Radical Chemistry and Reaction Mechanisms of Propane Oxidative Dehydrogenation over Hexagonal Boron Nitride Catalysts

Article abstract of DOI:10.1002/anie.202002440

Although hexagonal boron nitride (h-BN) has recently been identified as a highly efficient catalyst for the oxidative dehydrogenation of propane (ODHP) reaction, the reaction mechanisms, especially regarding radical chemistry of this system, remain elusive. Now, the first direct experimental evidence of gas-phase methyl radicals (CH₃^.) in the ODHP reaction over boron-based catalysts is achieved by using online synchrotron vacuum ultraviolet photoionization mass spectroscopy (SVUV-PIMS), which uncovers the existence of gas-phase radical pathways. Combined with density functional theory (DFT) calculations, the results demonstrate that propene is mainly generated on the catalyst surface from the C?H activation of propane, while C₂ and C₁ products can be formed via both surface-mediated and gas-phase pathways. These observations provide new insights towards understanding the ODHP reaction mechanisms over boron-based catalysts.

Full text of DOI:10.1002/anie.202002440

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Accepted Article
Title: Radical Chemistry and Reaction Mechanisms of Propane
Oxidative Dehydrogenation over Hexagonal Boron Nitride
Catalysts
Authors: Xuanyu Zhang, Rui You, Zeyue Wei, Xiao Jiang, Jiuzhong
Yang, Yang Pan, Peiwen Wu, Qingdong Jia, Zhenghong Bao,
Lei Bai, Mingzhou Jin, Bobby Sumpter, Victor Fung, Weixin
Huang, and Zili Wu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202002440
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10.1002/anie.202002440
Angewandte Chemie International Edition
COMMUNICATION
Radical Chemistry and Reaction Mechanisms of Propane
Oxidative Dehydrogenation over Hexagonal Boron Nitride
Catalysts
Xuanyu Zhang+, Rui You+, Zeyue Wei, Xiao Jiang, Jiuzhong Yang, Yang Pan, Peiwen Wu, Qingdong
Jia, Zhenghong Bao, Lei Bai, Mingzhou Jin, Bobby Sumpter, Victor Fung*, Weixin Huang* and Zili Wu *
Abstract: Although hexagonal boron nitride (h-BN) has recently been
identified as highly efficient catalyst for the oxidative
attention due to its thermodynamic favorability characterized by
low reaction temperatures and suppression of coking.[2] However,
the commercialization of the ODHP process has not been
achieved because of the runaway over-oxidation of olefins
leading to significant amounts of COx. Recently, several research
groups have reported that hexagonal boron nitride (h-BN) is an
outstanding and selective catalyst for the ODH of alkanes to
corresponding olefins with only negligible COx formation.[3] It is
generally proposed that the formed oxidized B sites (e.g., B-O, B-
OH and B(OH)xO3−x) act as the active centers under ODHP
reaction conditions.[4] Different from the traditional vanadium and
molybdenum-based redox metal oxide catalysts in ODHP
reaction, h-BN shows very low selectivity to carbon oxides and
relatively high selectivity to ethylene, compared to V-based
catalysts where the main by-products are COx.[5] The drastically
different product distributions observed in the h-BN and metal
oxide-based catalysts imply that the reaction mechanisms are
different in these two types of catalysts. Generally, the ODHP
reaction catalyzed by transition metal oxide catalysts was
demonstrated to follow the redox mechanism, in which the
oxidation of propane by surface lattice oxygen of transitional metal
oxides is the rate-limiting step.[1a,5a,5c]
a
dehydrogenation of propane (ODHP) reaction, the reaction
mechanisms, especially regarding radical chemistry of this system,
remain elusive. Herein we report the first direct experimental evidence
of gas-phase methyl radicals (CH3∙) in the ODHP reaction over boron-
based catalysts by using an online synchrotron vacuum ultraviolet
photoionization mass spectroscopy (SVUV-PIMS), which uncovers
the existence of gas-phase radical pathways. Combined with density
functional theory (DFT) calculations, our results demonstrate that
propene is mainly generated on the catalyst surface from the C-H
activation of propane while C2 and C1 products can be formed via both
surface-mediated and gas-phase pathways. These observations
provide new insights towards understanding the ODHP reaction
mechanisms over boron-based catalysts.
