Cleavage of aryl-ether bonds in lignin model compounds using a Co-Zn-beta catalyst

Article abstract of DOI:10.1039/d0ra08121c

Efficient cleavage of aryl-ether linkages is a key strategy for generating aromatic chemicals and fuels from lignin. Currently, a popular method to depolymerize native/technical lignin employs a combination of Lewis acid and hydrogenation metal. However, a clear mechanistic understanding of the process is lacking. Thus, a more thorough understanding of the mechanism of lignin depolymerization in this system is essential. Herein, we propose a detailed mechanistic study conducted with lignin model compounds (LMC) via a synergistic Co-Zn/Off-Al H-beta catalyst that mirrors the hydrogenolysis process of lignin. The results suggest that the main reaction paths for the phenolic dimers exhibiting α-O-4 and β-O-4 ether linkages are the cleavage of aryl-ether linkages. Particularly, the conversion was readily completed using a Co-Zn/Off-Al H-beta catalyst, but 40% of α-O-4 was converted and β-O-4 did not react in the absence of a catalyst under the same conditions. In addition, it was found that the presence of hydroxyl groups on the side chain, commonly found in native lignin, greatly promotes the cleavage of aryl-ether linkages activated by Zn Lewis acid, which was attributed to the adsorption between Zn and the hydroxyl group. Followed by the cobalt catalyzed hydrogenation reaction, the phenolic dimers are degraded into monomers that maintain aromaticity. This journal is

Full text of DOI:10.1039/d0ra08121c

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Cleavage of aryl–ether bonds in lignin model
compounds using a Co–Zn-beta catalyst†
Cite this: RSC Adv., 2020, 10, 43599
a
a
b
c
c
Xiaomeng Dou, Wenzhi Li, * Chaofeng Zhu, Xiao Jiang,
Hou-min Chang
c
and Hasan Jameel
Efficient cleavage of aryl–ether linkages is a key strategy for generating aromatic chemicals and fuels from
lignin. Currently, a popular method to depolymerize native/technical lignin employs a combination of Lewis
acid and hydrogenation metal. However, a clear mechanistic understanding of the process is lacking. Thus,
a more thorough understanding of the mechanism of lignin depolymerization in this system is essential.
Herein, we propose a detailed mechanistic study conducted with lignin model compounds (LMC) via
a synergistic Co–Zn/Off-Al H-beta catalyst that mirrors the hydrogenolysis process of lignin. The results
suggest that the main reaction paths for the phenolic dimers exhibiting a-O-4 and b-O-4 ether linkages
are the cleavage of aryl–ether linkages. Particularly, the conversion was readily completed using a Co–
Zn/Off-Al H-beta catalyst, but 40% of a-O-4 was converted and b-O-4 did not react in the absence of
a catalyst under the same conditions. In addition, it was found that the presence of hydroxyl groups on
the side chain, commonly found in native lignin, greatly promotes the cleavage of aryl–ether linkages
activated by Zn Lewis acid, which was attributed to the adsorption between Zn and the hydroxyl group.
Followed by the cobalt catalyzed hydrogenation reaction, the phenolic dimers are degraded into
monomers that maintain aromaticity.
