Unprecedented NH2-MIL-101(Al)/n-Bu4NBr system as solvent-free heterogeneous catalyst for efficient synthesis of cyclic carbonates via CO2 cycloaddition

Article abstract of DOI:10.1039/c7dt03754f

Amine-functionalised framework NH₂-MIL-101(Al) was synthesized using a solvothermal and microwave method and characterized by PXRD, FT-IR, TGA, SEM-EDX, and BET surface area analysis. The desolvated framework, in the presence of co-catalyst tet

Full text of DOI:10.1039/c7dt03754f

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Unprecedented NH2‒MIL‒101(Al)/n‒Bu4NBr system as solvent free
heterogeneous catalyst for efficient synthesis of cyclic carbonate via CO2
cycloaddition
Received 00th January 20xx,
Accepted 00th January 20xx
S. Senthilkumar, Minaxi S. Maru, R. S. Somani, H. C. Bajaj and Subhadip Neogi*
DOI: 10.1039/x0xx00000x
The amine functionalised framework NH2-MIL-101(Al) is synthesized by solvothermal as well as microwave method and
characterized by PXRD, FT-IR, TGA, SEM-EDX and BET surface area analysis. The desolvated framework, in the presence of
co-catalyst tetra butyl ammonium bromide (TBAB), acts as excellent heterogeneous catalyst for the solvent free
cycloaddition of carbon dioxide (CO2) with epoxides, affording five-membered cyclic carbonates. Using styrene oxide, the
NH2-MIL-101(Al)/TBAB system shows more than 99% conversion with 96% yield, 99% selectivity, having turn over
frequency (TOF) of 23.5 h-1, and validates synergistic effect of quaternary ammonium salt during CO2 cycloaddition. The
catalyst could be recycled at least five times without noticeable loss of activity, while leaching test shows no leached Al3+
ion throughout the reaction. Thorough analysis of reaction parameters revealed optimum conditions for obtaining
maximal yield with highest selectivity are: 6 h duration, 120 °C temperature, and 18 bar of CO2 pressures. The outstanding
conversion and selectivity is maintained for a range of aliphatic and aromatic epoxides, corroborating the duel benefit of
micro-mesoporous system with amine functionality that offers easy accessibility of reactant molecules with diverse sizes,
and provides an inspiration to future catalytic system for CO2 cycloaddition. We also propose a rationalized mechanism for
the cycloaddition reaction, mediated by NH2-MIL-101(Al) and TBAB, on the basis of literature and experimental outcome.
www.rsc.org/
titanosilicates,12 organic and metal complexes,13 organic
Introduction
networks,14 and microporous polymers15 mostly demand very
high temperatures and/or pressures with multiple purification
steps that count for a given chemical process in industry.16
While traditional catalysts17 like zeolites,18 mesoporous silica
(MCM-41 and SBA-15), having both acidic and basic pairs, are
effective for the cycloaddition reaction, yields from these
reactions are often low, and include multiple separation and
recycling steps. Therefore, development of efficient catalysts
for the synthesis of cyclic carbonates from CO2 is critical and
challenging.19
The rising level of carbon dioxide (CO2)1 in atmosphere is a
major threat to global warming that has initiated several
attempts for CO2 capture processes. The more abundance of
non-toxic and non-flammable CO2 has inspired the scientific
community to invent technologies of using CO2 as C1 feed
stock for multipurpose synthesis. In this regard, the
cycloaddition reaction of CO2 with epoxide is a 100% atom
economic reaction, constituting one of the most efficient ways
of artificial CO2 fixation. This chemistry is greener compared to
the traditional synthesis, involving highly toxic and corrosive
phosgene or isocyanates.2 Furthermore, cyclic carbonate is of
great industrial interest3 because it finds worthy application4
as green solvents,5 electrolytes in lithium ion batteries6 or in
the pharmaceutical and fine chemical industries.7 However,
preparation of cyclic carbonate by homogeneous catalysis8
requires high temperature and pressure, besides additional
difficulties related to product separation. Alternatively,
heterogeneous catalysts for CO2 fixation, including ionic
In this regard, CO2 adsorption with in situ conversion,
instigating from porous metal-organic frameworks (MOFs),
should prove to be a worthy strategy for efficient and
profitable reduction of CO2 emission. Compared to the
traditional catalysts (vide supra), MOFs show (i) well-defined
