Stable and Surface-active Co Nanoparticles Formed from Cation (x) Promoted Au/x-Co3O4 (x=Cs) as Selective Catalyst for [2+2+1] Cyclization Reactions

Article abstract of DOI:10.1002/cctc.202001841

Surface-active and highly stable cobalt nanoparticles generated from alkali ion-promoted gold catalyst for catalyzed carbonylative [2+2+1] cyclization reaction, is described. The gold nanoparticle‘s (AuNPs) role was assumed to dissociate the CO and H

Full text of DOI:10.1002/cctc.202001841

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doi.org/10.1002/cctc.202001841
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Stable and Surface-active Co Nanoparticles Formed from
Cation (x) Promoted Au/x-Co3O4 (x=Cs) as Selective
Catalyst for [2+2+1] Cyclization Reactions
Charles O. Oseghale,[a] Batsile M. Mogudi,[a] Oluwatayo Racheal Onisuru,[a]
Christianah Aarinola Akinnawo,[a] Dele Peter Fapojuwo,[a] and Reinout Meijboom*[a]
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tions of the resulting materials.[2] Aside from the use of pure
Surface-active and highly stable cobalt nanoparticles generated
cobalt carbonyl as a homogeneous catalyst, heterobimetallic
cobalt supported on ruthenium,[8] rhodium,[9] and charcoal[10]
has been widely reported for Pauson-Khand reactions. Muller
and co-workers prepared a Raney cobalt, which acts as an
active catalyst for inter- and intramolecular Pauson-Khand
reaction.[11] However, the stabilization and preparation of these
cobalt carbonyl complexes require a pressure of >20 MPa and
higher temperatures; this process often requires sublimation
and recrystallization before use to obtain an excellent product
yield and reusability due to carbon monoxide dissociation
under an inert gas atmosphere during storage and below
from alkali ion-promoted gold catalyst for catalyzed carbon-
ylative [2+2+1] cyclization reaction, is described. The gold
nanoparticle‘s (AuNPs) role was assumed to dissociate the CO
and H2 into atomic species on the catalyst surface by spillover,
which in-situ reduces the robust mesoporous cobalt oxide to
metallic cobalt (Co3+!Co2+!Co), as the active catalytic species
that catalyzed the reaction; thereby providing up to 93% yield
of cyclopentenone adducts. Prior to this, catalyst pre-treatment
°
with H2 gas (130 C, 3 h, 20 atm) was performed to reduce the
catalyst. It appeared that the low reducibility temperature and
increased surface basicity ascribed to the presence of alkali ion-
promoters in the catalyst revealed a strong correlation with the
catalyst activity, for the intra- and intermolecular reactions
under milder reaction conditions. Thus, a sustainable, highly
reusable, and environmentally friendly green catalyst for the
carbonylation reaction, such as Pauson-Khand, was developed.
[12]
°
0 C.
Besides, the functional efficiency in the synthesis of
these Co2(CO)8 complexes is still limited by problems of
separation of the catalyst from the reaction mixture, toxicity,
and volatility.[13,14] Homogeneous cobalt carbonyl complexes are
quite toxic and can precipitate during the catalytic reactions.
The best possible way to overcome this, is to heterogenize the
catalyst by depositing it on metal oxide supports, such as
SiO2,[15] activated carbon,[16] and resins.[17,18] Incorporating these
active catalytic sites on the metal oxides through chemical and
physical exchanges can create a stable heterogeneous catalyst,
leading to an economical and environmentally friendly “green”
process chemistry.
Cycloaddition [2+2+1] of alkenes, alkynes in the presence of
dicobalt octacarbonyl Co2(CO)8 complexes and carbon mon-
oxide to cyclopentenones (known as Pauson-Khand reactions
(PKR)) have been actively recognized and investigated for
decades as an essential synthetic protocol for the preparation
of cyclopentenone derivatives. The products of these reactions
have found usage in several biologically active pharmaceuticals
and vital natural products.[1,2] Dicobalt octacarbonyl has been
broadly used in amidocarbonylation,[3] carbonylation,[4] Pauson-
Khand,[5] and hydroformylation[6] reactions. These cobalt
carbonyl compounds are versatile reagents among metal
carbonyls for organic and inorganic syntheses. Cobalt carbonyl
complexes have been reported as homogeneous catalysts for
Pauson-Khand reactions with poor catalytic performance (low
product yield).[7] Similarly, cobalt metal deposited by thermal
decomposition of Co2(CO)8 complexes on various supports
demonstrated great catalytic potential for Pauson-Khand reac-
The utilization of heterogeneous catalytic systems for
carbonylation reactions such as Pauson-Khand has been a
subject of intense interest in academia and chemical industries.
