Breaking the Limit of Lignin Monomer Production via Cleavage of Interunit Carbon–Carbon Linkages

Article abstract of DOI:10.1016/j.chempr.2019.03.007

Conversion of lignin into monocyclic hydrocarbons as commodity chemicals and drop-in fuels is a highly desirable target for biorefineries. However, this is severely hindered by the presence of stable interunit carbon–carbon linkages in native lignin and those formed during lignin extraction. Herein, we report a new multifunctional catalyst, Ru/NbOPO₄, that achieves the first example of catalytic cleavage of both interunit C–C and C–O bonds in one-pot lignin conversions to yield 124%–153% of monocyclic hydrocarbons, which is 1.2–1.5 times the yields obtained from the established nitrobenzene oxidation method. This catalyst also exhibits high stability and selectivity (up to 68%) to monocyclic arenes over repeated cycles. The mechanism of the activation and cleavage of 5–5 C–C bonds in biphenyl, as a lignin model adopting the most robust C–C linkages, has been revealed via in situ inelastic neutron scattering coupled with modeling. This study breaks the conventional theoretical limit on lignin monomer production. The conversion of lignin into monocyclic hydrocarbons as commodity chemicals and drop-in fuels is essential for the future of biorefineries. State-of-the-art lignin depolymerization is primarily achieved via cleavage of interunit C–O bonds to form low-molecular-weight feedstocks. However, these processes can hardly cleave interunit C–C bonds in lignin, and thus, the yields of lignin monomers are heavily restricted. Here, we report a multifunctional catalyst, Ru/NbOPO₄, that achieves the first example of catalytic cleavage of both interunit C–C and C–O bonds in lignin in one-pot reactions to yield 153% of monocyclic C₆–C₉ hydrocarbons from Kraft lignin, which is 1.5 times the theoretical yield obtained from the established nitrobenzene oxidation (NBO) method. Thus, significantly, this study successfully breaks the conventional limit on lignin monomer production. Lignin, containing a large volume of aromatic functionalities, is the most energy-dense fraction of renewable biomass. Particularly, conversion of lignin into monocyclic hydrocarbons as commodity chemicals is a highly desirable target. However, this is severally hindered by the presence of stable interunit carbon–carbon linkages in native lignin and those formed during lignin extraction. Here, we report a multifunctional Ru/NbOPO₄ catalyst that achieves the first example of catalytic cleavage of both interunit C–C and C–O bonds in lignin in one-pot reactions.

Full text of DOI:10.1016/j.chempr.2019.03.007

Article
Breaking the Limit of Lignin Monomer
Production via Cleavage of Interunit Carbon–
Carbon Linkages
Lin Dong, Longfei Lin, Xue
Han, ..., Stewart F. Parker, Sihai
Yang, Yanqin Wang
guoyong@ecust.edu.cn (Y.G.)
sihai.yang@manchester.ac.uk (S.Y.)
wangyanqin@ecust.edu.cn (Y.W.)
HIGHLIGHTS
Catalytic cleavage of both C–C
and C–O bonds in lignin is
achieved
The conventional theoretical
limitation on lignin monomer
production is broken
Excellent activity for C–C bond
breakage depends on the unique
property of Ru/NbOPO4
A new method for the
hydrogenolysis of lignin into
monocyclic hydrocarbons is
proposed
Lignin, containing a large volume of aromatic functionalities, is the most energy-
dense fraction of renewable biomass. Particularly, conversion of lignin into
monocyclic hydrocarbons as commodity chemicals is a highly desirable target.
However, this is severally hindered by the presence of stable interunit carbon–
carbon linkages in native lignin and those formed during lignin extraction. Here,
we report a multifunctional Ru/NbOPO4 catalyst that achieves the first example of
catalytic cleavage of both interunit C–C and C–O bonds in lignin in one-pot
reactions.
Dong et al., Chem 5, 1–16
June 13, 2019 ª 2019 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2019.03.007

Please cite this article in press as: Dong et al., Breaking the Limit of Lignin Monomer Production via Cleavage of Interunit Carbon–Carbon Link-
ages, Chem (2019), https://doi.org/10.1016/j.chempr.2019.03.007
Article
Breaking the Limit of Lignin Monomer
Production via Cleavage of Interunit
Carbon–Carbon Linkages
3
Lin Dong,1,5 Longfei Lin,2,5 Xue Han,2 Xiaoqin Si,3 Xiaohui Liu,1 Yong Guo,1, Fang Lu, Svemir Rudic,
*
ꢀ 4
1,6,
Stewart F. Parker,4 Sihai Yang,2, and Yanqin Wang
*
*
SUMMARY
The Bigger Picture
The conversion of lignin into
monocyclic hydrocarbons as
commodity chemicals and drop-in
fuels is essential for the future of
biorefineries. State-of-the-art
lignin depolymerization is
Conversion of lignin into monocyclic hydrocarbons as commodity chemicals and
drop-in fuels is a highly desirable target for biorefineries. However, this is
severely hindered by the presence of stable interunit carbon–carbon linkages
in native lignin and those formed during lignin extraction. Herein, we report a
new multifunctional catalyst, Ru/NbOPO4, that achieves the first example of cat-
alytic cleavage of both interunit C–C and C–O bonds in one-pot lignin conver-
sions to yield 124%–153% of monocyclic hydrocarbons, which is 1.2–1.5 times
the yields obtained from the established nitrobenzene oxidation method. This
catalyst also exhibits high stability and selectivity (up to 68%) to monocyclic are-
nes over repeated cycles. The mechanism of the activation and cleavage of 5–5
C–C bonds in biphenyl, as a lignin model adopting the most robust C–C linkages,
has been revealed via in situ inelastic neutron scattering coupled with modeling.
This study breaks the conventional theoretical limit on lignin monomer produc-
tion.
primarily achieved via cleavage of
interunit C–O bonds to form low-
molecular-weight feedstocks.
