Covalent grafting of cobalt carbonyl cluster on functionalized mesoporous SBA- 15 molecular sieve and its applications towards hydroformylation of 1-octene

Article abstract of DOI:10.1016/j.molcata.2016.08.016

Cobalt carbonyl cluster was anchored through a facile route on the surface of organo-functionalized SBA-15 molecular sieve by post-synthetic approach. The successful grafting of organofunctional ligand and cobalt carbonyl cluster was evident thorough ²⁹Si-MAS NMR, ¹³C-MAS NMR and FT-IR studies. The resultant cobalt clusters anchored functionalized SBA-15 material (SBA-15-RCo) showed promising catalytic activity on hydroformylation of 1-octene (97% conversion) with excellent selective towards hydroformylated products (90%).

Full text of DOI:10.1016/j.molcata.2016.08.016

Accepted Manuscript
Title: Covalent grafting of cobalt carbonyl cluster on
functionalized mesoporous SBA- 15 molecular sieve and its
applications towards hydroformylation of 1-octene
Author: Maqsood Ahmed Ayyamperumal Sakthivel
PII:
S1381-1169(16)30340-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.08.016
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Journal of Molecular Catalysis A: Chemical
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Accepted date:
20-4-2016
4-8-2016
14-8-2016
Please cite this article as: Maqsood Ahmed, Ayyamperumal Sakthivel, Covalent grafting
of cobalt carbonyl cluster on functionalized mesoporous SBA- 15 molecular sieve and
its applications towards hydroformylation of 1-octene, Journal of Molecular Catalysis
A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.08.016
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Covalent grafting of cobalt carbonyl cluster on functionalized
mesoporous SBA- 15 molecular sieve and its applications towards
hydroformylation of 1-octene
a
a,b
Maqsood Ahmed, Ayyamperumal Sakthivel *
aDepartment of Chemistry, Inorganic Materials & Catalysis Laboratory,
University of Delhi (North Campus) Delhi-110007, India
b
Department of Chemistry, School of Mathematical & Physical Sciences, Riverside Transit
Campus, Padnekkad, Nileshwar, Kasaragod District - 671314
Email: asakthivel@chemistry.du.ac.in / sakthivelcuk@cukerala.ac.in
Graphical abstract
A new method was developed for the grafting of cobalt carbonyl clusters on the surface of SBA-
1
5 using [(2-propynylcarbonate)-propyl]triethoxysilane organosilane ligand as linker. The
resulting cobalt carbonyl clusters grafted material showed a promising catalytic activity (97 %
conversion) towards hydroformylation of 1-octene with 90% selectivity for hydroformylated
products.

Highlights


Cobalt carbonyl cluster was covalently grafted on the surface of SBA-15.
Organosilane linker (2-propynylcarbonate)-propyl]triethoxysilane used for
grafting.


Covalent grafting evident from FT-IR and solid state NMR studies.
Resultant Materials showed promising activity for hydroformylation of 1-octene.
Abstract
Cobalt carbonyl cluster was anchored through a facile route on the surface of organo-
functionalized SBA-15 molecular sieve by post-synthetic approach. The successful grafting of
2
9
13
organofunctional ligand and cobalt carbonyl cluster was evident thorough Si-MAS NMR, C-
MAS NMR and FT-IR studies. The resultant cobalt clusters anchored functionalized SBA-15
material (SBA-15-RCo) showed promising catalytic activity on hydroformylation of 1-octene
(97% conversion) with excellent selective towards hydroformylated products (90 %).
Keywords: Cobalt carbonyl cluster, organosilane grafting, hydroformylation, heterogenization
1
. Introduction
Hydroformylation is one of the important industrial process involve by the addition of formyl
group to olefins for the construction of aldehydes functionality. [1-2]. Transition metal ions
compounds such as cobalt [3] and rhodium [4] based complexes are widely used as a

