Selective carbon-chain increasing of renewable furfural utilizing oxidative condensation reaction catalyzed by mono-dispersed palladium oxide

Article abstract of DOI:10.1016/j.mcat.2019.110545

A novel carbon-chain increasing valorization of furfural (FUR), a cheap bio-based platform compound, has been successfully performed via the catalytic oxidative condensation process with mono-dispersed palladium oxide catalyst. For the reaction of furfural, n-propanol and O₂, the PdO?TiO₂ exhibited a prominent catalytic activity in which a 77.8% conversion of FUR with 89.2% selectivity of 3-(furan-2-yl)-2-methylacrylaldehyde was obtained. Based on the XRD, SEM, TEM, HRTEM, XPS, and UV–vis results of catalysts, it was concluded that high activity is associated with particle size, uniform distribution of palladium oxide and surface area of the support. Moreover, the recycling experiments confirmed that mono-dispersed palladium oxide catalyst was stable, in which it still kept a good catalytic performance after being recycled for five times. This provided a new route for the selective transformation of the hemicellulose-derived platform compounds.

Full text of DOI:10.1016/j.mcat.2019.110545

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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Selective carbon-chain increasing of renewable furfural utilizing oxidative
condensation reaction catalyzed by mono-dispersed palladium oxide
Xinli Tonga,⁎, Zhenya Zhanga, Yiqi Gaoa, Yue Zhanga,⁎, Linhao Yua, Yongdan Lib
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 School of Chemical Engineering, Tianjin University, Tianjin, 300072, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Oxidation
Furfural
Molecular oxygen
Palladium
A novel carbon-chain increasing valorization of furfural (FUR), a cheap bio-based platform compound, has been
successfully performed via the catalytic oxidative condensation process with mono-dispersed palladium oxide
catalyst. For the reaction of furfural, n-propanol and O2, the PdO@TiO2 exhibited a prominent catalytic activity
in which a 77.8% conversion of FUR with 89.2% selectivity of 3-(furan-2-yl)-2-methylacrylaldehyde was ob-
tained. Based on the XRD, SEM, TEM, HRTEM, XPS, and UV–vis results of catalysts, it was concluded that high
activity is associated with particle size, uniform distribution of palladium oxide and surface area of the support.
Moreover, the recycling experiments confirmed that mono-dispersed palladium oxide catalyst was stable, in
which it still kept a good catalytic performance after being recycled for five times. This provided a new route for
the selective transformation of the hemicellulose-derived platform compounds.
Biomass transformation
1. Introduction
[10] performed FUR oxidation in aqueous media with H2O2 as terminal
oxidant, and a 74.2% yield of succinic acid was obtained at 80 °C. In
The excessive development and utilization of fossil energy have
caused more and more worldwide problems, such as environmental
pollution and the greenhouse effect [1,2]. Thus, exploring a new, green,
low-carbon, and recycling energy has gradually become the hot topic
on the present chemical industry and engineering research. In all re-
newable energy substitutes, biomass feedstock has attracted extensive
attention from researchers because of its nature of renewable, clean,
abundant and sustainable application [3–5]. Therein, a feasible tech-
nology to resolve the energy problem is the efficient conversion of
biomass feedstocks to the fine chemicals and the high-quality fuels
through the catalytic processes.
Furfual (FUR), derived from the hemicelluose of lignocellulose, is a
very important bio-based platform compound that can be converted to
numerous valuable chemicals by catalytic reduction [6,7], oxidative
transformation [8–10], decarboxylation [11] acetalization [12,13] or
aldol condensation [14–16], etc. For example, Taarning’s research
group found that the oxidative esterification of FUR was achieved in
methanol using the Au/TiO2 as the solid catalyst, and a 99% yield of
methyl fuorate was acquired under the optimized conditions [8]. Gupta
et al. [9] conducted aerobic oxidation of FUR to furoic acid using an N-
heterocyclic carbene as a homogeneous catalyst and obtained a 99%
product yield at 40 °C in dimethyl sulfoxide for 4 h. Choudhary’s group
addition, aldol condensation of FUR and ketones was also studied
widely to produce the value-added compounds as the feedstock of
biofuel, in which the basic sodium hydroxide [17] and Mg-Zr mixed
oxides [18] are often employed as the catalysts.
Furthermore, FUR can also be used to synthesize the carbon-chain
growing products by carrying out the oxidative condensation with ali-
phatic alcohol, which is a very important approach to prepare high
quality aviation kerosene [19,20]. During the oxidative condensation,
generally used catalysts are Au, Pt, Co, Fe and Cu metals [21–27]. For
example, using Au/FH as the catalyst, the condensation reaction of FUR
with n-propanol was performed successfully, in which ca. 91.2% yield
of main product 3-(furan-2-yl-)-2-methylacrylaldehyde was attained
[21]. Moreover, in the FUR-ethanol-O2 system, a more than 63.7% yield
of the target product furan-2-acrolein could be obtained with the Pt/FH
as catalyst under the suitable conditions [22]. Besides, it was also
presented that metallic Co-based catalyst derived from a two dimen-
sional metal-organic framework could effectively promote selective
transformation of FUR with n-propanol under the oxygen atmosphere.
Correspondingly, a near to 85.0% yield of 3-(furan-2-yl-)-2-methylacryl
aldehyde was acquired [23]. In particular, our group found a novel
versatile catalyst of iron, which offered a 70% yield of the main product
furan-2-acrolein during the oxidation-condensation reaction of FUR,
⁎ Corresponding authors.
E-mail addresses: tongxinli@tju.edu.cn, tongxli@sohu.com (X. Tong), 273551472@qq.com (Y. Zhang).
https://doi.org/10.1016/j.mcat.2019.110545
Received 7 April 2019; Received in revised form 29 July 2019; Accepted 31 July 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

