A New Route to Cyclohexanone using H2CO3 as a Molecular Catalytic Ligand to Boost the Thorough Hydrogenation of Nitroarenes over Pd Nanocatalysts

Article abstract of DOI:10.1002/cctc.201900389

Carbon dioxide has been important in green chemistry, especially in catalytic and chemical engineering applications. While exploring CO₂ to produce cyclohexanone for nylon or nylon 66 that is currently produced with low yields using harsh catalytic methods, we made the exciting discovery that carbonic acid, generated from dissolved CO₂ in water, was utilized as molecular catalytic ligand to produce cyclohexanone via the hydrogenation of nitrobenzene in aqueous solution that uses Pd catalysts with a total yield higher than 90 %. Importantly, the gaseous nature of catalytic ligand H₂CO₃ profoundly simplifies post-catalysis cleanup unlike liquid or solid catalysts. This new green catalysis strategy demonstrated the universality for hydrogenation of aromatic compounds like aniline and N-methylaniline and could be broadly applicable in other catalytic field like artificial photosynthesis and electrocatalytic organic synthesis.

Full text of DOI:10.1002/cctc.201900389

Accepted Article
Title: A New Route to Cyclohexanone using H₂CO₃ as a Molecular
Catalytic Ligand to Boost the Thorough Hydrogenation of
Nitroarenes over Pd Nanocatalysts
Authors: Tian-Jian Zhao, Jun-Jun Zhang, Bing Zhang, Yong-Xing Liu,
Yun-Xiao Lin, Hong-Hui Wang, Hui Su, Xin-Hao Li, and Jie-
Sheng Chen
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A New Route to Cyclohexanone using H2CO3 as a Molecular
Catalytic Ligand to Boost the Thorough Hydrogenation of
Nitroarenes over Pd Nanocatalysts
Tian-Jian Zhao, Jun-Jun Zhang, Dr. Bing Zhang, Yong-Xing Liu, Yun-Xiao Lin, Hong-Hui Wang, Hui Su,
Prof. Dr. Xin-Hao Li*, and Prof. Dr. Jie-Sheng Chen*
Dedication ((optional))
Abstract: Carbon dioxide has been important in green chemistry,
especially in catalytic and chemical engineering applications. While
exploring CO2 to produce cyclohexanone for nylon or nylon 66 that is
currently produced with low yields using harsh catalytic methods, we
made the exciting discovery that carbonic acid, generated from
dissolved CO2 in water, was utilized as molecular catalytic ligand to
produce cyclohexanone via the hydrogenation of nitrobenzene in
aqueous solution that uses Pd catalysts with a total yield higher than
90%. Importantly, the gaseous nature of catalytic ligand H2CO3
profoundly simplifies post-catalysis cleanup unlike liquid or solid
catalysts. This new green catalysis strategy demonstrated the
universality for hydrogenation of aromatic compounds like aniline
and N-methylaniline and could be broadly applicable in other
catalytic field like artificial photosynthesis and electrocatalytic
organic synthesis.
needed to manufacture nylon 6 and nylon 66. [8,9] The production
of cyclohexanone at an industrial scale is typically achieved via
[10-13]
the oxidation of cyclohexane
hydrogenation of phenol
(Figure 1a) or the
[14-23]
(Figure 1b). Benzene can be
hydrogenated into cyclohexane (Figure 1a) under harsh and
high energy conditions, but the oxidation of cyclohexane to
cyclohexanone is inefficient, even when using hydrogen
peroxide as the oxidant. [10, 11] The conversion rates of phenol
from benzene are low (Figure 1b) via cumene peroxidation or via
direct hydroxylation of benzene even by the aid of noble metal-
based catalysts under harsh conditions. [14-23]
Introduction
Waste gas-carbon dioxide (CO2) reutilization has for some time
been understood as an important and desirable part of green
[1]
chemistry.
