Au-catalyzed oxidative condensation of renewable furfural and ethanol to produce furan-2-acrolein in the presence of molecular oxygen

Article abstract of DOI:10.1016/j.apcata.2015.11.025

A specialty chemical compound, furan-2-acrolein, can be produced from renewable feedstock via furfural. An efficient catalytic process related to oxidative condensation of furfural and ethanol using supported Au catalyst has been developed. A series of solid catalysts including Au/Al₂O₃, Au/CeO₂, Au/Fe₃O₄, Au/HTc (hydrotalcite calcined) and Au/Nb₂O₅ are prepared and their catalytic activities in oxidative condensation of furfural with ethanol are compared. Moreover, the influence of the presence of base promoters, such as K₂CO₃, on Au catalyst performance is investigated. The physical properties of these catalysts are characterized by XRD, TEM, SEM, UV-vis and BET techniques. The impact of reaction variables on catalyst performance is measured. It has been found that, in the presence of Au/Al₂O₃ and potassium carbonate, 94.1% conversion of furfural and 75.0% selectivity to furan-2-acrolein are obtained under the conditions of 140 °C, 0.3 MPa of O₂ and reaction time of 4 h.

Full text of DOI:10.1016/j.apcata.2015.11.025

Accepted Manuscript
Title: Au-Catalyzed Oxidative Condensation of Renewable
Furfural and Ethanol to Produce Furan-2-acrolein in the
Presence of Molecular Oxygen
Author: Xinli Tong Zonghui Liu Jianli Hu Shengyun Liao
PII:
S0926-860X(15)30253-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.11.025
APCATA 15652
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
12-8-2015
20-10-2015
14-11-2015
Please cite this article as: Xinli Tong, Zonghui Liu, Jianli Hu, Shengyun Liao, Au-
Catalyzed Oxidative Condensation of Renewable Furfural and Ethanol to Produce
Furan-2-acrolein in the Presence of Molecular Oxygen, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2015.11.025
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Au-Catalyzed Oxidative Condensation of Renewable Furfural and Ethanol
to Produce Furan-2-acrolein in the Presence of Molecular Oxygen
Xinli Tong* tongxinli@tju.edu.cn or tongxli@sohu.com, Zonghui Liu, Jianli Hu*
jianlihu@hotmail.com, Shengyun Liao
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry
and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P. R. China
*
Corresponding author. Tel.: +86-22-60214259; Fax: +86-22-60214259.
1

Graphical Abstract
The efficient aerobic oxidative condensation of renewable furfural and ethanol in the presence of a nano Au
catalyst and potassium carbonate has been developed and a 70.6% yield of furan-2-acrolein is obtained.
2

Highlights




A catalytic system is developed for the volarization of renewable furfural.
Oxidative condensation of furfural with ethanol to furan-2-acrolein is achieved.
The catalyst system is composed of Au nanoparticle and potassium carbonate.
A 94.1% furfural conversion in 75.0% selectivity of furan-2-acrolein is obtained.
3

Abstract
A specialty chemical compound, furan-2-acrolein, can be produced from renewable feedstock via
furfural. An efficient catalytic process related to oxidative condensation of furfural and ethanol using
supported Au catalyst has been developed. A series of solid catalysts including Au/Al
Au/Fe , Au/HTc (hydrotalcite calcined) and Au/Nb are prepared and their catalytic activities in
oxidative condensation of furfural with ethanol are compared. Moreover, the influence of the
presence of base promoters, such as K CO , on Au catalyst performance is investigated. The physical
2 3 2
O , Au/CeO ,
3
O
4
2 5
O
2
3
properties of these catalysts are characterized by XRD, TEM, SEM, UV-vis and BET techniques.
The impact of reaction variables on catalyst performance is measured. It has been found that, in the
2 3
presence of Au/Al O and potassium carbonate, 94.1% conversion of furfural and 75.0% selectivity
to furan-2-acrolein are obtained under the conditions of 140 ºC, 0.3 MPa of O
hours.
2
and reaction time of 4
Keywords: Furfural; Ethanol; Gold; Furan-2-acrolein; Biomass conversion.
4

