Conversion of furfural into cyclopentanone over Ni-Cu bimetallic catalysts

Article abstract of DOI:10.1039/c3gc37133f

The conversion of furfural into cyclopentanone over Ni-Cu bimetallic catalysts was studied under hydrogen atmosphere. Furfuryl alcohol, 4-hydroxy-2-cyclopentenone and 2-cyclopentenone were verified as three key intermediates. Rearrangement of the furan ring was independent of hydrogenation, starting from furfuryl alcohol rather than furfural. The opening and closure of the furan ring were closely related to the attack of a H₂O molecule on the 5-position of furfuryl alcohol. Prior hydrogenation of the aldehyde group accounted for the different reactivity of furfural and furfuryl alcohol. The high selectivity of cyclopentanone was ascribed to the presence of 2-cyclopentenone.

Full text of DOI:10.1039/c3gc37133f

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Conversion of furfural into cyclopentanone over Ni–Cu
bimetallic catalysts†
Cite this: Green Chem., 2013, 15, 1932
Yanliang Yang,‡a,b Zhongtian Du,‡a Yizheng Huang,a Fang Lu,a Feng Wang,a
Jin Gaoa and Jie Xu*a
The conversion of furfural into cyclopentanone over Ni–Cu bimetallic catalysts was studied under hydro-
gen atmosphere. Furfuryl alcohol, 4-hydroxy-2-cyclopentenone and 2-cyclopentenone were verified as
three key intermediates. Rearrangement of the furan ring was independent of hydrogenation, starting
from furfuryl alcohol rather than furfural. The opening and closure of the furan ring were closely related
to the attack of a H2O molecule on the 5-position of furfuryl alcohol. Prior hydrogenation of the alde-
hyde group accounted for the different reactivity of furfural and furfuryl alcohol. The high selectivity of
cyclopentanone was ascribed to the presence of 2-cyclopentenone.
Received 30th December 2012,
Accepted 11th April 2013
DOI: 10.1039/c3gc37133f
www.rsc.org/greenchem
chemicals. Currently, most CPO is produced through decarboxy-
lation of adipic acid at high temperature.11 Considering that
Introduction
Recently, transformation of biomass into useful chemicals has both CPO and furfural have five carbon atoms, we are inter-
attracted much attention.1 Carbohydrate biomass could be ested in the conversion of furfural into CPO. During the prepa-
converted into furanics via catalytic dehydration process.2–4 ration of this manuscript, Hronec et al. described that furfural
For example, 5-hydroxymethylfurfural (HMF) could be pre- could be converted into CPO through hydrogenation over Pt,
pared from hexoses, such as fructose and glucose,2 while fur- Pd, Ni, and Ru catalysts.12 Some results were in good agree-
fural could be obtained from pentoses.3,4 Unlike hydrocarbons, ment with our observation. However, several important issues
carbohydrates and their furan derivatives readily coexist with need further investigation. For example, the formation pathway
water due to oxygen atoms in their molecular structures. Water through 4-hydroxy-2-cyclopentenone (HCP), the different reac-
is also a cheap, non-toxic and non-flammable solvent,5 thus tivities of furfural and FA, the high selectivity for CPO, and the
transformation of furanics into valuable oxygenated com- reaction activity of other furan compounds. This transform-
pounds in water is of great importance.
ation over other catalysts also needs further study.
Furfural is viewed as one of the key platform molecules in
Ni–Cu bimetallic catalysts are known for gas-phase hydro-
biomass conversion.1 In industry, furfural is produced from genation of furfural; however, they exhibit a high selectivity
agricultural raw materials rich in pentosan, such as corncobs, towards the formation of FA rather than CPO.10 Both nickel
oat hulls, bagasse, etc.4 Generally, furfural could be converted and copper have face-centered cubic structures with similar
into useful chemicals through oxidation, hydrogenation and lattice parameters, and a copper–nickel binary alloy would
resinification.4,6–10 Furfuryl alcohol (FA), tetrahydrofurfuryl exhibit significantly different catalytic activity compared with
alcohol (THFA) and 2-methylfuran (MF), which contain a five- Ni or Cu monometallic catalysts.13 On the basis of our recent
membered heterocyclic ring with one oxygen atom, are typical work on bimetallic catalysts,14 herein we focused on Ni–Cu
products through selective hydrogenation.7–10 In contrast, con- bimetallic catalysts supported on SBA-15 for the conversion of fur-
version of furfural into alicyclics containing five-membered fural into CPO in an aqueous medium under a H2 atmosphere.
