Aerobic oxidative esterification of 5-hydroxymethylfurfural to dimethyl furan-2,5-dicarboxylate by using homogeneous and heterogeneous PdCoBi/C catalysts under atmospheric oxygen

Article abstract of DOI:10.1039/c8gc01393d

The conversion of platform molecule 5-hydroxymethylfurfural (HMF) into many value-added derivatives has attracted significant interest. FDCA and its esters are important derivatives of HMF, which can be used as polyester monomers and pharmaceutical intermediates. In this paper, oxidative esterification of 5-HMF has been carried out by using homogeneous and heterogeneous PdCoBi/C catalysts under atmospheric oxygen. The effect of reaction conditions on product distribution has been studied under both homogeneous and heterogeneous catalytic conditions. The highest yields of oxidative esterification products are obtained at 93% and 96% by using homogeneous and heterogeneous PdCoBi/C catalysts, respectively. The catalysts are characterized by X-ray photoelectron spectroscopy (XPS) and powder X-ray diffraction (XRD). The catalytic system has better compatibility according to the expansion of the substrate. A reaction mechanism is proposed, and recycle experiments are also conducted.

Full text of DOI:10.1039/c8gc01393d

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Aerobic oxidative esterification of 5-hydroxymethylfurfural to
dimethyl furan-2, 5-dicarboxylate by using homogeneous and
heterogeneous PdCoBi/C catalysts under atmospheric oxygen
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Feng Li, Xing-Long Li, Chuang Li, Jing Shi and Yao Fu *
www.rsc.org/
Conversion of platform molecule 5-hydroxymethylfurfural (HMF) into highvalue-added derivatives has attracted significant
interest. FDCA and its esters are important derivatives of HMF, which can be used as polyester monomers and
pharmaceutical intermediates. In this paper, the oxidative esterification of 5-HMF have been carried out by using
homogeneous and heterogeneous PdCoBi/C catalysts under atmospheric oxygen. The effect of reaction conditions on
product distribution have been studied under both homogeneous and heterogeneous catalytic conditions. The highest
yield of oxidative esterification products were 93% and 96% by using homogeneous and heterogeneous PdCoBi/C
catalysts, respectively. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS), powder X-ray
diffraction (XRD). The catalytic system had better compatibility according to the substrate extension. A proposed reaction
mechanism and the recycle experiments were also carried out.
(
FDCA) or its esters and ethylene glycol had excellent material
properties. The oxygen isolation rate of PEF was six times higher
than that of PET, and the isolation rate of CO and water of PEF was
Introduction
2
The environmental pollution and energy crisis caused by the
consumption of fossil resources were imminent. Exploring
sustainable green energies was very urgent and necessary. Biomass
resources, as the only renewable carbon resources, could be
converted into high value-added fuels and chemicals which was an
two times higher than that of PET. The glass transition temperature
of PEF was 11 degrees higher than that of PET, and the melting
[4]
temperature of PEF was 40 degrees lower than that of PET. FDCA
had an aromatic ring system similar to PTA and a diacid structure
1] needed for the synthesis of polyester. It was expected to be an
[
important way to solve the problems of energy and environment.
important biomass based polyester material for the synthesis of
Biomass-based polymer materials have dual functions of resource
conservation and environmental protection. At present, the widely
used biomass-based polymer materials were mainly fatty polymers
[5]
green, degradable plastics and non-toxic plasticizers. Therefore, it
is of great significance to study the green and efficient synthesis
[2b] method of FDCA and its esters.
[2a]
such as polylactic acid (PLA),
polyhydroxy fatty acid (PHA),
polybutylene glycol succinate (PBS)
etc. Their mechanical properties and heat resistance were
[2c]
According to the different sources of raw materials, the
polyglycolic acid (PGA),
[2d]
methods for preparation of 2, 5-FDCA were mainly divided into the
following categories: 1) Preparation by furoic acid through
,
significantly lower than those of petroleum based polymers, such as
aromatic polyethylene terephthalate (PET), bisphenol A epoxy resin
disproportionation reaction or chloromethylation reaction or CO
2
[
6]
solid-phase reaction.
2) Preparation by hexaric acid through
(
Epoxy), which severely limited their application. 2,5-
[7]
oxidation dehydration reaction.
The raw materials were easily
furandicarboxylic acid (FDCA) was one of the potential bio-based
platform compounds evaluated by the U.S. Department of Energy.
It is considered as a "sleeping giant" by DuPont and DSM
isomerized and carbonized under acidic conditions which resulting
in low yield; 3) The diglycolic acid was used as the raw material to
react with methanol and thionyl chloride to obtain the diglycolic
acid dimethyl ester, which was then condensed with trimeric
glyoxal dehydrate. But it was expensive and not suitable for large-
[3]
corporations.
