One-step conversion of furfural into 2-methyltetrahydrofuran under mild conditions

Article abstract of DOI:10.1002/cssc.201500178

One-step direct conversion of biomass-derived furfural to 2-methyltetrahydrofuran was realized under atmospheric pressure over a dual solid catalyst based on two-stage-packed Cu-Pd in a reactor; this is the first report that one-step conversion of furfural resulted in high yield of 2-methyltetrahydrofuran (97.1 %) under atmospheric pressure. This strategy provided a successive hydrogenation process, which avoids high H₂ pressure, uses the reactor efficiently, and eliminates the product-separation step. Therefore, it could enhance the overall efficiency as a result of low cost and high yield.

Full text of DOI:10.1002/cssc.201500178

DOI: 10.1002/cssc.201500178
Communications
One-step Conversion of Furfural into 2-Methyl-
tetrahydrofuran under Mild Conditions
Fang Dong,[a, b] Yulei Zhu,*[a, c] Guoqiang Ding,[c] Jinglei Cui,[a, b] Xianqing Li,[c] and
Yongwang Li[a, c]
One-step direct conversion of biomass-derived furfural to 2-
methyltetrahydrofuran was realized under atmospheric pres-
sure over a dual solid catalyst based on two-stage-packed Cu–
Pd in a reactor; this is the first report that one-step conversion
of furfural resulted in high yield of 2-methyltetrahydrofuran
(97.1%) under atmospheric pressure. This strategy provided
a successive hydrogenation process, which avoids high H2
pressure, uses the reactor efficiently, and eliminates the prod-
uct-separation step. Therefore, it could enhance the overall ef-
ficiency as a result of low cost and high yield.
and the process was commercialized.[8,12–14] However, the fol-
lowing two steps faced several problems, that is, limited large-
scale production of 2-MTHF [including the direct conversion of
furfural to 2-MTHF necessitating
a
high H2 pressure
(>8.0 MPa)] and the multi-step conversion requiring multi-
component catalysts, at least two different reactors, and the
isolation systems for the intermediates.[15,16] In a patent, it was
reported that the conversion could also be realized by a two
step conversion of furfural, for which two reactors and two cat-
alysts are required.[7] In the first reactor, furfural was converted
into 2-MF, then the crude product 2-MF was isolated and puri-
fied through a separation system. Following the second step,
hydrogenation was performed in another reactor with the puri-
fied intermediate 2-MF as feed (reactant). Although this pro-
Currently, the development and utilization of the biomass-
based carbon resources is vitally critical for the sustainable pro-
duction of transportation fuels and chemical substan-
ces.[1–3] Biomass-based synthesis of 2-methyltetrahy-
drofuran (2-MTHF) has gained significant attention as
an approved gasoline additive with excellent com-
bustion properties, as a versatile new-style solvent
with broad application in organic chemistry and in
the commercial resin industry, and also as an impor-
tant medical intermediate.[4,5] Generally, 2-MTHF is de-
rived from lignocellulose, which is a promising bio-
mass-based alternative in the search for environmen-
tally benign synthesis strategies.[6,7] The conversion of
lignocellulose into 2-MTHF is achieved by a multistep
conversion, including an acid-catalyzed hydrolysis of
lignocellulose (pentose) to furfural, followed by hy-
drogenation–deoxygenation of furfural to 2-methyl-
furan (2-MF) and further hydrogenation of 2-MF to 2-
MTHF in several separated reaction systems
(Scheme 1).[8–11] For the conversion of lignocellulose
into furfural (the first step reaction for the conversion
Scheme 1. Synthesis pathways of 2-methyltetrahydrofuran through the conversion of
lignocellulose.
of lignocellulose to 2-methyltetrahydrofuran), a high
yield of furfural was achieved over various catalysts,
cess could avoid high H2 pressures, it would consume a great
deal of energy and material and reduce the yield of the overall
process. Thus, it would produce some negative effects on the
economy of the 2-MTHF industry.
