Design of a bio-inspired copper (II) Schiff base complex grafted in mesoporous silica for catalytic oxidation Dedicated to Georges Christou for his 60th birthday.

Article abstract of DOI:10.1016/j.poly.2013.06.039

Controlled grafting of a copper (II) complex with a Schiff base ligand (L_A = N-(salicylaldimine)-(N′-propyltrimethoxysilane)- diethylenetriamine) on a mesoporous MCM-41 type of silica (LUS) was accomplished using a pattering technique to allow

Full text of DOI:10.1016/j.poly.2013.06.039

Polyhedron xxx (2013) xxx–xxx
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Design of a bio-inspired copper (II) Schiff base complex grafted in
mesoporous silica for catalytic oxidation
b
a,
Wen-Juan Zhou a,b,1, Belén Albela a, , Ming-Yuan He , Laurent Bonneviot
⇑
⇑
a Laboratoire de Chimie, Ecole Normale Supérieure de Lyon, University of Lyon, 46, Allée d’Italie, 69364 Lyon Cedex 07, France
b Shanghai Key Lab of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai, China
a r t i c l e i n f o
a b s t r a c t
Controlled grafting of a copper (II) complex with a Schiff base ligand (LA = N-(salicylaldimine)-(N0-propyl-
trimethoxysilane)-diethylenetriamine) on a mesoporous MCM-41 type of silica (LUS) was accomplished
using a pattering technique to allow a homogeneous distribution of isolated copper sites on the solid
using trimethylsilyl groups as dispersing function. XRD patterns suggest that the final material LUS-CuLA
exhibits a well-ordered 2D hexagonal structure. Elemental analysis, solid UV–Vis and EPR spectroscopies
indicate that the copper (II) coordination sphere is most likely of a 3N1O type with an additional acetate
ion as a ligand. Almost all the Cu(II) species grafted on the solid were EPR active (2.6 wt% active species
for 2.8 wt% total copper measured from ICP-MS analysis) confirming the presence of monomeric copper
species. Preliminary catalytic tests for phenol hydroxylation show that the supported system presents
similar activity as the molecular analogue.
Article history:
Available online xxxx
Dedicated to Georges Christou for his 60th
birthday.
Keywords:
MCM-41
Mesoporous silica
Copper complexes
Schiff base
Phenol hydroxylation
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
successful using such approach, several conditions should be ful-
filled: (i) a homogeneous distribution of the grafted species, (ii) a
The design of a heterogeneous catalyst with equivalent active
sites and molecular control of the active species is a matter of con-
cern. Indeed, better knowledge of the nature of the active sites can
allow fine-tuning of the material to improve its reactivity proper-
ties and potential applications. Analogy with metalloproteins
motivates research for the control of both structure of the metal
sites and hydrophobicity at their vicinity. An obvious approach is
to take advantage of recent progress in the grafting of
homogeneous metal catalysts on inorganic supports to develop a
bottom-up approach [1–7]. In particular, the use of a mesoporous
silica matrix such as MCM-41 [8], MCM-48 [8] and SBA-15 [9],
has allowed the synthesis of new heterogeneous catalysts
[10–13]. Indeed, these solids are robust supports with a high
specific surface area (600–1000 m2 gꢀ1) and small pore size distri-
bution (pore diameter ꢁ3–10 nm). In addition, they present the
advantage of an easy functionalization with organic groups by
grafting alkoxysilane derivatives on the silanol or silanolate func-
tions (depending on the pH) at the pore-wall interface.
controlled environment of the metal ion, and (iii) a homogeneous
confinement for the metal active sites. If these conditions are gath-
ered, straightforward relations between structure and reactivity of
the active sites can be stated and therefore an easy improvement of
the material should be possible.