1. Introduction
Nowadays, growing concerns about depletion of petroleum reserve and climate change have been a
driving force for the exploration of alternative resources for energy and chemical production. Therein,
the applications of photosynthesis, wind-based, geothermal, and biomass-based energy source appear
to be promising in the future. Particularly, renewable biomass is generally considered as the only
renewable resource with the potential to replace the fossil fuels for the production of both chemicals
and liquid fuels [1-3]. Obviously, the selective transformation of biomass or bio-based platform
compound via catalytic reaction is beneficial to the increase in efficiency and reduction in energy
consumption in a sustainable industrial process [4]. Furfural (FUR) is one of the most common
chemicals derived from lignocellulose with an annual production volume of more than 200 000 ton
[5]. FUR is often obtained from C5 carbohydrates and its industrial application is critical in
valorization of hemicellulose in biomass [6-7]. A variety of chemicals can be synthesized from
FUR[8], wherein FUR is converted by hydrogenation, oxidation, reductive amination,
decarbonylation, nitration, condensations, etc. [9]. For example, aldol condensation of FUR with
acetone has been proposed as an intermediate step to synthesize second generation biofuels from
lignocellulosic biomass [10-14]. In organic chemistry, the condensation of FUR and aldehydes is a
feasible pathway to form longer chain but low volatile liquid transport fuels. Furthermore, acetone
and aldehydes can be produced from the oxidation of secondary and primary alcohols in the presence
of molecular oxygen [15].
On the other hand, ethanol is considered as one of important molecular building blocks, having
established production volume from renewable feedstock [16, 17]. The conversion of bioethanol to
commodity chemicals via chemical or biochemical methods has been studied [18-20]. Thus, the
5

cascade transformation of alcohol oxidation and aldol condensation for the formation of furanic
derivatives prove to be effective utilization of renewable chemical intermediates. However, it is
2
inevitable that many other reaction pathways can take place in the FUR-ethanol-O (FEO) system
when molecular oxygen is chosen as an oxidant. As reported in our previous publication [21], the
possible reaction pathways are given in Scheme 1. It is shown that the first reaction may be semi
2 2
acetal reaction of FUR with ethanol, oxidation of FUR with O or oxidation of ethanol with O ,
where the condensation of FUR with ethanol hardly happens. In the following, the product ethyl
furoate can be obtained via the oxidation of ethoxy(furan-2-yl)methanol or the esterification of
2
-furoic acid, while the product furan-2-acrolein can be attained from the condensation of FUR with
acetaldehyde. It is obvious that the oxidative condensation reaction and the oxidative esterification
reaction are two competing pathways.
In this article, we propose a selective oxidative condensation of FUR and ethanol using supported Au
and base promoter as the catalyst in the presence of molecular oxygen. It is found that the main
product is furan-2-acrolein where a 94.1% conversion of FUR and a 75.0% selectivity is obtained at
1
2
40 ºC under 0.3 MPa of O for 4 hours.
2
. Experimental
2
.1 Reagents and equipment
The following analytical grade chemicals were purchased from commercial venders: furfural, ethanol,
HAuCl , NaOH, KOH, Fe (SO , FeSO , Mg(NO ·H O, Al(NO ·9H O, Ce(NO ·6H O, urea,
Nb , NaCO , NaHCO , KHCO , Cs CO , Ca(OH) , Li CO , H PO , CaCO , MgO and CaO. The
4
2
4
)
3
4
3
)
2
2
3
)
3
2
3
)
3
2
2
O
5
3
3
3
2
3
2
2
3
3
4
3
high purity oxygen gas supplied in a high-pressure cylinder was used without further treatment.
X-ray diffraction (XRD) measurement was performed by diffractmeter with Cu Ka radiation (0.02 º
6

