Conversion of HMF to methyl cyclopentenolone using Pd/Nb2O5 and Ca-Al catalysts: Via a two-step procedure

Article abstract of DOI:10.1039/c7gc02310c

The catalytic conversion of HMF to 2-hydroxy-3-methyl-2-cyclopenten-1-one (MCP), which is a valuable edible essence that has traditionally been obtained from adipic acid, was achieved with an isolated yield of 58%. This procedure comprised two steps: the hydrogenation of 5-hydroxymethylfurfural (HMF) to 1-hydroxy-2,5-hexanedione (HHD) in water on Pd/Nb₂O₅ catalysts and then the isomerization of HHD to MCP in the presence of a base. The Nb₂O₅ supports, which were acidic, were characterized by FTIR, XRD and NH₃-TPD. The supported Pd/Nb₂O₅ catalysts, in which Pd was highly dispersed, were synthesized employing cyclohexene as a reductant and were characterized by XRD, TEM, ICP-AES, XPS, EDX and CO pulse chemisorption. The high conversion of HMF was attributed to the high dispersion of Pd, and the acidity of the supports led to high selectivity for HHD. The conversion of HHD to MCP was an intramolecular aldol condensation reaction, and the protonic solvent favored this reaction. Ca-Al was proved to be an effective solid base for the conversion of HHD to MCP in water.

Full text of DOI:10.1039/c7gc02310c

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Conversion of HMF to Methyl Cyclopentenolone by the Pd/Nb O5
2
and Ca-Al Catalysts via Two-steps Procedure
Received 00th January 20x1x,
Accepted 00th January 20xx
ab
a
a
a
a
a
Ying Duan, Min Zheng, Dongmi Li, Dongsheng Deng, Lu-Fang Ma and Yanliang Yang*
DOI: 10.1039/x0xx00000x
The catalytic conversion of HMF to 2-hydroxy-3-methyl-2-cyclopenten-1-one (MCP), which was a valuable edible essence
traditionally obtained from adipinic acid, was achieved with 58% isolated yield. This procedure contained two steps: the
www.rsc.org/
hydrogenation of 5-hydroxymethylfurfural (HMF) to 1-hydroxy-2,5-hexanedione (HHD) in water on Pd/Nb
then the HHD isomerized to MCP in the presence of base. The Nb supports, possessing acidity, were characterized by
FTIR, XRD and NH -TPD. The supported Pd/Nb catalysts, of highly dispersed Pd, were synthesized employing
2 5
O catalysts and
2
O
5
3
2 5
O
cyclohexene as reductant and characterized by XRD, TEM, ICP-AES, XPS, EDX and CO pulse chemisorption. The high
conversion of HMF was attributed to high disperse of Pd and the acidity of supports led to high selectivity of HHD. The
conversion of HHD to MCP was an intramolecular aldol condensation reaction and the protonic solvent was in favor of this
reaction. The Ca-Al was proved to be an effective solid base for the conversion of HHD to MCP in water.
also demonstrated to be efficient for the hydrogenation of
3
HMF to HHD. For example, Jérôme et al. reported a 77% yield
1
Introduciton
of HHD from HMF by the combination of Pd/C and Amberlyst-
Production of chemicals and fuels from bio-derived furans is of
great significance as the diminishing fossil resources. The
furans, including furfural and HMF, could be readily obtained
by the dehydration of carbohydrates and converted via various
1
5 in THF. Considering the catalysts mentioned above were
mainly noble metals, how to elevate the efficiency of these
catalysts involving conversion and selectivity is a key topic.
Moreover, the transformation of HHD to valuable chemicals is
also a significant problem due to the utility of the HHD.
1
-8
9, 10
of catalytic processes. Tetrahydrofurfuryl alcohol,
1
furfuryl
1-14
15,
16
alcohol,
furandimethanol,
2,5-dimethylfuran,
tetrahydro-2,5-
1
7, 18
19, 20
Cyclopentanone (CPO),
a
typical aliphatic cyclic
furan-2,5-dicarboxylic acid
are regular products from furfural and
Among these circumstances, the structure of the
and 2,5-
2
1, 22
compound, obtained from furfural had been reported by
Hronec et al. in 2012. In this procedure, the structure of the
furan ring was hydrogenated then isomerized to aliphatic
diformylfuran
2
3-26
HMF.
furan ring was kept in most of the common products from
furfural and HMF. Another attractive research area is to breach
the furan structure to get more categories of products which
can mainly be classified as the chain compounds and aliphatic
cyclic compounds. The levulinic acid was the representative
3
.
9
cyclic in water in the presence of H
2
And Xu el al. also
reported the same results and demonstrated that this process
conduct through furfuryl alcohol, then 4-hydroxy-2-
4
0
cyclopentenone and finally 2-cyclopentenone. Then several
7- groups had focused on this reaction and the best yield of
cyclopentanone from furfural reached up to 99%. The catalysts
used in these articles including Ni-Cu, Cu-Co, Au, Pd, Pt, Ru etc.
2
chain compound through the ring-opening of the furan rings.
