A covalent triazine framework-based heterogenized Al-Co bimetallic catalyst for the ring-expansion carbonylation of epoxide to β-lactone

Article abstract of DOI:10.1039/c6ra26828e

Difficulties in product separation and ineffective recycling of the homogenous catalyst deter the mass production of β-lactones via carbonylation of epoxides. Herein, we address these issues, for the first time, using a recyclable heterogenized catalyst [bpy-CTF-Al(OTf)₂][Co(CO)₄] that efficiently converts propylene oxide (PO) to β-butyrolactone with high selectivity.

Full text of DOI:10.1039/c6ra26828e

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A covalent triazine framework-based
heterogenized Al–Co bimetallic catalyst for the
ring-expansion carbonylation of epoxide to b-
lactone†
Cite this: RSC Adv., 2017, 7, 4635
Senkuttuvan Rajendiran, Prakash Natarajan and Sungho Yoon*
Received 15th November 2016
Accepted 3rd January 2017
Difficulties in product separation and ineffective recycling of the homogenous catalyst deter the mass
production of b-lactones via carbonylation of epoxides. Herein, we address these issues, for the first
DOI: 10.1039/c6ra26828e
2 4
time, using a recyclable heterogenized catalyst [bpy-CTF-Al(OTf) ][Co(CO) ] that efficiently converts
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propylene oxide (PO) to b-butyrolactone with high selectivity.
b-Lactones are important intermediates for ring-opening poly- shown in Fig. 1b, would provide similar N
merization to form poly(b-hydroxyalkanoates), a naturally environment for Al cation and Co anion as those provided by
2
O
2
coordination
3
+
1
2
occurring biodegradable and thermoplastic polyester. Many the [(salph)Al(THF)
2
][Co(CO)
(Fig. 1a). Herein, we report the synthesis and excellent catalytic
among which catalytic ring-opening carbonylation of epoxides conversion of a heterogenized catalyst [bpy-CTF-Al(OTf)
4
]
(4) homogeneous catalyst
3
synthetic methods are available for preparing b-lactones,
2
]
has received considerable attention because of its ability to [Co(CO) ] for the carbonylation of PO into b-butyrolactone. In
4
4
produce enantiopure b-lactones. Over the past several decades,
it has been reported that Co (CO) precursors catalyze the
2
8
5
carbonylation of epoxides to lactones. Particularly, Coates et al.
employed homogenous bimetallic Lewis acid–base salts [(salph)
0
+
M(THF)
2
][Co(CO)
4
] (salph ¼ N,N -o-phenylene bis(3,5-di-tert-
3
3+
butylsalicylideneimine), M ¼ Al , Cr ), as catalysts for the
conversion of epoxides to lactones, which are the best catalysts
4
a,6
to date for epoxide carbonylation.
However, difficulty in
product separation and poor catalyst recycling may signicantly
limit their practical utilization. To overcome these drawbacks,
heterogenization of the active sites of previously reported
homogenous catalysts on the solid support, guarantying high
catalytic efficiency with facile separation, is highly desired.
In recent years, covalent triazine frameworks (CTFs) have
7
attracted much attention as heterogeneous catalytic supports,
because they have a high surface area, stability, large pore
8,9
volumes and structural tunability. In particular, bipyridine
bpy)-based CTF (bpy-CTF, represented as 1), which contains
(
bipyridine ligand motifs in pore walls of the framework as
shown in Scheme S1 (ESI†), has potential to form complexes
10
with N–N coordination environment. Hence, we hypothesized
that immobilization of an {Al(OTf) }[Co(CO) (OTf
2
4
]
¼
triuoromethane sulfonate) motif on the bpy-CTF system, as
Department of Bio & Nano Chemistry, Kookmin University, 861-1 Jeongneung-dong,
Seongbuk-gu, Republic of Korea. E-mail: yoona@kookmin.ac.kr
Fig. 1 Structural representation of salph-based (a) homogeneous
[(salph)Al(THF) ][Co(CO) ] (4) and (b) relevant structural features of the
heterogenized catalyst [bpy-CTF-Al(OTf) ][Co(CO) (3) (white:
2
4
†
Electronic supplementary information (ESI) available: Detailed synthetic
2
4
]
procedures, characterization of 1, 2 & 4 and detailed experimental procedure of
carbon, blue: nitrogen, pink: aluminum, red: (OTf), maroon: cobalt,
and green: oxygen; hydrogen atoms are omitted for clarity).
