Highly efficient Nb2O5 catalyst for aldol condensation of biomass-derived carbonyl molecules to fuel precursors

Article abstract of DOI:10.1016/S1872-2067(19)63371-1

Aldol condensation is of significant importance for the production of fuel precursors from biomass-derived chemicals and has received increasing attention. Here we report a Nb₂O₅ catalyst with excellent activity and stability in the aldol condensation of biomass-derived carbonyl molecules. It is found that in the aldol condensation of furfural with 4-heptanone, Nb₂O₅ has obviously superior activity, which is not only better than that of other common solid acid catalysts (ZrO₂ and Al₂O₃), more importantly, but also better than that of solid base catalysts (MgO, CaO, and magnesium-aluminum hydrotalcite). The detailed characterizations by N₂ sorption/desorption, NH₃-TPD, Py-FTIR and DRIFTS study of acetone adsorption reveal that Nb₂O₅ has a strong ability to activate the C=O bond in carbonyl molecules, which helps to generate a metal enolate intermediate and undergo the nucleophilic addition to form a new C–C bond. Furthermore, the applicability of Nb₂O₅ to aldol condensation is extended to other biomass-derived carbonyl molecules and high yields of target fuel precursors are obtained. Finally, a multifunctional Pd/Nb₂O₅ catalyst is prepared and successfully used in the one-pot synthesis of liquid alkanes from biomass-derived carbonyl molecules by combining the aldol condensation with the sequential hydrodeoxygenation.

Full text of DOI:10.1016/S1872-2067(19)63371-1

Chinese Journal of Catalysis 40 (2019) 1168–1177
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Highly efficient Nb2O5 catalyst for aldol condensation of
biomass-derived carbonyl molecules to fuel precursors
Yaxuan Jing †, Yu Xin †, Yong Guo #, Xiaohui Liu, Yanqin Wang *
Shanghai Key Laboratory of Functional Materials Chemistry, Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering,
East China University of Science and Technology, Shanghai 200237, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Aldol condensation is of significant importance for the production of fuel precursors from bio-
mass-derived chemicals and has received increasing attention. Here we report a Nb2O5 catalyst with
excellent activity and stability in the aldol condensation of biomass-derived carbonyl molecules. It is
found that in the aldol condensation of furfural with 4-heptanone, Nb2O5 has obviously superior
activity, which is not only better than that of other common solid acid catalysts (ZrO2 and Al2O3),
more importantly, but also better than that of solid base catalysts (MgO, CaO, and magnesi-
um-aluminum hydrotalcite). The detailed characterizations by N2 sorption/desorption, NH3-TPD,
Py-FTIR and DRIFTS study of acetone adsorption reveal that Nb2O5 has a strong ability to activate
the C=O bond in carbonyl molecules, which helps to generate a metal enolate intermediate and
undergo the nucleophilic addition to form a new C–C bond. Furthermore, the applicability of Nb2O5
to aldol condensation is extended to other biomass-derived carbonyl molecules and high yields of
target fuel precursors are obtained. Finally, a multifunctional Pd/Nb2O5 catalyst is prepared and
successfully used in the one-pot synthesis of liquid alkanes from biomass-derived carbonyl mole-
cules by combining the aldol condensation with the sequential hydrodeoxygenation.
Received 26 February 2019
Accepted 6 April 2019
Published 5 August 2019
Keywords:
Aldol condensation
Nb2O5
C=O activation
Fuel precursor
Bio-liquid alkane
One-pot process
© 2019, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
high carbon fuel precursors [3]. After total hydrodeoxygena-
tion, thus-prepared oxygen-rich fuel precursors can be con-
As a solution to the environmental and energy problems, the
catalytic upgrading of renewable biomass to value-added
transportation fuels and chemicals is of key interest [1]. The
production of high carbon diesel (C12–C22) and jet-fuel (C8–C16)
range alkanes, which are widely used as transportation fuels, is
drawing immense attention [2]. Biomass-derived platform
compounds are light molecules with up to six carbon atoms,
therefore, a chain growth process is imperative to synthesize
verted to alkanes and used as transportation fuels [4].
Following the pioneering work of Dumesic et al. [5], several
types of C–C bond coupling reactions, such as aldol condensa-
tion [6,7], hydroxyalkylation/alkylation [8–11], Diels-Alder
reaction [12], benzoin coupling [13], ketonization [14], Michael
addition and Robinson annulation [15,16], were developed to
produce fuel precursors. Among the C–C bond coupling reac-
tions, aldol condensation is the best known C–C forming syn-
* Corresponding author. Tel/Fax: +86-21-64253824; E-mail: wangyanqin@ecust.edu.cn
# Corresponding author. Tel/Fax: +86-21-64253824; E-mail: guoyong@ecust.edu.cn
† These authors contributed equally to this work.
This work was financially supported by the National Natural Science Foundation of China (No. 21832002, 21872050, and 21808063), the Funda-
mental Research Funds for the Central Universities (222201718003), the Science and Technology Commission of Shanghai Municipality
(18ZR1408500, 10dz2220500), and the “Zhang Jiangshu” Excellent Ph.D. Project of ECUST.
DOI: S1872-2067(19)63371-1 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 40, No. 8, August 2019

Yaxuan Jing et al. / Chinese Journal of Catalysis 40 (2019) 1168–1177
1169
thetic reaction, which involves the transformation of two car-
bonyl compounds and requires at least one carbonyl compound
containing active -hydrogen atoms [17]. Since bio-
mass-derived carbonyl compounds are numerous, as shown in
Scheme 1, employing aldol condensation reaction to increase
carbon chain length has attracted considerable interest [18].
