Application of zeolite-encapsulated Cu(ii) [H4]salen derived from [H2]salen in oxidative delignification of pulp

Article abstract of DOI:10.1039/c4ra04175e

Since the disclosure that salen complexes were potent low-temperature oxidation catalysts with exceptional selectivity, considerable efforts by our group have focussed on their development towards the efficient catalytic delignification of pulp, principal

Full text of DOI:10.1039/c4ra04175e

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Application of zeolite-encapsulated Cu(II) [H4]salen
derived from [H2]salen in oxidative delignification
of pulp
Cite this: RSC Adv., 2014, 4, 28029
*
Xue-Fei Zhou
Since the disclosure that salen complexes were potent low-temperature oxidation catalysts with
exceptional selectivity, considerable efforts by our group have focussed on their development towards
the efficient catalytic delignification of pulp, principally with environmentally benign oxygenic chemicals.
These efforts have resulted in additional developments including the heterogenisation of catalytic
systems as well as the hydrogenation for application in catalytic delignification of pulp. Herein,
heterogeneous complexes of two ligands: [H2]salen and [H4]salen with copper as a transition metal were
synthesized and encapsulated in zeolite cages by the impregnation (IM) and ship-in-a-bottle (SB)
method. The resulting complexes were employed as catalysts for oxidative delignification of pulp, using
peracetic acid (CH3COOOH) as an oxidant. The observed trends in catalytic activity showed that the
encapsulation and hydrogenation of complexes played a key role in the selective delignification of pulp.
By encapsulating and hydrogenating the complexes, their spatial separation increased which strongly
enhanced the activity of complexes. The encapsulation and hydrogenation of complexes resulted in a
significant increase in delignification and properties of pulp. The delignification was dependent on the
nature of the complexes. The reasons were investigated by spectroscopic techniques.
Received 11th April 2014
Accepted 29th May 2014
DOI: 10.1039/c4ra04175e
www.rsc.org/advances
chlorine at all but only free from chlorine gas. Instead chlorine
dioxide is used. ECF pulp production results in the release of
1 Introduction
high levels of chlorinated organic pollutants into the environ-
ment. Many of these chlorinated pollutants have been demon-
strated to cause numerous health effects on humans including
reproductive, developmental, immune and hormonal prob-
lems. Totally chlorine-free or TCF bleaching is one which uses
normally oxygenic chemicals. The use of TCF bleaching is the
safest process for the environment as it does not release toxins
into the environment. For this reason, the pulp and paper
industry has directed pulp practices to TCF bleaching
processes.3–6
TCF pulps tend to have lower brightness and reduced
strength properties compared with ECF pulps. Selective
delignication with TCF bleaching processes has received much
attention recently but it is still difficult to activate oxygenic
agent with available catalysts. Increasing restrictions on pulp
selective delignication demand the use of sustainable and
environmentally friendly processes.7,8 At the same time, the
biomimetic catalysts offer an alternative for conventional bio-
logical and chemical processes and have many advantages over
natural enzyme system, allowing development of more robust
catalysts at relative low cost. Such catalysts are pH and ther-
mally more stable, may expand the scope of substrates and
increase the scale of production. Moreover, their stability and
selectivity may be improved by chemical modications of the
active sites by altering the immobilization procedures. The
Lignin is a complex and heterogeneous mixture of polymers
resulting from the oxidative combinatorial coupling of 4-
hydroxyphenylpropanoids. The main building blocks of lignin
are the coniferyl alcohol and sinapyl alcohol, with typically
minor amounts of p-coumaryl alcohol. Lignin is the main
source of color in pulp due to the presence of a variety of
chromophores naturally present in the wood or created in the
pulp mill. Chemical pulps (kra and sulphite) contain residual
lignin less than 5%. Subsequent bleaching is required to
remove essentially all of the residual lignin from the pulp to
meet brightness requirements. The bleaching of pulp is the
chemical processing carried out on various types of pulp to
decrease the color of the pulp, so that it becomes whiter.
Bleaching of pulp is extremely important and useful reaction in
pulp and paper industry.1,2
In making pulp products, elemental chlorine bleaching is
the chlorine bleaching process that produces and releases into
the environment, large amounts of chlorinated organic
compounds including dioxin and furans. It is a cheap gas to
produce and is extensively used in pulping industries.
