Facile synthesis of furfuryl ethyl ether in high yield: Via the reductive etherification of furfural in ethanol over Pd/C under mild conditions

Article abstract of DOI:10.1039/c7gc03887a

The one-pot synthesis of furfuryl ethyl ether (FEE) over Pd nanoparticles supported on TiO₂, Al₂O₃, SiO₂, and active carbon via the catalytic reductive etherification of furfural in ethanol was systematically studied. The Pd nanoparticles supported on SiO₂, TiO₂ and active carbon are all active for this novel process under mild reaction conditions, with Pd/C showing the highest selectivity to FEE. The effects of palladium loading, reaction temperature, and hydrogen pressure on the activity and selectivity of Pd/C have been investigated in detail. The results demonstrate that suitable Pd amount, low reaction temperature of about 60 °C, and low H₂ pressure of about 0.3 MPa are favorable for the formation of the desired ether product. Under the optimized conditions, an unprecedented high yield of up to 81% of FEE was firstly obtained with the major by-products being furfuryl alcohol and 2-methyltetrahydrofuran. Compared with the conventional hydrogenation-etherification route via furfural alcohol as a reaction intermediate, the reductive etherification shows significant advantage in product yield because of its much lower reaction temperature that is required.

Full text of DOI:10.1039/c7gc03887a

View Article Online
View Journal
Green
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Y. Wang, Q. Cui,
Y. J. Guan and P. WU, Green Chem., 2018, DOI: 10.1039/C7GC03887A.
This is an Accepted Manuscript, which has been through the
Volume 18 Number
7 7 April 2016 Pages 1821–2242
Royal Society of Chemistry peer review process and has been
accepted for publication.
Green
Chemistry
Accepted Manuscripts are published online shortly after
Cutting-edge research for a greener sustainable future
www.rsc.org/greenchem
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1463-9262
CRITICAL REVIEW
G. Chatel et al.
Heterogeneous catalytic oxidation for lignin valorization into valuable
chemicals: what results? What limitations? What trends?
rsc.li/green-chem

Please do not adjust margins
Green Chemistry
Page 1 of 9
View Article Online
DOI: 10.1039/C7GC03887A
Journal Name
ARTICLE
Facile synthesis of furfuryl ethyl ether in high yield via reductive
etherification of furfural in ethanol over Pd/C under mild
conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Yun Wang, Qianqian Cui, Yejun Guan* and Peng Wu
DOI: 10.1039/x0xx00000x
The one-pot synthesis of furfuryl ethyl ether (FEE) over Pd nanoparticles supported on TiO2, Al2O3, SiO2, and active carbon
via catalytic reductive etherification of furfural in ethanol was systematically studied. Pd supported on SiO2, TiO2 and active
carbon are all active for this novel process under mild reaction conditions, with Pd/C showing the highest selectivity to
FEE. The effects of palladium loading, reaction temperature, hydrogen pressure on the activity and selectivity of Pd/C have
been investigated in detail. The results demonstrate that suitable Pd amount, low reaction temperature about 60 oC, and
low H2 pressure about 0.3 MPa are favorable to the formation of desired ether product. Under the optimized conditions,
unprecedented high yield up to 81% of FEE was firstly obtained with the major by-products being furfuryl alcohol and 2-
methyltetrahydrofuran. Compared with the conventional hydrogenation-etherification route via furfural alcohol as
reaction intermediate, the reductive etherification shows significant advantage in product yield because of its much lower
www.rsc.org/
reaction
temperature
required.
such as furfuryl alcohol (FA), tetrahydrofurfuryl alcohol (TFA),
2-methylfuran (MF), 2-methyl tetrahydrofuran (MTF), furan
and tetrahydrofuran (THF). Recently, another kind of furfural
derivatives, furfuryl ethers, has attracted wide attention
because it is one of the promising components of gasoline due
to their high stability and high octane numbers24-28. The
current production of furfuryl ethers generally includes two
routes (Scheme1): one is a two-step process (a), wherein
furfural is firstly hydrogenated to FA, which then undergoes
etherification with alcohols. This route requires a bifunctional
hydrogenation catalyst and a strong Bronsted acid catalyst.
Lange and co-workers disclosed the production of furfuryl
ethyl ether (FEE) by etherification of FA using zeolites, with
FEE yield of 50 mol% at 80% furfural conversion using the
HZSM-5 zeolite at 125 °C24,29. Further increase of FEE yield is a
challenge task in route (a). As the etherification normally
requires high reaction temperature (>100 °C), under such
reaction conditions ring opening of furan-group of FEE
inevitably takes place resulting in the formation of levunic acid
Introduction
Due to the fast depletion of fossil fuels and environmental
concerns about climate change, paradigm change from fossil
to biomass resources for the manufacture of commodity and
fine chemicals will become inevitable in the near future1-3
.
However, platform compounds obtained from biomass contain
high oxygen content; hence, many catalytic processes i.e.,
hydrogenation, dehydration, decarboxylation, esterification,
etherification, acetalization, and C−C coupling have been
developed for selectively tailoring their oxygen content to
achieve the desired chemicals and fuels4-10. Among these
processes, etherification is one of the promising means for the
production of oxygenated fuels, as it reduces the amount of
hygroscopic alcohol groups, and increases both the energy
content and cetane number11,12. Several ether productions
obtained from platform compounds has been recently
reported, eg., ethyl-4-ethoxy pentanoate13, ethylene glycol
ether14,
glycerol
ethers15,
,
isosorbide tert-butyl ethers22 and β-
5-(ethoxymethyl)furfan-2-
or its esters30,31
.
carbaldehyde16-21
Citronellene ethers23.
