Green catalytic synthesis of 5-methylfurfural by selective hydrogenolysis of 5-hydroxymethylfurfural over size-controlled Pd nanoparticle catalysts

Article abstract of DOI:10.1039/c9cy00039a

A green approach for the conversion of 5-(hydroxymethyl)furfural (HMF) to 5-methylfurfural (MF) by using size-controlled palladium catalysts has been developed. Palladium nanoparticles (Pd NPs) with various sizes supported on activated carbon were prepared with polyvinylpyrrolidone (PVP) as the capping agent. The reaction results showed that all the PVP-assisted Pd catalysts achieved high selectivity whilst the hydrogenation ability of Pd NPs could be rationally tuned by varying the mole ratio of Pd/PVP. 2.5% Pd-PVP/C (1:2) presented a satisfactory activity with 80% MF yield and 90% selectivity. The reaction kinetics study showed that the transformation of HMF into MF over bifunctional PVP-assisted Pd NPs underwent an acid-catalyzed esterification followed by a Pd-catalyzed hydrogenolysis procedure. The role of formic acid in the transformation is not only as a hydrogen-donating agent but also as a reactant to form the key intermediate. This work provides a novel and environmentally-friendly method for the selective hydrogenation of bio-based HMF to MF.

Full text of DOI:10.1039/c9cy00039a

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DOI: 10.1039/C9CY00039A
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ARTICLE
Green Catalytic Synthesis of 5-Methylfurfural by Selective
Hydrogenolysis of 5-Hydroxymethylfurfural over Size-
controlled Pd Nanoparticles Catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Guohan Sun[a], Jiahuan An[a], Hong Hu[a], Changzhi Li[b]*, SonglinZuo[a], Haian Xia[a]*
www.rsc.org/
A green approach for the conversion of 5-(hydroxymethyl)furfural (HMF) to 5-methylfurfural (MF) by using size-
controlled palladium catalyst has been developed. Palladium nanoparticles (Pd NPs) with various sizes
supported on activated carbon were prepared with the capping agent of polyvinylpyrrolidone (PVP). The reaction
results showed that all the PVP-assisted Pd catalysts achieved high selectivity whilst the hydrogenation ability of
Pd NPs could be rationally tuned by varying the mole ratio of Pd/PVP. 2.5%Pd-PVP/C(1:2) presented a
satisfactory activity with 80% MF yield and 90% selectivity. Reaction kinetic study showed that the transformation
of HMF to MF over bifunctional PVP-assisted Pd NPs underwent an acid-catalyzed esterification followed by Pd-
catalyzed hydrogenolysis procedure. The role of formic acid in the transformation is not only the hydrogen-
donating agent but also the reactant to form the key intermediate. This work provides a novel and
envronmentally-friendly method for the selective hydrogenation of bio-based HMF to MF.
then hydrolyzing the reaction mixtures (Route 1).12 The pathway
has a drawback that phosgene is a very toxic agent, which is an
environmentally unfriendly process. Sen and co-workers reported
Introduction
5-Methylfurfural (MF),
a
typical derivative of 5-
an efficient method to synthesize MF directly using fructose as
starting material (Route 2).15-17 A MF yield up to 69% was achieved
from fructose using Pd/C as a catalyst with the addition of HI.17 They
proposed that the formation of 5-iodomethylfurfural as an
intermediate is very critical to obtain a high MF yield because iodide
is a good leaving group as compared to chloride.17 Besides, two
strategies for the transformation of glucose, cellulose and cellulosic
biomass into MF are reported by Karl18 and Mascal19(Route 3). One
involves a three-step process: firstly, during an acid-catalyzed
hydrolysis procedure, the polysaccaharides in biomass have been
cracked into their corresponding monosaccaharides which will
subsequently occur a dehydration resulting in the production of
HMF. Subsequently, HMF is subjected to the formation of 5-
chloromethylfurfural (CMF) in a biphasic reaction system, where
the high-concentrated HCl solution is responsible for the
dehydration and halogenation process. Finally, MF could be formed
by the hydrodechlorination of CMF using palladium as catalyst
under mild conditions.
