Photocatalytic Cleavage of C-C Bond in Lignin Models under Visible Light on Mesoporous Graphitic Carbon Nitride through π-π Stacking Interaction

Article abstract of DOI:10.1021/acscatal.8b00022

Photocatalysis is a potentially promising approach to harvest aromatic compounds from lignin. However, the development of an active and selective solid photocatalyst is still challenging for lignin transformation under ambient conditions. We herein report a mild photocatalytic oxidative strategy for C-C bond cleavage of lignin β-O-4 and β-1 linkages using a mesoporous graphitic carbon nitride catalyst. Identifications by solid-state NMR techniques and density functional theory (DFT) calculations indicate that π-π stacking interactions are most likely present between the flexible carbon nitride surface and lignin model molecule. Besides, low charge recombination efficiency and high specific surface area (206.5 m² g^-1) of the catalyst also contribute to its high catalytic activity. Mechanistic investigations reveal that photogenerated holes, as the main active species, trigger the oxidation and C-C bond cleavage of lignin models. This study sheds light on the interaction between complex lignin structures and the catalyst surface and provides a new strategy of photocatalytic cleavage of lignin models with heterogeneous photocatalysts.

Full text of DOI:10.1021/acscatal.8b00022

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Photocatalytic Cleavage of C–C Bond in Lignin Models under Visible Light
on Mesoporous Graphitic Carbon Nitride through #-# Stacking Interaction
Huifang Liu, Hongji Li, Jianmin Lu, Shu Zeng, Min Wang, Nengchao Luo, Shutao Xu, and Feng Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00022 • Publication Date (Web): 13 Apr 2018
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Photocatalytic Cleavage of C–C Bond in Lignin
Models under Visible Light on Mesoporous
Graphitic Carbon Nitride through πꢀπ Stacking
Interaction
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Huifang Liu a,b, Hongji Li a,b, Jianmin Lu a, Shu Zeng b,c, Min Wang a, Nengchao Luo a,b, Shutao
Xu c, and Feng Wang a,*
a State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian
Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China.
b University of Chinese Academy of Sciences, Beijing 100049, P.R. China.
c National Engineering Laboratory for Methanol to Olefins, Dalian National Laboratory for
Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian
116023, P.R. China.
*Corresponding author
Eꢀmail: wangfeng@dicp.ac.cn
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ABSTRACT
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Photocatalysis is a potentially promising approach to harvest aromatic compounds from lignin.
However, the development of an active and selective solid photocatalyst is still challenging for
lignin transformation under ambient conditions. We herein report a mild photocatalytic oxidative
strategy for C−C bond cleavage of lignin βꢀOꢀ4 and βꢀ1 linkages using a mesoporous graphitic
carbon nitride catalyst. Identifications by solidꢀstate NMR techniques and DFT calculations
indicate that πꢀπ stacking interactions are most likely present between the flexible carbon nitride
surface and lignin model molecule. Besides, low charge recombination efficiency and high
specific surface area (206.5 m2 gꢀ1) of the catalyst also contribute to its high catalytic activity.
Mechanistic investigations reveal that photogenerated holes, as the main active species, trigger
the oxidation and C–C bond cleavage of lignin models. This study sheds light on the interaction
between complex lignin structures and the catalyst surface, and provides a new strategy of
photocatalytic cleavage of lignin models with heterogeneous photocatalysts.
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KEYWORDS: lignin model, photocatalysis, C–C bond cleavage, mesoporous carbon nitride,
visibleꢀlight
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INTRODUCTION
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Lignin, as a kind of sustainable aromatic polymers in nature, can provide high valueꢀadded
aromatic chemicals or fuels as an alternative to fossil resources.1ꢀ7 The interlinkages of lignin
mainly include C−O bonds and C−C bonds. Despite some progress has been made in the
transformation and fragmentation of lignin through C−O bond cleavage to aromatic
monomers,8ꢀ15 catalytic activity and selectivity should be further increased toward C−C bond
cleavage.16ꢀ22 However, the bond dissociation energies of C−C bonds in lignin are generally
higher than that of C−O bonds.23ꢀ24 And the C−C bonds reside between bulky aromatic groups,
which brings the cleavage difficulty of lignin linkages on the solid surface under ambient
condition due to the inefficient activation. That is the reason why most excellent results in the
oxidative lignin valorization involve homogeneous metal complexes.18 But the product
separation and recycling of homogeneous catalysts can be difficult. Therefore, studies and
fundamental understanding on the C−C bond cleavage of lignin models like the most common
βꢀOꢀ4 linkages with heterogeneous catalysts are desirable for the lignin conversion.
