Biomimetic oxidase based on functionalized mesoporous SiO2 and metalloporphyrin for 5-hydroxymethylfurfural conversion

Article abstract of DOI:10.1016/j.procbio.2019.07.020

2,5-Furandicarboxylic acid (FDCA) has the potential to replace terephthalic acid (TPA) as a building block in the preparation of bio-based and degradable polyester materials. However, the lack of efficient and stable catalysts limits the commercial viability of FDCA production. In this work, metalloporphyrins were immobilized on mesoporous SiO₂ and used as a novel oxidase to convert 5-hydroxymethylfurfural (HMF) to FDCA. As the catalytic active center of P450 monooxygenases and horseradish peroxidase, supported palladium, manganese, ferri, and cobalt porphyrins via ionic-liquid-functionalized SiO₂ (PdPOP-SiO₂, MgPOP-SiO₂, FePOP-SiO₂ and CoPOP-SiO₂) were able to simulate oxidase alone under mild conditions. 100% HMF (0.02 M) conversion and 94.94% FDCA yield were achieved using PdPOP-SiO₂ as a biomimetic oxidase after a 12 h reaction at 373 K in water at atmospheric pressure. This type of mimetic oxidase is stable, inexpensive, and readily available, and the method developed in this work can potentially be used to oxidize HMF to FDCA in an industrial setting.

Full text of DOI:10.1016/j.procbio.2019.07.020

Accepted Manuscript
Title: Biomimetic oxidase based on functionalized
mesoporous SiO and metalloporphyrin for
2
5-hydroxymethylfurfural conversion
Authors: Zou Bin, Wang Pengyun, Liu Zhenjiang, Xia
Jiaojiao, Chen baihan, Tan Zhiyuan
PII:
S1359-5113(19)30499-4
DOI:
Reference:
https://doi.org/10.1016/j.procbio.2019.07.020
PRBI 11722
To appear in:
Process Biochemistry
Received date:
Revised date:
Accepted date:
2 April 2019
24 July 2019
25 July 2019
Please cite this article as: Zou B, Wang P, Liu Z, Xia J, Chen b,
Tan Z, Biomimetic oxidase based on functionalized mesoporous SiO2 and
metalloporphyrin for 5-hydroxymethylfurfural conversion, Process Biochemistry
(2019), https://doi.org/10.1016/j.procbio.2019.07.020
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Biomimetic oxidase based on functionalized mesoporous SiO
metalloporphyrin for 5-hydroxymethylfurfural conversion
2
and
a*
a
b
a*
a
a
Zou Bin , Wang Pengyun , Liu Zhenjiang , Xia Jiaojiao , Chen baihan , Tan Zhiyuan
a
School of Food and Biological Engineering,
b
School of Environmental and Safety Engineering
Jiangsu University, Zhenjiang 212013, China.

To whom correspondence should be addressed. The postal address: Jiangsu University, No.301
Xuefu Road, Zhenjiang 212013, China. Tel /Fax.: +86-0511-88780201; E-mail:
binzou2009@ujs.edu.cn, jiaojiaoxia@ujs.edu.cn
Graphical abstract
Highlights



5-hydroxymethylfurfural(HMF) oxidation was carried out in aqueous phase.
Metal porphyrin as biomimetic oxidase catalyzed HMF conversion.
Excellent HMF conversion of 100% and FDCA yield of 94.94% were achieved.
1

Abstract:
2
,5-furandicarboxylic acid (FDCA) has the potential to replace terephthalic acid
(TPA) as a building block in the preparation of bio-based and degradable polyester
materials. However, the lack of efficient and stable catalysts limits the commercial
viability of FDCA production. In this work, metalloporphyrins were immobilized on
mesoporous SiO
HMF) to FDCA. As the catalytic active center of P450 monooxygenases and
horseradish peroxidase, supported palladium, manganese, ferri, and cobalt porphyrins
via ionic-liquid-functionalized SiO (PdPOP-SiO , MgPOP-SiO , FePOP-SiO and
2
and used as a novel oxidase to convert 5-hydroxymethylfurfural
(
2
2
2
2
CoPOP-SiO
2
) were able to simulate oxidase alone under mild conditions. 100% HMF
as a
(
0.02 M) conversion and 94.94% FDCA yield were achieved using PdPOP-SiO
2
biomimetic oxidase after a 12 h reaction at 373 K in water at atmospheric pressure.
This type of mimetic oxidase is stable, inexpensive, and readily available, and the
method developed in this work can potentially be used to oxidize HMF to FDCA in
an industrial setting.
Keywords:
mimetic
oxidase,
functionalized
surface,
mesoporous,
5
-hydroxymethylfurfural, 2,5-furandicarboxylic acid.
2

1
. Introduction
Renewable resources have received increasing attention to replace fossil fuels.
Cellulose, one of the most abundant natural resources, can be transformed into
-hydroxymethylfurfural (HMF) whose chemical structure contains a methylol group,
5
[
1]
a formyl group, and a furan ring . HMF has been selected as a platform compound to
generate more valuable chemicals through ring-opening, reduction, and oxidation
reactions.[2, 3] Of the products derived from HMF, 2,5-furan dicarboxylic acid (FDCA)
can be prepared through the full oxidation of HMF.[4] The molecular structure of
FDCA is similar to that of terephthalic acid (TPA), which is derived from fossil oils
and used to prepare polyethylene terephthalate (PET) plastics. Degradable FDCA
from biomass could be used as a renewable replacement for TPA, which is generated
at a rate of 45.2 million tons per year.[5, 6] Biomass-based FDCA could save energy,
reduce trash, prevent pollution, and protect the environment, so it is very meaningful
to investigate the oxidation of HMF into FDCA (Scheme 1).
The catalysts used in HMF aerobic oxidation have received significant attention
from researchers. As early as 2012, Saha et al. catalyzed HMF oxidation using
Co(OAc)
2
/Zn(OAc) /NaBr as a homogeneous catalyst and obtained 60% FDCA
2
yield.[7] Recently, Zuo et al. reported that Co/Mn/Br homogeneous catalysts boosted
HMF oxidation with over 90% FDCA yield.[8] However, it is difficult to separate
homogeneous catalysts from reaction systems for recycling, which greatly increases
the production cost of FDCA. Hence it is preferable to use heterogenous catalysts
during the oxidation of HMF into FDCA. Among heterogeneous catalysts, the
catalytic efficiency of supported Au, Pd, Ru, and Pt have been shown to be
3

