Synthesis, characterization and catalytic properties of heterogeneous iron(III) tetradentate Schiff base complexes for the aerobic epoxidation of styrene

Article abstract of DOI:10.1007/s11243-009-9322-9

A series of hybrid mesoporous SBA-15 materials containing four iron(III) Schiff base complexes of the type [FeL^x(NO₃)] (x = 4-7, L = N,N′-bis(salicylidene) ethylenediamine, N,N′-bis(salicylidene) diethylenetriamine, N,N′-bis(salicylidene)o-phenylenediamine, N,N′-bis(3-nitro-salicylidene)ethylenediamine) was synthesized by a post-grafting route. The XRD, N₂ adsorption/desorption and TEM measurements confirmed the structural integrity of the mes-oporous hosts, and the spectroscopic characterization tech-niques(FT-IR, UV-visspectroscopy, ¹HNMR)confirmedthe ligands and the successful anchoring of iron(III) Schiff base complexes over the modified mesoporous support. Quantification of the supported ligand and metal was carried out by TG/DSC and ICP-AES techniques. The catalyst FeL⁷-SBA resulting from N,N′-bis(3-nitro- salicylidene)ethylenediamine) ligand was considerably active for the aerobic epoxidation of styrene, in which the highest conversion of styrene reached 83.6%, and the selectivity to styrene oxide was 83.0%. Moreover, it was also found that the catalytic activity increases with the decrease inthe electron-donating ability of the Schiff bases, and the selectivity varies according to the types of substituents in the ligands. Springer Science+Business Media B.V. 2009.

Full text of DOI:10.1007/s11243-009-9322-9

Transition Met Chem (2010) 35:263–270
DOI 10.1007/s11243-009-9322-9
Synthesis, characterization and catalytic properties
of heterogeneous iron(III) tetradentate Schiff base
complexes for the aerobic epoxidation of styrene
•
•
•
Ying Yang Jingqi Guan Pengpeng Qiu
Qiubin Kan
Received: 1 November 2009 / Accepted: 4 December 2009 / Published online: 24 December 2009
Ó Springer Science+Business Media B.V. 2009
Abstract A series of hybrid mesoporous SBA-15 materials
containing four iron(III) Schiff base complexes of the
type [FeLx(NO3)] (x = 4–7, L = N,N0-bis(salicylidene)
ethylenediamine, N,N0-bis(salicylidene)diethylenetriamine,
N,N0-bis(salicylidene)o-phenylenediamine, N,N0-bis(3-nitro-
salicylidene)ethylenediamine) was synthesized by a post-
grafting route. The XRD, N2 adsorption/desorption and TEM
measurements confirmed the structural integrity of the mes-
oporous hosts, and the spectroscopic characterization tech-
niques(FT-IR,UV–visspectroscopy, 1HNMR)confirmedthe
ligands and the successful anchoring of iron(III) Schiff base
complexes over the modified mesoporous support. Quantifi-
cation of the supported ligand and metal was carried out by
TG/DSC and ICP-AES techniques. The catalyst FeL7-SBA
resulting from N,N0-bis(3-nitro-salicylidene)ethylenediamine)
ligand was considerably active for the aerobic epoxidation of
styrene, in which the highest conversion of styrene reached
83.6%, and the selectivity to styrene oxide was 83.0%.
Moreover, it was also found that the catalytic activity
increases with the decrease in the electron-donating ability of
the Schiff bases, and the selectivity varies according to the
types of substituents in the ligands.
VO(II), Mn(III), Cr(III), Bi(III), Mo(VI) and Co(II) com-
plexes have been widely used for the oxidation of cyclo-
hexanol [2], selective epoxidation of olefins [3–7] or
decomposition of H2O2 [8]. The use of dioxygen or air as
the oxidant for such processes is particularly attractive
from both economic and environmental standpoints.