There are numerous “trash-to-treasure” examples
of CO2 catalytic and chemical engineering applications, and
there has been recently been renewed interest in energy storage
via reduction of CO2 [2] and in the chemical fixation of CO2 [3] into
high-valuable chemicals. However, until now, H2CO3, generated
from dissolved CO2 in water, has not been employed as a gas-
ligand additive for catalytic and chemical engineering
Figure 1. Oxidation of cyclohexane (a), hydrogenation of phenol (b), and the
hydrogenation of nitrobenzene featured in the present study (c).
applications. Whereas traditional organic ligands or other
[4-7]
additives
that are typically used to activate heterogeneous
Here, we describe the use of carbonic acid, generated from
dissolved CO2 in water, as unique catalytic “binder” ligands to
boost a mild and efficient synthetic process for transforming
benzene via nitrobenzene to cyclohexanone in high yields
(Figure 1c). As compared to other substrates (cyclohexane and
phenol), the synthesis of nitrobenzene from the complete
nitrification of benzene is only usually prepared using zeolite or
solid acid under relatively mild conditions with much lower
energy consumption (Figure 1a and 1b).[24-27] Besides, H2CO3
can activate the supported Pd nanocatalysts and thereby boost
the formation of cyclohexanone as the main product from the
hydrogenation of nitrobenzene in water at room temperature,
yielding cyclohexanone in excess of 90%; note that
cyclohexanone is usually the minor byproduct under pure H2
nanocatalysts incur additional costs from the need for separation
and purification of the target products, the reutilization of
gaseous state CO2 not only facilitates fast separations following
reactions, but also efficiently reduced CO2 emission in the
atmosphere which human activity contributes about 35 GT per
year. [1]
Our research group has been exploring the industrially
important synthesis of cyclohexanone, an important intermediate
for the production of the caprolactame and adipic acid that is
[*]
Tian-Jian Zhao, Jun-Jun Zhang, Dr. Bing Zhang, Yong-Xing Liu,
Yun-Xiao Lin, Hong-Hui Wang, Hui Su, Prof. Dr. Xin-hao Li and
Prof. Dr. Jie-sheng Chen
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
atmosphere.[28-30] Beyond being
a
new trash-to-treasure
application for it in green chemistry, this new highly efficient
catalytic system marked facilitates the separation process after
the reaction compared to systems that use a liquid or solid
catalyst. Moreover, our demonstration that H2CO3 can be used
Shanghai 200240 (People's Republic of China)
E-mail: xinhaoli@sjtu.edu.cn (X.H.Li); chemcj@sjtu.edu.cn
(J.S.Chen)
Supporting information for this article is available on the WWW
under https://doi.org/10.1002/cctc.201900389R2
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10.1002/cctc.201900389
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FULL PAPER
similarly as a catalytic ligand for the synthesis of many other
intermediates such as cyclohexanecarboxylic acid, substituted
detected when the water was replaced with anhydrous methanol
or tetrahydrofuran (THF) as the solvent (Figure 2c and Table S1).
Monitoring the hydrogenation of nitrobenzene over Pd/C in
CO2-H2 atmosphere throughout a time course (Figure 2e)
revealed a gradual increase in the yield of cyclohexanone, from
6% to 90%, dramatically outpacing the yield of the same
reaction with a pure H2 atmosphere (Figure 2d and 2e) by more
than 14 times.
methylcyclohexanone,
tetrahydro-1-naphthylamine,
tetrahydronaphthalene, and tetrahydroquinoline, the conceptual
door to a truly vast domain of potential applications in synthetic
chemistry and chemical engineering.