Typically, a series of fuel precursors have been obtained by the
aldol condensation of furfural with biomass-derived ketones,
such as acetone, methyl isobutyl ketone (MIBK), 2-pentanone,
2-heptanone, 3-pentanone, cyclopentanone, acetylacetone,
angelica lactone and LA ester [5,7,16,19–25]. In addition, ace-
tone, MIBK and cyclopentanone can be selectively converted to
fuel precursors by its self–aldol condensation [26–28]. Specifi-
cally, some biomass-derived carbonyl compounds (furfural,
acetone, MIBK and cyclohexanone) can be currently produced
in industrial scale [18,29–32], making the promising produc-
tion of fuel precursors through aldol condensation of bio-
mass-derived carbonyl compounds possible. Besides its appli-
cation to biomass transformation, the aldol reaction is also
important in industry, either in polymers, or in fine chemical
and pharmaceutical industry [17,33,34]. In short, aldol con-
densation is a cornerstone of synthetic chemistry and has re-
ceived immense attention.
aldol condensation are still few. In addition, the hydrodeoxy-
genation of bio-fuel precursors is catalyzed by bifunctional
metal/solid acid catalysts, so it would be promising if an excel-
lent solid acid is found for aldol condensation, in this case, the
aldol condensation and following hydrodeoxygenation can be
integrated over bifunctional metal-acid catalysts and the sepa-
ration can be omitted. Therefore, it is highly desirable to de-
velop efficient solid acid catalysts to promote aldol condensa-
tion.
Nb2O5 is an extremely promising catalyst owing to its excel-
lent functionality, such as water resistance, high stability, suita-
ble acidity and C–O bond cleavage ability [49–53]. Therefore,
increasing attention has been paid to use Nb2O5 as catalyst for
the catalytic upgrading of biomass to value-added transporta-
tion fuels and chemicals. These excellent properties are fully
exploited in the hydrodeoxygenation of biomass and bio-
mass-derived oxygenates. Nb-based catalysts have a unique
feature, that is, NbOx species play a key role in C–O bond cleav-
age, especially in the cleavage of tetrahydrofuran ring, leading
to the excellent performance in the hydrodeoxygenation reac-
tion [52]. Obviously, Nb2O5 catalyst exhibited the superior per-
formance in comparison with other solid acid catalysts (ZrO2,
Al2O3, TiO2 and H-ZSM-5) [51–53].
Generally, aldol condensation can proceed in the presence of
base, acid and acid-base bifunctional catalysts. In most cases,
basic catalysts are applied because they can directly abstract
α-protons, forming carbanions to attack –C=O bond and then
promote C–C bond formation. Homogeneous base catalysts,
such as NaOH and Ca(OH)2 [35–39], are generally employed to
catalyze aldol condensation, but have significant disadvantages,
including non-recyclability, complex separation, equipment
corrosion and waste water production. To overcome these
drawbacks, several heterogeneous basic catalysts were used to
push aldol condensation. For example, CaO, CaAl-hydrotalcite
and MgAl-hydrotalcite were considered as a promising alterna-
tive to avoid the above problems [19,27,40,41]. As we know,
basic metal oxides have high sensitivity to the ambient CO2 and
face a challenge for stability in the presence of water, while
aldol adducts can easily dehydrate to enone groups and pro-
duce water. Solid acid catalysts, such as TiO2, Hf-, Sn-, and
Zr-Beta zeolites, also emerged to promote aldol condensation
owing to their recyclability and water resistance [42–48].
However, the examples of using acidic materials as catalysts for
Herein, we report that Nb2O5 is taken as a solid acid catalyst
to catalyze aldol condensation of biomass-derived carbonyl
chemicals and exhibits excellent activity and stability. Investi-
gations on the originate of high activity reveal that Nb2O5 cata-
lyst can efficiently activate C=O bond of carbonyl molecules to
form a metal enolate intermediate, leading to the excellent
performance. We also demonstrate the scope of its application
to other biomass-derived carbonyl compounds. Finally, we
combine the two functions of Nb2O5, namely, cleavage of C–O
bond which is reported before [52] and activation of C=O bond,
and hydrogenation function of palladium, to develop a one-pot
process of the direct production of liquid alkanes from bio-
mass-derived carbonyl molecules over a multifunctional
Pd/Nb2O5 catalyst.
2. Experimental
2.1. Materials
Furfural, 4-heptanone, MIBK, cyclopentanone, acetone and
Scheme 1. Aldol condensation of biomass-derived carbonyl molecules.

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Yaxuan Jing et al. / Chinese Journal of Catalysis 40 (2019) 1168–1177
2,4-pentanedione were purchased from Shanghai Titan Tech-
nology Co., Ltd. Pd(NO3)2 aqueous solution was purchased from
Heraeus Materials Technology Shanghai Co., Ltd. The MgO and
CaO catalysts were purchased from Aladdin Industrial Inc. Be-
fore the tests, the metal oxide catalysts were pretreated in N2
flow at 500 °C for 4 h. All other chemicals were purchased from
Sinopharm Chemical Reagent Co., Ltd.
furfural and yield of products were calculated by using the
equation: furfural conversion = (moles of furfural reacted) /
(moles of starting furfural) × 100%, single aldol adduct yield =
(moles of single aldol adduct) / (moles of starting furfural) ×
100%, double aldol adduct yield = (moles of double aldol ad-
duct) / (moles of starting furfural / 2) × 100%. The experi-
mental section of aldol condensation involving other molecules
is the same as above process.
