One-pot selective conversion of furfural into 1,5-pentanediol over a Pd-added Ir-ReOx/SiO2 bifunctional catalyst

Article abstract of DOI:10.1039/c3gc41335g

One-pot selective conversion of furfural into 1,5-pentanediol (1,5-PeD) was carried out over Pd-added Ir-ReO_x/SiO₂ catalysts through two-step reaction temperatures. The Pd(0.66 wt%)-Ir-ReO_x/SiO ₂ catalyst showed the best performance in the production of 1,5-PeD from furfural. The maximum yield of 1,5-PeD was 71.4%. The furfural conversion and yield of 1,5-PeD was almost maintained during four repeated tests when the catalyst was calcined again. The characterization results from TPR, XRD, XANES, EXAFS and FT-IR of adsorbed CO indicated that Pd-Ir-ReO_x/SiO ₂ catalysts consisted of ReO_x-modified Pd metal particles and ReO_x-modified Ir metal particles. The lower-temperature reaction step was very crucial for the total hydrogenation of furfural into a tetrahydrofurfuryl alcohol intermediate, which was converted into 1,5-PeD by hydrogenolysis during the high temperature step over the ReO_x- modified Ir metal particles.

Full text of DOI:10.1039/c3gc41335g

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DOI: 10.1039/C3GC41335G
Table Contents
One-pot conversion of furfural over Pd-added Ir-ReOx/SiO2 via two-step controlled temperature
reaction is very effective for the production of 1,5-pentanediol with the highest yield of 71.4%.

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DOI: 10.1039/C3GC41335G
One-pot selective conversion of furfural into 1,5-pentanediol
over Pd-added Ir-ReOx/SiO2 bifunctional catalyst
Sibao Liu, Yasushi Amada, Masazumi Tamura, Yoshinao Nakagawa* and Keiichi Tomishige*
Department of Applied Chemistry, School of Engineering, Tohoku University, 6-6-07, Aoba,
Aramaki, Aoba-ku, Sendai 980-8579, Japan
*Corresponding authors: Keiichi Tomishige and Yashinao Nakagawa
Department of Applied Chemistry, School of Engineering, Tohoku University
Tel + Fax : +81-22-795-7214
E-mail : tomi@erec.che.tohoku.ac.jp and yoshinao@erec.che.tohoku.ac.jp

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DOI: 10.1039/C3GC41335G
Abstract
One-pot selective conversion of furfural to 1,5-pentanediol (1,5-PeD) was carried out over
Pd-added Ir-ReOx/SiO2 catalysts through two-step reaction temperatures. Pd(0.66 wt%)-
Ir-ReOx/SiO2 catalyst showed the best performance in the production of 1,5-PeD from furfural. The
maximum yield of 1,5-PeD was 71.4%. The furfural conversion and yield of 1,5-PeD was almost
maintained during four repeating tests when the catalyst was calcined again. The characterization
results of TPR, XRD, XANES, EXAFS and FT-IR of adsorbed CO indicated that Pd-Ir-ReOx/SiO2
catalysts consisted of ReOx-modified Pd metal particles and ReOx-modified Ir metal particles. The
lower-temperature reaction step was very crucial for the total hydrogenation of furfural into
tetrahydrofurfuryl alcohol intermediate, which was converted to 1,5-PeD by hydrogenolysis during
the high temperature step over the ReOx-modified Ir metal particles.
Keywords
hydrogenation, hydrogenolysis, furfural, palladium, iridium
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DOI: 10.1039/C3GC41335G
1. Introduction
Biomass and its derived substrates are playing a more and more important role in replacing
non-renewable fossil resources in the production of transportation fuels and chemical products
because of the dwindling fossil resources and global warming.1-7 Furfural is a key important
platform molecule that is produced by hydration + dehydration of hemicellulose from agricultural
wastes and forest residues on an industrial scale.8-12 Generally, furfural can be converted into useful
fuels and chemicals through oxidation, hydrogenation, hydrogenolysis, condensation and so on.13-23
Among them, hydrogenation and hydrogenolysis are the promising ways because of the high
oxygen content of furfural (Scheme 1). Hydrogenation of furfural can give furfuryl alcohol (FFA)
and tetrahydrofurfuryl alcohol (THFA). Hydrogenolysis can also proceed by the reaction of furfural
with hydrogen, which can produce 2-methylfuran, 2-methyltetrahydrofuran (2-MTHF),
cyclopentanone or polyols especially diols. Hydrogenolysis of hydrogenation products of furfural
also produces valuable compounds such as 1,5-pentanediol (1,5-PeD) and 1,2-pentanediol
(1,2-PeD) that can be used as monomers of polyesters and polyurethanes.24 Adkins et al.25,
26 conducted hydrogenolysis of furfuryl alcohol (FFA) using CuCr2O4 catalyst at 448 K and 10-15
MPa with high conversion, and 30% of 1,5-PeD and 40% of 1,2-PeD were obtained. Li et al.27 also
performed hydrogenolysis of furfuryl alcohol using Ru/MnOx catalyst at 423 K and 1.5 MPa. The
yield of 1,2-PeD was 42.1% and no 1,5-PeD was obtained. Recently, our group has reported
hydrogenolysis of THFA into 1,5-PeD using Rh-ReOx/SiO2, Rh-MoOx/SiO2, Rh-ReOx/C or
Ir-ReOx/SiO2 at lower temperature of 373-393 K, 8 MPa. The highest yield of 1,5-PeD reached
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94%.28-34 Among these Rh- or Ir-based catalysts, Ir-ReOx/SiO2 showed the highest turnover
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DOI: 10.1039/C3GC41335G
frequency value (TOF) per the surface active metal (Rh or Ir) atoms.34 In addition, other groups also
reported hydrogenolysis of THFA with good performance in the production of 1,5-PeD using
35
Rh/MCM-41 in the supercritical CO2
and Rh-ReOx/C (Vulcan XC-72) 36. Recently, Lu et
al.37 have applied Pt-Li/Co2AlO4 catalyst to convert furfural into 1,5-PeD directly in an alcohol
solvent, however the highest yield of 1,5-PeD was only 34.9%. It’s still a challenge to perform
one-pot selective hydrogenolysis of furfural into 1,5-PeD with high yield. According to our recent
report, Ir-ReOx/SiO2 was a very effective catalyst for selective hydrogenation of C=O bond of
unsaturated aldehydes.20 So, improving the hydrogenation activity of C=C bond may be the key of
the direct conversion of furfural into 1,5-PeD.