However, these processes can
hardly cleave interunit C–C bonds
in lignin, and thus, the yields of
lignin monomers are heavily
restricted. Here, we report a
multifunctional catalyst, Ru/
NbOPO4, that achieves the first
example of catalytic cleavage of
both interunit C–C and C–O
bonds in lignin in one-pot
INTRODUCTION
Lignin, accounting for 15–40 wt % of lignocellulosic biomass, is the most abundant
source of renewable aromatics on earth.1–4 The heavily branched 3D network of
lignin is constructed from methoxylated phenylpropanoid subunits bridged by
various interunit C–O and C–C linkages (accounting for approximately 70% and
30%, respectively) in a completely random order (Figures 1 and S1).5–7 C–C bonds
have notably higher dissociation energy (226–494 kJ molÀ1) than those of C–O
bonds (209–348 kJ molÀ1) in lignin (Table S1).6 Because of its highly robust polymeric
structure, the conversion of lignin to small-molecule arenes as commodity chemicals
and fuel additives represents a significant challenge for industries and biorefineries.
reactions to yield 153% of
monocyclic C6–C9 hydrocarbons
from Kraft lignin, which is 1.5
times the theoretical yield
obtained from the established
nitrobenzene oxidation (NBO)
method. Thus, significantly, this
study successfully breaks the
conventional limit on lignin
monomer production.
State-of-the-art processes usually undergo lignin depolymerization via the cleavage
of C–O bonds to obtain low-molecular-weight monomers, which can be sequentially
upgraded to useful chemicals and fuels.1,2 A great deal of effort has been devoted to
developing strategies for the depolymerization of lignin.8–22 However, the primary
products from these processes are mixtures of oxygen-containing compounds
with high boiling points, which can hardly be separated by conventional distillation
techniques. Moreover, the yields of lignin monomers are limited because of the
presence of stable interunit C–C bonds within native lignin or those formed during
lignin extraction.23 Importantly, adding formaldehyde during the pre-treatment of
biomass can effectively improve the yield of lignin monomers by preventing the
interunit C–C coupling.23,24 More interestingly, the emerging ‘‘lignin-first’’
Chem 5, 1–16, June 13, 2019 ª 2019 Elsevier Inc.
1

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Figure 1. View of a Representative Structure of a Lignin Fragment Shows Various Intramolecular Linkages and Schematic Representation of the One-
Pot Depolymerization and Hydrodeoxygenation of Lignin into Monocyclic Aromatic Hydrocarbons
The route above the green arrow shows the formation of monocyclic arenes via cleavage of both C–O and C–C linkages.
approaches have shown productions of (near)theoretical amounts of phenolic
monomers via solvolytic delignification, offering great promise for high-yield lignin
depolymerization.4,21,22 However, the cleavage of intrinsic C–C bonds within native
and technical lignin remains a long-standing challenge. Very recently, a CoS2 cata-
lyst has enabled the cleavage of C–C bonds in methylene-linked lignin models.25
However, this catalyst has a limited structural stability and is inefficient to cleave
the most robust 5–5 C–C linkages in lignin. Although lignin can be directly converted
into volatile hydrocarbons via catalytic hydropyrolysis at typically 650ꢀC,26–28 these
processes are very energy consuming, and deactivation of catalysts often occurs
rapidly because of the coke formation (up to 40%).29,30
Efficient catalytic cleavage of both interunit C–C and C–O linkages in lignin can maxi-
mize the lignin monomer production and thus is highly desirable. This is, however, a
very challenging task and requires solutions based upon new multifunctional cata-
lysts. Zeolite materials incorporating Brønsted acid sites can catalyze hydrocarbon
cracking in petroleum refineries.31,32 Phosphate-based materials containing strong
Brønsted acid sites can protonate benzene rings and thus facilitate the cleavage
of C–C bonds.33,34 However, these materials alone have poor activity in cleaving
1Key Laboratory for Advanced Materials and Joint
C–O bonds in lignin.35 Meanwhile, the emerging NbOx-supported catalysts have ex-
hibited unique activities in cleaving C–O bonds in biomass to yield fully deoxygen-
ated compounds.36
International Research Laboratory of Precision
Chemistry and Molecular Engineering, Feringa
Nobel Prize Scientist Joint Research Center,
Research Institute of Industrial Catalysis, School
of Chemistry and Molecular Engineering, East
China University of Science and Technology,
Shanghai 200237, China
Here, we report a mesoporous multifunctional catalyst, Ru/NbOPO4, that combines
the NbOx species and phosphates containing strong Brønsted acid sites and thus
enables the efficient cleavage of both interunit C–O and C–C linkages in lignin.
Significantly, the one-pot conversion of lignin over Ru/NbOPO4 integrates the
depolymerization of lignin, the hydrogenolysis of depolymerized compounds, and
the cleavage of interunit C–C linkages and achieves optimal production of monocy-
clic hydrocarbons. Theoretical yields of intrinsic lignin monomers have been deter-
mined from the established nitrobenzene oxidation (NBO) method (Note S1)23,37,38
and serve as the 100% standard in this report. Here, the ratio of molar yields (RMY) is
defined as (molar yield of monocyclic compounds over Ru/NbOPO4)/(molar yield of
monocyclic compounds from NBO) 3 100%. When RMY above 100% is obtained, it
suggests the recovery of additional monocyclic compounds from depolymerized
dimer and oligomer products via the cleavage of interunit C–C linkages (Figure 1).
The one-pot conversion of lignin over the Ru/NbOPO4 catalyst has produced liquid
2School of Chemistry, University of Manchester,
Manchester M13 9PL, UK
3State Key Laboratory of Catalysis, Dalian
Institute of Chemical Physics, Chinese Academy
of Sciences, Dalian National Laboratory for Clean
Energy, Dalian 116023, China
4ISIS Facility, STFC Rutherford Appleton
Laboratory, Chilton, Oxfordshire OX11 0QX, UK
5These authors contributed equally
6Lead Contact
*Correspondence:
guoyong@ecust.edu.cn (Y.G.),
sihai.yang@manchester.ac.uk (S.Y.),
wangyanqin@ecust.edu.cn (Y.W.)
https://doi.org/10.1016/j.chempr.2019.03.007
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monocyclic hydrocarbons with an RMY up to 153% as well as an exceptional arene
selectivity of 68%. Furthermore, the unique activity of Ru/NbOPO4 to crack the inter-
unit C–C bonds has been confirmed in lignin dimer models (e.g., biphenyl, diphenyl-
methane, and diphenylethane), where high yields of monocyclic aromatics have
been obtained. A combined inelastic neutron scattering (INS) and density functional
theory (DFT) analysis unambiguously confirmed the strong adsorption with
preferred orientation, activation, and hydrogenolysis of biphenyl on the surface of
Ru/NbOPO4. To the best of our knowledge, this is the first example of using INS
and DFT to study the mechanism of C–C bond activation in biomass conversions.
