Paving the Way for the Lignin Hydrogenolysis Mechanism by Deuterium-Incorporated β-O-4 Mimics

Article abstract of DOI:10.1021/acscatal.0c02339

Gaining more insight into the lignin hydrogenolysis mechanism is of great importance for developing next-generation catalysts and regulating product distribution because lignin is the only renewable aromatic source leading to aromatic chemicals. However, the inherent complexity of the lignin structure and the multiple pathways in lignin hydrogenolysis make gaining insight into the lignin hydrogenolysis mechanism even more challenging. In this report, a β-O-4 polymer with deuterium incorporated at the α, β, and γpositions which can better model the plant lignin structure was prepared for mechanistic study. The location and retention of deuterium in the monomers resulting from the hydrogenolysis and transfer hydrogenolysis of the deuterated β-O-4 mimics with Ru/C, Pd/C, and Pd/Zn/C in MeOH indicated that the β-O-4 linkage protons did not participate in the cleavage process, which indicated that pathways involving dehydrogenation and/or dehydration reactions are infeasible for these catalysts under these conditions. A concerted process, wherein the Cα-O and Cβ-O bonds in β-O-4 structures were cleaved concurrently, was proposed as a potential hydrogenolysis mechanism in the Pd/C and Ru/C systems. Dehydroxylation at the α position was identified as a side reaction in the Pd/C-catalyzed hydrogenolysis of dimer models, from which the same concurrent cleavage mechanism was also inferred. The introduction of a Lewis acid center in Pd/C was conducive to β-O-4 hydrogenolysis, as confirmed by the abatement of the side reaction and the enhanced monomer yield in the polymeric model reactions. The use of deuterium-incorporated β-O-4 mimics paved the way for clearly elucidating the lignin hydrogenolysis mechanism.

Full text of DOI:10.1021/acscatal.0c02339

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Article
Paving the Way for the Lignin Hydrogenolysis
Mechanism by Deuterium-Incorporated #-O-4 Mimics
Helong Li, and Guoyong Song
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c02339 • Publication Date (Web): 23 Sep 2020
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Paving the Way for the Lignin Hydrogenolysis
Mechanism by Deuterium-Incorporated β-O-4
Mimics
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Helong Li and Guoyong Song*
Beijing Advanced Innovation Center for Tree Breeding by Molecular Design, Beijing
Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, No.35
Tsinghua East Road, Beijing 100083, China.
KEYWORDS: lignin, β-O-4 polymer, deuterium, hydrogenolysis, palladium, C−O
cleavage
ABSTRACT: Gaining more insight into the lignin hydrogenolysis mechanism is of
great importance for developing next-generation catalysts and regulating the product
distribution because lignin is the only renewable aromatic source leading to aromatic
chemicals. However, the inherent complexity of the lignin structure and the multiple
pathways in lignin hydrogenolysis make gaining insight into the lignin hydrogenolysis
mechanism even more challenging. In this report, a β-O-4 polymer with deuterium
incorporated at the α, β and γ positions that can better model the plant lignin structure
was prepared for mechanistic study. The location and retention of deuterium in the
monomers resulting from the hydrogenolysis and transfer hydrogenolysis of the
deuterated β-O-4 mimics with Ru/C, Pd/C and Pd/Zn/C in MeOH indicated that the
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β-O-4 linkage protons did not participate in the cleavage process, which indicated that
pathways involving dehydrogenation and/or dehydration reactions are infeasible for
these catalysts under these conditions. A concerted process, wherein the C −O and
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C –O bonds in β-O-4 structures were cleaved concurrently, was proposed as a
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potential hydrogenolysis mechanism in the Pd/C and Ru/C systems. Dehydroxylation
at the α position was identified as a side reaction in the Pd/C-catalyzed
hydrogenolysis of dimer models, from which the same concurrent cleavage
mechanism was also inferred. The introduction of a Lewis acid center in Pd/C was
conducive to β-O-4 hydrogenolysis, as confirmed by the abatement of the side
reaction and the enhanced monomer yield in the polymeric model reactions. The use
of deuterium-incorporated β-O-4 mimics paved the way for clearly elucidating the
lignin transfer hydrogenolysis mechanism.
1
. INTRODUCTION
Lignin accounts for nearly 25% of plant biomass by weight and is one of few
renewable aromatic hydrocarbon sources.1,2 The upgrading of lignin, which is
increasingly noted to be crucial to the economic viability of biorefineries, represents
one of the most challenging aspects of the entire utilization of all biomass
3
components. In principle, the depolymerization of lignin leading to valuable aromatic
chemicals suitable for downstream processing is an important starting point for lignin
valorization.4-7 For this purpose, various methods, such as reduction,8-10
oxidative-reductive processes,11-15 biodegradation,16-19 and electro-20-21 and
photo-driven22-25 depolymerization, have been developed. Of the possible approaches,
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catalytic hydrogenolysis of lignin (reductive method), the converse process of lignin
biosynthesis, takes advantage of high monomer yields, controllable selectivity, a
narrow product distribution and readily purified products.26-28 A series of
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heterogeneous metal catalysts based on Ru,29-33 Pd,34-38 Ni,39-47 Cu,48,49 Mo50-54 and
Co55,56 have been utilized for lignin hydrogenolysis and transfer hydrogenolysis.
In protolignin, a β-O-4 alkyl–aryl ether motif featuring a 2° benzylic alcohol
(α-OH) and 1° aliphatic alcohol (γ-OH) is the most abundant connection linking the
monomeric phenylpropanoid units (Scheme 1).5,6 Metal catalysts are usually
employed to rupture the C −O bond in β-O-4 motifs to afford monomeric products
β
during lignin hydrogenolysis. In addition, the catalysts stabilize the formed
monomeric products via hydrogenation and/or dehydroxylation reactions.9,30,39,57
A
deep understanding of the lignin hydrogenolysis mechanism is therefore of great
importance and interest for developing novel catalysts and regulating the product
selectivity. However, due to the complexity of the inherent lignin biopolymer
structure and the multiple pathways of β-O-4 motif cleavage and the subsequent
stabilization processes, it is extremely challenging to clearly elucidate the
hydrogenolysis mechanism.58,59 During the past two decades, several hydrogenolysis
and transfer hydrogenolysis pathways have been proposed (Scheme 1, see also Figure
S1). 1) The direct hydrogenolysis of the C −O bond in the β-O-4 motif leads to the
β
formation of propanediol-substituted monophenols, which undergoes either sequential
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dehydration and hydrogenolysis (Ni/C) or the hydrogenolysis of α−OH (Pd/C) to
give the final products. 2) A C ketone species is initially formed through a
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dehydrogenation reaction, and subsequent C −O bond hydrogenolysis, hydrogenation
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and dehydration results in monophenols.48,61,62 3) An enol ether intermediate is
generated via a dehydration reaction, and then, the C −O bond is cleaved by H O
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(hydrolysis) or H (hydrogenolysis), thus forming the Hibbert ketone or 4-propanol
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phenols.63-65 4) Homolysis of the β-O-4 units, starting from the free phenolic OH
group and going through a quinone radical species, was also predicted.41,66
Scheme 1. Reported Pathways for the Hydrogenolysis of β-O-4 Lignin into
Monomeric Phenols over Late Transition Metal Catalysts39,41,48,60-67
1)
H2O
RO
MeO
OH
MeO
dehydration/hydrogenolysis
O
O
RO
OH
OH
HO