(hydroxymethyl)furfural (HMF),1-3 has been paid increasing
attention recently because of its extensive usage in both industry
and academia, e.g. as a precursor of certain anti-cancer natural
products and pharmaceuticals,4,
as a substrate for organic
5
synthesis research,6-11 and as an intermediate in the production of
agrochemicals.12 Moreover, MF is a key intermediate to produce
biofuel candidates, for instance, 2,5-dimethylfuran (DMF) and 5,5’-
di(hydroxymethyl)furoin (DHMF). DMF has been recognized as a
high-energy-density, high-boiling-point and high-octane biofuel,13
while DHMF is a promising C12 fuel intermediate which can be
transformed into the liquid biodiesel through
a
simple
hydrogenation reaction.14
Valorization of biomass feedstock including sugars and cellulose
to produce MF has received much attention recently. Typically, the
transformation of biomass into MF involves a multiple-step process.
Hirota et al. developed a process for preparing MF in a high yield by
reacting 2-methylfuran, phosgene and N,N-dimethylformamide and
As mentioned above, there remain great challenges for the MF
synthesis from HMF or sugars. For one thing, it is very urgent to
develop a novel and green reaction pathway without the addition
of HCl or HI. In addition, a challenge is that how to avoid the
hydrogenation of aldehyde group with high activity and to remove
the hydroxyl group of HMF without the addition of leaving groups.
These challenges require the development of selective
a
Jiangsu provincial key lab for the chemistry and utilization of agro-forest biomass,
College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037,
China, E-mail: haxia@dicp.ac.cn
bState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, E-mail: licz@dicp.ac.cn
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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hydrogenation catalyst by tuning its hydrogenation activity. Pd NPs
are always of great interest for heterogeneous catalysis due to their were used to characterize these catalysts. Figure 1 shows the XRD
patterns of the catalysts with variable Pd to PVP mole ratios before
Several characterization techniques, including XRD,VTieEwMAr,tiaclnedOnXlPinSe
DOI: 10.1039/C9CY00039A
and after the pyrolysis process. As shown in Figure 1A, two broad
Route 1
diffraction peaks are observed at 25° and 45° due to (002) and (100)
O
COCl2
O
N
O
+
of typical graphite of the amorphous carbon support.30 For the
catalysts with 1% Pd loading, the XRD patterns of the as-synthesized
catalysts are almost the same, except a weak peak associated with
Pd (111) for 1%Pd-PVP/C(1:1), showing that the Pd NPs are highly
dispersed (Figure 1A). When increasing the Pd loading amount,
several diffraction peaks are observed at 27.2o, 31.6o, 40.5o, 45.4o,
56.4o, 66.4o, and 75.2o. The diffraciton peaks at 40.5o, 45.4o, and
66.4o are ascribed to (111), (200), and (220) reflection of Pd(0)
metal particles, respectively.31 The peaks at 27.2o, 31.6o, 56.4o,
66.4o, and 75.2o are attributed to (111), (200), (222), (400), and
(420)of PdO particles, respectively.32 This result clearly
demonstrates that the catalysts contain the Pd and PdO which
could be formed by reoxidation of Pd metal upon its exposure to air.
After a carbonization process, the peak intensity of Pd metal
particles gets stronger (Figure 1B), indicating that the Pd NPs have
been further aggregated. It is reasonable that the particle size
would became bigger after high temperature pyrolysis due to the
removal of the cagging agent PVP and the migration of small Pd
particles. The inset in Figure 1B is an enlargement of line f. It can be
seen from the curve f that PdO was observed.
O
Toluene, 12 h, 60 °C
H
Route 2
O
HOH2C
OH
H2, HI, RuCl3 or Pd/C
90 ºC, 0.5-1 h
O
CH2OH
OH
O
HO
Route 3
glucose
cellulose
cellulosic biomass
O
O
O
H2, PdCl2
4 h, 40 °C
LiCl, HCl
Cl
O
O
DCE, 18 h, 65 °C
DCE, 24 h
O
HCl
1.acid-catalyzed hydrolysis
2.dehydration
This work
HO
Scheme 1 Synthetic routes for the production of MF[4-13]
.