Elemental chlorine-free bleaching (ECF bleaching) is not free of
Kunming University of Science and Technology, P.O. Box A302-12, Building No. 5,
Xinying Yuan, No. 50, Huancheng East Road, Kunming, 650051, China. E-mail:
lgdx602@tom.com
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immobilization of active sites within solid host materials can
Laboratory oxygen delignied eucalyptus (E. urophylla ꢀ E.
lead to the creation of highly active and selective catalysts that grandis) kra pulp (46.4% ISO brightness, 13.0 kappa number,
can facilitate highly selective transformation.9–11 In this respect, 1130 mL gꢁ1 viscosity and 83.34 N m gꢁ1 tensile index) was used
a well studied zeolite-encapsulated system involves the salen for the catalytic TCF bleaching. Before bleaching, the pulp was
ligand
(N,N0-bis
(salicylaldehyde)ethylenediimine)
that thoroughly washed with tapwater and distilled water to remove
ꢂ
possesses two oxygen and two nitrogen atoms and can form all residual black liquor and stored at 0 C.
stable complexes with transition metal ions. Salen-type
complexes are a fundamental class of compounds in coordi-
nation chemistry. An extremely wide variety of reactions cata-
lysed by salen complexes has been investigated.12 Oxidations of
lignin and lignin model compound by the use of such salen
complexes with oxidants like O2 and H2O2 have become of
interest as models for biological oxidations, since salen
complexes can mimic the lignin degrading enzymes. Co(salen)/
O2 oxidation of phenolic and non-phenolic b-O-4 aryl ether
lignin model compounds showed very high conversion values
within 30 min.13–15 A series of biomimetic oxidation of lignin
and lignin model compound using Co(salen) have been
successfully studied in our laboratory.16–19 Ambrose et al.
reported the ionic liquid tagged Co(salen) with which veratryl
alcohol, a lignin model compound, was selectively oxidized to
veratraldehyde using air or pure oxygen as the source of oxygen.9
Co(salen) supported in SBA-15 showed higher activity for the
oxidation of a lignin model phenolic dimmer [1-(4-hydroxy-3-
methoxyphenoxy)-2-(2-methoxyphenoxy)-propane-1,3-diol] than
that unsupported.20
2.2 Instrumentation
X-ray power diffraction (XRD) patterns of the samples
NaY, Cu([H2]salen)/IM, Cu([H2]salen)/SB, Cu([H4]salen)/IM,
Cu([H4]salen)/SB were recorded on a Rigaku Dmax X-ray
diffractometer (Ni-ltered, CuKa radiation). The Fourier trans-
form infrared (FTIR) spectra of the samples NaY, Cu([H2]salen),
Cu([H2]salen)/IM, Cu([H2]salen)/SB, Cu([H4]salen), Cu([H4]salen)/
IM, Cu([H4]salen)/SB were recorded on Equinox 55 FT-IR spec-
trometer in KBr. Atomic absorption spectrometer, Shimadzu AA-
6800 was used for the estimation of copper of the samples
Cu([H2]salen), Cu([H2]salen)/IM, Cu([H2]salen)/SB, Cu([H4]salen),
Cu([H4]salen)/IM, Cu([H4]salen)/SB. Diffuse reectance (DR) UV-
vis measurements of the samples Cu([H2]salen), Cu([H2]salen)/
SB,
Cu([H2]salen)/IM,
Cu([H4]salen),
Cu([H4]salen)/SB,
Cu([H4]salen)/IM 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) of the
samples Cu([H2]salen)/IM, Cu([H2]salen)/SB, Cu([H4]salen)/IM,
Cu([H4]salen)/SB was determined by adsorption and desorption
of nitrogen at the temperature of liquid nitrogen (77 K) using
volumetric adsorption set-up (Micromeritics ASAP-2020, USA).
Salen complex catalysed oxidation of lignin offers a way to
solve the difficult to activate oxygenic agent on pulp selective
delignication with TCF bleaching processes. Catalytic
delignication also could make pulp bleaching more environ-
mentally friendly and more cost effective. Lignin and model
compound studies enable us to develop the neat and encapsu-
2.3 Preparation of zeolite-encapsulated Cu(II) [H4]salen and
[H2]salen complexes
lated salen complex ([H2]salen complex) used as an effective Cu(II) [H4]salen and [H2]salen complexes were encapsulated in
catalyst for the selective delignication of kra pulp.21 From NaY by the impregnation method (IM) and ship-in-a-bottle
this and related studies, in the present work, in order to method (SB). The encapsulated catalysts were designated by the
improve the cost-effectiveness and performance of following notation: complex/encapsulation method.