2.2. Catalyst preparation
The one-pot conversion of furfural and 4-heptanone into
branched alkanes was conducted in a 50 mL Teflon-lined
stainless-steel autoclave. Furfural (0.96 g, 10 mmol),
4-heptanone (0.57 g, 5 mmol), cyclohexane (5.00 g) and
2%Pd/Nb2O5 (0.3 g) were transferred into the autoclave. The
reactor was charged to 1 MPa N2 pressure. The reaction was
then performed at 130 C under magnetic stirring for 24 h.
After the aldol reaction, the reactor was purged with H2 three
times, and charged to 6 MPa H2 pressure. The reaction was then
performed at 220 °C and 6 MPa for 24 h. After the HDO reac-
tion, the autoclave was quenched in an ice-water bath to room
temperature. The liquid solution was separated from the solid
catalyst by centrifugation and analyzed qualitatively by GC-MS
(Agilent 7890A-5975C) and quantitatively by GC-FID (Agilent
7890) using C15 chain alkane as the internal standard. Here, we
employed pure straight chain alkanes (C12, C17) to replace tar-
geted alkanes as standards, because pure targeted alkanes
cannot be purchased or synthesized at present. The product
yields were calculated by using the equation: yield = (moles of
product) / (moles of starting 4-heptanone) × 100%.
The Nb precursor (niobium oxalate) was prepared accord-
ing to the literature procedure [54]. Niobium oxalate solutions
with a Nb concentration of 0.4 mol L–1 were prepared for use.
The layered Nb2O5 was synthesized by changing the substrates
and reaction conditions following Murayama’s work [55]. Typ-
ically, 19.2 g of prepared niobium oxalate and 0.711 g of am-
monium oxalate were dissolved in 50 mL deionized water, and
then the transparent solution was aged in a Teflon-lined auto-
clave for 24 h at 180 °C. After cooling down, the solid was fil-
tered, washed with distilled water and then dried at 50 °C
overnight. Finally, the Nb2O5 sample was obtained by calcina-
tion at 400 °C for 4 h in air with a linear heating ramp of 10 °C
min–1.
2% Pd/Nb2O5 catalyst was prepared by incipient wetness
impregnation of L-Nb2O5 with the aqueous solution of
Pd(NO3)2. After impregnation, the catalyst was dried at 100 °C
for 12 h, followed by calcination in air at 400 C for 5 h with a
linear heating ramp of 2 C min–1.
Magnesium-aluminium hydrotalcite (MgAl-HT) with Mg/Al
atomic ratio of 2:1 was prepared according to the literature
[56]. Typically, an aqueous solution was prepared by adding
0.093 mol Mg(NO3)2·6H2O and 0.0465 mol Al(NO3)3·9H2O to
100 g of H2O. A second solution containing 0.219 mol NaOH and
0.0565 mol Na2CO3 in 100 g of H2O was slowly added to the
Mg/Al aqueous solution under constant stirring at 70 °C. Sub-
sequently, the precipitate as obtained was aged at 70 °C for 20
h, filtered and washed with deionized water until pH = 7. The
product was dried in an oven at 80 °C overnight and calcined in
nitrogen flow at 400 °C for 8 h. The mixed oxides were rehy-
drated in decarbonated water at 80 °C for 3 h. Then the acti-
vated catalyst was obtained by drying under vacuum at 100 °C
and denominated as MgAl-HT.
2.4. Characterization
DRIFTS of acetone adsorption were collected with a
NICOLET iS50 FT-IR spectrometer equipped with an MCT/A
detector. Firstly, the catalysts were pretreated in situ in the cell
in N2 at 400 °C for 40 min and background spectra were rec-
orded at 200, 150, 100, and 50 °C, respectively. Then acetone
with N2 was bubbled into the in situ cell for 40 min. Next, the
cell was purged with N2 at 35 °C for 30 min and the adsorption
spectra of acetone was recorded. Finally, the temperature was
increased to 70 °C and 100 °C, respectively, and the adsorption
spectra of acetone were recorded with increasing the temper-
ature.
Powder XRD patterns were recorded with a Rigaku
D/max-2550VB/PC diffractometer by using Cu K (λ = 0.15406
nm) radiation that was operated at 40 kV and 40 mA.
N2 adsorption-desorption isotherms were measured at –196
°C by using a Micromeritics ASAP 2020 M sorption analyzer.
The BET method was used to calculate the specific surface area.
Scanning electron microscopy (SEM) images of samples
were recorded on a Nova NanoSEM 450 microscopy at 3 kV.
Transmission electron microscopy (TEM) images of samples
were obtained on a JEOL Model 2100F electron microscopy at
200 kV.
2.3. Catalytic reactions
The aldol condensation of furfural and 4-heptanone was
performed in a 50 mL Teflon-lined stainless-steel autoclave.
Typically, furfural (0.48 g, 5 mmol), 4-heptanone (2.28 g, 20
mmol) and catalyst (0.1 g) were transferred into the autoclave.
The reactor was charged to 1 MPa N2 pressure. The reaction
was then performed at 110–140 C under magnetic stirring for
4–7 h. The reactor was quenched in an ice-water bath to stop
the reaction immediately. The liquid solution was separated
from the solid catalyst by centrifugation and analyzed qualita-
tively by GC-MS (Agilent 7890A-5975C) and quantitatively by
GC equipped with a flame ionization detector (FID, Agilent
7890) using 2-octanone as the internal standard. Conversion of
The actual Pd loading in the sample was detected by induc-
tively coupled plasma-atomic emission spectroscopy (ICP-AES)
on a Perkin-Elmer Optima 2100 DV spectrometer.