and tunable pores, (ii) versatile structures, and (iii) possibilities
of incorporating acid-base pairs.20 The definite 3D structure in
MOFs offer rational separation between the base
functionalities in the organic linkers with adjacent open metal
nodes (Lewis acidic) that allow intramolecular reaction
between the adsorbed species in these two sites, and prevent
catalyst poisoning. Using MOFs, several tetrabutyl ammonium
bromide (Bu4NBr) salt co-catalyzed CO2 cycloaddition have so
far been studied.21,22 Han et al. studied the performance of
MOF-523 while Ahn et al. compared various popular MOFs for
the cycloaddition of CO2 with styrene oxide (SO) in polar
liquids,9
silica-supported
salts,10
metal
oxides,11
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solvents.24 However, a serious of drawback in many MOFs that
limits their application in catalysis is non availability of active
sites, specifically Lewis acidic centres. Though several famous
MOFs, including MIL-101, MIL-53, HKUST-1, NU-1000 and UiO-
66, have been applied for cycloaddition, these still suffer from
lack of functional sites, especially basic sites. Taking advantage
of their tunabilities, MOFs can be easily synthesized with Lewis
acidic character in the metal nodes, so that they can rapidly
activate epoxides. Likewise, incorporation of amine group in
MOF has dual advantage as they can act as electron donor
(Lewis base) to CO2 molecule and increase the local
concentration of CO2 near the catalytic centres through high
CO2 adsorption due to the amino effect.25 In fact, the
concerted reaction concepts on acid-base MOFs has been
reported by Baiker et al. in the synthesis of propylene
carbonate with amine-containing mixed-linker.21a Based on
these rationales, we anticipated that MOF having both, open
metal sites and amine functionalization should be suitable for
the CO2 cycloaddition reaction. In this regard, the well-known
NH2-MIL-53(Al)26 could be a starting point. However, this MOF
exhibits a very narrow pore (vnp) configuration after solvent
removal, as a result of hydrogen bonding interactions between
the -NH2 group with the aluminium cluster,27 ultimately
leading to poor condensations reactions.28 In contrast, the
kinetically favoured NH2-MIL-101(Al) framework29 propagates
via sharing the large tetrahedron (also called super
tetrahedron, ST), made from trimeric Al3+ secondary building
units (SBUs), and allows the advantage of generating
coordinatively unsaturated metal centres upon desolvation.
All the reagents and solvents were purchased and used
without further purification. The powder X-ray diffraction
(PXRD) for NH2-MIL-101(Al) were recorded using X’Pert-MPD
Diffractometer with Cu-Kα (λ =1.54056 Å) radiation in the 2θ
range 2-50° at a scan speed of 0.1°sec-1. Thermogravimetric
analysis (TGA) was studied using 209 F1-Libra (Netzsch,
Germany) thermal analyser at a heating rate of 10 °C/minute
under N2 atmosphere. The temperature is ramped from room
temperature to 600 °C and percentage mass change (Δm) is
plotted as a function of temperature. The FT-IR spectra were
recorded using the Perkin Elmer GX-FTIR spectrometer with
the wave range of 4000-400 cm–1. The resolution was set to 4
cm–1, and about 200 scans were averaged to one spectrum.
The field emission-scanning electron microscopy (FE-SEM)
micrographs were recorded on a JSM-7100F scanning electron
microscope equipped with energy-dispersive X-ray detector
(EDX). The N2 adsorption isotherm on the desolvated samples
were performed using Micromeritics ASAP 2020 analyser,
while high pressure gas adsorption was carried out using
BELSORP-HP, (BEL Inc. Japan). Prior to adsorption
measurements, the as-synthesized frameworks were heated to
120 °C for 5 h under vacuum to produce guest free samples.
For dynamic light scattering (DLS) measurements, NaBiTec
SpectroSize300 light scattering apparatus (NaBiTec, Germany)
with
a He−Ne laser (633 nm,
4 mW) was used. The
temperature programmed desorption (TPD) were performed
using Micromeritics Autochem II. The NMR spectra were
recorded using 500 and 600 MHz JEOL NMR spectrometer at
ambient temperature in CDCl3 as solvent. The chemical shifts
are reported in parts per million (ppm) and the coupling
constants J are given in hertz (Hz). Data are reported as
follows: chemical shift, multiplicity (s-singlet, d-doublet, t-
triplet), integration and coupling constant. The residual signal
of CDCl3 is δH = 7.26 ppm.
