Room-temperature transfer hydrogenation and fast separation of unsaturated compounds over heterogeneous catalysts in an aqueous solution of formic acid

Article abstract of DOI:10.1039/c4gc00981a

The facile conversion of olefins and unsaturated biomass to saturated compounds is achieved over heterogeneous catalysts composed of noble metal nanoparticles and carbon nitride. Reactions could proceed smoothly at room temperature in water using formic acid as the hydrogen source. The reusability of such a hybrid catalyst is high due to the strong Mott-Schottky effect between the metal nanoparticles and the carbon nitride support. The fast and automatic separation of the as-formed saturated hydrocarbons from water combined with the mild reaction conditions and the excellent reusability of catalysts make the catalytic process a highly "green" path for hydrogenation of unsaturated compounds and biofuel upgrading. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00981a

Green Chemistry
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Room-temperature transfer hydrogenation and
fast separation of unsaturated compounds over
heterogeneous catalysts in an aqueous solution
of formic acid†
Cite this: Green Chem., 2014, 16,
3746
Received 28th May 2014,
Accepted 16th June 2014
DOI: 10.1039/c4gc00981a
Ling-Hong Gong, Yi-Yu Cai, Xin-Hao Li,* Ya-Nan Zhang, Juan Su and
Jie-Sheng Chen*
www.rsc.org/greenchem
The facile conversion of olefins and unsaturated biomass to biomasses, because it may be used to produce furan chemi-
saturated compounds is achieved over heterogeneous catalysts cals, chemical solvents, and liquid biofuels.4 However, furfural
composed of noble metal nanoparticles and carbon nitride. by itself is toxic and thus not suitable for large-scale appli-
Reactions could proceed smoothly at room temperature in water cations as fuels or solvents. In addition, furfural and its corres-
using formic acid as the hydrogen source. The reusability of such a ponding derivative compounds, including furfuryl alcohol and
hybrid catalyst is high due to the strong Mott–Schottky effect 2-methylfuran, cannot be directly used as motor fuel because
between the metal nanoparticles and the carbon nitride support. they tend to polymerize into solid byproducts at elevated temp-
The fast and automatic separation of the as-formed saturated eratures due to the presence of unsaturated carbon–carbon
hydrocarbons from water combined with the mild reaction con- bonds.5
ditions and the excellent reusability of catalysts make the catalytic
The main purpose of biofuel upgrading is to increase the
process a highly “green” path for hydrogenation of unsaturated quality of the fuels by removing the oxygen and increasing the
compounds and biofuel upgrading.
hydrogen/carbon ratios. Taking furfural for example, hydro-
deoxygenation6 (HDO) and hydrogenation are the two essential
processes (Fig. 1) to gain high quality biofuels, which can be
used directly as a motor fuel or a gasoline additive in P Series
type fuels.5 Again, green and cost-effective methods for hydro-
genation of furfural, furfuryl alcohol or 2-methylfuran are
highly required for the real use of biofuels at low cost.
The science of green chemistry was developed to meet
the increasing demand for environmentally benign chemical
processes.7–12 In this regard, the combination of efficient
and reusable catalytic protocols for transfer hydrogenation
Introduction
Hydrogenation of unsaturated carbon–carbon bonds is a vital
reaction in organic chemistry, which has been used in the syn-
thesis of fine chemicals, particularly in flavor and fragrance
chemistry and pharmaceuticals.1 In conventional methods,
hydrogenation of the CvC bonds was mainly conducted in the
gas phase with an excess amount of hydrogen gas (>25 times).
Gaseous hydrogen is difficult and dangerous to handle due to
its hazardous properties. As an alternative, transfer hydrogen-
ation of unsaturated bonds using stable and abundant hydro-
gen sources is safer and environmentally benign.
Among the hydrogenation of unsaturated CvC bonds,
upgrading furfural and its unsaturated derivatives into satu-
rated hydrocarbons with better thermal stability and low
toxicity is the key step to realize the practical application of
furfural in the realm of biofuels and sustainable energy
supplies.2–5 Furfural is an important intermediate from
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University,
Fig. 1 Typical conversion paths of transforming furfural to 2-methyl-
tetrahydrofuran (MTHF) via hydrodeoxygenation (HDO) and ring hydro-
genation (RH). Aldehyde hydrogenation (AH) of furfural was sometimes
involved.