Results and Discussion
Heterogeneous Pd catalysts, especially Pd carbon supported
(Pd/C) catalysts (The characterizations of Pd/C were presented
as Figure S1 and S2), are currently preferred in industry for
cyclohexanone production due to their excellent universality for
hydrogenation of nitrobenzene with aniline as the main product
(Figure 2a-b).[31] In the absence of CO2, we found that only 6%
of nitrobenzene was transformed into cyclohexanone (Figure 2b),
with aniline being the main product. Strikingly, unexpectedly high
selectivity (Figure 2b) to cyclohexanone was achieved when a
mixed gas of CO2 and H2 was used to replace pure H2 gas: 90%
yields were achieved within 6 h when we optimized the molar
ratios of CO2 and H2 (CO2/H2) to 1:3 and utilized water as the
solvent (Figure 2c).
Figure 3. CO2-related additives (carbonates) (a) for optimized yields of
cyclohexanone (the molar ratios of carbonates (e.g. NaHCO3, Na2CO3) and
nitrobenzene were 10:1). Proposed structure of as-formed H2CO3 (b) from the
dissolved CO2 in water on the Pd surface for the proposed activation of
nitrobenzene. Optimized structures of nitrobenzene (c) and H2CO3-coupled
nitrobenzene in the presence of CO2 (d) on the Pd surface, and the
corresponding adsorption energies (e). Temperature Programmed Desorption
profiles of CO2/H2 (red line), H2 (black line), and CO2 (navy blue line) over 10%
Pd/C catalyst (f). Charge density difference stereogram of H2CO3-coupled
nitrobenzene on the Pd surface (g).
Figure 2. Proposed reaction routes (a) for the hydrogenation of nitrobenzene
into cyclohexanone in the presence of CO2 gas. Screening molar ratios of CO2
and H2 (b), solvents (c). Typical time courses of nitrobenzene hydrogenation to
cyclohexanone over Pd/C under pure H2 (d) or H2/CO2 (e) atmosphere.
Reaction conditions: 0.25 mmol nitrobenzene, 10 mg of catalyst, 5 mL of H2O,
pressure (1 bar), 6 h, room temperature. (black, green, red and gray lines
represent nitrobenzene, aniline, cyclohexanone and diamines, respectively).
The role of H2CO3 molecules in promoting the observed
selectivity to cyclohexanone over Pd nanocatalysts was further
examined via both experimental and theoretical methods. The
-
2-
promoting effect of HCO3 or CO3 on the formation of
cyclohexanone was negligible, yielding cyclohexanone at only
10% and 8%, respectively (Figure 3a and Table S2). Density-
functional-theory (DFT) calculations (The (111) plane of Pd was
chose in DFT calculation)[35,36] were performed to elucidate the
possible states of the nitrobenzene in the presence of CO2-
Water is essential for the hydration of enamine and also
essential following the deamination process to yield
cyclohexanone[32] (Figure 2a), as cyclohexanone was not
-
based complexes, including H2CO3, HCO3 CO32-, and CO2
(Figure 3c-d, Figure S3 and Table S3), on the surface of a Pd
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nanocatalyst.[33-35] Uniquely among these carbonates, the
simulations indicated that only H2CO3 can function to
significantly promote the adsorption of nitrobenzene on the Pd
surface; this results from its ability to form two hydrogen bonds
(1.61 Å and 1.67 Å) that increase nitrobenzene’s adsorption
energy on Pd from 2.13 to 3.19 eV. We noted only slight
changes in the adsorption energies of nitrobenzene when using
Similarly, the hydrogenation of benzene ring in methylbenzene
or styrene with weak coupling effect with the H2CO3 molecule
was not observed under fixed conditions (Table S4 and Figure
4d). Collectively, these results experimentally confirm the
essential contribution of the H2CO3 ligand to promoting the
hydrogenation of the meta carbons of the benzene ring.
-
HCO3 , CO32-, or CO2 species as possible ligands (Figure S3
and Table S3), underscoring their relatively weak interactions
with nitrobenzene molecules. Beyond elucidating the energetics
of the mechanism through which nitrobenzene binds to the Pd
catalyst, the specific functional role for H2CO3 that was revealed
by these theoretical simulations underscores the important fact
that water—which facilitates H2CO3 formation from CO2—is an
essential reaction component for the use of H2CO3 as a catalytic
ligand (Figure 3a and 3b). Similarly, the simulations revealed
that H2CO3 can also function to promote intermediate
hydrogenation product-aniline adsorpted on the Pd surface by
one hydrogen bonds (1.73 Å) that increase aniline’s adsorption
energy on Pd from 2.62 to 3.93 eV (Figure S4).
