Selective Conversion of 5-Hydroxymethylfuraldehyde Using Cp?Ir Catalysts in Aqueous Formate Buffer Solution

Article abstract of DOI:10.1002/cssc.201501625

The highly selective hydrogenation/hydrolytic ring-opening reaction of 5-hydroxymethylfuraldehyde (5-HMF) was catalyzed by homogeneous Cp?Ir^III half-sandwich complexes to produce 1-hydroxy-2,5-hexanedione (HHD). Adjustment of pH was found to regulate the distribution of products and reaction selectivity, and full conversion of 5-HMF to HHD with 99 % selectivity was achieved at pH 2.5. A mechanistic study revealed that the hydrolysis/ring-opening reaction of 2,5-bis-(hydroxymethyl)furan is the important intermediate reaction step. In addition, an isolated yield of 85 % for HHD was obtained in a 10 g-scale experiment, and the reaction with fructose as the starting material also led to a 98 % GC yield (71.9 % to fructose) of HHD owing to the excellent tolerance of the catalyst under acidic conditions. pH dependent: A catalytic system is developed for the selective conversion of 5-hydroxymethylfuraldehyde to 1-hydroxy-2,5-hexanedione in high yield and selectivity. The Cp?Ir^III half-sandwich catalysts have an excellent tolerance to acidic aqueous conditions and can transform 5-HMF in the hydrolysis solution of fructose in excellent yield, demonstrating a potential for a large-scale production.

Full text of DOI:10.1002/cssc.201501625

DOI: 10.1002/cssc.201501625
Full Papers
Selective Conversion of 5-Hydroxymethylfuraldehyde
Using Cp*Ir Catalysts in Aqueous Formate Buffer Solution
[a]
[a]
[a]
[a, b]
[a, c]
[a]
Wei-Peng Wu, Yong-Jian Xu, Rui Zhu, Min-Shu Cui,
Yao Fu*
Xing-Long Li,
Jin Deng,* and
[a]
The highly selective hydrogenation/hydrolytic ring-opening re-
action of 5-hydroxymethylfuraldehyde (5-HMF) was catalyzed
that the hydrolysis/ring-opening reaction of 2,5-bis-(hydroxy-
methyl)furan is the important intermediate reaction step. In
addition, an isolated yield of 85% for HHD was obtained in
a 10 g-scale experiment, and the reaction with fructose as the
starting material also led to a 98% GC yield (71.9% to fructose)
of HHD owing to the excellent tolerance of the catalyst under
acidic conditions.
III
by homogeneous Cp*Ir half-sandwich complexes to produce
1
-hydroxy-2,5-hexanedione (HHD). Adjustment of pH was
found to regulate the distribution of products and reaction se-
lectivity, and full conversion of 5-HMF to HHD with 99% selec-
tivity was achieved at pH 2.5. A mechanistic study revealed
Introduction
[
10]
Rapid depletion of fossil resources becomes an inevitable issue
for the development of the chemical industry, which relies
heavily on unrenewable resources. To pursue sustainable de-
velopment, research on biomass refining, which is a promising
replacement for petroleum-derived chemicals, has grown in
1-hydroxy-2,5-hexanedione (HHD).
As 5-HMF is obtained
from the acidic catalytic dehydration of hexoses, in situ further
transformation process for 5-HMF seem more cost-effective
and environment-friendly than those following a separation
process from acidic aqueous phase. Also, water is the primary
[
1]
[11]
popularity. Biomass is considered to be the sole renewable
solvent in many biorefinery processes, and the Brønsted am-
[2]
organic carbon source, and lignocellulose is the most abun-
photeric property of water makes it possible to control the
rate and selectivity of aqueous reactions by simply regulating
[3]
dant biomass resource. To achieve the effective utilization of
biomass, it is necessary to transform lignocellulose into funda-
[
12]
pH values.
Therefore, looking into the role of pH in the
[
4]
mental organic molecules, known as platform molecules.
transformation of 5-HMF in the aqueous phase has significant
meaning for both theoretical and practical studies.
