Direct synthesis of 1,6-hexanediol from HMF over a heterogeneous Pd/ZrP catalyst using formic acid as hydrogen source

Article abstract of DOI:10.1002/cssc.201300832

A new approach is developed for hydrogenolytic ring opening of biobased 5-hydroxymethylfurfural (HMF), dehydration product of hexoses, towards 1,6-hexanediol (HDO) under atmospheric pressure. The highest yield of HDO, 43 %, is achieved over reusable Pd/zirconium phosphate (ZrP) catalyst at 413 K in the presence of formic acid as hydrogen source. In comparison with various Bronsted and/or Lewis acidic supports, the specific Bronsted acidity on ZrP support effectively accelerated the cleavage of C-O bond in a furan ring.

Full text of DOI:10.1002/cssc.201300832

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DOI: 10.1002/cssc.201300832
Direct Synthesis of 1,6-Hexanediol from HMF over
a Heterogeneous Pd/ZrP Catalyst using Formic Acid as
Hydrogen Source
Jaya Tuteja, Hemant Choudhary, Shun Nishimura, and Kohki Ebitani*[a]
A new approach is developed for hydrogenolytic ring opening
of biobased 5-hydroxymethylfurfural (HMF), dehydration prod-
uct of hexoses, towards 1,6-hexanediol (HDO) under atmos-
pheric pressure. The highest yield of HDO, 43%, is achieved
over reusable Pd/zirconium phosphate (ZrP) catalyst at 413 K
in the presence of formic acid as hydrogen source. In compari-
son with various Brønsted and/or Lewis acidic supports, the
specific Brønsted acidity on ZrP support effectively accelerated
the cleavage of CÀO bond in a furan ring.
number of steps and the need to purify the product of each
step render the overall process environmentally and economi-
cally undesirable. An effort towards one-step hydrogenation of
HMF employing severe conditions (543 K, 150 bar H2) afforded
less than 4% yield of HDO, whereas a two-step process cou-
pled with Nafion SAC13 gave overall yield of 86% HDO in
water.[14–16] Two other research groups, Chen et al.[17] and Chia
et al.,[18] produced HDO by utilizing tetrahydro-2H-pyran-2-yl-
methanol (THPM) as substrate (HMF derivative), using the
same catalyst at 393 K with 34 bar H2. High-pressure H2, how-
ever, is difficult to handle and requires an expensive facility.
Thus, an inexpensive, more practical alternative to high-pres-
sure H2 is needed.
The concept of producing materials from renewable natural
sources is well-known: people have been wearing woolen
clothing since many generations, and houses have been built
using wood since thousands of years. In this context, lignocel-
lulosic biomass is a promising renewable feedstock because it
is abundant, cheaper, and potentially more sustainable than
conventional fossil resources.[1–3] 5-Hydroxymethylfurfural
(HMF), which can be synthesized through dehydration of hexo-
ses by using various homogeneous and heterogeneous cata-
lysts, is one of the most promising platform chemicals in the
biorefinery area and is regarded as a “sleeping giant” in the
field of intermediate chemicals from renewable resources.[4,5]
HMF has been successfully transformed into various useful
chemicals that may serve as building blocks in advanced poly-
mers, such as 2,5-furandicarboxylic acid,[6] levulinic acid,[7] 2,5-
diformylfuran,[8] succinic acid,[9] and 2,5-dimethylfuran.[10]
Among other organic compounds such as tetrahydroquino-
line, methylpyrrolidine, and cyclohexene, formic acid (FA;
HCOOH) is a hydrogen donor with much potential because it
is safe and cheap.[19] It can be traditionally obtained from bio-
mass processing and produces no waste products.[20–22] FA has
been employed by many researchers for various organic trans-
formations.[10a,23–25]
The hydrogenolysis of HMF to HDO may proceed through
two key reactions: (1) deoxygenation of the furan ring (CÀO
bond cleavage) by Brønsted-acid catalytic sites, and (2) hydro-
genation of C=O and C=C bonds by the metal catalytic sites.
In this study, we use zirconium phosphate (ZrP) as acidic sup-
port; ZrP has already attracted much attention as potential
solid-acid catalyst[26–28] for the dehydration of alcohols,[29,30] iso-
merization of olefins,[31,32] hydrogenation[33] and hydrodeoxyge-
nation reactions.[34,35]
Using earlier reports as basis,[14,36] we surveyed acid-based
supports for ring cleavage and palladium as metal for hydroge-
nation. Table 1 summarizes the results for hydrogenolysis of
HMF over various Pd/support catalysts. The products obtained
from the hydrogenolysis of HMF with Pd/ZrP catalyst in the
presence of FA include 5-methylfurfural (MF), 2,5-hexanedione
(HDN), HDO, tetrahydrofuran-2,5-dimethanol (THFDM), and 2,5-
dimethylfuran (DMF). The selectivity for the formation of HDO
varies considerably among different acidic supports, and Pd/
ZrP exhibits the best catalytic activity for HDO in high yield
(42.5%) (entries 1–10). The Brønsted/Lewis acid site ratio of dif-
ferent solid acid catalysts was compared by Huber et al.[37]
Among them, ZrP has the highest ratio. Accordingly, the
reason for the high activity of ZrP is its high Brønsted/Lewis
acid site ratio as compared to zeolites, Al2O3, and SiO2–Al2O3,
thus decreasing the carbon mass balance for these catalysts.
Post-synthesis treatment of ZrP with dilute nitric acid (see Sup-
porting Information) enhances the activity, leading to an in-
Herein, we report a new approach for the production of 1,6-
hexanediol (HDO; a linear diol with two primary hydroxyl
groups) from inedible biomass. HDO is extensively used in the
production of polyesters for polyurethane elastomers, coatings,
adhesives, and polymeric plasticizers.[11] The major route for
HDO synthesis involves the hydrogenation of adipic acid or its
esters by various homogeneous or heterogeneous cata-
lysts.[12,13] Very recently, Buntara et al. employed a novel meth-
odology for converting HMF to HDO in a two- to five-step pro-
cess under high H2 pressure (10–80 bar) by using a ReOx-modi-
fied rhodium-based catalyst. The five-step process has high se-
lectivity for HDO, but a low yield (4%). Moreover, the large
[a] J. Tuteja, H. Choudhary, Dr. S. Nishimura, Prof. Dr. K. Ebitani
School of Materials Science
Japan Advanced Institute of Science and Technology
1-1 Asahidai, Nomi, Ishikawa 923-1292 (Japan)
Fax: (+81)761-51-1610
E-mail: ebitani@jaist.ac.jp
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201300832.
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active (entry 11). PdO exhibits
a high conversion of HMF but
no yield of HDO due to the for-
mation of levulinate ethyl ester
in the presence of FA (entries 11
and 12). A control experiment
without Pd catalyst affords high
HMF conversion and humin for-
mation in the presence of only
FA (entry 13). In the absence of
FA, the acidic proton on Pd/ZrP
affords HDO in 7.5% yield. On
the other hand, the absence of
both catalyst and FA fails to ach-
ieve the hydrogenolysis of HMF
to HDO (entries 14 and 15).
These results clarify the signifi-
cance of both Pd/ZrP and FA for
an efficient single-step synthesis
of HDO from HMF.[40]
Table 1. Hydrogenolysis of HMF over various Pd/support catalysts.[a]
Entry Catalyst
Mean particle HMF
HDO
HDN
THFDM MF
DMF
Carbon mass
size [nm][b]
conv. [%] yield [%] yield [%] yield [%] yield [%] yield [%] balance [%][c]
1
Pd/ZrP
11.9
92.5
96.9[d]
72
70.4
93.3
90.4
90.9
37.8 15.2
42.5[d] 16.2[d]
26.9 12.7
12.1
15.6[d]
0.0
5.1
0.0
38.2
30.1
30.6
0.0
0.0
0.0
4.6
4.8[d]
4.3
0.0
0.0
4.5
2.3
0.0
0.0
0.0
0.0
0.0
2.4 77.9
2.5[d] 84.2[d]
1.6 63.2
0.0 46.2
0.0 35.4
0.0 77.8
1.9 74.4
2
3
4
5
6
7
8
9
10
11
12
13
Pd/ZrP[e]
Pd/HY zeolite[f] 4.3
4.0
27.4
0.0
[k]
Pd/Nb2O5
Pd/ZSM-5[g]
Pd/SiO2–Al2O3
Pd/Al2O3
Pd/SO4/ZrO2
Pd/ZSM-5[i]
ZrP
Pd(NO3)2
PdO
blank
–
–
20.7 12.3
17.3 10.3
15.8 17.5
13.3 25.5
[h]
3.3
10.4
3.1
3.5
–
–
–
–
–
>99
16
85.5
39.1
54.1
50.4
61.6
86.2
78.7
84.3
79.6
5.1
3.2
0.0
0.0
1.8
0.0
0.0
7.5
0.0
0.0 13.0
0.0
0.0
0.0
9.2
0.0
0.2
25.4
0.0
trace
14.3
0.0
0.0
0.0
trace
trace
0.0
0.0
0.0
0.0
0.0
0.0
0.0
14[j] Pd/ZrP
15[j] blank
7.5 33.7
0.0 0.0
0.0 48.9
0.0 0.0
–
The course of the HMF hydro-
genolysis was monitored in
order to understand the reaction
pathway. Figure 1 is a plot of the
conversion of HMF and yields of
various products as a function of
time. After 3 h, an 85% conver-
[a] Reaction conditions: HMF (1 mmol), EtOH (3 mL), FA (22 mmol), T=413 K, t=21 h, 5 wt% Pd catalyst
(50 mg). [b] Determined by transmission electron microscopy. [c] Determined on the basis of observed HDO,
HDN, THFDM, MF, and DMF products. [d] 7 wt% Pd catalyst. [e] Without post acid treatment. [f] JRC-Z-HY5.5
(Si/Al=2.8). [g] Si/Al=12. [h] JRC-SAH-1 (Si/Al=2.1). [i] JRC-Z5-90H(1) (Si/Al=45). [j] Without FA. [k] The parti-
cles could hardly be distinguished from the support.
crease in the carbon mass balance compared to nontreated
ZrP (entries 1 and 2). This is because treating the material with
dilute acid increases the number of Brønsted sites.[38] The loss
of framework SiAlOH units with increasing Si/Al ratio is attrib-
uted to a decrease of (Brønsted+Lewis) acid site density for
ZSM-5 (Si/Al=45 or 12), HY zeolite (Si/Al=2.8), SiO2–Al2O3, (Si/
Al=2.1), and it disfavors HDO formation (entries 3, 5, 6, and 9).
Nb2O5 and sulfated ZrO2 show lower activity for hydrogenolysis
and/or hydrogenation than ZrP, therefore the carbon mass bal-
ances are very low (entries 4 and 8). On the contrary, Pd/Al2O3
catalyzes the ring hydrogenation reaction over ring hydroge-
nolysis to afford THFDM as the major product, which in turn
increases the carbon mass balance (entry 7).
sion of HMF is achieved and the primary products (HDO and
HDN) are formed. Competitive products such as MF and
THFDM are produced after 6 h, whereas DMF is obtained in
the reaction mixture after 9 h. The yield of HDO reaches a maxi-
mum of 42.5% (HMF conv. 96.9%) after 21 h.[41] Scheme 1
shows a proposed reaction pathway for hydrogenolysis of
HMF over the Pd/ZrP, based on the time course displayed in
Figure 1 that clearly shows the simultaneous conversion of
HMF into HDO, THFDM, and HDN.
Moreover, even after longer reaction times, no significant
change in product distribution is observed, indicating that ring
Particle size distributions, determined from transmission
electron microscopy (TEM) images (see Figure S6), show that
only a few different supports, such as HY zeolite, ZSM-5, SiO2–
Al2O3, and sulfated ZrO2, lead to catalysts with small-sized
(mean diameter 3.1–3.3 nm); however, the activities of these
catalysts are not so high. Accordingly, we find that the activity
has no correlation with the size of the palladium nanoparticles.
It is well-known that hydrogenation reactions do not depend
on the size of the metal particles.[39]
The hydrogenolytic ring opening of HMF demands a high
Brønsted/Lewis acid ratio, whereas the role of palladium is to
dissociate formic acid, leading to the hydrogenation. The activ-
ity of the Pd/ZrP catalyst was studied in more detail. Almost
half of the HMF could be converted with no yield of HDO in
the presence of only support (ZrP; Table 1, entry 10), probably
due to adsorption of the substrate onto the support. Pd(NO3)2,
the metal source used to prepare the active catalyst, itself is in-
Figure 1. Time course for hydrogenolysis of HMF over Pd/ZrP. Conditions:
HMF (1 mmol), FA (22 mmol), EtOH (3 mL), 7 wt% Pd/ZrP (50 mg), 413 K.
~
~
&
&
*
*
HMF Conv. ( ), Yields of HDO ( ), HDN ( ), THFDM ( ), MF ( ) and DMF ( ).
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the acidity of the ZrP surface. On addition of FA, HDO forma-
tion is enhanced with a decrease of HDN. A large amount of
acid favors a ring hydrogenation to THFDM, which can not be
cleaved further into HDO because tetrahydrofuran rings are
rather stable. This is why the reaction conditions, favoring step
B, diminish the progress of reaction step A. On the other hand,
it is known that HDN formation is favored in acidic environ-
ments,[37] but a high FA content lowers the yield of HDN be-
cause of competitive THFDM formation.
The present hydrogenolysis of HMF to HDO is proposed to
consist of 6 major steps, marked as steps 1–6 in Figure 2.
In step 1, HMF is adsorbed onto the catalyst surface by electro-
static interactions with both metal as well as the acidic ZrP
support. Step 2 is proposed based on previous literature,[37,38]
in which ZrP is held responsible for the furan ring opening
with the loss of ring oxygen. In this case, scissoring of the
furan ring would most probably form hex-1,3,5-triene-1,6-diol
(steps 2 and 3). This expected intermediate (hex-1,3,5-triene-
1,6-diol) is not commercially available but is assumed to
appear at a retention time of 6.96 min in the GC chromato-
gram. The same peak, with a high intensity, is observed in re-
actions performed without FA. The dissociation of FA (step 4)
may occur at the initial stages of the reaction, however, in
order to maintain a reaction cascade it is shown as step 4. The
product generated in step 3 can undergo keto–enol tautomer-
ism (step 5). The enol form is expected to dominate in polar
protic solvent (ethanol). The ratio of the intensity of the peak
at 6.96 min to that of an internal standard (naphthalene) to de-
creased with increasing metal loading on the catalyst. The
highest ratio occurs in the absence of FA, which is possibly
due to the absence of hydrogen source for hydrogenation of
double bonds. The reason for the appearance of the peak for
low metal loading could be explained by the inefficient dissoci-
ation of FA with low metal content on the catalyst surface. The
final step was the hydrogenation of the double bonds to form
HDO using Pd/ZrP with FA as the hydrogen source (Figure 2,
step 6).
Scheme 1. Various possible reaction pathways from HMF.
hydrogenolysis to HDO (Scheme 1, step A) or HDN (steps C, D,
and E), or ring hydrogenation to THFDM (step B) progress as
competitive reactions. MF and DMF are formed in small
amounts from HMF as hydrogenation products (steps D and E).
This proposed reaction scheme is in agreement with the ob-
served trends for THFDM, MF, DMF, and HDN as substrates
under similar reaction conditions (see Supporting Information,
Table S1); HDO formation is not noticed from the hydrogenoly-
sis of MF, DMF, and THFDM.
To certify the formation route of HDO is from hydrogenolysis
of the HMF ring or THFDM ring, the hydrogenolysis of THFDM
on Pd/ZrP was investigated under the same reaction condi-
tions. No HDO or HDN was detected (Table S1, entry 5). This
finding indicates that the reaction route from THFDM to HDO
can be practically neglected. These results are in correlation
with previous studies showing that the hydrogenolysis of a tet-
rahydrofuran ring is not possible under conditions in which
furan ring hydrogenolysis occurs.[42,43] The results of MF and
DMF as a substrate (Table S1, entries 2 and 3) suggest that MF
is hydrogenated to DMF and successively to HDN. Under the
present conditions, the rate of ring cleavage of DMF to HDN is
faster than the side chain hydrogenation of HMF to MF and
then to DMF. This is the reason why MF and DMF appears late
in the reaction progress.
Recyclability is an important property of heterogeneous cat-
alyst, so the reusability of the catalyst Pd/ZrP was investigated.
The catalyst was simply recovered by centrifugation, through
washing with ethanol, and drying in vacuum overnight, fol-
lowed by calcination at 773 K for 6 h. The catalyst could be re-
cycled without any significant loss of activity even after
5 cycles (see Supporting Information, Figure S3). Furthermore,
the product was isolated by using silica gel column chroma-
tography, eluted with hexane–ethyl acetate solvent system
(8:2–2:8) to yield HDO in 39.6% yield (0.467 g from 10 mmol
of HMF, in a set of 5 batch reactions of 2 mmol each) as
According to stoichiometry, 8 hydrogen atoms are required
for the conversion of HMF to HDO. To meet the requirement,
experiments with different amounts of FA were performed and
compiled (see Supporting Information, Table S2). In fact,
22 mmol of FA produces HDO in highest yield over 7 wt% Pd/
ZrP, whereas 8.2 mmol of FA is consumed. The highest efficien-
cy of FA is observed when using 11 mmol of FA with high
metal content that is, 10 wt% Pd/ZrP, to achieve the highest
yield of HDO (43.6%). A high-metal-content catalyst can break
the FA more efficiently than a low-metal-content catalyst.
However, the same yield of HDO can also be obtained by
using a low-metal-content (7 wt%) catalyst and a high amount
of FA. Therefore, we discourage the use of a catalyst with high
metal content. In the absence of FA, HDN is formed in large
amounts while HDO is also detected in small quantities due to
1
a white to pale-yellow solid. Analysis by H- and 13C NMR con-
firmed the HDO structure (see Supporting Information, Figur-
es S4 and S5).
In summary, we report a new, safe, economical, and environ-
mentally benign pathway for the formation of HDO from HMF;
a renewable source that can be obtained from hexoses. Using
the one-step direct reaction, we convert HMF to yield 43%
HDO over reusable Pd/ZrP catalyst and HCOOH as hydrogen
source at 413 K for 21 h under an atmospheric pressure. To the
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Figure 2. Proposed mechanistic steps demonstrating the role of Pd/ZrP and FA in hydrogenolysis of HMF to HDO in the presence of FA.
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are indispensable for the formation of HDO. It is suggested
that the acidity of the surface, for ring cleavage; the presence
of a transition metal, for FA dissociation; and thereby utiliza-
tion of in situ generated hydrogen for hydrogenation are the
important aspects governing HDO formation from HMF.
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Keywords: 1,6-hexanediol
catalysis · renewable resources · transfer hydrogenation
· formic acid · heterogeneous
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[41] These phenomena were also observed in recycling runs. A conversion
of 45% was observed even at 0.5 h of reaction progress, demonstrating
the facile adsorption of HMF onto the catalyst surface. Subsequently,
adsorbed HMF is converted into products. A significant effect on con-
version rate of HMF and HDO was noticed with additional experiments.
The rates of consumption of both HMF and HDO were suppressed in
the presence of one another, which may explain the maxima for HDO
yield (for a detailed explanation, see Supporting Information, Figure S5).
A detailed product analysis by GC-MS revealed that HDO is slowly con-
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Received: August 8, 2013
Revised: October 8, 2013
Published online on November 20, 2013
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