Selective reductive cleavage of C–O bond in lignin model compounds over nitrogen-doped carbon-supported iron catalysts

Article abstract of DOI:10.1016/j.mcat.2018.03.014

Lignin has recently attracted much attention as a promising resource to produce fuels and aromatic chemicals. The selective cleavage of C–O bond while preserving the aromatic nature has become one of the major challenges in the catalytic valorization of lignin to aromatic chemicals. In this work, we report that the selective reductive cleavage of C–O bond in lignin model compounds can be successfully achieved through heterogeneous iron catalysis. The hydrogenolysis of α-O-4 model linkage shows that the iron catalyst prepared by the simultaneous pyrolysis of iron acetate and 1,10-phenanthroline on activated carbon at 800 °C is the most active iron catalyst, affording phenol and toluene with yields of 95% and 90%, respectively. This aromatics selectivity is found to be much higher than that obtained over noble metal catalysts. The presence of N?Fe species as the active center of heterogeneous iron catalyst was confirmed by various technologies especially XPS and H₂-TPR. For the β-O-4 model linkage, the vicinal –OH group was essential for the iron-catalyzed hydrogenolysis of ether linkage. The oxidation of the α-carbon in the β-O-4 model compounds can significantly decrease the bond dissociation energy of ether linkage, giving depolymerization products in moderate to excellent yields.

Full text of DOI:10.1016/j.mcat.2018.03.014

Molecular Catalysis 452 (2018) 36–45
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Selective reductive cleavage of CeO bond in lignin model compounds over
nitrogen-doped carbon-supported iron catalysts
a,⁎
a
a
a
a
b
Jiang Li , Hui Sun , Jia-xing Liu , Jun-jie Zhang , Zhen-xing Li , Yao Fu
a
State Key Laboratory of Heavy Oil Processing, Institute of New Energy, China University of Petroleum (Beijing), Beijing, 102249, China
Anhui Province Key Laboratory of Biomass Clean Energy, Department of Chemistry, University of Science and Technology of China, Hefei, 230026, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Biomass conversion
Lignin
CeO bond cleavage
Heterogeneous catalysis
Iron
Lignin has recently attracted much attention as a promising resource to produce fuels and aromatic chemicals.
The selective cleavage of CeO bond while preserving the aromatic nature has become one of the major chal-
lenges in the catalytic valorization of lignin to aromatic chemicals. In this work, we report that the selective
reductive cleavage of CeO bond in lignin model compounds can be successfully achieved through heterogeneous
iron catalysis. The hydrogenolysis of α-O-4 model linkage shows that the iron catalyst prepared by the si-
multaneous pyrolysis of iron acetate and 1,10-phenanthroline on activated carbon at 800 °C is the most active
iron catalyst, affording phenol and toluene with yields of 95% and 90%, respectively. This aromatics selectivity
is found to be much higher than that obtained over noble metal catalysts. The presence of N⋯Fe species as the
2
active center of heterogeneous iron catalyst was confirmed by various technologies especially XPS and H -TPR.
For the β-O-4 model linkage, the vicinal eOH group was essential for the iron-catalyzed hydrogenolysis of ether
linkage. The oxidation of the α-carbon in the β-O-4 model compounds can significantly decrease the bond
dissociation energy of ether linkage, giving depolymerization products in moderate to excellent yields.
1. Introduction
selective cleavage of ether linkage in lignin. For example, some noble
metal catalysts based on Pd [14–23], Ru [24,25], and Rh [26] have
The rapid decline of fossil reserves and increasing demand for en-
ergy have prompted researchers to utilize alternative sustainable re-
sources, such as biomass, for the production of fuels and chemicals
been proved efficient for the cleavage of CeO bond in various lignin
model linkages. Whereas the scarcity and high cost of noble metals,
non-noble metal based heterogeneous catalysts were perhaps more
preferable for lignin conversion. A series of nickel-based catalysts such
as heterogeneous Ni catalyst in-situ generated from Ni
[
1,2]. As one of the three main components of lignocellulosic biomass,
lignin has been regarded as a promising resource to produce valuable
aromatic compounds [3]. It is composed of methoxy-substituted phenyl
and phenolic subunits linked together through CeO bonds of α- and β-
arylalkyl ethers. Currently, selective cleavage of CeO bond while pre-
serving its aromatic nature has become one of the major challenges in
the catalytic valorization of lignin to aromatic chemicals.
2 2 2
(CH TMS) (TMEDA) [27], Ni/SiO [28–30], Ni/C [31–33], Ni/hydro-
talcite (HTC) [34], raney Ni [35,36], titanium nitride-nickel nano-
composite (TiNeNi) [37], NiMo (with or without sulfide) [38,39], Ni/
niobic acid-activated carbon composites [40], NiAlOx [41], Ni/amor-
2
phous silica-alumina [42], Ni-based alloy catalysts [43–45], W C [46],
Well-defined molecular catalysts were first used for the selective
cleavage of lignin-derived aryl ether linkages. Hartwig and co-workers
reported an effective nickel carbene complex for the hydrogenolysis of
aromatic CeO bonds in alkyl aryl and diaryl ethers [4]. Some other
molecular catalysts based on Ru [5–7], V [8,9], Fe [10], Ir [11,12] and
Re [13] complexes have also been reported for the selective CeO bond
cleavage. Although the molecular catalysts can be easily accessible to
react with individual ether linkage, they also suffer from sensible to
water, which is costly to remove during biomass conversion. Hence,
various heterogeneous catalysts have been developed to achieve the
and Mo-based catalyst [47,48] have been reported for reductive clea-
vage of ether linkage. However, the hydrogenation of the aromatic ring
of the depolymerization products especially the phenolic fragments are
usually inevitable to produce cyclohexanol or cycloalkanes, leading to
unsatisfactory aromatic selectivity. Thus, more selective heterogeneous
catalyst systems still need to be developed.