The hydrolysis product of lignocellulose-based hexoses (glu-
cose and mannose) is 5-hydroxymethylfuraldehyde (5-HMF),
The hydrolytic ring opening of furan-containing 5-HMF leads
to the formation of many important building block molecules.
[4,5]
a promising platform molecule.
5-HMF has the potential to
[
5b]
be upgraded into many important bio-derived chemicals in-
Typically, this process is carried out under acidic conditions.
[6]
cluding 2,5-dimethylfuran (biofuel molecule), 2,5-bis-(hydrox-
A common ring-opened product of 5-HMF is levulinic acid. Re-
cently, increasing attention is paid to another ring-opened
product, 1-hydroxy-2,5-hexanedione (HHD). Mentech and cow-
[
7]
ymethyl)furan (BHMF, organic synthesis building block), 2,5-
[8]
[9]
furandicarboxylicacid(polymer monomer), levulinic acid and
[
10]
orkers reported the synthesis of HHD in 60% yield by reduc-
ing 5-HMF in oxalic acid solution using Pt/C as catalyst at
[
a] W.-P. Wu, Y.-J. Xu, R. Zhu, M.-S. Cui, X.-L. Li, Dr. J. Deng, Prof. Dr. Y. Fu
iChEM, CAS Key Laboratory of Urban Pollutant Conversion
Anhui Province Key Laboratory of Biomass Clean Energy
Department of Chemistry
[
13]
1408C under 3 MPa H . Heeres and coworkers
a two-step process in which an aqueous solution of 5-HMF
reported
2
was reduced under 10 bar H for 1 h using 10 mol% of Rh–Re/
2
University of Science and Technology of China
Hefei 230026 (P. R. China)
SiO catalyst at 1208C, followed by 80 bar H for 17 h, led to
2
2
Fax: (+86)551-6360-6689
dengjin@ustc,edu.cn
E-mail: fuyao@ustc.edu.cn
full conversion of 5-HMF and the formation of HHD with 81%
[
14]
selectivity. JØrôme and coworkers reported the conversion of
fructose to HHD, which was performed in 0.3 wt% DMSO and
[
b] M.-S. Cui
Nano Science and Technology Institute
THF solvent under 20 bar H , with a combined catalyst of Pd/C
2
and Amberlyst-15; HHD in 55% yield was obtained. Later,
University of Science and Technology of China
Suzhou 215123 (P. R. China)
[
15]
JØrôme and coworkers reported that inulin, fructose, and 5-
[
c] X.-L. Li
HMF can be converted to HHD in water under CO pressure
2
School of Medical Engineering
and Key Laboratory of Advanced Functional Materials and Devices
Hefei University of Technology
over Pd/C in a one-pot process, and a 70% average yield for
each step was obtained in the conversion of inulin to HHD.
[
16]
Satsuma and coworkers reported a procedure for the pro-
duction of HHD in a yield of 60% starting from 0.067m 5-HMF
aqueous solution in the presence of 8.5 mm H PO , Au/Nb O
5
Hefei 230009 (P. R. China)
Supporting Information for this article can be found under
http://dx.doi.org/10.1002/cssc.201501625.
3
4
2
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Full Papers
[
17]
catalyst, and 8 MPa H at 1408C. Singh and coworkers per-
2
formed the catalytic reaction under the following conditions:
6
1
.0 mmol 5-HMF, 1 mol% [(h -p-cymene)RuCl(8-aminoquinoli-
ne)]Cl catalyst, 12 equivalents formic acid, and 10 mL water,
which achieved a yield of 54% HHD at 808C. Zhang and cow-
[
18]
orkers studied the hydrogenation/hydrolytic ring-opening re-
2
+
action of 5-HMF with [Cp*-Ir(2,2’-bipyridine)(H O)] catalyst to
2
produce HHD under H , and a yield of 85.5% was obtained.
