Soluble asphaltene oxide: A homogeneous carbocatalyst that promotes synthetic transformations

Article abstract of DOI:10.1039/d0ra01762k

Carbocatalysts, materials which are predominantly composed of carbon and catalyze the synthesis of organic or inorganic compounds, are promising alternatives to metal-based analogues. Even though current carbocatalysts have been successfully employed in a broad range of synthetic transformations, they suffer from a number of drawbacks in part due to their heterogeneous nature. For example, the insolubility of prototypical carbocatalysts, such as graphene oxide (GO), may restrict access to catalytically-active sites in a manner that limits performance and/or challenges optimization. Herein we describe the preparation and utilization of soluble asphaltene oxide (sAO), which is a novel material that is composed of oxidized polycyclic aromatic hydrocarbons and is soluble in a wide range of organic solvents as well as in aqueous media. sAO promotes an array of synthetically useful transformations, including esterifications, cyclizations, multicomponent reactions, and cationic polymerizations. In many cases, sAO was found to exhibit higher catalytic activities than its heterogeneous analogues and was repeatedly and conveniently recycled, features that were attributed to its ability to form homogeneous phases.

Full text of DOI:10.1039/d0ra01762k

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Soluble asphaltene oxide: a homogeneous
carbocatalyst that promotes synthetic
Cite this: RSC Adv., 2020, 10, 15598
transformations†
ab
abc
*
Hyosic Jung
and Christopher W. Bielawski
Carbocatalysts, materials which are predominantly composed of carbon and catalyze the synthesis of
organic or inorganic compounds, are promising alternatives to metal-based analogues. Even though
current carbocatalysts have been successfully employed in a broad range of synthetic transformations,
they suffer from a number of drawbacks in part due to their heterogeneous nature. For example, the
insolubility of prototypical carbocatalysts, such as graphene oxide (GO), may restrict access to
catalytically-active sites in a manner that limits performance and/or challenges optimization. Herein we
describe the preparation and utilization of soluble asphaltene oxide (sAO), which is a novel material that
is composed of oxidized polycyclic aromatic hydrocarbons and is soluble in a wide range of organic
solvents as well as in aqueous media. sAO promotes an array of synthetically useful transformations,
including esterifications, cyclizations, multicomponent reactions, and cationic polymerizations. In many
cases, sAO was found to exhibit higher catalytic activities than its heterogeneous analogues and was
repeatedly and conveniently recycled, features that were attributed to its ability to form homogeneous
phases.
Received 24th February 2020
Accepted 8th April 2020
DOI: 10.1039/d0ra01762k
rsc.li/rsc-advances
size, GO can undergo rapid decomposition when exposed to
high temperatures, light or microwaves.14 Although heteroge-
Introduction
neity can facilitate purications, the catalytically-active sites
present on GO may be inaccessible due to its heterogeneous
nature and, as such, performance may be intrinsically limited.
While enhanced solvent dispersibility has been reported to
increase the performance and material quality of GO,15,16 a fully
soluble derivative has hitherto remained unknown.
