Protein powder derived nitrogen-doped carbon supported atomically dispersed iron sites for selective oxidation of ethylbenzene

Article abstract of DOI:10.1039/d1dt02001c

Atomically dispersed Fe species embedded in the nitrogen-containing carbon supports (Fe₁/NC) are successfully synthesized using a ball milling approach, with commercial protein powder as the nitrogen source. The catalyst exhibits outstanding performance in the oxidation of aromatic compounds containing saturated C-H bonds into corresponding ketones under ambient conditions, which is superior to those of a nanoparticle catalyst (Fe_n/NC) and a metal-free catalyst (NC).

Full text of DOI:10.1039/d1dt02001c

Volume 50
Number 34
1
4 September 2021
Pages 11653-12018
Dalton
Transactions
An international journal of inorganic chemistry
rsc.li/dalton
ISSN 1477-9226
COMMUNICATION
Xiaohui He, Hongbing Ji et al.
Protein powder derived nitrogen-doped carbon supported
atomically dispersed iron sites for selective oxidation of
ethylbenzene
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Volume

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Protein powder derived nitrogen-doped carbon
supported atomically dispersed iron sites for
selective oxidation of ethylbenzene†
Cite this: Dalton Trans., 2021, 50,
11711
Received 16th June 2021,
Accepted 20th July 2021
a
a
a
a
a
b
Yujie Cheng, Qingdi Sun, Liyun Huang, Qian He, Hao Zhang, Pengbo Wang,
c
b
a
a
a,c
DOI: 10.1039/d1dt02001c
Ying Zhang, Shaolin Shi, Xingcong Zhang, Tao Gan, Xiaohui He* and
a,b,c,d
rsc.li/dalton
Hongbing Ji
*
Atomically dispersed Fe species embedded in the nitrogen-con- tate reactions, and exhibit excellent stability under harsh reac-
1
5
taining carbon supports (Fe /NC) are successfully synthesized tion conditions. Furthermore, in order to hold metal centres
1
using a ball milling approach, with commercial protein powder as stably, heteroatoms are needed in carbon-based supports,
the nitrogen source. The catalyst exhibits outstanding performance such as N, S, and P, which can provide abundant anchor sites
in the oxidation of aromatic compounds containing saturated C–H to coordinate with metal centres to form a stable structure
bonds into corresponding ketones under ambient conditions, through electron transfer, and then effectively inhibit metal
1
6,17
which is superior to those of a nanoparticle catalyst (Fe
n
/NC) and a migration and sintering during the pyrolysis process.
metal-free catalyst (NC).
However, the surfaces of conventional carbon-based supports,
such as activated carbon and carbon black, usually do not
contain these heteroatoms. Therefore, it is highly desirable to
find a cheap and readily available source of heteroatoms
together with a facile introduction strategy.
Single atom catalysts (SACs), with their extremely high atomic
utilization (∼100%), unique geometric-electronic structure and
easy separation operation, have emerged as a rising star in
heterogeneous catalysis.
properties in a series of industrially important reactions, e.g.,
selective oxidation, such as catalytic oxidation of alkanes,
aromatic hydrocarbons and alcohols.
defined active sites of SACs can provide a clear analysis of the
structure–property relationship, and thus shed light on the
1
–4
SACs exhibit remarkable catalytic
From biological systems, we come to know that plenty of
organic biomass possessing the merits of abundant and
renewable properties contains various heteroatoms, e.g. nitro-
gen. Therefore, biomass materials have received extensive
attention for their possible utility as the nitrogen source of
carbon-based supports after simple treatments. For instance,
5
,6
7
,8
9,10
Moreover, the well-
1
1
reaction mechanisms at the molecular/atomic scale.
15
Li et al. selected cocoon silk to prepare a N-doped carbon-
A considerable number of studies have shown that in SACs,
the underlying supports not only play a significant part in the
stability of the metal atoms, but also have an essential impact
based support by a degumming, regenerating and pyrolysis
procedure and obtained Co SACs (Co-ISA/CNS). Jiang and co-
18
workers synthesized Fe SACs (Fe–N–C) using animal blood as
1
2–14
on the catalytic performance.