Shanghai, 200240, People’s Republic of China. E-mail: xinhaoli@sjtu.edu.cn,
chemcj@sjtu.edu.cn; Fax: (+86)-21-5474-1297
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c4gc00981a
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Table 1 Study of the reaction conditionsa
reactions under mild conditions and in environmentally friendly
solvents is the most important issue. Herein, we report the
application of the Mott–Schottky catalyst based on g-C3N4 sup-
ported noble metal nanoparticles (M/CN) in room-temperature
hydrogenation of 2-methylfuran and other compounds with
CvC bonds. All reactions were conducted in an aqueous
solution of formic acid (FA) under ambient atmosphere. The
heterogeneous Mott–Schottky catalyst can be reused for tens of
cycles without any obvious loss of its catalytic activity.
With respect to catalytic reactions, metal nanoparticle-
based heterogeneous catalysts have good reusability as com-
pared with homogeneous ones. Current heterogeneous cata-
lysts8 available for transfer hydrogenation of CvC bonds are
mainly carbon supported noble metal catalysts9 with low
efficiency. Most examples of heterogeneous catalytic reactions
were conducted in organic solvents, whilst high temperatures
and/or sonication were usually required to ensure a high con-
version.1,8,9 In order to improve the catalytic activity of noble
metal nanoparticle based catalysts, there have been many
attempts to rationally control the size, shape and/or content of
alloying metals.10 Critical control of the structure and content
on the nanometer scale required very complicated fabrication
processes, which are not suitable for industrial applications.
Thus, current efforts have been turned towards exploiting
useful support effects to improve both the dispersion and the
activity of the embedded metal nanoparticles.7
Entry
Catalyst
Solvent
C.b (%)
S.b (%)
1
2
3
4
—
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Ethanol
CH3CN
Ethyl acetate
DMF
THF
—
Trace
—
—
>99.9
—
>99.9
—
—
4.44
—
—
57.98
>99.9
>99.9
32.70
76.02
—
—
—
—
>99.9
—
>99.9
—
—
>99.9
—
Pt/CN
Au/CN
Ag/CN
Pd/CN
Pd/CN
Pd/CN
Pd/CB
Pd/N-LC
Pd/SiO2
Pd2+/CN
CN
Pd/CN
Pd/CN
Pd/CN
Pd/CN
Pd/CN
5
6c
7d
8e
9f
10
11
12
13
14
15
16
17
—
>99.9
>99.9
>99.9
>99.9
>99.9
a Standard conditions: 2.5 mL of H2O, 0.1 mmol of styrene, 0.3 mmol
of FA, 10 mg of catalyst, 15 min, T = 298 K. b Conversions (C.) and
selectivity (S.) were determined by GC by using a FID detector.
c Without FA. d The reaction was conducted in N2. e CB: carbon black.
f N-LC: Nitrogen-doped layered carbon.
theoretical results.7 Pd/CN as the best catalyst here was thus
selected for the following reactions.
We further optimized the molecular ratios of FA and sub-
strate for hydrogenation of styrene over Pd/CN in water at
room temperature. The total conversion of styrene to ethylben-
Results and discussion
We recently introduced a series of M/CN dyads as Mott– zene was achieved when three equivalents of FA or more were
Schottky catalysts, the activity of which was significantly pro- used (Fig. S3†). Blank experiments indicated that the reduction
moted by the varied electron density at the interface of rectify- of styrene could not proceed in the absence of a catalyst or FA
ing metal–semiconductor contact.7,14 Extending the concept of (Table 1, entries 1, 6). Various solvents were screened over Pd/
the Mott–Schottky catalyst to organic reactions is of great CN (Table 1, entries 13–17) to test the solvent effect. The
importance in the search for both laboratory-scale and indus- survey shows that water, CH3CN and acetic ester provided the
trial synthesis. FA was selected as a possible hydrogen source same outcome, which is better than ethanol, DMF and THF.
here due to its low toxicity, low cost and high stability. More However, as compared with CH3CN (or ethyl acetate) which is
importantly, FA can be obtained in large quantities via hydro- stimulating and nocuous, water is more environmentally
genation of waste carbon dioxide from industry, biomass pro- friendly and cheaper, meeting the requirements of green
cessing and artificial photosynthesis.1,7 Consequently, FA can chemistry. It is worth noting that the transfer hydrogenation
be considered as a sustainable medium for hydrogen storage, reactions of CvC bonds over heterogeneous catalysts reported
promising its application as a sustainable choice of hydrogen in the literature could only be conducted in toluene or a
resource for organic synthesis.
