In-Situ generation of surface-active HCo(CO)y like intermediate from gold supported on ion-promoted Co3O4 for induced hydroformylation-hydrogenation of alkenes to alcohols

Article abstract of DOI:10.1016/j.apcata.2020.117735

In this study, a greener and stable surface-active cobalt-carbonyl like specie [HCo(CO)_y] was generated via H₂ and CO spillover by gold on ion-promoted cobalt oxide. The supports and catalysts syntheses were based on inverse micelle and deposition-precipitation methods, respectively. The temperature-programmed reduction was used for optimization to obtain the best supports. The catalysts with activity (Co₃O₄ < Cs-Co₃O₄ < Au/Cs-Co₃O₄ and Au loadings 10 percent < 2 percent < 5 percent) were evaluated for the hydroformylation-hydrogenation of alkenes to alcohols, with the 5 percent Au/Cs-Co₃O₄ catalyst more active than the others and displayed excellent alcohol chemoselectivity with varying regioselectivity under milder reaction conditions. The reaction was assumed to take place via the formation of [HCo(CO)_y] specie, as the active catalytic site of the catalyst. The enhanced catalytic performance was also ascribed to the low-temperature reducibility and surface basicity of the nanomaterials. The stability of the catalyst was evaluated by recycling, with its mesostructure retained after four cycles.

Full text of DOI:10.1016/j.apcata.2020.117735

Journal Pre-proof
in-situ generation of surface-active HCo(CO)y like intermediate from gold
supported on ion-promoted Co O for induced
3
4
hydroformylation-hydrogenation of alkenes to alcohols
Charles O. Oseghale (Investigation) (Formal analysis)
(Methodology) (Writing - original draft) (Writing - review and editing),
Batsile M. Mogudi (Formal analysis) (Methodology), Christianah A.
Akinnawo (Formal analysis) (Methodology), Reinout
Meijboom<ce:contributor-role>Conceptualisation) (Methodology)
(Resources) (Writing - review and editing) (Supervision) (Project
administration) (Funding acquisition)
PII:
S0926-860X(20)30328-8
DOI:
https://doi.org/10.1016/j.apcata.2020.117735
APCATA 117735
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
21 April 2020
3 July 2020
7 July 2020
Please cite this article as: Oseghale CO, Mogudi BM, Akinnawo CA, Meijboom R, in-situ
generation of surface-active HCo(CO)y like intermediate from gold supported on
ion-promoted Co O for induced hydroformylation-hydrogenation of alkenes to alcohols,
3
4
Applied Catalysis A, General (2020), doi: https://doi.org/10.1016/j.apcata.2020.117735

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in-situ generation of surface-active HCo(CO)
y
like intermediate from gold
supported on ion-promoted Co for induced hydroformylation-
3
O
4
hydrogenation of alkenes to alcohols
Charles O. Oseghale, Batsile M. Mogudi, Christianah A. Akinnawo and Reinout Meijboom*
Research Center for Synthesis and Catalysis, Department of Chemical Sciences, University of Johannesburg, PO Box
5
24, Auckland Park 2006, Johannesburg, South Africa Tel.: +27 (0)72 894 0293, Fax: +27 (0)11 559 2819
Corresponding Author E-mail: rmeijboom@uj.ac.za (R.M.)
*
Graphical abstract
Highlights

Gold nanoparticles supported on alkali ion-promoted mesoporous Co
and characterized using various instrumental techniques.
3
4
O were synthesized

The catalysts were evaluated for the hydroformylation-hydrogenation of 1-octene to nonanol
under syngas H /CO (2:1) pressure.
2


The products of the 1-octene hydroformylation-hydrogenation reaction were analyzed by
GC-MS and GC-FID analyses.
Abstract
In this study, a greener and stable surface-active cobalt-carbonyl like specie [HCo(CO) ] was
y
generated via H and CO spillover by gold on ion-promoted cobalt oxide. The supports and catalysts
2
syntheses were based on inverse micelle and deposition-precipitation methods, respectively. The
temperature-programmed reduction was used for optimization to obtain the best supports. The
catalysts with activity (Co
3
O
4
< Cs-Co
3
O
4
< Au/Cs-Co
3
4
O and Au loadings 10% < 2% < 5%) were
evaluated for the hydroformylation-hydrogenation of alkenes to alcohols, with the 5% Au/Cs-Co
3
O
4
catalyst more active than the others and displayed excellent alcohol chemoselectivity with varying
regioselectivity under milder reaction conditions. The reaction was assumed to take place via the
formation of [HCo(CO) ] specie, as the active catalytic site of the catalyst. The enhanced catalytic
y
performance was also ascribed to the low-temperature reducibility and surface basicity of the
nanomaterials. The stability of the catalyst was evaluated by recycling, with its mesostructure
retained after four cycles.
Keywords: Hydroformylation-Hydrogenation; Mesoporous cobalt; Gold catalyst; Ion-promoters;
1
-octene; Alcohol.
1
.0
Introduction:
Hydroformylation, also known as oxo reaction, remains a vital process in academic and industrial
perspectives for the synthesis of alcohols and aldehydes from alkenes. In 1938, Otto Roelen
discovered hydroformylation reaction and patented it in 1943 [1]. Approximately eight (8) million
metric tons of alcohols and aldehydes are produced via hydroformylation reactions. These products
have found usage in the manufacturing of surfactants, detergents, adhesives, soaps, solvents, and
plasticizers. Commercially, phosphine modified rhodium-based catalysts have been widely reported
for the hydroformylation of alkenes with shorter chain length (C
2
-C ) under milder reaction
6
conditions. However, this homogeneous catalyst cannot be employed for longer chain alkenes due
to the decomposition of the rhodium catalyst during separation from the mixture and for reuse in
subsequent reactions [2,3].
Although, homogeneous catalysts give higher conversion and product selectivity in shorter reaction
time compared to its heterogeneous counterpart, but with problems of separating the catalyst from