Iron has been featured as a very promising alternative to precious
metals in catalysis because it is abundant, eco- friendly, relatively
nontoxic, and inexpensive. Homogeneous iron catalysts such as ferric
acetylacetonate [10] and fenton catalysts [49,50] have been used for
⁎
Corresponding author.
E-mail addresses: lijiang@cup.edu.cn (J. Li), fuyao@ustc.edu.cn (Y. Fu).
https://doi.org/10.1016/j.mcat.2018.03.014
Received 13 January 2018; Received in revised form 18 March 2018; Accepted 19 March 2018
2468-8231/ © 2018 Elsevier B.V. All rights reserved.

J. Li et al.
Molecular Catalysis 452 (2018) 36–45
(
BDE) of the ether linkage, giving hydrogenolysis products in moderate
to excellent yields.
2. Experimental
2.1. List of chemicals
Benzyl phenyl ether (98%), guaiacol (98%), syringol (98%), ethy-
lenzene (99%), acetophenone (98.5%), 4′-methoxyacetophenone
99%), 1-ethyl-4-methoxybenzene (98%), 4-allylanisole (98%), 1-(4-
methoxyphenyl)propan-1-one (97%), 1-phenylethane-1,2-diol (98%),
-phenylethyl alcohol (98%), 1-phenylethanol (98%), styrene (99%), 2-
hydroxy-1-phenylethanone (98%), iron(II) acetate anhydrous (90%),
,10-phenanthroline (99%), 2,2′-bipyridine (99%), 2,2′:6′,2′’-
(
2
1
Terpyridine (98%), hemin (95%), and activated carbon were purchased
from TCI. Iron(III) acetylacetonate (98%) was purchased from J&K
Chemical. 8-hydroxyquinoline (99.5%), phenylglycine (97%), cobalt(II)
acetate tetrahydrate (99.5%), nickel(II) acetate tetrahydrate (98%),
toluene (99.5%), phenol (99%), benzyl alcohol (98%) were purchased
from Sinopharm Chemical Reagent Co. Ltd. Metal-oxide supports such
as TiO
from Aladdin Reagent Co. Ltd. Ru/C and Pd/C with 5% metal loading,
and PtCl ·6H (metal content > 38%) were purchased from
2 2 2 3
(99.99%), SiO (99.8%), and Al O (99.9%) were purchased
H
2
6
2
O
Shanghai Jiuling Chemical Co. Ltd. The β-O-4 lignin model compounds
were synthesized according to previous literature. The synthetic pro-
cedure and the 1H NMR spectra can be found in the Supporting
Information.
2.2. Catalyst preparation
The catalyst was prepared by a simultaneous pyrolysis of metal
precursors and nitrogen precursors on supports at a pyrolysis tem-
perature ranged from 400 to 1000 °C as previous literature [61–63]. A
typical procedure for catalyst preparation is described as follows: iron
precursor (0.5 mmol) and corresponding ligands were added to ethanol
Scheme 1. Selective hydrogenolysis of CeO bond of α-O-4 and β-O-4 lignin
model linkages over heterogeneous iron catalysts. The oxidation of the α-
carbon in the β-O-4 model compounds can generally lead to higher aromatics
yields.
(
50 mL) under vigorously stirring at room temperature. Then activated
carbon (1 g) was added into the solution, and the mixture was stirred at
0 °C for 15 h, followed by rotary evaporation at 30 °C. The obtained
solid was ground into fine powder and then pyrolyzed under an Ar
6
lignin depolymerization through reductive and oxidative cleavage, re-
spectively. To date, the only heterogeneous iron catalyst used for lignin-
derived CeO bond cleavage was bimetallic FeMoP catalyst, which was
reported by Hicks et al. [51]. However, it suffered from very high re-
action temperature (400 °C), and limited substrate scope was reported
in their work.
−
1
atmosphere with a gas flow rate of 100 mLmin in a tubular furnace.
The as-prepared catalysts were denoted as Fe-Ln/S-T, in which Ln re-
presents the type of nitrogen precursor, S represents the type of support,
and T represents the pyrolysis temperature. The catalyst prepared from
hemin was denoted as 6/C-800 (see Fig. S1 in the Supporting In-
formation for more details).
Nitrogen-doped carbon material-supported iron catalysts have re-
cently attracted much attention as ideal alternatives to noble metal
catalysts in some important chemical reactions such as the oxygen re-
duction reaction (ORR) or nitroarene hydrogenation [52–54]. Notably,
Beller et al. reported some pioneer work using heterogeneous iron
catalysts for organic synthesis [54–60]. Inspired by their work, we re-
cently successfully achieved some heterogeneous iron-catalyzed bio-
mass conversions such as catalytic transfer hydrogenation of furfural to
furfuryl alcohol [61] and selective hydrodeoxygenation of 5-hydro-
xymethylfurfural to 2,5-dimethylfuran [62]. In this work, selective
cleavage of CeO bond of α-O-4 and β-O-4 lignin model linkages while
preserving the aromatic nature was successfully achieved over hetero-
geneous iron catalysts (Scheme 1). The hydrogenolysis of α-O-4 model
linkage showed that the catalyst prepared by the pyrolysis of iron(II)
actate and 1,10-phenanthroline on activated carbon at 800 °C was the
most active heterogeneous iron catalyst, affording phenol and toluene
Pt/C catalyst with 5% metal loading were prepared by an impreg-
nation method using H PtCl ·6H O as precursor. 133.5 mg
2
6
2
H PtCl ·6H O was first dissolved in 50 mL ethanol and then 1 g acti-
2
6
2
vated carbon was added. The mixture was stirred at 50 °C for 4 h fol-
lowed by rotary evaporation at 30 °C. Obtained solid was ground into
fine powder, and then pyrolyzed under a flowing gas mixture of 5 vol%
−
1
2
H /Ar with a gas flow rate of 100 mL min
in a tubular furnace at
280 °C for 2 h.