However, triplet ground state of O2 disfavors reactions
with singlet organic compounds. Thus, O2 combined with a
sacrificial co-reductant, e.g., O2–H2 [9], O2–Zn powder
[10] and O2-organic reductant such as alcohol or aldehyde
[11] are required in the reaction mixture. Worthy of special
mention is the work of Lyons and Ellis employing per-
halogenated metal porphyrins as active catalysts for the
oxidation of hydrocarbons by O2 under mild conditions
[12]. However, the difficulty and high cost of synthesis are
problems limiting the use of perhalogenated metal por-
phyrins as catalysts in hydrocarbon oxidations. Therefore,
we have developed a series of iron(III) tetradentate Schiff
base catalysts that are more easily prepared and have fea-
tures in common with metalloporphyrins with respect to
their structure and catalytic activity. Bell and coworkers
have systematically studied the detailed factors controlling
activity and selectivity of perhalogenated metal porphyrins
as active catalysts for olefin epoxidation from the structural
standpoint [13]. Mohebi [14] and Mohebbi [15] also
studied the catalytic properties of vanadyl tetradentate
Schiff base complexes for aerobic olefin epoxidation based
on ancillary ligand modification.
Introduction
Covalent immobilization of olefin epoxidation catalysts
onto solid supports can increase catalyst stability and allow
for product separation [1]. In this field, polystyrene (PS),
zeolite-Y, M41S and mesoporous carbon-anchored Cu(II),
It is generally recognized that iron complexes are less
environmentally damaging than other transition-metal
complexes and such complexes have received considerable
attention as catalysts [16, 17]. Iron complexes consisting of
porphyrin [18], phthalocyanine [19] and quinoline [20] are
involved in many oxidation reactions. However, little is
known about the catalytic role of iron(III) Schiff base
Y. Yang ꢀ J. Guan ꢀ P. Qiu ꢀ Q. Kan (&)
College of Chemistry, Jilin University, Jiefang Road 2519,
130023 Changchun, Jilin, People’s Republic of China
e-mail: qkan@mail.jlu.edu.cn
123

264
Transition Met Chem (2010) 35:263–270
complexes in aerobic olefin epoxidations. The present
study seems to be the first attempt to use iron(III) tetra-
dentate Schiff base complexes for the aerobic oxidation of
styrene.
120 °C for 2 h. After cooling, the 3-aminopropyltriethox-
ysilane (1.6 mmol in 20 mL of dry toluene) was added under
N2 atmosphere, and the mixture was stirred at room tempera-
ture for 24 h. The resulting solid was filtered off, washed,
Soxhlet-extracted with CH2Cl2 for 24 h and dried under vac-
uum. Anal. found for APS-SBA-15: C, 5.4; H, 1.4; N, 1.4%.
Giving the increasing interest in the structure and cata-
lytic chemistry of supported iron complexes, here we have
heterogenized a novel series of iron(III) Schiff base com-
plexes onto a silica surface via the linker 3-aminopropyl-
triethyloxysilane. The catalytic properties of these hybrid
materials were investigated for the epoxidation of styrene
using air combined with isobutyraldehyde as the oxidant.
Synthesis of substituted salicylaldehyde
5-chloromethylsalicylaldehyde (5-CM-Sal) was synthesized
from salicylaldehyde by the classical chloromethylation
method. In a typical synthesis, 17.5 g (160 mmol) of sali-
cylaldehyde was treated with 24 mL of 38% aqueous
formaldehyde and 1.2 g of ZnCl2 in 100 mL of conc. HCl.
The mixture was stirred at room temperature under N2
atmosphere for 24 h. The resulting white solid was filtered
off and repeatedly extracted with diethyl ether. The com-
bined organic phases were washed with saturated aqueous
NaHCO3 and water and then dried over MgSO4. The vis-
cous oil was obtained by distillation and then subjected to
crystallization in petroleum ether. The extract furnished
5-chloromethylsalicylaldehyde as white-colored needles.