Received 23rd September 2020
Accepted 24th November 2020
DOI: 10.1039/d0ra08121c
rsc.li/rsc-advances
A great deal of research has been carried out on the native
lignin depolymerization. The analysis of lignin structure
Introduction
Lignin, a heterogeneous phenolic polymer which constitutes changes (rearrangement and repolymerization) during its
5–30% by weight of biomass and 40% by energy, is by far the extraction has provided many useful guidelines in lignin
1
1
6–8
most abundant renewable source of aromatics. And also transformation. For example, Li et al. proposed that a strategy
because of its abundant functional groups, lignin is oen pre- for minimizing condensation by reacting formaldehyde with
sented as a sustainable feedstock for aromatic chemicals and the a,g-diol group on lignin side-chains to form a 1,3-dioxa-
fuels. The key issue for the valorization of this complex ne(acetal) structure during lignin extraction. Lan et al. further
natural polymer lies in the development of highly active and compared different diol protection reagents, and found that
2,3
9
4
selective catalysts to effectively crack the typical ether bonds. In under the condition of propionaldehyde and acetaldehyde, the
addition, the repolymerization of degradation intermediates, yield of monomers was close to that obtained with formalde-
10
known as condensation reactions that lead to the undesired hyde, and a higher selectivity was obtained. These improved
products with high molecular weight, should also be mini- options for lignin utilization has led to the approach known as
5
mized. Despite the signicant advances that have been made in “lignin-rst” and include methods to improve lignin yield and
the past few decades, the conversion of lignin into useful the available functional groups. In addition, a method termed
products remains a big challenge.
“bottom-up” lays stress on the mechanism of lignin model
compound (LMC) conversion as it applies to lignin conver-
11
sion. Although there is a huge difference between studies on
LMC and studies on real lignin due to the recalcitrant nature
Laboratory of Basic Research in Biomass Conversion and Utilization, University of and heterogeneous structure of the latter, the “bottom-up”
a
Science and Technology of China, Hefei 230026, PR China. E-mail: liwenzhi@ustc.
method has provided many effective strategies for lignin
conversion, such as catalytic systems and active metals. Aer
all, each individual linkage type in lignin has its own reaction
edu.cn; Tel: +86-551-63600786
b
Hefei National Laboratory for Physics Science at Microscale, School of Chemistry and
Materials Science, University of Science and Technology of China, Hefei 230026, PR
path and product distribution, resulting in a nal complex
Department of Forest Biomaterials, North Carolina State University, Raleigh, NC mixture. Understanding and predicting the mechanism and
China
c
2
†
1
7695-8005, USA
products produced are necessary to optimize reaction parame-
ters and design catalysts for more efficient depolymerization of
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra08121c
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lignin. However, the direct use of real lignin in mechanism
Recently, the bifunctional catalysts containing a balance of
research is fairly challenging, because the chemical reaction Lewis acids and hydrogenation metals for a one-step catalytic
network on the complete lignin polymer and product distribu- depolymerization of lignin into chemicals and fuels have been
16,38–41
tion are exceedingly complex. Hence, it is practical to focus on extensively studied by several groups including our own.
LMC with only a single representative type of linkage, using the In our previous study, we discovered that the Co-Zn/Off-Al H-
most basic relevant units to simplify our understanding of the beta catalyst displayed superior activity in Kra lignin depoly-
overall reaction.
merization. This process gave a 81 wt% yield of petroleum ether
ꢁ1
The b-aryl ether (b-O-4) linkage is the ubiquitous interunit soluble product with a low molecular weight of ca. 330 g mol
linkages in native lignin, the content of which is reported to at 320 C for 24 h. In this work, we investigated the effective-
ꢀ
41
1
2
range from ca. 43% (sowood) to ca. 65% (hardwood). Hence, ness of Co-Zn/Off-Al H-beta catalyst on the hydrogenolysis of
it is the main target structure for cracking during lignin depo- aryl–ether linkages in LCM.
13,14
lymerization.
Additionally, a-O-4 and 4-O-5 bonds account
At the completion of the preparation this manuscript, two
42,43
for around 7% and 5% of interunit linkages with the balance publications appeared.
