Metallo-deuteroporphyrin as a novel catalyst for p-xylene oxidation using molecular oxygen

Article abstract of DOI:10.1016/j.ica.2011.12.039

The liquid phase oxidation of p-xylene with air as oxygen source catalyzed by metallo-deuteroporphyins [M(DPDME)], where M = Fe(III)Cl, Co(II), or Mn(III)Cl, were carried out in the absence of additives and solvents. Oxidation of p-xylene by M(DPDME) resulted in the formation of p-methylbenzaldehyde, p-methylbenzyl alcohol and p-methylbenzoic acid as major products. The complexed metals have an important effect on the activities of these metallo complexes and the distribution of the products. The oxidation reaction initiated by M(DPDME) was deemed to undergo a different path from metallo- tetraphenylporphyrins, and most of M(DPDME) may convert to μ-oxo dimmer as the active intermediate during the oxidation of p-xylene.

Full text of DOI:10.1016/j.ica.2011.12.039

Inorganica Chimica Acta 382 (2012) 167–170
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Note
Metallo-deuteroporphyrin as a novel catalyst for p-xylene oxidation using
molecular oxygen
a,
⇑
b
b
b,
⇑
Weiyou Zhou , Chengguo Sun , Shichao Xu , Bingcheng Hu
a
School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China
College of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2011
Received in revised form 19 December 2011
Accepted 21 December 2011
Available online 29 December 2011
The liquid phase oxidation of p-xylene with air as oxygen source catalyzed by metallo-deuteroporphyins
M(DPDME)], where M = Fe(III)Cl, Co(II), or Mn(III)Cl, were carried out in the absence of additives and sol-
vents. Oxidation of p-xylene by M(DPDME) resulted in the formation of p-methylbenzaldehyde, p-meth-
ylbenzyl alcohol and p-methylbenzoic acid as major products. The complexed metals have an important
effect on the activities of these metallo complexes and the distribution of the products. The oxidation
reaction initiated by M(DPDME) was deemed to undergo a different path from metallo-tetraphenylpor-
[
Keywords:
phyrins, and most of M(DPDME) may convert to
l
-oxo dimmer as the active intermediate during the oxi-
Deuteroporphyrins
p-Xylene oxidation
Air
dation of p-xylene.
Ó 2011 Elsevier B.V. All rights reserved.
Biomimetic catalysis
1
. Introduction
In our reported literature, we have found the high catalytic
activities of M(DPDME) (M = Co(II), Cu(II), Zn(II) and Ni(II)) in the
oxidation of cyclohexane with air as the oxidant [13,14]. In the
present work, the authors have applied the metallo-deuteropor-
phyrins, where metals are Fe(III)Cl, Co(II), Mn(III)Cl (Fig. 1), as
the catalyst for the oxidation of p-xylene by molecular oxygen in
a solvent-free homogeneous system. The catalytic activities of
the corresponding metallo-tetraphenylporphyrins have also been
investigated as the comparison.
The selective oxidation of hydrocarbon with mild conditions con-
tinues to be a challenge [1]. Varied catalysts have been developed
and applied in the reaction, however, these catalysts often operate
under harsh conditions and produce large amounts of heavy-metal
waste [2,3]. Metallo-porphyrins as the biomimetic catalysts have
been attracting much attention from organic and industrial chem-
ists because they are robust catalyst and cause less harm to the envi-
ronment [4–6]. The method has become the most likely strategy to
replace the traditional process. For example, the simple metallo-
porphyrins system has been reported as a promising candidate for
the selective oxidation of cyclohexane [7]. Although a huge amount
of work has been done during the last decades, nearly all of the
metallo-porphyrins used as oxidation catalysts were based on the
system of synthetic meso-tetraphenylporphyrin (TPP) [8–11]. There
are few reports about the models derived from heme, the high acces-
sibility of deuteroporphyrins together with its excellent stability
and the close relationship to the naturally occurring hemes makes
it an ideal compound to mimic enzymatic systems based on hemes.
Additionally, deuteroporphyrin dimethyl ester and its metal
complexes can be prepared in high yield (Y > 90% [12]) from the
red blood pigment heme which is available in almost any desired
amount from slaughterhouse wastes.
2
. Experimental
2.1. Instruments and reagents
All reagents and solvents were of analytical grade and were ob-
tained commercially. DPDME was synthesized from heme according
to published procedures [12] and purified by column chromatogra-
phy on silica gel with dichloromethane/ethyl ether (10:1). Iron, co-
balt and manganese were then introduced by a standard metal
insertion method [15,16]. No impurities were found in the p-xylene
by GC–MS analysis before use.
A Bruker 500 MHz spectrometer was used for 1H NMR spectra
of deuteroporphyrins and those molecular complexes in CDCl
3
sol-
vent. For FTIR spectra, a Shimadzu IR-435 instrument was applied
+
(
using KBr pellets). ESI -MS results were obtained from a Finnigan
⇑
Corresponding authors. Tel.: +86 0519 86330521 (W. Zhou), +86 025 84315030
B. Hu).
E-mail addresses: zhouwy426@yahoo.cn (W. Zhou), hubingcheng@yahoo.com
B. Hu).
TSQ Quantum Mass Spectrometer. The UV–Vis spectra were
recorded in DMF solution on a Shimadzu UV-240 spectrophotom-
eter (a 1 cm optical path length was employed).
(
(
0
020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.039

