Selective hydrogenation of 5-hydroxymethylfurfural and its acetal with 1,3-propanediol to 2,5-bis(hydroxymethyl)furan using supported rhenium-promoted nickel catalysts in water

Article abstract of DOI:10.1039/c9gc03856f

The high reactivity of the formyl group of 5-hydroxymethylfurfural (5-HMF) is problematic, because it leads to undesired oligomerization reactions. This is usually countered by working in dilute non-aqueous solutions. Here, we present a novel approach to convert concentrated aqueous solutions of 5-HMF to 2,5-bishydroxymethylfuran (BHMF), which is a prospective monomer for polyesters and self-healing polymers. Our approach is based on the protection of the formyl group of 5-HMF using acetalization with 1,3-propanediol. Hydrogenation is carried out using an optimized bimetallic Ni-Re catalyst supported on TiO₂ at a carefully controlled pH, resulting in balanced rates of deprotection and hydrogenation and high BHMF yield. Under optimized conditions at a benign temperature of 40 °C, hydrogenation of concentrated solutions (10-20 wt%) of protected 5-HMF in water gave 81-89% yields of BHMF without having to resort to platinum-group metals such as palladium or platinum.

Full text of DOI:10.1039/c9gc03856f

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Selective hydrogenation of 5-hydroxymethylfurfural and its acetal with 1,3-propanediol to
2,5-bis(hydroxymethyl)furan using supported rhenium-promoted nickel catalysts in water
Jan J. Wiesfelda, Minjune Kimb, Kiyotaka Nakajimab,c*, Emiel J.M Hensena*
a Laboratory of Inorganic Materials and Catalysis, Department of Chemical Engineering and
Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The
Netherlands
b Institute for Catalysis, Hokkaido University, Kita 21 Nishi 10, Kita-ku, Sapporo 0010021,
Japan
c Advanced Low Carbon Technology Research and Development Program (ALCA), Japan
Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi 332-0012, Japan
* To whom correspondence should be addressed, e.j.m.hensen@tue.nl,
nakajima@cat.hokudai.ac.jp
Abstract
The high reactivity of the formyl group of 5-hydroxymethylfurfural (5-HMF) is problematic,
because it leads to undesired oligomerization reactions. This is usually countered by working in
dilute non-aqueous solutions. Here, we present a novel approach to convert concentrated aqueous
solutions of 5-HMF to 2,5-bishydroxymethylfuran (BHMF), which is a prospective monomer for
polyesters and self-healing polymers. Our approach is based on the protection of the formyl group
of 5-HMF using acetalization with 1,3-propanediol. Hydrogenation is carried out using an
optimized Ni-Re supported on TiO2 at a carefully controlled pH, resulting in balanced rates of
deprotection and hydrogenation and high BHMF yield. Under optimized conditions at a benign
temperature of 40 °C, hydrogenation of concentrated solutions (10 - 20 wt%) of protected 5-HMF
in water gave 81-89 % yields of BHMF without having to resort to platinum-group metals such as
palladium or platinum.
Keywords: biomass conversion; selective hydrogenation; 5-hydroxymethylfurfural; Ni-Re; pH
control
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Introduction
Climate change is a main driver for the decarbonization of the energy industry. Chemical industry
cannot decarbonize like the energy sector, as materials are mostly based on carbon. Therefore,
there is a great need to substitute or supplement petroleum-based feedstock of the chemical
industry by renewable sources.1–3 Lignocellulosic biomass is considered a treasure trove of
industrially relevant chemicals, accessible through digestion to a selection of small compounds
called platform molecules.4,5 Among a variety of platform molecules, 5-hydroxymethylfurfural (5-
HMF) is especially important, as it can be obtained by acid-catalyzed dehydration of hexoses,
which form a dominant part of lignocellulose.6,7
From a chemical point of view, 5-HMF is interesting because it possesses aldehyde and alcohol
functionalities as well as a rigid unsaturated furan ring. As a result, it can be converted to many
high value-added compounds. Especially, oxidation to 2,5-furandicarboxylic acid (FDCA) is
currently at the center of the attention of many researchers, because it can be used as a monomer
for the production of polyethylene 2,5-furandicarboxylate, a biobased analogue of polyethylene
terephthalate (PET).8–10 Two other prospective monomers that can be obtained from 5-HMF by
hydrogenation
are
2,5-bis(hydroxymethyl)furan
(BHMF)
and
2,5-bis
(hydroxymethyl)tetrahydrofuran (BHMTHF). Both diol monomers can be used to produce
polyesters, but have other uses as well. BHMTHF has received most attention, because it can be
ring-opened. In this way, a biorenewable route to 1,6-hexanediol can be established to replace
current fossil resource based pathways for the large scale production of plastics.11,12 BHMF is more
difficult to obtain, but interesting as a monomer for thermosets and self-healing polymers because
of its two conjugated double bonds.13,14
Recent literature has shown that rhenium-promoted iridium, palladium and platinum are
excellent catalysts for the selective hydrogenation of aldehydes, carboxylic acids and esters,
including the aldehyde moiety of HMF. Hydrogenation of 5-HMF has been studied intensively
using heterogeneous catalysts, which are usually based on one or more platinum group metals
(PGMs), yielding products such as BHMF, BHMTHF, and/or ring-opened, deoxygenated and
decarbonylated products depending on the harshness of the conditions employed.15 A comparison
of several catalytic systems for the hydrogenation of 5-HMF to BHMF and BHMTHF is listed in
Table S1 of the supporting information. The oxophilic nature of rhenium promoter helps to activate