carbon rings remains to be explored.
We presented a clear formation pathway of CPO from furfural,
Cyclopentanone (CPO) is an important chemical intermedi- studied the different reactivity of furfural and FA, and eluci-
ate to produce pharmaceuticals, insecticides and rubber dated the higher selectivity for CPO than cyclopentanol (CPL).
aState Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
Experimental
P. R. China. E-mail: xujie@dicp.ac.cn; Fax: (+) 86-411-84379245
bUniversity of Chinese Academy of Sciences, Beijing 100049, P. R. China
†Electronic supplementary information (ESI) available: Catalyst characterization,
Materials
Furfural and furfuryl alcohol were of analytical grade and dis-
tilled under reduced pressure before use. Deionized water in
additional results and GC traces. See DOI: 10.1039/C3GC37133F
‡YL Yang and ZT Du contributed equally to this work.
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all experiments was purified by a Milli-Q system (Millipore). of the column was initially kept at 80 °C for 3 min, and
H218O (97% 18O abundance) was obtained from Shanghai then was increased at a rate of 20 °C min−1 to 220 °C. 1,2,4,5-
Research Institute of Chemical Industry. D2O (99.9% D abun- Tetramethylbenzene (TMB) and cyclopentane were used as the
dance)) was purchased from Beijing SeaskyBio Technology Co., internal standard for liquid and gas phase products, res-
Ltd. Tetraethyl orthosilicate was obtained from J&K Chemical pectively. The products were identified by Agilent 6890N
Ltd, and poly(ethylene glycol)-block-poly(propylene glycol)- GC/5973MS as well as by comparison with the retention times
block-poly(ethylene glycol) (P123, AR) from Sigma-Aldrich. of the respective standards in GC traces.
Nickel nitrate (Ni(NO3)2·6H2O) and cupric nitrate (Cu(NO3)2·
The amounts of products were determined based on GC
3H2O) of analytical grade were obtained from Tianjin Kermel data using the internal standard method. All results were evalu-
Chemical Reagent Development Center and used as received. ated on the basis of the amount of furfural. The conversion
All other reagents were commercially available.
of furfural (mol%), the yield (mol%) and selectivity (mol%) of
main products were calculated as below.