3
0
HHD, another chain compound which was obtained
4
1-48
and were proved to be effective for this procedure.
through the hydrogenation of HMF with the homogeneous or
heterogeneous catalysts, had gained many attentions
Due to the same skeleton of HMF and furfural, the
conversion of HMF to a cyclopentanone derivative had been
gaining increasing interest recently. Different from the
rearrangement of furfural to cyclopentanone, when HMF was
3
1-38
33
Fu et al. reported the Cp*Ir(III) half-sandwich
recently.
complexes were effective catalysts for the hydrogenation of
HMF in aqueous formate buffer solution and 85% yield of HHD
could be achieved. The supported Pd, Pt or Au catalysts were
used as feedstocks instead of furfural,
a series of
cyclopentanone derivatives were obtained varied on the
catalysts and reaction conditions. For instance, Ohyama et
4
9-51
al.
from HMF by using Au/Nb
mainly depended on the acid-base properties of the support.
obtained 3-hydroxymethylcyclop-entanone (HCPN)
2 5
O
in water and the yield of HCPN
3
-Methyl-2-cyclopenten-1-one could be acquired from HMF by
5
water splitting with Zn at high temperature by Huo group.
2
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3
53 K for 24 h. Then ammonia water was added drop wise into
the clear solution to adjust the pH 8-9 under drastic stirring.
The precipitation was centrifuged and washed first with
ammonia water and then water for 3 times. The solid was
dried at 373 K overnight to afford niobic acid. The niobic acid
was then calcined in air at different temperature to afford
Scheme 1 The conversion of HMF to MCP.
Nb
Pd/Nb
by cyclohexene. The mixture was stirred for 30 min at room
temperature. Then PdCl (Pd: 5.0 mg) dissolved in dilute
2
O
5
-x where x referred to the calcined Celsius temperature.
2
O
5
-x. Nb -x (1.00 g) was added into water saturated
2 5
O
MCP is a valuable edible essence, widely used in foods,
5
3
beverages and flavors. The MCP was mainly produced from
adipinic acid via multistep reactions and usually employed
2
hydrochloric acid was added drop wise in the mixture under
stirring. After stirring for 12 h at room temperature the
mixture was centrifuged and washed first with water and then
ethanol. The solid was dried at 323 K under the vacuum for 24
h to afford the Pd/Nb O -x. The Pd/Nb O -x with different
5
.
4
organic solvent and toxic reagent such as Cl
gave a report on the catalytic conversion of HMF to MCP on
Pd/Nb in water. The procedure contained two steps as
shown in Scheme 1. The first step was the hydrogenation of
HMF to HHD over Pd/Nb O and then HHD to MCP in alkaline
2
Herein, we
2 5
O
2
5
2 5
2
5
content of Pd were prepared using a similar procedure with
aqueous solution. No organic solvent and toxic reagent was
used in this procedure. The niobium oxide was a water-
tolerant acid and widely used as catalyst or support in the
different amount of PdCl and cyclohexene.
2
Pd/Nb O -400-IP. Nb O -400 (1.00 g) was added in dilute
2
5
2 5
hydrochloric acid solution of PdCl (Pd: 5.0 mg). The mixture
2
5
5-60
conversion of biomass.
The Pd/Nb O was synthesized
2 5
was stirred for five minutes and kept stand for 24 h under
room temperature. Then the solid was dried at 373 K
overnight and reduced at 573 K by H2.
through a reduction reaction by stirring the mixture of PdCl2,
Nb O and water saturated by cyclohexene which was a part of
2
5
the Wacker Oxidation process as shown in eq. 1. This process
was first used for the preparation of Pd/SBA-15 employing
surface bonded vinyl as the reductant by our group short
Pd/Nb O -400-B. Nb O -400 (1.00 g) was dispersed in water
2
5
2 5
containing PdCl (Pd: 5.0 mg) dissolved in dilute hydrochloric
2
acid. Then a solution of NaBH (37.8 mg) was added drop wise
4
6
1
recently.
However, this procedure was complex and
under stirring in N2 at 273 K. The mixture was stirred for
another 30 min and washed first with water and then ethanol.
The resulted solid was dried at 323 K under vacuum for 24 h.
Ca-Al. The Ca-Al was prepared according to our previous
contained multisteps to obtain vinyl-functionalized SBA-15. In
this paper, water saturated by cyclohexene was proved to be a
preferable reductant for Pd catalysts given to the simple
1
0
procedure and effective usage. The as-synthesized Pd/Nb O ,
2
5
report. Typically, Al(NO ) ·9H O (37.5 g) and Ca(NO ) ·4H O
3 3 2 3 2 2
with low Pd loading (0.61 wt%) and highly dispersed Pd,
showed much higher conversion and selectivity in the
hydrogenation of HMF to HHD than traditional catalysts which
(
23.6 g) was dissolved in the deionized water. Then ammonium
-1
carbonate aqueous solution (600 mL, 0.5 mol·L ) was added
dropwise under vigorous agitation. The mixture was stirred for
30 min at room temperature, and then kept stand for 24 h at
the same temperature. The resulted precipitate was washed
by deionized water for 3 times and then dried overnight at 373
K. After grinded and sieved and the solid was then calcinated
under air to afford the Ca-Al.
were obtained by H or NaBH reduction.