polar bonds and improves the reaction rate of the expensive PGM-based active phase.16–19 Ir-
2

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ReOx/TiO2 hydrogenated HMF to BHMF quantitatively (BHMF yield > 99%), whereas bimetallic
ReOx-modified Rh-Ir/SiO2 catalyst converted furfural to 1,5-pentanediol at high concentration (50
wt%).17,20 Despite the high activities, escalating prices of and increasing demand for PGMs
inspired us to look for cheaper, non-critical metals capable of efficient hydrogenation. These
scarcity aspects are less pressing for rhenium. Herein, we present an approach to convert 5-HMF
in high yield to BHMF in water using a heterogeneous catalyst based on the earth-abundant metal
nickel using rhenium as a promoter. To achieve practically interesting yields in water, we protect
the formyl group of 5-HMF by acetalization with 1,3-propanediol, another biobased platform
chemical. Direct oxidation of the acetal form of 5-HMF, propanediol-HMF (PDHMF), is possible
and provides an attractive route to obtain FDCA and its carboxylates in high yield from
concentrated solutions.8,21 Acetal protection of the highly reactive formyl group suppressed not
only heavy by-product formation occurring predominantly during oxidation reactions in
concentrated reaction mixtures, but also simple self-degradation of pure HMF itself during thermal
treatment. The protection strategy with minimizing side reactions can be potentially applicable to
a variety of HMF-based reactions at commercially relevant concentrations, which includes BHMF
formation. However, direct hydrogenation of an acetal is not possible and requires deprotection
prior to hydrogenation.22
Scheme 1. Reaction routes of hydrogenation of 5-HMF and PDHMF
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This work is organized in the following manner. Firstly, the activity of the catalysts and their
selectivity towards different hydrogenated products is investigated using dilute and concentrated
solutions of 5-HMF to study the effects of the rhenium promotor on conversion and selectivity of
Ni/TiO2 catalyst towards hydrogenated products (Scheme 1a). The catalysts were then screened
for highest BHMF yield from concentrated solutions of PDHMF, which liberates 5-HMF during
the course of the reaction as acetals are water-labile (Scheme 1b). The innovation in this work is
the balancing of the deprotection rate of PDHMF by control of the pH and the hydrogenation rate
of the liberated 5-HMF to maximize the BHMF yield and avoid undesirable humin formation even
in a concentrated solution. This approach is generic and can be applied to other catalytic reactions
in which biobased platform molecules that often carry a reactive formyl moiety need to be
converted.
Methods
Chemicals
TiO2 (P25) was procured from Degussa. Nickel nitrate hexahydrate (Ni(NO3)2.6H2O, ≥98.5%),
perrhenic acid (HReO4, 75-80 wt. % aqueous), 5-hydroxymethylfurfural (5-HMF, ≥99%), 1-
butanol (1-BuOH, 98%), 1,3-propanediol (PDO, 98%) and CDCl3 (99.96% deuterated, silver
stabilized) were purchased from Sigma Aldrich. 2,5-bis(hydroxymethyl)-furan (BHMF, 95%) was
acquired from Wako Pure Chemical Industries Ltd. 2,5-bis(hydroxymethyl)-tetrahydrofuran
(BHMTHF, 95%) was obtained from Carbosynth. All chemicals were used without further
purification.
All supported catalysts were prepared by wet-impregnation with quantities listed per gram of
catalyst. Metal loading of Ni/TiO2 and Re/TiO2 was fixed at 0.5 mmol/g support, resulting in
weight loadings of respectively 3 wt.% and 9.5 wt.%. Nickel loading of bimetallic NiRex catalysts
was equally fixed at 3 wt.%. x denotes the intended Ni:Re atomic ratios of 0.5, 1 and 2, which
corresponds with Re weight loadings of 4.8 wt.%, 9.5 wt.% and 19 wt.% respectively.
In a typical procedure HReO4 (40-159 μL) was added to a 5 mL aqueous solution of
Ni(NO3)2.6H2O (148.6 mg). The appropriate amount of TiO2 was added slowly and the resulting
paste was homogenized for three hours after which the water was evaporated at 85 °C. The
catalysts were dried further at 110 °C under vacuum, thoroughly ground in an agate mortar and
sieve fractioned (125-500 μm). The catalysts were reduced under a flow of 10% H2/He (100
4

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mL/min) whilst heating at ramp rate of 2 °C/min to final temperatures of 300 °C for Ni/TiO2 and
NiRex/TiO2, and 250 °C for Re/TiO2 to prevent loss of volatile ReOx species. Final temperatures
were maintained for 3 hours in all cases. Since all catalysts share the same support (TiO2), they
will be referred to by the metals present and ratio when appropriate for the sake of brevity.
PDHMF was prepared by thermal treatment of HMF in pure 1,3-propanediol in the absence of
any acid catalyst. Briefly, the mixture of HMF (0.53 g, 4.2 mmol) and 1,3-propanediol (2 mL, 28
mmol) was stirred at 333 K for 6 hours, which gave both high HMF conversion (76.7%) and
PDHMF selectivity (94.1%). The catalyst-free environment necessitates the use of a considerable
excess of 1,3-PDO as formation of an acetal is an equilibrium reaction. After conventional work-
up procedures, PDHMF with 90-97% purity can be obtained, and we then used it as a substrate for
hydrogenation reactions. 1H NMR spectra of PD-HMF were recorded on a Bruker Avance III HD.
1
(5-(1,3-dioxan-2-yl) furan-2-yl)methanol (PDHMF): H NMR (400
MHz, CDCl3): 6.39 (d, 1H), 6.27 (d, 1H), 5.56 (s, 1H), 4.60 (s, 1H), 4.25
(m, 2H), 3.95 (m, 2H), 2.24 (m, 1H), 1.45 (m, 1H)
Characterization
Elemental analysis was performed on a Spectroblue EOP ICP optical emission spectrometer with
axial plasma viewing, equipped with a free-running 27.12 MHz generator operating at 1400 W.