Homogeneous distribution of active sites implies isolation of
the grafted species in the heterogeneous catalyst. Among the de-
scribed strategies to isolate functions on a grafted porous silica,
the so-called Molecular Stencil Pattering (MPS) approach offers
the advantage of affording an homogeneous dispersion of one
function at high loading [14–17]. The MPS has been developed
for cationic templated mesoporous silica of MCM-41 type
[18,19]. It is based on the use of the cationic template as stencil
masking the surface according to a regular pattern generated by
electrostatic self-repulsion. Then a sequential introduction of
two different functions is performed: the first function is usually
a trimethylsilyl (TMS) group, which acts as hydrophobic isolating
function that is incorporated in the presence of some of the tem-
plate molecules. Then, after removal of the remaining template
molecules, the second function is introduced in the appropriate
ratio to be totally surrounded by the first function, and therefore
isolated. This approach has been already described for the synthe-
sis of a heterogeneous ruthenium catalyst for sulfur oxidation
[16].
The most common approach consists in the grafting of either a
ligand to be further complexed to the metal ion or a metal complex
possessing one or two alkoxysilane arms [2–4,11,1]. In order to be
⇑
Corresponding authors. Tel.: +33 472 88 56/fax: +33 472 72 88 60.
E-mail address: belen.albela@ens-lyon.fr (B. Albela).
Present address: Eco-Efficient Products and Processes (E2P2) Laboratory, 3966 Jin
Herein, we describe the grafting of a copper (II) Schiff base com-
plex with the N-(salicylaldimine)-(N0-propyltrimethoxysilane)-
diethylenetriamine (LA) ligand, using the MSP technique (Scheme 1).
1
Du Rd., Xin Zhuang Industrial Zone, Shanghai 201108, China. Tel.: +86 21 24089535;
fax: +86 21 54424351.
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.06.039
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W.-J. Zhou et al. / Polyhedron xxx (2013) xxx–xxx
This ligand has been chosen since it can mimic the metal environ-
ment of some oxidation copper metalloproteins such as catechol
oxidase (GO) or copper amine oxidase (CAO) [20]. The aim of our
work was the design of a heterogeneous catalyst for phenol hydrox-
ylation. The challenge for this process is to be able to tune selectivity
between catechol and hydroquinone, which are the desired prod-
ucts, and to diminish the formation of by-products such as tars.
uumed and washed with a mixture of petroleum ether and ethyl
acetate (7:2) and then vacuumed. The obtained yellow products
were analyzed using 1H NMR spectroscopy (Table S1).
2.3. Synthesis of complexes CuLA and CuLB
An ethanolic solution (10 mL) of HLA (0.308 g, 0.73 mmol) or
HLB (0.199 g, 0.81 mmol) was introduced into a round flask. Then,
copper acetate monohydrate (0.15 g, 0.74 mmol) in absolute etha-
nol (15 mL) was dropwise added. The resulting mixture was stirred
under reflux for 2 h under argon atmosphere. Dark blue solutions
containing CuLA or CuLB complexes were formed.
2. Materials and methods
2.1. Chemicals and solutions
Copper (II) acetate monohydrate (99%, Acros), N1-(3-trimethoxy-
silylpropyl) diethylenetriamine (LC, 95%, ABCR), N0-isopropyldiethy-
lenetriamine (tech., 75%, Aldrich), hexadecyltrimethylammonium-
p-toluene-sulfonate, (CTATos; >99% Merck), Ludox HS-40 (40%
SiO2; Aldrich), hexamethyldisilazane (HMDSA, 98% Acros), sodium
hydroxide (Acros), salicylaldehyde (98%, Avocado), ethyl alcohol
anhydrous (plus for HPLC, Carlo Erba), hydrochloric acid (1 N stan-
dard solution, Acros), hydrogen peroxide (50% wt% solution in
water, Aldrich), phenol (P99%, Sigma–Aldrich), catechol (99%,
Acros), hydroquinone (P99%, Fluka), 1,4-benzoquinone (>99.5%,
Fluka), potassium phosphate monobasic (99%, Acros), di-sodium
hydrogen phosphate (anhydrous, Merck), sulfuric acid (95–97%,
Fluka), Iotect (99%, Prolabo), potassium iodide (99%, Aldrich), potas-
sium dichromate (99%, Avocado).
2.4. Synthesis of hybrids materials LUS-CuLA and LUS-CuLC
They were synthesized starting from a 2D hexagonal LUS silica
prepared at 130 °C [18,19]. The Molecular Stencil Patterning (MSP)
technique [14,16] was used to homogeneously distribute the
grafted complexes, using trimethylsilyl functions (TMS) as isolat-
ing groups for the complexes (Fig. 1).
