Single step production of styrene from benzene by alkenylation over palladium-anchored thermal defect rich graphitic carbon nitride catalyst

Article abstract of DOI:10.1016/j.mcat.2021.111844

Styrene is a widely used monomer in polymer industries for the manufacturing of plastics, rubbers and resins. The commercial styrene production process suffers from disadvantages such as requiring multiple steps, catalyst deactivation, high energy consump

Full text of DOI:10.1016/j.mcat.2021.111844

Molecular Catalysis 514 (2021) 111844
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Single step production of styrene from benzene by alkenylation over
palladium-anchored thermal defect rich graphitic carbon nitride catalyst
a
a,
*
a
b
c
a
Kalpana Avasthi , Ashish Bohre , Janvit Ter ˇz an , Ivo Jerman , Janez Kova ˇc , Bla ˇz Likozar
a
Department of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia
b
Department of Material Chemistry, National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia
Department of Surface Technology, Jo ˇz ef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
c
A R T I C L E I N F O
Editor by Prof. I.K. Song
A B S T R A C T
Styrene is a widely used monomer in polymer industries for the manufacturing of plastics, rubbers and resins.
The commercial styrene production process suffers from disadvantages such as requiring multiple steps, catalyst
deactivation, high energy consumption and low yield. To address the aforementioned problems, metal complex
catalysed single-step oxidative coupling of benzene and ethylene to styrene have been proposed. The attempts so
far for the heterogenization of metal complexes lead to worsening the catalytic performance due to metal
leaching or clustering. Here, we have presented a first step towards the development of a heterogeneous catalyst
to synthesize styrene directly from benzene and ethylene in a single- step. We have anchored palladium nano-
particles on the thermal defect rich surface of graphitic carbon nitride and prepared a heterogeneous catalyst
Keywords:
Styrene
Alkylation
Monomer
Sustainable process
Catalysis
(
3 4
Pd@d-C N ) that is active for the single-step styrene production. The product and side-products were analysed
by gas chromatography coupled with mass spectrometry. To understand the role of active sites, the catalysts were
characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high resolution transmission
electron microscopy (HRTEM), atomic force microscopy (AFM), Brunauer–Emmett–Teller (BET) and Fourier-
transform infrared (FT-IR) spectroscopy techniques. A combination of microscopic and spectroscopic charac-
3 4
terization supports the hypothesis that the activity of Pd@d-C N catalyst is associated with their layered
structure, surface defect site, and well-stabilized palladium nanoparticles.
Introduction
is mainly conducted in a batch reactor over homogeneous catalysts and
styrene is produced by the C-H bond activation of benzene [7,8]. Using
direct oxidative conversion of benzene and ethylene, high styrene yield,
Styrene is an industrially important monomer produced annually on
a scale of 20 million tons with a market value of approximately $43.1
billion [1]. The majority of styrene is used in the production of plastics,
elastomers, and fine chemicals [2]. Commercially styrene is produced by
the alkylation of benzene and ethylene to ethylbenzene followed by
dehydrogenation of ethylbenzene to styrene using large feeding streams
over the iron oxide-based catalyst [3,4]. This process is extremely
endothermic in nature and suffers from thermodynamic limitations. In
addition, low atom economy, short catalyst lifetimes and the net emis-
sion of greenhouse gases are other drawbacks, associated with this in-
dustrial process [5]. In this context, the direct production of styrene
from benzene in a one-pot process under moderate reaction conditions is
highly desirable [6].
◦
above 95 mol% styrene yield was claimed at 180 C by Jia et al. The
author’s proposed a palladium acetate catalyst with the in situ
air-recyclable oxidant copper(II) pivalate [9]. The selectivity of styrene
can be further enhanced by the addition of ligands such as pyridine or
heteropolyacids with palladium acetate catalyst [10,11].
Rh-mediated arylation has also attracted the attention of researchers.
Matsumoto and Yoshida have reported significant work on the oxidative
arylation of ethylene with benzene to produce styrene by Rh (I) com-
2
plexes [12,13]. In one such work, the author reveals that Rh(ppy) (OAc)
complex provide better selectivity to styrene (77 mol%) compared to Pd
(II) acetate (47 mol%) without the addition of oxidizing agents such as
Cu or Ag salts under aerobic conditions [12]. In 2015, Gunnoe et al.
pioneered the field of styrene production by introducing rhodium cat-
The oxidative coupling of ethylene with benzene is considered a
promising alternative process for the production of styrene. This process
Fl
2
2 4
alysts ( DAB)Rh(TFA)(h -C H )[14]. The developed metal complex
*
Corresponding author.
E-mail addresses: ashish.bohre@ki.si (A. Bohre), blaz.likozar@ki.si (B. Likozar).
https://doi.org/10.1016/j.mcat.2021.111844
Received 24 April 2021; Received in revised form 18 August 2021; Accepted 18 August 2021
Available online 29 August 2021
2
468-8231/© 2021 Elsevier B.V. All rights reserved.

