Iron-based metalloporphyrins as efficient catalysts for aerobic oxidation of biomass derived furfural into maleic acid

Article abstract of DOI:10.1016/j.mcat.2018.03.018

A series of porphyrin type catalysts with the metal active sites of Fe were prepared and investigated in aerobic oxidation of biomass-based furfural to maleic acid (MAD) in aqueous phase. The catalytic performance of meso-tetrakis(4-bromophenyl)porphyrin iron (III) chloride (FeT(p-Br)PPCl) immobilized on different supports was evaluated. It was interesting to find that the catalytic activity varied with the supports and followed the trend: FeT(p-Br)PPCl/SBA–15 > FeT(p-Br)PPCl/meso-ZSM–5 > FeT(p-Br)PPCl/MCM-41. The effect of reaction conditions were discussed in detail over FeT(p-Br)PPCl/SBA-15 catalyst, and 56.1% yield and 73.8% selectivity of MAD were obtained from renewable furfural under the optimal conditions. Moreover, the FeT(p-Br)PPCl/SBA-15 catalyst could be reused five times without a significant decrease of activity in recycling examinations.

Full text of DOI:10.1016/j.mcat.2018.03.018

MolecularCatalysis452(2018)20–27
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Iron-based metalloporphyrins as efficient catalysts for aerobic oxidation of
biomass derived furfural into maleic acid
Yongdi Xiea, Yi Huanga, Chunyan Wua, Wenwen Yuana, Yongmei Xiab, Xiang Liua,
Haijun Wanga,⁎
a
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, 214122, China
State Key Laboratory of Food Science & Technology, Wuxi, 214122, Jiangsu, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of porphyrin type catalysts with the metal active sites of Fe were prepared and investigated in aerobic
oxidation of biomass-based furfural to maleic acid (MAD) in aqueous phase. The catalytic performance of meso-
tetrakis(4-bromophenyl)porphyrin iron (III) chloride (FeT(p-Br)PPCl) immobilized on different supports was
evaluated. It was interesting to find that the catalytic activity varied with the supports and followed the trend:
FeT(p-Br)PPCl/SBA–15 > FeT(p-Br)PPCl/meso-ZSM–5 > FeT(p-Br)PPCl/MCM-41. The effect of reaction con-
ditions were discussed in detail over FeT(p-Br)PPCl/SBA-15 catalyst, and 56.1% yield and 73.8% selectivity of
MAD were obtained from renewable furfural under the optimal conditions. Moreover, the FeT(p-Br)PPCl/SBA-15
catalyst could be reused five times without a significant decrease of activity in recycling examinations.
Iron porphyrins
Supported catalysts
Biomass
Furfural
1. Introduction
Supported metalloporphyrin catalysts show a good application
prospect in oxidation of organic compounds, but only a few studies
As the biomimetic models of the cytochrome P-450 monooxygenase
enzyme, metalloporphyrins have been extensively studied in the past
decades for their efficient catalytic ability in hydroxylation and epox-
idation reactions of hydrocarbons [1–4]. Many reactions are performed
with metalloporphyrins under mild conditions due to the inherent
properties of these models. However, metalloporphyrins employed as
catalysts in homogeneous reaction systems have several drawbacks. For
instance, they are not stable enough due to the aggregation of por-
phyrin rings caused by π–π interaction [5], and it’s difficult to realize
the recovery of these expensive catalysts from the homogeneous media
at the end of the reaction, which limit the practical applications of them
in industrial production. To overcome the disadvantages, immobiliza-
tion of metalloporphyrin complexes on suitable and stable supports is
referred to the utilization of them in oxidation of biomass materials,
which are the most abundant renewable resources on the earth [12,13].
As the potential alternative feedstock of traditional fossil resources,
biomass can be used to produce many platform chemicals and fuel
products [14–16]. Furfural is one of the biomass derived 5-carbon
product and abundant in many agricultural materials such as wheat,
corncobs and sawdust, which are not competitive with human beings.
Thus, it’s very attractive to convert furfural to value added products as
the replacements of fossil-derived chemicals. In fact, more than 1600
commercial chemicals can be produced from furfural [17]. Compared
with furfural, maleic acid (MAD) is currently a fossil-derived product,
which can be manufactured from the route of aerobic oxidation of n-
butane or benzene under hard reaction conditions in industry [18].
