Selective hydrogenation of aromatic furfurals into aliphatic tetrahydrofurfural derivatives

Article abstract of DOI:10.1039/d0gc01587c

Tetrahydrofurfural (THFF) and 5-hydroxymethyltetrahydro-2-furaldehyde (5-HMTHFF) are important chemicals. Synthesis of THFF and 5-HMTHFF from the selective hydrogenation of furfural (FF) and 5-hydroxymethylfurfural (HMF) is highly desirable. However, it is a great challenge to hydrogenate furanyl rings while keeping CO intact. Herein, we found that Pd/LDH-MgAl-NO3 could efficiently catalyze the hydrogenation of FF to THFF and HMF to 5-HMTHFF in water. At near complete conversion of FF and HMF, the selectivities of THFF and 5-HMTHFF could reach 92.6% and 83.7%, respectively. A series of control experiments showed that both the LDH-MgAl-NO3 support and water solvent played an important role in the unusual performance of the catalytic system. The hydrogenation of the furanyl ring occurred on the surface of Pd. Water prohibited the hydrogenation of the CO group, and the special nature of LDH-MgAl-NO3 prevented hydrogenation of the CO group on the support by the hydrogen spillover. Thus, the furanyl ring was selectively hydrogenated, and high selectivity of the desired product was successfully achieved. As far as we know, efficient hydrogenation of FF to THFF or HMF to 5-HMTHFF has not been reported. This work opens the way to selectively hydrogenate the furanyl ring while keeping CO in the same molecule unchanged. This journal is

Full text of DOI:10.1039/d0gc01587c

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DOI: 10.1039/D0GC01587C
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ARTICLE
Selective Hydrogenation of Aromatic Furfurals into Aliphatic
Tetrahydrofurfural Derivatives
Received 00th January 20xx,
Accepted 00th January 20xx
Youdi Yang,a,b,c,d Yanyan Wang,a,b,c Shaopeng Li,a,b,c Xiaojun Shen,a,b,c Bingfeng Chen,a Huizhen
Liu*a,b,c and Buxing Hana,b,c
DOI: 10.1039/x0xx00000x
Tetrahydrofurfural (THFF) and 5-hydroxymethyltetrahydro-2-furaldehyde (5-HMTHFF) are important chemicals. Synthesis
of THFF and 5-HMTHFF from the selective hydrogenation of furfural (FF) and 5-hydroxymethylfurfural (HMF) is highly
desirable. However, it is a great challenge to hydrogenate furanyl ring while keeping C=O intact. Herein, we found that
Pd/LDH-MgAl-NO3 could efficiently catalyze the hydrogenation of FF to THFF and HMF to 5-HMTHFF in water. At near
complete conversion of FF and HMF, the selectivities of THFF and 5-HMTHFF could reach 92.6% and 83.7%, respectively. A
series of control experiments showed that both LDH-MgAl-NO3 support and water solvent played important role for the
unusual performance of the catalytic system. The hydrogenation of furanyl ring occurred on the surface of Pd. Water
prohibited the hydrogenation of C=O group, and the special nature of LDH-MgAl-NO3 prevented hydrogenation of C=O
group on the support by the hydrogen spillover. Thus, the furanyl ring was selectively hydrogenated, and high selectivity of
the desired product was successfully achieved. As far as we know, efficient hydrogenation of FF to THFF or HMF to 5-
HMTHFF has not been reported. This work opens the way to selectively hydrogenate the furanyl ring while keeping C=O in
the same molecule unchanged.
www.rsc.org/
the hydrogenation of aldehyde group was dominant (Scheme 1a).
For example, Hronec and his co-workers reported the
hydrogenation of FF to furfuryl alcohol (FA) over PdCu bimetal
Introduction
Biomass is a very important renewable carbon resource and can
be used as feedstocks to prepare many valuable chemicals4,5
Selective hydrogenation6-9 is a common strategy to achieve the
transformation of biomass and its derivatives to valuable
compounds. Since most biomass and its derivatives contain
different kind of reducible functional groups in one compound, it is
often a great challenge to control the selectivity for a desired
product.