Route (a): hydrogenation and etherification
Furfural (FF) as one of the platform molecules can be
transformed to a variety of value-added chemicals and fuels
O
OH
(FA)
O
O
O
O
(FEE)
(FF)
Route (b): reductive etherification
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 2 of 9
View Article Online
DOI: 10.1039/C7GC03887A
ARTICLE
Journal Name
Scheme 1: Reaction parthways for the synthesis of furfuryl ethyl ether from
fufural in ethanol.
1.4 wt.%). The average particle size of palladium in 0.3Pd/C,
0.7Pd/C and 1.4Pd/C according to TEM analysis is 6.4, 3.6 and
3.0 nm, respectively. For 0.3Pd/C, we see few Pd nanoparticles
less than 2 nm in TEM. Figs.2d–f show the images of 0.8
Pd/SiO2, 0.9 Pd/Al2O3, and 0.7 Pd/TiO2, respectively. The
average Pd particle size of 0.8 Pd/SiO2, 0.9 Pd/Al2O3 and 0.7
Pd/TiO2 is estimated to be 7.1, 3.1 and 4.7 nm, respectively.
Serious aggregation of Pd nanoparticles on SiO2 is noticed,
which is consistent with the XRD diffraction that Pd/SiO2 has
the largest Pd particle size. The CO chemisorption results are
also listed in Table 1. The Pd dispersion for 0.3Pd/C, 0.7Pd/C
and 1.4Pd/C is 11%, 35%, and 40%, respectively. This trend is
consistent with the TEM analysis. For oxides supported Pd
catalysts, the Pd dispersion showed distinct results.
Another strategy called reductive etherification (b), allows for
the synthesis of ethers from an alcohol and an aldehyde
substrate in one-step32. This methodology has been found to
be very selective for the etherification of some commonly seen
carbonyl compounds such as cyclohexanone, octanal,
benzaldehyde, and etc. under hydrogen atmosphere33-41
.
However, few reports have been made in the synthesis of
furfural-derived ethers by this robust way27,28. Recently, a
maximum selectivity of 77% of furfuryl methyl ether (FME) has
been noted on palladium charcoal catalysts under high
hydrogen pressure (5 MPa) at about 100 °C27. Compared with
the hydrogenation-etherification process, the reductive
etherification path occurs at relatively lower temperature
therefore inhibiting the decomposition of produced ether. In
this study, we found that high yield synthesis of furfuryl ethyl
ether (FEE) via reductive etherification of furfural in ethanol
could be achieved under even more mild conditions, e.g., H2
pressure as low as 0.3 MPa and temperature close to 60 °C.
Under this reaction conditions, the decomposition of FEE is
completely inhibited therefore high yield of 81% FEE was
obtained on 0.7 wt.% Pd/C catalyst. The detailed kinetics
results suggest that the Pd loading, reaction temperature
together with hydrogen pressure all govern the FEE yield by
tuning the competition between hydrogenation and
hydrogenolysis activity of PdH hydride species.
0.9Pd/Al2O3 catalyst gave
a dispersion of 31%, while
0.8Pd/SiO2 and 0.7Pd/TiO2 both showed lower dispersion
about 6%. This very low Pd dispersion on SiO2 and TiO2 has
been found previously and might be due to a support effect42.
Table 1. Structural properties of supported Pd catalysts
c
e
Pda
SSAb
(m2 g-1)
Vtotal
Dd
dTEM
Catalyst
(wt.%)
( cm3 g-1)
(%)
(nm)
0.3Pd/C
0.7Pd/C
0.29
0.68
1.4
1340
1335
1295
53
0.95
0.91
0.87
0.40
0.48
1.44
11
35
40
6
6.4
3.6
3.0
4.7
3.1
7.1
1.4Pd/C
0.7Pd/TiO2
0.9Pd/AI2O3
0.8Pd/SiO2
0.7
0.91
0.79
204
31
6
174
a Determined by ICP-AES. b Calculated using BET method. c Calculated from the
adsorption capacity at P/P0 of 0.99. d According to pulse CO chemisorption. e
Determined by TEM
Results and discussion
Catalyst characterization
The structural properties of the supported Pd catalysts,
including BET specific surface area (SSA) and total pore volume
(Vtotal) are summarized in Table 1. The surface areas of active
carbon supported Pd (Pd/C) catalyst are all close to 1300 m2
g−1. With the Pd loading increasing from 0.3 to 1.4 wt.%, the
surface area slightly decreased. The surface area of Pd/Al2O3,
Pd/SiO2, and Pd/TiO2 is 204, 174.5, and 53 m2 g−1, respectively.
Figure 1 shows the XRD patterns of the as-prepared Pd
catalysts. No diffraction peaks due to Pd nanoparticles are
visible for the 0.7Pd/TiO2, 0.9Pd/Al2O3 and Pd/C catalysts,
meaning that Pd is highly dispersed on these supports at
loadings less than 1.5 wt%. The high dispersion of Pd on TiO2
and Al2O3 is likely due to the strong interaction between Pd
and TiO2 and Al2O3 surface. For Pd/C catalyst, the high surface
area unambiguously benefits the loading of Pd species. By
contrast, weak diffraction peak at 40o is observed for
0.8Pd/SiO2 catalyst (JCPDS [46-1043]), suggesting the presence
of larger Pd particles on SiO2 because of the weak interaction
between Pd species and silica surface.