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X. Tong, et al.
MolecularCatalysis477(2019)110545
ethanol and molecular oxygen [27]. Although the precious metal cat-
alysts have showed excellent catalytic performance, the less reserve,
high price and agglomeration effect under harsh conditions hinder the
development and spreading of noble metal catalysts. In particular, most
noble metal catalysts usually have high metal loading which may in-
crease the cost and cause aggregation of the particles. Thus, a lot of
effort has been made to design and to develop of novel noble catalysts
by researchers with improved atomic utilization. Mono-dispersed cat-
alyst, with the atomic utilization of up to 100%, is considered to be a
promising catalyst to meet the requirement [28–31].
In this work, a novel PdO@TiO2 nanomaterial was fabricated, by a
facile method, and pioneered application of the catalyst to promote the
tandem oxidative condensation of FUR with aliphatic alcohol. Herein,
homogenous Pd nanoparticles were evenly anchored on the special 2D
TiO2 support by a simple photochemical reduction method. As a result,
the high conversion of FUR and high selectivity of target product were
acquired in the reaction of FUR with n-propanol-O2 with Pd-based
catalysts. Moreover, the catalytic reaction mechanism is proposed ac-
cording to the control experiment results in which Pd(II) species play a
crucial role in the catalytic reaction.
centrifugation. Finally, the product was dried in a vacuum oven.
2.2.2. Preparation of palladium oxide supported catalysts
The above TiO2 nanosheet (17.4 mg) was added to a certain amount
of water, and then 0.5 mL H2PdCl4 solution (5 mM, 2.5 μmol Pd) was
added with a magnetic stirring. The mixture was then treated under the
UV light (365 nm wavelength) for 5 min. After that, the obtained grey
catalytic material was further washed by the diluted water. Therein, the
product was collected after the release of Cl− was confirmed (The
amount of Cl− ions was measured by the Mohr titration method which
uses chromate ions as an indicator, which is provided in the supporting
information). After being dried on vacuum, the powder was grinded
and named as Pd@TiO2. Then, the solid was further calcined in air at
350 °C for 2 h, and named as PdO@TiO2 catalyst.
The light yellow PdO@TiO2 powder was further treated in a stream
of H2 (flowing rate: 10 mL/min) at 120 °C for 2 h. The product was
named as r-Pd@TiO2.
As comparison, the n-TiO2 material was firstly prepared by simple
ammonia precipitation method from TiCl4. In the following, using n-
TiO2 as the solid support, the PdO/n-TiO2 catalyst was prepared by
reduction loading method, which is similar with above UV treatment on
the PdO@TiO2 synthesis.
2. Experimental
2.2.3. General conditions for oxidative condensation of FUR with alcohol
The selective transformation of FUR with alcohol was carried out in
the stainless steel reactor. A general testing procedure is presented as
follows: 0.200 g of FUR, 0.050 g of PdO@TiO2 catalyst, 0.050 g of
K2CO3 and 15 mL of n-propanol were added to a 120 mL autoclave. The
atmosphere in the reactor was exchanged by pure oxygen for three
times, and then the autoclave was sealed. Before the heating, the
pressure of oxygen was controlled to 0.3 MPa for the oxidation process.
Next, the temperature was raised to 140 °C, and the time was 4 h. After
the oxidative condensation, the mixture was slowly cooled and the
excessive gas was discharged. Finally, the solid PdO@TiO2 catalyst was
separated, and the products were detected by gas chromatography -
mass spectrometry (GC–MS) instrument.
2.1. Reagents and equipment
All reagents and chemicals used were of analytical grade unless
otherwise specified. Palladium chloride (PdCl2, 59.5%) was purchased
from Energy Chemical (Shanghai, China), Titanium tetrachloride
(TiCl4), CeO2, gama-Al2O3, Pd/C, ammonia water, ethylene glycol (EG)
and absolute ethanol was purchased from Aladdin Chemistry Co. ltd.
(Shanghai, China). Deionized water was used from commercial during
this study.
The morphology and particle size of catalytic materials were re-
spectively detected by SEM and TEM instruments. In addition, the XRD
patterns were carried out using a Rigaku D/max-IIIA diffractometer
(Cu-Kα, λ =1.54056 Å). The IR spectrum was characterized with a
Bruker Vertex-70 Spectrometer. The UV–vis spectrum was investigated
with Hitachi-3900 UV–vis Spectrophotometer. For the characterization
of surface acidity of catalysts, the NH3-TPD technique was also per-
formed. The as-prepared samples were treated at 650 °C for 1 h, and
purged with NH3 gas to saturation and kept at 50 °C for 1 h, and then
streamed with Ar for 0.5 h at 120 °C before being cooled to room
temperature, and then warmed up to 800 °C with the temperature
control program (10 °C /min). The pore structures of catalytic materials
were tested by the Micromeritics ASAP2020 M system. Next, the TG
results were recorded from 40 °C up to 800 °C with a 10 °C/min heating
rate. The oxidation states of Pd in the catalysts were studied by XPS
technique. The UV treatment of the catalysts were performed using a
Xenon-lamp light source with filter at 365 nm, in which a power density
of about 1.94 mW/cm2 was used. The quantitative analysis of the pro-
ducts was performed on a Gas Chromatography (GC) with a hydrogen
ion flame detector. The molecular structures of products were de-
termined with gas chromatography and mass spectrometry (Agilent
7890/5975C) apparatus.
3. Results and discussion
3.1. Physical properties of catalytic materials
The formation and crystalline structure of the product was con-
firmed by XRD. As shown in Fig. 1, the diffraction peaks at the 2θ values
of 14.38°, 24.86°, 27.72°, and 48.31° can be attributed to (001), (110),
(002), and (020) facets of TiO2 phase with the PDF card No. 74-1940,
which is consistent with the literature reported.21 The diffraction peak
at around 7.2° can be attributed to the 2D layered structure of TiO2
2.2. Synthetic procedure of different catalytic materials
2.2.1. The preparation of ultrathin TiO2 nanosheet
The general preparation route of TiO2 nanosheet is as follows: about
1 mL of TiCl4 solution was slowly added into 30 mL EG, and then the
mixture was placed at room temperature for more than 2 h to verify the
release of HCl gas; next, 1 mL deionized water (˜18 MΩ cm) was ap-
pended to the mixture under stirring. The obtained colorless solution
was transferred into the stainless-steel autoclave for further hydro-
thermal treatment for 6 h at 150 °C. After being cooled to the room
temperature, the solid products were attained through the
Fig. 1. XRD patterns of different catalytic materials.
2