resolution) and the diffraction spectra were collected from a range of 20 to 80 º [2θ]. Moreover, the
surface morphology and particle size of catalytic materials were characterized by scanning electron
microscope (SEM: JSM-6301F, JEOL) and transmission electron microscope (TEM: JEM-2100,
JEOL). The solid UV characterization was carried out on a U-3310 apparatus from HITACHI
corporation. BET surface areas, pore volumes, and average pore diameters of the catalyst samples
2
were measured by N (77 K) adsorption using a Micromeritics ASAP2020M system. Therein, the
catalyst samples were pretreated under vacuum at 250 ºC for 4 hours before the measurement. The
average pore diameter data were calculated according to the Barrett-Joyner-Halenda (BJH) model in
absorption and desorption period.
The quantitative analysis of the products was performed on a GC with hydrogen ion flame detector.
GC-MS analysis was carried out to identify chemical structures of the products (Agilent 6890/5973)
In addition, nuclear magnetic resonance (NMR) measurement was performed on a Bruker 400 MHZ
spectrometer.
2
2
.2 Synthesis of Different Metal Oxides as the Supports
.2.1 The Preparation of Alumina
3 3 2 4 2 3
A certain amount of Al(NO ) ·9H O and (NH ) CO was dissolved in deionized water，then
transferred into a 250 ml three-neck flask. The pH value of the solution was controlled at about 8.5.
After the precipitation is completed，the solution was stirred for another 2h. The precipitates were
filtered and washed repeatedly with the deionized water. After drying at 373 K for 24 hours, the
catalyst sample was calcined at 873 K for 2 hours.
2
.2.2 The Preparation of Iron Oxide
Fe (SO (0.01mol) and FeSO
2
4
)
3
4
(0.01mol) were dissolved in water in a three-neck flask. The
7

o
flask was placed in water bath at 45 C, in which ammonia was added dropwise until pH=10-11. The
reaction was continued for 1 hour, after which the substrate was separated using magnetic separation
o
technique. The substrate was washed with distilled water until pH=7, then vacuum dried at 80 C
overnight.
2
.2.3 The Preparation of Hydrotalcite Calcined (HTc)
To prepare HTc, 30.76 g of Mg(NO ·6H O and 15 g Al(NO ) ·9H O were dissolved in 400ml of
3
)
2
2
3 3 2
water, followed by the addition of 72 g urea under agitation. The solution was maintained at boiling
point for 8 hours, then cooled to room temperature under which precipitation took 12 hours to finish.
o
o
The precipitates were washed with distilled water at 60 C overnight and calcined at 500 C for 3
hours. The catalyst sample was designated as HTc with molar ratio of Mg/Al being 3.
2
.2.4 The Preparation of Cerium Oxide
To synthesis nanorods, 0.868 g of Ce(NO ) ·6H O and 9.6 g of NaOH were dissolved in 5 and 35 ml
3 3 2
of deionized water, respectively. These two solutions were mixed in a Teflon bottle under agitation
for 30 min. The formation of a milky slurry was observed. Subsequently, the mixed solution was
o
transferred to a stainless steel autoclave, heated to 373 C and held for 24 hours to form nanorods.
After the hydrothermal treatment, white precipitates were separated by centrifugation, washed with
deionized water and ethanol several times, followed by drying at 60 ºC in air overnight.
2
.3 The Preparation of Supported Au Catalysts
Au was deposited onto the above metal oxide supports via a precipitative deposition method.
Typically, 40 ml HAuCl aqueous solution (1.25 mg Au/ml) was charged into a beaker. The pH of
the solution was adjusted to 9 by dropwise addition of 0.1 M NaOH solution under vigorous
4
2 3
agitation and monitored with a pH meter. One gram of each support prepared above (Al O , HTc,
8