RESULTS
One-Pot Conversion of Lignin into Monocyclic Hydrocarbons
Mesoporous NbOPO4 was prepared from a hydrothermal reaction and a loading of
5 wt % Ru conducted via wetness impregnation (Note S2). The total acid amount and
Brunauer-Emmett-Teller (BET) surface area of activated Ru/NbOPO4 were deter-
mined to be 980 mmol gÀ1 and 235 m2 gÀ1, respectively, and the pore size distribu-
tion was centered at 3.5 nm (Figure S2). The mesopores and high porosity of the
catalyst can promote reactions taking place at the solid-solid interface and have a
positive effect on the mass transfer of substrates and reaction intermediates. Kraft
lignin, typically produced from chemical pulping processes, has a highly condensed
and cross-linked (via interunit C–C bonds) network. Its intrinsic monomer units were
determined to be 1,040 mmol gÀ1 from the NBO method (Tables S2 and S3), which is
widely used in the literature as a standard protocol for analyzinig lignin monomers
connected by C–O bonds (including but not limited to b–O–4 linkage).23,38,39 The
C–C linkage in lignin cannot be cleaved in the NBO method, thus making the com-
parison straightforward for the present study. It has been reported that the theoret-
ical amount of lignin monomers coincides with the square of the fraction of ether
bonds in the lignin structure, and that of birch lignin varies from 45% to 58%.7,22
With the NBO method, on the basis of the molar content of S, G, and H units in birch
lignin, the maximum amount of lignin monomers was found to be within the range of
52%–57% (Table S2, entry 4), highly consistent with the reported results. The one-
pot conversion of Kraft lignin was first tested over the Ru/NbOPO4 catalyst in dodec-
ane at 310ꢀC under 0.5 MPa H2 for 40 h. Interestingly and surprisingly, a very high
yield of monocyclic hydrocarbons (1,594 mmol gÀ1/14.5 wt %) was obtained with a
C6–C9 arene selectivity of 68% (Table 1, entry 1; Table S4, entry 4; Table S5). This
gives an RMY of 153%, indicating a 53% additional recovery of monocyclic hydrocar-
bons via the cleavage of interunit C–C linkages in Kraft lignin. Next, enzymic corn-
cob, pine, and birch lignin (representing grasses, softwood, and hardwood lignin,
respectively, Table S2 and Note S3) were employed as feedstocks (Table 1, entries
3–5; Table S6). The RMY for conversions of enzymic, pine, and birch lignin were
147%, 151%, and 124%, respectively, confirming the cleavage of interunit C–C link-
ages in all cases. Importantly, high selectivities to monocyclic arenes ranging from
62% to 67% were obtained throughout, demonstrating the general applicability of
this multifunctional catalyst and one-pot approach to producing monocyclic arenes.
In addition, the comparison of the activity and selectivity with early reported litera-
ture in the one-pot conversion of lignin is also summarized in Figure S3 and Note S4.
Stability Tests
To investigate the stability of Ru/NbOPO4, we conducted three consecutive conver-
sions of Kraft lignin by using the recycled catalyst. Negligible changes in reaction
yield or arene selectivity were observed (Figure 2; Table S7). For example, the yield
of monocyclic hydrocarbons and arene selectivity on the third run were
1,588 mmol gÀ1 and 66%, comparable to those (1,594 mmol gÀ1 and 68%) achieved
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Table 1. Summary of the Products Distribution from the One-Pot Conversion of Lignin over the Ru/NbOPO4 Catalysta
Entry Lignin
Product Distribution (mmol,gÀ1
)
Total Amount
(mmol gÀ1/wt %) Arenes (%)b
Selectivity to RMY (%)c
C6–C9 Arenes
C6–C9 Cycloalkanes
1
Kraft
Kraft
524
204
389
262
323
640
877
103
146
387
177
251
67
42
77
209
56
42
167
150
203
448
487
67
26
19
1,594/14.5
1,055/10.1
2,135/20.3
3,254/29.3
3,394/31.9
68
62
67
66
62
153
101
147
151
124
2d
3
88
88
enzymic 651
69
98
266
111
189
61
4
pine
1,179
796
138
191
285
254
211
191
65
5
birch
158
aReaction conditions: 0.1 g lignin, 0.2 g 5% Ru/NbOPO4, 5 mL dodecane, and 0.5 MPa H2 at 310ꢀC for 40 h.
bSelectivity to arenes = (total amount of arenes)/(total amount of hydrocarbons) 3 100%.
cRatio of molar yields (RMY) = (molar yield of monocyclic compounds over Ru/NbOPO4)/(molar yield of monocyclic compounds from NBO) 3 100% = (total
amount of monocyclic hydrocarbons over Ru/NbOPO4)/(theoretical amount of monocyclic hydrocarbons from NBO) 3 100%. The theoretical hydrocarbon
amounts of Kraft lignin, enzymic lignin, pine lignin, and birch lignin are 1,040, 1,448, 2,162, and 2,741 mmol gÀ1, respectively. Theoretical hydrocarbon amounts
were calculated by the NBO method as described in Table S2 and Note S1.
dReaction conditions: 0.1 g lignin, 0.2 g 5% Ru/NbOPO4, 5 mL dodecane, and 0.5 MPa H2 at 250ꢀC for 20 h.
with a fresh catalyst. The stability test was also conducted with 100 wt % catalyst
loading in four consecutive recycling runs (Table S8; Note S5), where the catalytic
activity reduced after the first run and remained the same for the following runs.
X-ray diffraction (XRD), transmission electron microscopy (TEM), and chemical anal-
ysis further confirmed the absence of notable structural change, aggregation of Ru
particles, and leaching of Ru, respectively, for the used catalyst (Figures S4 and S5;
Table S9). These results demonstrate the excellent stability of Ru/NbOPO4 in this
one-pot process. In addition, the effect of S element in Kraft lignin was also studied
by control experiments, suggesting that the S element in Kraft lignin has little effect
on the catalytic activity (Table S10; Note S6).