In this work, we investigated the additive effect of another noble metal such as Ru, Rh, Pt and
Pd to Ir-ReOx/SiO2 catalyst for the one-pot aqueous phase conversion of furfural into 1,5-PeD. As a
result, it was found that Pd-added Ir-ReOx/SiO2 (Pd-Ir-ReOx/SiO2) is the most effective. We also
characterized Pd-Ir-ReOx/SiO2 with XRD, TPR, XANES, EXAFS, and FT-IR of adsorbed CO.
2. Experimental
2.1. Catalyst preparation
The SiO2 (G-6, BET surface area 535 m2/g) supplied by Fuji Silysia Chemical Ltd. was used as a
support of the catalysts. M/SiO2 (M=Pd, Rh, Pt, Ru) catalysts were prepared by impregnating SiO2
with an aqueous solution of Pd(NO3)2 (N. E. Chemcat Co., Ltd.), Rh(NO3)3 (Wako Pure Chemical
Industries, Ltd.), Pt(NH3)4(NO3)2 (Soekawa Chemical Co., Ltd.), and Ru(NO)(NO3)3 (Strem
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Chemicals Co., Ltd.). After evaporating the solvent and drying at 383 K for 12 h, they were
DOI: 10.1039/C3GC41335G
calcined in air at 773 K for 3 h. M-Ir/SiO2 (M=Pd, Rh, Pt, Ru) were prepared by impregnating
M/SiO2 after the calcination procedure with an aqueous solution of H2IrCl6 (Furuya Metals Co.,
Ltd), and dried at 383 K for 12 h. M-Ir-ReOx/SiO2 (M=Pd, Rh, Pt, Ru) were prepared by
impregnating M-Ir/SiO2 after the drying procedure with an aqueous solution of NH4ReO4
(Soekawa Chemical Co., Ltd.). These catalysts were calcined in air at 773 K for 3 h after drying at
383 K for 12 h. The loading amounts of Ir and Re were 4.0 and 7.8 wt%, respectively (Re/Ir molar
ratio = 2), and that of additive was represented by the weight percent of the additives to the total
catalyst in parenthesis like M(x)-Ir- ReOx/SiO2 (M=Pd, Rh, Pt, Ru). Rh-ReOx/SiO2 catalysts were
prepared by the sequential impregnation of Fuji Silysia G-6 SiO2 with aqueous solution of
RhCl3•H2O and NH4ReO4 (Soekawa Chemical Co., Ltd.) followed by drying and calcination at
773 K as described previously.28 The preparation method of Pd(0.66)-Rh-ReOx/SiO2 is the same as
M-Ir-ReOx/SiO2. The loading amount of Rh was 4 wt% and the molar ratio of Re to Rh was 0.5
(2.6 wt% Re). All the catalysts were used in powdered form with a granule size of <100 mesh.
2.2. Activity tests
Activity tests were performed in a 190-ml stainless steel autoclave with an inserted glass vessel.
The catalyst was put into an autoclave together with a spinner and water with the amount required
for the reaction and heated at 473 K with 8 MPa H2 for 1 h for the reduction pretreatment. After the
pretreatment, the autoclave was cooled down, and hydrogen was removed. Furfural (Wako Pure
Chemical Industries, Ltd., >95%) was put into the autoclave. After sealing the reactor, the air
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content was purged by flushing three times with H2 (1 MPa, 99.99%; Nippon Peroxide Co., Ltd.).
DOI: 10.1039/C3GC41335G
The reactor was pressurized with H2 to 6.0 MPa and then heated to set temperature T1 (303-333 K).
The temperature (T1) was kept for 2-8 h (time 1). After this step (low temperature step), the reaction
temperature was increased to another set temperature T2 (353-413 K). The temperature increase
took us about 30 min. The temperature (T2) was kept for 0-120 h (time 2). These parameters (T1,
time 1, T2, and time 2) were described for each result. After reaction, the reactor was cooled down
and the gases were collected in a gas bag. The autoclave contents were transferred to a vial, and the
catalyst was separated by centrifugation and filtration. The products were analyzed using two kinds
of gas chromatographs (Shimadzu GC-2014) equipped with FID. A TC-WAX capillary column
(diameter 0.25 mmφ, 30 m) or a Rtx-1-PONA capillary column (diameter 0.25 mmφ, 100 m) were
used for separation. The products were also identified using GC-MS (QP5050, Shimadzu). The
products were furfuryl alcohol (FFA) and tetrahydrofurfuryl alcohol (THFA), 1,5-pentanediol
(1,5-PeD), 1,4-pentanediol (1,4-PeD), 1,2-pentanediol (1,2-PeD), 1-pentanol (1-PeOH), 2-pentanol
(2-PeOH), 2-methyltetrahydrofuran (2-MTHF), n-pentane, C-C cracking products (such as
methane) and other products that could not be identified (“Others”). The amount of gas products
(n-pentane and methane) was very small and they were included in “Others”. The products were
quantified by the external standard method using an aqueous solution of each product with known
concentration. The conversion and selectivity were calculated on carbon basis similar to that
reported previously.38 The used catalyst was washed with excess water and dried in air, and then
calcined at 773 K at 3 h. A slight loss (<8% in weight) was observed during the recovery process
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and was compensated with fresh catalyst in each reuse experiment. The amount of eluted metal
DOI: 10.1039/C3GC41335G
during the reaction was analyzed with inductively coupled plasma atomic emission spectrometry
(ICP-AES, Thermo Scientific iCAP 6500).