MAD is a key downstream chemical for its high applied value in
polyester resins, surface coatings, lubricant additives, plasticizers,
agrochemicals, pharmaceuticals, etc [19]. From the view point of green
chemistry, establishing the biomass-based MAD formation route by
oxidation of renewable furfural over metalloporphyrin type catalysts
under mild conditions is both environmental friendly and cost effective.
In this paper, different iron porphyrins were prepared. Meso-tetrakis
(4-bromophenyl)porphyrin iron (III) chloride complex (FeT(p-Br)PPCl)
was successfully immobilized on three different mesoporous materials:
SBA-15, MCM-41 and meso-ZSM-5. We chose these three mesoporous
an effective method.
A great deal of attempts towards the im-
mobilization of metalloporphyrins on various supports including silica,
zeolite, porous glass, chitosan and resins have been investigated [6–10].
In comparison with homogeneous porphyrins, immobilized porphyrins
exhibit higher stability and can be easily separated from the hetero-
geneous system for the next use. Furthermore, the special structure of
supports provide a better microenvironment for the substrates to in-
teract with the active sites of catalysts, which not only keep the high
catalytic activity of metalloporphyrins, but also enhance the selectivity
of the target products [11].
⁎
Corresponding author.
E-mail address: wanghj329@outlook.com (H. Wang).
https://doi.org/10.1016/j.mcat.2018.03.018
Received 18 January 2018; Received in revised form 21 March 2018; Accepted 23 March 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

Y. Xie et al.
MolecularCatalysis452(2018)20–27
2.2.3. Preparation of meso-ZSM-5
We first synthesized microporous ZSM-5 zeolite according to the
previous report [23]. The synthesis steps were as follows: 3 g of TEAOH
as organic template was dissolved in 45 mL of deionized water, and
then NaOH was added to the mixture followed by adding a measured
amount of AIP as aluminum source. The mixed solution was stirred at
room temperature for 1 h. Then 10.4 g of TEOS was added dropwise and
the resultant solution (Si/Al = 60, molar ratio) was stirred for several
hours. Subsequently, the mixture obtained was transferred to the Te-
flon-lined autoclave and crystallized by hydrothermal treatment at
170 °C for 72 h. Then the solid product was separated via filtration and
washed with deionized water and ethanol, and dried at 80 °C for 8 h.
Finally, the collected solid was calcined at 550 °C for 6 h to obtain ZSM-
5. Meso-ZSM-5 was prepared by a simple alkaline treatment [24]. The
obtained ZSM-5 was added to a 250 mL round-baker and then refluxed
in a reflux condenser filled with a 0.5 M aqueous NaOH solution at
70 °C for 30 min. Then the solid zeolite was recovered by filtration,
washing with deionized water, drying, and calcination in air at 300 °C
for 5 h to obtain meso-ZSM-5.
Scheme 1. Oxidation of furfural to MAD catalyzed by iron-based metallopor-
phyrins.
materials mainly because they were common supports in supported
catalysts, and each had different structural characteristics. These dif-
ferences were exactly what we need in order to find the most suitable
support for immobilized iron porphyrins. The catalysts synthesized
were characterized by FT-IR, UV-vis, 1HNMR, XRD, N2-adsorption-
desorption, SEM, TEM and TGA techniques. Both unsupported and
supported porphyrin catalysts were used to catalytic the oxidation of
furfural into MAD in the presence of dioxygen (1 MPa) in aqueous phase
(Scheme 1). In addition, the effect of reaction parameters (reaction
temperature, reaction time and catalyst amount) on the catalytic ac-
tivity and the recycle tests of catalyst were also investigated in detail.
2.2.4. Preparation of different porphyrin ligands
Porphyrin ligands were synthesized by the method of Alder with
some modifications [25]. In a 100 mL of flask with three necks,
0.015 mol of corresponding benzaldehyde was dissolved in 30 mL of
propionic acid solution. The mixture was heated at reflux temperature
with vigorous stirring. Subsequently, 0.015 mol of freshly distilled
pyrrole solved in propionic acid solution (5 mL) was slowly added into
the above mixture. After a period of time of reaction, the mixture so-
lution was cooled to room temperature and placed in the refrigerator
overnight. Then the purple solid was filtered and washed with hot
water and ethanol and dried at 80 °C for 8 h. The crude product was
purified via column chromatography using neutral alumina
(100–200 mesh size) with chloroform or dichloromethane as eluent.