selective hydrogenation performance.20 However, there are two
problems for the nano-palladium catalyst. Firstly, Pd NPs are easy
to accumulate by the way of Ostwald ripening because of their
inherent properties, such as high energy of the surface and low
Tammann temperature.21 Secondly, because of the strong
hydrogenation ability of Pd NPs, HMF can be over hydrogenated
leading to other byproducts rather than MF. Accordingly, the
stabilizers or the inhibitors, such as polymers and surfactants, are
widely used to prepare Pd colloids as the precursors in order to
compensate the high surface energy, reduce the possibility of
agglomeration22-24 and restrain the over hydrogenation so as to
enhance the selectivity. Polyvinylpyrrolidone (PVP) is one of the
most prevalent stabilizers which has been verified to transfer the
electrons between the Pd NPs surface and long carbon chains with
the polymer functional groups,25-27 making catalytic performances
of the Pd NPs drastically different from that of bulk palladium
particles owing to their relatively high dispersion and stabilization.
28 It has also been reported that PVP can modify the morphology of
Pd NPs, which enhances the interaction between Pd NPs and the
supports or even covers the high active spots to avoid the excessive
hydrogenation.22, 29
Figure 2 shows the TEM micrographs of these catalysts with
different Pd/PVP ratio and Pd loading as well as their corresponding
particle size distributions obtained from the counts with more than
300 particles (see TEM micrographs of other catalysts in Figures S8-
S13, Supporting Information). Table 1 summarizes the average
particle sizes of the as-synthesized catalysts. The mean sizes of Pd
NPs on as-synthesized catalysts depend on the adding amount of
PVP: 8.3 nm for 1%Pd-PVP/C(1:1), 3.8 nm for 1%Pd-PVP/C(1:2), and
3.2 nm for 1%Pd-PVP/C(1:4). The particle sizes of 5%Pd-PVP/C(1:2)
and 2.5%Pd-PVP/C(1:2) increaseslightly because of the high loading
amount - 4.7nm for 2.5%Pd-PVP/C(1:2) and 6.6 nm for 5%Pd-
PVP/C(1:2). So it is reasonable that the lower PVP amount was
added, the larger Pd NPs would appear. The particle size
distributions of Pd NPs of all the catalysts before the carbonization
process show a narrow and uniform distribution. CO chemisorption
results clearly show that the PVP-stabilized Pd NPs catalysts have
less accessible active sites as compared to the commercial Pd/C
catalyst albeit it exhibits bigger particle size (Table 1). As expected,
the particle size of 1%Pd-PVP/C(1:1) is relatively larger while 1%Pd-
PVP/C(1:4) has less accessible active sites due to the excessive
amount of PVP.
Herein, we develop a novel approach to convert HMF to MF
directly in one pot by using PVP-assisted palladium nanoparticles as
catalyst and formic acid as hydrogen-donating agent under a
relatively mild condition with high yield and selectivity (Scheme 2).
To the best of our knowledge, it is the first time that this
environmental-friendly procedure has been proposed to produce
MF during which no poisonous components (phosgene, N,N-
dimethylformamide or hydrogen halides) have been employed.
Pd (111)
(B)
(A)
a
Pd (200)
PdO (220)
PdO (200)
b
c
30
33
36
39
42
45
48
d
O
Pd (111)
HCOOH, PVP-assisted Pd catalyst
THF, 200-220 ºC, 7.5 h
O
Pd (200)
HO
Pd (220)
e
O
O
f
g
h
Scheme 2. Conversion of HMF into MF by using PVP-assisted Pd
catalyst.
10
20
30
40
50
60
70
80
10
20
30
40
50
60
70
80
2 / Degree
2 (Degree)
Figure 1 XRD patterns of the catalysts: (A) catalysts before the
carbonization process: a.1%Pd-PVP/C(1:1), b.1%Pd-PVP/C(1:2),
c.1%Pd-PVP/C(1:4); d. 2.5%Pd-PVP/C(1:2); e.5%Pd-PVP/C(1:2). (B)
Results and discussion
2 | J. Name., 2012, 00, 1-3
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catalysts after the carbonization process: f.1%Pd-PVP/C(1:1, 550°C),
g.1%Pd-PVP/C(1:2, 550°C), h.1%Pd-PVP/C(1:4, 550°C).
