Palladium/Graphitic Carbon Nitride (g-C3N4) Stabilized Emulsion Microreactor as a Store for Hydrogen from Ammonia Borane for Use in Alkene Hydrogenation

Article abstract of DOI:10.1002/anie.201809882

Direct hydrogenation of C=C double bonds is a basic transformation in organic chemistry which is vanishing from simple practice because of the need for pressurized hydrogen. Ammonia borane (AB) has emerged as a hydrogen source through its safety and high hydrogen content. However, in conventional systems the hydrogen liberated from the high-cost AB cannot be fully utilized. Herein, we develop a novel Pd/g-C₃N₄ stabilized Pickering emulsion microreactor, in which alkenes are hydrogenated in the oil phase with hydrogen originating from AB in the water phase, catalysed by the Pd nanoparticles at the interfaces. This approach is advantageous for more economical hydrogen utilization over conventional systems. The emulsion microreactor can be applied to a range of alkene substrates, with the conversion rates achieving >95 % by a simple modification.

Full text of DOI:10.1002/anie.201809882

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Pd/g-C3N4 stabilized emulsion microreactor as a temporal
hydrogen storage for full use of hydrogen from ammonia borane
toward efficient alkene hydrogenation
Authors: Jingsan Xu, Chenhui Han, Peng Meng, Eric Waclawik, Chao
Zhang, Xin-Hao Li, Hengquan Yang, and Markus Antonietti
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201809882
Angew. Chem. 10.1002/ange.201809882
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Angewandte Chemie International Edition
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COMMUNICATION
Pd/g-C
3
N
4
stabilized emulsion microreactor as a temporal
hydrogen storage for full use of hydrogen from ammonia borane
toward efficient alkene hydrogenation
[a]
[a]
[a]
[b]
[c]
[d]
Chenhui Han , Peng Meng , Eric R. Waclawik , Chao Zhang , Xin-Hao Li , Hengquan Yang ,
[e]
[a]
Markus Antonietti , and Jingsan Xu*
Abstract: Direct hydrogenation of C=C double bonds is a basic
transformation in organic chemistry which is vanishing from simple
practice due to the need for pressurized hydrogen. Ammonia borane
stability, aqueous solubility and high hydrogen content (19.4
wt%).[6] It is convenient and useful for the efficient hydrogenation
of C=C, C=N and N=O in the presence of Pd catalysts, where
the Pd catalyses both the AB hydrolysis and the
(AB) has emerged as a unique hydrogen source due to its safety
[
7]
and high hydrogen content. However, in conventional systems the
hydrogen liberated from the high-cost AB cannot be fully utilized.
hydrogenation. One potential drawback with using AB is that
the cost is still quite high.[ Another drawback that needs
8]
Herein, we develop a novel Pd/g-C
3
N
4
stabilized Pickering emulsion
redressing is that the H
the conventional reaction system due to local H
resulting in a waste of the reagent. In this context, the full use of
AB for organic transformations is a significant task.
Pickering emulsions are emulsions stabilized by solid
particles rather than surfactant molecules, in which oil/organic
solvent or water is dispersed into the other phase in the form of
micrometre-sized droplets.[9] Taking advantage of the unique
organic-solid-water morphology, Pickering emulsions can
2
produced from AB partly escapes from
overproduction,
microreactor, in which alkenes are hydrogenated in the oil phase
with hydrogen originating from AB in the water phase, catalysed by
the Pd nanoparticles at the interfaces. Notably, unit efficiency of
hydrogen utilization is achieved within this microreactor, benefiting
from the hydrogen storage capability of the emulsion via hydrogen
concentrating at the Pd sites and in the liquid phases, whilst availabe
for further hydrogenation until depletion. This buffer effect proves
advantageous for more economical hydrogen utilization over
conventional systems. The emulsion microreactor can be applied to
a range of alkene substrates, with the conversion rates achieving >
2
convert traditional biphasic reactions to
a macroscopically
homogeneous emulsion phase, with the incompatible reactants
confined in either oil or the aqueous phase.[10] This approach
exposes an abundant reactive interface surface area and is free
of extra solvents or surfactants/transfer agents often required to
make the reaction system potentially miscible. In a recent study,
95% by a simple modification.