The novel polyethylene-2, 5-furandicarboxylate
(
PEF) prepared by polymerization of 2, 5-furandicarboxylic acid
[
8]
scale industrial production. 4) Preparation by furan, 2, 5-dimethyl
furan, 2-methyl-5-acetyl furan, 5-methyl-2-furfural and other furan
based materials through oxidative reaction. Under acidic conditions,
the opening of the furan ring was more likely to cause incomplete
oxidation, and the synthesis of the raw materials was difficult,
[9]
which greatly reduced the selectivity of 2, 5-FDCA. 5) Preparation
by 5-HMF obtained from the hydrolysis of hexoses. HMF could be
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[
22]
further transformed into a wide variety of value-added derivatives.
Stahl et al. reported that alcohols could be oxidized to esters or
[
10]
[23]
acids with high selectivity by using PdBiTe catalysts.
Overall, the
The oxidation of HMF was the most studied reaction process at
selective aerobic oxidative esterification of HMF was still under high
temperature or certain pressure. Therefore, it was still a challenge
to develop a high selectivity catalytic system for oxidizing
present due to an analogous structure with FDCA, namely similar
functional group position.
Traditional pathway for the oxidizing of HMF to FDCA involved esterification of 5-HMF to DMFDCA under mild conditions with both
[
11a]
an equivalent oxidizing agents such as nitric acid,
potassium normal pressure and low temperature.
[11b]
hypermanganate,
etc. However, more waste salt and lower
Herein, a homogeneous catalytic system and a heterogeneous
reaction yield were main problems which were not suitable for catalytic system were designed for the selective oxidative
industrial production. In this direction, homogeneous catalytic esterification of HMF to DMFDCA. The effect of various additives on
[
12a]
[12b]
system such as Co/Mn/Br catalyst
and TEMPO catalysts
the product distribution was investigated initially. Based on these
were developed for the improvement of reaction conditions. studies, the reaction conditions such as catalyst loadings, catalyst
However, problems still remained such as reactor corrosion and proportion, alkali type and alkali amount, etc. were screened
catalyst recovery difficulties. Therefore, numerous supported metal subsequently. In order to realize the recyclability of catalytic
or metal oxide heterogeneous catalytic systems have been system, the heterogeneous catalyst was prepared. Meanwhile, the
[13]
[14]
[15]
[16]
developed (e.g. Au,
Pd,
Ru,
Fe,
etc.) for the conversion characterization of XRD and XPS, etc. were carried out for the
of HMF to FDCA and higher selectivity and yield were obtained. investigation of structure activity relationship of catalysts. The
However, due to the special physical and chemical properties of possible reaction mechanism was also proposed and supported by
1
FDCA, its solubility was poor in water and most organic solvents.
Purification by conventional crystallization and rectification was
difficult, which limited the application scale of FDCA. Furan
H-NMR and infrared spectra (IR) analysis of the reaction solutions.
dicarboxylic acid esters, especially dimethyl furan dicarboxylate Experimental Section
DMFDCA), could be easily separated and purified from reaction
(
due to the lower boiling point and could be used as ideal substitute Materials and reagents
of FDCA. Therefore, it was of great significance to study the
All the chemicals used in the experiments were commercially
selective one-step oxidation of 5-HMF to DMFDCA. The oxidative
esterification of HMF to DMFDCA undergone two reaction
pathways: 1) The aldehyde group was converted to 5-
hydroxymethyl-2-methyl-furoate through oxidative esterification,
then the hydroxyl is oxidized to 5-formyl-2-methyl-furoate; 2) The
hydroxyl group was first oxidized to DFF, and then aldehyde group
was esterified to 5-formyl-2-methyl-furoate. Finally, aldehyde group
was oxidized and esterified to obtain DMFDCA.
available. 5-Hydroxymethylfurfural was sponsored by Hefei Leaf
Biotech Co., Ltd. Pd/C (10 wt %) was purchased from Alfa Aesar
(
China) Chemicals Co., Ltd. ZrO(NO ) , Fe(NO ) .9H O,
3 2 3 3 2
Ni(NO ) .6H O, Ce(NO ) , Se, Co(NO ) .6H O, Pb(NO ) , Bi(NO )
3 3,
3
2
2
3 3
3 2
2
3 2
K CO , NaOAc, NaHCO , Na CO NaOH, KOH, Et N, KH PO , Ethyl
2
3
3
2
3,
3
2
3
benzoate , Conc HCl ,Conc HNO and SOCl were purchased from
3
2
Sinopharm Chemical Reagent Co.,Ltd. t-BuOLi, t-PenONa were
purchased from Meryer (Shanghai) Chemical Technology Co., Ltd .
