One-pot reduction of 5-hydroxymethylfurfural via hydrogen transfer from supercritical methanol

Article abstract of DOI:10.1039/c2gc35667h

Catalytic conversion of HMF to valuable chemicals was achieved over a Cu-doped porous metal oxide in supercritical methanol. The hydrotalcite catalyst precursor is prepared following simple synthetic procedures, using inexpensive and earth-abundant starting materials in aqueous solutions. The hydrogen equivalents needed for the reductive deoxygenation of HMF originate from the solvent itself upon its reforming. Dimethylfuran, dimethyltetrahydrofuran and 2-hexanol were obtained in good yields. At milder reaction temperatures, a combined yield (DMF + DMTHF) of 58% was achieved. Notably, the formation of higher boiling side products and undesired char from HMF is not detected under these reaction conditions.

Full text of DOI:10.1039/c2gc35667h

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PAPER
One-pot reduction of 5-hydroxymethylfurfural via hydrogen transfer from
supercritical methanol†
Thomas S. Hansen,a,b Katalin Barta,*a Paul T. Anastas,a Peter C. Fordc and Anders Riisager*b
Received 2nd May 2012, Accepted 9th July 2012
DOI: 10.1039/c2gc35667h
Catalytic conversion of HMF to valuable chemicals was achieved over a Cu-doped porous metal oxide in
supercritical methanol. The hydrotalcite catalyst precursor is prepared following simple synthetic
procedures, using inexpensive and earth-abundant starting materials in aqueous solutions. The hydrogen
equivalents needed for the reductive deoxygenation of HMF originate from the solvent itself upon its
reforming. Dimethylfuran, dimethyltetrahydrofuran and 2-hexanol were obtained in good yields. At
milder reaction temperatures, a combined yield (DMF + DMTHF) of 58% was achieved. Notably, the
formation of higher boiling side products and undesired char from HMF is not detected under these
reaction conditions.
copper–ruthenium catalyst.10 Leitner and coworkers have devel-
Introduction
oped highly selective iridium catalyzed decarbonylation of HMF
to furfuryl alcohol in compressed carbon dioxide.12
Obtaining transportation fuels and chemicals from renewable
feedstocks is highly desired, since the availability of fossil fuels
is limited and they release large quantities of CO2 when
combusted.1–3 One key platform compound identified for the
renewable chemicals and fuel industry4 is 5-hydroxymethyl-
furfural (HMF) which can be readily obtained from hexose
sugars.5–7 Among several options, HMF can be oxidized to
monomeric building blocks that serve as precursors for polymer
synthesis.8,9 It can also undergo reductive deoxygenation to
2,5-dimethylfuran (DMF)10 or more extensive reduction to
2,5-dimethyltetrahydrofuran (DMTHF). Both of these products
have high energy density and low volatility which makes them
suitable fuel replacements or additives.10,11 DMTHF could
further serve as a potential solvent substitute for tetrahydrofuran
(THF).7 Although this approach is attractive, selective transform-
ation of HMF to its reduced counterparts remains a challenge as
it requires a series of particular chemical transformations, invol-
ving C–O bond cleavage. Moreover formation of undesired side
products from the reactive HMF molecule should be prevented.
Recent efforts towards selective conversion of HMF involve
both heterogeneous and homogeneous catalysts. Dumesic and
coworkers presented an efficient strategy for the conversion of
fructose derived HMF to DMF by hydrogenolysis over a
Porous metal oxides (PMOs) derived from hydrotalcite-like
precursors have been successfully used in a variety of chemical
transformations, such as reductions, and can be conveniently
doped with suitable metal-ions, for example Cu2+, Ni2+, Fe3+,
Ga3+ etc.13–17 Previously, Cu-doped PMOs have proven promis-
ing in the one step–one pot depolymerization of organosolv
lignin18 by extensive deoxygenation/hydrogenation in super-
critical MeOH (Sc-MeOH).19,20 It was also demonstrated that
raw biomass can be efficiently converted to a mixture of com-
bustible liquids and gases without the formation of undesired
biochar.21 In this work we apply the same approach for the
reductive deoxygenation of HMF to DMF and DMTHF by
in situ generated hydrogen over a multifunctional Cu-PMO cata-
lyst. We demonstrate that the formation of higher boiling side
products will be similarly suppressed when using HMF as a sub-
strate. At 300 °C rapid conversion of HMF to volatile products is
achieved, affording a combined yield of 61% to DMF, DMTHF
and 2-hexanol. At milder reaction temperatures 50% DMF yield
was achieved.