4
2
O
H+
+
Pd(II)
+
H O
+
+
Pd(0)
eq. 1
2
Characterization of catalysts
X-ray diffraction (XRD). The crystal structures of supports and
catalysts were characterized by powder X-ray diffraction (XRD)
using Rigaku D/Max 2500/PC powder diffractometer with Cu
Kα radiation (λ = 0.15418 nm) at 40 kV and 40 mA in a
Experimental
Materials
HMF, 5-methylfuran-2-carbaldehyde, Na PO , MgO, MCP, Ru/C
4
3
o
scanning rate of 5 /min.
and Pt/C were purchased from Aladdin Chemistry Co. Ltd..
Palladium chloride (PdCl ), Pd/C, (5-methylfuran-2-yl)methanol
and 2,5-di(hydroxymethyl)furan were obtained from J&K
Chemical Ltd.. Nb was purchased from Sinopharm Chemical
Reagent Co., Ltd.. HF, cyclohexene, triethylamine, Na CO and
Elemental analysis. The content of Pd on the catalysts was
measured by inductively coupled plasma atomic emission
spectroscopy (ICP-AES) conducted on CPS-8100, Shimadzu.
Fourier transform infrared (FT-IR). The spectra were collected
on a Bruker Tensor 27 FT-IR spectrometer in KBr media.
Samples were thoroughly dried before measured.
2
2 5
O
2
3
NaOH were obtained from Tianjin Kermel Chemical Reagent
Development Center. Deionized water in all experiments was
purified by a Milli-Q system. All other reagents were
commercially available and used as received.
Temperature-programmed desorption of NH
experiments were performed on a Micromeritics AutoChem II
920 Instrument with a thermal conductivity detector (TCD)
and MS detector to monitor the desorbed NH . Samples was
first pretreated at 473 K and then NH was adsorbed at 373 K
in a flow of He. After the physical adsorpted NH was removed,
3 3
(NH -TPD). The
2
Preparation of catalysts
3
Nb
report. Typically, Nb
aqueous solution (45 mL, 40 wt%) by stirring the mixture at
2
O
5
-x. The Nb
2
O
5
was synthesised according to previous
4
0
3
2 5
O
(10.00 g) was dissolved in HF
3
2
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the temperature was increased from 373 to 1073 K in an 1,2,4,5-tetramethylbenzene (TMB) or tetrahydrofurfuryl
atmosphere of He and the desorbed NH
and MS.
3
was recorded by TCD alcohol was used as the internal standard. The products were
identified by Agilent 6890N GC/5973MS as well as by
X-ray photoelectron spectroscopy (XPS). The measurements comparison with the retention times of the respective
were conducted on the Thermo ESCALAB 250Xi by using Mg Kα standards in GC traces.
(
1253.6 eV) radiation source and a chamber pressure lower The quantification of products were determined based on GC
−
10
than 5×10 mbar. The binding energy (BE) was adjusted by data using internal standard method and the mass of isolated
the binding energy of C1s. products by column chromatography method. The conversion
Transmission electron microscopy and Energy dispersive X- of (%), selectivity (%) and isolated yield (%) were calculated as
ray spectroscopy (TEM-EDX). The microstructure of the below.
materials was examined by transmission electron microscopy
TEM) on a FEI Tecnai G2 F20 operating at 200 kV with an EDX
Molesof substrate
Conversion(%) = (1-
)×100%
(
Molesof substrateloadedinitially
detector.
CO pulse chemisorption. The measurements were conducted
on Micromeritics AutoChem 2920 apparatus. The sample were
pretreated at 393 K under flowing of He and then reduced at
Moles of products
Selectivit y (%) =
×100%
Moles of substrate converted
Moles of isolated products
2
373 K by H . After that, 1 vol.% CO/He were pulsed over the
Isolated yield (%) =
× 100%
catalyst bed every 5 min at 323 K.
Moles of substrate
Catalytic reactions
Hydrogenation of HMF. The catalytic reactions were
performed in a 20 mL stainless steel autoclave equipped with a
magnetic stirrer, a pressure gauge and automatic temperature
Results and Discussion
Characterization of catalyst
control apparatus. In a typical procedure, catalyst (20 mg), The XRD patterns of Nb
HMF aqueous solution (2.0 g, HMF: 1 mmol) and magnetic samples showed characteristic diffraction peaks of Nb
dipole was put in the reactor. After the reactor was sealed and according to the standard value (PDF#30-0873). The
purged with H
for 4 times to exclude air, the reactor was commercial Nb had sharp diffraction peaks due to the high
charged with H
to desire pressure. Then the autoclave was degree of crystallization. The crystallization degree of as-
heated to the desired temperature and the reaction began by synthesized Nb -x increased with the calcined temperature
starting the stirring. After reaction, the reaction mixture was revealed by the XRD patterns. Nb -200 had weak broad
centrifugated and the liquid phase was diluted and analyzed by diffraction peaks corresponding to Nb . The intensity
GC after adding the internal standard (1,2,4,5- increased and the width reduced as the calcination
2 5
O were shown in Fig. 1. All the
2 5
O
2 5
O
2
2
2 5
O
2 5
O
2 5
O
tetramethylbenzene).