Catalyst preparation and characterization
Co(NO
mmol) were dissolved in a mixture of 2 mL HCl and 0.2 mL
conc HNO while stirred at room temperature for 5 minutes,
3 2 2 3 3
) .6H O (146 mg , 5 mmol) and Bi(NO ) (198mg ,5
3
then added to a flask which mixed with 32 mL of deionized
water and 530 mg 10 wt % Pd/C . Reaction was kept at 50 °C
for 3 h. After 3 h, 1.2 mL of 30% NaOH solution was added to
adjust the mixture alkaline. Then, 0.4 mL of 37% formaldehyde
Scheme 1. The reaction pathways of HMF under the oxidative
esterification conditions.
In 2008, Christensen et al. realized the catalytic oxidation of 5-HMF
2
was added, and the mixture was heated to 80 °C under Ar for
to DMFDCA in the presence of sodium methoxide at 130 °C and 4
[17]
16 h. After the mixture was cooled to room temperature, the
catalyst was filtered, washed with deionized water for several
atm via Au/TiO
reported the oxidative esterification of 5-HMF with Au/CeO
catalyst at 130 °C and 10 atm O to obtain a 99% yield.
Subsequently, Maela Manzoli developed a catalytic system for
oxidative esterification of furfural and HMF over Au/ZrO catalyst
under 1 atm O that high yields and selectivity were obtained. In
addition to the Au-based catalysts, Fu et al. reported that DMFDCA
could be prepared via CoxOy-N@C catalyst at (100 °C, 10 atm O
2
catalyst with a yield of 98%.
Corma et al.
2
as a
[
18] times until neutral, and dried under vacuum at 70 °C for 48 h
2
to afford heterogeneous catalyst (PdCoBi/C).
Catalysts were characterized by X-ray photoelectron
2
spectroscopy (XPS), X-ray power diffraction (XRD). X-ray
photoelectron spectroscopy (XPS) data were obtained with a
[19]
2
Thermo
Scientific
Escalab
250-X-ray
photoelectron
2
)
spectrometer which equipped
a
hemispherical electron
[20]
with
dimethylcarboxylate (FDMC, 95.6 %) without basic additive was
obtained by using CoOx-N/C and α-MnO
a
96% yield.
In 2016, high yield of furan-2,5-
analyzer and an Al Kα X-ray source. All binding energies were
referenced to C 1s line at 284.7 eV. X-ray power diffraction
catalytic system. (6 atm O2, (XRD) patterns were obtained on X’ pert (PANalytical)
00 °C). Recently, Tong et al. used MnO -Cu catalyst (3 atm O , diffractometer with Ni-filtered Cu-Kα radiation, at 40 kV and
1 13
40 °C), and found that the selectivity of DMFDCA was only 26.5%. 40 mA. 2θ range was 20°~80°. H-NMR and C-NMR spectra
2
[
21]
1
1
2
2
2
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were recorded on a Bruker Avance 400 spectrometer at that the yield of DMFDCA was increased to 42% by using
ambient temperature. NMR spectra were analysed with cobalt nitrate and bismuth nitrate as co-additives (Table 1,
MestReNova software.
Entry 10). With above reaction results, it was found that
bismuth nitrate and cobalt nitrate have a great effect on this
reaction. This might be caused by the formation of geminal
diol intermediate state during the reaction, while bismuth
atom had strong affinity for oxygen. The formation of Bi-O
bond could promote the adsorption of substrate on the
catalyst surface to form geminal diol intermediates, which was
Typical experiment and product analysis
20 mol% K CO , 5 mol% Pd(10 wt% Pd/C), 5 mol% Co(NO ) , 5
2 3 3 2
mol% Bi(NO
equipped with a stir bar. The vessel was evacuated and
agitated under 1 atm O . After the mixture was heated to 60 °C
3 3
) , 2 mL MeOH were added to a Schlenk tube
2
，
0.5 mmol 5-HMF was added to the vessel, then reaction was favorable for the dehydrogenation of geminal diol
kept at 60 °C under 1 atm for 14 h. After reaction, the mixture intermediates to obtain acid by using Pd-based catalysts.
was cooled to room temperature, SOCl was added and the Meanwhile, the formation of complexes adsorbed on the
2
[
24]
mixture was stirred at room temperature for 2 h, 0.5 mmol metal surface could also inhibit the Pd catalyst acidosis.
ethyl benzoate was added as an internal standard. Then the
solution was analyzed with HPLC. The conversion of 5-HMF
and the yield of products were calculated according to the
following formula:
It
was found that the conversion of HMF and the selectivity of
products were still poor, which might be related to the
reaction conditions.