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Photocatalysis has been recently reported to be a potentially promising approach to harvest
aromatic compounds from lignin. For example, a twoꢀstep oxidationꢀreduction strategy has been
developed for the reaction of lignin βꢀOꢀ4 models through C−O bond cleavage by photoredox
and Ir (Pd) catalysis.25ꢀ26 This process has also been achieved using heterogeneous catalysts like
27ꢀ28
ZnIn2S4
or a carbazolic porous organic framework.29 In addition, photocatalytic C−C bond
cleavage in one step is also a fascinating strategy for lignin conversion. For instance, the
photochemical oxidation of lignin models using benzoquinone and copper achieved the C−C
bond cleavage, but with only 14−62% total yields of the cleaved products.30 Besides, it has been
reported that homogeneous vanadium catalysts are active for C−C bond cleavage of βꢀOꢀ4
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linkages under visible light31ꢀ32, while difficulties associated with separation and recycling still
exist. Based on our previous work about the photocatalytic oxidation of βꢀ1 linkages by
CuOx/ceria/anatase nanotube33, we want to develop new strategies toward C−C bond cleavage of
lignin βꢀOꢀ4 models using an active solid photocatalyst and to study the interaction between a
complex substrate molecule and the catalyst surface.
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Graphitic carbon nitride (gꢀC3N4), a visibleꢀlightꢀresponsive and metalꢀfree semiconductor,
has been extensively studied in environmental remediation and many organic reactions with solar
energy.34ꢀ36 Properties of large surface area, lowꢀcost, visible light adsorption, and plenty of
nitrogen atoms or vacancies, make gꢀC3N4 one of the most promising catalysts for visibleꢀlight
photoredox catalysis.37ꢀ41 As for the heterogeneously catalytic conversion of lignin models,
adsorption of the molecule with bulky benzene rings on a rigid catalyst surface may need to
overcome high energy barrier due to the structural complexity and large steric hindrance, while
the flexible organo 2D character of gꢀC3N442 may provide an ideal surface here. This brings out a
critical issue of substrateꢀcatalyst interaction. Incidentally, the specific interaction between
gꢀC3N4 and a complex aromatic molecule with lignin linkages has not been studied previously.
In the present work, we report an oxidative method that cleaves C−C bond in lignin models
with βꢀOꢀ4 or βꢀ1 linkages under visible light by using a mesoporous graphitic carbon nitride
(mpgꢀC3N4). Lignin models with two benzene rings and methoxy substituents were oxidized to
aromatic oxygenates with molecular oxygen at room temperature. Photogenerated holes were
demonstrated to be the main active species for this transformation. Possible interactions between
a substrate molecule and flexible C3N4 surface were studied using solidꢀstate NMR techniques
and the adsorption patterns were explored with DFT calculations.
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EXPERIMENTAL SECTION
Chemicals and reagents
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Urea (AR), melamine (99%), Bi(NO3)3·5H2O (> 98%), Na2WO4·2H2O (99.5%), tetraethyl
orthosilicate (TEOS, 99.0%) and hydrofluoric acid (HF, AR, ≥ 40%) were purchased from
Aladdin Chemicals. Ethanol (AR), acetonitrile (AR), acetone (AR) and dichloroethane (AR)
were purchased from Sinopharm Chemical Reagent Co., Ltd. Hydrochloric acid (HCl,
36%−38%) and nitric acid (HNO3, AR) were from Tianjin Kemiou Chemical Reagent Co., Ltd.
O2 (99.9%) and Ar (99.9%) were obtained from Dalian Institute of Chemical Physics. A
Synergy UV system from Millipore was used to deionize the distilled water (18 mꢁ).
Preparation of C3N4 catalysts
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The mesoporous graphitic carbon nitride was synthesized according to a modified method in
previous work.43 Briefly, urea (10 g) was dissolved in a solution of 0.2 M HCl solution (15 mL)
and ethanol (13 mL) under stirring. And tetraethyl orthosilicate (TEOS, 8 mL) was then slowly
added dropwise to the above solution. After stirring vigorously at room temperature for 3 h, the
mixture was heated on a plate for the solvent evaporation and then dried at 100 °C for 10 h. The
obtained white solid was calcined in a muffle furnace at 550 °C for 4 h (heating rate: 2.5
°C/min). Subsequently, hydrofluoric acid was used to remove SiO2 through stirring with the
above material for 20 h. Then after the following filtration, water and ethanol washing for several
times, and drying at 80 °C for 12 h, the paleꢀyellow solid was finally obtained and denoted as
mpgꢀC3N4.
For comparison, urea (10 g) was directly heated at 550 °C for 4 h (the same heating rate: 2.5
°C/min) in the muffle furnace, giving a paleꢀyellow solid denoted as C3N4ꢀU. Similarly,
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melamine as the precursor was directly heated through the same procedures, giving a yellow
sample denoted as C3N4ꢀM.
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Preparation of other semiconductors
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Semiconductors like TiO2ꢀA (AnataseꢀTiO2, 99.8% metals basis, 40 nm, Aladdin Chemicals)
and P25 (Degussa, 20 nm) were obtained commercially and directly used without further
treatment in this work. Bi2WO6 was synthesized according to the literature report.44 The general
procedures are as follows: Firstly, Bi(NO3)3·5H2O (2 mmol, 0.97 g) was dissolved in 1.0 M
HNO3 aqueous solution (15 mL), and Na2WO4·2H2O (1 mmol, 0.33 g) was dissolved in 15 mL
of H2O. Then, the Na2WO4 solution was added into the Bi(NO3)3 solution and stirred for 30 min,
after which the mixture was transferred to a 50 mL autoclave and heated at 180 °C for 24 h in an
oven. After the hydrothermal crystallization process, the mixture was filtered, washed with
deionized water for several times and dried at 100 °C for 12 h, giving the final solid Bi2WO6
sample.