2.4.1. Synthesis of LUS
A sodium silicate solution (320 mL) was stirred at 60 °C for 1 h.
A second solution of hexadecyltrimethylammonium p-tolunensulf-
onate (CTATos, 12.842 g) in deionized water (460 mL) was stirred
during 1 h at 60 °C. The resulting solution was dropwise added to
the second one during 20 min. The formed sol–gel was stirred at
60 °C for 24 h. After filtration and washing with deionized water
(200–300 mL), the as-made solid was dried at 100 °C overnight,
obtaining 19.8 g of LUS. Elemental analysis: C, 33.89; H, 6.72; N,
1.99%, weight loss at 1000 °C (48.38%).
2.1.1. Sodium silicate solution
It was prepared as follows: Ludox (187 mL) was added to a solu-
tion of sodium hydroxide (32 g) in deionized water (800 mL) and
stirred at 40 °C until clear.
2.1.2. Phosphate buffer solutions
(1) pH = 7: 0.68 g of KH2PO4 and 40 mL of 0.1 mol/L NaOH were
introduced in a 100 mL volumetric flask. Distilled water was added
until a total volume of 100 mL. [KH2PO4] = 50 mmol/L; (2) pH = 6:
2.085 g of KH2PO4 and 0.353 g of Na2HPO4 were introduced in a
250 mL volumetric flask. Distilled water was added until a total
volume of 250 mL. [KH2PO4] = 60 mmol/L; (3) pH = 5: 0.2 mol/L of
NaOH was added into 100 mL of KH2PO4 (0.2 mol/L); distilled
water was added to adjust the pH until 5 controlled by pH-meter.
2.4.2. Partial surfactant extraction
LUS (10 g) was placed in a round flask, and then ethanol
(400 mL, 96%) and hydrochloric acid 1 mol Lꢀ1 (6.8 mL, 0.5 eq)
were added. The mixture was stirred at 40 °C for 1 h. After filtra-
tion and washing with ethanol (100 mL ꢂ 2) and acetone
(50 mL ꢂ 2) the solid was dried at 80 °C for 20 h. Material LUS-
PE (7.1 g) was obtained.
2.2. Synthesis of ligands HLA and HLB
2.4.3. Partial silylation
LUS-PE (6.8 g) was introduced in a round bottom three-necked
flask, and then dried at 130 °C for 1 h under argon flow and during
2 h under vacuum. Cyclohexane (170 mL) and HMDSA (30 mL,
30 eq) were added under argon. The mixture was refluxed for
18 h. The obtained solid was finally washed with cyclohexane
(2 ꢂ 30 mL), ethanol (2 ꢂ 60 mL) and acetone (2 ꢂ 60 mL), and
then dried at 80 °C for 18 h. This sequence of steps was repeated
twice. Finally, partially silylated material LUS-PES (7.1 g) was
obtained.
A closed two-necked round flask with a condenser was firstly
vacuumed at room temperature and then filled with argon. The
same procedure was repeated for three times. Salicylaldehyde
(1.09 mL, 10 mmol or 0.11 mL, 1 mmol) and absolute ethanol
(50 mL) were injected in the flask. Then LC (2.71 mL, 10 mmol) or
N0-isopropyldiethylenetriamine (0.22 mL, 1 mmol) was dropwise
added into the flask. The mixture was stirred under reflux for 6 h
under argon atmosphere. The obtained yellow products were vac-
H
H
H
N
N
O
N
N
O
N
NH2
Cu
Cu
N
N
N
H
H
H
Si OH
Si OH
SiO2
SiO2
LUS-CuLA
LUS-LC
CuLB
Scheme 1. Structure of the grafted species LUS-CuLA and LUS-LC and the molecular analogue CuLB.