Experimental
Materials
aCenter for Green Chemistry and Green Engineering at Yale,
Department of Chemistry, Yale University, 225 Prospect Street, 06520
New Haven, CT, USA. E-mail: katalin.barta@yale.edu,
kbarta.chem@gmail.com
5-Hydroxymethylfurfural (99%), Mg(CH3COO)2·4H2O (≥99.0%),
Cu(NO3)2·2.5H2O (98%) were purchased from Sigma Aldrich,
MeOH (99.8%, anhydrous) used as a reaction medium was from
Acros, MeOH (99.9%) used for washing the catalyst was from
Merck, Al(NO3)3·9H2O (98.7%), Na2CO3·H2O (p.a.) were
from J. T. Baker and NaOH (98.6%) was purchased from Mal-
linckrodt Chemicals. All chemicals were used as received
without further purification.
bCentre for Catalysis and Sustainable Chemistry, Department of
Chemistry, Technical University of Denmark, Kemitorvet, Building 207,
2800 Kgs. Lyngby, Denmark. E-mail: ar@kemi.dtu.dk
cDepartment of Chemistry and Biochemistry, UC Santa Barbara, 93106
Santa Barbara, CA, USA
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2gc35667h
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Green Chem., 2012, 14, 2457–2461 | 2457

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Cu–PMO synthesis
(30 m, 0.25 mm ID, 0.25 μm). The carrier gas was helium with a
total flow of 14 mL min−1, column pressure was 49.7 kPa.
Linear velocity was 36.1 cm s−1 and split ratio was kept at 10.
The temperature program for analysis was: 40 °C kept for 6 min,
15 °C min−1 to 60 °C kept for 2 min, 20 °C min−1 to 150 °C
and kept for 2 min and finally heated at 25 °C min−1 to 260 °C
and kept for 5 min.
X-ray powder diffraction (XRPD) experiments were per-
formed on a Bruker D8-focus X-ray diffractometer equipped
with a Cu line-focus sealed tube, a divergent beam geometer and
a NaI scintillation detector. Measurements were made with a
40 kV, 40 mA beam in the range 2θ from 3° to 80° locked
couple scan type, a step size of 0.05° and a scan speed of 10 s.
The Cu–PMO compound was measured with a scan speed of 5 s.
Fourier transformed infrared spectroscopy (FT-IR) measure-
ments were conducted on a Thermo Scientific Nicolet 6700
FT-IR instrument with a Thermo Scientific Smart Orbit,
diamond 30 000 to 200 cm−1 accessory.
The copper-containing porous metal oxide (Cu–PMO) was
synthesized by slowly adding aliquots (10 mL) of an aqueous
solution containing Al(NO)3·9H2O (18.8 g, 0.05 mol), Mg-
(CH3COO)2·4H2O (25.7 g, 0.12 mol) and Cu(NO3)2·2.5H2O
(7.0 g, 0.03 mol) in demineralized water (300 mL) along with
aliquots of 1 M NaOH (10 mL) over a period of 4 h to an
aqueous solution of Na2CO3·H2O (6.2 g, 0.05 mol) in deminera-
lized water (375 mL) preheated to 65 °C and under vigorous stir-
ring. The addition was conducted so the pH after the NaOH
addition was kept relatively constant at pH ∼ 10. Subsequently,
the mixture was left stirring for 3 days. The resulting light blue
slurry was collected by vacuum filtration and the solids were
washed with water (∼1.5 L). The filter cake was then re-
suspended in 2 M warm aqueous Na2CO3·H2O (62 g, 250 mL)
and left overnight before harvesting the Cu-doped hydrotalcite-
like compound by vacuum filtration followed by washing with
demineralized water (∼2.5 L). The resulting blue compound was
dried overnight in an oven at 100 °C and then calcined at 460 °C
for 24 h in air to yield the final Cu–PMO.
