Dehydrogenation of 5-hydroxymethylfurfural to diformylfuran in compressed carbon dioxide: An oxidant free approach

Article abstract of DOI:10.1039/c6gc02977a

The dehydrogenation of biomass-based 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) was achieved utilizing an activated carbon supported rhodium (Rh/C) catalyst under mild reaction conditions. The developed method successfully afforded complete conversion and the highest selectivity of DFF (>99%) without any additive, conventional hydrogen acceptor and oxidant. The efficiency of the method is achieved by the addition of compressed carbon dioxide (scCO₂) and the synergistic effect of scCO₂ and Rh/C, where scCO₂ plays a pivotal role in accelerating the reaction by removing hydrogen, and consequently shifting the equilibrium to the forward direction. Optimization of different reaction parameters ensures the achievement of high conversion and selectivity. Characterization of the catalyst using different spectroscopic techniques suggests an interaction between the substrate and the catalyst and provides an indication of the possible reaction pathway, thus a mechanism would be outlined. The rate determining step of the reaction was calculated through mechanistic investigations involving theoretical calculations together with experimental analysis. One of the most attractive features of the method developed in this study is the reverse reaction of DFF, which can be achieved in one-pot without the addition of any external hydrogen. This process has successful application to the dehydrogenation of a variety of alcohols with different substituents.

Full text of DOI:10.1039/c6gc02977a

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Dehydrogenation of 5-hydroxymethylfurfural to diformylfuran in
compressed carbon dioxide: An oxidant free approach
a
a
c
a, b
Maya Chatterjee* , Takayuki Ishizaka Abhijit Chatterjee and Hajime Kawanami*
a
Microflow Chemistry Group, Research Institute for Chemical Process Technology
AIST Tohoku, 4-2-1, Nigatake, Miyagino-ku, Sendai, 983-8551, Japan, Tel: 81 22 237
5
213; Fax: 81 22 237 5388
b
CREST, Japan Science and Technology (JST), 4-1-8, Honcho, Kawaguchi, Saitama,332-
012, Japan
0
c
Materials Science, Dassault systemes, BIOVIA K.K. Tokyo, Think Park Tower, 2-1-1
Osaki Shinagawa-ku, 141-6020, Japan
E-mail: c-maya@aist.go.jp; h-kawanami@aist.go.jp
Abstract
The dehydrogenation of biomass-based 5-hydroxymethylfurfural (HMF) to 2, 5-
diformylfuran (DFF) was achieved utilizing an activated carbon supported rhodium (Rh/C)
catalyst under a mild reaction condition. The developed method successfully afforded
complete conversion and highest selectivity of DFF (> 99 %) without any additive,
conventional hydrogen acceptor and oxidant. The efficiency of the method lies beneath the
addition of compressed carbon dioxide (scCO ) and the synergistic effect of scCO and
2
2
Rh/C, where scCO played a pivotal role to accelerate the reaction by removing hydrogen,
2
consequently shifted the equilibrium to the forward direction. Optimization of different
reaction parameters ensures to achieve high conversion and selectivity. Characterization of

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the catalyst using different spectroscopic techniques suggested an interaction between the
substrate and the catalyst and provides an indication of the possible reaction path, thus a
mechanism would be outlined. Rate determining step of the reaction was calculated through
the mechanistic investigations involving theoretical calculations together with experimental
analysis. One of the most attracting features of the method developed in this study was the
reverse reaction of DFF, which could be achieved in one-pot without the addition of any
external hydrogen. This process has successful application to the dehydrogenation of a
variety of alcohols with different substituents.

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Introduction
Depletion of fossil fuel-based resources and concern over environmental degradation
necessitated research to develop sustainable technology for the transformation of alternative
raw materials into chemical building blocks with different fields of application. In this
context, abundant and inexpensive lignocellulosic biomass have the potential to be an ideal
replacement of fossil fuel resource and have attracted considerable attention. One of the
most important aspects of this bio-feedstock is that it already contains different
functionalized compounds, which can be converted into simple “platform molecules”.
These “platform molecules” are highly stable, active, retain their functionality and can be
upgraded to other essential chemicals. 5-hydroxymethylfurfural (HMF) is one of the most
versatile “platform molecules” obtained by the acid catalyzed dehydrogenation of glucose
1
or fructose . There are several reviews on the synthesis chemistry of HMF and its
2
derivatives . The most interesting feature of HMF is its potential availability and the
presence of multiple functional groups. Those active functional groups and the furan ring
can undergoes different type of reactions such as hydrogenation, oxidation, hydrogenolysis
of C-O bond, rearrangement, dissociation and polymerization to produce a wide array of
3
derivatives .
The oxidation of HMF produces a variety of compounds including 5-formyl-2-
furancarboxylic acid, 5-hydroxymethyl-2-furancarboxylic acid and 2, 5-furandicarboxylic
4
acid has strong market value in polymer industry . However, selective oxidation of the –
OH group without affecting the other functionalities as well as the furan ring results in DFF,