Propene (C3H6) is mainly produced by the steam cracking of
oil-derived naphtha or the direct dehydrogenation of propane
(PDH).[1] As
a
promising alternative, the oxidative
dehydrogenation of propane (ODHP), has gathered significant
Interestingly, a recent work implied a possible influence of gas-
phase radical chemistry of h-BN catalyst in ODHP reaction,[6] in
which no expected relationship was found between the catalytic
activity and contact time. Wang et.al.[7] have proposed that methyl
radicals are generated on the h-BN surface from the C-C bond
cleavage, leading to various secondary reaction pathways
involving methyl radicals during ODHP. However, to date there is
no direct experimental evidence for the presence of radicals in the
ODHP reaction over h-BN catalyst, possibly due to the high
reactivity and short lifetime of the gas-phase radicals. In this
regard, synchrotron radiation vacuum ultraviolet photoionization
mass spectroscopy (SVUV-PIMS) is an emerging powerful tool
for direct online detection of reactive, unstable, and short-lived
gas-phase radicals and intermediates for various heterogeneous
catalytic reactions.[8] Herein, we report for the first time that gas-
phase methyl radicals (CH3∙) can be directly observed in the
ODHP reaction over h-BN and suppoted boron oxide catalysts.
Furthermore, the complete reaction pathways and mechanisms of
ODHP reaction over the h-BN catalysts are proposed with the aid
of combined kinetic study, SVUV-PIMS measurement, and
density functional theory (DFT) calculations.
[*]
X, Zhang,[+] R, You,[+] Z, Wei, W. Huang
Hefei National Laboratory for Physical Sciences at the Microscale,
Key Laboratory of Surface and Interface Chemistry and Energy
Catalysis of Anhui Higher Education Institutes, CAS key Laboratory
of Materials for Energy Conversion and Department of Chemical
Physics, University of Science and Technology of China
Heifei 230026 (P. R. China)
E-mail: huangwx@ustc.edu.cn
X, Zhang,[+] X, Jiang, Z, Bao, L, Bai, Z, Wu
Chemical Sciences Division and Center for Nanophase Materials
Science, Oak Ridge National Laboratory
Oak Ridge, TN 37831 (USA)
E-mail: wuz1@ornl.gov
B, Sumpter, V, Fung.
Center for Nanophase Materials Science and Computational
Sciences & Engineering Division, Oak Ridge National Laboratory
Oak Ridge, TN 37831 (USA)
E-mail: fungv@ornl.gov
J, Yang, Y, Pan
National Synchrotron Radiation Laboratory, University of Science
and Technology of China
Heifei 230026 (P. R. China)
P, Wu, Q, Jia
School of Chemistry and Chemical Engineering, Jiang Su University
Zhenjiang 212013 (P.R. China)
M, Jin
Institute of a Secure and Sustainable Environment
The University of Tennessee, Knoxville
Knoxville, TN 37996 (USA)
A high BET surface area h-BN catalyst (72.7 m2 g-1), denoted
as BN-high, was prepared via a novel gas exfoliation of bulk h-BN
in liquid N2.[9] The commercial h-BN catalyst (BN-low) was
purchased from Johnson Matthey INC. with a low BET surface
area (3.7 m2 g-1). Before ODHP tests, all the h-BN catalysts were
treated by reaction feed gas (C3H8:O2:He = 1:1:38, total flow rate
[+]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/anie.202002440
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COMMUNICATION
30 mL min-1) at 500 oC for 12 h (denoted as BN-T). There was a
negligible change of BET surface area after the ODHP reaction
condition treatment (See Table S1). The X-ray diffraction (XRD)
patterns only showed peaks arising from the hexagonal boron
nitride in fresh and treated catalysts (Figure S1A). Raman spectra
of these samples exhibited a strong B-N E2g mode around 1370
cm-1 (Figure S1B) with no obvious change before and after the
treatment.[10] The E2g mode was broader for the BN-high catalysts,
due to the smaller crystal grain size[10b] than the BN-low samples.
The morphology of the fresh and treated h-BN catalysts showed
no significant changes from scanning electron microscopy (Figure
S2). The XRD, Raman and SEM results indicate no evident
changes to the bulk structure and physical property of BN after
the treatment.