Prior to the measurement, the samples were digested in mixtures of concentrated HNO3 and H2SO4
(2:5 parts by volume) and heated to 250 °C until full dissolution was observed.
X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific K-alpha
equipped with a monochromatic small-spot X-ray source and a 180° double focusing
hemispherical analyzer with a 128-channel detector. Initial pressure was 8·10-8 mbar or less which
increased to 2·10-7 mbar due to the active argon charge compensation dual beam source during
measurement. For a typical sample preparation, freshly reduced catalyst was pressed down on
carbon tape supported by an aluminium sample plate under inert atmosphere. The inert conditions
were retained by using a vacuum transfer module. Spectra were recorded using an Al Kα X-ray
source (1486.6 eV, 72 W) and a spot size of 400 μm. Survey scans were taken at a constant pass
energy of 200 eV, 0.5 eV step size, region scans at 50 eV constant pass energy with a step size of
0.1 eV. XP spectra were calibrated to the C-C carbon signal (284.8 eV) obtained from adventitious
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carbon and deconvoluted with CasaXPS. The peak areas thus obtained were used to estimate
surface chemical composition.
Temperature programmed reduction (TPR) plots were recorded on a Micromeritics Autochem
II 2920. Powdered catalyst (100 mg - 150 mg) was loaded between two quartz wool plugs in a
quartz glass U-tube. Samples were pretreated in a 10 mL/min He flow at 130 °C for 2 hours to
remove physisorbed water and other adsorbates. TPR was then performed in a 50 mL/min flow of
4% H2 in N2 in a temperature range of 50 °C to 750 °C, 10 °C/min. The H2 signal was calibrated
using a CuO/SiO2 reference material.
CO chemisorption data was obtained from a Micromeritics ASAP 2020 to determine the number
of active sites. Powdered catalyst (100 mg - 150 mg) was loaded between two quartz wool plugs
in a quartz glass U-tube. Prior to measurement, the samples were reduced in situ at 300 °C for 4
hours, ramp rate 2 °C/min in a flow of pure H2. CO chemisorption was then performed at 35 °C.
Active sites were determined by extrapolating the linear part of the isotherm to the ordinate.
Particle size calculations were based on total metal content assuming complete reduction of the
metals and using a stoichiometry factor of 1.5 CO/active site.
HAADF-STEM images were acquired on an aberration corrected JEOL ARM-200CF
TEM/STEM operating at 200 kV. This microscope was also equipped with a Centurio silicon drift
detector (SDD) system for X-ray energy dispersive spectroscopy (XEDS) analysis. Quantification
of the XEDS spectra was carried out via the Thermo NORAN System SIX (NSS) software using
the Cliff-Lorimer method assuming no X-ray absorption.23 The Ni Kα (peak range: 7331 eV - 7623
eV) and Re Lα (peak range: 8496 eV - 8806 eV) were used to quantify the relative proportions of
the two elements in the areas analyzed. Standard-less Gaussian fitting was applied to the two
characteristic peaks, with the k-factors pre-determined by the NSS software (kNi Kα =1.217, kRe Lα
= 2.652).
Catalytic activity measurements
All hydrogenation experiments were conducted in 10 mL autoclaves equipped with a gas delivery
system, and Teflon insert and magnetic stirring bar. The autoclaves were loaded with appropriate
amounts of substrate (20 mg, 200 mg or 400 mg) and catalyst (10 mg or 100 mg) in a glovebox.
Premixed solutions of Na2CO3 in degassed water (2.0 mL) and internal standard 1-butanol (1-
BuOH, 1 L) was added externally through a septum in order to retain the inert atmosphere in the
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autoclave. The autoclaves were then flushed 6 times with H2 or N2 prior to pressurizing the vessel
to the desired pressure (5 MPa). Stirring was provided by a magnetic stirring plate set to 750 rpm
and the autoclaves were heated to the appropriate temperature with a band heater connected to a
programmable controller (Eurotherm 2408). Heating and stirring were stopped after completion of
the reaction and pressure was released carefully. Catalyst was separated by centrifugation and the
reaction liquor was analyzed by an Interscience Focus GC-FID over a Stabilwax-DA column (30
m x 0.53 mm I.D., df = 1.00 μm). Product quantification was based on mass response factors
determined by five-point calibration lines of commercial compounds, referenced to 1-BuOH as
internal standard. PDHMF calibration was performed using the as-synthesized compound.
Results and discussion
Effect of catalyst composition on 5-HMF hydrogenation in dilute solutions
To address PGM scarcity, we evaluated nickel catalysts for the hydrogenation of 5-HMF. Titania
is a useful support in this kind of catalytic chemistry, as it can improve the hydrogenation
performance of supported metals with respect to aldehydes, carboxylic acids and esters.24–26 We
prepared a range of nickel-rhenium catalysts supported on titania by straightforward wetness
impregnation followed by drying and reduction (see Experimental methods, Supplementary Tables
S2 – S4, and Supplementary Figures S1 – S3 in the Supporting Information). The resulting
materials will be denoted by the supported metal(s) for the sake of brevity. Under benign
conditions (5-HMF, 20 mg; H2O, 2 mL; catalyst, 10 mg catalyst, molar ratio 5-HMF/Ni = 31);
temperature 40 °C; P 5 MPa; time 4 hours), the nickel-containing catalysts can almost completely
2
H
convert 1 wt% 5-HMF in water into BHMF and BHMTHF. The product distribution depends on
the atomic Re/Ni ratio (Figure 1). Ni and NiRe0.5 predominantly lead to BHMF formation, while
NiRe1 and NiRe2 are significantly more selective to BHMTHF. Re is poorly active in comparison
to the other catalysts and only gives BHMF.