nꢀꢁꢂꢃꢄ − nꢅ
ꢀꢁꢂꢃꢄ
a
Conv/% =
× 100%
× 100%
Table 1. The oxidative esterification of HMF with different catalysts
n
ꢀ
ꢁꢂꢃꢄ
n(
ꢆꢇꢈꢉꢊꢇꢋꢊꢌꢍꢇꢎꢏꢐꢑꢊ)
Y/% =
nꢀꢁꢂꢃꢄ
nꢀ
n
: mol of 5-HMF before reaction
: mol of 5-HMF after reaction
b
ꢁꢂꢃꢄ
Additives
A/B
b
Yield [%]
ꢅ
Entry
Conversion [%]
ꢀ
ꢁꢂꢃꢄ
HMMF FMF DMFDCA
ZrO(NO
/Fe(NO3)3
ZrO(NO /Ni(NO
ZrO(NO
3 2
)
1
2
3
4
5
32
42
42
39
29
18
25
18
10
12
7
6
Typical heterogeneous experiment and product analysis
0 mol% K CO , 5 mol% PdCoBi/C catalyst, and 2 mL MeOH
were added to a Schlenk tube equipped with a stir bar. The
vessel was evacuated and agitated under 1 atm O . After the
mixture was heated to 60 °C 0.5 mmol 5-HMF was added to
the vessel, then reaction was kept at 60 °C under 1 atm O for
2
2
3
3
)
2
)
3 2
7
8
4
3
9
3 2
)
13
22
13
2
/Ce(NO3)2
，
ZrO(NO ) /Se
3
2
2
ZrO(NO ) /
3
2
14 h. After reaction, the mixture was cooled to room
3
Co(NO )
2
temperature, 0.5 mmol ethyl benzoate was added as an
internal standard. Then the solution was analyzed with HPLC.
The conversion of 5-HMF and the yield of products were
calculated according to the following formula:
6
7
3 2
Fe(NO ) /Ni(NO )
28
30
24
42
70
48
42
39
10
18
6
4
2
3
4
3
14
9
3
3
Fe(NO
Fe(NO
Co(NO
Co(NO / Bi(NO
/ Ce(NO
/ Fe(NO
/ Ni(NO
3
)
3
/Ce(NO
/Se
/Se
3 3
)
8
3
)
3
14
23
42
12
16
13
9
3
)
2
13
25
25
17
20
nꢀꢁꢂꢃꢄ − nꢅ
10
11
3
)
2
3 3
)
ꢀꢁꢂꢃꢄ
Conv/% =
× 100%
n
Co(NO
3
)
2
3
)
2
10
6
ꢀ
ꢁꢂꢃꢄ
n(
ꢆꢇꢈꢉꢊꢇꢋꢊꢌꢍꢇꢎꢏꢐꢑꢊ)
1
1
2
3
Co(NO
3
)
2
3 3
)
Y/% =
× 100%
nꢀꢁꢂꢃꢄ
Co(NO
3
)
2
3
)
3
4
nꢀ
n
: mol of 5-HMF before reaction
: mol of 5-HMF after reaction
ꢁꢂꢃꢄ
ꢁꢂꢃꢄ
[
a] Reactions conditions : 0.5 mmol HMF, 2.5 mol% Pd/C catalyst, 2.5
mol% A, 2.5 mol% B, 20 mol% K CO as base, 2 mL MeOH, under 1 atm
, 14 h, 60 C. [b] The yields were determined by HPLC, ethyl
ꢅ
ꢀ
2
3
o
O
2
benzoate was used as internal standard. HMMF: 5-hydroxymethyl-2-
methyl-furoate). FMF: 5-formyl-2-methyl-furoate. DMFDCA: dimethyl
furandicarboxylate.
Results and Discussion
The selective oxidative esterification of HMF to DMFDCA were
carried out in the presence of Pd/C catalyst, additives and
alkali in methanol solvent under atmospheric pressure oxygen
To gain further insight into our reaction, the detailed analysis
of reaction conditions were carried out.
(
Table 1). Additives including transition metals, non-metallic
elements and metal nitrate were screened initially. Firstly, the
additives of zirconyl nitrate with iron nitrate, nickel nitrate,
cerium nitrate, selenium, and cobalt nitrate were carried out,
Effect of catalyst ratio
The effect of catalyst ratio on product distribution was
investigated and the results were shown in Figure 1. Without
the addition of Pd/C catalysts, no reaction was occurred. It has
been reported that Pd/C catalyst could catalyse the conversion
of alcohol to carboxylic acid, but the reaction needed an
but they did not give satisfactory results (Table 1, Entries 1-5)
.