2.3. Catalyst characterization
Temperature-programmed reduction (TPR) was carried out in a fixed-bed reactor equipped
with a thermal conductivity detector using 5% H2 diluted with Ar (30 ml/min). The amount of
catalyst was 0.05 g, and the temperature was increased from room temperature to 1123 K at a
heating rate of 10 K/min. X-ray diffraction (XRD) patterns were recorded by a diffractometer
(Rigaku MiniFlex 600).
The extended X-ray absorption fine structure (EXAFS) spectra were measured at the BL01B1
station at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI;
Proposal No. 2013A1048). The storage ring was operated at 8 GeV, and a Si (111) single crystal
was used to obtain a monochromatic X-ray beam. Two ion chambers for I0 and I were filled with
85% N2+15% Ar and 50% N2+50% Ar, respectively, for Re L3-edge and Ir L3-edge measurements,
and 100% Ar and 100% Ar for Pd K-edge measurement. We prepared the sample after the catalytic
use as follows. The catalytic reaction was carried out in an autoclave. The reaction conditions were
the same as in activity tests. After cooling, the wet catalyst powder was transferred to the
measurement cell in a glove bag filled with nitrogen. The thickness of the cell filled with the
powder was 2 mm to give an edge jump of 0.7, 0.9 and 0.6 for Ir L3-edge, Re L3-edge and Pd
K-edge measurement, respectively, in the case of Pd(1.8)-Ir-ReOx/SiO2. The EXAFS data were
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collected in a transmission mode. For EXAFS analysis, the oscillation was first extracted from the
DOI: 10.1039/C3GC41335G
EXAFS data using a spline smoothing method.39 Fourier transformation of the k3-weighted EXAFS
oscillation from the k space to the r space was performed to obtain a radial distribution function.
The inversely Fourier filtered data were analyzed using a usual curve fitting method.40, 41 For curve
fitting analysis, the empirical phase shift and amplitude functions for the Re-O, Ir-Ir and Pd-Pd
bonds were extracted from data for NH4ReO4, Ir powder and Pd foil, respectively. Theoretical
functions for the Re-Ir, Re-Pd and Pd-Re bond were calculated using the FEFF8.2 program.42 The
Re-Ir, Ir-Ir and Pd-Re bonds are represented by the Re-Ir (or -Re), Ir-Ir (or -Re) and Pd-Re (or -Ir),
respectively, in the curve fitting results. This is because it is very difficult to distinguish between Ir
and Re as a backscattering atom. Analyses of EXAFS data were performed using a computer
program (REX2000, ver. 2.5.9; Rigaku Corp.). Error bars for each parameter were estimated by
stepping each parameter, while optimizing the others parameter, until the residual factor becomes
two times as its minimum value.43
FT-IR spectra were recorded by a Nicolet 6700 FT-IR with a resolution of 4 cm-1 in a diffuse
reflectance mode, using an in situ cell (Thermo Spectra-Tech) equipped with a sample cup, heater
and ZnSe windows. The powder of catalyst was placed in the sample cup. The cell was connected
to the gas flow system. Pretreatment was carried out in H2 flow at 473 K for 1 h. CO was
introduced by pulses with N2 as a carrier gas at room temperature until the CO coverage reached
the saturation level. The FT-IR spectra were collected at room temperature. The amount of CO
chemisorption was measured in a high-vacuum system using a volumetric method (Micrometrics
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ASAP 2020). Before adsorption measurements, the catalysts were in H2 at 473 K for 1 h.
DOI: 10.1039/C3GC41335G
Subsequently the adsorption equilibrium, the sample weight and the dead volume of the apparatus
was about 1.1 kPa, 0.15 g and 43 cm3, respectively. Adsorption amount of CO is represented as the
molar ratio to Pd (Pd/SiO2), Ir (Ir-ReOx/SiO2) and Ir and Pd (Pd-Ir-ReOx/SiO2).
3. Results and discussion
3.1 Screening of catalysts for the conversion of furfural
The catalytic performances of various catalysts in the conversion of furfural are compared in
Table 1. In our previous research, Ir-ReOx/SiO2 (Re/Ir = 2) showed good catalytic performance in
hydrogenolysis of THFA into 1,5-PeD with high TOF.34 However, when Ir-ReOx/SiO2 (Re/Ir = 2)
was applied in conversion of furfural (Entry 1) at one-step reaction, furfural was polymerized
severely and almost no 1,5-PeD was produced. The main detected product was 1,4-PeD, which is
formed via rearrangement as well as hydrogenation.14 Furan compounds are known to be easily
polymerized.44 We added another noble metal to Ir-ReOx/SiO2 (Re/Ir = 2) in order to enhance the
hydrogenation activity of the furan ring and decrease the rate of the polymerization relatively. The
performances of Ru, Pt or Rh-added catalysts were similar to that of Ir-ReOx/SiO2 and furfural was
also polymerized severely (Entries 2-4). Almost no 1,5-PeD was formed. When Pd-modified
catalyst was used, the yield of 1,5-PeD increased to 20.4% and polymerization decreased
dramatically (Entry 5). It is known that furfural was easily polymerized, especially at high
temperature.44 Nakagawa et al.45 conducted total hydrogenation of furfural using Ni-Pd/SiO2 as a
catalyst at low temperature (313 K) and obtained high THFA yield (96%). In order to suppress
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side-reactions, two-step controlled temperature reaction conditions were utilized. The former step at
DOI: 10.1039/C3GC41335G
low temperature (T1, 323 K) was for the total hydrogenation of furfural into THFA and preventing
furfural from polymerizing. The later step at high temperature (T2, 393 K) is for the hydrogenolysis
of THFA into 1,5-PeD. The yield of 1,5-PeD increased remarkably (54.5%, Entry 6) to more than
two-fold from that obtained in the one-step reaction.
