Catalytic Aerobic Oxidation of Biomass-based Furfural into Maleic Acid in Aqueous Phase with Metalloporphyrin Catalysts

Article abstract of DOI:10.1002/jccs.201700004

Catalytic oxidation of renewable furfural into valuable maleic acid in aqueous solutions using metalloporphyrin catalysts was investigated for the first time. The synthesized catalysts were characterized by FT-IR, UV–vis, ¹H NMR, elemental analysis, and TGA. The catalysts varied in metal active sites and functional groups, which had different effects on their catalytic activity. Furthermore, the effects of temperatures, reaction time, catalyst loading, and oxygen pressure were studied in detail. Maleic acid could be achieved in 44% yield by using FeT(p-Cl)PPCl as catalyst under optimal conditions. Finally, FeT(p-Cl)PPCl could be reused in five consecutive runs without a significant loss of activity.

Full text of DOI:10.1002/jccs.201700004

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Catalytic Aerobic Oxidation of Biomass-based Furfural into Maleic Acid in Aqueous
Phase with Metalloporphyrin Catalysts
a
a
a
b
a
c
*
Yi Huang, Chunyan Wu, Wenwen Yuan, Yongmei Xia, Xiang Liu, Huamei Yang
a
*
and Haijun Wang
aKey Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material
Engineering, Jiangnan University, Wuxi 214122, China
bState Key Laboratory of Food Science & Technology, Wuxi 214122, China
cSchool of Chemistry and Chemical Engineering, Xuzhou University of Technology, Xuzhou 221000, China
(Received: January 3, 2017; Accepted: April 8, 2017; DOI: 10.1002/jccs.201700004)
Catalytic oxidation of renewable furfural into valuable maleic acid in aqueous solutions using
metalloporphyrin catalysts was investigated for the first time. The synthesized catalysts were char-
1
acterized by FT-IR, UV–vis, H NMR, elemental analysis, and TGA. The catalysts varied in
metal active sites and functional groups, which had different effects on their catalytic activity. Fur-
thermore, the effects of temperatures, reaction time, catalyst loading, and oxygen pressure were
studied in detail. Maleic acid could be achieved in 44% yield by using FeT(p-Cl)PPCl as catalyst
under optimal conditions. Finally, FeT(p-Cl)PPCl could be reused in five consecutive runs without
a significant loss of activity.
Keywords: Furfural; Biomass catalytic conversion; Metalloporphyrins; Maleic acid.
INTRODUCTION
Since the problems of energy depletion and environ-
In 2011, the route to produce maleic acid from
renewable furfural was first reported. Copper and iron
salts displayed potential catalytic activity to the reaction,
and the combination of copper nitrate with phosphomo-
lybdic acid achieved a yield of 49.2% maleic acid under
2 MPa O2 pressure after 14 h of reaction.11 Later, Guo
et al.12 developed a new strategy for the aerobic oxidation
of furfural to maleic acid in an aqueous/organic biphase
system with improved product selectivity, and explored
the product separation and reactant recycle. A yield of
34.5% maleic acid with 68.6% selectivity was achieved in a
water/tetrachloroethane biphase system (v/v, 2:0.8) with a
phosphomolybdic catalyst. Lan et al.13 described the
transformation of furfural into maleic anhydride over
H5PV2Mo10O40 mixed with Cu(CF3SO3)2 and obtained a
yield of 54% maleic anhydride under optimized condi-
tions. Besides molecular oxygen, hydrogen peroxide was
also employed as an oxidant to catalyze the oxidation of
furfural.14 Titanium silicalite was used as the catalyst. As
for the reaction mechanism, the route of furfural oxida-
tion to maleic acid might involve epoxidation and rear-
rangement. Very recently, Li et al.15 demonstrated a
heterogeneous catalytic system of Mo–V oxides as
mental pollution have intensified in the past decade, there
is an urgent need to seek potential alternatives to feed-
stock based on nonrenewable fossil resources. Biomass,
as the most abundant renewable resource, has drawn
growing interest from people and been used as the start-
ing material for the synthesis of chemical intermediates
and fuels.1–4 Furfural, a biomass-derived five-carbon
product, originates from many agricultural materials that
are not competitive with human needs (e.g. wheat, corn-
cobs, sawdust) and could be synthesized to maleic acid
and other important platform chemicals.5,6 Maleic acid is
a key downstream product because of its high applied
value in polyester resins, surface coatings, lubricant addi-
tives, plasticizers, agrochemicals, and pharmaceuticals.7,8
But it is still manufactured from aerobic oxidation of
petroleum-based n-butane or benzene under difficult con-
ditions in the industry,9,10 which can hardly decrease the
consumption of energy or improve economy. Thus,
efforts are needed to convert the pathway of producing
maleic acid and its derivatives from petroleum-based
resources to biomass-based resources.