NMR experiments were conducted on a Brucker Avance
500 MHz spectrometer running Topspin 1.3 with a broadband
observe probe.
Catalytic reduction of HMF
In a typical experiment, HMF (100 mg, 0.8 mmol), Cu–PMO
(100 mg) and MeOH (3 mL) along with an internal standard
(decane, 5 μL, 25 μmol) were placed in a stainless steel bomb
(SwagelokTM, 10 mL, see picture in ESI†) and heated to 300 °C
for a specified time. After the reaction, the stainless steel bomb
was rapidly cooled in an ice bath. The bomb was then carefully
opened and the contents were filtered through a glass filter
funnel and the filter washed with MeOH (10 mL). Finally, the
liquid was analyzed by GC-FID and GC-MS.
Results and discussion
A 20 mol% copper-doped hydrotalcite (HTC) was synthesized
by co-precipitation of a mixture of Al(NO3)3, Mg(CH3COO)2
and Cu(NO3)2 with Na2CO3 and NaOH using a 3 : 1 molar ratio
of M2+ with respect to M3+. The blue colored HTC turned green
during calcination (460 °C, 24 h, see picture in ESI†) resulting
in the active porous Cu–PMO catalyst (BET surface area:
142 m2 g−1).
The HTC catalyst precursor was analyzed by XRPD (Fig. 1)
and FT-IR (ESI, Fig. S4 and S5†). XRPD revealed a highly
ordered crystalline structure with distinct hydrotalcite features
consistent with the literature.22 After calcination, a porous near-
amorphous material was identified by the lack of any charac-
teristic HTC reflections and the appearance of weak MgO and
CuO reflections (Fig. 1).22
Catalyst recycling
A typical catalytic run was conducted with 100 mg Cu–PMO,
and 100 mg HMF using 3 mL methanol. After the first run, the
reaction was stopped, and the content of the reactor was trans-
ferred to a centrifuge tube using 10 mL methanol in total. The
liquid phase was decanted, analyzed by GC-FID, and the catalyst
was additionally washed 2 times with 5 mL methanol. After cen-
trifugation, the methanol washings were discarded and the solid
was dried in a desiccator in vacuo. The dry, purple solid was
used in the next experiment. This procedure was repeated four
more times.
Catalytic runs were carried out in a 10 mL stainless steel
bomb (see ESI† for details) containing Cu–PMO (100 mg) and
HMF (100 mg) in methanol (3 mL) at 300 °C. Three main pro-
ducts were observed: DMF, DMTHF and 2-hexanol (Scheme 1).
Analysis
GC-FID measurements were performed on a Shimadzu GC 2010
plus instrument equipped with a Shimadzu SHRIX-5MS column
(30 m, 0.25 mm ID, 0.25 μm) and a Shimadzu AOC-20i auto
injector. The carrier gas was helium with a total flow of
30.9 mL min−1. The temperature program for analysis was: 30 °C
kept for 6.5 min, 15 °C min−1 to 50 °C kept for 0.5 min,
20 °C min−1 to 100 °C and 35 °C min−1 to 300 °C kept for
5 min.
GC-MS measurements were made on
GCMS-QP2010S equipped with a Shimadzu DB-5ms column
a
Shimadzu
Fig. 1 XRPD of 20 mol% Cu–HTC (top) and the corresponding cal-
cined Cu–PMO (bottom).
2458 | Green Chem., 2012, 14, 2457–2461
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Scheme 1 Proposed reaction pathway from HMF to the three main
products DMF, DMTHF and 2-hexanol.
These compounds were quantified by GC-FID‡ using calibration
curves of the pure compounds and an internal standard.
