Salen copper (II) complex encapsulated in y zeolite: An effective heterogeneous catalyst for TCF pulp bleaching using peracetic acid

Article abstract of DOI:10.1016/j.molcata.2012.08.010

Cu(salen) and NaY encapsulated complexes, were synthesized by the "impregnation" (Cu(salen)/Y-IM) and flexible ligand "ship-in-a-botttle" (Cu(salen)/Y-SB) method. The resulting compounds were characterized by XRD, FTIR, DR UV-vis, BET. The encapsulated co

Full text of DOI:10.1016/j.molcata.2012.08.010

Journal of Molecular Catalysis A: Chemical 365 (2012) 66–72
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Salen copper (II) complex encapsulated in Y zeolite: An effective heterogeneous
catalyst for TCF pulp bleaching using peracetic acid
∗
Na Zhang, Xue-Fei Zhou
Kunming University of Science and Technology, P.O. Box A302-12, Building No. 5, Xinying Yuan, No. 50, Huancheng East Road, Kunming, 650051 Yunnan Province, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 July 2012
Received in revised form 10 August 2012
Accepted 15 August 2012
Available online 24 August 2012
Cu(salen) and NaY encapsulated complexes, were synthesized by the “impregnation” (Cu(salen)/Y-IM)
and flexible ligand “ship-in-a-botttle” (Cu(salen)/Y-SB) method. The resulting compounds were char-
acterized by XRD, FTIR, DR UV–vis, BET. The encapsulated complexes show structural properties and
chemical behaviour which is different from those of the neat complex. The catalytic bleaching by Cu(salen)
and encapsulated Cu(salen) was tested for the eucalyptus oxygen delignified kraft pulp using peracetic
acid as an oxidant. Influence of catalysis on TCF (total chlorine free) bleaching viz. delignification, property
of bleached pulp was studied. Under the catalytic conditions, the encapsulated copper complexes exhibit
enhanced activity or selectivity compared to the neat complex. The reasons for the enhanced activity of
Cu(salen) upon encapsulation in NaY are reported by UV–vis techniques. The catalytic performance of
encapsulated complex could be attributed to the formation of copper–hydroperoxide species.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
TCF bleaching
Catalyst
Cu(salen)
Encapsulated Cu(salen)
Peracetic acid
1
. Introduction
The bleaching of pulp is the chemical processing carried out on
pulp and paper industry has directed bleaching practices to mini-
mize the use of chlorine and chlorine compounds in order to satisfy
regulatory and market demands. From the 1990 onwards the use
of elemental chlorine in the delignification process was substan-
tially reduced and replaced with ECF (elemental chlorine free) and
TCF (total chlorine free) bleaching processes. ECF bleaching can
substantially reduce but not fully eliminate chlorinated organic
compounds, including dioxins, from effluent. TCF bleaching, by
removing chlorine from the process, reduces chlorinated organic
compounds to background levels in pulp mill effluent.
At industrial scale, TCF delignification is performed by oxygen,
hydrogen peroxide, ozone and peracetic acid. Hydrogen peroxide is
the most commonly used bleaching agent for pulp. Peracetic acid
looks attractive when quantity of electrons exchanged to obtain
a one unit kappa drop in the most common bleaching stages are
various types of pulp to decrease the colour of the pulp, so that
it becomes whiter. Bleaching of pulp is extremely important and
useful reaction in both chemical and pulp industries [1].
Lignin is a complex and heterogeneous mixture of poly-
mers resulting from the oxidative combinatorial coupling of
4
-hydroxyphenylpropanoids (Figs. 1 and 2). The main building
blocks of lignin are the hydroxycinnamyl alcohols (or monolignols),
coniferyl alcohol and sinapyl alcohol, with typically minor amounts
of p-coumaryl alcohol. Lignin is the main source of colour in pulp
due to the presence of a variety of chromophores naturally present
in the wood or created in the pulp mill [2].