The effect of Pd amount of Pd-Ir-ReOx/SiO2 in conversion of furfural was investigated (Entries
6-12). In these conditions, all the catalysts achieved >99.9% furfural conversion. Both the yields of
the intermediate THFA and overhydrogenolysis products (pentanols) were less than 15%. The
dependence of 1,5-PeD yield on the Pd loading amount is volcano-type. Among them,
Pd(0.66)-Ir-ReOx/SiO2 showed the best performance and 1,5-PeD yield reached 62.4%. Catalysts
with smaller Pd amount gave higher selectivity to 1,4-PeD, suggesting lower hydrogenation activity
of furfural. On other hand, catalysts with larger Pd amount gave higher yield of THFA and lower
yield of 1,5-PeD, and these behaviors can be explained by lower hydrogenolysis activity of THFA.
Moreover, Pd(1.8)-Ir-ReOx/SiO2 showed relatively higher 2-MTHF selectivity. Physical mixture of
Pd(0.75)/SiO2 and Ir-ReOx/SiO2 showed only 53.3% yield of 1,5-PeD (Entry 13), indicating the
importance of the interaction of Pd with Ir or ReOx.
Rh-ReOx/SiO2 is also a good catalyst for hydrogenolysis of THFA into 1,5-PeD,28-33 however,
it showed very low 1,5-PeD yield (6.2%, Entry 14) in the conversion of furfural. In addition, the
yield of 1,5-PeD on Pd(0.66)-Rh-ReOx/SiO2 was only 26.8% (Entry 15), which was much lower
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than that of Pd(0.66)-Ir-ReOx/SiO2 (Entry 11). Large amount of 2-MTHF and unidentified
DOI: 10.1039/C3GC41335G
by-products were formed over Rh-based catalysts.
In addition, Ir-ReOx/SiO2 catalyst gave furfuryl alcohol very selectively (Entry 16) only at
lower temperature (T1=323 K). This result agreed well with that in our previous report. 20 As the
temperature increased to T2, the yields of THFA and hydrogenolysis products of THFA on
Ir-ReOx/SiO2 were still very low (Entry 18), and side-reactions such as polymerization began to
start. In the case of Pd(0.66)-Ir-ReOx/SiO2, good yield of THFA was obtained after the
low-temperature step (Entry 17). The product distributions were almost unchanged during the
heating to 393 K (Entry 19). As compared with Entry 11, the hydrogenolysis of THFA to 1,5-PeD
proceeds at the higher-temperature step. These data showed that low performance of Ir-ReOx/SiO2
in the conversion of furfural is due to the low activity in hydrogenation of furan ring.
3.2 Effect of reaction conditions
Table 2 lists the dependence of furfural conversion over Pd(0.66)-Ir-ReOx/SiO2 on reaction
temperatures of T1 and T2. As the temperature of lower-temperature step (T1) increased from 313 K
to 333 K, the yield of 1,5-PeD decreased and the yield of 1,4-PeD increased. This indicates that
higher T1 can be more favorable for rearrangement side reaction to 1,4-PeD. In the case of low T1
of 303 K, the yield of 1,5-PeD was slightly lower than in the case of T1=313 K. It is concluded that
the optimize temperature of T1 was 313 K. As the temperature of high-temperature step (T2)
increased from 353 K to 383 K, the yield of 1,5-PeD increased and that of THFA decreased
gradually. However, the yield of 1,5-PeD decreased at higher T2 like 413 K, and this is due to the
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formation of 1-petanol by the overhydrogenolysis. As a result, a moderate reaction temperature
DOI: 10.1039/C3GC41335G
(373~393 K) is more favorable to the synthesis of 1,5-PeD.
The effect of H2 pressure on the reaction over Pd(0.66)-Ir-ReOx/SiO2 is listed in Table 3. The
yield of 1,5-PeD were gradually increased with H2 pressure except the high pressure range (> 6
MPa). Lower H2 pressure gave higher 1,4-PeD yield, probably because of the low rate of
hydrogenation of furan ring. The overhydrogenolysis product 1-pentanol increased significantly
above 6 MPa. Totally, 6 MPa was optimum.
We also investigated the effect of furfural concentration in the conversion of furfural over
Pd(0.66)-Ir-ReOx/SiO2. In this experiment, the total amount of furfural was fixed and the
concentration of furfural was adjusted by the amount of H2O as a solvent. In the case of low
concentration (below 20%), the yield of 1,5-PeD was almost constant. However, as furfural
concentration increased to 50%, the yield of 1,5-PeD decreased significantly and furfural was
polymerized to some extent. The detailed data are listed in Table S1.
Table 4 shows the effect of reaction times on the conversion of furfural over
Pd(0.66)-Ir-ReOx/SiO2. Entries 1-8 show the dependence on the reaction time (time 2) at
high-temperature step (T2). When the temperature reached T2 (corresponding to time 2 = 0 h), the
main product was THFA (Entry 1). The yield of 1,5-PeD increased and that of THFA decreased
(Entries 2-6) at longer time 2 and the yield of 1,5-PeD reached the maximum of 68.4% at time 2 =
72 h. At further longer time 2 (Entries 7 and 8), the yield of 1,5-PeD decreased by the
overhydrogenolysis. In order to obtain higher 1,5-PeD yields, longer reaction time (time 1) of
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low-temperature step was applied (Entries 10-12). Indeed, for Entries 1-8, the hydrogenation of
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furfural was not completed during the low-temperature step (Entry 9). Longer time 1 gave slightly
higher 1,5-PeD yields. The highest yield of 1,5-PeD reached 71.4% (Entry 12). This value is
much higher than that reported using Pt-Li/Co2AlO4 (34.9%) catalysts.37
Table 5 lists the results of the activity test of the reused Pd(0.66)-Ir-ReOx/SiO2. The method of
the catalyst reuse was as follows: the catalyst was washed with water and dried at 383 K for 12 h
and calcined in air at 773 K for 3 h. Before the activity test, the reduction pretreatment was carried
out in the same way for the fresh catalyst. The yield of 1,5-PeD slightly decreased from 63.2% to
54.8% after four consecutive runs and the yield of THFA increased from 10.2 to 27.4%. Inductively
coupled plasma (ICP) analysis of the reaction solution after the catalyst had been removed by
filtration showed that almost no Pd, Ir and Re (<0.01%) leached, indicating that the catalyst can be
reused to a large extent without leaching. The decrease of the yield of 1,5-PeD could be attributed
to aggregation of metals during recalcination that lowered the hydrogenolysis activity of the
catalysts. The XRD patterns of the catalyst after the 4th run became sharper than that of the catalyst
after reduction (Fig. S1).
