Effect of Cp*Iridium(III) Complex and acid co-catalyst on conversion of furfural compounds to cyclopentanones or straight chain ketones

Article abstract of DOI:10.1016/j.apcata.2017.07.004

In this paper, Cp*Ir (III) Complex and acid co-catalyst system was developed. By using Cp*Ir and γ-Al₂O₃ (Lewis acid), 5-hydroxymethylfurfural (5-HMF) can be converted efficiently to 3-hydroxymethyl cyclopentanone (HCPN). Meanwhile, Cp*Ir and Br?nsted acid can promote conversion of 5-HMF to 1-Hydroxy-2,5-hexanedione (HHD). The effect of Lewis acid and Br?nsted acid on the hydrogenation of furan derivatives was studied. Mechanism of conversion of 5-HMF to HCPN was discussed in detail and mechanism proposed by our predecessors was revised. Instead of being an intermediate for the formation of HCPN, it is believed that, HHD is a product of another reaction pathway. HHD condensed via Aldol reaction to produce 3-methylcyclopenten-2-ol-1-one (MCP) instead of HCPN. Under the promotion of Lewis acid, 5-HMF firstly convert to the precursor of HHD. After that, the reaction is through 4 π-electrocyclic ring closure process and HCPN was formed ultimately. Furthermore, we found that our Cp*Ir and acid co-catalyst system is suitable for a variety of furfural compounds. By using Cp*Ir, Br?nsted acid can promote conversion of furfural compounds to straight chain ketones and Lewis acid can promote the rearrangement of furfural compounds to cyclopentanone derivatives.

Full text of DOI:10.1016/j.apcata.2017.07.004

Accepted Manuscript
Title: Effect of Cp*Iridium(III) Complex and acid co-catalyst
on conversion of furfural compounds to cyclopentanones or
straight chain ketones
Authors: Yong-Jian Xu, Jing Shi, Wei-Peng Wu, Rui Zhu,
Xing-Long Li, Jin Deng, Yao Fu
PII:
S0926-860X(17)30304-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.07.004
APCATA 16309
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
18-5-2017
30-6-2017
3-7-2017
Pleasecite thisarticle as:Yong-JianXu, JingShi, Wei-PengWu, RuiZhu, Xing-Long Li,
Jin Deng, Yao Fu, Effect of Cp*Iridium(III) Complex and acid co-catalyst on conversion
of furfural compounds to cyclopentanones or straight chain ketones, Applied Catalysis
A, Generalhttp://dx.doi.org/10.1016/j.apcata.2017.07.004
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Effect of Cp*Iridium(III) Complex and acid co-catalyst on
conversion of furfural compounds to cyclopentanones or
straight chain ketones
Yong-Jian Xu,[a] Jing Shi,[a] Wei-Peng Wu,[a] Rui Zhu,[a] Xing-Long Li,[a] Jin Deng,*[a] and Yao Fu*[a]
Hefei National Laboratory for Physical Sciences at the Microscale, iChEM, CAS Key laboratory of
Urban Pollutant Conversion, Anhui Province Key Laboratory of Biomass Clean Energy,
Department of Chemistry, University of Science and Technology of China, Hefei 230026 (P. R. China)
Author Information
Corresponding Author
E-mai address: dengjin@ustc.edu.cn (J. Deng)
fuyao@ustc.edu.cn (Y. Fu)
Fax: (+86)551-6360-6689
Graphical Abstracts
By using Cp*Ir , Brønsted acid and Lewis acid can promote conversion of furfural compounds to straight chain
ketones and cyclopentanone derivatives, respectively.
Highlights



A highly efficient Cp*Ir (III) Complex and acid co-catalyst system to conversion of furfural compounds
to cyclopentanones or straight chain ketones.
Mechanism of conversion of 5-HMF to HCPN proposed by our predecessors was revised and
supplemented. We found that HHD condensed via Aldol reaction to produce MCP instead of HCPN.
Brønsted acid can promote conversion of furfural compounds to straight chain ketones and Lewis acid
can promote rearrangement of furfural compounds to cyclopentanones.