homogeneous catalyst in hydroformylation reaction. Despite homogeneous catalysts show better
catalytic activity and selectivity, but limits there applications owing to the problems associated
with them such as separation of products from catalyst and regeneration of expensive transition
metal complexes, etc. In order to overcome these drawbacks, heterogenization of homogeneous
catalyst have been developed through immobilization of inorganic complexes on the surface of
solid support like microporous and mesoporous molecular sieves such as zeolite-Y, MCM-41,
MCM-48 and SBA-15 [5-14], hydrotalcite materials [15-17] and some metal oxides viz., ZnO
and MgO [18-20]. New approaches have been made to promote the covalent interaction of
homogeneous catalysts on the surface of mesoporous molecular sieves by prior functionalization
using organic ligands such as amines [7, 14, 15, 21], thiols, [16] and phosphanes [22-23,]
through alkoxysilanes as a linker. The resultant materials was extensively studied for the various
organic transformations such as Grubbs catalyst for olefin metathesis [24], molybdenum
carbonyl for alkene epoxidation [6], Rh immobilization for hydroformylation reaction [25, 26-
2
8] etc. In particular, the rhodium based organometallic complexes have been extensively
studied on heterogenization using various supports [7, 9, 11, 13-14, 21-23, 29-32]. These
heterogenized homogeneous catalysis quite often possess intrinsic properties of homogeneous
catalysts and the higher surface area, pore volume and better stability derived from
heterogeneous support. For example, the functionalized SBA-15 type molecular sieve showed
the promising catalytic activity in various organic transformation like olefin epoxidation [6, 26-
2
8], olefin metathesis [24], Fischer–Tropsch synthesis [33] etc. Among the various supported
material’s rhodium based organometallic catalysis loaded on SBA-15 mesoporous silica showed
good activity for olefin hydroformylation [14, 25]. Although rhodium based catalyst showed
significantly better activity and selectivity in presence of phosphine or modified phosphine

ligand, attention still focused on development of phosphine free cobalt carbonyl complexes.
However, to the best of knowledge there is no report on heterogenization of cobalt carbonyl
cluster (Co
2
(CO) ). Among the various support known in the literature mesoporous molecular
8
sieve SBA-15 is chosen as support for the present study due to its excellent thermal stability,
large surface area and pore diameter, availability of more free silanol groups (-OH) on surfaces
which facilitate effective grafting of organosilane linker [14,25]. Here with first time we are
reporting the heterogenization of Co
2
(CO) cluster on the surface of SBA-15 possessing
8
covalently connected organosilane ligand with terminal alkyne group. The organosilane ligand
was grafted onto the surface of mesoporous SBA-15 molecular sieves by simple post synthesis
grafting technique (Scheme 1) and subsequently Co
2
(CO) was anchored. The resulting material
8
was systematically characterized by various analytical and spectroscopic techniques. The
catalytic activity of the cobalt cluster anchored material was investigated for hydroformylation of
1
-octene using syn-gas mixture in liquid phase medium.
2
. Experimental procedure:
Mesoporous SBA-15 molecular sieve was synthesized as per the procedure described elsewhere
34]. The organosilane ligand (R) was prepared as per the procedure described in literature (see
[
Fig. S1) [35, 36]. About 3 mmol of organosilane was introduced into 1 g of calcined SBA-15
molecular sieves and the grafting was carried out by using standard Schlenk techniques under
1
nitrogen atmosphere [34]. The structure of organo-silane ligand was confirmed with HNMR
(Fig. S2), the organosilane ligand grafted SBA-15 was represented as SBA-15-R. The organo-
functionalized material (SBA-15-R) (1g) was treated with 1 mmol of Co (CO) clusters in dry
2
8
dichloromethane (DCM) for 1 hour under nitrogen atmosphere. Binuclear cobalt cluster
2
coordinated to the terminal alkyne group of the grafted ligand R in µ
2
˗Ƞ mode (Scheme 1).