0.7Pd/TiO2
0.9Pd/Al2O3
0.8Pd/SiO2
1.4Pd/C
0.7Pd/C
0.3Pd/C
20
30
40
50
60
2Theta (Degree)
Fig. 1 XRD patterns of supported Pd catalysts.
We further measured the particle size and metal dispersion of
palladium on active carbon by TEM and CO chemisorption,
respectively. Typical TEM images of the supported Pd catalysts
are shown in Figure 2. Narrow distribution of palladium
particle size (1–9 nm) was obtained for all catalysts. Figs.2a–c
shows the images of Pd/C with various Pd loading (from 0.3 to
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 3 of 9
View Article Online
DOI: 10.1039/C7GC03887A
Journal Name
ARTICLE
for 2 h, the reaction mixtures were withdrawn and analysed.
Furfural conversion over 0.3, 0.7, and 1.4 Pd/C catalysts (Table
2, Entries 1–3) are 77, 98, and 100%, respectively. The three
catalysts showed distinct product distribution. For 0.3Pd/C, the
predominant product was 2-(diethoxymethyl)furan (DOF), with
selectivity of 78%. The mass spectra of some of the products
were shown in Fig. S1. The rest products were furfuryl ethyl
ether (FEE, Sel. 15%) and furfuryl alcohol (FA, Sel. 7%). For
0.7Pd/C, the dominating product was FEE with selectivity of
83%. Other by-products included FA (10%), DOF (4%) and
tetrahydrofurfuryl alcohol (TFA, 3%). For 1.4Pd/C, the major
products contained 50% of FEE, 38% of FA and 6% of TFA. It
should be noted that by decreasing the catalyst amount of
1.4Pd/C from 100 mg to 20 mg while keeping the rest reaction
conditions unchanged, the FEE selectivity could be increased
from 50% to 70%. This result can be explained by the similar
particle size distribution of 0.7Pd/C and 1.4Pd/C. We may
conclude that by using suitable furfural/Pd ratio, the Pd/C
catalysts with particle size around 3 nm may selectively
catalyse the formation of FEE.
Figure 2 TEM images of different Pd catalysts. a) 0.3Pd/C; b) 0.7Pd/C; c) 1.4Pd/C; d)
0.8Pd/SiO2; e) 0.9Pd/Al2O3; f) 0.7Pd/TiO2.
Reductive etherification activity of Pd/C catalysts
Table 2 shows the catalytic performance of Pd/C catalysts in
reductive etherification of furfural in ethanol in terms of
furfural conversion and product distribution. As an initial
study, we started the reaction at 60 oC and 0.3 MPa H2
pressure. For each experiment, 10 mL of 0.24 M furfural in
ethanol solution and 100 mg catalyst was used. After reaction
Table 2. Reductive etherification of furfural with ethanol on different catalystsa
Sel.%
Entry
Catalyst
Conv.%
FEE DOF FA TFA DOTF TFF MTF TFEE
1
2
3
4
5
6
7
8
9
0.3Pd/C
0.7Pd/C
77
98
15
83
50
19
78
4
7
10
38
2
3
6
1.4Pd/C
100
86
1
5
3
2
63
11
0.8Pd/SiO2
0.9Pd/Al2O3
0.7Pd/TiO2
0.7Pd/Cb
0.7Pd/Cc
98
98
100
35
0
11
3
5
25
8
34
2
16
10
6
34
55
76
70
30
0.7Pd/Cd
a Reaction conditions: 10 mL of 0.24 M furfural (FF) ethanol solution; 60 oC; 0.3 MPa H2; 100 mg catalyst; 2 h. FEE: Furfuryl ethyl ether ; DOF: 2-(diethoxymethyl)furan;
FA: furfuryl alcohol; TFA: tetrahydrofurfuryl alcohol; DOTF: 2-(diethoxymethyl)tetrahydrofuran ; TFF: tetrahydrofurfural; MTF: 2-methyltetrahydrofuran; TFEE:
tetrahydrofurfuryl ethyl ether .
b DOF as reactants.
c Reactant: Furfuryl alcohol.
d Reaction conditions: 10 mL of 0.24 M furfural (FF) ethanol solution; 60 oC; 0.5 MPa N2; 100 mg catalyst; 2 h.
Meanwhile, total hydrogenation of DOF and furfural also took
Reductive etherification of furfural over Pd/Oxides
place on this catalyst leading to the formation of 2-
For comparison, the reductive etherification of furfural over (diethoxymethyl)tetrahydrofuran
(DOTF)
and
oxide (SiO2, Al2O3, and TiO2) supported Pd catalysts was also tetrahydrofurfural (TFF), respectively. No FEE was found for
investigated and the results are shown in Table 2, entries 4-6. 0.9Pd/Al2O3 catalyst and the products were mainly consisted
Pd catalysts supported on three oxides all showed very high of FA, TFA, DOTF and TFF. These products were formed via the
furfural conversion (>86%). For 0.8Pd/SiO2, 63% selectivity of partial or total hydrogenation of furfural, pointing to the
2-(diethoxymethyl)furan (DOF) was observed and 19% hydrogenation nature of Pd species on alumina. In the case of
selectivity of furfuryl ethyl ether (FEE) was noticed. Previous 0.7Pd/TiO2 catalyst, a 55% selectivity of FEE was noticed. The
report has shown that Pd/SiO2 catalyst was active for the other main by-product was FA (Sel. 25%).
etherification of 2-methylpentanal though with much higher
loading (16 wt.%) and at temperature above 125 oC40.