Preparation of catalysts
ꢀ
ꢁ
ꢁ
Moles of furfural
Moles of furfural loaded initially
Conversion ¼ 1 ꢀ
ꢁ 100%
ꢁ 100%
ꢁ 100%
SBA-15. The mesoporous silica SBA-15 was synthesized
according to the previous report by Zhao et al.15 The P123 was
removed by calcination in the air at 500 °C for 4 h. The corres-
ponding XRD, BET and TEM results are shown in ESI Fig. S1–
S6 and Table S1.†
ꢀ
Moles of product
Moles of furfural converted
Selectivity of product ¼
ꢀ
ꢁ
NiCu/SBA-15. The Ni–Cu bimetallic catalysts were prepared
by a method reported previously,16 and named NiCu-n/SBA-15
(Ni metal loading: 10 wt%, Cu : Ni = n% in atomic ratio). Take
the preparation of NiCu-50/SBA-15 for example: 1.00 g of
SBA-15 was added to 30 mL of cyclohexane under stirring at
room temperature. After the SBA-15 was dispersed, 2.85 g
aqueous solution of Ni(NO3)2·6H2O (0.4905 g, 1.70 mmol) and
Moles of product
Moles of furfural loaded initially
Yield of product ¼
Results and discussion
Cu(NO3)2·3H2O (0.2054 g, 0.85 mmol) were added dropwise Hydrogenation of furfural over different catalysts in water
under stirring. The mixture was kept stirring for 30 min, and
We began our study of hydrogenation of furfural using water
the solid residue was centrifuged and dried at 120 °C over-
as a solvent (Scheme 1). 39% selectivity of CPO was detected
night, then calcined at 500 °C for 4 h. The catalyst was named
with 46% conversion of furfural over Ni/SBA-15 within 4 h
NiCu-50/SBA-15, where 50% is the molar ratio of Cu/Ni. Before
under 4 MPa H2. After optimizing the supports and compo-
use, the catalyst was reduced by H2 at 400 °C for 2 h and
sition of catalysts (Fig. 1 and Table S2†), we found that Ni–Cu
bimetallic catalysts supported on SBA-15 enhanced both the
cooled to room temperature under a H2 atmosphere. The
corresponding XRD, BET and TEM results are shown in
conversion of furfural and selectivity of CPO. Over NiCu-50/
Fig. S1–S6 and Table S1.† Other Ni-based catalysts were pre-
SBA-15 catalyst, nearly complete conversion of furfural was
achieved with 62% selectivity of CPO and 3% of CPL within
pared by incipient wetness impregnation.
4 h. Besides these alicyclic compounds, 17% of 2-methyltetra-
Typical procedure for catalytic hydrogenation of furfural
hydrofuran (MTHF) was also detected. The carbon loss was
Catalytic hydrogenation of furfural was performed in a 60 mL
observed and this was attributed to the inherent properties of
stainless steel autoclave equipped with a magnetic stirrer, a
furfural and FA, which will be discussed in the following
pressure gauge and automatic temperature control apparatus.
section. However, little amounts of FA and THFA were detected
The reactor was connected to a hydrogen cylinder of the reac-
by GC after 4 h (Fig. 1; Fig. 4, left line a). Thus the product
tion pressure. In a typical experiment, 10.0 g aqueous solution
of furfural (0.5 g, 5.2 mmol) and catalyst (0.2 g) were loaded
into the reactor. Then it was sealed and purged with H2 for 4
times to exclude air. When the autoclave was heated to the
desired temperature, H2 (4 MPa) was charged into the reactor.
After reaction, the autoclave was cooled. The gas was released
and collected in a gas bag for subsequent GC analysis. After
centrifugation, the liquid reaction mixture was diluted and
analyzed by GC after adding the internal standard.
Products analysis
Gas chromatography measurements were conducted on
Agilent 7890A GC with autosampler and a flame ionization
detector. A HP-5 capillary column (30 m × 530 μm × 1.5 μm)
Scheme 1 Main products during the hydrogenation of furfural in water under
was used for the separation of reaction mixtures. The temperature a hydrogen atmosphere.
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meanwhile only 9% of FA was detected. At 160 °C, the main
products were CPO and MTHF, but no FA or THFA was
observed after 4 h. Carbon balance became worse at higher
temperature (180 °C). Therefore the reaction temperature had
a great impact on the product distributions. The hydrogen-
ation of furfural to FA and THFA could proceed smoothly over
NiCu-50/SBA-15 even below 100 °C, meanwhile the transform-
ation of the furan ring into an alicyclic ring probably depends
significantly on the higher reaction temperature in this work.
Effect of different solvents and acidity of water
About 62% selectivity of CPO was observed in water with
nearly complete conversion of furfural. However, there was no
trace of CPO or CPL when CH2Cl2, CH3OH and 1,4-dioxane
were used (ESI, Fig. S7†). It is interesting that CPO appeared
when water–1,4-dioxane was used as a mixed solvent. Thus
water was the key solvent in the transformation of furfural to
CPO. This observation was in good agreement with recent
reports,12 and the role of water will be discussed in the follow-
ing section.