Several intermediates were identified by GC-MS especially in
the earlier stages of the reaction (ESI, Scheme S1†). These inter-
mediates were converted to the main products over time. Con-
siderable deoxygenation starting from HMF takes place, finally
resulting in the main products that are monooxygenates. Based
on these intermediates we hypothesize that the reaction proceeds
by rapid reduction of the aldehyde group in HMF followed by
hydrogenolysis of the hydroxyl functionalities. Subsequent
reduction of the furan ring of DMF results in the formation of
cis- and trans-DMTHF while ring-opening of furan under hydro-
genolysis and hydrogenation leads to 2-hexanol. Experiments
elucidating the product distribution as a function of time revealed
that the DMF yield peaked after only 30 min. All HMF was con-
sumed in 45 min resulting in a product mixture consisting of
34% DMF, 8% DMTHF and 2% 2-hexanol. Prolonged reaction
times gave full conversion of DMF to DMTHF and 2-hexanol
where yield of the latter reached its maximum after 300 min.
After 720 min, the DMF yield was <1% and DMTHF was
observed as the major product in 38% yield and 2-hexanol in
15% yield (Fig. 2). Surprisingly, 2-hexanol did not seem to arise
from the ring-opening of DMTHF but rather from DMF. Experi-
ments using DMF and DMTHF as substrates supported this
observation (ESI, Table S3†).
The three quantified products did not fully account for the
consumed starting material in terms of mass balance. A series of
volatile side-products were also found during GC analysis,
although present in low quantities. Additional GC-MS analysis
revealed that the nature of these intermediates is similar to the
main products, comprising mainly isomers of aliphatic alcohols
and tetrahydrofuran or furan derivatives. Thus, HMF was fully
converted without the formation of non-volatile side products
(ESI, Table S1†). Likely sources of these compounds are reac-
tions of main products or by-products which engage in chain
prolongation via alkylation through the in situ formed reactive
intermediates. Similar chain prolongation/alkylation reactions of
the formed primary products were observed using cellulose21
and organosolv lignin20 as substrates. To confirm that there were
no higher boiling side-products originating from the inherent
reactivity of HMF, a reaction mixture obtained after 2 h‡ was
evaporated at reduced pressure. Indeed, >98% of the products
Fig. 2 The conversion of HMF to DMF, DMTHF and 2-hexanol over
the Cu–PMO as a function of time. Conditions: HMF (100 mg,
0.8 mmol), Cu–PMO (100 mg), MeOH (3 mL), 300 °C. Each point on
the graph corresponds to a separate experiment.
Scheme 2 Proposed mechanism for the preferential formation of the
cis-DMTHF isomer over the trans-DMTHF isomer.
were volatile. In contrast, a control experiment conducted with a
Mg/Al PMO (Mg : Al = 3 : 1) not containing any Cu resulted in
<1% DMF while neither DMTHF nor 2-hexanol were detected.
However, a significant amount of high-boiling compounds was
found (ESI, Table S4†) in the form of a brownish tar-like
residue. A blank reaction with HMF in supercritical methanol in
the absence of a catalyst resulted in a similar product mixture.
Most likely, rapid deoxygenation of HMF was the main reason
why undesired side-reactions such as polymerizations and con-
densations were suppressed in the presence of Cu-doped PMO.
The cis- and trans-DMTHF isomers have distinct physical
properties, e.g. boiling points.23 Consequently, the isomers could
be distinguished by NMR experiments (ESI, Fig. S6–S8†) and
quantified separately by GC-FID. The cis-DMTHF isomer was
favored in all experiments over the trans-DMTHF isomer, which
can be explained by steric reasons. During the reduction,
addition of the second H2 molecule occurring to the same face
as the first would result in less steric hindrance, whereas the
methyl group restricts the formation of the trans-DMTHF
(Scheme 2). A molar ratio of ∼7 : 1 (cis : trans) was observed
after 45 min at 300 °C which slowly changed to ∼3 : 1 after
720 min. We speculate that this might be due to isomerization
caused by the basic catalyst through a non-ring-opened inter-
mediate. However, a more thorough study will be required to
understand this aspect in more detail.
‡DMTHF was calibrated using a 1 : 1 mixture of the cis- and trans-
isomer, which were separable by GC-FID. After 2 h of reaction with
HMF most of the reaction intermediates identified by GC-MS capable of
converting into the major products were consumed.