2
The above works show that so far the hydrogenation/ring-
opening reaction of 5-HMF to HHD has not obtained a satisfied
selectivity. Moreover, the mechanism of acid facilitating 5-HMF
ring opening have not yet been studied in detail.
Therefore, it is necessary to study the mechanism
and modulating factors of the conversion of 5-HMF
to achieve high selectivity for HHD, and develop the
hydrogenation/hydrolytic ring-opening reaction of 5-
III
Figure 1. Cp*Ir half-sandwich complexes used in this work.
III
Scheme 1. Reaction of 5-HMF in aqueous FBS catalyzed by different Cp*Ir half-sandwich
catalysts. Reaction conditions: 5-HMF (1 mmol); catalyst (0.1 mmol, 0.01 mol% with re-
HMF with high selectivity and excellent compatibility
with the acidic aqueous phase system.
À1
spect to 5-HMF); and either a) FBS (1 molL , 5.0 mL) at 1208C for 2.0 h, b) FBS
The transformation of 5-HMF to HHD contains
a double-bond saturation process and furan ring-
opening process. Therefore, the catalytic system
should have excellent reduction ability as well as
À1
À1
(
1 molL , 5.0 mL) at 1308C for 2.0 h, or c) 1 MPa H
2
and PBS (0.1 molL , 5.0 mL) at
1308C for 2.0 h.
[19]
III
acidity. We hve previously reported that Cp*Ir half-sandwich
catalysts could bring about outstanding catalytic efficiency for
hydrogenation of furan derivatives and withstand the acidity
served starting at pH>3.0, to a peak at pH 4.5, and then de-
creased at higher pH values. The yield of HHD using different
catalysts (i.e., catalytic efficiency of different catalyst) at pH 2.5
III
of the hydrolysis system. Hence, Cp*Ir half-sandwich catalysts
are appropriate candidates for the catalytic selective conver-
sion of 5-HMF to HHD.
III
Herein, we investigated homogeneous Cp*Ir half-sandwich
complexes for the highly selective hydrogenation/hydrolytic
ring-opening reaction of 5-HMF to HHD in aqueous formate
buffer solution (FBS) under mild reaction conditions. We found
that system acidity is a very important factor that could affect
catalytic efficiency and regulate distribution of products for
a better selectivity. Also, our reaction system shows an excel-
lent tolerance to the acidic aqueous phase system, which
makes it suitable for the conversion of the hydrolysis reaction
solution of fructose. Mechanistic studies revealed the transfor-
mation path of 5-HMF and the effect of acidity on the reaction,
and provided a new research idea for effective and selective
hydrolysis of 5-HMF.
Results and Discussion
III
We evaluated the catalytic efficiency of a series of Cp*Ir cata-
lysts with different electronic effect and steric effect of sub-
stituent group on the dipyridine ligand. The catalysts men-
tioned in this work (1–7) are shown in Figure 1. Typically, 5-
HMF (1 mmol) and the catalyst (0.01 mol% ) were dissolved in
FBS (5 mL, 1 m) and reacted at 1208C for 2 h (Scheme 1, condi-
tion a). The pH of buffer solution, which varied from 0.0 to 7.0,
was found to have a notable effect on the product distribu-
tion. The major product was HHD at pH 1.5–3.5 (Figure 2, top).
The yield of HHD increases with increasing pH value from 0.5,
to a peak at pH 2.5, and then decreased at higher pH values.
HHD was not observed when pH>4.5 and the major product
was BHMF at pH 4.0–6.5 (Figure 2, bottom). BHMF was ob-
III
Figure 2. Yield of (top) HHD and (bottom) BHMF using different Cp*Ir half-
sandwich catalysts. Reaction conditions: 5-HMF (1 mmol), catalyst (0.1 mmol,
.01 mol% to 5-HMF), and FBS (1 m, 5.0 mL) at 1208C for 2.0 h. GC yield, di-
0
methyl phthalate was used as an internal standard.