particularly effective.[9, 10] Rass et al. reported that 100% HMF conversion was
obtained using a Pt/C catalyst with 70% FDCA yield at 373 K under 40 bar O
pressure.[11] Mei et al. obtained 52.7% HMF conversion and 45.8% FDCA yield using
-Pd catalyst at 373 K in dimethyl sulfoxide.[12] In addition, Wang et al.
2
C-Fe
3
O
4
reported that nano-Fe
3
4
O -CoOx catalyzed HMF oxidation with 68.6% FDCA yield
and 98.7% HMF conversion in dimethyl sulfoxide in 12 h, using tert-butyl
hydroperoxide as the oxidant.[13] However, the above catalysts required high pressure,
organic solvents, and strong oxidants, which resulted in environmental pollution.
Therefore, the design of green, environmentally-friendly catalysts is crucial.
Porphyrin is a macrocyclic compound containing four pyrrole molecules, and
when the pyrrole proton is replaced by a metal ion, it becomes a metalloporphyrin.[14]
Metalloporphyrin, the catalytic active center of cytochrome P450 monooxygenase and
horseradish peroxidase, is stable, has a strong specificity, is economical, and is readily
available. It can simulate oxidase to activate molecular oxygen under mild
conditions.[15-17] Currently, only two studies have reported the use of
metalloporphyrins for HMF oxidation.[18-19] Saha et al reported that 100% HMF
conversion was obtained using a porous polymer-immobilized Fe(III) porphyrin
catalyst and oxygen as the oxidant to achieve a 79% FDCA yield at 373 K under 10
bar O
pressure in water after 10 h reaction.[18] Metalloporphyrins have been shown to
2
be excellent catalysts for HMF oxidation, but the high pressure and low FDCA yield
need further improvements. Gao et al. obtained 95.6% HMF conversion and 90.4%
FDCA yield using a Merrifield-resin-supported Co(II) porphyrin catalyst and a
tert-butyl hydroperoxide oxidant at 373 K in acetonitrile after 24 h reaction at
atmospheric pressure.[19] However, the use of expensive tert-butyl hydroperoxide
oxidant, organic solvents, and the long reaction time restrict the applications of Co(II)
4

porphyrin catalysts. Therefore, we envisage a novel catalyst based on
metalloporphyrin that combines the advantages of both Fe(III) and Co(II) porphyrins.
In this study, palladium porphyrin, manganese porphyrin, ferriporphyrin, and
cobalt porphyrin are investigated for use as biomimetic oxidases for HMF oxidation.
Since homogeneous metalloporphyrins easily form dimers that are difficult to separate,
recover, and reuse,[20, 21] metalloporphyrins were loaded onto mesoporous SiO
to its large specific surface area, tunable pore size, and abundant modifiable -OH
. Due
2
[
22]
groups, mesoporous SiO
2
is an excellent support material for metalloporphyrins.
The oxidation activity of oxidase is the result of the combination of metalloporphyrins
and glycoproteins. Individual metalloporphyrin activity usually was low due to their
naturally hydrophobic environment loss. When functionalized ionic liquids, one kind
of designable ionic solvent, which could mimic glycoproteins, were introduced into
the mesoporous SiO
2
surface, it provided the metalloporphyrin with a biomimetic
.
microenvironment [23, 24]
Therefore, metalloporphyrins were chosen as the active centers, mesoporous
as the carrier, and an ionic liquid as a modifier for the oxidation of HMF
SiO
2
(
Scheme 2). The effect of four different metal ions (iron, cobalt, manganese, and
palladium) on the catalytic activity of biomimetic oxidases was investigated. Then,
the optimal catalyst was selected to investigate the effect of catalyst dosage,
temperature, time, oxidant type, and amount of base on the catalytic conversion of
HMF to FDCA. Furthermore, the reusability of this novel biomimetic oxidase was
investigated to identify an excellent and green catalyst for HMF oxidation.
2
2
Experiment
.1 Materials
5

5
-hydroxymethyl-2-furancarboxylic acid (HFCA), 5-formyl-2-furancarboxylic
acid (FFCA), FDCA, and HMF were purchased from J&K Chemical Co. Ltd.
,10,15,20-Tetra(4-pyridyl)porphyrin and palladium(II) chloride (PdCl2, 59%Pd) were
5
purchased from Aladdin Chemicals Co. Ltd. Mesoporous silica was purchased from
Qingdao Mei Gao Group Co. Ltd. (particle size 220 nm, pore diameter 11.5 nm). All
other reagents, including cobalt acetate, anhydrous ferric chloride, manganese acetate,
chloropropyltriethoxysilane, N,N-dimethylformamide (DMF), cyclohexane, and
absolute ethanol were purchased from Sinopharm Chemical Reagents.
2
.2 Preparation of metalloporphyrins
Excess metal salts (cobalt acetate, manganese acetate, anhydrous ferric chloride,
and palladium chloride) and 1 g 5,10,15,20-Tetra(4-pyridyl)porphyrin were added
into a 250 mL flask with 50 mL DMF. The molar ratio of metal salt to porphyrin was
5
: 1. The mixed solution was then heated in an oil bath at 433 K with 1000 rpm
magnetic stirring for 12 h. Afterwards, the mixture was cooled to room temperature,
and a rotary evaporator removed most of the solvent, and then 50 mL deionized water
was added into the mixture. Finally, the mixture was filtered to obtain a solid catalyst,
and the metal salts that did not participate in the reaction was washed with DMF.[18]
The resulting solid catalysts (denoted as PdPOP, FePOP, MnPOP, and CoPOP) were
dried at 353 K for 24 h and reserved for further use.
2
.3 Preparation of modified SiO
First, pure mesoporous SiO
2
2
was activated in a vacuum oven at 473 K for 4 h for
further use. Then, 5 g chloropropyl triethoxysilane was added into a 150 mL
Erlenmeyer flask with 30 mL cyclohexane. After that, 5 g activated mesoporous silica
was added into the solution, which was then sealed and stirred at 298 K with 1000
rpm magnetic stirring for 24 h. Finally, the reaction mixture was washed, filtered with
6