Effects of reaction temperature, H2 pressure on the reductive
etherification activity of 0.7Pd/C catalyst
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 4 of 9
View Article Online
DOI: 10.1039/C7GC03887A
ARTICLE
Journal Name
The above mentioned results unambiguously demonstrate that
Pd/C catalyst showed unique performance in converting
furfural to furfuryl ethyl ether (FEE) via reductive
etherification. We next investigated the effect of reaction
parameters on the activity of Pd/C catalyst. 0.7Pd/C was
chosen because it showed the highest FEE yield among the
catalysts studied. In these studies, the reactions were
conducted under 0.5 MPa H2 pressure at temperatures of 40,
60, or 80 °C for 2 h (Figure 3). 0.7Pd/C catalyst showed very
high furfural conversion at all temperatures following the
order of 80 °C (100%) > 60 °C (98%) > 40°C (85%). When the
reaction was conducted at lower temperature (40 °C), the
selectivity of FEE was 21%, while DOF was found to be the
predominant product with 60% selectivity. This result means
that the Pd/C catalyst is very active for DOF formation at low
reaction temperature. Increasing the reaction temperature to
60 °C, 71% selectivity to FEE was observed along with the
disappearance of DOF. This phenomenon suggests that the FEE
was mainly formed via the hydrogenolysis of DOF under
elevated temperature. Further increasing the reaction
temperature to 80 °C, the selectivity to FEE decreased to 19%
and the byproducts were consisted of hydrogenation
compounds such as FA (23%), TFA (10%) and DOTF (25%). This
result clearly emphases that Pd/C is highly active for the
hydrogenation of both C=O and furan ring at higher
temperatures. We conclude that the reductive etherification of
furfural in ethanol occurs at very narrow reaction temperature
window on Pd/C catalyst, i.e. higher than 40 oC and lower than
80 °C. It should be noted that under this reaction condition,
neither levulinic acid nor ethyl levulinate was observed
because the reaction temperature was not high enough for the
decomposition of FEE. In this regard, reductive etherification
shows remarkable advantage leading to the high yield
synthesis of FEE compared with conventional hydrogenation-
etherification route.
100
80
60
40
20
0
98
83
98
81
100
73
71
54
20
17
17
10
7
4 4
4
4
3
3
2
0
0
0.1 MPa
0.3 MPa
0.5 MPa
0.8 MPa
Conv. FEE DOF FA TFA DOTF TFF MTF
Figure 4 Reductive etherification of furfural (FF) in ethanol on 0.7Pd/C at different H2
pressures. Reaction conditions: 10 mL of 0.24 M FF ethanol solution; 60 oC; 100 mg
catalyst; 2 h.
We next investigated the effect of H2 pressure on the ether
(FEE) yield at 60 °C and the results are shown in Figure 4.
Under 0.1 MPa H2, 73% conversion of furfural is obtained, and
the selectivity of FEE is only 17% with DOF (Sel. 81%) as the
dominant by-product. high selectivity of 83% to FEE is achieved
with 98% conversion of furfural under 0.3 MPa H2. The overall
FEE yield at 0.5 MPa (70%) is lower than that (83%) operated
under 0.3 MPa. As the H2 pressure further increasing to 0.8
MPa, full conversion of furfural was observed while the
selectivity to FEE is only 4% and the main by-product is found
to be TFA (54%). Apparently, higher H2 pressure favours the
C=O and furan hydrogenation reaction pathways, and
therefore relatively low hydrogen pressure is preferred for the
synthesis of desired ether product. Taking into account of the
above results, one can clearly see that several reactions
including
acetalization,
hydrogenolysis,
and
deep
hydrogenation are all involved in the reductive etherification
process. The selectivity to FEE is mainly determined by the
competition between hydrogenolysis and hydrogenation. In
principle, higher Pd loading, higher reaction temperature and
higher H2 pressure favours the deep hydrogenation pathways,
therefore lowering the overall furfuryl ethyl ether yield. It
should also be mentioned that over-reduction of FEE may also
take place on 0.7Pd/C catalyst. We have tested the activity of
0.7Pd/C for FEE hydrogenation by using FEE/ethanol solution
as starting reagent at 60 oC and 0.3 MPa H2. After reaction for
2 h, FEE was fully converted, with 75% selectivity towards
tetrahydrofurfuryl ethyl ether 75% FEE and the rest product
was mainly 2-methyl tetrahydrofuran (Table S1). The
hydrogenation of FEE was hardly perturbed in presence of
furfural or furfuryl alcohol over 0.7Pd/C (Table S1).
100
85
98
100
71
80
60
40
20
0
60
23
21
25
18
20
19
17
10
3
1
1
0
0
0
80 oC
40 oC
60 oC
Conv. FEE DOF FA TFA DOTF TFF MTF
Figure 3. Reductive etherification of furfural (FF) in ethanol on 0.7Pd/C at different
reaction temperatures (40, 60 and 80 oC). Reaction conditions: 10 mL of 0.24 M FF
ethanol solution; 0.5 MPa H2; 100 mg catalyst; 2 h.