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W.-J. Zhou et al. / Polyhedron xxx (2013) xxx–xxx
3
TMS
Si
Si
O
O
Si
Si
O
O
H2
OH
O
H2
O
Si
OH
Si
O
OH
Si
O
Si
O
HO
H2
OH
O
O
SiO2
CuLA
LUS-PSE
N
N
H
N
H
CTA+
N
Cu
HN
O
Cu
HN
O
Si
Si
O
O
Si
O
O
Si
O
O
Si
Si
Si
Si
O
O
Si
O
H2
O
Si
Si
Si
Si
O
O
H
2O
Si
O
Si
O
Si
Si Si
O
OH
O
O
O
OH
O
OH
O
O
H
2O
O
SiO2
SiO2
LUS-PES
LUS-CuLA
Fig. 1. Synthesis route to prepare LUS-CuLA using the MSP approach. A perpendicular section of the pore of the mesostructured silica LUS is represented.
2.4.4. Extraction of the remaining surfactant
2.6. Analytical techniques
LUS-PES (2 g) was placed in a round flask, and then ethanol
(86 mL, 96%) and hydrochloric acid (1.28 mL, 1 eq, 1 mol/L) were
added. The mixture was stirred at 0 °C for 1 h. After filtration
and washing with ethanol (50 mL) and acetone (50 mL) the ob-
tained solid was dried at 80 °C for 20 h. The same procedure was
repeated again. Finally, material LUS-PSE (1.4 g) was obtained. Ele-
mental analysis: C, 7.72; H, 2.40; N, <0.1%, weight loss at 1000 °C
(10.33%).
Low angle X-ray powder diffraction (XRD) experiments were
carried out using a Bruker (Siemens) D5005 diffractometer using
Cu K monochromatic radiation. Infrared (IR) spectra were recorded
from KBr pellets using a Mattson 3000 IRTF spectrometer. Liquid
UV–Vis spectra were recorded using a Vector 550 Bruker spec-
trometer; solid diffuse reflectance UV–Vis spectra were recorded
from aluminum cells with Suprasil 300 quartz windows, using a
PerkinElmer Lambda 950 spectrophotometer and PE Winlab soft-
ware. Nitrogen sorption isotherms at 77 K were determined with
a volume device Micromeritics ASAP 2010 M on solids that were
dried at 80 °C under vacuum overnight. TGA measurements were
collected from Al2O3 crucibles on a DTA-TG Netzsch STA 409 PC/
PG instrument, under air (30 mL/min), with a 25–1000 °C (10 °C /
min) temperature increase. pH-meter (Meterlab pHM210) was cal-
ibrated in aqueous solution. EPR spectra were recorded using a
Brucker Elexsys e500 X-band (9.4 GHz) spectrometer with a stan-
dard cavity. Quantification of Cu(II) species was performed using
crystals of CuSO4ꢃ5H2O as calibration reference. 1H NMR spectra
were recorded on a Brucker Advance 300 NMR spectrometer. HPLC
analyses were performed on a LC/MS Agilent 1100SLT. The working
conditions were the following: column, ZORBAX-SB-C18 of dimen-
2.4.5. Grafting of the complex Cu-LA
LUS-PSE (0.5 g) was dried at 130 °C under argon flow during
1 h, and then vacuumed at 130 °C during 2 h. A solution of metal
complex (0.74 mmol of Cu-LA complex) in 2 mL anhydrous ethanol
was added into the flask together with 40 mL of toluene. The
resulting mixture was stirred at 60 °C for 18 h under argon. The ob-
tained solid was washed with toluene (50 mL) and ethanol
(25 mL), and then dried at 60 °C overnight, obtaining material
LUS-CuLA (0.55 g). Elemental analysis: C, 13.82; N, 1.92; Cu, 2.8%,
weight loss at 1000 °C (18.12%).
2.4.6. Grafting of ligand LC
sion 4.6 ꢂ 250 mm; protect, 5 microns; loop size, 8
lL; tempera-
LUS-PSE (0.3 g) was dried at 130 °C under argon flow during
1 h, and then vacuumed at 130 °C during 2 h. Ligand LC (0.07 mL)
was added into the flask together with 50 mL of toluene. The
resulting mixture was stirred at 60 °C for 18 h under argon. The ob-
tained solid was washed with toluene (50 mL) and ethanol
(25 mL), and then dried at 60 °C overnight, obtaining material
LUS-LC (0.33 g).
ture of the column, room temperature; UV detector, k = 245 nm
and 280 nm; mobile phase, water (10 mmol Lꢀ1 formic acid, 60%)
and acetonitrile (40%); flow rate, 0.2 mL minꢀ1
.