Experimental
The synthesis procedures for various neat or heterogeneous
iron(III) complexes are depicted in Scheme 1.
Synthesis of aminofunctionalized SBA-15
The mesoporous support SBA-15 (1.0 g), prepared by a
literature method [21, 22], was pre-activated by heating to
NO2
OH
OH
OH
HCl, HCHO,
(a)
HNO3
CH3COOH
ZnCl2
CHO
CHO
CHO
CH2Cl
CH2Cl
5-CM-Sal
3-N-5-CM-Sal
B
B
B
OH
OH
OH
APTES
OH
EtOH
OH
OH
OH
O
O
O
Si
+
NH2RNH2
NH2
(b)
+
A
Reflux
Toluene
CHO
N
N
A
A
R
SAB-15
APS-SBA-15
5-CM-Sal, A = CH2Cl, B = H
3-N-5-CM-Sal, A = CH2Cl, B = NO2
H2Lx (x = 1-7)
B
B
NO3
O
O
N
Fe
OH
O
O
O
Si
NH
A
CH2
N
R
FeLx (x = 4-7)
NO3
Fe
o
O
OH
OH
Fe(NO3)3.9H2O
MeOH
(c)
N
N
N
N
R
R
FeLx (x = 1-3)
Scheme 1 Overall synthesis path for the preparation of a substituted
salicylaldehyde, b SBA-15 anchored iron(III) complexes and c homo-
geneous iron(III) complexes. H2Lx (x = 1–7): H2L1, R = CH2CH2,
A = H, B = H; H2L2, R = CH2CH2NHCH2CH2, A = H, B = H;
H2L3, R = Ph, A = H, B = H; H2L4, R = CH2CH2, A = CH2Cl,
B = H; H2L5, R = CH2CH2NHCH2CH2, A = CH2Cl, B = H; H2L6,
R = Ph, A = CH2Cl, B = H; H2L7, R = CH2CH2, A = CH2Cl,
B = NO2. FeLx (x = 4–7): FeL4-SBA, R = CH2CH2, A = CH2Cl,
B = H; FeL5-SBA, R = CH2CH2NHCH2CH2, A = CH2Cl, B = H;
FeL6-SBA, R = Ph, A = CH2Cl, B = H; FeL7-SBA, R = Ph, A =
CH2Cl, B = NO2. FeLx (x = 1–3): FeL1, R = CH2CH2, A = H,
B = H; FeL2, R = CH2CH2NHCH2CH2, A = H, B = H; FeL3,
R = Ph, A = H, B = H
123

Transition Met Chem (2010) 35:263–270
265
Found: C, 57.6; H, 4.1%. Anal. Calc. for C8H7O2Cl: C,
56.3; H, 4.1%. FTIR (KBr pellets, cm-1): 3240 (OH),
2925, 2873, (Aliph-H), 1659 (C=O), 1260 (CH2Cl), 772
Table 2 IR and electronic spectral data of compounds
Compound Selective IR bands (cm-1
Transition frequency
)
(kmax/nm)
1
C=N C–O C–Cl Fe–O/
Fe–N
(C–Cl). UV–vis, k (nm): 224, 257, 332. H NMR (CDCl3,
TMS, d ppm): 11.07 (1H, s, OH), 9.90 (1H, s, CHO), 7.59
(1H, s, Ar–H), 7.26–7.19 (1H, d, Ar–H), 7.02–6.08 (1H, d,
Ar–H), 4.59 (1H, s, CH2Cl).