cyclohexane, and dried at 353 K for 24 h to give a white solid powder which was
extracted with ethanol for to remove physically-adsorbed
8
h
chloropropyl-functionalized silica.[25] The powder was dried at 353 K for 24 h to
obtain the final functionalized silica sample for further use.
2
.4 Immobilization of metalloporphyrins on modified SiO
2
Metalloporphyrins (iron porphyrin, cobalt porphyrin, manganese porphyrin, and
palladium porphyrin) were mixed with 1 g chloropropyl functional mesoporous silica
at a ratio of metalloporphyrins to chloropropyl-functionalized mesoporous silica of
-1
0
1
.74 mmol·g . Then, they were dissolved in 50 mL DMF and heated at 433 K with
000 rpm magnetic stirring for 24 h.[26] Afterwards, the reaction solution was cooled
to 298 K and filtered. Finally, the solid catalyst was washed with water and DMF and
dried in a vacuum oven at 353 K for 24 h for further use. These samples were denoted
as PdPOP-SiO
.5 Aerobic oxidation process of HMF over biomimetic oxidase catalysts
To test the effect of solvent, HMF (0.08 mmol, 10 mg) was first dissolved in 4 g
2
, FePOP-SiO
2
, MnPOP-SiO
2
, and CoPOP-SiO
2
(Scheme 2).
2
solvent (deionized water, acetonitrile, ethanol, toluene, or dimethyl sulfoxide) in a
reaction flask. A certain amount (20 mg, 40 mg, 60 mg, 80 mg, 100 mg) of
immobilized metalloporphyrin mimetic enzyme catalysts (PdPOP-SiO
2
, FePOP-SiO ,
2
MnPOP-SiO , and CoPOP-SiO ) and NaOH (0 mg, 2 mg, 4 mg, or 6 mg) were
2
2
quickly added to the reaction system, and oxygen was passed through the reaction
mixture(10 ml/min). The reaction mixture was sealed immediately and was as
immersed in an oil bath at the desired temperature (313 K, 333 K, 353 K, 373 K, or 393
K) with stirring at 500 rpm. After a desired reaction time (2 h, 4 h, 8 h, 12 h, or 24 h), a
1
0 μL aliquot was collected from the reaction solution and analyzed by
high-performance liquid chromatography (HPLC). The catalyst used in the experiment
7

was filtered to remove the supernatant. Used catalysts were washed twice with
deionized water and dried at 353 K for 24 h to recover the catalyst for reusability
testing.
Reactants and products were analyzed by HPLC (Shimadzu Technology, model
2
0AB) equipped with an ultraviolet (UV) detector. Samples were measured at a
wavelength of 280 nm using a reverse-phase Sapphire C18 column from Sepax
Technologies, inc. (200 mm × 4.6 mm, PN 801185-4625). The mobile phase for
gradient elution analysis was composed of acetonitrile and 0.1wt% acetic acid aqueous
-1
solution at a 0.5 mL·min flow rate. The concentration of FDCA, HFCA, FFCA, and
HMF (retention time was 10.2, 11.1, 12.3, 15.1 min respectively.) were calculated from
the calibration curves of standards as follows:
HMF conversion = converted HMF moles/initial HMF moles × 100%.
HFCA yield = HFCA moles/ initial HMF moles × 100%.
FFCA yield = FFCA moles/initial HMF moles × 100%.
FDCA yield = FDCA moles/initial HMF moles × 100%.
3
3
. Results and discussion
.1. Catalyst characterization
In this study, the preparation of biomimetic oxidases included three reaction
steps. Firstly, the metalloporphyrins were synthesized by reacting metal salts with
,10,15,20-Tetra(4-pyridyl)porphyrin. Secondly, mesoporous SiO was modified by
chloropropyl triethoxysilane. Finally, metalloporphyrins were grafted onto modified
SiO via ionic liquid bonding (Scheme 2). Scanning electron microscopy (SEM) was
used to analyze the particle size and morphological changes of the SiO carrier and
metalloporphyrin-loaded SiO . The results in Fig 1 show that both the SiO support
and metalloporphyrin-loaded SiO
5
2
2
2
2
2
2
catalysts have irregular spherical structures. The
8

particle size and morphology of the SiO
2
support did not greatly change,[27] which
showed the SiO is relatively stable, and its structure was not damaged during
2
metalloporphyrin immobilization.
Fourier-transform infrared spectroscopy (FT-IR) results are shown in Fig 2 (A).
-1
The absorption peak at 3325 cm belongs to the pyridyl N-H stretching vibration in
-1
porphyrin, and the absorption peak at 968 cm was assigned to the pyridyl bending
-1
-1
vibration in porphyrin. The peak from 2922 cm - 3117 cm was attributed to the C-H
stretching vibration of porphyrin and pyrrole rings, and the peak at 1344 cm-1
belonged to the pyridyl C=N stretching vibration in porphyrin. FePOP, MnPOP,
FePOP-SiO
2
, and MnPOP-SiO
2
all have these characteristic absorption peaks.[28, 29]
-1
3
411 cm was the -OH vibrational absorption peak from the silicon support, while the
-1
peak at 1087 cm belonged to the Si-O vibration of the support. SiO
2
, PdPOP-SiO ,
2
FePOP-SiO , MnPOP-SiO , and CoPOP-SiO were all found to have these
2
2
2
characteristic absorption peaks.[30] It can be concluded that the immobilized
metalloporphyrin mimetic enzyme catalysts were successfully synthesized.
As shown in Fig 2 (B), the first weight loss of immobilized metalloporphyrins
and SiO
the loss of water adsorbed on the SiO
is attributed to the evaporation of DMF on the catalyst surface. Finally, the weight
loss of chloropropyl functional silica, PdPOP-SiO , FePOP-SiO , MnPOP-SiO , and
CoPOP-SiO from 708 K - 808 K were 2.18%, 6.21%, 13.28% 11.76%, and 11.88%,
respectively. These weight losses are likely due to the weight loss of ionic liquids and
the pyrolysis of metalloporphyrins on the catalyst surfaces. Moreover, the SiO
2
support that occurs near 373 K in the thermogravimetry (TG) curve is likely
2
surface. The weight loss around 433 K - 473 K
2
2
2
2
2
support has good thermal stability up to 1273 K,[31] and since the highest reaction
temperature in this study was 393 K, it can assumed that the catalysts will have good
9