the reaction mixture. On the other hand, cobalt metal is less expensive than rhodium, and its
carbonyl-based catalysts ([Co (CO) ] and [Co(CO) PR ]) have been reported for higher alkene
2
8
3
3
hydroformylation under homogeneous conditions. Alternative metals like Pt, Pd, Fe, and Ru have
also been reported as hydroformylation catalysts [4]. However, the use of these cobalt carbonyl
complexes requires a rather harsh condition of temperature, syngas pressure of >24 MPa, and longer
reaction time to stabilize the catalyst. This method often involves recrystallization and sublimation
before use to obtain a good product yield and reusability, due to the dissociation of CO during storage
below 0 °C and under an inert gas atmosphere [5]. The practical utility in the synthesis of these
cobalt carbonyl-based catalysts is still limited to problems of volatility, toxicity, the formation of
insoluble carbonyl clusters, and separation of the carbonyls from the reaction mixture [5,6]. One
possible solution is to heterogenize this cobalt catalyst by depositing it on supports such as resins
[
7,8], activated carbon [9], and SiO [10]. Immobilization of active sites on these supports through
2
physical and chemical interactions can form heterogeneous catalysts, which in turn leads to an
environmental and economically friendly “green” process chemistry. Given this, developments of
heterogeneous catalytic systems are gaining broad research interest in various carbonylative
transformations, such as hydroformylation reaction. In recent time, carbonylation reactions
involving noble metal nanoparticles are quite useful, efficient, and gives excellent product yields.
Metal nanoparticles are the subject of intense interest in materials science and chemistry because
they display unusual chemical and physical properties, which are distinct from those of both isolated
molecule and bulk phase [11,12]. Particularly, gold catalysis has led to alternative, competitive
strategies and highly innovative discoveries that have laid down a foundation for the implementation
of various catalytic transformations. A few decades ago, gold was considered to be chemically inert
and catalytically inactive as a catalyst for hydrogenation or oxidation reactions at ambient
temperature. To contradict these ideas, Sermon and Bond in 1973 reported that highly dispersed
supported gold nanoparticles can chemisorb hydrogen gas at a temperature as low as 120 °C, and
can catalyze hydro-cracking and hydrogenation reactions at ambient temperatures [13]. Gold
nanoparticles (AuNPs) are well-known for their enhanced catalytic activity in numerous
heterogeneous and homogeneous reactions, such cycloaddition [14], C-C coupling [15],
hydroamination [16–18], amidocarbonylation [19], oxidative cyclization [20,21], hydroalkoxylation
[
22], oxidation [23] and reduction [24]. The most important key is to support AuNPs on selected
mesoporous metal oxides (MMOs) to afford a bimetallic structure. Although the choice of this
support affects the durability and performance of the catalysts, and the supported gold nanoparticles
on mesoporous metal oxides can moderately absorb CO gas [25]. Mesoporous metal oxides have

been of intense research interest due to its applications in separation, adsorption, gas storage,
electrochemistry, and catalysis [26–28]. These mesoporous materials have tunable pore size
diameters within the range of 2 to 50 nm, large surface area, and their crystal structures are the most
intrinsic features supporting their numerous applications [29,30].
Several works of literature on heterogeneous gold supported on MMOs have been reported for
reactions involving CO gas. Investigation of the oxygen storage capacity of mesoporous titania
supported gold catalysts for CO oxidation has been widely reported. Notably, the influence of the
nano-sized gold in the CO spillover mechanism [31–35]. Similarly, MMOs supported gold catalyst
has been reported for other types of carbonylation reactions, such as amidocarbonylation [19], and
cyclocarbonylation [36]. Tokunaga and co-workers reported that hydroformylation reactions could
proceed smoothly on supported gold catalysts under the syngas atmosphere. In these reactions, the
investigation of the role of AuNPs in providing spillover of carbon monoxide and hydrogen gas was
reported [37]. Therefore, heterogeneous supported gold catalysts has proven to be of great alternative
to homogeneous metal carbonyls, and are ideal choice, in terms of catalyst separation, easy
preparatory steps and reusability.
The applicability of this noble-transition metal catalyst for the hydroformylation-hydrogenation of
alkenes to alcohols, to better understand the synergistic effects is limited. Although, few researchers
have reported the reductive hydroformylation reaction using a combination of transition metals in
the form of Co-Rh [38] and Fe-Cu [39] under homogeneous conditions. Alcohol synthesis via
reductive hydroformylation is a vital reaction sequence, which directly converts alkene compounds
to alcohols. The use of syngas (H /CO), in varying ratios, is more advantageous and eliminates the
2
risk of using hydrogen for aldehyde hydrogenation in a multi-step process. To date,
hydroformylation-hydrogenation reactions using modified palladium, cobalt, and ruthenium catalyst
have been reported [40]. Recently, a new modified homogeneous cobalt complex with phosphine
ligand has been reported for the reductive hydroformylation of higher alkenes to alcohol [41]. Liu
and co-workers extensively studied the hydroformylation-hydrogenation of alkenes to alcohol via a
reverse water-gas shift [42]. The reductive hydroformylation of alkenes to alcohols using a fibrous
cobalt oxide catalyst has also been reported [43], and the reaction proceeded at temperatures ranging
from 150-170 °C. Llorca and co-workers reported the alcohol synthesis by hydroformylation-
hydrogenation of ethene, using a homogeneous cobalt carbonyl cluster [44]. Yang et al. have
reported a supported gold catalyst modified with phosphine ligand for the one-pot synthesis of
tricyclodecanedimethylol (TDDMO) [45]. The AuNPs assisted in the formation of a phosphine
complex, as active catalytic sites on the catalyst. The poor chemo- and regio-selectivity, harsh

reaction conditions, catalyst separation, and cost of the phosphine ligands, were the major limitations
associated with this multi-step synthetic protocol. Also, it was impossible to investigate the activity
and structural relationship in the mixed catalytic systems. Therefore, on an economic larger scale,
higher alkene hydroformylation-hydrogenation processes with these catalytic systems are not
applicable. For this reason, the development of an effective, stable and highly reusable
heterogeneous catalysts for reactions involving CO, and to better understand their activity-structural
relationship is desirable. Hence, heterogeneous catalysts present more advantages to its
homogeneous counterparts, in terms of catalyst recovery and reusability.
By putting the advantages of heterogeneous catalytic systems into consideration, and to continue our
ongoing research on the use of the gold catalyst in heterogeneous carbonylation reactions, herein,
we aim to synthesize mesoporous Co
3
4
O support by dopping with alkali metals (ion-promoters) via
inverse micelle approach [46]. Subsequently, the immobilization of gold nanoparticles on the
modified support to afford a bimetallic structure. As earlier reported in our group, alkali metals on
mesoporous Co
3
4
O tend to improve catalytic activities of various industrial chemical reactions to a
great extent [47]. Although several works have reported the enhanced catalytic efficiency of alkali-
doped metal oxides [48–50], this is due to the dopant’s structural [51] and electronic [49,50] effects
on the mesoporous metal oxides. The synergy, parameters effects (catalyst loading, partial pressure,
temperature, and time), kinetics, reusability, and catalytic activity in terms of low-temperature
reducibility and increase in surface basicity of the catalysts were studied in detail. The supported
gold catalyst was applied for the hydroformylation-hydrogenation reaction (Scheme 1) of various
alkenes, by investigating the role and effects of the AuNPs and alkali metals on the catalyst.
Scheme 1. 5% Au/Cs-Co
3
4
O catalyzed hydroformylation-hydrogenation of 1-octene.
2
.0
Experimental

