Alkanethiolate-capped palladium nanoparticles for selective catalytic hydrogenation of dienes and trienes

Article abstract of DOI:10.1039/c7cy01880k

Selective hydrogenation of dienes and trienes is an important process in the pharmaceutical and chemical industries. Our group previously reported that the thiosulfate protocol using a sodium S-alkylthiosulfate ligand could generate catalytically active Pd nanoparticles (PdNP) capped with a lower density of alkanethiolate ligands. This homogeneously soluble PdNP catalyst offers several advantages such as little contamination via Pd leaching and easy separation and recycling. In addition, the high activity of PdNP allows the reactions to be completed under mild conditions, at room temperature and atmospheric pressure. Herein, a PdNP catalyst capped with octanethiolate ligands (C8 PdNP) is investigated for the selective hydrogenation of conjugated dienes into monoenes. The strong influence of the thiolate ligands on the chemical and electronic properties of the Pd surface is confirmed by mechanistic studies and highly selective catalysis results. The studies also suggest two major routes for the conjugated diene hydrogenation: the 1,2-addition and 1,4-addition of hydrogen. The selectivity between two mono-hydrogenation products is controlled by the steric interaction of substrates and the thermodynamic stability of products. The catalytic hydrogenation of trienes also results in the almost quantitative formation of mono-hydrogenation products, the isolated dienes, from both ocimene and myrcene.

Full text of DOI:10.1039/c7cy01880k

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Alkanethiolate-capped palladium nanoparticles for
selective catalytic hydrogenation of dienes and
trienes†
Cite this: DOI: 10.1039/c7cy01880k
*
Ting-An Chen and Young-Seok Shon
Selective hydrogenation of dienes and trienes is an important process in the pharmaceutical and chemical
industries. Our group previously reported that the thiosulfate protocol using a sodium S-alkylthiosulfate li-
gand could generate catalytically active Pd nanoparticles (PdNP) capped with a lower density of alkanethio-
late ligands. This homogeneously soluble PdNP catalyst offers several advantages such as little contamina-
tion via Pd leaching and easy separation and recycling. In addition, the high activity of PdNP allows the
reactions to be completed under mild conditions, at room temperature and atmospheric pressure. Herein,
a PdNP catalyst capped with octanethiolate ligands (C8 PdNP) is investigated for the selective hydrogena-
tion of conjugated dienes into monoenes. The strong influence of the thiolate ligands on the chemical and
electronic properties of the Pd surface is confirmed by mechanistic studies and highly selective catalysis re-
sults. The studies also suggest two major routes for the conjugated diene hydrogenation: the 1,2-addition
and 1,4-addition of hydrogen. The selectivity between two mono-hydrogenation products is controlled by
the steric interaction of substrates and the thermodynamic stability of products. The catalytic hydrogena-
tion of trienes also results in the almost quantitative formation of mono-hydrogenation products, the iso-
lated dienes, from both ocimene and myrcene.
Received 13th September 2017,
Accepted 19th September 2017
DOI: 10.1039/c7cy01880k
rsc.li/catalysis
highly selective catalytic reactions. Palladium nanoparticles
are still found to be stable enough to maintain homogeneous
Introduction
Many different homogeneous or heterogeneous catalysts have
been studied for various organic reactions including hydroge-
nation and carbon–carbon coupling reaction.1–3 Group VIII
transition metals are usually the most common catalysts for
these reactions. Moreover, palladium is more accessible and
suitable for selective hydrogenation compared to other metal
catalysts such as platinum and rhodium.4,5 It is because the
lower metal–hydrogen bonding energy of platinum or rho-
dium makes these metal catalysts readily hydrogenate
multiene substrates to alkanes.6–8 Therefore, more attention
has been paid to developing palladium nanoparticles as selec-
tive hydrogenation catalysts.9–13
Based on previous research work from our group,
alkanethiolate-capped palladium nanoparticles generated
using the thiosulfate protocol have shown good catalytic reac-
tivity for the hydrogenation or isomerization of alkenes.14–17
The lower surface coverage of alkanethiolate ligands on palla-
dium nanoparticles could provide controlled poisoning for
solubility for multiple recycling. Ligand-capped colloidal Pd
nanoparticles are considered as semi-heterogeneous catalysts
because the core contains various surface adsorption sites
with different activities such as corners, edges, steps, and ter-
races. This characteristic of colloidal Pd nanoparticles led to
their good reactivity, selectivity, facile separation and reus-
ability, displaying the advantages of both homogenous and
heterogeneous catalysts.