Chemical pulps (kraft and sulphite) contain residual lignin
less than 5%. Subsequent bleaching is required to remove essen-
tially all of the residual lignin from the pulp to meet brightness
requirements. However, conventional bleaching has the potential
to cause large amounts of chlorinated organic compounds includ-
ing chlorinated dioxins into waterways when chlorine and chlorine
compounds are used in the pulp bleaching process. Dioxins are
recognized as a regulated internationally persistent environmen-
tal pollutant. Dioxins are highly toxic, and health effects on humans
include reproductive, developmental, immune and hormonal prob-
lems. They are known to be carcinogenic [3]. For this reason, the
considered. The exchange of electrons produced by CH COOOH
3
is efficient and lignin reactivity does not decrease during delig-
nification to the same extent as for H O . It is also shown that
2
2
when compared to chlorine, chlorine dioxide and ozone for the
same kappa number drop, with oxygen, hydrogen peroxide and
peracetic acid much more electrons have to be exchanged and,
most of these reported TCF bleaching processes are either not
enough effective or not enough selective [4–6]. The effective degra-
dation of lignin and its selective removal from the carbohydrate
component of wood is a key process in the pulp and paper indus-
try [7], which justifies the search for catalysts. Therefore, the
development of an effective, selective, and robust bleaching agent
remains a major challenge in pulp bleaching processes. Alterna-
tive pathways available for pulp bleaching also include the use of
∗ Corresponding author.
E-mail address: lgdx602@tom.com (X.-F. Zhou).
1
381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.08.010

N. Zhang, X.-F. Zhou / Journal of Molecular Catalysis A: Chemical 365 (2012) 66–72
67
OH
OH
1
)
H O
+
2
H+
+
-
,
- e-
2) - H
coupling
.
Softwood lignin
R2
R1
R1
R2
OH
O
R =R =H, coumaryl alcohol
1
2
R =OMe, R =H, coniferyl alcohol
1
2
R =R =OMe, sinapyl alcohol
1
2
Fig. 1. Formation of softwood lignin via oxidative enzymatic coupling of para-hydroxycinnamyl alcohols.
white-rot fungi and enzymatic treatment associated with the sub-
sequent incorporation into bleach sequences, it may be related to
replacement of conventional polluting chlorine-based procedures.
In principle, the use of an enzymatic system for delignification
is not economically convenient with respect to simpler catalysts,
due to the costs of purification. Moreover, since ligninase is sensi-
tive to peroxide excess, its practical utilization in pulp and paper
is difficult to develop. Hence the need for the design of suitable
ligninase models have received considerable attention as alterna-
tives to ligninolytic enzymes, since they could be more economical
and easier to use. These biomimetic systems are also helpful for
the understanding the mechanisms of complex lignin degradation
model phenolic dimer 1-(4-hydroxy-3-methoxyphenoxy)-2-(2-
methoxyphenoxy)-propane-1,3-diol using Co(salen)/SBA-15
a
based heterogeneous catalytic system in the presence of hydrogen
peroxide as oxidant. The heterogeneously catalysed reaction in the
presence of Co(salen)/SBA-15 gave a remarkably superior TON and
therefore much increased conversion to 2-methoxyphenol as com-
pared to the homogeneous Co(salen). From this and related studies,
molecular sieve based heterogeneous catalytic systems appear to
be a promising alternative for the selective degradation of lignin
structures.
In our previous work on salen complexes we have reported the
catalytic oxidation of lignin, lignin model compound and pulp in the
presence of H O and O [15,16]. Our observations reveal that salen
[
8–10]. In this respect, homogeneous metal schiff base complexes
2
2
2
ꢀ
of N,N -ethylenebis(salicylidenaminate) (referred to as salen) have
been widely studied as functional mimics of metalloproteins in the
field of catalysis in dioxygen binding and oxidation reactions.
However, these reported complexes often deactive with time
due to easy formation of dimeric oxo- and peroxo-bridged
species. Therefore, the heterogenization of the homogeneous
complexes remains a major challenge in lignin degradation
processes to increase their stability and the synergistic interac-
tion between the complexes and the support [11–13]. Badamali
et al. [14] have studied the oxidation behaviour of a lignin
complex-oxidant is desirable for lignin oxidation and pulp deligni-
fication compared to the reported enzymatic catalysts, in that, it is
efficient even at extreme conditions without causing unacceptable
damage to pulp fibers. These results encouraged us to extend our
studies to the encapsulation of salen complexes and their uses in
field of lignin degradation and pulp bleaching.
