Decomposition of a Β-O-4 lignin model compound over solid Cs-substituted polyoxometalates in anhydrous ethanol: acidity or redox property dependence?

Article abstract of DOI:10.1016/S1872-2067(17)62854-7

Production of aromatics from lignin has attracted much attention. Because of the coexistence of C–O and C–C bonds and their complex combinations in the lignin macromolecular network, a plausible roadmap for developing a lignin catalytic decomposition process could be developed by exploring the transformation mechanisms of various model compounds. Herein, decomposition of a lignin model compound, 2-phenoxyacetophenone (2-PAP), was investigated over several cesium-exchanged polyoxometalate (Cs-POM) catalysts. Decomposition of 2-PAP can follow two different mechanisms: an active hydrogen transfer mechanism or an oxonium cation mechanism. The mechanism for most reactions depends on the competition between the acidity and redox properties of the catalysts. The catalysts of POMs perform the following functions: promoting active hydrogen liberated from ethanol and causing formation of and then temporarily stabilizing oxonium cations from 2-PAP. The use of Cs-PMo, which with strong redox ability, enhances hydrogen liberation and promotes liberated hydrogen transfer to the reaction intermediates. As a consequence, complete conversion of 2-PAP (>99%) with excellent selectivities to the desired products (98.6% for phenol and 91.1% for acetophenone) can be achieved.

Full text of DOI:10.1016/S1872-2067(17)62854-7

Chinese Journal of Catalysis 38 (2017) 1216–1228
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Decomposition of a β‐O‐4 lignin model compound over solid
Cs‐substituted polyoxometalates in anhydrous ethanol:
acidity or redox property dependence?
Xuezhong Wu a,b, Wenqian Jiao a, Bing‐Zheng Li b, Yanming Li b, Yahong Zhang a, Quanrui Wang c,
Yi Tang a,*
a Department of Chemistry, Laboratory of Advanced Materials, Collaborative Innovation Center of Chemistry for Energy Materials and Shanghai Key
Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, China
b State Key Laboratory of Non‐Food Biomass Enzyme Technology, National Engineering Research Center for Non‐Food Biorefinery, Guangxi Key
Laboratory of Biorefinery, Guangxi Academy of Sciences, Nanning 530007, Guangxi, China
c Department of Chemistry, Fudan University, Shanghai 200433, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Production of aromatics from lignin has attracted much attention. Because of the coexistence of C–O
and C–C bonds and their complex combinations in the lignin macromolecular network, a plausible
roadmap for developing a lignin catalytic decomposition process could be developed by exploring
the transformation mechanisms of various model compounds. Herein, decomposition of a lignin
model compound, 2‐phenoxyacetophenone (2‐PAP), was investigated over several ce‐
sium‐exchanged polyoxometalate (Cs‐POM) catalysts. Decomposition of 2‐PAP can follow two dif‐
ferent mechanisms: an active hydrogen transfer mechanism or an oxonium cation mechanism. The
mechanism for most reactions depends on the competition between the acidity and redox proper‐
ties of the catalysts. The catalysts of POMs perform the following functions: promoting active hy‐
drogen liberated from ethanol and causing formation of and then temporarily stabilizing oxonium
cations from 2‐PAP. The use of Cs‐PMo, which with strong redox ability, enhances hydrogen libera‐
tion and promotes liberated hydrogen transfer to the reaction intermediates. As a consequence,
complete conversion of 2‐PAP (>99%) with excellent selectivities to the desired products (98.6%
for phenol and 91.1% for acetophenone) can be achieved.
Received 26 March 2017
Accepted 30 April 2017
Published 5 July 2017
Keywords:
Lignin model compound
β‐O‐4 ether bond
Polyoxometalate
Hydrogen transfer mechanism
Oxonium cation mechanism
Anhydrous ethanol
© 2017, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
[3–5]. Lignocellulose is an inedible, almost inexhaustible [6],
and consistently renewable bioresource that can be used to
With depletion of fossil fuels and exacerbation of haze
weather and globe warming, sustainable development has been
highlighted by many governments [1,2]. Production of fuels and
chemicals from bioresources is reasonable and practicable
produce energy and chemicals. As the second principal com‐
ponent of lignocellulose, lignin can be used as a precursor for
fuels and aromatic chemicals because it is the only naturally
occurring aromatic polymer [7]. However, the potential of lig‐
* Corresponding author. Tel: +86‐21‐55664125; Fax: +86‐21‐65641740; E‐mail: yitang@fudan.edu.cn
This work was supported by the National Key Basic Research Program of China (973 program, 2013CB934101), National Natural Science Foundation
of China (21433002, 21573046), China Postdoctoral Science Foundation (2016M601492), and International Science and Technology Cooperation
Projects of Guangxi (15104001‐5).
DOI: 10.1016/S1872‐2067(17)62854‐7 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 38, No. 7, July 2017

Xuezhong Wu et al. / Chinese Journal of Catalysis 38 (2017) 1216–1228
1217
nin has not been fully realized. Most lignin is directly burned in
alkali recovery systems. Only a small amount of lignin is sepa‐
rated and used to produce dispersants or adhesives, and an
even smaller amount is used to produce value‐added chemicals
[8–10]. With the development of catalytic technologies, prepa‐
ration of bulk or fine aromatics from lignin is essential and
perhaps the most promising use of lignin, and this ambitious
objective has attracted tremendous research interest [11–13].
Because of its stable three‐dimensional network structure
and the robust complicated chemical associations among the
monolignans, it is still a great challenge to selectively convert
lignin to value‐added chemicals [11,14]. Some achievements
have been made in recent years [15–24], and many scientists
have studied hydrogenolysis of C–O linkages in lignin with var‐
ious hydrogen sources over metal catalysts [17,25–28]. Song et
al. [15] used a Ni/C catalyst to depolymerize native lignin in
methanol dispersant. They achieved high conversion and selec‐
tivity, which opens up a new pathway for lignin transformation
and has methodological significance. Ma et al. [16] completely
ethanolyzed Kraft lignin over an α‐MoC1−x/AC catalyst in pure
ethanol at 280 °C to give low molecular mass chemicals. Kon‐
nerth et al. [29] used bimetallic Ni7Au3 nanoparticles to hydro‐
genolyze a lignin model compound and organosolv lignin in
alkaline surroundings. Various acids have also been used as
catalysts to produce phenols from alkaline lignin [20,30].
To take full advantage of lignin and prepare valuable mul‐
ti‐substituted benzenes or their derivatives with reasonable
selectivity, a rational catalytic process to selectively cleave the
linkages between the monolignans needs to be developed.
Nevertheless, owing to the coexistence of complex C–O and C–C
bonds and the complicated combinations in the macromolecu‐
lar structure, it is very challenging to directly investigate the
mechanism of the decomposition process of technical lignin. It
is more reasonable to first investigate the cleavage mechanisms
of various lignin model compounds with specific linkages, and
then consider extension from micro‐ to macromolecule sys‐
tems [15,31] to finally achieve the objective of rational design
of catalytic materials and development of the catalytic process
for decomposition of technical lignin to valuable aromatics.
Among the linkages of monolignans, the β‐O‐4 linkage accounts
for the majority of the linkages in natural lignin and even in
some technical lignins, such as lignin obtained from bioethanol
refinery where β‐O‐4 linkages are still largely preserved
[33,34]. 2‐Phenoxyacetophenone (2‐PAP) was selected as the
model for the β‐O‐4 linkage because this linkage represents
some partially oxidized lignins [11,35,36] and the amount of
partially oxidized lignin has increased as C–C bond cleavage
technology roadmaps have recently emerged [37–40].
tungstic acid (Cs‐PW) catalyst to cleave an α‐O‐4 model com‐
pound to aromatics. Although some great achievements have
been made in hydrogenolysis and acidolysis cleavage of the
β‐O‐4 ether bond in lignin or its model compounds, the effect of
cooperation or competition between the acidity and redox
properties of the catalyst on the cleavage and its mechanism
are not clear.
Herein, several Cs‐substituted POMs were selected to sys‐
tematically investigate the effects of the acidity and redox
properties of the acid catalyst on the cleavage of the lignin
model compound 2‐PAP. A comprehensive hydrogen transfer
and acid‐catalyzed mechanism are proposed based on the cat‐
alytic performance over these catalysts. These results will be
useful for understanding the reaction phenomena and design‐
ing a practicable process for decomposition of technical lignin.
2. Experimental
2.1. Materials
The model compound 2‐PAP was purchased from TCI
Chemicals (Shanghai, China). Anhydrous ethanol (AE), phenol,
acetophenone,
H3PMo12O40·xH2O,
H3PW12O40·xH2O,
H4SiW12O40·xH2O, and cesium nitrate (CsNO3) (all of analytical
purity) were purchased from Sinopharm Chemical Reagent Co.,
Ltd. (Shanghai, China). They were used as received without
further purification.
2.2. Preparation of the solid POMs
Cs‐substituted POMs were prepared by the ion exchange
method described by Park et al. [44]. The phase structures of
the POM salts were determined by powder X‐ray diffraction
(XRD), Fourier transform infrared (FT‐IR) spectroscopy, and
Raman spectroscopy (Figs. 1–3). These characterizations of the
Cs‐POMs verified that crystals were successfully prepared with
the phase of the Keggin structure [10,45–52]. All of products
were kept in a desiccator before use.
(220)
Cs-PW
Cs-PMo
Cs-SiW
Polyoxometalates (POMs) are widely applied as acid cata‐
lysts [41]. Recently, several POMs have been used for homoge‐
neous or heterogeneous catalytic conversion of lignin or its
model compounds [30,42]. Soluble lignin and model com‐
pounds have also been oxidized to benzoquinones over several
soluble POM catalysts with H2O2 as the oxidant [42]. Li and
co‐workers [30] used soluble and supported H4PW12O40 cata‐
lysts to depolymerize Kraft lignin to phenols with a high yield
of 67 mg/g. Park et al. [43] used a cesium‐exchanged phospho‐
Cs1.7H1.3PW12O40 -PDF#51-0417
10
20
30
2 (°)
40
50
Fig. 1. XRD patterns of the Cs‐POMs.