Then the combination of iron nitrate with nickel nitrate,
cerium nitrate and selenium were tested, and only small
amounts of oxidative esterification products were obtained
(Table 1, Entries 6-8). Then cobalt nitrate was combined with
[25]
equivalent amount of base. The yield of DMFDCA was only
selenium or metal salts (Table 1, Entries 9-13). It was found
3
0%, and the conversion of HMF was 65% with Pd/C catalyst
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alone. It showed that Pd/C could catalyse the reaction but the Effect of alkaline species
yield was lower. There was no oxidative esterification products
obtained by using a single Co(NO
and Bi(NO co-additives. The conversion of substrate was oxidative esterification of alcohols. The effect of bases on the
significantly improved to 94% and 92% by using Pd/C+ Bi(NO product distribution was investigated and the results were
or Pd/C+ Co(NO catalyst system, respectively. This showed showed in the Figure 2. The weak bases such as sodium
that the presence of Co or Bi both promoted the oxidative acetate, sodium bicarbonate and sodium carbonate were
3
)
2
or Bi(NO
3
)
3
or Co(NO
3
)
2
The alkali strength and alkalinity have important effect on
3 3
)
3 3
)
3 2
)
[26]
esterification reaction.
investigated. Only 21% conversion of HMF and 7% yield of the
oxidative esterification products were obtained by using
sodium acetate as alkali. The yield of DMFDCA increased to
3
2% by using sodium bicarbonate as alkali. It showed that the
rate of oxidation increased with the increase of alkalinity. This
was mainly caused by the presence of alkali, which could
promote in the hydration of the aldehyde. The hydration of
aldehyde group to geminal diol group was lowered with lower
[24a]
reaction pH.
As a result, the strong base potassium
carbonate was used, and the yield of DMFDCA was significantly
increased to 93%. The use of potassium hydroxide only
generated small amount of DMFDCA. This might be due to the
gradual degradation of HMF (Cannizzaro reaction) under high
[28]
pH.
The better oxidative esterification yield was obtained
by using the stronger organic bases (t-BuOLi, t-PenONa). The
weak organic base such as triethylamine, almost could not
promote the reaction. Finally, the weakly acidic potassium
dihydrogen phosphate (KH PO ) was further investigated and
2
4
Figure 1. Reactions conditions: 0.5 mmol HMF, 20 mol% K
Pd/C as catalyst, Co(NO and Bi(NO as additives, 2 mL MeOH, 1 atm O
4 h, 60 C. The yields was determined by HPLC, ethyl benzoate was used as
2 3
CO as base, 5%
no product was observed. Therefore, the suitable strength of
alkali was beneficial for the oxidative esterification of 5-HMF.
)
3 2
)
3 3
2
,
o
1
internal standard. HMMF: 5-hydroxymethyl-2-methyl-furoate. FMF: 5-
formyl-2-methyl-furoate. DMFDCA: dimethyl furandicarboxylate.
It was found that HMMF obtained by using Pd/C+ Bi(NO
3 3
) co-
catalysts was lower than that obtained by using Pd/C+ Co(NO ) co-
3
2
catalysts, and the yield of DMFDCA was higher than those of Pd/C+
Co(NO co-catalysts. This indicated that the presence of Bi was not
only conducive to the conversion of substrate, but also the
3 2
)
[27]
promotion of oxidative esterification.
3 3
The use of Bi(NO ) alone
had no effect on the reaction, but it could greatly increase the
catalytic activity and product selectivity combined with Pd/C+
Co(NO
selectivity of products were still poor by using Pd/C+ Bi(NO
Co(NO ) co-catalysts and the amount of catalysts were 2.5 mol%. Figure 2. Reactions conditions : 0.5 mmol HMF, 5 mol% Pd/C catalyst, 5
3 2
) . Comparing with the experimental results in Table 1, the
3 3
) +
3
2
The yield of DMFDCA increased greatly with a yield of 93% and no mol% Co(NO
3 2 3 3 2
) , 5 mol% Bi(NO ) , 20 mol% base, 2 mL MeOH, 1 atm O , 14 h,
o
FMF was observed while increased the amount of catalysts to 5 60 C. The product yields were determined by HPLC, ethyl benzoate was
used as internal standard. HMMF: 5-hydroxymethyl-2-methyl-furoate. FMF:
mol%. On this basis, the catalyst ratio was further investigated
5
-formyl-2-methyl-furoate. DMFDCA: dimethyl furandicarboxylate.
