Reductive conversion of 5-hydroxymethylfurfural to 1,2,6-hexanetriol in water solvent using supported Pt catalysts

Article abstract of DOI:10.1016/j.cattod.2019.12.002

One-pot conversion of biomass derived 5-hydroxymethylfurfural (HMF) to 1,2,6-hexanetriol (1,2,6-HT) in water solvent was performed using Pt catalysts supported on various acid-base metal oxides. Pt catalysts supported on hydrotalcite, MgO, and CeO₂ showed better yield of 1,2,6-HT and 2,5-bis(hydroxymethyl)-tetrahydrofuran (BHF), while ring-rearranged cyclopentanol derivatives were predominant products on the other Pt catalysts. The product distribution with time course on Pt/hydrotalcite revealed that HMF is at first hydrogenated to BHF, then the following parallel reactions proceed; ring-rearrangement to cyclopentanol derivatives, ring-hydrogenation to BHF, and hydrogenolysis to 1,2,6-HT. When pure hydrotalcite, MgO and CeO₂ were physically mixed with Pt/SiO₂, the selectivity to 1,2,6-HT was almost zero or less than 10 %. It was suggested that the formation of 1,2,6-HT proceeds at metal-support interface. The effect of metal-support interface was examined by means of IR spectra of adsorbed methanol. It was indicated that both basic property of supports and surface monodentate alkoxide formation are essential for the production of 1,2,6-HT. The maximum yield of 1,2,6-HT (42 %) was obtained using Co-promoted Pt/CeO₂ catalysts pre-reduced at 200 °C.

Full text of DOI:10.1016/j.cattod.2019.12.002

Journal Pre-proof
Reductive Conversion of 5-Hydroxymethylfurfural to 1,2,6-Hexanetriol in
Water Solvent Using Supported Pt Catalysts
Hiroto Kataoka, Daichi Kosuge, Keiji Ogura, Junya Ohyama, Atsushi
Satsuma
PII:
S0920-5861(19)30651-0
DOI:
https://doi.org/10.1016/j.cattod.2019.12.002
CATTOD 12580
Reference:
To appear in:
Catalysis Today
Received Date:
Revised Date:
Accepted Date:
23 June 2019
3 October 2019
2 December 2019
Please cite this article as: Kataoka H, Kosuge D, Ogura K, Ohyama J, Satsuma A, Reductive
Conversion of 5-Hydroxymethylfurfural to 1,2,6-Hexanetriol in Water Solvent Using Supported
Pt Catalysts, Catalysis Today (2019), doi: https://doi.org/10.1016/j.cattod.2019.12.002
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Reductive Conversion of 5-Hydroxymethylfurfural to 1,2,6-Hexanetriol in Water
Solvent Using Supported Pt Catalysts
Hiroto Kataoka1, Daichi Kosuge1, Keiji Ogura1, Junya Ohyama1,2,3, and Atsushi Satsuma1,2*
1Department of Materials Chemistry, Graduate school of Engineering, Nagoya University, Nagoya,
464-8603, Japan.
2Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Kyoto, 615-8245, Japan.
3Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto, 860-8555,
Japan.
*E-mail: satsuma@chembio.nagoya-u.ac.jp
Graphical abstarct
Highlights:
 Pt catalysts supported on hydroctalcite, MgO, and CeO2 are effective for one-pot
conversion of 5-hydroxymethylfurfural to 1,2,6-hexanetriol in water.
 The maximum yield of 1,2,6-hexanetriol (42%) was obtained using Co-promoted
Pt/CeO2 catalyst.
 The essential factors are monodentate alkoxide adsorption, basic property of supports,
and C=C bond adsorption on CoO particles.
1

One-pot conversion of biomass derived 5-hydroxymethylfurfural (HMF) to 1,2,6-hexanetriol
(1,2,6-HT) in water solvent was performed using Pt catalysts supported on various acid-base metal
oxides. Pt catalysts supported on hydrotalcite, MgO, and CeO2 showed better yield of 1,2,6-HT and
2,5-bis(hydroxymethyl)-tetrahydrofuran (BHF), while ring-rearranged cyclopentanol derivatives
were predominant products on the other Pt catalysts. The product distribution with time course on
Pt/hydrotalcite revealed that HMF is at first hydrogenated to BHF, then the following parallel
reactions proceed; ring-rearrangement to cyclopentanol derivatives, ring-hydrogenation to BHF, and
hydrogenolysis to 1,2,6-HT. When pure hydrotalcite, MgO and CeO2 were physically mixed with
Pt/SiO2, the selectivity to 1,2,6-HT was almost zero or less than 10%. It was suggested that the
formation of 1,2,6-HT proceeds at metal-support interface. The effect of metal-support interface
was examined by means of IR spectra of adsorbed methanol. It was indicated that both basic
property of supports and surface monodentate alkoxide formation are essential for the production of
1,2,6-HT. The maximum yield of 1,2,6-HT (42%) was obtained using Co-promoted Pt/CeO2
catalysts pre-reduced at 200 ºC.
Keywords: 5-Hydroxymethylfurfural, 1,2,6-Hexanetriol, Supported Pt Catalysts, Ring-opening
reaction, Water Solvent
1. Introduction
Biomass derived 5-hydroxymethylfurfural (HMF) is known to be one of the key platforms of
bio-refinery for production of various chemicals and fuels [1-5]. For examples,
2,5-bis-(hydroxymethyl)-furan (BHF) and 2,5-bis (hydroxymethyl)-tetrahydrofuran (BHTHF) can
be obtained by hydrogenation of HMF and used as resin additives, solvents, feed stocks for
polymers, drugs, and so on [6-12]. Oxidation of HMF produces 2,5-furandicarboxylic acid, which
can be used as a monomer for engineering plastics [13]. Hydrogenolysis of HMF results in
production of 2,5-dimethylfuran as a liquid fuel having high energy density. Straight chain alcohols
and acids are also synthesized from HMF. 1,2,6-Hexanetriol (1,2,6-HT) is an intermediate for
wetting agents, solvents, and pharmaceuticals, and is also used in the cosmetics and resin industries
[14].
Since HMF is produced via hydrolysis of cellulose followed by isomerization and dehydration,
selective conversion of HMF in water is advantageous in terms of energy saving and green
chemistry [15]. However, compared to organic solvents, the selective conversion of HMF in water
is difficult because of interference of various side reactions; hydrolysis, ring rearrangement, and
polymerization. However, there are some papers reporting selective conversion of HMF in water.
Recently, our research group reported the selective conversion of HMF: Selective hydrogenation to
BHF (96% yield) over Au nano-cluster supported on Al2O3 [8], and one-pot hydrogenation and
rearrangement of HMF to 3-hydroxymethylcyclopentanol (88% yield) [16]. For the conversion of
HMF to straight-chain alcohols, two reaction pathways have been reported. One is two-step
hydrogenation of HMF to BHTHF followed by hydrogenolysis to 1,2,6-HT. Buntara et al.
2

selectively converted BHTHF to 1,2,6-HT at lower conversion level (Conv. 21%, Select. 97%,)
using Rh-Re/SiO2 catalyst in water at 80 ºC [17]. He et al. examined the selective production of
1,6-hexanediol from BHTHF via 1,2,6-HT. Using 10wt%Pt-10wt%WOx/TiO2 catalyst, consecutive
production of 1,6-hexanediol via 1,2,6-HT proceeds when the catalyst amount was 0.50 g, while
more than 95% selectivity of 1,2,6-HT was achieved with BHTHF conversion of 22% by use of
0.10 g catalyst in water [18].
Another pathway is direct one-pot conversion of HMF to diols or triol. Mizugaki et al. reported
that Pt/hydrotalcite showed high performance for furfural conversion to 1,2-pentanediol (Conv. >
99%, Select. 73%) in 2-propanol [19]. In water solvent, Chen et al. reported that furfural conversion
to 1,5-pentanediol can successfully proceed using Ir-Re/SiO2 (Conv. 60.3%, Select. 94.2%) [20]. On
the other hand, as a conversion of HMF in water, the selectivity of 1,2,6-HT was still less than 40%
as Yao and co-workers reported (Ni-Co-Al catalyst, Conv. 100 %, Select. 37.4%) [14]. For the
strategy of catalyst design, both Mizugaki et al. [19] and Yao et al. [20] pointed out the importance
of metal-support interface and adsorption mode of substrate for furan ring-opening reaction.
In the present study, we focus on hydrogenation of HMF with direct cleavage of furan ring to
1,2,6-HT using supported Pt catalysts. Taking the importance of adsorption state of substrate into
account, the effects of various supports and the addition of cobalt oxide are investigated. Since the
adsorption state of substrate was not experimentally investigated, IR spectra of adsorbed alcohol on
metal oxide supports are measured. As a model molecule of HMF or BHF, methanol adsorption was
conducted. The roles of metal-support interface and metal-CoOx interface are discussed.
2. Experimental
2.1. Catalyst preparation. 5 wt% Pt/MOx catalysts (MOx = CeO2, hydrotalcite, MgO, La2O3,
Nd2O3, Dy2O3, Al2O3, ZrO2, Ta2O5, Nb2O5, and SiO2) were prepared by an impregnation method. A
aqueous suspension containing Pt(NO3)2 (Pt: 0.05 g), MOx (0.95 g), and water (100 mL) was stirred
for 1 h at room temperature. After evaporation in a rotary evaporator at 60˚C, the obtained cake was
dried at 80˚C overnight, and calcined in air at 500˚C for 3 h. PtCo/MOx catalysts were prepared by
impregnation of an aqueous solution of Co(NO3)2ˑ6H2O to Pt/MOx, followed by evaporation at
60˚C, dryness at 80˚C overnight, and calcination in air at 500˚C for 3 h. The molar ratio of Pt:Co
was varied from 1:9 to 9:1, and the content of Pt + Co was 5wt%.
Pt(NO3)2 aqueous solution was obtained from Cataler Co. Ltd. The metal oxides for supports were
obtained as follows: CeO2 from Solvay, hydrotalcite from Kyowa Chemical Industry Co., Ltd.,
MgO from Ube Industries, Ltd., and Nb2O5ˑnH2O from CBMM. La2O3, Nd2O3, Dy2O3 and Ta2O5
were purchased from Shin-Etsu Chemical Co., Ltd. SiO2 (JRC-SIO-5), ZrO2 (JRC-ZRO-5), and
Al2O3 (JRC-ALO-8) were supplied from Catalysis Society of Japan. Co(NO3)2ˑ6H2O (> 98 %) was
purchased from Kishida Chemical Co., Ltd.
Pt5Co5/CeO2 by colloid method was prepared as follows [21]. 0.5 mmol of Pt(acac)2 (acac =
aceylacetonate), 0.5 mmol of Co(acac)2, 20 mL of oleylamine were mixed at room temperature.
Then the solution was further heated to 300 °C at a heating rate of 5 °C/min, the temperature was
3

kept at 300 °C for 1 h, cooled down to room temperature. A black product was precipitated by
adding 40 mL of ethanol, and separated by centrifugation. The product was dispersed in hexane, and
then supported on CeO2 by impregnating the solution to CeO2. After evaporation in a rotary
evaporator, the obtained cake was dried at 80˚C overnight, and calcined in air at 700 ˚C for 2 min.
2.2. Catalytic tests. HMF was purchased from Tokyo Chemical Industry Co., Ltd. The other
chemicals were purchased from Kishida Chemical Co., Ltd. Before the catalytic tests, the catalysts
were reduced in H2 at 200 °C for 1 h. A 10 mg of pretreated Pt/MOx, 0.2 mmol of HMF and 3 mL
of distilled water were put into an autoclave (30 mL, Taiatsu Techno Co., TVS-1 type). The gas
phase in the autoclave was replaced with ca. 1 MPa of H2 for 5 times, and finally 3 MPa of H2 was
introduced. The conversion of HMF was carried out at 135 ˚C for 24 h. The reaction solution was
analyzed using a gas chromatograph (GC, SHIMADZU GC-14A) with FID. The products were also
1
identified using GC-MS (SHIMADZU GC-17A with GC-MS QP-5000) and H-NMR (Bruker 500
Ultra Shield.).
2.3. Characterizations. Fourier transform Infrared (FT-IR) spectra of the metal oxide supports
were obtained on a JASCO FT/IR-6100 (JASCO Co.) equipped with a MCT detector at 4 cm-1 of
spectral resolution of. Metal oxides (30‒200 mg) were pressed into wafers with 20 mm diameter.
The wafer was pretreated under 10%H2/Ar flow (100 mL min-1) at 200˚C for 30 min, and
background spectrum was recorded at 100˚C. Then, the wafer was exposed to methanol vapor (2
L) under Ar flow (90 mL min-1) at 100˚C. After purging by Ar at 100 ˚C for 3 min, FT-IR spectra
of adsorbed methanol on metal oxides were recorded. Scanning transmission electron microscopy
(STEM) and energy dispersive X-ray spectrometry (EDS) mapping were conducted on a
JEM-2100F (JEOL) instrument operated at 200 kV. H2-temperature programmed reduction
(H2-TPR) profiles were recorded using Microtrac-Bel BELCAT-B. A 50 mg catalyst was exposed in
a flow of 5%H2/Ar at 40 mL min-1 and temperature raised from 50 to 500 °C at a rate of 5 °C min-1.
The outlet flow was analyzed by a TCD detector.
3. Results and Discussion
3.1. Effect of supports on product selectivity.
Table 1 shows the product yields in HMF conversion using Pt catalysts supported on various
metal oxides. These catalysts were classified into two groups. When hydrotalcite, MgO, and CeO2
were used as supports, 1,2,6-HT and BHTHF were preferentially produced. On the other supports,
the main products were ring rearrangement products (HCPN, HCPO). Small amount of
1-hydroxyhexane-2,5-dione (HDD) was obtained on Pt/Ta2O5, Pt/Nb2O5, and Pt/SiO2, and
4-hydroxymethyl-2-cyclopentenone (HCPEN) was detected on Pt/Ta2O5. The other products are not
detected in GC, and considered to be polymerized products such as humic materials. It should be
noted that, in addition to supported Pt catalysts, CeO2 supported Ir, Ru, Pd, Ni, and Co catalysts
were also examined, but the production of 1,2,6-HT was only observed on Pt/CeO2.
Table 1 Product yields in HMF conversion using supported Pt catalysts.
4

HCPN+HCPO
Catalyst
1,2,6-HT (%)
BHTHF (%)
HHD (%)
others (%)
(%)
1
Pt/hydrotalcite
Pt/MgO
32
14
27
0
36
34
63
0
0
0
31
50
8
2
Pt/CeO2
2
0
Pt/La2O3
Pt/Nd2O3
Pt/Dy2O3
Pt/Al2O3
Pt/ZrO2
84
80
85
70
48
19
24
31
0
16
17
15
30
52
65
66
58
0
3
0
0
0
0
0
0
0
0
0
0
Pt/Ta2O5
Pt/Nb2O5
Pt/SiO2
0
0
16
10
11
0
0
0
0
Reaction conditions: HMF 0.2 mmol, H2O 3 mL, catalyst 10mg, H2 pressure 3MPa, 135 ºC, 24 h.
1,2,6-HT = 1,2,6-hexanetriol, BHTHF = 2,5-bis(hydroxymethyl)-tetrahydrofuran, HCPN =
3-hydroxymethylcyclopentanone,
HCPO
=
3-hydroxymethylcyclopentanol,
HHD
=
1-hydroxyhexane-2,5-dione. Others include 4-hydroxymethyl-2-cyclopentenone (HCPEN, 18%
detected only on Pt/Ta2O5) and undetectable products such as humic materials.
Table 2 Product yields in HMF conversion using physical mixtures of Pt/SiO2 and metal oxides.
HCPN+HCPO
Catalyst
1,2,6-HT (%) BHTHF (%)
BHF (%) others (%)
(%)
37
Pt/SiO2+hydrotalcite
Pt/SiO2+MgO
0
6
0
4
19
0
19
5
40
59
6
11
Pt/SiO2+CeO2
94
0
Reaction conditions: HMF 0.2 mmol, H2O 3 mL, catalyst 10mg, H2 pressure 3MPa, 135 ºC, 24 h.
Weight ratio of 5wt%Pt/SiO2: metal oxide = 10 mg: 10 mg. BHF = 2,5-bis(hydroxymethyl)furan.
The effect of Pt-support interface was examined by comparing physical mixture of Pt/SiO2 and
pure supports (Table 2). SiO2 was used as a chemically inert support. Comparing with Pt supported
catalysts (Table 1), the products distributions are entirely different. Even when pure hydrotalcite,
MgO and CeO2 were physically mixed with Pt/SiO2, 1,2,6-HT was not produced (hydrotalcite and
CeO2) or was far smaller than the supported catalyst (MgO). The essential role of metal-support
interface on 1,2,6-HT formation was demonstrated.
The reaction network was then investigated using Pt/hydrotalcite as a model catalyst. Figure 1
shows the dependence product distribution on reaction time for HMF conversion using
Pt/hydrotalcite. At first, substrate HMF steeply decreased, and BHF yield increased. BHF yield then
5

decreased after 2 h, instead, the yield of other products monotonously increased and further
decrease was not observed. The result indicates that BHF is an intermediate, and parallel reactions
proceed from BHF to other products, i.e., BHTHF, 1,2,6-HT, and cyclopentanone derivatives
(HCPN, HCPO). In the separate experiment, consecutive reaction of BHTHF to 1,2,6-HT was
examined. HMF was converted to BHF using Ni/SiO2 at 100 °C, for 2 h in ethanol (Yield >99%).
After isolation of BHTHF by evaporation at 45 °C, the obtained BHTHF was dissolved in water.
Using 10 mg of Pt/hydrotalcite, the conversion of BHTHF was conducted under the same
conditions (135 °C, 3MPa H2, 24 h). BHTHF did not convert to any products. No consecutive
conversion of BHTHF to 1,2,6-HT was confirmed under the present reaction conditions. The
reaction network can be summarized in Scheme 1: HMF is hydrogenated to BHF, then various
major products, 1,2,6-HT, BHTHF, HCPN, and HCPO are parallelly produced from BHF as an
intermediate. Therefore, the selective conversion of BHF is the important key for the product
selectivity.
Figure 1. Effect of reaction time on yield of substrate and products on Pt/hydrotalcite.
6

Scheme 1 Reaction network of HMF hydrogenation on Pt/hydrotalcite.
3.2. Structure-Activity correlation on Pt/MOx for 1,2,6-hexanetriol production.
In order to investigate the higher selectivity for 1,2,6-HT on hydrotalcite, MgO, and CeO2
supported catalysts, correlations between 1,2,6-HT selectivity and various chemical properties of
the supports are examined. Table 3 shows indicators of acid-base characters of metal oxide supports.
IR band position of ring vibration mode of (8a) of adsorbed pyridine is a good indicator of the
strength of Lewis acidity, which shifts to higher wavenumber with the increase in Lewis acid
strength [22]. The IR band position of C-H stretching mode of adsorbed chloroform shifts to lower
wavenumber with the increase in basicity [22]. The selectivity of 1,2,6-HT can be roughly
correlated to basicity of metal oxide supports, however, there are several exceptions. For examples,
pH of the aqueous solution of CeO2 was acidic, and Lewis acid strength and base strength of
hydrotalcite were medium among the supports examined. In order to estimate pH of the aqueous
solution after the conversion of HMF, we also measured pH of the aqueous solution including
Pt/MOx catalysts after reduction in H2 at 135 ºC for 1 h. MgO, hydrotalcite, and CeO2 showed the
highest pH, suggesting basic property is favorable for the production of 1,2,6-HT. However, the
difference in pH between CeO2 and ZrO2 is too small to explain the selectivity of 1,2,6-HT of these
catalysts. The 1,2,6-HT selectivity cannot be rationalized only by these acid-base properties of the
supports. Contribution of another factor should be considered.
For the selective diols and triol production, the importance of adsorption mode of substrate has
been discussed. Mizugaki et al. reported the selective conversion of furfural to 1,2-pentanediol
using Pt/hydrotalcite with a yield of 73% in 2-propanol [19]. In their report, the reaction mechanism
is proposed as follows; C=O bond is at first hydrogenated to form furfuryl alcohol followed by
adsorption of alkoxide species on support nearby Pt particle, and ring-opening reaction proceeds
with dissociation of C-O bond in furan ring on Pt. Alkoxide species on the support is proposed to
7

play an important role of ring-opening. The important role of alkoxide on metal-support interface is
also proposed in other reports [23-25]. However, this proposal is not experimentally supported.
Therefore, the state of adsorbed alcohol was investigated in the present study. Due to very high
boiling point (275 ºC) and low vapor pressure of BHF, we applied methanol as the simplest model
of alcohol for IR measurement.
Table 3 Acid-base character of metal oxide supports estimated by pH of the aqueous solution before
the reaction, pH of the aqueous solution with pre-reduced (in H2 at 135 ºC for 1 h) Pt/MOx, IR band
positions of ring vibration mode of (8a) of pyridine and C-H stretching mode of chloroform.
pH with
reduced
Pt/MOx
10.3
(8a) of
pyridine
/ cm-1
1588
(C-H) of
chloroform /
cm-1
pH with
MOx
MOx
MOx
MOx
MOx
MgO
hydrotalcite
CeO2
MgO
hydrotalcite
La2O3
10.7
9.8
7.0
6.7
6.7
5.7
MgO
La2O3
La2O3
Ta2O5
Nd2O3
CeO2
Al2O3
MgO
2962
9.9
1594
2964
8.7
Nd2O3
1597
2965
ZrO2
8.5
Nd2O3
CeO2
1598
2970
La2O3
8.4
Dy2O3
Dy2O3
1602
2972
Nb2O5
8.3
CeO2
hydrotalcite
1602
2974
Nd2O3
8.2
Al2O3
ZrO2
5.3
4.9
4.4
4.4
4.3
ZrO2
Nb2O5
Ta2O5
Al2O3
SiO2
1603
1604
1607
1614
N.D.
hydrotalcite
Dy2O3
ZrO2
2975
2980
3004
3020
N.D.
Al2O3
SiO2
7.9
7.5
7.5
7.3
Ta2O5
SiO2
Dy2O3
Ta2O5
Nb2O5
SiO2
Nb2O5
Figure 2 shows IR spectra of adsorbed methanol on pure supports at 100 °C after purging gas
phase methanol in a flow of Ar for 3 min. The spectra shows three types of (O-C) vibration bands:
The bands around 1160 cm−1 for ZrO2, Nb2O5, and SiO2, the bands around 1100 cm−1 for
hydrotalcite, MgO, and CeO2, and the bands below 1060 cm−1 for CeO2, La2O3, Nd2O3, Dy2O3,
Al2O3, and ZrO2. The bands around 1160 cm−1 and 1100 cm−1 are assigned to monodentate surface
methoxide, and the bands below 1060 cm−1 are assigned to bidentate surface methoxide,
respectively [26–31]. The broadening of the band below 1060 cm−1 to lower wavenumber suggests
the presence of other species: bidentate species with different coordination (1045 cm−1) and triply
coordinated methoxy species (1012 cm−1). Interestingly, the monodentate methoxide band at 1100
cm−1 was observed only on hydrotalcite, MgO, and CeO2, on which 1,2,6-HT and BHTHF were
selectively produced. The selective production of 1,2,6-HT can be correlated to the monodentate
adsorption mode on the supports. However, another monodentate species around 1160 cm−1 was
8

observed on ZrO2, Nb2O5, and SiO2, on which 1,2,6-HT was not produced. It should be noted only
these catalysts produced 1-hydroxyhexane-2,5-dione (HDD) which suggests direct ring-opening of
HMF (Table 1). Without monodentate adsorption on La2O3, Nd2O3, Dy2O3, and Al2O3, the ring
rearrangement reaction to form cyclopentanone derivatives (HCPN+HCPO) preferentially
proceeded. The monodentate alkoxide adsorption is essential for ring-opening reaction, however,
another factor is necessary for 1,2,6-HT production. The acid-base properties of the supports (Table
2) indicates that Pt/MOx on basic supports (hydrotalcite, MgO, and CeO2) produced 1,2,6-HT,
while Pt/MOx on acidic or inert supports (Ta2O5, Nb2O5, SiO2) produced HDD. These results
indicate the importance of both basic property of supports and monodentate adsorption of BHF on
Pt-support interface for the selective production of 1,2,6-HT. Hydrotalcite, MgO, and CeO2 gave
preferential metal-support interface to from monodentate alkoxide on basic supports, however, the
yield of 1,2,6-HT is still lower than 40%. Then we examined the addition of CoOx as a promoter.
Figure 2. (O-C) vibration region of IR spectra of adsorbed methanol on various metal oxide
supports.
3.3. Addition of CoOx as a promoter
Although the strong effect of supports on the product selectivity was observed in the previous
section, the yield of 1,2,6-HT was still lower than 40%. Then, as another strategy to promote
9

hexanetriol selectivity, we examined the addition of CoOx which is reported to be effective for the
hydrogen ring-opening reaction in organic solvents [25, 32-24]. Table 4 shows the effect of the
supports on the product distributions using PtCo/MOx for HMF conversion in water. When loading
amount was Pt:Co =5:5, CeO2, hydrotalcite, MgO showed high selectivity to hexanetriol, and
1,2,6-HT yield exceeded 40% only when CeO2 was used as the support (42%). Comparing
PtCo/CeO2, and Pt/CeO2, the addition of CoOx improved the selectivity of 1,2,6-HT by the sacrifice
of BHTHF selectivity, which means that CoOx enhanced the C-O cleavage of furan ring by
suppressing hydrogenation of furan ring [14]. Table 3 also shows the effect of Pt:Co ratio. The best
selectivity for 1,2,6-HT was observed on Pt:Co = 5:5.
The effect of the contact between Pt and Co was also examined. Alloying of Pt and Co was
examined using Pt5Co5/CeO2 catalyst prepared by colloid method as reported by Yu et al. [21]. The
total yield of 1,2,6-HT and BHTHF was less than 10%, while cyclohexanone derivatives
(HCPN+HCPO) were the main products. The alloying of Pt-Co is not effective for the target
reaction, and Pt and CoOx should be exist as individual particles. However, a physical mixture of
Pt/CeO2 and Co3O4 showed almost the same yield to Pt/CeO2. Pt/Co3O4 showed low selectivity to
1,2,6-HT. These results suggest that the added CoOx should be exist as small particles, nearby Pt
particle on CeO2, controlling parallel reaction from BHF to 1,2,6-HT or BHTHF.
Table 4 Product yields in HMF conversion using Co-promoted supported Pt catalysts.
Catalyst
1,2,6-HT (%) BHTHF (%) HCPN+HCPO (%)
others (%)
Pt5Co5/CeO2
Pt5Co5/hydrotalcite
Pt5Co5/MgO
42
31
23
0
41
42
52
0
7
0
10
27
25
9
0
Pt5Co5/SiO2
91
28
Pt5Co5/Al2O3
11
15
46
Pt/CeO2
27
30
33
42
30
32
0
63
54
48
41
42
42
0
2
1
8
Pt9Co1/CeO2
Pt7Co3/CeO2
Pt5Co5/CeO2
Pt3Co7/CeO2
Pt1Co9/CeO2
Co/CeO2
15
17
10
17
20
35
67
2
7
11
6
0
Pt/Co3O4
3
13
17
Physical mixture of
Pt/CeO2 + Co3O4
26
2
34
8
8
32
29
Pt5Co5/CeO2
61
10

(colloid method)
Reaction conditions: HMF 0.2 mmol, H2O 3 mL, catalyst 10mg, H2 pressure 3MPa, 135 ºC, 24 h.
For the physical mixture, a mixture of 10 mg Pt/CeO2 and 10 mg Co3O4 is used.
The dependence product distribution on reaction time for reductive conversion of HMF on
Pt5Co5/CeO2 is shown Figure 3. The trends in the product distribution were almost the same to
Pt/hydrotalcite, i.e., the substrate HMF at first steeply decreased, BHF yield showed the maximum
at 3 h, and then the yield of 1,2,6-HT and BHTHF monotonously increased without further decrease.
The cyclopentanone derivatives (HCPN, HCPO) were negligible. Although the reaction scheme was
almost the same, the higher selectivity of 1,2,6-HT with the sacrifice of BHTHF was obtained. The
reaction was stopped completely by the removal of Pt5Co5/CeO2 from the reaction solution. After
the reaction mixture had been stirred for 3 h (HMF conversion at 100 % and yields of BHF, BHTHF,
and 1,2,6-HT are 50%, 23%, 5.3%, respectively), the catalyst was removed by filtration. The
reaction did not proceed with the obtained solution by heating under 1 MPa H2 at 135 ºC for 24 h.
The result excludes a possible contribution of homogeneous catalysis by leached species.
Figure 3. Effect of reaction time on yield of substrate and products on Pt/hydrotalcite.
11

The product selectivity was strongly affected by the reduction temperature of Pt5Co5/CeO2, as
shown in Table 5. Without pre-reduction in H2, the hexanetriol yield was below 40%. The
maximum yield of 1,2,6-HT was obtained when the catalyst was reduced in H2 at 200 ºC (42%),
while further increase in the reduction temperature decreased the hexanetriol yield.
Table 5 Effect of pre-reduction temperature of Pt5Co5/CeO2 on product yields in HMF conversion.
Pre-reduction
1,2,6-HT (%) BHTHF (%) HCPN+HCPO (%)
others (%)
As prepared
37
42
30
33
30
43
41
45
44
47
3
7
3
4
4
17
10
22
19
19
Reduced in H2 at 200 ºC
Reduced in H2 at 300 ºC
Reduced in H2 at 400 ºC
Reduced in H2 at 500 ºC
Reaction conditions: HMF 0.2 mmol, H2O 3 mL, catalyst 10mg, H2 pressure 3MPa, 135 ºC, 24 h.
The structure of Pt5Co5/CeO2 was characterized. Figure 4 shows STEM-EDX images of
Pt5Co5/CeO2. In the elemental mapping, red, green, and yellow dots show the presence of Pt, Co,
and the mixed region, respectively. The positions of red dots agree well with those of green dots,
indicating Pt and Co were well mixed, in other words, Co-species are positioned nearby Pt particles.
The particle size of Pt was around 3-5 nm from STEM. The formation of Pt-Co alloy can be
rejected because Pt5Co5/CeO2 prepared by the colloid method showed very low selectivity to
1,2,6-HT (Table 5). The Co-species exists as small particles nearby Pt particles. In order to examine
the effect of pre-reduction temperature, H2-TPR was conducted and the profiles are shown in Figure
5. The profile of Pt/CeO2 showed two reduction peaks, assignable to Pt oxide reduction to Pt0 at
80‒170 °C and the reduction of CeO2 surface at 300‒400 °C [35]. In the profile of Pt5Co5/CeO2, an
additional small reduction peak was also observed at 200‒260 °C. Luo et al. reported detailed
assignment of H2-TPR profile of PdCo/CeO2 [36]. H2-TPR of supported CoOx gives two step
reduction profiles: The first reduction of Co3O4 to CoO at 154 °C, and the second reduction of CoO
to metallic Co at 368 °C [36]. The lower reduction temperature than metal-free CoOx/CeO2 is due to
hydrogen spillover [37]. According to their report, the reduction peak above 200 ºC is assigned to
the second reduction of CoO to metallic Co, i.e., the H2 pre-treatment at 200 ºC results in the first
reduction of Co3O4 to CoO, and the highest yield of 1,2,6-HT was obtained when the state of
supported cobalt is CoO.
12

Figure 4. STEM-EDX images of Pt5Co5/CeO2.
Figure 5. Profiles of H2-temperature programmed reduction of Pt/CeO2 and Pt5Co5/CeO2.
13

It is reported that CoOx provides adsorption sites for C=C bond [38], which promotes C-O bond
cleavage of furan ring [14, 39]. Yao et al. correlated the promoted ring-opening reaction over
Ni-Co-Al mixed oxide to tilted adsorption of BHF by the effect of CoO [14]. The adsorption mode
of BHF determine the selectivity to 1,2,6-HT or BHTHF: Tilted adsorption of BHF proceed ring
opening reaction by C-O cleavage to form 1,2,6-HT, while parallel adsorption of BHF proceeds
hydrogenation of furan ring to form BHTHF. The promotion of ring-opening reaction over
Pt5Co5/CeO2 can be rationalized as follows: BHF is adsorbed on CeO2 by forming surface
methoxide, leading adsorption of C=C bond of BHF on CoO nearby Pt particle by forming tilted
BHF adsorption, which results in the selective C-O bond cleavage to from 1,2,6-HT.
Conclusion
The effects of supports and CoOx addition on one-pot conversion of 5-hydroxymethylfurfural to
1,2,6-hexanetriol in water solvent were investigated using supported Pt catalysts. By using Pt
catalysts supported on hydroctalcite, MgO, and CeO2, 1,2,6-hexanetriol was obtained. The essential
role of metal-support interface, i.e., monodentate alkoxide adsorption and basic property of supports,
are clarified. Further improvement of 1,2,6-hexanetriol yield was achieved by the addition of cobalt
oxide. The maximum yield of 1,2,6-hexanetriol (42%) was obtained using Pt5Co5/CeO2 catalyst
pre-reduced at 200 °C. The interface of Pt particles and CoO particles on CeO2 was suggested to be
important for the selective 1,2,6-hexanetriol formation.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by Grant-in-Aids from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT), Japan – "Elements Strategy Initiative to Form Core Research Center"
program (since 2012), Grants-in-Aid for Scientific Research KAKENHI (B) (18H01787), and
Grant-in-Aid for Young Scientists (A) (16H06131).
14

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