Reaction mechanism and kinetics study
The above results have highlighted the importance of 2-
(diethoxymethyl)furan DOF formation in the reductive
etherification. To understand the furfuryl ethyl ether
formation mechanism in more detail, we have followed the
reaction progress carefully on various Pd/C catalysts, namely,
0.7Pd/C and 1.4Pd/C. Figure 5A shows the furfural conversion
and product distribution during the reaction course over
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 5 of 9
View Article Online
DOI: 10.1039/C7GC03887A
Journal Name
ARTICLE
0.7Pd/C at 60 °C and 0.3 MPa H2 pressure. After reaction for 5 hydrogenation of furfural, which is a competitive reaction with
min, 70 % of furfural conversion was already observed with the acetalization. Moreover, the furfural conversion rate for
DOF yield of 61% and FA yield of 8%, suggesting that 1.4Pd/C when hydrogenation occurs is much lower than that
acetalization of furfural with ethanol to DOF proceeds very fast for 0.7Pd/C when acetalization takes place dominantly at the
(Scheme 2). As the reaction further going on, the yield of beginning of the reaction. This phenomenon again points to
furfuryl ethyl ether increased, and the yield of DOF decreased the importance of DOF formation as an intermediate for the
simultaneously. After 2 h reaction, almost no DOF was production of furfuryl ethyl ether.
detected, and the yield of furfuryl ethyl ether reached 81%. To
check whether DOF can serve as the precursor of furfuryl ethyl
ether, the hydrogenolysis of commercially purchased DOF
substrate was tested over 0.7Pd/C under the same condition
(60 °C, 0.3MPa H2). The result (Table 2, entry 7) shows that
DOF is highly reactive towards furfuryl ethyl ether (yield of
76%), firmly suggesting that the furfuryl ethyl ether is formed
by the hydrogenolysis of DOF, which is consistent with the
previous findings as observed in the synthesis of other
ethers35,37. It should be noted that presence of tiny amount of
water will significantly inhibit the production of DOF and
furfuryl ethyl ether. When 100
µL deionized water was
introduced, the 0.7Pd/C catalyst gave FEE yield of 7%, with
65% selectivity toward FA and other hydrogenation byproducts
(see ESI Table S1). When FA is used as the substrate (Table 2,
entry 8), no FEE was formed under otherwise the same
conditions. We therefore safely conclude that DOF formed in
furfural/ethanol solution via the acetalization is the precursor
of FEE.
Scheme 2. Reaction pathways during the reductive etherification of furfural (FF) in
ethanol over supported Pd catalysts.
The acetalization in general is a typical acid-catalyzed reaction.
Solid Bronsted acids such as zeolites or soluble Lewis acids are
both active for this reaction43,44. By contrast, the true active
centre for the acetalization reaction in reductive etherification
is still under debating. One assumption is that the acid sites on
active carbon can catalyse the acetalization as observed for
the acetalization of 1-butanol and octanal at 100 oC37.
However, this possibility can be excluded in our study. We
carried out the reaction under 0.5 MPa N2 under otherwise the
same reaction conditions (Table 2, entry 9). No reaction was
seen at all, meaning that the support acidity is not strong
100
100
A
B
Conv.
FEE
DOF
FA
TFA
TFEE
MTF
TFF
80
60
40
20
0
80
60
40
20
0
Conv.
FEE
DOF
FA
o
enough for the acetalization at 60 C. Another assumption is
TFA
that the acetalization is likely ascribed to the palladium
hydride formed in H2 atmosphere34, 45. The formation of
palladium hydride in aqueous solution on supported Pd
catalysts have been extensively studied46, 47. Certain forms of
these hydrides have been proposed to be to some extent
protonic in character, which can catalyse several reactions
usually catalysed with protonic catalysts48. We noticed that
furfuryl ethyl ether always formed along with DOF over the
catalysts being selective for furfuryl ethyl ether synthesis
(Table 2) in this study, namely, Pd/SiO2, Pd/TiO2, and Pd/C. We
therefore may conclude that Pd hydrides on Pd/Al2O3, Pd/SiO2,
Pd/TiO2, and Pd/C are all active for the acetalization reaction,
whereas with different activity. The difference in acetalization
activity can be explained by that PdH species may show
support-dependent protonic character.
0
30
60
90
120
0
30
60
90
120
Reaction time (min)
Reaction time (min)
Fig. 5 Furfural (FF) conversion and products distribution as a function of reaction time
for the reductive etherification of FF in ethanol over 0.7Pd/C (A) and 1.4Pd/C (B).
Reaction conditions:10 mL of 0.24 M FF ethanol solution; 60 oC;0.3 MPa H2; 100 mg
catalyst.
Figure 5B also shows the product distribution with time on
stream over 1.4Pd/C at 60 °C and 0.3 MPa H2 pressure. For the
first 5 min, 1.4Pd/C gave furfural conversion of 41% with DOF
yield of 16%. The other predominant product was FA with yield
of 23%. It can be seen that after reaction for 5 min, the yield of
furfuryl ethyl ether (FEE) is as low as 2%. As the reaction going
on, the yield of both FA and FEE increased, and the yield of
DOF went through a maximum for a 15-min reaction time,
then decreased. Complete conversion of furfural was achieved
after 60 min reaction, with FEE yield of 52% and FA yield of
41%. We hypothesize that the FA is produced by direct
It has been shown that the nature of support or Pd loading
may significantly affect the catalytic role of Pd hydride formed
on it48,49
. To elucidate the effect of Pd loading on
hydrogenolysis and hydrogenation, we tested the catalytic
performances of 0.7Pd/C and 1.4Pd/C under H2 by using DOF
directly as
a
starting reactant. We followed the DOF
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 6 of 9
View Article Online
DOI: 10.1039/C7GC03887A
ARTICLE
Journal Name
conversion and products distribution during the reaction
course and results are shown in Figure 6. For 0.7Pd/C (Figure
6A), DOF conversion of 97% was achieved with FEE yield of
53% as main product. For 1.4Pd/C (Figure 6B), one can see
that after reaction for 30 min, full conversion of furfural was
achieved with tetrahydrofurfuryl ethyl ether (TFEE) (yield of
50%) and DOTF (yield of 31%) as the dominant by-product, and
no furfuryl ethyl ether is detected. These results
unambiguously suggest that higher Pd content on 1.4Pd/C
prefers to catalyze the deep hydrogenation of furfuryl ethyl
ether and DOF to TFEE and DOTF, respectively. When
decreasing the amount of 1.4Pd/C catalyst used to 20 mg, the
FEE yield was increased to 71%. Hence we conclude that the
ability of Pd hydride towards hydrogenation or hydrogenolysis
is likely associated with the reaction conditions employed,
Fig. 7 Reusability of 0.7Pd/C catalyst for reductive etherification of furfural.