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which is an important compound and is used as a monomer, starting material for the
5
6
7
8
adhesives, binder, foams , antifungal agents , heterocyclic ligands , furan-urea resins as
9
well as in pharmaceuticals . Several studies were reported on the oxidation of HMF to
1
0
DFF in the presence of different classical oxidants such as KMnO but the main
4
disadvantages are the requirement of the stoichiometric amount of oxidant and the
generation of large amount of hazardous wastes, which is undesirable environmentally and
economically. Hence, instead of those conventional oxidant, Sheldon reported the oxidation
of HMF over TS-1 and hydrogen peroxide as a catalyst and an oxidizing agent, respectively.
However, the yield of DFF was only 25 % and hydrogen peroxide is a hazardous,
1
1
potentially explosive substance . Thus, many efforts were made to achieve quantitative
yield of DFF through the oxidation of HMF. Hu et al. provided a comprehensive review,
which includes the different aspects of the synthesis of DFF starting from fructose or HMF
1
2
.
Progressing towards the development of environmentally friendly process, the use of
molecular oxygen is one of the best choices to replace the stoichiometric reactions
involving expensive and toxic oxidants. Different noble metal catalysts were adopted to
accomplish the synthesis of DFF in an efficient way. For instance, Takagaki et al.
conducted aerobic oxidation of HMF over Ru/hydrotalcite catalyst and obtained 92 % yield
1
3
at 120 ºC in dimethylformamide within the reaction time of 6 h . In addition, Nie et al.
1
4
reported 96 % selectivity of DFF, while using Ru/C and 2.0 MPa of oxygen in toluene .
Instead of organic solvents, when the reaction was conducted in water, depending on the
1
5
pH of the medium, HMF was oxidized to acid in the presence of Pt and Au catalysts .

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Recently, different transition metal catalysts were also emphasized for the oxidation of
1
6
HMF because of their low cost , but in most of the cases, initiator was required to achieve
high yield of DFF, thus, extra effort on product separation and waste management was
inevitable.
Despite the fact that oxidation with molecular oxygen is an environmentally friendly
process, it raises a few unavoidable problems such as over oxidation, and health and
process safety (flammability, explosion). Furthermore, water was formed as a by-product,
which often deactivates the catalyst and requires tedious work for product purification. To
circumvent these problems, oxygen-free methodologies (dehydrogenation) were introduced,
which features number of advantageous points such as (i) it eliminates the formation of
water, (ii) it is tolerant towards different functional groups, (iii) it suppress over oxidation
1
7
and (iv) it forms hydrogen, which is an attractive feedstock . As a result, liquid phase
1
8
19
oxygen-free dehydrogenation of alcohols using homogeneous
and heterogeneous
catalysts attracted considerable attention from the practical and environmental view point.
The major disadvantages of homogeneous catalyst are their stability only in relatively mild
condition and the separation/recovery at end of the process is difficult and expensive. On the
contrary, the use of heterogeneous catalyst has several advantages including wide range of
operating condition (temperature, pressure), high industrial relevance and most important is
the ease of separation and recovery.
Here, we have attempted the selective dehydrogenation of the –OH group of HMF (behaves
typically as primary alcohol) using a heterogeneous Rh/C catalyst in scCO . The reaction
2
was performed without any oxidizing agent or additive or any conventional hydrogen

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acceptor. Generally, dehydrogenation of organic compounds is thermodynamically
unfavorable, so it is challenging when conducted in the absence of any oxidant and
hydrogen acceptor. The gas phase oxidative dehydrogenation using CO as a soft oxidant
2
2
0
for ethylbenzene to styrene was investigated , but, dehydrogenation in scCO is extremely
2
rare. Bergner et al. reported a preliminary study of the dehydrogenation of 1-phenylethanol
21
in scCO and obtained 58 % yield of the corresponding carbonyl compound . Our target is
2
to exploit the beneficial effects of the utilization of scCO such as its miscibility with
2
hydrogen, it can act as soft oxidant and easy to separate from the product through
depressurization in combination with a heterogeneous catalyst to overcome the challenges
associated with the oxidant free acceptor-less dehydrogenation of primary alcohols in an
efficient way.
Results and discussion
The dehydrogenation of HMF was carried out in scCO to obtain DFF in the absence of any
2
oxidant and additives. Scheme 1 represents the possible reaction path of the transformation
of HMF to DFF.
Screening for efficient catalyst
Different noble metal catalysts such as Ru, Rh, Pd and Pt supported on activated carbon (C),
alumina (Al O ) and mesoporous MCM-41 were screened for the reaction and the results
2
3
are shown in the Table 1. A combination of metal dispersion and catalytic performance
clearly suggested that the dispersion is not the determining factor of catalytic activity.
Under identical conditions (temperature= 150 °C, PCO2= 8 MPa and reaction time= 2 h), the