Table 1 Average particle size of all the as-syntVhieewsiAzreticdle Online
DOI: 10.1039/C9CY00039A
catalysts, metal dispersion and electronic state of Pd.
Catalyst
Average
Metallic
Electronic
diameter[a] dispersion[b] state of Pd[c]
However, after a carbonization treatment, significant changes of
Pd particle sizes have taken place. As can be seen from Table 1, the
average sizes of 1%Pd-PVP/C(1:1, 550 °C), 1%Pd-PVP/C(1:2, 550 °C)
and 1%Pd-PVP/C(1:4, 550 °C) are 22.6 nm, 16.0 nm and 11.4 nm,
respectively. The pyrolysis process destroyed the capping agent
around the Pd NPs and then the particle agglomeration occurred
under high temperature, which contributed to the large sizes and
heterogeneity of nanoparticles compared with the as-synthesized
counterparts. Moreover, it seems that more PVP amount can more
effectively prevent the particle size from bigger size due to its
stabilizing role.
[nm]
[%]
[%]
Pd(0) Pd(II)
1%Pd-
8.3 ± 1.9
3.8 ± 1.1
3.2 ± 0.7
4.7 ± 1.1
6.6 ± 2.5
1.6940
1.0240
0.8506
0.1689
0.0239
66
68
73
40
33
34
32
27
60
67
PVP/C(1:1)
1%Pd-
PVP/C(1:2)
1%Pd-
PVP/C(1:4)
2.5%Pd-
PVP/C(1:2)
50
(A)
5%Pd-
PVP/C(1:2)
d = 8.3  1.9 nm
40
> 300 counts
30
20
10
0
5%Pd/C(comm.)
5.7 ± 1.9
8.7190
-
-
-
1%Pd-PVP/C
(1:1, 550 °C)
22.6 ± 7.2
59
41
0
5
10
15
20
25
1%Pd-PVP/C
(1:2, 550 °C)
16.0 ± 9.5
11.4 ± 3.9
-
-
62
68
38
32
Particle Size (nm)
50
40
30
20
10
0
(B)
1%Pd-PVP/C
(1:4, 550 °C)
d = 22.3  7.2 nm
> 300 counts
[a] The average particle size of each catalyst is based on a count of
more than 300 particles.
[b] Metal dispersion of the catalysts were tested by CO
chemisorption.
10
20
30
40
50
[c] Electronic state of Pd was calculated by XPS results.
Particle Size (nm)
XPS analyses were performed both on the as-synthesized
catalysts and the carbonized samples to further identify the Pd
electronic states. As can be seen in Figure 3, Pd 3d XPS spectra
consist of two bands corresponding to the 3d3/2 and 3d5/2
transitions. After the deconvolution, two peaks at 335.8 and 341.2
eV assigned to Pd(0) metal are observed, while the peaks at 337.2
and 343.0 eV are associated with the Pd(II) species due to the
generation of PdO.33 The result is in good agreement with the XRD
patterns in Figure 1 that Pd(0) and Pd(II) species are presented in
these sample. Compared to the commerical Pd/C catalyst, the
binding energy of Pd(0) 3d5/2/3d3/2 shifts from 366.3/341.5 eV (the
commerical Pd/C) (Supporting information, Fig. S15(C)) to lower
energy 335.8/341.2 eV (1%Pd-PVP/C(1:1)) , meaning that electron
transfers from N atoms to Pd atoms. Thus, in addition to its
stabilizing function, PVP may also serve as an electron donor. N 1s
XPS results contain only one peak located at 400.4 eV, which is
ascribed to the pyrrolidone groups.25
40
30
20
10
0
(C)
d = 3.8  1.1 nm
> 300 counts
0
2
4
6
8
10
Particle Size (nm)
50
40
30
20
10
0
(D)
d = 16.0  9.5 nm
> 300 counts
0
10
20
30
40
Particle Size (nm)
Figure
2 TEM images and particle size distributions: (A)
Pd 3d3/2
Pd 3d5/2
400.4 eV
N 1s
Pd 3d
(A)
335.8 eV
1%PVP/C(1:1); (B) 1%PVP/C(1:1, 550°C); (C) 1%PVP/C(1:2); (D)
1%PVP/C(1:2, 550°C).