3. Results and discussion
3.1. Synthesis of the hybrid materials containing the copper complex
2.5. Catalytic tests
The ligand HLA was synthesized starting from N1-(3-trimeth-
oxysilylpropyl) diethylenetriamine (LC), and it was complexed
with copper (II) acetate prior to the grafting. The hybrid material
possessing the desired CuLA sites (Scheme 1) was prepared using
LUS silica as inorganic support, which is a 2D hexagonal meso-
structured porous silica (MCM-41 type) that is synthesized in basic
conditions using cetyltrimethylammonium tosylate (CTATos) as
template to generate the mesoporosity. Then the molecular stencil
patterning (MSP) technique was used to introduce, first trimethyl-
silyl (TMS) groups as isolating functions, and second, the copper
(II) complex CuLA precursor [6,16]. In the first step, the cationic
template (CTA+) that covers the surface of LUS was partially
removed by adding an appropriate amount of HCl, obtaining
LUS-PE material (Fig. 1). In the second step, TMS groups were
grafted in the empty space without removal of the remaining
Phenol hydroxylation experiments were run in a 50 mL two-
necked flask with
a condenser. In a standard run, phenol
(0.1 g, 1.1 mmol), catalyst [Cu:phenol = 1:100, LUS-CuLA] and
buffer solution (see above for composition, 7 mL) and H2O2
(50% aqueous) were orderly added. The reaction was carried
out at 80 °C for 2 h. Phenol and the possible products, catechol
(CAT), hydroquinone (HQ) and 1,4-benzoquinone (BQ), were
analyzed by HPLC. H2O2 conversion was determined by the fol-
lowing methods: (i) the H2O2 conversion was determined by
iodometry method, and (ii) the efficiency conversion of H2O2
was calculated as follows: H2O2 eff. conv. = 100 ꢂ H2O2 (mol)
consumed in formation of diphenols and benzoquinone/H2O2
(mol) converted.
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W.-J. Zhou et al. / Polyhedron xxx (2013) xxx–xxx
template molecules that act as stencil, leading to LUS-PES. In the
third step, the remaining template molecules were extracted by
adding a suitable amount of HCl in gentle conditions (1 h at 0 °C)
to afford LUS-PSE. Finally, the CuLA complex was grafted on the
partially silylated mesoporous material (LUS-CuLA). A molecular
analogue without the silane arm, i.e. CuLB, and the corresponding
hybrid material with LC ligand (precursor of HLA), i.e. LUS-LC, were
also synthesized to better characterize the desired material LUS-
CuLA (Scheme 1).
3.2. Characterization of the copper materials
An appropriate TMS/CuLA ratio was required to lead to isolated
metal complexes totally surrounded by TMS functions. In the final
material LUS-CuLA this ratio was of 3.5 (Table S2), as deduced from
chemical analysis and quantitative IR spectroscopy, with ꢁ80% of
the surface covered with organic functions (TMS + LA). The LA/Cu
molar ratio of 1.0 in the initial synthesis gel was conserved in
the final material. Almost all the Cu(II) species grafted on the solid
were EPR active (2.6 wt% active species for 2.8 wt% total copper
measured from ICP-MS analysis, Table S2, vide infra). IR spectros-
copy confirmed the presence of LA ligand and suggested the reten-
tion of acetate ions in the solid (Fig. S1). Indeed, a phenolate
stretching vibration band was observed at 1542 cmꢀ1 characteris-
tic of metal-salen complexes [21,22] and a band at 1451 cmꢀ1
likely assigned to the deformation vibration of the aromatic ring
[23]. An additional band was observed at 1640 cmꢀ1 compared to
the reference material LUS-LC, which is likely attributed to the
Fig. 2. Liquid UV–Vis spectra of HLA and Cu-LA in absolute ethanol and solid diffuse
reflectance UV–Vis spectra of LUS-CuLA.