Reaction conditions:10 mL of 0.24 M furfural/ethanol solution; 60 oC; 0.3 MPa
which affect the hydrogen dissociation and adsorption energy H2; 100 mg catalyst
of surface H species. In general, low H2 pressure and lower
reaction temperature benefits to the hydrogenolysis reaction
pathway. This result is different from the furfuryl methyl ether
， 2 h.
Catalyst reusability
synthesis over Pd/C in methanol as reported by Pizzi et al.27,
wherein high H2 pressure up to 5 MPa was used. We surmise
that the nature of solvent may also play an important role. We
have done preliminary study on solvent effect on the ether
yield by using methanol and 1-propanol as solvent. The results
shows in Table S2 clearly shows that only 18% and 41% yield
towards ethers were obtained in methanol and 1-propanol,
respectively. In general the alcoholic solvent may affect the
catalytic performance or substrate adsorption on Pd
nanoparticles. Further study on the solvent effect on the
Given the excellent performance of 0.7Pd/C in the synthesis of
furfuryl ethyl ether, its reusability is also of interest from the
viewpoint of practical application. We have tested the
reusability of 0.7Pd/C catalyst and the results are shown in
Figure 7. We found that the furfural conversion over this
catalyst decreased slightly within 4 consecutive runs. It should
be noticed that the selectivity of furfuryl ethyl ether (FEE)
decreased remarkably from 83% to 55% after 4 runs, and the
selectivity of FA increased from 10% to 38% simultaneously.
Two possible reasons may be responsible for this selectivity
decline. First the nature of PdH changed after several runs,
which led to the lost in hydrogenolysis activity to some extent
while the hydrogenation activity was enhanced. Theoretical
studies on the electronic property and catalytic mechanism of
PdH under hydrogen atmosphere will help understanding this
complex behaviour. Second, the deposited carboneous species
may alter the hydrophilicity of the carbon surface, which may
change the hydrogenation and hydrogenolysis reaction
pathway. Further study on improving the stability of this
catalyst is ongoing.
reductive
etherification
performance
need
deeper
investigation.
100
B
100
A
Conv.
FEE
FA
TFA
TFEE
MTF
DOTF
Conv.
FEE
FA
80
60
40
20
0
80
60
40
20
TFA
TFEE
MTF
DOTF
Experimental
Materials.
0
0
0
20
40
60
Palladium (II) chloride (99.9%, J&K Chemical), TiO2 (Anatase,
Acros) and active carbon (surface area about 1300 m2/g,
STREM Chemicals) were purchased from J&K Chemical Ltd. The
SiO2 support was Evonik Degussa Aerosil® 200 with surface
area of 208 m2 g-1.
20
40
60
Reaction time(min)
Reaction time(min)
Fig. 6 2-(diethoxymethyl)furan (DOF) conversion and products distribution as a
function of reaction time over 0.7Pd/C (A) and 1.4Pd/C(B). Reaction conditions: 10 mL
of 0.24 M DOF ethanol solution; 60 oC; 0.3 MPa H2; 100 mg catalyst.
Preparation of catalysts
Pd/C samples were prepared by
a typical deposition–
precipitation–reduction method50. In a typical preparation, 0.5
g of active carbon was dispersed in H2O (60 mL) with stirring.
0.116 mL of H2PdCl4 aqueous solution (21.512 gPd L-1) was
added to the mixture, and then the mixture was stirred for 3 h.
The final pH value of the suspension was adjusted to 10 by
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 7 of 9
View Article Online
DOI: 10.1039/C7GC03887A
Journal Name
ARTICLE
adding NaOH solution (1M). Then, NaBH4 aqueous solution etherification, which involves the acetalization/hydrogenolysis
(NaBH4/Pd = 15, molar ratio) was added into the suspension pathways in one-pot. A high yield of 81% furfuryl ethyl ether
and the mixture was stirred for another 0.5 h allowing for the was achieved over 0.7 wt.% Pd/C under mild reaction
full reduction of Pd2+ species. Thus obtained catalyst was dried conditions, i.e., 60 oC and 0.3 MPa H2. The remarkable
at 110 °C overnight. The Pd loading was determined by advantage of this strategy compared with previous
inductively coupled plasma atomic emission spectrometer hydrogenation/etherification process lies in that much lower
(ICP-AES). The samples were denoted as xPd/C (x = 0.3, 0.7, reaction temperature is required in this route thus preventing
and 1.4 in wt.%). For comparison, Pd supported on some the sequential decomposition of furfuryl ethyl ether to other
commonly used oxides were also prepared by the same by-products such as levulinic acid or its esters. Detailed
method, namely, 0.8Pd/SiO2, 0.7Pd/TiO2 and 0.9Pd/Al2O3.
