Efficient and selective transformation of biomass-derived furfural with aliphatic alcohols catalyzed by a binary Cu-Ce oxide

Article abstract of DOI:10.1016/j.cattod.2017.04.057

The efficient transformation of furfural (FUR) with aliphatic alcohols to achieve the carbon-chain growth has been developed using a binary Cu-Ce oxide as the catalyst. In the presence of molecular oxygen, the tandem oxidative condensation of FUR with n-propanol is successfully performed, in which an 85.4% conversion of FUR in 95.3% selectivity of 3-(furan-2-yl-)-2-methylacryaldehyde was obtained. The effects of different Cu/Ce ratios and base additives were investigated in detail. As a result, it is found that the CuO-CeO₂ (1: 9) catalyst is optimal and potassium carbonate is a suitable additive. Next, the recycling of CuO-CeO₂ catalyst was tested and there is no obvious activity loss after being reused five times. Moreover, the oxidative condensation of FUR with various aliphatic alcohols including ethanol, isopropanol, n-butanol and n-hexanol was studied where the long chain alcoholic molecule hinders the proceeding of reaction. Finally, based on the experimental results and reaction phenomena, a possible mechanism for the oxidative condensation of FUR with n-propanol-O₂ is proposed.

Full text of DOI:10.1016/j.cattod.2017.04.057

CatalysisTodayxxx(xxxx)xxx–xxx
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Efficient and selective transformation of biomass-derived furfural with
aliphatic alcohols catalyzed by a binary Cu-Ce oxide
Xinli Tonga,⁎, Linhao Yua, Xuan Luob, Xuli Zhuanga, Shengyun Liaoa, Song Xuea
a
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin
300384, PR China
b
Technical Center for Safety of Industrial Products, Tianjin Entry-Exit Inspection and Quarantine Bureau, No. 33 Youyi Road, Tianjin 300201, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Furfural
Oxidative condensation
Aliphatic alcohols
Molecular oxygen
Biomass conversion
The efficient transformation of furfural (FUR) with aliphatic alcohols to achieve the carbon-chain growth has
been developed using a binary Cu-Ce oxide as the catalyst. In the presence of molecular oxygen, the tandem
oxidative condensation of FUR with n-propanol is successfully performed, in which an 85.4% conversion of FUR
in 95.3% selectivity of 3-(furan-2-yl-)-2-methylacryaldehyde was obtained. The effects of different Cu/Ce ratios
and base additives were investigated in detail. As a result, it is found that the CuO-CeO2 (1: 9) catalyst is optimal
and potassium carbonate is a suitable additive. Next, the recycling of CuO-CeO2 catalyst was tested and there is
no obvious activity loss after being reused five times. Moreover, the oxidative condensation of FUR with various
aliphatic alcohols including ethanol, isopropanol, n-butanol and n-hexanol was studied where the long chain
alcoholic molecule hinders the proceeding of reaction. Finally, based on the experimental results and reaction
phenomena, a possible mechanism for the oxidative condensation of FUR with n-propanol-O2 is proposed.
1. Introduction
reaction conditions. In addition, aldol condensation of furfural with
acetone was extensively researched because the produced compound is
Serious environment problems and the diminishing fossil resources
have driven the search for new forms of alternative and renewable
energy [1,2]. The efficient use of biomass resources can supply common
and sustainable chemical intermediates and liquid fuels [3–5]. Parti-
cularly, the selective conversion of a biomass-derived platform com-
pound using a catalytic cascade reaction is beneficial to improve the
efficiency and energy consumption in large-scale industrial applications
[6]. Furfural (FUR) is one of the most common platform chemicals
derived from the lignocellulosic biomass and has an annual production
of more than 200,000 tons [7]. FUR can be obtained from C5
carbohydrates and is critical in the valorization of the hemicellulose
contained in biomass feedstocks [8]. Numerous studies showed that
furfural, as the starting material, could be used to synthesize a variety
of commercial products [9], including the hydrogenation, oxidation,
reductive amination, decarbonylation, nitration, and condensation
reaction of furfural [10]. For example, in the presence of an Au/TiO2
catalyst with NaCH3O as the additive, the oxidative esterification of
furfural yields high amounts of methyl furoate [11]. Pinna et al.