OH
MeO
from C-O hydrogenolysis
O

H2
MeO
RO
MeO
OH
OH

OH

O
HO
O
OMe
2)
O
OH
MeO
OMe
RO
Ar
O
p-coumaryl alcohol

RO
MeO
OH
O
O
HO
HO
HO
OH
MeO HO
C-ketone

OH
O
OMe
O
OH
Lignin
metal catalyst
HO
MeO
Lignin peroxidase
OH
MeO
OMe
OH
MeO
hydrogen donor
3)
O
RO
Ar
H2O
RO
MeO
OH
OH
oxidative
polymerization
reductive

MeO
depolymerization
OMe
MeO HO
enol ether

O
OH
HO
HO
O
coniferyl alcohol
O
OMe
H2
OH
RO
MeO
4)
MeO
HO
O
O
OH
O
HO
OH
MeO
radical pathways
MeO
OMe
OH
OH
sinapyl alcohol
MeO
5)
OH

O
RO
Ar
RO
MeO
OH
O
O
MeO HO
Recently, we reported a mechanistic study of the Ru/C-catalyzed hydrogenolysis of
67
lignin using deuterium-incorporated β-O-4 dimers (GG-D and VG-D). The location
and retention of deuteriums offered solid evidence that excluded the possibility of the
above pathways in the Ru/C catalytic system. It was proposed that the monolignols
are formed primarily through a concerted hydrogenolysis process, in which the C −O
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and C –O bonds are simultaneously ruptured (Scheme 1). However, the reaction
β
pathway of dimer model compounds with only one β-O-4 unit might differ from that
of actual lignin with polymeric β-O-4 units.68 For example, the interaction of the
catalyst with multiple hydroxy groups on lignin biopolymer would influence the
efficiency of the catalytic reaction, which is difficult to evaluate in dimer model
reactions. To link the studies of dimer β-O-4 mimics to actual plant lignin, an artificial
lignin polymer composed of the β-O-4 substructure with deuteriums incorporated
exclusively at the α, β and γ positions was prepared in this study. Applying Ru/C,
Pd/C and Pd/Zn/C catalysts to this deuterium-incorporated β-O-4 polymer resulted in
explicit evidence for identifying reaction pathways. A comparison of the homology
and dissimilarity of the β-O-4 dimers and polymer provided more insight for
understanding the lignin hydrogenolysis mechanism.
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. RESULTS AND DISCUSSION
Preparation of the Deuterated β-O-4 Polymer. The preparation of the deuterated
lignin polymer (LP-D) was based on the synthesis methodology for the β-O-4
polymer (Scheme 2).69,70 Deuteriums were incorporated at the linkages in two stages.
Stirring compound 1 in CH OD led to the complete H/D exchange of the phenolic OH
3
and methylene proton, thus giving 1-D. Polymerization of 1-D in anhydrous DMF
with K CO resulted in a quantitative yield of a carbonyl polymer (CP-D). Upon
2
3
treatment with NaBD in CH OD, the carbonyl group at the α position and the ester
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3
group at the γ position of CP-D were reduced to benzylic and aliphatic alcohols,
respectively. Precipitation with an HCl solution afforded the deuterated lignin
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polymer (LP-D) in 43% yield. In contrast to CP-D, in which β-D could be readily
exchanged by an acidic proton, the incorporated deuteriums in LP-D were stable
against acidic protons.
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Scheme 2. Synthesis of the Deuterated β-O-4 Lignin Polymer
O
H
OMe
Br
O
D
OMe
Br
O
OMe
MeO
O
O
D
O
O
O
CH OD
3
2 3
K CO
25 oC, 12 h
DMF, 25 oC, 24 h
O
MeO
D
OMe
O
n/2
OMe
OMe
OD 1-D
CP-D, 95%
OH
1
,
99%
OH
OMe

D
NaBD4
CH3OD, 25 oC, 24 h
O
D
HO
D
D

D
D

O
HO
D
D
OMe
OH
n/2
LP-D, 43 %
D% at ,  and : > 99%
-1
Mw = 4200 g mol
-1
The average molecular weight (M ) of LP-D was measured to be 4200 g mol by
w
gel permeation chromatography (GPC), from which the degree of polymerization
(DP ) was established as 20.5. The degree of polymerization of LP-D was higher than
w
-1
that of the nondeuterated synthetic β-O-4 polymer (M = 3200 g mol , DP = 15, see
w
w
1