3.3 Catalyst characterization
The profiles of temperature-programmed reduction (TPR) of Pd-Ir-ReOx/SiO2, Ir-ReOx/SiO2,
Pd-ReOx/SiO2, ReOx/SiO2, Ir/SiO2 and Pd/SiO2 after calcination are shown in Fig. 1 and the result
of the valence of Re and the consumption of H2 of the catalysts are listed in Table S2. No H2
consumption peak was detected in the case of Pd/SiO2, and this can be interpreted by the reduction
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of Pd at lower temperature than the measurement temperature range (Fig. 1a). Ir/SiO2 was
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completely reduced at 600 K (Fig. 1b) while ReOx/SiO2 was partially reduced at 800 K to the
valence of +1.6 which was calculated by stoichiometry of H2 uptake in the TPR profile (Fig. 1c).46
In the case of the bimetallic catalysts, Ir-ReOx/SiO2 shows only one large peak centered at around
460 K (Fig. 1e).46, 47 Pd-ReOx/SiO2 showed two broad signals between 342 K and 445 K (Fig. 1d),
which was similar to the result reported before.48 In the case of trimetallic catalysts,
Pd(0.09)-Ir-ReOx/SiO2 (Fig. 1f) showed a broader signals than Ir-ReOx/SiO2 and the reduction
temperature was lower than Ir-ReOx/SiO2 indicating the presence of Pd promoted the reduction of
Ir-ReOx/SiO2. At larger loading amount of Pd than 0.09 (Fig. 1g-k), it is characteristic that three
signals appeared. In particular, the peak patterns can be explained by the combination of
Pd-ReOx/SiO2 and Ir-ReOx/SiO2. The former two signals may be assigned to Pd-ReOx/SiO2
reduction patterns and the last one may be assigned to the patterns of Ir-ReOx/SiO2 suggesting that
trimetallic catalysts’ structure is the mixture of Ir-ReOx/SiO2 and Pd-ReOx/SiO2 structures. The
peak areas in the profiles of Ir-ReOx/SiO2 and Pd-Ir-ReOx/SiO2 corresponded to the reduction of
Re species to the valence of 3-5 in addition to the total reduction of Ir while Pd was assumed to be
reduced before heating (Table S2). The valence of Re species after reaction was also determined by
Re L3-XANES (3.1 for both Ir-ReOx/SiO2 and Pd (1.8)-Ir-ReOx/SiO2). Similarly to TPR results,
the valence of Re species in Ir-ReOx/SiO2 and Pd-Ir-ReOx/SiO2 was almost the same. The
difference in the valence from the XANES analysis and TPR can be explained by the difference of
pretreatment conditions between PH2 = 8 MPa in water in XANES/EXAFS measurement and PH2
=
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Green Chemistry
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5.05 kPa in N2 flow in TPR. Similar tendency in the valence difference was also observed in the
DOI: 10.1039/C3GC41335G
46, 47
cases of Rh-ReOx/SiO2,49 Ir-ReOx/SiO2
and Pt-ReOx/SiO2.50, 51 The spectra and the detailed
analysis of Re L3-XANES are shown in Fig. S2.
Figure 2 shows XRD patterns of Pd-Ir-ReOx/SiO2, Ir-ReOx/SiO2, Pd-ReOx/SiO2, ReOx/SiO2,
Ir/SiO2 and Pd/SiO2 after reduction with 8 MPa H2 at 473 K for 1 h. In the case of monometallic
catalysts, Ir/SiO2 (Fig. 2a) showed the peaks at 2θ = 41.0o, 47.3o and 69.1o which were assigned to
Ir metal, 52 Pd/SiO2 (Fig. 2b) showed the peaks at 2θ = 40.1o and 46.6o which were assigned to Pd
metal53 and ReOx/SiO2 (Fig. 2c) showed the peaks at 2θ = 37.5o, 40.5o and 42.9o which were
assigned to Re metal54. It should be noted that most Re species were in the oxidized state based on
the TPR result. In the case of bimetallic catalysts, Pd-ReOx/SiO2 (Fig. 2d) the peaks at 2θ = 37.5o,
40.5o and 42.9o became sharper indicating the presence of Pd promoted the reduction of ReOx/SiO2.
Ir-ReOx/SiO2 (Fig. 2e) showed the pattern whose position was almost the same as that of Ir/SiO2
and it was assigned to Ir metal as reported earlier.46 The peaks corresponding to Re metal or ReOx
were not detected at all, indicating that Re species was highly dispersed. In the case of Pd-added
Ir-ReOx/SiO2 catalysts, the XRD patterns of Pd-Ir-ReOx/SiO2 (Fig. 2f-k) were very similar to that
of Ir-ReOx/SiO2 indicating that the addition of Pd did not change the dispersion of Ir and ReOx. No
peak for Re metal was observed. The peak of metal (111) plane (around 41o) was located between
pure Pd and pure Ir peaks. However, it is difficult to distinguish whether Ir and Pd were alloyed or
existed separately. At present, it is difficult to estimate the particle sizes of Pd and Ir.
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Further characterization of Pd-Ir-ReOx/SiO2 catalyst was carried out using Ir L3-edge, Re L3
DOI: 10.1039/C3GC41335G
edge and Pd K-edge EXAFS (Table 6) and the detailed spectra and analyses are shown in Figs.