2. Experimental
2.1. Reagents and materials
Pyrrole, propionic acid, ferrous chloride tetrahydrate, furfural,
ammonia (28 wt%), cetyltrimethylammonium bromide (CTAB) and
tetraethyl orthosilicate (TEOS) were purchased from Sinopharm
Chemical Reagent Co. Ltd.; Maleic acid, P123, tetraethyl ammonium
hydroxide (TPAOH, wt%) and aluminum isopropoxide (AIP) were
purchased from Shanghai Aladdin Industrial Inc.; Other main reagents
used in the work were obtained from Energy Chemical Reagent Co. Ltd.
2.2.5. Preparation of iron-based porphyrins
In a typical synthetic process, 0.16 g of porphyrin ligand synthesized
above was dissolved in 30 mL of DMF. The mixed solution was heated at
reflux temperature under magnetic stirring. Then FeCl2·4H2O (four
times the molar quantities of the ligand) was added into the solution in
three batches. The reaction was carried out for 4 h and the solvent was
removed by reduced pressure distillation, then hydrochloric acid was
added slowly until the brown solid on the reactor wall was full dis-
solved. The mixture was immersed in deionized water overnight, col-
lected via filtration and washing with hydrochloric acid and deionized
water, and dried in oven at 80 °C for 8 h.
2.2. Catalyst preparation
2.2.1. Preparation of MCM-41
MCM-41 molecular sieves were prepared by the following literature
procedures [20,21]. 1.37 g of CTAB was added into 50 mL of deionized
water with gentle stirring at 40 °C. After a clear solution was formed,
TEOS (5.2 mL) was added dropwise and aqueous ammonia was added
until the pH of the mixed solution was adjusted to 10.5. With continued
stirring for 3 h, the mixture was transferred to an autoclave equipped
with a Teflon liner and heated at 105 °C for 24 h. Then the reaction was
stopped and the gel was filtered, washed with deionized water and
ethanol, dried in oven at 80 °C for 8 h. Finally, the solid was calcined at
550 °C for 6 h.
2.2.6. Preparation of supported feT(p-Br)PPCl catalysts
In a 100 mL round bottom flask, 0.5 g of support (MCM-41, SBA-15
and meso-ZSM-5) was dispersed in 15 mL of DMF. The mixture solution
was heated to 120 °C with vigorous stirring. Then 0.1 g of FeT(p-Br)
PPCl DMF solution were slowly added into the above mixture. After
24 h of reaction, the brown solid was filtered and exhaustively washed
with solvent to remove the weakly adsorbed iron porphyrins.
2.3. Catalysts characterization
2.2.2. Preparation of SBA-15
FT-IR spectra were measured with a Nicolet 360 FT-IR instrument
SBA-15 mesoporous material was synthesized according to the
method described in literature with a slight change [22]. In a typical
synthesis, 4 g of P123 as organic template was dissolved in 30 mL of
deionized water. After stirring at 42 °C for 1 h, 100 mL of HCl solution
(2 M) was added. Then 8.5 g of TEOS was added dropwise and the re-
sultant solution was stirred for 22 h, followed by hydrothermal treat-
ment at 100 °C for 24 h. The solid product was collect by filtration,
washed with deionized water and ethanol, dried in oven at 80 °C for 8 h,
and then calcined at 550 °C for 6 h.
(KBr discs) in the 4000–500 cm−1 region. UV–vis spectra were obtained
using
a TU-1901 dual-beam UV–vis spectrophotometer over the
300–700 nm range. 1H NMR spectra were recorded in CDCl3 with a
Bruker DPX 300 spectrometer. X-ray diffraction (XRD) patterns of
samples were performed on
a Bruker D8 Advance powder dif-
fractometer with a Ni-filtered Cu/Kα radiation source at 40 kV and
20 mA in the 2θ range of 0.5–50° at the rate of 0.5° min−1. The scanning
electron microscope (SEM) images were taken on a HITACHI S-4800
emission scanning microscope. Transmission electron microscope
21

Y. Xie et al.
MolecularCatalysis452(2018)20–27
(TEM) images were obtained with a JEOL JEM model 2100 microscope
operated at 200 kV. The Brunauer–Emmett–Teller (BET) surface areas
were determined by N2 adsorption–desorption measurements
(Micromeritics ASAP 2020) at liquid N2 temperature. Surface areas
were calculated by using the BET method. Pore size distributions were
calculated by the BJH model. Thermal gravimetric analysis (TGA) of
samples were recorded on a STA409 instrument under a nitrogen at-
mosphere. The temperature was between 25 and 600 °C at a heating
rate of 20 °C min−1. The amount of iron on per gram of supports was
determined by means of ICP-OES with an Optima 7300DV
(PerkinElmer) spectrometer.