Hydrogenation of C=C double bonds is one of the most
important transformations in organic chemistry and is widely
applied in pharmaceutical and fine chemical industries.[1] This
transformation can be realized by direct hydrogenation with
it has been demonstrated that graphitic carbon nitride (g-C
can serve as a low cost Pickering stabilizer.[11] Unlike the
commonly used SiO emulsifier, no surface modification of g-
is required. In addition, g-C is a well-known support for
3 4
N )
2
gaseous H
2
,[2] requiring elevated hydrogen pressure, safety
C
3
N
4
3 4
N
measures and specialist reactors.[3] Alternatively, the
transformation can be achieved by transfer hydrogenation,
which refers to the transfer of hydrogen to the target molecule
from a liquid hydrogen donor, such as an alcohol.[4] This
approach is environmentally more benign and is recieving
the immobilization of metal nanoparticles and molecular
catalysts.[ It also posseses excellent stability across a broad
12]
[13]
range of conditions and solvents.
3 4
Herein, Pd nanoparticles (NPs) were loaded onto g-C N
sheets by simple photo-deposition to endow the material specific
catalytic activity while keeping the emulsifying capability. The
increasing attention.[4b,5] Recently, ammonia borane (NH
AB) has proven to be an excellent hydrogen source owing to its
3
3
BH ,
3 4
obtained Pd/g-C N was then employed to stabilize Pickering
emulsion for conducting an efficient alkene hydrogenation in the
oil phase, where the hydrogen was generated in situ from AB in
water, with Pd NPs acting as a bifunctional catalyst located at
the interfaces. It is shown that the efficiency of hydrogen
utilization in this Pickering emulsion system can reach unity, i.e.
the added AB is used without loss for the hydrogenation
occurring in the other phase. Kinetic insights and in situ
observation show that the emulsion is capable of temporally
storing hydrogen that is not directly consumed, and the stored
hydrogen reacts with the alkene afterward until depleted.
[
a]
C. Han, P. Meng, Prof. E. R. Waclawik, Dr. J. Xu
School of Chemistry, Physics and Mechanical Engineering,
Queensland University of Technology,
Brisbane, QLD 4001, Australia
E-mail: Jingsan.Xu@qut.edu.au
Dr. C. Zhang
College of Materials Science and Engineering,
Donghua University,
Shanghai 201620, China
[b]
[c]
[d]
[e]
Dr. X.-H. Li
School of Chemistry and Chemical Engineering,
Shanghai Jiao Tong University,
Shanghai 200240, China
Figure 1a provides a TEM image of 2 wt% Pd/g-C
NPs have a size distribution centered at 2~6 nm and were
uniformly dispersed on the g-C support. HRTEM imaging
Figure 1b) of a single Pd nanoparticle measured a fringe
spacing of 0.225 nm, which corresponds to the (111) planes of
face-centered cubic palladium. Measurments of the g-C sheet
3 4
N . Pd
Prof. H. Yang
3
N
4
School of Chemistry and Chemical Engineering,
Institute of Molecular Science, Shanxi University,
Taiyuan 030006, China
Dr. Prof. M. Antonietti
Max Planck Institute of Colloids and Interfaces,
(
3 4
N
supports reveal that they were typically weakly ordered.
Elemental line-scans across two different locations shows the
distribution of the Pd element, which corresponds with the dark-
field TEM image (Figure 1c). The chemical nature of the
14476 Potsdam, Germany
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
10.1002/anie.201809882
COMMUNICATION
supported Pd NPs was studied by X-ray photoelectron
spectroscopy (XPS). Fitted XPS spectra (Figure 1d) of Pd 3d3/2
and 3d5/2 orbitals are strong evidence that the Pd NPs were
predominantly in metallic state (Pd ), with the presence of a
2 3 4
small proportion of PdO and PdO N
.[14] Importantly, the Pd/g-C
unity (>99%). Control experiments showed that no ethylbenzene
was detected when only g-C N was used, verifying that Pd
3 4
provides the catalytic sites for the coupled reactions.