2.3. Lignin extraction and oxidation
The procedure for the extraction of organosolv lignin from birch
sawdust was described as follows: 50 g birch sawdust was mixed with
1:1 ethanol-H O (300 mL), and then the mixture was transferred to the
2
autoclave. After purging the reactor with N , the reactor was heated to
2
−
1
with yields of 95% and 90%, respectively at 240 °C and 2 MPa H
2
. The
178 °C with a ramp rate of 1.5 °C min , and then held for 3.3 h. The
result mixture was filtered, and then the filtrate was transferred to ro-
tary evaporation to give organosolv lignin.
aromatics selectivity is found to be much higher than that obtained over
noble metal catalysts. For the β-O-4 linkage, we found that the vicinal
−
OH group was essential for the iron-catalyzed hydrogenolysis of CeO
The oxidation of organosolv lignin was performed as previous lit-
erature [64]. To a 50 mL autoclave, 35 mg of organosolv lignin and
1 mg of 4-acetamido-TEMPO were added. Subsequently, solutions of
bond. In addition, the oxidation of the α-carbon in the β-O-4 model
compounds could significantly decrease the bond dissociation energy
37

J. Li et al.
Molecular Catalysis 452 (2018) 36–45
1
.5 μL of nitric acid (67%) in 1 mL acetonitrile, 1 μL of hydrochloric
representative GC spectrum of gaseous products, the gaseous phase was
collected and injected into a Fuli 9790II Gas Chromatograph equipped
with a TDX-01 packed column and a thermal conductivity detector
(TCD) through a six-way valve to analyze the composition.
The CeO cleavage of the lignin model compounds generally pro-
duce two fragments. One is aromatic fragment such as ethylbenzene or
acetophenone. The other one is phenolic fragment. The carbon balance
of phenolic fragment is always higher than aromatic fragment.
In the case of the conversion of α-O-4 lignin model compounds, the
conversion and the yield were calculated as follows,
acid (37%) in 1 mL of acetonitrile, and 10 mL of acetonitrile and 100 μL
of water were added. The autoclave was purged with oxygen gas. The
oxygen pressure was adjusted to 1 MPa. The reaction mixture was
stirred at 65 °C for 48 h, and then solvent was evaporated by rotary
evaporation. The residue was washed by 1:1 water-THF solvent mix-
ture, and then transferred to the autoclave for subsequent iron-cata-
lyzed hydrogenolysis reaction.
2.4. Catalyst characterization
Conversion = (1-(moles of substrates after reaction)/(moles of
substrates in the starting materials))*100%
XPS was obtained with an X-ray photoelectron spectrometer
(
ESCALAB250, Thermo-VG Scientific, USA) using monochromatized
Yield = (mole of each fragment product after reaction)/(moles of
substrates in the starting materials)*100%
In the case of the conversion of β-O-4 lignin model compound, the
boiling point of the substrates are too high to be detected in the GC-
spectrum. Thus, the conversion of the substrates is not determined. The
yield was calculated as follows,
AlKa radiation (1486.92 eV). For the fitting of the XPS results, shirley
type background subtraction is applied and charge correction is per-
formed with reference to C 1s at 284.6 eV. e(N⋯Fe) at 399.6 eV, pyr-
rolic N at 400.8 eV, and a combination of quaternary and graphitic N at
4
02 eV, N-Ti species at 396.8 eV. The FWHM is fixed as 1.5 eV except N-
Ti species (about 2 eV). FT-IR spectra were recorded on a Nicolet 8700
FT-IR spectrometer, and the samples were prepared by the KBr pellet
method. XRD analysis was conducted on an X-ray diffractometer (TTR-
III, Rigaku Corp., Japan) using Cu Kα radiation (λ = 1.54056 Å). The
data were recorded over 2θ ranges of 10–70°. Nitrogen adsorption
measurements were performed using an ASAP2020 M adsorption ana-
lyzer which reports adsorption isotherm, specific surface area and pore
volume automatically. The Brunauer-Emmett-Teller (BET) equation
was used to calculate the surface area in the range of relative pressures
between 0.05 and 0.20. The pore size was calculated from the ad-
sorption branch of the isotherms using the thermodynamic based
Yield = (mole of each fragment product after reaction)/(moles of
substrates in the starting materials)*100%
2.6. DFT calculation method
Lignin model compounds and their corresponding radicals were
fully optimized with the 6–311 + +G(d,p) basis set using Gaussian 03
software. Single-point energy calculations were performed at the same
level of theory. The homolytic CeO or CeC bond-dissociation energies
(BDEs) of the model compounds were estimated from the expression:
⋅
⋅
C
orC
x
−
O
y
→ C + O
x
y
2
Barrett-Joyner-Halenda (BJH) method. H -TPR was conducted on a
⋅
⋅
−
C
→ C + C
x
y
x
y
Quanta Chembet chemisorption instrument with a thermal conductivity
detector (TCD). Approximately 100 mg sample was loaded in a quartz
reactor and then heated to 800 °C with a heating ramp rate of
The CeO or CeC BDEs were calculated as follows:
−
1
1
5
0 °C min
in a stream of 5% H
2
/Ar with a total flow rate of
·
·
·
BDE = H(C ) + H(O orC ) − H(C − O orC − C )
y
y
y
x
y
x
y
−
1
0 mL min . More details about catalyst characterization results can
be found in Supporting Information.
where H(i)s are the single-point energies of different species i at 298 K.