Moreover, NH2-MIL-101(Al) is
a combination of micro-
mesopores that is expected to combine the advantages of
each pore size regime,30 and overcome the limitations of
purely microporous or purely mesoporous structures (vide
infra). To the best of our knowledge, no systematic analysis of
the acid-base property in NH2-MIL-101(Al), specifically
cycloaddition reaction of epoxides with CO2, has yet been
reported.
Synthesis of NH2-MIL-101(Al)
The NH2-MIL-101(Al) was solvothermally synthesised using the
improved method developed by Chmielewski et al.31 based on
the earlier report by Gascon et al.32 followed by activation, and
was denoted as 1S. This synthetic method was also carried out
under microwave irradiation as an alternative energy source at
130 °C for 1h at 800W with same starting material to yield
similar material, denoted as 1M. The detailed experimental
In the light of aforementioned annotations, we synthesized
micro-mesoporous NH2-MIL-101(Al) with well-isolated acid-
base pairs and explored its catalytic activity in the solvent-free
cycloaddition of CO2 with styrene oxide in the presence of co-
catalyst TBAB to yield styrene carbonates (SC). More than 99%
conversion with 96% yield and 99% selectivity with turn over
frequency (TOF) of 23.5 h-1 was achieved with NH2-MIL-
101(Al)/TBAB system, indicating a synergistic catalysis during
CO2 cycloaddition. The high conversion and selectivity of the
catalyst is maintained throughout five cycles, while leaching
test does not show any leached Al3+ ion. Furthermore, the
effects of various reaction parameters like catalyst-cocatalyst
ratio, reaction time, temperature and pressure have been
investigated in detail for diverse substrates that exhibit
outstanding conversions with excellent selectivity is
maintained for a range of aliphatic and aromatic epoxide,
providing the benefit of micro-mesoporous system.
procedures are provided in the ESI.
†
CO2 fixation with Epoxides
In a typical reaction, 50 mL stainless steel autoclave reactor
was equipped with a magnetic stirring bar. The autoclave was
first purged with CO2 (99.99% purity) to remove air and
charged with fixed amounts of the catalyst (1S, 0.17 mol%),
and cocatalyst (TBAB. 0.14 mol%). Next, 105 mmol of SO was
charged into the autoclave. The reactor was pressurized with
CO2 up to 18 bar at room temperature, stirred at 800 rpm and
maintained the temperature at desired level (120 °C) for 6 h.
After completion of reaction, the reactor was allowed to reach
room temperature and excess CO2 was carefully vented off.
The above catalytic procedure, with identical quantity of
catalyst/co-catalyst and starting materials, was performed
under similar reaction condition for the microwave
synthesised catalyst 1M as well. The final filtrate was analysed
Experimental section
Materials and methods
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using NMR spectroscopy to determine the conversion as well was collected to calculate the ratio of elements present in the
as selectivity. The details of NMR spectra of all the synthesised framework (Fig.S6 in ESI ).
cyclic carbonates are provided in Fig. S11-S23 in ESI.†
†
Alongside, the N2 adsorption isotherm at 77 K for 1S shows
BET surface area of 2074 m2/g, with the well-known distinctive
Safety Note: The autoclave experiments described in this steps as found for MIL-101 framework (Fig.S7 in ESI†). These
paper involve the use of high pressure and require equipment two steps are attributed to the filling of two different types of
with an appropriate pressure rating.
cavities,38 where the supertetrahedra are filled at low relative
pressures (P/P0 < 0.05), while the larger cavities are filled only
as pressure increases. It should be noted that microwave
synthesized material 1M also exhibits comparable surface area
(2025 m2/g) to that of 1S. Obviously, the total pore volume of
550 cm3 is lower than that of the un-functionalized material
because of the free -NH2 groups residing inside the cavities.