Due to increasing costs, high toxicity, and limited resources,
alternatives to transition metal-based catalysts used to promote
the preparation of organic and inorganic compounds have been
widely studied. Among these, carbocatalysts, which are mate-
rials that are composed primarily of carbon, have attracted
attention for use in a variety of synthetic applications.1–3 The
prototypical example of a carbocatalyst is graphene oxide (GO),
which was rst introduced as a catalyst for the selective oxida-
tion of benzyl alcohol in 2010.4 Since the initial disclosure, GO
has been reported to catalyze a variety of synthetic reactions,
including the oxidations of thiols and suldes,5 aza-Michael
additions,6 aldol coupling reactions,7 C–H activations/
coupling reactions,8–10 and others.11 GO has also been shown
to facilitate the polymerization of various monomers through
dehydration12 or cationic mechanisms.13 Despite these
advances, GO suffers from a number of fundamental and
practical drawbacks. For example, due to the its large particle
We recently reported a new carbocatalyst based on asphal-
tene.17 While the structure of asphaltene is reminiscent of
a graphene sheet, it is relatively small.18 Subjecting asphaltene
to a modied Hummers' method afforded asphaltene oxide
(AO), which was found to exhibit catalytic activities similar to
that of GO but under a wider range of conditions. Indeed, AO
facilitated condensations, C–C cross couplings, etherications
and heterocyclizations, and the use of microwaves were found
to promote some of these transformations. Building on these
results, we posited that asphaltene may be converted to
homogeneous carbocatalyst by enhancing the degree of oxida-
tion. Our supposition was inspired by a previous report,19 which
showed that exposing asphaltene to 70% nitric acid at elevated
temperatures facilitates its removal from crude oil due to an
enhanced solubility. Although this procedure was used in oil
renery applications, we hypothesized that the soluble product
may function as a homogeneous carbocatalyst. Moreover, the
enhanced solubility was expected to promote catalytic function
and result in higher activities when compared to heterogeneous
counterparts. Herein we report the synthesis and
aCenter for Multidimensional Carbon Materials (CMCM), Institute for Basic Science
(IBS), Ulsan 44919, Republic of Korea. E-mail: bielawski@unist.ac.kr
bDepartment of Chemistry, Ulsan National Institute of Science and Technology
(UNIST), Ulsan 44919, Republic of Korea
cDepartment of Energy Engineering, Ulsan National Institute of Science and
Technology (UNIST), Ulsan 44919, Republic of Korea
† Electronic supplementary information (ESI) available: Additional synthetic
details and characterization data. See DOI: 10.1039/d0ra01762k
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characterization of soluble asphaltene oxide (sAO) and describe vacuum (100 torr) at 100 ꢂC. Aer removal of the residual water
the utility of this material in facilitating synthetic trans- and nitric acid via distillation, the remaining solid products
formations, including relatively complex syntheses such as were collected and dried under vacuum at 50 ꢂC for 2 days. The
multicomponent reactions and cationic polymerizations.
nal product (2.1 g) was obtained as a brown powder.
Experimental section
General methodology
Results and discussion
Asphaltene was rst washed with n-heptane to remove the
residual oil contaminants and then subjected to a modied
version of a previously reported oxidation method.19 Briey,
All chemicals were purchased from Sigma Aldrich, TCI or
Thermo Fisher Scientic Chemicals, and were used as received.
Asphaltene (blown asphalt 5–10) was kindly supplied by Korea
Petroleum. Unless otherwise noted, all experiments were per-
formed under ambient conditions. NMR spectra were recorded
on a Bruker Ascend 400 MHz spectrometer. Chemical shis are
reported in parts per million (d). NMR data were collected in
CDCl3, DMSO-d6 or D2O and the residual solvent was referenced
to 7.26, 2.50 and 4.79 ppm, respectively. The following abbre-
viations apply: s, singlet; d, doublet; t, triplet; q, quartet; m,
multiplet; br, broad. FT-IR spectra were recorded using KBr
pellets on a PerkinElmer Frontier MIR spectrometer. Elemental
analyses were performed using a Thermo Scientic Flash 2000
Organic Elemental Analyzer that was calibrated with 2,5-bis(5-
tert-butyl-benzoxazol-2-yl)thiophene (BBOT). Matrix-assisted
laser desorption/ionization (MALDI) spectra were recorded on
a Bruker Ultraex III with 2,5-dihydroxybenzoic acid (DHB) as
a matrix. THF solutions of sAO and DHB were combined and
then evaporated prior to mass analysis. Polystyrene equivalent
molecular weights and polydispersity index (Đ) values were
measured by gel permeation chromatography (GPC) using
a Malvern VISCOTEK GPCmax system. All samples were
ꢂ
asphaltene was reuxed in 60% nitric acid at 120 C for 24 h,
and then diluted with deionized water. Aer ltering the diluted
solution to remove the insoluble particulates, the ltrate was
dried by heating to 100 ꢂC under low pressure and then placed
in a vacuum oven that was set to 50 ꢂC for two days. The product
obtained from this procedure was a brown powder and found to
be soluble in methanol (up to 200 mg mLꢀ1), ethanol (140 mg
mLꢀ1), THF (200 mg mLꢀ1), DMSO (80 mg mLꢀ1), DMF (80 mg
mLꢀ1), acetone (200 mg mLꢀ1) and water (100 mg mLꢀ1) (see
Fig. S1†). Compared to asphaltene oxide (AO), which was
synthesized using a modied Hummers' method (conditions:
KMnO4, H2SO4, and H2O2), sAO contained a relatively high
oxygen content (13.1 wt%, C/O ratio ꢃ 6.67 vs. 42.0 wt%, C/O
ratio ꢃ 1.23, respectively), as determined by elemental anal-
ysis (see Fig. 1a). The FT-IR spectrum recorded for the material
featured a strong, broad hydroxyl signal (nO–H ¼ 3500–
2800 cmꢀ1), a strong carbonyl signal (nC]O ¼ 1720 cmꢀ1), and
analyzed using THF as the eluent at a ow rate of 0.8 mL minꢀ1
.