[12–15] found that gold-supported sulfated zirconia was also active
for the oxidative esterification, in which the highest conversion of FUR
and selectivity of methyl furoate was 94% and 99% under the optimal
a useful intermediate in the synthesis of the second-generation biofuels
[16–20]. Fakhfakh et al. [21] further reported that sodium hydroxide
could promote the aldol-condensation reaction of furfural and acetone
where the yield of 4-(furan-2-yl)but-3-en-2-one arrived at 50% in
ethanol-water solvent. Moreover, Sádaba et al. [22] found that co-
precipitated Mg-Zr mixed oxides were an efficient catalyst for the aldol
condensation of furfural and acetone. Kikhtyanina et al. [23] reported
that a 27% conversion of furfural and a 71% selectivity of 4-(furan-2-yl)
but-3-en-2-one were obtained with MOF and zeolite catalysts. Further-
more, Thanh et al. [24] investigated the catalytic performance of
nanosized TiO2 in the aldol condensation, which yielded a 25%
conversion of furfural in a 72% of selectivity of 4-(furan-2-yl)but-3-
en-2-one as the main product.
Generally, the condensation reaction of furfural and aldehyde can
drive two carbon molecules together to produce longer hydrocarbon
chains and low volatile liquid transport fuels. Previous reports con-
firmed that acetone and aldehydes could be produced from the
oxidation of secondary and primary alcohols in the presence of
molecular oxygen [25]. Therefore, from the standpoint of green and
sustainable chemistry, the cascade transformation of alcohol oxidation
and aldol condensation in a ternary FUR-alcohol-O2 (FAO) system may
⁎
Corresponding author.
E-mail addresses: tongxinli@tju.edu.cn, tongxli@hotmail.com (X. Tong).
http://dx.doi.org/10.1016/j.cattod.2017.04.057
Received 17 November 2016; Received in revised form 24 March 2017; Accepted 29 April 2017
0920-5861/©2017ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:Tong,X.,CatalysisToday(2017),http://dx.doi.org/10.1016/j.cattod.2017.04.057

X. Tong et al.
CatalysisTodayxxx(xxxx)xxx–xxx
be an efficient use of FUR.
solution was mixed with copper nitrate and (NH4)2Ce(NO3)3 (1:9) by
dissolving 1 mmol of Cu(NO3)2·3H2O and 9 mmol of (NH4)2Ce(NO3)3 in
30 mL de-ionized water. Then, 2 g of urea were added into the solution
and the mixture was heated to 100 °C in oil bath pot with vigorous
stirring for 8 h. A dark brown precipitate was obtained, separated by
filtration and washed with of de-ionized water at 70 °C. The cake was
dried in a vacuum oven at 100 °C over night. After grinding the solid
precursor to powder, the material was calcined in a muffle furnace at
650 °C for 4 h in air. The final powder was designated as the CuO-CeO2
catalyst and characterized by XRD, SEM, TEM, and NH3-TPD techni-
ques. The surface area and pore structure of catalyst are detected by
BET method. In addition, Fe2O3-CeO2, NiO-CeO2, CaO-CeO2, and ZnO-
CeO2catalysts were synthesized for comparison using the same method.
The characterizations of catalysts including XRD patterns, BET
detection, TEM images and TPD measurement were presented in the
supporting information, respectively.
In the previous study, our group has reported the oxidative
condensation of FUR and aliphatic alcohols in the presence of mole-
cular oxygen using the Au/FH, Pt/FH, and CoxOy-N/Kaolin as catalyst
[26–29]. In this work, we employ the simple and low cost mixed Cu-Ce
oxides as the catalysts to perform the oxidative condensation of FUR
with aliphatic alcohols and further hydrogenation reaction. Based on
the experimental results, it was found that 85.4% conversion of FUR
and 95.3% selectivity to 3-(furan-2-yl)-2-methylacrylaldehyde (1) was
obtained in the furfural-n-propanol-O2 (FPO) system in the presence of
CuO-CeO2 and potassium carbonate. Besides, in the catalytic hydro-
genation of 1, a 95.8% conversion and 94.7% selectivity of 3-(furan-2-
yl-)-2-methylpropanol was attained under H2 atmosphere with similar
catalyst system.