catalysts the reduction and encapsulation have been used for
The preparation of Cu(II) [H4]salen and [H2]salen complexes
the Cu([H2]salen) complex. It would be more desirable to have and the following encapsulations on NaY via ‘IM’ and ‘SB’
more exible complexes where [H2]salen ligands are converted process were depicted in Scheme 1.
to tetrahydro-salen ligand ([H4]salen) by the hydrogenation of
2.3.1 Preparation of [H2] salen and [H4]salen ligands.
C]N to C–N in the present of NaBH4. The reduced derivatives [H4] salen and [H2]salen ligands were prepared by following the
[H4]salen complexes have showed higher activity than the procedures reported in the literature.24,25
parent [H2]salen complexes and are an alternative ligand system
[H2]salen
(N,N0-bis(salicylidene)-ethylenediamine)
was
for oxidation catalysis.22,23 Cu([H4]salen) was encapsulated synthesized by dropwise addition of ethylenediamine (4.5
inside the pores of NaY by the impregnation (IM) and ship-in-a- mmol), to a methanolic solution (18 mL) of salicylaldehyde (9
bottle (SB) method. Furthermore, pulp was delignied with two mmol). The reaction mixture was stirred for 30 min in a ice-
NaY-encapsulated Cu([H4]salen) in the presence of water bath with a condenser. Aer standing for 15 min, the
CH3COOOH. Very little is known on the catalytic delignication yellow precipitate of [H2]salen was formed which was ltered
of pulp with encapsulated [H4]salen complex, which was sug- out, washed with petroleum ether, and dried in vacuum.
gested by us as a promising catalyst for the biomimetic [H4]salen was obtained by the stirring of 0.011 mol NaBH4 with
bleaching of pulp.
0.01 mol [H2]salen in CH3OH at ambient temperature for 2 h.
The solid product was washed with water and dried in vacuum.
The purity of the ligands were conrmed by IR and H-1 NMR
before coordination to Cu(II) cation.
2 Experimental
2.1 Materials
2.3.2 Preparation of Cu([H2]salen) and Cu([H4]salen).
All chemicals are commercially available and were used as Cu([H2]salen) and Cu([H4]salen) were prepared followed by a
received without further purication, unless otherwise noted.
28030 | RSC Adv., 2014, 4, 28029–28035
literature method.26
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Scheme 1 The preparation of the Cu(II) [H4]salen and [H2]salen complexes and the immobilizations on NaY.
Cu([H2]salen) (Cu([H4]salen)) was prepared by the bleaching stages such as alkaline extraction stage with
following method: 1.16 g of [H2]salen ([H4]salen) was dissolved hydrogen peroxide addition and two hydrogen peroxide stages,
in 50 mL of hot methanol, followed by the addition of 1.08 g designated Cat, Ep, P1 and P2, respectively. Aer each stage the
¨
Cu(CH3COO)2 in 6.7 mL of hot deionised water. Then the pulp was washed thoroughly with deionized water in a Buchner
mixture was reuxed for 60 min. The precipitate obtained was funnel and then pressed to 30% consistency.
ltered out, washed with methanol, vacuum dried.
A
series of catalytic bleaching trials (Cat) were
2.3.3 Preparation of Cu/Y. Cu/Y was obtained by ion- performed using Cu([H2]salen), Cu([H4]salen), Cu([H2]salen)/
exchanging NaY with Cu2+ ions at room temperature in an SB, Cu([H2]salen)/IM, Cu([H4]salen)/SB, Cu([H4]salen)/IM for
aqueous solution of copper acetate (0.08 M) with a liquid/solid 120 min at 70 ꢂC: oxygen delignied eucalyptus kra pulp (15 g,
ratio of 20 (mL gꢁ1).
o. d. p), obtained as a 30% consistency, was combined with
2.3.4 Preparation of Cu([H2]salen)/IM and Cu([H4]salen)/ catalyst (0.03% on o. d. p), CH3COOOH (0.5% on o. d. p), and
IM. The Cu([H2]salen) (Cu([H4]salen)) was supported on the NaY by some amount of deionized water to obtain a 5% pulp consis-
impregnation: 150 mg of Cu([H2]salen) (Cu([H4]salen)) complex was tency. Control experiments were carried out also under identical
dissolved in t-butanol and added to 300 mg of the NaY molecular conditions to examine the ability of catalyst.
sieve. The amount of solvent was just enough to cover the mixture.
