Simultaneous High Conversion and Selectivity in Olefin Oxidation with Oxygen Through Solid/Liquid/Gas Three-Phase Interface Design

Article abstract of DOI:10.1002/cctc.201900918

In oxidation reactions with oxygen as oxidant, low O₂ concentration on catalyst surface under ambient pressure is a major kinetic limitation for selective oxidation of olefins; while increasing O₂ pressure to enhance the reaction rate suffers lower selectivity due to thermodynamic factors. In this study, we demonstrate that simultaneous enhancement of conversion and selectivity can be achieved under ambient pressure by solid/liquid/gas three-phase interface design. We produced Au nanoparticles supported on graphene aerogel with superaerophilic surface (denoted as Au/GA) as the catalyst for demonstration. The results show that O₂ gas shows “bursting” behavior on the surface of Au/GA under water, resulting in rapid adsorbing of O₂ in 98 ms and therefore high O₂ concentration on the surface of catalyst. Thus, solid/liquid/gas three-phase interface is formed under this condition, leading to high conversion as well as high selectivity at ambient O₂ pressure. Such strategy is expected to find wide applications in many other selective oxidation reactions that traditionally needed high pressure.

Full text of DOI:10.1002/cctc.201900918

Accepted Article
Title: Simultaneous high conversion and selectivity in olefin oxidation
with oxygen through solid/liquid/gas three-phase interface design
Authors: Wei-Guo Song, Zhaohua Li, Changyan Cao, Zhongpeng Zhu,
and Lei Jiang
This manuscript has been accepted after peer review and appears as an
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To be cited as: ChemCatChem 10.1002/cctc.201900918
Link to VoR: http://dx.doi.org/10.1002/cctc.201900918
A Journal of
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ChemCatChem
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Simultaneous high conversion and selectivity in olefin oxidation
with oxygen through solid/liquid/gas three-phase interface design
Zhaohua Li,[a, c] Changyan Cao,*[a, c] Zhongpeng Zhu,[b, c] Lei Jiang[b, c] and Weiguo Song*[a, c]
Abstract: In oxidation reactions with oxygen as oxidant, low O2
concentration on catalyst surface under ambient pressure is a major
kinetic limitation for selective oxidation of olefins; while increasing O2
pressure to enhance the reaction rate suffers lower selectivity due to
thermodynamic factors. In this study, we demonstrate that
simultaneous enhancement of conversion and selectivity can be
achieved under ambient pressure by solid/liquid/gas three-phase
interface design. We produced Au nanoparticles supported on
graphene aerogel with superaerophilic surface (denoted as Au/GA)
as the catalyst for demonstration. The results show that O2 gas shows
“bursting” behavior on the surface of Au/GA under water, resulting in
rapid adsorbing of O2 in 98 ms and therefore high O2 concentration on
the surface of catalyst. Thus, solid/liquid/gas three-phase interface is
formed under this condition, leading to high conversion as well as high
selectivity at ambient O2 pressure. Such strategy is expected to find
wide applications in many other selective oxidation reactions that
traditionally needed high pressure.
found that using thiol-treated ultrathin Pd nanosheets, the
catalytic performance for semihydrogenation of internal alkynes
was enhanced.[4l] However, selective oxidation at high conversion
is still elluding.
Selective oxidation of olefins to aldehydes is a very useful
reaction in pharmaceutical, perfume, dye and other fine chemical
industries.[5] Compared to conventional oxidants such as tert-butyl
hydroperoxide (TBHP) or hydrogen peroxide (H2O2), O2 is a
greener oxidant. However, its intrinsic activation energy barrier
restricts activity of heterogeneous catalysts. In liquid phase
oxidation reactions, concentration of O2 in the solution and on the
surface of heterogeneous catalysts is very low under ambient O2
pressure, further reducing the reaction rate.[6] Therefore,
increasing O2 pressure is often required to enhance O2
concentration and the conversion. However, high oxygen
pressure usually leads to extensive oxidation by-products (e.g.
ketones or carboxylic acids) and thus reduces selectivity.[7]
In our previous study, we found that superaerophilic materials
could enhance H2 gas concentration on the surface of the
catalysts, and enhanced the reaction rate under ambient
hydrogen pressure.[8] We envisioned that this strategy could be
suitable for oxidation reactions under ambient O2 pressure. In this
case, both high conversion and high selectivity might be achieved
with the superaerophilic catalyst.