H2L1
FeL1
H2L2
FeL2
H2L3
FeL3
H2L4
1635 1280
1623 1298
1629 1268
1618 1305
1610 1274
1605 1309
–
–
–
–
–
–
–
249, 281, 334, 401
539/413 233, 312, 362, 521
240, 275, 320, 389
582/419 235, 321, 375, 515
225, 267, 289, 390
532/454 252, 283, 356, 505
3-nitro-5-chloromethylsalicylaldehyde (3-N-5-CM-Sal)
was synthesized as follows: 0.72 mL of fuming nitric acid
was added dropwise to a vigorously stirred mixture of
5-chloromethylsalicylaldehyde (2.88 g) and acetic acid
(3.36 mL). After the addition was completed, the mixture
was stirred at 35 °C for 3 h. The yellowish product was
collected by filtration and recrystallization from benzene.
Found: C, 44.3; H, 2.8; N, 6.4%. Anal. Calc. for C8H6ClNO4:
C, 44.6; H, 2.8; N, 6.5%. FTIR (KBr pellets, cm-1):
3246 (OH), 2932, 2876, (Aliph-H), 1666 (C=O), 1265
–
–
1638 1290 698
–
236, 287, 380
H2L4-SBA 1635 1280
FeL4-SBA 1629 1305
H2L5
H2L5-SBA 1630 1286
FeL5-SBA 1628 1292
H2L6
H2L6-SBA 1614 1284
FeL6-SBA 1612 1305
H2L7
H2L7-SBA 1641 1285
FeL7-SBA 1636 1308
–
–
–
254, 281, 313, 378, 430
256, 280, 315, 401, 526
241, 290, 334, 400, 421
229, 247, 280, 313, 404
210, 259, 329, 398, 529
250, 270, 292, 349
554
–
1631 1279 766
–
–
–
543
–
1
(CH2Cl), 783 (C–Cl). H NMR (CDCl3, TMS, d ppm):
11.27 (1H, s, OH), 10.42 (1H, s, CHO), 7.90 (1H, s, Ar–H),
1617 1276 751
–
–
–
247, 278, 311, 393
8.15 (1H, s, Ar–H), 4.60 (1H, s, CH2Cl).
563
–
280, 329, 391, 565
1644 1296 702
238, 278, 318, 360, 454
241, 277, 316, 359, 455
234, 331, 364, 398, 522
–
–
–
Synthesis of ligands: H2Lx and iron(III) complexes:
FeLx-SBA (x = 4–7)
543
functionalized SBA-15. In a typical synthesis, 0.8 mmol of
ligand was added to 0.5 g of APS-SBA-15 in dry toluene and
stirred at reflux temperature for 24 h to obtain the hybrid
materials H2Lx-SBA (x = 4–7). FeLx-SBA was obtained by
stirring 0.41 g of H2Lx-SBA with 0.11 g of Fe(NO3)3ꢀ9H2O
in MeOH at room temperature for 12 h. The resulting solid
was filtered and Soxhlet-extracted with CH2Cl2 to remove
untethered species and then dried in vacuum.
The modified Schiff bases were obtained by the condensa-
tion of appropriate diamine and substituted salicylaldehyde.
H2Lx (x = 4–6) were prepared by the treatment of a
dichloromethane solution of 5-chloromethylsalicylaldehyde
(20 mmol) with ethylene diamine, diethylenetriamine or
o-phenylenediamine (10 mmol). H2L7 was obtained by the
condensation of 3-nitro-5-chloromethylsalicylaldehyde and
ethylene diamine in CH2Cl2. The mixture was stirred under
N2 atmosphere at room temperature for 24 h. The ligands
were isolated by filtration, washed with CH2Cl2 and char-
acterized by FT-IR, UV–vis spectroscopy and 1H NMR (see
Tables 1 and 2).
Synthesis of ligands: H2Lx and iron(III) complexes:
FeLx (x = 1–3)
Schiff base ligands H2Lx (x = 1–3) and the corresponding
iron(III) complexes FeLx (x = 1–3) were prepared by a
similar manner to that reported in the literature [23].