*Corresponding author. Email: wanghj329@outlok.com; yhmcumt@yeah.net
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Article
Huang et al.
catalysts for the production of maleic anhydride from fur-
fural, and reached a yield of 65% over the Mo–V–O cata-
lyst (Mo/V, 4:1) under optimal conditions, and the
catalyst could be recycled more than three times. The use
of other biomass substrates generated a yield of 71%
maleic anhydride from levulinic acid16 with supported
vanadium oxides, and 5-hydroxymethyl furfural (5-
HMF)17,18 was also employed over the corresponding
catalysts.
All the studies above were devoted to developing a
new route for producing maleic acid from biomass mate-
rials and for replacing the current industrial fossil fuel
routes, bringing out much progress. Nevertheless, further
efforts are needed to improve the reaction conditions
under high oxygen pressure (2 MPa or higher) and long
reaction time, and to expand the use of catalysts beyond
phosphomolybdic acid type or V-based mostly. As the
biomimetic models of cytochrome P450, metallopor-
phyrin (MP) catalysts exhibit efficient activity for many
oxidation reactions of organic compounds under mild
conditions, such as alkanes,19 cycloalkanes,20,21
alkenes,22 alkylaromatics,23,24 and alcohols.25,26 A few
studies also reported the oxidation of carbohydrates 5-
HMF27,28 over supported MPs. However, to the best of
our knowledge, there is no investigation on the oxidation
of furfural to maleic acid using MPs as catalysts.
In this work, the catalytic performance of MPs for
the oxidation of furfural to maleic acid in aqueous solu-
tions was studied (Equation 1). The optimum reaction
conditions were also investigated, including the reaction
temperature, reaction time, catalyst loading, and oxy-
gen pressure. Additionally, the catalysts were character-
ized via Fourier transform infrared (FT-IR), UV–vis,
1H NMR, elemental analysis (EA), and thermogravi-
metric analysis (TGA). It is hoped that metalloporphyr-
ins will be widely employed in the field of biomass
materials catalysis.
Fig. 1. FT-IR spectra of (a) FeTPPCl, (b) TPP,
(c) T(p-Cl)PP, (d) Na4TSPP, and (e) T(p-
CH3)PP.
absorption bands of TPP, T(p-Cl)PP, and T(p-CH3)PP
are roughly similar. The band at about 3317 cm−1 is
attributed to the stretching vibration of N–H, and the
bands near 960 cm−1 are related to the bending vibra-
tions of N–H. The bands in the region 3060–2900 cm−1
are assigned to the stretching vibration of C–H of por-
phyrins. The band at about 1344 cm−1 represents the
vibration of C N in pyrrole rings. Moreover, the
strong absorption bands at around 798 and 698 cm−1
are attributed to the skeletal vibration of the porphyrin
rings. In the spectrum of Na4TSPP, the bands at 1192
and 1040 cm−1 represent the stretching vibration of
S O of sulfonate, indicating that the porphyrin had
been sulfonated. After the metalation reaction
(e.g. FeTPPCl in Figure 1), new sharp absorption bands
appear at approximately 998 cm−1, which are due to
the vibration of N–metal bonds. For other metal tetra-
phenylporphyrins, the N–metal absorption bands tend
to shift to lower wavenumbers with the increase of the
atomic number. In addition, due to the coordination of
O
H
COO
H
C
O
+
s
MP
2
O2
+
CO2
(1)
H
COO
M
2+, the vibrations of N–H at around 3317 and
960 cm−1 disappear, which indicates the successful
insertion of the metal ions into free-base porphyrins.
