Nickel phosphide nanoalloy catalyst for the selective deoxygenation of sulfoxides to sulfides under ambient H2pressure

Article abstract of DOI:10.1039/d0ob01603a

Exploring novel catalysis by less common, metal-non-metal nanoalloys is of great interest in organic synthesis. We herein report a titanium-dioxide-supported nickel phosphide nanoalloy (nano-Ni2P/TiO2) that exhibits high catalytic activity for the deoxygenation of sulfoxides. nano-Ni2P/TiO2 deoxygenated various sulfoxides to sulfides under 1 bar of H2, representing the first non-noble metal catalyst for sulfoxide deoxygenation under ambient H2 pressure. Spectroscopic analyses revealed that this high activity is due to cooperative catalysis by nano-Ni2P and TiO2. This journal is

Full text of DOI:10.1039/d0ob01603a

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Nickel phosphide nanoalloy catalyst for the
selective deoxygenation of sulfoxides to sulfides
under ambient H2 pressure†
Cite this: DOI: 10.1039/d0ob01603a
Received 4th August 2020,
Accepted 29th August 2020
Shu Fujita,a Sho Yamaguchi,a Seiji Yamazoe,b Jun Yamasaki,c Tomoo Mizugaki
a
a
DOI: 10.1039/d0ob01603a
and Takato Mitsudome
*
rsc.li/obc
Exploring novel catalysis by less common, metal–non-metal organic synthesis of fine and bulk chemicals are less
nanoalloys is of great interest in organic synthesis. We herein developed.12–18 In this context, our research group has recently
report a titanium-dioxide-supported nickel phosphide nanoalloy focused on exploring the novel catalysis by metal phosphide
(nano-Ni2P/TiO2) that exhibits high catalytic activity for the deoxy- nanoalloys, finding that non-noble metal phosphide nanoalloy
genation of sulfoxides. nano-Ni2P/TiO2 deoxygenated various sulf- catalysts can outperform conventional metal nanoparticles for
oxides to sulfides under 1 bar of H2, representing the first non- hydrogenation reactions.19,20 For instance, a nickel phosphide
noble metal catalyst for sulfoxide deoxygenation under ambient nanoalloy (nano-Ni2P) represents the first example of a non-
H2 pressure. Spectroscopic analyses revealed that this high activity noble metal catalyst for the selective hydrogenation of biofura-
is due to cooperative catalysis by nano-Ni2P and TiO2.
nic aldehydes to diketones in water.19 In addition, a cobalt
phosphide nanoalloy (nano-Co2P) exhibits outstanding cata-
lytic performance for nitrile hydrogenation, with 20–500-fold
greater activity than previously reported non-noble metal cata-
lysts.20 Thus, catalysis by metal phosphide nanoalloys for
liquid-phase molecular transformations represents an exciting
research area that still has unexplored possibilities, especially
for organic synthesis.
Introduction
It is well known that metal–metal nanoalloys often exhibit
unique physicochemical properties that are superior to those
of nanocrystals comprised of individual metals.1,2 In the field
of catalysis, significant enhancements in activity, selectivity,
and/or durability have been made by alloying two or more
metals.3–5 In contrast, further extension beyond the use of tra-
ditional metal–metal nanoalloys to novel catalysis by metal–
non-metal nanoalloys has not yet been widely explored.
Recently, the rapid advance of nanoengineering technology
has enabled the precise synthesis and catalytic application of
metal–non-metal nanoalloys. Accordingly, metal–non-metal
nanoalloys, e.g., metal phosphide nanoalloys, have been
gaining attention as electrocatalysts for energy conversion
reactions6–8 and petroleum-reforming catalysts for hydrotreat-
ing reactions.9–11 However, despite this catalytic potential, the
catalytic applications of metal phosphide nanoalloys for the
The deoxygenation of sulfoxides is an important transform-
ation in organic synthesis because sulfoxides are utilized as
chirons in asymmetric transformations.21–24 A target chiral
compound can be synthesized through a series of reactions
including the introduction of a chiral sulfoxide and its sub-
sequent deoxygenation. To date, many stoichiometric methods
using metal hydrides,25–27 hydrogen halides,28–30 thiols,31,32
and phosphines33–35 have been reported for the deoxygenation
of sulfoxides. Alternative catalytic methods employing Mo, Re,
Cu, Au, and Ru combined with phosphorus compounds,36–38
hydrosilanes,39–41 BH3,42–44 and alcohols45–47 have been also
developed. However, these catalytic systems still suffer from
low atom efficiency. In contrast, the catalytic deoxygenation of
sulfoxides with H2 represents the most environmentally
friendly method for the synthesis of sulfides with high atom
efficiency because water is formed as the sole by-product.48–53
Our research group recently found that Ru/TiO2 has a high
catalytic activity for the deoxygenation of various sulfoxides
aDepartment of Materials Engineering Science, Graduate School of Engineering
Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan.