1
68
W. Zhou et al. / Inorganica Chimica Acta 382 (2012) 167–170
H C
396
496
3
1
1
0
.5
.0
.5
5
28
0
.3
.2
564
a
618
N
M
N
H C
CH3
3
0
5
00
600
N
N
H COOC
0.2
0.1
518 544
545
3
558
1.0
365 391
3
94 457
CH3
b
c
d
H COOC
0.5
3
500
600
M = Mn(III)Cl, Co(II), Fe(III)Cl
0.0
Fig. 1. Formula of the metallo-deuteroporphyrins mentioned in the text.
400
500
600
700
Wavelength (nm)
Fig. 2. UV–Vis absorption spectra of DPDME and its metallo complexes in DMF. (a)
DPDME; (b) ClMn(III) (DPDME); (c) ClFe(III) (DPDME); (d) Co(II) (DPDME)
2.2. General procedure for p-xylene oxidation
In the standard procedure, the catalytic oxidation of p-xylene
has been carried out in a stainless steel reactor of 2 L volume
equipped with a mechanical stirrer, an internal thermocouple
and cooling coils for regulating reaction temperature. In a typical
experiment, the amount of p-xylene was 1000 ml, the optimum
temperature was set at 150 °C, the air pressure was 0.8 MPa and
the amount of catalyst was 0.02 mmol. We sampled during the
reaction every 30 min until the yield decreased markedly. After
the reaction, the oxidation was stopped and the products were
analyzed by a GC–MS.
CH3
CH OH
CHO
COOH
2
M(DPDME)
air
+
other product
+
+
CH3
CH3
CH3
CH
3
p-xylene
TALC
TALD
TA
Scheme 1. Oxidation of p-xylene catalyzed by M(DPDME).
2.3. Product analysis
3.2. Oxidation of p-xylene catalyzed by M(DPDME)
GC–MS analysis of the catalytic oxidation products was per-
These metallo-deuteroporphyrins can smoothly catalyze
p-xylene oxidation at temperatureof 150 °C and air partial pressures
of 0.8 MPa in the absence of co-catalysts and solvents. The main
products of p-xylene oxidation by molecular oxygen (as air) are p-
methylbenzaldehyde (TALD), p-methylbenzyl alcohol (TALC) and
p-methylbenzoic acid (TA) as presented in Scheme 1, and aldehyde
is the predominant product of the reaction. GC–MS analysis data
showed that there were small amounts of ester and other by-prod-
ucts; however, there was no evidence for the oxidation of the second
methyl group. The result was identical to the case catalyzed by
formed using a Finnigan DSQ Quantum Mass Spectrometer with
a 0.5 mm i.d. ꢀ 15 m PEG2000 capillary column; helium was the
carrier gas. The chromatographic conditions were as follows: initial
temperature: 50 °C, keep for 1 min; then increase the temperature
to 260 °C with a rate of 15 °C/min, keep for 1 min; Injector volume:
1
lL; Split ratio: 1:0.8.
3
. Results and discussion
2
0
5
3
.1. Synthesis and characterization of catalysts
M(DPDME), where M = Fe(III)Cl, Co(II), Mn(III)Cl, were synthe-
1
+
sized from heme and confirmed by FT-IR, H NMR and ESI -MS. Addi-
tionally, the UV–Vis was used to characterize these catalysts and the
absorption spectra of DPDME as well as its metallo complexes are
shown in Fig. 2. From such spectra we can find that insertion of a me-
tal into the deuteroporphyrin results in the well-known reduction
from four to two Q bands in the UV–Vis spectra [17] except Mn(III)Cl,
in which two new Sorets emerged at 365 and 457 nm, respectively.
The result was accordance to the spectra of hypertrophic type metal-
lo-porphyrins reported by Hanso and coworkers [18]. The phenom-
1
10
5
enon is due to the intensively interaction between a1u (p) ? e.g. (dp)
and Soret band [19,20]. The absorption spectrum of the protonated
species consists of only two bands in the Q-band region of the spec-
0
trum because of the degeneracy of the excited state [21]. The Q band
0
1
2
3
4
5
6
7
⁄
in M(DPDME) has been attributed to the
p
?
p
electronic transition
Time (h)
from the HOMO (highest occupied molecular orbital) to the LUMO
Fig. 3. Conversion and product percentages in the course of the oxidation of p-
xylene catalyzed by Co(II) (DPDME) [(s) conversion; (.) TALD; (}) TALC; (d) TA;
(
lowest unoccupied molecular orbital) of the ring of porphyrin,
and the B band has been attributed to the deeper
transitions [22].
p levels ? LUMO
(
4) by-product]. Reaction conditions: p-xylene 1000 mL (8.9 mol), the catalyst
12 mg (0.02 mmol), air pressure 0.8 MPa, temperature 150 °C.