K. Avasthi et al.
Molecular Catalysis 514 (2021) 111844
converts benzene and ethylene to styrene with 100 mol% selectivity and
Aluminum nitrate nonahydrate (98%, Sigma Aldrich) and ammonium
hydroxide (99%, Honeywell). All the reagents were used as received
from commercial suppliers without further purification.
≥
95 mol% yields. In this process Cu(II) acetate was utilized as an
oxidant similar to the commercialized Wacker-Hoechst process for
ethylene oxidation [15]. Cu(II) oxidant can be easily regenerated in the
presence of air. However, in this process regeneration was not demon-
strated because Rh catalysts can only maintain catalytic activity under
Catalysts synthesis
an inert atmosphere [16]. To overcome this difficulty, the research
Synthesis of g-C N4 and d-C N4 supports
3
3
2
group has developed an air-stable (5-FP)Rh(TFA)(
η
2
-C H
4
) complex and
3 4
Graphitic carbon nitride (g-C N ) was prepared by loading 10 g
◦
◦
achieved 91 mol% styrene yield under milder reaction conditions
compared to the previous study [17].
melamine in a muffle furnace and heated to 500 C (g-C N -500), 550 C
3 4
◦
(g-C N -550) and 600 C (g-C N -600), respectively for 4 h. After
3
4
3 4
The literature review demonstrated that to date only metal complex-
based homogenous catalysts have been developed for single-step styrene
synthesis. Although their catalytic activities are excellent, the develop-
ment of low-cost, non-toxic and environmentally benign heterogeneous
catalysts is essential to avoid the limitations of a homogenous catalytic
system [18].
cooling down to room temperature, the obtained powder was washed in
–
1
succession with hydrochloric acid (1 mol L ) and ultrapure water to
remove any alkaline species (e.g., ammonia) adsorbed on the surface of
the supports. The washed g-C N4 was collected by centrifugation and
3
◦
dried at 80 C for 12 h. To remove the salts that possibly formed during
◦
the washing; the catalyst was post-purified by heating at 400 C for 3 h.
Graphitic carbon nitride, g-C
3
N
4
, is an interesting nitrogen anchored
The d-C N4 support was prepared from the same method by calcining
3
◦
carbon material widely used as a catalyst and catalyst support for pho-
tocatalysis, oxidation, hydrogenation, cycloaddition, and esterification
reactions [19,20]. The unique two-dimensional layered structure of
the melamine at 640 C.
Synthesis of Pd@d-C N4 catalyst
3
g-C
3
N
4
is flexible, and consists of defective tri-s-triazine units in which
Pd@d-C N4 catalyst was synthesized by the incipient-wetness
3
2
the carbon and nitrogen atoms are in stable conjugating sp configura-
tion. The presence of an excess amount of nitrogen and strong covalent
bonds between nitrogen and carbon atoms provides this allotrope
thermal and chemical stability. The surface modification of the parent
graphitic framework by the introduction of many organic and inorganic
impregnation method. Typically, 1 g of d-C N4 support was added to
3
20 mL aqueous solution of palladium nitrate under stirring for 20 min.
Subsequently, in order to adjust the pH up to 9, an appropriate amount
of ammonium hydroxide (0.2 M) was added dropwise under vigorous
◦
stirring. The mixture was then stirred for 1 h at 60 C. After cooling to
compounds or metals could improve the reactivity of g-C
vide novel material for the development of new chemistry [21,22].
The formation of thermal defects in the g-C framework is an
3
N
4
and pro-
room temperature, the solid was recovered by filtration and washed
◦
with ultrapure water and dried in an oven at 60 C for 12 h. The
3
N
4
reduction of the sample was performed in a tubular furnace. The sample
◦
effective way to significantly enhance its catalytic activity [23]. It was
demonstrated that the thermal defects can modify the electronic struc-
ture and also act as the reactive sites in the reaction system [24]. In
addition, the existence of such defects serves as an ideal surface for the
adsorption and activation of the reactants because of their abundant
localized electrons and interfacial electron transfer mechanism [25].
was initially heated to 200 C in He, followed by heating at the same
–
1
temperature in 10 vol.% H in He (100 mL min total flow) for 2 h. The
2
Pd@d-C N4 catalyst was subsequently cooled to room temperature
3
under flowing helium. Similarly, Pd-g-C N -500, Pd-g-C N -550 and Pd-
3
4
3 4
g-C N -600 catalysts were also prepared by this method. Graphene
3
4
oxide, reduced graphene oxide, Mn-rGO, Mn/Ce-rGO, Pd/C, Pd/Al O
2
3
The thermal defects enhance the content of NH/-NH
surface of g-C [26]. The nitrogen-rich surface provides abundant
homogenously distributed anchoring centers that lead to stabilize the
anchored metal nanoparticles when g-C is used as a support in het-
erogeneous catalysis [27]. More importantly, the strong coordination
between the metal nanoparticles and g-C support induced the elec-
tron transfer from NH/-NH groups to metal. This electron transfer
mechanism makes the metal nanoparticles more electron-rich and
consequently makes the metal-g-C system more reactive [28]. The
high temperature calcination is an interesting process to incorporate
thermal defects in the g-C structure. This process avoids the use of
2
groups on the
and Pd-rGO catalysts were prepared by adopting the reported methods
[30].