Abstract: In this paper, Cp*Ir (III) Complex and acid co-catalyst system was developed. By using Cp*Ir and γ-
Al2O3 (Lewis acid), 5-hydroxymethylfurfural (5-HMF) can be converted efficiently to 3-hydroxymethyl
cyclopentanone (HCPN). Meanwhile, Cp*Ir and Brønsted acid can promote conversion of 5-HMF to 1-Hydroxy-
2,5-hexanedione (HHD). The effect of Lewis acid and Brønsted acid on the hydrogenation of furan derivatives
was studied. Mechanism of conversion of 5-HMF to HCPN was discussed in detail and mechanism proposed
by our predecessors was revised. Instead of being an intermediate for the formation of HCPN, it is believed that,
HHD is a product of another reaction pathway. HHD condensed via Aldol reaction to produce 3-
methylcyclopenten-2-ol-1-one (MCP) instead of HCPN. Under the promotion of Lewis acid, 5-HMF firstly convert
to the precursor of HHD. After that, the reaction is through 4 π-electrocyclic ring closure process and HCPN was
formed ultimately. Furthermore, we found that our Cp*Ir and acid co-catalyst system is suitable for a variety of
furfural compounds. By using Cp*Ir, Brønsted acid can promote conversion of furfural compounds to straight
chain ketones and Lewis acid can promote the rearrangement of furfural compounds to cyclopentanone
derivatives.
Keywords: furfural compounds, Lewis acid, Brønsted acid, reaction mechanism, ketones
1. Introduction
The depletion of fossil resources has severely restricted the development of chemical industry that relies on
non-renewable resources. Thus, for the sake of sustainable development, attention has been drawn to
biorefinery. Lignocellulose, as the most abundant renewable organic carbon source, is an important raw material
for biorefinery.1
5-hydroxymethylfurfural (5-HMF), as one of the most promising biomass-based platform molecules, can be
synthesized from the downstream products of lignocellulose, such as glucose and fructose.2 5-HMF can be
converted to many valuable bio-derived chemicals, including 2,5-furan dicarboxylic acid,3 2,5-furan
carboxaldehyde,4 2,5-dimethyl furan,5 2,5-dihydroxymethyllfuran (BHMF),6 2,5-dimethyl tetrahydrofuran,7 1-
hydroxy-2,5-hexanedione (HHD)8 and 3-hydroxymethyl cyclopentanone (HCPN).9
3-hydroxymethylcyclopentanone (HCPN) is a valuable raw material for fine chemicals, which can be used to
prepare pharmaceuticals, polymers, spices, and fuels, such as methyl jasmonate10 and carbocyclic nucleoside11
(Figure 1). Thus, studying the synthesis of HCPN has important scientific and economic value.
The reaction of HCPN from 5-HMF resembles a widespread concerned reaction, furfural rearrangement to
cyclopentanone.12 They both follow the mechanism of Piancatelli rearrangement reaction, which is mainly
experienced by a cascade sequence that terminates with a 4 π-electrocyclic ring closure to give a pentadienyl
cation, a step that is analogous to the Nazarov reaction.13 The substitution at 5-position of the furan ring blocks
Piancatelli rearrangement.12a-12b,13c So even though HCPN is a potential biomass-based platform molecule, the
transformation of 5-HMF hydrogenation to HCPN has been rarely reported.
In 2014 and 2015, Satsuma developed Au/Nb2O59a and Pt/SiO2 with Ta2O59b catalyst system to achieve the
conversion of HCPN from 5-HMF. Recently, Rosseinsky obtained HCPN from 5-HMF by using Ni/Al2O3 metal
hydrate9c and Cu/Al2O3 metal hydrate9d as catalyst. However, these studies all required long reaction time and