thermal stabilities in this study. Thus, temperature has no obvious effect on the
catalyst structure during the experiment.
X-ray powder diffraction (XRD) patterns of the SiO
2
support, PdPOP-SiO ,
2
FePOP-SiO
2
, MnPOP-SiO
2
, and CoPOP-SiO
2
are shown in Fig 2 (C). It was observed
o
that the same characteristic diffraction peak appeared at 23 in all samples, and due to
its broad shape, it can be inferred that it was the characteristic diffraction peak of
amorphous SiO
2
.[32] The presence of this peak indicated that all samples have an SiO
2
skeleton which was not significantly degraded during metalloporphyrin
o
immobilization. Moreover, the peak at 40 was the characteristic diffraction peak of the
Pd(II) nanoparticles in the PdPOP-SiO
2
XRD patterns. In addition, the XRD patterns
of immobilized metalloporphyrin catalysts showed characteristic diffraction peaks at
o
o
o
1
9 , 21 , and 26 which may be due to π-π interactions between adjacent stacked
porphyrins during metalloporphyrin immobilization.[33] Thus, the order and regularity
of the molecular sieve pores were changed, which indicated that metalloporphyrins
had been successfully loaded onto the SiO support.
2
The immobilized palladiumporphyrin catalyst was also measured by X-ray
photoelectron spectroscopy (XPS), and the results are shown in Fig 2 (D). The peaks
at 102.08 eV, 283.08 eV, 339.08 eV, 401.08 eV, and 531.08 eV were assigned to Si 2p,
C 1s, Pd 3d, N 1s, and O 1s, respectively. These data demonstrate that the surface
elements of the immobilized palladiumporphyrin were Si, C, Pd, N, and O.[34] This
also indicated the successful synthesis of the immobilized palladiumporphyrin
catalyst.
The physical adsorption isotherm and pore size distribution of SiO
2
, PdPOP-SiO ,
2
10

FePOP-SiO
2
, MnPOP-SiO
2
, and CoPOP-SiO are shown in Fig S1, which showed
2
that they are all typical mesoporous materials. The typical characteristics of
mesoporous materials are an adsorption curve that is not consistent with the
desorption curve belonging to an H
the pore size distribution, the pore diameter of SiO
MnPOP-SiO , and CoPOP-SiO were 11.50 nm, 6.31 nm, 11.42 nm, 11.46 nm, and
.60 nm, respectively. Hence, it can be concluded that immobilized metalloporphyrin
catalysts were successfully synthesized. In addition, the Brunner−Emmet−Teller
BET) measurements data of specific surface area, pore volume, and pore size are also
1
-type hysteresis loop.[35] In addition, according to
2
, PdPOP-SiO , FePOP-SiO ,
2
2
2
2
7
(
shown in Table 1. According to this data, the chemical modification of the surface of
the porous materials appears to affect their surface properties. The surface area, pore
volume, and pore size of immobilized metalloporphyrins were all lower compared
with the SiO
on the surface of SiO
Organic and metal elements of the catalysts are shown in Table S1. The content
of C, H, and N of the unmodified SiO and chloropropyl-functionalized silica were
2
support, likely due to the loading of ionic liquids and metalloporphyrins
2
.
2
-1
analyzed, which showed that 0.56 mmol·g of the silane modifier was loaded onto
.[36] The amount of C, H, N, and metal elements of immobilized metalloporphyrin
SiO
2
catalysts were also analyzed. The results showed that the loading of metalloporphyrin
-1
of PdPOP-SiO
2
, FePOP-SiO
2
, MnPOP-SiO
2
, and CoPOP-SiO
2
were 0.49 mmol·g ,
-1
-3
-1
-1
[37]
0
.02 mmol·g , 0.66 × 10 mmol·g , and 0.13 mmol·g , respectively.
The
PdPOP-SiO
the SiO support.
.2. Comparison of catalytic activity of different biomimetic oxidase
To identify the best biomimetic oxidase for HMF oxidation, a series of
2
catalyst contained the highest amount of metalloporphyrin loaded onto
2
3
11

experiments were carried out to catalyze HMF oxidation with different catalysts. The
catalytic activities of the SiO carrier and no catalyst under the same reaction
2
conditions are shown in Table 1, entries 6-7. When heated for 12 h at 363 K without
catalyst, small amount of HMF was oxidized and decomposed in the control group.
The experimental results in Table 1 showed that the catalytic activity of immobilized
palladiumporphyrin was much better than that of the other catalysts. The HMF
conversion and FDCA yield using the palladiumporphyrin catalyst were 48.98% and
3
8.91%, respectively. Moreover, the pore size, pore volume, and specific surface area
of the palladiumporphyrin catalyst in Table 1 are the smallest among the
metalloporphyrin catalysts. This indicates that the highest amount of
metalloporphyrin was loaded onto SiO
2
support in the PdPOP-SiO
2
catalyst,
according to Table S1. Moreover, PdPOP-SiO
2
is not prone to π-π stacking which
prevented auto-oxidation and deactivation of catalytic center. Hence, it can be
inferred that the surface modification of the support and the successful grafting of
metalloporphyrins gave the immobilized palladiumporphyrin catalyst ideal catalytic
activity. Thus, immobilized palladiumporphyrin catalysts were selected for further
HMF oxidation studies.
3
.3 Effect of different solvents on HMF oxidation
Solvents play very important roles during chemical reactions due to different
polarities, conductivities, dielectric constants, and pH values.[38] To determine the best
solvent for HMF oxidation, a series of experiments were carried out in various solvents,
such as ethanol, toluene, acetonitrile, water, and dimethyl sulfoxide, and the results are
summarized in Fig 3. Water was the best solvent for HMF oxidation, since the HMF
conversion and FDCA yield were 75.44% and 76.26%, respectively. These results
may be due to the high polarity of water. The polarity of toluene, ethanol, acetonitrile,
12