temperature increased. The Nb
2 5
O -x calcined at lower
o
O (10.0 g), HHD (130 mg, temperature had an additional peak at 13-14 . This should be
Conversion of HHD to MCP.
mmol) and tetrahydrofurfuryl alcohol (60 mg, internal ascribed to (120) crystalline planes of the niobium hydrogen
H
2
1
standard) was added in a 50 mL round flask. Then the bases oxide (HNb , PDF#30-083). This peak decreased as the
3
O
8
were added and the reaction started by conducting under calcination temperature increased and disappeared when the
reflux or at 298 K. After reaction, the liquid was neutralized temperature was 773 K. This should be ascribed to the
dehydration of niobium hydrogen oxide at evaluated
(not needed for MgO and Ca-Al) and diluted for GC analysis.
Conversion of HMF to MCP. The first step was the same temperature.
procedure as the hydrogenation of mentioned above. After
reaction, the liquid phase was divided in two parts. One part
was diluted and analyzed by GC after adding the internal
standard (1,2,4,5-tetramethylbenzene). The other part (1.00 g)
was added into H
added the reaction started by conducting under reflux or at
98 K. After reaction, the liquid was neutralized by HAc (not
needed for Ca-Al)and extracted by CH Cl . The mixture was
2
O (10 mL). After base (NaOH or Ca-Al) was
2
2
2
separated by column chromatography to give the white
powder final products (MCP).
Products analysis
Gas chromatography measurements were conducted on
Agilent 7890A GC with autosampler and a flame ionization
detector. HP-5 capillary column (30 m × 320 μm × 0.25 μm)
was used for the separation of reaction mixtures. The
temperature of the column was initially kept at 393 K for 3 min,
-1
and then was increased at a rate of 20 K min to 493 K.
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3 2 5 2 5
Fig. 2 The NH -TPD profiles of Nb O (a) and Nb O -400 (b).
The Nb O supported Pd catalysts were prepared through
2
5
the reduction of PdCl by cyclohexene saturated water. The
2
surface bonded vinyl had been proved to be an effective
1
6
reductant for the reduction of PdCl in our earlier work.
2
Herein, the surface bonded vinyl was replaced by cyclohexene
saturated water. For comparison, Pd/Nb O -400-IP and
2
5
Pd/Nb O -400-B were also prepared by H and NaBH4
2
5
2
reduction method respectively. The XRD patterns of the Pd
catalysts were shown in Fig. 3. No obvious changes were
observed on the supports after the introduction of Pd by the
comparison of Pd catalysts (Fig. 3) and the supports (Fig. 1).
The crystal structure of the supports was well kept after the
introduction of palladium. All the catalysts showed no
characteristic diffraction peaks of palladium. These could be
attributed to the low Pd loading (Table S2) and well dispersed
Pd on the surface of the supports.
2 5
Fig. 1 The XRD patterns of Nb O .
The surface acidity of the commercial Nb O and as-
5
2
synthesized Nb O -400 was detected by the NH -TPD (Fig. 2).
2
5
3
Both the TCD signal and the mass spectrum signal were
collected. No desorption of NH3 was observed during the
whole temperature range for the commercial Nb O (Fig. 2a).
2
5
This result indicated that the commercial Nb
acidity. In contrast, the Nb -400 had a desorption peak
around 630 K (Fig. 2b) and the MS detector showed the main
desorption substance was NH = 17) as well as a little H
= 18). These results indicated that the Nb -400 had
2 5
O did not have
2 5
O
3
(
m
/z
2
O
(m
/z
2 5
O
strong acidity which was introduced during the preparation
process.
4
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ARTICLE
Fig. 3 The XRD patterns of Pd catalysts.
spectra also gave the clue for the existence of Pd. This should
be due to the high disperse of Pd on the support. The high
disperse of Pd was also supported by the CO pulse
chemisorption experiments (Fig. S3). The calculated Pd
dispersion was 34% based on the CO chemisorption on the
catalysts. The The employing of cyclohexene as reductant was
responded for this high disperse. Firstly, the cyclohexene had
weaker reducing power comparing to other reductant such as
The presence of palladium species as well as the oxidation
state of palladium on the supports could be testified by the X-
ray photoelectron spectroscopy (XPS). The 3d spectra of Pd
had been collected and analyzed and the results were
summarized in Fig. 4 and Table S1. All the samples showed
binding energy peaks at 335 eV and 340 eV, which are the
NaBH
As a result, the reduction reaction conducted too slow to form
TEM observable Pd nanoparticles. In contrast, the Pd/Nb -B
and Pd/Nb -IP which had similar Pd loading to Pd/Nb -400
4
. And the solubility of cyclohexene in water was very low.
characteristic peaks of Pd 3
Curve fitting analysis showed that the main binding energy
peaks appeared at 334.7 eV and 339.9 eV for Pd/Nb -400.
and these peaks should be attributed to Pd(0) according to the
d5/2 and Pd 3d3/2, respectively.