H2-TPR was carried out on a Micromeritics AutoChem II

Yaxuan Jing et al. / Chinese Journal of Catalysis 40 (2019) 1168–1177
1171
2920 System in 10%H2/He, and 0.10 g sample was loaded.
Infrared (IR) spectra of pyridine adsorption were recorded
on a Nicolet NEXUS 670 FT-IR spectrometer. The samples were
pressed into self-supporting disks and placed in an IR cell at-
tached to a closed glass-circulation system. The disk was dehy-
drated by heating at 400 °C for 1 h under vacuum in order to
remove the physically adsorbed water. After the cell was cooled
to room temperature, the IR spectrum was recorded as back-
ground. Pyridine vapor was then introduced into the cell at
room temperature until equilibrium was reached, and then a
second spectrum was recorded. Subsequent evacuation was
performed at 100 °C for 10 min followed by spectral acquisi-
tions. The spectra presented were obtained by subtracting the
spectra recorded before and after pyridine adsorption.
NH3-TPD was carried out in an apparatus (PX200, Tianjin
Golden Eagle Technology Co. Ltd.) equipped with a thermal
conductivity detector (TCD). The samples (100 mg) were
loaded into a U-shaped quartz tube. Prior to TPD measure-
ments, the samples were pretreated in flowing N2 (45 mL min–1
) for 1 h at 500 C, and then cooled to 50 C. NH3 was adsorbed
onto the samples by exposure to flowing 10% NH3 in N2 gas
mixture (50 mL min–1 ) for 45 min at 50 C. Residual and phys-
ical adsorbed NH3 was removed by purging the samples with
flowing N2 (45 mL min–1) at 90 C for 1 h. Desorption of NH3
was performed by heating the samples at a rate of 10 C min–1
under flowing N2 (45 mL min–1) from 90 C to 600 C.
Temperature-programmed desorption of carbon dioxide
(CO2-TPD) as a probe molecule was carried out on a Mi-
cromeritics AutoChem 2920 equipped with mass spectrometry
using a quadruple spectrometer (MS OmniStar). 100 mg of the
catalyst was placed in a quartz sample tube. Before TPD ex-
periments the catalysts were outgassed at 300 °C for 90 min in
a flow of helium. Subsequently, the catalysts were cooled down
to 50 °C and treated under a CO2 flow for 40 min. Weakly ad-
sorbed CO2 was removed by flushing with He at 50 °C for 30
min. The desorption of CO2 was measured by heating the cata-
lyst from 50 °C to 800 °C at a heating rate of 10 °C min–1 in a He
flow. The desorbed products were analyzed by mass spectrom-
etry.
condensation of ABE fermentation mixture [57–59]. There are
three reasons for the selection of this probe reaction. (1) The
aldol condensation of furfural with biomass-derived ketones is
the most common according to previous reports [3]. (2) Com-
pared with common biomass-derived ketones, the steric hin-
drance of 4-heptanone is more severe, so the aldol reaction
would be more challenging. In this way, the catalyst selected in
this reaction can be easily used to other aldol reactions. (3) To
our knowledge, there is no report on the production of
branched alkanes by using 4-heptanone and furfural as pre-
cursors, so we likely provide an alternative strategy for the
synthesis of branched alkanes. Generally, aldol condensation is
carried out in the presence of basic catalysts. Therefore, com-
mon solid-base catalysts, such as MgO, CaO and magnesi-
um-aluminum hydrotalcite (MgAl-HT) (the powder XRD pat-
tern of as-synthesized MgAl-HT see Fig. S1), were tested for the
aldol condensation of furfural with 4-heptanone and compared
with Nb2O5, a solid acid catalyst, the results are shown in Table
1. Surprisingly, Nb2O5 has obviously superior activity over the
solid-base catalysts; the yield of aldol adducts (FH and FHF)
reached 70.8% over Nb2O5 catalyst, it was lower than 30% over
each solid-base catalyst. Given these results, we first suspected
that Nb2O5 is an amphoteric metal oxide that may have both
acidic and basic sites. It is recognized by all that Nb2O5 is an
acid catalyst. To check whether Nb2O5 has basic sites or not,
CO2-TPD was conducted and the result is presented in Fig. S2. It
can be seen that there are no obvious CO2 desorption peaks,
indicating that Nb2O5 has no basic sites. As common Lewis acid
catalysts, ZrO2 and Al2O3 were also used for the aldol condensa-
tion of furfural with 4-heptanone but have relatively poor cata-
lytic performance, and the yields of aldol adducts were only
5.5% and 29.8%, respectively. The above results show that
Nb2O5 is an excellent catalyst for the aldol condensation of fur-
fural with 4-heptanone. The influence of reaction conditions on
the yields of the aldol products was investigated and the results
(Fig. S3) show that a reaction at 130 C for 6 h is optimal for the
aldol condensation of furfural with 4-heptanone. Theoretically,
the prior aldol condensation product should be
a
β-hydroxyketone. However, the GC result of the aldol conden-
sation of furfural with 4-heptanone (Fig. S4) shows that the
obtained products are mainly α,β-unsaturated carbonyl com-
pounds (FH and FHF), rather than β-hydroxyketones. Driven by
the conjugated structure of furan ring and C=C bond, the
β-hydroxyketone can be easily converted into the stable
α,β-unsaturated carbonyl compound. Besides this, it is well
known that acid catalysts can promote the dehydration reac-
3. Results and discussion
3.1. Catalyst screening
A suitable probe reaction is of significant importance for the
catalyst screening of aldol condensation. Here, we employed
the aldol condensation of furfural with 4-heptanone (Scheme 2)
as a probe reaction to screen catalysts. 4-Heptanone is also a
biomass-derived molecule and can be synthesized by the ester-
ification-ketonization of biomass-derived n-butyric acid or
Table 1
Aldol condensation of furfural with 4-heptanone over various catalysts.