In the present work, in order to improve the cost-effectiveness
and performance of catalysts “Impregnation method” (IM) and
“ship-in-a-bottle method” (SB) have been developed for the immo-
bilization of catalyst in and on solid supports. The project described
here makes use of zeolite Y (NaY) as porous support, and one type
of oxygen-activating catalyst, copper salen [Cu(salen)], is synthe-
sized inside the pores and trapped there irreversibly. Cu(salen)
complex encapsulated into molecular sieves with suitable pore
sizes such as zeolite Y, via covalent or ionic bonding, is expected
to be as active as that present in metaloenzyme. Furthermore, the
hybrid complex formed is structurally and thermally more stable,
remain unchanged during reactions and gives higher conversions.
HO
OCH3
-O
O-
OH
OH
CH O
CH O
3
3
O
The lignin and CH COOOH are able to diffuse through the pore
3
OCH3 CH O
O
3
channels and react with the catalyst and the products formed can
subsequently diffuse out (Fig. 3).
HO
O
OH
O
O
O
OH
We wish to report the delignification of pulp bleaching with
OCH3
OH
OH
two NaY-encapsulated Cu(salen) in the presence of CH COOOH.
3
-O
OH
OCH3
OH
OH
O
CH O
O
3
HO
O
O
CH O
CH COOH, O2
CH COOOH
3
3
HO
O
OCH3
O
N
O
N
3
O
Cu
OH
O
O
sugar
O
Lignin
LigninOx
O
OH
CH O
OH
OCH3
3
OH
Fig. 3. Zeolite-encapsulated copper salen as an oxidation catalyst. Reagents
CH3COOOH and lignin can diffuse into the zeolite pores and react with the catalyst.
After the reaction, the products diffuse out.
Fig. 2. Partial structural representation of softwood lignin.

6
8
N. Zhang, X.-F. Zhou / Journal of Molecular Catalysis A: Chemical 365 (2012) 66–72
Very little is known on the catalytic delignification of pulp with
encapsulated salen complex, which was suggested as a promising
catalyst for the biomimetic bleaching of pulp. In addition, in spite
of appreciable progress in recent years, there are very few which
mixture was refluxed for 60 min. The precipitate obtained was fil-
tered out, washed with methanol, vacuum dried and characterized.
2.3.3. Preparation of Cu/Y
use CH COOOH as a source of oxygen in the organic C H oxidation
Cu/Y was obtained by ion-exchanging NaY with Cu2+ ions
at room temperature in an aqueous solution of copper acetate
3
by these catalysts [17,18].
(0.08 mol/L) with a liquid/solid ratio of 20 (mL/g).
2
. Experimental
2.3.4. Preparation of Cu(salen)/Y-IM
The Cu(salen) was supported on the NaY by impregnation:
50 mg of Cu(salen) complex was dissolved in t-butanol and added
2.1. Materials
1
to 300 mg of the NaY molecular sieve. The amount of solvent was
just enough to cover the mixture. Then refluxed for 6 h under nitro-
gen gas flow. The resulting solid after filtering was extracted with
t-butanol and acetonitrile using soxhlet extractor to the colourless,
if any. Then the solid was allowed to vacuum dry at 110 C to obtain
the final encapsulated complex.
All chemicals are commercially availabe and were used as
received without further purification, unless otherwise noted.
The raw material for pulping was hybrid eucalyptus chips (E.
urophylla × E. grandis) supplied by Yunjing Forestry Development
Co. Ltd. of Yunnan Province. The chips were cooked in a laboratory
digester using kraft pulping method. Oxygen delignification was
performed in a pressure reactor. The pulp was washed exhaustively
with distilled water and finally with deionized water. The charac-
teristics of kraft pulp (kappa value 20.52) and oxygen delignified
kraft pulp (kappa value 13.05) were given in Table 2. The oxygen
delignified kraft pulp was used for the catalytic TCF bleaching.
◦
2.3.5. Preparation of Cu(salen)/Y-SB
CuY was mixed well with excessive salen ligand (lig-
and/metal = 3, M/M), and sealed into a round flask. Then the mixture
◦
was heated for 24 h at 150 C under high vacuum condition. Uncom-
plexed ligand and complex were removed by extraction with
acetone, and uncoordinated Cu2+ ions by ion-exchange with NaCl
aqueous solution (0.1 mol/L). The sample was further washed thor-
oughly with deionized water, and air-dried for 60 min to give the
Cu(salen)/Y-SB.
