A cobalt catalyst for reductive etherification of 5-hydroxymethyl-furfural to 2,5-bis(methoxymethyl)furan under mild conditions

Article abstract of DOI:10.1039/c7gc03072j

Conversion of platform molecule 5-hydroxymethylfurfural (HMF) into high-value-added derivatives has attracted significant interest. In this paper, a metallic cobalt catalyst was prepared by the simple reduction of commercially available Co₃O₄, and used to catalyze the reductive etherification of HMF to 2,5-bis(methoxymethyl)furan (BMMF) under mild conditions. A yield of 93% 2,5-bis(hydroxymethyl)furan (BHMF) was obtained (90 °C, 2 MPa H₂, 1 h) and 98.5% yield of BMMF was achieved (140 °C, 2 MPa H₂, 1 h) by using a Co-400 catalyst. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), ammonia-temperature-programmed-desorption (NH₃-TPD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). It was found that the Co⁰ and Co^2+/3+ species coexist on the surface of the catalyst and the catalyst became more porous and rougher after reduction at high temperature. This may be more beneficial for enhancing the selectivity of the etherification product and the mass transfer of reaction species. A possible reaction mechanism was proposed based on GC and ¹H-NMR analyses. The cobalt catalyst was reused five times with a slight decrease in activity.

Full text of DOI:10.1039/c7gc03072j

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Cobalt catalyst for reductive etherification of 5-hydroxymethyl-
furfural to 2,5-bis(methoxymethyl)furan under mild conditions
Received 00th January 20xx,
Accepted 00th January 20xx
a
a, b
a, b
a
a
c
a
Xing-Long Li, Kun Zhang, Shi-Yan Chen, Chuang Li, Feng Li, Hua-Jian Xu, * and Yao Fu *
DOI: 10.1039/x0xx00000x
Conversion of platform molecule 5-hydroxymethylfurfural (HMF) into high-value-added derivatives has attracted
significant interest. In this paper, metallic cobalt catalyst was prepared by simple reduction of commercially available
www.rsc.org/
Co
conditions. A yield of 93% 2,5-bis(hydroxymethyl)furan (BHMF) was obtained (90 °C, 2 MPa H
BMMF was achieved (140 °C, 2 MPa H , 1 h) by using Co-400 catalyst. The catalysts were characterized by X-ray
photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), and ammonia-temperature-programmed-desorption
NH -TPD), Scanning electron micrographs (SEM), Transmission Electron Microscopy (TEM) and Inductively Coupled Plasma
Atomic Emission Spectroscopy (ICP-AES). It was found that the Co and Co
3
O
4
, and used to catalyze the reductive etherification of HMF to 2,5-bis(methoxymethyl)furan (BMMF) under mild
2
, 1 h) and 98.5% yield of
2
(
3
0
2+/3+
species were coexisted on the surface of
catalyst and the catalyst became more porous and rougher after reduction at high temperature. This may be more
beneficial for enhancing the selectivity of etherification product and the mass transfer of reaction species. A possible
1
reaction mechanism was proposed based on GC and H-NMR analyses. The cobalt catalyst was reused five times with a
slight
decrease
in
activity.
as a diesel additive in a six-cylinder heavy duty engine and no
significant difference in engine operation was found for any of the
Introduction
1
1
12a
tested blending ratios.
alkoxymethyl)tetrahydrofuran,
2-(Alkoxymethyl)furan,
2-
Severe resource crises and environmental problems have been
created along with the depletion of fossil fuels. In this context,
researchers have focused more attention on the conversion of
renewable resources into valuable products. Given that biomass
resources constitute the only renewable carbon source, it is of great
significance to synthesize high-value-added fuels and chemicals from
12b
(
1
and bis(tetrahydrofurfuryl)-ether,
2c
etc. are also potential biodiesel candidates.
O
HOOC
COOH
O
HOOC
COOH
HO
OH
FDCA
1
biomass resources.
cy
nation
BHMF
O
PTA
cl
iz
oxidation
at
io
oge
5
-Hydroxymethylfurfural (5-HMF), obtained from the hydrolysis
n
ydr
h
O
OH
reductive
etherification
of lignocellulose, is recognized as an important potential platform
molecule that can be converted into a range of high-value-added
O
N
amination
tion
MeO
OMe
R
HO
O
This work
AHHMP
BMMF
HMF
e
2
3
th
za
ri
erification
chemicals through the application of etherification, oxidation,
ome
4
5
6
7
is
condensation
hydrogenation, amination, isomerization, condensation, and
O
O
OH
OR
8
9
O
O
O
cyclization reactions, etc. (Scheme 1). Furfuryl alkyl ethers and
their derivatives are important chemical raw materials that can be
used as pharmaceuticals, pesticides, spices, food additives,
surfactants, fuel additives, paint remover, and rubber modifiers, etc.
HCPN
OH
5-AMF
HO
O
HO
DHMFO
1
0
Scheme 1. The conversion of HMF into high-value-added products through
different reaction pathways.