Atomic force microscopy measurements were performed using
a Multimode 8 Nanoscope® V (Bruker). Samples were dissolved
in THF (1.0 mg mLꢀ1) and spin coated onto silica at 3000 rpm
for 1 min. Mass spectrometry (MS) data were recorded on
a Thermo LCQ Fleet Quadrupole Ion Trap Mass Spectrometer in
Atmospheric Pressure Chemical Ionization (APCI) and Electro-
spray Ionization (ESI) mode (electrospray voltage 3.5 kV).
Microwave reactions were conducted using an Anton Paar
GmbH-Monowave 300 microwave reactor.
Separation of asphaltene
Asphaltene (10.0 g) was mixed with n-heptane (2.0 L) and then
sonicated. The mixture was ltered through a 0.22 mm nylon
membrane and washed with n-heptane (0.5 L ꢁ 3). The ltered
asphaltene particles were collected, dried at 80 ꢂC under
vacuum for 1 day, and then directly used.
Preparation of soluble asphaltene oxide (sAO)
A 500 mL ask was charged with asphaltene (4.0 g), nitriꢂc acid
(0.1 L), and a stir bar. The mixture was reuxed at 120 C for
24 h. CAUTION: a brown gas, presumably NO2, was generated
during the reaction. Aerward, the mixture was slowly poured
into deionized water (0.5 L). The resulting solution was ltered
through a 0.22 mm nylon membrane to remove the unreacted
Fig. 1 Summary of characterization data. (a) Elemental analysis data
recorded for asphaltene, AO and sAO. (b) FT-IR spectra recorded for
asphaltene particles. The ltrate was collected and dried under asphaltene (blue line), asphaltene oxide (red line) and sAO (black line).
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epoxide signals (nC–O ¼ 1240 cmꢀ1 and 1050 cmꢀ1), and signicantly higher than those measured for AO and GO (see
resembled a spectrum recorded for AO (see Fig. 1b). The pH of Table S2†), which may rationalize the relatively high solubility
a solution of sAO in water (1.0 mg mLꢀ1) was measured to be and high catalytic activity (vide infra) exhibited by sAO.
2.7; for comparison, the pH of an AO suspension was measured
to be 3.5 under similar conditions.