2 5
O
2 5
O
2
O
5
2 5
O
6
1, 62
(Table S2) showed clear Pd nanoparticles (Fig. 5b and 5c). In
addition, the HRTEM inset and the EDX spectra also verified
the Pd nanoparticles on the Pd/Nb O -B and Pd/Nb O -IP.
literature reports.
peaks at 336.7 eV and 341.8 eV were observed, with the
ownership of 3 3/2 of Pd(II). The results indicated
that the palladium species for Pd/Nb -400 was primarily
Additionally, two weak binding energy
2
5
2 5
d
5/2 and 3d
2 5
O
Pd(0) while the Pd(II) should be ascribed to the oxidation of
Pd(0) in the air atmosphere. Similar results could be observed
2 5 2 5
on the Pd/Nb O -B and Pd/Nb O -IP.
2 5 2 5 2 5
Fig. 4 The XPS spectra of Pd/Nb O -400,Pd/Nb O -B and Pd/Nb O -IP.
2 5 2 5 2 5
The morphology of Pd/Nb O -400, Pd/Nb O -B and Pd/Nb O -
IP was characterized by the TEM images as well as HRTEM. As
the results of XPS showed the presence of Pd, it is surprising
that no Pd nanoparticle was observed after 50 TEM or HRTEM
images were checked for Pd/Nb
2 5
O -400 (Fig. 5a, 5d). The
HRTEM images only exhibited the lattice for Nb O . The EDX
2
5
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a)
b)
c)
2 5 2 5 2 5
Fig. 5 The TEM images and EDX spetra of Pd/Nb O -400 (a, d), Pd/Nb O -B (b, e) and Pd/Nb O -IP(c, f). Insets showed HRTEM images for the corresponding
samples.
The Hydrogenation of HMF to HHD
The hydrogenation of HMF to HHD was conducted on the Pd/C as the catalyst by the addition of acid or base additives.
catalyst prepared above in water in the presence of H and the After 1 mmol of HAc was introduced, the selectivity of HHD
results were summarized in Table 1. When the commercial increased from 30% to 61%. On the contrary, no HHD was
Nb was used as support, both the conversion of HMF and detected after 1 mmol NaOH was added at the same reaction
the selectivity of HHD were very low (Table 1, Entry 1). The by- conditions.
products were chiefly 2,5-hexanedione, 2,5- For comparison, the Pd/Nb
di(hydroxymethyl)furan (DHMF) and tetrahydro-2,5- Pt/C catalysts were also tested for this reaction. The Pd/Nb
furandimethanol (THFDM). When Nb -x were used as B and Pd/Nb -IP gained 67% and 65% selectivity of HHD
supports, the selectivity of HHD increased from 31% to 61-85% respectively while the Pd/C gave 30% selectivity (Table 1,
Table 1, Entries 2-5). The enhanced selectivity should be Entries 6-8). These results revealed the advantage of Nb -x
ascribed to the acidity of the Nb -x. The addition of acid was as support for the hydrogenation of HMF to HHD. However,
necessary for the reported homogeneous or heterogeneous the conversion of HMF was 19% and 18% for Pd/Nb -B and
-IP respectively which were much lower than that of
-400 (56%) (Table 1, Entries 6 and 7 vs Entry 4). As
2
2 5
O
2 5 2 5
O -B, Pd/Nb O -IP and Pd/C, Ru/C,
2
5
O -
2
O
5
2 5
O
(
2 5
O
2 5
O
2 5
O
catalytic system for this reaction. It is widely accepted that the Pd/Nb
acid facilitates the ring-opening procedure during the Pd/Nb
2
2
O
O
5
5
3
8
reaction. The NH
x had acidity after the prepared procedure. As a result, the catalysts (Table S2), the main reason for this difference in
Nb -x worked both as support for Pd and as the acid catalyst. conversion was the high disperse of Pd on the Pd/Nb -400
And no additional acid was needed for the high selectivity of prepared by the cyclohexene reduction method. The Ru/C and
HHD when Pd/Nb -x was used as the catalyst. To verify the Pt/C did not give good results for this reaction with the
3
-TPD results (Fig. 2) showed that the Nb
2
O
5
-
the Pd loading determined by ICP-AES was similar for the three
2
O
5
2 5
O
2 5
O
importance of acid site, the reaction was performed employing selectivity of 13% and 10% respectively.
a
Table 1 Hydrogenation of HMF to HHD over different catalysts.
6
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ARTICLE
Selectivity of Products (%)
DHMF
Entry
catalysts
Pd/Nb
Conversion (%)
b
HHD
HD
THFDM
Others
1
2
3
4
5
6
7
8
9
2
O
5
35
14
39
56
45
18
19
43
80
98
98
99
31
71
67
85
71
65
67
10
13
30
61
N.D.