Figure 2 presents the UV–vis spectral results of
TPP and FeTPPCl in the range 700–300 nm. The spec-
tra of TPP shows one intense Soret band at 417 nm
generated by the transition from a1u(π) to eg*(π) and
four less intense Q bands in the 650–510 nm region
RESULTS AND DISCUSSION
Characterization of catalysts
Figure 1 shows the FT-IR spectra of 5,10,15,20-
tetraphenylporphyrin (TPP), T(p-Cl)PP, T(p-CH3)PP,
Na4TSPP, and FeTPPCl. The main characteristic
2
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Oxidation of Furfural to MA with Metalloporphyrins
under a N2 atmosphere. As shown in Figure 3, a weight
loss of 4.28% within 358–415ꢀC is attributed to the disso-
ciation of axial chloride of the complex. A significant
weight loss (12.22%) within 415–520ꢀC is caused by the
decomposition of porphyrin rings. Clearly, the FeT(p-
Cl)PPCl catalyst is stable up to 358ꢀC.
Catalytic oxidation of furfural into maleic acid with
different MP catalysts
Metallic properties of catalysts often play a deci-
sive role in most oxidation reactions of organic
compounds,31 including furfural. Only 30% of substrate
converted as a result of the polymerization and little
maleic acid could be detected when no catalyst was
added to the reaction system (Table 1, entry 1), which
confirms that the oxidation reaction to maleic acid
hardly occurs without any catalyst. In order to identify
the potential catalyst, we synthesized a series of metal
tetraphenylporphyrins and used them in the reactions
(Table 1 entries 2–7). As reported, copper-based salts
are the most potential catalysts for the oxidation of fur-
fural into maleic acid.11 However, in this reaction sys-
tem, CuTPP displayed poor catalytic activity and
produced only 1.9% maleic acid (Table 1, entry 2). In
comparison, ZnTPP improved the catalytic efficiency
slightly and gave a yield of 8.6% maleic acid and a con-
version of 37.7% furfural, while CoTPP generated a
3.4% yield of maleic acid and a 26.5% conversion of
furfural. FeTPPCl gave a yield of 20.4% maleic acid
and a conversion of 47.8% furfural under the same con-
ditions, indicating that FeTPPCl has the highest cata-
lytic activity for the oxidation of furfural to maleic acid
among these metal porphyrins. Thus, iron-based MP
was the most effective catalyst in this system. The other
metal teraphenylporphyrins (MnTPPCl, NiTPP)
showed little catalytic efficiency for the reaction and
were not considered potential catalysts.
Fig. 2. UV–vis spectra of TPP and FeTPPCl at room
temperature in dichloromethane solution.
induced by the a2u(π) –eg*(π) transitions.29 After meta-
lation, the Soret band of metal porphyrins slightly shifts
(red, or in case of FeTPPCl blue), and the peaks of the
Q bands are fewer compared to free-base porphyrins
because of the increased molecular symmetry,30 which
are important signs indicating the formation of MPs.
Other porphyrin ligands and their metal complexes
show similar characteristics as TPP and FeTPPCl.
The EA of the MPs used in catalytic experiments
was performed. The results [in the electrospray ioniza-
tion (ESI) mode] show that the experimental values are
basically consistent with the calculated ones.
Thermogravimetry/differential thermogravimetry
(TGA/DTG) analysis of FeT(p-Cl)PPCl was carried out
The following work was conducted to study the
impact that the different functional groups on iron por-
phyrins made on the target reaction. Thus, the same
mole of FeT(p-Cl)PPCl and FeT(p-CH3)PPCl was used
in the furfural oxidation reaction, including the
electron-withdrawing group
Cl and the electron-
donating group CH3, respectively. FeTSPPCl contain-
ing the water-soluble substituent SO3Na on the por-
phyrin macrocycle was also investigated as
a
Fig. 3. TG/DTG curves of FeT(p-Cl)PPCl.
comparison. Differences in the activity of these iron-
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Huang et al.