For instance, 2,5-bis(ethoxymethyl)furan (BEMF) has been assessed
Considering the multiple applications and high value of
etherified products, it is of great interest to investigate the
conversion of biomass platform molecules into furfuryl alkyl ether. To
date, the etherification of HMF has mostly resulted in either the
1
3
monoether derivative or dimers.
Approaches used for the
preparation of furfuryl alkyl ethers can be classified as follows. 1)
Halogenation of furan alcohols followed by etherification with
aliphatic alcohols; however, the use of equivalent halogenated agent
1
4a
was poor in terms of atom economy.
2) The use of 5-
H-SBA-15 as the
chloromethylfurfural as an intermediate and Ar-SO
3
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catalyst to obtain EMF. The obtained ethyl levulinate, formed in the
etherification reaction, was attributed to the presence of strong
Experimental Section
14b
Brønsted acid.
3) The most common method is the direct
Materials and reagents
etherification of furan alcohols and monohydric alcohols in the
presence of acid catalyst. For instance, acidic molecular sieve ZSM-5
Co
3 4 3 4
O , Fe O , CuO, Raney Ni, methanol (AR), N,N-dimethylformamide
(
AR), hydrochloric acid (AR), sulfuric acid (AR), sodium hydroxide (AR),
(
Si/Al=25) was used as catalyst to catalyze the etherification of
sodium bicarbonate (AR), sodium borohydride (AR), dichloromethane
(AR), isopropyl alcohol (AR) were purchased from Sinopharm
Chemical Reagent Co. Ltd. 5-hydroxymethylfurfural was supplied by
Hefei Leaf Biotech Co., Ltd (www.leafresource.com). Pure water was
purchased from Wahaha, Hangzhou. The preparation methods of the
possible intermediates and products were listed in Supporting
Information.
furfuryl alcohol and ethanol. However, the selectivity of furfuryl ethyl
1
2b
ether was only 44.8%.
The synthesis of 2, 5-bis (alkoxymethyl) furan required
hydrogenation of HMF to BHMF first, followed by etherification
mediated by acidic catalyst to obtain 2, 5-bis (alkoxymethyl) furan.
The reductive etherification has been catalyzed by transition-metal
1
5
16
17
catalysts,
organocatalyst. To date, the reductive etherification of HMF has Catalyst preparation and characterization
proceeded in low yield, mainly because of competing reactions such Co O was reduced by increasing the temperature from 30 C to
Lewis acids,
Brønsted acids,
and thiourea
1
8
°
3
4
°
as the formation of levulinic acid, or the reductive homocoupling of 400 C and the gas composition was H : N (10:90, 100 mL/min),
2
2
1
9
°
°
the aldehyde component as well as the formation of humin.
Balakrishnan et al. studied reductive etherification of HMF to 2, 5-bis the hydrogen and dropped to room temperature under
alkoxymethyl) furan by using a PtSn/Al and Amberlyst-15 co- nitrogen, then took out the catalyst, bottled it and marked as
at the rate of 2 C/min. After holding for 2 h at 400 C, turned off
(
2 3
O
2
0
catalyst system under hydrogen pressure in one pot. They obtained Co-400 catalyst. Other Co catalysts with different reduction
the corresponding 2, 5-bis (alkoxymethyl) furan in yields of 64% and temperature were reduced and labelled with similar methods.
4
7% by using ethanol and n-butanol, respectively. Gruter et al. Fe O was reduced at atmospheric pressure by increasing the
3 4
°
°
investigated the continuous hydrogenation and etherification of HMF temperature from 30 C to 500 C and the gas composition was
to 2, 5-bis (ethoxymethyl) furan. HMF was hydrogenated to 2, 5-bis H : N (10:90, 100 mL/min), at the rate of 2 C/min. After holding
°
2
2
°
(
hydroxymethyl) furan (BHMF) over Pt/C catalyst first and then for 2 h at 500 C, turned off the hydrogen and dropped to room
etherified to 2, 5-bis (ethoxymethyl) furan at 348 K without hydrogen. temperature under nitrogen, then took out the catalyst and
2
1
The yield of 2, 5-bis (ethoxymethyl) furan reached 75%. Mu et al. bottled it. CuO was reduced at atmospheric pressure by
°
°
applied a two-step method to obtain 70% yield of BMMF from HMF increasing the temperature from 30 C to 240 C and the gas
by using a Cu/SiO
2
2
°
2
and HZSM-5 co-catalyst system.