Subsequently, the solvent was removed under vacuum and the resultant cobalt cluster grafted
2
sample was represented as SBA-15-RCo. For comparison and to confirm the µ
2
˗Ƞ mode of
bonding of cluster complex, homogeneous catalyst (RCo; Fig. S1) was also prepared by reacting
Co (CO) cluster with organosilane ligand (R) in DCM forming siloxane functionalized cobalt
2
8
cluster with the modified procedure described in literature [36] and was represented as RCo. The
1
homogeneous complex (RCo) was confirmed by H NMR (Fig. S3), which is in agreement with
literature value. More details experimental protocols are summarized in the supplementary
information (ESI). The cobalt content in the resultant material was calculated with the help of
atomic absorption spectroscopy and was found to be 3.6 wt. %, which is similar concentration of
the complex introduced in the silane linker modified samples.
The prepared catalyst was utilized for the hydroformylation of olefin viz., 1-octene in a
stainless steel autoclave (Amar equipment India Pvt. Ltd.). In a typical procedure, required
amount of catalyst SBA-15-RCo (100 mg) containing 0.1 mmol of Co, 1-octene (0.0191 M; 3ml)
and the solvent (Toluene 3ml ) were charged into the reactor under the nitrogen atmosphere
and was stirred at 250 rpm. The reactor was purged with syngas with CO and H ratio of 1:1 with
2
desired pressure and the temperature (100⁰C) with constant stirring. After the completion of
reaction the reactor was cooled to room temperature and depressurized. The product was
separated and was analyzed by gas chromatography equipped with FID detector connected to
HP-5 capillary column (3m; HP-5). Different products such as isomerized, hydrogenated and
hydroformylated was confirmed using GC-MS. The recyclability of the catalyst was studied after
the reaction catalyst was washed with toluene and reaction at same condition after washing with
toluene.

3
. Results and Discussion
Powder XRD pattern of the calcined SBA-15, organosilane linker grafted SBA-15-R and
cobalt carbonyl anchored SBA-15-RCo exhibit well resolved X-ray reflections characteristics of
the hexagonal mesoporous SBA-15 in 2θ range of 0.5‒2.0⁰ (Fig. 1) [34, 37]. The broadening
peak intensities corresponding to (100) plane is evident after grafting of ligands and cobalt
cluster suggesting the grafting of the organosilane ligand R and cobalt carbonyl cluster present in
the channel of SBA-15 framework. The mesoporous SBA-15 structure of the material was found
to be remain intact ever after several catalytic runs reaction (used SBA-15-RCo). There is shift in
peaks towards higher angle in the XRD spectra of used catalyst, which might be due to presence
organic reactant molecules in the channels. The FT-IR spectra of SBA-15, SBA-15-R, SBA-15-
RCo and used SBA-15-RCo samples are shown in Fig. 2. All the samples exhibit vibrational
bands characteristic of stretching frequency of framework mesoporous silica [38]. Two
‒
1
additional bands were observed in SBA-15-R and SBA-15-RCo at 2848 and 2927 cm , which is
typical for the C‒H stretching vibration of the covalently grafted organosilane ligand (R). The
-1
additional vibrational band appeared at 2983 cm in case of SBA-15R corresponding to C‒H
stretching frequency of the sp hybridized alkyne functionality present in the organosilane linker.
The above vibrational band corresponds to alkyne moiety was reduced in cobalt cluster anchored
(SBA-15-RCo) sample suggesting the cross coupling of cobalt carbonyl complex with alkyne
group of the grafted organosilane ligand (R) as similar to the homogeneous complex (RCo)
‒1
reported in literature [36]. The peaks around 2029, 2060, and 2100 cm appeared in SBA-15-
RCo are attributed to CO stretching of the grafted cobalt carbonyl [36, 39]. Two strong
‒1
vibrational peaks observed at 1710 and 1540 cm in SBA-15R and SBA-15-RCo are due to the
CO stretching and NH bending mode of the functional group present in the grafted organosilane

ligand. The bands corresponding to the CO stretching of cobalt carbonyl retained in the IR
spectra of used catalyst suggesting the presence of grafted cobalt carbonyl present in the organo-
functionalized SBA-15 framework even after several catalytic runs.
The N
2
sorption isotherms of all the samples are displayed in Fig. 3. The samples SBA-15, SBA-
hysteresis loop due to the
1
5-R and SBA-15RCo all show the typical type VI isotherms with H
1
capillary condensation of nitrogen in the mesopores channel of ordered mesoporous material.
The pure SBA-15 sample shows multi-layer adsorption with sharp increase in the volume
adsorbed in the relative pressure (p/p ) of 0.7, which is characteristic of mesopores. On the other
o
hand SBA-15-R and SBA-15RCo display drastic decrease in the capillary condensation of
nitrogen and further shift in the isotherms towards low relative pressure suggesting presence of
organosilane ligand (R) and cobalt carbonyl cluster complex grafted on the internal surface of
SBA-15. Further the hysteresis loop in the isotherms of used catalyst (after three runs, used
SBA-15-RCo) is considerably reduced suggesting accumulation of some organic molecules in
the channel of SBA-15 (Fig. 3). The textural properties such as BET surface area, micropore
volume and total pore volume are summarized in Table 1. The pure sample of SBA-15 shows the
2
-1
2 -1
BET surface area around 769 m g which decrease drastically in the SBA-15-R (347.7 m g )
2
-1
and SBA-15-RCo (178.4 m g ), which is in agreement with FT-IR spectra. The pore diameter of
pure SBA-15 was decreased after loading of ligand and cobalt carbonyl complex.
The grafting of organosilane on the surface of SBA-15 was further confirmed through solid state
2
9
13
Si MAS-NMR and C MAS-NMR spectroscopy. The parent SBA-15 and grafted SBA-15-R
exhibits three broad peaks at -111, -103 and -97 ppm (Fig. 4) which are attributed to Q , Q and
sites of the silicon present in the framework (Q = Si(OSi) (OH)4-n ) [40]. Three additional
peaks evident for the SBA-15-R sample around -68, -60 and -52 ppm which can be assign to the
4
3
Q
2
n
n