showed low catalyst activity according to calculated turnover frequency (TOF)8g based on the reported results
(Table 1). They all did not discuss the mechanism in detail and thought that 1-hydroxy-2,5-hexanedione (HHD)
may be an important intermediate in the reaction. The effect of Lewis acid on the reaction was discussed.
Our study reported half-sandwich Ir complexes8a,14 have shown excellent hydrogenation activity, selectivity
and acid resistance for hydrogenation of furan ring derivatives. Recently, we discussed the effect of Brønsted
acid on conversion of 5-HMF to HHD and 2,5-dihydroxymethylfuran (BHMF)8a. To research the effect of Lewis
acid on the reaction and to further develop half-sandwich Ir complexes and acid co-catalyst system, we chose
the most efficient and acid-resistant half-sandwiched Ir complex, [Cp*Ir(4,4'-(OH)2-bpy)(OH2)]SO4 (Figure 2,
Table 2, hereinafter referred to as Cp*Ir)8a,14b and metal oxides (Lewis acid) as catalyst in hydrogenation of 5-
HMF. γ-Al2O3 showed the highest catalytic activity in all metal oxides used in the reaction.
Herein, we used Cp*Ir and γ-Al2O3 as the catalyst in hydrogenation of 5-HMF to HCPN. The yield of HCPN
can reach 82% at 130 °C and 3 MPa H2 for 4 h (TOF reach 205 h-1, Table 1). The effect of Brønsted acid and
Lewis acid on the hydrogenation of furfural compounds was studied. Under the catalysis of Brønsted acid and
Lewis acid, HHD and HCPN can be obtained with high selectivity respectively. In our catalytic system, this rule
is applicable to many furfural compounds (Scheme 1). In the article, the mechanism of the reaction was
discussed in detail and mechanism proposed by our predecessors was revised and supplemented.
2. Experimental Section
2.1 Materials and reagents
5-hydroxymethylfurfural, 5-methyl furfural, furfural and fructose were generously gifted by Hefei Leaf Energy
Biotechnology Co., Ltd (www. leafresource. com). 3-methly-2-cyclopenten-1-one (97.0%), 2-cyclopenten-1-one
(> 98.0%) were purchased from Adamas Reagent Co., Ltd. 3-methylcyclopentanone (99.0%) was purchased
from Shanghai yuanye Bio-Technology Co., Ltd. IrCl3·nH2O (Ir ≥ 60.0%) was purchased from Shaanxi Kaida
Chemical Engineering Co. Ltd. Pentamethylcyclopentadiene was purchased from Energy Chemical. 4,4’-
dicarboxy-2,2’-bipyridine were purchased from TCI. γ-Al2O3-1(99.9%, 30nm)was purchased from Xiya Reagent
Co., Ltd. TiO2 (99.8%, Anatase, 5-10nm), ZrO2 ( > 99.9%, 50 nm), WO3 (99.9%, 50 nm), γ-Al2O3-2 (99.9%, 10
nm), α-Al2O3 (99.9%, 200 nm) were purchased from Aladdin Chemistry Co., Ltd. Ta2O5 (> 99.9%), Al2(SO4)3
(98%),Ag2SO4 (99.7 %), ethyl acetate (99.5 %), petroleum ether (60-90oC), methanol (99.5 %), acetonitrile
(99.0%), dichloromethane (99.5%), Na2HPO4·12 H2O (99.0 %), and NaH2PO4·2 H2O (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. All of the metal oxides were calcined at 500 ℃ for 4
h before used.
2.2 Typical Catalyst preparation
Iridium trichloride hydrate (10 g, 0.026 mol) and pentamethylcyclopentadiene (5.0 g, 0.036 mol) were added
to 300 mL methanol in a 500mL round-bottom flask, purged with nitrogen for 5 min. Then the mixture reacted
under nitrogen for 48 h at room temperature. Recrystallization of the product from chloroform-hexane obtained
[IrCp*Cl2]2, an orange microcrystalline solid.15
[Cp*IrCl2]2 (480 mg, 0.60 mmol) and Ag2SO4 (374 mg, 1.20 mmol) were added to 4 mL of water under argon
atmosphere. The solution was stirred at room temperature for 12 h and then filtered to remove AgCl. The filtrate
was evaporated and dried in vacuum to get yellow powder [Cp*Ir(H2O)3]SO4. [Cp*Ir(H2O)3]SO4 (48 mg, 0.10
mmol) was dissolved in 12.5 mL of water under argon and 0.10 mmol of 4,4'-dihydroxy-2,2'-bipyridine was added.
The solution was stirred under reflux for 12 h. After the reaction, the aqueous solution was then distillated under
reduced pressure to remove water and got a pale yellow powder. The solid was dried under vacuum at 55 °C
for 12 h and got [Cp*Ir(4,4'-(OH)2-bpy)(OH2)]SO4. The catalyst was analyzed by NMR (supporting information
part 12). The catalyst can be further purified by recrystallization from water.
2.3 Catalyst characterization
The acidity of the samples was measured by NH3-TPD. A gas chromatograph (GC-SP6890, Shandong Lu-
nan Ruihong Chemical Instrument Co., Ltd., Tengzhou China) was used with an inline thermal conductivity

