Design and synthesis of a new magnetic metal organic framework as a versatile platform for immobilization of acidic catalysts and CO2fixation reaction

Article abstract of DOI:10.1039/d1nj02140k

In this study, a new magnetic metal organic framework (MNP@MOF) with a core-shell structure has been introduced as an efficient and versatile platform for immobilization of Preyssler (H₁₄[NaP₅W₃₀O₁₁₀]) heteropolyacid (PR HPA). The chemical structure of the nanocatalyst was analyzed by using different techniques, including HRTEM, TEM, HRTEM mapping, SEM, EDX, TGA, XRD, VSM, BET and ICP. These analyses confirmed the core-shell and spherical structure of the catalyst and successful immobilization of PR HPA on its surface. After complete characterization, the efficiency of the catalyst was tested for the synthesis of cyclic carbonates by the chemical fixation of carbon dioxide. Different epoxides were converted to cyclic carbonates in the presence of 0.4 mol% of catalyst and a 0.3 MPa CO₂pressure under solvent-free conditions. To date, this reaction has been performed with various heterogeneous catalysts, but this is the first report on the use of PR HPA and MNP@MOF for this reaction.

Full text of DOI:10.1039/d1nj02140k

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Design and synthesis of a new magnetic metal
organic framework as a versatile platform for
immobilization of acidic catalysts and CO2
fixation reaction†
Cite this: DOI: 10.1039/d1nj02140k
ab
ab
c
Faezeh Taghavi,‡ Amir Khojastehnezhad, ‡* Reza Khalifeh,
c
c
ab
Maryam Rajabzadeh, Fahimeh Rezaei, Khalil Abnous
Seyed Mohammad Taghdisi
*
and
de
*
In this study, a new magnetic metal organic framework (MNP@MOF) with a core–shell structure has
30
been introduced as an efficient and versatile platform for immobilization of Preyssler (H14[NaP W O110])
5
heteropolyacid (PR HPA). The chemical structure of the nanocatalyst was analyzed by using different
techniques, including HRTEM, TEM, HRTEM mapping, SEM, EDX, TGA, XRD, VSM, BET and ICP. These
analyses confirmed the core–shell and spherical structure of the catalyst and successful immobilization
of PR HPA on its surface. After complete characterization, the efficiency of the catalyst was tested for
the synthesis of cyclic carbonates by the chemical fixation of carbon dioxide. Different epoxides were
Received 2nd May 2021,
Accepted 15th July 2021
2
converted to cyclic carbonates in the presence of 0.4 mol% of catalyst and a 0.3 MPa CO pressure
DOI: 10.1039/d1nj02140k
under solvent-free conditions. To date, this reaction has been performed with various heterogeneous
catalysts, but this is the first report on the use of PR HPA and MNP@MOF for this reaction.
rsc.li/njc
performed to present new catalytic systems for the chemical
Introduction
9–13
fixation of CO
2
,
although owing to the high importance of this
Nowadays, ‘‘global warming’’ has become a serious problem due reaction, the development of new methods is highly recommended.
1,2
to the increase of the CO
2
content in the atmosphere. So, the
3 4
Currently, Fe O magnetic nanoparticles (MNPs) have received
conversion of carbon dioxide into the useful chemistry materials is widespread attention due to their superparamagnetic properties,
3
one of the main goals in environmental chemistry. In this regard, high chemical stability, high Curie temperature, and low
1
4,15
the incorporation of carbon dioxide into the epoxide compounds toxicity.
and, consequently, the production of 5-membered cyclic carbo- and electrical features and also easy separation process, they have
On the other hand, due to their magnetic, mechanical
4
nates is the best way to solve the above-mentioned problem.
great potential for use in different fields of science and technology,
Carbonate derivatives are found in natural products and possess especially heterogeneous catalysis.
16–23
However, because of the
different worthwhile applications such as building blocks in high magnetic capacities, serious aggregations are observed in
organic synthesis, electrolytes in batteries, raw materials for these unique materials, which is one of the most significant
5
–8
24,25
cosmetics, green solvents, polymers, and pharmaceuticals.
limitations.
In order to overcome these shortcomings, a coat-
During the past few decades, extensive attempts have been ing process of MNPs can be an excellent approach for the synthesis
26–30
of well-dispersed MNPs with high magnetic properties.