Figure S3). In H NMR spectra of LP-D (DMSO-d with one drop of D O), side chain
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signals of the nondeuterated β-O-4 polymer (LP) corresponding to α-H (br, 4.76
ppm), β-H (br, 4.30 ppm) and γ-H (3.52, 3.60 ppm) were not observed (Figure S2).
1
3
This result indicated perdeuteration of the β-O-4 linkages. C NMR spectroscopy is a
powerful tool for elucidating complicated lignin structures, and most of the signals in
lignin have been previously assigned.69,71 Both the erythro and threo isomers derived
from the chiral α and β carbons were present in LP-D and could be identified by
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comparing its C NMR spectrum with those of the phenolic (GG) and nonphenolic
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VG) lignin model dimers. The signals at 59.5, 71.4 and 83.4 ppm were attributed to
the γ, α and β carbons, respectively, of the erythro form; the α and γ carbon peaks of
the threo form were observed at 70.7 and 84.0 ppm in an inferior fashion. No clear
C−D coupling patterns were observed for the linkage carbons in the 13C NMR
spectrum, probably because C−D coupling through more than one bond resulted in
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multiple patterns. The aromatic carbons were also assigned by comparing the results
of the lignin model dimers and polymer. These results clearly suggested that polymer
LP-D had β-O-4 linkages between guaiacyl monomer units.
Hydrogenolysis of LP-D with a Ru/C Catalyst. In lignin hydrogenolysis, phenolic
groups would be exposed after the cleavage of internal β-O-4 units, which would
significantly influence the reactivity. Two dimer models with a complete β-O-4
structure, veratrylglycerol-β-guaiacyl ether (VG) and guaiacyl-glycerol-β-guaiacyl
ether (GG), have often been used to model the internal and phenolic units of lignin,
respectively.39,44,59 Compared with the dimer β-O-4 models, the polymeric β-O-4
model can better simulate lignin biopolymer as a whole in depolymerization reactions.
The deuterium-incorporated polymeric lignin mimic LP-D was initially treated with a
Ru/C catalyst (10 wt%) in MeOH at 180 °C for 12 h (Scheme 3). The yields of the
depolymerized monomers were normalized to the total molar amount of guaiacol
units in LP-D by comparison with their authorized samples. This reaction gave
4-propyl guaiacol (4, 22%) and 4-propanol guaiacol (5, 45%) corresponding to 67%
of the carbon balance (C.B.) of LP-D. These two monomers could be isolated from
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the reaction mixture by silica gel column chromatography. H NMR and mass
analyses indicated that the deuteriums at the α, β and γ positions in 4 and 5 remained
intact after hydrogenolysis (Figure S4). The locations of the deuteriums and the
product distribution were in agreement with a previous report that used deuterated
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dimer models (VG-D and GG-D, Scheme 3). The consistency between the β-O-4
dimers and polymer in the Ru/C catalytic system indicated they can feasibly emulate
the lignin biopolymer. The complete retention of deuterium suggested that the linkage
protons in the β-O-4 units did not participate in the deconstruction of the linkages and
subsequent functionalization, thus ruling out the feasibility of reported pathways
involving
Cα
ketones,26,48,61
enol
ethers63-65
or
consecutive
dehydration/hydrogenolysis in the Ru/C-catalyzed hydrogenolysis process.39,60 The
cleavage of C –OH, C –OAr and C –OH (for 4) should follow a hydrogenolysis
α
β
γ
pathway rather than undergoing dehydration or dehydrogenation reactions.
Scheme 3. Hydrogenolysis of Deuterated Lignin Mimics with Ru/C
previous results
OH

