Synthesis of 1,6-hexanediol from HMF over double-layered catalysts of Pd/SiO2 + Ir-ReOx/SiO2 in a fixed-bed reactor

Article abstract of DOI:10.1039/c5gc02228b

1,6-Hexanediol (1,6-HDO) was effectively prepared from 5-hydroxymethylfurfural (HMF) over double-layered catalysts of Pd/SiO₂ + Ir-ReO_x/SiO₂ in a fixed-bed reactor. Under optimal reaction conditions (373 K, 7.0 MPa H₂, in solvent mixtures of 40% water and 60% tetrahydrofuran (THF)), 57.8% yield of 1,6-HDO was obtained. The double-layered catalysts loaded in double-layered beds showed much superior performance compared to that of a single catalyst of Pd-Ir-ReO_x/SiO₂, even when the same amount of active components were used in the catalysts. The reaction solvent significantly affected product distributions, giving a volcano-shape plot for the 1,6-HDO yield as a function of the ratio of water to THF. Br?nsted acidic sites were generated on the catalyst in the presence of water which played determining roles in 1,6-HDO formation. A high pressure of H₂ contributed to 1,6-HDO formation by depressing the over-hydrogenolysis of reaction intermediates and products to form hexane and hexanol. The reaction route was proposed for HMF conversion to 1,6-HDO on the basis of conditional experiments.

Full text of DOI:10.1039/c5gc02228b

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Synthesis of 1,6-hexanediol from HMF over
double-layered catalysts of Pd/SiO2 + Ir–ReOx/
SiO2 in a fixed-bed reactor†
Cite this: DOI: 10.1039/c5gc02228b
Bin Xiao,a Mingyuan Zheng,*a Xinsheng Li,a,b Jifeng Pang,a Ruiyan Sun,a,b
Hua Wang,a Xiaoli Pang,a Aiqin Wang,a Xiaodong Wanga and Tao Zhang*a
1,6-Hexanediol (1,6-HDO) was effectively prepared from 5-hydroxymethylfurfural (HMF) over double-
layered catalysts of Pd/SiO2 + Ir–ReOx/SiO2 in a fixed-bed reactor. Under optimal reaction conditions
(373 K, 7.0 MPa H2, in solvent mixtures of 40% water and 60% tetrahydrofuran (THF)), 57.8% yield of
1,6-HDO was obtained. The double-layered catalysts loaded in double-layered beds showed much
superior performance compared to that of a single catalyst of Pd–Ir–ReOx/SiO2, even when the same
amount of active components were used in the catalysts. The reaction solvent significantly affected
product distributions, giving a volcano-shape plot for the 1,6-HDO yield as a function of the ratio of water
to THF. Brønsted acidic sites were generated on the catalyst in the presence of water which played deter-
mining roles in 1,6-HDO formation. A high pressure of H2 contributed to 1,6-HDO formation by depressing
the over-hydrogenolysis of reaction intermediates and products to form hexane and hexanol. The reaction
route was proposed for HMF conversion to 1,6-HDO on the basis of conditional experiments.
Received 17th September 2015,
Accepted 27th November 2015
DOI: 10.1039/c5gc02228b
www.rsc.org/greenchem
be used as precursors for the synthesis of renewable
monomers.11–13 5-Hydroxymethylfurfural (HMF) is a typical
1. Introduction
Stimulated by ever-increasing concerns over the decline of versatile platform chemical that can be obtained from
fossil resources and challenges in the sustainable development sugars, cellulose or lignocellulosic biomass by chemical
of economy, synthesis of high value and bulk materials from transformations.14–18 Besides being used for the synthesis of
renewable feedstock has attracted great interests in the past fuels,19,20 it is also an important intermediate for production
decade.1,2 1,6-Hexanediol (1,6-HDO) is a high-value and impor- of polymer monomers, such as 2,5-furandicarboxylic acid,21–23
tant diol with dual hydroxyls at the molecule terminals. This para-xylene,24,25 and 1,6-HDO.26–30 Buntara et al. employed
structure makes it an ideal monomer for the synthesis of poly- 2,5-bis(hydroxymethyl)-tetrahydrofuran (DHMTHF), a product
mers such as polyester,3 polyurethane,4 adhesives5 and unsatu- of selective hydrogenation of HMF, as feedstock for the
rated polyesters.6 Compared with widely-used polymers such synthesis of 1,6-HDO.26 In the presence of mixed catalysts of
as polyethylene terephthalate (PET) which is made from ethyl- Nafion SAC-13 and Rh-ReOx/SiO2, 86% yield of 1,6-HDO was
ene glycol,7–9 polyesters made from 1,6-HDO possess superior obtained after a 20 h reaction at 120 °C. They also investigated
properties in flexibility, caustic resistance and hydrolytic stabi- the conversion of 1,2,6-hexanetriol (1,2,6-HTO) to 1,6-HDO
lity thanks to the longer carbon chain of 1,6-HDO.10 Currently, with different catalysts and proposed reaction pathways.27
1,6-HDO is
a petrochemical product obtained through Tuteja et al. developed one-pot conversion of HMF to 1,6-HDO
multiple reactions including cyclohexane oxidation, esterifica- over a Pd/ZrP catalyst using formic acid as a hydrogen source,
tion and hydrogenation.10
and obtain a 43% yield of 1,6-HDO after a 21 h reaction at
Catalytic conversion of biomass produces a variety of 413 K.28 2-(Hydroxymethyl)tetrahydropyran was also used as a
important platform chemicals, some of which are suitable to precursor for 1,6-HDO production.26,29,30 The catalysts contain-
ing hydrogenation sites and acid sites, typically consisting of
ReOx and noble metal Rh or Ir, are found to be effective for
aDalian Institute of Chemical Physics, Chinese Academy of Sciences, State Key
Laboratory of Catalysis, Zhongshan Road 457, Dalian 116023, China.