The BDEs represent the bond strength of the CeC or CeO linkages,
so that they are compared to explain the different reactivities of the
model compounds [28].
2.5. Experimental procedure
The catalytic hydrogenolysis of lignin model compounds was per-
formed using a 50 mL Zr-alloy autoclave provided by Anhui Kemi
Machinery Technology Co., Ltd. For a typical procedure, lignin model
compounds (0.5 mmol or 1 mmol) or organosolv lignin (35 mg), het-
erogeneous Fe catalyst (100 mg), and solvent (20 mL) were added into
3. Results and discussion
3.1. Catalyst screening for reductive cleavage of CeO bond in α-O-4 lignin
model compounds
the autoclave with a quartz lining. After purging the reactor with H
the reaction was conducted with 1 MPa H (at room temperature) at
40 °C for 12 h with a stirring speed of 800 rpm. After reaction, internal
2
,
2
The hydrogenolysis of α-O-4 lignin model compound, benzyl phenyl
ether, in a water-tetrahydrofuran (THF) solvent mixture was selected as
a model reaction to screen the best heterogeneous iron catalyst
(Table 1). A glass vial was used to exclude the influence of the reactor
body. In the blank test, phenol and benzyl alcohol were generated via
hydrolysis pathway [29]. The protons existed in the hot water-THF
system could be the acid catalyst for the hydrolysis reaction. Despite the
2
standards are added to the product solution, and then the liquid pro-
ducts were analyzed by using both GC and GC–MS. For the conversion
of α-O-4 lignin model compound, 2-phenylethanol is used as internal
standard to determine the yields of benzyl alcohol and phenol, and
dodecane is used to determine the yield of toluene. For the conversion
of β-O-4 lignin model compounds, benzyl alcohol and dodecane are
used as internal standards to determine the yields of phenolic fragments
and aromatic fragments, respectively. A representative GC spectrum
can be seen in Fig. S11 in supporting information. GC–MS analyses were
performed on an Agilent 7890 Gas Chromatograph equipped with a DB-
WAXETR 30 m × 0.25 mm × 0.25 mm capillary column (Agilent) or a
HP-5MS 30 m × 0.25 mm × 0.25 mm capillary column (Agilent).
Although HP-5MS column is unsuitable for the determination of pro-
ducts yields due to the low polarity, it can be used to confirm whether
some complex lignin model compounds were completely converted.
The GC was directly interfaced to an Agilent 5977 mass selective de-
tector (EI, 70 eV). The following GC oven temperature programs were
2
reaction was performed under H , no toluene was detected, indicating
that the hydrogenolysis reaction was unlikely to occur without any
catalyst. To our delight, the iron catalyst prepared from Fe-phenan-
throline (L1) complex exhibited excellent catalytic performance to-
wards the hydrogenolysis reaction. The yields of phenol and toluene
reached 95% and 90%, respectively, and the yield of hydrolysis pro-
2
duct, benzyl alcohol, was only 5%. Only trace yield of CO could be
detected in the GC spectrum of gaseous phase (Fig. S12), indicating that
very small amount of carbon is converted into gas phase. Under iden-
tical reaction condition, the hydrogenolysis of benzyl alcohol (11) af-
forded 25% yield of toluene and 10% yield of benzaldehyde at 40%
conversion (see entry 1, Table 4), suggesting that toluene was more
likely to be generated by direct hydrogenolysis of benzyl phenyl ether
rather than the hydrolysis of benzyl phenyl ether to benzyl alcohol
−1
used: 40 °C hold for 1 min, ramp 5 °C min to a temperature of 120 °C,
−1
and then ramp 10 °C min
to 300 °C and hold for 5 min. To get the
38

J. Li et al.
Molecular Catalysis 452 (2018) 36–45
Table 1
a
Catalyst screening for CeO bond cleavage of α-O-4 lignin model compound .
Entry Catalysts
T °C P MPa Conversion [%] Yield [%]b
a
b
c
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
–
240
240
220
200
240
240
240
240
240
240
2
2
2
2
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
70
99
84
34
98
75
96
66
69
99
95
86
89
42
17
68
72
90
71
83
10
9
61
95
65
30
90
71
93
59
58
94
88
72
85
28
15
55
59
80
67
73
10
8
0
53
5
3
Fe-L1/C-800
Fe-L1/C-800
Fe-L1/C-800
Fe-L1/C-800
Fe-L2/C-800
Fe-L3/C-800
Fe-L4/C-800
Fe-L5/C-800
6/C-800
90
68
21
57
44
73
38
28
89
84
64
53
12
6
40
13
9
19
3
3
3
5
60
60
1
23
20
14
15
19
5
5
6
25
9
11
9
42
68
31
65
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
Fe(III)-L1/C−800c 240
Fe-L1/C-1000
Fe-L1/C-600
Fe-L1/C-400
Fe-L1/TiO
Fe-L1/SiO
Fe-L1/Al
L1/C-800
Fe/C-800
Fe
L1
240
240
240
240
240
240
240
240
240
240
240
240
240
240
240
240
240
-800
-800
Fig. 1. N 1 s XPS spectra of Fe-L1/C-800, Fe-L2/C-800, Fe-L5/C-800, and Fe-
L1/TiO -800 catalysts.