General characterizations
N2 adsorption and desorption of the C3N4 catalysts were measured on a Quadrasorb SI
instrument (Quantachrome, USA) at 77.3 K. Nitrogen and hydrogen contents were determined
through elemental analyses on a HORIBA EMGAꢀ930 analyzer, and carbon content analysis was
performed on a HORIBA EMIA analyzer. Fourier transform infrared spectra (FTIR) were
collected on a Bruker Tensor 27 instrument. Powder Xꢀray diffraction patterns were obtained on
a PANalytical XꢀPert PRO diffractometer using Cu Kα radiation at 40 kV and 20 mA, with
continuous scans in a 2θ range of 5−80°. The morphology of mpgꢀC3N4 was examined by
scanning electron microscopy (SEM, JSMꢀ7800F) and transmission electron microscopy (TEM,
JEMꢀ2100). UVꢀVis diffuse reflectance spectra of the C3N4 catalysts were recorded on a
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SHIMADZU UVꢀ2600 Spectrophotometer. Photoluminescence (PL) measurements were
performed on a Fluorescence Spectrophotometer (Photon Technology International, QM 400)
with the excitation wavelength at 375 nm.
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Photocatalytic reactions
Typical reactions in this work were carried out in a homemade quartz tube under 455 nm LED
light with a total power of 6 W. Typically, 0.05 mmol of lignin model, 10 mg of catalyst and 1
mL of solvent (generally CH3CN, unless other solvents stated in the text) were firstly added into
the reactor. And the required gas (Ar or O2) was charged in, after which the tube was sealed and
placed under light illumination. The reaction mixture was stirred for a desired time, with the air
conditioning cooling the photocatalytic system. CAUTIOUS! Wearing eye goggle of shielding
455 nm light is mandatory. After reaction, pꢀmethylacetophenone was added into the reaction
mixture as an analytical internal standard. Then after filtering through a 0.22 ꢂm filter, the liquid
sample was diluted and analyzed by gas chromatographyꢀmass spectrometry (GCꢀMS) and high
performance liquid chromatography (HPLC). Conditions of gas chromatography: instruments:
Agilent 7890A/5975C, HPꢀ5 column; carrier gas: helium; split injection; inlet temperature: 260
°C; temperature program: 80~270 °C 10 °C/min, 270 °C isothermal 2 min; mass spectrometer:
quadrupole detector. Conditions of HPLC: instruments: Agilent 1260 Infinity; injection: 10 µL;
mobile phase: acetonitrile/water (v/v of 40/60~60/40); flow rate: 1.0 mL/min; column: Agilent
Eclipse Zorbax C18 column; TCC temperature: 35 °C; detector: UV 230 nm.
Operations and results of a largeꢀscale photocatalytic experiment using a xenon lamp are
attached in the Supporting Information.
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Solid-state NMR
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A spectrometer (Bruker Avance III 600) equipped with a 14.1 T wideꢀbore magnet was used
here for all the solidꢀstate magic angle spinning (MAS) NMR experiments. The resonance
frequencies for 1H, 13C were 600.1 and 150.9 MHz, respectively. For the 1H NMR experiments, a
4 mm HꢀX WVT probe was used with a spinning rate of 12 kHz, and the spin echo pulse
sequence with a π/2 pulse length of 4.35 µs and π pulse length of 8.7 µs were used. Both the
1Hꢀ13C CP/MAS and heteronuclear correlation (HETCOR) experiments were performed with a
spinning rate of 12 kHz. A recycle delay of 2 s and a contact time of 1 ms were recorded in
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1Hꢀ13C CP/MAS NMR spectra. For the twoꢀdimensional (2D) Hꢀ13C HETCOR NMR
experiment, the increment interval was set to 41.67 µs in the indirect dimension. Typically, 256
scans were acquired for each t1 increment, and twoꢀdimensional data sets consisted of 256 t1 ×
2048. A 1H rf field of 57.5 kHz, a 13C rf field of 33.5 kHz, and a contact time of 1 ms were set to
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the sequence. The H and C chemical shifts were both referenced to adamantane at 1.74 ppm
and the upfield methine peak at 29.5 ppm, respectively. 1H double quantum (DQ) NMR spectra
were recorded on a 3.2 mm HꢀXꢀY WVT probe with a spinning rate of 15 kHz using a double
resonance mode. Following the general scheme of 2D multipleꢀquantum spectroscopy, 1Hꢀ1H DQ
coherences were excited and reconverted with a POSTꢀC7 pulse sequence. And for each of the
128 experiments, the spectra were acquired using 16 scans with t1 increment of 33.33 ꢂs.