mixture of toluene and other solvents, where high temperature
A series of M/CN catalysts (M = Ag, Au, Pd and Pt) and (or sonication) and protection gas were required to ensure the
control samples were fabricated via a modified wet impreg- catalytic performance.1,11
nation method (for detailed methods, see ESI†).12–14 The
For practical applications, fast separation of final products
powder X-ray diffraction (PXRD) patterns (Fig. S1†) and trans- and catalysts from mother solution is an important aspect to
mission electron microscopy images (TEM, Fig. S2†) of the be considered for reducing the additional cost of purification
M/CN materials confirmed the formation of metal nanoparticles of the final products. The target products here are saturated
on the surface of CN. The work functions of most noble metals hydrocarbon with rather low solubility in water, making it
are lower than that of g-C3N4, making it possible to construct possible to separate the oil phase from the aqueous solution
rectifying contact between g-C3N4 and noble metals. However, used here. The as-formed ethyl benzene (Fig. 2 and Fig. S4†)
Pd/CN offered the highest conversion (entries 2–5, Table 1) can be separated automatically after the reaction, again reveal-
due to the high activity of Pd in activating the hydrogen source ing the advantage of our green catalysis system by using only
FA, which has been demonstrated by both experimental and water and FA as the reactants. Moreover, the heterogeneous
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Fig. 2 The fast separation of ethylbenzene after the hydrogenation
reaction of styrene. The as-formed ethylbenzene was separated auto-
matically from the water phase after reaction within 10 minutes. The
solid catalyst was precipitated automatically at the bottom and separ-
ated facilely from the oil phase via filtration or decantation. Reaction
conditions: 10 mmol of styrene, 250 mL of H2O, 30 mmol of FA, 500 mg
of Pd/CN, 298 K.
catalyst can also be separated from the mother solution via
simple decantation for further use in the following reactions.
After knowing the activity of Pd/CN in reducing CvC bonds
by FA, we thus turned towards the hydrogenation of 2-methyl-
furan to 2-methyltetrahydrofuran (MTHF), which is the key
step for increasing the hydrogen/carbon ratios and thus trans-
forming the biomass (furfural) to renewable biofuel (MTHF)
(Fig. 1). Recently, Pd nanoparticles modified with certain
organic tail ligands were utilized as high selective catalysts for
the formation of methylfuran.15 Interestingly, Pd/CN nanocata-
lyst can further catalyze the hydrogenation of 2-methylfuran
which is a conjugated diene and gave a high yield (Fig. 3A) at
room temperature. Such a yield value far exceeds those over
the carbon-supported Pd catalysts (Pd/N-LC12 and Pd/C) and
the SBA-15-supported Pd catalyst (Pd/SiO2 13) under the same
conditions (Fig. S5†). For Pd/CN and Pd/N-LC, the surface
area, types of nitrogen functional groups, and sizes of PdNPs
and Pd loadings (8 wt%) are similar (Fig. S6 and Table S1†),
Fig. 3 (A) The path of hydrogenation of 2-methylfuran to 2-methyl-
tetrahydrofuran (MTHF). Reaction conditions: 0.1 mmol of 2-methylfuran,
2.5 mL of H2O, 0.5 mmol of FA, 10 mg of Pd/CN, 298 K. (B) Reuse of the
Pd/CN catalyst for transfer hydrogenation of methylfuran. Reaction con-
ditions: 0.1 mmol of methyl furan, 5 mL of H2O, 0.5 mmol of FA, 20 mg
of catalyst, 298 K, 30 min. Regeneration of the Pd/CN-used catalyst was
conducted after 31st run by dispersing the separated catalyst in 0.1 M
NaOH for 12 h followed by completely washing with water. (C) The
EXAFS Pd K-edge spectra of Pd/CN and Pd/CN-used; the intensity
of the white line (arrow) reflected the enriched electron intensity of
supported Pd NPs. Inset (B): the TEM image of the deactivated catalyst
(Pd/CN-used) after 31 runs.