E-mail: myzheng@dicp.ac.cn, taozhang@dicp.ac.cn; Fax: +86 411 84691570;
the selective catalytic breakage of C–O in the furan ring or to
obtain terminal diols from biomass.26,31,32 All of these studies
have accumulated much valuable knowledge and afford gui-
dance for the synthesis of 1,6-HDO from biomass. On the
other hand, to date the yield of 1,6-HDO is not very high yet
when using HMF as feedstock (ca. 40%). The reaction
Tel: +86 411 84379738
bGraduate University of Chinese Academy of Sciences, Beijing 100049, China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5gc02228b
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efficiency of these processes is low, usually needing more than 2.3. Catalyst characterization
20 h to reach 100% conversion of feedstock. In addition, the
X-ray photoelectron spectra (XPS) were conducted on an
ESCALAB 250 X-ray photoelectron spectrometer with a mono-
chromated Al Kα anode. Before the experiment, the sample
was reduced at 573 K in flowing hydrogen for 2 h. All binding
energies were calibrated for surface charging by referencing
them to the energy of the C 1s peak at 284.5 eV.
The metal loadings of Ir–ReOx/SiO2 catalysts before and after
24 h of reaction were determined by an inductively coupled
plasma atomic emission spectrometer (ICP-AES) on an IRIS
Intrepid II XSP instrument (Thermo Electron Corporation).
The FT-IR spectra of pyridine adsorption on the Ir–ReOx/
SiO2 catalyst after water vapour treatment were collected on a
Bruker Equinox 55 spectrometer equipped with a deuterated
triglycine sulphate (DTGS) detector in the transmittance
mode. In detail, the Ir–ReOx/SiO2 catalyst was pressed into
reactions were conducted in batch reactors or multiple steps,
the operation of which is inefficient from the point of view of
industrial applications.33 Herein, we studied the transform-
ation of HMF into 1,6-HDO in a fixed-bed reactor. A variety of
noble metal–rhenium catalysts were screened, and the effects
of reaction conditions on the 1,6-HDO yield were investigated.
A high yield of 1,6-HDO was effectively obtained over double-
layered catalysts under optimized reaction conditions. The
reaction route was proposed based on the conditional experi-
ment results. This work may provide inspiration for developing
efficient catalysts and reaction systems for 1,6-HDO synthesis
from biomass.
self-supporting wafers and reduced under
a H2 stream
2. Experimental
2.1. Materials
(40 mL min−1) at 573 K for 1 h. Then, it was cooled to 373 K
and evacuated, followed by the introduction of water vapour
HMF (99%) was provided by Ningbo Institute of Industrial (saturated at ambient temperature) for 30 min. After cooling to
Technology. Standard samples (purity >99%), including room temperature and thorough evacuation, pyridine was
DHMTHF, 1,6-HDO, and 1,2,6-hexanetriol (1,2,6-HTO) were absorbed until saturation. Finally, the IR cell was evacuated at
purchased from J&K Scientific Ltd. The solvent of tetrahydro- 423 K and IR spectra of the sample were recorded. For com-
furan (THF) was purchased from Tianjin Kermel Chemical parison, FT-IR spectra of pyridine adsorption on the Ir–ReOx/
Reagent Co., Ltd. Silica (99%, Qingdao Ocean Chemical Ltd, SiO2 catalyst without water treatment were measured by a
BET surface area 509 m2 g−1), activated carbon (99%, Beijing similar procedure as mentioned above, except for the absence
Guanghua-Jingke Activated Carbon Co., Ltd, BET surface area of the water treatment step.
1203 m2 g−1), TiO2 (Degussa, P25), SiO2–Al2O3 (Si/Al = 1 : 7,
Sinopharm Chemical Reagent Co., Ltd), HZSM-5 (Si/Al = 50, 2.4. Catalytic activity measurement
The Catalyst Plant of Nankai University) and perrhenic acid
The catalytic conversion of HMF was conducted in a vertical
(99%, Alfa Aesar) were purchased from commercial sources
fixed-bed reactor, which was made from a stainless steel tube
and used without further purification. Al2O3 is home-made
with an inner diameter of 5 mm. For evaluation of a single
with a surface area of 250 m2 g−1
.
catalyst, 1.0 g catalyst (60–80 mesh) was loaded at the centre of
the reactor, and quartz granules (60–80 mesh) were filled from
both sides. For evaluation of binary catalysts in the double-
layered catalyst bed, 1.0 g Pd/SiO2 was loaded in the upper
layer and 1.0 g of the Re-based catalyst was loaded in the
bottom layer (the two layers were separated by a thin layer of
quartz wool). Prior to reaction, the catalyst was reduced at
573 K for 2 h in flowing H2 at a rate of 60 mL min−1 under
3 MPa pressure. After the temperature decreased to the reaction
temperature, the H2 pressure was modulated to the desired
value with a back pressure valve, and then 1% HMF solution
was fed into the reactor with an HPLC pump (Model LC-20A,
SHIMADZU Company) at a LHSV of 6 h−1 (hourly volume of
reactant solution/weight of a single catalyst) in flowing hydrogen
at a rate of 60 mL min−1. The product solution was collected in
a 100 mL tank under the reaction pressure, and periodically
released for analysis. The gas products in H2 were analysed by
in situ gas chromatography. Typically, the results at 2 h of reac-
tion were adopted in the present contribution.