DFT calculation settings
We used the Vienna Ab Initio Simulation Package (VASP)45ꢀ46 for all the DFT calculations
within the generalized gradient approximation (GGA) using the PBE47 functional formulation.
Projected augmented wave (PAW) pseudopotentials describe the interactions between ionic
cores and valence electrons.48ꢀ49 1 (H), 4 (C), 5 (N), and 6 (O) valence electrons were explicitly
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taken into account. Plane wave basis set with a kinetic energy cutoff of 400 eV was included.
Partial occupancies of electronic bands were allowed with the Gaussian smearing method and a
width of 0.10 eV. Brillouin zone integration was performed using a 6 × 6 × 1 MonkhorstꢀPack
grid for the periodic slab with one singleꢀlayered planar gꢀC3N4 sheet and a 20 Å vacuum
between the sheet and its periodic images; and a 3 × 3 × 1 MonkhorstꢀPack grid for the slab with
a singleꢀlayered corrugated gꢀC3N4 sheet and a 20 Å vacuum. The convergence criterion for the
electronic selfꢀconsistent cycles was set to 10ꢀ7 eV and that for geometry optimization was 10ꢀ6
eV.
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RESULTS AND DISCUSSION
Photocatalytic reaction of 2-Phenoxy-1-phenylethanol
As a representative of lignin βꢀOꢀ4 linkage, 2ꢀPhenoxyꢀ1ꢀphenylethanol (1) with Cα–OH, Cα–
Cβ and Cβ−O bonds was initially used as a model substrate in this work. Other studies have
reported that the oxidized products of 1 generally include a ketone (2) via Cα–H oxidation and
the degraded products (3-6) through Cα–Cβ or Cβ−O bond cleavage (Scheme 1).17, 21, 26, 30, 50ꢀ51
Scheme 1. General oxidative products of 2ꢀphenoxyꢀ1ꢀphenylethanol (1)
To test the photocatalytic reaction of model 1, various semiconductors have been used as the
catalysts. Results with different catalysts under various conditions are summarized in Table 1.
The reaction did not occur without a catalyst (Table 1, entry 1). Typical photocatalysts like
TiO2ꢀA, P25 and Bi2WO6 showed low activity (conversions of 10−28%) for this reaction (Table
1, entries 2−4). The most common gꢀC3N4 prepared from melamine (C3N4ꢀM) exhibited
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relatively higher activity, with a 46% conversion and 63% C−C cleavage selectivity (Table 1,
entry 5). As different precursors and preparation methods of gꢀC3N4 would greatly affect their
catalytic activity, two other gꢀC3N4 samples (C3N4ꢀU and mpgꢀC3N4) have also been synthesized
and tested in the reaction. Herein, C3N4ꢀU prepared from urea (Table 1, entry 6) was more active
than C3N4ꢀM with the conversion of 67% and C−C cleavage selectivity of 81%, which can be
attributed to the faster carrier transport and higher surface area of C3N4ꢀU52. When mpgꢀC3N4
was used (Table 1, entry 7), substrate 1 was nearly completely transformed with a relatively high
selectivity (91%) of C−C bond cleaved products, including benzaldehyde (3), phenyl formate (4)
and benzoic acid (5).
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Table 1. Photocatalytic transformation of model 1 under various reaction conditions a
Yield (%) b
Conversion
C–C cleavage
selectivity (%) c
Entry
Catalyst
(%)
0
2
ꢀ
3
ꢀ
4
ꢀ
5
ꢀ
1
None
ꢀ
ꢀ
2
TiO2ꢀA
12
28
10
46
67
96
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1
ꢀ
ꢀ
ꢀ
3
P25
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19
ꢀ
9
ꢀ
70
ꢀ
4
Bi2WO6
C3N4ꢀM
C3N4ꢀU
ꢀ
ꢀ
ꢀ
5
11
10
7
18
20
51
ꢀ
17
17
30
ꢀ
ꢀ
63
81
91
ꢀ
6
25
21
ꢀ
7
mpgꢀC3N4
mpgꢀC3N4
mpgꢀC3N4
mpgꢀC3N4
mpgꢀC3N4
mpgꢀC3N4
8 d
9 e
10 f
11 g
12 h
ꢀ
5
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
27
79
86
6
14
16
45
15
18
20
8i
15
18
78
52
83
29
13
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Standard reaction conditions: 0.05 mmol of substrate 1, 10 mg of catalyst, 1 mL of CH3CN,
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moles of product 2ꢀ5 formed
moles of substrate 1 input
b
c
455 nm LED (6 W), O2 (1 atm), 10 h. GC yield of 2−5 (%) =
× 100%.
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C−C bond cleavage selectivity was presented as the molar ratio of (3+5)/(2+3+5) in product
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d
e
f
distribution. Reaction conducted in the dark. Reaction under 1 atm of argon. Reaction in
g
h
i
C2H5OH. Reaction in C2H4Cl2. Reaction in acetone. Yield of ethyl benzoate. “ꢀ” means
not detected.