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Xuezhong Wu et al. / Chinese Journal of Catalysis 38 (2017) 1216–1228
speed was 10 °C/min. Before the experiments, the catalysts
were activated in helium at 250 °C for 2 h. The H2‐TPR profiles
were obtained with an Autochem II chemisorption instrument
(Micromeritics, USA) equipped with a thermal conductivity
detector and a cold trap. Before the experiments, the catalysts
were activated in argon 250 °C for 2 h. The XPS data were ac‐
quired with an Axis Ultra Dld spectrometer (Shimadzu, Japan)
with Mg Kα radiation as the excitation source. All of binding
energy values were referenced to the C 1s peak at 284.6 eV.
Cs-SiW
Cs-PMo
2.4. Decomposition of the lignin model compound
The decomposition reactions were performed in a T316
stainless‐steel autoclave equipped with a controller, a thermo‐
couple, and a mechanical agitator (Model 4598, Parr Instru‐
ment Company, Moline, USA). The reactions were typically
performed by first introducing 0.100 g of 2‐PAP and 0.010 g of
the catalyst (or no catalyst) into the dry autoclave, and then 50
mL of AE was introduced. The autoclave was quickly closed to
prevent adsorption of water and then purged with N2 several
times. The reaction was performed at 280 °C for 4 h with a stir‐
ring speed of 300 r/min. After a predetermined time, the reac‐
tor was cooled to room temperature and the products were
then collected for further analysis [53].
Cs-PW
400
600
800
1000
1200
Wavenumber (cm–1)
Fig. 2. Raman spectra of the POMs (Oa central tetrahedron oxygen, Ob
corner oxygen, Od terminal oxygen, Oe edge‐shared oxygen).
Cs-PMo
2.5. Product analysis
Cs-SiW
Cs-PW
The products were first analyzed by gas chromatog‐
raphy‐mass spectroscopy (GC‐MS, Model DSQ II, Thermo Fish‐
er, Austin, USA), and then analyzed using a GC flame ionization
detector (GC‐FID) equipped with a HP‐5 column (GC‐2010,
Shimadzu, Japan). The concentrations of 2‐PAP, POPE, phenol,
and acetophenone were determined by the external standard
method. The 2‐PAP conversion, selectivities for phenol and
acetophenone, and total mass yield of all other products (wt%)
were calculated by the following equations:
1200 1100 1000
900
800
700
600
Wavenumber (cm–1)
Conv. (%) = (c
substrate_before – csubstrate_post)/csubstrate_before × 100
Fig. 3. FT‐IR spectra of the POMs.
SP (%) = cphenol/(csubstrate_before × Conv.) × 100
SA (%) = cacetophenone/(csubstrate_before × Conv.) × 100
YP (%) = Conv. × SP
2.3. Characterization of the solid POMs
YA (%) = Conv. × SA
others (%) = (ωsubstrate_before − ωsubstrate_post – ωphenol
The solid POMs were characterized by XRD, FT‐IR spec‐
troscopy, Raman spectroscopy, NH3‐temperature programmed
reduction (NH3‐TPD), H2 temperature programmed reduction
(H2‐TPR), and X‐ray photoelectron spectroscopy (XPS). The
XRD patterns were collected with a D8 Advance Plus X‐ray dif‐
fractometer (Bruker, Germany) using a Cu Kα radiation source
and recorded in the 2θ range 5°–50° at a scanning speed of 10
°/min. The FT‐IR spectra of the POMs were recorded with a
Nicolet IS10 infrared spectrometer (Thermo Scientific, USA)
and acquired by the transmission method. The Raman spectra
were collected using an XploRA Raman spectrometer (Horiba
Jobin Yvon, France) with 532 nm laser excitation under ambi‐
ent conditions. The NH3‐TPD profiles were collected with an
Autochem II chemisorption instrument (Micromeritics, USA)
with a thermal conductivity detector and a cold trap. The ad‐
sorption temperature of ammonia was 100 °C and the ramp
Ω
– ωacetophenone)/ωsubstrate_before × 100
where Conv. is conversion of the 2‐PAP substrate, SP and SA are
the selectivities of phenol and acetophenone, respectively, YP
and YA are the productivities of phenol and acetophenone, re‐
spectively, Ωothers is the total mass yield of products other than
phenol and acetophenone, c is the molar concentration of the
specific compound calculated by the chromatographic peak
area in the GC‐FID chart of the reaction products that corre‐
spond to the external standards of a specific compound
(mol/L), and ω is the mass concentration of a specified com‐
pound (g/L).
2.6. Determination of the loss of solid POMs in the solvent
Loss of the solid POMs in the AE solvent was analyzed by inductively