organosilane sites such as T
3
, T
2
and T
1
sites (T
m
= RSi(OSi) (OR)3-m) ) [6, 40,] which support
m
that grafting of organosilane on the surface of SBA-15 molecular sieve. The grafted ligand
having cobalt cluster remains intact in used catalyst, which was confirmed through the presence
of T sites in spectrum of used SBA-15-RCo. Further 13C MAS- NMR spectrum (Fig. 5) of
organosilane ligand grafted material (SBA-15-R) showed prominent peaks around 73, 57, 51, 42,
2
2 and 7 ppm for various aliphatic carbons present in the grafted organosilane ligand. The peak
at 155 ppm is corresponding to the carbonyl carbon derived from the ligand, further an additional
peak appeared around 17 ppm might be due the unreacted ethoxy groups. It is well evident from
1
3
the C MAS NMR studies that the organosilane ligand is covalently grafted on the surface of
SBA-15 molecular sieve which is in agreement with FT-IR studies.
The thermal Stability of the SBA-15-RCo sample was followed using thermogravimetric
analysis (Fig. 6). The TGA showed total weight loss of around 35% in the cobalt carbonyl
anchored sample due to the loss of carbonyl groups and methyl, methylene, amide and carbonyl
functionality derived from ligand and grafted cluster respectively.
Thoroughly characterized SBA-15-RCo was studied for hydroformylation of 1-octene (Scheme-
2
1
). The effect of syn-gas pressure on hydroformylation of 1-octene over SBA-15-RCo studied at
00ºC and the results are summarized in Table 2. The increase in syn-gas pressure favor increase
in 1-octene conversion and maximum conversion (> 95) was observed at 65 and 85 bar pressure
Table 3). Further, at low pressure hydro-isomerized products were obtained as major products,
(
when the syn-gas pressure increase exclusive formation of hydroformylated products was
evident. In addition the normal / iso- aldehyde ratio is found to increase with increase in syn-gas
pressure. Thus subsequent reactions were carried out at 65 bar pressure of syn-gas. The optimum
condition SBA-15-RCo showed conversion of 1-octene around 97% (Table 3). More than 90%

selectivity towards hydroformylated products was observed in which 53.5 % was linear aldehyde
1-nonanal). The other products were alcohols and isomerized product of 1-octene. The
(
-1
calculated turn over frequency (TOF) at optimum reaction conditions is about 23 h which is
almost similar to conventional catalyst [41, 23]. The recyclability of the catalyst was further
studied for three runs by separating the reaction mixture under nitrogen atmosphere and
subsequently introducing reactant molecules. It was found that the catalytic activity of the re-
cycled catalyst remains intact even after three runs with 91.2 % conversion. Considerable
decrease in selectivity of hydroformylated products were obtained, however, the ratio of terminal
/
iso-aldehyde remains intact. For comparison reaction with homogeneous catalyst (RCo) was
also studied showing maximum 96.8 % conversion at 100 bar pressure with the selectivity for
hydroformylated products of 81.2%. Although homogeneous catalysts showed the similar
activity and TOF but subsequent run using homogeneous catalyst doesn’t show any activity.
Reaction with pure SBA-15 and without catalyst also didn’t yield any appreciable conversion
(Table 3).
Conclusion
In conclusion the present work established a simple and convenient route to anchored cobalt
carbonyl cluster on the surface of SBA-15. Powder XRD studies revealed the mesoporous SBA-
1
5 structure intactness after bulk cluster molecules. The presence of covalently grafted
2
9
13
organosilane ligand (R) and cobalt carbonyl cluster was evident through FT-IR, Si and C
MAS NMR studies. The resultant cobalt carbonyl cluster grafted material shows the promising
catalytic activity towards hydroformylation of 1-octene.
Acknowledgement