On
a
b
Pharmaceutical Research Center, Pharmaceutical Technology Institute, Mashhad
University of Medical Sciences, Mashhad, Iran.
this basis, MNPs are coated with different organic and inorganic
materials such as silica, polymers, carbon materials, covalent
E-mail: akhojastehnezhad@yahoo.com, abnouskh@mums.ac.ir
Department of Medicinal Chemistry, School of Pharmacy, Mashhad University of
Medical Sciences, Mashhad, Iran
31–36
organic frameworks (COFs) or especially MOFs.
These
materials are used as shell structures to control the growth
and better dispersion of MNPs. More importantly, the design of
core–shell magnetic microspheres with nucleophilic or electro-
c
Department of Chemistry, Shiraz University of Technology, Shiraz, 71555-313, Iran
Targeted Drug Delivery Research Center, Pharmaceutical Technology Institute,
Mashhad University of Medical Sciences, Mashhad, Iran.
E-mail: taghdisihm@mums.ac.ir
d
philic functional groups on the shell can lead to the preparation
37–41
of various acidic or basic catalysts,
because these functional
e
Department of Pharmaceutical Biotechnology, School of Pharmacy, Mashhad
University of Medical Sciences, Mashhad, Iran
groups, such as hydroxyl, amine, carboxyl or halide groups, have
the potential to connect to a wide variety of organic and
†
‡
Electronic supplementary information (ESI) available. See DOI: 10.1039/d1nj02140k
These authors contributed equally to this work.
4
2–44
inorganic ligands by physical and chemical interactions.
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Scheme 1 Preparation of the catalyst (MNP@MOF–PR).
In the traditional heterogeneous catalysts, SiO
carbon materials and polymer substances have been extensively
employed as shell structures and have linked covalently to a number
2
, Al
2
O
3
or TiO
2
,
Results and discussion
Briefly, in this project, a new MNP@MOF with a core–shell
structure has been introduced as an efficient and versatile plat-
form for immobilization of PR HPA, an either Brønsted or Lewis
acid catalyst (Scheme 1). In a previous report, Tang and co-workers
reported these MOF structures (IRMOF-3) for coating palladium
45–51
of ligands and been exploited in organic synthesis.
Thus, it is of
prime importance to offer new magnetic shell platforms with
different functional groups to develop the heterogeneous catalysts.
MOFs are reputed as coordination crystalline materials, in
which the frameworks are constructed based on polyfunctional
65
NPs. Afterwards, Zhang et al. and also Movahedi and colleagues
developed the synthesis of magnetic IRMOF-3 for catalytic applica-
tions; however, they were not able to achieve the synthesis of core–
5
2,53
organic linkers and inorganic nodes.
In recent decades,
these porous materials have attracted considerable research
attention, not only due to their huge internal surface areas,
large density of active sites, high porosity and tailorable proper-
ties, but also for wide noteworthy applications in biomedical
technology, sensing platforms, energy storage, environmental
6
4,66
shell magnetic MOFs, especially IRMOF-3,
synthesized magnetic MOFs with composite structures. Thus, in
this research, at first, Fe MNPs were synthesized by using FeCl
and they only
3
O
4
3
67
and a hydrothermal process, and then, these synthesized MNPs
with a size below 50 nm were coated with amino-functionalized
MOFs (IRMOF-3) by the solvothermal reaction of zinc nitrate and
5
4–61
pollution, and particularly catalysis.
reported the synthesis of core–shell magnetic MOFs (Fe
MIL-100(Fe)) as an excellent catalyst for the Claisen–Schmidt
For example, Ke et al.
3 4
O @
65
2
-aminoterephthalic acid. Eventually, PR HPA was immobilized
6
2
into the amine groups on the surface of the core–shell
MNP@MOFs by ionic bonds and the final catalyst (MNP@MOF–
condensation reaction. Chang and co-workers developed the
preparation of AgNP-embedded Fe O @MOFs (MIL-100(Fe))
3
4
68–70
PR) was prepared (Scheme 1).
The structure of the catalyst was
with core–shell structures for the catalytic reduction of 4-nitro-
63
characterized by using different techniques, including HRTEM,
TEM, HRTEM mapping, SEM, EDX, TGA, XRD, VSM, BET, and
ICP, and explained in the following sections.
First, all the steps for the preparation of magnetic core–shell
catalysts were analyzed by FT-IR spectroscopy (Fig. 1 and
Fig. S1–S5, ESI†). In the first spectrum related to Fe O MNPs
phenol. In another study, Movahedi and colleagues introduced a
magnetic and acidic IRMOF catalyst. They designed a sulfamic acid-
functionalized Fe
benzopyranopyrimidine derivatives.