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Figure 1. Evaluation of titania-supported Ni, NiRex and Re catalysts in the hydrogenation of
aqueous 1 wt.% 5-HMF solutions.
From these results, we conclude that ring hydrogenation by nickel is greatly enhanced in the
presence of rhenium promoter when the atomic Re to Ni ratio exceeds 0.5. The difference among
Ni, Re and NiRex catalysts towards hydrogenation of the formyl group and the furan ring can be
attributed to the activation behavior of 5-HMF and BHMF. Ni and Re sites in respectively pure Ni
and Re catalysts only activate the formyl group of 5-HMF, which leads to selective BHMF
formation. Considering the potential mechanism, the formyl group is activated through its
association to nickel, forming an η2(C,O)-aldehyde complex (Figure 2, blue box).27,28 The actual
hydrogenation is proposed to proceed through sequential addition of a hydride and a proton. This
infers that molecular hydrogen dissociates heterolytically, which has been postulated for these
catalysts in several earlier works.29,30 The oxidic species present on the catalyst are expected to aid
in the heterolytic dissociation of hydrogen This effect has been well-investigated by the group of
Tomishige for a variety of metal oxide-Ir/SiO2 catalysts.31 Side-reactions arising from the nickel-
formyl interaction include decarbonylation and hydrodeoxygenation. These respectively occur
after forming a η1(C)-acyl species through cleavage of the formyl C-H bond (Figure 2, top red
box) or after breaking the formyl C-O bond succeeding its hydrogenation (Figure 2, rightmost red
box). Both side-reactions only take place at higher temperature and were not encountered in our
test reactions.32–35 For rhenium, the exact binding mode is unknown. However, the group of
Shimizu found a nearly quantitative hydrogenation of 3-phenylpropionic acid towards 3-
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phenylpropanol with titania-supported rhenium as catalyst and postulated the preferential
adsorption of the carboxylic acid group over the phenyl moiety on oxophilic rhenium as one of the
reasons for the high selectivity towards the aromatic alcohol.36,37 The mechanism assisted by
oxophilic rhenium was fully supported by DFT studies.
Figure 2. Proposed pathways for C-O hydrogenation (blue), furanic ring hydrogenation (yellow),
and high-temperature decarbonylation and deoxygenation (red).
The promoting effect of rhenium therefore lies in its oxophilic nature, readily forming
complexes with the formyl oxygen and, potentially, the furanic oxygen.38 Unlike 3-
phenylpropionic acid, the formyl and furan ring are conjugated. Strong polarization of the formyl
group by rhenium can thus activate the furan ring, promoting ring hydrogenation by forcing the
ring in the required orientation. This effect only occurs on the nickel-containing catalysts,
however, and this underlines the promoting role rhenium displays under the employed conditions
as rhenium by itself shows poor activity compared to the other materials and is unable to
hydrogenate the furan ring. Studies performed by the group of Rosseinsky conclude that the
formation from BHMTHF proceeds through BHMF.39 This is in line with our findings, as
BHMTHF was never encountered in absence of BHMF in any of our reactions starting from 5-
HMF. A similar pathway has been found by the group of Tomishige for hydrogenation of furfural
to tetrahydrofurfuryl alcohol, which proceeds through furfuryl alcohol as intermediate.40 Both
groups found that the selectivity towards the saturated alcohol depends on the conversion of the
unsaturated aldehyde. With our materials we find that both the furan ring and the formyl group in
5-HMF can be stabilized and activated simultaneously only on bimetallic NiRex catalysts and with
a high Re loading (Re/Ni atomic ratio > 0.5), thus promoting the hydrogenation of both of these
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functional groups. As a consequence, BHMTHF is the dominant reaction product over NiRe1 and
NiRe2 catalysts (Figure 2, yellow box). This behavior is comparable to that of the Ir-ReOx/SiO2
catalysts investigated by Tomishige et al. In their work, which involves the selective
hydrogenation of crotonaldehyde to crotyl alcohol among other α,β unsaturated alcohols, they
propose that oxidic species activate the aldehyde group of the reactant and aid in the heterolytic
dissociation of hydrogen into protons and hydrides.17,31 The hydrides formed in this manner are
especially reactive towards polar unsaturated groups such as aldehydes, leading to the formation
of alcohols. Their family of MOx-Ir/SiO2 catalysts, however, are not particularly active for C=C
hydrogenation in contrast to the materials studied in this work. We postulate that this is due to the
presence of nickel instead of iridium, as nickel, palladium and platinum-based catalysts generally
excel at C=C hydrogenation. However, supported monometallic Ni, Pd and Pt catalysts are not
overly active towards 5-HMF furan hydrogenation at moderate temperatures (Table S1, entries 1,
3 and 5) and selectivity towards BHMTHF strongly benefits from synergistic effects resulting from
bimetallic systems (Table S1, entries 2 and 4). A combination of these effects likely explains our
findings in Figure 1.
Table 1. Evaluation of Ni/TiO2, NiRe1/TiO2 and Re/TiO2 catalysts in the hydrogenation of
aqueous 1 wt% BHMF solutions. [a]
Catalyst
Temp.
XBHMF
YBHMTHF
Yunknown
C.B. [b]
(°C)
(%)
(%)
(%)
(%)
Ni
40
40
90
40
5.1
4.6
n.d.
2.9
5.1
1.7
94.9
98.3
89.5
95.4
NiRe1
NiRe1
Re
67.8
4.6
57.3
n.d.
10.5
4.6
[a] Reagents and conditions: Catalyst (10 mg), BHMF (20 mg, molar ratio BHMF/Ni = 31), H2O (2 mL), 1-butanol
(1 μL), H2 (5 MPa), 40 °C, 4 hours, 750 rpm.
[b] Carbon balance.