Table 1. Conversion furfural into maleic acid over different
metalloporphyrins and iron saltsa
Conversion
(%)
Yield
(%)
Selectivity
(%)
Entry Catalyst
1
2
3
4
5
6
7
8
9
–
29.7
28.4
37.7
26.5
24.6
25.9
47.8
43.6
49.6
49.3
45.5
41.2
35.2
0.6
1.9
8.6
3.4
0.3
2
6.9
22.7
12.6
1.3
CuTPP
ZnTPP
CoTPP
MnTPPCl
NiTPP
FeTPPCl
FeT(p-CH3)PPCl
FeT(p-Cl)PPCl
0.4
1.6
20.4
26.3
33
25.2
18.5
13.4
11.1
42.7
60.3
66.7
51.1
40.7
32.5
31.5
10 FeTSPPCl
11 FeSO4
12 FeCl3
13 Fe(NO3)3
a Reaction conditions: catalyst, 3 × 10−5 mol; furfural,
Fig. 4. Influence of the reaction temperature on the
2.5 mmol; H2O, 4 mL; temperature, 90ꢀC; 6 h; O2, 1 Mpa.
oxidation of furfural to maleic acid. Reaction
conditions: FeT(p-Cl)PPCl, 20 mg; furfural,
2.5 mmol; H2O, 4 mL; 6 h; O2, 1 MPa.
based porphyrins with substituents could be observed
and the reactivity followed the following order:
FeTPPCl < FeTSPPCl ≈ FeT(p-CH3)PPCl < FeT(p-
Cl)PPCl. It is interesting that the yield of maleic acid
could be improved compared with FeTPPCl depending
on whether the iron porphyrins contained an electron-
withdrawing or electron-donating group. The possible
reason is that the introduction of substituents provided
the steric effects necessary to reduce the oxidative deg-
radation of porphyrins, which enhanced the stability of
the catalysts.32 The catalytic activity of iron salts
(entries 11–13) was also investigated in the experiment.
The different iron salts displayed some activity to the
reaction, but it was relatively low compared to that of
iron porphyrins, indicating that the large π-conjugated
structure of porphyrin ligands could improve the cata-
lytic performance of the metal sites. FeT(p-Cl)PPCl
showed a yield of 33% maleic acid and a conversion of
50% furfural under the reaction conditions, which indi-
cated the best catalytic efficiency in this system. There-
fore, FeT(p-Cl)PPCl was chosen and used in the
following optimization of the reaction conditions.
to maleic acid could hardly occur. The yield of maleic
acid increased from 6.1% at 70ꢀC to 21.9% at 90ꢀC.
Then, the yield of maleic acid rose slightly with the tem-
perature and reached 27.5% at 110ꢀC. However, the
selectivity of maleic acid, which increased sharply from
70 to 90ꢀC, began to decrease, reaching a maximum
(47.9%) at 90ꢀC. This may be attributed to the domi-
nant side reaction of polymerization of furfural, with
the appearance of a black precipitate clearly after the
reaction. A higher reaction temperature would benefit
the conversion of furfural at all times, because with the
temperature rising from 60 to 110ꢀC, it still maintains
furfural catalytic conversion efficiency to maleic acid
over FeT(p-Cl)PPCl when above 90ꢀC, but the selectiv-
ity of the side product becomes dominant in the system.
Thus, 90ꢀC is taken the best reaction temperature.
Time course of the oxidation of furfural into maleic acid
The effect of the reaction time is shown in Figure 5.