Figure 4. Control experiments using substituted nitrobenzene (a-c) and
styrene (d) as the substrates.
The promoting effect of H2CO3 for nitrobenzene was further
confirmed via Diffuse reflectance FT-IR spectroscopy (DRIFT)
analysis (Figure S9).[42-43] Typical weakened vibration signals of -
NO2 group of nitrobenzen-H2CO3 complex was observed as
compared to pure nitrobenzene sample, directly indicating the
adsorption and stablization of nitrobenzene by H2CO3. Most
importantly, a blue shift of the -NO2 DRIFT peaks from 1518 and
1344 cm-1 for the pure nitrobenzen to 1526 and 1347 cm-1
respectively for the nitrobenzen-H2CO3 complex further
confirmed the interaction between nitrobenzen and H2CO3 with
hydrogen bond.
Importantly, the binding effect of the absorbed CO2 molecules
doesn’t disturb the absorbance of H2 to supported Pd for further
reactions. Temperature programmed desorption (TPD) analysis
results (Figure 3f and Figure S10) indicate that CO2 showed no
obvious influence on the adsorption uptakes of H2 with
approximated desorption peak location and desorption volume
over 10% Pd/C in H2+CO2 (red line) and pure H2 (black line)
atmospheres even though a significant amount of CO2 could
also be adsorbed on Pd (Figure 3f bottom). Again, water is
essential to transform CO2 into H2CO3 to enhance the preferred
adsorption and further activation of nitrobenzene.
The DFT simulations revealed and confirmatory experiments
then confirmed the coupling effect of nitrobenzene with H2CO3
on the formation a divergent electron distribution of its benzene
ring (Figure 3g). The prediction that the hydrogenation of the
benzene ring becomes more favorable via the addition of the
first H radical to electron-deficient meta carbons (marked by red
rings in Figure 3g) was confirmed by a series of control reactions
(Figure 4a-d).
The substrate with all of its meta carbons blocked by methyl
groups (3,5-dimethylnitrobenzene) could only give a trace yield
(0.5%) of 3,5-dimethylcyclohexanone (Figure 4a), even with
higher loading of the catalysts and with longer reaction times.
The 4-nitrobenzotrifluoride-with its strong electron withdrawing
group (-CF3) adjacent to the meta carbons (Figure 4b) could also
completely quench the hydrogenation reaction of the benzene
ring; the p-nitroaniline was only hydrogenated into 1,4-
diaminobenzene, which could couple with two H2CO3 molecules
and thus destroy the divergent electron distribution of the
benzene ring, preventing any further hydrogenation (Figure 4c).
Moving beyond nitrobenzene, we excitingly found that the
enhancing effect of the H2CO3 catalytic ligand is also general for
the hydrogenation of various nitroarenes that possess at least
one exposed exposable meta carbon and lack strong electron-
attraction groups at their para position; we even observed the
enhancing effect when using Pd nanocatalysts immobilized on
other supports (Figure 5a-b). Indeed, the yields (94-97%) of
cyclohexanone over variously supported Pd nanoparticles
(Pd/TiO2, Pd/g-C3N4, Pd/γ-Al2O3 and Pd/C) were increased by
14–45.5 times with the assistance of H2CO3 as a molecular
catalytic ligand (Figure 5a). Further expanding the reaction
scope, in addition to H2CO3’s ability to promote hydrogenation of
various nitroarenes (Figure 5b) into their corresponding
cyclohexanone compounds, coupling of H2CO3 with the
functional groups of other aromatic compounds also boosted
hydrogenation of benzene rings (e.g., anisole, benzoic acid,
aniline, N-methylaniline, N, N-dimethylaniline, and 1-
nitronaphthalene). Indeed, the simple addition of the CO2 gas
elevated the turnover frequency (TOF) values of the Pd
nanocatlysts for the hydrogenation of the benzene rings of these
aromatic substrates by 4-29 times (Figure 5b-c and Table S4).