PO carbonylation. See DOI: 10.1039/c6ra26828e
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addition, facile separation and sequential recycling of the
catalyst was demonstrated.
The stacked 2D layers of porous 1 material was synthesized
9,10
by following the reported procedures. Pore size distribution
of as prepared 1 was analyzed using Barrett–Joyner–Halenda
method, elucidating a predominant presence of pores with an
average pore size of 2.4 nm (Fig. S2 and Table S1 in ESI†). Upon
treatment of Al(OTf)
3
with 1 in methanol solution and
sequential reuxing leads to only 0.57 wt% of metal immobili-
zation (Table S2, ESI†). To increase the amount of Al(OTf)3
immobilization on 1, a hydrothermal method was used for
much vigorous reaction condition, and yielded [bpy-CTF-
3
Al(OTf) ] (2) as a black precipitate. Scanning electron micros-
copy (SEM) images of 2 show irregularly shaped blocks with the
mean size of >35 (5) mm (Fig. S3a, ESI†). The energy-dispersive
spectroscopy (EDS) mapping demonstrates the uniform distri-
bution of aluminum, sulfur and uorine atoms throughout the
framework while maintaining the ratio of ca. 1 : 3 : 9 (Fig. S3b–
d and Table S3, ESI†), which suggested the formation of 2 as
expected (Scheme S1, ESI†). In addition, inductively coupled
plasma-optical emission spectroscopy (ICP-OES) analysis
revealed that the Al content is 3.78 wt%.
ꢀ
ꢀ
4
] anions,
To exchange OTf anions with exogenous [Co(CO)
a freshly prepared KCo(CO) solution was treated with a meth-
anolic solution of 2 at ambient temperature, resulting in a black
4
Fig. 2 SEM images of (a) 1 and (b) complex 3. (c) Al and (d) Co
elemental mappings of 3. High-resolution XPS spectra of complex ( )
precipitate (Fig. S4, ESI†). The infrared (IR) spectrum of [bpy- 4 and ( ) 3 (e) Al 2p peaks and (f) Co 2p peaks.
CTF-Al(OTf) Co(CO) ] (3) showed a CO stretching peak at 1878
2
4
ꢀ
1
cm , corresponding to the carbonyl groups in the tetrahedral
] (Fig. S5, ESI†), indicating the successful exchange of employed with special care to study the coordination environ-
OTf with [Co(CO)
ꢀ
[
Co(CO)
4
ꢀ
ꢀ
3+
1ꢀ
4
] . The SEM images of the isolated 3 as ment of Al and Co in 3. For comparison, the homogeneous
depicted in Fig. 2a and b, indicates that the irregular block catalyst 4 was analyzed and the binding energies (BEs) corre-
3
+
3/2
1ꢀ
1/2
1ꢀ
3/2
shape of the 1 was maintained. A uniform distribution of sponding to Al 2p , Co 2p and Co 2p peaks were
aluminum, uorine, sulfur and cobalt atoms throughout the found to be 73.8, 780.7 and 796.4 eV, respectively (Fig. 2e and f).
framework was conrmed via EDS mapping (Fig. 2c and d). As shown in the Fig. 2e and f, the heterogeneous catalyst 3 also
3
+
3/2
1ꢀ
1/2
1ꢀ
3/2
Using ICP-OES, the precise contents of Al and Co were has the Al 2p , Co 2p and Co 2p ions with BE values
measured to be 3.76 and 2.67 wt%, respectively. (The Al and Co of 73.8, 780.7 and 796.4 eV respectively, which was well matched
+
3+
contents calculated based on full occupancy of {Al(OTF)
2
} and with homogeneous catalyst 4. (The additional Al peak at
units on the 1 are 3.82 and 8.34 wt%, respectively.) 74.3 eV in Fig. 2e of catalyst 3 may originate from the Al(OTf)3
ꢀ
[Co(CO)
4
]
Interestingly, these values are lower than the calculated ones. It species.) These results collectively reiterate that the heteroge-
indicates that each CTF ring was immobilized with one neous 3 has similar environment to that of homogeneous
[
(
Al(OTf)
ESI†).
To understand the change of pore volume and surface area a stainless steel tube reactor using the weakly coordinating
upon immobilization, nitrogen adsorption/desorption polar solvent (dimethoxy ethane, DME), which is the best
2
Co(CO)
4
] and two Al(OTf)
3
unit as depicted in Fig. S6 catalyst 4.