Next, a series of reference and blind experiments were
conducted for better understanding the reactions in the Pickering
emulsion microreactors. As shown in Figure 2d, when AB was
0
composite can act as an efficient stabilizer for Pickering
emulsions, an effect originating from the interfacial activities of
2
replaced by 0.35 mmol of gaseous H (outside the emulsion
g-C
generated by shaking the mixture of styrene and aqueous
dispersions of Pd/g-C manually. By dyeing the water phase
with methylene blue it was confirmed that an oil-in-water
emulsion is formed, i.e. styrene constituted the dispersed
droplets and water as the continuous phase (Figure S1).
3
N
4
. As shown in Figure 1e, Pickering emulsions were readily
system), very low amount of ethylbenzene and HUE were
observed, due to limited mass transfer from gas to the liquid
phases (entry 2). Additionally, when AB was used but no
emulsification was applied, 0.11 mmol of ethylbenzene and a
HUE of 33% was obtained, resulting from the limited macro-
interface between the two immiscible phases (entry 3). Applying
stirring (500 rpm) led to slightly increased interface area and
thus the catalytic performance slightly improved (entry 4). In
another experiment, ethanol was added to generate a miscible,
homogenous solvent for the reactions. In this condition, the
catalytic performances were still low, achieving only 0.15 mmol
of ethylbenzene and a HUE of 45% (entry 5). These comparison
studies confirmed the efficacy of a combined dehydrogenation-
hydrogenation in the emulsion microreactor. This statement can
also be verified by using water-soluble alkenes (allylamine and
2,5-dihydrofuran) for more efficient hydrogenation in Pickering
emulsions than in normal aqueous solutions (Figure S3).
3
N
4
40
30
20
10
0
(a) (b)
(d)
1
2
3
4
5
6
7
Particle size (nm)
(
c)
Pd
(e)
0
5
10
15
20
25
30 nm
3 4
Figure 1. Characterizations of 2 wt% Pd/g-C N : (a) TEM image and size
distribution, (b) HRTEM image, (c) elemental line profile of Pd, and (d) XPS
spectrum of Pd 3d3/2 and 3d5/2 orbitals. (e) Photograph of styrene-water
(a)
(b)
Pickering emulsion stabilized by 2wt% Pd/g-C
emulsifying.
3 4
N before (left) and after (right)
Figure 2a shows a light microscopy image of the as-
prepared emulsion, exhibiting numerous oil droplets with sizes in
the range of 100–400 μm. The droplet surfaces were densely
covered by the Pd/g-C
3 4
N particles. Employing a styrene-Pd/g-
0
.4
.3
.2
0.4
0.3
0.2
0.1
0.0
3 4
C N
-water emulsion system, we then conducted a hydrogen
Ethylbenzene
HUE
(
c)
AB depletion
(d)
100
liberation-hydrogenation tandem reaction using AB as the
hydrogen source, as schematically displayed in Figure 2b. In a
typical procedure, 2 mg of Pd/g-C
deionized water by sonication, and then 1 ml of styrene was
added and emulsified by manual shaking. Afterwards, 0.5 ml of
AB aqueous solution (8 mg/ml) was injected into the emulsion to
0
0
80
3
N
4
was dispersed in 1 ml of
60
40
20
0
0
.5 wt% Pd/C3N4
2 wt% Pd/C3N4
wt% Pd/C3N4
0.1
0.0
5
o
initiate the reaction at 40 C. Note that styrene was in far excess
compared to AB in this reaction. In this case, a maximum 0.35
0
3
6
9
12
15
18
1
2
3
4
5
6
Reaction Time (h)
mmol of H
2
(H
2
is used for convenience purposes to denote the
Figure 2. (a) LM image of Pickering emulsions made from 1 ml of styrene and
1 ml of 2 mg/ml 2 wt% Pd/g-C dispersion. (b) Schematic illustration of AB
hydrolysis combined with styrene hydrogenation in the Pickering emulsion
system, catalyzed by the Pd NPs at the droplet interfaces. (c) Time-dependent
hydrogen from AB and does not necessarily mean gaseous H
unless noted) could be produced by AB, ideally leading to the
generation of 0.35 mmol of ethylbenzene. Different Pd loadings
2
3 4
N
ethylbenzene generation over 0.5, 2 and 5 wt% Pd/g-C
3 4
N , respectively. (d)
3 4
were examined to assess the catalytic activity of the Pd/g-C N
Control group experiments for hydrogenation: entry 1. g-C N4 as catalyst; 2.