Herein, we demonstrate that the superaerophilic surface can be
used to simultaneously enhance conversion and selectivity in
selective oxidation of olefins with oxygen under ambient pressure.
Au nanoparticles loaded on graphene aerogels with different
degrees of aerophilic/aerophobic surfaces (denoted as Au/GA
and Au/NGA, respectively) were prepared as model catalysts for
oxidation of olefins. O2 contact angle under water analysis
confirmed that O2 bubbles exhibited “bursting” state and can be
quickly adsorbed within 98 ms on Au/GA, suggesting the
superaerophilic surface of Au/GA; while O2 bubbles kept “pinning”
state and cannot be spread across the aerophobic surface of
Au/NGA. As a result, Au/GA showed higher conversion of styrene
(~ 87 %) as well as high selectivity of benzaldehyde (~ 90 %)
under ambient pressure. When O2 pressure was increased to 20
bar, nearly the same conversion can be achieved with
aerophobic Au/NGA under the same reaction time, but the
selectivity of benzaldehyde decreased to ~ 62 %.
Activity and selectivity are usually at odds for heterogeneous
catalysis. Catalysts with high activity often suffer side reactions
and reduced selectivity due to the scaling relationship.[1] Thus in
order to achieve high selectivity, activity is often sacrificed.[2] One
example is Lindlar catalyst for semi-hydrogenation of alkynes.[3]
Thus, developing a catalyst with both high activity and selectivity
is always a goal for researchers. In recent years, surface
modification or chemical micro-environment control was found to
be an efficient route to enhance both the activity and selectivity.[4]
For example, Kunz et al. presented that through functionalized Pt
nanoparticles with hydrophilic L-proline, both the activity and
selectivity were enhanced in selective hydrogenation of
acetophenone.[4c] Yaghi et al. reported that Pt nanopartciles
embedded in UiO-66 with sulfonic acid functional groups as
organic linkers showed enhanced activity and selectivity in the
gas-phase conversion of methylcyclopentane.[4b] Zheng et al.
[a]
Z. Li, Dr C. Cao, Prof. W. Song
Beijing National Laboratory for Molecular Sciences, CAS
Research/Education Center for Excellence in Molecular Sciences,
CAS Key Laboratory of Molecular Nanostructure and
Nanotechnology, Institute of Chemistry, Chinese Academy of
Sciences
Au/GA and Au/NGA were prepared according to our previous
work.[8] Scanning electron microscopy (SEM) and Transmission
electron microscopy (TEM) images (Figure 1) showed that Au
nanoparticles with the same average sizes were dispersed on
graphene nanosheets, which were interconnected to form
channels in the aerogels. The contents and dispersion of Au were
determined by inductively coupled plasma atomic emission
spectroscopy (ICP-AES) and CO puls chemisorption,
respectively. The results were summarized in Table S1. Au 4f X-
ray photoelectron spectroscopy (XPS) spectra indicated the
chemical property of Au nanoparticles maintained nearly the
Zhongguancun North First Street 2, 100190 Beijing (P.R. China)
E-mail: cycao@iccas.ac.cn; wsong@iccas.ac.cn
Z. Zhu, Prof. L. Jiang
Key Laboratory of Bio-inspired Materials and Interfacial Science,
Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences
Zhongguancun East Road 29, 100190 Beijing (P.R. China)
Z. Li, Dr C. Cao, Z. Zhu, Prof. L. Jiang, Prof. W. Song
University of Chinese Academy of Sciences
100049 Beijing (P.R. China)
[b]
[c]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201900918
ChemCatChem
COMMUNICATION
same in both of Au/GA and Au/NGA samples (Figure S1). In
addition, N2 adsorption-desorption isotherms and pore size
distributions also showed nearly the same (Figure S2). All above
results suggested that the state of Au nanoparticles on both
catalysts were nearly the same, which was crucial for
investigating the effect of surface property for catalysis.