The reaction is photoꢀdriven and light irradiation is necessary for this transformation, as the
reaction did not occur under dark conditions (Table 1, entry 8). Besides, no product was
detected under argon (Ar) atmosphere (Table 1, entry 9) with a significantly decreased
conversion (5%), indicating that O2 is indispensable for this reaction. In addition, solvents
also have great effects on the catalytic performance. Ethanol was not a suitable solvent for
this transformation with a low conversion of 27% (Table 1, entry 10). Reactions in
C2H4Cl2 and acetone showed medium conversions of 79% and 86% (Table 1, entries
11−12), respectively. Acetonitrile was the best solvent in this study, with a nearly total
conversion of the substrate and a 91% selectivity of C−C bond cleavage (entry 7).
Characterizations of C3N4 catalysts
Table 2. Structural properties and compositions of the asꢀprepared C3N4 catalysts
SBET
Vpore
dpore
C
N
H
C/N
Sample
(m2 gꢀ1) a (cm3 g−1) (nm) b (wt %)
(wt %)
62.95
57.16
56.57
(wt %) atomic ratio
C3N4ꢀM
C3N4ꢀU
6.6
48.2
206.5
0.04
0.15
0.42
2.7
2.9
3.6
32.13
28.13
26.11
1.81
1.87
2.33
0.60
0.57
0.54
mpgꢀC3N4
a Multipoint BET surface area. b Average pore width determined by the DFT method.
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Table 2 shows the physicochemical properties and elemental compositions of the asꢀprepared
C3N4 catalysts. According to the N2 physical adsorption results (Figure S1), the BET specific
surface area of C3N4ꢀM is only 6.6 m2 gꢀ1, while it is 48.2 m2 gꢀ1 for C3N4ꢀU, indicating that
precursor types affect the structure of polymeric graphitic C3N4 materials. Meanwhile, mpgꢀC3N4
has a much higher surface area (206.5 m2 gꢀ1) and is verified to be mesoporous with the average
pore size of 3.6 nm. From the elemental analysis, the presence of hydrogen in C3N4 materials
(1.81–2.33 wt %) means the incomplete condensation of precursors leaving NHx groups
remained in the catalysts. And for the mpgꢀC3N4, residual trace silica in the catalyst can also
enhance the charge separation of electronꢀhole pairs53.
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Figure 1. (a) FTIR spectra and (b) XRD patterns of the asꢀprepared C3N4 catalysts. (c) HRTEM
image of the mpgꢀC3N4 catalyst.
FTIR spectra (Figure 1a) exhibit typical characters of molecular structure of C3N4 materials for
all three asꢀsynthesized catalysts.54ꢀ55 We can see the sharp peak at 810 cm−1 and strong bands in
the 1200−1650 cm−1 region, which are assigned to the breathing mode of triꢀsꢀtriazine units and
the stretching vibration modes of CꢀN heterocycles, respectively. Besides, broad peaks in the
region of 3000−3500 cm−1 are ascribed to −NHx and –OH residues caused by precursors or air
exposure.
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In general, two typical diffraction peaks at 12.9° and 27.3° are shown in the XRD patterns
(Figure 1b) of the asꢀprepared C3N4 catalysts, corresponding to the intralayer longꢀrange order
and interlayer periodic stacking controlled by van der Waals forces, respectively.56 C3N4ꢀU and
mpgꢀC3N4 have a much weaker peak at 12.9° than C3N4ꢀM, indicating that C3N4 synthesized
from urea has less intralayer hydrogen bonds. A previous report has indicated that carbon nitride
with less hydrogen bonds may show superior photoactivity because of the fast charge transfer
between interlayers.52 Besides, the relatively weak and broadened peaks at 27.3° of C3N4ꢀU and
mpgꢀC3N4 indicate that polymerization of urea gives the C3N4 structure with less layers.
Moreover, the layerꢀstacked structure of mpgꢀC3N4 with slit pores is further clearly evidenced by
the high resolution transmission electron microscopy image of it (Figure 1c and Figure S2).
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Figure 2. The plots of (αhυ)1/2 versus photon energy (a) and photoluminescence spectra (b) of
the asꢀprepared C3N4 catalysts.
Moreover, UVꢀVis absorption and photoluminescence spectra of these C3N4 catalysts were
also performed to study their optical properties. The band gaps of C3N4ꢀM, C3N4ꢀU and
mpgꢀC3N4 are calculated to be 2.71, 2.78 and 2.74 eV via KubelkaꢀMunk plot (Figure 2a),
respectively. Thus the similar light absorption of these C3N4 samples contributes little to the
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activity difference among them. On the other hand, compared to C3N4ꢀM and C3N4ꢀU, mpgꢀC3N4
has a much lower photoluminescence intensity (Figure 2b), meaning that mpgꢀC3N4 has the
lowest electronꢀhole recombination efficiency among them. Therefore, large surface area and
low charge recombination efficiency from the introduced mesopores may contribute to the high
activity of mpgꢀC3N4 in this photocatalytic reaction.