Analysis of the Reaction Pathways
To investigate the reaction pathways, we converted Kraft lignin at 250ꢀC for 20 h
(Table 1, entry 2; Table S4; Note S7) and isolated and analyzed reaction intermedi-
ates by 2D heteronuclear single-quantum coherence nuclear magnetic resonance
(HSQC NMR). Compared with NMR signals for the raw lignin, those for A (b–O–4 link-
ages), B (b–5 linkages), and C (b–b linkages) decreased significantly in the side-chain
regions after the reaction, and the presence of G and H subunits in the reaction
residue was negligible (Figure S6). Elemental analysis shows only a minor oxygen
content (<0.8 wt %) in the reaction intermediates, further suggesting the near-com-
plete oxygen removal during the initial reaction. Meanwhile, bicyclic aromatic
hydrocarbons (e.g., biphenyl and diphenylethane) were also detected (Figure S7),
indicating that the activation and cleavage of interunit C–C bonds in depolymerized
compounds require additional energy input, consistent with the difference in their
bond dissociation energies (Table S1). The mass yield of total monocyclic com-
pounds at 250ꢀC for 20 h was only 10.1 wt % (Table 1, entry 2), and that can be
increased to 14.5 wt % at 310ꢀC for 40 h via further cleavage of C–C linkages in de-
polymerized compounds.
Catalytic Cleavage of C5–C5 Linkages in Lignin Model Compounds
Among all interunit C–C linkages in lignin, the 5–5 bond between two phenyl groups
accounts for the highest portion in lignin (40%–50% in softwood and 25%–40% in
4
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Figure 2. Comparison of C6–C9 Reaction
Products and Arene Selectivity in Three
Consecutive Recycling Runs of One-Pot
Conversion of Kraft Lignin over the Ru/
NbOPO4 Catalyst
Reaction conditions: 0.1 g Kraft lignin, 0.2 g 5%
Ru/NbOPO4, 5 mL dodecane, and 0.5 MPa H2 at
310ꢀC for 40 h.
hardwood on the basis of all C–C linkages) and has the strongest dissociation en-
ergies of 481–494 kJ molÀ1. Therefore, biphenyl is selected for in-depth investiga-
tion as a model compound for C–C cleavage.7,40 To closely monitor the intermedi-
ates, we conducted the reaction at a reduced temperature of 280ꢀC (Figure 3).
Initially, phenylcyclohexane (1) was found to accumulate rapidly to 73% within 1 h
and then gradually convert to benzene (2) and cyclohexane (3) as the reaction pro-
ceeded. This indicates that the partial hydrogenation of biphenyl occurred to acti-
vate the C5–C5 linkage from the sp2–sp2 to sp2–sp3 bond, thus effectively reducing
the bond dissociation energy. Similar product distribution was obtained when 1 was
used as the substrate, confirming 1 as the primary intermediate in the conversion of
biphenyl (Figure S8). Interestingly, a small amount of bicyclohexyl (4) was observed
after 2 h, and no further conversion of 4 occurred with prolonged reaction time.
Indeed, no product was detected in the hydrogenolysis of 4 or dodecane under
the same reaction conditions (Figures 4A and 4B), indicating that Ru/NbOPO4 has
a poor activity to cleave the Caliphatic–Caliphatic bonds, although they have consider-
ably lower bond dissociation energy. Comparison of the Fourier transform infrared
spectroscopy (FTIR) of adsorbed biphenyl, 1, 4, and dodecane on Ru/NbOPO4
showed that the catalyst had much weaker binding to aliphatic hydrocarbons than
that to aromatic ones (Figure S9; Note S8), indicating that the adsorption of sub-
strates on the catalyst surface plays an important role in its activation.
Interestingly, small amounts of (1-methylcyclopentyl)-benzene (5) and (1-methylcy-
clopentyl)-cyclohexane (6) were also detected, indicating the presence of isomeriza-
tions in this one-pot process. The hydrogenation of biphenyl can generate phenyl-
cyclohexene, which undergoes isomerization to give 5. 5 can be further cracked
into 2 and methylcyclopentane (7) (Figure 5, route 2). On the other hand, 7 can
also be obtained from the isomerization of 3 (Figure 5, route 1; Figure S10). Thus,
the conversion of biphenyl over Ru/NbOPO4 first undergoes partial hydrogenation
to 1 or phenylcyclohexene, which is then converted via three plausible routes (Fig-
ure 5). In route 1, 1 is directly cracked into 2 and 3; the minority of the latter is iso-
merized to 7. In route 2, biphenyl is first hydrogenated to phenylcyclohexene and
then isomerized and hydrogenated to produce 5, which is finally cracked to 2 and
7. In route 3, further hydrogenation of 1 produces 4, which cannot undergo further
conversion. It is worth noting that the isomerization in routes 1 and 2 has a very minor
contribution to the final products, and the direct cracking of 1 in route 1 is the domi-
nant pathway, resulting in the efficient production of monocyclic hydrocarbons.
To investigate the role of the density of acid sites, we converted biphenyl over Ru/
Nb2O5 (Table 2, entries 2 and 3). Compared with Ru/NbOPO4, Ru/Nb2O5 showed
significantly reduced activity for C–C cleavage, with only 3.5% yield of monocyclic
products, whereas upon the addition of trifluoromethanesulfonic acid, the yield of
monocyclic greatly increased to 38.0%, confirming the important role of Brønsted
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Figure 3. Product Distributions versus Reaction Time for the Conversion of Biphenyl over the Ru/
NbOPO4 Catalyst
Views of variation of the yield (A) and selectivity (B) for all possible intermediates and products as
shown in (C). Reaction conditions: 0.2 g biphenyl, 0.1 g 5% Ru/NbOPO4, 2 g dodecane, and 0.5 MPa
H2 at 280ꢀC.
acids. Another control experiment of the hydrogenolysis of biphenyl over the cata-
lyst support, NbOPO4, was conducted under the same reaction condition, and no
product was detected (Table 2, entry 4). This result indicates that Ru nanoparticles
serving as active sites for the dissociation of hydrogen molecules are also important,
consistent with those reported in literature.41,42
The performance of Pd/NbOPO4, Pt/NbOPO4, and Rh/NbOPO4 on C–C cleavage of
biphenyl was also tested (Table 2, entries 5–7). Over Pd/NbOPO4, the yield of ben-
zene and cyclohexane was 16.3% and 12.3%, respectively, lower than that of Ru/
NbOPO4 (24.4% and 19.1%, respectively), and the main product was bicyclohexyl
(42.1%). Over Pt/NbOPO4, the main product was also bicyclohexyl (54.2%), and
cyclohexane and benzene were produced in 18.5% and 1.3% yields, respectively.