[a] Dr. F. Dong, Prof. Y. Zhu, Dr. J. Cui, Prof. Y. Li
State Key Laboratory of Coal Conversion
Institute of Coal Chemistry, Chinese Academy of Sciences
Taiyuan 030001 (PR China)
To the best our knowledge, there are only few reports on
the one-step conversion of furfural to 2-MTHF in a reactor
under atmospheric pressure. Especially high H2 pressure and
the multiple reaction systems are the reasons limiting the
large-scale production of 2-MTHF via biomass conversion be-
cause the whole process is difficult to control and requires
E-mail: zhuyulei@sxicc.ac.cn
[b] Dr. F. Dong, Dr. J. Cui
Graduate University of Chinese Academy of Sciences
Beijing 100039 (PR China)
[c] Prof. Y. Zhu, Dr. G. Ding, X. Li, Prof. Y. Li
Synfuels China Co. Ltd.
Taiyuan 030032 (PR China)
a great deal of energy to complete, thus reducing the overall
efficiency and yield of hydrogenation reaction.
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500178.
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Herein, a new continuous process is realized for the synthe-
sis of 2-MTHF via the direct conversion of furfural under mild
conditions that avoids the use of multiple reactors. We de-
signed a successive process for the synthesis of 2-MTHF under
atmospheric pressure, in which the conversion of intermedi-
ates can be controlled efficiently on the basis of the nature of
the functional groups in furfural and especially through the
method of packing the reactor with the catalyst. Interestingly,
when the continuous fixed-bed reactor was fed with pure fur-
fural, the yield of desired product 2-MTHF was high (up to
97.1%) for an optimized packing of the dual solid catalysts
Cu–Pd under atmospheric pressure. From the perspective of
energy consumption, this technology is also economical and
greatly reduces the cost of product separation as the products
are automatically isolated in the two-phase oil–water-based
system (low solubility of 2-MTHF in water; Figure S1 in the
Supporting Information). Furthermore, we also performed a de-
tailed investigation of the performance of various catalysts and
the influence of reaction conditions on the synthesis of 2-
MTHF.
duction temperature of Cu and Ni metals is discordant, so Cu-
and Pd-based catalysts would be the better choice for the suc-
cessive hydrogenation of furfural to 2-MTHF. Our previous
work suggested that Cu/SiO2 catalyst performs excellently in
the hydrogenation–deoxygenation of the C=O bonds in furfu-
ral (Figure S2).[22] Herein, we investigated the operation of Cu-
based (copper phyllosilicate=Cu2Si2O5(OH)2) and Pd-based
(Pd/SiO2) catalysts; the results indicated that both
Cu2Si2O5(OH)2 and Pd/SiO2 had a poor selectivity towards the
desired product 2-MTHF (Table 1, entries 1 and 2). For
Cu2Si2O5(OH)2, the main product was 2-MF (yield=84.6%)
above 1708C, which derived from the hydrogenation–deoxyge-
nation of the ÀCH=O bonds in furfural and exhibited an out-
standing hydrogenation ability of the ÀCH=O bonds (Fig-
ure S3). However, on Pd/SiO2, the major products were tetrahy-
drofurfuryl alcohol at low temperature and furan at high tem-
perature, which derived from the hydrogenation of the C=C
bonds and the breaking of CÀC bands in furfural (the breaking
of furan ring and ÀCH=O; Figure S4). To verify the outstanding
hydrogenation performance of Pd/SiO2 for the C=C bonds, 2-
MF was used as a reactant on Pd/SiO2 as only C=C bonds exist
in 2-MF, which could be efficiently converted into 2-MTHF with
a 98.6% selectivity (Table 1, entry 3). These results verified that
the Cu-based catalyst performed well in the hydrogenation–
deoxygenation of the ÀCH=O bonds whereas the Pd-based
catalyst efficiently hydrogenated the C=C bonds. Afterwards,
the bimetallic CuPd (supported on SiO2) catalyst was synthe-
tized and tested for the direct conversion of furfural into 2-
MTHF. Unfortunately, CuPd/SiO2 had a high selectivity for 2-MF
and furfuryl alcohol due to the conversion of the ÀCH=O
bonds (Table 1, entry 4), suggesting that the hydrogenation of
the ÀCH=O bonds is superior to that of the C=C bonds of the
The hydrogenation process from furfural to 2-MTHF was in-
vestigated in a continuous fixed-bed reactor over various cata-
lysts (Figure S2 and Table 1). Furfural possesses both C=C and
C=O bonds and is a type of a,b-unsaturated compound, which
was treated as a good model compound for studying the se-
lective hydrogenation of C=C and C=O bonds. Therefore, the
selective conversion of functional groups from furfural plays an
important role in the field of biomass conversion.[17] Crucially,
the conversion of furfural to 2-MTHF demands the hydrogena-
tion–deoxygenation of the ÀCH=O bonds and the hydrogena-
tion of the C=C bonds in the furan ring (Scheme 1). Generally,
Cu-based catalysts are superior to some conventional hydroge-
nation catalysts in the hydrogenation–deoxygenation of the À furan ring in furfural. Based on the above results, we designed
CH=O bonds,[18,19] and Ni/Pd-based catalysts play a significant
role in the selective hydrogenation of C=C bonds.[20,21] There-
fore, Cu/Ni/Pd-based catalysts might be excellent catalysts for
the direct conversion of furfural to 2-MTHF. However, the re-
an integral process using two-stage packing of Cu2Si2O5(OH)2
and Pd/SiO2 as catalysts (dual solid Cu–Pd catalysts) in a reactor,
which depends mainly on the good catalytic performance of
the Cu-based catalyst in the synthesis of 2-MF and the subse-
quent C=C bond hydrogenation
of 2-MF to 2-MTHF on Pd/SiO2
(Scheme S1).