(
Figure 1).The yield of DMFDCA were 62% and 89% as the ratio of
Pd: Co: Bi was 5:5:2.5 and 5:5:7.5, respectively. The yield of
DMFDCA was 63% and 87% as the ratio of Pd: Co: Bi was 5:2.5:5 and
Effect of alkali amounts
5
3 2 3 3
:7.5:5, respectively. The decrease amount of Co(NO ) or Bi(NO )
The effect of alkali amounts on product distribution was
investigated and the results were listed in Figure 3. No
conversion of HMF was observed in the absence of base,
which indicated that alkali played an important role in the
oxidative esterification reaction. The conversion of HMF was
occurred (25% of conversion) as the alkali amount was
increased to 10 mol%. Meanwhile, the yield of oxidative
esterification product DMFDCA were 15%, and the yield of
was not conducive to the increased yield of DMFDCA. The yield of
DMFDCA extremely decreased to 53% as the proportion of Pd
decreased to 2.5% (Pd: Co: Bi = 2.5:5:5). The results showed that
increasing the amount of a single additive could promote the
reaction. The yield of DMFDCA was similar as the ratio of Pd: Co: Bi
increased from 5:5:5 to 7.5:7.5:7.5, and the optimal ratio of
catalysts was 5: 5: 5.
4
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intermediate HMMF and FMF was 5% severally. It showed that that the intermediate FMF could hardly been detected. It also
showed that the oxidative esterification of aldehyde groups was a
fast step. The lower yield of MFDCA also showed that the aldehyde
group was not easily oxidized in the catalytic system to obtain a
carboxyl product. Along with the extension of reaction time, the
conversion of 5-HMF was completed, the yield of intermediate
HMMF gradually decreased and the yield of DMFDCA increased
gradually. The highest yield of oxidative esterification product
reached to 96% after 16 h.
the alkali promoted the oxidative reaction, but the rate of
reaction was slower due to the small amount of alkali.
Increasing the alkali amount to 20 mol%, HMF was completely
converted and the main product was DMFDCA.
Figure 3. Reactions conditions : 0.5 mmol HMF, 5 mol% Pd/C catalyst, 5
mol% Co(NO
3
)
2
, 5 mol% Bi(NO
C. The product yields were determined by HPLC, ethyl benzoate was
used as internal standard. HMMF: 5-hydroxymethyl-2-methyl-furoate. FMF:
-formyl-2-methyl-furoate. DMFDCA: dimethyl furandicarboxylate.
3 3 2 3 2
) , K CO as base, 2 mL MeOH, 1 atm O , 14 h,
o
6
0
Figure 4. Reactions conditions : 0.5 mmol HMF, 5 mol% Pd/C catalyst, 5
5
mol% Co(NO
3
)
2
,5 mol% Bi(NO
3 3 2 3 2
) , 20 mol% K CO , 2 mL MeOH, 1 atm O , 60
o
The content of FMF in the mixture was always low. This might
C. The yields were determined by HPLC, ethyl benzoate was used as
mainly due to the fast conversion rate of aldehyde group in the internal standard. HMMF: 5-hydroxymethyl-2-methyl-furoate. FMF: 5-
reaction system, which led to the rapid conversion of the formyl-2-methyl-furoate. DMFDCA: dimethyl furandicarboxylate.
intermediate FMF into DMFDCA. The conversion of HMF was
complete, but the yield of the oxidative esterification products
Effect of heterogeneous PdCoBi/C catalysts
decreased obviously as the amount of alkali was increased to 50
mol%. It might be that the aldehyde groups belonged to HMF and
intermediate FMF were disproportionated with increasing the alkali
Table 2. The optimization of reaction conditions by using heterogeneous
a
PdCoBi/C catalysts
amount, which led to the reduced selectivity and yield of DMFDCA.
b
[28]
Pd: Co:
Bi
Yield [%]
b
The yield of oxidative esterification product DMFDCA almost
Entry
Conversion [%]
HMMF FMF DMFDCA
unchanged with the alkali amounts increased to 100 mol%. The
yield of DMFDCA increased firstly and then decreased with the
alkali amounts increased from 10 mol% to 100 mol%. Increasing the
alkali amount not only promoted the oxidative esterification
reaction of hydroxyl and aldehyde groups, but also promoted the
saponification reaction. Therefore, a suitable alkali amount was
beneficial to obtain the higher selectivity of DMFDCA, and an alkali
amount of 20 mol% was suitable herein.