2
2
2
2
O
3
-800
XPS spectra can be divided into four species: pyridinic N at 398.6 eV, N
coordinated to Fe (N⋯Fe) at 399.6 eV, pyrrolic N at 400.8 eV, and a
combination of quaternary and graphitic N at 402 eV [67,68]. For the
Fe-L1
C
Co-L1/C-800
Ni-L1/C-800
4
2
Fe-L1/TiO -800 catalyst, N-Ti species can be detected at 396.8 eV [69].
100
92
86
97
98
77
84
87
81
74
18
4
The results for N speciation of various iron catalysts are listed in table
S1 (see supporting information). For iron catalysts prepared from dif-
ferent ligands, it can be clearly seen that a certain amount of N⋯Fe
species can be found in relatively more active iron catalysts such as Fe-
L1/C-800, Fe-L3/C-800, and 6/C-800 catalysts, and it reached the
highest value for Fe-L1/C-800 catalyst. By contrast, extremely low
percentage content (2.6% of total N contents) of N⋯Fe species were
found in Fe-L5/C-800 catalyst, and no N⋯Fe species were found in the
XPS spectra of Fe-L2/C-800 and Fe-L4/C-800 catalysts. These results
confirm that the presence of N⋯Fe species is important for the HDO
activities of iron catalysts. In addition, for the Fe-L5/C-800 catalyst,
FeOx species, which are adverse for the catalytic performance of iron
catalysts [61], can be detected as shown in its XRD pattern (Fig. 2).
d
e
f
g
n.d.h n.d. n.d.
Ru/C
g
Pd/C
n.d.
53
n.d. n.d.
71 n.d.
Pt/Cg
a
Reaction conditions: 1 mmol benzyl phenyl ether, 1:1 water-THF (20 mL),
0
.1 g iron catalyst, t = 12 h. The molar ratio of metal/benzyl phenyl ether was
kept at 5.6 mol%.
b
GC yield.
c
Iron(III) acetylacetonate was used as iron precursor.
Ring-hydrogenation products are detected as major by-products: cyclo-
d
hexanol (13%), cyclohexanone (20%), cyclohexane (12%), methylcyclohexane
(
34%), cyclohexylmethanol (31%).
e
Major by-products: cyclohexanol (82%), cyclohexanone (7%), cyclohexane
3
Extremely weak peaks of FeNx, Fe C, and metallic Fe can be found in
(
3%), methylcyclohexane (63%).
the XRD pattern of Fe-L1/C-800 catalyst, indicating that all the iron
species are homogeneously dispersed on the support. The excellent
catalytic performance of Fe-L1/C-800 catalyst suggested that this well-
dispersion of iron species onto carbon support is favorable for the HDO
reaction.
f
2% yield of cyclohexanol, 4% yield of methylcyclohexane, and 8% yield of
cyclohexane are obtained as major-products.
g
The metal loading was 5 wt%.
n.d.= not detected.
h
The iron precursor iron(II) acetate could be replaced by iron(III)
acetylacetonate without significant loss of catalytic activity (entry 11).
0.88 at% of N⋯Fe species can be observed in its XPS spectrum. The
effect of pyrolysis temperature indicated that the optimal pyrolysis
temperature was 800 °C (entries 12–14). At pyrolysis temperature lower
than 800 °C, the iron precursors are not incorporated into carbon sup-
port to form active sites [54]. However, when the pyrolysis temperature
was raised to 1000 °C, the total N content decreased sharply to 0.34 at%
(2.65 at% at 800 °C), suggesting that the N⋯Fe species were collapsed
to release nitrogen. In addition, inactive Fe O phase is found in its XRD
followed by deoxygenation. Moreover, we did not observe any ring-
hydrogenated products such as cyclohexanol or methyl cyclohexane,
suggesting that the aromatic rings were well reserved during the iron-
catalyzed CeO bond cleavage. This chemoselectivity was highly ad-
vantageous for the concept of catalytic valorization of lignin to aro-
matic chemicals.
Significant decreases in ether conversion and hydrogenolysis pro-
ducts yields were observed when experiments were performed at lower
reaction temperatures and hydrogen pressures (entries 2–5). The iron
catalyst prepared from the native iron complex (6/C-800) afforded si-
milar yields of hydrogenolysis products, but catalysts prepared from
other nitrogen precursors exhibited much more inferior hydrogenolysis
activities (entries 6–10).
To explore the reason for the outstanding catalytic performance of
Fe-L1/C-800 catalyst, the iron catalysts have been characterized by
2
various technologies such as XPS, XRD, BET, and H -TPR. As this kind
of iron catalysts has represented excellent activity for various chemical
conversions, the presence of an Fe-Nx type structure as the active center
is generally accepted [65,66], which can be realized by the XPS ana-
lysis. The N 1s XPS spectra of Fe-L1/C-800 catalyst, and some other
inferior iron catalysts such as Fe-L2/C-800, Fe-L5/C-800, and Fe-L1/
2
3
pattern (see Fig. S5 in the supporting information).