1-3
catalyst 14. CuCo catalyst was used to catalyze the hydrogenation of
FF to 2-methylfuran (2-MF) and the intermediate was FA15. The
hydrogenation of HMF to 2,5-dimethylfuran (DMF) could proceed
via hydrogenation of HMF to bis(hydroxymethyl)furan (BHMF) and
.
then the hydrogenolysis of BHMF to DMF 16,17
.
Tetrahydrofurfural (THFF) and 5-hydroxymethyltetrahydro-2-
furaldehyde (5-HMTHFF) are important intermediates for the furan-
based drugs, such as ranitidine, prazosin, furosemide, and
cefuroxime, and have been widely used in pharmaceutical
Furfural (FF) and 5-hydroxymethylfurfural (HMF), which can be
produced from carbohydrates via hydrolysis and dehydration
are considered as platform chemicals for biomass conversion.
Plenty of products have been obtained from the selective
hydrogenation of FF and HMF 12-17. However, in the reported work,
10,11
,
industry18,19
hydroformylation
.
Traditionally, THFF was synthesized by the
of dihydrofurans using Rh(acac)CO2,
HRh(CO)(PPh3)3 or RhCl(CO)(PPh3)2 as the catalysts20-22. In addition,
THFF could also be obtained by the reaction of lithium derivatives
of furfural with CO23. 5-HMTHFF was synthesized by the oxidation
of 4-acetoxy-1,5-dibromopentane derivatives with silver nitrate24.
Obviously, the traditional methods are complex and not
environmentally benign (Scheme 1b). It is highly desirable to obtain
THFF from FF and 5-HMTHFF from HMF by selective hydrogenation,
but this is a great challenge because hydrogenation of C=O is easier
than furanyl ring over commonly used catalysts.
a. Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid
and Interface and Thermodynamics, CAS Research/Education Centre for
Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China.
b. School of Chemistry and Chemical Engineering, University of Chinese Academy of
Sciences, Beijing 100049, China.
c. Physical Science Laboratory, Huairou National Comprehensive Science Centre,
Beijing 101407, P. R. China.
d. College of Science, Beijing Forestry University, Beijing 100083, China.
*Correspondence to: liuhz@iccas.ac.cn (Huizhen Liu).
Electronic Supplementary Information (ESI) available: [Materials and Methods,
Table S1-S2 and Fig. S1-S4]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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image of the Pd/LDH-MgAl-NO3 catalyst is presented ViniewFiAgr.tic1leaO. nTlhinee
DOI: 10.1039/D0GC01587C
Pd nanoparticles about 2.3 nm were immobilized on the support.
The catalyst was also characterized by X-ray diffraction (XRD)
method. No peaks of Pd were observed because of the small size of
the particles (Fig. 1b). A series of diffraction peaks with 2θ values at
11.4°, 22.8°, 34.6°, 38.7°, 45.2°, 60.6° and 61.8° are related to the
(003), (006), (012), (015), (018), (110) and (113) crystal planes of the
MgAl-LDH. The basal spacing of Pd/LDH-MgAl-NO3 calculated from
(003) peak is 8.16 Å. Considering the thickness of brucite-like sheet
is about 4.8 Å, the interlayer spacing of the LDH-MgAl-NO3 was
known to be 3.36 Å 34. Moreover, it can be known from TEM and
XRD characterizations that the lamellar structure of LDH was
maintained after loading Pd nanoparticles. The Pd content obtained
by ICP was 2.76%. Besides Pd/LDH-MgAl-NO3 catalyst, Pd/SiO2,
Pd/TiO2 and Pd/ZrO2 were also fabricated and the TEM
characterizations are shown in Fig S1. The Pd contents obtained by
ICP were 2.58%, 2.66% and 2.98%, respectively (Table S1).
Performances of the catalysts for selective hydrogenation of
FF and HMF. The performances of the catalysts were studied by
selective hydrogenation of FF and the results are shown in Table 1.