2 3 4 2 5
CeO , Fe O and Nb O ) was separately added to the solution under agitation, and the pH values of
the suspension were readjusted to 9 using 0.1 M NaOH solution. The suspension was heated to 65 ºC
under agitation and held at the temperature for 1 hour. The precipitates were filtered, washed with
deionized water repeatedly and dried at 60 ºC overnight. The catalyst samples were dried at 60 ºC
then calcined at 400 ºC for 3 hours. These catalysts were designated as Au/Al
Au/Fe and Au/Nb catalyst, respectively.
.4 General Procedure for the Reaction of FUR and Ethanol in the Presence of Molecular
2 3 2
O , Au/HTc, Au/CeO ,
3
O
4
2 5
O
2
Oxygen
Catalytic experiments were carried out in a 120 ml autoclave equipped with a magnetic agitator and
automatic temperature control. A typical procedure for the reaction of FUR with ethanol is described
2 3 2 3
as follow: 15 ml ethanol, 0.2 g FUR, 0.05 g Au/Al O and 0.05 g K CO are charged into the
autoclave. The autoclaved is sealed and purged. Oxygen is introduced at room temperature till P=0.3
o
MPa, then heated to 140 C and kept for 4 hours. After the reaction, the autoclave was cooled, purged
and the products were analyzed by GC and GC-MS.
3. Results and Discussion
3
3
.1 The Characterization of Catalysts
.1.1 XRD Patterns
Illustrated in Figure 1 are XRD patterns of different Au supported catalysts. The peak at 2θ=38.2º,
4.3º, 64.6ºor 77.5ºin the Au/Fe , Au/HTc, Au/Al and Au/CeO catalysts are attributed to the
4
3
O
4
2
O
3
2
representative diffraction of planes of (111), (200), (220) and (311) of the Au(0) crystalline, which
indicates the formation of Au crystal particles on these supports. In contrast, only the peaks
9

attributed to supports is exhibited in the catalyst Au/Nb
concentration of Au particles on support or irregular crystal structure of the Au nano particles.
.1.2 BET Surface Measurement
Textural properties of the supported Au catalysts measured by nitrogen physisorption are shown in
2 5
O , which could be due to the low
3
2
-1
Table 1. The results indicate that the surface areas of Au/Al
2
O
3
and Au/HTc catalysts are 314.1 m ·g
2
-1
2
-1
and 216.6 m ·g , respectively. The BET surface areas of Au/CeO
2
and Au/Fe
and 37.45 m ·g , respectively. However, the surface area of Au/Nb
Moreover, the pore volume of Au/Al catalyst is much larger than that of other catalysts which
3
O
4
are 93.78 m ·g
2
-1
2
-1
2
5
O is only 0.9031 m g .
2
O
3
3
-1
3
-1
reaches about 0.5541 cm g , whereas the pore volume of Au/Nb
2 5
O is only 0.0004 cm g . Besides,
the average pore diameters of these supported Au catalysts are between 39.58 nm to116.6 nm.
3
.1.3 SEM Images
The morphology of different catalysts was characterized by SEM. Figures 2a-2d show SEM images
of 4 catalysts. Loosely connected surface is observed on Au/Al catalyst. In contrast, on Au/HTc,
irregular edges are observed, on top of which Au particles are uniformly distributed. Interestingly,
tubular structure is formed on Au/CeO catalyst whereas the surface of Au/Nb catalyst exhibits a
smooth and oval-shaped structure which is consistent with low BET surface area.
2
O
3
2
2 5
O
3
.1.5 TEM Images
These supported Au catalysts are also characterized by TEM technique. As shown in Figure 3, Au
nano particles with average size of 6-7 nm are found in the image of the Au/Al catalyst.
Moreover, the Au particle size between 7 nm to 8 nm is measured on Au/CeO catalyst where the
2
O
3
2
1
0