OMe
D
D
OH
+
OMe
D
O

Ru/C (10 wt%)
MeO
RO
MeO
+
OH
D
H2 (20 atm), MeOH
HO

D
D
RO
RO
OMe
VG-D (R = Me )
GG-D (R = H )
240 oC, 4 h (C.B., 40%)
(C.B., 83%)
2-D, 19%
4-D, 18%
3-D, 21%
5-D, 65%
45%
79%
60 oC, 3 h
1
100% D retention at  and 
this work
OH

OMe
D
D
D
D
D
D
D
H
O
Ru/C (10 wt%)

+
OH
D
H2 (30 atm), MeOH
180 oC, 12 h
n
HO  DD
D
D
O
HO
HO
OMe
C.B., 67%
OMe 4-D', 22%
OMe 5-D'
, 45%
LP-D
C.B., carbon balance
100% D retention at ,  and 
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Pd/C Catalytic System. The commercially available catalyst Pd/C worked well in the
hydrogenolysis of lignin derived from various plants,36-38 which generally resulted in
62,73,74
4
-propanol-substituted phenols from β-O-4 units under an H atmosphere.
2
1
1
1
1
1
1
1
1
1
1
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9
0
Abu-Omar and coworkers proposed a mechanism for the Pd/C catalytic system
involving the generation of a propanediol-substituted monophenol via C −O bond
β
6
0
cleavage, followed by the hydrogenolysis of C −OH. Marks et al. suggested that
α
enol ethers and the corresponding hydrogenated products (similar to 6 and 7, vide
infra, see also Figure S1) act as intermediates in the depolymerization of lignin
dimetric models in the presence of Pd/C and a Lewis acid catalyst.63 Based on a
kinetic study, Dumesic and Ralph recently proposed that the deconstruction of β-O-4
5
9
motifs with and without phenolic end units follows different reaction pathways. To
gain more insight into the mechanism of lignin hydrogenolysis, the hydrogenolysis of
deuterium-incorporated β-O-4 mimics was performed with Pd/C. The nonphenolic
dimer VG-D was initially treated with 5 wt% of Pd/C in MeOH at 180 °C under H2
(30 atm), and propanol-substituted veratrole 3 and guaiacol were formed in low yields
(12% and 14%, respectively). In addition, two dimeric products, i.e., the
α-methoxylated dimer VG-OMe (69%) and α-dehydroxylation product 6 (6%), were
observed (Figure S5). The VG-OMe dimer has been identified as an intermediate of
6
7
depolymerization in the Ru/C system. Compound 6, generated via the cleavage of
C −OH bond with the reservation C −O bond, has been previously reported in the
α
β
Pd/C-catalyzed hydrogenolysis of VG.59,63 The low yield of 3-D under this condition
made it difficult to isolate for further NMR analysis. The Pd/C dose was then
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increased to 30 wt%, which led to the formation of 3-D in 47% yield, VG-OMe in
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2% yield and 6 in 9% yield, corresponding to an 88% carbon balance (Scheme 4).
1
Analysis of the H NMR spectrum of isolated 3-D indicated that 27% of C -D was
α
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7
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9
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5
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7
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4
5
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0
lost, whereas C -D remained (Figure S5).
β
Scheme 4. Depolymerization of VG-D over Pd/C.
H/D
OMe
MeO
MeO


OH
OH
+
D
3-D, 47%
4
9%
HO

D retention: for , 73%
for , 100%
D
D
O
HO

Pd/C (30 wt%)