At present, the commonly
the precursor, which contained much haemoglobin. Shi’s
used supports in SACs mainly include metal oxides and
carbon-based materials. Among them, carbon-based materials
are widely applied in various industrial reactions, because of
their irregular porous structures with high specific surface
areas, which can enhance mass transfer effectively and facili-
19
group used chlorella as a precursor, synthesizing carbon-sup-
4
ported atomically dispersed CoN sites by salt melt synthesis.
Due to the plentiful content of Fe in Porphyra and Auricularia,
Liang’s team used the two kinds of biomass adsorbed urea as
20,21
1 1
precursors, preparing two SACs, Fe-ISA/NC and Fe Pt /NC.
Commercially available protein powder, as an inexpensive
and readily available biomaterial with plenty of protein, pos-
sesses rich N content (as high as 12%). It can be mixed uni-
formly with metal precursors and carbon-based supports
a
Fine Chemical Industry Research Institute, School of Chemistry,
Sun Yat-Sen University, Guangzhou 510275, Guangdong, China.
E-mail: jihb@mail.sysu.edu.cn, hexiaohui@mail.sysu.edu.cn
b
22
School of Chemistry and Chemical Engineering, Guangxi University,
through a facile ball milling process. During carbonization,
Nanning 530004, China
protein powder may introduce sufficient N atoms to the
support. The dense and uniform N atoms can facilitate the for-
mation of metal–N bonds, which lead to the stability of the iso-
lated metal sites during the pyrolysis and reaction process.
Usually, commercial protein powder contains some other
c
Huizhou Research Institute of Sun Yat-sen University, Huizhou 516216, China
d
School of Chemical Engineering, Guangdong University of Petrochemical
Technology, Maoming 525000, P.R.China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1dt02001c
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heteroatoms such as S and P, which may also benefit the fix-
ation of atomically dispersed metal species, but the contents
are small, so the tiny influence can be ignored.
Herein, combining the merits of biomaterials and the
advantages of the ball milling strategy, without any templates,
2
3–25
solvents, and expensive equipment,
we propose a facile
and efficient method to synthesize SACs. Based on the pre-
3
,22,26
vious work on SACs,
a commercially available protein
powder containing high protein is selected as the nitrogen
source, and carbon black is used as the support. The two
materials are mechanically ball milled with Fe(NO ) ·9H O
3
3
2
1
and then pyrolyzed to obtain Fe SACs (Fe /NC). Conversely, if
the protein powder is not added, dense Fe-based particles
show up in the Fe/C catalyst under the same conditions
(
Fig. S1†), indicative of the necessity of adding the protein
powder. In addition, Fe /NC exhibits outstanding activity
95%) and selectivity of acetophenone (99%) in the oxidation
1
(
of ethylbenzene at 35 °C after 48 hours, and it also shows
superb stability, i.e. the activity and the selectivity almost do
not decrease after five cycles of reactions. Furthermore, out-
1
standing expandability is shown when Fe /NC is used in the
oxidation reactions of other aromatic compounds saturated
with C–H bonds under ambient conditions. In sum, the syn-
thesis method avoids the cumbersome and complicated tra-
ditional preparation process of SACs and provides an efficient
way to construct single metal site catalysts at the atomic scale.
Fig. 1a illustrates the synthesis process of Fe /NC by
1
mechanical ball milling, and the details are described in the
Experimental section of the ESI.† Typically, 145 mg of Fe
(
NO
carbon black were subjected to mechanical ball milling at 400 TEM image, (c) STEM image, (d) AC HAADF-STEM image, and (e) EDS
elemental mapping of Fe /NC.