The chemical structure was further characterized with FTIR
spectroscopy (Figure S3) and XPS (Figures S4 and S5). The IR
band at around 1325 cm-1 can be assigned to the in-plane B-N
Transverse Optical (TO) modes of h-BN, while the band at 815
cm-1 corresponded to the out-of-plane B-N-B bending
vibration.[3d,11] A new IR peak at 690 cm-1 and a broad one at 3200
cm-1 appeared on the treated h-BN samples, and were attributed
to the O-B-O and B-OH modes, respectively.[3e,11b] Further, the B
1s XPS spectra of treated BN samples (Figure S4 and S5)
exhibited major peaks at 190.6 and 192.2 eV, corresponding to B-
N and B-O bonds, respectively.[3d,4a,11b,11d] The proportion of B-O
bonds was calculated from the B 1s spectra, and the ratio of O/B
increased when BN was treated in ODHP reaction atmosphere at
500 oC (Table S1), which indicates the increment of B-OH and B-
O species.[11c] The results from FTIR and XPS agree well with
previous studies, showing that the BN surface is oxidized and
hydrolyzed to form B-O and B-OH species under ODH reaction
conditions.[4c,4f,11b]
low BET surface area and fewer surface B-O and B-OH sites on
the BN-T-low sample. The stability test was evaluated for BN-T-
high catalyst at 600 oC for 24 hours and the conversion of propane
is stable at ~43% with selectivity remained at around 75% for
C2+C3 alkenes (Figure S7). A re-run of the 600 oC-tested BN-high
sample showed similar catalytic performance (Figure S7) to the
500 oC-treated one (Figure 1), indicating similar surface sites on
BN after the two temperature treatments.
The product distributions of BN catalysts were quite different
from the traditional metal oxide catalysts, where the main by-
product is C2H4, and its selectivity increased with C3H8 conversion
especially at high temperatures. As shown in Figure S6B, the
BOx/SiO2 catalyst showed similar product distributions in the
ODHP reaction as on the BN samples. This similarity suggests
similar surface sites for ODHP over the two type of catalysts. We
detected traces of nitric oxide (NO) on BN-T-high catalyst at high
temperatures at high propane conversion (Figure S8). The
catalytic conversion of ODHP over both BN-T-high and BN-T-low
did not change linearly with the different gas space velocities
(Figure S9). The absence of mass and heat transfer limitation in
the ODHP tests was verified by Weisz-Prater analysis and Mears
analysis, respectively (see SI for detailed calculation). Meanwhile,
the high Ea values (>170 kJ/mol) over the different boron-based
catlaysts were addtional evidence for the absence of diffusion
limitation (Figure S10). This observation is consistent with
previous research.[6] Specifically, the gas-phase radicals [6-7] were
proposed as the key reaction intermediates in ODHP reaction
over BN catalysts, however the direct experimental evidence for
the gas-phase radicals is still lacking.
To investigate the possible radical chemistry, the gas-phase
components of the ODHP reaction over h-BN-T catalysts were
analyzed online by synchrotron radiation vacuum ultraviolet
photoionization mass spectroscopy (SVUV-PIMS) (Figure S11-
S18, and see the SI for the detailed gas-phase components
analysis). Figure 2A shows the SVUV-PIMS spectrum of gas-
phase intermediates catalyzed by BN-T-high during ODHP
The temperature-dependent catalytic performance of the
ODHP reaction over the various treated h-BN and BOx/SiO2
catalysts is shown in Figure 1 and Figure S6, respectively. On the
BN-T-high sample, the reaction started from the temperature of
o
o
500 C with a low propane conversion of 0.3% and reached the
reaction at 600 C ionized at a photon energy of 10.0 eV. The
highest conversion (38.2%) at 600 oC. Compared with the activity
of BN-T-low sample (Figure S6A), the propane conversion is
almost four times higher over the BN-T-high catalyst with
obvious signals at m/z values of 15.03 and 42.08 were assigned
to methyl radicals (CH3∙) and propene (C3H6), respectively, with
reference to online standard database[12] and our previous
work.[8d-f] In addition, we also detected the methyl radicals (CH3∙)
in BN-T-low and BOx/SiO2 catalysts (Figure S11, S15, and S19).