Recently, metal nanoparticles have been quite efficient, useful
and give excellent product yields for carbonylation reactions.
These metal nanoparticles are gaining broad research interest
in material science and chemistry because they display unusual
chemical and physical properties, which are distinct from those
of isolated molecule and bulk phase.[19,20] Notably, supported
gold catalysts are known for their superior catalytic perform-
ance in several homogeneous and heterogeneous reactions,
such as oxidation,[21] CÀ C coupling,[22] cycloaddition,[23] and
reduction.[24] To afford a bimetallic structure, it essential to
support gold nanoparticles on selected metal oxides, even
though the support‘s choice affects the catalyst‘s performance
and durability. Gold nanoparticles supported on metal oxides
have been reported for reactions such as alkoxy-carbonylation
of epoxide and hydroformylation.[25,26] In these reactions, the
metal nanoparticles assisted in the spillover of H2 and CO on
the catalyst surface, thereby forming a cobalt carbonyl complex
as the active catalytic site that catalyzed the reaction. For the
intra- and intermolecular reaction reported by Hamasaki and
[a] C. O. Oseghale, B. M. Mogudi, O. R. Onisuru, C. A. Akinnawo,
D. P. Fapojuwo, Prof. Dr. R. Meijboom
Department of Chemical Sciences, Research Center for Synthesis and
Catalysis
University of Johannesburg
Auckland Park, 2006, Johannesburg (South Africa)
E-mail: meijboom@uj.ac.za
Supporting information for this article is available on the WWW under
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co-workers,[27] the dissolution of the active cobalt carbonyl
species is necessary for the reaction to proceed smoothly and
at a longer reaction time; the active cobalt species leached out
of the catalyst into the reaction mixture, and hence, recyclability
was not achieved. Given the drawbacks associated with these
catalytic systems, in terms of catalyst stability and reusability,
much attention has been drawn to developing suitable
supports and catalysts for carbonylation reactions, such as PKR.
However, the industrial applications are still undeveloped
despite the successes recorded for transition metal-catalyzed
synthesis of cyclopentenone products from their readily avail-
able substrates. On a sizeable economic scale, developing an
active heterogeneous catalyst for these reactions is highly
desirable.
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Scheme 1. (a) Intra- and (b) intermolecular PK reactions.
Recently, we reported that alkali-promoted catalysts are
The cation-promoted catalysts were synthesized via the
deposition-precipitation method with urea and employed as an
active catalyst for inter- and intramolecular PK reactions. The
reactions were carried out with excellent selectivity and yield to
the desired cyclopentenone products under CO atmosphere;
though more favorable for the intra PKR, with catalytic activity
in order of (5 wt.%) Au/Co3O4 (Xc)<Au/CaÀ Co3O4 (Xd4)<Au/
KÀ Co3O4 (Xd3)<Au/NaÀ Co3O4 (Xd2)<Au/LiÀ Co3O4 (Xd1)<Au/
CsÀ Co3O4 (Xd*). We assumed that the notable enhanced
catalytic performance of the catalyst could be attributed to the
cobalt nanoparticles formed on the catalyst surface by spillover
H2 from the AuNPs. The activity was also linked to the increased
surface basicity of the nanomaterials.
The nanomaterial‘s surface basicity was evaluated by CO2-
TPD measurements because of the substantial role of basicity in
the carbonylation reactions involving CO gas[25] (Figure 1a and
Figure S4). The significant role of ion-promoters in catalytic
reactions is still unclear, despite it being mentioned in literature.
We believe the alteration of the surface basicity could explain
these promoter‘s role in the enhanced catalytic performance.