Control experiments using BHMF gave us an important insight into these reaction mechanisms
(Table 1). Ni, Re and NiRe1 catalysts are inactive for ring-hydrogenation of BHMF at 40 °C in a
dilute solution (1 wt%), whereas the NiRe1 catalyst can promote hydrogenation of both the furan
ring and the formyl group in 5-HMF under the same reaction conditions, yielding BHMTHF
(Figure 1). Therefore, the interaction of the formyl group in 5-HMF with oxophilic Re site could
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induce additional interaction between the furan ring and active Ni site of NiRe1, which leads to
simultaneous hydrogenation of these two functional groups. Due to a lower affinity of the furan
ring with both Ni and Re species, the interaction between the nucleophilic oxygen atom in the
formyl group and oxophilic Re promoter present in close proximity with active Ni, confirmed with
HAADF-STEM and EDS (Supplementary Figure S3), can be identified as the main driving force
for ring hydrogenation. One exception is noted during a high-temperature reaction: direct ring-
hydrogenation of BHMF with NiRe1 can be achieved at 90 °C (Figure 2, yellow box). In order to
develop a more efficient process for BHMF production with high productivity, we next increased
the initial 5-HMF concentration to 10 wt% and conducted the reactions without changing other
parameters including reaction temperature, catalyst weight, hydrogen pressure, and so on. This
means that the reactions were carried out at a ten-fold ratio of substrate to catalyst (5-HMF/Ni).
Table 2 summarizes the activities in the hydrogenation of 10 wt% 5-HMF solution (5-HMF, 200
mg; H O, 2 mL; catalyst, 10 mg catalyst, molar ratio 5-HMF/Ni = 310; temperature 40 °C; P 5
2
H
2
MPa; time 4 hours).
Activity tests using concentrated 5-HMF solutions
Table 2. Evaluation of Ni/TiO2, NiRex/TiO2 and Re/TiO2 catalysts in the hydrogenation of
aqueous 10 wt% 5-HMF solutions. [a]
Entry
Catalyst
Cat.
X5-HMF
YBHMF
YBHMTHF
C.B. [b]
(mg)
(%)
(%)
(%)
(%)
1
2
3
4
5
6
7
8
Ni
10
10
10
10
10
10
100
10
11.2
21.5
9.87
18.1
25.5
2.1
< 0.1
< 0.1
< 0.1
34.5
< 0.1
<0.1
2.6
87.7
84.3
85.5
66.3
84.7
73.9
88.8
85.5
NiRe0.5
NiRe1
NiRe1
NiRe2
NiRe2
NiRe2
Re
29.9
70.5 [c]
53.8
45.6
72.1
83.3
4.5
98.2 [d]
97.2
5.3
< 0.1
[a] Reagents and conditions: 5-HMF (200 mg, molar ratio 5-HMF/Ni = 312 or 31), H2O (2 mL), 1-butanol (1 μL),
H2 (5 MPa), 40 °C, 4 hours, 750 rpm.
[b] Carbon balance.
[c] Conditions as [a] but reaction was extended for 8 hours and heated to 90 °C from 40 °C for this period
[d] Conditions as [a] but with addition of 0.5 eq. of Na2CO3
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A drastic increase in 5-HMF/Ni ratio to 312 resulted in a decrease in the performance of all
studied catalysts (entries 1, 2, 3, 5, and 8) compared to those tested at a 5-HMF/Ni ratio of 31 in
Figure 1. Monometallic Ni and Re showed lower activity than bimetallic catalysts with respect to
5-HMF conversion and BHMF yield. NiRe2 catalyst showed the highest activity (46% of BHMF
yield at 54% of 5-HMF conversion) among the catalysts tested. Interestingly, NiRe1 and NiRe2
gave only BHMF in concentrated 5-HMF solution (hydrogenation limited to the formyl group),
while simultaneous hydrogenation of the furan ring and the formyl group leading to BHMTHF
was predominant in diluted 5-HMF solution (Figure 1). This tendency for no-BHMTHF formation
did not change, even after the increase of the 5-HMF/Ni ratio to 31 in a NiRe2-catalyzed reaction.
In this case an acceptable BHMF yield of 83% was obtained at nearly complete 5-HMF conversion
(97%, entry 7). We hypothesize this change is due to the molecular orientation of adsorbed 5-HMF
on the surface of NiRe1 and NiRe2 in concentrated solution. Interaction of the furan ring with the
active Ni sites is limited due to steric hindrance, i.e. when the NiRex surface is fully covered with
5-HMF and/or BHMF in concentrated solution. This prevents the formation of BHMTHF by
simultaneous hydrogenation of the furan ring and the formyl group (Figure 3).
Figure 3. Proposed 5-HMF orientations in dilute (1 wt.%) and concentrated (10 wt.%) solutions
over NiRe1 and NiRe2 bimetallic catalysts.
The reaction time can be shortened by increasing the reaction temperature. However, a high
temperature reaction at 90 °C using NiRe1 is not efficient for BHMF formation because of the
simultaneous hydrogenation of furan and formyl moieties, resulting in a moderate BHMTHF yield
(34.5%) (entry 4). It should be noted that carbon balances for entries 1, 2, 3, 5, 7, and 8 of Table 2
are in the 84-88% range, which is significantly lower than those in the 97-99% range obtained
from the hydrogenation of 1 wt% 5-HMF solution (Figure 1). Low carbon balances can be
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explained by the formation of undesired humin oligomers in concentrated aqueous solutions of 5-
HMF, which was also evident from the yellow coloration of reaction mixtures and the formation
of a brown solid residue (insoluble humins), even without catalyst or in the presence of the bare
titania support. Presence of humins on the catalytic surface was verified with TGA (Figure 4).