The control and blank tests (Figure S20) showed no evident gas-
phase methyl radicals signal (m/z=15) with the presence of
catalyst at room temperature and without catalyst at 600 oC,
implying that all detected gas-phase methyl radicals should be
produced from the activation of propane on the catalyst surface
during ODHP reactions. Furthermore, the generation of methyl
radicals was observed throughout the ODHP reaction even at low
temperatures with low C3H8 conversions over the h-BN and
BOx/SiO2 catalysts (Figure S21). This suggests that the working
BN catalysts and the BOx/SiO2 indeed may have similar surface
active sites for the ODHP reaction. Our SVUV-PIMS results
provided direct and unambiguous detection of gas-phase methyl
radicals in the ODHP reaction catalyzed by h-BN and BOx/SiO2
catalysts. To our knowledge, this is the first report of direct
experimental evidence of methyl radical generation in ODHP
reaction over boron-based catalysts. Meanwhile, any gas-phase
ethyl (C2H5∙) and propyl (C3H7∙) radicals were not detected in this
system. In our recent work, we have successfully detected
propyl radicals by the same SVUV-PIMS with high sensitivity and
comparable olefin selectivity, owing to
a
relatively
Figure 1. Influence of temperature on the catalytic performance of ODHP
reaction over BN-T-high catalyst. Reaction condition: 100 mg of catalyst; gas
feed, 2.5 vol% C3H8, 2.5 vol% O2, He balance; flow rate 30 mL min-1.
2
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10.1002/anie.202002440
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mass resolution in ODHP reaction, even at very low concentration
of gas-phase radicals.[8e] Combined with this experimental
phenomenon, we can speculate that propyl radicals are not
formed, or the contribution is very limit in this reaction system.
The integrated peak intensity of signals of various gas-phase
components in the SVUV-PIMS spectra of the ODHP reaction
catalyzed by BN-T-high sample was plotted as a function of
temperature and shown in Figure 2B. With the increasing of
ODHP reaction over the BN catalysts warrants more attention,
and further investigation is currently ongoing in our lab. However,
the detection of methyl radical over BOx/SiO2 (Figure S21) in the
absence of NO suggests that radical chemistry over boron-based
catalysts for ODHP reaction is indeed present regardless of NO
presence.
DFT calculations were performed to construct the reaction
mechanism including pathways and radical chemistry of ODHP
on the boron nitride catalyst. Based on our experimental
characterizations (IR and XPS) and previous studies of boron
nitride catalysts under ODHP reaction conditions, the active
phase of the catalyst can be characterized as an amporphous
boron oxide layer with B(OH)xO3-x sites.[4f] In our DFT study, a
surface model was constructed from B2O3 containing surface BO3
units (Figure S23, further discussion of surface model in SI).
As shown in Figure 3, the first step in ODHP is the C-H
activation of propane, which we find to most likely occur via a
heterolytic cleavage over a B-O pair in a BO3 active site (Figure
S28). This step occurs with a barrier of 1.73 eV to form a co-
adsorbed B-C3H7 on the boron and a hydrogen on the B-O-B
oxygen (steps A-C in Figure 3). To drive the formation of propene
from propyl on the surface, O2 is required by first abstracting the
hydrogen from the surface O-H with a barrier of 0.73 eV (steps D-
F in Figure 3), which reduces the O2 to HO2. This process results
in a weakly bound B-HO2 and B-C3H7 on adjacent BO3 sites. This
is followed by a reaction of the two surface species by HO2
abstracting the hydrogen from the propyl to form H2O2 (steps F-G
in Figure 3), a thermodynamically downhill step which can occur
with little to no energy barrier. Both propene and H2O2 then desorb
readily from the surface and H2O2 decomposes to form water,
closing the cycle and regenerating the BO3 site. We have also
explored the formation of C1 and C2 products which start to form
in larger proportions at higher temperatures (Figure 1). We find
the formation of such as ethylene and methyl radicals can occur
competitively over either the surface or the gas-phase (Figures
S30, 31) with similar barriers.
o
reaction temperature from 500 to 600 C, there was an obvious
growth of the signals for C2H4, C2H6, and HCHO, whereas the
integrated ion intensities of CH3∙, CH4, and COx increased slowly
with the reaction temperature. This similar growth trend of
integrated ion intensities suggests that some of C1 and C2
products should be formed by the secondary gas-phase reaction
of surface-generated methyl radicals. [7,8f] Meanwhile, the main
product, propene, is more likely formed on the catalyst surface,
since no gas-phase propyl radicals were observed.