Fig. 1 Conversion of furfural over metal/SBA-15. Reaction conditions: 10 g fur-
fural aqueous solution (5.2 mmol), catalyst (0.2 g), 160 °C, 4 MPa H2, 4 h. Others
are mainly products due to resinification.
distributions via selective hydrogenation in aqueous solution
were quite different from those in vapour-phase reactions.9,10
The results were also interesting when the selective hydro-
genation of furfural was performed in water using acid or base
as an additive (Table 1). When Na2CO3 was added, no CPO was
detected, but FA and THFA were observed as the main pro-
ducts instead (Table 1, entry 1). When the initial aqueous solu-
tion was slightly basic, adjusted by Na2HPO4, FA and THFA
were still preferred, rather than CPO (Table 1, entry 2).
However, about 40% selectivity of CPO was obtained in
NaH2PO4 and acetic acid aqueous solutions (Table 1, entries 4
and 5). Resinification prevailed when the experiment was
carried out in strongly acidic aqueous solution at 160 °C, and
a black mixture was observed (Table 1, entry 6; Fig. S39†),
which could not be analyzed by GC. These results indicate that
weak acidic or neutral aqueous solutions are preferred for the
formation of CPO.
Products distribution at different times
We further studied the reaction progress profiles of the hydro-
genation of furfural in water on NiCu-50/SBA-15 catalyst
(Fig. 2). It is notable that more than 20% selectivity of FA was
detected within 30 min, and then diminished, finally dis-
appearing after 2 h. This suggested that FA was probably an
important intermediate. Selectivity of CPL was very low before
furfural was completely converted; and then it increased with
the decrease of CPO when the reaction time was prolonged.
Effect of reaction temperature
When the reaction temperature was 80 °C, no CPO was
detected (ESI Fig. S8;† Fig. 4, left, line b); the main products
were FA and THFA (total selectivity >80%). When the reaction
was performed at 120 °C, 43% selectivity of CPO was obtained;
Formation pathway of cyclopentanone from furfural
When a five-membered carbon ring is formed from furfural, a
rearrangement reaction is inevitable. However, when the reac-
tion of furfural was performed under N2 or H2 atmosphere
without catalyst, no CPO or other alicyclics were detected
(Table S4,† entries 3 and 4), which indicated that the
rearrangement of the furan ring does not occur directly from
furfural. It should be mentioned again that FA was detected in
the beginning (Fig. 2), and FA was probably one of the key
intermediates to form five-membered carbon ring products.
When FA was tested as a substrate under H2 atmosphere, CPO
and CPL were obtained with a total selectivity of 36% (Fig. 3).
Then HCP was observed and isolated (Fig. S10†). 59% yield of
HCP was obtained from FA under an N2 atmosphere in the
presence of catalyst, but no CPO was observed (Fig. 3). More-
over, without catalyst, FA was also converted into HCP under
either N2 or H2 atmosphere, and there was no trace of CPO or
CPL (Fig. 3). Conversion of FA into HCP should proceed via
ring opening and closure, which could partly be explained by a
Piancatelli type rearrangement.17 However, the rearrangement
Fig. 2 Reaction progress profiles. Reaction conditions: 10 g furfural aqueous
solution (5.2 mmol), NiCu-50/SBA-15 (0.2 g), 160 °C, 4 MPa H2.
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Table 1 Effect of additives on the selective hydrogenation of furfurala
Selectivity of products (%)
Entry
Additive
Conv. (%)
1
2
3
4
Na2CO3
Na2HPO4
None
NaH2PO4
AcOH
>99
>99
>99
>99
94
n.d.
2
62
40
40
—
n.d.
1
3
1
2
n.d.
n.d.
17
6
3
—
40
36
n.d.
n.d.
n.d.
—
36
28
n.d.
n.d.
n.d.