The notably stronger activity for hydrogenation over Pd and Pt catalysts than Ru cat-
alysts14,43 greatly promoted further hydrogenation of the primary reaction interme-
diate (phenylcyclohexane) and thus hindered the cleavage of C–C bonds, thereby
leading to low yields of monocyclic products. Over the Rh/NbOPO4 catalyst, the
yield of monocyclic products reached 39.4%, but the arene selectivity was signifi-
cantly lower than that of Ru/NbOPO4, indicating that Rh/NbOPO4 can promote
further hydrogenation of benzene into alkanes. Thus, the unique activity of Ru-based
analog is attributed to its moderate ability for hydrogenation to enable the partial
hydrogenation of biphenyl to phenylcyclohexane while preventing any further
hydrogenation of the intermediate and product. It is worth noting that all catalysts
(Ru, Pd, Pt, or Rh/NbOPO4) are inactive for C–C cracking of bicyclohexyl under
the same conditions.
HZSM-5 is widely used for cracking and hydrocracking processes in petroleum refin-
eries,44 and Ru/HZSM-5 has also been studied for the conversion of biphenyl
(Table 2, entry 8). The yield of monocyclic hydrocarbons over Ru/HZSM-5 was only
8.8%, but phenylcyclohexane was produced in 57.3% yield together with small
amounts of C1–C4 products in the gas phases. This result gives sharp comparisons
to Ru/NbOPO4, which can effectively prevent the deep cracking of substrate and in-
termediates and thus lead to the optimal production of monocyclic arenes.
6
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A
B
C
D
E
Figure 4. The Conversion of Hydrocarbons over the Ru/NbOPO4 Catalyst
(A–D) Product distributions for the conversion of bicyclohexyl (A), solvent dodecane (B),
diphenylmethane (C), and diphenylethane (D).
(E) Schemes of selective hydrogenolysis of three lignin model compounds over the Ru/NbOPO4
catalyst.
Reaction conditions: 0.2 g reactant, 0.1 g 5% Ru/NbOPO4, 2 g dodecane, and 0.5 MPa H2 at 280ꢀC
for 12 h. Product yield was calculated with the following equations: yield of the monocyclic
compound = (molar amount of the monocyclic compound)/(molar amount of the substrate)/2 3
100%; yield of the bicyclic compound = (molar amount of the bicyclic compound)/(molar amount of
the substrate) 3 100%.
Catalytic Cleavage of Other C–C Linkages in Lignin Model Dimers
Conversions of diphenylmethane and diphenylethane, as the a-1 and b-1/b-5 lignin
models, respectively, were also studied. The conversion of diphenylmethane gave
benzene (31.7%) and toluene (10.8%) as two main products together with the pres-
ence of a small amount of ring-saturated products (Figure 4C). For diphenylethane,
the main products were benzene and ethylbenzene (12.4% and 6.7%, respectively),
and there was a small amount of toluene (2.7%) (Figure 4D). These results confirm
that Ru/NbOPO4 truly has activity for the selective and direct hydrogenolysis of
C
aromatic–C bonds (Figure 4E). The carbon balance was 88%ꢁ92%, and the discrep-
ancies are due to (1) the adsorption of substrate, intermediate, and product on the
catalyst surface, (2) a small amount of alkane products in the gas phase, and (3) a
small loss of product during the extraction and transfer of product during analysis.
INS Studies on Adsorption, Activation, and Hydrogenolysis of Biphenyl
The selective cleavage of interunit C–C linkages in depolymerized lignin compo-
nents and simultaneous maintenance of the aromatic functionalities of phenyl rings
leads to the optimal production of monocyclic arenes in this study. Direct
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Figure 5. Proposed Reaction Network of the Conversion of Biphenyl over the Ru/NbOPO4
Catalyst
The direct cracking of phenylcyclohexane in route 1 is the dominant pathway.
visualization of the interaction between adsorbed biphenyl and the catalyst (Ru/
NbOPO4) surface is crucial to understanding the molecular details of adsorption,
activation and hydrogenolysis of biphenyl into monocyclic hydrocarbons. INS is a
powerful neutron spectroscopy technique for investigating the dynamics (particu-
larly for the deformational and conformational modes) of biphenyl, and the DFT
calculation of INS spectra is straightforward. Therefore, we combined in situ INS
and DFT calculations to investigate the molecular-binding properties of the
biphenyl-Ru/NbOPO4 system to reveal the conversion mechanism of biphenyl.
The INS spectrum of bare Ru/NbOPO4 catalyst gave a clean background with a
broad feature centered at 1,100 cmÀ1, indicating the presence of surface hydroxyl
groups (Figures S11 and S12; Note S9). In comparison, on the adsorption of biphenyl
at 220ꢀC, the INS spectrum showed a significant increase in total intensity, demon-
strating the binding of biphenyl to the catalyst surface (Figure S11). The INS spec-
trum of solid biphenyl was also collected and analyzed via DFT calculations (Fig-
ure S13; Table S11), allowing a full assignment of the spectral features for biphenyl.
Comparison of the difference of INS spectra before and after biphenyl adsorption on
the catalyst (that is, signals for adsorbed biphenyl) and that of the solid biphenyl
showed a number of marked changes (Figure 6A). Peaks at low energy (below
120 cmÀ1), assigned to the translational and rotational modes of biphenyl, shifted
to lower energy with a continuum profile, suggesting that adsorbed biphenyl mole-
cules are disordered over the catalyst surface with hindered motions as a result of the
strong binding to the catalyst. The modes of C5–C5 out-of-plane and in-plane
bending (135 and 189 cmÀ1, respectively) disappeared completely upon adsorption
on the catalyst surface, suggesting that the biphenyl molecule adsorbed onto the
catalyst with a flat position. Meanwhile, the peak at 443 cmÀ1 (assigned to C2 and
C5 out-of-plane wagging) decreased significantly and shifted to higher energy at
488 cmÀ1 (D = 45 cmÀ1); this notable blue shift is consistent with the strong binding
of phenyl plane on the catalyst surface. The intensities of deformational modes of
benzene rings at 628, 736, and 986 cmÀ1 also reduced dramatically. These results
strongly indicate that both benzene rings of biphenyl adsorbed on the catalyst.