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is in the order of 5>4>2>6@3>7~1. The yield of BHMF
using different catalysts at pH 4.5 is in the order of 5~6>4>
2
>1>3>7.
This result indicates that the position and electron donating
ability of the substituent have significant effects on catalyst ac-
+
tivity. The Hammett constants of the substituents (s ; i.e., the
p
[
20]
+
electron-donating ability),
is in the order of: ÀNH (s
=
2
p
+
+
À1.30)>, ÀOH (s =À0.92)>, ÀOMe (s =À0.78)>, ÀH
p
p
+
+
(
s
p
=0.00)>, and ÀCOOH (s =0.42). Therefore, strong elec-
p
tron-donating groups lead to higher catalytic efficiency. The
low catalytic effect of 7 is a result of the electron withdrawing
ability of its 4,4-dicarboxyl substituent. The activity of 2 is
lower than 4 owing to increased steric hindrance of the 2,2-
versus 4,4-dimethoxy substitution. Activity of the 2,2-dihy-
droxy-substituted catalyst 3 is lower even than 2,2-dimethoxy-
substituted catalyst 2 owing to the effect of ortho-hydroxyl
[
20a]
group.
4,4-diamido-substituted catalyst 6 provides the best
performance at pH 4.5. The rapid decline of catalyst activity at
pH<3.0 is a result of protonation of the amino groups. Under
the same conditions, the best catalysts for selective conversion
of 5-HMF to HHD are catalysts 4 and 5.
Upon further increasing the reaction temperature to 1308C
(
Scheme 1, condition b), full conversion of 5-HMF to HHD was
achieved at pH 2.5 without any by-products (Figure 3). This
demonstrates the excellent catalytic efficiency and selectivity
of our reaction system.
The effect of hydrogen sources on the reaction at 1308C
was investigated by using H as hydrogen source, for compari-
2
Figure 4. Reaction of 5-HMF to (top) HHD and (bottom) BHMF catalyzed by
son with HCOOH, and by varying the pH from 0.0 to 7.0
4
and 5. Reaction conditions: 5-HMF (1 mmol); catalyst (0.1 mmol, 0.01 mol%
to 5-HMF); and either a) FBS (1 m, 5.0 mL) or b) under 1 MPa H and PBS
0.1 m, 5.0 mL) at 1308C for 2.0 h. GC yield, dimethyl phthalate was used as
an internal standard.
(
Scheme 1, condition c). Dissolving 5-HMF (1 mmol) and cata-
2
(
lyst (0.01 mol%) in phosphoric acid/sodium phosphate buffer
solution (PBS, 5 mL, 0.1 m, pH 0.0–7.0) under 1 MPa H for 2 h
2
(
Figure 4), we found that the highest yield of HHD is lower
than half of that value when H was used as the source of hy-
improved to some extent, but it is still unsatisfactory. Consider-
2
drogen. As H is poorly water soluble, we improved the H2
ing HCOOH is decomposed to produce CO in a FBS, and the
2
2
pressure in the system. The results show that the yield can be
high pressure CO was reported to promote the hydrogena-
2
tion/hydrolysis reaction and restrain humin in the acid hydroly-
[
15,21]
sis system,
we tried to improve the yield of HHD by intro-
ducing CO (partial pressure of CO equal to H ) in this system,
2
2
2
but the results were not significantly improved (Figure S6 and
S7).
Later, we investigated the reason that leads to low yield of
HHD in PBS by detecting the change of pH with different hy-
drogen sources. When using HCOOH as hydrogen source, the
pH raised from 2.5 to 6.5 owing to the reaction of HCOOH at
1
308C for 2 h. When using H as hydrogen source, the buffer
2
solution is PBS and the pH value shows no change after reac-
tion. We investigated the stability of HHD and BHMF in differ-
ent buffer solutions. Degradation rates of BHMF are higher
than degradation rates of HHD under the same conditions,
and both degradation rates in PBS are higher than degradation
rates in FBS. The results show that the continuously increased
pH slows down the degradation of BHMF and results in
a better yield when using HCOOH as hydrogen source
Figure 3. Reaction of 5-HMF in aqueous FBS catalyzed by 4 and 5. Reaction
conditions: 5-HMF (1 mmol), catalyst (0.1 mmol, 0.01 mol% to 5-HMF), and
FBS (1 m, 5.0 mL) at 1308C for 2.0 h. GC yield, dimethyl phthalate was used
as an internal standard.