dimethyl sulfoxide, and water are 2.4, 6.0, 6.2, 7.2, and 10.2, respectively. According
to the principle of similar phase dissolution, since the polarity of the substrate HMF
and product FDCA were also relatively high, water was the optimal solvent to
dissolve the reaction substrate and products. The polarities of ethanol and toluene are
relatively low, and the product FDCA is insoluble in the reaction system, which
explains why the FDCA yield in ethanol and toluene were only 0.97% and 1.23%,
respectively. Relatively higher HMF conversions of 37.78% and 49.17% and FDCA
yields of 21.04% and 45.33% in acetonitrile and dimethyl sulfoxide were obtained,
respectively, which are lower than those of water. These differences may be because
water is the most polar solvent, and the dispersion of catalyst in organic solvents is
not good, which reduces catalytic activity. Thus, water was the best reaction solvent
and is both cheap and environmentally-friendly.
3
.4. Effect of catalyst dosage on HMF oxidation
In this study, different catalyst amounts were used to determine the effect of
dosage on HMF oxidation. A series of experiments were carried out with 20 mg, 40
mg, 60 mg, 80 mg, and 100 mg catalyst. In Fig 4, the HMF conversion and FDCA
yield initially increased as the catalyst amount increased. For example, 64.14% and
1
00% HMF conversion were acquired after 12 h reaction using 20 mg and 80 mg of
catalyst, and the corresponding FDCA yields were 61.22% and 90.48%. This increase
was attributed to an increase in the number of catalytic active sites as the catalyst
amount increased. Further increasing the catalyst amount did not change the HMF
conversion and FDCA yield they were in equilibrium.[39] This may be due to the
saturated adsorption of HMF substrate onto the catalyst active sites, which effectively
reduced the amount of catalyst available to convert substrate into product. However,
when the catalyst amount reached to 100 mg, we found that nearly 10% compounds
13

was not detected. Since the temperature and base dosage were not high, the substrate
should not have been degraded. Thus, it is likely that products were absorbed onto the
catalyst so that a portion of the products were not detected. When the catalyst was
ultrasonicated for 20 min, 5.72% FFCA and 0.38% FDCA were obtained. Therefore,
to ensure sufficient catalytic active sites in the reaction system to catalyze the
oxidation of HMF and the catalysts did not seriously adsorb the products. Thus, 80
mg of catalyst was selected for the next experiment.
3
.5 Effect of base and oxidant on HMF oxidation
Base played an important role in HMF oxidation into FDCA since bases can
neutralize the FDCA and prevent it from being adsorbed onto the catalyst surface.[40]
Therefore, we explored the optimum amount of base in the HMF oxidation system,
and the results in Table 2 show that HMF conversion (33.06%) and FDCA yield
(14.24%) were relatively low without base. Further increasing the amount of base
increased both the HMF conversion and FDCA yield, but when the base amount was
increased to 6 mg, a side reaction occurred. It is proposed that high base
concentrations degrade the HMF, thus 4 mg base was selected for the next experiment.
The oxidizing effects of tert-butylhydroperoxide and oxygen were compared, and it
was found that oxygen was better. Since oxygen is a green oxidant, it is potentially a
better choice due to its abundant resources and lack of toxic pollution.
3
.6. Effects of temperature on HMF Oxidation
Experiments were performed at different temperatures from 313 K to 393 K, and
the results in Fig 5 show that temperature had an important effect on HMF conversion
and FDCA yield. Generally speaking, HMF conversion and FDCA yield were
improved as the reaction temperature increased. At 313 K, the HMF conversion and
FDCA yields were 69.08% and 62.41%, respectively. When the reaction temperature
14

was increased to 333 K, 353 K, and 373 K, HMF conversions were 82.81%, 91.15%,
and 100%, and FDCA yields were 74.06%, 82.38%, and 92.94%, respectively.
However, when the temperature was further increased to 393 K, the FDCA yield was
9
1.40%, and no other intermediate product was detected. At 393 K, the temperature
was likely too high, and the HMF substrate was degraded into other by-products.[41]
Therefore, 373 K was selected for the following experiment.
3
.7 Effect of time course on HMF conversion
To study optimal HMF oxidation reaction time using PdPOP-SiO , different time
2
points were recorded during the oxidation reaction, and the results are shown in Fig 6.
The HMF conversion and FDCA yield gradually increased during the oxidation
process,[42] and complete HMF conversion was achieved between 8 h and 12 h. The
FDCA yield also gradually increased at longer reaction times, and the FDCA yield was
9
7.34% after 24 h. However, the FDCA yield reached equilibrium at 12 h with a yield
of 94.94%.
3
.8 Kinetics of HMF oxidation via biomimetic oxidase catalyst
The reaction kinetics model was established based on the catalyst concentration
and reaction temperature (E-supplementary data). A reaction mechanism model for the
catalytic oxidation of HMF by immobilized metal palladium porphyrin was proposed
(
Fig. S2). The kinetic reaction constants are shown in Table 3, where K
1
and K
3
represent the oxidation of the aldehyde group, K represents the oxidation of the
2
hydroxyl group. The oxidation experiment was conducted at 343 K, 353 K, and 363 K,
and the catalyst dosage was 40 mg, 60 mg, or 80 mg. It was demonstrated from Table 3
that the rate constants were in the order K
1
> K
2
> K in the reaction route
3
HMF→HFCA→FFCA→FDCA, indicating the oxidation of FFCA to FDCA was the
rate-limiting step. A possible explanation is that electrons in the furan were attracted to
15