2
.2. Instrumentation
X-ray power diffraction (XRD) patterns of the samples were
recorded on a Rigaku Dmax X-ray diffractometer (Ni-filtered, CuKa
radiation). The Fourier transform infrared (FTIR) spectra were
recorded on Equinox 55 FT-IR spectrometer in KBr. Atomic absorp-
tion spectrometer, Shimadzu AA-6800 was used for the estimation
of copper. Diffuse reflectance (DR) UV–vis measurements in the
range of 200–800 nm were performed on a Perkin-Elmer Lambda
Bio40 spectrophotometer equipped with an integration sphere.
The surface area (by BET method) was determined by adsorption
and desorption of nitrogen at the temperature of liquid nitrogen
2.4. Catalytic TCF bleaching trials
In this paper, catalytic TCF bleaching sequences consisting of
four stages included a treatment with catalytic agent and several
bleaching stages such as alkaline extraction stage with hydrogen
peroxide addition and two hydrogen peroxide stages, designated
Cat, Ep, P1 and P , respectively. After each stage the pulp was
2
washed thoroughly with deionized water in a Büchner funnel and
then pressed to 30% consistency.
(
77 K) using volumetric adsorption set-up (Micromeritics ASAP-
2
020, USA).
A series of catalytic bleaching trials (Cat) were performed using
◦
Cu(salen), Cu(salen)/Y-IM and Cu(salen)/Y-SB for 120 min at 70 C:
2.3. Preparation of Cu(salen)/Y-IM and Cu(salen)/Y-SB
oxygen delignified eucalyptus kraft pulp (15 g, o.d.p.), obtained as
a 30% consistency, was combined with catalyst (0.03% on o.d.p.),
Cu(salen) complex was encapsulated in NaY by the “impreg-
CH COOOH (0.5% on o.d.p.), and some amount of deionized water
3
nation method” (IM) and “ship-in-a-bottle method” (SB). The
encapsulated catalyst is designated by the following notation:
to obtain a 5% pulp consistency. Deionized water was replaced by
addition of methanol to evaluate its effect on catalysis. Control
experiments were carried out also under identical conditions to
examine the ability of catalyst.
[
(complex)/(zeolite)-(encapsulation method)]. Thus Cu(salen)/Y-
IM designates a NaY zeolite containing Cu(salen) encapsulated in
its voids by the impregnation method, and Cu(salen)/Y-SB by ship-
in-a-bottle method.
The preparation of the Cu(salen) and the following immobiliza-
tions on NaY via “IM” and “SB” process are depicted in Scheme 1.
Alkaline extraction with 0.5% (on o.d.p.) addition of H O2 (Ep)
2
◦
was performed by heating the pulp at a consistency of 10% at 70 C
for 60 min in 0.0375 M NaOH and using MgSO4 (0.1% on o.d.p.) and
DTPA (0.05% on o.d.p.) as stabilizer at this stage.
The consistency of pulp in P1 and P2 stage was maintained at
a value of 10%. NaOH, MgSO , Na SiO , DTPA, H O were used as
2
.3.1. Preparation of salen
N,N -bis(salicylidene)-ethylenediamine (salen) was synthe-
4
2
3
2
2
ꢀ
bleaching chemicals. P -stage: NaOH 1.5% (on o.d.p.), MgSO4 0.1%
1
sized by dropwise addition of ethylenediamine (4.5 mmol), to a
methanolic solution (18 mL) of salicylaldehyde (9 mmol). The reac-
tion mixture was stirred for 30 min in a ice–water bath with a
condenser. After standing for 15 min, the yellow precipitate of salen
was formed which was filtered out, washed with petroleum ether,
dried and characterized. The purity of the ligand was confirmed by
IR and 1H NMR before coordination to Cu(II) cation.
(
2
on o.d.p.), Na SiO3 1.0% (on o.d.p.), DTPA 0.1% (on o.d.p.), H O
2
2
2
◦
.0% (on o.d.p.), and 80 C for 150 min; P -stage: NaOH 1.0% (on
2
o.d.p.), MgSO4 0.05% (on o.d.p.), DTPA 0.1% (on o.d.p.), H O 1.5%
2
2
◦
(
on o.d.p.), and 80 C for 90 min.
All experiments were run at least three. The relative error in the
parallel experiments was lower than the standard level.