S3-S6. Pd(1.8)-Ir-ReOx/SiO2 ( the molar ratio of Pd/Ir = 0.8) was used because the curve fitting is
easier for higher palladium content. The Ir L3-edge curve fitting results of Pd(1.8)-Ir-ReOx/SiO2
after reaction showed the presence of Ir-Ir (or -Re) bond with the length of 0.275 nm, and that of
Ir-Pd bond was not observed. This result suggests that Ir was in metallic state, and no interaction
between Ir and Pd was present on Pd(1.8)-Ir-ReOx/SiO2. The coordination number (CN) of
Pd(1.8)-Ir-ReOx/SiO2 was 10.8 which is comparable to that of Ir-ReOx/SiO2, suggesting that Ir
metal particle size of Pd(1.8)-Ir-ReOx/SiO2 is almost the same as that of Ir-ReOx/SiO2 which was
determined to be 1.9 nm.46
The Re L3-edge EXAFS curve fitting analysis of Pd(1.8)-Ir-ReOx/SiO2 indicates the presence
of the Re-O, Re-Ir (or -Re) and Re-Pd bonds with bond length (R) of 0.205, 0.269 and 0.268 nm,
respectively, and CN of 1.7, 4.6 and 1.4, respectively. If the curve fitting was carried out without
Re−Pd bond, the fitting was much worse than that with Re-Pd bond (Table S3 and Fig. S6). The
existence of Re-Pd bond is verified by the curve fitting analysis. An important point is that the
presence of Re-Pd bond indicates that the interaction between ReOx species and Pd species exists.
Compared with the results of Ir-ReOx/SiO2, CN of Re-O of Pd(1.8)-Ir-ReOx/SiO2 was as same as
that of Ir-ReOx/SiO2, which can be explained by the similar oxidation state of Re on
Pd(1.8)-Ir-ReOx/SiO2 to that on Ir-ReOx/SiO2.
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The Pd K-edge EXAFS curve fitting analysis of Pd(1.8)-Ir-ReOx/SiO2 showed that Pd was in
DOI: 10.1039/C3GC41335G
metallic state. The presence of the Pd-Pd bond and Pd-Re (or -Ir) bond was verified. Considering
the results of Ir L3-edge EXAFS and Re L3-edge EXAFS of Pd(1.8)-Ir-ReOx/SiO2, Pd-Re (or -Ir)
bond can be mainly derived from Pd-Re bond: Ir-Pd bond was not observed from Ir L3-edge
EXAFS. The CN of Re-Pd and Pd-Re on Pd(1.8)-Ir-ReOx/SiO2 was 1.4 and 2.0, respectively,
giving CNPd-Re (or -Ir)/CNRe-Pd of 1.4, where the ratio of the coordination numbers should be related to
the molar ratio of two components like CNPd-Re/CNRe-Pd = Re/Pd.33, 55, 56 Considering the molar ratio
of Re/Pd (1.3), it is concluded that the shell of Pd-Re (or -Ir) was derived mainly from Pd-Re bond.
On the other hand, the distance of the Pd-Re bond was 0.268 nm and this is shorter than that of
Pd-Re alloy (0.273 nm, molar ratio of Re/Pd = 1),57 suggesting the existence of the interaction
between cationic species of Re and Pd species. This interpretation is also supported by the structural
analysis of Rh-ReOx, Ir-ReOx and Pt-ReOx catalysts.28, 46, 50 Based on the above results on the
structural analysis, Ir metal and Pd metal of Pd-Ir-ReOx/SiO2 exist separately, and ReOx species are
located on both Ir and Pd metals, showing that Ir-ReOx species and Pd-ReOx species can be formed
separately on SiO2 (Scheme 2). Taking CN of Ir-Ir (or -Re) and Re-Ir bonds and oxidation state of
Re into account, the structure of Ir-ReOx species on Pd-Ir-ReOx/SiO2 will be similar to that of
Ir-ReOx/SiO2, which is reported as follows: supported Ir is in the metallic state and the Re species
form low-valent oxide clusters (ReOx) that partially cover the Ir metal surface.49, 50, 58 The Ir-ReOx
species can catalyze the hydrogenolysis of THFA to 1,5-PeD (Scheme 2).
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Page 19 of 36
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We also measured FT-IR spectra of adsorbed CO on the reduced catalysts (Fig. 3).
DOI: 10.1039/C3GC41335G
Pd(0.66)/SiO2 showed one broad spectral pattern at lower wavenumber than 2000 cm-1 which is
attributed to the bridge CO adsorption on Pd metal surface.59 In the case of Pd(0.66)-Ir-ReOx /SiO2,
the peak assigned to the bridge CO adsorption on Pd metal surface was not seen and it showed
similar spectral patterns to that of Ir-ReOx/SiO2. The position of the peak top was located between
2066 and 2069 cm-1 which can be assigned to the linear CO adsorbed on Ir metal surface. This
result suggests that metal Pd is partially covered with the low-valent ReOx species which could not
adsorb CO and the property of Pd is modified with the ReOx species. The amount of CO adsorption
on the reduced catalyst was also measured and the results are listed in Table. S2. The amount of CO
-1
adsorption on Ir-ReOx/SiO2 and Pd(0.66)-Ir-ReOx/SiO2 were 0.037 and 0.058 mmol·gcat
,
respectively. The results of EXAFS analysis suggest that Ir metal particle size of Pd-Ir-ReOx/SiO2
is comparable to that of Ir-ReOx/SiO2. Therefore, it is thought that the amount of CO adsorption on
Ir-ReOx species could be almost the same as that of Ir-ReOx/SiO2, because the CO adsorption
amount was almost constant on Ir-ReOx/SiO2 in the range of Re/Ir=0.5-2.45 The amount of CO
-1
adsorption on Pd-ReOx species could be 0.021 mmol·gcat which was larger than that of Pd/SiO2
(0.009 mmol·gcat-1), suggesting that ReOx species decreases Pd metal average particle size and
increases the dispersion of Pd metal. This effect of ReOx is also connected to high performance in
the hydrogenation of furfural to THFA (Scheme 2).
4. Conclusions
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Addition of Pd to Ir-ReOx/SiO2 catalyst was effective in the formation of 1,5-pentanediol by
DOI: 10.1039/C3GC41335G
hydrogenation + hydrogenolysis of furfural. Addition of Ru, Rh and Pt had much lower effect.