Moreover, polar CO2 molecules (quadrupole moment = 13.4 ×
10–40 C m2; polarizability = 26.3 × 10-25 cm3) can easily enter
into the pores of this framework and shows 21 wt% (106.66
cm3/g for 1S), and 20.16 wt% (102.25 cm3/g for 1M) of CO2
Results and discussion
While MIL-53(Al) framework is thermodynamically favoured
product, MIL-101(Al) is kinetically favoured. The crucial
observation is the occurrence of an intermediate MOF-
235(Al)29 structure, for which analogous species were earlier
recognised during the crystallization of iron carboxylates.33
Importantly, kinetics can be altered by changing the solvent
and metal precursor, resulting in the selective formation of
either MIL−53(Al) or MIL-101(Al). Given the presence of -NH2
functionality in MOF leads to higher CO2 adsorption and
increased hydrolytic stability, as clearly evidenced by the
higher stability of IRMOF-3 over IRMOF-1,34 the amino-
substituted NH2-MIL-101(Al) is prepared (1S) by proper tuning
of the synthetic conditions.35 Moreover, NH2-MIL-101(Al)
shows presence of both micro as well as mesopores in a 3D
adsorption up to 15 bar at 298 K (Fig. S8 in ESI†).
network (Fig. 1 and Fig. S25 in ESI†), which may benefit the
free diffusion of substrates, products and solvents in the pores
of the framework. In addition, we also synthesized the same
MOF (1M) by microwave method (ESI†).
(a)
(b)
(c)
Fig. 2 FE‒SEM images of 1S (a, b) and 1M (c, d)
(d)
(a)
(b)
(c)
Fig. 1 Visualization of three different cage structures in NH2‒MIL‒101(Al), the super
To understand the catalytic activity levels, presence of acidic as
well as basic sites were accessed by NH3 and CO2 temperature
programmed desorption (TPD) profiles (Fig. S9 and Table S1 in
tetrahedra (ST) (a) along with pentagonal (b), and hexagonal voids (c).
ESI†). Finally, the particle size of the framework was measured
The phase purity and structural integrity of both solvothermal
and microwave synthesized NH2-MIL-101(Al) were cross-
checked by comparing their PXRD patterns with the previous
reports32,36 that clearly verifies the formation of pure phase in
using dynamic light scattering (DLS) experiment and the
hydrodynamic radii (Dh) was found to be 480 nm (Fig. S10 in
ESI†).
both the cases (Fig. S1 and S2 in ESI†). The as-synthesized
Catalytic cycloaddition of CO2 and epoxides
framework pores are occupied by solvent molecules, which are
completely removed under vacuum. Thermogravimetric
analysis (TGA) of the individual desolvated framework shows
Together, (i) the presence of exposed Al3+ metal sites, (ii)
pendent amino groups, (iii) high surface area (iv) duel
existence of micro and meso-pores, as well as (v) sufficient CO2
uptake ensures that this framework can be used for CO2
fixation reaction using epoxides. Prior to the catalytic reaction,
the catalyst was activated to remove any physically adsorbed
moisture or other guest solvent molecules by vacuum drying at
120 °C for 5 h and finally crushed to fine powder. In the
absence of catalyst, the cycloaddition reaction did not afford
any product using styrene oxide (SO) as a model substrate, at
18 bar CO2 pressure and 120 °C even after 12 h (Table 1, entry
1). Alternatively, starting materials (aluminium chloride
hexahydrate and 2-amino terephthalic acid) yielded only a
trace amount of styrene carbonate (SC) (Table 1, entry 2 and
3). Given that nucleophilic co-catalyst in conjunction with a
no mass loss till 350 °C (Fig. S3 in ESI
stable up to 400 °C. Moreover, the resemblance in FT-IR
spectra for 1S and 1M (Fig. S4 and Fig.S5 in ESI ) to that of
†) and the framework is
†
reported ones32 confirms that compositional similarity is
preserved in both the cases. Field emission scanning electron
microscopy (FE-SEM) was used to study the morphology and
particle dimensions of 1S and 1M, which revealed (Fig. 2)
comparable surface arrangement with rod and plate like
morphologies in both the cases.31 The decomposition of DMF
+
into dimethylammonium (H2NMe2 ) and carbonate may have
played a role in influencing such morphology of the resulting
framework.37 SEM-Energy-dispersive X-ray spectroscopy (EDX)
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Lewis acid is beneficial to produce cyclic carbonates from changes are observed in the conversion with further catalyst
epoxides and CO2, tetrabutylammonium bromide (TBAB) was loading (0.2 and 0.25 mol%). Such an observation possibly
chosen as a suitable candidate. TBAB alone as a catalyst shows arises because of the limitation in the mass transfer between
only 47% conversion of SO with 99% SC selectivity (Table 1, the catalyst active sites and reagent, caused by the less
entry 4). However, TBAB mixed with the aforesaid starting dispersion of excess catalyst in the reaction mixture,39 or due
materials produces 48% conversion with 98% selectivity under to the achievement of reaction equilibrium.