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catalytic activity was strongly depended on the metal as well as the support used. Generally,
support strongly influences the catalytic behavior as consequence of different related
parameters such as dispersion, electronic and geometric state of metal etc. Among the
studied materials, Rh/C and Pd/C were highly active and complete conversion was
achieved within the reaction time of 2 h (Table 1; Entry 5 and 8). On the other hand, Ru
and Pt exhibited comparatively lower activity; the conversion of HMF was ∼ 20 % and
4
0 %, respectively (Table 1; Entry 2 and 11). For Al O supported metals, the activity order
2 3
was Pd> Rh> Pt∼ Ru, which varies from 100 % to 20 % (Table 1; Entry 1, 4, 7 and 10).
Furthermore, metal supported MCM-41 show very poor conversion in the range of 12 % to
1
7 % (Table 1; Entry 3, 6, 9 and 12). Therefore, in each case, the lowest activity was
detected on neutral support like MCM-41 and then increased with the acidity of the support
material, thus, a critical role of support acidity might be suggested.
Regarding the aldehyde selectivity, it is interesting to note that HMF was dehydrogenated
to DFF as a sole product only on Ru and Rh, independent of the support used (Table 1;
Entry 1- 6). On the other hand, no DFF was detected in the resultant product mixture of
Pd/Al O catalyst. It consisted of 57.5 % of furfuryl alcohol (FA) as a major product along
2
3
with 21.3 % of 2-methyl furfural (MF) and the rest was 2,2'-oxybis(methylene)difuran; a
condensation product of MF (Table 1; Entry 7). However, DFF was formed on Pd/C
(selectivity = 23.3 %) and Pd/MCM-41 (selectivity =26.0 %) despite the poor selectivity. In
addition, FA (> 50 %) and MF (> 70 %) were also detected as major products on Pd/C and
Pd/MCM-41, respectively (Table 1; Entry 8 and 9). In the case of Pt, the selectivity of DFF
was varied from 71 to 90 %, followed the order of Al O > C > MCM-41 (Table 1; Entry
2
3

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1
0-12) along with the other products such as FA, MF and 2,5-Bis(hydroxymethyl)furan.
Based on the above results, Rh/C and Pd/C were emerged as two efficient catalysts in terms
of HMF conversion. A time-dependent product distribution on Pd/C evidences the initial
formation of DFF, FA and MF with the selectivity of 53 %, 46 % and 0.6 % respectively.
As the time progressed, DFF was decreased to 23 % with the increased selectivity of FA
(52.0 %) and MF (24.7 %); decarbonylation became the prominent path of the conversion
of HMF (supporting information; Figure S1). These results might be attributed to strong
22
hydrogen adsorption capability of Pd, compared to Rh . As soon as hydrogen was formed
from the dehydrogenation of the –OH group, it adsorbed on the metal site and thought to be
the possible cause of hydrogenation and decarbonylation of HMF, although further studies
are needed. Hence, the path of transformation of HMF was significantly different on Pd,
which involves decarbonylation and hydrogenolysis in addition to the dehydrogenation.
Therefore, Rh/C was considered as a most suitable catalyst under the applied condition to
achieve our targeted high selectivity of DFF with complete conversion. The efficiency of
the catalyst could be related to the enhancement of the adsorption of reactant and
desorption of product and Rh/C potentially fulfilled the criteria of an efficient catalyst
Furthermore, use of Rh catalyst could avoid the possible side reactions such as
hydrogenation, hydrogenolysis generally occur during dehydrogenation. In this context,
previously, we have shown that hydrogenation of HMF in scCO was unfavorable at high
2
2
3
temperature as well as on Rh . On the other hand, hydrogenolysis requires presence of
2
4
water along with CO2 . Thus, present reaction condition was unsuitable for other possible

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side reactions of transformation of HMF. Noticeably, there was no reaction in the absence
of catalyst.
Optimization of reaction parameters
Effect of CO pressure
2
To develop an efficient process, it is necessary to optimize the reaction conditions. The
effect of CO pressure on the reaction was studied at 150 ºC and the reaction time of 2 h
2
using Rh/C. Figure 1 shows the result of the dehydrogenation of HMF conducted with
different CO pressure ranging from 0 to 16 MPa. In the absence of CO (PCO2= 0 MPa),
2
2
the conversion of HMF was low of ∼15 %, which could be mediated by air present inside
the reactor or was contributed by the catalyst. Hence, the same experiment was conducted
in N to replace air from the reactor and to create an inert atmosphere. Under this condition,
2
the detected conversion was 13.2 % (Table 1; Entry 13), which corresponds well with the
results of 0 MPa of CO and confirmed the role of the catalyst.
2
The conversion of HMF was improved significantly in the presence of CO . For instance,
2
4
7 % conversion was detected after the incorporation of 4 MPa of CO , and then reached a
2
maximum of 100 % as the pressure increased to 8 MPa. The conversion remained constant
even after exposing at the higher pressure of 16 MPa. From the observed results it is
confirmed that the dehydrogenation of HMF occurred with poor conversion in the absence
of CO , but introduction of CO to the system substantially enhanced the reaction rate. We
2
2
suspected that (i) through the direct interaction with CO , the substrate can be converted
2
into the product or (ii) the forward shift of the reaction equilibrium due to the removal of
the co-product (hydrogen). Mirzabekova et al. reported that CO can act as an oxidizing
2