support exhibits the obvious tubular structure from the TEM image. In contrast, the aggregation of
Au particles is observed in the images of the Au/HTc catalyst and Au/Nb catalyst. So we can
2
O
5
conclude that the small particle and regular distribution of gold nanoparticles were helpful to
catalytic reaction in the FEO system.
3
.1.6 UV-vis Characterization
Figure 4 presents the UV-vis spectra of different supported Au catalysts. In the spectra, Au particles
can be detected by the appearance of well resolved plasmon peak at the wavelength of ca. 530 nm,
which is a typical characteristic of metallic Au particles [22-24]. It can be observed that freshly
prepared Au/Al
values of Au/Al
Au/Nb catalyst appears at 550-560 nm. The shift of peak in the Au/Nb
larger nano particles on the surface of the catalyst [25]. The peaks associated with Au/Fe
Au/CeO catalysts are not very obvious in the spectra which is attributed to the Au particles entering
into the pore channel of catalysts.
2
O
3
, Au/HTc and Au/Nb
and Au/HTc catalysts appear at 530-535 nm, whereas the peak associated with
is attributed to the
and
2 5
O catalysts contain metallic Au particles, in which the peak
2
O
3
2
O
5
2 5
O
3
O
4
2
3
.1.7 TPD Characterization
In order to determine acidic properties of the catalysts, the NH
3
-TPD was performed from 100 to 950
, Au/HTc and Au/CeO
o
C, and the results were shown in Figure 5. In the TPD patterns of Au/Al
2
O
3
2
o
o
catalysts, two signals observed at 260 C and 590 C were due to the presence of two different acidic
o
o
sites, that of the Au/Fe
3
O
4
catalyst was found at 210 C and 650 C, and the Au/Nb
2
O
5
catalyst
o
o
appeared at 590 C and 850 C; Obviously, Au/Al
2
O
3
, Au/HTc, Au/Fe
3
O
4
and Au/CeO
2
mainly have
11

weak and middle acid sites, and Au/Nb
concluded that the numerous weak acid sites are helpful to catalytic reaction in FEO system.
.2 The Reaction of FUR with Ethanol in the Presence of Molecular Oxygen
Initially, the catalytic reaction of FUR with ethanol in the presence of O was carried out using
2 5
O mainly has middle and strong acid sites. Thus, it can be
3
2
different catalyst under the same reaction conditions. The reaction equation is shown in Scheme 2,
where the products include furan-2-acrolein (Compound 1), ethyl furoate (Compound 2),
2
-(diethoxymethyl)furan (Compound 3), and others. The experimental results with different catalytic
systems are summarized in Table 2.
3 4
In the absence of catalyst, only 9.4% conversion of FUR is obtained. Using Au/Fe O as the catalyst
o
operated at 130 C, reaction time of 4 hours, about 29.5% conversion of FUR with 99% selectivity to
2 3
Compound 3 are obtained. When a base catalyst promoter K CO was added, the activity of Au
catalyst in oxidative condensation of FUR with ethanol is obviously improved, showing 68.4%
conversion of FUR and 91.3% selectivity towards Compound 1 (Entry 3). Moreover, three control
experiments (Table 2, Entries 4-6) were performed in the presence of catalysts Fe
and Fe . In these experiments, the conversions of FUR are 25.6%, 21.2% or 23.2%, respectively.
When Fe -K CO catalyst is used, only Compound 3 and others are formed, Compounds 1 and 2
are not observed, whereas K CO and Fe are not active in the formation of Compounds 1-3.
Compared with the results shown in Entries 2&3, synergistic effect between Au and K CO is
responsible for the entire reaction shown in Scheme 2, especially for the formation of Compounds 1
2. Furthermore, the catalytic performances of the Au/HTc-K CO , Au/Co -K CO and
Au/Al -K CO catalysts were also investigated (Entries 7-9). As shown in Table 2, the conversions
of FUR are 78%, 80.1% and 84.9% whereas selectivities towards Compound 1 are 81%, 83.2% and
3 4 2 3 2 3
O -K CO , K CO
3
O
4
3
O
4
2
3
2
3
3 4
O
2
3
&
2
3
3
O
4
2
3
2
O
3
2
3
1
2