These Schiff base ligands were immobilized on meso-
porous SBA-15 based on the nucleophilic reaction between
the chloromethyl modified Schiff bases and the aminopropyl
Table 1 1H NMR chemical shift (d in ppm) of compounds H2Lx (x = 1–7)
Compounda
O–H
CH=N
H (aromatic)
CH2Cl
Methylene
b
H2L1
H2L2
H2L3
H2L4
H2L5
H2L6
H2L7
a
13.20 (s)
13.22 (s)
13.08 (s)
11.12 (br)
12.08 (br)
13.04 (br)
13.40 (br)
8.35 (s)
7.30–6.80 (m)
7.10–6.40 (m)
7.80–6.80 (m)
7.83–6.98 (m)
7.99–7.00 (m)
7.83–6.89 (m)
8.03–7.12 (m)
–
3.91 (t)
8.50 (s)
–
4.0–2.5 (m)
–
8.52 (s)
–
8.44, 8.40 (s)
8.39, 8.34 (s)
8.42 (s)
4.44 (s)
4.43 (s)
4.25 (s)
4.56 (s)
3.19–3.10 (t)
3.40–3.20 (m)
–
8.65 (s)
3.45–3.6 (m)
br broad, s singlet, t triplet and m multiplet
b
Not detected
123

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Transition Met Chem (2010) 35:263–270
1.0
Catalytic oxidations
100
96
92
88
84
80
0.5
The epoxidation of styrene with air was carried out in a
batch reactor. In a typical run, 10 mmol of styrene and
50 mg of catalyst (5 mg for the neat iron catalyst) were
charged into a 100 mL two-necked flask equipped with a
condenser and with an air pump. The dry air with a stable
flow rate of 80 mL/min controlled by a flowmeter was
bubbled into the bottom of the reactor. The liquid organic
products were quantified by using a gas chromatograph
(Shimadzu, GC-8A) equipped with a flame detector and an
HP-5 capillary column. The liquid organic products were
identified by comparison with authentic samples and GC–
MS coupling.
0.0
-0.5
-1.0
-1.5
-2.0
0
200
400
600
800
Tempreature (oC)
Fig. 1 TG and DSC profile of FeL4-SBA
below 190 °C is due to the physically adsorbed water and
solvent inside the pores. The 7.8% weight loss at 190–357 °C
is mainly related to the combustion of amine. Larger loss of
8.4% in the temperature range from 357 to 714 °C is prob-
ably attributed to the decomposition of Schiff base groups
[23]. The loading of Schiff base is 0.23 mmol/g based on the
8.4% of weight loss, and iron content is 0.35 mmol/g esti-
mated by ICP-AES. The molar ratio of iron to ligand is
approximately 1.5:1 and deviates from the theoretical value
of 1:1, indicating that partial uncoordinated iron is captured
and not released after the elution by CH2Cl2. The DSC curve
agrees with the TG result. An intense exothermal peak at
605 °C is observed. The much higher temperature peak of
FeL4-SBA than that of H2L4-SBA (586 °C) gives another
piece of evidence for the existence of iron complexes in the
hybrid materials. TG/DSC analysis data of other supported
iron complexes are listed in Table 3. It is clear that all ligand
loadings are similar, which facilitates comparative study of
the role of iron(III) compounds with different substituted
ligands on catalytic activity.
Results and discussion
Spectroscopic characterization
1H NMR spectra (recorded in DMSO-d6) of the tetradentate
Schiff base ligands of N2O2 donor sets possess two phenolic
1
and azomethine groups (Table 1). H NMR spectra of all
the Schiff bases H2Lx show the aromatic protons as multi-
plet in the range 7.99–6.80 ppm and O–H protons of
phenolic groups ranging from 13.20 to 10.04 ppm [14].