1
1:1:0.5
1:0.5:1
0.5:1:1
1:1:1
91
95
94
99
28
19
15
3
7
3
55
71
68
96
2
3
4
5
6
7
10
0
1:1:1.5
1:1.5:1
97
97
98
88
90
97
5
6
2
3
5
1
5
1
89
86
82
86
20
76
1
.5:1:1
9
c
8
1:1:1
1:1:1
1:1:1
1
Effect of reaction time
d
9
62
19
e
1
0
The effect of reaction time on product distribution was studied to
better analysis the reaction pathway. (Figure 4) The conversion of
HMF was about 80% and the generation of HMMF and DMFDCA
were observed after 1 h. Among them, the product HMMF, which
[
a] Reactions conditions: 0.5mmol HMF, 20 mol% K
2
CO
3
, 10 mol%
, 14 h, 60 C. [b]The yields
were determined by HPLC, ethyl benzoate was used as internal
o
PdCoBi/C catalyst, 2 mL MeOH, 1 atm O
2
obtained from the esterification of aldehyde group on HMF, was standard. [c]. 5 mol% PdCoBi/C catalyst. [d]. no base was used. [e] Air
the major product with a yield of 53%. The yield of DMFDCA was was used instead of O . HMMF: 5-hydroxymethyl-2-methyl-furoate.
4%, while the yield of FMF was much lower. This indicated that the FMF:5-formyl-2-methyl-furoate.DMFDCA:
dimethyl
2
2
aldehyde group of HMF was easier to oxidize and esterify at the
initial stage of reaction. This was probably due to the fact that the
hemiacetal was easily formed from the aldehyde group in the
furandicarboxylate.
In order to better study the interaction of the various
presence of methanol. It have been reported that the hemiacetal at components in catalyst, the heterogeneous catalysts were
[
20c]
the benzyl group was more easily oxidized than that of aldehyde prepared according to the literature
and the effect of
[29]
group to obtain an esterified product. This might be the reason catalyst ratio on product distribution was investigated and the
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results were listed in Table 2. The effect of catalyst ratios had a and used directly for the next cycle. The effect of catalyst circulation
great effect on product distribution according to the above on product distribution was listed in Figure 5. The conversion of raw
results. Similar results were found by using heterogeneous materials and the yield of oxidative esterification product gradually
catalysts. The highest yield of DMFDCA was 96% as the ratio of decreased with the increased catalyst recycling times. However,
Pd: Co: Bi=1:1:1 (Table 2, Entry 4). The yield of DMFDCA only the yield of product HMMF was gradually increased. This might
decreased to 55% with the ratio of Bi decreased from 1 to 0.5 be caused by the loss of the catalyst during the treatment process.
(Table 2, Entry1), the yield of DMFDCA decreased from 96% to After the catalyst was recycled four times, the amount of recycled
89% as the ratio of Bi increased from 1 to 1.5 (Table 2, Entry catalyst was weighed and fresh heterogeneous catalyst was added
5)
. It showed that the ratio of Bi had a great effect on product to 10 mol%. A good catalytic activity and 91% yield of DMFDCA
distribution. Similarly, the ratio of Pd and Co also affected the were obtained. Characterization with XRD and XPS analyses of both
product yield. The yield of DMFDCA decreased to 71% and 86% fresh and used catalysts were conducted. (Figure S1- Figure S4) No
with the ratio of Co decreased to 0.5 and increased to 1.5, significant changes of the fresh and used catalysts were found
respectively (Table 2, Entries 2 and 6). The yield of DMFDCA according to the analysis of the XRD spectra and XPS spectra. The
decreased to 68% and 82% with the ratio of Pd decreased to catalyst exhibited a better stability.
0
.5 and increased to 1.5, respectively (Table 2, Entries 3 and
Scope of substrates
7)
. It was shown that increasing the proportion of one metal
could not obtain the best reaction effect. The optimal ratio of
Pd: Co: Bi was 1:1:1.
Oxidative esterification of different alcohols in the presence of
catalysts were also investigated. It was found that good yields could
The yield of DMFDCA were 86% and 96% with 5 mol% and be obtained with aryl substrates. Substrates containing electron-
10 mol% catalyst loading, respectively (Table 2, Entries 8 and donating groups (e.g., methoxy, methyl) could be converted to the
4). Meanwhile, the effect of alkali on product distribution was desired products smoothly with good to excellent isolated yields,
further investigated in the heterogeneous system. Under the while the yield of substrates with electron-withdrawing groups (e.g.,
homogeneous system, no oxidative esterification products nitro) were slightly decreased. With p-chlorobenzyl alcohol as
were formed without alkali (Figure 3). But under the condition substrate, the oxidative esterification product was obtained with a
of heterogeneous catalysts, 20% yield of DMFDCA was much lower 70% yield, which might be caused by the
obtained without the addition of alkali (Table 2, Entry 9). This dehalogenation reaction in the presence of a strong base. High yield
showed that the interaction between the three metals was was obtained with furfuryl alcohol as a substrate, indicated that this
conducive to the occurrence of oxidative esterification catalytic system has an excellent compatibility with furan
reaction. 76% yield of DMFDCA was obtained by replacing derivatives. Regarding to the fatty alcohols, such as octanol,
oxygen with air (Table 2, Entry 10)
.
moderate yield was obtained.
