Wet aerobic oxidation of lignin into aromatic aldehydes catalysed by a perovskite-type oxide: LaFe1-xCuxO3 (x=0, 0.1, 0.2)

Article abstract of DOI:10.3390/molecules14082747

The perovskite-type oxide catalyst LaFe_1-xCu_xO ₃ (x=0, 0.1, 0.2) was prepared by the solgel method, and tested as a catalyst in the wet aerobic oxidation (WAO) of lignin into aromatic aldehydes. The lignin conversion and t

Full text of DOI:10.3390/molecules14082747

Molecules 2009, 14, 2747-2757; doi:10.3390/molecules14082747
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Wet Aerobic Oxidation of Lignin into Aromatic Aldehydes
Catalysed by a Perovskite-type Oxide: LaFe Cu O
1
-x
x
3
(
x=0, 0.1, 0.2)
1
2
1,
Junhua Zhang , Haibo Deng and Lu Lin *
1
Department of Resources Science and Engineering, State Key Laboratory of Pulp and Paper
Engineering, South China University of Technology, Guangzhou 510640, Guangdong Province,
China
2
College of Textile and Clothing, Jiangnan University, Wuxi 214122, Jiangsu Province, China
*
Author to whom correspondence should be addressed; E-mail: lclulin@scut.edu.cn
Received: 8 June 2009; in revised form: 7 July 2009 / Accepted: 23 July 2009 /
Published: 27 July 2009
x 3
Abstract: The perovskite-type oxide catalyst LaFe1-xCu O (x=0, 0.1, 0.2) was prepared by
the sol–gel method, and tested as a catalyst in the wet aerobic oxidation (WAO) of lignin
into aromatic aldehydes. The lignin conversion and the yield of each aromatic aldehyde
were significantly enhanced in the catalytic process, compared with the non-catalyzed
process. Moreover, it was shown that the stability of activity and structure of LaFe1-xCu
x 3
O
(
x=0, 0.1, 0.2) remained nearly unchanged after a series of successive recyclings of the
catalytic reactions, indicating it was an efficient and recyclable heterogeneous catalyst for
the conversion of lignin into aromatic aldehydes in the WAO process.
x 3
Keywords: perovskite-type oxide; LaFe1-xCu O ; heterogeneous; wet aerobic oxidation;
lignin; aromatic aldehydes
1
. Introduction
With the gradual diminishing of fossil fuel reserves and growing concerns about global warming,
finding ways of exploiting feasible pathways for the replacement of the petroleum-based chemicals is
highly desirable [1]. To meet the growing demand for this, biomass can serve as a sustainable source
of renewable fuels and chemicals. It was estimated that a sustainable production of 1.3 billion dry

Molecules 2009, 14
2748
tonnes of biomass per year can be achieved without significant changes in agricultural practices and
food supplies, as reported by the U.S. Department of Agriculture (USDA) and U.S. Department of
Energy (DOE) recently [2-3]. However, lignin, one of the main components of biomass in the bio-
refining process, is usually discarded as non-cellulosic waste. Lignin is an extremely complex three-
dimensional macromolecule with irregular structure, which results from random dehydrogenative
polymerization of phenylpropane building units, including coniferylic, sinapylic and р-coumarylic
alcohols (Figure 1), in the presence of peroxidase enzymes [4], and it can be converted into aromatic
aldehydes such as p-hydroxybenzaldehyde, vanillaldehyde and syringaldehyde (Figure 2). These
aromatic aldehydes have wide applications in flavoring, as chemical intermediates for pharmaceutical
drugs and agricultural pesticides [5-6]. So the study of the conversion of aromatic aldehydes from
lignin is of significant importance.
Figure 1. Lignin building units.
CH OH
CH OH
CH OH
2
2
2
CH
CH
CH
CH
CH
CH
OMe
MeO
OMe
OH
OH
OH
Figure 2. Aromatic aldehydes resulting from the lignin WAO process.
CHO
CHO
CHO
OMe
MeO
OMe
OH
OH
OH
syringaldehyde
p-hydroxybenzaldehyde
vanillaldehyde
A routine way to convert lignin into aromatic aldehydes is Wet Aerobic Oxidation (WAO) process,
which often uses oxygen and catalysts to increase the yields of aldehydes. Some noble metals have
been used as catalysts in the lignin WAO process [6-8], but they are expensive, which is an
impediment for their commercial applications. Alternatively, some inexpensive metal ions such as
iron, copper, and cobalt have also shown activity in the lignin oxidation process [9-13]. Nevertheless,
these homogeneous catalysts can lead to secondary pollution, result in high recycling costs, and
restricting their industrial utilization.
It is well known that perovskite-type oxides of general formula ABO have high activity and
3
stability in catalytic hydrocarbon oxidation processes, and they are a promising alternative to noble
metal catalysts for the wet hydrocarbon catalytic oxidation process [14-17]. Nevertheless, the activity
of perovskite-type oxides as catalysts in the WAO reaction has rarely been studied. To the best of our