OMe
H2 (30 atm), MeOH
HO

HO
1
80 oC, 3 h
D
D
O
D
O
OMe
MeO
C.B., 88%



OMe
+
VG-D
OMe
OMe
OMe
OMe
OMe
OMe
6, 9%
VG-D-OMe, 32%
5
9
Then, GG-D, a more reactive β-O-4 dimer bearing a phenolic group, was treated
with Pd/C in MeOH under various conditions (Figure 1). At 150 °C under H (30
2
atm), GG-D was fully converted by 5 wt% Pd/C, which was consistent with the
results for the nondeuterated version.36,37,59 4-Propanol guaiacol 5 was generated as a
major depolymerization product (43%) with the same selectivity achieved with plant
7
4
protolignin (Figure 1a). Dimeric compound 7 (6 analogue), generated through the
hydrogenolysis of C −OH, was also observed in 40% yield. The yield of 7, along with
α
those of 5 and 4-propyl guaiacol 4 corresponded to 87% of the linkage carbons in
GG-D. With increasing reaction temperature, the depolymerized products (5 and
guaiacol) increased, and dimeric compound 7 decreased (Figure 1b). At 250 °C, only
1
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8% of compound 7 was detected, whereas 5 and guaiacol were formed in 78% and
8
3% yields, respectively. There existed different opinions on the role of 7 and its
analogues in Pd-catalyzed β-O-4 hydrogenolysis. Marks et al. proposed that they are
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0
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3
intermediates leading to the depolymerized products, whereas Ralph and coworkers
7
5
very recently described them as nonreactive. Then, compound 7 was independently
treated with Pd/C. No depolymerization reaction occurred at 220 °C or 250 °C (Figure
1
c). Therefore, it was concluded that 7 is a side product rather than an intermediate,
ruling out the possibility that the hydrogenolysis of β-O-4 motifs starts with C −OH
α
bond cleavage in the Pd/C catalytic system. The fact that the formation of 7 was
favored by a low reaction temperature implied that it is a kinetically controlled
product and should have a relative low energy barrier (Figure 1b). Therefore, it is
difficult to cleave the C −O bond before cleaving the C −OH bond. Because
β
α
pathways starting with the cleavage of the C −OH or C −O bond are not compatible
α
β
with the current β-O-4 hydrogenolysis, it was deduced that these two C−O bonds are
cleaved instantaneously by the Pd/C catalyst. The same conclusion was previously
drawn for the Ru/C catalytic system to account for the simultaneous formation of the
6
7
C =C bond and β-O-4 cleavage without the participation of α-H or β-H. From the
α
β
viewpoint of the cleavage of lignin β-O-4 structures, the same conclusion was reached
independently for the Pd/C and Ru/C catalysts.
The deuterium retentions of the GG-D derivatives were measured based on the
1
analysis of the H NMR spectra (Figure 1, see also Figure S6). At relatively low
temperatures (150 °C and 180 °C), both α-D and β-D were well preserved in 5, which
1
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was consistent with the observation for the Ru/C catalytic system. These results
confirmed that α-D and β-D are not involved in the depolymerization reactions under
these conditions. For compound 7, a partial loss of α-D was observed (50% and 15%
retentions at 150 °C and 180 °C, respectively), whereas β-D was well retained. The
intactness of β-D implied that it is infeasible for 7 to be formed via a dehydration
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3
reaction (to form an enol ether) followed by a hydrogenation reaction under these
conditions. At higher temperatures, the hydrogenolysis of GG-D led to the partial loss
of α-D in 5, as observed at 220 °C (92% D retention) and 250 °C (37% D retention),
but β-D was still well retained. There are two possible explanations for the partial loss
of α-D. 1) The Pd-catalyzed H/D exchange of 5 or 7 leads to the loss of deuterium, or
2) α-D participates in the deconstruction of the β-O-4 motif at high temperatures and
is thus consumed. To probe the feasibility of H/D exchange, 5-D was treated with
Pd/C under H and MeOH, and 10% and 59% of α-deuterium were replaced with
2
hydrogen at 220 °C and 250 °C, respectively (Figures 1d and S7). The similar
deuterium retentions obtained by the hydrogenolysis of GG-D and the H/D exchange
reactions indicated that H/D exchange might cause the loss of α-deuterium after β-O-4
depolymerization. An independent experiment with 7-D1 over Pd/C led to the
complete loss of α-deuterium at 220 °C, again confirming the feasibility of the H/D
exchange reaction (Figures 1c and S8).
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(b)
OH
OMe
OMe

D
OH
MeO
HO
OH
O
80
70
60
O
Pd/C (5 wt%)
guaiacol
OMe

D
+
+

MeOH, 3 h
HO
HO
HO
OH
5
5
7
OMe
GG-D
H2 (30 atm) or N2 (1 atm)
OMe
side & kinetic product
50
(a)
H/D
H
D
O
OMe
OH
MeO
HO
D
OH
OMe
40
30
20
10
5
HO
OH
7
H/D
OMe
yield: 43%
7
yield: 54%
IS -D retention: 100%
-D retention: 100%
yield: 40%
-D retention: 50%
-D retention: 100%
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5
6
0
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2
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6
7
8
9
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5
6
7
8
9
0
50 oC, H2
1