2. Experimental sections
2.1. Reagents and instruments
2.3. General procedure for the oxidative condensation of furfural and
aliphatic alcohols
(NH4)2Ce(NO3)6, Cu(NO3)2·3H2O, Fe(NO3)3·9H2O, Ni(NO3)2·6H2O,
Zn(NO3)2·6H2O, and Ca(NO3)2·4H2O were of analytical grade and
purchased from the Aladdin Reagent Co., Ltd. (Shanghai, China). The
3-(furan-2-yl-)-2-methylacryaldehyde as the standard sample was ob-
tained from Alfa Aesar, Thermo Fisher Scientific, Inc. (Ward Hill, MA,
U.S.A.). Methanol, ethanol, n-propanol, isopropanol, n-propanal, n-
butanol, n-hexanol, n-hexane, n-octane and other solvents were purified
by distillation before use. All other chemicals were used without further
purification.
The measurement of X-ray diffraction (XRD) was performed by
diffractometer with Cu Ka radiation (0.02° resolution) and was
collected from 20 to 80° [20]. BET surface areas, pore volumes, and
average pore diameters of the prepared samples are obtained from N2
(77 K) adsorption measurement using a Micromeritics ASAP2020 M
system, in which the samples are pretreated under vacuum at 250 °C for
4 h before the measurement. The average pore diameters are calculated
according to Barrett-Joyner-Halenda (BJH) model to absorption and
desorption data. Scanning electron microscopy (SEM) was used by a
JSM-6301F, JEOL to obtain the surface morphology of the catalytic
materials. A transmission electron microscope (TEM: JEM-2100, JEOL)
was also used to characterize the catalyst. The solid catalyst was
analysed by temperature programmed desorption (TPD) using a Micro-
meritics 2920 Autochem II Chemisorption Analyzer. The samples were
first pretreated at 500 °C for 1 h in the presence of Ar with a flow rate of
25 mL min−1, then cooled to room temperature and ammonia was
absorbed with a flow rate of about 25 mL min−1. During the TPD
experiments, the temperature was set at a heating rate of 10 °C −min
rising to 900 °C in the presence of He with a flow rate of 60ml-min.
Effluent gas was dried by powdered KOH, and the concentrations of
ammonia were recorded by a thermal conductivity detector.
In
a typical oxidative condensation-hydrogenation reaction of
furfural with n-pronanol, 15 mL solution of n-propanol is thoroughly
mixed with 0.05 g CuO-CeO2 catalyst, 0.05 g K2CO3, and 0.2 g furfural.
The mixture was charged into a 120 mL autoclave equipped with the
magnetic stirring and automatic temperature control. After reactor is
sealed, the solution was then stirred for 4 h at 140 °C under 0.3 MPa of
O2 for the oxidative condensation process. After the reaction, the
autoclave was cooled to room temperature and a certain amount of
solution was diluted with CH3CN solvent and analyzed by GC and
GC–MS after the excess gas is purged.