2.5. Analytical methods of pulp
2
.3.2. Preparation of Cu(salen)
Cu(salen) (N,N -bis(salicylidene)-ethylenediamine copper salt)
ꢀ
Kappa numbers of pulp were determined using TAPPI Test
was prepared by the following method: 1.16 g of salen was dis-
solved in 50 mL of hot methanol, followed by the addition of
Method T 236 om-06. The kappa number of a pulp is a standard
pulp and paper industry index of its lignin content. Pulp viscosity
(T 230 om-08), a measure of its strength properties depending on
1
.08 g Cu(CH COO) ·H O in 6.7 mL of hot distilled water. Then the
3
2
2

N. Zhang, X.-F. Zhou / Journal of Molecular Catalysis A: Chemical 365 (2012) 66–72
69
NH CH CH NH
2
Cu(AcO)2
CHO
OH
2
2
2
OH HO
O
N
O
N
CH OH ¦¤
CH OH ¦¤
,
3
,
3
Cu
N
N
Cu(AcO)2
NaY
CuY
CuY ¦¤
NaY ¦¤
t-butanol
2
H O
,
¦¤
O
N
O
N
O
Cu
N
O
N
Cu
Si
O
Si
O
Si
O
Si
O
O
O
O
O
O
O
O
O
Cu(salen)/-IM
Cu(salen)/-SB
Scheme 1. The preparation of the Cu(salen) and the immobilizations on NaY.
molecular mass, were obtained using capillary viscometer method.
Brightness of pulp (T452) was measured for samples obtained in
each bleaching sequence. Sample sheets (T 205 sp-06) for physical
tests of pulp were formed by means of a handsheet former. The
Nevertheless, compared with free Cu(salen), a small shift to long
wavelength was observed in the DR UV–vis spectra of the encapsu-
lated Cu complexes, the band at 320 nm for the neat complex shifts
to 350 nm for the encapsulated complex and 440–480 nm. This
can be explained by the small change in metal–ligand geometry
of Cu(salen) due to encapsulation in NaY [21,22].
2
handsheet weight varied from 60 to 65 g/m . Physical properties (T
2
20 sp-10) of pulp handsheets were measured for tensile, tear and
burst index.
The UV–vis spectra of the neat and encapsulated Cu(salen) were
recorded in CHCl3 due to the low solubility of the complexes in
other organic solvents. The complexes spectra were similar in the
position and intensity of the bands characteristic to free complex,
further confirming the presence and change of Cu(salen) in NaY
3
3
3
. Results and discussion
.1. Characterization of the catalyst
(Fig. 5).
.1.1. Estimation of metal contents
3.1.3. FTIR
The analytical data of catalysts, viz. NaY, Cu(salen), Cu(salen)/Y-
The infrared spectra are depicted in Fig. 6. The peaks at
IM and Cu(salen)/Y-SB obtained at different stages of synthesis
of catalyst, are given in Table 1. The colours of NaY, Cu(salen),
Cu(salen)/Y-IM and Cu(salen)/Y-SB compounds were different due
to change in copper content, which was determined by atomic
absorption analysis. There was no copper in NaY and its colour was
white. The colour of Cu(salen) was blue due to the presence of cop-
per by 19.00% while that of Cu(salen)/Y-IM and Cu(salen)/Y-SB was
green due to the presence of Cu(salen) in which copper was 1.88%,
−
1
3
500 cm
in the spectrum of samples confirm the presence of
1
.21%. The colour of the complexes changed upon encapsulation in
NaY.
The amount of copper is higher in sample obtained by “IM”
rather than the “SB” method due to the low, but finite, solubility
of the copper salen in the synthesis medium. A another probable
explanation is that the probability of retention of the salen com-
plex inside the supercage during the synthesis is higher for complex
obtained by “IM”.
The encapsulation of Cu(salen) led to a decrease of surface area
2
2
(
5
SBET) of NaY from 584.38 m /g to 270.10 m /g (Cu(salen)/Y-IM),
2
81.77 m /g (Cu(salen)/Y-SB) since the complex molecules occu-
pied part of supercages. It was, however, noted that the changes in
SBET were smaller observed for Cu(salen)/Y-SB.
3.1.2. DR UV–vis
The DR UV–vis spectra are presented in Fig. 4. The encapsulated
comlexes exhibit adsorption bands at 200–310 nm attributable to
ligand transition. This indicates the existence of salen complex in
the supercages. The DR UV–vis spectra of the free and encapsulated
Cu complexes show intense ligand–metal charge transfer adsorp-
tions in the range of 310–420 nm. All the samples exhibit a broad
d–d electron transitions adsorption at 430–530 nm [19,20].