3
nanocomposite. As shown in Figure 2c, experiments with 0.5
wt% Pd loading (Figure S2a) show relatively low activity, and
2
AB replaced by 0.35 mmol of gaseous H ; 3. without emulsifying; 4. no
emulsifying but stirred at 500 rpm; 5. dissolving whole emulsion system in
ethanol; 6. typical procedure. Note that all these experiments were conducted
for 18 h.
0
.31 mmol of ethylbenzene was obtained after 18 h reaction
time. For the nanocomposites with 2 and 5 wt% Pd loading
TEM images shown in Figure S2b), the reactions proceeded
(
In the next set of experiments, the detailed kinetics of the
dehydrogenation and hydrogenation reaction were monitored
separately at different temperatures (30, 40 and 50 oC, Figure
much more quickly, and 0.32 mmol of ethylbenzene was already
reached in the first 8 h. Then the reactions slowed down owing
to the consumption of AB, and 0.35 mmol of ethylbenzene was
3a-c). Both the dehydrogenation and hydrogenation became
eventually generated. In this regard, the H
equiv. of AB) fully reacted with styrene; in other words, the
hydrogen utilization efficiency (HUE), defined as
HUE= [푚표푙 표푓 푒푡ℎ푦푏푒푛푧푒푛푒⁄3 × (푚표푙 표푓 퐴퐵)] × 100% , reached
2
released from AB (3
faster at higher temperatures, whilst all the evolved H from AB
2
was consumed by styrene, generating 0.35 mmol of
ethylbenzene at each temperature. It is very interesting to note
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Angewandte Chemie International Edition
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COMMUNICATION
that the hydrogenation lags behind the hydrogen evolution in the
middle of the reaction, especially at 30 and 40 oC, leading to
varied HUE against reaction time (Figure S4). We can derive
from the profiles that the hydrogenation reaction was slower
than the hydrogen evolution at the initial stage of the reactions.
After then, during the middle and final stage of the reactions, the
hydrogenation rate turned faster than the hydrogen evolution
rate, according to the derivatives of the profiles. This should be
owing to the consumption of AB and accumulation of hydrogen.
Moreover, the gap between the kinetic profiles became narrower
styrene droplets after injection of AB, and the bubbles grew a bit
larger in the next 10 min. Interestingly, the bubbles shrink
afterwards and then completely vanished, owing to the reaction
with styrene. The hydrogen buffer effect of the Pd/g-C N
3 4
stabilized Pickering emulsion is schematically shown in Figure
3d.
Further experiments prove that the hydrogen storage
capacity of the emulsion is related to the amount of the Pd/g-
3 4
C N stabilizer. As shown in Figure 4a, the ethylbenzene
amount decreased to 0.19 and 0.09 mmol when 1 mg and 0.5
mg of catalyst was used, respectively, which was expected due
to the reduced catalyst amounts. More importantly, the HUE
concurrently reduced to 54% and 27%, respectively, even when
the reaction was prolonged to 45 h (Figure S7). These results
o
when the temperature was raised to 50 C, with the reaction rate
of dehydrogenation and hydrogenation becoming very close.
These results indicate that in this emulsion-based biphasic
reaction, the majority of the hydrogen from AB is consumed by
the subsequent styrene hydrogenation, catalysed by the
interfacial Pd sites via direct hydrogen transfer (Equation S1).
The abundant reactive and catalytic sites at the droplet surfaces
are highly beneficial to the coupled reactions. Nevertheless, a
small amount of locally overproduced hydrogen is not available
2
strongly imply that in these conditions, the liberated H escaped
from the emulsion system and did not participate further in the
hydrogenation, most likely resulting from insufficient Pd
adsorption sites and emulsion stabilizer (Figure S8). Moreover,
the hydrogen storage capability can be taken advantage of to
improving the transformation rate of styrene, and meanwhile
maintaining unity HUE. As shown in Figure 4b, when the AB
loading was increased to 8 and 12 mg, 0.7 and 1.05 mmol of
ethylbenzene was obtained, respectively. It is worth pointing out
that the gap between the kinetic profiles of hydrogen evolution
and ethylbenzene generation became larger at higher AB
amounts, which is reasonable as more hydrogen was released.