2
.1. Chemicals and reagents
Gold(III) chloride trihydrate (99.9%), potassium tetrachloroplatinate(II) (≥99.9%), nitric acid
HNO ) (70%), cobalt(II) nitrate hexahydrate (98.8%), 1-butanol (≥99.8%), tetrahydrofuran (99%),
(
3
acetonitrile (99%), hexane (99.8%), toluene (99.9%), poly(ethylene glycol)-block-poly(propylene
glycol)-block-poly(ethylene glycol) Pluronic (P-123), 1-octene (98%), pentene (98%), hexene
(98.8%), heptene (98%), styrene (98.8%), nitrate salts of sodium (98%), cesium (98%), lithium
(98.8%), sodium borohydride (≥96%) and urea (≥99.9%) were purchased from Sigma-Aldrich. The
nitrate salts of calcium (≥98%) and potassium (99%) were purchased from Associated Chemical
Enterprises. The synthesis gas H /CO (2:1) was purchased from AFROX Chemicals, RSA. Milli-Q
2
water was used in the synthesis of the catalysts. All chemicals were of analytical grades and used
without further purification.
2
.2. Synthesis of the supports and catalysts
Mesoporous Co at different temperatures (150-550 °C) were prepared via the inverse micelle
approach with modification [46]. In a typical synthesis, 5.82 g (20 mmol) of Co(NO ·6H O was
dissolved in a solution containing 17.05 g (230 mmol) 1-butanol, 2.39 g (38 mmol) HNO and 2.37
3
O
4
3
)
2
2
3
-5
g (4 × 10 mmol) Pluronic P-123. The mixture was stirred overnight at room temperature (RT) and
dried in an oven at 120 °C for 6 h. The gel obtained was washed thrice with ethanol and centrifuged
at 4200 rpm for 15 min interval, until the supernatant was clear, and subsequently, dried overnight
in a vacuum oven at 70 °C. The powder obtained was subjected to various calcination cycles at 150
°
C/12 h, 250 °C/4 h, 350 °C/3 h, 450 °C/2 h, and 550 °C/1 h. To obtain the best calcination
temperature for the synthesis of the alkali doped supports, the H -TPR, and CO-TPR measurements
were performed. Based on their respective low-temperature reducibility, the order follows thus:
Co -550 °C < Co -450 °C < Co -150 °C < Co -250 °C < Co -350 °C.
2
3
O
4
3
O
4
3
O
4
3
O
4
3
O
4
The synthesis of the alkali metal-doped Co
An appropriate amount of x(NO ) (x = K, Na, Li, Ca, and Cs) 2% (mol/mol) were added to a solution
containing 5.82 g (20 mmol) of Co(NO ·6H O, 17.05 g (230 mmol) 1-butanol, 2.39 g (38 mmol)
and 2.37 g (4 × 10 mmol) Pluronic P-123. The mixture was stirred overnight at RT and dried
3
O
4
was carried out by a procedure earlier reported above.
3
3
)
2
2
-
5
HNO
3
in an oven at 120 °C for 6 h. The product obtained was washed thrice with ethanol and centrifuged.
The resulting powder was calcined at 350 °C for 3 h. Similarly, the temperature-programmed
reduction measurements were used to obtain the best support for the synthesis of the catalysts. The
order in their reduction temperature follows thus: Ca-Co
3
O
4
< Li-Co
3
O
4
< Na-Co
3
O
4
< K-Co
3
4
O <
Cs-Co
3
4
O .

The gold nanoparticles were deposited on the mesoporous Cs-Co
method with urea [52]. In a typical reaction procedure, 1 g of the Cs-Co
solution of HAuCl .3H O and stirred for 1 h at room temperature. Urea in (2:1) molar ratio was
added to the reaction mixture and heated to 80 °C. The solution was allowed to cool down, and
freshly prepared NaBH in (10:1) molar excess was added to the mixture and stirred for another 3 h.
3
O
4
via the deposition-precipitation
3
O
4
was added to an aqueous
4
2
4
The resulting precipitate was filtered and washed with excess distilled water to remove the chloride.
The catalyst was calcined at 350 °C for 3 h, and the initial Au loading was set at 5 wt.%.
2
.3. Characterization of the catalyst
Scanning electron microscopy (SEM) was performed using a Tescan Vega 3 LMH instrument,
operated at a high voltage of 20.0 kV. Before the analysis, the samples were carbon-coated in an
Agar Turbo coater. Energy disperse X-ray spectroscopy (EDXS) on SEM, was used to determine
the quantity of the dopants. Transmission electron microscopy (TEM) was carried out using a JEOL
TEM-2100F, with an accelerating voltage of 200 kV. Prior to this, the catalysts were placed on the
TEM-grids after ultrasonication in ethanol for 1 h. Powder X-ray diffraction (p-XRD) measurements
were performed on a Philips XPERT-PRO diffractometer, operating with Cu Kα1 and Kα2 (λ-
1
.5406 and 1.54443), respectively. The scan ranges from 0.5-2° and 10-80° for the low and wide 2θ
angles, respectively. Nitrogen sorption measurements were performed on a Micrometrics ASAP
460 instrument at 77 K. Before the analysis; the samples were degassed at 80 °C under a nitrogen
atmosphere and vacuum for 12 h and 3 h, respectively. The surface areas were calculated using a
Brunauer-Emmett-Teller (BET) method. Temperature-programmed reduction (CO-TPR and H
2
2
-
TPR) were performed on a Micromeritics AutoChem II. The quartz tube reactor was loaded with
about 45 mg of the catalyst and pretreated under argon/helium flow at 200 °C for 1 h. The materials
were cooled and probed by passing 10% H
2
/Ar (H
2
-TPR) and 10% CO/He (CO-TPR) at different
-1
times through the catalyst bed at the flow rate of (50 mL min ), from room temperature to 700 °C
-1
(
10 °C min ). Temperature-programmed desorption (CO -TPD) was performed on a Micromeritics
2
AutoChem II, to determine the basicity of the catalysts. About 0.2 g of the catalyst was packed into
a quartz tube reactor and pretreated under an inert gas (Ar) flow at 200 °C for 1 h. The materials
were probed by passing 10% CO /He at a different time of measurements in the temperature range
2
of 20-600 °C. A Spectro Arcos ICP-OES instrument was used to determine the metal loading (wt.%)
of the nanomaterials. A TGA Perkin-Elmer STA 6000 instrument was used to carry out the
-1
thermogravimetric analyses with a ramping rate of 10 °C min , from 25 °C to 1000 °C under
-1
nitrogen flow (100 mL min ). Fourier transform infrared spectroscopy (FT-IR) was conducted on a