Solid acid composition was H : N (10: 90, 100 mL/min), at the rate of 2 C
2 2
°
catalysts, such as Sn-Beta and Zr-Beta, were used for the reductive /min. After holding for 2 h at 240 C, turned off the hydrogen
etherification of HMF to BMMF through the Meerwein–Ponndorf– and dropped to room temperature under nitrogen condition,
Verley reaction (MPV reaction). The authors found that appropriate then took out the catalyst and bottled it.
substrate concentration and sufficient numbers of acid–base sites
Catalysts were characterized by X-ray phot-oelectron
were both important to obtain high yields of the etherified products. spectroscopy (XPS), X-ray diffraction (XRD), Ammonia-
2
3
Nowadays, the conversion of HMF into BMMF is generally realized Temperature-Programmed-Desorption (NH -TPD), Scanning
3
by using a catalyst system containing hydrogenation catalyst and acid electron micrographs (SEM), Transmission Electron Microscopy
catalyst. However, it remains challenging to obtain high yields of (TEM) and Inductively Coupled Plasma Atomic Emission
reductive etherification products under mild conditions using a Spectroscopy (ICP-AES). X-ray power diffraction (XRD) patterns
bifunctional catalyst containing both hydrogenation activity and of catalysts were obtained on X’ pert (PANalytical)
etherification activity.
diffractometer with Ni-filtered Cu-Kα radiation, at 40 kV and 40
~80 . X-ray photoelectron spectra (XPS)
Cobalt-based catalysts have many important applications, and mA. 2θ range was 20
°
°
they can catalyze oxidation, hydrogenation, and isomerization data were obtained with a Thermo Scientific Escalab 250-X-ray
reactions, etc. under mild conditions. Cobalt-based catalysts retain photoelectron spectrometer which equipped a hemispherical
both hydrogenation activity and Lewis acid acidity even after electron analyzer and an Al Kα X-ray source. All binding energies
0
2+/3+
24
reduction due to the coexistence of Co and Co
species. Indeed, were referenced to C1s line at 284.7eV. Ammonia-Temperature-
in our previous work, we also found that the reduced cobalt-based Programmed-Desorption (NH -TPD) tests: the catalysts were
3
0
2+/3+
25
-
°
catalyst contains both Co and Co
species.
treated at 500 C under helium flow (ultrahigh purity, 20 mL·min
1
) for 2 h, and the adsorption of ammonia was carried out at 40
convert 5-HMF into BMMF by using a Co catalyst. The Co catalyst was C for 1 h. After that, the catalysts were flushed with helium at
In this work, we wanted to develop an effective approach to
°
°
obtained by simple reduction of commercially available Co
3
O
4
.
40 C for 1 h, and the programmed-desorption of NH was run
3
Parameters such as reaction temperature, reaction time, reaction from 40 to 800 C at a heating rate of 10 C/min. The desorbed
°
°
pressure, and substrate concentration, etc. were investigated in ammonia was measured by a gas chromatograph (GC-SP6890,
1
detail. H-NMR analysis and reaction dynamic studies were also Shandong Lunan Ruihong Chemical Instrument Co. Ltd,
conducted to help clarify the reaction mechanism, and a possible Tengzhou China) with a thermal conductivity detector (TCD).
reaction pathway is proposed. The recycling and reuse of the Co Transmission Electron Microscopy (TEM) microphotographs
catalyst are also discussed. The preparation of BMMF from fructose were operated on JEOL-2010 electron microscope at 200 kV. The
in two-steps is also studied.
samples were suspended in ethanol. The loss of Co in the
2
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reaction solutions was measured by Inductively Coupled Plasma subsequent loss of activity. Selective conversion of HMF into
Atomic Emission Spectroscopy (ICP-AES, Thermo-Jarrell ASH- BHMF was achieved by using both metallic Cu catalyst, Raney Ni,
Atom Scan Advantage). ICP-AES tests: after reaction, the Raney Co catalysts (Table 1, Entries 1, 4 and 5). However,
reaction solution was centrifuged and evaporated to dryness etherified products were not obtained. Metallic Co catalyst (Co-
under reduced pressure. The residue were dissolved with 400) not only catalyzed the hydrogenation of HMF, but also
concentrated nitric acid and diluted with pure water. Scanning promoted the etherification reaction, and mixtures containing
electron micrographs (SEM) of Co catalysts were taken using a significant amounts of hydrogenated and etherified product
scanning electron microscope (SEM, Sirion 200, FEI Electron were obtained (Table 1, Entry 3). The unobserved formation of
Optics Company, USA).
hydrolysis product methyl levulinate indicated that the catalyst
H-NMR and C-NMR spectra were recorded on a Bruker had moderate Lewis acidity. Weaker Lewis acidity was observed
Avance 400 spectrometer at ambient temperature. NMR spectra according to NH -TPD analysis (Figure S1). It is known that
1
13
3
were analysed with MestReNova software.
Brønsted acids and strong acid sites are more conducive to the
production of alkyl levulinates, whereas the production of
furanic ethers seems to be less demanding in terms of catalyst
Typical experiment and product analysis
2
6
acidity. Gorte et al. also reported that Sn-Beta catalyst, which
only contains Lewis acid sites, was the most active catalyst for
transfer hydrogenation of HMF and formation of etherified
If not stated otherwise, the reaction was carried out in an
autoclave sponsored by Anhui Kemi Machinery Technology Co.,
°
Ltd. Usually, the initial temperature was 30 C, then heating to
27
product. The activity of several metal oxides was low, and no
the required reaction temperature through intelligent
temperature controller in 0.5 h. After that the reaction was
heating preservation with a certain period of time.
