Selective Copper-N-Heterocyclic Carbene (Copper-NHC)-Catalyzed Aerobic Cleavage of β-1 Lignin Models to Aldehydes

Article abstract of DOI:10.1021/acscatal.7b00565

As fossil resources are undergoing fast depletion, the harvesting of sustainable aromatic compounds from biorenewable lignin has become highly appealing nowadays. However, the development of efficient catalysts for lignin depolymerization to high value-added chemicals is challenging. In this paper, a selective copper-N-heterocyclic carbene (copper-NHC)-catalyzed aerobic cleavage is reported for β-1 lignin model compound, which represents an important family of natural wood lignin. Using this catalyst system, the cleavage of β-1 model compounds gave >99% conversions and satisfactory yields of the corresponding aldehydes, which can serve as versatile starting materials for the chemical industry.

Full text of DOI:10.1021/acscatal.7b00565

Subscriber access provided by Fudan University
Letter
Selective Copper-NHC Catalyzed Aerobic
Cleavage of #-1 Lignin Models to Aldehydes
Zhong-zhen Zhou, Mingxin Liu, and Chao-Jun Li
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00565 • Publication Date (Web): 06 Apr 2017
Downloaded from http://pubs.acs.org on April 6, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 6
ACS Catalysis
1
2
3
4
5
6
7
8
9
Selective CopperꢀNHC Catalyzed Aerobic Cleavage of βꢀ1
Lignin Models to Aldehydes
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Zhong-zhen Zhou, †, §, ‡ Mingxin Liu †, ‡ and Chao-Jun Li †, *
†Department of Chemistry and FQRNT Centre for Green Chemistry Catalysis, McGill University 801 Sherbrooke St.
W., Montreal, Quebec H3A 0B8 (Canada).
§School of Pharmaceutical Sciences, Southern Medical University, Guangzhou 510515, China.
ABSTRACT: As fossil resources are undergoing fast depletion, harvesting sustainable aromatic compounds from bio-
renewable lignin becomes highly appealing nowadays. However, the development of efficient catalyst for lignin depoly-
merization to high value-added chemicals is challenging. In this paper, a selective copper-NHC catalyzed aerobic cleavage
is reported for β-1 lignin model compound, which represents an important family of natural wood lignin. Using this cata-
lyst system, the cleavage of β-1 model compounds gave >99% conversions and satisfactory yields of the corresponding
aldehydes, which can serve as versatile starting materials for chemical industry.
KEYWORDS: aerobic cleavage, copper-NHC,
β
-1 lignin models, C-C bond cleavage, aldehydes
(a) Baker and Hanson (2013)
Efficient cleavage of lignin linkages for harvesting high
value-added aromatic material remains highly challenging
nowadays1-4. Among those linkages, the β-1 linkage is a
significant component5-6. According to thioacidolysis and
2D-HSQC-NMR studies of the lignin oligomers from cryp-
tomeria japonica (softwood) and eucalyptus globulus
(hardwood), β-1 abundancy is up to 30% for the former,
whereas up to 50% was observed for the later7. Moreover,
other linkages, for example β-O-4 in softwood and hard-
wood lignin can also be readily converted into β-1 by oxi-
dative cross-linking reaction1, 8. Nevertheless, considera-
ble interests were paid to the cleavage of β-O-4 linkage2, 9-
15. The cleavage method for β-1 linkage remains very
scarce16-21, especially examples utilizing oxygen as a natu-
rally abundant and environmentally benign terminal oxi-
dant.
OMe
HO
MeO
1 equiv Cu (OTf)/TEMPO
O
10 equiv 2,6-lutidine
O2 (1 atm)
MeO
O
X
H
+
toluene, 100 OC, 48h
100% conversion
O
MeO
OH
OMe
82 %
HO
25% (X = H)
1% (X = OH)
O
H
OMe
MeO
HO
+
O
MeO
OMe
6%
9%
(
b) Mariano (2015)
OH
O
R2
OH
h
R1
R1
+
9,10-Dicyanoanthracene
OH
OH
R1
5%H2O-MeCN, O2
100% conversion
OH
O
O
In 2013, Baker and Hanson16 reported an aerobic cleavage
of β-1 linkage using copper(I) trifluoromethane-sulfonate
(CuOTf)/TEMPO system. This discovery is highly im-
portant, however the method requires excessive alkali (10
equiv 2,6-lutidine) and even excessive catalyst for certain
cases (Figure 1a). The substrate scope was very limited.