2.4. Catalytic test
The oxidation reaction was carried out in a 100 mL of autoclave
reactor equipped with a magnetic stirrer. Typically, porphyrin catalysts
and furfural mixed water were added into the autoclave reactor, and
the reaction mixture was heated to the setting temperature with mag-
netic stirring at a speed of 500 rpm. Then the mixture was pressurized
to 1 MPa of pressure with molecular oxygen. After a setting time of
reaction, the autoclave reactor was cooled to room temperature and
depressurized carefully to atmosphere pressure. The catalysts were se-
parated by filtration and the sample was diluted with deionized water
and analyzed by HPLC using an Agilent Alliance System instrument
(1100 series), equipped with a refraction index detector and a Bio-Rad
Aminex HPX-87H column (300 mm × 7.8 mm). A 0.005 M H2SO4 so-
lution was employed as the eluent phase at 313 K and a 0.55 mL min−1
flow rate. The main product (MAD) of the reaction was separated via
crystallization and identified by FT-IR, 1H NMR and 13C NMR techni-
ques [in the Electronic Supporting Information (ESI)]. The conversion
of furfural, the yield and the selectivity of MAD were determined using
the external standard method.
Fig. 2. UV–vis spectra of T(p-Br)PP and FeT(p-Br)PPCl at room temperature in
dichloromethane solution.
CeH bonds. Meanwhile, the band at 1345 cm−1 represented the vi-
bration of C]N in pyrrole rings. After the metallation reaction, a new
sharp absorption band at 997 cm−1 could be observed due to the vi-
bration of N-Metal bond. In addition, the vibration of NeH at 3311 and
964 cm−1 disappeared. The sharp peak observed at 1086 cm−1 was
assigned to the SieOeSi stretching vibration (Fig. 1C). The broad band
at 3425 cm−1 could be attributed to stretching vibration of SieOH
groups and HeOeH of absorbed water. The weak band at 1656 cm−1
corresponded to the bending vibration of the trapped water in the silica
matrix. After the immobilization process, brown color was observed in
FeT(p-Br)PPCl/SBA-15 catalyst, which indicated that FeT(p-Br)PPCl
complexes had been successfully immobilized on the support. However,
the main characteristic peaks of porphyrin complexes could not be
clearly observed in Fig. 1D, the possible reason in that the complexes
were highly dispersed on SBA-15 molecular sieves [26]. Other catalysts
were also investigated by FT-IR spectroscopy (in the ESI).
Fig. 2 depicted the UV–vis results of T(p-Br)PP and FeT(p-Br)PPCl.
In the spectrum of T(p-Br)PP, one intense Soret band at 419 nm ac-
companied by four less intense Q-bands in the 510–650 nm region
could be observed. After coordination with Fe2+, the Soret band of
porphyrin had a minor degree of blue shift. In addition, the peaks of Q-
bands were relatively fewer compared to T(p-Br)PP due to the increased
symmetry of molecule structure [27]. The decrease of the number of
absorption peaks in the Q-band and the displacement of the Soret band
were important signs of the formation of metal complexes, indicating
that the metal ions were successfully coordinated to the porphyrin li-
gands. Other UV spectrums of porphyrin ligands exhibited the similar
features (in Figs. S14–19).