Data
Fit
Pd(0)
Pd(II)
341.2 eV
Data
Fit
337.2 eV
343.0 eV
406
404
402
400
398
Binding Energy (eV)
396
394
352
340
332
328
348Bin3d4in4g Energy3(3e6V)
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changed its structure during the pyrolysis.34 Similarly, ViniewcoAmrticpleaOrinsloinne
Pd 3d
(B)
N 1s
DOI: 10.1039/C9CY00039A
Data
Fit
pyrrolic N
pyridinc N
graphitic N
oxidized N
with the commerial Pd/C catalyst, after the carbonnization, the
Data
399.1 eV
401.0 eV
Fit
Pd(0)
Pd(II)
binding energy of Pd(0) 3d5/2/3d3/2 also shifts to lower energy,
indicating that electron transfers from N-containing groups to Pd
atoms.
402.0 eV
404.8 eV
Table 2 summarizes the results on the catalytic reaction of HMF
to form MF using different palladium catalysts. As can be seen from
Table 2, when 1%Pd-PVP/C samples with different mole ratios of Pd
to PVP were used as the catalysts, good HMF conversion and MF
selectivitiy could be obtained (entries 3-11). When increasing
temperature (entries 3-6) or reaction time (entries 7-9), MF yield
decreased, which is attributed to the hydrogenation of HMF to
other products, such as 2,5-dimethylfuran and (5-
methyltetrahydrofuran-2-yl)methanol detected by GC-MS (Figure
S6, Supporting Information). There is an esterified product, (5-
formylfuran-2-yl)methyl formate (FFMF) emerging unexpectedly
(Figures S2-S5, Supporting Information). A 73% yield of MF with
94% selectivity can be obtained under the temperature of 220 °C
for 7.5 h using the 1%Pd-PVP/C(1:2) catalyst (Entry 8). As increasing
the ratio of Pd/PVP from 1:2 to 1:4, the yield of MF has declined
from 73% to 46% (entries 8 and 11), indicating that PVP plays a role
of inhibitor on the catalysts due to it covers more Pd active sites
determined from CO chemisorption. The catalysts 1%Pd-PVP/C(1:2)
presents higher MF yield and selectivity compared with other mole
ratios of Pd to PVP (entris 8, 10 and 11). These results seem that the
particle size and accessible active sites are not a key factor for the
formation of MF. With the exception of a capping agent and a cover
agent blocking Pd active sites with high hydrogenation activity, the
PVP could act as an electron donor to interact with Pd active sites,
thereby mediating the hydrogenolysis performance and increasing
the selectivity toward MF. A blank experiment without the catalyst
was conducted, less MF was generated at the HMF conversion of
46%, showing that formic acid cannot not catalyze the conversion
of HMF into MF (entry 1). Interestingly, a commercial 5%Pd/C
presents a very high catalytic activity but does not afford any MF
(entry 2), which is attributed to its strong hydrogenation
performance to produce some overhydrogenated products. This result
further showes that PVP play a key role in the formation of MF,
especially for the blocking role of the Pd active sites with high
overhydrogenation acitvitiy.
352
348
344
340
336
332
328
408 406 404 402 400 398 396 394 392 390
Binding Energy (eV)
Binding Energy (eV)
Figure 3 X-ray photoelectron spectra for Pd (3d) and N (1s) of the
catalysts: (A) 1%Pd-PVP/C(1:1); (B) 1%Pd-PVP/C(1:1, 550°C).
After the carbonization treatment, a little change for the
percentage of Pd(0) and Pd(II) was observed, while significant
changes had taken place on the N 1s XPS (Fig. 3 and Table 1). In
Figure 3B, the deconvolution of N 1s spectrum gives rise to four
individual peaks for the catalyst 1%Pd-PVP/C(1:1, 550°C): 399.1 eV,
401.0 eV, 402.0 eV and 404.8 eV, which represented pyridinc N,
Table 2 Conversion of HMF to MF using the fresh catalysts.
Entr Catalyst
y
Temp Tim
Conv.
[%]
Yiel
d
[%]
Selec.