Selective oxidation of styrene was first chosen to study the
catalytic property of these catalysts. Considering that the
selective oxidation of olefins is hard to carry out when only O2 is
used as an oxidant, a small amount of TBHP is used as an initiator
to initiate the reaction. As shown in Figure 3a and Table S2, ~
87 % conversion of styrene and ~ 90 % benzaldehyde selectivity
were observed at 22 h under ambient O2 pressure with
Hydrophilic/hydrophobic and aerophilic/aerophobic properties
of the surface of Au/GA and Au/NGA were then studied. The
results were shown in Figure 2. Au/GA exhibited hydrophobic
property in the air with water contact angle of about 130º (Figure
2a). Due to the N-doping (Figure S1), Au/NGA showed
superhydrophilic with water contact angle of 0º (Figure 2c). The
different surface hydrophilicity/hydrophobicity of Au/NA and
Au/NGA led to different aerophilic/aerophobic properties under
water. Figure 2b and Figure 2d showed the O2 bubble adhesion
behaviors under water on Au/GA and Au/NGA, respectively. It can
be seen that O2 bubble exhibited “bursting” behavior on the
surface of Au/GA under water, indicating the superaerophilic
surface of Au/GA.[9] The time for O2 bubble to be completely
adsorbed was only 98 ms. In contrast, Au/NGA with
superhydrophilic surface exhibited the “pinning” state for O2
bubble under water and stayed the “pinning” state after 10 s
(Figure S3), suggesting the aerophobic surface of Au/NGA under
water.[10]
superaerophilic Au/GA catalyst. For comparison,
~ 66 %
conversion and ~ 89 % benzaldehyde selectivity were obtained
with aerophobic Au/NGA under the same reaction conditions. In
order to further investigating the effect of O2 pressure on the
conversion and selectivity, O2 pressure was increased while other
reaction conditions were kept the same with aerophobic Au/NGA
catalyst. As shown in Table S2, the conversion of styrene indeed
rose from ~ 66 % to ~ 75 % at 10 bar O2 pressure (Figure 3b). At
the same time, the selectivity of benzaldehyde decreased from ~
89 % to ~ 71 %. When O2 pressure was further increased to 20
bar, the conversion was up to ~ 87 %, which was the same as that
of reaction catalyzed by Au/GA under ambient pressure. However,
the selectivity of benzaldehyde was further decreased to ~ 62 %
(Figure 3a). The side products were ketones and carboxylic acids,
as shown in Table S2. Carboxylic acids came from the deep
oxidation of target products aldehydes. For catalysts with different
supports, the selectivity was only slightly different. Therefore, the
influence of supports can be ignored. However, once the reaction
pressure increased, aldehydes were oxidized to acids, resulting
in the significantly decrement of selectivity. Therefore, it is better
to conduct the reaction under ambient pressure. Using
superaerophilic Au/GA as catalyst makes it possible and high
activity and selectivity can be obtained simultaneously.
Similar results were observed for selective oxidation of other
alkenes, including 4-methoxystyrene, 4-methylstyrene and 4-
chlorostyrene. As shown in Figures 3c-3e, Au/NGA with
aerophobic surface exhibited low activity for these substrates
under ambient oxygen pressure. when the O2 pressure was
increased to 10 bar, all substrates can be converted with above ~
95 % conversions, while the selectivity decreased significantly.
Impressively, ~ 90 % conversions and ~ 90 % selectivity were
achieved over Au/GA even under ambient O2 pressure.
Figure 1. (a) SEM image and (b) TEM image of Au/GA; (c) SEM
image and (d) TEM image of Au/NGA.
Figure 2. (a) Water contact angle and (b) O2 bubble adhesion
behavior of Au/GA; (c) Water contact angle and (d) O2 bubble
adhesion behavior of Au/NGA.
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10.1002/cctc.201900918
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this study. In addition, reactions under ambient pressure is much
safer and cost-less for reactions involving gas as reactants.
In contrast, when using aerophobic Au/NGA as catalyst,
because O2 gas could not be spread on the surface of Au/NGA
under water due to the “pinning” state, no solid/liquid/gas three-
phase interface was established, and oxidation reactions could
only proceed with dissolved O2 gas in water, which has very low
O2 concentration and thereby lower reaction rate. Conversion
could be indeed enhanced through increasing O2 gas pressure.