Xuezhong Wu et al. / Chinese Journal of Catalysis 38 (2017) 1216–1228
1219
Table 1
Loss of POMs in the AE solvent determined by ICP‐AES.
Table 2
Total amounts of acid sites of the POMs calculated by NH3‐TPD.
Theoretical loss ratio a
Detected concentration (mg/L)
Total acidic amount
Catalyst
Catalyst
(%)
0.22
0.27
<0.023
Mo
0.05 <0.01 <0.02
0.03 0.12 0.02
W
P
Si
(mmol/g)
0.43
(mol/mol)
0.92
Cs‐PMo
Cs‐PW
0.92
0.55
0.25
Cs‐PMo
Cs‐PW
Cs‐SiW
0.18
0.29
0.57
0.92
Cs‐SiW <0.01 <0.01 <0.02
a Theoretical loss ratio (%) = cdetected/ctheoretical, where cdetected is the de‐
tected concentration of the coordination atoms in the post‐reaction
solution and ctheoretical is the concentration of the coordination atoms in
the POM, which is assumed to completely dissolve in the solution be‐
fore the reaction.
that the acidities of Cs‐PMo and Cs‐SiW are much higher than
that of Cs‐PW. Furthermore, the amount of acid sites of Cs‐PMo
exceeds that of Cs‐SiW (Table 2). According to the molar mass,
the amount of acid sites of Cs‐PMo is the same as that of
Cs‐SiW, but nearly twice that of Cs‐PW.
coupled plasma‐atomic emission spectrometry (ICP‐AES; Model
iCAP6300, Thermo Fisher, USA). About 20 mL of the AE solvent was
withdrawn in a clean crucible before and after the reaction. The solvent
samples were first acidified by HNO3 solvent and then air‐dried at room
temperature. The crucible was then calcined at 500 °C for 6 h in air.
HNO3 solvent (20 mL, 1 mol/L) was then introduced into the crucible
and the crucible was kept in an ultrasonic machine for 30 min. Finally,
the metals in this ultrasonic treated solvent were determined by
ICP‐AES. The results are given in Table 1. Almost none of the solid
POMs are lost in the solvent during the reactions.
3.1.2. Redox properties of the POMs
The redox properties of the three POMs were evaluated by
H2‐TPR, and the profiles are shown in Fig. 5. There are no con‐
sumption peaks below 300 °C, indicating no adsorption of H2 to
the POM catalysts in this temperature range. The TPR profile of
Cs‐PMo shows distinct H2 consumption peaks in the tempera‐
ture range 350 to 500 °C. This indicates that there is considera‐
ble reduction from MoO3 to MoO2 in this temperature range
because molybdena can be partly formed at a TPR temperature
of about 400 °C [55,56]. The TPR profiles of Cs‐PW and Cs‐SiW
show an increasing tendency as the TPR temperature increases
above 350 °C and their response signals are much weaker than
that of Cs‐PMo, indicating that Cs‐PW and Cs‐SiW are much
more difficult to be reduced than Cs‐PMo [41,54].
The redox properties of the catalysts were further investi‐
gated by XPS experiments. Fig. 6 shows the high‐resolution Mo
3d and W 4f XPS spectra of Cs‐PMo and Cs‐PW before and after
treatment in AE at 280 °C for 4 h, respectively. The curve‐fitting
procedure (Fig. 6(a)) reveals a doublet peak of Mo(VI)
(3d5/2/3d3/2) in untreated Cs‐PMo at 232.9/236.2 eV, and dou‐
blet peaks at 232.9/236.2 and 231.0/234.8 eV for Mo(VI) and
Mo(IV) (3d5/2/3d3/2) in treated Cs‐PMo, respectively [57,58].
These peaks indicate that Mo(VI) in the crystals of Cs‐PMo is
partially reduced during treatment in AE. Fig. 6(b) shows the
results for Cs‐PW before and after treatment under the same
3. Results and discussion
3.1. Characterization of the Cs‐POMs
3.1.1. Acidity of the Cs‐POMs
The NH3‐TPD method was used to evaluate the acidity of the
POMs. Their profiles are shown in Fig. 4. The acid amounts and
strengths of the three samples are very different. Cs‐PMo
shows reasonably strong acidity with an ammonia desorption
peak in the temperature range 250–400 °C, while Cs‐SiW
shows stronger acidity with the ammonia desorption temper‐
ature mainly in the range 450–550 °C. Cs‐PW shows the
strongest acidity among the three samples, and part of the ad‐
sorbed ammonia did not desorb even at a temperature higher
than 550 °C. These results are consistent with the literature
[41,54]. Table 2 lists the amounts of acid sites calculated from
the peak areas in Fig. 4. The acid site amounts per gram verify
Cs-PMo
Cs-SiW
Cs-PW
Cs-SiW
Cs-PW
Cs-PMo
100
200
300
400
500
600
100 150 200 250 300 350 400 450 500 550 600
Temperature (°C)
Temperature (°C)
Fig. 4. NH3‐TPD profiles of the POMs.
Fig. 5. H2‐TPR profiles of the POMs.

1220
Xuezhong Wu et al. / Chinese Journal of Catalysis 38 (2017) 1216–1228
Mo6+
Mo4+
W6+ 4f
Mo4+ 3d3/2
W6+ 4f
(a)
(b)
7/2
5/2
Mo4+ 3d5/2
(2)
(2)
(1)
Mo6+ 3d
Mo6+ 3d
3/2
5/2
(1)
240
237
234
231
228
225
42
40
38
36
34
32
30
28
Binding eneygy (eV)
Binding energy (eV)
Fig. 6. High‐resolution Mo 3d and W 4f XPS spectra with signal fitting analysis: (a) Cs‐PMo and (b) Cs‐PW before (1) and after (2) treatment in AE at
280 °C for 4 h.
conditions. The curve‐fitting procedure reveals that the spectra
of Cs‐PW show a doublet at 34.8/37.0 eV for W(VI) (4f7/2, 4f5/2
ly. The corresponding reaction mechanism has been described
in our previous paper [53].
)
components both before and after treatment [59,60]. This in‐
dicates that W(VI) in the crystals of Cs‐PW is not reduced dur‐
ing the treatment in AE. Therefore, the Cs‐PMo catalyst is a
bifunctional catalyst with redox and acidic properties, while the
Cs‐PW catalyst only acts as a monofunctional acid catalyst in
our reactions.
When Cs‐PMo was added into the reaction system (row 2 of
Table 3), 2‐PAP was completely converted and the selectivities
of phenol and acetophenone were 98.6% and 91.1%, respec‐
tively. Only a small amount of others products (<5 wt%) was
produced. This result suggests that Cs‐PMo has excellent cata‐
lytic activity for decomposition of 2‐PAP to the target products
in AE. However, it is surprising that conversion of 2‐PAP was
surprisingly low (89%) when Cs‐PW was used as a catalyst
(row 3), and the selectivities for phenol and acetophenone also
decreased to 65.5% and 47.7%, respectively. These values are
even lower than those without adding a catalyst. Cs‐SiW per‐
formed even worse for this reaction, with selectivities for phe‐
nol and acetophenone of 40.7% and 11.3%, respectively, and
2‐PAP conversion of only 37.6%.
3.2. Conversion of 2‐PAP over the POMs
3.2.1. Conversion and selectivity over the POM catalysts
Cs‐SiW, Cs‐PW, and Cs‐PMo were used to investigate their
catalytic effect on decomposition of 2‐PAP in AE at 280 °C for 4
h in a T316SS autoclave with the stirring speed of 300 r/min.
Conversion of 2‐PAP and the selectivities to the main products
were determined by GC‐FID with external standard methods.
The results are given in Table 3 along with the results of the
reaction without adding a catalyst for reference. It has been
reported that 2‐PAP can be decomposed in AE at high temper‐
ature in a T316SS autoclave. The stainless‐steel wall of the
T316SS autoclave could promote transfer of the hydrogen lib‐
erated from AE to 2‐PAP and various decomposed intermedi‐
ates. As shown in row 1 of Table 3, 2‐PAP is almost completely
converted even without adding a catalyst, and the selectivities
of phenol and acetophenone are 81.6% and 54.4%, respective‐
3.2.2. Product distribution of 2‐PAP decomposition in AE
catalyzed by the POMs
Analysis of the product distribution can reveal the details of
the reaction, which are very useful for investigating the reac‐
tion mechanism. The GC‐MS results of the reaction products
with and without addition of a POM catalyst are shown in Table
4, and their corresponding total ion current (TIC) profiles are
shown in Fig. 7. As shown in rows 1–4 of Table 4, the products
from dehydrogenation and/or intermolecular dehydration
Table 3
Product distributions of 2‐PAP in AE with and without the Cs‐POM catalysts a.
Phenol
Acetophenone
Catalyst
Temp. (°C)
Conv. (%)
SP/ SA
Ωothers (%)
Total areab
YP (%)
81.2
98.6
58.2
15.3
YA (%)
54.1
91.1
42.4
4.2
SP (%)
81.6
98.6
65.5
40.7
SA (%)
54.4
91.1
47.7
11.3
No catalyst
Cs‐PMo
Cs‐PW
280
280
280
280
>99
>99
88.8
37.6
1.5
1.1
1.4
3.6
33.0
4.6
39.0
847253
5888967
1145881
1257376
Cs‐SiW
28.4
a Reaction conditions: 0.100 g 2‐PAP, 50 mL AE, 280 °C, N2 atmosphere, reaction time 4 h, 0.010 g Cs‐POM catalyst, stirring speed 300 r/min.
b Total area = areaethoxyethene + areaacetal. The areas are the peak areas of the chromatogram of GC‐FID, and these areas are only used to reflect their
corresponding tendencies with changing reaction conditions. Thus, they are not quantitatively useful.