One-pot two-step controlled temperature reaction was beneficial for the synthesis of 1,5-pentanediol
over Pd-Ir-ReOx/SiO2 catalysts. The yield of 1,5-PeD reached 71.4% when using Pd (0.66
wt%)-Ir-ReOx/SiO2 at the conditions of low temperature step at 313 K for 8 h and high temperature
step at 373 K for 72 h. Characterization with TPR, XRD, XANES, EXAFS and FT-IR of adsorbed
CO indicated that Pd-Ir-ReOx/SiO2 catalysts consisted of Pd metal particles with ReOx species
(Pd-ReOx) and Ir metal particles with ReOx species (Ir-ReOx). The Pd-ReOx species catalyze total
hydrogenation performance of furfural into tetrahydrofurfuryl alcohol in low temperature reaction
step and Ir-ReOx species catalyze promoted the hydrogenolysis of tetrahydrofurfuryl alcohol to
1,5-pentanediol.
Acknowledgements
This work was in part supported by the Cabinet Office, Government of Japan through it’s
“Funding Program for Next Generation World-Leading Researchers”. Sibao Liu thanks the China
Scholarship Council (CSC) for the financial support.
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Page 24 of 36
Table 1. Conversion of furfural over different catalysts
Yield / %
2-PeOH
T1
Time 1
/ h
T2
Time 2 Conversion
Entry
Catalysts
/ K
/ K
/ h
/ %
1,5-PeD
0.2
1,4-PeD
17.0
20.3
22.5
23.4
21.3
7.8
1,2-PeD
0.0
0.6
0.5
0.2
1.5
1.3
1.8
1.9
2.1
1.8
1.4
1.4
1.4
1.2
4.2
0.3
0.4
0.6
0.4
1-PeOH
0.1
THFA
0.0
FFA
0.1
0.7
0.4
0.5
0.2
0.0
0.5
0.4
0.0
0.0
0.0
0.0
0.0
0.1
0.0
97.0
7.6
78.8
0.6
2-MTHF
4.2
Others
a
a
1
2
Ir-ReOx/SiO2
393
393
393
393
393
323
323
323
323
323
323
323
323
323
323
323
323
323
323
24
24
24
24
24
2
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
0.4
0.7
1.0
0.8
1.7
0.7
1.4
1.8
2.9
1.6
0.7
0.8
1.2
0.8
0.9
0.0
0.0
0.0
0.0
78.0
70.7
69.5
67.5
33.6
11.5
67.3
65.4
26.7
5.0
–
–
a
a
Ru(0.89)-Ir-ReOx/SiO2
Pt(0.89)-Ir-ReOx/SiO2
Rh(0.89)-Ir-ReOx/SiO2
Pd(0.89)-Ir-ReOx/SiO2
Pd(0.89)-Ir-ReOx/SiO2
Ir-ReOx/SiO2
0.5
0.2
2.3
3.9
–
–
a
a
3
0.3
0.3
0.4
5.0
–
–
a
a
4
1.2
0.3
2.1
4.2
–
–
a
a
5
20.4
54.5
2.3
3.5
5.8
11.9
4.4
–
–
6
393
393
393
393
393
393
393
393
393
393
24
24
24
24
24
24
24
24
24
24
8.6
11.2
1.2
7
2
22.1
22.8
20.1
9.4
0.6
2.8
8
Pd(0.09)-Ir-ReOx/SiO2
Pd(0.26)-Ir-ReOx/SiO2
Pd(0.44)-Ir-ReOx/SiO2
2
1.9
0.7
2.2
3.0
9
2
29.0
58.5
62.4
50.4
53.3
6.2
5.0
6.9
7.0
10
11
12
13
14
15
16
17
18
19
2
12.6
11.2
7.8
7.0
4.1
Pd(0.66)-Ir-ReOx/SiO2
Pd(1.8)-Ir-ReOx/SiO2
Pd(0.75)/SiO2+Ir-ReOx/SiO2
Rh-ReOx/SiO2(Rh/Re=0.5)
Pd(0.66)-Rh-ReOx/SiO2
Ir-ReOx/SiO2
2
6.7
11.5
14.7
3.0
3.7
2.5
2
10.3
8.4
9.3
5.3
b
2
13.7
2.7
5.0
13.9
48.2
29.9
1.7
2
5.3
8.7
26.9
10.8
0.1
2
26.8
0.1
3.7
4.8
18.8
0.6
a
a
2
0.0
0.1
–
–
a
a
Pd(0.66)-Ir-ReOx/SiO2
Ir-ReOx/SiO2
2
0.3
0.2
0.1
78.0
1.0
1.2
12.3
18.3
16.4
–
–
2
393c
0 c
0.5
0.1
0.2
0.4
393c
0 c
0.6
4.6
0.1
75.2
2.0
Pd(0.66)-Ir-ReOx/SiO2
2
PeD: pentanediol; PeOH: pentanol; THFA: tetrahydrofurfuryl alcohol; FFA: furfuryl alcohol; 2-MTHF: 2-methyltetrahydrofuran.
Pretreatment: 473 K, H2 (8 MPa), 1 h.
Reaction conditions: furfural (1 g), H2O (9 g), catalyst (Re/Ir=2; 100 mg), initial H2 (6 MPa), 500 rpm. a: One-step reaction. b: Physical mixture of Pd(0.75)/SiO2 and Ir-ReOx/SiO2 with the same metal
23

Page 25 of 36
Green Chemistry
amount as Pd(0.66)-Ir-ReOx/SiO2, c: the reaction was stopped just after the temperature reached T2.