(
Table S1).
To investigate the ring-opening mechanism of 5-HMF under
acidic conditions, we proposed a possible reaction mechanism
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Scheme 2. Possible reaction paths for the ring-opening of 5-HMF under acidic conditions.
(
Scheme 2). The aldehyde group of 5-HMF was first saturated
to obtain BHMF. Then BHMF had two possible reaction paths:
A) BHMF was hydrolyzed/ring opened to produce 1-hydroxyl-
-en-2,5-hexanedione, then the C=C was saturated to obtain
HHD; or (B) BHMF underwent hydrogenolysis to produce
bond species are generated as intermediates in the reduction
process of 5-HMF, which confirm our conclusion.
(
There are three main steps in the reaction process: 1) reduc-
tion of 5-HMF to BHMF, 2) hydrolysis/ring-opening reaction of
BHMF, and 3) reduction of the ketene intermediate. The major
competitive reactions are polymerization of BHMF, decomposi-
tion reaction of HHD, and decomposition reaction of HCOOH.
Reduction of the aldehyde group, hydrolysis/ring opening of
BHMF, and polymerization of BHMF are promoted by strong
acidity, whereas catalyst 4 and 5 are more active in weakly
acidic media. At pH 4.5–7.0, reduction of the aldehyde groups
can be carried out smoothly, which is different from hydrolysis/
ring-opening reaction of the furan ring, hence leading to the
generation of BHMF as the major product. At pH<4.0, BHMF
can further react to open the furan ring, thus PBS can catalyze
the hydrolysis of furan ring with sustained strong acidity
(pH 2.5), and also lead to the decomposition of BHMF, whereas
for FBS, the pH is rising with the continuous decomposition of
HCOOH. The strong acidity in prophase of reaction facilitates
the hydrolysis/ring-opening reaction of BHMF, reducing the
concentration of BHMF and avoiding polymerization. The hy-
drogenation process was accelerated by the increase in the
catalytic activity with the increase in pH. The weak acidity in
the later stage of reaction also avoids the decomposition reac-
tion of BHMF. This balance is the key to transfer 5-HMF to HHD
in high efficiency and selectivity in FBS.
3
5
-methylfurfuryl alcohol, and then ring opening to generate
[
22]
HHD.
An important distinction between the two reaction paths is
the generation of 5-methylfurfuryl alcohol as an intermediate.
Therefore, we carried out the reaction using 5-methylfurfuryl
alcohol as the starting material to check our reaction path. If
5
-methylfurfuryl alcohol reacted to form 2,5-hexanedione
through the hydrolytic ring-opening reaction followed by hy-
drogenation, it indicates that path B is our reaction path.,
whereas if 5-methylfurfuryl alcohol reacts to form HHD via
ring-opening reaction, it indicates that path A is our reaction
path. Analysis of the product solution of 5-methylfurfuryl alco-
hol by GC–MS revealed the only product was 2,5-hexanedione
(
Scheme 3). Hence, path A is our reaction mechanism. More-
over, to confirm that BHMF is the intermediate, the hydrolysis/
ring-opening reaction of BHMF was carried out under the opti-
mal conditions for HHD (1308C, 1 m, pH 2.5 FBS, catalyst 5,
2
h). A red solid that is insoluble in water and methanol was
formed. The detection of 24.5% yield of HHD in the GC spec-
trum supports that BHMF is the intermediate of reaction. Du-
[7a]
mesic and coworkers reported that BHMF can polymerize
easily under acidic conditions. Generation of BHMF polymers
leads to a poor yield of HHD. Higher HHD yields (66.4%) were
obtained at lower temperature (1008C) owing to inhibition of
To test the recyclability and stability of the catalyst, a large-
scale experiment was performed in a 1 L autoclave (Figure 5).