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agent in the dehydrogenation of methanol to formaldehyde at 580 to 700 °C and expected
25
an interaction with CO could facilitated the reaction . Here, an interaction between the –
2
OH group of HMF with CO can be ignored as the process requires a very high temperature.
2
In the present system, CO , existed as a supercritical state and we examined the solubility
2
of HMF in scCO . Naked eye observation of phase (supporting information; Figure S2)
2
confirmed a separate phase for HMF and scCO . We also observed that, dehydrogenation
2
of HMF occurred on the catalyst surface, although, with very low conversion, which
improved in the presence of CO . Equilibrium should be achieved between the reactant and
2
products unless hydrogen is being consumed by scCO to drive the equilibrium to product
2
side and accelerated the reaction. From the analysis of the liberated gas, we detected
hydrogen, in the large volume of CO (supporting information: Figure S3). Hence, based on
2
the obtained results, at this stage it could be confirmed that here CO played the role of a
2
hydrogen acceptor. Therefore, it would be logical to consider that CO eliminates hydrogen,
2
2
1
and favored the formation of DFF .
Regarding the product selectivity, DFF was the only product in detectable range, but the
selectivity of DFF dropped to 46.2 % without CO due to the formation of by-products
2
(35.5 % MF, 16.2 % 2, 5- Bis- (hydroxymethyl)-furan and 2.1 % 2,2'-
oxybis(methylene)difuran through hydrogenolysis and hydrogenation. Considering the
results, an optimum pressure of 8 MPa necessary to achieve complete conversion and
selectivity of the targeted DFF, chosen for further optimization of the other reaction
parameters.
Variation of temperature

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The reaction temperature also has significant effect on the catalytic activity. Thus, we
started to explore the influence of temperature on the dehydrogenation of HMF. Figure 2
exhibit the results of the variation of temperature from 35 to 150 ºC at the fixed CO2
pressure of 8 MPa. As shown in Figure 2, there was no catalytic performance observed at
3
5 ºC, but the activity was increased with temperature. At 60 ºC, only 6 %, conversion was
detected, which was then steadily increased and a complete conversion of HMF was
achieved at 150 ºC within the reaction time of 2 h. Thus, at the lower temperature, the
dehydrogenation rate was very slow and confirmed the requirement of higher temperature
to increase the conversion. However, it should be remember that here we deals with a
system, where temperature not only affect the reaction rate, but also influence the other
temperature-sensitive parameters like density, diffusivity and mass-transfer coefficient.
Thus, overall consequences of temperature tends to be more complicated. Noticeably, the
change in temperature had no effect on the DFF selectivity, no by-products or over-
hydrogenated product was detected in any of the cases.
Reaction time vs. catalytic activity
To monitor the progress of the reaction under the optimized condition (temperature=150 ºC
and PCO2= 8 MPa), a time dependent catalytic performance was studied by varying the
reaction time from 15 min. to 18 h (Figure 3). It has to be mentioned that there was no
catalytic activity observed at the beginning of the reaction (0 min.), but monitoring the time
course revealed that the conversion of HMF increased from 24. 4 % (15 min.) to 100 % (2
h) and then remained constant. The selectivity of DFF was unaltered throughout the
reaction as there was no other product/products in the detectable range. In order to check

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the over-hydrogenation of DFF, the reaction time was further extended to 4 h, but DFF still
continued as the sole product. The conversion of DFF to the corresponding acid was not
observed even after the prolonged reaction time of 18 h, surprisingly, HMF detected in the
product mixture with the selectivity of 35 % along with DFF. To understand this
observation, we monitored the reaction within a short interval of time. After 6 h, HMF
started to form and then increase slowly and reached a maximum level of 35 % as the
reaction time extended to 18 h. So, we became interested about the reverse reaction;
hydrogenation of DFF, due to the sudden appearance of HMF in the product mixture. Since,
2
6
Rh is an effective catalyst for hydrogenation
and CO also proved its efficiency as a
2
2
7
reaction medium to conduct the hydrogenation reaction . Hence, there was every
possibility of hydrogenation of the resultant aldehyde because of the presence of hydrogen
in the system. Therefore, we next examined whether the reverse reaction (hydrogenation) of
DFF to HMF could be accomplished under the applied reaction condition. The reaction was
conducted separately using the same reaction condition except the addition of hydrogen
along with 8 MPa of CO . The results are summarized in Table 2. DFF was hydrogenated
2
to HMF in the presence of hydrogen and the reaction was strongly dependent on the
hydrogen pressure as well as the temperature used. Applying the hydrogen pressure of 0.1
MPa, the yield of HMF was ∼ 20 % within the reaction time of 6 h (Table 2; Entry 1).
Moreover, the change in hydrogen pressure from 0.1 to 0.5 MPa, the conversion of DFF
was increased to 35.3 %, but selectivity was decreased due to the formation of MF (Table
2
; Entry 3). As the hydrogenation is an exothermic process, higher temperature will
decrease the yield. Thus, we reduced the temperature to 50 ºC, while the hydrogen pressure