E-mail: mitsudom@cheng.es.osaka-u.ac.jp
bDepartment of Chemistry, Tokyo Metropolitan University, 1-1 Minami Osawa,
Hachioji, Tokyo 192-0397, Japan
cResearch Center for Ultra-High Voltage Electron Microscopy, Osaka University, 7-1,
Mihogaoka, Ibaraki, Osaka 567-0047, Japan
under ambient H2 pressure.48 Subsequently, Pt-MoOx/TiO2,49
Pt/V0.7Cr0.3-Hol,50 and Pt/HxMoO3−y
catalysts were devel-
51
oped. As alternatives to expensive and rare noble-metal-based
catalysts, low-cost and earth-abundant non-noble metal cata-
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0ob01603a
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lysts for the deoxygenation of sulfoxides have been also devel-
oped. In this context, only two non-precious metal catalysts
Results and discussion
have been reported for the deoxygenation of sulfoxides using nano-Ni2P was synthesized by a solvothermal method using
H2.52,53 One is a high-valent oxo-molybdenum(VI) complex nickel(II) chloride (NiCl2) and triphenyl phosphite as the nickel
catalyst which requires harsh reaction conditions (50 bar H2, and phosphorous precursors, respectively.19 For comparison,
120 °C).52 The other is a Co–Mo/NC catalyst which operates nano-Co2P and nano-Fe2P were prepared by similar methods
well at room temperature but requires pressurized H2 (10 bar) (Fig. S1 and S2†). The formation of nano-Ni2P was evidenced
and a long reaction time (60 h) to obtain sufficient sulfide by X-ray diffraction (XRD). In the XRD pattern of nano-Ni2P
yields.53 Therefore, the development of a new and efficient (Fig. S1†), the diffraction peaks located at 2θ = 40.8°, 44.7°,
catalytic system based on a non-precious metal catalyst operat- 47.3°, and 54.1° can be ascribed to the (111), (201), (210), and
ing under mild conditions would greatly advance the utility of (300) planes of Ni2P (JCPDS card no. 03-0953), respectively.54
sulfoxide deoxygenation. Herein, we report that a titanium- Fig. 1a shows a transmission electron microscopy (TEM) image
dioxide-supported nickel phosphide nanoalloy (nano-Ni2P/ of a collection of spherical nano-Ni2P particles, which have a
TiO2) exhibits high catalytic activity for the deoxygenation of mean diameter of 5.2 nm. A high-magnification TEM image
sulfoxides to sulfides using H2 (Scheme 1). In contrast to the shows the well-resolved lattice fringes of nano-Ni2P (Fig. 1b),
previously reported non-noble metal catalysts requiring high where the lattice spacings of 0.236 and 0.224 nm correspond
H2 pressures, nano-Ni2P/TiO2 is the first example of a non- to the (102) and (111) planes of hexagonal Ni2P, respectively
noble metal catalyst that promotes the selective deoxygenation (Fig. 1c and d).55 Moreover, the selected area electron diffrac-
of various sulfoxides to the corresponding sulfides under tion (SAED) pattern is consistent with that of hexagonal Ni2P
ambient H2 pressure.
(inset, Fig. 1b), which further verifies the formation of crystal-
line Ni2P.56 Next, nano-Ni2P was immobilized on TiO2. Nano-
Ni2P was dispersed in hexane and then stirred with TiO2,
giving nano-Ni2P/TiO2. The high dispersion of nano-Ni2P on
TiO2 was also confirmed, as depicted in Fig. 1e. These results
clearly showed the successful immobilization of uniform and
crystalline nano-Ni2P on TiO2.