Table 1. The hydrogenation of furfural over various catalysts.[a]
Notably, the method of pack-
ing of the dual solid Cu–Pd cata-
lyst significantly affected the hy-
drogenation sequence of the
ÀCH=O and C=C bonds and fur-
ther influences product selectivi-
ty. It was discovered that when
the upper of reactor was packed
with Cu2Si2O5(OH)2 and catalyst
and Pd/SiO2 was at the bottom
of the reactor, a 85.5% selectivi-
ty to 2-MTHF was obtained
Catalysts
Conv.
[%]
Selectivity [%]
THF+FU
others
Cu2Si2O5(OH)2
Pd/SiO2
94.4
99.3
88.9
88.7
97.5
99.5
99.1
0.8
0.4
98.6
1.6
85.5
1.6
14.9
84.6
0.1
0
66.1
4.1
43.4
65.6
0.1
1.3
0
5.4
12.9
0
0
84.8
0
0.1
0
9.1
0.5
1.4
6.6
0.3
5.1
4.6
[b]
Pd/SiO2
[c]
CuPd/SiO2
20.9
0.3
0
4.7
9.8
21
[d]
[e]
[f]
Cu2Si2O5(OH)2 +Pd/SiO2
Pd/SiO2 +Cu2Si2O5(OH)2
Cu2Si2O5(OH)2 +Pd/SiO2
28.9
1.2
0.8
12.9
[a] Reaction conditions: atmospheric pressure, 1708C, H2/furfural (molar ratio)=29, weight hourly space veloci-
ty (WHSV)=0.19 hÀ1, Cu2Si2O5(OH)2 =1.5 g, Pd/SiO2 =1.5 g, FU: furan, THF: tetrahydrofuran, others: 2-potanone
(2-PO), 2-pentanol (2-PL), 1-pentanol (1-PL), tetrahydrofurfural, furfural polymers. [b] Reactant is 2-MF. [c] CuPd
bimetal catalyst: palladium was impregnated on Cu phyllosilicate precursor. [d] The upper reactor was packed
with Cu phyllosilicate as catalyst, and the bottom reactor was packed with Pd/SiO2 as catalyst. [e] The upper re-
actor was packed with Pd/SiO2 as catalyst, and the bottom reactor was packed with Cu phyllosilicate as cata-
lyst. [f] Cu phyllosilicate and Pd/SiO2 catalysts were mechanically mixed.
(Table 1,
entry 5). Contrarily,
when Pd/SiO2 catalyst was in the
upper part of the reactor, tetra-
hydrofurfuryl alcohol and tetra-
hydrofuran were selectively ob-
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tained through the hydrogenation of the C=C bonds and the
breaking of CÀC bonds (Table 1, entry 6). To further illustrate
the importance of the packing method of the catalysts,
Cu2Si2O5(OH)2 and Pd/SiO2 catalysts were mechanically mixed,
and it was found that the selectivity reaction to 2-MTHF was
much lower than for two-stage packing of the dual solid Cu–
Pd catalyst (Table 1, entry 7). The molar ratio of H2/furfural
(mol/mol) was also investigated in our system. When the molar
ratio of H2/furfural was 29, the selective formation of 2-MTHF
was the highest (Figure S5), which might be ascribed to the
number of activated hydrogen atoms on the catalyst surface.