Freund et al. found out that the catalytic reaction of a ter-
minal alkene would produce an alkane, the hydrogenation
product, in a higher yield by increasing the size of Pd nano-
particles.18,19 This positive correlation strongly indicates that
the hydrogenation reaction is affected by the number of ter-
race sites in Pd nanoparticles, which also relates to the for-
mation of a di-σ bonded Pd–alkyl intermediate. The poison-
ing effect of Pd nanoparticles by thiolate ligands would play
an important role in the adsorption of alkene on the metal
surface. This is because the available sites in the terrace sur-
face would decrease more for ligand-passivated nano-
particles, making the hydrogenation of alkenes to alkanes
more difficult to occur.
Department of Chemistry and Biochemistry and Keck Energy Materials Program,
California State University, Long Beach, 1250 Bellflower Blvd., Long Beach,
California 90840, USA. E-mail: ys.shon@csulb.edu
Selective hydrogenation of small dienes is an important
reaction for the petroleum industry, because dienes could act
as monomers for polymerization that would deteriorate fuels
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7cy01880k
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during pyrolysis and storage processes.20–22 Fully saturated
alkane petroleum products in a pure form are also not the
most ideal since their lower boiling points and higher gas
pressures would require containers with larger storage vol-
umes. The naturally occurring trienes, ocimene and myrcene,
are frequently used in the perfume industry, and the selective
hydrogenation of natural triene products is also important in
the pharmaceutical industry. It is because the primary prod-
uct has the potential to be the precursor for functionalized
compounds such as oxygenated derivatives.23,24 Many re-
search groups have utilized different metal complexes and
hydrogen sources to reduce diene and triene compounds in
various solvents using either homogeneous or heterogeneous
conditions.
An octanethiolate-passivated palladium nanoparticle (C8
PdNP) catalyst has shown good reactivity for the selective hy-
drogenation of penta-1,4-diene, an isolated diene, to mono-
enes under mild reaction conditions in our previous work.25
Building upon this previous work, in this article we investi-
gate the catalytic applications of C8 PdNP for the selective hy-
drogenation of various conjugated dienes and trienes. The
mechanism for the high selectivity of C8 PdNP is proposed
based on kinetic studies and catalysis results for various sub-
strates. Comprehensive comparisons with other catalytic sys-
tems on the selective hydrogenation of conjugated dienes
and trienes are also presented.12,26,27
Table
1
Catalysis results of various dienes and trienes with the
octanethiolate-capped Pd nanoparticle catalyst, Pd on carbon, and Pd on
alumina in CDCl3 at 1 atm H2 after 24 ha
C8 PdNP
MHb
Pd/C
MHb
Pd/Al2O3
MHb
Substrate
DHb
0%
DHb
DHb
100%
0%
100%
0%
100%
1
100%
0%
0%
100%
0%
100%
2
3
4
c
c
100%
100%
97%
0%
0%
3%
0%
0%
9%
100%
100%
91%
—
—
0%
6%
100%
94%
5
6
95%
0%
0%
0%
0%
100%
100%
0%
0%
100%
100%
100%
7
92%
0%
4%
96%
10%
90%
8
a
Reaction conditions: 0.5 mmol diene, 5 mol% Pd catalyst, 2.5 mL
CDCl3, 1 atm H2, 24 h reaction. Other heterogeneous catalysts, Pd/C
Results and discussion
and Pd/Al2O3, were also examined under the same reaction
b
conditions. Yields were obtained by 400 MHz NMR.
Mono-
An octanethiolate-capped Pd nanoparticle (C8 PdNP) catalyst
with an average core size of ∼2.3 nm is synthesized and its
composition and structure are confirmed using various instru-
ments as shown in our previous article and in the ESI.†25 C8
PdNP is used as the catalyst for the hydrogenation of various
conjugated diene and triene substrates as shown in Table 1.
The catalytic activity of C8 PdNP is directly compared with
other Pd-based heterogeneous catalysts (Pd/C and Pd/Al2O3).