We initially assessed the catalytic potential of various metal
phosphide nanoalloys for the deoxygenation of diphenyl sulf-
oxide (1a) under 10 bar of H2 at 120 °C for 1 h, as summarized
in Table 1. nano-Ni2P promoted the deoxygenation of 1a,
affording diphenyl sulfide (2a) in 14% yield (entry 1). In con-
trast, other non-noble metal phosphides, namely, nano-Co2P,
nano-Fe2P, and bulk Ni2P, showed almost no activity
(entries 2–4), revealing the unique deoxygenation catalysis by
nano-Ni2P. Next, active nano-Ni2P immobilized on various sup-
ports, such as TiO2, ZrO2, Nb2O5, SiO2, hydrotalcite (HT), and
Al2O3, were tested for the deoxygenation of 1a (entries 5
and 9–13). While most of the supported Ni2P alloys provided
Scheme 1 Catalytic deoxygenation of sulfoxides to sulfides using H2.
Fig. 1 (a) TEM image and histogram of nano-Ni2P. (b) High-magnification TEM image of nano-Ni2P (the inset in (b) shows the corresponding SAED
pattern). (c) and (d) Enlarged views of the area selected by the red and yellow square in (b), respectively. (e) TEM image of nano-Ni2P/TiO2 with
nano-Ni2P marked in yellow circles.
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Table 1 Deoxygenation of diphenyl sulfoxide (1a) catalyzed by metal
generated during the reaction, the yield of 2a was further
improved to 56% with a reaction time of 1 h and reached 98%
when the reaction time was extended to 2.5 h (Table 1, entries 6
and 7). Notably, with nano-Ni2P/TiO2, the reaction proceeded
under just 1 bar of H2, providing 2a in 97% yield (Table 1, entry
8). This is the first example of sulfoxide deoxygenation using a
non-precious metal catalyst under ambient H2 pressure.
phosphide catalystsa
Entry
Catalyst
Time (h)
Yieldb (%)
Next, we carried out a hot filtration experiment using nano-
Ni2P/TiO2 to confirm the occurrence of hydrogenation on the
catalyst surface. After the removal of nano-Ni2P/TiO2 by fil-
tration at approximately 50% conversion of 1a, no further con-
version was observed (Fig. S3†). In addition, the concentration
of Ni in the filtrate was below the detection limit of inductively
coupled plasma atomic emission spectrometry (ICP-AES), and
the Ni loading amount of used nano-Ni2P/TiO2 was almost the
same as that of the fresh catalyst (Table S1†). These results
showed that nano-Ni2P was strongly immobilized on the TiO2
support and there was no leaching of Ni species from the cata-
lyst, indicating that this is the active species for the deoxygena-
tion of sulfoxides.
1
2
3
4
nano-Ni2P
nano-Co2P
nano-Fe2P
Bulk Ni2P
nano-Ni2P/TiO2
nano-Ni2P/TiO2
nano-Ni2P/TiO2
nano-Ni2P/TiO2
nano-Ni2P/ZrO2
nano-Ni2P/Nb2O5
nano-Ni2P/SiO2
nano-Ni2P/HT
nano-Ni2P/Al2O3
1
1
1
1
1
1
2.5
12
1
1
1
1
1
14
<1
<1
<1
33
56
98
97
28
15
11
10
8
5
6c
7c
8d
9
10
11
12
13
a Reaction conditions: Catalyst (5 mol% metal), 1a (0.5 mmol), toluene
(3 mL), H2 (10 bar), 120 °C. b Determined by gas chromatography-mass
spectrometry (GC-MS) using naphthalene as an internal standard. c 4 Å
M.S. (0.1 g). d 4 Å M.S. (0.1 g), H2 (1 bar), 160 °C.