Xuezhong Wu et al. / Chinese Journal of Catalysis 38 (2017) 1216–1228
1221
Table 4
Distribution of the main products from decomposition of 2‐PAP in AE with POM catalystsa.
No
Product
Cs‐PMo Cs‐PW Cs‐SiW H4SiW12O40 Product
No catalyst Cs‐PMo
Cs‐PW
d.
Cs‐SiW H4SiW12O40
catalyst
trace
trace trace
trace
d.
trace
d.
u.d.
u.d.
u.d.
u.d.
d.
d.
d.
d.
O
d.b
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
u.d.
u.d.
u.d.
u.d.
u.d.
d.
u.d.
d.(Greatly
increased)
d.
d.
d.
u.d.
u.d.c
d.
d.
d.
d.
u.d.
d.
u.d.
d.
u.d.
d.
u.d.
u.d.
u.d.
u.d.
u.d.
u.d.
u.d.
u.d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
u.d.
u.d.
d.
u.d.
d.
d.
d.
d.
d.
d.
d.
d.
d.
u.d.
d.
d.
d.
d.
u.d.
u.d.
u.d.
u.d.
u.d.
d.
a Reaction conditions: 0.100 g 2‐PAP, 50 mL AE, 280 °C, N2 atmosphere, reaction time 4 h, 0.010 g of the POM catalyst, stirring speed 300 r/min. Veri‐
fied by GC‐MS.
b Detected.
c Undetected.
reactions of the AE solvent are all detected, such as ethylal,
ethoxyethene, diethyl ether, and acetal [53,61,62].
our previous report as the result of hydrogen transfer reactions
[53]. Production of nitrides can be ascribed to the reactions of
the 2‐PAP cleavage intermediates with the activated nitrogen in
the autoclave.
When the reaction is catalyzed by Cs‐PMo, in addition to
similar products to those without a catalyst, a small amount of
ethyl benzene is generated (column 3 of Table 4), while there
are no products of nitrides, such as (2,4‐dimethylphenyl)‐2‐
(4‐methoxyphenyl)diazene. Generation of ethyl benzene can be
attributed to excessive hydrogenation of styrene along route
Except for the similar products from the ethanol solvent,
there are large differences in the products from decomposition
of 2‐PAP. The products from decomposition of 2‐PAP without a
catalyst are styrene, phenol, acetophenone, ethyl benzoate,
(1‐ethoxyethyl)benzene, 4‐(1‐hydroxyethyl) benzaldehyde,
2‐phenoxy‐1‐phenylethan‐1‐ol, and (2,4‐dimethylphenyl)‐2‐
(4‐methoxyphenyl) diazene (nitrides) (column 2 of Table 4 and
Fig. 7). Their generation mechanisms have been described in

1222
Xuezhong Wu et al. / Chinese Journal of Catalysis 38 (2017) 1216–1228
Fig. 7. TIC profiles of the reaction products of 2‐PAP in AE with a stirring speed of 300 r/min with POM catalysts from GC‐MS. (a) no catalyst; (b)
Cs‐PMo; (c) Cs‐PW; (d) Cs‐SiW.
Ia‐1 in Scheme 2 in hydrogen‐enriched surroundings [53],
whereas the lack of nitrides can be attributed to greatly en‐
hanced hydrogen donation (row 2 of Table 3). When the reac‐
tion is catalyzed by Cs‐PW (row 3 of Table 3) or Cs‐SiW (row 4
of Table 3), some other by‐products are generated, such as
(diethoxymethyl)benzene, 1,2‐diphenylethene, 2,3‐dipheny‐
loxirane, and 1‐(3,5‐dimethoxyphenyl)propan‐1‐one. The slight
difference between these two catalysts is generation of
3‐phenylbenzofuran when the catalyst is changed from Cs‐PW
to Cs‐SiW.
ment, ethoxyethene and acetal are detected (rows 3 and 4).
Furthermore, the total area of generated ethoxyethene/acetal
increases as the temperature increases (column 4). Remarka‐
bly, when the temperature is higher than 260 °C, the total
GC‐FID peak area of ethoxyethene and acetal significantly in‐
creases.
After addition of Cs‐PMo to the system, the amounts of eth‐
oxyethene and acetal significantly increase (Table 6). The total
peak area of ethoxyethene and acetal in the GC‐FID chromato‐
gram increases to 1054547 in the post‐reaction solvent com‐
pared with 165743 (Table 5) for the case without a catalyst at
200 °C. In addition, the value at 280 °C is 4 times higher than
that without Cs‐PMo, meaning that Cs‐PMo greatly accelerates
dehydrogenation of ethanol. The Cs‐SiW catalyst, which has
weaker redox properties and a higher acid amount than
Cs‐PMo, was used to further investigate the effect of the acidity
on the dehydrogenation reactions (row 5 of Table 6). Cs‐SiW
also increases the total amount of generated ethoxye‐
thene/acetal compared with the data without a POM catalyst
(row 5 of Table 5), and the increase is mainly because of en‐
hancement of production of ethoxyethene.
3.3. Hydrogen donation reaction of the AE solvent
3.3.1. Reactions of pure AE
As well as all of the reactions of 2‐PAP with and without a
catalyst, the products of dehydrogenation/dehydration of AE
can also be generated, such as ethylal, ethoxyethene, diethyl
ether, and acetal, accompanied by active hydrogen liberation
[15,53,63]. Active hydrogen plays a crucial role in conversion of
2‐PAP [53]. Therefore, promotion of conversion of AE by the
POM catalysts was first investigated in the absence of 2‐PAP.
Table 5 shows generation of ethoxyethene/acetal from pure
AE with increasing temperature at a stirring speed of 300
r/min. As shown in Table 5, no dehydrogenated species of AE
are detected before treatment (row 1). However, after treat‐
The proposed mechanism is summarized in Scheme A
[64,65]. First, ethanol is dehydrogenated to generate ethylal by
a catalytic redox or self‐redox reaction. Ethylal is then tautom‐
erized to vinyl alcohol, which then reacts with ethanol to give
Table 5
Table 6
Generation of ethoxyethene/acetal with increasing temperature ac‐
companied by liberation of active hydrogen without a catalyst a.
Generation of ethoxyethene and acetal accompanied by liberation of
active hydrogen with Cs‐POM catalysts a.
Temperature (°C)
Untreated
200
240
260
Areaethoxyethene
n.d.
n.d.
n.d.
27411
143567
Areaacetal
n.d.
165743
177070
304464
519981
Total area b
0
165743
177070
331875
663548
Catalyst Temperature (°C) Areaethoxyethene
Areaacetal Total area b
Cs‐PMo
Cs‐PMo
Cs‐PMo
Cs‐PMo
Cs‐SiW
200
240
260
280
280
103972
222500
341989
782342
357394
950575
1013524
2011751
1966455
482593
1054547
1236024
2353740
2748797
839987
280
a Reaction conditions: 50 mL pure AE, N2 atmosphere, reaction time 4 h,
no catalyst, stirring speed 300 r/min.
a Reaction conditions: 50 mL pure AE, reaction time 4 h, 0.010 g Cs‐POM
catalyst, stirring speed 300 r/min.
b Total area = areaethoxyethene + areaacetal
b Total area = areaethoxyethene + areaacetal
.
.