Alkaline extraction with 0.5% (on o. d. p) addition of H2O2
Then reuxed for 6 h under nitrogen gas ow. The resulting solid (Ep) was performed by heating the pulp at a consistency of 10%
aer ltering was extracted with t-butanol and acetonitrile using at 70 ꢂC for 60 min in 0.0375 M NaOH and using MgSO4 (0.1%
soxhlet extractor to the colorless. Then the solid was allowed to on o. d. p) and DTPA (0.05% on o. d. p) as stabilizer at this stage.
vacuum dry to obtain the nal encapsulated complex.
The consistency of pulp in P1 and P2 stage was maintained at
2.3.5 Preparation of Cu([H2]salen)/SB and Cu([H4]salen)/ a value of 10%. NaOH, MgSO4, Na2SiO3, DTPA, H2O2 were used
SB. Cu/Y was mixed well with excessive [H2]salen ligand as bleaching chemicals. P1-stage: NaOH 1.5% (on o. d. p),
([H4]salen) (ligand/metal ¼ 3, m/m), and sealed into a round MgSO4 0.1% (on o. d. p), Na2SiO3 1.0% (on o. d. p), DTPA 0.1%
ꢂ
ask. Then the mixture was heated for 24 h at 150 ꢂC under high (on o. d. p), H2O2 2.0% (on o. d. p), and 80 C for 150 min; P2-
vacuum condition. Uncomplexed ligand and complex were stage: NaOH 1.0% (on o. d. p), MgSO4 0.05% (on o. d. p), DTPA
removed by extraction with acetone, and uncoordinated Cu2+ 0.1% (on o. d. p), H2O2 1.5% (on o. d. p), and 80 ꢂC for 90 min.
ions were removed by ion-exchange with NaCl aqueous solu-
All experiments were run at least three. The relative error in
tion (0.1 M). The sample was further washed thoroughly the parallel experiments was lower than the standard level.
with deionized water, and air-dried for 60 min to give the
Cu([H2]salen)/SB (Cu([H4]salen)/SB).
2.5 Analytical methods of pulp
Kappa numbers of pulp were determined using TAPPI Test
Method T 236 om-06. The kappa number of a pulp is a standard
2.4 Catalytic TCF bleaching trials
In this paper, catalytic TCF bleaching sequences consisting of pulp and paper industry index of its lignin content. Pulp
four stages included a treatment with catalytic agent and several viscosity (T 230 om-08), a measure of its strength properties
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depending on 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 handsheet weight varied from 60 to 65 g
mꢁ2. Physical properties (T 220 sp-10) of pulp handsheets were
measured for tensile index.
Fig. 1 XRD spectra of (a) NaY, (b) Cu([H2]salen)/IM, (c) Cu([H2]salen)/
SB, (d) Cu([H4]salen)/IM, (e) Cu([H4]salen)/SB.
3 Results and discussion
3.1 Characterization of the catalyst
3.1.1 Chemical analysis and XRD. The analytical data of
neat and encapsulated Cu(II) [H4]salen and [H2]salen complexes
were given in Table 1. The encapsulation methods employed led
to the decrease in copper amount of the complex. However, it
maybe noted that in Table 1 the amount of copper was higher in
sample obtained by ‘IM’ rather than the ‘SB’ method. This
suggests that the probability of retention of the complex inside
the supercage during the synthesis was higher for complex
obtained by ‘IM’.12
Cu(II) [H4]salen and [H2]salen complexes encapsulated in
NaY showed lower surface area (SBET) and pore volume (VBJH) as
well as pore diameter (DBET) compared with pure NaY. The
lowering of SBET, VBJH and DBET indicated the presence of the
Cu(II) [H4]salen and [H2]salen complex within the supercages of
the NaY structure (Table 1).27,28
XRD patterns of Cu(II) [H4]salen and [H2]salen complexes
encapsulated within NaY conrmed that the complexes were
structurally unaltered aer the encapsulation procedure
(Fig. 1).29
Fig. 2 FTIR spectra of (a) NaY, (b) Cu([H2]salen), (c) Cu([H2]salen)/IM,
(d) Cu([H2]salen)/SB, (e) Cu([H4]salen), (f) Cu([H4]salen)/IM, (g)
Cu([H4]salen)/SB.