Fresh materials showed practically no mass loss, while catalysts used in the hydrogenation of 1
wt% 5-HMF and 10 wt% 5-HMF lost roughly 10% and 20% of their mass, respectively. Therefore,
we conclude that the hydrogenation of concentrated 5-HMF solution with NiRe bimetallic
catalysts is not applicable for efficient BHMF production, because the catalysts most probably
deactivate before reaching full 5-HMF conversion in these runs due to deposition of humins.
Figure 4. TG curves showing relative mass loss of fresh and spent Ni, NiRex and Re used for the
hydrogenation of 1 wt.% and 10 wt.% 5-HMF. TG curve sets are offset by -5% (1 wt.% 5-HMF)
and -10% (10 wt.% 5-HMF) for clarity.
Catalyst screening using concentrated PDHMF solutions
We extended 5-HMF hydrogenation using NiRe bimetallic catalysts to PDHMF and aimed for
a more sophisticated BHMF process. Initial screening using 10 wt% PDHMF was performed in
pH-neutral environment in order to facilitate the required deprotection. The results are summarized
in Table 3. The strategy worked very well as we were able to reach high conversion of both
protected 5-HMF (as PDHMF) and 5-HMF itself, which is now considered an intermediate. The
bimetallic catalysts outperform both Ni/TiO2 and Re/TiO2 monometallic catalysts, which produce
only small amounts of hydrogenated products.
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Table 3. Evaluation of Ni, NiRex and Re catalysts in the hydrogenation of aqueous 10 wt.%
PDHMF solutions.[a]
Catalyst
XPDHMF
Y5-HMF YBHMF YBHMTHF C.B. [b] PDO Rec. [d]
(%)
(%)
57.5
8.5
(%)
(%)
< 0.1
5.0
(%)
(%)
Ni
98.3
94.4
94.7
99.2
98.2
18.7
78.7
77.1
78.8
5.87
78.0
97.8
91.7
87.4
74.7
93.8
91.8
83.0
76.5
92.3
NiRe0.5
NiRe1
NiRe2
Re
4.5
4.7
1.1
6.1
66.5
< 0.1
[a] Reagents and conditions: Catalyst (10 mg), PDHMF (200 mg, molar ratio PDHMF/Ni = 213), H2O (2 mL), 1-
butanol (1 μL), H2 (5 MPa), 40 °C, 4 hours, 750 rpm.
[b] Carbon balance.
[c] PDO recovery presented as a sum of unreacted PDHMF and free 1,3-propanediol. PDO originates from
deprotection of PDHMF during the course of the reaction. It is excluded from the carbon balance.
The bimetallic catalysts are very selective to BHMF and only small amounts of BHMTHF are
produced in all cases, and similar results were also obtained in the test reactions with concentrated
5-HMF solutions (Table 2). However, we do observe a decaying trend in both carbon balance and
1,3-propanediol (PDO) recovery. Peculiar here is that the selectivities to the hydrogenation
products are very similar even with decaying carbon balances. However, a viable explanation is
that rhenium oxides tend to be Brønsted acidic or, alternatively, Brønsted acid sites can be
generated from metallic rhenium in contact with water, which yields acidic hydroxyl species due
to the oxophilicity of rhenium.38,41,42 It is therefore possible that higher levels of rhenium will
increase deprotection rates, as acetals are acid labile. This makes the deprotected 5-HMF
vulnerable to degradation, a process which itself is enhanced by higher levels of Brønsted acidity.
Additionally, PDO by itself does not degrade when in contact with NiRe2 catalyst, although up to
35% of the PDO deteriorates during hydrogenation of 5-HMF (Supplementary Table S5).
However, no strong PDO deterioration was detected when replacing 5-HMF with BHMF. This
indicates that PDO can form side-products with 5-HMF when exposed to elevated levels of
Brønsted acidity, but not with BHMF.
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Optimization of acetal deprotection rate by varying pH
The screening above showed that NiRe0.5 is the preferred catalyst to produce BHMF, as it
combines a decent BHMF selectivity with very good carbon balance and PDO recovery (Table 3).
However, a fair amount of 5-HMF is also recovered, which means the rate of deprotection is too
high. This is not optimal as this liberated 5-HMF can still engage in side-reactions. As acetal
stability depends on the alkalinity of the solution, the rate of deprotection can be controlled by
careful adjustment of the pH. We tested the addition of various amounts of Na2CO3 to the reaction
liquor, expressed as equivalents (eq.) to PDHMF. The results are summarized in Figure 5 and
complete data is provided in Supplementary Table S6. The influence of base is significant. At 0.1
eq. of Na2CO3, we were only able to reach a PDHMF conversion of 53%. No trace of 5-HMF was
found, meaning that any liberated 5-HMF is immediately hydrogenated, but the yields of BHMF
and BHMTHF were very low. Curiously, we discovered the emergence of an unexpected side-
product at high Na2CO3 eq., which we identified as the 1,3-propanediol acetal of 5-
hydroxymethyltetrahydrofuran (PDHMTHF) by NMR (Supplementary Table S6 and Fig. S4). It
appears that under these conditions, ring hydrogenation is possible but not preferred, surmised
from the low conversion. This is in contrast to our verification experiments to hydrogenate pure
BHMF (Table 1). This indicates that the activation barrier for PDHMF ring-hydrogenation might
be lower than for BHMF hydrogenation, but is still difficult to overcome. A plausible mechanism
for the PDHMF ring-hydrogenation is that the molecule can freely rotate over the furan-acetal
bond, which might aid in finding an orientation where the ring can be hydrogenated. In contrast,
5-HMF cannot freely rotate over the furan-formyl bond as the formyl C=O bond is conjugated to
the furan ring, which forces the formyl group planar to the furan ring through resonance.
The high stability of the acetal group towards deprotection at higher pH thus suppressed the
deprotection step which causes the reactive formyl group to be unavailable for hydrogenation.