These publications, especially the
1
2
42
being various C–C linkages, such as b-1, b-b, b-5, and 5-5. To one by Li and Song, are relevant to the present results and will
fully unlock lignin's potential, a series of methods including be included in the discussion below.
acidolysis, alkaline hydrolysis, redox-neutral, reduction, and
oxidation have been used to break these lignin linkages.
use of homogeneous catalysts has been the focus of initial
investigations on the transformation of lignin models. The
1
5–17
The
Experimental section
Materials
addition of homogeneous bases or acids promotes the cleavage Benzyl phenyl ether (BPE, 1 in Fig. 1) was purchased from
1
8–20
of the ether linkages into smaller fragments.
presented that zinc activates the decomposition of b-O-4 ether in Fig. 1) and 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)
linkage in LMC by binding the hydroxyl group at the C
posi- propane-1,3-diol (PPPD, 3 in Fig. 1) were purchased from
Parsell et al. Aladdin (Shanghai, China). 2-Phenoxy-1-phenylethanol (PPE, 2
a
21
tion. Deuss et al. proposed a triic acid-catalyzed method Maclean Biochemical Technology Co., Ltd. (Shanghai, China).
based on the in situ stabilization of the aldehyde intermediates Analytical grade methanol was purchased from Sinopharm
2
2
to achieve the cleavage of lignin b-O-4 linkages. Homogeneous Chemical Reagent Co., Ltd. (Shanghai, China).
catalysts with well-dened structures including Ru, Ni, Mn, V,
and Fe complexes for C–O bond scission under relatively mild Catalyst preparation
23–27
conditions have also been studied.
The soluble homogeneous
Co:Zn ¼ 1:3/Off-Al H-beta was synthesized using our previously
catalysts allows them to closely contact the C–O bonds. However,
homogeneous catalytic systems have poor recyclability and may
cause complication in the purication of products.
41
published synthesis processes. Briey, H-beta was rst treated
ꢀ
with 13 M HNO at 100 C for 20 h to obtain dealuminized beta
3
material (Off-Al H-beta). Then the Off-Al H-beta powder was
From a green and sustainable chemistry perspective,

nely ground with cobalt acetate and zinc acetate to get an
28
heterogeneous catalysts are a better choice. Heterogeneous
catalysts do not have many of the problems of homogenous
catalysis and therefore have attracted much attention in LMC
conversion. The introduction of noble metals signicantly
increased the hydrogenolysis activity of C–O bonds, but inevi-
tably caused the excessive hydrogenation of the aromatic ring,
while the non-precious metals catalysts, which retain aromatic
structures during depolymerization, generally require harsh
intimate mixture. The total amount of Co and Zn is 0.844 mmol
ꢁ1
g
5
zeolite , and the molar ratio is 1 : 3. Finally, it was calcined at
ꢀ
50 C for 5 h in air. A comprehensive characterization of the
41
catalyst has also been reported by the previous research.
Catalytic hydrogenolysis of LMC
ꢁ
1
For the reaction of all LMC, 0.01 g mL of model compound,
29,30
10 mL of methanol, 0.1 g of Co:Zn ¼ 1:3/Off Al H-beta and 4 MPa
conditions.
For instance, Rensel et al. reported the decom-
ꢀ
of H were reacted in a pressurized 25 mL reactor (Anhui Kemi
position of b-O-4 linkages in decane at 400 C using FeMoP
2
31
ꢀ
Machinery Technology Co., Ltd) at temperature ranging from
catalyst. Song et al. achieved the goal at 300 C by using
ꢀ
32
140–260 C for 6 h. Aer reaction nished, the reactor was
a sulfated ZrO
2
supported CoMo catalyst in decalin. At lower
ꢀ
cooled down by submerging in an ice water bath. The content of
the reactor was ltered to remove the catalyst. 1 mL of the ltrate
was quantitatively analyzed by GC/MS (7890A GC/5975C MS,
Agilent, USA) aer adding acetophenone as an internal stan-
dard (IS). The oven temperature programs of GC were: for BPE
temperatures (80–140 C), Pd/C and Ni–M (M ¼ Pd and Ag) were
found to be active for the transformation of b-O-4 model
3
3–36
compounds.
Recently, an oxidation–hydrogenation two-step
strategy has also been studied as an efficient approach to the
11,15
transformation of LMC.