LDH-MgAl-NO3 could not catalyze the hydrogenation of FF and no
product was detected (Table 1, entry 1). Using water as the solvent,
the conversion of FF could reach 97.5% in 2 h at room temperature
with 92.6% selectivity of THFF over Pd/LDH-MgAl-NO3 (Table 1,
entry 2). The conversion of FF and the selectivity of THFF were both
very low over Pd/ZrO2 and Pd/SiO2 and the yield of THFF was only
3.3% and 1.5%, respectively (Table 1, entries 3 and 4). Although the
activity of Pd/TiO2 was very high with 95.2% conversion of FF in 3 h,
the selectivity of THFF was only 19.4%, and much coupling product
was formed (Table 1, entry 5). Very interestingly, no FA, which was
produced by hydrogenating the C=O prior to furanyl ring, was
detected over Pd/LDH-MgAl-NO3 catalyst, while it was produced
over Pd/SiO2, Pd/ZrO2, and Pd/TiO2. These results indicate that the
supports of the catalysts played an important role for the reaction,
and the activity of Pd/LDH-MgAl-NO3 for the hydrogenation of
furanyl ring was much higher than that for C=O. The effect of time
on the reaction was also investigated (Fig. S2). No FA was detected
at lower conversion. Only 3.3% of FF was converted in 0.25 h and
the selectivity of THFF was 100% (Table 1, entry 6). It further
indicates that the hydrogenation of furanyl ring was prior to C=O.
However, the C=O group in the desired product THFF could be
hydrogenated to form tetrahydrofurfuryl alcohol (THFA) with longer
time, and the selectivity of THFF dropped to 75.6% in 3 h (Table 1,
entry 7).
Scheme
1
The reaction routes of (a) reported work for
hydrogenation FF and HMF, (b) traditional methods to synthesize
THFF and 5-HMTHFF, (c) this work to synthesize THFF and 5-
HMTHFF.
Layered double hydroxides (LDHs) are a class of highly ordered
two-dimensional anionic clays and their properties can be adjusted
by changing the metallic ions and the interlaminar anion, which
makes it a tunable heterogeneous catalyst support25-27. On the
other hand, Pd based catalyst has been widely used to catalyze
hydrogenation reactions6-7, 28-29 In addition, it has been reported
that support and solvent could significantly influence the
performance of the catalyst30-32. In this work, we discover that
Pd/LDH-MgAl-NO3 could efficiently catalyze the selective
hydrogenation of FF to THFF and HMF to 5-HMTHFF in water
(Scheme 1c). At near complete conversion, the selectivities of THFF
and 5-HMTHFF could reach 92.6% and 83.7%, respectively. Detailed
study indicates that the solvent water and LDH-MgAl-NO3
synergistically promotes the selectivity of the reaction. The reaction
occurred on the surface of Pd in Pd/LDH-MgAl-NO3. Water
prohibited the adsorption of C=O on the metal surface that made
the active sites unfavorable for the hydrogenation of C=O.
Results and discussion
Preparation and characterization of Pd based catalysts. MgAl-
-
LDH with anion NO3 was obtained according to the procedure
reported33. The Pd/LDH-MgAl catalyst was prepared by
impregnation method and the procedure is shown in the
Supporting Information. Transmission electron microscopy (TEM)
Fig. 1 (a) The TEM image of Pd/LDH-MgAl-NO3; (b) The XRD
patterns of LDH-MgAl-NO3 (dash line) and Pd/LDH-MgAl-NO3 (solid
line).
2 | J. Name., 2012, 00, 1-3
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The solvent affected the performance of the catalyst
This catalytic system also could catalyze Vtihewe ArsticelleeOcntilivnee
significantly (Table 1, entries 2 and 8-10). The selectivity of THFF hydrogenation of HMF to 5-HMTHFF, anDdOIt:h1e0.1s0e3l9e/cDti0vGitCy01o5f875C-
was 69.2%, 39.8% and 8.2% in EtOH, THF and n-hexane, respectively. HMTHFF could reach 83.7% when HMF conversion was 97.2%
Moreover, FA was detected in EtOH, THF and n-hexane. Thus, it can (Table 1, entries 13). In addition, no BHMF was detected, indicating
be deduced that in the organic solvents examined, the C=O could be that the hydrogenation of furanyl ring also occurred prior to the
hydrogenated prior to the furanyl ring, which was obviously hydrogenation of C=O for the selective hydrogenation of HMF over
different from the reaction in water.