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was increased to 1 MPa; as a result conversion enhanced to 60 % resulting in 97 % of HMF
and ∼3 % of other products (Table 2; Entry 3). From the results of the extended experiment,
it was confirmed that the hydrogenation of DFF to HMF could be achieved with the
2
8
utilization of surface hydrogen
. Most interestingly, the dehydrogenation and
hydrogenation reaction between HMF and DFF occurred in a one-pot batch system without
addition of any external hydrogen using a unique combination of scCO and Rh, which
2
created a greatest opportunity to achieve dehydrogenation-hydrogenation reaction in a very
simple and effective way.
Variation of catalyst: substrate ratio
Upon further investigation, we found that the reaction cannot be driven without any catalyst.
After finding the optimum reaction conditions (PCO2= 8 MPa, temperature = 150 ºC and
reaction time= 2 h) to achieve maximum conversion and selectivity of the targeted DFF, we
studied the influence of the catalyst: substrate weight ratio on the dehydrogenation process.
The catalyst: substrate ratio was varied from 1: 5 to 1: 30 after keeping the other parameters
constant (Figure 4). Interestingly, the increase in the catalyst: substrate ratio did not
negatively affect the progress of the reaction; HMF was successfully converted to DFF
without any by-product formation even if the large amount of substrate was used. At low
substrate concentration (catalyst: substrate= 1: 5 and 1: 10), complete conversion of HMF
was accomplished within the reaction time of 2 h. Moreover, when the ratio was increased
to 1: 20, the process became slower and the conversion reached to ∼ 41 % after 2 h, which
was slightly improved to 45 % as the reaction time was extended to 6 h. When the ratio was
further increased to 1: 30, the conversion of HMF again dropped to 21.8 % and then

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remained almost constant even after changing the reaction time. We conducted controlled
experiments to check the substrate concentration dependence of the initial reaction rate
after shortened the reaction time to 15 min. In each case, the initial rate of the reaction was
-1
calculated in terms of turnover frequency (TOF). The calculated TOF s were 2017 h , 2117
-1
-1
h and 2282 h for 1: 10, 1: 20 and 1: 30, respectively. Hence, from the TOF it was
observed that the effect of the increase in the amount of substrate on the initial reaction rate
was minimal. However, in each case, same trend was found; the conversion of HMF was
increased with time and reached a maximum value, which was strongly related to the
substrate concentration used such as 100 %, 41% and 21.8 % for 1: 10, 1: 20 and 1: 30,
respectively within the reaction time of 2 h. The reason might be the deactivation of the
catalyst or the amount of catalyst was not enough for the conversion of the excess amount
of substrate under the present reaction condition. As the catalyst can be recycled
successfully (please see recycling of the catalyst), the deactivation can be ignored. Thus,
considering the large amount of substrate, initially the substrate can adsorb easily to the
catalyst surface and the dehydrogenation occur, but an excess amount of the substrate
enhanced the concentration on the metal surface and this coverage could be the responsible
factor for the reduction of reaction rate. From the experimental results it was confirmed that
an optimum amount of substrate was necessary to achieve complete conversion and
selectivity of DFF. Hence, catalyst: substrate ratio of 1: 10 was used throughout the study.
Dehydrogenation of HMF in organic solvents
Instead of CO , the dehydrogenation of HMF was carried out in different organic solvents
2
and the results are shown in Figure 5. Under identical condition, there was a significant

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difference of the performance in organic solvents. A clear trend of catalytic activity
depending on the solvent polarity was evident. The conversion of HMF was increased with
the polarity of solvent, but selectivity of DFF decreased significantly. Solvents with low
dielectric constant (hexane and toluene) showed very poor conversion of ∼ 10 %, which
was enhanced slightly in THF (27.4 %). On the contrary, the conversion of HMF was
significantly improved to 55 % and 80 % in methanol and water respectively. Considering
the selectivity of DFF, solvents with low dielectric constant such as hexane and toluene
exhibited 100 % selectivity. However, the selectivity of DFF was dropped substantially in
methanol (33.2 %). Moreover, instead of DFF, 95 % of 1-(furan-2-yl)ethane-1,2-diol was
detected in water, despite the highest conversion of HMF. Thus, solvents have pronounced
effect on the catalytic activity because it is related to complex issues such as reaction
2
9
equilibria and reaction rate, which needs separate attention . Based on our experimental
results it can be inferred that the studied organic solvents failed to provide effective
contribution to the dehydrogenation of the –OH group of HMF under the applied reaction
condition.
Expansion of the substrate scope
To investigate the scope of the developed method, different substrates with various
functional groups and electronic properties were dehydrogenated using Rh/C catalyst at 150
ºC and 8 MPa of CO (Table 3). At first, our target was biomass-based simple furanic
2
compounds such as furfuryl alcohol and 2-nitrofurfuryl alcohol. Both of the compounds
were successfully dehydrogenated to their corresponding aldehydes with 100 % selectivity
within the reaction time of 4 h, but the conversion was 81.2 % and 45.3 % for furfuryl