12
13
6
13
3
17
5
27
8
Pd/Nb
Pd/Nb
Pd/Nb
Pd/Nb
2
2
2
2
O
5
5
5
5
-200
-300
-400
-500
O
O
O
8
2
17
9
4
1
<1
2
9
4
14
16
18
38
23
39
20
60
Pd/Nb
2
O
5
-400-IP
-400-B
12
11
7
4
3
Pd/Nb
2
O
5
2
2
Pt/C
Ru/C
44
2
1
2
60
20
6
1
1
1
0
1
2
Pd/C
5
6
c
Pd/C + HAc
12
<1
c
Pd/C + NaOH
N.D.
7
33
a
b
Reaction conditions: catalyst (20 mg), 2.00 g HMF aqueous solution (HMF: 1 mmol), 413 K, 4 MPa H2, 2 h. The others referred to 2,5-hexanedione, 2,5-
c
dimethyltetrahydrofuran and unidentified products, 1 mmol, N.D. not detected.
Effect of H pressure
Effect of Temperature
2
The effect of H pressure on the hydrogenation of HMF to HHD
The effect of temperature on the hydrogenation of HMF
2
in water was shown in Fig. 6. The selectivity of HHD were between in water was also investigated and the results were shown in
7% and 90% when the pressure varied from 1 MPa to 5 MPa. It is Table 2. The reaction could conduct at a temperature as low as
7
noteworthy that 90% selectivity of HHD could be obtained when 393 K, and 37% conversion as well as 82% selectivity of HHD
the H pressure was 1 MPa. This indicated that the H pressure did were obtained after 2 h at 4 MPa (Table 2, Entry 1). The
2
2
not influence the selectivity of HHD in the pressure range. However, conversion increased with the temperature increased which
the conversion of HMF were promoted from 21% to 57% when the was accordance to our expectations. The selectivity of HHD
H
2
pressure increased from 1 MPa to 5 MPa.
decreased when the temperature was higher than 413 K. This
should be attributed to the more side reaction at higher
temperature.
a
Table 2 Hydrogenation of HMF to HHD under different temperature.
Selectivity of products (%)
Entry
Temperature (K)
Conv. (%)
HHD
Others
1
2
3
4
5
393
403
413
423
433
37
42
56
71
82
82
75
85
54
61
18
25
15
46
39
a
2 5
Reaction conditions: Pd/Nb O -400 (20 mg), 2.00 g HMF aqueous solution
(
1 mmol), 4 MPa H , 2 h.
2
Effect of reaction time
Fig. 6 Hydrogenation of HMF to HHD at different H
conditions: Pd/Nb -400 (20 mg), 2.00 g HMF aqueous solution (1 mmol)
13 K, 2 h.
2
pressure. Reaction
The influence of time on the hydrogenation reaction was
studied by conducting the reaction for a specified time. The
results were shown in Fig. 7. It is logical that the conversion of
HMF increased as the time prolonged. The conversion of HMF
was 28% when the reaction time was 1 h. The conversion
2
O
5
4
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Journal Name
a
Table 3 Hydrogenation of HMF on Pd/Nb
2
O
5
-400 with different Pd contents.
increased steadily as the reaction time prolonged. The
conversion of HMF reached 93% when the reaction lasted for 6
h. However, the selectivity of HHD remained stable around 80% Entry
as the reaction time changed from 1 h to 6 h. The selectivity of
Selectivity of products (%)
b
Pd contents (wt%)
Conv. (%)
HHD
Others
1
0.61
0.74
1.94
2.48
4.62
4.62
56
78
85
74
77
73
69
71
15
26
23
27
31
29
HHD was 83% at low conversion of 28%. When the conversion
2
of HMF reached 93% the selectivity of HHD was still as high as
3
84
73%.
4
5
97
Based on the study of the effect of H
temperature and the reaction time, it had been found that the
reaction could conduct at mild condition on Pd/Nb -400.
2
pressure,
>99
91
c
6
2 5
O
a
Reaction conditions: Pd/Nb
2
O
5
-400 (20 mg), 2.00 g HMF aqueous solution
And the selectivity of HHD kept high even at the conversion of
HMF over 90%.
b
c
(
1 mmol), 4 MPa H
2
, 2 h, determined by ICP-AES, 0.5 MPa H
2
, 8 h.
Catalyst recycling
The recyclability of the Pd/Nb O -400 was elucidated in Fig. 8
2
5
and the reaction time was fixed at 6 h to achieve the high
conversion of HMF over 90%. The catalyst was reused for 4
times. The catalyst was centrifuged and fully washed after
each cycle. And then the catalyst was dried at 323 K under
vacuum and used for the next run. It was found that the
conversion of HMF and selectivity of HHD kept stable in all the
four cycles from Fig. 8. The selectivity of HHD was 71% even in
the fourth cycle with 89% conversion. This examination
illustrated the high stability of the catalyst under the reaction
conditions.