Iron catalysts prepared on metal oxides exhibited inferior catalytic
performance than that on activated carbon (entries 15–17). Carbon is
usually the best support to prepare highly active Fe-N catalysts. The
reasons include: 1) catalysts prepared from carbon usually have the
highest BET surface area (Table 2), thus leading to the exposure of more
active sites on the catalyst surface to catalyze reactions; 2) the doping
nitrogen donates more electrons to the carbon support to facilitate
redox reactions [70]; 3) N-doped carbon materials are recommended as
“solid ligands” to bind metal nanoparticles to form structures mimicked
homogeneous metal complexes with N ligands [71].
The poor catalytic performances of L1/C-800, Fe/C-800, iron(II)
2
TiO -800 catalysts are shown in Fig. 1. In general, the peaks in the N1 s
39

J. Li et al.
Molecular Catalysis 452 (2018) 36–45
Fig. 2. The XRD patterns of various iron catalysts prepared from different nitrogen precursors.
Table 2
Textural properties of iron catalysts prepared from different supports.
2
3
Entry
Catalysts
S
BET [m /g]
V
p
[cm /g]
Pore Size [nm]
1
2
3
4
Fe-L1/C-800
830
52
245
43
0.89
0.05
0.70
0.08
4.27
3.62
11.40
6.97
Fe-L1/TiO
Fe-L1/SiO
Fe-L1/Al
2
-800
-800
2
2
O
3
-800
acetate (Fe), 1,10-phenanthroline (L1), iron complex (Fe-L1), and
carbon support confirmed that the simultaneous pyrolysis of iron pre-
cursor and nitrogen precursor on a carbon support was critical to the
hydrogenolysis activity of the iron catalysts. The replacement of the
metal center Fe with Co or Ni would lead to inferior catalytic activity.
The catalytic performance of iron catalysts are also compared with
noble metal catalysts (entries 26–28). Ring-hydrogenation products are
detected as major by-products when Ru/C and Pd/C catalysts are used.
Pt/C catalyst afforded 53% yield of phenol and 71% yield of toluene,
which is lower than that of Fe-L1/C-800 catalyst. In addition, the sol-
vent THF will be converted to ring-opening products such as butanol or
butanediol over noble metal catalysts. By contrast, the cyclic ether
solvent THF remained stable over iron catalysts. Thus, the iron catalyst
represented better catalytic performance towards the hydrogenolysis of
α-O-4 lignin model compound than noble metal catalysts.
Fig. 3. The time course for the hydrogenolysis of benzyl phenyl ether over Fe-
L1/C-800 catalyst. Reaction condition: 1 mmol benzyl phenyl ether, 1:1 water-
THF (20 mL), 0.1 g Fe-L1/C-800 catalyst, 2 MPa H , T = 240 °C.
2
To explore the reaction pathway for the conversion of α-O-4 lignin
model compound, the time course for the hydrogenolysis of benzyl
phenyl ether over Fe-L1/C-800 catalyst was investigated (Fig. 3). It was
shown that the substrate benzyl phenyl ether was completely consumed
after 12 h, and highly selective hydrogenolysis of the α-O-4 linkage was
achieved to afford phenol and toluene as main products. The yield of
benzyl alcohol was below 5% during the reaction. Owing to the large
excess of hydrogen, the hydrogenolysis of benzyl phenyl ether was a
pseudo-first order reaction with a reaction rate constant of 0.30825 at
Scheme 2. Different reaction pathways for the conversion of α-O-4 lignin
model compound with or without iron catalysts in a H O-THF solvent mixture.
2
5
13 K (Fig. S13).
3.2. Reductive cleavage of CeO bond in β-O-4 lignin model compounds
To sum up, the reaction pathway for the conversion of α-O-4 lignin
model compound is shown in Scheme 2. When the reaction is per-
formed without iron catalysts, the α-O-4 lignin model compound will
be hydrolyzed to phenol and benzyl alcohol catalyzed by the protons
existed in the hot water-THF system. In contrast, the use of Fe-L1/C-800
catalyst led to the generation of phenol and toluene by direct hydro-
genolysis of ether linkage. Toluene is unlikely to be generated by the
hydrolysis of α-O-4 lignin model compound followed by hydrogenolysis
because Fe-L1/C-800 catalyst is ineffective at catalyzing the conversion
of benzyl alcohol to toluene.
The excellent catalytic performance of Fe-L1/C-800 catalyst in the
hydrogenolysis of α-O-4 model linkage inspired us to further explore
the heterogeneous iron-catalyzed hydrogenolysis of β-O-4 linkage
(Table 3), which is the most predominant type of ether linkages in
native lignin. The simplest model compound of β-O-4 linkage, 2-phe-
noxy-1-phenylethanol (compound 1), could be cleaved to phenol,
ethylbenzene, and acetophenone with yields of 43%, 40%, and 3%,
respectively. The unsatisfactory yields of hydrogenolysis products could
be patially attributed to the generation of phenethoxybenzene (com-
pound 12, see Table 4) as the by-product with 20% yield, for which the
40

J. Li et al.
Molecular Catalysis 452 (2018) 36–45
Table 3
a
The hydrogenolysis of β-O-4 lignin model compounds over Fe-L1/C-800 catalyst .
a
Reaction conditions: 0.5 mmol β-O-4 lignin model compounds, 1:1 water-THF (20 mL), 0.1 g Fe-L1/C-800 catalyst, 1 MPa H2, T = 240 °C, t = 12 h. N. D. = not
determined.
b
Determined by GC.
c
Phenethoxybenzene could be observed as the by-product with 20% yield.