[%]
.
e
[h]
[°C]
1[a]
2[b]
3
-
220
220
200
220
240
260
220
220
220
220
220
190
7.5
7.5
5
46
100
57
68
90
100
61
78
79
80
63
47
2
4
5%Pd/C(comm.)
1%Pd-PVP/C(1:2)
1%Pd-PVP/C(1:2)
1%Pd-PVP/C(1:2)
1%Pd-PVP/C(1:2)
1%Pd-PVP/C(1:2)
1%Pd-PVP/C(1:2)
1%Pd-PVP/C(1:2)
1%Pd-PVP/C(1:1)
1%Pd-PVP/C(1:4)
/
/
9
16
99
73
30
90
94
81
58
73
62
4
5
67
66
30
55
73
64
46
46
29
5
5
6
5
7
2.5
7.5
10
7.5
7.5
7.5
8
9
10
11
12
To further increasing the MF yield, the catalysts with high Pd
loading amount (2.5 wt% and 5 wt%) stablized by the capping agent
PVP have also been prepared and evaluated (entries 12-17). The
high-loading amount is beneficial since the yield of MF has greatly
been improved from 73% to 80% compared with the 1%Pd-
PVP/C(1:2) catalyst (entries 8 vs 13). Under the optimal condition,
2.5%Pd-PVP/C(1:2) achieved a high MF yield and selectivity because
it can reduce the formation of side-products during the
hydrogenolysis of HMF (entry 13).
2.5%Pd-
PVP/C(1:2)
13
14
2.5%Pd-
PVP/C(1:2)
200
220
7.5
7.5
87
80
90
2.5%Pd-
PVP/C(1:2)
89
37
42
Considering PVP covers some surface active sites of the Pd NPs,
we used carbonization method at high tempreture to remove part
of the PVP. Table 3 shows the conversion of HMF to MF using
catalysts after the pyrolysis. As shown in entry 2, the catalyst 1%Pd-
PVP/C(1:2, 550°C) presents a satisfactory activity with 77% MF yield
and 94% selectivity. However, the promotion impact of the
pyrolysis process doesn’t work on the catalysts 1%Pd-
PVP/C(1:1,550°C) (entry 1) and 1%Pd-PVP/C(1:4,550°C) (entry 3). A
possible explanation is that the mean particle size is too big for
1%Pd-PVP/C(1:1,550°C) (22.6 ± 7.2 nm) to exhibit good selectivity
toward MF, whereas the surface Pd active sites was excessively
covered by the carbon shell generating from the pyrolysis for the
15
16
17
5%Pd-PVP/C(1:2)
5%Pd-PVP/C(1:2)
5%Pd-PVP/C(1:2)
160
190
220
7.5
7.5
7.5
60
70
93
24
64
17
40
91
18
[a] No catalyst was added to the reaction mixture. [b] The catalyst
used in entry 2 is commercially available which is produced by J&K
Scientific Ltd, Beijing. Reaction conditions: 250 mg HMF, 50 mg
catalyst, 10 mL THF, 1 mL HCOOH, 5 bar N2.
pyrrolic N, graphitic N and oxidized N, respectively. The exsitences
of pyridinc N and graphitic N indicate that the polymer PVP had
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Pd-PVP/C(1:4,550°C) catalyst although it has smaller average value between the activity and the selectivity toward MF, which
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DOI: 10.1039/C9CY00039A
particle size.[14b] Moreover, there remains an interesting
Table 3 Conversion of HMF to MF using the carbonized catalysts.
leads to the relatively high MF yield and selectivity.
To study the reaction mechanism,
a reaction kinetic was
conducted. Because the pure FFMF product cannot be commercially
achieved, we used peak area instead of the yield of FFMF to analyze
this reaction kinetic. As can be seen in Figure 4, when reaction time
was 1.5 h, much FFMF was formed and there was a small quantity
of MF in the product. As prolonging reaction time, the FFMF
gradually disappeared accomanied by an increase in the yield of MF,
which is a typically pattern of cascade reaction. Therefore, we
propose that FFMF is a reaction intermediate in the hydrogenolysis
of HMF. To further investigate the reaction mechinasm, other
hydrogen sources including hydrogen and isopropanol, had also
been used to conduct the reaction under identical reaction
Entry
Catalyst[a]
Temp. Time
Conv.