Shimadzu IRAffinity-1S, equipped with a QATR-10 single reflectance accessory, using KBr pellets
-1
in the region of 4000-500 cm .
2
.4. Catalytic reaction
Hydroformylation-hydrogenation reaction was conducted in a 100 mL Teflon tube inside a stainless-
steel Parr stirred reactor. In a typical procedure, 8 mmol of 1-octene, 8 mmol of decane (internal
standard), and 40 mg of the catalyst were dissolved in 10 mL of the solvent and introduced into the
reactor. The syngas H /CO (2:1) was set at 40 bar constant pressure, maintained throughout the
2
reaction. The temperature was raised to 140 °C, with stirring and heating for 12 h. On completion
of the reaction, the reactor was allowed to cool down. The products obtained were analyzed using
gas chromatography (GC) mass spectroscopy (GC-MS) and a GC equipped with a flame ionization
detector (FID). For recycling purposes, the catalyst was removed by centrifugation, washed
successively with tetrahydrofuran (THF), and dried in a vacuum before reuse.
3
.0
Results
3
.1. Textural characterization of the mesoporous supports and catalysts
The structural properties of the nanomaterials are presented in Table 1 and Table S1. Nitrogen
adsorption-desorption isotherms of the supports and catalysts are shown in Fig. 1, and Fig. S1,
respectively. A Type IV isotherm and H1 hysteresis loop were observed for the nanomaterials,
indicating a well ordered and regular uniform mesoporous structure of the materials [53], as
observed from the quantity adsorbed at STP to the relative pressure (P/P ) (Fig. 1a). As shown in
0
Table 1, it is essential to note that upon doping the pure cobalt oxide, the average pore diameter
increased, with a decrease in pore volume and surface area. Immobilization of the gold nanoparticles
on the support (Cs-Co
3
O
4
) matrix further decreased the BET surface areas and pore sizes for the
, and ascribed to the partial blockage of the
catalysts, with the least observed for 10% Au/Cs-Co
3
O
4
mesopores due to the high metal loading content. The synthesized nanomaterials, as shown in Fig.1b,
displayed a monomodal pore size distribution.
The powder X-ray diffraction patterns (p-XRD) for the nanomaterials are given in Fig. 1, and Fig.
S2, and displayed a typical Co
3
O
4
spinel pattern with cubic crystal structure [54]. The characteristics
could be well indexed in the JCPDS database (74-1657),
peaks and planes of the spinel Co
3
O
4
indicating the purity of materials. Similar spectra patterns were observed for the cobalt oxide
supports and catalysts at the different gold loadings (Fig. 1c). No distinct AuNPs reflection was seen
for all the catalysts, indicating a small size or low content of the nanoparticles incorporated on to

support matrix, which could not be detected by the XRD, with no phase transition observed. The
presence of sharp diffraction peaks in the low angle spectra is an indication of the existence of pores
in the nanoscale for the materials [55] (Fig. 1d). Scherrer’s equation was used to calculate the
crystallite sizes of the nanomaterials, as shown in Table 1 and Table S1. It’s important also to note
that the crystallite size of the pure cobalt oxide increased upon dopping with alkali metals, and
further increased on immobilization of the gold nanoparticles on to the support matrix.
Table 1 Textural parameters of supports and catalysts
c
Entry Sample
Metal
content
Crystallite Pore
Pore
SA BET
a
sizes b
(nm)
7.4
2
-1
Diameter
volume
(m g )
3
-1
(wt.%)
(nm)
12.8
26.2
24.9
24.2
9.1
(cm g )
1
2
3
4
5
Co
Cs-Co
2% Au/Cs-Co
5% Au/Cs-Co
10% Au/Cs-Co
3
O
4
-350 °C
-
0.36
0.32
0.29
0.28
0.21
64
58
52
49
22
3
O
4
0.77 (Cs)
0.82 (Au)
4.71 (Au)
8.26 (Au)
15.6
18.2
20.4
26.7
3
3
O
O
4
4
3
O
4
b
a
c
Determined by ICP-OES. Calculated using Scherrer’s formula. Determined by Nitrogen BET

Fig. 1. (a) Nitrogen-sorption isotherms, (b) BJH pore size distributions, (c) Wild angle, and (d) Low-
angle p-XRD diffraction patterns of the supports and catalysts.
The morphologies of the nanomaterials were viewed by TEM and SEM, with the representative
micrographs shown in Fig. 2. The Cs-Co
3
4
O showed an almost similar morphology to the 5% Au/Cs-
Co catalyst, with a nano-sized ball-like particle (Fig. 2a and 2b), indicating that the gold
3
O
4
nanoparticles did not alter the phase of the support. This was in good agreement with the p-XRD
data, where the AuNPs were fully incorporated into the support matrix with the absence of new
phases in the spectra. The HRTEM images in Fig. 2c and 2e, reveal the dispersed AuNPs with slight
agglomeration on the cobalt oxide support, except for the 10% Au/Cs-Co
3
4
O catalyst, whose
nanoparticles agglomerated more, due to the high metal loading on the support matrix (Fig. 2f). The
particle diameters for the catalysts ranges from 2.1-8.8 nm, with 4.2 ± 0.1 nm observed for 5%
Au/Cs-Co
3
4
O and its corresponding size distribution shown in Fig. 2d. The lattice fringes of 0.24

nm, as revealed by the HRTEM image, can be well indexed to the (311) phase of a typical Co
cubic spinel crystal structure. The representative AuNPs were indicated with a white circle.
3
O
4
Fig. 2. Representative SEM images of (a) Cs-Co
with particle size distribution of (c) (d) 5% Au/Cs-Co
Co
3
O
4
, and (b) 5% Au/Cs-Co
3
O
4
and HRTEM images
, and (f) 10% Au/Cs-
3
O
4
, (e) 2% Au/Cs-Co
3
O
4
3
4
O .
The temperature-programmed reduction is an efficient tool to determine the reducibility properties
of nanomaterials. The supports and catalysts were analyzed by CO-TPR and H -TPR, and their
respective profiles are presented in Fig. 3 and Fig. 4, with the data shown in Table 2. The CO-TPR
2