similar reaction condition (Table 1, entry 5). We speculate that
amino group of the ligand may adsorb some CO2 and further
promote the reaction with TBAB. On the other hand, the
pristine catalysts, 1S or 1M in the absence of any TBAB, gave
26% and 22% SO conversion, with 78% and 72% selectivity,
respectively (Table 1, entry 6 and 7). To our delight, 99%
conversion with 96% yield and 99% selectivity is achieved
when 1S/TBAB system is used under an employed reaction
condition of 120 °C, 18 bar CO2 pressure with 105 mmol of SO,
0.17 mol% of 1S and 0.14 mol% of TBAB in 6 h (Table 1, entry
9). Evidently, the heterogeneous catalyst 1S/TBAB offers a
multiple fold increase of SC yield compared to any of its
precursor materials, indicating the occurrence of a synergistic
catalysis during cycloaddition of CO2 with SO. Under similar
reaction condition, the catalytic performance of 1M with TBAB
shows almost equal conversion (93.6% SC) with 99% selectivity
(Table 1, entry 10). This result highlights the ability of
Fig. 3 Effect of catalyst (1S) and TBAB (inset table) in the yield of styrene carbonate
(SC). Reaction conditions: SO = 105 mmol, 18 bar CO2, 6 h, 120 °C. Conversions were
determined by 1H NMR spectra (ESI†).
microwave energy not just a rapid route for catalyst synthesis,
but also as an efficient means to produce catalysts that
maintain the qualities and activities of conventional synthesis.
However, room temperature reaction (vide infra) yielded only
a trace amount of SC (Table 1, entry 8).
Since a synergism is involved in this catalytic system between
the present catalyst/co-catalyst/substrate, the effects of
varying the catalyst concentrations with different amount of
co-catalyst were also studied. As tabulated in Fig. 3 (Inset),
0.14 mol% of TBAB with 0.17 mol% of 1S was identified as the
optimal amount of catalyst that offers maximum SO
conversion with 96% SC yield and 99% selectivity. The
influence of CO2 pressure on the conversion of SO was
monitored by increasing the pressure, which unveils that the
SO conversion is increased with an increase of CO2 pressure
from 2 bar to 22 bar (Fig.4). This could be attributed to an
increased solubility of CO2 at higher pressures and subsequent
improvement of CO2 concentration in the reaction mixture
that ultimately shifts the reaction equilibrium to favour the
formation of SC. Since 1S exhibits excellent CO2 adsorption
capacity, its highest catalytic activity is observed under 18 bar
of CO2 pressures and it is maintained up to 22 bar.
Table 1 Cycloaddition of styrene oxide with CO2 using various starting materials or
catalysts
Entry
Catalyst
Temperature Conversion Selectivity Yield
(°C)
120
120
120
(%)
0
(%)
0
(%)
0
1
2
3
None
AlCl3.6H2O
NH2-BDC
Trace
Trace
‒
‒
‒
‒
4
TBAB
120
47
99
‒
AlCl3.6H2O
+ NH2-BDC
+ TBAB
5
120
48
98
‒
6
7
1S
120
120
RT
26
22
78
72
‒
14
11
‒
1M
8
1S /TBAB
1S /TBAB
1M /TBAB
Trace
99
9
120
120
99
99
96
95
10
93.6
Effect of reaction parameters
In order to determine the ideal condition for the catalysis,
different reaction parameters such as temperature, catalyst
amount, CO2 pressure and reaction time were investigated in
detail. At the onset, a series of reactions were conducted with
various amount of catalyst loading to find the best SC yield. As
depicted in Fig. 3, a gradual increment in the yield was
observed from 0.05 to 0.17 mol% of catalyst and no significant
Fig. 4 Effect of CO2 pressure on the reactivity of SO and CO2 using 0.17 mol% catalyst at
120 °C in 6h.
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Given the lower reaction temperature enhances the possibility standard measures to evaluate the efficiency of a catalyst and
of side products formation (diols and dimers),40 while very high it is based on the ratio of mmol of substrate to total active
reaction temperature promotes the chances for collapsing of sites with the conversion.
catalyst active sites,45i an optimum temperature for catalytic
reactions should be evaluated for working multiple times.
Therefore, the effect of reaction temperature on the
conversion of cyclic carbonates was investigated in the range
up to 140 °C (every 20 °C gradients, Fig. 5a), using 18 bar CO2
pressure, 105 mmol of SO, 0.17 mol% of 1S and 0.14 mol% of
TBAB. A multi fold increase in conversion was observed when
the temperature gradually ramped from 40 to 120 °C that
articulate the effect of heating on cycloaddition reaction.