tero et al. have reported that 6-amino-l,3-dimethyl-5-nitrosourac-
ilato (N5, N6)-aqua-2,20-bipyridine (N,N0)-copper (II) perchlorate
hydrate and 2,20-bipyridine(N,N0)-chloro-l,3-dimethylviolurato(N5,
O6)-copper (II) hemihydrate complexes display axial EPR powder
spectra with g\ = 2.03 and g|| = 2.25 and g\ = 2.03 and g|| = 2.20,
respectively, proposing a (4N)xy + Oz and (3N1O)xy + Oz environ-
ment around the Cu(II) [28]. Abry et al. have observed that
N,N0-bis(2-pyridinylmethyl)-ethane-1,2-diamine copper (II)trifluo-
romethanesulfonate complex with tetradentate ligand grafted in
mesoporous silica exhibits low g values with g|| = 2.219 and
g\ = 1.994 [29]. Combining EPR spectra with EXAFS analysis, they
proposed a 2N2O environment for the grafted copper complexes.
For LUS-CuLA, it seems that a (3N1O)xy + Oz environment around
copper is more reasonable than (2N2O)xy and the coordination
geometry is rather pentacoordinated.
m(C@N) of the imine function after incorporation of CuLA, discard-
ing a possible hydrolysis of the imine group present in LA during
the synthesis of the hybrid material [24,25]. The presence of ace-
tate ions in the solid was stated from the observation of the two
ꢀ
characteristic symmetrical
m
s(CO2ꢀ) and asymmetrical
m
as(CO2
)
stretching vibrations of the carboxylate group at 1400 and
1605 cmꢀ1, respectively [26]. The separation between these two
bands (
D
m
ꢁ200 cmꢀ1) is much larger than for the free acetate
ion (Dm , m , m
= 144 cmꢀ1 a(CO2ꢀ): 1560 cmꢀ1 s(CO2ꢀ): 1416 cmꢀ1),
which is in accordance with a monodentate acetate ion coordi-
nated to the metal ion [26,27]. These results combined with those
from elemental analyses allowed us to conclude that only one ace-
tate ion was retained in LUS-CuLA material to neutralize the charge
of the Cu(II) complex.
The hybrid material LUS-CuLA possesses a highly ordered 2D
hexagonal mesostructure, as shown in the XRD pattern with the
presence of the characteristic (100), (110) and (200) diffractions
(a0 = 4.7 nm) (Fig. 4, left). Nitrogen sorption isotherms for
LUS-CuLA and the intermediate materials during the synthesis of
the hybrid catalyst were of type IV according to the IUPAC
The integrity of the copper complex in LUS-CuLA was confirmed
by UV–Vis spectroscopy (Fig. 2). Indeed, LUS-CuLA displays a broad
absorption band below 330 nm, which can be attributed to ligand-
based p–
p⁄ transitions. A second band at ca. 370 nm was observed,
which was also present for the molecular species CuLA in ethanol,
but that was absent fort the free ligand HLA. This band can be
assigned to a n–p⁄ transition of the imino group coordinated to
the Cu ion [24]. Finally, the broad absorption band at ca. 588 nm,
which was also observed in complex CuLA, corresponds to the
d–d transitions of the Cu(II) ion.
The EPR spectrum observed for LUS-CuLA is typical of d9 com-
plexes with dx2
ground state and elongated square pyramidal
ꢀy2
or elongated octahedral environment for Cu(II) (Fig. 3). Indeed, val-
ues of g||, g\ and A|| of LUS-CuLA were equal to 2.18, 1.99 and
19.4 mT at room temperature, and slightly changed to 2.20, 1.98
and 19.5 mT at 130 K, which are similar to those observed for the
CuLB complex. In addition, EPR spectra become broaden at low
temperature (130 K). The reason is most probably that at low tem-
perature the grafted complex possesses low mobility and is
blocked in various conformations due to the heterogeneity of the
silanol and TMS position in its vicinity. Different parameters of g||
and A|| of Cu(II) complexes in EPR spectra can be attributed to a
varying number of nitrogen and oxygen coordinating sites and to
a change in the symmetry around the metal center. Moreno-Carre-
Fig. 3. Solid state EPR spectra of Cu-LA and LUS-CuLA. The spectra were recorded at
130 K for the former sample, and at 130 K and room temperature respectively for
the latter. Frequency: 9.35 GHz; power: 10, 32 and 10 mW, respectively; modula-
tion amplitude: 6G, 15G and 1G respectively, and modulation frequency: 100 kHz.