Authors express their sincere thanks to DST-SERB (EMR/2014/001214) India, for their
financial support. Authors are also thankful to USIC, University of Delhi, for the support
on instrumentation facility. Maqsood Ahmed is grateful to CSIR, India for his research
fellowship respectively.
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Figure Captions
Fig 1. Powder XRD pattern of SBA-15 (a), SBA-15-R (b), and SBA-15-RCo (c). and Used SBA-
1
5-RCo (d)
Fig 2. FT-IR spectra of SBA-15 (a), SBA-15-R (b), SBA-15-RCo (c) and used SBA-15-RCo (d).
Fig 3. N sorption isotherms of SBA-15 (a), SBA-15-R (b) and SBA-15RCo (c), Used SBA-15-
2
RCo (d)
2
9
Fig 4. Si MAS NMR spectra of SBA-15 (a) and SBA-15-R (b), Used SBA-15-RCo (c)
1
3
Fig 5. C MAS NMR spectra of SBA-15-R.
Fig 6. TGA profile of SBA-15-RCo.

Fig 1. Maqsood et. al.

Fig. 2. Maqsood et. al.

Fig. 3. Maqsood et. al.

Fig. 4. Maqsood et. al.

Fig. 5. Maqsood et. al.

Fig. 6. Maqsood et. al.

Scheme 1. Schematic representation of cobalt carbonyl grafting onto SBA-15 surface through
organosilane ligand

Scheme 2. Hydroformylation of 1-octene in presence of SBA-15-RCo

Table 1. Textural data of various SBA-15 grafted molecular sieves
Samples
Surface area
BET (m g )
Total
Pore diameter (Å)
2
-1
pore volume (cm-3g-1)
SBA-15
769
347
178
189
1.10
0.58
0.15
0.12
58.4
52.7
38.5
39.3
SBA-15-R
SBA-15-RCo
Used-SBA-
1
5-RCo

Table 2. Hydroformylation of 1-Octene in presence of SBA-15RCo at different pressure
-1
Pressure
bar)
Conv.
(%)
Selectivity (%)
Distribution of
Aldehydes
TOF (h )
(
CHO #
OH#
2.9
Iso#
81.0
4.7
1.0
0.0
--
(n : iso. )
2
.2
1
3
6
8
2
5
5
5
9.4
48.0
97.4
96.4
0.9
16.0
72.3
91.2
86.2
--
0.8
23.0
7.7
1.0
1.4
1.7
--
11.4
23.2
23.0
--
17.7
--
9
0
#
Reaction conditions: temperature = 100 ⁰C; 1-octene: 0.0191 mol. (3ml); catalyst: 100 mg; Time 8h; CHO =
#
#
hydroformylated products ; OH = Alcoholic products ; Iso. = isomerized products of 1-Octene, n= linear
aldehyde, and iso = branched aldehyde. * Reaction studied under identical condition without using any catalyst.

Table 3. Hydroformylation of 1-Octene on various Catalysts
-1
Catalyst
Conv.
%)
Pressure
(bar)
Selectivity (%)
Distribution of
Aldehydes
TOF (h )
(
CHO #
OH#
Iso#
(n: iso )
SBA-15-RCo
97.4
91.2
65
65
91.2
76.5
7.7
1.0
1.7
1.4
23.2
21.7
Recycled
21.8
1.5
SBA-15-RCo‡
RCo
96.8
100
90
81.2
11.4
4.3
1.8
33.5
-
Without
catalyst
0
.9
-
-
-
-
-
-
-
-
SBA-15
2.0
90
-
#
Reaction conditions: temperature = 100 ⁰C; 1-Octene: 0.0191 mol. (3ml); catalyst: 100mg; time 8h; CHO =
#
#
hydroformylated products; OH = Alcoholic products; Iso = isomerized products of 1-Octene, n= linear
‡
rd
aldehyde, and iso = branched aldehyde. 3 recycled.