2.2. Catalyst preparation
Pd catalysts supported on different carriers, including SiO2,
Al2O3, active carbon (AC), TiO2, HZSM-5, SiO2–Al2O3, zirco-
nium phosphate, were prepared by incipient wetness impreg-
nation. Taking Pd/SiO2 as an example, 3.0 g SiO2 was
impregnated with 5.0 mL solution containing 0.14 mmol
PdCl2·2H2O and then dried at 393 K for 12 h. After calcination
at 573 K for 3 h, a 0.6 wt% Pd/SiO2 catalyst was obtained. The
zirconium phosphate support was prepared as described in
the literature.28 Differing from other Pd catalysts calcined in
air, the active carbon supported Pd catalyst was calcined under
a nitrogen atmosphere.
M–ReOx/SiO2 (M = Ir, Pd, Pt, Rh, or Pd–Ir, Pd–Rh) catalysts
were prepared by step-wise impregnation. M/SiO2 was first
prepared by incipient wetness impregnation with noble metal
salt solutions and dried at 393 K for 12 h. Then, it was re-
impregnated with an aqueous solution of perrhenic acid. After
drying at 393 K for 12 h and calcination at 573 K for 3 h,
M–ReOx/SiO2 was obtained. For catalysts using other supports,
2.5. Analysis of product
they were prepared by the same process except for the AC The gas and liquid products were analysed with Agilent 7890B
supported catalyst which was calcined under a N2 atmosphere. GC equipped with a Varian CP-WAX58 (FFAP) CB capillary
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column and a HP-Plot/Q capillary column, respectively. The HMF at room temperature but not in the total hydrogenation
concentration of HMF after the reaction was measured with an of HMF.41 Apparently, the reaction pathway of HMF conversion
Agilent HPLC which was equipped with a Biorad Aminex over Ir–ReOx/SiO2 strongly depended on the reaction
HPX-87H organic acid column and an ultraviolet detector temperatures.
(Agilent G 1314 A, operated at 284 nm). The mobile phase was
To improve the 1,6-HDO yield over the Ir–ReOx/SiO2 cata-
an aqueous solution of 140 mg L−1 H2SO4 at a flow rate of lyst, Pd was co-loaded with Ir–ReOx based on the consideration
0.5 mL min−1 and the temperature of the column was set at that Pd might promote the hydrogenation of HMF and facili-
318 K.
tate the subsequent reactions. However, the 1,6-HDO yield over
Pd–Ir–ReOx/SiO2(0.6–5–5) (Table 1, entry 5) turned out to be
very similar to that of Ir–ReOx/SiO2(5–5) (14.6% vs. 15.2%), and
gave a significant amount of 1,2,6-HTO (30.4% yield). Increas-
ing the Pd loading to 5% led to a slight improvement in the
1,6-HDO yield to 19.2%, which was accompanied by a notable
increase in the by-product of 1,5-HDO (entry 6). These results
suggest that Pd, Ir, and ReOx could be used as effective active
components for the 1,6-HDO preparation from HMF, but the
product selectivity over them needs to be improved through
rational design of catalysts and optimization of reaction
conditions.
3. Results and discussion
3.1. Single catalysts’ screening
ReOx based bimetallic catalysts were reported to be active for
breaking the C–O bond in the furan ring and hydrodeoxygena-
tion reactions.30,34–38 Therefore, we first synthesized a series of
Re based bimetallic or tri-metallic catalysts and tested them in
the one-step conversion of HMF to 1,6-HDO.
As shown in Table 1 (entries 1–4), different noble metal–
ReOx catalysts afforded remarkably different product distri-
butions. It should be mentioned that the solvent THF was
stable in the presence of catalysts studied herein, as evidenced
by the overall yield of C4 products (1,4-butanediol, butanol, or
butane) which was lower than 0.1% on the basis of THF. The
dominant product over Pd–ReOx/SiO2(5–5%) was DHMTHF
with a high yield of 73.0% but no 1,6-HDO was formed. This
suggests that Pd catalysts have high activities toward hydrogen-
ation of the aldehyde group and carbon–carbon double bonds
in the furan ring to form DHMTHF.39,40 However, the catalyst
showed a low activity in opening the ring of DHMTHF and did
In addition, it was found that catalyst supports affected the
product selectivity significantly. Differing from SiO2 supported
catalysts, active carbon supported Ir–ReOx did not produce 1,6-
HDO (Table 1, entry 7) but gave 36% yield of 2-hexanol and
many unknown products which were undetectable by GC or
HPLC analysis. ZrP and Al2O3 supported catalysts did not give
a high yield of 1,6-HDO either. Since the SiO2 supported cata-
lysts showed the best performance for HMF conversion to 1,6-
HDO, they were further studied in the following sections.