The relatively high solubility of sAO facilitated additional
characterization of the material. A H NMR spectrum recorded
1
As summarized in Scheme 1, a series of chemical tests were for the material exhibited broad signals at d 8.62–7.41 ppm and
deployed to elucidate the structure of sAO and to quantify the 2.31–1.00 ppm in D2O which were assigned to aromatic and
oxygen-containing functional groups decorated on the material. alkyl chain protons, respectively (see Fig. S3†). Similarly, 13C
The reagents utilized were designed to preferentially react with NMR signals attributed to carbonyl groups (d 179.0–176.9 ppm),
carboxylic acid, hydroxy or epoxide groups and contained aromatic groups (d 134.4–129.0 ppm) and alkyl chains (d 40.4–
nitriles to facilitate characterization via IR spectroscopy and 16.1 ppm) were also recorded (see Fig. S4†). Matrix assisted laser
elemental analysis (see Fig. S2 and Table S1,† respectively). desorption/ionization time of ight (MALDI-TOF) mass spec-
Treatment of sAO with 3-cyanobenzyl alcohol under Steglich trometry revealed that the molecular weight of sAO was in the
esterication conditions20 afforded the corresponding ester, as
determined in part by a bathochromic shi in the salient nC]O
signal (from 1720 cmꢀ1 to 1630 cmꢀ1) and the appearance of
Table 1 A summary of esterifications that were promoted with sAOa
a new signal which was assigned to a nitrile group (nC^N
¼
2230 cmꢀ1). Introducing sAO to 3-cyanophenyl isocyanate
resulted in a reduction in the intensity of the signal assigned to
the hydroxyl group (nO–H ¼ 3320 cmꢀ1) and was accompanied by
the formation of strong signals that were assigned to the nitrile
(nC^N ¼ 2230 cmꢀ1), amido (nN–H ¼ 3090 cmꢀ1) and carbamyl
(nC]O ¼ 1590 cmꢀ1) groups of the expected product. Finally,
using methodology that was previously reported to form C–C
bonds via the ring-opening of the epoxide groups on GO,21 sAO
was treated with malononitrile under basic conditions. The
disappearance of signals attributed to the epoxide groups (nC–O
¼ 1230 cmꢀ1) in the starting material was observed along with
the formation a new nC^N at 2180 cmꢀ1. Elemental analysis of
the products of the aforementioned reactions enabled quanti-
cation of the respective nitrogen contents and thus the cor-
responding functional groups: carboxylic acid (5.1 ꢁ 10ꢀ3 mol
gꢀ1), hydroxy (5.6 ꢁ 10ꢀ3 mol gꢀ1) and epoxide (7.5 ꢁ 10ꢀ3 mol
gꢀ1). The concentrations of these groups were measured to be
Entry
1
Carboxylic acid
Alcohol
MeOH
Temp. (ꢂC)
50
Conversionb (%)
>99
2
MeOH
50
>99
3
4
MeOH
MeOH
50
50
>99
>99
5
6
7
8
MeOH
MeOH
EtOH
50
50
70
70
>99
>99
91
n-BuOH
96
Scheme 1 Chemical tests that were used to quantify the functional
groups displayed on sAO. The starting material shown in the centre
represents a truncated form sAO and features pendant carboxylic acid,
hydroxy, and epoxide groups. Using the reagents shown, the
carboxylic acid groups were converted to esters, the hydroxyl groups
were converted to carbamates, and epoxide groups were ring-
opened. In all cases, the corresponding products feature nitrile groups
(CN) that could be quantified using IR spectroscopy as well as
elemental analysis.
9
IPA
85
85
a
Unless otherwise noted, all reactions were performed using 1.0 mmol
of carboxylic acid, 1.0 mL of alcohol, and 50 mg of catalyst. b Conversion
aer 24 h as calculated by gas chromatography against a standard
(anisole).
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transformations,22 these liquid reagents are toxic and volatile. For
comparison, sAO is a solid powder that is relatively straightfor-
ward to handle. As summarized in Table 1, sAO successfully
promoted the condensation of a wide range of carboxylic acids
and alcohols. Esterications involving carboxylic acids and
methyl alcohol quantitatively afforded the corresponding methyl
esters while other alcohols were converted to their respective
esters in yields of up to 96%. An intramolecular esterication was
also explored. As shown in Scheme 2, 6-hydroxycaproic acid (1.0
mmol) was successfully converted to the expected product (3-
caprolactone) in quantitative yield using sAO (conditions: [6-
hydroxycaproic acid]0 ¼ 1.0 M; 50 mg sAO).