3. Results and decision
3.1. Characterization of the catalysts
The FT-IR spectra of T(p-Br)PP, FeT(p-Br)PPCl, SBA-15 and FeT(p-
Br)PPCl/SBA-15 were given in Fig. 1. The main features of porphyrins
could be observed (Fig. 1A, B). The absorption bands at 3311 and
964 cm−1 were attributed to the vibrations of NeH bonds. The peaks in
the range of 3025–2854 cm−1 were due to the stretching vibrations of
The textural properties of both supports and catalysts were de-
termined via N2 adsorption-desorption analyses (Fig. 3 and Table 1). As
shown in Fig. 3, all samples displayed type IV isotherm according to the
classification of IUPAC. For MCM-41, a capillary condensation was
evidenced in the P/P0 range of 0.3–0.4, while the adsorption isotherms
of SBA-15 and meso-ZSM-5 exhibited H1 and H4 hysteresis loop re-
spectively, reflecting the microstructure difference of the three meso-
porous materials. No significant differences between the isotherms of
pure supports and supported metalloporphyrins were observed, in-
dicating that the catalysts basically maintained the porous features of
the supports. The surface areas, pore volumes, pore sizes and porphyrin
loadings of samples were summarized in Table 1. It was clear that
MCM-41 and SBA-15 exhibited a larger surface area and pore size,
while the specific surface area of meso-ZSM-5 was relatively small
Fig. 1. FT-IR spectra of (A) T(p-Br)PP, (B) FeT(p-Br)PPCl, (C) SBA-15 and (D)
FeT(p-Br)PPCl/SBA-15.
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Y. Xie et al.
MolecularCatalysis452(2018)20–27
Fig. 3. Nitrogen adsorption-desorption isotherms of samples: (A) MCM-41 and FeT(p-Br)PPCl/MCM-41, (B) SBA-15 and FeT(p-Br)PPCl/SBA-15, (C) meso-ZSM-5 and
FeT(p-Br)PPCl/meso-ZSM-5.
system, iron porphyrins could combine with oxygen molecules to form
the high-valent PorFeIV = O radical species, which was responsible for
the oxidation activity of Fe-porphyins [29,30]. It was not difficult to
find the iron content of FeT(p-Br)PP/meso-ZSM-5 catalyst was the
lowest compared to FeT(p-Br)PPCl/SBA-15 and FeT(p-Br)PPCl/MCM-
41 (Table 1, entries 2, 4, 6). Based on the above analysis, it could be
inferred that the catalytic activity of FeT(p-Br)PPCl/SBA-15 was better
than that of FeT(p-Br)PPCl/MCM-41 and FeT(p-Br)PPCl/meso-ZSM-5.
These results were consistent with the experimental results (Table 1,
entries 11–13), indicating that the nature of the supports had a sig-
nificant effect on the activity of the catalysts.
Table 1
Physicochemical properties for supported catalysts.
Entry Sample
Surface
Pore volumeb
Pore
sizec
(nm)
Fee
(wt%)
areaa(m2 g−1
)
(cm3 g−1
)
1
2
MCM-41
947.4
1.136
0.984
4.63
4.43
–
0.96
FeT(p-Br)PPCl/ 864.5
MCM-41
SBA-15
3
4
689.3
0.603
0.495
5.94
5.84
–
0.83
FeT(p-Br)PPCl/ 473.2
SBA-15
Meso-ZSM-5
5
6
232.9
0.031
0.019
3.91
3.70
–
0.63
The powder X-ray diffraction patterns for prepared catalysts were
shown in Fig. 4. It could be found that SBA-15 (Fig. 4A) showed three
typical diffraction peaks at 2θ values of 0.96, 1.56 and 1.82, corre-
sponding to the reflections of (100), (110) and (200) planes respec-
tively, indicating that SBA-15 was a well-ordered 2D hexagonal pore
structure and micropores existed between mesopores and mesopores
[31]. After the immobilization procedure, a lower intensity of reflection
accompanying with a slight shifting of diffraction lines to higher angle
in the (100) plane could be observed, suggesting that the introduction
of porphyrin ligand had an effect on the crystallinity while the order
mesoporous structure of SBA-15 remained almost unchanged. MCM-41
and FeT(p-Br)PPCl/MCM-41 (Fig. 4B) also displayed the typical char-
acteristic peaks of 2D hexagonal materials, which was consistent with
the typical patterns reported in the literatures [32]. However, reactants
and products could only flow in two-dimensional pipelines and could
not shuttled between pipelines. That was, the residence time of the
reactants inside the FeT(p-Br)PPCl/MCM-41 was relatively short, and
the contact with the FeT(p-Br)PPCl/MCM-41 was insufficient, resulting
in a decrease in the activity of the FeT(p-Br)PPCl/MCM-41. The XRD
patterns for meso-ZSM-5 (Fig. 4C) showed five distinct peaks at 2θ
values of 7.92, 8.83, 23.01, 23.78 and 24.33, corresponding to the
FeT(p-Br)PPCl/ 90.8
meso-ZSM-5
7
FeT(p-Br)PP/
451.9
0.491
5.01
0.74
SBA−15d
a
b
c
Surface areas were calculated by BET method.