The azomethine protons appear at 8.44–8.34 ppm. These
functional groups can also be identified by FT-IR and UV–vis
spectroscopy. As illustrated in Table 2, IR bands of H2Lx in
the 1644–1610 and 1308–1268 cm-1 can be attributed to
the C=N and C–O stretching vibrations [24]. These bands
register a redshift of 3–12 cm-1 and a blueshift of 6–37 cm-1
in the iron(III) complexes, respectively. The shifts support
the participation of the azomethine and phenolic oxygen of
these ligands in binding to the metal ions [24]. Electronic
spectra of FeLx-SBA show bands in the visible region
attributed to ligand-to-metal charge transfer (LMCT) O2p
or N2p ? Fe3d. The ultraviolet bands in the regions
of 250–280 nm and 312–360 nm due to the intraligand IL
(g–g*) localized predominantly on the phenyl rings and on C=N
fragments of the N2O2-ligands, respectively [25]. Expected
d–d transitions of iron coordination field at 522–565 nm in
combination with Fe–O vibrations at 556–584 cm-1 dem-
onstrate the formation of iron complex on SBA-15 host.
The similarity of the overall electronic spectral profiles for
FeLx-SAB suggests that they have similar coordination
environments.
Structural integrity studies
The powder XRD patterns for SBA-15, H2L4-SBA and
FeL4-SBA are depicted in Fig. 2. Three reflections, one
strong peak at (100) and two with low intensity at (110)
and (200) planes, are seen in the range 2h = 0.8–2° for the
mesoporous SBA-15 material, indicating a significant
degree of long-range ordering and well-formed hexagonal
pore arrays [26]. For the hybrid materials, the relative
intensities of the prominent diffraction peak (100)
decreased after the introduction of bulky organometallic
groups. The intensity reduction may be mainly due to
contrast matching between the silicate framework and
organic moieties that are located inside the channels of
SBA-15 [27]. TEM images provide a direct visualization of
well-ordered hexagonal arrays of 1D mesoporous channels
TG/DSC studies
The TG/DSC analysis result of FeL4-SBA is depicted in
Fig. 1. Quantification of weight loss at various steps is pos-
sible because of their nonoverlapping nature. Weight loss
123

Transition Met Chem (2010) 35:263–270
267
Table 3 TG/DSC analysis
data of samples
Materials
Temperature
range (°C)
Weight
loss (%)
Peak (°C)
Type of loss
Ligand loading
(mmol/g)
H2L4-SBA
FeL4-SBA
FeL5-SBA
FeL6-SBA
FeL7-SBA
26–250
250–386
450–707
33–192
8
68
270
586
79
Endothermic
Endothermic
Exothermic
Endothermic
Exothermic
Exothermic
Endothermic
Exothermic
Exothermic
Endothermic
Exothermic
Exothermic
Endothermic
Exothermic
Exothermic
–
8
–
7.3
2.7
7.8
8.4
3.6
6.7
7.1
3.4
6
0.21
–
192–357
357–714
27–260
250
605
80
–
0.23
–
260–310
310–740
36–180
265
625
80
–
0.25
–
180–314
314–750
35–200
267
646
82
–
10
0.24
–
2.3
9.2
5.9
200–324
324–720
282
650
–
0.22
hexagonal channels, and thus results in increased wall
thickness. A similar trend has also been observed previ-
ously [28]. Compared to H2Lx-SBA (x = 4, 5), however,
FeLx-SBA (x = 4, 5) exhibited larger textural parameters,
which is consistent with the idea reported in literature [29].
Aerobic epoxidation of styrene
Electronic effect and catalytic properties
(a)
To quantify the influence of electronic properties of iron(III)
Schiff base complexes on the aerobic epoxidation of styrene,
iron complexes with electron-donating and electron-with-
drawing groups have been evaluated. From the data present in
Table 5, FeL4-SBA shows the highest activity with 80.9%
conversion of styrene after 8 h (TOF 57.8 h-1) among the
threecatalystsFeLx-SBA (x = 4–6). Adefinitetrendtohigher
activity is displayed by diimine bridge substituents on the
Schiff base skeleton in the order: FeL4 [ FeL5 [ FeL6. From
the mechanistic point of view, the o-phenylenediimine bridge
is the strongest electron-donor among three diimines, which
stabilizes the positive iron(IV)-oxo species and decreases the
electrophilic attack to styrene. The introduction of nitro
groups into the Schiff base ligand leads to the much higher
conversion of 83.6%. Similarly, the electron-withdrawing
substituents are expected to destabilize the metal-oxo inter-
mediate, making it more reactive (Scheme 2).