The catalytic reactions of conjugated dienes with different
substitution patterns around CC bonds (1–6) using C8
PdNP produce almost exclusively mono-hydrogenation prod-
ucts (MH), without having double hydrogenation products
(DH). In addition, the hydrogenation of trienes (7 and 8)
using C8 PdNP also results in high yields of isolated dienes,
the mono-hydrogenation products (MH). This high selectivity
of C8 PdNP for mono-hydrogenation reaction is clearly distin-
guishable from the catalytic results of other Pd-based hetero-
geneous catalysts, Pd/C and Pd/Al2O3, that produce mostly
double-hydrogenation products (DH) from both dienes and
trienes. This different reactivity confirms the important role
of alkanethiolate ligands in controlling the activity of the
PdNP catalyst surface. With its surface passivated by alkane-
thiolate ligands, the PdNP has increased stability, so that it
avoids aggregation and keeps the large surface area intact.
The alkanethiolate surface ligand on the PdNP would block
more active sites (terrace surfaces) for hydrogenation and
promote the selective hydrogenation of dienes.
hydrogenation (MH), double-hydrogenation (DH). c Not attempted.
The product ratios of the 1,4- and 1,2-addition products in
addition to the overall conversion yields during the kinetic
study are summarized in Table 2. The catalytic reaction of C8
PdNP with 2,3-dimethyl-buta-1,3-diene 1 exhibits excellent se-
lectivity toward the mono-hydrogenation products with 100%
conversion yield. The analysis of the final monoene composi-
tion showed that the major product is 2,3-dimethyl-2-butene
1b, the 1,4-addition product, and the minor product is 2,3-
dimethyl-1-butene 1a, the 1,2-addition product. The high
yield of 1b is the result of both the initial 1,4-addition reac-
tion and the subsequent isomerization of the terminal al-
kene, the 1,2-addition product, into an internal alkene. This
is clearly evidenced by the kinetic study of dienes to mono-
1
enes, which is monitored by H NMR as shown in Fig. 1. The
decreasing signals at 1.9 and 4.9 ppm in Fig. 1 indicate the
consumption of compound 1, and the increased signals at
1.7 (or 1.0) and 1.6 ppm correspond to two mono-
hydrogenation products, which are the chemical shifts for 1a
and 1b, respectively. After the first hour of reaction, the con-
version of 1 reaches over 50% with the ratio of the 1,4-/1,2-
addition products at 3.43. The TOF at the first hour is calcu-
lated to be 375.1 molecules per Pd per h. The consumption
of reactant 1 is almost complete after 5 h of reaction with the
ratio of the 1,4-/1,2-addition products at 4.9. The result of the
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Table 2 Hydrogenation of different diene and triene substrates using C8 PdNPa
Substrate
1,2-Add
Yield (%)
9
1,4-Add
Yield (%)
91
1,4-Add/1,2-add
10.4
1
2
1b
2c
1a
2a
7
93
14.2
0
2b
3a
3b
4a
4b
92c E/Z = 4.0
—
—
3b
3a
4c
8
29
71 E/Z = 3.5
2.4
—
4
0
90c
—
5b
6b
7
5a
5b
5a
7
90c
—
—
6a
6b
6a
7c
5
41
59
1.5
7a
7b
8a
8b
0
3
69
3.0
8
7c
20
a
Reaction conditions: 0.5 mmol diene, 5 mol% Pd catalyst, 2.5 mL CDCl3, 1 atm H2, 24 h reaction. Yields were obtained by 400 MHz 1H NMR.
The catalytic reaction of this substrate yields the same product after 1,2- and 1,4-addition reactions. This yield is the combination of both
b
c
1,2- and 1,4-addition reactions.
24 h reaction indicates that the yield of 1b is still increasing
after 5 h of reaction with the ratio of the 1,4-/1,2-addition
products reaching 10.4. This is clear evidence suggesting that
the isomerization of the terminal alkene 1a to the internal al-
kene 1b continuously takes place after the 5th hour of reac-
tion. The good activity of C8 PdNP for the isomerization of
terminal alkenes to internal alkenes driven by a thermody-
namic effect has previously been reported by our group.25
The mechanistic explanation for the high selectivity to the
1,4-addition product in the 2,3-dimethyl-buta-1,3-diene 1 hy-
drogenation reaction is provided in Scheme 1.