Although HMF was rapidly converted to a mixture of DMF,
DMTHF and 2-hexanol at 300 °C with 60% total yield, the
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Table 1 Reduction of HMF at varying temperaturesa
DMF
yield (%)
DMTHF
yield (%)
cis : trans
ratio
2-Hexanol
yield (%)
Entry
T (°C)
1
2
3
4
5
6
7
8
320
310
300
290
280
270
260
240
32
36
34
40
35
31
26
20
11
8
8
5
3
2
<1
<1
6.5
6.2
6.8
6.8
6.6
6.4
n.a
n.a
6
2
2
1
1
<1
0
0
a Reaction conditions: HMF (100 mg, 0.8 mol), Cu–PMO (100 mg),
MeOH (3 mL), 45 min.
Table 2 Reduction of HMF at milder reaction temperatures and
varying reaction timesa
Fig. 3 Experiments with different HMF to catalyst ratios using differ-
ent reaction times. General conditions: Cu–PMO (100 mg), MeOH
(3 mL), 300 °C.
Yield DMF
Time DMF yield DMTHF Totalb + DMTHF
Entry T (°C) (h)
(%)
yield (%) (%)
(%)
1
2
3
4
5
6
7
240
240
240
240
260
270
270
0.75 21
0
0
0
2
10
15
11
58
73
79
77
84
79
81
21
26
41
41
58
55
57
1.5
3
26
41
39
48
40
46
identification of the major intermediates by GC-MS measure-
ments (ESI, Scheme S6†). The product of one step reduction and
hydrogenolysis of HMF, 5-methyl-2-furanmethanol, and its
methyl ether could be identified. Its reduced counterpart, isomers
of 5-methyl-2-tetrahydrofuran methanol were also detected,
suggesting that besides direct DMF reduction, alternative path-
ways may also be the source of DMTHF. No furfuryl alcohol
was observed. The selectivity of total identified products was
high in all cases, reaching 84% at 260 °C (Table 2, entry 5).
Quantification and additional information regarding reaction
intermediates are described in the ESI, Table S2 and
Scheme S6.† In summary, milder reaction conditions afforded
cleaner product mixtures, and a 48% DMF yield together with a
total fuel output of almost 60%.
The substrate to catalyst ratio was additionally varied at
300 °C with the Cu–PMO catalyst (100 mg) and MeOH (3 mL)
by changing the substrate amount and reaction time (Fig. 3).
Here, experiments revealed that the HMF to catalyst ratio could
be increased significantly compared to the initial experiments.
Although the total yield of the three main products decreased
with increasing HMF amount (when compared at 45 min),
yields became comparable at longer reaction times. The total
yield of the three products went from 44% with a 100 mg sub-
strate loading to 27% at 300 mg after 45 min. A maximum total
yield of 61% was obtained after 2 h using 100 mg substrate.
This value decreased to 44% at 200 mg HMF loading and
31% at 500 mg HMF loading. Subjecting 500 mg HMF to a 5 h
reaction time gave a slightly higher total yield of 34%. This
demonstrates that the substrate loading can be significantly
improved. The system proved capable of converting up to
1000 mg HMF in 5 h, though decomposition products of HMF
here partially covering the bottom of the reactor and catalyst
(Fig. 3).
4
3
3
2
a Reaction conditions: HMF (100 mg, 0.8 mmol), Cu–PMO (100 mg),
MeOH (3 mL). b Selectivity of total identified components based on
GC-FID. Products are depicted in ESI, Scheme S6.†
individual DMF yield was rather moderate at this reaction temp-
erature primarily due to over-reduction. Therefore, we were inter-
ested to determine whether HMF could be reduced at milder
reaction temperatures. First we conducted experiments at temp-
eratures ranging from 240 to 320 °C for 45 min. The results are
summarized in Table 1. The highest DMF yield (40%) was
achieved at 290 °C (Table 1, entry 4), but gratifyingly the reac-
tion took place at all reaction temperatures utilized. Entries 7 and
8 (Table 1) show that at 240 and 260 °C HMF reduction to DMF
was slower, resulting in 20 and 26% product yield, respectively,
but no DMTHF or 2-hexanol was obtained.