detector (TCD). Before testing, about 80 mg of sample was heated up to 500 °C for 2 h in a flow of argon to
remove adsorbed water or organic species. For NH3-TPD, the sample was adsorbed with NH3 until saturated at
80 °C for 1 h. After the catalyst surface became saturated, the sample was kept at 80 °C for 1 h in argon to
remove the excess base. NH3 was thermally desorbed by rising the temperature with a linear heating rate of
approximately 10 °C·min-1 from 80 °C to 800 °C. Then temperature was kept for 2 min. The total number of acid
sites was calculated based on the TCD signal of NH3.
The pyridine adsorption infrared was measured by Bruker Vertex 70 FT-IR spectrometer. The catalyst was
pressed and placed in an infrared spectroscopy cell at 300 °C for 1 h, vacuum of < 1 × 10-2 MPa. The temperature
was reduced to 30 °C, the pyridine vapor was adsorbed for 30 min and then heated to 150 °C and 250 °C.
Infrared spectra were recorded.
2.4 Typical experiment and product analysis
5-HMF (25.2 mg, 0.2 mmol), Cp*Ir catalyst (dissolved in H2O, 5 mmol·L-1, 0.04 mL, 0.1 mol%), γ-Al2O3-1 (10
mg) and H2O (3 mL) were loaded into a 10 mL stainless steel autoclave and stirred at a rate of 1000 rpm. The
mixture was heated to 130 °C under 3 MPa H2 for 4 h. The liquid products were diluted with acetonitrile and
analyzed by GC. Dimethyl phthalate was used as an internal standard (The typical GC charts were showed in
supporting information Figure S2 and Figure S3).
The liquid products were diluted with acetonitrile and analyzed by Shimadzu GC-2014 gas chromatograph
equipped with a capillary column (DM Wax 30 m × 0.25 mm) and a flame ionization detector. The vaporization
temperature was 250 °C and the detection temperature was 280 °C. An initial oven temperature of 60 °C was
kept for 2 min, and the temperature increased to 240 °C in steps of 10 °C·min-1. The column flow was 2.7
mL·min-1. The carrier gas was nitrogen and the split ratio was 50.
3. Results and Discussion
3.1. Effect of Lewis acid and Brønsted acid on conversion of 5-HMF to HCPN and HHD
Firstly, we investigated the effect of different Lewis acid on the reaction (Figure 3). Ta2O5, ZrO2, TiO2, Nb2O5,
Al2O3 were selected because of their good catalysis effect on the hydrogenation of HMF to HCPN, according to
previous reports.9 The highest yield of HCPN reached ca. 80% when γ-Al2O3 was added as Lewis acid. TiO2
and Ta2O5 can also promote the reaction to HCPN with the yield of 60%, while Nb2O5 has weak effect on the
reaction. In addition, 3-methylcyclopenten-2-ol-1-one (MCP), 1-hydroxy-2,5-hexanedione (HHD) and 3-
hydroxymethylcyclopentenone (HCPEN) were also detected.
Temperature programmed desorption of ammonia (NH3-TPD) show that the acidity of metal oxides can affect
the yield of HCPN (Figure 4). There is only one weak acid site under 200 °C in each metal oxide.16 Comparing
these acid sites, we found that acid strength is ordered as follow: γ-Al2O3-2 > γ-Al2O3-1 > Ta2O5 > TiO2 > α-Al2O3
> WO3 > ZrO2 > Nb2O5 and the order of acid amount is γ-Al2O3-2 > γ-Al2O3-1 > TiO2 > ZrO2 > Ta2O5 > WO3 > α-
Al2O3 > Nb2O5 (Figure 5). The higher acid strength, the more HCPN yield. Meanwhile, acid amount has a weak
effect on the yield. Acid strength of TiO2 is weaker than Ta2O5, but more likely to promote the formation of HCPN,
which is due to the acid amount of TiO2 being much larger than Ta2O5. The acid strength and acid amount of γ-
Al2O3-2 are both larger than γ-Al2O3-1, but the yield of HCPN is slightly reduced, which showed proper Lewis
acidity is important for the reaction.9b
Pyridine adsorption infrared (Figure 6) showed that there were no obvious Brønsted acid sites at about 1540
cm-1 in all kinds of metal oxides used in our reaction systems, but there were obvious Lewis acid absorption
peaks at about 1450 cm-1.17 The peaks suggest that the Lewis acid sites in the metal oxides play a major role in
the reaction of rearrangement from 5-HMF to HCPN. The higher wavenumber indicates the stronger Lewis
acidity of the metal oxides.9b,17 The wavenumbers of the peaks which indicates Lewis acid sites in the vicinity of
1450 cm-1 increase in the order of γ-Al2O3-2 > γ-Al2O3-1 > α-Al2O3 > ZrO2. The peaks of Ta2O5、α-Al2O3、WO3
and Nb2O5 are not obvious due to the low acid amount (supporting information Figure S11). It is consistent with
the rule of NH3-TPD. This further demonstrates that the Lewis acidity of the metal oxide promotes the formation
of HCPN.