target products were obtained (Table 1, Entries 6–8). Lower
conversion of HMF was obtained without catalyst, which
confirmed that the catalyst had an important effect on product
distribution (Table 1, Entry 9). Meanwhile, we found that the
The typical experimental procedure was listed as follows:
The conversion of HMF was carried out in a 25 mL parr
autoclave equipped with a magnetic stirrer, a temperature
probe and a programmed temperature controller. The typical
experiment was as follows: added 126 mg of substrate, 20 mg of
catalyst, and 10 mL of methanol to the parr autoclave. Then
tightened the screws and replaced the air with hydrogen four
times. Filling hydrogen to the desired pressure at room
temperature, Stirring and heating to the desired temperature
under 500 rpm/min stirring. After maintaining the reaction for a
2 4
addition of acid (eg. H SO ) had a weak inhibitory effect on the
reaction, which might be due to an increased side reactions such
as hydrolysis and oligomerization with an increased acidity
2 3
(Table S1, Entry 1). The addition of base (eg. NaOH & Na CO )
was not conducive to the production of BMMF, and the main
product was hydrogenation product BHMF. The presence of
bases inhibited the catalysis of Lewis acid, but did not inhibit the
hydrogenation reaction (Table S1, Entry 1, Entry 2).
a
certain period of time, decreased temperature and released Table 1. Reductive etherification of HMF with different catalysts.
pressure. The solution was transferred out with 20 mL of
methanol, and a certain amount of N, N-dimethylformamide
(
DMF) was added as an internal standard. After centrifugation,
the supernatant was analyzed by the Gas Chromatography
SHIMADZU, GC-2014C) equipped with the DM-Wax capillary
(
Yield /%
Entry
Catalyst
Conv. /%
column (30 m × 0.32 mm × 0.25 m). The conversion of HMF and
the yield of products were calculated as follows:
BHMF MMFA
BMMF
n.d
1
2
3
4
5
6
7
8
9
Metallic Cu
Metallic Fe
Co-400
100.0
4.0
87.0
2.0
n.d
n.d
28.4
n.d
n.d
n.d
n.d
n.d
n.d
n.d
100.0
100.0
100.0
3.0
8.4
62.3
n.d
Raney Ni
Raney Co
97.0
94.5
2.0
n.d
b
CuO
n.d
b
Fe
3
O
4
1.0
n.d
n.d
b
Co
3
O
4
3.0
n.d
n.d
-
1.0
n.d
n.d
[
a] Reaction conditions: 126 mg HMF, 20 mg catalyst, 10 mL
methanol, 120 C, 1 h, 2 MPa H , GC mole yield. [b] Commercial
oxides were used directly without reduction. BHMF: 2,5-
bis(hydroxymethyl)furan. MMFA: 5-methoxymethyl-furfuryl
°
2
Results and Discussion
The effects of metallic metals and metal oxide catalysts on alcohol. BMMF: 2,5-bis(methoxymethyl)furan. n.d: not detected.
Me: methyl.
product distribution were examined initially; the results are
summarized in Table 1.
Exploratory catalyst screening showed that metallic Fe
catalyst did not catalyze the reaction compared with other
metallic metals (Table 1, Entries 1–5). This might be because the
metallic Fe catalyst is more susceptible to oxidation and
To gain further insight into our reaction, we studied the reaction
conditions in more detail.
Effect of reaction temperature
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The reaction temperature had a significant effect on product
The preparation and characterization of Co catalysts with
distribution during the reaction, especially for the etherification different reduction temperature were also carried out and
process. We initially investigated the effect of different reaction investigated. The effect of catalytic reduction temperature on
temperatures on the product distribution (Figure 1). BHMF production distribution was carried out and the results was
could be obtained almost quantitatively (93% yield) from HMF listed in Figure S2. Figure S3 were the XPS spectra of Co catalysts
(
94% conversion) at a lower temperature (90
°C). It was shown with different reduction temperature. And the surface
that the Co-400 catalyst had a high hydrogenation activity at composition of Co catalysts obtained from XPS analysis were
2
8
lower temperature. In our previous work, we also reported listed in Table S5. The conversion of HMF and the yield of
that the Co catalyst was more selective in hydrogenation of etherified products were too low by using Co-200 catalyst. Along
2
5
carbonyl groups. At this point, no 5-methoxymethyl-furfuryl with the increased reduction temperature, the yield of
alcohol (MMFA) or 5-methoxymethyl-furfural (MFFA) was etherified products (MMFA, BMMF) increased initially and then
0
observed, which indicated that the catalytic system did not decreased. From the Table S5, we found that the amount of Co
+
catalyze the etherification reaction of BHMF with methanol at species were decreased and the amount of Co /Co species
lower temperature. It was reported that Co O , which has Lewis were increased along with the increase of reduction
2
3+
3
4
o
a lower temperature from 200 C to 500 C. The content of metal Co
o
acidity, did not catalyze the etherification at
2
9
0
temperature. With the reaction temperature increased from (Co ) in the surface was lower after reduction at high
00 C to 140 C, HMF was completely converted and the yield temperature. This may be due to the reoxidation of highly active
1
°
°
of BHMF decreased gradually. Meanwhile, the yield of metal state Co species in the catalyst surface with higher
monoether MMFA increased initially, then decreased while the reduction temperature. This indicated that the increase of
yield of BMMF gradually increased. The highest yield of BMMF substrate conversion and product yield was not just attributable
0
C and 1 h. This illustrated that the Co catalyst to the proportion of Co species on the surface. From Figure S4
was 98.5% at 140 °
exhibited a certain acidity and promoted the rapid etherification of TEM images, the surface of Co catalyst turned porous after
o
increase the reduction temperature from 200 C to 300 C
o
reaction at higher temperature.