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W.-J. Zhou et al. / Polyhedron xxx (2013) xxx–xxx
5
Table 1
Comparison of the catalytic activity of LUS-CuLA with other systems.
Entry Catalyst
Phenol
conv. (%)
Diphenols
selectivity (%) selectivity (%)
Product
CAT/
HQ
CAT HQ BQ
1
2
3
4
No catalyst
LUS-PSE
Cu-LB
11
12
52
48
0
0
70
76
0
0
61
63
0
0
39
37
0
0
0
0
0.2 1.6
0 1.7
LUS-CuLA
Fig. 4. XRD patterns of LUS, LUS-PSE and LUS-CuLA (left), and N2 sorption
isotherms of LUS-PSE and LUS-CuLA at 77 K (right).
All the reactions were performed at 80 °C and pH = 6 in a phosphate buffer (60 mM)
solution; reaction time 2 h; phenol: H2O2 = 1: 1.2 (molar ratio); H2O2 was added at
once. Products were analyzed using HPLC. [b] Diphenols selectivity = yield of CAT,
HQ and BQ/phenol conversion (mol%). Relative precision ꢁ10%. CAT: catechol; HQ:
hydroquinone; BQ: 1, 4-benzoquinone.
classification (Fig. 4, right) [30]. The steep step of the capillary con-
densation at about 0.38 P/P0 evidences a narrow pore size distribu-
tion (Table S3) [8,31]. The specific surface (SBET) and total pore
volume of LUS-CuLA are 660 m2 gꢀ1 and 0.45 cm3 gꢀ1, respectively.
These values are lower than those corresponding to the parent
support LUS-PSE (820 m2 gꢀ1 and 0.66 cm3 gꢀ1), indicating the
incorporation of the metal complexes inside the pores of the silica.
Consistently, the pore diameter /BdB (Broekhoff and de Boer mod-
el) was reduced from 3.2 nm for LUS-PSE to 2.8 nm for LUS-CuLA,
upon incorporation of the complex. This shows that the metal com-
plexes are effectively inside the pores while there is still enough
space for the catalytic reaction to take place.
selectivity, in agreement with previous works [10,12]. The turn-
over numbers (TON) were comprised between 11 and 38, increas-
ing for higher amounts of oxidant present in the reaction mixture
(Table 2). Conversely, the highest diphenol selectivity of ca. 76%
was observed when less oxidant was used up to a phenol/H2O2
molar ratio of 1.2 (entries 1–4, Table 2). The CAT/HQ ratio was
not very sensitive to this parameter, which differs from results
reported on TS-1 zeolite, where this ratio strongly decreases for
higher amount of H2O2 [12]. Adding H2O2 stepwise or at once,
was investigated since it was reported to affect both activity and
selectivity [10,13]. Sequential addition of H2O2 for 1 h at the reac-
tion temperature mainly impacted on diphenol selectivity which
decreased from 73% (one addition) down to 55% (sequential), while
phenol conversion and CAT/HQ ratio were practically not affected
(entries 2–2⁄, Table 2). These results suggest that sequential addi-
tion favors the formation of by-products at the expense of CAT and
HQ.
3.3. Catalytic activity
The catalytic activity of LUS-CuLA was evaluated for phenol
hydroxylation with H2O2 (50 wt%) in diluted aqueous conditions
with an average initial phenol concentration of 1.4 wt% in a phos-
phate buffer. A reaction time of 2 h at 80 °C with a H2O2/phenol
molar ratio of one, using 1 mol% of LUS-CuLA catalyst was investi-
gated. The formed products were analyzed using HPLC. In addition
to the catechol (CAT), hydroquinone (HQ) and 1,4-benzoquinone
(BQ), dark brown by-products were also observed [32,33]. These
products may be the result of over-oxidation, coupling of radical
intermediates and cleavage of oxidation products, for example
maleic acid, acrylic acid, acetic acid, oxalic acid, polyphenols and
other oligomers, that are usually called as tar [34].