Pd/LDH-MgAl-NO3.
Table 1 Results for the selective hydrogenation of FF and HMF over different catalysts.
O
O
OH
O
OH
O
O
O
+
+
+
+
FF
THFF
1
FA
2
THFA
3
OH
OH
OH
OH
O
O
OH
OH
O
O
O
O
HMF
5-HMTHFF
1
BHMF
2
BHMTHF
3
Yield (%)
t
(h)
Conversion
(%)
Entry
Catalyst
Solvent
Selectivity of 1 (%)
1
2
3
others
Substrate: FF
1
2
LDH-MgAl-NO3
Pd/LDH-MgAl-NO3
Pd/SiO2
H2O
H2O
H2O
H2O
H2O
H2O
H2O
EtOH
THF
Hex
H2O
H2O
2
2
0
0
0
0
0
0
0
-
97.5
39.1
13.6
95.2
3.3
90.3
3.3
7.1
92.6
8.5
3
3
18.0
6.5
11.1
0
4.4
1.6
19.9
0
13.4
4.0
45.7
0
4
Pd/ZrO2
3
1.5
10.9
19.4
100
75.6
69.2
39.8
8.2
5
Pd/TiO2
3
18.5
3.3
6
Pd/LDH-MgAl-NO3
Pd/LDH-MgAl-NO3
Pd/LDH-MgAl-NO3
Pd/LDH-MgAl-NO3
Pd/LDH-MgAl-NO3
0.25
3
7
100
73.3
6.3
75.6
50.8
2.5
0
24.3
9.4
1.2
5.4
4.4
1.6
0.1
1.9
0.1
0.1
2.0
5.7
8
2
11.1
2.5
3.6
7.1
4.3
9
2
10
2
9.9
0.8
11a
12b
Pd/LDH-MgAl-NO3+ ZrO2
Pd/ZrO2+LDH
2
57.1
24.6
43.6
13.0
76.3
52.8
3
Substrate: HMF
13 Pd/LDH-MgAl-NO3
H2O
4
97.2
81.4
0
8.8
7.0
83.7
Reaction conditions: FF: (1 mmol) or HMF (0.5 mmol), catalyst (10 mg), solvent (2 mL); 30°C; H2 pressure, 1 MPa; and stirring speed, 800
rpm. [a] ZrO2 (10 mg). [b] LDH-MgAl-NO3 (10 mg).
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DOI: 10.1039/D0GC01587C
The effect of the support. As discussed above, support of the
Hydrogen spillover on the catalysts. On the surface of
supported catalysts, it is a common phenomenon that the activated
hydrogen atoms on the surface of the metal particles can further
migrate to the support, which makes the support become the active
catalysts influences the reaction significantly. The mode of the
substrate adsorbed on different supports in H2O and n-hexane was
investigated by Fourier transform infrared spectroscopy (FT-IR)
according to the reported work38-40. Propyl aldehyde was used as
the model compound in order to verify the adsorption of C=O on
the surface of the supports. The support SiO2, ZrO2, TiO2 and LDH-
MgAl-NO3 were treated by the solution of propyl aldehyde in H2O or
n-hexane, and the physically adsorbed propyl aldehyde was
removed. As shown in Fig. 2a-2c, the peaks at 1157 and 1212 cm-1,
assigned to the ν(C-O) bands of the on-top and bridged sites,
respectively, were observed on the support SiO2, TiO2, ZrO2, while
these peaks were not observed on LDH-MgAl-NO3 support (Fig. 2d).
The results indicate that, C=O group could be chemically adsorbed
on SiO2, ZrO2, TiO2 in both water and n-hexane, but not on LDH-
MgAl-NO3.
center as well, which is called as hydrogen spillover. Hydrogen
35-37
spillover can be proved by mixing various catalysts with WO3
.