profile of the pure oxide Co
of the ion-promoter on Co
3
O
4
-350 °C shows two reduction peaks at 236 °C and 264 °C. On addition
3
O
4
-350 °C, a significant reduction in temperature to 171°C and 241 °C
was observed, indicating the easily reducible and electropositive nature of the alkali metal ions on
the pure oxides [49] (Fig. 3a). The first profile peak at 171 °C, was ascribed to the reduction of
Co
to the reduction of CoO to Co [56]. Fig. 3a shows a slight shift to lower reduction temperatures upon
immobilization of the gold nanoparticles on the cobalt oxide support, except for 10% Au/Cs-Co
which shifted to higher reduction temperatures at 274 and 334 °C, respectively. The inconsistency
observed in the increased temperature values for 10% Au/Cs-Co , was also in good agreement
3
O
4
to CoO, due to the reducible surface-active oxygen, and the peak at 241 °C can be attributed
3
O
4,
3
O
4
with BET data; we postulated that it might be due to the partial blockage of its mesopores from the
increase in gold loadings on the support (Table 1). The notable decrease in the reduction temperature
of the 5% Au/Cs-Co
3
O
4
catalyst compared to Cs-Co
3
4
O , shows that the gold nanoparticles promoted
the spillover of CO on the support [57].
A similar trend in the reduction temperatures was observed for the H -TPR profiles of the
2
nanomaterials (Fig. 4b). The supports and catalysts reduced in two stages, and are ascribed to the
reducible surface absorbed capping oxygen and bulk reduction at a higher temperature, respectively.
The shift to lower reduction temperature on the addition of ion-promoters and nanoparticles on the
support, suggests an altered electronic structure upon immobilization. In the presence of the ion-
promoters and nanoparticles, hydrogen atoms are more likely to be provided efficiently, and the
probable reduction will occur at lower temperatures, as compared to the pure metal oxides without
ion-promoters and nanoparticles in its matrix. This phenomenon is referred to as the hydrogen
spillover effect [58,59].

Fig. 3. (a) CO-TPR and (b) H -TPR profiles of the supports and catalysts.
2
A comparison between the ion-promoted catalyst (5% Au/Cs-Co
3
4
O ) and its non-promoted
counterpart (5% Au/Co ), shows the significant effect of the highly electropositive alkali metal
3
O
4
ion (Cs) in the reduction temperature of the catalysts (Fig. 5). The lower temperature shift in the
reduction peaks simply contributed to the high catalytic activity of the catalyst.
Fig. 4. Comparison of (a) CO-TPR and (b) H
2
-TPR profiles of 5% Au/Co
3
O
4
and 5% Au/Cs-Co
3
4
O .

Table 2 Reduction temperatures and surface basicity of the nanomaterials
Entry Samples
CO-TPR
°C)
H
2
-TPR
Total basicity
(
g-1) x 10-1
(°C)
(mmol CO
2
1st
2nd
1st
2nd
1
2
3
4
5
Co
3
O
4
-350 °C
236
171
164
159
274
264 239
241 218
198 208
186 191
334 282
291
271
267
237
319
2.1
8.7
9.3
10.7
4.3
Cs-Co
3
O
4
2% Au/Cs-Co
5% Au/Cs-Co
3
3
O
O
4
4
10% Au/Cs-Co
3
O
4
The relative basicity of nanomaterials is revealed by the CO -TPD analysis (Fig. 5), with the data
2
summarized in Table 2. Alkali metal ions are known to increase the basicity of metal oxides and can
enhance catalytic activity [49,60], due to their electropositive nature [61] and structural effects [51].
Strong broad peaks, high intensities, and more basic sites were observed for all the materials (2-5)
at a temperature range of 96-452 °C, except for the pure metal oxide (1), which gave lower peak
intensities. It should be noted that the immobilization of the AuNPs slightly improved the basic
properties of the catalysts. As shown in Table 2, the amount of base concentrations of these
-1
nanomaterials ranges from 0.21-1.07 mmol g , with the 5% Au/Cs-Co
3
4
O (5) showing the more
-1
pronounced base strength concentration of 1.07 mmol g , highest peak intensities, and robust basic
sites for probable activation of substrates. Therefore, we postulated that basicity played an essential
role in these catalysts, which directly correlates and is in good agreement with the CO-TPR and H
2
-
TPR data.

Fig. 5. CO
2
-TPD profiles of (1) Co
3
O
4
-350 °C, (2) Cs-Co
3
O
4
, (3) 2% Au/Cs-Co
3
4
O , (4) 5% Au/Cs-
Co and (5) 10% Au/Cs-Co
3
O
4
3
4
O .
Fig. 6a shows the thermal stability of the nanomaterials evaluated by thermogravimetric analysis
TGA). All materials show a minor weight (%) loss in the range of 1.5-4.0% below 150 °C due to
physically adsorbed moisture on the surface. At the second stage of degradation, a minimal weight
loss was observed for the catalysts, except for Co -350 °C and Cs-Co , which displayed weight
(
O
3 4
3
O
4
loss at 69% and 54%, respectively. The weight loss in the range of 162-502 °C, is assigned to the
thermal degradation of the organic matter in the catalysts, suggesting that these materials could
safely be used well below 162 °C. The weight loss observed for Co
3
4
O -350 °C at 654 °C could be
due to the decomposition of the inorganic residue.
The infrared spectra for the supports and catalysts are shown in Fig. 6b. All the as-synthesized
-1
nanomaterials gave the same peaks at around 659 and 561 cm , assigned to Co-O stretching
vibrations of the pure Co , which further confirms that the Cs and AuNPs did not alter the crystal
structure of the spinel Co [62,63], and are well incorporated in its matrix. The FT-IR and p-XRD
3
O
4
3
O
4
results unambiguously reveal that, after modification of the Co
3
4
O with Cs and AuNPs, its structure
is retained, due to excellent dispersion on the support.

Fig. 6. Representative (a) TGA and (b) FT-IR spectra of the supports and catalysts.
3
.2. Catalytic screening and activity for hydroformylation-hydrogenation of 1-octene.
The catalysts screening was performed under syngas H /CO (2:1) pressure for the hydroformylation-
2
hydrogenation of 1-octene (Table S2). The selected 1-octene was used as a screening molecule to
understand the optimization of reaction conditions better. To further justify the promotional effects
of the alkali metal ions on nanomaterials [49,51], the 5% Au/Co
ion-promoted counterpart (5% Au/Cs-Co ). The catalyst without alkali metal ions (5%
Au/Co ), gave lower 1-octene conversion and alcohol selectivity to the ion-promoted catalyst (5%
Au/Cs-Co ). Similarly, the pure metal oxide exhibited poor activity to the ion-promoted metal
3
4
O catalyst was compared with its
3
O
4
3
O
4
3
O
4
oxide. See Table S2 in the supplementary material. Furthermore, the effect of gold loadings on the
catalyst was investigated. Under the same reaction conditions; 1-octene (8 mmol), catalyst (60 mg),
H
2
/CO 2:1, 40 bar, 140 °C, 16 h, THF (10 mL), the 5% Au/Cs-Co
3
O
4
(1a) catalyst gave the best
activity (92% conversion and 94% selectivity) to 10% Au/Cs-Co
3
O
4
(58% conversion and 49%
selectivity). The series in the activity of the catalysts were also in good agreement with the low-
temperature reducibility and surface basicity of the nanomaterials (Fig. 3, 4, and 5), which has an
impact on the reaction turnover frequency (TOF). Thus, the observation due to the 1-octene
conversion and selectivity to the desired alcohol is similar to the turnover frequency of the respective
catalysts. The products of the 1-octene hydroformylation-hydrogenation were analyzed by GC-MS,
indicating the mass charge (m/z) corresponds to nonanol. According to the GC-MS library, the
fragmentation pattern matched the linear alcohol by 94%. Based on these results, the effect of
reaction parameters using 5% Au/Cs-Co
3
4
O catalyst was studied.