Xuezhong Wu et al. / Chinese Journal of Catalysis 38 (2017) 1216–1228
1223
Scheme A. Proposed mechanisms for generation of ethylal, ethoxyethene, acetal, and diethyl ether accompanied by generation of active hydrogen.
ethoxyethene by an intermolecular dehydration reaction (1 in
Scheme A). The carbonyl oxygen of ethylal can be attacked by a
proton (from the acid) to generate an oxonium cation, which is
susceptible to nucleophilic attack by the lone electron pair of
oxygen in an ethanol molecule [64,65]. Hemiacetal is then gen‐
erated after elimination of one water molecule and one proton.
Hemiacetal is reactive with ethanol and undergo further acety‐
lation to generate acetal, which is why no hemiacetal is detect‐
ed in our products (2 in Scheme A). A small amount of diethyl
ether is also detected in the post‐reaction solutions and its
generation mechanism is shown in 3 of Scheme A [61]. Because
generation of diethyl ether is only because of intermolecular
dehydration of two ethanol molecules and no active hydrogen
is liberated, the amount of diethyl ether is not included in the
“Total area” columns of Tables 3, 5, and 6 as the estimate of the
amount of liberated active hydrogen. This mechanism can ex‐
plain the above reaction results where both the acidity and
redox properties of the catalyst can accelerate conversion of AE
with an increase in the dehydrogenating/dehydrating products
of AE. Liberation of active hydrogen accompanying these reac‐
tions will be shown to be the key factor for decomposition of
2‐PAP (Scheme B).
Cs‐SiW > Cs‐PMo, and the order of the redox abilities is Cs‐PMo
> Cs‐PW ≥ Cs‐SiW. The first concern is whether the acidity or
the redox properties of the catalyst governs the dehydrogena‐
tion reaction of ethanol. Comparing the data given in column 5
of Table 5 and column 5 of Table 6, it could be inferred that the
acidity of the catalyst can lead to an increase of the total peak
area of ethoxyethene and acetal, and such enhancement mainly
comes from an increase in the amount of ethoxyethene. The
effect of the amount of acid sites was further investigated in the
system with 2‐PAP by comparing the results of Cs‐PW and
Cs‐SiW (column 8 of Table 3) because of the clear differences in
their acid strengths (Fig. 4) and redox properties (Fig. 5) is
slight. The difference in the total production of ethoxyethene
and acetal is not very large (~10%) even though the acid
amount of Cs‐SiW is 60% higher than that of Cs‐PW. However,
the amount of dehydrogenated solvent products over Cs‐PMo,
which has the highest redox ability, is remarkably higher than
(as much as 3.3 times) that of Cs‐SiW, even though the former
has the weakest acid strength and only 50% more acid sites
than the latter (Table 6). This indicates that the redox ability
might dominate generation of active hydrogen by dehydro‐
genation of AE.
The effect of hydrogen transfer on the self‐redox equilibri‐
um of dehydrogenation‐hydrogenation of ethanol was also
investigated. As shown in row 5 of Table 6 and row 4 of Table 3,
transfer of active hydrogen to 2‐PAP and its intermediates can
promote production of ethoxyethene and acetal through ethylal
3.3.2. Hydrogen generation in the system with 2‐PAP
By comparing the total GC‐FID peak areas after the reaction
(column 8 of Table 3), it is clear that addition of POMs in the
reaction system containing 2‐PAP and AE solvent increases
active hydrogen liberation. The total area of ethoxyethene and
acetal increases from 847253 without a POM catalyst to
1145881, 1257376, and 5888967 when Cs‐SiW, Cs‐PW, and
Cs‐PMo are added to the reaction system, respectively.
Considering the characterization results of the POMs (Sec‐
tion 3.1), the order of the amounts of acid sites is Cs‐PMo >
Cs‐SiW > Cs‐PW, the order of the acid strengths is Cs‐PW ≥
Scheme B. Summary of the self‐redox equilibrium reactions of AE.

1224
Xuezhong Wu et al. / Chinese Journal of Catalysis 38 (2017) 1216–1228
generation. The difference of the total areas of ethoxye‐
thene/acetal with and without a catalyst is about 30%. Genera‐
tion of ethoxyethene/acetal is remarkably enhanced when
Cs‐PMo is used (column 4 of Table 6 and column 2 of Table 3).
The resulting 120% increase in generation of ethoxye‐
thene/acetal can be ascribed to the high hydrogen transfer
ability of Cs‐PMo [58,66]. Scheme B summarizes promotion of
hydrogen transfer and acid catalysis in the redox reaction of
AE.
susceptible to attack by a proton from the acid catalyst to form
an oxonium cation [59] along route I (Scheme C). The formed
oxonium cation can be temporarily stabilized by coordination
to POMs anions [46,65,67,68], or even be attacked by a water
molecule (if water is present as a nucleophilic agent) to give
phenol
and
2‐hydroxy‐1‐phenylethan‐1‐one
[69].
2‐Hydroxy‐1‐phenylethan‐1‐one can then be hydrogenated and
dehydrated to 2‐phenylethan‐1‐ol over an acid catalyst [70]. If
the 2‐PAP molecule is attacked by a proton at the α‐carbonyl
group, conversion can proceed along route II in Scheme C to
form a 1‐hydroxy‐2‐phenoxy‐1‐phenylethan‐1‐ylium cation,
which can also be temporarily stabilized by coordination to
POM anions in an anhydrous environment [46,65,67,68], or
further cyclized and transform to 3‐phenylbenzofuran. Some
3.4. Reaction of 2‐PAP in AE catalyzed by POMs
3.4.1. Reaction mechanism of 2‐PAP in AE
2‐PAP would react with the liberated active hydrogen ac‐
companying dehydrogenation of AE to form various products
even if no other catalyst is added (Section 3.2). The hydrogen
transfer mechanism was proposed in our previous paper from
the product distribution in the system without adding a catalyst
other
products,
such
as
2,3‐diphenyloxirane,
1,2‐diphenylethene, and (2,2‐diethoxyethyl)benzene, can be
considered as the acid‐catalyzed products obtained from hy‐
drogen
transfer
reactions.
The
generated
[53].
However,
some
new
products,
such
as
and
2‐hydroxy‐1,2‐diphenylethan‐1‐one can be hydrogenated to a
1,2‐diphenylethane‐1,2‐diol (route IV) and further transform to
2,3‐diphenyloxirane along route IVa or 1,2‐diphenylethene
along route IVb. The product (2,2‐diethoxyethyl)benzene can
be considered to be generated along route V by reaction of
pre‐generated styrene (product of route III) with ethanol mol‐
3‐phenylbenzofuran,
2‐phenylethan‐1‐ol,
1,2‐diphenylethene, are observed when acidic POMs are used
as a catalyst (Table 4, Fig. 7). Therefore, we propose the ac‐
id‐catalyzed reaction pathways beyond the hydrogen transfer
mechanism in Scheme C.
When an acid catalyst exists in the reaction system, the lone
pair electrons on the oxygen of the ether linkage in 2‐PAP are
ecules
[71].
The
generation
mechanism
of
1‐(3,5‐dimethoxyphenyl)propan‐1‐one is not clear under our
OH
O
H
O
O
O
OH
H
O
OH
OH
OH
[H]
Lewis acid, H+
+[H], -H2O
H2O
BP: 166 °C
H+
BP:227.85 °C
Route I
O
H
O
O
OH
OH
O
O
d-
OH
O
H+
-H+
OH
Route II
BP: 376.42 °C
Route III
H+
-H2O
[H]
Refer to [53]
O
OH
O
O
O
OH
O
BP: 329.15 °C
...
O
OH
OH
O
O
[H]
O
Route IVa
H+
OH
OH
-H2O
-H2O
Route IV
BP: 329.15 °C
H+
Route IVb
BP: 304.29 °C
Route V
Lewis acid
OH
+
O
O
hydrogen transfer
BP: Boiling point
BP: 272.09 °C
Scheme C. Proposed mechanisms for generation of some of the main products of 2‐PAP in AE catalyzed by Cs‐PW or Cs‐SiW.