(a)
(a)
(b)
Fig. 6 Turnover frequency (TOF) calculated based on the yield of styrene carbonate at
different temperatures (a), recycling ability of the catalyst up to five cycles, showing
99% conversion is maintained throughout under the optimised reaction condition (b).
The TOF for moles of SO converted per mole of active sites of
catalyst concentration with 99% conversion was calculated to
be 23.5 h-1 (TON =141 in 6h) and it is comparable to the
previous report.41 The TOF for other epoxies are listed in Table
3. As shown in Fig. 6a, the TOF experiences almost three fold
upsurge, when the temperature is raised from RT to 120 °C. It
should be noted that 120 °C is still moderate temperature
compared to most of the previous reports45a for the synthesis
of SC.
(b)
Fig. 5 Formation of SC at different temperature (a), and time variable studies of
SC yield at different temperatures (b) using 105 mmol of SO, 18 bar of CO2
pressure, 0.17 mol% catalyst and 0.14 mol% TBAB.
Catalyst recycling and leaching test
Below 60 °C the SC yield was low (19%), and above that a
sudden increase in SC yield was observed with the maximum
of 96% yield at 120 °C. However, the yields remained the same
afterwards, inferring that further increase in temperature does
not show any major effect on the conversion. The regenerated
catalyst maintains its structural integrity, as confirmed from
Another essential factor to be considered in heterogeneous
catalysis for both laboratory as well as industrial processes is
the catalyst recyclability. For bulk scale production in industrial
level, catalyst separation via simple and easier way is
economic and environmentally essential to minimize waste
streams and develop possible catalyst recycling strategies.42 To
this end, we tested the recyclability of catalyst 1S /TBAB under
aforesaid reaction conditions. For this, 1S was recovered by
centrifugation after each catalytic cycle, rinsed with
chloroform and methanol, and dried in vacuum at 90 °C for the
next catalytic run under the similar reaction conditions. To our
delight, the high conversion and selectivity is maintained
throughout five recycles for the same catalyst (Fig. 6b). The
framework 1S preserved its structure throughout the recycling
process, as the characteristic peaks remain unchanged in PXRD
the PXRD pattern (Fig. S2 ESI†).
In order to better understand the catalytic system, we also
studied the yield of SC at various temperatures with different
time intervals and the results are summarised in Fig. 5b and
S24 in ESI†. The product conversion increased with respect to
reaction time and the highest conversion was obtained at 6 h.
The catalytic reaction performed at room temperature gives
only less than 20% of the product and offers 68% yield up to
60 °C. However, the SC yield was maximum at 120 °C up to 6h,
and therefore, it was considered the ideal time for reaction.
Thus, the optimum conditions for obtaining maximum yields
using catalyst 1S/TBAB were identified as 6 h duration, 120 °C
temperature, and 18 bar of CO2 pressures contributing to 99%
of SO conversion with 99% selectivity. The turnover number
(TON) and turnover frequency (TOF) of the catalysts are the
and FT-IR spectra (Fig. S2 and S5 in ESI†). Alternatively, though
hot filtration43 is considered as a very common method to test
the presence of leached metal ions in the reaction medium,
the same is not suitable here because of the presence of halide
ions in the catalytic system. Therefore, to have an accurate
determination of catalyst leaching after reaction, the filtrate
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was analysed using inductively coupled plasma optical atom of the polarized CO2 molecule attacks the β‒carbon of
emission spectrometry (ICP-OES) that shows no trace Al3+ ions the ring opened epoxide, producing an intermediate with the
in the solution even after fifth cycle of the catalyst. The NH3 elimination of bromide ion. Finally, the ring closure takes place
and CO2 TPD profiles were also examined for recycled catalyst, to produce SC that subsequently regenerate NH2-MIL-101(Al),
which showed virtually the same profiles as found for the fresh which is further transferred to the next cycle of cycloaddition
catalyst (Fig. S9 ESI
†).