Given this impressive catalytic performance, and considering the
well-appreciated reusability of Pd nanocatalysts, it is clear that
the use of molecular H2CO3 catalytic ligands can be viewed as a
highly attractive green catalytic system for the hydrogenation of
various aromatic compounds into high-value-added product.
We here described a new and mild synthetic process for the
high efficiency hydrogenation of nitrobenzene to cyclohexanone.
Mechanistically, it demonstrated that CO2 dissolved in water
generates H2CO3 which couples with and thus activates the
nitrobenzene molecules adsorbed on the Pd surface, giving high
yields of cyclohexanone (90-97%). An important point to
consider relates to the post-catalysis purification of gaseous-
H2CO3-catalyzed reaction products like cyclohexanone: unlike a
liquid or solid ligand (e.g. triethanolamine, ethylenediamine,
(R,R)-2,3-Bis(t-butylmethylphosphino)quinoxaline, 4,4 ′ di-(3-
pentyl)-2,2 ′ -dipyridyl, or 2,2 ′ -bipyridine-4,4 ′ -dicarboxylic
acid chloride),[4-7] gaseous CO2 can be easily sequestered upon
completion of a reaction, dramatically simplifying any required
clean-up steps.
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“binder” ligand to significantly promote the adsorption of
nitrobenzene and following aniline on the Pd surface mainly by
forming hydrogen bonds (Figure 6) rather than providing protons.
Unlike the typical role of supercritical CO2 as a unique solvent
in promoting the kinetic issues for specific catalytic reactions32,
the H2CO3 in water could however induce the pre-adsorption of
the substrate on the Pd surface via relatively strong inter-
molecule interactions and thus largely depress the activation
barrier to boost an innovative route for highly selective
production of cyclohexanone from nitrobenzene hydrogenation.
Conclusions
In summary, expanding beyond cyclohexanone, we
demonstrated a broader application scope for this new green
catalysis system with the successful hydrogenation of the
benzene rings of a large variety of aromatic compounds. It is
easy to imagine the use of similar approaches for other
applications such as transfer hydrogenation, C-C or C-N
coupling, and even selective oxidation, and one can imagine
possible extensions of the technology for applications like
artificial photosynthesis and electrocatalytic organic synthesis.
Indeed, given its ability to potentiate the absorption of substrates
onto catalytic surfaces, our study shows the way forward for a
green strategy that is in theory applicable for all heterogeneous
catalytic process involving the adsorption and activation of
reactant molecules.
Figure 5. (a) Yields of cyclohexanone from nitrobenzene over Pd nanocatalyts
on different supports in CO2/H2 (red bars) or pure H2 atmosphere (black bars).
(b-c) Turnover frequency (TOF) values for the hydrogenation of conjugated
aromatic compounds over Pd/C. All reactions were conducted under the
conditions described in Figure 2. The reaction time was 6 h for all catalysts in
(a), excepting the Pd/CN catalyst for which the time was prolonged to 16 h.
The time number represent TOF enhancement which was calculated on the
basis of conversions between 2-50%, as listed in Table S4.
Experimental Section
Synthesis of 10 wt. % Pd/P25: 135 mg P25 was dispersed into 40 mL
distilled water under sonication for 2 h, and 2.5 mL PdCl2 solution (6
mg/mL) was added and stirred overnight. 4 M NaOH solution was used
to adjust PH to 13. Then, the aqueous solution containing 60 mg NaBH4
was dropped quickly into as-preparation solution to reduce Pd2+ to Pd0.