Fig. 4. DR UV–vis spectra (a) Cu(salen)/Y-IM, (b) Cu(salen)/Y-SB and (c) Cu(salen).

7
0
N. Zhang, X.-F. Zhou / Journal of Molecular Catalysis A: Chemical 365 (2012) 66–72
Table 1
Cu content, surface area and pore size of NaY, Cu(salen), Cu(salen)/Y-IM, Cu(salen)/Y-SB samples.
2
Catalyst
Colour
Cu content (wt.%)
BET surface area (m /g)
BET pore size (nm)
NaY
Cu(salen)
Cu(salen)/Y-IM
Cu(salen)/Y-SB
White
Blue
Green
Green
–
584.38
–
270.10
581.77
2.2237
–
2.1707
2.2172
19.00
1.88
1.21
characteristic adsorptions of the encapsulated complexes in the
−
1
range of 1300–1550 cm , indicated the presence of Cu(salen) in
the NaY, of which no adsorptions were observed.
The comparison of the IR vibration spectra of the free and the
molecular sieve loaded complexes reveals that the complex is really
embedded. The appearance of the spectrum of the embedded com-
plex is very similar to that of the free complex. Obviously, the
structure of the complex is maintained in the immobilized state.
The wavenumber shifts and relative intensity changes of the vibra-
tion bands show significant guest/host interactions [20].
3.1.4. XRD
XRD measurements (Fig. 7) showed that diffraction peak
strength corresponding to NaY and Cu(salen)/Y-IM was
I1
◦
> I
◦
> I ,
◦
in which no change between
5.6 /3 3 1
10.2 /2 2 0
11.9 /3 1 1
+
NaY and Cu(salne)/Y-IM was observed indicating that Na at S2
and S4 in NaY was not replaced by other ions. While the change
of difraction peak strength corresponding to Cu(salen)/Y-SB
+
(
I3 3 1 > I3 1 1 > I2 2 0) suggested that some traces of Na ions were
replaced by Cu(salen). XRD measurements showed that the encap-
sulation of Cu(salen) in NaY had no obvious influence on the
structure of the zeolitic matrix.
Fig. 5. UV–vis spectra in CHCl3, (a) Cu(salen)/Y-IM, (b) Cu(salen) and (c) Cu(salen)/Y-
SB.
3
.2. Catalytic bleaching of pulp using neat and encapsulated
external water in addition to the strongly hydrogen-bonded OH
−
1
Cu(salen)
or extremely strongly coordinated H O. The band at 1628 cm
2
for salen, Cu(salen), Cu(salen)/Y-IM, and for Cu(salen)/Y-SB
are attributed to C=N adsorption. Vibrations in the range of
Catalytic bleaching trials of pulp using Cu(salen) in the neat state
−1
and when encapsulated in NaY were carried out using CH COOOH
1
500–1550 cm , which attributed to the aromatic ring, are
3
as the oxidant. The effect of the Cat-stage in the pulp bleaching has
been determined. The results, together with that obtained from a
control pulp, are displayed in Table 2 (Delignification degree, Phys-
ical property), Fig. 8 (Kappa value vs. pulp viscosity).
The effect of Cat-stage on the extent of delignification was
inferred from the delignification degree of pulp after the treatment
with the catalyst/peracetic acid plus the Ep stage (Table 2). Table 2
clearly showed that all the Cat-stages exhibit remarkable catalytic
activity in the delignification of the pulps, delignification degree
observed in all the samples. Broad bands in the range of
−1
1
150–1200 cm
for salen, Cu(salen), Cu(salen)/Y-IM, and for
Cu(salen)/Y-SB (formed from the reaction of ethylenediamine with
salicylaldehyde) are attributed to C-N adsorption. The vibrations
−1
−1
at the band of 568 cm (Cu N) and 470 cm (Cu O) recorded
in FT-IR spectrum of Cu(salen), Cu(salen)/Y-IM and Cu(salen)/Y-SB
are observed, which demonstrate a successful formation and
encapsulation of Cu(salen) in the NaY by the coordination. The
(a)
(
b)
c)
(
5
10
15
20
25
Fig. 6. FTIR spectra (a) salen, (b) Cu(salen), (c) NaY, (d) Cu(salen)/Y-IM and (e)
Cu(salen)/Y-SB.