3
.2.1. Optimization of reaction conditions and substrate scope
The catalyst amount was varied from 20-80 mg, keeping other reaction conditions constant (Table
). From observation, increasing the catalyst amount to 80 mg decreased the product selectivity
3
slightly (Table 3, entry 4). On the other hand, reducing the catalyst amount to 40 mg did not show
any significant effect on the reaction (Table 3, entry 2 ). A further decrease to 20 mg led to a
considerable decrease in 1-octene conversion and alcohol selectivity (Table 3, entry 1). The
temperature effect towards alcohol selectivity and 1-octene conversion was studied, respectively.
The reaction temperature was varied from 120-160 °C. Interestingly, an increase in temperature to
1
60 °C, led to a significant decrease in the alcohol selectivity, with the 1-octene conversion
remaining almost unchanged. The low product selectivity (77%) observed at a higher temperature
of 160 °C, was due to the formation of the more hydrogenated substrate (octane) (Table 3, entry 7).
At a lower temperature of 120 °C, the considerable decrease in alcohol selectivity (58%) was
ascribed to isomerization and branched aldehyde formations (Table 3, entry 5).
From the carbon monoxide temperature-programmed reduction (Fig. 3a), the effect of carbon
monoxide on the reduction behaviour of the catalysts suggests that CO improves the reducibility of
cobalt-based catalysts [64]. As shown in Table S3, the effect of H /CO partial pressure was
2
investigated. Lowering the pressure to 20 bar, gave poor 1-octene conversion and low alcohol
selectivity (Table S3, entry 1). At pressure 40-60 bar, the branched aldehydes were fully converted
to alcohol and hydrogenated substrates, with lower alcohol selectivity observed at 60 bar partial
pressure (Table S3, entries 2-4). The reaction was successful under syngas H /CO (2:1), indicating
2
that the excess hydrogen may have contributed to the formation of unsaturated [HCo(CO)
intermediates on the catalyst surface, for alcohol synthesis to proceed smoothly.
y
]
As shown in Table S3, entry 5, a significant decrease in 1-octene conversion, and alcohol selectivity
was observed at 8h. At 16 h reaction time, the selectivity of alcohol reached 95%, with the highest
-1
turnover frequency achieved at 183 h , for 1-octene hydroformylation-hydrogenation reaction
Table 3, entry 2). On further increase to 20 h (Table S3, entry 7), the conversion increased
(
considerably with a slight decrease in the alcohol selectivity (85%). For different reaction time, the
decrease in selectivity can be attributed to the formation of branched aldehyde and hydrogenated
substrate products. The reaction rates for the 5% Au/Cs-Co
shown in Fig. 7d. A first-order reaction with respect to 1-octene concentration was obtained from a
straight-line plot of Ln(1-C ) vs. time (t), with a -k slope, where Ln(1-C ) = -0.16t + 0.03 and k is
the first-order rate constant.
3
O
4
catalyst at 140 °C and 40 bar, is
x
x

Using tetrahydrofuran as solvent resulted in the best activity compared to other solvents used in this
study (Table S4, entry 3). The high activity could be ascribed to its ether functionality, which we
believed to have played a vital role in the stabilization of the active species on the surface of the
catalyst. Therefore, the hydroformylation-hydrogenation of 1-octene optimized reaction conditions
are: 5% Au/Cs-Co
THF.
3
O
4
(40 mg), 1-octene (8 mmol) H /CO (2:1, 40 bar), 16 hr at 140 °C in 10 mL
2
To study the effect of the substrate structure on reactivity and to establish extensive applicability of
this approach, the reactions of other alkenes were examined using the 5% Au/Cs-Co (1a)catalyst
3
O
4
with the optimized reaction conditions. The use of the 1a catalyst is due to the fact that it was the
most active from the catalysts screening under the same reaction conditions (Table S2). From the
data summarized in Table 3, the selectivity of alcohols decreased slightly with an increase in carbon
chain lengths. For example, a 97% conversion with 98% alcohol selectivity was achieved for the
hydroformylation-hydrogenation of 1-pentene in 16 hr at 140 °C (Table, entry 8). However, the
selectivity decreased slightly to 96% for 1-hexene (Entry 9) and can be ascribed to faster formation
of hydrogenated substrates. For styrene, a decrease in the product selectivity was observed (Entry
1
1).
The catalytic performance of 5% Au/Cs-Co
hydroformylation-hydrogenation of 1-octene (Table 4). The catalyst reported in [43], under syngas
/CO (2:1), gave 93% conversion with 87% selectivity to the desired alcohol at a pressure and
3
O was compared with other catalysts for the
4
H
2
temperature of 56 bar and 170 °C, respectively. The reusability of this catalyst reached three cycles,
with a significant drop in conversion and selectivity. However, the ion-promoted catalyst reported
in this present study was highly reusable, and exhibited higher alcohol selectivity under milder
reaction conditions (120-140 °C and 40 bar), compared to other supported and homogeneous
catalysts reported in the literature [41,43,65].

Table 3 Effect of catalyst amount, temperature and substrate scope
Entry
Cat. 1a Temp. Conversion
Selectivity
(%) a
TOF
(h )
a
-1
(mg)
(°C)
(%)
Aldehyde Alcohol
(i:h) c
b
b
(l:b)
(l:b)
1
2
3
4
5
6
7
8
9
20
40
60
80
40
40
40
40
40
40
40
140
140
140
140
120
150
160
140
140
140
140
78
92
92
95
63
94
96
97
91
88
83
29 (30:70) 42 (72:28)
07:22 81
00
00
00
95 (72:28)
94 (70:30)
88 (70:30)
00:05 183
00:06 181
05:07 169
08:12 111
00:09 175
00:23 148
00:02 189
00:04 185
00:04 185
00:18 177
22 (22:78) 58 (70:30)
00
00
00
00
00
00
91 (70:30)
77 (68:32)
98 (80:20)
96 (70:30)
96 (72:28)
92 (65:35)
1
1
0
1
Reaction conditions: - 1-octene (8 mmol), H /CO (2:1), 40 bar, 16 h and THF (10 mL).
2
Substrate scope: 40 bar, 16 h. 1-pentene (Entry 8), 1-hexene (Entry 9), 1-heptene (Entry 10), and
styrene (Entry 11).
a
Selectivity and conversion.
Linear (l) and branch (b).
Hydrogenated (h) and isomerized substrate (i)
b
c