3 3 2
) ·9H O, 600 mg of commercial protein powder and 2 g of Fig. 1 (a) Schematic illustration of the synthesis process of Fe /NC. (b)
1
1
rpm for 24 hours, and the resulting complex was pyrolysed at
00 °C under a nitrogen atmosphere for 2 hours to remove the
metal precursors and carbonized protein powder, and then to
anchor the metal atoms on the N sites of the support. For com- and clusters were observed on the amorphous structure of
parison, Fe
/NC (the nanoparticle catalyst, adding 432 mg of nitrogen-containing carbon materials. In addition, aberration-
Fe(NO ·9H
O), Fe/C (without protein powder), and NC (the corrected high-angle annular dark-field scanning transmission
metal-free catalyst, without adding Fe(NO ) ·9H O), were syn- electron microscopy (AC HAADF-STEM, Fig. 1d) revealed that
6
n
3
)
3
2
3
3
2
thesized by the same process. According to elemental analysis the Fe species were atomically dispersed on the surface (high-
EA), the raw protein powder contained up to 12.46% nitrogen lighted in yellow circles), and no nanoparticles were detected.
and the negligible S (0.53%), and enough nitrogen (1.16%) The energy dispersion spectrum (EDS) elemental mapping
still existed in Fe
/NC. The Brunauer–Emmett–Teller surface images further confirmed that iron atoms were uniformly dis-
(
1
2
−1
area (117.58 m g ) of Fe
/NC was detected, and the area was persed on the support (Fig. 1e). In contrast, due to the lack of
1
high enough to facilitate the dispersion of Fe. The pore anchoring atoms provided by protein powder, quantities of
3
−1
volume (0.19 cm g ) and the pore size (2.51 nm) of Fe
revealed that the catalyst maintained the porous structure. By similar iron loading to Fe
the inductively coupled plasma-optical emission spectrometry was 4.59 ± 1.37 nm (Fig. S1†). Besides, there were obvious
ICP-OES) measurements, the contents of Fe in Fe /NC, Fe
/NC nanoparticles on the surface of Fe /NC with an excess Fe
1
/NC nanoparticles appeared on the surface of Fe/C in spite of the
1
/NC, and the average particle size
(
1
n
n
and Fe/C were found to be 1.01 wt%, 2.94 wt% and 1.00 wt%, content, and the average particle size was 2.04 ± 1.00 nm
respectively (Table S1†). The actual iron contents of the three (Fig. S2†). The crystalline phases of catalysts were detected by
catalysts were similar to the theoretical values, which was one X-ray powder diffraction (XRD). As illustrated in Fig. 2a, there
of the outstanding advantages of the ball milling method. The were two obvious characteristic peaks at 25.5° and 43.8° in Fe
1
/
surface morphology and structural composition of Fe /NC NC, Fe /NC, Fe/C, and NC, which could be attributed to the
1
n
1
8
were observed using a transmission electron microscope (002) and (101) crystal faces of graphitic carbon. No Fe-con-
TEM, Fig. 1b) and a scanning transmission electron micro- taining characteristic peaks (44.7° and 65.0°) were detected in
(
scope (STEM, Fig. 1c), respectively; no obvious nanoparticles Fe
/NC, Fe
/NC and Fe/C, which probably indicated that the
1
n
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NC, which further demonstrated that no Fe particles existed in
the catalyst. The wavelet transforms (WT) of Fe /NC and Fe foil
1
are exhibited in Fig. 2e. Different from the contour map of Fe
foil, the maximum intensity of Fe
which was obviously distinct from the maximum intensity of
.7 Å in Fe foil. More detailed structural information was
obtained by quantitative EXAFS fitting of the Fe K-edge in Fe
1
/NC was only around 4.0 Å,
7
1
/
NC (Fig. 2f). The results suggested that each Fe atom co-
ordinated to four surrounding N atoms on average, and the
bond length was about 1.98 Å (Table S2†). Herein, the possible
structure is shown in Fig. 2f.
Selective oxidation is an essential method to synthesize
2
9,30
important compounds, e.g. sulfoxides,
and ketones.
aldehydes, epoxides
3
1–33
In chemical production, acetophenone is
one of the key intermediates with a great market demand,
which can be used to produce a series of products such as
1
alcohol, esters, perfumes and so on. However, due to the
inertness of the C–H bonds and the prone side reactions
caused by excessive oxidation, it is still a long-term challenge
to achieve ideal conversion and acetophenone selectivity under
ambient conditions. As a result, we chose the selective oxi-
dation of ethylbenzene to acetophenone with tert-butyl hydro-
peroxide (TBHP) as the oxidant to investigate the performance
1
of Fe /NC (detailed in the Experimental section of the ESI†).