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X. Tong, et al.
MolecularCatalysis477(2019)110545
Fig. 2. SEM images of TiO2 (a), the TiO2-calcined (b), Pd@TiO2 (c, d) and PdO@TiO2 (e, f).
nanosheet.28 While, after being calcinated at 350 °C for 2 h, the dif-
fraction peak at around 7.2° disappears which indicates a structure
change of TiO2 nanosheets to the aggregates of TiO2 nanoparticles.
Besides, it is noticeable that after calcination the intensity of the dif-
fraction peaks of PdO@TiO2 and TiO2 are increased, which indicates
that the crystallinity elevates by heating. Moreover, the diffraction
peaks of mono-metallic Pd nanoparticles cannot be seen from the XRD
images, which could also confirm the possibility of the existence of
mono-dispersed palladium.
The morphologies of catalytic materials have been characterized by
SEM and TEM techniques. It was shown from Fig. 2a and b that TiO2
shows the regularly flower-like nanospheres, in which thickness of the
surrounding curved flake was about several nanometers. In addition, it
can be seen that there exist numerous pores on the surface of TiO2
nanosheets, in which the well-distributed pores are helpful to the ad-
sorption of small molecules. After the anchor of mono-metallic Pd, the
morphology of Pd@TiO2 shows a little change with some aggregation of
TiO2 nanospheres (Fig. 2c and d). Then, after being calcinated in air at
350 °C for 2 h, the morphology of the following product PdO@TiO2
shows more aggregation. The flower like nanospheres that assembled
by TiO2 nanosheets disappear. TiO2 shows microsphere morphology
which is assembled by lots of nanoparticles (Fig. 2e and f).
The transformation of the TiO2 support can be further verified by
the TEM images of PdO@TiO2. It can be seen from Fig. 3a and b that the
morphology of TiO2 has been changed to nanoparticles where the
structure of 2D nanosheets disappeared. In TiO2 crystal, the interplanar
distances of 0.521 nm and 0.211 nm could be attributed to the (200)
and (003) facets of the TiO2 phase (shown in the HRTEM image of the
Fig. 3b). Meanwhile, mono dispersed Pd nanoparticles have been oxi-
dized and aggregated, which can be detected by HRTEM (Fig. 3c and d).
In PdO@TiO2 material, the corresponding interplanar distance of
0.304 nm can be attributed to the (100) facet of the PdO phase.
3