Noteworthily, we detected NO on BN-T-high catalyst during
ODHP reaction at 600 oC by using the SVUV-PIMS (Figure S22),
which was consistent with the GC-TCD results (Figure S8). The
presence of NO may also affect the formation of gas-phase
radicals in the alkane reaction system as demonstrated recently
in catalyst-free propane ODH in empty reactors in the presence
of O2 at around 500 oC.[14] Therefore, the effect of NO on
Figure 3. DFT calculated energy profile for propane oxidative hydrogenation to
propene over B-O-B site on B2O3(101) surface. From left to right, the initial state,
transition state and final states are shown. The models A – H show the side-
view of the geometries of varies steps. The BO3 active site is highlighted with
ball-and-stick models.
Figure 2. (A) SVUV-PIMS spectra of gas-phase components of the ODHP
reaction catalyzed by BN-T-high at 600 oC acquired with a photon energy of
10.0 eV. (B) Integrated ion intensities of various components detected during
the ODHP reaction catalyzed by BN-T-high at different temperatures.
3
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COMMUNICATION
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In the absence of an oxidant such as O2, the C3H7
intermediate is inert to surface-mediated C-H activation and
further oxidation (Figure S32), though gas-phase unimolecular
cleavage could still occur. The high thermodynamic (and kinetic)
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surface. The proposed mechanism bears similarities to other
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selective alkane conversions.
In summary, we have successfully demonstrated that the
ODHP reaction catalyzed by both h-BN and suppored boron oxide
catalysts involves the gas-phase radical mechanisms and
pathways with unambiguously identified gas-phase methyl
radicals by using the SVUV-PIMS. By coupling the results from
kinetic and SVUV-PIMS studies with DFT calculations, detailed
reaction pathways are proposed for the various products from
ODHP over boron-based catalysts. Propene is mainly formed
from surface reaction via the cleavage of C-H bonds of propane.
Both surface-mediated and gas-phase reactions pathways can
contribute to the C1 and C2 products. Our findings provide new
insights towards understanding the ODHP reaction mechanisms
and pathways over boron-based catalysts and are of significance
for developing highly selective catalysts for alkane ODH.
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This work was supported by the Center for Understanding and
Control of Acid Gas-Induced Evolution of Materials for Energy
(UNCAGE-ME), an Energy Frontier Research Center funded by
U.S. Department of Energy, Office of Science, Basic Energy
Sciences. X.Z, R.Y, Z.Y.W and W.H were supported by the
National Natural Science Foundation of China (21525313,
91745202, 21703227), the Chinese Academy of Sciences, the
Changjiang Scholars Program of Ministry of Education of China,
and the China Scholarship Council. Part of the work incluidng the
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Notice: This manuscript has been authored by UT-Battelle, LLC
under Contract No. DE-AC05-00OR22725 with the U.S.
Department of Energy. The United States Government retains
and the publisher, by accepting the article for publication,
acknowledges that the United States Government retains a non-
exclusive, paid-up, irrevocable, world-wide license to publish or
reproduce the published form of this manuscript, or allow others
to do so, for United States Government purposes. The
Department of Energy will provide public access to these results
of federally sponsored research in accordance with the DOE
Public Access Plan (http://energy.gov/downloads/doe-public-
access-plan).
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Entry for the Table of Contents (Please choose one layout)
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Propane ODH over h-BN undergoes
both surface-mediated and gas-phase
radical pathways where propene is
exclusively formed from surface
reaction while C1 and C2 products are
formed via both channels.
Xuanyu Zhang+, Rui You+, Zeyue Wei,
Xiao Jiang, Jiuzhong Yang, Yang Pan,
Peiwen Wu, Qingdong Jia, Zhenghong
Bao, Lei Bai, Bobby Sumpter, Victor
Fung*, Weixin Huang* and Zili Wu*
Page No. – Page No.
Radical Chemistry and Reaction
Mechanisms of Propane Oxidative
Dehydrogenation over Hexagonal
Boron Nitride Catalysts
6
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