Molecules 2009, 14
knowledge, only three recent studies deal with the activity of perovskite-type oxides La A BO
3
2749
1
-x
x
(
A=Sr, Ce; B=Co, Mn) for the WAO reaction [18-20], all claiming that this perovskite-type oxide
presented high activity for this reaction. However, metal ions such as Sr, Ce, Co and Mn can cause a
secondary pollution problem to a certain extent. For this reason, the WAO reaction of lignin is one
process where the demand for green chemistry and sustainable technology is stimulating the
replacement of these ‘‘toxic” metal ions by alternative “non-toxic” metal ions.
Iron (Fe) is the only transition metal which can be considered “non-toxic” to nature, so the study of
3
LaFeO as a catalyst in the catalytic WAO of lignin is an inviting prospect. However, in the past
decades, the effect of iron (Fe) on the hydrocarbon catalytic oxidation has rarely been studied due to
its poor oxidation ability. In this paper, Cu ion was loaded into LaFeO to partially replace the Fe ion
3
because the Cu ion has higher oxidation reaction activity [21-23], and a new catalyst: LaFe Cu O
3
1
-x
x
(
x=0, 0.1, 0.2) was thus obtained. The activity and stability of this species as a catalyst were tested in
x 3
the catalytic WAO of lignin into aromatic aldehydes, and LaFe1-xCu O (x=0, 0.1, 0.2) was also
characterized by XRD, TPR and XPS.
2
2
2
. Result and Discussion
.1. Catalyst characterization
.1.1. Surface oxygen species and Fe ion oxidation states
The O 1s, Fe 2p and Cu 2p XPS spectra of LaFe1-xCu
x
3
O (x=0, 0.1, 0.2) are depicted in Figure 3. It
can be seen from the O 1s spectrum (Figure 3a) that two contributions could be obtained by curve
2
-
fitting of O 1s, which are assigned to lattice oxygen in the form of O (metal oxygen bond) and
-
-
2-
adsorbed oxygen species (such as O , O
5]. From the integral of O 1s spectrum, it can be found that the content of oxygen increased as the x
value of LaFe1-xCu increased (x=0, 0.1, 0.2), indicating that the absorption ability of oxygen
2 2
or O ), respectively, according to the prior literature [24-
2
x
O
3
increases as the content of copper ion increases, a result similar to that obtained by Zhang et al. [26].
Moreover, in Figure 3(b) the appearance of two signals can be observed, which belong to Fe 2p3/2 and
3
+
Fe 2p1/2, respectively, indicating that there are Fe ions in the catalyst, as indicated in [27].
Furthermore, two signals at BE=930-935 and BE=940-955 ev can be also observed from Figure 3(c),
2
+
x 3
which belong to Cu according to the literature [28]. All these indicated that the LaFe1-xCu O (x=0,
0
.1, 0.2) catalyst was a typical perovskite-type oxide.
2.1.2. TPR profile of LaFe1-xCu
x
O
3
(x=0, 0.1, 0.2)
The reducibility of LaFe1-xCu
LaFe1-xCu
x
O (x=0, 0.1, 0.2) samples is depicted in Figure 4. An obvious
3
x
O
3
(x=0, 0.1, 0.2) reducing peak can be observed as the x value decreases from 0.2 to 0,
2
+
which is attributed to the reduction of Cu according to the literature cited [29-31], indicating that the
Cu has attached to the LaFe Cu O (x=0, 0.1, 0.2) catalyst as the x value increased.
1
-x
x
3