cleavage of ether linkage was unlikely to occur even at a reaction
temperature of 290 °C (Table 4). However, the BDE of ether linkage in
group was essential for the iron-catalyzed cleavage of CeO bond, which
can be further confirmed by the different reactivity of 1-phenylethane-
1,2-diol (compound 13, see Table 4) and 2-phenylethanol (compound
14, see Table 4). Total conversion of 13 could be achieved over Fe-L1/
C-800 catalyst, giving 29% yield of 14, 13% yield of benzaldehyde, and
12 (261 kJ/mol) was slightly lower than that in 1 (266 kJ/mol), sug-
gesting that the cleavage of ether linkage in 12 should be theoretically
easier to occur. This contradiction indicated that the vicinal −OH
41

J. Li et al.
Molecular Catalysis 452 (2018) 36–45
Table 4
a
Heterogeneous iron-catalyzed cleavage of various molecules to confirm the reaction pathway .
a
Reaction conditions: 0.5 mmol substrate, 1:1 water-THF (20 mL), 0.1 g Fe-L1/C-800 catalyst, 1 MPa H2, T = 240 °C, t = 12 h. N.D. = Not determined.
Determined by GC.
b
5
0% yield of ethylbenzene. In contrast, only 2% yield of ethylbenzene
was obtained from 14 with only 3% conversion. In addition, the CeC
bond became weaker in the pinacol structure, so that the cleavage of
CeC bond could be observed during the hydrogenolysis of 13.
further confirming that the dehydrogenation reaction definitely oc-
curred over heterogeneous iron catalysts under H . The high selectivity
2
of ethylbenzene from 15 also explained why the major product from the
hydrogenolysis of 1 is ethylbenzene. Owing to the high BDE for CeO
bond in 15 (335 kJ/mol), we proposed that the reaction pathway for
the conversion of 15 to ethylbenzene was the dehydration of 15 to
styrene (compound 16, see Table 4) followed by hydrogenation. This
hypothesis is confirmed by the time course for the hydrogenolysis of 15
(Fig. S14), where 19.4% yield of styrene can be detected at 2 h and then
be completely converted after 12 h.
The oxidation of the α-carbon in β-O-4 linkage has been previously
proved to significantly weaken the ether bond, thus leading to more
effective lignin degradation [12,64,72,73]. Hence, we further explored
the iron-catalyzed hydrogenolysis of oxidative β-O-4 model com-
pounds. The BDE of ether linkage decreased from 266 to 208 kJ/mol
after the oxidation of the α-carbon in 1, and the oxidative product
(compound 2) could be nearly quantitatively cleaved to yield aromatic
products. The yield of ethylbenzene was only 5%, suggesting that
The cleavage of ether linkage in 1 was more favorable at a higher
reaction temperature of 290 °C, giving 88% yield of phenol, 59% yield
of ethylbenzene, and 25% yield of acetophenone. Acetophenone was
probably formed by the dehydrogenation of 1 to compound 2 followed
by CeO bond cleavage rather than CeO bond cleavage followed by
dehydrogenation of 1-phenylethanol (compound 15, see Table 4) be-
cause 15 would be converted to ethylbenzene with 90% selectivity
under identical reaction condition. Although it is hard to imagine that
the dehydrogenation reaction can happen in a hydrogen atmosphere,
2
the iron-catalyzed dehydrogenation reaction under H has been proven
in our latest work, where considerable yield of 2,5-diformylfuran was
obtained from 5-hydroxymethylfurfural over Fe-L1/C-800 catalyst in
THF [62]. In addition, 10% yield of benzaldehyde can be obtained from
benzyl alcohol under identical reaction condition (entry 1, Table 4),
42

J. Li et al.
Molecular Catalysis 452 (2018) 36–45
acetophenone was more stable than 15 under identical reaction con-
dition. In addition, the calculation of BDE of CeC and CeO linkages in
1
, 2, 13 and 2-hydroxy-1-phenylethanone (compound 17, see Table 4)
suggested that the oxidation of the α-carbon led to weaker CeO linkage
and stronger CeC bond. Thus, the cleavage of CeO bond was more
preferable than the cleavage of CeC bond in the oxidative β-O-4 model
compounds, leading to higher carbon balance.
The iron-catalyzed CeO bond cleavage of some more complex β-O-4
model compounds was also explored. It was shown that the BDE of
ether linkage in guaiacol-based β-O-4 model compound (compound 3)
was 30 kJ/mol lower than that in phenol-based β-O-4 model compound
(
compound 1), so that 3 could be cleaved to aromatic compounds with
higher yields. In addition, for these more complex β-O-4 model com-
pounds, higher reaction temperature will lead to lower carbon balance.
The oxidative product of 3 (compound 4) can be cleaved to aromatic
compounds with excellent yields.
The iron-catalyzed CeO bond cleavage in the syringol-based β-O-4
model compounds (5) was less effective than 3, despite that the BDE of
the ether linkage was similar. In addition, unexpected low yields of
hydrogenolysis products were obtained for the conversion of the oxi-
dative product of 5 (compound 6), despite the BDE of the ether linkage
was only 171 kJ/mol. The steric hinderance caused by two o-methoxy
groups was probably the main reason for the ineffective hydrogenolysis
of the ether linkage. The presence of p-methoxy group in guaiacol-based
β-O-4 model compound (7) led to slightly higher BDE of CeO bond (as
compared to 3), thus affording lower hydrogenolysis products. In
comparison, the oxidative product of 7 (compound 8) could be cleaved
to aromatic compounds with excellent yields.