3. Results and discussion
3.1. The oxidative condensation of furfural with n-propanol using different
catalytic systems
To investigate the activities of different catalysts, a series of
catalysts were used in the FPO system where the reaction process is
shown in Scheme 1. As shown in Table 1, five types of metal (Cu, Ni, Fe,
Ca, and Zn) supported on cerium oxide were tested under the same
experimental conditions. In the presence of CuO-CeO2 catalyst, 85.4%
conversion of FUR and 95.3% selectivity of 1 were obtained when
K2CO3 was used as the additive (entry 1). However, the conversion of
FUR fell to 58.2%, 44.1%, 30.9%, 22.6% and the selectivity of 1 was
60.9%, 73.7%, 55.1%, 62.8%, respectively, when Fe2O3-CeO2, NiO-
CeO2, CaO-CeO2, and ZnO-CeO2 catalysts were employed instead of the
CuO-CeO2 catalyst. Based on the above experimental results, it is
concluded that copper oxide plays a crucial role on the oxidative
condensation of FUR with n-propanol in the presence of molecular
oxygen which leads to the CuO-CeO2 exhibiting a higher catalytic
activity than other mixed oxides. Furthermore, when a single CuO or
CeO2 was used as the catalyst (entries 6 and 7), the conversion of FUR
was decreased to 47.8% or 48.2%, and the selectivity of 1 was 89.5% or
57.9%, respectively. In addition, in the presence of physically-mixed
CuO and CeO2, the conversion of FUR and selectivity of 1 were
respectively 51.2% and 74.5% under similar reaction conditions.
Therefore, the cooperation effect between copper oxide and cerium
oxide favored the tandem oxidative condensation reaction in FPO
The quantitative analyses of the products are performed on a GC
apparatus with FID detector. The capillary column is HP-5,
30 m × 0.25 mm × 1.0 μm. In addition, the qualitative analysis for
the product is carried out on the Agilent 6890/5973 Gas
Chromatograph-Mass Spectrometer (GC–MS) instrument.
2.2. The preparation of catalyst
The CuO-CeO2 catalyst was prepared as follows. The aqueous
Scheme 1. The reaction of FUR with n-propanol in the presence of oxygen.
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Table 1
In order to further reveal the nature of copper oxide and cerium
oxide in CuO-CeO2 catalyst, the catalyst including different Cu/Ce
ratios was characterized by different techniques. The SEM images of
CeO2, CuO-CeO2 (1:9), CuO-CeO2 (1:1), CuO-CeO2 (9:1) and CuO were
shown in Fig. 1. It is found that the pure CeO2 material appears as the
loose and even nanobulk (Fig. 1a), while pure CuO material is the
flower-like spherical composed of numerous tight nanoflake parts
(Fig. 1e). Moreover, along with the increase of the amount of CuO,
the surface morphology of CuO-CeO2 was also gradually changed.
Therein, the morphology of CuO-CeO2 (1:9) is very similar with that of
CeO2, and a few CuO supports on the surface of CeO2; while, the
morphology of CuO-CeO2 (9:1) is so much like that of CuO, and few
CeO2 should be doped on the nanoflake CuO material. Correspondingly,
the difference of morphology leads to the distinction of BET surface, in
which the BET surface of CuO, CeO2 and CuO-CeO2 (1:9) is 4.3, 27.4
and 46.4 m2 g−1, respectively (shown in supporting information).
Oxidative condensation of FUR with n-propanol-O2 using different catalytic systems.a
Entry Catalytic system
Conversion (%)b Product distribution (%)b
1
2
Othersc
1
2
3
4
5
6
7
8
CuO-CeO2 + K2CO3
Fe2O3-CeO2 + K2CO3
NiO-CeO2 + K2CO3
CaO-CeO2 + K2CO3
ZnO-CeO2 + K2CO3
CuO + K2CO3
CeO2 + K2CO3
CuO
85.4
58.2
44.1
30.9
22.6
47.8
48.2
51.2
95.3
60.9
73.7
55.1
62.8
89.5
57.9
74.5
4.7
8.5
–
30.5
13.1
27.7
18.3
0
13.2
17.2
18.9
10.5
32.1
8.5
10.0
17.0
+ CeO2 + K2CO3
CuO-CeO2
9
25.0
8.7
2.1
98.3
16.1
1.7
81.8d
–
10
K2CO3
system. In addition, it needs to be mentioned that the selectivity of 1
could be improved with the participation of copper oxide on the
catalytic reaction. In order to reveal the action of K2CO3, the control
experiments were performed using only CuO-CeO2 or K2CO3 as
promoter. As a result, it is found that a mere 25.0% conversion of
FUR and 2.1% selectivity of 1 was obtained in the absence of K2CO3
(entry 9); meanwhile, only 8.7% conversion of FUR and 98.3%
selectivity of 1 was acquired in the absence of CuO-CeO2 catalyst
(entry 10).