Yield (%)
FHF
0.0
0.0
0.0
1.7
0.0
0.0
Conversion of furfural
Catalyst
(%)
30.6
19.4
41.9
82.2
16.9
35.3
FH
17.1
5.5
28.6
69.1
5.5
Total
17.1
5.5
28.6
70.8
5.5
CaO
MgO
MgAl–HT
Nb2O5
ZrO2
Al2O3
29.8
29.8
Reaction conditions: 4-heptanone (2.28 g, 20 mmol), furfural (0.48 g, 5
mmol), catalyst (0.10 g), 130 °C, 4 h.
Scheme 2. Aldol condensation of furfural with 4-heptanone.

1172
Yaxuan Jing et al. / Chinese Journal of Catalysis 40 (2019) 1168–1177
tion, leading to the conversion of β-hydroxyketones into
α,β-unsaturated carbonyl compounds.
ZrO2
3.2. Mechanism studies
Nb2O5
To understand the origin of the excellent activity of Nb2O5
catalyst, first of all, nitrogen adsorption/desorption was carried
out to measure the BET surface areas of Nb2O5, ZrO2 and Al2O3.
Obviously, the BET surface area of Nb2O5 (207 m2 g–1) is close
to that of Al2O3 (218 m2 g–1) and higher than that of ZrO2 (66 m2
g–1), but it is inconsistent with the trend of catalytic activity.
Therefore, we can conclude that the difference of the BET sur-
face area cannot significantly affect the catalytic activity of the
catalysts in the aldol condensation.
Al2O3
1560 1540 1520 1500 1480 1460 1440 1420
-1
Wavenumber (cm )
Fig. 2. Py-FTIR spectra collected after treatment at 100 °C under vac-
First, we guessed that the amount of acidity was responsible
for the catalytic performance and therefore NH3-TPD was car-
ried out to measure the amount of acid and was shown in Fig. 1.
It can be seen that Al2O3 catalyst has markedly higher acid
amount than Nb2O5 and ZrO2, but cannot follow the sequence of
catalytic activity. Interestingly, it is noticed that Nb2O5 has a
certain amount of Brønsted acid sites but ZrO2 and Al2O3 have
hardly any according to the result of Py-FTIR spectra (Fig. 2).
The clear difference offers a possibility, that is, the excellent
performance of Nb2O5 catalyst may be attributed to the
Brønsted acid sites on the catalyst. In order to verify this, we
employed Amberlyst-15 as catalyst, which is a typical Brønsted
acid catalyst, to conduct the aldol condensation under the same
conditions. The results showed that the conversion of furfural
and the yield of aldol adducts were 50.5% and 40.6%, respec-
tively, significantly lower than that (82.2% and 70.8%) over
Nb2O5 catalyst, which indicated that Brønsted acid sites are not
the main origin of the excellent activity of Nb2O5 catalyst.
For understanding the origin of high activity, DRIFTS ex-
periments of acetone adsorption were performed (Fig. 3) to
test the activation ability of Nb2O5, ZrO2 and Al2O3 to C=O bond.
Acetone, which is a simple biomass-derived carbonyl molecule,
was chosen as the probe molecule. For acetone, the peak ap-
peared at 1736 cm–1 region is ascribed to the C=O stretching
vibration, the band at 1213 and 1366 cm–1 are attributed to
C–CH3 deformation modes. After adsorption of acetone on the-
se three catalysts for 30 min and purging with N2 for 30 min at
35 °C, a red shift in the C=O stretching vibration was observed
uum of various catalysts.
in all three catalysts. But, the red shift for the Nb2O5 catalyst
(1695 cm–1, Fig. 3a) is much larger than that for ZrO2 (1703
cm–1, Fig. 3b) and Al2O3 catalysts (1703 cm–1, Fig. 3c). The red
shift of the C=O bond can be interpreted as the strong adsorp-
tion of C=O bond on catalyst, leading to weakness in the
strength of the C=O bond. This result indicates that Nb2O5 cata-
lyst has stronger activation ability for C=O bond than ZrO2 and
Al2O3, which would be the origin of the excellent activity of
Nb2O5 catalyst. To explore the activation behavior of the C=O
bond by reaction temperature, the cell was purged with N2 at
70 and 100 C, respectively, and the adsorption spectra of ace-
tone were recorded. It can be seen that the red shift is more
severe with the increase of temperature and the red shift on
Nb2O5 catalyst was also more evident (1681 cm–1) than that of
ZrO2 (1695 cm–1) and Al2O3 catalysts (1700 cm–1). The striking
difference further confirms that Nb2O5 has the unique ability
for C=O bond activation. It is worth noting that the intensity of
a new band at 1557 cm–1 gradually increased over Nb2O5 with
the rise of temperature, but it did not occur on ZrO2 and Al2O3.