The substrate scope of the sulfoxide deoxygenation reaction
catalyzed by nano-Ni2P/TiO2 under atmospheric H2 was investi-
gated (Scheme 2). nano-Ni2P/TiO2 efficiently promoted the
selective deoxygenation of various sulfoxides, including aro-
2a in yields similar to or lower than that with unsupported matic (2a–2e) and aliphatic (2f–2i) substrates, giving the corres-
nano-Ni2P, nano-Ni2P/TiO2 and nano-Ni2P/ZrO2 had 2–3-fold ponding sulfides in high yields. Notably, functional groups
higher activities, giving 2a in 33% and 28% yields, respectively such as halogens (2j and 2k), ether (2l), carbonyls (2m and
(entries 5 and 9). These results showed that the combination 2n), and alkene (2o) were tolerated because nano-Ni2P/TiO2
of the Ni2P nanoalloy with TiO2 provides the best activity operated under mild conditions. In particular, selective deoxy-
among the tested metal phosphide nanoalloy catalysts, genation of sulfoxide while retaining an aldehyde group was
suggesting that metal-support cooperation plays an important achieved for the first time, with nano-Ni2P/TiO2 chemoselec-
role in the present deoxygenation reaction. With nano-Ni2P/TiO2 tively deoxygenating 4-(methylsulfinyl)benzaldehyde to the
in hand, the reaction conditions were optimized. When 4 A corresponding 4-(methylthio)benzaldehyde (2m). Furthermore,
molecular sieves (4 Å M.S.) were added to remove the water nano-Ni2P/TiO2 was applicable to the gram-scale reaction:
Scheme 2 Deoxygenation of various sulfoxides catalyzed by nano-Ni2P/TiO2. Reaction conditions: nano-Ni2P/TiO2 (0.145 g, 5 mol% Ni), substrate
(0.5 mmol), toluene (3 mL), 4 Å M.S. (0.1 g), H2 (1 bar), 160 °C, 12 h. Yields were determined by GC-MS using naphthalene as an internal standard.
a 10 mol% Ni, 140 °C. b 10 mol% Ni. c 10 mol% Ni, 120 °C, 24 h.
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Fourier transform of the extended XAFS (FT-EXAFS) spectrum
of nano-Ni2P showed two peaks at 1.7 and 2.3 Å, which are
assigned to Ni–P and Ni–Ni bonds, respectively (Fig. S4†).58,59
The absence of Ni–O bonds indicated that nano-Ni2P is not
oxidized, which is consistent with the XANES analysis. Curve-
fitting analysis suggested that the Ni–Ni bond length of nano-
Ni2P (2.58 Å) was longer than that of Ni foil (2.48 Å) owing to
the formation of vertex-sharing NiP4 tetrahedra. Interestingly,
nano-Ni2P was significantly different from bulk Ni2P, in terms
of the coordination number (CN) ratio, with the CNNi–Ni/CNNi–P
ratio of nano-Ni2P (1.0) being smaller than the ideal value in
bulk Ni2P (3.3) (Fig. S5 and Table S2†). This small value for the
CNNi–Ni/CNNi–P ratio in nano-Ni2P indicates the presence of
highly coordinatively unsaturated Ni–Ni sites on the surface
which can activate H2.14,19 The crystal structure of Ni2P and an
exposed (300) plane are shown in Fig. S6.† We infer that the
lower CNNi–Ni/CNNi–P ratio of nano-Ni2P is due to the exposure
of the (300) plane because many Ni–Ni sites are arranged on
this plane. Hence, the difference between the deoxygenation
activities of nano-Ni2P and bulk Ni2P is derived from the
highly coordinatively unsaturated active sites for H2 on the
exposed (300) plane of nano-Ni2P.
Next, X-ray photoelectron spectroscopy (XPS) measurements
of nano-Ni2P were carried out. The Ni 2p XPS spectrum of
nano-Ni2P showed two peaks located at 853.1 and 870.2 eV,
which were similar to those of metallic Ni 2p3/2 (852.8 eV) and
Ni 2p1/2 (870.0 eV) (Fig. S7†), supporting the metallic nature of
the Ni species in nano-Ni2P. Furthermore, XPS measurements
of nano-Ni2P/TiO2 were performed to elucidate the role of TiO2
in the deoxygenation of sulfoxides. Fig. 3 shows the Ti 2p XPS
spectrum of nano-Ni2P/TiO2, where the Ti 2p3/2 peak located at
458.8 eV corresponds to Ti4+ of TiO2 (Fig. 3a). After treating
nano-Ni2P/TiO2 under deoxygenation conditions without sulf-
oxides (10 bar H2 at 120 °C for 1 h), a peak assigned to Ti3+
Scheme 3 Gram-scale reaction of diphenyl sulfoxide using nano-Ni2P/
TiO2.
1.0 g of 1a was successfully converted to 2a with a yield of
92%, where a turnover number (TON) reached 92 (Scheme 3).