According to the results of GC–MS, the main by-product is com-
pound 2, furfuryl alcohol and 2- (dipropoxymethyl)furan where the
compound 2 is obtianed from the oxidative esterification route. While,
the furfuryl alcohol is generated via the reduction of FUR by n-
propanol, and the 2- (dipropoxymethyl)furan is produced via the
acetalization of FUR with n-propanol in the absence of K2CO3. These
data showed that both the CuO-CeO2 and K2CO3 are necessary to the
proceeding of this tandem reaction. Moreover, based on catalytic data
of Table 1 and characterization of different catalysts, it can be
concluded that the conversion of FUR is mainly attributed to the
component of mixed oxide catalyst; while, the selectivity of compound
1 should be closely related to the surface area and amount of weak acid
sites of those catalysts.
3.3. The effect of various additives on the oxidative condensation
Considering the importance of K2CO3 as the additive in the reaction,
the effects of different basic additives including carbonates, hydroxides,
and ammonium salts were further investigated. The results are shown
in Table 3. It can be seen that, when Na2CO3 or Cs2CO3 were used as
additive, the conversion of FUR was 25.0% or 50.8%, and the
selectivity of 1 was 85.6% or 88.5%, respectively (entries 1 and 2).
Otherwise, with NaOH as the basic additive, a 95.0% conversion of FUR
in a 72.3% selectivity of 1 was attained in the presence of CuO-CeO2
catalyst (entry 3), in which the decrease of selectivity can be attributed
to the strong basicity of NaOH leading to the occurrence of more side-
reactions. Furthermore, when K3PO4 or Na2SiO3 was employed as
additive, only 28.6% and 15.8% conversion of FUR was obtained
where the selectivity of 1 was 95.3% and 79.3%, respectively (entries
4 and 5). On the other hand, a large amount of the Schiff bases were
produced when ammonium salts were used as the additives in this
reaction system (entries 6 and 7). In addition, seen from the data
displayed in Table 3, it is found that the strong basic additives are more
efficient than weak basic ones during the oxidative condensation of
FUR with n-propanol. However, the suitable basicity is more selective
for the generation of compound 1; meanwhile, the existence of
carbonate or phosphate ions is also helpful to the increase of selectivity.
Consequently, K2CO3 was considered as the best basic additive in the
FPO system, not only because of its promotion effect of the CuO-CeO2
catalyst, but also because it is environmentally friendly and cost
effective.
3.2. The effect of different Cu/Ce ratios on the oxidative condensation
The effects of different ratios on the oxidative condensation of FUR
with n-propanol-O2 was investigated. As shown in Table 2, it is found
that the catalytic activity of CuO-CeO2 was gradually decreased along
with the elevation of the amount of CuO component in mixed oxides.
Therein, a 70.5% and 67.6% conversion of FUR was respectively
obtained when CuO-CeO2 (1:3) and CuO-CeO2 (1:1) was employed as
catalyst. Especially, in case of CuO-CeO2 (9:1) catalyst, only 49.8%
conversion of FUR in 91.2% selectivity of 1 was attained, which is near
to that of using pure CuO as the catalyst (entry 6 in Table 1).
In the following, the effects of reaction time and reaction tempera-
ture have been also studied for the oxidative condensation of FUR with
n-propanol (the data are shown in supporting information). As a result,
the optimal temperature is 140 °C and the suitable reaction time is 4 h.
3.4. The investigation on the recycling of catalyst
The recycling experiment was carried out to investigate the stability
and reuse of CuO-CeO2 catalyst. After the catalytic reaction, the catalyst
was separated, and washed with anhydrous ethanol, and then dried at
80 °C for 12 h before being reused in the next run. As shown in Fig. 2,
the conversion of FUR still kept ca. 80% and the selectivity of 1 was as
high as 92% even after the CuO-CeO2 catalyst being recycled five times.
These results showed that the CuO-CeO2 catalyst was efficient and kept
stable in FPO system, which indicates that this oxidative condensation
is the heterogeneous catalytic process.