The band at 1557 cm–1 can be assigned to C=C stretching vibra-
tion. There are two possibilities for the origin of C=C stretching
vibration: (1) with the activation of the C=O bond, it is gradual-
ly converted into an enolate intermediate containing a C=C
bond, and thus-generated enolate can undergo nucleophilic
addition with another carbonyl molecule to get target product;
(2) with the increasing of temperature, the self-aldol condensa-
tion of acetone could happen over Nb2O5, thus-generated ad-
duct is a typical α,β-unsaturated carbonyl compound contain-
ing a C=C bond, so the band at 1557 cm–1 may be interpreted as
the C=C bond of the self-aldol adduct of acetone. The two possi-
bilities are both in good consistent with the observed catalytic
activity. Meanwhile, the enolate intermediate certainly exists in
every case, it is merely due to poor stability of the enolate in-
termediate and cannot be seized by the FT-IR spectra in the
second case. Overall, the DRIFTS study of acetone adsorption
reveals that Nb2O5 catalyst has excellent activation ability for
C=O bond and thus generates an enolate intermediate for aldol
condensation, leading to the high activity.
Al2O3
Nb2O5
ZrO2
100
200
300
400
500
Temperature (ºC)
3.3. Proposed reaction mechanism
Fig. 1. NH3-TPD profiles of various catalysts.

Yaxuan Jing et al. / Chinese Journal of Catalysis 40 (2019) 1168–1177
1173
-1
c
1703 cm
-1
a
-1
b
1695 cm
1703 cm
35 °C, N2 purging 30min
-1
1557 cm
35 °C, N2 purging 30min
-1
1685 cm
-1
35 °C, N2 purging 30min
1700 cm
70 °C, N2 purging 5min
100 °C, N2 purging 5min
-1
1702 cm
70 °C, N2 purging 5min
-1
1681 cm
70 °C, N2 purging 5min
-1
1695 cm
100 °C, N2 purging 5min
-1
1700 cm 100 °C, N2 purging 5min
1800 1700 1600 1500 1400 1300 1200
1800 1700 1600 1500 1400 1300 1200
-1
Wavenumber/cm
1800 1700 1600 1500 1400 1300 1200
-1
Wavenumber/cm
-1
Wavenumber/cm
Fig. 3. DRIFTS spectra of acetone adsorption on (a) Nb2O5, (b) ZrO2, and (c) Al2O3.
Based on the DRIFTS study of acetone adsorption, we pro-
pose a possible reaction mechanism of Nb2O5-catalyzed aldol
condensation for two carbonyl compounds, i.e., carbonyl com-
pound 1 containing active α-hydrogen atoms with carbonyl
compound 2, as shown in Scheme 3. First, the metal center of
Nb2O5 polarizes the C=O bond of 1, which forms a metal eno-
late. Subsequently, the metal enolate undergoes nucleophilic
addition to C=O bond in molecule 2 and form a new C–C bond,
which affords 3. Finally, 3 can be easily converted to an enone
compound 4 by the dehydration reaction in the presence of
acidic Nb2O5.
possible factor that higher yield of aldol adducts was obtained
in the 2,4-pentanedione system. We tried to increase reaction
time to improve the yield of aldol adducts and the results
(Scheme 4) show that extending the reaction time from 4 h to
12 h significantly increased the yield of the aldol adducts of
furfural with MIBK (from 60.2% to 81.7%). In addition, the
self-aldol condensation of cyclopentanone over Nb2O5 catalyst
was carried out and the result showed that a moderate yield of
the aldol adducts was obtained. Inspired by the MIBK system,
we extended reaction time to 12 h and achieved a high yield
(87.0%) of aldol adducts (Scheme 4). The above results fully
demonstrate that Nb2O5 is an efficient catalyst for bio-
mass-related aldol reactions.
3.4. Extension to other biomass-derived carbonyl molecules
We were interested to extend the applicability of Nb2O5 to
other biomass-derived carbonyl molecules-involved aldol con-
densation. To explore the scope of the reaction, we investigated
two typical aldol condensation, the cross condensation of two
different carbonyl molecules and the self-condensation of the
same carbonyl molecule (for a structural analysis, see Figs.
S5–S11 in the Supporting Information). Firstly, the aldol con-
densation of furfural with biomass-derived ketones, such as
acetone, methyl isobutyl ketone (MIBK) and 2,4-pentanedione,
were carried out over Nb2O5 catalyst under the same condi-
tions as the probe reaction system and the results are summa-
rized in Scheme S1. It was found that a high yield of aldol ad-
ducts was obtained in the acetone system and
2,4-pentanedione system. However, in the MIBK system, the
yield (60.2%) of aldol adducts is medium, indicating steric hin-
drance of MIBK could be more obvious than acetone and
2,4-pentanedione. Clearly, the alpha-H of 2,4-pentanedione is
more active than that of MIBK owing to the presence of two
electron withdrawing groups in 2,4-pentanedione, which is a
3.5. Recyclability test of Nb2O5
The recyclability of the Nb2O5 catalyst for the aldol conden-
sation of 4-heptanone with furfural was investigated by reusing
the catalyst in consecutive catalytic runs. After reaction, the
color change of the catalyst from original white to ca-
nary yellow was observed, and the conversion of furfural and
the yield of aldol adducts decreased from 55.0% to 20.3% and
from 48.9% to 17.9%, respectively. To discover the reason for
this catalyst deactivation, thermogravimetric analysis (TG) was
performed to characterize the deactivated catalyst. The result
O
O
0.1g Nb
O
, 2.1g (25mmol)
o
Yield: 29.2%
2
5
cyclopentanone, 130 C, 12h
O
Conversion: 94.2%
Yield: 57.8%
Yield: 70.8%
O
0.1g Nb
O
, 1.16g (20mmol) acetone
o
O
2
5
O
O
0.48g(5mmol) furfural, 130 C, 4h
O
O
O
O
Yield: 9.2%
Conversion: 88.9%
O
O
O
O
0.1g Nb
O , 2g (20mmol) methyl isobutyl ketone
2 5
O
Yield: 81.7%
o
0.48g(5mmol) furfural, 130 C, 12h
Conversion: 91.3%
O
O
0.1g Nb
O
, 2g (20mmol) 2,4-pentanedione,
o
2
5
O
O
O
0.48g(5mmol) furfural, 130 C, 4h
O
Yield: 80.5%
O
Conversion: 89.3%
Scheme 3. Proposed reaction mechanism for the aldol condensation of
biomass-derived carbonyl molecules over Nb2O5 catalyst.