(b)
(c)
1
2
3
4
5
Degrees (2θ)
Fig. 2 XRD patterns of a SBA-15, b H2L4-SBA and c FeL4-SBA
particularly along the direction of the pore axis or in the
direction perpendicular to the pore axis (Fig. 3). The N2
adsorption/desorption isotherms of SBA-15 and the hybrid
materials are shown in Fig. 4. Obviously, the hybrid
materials maintain the characteristics of type IV isotherms
and show a uniform pore size distribution in the meso-
porous region. However, compared to SBA-15, the ligand
and complex supported samples exhibit a drastic decrease
in N2 uptake due to the relatively high loading with the
comparatively bulky organometallic compounds on the
surface of the mesoporous channels. The textural properties
are listed in Table 4. Obviously, the hybrid materials dis-
play reduced specific surface areas, pore sizes and pore
volumes, which indirectly evidences that the ligands or
organometallic complexes are in the empty voids of the
It is clear that the activity decreases accompanied by the
elevation of iron loading for FeLx-SBA (x = 4–6), which
suggests that the efficiency of catalysts is strongly dependent
on the electronic properties of the Schiff bases. However,
are more iron species attributed to the higher activity for
FeL7-SBA? To probe this question, what is the truly active
123

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Transition Met Chem (2010) 35:263–270
Fig. 3 TEM images of
H2L4-SBA (a, b) and FeL4-SBA
(c, d): a and c are in the
direction of the pore axis,
b and d are perpendicular
to the pore axis
It turned out that the conversion of styrene was almost 0%
after 8 h. By contrast, the corresponding neat iron com-
plexes are highly active and display slightly lower selec-
tivity to styrene oxide than the corresponding heterogeneous
catalysts FeLx-SBA. These observations suggest that the
oxidations occur due to the catalytic nature of the anchored
iron(III) Schiff base complexes. It is reasonable to speculate
that the catalytic activity is related to ligand loading since
overall ligands are binding to iron(III). Therefore, the cata-
lytic activity of FeLx-SBA with similar ligand loading is
mainly determined by electro-withdrawing effect of substi-
tuted ligands. That is why FeL7-SBA is the most active
catalyst with 83.6% conversion of styrene and with a lower
TOF value (38.1 h-1).
(a)
(b)
(c)
(d)
(a)
(b)
(c)
(e)
(f)
(d)
(e)
(g)
(f)
(g)
0.0 0.2 0.4 0.6 0.8 1.0
4
8
12
16
20
Pore diameter (nm)
Relative pressure (P/P0)
Fig. 4 N2 adsorption/desorption isotherms and pore size distribution
profiles of a SBA-15, b H2L4-SBA, c FeL4-SBA, d H2L5-SBA,
e FeL5-SBA, f H2L6-SBA and g FeL6-SBA
Possible mechanism for the aerobic epoxidation
of styrene
species seems crucial. Blank reaction performed over SBA-
15 or H2L4-SBA under identical conditions shows only
negligible conversion. As well as coordination to a chelate
ligand, the iron(III) may also interact with the surface Si–
OH. Thus, the materials of blank SBA-15 that adsorbed
iron(III) were also investigated for epoxidation experiment.