Fig. 7 Time-course of HMF hydrogenation to HHD. Reaction conditions:
2 5
Pd/Nb O -400 (20 mg), 2.00 g HMF aqueous solution (1 mmol) 413 K, 4 Mpa
2
H .
Effect of Pd contents
The catalysts with different contents of Pd could be
obtained by changing the amount of water saturated by
cyclohexene and PdCl . The actual contents of Pd were
2
determined by ICP-AES and the results were shown in Table 3.
The Pd contents could be varied from 0.61 wt% to 4.62 wt%.
The catalysts were used for the hydrogenation of HMF. The
conversion of HMF increased from 56% to 99% as the contents
of Pd increased. This phenomenon should be ascribed to the
increase of active sites in catalysts with the increase of Pd-
loading amount. As the catalyst with 4.62 wt% contents of Pd
had the highest Pd loading, this catalyst could convert HMF to
Fig. 8 Cycle experiments for the hydrogenation of HMF in water. Reaction
2 5
conditions: Pd/Nb O -400 (20 mg), HMF aqueous solution (2.00 g, 1 mmol)
4
13 K, 4 MPa H , 6 h.
2
2
HHD at a very low H pressure (Table 3, Entry 6). The Pd
contents did not affect the selectivity to HHD obviously. All the
catalysts had high selectivity to HHD, and the catalyst with
Possible reaction pathway
Different substrates were used instead of HMF to find the
possible reaction pathway and the results were shown in Table
0.61 wt% Pd contents had the best selectivity to HHD (Table 3,
Entry 1).
4. 54% selectivity of HHD was achieved when DHMF was used
as the substrates (Table 4, Entry 2). The DHMF should be one
of the intermediates from HMF to HHD. When 5-methylfuran-
2
-carbaldehyde (MFA), (5-methylfuran-2-yl)methanol (MFOL)
and 2,5-dimethylfuran (DMF) was used as substrates
respectively, no HHD was detected after 2 h reaction. Instead,
HD was found to be the main products in these cases (Table 4,
8
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ARTICLE
Entries 3-5). The MFOL and DMF gave much lower selectivity
The conversion of HHD to MCP is an aldol condensation
to HD than the MFA. This should be caused by the increased reaction as illustrated in Scheme 3. Due to the more acidity of
side reactions at high concentration of MFOL or DMF. 5,5'- C1-H, C1 could attack C5 to accomplish a five-memebered ring
(oxybis(methylene))bis(2-methylfuran), the dimer of MFOL, II with the assistance of base. As the C5 of the intermediate III
was easy to be produced when MFOL was used as the had no H atom, the dehydroxylation should conduct at C1.
3, 64
6
substrate.
The 2,5-dimethyltetrahydrofuran emerged as Then, the dehydration took place to give more stable product
the by-product when DMF was the substrate. At the standard MCP and another dehydration product was not detected.
reaction conditions, the HHD kept stable and almost no HHD These must be the reasons for the high selectivity to MCP.
was transformed after 2 h reaction at 413 K (Table 4, Entry 6).
a
2 5
Table 4 Hydrogenation of different substrates on Pd/Nb O -400.
Selectivity of products (%)
Entry
Substrates
Conv. (%)
HHD
85
HD Others
1
2
3
56
78
78
4
2
15
44
28
54
Scheme 3 The conversion of HHD to MCP.
N.D.
72
4
5
>99
>99
N.D.
N.D.
26
35
74
65
Effect of bases
The effect of different bases was studied by performing
the aldol condensation reaction in water employing different
bases as catalysts and the results were summarized in Table 5.
Most of the alkaline substance was effective for this aldol
condensation reaction. The reaction could conduct smoothly
6
2
97
N.D.
3
a
Reaction conditions: Pd/Nb
2 5
O -400 (20 mg), 2.00 g aqueous solution of
substrate (1 mmol), 4 MPa H , 2 h, 413 K. N.D.: not detected.
2
-1
even at 298 K when the 0.15 mol L NaOH was used with full
Based on the results mentioned above and the conversion and 90% selectivity (Table 5, Entry 1). When
3
3, 35, 38
literatures
, we proposed the possible reaction pathway weaker base was empolyed for this reaction, higher
for the hydrogenation of HMF to HHD as shown in Scheme 2. temperature or/and longer time should be adopted. Satisfying
Firstly, the HMF was hydrogenated to DHMF on the Pd catalyst results were achieved in the case of Na CO and Na PO with
2
3
3
4
2
in the presence of H . And then the DHMF was converted to 87% and 91% yields under reflux (Table 5, Entries 3 and 4). The
HHD by the cooperation of Pd catalysts and acid sites. The acid organic bases such as triethylamine were also effective for this
sites were responsible for the opening of the furan rings of reaction with 89% selectivity of HHD and 83% conversion of
DHMF and the most likely product was 1-hydroxyhex-3-ene- HHD (Table 5, Enter 5). Solid bases should be a good options
2,5-dione while the Pd catalysts behaved as hydrogenation for the reaction. 74% selectivity of MCP was obtained with 47%
catalysts. One of the by-products was THFDM which was conversion of HHD when MgO was used (Table 5, Enter 6). To
produced through the hydrogenation of THFDM. Another by- our delight, 87% selectivity of MCP with full conversion of HHD
product detected was HD which should be produced by the was achieved when the Ca-Al was used as catalyst (Table 5,
hydrolysis of 2,5-dimethylfuran.