3.2. Screening of catalysts in a double-layered catalyst bed
not produce 1,6-HDO. Rh–ReOx/SiO2 afforded 6.1% yield of Since using a single catalyst met with great challenges in effec-
1,6-HDO. This low value is similar to that reported in the tively coupling the multiple reactions of HMF conversion to
literature, where 7% yield of 1,6-HDO was obtained when 1,6-HDO, we further employed binary catalysts and loaded
using HMF as feedstock.26 Iridium co-loaded with ReOx them in double layers to improve the selectivity of 1,6-HDO.
showed the best performance, over which 15.2% yield of The catalyst in the upper layer would primarily take charge of
1,6-HDO was obtained at the expense of DHMTHF yield which hydrogenation of HMF, and the catalyst in the bottom layer ful-
decreased to 10.9%. Tamura et al. reported that Ir–ReOx/SiO2 fills the function of subsequent hydrogenolysis of the furan
was very effective in the selective hydrogenation of CvO in ring to 1,6-HDO. It was reported that the Pd based catalyst
Table 1 Product yields in HMF hydrogenolysis over different catalystsa
Entry Catalysts
1,6-HDO/% 2,5-HDO/% 1,5-HDO/% Hexane/% 1-Hexanol/% DHMTHF/% MHMTHF/% 1,2,6-HTO/%
1
2
3
4
5
6
7
8
9
Pd–ReOx/SiO2(5–5)
0
0
0
0
0
0
0
0
0
0
0
4.6
0
13.7
11.5
22.0
0
8.0
23.9
13.3
18.2
8.5
24.6
14.2
13.5
14.8
0
0
0
11.7
0
4.0
36.0
0
72.9
32.6
42.9
10.9
18.8
6.8
0
0
42.1
8.4
9.1
0
0
0
28.1
12.2
30.4
0
0
0
6.0
Pt–ReOx/SiO2(5–5)
Rh–ReOx/SiO2(5–5)
Ir–ReOx/SiO2(5–5)
2.3
6.1
15.2
0
Pd–Ir–ReOx/SiO2(0.6–5–5) 14.6
6.4
2.7
0
5.0
10.5
Pd–Ir–ReOx/SiO2(5–5–5)
19.1
0
4.8
0
Ir–ReOx/AC(5–5)b
Ir–ReOx/ZrP(5–5)c
0
0
Ir–ReOx/Al2O3(5–5)d
0
a Reaction conditions: 373 K, 3 MPa H2, 1.0 g catalyst loaded in a fixed-bed reactor, mixed solvents of water and THF at a volume ratio of 2 : 3,
1 wt% HMF, LHSV = 6 h−1. HMF conversions were 100% in all experiments. b Ir–ReOx/C(5–5) gave a product of 2-hexanol but not 1-hexanol. c Ir–
ReOx/ZrP(5–5) gave a large amount of an unknown precipitate. d Ir–ReOx/Al2O3(5–5) gave a product of dihydroxylfurfuran but not DHMTHF. 1,6-
HDO, 2,5-HDO, 1,5-HDO, DHMTHF, MHMTHF, and 1,2,6-HTO represent 1,6-hexanediol, 2,5-hexanediol, 1,5-hexanediol, 2,5-bis(hydroxymethyl)-
tetrahydrofuran, 5-methyltetrahydrofurfuryl alcohol, and 1,2,6-hexanetriol, respectively.
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Table 2 lists the results of HMF transformation over
different double-layered catalysts in a fixed-bed reactor. Com-
pared to the performance of single catalysts listed in Table 1,
the 1,6-HDO yield was significantly improved. Over double-
layered catalysts of Pd/SiO2(0.6)
+
Ir–ReOx/SiO2(5–5),
36.1–46.0% yield of 1,6-HDO was obtained (Table 2, entries 1
and 2) in contrast to 14.6% yield of 1,6-HDO (Table 1, entry 5)
over a single catalyst of Pd–Ir–ReOx/SiO2(0.6–5–5), even though
the same loadings of active components were used in both
cases. In addition, the performance of Ir–ReOx/SiO2(5–5) is
notably superior to that of Rh–Re/SiO2(5–5) (9.7% yield,
Table 2, entry 3), which was reported to be active for 1,6-HDO
production from DHMTHF.26 The inferior performance of Rh–
Re/SiO2 might result from no use of an acidic catalyst such as
Nafion SAC-13 in the present study.
The 1,6-HDO yield over Pd/SiO2(0.6) + Ir–ReOx/SiO2 first
increased and then levelled off at 46.0% with the ReOx loading
increasing from 0.5 to 5%, accompanied by the decrease of the
DHMTHF yield from 11.4 to 1.4% (Table 2, entries 1 and 4–6).
This indicates that ReOx played an important role in selectively
catalyzing the ring opening of DMHTHF, which is consistent
Fig. 1 Results of HMF hydrogenation over Pd catalysts supported on
different carriers, the loadings of palladium in all catalysts were 0.6 wt%
except 5% Pd/SiO2. (HMF conversions were 100% in all experiments;
reaction conditions: 373 K, 3 MPa H2, mixed solvents of water and THF
at a volume ratio of 2 : 3, 1 wt% HMF, LHSV = 6 h−1.)
could transform HMF to DHMTHF with high selectivity.42 with the findings in the literature.26,29–32,38 In addition, ReOx
Therefore, the Pd catalysts supported on different carriers were slightly contributed to hexane formation as evidenced by more
first screened for obtaining a high selectivity. As shown in hexane being formed with the increase of ReOx loading.