The catalytic properties and electronic effects discussed
above are closely correlated with the mechanism. Two
major pathways involving either a high valent metal-peroxo/
oxo species or free radical intermediates have been proposed
for the oxidation of styrene by O2 [30–32]. The mechanistic
model of Figueiredo [33], Ramaraj [34] and Ghandi [35] can
123

Transition Met Chem (2010) 35:263–270
269
Table 4 Analysis results of textural characteristics
Materials
SBET (m2 g-1
)
Vp (cm3 g-1
)
Dap (nm)
Db100(nm)
Ao (nm)
Wall thicknessc (nm)
SBA-15
892
409
302
340
416
387
285
1.01
0.72
0.53
0.61
0.71
0.66
0.52
6.35
5.31
5.34
6.11
5.25
6.03
6.01
10.28
10.71
11.19
10.50
11.09
11.10
11.43
11.87
12.37
12.92
12.12
12.80
12.82
13.20
5.52
7.06
7.58
6.01
7.55
6.79
7.19
H2L4-SBA
FeL4-SBA
H2L5-SBA
FeL5-SBA
H2L6-SBA
FeL6-SBA
a
Obtained from the desorption branch
b
c
Calculated from XRD results
ꢀ
pffiffi
ꢁ
Wall thickness ¼ Ao ꢁ Dp Ao ¼ 2D100
=
3
Table 5 Catalytic results of homogeneous and heterogeneous iron(III) complexes in aerobic epoxidation of styrene
Catalysts
Metal loading
(mmol/g)
Styrene conversion
(%)a
TOF
(h-1 c
Product selectivity (mol%)d
)
So
Bza
Others
FeL4-SBA
FeL5-SBA
FeL6-SBA
FeL7-SBA
FeL1
0.35 (1.9)b
0.43 (2.4)
0.49 (2.7)
0.55 (3.1)
3.10 (17.3)
2.97 (16.5)
2.70 (15.1)
80.9
70.2
48.2
83.6
83.1
86.2
77.4
57.8
40.8
24.6
38.1
66.9
72.6
71.6
59.7
53.5
51.4
83.0
51.2
43.2
46.8
32.8
40.5
44.6
8.9
7.5
6.0
4.0
8.1
4.7
1.8
2.6
44.1
55.0
50.6
FeL2
FeL3
a
Reaction conditions: catalyst 50 mg for FeLx-SBA (x = 4–7), 5 mg for FeLx (x = 1–3), styrene 1.14 mL (10 mmol), CH3CN 10 mL, flow of
air 80 mL/min, isobutyraldehyde 2.28 mL (25 mmol), temperature 80 °C and duration 8 h
b
wt.%
TOF, h-1: (turnover frequency) moles of substrate converted per mole metal ion per hour
c
d
So: Styrene oxide, Bza: benzaldehyde and Others: including benzoic acid, phenylacetaldehyde and 1-phenyl-ethane-1,2-diol
Scheme 2 Proposed
mechanism for aerobic
epoxidation of styrene with
iron(III) complexes in the
presence of isobutyraldehyde
O2
*
FeLx-SBA
*
O
(i)
OO(O)C
(O)C
*
OO
O
OO(O)C
H
*
(ii)
H
*
*
*
(O)C
+
+
OO(O)C
+
HCHO
O=FeIVLx-SBA
2
(iii)
O2
(iv)
O
FeIIILx-SBA
1
O
O
N
Fe
3
N
O
CH
O
CH2
O
(v)
O
Fe
N
N
4
123

270
Transition Met Chem (2010) 35:263–270
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(1) Isopropylacyl radical produced through the electron
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Conclusions
We have succeeded in designing an active family of het-
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Schiff bases for styrene epoxidation by air under mild
conditions. Systematic ancillary ligand substitutions dem-
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the Schiff bases promotes reaction activity and the selec-
tivity to styrene oxide. The correlation between electronic
effect and activity seems to be subjected to subtle structural
effects that are not apparent in other heterogeneous sys-
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Acknowledgments Financial assistance from the National Basic
Research Program of China (2004CB217804) and the National Nat-
ural Science Foundation of China (20673046) is gratefully acknowl-
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