For example, Zhang et al. rst used
a biomimetic organic catalytic system O /NaNO /DDQ/NHPI to
2
2
oxidize the C H–OH in b-O-4 linkages to C ¼O, and then
a
a
11
hydrogenated the C –O–aryl ether bonds with NiMo. From the
b
atom-economy and green chemistry viewpoint, direct reductive
cleavage of b-O-4 linkages would be more desirable because it
avoids the use of additional oxidants or reductants, which are
37
oen harmful and wasteful.
Fig. 1 a-O-4 and b-O-4 model compounds.
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and PPE, the oven temperature was held at 40 C for 3 min, rearrangement that produces 7.
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ꢀ
44,45
The formation of the trace
ꢀ
ꢀ
ꢁ1
increased to 280 C (10 C min ) and held at the temperature amount of 6 is obviously the results of hydrolysis by the pres-
ꢀ
for 3 min. For PPPD, the oven was held at 40 C for 3 min, ence of trace amount of water. The formation of the trace
ꢀ
ꢀ
ꢁ1
increased to 85 C (10 C min ) and held for 2 min, then the amount of 8 could result from the condensation of 6 with 4 and/
ꢀ
ꢀ
ꢁ1
44,45
temperature was increased to 200 C (3 C min ) and held for or from rearrangement of 7.
min, and nally raised to 280 C (10 C min ) and held for overwhelming amount of ortho to para products, the rear-
0 min. The conversion (wt%) and the yield of product (mol%) rangement is a more likely pathway. The formation of 7 and 8
However, judging from the
ꢀ
ꢀ
ꢁ1
4
1
were calculated by the eqn (1) and (2), respectively.
could also arise from homolytic cleavage of benzyl ether bond
followed by recombination of the radical products as reported
in the literature.
Conversion % ¼
46–48
In the process of BPE degradation, the rearrangement is the
predominant reaction at temperatures below 220 C whereas
mðBPE=PPE=PPPDÞinitial ꢁ mðBPE=PPE=PPPDÞ
initial
residual
ꢂ 100
ꢀ
mðBPE=PPE=PPPDÞ
the methanolysis reaction becomes competitive at temperature
(
1)
ꢀ
above 220 C. It is also of interest to note that the rearrangement
ꢀ
requires a Lewis acid catalyst, as no 7 was found at 240 C in the
mole of product
Yield % ¼
ꢂ 100
(2)
absence of the catalyst for 6 h (Fig. S3†). In contrast, methanolysis
moleðBPE=PPE=PPPDÞinitial
ꢀ
reaction occurs at 240 C without the catalyst aer 6 h, but the
yields of 4 and 5 are lower (40%) than those with the catalyst (55%).
Reaction of 2-phenoxy-1-phenylethanol (PPE, 2)
Results and discussion
Reaction of benzyl phenyl ether (BPE, 1)
PPE (2), as a model for b-O-4 linkage in lignin, its alkyl–aryl
ether bond cleavage occurred under more stringent conditions,
First, we investigated the degradation of BPE (1) as a model for which is consistent with its higher dissociation energy than BPE
49
the a-O-4 linkage in lignin. BPE (1) was reacted in the presence (1). As can be seen in Table 2 and Fig. S5,† hardly any product
ꢀ
of Co:Zn ¼ 1:3/Off-Al H-beta for 6 h at temperatures ranging was formed with little conversion of PPE (2) at 140 C. From
ꢀ
ꢀ
from 140–260 C. Fig. S2† shows the GC chromatogram of the 160–200 C, p-hydroxystilbene (9) is the dominate product with
ꢀ
ꢀ
reaction products at 240 C, whereas Table 1 shows the distri- little phenol (4) formation. At 220 C, the formation of phenol
bution of reaction products at various temperatures. The reac- and 2-phenylethanol (11) begins to become competitive, but 9
tion products appear to be rather simple. The conversion of BPE remains the dominate product. In addition, a small amount of
is relatively slow at low temperature, reaching 100% conversion 1,2-dimethoxyethyl phenyl (12) was formed at 160 C and
only at 240 C. There were only three major products, phenol increases slowly as the temperature increases. Finally, at 220–
ꢀ
ꢀ
ꢀ
(
4), methyl benzyl ether (5) and o-benzylphenol (7) found along 260 C, a new product, ethylbenzene (13), is formed (Fig. S6†).