HFCA to form a carboxyl group during the oxidation of HMF, which generated an
unstable free radical with a resonance structure during the formation of FDCA[43]
.
Table 3 shows that the reaction rate constant increased as both the temperature and
catalyst dosage increased.
3
.9. Reusability test of catalyst
To verify the reusability of the heterogeneous catalysts, recycling experiments of
PdPOP-SiO were carried out at 373 K with 80 mg catalyst for 12 h. After the reaction,
2
the supernatant was discarded, and the recycled catalyst was washed with deionized
water twice and then dried at 353 K for the next cycle. The results in Fig 7 show that
after the catalysts were reused for 5 cycles, the HMF conversion was still 100%, and
the FDCA yield was slightly reduced from 94.94% to 93.13%. The experimental
results showed that the repeatability of the catalyst was remarkable, and the catalyst
can be reused.
4
．Conclusion
In summary, the PdPOP-SiO
catalysts were successfully prepared via covalent binding of metalloporphyrin
nanoparticles onto the functioned surface of mesoporous SiO and characterized by
SEM, XRD, TG, FT-IR, XPS, and BET techniques. The PdPOP-SiO catalyst
2
, MnPOP-SiO
2
, FePOP-SiO
2
, and CoPOP-SiO
2
2
2
demonstrated the highest catalytic activity during HMF oxidation into FDCA using
mild reaction conditions. The metal active center of the catalyst was critical for HMF
oxidation. In addition, the solvent, catalyst amount, base dosage, oxidant, temperature,
and reaction time also showed significant effects on HMF conversion and FDCA
selectivity. Under optimum reaction conditions, 100% HMF conversion and 94.94%
FDCA yield were archived after 12 h reaction at 373 K, using PdPOP-SiO
2
as the
catalyst. Compared with previously-reported, the our catalytic system shows the
16

following notable advantages: (a) PdPOP-SiO
concentrations; (b) the reaction could be carried out under atmosphere, and does not
require high oxygen pressure; (c) biomimetic oxidase PdPOP-SiO could be simply
separated from the reaction system and further recycled without severe loss of activity;
d) water was the optimal reaction solvent for HMF conversion and is both cheap and
2
catalyst avoids the need for high base
2
(
environmentally-friendly. Therefore, biomimetic oxidase developed in this study has
promising potential for large-scale FDCA production for use in renewable
biomass-based platform chemicals.
5
. Conflict of interest
The authors declare no conflict of interest.
Acknowledgments
The work was funded by the National Natural Science Foundation of China (No.
1406093), the Natural Science Foundation of Jiangsu province (BK20140529), Key
2
University Science Research Project of Jiangsu Province(14KJB530001), China
Postdoctoral Science Foundation (2014M550271), and the Project Funded by the
Priority Academic Program Development of Jiangsu Higher Education Institutions.
17

References
[
1] S.G. Wang, B. Liu, Z.L. Yuan, Z.H. Zhang, Aerobic oxidation of 5-hydroxymethylfurfural into
2 3
furan compounds over Mo-hydroxyapatite-encapsulated magnetic γ-Fe O . J. Taiwan. Inst. Chem.
E. 58 (2015) 92-96.
[
2] D.X. Yan, J.Y. Xin, C.Y. Shi, X.M. Lu, L.L. Ni, G.Y. Wang, S.J. Zhang, Base-free conversion
of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid in ionic liquids. Chem. Eng. J. 323
(2017) 473-482.
[
3] G.S. Yi, TS.P. eong, Y.G. Zhang, Base-free conversion of 5-hydroxymethylfurfural to
2
,5-furandicarboxylic acid over a Ru/C catalyst. Green. Chem. 18 (2016) 979-983.
[
4] V.N. Chi, Y.T. Liao, T.C. Kang, J.E. Chen, T. Yoshikawa, Y. Nakasaka, T. Masuda, K.C.W. Wu ,
A Metal-Free, High Nitrogen-Doped Nanoporous Graphitic Carbon Catalyst for an Effective
Aerobic HMF-to-FDCA Conversion. Green. Chem. 18 (2016 ) 5957-5961.
[
5] G.R. Dick, A.D. Frankhouser, A. Banerjee, M.W. Kanan, A scalable carboxylation route to
furan-2,5-dicarboxylic acid. Green. Chem. 19 (2017) 2966-2972.
6] X.W. Han, L.Geng, Y. Guo, R. Jia, X.H. Liu, Y.G. Zhang, Y.Q. Wang, Base-free aerobic
[
oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over a Pt/C–O–Mg catalyst.
Green. Chem. 18 (2015) 1597-1604.
[
7] B. Saha, S. Dutta, M.M. Abu-Omar, Aerobic oxidation of 5-hydroxylmethylfurfural with
homogeneous and nanoparticulate catalysts. Catal. Sci. Technol. 2 (2012) 79-81.
8] Y.B. Wang, K. Yu, D. Lei, W. Si, Y.j. Feng, L.L. Lou, S.X. Liu, Basicity-Tuned
[
Hydrotalcite-Supported Pd Catalysts for Aerobic Oxidation of 5-Hydroxymethyl-2-furfural under
Mild Conditions. Acs. Sustain. Chem. Eng. 4 (2016) 4752-4761.
[
9] W. Niu, D. Wang, G. Yang, J. Sun, M. Wu, Y. Yoneyama, N. Tsubaki, Pt Nanoparticles Loaded
on Reduced Graphene Oxide as an Effective Catalyst for the Direct Oxidation of
-Hydroxymethylfurfural (HMF) to Produce 2,5-Furandicarboxylic Acid (FDCA) under Mild
Conditions. B. Chem. Soc. Jpn. 87 (2014) 1124-1129.
10] D.K. Mishra, H.J. Lee, J. Kim, H.S. Lee, J.K. Cho, Y.W. Suh, Y. Yi, Y.J. Kim, MnCo
5
[
2 4
O
Spinels supported Ruthenium catalyst for air-oxidation of HMF to FDCA under aqueous phase and
base-free conditions. Green. Chem. 19 (2017) 1619-1623.
[
11] H.A. Rass, N. Essayem, M. Besson, Selective aqueous phase oxidation of
-hydroxymethylfurfural to 2,5-furandicarboxylic acid over Pt/C catalysts: influence of the base
and effect of bismuth promotion. Green. Chem. 15 (2013) 2240-2251.
12] N. Mei, B. Liu, J. Zheng, K. Lv, D. Tang, Z. Zhang, A novel magnetic palladium catalyst for
5
[
the mild aerobic oxidation of 5-hydroxymethylfurfural into 2,5-furandicarboxylic acid in water.
Catal. Sci. Technol. 5 (2015) 3194-3202.
[
13] S. Wang, Z. Zhang, B. Liu, Catalytic conversion of fructose and 5-hydroxymethylfurfural
18