3 4
N
3
N
4
Catalysts characterization
X-ray diffraction (XRD) measurements were carried out on PW3040/
60 X’Pert PRO MPD diffractometer which was operated at 45 kV and 40
3 4
N
2
mA with CuK
α radiation source (λ=0.154056 nm) at room temperature
◦
◦
◦
by a step size of 0.05 in 2θ range from 5 to 90 . Joint Committee on
Powder Diffraction Standards (JCPDS) database was used for the iden-
tification of the phases present in the XRD patterns of the catalysts.
Average crystallite sizes of the supports and catalysts were calculated by
applying the Debye-Scherrer equation.
3 4
N
3 4
N
surfactant and templating agents and defects can be easily engineered by
tuning the processing temperature [29].
X-ray photoelectron spectroscopy (XPS) analyses were carried out by
the PHI XPS spectrometer (Physical Electronics). Sample was deposited
on adhesive carbon tape and introduced into an ultra-high vacuum
In the present study, we have employed this method to obtain high
3 4
yield d-C N support, decorated with thermal defects. Using this mate-
–
9
rial, we were able to strongly anchor palladium nanoparticles into host
cavities, designing the first heterogeneous catalyst for the single-step
production of styrene by the oxidative coupling of benzene and
ethylene to styrene. This work highlights an important step toward the
application of heterogeneous catalyst for styrene production and ex-
pands the scope of inexpensive carbocatalysis.
spectrometer. The vacuum during XPS analyses was in the range of 10
mbar, as a high surface sensitivity is a general characteristic of the XPS
methods. Sample surfaces were excited by the X-ray radiation from the
monochromatic Al source at the photon energy of 1486.6 eV. High-
energy resolution spectra were acquired with the energy analyser,
operating at the resolution of about 0.6 eV and the pass energy of 29 eV.
During data processing, surface spectra were aligned by setting the C 1s
peak at 285.0 eV, the latter being characteristic for C–C bonds. The
accuracy of binding energies was about (±0.3 eV. XPS spectra were
analysed by the MultiPak software, version 9.9. (Physical Electronics).
Nitrogen adsorption-desorption measurements were performed by
using a Micromeritics ASAP 2020 MP/C apparatus. The measurements
Experimental
Chemicals
The source and purity of the chemicals used are benzene (99.8%,
Honeywell), ethylene (99.9%, Messer), copper (II) acetate (98%,
Merck), melamine (99%, Sigma Aldrich), palladium (II) acetate (98%,
Sigma Aldrich), manganese(II) nitrate tetrahydrate (99%, Sigma
Aldrich), cerium(III) nitrate hexahydrate (98%, Sigma Aldrich),
◦
were conducted at –196 C temperature. The specific surface areas of the
2
catalysts were calculated from the amount of N adsorbed at a relative
pressure P/P
0
using the Brunauer-Emmett-Teller (BET) equation. Prior
flow for 4 h at
to characterization, the samples were degassed under N
2
2

K. Avasthi et al.
Molecular Catalysis 514 (2021) 111844
2
00 ◦C. The pore width was estimated using Barret–Joyner–Halenda
In the XRD patterns of bulk g-C
3
N
4
samples the low angle peak at 2θ
◦
(
BJH) method.
= 13.5 was assigned to the in-plane structural packing of tri-s-triazine
◦
The Fourier-transform infrared (FT-IR) spectra of the samples were
and high angle peak appeared at 2θ = 28.5 corresponds to the inter-
recorded on Spectrum Two UATR FT-IR spectrometer (PerkinElmer,
planar graphitic stacking of the conjugated aromatic system (Fig. 1)
ꢀ 1
Waltham, USA) at the wavenumbers ranging from 4000 cm to 400
[32]. However, in the case of d-C
3
N
4
sample the high angle peak shift to
ꢀ
1
◦
◦
cm
.
27.7 , indicates that the high-temperature calcination (640 C) of car-
bon nitride leads to an increase the interplanar space [33]. After loading
with palladium nanoparticles this peak remains unchanged, confirming
that the graphitic interlayer remains constant and the original crystal
Transmission electron microscopy (TEM) images and EDS-mapping
were recorded on JEOL JEM-3010 instrument operated at 300 kV,
equipped with a LaB
6
filament coupled with energy-dispersive X-ray
probe.