Xuezhong Wu et al. / Chinese Journal of Catalysis 38 (2017) 1216–1228
1225
reaction conditions (Table 4 and Fig. 7).
3.4.2. Interpretation of 2‐PAP transformation over the POMs
The experimental results in Section 3.2 (Tables 3 and 4, Fig.
7) show the dramatic differences in 2‐PAP conversion, the
product selectivities, and the product distributions over differ‐
ent POM catalysts. This can be explained by associating the
reaction results with the reaction mechanism and the proper‐
ties of the POMs.
(1) Conversion of 2‐PAP in AE (column 3 of Table 3). As
shown in Table 2, the reaction of 2‐PAP in AE shows dramati‐
cally different conversion over different POM catalysts. Both
the reactions without a catalyst and with the Cs‐PMo catalyst
have high conversion of > 99%. However, addition of Cs‐PW
decreases the conversion of 2‐PAP to 88.8%, while it greatly
decreases to 37.6% when Cs‐SiW is added. We speculated that
Cs‐SiW and Cs‐PW can promote the reverse‐cleavage reaction,
that is, the condensation reaction of phenol with acetophenone.
Thus, a reaction was performed with these two reactants in AE
using the Cs‐SiW catalyst (Table 7 and Fig. 8). The reverse reac‐
tion did not occur. Both phenol and acetophenone were stable
and only a small part of acetophenone was hydrogenated to
1‐phenylethan‐1‐ol or its derivatives, such as styrene (Table 7
and Fig. 8). This reaction confirmed the mechanism for genera‐
tion of 1‐phenylethan‐1‐ol proposed in Scheme 2 in our previ‐
ous report [53], although the speculation about the reverse
reaction mechanism was false.
Considering the characterization results of the POMs and
combining them with previous results [41,46,54,65,68,72], it is
clear that these three POMs have very different acid strengths,
acid site amounts, and redox properties. Considering the order
of the active hydrogen amounts in Table 2 (Cs‐PMo > Cs‐SiW ≥
Cs‐PW) and the proposed scheme for conversion of 2‐PAP [53],
conversion of 2‐PAP is clearly enhanced as the active hydrogen
donating ability increases, whereas it is inhibited with increas‐
ing amount of acid sites and acid strength (Figs. 4–7, and Tables
2 and 3). The former was demonstrated in our previous report
[53], while the latter can be explained as follows. Under cataly‐
P+A After processed
P+A Before processed
2
4
6
8
10
12
14
Retention time (min)
Fig. 8. GC‐FID profiles of phenol and acetophenone in AE before and
after decomposition at 280 °C for 4 h with Cs‐SiW as the catalyst.
sis with a strong acid, the 2‐PAP molecule can form an oxonium
cation at the α‐carbonyl and/or ether bond oxygen atom (route
I or II in Scheme C) [64,65], and this oxonium cation can then
be stabilized by the anion of the POM [46,54,64,65,68,72,73].
The competition between this stabilization effect of the oxo‐
nium cation and the ability for active hydrogen transfer of the
catalyst [53] dominates conversion of 2‐PAP.
When the active hydrogen donating and transfer ability is
strong but there are few weak acid sites, conversion of 2‐PAP
would proceed along the hydrogen transfer route, similar to the
reaction of 2‐PAP in AE or IPA without a catalyst [53]. Similarly,
when the active hydrogen donating and transfer ability is very
strong but the acidity strength is not very strong, similar to the
case with the Cs‐PMo catalyst, conversion of 2‐PAP would also
proceed along the active hydrogen transfer route (Tables 3–6
and Figs. 4–7). However, if the strength of the acid sites is very
strong and the acid site amount is very large, conversion of
2‐PAP would proceed along the oxonium cation mechanism,
such as the reaction of 2‐PAP in AE with the Cs‐PW catalyst
(Tables 3–6 and Figs. 4–7) or Cs‐SiW catalyst (Tables 3 and 4,
and Fig. 7).
(2) Selectivities of the desired products. The selectivities of
the various products depend on their reaction routes and reac‐
tion rates. When 2‐PAP reacts in AE without adding a catalyst,
the selectivities of phenol and acetophenone are 81.6% and
54.4%, respectively, and the total mass yield of other byprod‐
ucts is 33.0% (Table 3). Addition of the Cs‐PMo catalyst further
enhances the selectivities to phenol and acetophenone to
98.6% and 91.1%, respectively, with 2‐PAP conversion of >
99% and the mass yield of other products of < 5% (Table 3).
This can be explained by our reaction mechanism and the de‐
tailed product distribution (columns 2 and 3 of Table 3). When
Cs‐PMo is added, the by‐products generated along route Ib in
Scheme 2 in [53] are not present, and the desired products of
phenol and acetophenone are produced with in small amounts
Table 7
Product distribution of phenol and acetophenone in AE before and after
treatment at 280 °C for 4 h with Cs‐SiW as the catalyst.
Before processed
area a
After processed
area
Product
u.d.
34734
u.d.
2391387
u.d.
85431
2535273
2406059
u.d.
498623
3190164
u.d.
2391387
u.d.
excessive
hydrogenated
products,
such
as
2‐PAP
(1‐ethoxyethyl)benzene,
styrene,
ethylbenzene,
and
a Areas of the corresponding peaks from GC‐FID.

1226
Xuezhong Wu et al. / Chinese Journal of Catalysis 38 (2017) 1216–1228
2‐phenoxy‐1‐phenylethan‐1‐ol. The by‐products of isomeriza‐
tion, rearrangement, and oxidization are also not present. Con‐
sidering Tables 3–6, active hydrogen generation greatly in‐
creases when Cs‐PMo is added, which can be attributed to its
strong redox properties. Cs‐PMo not only enhances generation
of active hydrogen, but it can also promote hydrogen transfer
to the intermediates [66,74–77]. Therefore, Cs‐PMo can pro‐
mote the reaction along route Ia to generate the desired prod‐
ucts (Scheme 2 in [53]). Moreover, the acid strength of Cs‐PMo
is not very high and isomerization and rearrangement of the
intermediates rarely occurs. As a result, Cs‐PMo exhibits excel‐
lent selectivity for the desired products with almost complete
conversion of 2‐PAP.
nol and 91.1% for acetophenone). An acid catalyst with very
strong acid strength and high amount of acid sites can have an
inhibitory effect on the decomposition reactions of 2‐PAP; that
is, it can inhibit conversion and lower the product selectivities.
References
[1] U.S. Department of Agriculture, USDA Announces Investments in
Bioenergy Research and Development to Spur New Markets, Inno‐
vation, and Unlimited Opportunity in Rural America, 2013.
[2] IRENA (2014): REmap 2030: A Renewable Energy Roadmap,
IRENA: Abu Dhabi, 2014.
[3] D. S. Argyropoulos, Materials, Chemicals, and Energy from Forest
Biomass, American Chemical Society; Oxford University Press,
2007.
[4] J. van Haveren, E. L. Scott, J. Sanders, Biofuels Bioprod. Bioref.,
2008, 2, 41–57.
[5] T. Werpy, G. Peterson, A. Aden, J. Bozell, J. Holladay, J. White, A.
Manheim, D. Eliot, L. Lasure, S. Jones, Top Value Added Chemicals
from Biomass, 2004, Vol. 1.
When the Cs‐PW catalyst is used, the selectivities to phenol
and acetophenone are both lowered and 2‐PAP conversion is
also lowered (Table 3). Analyzing the results in Tables 3 and 4,
some side products are generated by a pathways other than the
hydrogen transfer mechanism, such as (diethoxyme‐
thyl)benzene,
1‐(3,5‐dimethoxyphenyl)propan‐1‐one,
[6] T. E. Amidon, S. J. Liu, Biotechno. Adv., 2009, 27, 542–550.
[7] C. Heitner, D. R. Dimmel, J. A. Schmidt, Lignin & Lignans Advances
in Chemistry, CRC Press, 2010.
[8] S. M. Kang, X. L. Li, J. Fan, J. Chang, Renew. Sust. Energ. Rev., 2013,
27, 546–558.
[9] J. Zakzeski, A. L. Jongerius, P. C. A. Bruijnincx, B. M. Weckhuysen,
ChemSusChem, 2012, 5, 1602–1609.
[10] K. De Oliveira‐Vigier, N. Abatzoglou, F. Gitzhofer, Can. J. Chem. Eng.,
2005, 83, 978–984.
[11] P. J. Deuss, K. Barta, Coord. Chem. Rev., 2016, 306, 510–532.
[12] C. Z. Li, X. C. Zhao, A. Q. Wang, G. W. Huber, T. Zhang, Chem. Rev.,
2015, 115, 11559–11624.
[13] R. Rinaldi, R. Jastrzebski, P. C. A. Bruijnincx, B. M. Weckhuysen, M.
Kennema, M. T. Clough, J. Ralph, Angew. Chem. Int. Ed., 2016, 55,
8164–8215.
1,2‐diphenylethene, and 2,3‐diphenyloxirane (columns 2 and 4
of Table 4). This shows that a portion of the reaction of 2‐PAP
proceeds along the acid‐catalyzed route (routes II–IV in
Scheme C) [64,65]. Because the amount of acid sites is not too
much (Table 2), most of 2‐PAP can be converted by the hydro‐
gen transfer mechanism. The selectivities to the desired prod‐
ucts decreases with conversion. When the Cs‐SiW catalyst is
used, the selectivities to phenol and acetophenone are 40.7%
and 11.3% with 2‐PAP conversion of only 37.6% (row 4 of Ta‐
ble 3). From the product distribution given in Table 4, the
probable products generated by the hydrogen transfer mecha‐
nism are generated owing to enhanced hydrogen donation
from AE compared with the case without a catalyst (Table 3).
Some new products are also generated, such as (diethoxyme‐
[14] S. Dutta, K. C. W. Wu, B. Saha, Catal. Sci. Technol., 2014, 4,
3785–3799.
thyl)benzene,
1,2‐diphenylethene,
2,3‐diphenyloxirane,
3‐phenylbenzofuran, and 2‐phenylethan‐1‐ol. This can be at‐
tributed to an increased portion of the reaction of 2‐PAP pro‐
ceeding by the acid‐catalyzed mechanism. Careful analysis of
proposed Scheme C and the product distribution in Table 4
shows that there are considerably more side reactions derived
from acetophenone along route V and less reaction along route
III (Scheme C). These phenomena lead to a low yield of aceto‐
phenone and a very high SP/SA ratio (row 4 of Table 3).
[15] Q. Song, F. Wang, J. Y. Cai, Y. H. Wang, J. J. Zhang, W. Q. Yu, J. Xu,
Energy Environ. Sci., 2013, 6, 994–1007.
[16] R. Ma, W. Y. Hao, X. L. Ma, Y. Tian, Y. D. Li, Angew. Chem. Int. Ed.,
2014, 53, 7310–7315.
[17] X. L. Ma, R. Ma, W. Y. Hao, M. M. Chen, F. Iran, K. Cui, Y. Yan, Y. D. Li,
ACS Catal., 2015, 5, 4803–4813.
[18] A. Narani, R. K. Chowdari, C. Cannilla, G. Bonura, F. Frusteri, H. J.
Heeres, K. Barta, Green Chem., 2015, 17, 5046–5057.
[19] J. Mottweiler, M. Puche, C. Raeuber, T. Schmidt, P. Concepcion, A.
Corma, C. Bolm, ChemSusChem, 2015, 8, 2106–2113.
[20] L. Yang, Y. D. Li, P. E. Savage, Ind. Eng. Chem. Res., 2014, 53,
2633–2639.
4. Conclusions
Decomposition of the lignin model compound 2‐PAP in AE
has been investigated using three POM catalysts. Cleavage of
2‐PAP can follow either an active hydrogen transfer mecha‐
nism or an oxonium cation mechanism. The product distribu‐
tion depends on the competition between the hydrogen donat‐
ing ability and the acid strength/amount. The function of the
POM catalyst is to promote active hydrogen liberation and
promote formation and temporary stabilization of oxonium
cations. The Cs‐PMo catalyst can also increase the hydrogen
donating ability and promote hydrogen transfer to the reaction
intermediates, leading to high conversion of 2‐PAP (>99%) and
excellent selectivities to the desired products (98.6% for phe‐
[21] W. P. Deng, H. X. Zhang, L. Q. Xue, Q. H. Zhang, Y. Wang, Chin. J.
Catal., 2015,36, 1440–1460.
[22] Z. C. Jiang, C. W. Hu, J. Energy Chem., 2016, 25, 947–956.
[23] Y. Y. Ma, Z. T. Du, J. X. Liu, F. Xia, J. Xu, Green Chem., 2015, 17,
4968–4973.
[24] H. W. Guo, B. Zhang, Z. J. Qi, C. Z. Li, J. W. Ji, T. Dai, A. Q. Wang, T.
Zhang, ChemSusChem, 2017, 10, 523–532.
[25] J. G. Zhang, H. Asakura, J. van Rijn, J. Yang, P. Duchesne, B. Zhang, X.
Chen, P. Zhang, M. Saeys, N. Yan, Green Chem., 2014, 16,
2432–2437.
[26] M. Zaheer, R. Kempe, ACS Catal., 2015, 5, 1675–1684.
[27] C. Peng, Q. Chen, H. W. Guo, G. Hu, C. Z. Li, J. L. Wen, H. S. Wang, T.