W. Zhou et al. / Inorganica Chimica Acta 382 (2012) 167–170
169
cobalt naphthenate [23] and chloroperoxidase [24]. The contrasting
O
O
O2
a
1
2
CoII
CoIII
CoIII
O
O
CoIII
research showed that p-xylene could be oxidized in a very low yield
at the same temperature and pressure in the absence of M(DPDME).
This fact indicated that metallo-deuteroporphyrins acted as cata-
lysts during p-xylene oxidation by air.
CoIII
O
CoIII
O
CoIV
CoII
1
2
Fig. 3 shows the conversion of p-xylene and the molar percents of
products as the function of the reaction time. With the reaction pro-
ceeding, the molar contents of the major products varied markedly.
From the curve we can see that the contents of TALD and TALC in-
creased rapidly at the beginning 2 h and then changed a little. How-
ever, the content of TA and by-product kept on increasing but in a
low rate. From Fig. 3, it can be concluded that TA and by-product
are formed at the expense of compound TALD and TALC, and the
reaction time is an important factor concerning the selectivities
for these compounds. These results can be explained by the reaction
mechanism as following general free radical reactions [(4)–(8)]
which are commonly accepted [3,25,26].
b
c
RCH3
RCH3
RCH2
+
CoIII
OH
CoII
= Co(II) (DPDME)
Scheme 2. The proposed pathway of activation of the p-xylene molecule.
416
Å
RCH
3
þ O@MðIVÞ ! RCH þ HO—MðIIIÞ
ð1Þ
2
oxygen transfer
After the reaction
Å
RCH þ HO—MðIIIÞ ƒƒƒƒƒƒƒƒ! RCH
2
OH þ MðIIÞ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
ð8Þ
ð9Þ
2
Å
RCH þ O
2
! RCH
2
OOH
2
528 556
Å
ꢁ
390
RCH
2
OO þ MðIIÞ ! RCHO þ MðIIIÞ þ OH
Å
þ
RCHO þ MðIIÞ ! RCO þ MðIIIÞ þ H
Å
Å
Before the reaction
RCO þ O
2
! RCO
3
Å
Å
RCO þ RCH
3
! RCO
H þ RCHO ! 2RCO
3
H þ RCH
513
549
3
2
RCO
RCH
3
2
H
2
OH þ RCO
2
H ! RCO
2
CH
2
R þ H
2
O
350
400
450
500
Wave /nm
550
600
650
Different metallo-deuteroporphyrins have been used in the
oxidation to investigate the influence of central metallic ions
on the catalytic activities of these complexes. As is shown in Ta-
ble 1, the total yields of the three main products varied with the
change of the complexed metals, but the change of the selectiv-
ities of the products is very small. Co(II) (DPDME) exhibited the
highest activity in the reaction. When the central metal were
Fe(III)Cl and Mn(III)Cl, p-xylene was oxidized with lower conver-
sion and the following order of reactivity was observed:
Co(II) > Mn(III)Cl > Fe(III)Cl, in accord with the results of Guo’s
Fig. 4. UV–Vis absorption spectrum of the reaction solution.
been studied by UV–Vis spectrum. The UV–Vis spectra of reac-
tion solution before and after the catalytic oxidation of p-xylene
catalyzed by Co(II) (DPDME) were shown in Fig. 4. The starting
cobalt–deuteroporphyrin spectrum (390 nm) was transformed
into that of a new intermediate with a strong and relatively