Table 3. The scope of substrates
Catalyst recirculation
PdBiCo/C (10 mol%)
O
R
OH
R
O
2 3
20 mol % K CO , 60 oC
1
atm O
2
, MeOH,14 h
O
O
O
O
O
O
O
O
O
Cl
89%
97%
70%
88%
O
O
O
O
O
O
O
2
N
O
80%
96%
65%
2 3
Reactions conditions: 0.5 mmol HMF, 20 mol% K CO , 10 mol% PdCoBi/C
o
(
2
1:1:1) catalyst, 2 mL MeOH, 1atm O , 14 h, 60 C.
Figure 5
PdCoBi /C (1:1:1) catalyst, 2 mL MeOH, 1atm O
determined by HPLC, ethyl benzoate was used as internal standard. Number
: Fresh heterogeneous catalyst was added to 10 mol% after recycling 4
.
Reactions conditions: 0.5 mmol HMF, 20 mol% K
2
CO
3
, 10 mol%
o
2
, 14 h, 60 C. The yields were
Reaction mechanism
5
A proposed reaction mechanism was carried out according to
the previous study of the reaction process (Scheme 2). The
conversion of HMF might undergo three reaction pathways in
the presence of a catalyst, methanol and oxygen: a) the
times. HMMF: 5-hydroxymethyl-2-methyl-furoate. FMF: 5-formyl-2-methyl-
furoate. DMFDCA: dimethyl furandicarboxylate.
The recycling experiment was carried out subsequently.
hemiacetal intermediate
E
was obtained through hemiacetal
b) the etherified
(
Figure 5) The experimental procedure was carried out as follows:
reaction of the aldehyde group on HMF
；
after the reaction, the solution was centrifuged to separate the
catalyst, the catalyst was washed with methanol twice, centrifuged
intermediate
F
obtained through etherification of the hydroxyl
6
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ARTICLE
group on HMF
;
c) the product DFF obtained from the oxidation The efficient conversion of HMF to oxidative esterification
product DMFDCA was carried out with using homogeneous
of hydroxyl group on HMF. No production of intermediates
F
and DFF were observed through monitoring of the products. and heterogeneous PdCoBi/C catalysts under atmospheric
This was mainly due to the etherification reaction was difficult oxygen. The highest yield of DMFDCA was 93% with 10% Pd/C
to occur under alkaline conditions.
as catalyst, Co(NO
as base, 1 atm O
was 96% with 10 mol% PdCoBi/C (1:1:1) catalyst, 20 mol%
3
)
2
and Bi(NO
3
)
3
as additives, 20 mol% K
2 3
CO
o
2
, 14 h, 60 C. The highest yield of DMFDCA
o
2 3 2
K CO as base, 1 atm O , 14 h, 60 C. The catalytic system had
good compatibility according to the expansion of substrate. It
was found that the reduced reaction activity in the circulation
was caused by the loss during the treatment according to the
XRD spectra and XPS spectra characterization. The oxidation of
hydroxyl group to aldehyde group was the rate-limiting step of
reaction according to the study of reaction mechanism.
Scheme 2. Possible reaction pathways.
Conflicts of interest
The reason for the main obtained product intermediate
E was
that acetal and hemiacetal reactions of aldehyde group were
easier to carry out in alcoholic solvents. This was also the
reason that no DFF product could be detected. The
intermediate E was converted to intermediate HMMF more
easily due to the active hemiacetal structure. With the
occurrence of oxidation reaction, the hydroxyl group in
There are no conflicts of interest to declare.
Acknowledgements
intermediate HMMF undergone further oxidation reaction to This work was supported by the National Natural Science
Foundation of China (21732006 and 21572212), Ministry of
obtain the aldehyde product FMF. The intermediate FMF was
Science and Technology of China (2017YFA0303502), the
Fundamental Research Funds for the Central Universities
WK3530000001), Strategic Priority Research Program of the
CAS (XDA 21060101). The authors thank the Hefei Leaf Co., Ltd
and Anhui Kemi Machinery Technology Co., Ltd for free
samples and equipment that benefited our ability to conduct
this study.
mostly converted to the oxidative esterification product
under the catalytic system, and only small amount of the
aldehyde-based oxidized product was formed. This showed
D
(
C
that intermediate FMF was mainly converted through the
hemiacetalization and reoxidation of aldehyde groups, rather
than the direct oxidation process. During the reaction process,
the oxidation of hydroxyl group to aldehyde group was the
1
rate-limiting step. To further study the mechanism, the H-
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Conclusions
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