Scheme 2 Intramolecular condensation of 6-hydroxycaproic acid as
catalyzed by sAO.
range of 500–600 Da (see Fig. S5†). Presumably due to its rela-
tively high oxygen content, the material was found to aggregate
in the solid-state and formed particles with an average size of ca.
10 nm, as determined by atomic force microscopy (AFM) (see
Fig. S6†).
Since sAO was found to be acidic, initial efforts toward using
sAO as a carbocatalyst began with esterication chemistry.
Although inorganic acids such as hydrochloric acid, sulfuric acid
and phosphoric acid are commonly used to catalyze such
We previously showed that asphaltene-based carbocatalysts
feature an advantage over their graphene analogues in that they
can be utilized in microwave-assisted chemistry.17 Building on
these results, we focused on employing sAO in microwave-
assisted Fischer indole syntheses. Indole derivatives have
Table 2 A summary of microwave assisted Fischer indole synthesesa
Entry
1
Ketone
Product
Catalyst (mg)
100
Time (min)
30
Yieldb (%)
30
2
3
4
100
100
200
60
90
60
60
62
61
5
6
100
100
60
60
26
37
7
100
60
45
a
Unless otherwise noted, all reactions were performed using 1.2 mmol of phenylhydrazine, 1.0 mmol of cyclohexanone, 2.0 mL of water as solvent
and the indicated quantity of catalyst loading at 150 ꢂC in a microwave reactor. b Isolated yield aer purication using column chromatography.
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Table 3 A summary of microwave-assisted Biginelli reactionsa
Entry
Cat. loading (wt%)
Temp. (ꢂC)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
10
25
50
50
50
50
50
75
75
75
5
5
5
5
5
13
21
53
7
75
74
75
50
100
100
100
15
30
a
Unless otherwise noted, all reactions were performed with benzaldehyde (1.0 mmol), ethyl acetoacetate (1.0 mmol) and urea (1.5 mmol) using the
indicated catalyst loading. b Isolated yield aer purication using column chromatography.
been employed in numerous biological and pharmaceutical (50–100 ꢂC) and reaction periods (5–30 min). While DHPM
applications and the demand for these compounds continues to products were isolated from each experiment, optimal results
grow.23 As shown in Table 2, sAO successfully facilitated the were obtained when the reaction mixture was irradiated with
condensation of phenylhydrazine and cyclohexanone to afford microwaves for 5 min at 100 ꢂC (see entry 5). For comparison, an
the corresponding indole product in aqueous media. Similarly, analogous reaction that was performed with AO afforded a 33%
other ketones were converted to their expected indoles when yield of product under otherwise identical conditions. Other
irradiated with microwaves in the presence of sAO.
benzaldehydes, including those containing electron-rich or
A hallmark of many carbocatalysts is that they can be readily electron-poor functional groups, were also utilized and afforded
recycled. Since sAO is soluble in water, separation of the catalyst the expected products in good yields (see Table S4†).
from various product mixtures was conveniently accomplished
Finally, we explored the ability of sAO to catalyze cationic
via extraction. To quantify the recyclability of sAO, an aqueous polymerizations. Vinyl carbazole and vinyl ethers are ideal
solution of the catalyst was collected aer a Fischer indole monomers for such chemistry since the nitrogen or oxygen
synthesis and re-used without further purication. The starting heteroatoms effectively stabilize propagating carbocations. As
reagents (phenylhydrazine and cyclohexanone) were added to summarized in Table 4, the aforementioned monomers were
a solution of the catalyst in water ([sAO]0 ¼ 50 mg mLꢀ1) and found to undergo polymerization when subjected to AO (1 wt%)
then irradiated with microwaves for 1 h at 150 ꢂC. Aer in THF at ꢀ20 ꢂC, and the corresponding polymers were
extraction of the corresponding indole products with dichloro-
methane, the aqueous phase containing the catalyst was
collected, introduced to a new bolus of starting reagents and
Table 4 A summary of cationic polymerization that were performed
with sAOa
irradiated again. Using this methodology, the catalyst solution
was successfully and repeatedly used over multiple cycles to
facilitate condensations with minimal changes catalytic activity.