Fig. 7. Effect of (a) Catalyst amount, (b) Temperature, (c) Time and (d) Reaction rates for 1-octene
hydroformylation-hydrogenation (where C = % conversion). Reaction conditions:- 1-octene (8
mmol), catalyst 1a, H /CO (2:1), 40 bar, 16 h and THF (10 mL).
x
2
Table 4 Catalytic activity for hydroformylation-hydrogenation of alkenes with other catalysts
Catalyst
Conversion
%)
Selectivity Press.(bar)/Syngas
References
(
(%)
(H /CO)/ Temp. (°C)
2
Aldehyde Alcohol
Co-SiO
Co-Au/SiO
2
66.0
70.6
99.5
93.0
100
100
81
84.0
85.4
83.9
-
3.2
3.1
4.4
87.0
41.0
61
77.0
95.0
40/1:1/400
40/1:1/400
40/1:1/130
58/1:2/170
90/1:2/120
41/1:2/110
50/1:2/120
40/1:2/140
[58]
[58]
[58]
[43]
[45]
[41]
[65]
Present study
2
Au/Co
Fibrous Co
Au/Co /SiO
Co (CO)
Rh(CO)2acac]
% Au/Cs-Co
3
O
4
3
O
4
3
O
4
2
28.0
2
8
-
-
-
[
5
3
O
4
92.0

3
.2.2. Reusability study
The stability of a catalyst is a crucial requirement for its practical applications. Thus, the recycling
of the 5% Au/Cs-Co catalyst for the hydroformylation-hydrogenation of 1-octene with syngas
/CO (2:1) was conducted. The 1-octene was used as the model molecule for the reaction. Catalyst
separation was performed using simple filtration, washed thrice with THF, and heated in an oven at
50 °C for 1 h. The heating was done to remove the supernatant and reactivate the catalyst by
3
O
4
H
2
2
reopening of the mesopores. The procedure was repeated before reusing the catalyst; this is in
contrast to industrial homogeneous cobalt carbonyl-based catalysts, which forms insoluble cobalt
precipitates under syngas pressure, due to its instability. As shown in Fig. 8a, the activity of the
catalyst remained almost unchanged after four consecutive cycles, with the conversion and
selectivity sustained at 88% and 83%, respectively, indicating its high reusability. Although, the
slight decrease in alcohol selectivity observed was due to the formation of branched aldehyde and
hydrogenated substrate products. The p-XRD spectra (before and after use) were also used to
confirm these observations with not much difference observed, implying that the crystal structure of
the used catalyst was preserved (Fig. 8b). To further confirm the presence of the absorbed species
in the catalyst, the FT-IR analysis was carried out. Before this, the catalyst was removed, washed
once with 15 mL THF and dried in a vacuum oven at 70 °C for 2 h. Fig. 8c showed the appearance
-1
of broad peaks at 3498 and 1722 cm , assigned to alcohol (OH) and aldehyde (-C=O) stretching
-1
vibrations, respectively. The C-O bands at 2167 cm appeared in spectra, further confirming the
presence of carbonyl like species on the catalyst surface [66]. Weak peaks at the range of 2978-
-1
3
211 cm can be ascribed to the isomerized alkene (=C-H), alkane (-C-H), and aldehydic (-C-H)
-1
stretching bands. The metal bands (M-Co) at 561 and 659 cm also appeared in the spectra.

Fig. 8. (a) Reusability, (b) p-XRD (before and after use), and (c) FT-IR (before and after use) of 5%
Au/Cs-Co catalyst.
3
O
4
4
.0
Mesoporous Co
Co < Cs-Co
Discussion
3
O
4
and its ion-promoted counterparts (Ca-Co
3
O
4
< Li-Co
3
O
4
< Na-Co
3
4
O < K-
3
O
4
3
O
4
) were synthesized via inverse-micelle templated approach [46]. The hydrogen
and carbon monoxide temperature-programmed reduction were used for optimization to obtain the
best supports. The deposition-precipitation method was used for catalyst synthesis [52]. The
HRTEM images reveal a slight agglomeration of the AuNPs on Cs-Co
3
O
4
. Irrespective of the gold
loadings, the Cs-Co particles displayed an almost similar size around 15-30 nm, while the AuNPs
3
O
4
sizes were markedly affected by gold loadings (Fig. 2). The 2 and 5% gold loadings, led to a low
population of the AuNPs with sizes ranging from 2.1-6.0 nm on the support surface. In the case of

the 10% gold loading, the population of the AuNPs on the support surface increases (Fig. 2f). The
pore structure and BET surface areas of the catalysts were also markedly affected by gold loadings,
at 2, 5 and 10%, respectively (Table 1). The surface areas and pore sizes were observed to decrease
on increasing the Au loadings from 2-5%, with the least observed for 10%, due to partial blockage
of the pores. The catalytic data also reveals that an increase in gold loading up to 10 wt.%,
significantly lower the activity for 1-octene hydroformylation-hydrogenation, and could be ascribed
to the excess AuNPs in support matrix blocking the mesopores.
Fig. 8a shows that the insignificant decrease in 1-octene conversion and alcohol selectivity observed
for each cycle can be ascribed to handling loss of the catalyst. To further determine the provisional
release of the active sites from 5% Au/Cs-Co
3
4
O , the heterogeneity test on the composite catalyst
was performed for the hydroformylation-hydrogenation of 1-octene. Firstly, for the hot filtration
test, the solid catalyst was filtered upon completion of the reaction, and the supernatant was returned
to the reactor for 4 h keeping the optimized reaction conditions constant. The GC-FID results show
that the 1-octene conversion after filtration increased, indicating slight leaching of the metallic cobalt
into the reaction mixture, forming [HCo(CO) ] specie that re-catalyzed the hydroformylation-
y
hydrogenation reaction (Fig. S8). In addition, the ICP-OES was used to analyze the filtrate after its
fourth run and detected about 12.1 wt.% of the cobalt species in the solution. A comparison between
the 5% Au/Co
3
O
4
catalyst and its ion-promoted counterpart (5% Au/Cs-Co
3
4
O ) shows that the later
was highly reusable and stable after its fourth run, despite both catalysts leached in the process. This
further confirms the promotional effect of the alkali metal ions on the catalyst. See Fig.S9 in the
supplementary material for the reusability studies. Therefore, a combination of the hot filtration test
and reusability results reveal a little homogeneous contribution to the reaction due to the leaching
observed. The cyclic ether functionality of the tetrahydrofuran solvent was believed to have played
a significant role in the stabilization of the active species on the catalyst surface, which contributed
to the conversion and selectivity. Another hypothesis that the active sites may be [HCo(CO)
was confirmed by an immediate infrared analysis (Fig. 8c). A comparison between the fresh and
used catalyst (5% Au/Cs-Co ) under the same reaction condition reveals that a new peak at 2167
y
] specie
3
O
4
-1
cm due to the free C-O stretching vibrations appeared in the used catalyst, and can be assigned to
cobalt carbonyl species [66]. This further supports the possibility that the active specie on the catalyst
surface maybe [HCo(CO) ].
y
In this study, we believe the markedly enhanced catalytic activity of the catalyst, can be attributed
to the generation of coordinatively unsaturated [HCo(CO) ] carbonyl-like species, formed via
y