This value of TON is significantly larger than those of pre-
viously reported non-noble metal catalyst systems (Table S3†).
This result demonstrates that the nano-Ni2P/TiO2 shows high
catalytic activity and durability for the deoxygenation of sulfox-
ides. Conclusively, the use of nano-Ni2P/TiO2 provides a power-
ful method for the selective deoxygenation of various functio-
nalized sulfoxides to the corresponding sulfides under
ambient H2 conditions.
X-ray absorption fine structure (XAFS) analysis was used to
investigate the structure–activity relationship of nano-Ni2P/
TiO2. The X-ray absorption near-edge structure (XANES)
spectra of Ni foil, NiO, nano-Ni2P, and nano-Ni2P/TiO2
measured in air are shown in Fig. 2. The absorption edge
energy of nano-Ni2P (Fig. 2, red line) is close to that of Ni foil
(Fig. 2, blue line), which indicates that the Ni species in nano-
Ni2P are in a metallic state.19 Further, the absorption edge
energy of nano-Ni2P/TiO2 (Fig. 2, green line) is similar to that
of nano-Ni2P, suggesting that the Ni species in nano-Ni2P
retain their metallic nature after immobilization on TiO2.
However, slight changes in the spectral features of nano-Ni2P/
TiO2 suggest that TiO2 affects the local structure and/or elec-
tronic state of nano-Ni2P. Hence, the nano-Ni2P/TiO2 catalyst
has an air-stable metallic nature that is active for hydrogen-
ation, unlike conventional nickel(0) catalysts such as RANEY®
Ni which are oxidatively degraded in the atmosphere.57 The
Fig. 3 Ti 2p XPS spectra of (a) nano-Ni2P/TiO2, (b), (a) after treatment
with H2, and (c), (b) after treatment with 1a. The curve-fitted data are
shown as dash lines in (b).
Fig. 2 Ni K-edge XANES spectra of Ni foil, NiO, nano-Ni2P, and nano-
Ni2P/TiO2.
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the functional groups. XAFS and XPS analyses revealed that
the Ni2P nanoalloy and TiO2 efficiently and cooperatively func-
tioned to activate H2 and deoxygenate the sulfoxide, respect-
ively, leading to high catalytic performance. These results
confirm the potential of metal phosphide nanoalloys for cata-
lysing a wide variety of molecular transformations under mild
conditions. We envisage that our study will make a significant
contribution to expand the unexplored catalytic potential of
metal phosphide nanoalloys for organic synthesis.
Experimental section
General information
X-ray diffraction (XRD) studies were conducted on a Philips
X’Pert-MPD diffractometer with Cu-Kα radiation. Transmission
electron microscopy (TEM) was carried out FEI Tecnai G2 20ST
instruments operating at 200 kV. Gas chromatography-mass
spectrometry (GC-MS) was performed using a GCMS-QP2010
SE instrument equipped with an inert Cap WAX-HT capillary
column (30 m × 0.25 mm i.d.). The oven temperature was pro-
grammed as follows: starting temperature of 100 °C held for
2 min, then ramped to 280 °C at 20 °C min−1. Ni K-edge X-ray
absorption spectra were recorded at room temperature using a
Si (111) monochromator at the BL01B1 line of SPring-8 at the
Japan Synchrotron Radiation Research Institute (JASRI),
Harima, Japan. The obtained spectra were analyzed using
Athena software. After normalization at edge height, the k3-
weighted χ spectra were extracted. XPS analysis was performed
on a KRATOS system equipped with a mono Al X-ray source
and a hemispherical analyzer operating in the fixed analyzer
transmission mode. The spectra were obtained at a pass
energy of 40.0 eV with an Al–K X-ray source operating at 50 W
and 154 kV. The analysis area was 0.7 × 0.3 mm2 while the
working pressure in the analysis chamber was less than 1 ×
10−8 Pa. The C 1s peak at a binding energy of 285.0 eV was
used as the internal reference.