Molecules 2009, 14
2750
Figure 3. The XPS spectra of LaFe Cu O (x=0, 0.1, 0.2) (a. O 1s; b. Fe 2p; c. Cu 2p).
1
-x
x
3
a
b
c
LaFe Cu O
.8 0.2 3
0
LaFe Cu O
.8 0.2 3
0
LaFe Cu O
.8 0.2 3
0
LaFe Cu O
.9 0.1 3
0
LaFe Cu O
.9 0.1 3
0
LaFe Cu O
0.9 0.1
3
LaFeO3
LaFeO3
538 536 534 532 530 528 526 524
730
725
720
715
710
705
950
945
940
935
930
925
Bi n d i n g e r n e r g y , ev
Bi n d i n g e r n e r g y , ev
Bi n d i n g e r n e r g y , ev
Figure 4. TPR profile of the catalysts LaFe1-xCu
x
O
3
(x=0, 0.1, 0.2).
LaFe Cu O
0
.8
0.2
3
LaFe Cu O
0
.9
0.1
3
LaFeO3
0
150
300
450
600
750
900
Temperture, °C
.2. Catalytic oxidation lignin into aromatic aldehydes
The aromatic aldehydes obtained from lignin in the catalyzed WAO process can be further oxidized
2
to aromatic acids, and even to carbon dioxide and water under certain conditions. In our experiment,
the catalytic activity was evaluated with the rate of lignin conversion and the yield of aldehydes, which
was shown in Figure 5. It can be seen from Figure 5(a) that the lignin conversion was 41.8% at
3
.0 hours in the absence of a catalyst, and the conversion gradually increased when LaFeO3,
were added, respectively. The maximum lignin conversion (66.6%)
could be reached after 3.0 hours when lignin oxidation was catalysed by LaFe0.8Cu0.2 , which was
.59 times the yield obtained in the non-catalytic process. Similar concentrations of LaCl and FeCl
did not present any catalytic activity under these conditions.
3 3
LaFe0.9Cu0.1O and LaFe0.8Cu0.2O
O
3
1
3
3

Molecules 2009, 14
2751
Figure 5. Lignin conversion (a), yield of p-hydroxybenzaldehyde (b), vanillaldehyde (c)
and syringaldehyde (d) with reaction time (the reaction conditions were as follows: NaOH
(
2 mol/L), 120 °C, 5 bar partial pressure of oxygen in 20 bar total pressure,
3
C =60.00 kg/m , C
catalyst 0 t 0 0
=3 g/L and the lignin conversion=(C -C )/C , where C is the
L0
initial concentration of lignin, and C
t
is the concentration of lignin at any reaction time).
70
2.8
b
60
2.4
50
2.0
40
1.6
LaCl
3
3
0
non-catalyst
LaCl3
1.2
0.8
0.4
FeCl
3
20
FeCl3
LaFeO
3
LaFeO3
LaFe Cu O3
.9 0.1
0
10
LaFe0.9Cu0.1O3
LaFe Cu O3
.8 0.2
0
LaFe0.8Cu0.2O3
Non-Catalyst
0
0.0
0
20 40 60 80 100 120 140 160 180 200
0
20 40 60 80 100 120 140 160 180 200
Treatment time, min
Treatment time, min
1
4
4
.8
LaCl
3
d
c
FeCl
12
3
4
.4
.0
.6
.2
.8
.4
.0
LaFeO
3
10
LaFe Cu
O
3
4
0.9 0.1
LaFe Cu
O
3
0.8 0.2
8
6
4
2
0
3
Non-Catalyst
3
LaFeO
3
2
LaFe Cu
O
0.9 0.1
3
LaCl
FeCl
2
3
LaFe Cu
O
0.8 0.2
3
3
Non-Catalyst
2
0
20 40 60 80 100 120 140 160 180 200
0
20 40 60 80 100 120 140 160 180 200
Treatment time, min
Treatment time, min
The yields of p-hydroxybenzaldehyde, vanillaldehyde and syringaldehyde are shown in Figures
(b-d). The same results were obtained when LaCl , FeCl , LaFeO
5
3
3
3 3 3
, LaFe0.9Cu0.1O and LaFe0.8Cu0.2O
were added, respectively, and the yields increased gradually. The maximum yields of p-hydroxy-
benzaldehyde, vanillaldehyde and syringaldehyde were 2.49% (at 120 min), 4.56% (at 60 min) and
1
3
1.51% (at 30 min) in the LaFe0.8Cu0.2O catalytic process, which were 1.66, 1.42 and 2.51 times those
obtained in the non-catalytic process, respectively. All results indicate that the catalyst activity will be
enhanced by an increase in Cu content.
Although it is premature to discuss the precise role of the perovskite oxide LaFe1-xCu
x=0, 0.1, 0.2) in the catalytic mechanism of the lignin oxidation process at present, it is noteworthy
that two aspects can account for the activity of LaFe1-xCu (x=0, 0.1, 0.2) in the CWAO as reported
-Fe(surf) -lignin intermediate was generated on the surface of LaFe1-
(x=0, 0.1, 0.2), which will enhance the oxygen contact of lignin and accelerate the generation
x 3
O
(
x
O
3
3
+
[
18]: On the one hand, an O
2
x
Cu
x 3
O
of the intermediate of quinine methide radical: (3) or (3a); On the other hand, the oxygen space will be
3
+
2+
enhanced with the partial replacement of Fe by Cu , according to the report of Zhang [17], which
would accelerate the oxygen surface absorption ability of the catalyst and the intermediate content of