C.B., 87%
140
160
180
200
220
240
260
yield: 64%

yield: 26%
o
Temperature( C)
-D retention: 100%
-D retention: 100%
-D retention: 15%
-D retention: 100%
yield: 67%
yield: 76%
80 oC, H2
1
(
c) HO
C.B., 91%
O
OMe
X
depolymerized products
Pd/C (10 wt%)
(96% of 7 recovered)
yield:70%
OMe
H2 (30 atm), MeOH
220 or 250 oC , 3 h
2
20 oC, H2
C.B., 90%
-D retention: 92%
-D retention: 100%
7
OH
yield: 14%
HO
H
HO
H
O
Pd/C (10 wt%)
H
O
D
yield: 83%
yield: 87%
yield:78%
OMe
OMe
50 oC, H2
-D retention: 37%
-D retention: 100%
H2 (30 atm)
MeOH, 220 oC, 3 h
2

yield: 8%
OMe
C.B., 87%
OMe
7
-D
OH
OH

-D retention: 0
MeO
HO
D
D
OH
H/D OH
(
d)
MeO
HO
yield: 50%
-D retention: 100%
-D retention: 100%
Pd/C (10 wt%)
2
20 oC, N2
C.B., 78%
D
H2 (30 atm)
MeOH
8
D
D
HO
HO
OMe 5-D
OMe
-D retention, 220 oC, 3 h, 90%
2
50 oC, 3 h, 41%
220 oC, 12 h, 37%

-D retention, 100% in all cases
Figure 1. Pd/C-catalyzed hydrogenolysis of GG-D and its corresponding derivatives.
(a) Product distributions and deuterium retentions. Reaction conditions: GG-D (60
mg), Pd/C (3 mg), MeOH (10 mL), 3 h, H (30 atm) or N (1 atm), at the desired
2
2
reaction temperatures. The yields were determined by GC, and the retentions of
1
deuterium were measured by analyzing the H NMR spectra. (b) Temperature profile
of the hydrogenolysis of GG-D. (c) Treatment of 7 and 7-D with Pd/C. (d)
Pd/C-catalyzed H/D exchange reaction of 5-D.
Recently, Samec and coworkers reported the Pd/C-catalyzed transfer
hydrogenolysis of lignin samples from birch and pine trees, in which EtOH/H O was
2
used as the hydrogen source instead of H and monophenols with a propenyl end
2
3
7
group were generated as major products. Because it is difficult for the H/D exchange
reaction to occur at a C=C bond, the retention of deuterium at the propenyl group
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could be used to examine whether α-H participates in the cleavage of the β-O-4 motif.
GG-D was treated with Pd/C in MeOH under H -free conditions, and 4-propenyl
2
guaiacol 8 was obtained as a major product (50% yield) (Figure 1a). A decrease in the
0
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carbon balance (78%) was noted due to observable recondensation during the H -free
2
reaction. The selectivity to 8 was consistent with the transfer hydrogenolysis of plant
protolignin samples,37 in contrast to previous reports that used incomplete β-O-4
models36,76 or employed formic acid as the hydrogen source for the depolymerization
37
of nonphenolic models , where C ketone species were generated as major products
α
1
in the presence of a Pd/C catalyst. Further H NMR and mass spectral analyses of 8
obtained from GG-D suggested that α-D and β-D were well preserved (Figure S9),
which clearly showed that the reaction does not involve a dehydrogenation
3
7
mechanism and C ketone species are not intermediates of propenyl phenols. Based
α
on the retained deuteriums, it was concluded that the linkage protons are not involved
in the cleavage process, which, together with the formed C=C bond, supported the
concurrent cleavage of the C −O and C –O bonds in the Pd/C-catalyzed transfer
α
β
hydrogenolysis of the β-O-4 structure under these conditions.
To obtain further insight into lignin hydrogenolysis and transfer hydrogenolysis,
the deuterium-incorporated lignin polymer LP-D was treated with Pd/C (10 wt%)
under various conditions. The monomer yields and deuterium retentions were
summarized in Table 1. In an H atmosphere, LP-D was depolymerized into
2
4-propanol guaiacol 5 in 60-72% yields with high selectivity (>95%) (entries 1-3).
The characterization of isolated 5 showed that the loss of α-D increased with
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increasing reaction temperature, which was consistent with the dimer model results.
The α-D abundances from LP-D depolymerization (32%, at 220 °C for 12 h) were
consistent with the H/D exchange reaction of 5-D (37%, at 220 °C for 12 h, Figures
1d and S10), again confirming that the partial loss of α-D was caused by an H/D
1
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5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
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2
3
4
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7
8
9
0
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2
3
4
5
6
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9
0
exchange reaction. In the absence of H , the transfer hydrogenolysis of LP-D over
2
Pd/C led to the formation of 4-propenyl guaiacol 8 as the major product (50% yield),
and the selectivity was consistent with the dimer model and actual lignin biopolymer
results (entry 4). Complete preservation of all deuteriums was observed in 8-D’
(Figure S11), again suggesting that the cleavage of the C −O, C −O and C −O bonds
α
β
γ
do not proceed via dehydrogenation and/or dehydration processes.
Table 1. Pd/C-catalyzed Hydrogenolysis of LP-D
OH
OMe
H/D D D
OH
D
D
D
H