Xuezhong Wu et al. / Chinese Journal of Catalysis 38 (2017) 1216–1228
1227
Graphical Abstract
Chin. J. Catal., 2017, 38: 1216–1228 doi: 10.1016/S1872‐2067(17)62854‐7
Decomposition of a β‐O‐4 lignin model compound over solid Cs‐substituted polyoxometalates in anhydrous ethanol: acidity or
redox property dependence?
Xuezhong Wu, Wenqian Jiao, Bing‐Zheng Li, Yanming Li, Yahong Zhang, Quanrui Wang, Yi Tang *
Fudan University; Guangxi Academy of Sciences
CH2OH
High Redox Property
CHOH
CH2OH
OCH3
O
O
CH2OH
O
CH2OH
O
CH
OCH3
H3CO
O
O
H3CO
O
O
O
O
OCH3CH2OH
CHOH
CH2OH
CH
CH
O
O
>99% >91%
>99% conversion
O
OCH3
O
O
O
CHOH
H
C
OCH3
OCH3
CH2OH
O
H3CO
CH2OH
O
O
OCH3
CH2OH
CHOH
O
HOH2C
O
CHOH
Cane
H3CO
H3CO
OCH3
H
O
O
HOHC
CH2
O
OCH3
OCH3
O
O
O
HOH2C
HOH2
C
HC
O
CHOH
CH
OCH3
H3CO
OCH3
CH2OH
O
O
O
C
O
H
Bagasse
Lignin
The lignin model compound 2‐phenoxyacetophenone can be completely cleaved into target products in a hydrogen‐donating atmosphere
over a catalyst with strong redox properties by the hydrogen transfer mechanism.
Zhang, Z. B. K. Zhao, R. C. Sun, H. B. Xie, ChemCatChem, 2017, 9,
1135–1143.
[28] H. W. Guo, B. Zhang, C. Z. Li, C. Peng, T. Dai, H. B. Xie, A. Q. Wang, T.
Zhang, ChemSusChem, 2016, 9, 3220–3229.
[29] H. Konnerth, J. G. Zhang, D. Ma, M. H. G. Prechtl, N. Yan, Chem. Eng.
Sci., 2015, 123, 155–163.
[30] X. L. Ma, Y. Tian, W. Y. Hao, R. Ma, Y. D. Li, Appl. Catal. A, 2014, 481,
64–70.
Eng., 2011, 28, 1177–1180.
[44] J. K. Kim, H. W. Park, U. G. Hong, J. H. Choi, I. K. Song, J. Nanosci.
Nanotechno., 2014, 14, 8884–8890.
[45] Y. T. Cheng, J. Jae, J. Shi, W. Fan, G. W. Huber, Angew. Chem. Int. Ed.,
2012, 51, 1387–1390.
[46] M. T. Pope, A. Müller, Polyoxometalate Chemistry from Topology via
Self‐Assembly to Applications, Kluwer Academic Publishers, Bos‐
ton, 2001.
[31] N. Yan, Y. A. Yuan, R. Dykeman, Y. A. Kou, P. J. Dyson, Angew. Chem.
Int. Ed., 2010, 49, 5549–5553.
[32] Z. C. Luo, Y. M. Wang, M. Y. He, C. Zhao, Green Chem., 2016, 18,
433–441.
[47] A. J. Bridgeman, Chem. Phys., 2003, 287, 55–69.
[48] A. J. Bridgeman, Chem. Eur. J., 2004, 10, 2935–2941.
[49] K. N. Rao, K. M. Reddy, N. Lingaiah, I. Suryanarayana, P. S. S. Pra‐
sad, Appl. Catal. A, 2006, 300, 139–146.
[33] A. Berlin, M. Balakshin, N. Gilkes, J. Kadla, V. Maximenko, S. Kubo, J.
Saddler, J. Biotechnol., 2006, 125, 198–209.
[50] F. J. Berry, G. R. Derrick, M. Mortimer, Polyhedron, 2014, 68,
17–22.
[34] H. J. Li, Y. Q. Pu, R. Kumar, A. J. Ragauskas, C. E. Wyman, Biotechnol.
Bioeng., 2014, 111, 485–492.
[51] M. Langpape, J. M. M. Millet, U. S. Ozkan, M. Boudeulle, J. Catal.,
1999, 181, 80–90.
[35] Z. Strassberger, A. H. Alberts, M. J. Louwerse, S. Tanase, G.
Rothenberg, Green Chem., 2013,15, 768–774.
[36] J. Zhang, Y. Liu, S. Chiba, T. P. Loh, Chem. Commun., 2013, 49,
11439–11441.
[37] Y. Y. Jiang, L. Yan, H. Z. Yu, Q. Zhang, Y. Fu, ACS Catal., 2016, 6,
4399–4410.
[38] H. F. Liu, M. Wang, H. J. Li, N.C. Luo, S. T. Xu, F. Wang, J. Catal., 2017,
346, 170–179.
[39] M. Wang, L. H. Li, J. M. Lu, H. J. Li, X. C. Zhang, H. F. Liu, N. C. Luo, F.
Wang, Green Chem., 2017, 19, 702–706.
[52] B. M. Devassy, S. B. Halligudi, J. Mol. Catal. A, 2006, 253, 8–15.
[53] X. Z. Wu, W. Q. Jiao, Y. M. Li, B. Z. Li, Y. H. Huang, H. B. Zhang, Y. H.
Zhang, Q. R. Wang, Y. Tang, Bioresources, 2016, 11, 10349–10377.
[54] I. V. Kozhevnikov, Chem. Rev., 1998, 98, 171–198.
[55] J. L. Brito, J. Laine, K. C. Pratt, J. Mater. Sci., 1989, 24, 425–431.
[56] S. Rajagopal, H. J. Marini, J. A. Marzari, R. Miranda, J. Catal., 1994,
147, 417–428.
[57] D. Gazzoli, F. Prinetto, M. C. Campa, A. Cimino, G. Ghiotti, V. Indo‐
vina, M. Valigi, Surf. Interface Anal., 1994, 22, 398–402.
[58] D. P. Debecker, B. Schimmoeller, M. Stoyanova, C. Poleunis, P.
Bertrand, U. Rodemerck, E. M. Gaigneaux, J. Catal., 2011, 277,
154–163.
[40] M. Wang, J. M. Lu, X. C. Zhang, L. H. Li, H. J. Li, N. C. Luo, F. Wang,
ACS Catal., 2016, 6, 6086–6090.
[41] N. Mizuno, M. Misono, Chem. Rev., 1998, 98, 199–218.
[42] J. J. Bozell, J. O. Hoberg, D. R. Dimmel, J. Wood Chem. Technol.,
2000, 20, 19–41.
[59] Y. Sun, W. Wang, J. W. Qin, D. Zhao, B. G. H. Mao, Y. Xiao, M. Cao,
Electrochim. Acta, 2016, 187, 329–339.
[60] A. P. Shpak, A. M. Korduban, M. M. Medvedskij, V. O. Kandyba, J.
Electron Spectrosc., 2007, 156–158, 172–175.
[43] H. W. Park, S. Park, D. R. Park, J. H. Choi, I. K. Song, Korean J. Chem.