A relatively high acetophenone yield (95%) was achieved by
Fe /NC after 48 h at 35 °C (Fig. S4†), and the metal free NC
was inert (yield < 1%), which indicated that the Fe species
played a key role in catalyzing the selective oxidation of ethyl-
1
Fig. 2 (a) XRD images of Fe
1
/NC, Fe
n
/NC, Fe/C, and NC with standard
/NC. (c) XANES spectra,
, and FePc. (e) Wavelet
/NC. (f) Corresponding R-space
1
/NC, and the inset exhibits the atomic struc-
PDF of C and Fe. (b) XPS spectra of Fe 2p in Fe
d) FT-EXAFS spectra for Fe /NC, Fe foil, Fe
transform for the Fe K-edge of Fe
1
(
1
2 3
O
benzene. As illustrated in Fig. 3a, when the reactions lasted
1
1
6
hours, the activity of Fe
1
/NC (3.6 h− ) was 12 times that of
EXAFS fitting curves for Fe
ture model.
Fe
n
/NC (0.3 h− ) and three times that of Fe/C (1.2 h ). These
1
−1
results suggested that the isolated Fe sites showed a higher
catalytic activity than agglomerated Fe particles, which was
iron contents were too low to show the characteristic peaks. As due to that the well dispersed Fe species maximized the exposed
shown in Fig. 2b, X-ray photoelectron spectroscopy (XPS) was active interface of Fe /NC, but the agglomerated Fe particles hin-
1
implemented. The peaks at about 725.6 eV and 711.3 eV in dered the contact of most active centres of Fe /NC and Fe/C with
n
XPS images corresponded to Fe 2p1/2 and Fe 2p3/2, respectively, reactants. In addition, the selectivity of Fe /NC and Fe /NC
1
n
indicating that the electron transfer effect due to the coordi- exceeds 90% after 6 hours, far more than Fe/C (54%, pheny-
nation of Fe and N atoms induced charge redistribution and lethanol was the main by-product), which could be speculated
2
7
made Fe species in the positive valence. The binding energy that the selective oxidation of internal saturated C–H rather
of N in Fig. S3† indicated that pyridinic N (398.4 eV), metallic than terminal C–H was more inclined to take place on the Fe–N
N (399.5 eV), pyrrolic N (400.8 eV) and graphitic N (401.2 eV) sites. Moreover, the effect of temperature on the reaction was
2
8
existed in N 1s at the same time.
also considered. As shown in Fig. 3b, the experiments were
In order to obtain detailed messages about the chemical carried out at three different temperatures. When the ethylben-
1
valence and coordination environment of Fe /NC, the X-ray zene conversion was 20%, the corresponding activities of 25 °C,
absorption near-edge structure (XANES) and extended X-ray 35 °C and 45 °C were 1.9 h− , 3.6 h and 5.6 h , respectively,
1
−1
−1
absorption fine structure (EXAFS) measurements were per- which indicated that the reaction was sensitive to temperature,
formed. The Fe K-edge XANES spectra of Fe
Fe , and Fe phthalocyanine (FePc) are shown in Fig. 2c, to generate more active oxygen, thereby enhancing the efficiency
which testify that the oxidation valence of Fe in Fe /NC was a of the reaction. In addition, the stability of Fe /NC was also
1
/NC, Fe foil, and a higher temperature may promote the activation of TBHP
2 3
O
1
1
positive state, because the intensity of Fe
1
/NC was stronger investigated. As shown in Fig. 3c, after the first 48 h of reaction,
than FePc and similar to Fe , which was in line with the XPS the catalyst was filtered and washed with ethanol and deionized
2 3
O
results. The Fourier transform (FT) curve of Fe /NC showed a water five times, dried at 60 °C overnight, and then reused in
1
prime peak at nearly 1.5 Å, which can be ascribed to the Fe–N another cycle reaction under the same reaction conditions. After
bonds (Fig. 2d). In addition, the absence of the Fe–Fe peak five cycles, both the conversion of ethylbenzene and the selecti-
illustrated that no metallic Fe–Fe coordination existed in Fe1/ vity of acetophenone were not decreased. What is more, no
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1
Table 1 Selective oxidation of aromatic substrates by Fe /NC
Entry
1
Substrate
Product
Conv. (%)
Selec. (%)
93
99
2
3
99
98
>99
91
4
5
99
99
>99
>99
Reaction conditions: catalyst dosage = 20 mg, substrate dosage =
0
4
.25 mmol, TBHP = 1 ml, H
8 h.