—
5
6b
H3PO4
—
—
a Reaction conditions: 10 g furfural aqueous solution (5.2 mmol), NiCu-50/SBA-15 (0.2 g), additive (0.1 mmol), 160 °C, 4 MPa H2, 4 h. AcOH:
acetic acid. Results are calculated on the basis of the amount of furfural. n.d., not detected. The initial pH values are shown in ESI Table S3.†
b 1 mmol H3PO4, black solid was observed.
conversion of HCP into CPO. When furfural and FA were exam-
ined via the same two-step procedure, a similar phenomenon
was observed (Table 2): 2-cyclopentenone was detected in the
presence of a limited amount of H2, and then vanished with
the increased amount of CPO and CPL under an excess amount
of H2. These results suggested that 2-cyclopentenone was
another key intermediate in the formation of CPO from HCP.
We wondered whether the reaction pathway via HCP and
2-cyclopentenone was the only formation route of CPO. Then a
series of control experiments were performed using FA and
furfural as substrates. The GC traces are shown in Fig. 4. The
rearrangement of FA and the hydrogenation of furfural were
performed under different reaction conditions (“optimized
standard reaction conditions”, at 80 °C, Na2CO3 as additive, in
1,4-dioxane and in 1,4-dioxane–water mixed solvent). When
HCP was observed (Fig. 4, right), CPO was correspondingly
detected (Fig. 4, left). The main products were FA and THFA
while HCP disappeared (Fig. 4, b–d). Obviously, the formation
Fig. 3 Conversion of furfuryl alcohol under different reaction conditions (CPO:
cyclopentanone, CPL: cyclopentanol, HCP: 4-hydroxy-2-cyclopentenone).
Reaction conditions: 0.05 g mL−1 aqueous solution of furfuryl alcohol, 160 °C,
2 MPa H2 or 0.1 MPa N2. Catalyst: NiCu-50/SBA-15.
of HCP was in good agreement with the effects of solvent,
temperature and additive on furfural hydrogenation. Thus,
transformation of FA into HCP was closely related to the for-
mation of CPO.
We further excluded the possibility of THFA and 2-methyl-
of FA into HCP occurs spontaneously in water in this work, furan (MF) conversion into CPO. When THFA was tested, 98%
and was not immediately correlated to the catalyst or H2. of THFA remained intact under 4 MPa H2 over NiCu-50/SBA-15
The following control experiments further confirmed that after 4 h, and no rearrangement reaction was observed either
HCP was an important intermediate in the formation of CPO. (ESI Table S4,† entry 13). When MF was used as as substrate
HCP could be easily converted into CPO in 73% yield in the under the same conditions, the main product was 2-methyl-
presence of catalyst in 4 h (160 °C, 4 MPa H2). We further tetrahydrofuran, and no alicyclics were detected (ESI Table S4,†
examined the reaction of HCP in two steps (Table 2): Firstly, entry 14). All these experiments indicated that rearrangement
when a limited amount of H2 (4.1 mmol H2 in the presence of of the furan ring probably took place starting from FA after
5.2 mmol HCP) was charged, 38% of 2-cyclopentenone was selective hydrogenation of furfural; the reaction temperature
obtained after 4 h, besides a 4% yield of CPO. More interest- and solvent governed the reaction direction through the
ingly, after excess H2 was introduced into the reactor, 2-cyclo- control of FA rearrangement. The reaction pathway through
pentenone disappeared, meanwhile the total yield of CPO and HCP and 2-cyclopentenone should be the only formation route
CPL significantly increased to 39%. In contrast, without cata- of CPO over Ni–Cu bimetallic catalysts under optimized reac-
lyst or H2, no apparent CPO or 2-cyclopentenone could be tion conditions.
obtained from HCP. Unlike the rearrangement of FA, the
On the basis of the results mentioned above, the whole
NiCu/SBA-15 catalyst played an important role in the reaction pathway from furfural into CPO is proposed in
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Table 2 Hydrogenation reactions in two stepsa
Step 1
Step 2
Selectivity of alicyclics (%)
Selectivity of alicyclics (%)
Substrate
Conv. (%)
>99
Conv. (%)
>99
38
4
n.d.
n.d.