Moreover, the cleavage resulted in different oxidation
product mixture with dramatically varying composition
between different substrates. In 2015, Mariano developed
a photochemical and enzymatic SET (single electron
transfer) promoted C-C bond aerobic cleavage for lignin
β-1 model compounds (Figure 1b). Though conversions
were high, mixture of products was still obtained using
this method. In 2016, Wang developed an acid-promoted
C–C bond aerobic cleavage catalyzed by copper catalyst.
(Figure 1c)21 However, only one β-1 ketone example was
reported. A more efficient and product-selective method
towards β-1 cleavage is still highly desirable.
H
H
1
R2
R
+
R2
O
+
R1
OH
(c) Wang (2016)
O
O
O
20% Cu(OAc)2/BF3 OEt2
H
OMe
+
MeOH, 0.4 MPa O2
100 OC, 3h
OH
87 %
0%
(d) this work
20% CuCl/SIMes
3 equiv quinoline
O2 (1 atm)
OMe
O
O
OH
H
H
+
R
R
dioxane, 100 oC, 24h
MeO
OH
1
2aꢀe
2a
Figure 1. Catalyzed aerobic cleavage of β-1 lignin models to
aldehydes.
ACS Paragon Plus Environment

ACS Catalysis
Page 2 of 6
Recently, our group has shown that homogeneous cop- ducted in argon (entry 16). In summary, the optimized
1
2
3
4
5
6
7
per-NHC-catalysts can catalyze aerobic oxidation of alde-
hydes via a unique mechanism.22 Motivated by this work
and to further explore copper-NHC (Figure 2) catalyzed
aerobic oxidation, herein, we would like to report an effi-
cient and highly selective copper-NHC catalyzed aerobic
cleavage of β-1 lignin models to aldehydes without using
TEMPO.
conditions for the cleavage of β-1 lignin model com-
pounds are (SIMes)CuCl (20%) with quinoline (3 equiv) at
100 oC under O2 (1 atm) for 24 h using dioxane as solvent.
Table 1 Optimization of reaction conditions
8
i-Pr
i-Pr
9
N
N
N
N
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Cu
Cl
Cu
Cl
i-Pr
i-Pr
Entry
1
Catalyst/ mol %
(SIMes)CuCl/10
CuCl/10
Conv. (%)a
Yield (%)a
(SIMes)CuCl
(IPr)CuCl
71
52
0
50
2
21
3
SIMes/10
0
N
N
N
N
4
CuCl + SIMes/10
(DPSIMes)CuCl/10
(MOPI)CuCl/10
(OSIMes)CuCl/10
(IPr)CuCl/10
10
7
Cu
Cl
Cu
Cl
5
33
0
14
(DPSIMes)CuCl
MeO
(OSIMes)CuCl
6
0
7
42
16
17
N
N
OMe
8
7
Cu
Cl
9
(SIMes)CuCl/20
(SIMes)CuCl/20
(SIMes)CuCl/20
(SIMes)CuCl/20
(SIMes)CuCl/20
(SIMes)CuCl/20
(SIMes)CuCl/20
(SIMes)CuCl/20
100
100
6
90 (89) b
(MOPI)CuCl
10c
11d
12e
13f
14h
15i
68
0
60
64 (17) g
Figure 2. The structures of copper-NHCs.
66
100
71
Initially, we examined aerobic cleavage of β-1 lignin
model compound 1aE using pre-synthesized (SIMes)CuCl
as catalyst in dioxane with added quinoline (3 equiv with
respect to substrate) under O2 (1 atm) at 100 °C for 24 h
(Table 1). To our delight, the yield of 4-
methoxybenzaldehyde and the conversion of substrate
1aE were 50% and 71%, respectively (entry 1). However,
using CuCl (10% mol) without NHC-ligand as catalyst, the
yield of 4-methoxy-benzaldehyde was only 21%, and the
conversion of substrate 1aE was 52% (entry 2). Using
SIMes alone or CuCl and SIMes (without pre-mixing) as
catalysts also gave lower conversions and yields.
54
100
1
70
16j
0
a
The yields were determined by 1H NMR using 1,3,5-
b
trimethoxybenzene as the internal standard; Isolated yield;
c
The amount of quinoline is 1.5 equiv with respect to sub-
d
e
strate. The reaction temperature is rt; The reaction tem-
perature is 80 C; The reaction temperature is 120 C; The
yield of 4-methoxybenzoic acid; The reaction time is 10 h;
The reaction time is 20 h; The reaction was performed un-
der argon.
o
f
o
g
h
i
j
Encouraged by these results, we then examined other
NHCs (including IPr, OSIMes, DPSIMes and MOPI), but
all resulted in decreased yields (entries 5-8). By increasing
the amount of (SIMes)CuCl to 20%, the substrate conver-
sion and the yield were increased to 100% and 90%, re-
spectively (entry 9). The yield of 4-methoxybenzaldehyde
was also shown to decrease along with lower amount of
quinoline (entry 10). The replacement of either quinoline
or dioxane with other bases and solvents also led to lower
conversion and yield. Less polar solvents seem preferable
to this reaction (Table S1 in the Supporting Information),
possibly related to the solubility of β-1 model compound.