1228
Xuezhong Wu et al. / Chinese Journal of Catalysis 38 (2017) 1216–1228
[61] D. Varisli, T. Dogu, G. Dogu, Chem. Eng. Sci., 2007, 62, 5349–5352.
[62] M. Iwamoto, Catal. Today, 2015, 242, 243–248.
[63] M. J. Gilkey, B. J. Xu, ACS Catal., 2016, 6, 1420–1436.
[64] F. A. Carey, R. J. Sundberg, Advanced Organic Chemistry, 5th ed.,
Springer, New York, 2007.
[65] P. Vollhardt, N. Schore, Organic Chemistry: Structure and Function,
W. H. Freeman, 2011.
[66] X. W. Sha, L. Chen, A. C. Cooper, G. P. Pez, H. S. Cheng, J. Phys. Chem.
C, 2009, 113, 11399–11407.
[70] M. Orfanopoulos, I. Smonou, Synth. Commun., 1988, 18, 833–839.
[71] A. D. Chowdhury, G. K. Lahiri, Chem. Commun., 2012, 48,
3448–3450.
[72] M. L. Sarazen, E. Doskocil, E. Iglesia, J. Catal., 2016, 344, 553–569.
[73] R. Neumann, Polyoxometalate Complexes in Organic Oxidation
Chemistry, Wiley, Weinheim, 1998.
[74] L. Chen, A. C. Cooper, G. P. Pez, H. S. Cheng, J. Phys. Chem. C, 2008,
112, 1755–1758.
[75] H. S. Cheng, L. Chen, A. C. Cooper, X. W. Sha, G. P. Pez, Energy Envi‐
ron. Sci., 2008, 1, 338–354.
[67] H. Hattori, Y. Ono, Solid Acid Catalysis: from Fundamentals to Ap‐
plications, CRC Press, Boca Raton, FL, 2015.
[76] A. I. Gavrilyuk, Appl. Surf. Sci., 2013, 273, 735–747.
[77] A. I. Gavrilyuk, M. M. Afanasiev, Sol. Energy Mater. Sol. Cells, 2009,
93, 280–288.
[68] M. Misono, Catal. Rev. Sci. Eng., 1987, 29, 269–321.
[69] D. S. Matharu, D. J. Morris, A. M. Kawamoto, G. J. Clarkson, M. Wills,
Org. Lett., 2005, 7, 5489–5491.
无水乙醇中β-O-4型木素模型物在铯取代的多氧金属盐上的降解:
酸性和氧化还原性的影响
吴学众a,b, 焦文千a, 李秉正b, 黎演明b, 张亚红a, 王全瑞c, 唐 颐a,*
a复旦大学化学系, 能原材料协同创新中心, 先进材料实验室, 上海分子催化与创新材料重点实验室, 上海200433
b广西科学院国家非粮生物质能源工程技术研究中心, 非粮生物质酶解国家重点实验室, 广西生物质工程技术研究中心,
广西生物质产业化工程院, 广西生物炼制重点实验室, 广西南宁530007
c复旦大学化学系, 上海200433
摘要: 随着化石能源的日益减少, 从木质生物质获得能源、燃料和化学品变得至关重要. 木素是木质生物质的第二大主要
组分, 但是目前远未得到充分利用. 随着对木素结构的充分认识和相关催化科学技术的发展, 由木素制得大宗燃料或精细
化学品, 特别是芳香类化合物显示出越来越具有技术和经济可行性. 由于木质素大分子中复杂的C–O和C–C连接, 先研究
模型物的断裂机理并同时考虑从木素模型物小分子迁移到木质素大分子的问题, 然后设计出合适的催化材料并开发出可
行的工艺过程, 这条技术路线看起来更具有可行性.
近年来, 几种均相或非均相多氧金属盐(Polyoxometalates (POMs), 或称杂多酸)用于降解木素或者木素模型物, 但是
β-O-4醚键断裂的氢解还是酸解机理及其竞争合作作用尚不清晰. 我们在几种多氧金属盐(POMs)的催化下研究了β-O-4模
型物2-phenoxyacetophenone (2-PAP)在以无水乙醇作为供氢溶剂体系下的催化断裂机理和行为. 结果表明, 随着无水乙醇
溶剂处理温度的提高, 溶剂的供氢能力增强. 酸性催化剂的加入提高了溶剂供氢能力. 原因是催化剂的酸性改变了乙醇自
氧化还原反应的平衡, 使平衡向生成乙醛并释放出活性氢的方向进行. 我们还发现, Cs-PMo的氧化还原性, 对促进活性氢
的释放起更大的作用. 2-PAP反应底物的加入消耗了活性氢, 从而促使乙醇自氧化还原平衡向右移动.
在酸性催化剂的作用下, 2-PAP的转化裂解可以按照氢转移机制或酸催化的氧鎓离子机制进行. 大部分转化反应按照
哪个机制进行, 取决于所采用体系的供氢能力和酸强度/数量的竞争关系, 大部分反应将屈从于占竞争优势的机制. 在强供
氢及转移能力占优势, 而酸强较低酸量较少时, 反应主要按氢转移机制进行. 在酸强很强且数量较多, 反应将主要按酸催
化氧鎓离子机制进行. Cs-PMo这个拥有酸性和强氧化还原性的双功能催化剂的使用, 既促进了活性氢的释放, 又增强了活
性氢的还原能力及转移能力, 因而导致了在极高转化率(>99%)的下极佳的选择性(98.6%苯酚和91.1%苯乙酮).
这些发现将对理解木质素中醚键的断裂结果和机理提供启示, 为设计开发出木质素选择性地催化裂解为芳香小分子
的可行的工业过程打下初步理论基础.
关键词: 木素模型物; β-O-4醚键; 多氧金属盐; 氢转移机理; 氧鎓离子机理; 无水乙醇
收稿日期: 2017-03-26. 接受日期: 2017-04-30. 出版日期: 2017-07-05.
*通讯联系人. 电话: (021)55664125; 传真: (021)65641740; 电子信箱: yitang@fudan.edu.cn
基金来源: 国家重点基础研究发展计划(973计划, 2013CB934101); 国家自然科学基金(21433002, 21573046); 中国博士后科学基
金(2016M601492); 广西科学研究与技术开发计划(15104001-5).
本文的英文电子版由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).