Further study showed that γ-Al2O3-1 can promote the formation of HCPN efficiently (Figure 7). With the
increasing addition of γ-Al2O3-1, the selectivity of HCPN increased significantly at the beginning，followed by a
slight drop. This phenomenon can be explained by the variation of acidity. Suitable Lewis acid can promote the
foration of HCPN. However, when the γ-Al2O3-1 dosage exceeded, the system acidity increased, then the
selectivity of HCPN decreased.
In water-soluble hydrogenation catalytic system, using water-soluble Lewis acids may lead to better HCPN
selectivity. Based on the results above, Al2(SO4)3 was chosen as acid catalyst and found to give the reaction a
high selectivity of 1-Hydroxy-2,5-hexanedione (HHD). The selectivity of HHD reached the highest yield (77%)
when using 5 wt% Al2(SO4)3 (supporting information Figure S12). Under such condition, the pH of the reaction
system was 3.84. According to our previous study,8a proton acid can promote the hydrogenation of 5-HMF to
HHD. Al2(SO4)3 in aqueous solution will generate H+ and promote the formation of HHD.
BET analysis and x-ray diffraction (XRD) showed that the specific surface area, pore volume, pore size and
particle size of the metal oxides have a weak effect on the reaction of rearrangement from 5-HMF to HCPN
(Supporting information part 6 and part 7).
3.2. Effect of half-sandwich complex catalysts on conversion of 5-HMF to HCPN
We evaluated the catalytic efficiency of a series of half-sandwich complex catalysts by discussing the effects
of substitutes, hydrogenation activity metals and ligands (Table 2). Half-sandwich catalyst exhibited
hydrogenation activity by forming a metal hydride intermidates, which can be promoted by high hydrogenation
pressure.14b,18a Comparing catalyst 1, 2 and 3, iridium showed better activity than rhodium and ruthenium, though
rhodium and ruthenium were reported to show a high activity towards the hydrogenation of ketones.18a-18b It may
be explained by the fact that iridium hydride intermidate was more stable and showed high activity in lower
hydrogenation pressure illuminated by saturation kinetic.18a The hydroxyl-substituted catalyst activity is greater
than the methoxy-substituted catalyst, which can be explained by Hammett constants of the substituents (-OH
(σp+ = -0.92) >, -OMe (σp+ =-0.78) >, -H (σp+ = 0.00)).8a Strong electron-donating groups lead to higher catalytic
activity. Ortho-substituent Cp*Ir was reported to exhibit good performance in the hydrogenation of carbon dioxide
to formic acid17c. However, according to previous reports18d-18e and our study18f, compared with para-substituents,
ortho-substituents showed dehydrogenation activity to carbonyl compounds. Activity of catalyst 4-OH is lower
than 4-OMe due to the high dehydrogenation activity of 4-OH18d-18f. Moreover, steric effect showed an important
influence of catalyst activity. Ortho-substituents may hinder the formation of metal hydride intermediates and
weaken the effect of active intermediates on substrates. Thus, catalyst 1 had higher catalytic activity than
catalyst 4 and 5.
3.3. Reaction mechanism of 5-HMF to HCPN and HHD
Through the discussion of the effect of various Brønsted acids and Lewis acids, we have found that Brønsted
acid is not effective in promoting the hydrogenation of 5-HMF to 3-hydroxymethylcyclopentanone (HCPN).
Meanwhile, Lewis acid can greatly promote the formation of HCPN. Previous studies9 suggested that Brønsted
acid inhibited further cyclization of HHD. The studies also claimed that HHD was an important reaction
intermediate in the transformation from 5-HMF to HCPN. However, their conjectures were not well supported by
concrete evidence. HHD was hydrogenated in our catalytic system (supporting information part 2 and Figure
S4). HCPN and HCPEN were almost undetected after the reaction. HHD is not a reaction intermediate for
formation of HCPN in our catalytic system. Furthermore, we found that Brønsted acid can promote
hydrogenation ring-opening of 5-HMF to HHD8a and Lewis acid can promote the rearrangement of 5-HMF to
HCPN.
In addition, we detected a small amount of methylcyclopenten-2-ol-1-one (MCP) in the reaction, which was
generated from Aldol condensation of HHD. HHD condensed via Aldol reaction to produce MCP instead of