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alcohol and nitro-furfuryl alcohol, respectively (Table 3; Entry 1 and 2). Upon extension of
the reaction time to improve the conversion, nitro-furfuryl alcohol results in the formation
of some black sticky compounds, which was difficult to analyze. Furthermore, under the
same condition, 2, 5-furandimethanol was also converted to DFF with 100 % selectivity
(Table 3; Entry 3). Unfortunately, < 10 % conversion was detected for tetrahydrofurfuryl
alcohol (Table 3; Entry 4). We subsequently explored the most studied dehydrogenation of
benzyl alcohol and its p-substituted derivatives afforded corresponding aldehydes with
excellent conversion and selectivity (Table 3; Entry 5 to 9). Benzyl alcohol was
dehydrogenated to benzaldehyde with the conversion of > 99 % and the complete
selectivity to benzyl alcohol was attained within the reaction time of 4 h (Table 3; Entry 5).
Depending on the nature of the functional group, catalytic activity varies. A change in the
substrate conversion was observed based on the presence of electron donating or
withdrawing substituents in benzyl alcohol, which followed the order of –OMe (100 %) > -
Cl (75.6 % ) > -NO (45.6%) (Table 3; Entry 6, 8 and 9) This result clearly indicates that
2
.
substrates with electron donating group are more reactive than those with the electron
withdrawing group. Moreover, the conversion of dimethoxy benzyl alcohol was slightly
reduced (81.1 %) in comparison with the p-methoxy benzyl alcohol possibly due to the
steric effect, (Table 3; Entry 7). It has to be mentioned that the substituents remain
unaffected under the condition used. In addition, 2-phenylethanol was also successfully
dehydrogenated to 2-phenylacedtaldehyde (selectivity= 97.9 %) with the conversion of >
9
6 % and the reaction time of 4 h (Table 3: Entry 10). Applying the same condition, the
dehydrogenation of 1, 2, 3, 4-tetrahydronaphthol, as a representative of multi-cyclic

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compound, was also attempted. The conversion reached to 95 % after 6 h and a mixture of
tetralone (55.5 %) and tetralin (44.5 %) was formed (Table 3: Entry 11). Cyclohexanol is an
example of saturated ring compound was also transformed to cyclohexanone with the
moderate conversion of 58 % (Table 3; Entry 12). Again, we tested the versatility of the
method on the selective dehydrogenation of different allylic alcohols for production of α -
β unsaturated aldehydes. Cinnamyl alcohol, and geraniol were successfully dehydrogenated
to cinnamaldehyde (selectivity= 98 %) and citral (selectivity= 75.2 %), respectively (Table
3
: Entry 13 and 14). The dehydrogenation of diols were also important to check for the
present reaction system. When 1, 4 butanediol and 1, 5-pentanediol were subjected to the
dehydrogenation, both of them were converted mainly to lactone with the selectivity of
1
00 % and 90 %, respectively (Table 3; Entry 15, 16). A nominal amount of 5-
hydroxypentanal (selectivity= 9.6 %) was also detected (Table 3: Entry 15). On the other
hand, 1-hydroxy-2-pentanone was the sole product from the dehydrogenation of 1, 2
pentanediol (Table 3; Entry 17) obtained after 18 h of reaction with ∼ 60 % of conversion.
Catalyst recycling
As a heterogeneous catalyst, recycling of Rh/C is one of the most important criteria
considering the environmental and economic sustainability of the process. The used Rh/C
catalyst was subjected to recycling and the results are shown in Figure 6. The used catalyst
th
retained its catalytic activity until the 4 recycle and then conversion decreased slightly, but
selectivity of DFF remain unaltered. Different spectroscopic characterization techniques
were employed to find out the possible reasons for reduced catalytic activity after
confirming the stability of the metal by hot filtration test. A transmission electron

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microscopy (TEM) study of the catalyst before (supporting information; Figure S4 a) and
after (Figure S4; b) the reaction did not reveal any sign of agglomeration of metal particles
or deposition of carbonaceous materials. It is reasonable to believe that catalyst life
generally improved when the reaction was conducted in scCO2.
-1
The FTIR spectra of fresh and used catalysts were recorded in the range of 4000- 400 cm
and depicted in Figure 7a and 7b, respectively. The spectrum of the fresh catalyst shows a
-1
broad band of –OH vibration in the region of 3500- 3300 cm due to the existence of
-1
surface hydroxyl group and chemisorbed water. A peak at 3110 cm corresponds to the
-1
=
C-H stretching was also evident. In addition, a very sharp peak at 1395 cm and a slightly
-1
broad peak at 1500 cm can be attributed to the aliphatic C-H bending and aromatic C-C
stretching, respectively were present in the spectrum. A very small peak corresponds to the
-1 30a, b
C-O asymmetric and symmetric stretching vibration was detected at 1160 cm
. Figure
7
b shows the FTIR spectra of the used catalyst, which revealed that the broad peak in the
-
1
region of 3500-3000 cm present in the fresh catalyst became very broad with a maxima at
-1
-1
3
392 cm and a small new peak was visible at 2800 cm corresponds to the aldehyde
-1
group. Furthermore, peaks at 1672, 1570, 1513 and 1362 cm , characteristic of HMF in the
fingerprint region of C=O and C=C as observed from the overlay spectra of HMF and the
3
0c
used catalyst (Figure S5) . The IR absorption peak associated with the O-H and C-O
-1
stretching vibration of alcohol appears in the region of 1260-1050 cm . The peak assigned
-1
to C-O stretching vibration at 1190 cm was slightly shifted to the lower wavenumber of
-1
-1
1
182 cm . On the other hand, a large shift of ∼ 20 cm towards the lower wavenumber
-1
-1
(
1060 cm ) was observed for the band at 1080 cm (O-H stretching vibration) of HMF,