Another interesting observation is the increase in catalytic
activity, which correlated with the cation‘s charge/size ratio; this
particular observation has not been reported for carbonylation
reactions. Helwani et al. reported that alkali metal ions on the
surface of catalysts are known to improve the nanomaterial‘s
surface basicity due to their highly electropositive nature.[34] In
this work, the alkali metal ions caused a remarkable increase in
the catalytic performance towards the Pauson-Khand reaction.
effective
for
carbonylation
reactions,
such
as
hydroformylation.[25] The alkali ions were used to modify and
stabilize the mesoporous cobalt oxide, which minimized
leaching of the active metallic Co species and improve the
catalyst‘s overall binding properties and performance. Also, an
evident correlation was found between the ion-promoted
catalyst‘s reduction temperature and surface basicity with its
catalytic performance. Before this, our group had earlier
reported that dopants improve catalytic activities on mesopo-
rous Co3O4 considerably.[28] Other works of literature have
published the enhanced catalytic efficiency of alkali-promoted
catalysts[29–31] due to the dopants electronic[32] and structural[29,31]
effects on the metal oxides. As a continuous study of carbon-
ylation reactions involving CO gas, we are interested in
exploring the capability of this ion-promoted gold-cobalt based
catalyst for [2+2+1] cyclization reactions. Motivated by this
outcome, and hence the need to develop a more active and
stable heterogeneous catalyst for carbonylative reactions, here-
in, we prepared mesoporous Co3O4 by the inverse micelle
method,[33] modified by doping with alkali metals (x) ( where x=
Cs, Li, K, Ca and Na), to act as ion-promoters and stabilizers.
Subsequently, immobilization of the AuNPs on the reducible
metal oxide to afford a bimetallic structure. Under these
conditions, we assumed that the ion-promoted catalyst would
catalyze the Pauson-Khand reactions, based on the unique
structural properties and easy preparatory steps of the catalysts,
which was entirely different from others reported. To the best
of our knowledge, attributing catalytic performance to the
concentration of basic sites and low reducibility temperature of
a catalyst for Pauson-Khand reactions have not been reported.
The temperature-programmed measurements were performed,
and the catalytic performance of the supported catalyst was
studied in detail with respect to the pre-treatment of the
catalyst with hydrogen, recyclability, and parameter effects
(catalyst loading, temperature, solvent, and pressure). The
catalyst was applied for the inter -and intramolecular PK
reactions (Scheme 1) by investigating the alkali ions and the
gold nanoparticle‘s role in the overall catalyst. Also, the
catalyst‘s reusability makes our catalytic system more viable
and could be used as an easy handling alternative to other
carbonyl catalysts, which requires a tedious purification process.
°
Under the reaction conditions of CO (20 atm), 130 C, and 6 h, it
was observed that the Cs-promoted gold catalyst was the most
active to the nonpromoted Xc catalyst and others for the
oxygen-tethered cycloaddition reaction, with the highest yield
of the cyclopentenone product achieved at 87.2% (Figure S5).
This was ascribed to the highly electropositive nature of the Cs
metal, which increased the basic site concentration in the
catalyst and enhanced its catalytic efficiency for the PKR; further
confirming the induced promotional effect of these cations on
the catalysts (Figure 1a and Figure S4). It is important to note
that incorporating the Au metal on the support matrix slightly
improved the catalyst‘s basic strength. Therefore, basicity
played a crucial role in the higher catalytic performance for the
PKR. The improved catalytic strength and activity were also
linked to the nanomaterial‘s low reduction temperatures at
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Figure 1. Comparison of (a) CO2-TPD, (b) CO-TPR and (c) H2-TPR profiles to justify the promotional effect of the Cs+ on the nanomaterials.
Table 1. Effect of reaction conditions using 5% Au/CsÀ Co3O4 (Xd*).[a]
higher H2 and CO consumption (Figure 1b and Figure 1c) (see
Supplementary Figure S4 for more details).
°
Entry
Solvents
Cat.
Xd*[g]
T [ C]
Press.
[atm]
Conv.
[%][b]
Sel.