Pore volumes were calculated by BJH method.
Pore sizes were calculated by BJH method.
The recovered FeT(p-Br)PP/SBA-15 catalyst after five times use.
The iron content of FeT(p-Br)PP/SBA-15 catalysts were determined by ICP-
d
e
OES.
(Table 1, entries 1, 3, 5). Although the specific surface area of SBA-15
was smaller than that of MCM-41, the pore wall was thicker than MCM-
41, resulting in better hydrothermal stability of SBA-15 than that of
MCM-41 [22]. After the incorporation of iron porphyrins into supports,
both surface areas and pore sizes showed a decreasing trend, which
might be caused by the blockage of some pores [28]. It was well known
that the specific surface area, pore size and hydrothermal stability for
immobilized catalysts greatly affect the catalytic activity of the catalyst.
In addition, the structural differences of different supports were also
shown on the loading of iron porphyrin. In the metalloporphyrins/O2
23

Y. Xie et al.
MolecularCatalysis452(2018)20–27
Fig. 4. XRD patterns of (A) SBA-15 and FeT(p-Br)PPCl/SBA-15, (B) MCM-41 and FeT(p-Br)PPCl/MCM-41, (C) meso-ZSM-5 and FeT(p-Br)PPCl/meso-ZSM-5.
characteristics of MFI structure. In contrast, the significant decrease in
the diffraction peak intensity of the FeT(p-Br)PPCl/meso-ZSM-5 com-
pared to meso-ZSM-5 was due to the decrease of crystallinity after
supported by porphyrins, which could also be seen from the results of
SEM of FeT(p-Br)PPCl/meso-ZSM-5 and meso-ZSM-5.
morphology of different samples. As shown in Fig. 5, the images of SBA-
15 and MCM-41 particles showed sphere-like and bar-type structures
respectively with a uniform size, while meso-ZSM-5 particles were si-
milar to hexagon in nature. After incorporation of porphyrin complexes,
the morphology structures of catalysts remained approximately con-
stant. However, some particles of FeT(p-Br)PPCl/MCM-41 generated a
The SEM analysis was carried out in order to investigate the
Fig. 5. SEM images of (A) SBA-15, (B) FeT(p-Br)PPCl/SBA-15, (C) MCM-41, (D) FeT(p-Br)PPCl/MCM-41 (E) meso-ZSM-5 and (F) FeT(p-Br)PPCl/meso-ZSM-5.
24

Y. Xie et al.
MolecularCatalysis452(2018)20–27
Fig. 6. TEM images of FeT(p-Br)PPCl/SBA-15 catalyst.
slight degree of aggregation, which was responsible for the severe de-
crease in catalyst activity. On the contrary, only some fine agglomer-
ated sphere particles were observed on the surface of FeT(p-Br)PPCl/
SBA-15 and FeT(p-Br)PPCl/meso-ZSM-5, which probably because the
excess FeT(p-Br)PPCl complexes adhered to the outer surface of the
supports during the immobilization process. The TEM images of FeT(p-
Br)PPCl/SBA-15 catalyst (Fig. 6) from different angles witnessed a well-
ordered hexagonal arrangement of channels, which were consistent
with the XRD results. From the degree of catalyst surface agglomera-
tion, the surface structures of FeT(p-Br)PPCl/SBA-15 and FeT(p-Br)
PPCl/meso-ZSM-5 were better than FeT(p-Br)PPCl/MCM-41, which was
also an important parameter to measure the performance of a catalyst.
The TG/DTG analysis of FeT(p-Br)PPCl/SBA-15 was performed to
determine the thermal stability of the catalysts. As presented in Fig. 7,
the initial weight loss of 3.2% occured at the temperature range of
25–100 °C, which could be related to adsorbed water on the structure.
The decomposition of porphyrin complex took place between 250 and
350 °C, corresponding to the weight loss of about 10.6% in Figure.