3 4
O –IRMOF nanocomposite for the synthesis of
64
Thereby, in this study, a magnetic MOF with amine functional
groups (IRMOF-3) on its shell structure was developed as a new
platform for immobilization of PR (H [NaP W O110]) HPA
3
4
(
Fig. 1a), there are two distinct sharp peaks at 3414 and 569
14
5
30
À1
cm that are attributed to the O–H stretching bands of the
hydroxyl functional groups on the surface of MNPs and Fe–O
(
Scheme 1). This acidic and novel catalyst can convert different
aromatic and aliphatic epoxides to cyclic carbonates in the
presence of CO pressure of 0.3 MPa and under solvent-free
6
7
bonds into the structure of MNPs, respectively. In the next
spectrum corresponding to MOFs (Fig. 1b), the existence of two
characteristic peaks at 3448 and 3337 cm is assigned to the
asymmetric and symmetric stretching vibrations of amine groups
and the strong peaks at 1577 and 1257 cm are ascribed to the
2
conditions (Scheme 2). It is worth mentioning that, to the best
of our knowledge, there are no published reports for the chemical
fixation of carbon dioxide with the use of PR HPA-immobilized
magnetic MOFs.
À1
À1
64,66
stretching vibrations of –COOH and C–N groups, respectively.
Interestingly, the third spectrum related to MNP@MOF exhibited
all characteristic peaks of MOF structures and due to the inter-
actions between MNPs and MOFs, all peaks are shifted to lower
wavenumber (blue shift) (for example, 3448–3431 and 3337–3322
in the amine moieties). These observations show that the amino
functional groups in the MOF structures are completely retained.
So, the use of the simple mixed solvothermal method is a great
route to build the well-defined core–shell MNP@MOF structures.
Scheme 2 Synthesis of cyclic carbonates in the presence of MNP@MOF–PR. Afterwards, the fourth spectrum of PR HPA (Fig. 1d) exhibited
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to one sharp peak that confirmed the successful ionic bonding of
71,72
PR HPA on the amine groups of the MOF structures.
HRTEM, TEM, HRTEM mapping, SEM and also EDX analyses
were used to evaluate the morphology and size and investigate
the core–shell structures and PR HPA immobilization (Fig. 2–4).
As shown in Fig. 1, all the electron microscopic analyses
confirmed the spherical and core–shell structures for the mag-
netic MOF catalyst (Fig. 2). As it is evident from the SEM images
(Fig. 1a and b), the morphology of the nanomagnetic structures
was somewhat changed after the coating process, and also, the
TEM and HRTEM images (Fig. 2c–e) confirmed the successful
formation of core–shell structures and proper linkage of PR HPA
on the surface of the MNP@MOF structures (darker points in
7
3,74
Fig. 2e).
These observations were similar to the previous
study about the immobilization of heteropolytungstate on amino-
75
functionalized core–shell MNPs. Besides, for further confirmation
of the core–shell nanostructures, selected area electron diffraction
76
(SAED) analysis was performed and is shown in Fig. 2f.
Another great analysis to confirm the core–shell structures
and elemental analysis is HRTEM mapping (Fig. 3). This figure
obviously showed the core–shell structures of magnetic MOFs
with all elements including iron and oxygen for the core
moieties and carbon, zinc and nitrogen related to the shell
parts. As we observed in this image, the morphology of the core
Fig. 1 FT-IR spectra of (a) Fe
HPA and (e) MNP@MOF–PR.
3
O
4
MNPs, (b) MOF, (c) MNP@MOF, (d) PR
À1
À1
distinct vibrations at 527 and 1091 cm for P–O bending and parts (Fe and O) is changed after coating with MOFs (C, Zn, and
77–79
stretching, respectively, 961 and 912 cm for W–O–W stretching N).
For further confirmation of the catalyst preparation, the
À1
70
and 781 cm for WQO stretching in the PR structures. Finally, EDX analysis of bare Fe O MNPs and MNP@MOF–PR is
3
4
in the final spectrum of PR HPA immobilized on MNP@MOF performed (Fig. 4). As observed in these figures, after coating
MNP@MOF–PR, Fig. 1e), there are all the characteristic peaks of the Fe O MNPs with MOFs and immobilization of PR HPA,
(
3
4
PR and MOFs and it is worth mentioning that the two peaks some new peaks corresponding to the MOF structures (C, Zn,
related to the amine groups in the third spectrum are converted and N) and PR HPA (Na, W, P) are observed in the EDX pattern
Fig. 2 SEM images of (a) Fe
3 4 3 4
O MNPs and (b) MNP@MOF, TEM images of (c) Fe O MNPs and (d) MNP@MOF, HRTEM image of (e) MNP@MOF–PR and
SAED pattern of (f) MNP@MOF.