D
Pd/C
(10 wt%)
O
D

or
HO
D
D
D
MeOH,12 h
HO
O
HO

D
5-D'
8-D'
n
OMe
OMe
under N2 (1 atm)
OMe
LP-D
under H2 (30 atm)
Monomer
yield
D retention (%)
Entry Conditions
(mol%)
α
β
γ
1
2
3
4
H , 180 °C
60 (5-D’)
68 (5-D’)
72 (5-D’)
50 (8-D’)
76
32
100
100
2
H , 220 °C
2
100
100
100
100
100
100
H , 250 °C
2
29
N , 250 °C
2
100
Reaction conditions: LP-D (60 mg), Pd/C (6 mg), MeOH (10 mL), H (30 atm), 12 h.
2
1
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Pd/Zn/C Catalytic System. Recent results have indicated that the binding of the
OH groups in lignin to the catalyst plays an essential role in its depolymerization.35,75
Compared to C −OH, C −OH may play a more critical role in the cleavage of the
γ
α
0
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C −O bond. For instance, lignin dimers without C −OH (such as phenethoxybenzene)
β
α
were difficult to depolymerization,77,78 whereas C −O bond cleavage also occurred for
β
4
4,62
dimer models without C −OH.
We hypothesized that introducing a Lewis acid
γ
center with an affinity for oxygen atoms79 into Pd/C may be beneficial to lignin
hydrogenolysis. To verify this assumption, Pd/Zn/C, which was used for lignin
hydrogenolysis by Abu-Omar and coworkers,35,60 was chosen for further study. This
catalyst was prepared by mixing Pd/C with ZnCl (Pd/Zn molar ratio = 1:2) at 150 °C
2
6
0
in MeOH under H and subsequently drying the mixture under vacuum. TEM, XRD,
2
XPS and XAS characterizations indicated that Pd(0) nanoparticles were distributed on
the carbon, and no Pd-Zn alloy species were formed (Figure S13), which was
consistent with previous conclusions.60
After treating GG-D with Pd/Zn/C in MeOH at 180 °C under H (30 atm) for 3 h,
2
monomeric phenols, including 4-propyl guaiacol (4, 52%), 4-propanol guaiacol (5,
2
5%) and guaiacol (80%), were obtained (Scheme 5). Increasing the reaction
temperature to 250 °C led to a slight increase in the depolymerized products. These
results were consistent with previous reports in terms of the activity and selectivity.60
Unlike Pd/C, side product 7 was no longer detected in the Pd/Zn/C-catalyzed
hydrogenolysis of GG-D. The characterization of compounds 4 and 5 implied that
α-D and β-D remained intact during the Pd/Zn/C-catalyzed depolymerization reaction
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(Figure S14), again confirming that α-H(D) and β-H(D) dissociated during the
deconstruction of the β-O-4 motif and the subsequent functionalization. Obviously,
the H/D exchange reaction no longer occurred at the α position in 4 and 5 in the
presence of the Pd/Zn/C catalyst.
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In the case of the catalytic hydrogenolysis of LP-D, the Pd/Zn/C catalyst produced
monomeric phenols in 90% yield at 180 °C and 92% yield at 250 °C, and these yields
were higher than those obtained over the Pd/C catalyst under the same conditions
(60% and 72%, respectively, see Table 1). The deuterium content of 5 was determined
1
by H NMR and mass spectral analyses. No loss of deuterium was detected, which
was consistent with the GG-D reactions (Figure S15). The NMR characterization of 4
was difficult to achieve because of its low yield, and the mass analysis suggested that
all the deuteriums were well retained by 4 compared with the standard sample.
Therefore, the experimental results showed that α-H(D), β−H(D) and γ−H(D) do not
participate in the cleavage of lignin during Pd/Zn/C-catalyzed hydrogenolysis.
Scheme 5. Hydrogenolysis of D-incorporated β-O-4 Mimics with Pd/Zn/C
D
D
OH
OMe

MeO
HO

MeO
HO

Pd/Zn/C (5 wt%)


OH
GG-D
+
+
H2 (30 atm)
MeOH, 3 h
D
D
4-D
5-D
80 oC (C.B., 77%)
1
52%
63%
25%
19%
80%
85%
50 oC (C.B., 82%)
2
1
00% D retention at  and 
Compound 7 was not detected.
D
D
D
OH
D

D

D
H

MeO
MeO
HO

Pd/Zn/C (5 wt%)