Meanwhile, the peak of DTG curve demonstrated that the fastest weight
loss of the catalyst appeared at about 310 °C. Therefore, it could be
concluded that the FeT(p-Br)PPCl/SBA-15 catalyst was stable up to
250 °C.
Table 2
Catalytic performance of different iron porphyrins for oxidation of furfural.a
Entry Catalyst
Conb(%) Yb (%) Sb (%) C of Porb (mmol)
1
2
3
4
5
6
7
8
FeTPPCl
34.6
36.8
47.7
35.5
32.6
37.7
38.6
37.8
35.7
37.5
41.1
53.7
47.2
10.2
15.4
19.7
10.7
13.0
14.2
15.1
1.5
2.9
2.6
27.8
38
31.5
29.5
41.8
41.3
30.1
39.9
37.7
39.1
4.0
18.1
6.9
67.6
70.8
66.7
7.4 × 10−3
7.4 × 10−3
7.4 × 10−3
7.4 × 10−3
7.4 × 10−3
7.4 × 10−3
7.4 × 10−3
0
FeT(p-Cl)PPCl
FeT(p-Br)PPCl
FeT(o-F)PPCl
FeT(p-CH3)PPCl
FeT(p-OCH3)PPCl
FeT(p-COOMe)PPCl
MCM-41
SBA-15
Meso-ZSM-5
FeT(p-Br)PPCl/MCM-41
FeT(p-Br)PPCl/SBA-15
FeT(p-Br)PPCl/meso-
ZSM-5
9
0
0
10
11
12
13
8.6 × 10−3c
7.4 × 10−3c
5.6 × 10−3c
a
Reaction conditions: 3 mmol furfural; 50 mg catalyst; 6 mL H2O; 90 °C; 6 h;
1 MPa O2.
b
Con = Conversion; Y = Yield; S = Selectivity;
C of Por = Content of
Porphyrin.
c
The amount of porphyrins on catalysts were calculated according to ICP-
OES.
3.2. Catalytic oxidation of furfural into MAD with different iron-based
porphyrins
macrocycle have a remarkable impact on catalytic activity. The yield of
MAD was improved in comparison with unsubstituted FeTPPCl de-
pending on whatever types of anchoring groups they are. The reason
was that the introduction of substituents provided the steric effects
necessary to reduce the oxidative inactivation of porphyrins [33]. Ad-
ditionally, the yield increased little (10.7%) with FeT(o-F)PPCl as cat-
alyst. The introduction of the o-position substituents could lead to more
displacement of Fe metal center, which had a negative effect on the
reactivity [34]. The catalytic performance of different supported iron
porphyrins was next investigated. It was evident that pure supports
showed almost inactive in the reaction and only a small amount of
substrates were converted due to the polymerization (Table 2, entries
8–10). In addition, it was known that MCM-41 and SBA-15 were neutral
supports, while meso-ZSM-5 had a certain degree of acidity. However,
the catalytic effects were very poor when three pure supports were used
as catalysts for the reaction, indicating that the acidity of the catalysts
did not play a role for converting furfural into maleic acid. Never-
theless, MAD yield increased sharply when three supported catalysts
were used (Table 2, entries 11–13), indicating that the catalytic activity
of the catalysts was related to the component of porphyrin complex.
FeT(p-Br)PPCl/SBA-15 catalyst exhibited the best catalytic performance
in three kinds of supported catalysts under the same reaction condi-
tions, giving a conversion of 53.7% furfural and a selectivity of 70.8%
MAD, suggesting that the special ordered channel structure of SBA-15
molecular sieves could be more favorable for the overall diffusion and
transfer of furfural into MAD. Moreover, the catalytic results revealed
that FeT(p-Br)PPCl/SBA-15 displayed much better catalytic behaviors
than the unsupported porphyrins used in experiments, especially in
Firstly, a range of unsupported iron (III) porphyrins were employed
in the catalytic experiments (Table 2, entries 1–7). All of them were
active on the reaction, and the substituent groups on the porphyrin
Fig. 7. TG/DTG curves of FeT(p-Br)PP/SBA-15.