33
6
90
66
29
16
7
n.d.
n.d.
>99
>99
n.d.
n.d.
31
14
10
24
19
a Reaction conditions: 10 g aqueous solution of substrate (5.2 mmol), NiCu-50/SBA-15 (0.20 g), 160 °C. Step 1: limited amount of H2 (4.1 mmol),
4 h. Step 2: 4 MPa H2, 4 h.
Fig. 4 GC traces for conversion of furfural and FA. Standard reaction conditions unless otherwise specified: 10 g 5% FA or furfural aqueous solution, 0.2 g NiCu-
50/SBA-15, 160 °C, 4 MPa H2, 4 h. TMB (1,2,4,5-tetramethylbenzene) was used as an internal standard. The compounds correspond to retention time: 3.38 min
(CPO), 4.10 min (CPL), 5.40 min (TMB), 5.79 min (furfural), 6.14 min (THFA), 7.79 min (FA), 12.67 min (HCP).
Scheme 2. Furfural is firstly hydrogenated to FA catalyzed by
NiCu/SBA-15; and then rearrangement of FA into HCP occurs
spontaneously under proper reaction conditions in water,
independent of catalytic hydrogenation. Subsequently HCP is
converted into 2-cyclopentenone over NiCu/SBA-15, and finally
CPO is formed via further hydrogenation of 2-cyclopentenone.
These reactions proceed successively, and furfural is directly
transformed into CPO in a one-pot procedure. The swift con-
version of HCP and 2-cyclopentenone under reaction con-
ditions may be the obstacle for their detection. Without water
or at a low temperature, the rearrangement reaction is
impeded; and FA could be further hydrogenated into those
compounds that contain one oxygen atom within the ring
structure.
Scheme 2 The proposed reaction pathway during hydrogenation of furfural in
In this catalytic system, there were several parallel and
cascade reactions, including hydrogenation of CvO/CvC (5.2 mmol), NiCu-50/SBA-15 (0.2 g), 160 °C, 4 MPa H2, 4 h.
water over NiCu-50/SBA-15. Reaction conditions: 10 g furfural aqueous solution
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bonds, resinification, ring opening/closure and dehydration. proposed reaction pathway, and were in good agreement with
In the formation pathway of CPO, two stages occurred over the previous studies on the Piancatelli type rearrangement.17
Ni–Cu catalyst: the selective hydrogenation of furfural into FA, No substituent at the 5-position was necessary for the
conversion of HCP into CPO through dehydration and rearrangement of the furan ring.
hydrogenation.
Role of water in rearrangement reaction
As mentioned above, the water was indispensable for the for-
mation of CPO. When 18O-enriched water was tested as a
solvent, 95% abundance of 18O was observed in CPO
(Fig. S16†). Thus, the oxygen in CPO comes from water.
However, the exchange of oxygen between the CPO and water
did occur.18 In order to confirm that water was involved in the
rearrangement reaction of FA in this work, kinetic experiments
were carried out. The kinetic plots for the rearrangement of FA
in ordinary water and heavy water are shown in Fig. 5. The
graphs of ln(C/C0) against time gave good straight-line plots
and this revealed that the rearrangement reaction of FA fol-
lowed quasi first-order kinetics. The rate constants could be
obtained by linear regression with a 95% confidence interval.