Until no H2 was released, the catalyst was centrifuged and washed by
water twice and ethanol once which was finally dried at 333 K overnight.
Synthesis of 10 wt% Pd/γ-Al2O3: 135 mg γ-Al2O3 was dispersed into 40
mL distilled water under sonication for 2 h, and 2.5 mL PdCl2 solution (6
mg/mL) was added and stirred overnight. 4 M NaOH solution was used
to adjust PH to 13. Then, the aqueous solution containing 60 mg NaBH4
was dropped quickly into as-preparation solution to reduce Pd2+ to Pd0.
Until no H2 was released, the catalyst was centrifuged and washed by
water twice and ethanol once which was finally dried at 333 K overnight.
Synthesis of 10 wt% Pd/g-C3N4: 135 mg g-C3N4 was dispersed into 40
mL distilled water under sonication for 2 h, and 2.5 mL PdCl2 solution (6
mg/mL) was added and stirred overnight. 4 M NaOH solution was used
to adjust PH to 13. Then, the aqueous solution containing 60 mg NaBH4
was dropped quickly into as-preparation solution to reduce Pd2+ to Pd0.
Until no H2 was released, the catalyst was centrifuged and washed by
water twice and ethanol once which was finally dried at 333 K overnight.
Figure 6. Proposed reaction pathway for the high selectivity to cyclohexanone
from nitrobenzene over Pd catalyst with promotion effect H2CO3.
In order to verifying the promotion effect, other Bronsted acid-
type additives (Table S7), including HNO3, HCl, H3PO4 and
H3BO3, were tested instead of CO2, till offering a poor selecivity
to cyclohexanone. Again, the carbonic acid molecule, generated
from dissolved CO2 in water, can function as a unique catalytic
Temperature Programmed Desorption (TPD) test
The TPD measurements were conducted with AutoChem-Disvovery
2920 equipped with TCD signal. Firstly, 250 mg Pd/C catalyst was
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pretreated under N2 atmosphere at 393 K for 120 min to remove H2O and
O2. After pre-treatment, object gas adsorption was performed under 298
K for 2 h (50 mL/min). Then catalyst was purged with N2 for 60 min to
remove physical absorption gas. Furtherly, the process of desorption was
set up from 298 K to 550 K with the heat rate 15 K/min. Due to the similar
thermal conductivity between N2 (0.024 W/(m*C)) and CO2 (0.0146
W/(m*C), CO2 showed slight signal even though a significant amount of
CO2 could also be adsorbed on Pd surface.
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This work was supported by National Nature Science
Foundation of China (21722103, 21720102002 and 21673140),
Shanghai Basic Research Program (16JC1401600), SJTU-MPI
partner group, Shanghai Eastern Scholar Program and
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Keywords: nitroarenes • cyclohexanone • hydrogenation •
H2CO3 • palladium nanocatalysts
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10.1002/cctc.201900389
ChemCatChem
FULL PAPER
FULL PAPER
Molecular catalytic ligand CO2,
generating carbonic acid (H2CO3) in
water, is applied to accelerated the
adsorption and following
Tian-Jian Zhao, Wei-Jie Feng, Jun-Jun
Zhang, Bing Zhang, Yong-Xing Liu,
Yun-Xiao Lin, Hong-Hui Wang, Hui Su,
Xin-Hao Li*, and Jie-Sheng Chen*
hydrogenation of nitrobenzene on Pd
surface by forming two hydrogen
bonds. Such an enhanced pre-
adsorption of nitrobenzene boosts the
yield of cyclohexanone excess of 90%
on Pd nanocatalysts, promising a
new, mild and highly efficient route for
the cyclohexanone production than
other routes in industrial scale using a
reusable molecular ligand for facile
separation.
Page No. – Page No.
A New Route to Cyclohexanone
using H2CO3 as a Molecular Catalytic
Ligand to Boost the Thorough
Hydrogenation of Nitroarenes over
Pd Nanocatalysts
This article is protected by copyright. All rights reserved.