8
0.1%, respectively. When the Au/CeO
but the selectivity to Compound 1 is decreased to 54.9%. In the presence of Au/Nb
9.6% conversion of FUR and 84.6% selectivity of Compound 1 is attained under the same reaction
2
is used as a catalyst, the conversion is increased to 97.3%
2
O
5
- K CO , only
2
3
3
conditions. The activity test is correlated to BET, XRD, SEM and TEM characterization data shown
above, it is concluded that the high activity of supported Au catalyst is attributed to its large surface
area, large pore volume, and small Au nanoparticles. Further investigation by
temperature-programmed-desorption (TPD) measurement indicates weak acid sites play key role in
the condensation reaction.
3
.3 The Effect of Temperature
The effect of temperature on the reaction of FUR with C
2
5
H OH was investigated with
Au/Al
2
O
3
-K
2
CO
3
system and the results are shown in Figure 6. The conversion of FUR is gradually
o
increased and starts to level off at 140 C, meanwhile, the selectivity to Compound 1 is decreased on
raising temperature from 110-140 ºC, after which it starts to level off. The changes of selectivity to
o
Compounds 2 and 3 show a similar trend, both leveling off at 140 C.
3
.4 The Effect of Reaction Time
The effect of reaction time on oxidative condensation of FUR with ethanol is presented in Figure 7.
The conversion of FUR increases as reaction proceeds from 15 min to 4 hours, and then starts to
level off. Meanwhile, the selectivity to Compound 1 increases gradually in the first 2 hours then
declines afterwards. The highest yield to Compound 1 is reached at 70.6% in 4 hours reaction time in
the presence of the Au/Al
2
O
3
-K
2
CO
3
catalyst. Thus, under reaction system chosen, the optimal
OH.
reaction time is 4 hours for the oxidative condensation of FUR with C
2
H
5
1
3

3
.5 The Influence of Bases or Acids as Promotors
In order to further reveal the influence of base promoters on supported Au catalyst performance,
several base promoters including Na
were chosen in the reaction of FUR with C
when Na CO
achieved (Entry 1). Moreover, 84.8% conversion and 79.8% selectivity to Compound 1 are attained
when Cs CO is used as a promotor (Entry 2). However, when promoter is switched to Li
conversion of FUR drops to 14.3%, selectivity to Compound 1 is 10.1% but selectivity to Compound
is increased to 81.9% (Entry 3). The result seems to indicate when the promoter’s basicity is
decreased, the reaction pathways favor the esterification reaction to form Compound 2, instead of
condensation reaction which favors Compound 1. Furthermore, when KHCO or NaHCO are added,
2
CO
3
, Cs
2
CO3, Li
2
CO
3
3 3
, KHCO , NaHCO , KOH and NaOH
2
H
5
OH using Au/Al
2 3
O
catalyst. As shown in Table 3,
2
3
is used as a promoter, 61.7% conversion and 92.2% selectivity to Compound 1 are
2
3
2
CO3, the
2
3
3
a 75.3% or 74% conversion of FUR are obtained in which the selectivities to Compound 1 are 86%
and 80.5%, respectively (Entries 4-5). In the case of KOH, 86.3% conversion and 69.4% selectivity
to Compound 1 are obtained (Entry 6). Similarly, 79.0% conversion and 81.8% selectivity to
Compound 1 are attained using NaOH as a promotor (Entry 7). In addition, Ca(OH)
and MgO are also employed as the promotors for the oxidative condensation of FUR with ethanol.
When Ca(OH) or CaCO is added as a promoter, only 27.5% or 14.8% conversion is obtained
Entry 8-9). With the promotion of CaO, the conversion and the selectivity to Compound 1 are
increased to 61.3% and 68.3%, respectively (Entry 10). In contrast, when H PO is used as an acidic
additive, the selectivity to Compoud 1 decreases to almost zero Compound 3 becomes dominant
Entry 11). Considering catalytic effectiveness and the cost, K CO is chosen as a most effictive base
promoter for Au/Al catalyst.
2 3
, CaCO , CaO
2
3
(
3
4
(
2
3
2
O
3
1
4