24

Green Chemistry
Page 26 of 36
Table 2. Conversion of furfural over Pd(0.66)-Ir-ReOx/SiO2 at different temperatures
Yield / %
2-PeOH
T1
time 1
/ h
T2
time 2
/ h
Conversion
/ %
Entry
/ K
/ K
1,5-PeD
60.3
63.2
62.4
56.1
15.1
55.6
63.8
63.2
45.6
1,4-PeD
7.7
1,2-PeD
1-PeOH
14.3
12.5
11.2
14.2
0.5
THFA
7.4
FFA
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
2-MTHF
4.7
Others
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
1
2
3
4
5
6
7
8
9
303
313
323
333
313
313
313
313
313
2
2
2
2
2
2
2
2
2
393
393
393
393
353
373
383
393
413
24
24
24
24
24
24
24
24
24
1.5
1.3
1.4
1.5
1.3
1.3
1.4
1.3
1.2
1.1
0.6
0.7
1.4
0.1
0.2
0.5
0.6
2.0
3.1
3.9
2.5
5.1
8.0
4.6
3.5
3.9
3.3
5.3
10.2
11.5
5.1
3.0
6.7
3.7
10.6
6.2
6.0
65.7
26.0
13.6
10.2
2.1
3.2
5.7
3.6
3.1
6.0
8.0
3.3
5.3
12.5
36.1
3.0
4.6
5.1
PeD: pentanediol; PeOH: pentanol; THFA: tetrahydrofurfuryl alcohol; FFA: furfuryl alcohol; 2-MTHF: 2-methyltetrahydrofuran.
Pretreatment: 473 K, H2 (8 MPa), 1 h.
Reaction conditions: Furfural (1 g), H2O (9 g), Pd(0.66)-Ir-ReOx/SiO2 (100 mg), initial H2 (6 MPa), 500 rpm.
25

Page 27 of 36
Green Chemistry
Table 3. Conversion of furfural over Pd(0.66)-Ir-ReOx/SiO2 at different H2 pressure
Yield / %
2-PeOH
H2 pressure
/ MPa
Conversion
/ %
Entry
1,5-PeD
11.1
1,4-PeD
1,2-PeD
1.2
1-PeOH
1.2
THFA
16.3
10.8
10.2
4.9
FFA
0.1
0.0
0.0
0.0
2-MTHF
12.5
5.8
Others
26.1
1.1
>99.9
>99.9
>99.9
>99.9
1
2
3
4
2
4
6
8
30.6
9.3
1.0
1.1
0.6
0.9
60.1
1.6
10.3
63.2
5.3
1.3
12.5
3.0
3.9
63.1
5.1
1.3
20.4
3.5
0.8
PeD: pentanediol; PeOH: pentanol; THFA: tetrahydrofurfuryl alcohol; FFA: furfuryl alcohol; 2-MTHF: 2-methyltetrahydrofuran.
Pretreatment: 473 K, H2 (8 MPa), 1 h.
Reaction conditions: Furfural (1 g), H2O (9 g), Pd(0.66)-Ir-ReOx/SiO2 (100 mg), initial H2 (2-8 MPa), T1: 313 K, time 1: 2 h, T2: 393 K, time 2: 24 h, 500 rpm.
26

Green Chemistry
Page 28 of 36
Table 4. Conversion of furfural over Pd(0.66)-Ir-ReOx/SiO2 at different reaction time
Yield / %
2-PeOH
T1
time 1
/ h
T2
time 2
/ h
Conversion
/ %
Entry
/ K
/ K
1,5-PeD
1,4-PeD
1,2-PeD
1-PeOH
THFA
FFA
2-MTHF
Others
1
2
313
313
313
313
313
313
313
313
313
313
313
313
2
2
2
2
2
2
2
2
2
4
6
8
373
373
373
373
373
373
373
373
0
12
24
36
48
72
96
120
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
97.6
2.2
4.9
5.4
1.4
1.4
0.4
1.9
0.0
0.1
80.7
49.6
1.5
0.0
3.1
3.1
5.6
6.7
31.9
3
55.6
62.3
64.7
68.4
64.9
63.4
1.5
5.7
5.5
6.3
7.2
8.7
7.3
0.8
5.6
5.7
5.8
1.3
1.3
1.4
1.3
1.5
1.2
1.3
1.5
1.5
1.4
3.6
4.8
0.2
0.3
0.4
0.5
0.9
1.1
0.0
0.4
0.5
0.6
26.0
18.8
13.2
8.4
0.0
0.0
0.0
0.0
0.0
0.0
49.9
0.0
0.0
0.0
3.1
2.7
2.9
3.1
3.9
4.1
1.2
2.7
2.7
2.8
4.6
4.4
4.6
3.0
2.6
1.5
8.4
4.0
2.7
0.5
4
5
6.4
6
8.0
7
12.0
19.4
1.0
5.6
8
1.9
a
a
9
–
–
36.0
8.2
10
11
12
373
373
373
72
72
72
>99.9
>99.9
>99.9
69.1
70.0
71.4
8.5
12.1
13.1
4.9
4.4
PeD: pentanediol; PeOH: pentanol; THFA: tetrahydrofurfuryl alcohol; FFA: furfuryl alcohol; 2-MTHF: 2-methyltetrahydrofuran.
Pretreatment: 473 K, H2 (8 MPa), 1 h.
Reaction conditions: furfural (1 g), H2O (9 g), Pd(0.66)-Ir-ReOx/SiO2 (100 mg), initial H2 (6 MPa), 500 rpm. a: One-step reaction.
27

Page 29 of 36
Green Chemistry
Table 5. Reuse of Pd(0.66)-Ir-ReOx/SiO2 for conversion of furfural
Yield / %
Conversion
/ %
Usage times
1,5-PeD
63.2
1,4-PeD
5.3
1,2-PeD
1.3
1-PeOH
12.5
8.9
2-PeOH
THFA FFA
2-MTHF
3.0
Others
3.9
>99.9
>99.9
>99.9
>99.9
1
2
3
4
0.6
0.5
0.2
0.4
10.2
16.6
27.0
27.4
0.0
0.0
0.0
0.0
62.2
6.0
1.7
3.1
1.0
55.6
6.1
1.7
4.2
2.9
2.3
54.8
5.5
1.8
5.6
2.8
1.7
PeD: pentanediol; PeOH: pentanol; THFA: tetrahydrofurfuryl alcohol; FFA: furfuryl alcohol; 2-MTHF: 2-methyltetrahydrofuran.
Pretreatment: 473 K, H2 (8 MPa), 1 h.
Reaction conditions: furfural (1 g), H2O (9 g), Pd(0.66)-Ir-ReOx/SiO2 (100 mg), initial H2 (6 MPa), T1: 313 K, time 1: 2 h, T2: 393 K, time 2: 24 h,
500 rpm.
28