With the optimized conditions in hand, we examined our
method for the cleavage of other β-1 lignin model com-
pounds. The results in Table 2 indicate that for all β-1
model compounds, substrate conversions were always
>99% under the optimized reaction conditions. The yields
of the cleavage aldehyde products are good to excellent
for a broad range of substrates. Compared to 1aE
(erythro), its threo-diastereomer 1bT (entry 2) gave 4-
methoxybenzaldehyde in 70% yield. The yields of alde-
hydes from other erythro-substrates were also higher
than their threo- diastereomers, which is consistent with
o
o
Lowering the reaction temperature from 100 C to 80 C
or room temperature led to decreased reaction efficiency
(entries 11 and 12). Raising the reaction temperature from
100 C to 120 C gave rise to some 4-methoxybenzoic acid
(entry 13). Moreover, shortening the reaction time also
reduced the yield considerably (entries 14 and 15). The
cleavage was not observed when the reaction was con-
12
previous study. In addition, erythro- substrates 1c, 1e,
1g and 1j, bearing one or two MeO- groups at other posi-
tions of benzene ring A, gave lower yield of the cleavage
products than 1aE.
o
o
ACS Paragon Plus Environment

Page 3 of 6
ACS Catalysis
Table 2 Copper-NHC catalyzed aerobic cleavage of β-1
lignin model compounds
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 2. The cleavage of β-O-4 lignin model com-
pound 1kE using (SIMes)CuCl catalyst system
To further demonstrate the practicality of the
(SIMes)CuCl catalyst system, 1 mmol reaction of com-
pound 1aE was carried using pure O2 to give 217.5 mg of
compound 2a in 80% isolated yield (Scheme 1). In addi-
tion, we also performed the cleavage of β-O-4 model
compound 3 using (SIMes)CuCl catalyst system. The re-
sult showed that the ketone 4 is main product, and the
rate of cleavage of compound 3 is only 12% (Scheme 2).
Furthermore, under the same conditions, using 1:1 mix-
ture of 1aE (scheme 1) and 3 (Scheme 2) only resulted
cleavage of the former together with a small amount of
ketone 4.
Figure 3 Proposed mechanism of copper-NHC catalyzed aer-
obic cleavage of β-1 lignin model compounds.
a
The yields were determined by 1H-NMR using 1,3,5-
trimethoxybenzene as the internal standard.
For the (SIMes)CuCl catalyst system, these reactions af-
forded aldehydes as major products resulted from Cα−Cβ
bond cleavage (shown in Figure 3), but no ketone product
1
(see the crude H NMR (Figure S1) in Supplementary In-
formation) was observed. Furthermore, the results of en-
tries 3-10 showed that, for asymmetric substrates, the
yields of aldehyde 2a were always lower than their cleav-
age counterparts (2b-2e). Similar data was also observed
by Baker and Hanson16 using Cu(I) OTf /TEMPO as cata-
lyst, suggesting that the reaction may undergo similar
initial oxidation of the primary alcohol in compounds 1,
followed by a retro-aldol reaction to break the Cα−Cβ
bond and generate the first product 2a-e, which come
Scheme 1. The cleavage of compound 1aE (1 mmol)
using (SIMes)CuCl catalyst system
ACS Paragon Plus Environment

ACS Catalysis
Page 4 of 6
perb conversions. For the (SIMes)CuCl catalyst system, the
Cα−Cβ bond cleavage reaction may proceed by a retro-aldol
reaction after the selective oxidation of the primary alcohol.
from A-ring. In our study, it is also possible that the quin-
oline was oxidized into quinoline-N-oxide and served as
TEMPO-like additive. The resulting fragment 7 was then
oxidized to 8 (The NMR evidence for 8 presence was at-
tached in Figure S2 in supplementary information). Then
8 was further cleaved to give 2a, which come from B-ring.