Sitthisa et al.[23] and Zhang et al.[24] reported that Pd-based
catalysts could hydrogenate the C=C and C=O bonds at low
temperature but would eliminate the C=O bonds (through the
breaking of CÀC bonds) from furfural at high temperature. In
this work, we also investigated in detail the catalytic per-
formance of copper phyllosilicate and Pd/SiO2 at different reac-
tion temperatures using furfural as feed (Figures S3 and S4),
and the result is in accordance with the former reports. There-
fore, it was considered that the reaction temperature is vitally
important for the hydrogenation–deoxygenation of the ÀCH=
O bonds and the hydrogenation of the C=C bonds on the dif-
ferent metals. At low temperature, the Cu-based catalyst was
responsible for the hydrogenation of the ÀCH=O bonds, but
was hardly active for the breaking (deoxygenation) of the
ÀCH=O bonds. Similarly, Pd/SiO2 performed well in the hydro-
genation of the C=C and ÀCH=O bonds at low temperature,
with the main product being tetrahydrofurfuryl alcohol. Naka-
gawa and Tomishige[25] also reported that the yield of 2,5-bis
(hydroxymethyl) tetrahydrofuran obtained from the hydroge-
nation of the ÀCH=O and C=C bonds in 5-hydroxymethyl-2-fur-
aldehyde reached 96% over a co-impregnation of Ni/Pd cata-
lyst. Contrarily, at high temperature, Cu-based catalysts pro-
moted the breaking (deoxygenation) of the ÀCH=O bonds,
which is advantageous for the selective conversion of furfural
into 2-MF.[18,23] However, the high reaction temperature is un-
favorable for the hydrogenation of C=C bonds over the Pd-
based catalyst as a result of side reactions occurring, such as
the formation of furan and tetrahydrofuran, which stemmed
from the breaking of CÀC bonds (Figure 1). It was discovered
that the best reaction temperature was 1808C, at which the se-
lectivity towards 2-MTHF formation reached up to 93.3%.
Moreover, a decrease in selectivity at a higher temperature
(2008C) was related to the for-
Figure 1. Influence of reaction temperature on catalytic performance of Cu–
Pd dual solid catalysts. Reaction conditions: atmospheric pressure, H2/Furfu-
ral=29, WHSV=0.19 hÀ1, the upper of reactor: Cu2Si2O5(OH)2 =1.5 g, the
bottom of reactor: Pd/SiO2 =1.5 g, THFA: tetrahydrofurfuryl alcohol.
2.0 g Pd/SiO2 at the bottom of the reactor. This is related to
the number of active sites and the different selectivities of the
different metals for the conversion of functional groups in fur-
fural. Furfural was first converted to 2-MF over Cu2Si2O5(OH)2 in
the upper part of the reactor and then further converted to 2-
MTHF over Pd/SiO2 catalyst at the bottom of reactor. When the
number of Pd sites is too small for the complete hydrogena-
tion of the C=C bonds, the intermediate 2-MF could not be
completely converted into 2-MTHF. It was found that 2-MF se-
lectivity was roughly 23.4% at 0.5 g packing of Pd/SiO2
(Table 2, entry 1). On the other hand, when there are not
enough Cu sites (0.5 g Cu2Si2O5(OH)2 +2.5 g Pd/SiO2), furfural
could be not completely converted to 2-MF, which is then fur-
ther transformed to other products, such as furan and tetrahy-
drofuran, by the breaking of the CÀC bonds in furfural in
(Table 2, entry 4). Therefore, a suitable packing mass of the
two-stage catalyst is extremely important for improving the se-
lectivity of desired product 2-MTHF.
In summary, a highly efficiency process for the synthesis of
2-methyltetrahydrofuran (2-MTHF) from biomass-derived furfu-
ral was achieved by means of a two-stage packing of a dual
solid Cu–Pd catalyst under atmospheric pressure. Compared
mation of undesired by-prod-
ucts, such as pentanol and pen-
tanone, which were formed
through the hydrogenolysis of 2-
MF and 2-MTHF.[26]
Table 2. The effect of packing mass on product selectivity over dual solid catalysts based on two-stage-packed
Cu–Pd.[a]
Packing mass [g+g]
Conv.