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Fig. 5 XRD patterns of (a) Fe
3
O
4
MNPs, (b) MOF, and (c) MNP@MOF.
Fig. 3 HRTEM mapping of core–shell MNP@MOF.
is of prime importance to note that there are XRD peaks of
either Fe O MNPs or MOFs in the final XRD pattern of the
3
4
core–shell MNP@MOF materials, which confirm the proper
preparation of MNP@MOF nanomaterials (Fig. 5c); however,
some moderate shifts are observed in the MOF peaks which
probably are attributed to the physical interactions between
Fe
3
O
4
MNPs and MOF shell structures.
TGA analysis was performed to study the thermal stability of
Fe O MNPs, core–shell MNP@MOF and PR HPA-immobilized
3
4
core–shell MNP@MOF (MNP@MOF–PR) under a N flow and
2
running from room temperature to 600 1C (Fig. 6 and Fig. S6–S8,
3 4
ESI†). The first graph (Fig. 6a) related to Fe O MNPs exhibits
2.27% weigh loss at more than 100 1C, which can be attributed
to decomposition of hydroxyl functional groups on the surface
of MNPs. The second graph (Fig. 6b) corresponding to the core–
shell MNP@MOFs exhibits two step weight losses, in which the
first step that starts at about 100 1C is assigned to removal of
the remaining trapped solvents and the second weight loss that
begins at about 250 1C and stretches to 500 1C is attributed to
decomposition of MOF core structures. The total amount of
3 4
Fig. 4 EDX patterns of (a) Fe O MNPs and (b) the final catalyst (MNP@
MOF–PR).
of Fe
3 4
O MNPs (Fig. 4a) that confirmed the proper synthesis of
shell structures and excellent ionic linkage of PR HPA on the
8
0,81
surface of the MOF shells.
Fig. 5 exhibits the XRD patterns for the bare Fe
3 4
O MNPs,
pure MOF and core–shell MNP@MOF nanomaterials. The XRD
pattern of Fe O MNPs (Fig. 5a) shows the crystalline phase
3
4
characteristics with assignments such as (220), (311), (400),
(
6
422), (511) and (440) at 2y = 30.1, 35.6, 43.3, 53.3, 57.11 and
2.10 (JCPDS card no. 19–0629). The next XRD pattern corres-
8
2
ponding to the pure MOF materials with two sharp peaks at
around 2y = 6.8 and 9.6 with the lattice planes of (200) and (220)
6
5
3 4
confirms the successful synthesis of MOFs (Fig. 5b). Finally, it Fig. 6 TGA graph of (a) Fe O MNPs, (b) MNP@MOF, and (c) MNP@MOF–PR.
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weight loss in this graph is 12.61% and these observations showed a large decrease after immobilization of the PR HPA
showed that approximately 10% of the catalyst weight is MOF into the MNP@MOF structures. The surface area decreased
2
À1
2
À1
core structures. Eventually, the final TGA graph of PR HPA from 301.2 m g to 121.0 m g and the average pore volume
3
À1
3
À1
decorated MNP@MOF (Fig. 6c) is similar to the previous graph decreased from 0.483 cm g to 0.261 cm g , respectively.
and shows about 14.38% weight loss, which confirms that These observations confirmed the successful immobilization of
approximately 2.0% of PR HPA was immobilized on the surface PR HPA on the surface of the MNP@MOF structures.
6
4
6
4
of MNP@MOF. These results confirm the successful synthesis
of the catalyst and they are in agreement with the TEM and XRD tion was examined for the synthesis of cyclic carbonates by the
results. chemical fixation of carbon dioxide. In this regard, the reaction
After complete characterization of the catalyst, its applica-
The VSM technique was the next method for the evaluation of styrene oxide and carbon dioxide was selected as the model
of the magnetic properties of the synthesized materials. reaction. First, the CO2 was reacted with styrene oxide at a
Thereby, this analysis was also employed for the bare Fe O
pressure of 0.5 MPa without the catalyst and no product was
MNPs and the final catalyst including PR HPA decorated observed after 6 h of the reaction (Table 1, entry 1). Second, the
MNP@MOF. The Fe MNPs exhibited superparamagnetic model reaction was carried out in the presence of 0.8 mol% of
properties with a high saturation magnetization at about the MNP@MOF–PR catalyst at different temperatures (25, 50,
3
4
3 4
O
À1
À1
8
6 emu g . Interestingly, it was decreased to about 60 emu g
80 and 100 1C) (Table 1, entries 1–5). The highest amount of
after coating the MNPs with MOF core structures and immobi- 4-phenyl-1,3-dioxolan-2-one (98%) was obtained at 80 1C
7
3,83
lization of PR HPA.
synthesis of the catalyst and they are in agreement with TEM, have a significant effect on the reaction yield (Table 1, entry 5).
XRD and TGA results (Fig. 7). Subsequently, different amounts of the catalyst (0.4, 0.2 and
These outcomes confirm the successful (Table 1, entry 4) and the upper temperature (100 1C) did not
ICP analysis was examined for the evaluation of both the 0.08 mol%) were used in the model reaction at 80 1C (Table 1,
MOF and the PR HPA catalyst. On this basis, at first, the amount entries 6–8). The yields of the reaction were decreased with
of zinc obtained in the MNP@MOF sample was 5.261 wt%, these amounts and it can be concluded that the best amount is
which firstly confirms again the presence of Zn in the MOF core 0.8 mol% that was used in the first experiment. Then, the lower
2
structures and second, it is in accordance with HRTEM map- CO pressures were also investigated on the synthesis of
ping, EDX analysis and the TGA results. Next, the amount of W 4-phenyl-1,3-dioxolan-2-one (Table 1, entries 9 and 10). With a
in the fresh and reused catalysts (after five runs) was also decrease of the CO pressure to 0.3 and 0.1 MPa, the yields of
2
studied with this technique. It was 1.517 and 1.488 wt% for the reaction were decreased dramatically. To improve the
the fresh and reused catalysts, respectively. These outcomes are reaction conditions and increase the yields of the reaction,
in accordance with the TGA results, considering the total molecular the co-catalyst of tetrabutylammonium bromide (TBAB) was
weight of PR HPA. Besides, the insignificant difference of the exploited in the model reaction (entries 11–14). Interestingly,
amount of W between the fresh and reused catalysts confirms the 100% of the product yield was isolated in the presence of
2
stable structure of the catalyst and reveals that approximately no 10 mol% of TBAB and 0.5 MPa of CO pressure (Table 1, entry 11).
leaching of tungsten occurred during the CO fixation reaction. Afterwards, only two percent of the yield of the reaction was
2
The pore volume and BET surface area were analyzed using decreased with half the amount of TBAB (5.0 mol%, Table 1, entry
N adsorption/desorption under nitrogen gas (Fig. S9 and S10, 12). It is worth noting that, even with a decrease of the CO pressure
2
2
ESI†). Both the specific surface area and the pore volume to 0.3 MPa and of the catalyst amount to 0.4 mol%, the yield of the
reaction was excellent (96%) and this condition was chosen as the
a
Table 1 Screening of the reaction conditions for the synthesis of 3a
b
c
Temperature Catalyst
Co-catalyst Pressure Time Yield
Entry (1C) (mg, mol%) (mol%)
(bar)
(h)
(%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
25
25
50
80
100
80
80
80
80
80
80
80
80
80
—
—
—
—
—
—
—
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
3.0
1.0
5.0
5.0
3.0
5.0
6.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
4.0
2.0
2.0
2.0
2.0
—
(0.01, 0.8)
(0.01, 0.8)
(0.01, 0.8)
(0.01, 0.8)
(0.005, 0.4)
(0.0025, 0.2) —
(0.001, 0.08) —
(0.01, 0.8)
(0.01, 0.8)
(0.01, 0.8)
(0.01, 0.8)
66
87
98
98
92
76
52
70
68
100
98
96
82
—
—
10
5.0
0
1
2
3
4
(0.005, 0.4) 5.0
(0.001, 0.08) 5.0
a
b
c
Styrene oxide (10 mmol). Tetrabutylammonium bromide. Based
Fig. 7 VSM plot of (a) Fe
3
O
4
MNPs and (b) MNP@MOF–PR.
on the isolated yields.
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a
final reaction condition (Table 1, entry 13). However, with a decrease Table 3 Synthesis of cyclic carbonates in the presence of the catalyst
of the catalyst amount to 0.08 mol%, the yield of the reaction was
not satisfactory (Table 1, entry 14). Therefore, the best reaction
condition was the use of 0.4 mol% of the catalyst, at 80 1C and
b
c
Entry Epoxide
Product
Time (min) Yield (%)
0
.3 MPa of CO pressure and eventually 5.0 mol% of TBAB (Table 1,
1
120
150
96
97
2
entry 13).
Due to the significant effect of the co-catalyst on the cyclo-
addition reaction of epoxides with CO , the performance of
other co-catalysts such as tetrabutylammonium iodide (TABI),
tetrabutylammonium chloride (TBAC) and trimethylamine
2
2
3
4
(TEA) was compared with that of TBAB in the model reaction
at 80 1C and 3.0 bar pressure (Table 2). In the absence of the
co-catalyst, only 62% of the styrene carbonate was formed
under the reaction conditions. Hence, styrene carbonate 3a
was obtained with 90, 96, 76, and 59% yields, in the presence of
TABI, TBAB, TBAC and TEA, respectively. TBAB showed a
superior activity compared with other co-catalysts under similar
conditions (Table 2, entry 3).
Encouraged by the excellent activity of our catalyst (MNP@
MOF–PR) for the chemical fixation of carbon dioxide in the
lower CO2 pressure (0.3 MPa) and a higher yield of styrene
carbonate (96%), in the next step, the efficiency of the catalyst
was investigated with other epoxides (Table 3). The catalyst was
able to prepare the various epoxides with good to excellent
yields (87–98%) and in the presence of 5.0 mol% of TBAB and
finally without using any solvent.
150
98
90
96
97
5
6
120
1
1,84
Based on our previous studies on the fixation of CO2,
a
270
120
89
91
suggested mechanism for the synthesis of cyclic carbonates in
the presence of the catalyst (MNP@MOF–PR) is presented
(Scheme 3). As mentioned earlier, the PR HPA catalyst can act
8
5,86
87,88
7
8
as both Brønsted
and Lewis acid catalysts
due to either
proton sites or tungsten elements on its structure (Scheme 3,
routes 1 and 2). Hence, as depicted in route 1, owing to the W
sites on the catalyst, they can operate as the Lewis acid catalyst
and can coordinate to the epoxide and consequently, the
150
240
90
87
activated epoxide is able to react with Br and then CO , and
2
9
finally, cyclic carbonate is formed accordingly. The second path
is similar to the previous one, with the difference that the
proton sites of the catalyst (Brønsted acid) can activate the
reagents and produce the final products (route 2).
a
Expoxides (10 mmol), TBAB (5.0 mol%), catalyst (0.005 mg, 0.4 mol%)
b
at 0.3 MPa and 80 1C. All the products were charchetrized by HNMR
and CNMR (see the ESI for details). Isolated yields.
c
In addition, a plausible mechanism of the cycloaddition
2
reaction of epoxy styrene and CO catalyzed by MNP@MOF–PR
in the absence of TBAB is shown in Scheme 4. First, the epoxy
ring was activated by both the Brønsted and Lewis acid sites of
Table 2 Screening of the effect of other co-catalysts on the synthesis of
3
a
a
Temperature Catalyst
Co-catalyst Pressure Time Yield
Entry (1C)
(mg, mol%) (mol%)
(bar)
(h)
(%)
Scheme 3 The proposed mechanism for the synthesis of cyclic carbonate
in the presence of MNP@MOF–PR as both Lewis (route 1) and Brønsted
1
(0.005, 0.4)
—
3.0
3.0
3.0
3.0
3.0
2
2
2
2
2
62
90
96
76
59
(
route 2) acid catalysts with TBAB.
2
3
4
5
80
80
80
80
(0.005, 0.4) TBAI
(0.005, 0.4) TBAB
(0.005, 0.4) TBAC
(0.005, 0.4) TEA
the nanocatalyst, thereby leading to the formation of inter-
mediate (I). In the following step, the nucleophilic attack of
a
Based on the isolated yields.
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À
5 30
anion sites (H13[NaP W O110] ) of the nanocatalyst on the less
sterically hindered carbon atom of the epoxide (intermediate
I)) caused the ring-opening to form intermediate (II). Then,
(
acyclic carbonate (intermediate (III)) was formed by the nucleo-
philic attack of the oxygen atom of intermediate (II) to CO2.
Finally, the cyclic carbonate (IV) is achieved by intramolecular
cyclization without the co-catalyst.
The prepared nanomagnetic catalyst can be used in many
repeated model reactions. Thus, for the confirmation of this
claim, the stability and efficiency of the heterogeneous catalyst
were studied in the five repeated model reactions. On this
basis, after finalization of the CO2 fixation reaction, ethyl Fig. 8 Reusability of the catalyst for the model reaction.
acetate was added to the reaction mixture and MNP@MOF–
PR was separated from the reaction mixture by an external
no leaching of PR HPA as the catalyst occurred during the
reaction.
magnet and then it was rinsed many times with ethyl acetate
and H O and dried in a vacuum oven at 75 1C for 6 h for further
2
The hot filtration test was another analysis to confirm the
heterogeneous nature of MNP@MOF–PR in the chemical fixation of
carbon dioxide. Hence, the model reaction was studied again under
the optimized reaction conditions. After 60 min of the reaction
re-use. As it is evident from Fig. 8, the catalyst is able to
synthesize the carbonates after reuse five times without any
significant decrease in catalytic activity. Also, as mentioned
before, the amount of W was also studied by ICP analysis after
five times reusing the catalyst, and the ICP results did not show
any significant difference between the fresh and reused cata-
lysts. Furthermore, any structural changes of the catalyst were
studided by FT-IR, TGA and SEM techniques. It is clearly
evident from the FT-IR spectrum of the 5th reused catalyst that
the PR HPA ions were strongly bound to the surface of
MNP@MOF through the ionic linkage, and no important
changes in the intensities, frequencies, and shapes of the
absorption bands were observed (Fig. S11, ESI†). Interestingly,
comparing the TGA plots of the fresh (Fig. S8, ESI†) and 5th
reused catalysts (Fig. S12, ESI†) showed that no significant
changes in weight loss were observed and important weight
losses were similar in the two plots. In addition, the size and
morphology of the reused catalyst were investigated by the SEM
technique (Fig. S13, ESI†). This image was aproximately similar
to the SEM image of the fresh catalyst (Fig. 2b) and there are no
significant differences in the size and morphology. These out-
comes confirm that the PR HPA strongly connected to the
amine groups of the MOF shell structures by ionic bonding
and during the reaction, they do not separate from the MOF
structures and the structure of the catalyst remained stable and
(50% conversion), the reactor was allowed to cool down to room
temperature and CO was vented gradually. After this, ethyl acetate
2
was added to the reaction mixture and the MNP@MOF–PR was
separated from the reaction mixture by an external magnet. Next, the
rest of the reaction was carried out in the absence of the catalyst for a
further 60 min. Observations showed that the reaction did not
intend to move forward and confirmed that no leaching of PR
HPA occurred during the reaction. These results are in accordance
with the reusability test of the catalyst and ICP results as well.
The efficiency and capability of the PR HPA-immobilized
MNP@MOF catalyst were compared with other reported catalysts
for the cyclic synthesis of styrene carbonate via the chemical
fixation of carbon dioxide (Table 4). The results show that the
catalyst can catalyze the synthesis of styrene carbonate in a shorter
reaction time (2.0 h) compared with other catalysts (3.0–48 h) and
also a higher amount of yield (Table 4, entries 1–4, 6 and 8).
However, the CO pressure (0.3 MPa) was lower compared to some
2
methods (Table 3, entries 1–4), but it was higher compared to
others (Table 4, entries 5–8).
Experimental
General
All the materials and reagents were purchased from Sigma-
Aldrich and Merck and used without any further purification.
Preparation of the catalyst (MNP@MOF–PR)
For preparation of the catalyst, first, Fe
3 4
O MNPs were synthe-
6
7
sized by using FeCl and the hydrothermal process, and then
3
these synthesized MNPs with size below 50 nm were coated
with amino-functionalized MOF (IRMOF-3) by the solvothermal
6
5
reaction of zinc nitrate and 2-aminoterephthalic acid (ATA).
For this purpose, 1.0 g of polyvinyl pyrrolidone (PVP) was
dissolved in a mixture of 20 mL of ethanol and 30 mL of
Scheme 4 The proposed mechanism for the synthesis of cyclic carbo-
nate in the presence of MNP@MOF–PR as both Lewis (route 1) and
Brønsted (route 2) acid catalysts and without TBAB.
DMF. On the other hand, 40 mg of Fe
3 4
O MNPs were dispersed
in 2.0 mL of ethanol ultrasonically for 20 min and then this
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Table 4 Comparison of the MNP@MOF–PR catalyst with other catalysts for the synthesis of styrene carbonate 3a
Entry
Catalyst
Nb
La–Zr oxide
Zn–Mg–Al
Condition
Co-catalyst
Temperature (1C)
Pressure (MPa)
Time (h)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
9
2
O
5
DMF
DMF
TEA
Sol. free
Sol. free
Sol. free
—
—
—
135
130
140
80
60
40
60
80
85
80
5.0
2.5
2.5
2.0
0.1
0.1
0.1
0.1
0.3
0.3
15
80
90
89
88
99
88
98
90
98
96
89
90
91
92
93
94
95
96
11
—
3.0
3.0
3.0
8.0
48
48
4.0
10
a
CuCo
2
O
4
TBAI
Zn MOF
COF-JLU7
ZnO@NPC-Ox
Cationic MOF
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
CH
3
CN
Sol. free
Sol. free
Sol. free
CuAl
2
O
4
b
1
0
MNP@MOF-PR
2.0
a
b
Tetrabutylammonium iodide. Present study.
mixture was added drop by drop to the previously prepared mixture to remove the co-catalyst and unreacted reagents and
solution under stirring. Subsequently, 111.55 mg of zinc nitrate finally, the organic layers were dried with sodium sulfate. The
(
(
0.375 mmol) and 27.15 mg of 2-aminoterephthalic acid obtained carbonates were purified by column chromatography
0.15 mmol) were dissolved in 10 mL of DMF and added to using ethyl acetate–hexane. The structures of all the products
1
13
the previous solution. Afterwards, the mixture was dispersed were characterized by using H-NMR and C-NMR (see ESI†).
ultrasonically for 15 min and then it was transferred to a 100
mL Teflon-lined stainless steel autoclave and heated at 100 1C
Conclusion
for 5 h. After this, the synthesized core–shell MNP@MOF
materials were collected by an external magnet and in order
to remove the unreacted ATA and PVP, the synthesized solid
product was re-dispersed in 30 mL of DMF and heated in an
autoclave at 100 1C for 12 h. Finally, the pure core–shell MNPs
were separated from the solution by an external magnet and
then it was rinsed with DMF and ethanol and dried in a vacuum
oven at 50 1C for 12 h. Eventually, the PR HPA was immobilized
to the amine groups on the surface of the core–shell
MNP@MOFs by the ionic bonds with the following procedure.
Hence, 50 mg of PR HPA was dissolved in 5.0 mL of methanol
and then it was added dropwise to a suspension of 100 mg of
the prepared core–shell MNP@MOF material in 5.0 mL of
methanol. This mixture was stirred for 12 h and then the solid
PR HPA-immobilized MNP@MOF was separated from the
solution by an external magnet and dried in a vacuum oven at
In summary, a new magnetic MOF platform was introduced for
ionic immobilization of PR HPA. On this basis, the core–shell
magnetic MOFs were synthesized and then the PR HPA was
immobilized on the surface of the MOF shell structures by the
ionic reaction between the proton sites of PR HPA and the
amine groups of the MOF. The electronic microscopic observa-
tions, IR, TGA and ICP analysis confirmed the core–shell
structures and the proper decoration of PR HPA on the MOF
materials. This new supporting magnetic platform can be used in
the post-synthesis modification in the design and preparation of
various catalysts by chemical and physical immobilization of
different organic ligands and complexes and also homogeneous
catalysts. Moreover, this catalyst can be operated as both Lewis
and Brønsted acid catalysts for the synthesis of cyclic carbonates
via the chemical fixation of carbon dioxide. Eventually, due to the
excellent paramagnetic properties, it can be reused in several
repeated reactions without a significant decrease in catalytic
activity and leaching behavior.
7
0
50 1C for 12 h to obtain the final catalyst (MNP@MOF@PR).
General procedure for the chemical fixation of CO with
2
different epoxides
First, a stainless steel high pressure batch reactor was prepared Conflicts of interest
for the reaction. Then, 10 mmol of epoxide was loaded onto the
reactor and subsequently, 5.0 mmol of TBAB (co-catalyst) and
There are no conflicts to declare.
an appropriate amount of the catalyst were added to the reactor
and the mixture was heated at 80 1C. The reaction was carried
out for the specified durations as shown in Table 2, while the
Acknowledgements
The authors gratefully acknowledge partial support of this
study by the Mashhad University of Medical Sciences and the
Shiraz University of Technology.
2
reactor pressure was kept constant with CO at a reaction
pressure in the range of 0.1–0.5 MPa. Eventually, after completion
of the reaction, the reactor was allowed to cool down to room
temperature and CO2 was vented gradually. After this, ethyl
acetate was added to the reaction mixture and the MNP@MOF–
PR was separated from the reaction mixture by an external
magnet and then it was rinsed many times with ethyl acetate
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