Table 2
The effect of different ratios of Cu and Ce on the oxidative condensation with CuO-CeO2.a
Entry
Catalyst
Conversion (%)b
Product distribution (%)b
1
2
1
2
3
4
5
CuO-CeO2 (1:9)
CuO-CeO2 (1:3)
CuO-CeO2 (1:1)
CuO-CeO2 (3:1)
CuO-CeO2 (9:1)
85.4
70.5
67.6
63.2
49.8
95.3
88.1
91.5
89.2
91.2
4.7
11.9
8.5
10.8
9.8
3.5. The oxidative condensation reactions of FUR with various aliphatic
alcohols
a
Reaction conditions: 0.2 g of FUR, CuO-CeO2 catalyst 0.05 g and 0.05 g K2CO3 in
15 mL n-propanol, under 0.3 MPa of O2, at 140 °C for 4 h.
The result was obtained using GC with the internal standard technique where 1, 3-
dichlorobenzene is chosen as the internal standard.
b
In the following experiments, various alcohols were used to study
the oxidative condensation reaction of FUR in the presence of dioxygen.
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Fig. 1. SEM images of different catalysts [a. CeO2; b·CuO-CeO2 (1:9); c. CuO-CeO2 (1:1); d. CuO-CeO2 (9:1); e. CuO].
Table 3
Effects of various additives on the oxidative condensation of FUR with n-propanol.a
entry
additive
Conversion (%)b
Production distribution (%)b
1
2
Othersc
1
2
3
4
5
6
7
Na2CO3
Cs2CO3
NaOH
K3PO4
Na2SiO3
C2H7N
25.0
50.8
95.0
28.6
15.8
24.9
8.7
85.6
88.5
72.3
95.3
79.3
9.8
14.4
11.5
3.7
4.7
20.7
–
–
–
24.0
–
–
90.2d
91.8c
C4H11
N
8.2
–
a
Reaction conditions: 0.2 g of FUR, CuO-CeO2 catalyst 0.05 g and 0.05 g additive in
15 mL n-propanol, under 0.3 MPa of O2, at 140 °C for 4 h.
b
The result was obtained using GC with the internal standard technique where 1, 3-
dichlorobenzene is chosen as the internal standard.
c
The selectivity of others is generally referred to the selectivity of furfuryl alcohol.
The main product is the corresponding Schiff base in the reaction system.
d
Fig. 3. The oxidative condensation of FUR with various aliphatic alcohols using CuO-
CeO2 + K2CO3 catalytic system. [Reaction conditions: 0.2 g of FUR, 0.05 g CuO-CeO2
catalyst and 0.05 g K2CO3 in 15 mL alcohol, under 0.3 MPa of O2, at 140 °C for 4 h;
Compound X is the product from the oxidative condensation process that can be 3-(furan-
2-yl-)-2-R-acrylaldehyde (R = H, methyl, ethyl and butyl) or 4-(furan-2-yl)but-3-en-2-
one].
Table 4
The results of control experiments for studying the catalytic reaction mechanism.a
Entry Catalyst
Reactants
Conversion (%)b Selectivity (%)b
1
2
–
–
–
Othersc
53.6
8.3
1
CuO-
CeO2 + K2CO3
CuO-CeO2
FUR + n-
propanal
FUR + n-
propanal
FUR + n-
propanal
FUR + n-
propanol
FUR + n-
propanol
FUR + n-
propanolf
FUR + n-
propanalf
93.2
18.2
92.3
26.9
21.2
42.2
89.5
46.4
91.7
45.1
2
3
K2CO3
54.9
Fig. 2. The recycling of the CuO-CeO2 catalyst in the FPO system.
4d
5d
6
CuO-
CeO2 + K2CO3
CuO-CeO2
45.6 4.20 50.2
1.7
8.5
–
–
98.3e
91.5
–
CuO-
CeO2 + K2CO3
CuO-
As shown in Fig. 3, an 83.2% conversion of FUR and 92.1% selectivity
of 1 were obtained in ethanol, which trended similar to that in the n-
propanol. Otherwise, when isopropanol was used as reaction medium,
the conversion of and the selectivity of 4-(furan-2-yl)but-3-en-2-one are
respectively 20.3% and 52.1% in the oxidative condensation, which is
probably result of the steric hindrance during the catalytic reaction.