In order to prove that the direct 1,4-addition is the major
reaction route instead of the isomerization of the 1,2-addi-
tion product, the catalytic hydrogenation of commercially
available compound 1a is studied. The result shows the low
conversion efficiency of isomerization from 1a to 1b with only
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Fig. 1 (A) Kinetic study of 2,3-dimethylbuta-1,3-diene 1 hydrogenation using C8 PdNP at room temperature under 1 atm H2 in CDCl3. (B) The
zoom is for the area of the 5 and 24 h spectra. Chemical shifts of compound 1a are at 1.02 ppm (–CH3) and 1.71 ppm (allylic-CH3). Chemical shift
for compound 1b is at 1.64 ppm (–CH3).
52% of 1a being isomerized to 1b after 24 h of reaction
(Table 3). The slow conversion of 1a to 1b is partly due to the
lower number of α hydrogen atoms in the mono-σ-bonded pal-
ladium alkyl intermediate generated from the adsorption of al-
kene 1a on the PdNP surface. Since the terminal carbon has
more hydrogen atoms than the internal α carbon, the mono-σ-
bonded intermediate would most likely take the hydrogen from
the terminal carbon. It would cause the intermediate to equili-
brate back to the starting material, the terminal alkene. The ki-
netic study showing the low conversion rate of 1a to 1b proves
that the initial direct 1,4-addition route for 1 would be the ma-
jor route for the formation of 1b and supplemented by the
isomerization of the 1,2-addition product 1a.
The selectivity between the 1,4- and 1,2-addition products
is also summarized for other diene and triene substrates in
Table 2. Due to the structural characteristics of substrates 3,
5, and 6, generating the same products for the 1,2-addition
and 1,4-addition reactions, the major reaction route for these
Table 3 Hydrogenation of 2,3-dimethylbut-1-ene with the C8 PdNP
catalysta
1b
1c
1a
Time
Terminal alkene (%)
Internal alkene (%)
Alkane (%)
1 h
3 h
6 h
24 h
88%
78%
66%
48%
12%
22%
34%
52%
0%
0%
0%
0%
a
Scheme
2,3-dimethylbuta-1,3-diene
nanoparticles.
1 Proposed mechanism for the catalytic reaction of
Reaction conditions: 0.5 mmol diene, 5 mol% Pd catalyst, 2.5 mL
CDCl3, 1 atm H2, 24 h reaction. Yields were obtained by 400 MHz 1H
NMR.
1
with
octanethiolate-capped
Pd
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substrates could not be identified by analysing the products.
However, C8 PdNP clearly shows high selectivity for the
1,4-addition product from the reactions of 2-methylbuta-1,3-
diene 2 and hexa-1,3-diene 4. After the direct formation of
both 1,4-addition product 2c and 1,2-addition product 2a, the
isomerization of 2a would further take place readily because
of the presence of two hydrogens that can be removed from
its mono-σ-bonded Pd alkyl intermediate. Though a trace
amount of 2b is observed during the first few hours of reac-
tion, the thermodynamic stability of the isomerized product
2c makes 2b even easier to isomerize, completely depleting
the presence of 2b after 24 h of reaction.
sterically more accessible than that of other substrates with
substituent groups on the 1,4-position. In addition, the isom-
erization of the 1,2-addition product occurs more easily be-
cause the equilibrium of isomerization would be driven by
forming a more thermodynamically stable product. For the
catalytic reaction of hexa-1,3-diene 4, however, the 1,4-/1,2-
addition product ratio is relatively lower than that of
substituted buta-1,3-dienes. The 1,4-di-σ-bonded intermediate
generated from 4 is less accessible due to the presence of an
ethyl substituent group causing steric hindrance for the ini-
tial hydrogen addition. Moreover, the isomerization of 4a,
one internal di-substituted alkene, to 4c, another internal di-
substituted alkene, does not provide any additional thermo-
dynamic enthalpy reduction eliminating a driving force for
the isomerization.