spillover of H and CO by gold nanoparticles on the catalyst surface. Furthermore, to determine the
reasons for the difference in the catalytic activity of the catalysts, the effect of reducibility
temperature on the nanomaterials was studied in detail (Fig. 3). The catalyst with the highest H
CO consumption at lower temperatures presented the best activity (Table 2, entry 4). Thus, the
correlation observed indicates that the high consumption of CO and H gas by the 5% Au/Cs-Co
catalyst might be contributed to its high yield of alcohol (95%) formed for each of the studied
catalysts. However, considering that the catalysts (5% Au/Cs-Co and 2% Au/Cs-Co ), almost
possessed close reducibility temperatures, correlation with the high catalytic activities and high CO
and H consumption is not inevitable. Therefore, other determining factors for the high impact on
catalytic activity is necessary. An evident correlation was found between the basicity (TPR-CO
and the reducibility temperature of the nanomaterials. The 5% Au/Cs-Co catalyst with more basic
2
and
2
3
O
4
3
O
4
3
O
4
2
2
)
3
O
4
sites, presented the best activity to the other catalysts with smaller basic sites (Table 2). Thus, these
findings further indicate that the AuNPs and alkali metal ions on the mesoporous support may have
played an essential role in the increased basicity and temperature reducibility of the catalyst. We
believe that the increase in the basicity of the ion-promoted catalyst, also facilitates the increase in
adsorption sites of the materials [49], thereby promoting the rapid activation of the syngas and
substrate on the catalyst, for hydroformylation-hydrogenation reaction to proceed smoothly. In the
case of the pure oxide (Co
be ascribed to its higher reducibility temperatures (Table 2, entry 1), poor basic sites and absence of
AuNPs to promote H/CO spillover in the formation of the surface-active [HCo(CO) ] specie. Thus,
with the correlation between the surface basicity, temperature reducibility of the catalyst, and the
formation of [HCo(CO) ] specie, we propose a reaction mechanism for 1-octene hydroformylation-
hydrogenation with H /CO (2:1) (Fig. 9).
3
O
4
-350 °C) investigated under this study, its low catalytic activity may
y
y
2
The mechanism involves different steps. In the first step (1), the significant role of the AuNPs in the
catalyst is assumed to provide spillover of hydrogen and carbon monoxide on its surface, in the in-
situ reduction of the Co
from the temperature reducibility experiments (Fig. 3a and b). Furthermore, the Co metal combines
with the hydrogen and carbon monoxide to form the surface-active species [HCo(CO) ] (1a). At step
2), an alkyl-metal complex (2a) is formed via alkene-metal complexation from the 1-octene (1a1)
O
3 4
to Co metal. The reduction behaviour of the H and CO was also evident
y
(
interaction with the active surface. The alkyl-metal undergoes hydrogenation via reduction to form
1
a hydrogenated substrate (alkane), as a side product (2a ). In the next step (3), the CO from the
surface-active catalyst interacts via insertion with the alkyl-metal to form an acyl-metal complex
(3a). If the absorbed CO on the active surface does not interact with the alkyl group to form the acyl

complex, the 1-octene hydroformylation-hydrogenation reaction may not have progressed smoothly.
In step (4), hydride transfer with the acyl-metal complex gives an aldehyde (4a), which undergoes
1
hydrogenation to form the linear alcohol product (nonanol) (4a ). Branched alcohol (3-
methyloctanol), was assumed to be formed via similar reaction pathway in Fig. 9.
Fig. 9. The plausible reaction mechanism for hydroformylation-hydrogenation of 1-octene on 5%
Au/Cs-Co
3
4
O catalyst
5
.0
Mesoporous M-Co
synthesized via inverse-micelle and deposition-precipitation methods, respectively, for the
hydroformylation-hydrogenation of 1-octene to nonanol, under syngas H /CO (2:1) pressure. The
catalytic activity was markedly affected by gold loadings (2, 5 and 10%) and the presence of alkali
ion-promoters on the support matrix. The 5% Au/Cs-Co catalyst was more active than the other
Conclusion
3
O
4
and Au/M-Co
3
O catalysts (M= Ca, Li, Na, K, and Cs) have been successfully
4
2
3
O
4
synthesized catalysts, with selectivity to the desired alcohol achieved at above 90% under milder
reaction conditions (120-140 °C and 20-40 bar). The significant role of the AuNPs was assumed to
provide CO and H spillover in the in-situ reduction of the mesoporous cobalt oxides to metallic
cobalt forming carbonyl complex specie [HCo(CO) ], that catalyzed the reaction. The
y

tetrahydrofuran gave the best activity as compared to the other solvents and believed to have played
a vital role in the stabilization of the active species on the catalyst surface. The stability of the catalyst
was retained after four consecutive cycles and could be used as a natural handling alternative to
industrial cobalt carbonyl complexes, which requires rigorous purification processes. In summary,
the synthesized cobalt-based catalyst has proven to be more advantageous to its homogeneous
counterpart, in terms of activity, reusability, process handling, catalyst recovery, and could
effectively carry out the hydroformylation-hydrogenation of alkenes to alcohols. The various
characterization tools employed for the catalysts provided an extensive understanding of the reaction
mechanism and catalyst evolution, which could find applications in the conversions of other alkene-
related compounds for value-added chemical synthesis.
CRediT statement
COO: Investigation; Formal analysis; Methodology; Writing – original draft; Writing – review & editing
BMM: Formal analysis; Methodology
CAA: Formal analysis; Methodology
RM: Conceptualisation; Methodology; Resources; Writing – review & editing; Supervision; Project
administration; Funding acquisition.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that
could have appeared to influence the work reported in this paper.
Conflict of interest
The authors declare no competing financial interest
Acknowledgements
We acknowledge The World Academy of Science (TWAS) and the National Research Foundation
(NRF) with unique grant number 116088 for the funding. Special thanks to the University of
Johannesburg and SHIMADZU, South Africa, for the use of their instruments.
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