Carbonylation of PO using catalysts was performed in
measurements were performed (Fig. S7, Table S1, ESI†). In the solvent reported for carbonylation of an epoxide into
2
4,6
case of 1, surface area and pore volume were found to be 684 m
g
a lactone. Initially, carbonylation of PO was studied using the
ꢀ1
3
ꢀ1
ꢁ
and 0.40 cm g , respectively. Whereas aer immobiliza- reference complex 4 (1 mol% to substrate) in DME, at 50 C
tion, the surface area and the pore volume were reduced to 203 under 6.0 MPa of CO which resulted in 91% conversion to b-
2
ꢀ1
3
ꢀ1
m
g
and 0.14 cm g , respectively. It possibly originates butyrolactone and acetone in 9 : 1 ratio, which is very close to
ꢀ
4
from the successful immobilization of [Al(OTf)2]
Co(CO) ] unit on the 1, as we planned. It is worth to note that, study with 4, the activity of the as-prepared heterogeneous
as depicted in Fig. S6 (ESI†), the presence of [Al(OTf) ] and catalyst 3 was evaluated, using the same reaction conditions of
Co(CO)
and that of the reported result (Table 1, entry 1). Aer the initial
ꢀ
[
4
ꢀ
2
ꢀ
[
4
]
ions did not completely block the cavities of 1 entry 1. The catalyst 3 carbonylated PO to b-butyrolactone with
leaving possible channels for small molecules such, as PO and 46% conversion and 83% selectivity (entry 2). When decreasing
CO, to reach the active catalytic sites. the ratio of the substrate to catalyst (S/C) to 30, complete
To conrm whether 3 has similar active site as homogenous conversion was achieved, and interestingly, catalyst 3 was able
catalyst 4, X-ray photoelectron spectroscopy (XPS) analysis was to convert PO to b-butyrolactone with 90% selectivity (entry 3). It
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a
Table 1 Catalytic ring expansion carbonylation of PO using 3 and 4
d
Selectivity (%)
ꢁ
d
Entry
Catalyst
Temp ( C)
Pressure (MPa)
Time (h)
Conversion (%)
b-Butyrolactone
Acetone
b
e
1
2
3
4
5
6
7
8
4
50
50
50
70
90
34
34
50
6.0
6.0
6.0
6.0
6.0
6.0
0.1
6.0
24
24
24
12
12
24
2
91
90
83
90
26
17
48
27
—
10
17
10
74
83
52
73
—
b
e
3
46
c
3
>99
c
e
3
92
c
3
>99
c
e
3
51
c
e
3
47
c,f
e
3
24
<1
a
b
c
d
1
Reaction performed in 2.5 mL of DME pressurized using CO at room temperature. S/C ¼ 100. S/C ¼ 30. Determined by H-NMR spectroscopy
e
f
with naphthalene as an internal standard. Remaining were unreacted epoxide. Hot-ltration test.
is noteworthy that the selectivity for b-butyrolactone was higher temperature the availability of CO in the pores of 3 may
increased when increasing the catalyst concentration.
be limited, which slow down the CO insertion into the Co–alkyl
To increase the rate of reaction and reduce the acetone bond b. The delayed CO insertion may lead to b-hydride elim-
formation, the catalytic activity of 3 was studied at different ination, followed by enolate protonation d and tautomerization
ꢁ
temperatures. Upon increasing the temperature to 70 C and to form acetone as the major product (Table 1, entry 4 & 5).
ꢁ
9
0 C, (Table 1, entry 4 & 5) the acetone formation was more
Aer the initial run, catalyst 3 was separated from the reac-
dominant than lactone. This may be because of the facile b- tion mixture via simple ltration and was washed with DME.
hydride elimination at higher temperatures that facilitates the Interestingly, FT-IR analysis of the recovered catalyst shows
ꢀ1
formation of acetone aer rearrangement (vide infra). To enable a carbonyl peak at 1878 cm , indicating that the catalyst is still
the commercial applicability of the heterogeneous catalyst 3, PO active for further carbonylation (Fig. S8, ESI†). SEM together
carbonylation was performed at ambient temperature with with EDS analysis, supports the uniform distribution of
various pressure (entry 6 & 7), surprisingly large quantity of aluminum, uorine, sulfur and cobalt atoms throughout the
acetone was formed.
whole sample (Fig. S9, ESI†). However, ICP-OES analysis
Based on the proposed mechanism of PO carbonylation revealed that Al and Co contents are 3.72 and 2.48 wt%,
Fig. 3), the catalytic cycle initiates the activation of PO by respectively, indicating the leaching of a small amount of
(
3
+
ꢀ
immobilized Al cation, followed by a nucleophilic attack of [Co(CO)
4
] anion from the catalyst 3 to the ltrate. Hot-ltration
ꢀ
[
Co(CO)
4
]
at the less hindered carbon of PO to form both experiment was performed to test for homogeneous contribu-
4
a,5f
aluminum-alkoxide and cobalt alkyl species b.