Consequently, this resulted in gradual and direct ring-hydrogenation of PDHMF to PDHMTHF.
We conducted the hydrogenation of 5-HMF in the presence of Na2CO3 (0.5 eq.) as a control
experiment (Table 2, entry 6) in order to measure the effect of Na2CO3 on ring-hydrogenation of
5-HMF. Compared to the result without addition of Na2CO3 (entry 5), BHMTHF selectivity was
not increased, although the BHMF yield was significantly improved (72%, Table 2, entries 5 and
6). The carbon balance was negatively affected, dropping from 85% to 74%, which indicates that
addition of base did not suppress degradation of 5-HMF.
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From 0.1 to 5 x 10-3 eq. Na2CO3, the conversion remained relatively stable, but formyl
hydrogenation is gradually favored over furan ring-hydrogenation as we could observe increasing
BHMF and decreasing PDHMTHF yields. This indicates how sensitive the acetal is to the pH,
which only drops from 11.2 to 10.7 over this range. Near-full conversion was achieved (>99%) by
lowering the amount of Na2CO3 even further to 1 x 10-3 eq., which also significantly increased the
BHMF yield (89%). The small amount of base was, however, still able to mediate the deprotection
rate, as far less 5-HMF was recovered compared to the uncontrolled reaction. This translates in a
slightly higher carbon balance. The promoting role of rhenium under the optimized conditions was
verified by replacing NiRe0.5 by Ni. Using the same conditions, we recovered 56% 5-HMF and
obtained a BHMF yield of just 34%. There are several reports on selective BHMF formation with
> 98% yield at commercially relevant concentrations as summarized in Table S1, but most of them
requires Pd, Pt, and Ir-based catalysts. The use of PDHMF enables selective production of BHMF
over a PGM-free catalyst (Ni-ReOx) in concentrated solutions, which is clearly distinct from other
approaches reported in previous papers.
HO
O
O
High x eq.
Low x eq.
O
O
HO
O
HO
HO
OH
HO
OH
O
O
O
O
PDHMF
(200 mg)
+
O
PDHMTHF
5-HMF
BHMF
BHMTHF
NiRe0.5 (10 mg); x eq. Na2CO3
H2O (2 mL); H2 (5 MPa); 40 °C; 4 h
Figure 5. Conversion and product distribution as function of Na2CO3 equivalencies for the
hydrogenation of aqueous 10 wt.% PDHMF solutions using NiRe0.5 as catalyst. Carbon balances
are marked with black squares. Teal diamond markers indicate 1,3-PDO recovery as a sum of
unreacted PDHMF, PDHMTHF and free 1,3-propanediol. 1,3-PDO originates from deprotection
of PDHMF during the course of the reaction. It is excluded from the carbon balance.
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Tuning the acetal deprotection rate to the rate of hydrogenation is paramount to obtain high
conversion and optimal BHMF yield and selectivity. To gain more insight in these rates, we
conducted a kinetic study at 2 different Na2CO3 eq. The results are presented in Figures 6(a) and
6(b). For the optimized amount of base (1 x 10-3 eq. Na2CO3, Figure 6(a)), PDHMF is gradually
deprotected over the entire duration of the reaction and liberated 5-HMF is almost immediately
consumed. After 3 hours, the rate of hydrogenation starts to lag behind the deprotection rate as the
amount of free 5-HMF approaches 8%. However, at this point nearly 80% of the PDHMF is
consumed, allowing the hydrogenation rate to catch up again and, after 4 hours, PDHMF is fully
consumed and about 1% of 5-HMF remains. BHMF hydrogenation under these conditions is very
unfavorable, as already mentioned, and this explains the very high BHMF selectivity. Removal of
the catalyst after 2 hours of reaction ceases production of BHMF, confirming the heterogeneous
nature of the reaction. A higher base amount of Na2CO3 (0.05 eq.) significantly attenuates the
PDHMF conversion rate (Figure 6(b)). BHMF and BHMTHF yields are almost negligible and no
5-HMF is produced. Instead, the main product is PDHMTHF, formed by ring hydrogenation of
PDHMF. Here, we can conclude that furan ring-hydrogenation can occur even when the acetal is
still present. However, this reaction is not preferred and no acetal deprotection occurs, even after
the ring has been hydrogenated as neither BHMTHF or 5-HMTHF were detected. In absence of
H2, PDHMF conversion is very low and 5-HMF is in equilibrium with PDHMF (Supplementary
Fig. S5). Initially, the amount of 5-HMF recovered vs. the amount of converted PDHMF is fairly
similar. However, after 4 hours, we can see the 5-HMF yield leveling off, suggesting formation of
side-products.
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(a)
(b)
HO
O
O
O
HO
OH
O
HO
HO
O
O
O
O
O
HO
OH
O
Figure 6. Time courses of BHMF formation from PDHMF with NiRe0.5 using (a) 1 × 10-3, and (b)
5 × 10-2 eq. Na2CO3. Lines provide guidance for the reader’s eye. Reagents and conditions:
Catalyst (10 mg), PDHMF (200 mg, molar ratio PDHMF/Ni = 214), H2O (2 mL), 1-butanol (1
μL), H2 (5 MPa), 40 °C, 750 rpm. Dashed line and transparent symbols (a) show BHMF yield after
catalyst removal.