3 4
structure of d-C N support is largely retained. The calculated interlayer
The atomic force microscopy (AFM) analyses were conducted on a
WiTec Alpha300 instrument operating in tapping mode. Three different
spots of each sample were selected for the determination of the root-
mean-square (RMS) roughness.
distances of graphitic carbon nitrides are similar to those of supported
palladium nanoparticles samples (Table S1). According to the literature,
the interlayer distance changes when noble metals are present on the
surface of the graphitic sheets and remain constant when metals are
located within the six-fold cavities [34]. In addition, five well-resolved
The ninhydrin test was performed to identify the surface concen-
tration of amine functional groups. [31] Ninhydrin reagent (2,2-dihy-
droxy-1,3-indanedione) reacts with the primary amino groups of the
samples to form a purple-coloured ninhydrin-amino complex called
3 4
diffraction peaks in the Pd@d-C N sample are attributed to palladium
nanoparticles. The average crystallite size of Pd@d-C
between 14 and 17 nm.
3 4
N catalyst ranges
‘
Ruhemann’s purple’ (Scheme S1). In this test 1 mL of solution of
In order to identify the electronic state of palladium, surface defects
and surface chemical composition, we have performed X-ray photo-
electron spectroscopy (XPS) measurements. The elemental survey
–1
ninhydrin acetone solution (80 mg mL ) was added to 2 mL suspension
◦
of sample (2.0 mg). The solution was heating in an oil bath at 70 C for
1
5 min. The absorption spectra of the heated samples were measured on
3 4
spectra of the Pd@d-C N shows peaks at 284.70 eV, 336.19 eV and
the Perkin Elmer Lambda 950 UV/VIS/NIR spectrophotometer.
398.74 eV, due to the presence of carbon (C) palladium (Pd) and ni-
trogen (N) (Fig. 2a).
Catalytic activity measurement
3 4
The XPS spectra of C 1s for Pd@d-C N can be deconvoluted to four
peaks (Fig. 2b). The peak at 286.0 eV is assigned to disordered carbons
of both sp3-like configuration and a defective graphitic carbon surface,
whereas the other peak at 284.8 corresponds to graphitic carbon, which
is typically observed during the XPS analysis of carbon nitride [35]. The
The activity measurements were carried out in a batch autoclave
reactor system which consisted of 6 parallel independent batch reactors
(
system of 6 parallel batch reactors, each of 50 mL, Parr Model 5000)
2
equipped with a magnetic stirring bar, a thermocouple, a pressure
gauge, a rupture disk, gas and liquid sampling lines. In a typical
major C 1s peak at 288.1 eV ascribed to sp -bonded C (N-C-N) [36].
Three distinguished peaks appeared in the high-resolution N 1s
experiment, Cu(OAc)
The solution was loaded into the reactor with a catalyst (500 mg) and
sealed. The reactor was pressurized with N to 10 bar and vented three
2
(1 g) oxidant was dissolved in 10 mL of benzene.
spectrum of Pd@d-C
400.0 eV were attributed to sp -hybridized nitrogen in triazine rings (C-
N-C) and the tertiary nitrogen N-(C) , respectively. The small peak at
3 4
N (Fig. 2c). The peaks centered at 398.7 eV and
2
2
3
times to remove any residual oxygen atmosphere. Finally, the reactor
was pressurized to 15 bar above ambient pressure with ethylene, stirred
404.4 eV is due to the terminal nitrate groups with charging effects or
positive charge localization in heterocyclic molecules.
◦
magnetically (600 rpm) and heated-up to 200 C. The internal pressure
Interestingly, the C 1s and N 1s spectra of Pd@d-C
gible changes after the anchoring of palladium nanoparticles compared
to bare d-C , suggesting that palladium present in the six-fold cavities
3 4
N shows negli-
reached 25 bar in the heating-up time of 35 minutes. The recorded re-
action time started when the temperature reached the desired set value.
Once the reaction was completed the reactor was cool-down to room
3 4
N
(Fig. S1) [37]. The high-resolution spectra of the Pd 3d region are
depicted in Fig. 2d, exhibit two types of palladium species. The strong
3d5/2 peak at lower binding energy (336.2 eV) and 3d3/2 peak located at
◦
temperature (20 C).
Final liquid solution (product) was analyzed by gas chromatography
coupled with mass spectrometry (GCMS–QP 2010 Ultra, Shimadzu,
Kyoto, Japan) equipped with a nonpolar column (Zebron ZB–5MS,
length 60m, diameter 0.25 mm, film thickness 0.25 m). Compounds
μ
were identified by MS (mass spectrometry; fragments were scanned in
the range from 35 to 600 m/z and compared to the NIST 17 (National
Institute of Standards and Technology) library) and quantified by FID
(
flame ionization detector). Analysis method followed the temperature
programmed method, initially kept at 323 K for 5.5 min, then increased
–1
to 563 K with a heating rate of 20 K min , where the temperature was
kept constant for an additional 12.5 min. Injection volume was 1.0
μL
and the split ratio was set to 30. The injector temperature was main-
tained at 563 K and FID detector temperature at 573 K. The 5-point
calibration curves of reactant and product were prepared in order to
obtain reactant conversion and product yield.