At moderate tions to the rate; the catalyst 3 was heated in 2.5 mL of DME at
ꢁ
reaction temperature, the available CO in the pores of 3 may 50 C for 24 h and the resulting mixture was ltered to recover
facilitate the migratory insertion of CO into the cobalt–alkyl the solution part. Carbonylation of PO was performed with the
ꢁ
bond and produces cobalt acyl intermediate c, which further ltrate at 50 C under 6.0 MPa of CO which resulted in >99% of
undergo ring closing to form b-butyrolactone. However, at PO exists in the solution, indicating that no catalytically active
Co species in the ltrate for PO carbonylation (Table 1, entry 8).
The recovered catalyst was used at least three times for
carbonylation at the same reaction condition as the rst cycle
and the results are shown in Table 2. Compared to the rst
cycle, the catalytic activity in recycle runs was maintained at ca.
9
1%. Slight reduction of activity may originate from partial
ꢀ
discharge of the [Co(CO)
4
]
anion from the catalyst in the
course of separation. It is noteworthy that the relative amount of
acetone compared to lactone in recycling runs increases. It may
ꢀ
4
originate from previously described discharge of [Co(CO) ]
3
+
anion while maintaining the amount of Al cation unaffected,
which leads to b-hydride elimination augmented to form more
acetone. To check the possibility of the regeneration of the
recovered catalyst with comparatively reduced amount of
Fig. 3 A plausible mechanism of PO carbonylation and acetone
formation from a common intermediate.
ꢀ
[Co(CO)
4
] , it was subsequently treated with fresh K[Co(CO)
4
]
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a
Table 2 Recycling ability of catalyst 3
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Selectivity (%)
3
b
Cycle
Time (h)
Conversion (%)
b-Butyrolactone
Acetone
1
2
3
24
24
24
24
>99
92
81
90
88
84
90
10
12
16
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4
96
a
Reaction conditions: DME ¼ 2.5 mL, S/C ¼ 30, CO ¼ 6.0 Mpa,
ꢁ
b
1
c
temperature ¼ 50 C. Determined by H-NMR spectroscopy. Aer
rd
the 3 run, the catalyst 3 was regenerated with fresh K[Co(CO)
4
] (see
ESI).
aer the third run (see the procedure in ESI†). Interestingly, it is
found that the regenerated catalyst converts PO into the b-
butyrolactone (90%) with over all conversion efficiency of 96%
entry 4). These results show that the heterogeneous catalyst 3
has an ability to retain their activity for effective carbonylation,
(
which make
application.
3 an attractive catalyst towards industrial
In summary, the rst heterogenized complex [bpy-CTF-
Al(OTf) ][Co(CO) ] was synthesized by immobilizing Al(OTf) on
the covalent organic framework and subsequent exchanging of
2
4
3
ꢀ
ꢀ
the [OTf] anion with the [Co(CO) ] anion. This heterogenized
4
catalyst 3 exhibited excellent activity under mild conditions
6.0 MPa and 50 C), with a high selectivity of 90% for the ring
ꢁ
(
expansion carbonylation of PO to b-butyrolactone. More
importantly, the activity and selectivity of the heterogeneous
catalyst were comparable to those of the homogeneous catalyst.
Furthermore, the catalyst was readily separated from the
product and repeatedly used for several runs. These results
demonstrate that the heterogenized catalyst 3 might pave the
way to achieve sustainable epoxide carbonylation on an indus-
trial scale. Further studies focused on increasing the rate,
selectivity for b-lactone derivatives using heterogenized cata-
lysts are under progress.
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Acknowledgements
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We acknowledged nancial support by the C1 Gas Renery
Program through the National Research Foundation of Korea
(
(
NRF) funded by the Ministry of Science, ICT & Future Planning
2015M3D3A1A01064879).
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4638 | RSC Adv., 2017, 7, 4635–4638
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