[%]
Yield
[%]
Selec.
[%]
[°C]
220
[h]
1
2
3
4
1%Pd-PVP/C
(1:1, 550°C)
7.5
78
21
77
2
27
1%Pd-PVP/C
(1:2, 550°C)
220
220
7.5
7.5
7.5
82
62
72
94
3
5 X104
100
1%Pd-PVP/C
(1:4, 550°C)
80
60
40
20
0
4
3
2
1
Conversion of HMF
Yield of MF
Peak area of FFMF
2.5%Pd-PVP/C 200
(1:2, 550°C)
48
67
0
2
4
6
8
[a] The catalysts were carbonized in N2 flow under the
temperature of 550 °C. Reaction conditions: 250 mg HMF, 50 mg
catalyst, 10 mL THF, 1 mL HCOOH, 5 bar N2.
Time (h)
Figure 4 The reaction kinetics of HMF to MF using 2.5%Pd-
PVP/C(1:2) under the temperature of 200 ℃ : -■- (Conversion of
HMF); -●- (Yield of MF); -▲- (Peak area of FFMF).
observation that the yied and selectivty of MF is higher when using
1%Pd-PVP/C(1:2,550°C) compared with the as-prepared
counterpart. For the samples derived from carbonization of as-
synthesized 1%Pd-PVP/C (1:2) , the HMF conversion all increased
(Table S1, entries 4-6) in comparison with the as-prepared catalyst but
higher temperature carbonization resulted in a lower conversion (Table
S1, entry 4 VS 6). The possible interpretation is because higher
temperature carbonization is prone to promote the agglomeration of
nanoparticles into bigger particles, thereby reducing their catalytic
activity. However, when increasing the Pd loading to 2.5 wt% (entry
4), the opposit circumstance occured, i.e. the catalytic activity of
the as-prepared catalyst is better. Thus, the carbon shell and PVP
have the same role in the blocking the Pd active sites and inhibiting
the over-hydrogenation/hydrogenolysis, thereby increasing the MF
selectivity. It should be noted that the electronic effect on the
catalytic performances should be also considered for the
introduction of PVP or N-containing groups dervied from the high-
temperature carbonization. In generally, the nagative charge
density on the metal surface sites donored by PVP molecules may
enhance the catalytic performances, but it is more likely that the
blocking role of PVP is the most important factors cotrolling the
catalytic activity albeit the electronic effect also affect their catlaytic
activity.
O
O
O
O
O
O
esterification
HCOOH
hydrogenolysis
HCOOH
FFMF
MF
O
O
H2 or isopropanol
HO
overhydrogenation
only hydrogen-donating agent
HMF
acertic acid
only acid
very low yield and selectivity
Scheme 2 Reaction pathway of HMF to MF using PVP-assisted Pd
catalyst.
conditions. However, no MF was detected and many side-products
were produced in the products albeit HMF was fully converted as H2
was used, which verifies that the transformation of HMF to MF
must go through an esterification procedure using acid (Table S2,
Supporting Information). When using acetic acid to replace formic
acid as th reacants, only a low MF yield (8%) could be obtained,
which proves that the transformation of the esterified intermediate
to MF is mainly through hydrogenolysis, not decarboxylation (Table
S2, Supporting Information). Therefore, formic acid is not only the
hydrogen donor but also the reactant to form the esterification
intermediate. Thus, we prospect that the reaction pathway of HMF
to MF as following (Scheme 2): formic acid firstly reacts with HMF
On the basis of above reaction and characterization results, it
seems that there are the three obvious factors – PVP/Pd ratio,
mean particle size, and the Pd loading – affecting the MF yield and
selectivity drastically. We hypothesize that the selective blocking of
the Pd active sites with over-hydrogenation/hydrogenolysis activity
is the most important factor for the production of MF in the
conversion of HMF. 2.5%Pd-PVP/C(1:2) could reach an optimal
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HMF (250 mg,, Aladdin) and 50 mg catalyst were introVdieuwceAdrticinle tOhneline
reactor with tetrahydrofuran (THF, 10 mL, ADRO, NI:a1n0j.1in0g39C/hCe9mCYic0a0l039A
Reagent Co. LTD) as solvent and formic acid (HCOOH, 1 mL, equal to
20 mmol, AR, 88 wt%, Nanjing Chemical Reagent Co. LTD) as
hydrogen donor. All the reactions were conducted under N2
atmosphere (5 bar) with the heating rate of 5 °C/min.
to form the esterificated intermediate (FFMF) through esterification
followed by the hydrogenolysis of FFMF to form MF.