25

Y. Xie et al.
MolecularCatalysis452(2018)20–27
Fig. 8. Conversion of furfural to MAD over FeT(p-Br)PPCl/SBA-15 catalyst. Reaction conditions: 3 mmol furfural, 6 mL H2O, 1 MPa O2. (A) 50 mg FeT(p-Br)PPCl/
SBA-15, 6 h. (B) 50 mg FeT(p-Br)PPCl/SBA-15; 100 °C. (C) 100 °C; 6 h.
selectivity respect (70.8%). Although FeT(p-Br)PPCl was the most ac-
tive catalyst in all unsupported porphyrins, the selectivity of MAD was
relatively poor (41.3%). Previous reports had demonstrated that high-
valent PorFeIV = O radical species formed by the combination of iron
and oxygen molecules were the active sites for oxidation reactions
[29,30]. Even though pure supports could hardly convert furfural into
MAD in the reaction system, they provide a fine microenvironment
where the oxidation reaction between furfural substrates and Por-
FeIV = O active species could be performed more easily, enhancing the
catalytic performance of porphyrins [35].
amount on the reaction was also significant (Fig. 8C). The conversion of
furfural and yield of MAD increased greatly with the catalyst amount at
first, suggesting that increasing active sites of catalyst could benefit to
the conversion of furfural into MAD. When 60 mg of FeT(p-Br)PPCl/
SBA-15 was used, a yield of 56.1% MAD and a conversion of 76%
furfural were obtained. However, both the conversion of furfural and
yield of MAD decreased when the amount of FeT(p-Br)PPCl/SBA-15
was beyond 60 mg. These results demonstrated that the excessive
amount of catalyst hindered the conversion of furfural into MAD.
Thereform, 100 °C, 6 h and 60 mg FeT(p-Br)PPCl/SBA-15 were de-
termined as the best reaction conditions.
3.3. Effect of reaction conditions on the oxidation of furfural over feT(p-Br)
PPCl/SBA-15
3.4. Recycle of the feT(p-Br)PPCl/SBA-15 catalyst
The effects of reaction conditions containing of reaction tempera-
ture, reaction time and catalysts loading were studied in detail over
FeT(p-Br)PPCl/SBA-15 catalysts. The results were presented in Fig. 8. It
was not difficult to find that the conversion of furfural increased with
the temperature from 70 to 120 °C (Fig. 8A) and reached 81% at 120 °C.
However, the maximum yield (48.3%) and selectivity (76.1%) of MAD
were achieved at 100 °C. Afterward, the decrease in MAD yield and
selectivity with increasing temperature from 100 °C to 120 °C was
partly attributed to the oxidative degradation of MAD to other mole-
cules. In addition, the side reaction (polymerization of furfural) was
more likely to occur at higher temperatures, which was one of the
reasons leading to a decrease in MAD yield. The effect of the reaction
time was shown in Fig. 8B. The yield of MAD increased gradually in the
first 6 h and reached a maximum (48.3%) at 6 h. Then the product
began to decompose to small compounds as the yield presented a
downtrend [36]. The selectivity of MAD maximized (79.5%) at 4 h and
then decreased sharply, implying that the competitive side reaction
became serious upon a prolonged reaction time. The effect of catalyst
The recyclability of FeT(p-Br)PPCl/SBA-15 was investigated under
the optimal reaction conditions, and the results were shown in Fig. 9. As
the heterogeneous reaction system, the catalyst could be easily sepa-
rated by filtration, washed with hot water and ethanol, and dried in
oven overnight after each run. It could be observed that the yield of
MAD was almost unchanged (56%) in the first four runs and dropped a
little in the fifth cycle. The BET (Table 1, entry 7) results of the re-
covered FeT(p-Br)PPCl/SBA-15 after five cycles reflected that more
furfural polymers adsorb on the surface of catalyst, hindering the cat-
alytic activity. Additionally, the small leaching of iron (from 0.83 wt%
to 0.74 wt%) in the catalyst was also responsible for the slight loss of
activity. In addition, the SEM and TEM (Fig. S20) characterizations of
the recovered FeT(p-Br)PPCl/SBA-15 catalyst after five cycles showed
that the structure remained intact, indicating that the catalyst has well
catalytic stability.
26

Y. Xie et al.
MolecularCatalysis452(2018)20–27
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.mcat.2018.03.018.
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Acknowledgments
This work was financial supported by the MOE & SAFEA for the 111
Project (B13025).
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