Fig. 7. XRD spectra (a) Cu(salen)/Y-IM, (b) Cu(salen)/YSB and (c) NaY.

N. Zhang, X.-F. Zhou / Journal of Molecular Catalysis A: Chemical 365 (2012) 66–72
71
Table 2
Delignification degree and physical properties of TCF bleached pulp using the CatEpP1P2 as a sequence.
Cat
Delignification
degree (%)a
Physical properties
Brightness
Viscosity
(mL/g)
Tensile index
(N m/g)
Tear index
(mN m /g)
Burst index
(kPa m /g)
2
2
(
% ISO)
Cu(salen) + CH3COOOH
47.47
43.56
47.93
41.41
45.17
44.02
31.37
–
86.51
85.55
86.63
86.12
87.16
88.90
64.09
32.32
46.42
475
598
527
608
651
736
840
1293
1130
68.02
72.12
74.11
65.27
65.69
67.12
73.66
86.39
83.34
5.47
6.62
6.31
6.53
7.14
7.07
7.04
9.09
6.52
4.86
5.41
5.14
2.23
4.79
4.65
4.94
5.93
5.11
Cu(salen) + CH3COOOH + CH3OH
Cu(salen)/Y-SB + CH3COOOH
Cu(salen)/Y-SB + CH3COOOH + CH3OH
Cu(salen)/Y-IM + CH3COOOH
Cu(salen)/Y-IM + CH3COOOH + CH3OH
Control
Kraft pulp
Oxygen delignified kraft pulp
–
kappa valueoxygen delignified kraft pulp−kappa valueCatEp pulp
a
Delignification degree of CatEp-pulp =
.
kappa valueoxygen delignified kraft pulp
ranging between 43.4 and 47.5% after the treatment with the
catalyst/peracetic acid plus the Ep stage. It is interesting to note
that the reduction of the kappa number in the final Ep-stage
after the catalyst/peracetic acid treatment (ꢀK ≈ 6) was higher
than that observed in control experiment (ꢀK ≈ 4). Thus, the
catalyst/peracetic acid treatment induced a chemical modification
of the oxygen delignified pulp that has a beneficial effect in the
following oxidative extraction step.
Different activities of these complexes in neat and encap-
sulated form in delignification were also analysed reported in
Table 2. The encapsulated catalyst Cu(salen)/Y-IM, in delignifica-
tion degree, was even less active (45.2% delignification degree for
the pulp) than the neat catalyst Cu(salen) (47.5% delignification
degree for the pulp). This is probably due to diffusional limitations
in the NaY-catalyst. The addition of methanol to system probably
retards radicals [23], thus the delignification degree was obviously
decreased from 48.0% to 41.0%.
kraft pulps demonstrate that, in our series of biomimetic- and bio-
bleaching, the use of salen complexes is well suited to mimic the
behaviour of enzymatic oxidative degradation [15,16,24,25].
The effect of catalysts on the selectivity was evaluated by
the viscosity vs. kappa value parameter (Fig. 8) since the lower
is this value after the Cu(salen)/peracetic acid treatment, the
more extensive degradation of the carbohydrates has occurred.
The property of these catalysts was also evaluated by the
brightness and physical property of CatEpP1P2 bleached pulp
(Table 2).
The data presented in Table 2 demonstrate that adding a tiny
quantity of complex results in significantly improved bleaching
effects, with a concomitant level of fiber damage equivalent
to the one of the established standard technology. The Cat-
delignified pulps possess papermaking properties similar or even
higher than those of the control sample (Table 2). The degree of
improvement in delignification selectivity and pulp brightness
depends strongly on the catalytic system in use. It is noticed
that the selectivity in delignification increased rapidly taking
place during the total TCF bleaching. The NaY and methanol
had a pronounced effect on delignification selectivity and pulp
These results differ from those obtained in the delignification of
eucalyptus oxygen delignified kraft pulp samples promoted by the
GIF/H O /O and Co(salen)/H O /O systems, where a significant
2
2
2
2
2
2
lower delignification degree (9–15%) of the catalytic efficiency was
observed by the introduction of either GIF or Co(salen) catalyst than
brightness. For example, Cu(salen)/Y-IM+CH COOOH+CH OH
3
3
that (38–41%) by Cu(salen)/CH COOOH systems used in this paper
gives a viscosity of 736 mL/g and a brightness of 88.9% ISO,
3
[
24].