However, high chemical potential of O2 led to extensive oxidation
and more side products were produced, leading to decreased
selectivity.
The recyclability of catalyst is another important criteria for
catalysis. To investigate the recycling performance of
superaerophilic Au/GA, the oxidation of styrene was chosen as a
test reaction. After each experiment, the reaction mixture was
extracted with ethyl acetate and the catalyst was separated from
the system by centrifugation. After thoroughly washed with ethyl
acetate, the catalyst was freeze-dried before being used in the
next run. As shown in Figure S4, no obvious loss of activity and
selectivity was observed after five consecutive runs. The SEM
image of the recovered catalyst (Figure S5a) was similar to the
fresh one and the TEM image revealed that Au nanoparticles still
dispersed homogenously on graphene sheets with average
diameter of 2.6 nm (Figures S5b-5c). The recovered catalyst
maintained the surface hydrophobic and superaerophilic (Figures
S5d-S5e). All these results suggested the high stablity of Au/GA.
In summary, we demonstrated that both high conversion and
selectivity could be achieved simultaneously in the oxidation of
olefins by a solid/liquid/gas three-phase interface design. By
producing the catalyst with superaerophilic surface, fast O2
adsorption on the catalyst occurred, resulting in high O2
concentration for high conversion and high selectivity at ambient
pressure. Considering high O2 pressure is needed in many other
selective oxidation reactions, it is expected that the finding in this
work could be helpful to design catalysts and reaction systems
that are more efficient under low pressure to achieve high yields.
Figure 3. (a) Curves of conversion and selectivity vs time on different
catalysts in oxidation of styrene; Reaction conditions: Au/GA (1 mg),
Au/NGA (0.7 mg), substrate (0.4 mmol), anisole (0.4 mmol), TBHP
(0.01 mmol), H2O (4 mL), 100 ºC. (b-e) Catalytic activity and selectivity
of catalysts in oxidation of several alkenes after reaction at 22 h. a O2
pressure: 10 bar, b O2 pressure: 20 bar.
Experimental Section
Materials: Graphite powder, tert-butyl hydroperoxide (70%), 4-
methoxystyrene (98%), 4-methylstyrene (98%), anisole (99%), HAuCl4
were purchased from Alfa Aesar. Styrene (extra pure) was purchased from
Acros. 4-chlorostyrene was produced by J&K scientific Co., Ltd. Sulfuric
acid (98 wt%), phosphoric acid (85 wt%), potassium permanganate (A.R.),
hydrogen peroxide (30%), hydrochloric acid (37 wt%), ethyl acetate (A.R.)
and anhydrous ethanol (A.R.) were provided by Beijing Chemical Works.
KI was provided by Sinopharm Chemical Reagent Co., Ltd.
Scheme 1. Schematic illustration of solid/liquid/gas three-phase
interface for reaction with superaerophilic and aerophobic surface.
Above results proved that high conversion and selectivity could
be achieved simultaneously through creating superaerophilic
surface for selective oxidation using O2 gas under ambient
pressure. Solid/liquid/gas three-phase interface design was
critical for this catalysis system (Scheme 1). At three-phase
interface, the catalytic active sites on solid catalyst could be
accessed by alkenes in the solution and O2 gas in gas phase. O2
gas showed “bursting” behaviour on the surface of Au/GA under
water, resulting in quick adsorbing of O2 and therefore high O2
concentration on the surface of catalysts. Thus, high conversion
could be obtained even under ambient O2 pressure, and low O2
chemical potential favoured the selectivity of targeted product in
Synthesis of graphene oxide (GO). GO was synthesized according to
the report reported before with small improvements. In brief, 1.5 g of
graphite powder was mixed with 100 mL of H2SO4 in an ice bath and stirred
mechanically for 5 min. 15 mL of H3PO4 was added to the mixture drop by
drop. After 10 min, 6 g of KMnO4 was added slowly and the mixture was
kept in an ice bath for 1 h. Then, the flask was transferred into a 35 ºC
water bath for 24 h. After the mixture was cooled to room temperature, 160
mL of deionized water was added slowly in an ice bath and stirred for 5
min. Then, 10 mL of H2O2 was added dropwise. The resulting golden
suspension was centrifuged and washed to remove residual salts.
