 27 Environment ACS Paragon Plus

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This work was supported by the National Natural Science Foundation of China (No. 21776020), Fundamental Research Funds for the Central Universities (No. 2018BLRD12) and the National Program for Thousand Young Talents of China. We thank Dr. Gen Luo (Dalian University of Technology) for insightful discussions.

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Part 1: General Concept and

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TOC OH

OH OMe

HO O

MeO

OH O

O

O

OMe OMe MeO Lignin

O

O

OMe

O

Ru/C Cs2CO3

Ru/C OH

O

Ar O

MeO HO quinone methide

HO

Ar O MeO HO concerted process

OMe HO

OMe HO

MeO

R

MeO C2 phenols

C3 phenols


Ru-Catalyzed Hydrogenolysis of Lignin: Base-dependent Tunability of

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