into 2,5-furandicarboxylic acid over a recyclable Fe
Sustain. Chem. Eng. 3 (2015) 406-412.
3 4
O -CoOx, magnetite nanocatalyst. Acs.
[
14] M.I. Vohra, D.J. Li, Z.G. Gu, J. Zhang, Insight into the epitaxial encapsulation of Pd catalysts
in an oriented metalloporphyrin network thin film for tandem catalysis. Nanoscale. 9 (2017)
734-7738.
15] Y. Yao, Y. Du, J. Li, C. Wang, Z.Q. Zhang, X. Zhao, Crystal structure of meso-substituted
7
[
pyrazolyl porphyrin complexes and their highly active catalyst for oxidation of alkylbenzenes.
Appl. Organomet. Chem. 32 (2017) 4184.
[
16] X. Guo, D.H. Shen, Y.Y. Li , M. Tian, Q. Liu, C. Guo, Z. Li, Immobilization of
metalloporphyrin on organosilicon microsphere mixed with ceria as a new catalyst for oxidation
of cyclohexane. J. Mol. Catal. A. Chem. 351 (2011) 174-178.
[
17] B. Sabour, K. Mehrani , A. Mehrani, M.H. Peyrovi, Immobilization of Metalloporphyrin on
Functionalized MCM-48 Nanoporous Silica as a Catalyst in Oxidation of Cyclohexene: The
Effects of Nanoporous Structure of MCM-48 on Its Catalytic Efficiency. J. Inorg. Organomet. P.
2
3 (2013) 1183-1188.
18] Saha, B., Gupta, D., Abu-Omar, M. M., Modak, A., & Bhaumik, A. Porphyrin-based porous
organic polymer-supported iron(iii) catalyst for efficient aerobic oxidation of
-hydroxymethyl-furfural into 2,5-furandicarboxylic acid. Journal of
Catalysis, 299(2013)316-320.
19] Gao, L. , Deng, K. , Zheng, J. , Liu, B. , & Zhang, Z. . (2015). Efficient oxidation of biomass
[
5
[
derived 5-hydroxymethylfurfural into 2,5-furandicarboxylic acid catalyzed by merrifield resin
supported cobalt porphyrin. Chemical Engineering Journal,270(2015)444-449.
[
20] J. Santos, V. Palaretti, A. Faria, E. Crevelin, L. Moraes, M. Assis, Biomimetic simazine
oxidation catalyzed by metalloporphyrins. Appl. Catal. A. Gen. 408 (2011) 163-170.
21] X. Yang, M. Xie , C. Zou, Y. He, B. Chen, M. O’Keeffe, J. Porous Metalloporphyrinic
[
Frameworks Constructed from Metal 5, 10, 15, 20-Tetrakis (3, 5-biscarboxylphenyl) porphyrin for
Highly Efficient and Selective Catalytic Oxidation of Alkylbenzenes. Am. Chem. Soc. 134 (2012)
1
0638-10645.
[
22] B. Wang, L. Zhang, B. Li, Y. Li, Y. Shi, T. Shi, Synthesis, characterization, and oxygen
sensing properties of functionalized mesoporous silica SBA-15 and MCM-41 with a
Pt(II)–porphyrin complex. Sensor. Actuat. B. Chem. 190 (2014) 93-100.
[
23] S. Ahmad, K. Yadav, S. Chauhan, Role of hydroxyl functionalized ionic liquids in oxygenation
and oxidation of phenols with H catalyzed by iron(III) porphyrins. Curr. Org. Chem. 16 (2012)
989-3001.
24] L. Jameson, J. Kimball, Z. Gryczynski, M. Balaz, S. Dzyuba, Effect of ionic liquids on the
conformation of a porphyrin-based viscometer. RSC. Adv. 3 (2013) 18300-18304.
25] J. Li, X. Yan, Y. Zhang, B.L. Zhao, G.H. He, Enhanced hydroxide conductivity of
Imidazolium functionalized polysulfone anion exchange membrane by doping imidazolium
2
O
2
2
[
[
19