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Surface interaction and DFT calculations
The substrateꢀcatalyst interaction is a key point for heterogeneous reactions. For the carbon
nitride polymer, there are different types of nitrogen functionalities in its periodic structure,
which can act as the Lewis basic sites to anchor certain groups of the substrate molecule.57
Besides, the layered conjugate structure of C3N4 can also contribute to the coplanar interaction
with the reactants such as πꢀπ stacking or other noncovalent bonds.58
1
1
Figure 3. (a) Solidꢀstate Hꢀ13C CP/MAS NMR spectra and (b) H MAS NMR spectra of
mpgꢀC3N4 and R@mpgꢀC3N4. (* denotes spinning side band. R@mpgꢀC3N4 was prepared as
follows: 200 mg of mpgꢀC3N4 was dispersed in an acetonitrile solution containing 10 mg of
reactant 1, and the solvent was evaporated to obtain the dried solid R@mpgꢀC3N4.)
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To clarify their structures and interactions between lignin model molecule and the surface of
mpgꢀC3N4, solidꢀstate MAS NMR experiments were conducted, with the results shown in Figure
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3. In the 1D Hꢀ13C MAS NMR spectra (Figure 3a), only two typical carbon signals for
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mpgꢀC3N4 are observed with the peak maxima at 165 and 157 ppm, which are assigned to
CN2(NHx) and CN3 moieties59ꢀ60 respectively in C3N4. For R@mpgꢀC3N4 in Figure 3a, other
weak signals in the blue frame correspond to aromatic carbon signals of reactant 1. Besides, from
the 1H MAS NMR spectra (Figure 3b), we can see an apparent peak at 4.5 ppm and a broad peak
at 9.1 ppm for mpgꢀC3N4, with the former assigned to the adsorbed H2O molecules on C3N4.
1
Also in the 2D Hꢀ13C HECTOR MAS NMR spectrum of R@mpgꢀC3N4 (Figure S3), one cross
signal at (165, 9.1) ppm indicates that hydrogens at 9.1 ppm are only in close proximity to the
carbon at 165 ppm, which simultaneously demonstrates that peak at 9.1 ppm is assigned to NHx
groups on mpgꢀC3N4. Moreover, a new signal at 6.8 ppm for R@mpgꢀC3N4 corresponds to
hydrogens on the benzene rings of reactant 1 (Figure 3b). In comparison with the liquid 1H NMR
spectra of compound 1 (see the spectra in SI), peaks of phenyl hydrogen atoms at 7.2~7.5 ppm
are not prominent, which might come from some chemical shift change due to diamagnetic
anisotropy or the overlap with broad peaks at ~9.1 ppm. In addition, another small signal at ca.
2.0 ppm in Figure 3b is due to a small amount of residual acetonitrile solvent.
1
2D Hꢀ1H DQ MAS NMR spectroscopy was further used here to study the protonꢀproton
proximities (< 5 Å) / interactions of the reactant molecule and the catalyst surface.61 As shown in
1
Figure 4, the 2D Hꢀ1H DQ MAS NMR spectrum of R@mpgꢀC3N4 shows three diagonal peaks
at (1.9, 3.8), (6.8, 13.6) and (9.1, 18.2) ppm due to the autocorrelation of –CH3 from solvent,
benzene ring hydrogens from reactant 1 and NHx from mpgꢀC3N4, respectively. Another
offꢀdiagonal peak pair at (6.8, 15.9) ppm is also observable, indicating that aromatic hydrogens
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of reactant 1 (6.8 ppm) are in close spatial proximity to the NHx groups (9.1 ppm) on mpgꢀC3N4.
This spatial proximity between the reactant molecule and catalyst surface may promote the
transfer of photogenerated holes during the transformation,62 which is important for
photocatalytic reactions.
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Figure 4. 2D 1Hꢀ1H DQ MAS NMR spectrum of R@mpgꢀC3N4 (* denotes spinning side band).
Further, we explored the interaction model between C3N4 surface and reactant molecule 1
using DFT calculations. Firstly, calculated results show that a corrugated surface is much more
stable with the potential energy of 3.29 eV lower than that of a planar surface of the 2 × 2 C3N4
sheet (Figure S4), which is in accordance with previous report.63 Then structures of a corrugated
4 × 4 C3N4 sheet with different conformations were optimized and we found that the energy
differences between them (0.77 eV and 1.12 eV) were about one order less than the energy
difference between the corrugated and planar 4 × 4 C3N4 sheet (Figure S5 and discussion in the
supporting information). Therefore, conformational changes of a corrugated C3N4 surface might
be flexible and geometrically beneficial for the accommodation of complex biomass molecules.
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Figure 5. Optimized interaction patterns (top view and side view) and adsorption energies of
substrate molecule 1 adsorbed on a corrugated C3N4 surface. (a and d for adsorption pattern A; b
and e for pattern B; c and f for pattern C. Color scheme: H, white; C, gray; N, blue; O, red.)