Fig. 1, the Pd/SiO2(0.6%) catalyst showed the highest selectivity
The support of Ir–Re catalysts had remarkable effects on
(86%) toward DHMTHF with 13% hexane formation at 373 K. the product selectivity. Compared to the SiO2 supported cata-
Increasing the palladium loading from 0.6 to 5% did not lyst, Al2O3 and active carbon supported Ir–Re catalysts gave
improve DHMTHF selectivity over Pd/SiO2. The yield of much lower yields of 1,6-HDO but more DHMTHF formation.
DHMTHF over Pd/Al2O3 was slightly lower than that on Zirconium phosphate supported palladium catalysts were
Pd/SiO2. In contrast, over the Pd/AC catalyst, besides the recently reported to be active for the HMF transformation to
formation of DHMTHF and hexane, 22% yield of MHMTHF 1,6-HDO.28 However, Ir–ReOx/ZrP in combination with Pd/
was also obtained, which is the hydrodeoxygenation product of SiO2(0.6) did not produce 1,6-HDO, and the primary products
DMHTHF. Over acidic carriers of SiO2–Al2O3 and HZSM-5 were DHMTHF at a high yield of 73.8% (Table 2, entry 7). This
supported Pd catalysts, the yield of DHMTHF was rather low suggests that the activity of the Ir–ReOx/ZrP catalyst for the ring-
with a significant loss in carbon balance due to the formation opening of tetrahydrofuran is very low. Tuteja et al., proposed
of undetectable products. Pd/TiO2 afforded the worst perform- that over ZrP supported catalysts, HMF was transformed into 1,6-
ance without any DHMTHF production. Taken all together, the HDO through six-step reactions and an intermediate of hex-1,3,5-
0.6% Pd/SiO2 catalyst was the best one for use as an upper- triene-1,6-diol.28 This mechanism would be quite different from
layer catalyst in the following study.
that over Pd/SiO2(0.6) + Ir–ReOx/SiO2 studied herein.
Table 2 Product yields in HMF hydrogenolysis over double-layered catalystsa
Entry
Catalysts
1,6-HDO/%
1,5-HDO/%
Hexane/%
1-Hexanol/%
DHMTHF/%
MHMTHF/%
0
1
2
3
4
5
6
7
8
9
Pd/SiO2(0.6) + Ir–ReOx/SiO2(5–5)
Pd/SiO2(0.6) + Ir–ReOx/SiO2(5–5)b
Pd/SiO2(0.6) + Rh–ReOx/SiO2(5–5)
Pd/SiO2(0.6) + Ir–ReOx/SiO2(5–2.5)
Pd/SiO2(0.6) + Ir–ReOx/SiO2(5–1)
Pd/SiO2(0.6) + Ir–ReOx/SiO2(5–0.5)
Pd/SiO2(0.6) + Ir–ReOx/ZrP (5–5)
Pd/SiO2(0.6) + Ir–ReOx/C(5–5)
46.0
36.1
9.7
46.2
39.4
28.7
0
18.1
15.0
9.8
19.4
11.7
10.3
0
10.0
25.3
20.8
12.9
4.5
24.3
16.8
0
17.5
8.7
11.3
0
1.4
0
35.6
4.7
0
0
0
0
7.2
0
0
7.3
6.3
11.4
73.8
35.5
30.0
22.5
23.6
15.9
8.2
15.9
8.6
9.3
3.5
0
Pd/SiO2(0.6) + Ir–ReOx/Al2O3(5–5)
a Reaction conditions: 373 K, 5 MPa H2, 1.0 g Pd/SiO2 loaded in the upper layer and 1.0 g of the Re-based catalyst loaded in the bottom layer,
mixed solvents of water and THF at a volume ratio of 2 : 3, 1 wt% HMF, LHSV = 6 h−1 based on the amount of a single catalyst. HMF conversions
were 100% in all experiments. b Pd/SiO2(0.6) + Ir–ReOx/SiO2(5–5) was evaluated under 3 MPa H2. 1,6-HDO, 1,5-HDO, DHMTHF, and MHMTHF
represent 1,6-hexanediol, 1,5-hexanediol, 2,5-bis(hydroxymethyl)-tetrahydrofuran and 5-methyltetrahydrofurfuryl alcohol, respectively.
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Fig. 2 Results of DHMTHF hydrogenolysis over Ir–ReOx/SiO2(5–5) and
Rh–ReOx/SiO2(5–5) catalysts in a batch reactor, each metal loading on
catalysts is 5 wt%. (Reaction conditions: 20 h, 373 K, 5 MPa H2, 0.15 g
catalyst, 20 mL 1% DHMTHF in mixed solvents of water and THF at a
volume ratio of 2 : 3.)
Fig. 3 Effect of water content in the THF solution on the product yields
over Pd/SiO2(0.6%) + Ir–ReOx/SiO2(5%–5%) catalysts. (HMF conversions
were 100% in all experiments; reaction conditions: 373 K, 3 MPa H2,
1 wt% HMF, LHSV = 6 h−1.)