kinetics study suggests that 2-(diethoxymethyl)furan is the key
intermediate, the formation of which is likely catalysed by the
palladium hydride formed in situ. The hydrogenolysis of 2-
Materials Characterizations
The power X-ray diffraction (XRD) patterns were collected on a (diethoxymethyl)furan will lead to the formation of ether,
Rigaku Ultima IV X-ray diffractometer using Cu Ka radiation (λ while this desirable reaction pathway competes with
= 1.5405 Å) operated at 35 kV and 25 mA. Nitrogen hydrogenation reactions that lead to the formation of furfural
adsorption-desorption isotherms at -196 °C were obtained on alcohol, tetrahydrofurfural alcohol and other saturated
BELSORP-Max equipment. Prior to the measurement, the compounds. To achieve high yield of ether, a balanced
samples were first degassed at 300 °C under vacuum for 6 h. protonic and hydrogenolysis activity of palladium hydride is
Specific surface areas (SSA) were calculated according to the required, which is highly related to the Pd loading, reaction
BET method using five relative pressure points in the relative temperatures and hydrogen pressure.
pressure range of 0.05-0.30. Micropore volume (Vmicro) was
obtained by the t-plot method. Transmission electron
microscopy (TEM) images were taken on a FEI Tecnai G2 F30
microscope operating at 300 kV. The average Pd particle size
Conflicts of interest
There are no conflicts to declare.
was calculated by dTEM = (
∑ ∑
nidi3)/( nidi2) by measuring at
least 100 particles51. Pulse CO chemisorption was performed
on a Micromeritics AutoChem 2910 to determine the metal
dispersion of the reduced catalysts. Prior to measurement, the
catalyst was reduced in a flow of 80 mL/min 10 vol.% H2 in Ar
at 80 °C for 2h and then cooled to 40 °C by flushing He for 2 h.
Afterwards, CO gas pluses (5 vol.% in He) were introduced in a
flow of 110 mL/min. The gas phase CO concentration was
followed by thermal conductivity detector (TCD). The Pd
loading was quantified by inductively coupled plasma (ICP) on
a Thermo IRIS Intrepid II XSP atomic emission spectrometer.
About 25 mg catalysts were digested using 5 mL of aqua regia.
The obtained solutions were diluted with deionized water to
the desired Pd concentration.
Acknowledgements
We acknowledge the financial support from China Ministry of
Science and Technology under contract of 2016YFA0202804
and the National Natural Science Foundation of China (Grant
Nos. 21773067, 21533002).
Notes and references
1
C. H. Christensen, J. Rass-Hansen, C. C. Marsden, E. Taarning
and K. Egeblad, ChemSusChem, 2008, 1, 283.
M. Besson, P. Gallezot and C. Pinel, Chem. Rev., 2014, 114,
1827.
2
3
4
R. A. Sheldon, Catal. Today, 2011, 167, 3.
R. Mariscal, P. Maireles-Torres, M. Ojeda, I. Sádaba and M.
López Granados, Energy Environ. Sci., 2016, 9, 1144.
A. M. Robinson, J. E. Hensley and J. W. Medlin, ACS Catal.,
2016, 6, 5026.
K.Yan, G. Wu, T. Lafleur and C. Jarvis, Renewable Sustainable
Energy Rev., 2014, 38, 663.
L. Faba, E. Díaz and S. Ordóñez, Renewable Sustainable
Energy Rev., 2015, 51, 273.
Y. Nakagawa, M. Tamura and K. Tomishige, ACS Catal., 2013,
3, 2655.
A. Bohre, S. Dutta, B. Saha and M. M. Abu-Omar, ACS
Sustainable Chem. Eng., 2015, 3, 1263.
Catalytic tests
The reductive etherification of furfural was conducted in a
Teflon-lined (60 mL) steel batch reactor. No pretreatment on
the catalysts was conducted prior to each reaction. The reactor
was charged with 9.8 mL of ethanol, 0.2 mL of furfural and 100
mg of catalyst, and the mixture was stirred under 0.1-0.5 MPa
H2 pressure at 40-80 °C. After reaction for desired period, the
H2 was released and the liquid was diluted with ethanol before
analysis. The products were analyzed with flame ionization
detector (FID) and capillary column of DB-FFAP (30 m length
and 0.25 mm internal diameter). The mass spectra of some
products were recorded on Shimadzu GCMS-QP2010SE
equipped with Rxi-5Sil MS column (30 m*0.25 mm*0.25 um).
5
6
7
8
9
10 X. Hu, R. J. M. Westerhof, L. Wu, D. Dong and C.-Z. Li, Green
Chem., 2015, 17, 219.
11 R. S. Karinen, A. O. I. Krause, K. Ekman, M. Sundell and R.
Peltonen, Stud. Surf. Sci. Catal., 2000, 130, 3411.
12 W. Leitner, J. Klankermayer, S. Pischinger, H. Pitsch and K.
Kohse-Hoinghaus, Angew. Chem., Int. Ed., 2017, 56, 5412.
13 Y. Wang, P. Zhang, G. Huang, Q. Yuan, Y. Guan and P. Wu,
ACS Sustainable Chem. Eng., 2017, 5, 6645.
Conclusions
In summary, we have demonstrated the successful synthesis of
furfuryl ethyl ether from furfural and ethanol via reductive
14 W. Yu, F. Lu, Q. Huang, R. Lu, S. Chen and J. Xu, Green Chem.,
2017, 19, 3327.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 8 of 9
View Article Online
DOI: 10.1039/C7GC03887A
ARTICLE
Journal Name
15 A. M. Ruppert, A. N. Parvulescu, M. Arias, P. J. C. Hausoul, P. 49 M. A. Kulagina, P. A. Simonov, E. Yu. Gerasimov, R. I. Kvon, A.