Scheme 4. Results of aldol condensation of other biomass-derived
carbonyl molecules over Nb2O5.

1174
Yaxuan Jing et al. / Chinese Journal of Catalysis 40 (2019) 1168–1177
(Fig. S12) showed that ca. 12% carbon was deposited on the
deactivated catalyst, which was ascribed to polymerization of
the reactants and/or products in the presence of acid catalyst
[7,40]. After calcination at 400 C for 1 h, the catalyst was
re-generated and then recharged for the next run. It can be
seen from Fig. 4 that no obvious change was found in the cata-
lytic activity (58.6% conversion and 50.4% yield).
Re-measurement of XRD pattern (Fig. S13) shows that there
was no phase change after reaction, and N2 sorption shows that
the surface area was still high (170 m2 g–1). The above result
clearly demonstrates the excellent reusability of Nb2O5 catalyst
for aldol condensation.
the HDO reaction, the liquid solution was separated from the
solid catalyst by centrifugation and analyzed qualitatively by
GC-MS and quantitatively by GC. After the one-pot two-step
process, no oxygen-containing products were observed in the
reaction effluent. The yield of alkanes is shown in Fig. 5. As
expected, the yield of C12–17 alkanes reached up to 61.7%,
starting from 4-heptanone and furfural. A small amount of C–C
cleavage product (C10 alkane) was generated by the hy-
drocracking of the branched C2 alkyl groups because the hy-
drocracking is easier at the branched carbon atom [11]. In ad-
dition, C7 alkanes were produced owing to incomplete reaction
of 4-heptanone. The condensation product yield was 66.4%
after the first step. Compare for the total yield (61.7%) of
one-pot process, the above result indicates that the aldol con-
densation is the rate-determine step and the following hydro-
deoxygenation is easy to implement, which is consistent with
our previous results [48,49]. In short, the total yield of C10–17
alkanes reached up to 62.5% in the one-pot process. The above
results integrating the aldol condensation with the total HDO
over Pd/Nb2O5 are of interest to save considerable energy con-
sumption and time required for intermediate separation pro-
cesses, which might be an extremely promising reaction system
for the industrial-scale production of liquid alkanes from bio-
mass-derived carbonyl molecules.
3.6. One-pot process
Previously, our group demonstrated that metal-supported
Nb-based catalysts, such as Pd/NbOPO4, Pt/NbOPO4, Pd/Nb2O5
and Pd/Nb2O5/SiO2, are excellent catalysts for the total hydro-
deoxygenation of biomass and biomass-derived oxygenates
[48–53,60]. This is because of the abilities of noble metals for
H2 activation and dissociation, NbOx species for C–O bond
cleavage and acid sites on support for dehydration. Given this
and the above results, we speculated that a one-pot process for
the direct production of liquid alkanes from biomass-derived
carbonyl molecules would be possible over a multifunctional
metal-supported Nb2O5 by combining the aldol condensation
with the following total hydrodeoxygenation. Therefore, we
draw an experimental scheme to realize one-pot process by
using Pd/Nb2O5 as catalyst and the detailed process is pre-
sented in the experimental section. The details of the Pd/Nb2O5
catalyst characterization are provided in the Supporting Infor-
mation. A key in the one-pot two-step process is to change the
reaction atmosphere, as the aldol condensation and the follow-
ing HDO should be conducted in the atmosphere of nitrogen (as
a shielding gas) and hydrogen (as a reactant gas), respectively.
Firstly, furfural, 4-heptanone, cyclohexane and 2%Pd/Nb2O5
were transferred into the autoclave to conduct aldol condensa-
tion. The reactor was charged to 1 MPa N2 pressure as a
shielding gas. After the aldol reaction, the reactor charged to 6
MPa H2 pressure to perform the following HDO reaction. After
Based on the above results and previous reports [51,52],
Pd/Nb2O5 as a multifunctional catalyst plays four roles in the
conversion of biomass-derived carbonyl molecules into liquid
alkanes: (1) Nb2O5 helps to activate C=O bond of carbonyl mol-
ecules to promote aldol condensation; (2) the metal is used for
H2 activation and dissociation; (3) NbOx species promote C–O
bond cleavage; and (4) the niobium-based solid acid catalyzes
the dehydration.