5-HMF (12.6 g, 0.1 mol) and catalyst 5 (6.3 mg, 0.01 mol%)
were dissolved in FBS (500 mL, 1 m, pH 2.5) to react for 2 h at
1308C. After the system dropped to room temperature, the
gas produced by the reaction system was carefully released. A
[15]
BHMF polymerization. JØrôme and coworkers
and Zhang
[
18]
and coworkers pointed out that BHMF and exocyclic double-
9
2% GC yield of HHD was obtained. By adjusting the pH of
cooled solution to 2.5 with pure HCOOH, and then extracting
with dichloromethane, an isolated yield of 85% for HHD
(
(
11.0 g) was obtained by silica-gel column chromatography
Figure S1). The next cycle was performed by adding fresh
5
1
-HMF (12.6 g, 0.1 mol) to the extracted solution for 2 h at
308C. The treatment procedure of the second cycle was the
same as the first cycle, with an isolated yield of 70% for HHD.
To investigate what causes the decrease in yield, the organic
phase and aqueous phase were characterized by inductively
coupled plasma atomic emission spectrometry (ICP-AES). Ac-
Scheme 3. Mechanism for the conversion of 5-methylfurfuryl alcohol to 2,5-
hexanedione.
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[
24]
fructose with isopropanol and HCl in water at 1208C for 3 h.
After removing the isopropanol and water by reduced pressure
distillation, pure water was added and the solid was removed
by centrifugal separation. A light yellow solution of 5-HMF
with HPLC yield of 73.4% was obtained. Pure HCOOH was
added into the crude 5-HMF solution to adjust pH to 2.5. Acidi-
fied 5-HMF (5 mL) was mixed with the catalyst (0.01 mol%)
and produced HHD with 98% GC yield (71.9% to fructose) at
Figure 5. Catalyst recycling process for the hydrogenation of 5-HMF in aque-
ous FBS using phase separation: a) 1308C, 2 h; b) acidification; c) extract
with CH Cl ; d) phase separation.
2 2
1
308C for 2 h. These results support that our catalyst system
can yield the selective transformation of crude 5-HMF, which
was obtained from hydrolysis of fructose, to HHD in excellent
yield.
cording to the results of ICP-AES detection, the concentration
À1
of Ir fell from 3.84 to 2.36 mgL , whereas the TON rose from
Conclusions
9
200 to 11 388 (Table 1). This indicates that there is no loss in
III
We developed Cp*Ir half-sandwich catalysts for selective con-
version of 5-hydroxymethylfuraldehyde to 1-hydroxy-2,5-hexa-
nedione in high yield and selectivity in aqueous formate buffer
solution (FBS), which has an excellent tolerance to acidic aque-
ous conditions. We found that effective control of pH can in-
crease catalytic efficiency and regulate the distribution of prod-
ucts. A mechanistic study demonstrated the path of 5-HMF
transformation and the effect of acidity on the reaction
system. We found that increasing the pH in the FBS reaction
process can reduce competitive reactions and improve reac-
tion selectivity. Our catalytic system can transform 5-HMF in
the hydrolysis solution of fructose in excellent yield as well,
which showed a potential for a large-scale production.
[
a]
Table 1. Reuse of 5 for the hydrogenation of 5-HMF in aqueous FBS.
À1
À1
Cycle index
Ir conc. [mgL
]
GC yield
TON
TOF [h ]
1
2
3.84
2.36
92%
70%
9200
11 388
4600
5694
[
a] Reaction conditions: 5-HMF (0.1 mol), catalyst (0.01 mmol, 0.01 mol%
with respect to 5-HMF), and FBS (1 m, 500 mL) at 1308C for 2 h.
activity of the catalyst and a slight loss in the extraction pro-
cess causes the decrease of HHD yield.