These observations allowed for further optimization at milder
reaction temperatures. The results are summarized in Table 2. At
240 °C, the DMF yield gradually improved from 20% at 45 min
(Table 2, entry 1) to 25% at 1.5 h (Table 2, entry 2), and 41% at
3 h reaction time (Table 2, entry 3). After 4 h, slight over-
reduction to DMTHF took place and the total fuel output was
41% (Table 2, entry 4), identical to the 3 h run. At 260 °C the
DMF yield could be further improved to 48% and total fuel
output to 58% after 3 h (Table 2, entry 5). At 270 °C, 46% DMF
yield was achieved in 2 h, and a slightly lower, 40% yield in 3 h,
due to formation of 2-hexanol. Total fuel output was 53 and
57% at this reaction temperature (Table 2, entries 7 and 8).
While no 2-hexanol could be detected at 240 °C even after pro-
longed reaction times, it was present in lower quantities at 260
and 270 °C. In addition, the GC-FID traces of the product
mixtures only displayed a few main signals allowing for
Recycling experiments showed that it was possible to recycle
the catalyst over five runs using HMF with only a moderate
decrease in conversion (ESI, Table S5 and Fig. S11†). However,
to maintain the initial combined product yields of DMF and
2460 | Green Chem., 2012, 14, 2457–2461
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DMTHF longer reaction times might be needed for the latter
runs. In addition, our recent catalyst stability studies using
Cu–PMO in the conversion of lignocellulose and cellulose
revealed that no leaching occurs and that the catalyst is
Acknowledgements
This work was supported by the Danish National Advanced
Technology Foundation in collaboration with Novozymes A/S,
and the US Department of Agriculture.
sufficiently robust for
a variety of biomass conversion
reactions.21
Finally, we were interested in converting crude HMF syn-
thesized by dehydration of fructose as previously developed in
our laboratories.24 The reaction mixture was extracted with
MIBK (methyl-isobutyl ketone) as the only method of purifi-
cation, resulting in a brown sticky material after removal of the
solvent in vacuo. A slurry containing 0.5 g of this crude HMF
material in 3 mL MeOH was reacted for 5 h at 300 °C with
100 mg Cu–PMO. Impressively, the reaction proceeded and
resulted in DMF (12% yield) besides other volatile products
even though the conversion from fructose to HMF was not opti-
mized. Hence, with an optimized reaction process an even
higher DMF yield might be obtainable.
Notes and references
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In this work, we have shown that a Cu-doped PMO catalyst
obtained by the calcination of a hydrotalcite-like precursor is
capable of converting HMF into valuable and interesting chemi-
cals in supercritical MeOH, thus paving a new direction toward a
renewable chemical industry. Complete conversion of HMF to
volatile compounds could be achieved in only 45 min, without
the formation of higher boiling side products. Reduction of
HMF at 300 °C for 2 h resulted in 61% total yield of three main
products: DMF, DMTHF and 2-hexanol. The reaction conditions
were tunable and offer a degree of flexibility to the process such
that either DMF or DMTHF and 2-hexanol could be obtained as
the major reaction product. At 240 °C DMF yield reached 41%
after 3 h, and at 260 °C 48%. A total yield for DMF and
DMTHF of 58% was achieved at 260 °C after 3 h. Further in-
depth studies of the catalyst structure and optimization of reac-
tion conditions are desired to improve activity, and selectivity
toward DMF and DMTHF.
Notably, both the fructose dehydration24 as well as the
catalytic HMF conversion method described in this study could
be performed utilizing inexpensive and environment-friendly
materials (boric acid and earth abundant metals). The combi-
nation of these two novel routes would enable a simple two-step
production of DMF or DMTHF and 2-hexanol directly from
fructose. The Cu–PMO catalyst system was even able to convert
a tar-like slurry containing HMF. This could ultimately eliminate
one of the major challenges in the chemical infrastructure based
on HMF, namely purification.
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