4. Conclusions
We have presented the exceptional activity and reusability
of Nb2O5 catalyst for the aldol condensation of biomass-derived
carbonyl compounds. Among the investigated catalysts (CaO,
MgO, MgAl-HT, ZrO2, Al2O3 and Nb2O5), Nb2O5 showed superior
60
50
40
30
20
10
0
60
50
40
Conv. of furfural
Yield of FH
Yield of FHF
30
20
10
0
C10
C12
C7
C17
1
4
3
2
Fig. 5. Alkanes yield obtained from the one-pot conversion of furfural
and 4-heptanone over Pd/Nb2O5. Reaction conditions: 4-heptanone
(0.57 g, 5 mmol), furfural (0.96 g, 10 mmol), cyclohexane (5.00 g), 2%
Pd/Nb2O5 (0.3 g), 130 °C, 24 h, 1 MPa N2 for aldol condensation; 220 °C,
24 h, 6 MPa H2 for HDO reaction.
Run number
Fig. 4. Recyclability test of Nb2O5 for aldol condensation. Reaction con-
ditions: furfural (0.48 g), 4-Heptanone (2.28 g), Nb2O5 (0.1 g), 110 C, 4
h.

Yaxuan Jing et al. / Chinese Journal of Catalysis 40 (2019) 1168–1177
1175
activity for the aldol condensation of furfural with 4-heptanone.
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Graphical Abstract
Chin. J. Catal., 2019, 40: 1168–1177 doi: S1872-2067(19)63371-1
Highly efficient Nb2O5 catalyst for aldol condensation of biomass-derived carbonyl molecules to fuel precursors
Yaxuan Jing, Yu Xin, Yong Guo *, Xiaohui Liu, Yanqin Wang *
East China University of Science and Technology
We demonstrated the exceptional activity of Nb2O5 catalyst for aldol condensation and provided the originate of high activity.

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氧化铌高效催化生物质衍生物的羟醛缩合反应制备燃料前驱体
景亚轩†, 辛 宇†, 郭 勇#, 刘晓晖, 王艳芹*
华东理工大学化学与分子工程学院, 工业催化研究所, 上海市功能材料化学重点实验室, 上海200237
摘要: 生物质是自然界唯一可再生的有机碳来源, 其催化转化制备燃料和精细化学品受到广泛关注. 一般从生物质衍生的
小分子出发制备航空煤油和柴油等高碳数燃料包括两步: 首先通过碳链增长反应如羟醛缩合和烷基化等制备目标链结构
的燃料前驱体, 然后对所得燃料前驱体进行完全脱氧加氢得到燃料组分. 在目前报道的碳链增长反应中, 羟醛缩合反应应
用最多, 因为生物质衍生的羰基化合物众多, 而且丙酮、糠醛及环己酮等羰基化合物已经实现了工业化生产. 羟醛缩合反
应的催化剂以碱性材料为主, 但是碱性催化剂具有不抗二氧化碳、不抗水等缺点, 因此开发高活性的固体酸催化剂至关重
要.
本文选择糠醛和4-庚酮的羟醛缩合反应作为模型反应, 发现氧化铌材料对该反应具有优异的催化活性, 活性明显高于
常见的氧化钙、氧化镁、镁铝水滑石等固体碱和氧化铝、氧化锆等固体酸. 并采用N2吸附-脱附、二氧化碳程序升温脱附
(CO-TPD)、吡啶吸附红外光谱(Py-FTIR)、程序升温氨脱附(NH3-TPD)和丙酮的原位吸附傅立叶变换红外光谱等表征手段

Yaxuan Jing et al. / Chinese Journal of Catalysis 40 (2019) 1168–1177
1177
研究了氧化铝, 氧化锆和氧化铌催化性能差异的原因. 结果表明, 这三种催化剂的比表面积和酸性质与反应活性没有明显
关联. 为了解释反应活性的差异, 我们采用丙酮原位吸附红外光谱探究了这三种催化剂对羰基的活化. 发现相比于氧化铝
和氧化锆催化剂, 氧化铌对丙酮的C=O双键有更强的活化能力, 使C=O双键结构更容易转化为烯醇式结构, 进而促进羟醛
缩合反应的进行. 基于此, 我们提出了氧化铌催化羟醛缩合的反应机理. 另外, 底物拓展实验发现, 在其他生物质相关的羟
醛缩合反应体系中, 氧化铌催化依旧可以得到可观的收率. 循环套用实验表明, 由于积碳导致催化剂失活, 通过简单的煅
烧即可恢复活性.
我们之前的研究发现负载金属的铌基催化剂具有很好的断裂呋喃环上碳氧键的能力, 是很好的脱氧加氢催化剂. 基
于以上研究和我们之前的结论, 我们设计并制备了多功能的Pd/Nb2O5催化剂, 对该催化剂进行了结构表征, 并对羟醛缩合
和脱氧加氢过程进行了设计整合, 一锅实现了羟醛缩合和后续的脱氧加氢.
关键词: 羟醛缩合; 氧化铌; C=O活化; 燃料前驱体; 生物燃料; 一锅过程
收稿日期: 2019-02-26. 接受日期: 2019-04-06. 出版日期: 2019-08-05.
*通讯联系人. 电话/传真: (021)64253824; 电子信箱: wangyanqin@ecust.edu.cn
#通讯联系人. 电话/传真: (021)64253824; 电子信箱: guoyong@ecust.edu.cn
†共同第一作者.
基金来源: 国家自然科学基金(21832002, 21872050, 21808063); 中央高校基本科研业务费专项资金(222201718003); 上海市自然
科学基金(18ZR1408500, 10dz2220500); 华工理工大学“张江树优博”培育计划.
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).