(
Figure S4a-b). Further increase the reduction temperature to
o
00 C, the larger particles were formed due to the strong
4
magnetism property of catalysts after reduction (Figure S4c).
The particle size distribution of Co catalysts with different
reduction temperature was calculated according to the TEM
analysis and the results was listed in Figure S4d-e. It was found
that the average particle size was 35 nm and 52 nm in Co-200
and Co-300 catalysts, respectively. This indicated that the
particle size of Co catalyst was increased with the increase of
reduction temperature. This was similar to the results observed
in the TEM images. This was mainly due to the magnetic
properties of the catalyst after reduction. The increase of
reduction temperature led to the enhanced interaction of metal
and ions, which could be also observed from the changed
0
2+
3+
binding energy of Co and Co /Co species in XPS analysis
Figure S3). In order to better observe the morphology of the
catalyst, we carried out the SEM analysis. It was found that the
Figure 1. Reaction conditions: 126 mg HMF, 20 mg Co-400 catalyst, 10 catalyst became more porous and rougher after reduction at
(
2
mL methanol, 1 h, 2 MPa H , GC mole yield.
high temperature (Figure S5a-c). This may be more beneficial for
enhancing the selectivity of etherification product and the mass
0
It was found by XPS analysis that the Co and Co
2+/3+
species
3
2
transfer of the reaction species.
coexist on the surface of catalyst, which indicated that the
catalyst was only partially reduced during the reduction process
(
Figure 2a). According to XRD analysis, we found that only
0
diffraction peaks arising from Co species existed and no
diffraction peaks associated with Co O species were observed,
3
4
indicating that Co O was in an amorphous state in the Co
3
4
3
0
catalyst (Figure 2b, 2c). It was reported that more acidic sites
existed in the amorphous state of Co O than in the crystalline
3
4
state, which facilitates the etherification reaction. The
2+/3+
species on the surface of the
0
coexistence of Co and Co
catalyst was also more conducive to the electron transfer, which
endows the catalyst with not only hydrogenation activity, but
3
1
also with strong etherification activity.
4
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Figure 3. Reaction conditions: 126 mg HMF, 20 mg Co-400 catalyst, 10 mL
methanol, 1 h, 120 °C, GC mole yield.
Figure 2. The XPS and XRD pattern of catalysts. (a) XPS pattern of Co-400
3 4
catalyst; (b). XRD pattern of Commercial Co O . (c) XRD pattern of Co-400
catalyst.
Then we investigated the effect of hydrogen pressure on
product distribution (Figure 3). HMF could be completely
Throughout the reaction process, we only observed the converted at relatively low hydrogen pressure (0.5 MPa), and
aldehyde hydrogenated product and etherified products rather the main products were BHMF, MMFA, and BMMF. It was
than the products of over-hydrogenation of the furan ring shown that the hydrogenation reaction and the etherification
(Figure 1). This result was presumably due to strong adsorption reaction could be carried out even at a relatively low pressure by
of the polar unsaturated aldehyde to the Co catalyst, whereas using the Co-400 catalyst. With the hydrogen pressure increased
the unsaturated nonpolar double bonds on the furan rings are from 1.0 MPa to 3.0 MPa, the yield of BHMF gradually
repelled. After the aldehyde group was adsorbed on the surface decreased, the yield of monoether product MMFA decreased
of the Co catalyst and hydrogenated to hydroxyl groups, the gradually, and the yield of diether product BMMF gradually
3
2
substrate would detach from the catalyst surface. It has been increased. No significant change of carbon balance was
+
reported that H ions ionized from H
2
O or Lewis acid catalyst observed, which indicated that the products were stable in the
could promote the isomerization reaction of furfuryl alcohol or catalytic system under higher reaction pressure.
HMF. However, we did not observe the formation of ring-
3
3
opened products. In the reaction, no products of over-hydrolysis Effect of catalyst loading and substrate concentration
(
e.g. levulinic acid and its esters) were observed, indicating that
The effects of catalyst loading and substrate concentration on
product distribution were investigated; the results are shown in
Table 2. HMF could be completely converted and a mixture of
products was obtained at 10 mg catalyst loading (Table 2, Entry
the Co-400 catalyst had high selectivity for reductive
3
etherification.