Conclusions
In summary, we have successfully developed a novel and green
approach to convert HMF to MF by using carbon supported PVP-
assisted palladium nanoparticles as the catalysts and formic acid as
hydrogen-donating agent with high yield (80%) and selectivity (90%).
No poisonous components were employed during the reaction
procedure. It has been demonstrated that PVP plays a role of
inhibitor that reduces the activity of Pd NPs and thus prevent the
overhydrogenation of HMF to achieve a high selectivity of MF. PVP
can control the particle size of the Pd NPs by adjusting its amount,
thereby tuning the catalyst reactivity and the MF selectivity. The
role of formic acid is not only the hydrogen donor but also the
reactant to form the esterified intermediate. The work will benefit
the design and synthesis of catalysts for the production of bio-
based MF, and provides a new idea for the selective hydrogenation
of biomass-derivated platform molecules into value-added
chemicals.
Product analysis
The products were analyzed by gas chromatography mass
spectrometry (GC-Mass, Agilent 7890A associated with 5975C)
equipped with a DB-35MS column (Agilent) and high performance
liquid chromatography (HPLC, Agilent 1200) using a C18 column
(Elite) and a UV detector.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
This work was financially supported by the National Key R&D
Program of China (grant no. 2017YFD0601006), the Natural Science
Foundation of China (grant no. 31200445, 21473187, 21690083)
and the Natural Science Foundation of Jiangsu province (grant no.
BK20171452). This work was also financially supported by the
Natural Science Foundation of Jiangsu Higher Education Institutes
of China (16KJB220003).
Experimental
Catalyst preparation
The commercially activated carbon derived from coconut shells
was purchased from Mulinsen Activated Carbon Group, China. The
activated carbon which was crushed to 60-100 meshes after it was
pretreated with 1 wt% nitric acid (Nanjing Chemical Reagent Co.
LTD) at 368 K for 6 h and then was washed with deionized water
until pH=7. Polyvinylpyrrolidone (PVP, 41.8 mg equal to 0.376 mmol
monomer, K23-27, Aladdin) and potassium hexachloropalladate
(K2PdCl6, 74.7 mg, 0.188 mmol Aladdin) were dissolved in deionized
water (50 mL) at room temperature and the solution presented a
dark red colour. After 2 h stirring, sodium borohydride (NaBH4, 42.7
mg, 1.128 mmol, Aladdin) in the formation of 10 mL freshly
prepared aqueous solution was added dropwise with vigorous
stirring when the mixture of K2PdCl6 and PVP turned into light
orange. Then black Pd colloid solution was immediately formed.
This colloid solution was kept under stirring for another 2 h when
the NaBH4 solution had been used out. Once the colloid was
prepared, 2g support (pre-treated carbon, ZrO2 or SiO2) was
impregnated for 12 h under magnetic agitation. The water in
mixture was then removed by rotary evaporation to achieve a 1
wt% Pd loading catalyst with the 1:2 molar ratio of Pd and PVP. The
catalysts was denoted as follows: 1%Pd-PVP/C(1:2), where 1% is the
loading amount, C is the support and the mole ratio of PVP and Pd
is 2.The catalysts with different molar ratios of Pd and PVP were
synthesized by adding different amount of PVP.
Keywords: 5-Methylfurfural; 5-Hydroxymethylfurfural; PVP;
Hydrogenolysis; stabilizers
Notes and references
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DOI: 10.1039/C9CY00039A
O
O
O
O
FFMF
PVP
hydrogenolysis
HCOOH
O
O
HO
esterification
HCOOH
Pd
O
HMF
O
AC
MF
A green approach for the conversion of 5-(hydroxymethyl)furfural to 5-methylfurfural with a high
product yield and selectivity has been developed.