For substituted buta-1,3-dienes 1 and 2, the hydrogenation
results in a higher 1,4-/1,2-addition ratio because the forma-
tion of a 1,4-di-σ-bonded intermediate for these substrates is
Table 4 Comparison table for the catalytic hydrogenation selectivity of dienes and trienes
Reaction
Internal
Substrate Catalyst
C8 PdNP
pathwayb Convb
Semi-HET 100%
MHb
DHb 1,2-Add
1,4-Add alkene Reaction conditions
100%
0%
8.8%
91.2% 91.2%
R.T., CDCl3, 1 atm H2
PhCOOMeCrIJCO)3
(ref. 27)
Pd/sgPF6 (ref. 12)
C8 PdNP
HOM
100%
100%
0%
18%
82%
81%
82%
81%
160 °C, n-heptane,
30 atm H2
40 °C, CH2Cl2, 4 bar H2
R.T., CDCl3, 1 atm H2
Semi-HET >99%
Semi-HET 100%
100%
100%
0%
0%
19%
6.6%
93.4% 93.6%
Silica polyamine PdNP HET
composite21
100%
94%
6%
0%
0%
46.5% 2a:
24.5% 2b: 22%
25% 2a: 18%
2b: 7%
47.5% 47.5%
70 °C, toluene, 3 MPa H2
40 °C, CH2Cl2, 4 bar H2
R.T., CDCl3, 1 atm H2
Pd/sgPF6 (ref. 12)
Semi-HET >99%
100%
75%
—
75%
C8 PdNP
Semi-HET 100%
100% 3a: 92%
3b: 8%
39.3% 3a: 31.1% 0%
3b: 8.2%
—
98.2%
31.1%
0%a
Co3O4 (ref. 28)
HET
39.3%
100%
—
—
—
R.T., PC H = 50–60
mmHg,5PH8 /PC H = 2–3
2
8
[Cp2ZrHIJCH2PPh2)]n
(ref. 29)
HOM
0%
100%
—
80 °C, hexane/5THF,
40 bar H2
C8 PdNP
PhCOOMeCrIJCO)3
(ref. 27)
Semi-HET 100%
HOM 100%
100%
100%
0%
0%
29.3% 4a only 70.7% 100%
R.T., CDCl3, 1 atm H2
160 °C, n-heptane,
30 atm H2
24% 4a: 18%
4b: 6%
—
76%
94%
C8 PdNP
Semi-HET 100%
96.6% 5a: 90.3% 3.4%
5b: 6.3%
—
90.3%
80.1%
R.T., CDCl3, 1 atm H2
Polystyrene coated
PdNP on SiO2 (ref. 30)
HET
>99.7% >99.6% 5a:
0.1%
—
—
Supercritical CO2,
3 MPa H2
80.1% 5b:
19.8%
Dendrimer Pd
complex31
C8 PdNP
HOM
>99%
>99% 5a: 57%
5b: 43%
95% 6a: 90.6%
6b: 4.1%
82.4% 6a: 82.4% 0.7%
6b: 4%
100%
0%
0%
—
—
—
—
—
—
57%
25 °C, MeOH, 14 p.s.i. H2
R.T., CDCl3, 1 atm H2
R.T., acetone, 80 p.s.i. H2
R.T., CDCl3, 1 atm H2
Semi-HET 95%
90.6%
82.4%
Pd ferrocenyl complex32
C8 PdNP
—
87.1%
Semi-HET 100%
0%
40.8% 7a only 59.2% 100%
Copper–chromium
oxide33
HET
100%
85%
15% 78% 7a: 58%
7b: 20%
7%
65%
140 °C, trans-decalin,
1 atm pressure
C8 PdNP
Semi-HET 92.2%
92.2%
93.1%
0%
23.2% 8a: 3.1% 69%
8b: 20%
92.2%
R.T., CDCl3, 1 atm H2
Sol/gel 3% Pd/SiO2
(ref. 23)
HET
96%
2.9% 33.6% 8a:
19.2% 8b:
59.5% 59.5%
80 °C, 20 atm,
H2 cyclohexane
14.4%
Copper–chromium
HOM
100%
100%
0%
92% 8a: 23%
8b: 69%
8%
8%
140 °C, trans-decalin,
1 atm pressure
oxide33
a
b
The data are chosen from the best selectivity results for each type of catalyst. Abbreviations for the reaction pathway: semi-heterogeneous
(Semi-HET), homogeneous (HOM), heterogeneous (HET). Conversion (Conv). Mono-hydrogenation (MH), double-hydrogenation (DH).
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in the petroleum pyrolysis process. The comparison of C8
PdNP with other catalysts used for ocimene and myrcene hy-
drogenation is also summarized in Table 4.23,33 The results
again confirmed the excellent activity and selectivity of C8
PdNP for the selective mono-hydrogenation of trienes to
internal isolated dienes.