4
The current catalytic system thus offers the significant
advantage of allowing BHMF to be obtained by hydrogenation of
2
). Increasing the amount of catalyst to 20 mg led to a significant
HMF at low temperature (90 °C), and BMMF to be obtained by
decrease in the yield of BHMF and to a dramatic increase in the
yield of BMMF (Table 2, Entry 1). Further increases in the
amount of catalyst to 30 mg and 40 mg led to slight increases in
the yield of etherified product (Table 2, Entries 3 and 4). This
indicated that further increase in the amount of catalyst did not
have a significant effect on product distribution. A suitable
amount of catalyst was thus selected as 20 mg.
etherification of BHMF at high temperature (140 °C).
Effect of hydrogen pressure
Then we studied the effect of substrate concentration on
product distribution (Table 2, Entries 5–7). The data showed that
the yield of etherified products gradually reduced while the yield
of hydrogenated product BHMF gradually increased with an
increase in the substrate concentration from 126 mg to 1000
mg. This was possibly caused by the two-step etherification
reaction. Step one was the hydrogenation of HMF to BHMF, and
the second step was etherification of BHMF to BMMF. An
increase in the concentration of the substrate would allow more
of the starting materials to be involved in the reaction than
under the previous conditions (Table 2, Entry 1). If the
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hydrogenation reaction was extended, and the time allowed for
In the constant heating time of 0.17 h, the conversion of
the subsequent etherified reaction was reduced accordingly, this HMF increased significantly (89%), which indicated the effective
would contribute to an increase in the amount of hydrogenation hydrogenation activity of the Co-400 catalyst on the aldehyde
products. To verify this hypothesis, we extended the reaction group. It also showed that the etherification reaction was more
time to 4 h and found that most of the BHMF was converted difficult than the hydrogenation reaction because no monoether
into a mixture of etherified products (Table 2, Entry 8). This product was observed at a constant heating time of 0.17 h.
further illustrated that a suitable substrate concentration was Meanwhile, the temperature had a significant influence on both
beneficial for achieving the desired product distribution.
the hydrogenation and etherification reactions. With a further
extension of constant heating time to 0.33 h, the yield of BHMF
Effect of reaction time
Table 2. Effect of catalyst amounts and substrate amounts on product
distribution.
To understand the reaction mechanism involved in the reductive
etherification of HMF to BMMF, we investigated the effect of reaction
time on product distribution; the results are shown in Figure 4. The
product distribution during the heating process was investigated
a
Entr
y
catalyst
substrate
c
amounts
Conv./
%
Yield /%
c
amounts
BHMF MMFA
BMMF
62.3
27.8
62.8
65.0
20.3
8.4
1
2
3
4
5
6
7
20
10
30
40
20
20
20
20
126
126
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
8.4
30.2
7.9
28.4
34.6
28.6
27.5
40.7
32.5
14.3
25.2
initially. The reaction system was heated for 0.5 h from 30
°
C to 120
C, then the temperature was maintained for a defined period at 120
C until the reaction reached completion. We investigated the effect
°
°
126
126
7.2
of heating times on product distribution and the results were listed in
Figure S6 (Heating times of 0.17 h, 0.34 h, and 0.5 h, respectively).
HMF was not converted during the initial heating time of 0.17 h
200
38.0
57.6
84.1
7.4
300
1000
1000
0.5
(
internal temperature of ca. 60
indicated that the Co-400 catalyst did not catalyze the hydrogenation
reaction at 60 C. With the heating time extended to 0.33 h (internal
temperature of ca. 90 C), a small amount of HMF was converted and
°C), as shown in Figure S6. This
b
8
66.7
°
Reaction conditions: [a] 126 mg HMF, 20 mg Co-400 catalyst, 10 mL methanol,
20 °C, 1 h, 2 MPa H , GC mole yield. [b] 4 h. [c] The unit of catalyst amounts and
1
2
°
substrate amounts is mg.
hydrogenated product of BHMF was obtained quantitatively. The
hydrogenation activity of the catalyst was demonstrated at this
temperature, but the hydrogenation activity remained weak. With
the heating time extended to 0.5 h (internal temperature of ca. 120
decreased as it was gradually converted into MMFA. BMMF was
detected at a constant heating time of 0.33 h. At a constant
heating time of 1 h, the yield of BHMF reduced to 21.5%, and a
mixture of MMFA and BMMF ethers was found (yield of 33.4%
and 43.5%, respectively). This showed that the degree of
etherification gradually increased with extended reaction time
°
C), the conversion rate of HMF increased significantly (conversion of
6%), but only the hydrogenated product of BHMF was observed
yield of 35.7%). The reaction was then allowed to continue in the
constant heating stage at 120 C (Figure 4. “0 h” indicates the
3
(
°
at 120
°C. The yield of BMMF was obtained almost
beginning of constant heating). We found that the Co-400 catalyst
showed sufficient hydrogenation activity in the heating preservation
stage. To study the distribution of reaction intermediates, we initially
investigated the product distribution obtained from the constant
heating stage of 0.17 h to 0.5 h.
quantitatively (98.2%) when constant heating was extended to 6
h. We also found that 98.5% yield of BMMF could be obtained
upon heating at 140 °C for 1 h as noted previously (Figure 1).