The conversion of furfural gradually increases with pro-
longed reaction time. After 12 h of reaction, the conver-
sion of furfural with FeT(p-Cl)PPCl is 87.6%. Similar to
the trend in conversion, the yield of maleic acid rises
slowly in the first 10 h and reaches a maximum at 10 h
to 30.3%. The yield then decreases with longer reaction
times, probably because of the product degrading to
Catalytic oxidation of furfural at different temperatures
The reaction temperature has a significant effect
on furfural oxidation. As shown in Figure 4, both the
yield and selectivity of maleic acid are nearly 0% below
70ꢀC, indicating that the oxidation reaction of furfural
4
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Oxidation of Furfural to MA with Metalloporphyrins
Fig. 5. Effect of the reaction time on the oxidation
of furfural to maleic acid. Reaction condi-
tions: FeT(p-Cl)PPCl, 20 mg; furfural,
2.5 mmol; H2O, 4 mL; temperature, 90ꢀC;
O2, 1 MPa.
Fig. 6. Effect of different amounts of FeT(p-Cl)PPCl
on furfural oxidation. Reaction conditions:
furfural, 2.5 mmol; H2O, 4 mL; temperature,
90ꢀC; 10 h; O2, 1 MPa.
Effect of oxygen pressure on the oxidation of furfural
to maleic acid
The oxygen pressure also plays an important role
in the reaction. As illustrated in Figure 7, higher oxygen
pressure is favorable for furfural oxidation and
small molecules.15 The selectivity of maleic acid
increases at the beginning and reaches a maximum at 6 h
to 47.9%, but it decreases slowly beyond 6 h, suggesting
that the competitive side reaction becomes slightly more
serious with longer reaction times.
Effect of catalyst loading on the aerobic oxidation
of furfural to maleic acid
The effect of the catalyst amount is illustrated in
Figure 6. As described above, maleic acid could be
hardly produced without a catalyst in the reaction sys-
tem. Both furfural conversion and maleic acid yield
increase gradually with the increase of catalyst loading
at first, supporting the hypothesis that increasing the
number of active sites of the catalyst could support fur-
fural oxidation. When 40 mg of FeT(p-Cl)PPCl was
used, the yield (40.3%) and conversion (86.3%) reached
a maximum. With further increase of the catalyst load-
ing, the yield and conversion declined slowly, suggest-
ing that excessive loading could hinder the reaction.
The possible reason is that more byproducts cause
interactions and thus reduce the effect of the catalyst.
The selectivity of maleic acid also reached a maximum
(47.4%) at the load of 30 mg of FeT(p-Cl)PPCl.
Fig. 7. Influence of the oxygen pressure on transfor-
mation of furfural to maleic acid. Reaction
conditions: FeT(p-Cl)PPCl, 40 mg; furfural,
2.5 mmol;
H2O,
4 mL;
temperature,
90ꢀC; 10 h.
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Huang et al.
improves both the conversion and yield over FeT(p-Cl)
PPCl, supporting that the increase of pressure favors
the activation of oxygen and the generation of the cata-
lytic product. A yield of 44% maleic acid could be
achieved under 1.2 MPa. With further increase of pres-
sure, the yield only slightly changed, while nearly full
furfural conversion (98%) was obtained under 1.5 MPa.
It is interesting that the oxidant pressure employed to
produce maleic acid from furfural with iron porphyrins
is lower than that reported in the literature.11,12
Recycling of the FeT(p-Cl)PPCl catalyst
The reusability and stability of the FeT(p-Cl)PPCl
catalyst were investigated for several cycles under the
same conditions. After each run, the solid catalyst was
separated by filtration, washed with hot water and etha-
nol, and dried in oven at 70ꢀC overnight for the next
cycle. The particle size of the catalysts in the system
was measured by a granulometer, and the result showed
that the average diameter of the catalyst was 1.7 μm.
The test results are presented in Table 2. It could be
seen that the yield of maleic acid remained constant at
about 44% in the first four runs. The yield only
decreased slightly (41.2%) after the fifth cycle. This may
be because a small amount of the Fe (III) porphyrins
were destroyed by O2. In addition, no absorption bands
of porphyrins were detected in the UV–vis spectra of
the reaction solution after the tests, demonstrating that
the catalysts remained insoluble throughout the experi-
ments. Finally, the FT-IR and UV–vis spectra (see the
ESI) of the recovered FeT(p-Cl)PPCl showed that the
structure remained intact after five recycling steps, indi-
cating that the catalyst has good catalytic stability
(Scheme 1).