Enter 2). As a solid base, Ca-Al meets the green principle better
and could give a comparable good results. What is more, the
pH value of the reaction liquid was almost 7 after 2 h reaction
when Ca-Al was used. Therefore, no additional acids were
needed to neutralize the reaction mixture. These can be
accout for the Ca-Al to be chosen as the best candidate of base.
a
Table 5 The conversion of HHD to MCP by different bases
.
Concentration Temperature
Entry Catalsyts
Conv. (%) Select. (%)
-
mol L )
1
(
(K)
Scheme 2 The proposed reaction pathway for the hydrogenation of HMF to
HHD.
1
2
3
4
5
6
NaOH
b
Ca-Al
0.15
c
200
298
>99
>99
>99
>99
83
90
87
87
91
89
74
reflux
reflux
reflux
reflux
reflux
Na
Na
2
CO
3
4
0.10
0.10
0.10
3
PO
Et
3
N
c
Conversion of HHD to MCP
MgO
200
47
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Journal Name
a
2
Reaction conditions: HHD (130 mg), H O (10.0 g), 1 atm air, 298 K or
b
c
reflux, 2 h , 6 h, 200 mg.
Effect of solvent and reaction time
We also evaluated the reaction time for the conversion of
HHD to MCP in several solvents and the results were shown in
Fig. 9a. The HHD was completely transformed after 6 h when
the reaction was performed in water and in n-butanol. As the
reaction was performed under reflux condition, the low
conversion of HHD for ethanol might be attributed to the low
boiling point of ethanol (351 K). Though toluene and methyl
isobutyl ketone (MIBK) had similar boiling point to n-butanol,
the conversion was very low for both solvents (toluene and
MIBK) after 6 h. Similarly, the dioxane, though had a boiling
point close to water, gave very low conversion of HHD. As a
result, the protonic solvent should be a favour for this aldol
condensation reaction. However, the selectivity of MCP was
not affected sharply by the time and the nature of the solvent.
The selectivity of MCP kept between 80-90% in different
solvent as the time prolonged (Fig. 9b).
Fig. 9 Conversions (a) and selectivity (b) at different reaction time for
the conversion of HHD to MCP in different solvents.
Conversion of HMF to MCP
The conversion of HMF to MCP undergoes through two
steps as expatiated in the experimental section and in Scheme
1. And the results were summarized in Table 6. The first step
2 5
was the hydrogenation of HMF to HHD on Pd/Nb O -400 and
the next step was the conversion of HHD to MCP through aldol
condensation catalyzed by base in water. The conversion of
HMF varied from 90% to 93% while the selectivity changed
from 68% to 73% for the Step 1 showing the satisfying
repeatability (Table 6, Entries 1-5). It had been discussed
above that the selectivity of MCP from HHD was usually very
high. As a result, the isolated yield of MCP most depended on
the results of the Step 1. The isolated yield of MCP varied from
4
9% to 58% according to the base. The best isolated yield of
MCP was 58% when Ca-Al was used (Table 6, Entry 1).
a
Table 6 The Conversion of HMF to methyl cyclopentenolone via a two-steps peocedure
Step 1
.
Step 2
Entry
Selectivity
of HHD (%)
Concentration
-1
(mol L )
Isolated yield of
c
MCP (%)
Conv. (%)
b
catalysts
Temperature (K)
d
e
1
2
3
4
5
93
92
91
90
93
73
71
70
71
68
Ca-Al
200
reflux
reflux
298
58
55
57
53
49
d
e
Ca-Al
200
NaOH
0.15
0.10
0.10
Na
Et
2
CO
3
reflux
reflux
3
N
a
Reaction conditions: Pd/Nb
2 5 2 2
O -400 (20 mg), 2.00 g HMF aqueous solution (1 mmol), 413 K, 4 MPa H , 6 h for HMF hydrogenation and H O (10.0
b
g), reflux or 298 K for the conversion of HHD to MCP , The selectivity of HHD, The isolated yield of MCP based on HMF, 6 h, 200 mg.
c
d
e
cyclohexene reduction method with high disperse of Pd, were
used for the hydrogenation of HMF to HHD in water. The high
Conclusions
disperse of Pd was responsible for the high conversion of HMF
-x possessing acidity was and the acidity of Nb O -x leads to the high selectivity of HHD.
In summary, a series of Nb
2
O
5
2
5
synthesized and used as support for the preparation of Pd Finally, the HHD was converted to MCP, a kind of valuable
catalysts. The Pd catalysts, synthesized through the edible essence, catalyzed by a solid base Ca-Al in water with
1
0 | J. Name., 2012, 00, 1-3
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5
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This work was supported by the National Natural Science
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Green Chemistry
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DOI: 10.1039/C7GC02310C
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