The calculated values are kH = 0.0006721 0.0000046 s−1 in
ordinary water and kD = 0.0006169 0.0000051 s−1 in heavy
Transformation of furan derivatives in water
Besides furfural and FA, other furan derivatives were also
tested via catalytic hydrogenation in water. When 2-acetylfuran
was used as a substrate, 43% selectivity of 2-methyl-cyclopenta-
none was obtained (Table 3, entry 2). Thus, both formyl and
acetyl group of furan derivatives could lead to formation of
five-membered carbon rings (Table 3, entries 1 and 2). In con-
trast, when the 5-position was substituted with a methyl group
or hydroxymethyl group, no alicyclics were observed. Instead,
hydrogenation of the furan ring and aldehyde were observed
(Table 3, entries 3 and 4). These results confirmed the
Table 3 Conversion of furan derivatives over NiCu-50/SBA-15 in watera
Selectivity of main products
(%)
water. Secondary kinetic deuterium isotope effects (kH/kD
=
Entry
1
Substrate
Conv. (%)
>99
1.089 0.011) were observed in heavy water, which suggested
that water was involved in the rearrangement reaction as a
reactant. These results were also in good agreement with the
previous studies on Piancatelli type rearrangements of furan
compounds. According to those studies, the opening and
closure of a furan ring are related to the attack of a H2O mole-
cule in the 5-position.17
2
52
3
4
81
90
Prior hydrogenation of aldehyde group
One of the interesting matters was the absence of THFA when
CPO dominated as the main product (Table 1, entries 3–5). It
is clear that THFA could form when the reaction was per-
formed even at a low temperature like 100 °C (ESI, Fig. S8†). It
seemed reasonable to predict the formation of THFA at a
higher temperature, such as 160 °C. However, THFA was not
detected, accompanied by the formation of CPO, during the
a Reaction conditions: 10 g aqueous solution of substrates (5.2 mmol),
NiCu-50/SBA-15 (0.20 g), 160 °C, 4 MPa H2, 4 h.
Fig. 5 The kinetic plots for the rearrangement of furfuryl alcohol in H2O and D2O. Reaction conditions: 50 mL aqueous solution of furfuryl alcohol (0.05 g mL−1),
no catalyst, 120 °C, 4 MPa H2. C: concentration of furfuryl alcohol, g mL−1
.
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Table 4 Conversion of furfuryl alcohol in the presence of different aldehydesa
observed. When the same amount of FA is used directly as a
substrate, as there is no competition of hydrogenation
between the aldehyde group and furan ring, both hydrogen-
ation and rearrangement of the furan ring occur. As a result,
both THFA and CPO are detected. When the reaction con-
ditions are not proper for the rearrangement of FA into HCP,
such as in basic aqueous solution (Table 1, entries 1 and 2),
the concentration of FA in water rises, then hydrogenation of
FA into THFA would be inevitable.
Selectivity of products (%)
Entry Additiveb
Conv. (%)
1
2
None
>99
>99
10
38
26
1
n.d.
n.d.
16
n.d.
Prior selectivity of cyclopentanone
3
>99
42
3
n.d.
n.d.
Another important issue was the high selectivity for CPO,
rather than CPL, under a H2 atmosphere. Considering the reac-
tion pathway (Scheme 2), CPO was obtained by the hydrogen-
ation of 2-cyclopentenone. We assumed that the carbon–
carbon double bond is more readily able to react with H2 over
Ni–Cu bimetallic catalysts. In order to verify this hypothesis,
we studied the hydrogenation of 2-cyclopentenone over NiCu-
50/SBA-15. The time profiles are shown in Fig. 6. In the begin-
ning, the main product was CPO after selective hydrogenation
of the CvC bond in the 2-cyclopentenone molecule, at the
same time, less than 1% of CPL was detected. In contrast,
when the 2-cyclopentenone was converted nearly completely
after 2 h, CPL appeared in the reaction mixture. The selectivity
for CPO decreased while that for CPL increased when the reac-
tion time was prolonged. Thus, hydrogenation of CPO to CPL
was hampered by 2-cyclopentenone, CPL was obtained
through further hydrogenation of CPO after the disappearance
of 2-cyclopentenone. These results are in agreement with the
reaction progress profiles of catalytic hydrogenation of furfural
(Fig. 2).