Scheme 1. Molecular structure of prepared porphy-
rin catalysts.
EXPERIMENTAL
Materials and methods
Pyrrole, benzaldehyde, salicylic acid, furfural,
xylene, metal salts, propionic acid, and other solvents
were purchased from Sinopharm Chemical Reagent
Co. Ltd. (Shanghai, China) 4-Chlorobenzaldehyde was
provided by Shanghai Aladdin Industrial Inc. 4-Methyl
benzaldehyde was obtained from Energy Chemical Rea-
gent Co. Ltd. (Shanghai, China) FT-IR spectra were
measured with a Nicolet 360 FT-IR instrument (Madi-
son, Wisconsin, USA) (KBr disks) in the 4000–500 cm−1
region. UV–vis spectra were obtained using a TU-1901
dual-beam UV–vis spectrophotometer (Beijing Purkinje
General Instrument Co. Ltd., Beijing, China) over the
1
200–800 nm range. H NMR spectra were recorded in
D2O or CDCl3 with a Bruker DPX 300 spectrometer
(Billerica, MA, USA). Elemental analysis was obtained
with a Vario EL III CHNS instrument (Hanau, Ger-
many). Particle size of the catalysts was recorded using a
ZetaPALS granulometer (New York, USA). TGA anal-
ysis was performed on a STA409 instrument (Mettler
Toledo International Trading Co. Ltd., Shanghai,
China) at a heating rate of 20ꢀC/min from 25 to 800ꢀC.
Table 2. Recycling experiments of the catalysta
Runs
Conversion (%)
Yield (%)
Selectivity (%)
1
2
3
4
5
95.5
96
95.7
95.5
93.1
44.3
44.5
43.4
43.7
41.2
46.4
46.4
45.4
45.8
44.3
Catalyst preparation
Meso-tetraphenylporphyrin, meso-tetrakis(4-chlor-
ophenyl)porphyrin, and meso-tetrakis(4-methyl)porphy-
rin were prepared by the method of Adler with some
a Reaction conditions: FeT(p-Cl)PPCl, 40 mg; furfural,
2.5 mmol; H2O, 4 mL; temperature, 90ꢀC; 10 h; O2,
1.2 MPa.
6
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Oxidation of Furfural to MA with Metalloporphyrins
changes.33 Meso-tetrakis(4-sulfophenyl)porphyrin was
prepared by a reported procedure.34
crude product was purified via column chromatography
using neutral alumina (100–200 mesh size) with chloro-
form as eluent.
Synthesis of 5,10,15,20-tetraphenylporphyrin (TPP)
In a 100-mL flask with three necks, 0.9 g of sal-
icylic acid as an acidic catalyst was added into 30 mL of
xylene mixed with benzaldehyde (1.272 g, 0.012 mol).
The resulting mixture was heated at reflux temperature
under magnetic stirring. Then a xylene solution (6 mL)
of fresh distilled pyrrole (0.804 g, 0.012 mol) was added
dropwise to the mixture for 10 min. After 3 h of reac-
tion, the system was cooled down to room temperature.
Then 15 mL of ethyl alcohol was added, and the mixed
solution was placed in a refrigerator for 24 h. After fil-
tering, the precipitate was washed several times with
ethyl alcohol. The purple crude product was dried in
vacuum overnight and purified by a chromatographic
column using neutral alumina (100–200 mesh size) with
dichloromethane as eluent.