The CeO bond cleavage of the β-O-4 lignin linkage of 1,3-dilignol
model compounds was also examined. At a reaction temperature of
240 °C, 9 was cleaved to guaiacol, 1-ethyl-4-methoxybenzene, and 1-
methoxy-4-propylbenzene with yields of 84%, 5%, and 59%, respec-
tively. When the reaction temperature was raised to 290 °C, the yields
of aromatic fragments decreased significantly while the yields of phe-
nolic fragments remained nearly unchanged. The yields of aromatic
fragments were always lower than those of phenolic fragments during
the depolymerization of lignin-derived compounds, which was likely
due to condensation reactions [74]. The cleavage of CeC bond in the
pinacol structure could be observed in the hydrogenolysis of the oxi-
dative product of 9 (compound 10), giving 40% yield of 1-(4-methox-
yphenyl)ethanone and 13% yield of 1-(4-methoxyphenyl)propan-1-one.
The reaction pathways for the conversion of native and oxidative β-
O-4 lignin dimers 1 and 2 over Fe-L1/C-800 catalyst were further ex-
plored. The blank tests demonstrated that the iron catalyst played an
important role during the hydrogenolysis of β-O-4 lignin dimers (en-
tries 9 and 10, Table 4). In addition, the time course for the hydro-
genolysis of 1 and 2 over Fe-L1/C-800 catalyst was shown in Fig. 4. For
the conversion of native lignin dimer 1, the main pathway is the hy-
drogenolysis of 1 to form phenol and 1-phenylethanol, which was
further converted to ethylbenzene through deoxygenation. 1-pheny-
lethanol was not observed during the reaction, which was probably due
to the rapid hydrogenolysis of 1-phenylethanol to ethylbenzene (Fig.
S14). Two other pathways can also be observed: (a) the deoxygenation
of 1 to phenethoxybenzene with yield of ca. 20%, which was unlikely to
be further depolymerized to monomers over iron catalysts; (b) the de-
hydrogenation of 1 to 2 followed by hydrogenolysis to form phenol and
acetophenone with yield of < 10%. The direct hydrogenolysis of ether
linkage is more preferable in the depolymerization of more complex
native lignin dimers such as 3, 5, 7 and 9 due to the lower BDE of ether
likages, and thus led to higher yields of monomers. The reaction
pathway for the conversion of oxidative lignin dimer such as 2 is much
simpler. The dimers can be depolymerized to monomers with excellent
yields through direct hydrogenolysis. The as-mentioned reaction path-
ways are shown in Scheme 3.
Fig. 4. The time course for the hydrogenolysis of 1 and 2 over Fe-L1/C-800
catalyst. Reaction condition: 0.5 mmol substrate, 1:1 water-THF (20 mL), 0.1 g
Fe-L1/C-800 catalyst, 1 MPa H
2
, T = 240 °C. 2 was completely consumed after
6
h, but incomplete conversion of 1 was observed even after 12 h.
section, and the details for its extraction can be found in experimental
section. In addition, the oxidation of as-prepared lignin was also carried
out according to previous literature [64]. To our disappointment, no
monomers were detected after the reaction of two types of lignin with
Fe-L1/C-800 catalyst. The GPC analysis (Fig. S15) showed that the
native lignin treated by Fe-L1/C-800 catalyst tends to high molecular
scope, which probably due to the polymerization of native lignin
[73,74]. We proposed that the relatively low catalytic activity of Fe and
the steric effect observed in the iron-catalyzed hydrogenolysis of β-O-4
model linkage was the main reason for the poor reactivity during lignin
conversion, which led to difficult contact of the heterogeneous iron
catalyst with the ether linkage in the lignin. Nonetheless, we believe
that the unique chemoselectivity of the iron catalyst will hold great
potential for the conversion of lignin to aromatics. The preparation of
some iron-based alloy catalsyts such as PtFe or PdFe catalysts maybe
could improve the catalytic performance towards the conversion of
native lignin while preserving the chemoselectivity of iron. The re-
levant investigations are currently ongoing in our laboratory.
4. Conclusion
Finally, we also initiated some preliminary studies about iron-cat-
alyzed conversion of lignin. Organsolv lignin was first used in this
In summary, we report that the selective cleavage of CeO bond in
lignin model compounds without hydrogenation of aromatic ring can
43

J. Li et al.
Molecular Catalysis 452 (2018) 36–45
Scheme 3. Reaction pathways for the conversion of native and oxidative β-O-4 lignin dimers 1 and 2 over Fe-L1/C-800 catalyst.
be successfully achieved through heterogeneous iron catalysis. The
hydrogenolysis of α-O-4 model linkage shows that the iron catalyst
prepared by the simultaneous pyrolysis of iron acetate and 1,10-phe-
nanthroline on activated carbon at 800 °C is the most active iron cat-
alyst, affording phenol and toluene with yields of 95% and 90%, re-
spectively. For the β-O-4 linkage, the vicinal −OH group was essential
for the iron-catalyzed hydrogenolysis of ether linkage. Moreover, the
oxidation of the α-carbon in the β-O-4 model compounds can sig-
nificantly decrease the bond dissociation energy (BDE) of ether linkage,
leading to improved hydrogenolysis reactivity, thus giving depoly-
merization products in moderate to excellent yields.
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Acknowledgements
This work was supported by Science Foundation of China University
of Petroleum, Beijing (No. 2462014YJRC037, C201604), the National
Natural Science Foundation of China (21702227) and the National Key
Research and Development Program of China (No. 2016YFB0100201).
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