Yield
[%][b]
[%][b]
The pore structure and BET surface areas were noticeably
affected by the Au loadings (Table S1). Also, an increase in the
Au loadings up to 25 wt.% provided a lower yield of the
product (Table S3, entry 10). Although, the catalysts were
crystalline in their framework structures, as observed from the
acquired HR-TEM micrographs (Figure S1), as well as the well-
resolved p-XRD diffraction patterns (Figure S3a). A characteristic
Type IV isotherm was observed for Au catalyst and support,
indicating the material‘s mesoporous nature (Figure S3b). It is
noteworthy that the BJH pore diameter/volume and BET surface
area decreased after incorporating the AuNPs on the reducible
CsÀ Co3O4 (Table S1). From the TGA spectra, the decomposition
of the organic matter in 5% Au/CsÀ Co3O4, in the temperature
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THF
0.3
0.3
0.3
0.3
0.8*
0.1
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
130
130
130
130
130
130
110
90
150
130
130
130
110
110
20
20
20
20
20
20
20
20
20
10
5
96.5
93.1
62.7
81.9
98.2
83.3
79.8
36.2
98.5
67.9
51.3
99.8
60.2
81.2
90.4
55.8
24.3
68.3
95.6
77.1
68.6
51.4
90.2
54.6
18.9
31.3
49.3
25.3
87.2
51.9
15.4
55.7
93.8*
64.9
54.6
18.5
88.9
37.2
9.6
DMF
Toluene
Dioxane
THF
THF
THF
THF
THF
THF
THF
9
10
11
12[c]
13[d]
14[e]
THF
THF
THF
20
20
20
30.9
29.5
20.4
[a] Reaction conditions: Enyne 1a (0.3 mmol) and 6 h reaction time. Before
reaction, the catalyst (Xd*) was preheated with H2 gas (130 C, 20 atm, 2–
3 h) in 12 mL THF. [b] Selectivity, conversion, and yield (%). [c] 20 h, 1a
(0.3 mmol), THF (12 mL), Xd* (0.8 g), CO (20 atm) and temp. (130 C). [d] No
H2 pre-treatment (110 C, 20 atm, 6 h). [e] No H2 pre-treatment (110 C,
20 atm, 12 h).
°
°
range of 104–800 C, suggests the catalyst‘s safety when used
°
°
below 104 C (Figure S3d).
°
°
Next, we began our investigation by screening the newly
developed catalyst, using the oxygen-tethered enyne prototype
substrate (1a), in order to probe the feasibility of the gold-
°
cobalt based catalysts under the reaction conditions of 130 C,
6 h and CO (20 atm) (Figure S5). Before this, catalysts pre-
treatment with hydrogen gas were performed (H2 pressure
among the solvents screened in this study; presumably due to
its role in stabilizing the active Co species formed on the
catalyst surface and attributed to its cyclic ether functionality
and moderate polarity in its ability to dissolve a wide range of
compounds (Table 1, entry 1). Next, we varied the catalyst
loadings to see its effect on the cyclization reaction. Increasing
the catalyst amount from 0.3 to 0.8 g gave a better yield of the
product at 87.2% and 93.8%, respectively, compared to when
the amount was reduced to 0.1 g (64.9%) (Table 1, entry 6). It is
°
20 atm, 130 C, and 2–3 h). Our initial experiments reveal that
the more demanding pure cobalt support Co3O4 (Xa) showed
little or no beneficial effect for the [2+2+1] cyclization
reaction, and the series in the activity of the catalyst follows the
order Xc<Xd4 <Xd3 <Xd2 <Xd1 <Xd*. In this study, we as-
sumed that the metallic cobalt species formed on the catalyst
surface by the spillover process were shown to be the active
and stable heterogeneous catalyst that catalyzed the PK
reactions. Table 1 and Figure 3 lists the series of the cyclo-
pentenone products, which have been prepared by the Xd*
catalyst, promoted the [2+2+1] cyclization reaction under CO
pressure in THF and other solvents. All the cyclopentenones
synthesized in this work are in oily form and high boiling, which
were readily purified by flash column chromatography using
ethyl acetate, hexane, dichloromethane as eluents at different
times. For the optimization of reaction conditions, the cycliza-
tion of the enyne (1a) by Xd* in THF and other solvents were
attempted (Table 1, entries 1–4). The THF gave the best yield
°
noteworthy that reactions conducted below 130 C gave slightly
lower conversions and yields of the isolated product (Table 1,
entry 7 and 8). At higher temperatures up to 150 C (Table 1,
entry 9), only a slight decrease in the product yield was
observed after 6 h reaction time, with the substrate almost
consumed when the reaction time was increased to 20 h
(Table 1, entry 12). While some reactions will proceed with
better yields, we explored other milder reaction conditions,
such as reduced CO pressure, which might tolerate the enyne
substrate‘s functionality and sensitivity. To our surprise, ex-
°
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tremely poor substrate conversion and product yield was
obtained at 51.3% and 9.6%, respectively (Table 1, entry 11).