3.1.2 FTIR. The characterization of neat and encapsulated
complexes by FTIR spectroscopy was analysed in comparison
(Fig. 2).
3.1.3 DR UV-vis. DR UV-vis spectroscopy was used to
elucidate the nature of Cu species of neat and encapsulated
complexes within NaY (Fig. 3). The neat and encapsulated
complexes showed strong absorption bands at 200–250 nm
([H4]salen and [H2]salen), 300–350 nm (Cu [H4]salen and
[H2]salen), which indicate that the complexes were dispersed
within the zeolite supercages.31 However, the weak absorption
of encapsulated complexes and hydrogenated samples in the
DR UV-vis spectra conrmed the presence of small change in
FTIR spectra showed similarity upon encapsulation of the
complexes, providing an evidence that the complex structure
remained unchanged in the encapsulated state, in agreement
with the XRD analysis.30 The appearance of the bands at 568
cmꢁ1 (Co–N), 470 cmꢁ1 (Co–O), 1500 cmꢁ1 (aromatic ring),
1500–1300 cmꢁ1 (C–N for [H4]salen) provided
a further
conclusive evidence of the successful encapsulation of Cu(II)
[H4]salen and [H2]salen into the NaY through IM and SB
methods.31
Table 1 Characteristics of neat and encapsulated Cu(II) [H4]salen and
[H2]salen complexesa
Cu content
(wt%)
SBET
VBJH
DBET
(nm)
Sample
(m2 gꢁ1
)
(cm3 gꢁ1
)
NaY
—
584.38
—
270.10
581.77
—
0.32
—
0.15
0.26
—
3.22
—
2.17
2.22
—
Cu([H2]salen)
Cu([H2]salen)/IM
Cu([H2]salen)/SB
Cu([H4]salen)
Cu([H4]salen)/IM
Cu([H4]salen)/SB
19.00
1.88
1.21
19.22
3.99
0.86
308.38
528.03
0.17
0.28
2.21
2.28
Fig. 3 DR UV-vis spectra of (a) Cu([H2]salen), (b) Cu([H2]salen)/SB,
(c) Cu([H2]salen)/IM, (d) Cu([H4]salen), (e) Cu([H4]salen)/SB, (f)
Cu([H4]salen)/IM.
a
Specic surface area, SBET (m2 gꢁ1); pore volume, VBJH (cm3 gꢁ1); pore
diameter, DBET (nm).
28032 | RSC Adv., 2014, 4, 28029–28035
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geometry and stability of complexes due to encapsulation and [H2]salen complexes and their heterogeneous analogues
and hydrogenation, which is in agreement with the analyses by encapsulated in NaY using CH3COOOH as the oxidant.
Yang et al.32
From the data presented in Fig. 4 and 5, it is clear that those
heterogeneous catalysts which posses well isolated active sites
displayed higher activity than their neat homogeneous
analogues. In fact, while the encapsulated complexes afforded
high value of selectivity (Fig. 4), brightness (Fig. 5) compared to
the neat analogues, the [H4] complexes showed to be more
efficient than [H2] complexes in pulp delignication with
3.2 Catalytic bleaching of pulp
For closer evaluation of the role of encapsulation and hydro-
genation and their inuence on catalytic activity, catalytic
delignication of pulp was studied with neat Cu(II) [H4]salen
ðKoxygen delignified pulp ꢁ KCatEp pulpÞ=ðVoxygen delignified pulp ꢁ VCatEp pulp
Þ
; K: kappa number; V:
Fig. 4 Selectivity for the CatEp bleaching. Selectivity ¼
per mole metal ion
viscosity.
Fig. 5 Brightness for the CatEpP1P2 bleaching.
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complexes. Inuence of encapsulation method on catalytic
activity was studied. The SB-encapsulation of heterogeneous
Cu(II) [H4]salen and [H2]salen complexes resulted in increased
selectivity, but decreased brightness. Cu([H2]salen)/SB afforded
selectivity of 12 222 that decreased to 9722 when Cu([H2]salen)/
IM complex was employed. By hydrogenating the [H2]salen
complex for Cu([H4]salen)/SB complex the selectivity increased
to 19 789. However, SB-encapsulated Cu(II) [H4]salen and
[H2]salen complexes obtained a brightness of 87.8% ISO and
86.6% ISO that increased to 88.5% ISO and 87.1% ISO when IM-
encapsulated Cu(II) [H4]salen and [H2]salen complexes were
employed.
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