For example, an indole product was obtained in a 52% yield
using a catalyst solution that was subjected to ve (5) successive
cycles; for comparison, a yield of 59% was obtained aer the
rst cycle (see Table S3†).
Considering that the catalytic activity displayed by sAO
appeared to be relatively broad, we hypothesized that the
material may also facilitate multicomponent reactions (MCRs).
The Biginelli reaction is a typical example of a MCR and has
been used to obtain dihydropyrimidinone (DHPM) and other
valuable heterocyclic products.24 Since DHPM derivatives are
known to be effective calcium channel blockers, they have
found widespread utility in therapeutic and pharmacological
applications.25 As summarized in Table 3, a series of MCRs were
c
Entry
R
Conc. (mol Lꢀ1
)
Yieldb (%) Mn (kDa) Đc
1
2
3
–O(C4H9)
–Cbz
–O(C6H11
3
1
3
99
99
52
9.8
11.1
4.5
1.3
5.1
2.0
)
a
The reactions were performed by treating a solution of the monomer
in THF with 1 wt% of sAO at ꢀ20 ꢂC for 24 h under an atmosphere of
N2. b Isolated yield aer collection of the precipitate that formed upon
c
pouring the reaction mixture into excess methanol. The number-
conducted with a benzaldehyde derivative, ethyl acetoacetate average molecular weight (Mn) and polydispersity index (Đ) values
were determined by GPC and are reported as their polystyrene
equivalents. Cbz ¼ carbazole.
and urea at various catalyst loadings (10–50 wt%), temperatures
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isolated via ltration aer pouring the corresponding reaction
mixtures into excess methanol to induce precipitation. Poly(n-
butyl vinyl ether) was obtained in quantitative yield, and
featured a number average molecular weight (Mn) of 9.8 kDa
and a polydispersity index (Đ) of 1.3 as determined by gel
permeation chromatography. Similarly, N-vinyl carbazole was
3 N. M. Julkapli and S. Bagheri, Int. J. Hydrogen Energy, 2015,
40, 948–979.
4 D. R. Dreyer, H. P. Jia and C. W. Bielawski, Angew. Chem., Int.
Ed., 2010, 49, 6813–6816.
5 D. R. Dreyer, H. P. Jia, A. D. Todd, J. Geng and
C. W. Bielawski, Org. Biomol. Chem., 2011, 9, 7292–7295.
6 S. Verma, H. P. Mungse, N. Kumar, S. Choudhary, S. L. Jain,
B. Sain and O. P. Khatri, Chem. Commun., 2011, 47, 12673–
12675.
7 H. P. Jia, D. R. Dreyer and C. W. Bielawski, Adv. Synth. Catal.,
2011, 353, 528–532.
8 Y. Gao, P. Tang, H. Zhou, W. Zhang, H. Yang, N. Yan, G. Hu,
D. Mei, J. Wang and D. Ma, Angew. Chem., Int. Ed., 2016, 55,
3124–3128.
quantitatively converted to its corresponding polymer (Mn
¼
11.1 kDa). Although the latter exhibited a Đ value of 5.1, broad
polydispersity indices are oen observed in cationic polymeri-
zations due to the high reactivity of the cation during the
propagation step.26 Reduced distributions may be achieved
through further optimization (see Table S5†). While GO has also
used to promote cationic polymerizations,13 sAO afforded
polymers in higher yields, with better control, and within
shorter periods of time.
9 K. Morioku, N. Morimoto, Y. Takeuchi and Y. Nishina, Sci.
Rep., 2016, 6, 25824.
10 H. Wu, C. Su, R. Tandiana, C. Liu, C. Qiu, Y. Bao, J. Wu,
Y. Xu, J. Lu, D. Fan and K. P. Loh, Angew. Chem., Int. Ed.,
2018, 57, 10848–10853.