excluding the structure effect of the catalysts on the catalytic described in our previous work concerning photocatalytic
performance. The fact that the nitrogen-rich sample Pd/N-LC Suzuki coupling reaction and dehydrogenation of FA.7c,14
(N content: 10 at%) also offered no conversion rather
Besides high catalytic activity, the reusability and stability
suggested that the strong adsorption of formic acid is not are also important issues for practical applications of Mott–
enough to ensure the activity of Pd/N-LC for such a transfer Schottky catalysts in industry. We thus tested the catalytic per-
hydrogenation reaction here. An obvious decrease of photo- formance of the used Pd/CN catalyst for transfer hydrogen-
lumincence intensity of CN (Fig. S7†) after loading of Pd NPs ation of 2-methylfuran. In fact, the Pd/CN catalyst was
directly indicated the charge transfer from CN to the Pd metal successfully recycled without any loss of activity after at least
and thus an obvious Mott–Schottky effect at the Pd–CN inter- 26 times of reuse (Fig. 3B) with a simple base washing process
face. Consequently, the superb catalytic activity of Pd/CN is using 0.1 M NaOH solution after each run (for experimental
attributed to the Mott–Schottky effect, which has been details please see ESI†). Pd leaching was excluded in the
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current catalytic system by inductively coupled plasma atomic remained unchanged after 31 runs of catalytic reactions. We
emission spectrometry (ICP-AES) analysis with the concen- thus can exclude the effect of structural variation on the cataly-
tration of Pd in the filtrate below the detection level of the tic activity. As CN stays stable in FA solution, the band struc-
equipment (0.1 ppm).
ture of CN also did not change after multiple uses. Therefore,
To further verify the stability of our catalyst, another five it is presumable that the interface of Pd and CN may be
runs were conducted with a gradual decrease in activity. At the changed by the protonation of the surface of CN in FA solution
31st run, the yield drops to about 60%. However, the chemical (Fig. S10†). Such a protonation process is common in CN-
structure (Fig. S8a†) and morphology (Fig. 3B and S8b†) of based catalysis and photocatalysis due to the presence of termi-
both the Pd nanoparticles (Fig. S9†) and the CN support nal amino groups and bridging nitrogen on the surface of
Table 2 Transfer hydrogenation of various unsaturated compoundsa
Entry
1
Substrate
Product
T (min)
C. (%)
>99
S. (%)
>99
15
2
3
30
30
>99
98
>99
>99
4
30
94
>99
5
60
30
60
15
94
>99
60
>99
>99
>99
>99
6
7
8b
>99
9
60
70
56
>99
>99
10
180
11c
60
>99
21
12
13
180
180
52
>99
—
—
14d
120
64
>99
a Standard conditions: 2.5 mL of H2O, 0.1 mmol substrate, 0.3 mmol of FA, 10 mg of Pd/CN, 298 K. b 0.5 mmol of FA was added. c 79% of
C6H5CH2CH2COOH was detected. d 0.5 mmol of FA was added, 20 mg of Pd/CN, 285 K.
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CN.7,14 An adsorbed layer of protons as a “contact resistance” diphenylethylene (entry 13) has no reactivity even under more
leads to a loss of voltage or the coupled chemical potential at harsh conditions presumably due to its steric effect, whilst
the metal–semiconductor interface,7a which was unambigu- transfer hydrogenation of trans-1,2-diphenylethylene pro-
ously demonstrated by the extended X-ray absorption fine ceeded smoothly with very high selectivity under standard con-
structure spectra (EXAFS) analysis (Fig. 3C). The enriched elec- ditions (entry 12). As shown in Table 2, among mono-
tron intensity of Pd nanoparticles in Pd/CN was confirmed by substituted styrene (entries 2–4), the involvement of fluorine
the highest intensity of a white line in EXAFS K-edge spectra of substitution decreases the electron density of CvC bonds,
all samples (Fig. 3C). The deactivated Pd/CN (named Pd/CN- thus increasing the time for complete conversion. Moreover,
used in Fig. 3B) had a lower white line, suggesting a lower elec- para-fluorostyrene was more active than meta-fluorostyrene
tron intensity of Pd nanoparticles. Such a loss is however and ortho-fluorostyrene, which is in accordance with position-
unwanted in our Mott–Schottky catalysts as reflected by the ing regularity of the benzene ring.