The migrated activated hydrogen can react with WO3 to form
HxWO3, and the color of sample changes from yellow to dark blue.
We studied the hydrogen spillover of the catalysts in this work using
the reported method. In our experiment, 20 mg of catalyst and 1 g
of WO3 were mixed and ground, and then put into a transparent
double-pass quartz tube. The photos of the samples obtained after
treatment with H2 at room temperature for 10 min are shown in Fig.
S3. Pure WO3 exhibited pale yellow and the color remained
unchanged after hydrogen treatment. However, the treated
mixture of WO3 and Pd/LDH-MgAl-NO3 turned from pale yellow to
dark green. The same phenomenon occurred for the treated
mixtures consisting of WO3 and Pd/SiO2, Pd/TiO2, Pd/ZrO2. The
results indicate that WO3 reacted with activated hydrogen and
HxWO3 was formed, confirming the existence of hydrogen spillover
on the surface of the catalysts.
Hydrogen spillover affected the performance of the catalyst
significantly. The performance of the physically mixed Pd/LDH-
MgAl-NO3 and ZrO2 was checked. The yield of THFF decreased to
43.6%, and 7.1% of FA was generated (Table 1, entry 11). It suggests
that the active hydrogen generated on Pd of Pd/LDH-MgAl-NO3
migrated to ZrO2 because of hydrogen spillover, and the adsorbed
C=O group on ZrO2 was hydrogenated. The yield of THFF over
physically mixed catalysts of Pd/ZrO2 and LDH-MgAl-NO3 was
obviously higher than that over Pd/ZrO2 (Table 1, entries 4 and 12),
indicating that the active hydrogen generated on Pd of Pd/ZrO2
migrated to LDH-MgAl-NO3 because of hydrogen spillover, and the
C=O group could not be hydrogenated on LDH-MgAl-NO3 because it
cannot chemically adsorb the C=O group41. These results further
illustrate the existence of hydrogen spillover and the importance of
the support.
The special structure of LDH is the primary reason for the
unfavorable chemical adsorption of C=O. To get further information
on this, LDH-MgAl-NO3 was calcined to form the corresponding
o
metal oxides MgO-Al2O3 at 500 C. The XRD pattern, which is given
in Fig. S4a, is consistent with the pattern of periclase MgO. The
substrate adsorbed on the MgO-Al2O3 in H2O and n-hexane was also
checked by FT-IR using the method discussed above. It was found
that MgO-Al2O3 could chemically adsorb C=O bonds in n-hexane,
but not in water (Fig. S4b). It has been reported that the MgO-Al2O3
could restore the structure of LDHs in water.42 The material of
MgO-Al2O3 treated in water was characterized by XRD and the
characteristic peaks of LDHs appeared (Fig. S4a). The MgO-Al2O3
could not chemically adsorb C=O in water because it changed back
into LDHs. The catalytic performance of Pd/MgO-Al2O3 in water was
also checked (Table S2). The selectivity of THFF could reach 71.4%.
Fig. 2 (a-d) The FT-IR spectra of unmodified (dot line) and aldehyde-
modified supports (stirring in H2O, solid line; stirring in n-hexane,
dash line). (a) SiO2, (b) TiO2 (c) ZrO2 and (d) LDH-MgAl-NO3.
The effect of the solvent. The solvent influenced the reaction
significantly. In order to get some evidence to explain the solvent
effect, we studied the adsorption mode of C=O in different solvents
on Pd/LDH-MgAl-NO3 by FT-IR (Fig.3). Pd/LDH-MgAl-NO3 was
treated by the solution of propyl aldehyde in different solvents and
the physically adsorbed propyl aldehyde was removed by
Fig. 3 The FT-IR spectra of aldehyde-modified Pd/LDH-MgAl-NO3 in
H2O, EtOH, THF and n-hexane.