It should be noted that the catalysts loaded in the double-
layered fixed-bed reactor exhibited remarkably superior per-
formance compared to that in a batch reactor. As shown in
Fig. 2, when DHMTHF (the product of HMF hydrogenation
over Pd/SiO2) was used as feedstock in a batch rector in the
presence of Ir–ReOx/SiO2(5–5) or Rh–ReOx/SiO2(5–5), the con-
versions of DHMTHF were less than 20%. The main product
was 1,2,6-HTO and hexane without 1,6-HDO formation even
after 20 h of reaction. This suggests that the conversion of
DHMTHF was increased by ca. 5 folds together with high
selectivity to 1,6-HDO in a fixed-bed reactor as compared with
that in a batch reactor.
is still a lack of very clear understanding. For the performance
of the upper layer catalyst of Pd/SiO2 in HMF conversion in
different solvents, no big difference was observed with yields
of DHMTHF ranging from 89% to 100% (Fig. 4). Therefore, the
different selectivity of 1,6-HDO should be primarily ascribed to
the effect of the solvent on the catalytic conversion of
DHMTHF over the bottom layer Ir–ReOx/SiO2 catalyst.
The acidic property of the Ir–ReOx/SiO2 catalyst was probed
by IR spectroscopy of pyridine adsorption. As shown in Fig. 5,
the characteristic peaks occurring at 1450 cm−1 and 1544 cm−1
belong to Lewis and Brønsted acidic sites, respectively.46,47
The peak at 1489 cm−1 is associated with both Brønsted and
Lewis acids.48 It can be noticed that Brønsted acidic sites were
significantly generated on the Ir–ReOx/SiO2 catalyst after the
3.3. Effect of the solvent on 1,6-HDO production
The choice of the reactant solvent is an important factor that
often remarkably affects the product distribution.43–45 Two
kinds of solvents, i.e., a protic solvent of water and an aprotic
solvent of tetrahydrofuran (THF), were used at different
mixture ratios for the HMF transformation. As shown in Fig. 3,
when pure THF was used as the reaction solvent, the dominant
product was DHMTHF (76.8% yield) accompanied by 21.7%
yield of hexane, but no 1,6-HDO was formed. In contrast, in a
pure water solution, the primary products were 1,6-HDO and
hexanol (20.9% and 25.3% yields, respectively), while the yield
of DHMTHF decreased nearly to zero. Tuning the ratio of THF
to water led to a volcano-shaped curve for the 1,6-HDO yield,
which gave a maximum value of 36.1% at 40% water content
in the THF solution. A suitable ratio of THF to water benefited
HMF conversion to 1,6-HDO. Considering that a pure aqueous
solution would be detrimental to the stability of silica sup-
ported catalysts due to gradual collapse of the silica support
under a hydrothermal environment, a mixed solvent contain-
ing 40% water and 60% THF was used as a typical reaction
medium in the present study.
Fig. 4 Effect of water content in the THF solution on HMF conversion
to DHMTHF over the upper-layer catalyst of Pd/SiO2(0.6) without Ir–
ReOx/SiO2. (HMF conversions were 100% in all experiments; reaction
conditions: 373 K, 3 MPa H2, HMF concentration was 1 wt%, LHSV =
As for the reason why the 1,6-HDO selectivity was dramati-
cally different when using THF and water as the solvent, there 6 h−1.)
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coupled with metal-catalyzed hydrogenation. The hydroxyl
groups on Re atoms associated with metallic Rh are acidic.38
Tomishige’s group studied the Re-modified Ir catalyst and
identified that Ir metal particles were modified with ReOx
clusters regardless of whether the catalyst was being used in
water or in a n-heptane solvent or was freshly reduced under
dry conditions.49–51 Also, they proposed that the protonic acid
participated in the catalytic ring opening of tetrahydrofuran.52
Taken all together, it is rational to conjecture that ReOx
species on Ir–ReOx/SiO2 are partially hydrolyzed in the
presence of water to produce hydroxyl groups on the surface,
which provide Brønsted acidic sites for the catalytic opening of
the furan ring of DHMTHF. In contrast, in THF, a non-protonic
organic solvent, the amount of hydroxyl groups on the ReOx
surface would be much less, which led to the low activity of
DHMTHF conversion (lower than 25%). In addition, as
reported by Nakagawa et al., alcohol reactants in an alkane
solvent could adsorb on the Ir–ReOx/SiO2 catalyst more
strongly than that in water.49 Therefore, in the mixed solvents
of THF and water, the abundant amount of Brønsted acidic
sites and possibly suitable adsorption strength of reactants
could account for the maximum yield of 1,6-HDO in HMF
conversion.
Fig. 5 FT-IR spectra of pyridine adsorbed on the Ir–ReOx/SiO2(5–5)
catalyst: (A) catalyst treated with water vapour; (B) catalyst without water
treatment.
3.4. Effect of hydrogen pressure on product distribution
pretreatment with water vapour. The XPS characterization dis-
closed that a remarkable portion (ca. 30%) of Re existed in
oxide forms of Re4+ on Ir–ReOx/SiO2, which could account for
the origin of acidic sites (Fig. 6). The STEM and EDX images
demonstrated that Ir and Re elements had a very similar distri-
bution on the silica support, suggesting that Ir and Re co-
existed closely (ESI Fig. S1†). Chia et al. studied the selective
hydrogenolysis of secondary C–O bonds for a wide range cyclic
ethers and polyols over the ReOx–Rh/C catalyst.38 They found
that the catalyst realized selective hydrogenolysis of C–O bonds
by acid-catalyzed ring-opening and dehydration reactions
As shown in Fig. 7, the H2 pressure significantly affected the
product distributions. The yield of 1,6-HDO was merely 11.8%
but hexane yield reached 76.3% at 0.5 MPa H2. Increasing H2
pressure to 7 MPa greatly improved the yield of 1,6-HDO to
57.8% while depressing the hexane yield to 8.6%. The yield of
hexanol first increased from zero to 24% and then decreased
to 8.2% with H2 pressure increasing. The high pressure of
hydrogen seems to inhibit the further hydrogenolysis of
1,6-HDO to form hexanol and hexane.