LP-D

+
H2 (30 atm)
MeOH, 12 h
D
D
HO
4
-D
5-D
79%
77%
80 oC (C.B., 90%)
50 oC (C.B., 92%)
1
11%
15%
2
1
00% D retention at ,  and 
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The normal β-O-4 polymer LP was used as the substrate and treated with Pd/C and
Pd/Zn/C at 180 °C for 12 h, respectively. Similar to LP-D, Pd/Zn/C (82%) was
superior to Pd/C (58%) in term of the production of monomers yield. The resulted
oily products from LP were measured by two-dimensional HSQC NMR spectroscopy
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(Figure 2). In both cases, cross signals derived from β-O-4 units (A) completely
disappeared. The correlation signals resonated at δ δ = 36.1/2.81 (α), 80.2/4.29 (β)
C/ H
and 61.3/3.45 (γ) ppm were assigned as de-α-OH species (7 analogues) based on the
NMR spectra of compound 7 (Figure S12). These signals could be observed in
Pd/C-catalyzed hydrogenolysis reaction, whereas they were absence in the
Pd/Zn/C-catalyzed reaction, being in line with the observation in dimetric GG-D
o
reactions. An independent treatment of 7 with Pd/Zn/C at 250 C did not give any
monomeric phenols, suggesting that the absence of 7 and its analogues during β-O-4
hydrogenolysis may be caused by being suppressed rather than being consumed. We
2
+
proposed that the binding of Zn to hydroxy groups either on the catalyst surface or
in the solution60 might facilitate the cleavage of the C −O bond, preventing the
β
cleavage of only C −OH. The abatement of the side reactions and the increase in
α
monomeric products in polymeric model reactions suggested that the Pd/Zn/C catalyst
is better able to catalyze the hydrogenolysis of lignin, due to the oxygen atom affinity
of the Lewis acid zinc center.
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Pd/C
or Pd/Zn/C
10 wt%)
Depolymerized products (4 and 5)
(
LP
+
H2 (30 atm)
Side products (7 analogues)
o
MeOH, 180 C, 12 h
ppm
20
4
Pd/C
Pd/Zn/C
4
5
5
4
30
40
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DMSO
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7*
DMSO
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OMe
OMe
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5
60
7
*
70
7*
80
90
ppm
5
4
3
2
1
5
4
3
2
1



HO
HO

MeO
HO
HO

OH


 O


O
5
4

OMe
OMe

MeO
HO
MeO
MeO
7*
A
O
(-Aryl ether)
O
(7 analogues)
Figure 2. HSQC NMR spectra of the oily products from the hydrogenolysis LP with
Pd/C and Pd/Zn/C
3
. CONCLUSIONS
In summary, an artificial lignin polymer with a perdeuterated β-O-4 substructure,
which is a better model for the actual plant lignin structure, was synthesized and
characterized. Using this deuterated polymer and complementary dimer models, the
mechanism of lignin hydrogenolysis and transfer hydrogenolysis over Ru/C, Pd/C and
Pd/Zn/C catalysts was systematically studied. The location of deuterium after the
catalytic reactions demonstrated that the linkage protons do not participate in the
cleavage of the β-O-4 substructure and subsequent functionalization reactions, thus
negating the hypothesized routes including consecutive C−O cleavage, C ketones and
α
enol ethers for these catalytic systems. Using different points of view, a pathway
involving the concurrent cleavage of the C −O and C −O bonds was proposed for the
α
β
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hydrogenolysis of β-O-4 over the Pd/C catalyst. The sole α-dehydroxylation reaction,
which always occurred for the dimer models in the presence of Pd/C, was identified
as a side reaction. The introduction of Zn ions into Pd/C was conducive to β-O-4
hydrogenolysis, and this hypothesis was confirmed by using a Pd/Zn/C catalyst that
suppressed the side reactions and resulted in an improved monomer yield in the
polymeric model reactions. Because lowering the catalyst cost is one of the challenges
for lignin depolymerization, these results provided useful information for designing
new heterogeneous catalysts by reducing the amount of reducing precious metals
together with introducing cheap metals. Overall, these deuterium-incorporated β-O-4
mimics paved a definitive path for lignin hydrogenolysis mechanistic studies,
providing useful information for designing novel catalytic systems and controlling the
selectivity of lignin depolymerization.
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AUTHOR INFORMATION
Corresponding Author
Email: songg@bjfu.edu.cn
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
Supporting Information.
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The supporting information is available free of charge on the ACS Publications
website.
Details of experimental procedures, GC chromatograms, mass and NMR spectra
(PDF).
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ACKNOWLEDGMENT
This work was supported by the National Natural Science Foundation of China
(
21776020). We thank BioNMR facility of the Tsinghua University Branch of the
China National Center for Protein Sciences for NMR determination.
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For Table of Contents Only
OH
OMe
HO
OH
OMe
HO

D
D
O
D
O
D

D
D
D
D
D
D
HO  DD
D
O
O
HO
OMe
D-incorporated -O-4 lignin mimic
D
D
D
OMe
OH
OH
n/4
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D
D D
D
OH

MeO
HO
O
Ar


O
D
OH
MeO HO
D
Catalysts: Ru/C, Pd/C and Pd/Zn/C
Linkage protons: Uninvolved in the hydrogenolysis of -O-4
Mechanisium: Cocurrent cleavage of the CO and CO bonds
Deuterium-incorporated β-O-4 mimics can serve as definitive substrates for lignin
hydrogenolysis mechanistic study.
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