Scheme 4 Proposed reaction pathway for the deoxygenation of sulfox-
ides through the cooperative catalysis by nano-Ni2P and TiO2 support.
appeared at 458.2 eV, suggesting that Ti4+ species on TiO2 is
partially reduced to Ti3+ accompanied by the formation of
oxygen vacancy (Ov) sites (Fig. 3b).60,61 Furthermore, the Ti3+
peak disappeared by the subsequent treatment of nano-Ni2P/
TiO2 with 1a in toluene under an argon atmosphere (Fig. 3c),
resulting in the production of 2a (Scheme S1†). This
result indicated that the in situ generated Ti3+ species are oxi-
dized by sulfoxide, that is, the Ov sites on TiO2 could deoxygen-
ate 1a to 2a.
Based on these results, we proposed a reaction pathway for
the deoxygenation of sulfoxides catalyzed by nano-Ni2P/TiO2
through the cooperative behavior of nano-Ni2P and TiO2
(Scheme 4). First, H2 is activated at a coordinatively unsatu-
rated Ni–Ni site on nano-Ni2P (I). Subsequently, the spillover
hydrogen diffuses onto the TiO2 surface (II), where it reduces
TiO2 to form an Ov site (III).62,63 The Ov site in TiO2 deoxygen-
ates the sulfoxide to give a sulfide (IV), thereby completing the
catalytic cycle. The cooperative effects of nano-Ni2P and TiO2
play key roles in the activation of H2 followed by the formation
of Ov sites and the deoxygenation of sulfoxide, respectively.
This hybrid catalysis system leads to the high performance of
nano-Ni2P/TiO2 in the deoxygenation of sulfoxides.
General procedure for the synthesis of nano-Ni2P/TiO2
All reactions were conducted under an argon atmosphere
using standard Schlenk line techniques. In a typical synthesis,
NiCl2·6H2O (1.0 mmol) was combined with hexadecylamine
(10 mmol) and triphenyl phosphite (10 mmol) in a Schlenk
flask. The mixture was stirred at 120 °C for 1 h. Subsequently,
increasing the temperature to 315 °C while stirring gave a
black colloidal solution. The mixture was allowed to cool down
to room temperature and the black product was then collected
by centrifugation. The obtained powder was washed with a
chloroform–acetone mixture (1 : 1, v/v) and dried in vacuo over-
Conclusion
We found that a TiO2-supported Ni2P nanoalloy efficiently pro- night to give nano-Ni2P. Next, nano-Ni2P (22 mg) was dispersed
moted the selective deoxygenation of sulfoxides to sulfides in hexane (100 mL) with sonication for 1 h and then stirred
under mild conditions. This system is the first non-precious with TiO2 (1.0 g) for 6 h at room temperature. The obtained
metal catalyst to achieve sulfoxide deoxygenation to sulfides powder was dried in vacuo overnight to give nano-Ni2P/TiO2 as
under just 1 bar of H2. A wide variety of sulfoxides bearing a grey powder. Similar procedures were used to prepare the
reducible functional groups were chemoselectively converted other nano-Ni2P/support (ZrO2, Nb2O5, SiO2, HT, and Al2O3)
to the corresponding sulfides in high yields while retaining catalysts.
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Typical catalytic reaction
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As a representative catalytic reaction, the transformation of 1a
to 2a using nano-Ni2P/TiO2 was typically performed as follows.
First, nano-Ni2P/TiO2 (0.145 g) and 4 Å M.S. (0.1 g) were placed
in a 50 mL stainless-steel autoclave with a Teflon inner
cylinder, followed by the addition of 1a (0.5 mmol) and
toluene (3 mL). The reaction mixture was stirred vigorously at
120 °C under 10 bar of H2. The reaction solution was then
analyzed by GC-MS to determine the conversion and yield
using naphthalene as an internal standard method.
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Conflicts of interest
The authors declare that they have no competing interests.
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Acknowledgements
This work was supported by JSPS KAKENHI Grant No.
17H03456, 17H03457, 18H01790, and 20H02523. A part of this
work was supported by the Cooperative Research Program of
the Institute for Catalysis, Hokkaido University (20B1027) and
the Nanotechnology Open Facilities in Osaka University
(A-19-OS-0060), Ministry of Education, Culture, Sports, Science
and Technology (MEXT), Japan. We thank Dr Yoshikata
Nakajima (Institute for NanoScience Design, Osaka University)
for the TEM observation and Dr Toshiaki Ina (SPring-8) for the
XAFS measurements (2019A1390, 2019A1649, 2019B1560, and
2020A1487).
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