Molecules 2009, 14
2752
3
+
2+
.-
O -Fe(surf) -lignin will be enhanced. Moreover, the amount of activated species Cu
O will be
2
(surf)
2
3
+
2+
2+
increased with the partial replacement of Fe by Cu , which will result a cycling of Cu(surf)
→
+
2+
.-
2+
Cu(surf) →Cu(surf)
2
O →Cu(surf) and accelerate the generation of the intermediate quinone methide
radical: (3) or (3a) and quinone methide hydroperoxide : (4). With the combined effect of the above
mentioned factors, the catalytic activity of LaFe Cu O (x=0, 0.1, 0.2) is improved. The proposed
1
-x
x
3
x 3
mechanism of oxidation of lignin catalysted by LaFe1-xCu O (x=0, 0.1, 0.2) was depicted in Figure 6.
Figure 6. Proposed mechanism of oxidation of lignin in the LaFe Cu O (x=0, 0.1, 0.2)
1
-x
x
3
catalyst process.
+O2
Fe(surf)3+-lignin
R
C
C
O -Fe(surf)3+-lignin
2
R
C
C
R
C
C
-
O2
OH-
R1
R2
O-
R1
R1
-e-
R2
R2
-
-
O
O
(3)
(
1)
(2)
Phenolate anion
Phenolate anion
R , R =H or OMe
Cu(surf)2+
Cu(surf)+
1
2
R=H, Alk, Ar or OAr
+
e-
O2-
-
e- O2
-
R
C
R
C
C
R
R
Cu(surf)2+
Cu(surf)2+O
2
-
O
-O
O
O
C
C
O C H
-O C H
CHO
O-
C
+
O2-
R1
R2
R1
R2
R1
R2
R
R1
R2
1
O
(3a)
R2
O
-
(7)
O
O
(4)
(
5)
Aldehyde ions
(
6)
Quinone methide
hydroperoxide
Quinone methide
radical
Dioxethane
Intermediate
The catalytic stability of LaFeO
3
was investigated by repeatedly using LaFeO
3
for the lignin
conversion under the same reaction conditions; the lignin conversion and yield of aromatic aldehydes
from the non-catalyzed process and five successive runs is shown in Figure 7, where it can be seen that
the lignin conversion and the yield of each aromatic aldehyde were significantly higher in each of
successive catalyst run than the non-catalyzed process. Furthermore, it was clearly demonstrated that
the lignin conversion and the yield of aromatic aldehydes remained nearly the same in each run, which
indicated that the catalyst has better recovery ability.

Molecules 2009, 14
2753
Figure 7. Lignin conversion and yield of aromatic aldehydes in the absence of catalyst
process 0) and five successive runs of LaFeO for 30 minutes (other reaction conditions as
(
3
indicated in Figure 5).
30
25
20
15
10
5
0
p-hydroxybenzaldehyde
vanillin
syringaldehyde
lignin conversion
0
1
2
3
4
5
Reaction Cycle,Time
The crystalline phases of the catalysts before and after use were also determined by X-ray
diffraction, which is depicted in Figure 8. The XRD patterns of the LaFeO
and LaFeO which were used five times all showed characteristic reflections for the
perovskite-type oxide without other phases. No obvious changes can be seen in the structure of the
LaFeO catalyst used five times, which suggested that the LaFeO catalyst is stable in the
CWAO process.
3
3
, LaFe0.9Cu0.1O ,
LaFe0.8Cu0.2
O
3
3
3
3
3 3 3 3
Figure 8. XRD patterns of LaFeO , LaFe0.9Cu0.1O , LaFe0.8Cu0.2O and LaFeO which was
used five times.
LaFe Cu
O
0
.8 0.2 3
LaFe Cu
O
0
.9 0.1 3
LaFeO
3
LaFeO (Spent 5 times)
3
20
30
40
50
60
70
80
ο
2θ /
3
. Experimental
.1. Lignin and catalyst preparation
3
Lignin was obtained through the process of enzymatic hydrolysis of steam-explosion cornstalks
32]. The LaFe1-xCu (x=0, 0.1, 0.2) catalyst was prepared from citrate precursors [18,20,33]: a
concentrated solution of metal nitrates was mixed with an aqueous solution of citric acid, and the
[
x 3
O