[%]
Selectivity [%]
Furthermore, the packing
mass of the two-stage catalysts
was investigated (Table 2 by
varying the catalyst mass, and
the yield of 2-MTHF was as high
(Cu2Si2O5(OH)2 +Pd/SiO2)
THF+FU
others
2.5+0.5
1.5+1.5
1.0+2.0
0.5+2.5
99.1
98.4
100
100
68.6
93.3
97.1
76.1
23.4
4.7
1.4
6.5
0
0
0.6
0.4
0
0.1
0
0.6
1.2
0.8
1.6
0.9
1.0
21.2
0.5
0
as
97.1%
over
1.0 g
[a] Reaction conditions: atmospheric pressure, T=1808C, H2/furfural=29, WHSV=0.19 hÀ1, the upper reactor
was packed with Cu2Si2O5(OH)2 as catalyst, the bottom reactor was packed with Pd/SiO2 as catalyst.
Cu2Si2O5(OH)2 catalyst in the
upper part of the reactor and
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with the single reaction process, this strategy provided a suc-
cessive hydrogenation process by avoiding a high H2 pressure,
by using the reactor efficiently, and by eliminating the prod-
uct-separation step. Therefore, it could enhance the overall ef-
ficiency, resulting in low cost and high yield. Notably, 2-MTHF
yield is high (up to 97.1% at atmospheric pressure and at
1808C), which is the highest value reported so far. In this suc-
cessive process, the Cu2Si2O5(OH)2 catalyst located in the upper
part of the reactor was mainly responsible for the synthesis of
2-methylfuran (2-MF) through the hydrogenation–deoxygena-
tion of the ÀCH=O bonds in furfural, whereas Pd/SiO2 catalyst
at the bottom of the reactor facilitated the subsequent hydro-
genation of the C=C bonds from 2-MF. This strategy could also
be widely applied to other cascade reactions.
Keywords: biomass conversion · copper · deoxygenation ·
hydrogenation · palladium
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Experimental Section
Details about the preparation method of catalysts and the analysis
method of products are provided in the Supporting Information.
The one-step direct conversion of furfural to 2-MTHF was per-
formed in a continuous fixed-bed reactor. The reactor (600 mm
long, 12 mm i.d.) was packed with a suitable amount of catalysts
(20–40 mesh), which were located in the thermostat segment of
the fixed-bed reactor. Before reaction, 5% (v/v) H2/N2 was intro-
duced into the reactor to reduce the catalysts by increasing the
temperature from 25 to 2708C at a rate of 28CminÀ1 and then
keeping it at 2708C for 3 h. After reduction, distilled furfural was
continuously pumped to a preheater using a HPLC pump, and the
preheater was kept at 1408C to prevent condensation of vaporized
furfural. At the same time, pure H2 (99.99%) was introduced into
the top of the preheater using a mass-flow controller. Thereafter,
at atmospheric pressure, vaporized furfural and H2 were mixed
before introducing the mixture into the reactor to perform the hy-
drogenation. The final products were condensed and collected in
a gas–liquid separator.
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The particle morphology and crystal size were analyzed using
a JEM-1011 electron microscope operating at 80 kV. The reducibili-
ty of the catalysts was determined by means of H2-temperature
programmed reduction, which was performed using an Auto
Chem II2920 (Mircromeritics, USA). Temperature-programmed de-
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as H2-TPR (Supporting Information).
Acknowledgements
This work was supported by the Major State Basic Research De-
velopment Program of China (973 Program) (No.2012CB215305).
Received: February 3, 2015
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COMMUNICATIONS
Furfural in bed with two catalysts:
One-step conversion of biomass-derived
furfural to 2-methyltetrahydrofuran (2-
MTHF) is realized over dual solid cata-
lysts based on two-stage-packed Cu–Pd
in a reactor. 2-MTHF yield is as high as
97.1% at atmospheric pressure and
1808C, which is the highest value re-
ported so far. This strategy provides
a successive hydrogenation by avoiding
high H2 pressure, using the reactor effi-
ciently, and eliminating the product-sep-
aration step.
F. Dong, Y. Zhu,* G. Ding, J. Cui, X. Li, Y. Li
&& – &&
One-step Conversion of Furfural into
2-Methyl-
tetrahydrofuran under Mild
Conditions
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