Aldehyde 2a can be further oxidized into 9, which was
detected in trace amount in our experiment. The pro-
posed mechanism was supported by experiments depicted
in scheme 3. The cleavage of 1,2-diphenylethan-1-ol (com-
pound 10) did not occur under our catalyst conditions
(Scheme 3a). Under the same conditions, ketone 11
(scheme 3b) gave 52% 1-(3,4-dimethoxyphenyl)-2-(4-
methoxyphenyl)-ethane-1,2-dione (compound 13) and 2%
1-(3,4-dimethoxyphenyl)-2-(4-methoxyphenyl)-ethanone
(compound 12), which possibly generated from oxidation
of alcohol into carboxylic acid22 followed by decarboxyla-
tive process. Only 14% of cleavage products was obtained
with 1:1 ratio of 3,4-dimethoxybenzoic acid and 4-
1
2
3
4
5
6
7
8
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org.
Optimization of reaction conditions; Synthesis of dia-
stereomeric β-1 lignin model compounds; General pro-
cedures for aerobic cleavage of β-1 lignin models; The
crude NMR of copper-SIMes catalyzed aerobic cleavage
of 1aE and 2-(4-methoxyphenyl)acetic acid and ketone
11; Spectra of β-1 lignin model compounds and prod-
ucts. (PDF)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
AUTHOR INFORMATION
Corresponding Author
*Prof. Dr. C.-J. Li
Department of Chemistry and FQRNT Center for Green
Chemistry and Catalysis McGill University, Montreal, Que-
bec, H3A 0B8 (Canada)
E-mail: cj.li@mcgill.ca
Homepage: http://cjli.mcgill.ca/cjpage.htm
methoxybenzaldehyde,
dimethoxybenzaldehyde or 4-methoxybenzoic acid were
not detected in the reaction mixture [see the crude H
NMR (Figure S4) and crude GC-MS (Figure S5) in Sup-
plementary Information]. Finally, the oxidative cleavage
of 4-methoxybenzeneacetic acid (compound 8) was also
observed with our reaction conditions (scheme 3c).
a)
while
no
3,4-
1
Author Contributions
‡These authors contributed equally.
Notes
OH
(SIMes)CuCl (20 mol %)
O2 (1 atm)
The authors declare no competing financial interest.
No product
quilinone (3 equiv), dioxane
100oC, 24h
10
EXPERIMENTAL
0% conversion
b)
General procedures for aerobic cleavage of β-1 lignin
models. A reaction vessel, charged with (SIMes)CuCl
catalyst (8 mg, 0.02 mmol, 20 mol %) and a β-1 lignin
model compound (0.1 mmol), was gently flushed with
oxygen of ordinary purity using a balloon. After this, dis-
tilled dioxane (0.6 mL) and quinoine (36 μL, 3 equiv) were
added to the vessel. The reaction mixture was then
OMe
O
O
O
(SIMes)CuCl (20 mol %)
O2 (1 atm)
+
OH
H
MeO
MeO
MeO
OH
quilinone (3 equiv), dioxane
100oC, 24h
OMe
14%
OMe
14 %
11
75% conversion
O
O
H
OH
+
+
o
o
warmed up to 100 C, and kept at 100 C for 24h. After
this, water (5 mL) was added, and the pH of the aqueous
phase was then adjusted to 2 with 0.1 M HCl. Then, the
mixture was extracted with ethyl ether 3 times with a to-
tal ether volume of 15 mL and the combined ether phase
was dried over anhydrous sodium sulfate, and evaporated
in vacuo to obtain a residue, which was chromatographed
using a mixture of hexane and ethyl acetate (v/v = 20:1) to
give aldehydes.
MeO
MeO
OMe
N.D.
N.D.
OMe
O
+
MeO
12, 2%
OMe
OMe
OMe
O
+
O
MeO
13, 52%
c)
ACKNOWLEDGMENT
COOH
(SIMes)CuCl (20 mol %)
O2 (1 atm)
We are grateful to the Canada Research Chair Foundation (to
C.-J. L.), the Canadian Foundation for Innovation, FRQNT
Centre in Green Chemistry and Catalysis, and the Natural
Science and Engineering Research Council of Canada for
support of our research. Dr. Z. Z. Zhou is grateful for the
support from China Scholarship Council (CSC) and Scholar-
ship Council of Southern Medical University.
COOH
CHO
+
quilinone (3 equiv), dioxane
100oC, 24h
MeO
MeO
MeO
8
9
2a
isolated yield = 55%
10%
Scheme 3. Mechanism investigation
In summary, we developed an efficient (SIMes)CuCl catalyst
system for the cleavage of β-1 lignin linkages to give alde-
hydes with good to excellent yields, high selectivity, and su-
ABBREVIATIONS
ACS Paragon Plus Environment

Page 5 of 6
TEMPO,
ACS Catalysis
2,2,6,6-tetramethylpiperidine-1-oxyl;
NHC,
1
2
N heterocyclic carbene; rt, room temperature; N. D., not
detected.