The stability and recyclability of C8 PdNP are tested using
the catalytic reaction of 2,3-dimethylbuta-1,3-diene, 1. C8
PdNP is precipitated and separated from the reaction mix-
ture. The PdNP is further dried without any solvent before be-
ing recycled for additional catalysis reactions. As shown in
Fig. 2, the activity and selectivity of C8 PdNP are mostly
maintained especially without generating any double hydro-
genation product even after 7 cycles. The conversion of the
reactant after 7 cycles is still above 95% and the selectivity
for the 1,4-addition product remains over 80% after 7 cata-
lytic cycles. To examine the leaching of Pd atoms from C8
PdNP during the catalytic reaction, the presence of Pd2+ in
the crude solution is determined using ICP-AES. The results
suggest that the digested product solution has only ∼0.0546
ppm Pd2+, which shows the overall high stability of C8 PdNP
during the hydrogenation reaction.
Fig.
2
The recycle study of 2,3-dimethylbuta-1,3-diene
1
hydrogenation using C8 PdNP. Reaction conditions: 0.5 mmol diene, 5
mol% Pd catalyst, 2.5 mL CDCl3, 1 atm H2, 24 h reaction. Yields were
obtained by 400 MHz NMR.
The analyses of the mono-hydrogenation products gener-
ated from the hydrogenation of trienes 7 and 8 are also
shown in Table 2. The presence of a large alkyl substituent
group at the diene part of ocimene 7 decreases the ratio of
the 1,4- and 1,2-addition products. The yield for 7a, the
1,2-addition product, is relatively high due to the good ac-
cessibility of the terminal alkene group to the Pd surface.
Another possible 1,2-addition product 7b does not form be-
cause of the steric interference provided by the surrounding
substituent groups. The lack of a thermodynamic driving
force from products 7a to 7c as seen from the reaction of 4
is another main reason for the decreased yields of the
1,4-addition product. The ratio of the 1,4-/1,2-addition prod-
ucts for the catalytic reaction of myrcene 8 is, however,
more than twice higher than that of ocimene 7. This is be-
cause both terminal carbons of the diene group do not pos-
sess any substituent group resulting in less steric interfer-
ence for direct 1,4-hydrogen addition compared to ocimene
7. The thermodynamic stability of 7c is also higher than
that of 8a and 8b, the more likely 1,2-addition products,
making the isomerization more likely to occur. As shown
in Table 2, C8 PdNP is highly selective towards the mono-
hydrogenation products for both ocimene and myrcene and
would not hydrogenate the highly substituted and isolated
C6C7 double bond.
Conclusions
Octanethiolate-capped palladium nanoparticles show excel-
lent activity and selectivity for the mono-hydrogenation of di-
enes into internal alkenes and trienes into isolated internal
dienes. The kinetic study and comparison of various catalysis
reactions indicate that the conjugated diene hydrogenation
reactions mainly involve the direct 1,4-addition reaction in
addition to the isomerization of the 1,2-addition product that
further isomerizes terminal alkenes into internal alkenes.
The leaching examination and recycling study of the
octanethiolate-capped palladium nanoparticles show the high
stability of the catalyst. This octanethiolate-capped palladium
nanoparticle catalyst, therefore, has high technological poten-
tial for effective elimination of reactive monomers such as di-
enes and terminal alkenes for polymerization and prevention
against the deterioration of crude oil storage.
Conflicts of interest
C8 PdNP clearly exhibits excellent selectivity to internal al-
kenes, the mono-hydrogenation and 1,4-addition products,
compared to other reported catalytic systems summarized in
Table 4 for diene hydrogenation.12,21,27–32 Not only are the
conversion yields and selectivity superior but also the reac-
tion conditions (room temperature and atmospheric pres-
sure) are much friendlier than any known homogenous and
heterogeneous catalysts tested for diene hydrogenation. Since
both dienes and terminal alkenes are potential monomers
for the polymerization reaction, C8 PdNP could either hydro-
genate the dienes by a 1,4-addition reaction or isomerize ter-
minal alkenes to internal alkenes under mild reaction condi-
tions, so that the deterioration of petroleum products can be
effectively prevented by limiting the potential polymerization
There are no conflicts to declare.
Acknowledgements
This research was supported by the W. M. Keck Foundation
and the National Institute of General Medical Sciences
(SC3GM089562).
Notes and references
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X.-Y. Guo, Catal. Sci. Technol., 2016, 6, 7738–7743.
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