Correspondingly, the profiles almost totally matched when the
AB input was down to 2 mg.
to immediately react with styrene, which form gaseous H
2
and
then react with the styrene molecules (Equation S2 and S3).
(a) 0.4
(b)0.4
0.3
0.2
0.1
0.0
o
0.3
0.2
0.1
o
3
0 C
40 C
Hydrogen
Ethylbenzene
Hydrogen
Ethylbenzene
(
a)0.4
(b)
0.0
12 mg AB
0
3
6
9
12
15
18
0
3
6
9
12
15
18
100
Ethylbenzene
H2
Ethylbenzene
HUE
Reaction time (h)
Reaction time (h)
0.9
(c) 0.4
(d)
0.3
8
6
4
2
0
0
0
0
0
8
mg AB
0
0
0
.2
.1
.0
0.6
0.3
0.3
0.2
0.1
0.0
o
5
0 C
4 mg AB
2 mg AB
Hydrogen
Ethylbenzene
0.5
1
1.5
A
2
3
4
0
3
6
9
12
15
18
Amount of catalyst (mg)
Reaction time (h)
(
c) 0
(d)
.4
100
80
0.4
100
80
60
40
20
0
0
3
6
9
12
15
18
Reaction time (h)
0
.3
0.3
0.2
0.1
0.0
Figure 3. Kinetic study of hydrogen evolution and hydrogenation reaction at
60
o
o
o
0.2
(
a) 30 C, (b) 40 C, and (c) 50 C. (d) Schematic demonstration of the
4
2
0
0
hydrogen buffer effect of the Pd/g-C
enlarged for better visualization.
3 4
N Pickering system. Pd NPs are
0
.1
.0
0
0
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
Based on the above discussion and considering the overall
HUE reached unity at the end of the reactions, we propose that
a small fraction of the hydrogen produced at the beginning of the
reaction which is not instantly consumed by the slower
hydrogenation reaction did not escape, but conserved in the
system, i.e. within the Pickering hybrid system which is
effectively acting as a hydrogen buffer. It is well documented
that Pd is able to chemosorb a large number of hydrogen.[15]
These chemisorbed hydrogen are highly active and can react
with styrene afterwards. The second hydrogen storage
mechanism is hydrogen microbubbles, which – due to the
viscosity and roughness of the Pickering system – can stay
Reaction runs
Figure 4. (a) Ethylbenene production and HUE in the presence of different
amount of 2 wt% Pd/g-C
at 40 oC. (b) Kinetic study of dehydrogenation and
3 4
N
hydrogenation with different amount of AB over 2 mg of 2 wt% Pd/g-C
C. Dashed lines illustrate hydrogen amount and solid lines illustrate
3
N
4
at 40
o
ethylbenzene amount. (c) Reusability test of 2 wt% Pd/g-C
3
N
4
catalyst for 10
o
emulsification-catalyzation cycles at 40 C. (d) Hydrogenation of various
alkene substrates in the Pickering emulsion system. Reaction conditions: 0.35
mmol of substrates dissolved in 1ml of cyclohexane (for substrates 1, 2, 3) or
o
1 ml of toluene (for substrates 4, 5) as oil phase, 4 mg of AB, 18 h, 40 C (for
o
substrates 1, 2, 3,4) or 80 C (for substrate 5).
3 4
Furthermore, the recyclability of the Pd/g-C N catalyst in the
current Pickering emulsion microreactor was evaluated. After
simply washing, the nanomaterials can be reused for the next-
cycle emulsification and catalytic reaction (Figure S9). As
trapped at the g-C
phase (Figure S5) as well as in the dispersed organic droplets.
As shown in Figure S6, micro H bubbles were formed in the
3 4
N particle surface in the continuous aqueous
2
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Angewandte Chemie International Edition
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COMMUNICATION
shown in Figure 4c, high catalytic activity toward the hydrogen
evolution−hydrogenation reaction was well maintained up to 10
cycles, with no decline of HUE and ethylbenenze production.