Molecules 2009, 14
2754
molar ratio of citric acid to metal cations was fixed at 1.5. Water was evaporated from the solution at
8
0 °C until a viscous gel was obtained. The gel was kept at 100 °C overnight, and then calcined at
00 °C for 6 h.
7
3
.2. Catalyst characterization [20]
X-ray diffraction (XRD) measurements were performed on a Rigaku powder diffractometer
(
Rigaku, Japan) with CuK radiation. The tube voltage was 45 kV, and the current was 40 mA. The
α
XRD diffraction patterns were taken over 2 h in a range of 20° to 80° at a scan speed of 2°/min.
Temperature programmed reduction (TPR) of H was performed in a Micromeritics Autochem 2910
2
equipped with a thermal conductivity detector (TCD). The sample (50 mg) was pretreated in 10%
3
O /He flow at 800 °C for 30 min, and then it was reduced with 5 % H /Ar (30 cm /min) heating 5 °C
2
2
/
min from room temperature to 850 °C.
The X-ray photoelectron spectroscopy (XPS) analysis was performed on a Kratos Axis Ultra system
with 0.1 eV per step for detail scan and the binding energies for each spectrum were calibrated with a
C1s spectrum of 284.6 eV. The core levels of O 1s, Fe 2p and Cu 2p species were recorded and their
relative intensities determined.
3
.3. Catalytic experiments [20]
The non-catalyzed WAO and catalyzed WAO processes of lignin were carried out in a high-
pressure SS-316 Parr slurry reactor (model 4843) at 120 ± 1 °C. A 500 mL alkaline solution
NaOH, 2 mol/L) of lignin dissolved at a concentration of 60 g/L was introduced into the reactor and a
(
certain quantity of catalysts was added to the solution if necessary. The heating program was started
under a slight nitrogen pressure. When the solution in the reactor reached the desired temperature,
nitrogen was added until a total pressure of 15 bars was attained, and then, the oxygen was added until
the total pressure attained 20 bars. The pressure in the reactor was kept at 20 bars by continuous
flushing of oxygen as a supplement because of its consumption during the reaction. Time was recorded
from zero. During each reaction, sampling was conducted from the reactor and filtrated, and the
filtration was acidified to pH 2-3 with a dilute HCl solution. The resulting products were extracted
with chloroform until the chloroform layer appeared colorless. The residual lignin was obtained by
centrifugation of the suspensions after the extraction of resulted products.
The contents of p-hydroxybenzaldehyde, vanillaldehyde and syringaldehyde in the chloroform
extracts were analyzed by high performance liquid chromatography with a Hypersil ODS
2
column
(
4.6 mm × 250 mm) and a UV detector set at 280 nm, and a mixture of acetonitrile (10%), deionized
water (90%) and acetic acid (1.5%) as the mobile phase. The non-converted lignin was diluted in a
sodium hydroxide solution to dissolve the lignin and a UV spectrophotometer used to determine the
content of the non-converted lignin at a wavelength of 280 nm [34].
The experiment to test the recycling ability of LaFeO was performed as follows. Fresh catalyst and
3
lignin solution were added into the reactor and the process was performed under the same conditions
as mentioned above for 30 min. After the catalyzed WAO reaction, the reactor temperature was
quickly cooled to room temperature, and the resulting products were carefully poured out and filtered.
The catalyst was left in the reactor, and fresh lignin solution was added, then the process was

Molecules 2009, 14
2755
performed again under the same conditions for 30 minutes. This procedure was repeated for four times,
and the contents of p-hydroxybenzaldehyde, vanillaldehyde, syringaldehyde and lignin were analyzed
using the above methods.
4
. Conclusions
The perovskite-type oxide catalyst of LaFe Cu O (x=0, 0.1, 0.2) prepared by the sol–gel method
1
-x
x
3
exhibited high activity in the catalyzed WAO of lignin. Compared with the non-catalyzed process, the
lignin conversion and yield of each aromatic aldehyde were improved significantly in the catalyzed
x 3
process. It was shown that the perovskite-type oxide catalyst of LaFe1-xCu O (x=0, 0.1, 0.2) also
possesses distinctive stability of activity and structure in the catalyzed WAO of lignin, and is thus an
efficient and recyclable heterogeneous catalyst for the conversion of lignin in the catalyzed WAO
process of lignin.
Acknowledgements
The authors are grateful for the financial support from the Natural Science Foundation of China
(
50776035, U0733001), the Foundation of Scientific Research for Universities (20070561038) and the
Initiative Group Research Project (IRT0552) from the Ministry of Education of China, the National
High Technology Project (project 863) (2007AA05Z408) and the National Key R&D Program
(
2007BAD34B01) and National Basic Key Research Program (2010CB732201) from the Ministry of
Science and Technology of China.
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Sample Availability: Samples of the compounds are available from the authors.
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