8
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Table 2. Summary of the Products Yields from the Conversion of Biphenyl over Different Catalystsa
Entry
Catalyst
Yield (C %)b
Conversion (C %)
Monocyclic
Dicyclic
1
5% Ru/NbOPO4
5% Ru/Nb2O5
5% Ru/Nb2O5 TfOH
NbOPO4
24.4
1.1
20.1
0
19.1
2.3
3.3
0.1
12.9
0
25.4
55.8
29.8
0
13.6
24.2
15.7
0
0.9
0.5
0.2
0
5.7
4.9
2.0
0
97.0
97.8
98.2
0
2
3c
4
5.0
0
5
5% Pd/NbOPO4
5% Pt/NbOPO4
5% Rh/NbOPO4
5% Ru/HZSM-5
16.3
1.3
3.5
4.9
12.3
18.5
33.6
3.1
3.6
1.4
2.3
0.8
5.5
42.1
54.2
15.8
4.7
3.1
1.7
2.7
1.4
6.6
5.6
7.5
0.3
>99.9
>99.9
98.8
95.3
6
2.1
7
21.7
57.3
8d
aReaction conditions: 0.2 g biphenyl, 0.1 g catalyst, 2 g dodecane, and 0.5 MPa H2 at 280ꢀC for 12 h.
bProducts yields were calculated with the following equations: yield of the monocyclic compound = (molar amount of the monocyclic compound)/(molar amount
of the substrate)/2 3 100%; yield of the bicyclic compound = (molar amount of the bicyclic compound)/(molar amount of the substrate) 3 100%.
cReaction conditions: 0.2 g biphenyl, 0.1 g catalyst, 0.05 g trifluoromethanesulfonic acid, 2 g dodecane, and 0.5 MPa H2 at 280ꢀC for 12 h.
dC1–C4 alkanes were detected in the gas phases.
Meanwhile, the wagging (677, 1,100, and 1,281 cmÀ1), scissoring (1,215 cmÀ1), and
rocking (1,480 cmÀ1) modes of C–H groups on both benzene rings disappeared, and
INS peaks at 913 cmÀ1 (C–H wagging), 1,045 cmÀ1 (C–H rocking), and 1,178 cmÀ1
(C–H scissoring) notably decreased in intensity. The peaks at 245 and 392 cmÀ1, cor-
responding to the twisting mode of the C–C bond within benzene rings, shifted to
269 and 366 cmÀ1, respectively. The peak at 331 cmÀ1 (assigned to the intra-ring
stretching mode of benzene) shifted to lower energy at 313 cmÀ1 (D = 18 cmÀ1).
These changes suggest that the adsorbed biphenyl molecules are partially proton-
ated by the acid sites residing on the catalyst surface (e.g., hydroxyls) and the inter-
mediate carbocation formed (Figure S10).45 These results are in good agreement
with the in situ FTIR experiments, where a red shift (D = 28 cmÀ1) was observed
for the C5–C5 stretching mode on adsorption (Figure S14; Note S8).
The adsorbed biphenyl molecules on Ru/NbOPO4 underwent a first catalytic conver-
sion in H2 at 170ꢀC for 6 min. Comparison of the INS spectra of the first reacted and
adsorbed biphenyl showed a few changes (Figure 6B). The peaks at 736 and
986 cmÀ1 (ring deformational mode of phenyl group) disappeared upon reaction
in H2, and the peaks at 269, 313, and 366 cmÀ1 shifted to 275, 295, and
358 cmÀ1, respectively, indicating the formation of phenylcyclohexane via the
further hydrogenation of the intermediate carbocation. The spectrum for reacted
catalyst also showed the appearance of several new features at 242, 432, 460,
512, 895, 1,274, 1,348, and 1,456 cmÀ1, which are all consistent with the spectrum
of phenylcyclohexane. In contrast, the INS spectrum also confirmed the absence
of bicyclohexyl on catalyst surface (Figure S15), suggesting that only one benzene
ring on biphenyl is hydrogenated during the initial reaction.
To promote the further conversion of phenylcyclohexane, we carried out a second
hydrogenation by feeding a 5% H2/He stream at 270ꢀC for 6 min. The cell outlet
was monitored continuously via mass spectrometry (MS), which confirmed the
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Figure 6. Inelastic Neutron Scattering Spectra for Ru/NbOPO4 on the Adsorption and Catalytic
Conversion of Biphenyl
All spectra shown here are difference spectra; raw data are shown in Figures S9 and S10. INS
spectra of the reduced catalysts were used throughout for calculations of the corresponding
difference spectra. No abscissa scale factor was used throughout this report for INS calculations.
INS spectra for condensed biphenyl, phenylcyclohexane, and benzene in the solid state at 10 K
were included accordingly for a direct comparison of the vibrational modes between the adsorbed
or bound molecules and the free, intact molecules. Two hydrogenation reactions of adsorbed
biphenyl were conducted in pure and diluted (5%) H2 steams. Where no error bars are visible, these
are smaller than the symbols used to represent the data points.
(A) Comparison of INS spectra for solid and adsorbed biphenyl on Ru/NbOPO4.
(B and C) INS spectra for Ru/NbOPO4 during the first (B) and second (C) catalytic conversions of
biphenyl.
(D) Views of selected C–C vibration models of biphenyl.
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presence of benzene as the main product. Comparison of the INS spectra of the first
and second reacted biphenyl on Ru/NbOPO4 showed that the INS peaks for phenyl-
cyclohexane significantly reduced in intensity (436, 496, 1,348, and 1,453 cmÀ1) or
completely disappeared (242, 464, and 895 cmÀ1) on the second reaction (Fig-
ure 6C), indicating that high temperature greatly promoted the cleavage of C5–C5
bonds in adsorbed phenylcyclohexane on Ru/NbOPO4. New features appeared at
609 and 980 cmÀ1, and the peaks at 405, 702, 857, and 1,185 cmÀ1 increased in in-
tensity, fully consistent with the formation of benzene.
To confirm the adsorption domain, we also collected INS data for biphenyl-ad-
sorbed catalyst support NbOPO4 (Figure S16), and the INS spectra were fully consis-
tent with that of Ru/NbOPO4, indicating that the primary adsorption sites of Ru/
NbOPO4 reside on NbOPO4 and that Ru particles have little effect on adsorption.
Similar findings were also obtained from an in situ FTIR experiment (Figure S17).