sharp Soret maximum at 416 nm. Visible spectral changes after
the catalytic oxidation indicated that reactive intermediate
high-valent oxo species might be formed in the oxidation [30].
To investigate the catalytic behavior of M(DPDME) in the oxi-
dation of p-xylene, the TPPs complexed different metals have
also been studied in the paper. Table 1 summarizes the data ob-
tained from our catalytic experiments. It can be seen that the
investigated deuteroporphyrin complexes appeared suitable cata-
lysts for the oxidation of p-xylene under chosen conditions and
exhibited evidently higher activity than M(TPP) complexes. A
primary factor may be that the reaction catalyzed by Co(II)
[
27]. This phenomenon is opposite to the mechanism promoted
by Lyons [28], which deemed that the catalytic activity increases
with the increasing of redox potential of the metallo complexes,
because the redox potential of cobalt is more negative than that
of other metals. The exceptional case may be due to the fact that
2
Co(II) complexes is prone to combine O . Haber [29] has re-
ported that the lower redox potential of cobalt(II) complexes is
helpful to the reaction (Scheme 2, pathway a), and the oxygen
is prone to be activated. The stability of the metallo-deuteropor-
phyrins in the oxidation under the selected conditions has also
(
DPDME) undergoes a mechanism different from that proposed
for M(TPP) [14]. Since the steric effect reduced in comparison
with TPPs with four phenyl groups in the meso-site, in the oxi-
dation most of the metallo-deuteroporphyrins may convert to
Table 1
a
Oxidation of p-xylene with air catalyzed by metallo-deuteroporphyrin complexes .
b
b
Catalyst
t (h)
Conversion (%)
Selectivity (%)
the
l-oxo dimmer (Scheme 2, Co(II) (DPDME) is taken as the
Aldehyde
Alcohol
Acid
example) [13], which has been proven to be an active catalyst
in the oxidation [28]. On the other hand, the oxidation of p-
xylene starts with the activation of the hydrocarbon molecule
due to the abstraction of a proton by the axial ligand of the
porphyrin and simultaneous injection of an electron into the
No
8
0.8
9.6
4.8
15.0
8.2
12.2
6.5
72.8
59.8
63.9
48.9
50.7
51.1
60.4
14.6
29.6
28.4
32.7
35.4
29.8
30.1
11.7
11.3
7.2
13.6
11.2
15.7
8.5
ClFe(III) (DPDME)
ClFe(III) (TPP)
Co(II) (DPDME)
Co(II) (TPP)
ClMn(III) (DPDME)
ClMn(III) (TPP)
4.5
4.5
3.5
3
4
4
metallo-porphyrin complex [31]. In the case of l-oxo metallo-
deuteroporphyrins, the substitution of the electron-donating sub-
stituents weakens the M–O–M bond and facilitates splitting of
this bond in the course of activation of the hydrocarbon mole-
cule (Scheme 2, pathway c).
a
Reaction conditions: p-xylene 1000 mL (8.9 mol), the catalyst 12 mg (0.02
mmol), air pressure 0.8 MPa, temperature 150 °C.
b
Determined by GC–MS analyses.

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