Synthesis of 5,10,15,20-tetrakis(4-sulfophenyl)porphyrin
(Na4TSPP)
TPP (0.15 g) was added into a round-bottom flask
with 3 mL of H2SO4 (98%), and then the mixture was
heated at 100ꢀC with magnetic stirring. After 6 h of
reaction, the mixture was cooled down to room temper-
ature and poured into 20 mL of ice water carefully. A
solution of saturated sodium hydroxide was added
dropwise until the PH was 8–9 and the color of solution
turned from green to red. The solution was concen-
trated to about 10 mL, and added with hot methanol.
Then the mixed solution was filtered immediately to
remove Na2SO4, and the filter cake was washed with
hot methanol until it became colorless. Finally, the fil-
trate was dried by a rotary evaporator.
Synthesis of metal tetraphenylporphyrins (MTPP)
Synthesis of meso-substituted iron (III) porphyrins
The meso-substituted iron (III) porphyrins FeT(p-
Cl)PPCl, FeT(p-CH3)PPCl, and FeTSPPCl were synthe-
sized using the same method as FeTPPCl.
In a typical experiment, 0.16 g of the tetraphenyl-
porphyrin synthesized above was dissolved in DMF
(30 mL). The mixture was heated under magnetic stir-
ring. Then FeCl2ꢁ4H2O (five times the molar quantity of
TPP) was added into the solution in batches at reflux
temperature. After 4 h of reaction, the solvent was
removed by distillation, and hydrochloric acid was
added slowly until the brown solid on the reactor wall
dissolved. The mixture was immersed in deionized water
for 6 h. Then the product was filtered and washed with
hydrochloric acid and water, and dried unde vacuum
overnight. Other metal complexes were synthesized with
Catalytic experiments and analysis
In a typical reaction, a certain amount of catalyst
was added to 4 mL of water in a PTFE container; then
the furfural (0.242 g, 2.5 mmol) substrate was dissolved
in the mixture. The container was placed in a 100-mL
stainless autoclave charged with a certain pressure of
oxygen. The reaction mixture was magnetically stirred
at the desired temperature and then it was charged with
a specific pressure of oxygen. After the reaction, the
autoclave was cooled down to room temperature and
depressurized carefully to atmosphere pressure. A cer-
tain amount of reaction mixture was diluted 100 times
with deionized water as sample and quantitatively ana-
lyzed using an Agilent 1100 HPLC chromatograph
(Santa Clara, CA, USA) equipped with a refractive
index detector and a Bio-Rad Aminex HPX-87H
Cu(OAc)2ꢁH2O,
Zn(OAc)2ꢁ2H2O,
CoCl2ꢁ6H2O,
MnCl2ꢁ4H2O, and Ni(OAc)2ꢁ4H2O, respectively.
Synthesis of 5,10,15,20-tetrakis(4-chlorophenyl)
porphyrin (T(p-Cl)PP) and 5,10,15,20-tetrakis(4-methyl)
porphyrin (T(p-CH3)PP)
The meso-substituted porphyrins T(p-Cl)PP and
T(p-CH3)PP were prepared using a similar method as
TPP. A propionic acid solution (5 mL) of freshly dis-
tilled pyrrole (1.005 g, 0.015 mol) were added dropwise
to the mixed solution of propionic acid (30 mL) and 4-
(Shanghai, China) column (300 mm × 7.8 mm).
A
0.005 M H2SO4 solution was employed as the eluent
phase at 313 K and 0.55 mL/min flow rate. The main
product after the reactions was separated by crystalliza-
chlorobenzaldehyde
or
4-methyl
benzaldehyde
1
(0.015 mol) at reflux temperature under magnetic stir-
ring for 1 h. After the reaction, the precipitate was fil-
tered and washed with hot water and ethyl alcohol. The
tion and identified by H NMR and 13C NMR techni-
ques (in the ESI mode). The amount of maleic acid and
furfural was determined using the external standard
J. Chin. Chem. Soc. 2017
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Article
Huang et al.
method. The conversion of furfural as well as the yield
and the selectivity of maleic acid were calculated as
follows:
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furfural over MP catalysts was investigated for the first
time. The types of metal sites determined the catalytic
activity of porphyrins during the furfural oxidation reac-
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The peripheral substituents in MPs also had a remarkable
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