Moreover, we have calculated possible interaction models (Figure 5) between the substrate
molecule 1 and a corrugated C3N4 surface. Interaction patterns A and B correspond to the oxygen
atom in OH group of molecule 1 approaching different carbon atoms in triꢀsꢀtriazine cavity (also
the H atom in OH group of molecule 1 approaching N atoms in C3N4), and have similar
adsorption energies of ꢀ0.46 and ꢀ0.54 eV, respectively. While pattern C had a higher adsorption
energy of ꢀ0.92 eV, with two benzene rings of molecule 1 nearly parallel to the triazine rings of
the corrugated C3N4 surface. In addition, the distances between oxygen and carbon atoms were
about 2.60 and 2.73 Å respectively for patterns A and B (Figure S6a and S6b), indicating a
physical adsorption mode of this kind of interaction. For the adsorption pattern C, distances of
∼3.2–3.9 Å between benzene rings and triazine rings (Figure S6c) was in the range for πꢀπ
stacking interactions. As reported, πꢀπ interactions between the electron rich C3N4 and some
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aromatic molecules contribute to the molecule activation,64ꢀ65 which may also promote further
charge transfer between them and the transformation of lignin aromatic structures in this work.
In addition, we have conducted the control experiments using lignin models with tertꢀbutyl
substituted arenes (model S1 and S2). The conversions were lower compared to that of model 1
after reaction, and yields of benzaldehyde and benzoic acid as the representative cleaved
products also decreased significantly (Figure S7). Lignin models with tertꢀbutyl substituted
arenes might hardly take part in the most favorable interaction pattern with catalyst surface,
which implies the contribution and importance of surface interaction pattern for the cleavage of
reactants in this system.
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Reaction mechanism investigations
To explore the reaction routes, we conducted several control experiments (Scheme 2).
Benzaldehyde as one cleaved product could be oxidized to benzoic acid under reaction
conditions (Eq. 1), leading to a mixture of them after the reaction of molecule 1. Experiment
with phenyl formate as the substrate (Eq. 2) indicated that it was relatively stable under reaction
conditions, without considerable hydrolysis to phenol and HCOOH as reported31. Moreover, the
ketone 2 was partly cleaved at the Cα−Cβ bond with low yields of products (Eq. 3), indicating
that 2 is not the intermediate during the transformation of 1. Therefore, two routes including
benzylic alcohol oxidation and oxidative Cα−Cβ bond cleavage may coꢀexist during the reaction.
Besides, this reaction was greatly inhibited by TEMPO (2,2,6,6ꢀtetramethylꢀ1ꢀpiperidinyloxy)
(Eq. 4), indicating that the reaction proceeds through certain radical intermediates.
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Scheme 2. Control experiments
Scheme 3. Control experiments with deuterated substrates
In further experiments with deuterated substrates (Scheme 3), the conversion (29%) of 1'-D
with Cα–D was close to that of substrate 1 (30%), meaning that the hydrogen abstraction of Cα–H
is not the rateꢀdetermining step during the reaction process. Meanwhile, the conversion of 1''-D
with Cβ–D apparently decreased compared with that of 1, and the generation of cleaved products
was slowed down, indicating that oxidation of Cβ–H is pivotal for the substrate transformation
and Cα–Cβ bond cleavage process. Moreover, the presence of Cα–D and Cβ–D in deuterated
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aldehyde and phenyl formate separately (Figure S8 and S9) indicates a possible reaction
mechanism shown below.
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Figure 6. Conversion of 1 using mpgꢀC3N4 with different scavengers. Reaction conditions: 0.05
mmol of substrate 1, 10 mg of mpgꢀC3N4, 1 mL of CH3CN, 0.05 mmol of the scavenger, O2 (1
atm), 455 nm LED (6 W).
To identify the active species that orient this photocatalytic reaction, K2S2O8, (NH4)2C2O4,
pꢀbenzoquinone (pꢀBQ) and tꢀBuOH were adopted as the scavengers for photogenerated
electrons, photogenerated holes, O2 and OH, respectively (Figure 6).66ꢀ67 Addition of pꢀBQ
improved the substrate conversion (Figure S10), as pꢀBQ itself could induce part oxidation and
dissociation of the model substrate as reported before.30 Meanwhile, K2S2O8 or tꢀBuOH had little
•−
•
•
suppression effect on the substrate conversion, indicating that photogenerated electrons and OH
radicals contribute little in this transformation. Besides, the nonꢀpromotional effect on the initial
substrate conversion rate through increasing the O2 pressure (Figure S11), as well as the little
suppression effect of electron scavengers, also excludes that O2 is initially activated and
•−
subsequently induces the substrate transformation, as O2 comes from the reduction of O2 by
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photogenerated electrons in the conduction band of the catalyst. By contrast, addition of
(NH4)2C2O4 apparently inhibited the substrate conversion, revealing that photogenerated holes in
the valance band of mpgꢀC3N4 are main active species for the transformation of lignin model 1.