This confirmed that the increased temperature could lead to a
reduction in the etherification time and accelerate the reaction
rate. Simultaneously, we found that no over-hydrolysis product
methyl levulinate was observed in the reductive etherification
reaction. This might be because the generated etherification
products could inhibit the degradation reactions. After the
etherification of intermediates, acid sites on the Co-400 catalyst
may not catalyze the degradation reactions, thereby increasing
the selectivity of the reaction. We also investigated the effect of
reaction time on product distribution with high substrate
concentration (1000 mg HMF/10 mL methanol) at 120 °C and
40 C, respectively (Figure S7a, Figure 4). A similar product
1
°
distribution was obtained at high substrate concentration (1000
mg HMF/10 mL methanol) compared to the reaction at low
substrate concentration (126 mg HMF/10 mL methanol).
However, the reaction time was extended along with the
increased amount of substrate. The increase of temperature was
advantageous for the conversion of HMF and the obtainment of
ether products even at higher substrate concentration (Figure
Figure 4. Reaction conditions: 126 mg HMF, 20 mg Co-400 catalyst, 10 S7b).
mL methanol, 1 MPa H , 120 C, GC mole yield.
2
°
6
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HMF disappeared after 0.33 h (Figure 6e). With prolonged
constant heating, the intensity of the peaks arising from BHMF
gradually decreased and disappeared at 3 h (Figure 6i). The
intensity of peaks arising from MMFA increased first and then
decreased (from Figure 6d to Figure 6k, ca. 4.54 ppm). The peaks
arising from BMMF increased gradually with increased periods
of constant heating and a nearly quantitative yield of BMMF was
obtained at 6 h (Figure 6k, ca. 4.39 ppm).
1
Figure 5. H-NMR spectra of intermediates and products.
To analyze further the reaction process, the reaction
solution with different reaction times (details of the treatment
1
1
are given in the SI) was characterized by H-NMR spectroscopic
Figure 6. H-NMR analysis of the reaction solution with different reaction
times.
1
analysis. We initially assigned the characteristic peaks of H-
NMR for the possible intermediates in the reaction (Figure 5).
1
The H-NMR spectra of HMF, BMFOL, BHMF, MMFA, and BMMF,
Catalyst recirculation
respectively, are shown in Figure 5a–e. The characteristic peaks Given that the catalyst is magnetic, it could be adsorbed on a
of each material are indicated in red outlines. The characteristic suitable magnet. Recycling experiments were therefore
1
peaks at ca. 4.71 ppm (Figure 5a, HMF, H ) and ca. 6.51 ppm conducted as follows: after reaction, the magnet was taken out
2
(
Figure 5a, HMF, H ) arise from hydrogen atoms on the furan and washed with methanol three times before being put into
ring and from the methylene of the hydroxymethyl group, the next cycle. The solution was transferred out with methanol
respectively. The characteristic peaks at ca. 5.39 ppm (Figure 5b, and a defined amount of N, N-dimethylformamide (DMF) was
3
BMFOL, H ) arise from hydrogen atoms on the acetal. The added as an internal standard. After centrifugation, the
4
characteristic peaks at ca. 6.24 ppm (Figure 5c, BHMF, H ) supernatant was analyzed by gas chromatography. The catalyst
originate from hydrogen atoms on the furan ring. The adsorbed on the magnet was then put into the next cycle under
5
characteristic peaks at ca. 4.54 ppm (Figure 5d, MMFA, H ) and the same reaction conditions. The experimental results obtained
6
ca. 4.36 ppm (Figure 5d, MMFA, H ) arise from hydrogen atoms for catalyst recycling are summarized in Figure 7. The activity of
on the methylene of the hydroxymethyl group and from the the reused catalyst decreased slightly in terms of conversion and
methylene of the methoxy methyl group, respectively. The selectivity after five cycles. This was mainly due to the partial
7
characteristic peaks at ca. 4.39 ppm (Figure 5e, BMMF, H ) and loss of catalyst during the posttreatment. We also investigated
8
o
ca. 6.29 ppm (Figure 5e, BMMF, H ) arise from hydrogen atoms the leaching of metal Co in the reaction solution at 100 C and
o
on the methylene of the methoxy methyl group and on the 140 C, respectively (Table S4). The leaching of metal Co were
1
o
o
furan ring, respectively. Subsequently, H-NMR analysis of the 0.2% and 0.7% at 100 C (1h) and 140 C (1h), respectively. This
reaction solution with different reaction times was carried out; indicated that the leaching of Co in the reaction solution was
the results are presented in Figure 6. Likewise, we found that very small during the reaction process. The use of alcohol
1
the product distribution obtained by H-NMR analysis was solvent and partial neutral environment may be the reason for
similar to that of GC analysis. In the initial reaction process, only the less leaching of metal Co.