With the preferred reaction conditions in hand, we sought
to explore the catalyst versatility on this PKR protocol and to
establish an extensive understanding of the substrate structures
on reactivity, utilizing various substrates with varying degree of
functionality (Figure 2). First, we investigated the feasibility of
cyclizing the as-synthesized oxygen-tethered enynes 1a–1g
influenced the 2l and 2m yields, due to the phenyl and methyl
substituents attached to the 1l and 1m alkene moieties.
Hydroxyl containing terminal alkynes in the 1n–1p substrates
were attempted, and the 2n–2p product yields were also
affected by the substituents attached to their individual alkyne
moieties, apparently due to the steric effect.
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To further elucidate the active sites on Xd*, its catalytic
efficiency was investigated after hydrogen pre-treatment
°
°
under the optimized reaction conditions of 130 C, 6 h, and
(130 C, 2 h, 20 atm). Under the same reaction conditions
°
20 atm (Figure 3a). To our delight, the cyclopentenones 2a–2e
produced were of excellent yields. An attempt to synthesize
enones with N-heteroatoms were unsuccessful, and instead,
gave a trace amount of the product 2g, which were rather
unidentifiable. Likewise, a trace amount of the 2f was recovered
from the reaction mixture with the sterically congested alkyl
ortho-substituted enyne 1f. For the intermolecular PKR, cycliza-
tion of the substrates 1h–1p proceeded to moderate and
higher yields of the products 2h–2p in the range of 43.2–
80.9% (Figure 3b). Interestingly, increasing the alkene‘s carbon
chain length on the 1i substrate, provided a much better yield
of 2i (75.8%), compared to the alkene substrate 1h with shorter
chain length, which recorded a yield of 70.3% (2h). A similar
trend was also observed for 2j and 2k, though, with a more
improved product yield at 77.5% and 80.9%, respectively;
presumably due to the single triple bonds attached to alkyne in
the 1j and 1k substrates, compared to 1i and 1h with double
triple bonds in their molecules. Notably, steric hindrance also
(110 C, 20 atm), appreciable improvement in the cyclopente-
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none yield was observed at 54.6% after 6 h (pre-treated with
H2) (Table 1, entry 7), and decreased significantly to 29.5% (6 h)
and 20.4% (12 h) without H2 pre-treatment (Table 1, entry 13
and 14). The XRD of the pre-treated Xd* catalyst (spent) shows
the absence of Co3O4 peaks and the formation of metallic Co
species that catalyzed the [2+2+1] cyclization reaction (Figure.
4c). As shown in Figure 4a and 4b, it was established that the
active species was not Co2(CO)8 or HCo(CO)4 usually formed
from leaching that catalyzed the PKR, but the metallic Co
generated on the catalyst‘s surface. In practice, heterogeneous
catalytic systems are known to suffer severe leaching of their
active sites during reactions and eventually lose its catalytic
performance. Although the maximum reusability was not tested
in our system, but from our observation, no significant leaching
of the active sites was observed. Figure. 4a shows that the
product yield in the 1st cycle was still much higher than the
fresh catalyst, with a notable increase observed in the 6th cycle,
Figure 2. Intra- (1a–1g) and intermolecular (1h–1p) PK substrates.
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shown in Figure 4b, the promotional effect of the alkali-ions on
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the catalyst was also evident in the reusability of Xd* and its
nonpromoted counterpart (Xc), indicating that the Cs metal,
assisted in stabilizing the active Co species in the catalyst,
hence the insignificant leaching observed.
The heterogeneity test on Xd* was conducted for the [2+2
+1] cyclization reaction to determine if the active Co species
leached into the reaction mixture. Firstly, we ran the reaction at
the standard time; after that, the nanocomposite catalyst was
filtered off, and the filtrate was returned into the autoclave
reactor for 3 h, with stirring speed maintained at 450 rpm,
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°
under the optimized reaction conditions: 130 C, 20 atm.