surface-functionalized nanocomposites. Rsc. Adv. 6 (2016) 58380-58386.
26] G.M. Ucoski, V.H.A. Pinto, Defreitas-Silva., Manganese porphyrins immobilized on
[
magnetic SBA-15 mesoporous silica as selective and efficient catalysts for cyclic and linear
alkane oxidation. Micropor. Mesopor. Mat. 265 (2018) 84-97.
[
27] S.J. Hu, J.H. Zhu, Y.L. Wu, R.R. Xie, K.J. Wu, M.D. Yang, Preparation of packing type
catalysts AAO@Al/Meso-SiO -SO H for the dehydration of xylose into furfural. Micropor.
Mesopor. Micropor. Mesopor. Mat. 262 (2017) 112-121.
28] Y. Huang, C.Y. Wu, W.W. Yuan, Y.M. Xia, X. Liu, H.M. Yang, H.J. Wang, Catalytic aerobic
2
3
[
oxidation of biomass-based Furfural into maleic acid in aqueous phase with metalloporphyrin
Catalysts. J. Chin. Chem. Soc. Taip. 64 (2017) 786-794.
[
29] S. Rayati, S.E. Ruzbahani, F. Nejabat, A comparative study of catalytic activity of Fe, Mn and
Cu Porphyrins immobilized on mesoporous MCM-41 in oxidation of sulfides. Macroheterocycles.
Macroheterocycles. 10 (2017) 57-61.
[
30] X. Xu, E. Jiang, Z. Lei, Esterification of guaiacol with octanoic acid over functionalized
mesoporous silica. Renew. Enegr. 119 (2018) 439-446.
31] Z.G. Liu, L.T. Ji, J. Li, L.L. Fu, S.F. Zhao, Influence of heat treatment on catalytic performance
of Co-N-C/SiO , for selective oxidation of ethylbenzene. J. Mol. Catal. A. Chem. 395 (2014)
15-321.
32] L. Chen, P. Guo, M. Qiao, S. Yan, H. Li, W. Shen, H. Xu, Cu/SiO
[
2
3
[
2
catalysts prepared by the
ammonia-evaporation method: Texture, structure, and catalytic performance in hydrogenation of
dimethyl oxalate to ethylene glycol. J. Catal. 257(2008) 172-180.
[
33] S. Rayati, P. Nafarieh, M.M. Amini, The synthesis, characterization and catalytic application
of manganese porphyrins bonded to novel modified SBA-15. New. J. Chem. 42 (2018) 6464-6471.
34] A. Drelinkiewicz, D. Rutkowska, Nanospace constraints in mesoporous silica carriers—A
[
factor of critical importance in promoting the catalytic activity of supported ruthenium (II) complex
with hemilabile phosphine ligand. Appl. Catal. A. Gen. 427 (2012) 16-23.
[
35] A.P. Unnarkat, T. Sridhar, H. Wang, S.M. Mahajani, A.K. Suresh, Study of Cobalt
molybdenum oxide supported on mesoporous silica for liquid phase cyclohexane oxidation. Catal.
Catal. Today. 310 (2017) 116-129.
[
36] P. Shejwalkar, N.P. Rath, E.B. Bauer, ChemInform Abstract: New bis(imino)pyridine
complexes of Iron(II) and Iron(III), and their catalytic activity in the mukaiyama aldol reaction.
Cheminform. 45 (2014) 57-66.
[
37] N.A. Dare, L. Brammer, S.A. Bourne, Fe(III) Protoporphyrin IX encapsulated in a zinc
metal–organic framework shows dramatically enhanced peroxidatic activity. T.J. Egan, Inorg.
Chem. 57 (2018) 1171-1183.
[
38] R.R.S. And, T.W. Root, DFT study of solvent coordination effects on titanium-based
epoxidation catalysts. part one:ꢀ formation of the titanium hydroperoxo intermediate. J. Phys. Chem.
B. 107 (2003) 4090-4099.
20

[
39] Z. Zhang, Z. Yuan, D. Tang, Y. Ren, K. Lv, B. Liu, Iron oxide encapsulated by ruthenium
hydroxyapatite as heterogeneous catalyst for the synthesis of 2,5-diformylfuran. ChemSusChem. 7
(2014) 3496-3504.
[
40] L. Ardemani, G. Cibin, A.J. Dent, M.A. Lsaacs, G. Kyriakou, Lee. A.F., C.M.A. Parlett, S.A.
Parry, K. Wilson, Solid base catalysed 5-HMF oxidation to 2, 5-FDCA over Au/hydrotalcites: fact
or fiction?. Chem. Sci. 6 (2015) 4940-4945.
[
41] K.R. Vuyyuru, P. Strasser, Oxidation of biomass derived 5-hydroxymethylfurfural using
heterogeneous and electrochemical catalysis. Catal. Today. 195 (2012) 144-154.
42] L. Gao, K. Deng, J. Zheng, B. Liu, and Z. Zhang, Efficient oxidation of biomass derived
-hydroxymethylfurfural into 2,5-furandicarboxylic acid catalyzed by Merrifield resin supported
cobalt porphyrin. Chem. Eng. J. 270 (2015) 444-449.
[
5
[
43] S. E. Davis, A. D. Benavidez, R. W. Gosselink. Kinetics and mechanism of
-hydroxymethylfurfural oxidation and their implications for catalyst development. J. Mol. Catal.
A-Chem. 388-389 (2014)123-132.
5
21

Figure and table captions:
Scheme Reaction pathway from 5-hydroxymethylfurfural (HMF) to
,5-furandicarboxylic acid (FDCA).
Scheme 2 Immobilization process of metalloporphyrins onto ionic liquid modified
mesoporous SiO
Fig 1. SEM images of (a) SiO
e) PdPOP-SiO (5000 multiple), (f) PdPOP-SiO
1
2
2
.
2
, (b) CoPOP-SiO
2
, (c) MnPOP-SiO
2
, (d) FePOP-SiO ,
2
(
2
2
(500 multiple).
Fig 2. FT-IR spectra(A), TG spectra(B), XRD patterns(C) and XPS spectra (D) of
immobilized biomimetic oxidase.
Fig 3. Effect of the solvents on the HMF oxidation. (Reaction conditions:
PdPOP-SiO
Fig 4. The effect of catalyst dosage on the HMF oxidation. (Reaction conditions:
Solvent: H O, HMF (10 mg), 373 K, 12 h.)
Fig 5. The effect of reaction temperature on the HMF oxidation. (Reaction conditions:
PdPOP-SiO (80 mg), Solvent: H O, HMF (10 mg), 12 h.)
Fig 6. Effect of the reaction time on the HMF oxidation. (Reaction conditions:
PdPOP-SiO (80 mg), HMF (10 mg), Solvent: H O, 373 K.)
Fig 7. Recycle experiments of the biomimetic oxidase. (Reaction conditions:
PdPOP-SiO (80 mg), HMF (10 mg), Solvent: H O, 373 K, 12 h.)
2
(60 mg), HMF (10 mg), 373 K, 12 h.)
2
2
2
2
2
2
2
22

Fig. 1
23

Fig. 2
24

Fig. 3
25

Fig. 4
26

Fig. 5
27

Fig. 6
28

Fig. 7
29