with a trace amount of benzyl alcohol (6) and p-benzylphenol (8) None of these products, 4, 9, 11, 12 and 17, is formed in the
ꢀ
at all temperatures studied (Table 1). These results indicate that absence of the catalyst even at 260 C (Fig. S7†).
the reaction of BPE may involve two competitive reactions:
Three different reaction paths appear to operate for the
methanolysis that produces 4 and 5, and the alkyl–aryl ether reaction of PPE under the reaction conditions shown in Fig. 2.
Table 1 Catalytic conversion of BPE (1) over Co:Zn ¼ 1:3/Off-Al H-beta catalyst
Yield (%)
Conv.
ꢀ
Entry
T ( C)
(%)
4
5
6
7
8
1
2
3
4
5
6
7
140
160
180
200
220
240
260
6.3
—
2.0
0.8
5.4
0.2
1.0
2.5
2.8
2.7
3.4
3.7
3.6
5.7
0.5
0.7
1.1
2.0
3.0
3.8
2.5
16.0
29.5
50.2
66.3
100
100
13.2
15.8
28.2
55.4
57.5
10.7
13.0
25.5
51.0
53.7
14.0
32.4
35.1
40.8
39.7
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Table 2 Catalytic conversion of PPE (2) over Co:Zn ¼ 1:3/Off-Al H-beta catalyst
Yield (%)
Conv.
ꢀ
Entry
T ( C)
(%)
4
9
10
11
0.1
0.1
0.3
12
0.2
0.3
2.0
5.6
6.7
10.2
14.2
13
17
18
19
20
1
2
3
4
5
6
7
140
160
180
200
220
240
260
2.0
9.1
0.3
0.4
2.9
12.2
33.7
49.8
63.0
1.7
8.6
—
—
—
—
—
0.1
0.4
0.4
—
—
—
—
—
—
—
—
15.2
—
—
—
—
—
—
—
—
0.6
0.1
1.3
1.5
2.5
1.8
3.4
22.9
40.7
93.8
99.0
99.5
17.4
27.0
56.4
47.4
25.8
0.6
0.3
1.5
5.9
8.6
0.6
1.0
1.0
0.9
3.5
5.4
24.5
31.6
23.9
ꢀ
At the low temperatures of 160–180 C, only one major product, rearrangement (Route 1). The methanolysis pathway affords
9, dominates, whose formation is presumably via the alkyl–aryl phenol (4) and 2-methoxy-1-phenylethanol (16), the latter being
ether rearrangement to 2-(4-hydroxyphenyl)-1-phenylethanol an intermediate. Three products may be derived from inter-
44,45
(14) followed by dehydration to 9.
The formation of 14 mediate 16. Methylation of 16 gives 1,2-dimethoxylethylbenzene
from PPE via the rearrangement is of great interest, as it may go (12) whereas dehydration of 16 gives 2-methoxylvinylbenzene
through a direct [1, 5] shi (Route 1) or two consecutive [1, 2] (18). While direct hydrolysis of 18 gives 1-phenylacetaldehdye
shis (Route 2). Since the rearrangement is through a concerted (17), hydrogenation followed by hydrolysis gives 2-phenyl-
51
mechanism, the ability for the direct [1, 5] shi would require ethanol (11). As can be seen in Table 2, while 12 increases
a high distortion of the phenolic aromatic ring to bring the b- steadily from 2.0–14.2% as the temperature increases from 180–
ꢀ
carbon and the para carbon into close proximity (Fig. S8†). 260 C, 11 is the dominate product of Route 3 at temperatures
ꢀ
ꢀ
However, similar distorted aromatic transition state operates in 220–240 C. At 260 C, 17 increases at the expense of 11. The
50
the para-semiline rearrangement. On the base of the over- methylation product of 11, 19, was also detected with low yield
whelming amount of 9 over a tiny amount of its ortho isomer 10, from 180–260 C. Finally, at temperature of 260 C, a new
the direct [1, 5] shi is the most likely path. At temperature pathway, reductive cleavage of 2 (Route 4) appears to become
ꢀ
ꢀ
ꢀ
above 220 C, a second reaction pathway, the acid catalyzed competitive with the other two pathways. The pathway
methanolysis of PPE (Route 3) becomes competitive with the presumably goes through a direct transfer of hydride anion to
Fig. 2 Plausible reaction pathways of PPE.