Furthermore, the conventional C–C cracking catalyst, HZSM-5, was also selected
for the study of biphenyl adsorption by INS and diffuse reflectance infrared Fourier
transform (DRIFT) (Figures S18 and S19). The INS data confirmed the strong binding
of biphenyl on HZSM-5 surface, and small differences between adsorbed biphenyl
on HZSM-5 and NbOPO4 were observed. The ring deformation mode of adsorbed
biphenyl on NbOPO4 observed at 628 cmÀ1 disappeared upon adsorption on
HZSM-5. The C–H wagging mode of biphenyl at 677 cmÀ1 disappeared upon
adsorption on NbOPO4, whereas this was clearly visible for HZSM-5. In situ DRIFT
data revealed a large red shift (D = 100 cmÀ1) for the C–H stretching mode of ad-
sorbed biphenyl over NbOPO4, whereas the red shift was only by 10 cmÀ1 over
HZSM-5 (Figure S19). These results confirm that NbOPO4 shows stronger adsorption
of biphenyl than HZSM-5.
The adsorption of key intermediate (phenylcyclohexane) on NbOPO4 and HZSM-5
was also studied by INS to reveal the origin of their distinct catalytic activities. The
DFT-calculated INS spectrum of phenylcyclohexane agreed very well with the exper-
imental data (Figure S20; Table S12). Comparison of the difference in INS spectrum
before and after phenylcyclohexane adsorption on NbOPO4 (that is, signals for ad-
sorbed phenylcyclohexane) and that of the solid biphenyl showed a number of
changes (Figure S21). Peaks at low energy (below 120 cmÀ1), assigned to the trans-
lational and rotational modes of phenylcyclohexane, shifted to lower energy with a
continuum profile, suggesting that the adsorbed phenylcyclohexane molecules
were disordered over the catalyst surface with hindered motions as a result of the
strong binding to the catalyst. A number of INS peaks of phenylcyclohexane
decreased in intensity on adsorption on NbOPO4, including bands at 297 cmÀ1
(wagging of C2, C4, and C6 in benzene ring), 530 cmÀ1 (wagging of C2 and C5 in ben-
zene ring), and 848 and 996 cmÀ1 (wagging of C–H groups on benzene ring), indi-
cating that the adsorption of phenylcyclohexane was primarily via its benzene
ring. The wagging modes (247 cmÀ1) of C7 and C10 of the cyclohexane ring disap-
peared completely upon adsorption, and the intensities of C–C twisting modes
(237 cmÀ1) in the cyclohexane ring greatly reduced, indicating the hindrance of
motion of the cyclohexane ring on adsorption. In contrast, the comparison of INS
spectra of adsorbed phenylcyclohexane on HZSM-5 and that of solid phenylcyclo-
hexane showed only small decreases in the intensity of peaks at 247, 281, and
297 cmÀ1. This result suggests that adsorbed phenylcyclohexane was weakly bound
on the surface of HZSM-5 with similar motions to the solid state and could easily
desorb. This is in excellent agreement with the catalysis result, where phenylcyclo-
hexane was obtained as the main product over the Ru/HZSM-5 catalyst. Thus, the
INS study confirms that although biphenyl can strongly adsorb on both NbOPO4
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and HZSM-5, the former is also able to provide strong adsorption to the intermedi-
ate, phenylcyclohexane, thus leading to the selective C–C cleavage to produce
monocyclic products (Figure S22).
This INS study confirmed that (1) both benzene rings of biphenyl strongly adsorb on
the catalyst surface with a flat orientation, and on adsorption, benzene rings are pro-
tonated and the intermediate carbocation is formed; (2) hydrogenation of one ben-
zene ring of biphenyl occurs rapidly in the presence of H2 to give phenylcyclohexane
adsorbed on the catalyst via its phenyl group; and (3) C5–C5 bonds are then effi-
ciently cleaved on Ru/NbOPO4, and the volatile products readily desorb from the
catalyst surface, thus preventing the further hydrogenation of reaction intermediates
and products and driving the formation of arenes. Therefore, the catalytic conver-
sion of biphenyl follows adsorption, binding, protonation, partial hydrogenation
and cleavage of C5–C5 linkages, and the catalyst Ru/NbOPO4 plays an important
role in the optimal production of monocyclic aromatics.
DISCUSSION
Much current research on lignin conversion is centered on its depolymerization via
cleavage of C–O bonds. Interunit C–C bonds have higher dissociation energies,
but their cleavage is critical if the production of low-molecular-weight commodity
chemicals and jet fuels from lignin is to be improved. We designed a multifunctional
catalyst, Ru/NbOPO4, by integrating the Brønsted acid sites onto the emerging
NbOx support, coupled with the moderate hydrogenation ability of Ru centers, to
enable the cleavage of both interunit C–O and C–C linkages to maximize the lignin
monocyclic hydrocarbon production. Exceptional yields (up to 153% compared with
those of intrinsic lignin monomer units) of monocyclic hydrocarbons were obtained
from the one-pot conversion of various types of lignin. This approach also showed
complete removal of oxygen content from lignin and exhibits high selectivity to
arene products. Detailed studies using lignin models, including biphenyl, diphenyl-
methane, and diphenylethane, over multiple catalysts confirmed the unique activity
of Ru/NbOPO4 on the selective cleavage of interring C–C linkages. In situ INS and
modeling studies confirmed that the excellent activity of Ru/NbOPO4 originates
from a combination of strong adsorption and a synergistic effect between Ru,
NbOx species, and acid sites on the catalyst surface. Ru particles, NbOx species,
and acid sites promoted the dissociation of hydrogen, the strong adsorption of sub-
strates and intermediates, and the partial protonation and activation of adsorbed
substrates (e.g., biphenyl), respectively, thus driving the entire reaction.
Zeolite-based catalysts currently dominate the state-of-the-art petroleum refineries for
hydrocarbon cracking and pyrolysis-based biorefineries to produce small-molecule
feedstock chemicals.31 Significantly, the activity of Ru/NbOPO4 is distinct from that of
conventional zeolite-based catalysts. This new one-pot process for lignin conversion
offers efficient cleavage of both interunit C–C and C–O cleavages and effectively pre-
vents the deep C–C cracking of low-molecular-weight products. Thus, it results in the
isolation of desirable liquid C6–C9 monocyclic hydrocarbons under mild conditions
and successfully breaks the conventional theoretical limit on lignin monomer
production.