TEM observation and XPS measurement demonstrate that after
Keywords: graphitic carbon nitrides • Pd nanoparticles •
emulsion microreactors • hydrogenation
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1]
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Jacobsen, A. Pfaltz, H. Yamamoto, Comprehensive Asymmetric
Catalysis: Supplement 1, Vol. 1, Springer Science & Business Media,
1
0 cycles, the Pd NPs grew a bit larger, while their chemical
state remained unchanged (Figure S10). Turnover frequency
TOF) was
[(푚표푙 표푓 푒푡ℎ푦푙푏푒푛푧푒푛푒⁄푚표푙 표푓 푃푑) 푝푒푟 ℎ표푢푟]
2003; c) B. Chen, U. Dingerdissen, J. G. E. Krauter, H. G. J. Lansink
(
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determined and only showed slight decrease upon each
catalysis cycle (Figure S11), probably because of the slow
growth of Pd NPs, indicating the superior reusability of the Pd/g-
[
2]
C
3
N
4
catalyst.
Finally, the conversion rate of alkene can be substantially
promoted by simply modifying the Pickering emulsion
microreactor. Briefly, cyclohexane was used as a solvent to
dissolve 0.35 mmol of styrene and then emulsified with Pd/g-
[3]
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Chem. Rev. 2015, 115, 6621-6686.
3 4
C N aqueous dispersion, followed by addition of AB (0.117
mmol) as described above (see Supporting Information for
details). In this situation, the reagents can fully react with each
other and resulted in a final conversion of 97% (Figure 4d, entry
[
[
5]
6]
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extend the range of the alkene substrates, as some of the
educts cannot be directly emulsified. As shown in entry 2–5
(
Figure 4d), four substrates with very different types of C=C
double bonds were chosen for hydrogenation, and high
conversion rate and HUE (95~99%) were achieved.
2014, 50, 2142-2144; f) Z. Li, T. He, D. Matsumura, S. Miao, A. Wu, L.
In summary, we have presented
3 4
a novel Pd/g-C N
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generation from AB with hydrogenation of alkenes at the oil-
water interfaces of the droplets. The hydrogen utilization
efficiency can reach unity in such an emulsion system, mainly
due to the localized production, storage, and consumption of
hydrogen on the Pd NPs. Detailed analysis and observations
revealed that the Pickering emulsion can effectively store
hydrogen via adsorption in the Pd nanoparticles and
microbubble formation within the liquid phases. The stored
hydrogen then reacted with alkene molecules until complete
consumption. This hydrogen storage capability obviously played
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[
[
[
8]
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a) B. Peng, J. Chen, Energy Environ. Sci. 2008, 1, 479-483; b) The
price of ammonia borane complex for technical grade 90% is ~$150 per
10 g and for 97% is $390 per 10 g, according to the Sigma-Aldrich
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3 4
a key role for the efficient use of hydrogen. The Pd/g-C N
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catalyst illustrated excellent reusability and tunred out to be
rather generally applicable to various alkene molecules.
Importantly, the conversion rate of alkene can reach > 95% by
simply modifying the Pickering emulsion microreactors. This
work opens a new avenue for developing more economical and
efficient hydrogenation proccesses for the production of a wide
range of valuble chemicals.
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J.X. is grateful to the Discovery Early Career Researcher Award
(
DECRA) by Australian Research Council (DE160101488).
Science and Engineering Faculty and Central Analytical
Research Facility (CARF) at QUT are greatly acknowledged for
technical assistance.
This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201809882
COMMUNICATION
COMMUNICATION
A novel Pd/g-C
3
N
4
stabilized Pickering
Chenhui Han, Peng Meng, Eric R.
emulsion microreactor is developed to
conduct alkene hydrogenation using
ammonia borane as the hydrogen
source. This system can serve as a
general hydrogen buffer space,
Waclawik, Chao Zhang, Xin-Hao Li,
Hengquan Yang, Markus Antonietti and
Jingsan Xu*
Page No. – Page No.
leading to unit efficiency of hydrogen
utilization. This emulsion microreactor
provides more economical hydrogen
utilization over conventional systems.
3 4
Pd/g-C N stabilized emulsion
microreacter as a temporal hydrogen
storage for full use of hydrogen from
ammonia borane toward efficient
alkene hydrogenation
This article is protected by copyright. All rights reserved.