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Scheme 4. Proposed mechanism of mpgꢀC3N4 catalyzed transformation of molecule 1
Based on the above results, we proposed a possible mechanism of photoꢀdriven transformation
of lignin molecule 1 with mpgꢀC3N4 as the catalyst (Scheme 4). mpgꢀC3N4 is firstly excited by
visible light irradiation, generating holes and electrons separately in the valence and conduction
band. For the reactant molecule interacting with mpgꢀC3N4 surface, a hydrogen from Cβ is
abstracted under the role of photogenerated holes and surface basic sites on mpgꢀC3N4,
generating a Cβꢀcentered radical (A),33 which further combines with O2 and hydrogen to give a
peroxide intermediate C. The following electron transfer in C through a sixꢀmembered ring
transition state would induce the Cα–Cβ and O–O bond cleavage, forming aromatic aldehyde and
phenyl formate as the major products. Part of the aldehyde product can be further oxidized to
benzoic acid. On the other side, photogenerated electrons can also reduce O2 molecules to the
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•−
superoxide radical anion O2 , which would deprotonate benzylic alcohol to the alkoxide anion
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and finally result in the formation of byꢀproduct 2.41
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Oxidative cleavage of other lignin models
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We then tested the applicability of mpgꢀC3N4 for visible light driven transformations of other
lignin models (Table 3). βꢀ1 lignin models like 1,2ꢀdiphenylethanol (1a) and 1b were also
dissociated, with aromatic aldehydes and acids as the major cleaved products. Besides, other
βꢀOꢀ4 linkages with Cγ−OH (1c-1e) could also yield cleaved products under the same conditions.
Yields of phenolic part (with benzene ring B) with more methoxy groups were low due to the
overoxidation or polymerization of phenolic compounds or intermediates under light irradiation,
which would be problematic for hardwood lignins that mainly contain syringyl units.
Table 3. Oxidative cleavage of typical lignin models catalyzed by mpgꢀC3N4 under visible light a
a
Reaction conditions: 0.05 mmol of lignin model, 10 mg of mpgꢀC3N4, 1 mL of CH3CN, O2 (1
moles of product 3 (5) obtained
moles of substrate 1a input
b
atm), 455 nm LED light (6 W). For the reaction of 1a, yield of 3 (5) =
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moles of product 2a obtained
×
ꢀꢁ × 100%, yield of 2a =
× 100%. Unless otherwise specified, yields of
moles of substrate 1a input
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other products were calculated as the molar ratio of the corresponding product to the substrate
input.
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In addition, recycle experiments of model 1 using mpgꢀC3N4 were conducted, with the
catalyst showing relatively stable activity after four cycles (Figure S12). Moreover, a large scale
reaction was also conducted using a xenon illuminator by magnifying the reaction system
(Figure S13), achieving a 90% conversion of substrate 1 after 20 h and considerable amounts of
cleaved products (see the SI for details), which indicates the potential application of this
photocatalytic system. However, photocatalytic transformation of real lignin may be hampered
by its dark brown color and low solubility, which limit the light absorption by a photocatalyst.68
We performed a control experiment of model 1 in the presence of sodium lignosulfonate (Figure
S14), which resulted in a much lower conversion (from 96% to 43%) and less products after 10 h.
To address potential difficulties of visible light photocatalysis with real lignin extracts, using
some newly developed photocatalytic reactors with flow systems69ꢀ70, improving the solubility of
lignin, and reducing lignin color via UV irradiation first71ꢀ73 may be helpful to improve the light
throughput and reaction efficiency.
CONCLUSIONS
In conclusion, a mild oneꢀstep photocatalytic strategy has been developed for C–C bond
cleavage of lignin models using a mesoporous graphitic carbon nitride photocatalyst. βꢀOꢀ4 and
βꢀ1 linkages were effectively cleaved into aromatic aldehydes (acids) and phenolic esters.
Solidꢀstate NMR experiments identified the spatial proximity of aromatic hydrogens from model
molecule and the NHx groups on mpgꢀC3N4, with DFT calculations revealing the favorable πꢀπ
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stacking interactions between them. Photogenerated holes were primarily active for the substrate
transformation, according to the trapping experiments and mechanism studies. This study may
shed light on the holeꢀinduced direct conversion of lignin linkages on a solid catalyst surface and
inspire further development of visibleꢀlightꢀdriven heterogeneous catalysts for lignin conversion.
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Supporting Information
Quantification methods of the reaction, supplementary experimental results and discussion,
procedures of synthesizing different lignin models, NMR data and spectra
ACKNOWLEDGMENT
This work was supported by the National Natural Science Foundation of China (Projects
21690082, 21690080), the “Strategic Priority Research Program of the Chinese Academy of
Sciences” Grant No.XDB17020300, the Dalian Excellent Youth Foundation (2015J12JH203),
the CAS/SAFEA International Partnership Program for Creative Research Teams, and supported
by DICP (DICP DMTO201406).
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Valorization of Lignin for the Production of Renewable Chemicals. Chem. Rev. 2010, 110,
3552ꢀ3599.
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