HCPN（Scheme 2). We found that strong base and Lewis acid can promote the Aldol condensation reaction of
HHD to MCP (supporting information part 1 and part 2).
On the basis of previous reports9,13 and our speculation, the reaction is more likely to undergo Piancatelli
rearrangement reaction to produce 4-hydroxy-4-(hydroxymethyl)cyclopent-2-en-1-one (HHCPEN) and then
hydrogenate to HCPN. However, we almost did not detect HHCPEN in conversion to HCPN from 5-HMF.
HHCPEN was unstable in our catalytic system due to hydrogenation reaction. Therefore, we tried the reaction
of BHMF over metal oxide under N2. As expected, we had a high selectivity to get HHCPEN. In our catalytic
system, HHCPEN was almost completely hydrogenated to HCPN (supporting information part 4, Figure S7 and
Figure S8).
According to the research above, we proposed the following reaction mechanism (Scheme 2), which is
different from previous reports.9 Instead of being an intermediate for the formation of HCPN, we believe, HHD
is a product of another reaction pathway. HHD and HCPN are in a competitive relationship, Brønsted acid is
added to promote the formation of HHD, rather than inhibition of HHD continue cyclization. When Lewis acid
was used as the catalyst, the reaction firstly formed 4π-electrocyclic ring. According to frontier molecular orbital
theory, the highest occupied molecular orbital (HOMO) of the 4π intermediate undergoes conrotatory motion
under heating and closes ring to form a five-membered ring structure.19 This five-membered ring structure loses
a hydrogen ion to form a cyclopentanone structure and finally generate HCPN via hydrogenation.
3.4. Effect of reaction conditions on conversion of 5-HMF to HCPN
Hydrogen pressure, reaction time, half-sandwich Ir complex concentration and temperature of the reaction
system were investigated (Figure 8). Hydrogen pressure was found to be nearly unrelated to the yield of HCPN.
When H2 was greater than 2 MPa, the yield of HCPN was basically stabilized. As reaction proceeded, the yield
of intermediate products such as BHMF and HCPEN decreased, then transformed into HCPN. After 4 h, the
yield of HCPN did not increased. When the dosage of Cp*Ir was more than 0.1%, the yield of HCPN was almost
stable, the decrease of Cp*Ir could affect the selectivity of HCPN. When Cp*Ir was not used, almost no HCPN
was detected. Temperature changes show that the increase of temperature within a certain range can promote
the reaction. When reaction temperature was low, the main reaction product was 2,5-dihydroxymethylfuran. It
indicated that the reaction was not complete. As temperature rose up to 130 °C, the yield of HCPN increased to
80% or more. Then, as temperature increased, the yield of HCPN tended to be stable.
3.5. Effect of Lewis acid and Brønsted acid on conversion of furfural compounds to ketones
In addition to 5-HMF, other furfural derivatives can also rearrangement to cyclopentanone derivatives in our
catalyst systems. Interestingly, they are in line with the rule that Brønsted acid promotes the conversion to
straight chain ketones and Lewis acid promotes rearrangement to cyclopentanone derivative (Table 3). 5-methyl
furfural can be hydrogenated to 3-methyl-cyclopentanone and 3-methyl-cyclopent-2-en-1-ketone in the Lewis
acid catalysis, to 2,5-hexanedione under the Brønsted acid catalysis. Furfural can be hydrogenated to 2-
cyclopentenone and cyclopentanone under Lewis acid catalysis, to levulinic acid and γ-valerolactone under
Brønsted acid catalysis.
3.6. Conversion of fructose to HCPN
5-HMF can be obtained from hydrolysis of fructose in industry. In order to show the potential of our catalyst
system, we studied the reaction of producing HCPN with fructose as raw material (supporting information part
11). The yield of HCPN can reach 80% (59.2% to fructose). These results prove that our catalyst system can
convert crude 5-HMF from fructose hydrolysis to HCPN, with good yield.2i
4. Conclusions
In summary, a highly efficient Cp*Ir (III) Complex and acid co-catalyst system was developed. By using Cp*Ir
and γ-Al2O3 (Lewis acid), the hydrogenation of 5-HMF to HCPN can get the highest TOF reported. Meanwhile,
Cp*Ir and Brønsted acid can promote conversion of 5-HMF to HHD. More important, different from previous