3
.6 The Influence of Promoter Loading on Au Catalyst Performance
It is well known that the function of carbonates as a weak base in organic synthesis is to extract
proton from the substrate. Therefore, a stoichiometric excess of carbonate is generally necessary to
promote a high substrate conversion except for Au-catalyzed reaction [26-28]. Herein, the effect of
the amount of K
Figure 8. It can be seen that both the conversion of FUR and the selectivity to Compound 1 are
gradually increased with the increase in K CO loading from 0.01 to 0.06 g. The conversion remains
almost unchanged when more K CO is added. In contrast, the selectivity to Compound 1 is slowly
decreases with the increase of K CO loading from 0.01g to 0.07g, which is attributed to the
occurence of self-condensation of aldehyde. This is verified by GC-MS analysis.
2 3
CO in the FUR oxidation is further investigated and the results are presented in
2
3
2
3
2
3
3
.7 The recyclability of catalysts
Furthermore, the recycling of Au/Al
catalytic reaction, the catalyst was separated, and washed with anhydrous ethanol, and then dried at
0 ºC for 12 h before being reused in the next run. As shown in Figure 9. it was seen that the
conversion of FUR still kept above 90% and the selectivity of 1 was as high as ca. 74% even after
the Au/Al catalyst being recycled five times. These results showed that the Au/Al catalyst was
2
3
O catalyst has been examined in the FEO system. After the
8
2
O
3
2 3
O
efficient and kept stable in FEO system. In addition, the ICP-AES has been tested and the metal
content in the filtrate is 0.05494%, which may be negligible. This result indicates that this reaction is
the heterogeneous catalytic process.
3
.8 Possible Reaction Mechanism
1
5

The reaction mechanism is elucidated based on experimental study and organic synthesis theories.
Referring to the condensation of FUR with acetone [13, 14], the oxidation of alcohols [15] and a
similar literature report to the on the oxidation-condensation of FUR with n-propanol [21], we
postulate that the first step is the oxidation of ethanol to form an acetaldehyde intermediate followed
by a rapid condensation between FUR and the acetaldehyde intermediate in the solution. This
reaction pathway is illustrated in Scheme 3, route 1. Hydrogen transfer occurs between FUR and
ethanol, in which the acetaldehyde intermediate and furfuryl alcohol are produced. The next step is
the condensation reaction that takes place between FUR and the acetaldehyde intermediates.
Meanwhile, the furfuryl alcohol can be oxidized to FUR by oxygen as shown in route 2 in Scheme 3.
2 3
Correspondingly, throughout this study, it can be elucidated that the Au/Al O catalyst is responsible
2 3
for the selective oxidation of ethanol or furfuryl alcohol, whereas the role of base K CO promoter is
to catalyze condensation reaction to form desired product, Compound 1.
4. Conclusion
A highly efficient and selective oxidation-condensation of FUR with ethanol process that involves
using supported Au catalyst has been developed. In the presence of the Au/Al catalyst and
2
O
3
potassium carbonate, the main product of oxidative condensation is found to be Compound 1.
Synergistic effect between supported Au catalyst and base promoter is observed in the formation of
Compound 1. The importance of this study is to provide an effective approach for valorization of
biomass-derived hemi cellulose via catalytic process.
1
6

Acknowledgements
This work is financially supported from the Natural Science Foundation of China (No. 21003093)
and the State Key Program of National Natural Science of China (No.21336008).
1
7

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1
9

Figure Captions
2
Scheme 1. Possible reaction pathways in FUR-ethanol-O reaction system
Scheme 2. Possible product formation from condensation of FUR with ethanol
Scheme 3. Reaction mechanism involved in oxidative condensation of FUR with ethanol using
2 3 2 3
Au/Al O -K CO catalyst
2
0

Figure 1. The XRD patterns of different supported Au catalysts
Figure 2. The SEM images of the different nano Au catalysts (a) Au/Al
catalyst; (c) Au/CeO catalyst; (d) Au/Nb catalyst
2 3
O catalyst; (b) Au/HTc
2
2 5
O
2
1

Figure 3. TEM images of different catalysts (a) Au/Al
Au/CeO catalyst; (d) Au/Nb catalyst]
2 3
O catalyst; (b) Au/HTc catalyst; (c)
2
2 5
O
2
2

Figure 4. The UV-vis spectra of different nano Au catalysts
Figure 5. The TPD results for different gold supported catalysts
2
3

2 3
Figure 6. The effect of temperature in the oxidation of FUR with Au/Al O catalyst
2 3 2 3
Figure 7. The effect of time on the oxidation of furfural with Au/Al O -K CO catalyst
2
4

Figure 8. The effect of K
2
CO
3
promoters in the oxidative condensation of FUR (0.2 g FUR, 0.05g
OH, 0.3 MPa of O , 140 ºC, 4 hours reaction time)
5
%Au/Al catalyst, 15 ml C
2
O
3
2
H
5
2
Figure 9. The recycling of the Au/Al O catalyst in the FEO system. [Reaction conditions: 0.2 g of
2
3
FUR, 0.05 g of the Au/Al
2
O
3
2 3 2 5
(5 wt% Au loading) catalyst, 0.05 g K CO , in 15 mL of C H OH under
o
0
.3 MPa of O for 4 h at 140 C.]
2
2
5

Tables
Table 1. The physical property of different supported Au catalysts
Average pore diameter (nm)
BET surface
Pore volume
Catalysts
BJH
adsorption
59.89
BJH
desorption
63.23
2
-1
3 -1
area (m g )
(cm g )
Au/Al
Au/HTc
Au/CeO
Au/Fe
Au/Nb
2
O
3
314.1
216.6
93.78
37.45
0.9031
0.5541
0.2046
0.1224
0.0511
0.0004
37.45
39.58
2
54.02
67.50
3
O
4
53.07
82.27
2
O
5
62.45
116.6
2
6

Table 2. Performance comparison of different supported Au catalysts in oxidative condensation of
[
a]
furfural with ethanol
[
b]
[b]
Entry Catalytic System Conversion (%)
Selectivity (%)
1
0
0
2
0
0
3
Others
1
2
3
4
5
6
7
8
9
None (baseline)
Au/Fe
Au/Fe +K
Fe
9.4
>99%
0
3
O
4
29.5
68.4
25.6
21.2
23.2
78.0
80.1
84.9
97.3
39.6
>99
0
3
O
4
2
CO
3
91.3 2.9
0
79.6
0
5.8
3
O
4
+K
CO
Fe
Au/HTc + K
Au/Co +K
Au/Al +K
Au/CeO +K
Au/Nb +K
2
CO
3
0
0
0
0
0
0
20.5
100
100
13.6
14.0
15.0
27.4
11.5
K
2
3
3
O
4
0
2
CO
3
81.0 5.4
83.2 2.7
80.1 4.9
54.9 17.7
84.6 3.9
0
3
O
4
2
CO
CO
CO
CO
3
0
2
O
3
2
3
0
1
0
2
2
3
0
11
2
O
5
2
3
0
2 3
[a] Reaction conditions: 0.2 g FUR, 0.05g nano Au (5.0 wt.% loading) catalyst, 0.05 g K CO , in 15 ml of
2 5 2
C H OH, under 0.3 MPa of O , reaction time 4 hour, temperature 130 ºC. [b] Internal standard was used in GC
analysis. The compound 1 is furan-2-acrolein; the Compound 2 is ethyl furoate; the Compound 3 is
-(diethoxymethyl)furan.
2
2
7

[
a]
Table 3. The effects of different additive in the oxidative condensation of furfural with ethanol
b
Product distribution (%)
b
Entry
Promotor
Conversion (%)
1
2
3
others
5.0
1
2
3
4
5
6
Na
Cs
Li
KHCO
2
CO
CO
CO
3
61.7
84.8
14.3
75.3
74.0
86.3
92.2
79.8
10.3
86.0
80.5
69.4
2.8
3.7
81.9
2.6
9.7
2.8
0
2
3
0
16.4
0
2
3
7.9
0
3
14.4
9.8
NaHCO
3
0
KOH
0
27.7
7
NaOH
79.0
81.8
9.3
0
8.9
8
9
Ca(OH)
2
27.5
14.8
61.3
49.6
87.4
0
12.7
2.6
0
0
0
CaCO
3
73.8
0
23.6
30.3
0
1
0
CaO
68.3
0
11
H
3
PO
4
6.7
93.3
[
a] Reaction conditions: 0.2 g FUR, 0.05g Au/Al
5 ml of ethanol, P=0.3 MPa of O , reaction time 4 hours, T=140 ºC.
b] Internal standard is used in GC analysis
2 3
O (5 wt.% Au loading) catalyst, 0.05 g promotor,
1
2
[
2
8