V. Romanenko, Colloids and Surfaces A: Physicohemical and
Engineering Aspects, 2017, 526, 29
C. A. Bruijnincx, R. J. M. K. Gebbink and B. M. Weckhuysen, J.
Catal., 2009, 268, 251.
16 G. J. M. Gruter, US Patent 2010/0212217, 2010.
17 H. Wang, T. Deng, Y. Wang, Y. Qi, X. Hou and Y. Zhu,
Bioresource Technol., 2013, 136, 394.
18 Z. Yuan, Z. Zhang, J. Zheng and J. Lin, Fuel, 2015, 150, 236.
19 Z. Wang and Q. Chen, Green Chem., 2016, 18, 5884.
20 P. Lanzafame, K. Barbera, S. Perathoner, G. Centi, A. Aloise,
M. Migliori, A. Macario, J. B. Nagy and G. Giordano, J. Catal.,
2015, 330, 558.
50 P. Zhang, Q. Yuan, L. Chen, T. Xue, Y. Guan and P. Wu, RSC
Adv., 2016, 6, 65377.
51 J. M. M. Tengco, Y. K. Lugo-José, J. R. Monnier and J. R.
Regalbuto, Catal. Today, 2015, 246, 9.
21 B. Xiang, Y. Wang, T. Qi, H.-Q. Yang and C.-W. Hu, J. Catal.,
2017, 352, 586.
22 M. Rose, K. Thenert, R. Pfützenreuter and R. Palkovits, Catal.
Sci. Technol., 2013, 3, 938.
23 S. Radhakrishnan, P. J. Goossens, P. C. Magusin, S. P. Sree, C.
Detavernier, E. Breynaert, C. Martineau, F. Taulelle and J. A.
Martens, J. Am. Chem. Soc., 2016, 138, 2802.
24 R. J. Haan and J.-P. Lange, US Patent 2011/0035991 A1,
2011.
25 E. R. Sacia, M. Balakrishnan and A. T. Bell, J. Catal., 2014,
313, 70.
26 D. Padovan, A. Al-Nayili and C. Hammond, Green Chem.,
2017, 19, 2846.
27 R. Pizzi, R.-J. van Putten, H. Brust, S. Perathoner, G. Centi and
J. van der Waal, Catalysts, 2015, 5, 2244.
28 M. Balakrishnan, E. R. Sacia and A. T. Bell, Green Chem.,
2012, 14, 1626.
29 P. J. Lange, E. van der Heide, J. van Buijtenen and R. Price,
ChemSusChem, 2012, 5, 150.
30 G. M. Gonzalez Maldonado, R. S. Assary, J. Dumesic and L. A.
Curtiss, Energy Environ. Sci., 2012, 5, 8990.
31 V. E. Tarabanko, M. Y. Chernyak, I. L. Simakova, K. L.
Kaigorodov, Y. N. Bezborodov and N. F. Orlovskaya, Russ. J.
Appl. Chem., 2016, 88, 1778.
32 M. J. Climent, A. Corma, and S. Iborra, Chem. Rev., 2011,
111, 1072.
33 W. L. Howard and J. H. Brown, J. Org. Chem., 1961, 26, 1026.
34 S. Nishimura and T. Itaya, Chem. Commun., 1967, 232, 422.
35 V. Bethmont, F. Fache and M. Lemaire, Tetrahedron Lett.,
1995, 36, 4235.
36 M. P. Doyle, D. J. DeBruyn and D. A. Kooistra, J. Am. Chem.
Soc.,1972, 94, 3659.
37 V. Bethmont, C. Montassier and P. Marecot, J. Mol. Catal. A:
Chem., 2000, 152, 133.
38 S. S. Mochalov, A. N. Fedotov, E. V. rofimova and N. S.
Zefirov, Russ. J. Org. Chem., 2016, 52, 503.
39 Y. Fujii, H. Furugaki, E. Tamura, S. Yano and K. Kita, Bull.
Chem. Soc. Jpn., 2005, 78, 456.
40 T. T. Pham, S. P. Crossley, T. Sooknoi, L. L. Lobban, D. E.
Resasco and R. G. Mallinson, Appl. Catal. A: Gen., 2010, 379,
135.
41 M. Bakos, A. Gyomore, A. Domjan and T. Soos, Angew.
Chem., Int. Ed., 2017, 56, 5217.
42 P. Weerachawanasak, O. Mekasuwandumrong, M. Arai, S.-I.
Fujita, P. Praserthdam and J. Panpranot, J. Catal., 2009, 262,
199.
43 J. M. Rubio-Caballero, S. Saravanamurugan, P. Maireles-
Torres, and A. Riisager, Catal. Today, 2014, 234, 233
44 B. L. Wegenhar, S. Liu, M. Thom, D. Stanley, and M. M. Abu-
Omar, ACS Catal. 2012, 2, 2524
45 J.-I. Ishiyama, K. Esashika, Y. Senda and S. Imaizumi, Nippon
Kagaku Kaishi, 1988, 1988, 126.
46 A. Valcarcel, F. Morfin, L. Piccolo, J. Catal. 2009, 263, 315
47 Z. A. Chase, J. L. Fulton, D. M. Camaioni, D. Mei, M.
Balasubramanian, V.-T. Pham, C. Zhao, R. S. Weber, Y. Wang,
and J. A. Lercher, J. Phys. Chem. C 2013, 117, 17603
48 L. Cerveny, Chem. Eng. Comm. 1989, 83, 31
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 9
Green Chemistry
View Article Online
DOI: 10.1039/C7GC03887A
Furfuryl ethyl ether (FEE) was synthesized from furfural via reductive etherification
in 81% yield over Pd/C catalyst under mild conditions.