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10.1002/cctc.201900918
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Synthesis of graphene aerogels (GA). GA was prepared according to
previous literatures with minor modifications. In a typical experiment, 5 mL
of GO suspension was mixed with 5 mL of deionized water. Then, 0.5 g of
KI was added under stir, followed by adding 0.5 mL of HCl. After 5 min, the
mixture was treated under ultrasonication and heated for 8 h at 80 ºC. The
resultant was immersed in water for 3 days, freeze-dried and then heated
from room temperature to 140 ºC at a heating rate of 2 ºC min-1 under Ar
and held for 1 h, followed by heated from 140 ºC to 800 ºC at the same
heating rate and held for 2 h.
2100F electron microscope operated at 200 kV. XPS measurement was
performed on a VG Scientific ESCALab220i-XL electron spectrometer
using 300 W Al kα radiation. The loading content of Au was determined by
inductively coupled plasma atomic emission spectroscopy (ICP-AES,
Shimadzu ICPE-9000). The dispersion of Au was determined by a
chemisorption analyzer (AutoChem Ⅱ 2920). The water contact angle
(WCA) and O2 bubble adhesion behavior was measured by a contact
angle meter (OCA25, Dataphysics). The conversion and selectivity of
aromatic olefins were measured by a gas chromatograph (Shimadzu GC-
2010) equipped with a flame ionization detector (FID) and a Rtx-5 capillary
column (0.25 mm in diameter, 30 m in length).
Synthesis of N-doped graphene aerogels (NGA). NGA was synthesized
after heating GA from room temperature to 800 ºC at a heating rate of 2
ºC min-1 under NH3 and held for 4 h.
Acknowledgements
Synthesis of Au/(N)GA. 2 mL of anhydrous ethanol was added into a
certain amount of HAuCl4 aqueous solution (0.0564 M). Drip the mixture
to (N)GA and the samples were annealed from room temperature to 200 º
C at a heating rate of 2 ºC min-1 in H2 flow and held for 2 h.
The authors thank the National Natural Science Foundation of
China (NSFC 21573244, 21573245) and the Youth Innovation
Promotion Association of CAS (2017049).
Selective oxidation of aromatic olefins. A 25 mL steel autoclave was
charged with Au/GA (1 mg), substrate (0.4 mmol), anisole (internal
standard, 44 µL, 0.4 mmol), TBHP (0.01 mmol, 70 wt% in water) and 4 mL
water. The autoclave was evacuated and backfilled with O2 three times to
replace the air. Then, the reactor was sealed and adjusted to a certain O2
pressure. The autoclave was heated to 100 ºC and the temperature was
maintained for the desired time. After reaction, the mixture was extracted
with ethyl acetate and the catalyst was separated from the system by
centrifugation. The organic phase was analyzed by a GC (Shimadzu GC-
2010) equipped with a flame ionization detector (FID) and a Rtx-5 capillary
column (0.25 mm in diameter, 30 m in length). Au/NGA (0.7 mg) was used
for comparison.
Keywords: Superaerophilic • surface • selectivity •
heterogeneous • catalysis
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dissolved in 2 mL of the mixture of HCl and HNO3 (HCl and HNO3 v: v=3:1).
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Characterization: Scanning electron microscopy (SEM) images were
obtained on a JEOL-6701F scanning electron microscope at 10.0 kV.
Transmission electron microscopy (TEM) was carried out on a JEOL
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10.1002/cctc.201900918
ChemCatChem
COMMUNICATION
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COMMUNICATION
Zhaohua Li, Zhongpeng Zhu, Changyan
Cao,* Lei Jiang and Weiguo Song*
Simultaneous enhancement of
conversion and selectivity in olefin
oxidation with oxygen can be
achieved under ambient pressure by
solid/liquid/gas three-phase interface
design through creating
Page No. – Page No.
Simultaneous high conversion and
selectivity in olefin oxidation with
oxygen through solid/liquid/gas
three-phase interface design
superaerophilic surface.
This article is protected by copyright. All rights reserved.