reports, we find that HHD is not a reaction intermediate for the formation of HCPN in our catalytic system. HHD
condensed via Aldol reaction to produce MCP instead of HCPN. Furthermore, we found that our Cp*Ir and acid
co-catalyst system is suitable for various furfural compounds. Brønsted acid and Lewis acid can promote
conversion of furfural compounds to straight chain ketones and cyclopentanones respectively.
Acknowledgements
This work was supported by NSFC (21572212, 21402181, 21325208), CAS (XDB20000000, YZ201563),
FRFCU and PCSIRT. The authors appreciate the Hefei Leaf Energy Biotechnology Co., Ltd. and Anhui Kemi
Machinery Technology Co., Ltd. for free sample sponsoring in this study.
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Figure 1. Application of 3-hydroxymethyl cyclopentanone.
Figure 2. [CP*Ir(4,4'-(OH)2-bpy)(OH2)]SO4.
Figure 3. Product distributions obtained by the reaction using CP*Ir with various metal oxides. Reaction
condition: HMF aq. (0.067 M, 3 mL); Cp*Ir (0.1 mol%); a metal oxide (10 mg); H2 3 MPa; 130 °C; 4 h. γ-Al2O3-
1: 30 nm, γ-Al2O3-2: 10 nm.

Figure 4. NH3-TPD profiles of metal oxides used in the reaction.
Figure 5. The relation of desorption temperature of NH3, acid amount and yield of HCPN for metal oxides used
in the reaction.
Figure 6. The pyridine adsorption infrared in 150 °C of metal oxides used in the reaction.
Figure 7. Product distributions obtained by the reaction using Cp*Ir with increasing γ-Al2O3-1. Reaction condition:
HMF aq. (0.067 M, 3 mL); Cp*Ir (0.1 mol%); H2 3 MPa; 130 °C; 4 h.