Fig. 7 The product distribution under different hydrogen pressures
over Pd/SiO2(0.6%) + Ir–ReOx/SiO2(5%–5%) catalysts. (HMF conversions
were 100% in all experiments; reaction conditions: 373 K, mixed solvents
of water and THF at a volume ratio of 2 : 3, 1 wt% HMF, LHSV = 6 h−1.)
Fig. 6 XPS spectra of the Ir–ReOx/SiO2(5%–5%) catalyst.
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version. Meanwhile, the hexane yield decreased with the
increase of H2 pressure over the bottom layer catalyst.
It was reported that competitive adsorption between hydro-
gen and reactants on the catalyst surface could depress the
hydrogenolysis.53 With the increase of H2 pressure, more
hydrogen would occupy the metallic active sites and eliminate
the coverage of DHMTHF and 1,6-HDO on the upper-layer
Pd/SiO2 catalyst and the bottom-layer Ir–ReOx/SiO2 catalyst,
respectively, which consequently decreased the over-hydroge-
nolysis of 1,6-HDO to form hexane. In addition, as mentioned
in section 3.2, ReOx sites played roles in hexane formation.
Therefore, it can be conjectured that 1,6-HDO should be
formed at the interfacial sites between ReOx and hydrogen-
Fig. 8 Effect of hydrogen pressure on the product distribution over the ation metal sites, which is in agreement with the conclusion
reported by Dumesic et al.38 A high pressure of H2 would con-
tribute to the removal of 1,6-HDO from active sites and
Pd/SiO2(0.6%) catalyst. (HMF conversions were 100% in all experiments;
reaction conditions: 373 K, mixed solvents of water and THF at a volume
ratio of 2 : 3, 1 wt% HMF, LHSV = 6 h−1.)
decrease the opportunity of the further hydrodeoxygenation of
1,6-HDO to hexane.
To identify the reasons for the promoting effect of H2
3.5. Route for the conversion of HMF to 1,6-HDO
pressure on 1,6-HDO formation, the catalytic performance of
The reaction route of HMF conversion to 1,6-HDO was probed
by varying reaction temperatures and conditional experiments.
As shown in Fig. 10, the main product was 1,2,6-HTO with
42.4% yield at 333 K, accompanied by 27% yield of DHMTHF
and 22% yield of 1,6-HDO. Increasing the temperature led to a
continuous decrease in 1,2,6-HTO and DHMTHF yields, while
the yield of 1,6-HDO was raised to the highest value of 46% at
373 K. Similarly, as the space velocity of the reaction was
increased two and five times, the yield of 1,6-HDO decreased
but the yields of 1,2,6-HTO and DHMTHF increased remark-
ably (ESI Fig. S2†). These results strongly suggested that the
1,6-DHO formation underwent intermediates of DHMTHF and
1,2,6-HTO. In addition, as shown in Fig. 8, over the upper layer
Pd/SiO2 catalyst HMF was converted into DHMTHF at a yield
the upper-layer catalyst Pd/SiO2 was tested. As shown in Fig. 8,
the H2 pressure also imposed notable promoting effects on
DHMTHF formation over Pd/SiO2. With the hydrogen pressure
increasing from 0.5 to 7 MPa, the DHMTHF yield was
enhanced from 57 to 94% along with a remarkable decrease in
the hexane yield. Correlating the yield of DHMTHF in HMF
conversion over the upper-layer Pd/SiO2(0.6%) catalyst (Fig. 8)
and the yield of 1,6-HDO over the double-layered catalysts
(Fig. 7), the net selectivity of 1,6-HDO in DHMTHF conversion
over the bottom-layer Ir–ReOx/SiO2(5%–5%) catalyst was
obtained (Fig. 9). Evidently, H2 pressure imposed a remarkably
positive effect on the selectivity of 1,6-HDO in DHMTHF con-
Fig. 9 Net selectivity of 1,6-HDO and net yield of hexane in DHMTHF
conversion over the bottom layer catalyst of Ir–ReOx/SiO2(5%–5%)
under different hydrogen pressures. (The net selectivity of 1,6-HDO was
obtained by dividing 1,6-HDO yields in Fig. 7 by DHMTHF yields in Fig. 8;
the net yield of hexane was obtained by subtracting the yields in Fig. 7
with the yields in Fig. 8.)
Fig. 10 Effects of reaction temperature on the product distribution for
HMF conversion to 1,6-HDO over Pd/SiO2(0.6%) + Ir–ReOx/SiO2(5%–
5%) catalysts. (HMF conversions were 100% in all experiments; reaction
conditions: 5 MPa H2, mixed solvents of water and THF at a volume ratio
of 2 : 3, 1 wt% HMF, LHSV = 6 h−1.)
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Scheme 1 Reaction routes of HMF conversion to 1,6-HDO over Pd/
SiO2 + Ir–ReOx/SiO2 catalysts.