11 F. Hu, M. Patel, F. Luo, C. Flach, R. Mendelsohn,
E. Garfunkel, H. He and M. Szostak, J. Am. Chem. Soc.,
2015, 137, 14473–14480.
12 D. R. Dreyer, K. A. Jarvis, P. J. Ferreira and C. W. Bielawski,
Macromolecules, 2011, 44, 7659–7667.
13 D. R. Dreyer and C. W. Bielawski, Adv. Funct. Mater., 2012, 22,
3247–3253.
14 Y. M. Shulga, S. A. Baskakov, E. I. Knerelman, G. I. Davidova,
E. R. Badamshina, N. Y. Shulga, E. A. Skryleva, A. L. Agapov,
D. N. Voylov, A. P. Sokolov and V. M. Martynenko, RSC Adv.,
2014, 4, 587–592.
15 C. H. Chen, S. Hu, J. F. Shih, C. Y. Yang, Y. W. Luo,
R. H. Jhang, C. M. Chiang and Y. J. Hung, Sci. Rep., 2017,
7, 3908.
Conclusions
In summary, a homogeneous carbocatalyst was prepared
through the oxidation of asphaltene. To the best of our knowl-
edge, this is the rst report of a soluble carbocatalyst that
facilitates a broad range of synthetic transformations. Indeed,
sAO effectively promoted esterications, inter- and intra-
molecular condensations, multicomponent reactions and
cationic polymerizations, and was successfully used in micro-
wave reactors. Since sAO is soluble in water, it was found to be
straightforward to separate and recycle via extraction. Moreover,
due to its ability to form homogeneous phases, sAO exhibited
higher catalytic activities than its heterogeneous counterparts
(e.g., GO or AO). In a broader perspective, asphaltene-based
catalysts can be expected to expand the realization of metal-
free catalysts for facilitating synthetic reactions under hetero-
geneous and, now, homogeneous conditions, and thus poten-
tially usher further improvements in catalytic activity as well as
selectively.
16 J. Park, Y. S. Kim, S. J. Sung, T. Kim and C. R. Park,
Nanoscale, 2017, 9, 1699–1708.
17 H. Jung and C. W. Bielawski, Commun. Chem., 2019, 2, 113.
18 H. Groenzin and O. C. Mullins, J. Phys. Chem. A, 1999, 103,
11237–11245.
Conflicts of interest
19 S. E. Moschopedis and J. G. Speight, Fuel, 1971, 50, 34–40.
20 B. Neises and W. Steglich, Angew. Chem., Int. Ed. Engl., 1978,
17, 522–524.
21 W. R. Collins, E. Schmois and T. M. Swager, Chem. Commun.,
2011, 47, 8790–8792.
22 Y. J. Liu, E. Lotero and J. G. Goodwin, J. Mol. Catal. A: Chem.,
2006, 245, 132–140.
23 G. Mohammadi Ziarani, R. Moradi, T. Ahmadi and
N. Lashgari, RSC Adv., 2018, 8, 12069–12103.
24 H. Nagarajaiah, A. Mukhopadhyay and J. N. Moorthy,
Tetrahedron Lett., 2016, 57, 5135–5149.
There are no conicts to declare.
Acknowledgements
The Institute for Basic Science (IBS-R019-D1) and the BK21 Plus
Program as funded by the Ministry of Education and the
National Research Foundation of Korea are acknowledged for
support. We are grateful to Ms. Kyungha Kim and Dr Jinwon Seo
for assistance with graphic design.
Notes and references
25 L. H. S. Matos, F. T. Masson, L. A. Simeoni and M. Homem-
de-Mello, Eur. J. Med. Chem., 2018, 143, 1779–1789.
26 C. H. Lin, J. S. Xiang and K. Matyjaszewski, Macromolecules,
1993, 26, 2785–2790.
1 C. Su and K. P. Loh, Acc. Chem. Res., 2013, 46, 2275–2285.
2 D. R. Dreyer and C. W. Bielawski, Chem. Sci., 2011, 2, 1233–
1240.
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