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Table 3 Catalytic conversion of PPPD (3) over Co:Zn ¼ 1:3/Off-Al H-beta catalyst
Yield (%)
Conv.
ꢀ
Entry
T ( C)
(%)
21
22
5.0
10.1
15.0
31.0
59.5
50.1
39.6
23
1.1
5.4
12.8
19.7
16.2
21.5
30.0
24
25
a
1
140
160
180
200
220
240
260
13.0
55.1
82.9
8.0
—
—
7.8
9.5
11.9
19.0
26.3
—
—
a
2
35.0
55.2
85.7
90.0
95.1
a
3
3.0
2.2
2.4
3.5
4.1
a
4
100
a
5
100
100
100
a
6
7
100
a
Other products are not listed in the table, including many products with small amounts like dimers formed by repolymerization and trace
unknown product.
the phenolic oxygen and a concerted dehydration of the a- methyl isoeugenol (25) was also found. In the absence of the
hydroxyl group to form vinyl benzene (20) followed by hydro- catalyst, no disappearance of PPPD and no aforementioned
ꢀ
genation to form ethylbenzene (13).
reaction products were observed at 200 C (Fig. S11†).
Comparing Table 3 with Table 2, it is obvious that PPPD (3) is
much more reactive than PPE (2) under the reaction conditions
at any temperature studied. Aside from the presence of
methoxyl groups on both aromatic nuclei, the main structural
difference between PPPD and PPE is the presence of hydrox-
ymethyl group at the g-position of PPPD. This additional
hydroxyl group may contribute to a more extensive polarization
of the sidechain by the Lewis acid sites of the catalyst and
thereby facilitate the cleavage of the b-O-4 linkage. A six-
membered ring complex structure is formed by the interac-
Reaction of 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)
propane-1,3-diol (PPPD, 3)
PPPD (3), a model compound more closely related to the major
linkage in lignin than 1 and 2, allows us to isolate the effect of
various substituents by comparing PPE. Fig. S10† shows the GC
ꢀ
chromatogram of the reaction products at 200 C, whereas
Table 3 shows the distribution of reaction products at various
temperatures. As can be seen in Table 3, in addition to guaiacol
(
(
21) as an expected product, 3-(3,4-dimethoxyphenyl) propanol
22) is the major reaction product along with acetoveratrone (23)
tion between Zn and the hydroxyl groups at the Ca and C
g
52
positions of PPPD and supported by NMR spectroscopy.
and 4-propylveratrole (24). In addition, a small amount of
ꢀ
ꢀ
Fig. 3 Time-dependent profiles of the reaction products of PPPD at 180 C and 240 C.
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Furthermore, the higher reactivity of 3 may also be attributable 26 for Route I and 27 for Route II. While Route II was originally
52
to the three additional methoxyl substitutions on the two proposed by Klein, et al., we suggest that Route I is the
aromatic rings. This hypothesis is deduced from the recent dominant one as 22 derived from 26 is the dominant product.