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centrifugation and vacuum drying. The peaks observed at 1080 and
1002 cm−1 in Fig. 3 were assigned to the ν(C–O), which indicated the
chemically adsorption of C=O occurred on the surface of catalyst in
EtOH, THF and n-hexane. The peaks of the catalyst treated in n-
hexane were strongest, while that of the catalyst treated in water
were very weak. The results suggest that H2O prohibited the
adsorption of C=O group on Pd metal surfaces, which is not
favorable to the hydrogenation of C=O. The results were consistent
with the fact that the selectivity of THFF in water was much higher.
Conclusions
View Article Online
In summary, Pd/LDH-MgAl-NO3 could eDfOficI:ie10n.t1l0y39c/Dat0aGlyCz0e158t7hCe
selective hydrogenation of FF to THFF and HMF to 5-HMTHFF in
water, respectively. In water, the support and water worked
cooperatively to improve the selectivity. The furanyl ring was
hydrogenated on the surface of Pd and water prohibited the
reaction of C=O on the metal surface, and the support could not
activate C=O group, resulting in high selectivity to the desired
product. By comparison, in organic solvents, both the C=O and
furanyl ring could be hydrogenated and the selectivity was low. In
addition, the selectivity of reaction over Pd/SiO2, Pd/ZrO2 and
Pd/TiO2 was very low because the support could activate C=O and
hydrogenation of C=O took place on the support via hydrogen
spillover. This work makes the breakthrough in that furanyl ring is
selectively hydrogenated while the C=O in the same molecule
remains unchanged, which is a significant progress in hydrogenation
chemistry, and new, efficient, greener routes to prepare THFF and
5-HMTHFF are provided. We believe that the protocol can be used
to design other efficient catalytic systems for selective
hydrogenation of substances with two or more reducible groups.
The main reaction pathways. From the results above and the
knowledge in the literature, we can discuss the main reaction
pathways of the reaction over different catalysts in different
solvents, which are shown schematically in Scheme 2. On one hand,
the adsorption ability of support influences the selectivity
significantly. The activated hydrogen on Pd surface can migrate to
the supports and hydrogenate the substrates activated by the
supports43-45. In water, Pd/SiO2, Pd/TiO2 and Pd/ZrO2, Pd particles
promote the hydrogenation of furanyl ring to produce desired
product THFF. At the same time the supports could adsorb and
activate the C=O group (Figure 3a-3c), and the hydrogenation of
C=O took place on support to produce by-product FA via hydrogen
spillover. Thus, both THFF and FA were obtained (Scheme 2a). The
low selectivity of THFF (Table 1, entries 3-5) suggests that
hydrogenation of C=O group on the support via hydrogen spillover
is a dominant reaction. When the reaction occurred over the
Pd/LDH-MgAl-NO3 in water, the selectivity of the desired product
was very high because the ability of both Pd particle and the
support to adsorb C=O group was very weak (Figure 3d and 4), and
thus mainly the furanyl ring was hydrogenated to generate THFF
(Scheme 2b). When the reaction took place over the Pd/LDH-MgAl-
NO3 in organic solvents, the Pd particles could adsorb C=O group,
and thus both C=O group and furanyl ring could be hydrogenated
(Scheme 2c). Hence, the selectivity in the organic solvents was
lower than that in water. The ability of the catalyst to adsorb the
C=O in the organic solvents varied in the order of n-
hexane>THF>EtOH (Fig. 3), and selectivity of THFF decreased with
the increase of the adsorption ability. All the results are consistent
with the proposed reaction pathway, and both LDH-MgAl-NO3
support and water are crucial for the very high selectivity of THFF.
Conflicts of interest
The authors declare no competing financial interests.
Acknowledgements
The work was supported financially by National Natural Science
Foundation of China (21871277), Beijing Municipal Science &
Technology Commission (Z181100004218004) and Chinese
Academy of Sciences (QYZDY-SSW-SLH013).
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Scheme 2 The reaction pathways of the reaction over different
catalysts in different solvents. (a) Pd/ZrO2, Pd/TiO2 and Pd/SiO2 in
water. (b) Pd/LDH-MgAl-NO3 in water. (c) Pd/LDH-MgAl-NO3 in
organic solvents.
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