by coordinating with a new epoxide molecule. Here, it is
worthwhile to compare the efficiency of 1S with the earlier
reported MOFs towards CO2–SO fixation reaction. For this, we
matched the catalytic abilities of a series of carboxylate based
Reaction mechanism and comparison with reported MOFs
In principle, formation of cyclic carbonate is a concerted MOFs with that of 1S (Table 2). Though diverse reaction
mechanism, where activation of CO2 occurs by the nucleophilic conditions were employed, we attempted a comparison of the
attack of a basic group at the carbon atom of CO2, while the MOF catalysts at the closest reaction conditions. The
epoxide is activated by adjacent acidic sites. The ring opening microporous nature of gea-MOF-1, gave 85% conversion of SC
is favoured by the co-catalyst, which ultimately reacts with the up to 20 bar of CO2 pressure (Table 2, entry 1) and the non-
“activated CO2” to generate cyclic carbonate. During the porous nature of {Cu(Hip)2(Bpy)}n (CHB) (Table 2, entry 3)
catalytic reaction, substrates can diffuse into the pores of the afford around 70% conversion of SC up to 6 bar pressure of
framework that enhances interactions with the reactant CO2 at 120 °C in 6h with TBAB as co-catalyst. Under equivalent
molecules. Catalytic activity of NH2‒MIL‒101(Al) towards CO2 reaction conditions, 1S with TBAB as co-catalyst gave 99%
fixation with SO suggests that the presence of coordinatively conversion of SC with 96% yield (Table 2, entry 14). Clearly, 1S
-
unsaturated Al3+ centers in the SBUs act as Lewis acidic sites TBAB composite system exhibits better catalytic performance
that coordinate to the epoxides and activate the ring opening, compared to several reported MOFs. A notable observation in
while the amino groups can polarize thermodynamically stable this study is that the catalytic activity of MOF system,
CO2 molecule and facilitate CO2 insertion cycloaddition possessing either microporous or non-porous environment,
reaction.44 Based on these rationale, a possible mechanism for affords less styrene oxide conversion than the MOF with
the cycloaddition of SO in the presence of 1S and co-catalyst mesoporous system. This observation can be correlated with
TBAB is illustrated in Scheme 1.
the easy diffusion of epoxide molecules, having various sizes,
(Table 3) inside the mesopores of 1S, thereby affording
enhanced conversion and selectivity of the primary products
such as SC (vide infra).
Variation of substrates
The scope and generality of the present catalytic system for
CO2 cycloaddition is further extended with various epoxides
(both aliphatic and aromatic) under the aforesaid reaction
condition. To our pleasure, high yields were preserved almost
in all the cases (Table 3). Nevertheless, aliphatic epoxides gave
slight less yields than the aromatic epoxides, which might be
attributed to the weaker interactions between the aliphatic
chain and the catalyst.45h The phenyl glycidyl ether (PGE) gave
comparatively outstanding conversions (Table 3, entry 5) with
TON as 134 and TOF as 22.3 per mole of catalyst per hour, and
the results are essentially better than the reported one45c,i
under similar condition. Apart from that, aromatic epoxides
consume more time to pass through the pores of the
framework than the aliphatic epoxides. Therefore, a better
mass transfer between catalytic sites and the CO2 molecule is
anticipated. Such good catalytic performance is attributed to
the combined effects of available acidic and basic sites, as well
as presence of the micro-mesoporosity in 1S. However,
cyclohexene oxide inhibits the formation of corresponding
cyclic carbonate (Table 3, entry 6) and shows reasonably low
conversion (15%). This anomaly can be ascribed to the
hindrance in anionic attack posed by the steric crowding of the
cyclohexene ring. Overall, the present micro-mesoporous
material with high surface area and proper functionalization46
offers excellent catalytic CO2 fixation reaction under controlled
experimental condition, leading to significant yield in the
presence of the co-catalyst TBAB.
Scheme 1 The general (above) and proposed mechanism (below) for the cycloaddition
of SO and CO2 using 1S/TBAB system (A = acidic site; B = basic site).
At the onset, the oxygen atom of SO interacts with the acidic
sites (Al3+) of 1S, followed by attack of the bulky bromide ions
of TBAB to less hindered (β-carbon) carbon atom of epoxide,
leading to the opening of the ring. Subsequently, the oxygen
6 | J. Name., 2012, 00, 1-3
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Table 2 Comparison of the catalytic activities of the NH2-MIL-101(Al) catalyst with previously reported MOF catalysts along with the employed reaction conditions.