4
5
>99
94
37
21
10
27
1
1
n.d.
n.d.
a Reaction conditions: 10 g aqueous solution of substrates (5.2 mmol),
NiCu-50/SBA-15 (0.20 g), 160 °C, 4 MPa H2, 4 h. b The amount of
aldehyde was equimolar to that of FA.
hydrogenation of furfural. In contrast, THFA appeared when
FA was employed directly as the starting material (Table 4,
entry 1). Over Ni, Cu-based catalysts, the aldehyde group of fur-
fural could be easily hydrogenated into FA, as reported pre-
viously.9,10 It may hamper the formation of THFA during the
hydrogenation of furfural in this work. To verify this assump-
tion, a series of control experiments were performed using FA
as a substrate by adding different aldehydes (Table 4). In the
absence of aldehyde, the selectivity of CPO and CPL was 10%
and 26%, respectively; meanwhile 16% THFA was obtained
(Table 4, entry 1). In contrast, after aldehyde was added, no
THFA was detected in the products, regardless of furan deriva-
tives, benzaldehyde, or aliphatic aldehyde. The selectivity for
CPO and CPL increased significantly at the same time
(Table 4, entries 2–5), and the aldehyde group of these addi-
tives was hydrogenated rather than the aromatic ring. For
example, the presence of 5-methyl-2-furancarboxaldehyde
hampered the formation of THFA (Table 4, entry 2), mean-
while itself was converted into 5-methylfurfuryl alcohol. Benz-
aldehyde was hydrogenated into benzyl alcohol rather than
cyclohexylmethanol. The formyl group of furfural should affect
the formation of THFA during the hydrogenation of furfural in
water. In contrast, when ketones were used as an additive
instead, the conversion of FA was not as affected as with alde-
hydes (Table S5†).
The side reactions and carbon loss
The third key issue was the carbon loss. The conversion of fur-
fural into CPO in water has scarcely been studied previously,
and carbon loss was observed in this and a recent work.12 Due
The disappearance of THFA when furfural was used as the
substrate could be explained as follows: since the hydrogen-
ation of the aldehyde is preferred over a Ni, Cu based cata-
lyst,9,10 furfural is firstly converted into FA. Further
hydrogenation of the furan ring of FA is hampered by the alde-
hyde group of unconverted furfural; meanwhile rearrangement
of FA into HCP occurs in hot water, which leads to CPO. Thus
Fig. 6 Hydrogenation of 2-cyclopentenone over NiCu-50/SBA-15. Reaction
conditions: 10 g aqueous solution of cyclopentenone (5.2 mmol), NiCu-50/
FA could not be accumulated in water, and THFA is not SBA-15 (0.20 g), 80 °C, 1 MPa H2.
1938 | Green Chem., 2013, 15, 1932–1940
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of catalytic hydrogenation, starting from furfuryl alcohol
rather than furfural. The opening and closure of the furan ring
were closely related to the attack of the H2O molecule in the
5-position of furfuryl alcohol. Prior hydrogenation of the alde-
hyde group accounted for the different reaction behavior of
furfural/furfuryl alcohol. The high selectivity for cyclopenta-
none was ascribed to the presence of 2-cyclopentenone. This
work will be helpful to understand the transformation of fura-
nics into alicyclics in water under a hydrogen atmosphere.
Scheme 3 A schematic description for the production of cycloalkanes.
Acknowledgements
to their inherent properties, resinification of furfural or FA is
inevitable in hot water, especially in the presence of acids.4
Even when we carried out the hydrogenation of furfural in hot
water without catalyst or using SBA-15, 12% and 14% of fur-
fural was converted, respectively, however no products were
detected by GC and a yellow mixture was also observed. The
resinification reaction of FA occurred more or less at high
temperature, such as 160 °C, and prevailed under strong acidic
conditions (Fig. S39†). As a result, the mass loss was observed
when the products were analyzed by GC.
Unlike furfural and FA, CPO and CPL are much more stable
in hot water. Once CPO was formed, the carbon loss would be
reduced. A higher concentration of furfural and FA would lead
to higher carbon loss due to resinification. This is why a low
concentration of furfural aqueous solution was preferred in
this and a recent work.12
This work was supported by the National Natural Science
Foundation of China (21203180 and 21233008).
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