On the other side, the similar effects of salen complexes and
and Cu(salen)/Y-IM+CH COOOH 651 mL/g and 87.2% ISO,
3
whereas the neat complex (4.5 mg; more than tenfold the
0.44 mg of Cu(salen) present in 4.5 mg of Cu(salen)/Y-IM) used
in the reaction) 598 mL/g and 85.5% ISO, and 475 mL/g and
ligninases on the delignification efficiency observed by us in the
oxidation of both lignin model compounds and delignification of
8
6.5% ISO. The selectivity increased in the order: Cu(salen)/Y-
SB + CH COOOH < Cu(salen) + CH COOOH < Cu(salen) + CH COOOH
3
3
3
1000
+
+
CH OH < Cu(salen)/Y-SB + CH COOOH + CH OH < Cu(salen)/Y-IM
3 3 3
CH COOOH < Cu(salen)/Y-IM + CH COOOH + CH OH. The activ-
3
3
3
ity of the catalyst system in pulp brightness in TCF bleach-
9
8
7
6
5
00
00
00
00
00
ing varied in the following order: Cu(salen) + CH COOOH
3
+
CH OH < Cu(salen)/Y-SB + CH COOOH + CH OH<Cu(salen)
3 3 3
+ CH3COOOH < Cu(salen)/Y-SB + CH3COOOH < Cu(salen)/Y-IM
CH COOOH < Cu(salen)/Y-IM + CH COOOH + CH OH. To
+
our
3
3
3
delight, lignin degradation was selectively obtained under the
investigated conditions with relatively high pulp brightness. Pulp
brightness significantly increased from 64.1% ISO of control sample
to 85–89% ISO of Cat-samples under catalysed conditions. The data
suggest that activation of Cu(salen) molecules occurs when they
are encapsulated inside the supercages of NaY. The encapsulated
complexes exhibit a superior activity in selectivity and brightness
as compared to the neat catalysts. At the same time, due to the
low Cu(salen) content present in encapsulated complexes such
catalysts can be much more cost-competitive. The higher activity
of Cu(salen) located in NaY is probably a result of the highly
dispersed and/or structural integrity within themesoporous NaY
support (revealed by spectroscopic studies) [26,27].
6
.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0
Kappa value
Fig. 8. Kappa value vs. pulp viscosity for the CatEpP1P2 sequences and for a Cat-free
control sequence (ControlEpP1P2).

7
2
N. Zhang, X.-F. Zhou / Journal of Molecular Catalysis A: Chemical 365 (2012) 66–72
We conducted the leaching experiments of active sites during
possible reasons for the enhanced activity of different encapsulated
complexes were investigated using UV–vis spectroscopic meth-
ods. The catalytic performance of encapsulated catalyst could be
attributed to the formation of copper-hydroperoxide species. The
study reveals that encapsulated Cu(salen) complexes can truly
mimic the functionality of enzymes, possible applications of the
complexes, either for lignin oxidation or for the oxidative bleaching
of wood pulps, are thus worth to be considered.
reaction by spectroscopic method and only a trace amount of
copper was detected (0.16–0.7 ppm) in solution, which indicated
that no leaching of Cu ions from the interior of the zeolite to the
solution occurred and also that the external surface sites play a
very minute role in the overall catalytic process, the oxidation of
lignin occurred on the active sites of Cu(salen) encapsulated in the
pores rather than those adsorbed on the external surface of the
NaY host molecular sieves. Again, when methanol was added to
the system the selectivity of catalytic bleaching was significantly
improved under the identical catalytic condition. This suggests that
the addition of methanol reduced cellulose hydrolysis occurred
under the conditions of the bleaching reaction [23].
On the other hand, it appears that the method of encapsulation
has a marked effect on delignification by Cu(salen)/Y complexes.
The catalyst prepared by the “IM” is found to yield higher selectiv-
ity and brightness than by “SB”. The difference in the performance
of the catalysts arises because of differences in their relative activ-
Acknowledgements
The authors gratefully acknowledge the financial support of the
National Natural Science Foundation of P.R. China (nos. 21166011
and 20766002).
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