3
4
REFERENCES
5
6
7
8
((1) Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.;
Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.;
Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski,
T. J.; Tuskan, G. A.; Wyman, C. E., Science 2014, 344, 1246843.
((2) Xu, C.; Arancon, R. A. D.; Labidi, J.; Luque, R., Chem. Soc.
Rev. 2014, 43, 7485-7500.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
((3) Singh, A. K.; Jang, S.; Kim, J. Y.; Sharma, S.; Basavaraju, K.
C.; Kim, M.-G.; Kim, K.-R.; Lee, J. S.; Lee, H. H.; Kim, D.-P., ACS
Catal. 2015, 5, 6964-6972.
((4) Verma, S.; Nasir Baig, R. B.; Nadagouda, M. N.; Varma, R.
S., Green Chem. 2016, 18, 1327-1331.
((5) Azadi, P.; Inderwildi, O. R.; Farnood, R.; King, D. A.,
Renewable Sustainable Energy Rev. 2013, 21, 506-523.
((6) Crestini, C.; Crucianelli, M.; Orlandi, M.; Saladino, R.,
Catal. Today 2010, 156, 8-22.
((7) Saito, K.; Kaiho, A.; Sakai, R.; Nishimura, H.; Okada, H.;
Watanabe, T., J. Agric. Food Chem. 2016, 64, 9152-9160.
((8) Norberg, I.; Nordstroem, Y.; Drougge, R.; Gellerstedt, G.;
Sjoeholm, E., J. Appl. Polym. Sci. 2013, 128, 3824-3830.
((9) Rahimi, A.; Ulbrich, A.; Coon, J. J.; Stahl, S. S., Nature
2014, 515, 249-52.
((10) Deuss, P. J.; Barta, K., Coord. Chem. Rev. 2016, 306, 510-
532.
((11) Karkas, M. D.; Matsuura, B. S.; Monos, T. M.; Magallanes,
G.; Stephenson, C. R. J., Org. Biomol. Chem. 2016, 14, 1853-1914.
((12) Hanson, S. K.; Baker, R. T., Acc. Chem. Res. 2015, 48,
2037-2048.
((13) Mottweiler, J.; Puche, M.; Raeuber, C.; Schmidt, T.;
Concepcion, P.; Corma, A.; Bolm, C., ChemSusChem 2015, 8,
2106-2113.
((14) Jiang, Y.-Y.; Yan, L.; Yu, H.-Z.; Zhang, Q.; Fu, Y., ACS
Catal. 2016, 6, 4399-4410.
((15) Lohr, T. L.; Li, Z.; Marks, T. J., Acc. Chem. Res. 2016, 49,
824-834.
((16) Sedai, B.; Diaz-Urrutia, C.; Baker, R. T.; Wu, R.; Silks, L.
A. P.; Hanson, S. K., ACS Catal. 2013, 3, 3111-3122.
((17) Lim, S. H.; Lee, W. S.; Kim, Y.-I.; Sohn, Y.; Cho, D. W.;
Kim, C.; Kim, E.; Latham, J. A.; Dunaway-Mariano, D.; Mariano,
P. S., Tetrahedron 2015, 71, 4236-4247.
((18) Habe, T.; Shimada, M.; Okamoto, T.; Panijpan, B.;
Higuchi, T., J. Chem. Soc., Chem. Commun. 1985, 1323-1324.
((19) Shimada, M.; Habe, T.; Umezawa, T.; Higuchi, T.;
Okamoto, T., Biochem. Biophys. Res. Commun. 1984, 122, 1247-
1252.
((20) DiCosimo, R.; Szabo, H. C., J. Org. Chem. 1988, 53, 1673-
1679.
((21) Wang, M.; Li, L. H.; Lu, J. M.; Li, H. J.; Zhang, X. C.; Liu,
H. F.; Luo, N. C.; Wang, F., Green Chem. 2017, 19, 702-706.
((22)Liu, M.; Li, C. J., Angew Chem. Int. Ed. 2016, 55, 10806-
10810.
Authors are required to submit a graphic entry for the Table of Contents (TOC) that, in conjunction with the man-
uscript title, should give the reader a representative idea of one of the following: A key structure, reaction, equation,
concept, or theorem, etc., that is discussed in the manuscript. Consult the journal’s Instructions for Authors for
TOC graphic specifications.
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 6
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
6
ACS Paragon Plus Environment