Figure 7 shows the proposed reaction pathway. The acetalization reaction is an equilibrium
reaction. Deprotection of PDHMF proceeds gradually under optimized conditions as shown in
Figure 6(a). Although weakly Brønsted acidic hydroxyls and Lewis acid sites on the surface also
contribute to deprotection steps, we expect that the rates for initial and second hydrolysis steps,
giving hemiacetal of PDHMF and bare 5-HMF, are strongly dependent on the pH of the reaction
mixture. There are three compounds (i.e. PDHMF, the hemi-acetal of PDHMF and bare 5-HMF)
in the reaction mixture. The concentrations of these compounds approach an equilibrium under the
reaction conditions as confirmed in Supplementary Figure S5, when no hydrogenation takes place
in the absence of H2. However, the concentration of PDHMF decreases in the course of the
hydrogenation reaction, because 5-HMF is now involved in the hydrogenation. In theory, an attack
from a hydride species (depicted in gray) can break the C-O bond of the hemiacetal but
(hemi)acetals are only very weakly electrophilic, so this reaction is very unlikely; therefore,
hydrogenation of bare 5-HMF can be determined as a main path for efficient BHMF formation.
Due to a short life time of bare 5-HMF and high stability towards hydrogenation with NiRe0.5
catalyst, most of 1,3-propanediol used as protecting reagent can be recovered after the reaction
and used repeatedly for subsequent reactions.
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+1,3-PDO
+OH-
Z = Furanic moiety
H
O
H
HO
O
HO
OH
HO
O
O
H
H
Z
O
O
O
H+
H
O
O
O
H
H
PDHMF
OH-
+1,3-PDO
H
H
+OH-
H
O
+H+
M
H
Figure 7. Proposed reaction pathway for hydrogenation of PDHMF via 5-HMF as intermediate.
Table 4. Hydrogenation of 20% PDHMF with NiRe0.5 using optimized base loading. [a]
[b]
XPDHMF Y5-HMF YBHMF YBHMTHF YPDHMTHF
C.B. [c] PDO Rec. [d]
t
(h)
(%)
(%)
20.9
5.2
(%)
(%)
1.3
2.1
(%)
2.6
2.0
(%)
(%)
4
6
88.8
98.7
57.4
80.9
93.4
91.4
95.6
90.2
[a] Reagents and conditions: Catalyst (10 mg), PDHMF (400 mg, molar ratio PDHMF/Ni = 428), H2O (2 mL), 1-
butanol (1 μL), H2 (5 MPa), 40 °C, 1 × 10-3 equivalence of Na2CO3 to PDHMF, 750 rpm.
[b] 1,3-propanediol acetal of 5-hydroxymethyltetrahydrofurfural.
[c] Carbon balance.
[d] PDO recovery presented as a sum of unreacted PDHMF, PDHMTHF and free 1,3-propanediol. PDO originates
from deprotection of PDHMF during the course of the reaction. It is excluded from the carbon balance.
Inspired by these results, we decided to test the limits of the optimum catalyst and the protection
method by using a loading of 20 wt% PDHMF and an optimized Na2CO3 (1 x 10-3 eq.). The results
are presented in Table 4. After 4 hours of reaction we recovered 11% of unreacted PDHMF and a
substantial amount of 5-HMF (21%) and BHMF (57%). However, nearly all PDHMF was
converted (99%) and 5-HMF levels dropped to 5% after extending the reaction to a total of 6 hours.
More importantly, the BHMF yield increased to 81% with satisfactorily low BHMTHF and
PDHMTHF yields and a carbon balance of 91%.
Conclusions
We used titania-supported rhenium-promoted nickel catalysts for the selective hydrogenation of
5-HMF to BHMF and BHMTHF in water at low temperature. Rhenium adopts several promoting
roles in this catalytic chemistry. In dilute 5-HMF solutions (1 wt%), rhenium promotes the
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hydrogenation of the furan ring and full conversion can be reached with all nickel-containing
catalysts with carbon balances exceeding 95%. In more concentrated solutions of 5-HMF (10
wt%), higher loadings of rhenium increase the overall activity of the catalyst, although full
conversion was never reached due to humin buildup on the catalyst surface. With NiRe2, the most
active material, 54% conversion was obtained with a BHMF yield of 46% and a carbon balance of
85%. BHMTHF was not produced in this case, which we attribute to competitive adsorption
between unreacted 5-HMF and BHMF still adsorbed to the catalytic surface and it is expected that
this locally high concentration of 5-HMF also promotes the formation of humins. The benefit of
using rhenium on the overall activity is more prominent when a concentrated solution (10 wt.%)
of PDHMF, the acetal of 5-HMF with 1,3-propanediol, is hydrogenated, as all bimetallic NiRe
catalysts can fully convert the substrate unlike their monometallic counterparts. Although all
rhenium-promoted catalysts produce BHMF and BHMTHF in nearly equal amounts in pH-neutral
solutions, NiRe0.5 is identified as the optimal catalyst, as it produces the lowest amount of side-
products. The carbon balance, although improved compared to 10 wt.% 5-HMF (92% vs. 85%), is
still suboptimal, which is caused by degradation of the relatively high amounts of unprotected 5-
HMF (9%) formed during the reaction. The reason is that the deprotection rate was too high
compared to the hydrogenation rate at the neutral pH. We discovered that the deprotection rate can
be attenuated by increasing the pH. Too high pH leads to a too low deprotection rate, essentially
blocking the formation of the formyl and, instead, the slow and undesired hydrogenation of the
furan ring of PDHMF occurs. Careful balancing of the deprotection and hydrogenation rates by
judicious choice of the pH proved to be the key in selectively producing BHMF in high yields (81
– 89%) using concentrated solutions of PDHMF (10 – 20 wt%) in water.
Acknowledgements
The authors would like to thank M.G.W.M. Verhoeven for support with XPS processing. This
work was supported by The Netherlands Center for Multiscale Catalytic Energy Conversion
(MCEC), an NWO Gravitation programme funded by the Ministry of Education, Culture and
Science of the government of The Netherlands.
Competing interests
The authors declare no competing interests.
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89x51mm (600 x 600 DPI)