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which indicate that HMF remain adsorbed through the –OH group on the catalyst
3
0d
surface.
To get a further insight into the catalyst structure, X-ray photoelectron spectra (XPS)
measurement was conducted on fresh and recycled Rh/C (Figure 8). Figure 8a exhibits the
+3
de-convoluted spectra of Rh (3d ). The spectra of the fresh catalyst mainly contain Rh as
5/2
indicated by an intense peak at 308.5 eV. In addition to the dominant signal, a peak at 306.5
0
31a
and a spin-orbit splitting of 4.7 eV representing Rh can also be seen . In the spectrum of
the recycled catalyst, the intensity of the peak at 306.5 eV were increased followed by the
+
3
decreased intensity of Rh peak without any shifting of the peak position (Figure 8b). Thus,
generation of hydrogen by the dehydrogenation of HMF might be led to the reduction of
+
3
0 31b
Rh to Rh
. The O (1s) spectrum of the fresh catalyst evidences two peaks at 530.9 eV
3
2
and 532.5 eV, which could be relate to the O-anion in oxide and surface hydroxyl group,
3
3
respectively (Figure 8c). On the other hand, although the major components of the O (1s)
spectrum of the spent catalyst remain same, the intensity of the characteristics of oxide O
peak at 530.9 eV was reduced (Figure 8d). This might be attributed as a consequence of the
removal of the O bound to Rh. Furthermore, two other peaks were also present in the higher
binding energy region of the XP spectrum of the recycled catalyst. It is difficult to provide
the specific assignments of the O (1s) spectra related to Rh as there are number of
3
4
uncertainties in the binding energy values of the various O containing species . Figure 8e
and f represents the curve fitted spectra of C 1s of Rh/C before and after the reaction, which
exhibit a main peak at 284.2 eV assigned to the aliphatic and amorphous carbon and a peak
at 285.0 eV (C=C-C and C-H). However, additional peaks at 286. 0 eV (C-O-H or C-O-C)

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and 287.5 (C=O) related to the different O containing moieties were observed in the
3
1a
0
+3
spectrum of the used catalyst . From the calculation of the ratio of Rh /Rh suggested an
increase from 0.3 to 1.5 in the recycled catalyst which might explain the further loss of the
th
catalytic activity leading to the decreased conversion after the 4 recycle.
Proposed mechanism
Undoubtedly, the elucidation of the reaction mechanism will be helpful to predict the
reaction pathway and allow further optimization of the reaction process. For the present
case, a detailed mechanism is yet to be deduced, but we can propose an outline of the
possible mechanism (Scheme 2). It is widely accepted that the dehydrogenation reaction of
alcohol was proceed through the formation of alkoxy intermediate by the dissociation of
3
5
alcohol and this reaction known to occur on the noble metal surface . It is proposed that
in the catalytic cycle, adsorption of HMF occurred through the –OH group on the metal
surface forming a metal alkoxide intermediate (Scheme 2). The dissociative adsorption of
alcohol can be confirmed by the presence of C-O-H species from the spectroscopic analysis
of the used catalyst as described above. The -OH proton is adsorbed by neighboring surface
oxygen (Scheme 2). As a result, the C-O bond became stronger with the weakening of the
36
C-H bond . In the next step, C-H bond cleavage occurs leading to the formation of
4
b
aldehyde leaving behind a protonated metal surface . This is consistent with the
observation of Xu et al. who reported that the O atom from the metal oxide promotes the
Bronsted acid-base reaction with the hydrogen of the -OH group to dehydrogenate the
3
5c
reactant . From the time course of the reaction, experimentally we were unable to detect
any intermediate to confirm the reaction path of HMF dehydrogenation. Hence, we