Figure 8. The influence of H2, time, Cp*Ir concentration and temperature. Reaction condition: (a) (b) (c) (d)
5-HMF aq. (0.067 M, 3 mL), γ-Al2O3-1 (10 mg); (a) (b) (d) Cp*Ir (0.1 mol%); (b) (c) (d) H2 3 MPa; (a) (b) (c)
130 °C; (a) (c) (d) 4 h.
Scheme 1. The Influence of Lewis Acid and Brønsted Acid to Reaction

Sheme 2. The Mechanism of the Reaction We Proposed. LA= Lewis Acid.
Table 1. Hydrogenation of 5-HMF to HCPN Reported by Previous Study and This Work
Entry
Catalyst
Condition
Conv. [%]
Yield [%] TOF [h-1][a]
Ref.
9a
1
2
3
4
5
Au/Nb2O5
8MPa H2/ 140 °C/ 12 h >99
3MPa H2/ 140 °C/ 12 h 100
86
82
81
86
82
50
27
1
Pt/SiO2 and Ta2O5
Ni/Al2O3 metal hydrate
9b
2MPa H2/ 140 °C/ 6 h
100
100
100
9c
Cu/Al2O3 metal hydrate 2MPa H2/ 180 °C/ 6 h
Cp*Ir and γ-Al2O3 3MPa H2/ 130 °C/ 4 h
1
9d
205
this work
[a] TOF is calculated by molar ratio of product/catalyst. TOF is the average value of 1 h.
Table 2. The Hydrogenation of 5-HMF by Using Different Half-sandwich Complexes[a]

Selectivity [%]
Conv.
[%]
60
Cat.
MCP
7
HHD
0
HCPN HCPEN
BHMF
1-H
3
34
2
2
1-OH
100
98
5
1
82
22
15
0
0
1-OMe
2-OH
5
5
41
48
7
0
99
8
1
0
2-OMe
3-OH
45
2
1
0
99
16
3
2
13
3
44
6
0
3-OMe
4-OH
32
0
22
0
89
9
3
2
36
51
54
5
4-OMe
5-H[b]
93
14
10
1
2
7
1
100
98
4
20
40
24
0
5-OH[b]
5-OMe[b]
0
36
0
99
13
2
48
Reaction conditions: [a] 5-HMF, 5-methyl furfural (0.067 M, 3 mL), half-sandwich complexes (0.1 mol%), γ-
Al2O3-1 (10 mg), 3 MPa H2, 130 °C, 4 h; [b] half-sandwich complexes (0.05 mol%).

Table 3. Effect of acid on conversion of furfural compounds to ketones
Entry
Substrate
Acid
Conv. [%]
100
Yield [%]
10 mg γ-Al2O3-1[a]
2
82
[b]
1
5 wt% Al2(SO4)3
100
74
90
Trace
Trace
pH=2.5 formic acid
buffer solution[c]
100
10 mg γ-Al2O3-1[a]
100
100
100
100
30
39
60
41
24
2
3
pH=1.5 phosphate
buffer solution[d]
10 mg γ-Al2O3-1[a]
Trace
9
pH=1.0 phosphate
buffer solution[e]
Reaction conditions: [a] 5-HMF, 5-methyl furfural or furfural aq. (0.067 M, 3 mL), Cp*Ir (0.1 mol%), γ-Al2O3-1 (10
mg), H2 3 MPa, 130 °C, 4 h. [b] 5-HMF aq. (0.067 M, 3 mL), Cp*Ir (0.1 mol%), H2 3 MPa, 130 ˚C, 4 h. [c] 5-HMF
(0.2 M), pH = 2.5 formic acid buffer solution (1 M), Cp*Ir (0.01 mol%), 130 ˚C, 2 h.8a [d] 5-methyl furfural (0.067
M), 3mL pH = 1.5 phosphate buffer solution (0.1 M), Cp*Ir (0.1 mol%), H2 3 MPa, 130 °C, 4 h. [e] Furfural (0.60
mmol), 2 mL pH = 1.0 phosphate buffer solution (0.1 M), Cp*Ir (0.0083 mol%), 1 MPa H2, 120 °C, 4 h.14b