Fig. 11 Results of hydrogenolysis of typical intermediates over the Ir–
ReOx/SiO2 catalyst. (Reaction conditions: 373 K, 3 MPa H2, mixed sol-
vents of water and THF (H2O : THF = 2 : 3/v : v), reactant concentrations
were all 1 wt%, LHSV = 6 h−1.)
3.6. Catalyst stability
The stability of the optimized catalysts was tested and the
results are shown in Fig. 12. The yield of 1,6-HDO was ca. 60%
at the beginning and was maintained at higher than 50%
during 15 h of reaction. After running for 24 h, the 1,6-HDO
yield slightly decreased to ca. 40%, accompanied by the
decrease in yields of hexanol and 1,5-HDO. Conversely, the
yield of 1,2,6-HTO significantly increased from zero to 42%
after 24 h of reaction. As discussed above, 1,2,6-HTO was the
precursor of 1,6-HDO, and its conversion was closely related to
the ReOx species on the catalyst. Elemental analysis showed
that loadings of Ir or Pd on the Ir–ReOx/SiO2 and Pd/SiO2 cata-
lysts did not change before and after the reaction. However,
the ReOx loading decreased from 5% to 3% on the spent cata-
lyst. This is consistent with a previous report that Re oxides
could leach into the solution in a long running time.38 In
addition, comparing the surface areas and pore size distri-
butions of the catalysts before and after usage, no big differ-
of 86%. Therefore, it can be inferred that DHMTHF was con-
verted over the bottom layer Ir–ReOx/SiO2 catalyst to form
1,2,6-HTO which acted as the precursor of 1,6-HDO. The con-
ditional experiment shows that when 1,2,6-HTO was used as
the reaction substrate, 54% yield of 1,6-HDO was obtained,
further confirming that 1,2,6-HTO is the precursor of 1,6-HDO
(Fig. 11).
The main byproducts of HMF conversion to 1,6-HDO were
1,5-HDO, hexanol and hexane. At 373 K, the ratio of the 1,5-
HDO yield to the 1,6-HDO yield was 2/5 in the HMF conversion
over the double-layered catalysts Pd/SiO2(0.6%) + Ir–ReOx/
SiO2(5%–5%) (Table 2, entry 1). The conditional experiment
showed that 1,5-HDO can be produced in 1,2,6-HTO conver-
sion (Fig. 11), but its yield was merely 1/6th of that of 1,6-HDO,
notably lower than 2/5 in the case of HMF conversion. There-
fore, besides being formed from 1,2,6-HTO, nearly half the
amount of 1,5-HDO was formed from other precursors, poss-
ibly from 5-methyltetrahydrofurfuryl alcohol which is a hydro-
deoxygenation product of DHMTHF.
As for the byproduct hexanol, on the one hand it could be
derived from 1,6-HDO. As shown in Fig. 11, 6.7% yield of
hexanol was obtained when using 1,6-HDO as the reactant. On
the other hand, hexanol could also be produced from other
polyols, such as 1,5-HDO, according to the fact that the yield
of hexanol (16.8%) in HMF conversion is higher than that of
hexanol (6.7%) obtained in 1,6-HDO conversion.
Besides, it should be noted that although 1,6-HDO could be
hydrodeoxygenated to hexane and hexanol, the conversion of
1,6-HDO was much lower than the conversions of HMF and
1,2,6-HTO. This suggests that 1,6-HDO is more stable than
other reaction intermediates over the Ir–ReOx/SiO2 catalyst,
which is a virtue for obtaining 1,6-HDO at a high yield in HMF
conversion.
According to the above analysis, the reaction routes of HMF
conversion to 1,6-HDO were proposed and are shown in
Scheme 1.
Fig. 12 Results of stability evaluation of Pd/SiO2(0.6%)
+ Ir–ReOx/
SiO2(5%–5%) catalysts. (HMF conversion was 100%; reaction conditions:
373 K, 7 MPa H2, mixed solvents of water and THF at a volume ratio of
2 : 3, 1 wt% HMF, LHSV = 6 h−1.)
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ence was found (ca. 400 20 m2 g−1, 5.6 0.1 nm). It can be
concluded that the loss in 1,6-HDO was mainly caused by the
leaching of Re during the reaction. Therefore, for the practical
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4. Conclusions
In summary, biomass-derived HMF was effectively converted
into 1,6-HDO over double-layered catalysts in a fixed-bed
reactor. Under optimal reaction conditions of 373 K and
7 MPa H2, 57.8% yield of 1,6-HDO was obtained over Pd/SiO2 +
Ir–ReOx/SiO2. The double-layered composite catalyst showed a
much superior performance in improving the target product
selectivity as compared to that with a single catalyst. The
reaction solvent significantly effected 1,6-HDO formation, and
mixed solvents of water and THF at a volume ratio of 2 : 3 were
the most suitable to afford an enhanced 1,6-HDO yield. A high
pressure of H2 contributed to the formation of 1,6-HDO by
depressing the over-hydrogenolysis of reaction intermediates
and products to form hexane and hexanol. According to the
results of conditional experiments, the reaction route was
elucidated, wherein DHMTHF and 1,2,6-HTO were the succes-
sive intermediates for the formation of 1,6-HDO. The Pd/SiO2
+ Ir–ReOx/SiO2 catalysts showed reasonably good stability with
the 1,6-HDO yield in the range of 60% to 40% on running for
24 h. The catalyst deactivation was attributed to the gradual
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Acknowledgements
This work was supported by the National Nature Science Foun-
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