1
peaks arising from HMF were observed in the H-NMR spectra
(Figure 6a, ca. 6.51 ppm). With increased heating time, peaks
arising from HMF decreased gradually and peaks from BHMF
1
increased gradually according to the H-NMR spectra (Figure 6b
and Figure 6c, ca. 6.24 ppm). When the reaction was then
submitted to constant heating (Figure 6d–k), peaks arising from
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Figure 9. The XPS pattern of catalysts. (a) Fresh Co-400 catalyst. (b) Co-400
catalyst after recycling five times.
OMe
O
HO
OMe
BMFOL
O
route d×
HO
OH
O
MeO
H
]
[
H
te a
BHMF
O
O
O
O
rou
MeOH
MeO
HO
O
MeO
OH
OMe
MeO
]
[H
ro
H
u
×
t
MMFA
BMMF
HMF
e
MeO
b
MFFA
route c
Side Reactions
×hydrolysis
oligomerization
O
O
O
∗
∗
n
O
oligomers
ML
Scheme 2. Possible reaction pathways.
A possible pathway for the reductive etherification of HMF is
Figure 7. Reaction conditions: 126 mg HMF, 40 mg Co-400 catalyst, 10 mL shown in Scheme 2. Given that the Co-400 catalyst has both
methanol, 1 MPa H , 120 °C, GC mole yield.
2
hydrogenation activity and etherification activity, the conversion
of HMF might proceed in four ways: route a) the aldehyde group
on HMF may be initially hydrogenated to obtain BHMF in the
presence of hydrogenation catalyst and hydrogen; route b) the
hydroxyl group on HMF may be initially etherified to obtain
3
5
MFFA by the catalyst
; route c) HMF can be oligomerized to
obtain oligomers or hydrolyzed to obtain methyl levulinate (ML)
6
3
under catalysis
; route d) aldolization of HMF and methanol to
2
0
obtain BMFOL under catalysis. Based on the experimental
results and on characterization data, we have not found
evidence for the presence of BMFOL, MFFA, and ML. Therefore,
we suggest that route a) is the main reaction sequence. A
possible reaction pathway is as follows. 1) The hydrogenation
reaction of HMF to give BHMF occurred on the hydrogenation
sites of the Co-400 catalyst. Given the strong adsorption of C=O
3 4
O . (b) Fresh double bonds and the repellency of C=C double bonds of the
Figure 8. The XRD pattern of catalysts. (a) Commercial Co
Co-400 catalyst. (c) Co-400 catalyst after recycling five times.
metallic Co-400 catalyst, only the product BHMF, arising from
3
2
Characterization by XRD and XPS analyses of both fresh and used hydrogenation of the C=O double bonds, was obtained. 2) The
catalysts was conducted (Figure 8 and Figure 9). No significant etherification of BHMF to give MMFA occurred followed by
changes were found in the XRD pattern of the fresh and used further etherification to obtain the target product BMMF on the
catalysts. Compared with the fresh Co-400 catalyst, the proportion of Lewis acid sites of the Co-400 catalyst. The reaction required a
0
Co species was decreased in the used catalyst according to the XPS synergistic effect of hydrogenation activity and etherification
0
2+/3+
analysis. The Co-400 catalyst had a better stability and could maintain activity. The coexistence of both Co and Co
species on the
a high reaction activity even after five times reuse in the methanol surface of the Co-400 catalyst was more conducive to electron
system.
transfer and might be beneficial for high selectivity of the
reaction. However, quantitative carbon yield was not obtained,
perhaps because of oligomerization of HMF and intermediates
(Scheme 2, route c). Further studies to establish the details of
the mechanism are necessary.
HO
O
MeO
OMe
HCl
O
Co-catalyst
methanol
O
Fructose
isopropanol
5-HMF
BMMF
Scheme 3. The preparation of BMMF from fructose in two-steps.
Herein, we designed a two-steps method for the preparation of
BMMF from fructose (Scheme 3). Initial hydrolysis of fructose to
HMF was achieved by using HCl catalyst and isopropanol solvent
8
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3
7
according to a reported procedure. Reductive etherification of This work was supported by the NSFC (21572212, 21472033,
the obtained HMF by using the developed Co catalyst system and 21325208), MOST (2017YFA0303502), CAS (YZ201563),
was then undertaken to synthesize intermediate BMMF. The FRFCU and PCSIRT. The authors thank the Hefei Leaf Biotech Co.,
reaction was carried out in a 50 mL parr autoclave equipped Ltd. and Anhui Kemi Machinery Technology Co., Ltd. for free
with a magnetic stirrer, a temperature probe and a programmed samples and equipment that benefited our ability to conduct
temperature controller. 1 g of fructose, 10 μL concentrated this study.
hydrochloric acid and 10 mL of isopropanol were added to parr
°
autoclave. Stirring and heating to 120 C under 500 rpm/min
°
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yield of HMF was obtained. Subsequently the residue was
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0 °C, 2 MPa H
7
−
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2
and reaction time of 1 h, the yield of
8
−
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HMF was 94% by using Co-400 catalyst. HMF was converted
4
−
5
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°
5
−
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1
7
−
2
−
6
7
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1
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,
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91 −
−
6
−
Conflicts of interest
−
5
−
There are no conflicts to declare.
8
−
−
Acknowledgements
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