Moreover, the yield of the desired product from oxidative condensation
is decreased when the carbon chain length in alcoholic molecule is
increased. Therein, only 32.2% or 34.3% conversion of FUR was
attained when n-butanol or n-hexanol was used in FAO system, which
can be attributed to the slow rate of hydrogen transferring between FUR
and long chain alcoholic molecule.
7
98.3 1.7
CeO2 + K2CO3
a
Reaction conditions: 0.2 g of FUR, CuO-CeO2 catalyst 0.05 g and 0.05 g K2CO3 in
15 mL n-propanol or propanal, under 0.3 MPa of O2, at 140 °C for 4 h.
b
The results were obtained by GC with the internal standard technique where 1, 3-
dichlorobenzene is chosen as the internal standard.
c
The selectivity of others is generally referred to the selectivity of furfuryl alcohol.
d
The reaction was performed under a N2 atmosphere.
This datum is the selectivity of 2-(dipropoxymethyl) furan.
e
f
The ratio of FUR to n-propanol (or n-propanal) is 10: 1 (mol: mol), and the value of
conversion refers to that of n-propanol or n-propanal.
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Scheme 2. Possible reaction mechanism for oxidative condensation in FPO system.
3.6. The reaction mechanism for the oxidative condensation of FUR with n-
propanol
4. Conclusions
In summary, a highly efficient and selective oxidative condensation
furfural with aliphatic alcohols and further hydrogenation has been
achieved in the presence of CuO-CeO2 catalyst. In the oxidative
condensation reaction, an 85.4% conversion of FUR and 95.3%
selectivity of 1 was obtained when CuO-CeO2 and K2CO3 was employed
as the catalysts. The optimal reaction time and temperature was are 4 h
and 140 °C. The activity of catalyst has no obvious loss after being
recycled five times. This will provide a promising approach for efficient
valorization of biomass-derived hemicelluloses in biomass transforma-
tion.
The results of control experiments are provided in Table 4. The
interaction of FUR and n-propanal was first detected where numerous
products were obtained from different processes including FUR-propa-
nal condensation, n-propanal self condensation and FUR–FUR dispro-
portionation process, etc. Therefore, only 46.4% selectivity of 1 was got
although the conversion of FUR arrived at 93.2%. Furthermore, an
18.2% conversion and 97.1% selectivity for 1 was obtained in the
presence of single CuO-CeO2, and a 92.3% conversion in 45.1%
selectivity of 1 was attained in the presence of K2CO3. In addition, a
26.9% conversion and 45.6% selectivity of 1 was obtained using CuO-
CeO2 and K2CO3 as catalyst in FPO system under N2 atmosphere. It
should be noted that, in the obtained products, furfuryl alcohol
selectivity reaches 50.2% which exhibits that the hydrogen transferring
occurs between FUR and n-propanol. These results show that CuO-CeO2
and K2CO3 both contributed to the hydrogen transfer between FUR and
n-propanol. However, the acetalization process is dominant in FPO
system under N2 atmosphere when only CuO-CeO2 was employed.
These confirm that K2CO3 should act as weak bases in the reaction
involving proton extraction. Besides, the reaction of FUR with n-
propanol or n-propanal in the ratio of 10: 1 was performed, in which
a 42.2% conversion of n-propanol and very little 1 was acquired in FPO
system; however, the conversion of n-propanal and selectivity of 1 still
arrived at 89.5% and 98.3% under the similar conditions, respectively.
This shows that highly selective production of 1 can be achieved when
the reaction is carried out between a large number of FUR and little n-
propanal.
Acknowledgements
This research work was financially supported by the National
Natural Science Foundation of China (21003093) and the State Key
Program of the National Natural Science Foundation of China
(21336008).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.cattod.2017.04.057.
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