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incorporate theoretical investigations using density functional theory calculation for
preliminary understanding of the reaction (detail method is in the supporting information).
Figure 9a, b and c show the images of reactant (A), calculated transition state (TS ) and
AB
the product (B), respectively in the presence of a catalyst. Each structure was optimized and
the calculated reaction energies of each step are shown in Table 4. The reaction proceeds
through the transformation of reactant (A) to transition state TSAB corresponds to the
reaction energy change of -145.7 Kcal/mol (Table 4, Entry 1). The conversion of TSAB to
the product occur through the breakage of C-H bond, and requires an energy of 156.9
Kcal/mol (Table 4, Entry 2). Thus, based on the calculated energy, breaking of C-H bond
could be the slowest step and the possible rate determining step for this reaction.
Furthermore, the endothermic nature of the reaction was validated from the reaction energy
+
3
of 11.2 Kcal/mol (Table 4, Entry 3). XPS measurement, revealed that a part of the Rh in
0
the fresh catalyst was reduced to Rh after the reaction, which may arise from the
4
b
deprotonation of metal surface . As Rh has strong hydrogen adsorption capability, it may
hinder desorption of hydrogen from the catalyst surface and consequently the
dehydrogenation process. However, introduction of the large amount of scCO has different
2
beneficial effect such as it could eliminate the hydrogen because of high solubility of
hydrogen in scCO and shifted the equilibrium towards the product, thus, improved the
2
catalytic performance.
The reduction possibility of scCO , cannot be ignored because it can be hydrogenated to
2
37
methane in the presence of Rh/C . However, on the basis of current data, it is critical to

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confirm as there was not enough evidence, which needs further investigation and currently
under consideration.
Conclusion
In conclusion, we have developed an efficient catalytic dehydrogenation methodology free
from any kind of oxidant or additive for the selective dehydrogenation of biomass-based
platform molecule like HMF using Rh/C and scCO . The developed method is promising in
2
the conversion of HMF to DFF with very high selectivity. Indeed, the introduction of
scCO significantly improved the performance. Neither scCO , nor Rh alone appear as
2
2
suitable for the reaction, but a combination of both provides high catalytic performance and
excellent product yield through the proper control of the reaction variables. Excluding
hydrogen, no other products were detected along with DFF. Interestingly, the reverse
reaction of hydrogenation was also possible depending on the reaction condition. This
process allows an easy separation of product simply by depressurization. Based on the
characterization of the fresh and used catalyst, a reaction mechanism was proposed via
+
3
0
redox pathway of Rh to Rh .
The catalytic system provides appreciable reusability and is applicable for a series of
different types of substrates with various substituents, which generates corresponding
carbonyl compounds with good to excellent selectivity without affecting the substituent
present. Our future challenge is to improve the longevity of the catalyst. Further
development is necessary focusing on the determination of exact reaction mechanism for

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proper understanding of the status of scCO and also to extend the dehydrogenation of other
2
aliphatic alcohols.
Experimental section
Materials: 5-hydroxymethylfurfural (HMF) (Aldrich) was used as received. Carbon
dioxide (>99.99%) was supplied by Nippon Sanso Co. Ltd. Pd/C, Pd/Al O Pt/C, Pt/
2
3,
Al O , Rh/C and Ru/C were from Aldrich. Ru/ Al O and Rh/ Al O , were from Wako
2
3
2
3
2
3
Pure Chemicals. In each case, metal content was ∼5 %. Unless otherwise stated all the
chemicals were used without any further modification. Metal catalysts supported on
MCM-41 were synthesized in our laboratory using a modified method. Detail of catalyst
characterization techniques are in the supporting information section.
Phase observation: The phase behavior of HMF in scCO at 150 °C was studied separately
2
in a 10 ml high pressure view cell. HMF was introduced into the cell followed by the
introduction of CO after the stabilization of temperature. For phase observation during the
2
reaction, the content was stirred continuously. The image was recorded after stopping the
stirrer (Supporting information; Figure S2), where HMF and CO existed as a bi-phasic
2
system.
Catalytic performance: A 50 ml batch reactor was used for testing the
dehydrogenation of HMF in laboratory scale. The reactor was placed in a hot air
circulating oven. In a typical experiment, a specified amount of the catalyst and
substrate were introduced into the reactor and flushed with CO for 5 times to remove
2
the air inside the reactor. The reactor was heated for a specified amount of time to

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stabilize to the required temperature of 150 °C. After that CO , was charged into the
2
reactor using a high-pressure liquid pump and then compressed to the desired pressure.
The content of the reactor was stirred with a magnetic stirrer bar during the reaction.
The reactor was quenched using an ice bath after the reaction, followed by the
separation of solid catalyst from liquid product simply by filtration. The products were
identified by GC-MS against a standard, which was also used for qualitative analysis.
Detailed analytical method is given in the supporting information section. For all results
reported, conversion was calculated on the basis of unreacted substrate (obtained from
GC analysis) divided by total substrate reacted. Product selectivity was also calculated
from the desired product divided by the total products obtained. Catalytic performance
was also tested in N atmosphere and then similar procedure was followed for the
2
separation and analysis of the product as described for CO . In addition, when the
2
reaction was conducted in organic solvent medium, 10 ml of solvent was used instead
of CO2.
Acknowledgements
H. K. is funded by the Japan-U.S. cooperation project for research and standardization of
Clean Energy Technologies, The Ministry of Economy, Trade and Industry (METI), Japan
and CREST, Japan Science and Technology (JST).

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