Considering that no activity on phenol hydroxylation was re-
ported at pH >9 over hydrotalcite-like compounds (CuAlCO3-HTLc)
and ternary hydrotalcites (CuNiAl or CuCoAl) [11], pH was limited
to the range 5–7 for the catalyst here studied. The phenol conver-
sion increased from 21% to 43% and to 51% when the pH was
decreased from 7.0 to 6.0 and further to 5.0 (Table S4). However,
the highest diphenols selectivity was observed at pH = 6.0, i.e.,
73%. Therefore, a good compromise between phenol conversion
and diphenol selectivity was to run the reaction at pH 6.
Without catalyst or with TMS-modified mesoporous silica (LUS-
PSE) there was a low phenol conversion (ꢁ12%) and no diphenol
formation (entries 1–2, Table 1). The molecular analogue CuLB
and the supported catalyst exhibited much higher phenol conver-
sion (ꢁ50%) and a high selectivity in diphenol (ꢁ70–80%) with a
CAT/HQ molar ratio of ꢁ1.6 in both cases (entries 3–4, Table 1).
Next the variation of the substrate/oxidant molar ratio was
investigated. Increasing the amount of oxidant, improves phenol
conversion for a lower H2O2 efficiency and lower diphenols
Finally, a recycling test was performed on the catalyst filtered
and washed with ethanol and acetone, and dried overnight at
60 °C before the next run. After the second run, the catalyst retains
all the catalytic properties (entries 2–20, Table 2). However, at the
third run, the activity decreases by 20% (entry 200). Indeed, after the
Table 2
Effect of the phenol:H2O2 molar ratio on the catalytic activity of LUS-CuLA and
recycling test.
Entry Phenol/ Phenol
H2O2 eff.
Conv. (%)
Diphenols
selectivity (%)
CAT/ TON
HQ
H2O2
Conversion
(%)
1
1:1/3
1:1
1:1
1:1
1:1
15
43
41
43
33
48
73
34
31
n.d.
30
20
31
25
73
73
55
71
62
76
52
2.0
1.6
1.7
1.7
1.9
1.8
1.6
11
13
23
30
20
36
38
2
2⁄
a
20
200
3
a
1:1.2
1:2.2
4
The reactions were performed at 80 °C, using 1 mol% Cu catalyst (LUS-CuLA), phe-
nol: H2O2 = 1:1, reaction time 2 h, pH = 6 in phosphate buffer solution (60 mM).
H2O2 conversion = mol H2O2 consumed to form diphenols/mol H2O2 converted
(mol%). Diphenols selectivity = yield of CAT and HQ/phenol conversion (mol%). 2⁄:
Sequential addition of H2O2 for 1 h; in all the other cases it was added at once.
a
Recycling test of 2 (20: second run, 200: third run). N.d. = non determined. Rel-
ative precision ꢁ10%. CAT: catechol; HQ: hydroquinone. TON: turnover number.
Please cite this article in press as: W.-J. Zhou et al., Polyhedron (2013), http://dx.doi.org/10.1016/j.poly.2013.06.039

6
W.-J. Zhou et al. / Polyhedron xxx (2013) xxx–xxx
first run, most of the TMS functions (ꢁ99%) were lost though the
copper ions were retained according to elemental analysis and
they were complexed to LA according to EPR spectroscopy analysis
(Fig. S2). The mesostructure of the solid was also retained since no
modification of the XRD pattern was observed (Fig. S3). The partial
loss of hydrophobic functions (TMS) likely favors metal leaching,
which can explain the decrease of the catalytic activity after the
second run. In addition, a broad absorption band from 200 to
800 nm was observed in the UV–Vis spectrum of the recycled cat-
alysts, due to the presence of tar in the solid, consistent with the
dark brown coloration of the solid after the reaction. This tar also
contributes to the decrease of the catalytic activity of the solid
by plugging the pore entrance and therefore diminishing the access
to the active sites.
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Acknowledgments
JoRISS cooperative program between Ecole Normale Supérieure
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.06.039.
Please cite this article in press as: W.-J. Zhou et al., Polyhedron (2013), http://dx.doi.org/10.1016/j.poly.2013.06.039