significant decrease in the final catalytic activity. By following
It is worth noting that transfer hydrogenation of α,β-unsatu-
this principle, deprotonating the interfacial area of the used rated ketone, exemplified by chalcone here (entry 10, Table 2),
catalyst should be sufficient to recover the deactivated cata- was also achieved over the dyadic catalyst at room temperature
lysts. Indeed, the deprotonated catalyst offered total conver- in water with very high selectivity and good conversion. Such a
sion of the substrate again without obvious loss of activity in high conversion is comparable with that over benchmark
the following 2 runs (Fig. 3B and Fig. S11†). Again, this obser- homogeneous catalysts.17 Cinnamic acid benzyl ester (Table 2,
vation reveals the key role of the support effect, named the entry 11) gave a relatively low yield (21%) of the target product
Mott–Schottky effect as described in our previous work7a–c at due to the fracture of the ester group according to GC-MS ana-
the metal–CN interface for ensuring the catalytic performance lysis (Fig. S15†). Further investigations on the asymmetric reac-
of Pd nanoparticles. In order to demonstrate this effect, we tions of α,β-unsaturated ketones are in progress. Nevertheless,
also tested the reusability of Pd/SiO2, one of the catalysts the functional group tolerance and the high chemico-
without the Mott–Schottky effect, showing a lower reusability selectivity of metal/CN dyadic catalysts have already been
and stability (Fig. S12†). Such a Mott–Schottky effect can be demonstrated here.
further proved by the enhanced activity under photo-
irradiation (Fig. S13†) for transfer hydrogenation of nitrocom-
pounds, where the transfer hydrogenation reaction did not
consume electrons.7a–c,14 CN supported noble metal nanoparti-
cles can be thus considered as stable and reusable hetero-
geneous catalysts for practical applications.
Conclusions
In conclusion, we investigated the possibility of significantly
promoting the activity of metal NPs using the Mott–Schottky
catalyst for transfer hydrogenation of unsaturated carbon–
carbon bonds. FA was used as the liquid hydrogen resource.
This work is the first example of a heterogeneous catalyst for
chemicoselective transfer hydrogenation of unsaturated
organic compounds to saturated ones in water at room temp-
erature. Fast separation of the oil phase from the aqueous cata-
lytic media combined with the high stability of the Pd/CN
catalysts and low cost and toxicity of hydrogen source FA
renders the catalytic system rather suitable for industrial appli-
cations. Further engineering the interface and electronic struc-
ture of metal-NPs/CN dyads should generate new possibilities
to develop more efficient and sustainable catalysts for biofuels
and the green catalytic system. Efforts are also being devoted
to finding non-precious metal-based nanoparticles to con-
struct the Mott–Schottky catalyst for sustainable organic syn-
thesis in our laboratory.
Considering the possibility of dehydrogenation of FA over
Pd/CN to release H2, which can also be used to reduce CvC
bonds, further control experiments were conducted to investi-
gate the reduction path of 2-methylfuran in our catalytic
system. Under fixed conditions, the conversion of FA signifi-
cantly rose up to at least 2.5 times after the introduction of
2-methylfuran (Fig. S14†). This is to say that extracting the
hydrogen of FA to reduce 2-methylfuran over Pd/CN is much
faster and easier than to form H2 gas, which requires more
energy to overcome the overpotential of the chemisorbed H at
the noble metal surface.16 In other words, formation and acti-
vation of hydrogen gas was not essential for the reduction of
unsaturated carbon bonds here.
With the optimized reaction parameters, we investigated
the generality of the catalytic reactions over Pd/CN. In some
cases, slight adjustment of the reaction conditions was
required due to the distinct reactivity of reactants. As shown in
Table 2, a variety of unsaturated carbon–carbon bonds were
successfully hydrogenated in good yields. The transfer hydro-
genation of unsaturated CvC bonds was successfully achieved
even in the presence of reducible substituents, such as carbo-
Acknowledgements
nyl and ester groups (Table 2, entries 10–11). Both isolated This work was supported by the National Basic Research
olefins and conjugated olefins afforded the corresponding Program of China (2013CB934102, 2011CB808703) and the
alkanes in good yield (entries 1–9). Besides olefins, cycloalk- National Natural Science Foundation of China. The authors
enes were also studied (entries 8–9) with moderate to high con- thank Shanghai Synchrotron Radiation Facility for providing
versions and excellent selectivity. It is mentionable that cis-1,2- beam time (BL14W1) and H. Bao for data analysis.
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