Entry
Catalyst a
Structure
Pressure
(bar)
Temperature
(°C)
Time (h) Conversion (%) Yield (%) Ref 45
1
2
gea‒MOF‒1
20
12
120
RT
6
85.0
53.0
‒
a
b
UMCM‒1‒NH2
24
53
3
4
5
6
{Cu(Hip)2(Bpy)}n (CHB)
[Cu2L(H2O)2]∙4H2O∙2DMF
MIL‒68(In) ‒NH2
MOF‒5
12
10
8
120
100
150
50
6
6
69.8
64.1
74.0
‒
56
‒
c
d
e
f
8
‒
1
15
92
7
8
9
MIL‒101(Cr)
UiO‒67‒IL
ZIF‒67
8
1
25
90
48
12
15
98.0
‒
98
‒
g
h
i
‒
10
100
92.0
10
MOF‒205(M)
12
RT
24
58.0
58
j
11
12
13
3D‒CCB
ZIF‒8 b
1
7
100
100
100
12
10
4
65.9
‒
‒
55
‒
k
l
HKUST‒1
20
48.0
m
14
NH2‒MIL‒101(Al)
18
120
6
99.0
96
*
awith co‒catalyst = Tetrabutyl ammonium bromide (TBAB), bwithout co-catalyst, *present work.
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test does not show even a trace amount of leached Al3+ ion,
signifying that the present catalytic system is suitable in
extending its scope from academia and research, to industrial
perspectives. Moreover, outstanding conversion and
selectivity is maintained for both aliphatic and aromatic
epoxides under the similar reaction condition. This in turn
supplement that present micro-mesoporous hybrid material is
more efficient towards CO2 cycloaddition, with smaller as well
as bulkier epoxides, compared to many reported MOFs.
Overall, the hybrid nature of NH2-MIL-101(Al), having pendent
amino group, offers easy accessibility to a range of reactant
molecules inside its pores and represents a valuable family
member for MOFs to future heterogeneous catalysis in CO2
cycloaddition reaction.
Table 3 Cycloaddition of CO2 with various epoxide substrates using the NH2-MIL-
101(Al)/TBAB catalyst.a
Entry
Product
Conversionb Selectivity Yieldc TON TOFd
(%)
(%)
(%)
(h-1)
O
O
O
1
99
>99
96
23.5
141
130
O
O
O
2
3
4
96
98
94
99
99
99
95
96
94
21.7
22.2
21.2
133
127
Acknowledgements
S. N. acknowledges the financial support from DST-SERB
(Grant No. ECR/2016/000156), S. S. acknowledges CSIR
Network project (Grant No. CSC-0122). The analytical support
from ADCIF is greatly acknowledged. CSMCRI Communication
No. 141/2017
5
6
99
15
99
99
97
15
134
20
22.3
3.3
O
O
O
Notes and references
aReaction conditions: 120 °C, 18 bar CO2 pressure, solvent- free, epoxides –105
mmol, 1S–0.17 mol%, TBAB–0.14 mol%. bBased on 1H NMR spectrum of reaction
mixture aliquots without further purification. c Based on isolated product. dTOF
(turnover frequency): Moles of product formed per mole of catalyst per hour.
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In conclusion, the highly stable framework NH2-MIL-101(Al)
has been successfully synthesised by both solvothermal (1S) as
well as microwave energy (1M) and characterised through
PXRD, FT-IR, TGA, SEM-EDX and BET surface area analysis.
Presence of suitable acid/base pairs and the micro-
mesoporosity of the framework, together with excellent high
pressure CO2 adsorption provide a platform to utilize the MOF
in the chemical fixation of CO2 by cycloaddition reactions with
various epoxides. The solvent free cycloaddition reactions for
1S in the presence co-catalyst TBAB, using CO2 and styrene
oxide demonstrates a synergistic catalysis of 1S/TBAB system
that resulted more than 99% conversion with 96% yield and
99% selectivity, having turn over frequency (TOF) of 23.5 h-1.
Likewise, 1M gives 93.6% conversion with 95% yield and 99%
selectivity. Thorough analysis of various reaction parameters
revealed that optimum conditions for obtaining maximal yield
with highest selectivity for the cyclic carbonate are 6 h
duration, 120 °C temperature, and 18 bar of CO2 pressures.
The recyclability test of catalyst showed high conversion and
selectivity is maintained throughout five cycles, while leaching
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Table of Content
NH2‒MIL‒101(Al)/n‒Bu4NBr works as excellent solvent free catalyst for CO2 cycloaddition with
epoxides, and highlights the benefits of micro-mesoporous system with acidic and basic
functionalities.
1