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MolecularCatalysis477(2019)110545
Fig. 3. TEM (a, b) and HRTEM (c, d) images of PdO@TiO2.
Fig. 4. The XPS spectrum of PdO@TiO2 (a) and its high-resolution survey of Pd 3d region (b).
catalyst with Pd 3d5/2 binding energy values of 336.30 and 337.85 eV,
respectively. The peak of Pd species at 336.30 eV should be related to
the species of Pd2+ in the form of PdO on the surface of PdO/TiO2
catalyst, while the peak at 337.85 eV should be related to Pd species
with a higher oxidation valence in the form of PdO2; Moreover, the XPS
results of used PdO@TiO2 catalysts (presented in Fig. S4 of supporting
information) showed two states of Pd species on PdO/TiO2 catalyst still
existed, only the relative peak area were different, which indicates that
after the heating treatment under the air Pd has been oxidized into PdO
and PdO2 species. Furthermore, the ICP-AES result shows that the
loading content of PdO is as low as 1.35 wt.%, and it is similar to that
calculated according to the Pd/Ti molar ratio obtained from the XPS
characterization result. These further confirm the mono-dispersion of
Pd species on the TiO2 support.
Table 1
The physical property of the synthesized different catalysts.
Catalyst
BET surface
area
Pore Volume Average pore diameter (Å)
(cm3 g−1
)
(m2 g−1
)
BJH
BJH
adsorption
desorption
TiO2
343.6
0.3934
0.3631
0.2411
0.2281
33.82
66.89
40.09
66.11
30.47
65.17
35.95
63.81
TiO2-calcined 164.3
Pd@TiO2
PdO@TiO2
181.3
105.2
Moreover, the existence of Pd element and the PdO phase have also
been verified by performing the XPS characterization and ICP-AES de-
tection. As seen from Fig. 4a, the peaks of Ti, O, Pd, C and Cl elements
were respectively detected in the full XPS spectrum of PdO@TiO2 na-
nomaterial. In Fig. 4b, there are two states of Pd species on PdO/TiO2
In order to further investigate the solid surface characters of the
synthesized catalysts, BET surface measurement has been conducted.
Table 1 shows the textural properties of different catalysts measured by
4