Interestingly, no significant change in the conversion was
observed after 3 h reaction time, based on the GC-FID results
obtained (Figure. S7), indicating no possible bleeding of the
active metallic Co species into the solution. Although, the
elemental analysis by ICP-OES reveals <1.3 mg/L of the metallic
Co species was leached into the reaction mixture. We can infer
from the hot filtration test that there was a strong interaction
between the AuNPs, Cs-promoter, and the robust mesoporous
cobalt oxide support. The formation of this active metallic
cobalt species that catalyzed the intra- and intermolecular PKR
was also evident from the immediate FT-IR analysis of the spent
nanocomposite catalyst (Figure. S8). Prior to this, the nano-
composite catalyst was re-activated by drying in a vacuum oven
°
at 70 C for 1 hr. From the FT-IR spectra of the spent catalyst,
the MÀ Co metal bands due to 551 and 653 cmÀ 1 appeared in
the spectra. No peak assigned to Co2(CO)8 and HCo(CO)4 species
at 2050 and 2030 cmÀ 1, respectively, were seen.[35] This further
confirms the absence of CO-bond species or non-adsorptions of
the carbonyl vibrational bands in the catalyst‘s spectra,
indicating that the cobalt carbonyl complex was not respon-
sible for the PKR, but rather the active solid Co metal formed on
the catalyst surface. Interestingly, no trace amount of the cobalt
carbonyl species was observed in the NMR spectra analysis,
indicating an effective procedure in the catalyst separation and
product isolation.
Figure 3. (a) Intra- and (b) intermolecular PK reactions. Reaction conditions:
°
Catalyst Xd* (0.8 g), substrates (0.3 mmol), CO (20 atm), temp. (130 C), THF
(12 mL) and 6 h reaction time. The catalyst was pre-treated under the
°
conditions of 130 C, 20 atm (H2) and 2–3 h, before the experiments.
indicating its stability. The insignificant decrease observed in
the last two cycles could be due to the catalyst‘s slight
deactivation arising from the product‘s contamination. As
Figure 4. (a) Reusability studies of 5% Au/CsÀ Co3O4 and (b) Comparison between 5% Au/CsÀ Co3O4 (Xd*) and 5% Au/Co3O4 (Xc) catalysts, in their 1st and 6th
°
run. The reusability test was carried out using the as-synthesized oxygen-tethered enyne substrate (1a), with the optimized reaction conditions: 130 C, 6 h,
THF (12 mL), 0.80 g (Xd*), and 20 atm (CO). The catalyst was pre-treated with H2 gas (130 C, 2–3 h, 20 atm) before the 1st run; for subsequent runs, the
°
°
hydrogen pre-treatment was not repeated. (c) p-XRD of the fresh (before H2 pre-treatment) and spent (after H2 pre-treatment in 12 mL THF, 130 C, 20 atm,
2 h) 5% Au/CsÀ Co3O4 catalyst.
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[5] M. A. Mottalib, M. Haukka, E. Nordlander, Polyhedron 2016, 103, 275–
The effect of H2 and CO gas in the catalyst‘s reducibility
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282.
suggests that it improves the reduction behavior of Co3O4.[26,36]
In this work, the gold nanoparticle‘s role was believed to cause
a spillover effect of the H2 and CO gas on the catalyst surface.
The H2 gas, been more reducing (Figure 1c), reduces the
mesoporous Co3O4 to Co; the formed active Co species trans-
formed a variety as-synthesized oxygen-tethered 1, 6 enynes to
bicyclic cyclopentenones with excellent yields. It is essential to
note that Co metal was mainly responsible for the PK reaction.
The AuNPs neither promoted the catalytic activity by synergy
with the metallic cobalt nor interfered with the cyclization
reaction over the cobalt metal. Also, we attributed the superior
performance of this alkali-promoted catalyst (5% Au/CsÀ Co3O4)
in the cyclization reaction with carbon monoxide, to the
combined effects of the catalyst structure, increased surface
basicity, and low reduction temperatures. It’s noteworthy that
the alkali cations promoted and induced the robust Co3O4 and
catalyst stability, which minimized leaching of the active Co
species. Another more extensive feature of this catalytic system
is the reusability, in which the catalyst was stable after many
cycles, as well as the steady supply of the active Co species
during the reaction. Therefore, we concluded that the exper-
imental simplicity and catalyst reusability are incredibly attrac-
tive and could be useful in academia and chemical industries as
a stable and easy handling alternative to the homogeneous
catalytic systems.
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Acknowledgements
This work was supported by TWAS-NRF (grant number 116088).
The authors thank the University of Johannesburg and SHIMADZU,
South Africa, for the use of their instruments.
Conflict of Interest
The authors declare no conflict of interest.
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Keywords: Pauson-Khand
·
Mesoporous cobalt
· Cobalt
nanoparticles · Gold catalyst · Alkali ion-promoters
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Manuscript received: November 16, 2020
Revised manuscript received: December 21, 2020
Version of record online: January 27, 2021
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