2
O = 2 ml, T = 35 °C, and reaction time =
Fig. 3 (a) Catalytic performance for selective oxidation of ethylbenzene
to acetophenone by Fe
conditions: catalyst dosage
.25 mmol, TBHP = 1 ml, H
Activity under 25 °C, 35 °C and 45 °C respectively when the ethylben-
zene conversion was 20%. (c) Recycling performance of Fe /NC at
5 °C. (d) AC HAADF-STEM image of Fe /NC after five cycles.
1
/NC, Fe
n
/NC and Fe/C after 6 hours. Reaction
Conclusions
=
20 mg, ethylbenzene dosage
=
0
2
O = 2 ml, T = 35 °C, reaction time = 6 h. (b)
1
In general, we have successfully prepared SAC Fe /NC by a
mechanical ball milling strategy, using protein powder as the
nitrogen source and carbon black as the support. The iron
species are anchored stably by the N elements on the support
as atomically dispersed Fe–N sites. Besides, Fe /NC exhibits
1
3
1
4
1
superb activity, selectivity to corresponding ketones and
great stability in the selective oxidation of arylalkanes under
ambient conditions.
aggregation was observed by AC HAADF-STEM (Fig. 3d). These
results showed excellent stability of structure and great re-
usability of Fe /NC, which was of great significance for indus-
1
trial applications. It was noteworthy that the first reaction
activity was slightly lower than that of cycle reactions; it probably
was because those active sites were further activated during the
suitable reaction process, which made its catalytic performance
improved in the next reactions. Recently reported heterogeneous
catalysts and reaction conditions for the oxidation of ethylben-
zene are listed in Table S3.† Most of them required a high temp-
erature to obtain the satisfactory yield of acetophenone, and
Author contributions
In this work, X.H. and Y.C. developed the concept. Y.C.
designed these experiments and analysed experimental data.
Y.C., Q.S. and L.H. contributed to the synthesis of catalysts
and catalytic experiments. Y.C., Q.H., H.Z., P.W., and Y.Z. per-
formed characterization studies such as XRD, TEM, XPS, and
analysed the results. Y.C., S.S., X.Z. and T.G. performed the
EXAFS measurements and analysed the data. Y.C. and X.H.
wrote the paper. H.J. and X.H. directed the project. All authors
discussed the results and contributed to the scientific
interpretation.
3
5 °C was the most ambient temperature.
The superb catalytic performance of Fe /NC was also
1
achieved on the selective oxidation of saturated C–H bonds in
other aromatic compounds containing various functional
groups under the same reaction conditions. High conversion
(99%) and satisfactory selectivity (>90%) of corresponding
ketones were obtained under the catalysis of Fe /NC when the
1
substrates had steric hindrance, such as 4-ethylanisole,
N-propylbenzene and 4-ethylnitrobenzene (Table 1, entries
Conflicts of interest
1
–3). Besides, in the selective oxidation of compounds with the
There are no conflicts to declare.
steric hindrance of bulky substituents, such as 2-ethylnaphtha-
lene and diphenylmethane to 2′-acetonaphthone and benzo-
phenone under the catalysis of Fe
), superb conversion (99%) and selectivity (>99%) were
achieved. These results demonstrated that Fe
expandability with different aromatic substrates in the selec- Foundation of China (22078371, 21938001, and 21961160741),
tive oxidation of saturated C–H bonds.
the Guangdong Provincial Key R&D Programme
1
/NC (Table 1, entries 4 and
Acknowledgements
5
1
/NC had great This work was supported by the National Natural Science
11714 | Dalton Trans., 2021, 50, 11711–11715
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