nding that the rate of hydrogenolysis is much faster for This nding is reasonable since 26 is a more stable product
42,43
guaiacylglycerol-b-guaicyl ether than 3.
than 27 as the result of conjugation. It is most likely that 27 may
In order to elucidate the reaction pathways of PPPD degra- be converted to 26 under the acidic reaction conditions. With
dation, the time-dependent proles of reaction products were the exception of 23, all major reaction products, 22, 24 and 25, are
ꢀ
ꢀ
investigated at 180 C and 240 C under otherwise identical derived from Route I and II, with 22 being the predominant
reaction conditions. The results are shown in Fig. 3. At both product. It is also noteworthy that 24 may derive from Route I via 22
temperatures, the main products are 21–25. The prole at and from Route II via 25. Judging from the fact that 22 is unstable at
ꢀ
2
40 C is especially of interest (Fig. 3b). PPPD disappears within high temperatures and that the formation of 24 coincides almost
one hour of reaction with a concomitant formation of guaiacol quantitatively with the decreasing 22, most of 24 is formed via
21) to 69.8%, which increases to 95% aer 6 hours and even- Route I. The time and temperature proles for the formation of
tually reaches 100% aer 12 hours. The other major product is small quantities of 25 further support the above conclusions. Both
2, which is not stable under the reaction conditions and may be Routes I and II are totally consistent with the recent results of Li and
converted to 24. On the other hand, 23 is most likely formed in Song who found that deuterium on the a, b and g carbons of 3 were
(
2
4
2
competition with the formation of 22 and 24, as it forms steadily totally retained on hydrogenolysis with Pd/Zn/C.
at both 180 C and 240 C with only a small temperature effect.
ꢀ
ꢀ
The acid catalyzed cleavage of 3 (Route III), which competes
Based on the aforementioned results, the reaction of PPPD with the aforementioned reductive cleavage (Route I and II),
most likely involves two distinct pathways, the reductive produces only a single compound 23, likely via 29 through
cleavage (Route I and II) and the acid catalyzed cleavage (Route a reversed aldol reaction (Route IIIa). Alternatively, a vinyl ether
III) as shown in Fig. 4. Both pathways give rise to the degrada- intermediate (30) may formed via well-known elimination of
tion of the b-ether linkage and the formation of guaiacol (21). In formaldehyde (Route IIIb). The vinyl ether underwent addition
the reductive cleavage, the formation of a six-membered ring of water giving intermediated compound 31. The compound
complex structure between Zn and the two hydroxyl groups at further underwent reductive cleavage, forming the product
16,36,53
the C and C of PPPD facilitates the direct attack of a hydride 23.
Oxidative addition across the benzylic C–H bond of 31
a
g
anion on the phenolic oxygen with a concomitant elimination by a metal element would yield a b-phenoxyalkyl metal hydride
of the a-hydroxyl group (Route I) or the g-hydroxyl group (Route 32, which underwent transfer hydrogenation to give 23 as
53
II). These pathways result in the formation of, in addition to shown by Zhou, et al. using Pd/C. Since only a trace amount of
ꢀ
guaiacol, an intermediate with double bond on the side chain, acetophenone was found in the reaction with PPE (2) at 260 C
Fig. 4 Plausible reaction pathways of PPPD (3).
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using the Co-Zn/Off Al H-b catalyst system, the oxidative addi-
tion of metal to benzylic C–H bond, if occurs at all, would have
been activated by the methoxyl substitution at the para-posi-
tion. The acid catalyzed cleavage appears to compete well with
the reductive cleavage at temperatures below 200 C, above
which the reductive cleavage becomes the dominant reaction.
2 X. Huang, C. Atay, T. I. Kor ´a nyi, M. D. Boot and E. J. Hensen,
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ꢀ
It is noted that 23 was not found in the earlier studies using Pd/
5
2
42
C with Zn(OAc)
2
, Pd/Zn/C. We now conrm that 23 was also not
/HCl as the catalyst. In these studies, 3
found using Ru/C with ZnCl
2
went exclusively through Routes I and II to give 22 and 24 as major
products. It is hypothesized that Route III become competitive
when a weaker hydrogenolysis catalyst is used, as in the present
case with Co-Zn/Off-Al H-b zeolite. The role of Zn Lewis acid in the
52
hydrogenolysis of lignin is conrmed with Pd/C with Zn(OAc)
Pd/Zn/C, Ru/C with ZnCl and Co-Zn/H-b catalyst systems.
2
2
,
42
7
E. M. Anderson, M. L. Stone, R. Katahira, M. Reed,
W. Muchero, K. J. Ramirez, G. T. Beckham and Y. Rom ´a n-
Leshkov, Nat. Commun., 2019, 10, 1–10.
Conclusion
In this study, we propose a detailed experimental and mecha-
nism analysis of each reaction pathway to understand the
observed differences in product distributions on temperature
and time scales, aiming to use these model systems to explain
the experimental phenomena over “Lewis acid-metal” catalysts
in the process of lignin depolymerization. Results clearly
8 S. Van den Bosch, T. Renders, S. Kennis, S. F. Koelewijn,
G. Van den Bossche, T. Vangeel, A. Deneyer, D. Depuydt,
C. Courtin and J. Thevelein, Green Chem., 2017, 19, 3313–3326.
9 L. Shuai, M. T. Amiri, Y. M. Questell-Santiago, F. H ´e rogul,
Y. Li, H. Kim, R. Meilan, C. Chapple, J. Ralph and
J. S. Luterbacher, Science, 2016, 354, 329–333.
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expedites the reaction rate of C–O bonds cleavage in the a- or b-
Angew. Chem., Int. Ed., 2018, 57, 1356–1360.
ether linkages. In particular, no reaction of the b-O-4 linkage 11 C. Zhang, H. Li, J. Lu, X. Zhang, K. E. MacArthur, M. Heggen
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lytic hydrogenolysis reactions. In addition, the presence of 13 Y. Yang, H. Fan, J. Song, Q. Meng, H. Zhou, L. Wu, G. Yang
hydroxyl groups on the side chain, which are common in native and B. Han, Chem. Commun., 2015, 51, 4028–4031.
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Green Chem., 2015, 17, 5001–5008.
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H-beta catalyst in converting both LMC and lignin. More impor- 16 M. V. Galkin, C. Dahlstrand and J. S. M. Samec,
tantly, this study provides guidance for the design of bifunctional ChemSusChem, 2015, 8, 2187–2192.
catalysts including Lewis acids and hydrogenation metals for the 17 C. Liu, S. Wu, H. Zhang and R. Xiao, Fuel Process. Technol.,
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1
1
8 T. J. McDonough, Tappi J., 1993, 76, 186–193.
9 T. Kleine, J. Buendia and C. Bolm, Green Chem., 2013, 15,
Conflicts of interest
160–166.
There are no conicts to declare.
20 S. Jia, B. J. Cox, X. Guo, Z. C. Zhang and J. G. Ekerdt,
ChemSusChem, 2010, 3, 1078–1084.
2
1 T. H. Parsell, B. C. Owen, I. Klein, T. M. Jarrell, C. L. Marcum,
L. J. Haupert, L. M. Amundson, H. I. Kentt ¨a maa, F. Ribeiro
and J. T. Miller, Chem. Sci., 2013, 4, 806–813.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 51976212), Key research and 22 P. J. Deuss, M. Scott, F. Tran, N. J. Westwood, J. G. de Vries
development projects in Anhui Province (Grant No.
and K. Barta, J. Am. Chem. Soc., 2015, 137, 7456–7467.
02004a06020053), and the National Key Technology R&D 23 S. Son and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49, 3791–
2
Program of China (Grant No. 2018YFB1501601).
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4 C. Crestini, A. Pastorini and P. Tagliatesta, Eur. J. Inorg.
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5 T. vom Stein, T. den Hartog, J. Buendia, S. Stoychev,
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