EXPERIMENTAL PROCEDURES
Catalytic Reactions and Analysis of Products
The detailed reaction conditions are described in the figure captions and table foot-
notes. The one-pot conversion of lignin was conducted in a 25 mL stainless-steel
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autoclave reactor. In a typical reaction, lignin (0.10 g), catalyst (0.20 g), and dodec-
ane (5 mL) were filled into the reactor, which was then sealed, purged three times
with H2, and charged to an initial pressure of 0.5 MPa with H2. The reactor was heat-
ed to a target temperature, and the reaction conducted with magnetic stirring. After
the reaction, the reactor was quenched in an ice-water bath, and the reaction mixture
was centrifuged to separate the catalyst. The organic phase was qualitatively
analyzed by gas chromatography-mass spectrometry (GC-MS) (Agilent 7890A-
5975C) and quantitatively analyzed by a GC-flame ionization detector (Agilent
7890A) with an HP-5 column. The column temperature began at 50ꢀC (held for
5 min) and was then raised at 10ꢀC minÀ1 to 250ꢀC (held for 1 min); the total running
time was 26 min. Pentadecane was used as an internal standard for the quantification
of the liquid products. For the stability test, after each reaction, the used catalyst was
isolated by centrifugation, washed with ethanol to remove the unreacted lignin, and
dried at 80ꢀC in air overnight before the next run. The theoretical yield of lignin
monocyclic units was determined by the established NBO method (Note
S1).23,37,38 The conversion of lignin models and product analysis were conducted
with the same method.
INS Experiments
INS spectra were recorded on the TOSCA spectrometer at the ISIS Facility at the
STFC Rutherford Appleton Laboratory (UK). TOSCA is an indirect geometry crystal
analyzer instrument that provides a wide dynamic range (16–4,000 cmÀ1) with reso-
lution optimized in the 50–2,000 cmÀ1 range.46 All INS spectra for the catalysis sys-
tem were collected after the sample was cooled and stabilized at temperatures
below 30 K.
In a typical experiment, the Ru/NbOPO4 catalyst (22.84 g) was loaded into a flow-
type stainless-steel cell that can also be used as a static cell with all valves closed.
Similar amounts of catalysts were used for studies of NbOPO4 and HZSM-5. The
Ru/NbOPO4 catalyst used here had a 5 wt % loading of Ru. The sample was
heated at 300ꢀC (5ꢀC minÀ1 ramping) under He for 3 h to remove any remaining
trace water before the experiment. The sample was cooled to room temperature,
and a weight loss of 0.89 g was noted as a result of the loss of adsorbed water.
The activated catalyst was then reduced by heating under a H2 flow at 250ꢀC for
3 h. The samples were cooled to <30 K during data collection by a closed-cycle
refrigerator cryostat. The procedure of the in situ catalysis experiment with the
INS measurements is summarized in Figure S23. To study the reaction mecha-
nism, biphenyl was used as a 5–5 linked lignin model compound. Adsorption
of biphenyl was carried out by flowing hot biphenyl vapor diluted in He (1.1
bar, 0.15 L minÀ1; this flow condition was used throughout the study) over the
catalyst at 220ꢀC for 2.5 h, and the exhaust gas was monitored via MS. Before
the data collection, the cell was flushed with dry He to remove weakly bound
biphenyl molecules. The adsorbed biphenyl underwent the first catalytic conver-
sion in pure H2 flow for 6 min at 170ꢀC. The cell was then flushed with He to re-
move weakly bound products and free H2, sealed, and cooled for INS collection.
After the data collection, H2 (5%) was introduced to the cell for 6 min at 270ꢀC for
the second catalytic reaction to occur, and the production of benzene was
observed instantly by MS. The cell was then flushed with He again, sealed, and
cooled for INS collection to detect the presence of possible reaction intermedi-
ates. INS spectra of pure solid compounds for both starting material and reaction
products were collected at 10 K. The amount of each sample in the neutron beam
was 1.3 g (biphenyl), 2.6 g (phenylcyclohexane), 2.8 g (bicyclohexyl), and 8.1 g
(benzene).
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In Situ Diffuse Reflectance FTIR
The DRIFT spectra were recorded with a Nicolet NEXUS 670 FTIR spectrometer
equipped with an in situ reaction chamber and a liquid N2 cooled high-sensitivity
mercury cadmium telluride detector. Prior to the FTIR studies, about 20 mg of the
catalyst was finely ground and placed in the chamber. The procedure of the in
situ biphenyl adsorption experiment with the FTIR measurements is summarized
in Figure S24. Before adsorption, the catalysts were activated in flowing He at
300ꢀC for 1 h to remove any remaining trace water and then reduced in flowing
H2 at 250ꢀC for 1 h and decreased to 220ꢀC to collect the background spectra.
For the adsorption of biphenyl, it was fed into the chamber by flowing biphenyl
vapor in He for 0.5 h; then, the chamber was flushed with dry He at 220ꢀC for
1 h to remove physical or weakly adsorbed molecules, and finally the spectra
were recorded with a resolution of 4 cmÀ1 and an accumulation of 64 scans.
The adsorption of phenylcyclohexane, bicyclohexyl, and dodecane was conduct-
ed in the same method (Figure S9; Note S8). All spectra were used after subtract-
ing the background.
DATA AND SOFTWARE AVAILABILITY
The data supporting the findings of this study are available within the article or avail-
able from the authors upon reasonable request.
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2019.03.007.
ACKNOWLEDGMENTS
This work was supported financially by the National Natural Science Foundation of
China (21832002, 21872050, 21808063, and 91545103), the Science and Technol-
ogy Commission of Shanghai Municipality (2018SHZDZX03), the Programme of
Introducing Talents of Discipline to Universities (B16017) in China, the Fundamental
Research Funds for the Central Universities (222201718003), and the University of
Manchester, Royal Society, and Engineering and Physical Sciences Research Council
(EP/P011632/1) in the UK. We are especially grateful to the Science and Technology
Facilities Council (STFC) and the ISIS Neutron Facility for access to the Beamline
TOSCA. We thank S. Xu, Q. Xia, C. Goodway, and M. Kibble for their help at the
beamline. Computing resources (time on the SCARF cluster for the CASTEP calcula-
tions) were provided by STFC’s e-Science facility.
AUTHOR CONTRIBUTIONS
L.D., X.L., and Y.G. conducted catalytic reactions and material characterizations.
L.L., X.H., S.R., and S.F.P. contributed to the collection and analysis of INS data
and DFT calculations. X.S. and F.L. collected and analyzed 2D-HSQC-NMR data.
Y.G., S.Y., and Y.W. guided the direction of the project and contributed to the prep-
aration of the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: October 28, 2018
Revised: December 4, 2018
Accepted: March 14, 2019
Published: April 11, 2019
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