Results and discussion
We have synthesized four g-C
3
N
4
samples by changing the calcina-
◦
◦
tion temperatures (500 C to 640 C) and were used as a support to
impregnate palladium nanoparticles. The loading amount of palladium
nanoparticles was 3.0 wt% unless otherwise stated. The final materials
were denoted as Pd-g-C
Pd@d-C . The detailed experimental section is available in the sup-
porting information.
3 4 3 4 3 4
N -500, Pd-g-C N -550, Pd-g-C N -600 and
3 4
N
3 4 3 4 3 4
Fig. 1. XRD patterns of the Pd@d-C N , d-C N and bulk g-C N materials.
3

K. Avasthi et al.
Molecular Catalysis 514 (2021) 111844
3 4
Fig. 2. High-resolution XPS spectra of Pd@d-C N catalyst (a) survey spectrum, (b) C 1s, (c) N 1s and (d) Pd 3d.
higher binding energy (341.4 eV), originate from metallic Pd [38].
Another doublet originated from 3d5/2 and 3d3/2 with relatively low
intensity at 338.65 eV and 343.91 are well in agreement with the
lead to the formation of thermal defects [41]. It was evidently visible
that no obvious change in the FT-IR spectra of Pd@d-C has occurred
after the anchoring of palladium nanoparticles, indicating that the mo-
lecular structure of d-C is rather stable under the strong basic con-
3 4
N
2
+
+
2-oxidation state of palladium (Pd ) species. However, the fraction of
3 4
N
2
+
Pd phases in the catalyst is favourable for achieving better results in
our reaction [39].
ditions used for the preparation of the catalyst due to the strong van der
Waals forces between the single layers [42].
XPS analyses were performed on a couple of catalysts among them
Transmission electron microscopy (TEM) was used to further observe
were a fresh and a recycled catalyst (R-Pd@d-C
possible differences. The Pd-g-C -550 catalyst showing mediate con-
version efficiency of 9.0% and low selectivity 0.7 was analysed by XPS.
In addition, the d-C catalyst was also analysed. Normalize XPS
spectra C 1s, N 1s and Pd 3d from these catalysts are shown in Fig. 3
b–d). As one can see there are no significant differences between them.
3
N
4
) in order to see
the morphologies and structures of the Pd@d-C
ison with bulk g-C and d-C supports, as displayed in Fig. 5. Unlike
the bulk structure of g-C (Fig. 5a), the d-C support exhibited
thinner two-dimensional layered morphology (Fig. 5b), which was also
observed in a sample of Pd@d-C (Fig. 5c). This change in surface
3 4
N catalyst in compar-
N
3 4
N
3 4
3 4
N
3
N
4
3 4
N
3 4
N
3 4
N
(
morphology is in qualitative agreement with that in the specific surface
areas of the catalyst and supports. The high-magnification TEM image of
All of them have features described above for C 1s, N 1s and Pd 3d
spectra. To show in more details the Pd 3d spectra we plotted them also
on the same intensity scale (Fig. 3d). In these spectra again well resolved
Pd 3d5/2 and 3d3/2 doublet at 336.2 eV and 341.5 eV is observed related
Pd@d-C
of 15–20 nm are distinctly anchored on d-C
Fig. 5e, represents a high resolution TEM image of an individual palla-
dium nanoparticle on d-C surface with a lattice-fringe spacing of
3
N
4
catalyst reveals that palladium nanoparticles with the size
3 4
N
nanosheets (Fig. 5d).
to metallic Pd0 and Pd on all catalysts. No additional broadening
related with new oxidation states of the Pd atoms was observed. Small
differences in C 1 s spectra in Fig. 3a among the samples may be noted in
2+
3 4
N
0.25 nm, which is assigned to the (111) plane of Pd [38]. In the selected
area diffraction pattern (SAED) clear lattice fringes indicated the poly-
crystalline nature of the catalyst (Fig. 5f). The observed rings are well
indexed with the characteristic planes (002), (111), (200) and (220) of
XRD results. Additionally, EDX elemental analysis verifies the presence
a larger relative portion of graphitic carbon at 284.8 eV from g-C
sample with respect to sp2-bonded carbon (C-N-C) at 288 eV.
3 4
N
The Fourier-transform infrared (FT-IR) spectra of the supports and
catalyst is depicted in Fig. 4. All samples featured stretching vibration
3 4
of C, N, and Pd in the Pd@d-C N catalyst (Fig. 5g-j and Fig. S2).
–
1
modes of C=N and C–N bonds between 1500–1650 cm
and
250–1490 cm , respectively, which are associated with the aromatic
tri-s-triazine units [40]. The absorption peak appearing in the range of
More detailed information about the thickness and morphology was
offered by the atomic force microscopy (AFM) in tapping mode. The
–
1
1
AFM images further approved the layered structure of d-C
(Fig. 6a-b). The surface roughness of the g-C synthesized at different
temperatures was found to increase in the order of g-C -500 > g-C
550 > g-C -600 > d-C with the root-mean-square (RMS) rough-
3 4
N support
–
1
8
10 cm may be attributed to the breathing mode of heptazine units
3 4
N
[
38]. The peak located at around 3100–3400 cm– is ascribed to
1
3
N
4
3 4
N -
stretching vibration modes of N–H bonds. It was observed that in the
3
N
4
3 4
N
–1
–1
FT-IR spectra of d-C
3
N
4
the intensities of bands at 1210 cm , 1240 cm
and 1320 cm have increased which would mean that high temperature
calcination of melamine generates more –NH/NH groups. This in-
dicates that in the structure of d-C large portions contain only a few
heptazine units connected with more C–NH–C bridges. The enhance-
ment of the terminals –NH/NH groups in the small polymer segments
ness values of 64 nm, 49 nm, 35 nm and 25 nm, respectively (Fig. S3-S6).
The change of roughness probably related to the fact that the thermal
treatment induced important structural modifications that convert the
polymeric materials to more graphitic ones and make the surface smooth
[43].
–
1
2
3 4
N
2
The mesoporous nature of the samples was confirmed by the strong
4

K. Avasthi et al.
Molecular Catalysis 514 (2021) 111844
Fig. 3. High-resolution XPS spectra of different catalysts (a) C 1s, (b) N 1s, (c) Pd 3d, and (d) Normalize Pd 3d spectra.
adsorption and H3-type hysteresis loop in N
2
adsorption–desorption
the intensity of absorption peak (λmax = 570 nm) has increased with the
isotherm curves (Fig. 6c-d, Fig. S7-S8) [44–45]. It was observed that the
surface area and pore volume of the catalysts increases with the pro-
cessing temperature (Table 1) suggest that the introduction of thermal
defects unfolded the graphitic nanosheets, as confirmed by the HRTEM.
processing temperature, which confirmed that the surface concentration
of –NH/NH
2 3 4
groups in d-C N is temperature dependent (Fig. S9).
The result of the element analysis is a convincing proof of the exis-
tence of nitrogen defects. It was observed that the mass ratio of N:C for
The enhancement of the surface area for Pd@d-C
3
N
4
catalyst (52.78 m2
support is due to the fact that the
the Pd-g-C
3
N
4
-500 catalyst was 1.91 that significantly decreased to 0.97
catalyst (Table S2). The decrease of nitrogen
–
1
g
) in comparison to bare d-C
N
3 4
in case of Pd@d-C N
3 4
interlayer stacking forces between the graphitic nanosheets were
significantly reduced by the presence of palladium nanoparticles that
acts as a separator and increase the surface area, as previously reported
content indicates the existence of nitrogen vacancies after the thermal
treatment [49]. The XPS results further confirmed that the missing N
atoms mainly originate from amino groups and some two-coordinated
nitrogen.
[
46–48]. For all the catalysts, the diameter of the pores is in the ranges of
2
1–33 nm.
For a better understanding of the interaction between palladium
To further verify the surface concentration of amine functional
nanoparticles with surface defective sites of d-C , we have speculated
3 4
the structure of Pd@d-C . In general, g-C N has a Sierpinski sieve-
3
N
4
groups, we have employed the ninhydrin assay [31]. Ninhydrin reagent
2,2-dihydroxy-1,3-indanedione) reacts with the primary amino groups
3
N
4
(
like lattice structure made from tri-s-triazine units connected by nitro-
of the samples to form a purple-coloured ninhydrin-amino complex
called ‘Ruhemann’s purple’ (Scheme S1). The optical densities of the
samples were determined by UV-Vis spectrophotometer, which is pro-
portional to the concentration of amino groups. It became apparent that
gen atoms. However, the final structure, composition and morphology of
g-C
route. The high temperature calcination of melamine generates more
–NH/NH groups on the terminating edges of tri-s-triazine rings that
3 4
N depend upon calcination temperature, precursor and production
2
5

K. Avasthi et al.
Molecular Catalysis 514 (2021) 111844
change the color of g-C
3
N
4
from light yellow to brown and produce
thermal defects in the final material (Fig. S10) [27]. There were a total of
eleven adsorption sites including five non-equivalent atom sites of N and
C (N
interstitial sites (I
g-C , where dopant atoms can occupy substitutional or interstitial
positions (Scheme 1a) [50]. The four bond sites were corresponding to
1
2
, N , N
, C
3 1
and C
2
), four bond sites (B
1 2 3 4
, B , B , and B ) and two
2
and I
3
) available in the structure of defect free
3 4
N
the N
1
–C
1
, C
1
–N
2
, N
2
–C
2
and C
2
–N
3
3
bonds of triazine rings. One inter-
stitial site was delimited by six C
N
4
rings called “six-fold cavity” (I
2
)
3 4 3
and another interstitial site was the centre of the C N ring (I ).
Catalytic performance
Generally, the oxidative coupling of benzene and ethylene to styrene
requires either molecular oxygen or oxidizing agent to reoxidize the
transition metal that is reduced in the course of the reaction via
reductive elimination[12]. However, we have chosen a stoichiometric
amount of air-recyclable copper (II) acetate as the in situ oxidant to
regenerate Pd(II) because of the explosive nature of the ethylene and
oxygen mixture. The separation of copper(II) acetate is known for the
commercialized Wacker–Hoechst process to produce acetaldehyde by
the oxidation of ethylene over palladium(II) catalysts [51–53].
Fig. 4. FT-IR spectrum of bulk, defect rich and palladium anchored graphitic
carbon nitride samples.
In search of an active catalyst for the production of styrene, we have
Fig. 5. TEM images of (a) g-C
3
N
4
-550 (b) d-C
3
N
4
and (c) Pd@d-C
3
N
4 3 4
; (d-e) HRTEM images, (f) SAED pattern and (g-j) Elemental mapping images of Pd@d-C N .
6

K. Avasthi et al.
Molecular Catalysis 514 (2021) 111844
Fig. 6. Atomic force microscopy image (a) and height profile (b) of d-C
3
4
N support, (c-d) Nitrogen adsorption-desorption isotherm and pore size distribution (inset) of
3 4 3 4
d-C N and Pd@d-C N .
when reactions were conducted with bare supports (Table 2, entries 1-
Table 1
Textural properties of various catalysts.
4
). Upon anchoring with palladium nanoparticles all the supports
became active and exhibited some catalytic activity (Table 2, entries
Catalyst Surface area
Pore volume
Average Pore size
[nm]
5
–8). These results affirm that the anchoring of transition metal is
2
–1
3
–1
[
m
g
]
[cm
g ]
essential for this transformation as previously reported [14]. We have
achieved a maximum 8.9% conversion and 7.4 mol% selectivity over
g-C
g-C
g-C
3
3
3
N
N
N
4
4
4
-500
-550
-600
4.6
0.03
0.04
0.21
0.26
0.02
0.05
0.18
0.40
28.6
21.6
32.1
21.5
21.0
24.0
28.7
31.0
Pd@d-C
reaction [54]. Apart from the defects, other structural properties of
Pd@d-C catalyst were also responsible for the better activity
compared to Pd-g-C catalysts. In order to improve the selectivity of
styrene over the Pd@d-C catalyst, we have varied reaction time, and
temperature. Unfortunately, it did not improve the styrene selectivity
3 4
N catalyst, evident the necessity of defects to catalyse this
8.1
26.7
46.0
5.0
3 4
N
3 4
d-C N
Pd-g-C
Pd-g-C
Pd-g-C
3
N
3
N
3
N
4
-500
4
-550
4
-600
3 4
N
8.9
3 4
N
25.3
52.7
3 4
Pd@d-C N
(
Table 2, entry 9-10).
3 4
The catalytic efficiency of different Pd-g-C N materials was not
2
–1
tested different solid materials including CeO
rGO, Pd/C, Pd/Al
2
-rGO, Mn-rGO, Mn/Ce-
desirable due to their small specific surface areas (< 26 m
g
)
compared to Pd@d-C
N
3 4
catalyst (52.7 m2
g
–1
), which can limit the
2
O
3
and Pd-rGO knows for the effective oxidative
coupling reaction. However, no coupling product was detected over
these solid catalysts.
target-active site contact surface. Notably, the content of palladium
loading also affects the oxidative coupling of benzene and ethylene. The
In order to check the applicability of g-C
3
N
4
in our reaction, we have
3 4 3 4
Pd-g-C N -500 and Pd-g-C N -550 catalysts with excess palladium
conducted some preliminary experiments. It was confirmed from the FT-
IR and XPS analysis that the concentration of surface defects in poly-
loading (~ 2 wt%) are less selective for styrene production. This may be
due to the fact that the high metal content blocks the accessible active
sites of the catalysts [55].
3 4
meric g-C N has increased with the processing temperature, therefore,
it is sensible to investigate the influence of the surface defect sites on
styrene selectivity over bare and palladium nanoparticles anchored g-
We strongly believe that styrene selectivity is influenced by multiple
factors. Some formation of side products originating from benzene is
noticed, such as stilbene, γ Phenylbutyrolactone, 1-Phenyl-1,2-ethane-
diol diacetate and 4-Phenylbutanol (Scheme 2). However, aside from
C
N
3 4
prepared at four different temperatures.
As expected, we have not detected styrene in the reaction mixture
7

K. Avasthi et al.
Molecular Catalysis 514 (2021) 111844
Scheme 1. (a) Molecular structure of with possible adsorption sites (b) Positions of NH/NH
2
groups and palladium nanoparticles in a segment with six hepta-
zine units.
the next cycle directly under the identical reaction conditions. The re-
sults showed that the catalyst was stable, and its activity remained
almost constant throughout the recycling experiment (Table 2, entry
Table 2
Comparisons of the catalytic performance for the oxidative coupling of benzene
and ethylene to styrene over various catalysts.
1
1).
The Pd@d-C
[
a]
Entry
Catalyst
Conversion [%]
Selectivity [mol%]
3 4
N catalysed reaction closely follows the traditional
1
2
3
4
5
6
7
8
g-C
g-C
g-C
3
3
3
N
N
N
4
4
4
-500
-550
-600
2.8
-
homogeneous catalysis pathway with just a few differences [14]. In the
metal complex catalysed production of styrene electron rich ligands is
essential for the activation of benzene. The ligand with multiple coor-
dination sites also helps to suppress irreversible oxidation to inactive
metal centers in presence of an oxidant [57]. Although the coordination
of ligands is important in the homogeneous system, the deactivation of
the catalyst and rapid aggregation of the metal was observed in the re-
ported metal complexes [58]. In addition, the strong metal ligand
interaction makes the catalyst less active because the activation of the
catalyst requires ligand removal.
18.8
50.0
33.9
6.8
-
-
d-C
3
N
4
-
Pd-g-C
Pd-g-C
Pd-g-C
3
3
3
4
N -500
4
N -550
4
N -600
0.1
0.7
0.5
7.4
4.5
3.7
7.8
9.0
17.0
8.9
Pd@d-C
Pd@d-C
Pd@d-C
N
3 4
N
3 4
N
3 4
[
b]
9
1
1
7.0
[
c]
0
1
5.8
[
d]
R-Pd@d-C
3
N
4
9.5
[
a] Reaction conditions: Benzene (10 mL), ethylene (15 bar), catalyst (500 mg),
3 4
In the Pd@d-C N catalysed reaction defect abundant N-sites fulfil
◦
◦
Cu(OAc)
2
(1 g), t (3 h), T (200 C). [b] t (2 h), [c] T (150 C) and [d] recycled
the function of ligand and provide the necessary electron density
required for the activation of benzene. The presence of multiple coor-
dination sites in the fixable lattice enables transfer of charge continu-
ously to palladium during the reaction that enhances the stability of the
catalyst. The chemical environment and binding of the metal atom for
such materials remain constant during the reaction cycle [59,60].
catalyst. Values were determined by GCMS.
stilbene, all of these represent less than 5% of the area of styrene. The
only other product worth mentioning is stilbene which accounts less
than 50% of the area of styrene. Due to the colour of the reaction
mixture, we also suspect some polymerization to larger compounds
occurs (Fig. S11). These have a boiling point higher than the tempera-
ture of the liner, and they likely remain in the reactor (are not analysed).
Furthermore, there is a lot of acetic acid and its derivatives (such as
acetylacetone) present in the reaction mixture. For the latter products, it
is impossible to say if they are purely the result of copper acetate
decomposition, or if benzene also decomposes.
Conclusion
In the quest to achieve a single-step production of styrene by a het-
erogeneous catalyst that overcomes the limitations of state-of-the-art
homogeneous catalysts, we have designed and synthesized palladium
nanoparticles anchored on graphitic carbon nitride decorated with
Finally, we have conducted the recycling test to examine the stability
3 4
thermal defects. Among all the tested catalysts, Pd@d-C N yielded
3 4
of the (R-Pd@d-C N ) catalyst. After each run, the catalyst was isolated
styrene with 7.4 mol% selectivity under moderate reaction conditions.
The extensive spectroscopic measurements and structural analysis of the
catalyst reveal the pivotal role of the defect rich graphitic carbon nitride,
which as an electron abundant host stabilizes palladium nanoparticles,
promotes the adsorption and activation of the substrates. The insights
gained from this work open up wide technological potential of graphitic
carbon nitride for heterogenization of challenging chemical processes.
from the reaction mixture through centrifugation and filtration. The
obtained solid was thoroughly washed with aqueous ammonia solution
until a colourless aqueous layer is obtained. This washing procedure has
removed the impurities of copper in the form of blue copper-ammonia
complex [56]. The solid was then washed with hydrochloric acid (1
–
1
mol L ) and ultrapure water to remove ammonia adsorbed on the sur-
◦
face and dried in an oven at 80 C for 6 h. The dried catalyst was used for
8

K. Avasthi et al.
Molecular Catalysis 514 (2021) 111844
Scheme 2. Reaction network for the oxidative coupling of benzene and ethylene to styrene.
Our findings will inspire the design of cost-effective heterogeneous
catalysts for C-C bond coupling of other arenes with olefins that are
presently being conducted over metal complexes and update the basic
understanding of carbocatalysis.
acknowledged by all authors. Mr. Brett Pomeroy is acknowledged for
English proofreading.
Supplementary materials
In the present study, we have demonstrated for the first time that
heterogeneous catalysts can also be used for the one-pot styrene pro-
duction. We believe that our findings will motivate the researcher to
develop a more active and selective heterogeneous catalyst for styrene
production. Further studies are ongoing to improve styrene selectivity
by tuning the active sites and optimizing the reaction conditions. The
future work should be the focus on replacement of the expensive
palladium metal with more earth-abundant metals like nickel, copper or
cobalt to make this process industrially viable.
Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.mcat.2021.111844.
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