To investigate whether the supplement of pure HCOOH
causes loss of catalyst, we extracted the solution without ad-
justing pH. However, the solution exposed to air darkened in
color. The pH of the separated aqueous phase was adjusted
with pure HCOOH and then fresh 5-HMF was added for the
next cycle. The reaction produced less than 10% HHD after
Experimental Section
Materials
5
-HMF, BHMF, and 5-methylfurfuryl alcohol were generously gifted
by Hefei Leaf Energy Biotechnology Co., Ltd (www.leafresource.
com). IrCl ·nH O (Irꢀ60%) was purchased from Shaanxi Kaida
3
2
2
h. This result suggested that catalytically active [Ir–H] species
Chemical Engineering Co. Ltd. Pentamethylcyclopentadiene was
purchased from Energy Chemical. Fructose (99%) was purchased
from Alfa Aesar. 2,2’-Bipyridine, 4,4’-dicarboxy-2,2’-bipyridine, and
[23]
in the reaction process were instable towards O . Addition
2
of HCOOH allowed the air-instable [Ir–H] species to transform
into air-stable [Ir–OCOH] species (Scheme 4). Hence, delayed
supplement of HCOOH for pH adjustment to 2.5 leads to inef-
fectiveness of the catalyst and hinders subsequent extraction.
4
,4’-dimethoxy-2,2’-bipyridine were purchased from TCI. Ethyl ace-
tate (99.5%), petroleum ether (60~908C), methanol (99.5%), di-
chloromethane (99.5%), isopropanol (99.5%), hydrochloric acid
(
37%), silver sulfate (99.7%), formic acid (98.0%), sodium formate
5
-HMF can be obtained from hydrolysis of fructose in indus-
dihydrate (99.5%), Na HPO ·12H O (99.0%), and NaH PO ·2H O
2
4
2
2
4
2
try. To show the application potential of our catalyst system,
we studied the reaction to produce HHD with fructose as
a starting material. Initially, we carried out the hydrolysis of
(
99.0%) were purchased from Sinopharm Chemical Reagent Co.,
Ltd. Reagent water was purchased from Wahaha. The autoclave
was provided by Anhui Kemi Machinery Technology Co., Ltd.
[
25]
[26]
[
Cp*IrCl ] ,
4,4’-dihydroxy-2,2’-bipyridine,
4,4’-diami-
[28]
2
2
[27]
no-2,2’-bipyridine,
6,6’-dihydroxyl-2,2’-bipyridine,
[29]
and 6,6’-dimethoxyl-2,2’-bipyridine
were synthesized
according to the previously reported procedures.
General catalytic reaction
À1
5
0
-HMF (126 mg, 1 mmol), catalyst (0.50 mmolL , 200 mL,
.01 mol%) and FBS (5 mL, 1 m, pH range 0.0–7.0) were
loaded into a 35 mL sealed glass tube and stirred at
a rate of 700 rpm. The mixture was heated to 1308C for
2
h with an oil bath and cooled in water to room tem-
Scheme 4. Stabilization of the Cp*Ir catalyst in FBS.
perature after the reaction. The liquid products were di-
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luted with methanol and analyzed by GC using dimethyl phthalate
as an internal standard.
and PCSIRT. The authors thank the Hefei Leaf Energy Biotech-
nology Co., Ltd. and Anhui Kemi Machinery Technology Co., Ltd.
for free sample sponsoring in this study.
GC detection method
Keywords: 5-hmf
iridium · ring-opening
· biomass · homogeneous catalysis ·
The liquid products were diluted with methanol and analyzed by
using a Shimadzu GC-2014 gas chromatograph equipped with a ca-
pillary column (DM Wax 30 m”0.25 mm) and a flame ionization
detector. The vaporization temperature was 523 K and the detec-
tion temperature was 553 K. An initial oven temperature of 333 K
[
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim