Highly Active Ultrasmall Ni Nanoparticle Embedded Inside a Robust Metal-Organic Framework: Remarkably Improved Adsorption, Selectivity, and Solvent-Free Efficient Fixation of CO2

Article abstract of DOI:10.1021/acs.inorgchem.9b00833

We report integrating additional functionality in an amine decorated, robust metal-organic framework (MOF) by encapsulating Ni nanoparticles (NPs). In-depth characterization of the postmodified structure confirms well-dispersed and ultrasmall NPs inside t

Full text of DOI:10.1021/acs.inorgchem.9b00833

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Highly Active Ultrasmall Ni Nanoparticle Embedded Inside a Robust
Metal−Organic Framework: Remarkably Improved Adsorption,
Selectivity, and Solvent-Free Efficient Fixation of CO2
Manpreet Singh,‡,† Pratik Solanki,† Parth Patel,† Aniruddha Mondal,‡,† and Subhadip Neogi*,‡,†
†Inorganic Materials & Catalysis Division, CSIR-Central Salt and Marine Chemicals Research Institute (CSMCRI), Bhavnagar,
Gujarat 364002, India
‡Academy of Scientific and Innovative Research (AcSIR), CSIR-CSMCRI, G. B. Marg, Bhavnagar, Gujarat 364 002, India
S
* Supporting Information
ABSTRACT: We report integrating additional functionality in an amine decorated, robust metal−organic framework (MOF)
by encapsulating Ni nanoparticles (NPs). In-depth characterization of the postmodified structure confirms well-dispersed and
ultrasmall NPs inside the framework pores. Although, the surface area is more reduced than pristine MOF, the CO2 uptake
capacity is remarkably increased by 35% with a large 10 kJ/mol rise in adsorption enthalpy that validates favorable interactions
between CO2 and NPs. In particular, CO2 adsorption selectivity over N2 and CH4 displays significant improvement (CO2/N2 =
145.7, CO2/CH4 = 12.65), while multicycle CO2 uptake demonstrates outstanding sorption recurrence. Impressively, the
embedded NPs act as highly active functional sites toward solvent-free CO2 cycloaddition with epoxides in 98% yield and 99%
selectivity under relatively mild conditions. The catalyst shows high recyclability without leaching of any metal-ion/NPs and
greater pre-eminent activity than the unmodified analogue or contemporary reports. Of note is that outstanding conversion and
selectivity are maintained for a wide range of aliphatic and aromatic epoxides, while larger substrates exhibit insignificant
conversion, demonstrating admirable size selectivity. Based on the literature reports and experimental outcome, a rationalized
mechanism is proposed for the reaction. This study exclusively demonstrates how strategic encapsulation of Ni NPs influences
the inherent electronic properties in a MOF for highly selective CO2 adsorption and represents a step forward to sustainable
CO2 valorization in terms of abundant active sites, sufficient stability, and consistent usability.
surface. Alternatively, the nontoxicity and easy obtainability of
anthropogenic CO2 opens up its potential as a valid C1 source
in the manufacture of several value-added compounds.5−7 In
particular, the valorization of CO2 with epoxides to yield cyclic
carbonate is a 100% atom economic and attractive strategy8,9
due to the extensive application of the product in
pharmaceuticals,10 as precursors in fine chemicals,11 electrolytic
elements of batteries,12 polar aprotic solvents,13 and in polymer
synthesis.14 However, the major flaw in this reaction is the
chemical inertness of CO2 that demands backing of a catalyst.
In this connection, homogeneous catalysts are most effective,
yet they seriously suffer from separation difficulties.15
INTRODUCTION
■
The continued use of fossil fuels is anticipated for many years to
come, owing to the slow rate of transition toward renewable
and clean energy sources.1 Consequently, the concentration of
the central greenhouse pollutant carbon dioxide (CO2) has
already surpassed the level of 400 ppm and continues to
accumulate more in the atmosphere at an alarming rate.
Henceforth, CO2 capture, sequestration (CCS) and its
chemical transformation are the areas of intense current
interest.2−4 Although, postcombustion CO2 capture from flue
gas is considered as the easiest capture process, this feat is tricky
owing to the minimal differences in molecules present in the
main components of this gas (CO2 and N2). As such, their
effective separation requires tailor-made adsorbents with
molecule-specific chemical interactions on their internal
Received: March 22, 2019
© XXXX American Chemical Society
A
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Figure 1. (a) PXRD pattern of ZrOF (as synthesized) and Ni@ZrOF, (b) IR spectra of ZrOF and Ni@ZrOF, and (c) N2 adsorption isotherms of
ZrOF and Ni@ZrOF.
Alternatively, complications from heterogeneous catalysts
mostly stem from their inadequate activities, requirement for
solvent assistance, and harsh reaction conditions. Thus, rational
design of an acid−base multifunctional catalyst with good
recyclability is imperative that can work even under mild
conditions.16,17 Metal−organic frameworks (MOFs) are out-
standingly porous, crystalline materials with a tunable pore
surface and are highly promising candidates for a multitude of
potential applications.18,19 In fact, selective CO2 adsorption and
the catalytic cycloaddition between CO2 and epoxides in MOFs
is an expanding area of interest.20−23 However, most of the
MOF-based CO2 adsorption systems lack enough stability,
show moderate to low adsorbate−adsorbent interaction, and
fail to produce high CO2 adsorption selectivity. On the other
hand, the catalytic performance is relatively poor for epoxide-
based cycloaddition reactions in most of the MOFs, besides the
fact that such reactions often require employment of solvents,
very high temperature and pressure conditions, and extremely
expensive ligands. Given both selective CO2 adsorption and its
efficient transformation principally requires presence of active
functional sites, we focused on implementing additional
functionality to synthesize all in one MOFs that can adsorb
large amounts of CO2 and facilitate the cycloaddition reaction
under milder conditions.
Based on the above facts coupled with our quest to impart
multifunctionality in a single MOF system,24−26 the present
research strategy evolved by selecting Zr(IV) based frameworks
that have gained particular interest owing to their high thermal
as well as chemical stability, creation of open metal sites
(OMS), and presence of unique large pores.27−29 We used the
amine functionalized terephthalic acid linker to construct the
well-known framework UiO-66-NH2 (henceforth termed as
ZrOF).30 With a view to impart additional functionality and
manipulate electronic property of the system, we aimed to
incorporate metal nanoparticles (NPs) inside the pore via
postsynthetic modification (PSM). It should be mentioned that
encapsulation of metal NPs through PSM inside the large pores
of MOFs is an emergent area of research, where the modified
systems exhibit promising applications as diverse as gas
storage,31 organic transformation,32 photocatalysis,33 sensing,34
and so on.35 In this modus operandi, however, aggregation of
NPs on the crystal surface is often inevitable, while repeated
loading of precursors may also disrupt the targeted frameworks
even under solvent-free vapor deposition conditions. On the
other hand, most of such reports involve costly metal
precursors36,37 that prompted us to use low-cost transition
metal ions. Indeed, there is continued urge for the successful
loading of cheap metal NPs inside the pores of MOFs.38,39 This
observation led us to functionalize ZrOF with Ni NPs.
Essentially, Ni is a relatively inexpensive and versatile catalyst
that should allow the Ni NP embedded MOF to show unique
and/or improved properties, as illustrated for the Ni-supported
carbon or silica materials.40 Nevertheless, to our knowledge,
only a handful of examples exist involving the preparation of Ni
NPs inside the cavity of a MOF,41−43 while no report is
available for any Zr(IV)-based frameworks. The NP embedded
framework (Ni@ZrOF) was characterized by a battery of
experimental evidence including Fourier transform-infrared
(FT-IR), powder X-ray diffraction (PXRD), transmission
electron microscopy (TEM), scanning electron microscopy-
energy dispersive X-ray (SEM-EDAX), X-ray photoelectron
spectroscopy (XPS), Raman spectroscopy, and inductively
coupled plasma-atomic emission spectroscopy (ICP-AES),
which corresponds to the successful encapsulation of well-
dispersed and ultrasmall Ni NPs inside the pores. Although,
inclusion of Ni NPs decreases the Brunauer−Emmett−Teller
(BET) surface area compared to the pristine ZrOF, the CO2
uptake capacity, isosteric heats of CO2 adsorption, and CO2
adsorption selectivity over N2 and CH4 show remarkable
enhancement. These findings clearly validate improved
interactions between polarizable CO2 molecules and Ni NPs
in a postmodified framework. Furthermore, the Ni@ZrOF
system together with cocatalyst tetrabutyl ammonium bromide
(TBAB) acts as an efficient, recyclable catalyst toward solvent-
free cycloaddition of CO2 and epoxides under relatively mild
conditions. Impressively, the catalyst provided a much superior
yield of cyclic carbonate compared to the nonmodified
framework as well as most of the reported MOFs. Substrate
variation studies demonstrated excellent size selectivity of the
catalyst, where outstanding conversion and selectivity are
maintained for a wide range of aliphatic and aromatic epoxides,
while larger substrates showed negligible product yields.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Ni@ZrOF. The
established framework UiO-66-NH2 (hereafter termed as
ZrOF) was solvothermally synthesized,44 and formation of
the desired structure was clearly verified upon cross-checking
the experimental PXRD pattern (Figure 1a) to that of former
reports.45,46 Infrared spectroscopy (Figure 1b) and thermogra-
vimetric analyses (Figure S1) also confirm the formation of a
phase pure ZrOF. The pore occluded solvent molecules in the
as-synthesized framework were removed by solvent exchange
followed by heat treatment at 120 °C under vacuum for 4 h.
The permanent porosity was confirmed by the N2 adsorption
isotherms of guest-free ZrOF, which showed a BET surface area
of 992 m2/g (Figure 1c).47,48 The value is lower than
unfunctionalized UiO-66(Zr) because of the occupancy of
pendent −NH2 groups inside the MOF cavities. After complete
characterization of phase pure ZrOF, synthesis of NP
B
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Scheme 1. Synthesis of Ni@ZrOF Using the Solution Impregnation Method
Figure 2. SEM images of the framework (a) ZrOF and (b) Ni@ZrOF and TEM images of the (c) ZrOF and (d) Ni@ZrOF. Elemental mapping of
the Ni NP loaded framework (e−h) obtained from the SEM experiment.
embedded framework (Ni@ZrOF) was targeted by the solution
impregnation method, followed by reduction of the material
with a 10% H2/N2 gas mixture steam (Scheme 1). A
comparison of the PXRD pattern of the postmodified
framework to that of the parent material reveals exact
correspondence of peak positions (Figure 1a), which imply
that the addition of the Ni precursor and subsequent reduction
treatment did not affect the stability of the framework. No
characteristic diffraction peaks for metal nanoparticle (MNP)
were observed throughout the 2θ range that can be related to
the low metal contents in the postmodified material, as also
realized in metal NP loaded MOFs.49,50
Morphological investigation was performed for the corre-
sponding ZrOF and metallic Ni loaded framework using field
emission-scanning electron microscopy (FE-SEM), trans-
mission electron microscopy (TEM), and high-resolution
transmission electron microscopy (HRTEM). As shown in
Figure 2a, the particle size from the SEM images was initially
found in the range 100−130 nm. After incorporation of Ni
nanoparticles into the ZrOF cavity, there is no obvious
morphological change of the composite (Figure 2b). The
low-resolution TEM images of the Ni@ZrOF clearly indicate
the uniform size and shape (Figure 2d), with no apparent
changes in particle sizes and morphology to that of the pristine
framework. From HRTEM analysis, it was difficult to analyze
the presence of ultrasmall metallic Ni nanoparticles due to the
low Ni content (Figure S2). The presence of the corresponding
element (Zr, C, N, O, and Ni) could be identified from SEM
mapping on a selected area on the Ni@ZrOF composite
(Figure 2), which divulged a homogeneous distribution of Ni,
Zr, and N, suggesting that ultrasmall Ni nanoparticles were
effectively encapsulated inside the modified framework (Figure
2e−h). The dispersion was uniform, and no sign of aggregation
was observed. The SAED patterns (Figure S3) of ZrOF and
ultrasmall MNP loaded material proved the polycrystalline
nature in both the cases. Although, Raman spectroscopic
studies were carried out to understand the ultrasmall nature of
nickel nanoparticles, the presence of a highly fluorescent
environment did not allow us to recognize the spectrum (Figure
S4) unambiguously.51 The nickel content was determined by
ICP-AES analysis, which displayed a loading of 1.12% of metal
ions in the digested Ni@ZrOF framework. In addition, the
quantity of Ni2+ remaining in Ni2+@ZrOF was separately
calculated. For this, the washed out portion during successive
cleaning was collected every time and ICP analysis was carried
out. The difference between the supplied and removed Ni2+
(during washing steps) corresponds to the nickel amount
(1.15%) incorporated actually. Clearly, both the results are in
complete harmony with each other.
Thermal stability and robustness of the material were studied
by thermogravimetric analysis (TGA) under a N2 atmosphere
up to a temperature of 800 °C (Figure S1). Nearly 10% weight
loss ensues around 400 °C for Ni@ZrOF, showing high
stability of the postmodified framework. The sharp weight loss
above 700 °C is attributed to thermal degradation of the
material. Essentially, encapsulation of Ni nanoparticles in the
pores of ZrOF did not affect the thermal stability of the
framework. Besides, Fourier transform infrared spectrum
(FTIR) of Ni@ZrOF was found almost identical to that of
the pristine framework (Figure 1b), where characteristic
asymmetric stretching frequencies of the CO from
coordinated carboxylate groups are clearly observed at 1571
and 1497 cm−1, while symmetric stretching frequencies are
present at 1424 and 1381 cm−1. The C−N vibrations, related to
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amino groups, are also visible at 1335 and 1256 cm−1. In
addition, two peaks around 3470 and 3360 cm−1 are attributed
to the asymmetric and symmetric stretching absorptions of the
primary amine group that can be clearly discerned over the as
made and postmodified samples. These two peaks are down
shifted in comparison with pure NH2−H2BDC (3500 and 3400
cm−1, respectively), indicating that the amino groups are
retained.52
N, and C in the modified sample. The intensity of the Ni peak
was low due to the small nickel content in Ni@ZrOF (vide
supra). To further clarify the oxidation state of the embedded
Ni, the Ni2p spectrum was closely crosschecked (Figure 3),
which revealed that two peaks centered around 853 (2p3/2) and
871 (2p1/2) eV correspond to the Ni NPs. Besides, the N 1s
peak in Ni@ZrOF is a bit shifted compared to ZrOF (Figure
S5) that is most likely due to the noncovalent interaction
between Ni NPs and the Lewis basic −NH2 functionalities in
the pore structure. This observation additionally supports the
incorporation of small Ni nanoparticles inside the cavity only.53
The N2 adsorption−desorption isotherms at 77 K are
presented in Figure 1c. While the BET surface area of ZrOF
is found to be 992 m2/g (vide supra), the value for Ni@ZrOF
approximates to 898 m2/g. Although, both the materials exhibit
typical type I adsorption isotherms, indicating a microporous
nature, the sharp reduction in the surface area can be attributed
to the occupancy of the MNPs. Also, the total pore volume of
ZrOF and Ni@ZrOF are found to be 0.46 and 0.40 cm3/g,
respectively (Table S1). Such a reduction in (a) N2 adsorption
amounts, (b) BET surface area, as well as (c) pore volume of
Ni@ZrOF, as compared to the pristine ZrOF, collectively
validates that the Ni nanoparticles are occupied inside the pores
of framework. It should be noted that modified BET surface
area is still adequate to supply more surface active sites for
enhanced adsorption or catalysis.
To further explore the chemical composition and oxidation
states in ZrOF and Ni@ZrOF, XPS was adopted. The XPS
survey spectrum (Figure 3) confirms the existence of Zr, Ni, O,
CO2 Adsorption and Selectivity in Ni@ZrOF. Given the
importance to develop new protocols to reduce massive CO2
emission, together with the fact that CO2 adsorption capacity of
a framework can be developed by added functionality,54,55 we
aimed at probing Ni@ZrOF for CO2 adsorption measurements
Figure 3. XPS survey spectra of (a) ZrOF and (b) Ni@ZrOF. Inset
shows the Ni2p spectrum.
Figure 4. CO2 adsorption isotherms for (a) ZrOF and Ni@ZrOF at 273 K. (b) Qst curve of CO2 adsorption of ZrOF and Ni@ZrOF. (c) Sorption
recurrence during 10 cycles of CO2 uptake at 273 K for Ni@ZrOF.
D
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Figure 5. Gas adsorption isotherms for (a) Ni@ZrOF at 273 K and (b) IAST selectivity curves for CO2/N2 (red) and CO2/CH4 (black) for Ni@
ZrOF at 273 K.
at different temperatures. At the onset, a thorough literature
survey indicated no available reports of CO2 sorption studies in
metal NP loaded ZrOF and/or its isoreticular analogues (UiO-
67, UiO-68). In contrast to low N2 adsorption of postmodified
framework, CO2 adsorption capacity of the same material
showed a remarkable uptake of 93.3 cm3/g (4.16 mmol/g) at
273 K, which is approximately 35% superior to that of parent
ZrOF (Figure 4a). Notably, adsorption and desorption curves
indicate no hysteresis. Further, the regenerative feature and
nondestructive nature of Ni@ZrOF toward CO2 adsorption
was examined. To our delight, 10 cycles of CO2 uptake at 273 K
exhibit almost identical capacities with minor loss (Figure 4c),
validating remarkable sorption recurrence. Moreover, the
PXRD profile of Ni@ZrOF at the end of the adsorption−
desorption cycles shows complete agreement to that of its
original structural characteristics (Figure S7). No apparent
saturation was observed within the studied pressure range (1.0
bar), signifying that more gas can be adsorbed by a further
increase in pressure. The uptake value at 295 K (Figure S6)
approximates to 55.6 cm3/g (2.48 mmol/g), which is ∼11%
more than that of pristine ZrOF under similar measurement
conditions.56 Interestingly, it appears that the N2 adsorption
based BET surface area of the metal-loaded MOF is not directly
connected to its CO2 adsorption capacity. In fact, the quantity
of CO2 adsorbed in Ni@ZrOF (18.3 wt %) is much higher than
reported MOFs possessing similar surface areas and even
comparable with some highly porous MOFs (based on surface
areas). For instance, SNU-6, Y-FTZBP, rht-MOF-1, TTF-4,
BIF-9-Li, and JLU-Liu31 (Table S4, entries 1−6) showed CO2
uptake values of 9.9, 15.6, 17.7, 13.3, 6.6, and 6.86 cm3/g,
respectively, at 273 K. However, corresponding BET surface
areas of all those MOFs are far superior than the present system.
The sorption behaviors of Ni@ZrOF with respect to CH4 and
N2 were also studied. The result at 273 K shows (Figure 5a) a
maximum CH4 uptake of 23.4 cm3/g (1.04 mmol/g, 1.67 wt
%). However, the uptake of N2 is negligible. These data validate
that the Ni0 loaded pore structure is majorly available to CO2
(kinetic diameter = 3.3 Å), which can facilitate selective CO2
adsorption over other gases.
adsorption for Ni@ZrOF (36.20 kJ/mol) is found almost 10
kJ/mol higher than the framework (Figure 4b), devoid of
MNPs (26.30 kJ/mol). Such improved value evidently
highlights the significant role of encapsulated metal nano-
particles for providing additional interaction sites with
adsorbate molecules. The Qst value experiences a rise at low
loading because of enhanced interaction between the polar CO2
molecules and functional sites in the modified framework. The
value decreases thereafter, owing to the filling of the maximum
affinity sites and shows nearly maintenance of the plateau even
at higher coverages. As displayed in Table S4, the obtained Qst
value is superior to many of the known MOFs. It is worth
mentioning here that this reasonable enthalpy of adsorption
offers not only a good affinity to CO2 capture but also points to
the easy adsorbent restoration, which is a vital and anticipated
property for real-time applications.
To gain comprehensive understanding of the gas separation
performance on Ni@ZrOF, the CO2 gas selectivity (S) over N2
and CH4 were inspected by the ideal adsorbed solution theory
(IAST) from single-component isotherms at 273 K (Figure
5b).58,59 To our knowledge, no NP encapsulated framework has
so far been explored for CO2 selectivity. The CO2 selectivity
over N2 (CO2/N2 = 15:85 mole ratios) is calculated to be
145.7, which is more pre-eminent than the pristine MOF. Such
a high value is rarely reported, and only a handful of frameworks
have shown similar CO2/N2 selectivity (Table S3). The high
CO2/N2 selectivity is primarily ascribed to the strong
electrostatic interactions between diverse polar sites inside
the pore and CO2 molecules. We extended the IAST selectivity
to CO2/CH4 as well, which is found to be 12.65 (Figure 5b).
Although, CH4 has a larger kinetic diameter (3.8 Å) than N2
(3.64 Å), favorable adsorption of the former can be correlated
to the larger polarizability of CH4 (26 × 10−25 cm3) compared
to N2 (17.6 × 10−25 cm3). Even though, Ag-loaded porous
materials have shown enhanced adsorption of Xe over Kr,60 the
present work represents a previously unreported illustration of
how Ni0 can offer tremendous potential toward selective CO2
adsorption.61,62
Solvent Free Fixation of CO2 to Cyclic-Carbonate.
Given that loading of NPs offers potential auxiliary binding sites
to the CO2 molecules, we wondered if this exclusive
trifunctional system, encompassing open Brønsted-acid sites
(Zr−OH/Zr−OH2), Lewis basic −NH2 groups, as well as
highly active metal NPs, could promote chemical trans-
formation of adsorbed CO2 to cyclic carbonate. In a typical
experiment, solvent-free CO2 cycloaddition using model
substrate styrene oxide (SO) was evaluated for the heteroge-
neous catalytic performance (Scheme 2).
Clearly, the improved CO2 adsorption capacity in the Ni
nanoparticle incorporated structure can be attributed to the
strong affinity between the polarizable CO2 gas molecules
(quadruple moment, 13.4 × 10−40 C m2; polarizability, 26.3 ×
10−25 cm3) and auxiliary adsorption sites on the surface of Ni
NPs, besides free −NH2 groups. To corroborate this statement,
we further considered the isosteric heat of CO2 adsorption
(Qst) from the Clausius−Clapeyron equation, using the
isotherms at 273 and 295 K.57 The calculated heat of
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while some MOF catalysts involve expensive ligands, which are
unfavorable to practical usages. In contrast, the present system
is based on inexpensive raw materials, requires milder reaction
conditions, and exhibits better catalytic performance.
Scheme 2. Cycloaddition Reaction of Styrene Oxide and
CO2
Effect of Reaction Parameters. Given temperature,
pressure, time, and concentrations of reactants, catalyst, and
cocatalyst display profound roles in SC formation, and all these
parameters were thoroughly examined to determine the ideal
conditions for maximum product yield. At the onset, the effect
of catalyst amount in the cycloaddition reaction was
investigated under semibatch operation conditions. As shown
in Figure 6a, an increase in the yield of SC was evidenced when
the amount of Ni@ZrOF was increased from 0 to 0.3 mol %.
However, no promising development in the reaction was
observed after reaching the maximal yield of 98% at 0.45 mol %
of catalyst. This observation conceivably stems from the
limitation in mass transfer between the catalyst active sites
and reagent, caused by the lower dispersion of excess catalyst in
the reaction mixture for achieving the reaction equilibrium.64
As a part of the synergic Ni@ZrOF/TBAB system, the TBAB
amount was also varied from 0.03 to 0.14 mol %, which shows
98% yield at 0.11 mol % (Figure 6c). In addition, the effect of
anions in the tetraalkylammonium salt was studied by varying
the halide ion. To our surprise, TBAB as cocatalyst rendered
higher SC yield than its iodide analogue (TBAI). The result
(Table 1, entry 11) indicates that despite its higher
nucleophilicity, the diffusion of sterically demanding iodide
ions into the pore is hampered in Ni@ZrOF.65 In other words,
the Ni@ZrOF/TBAI catalyst must have had superior activities
than the Ni@ZrOF/TBAB system if the catalysis would occur
on the surface of the framework. This fact evidently
authenticates that the catalytic reaction occurs inside the
optimized pores of the framework, and not on its external
surface.
Apart from catalyst and cocatalyst, temperature is another
significant factor that governs conversion of reactant to form
thermodynamically and kinetically favored products. Besides,
high reaction temperature needs to be avoided for this
particular reaction to bypass the formation of unfavorable
side products (diol).66 As depicted in Figure 6b, the catalytic
activity progressively increased from room temperature to 50
°C, and setting the temperature at 70 °C produced a maximum
SC yield of 97%. A further increase in temperature had not
much effect on the yield (Table 1, entry 12), which led us to
execute all the reactions at 70 °C. Being the part of a synergic
catalyst/cocatalyst/substrate system, the effects of varying
pressure and time were also studied. As demonstrated in
Figure 6d, yield of the product continuously increases by
increasing pressure up to 1.0 MPa, and a further rise in pressure
shows a trivial effect. This could be ascribed to the increasing
solubility of CO2 and consequent enrichment of CO2
concentration in the reaction mixture up to the actual pressure
that favors the equilibrium to shift to SC formation.67 However,
higher CO2 pressure (1.2 MPa) decreases the concentration of
SO in the liquid state. To check the yield of styrene carbonate
as a function of time, study was conducted at different time
intervals, which showed a maximum yield of 97.6% at 6 h.
Further reaction continued up to 8 h and showed almost 99%
yield (Figure 6e), with no catalyst poisoning effect. Therefore,
the optimum conditions to obtain 97.6% yield of SC with 99%
selectivity were identified as 1.0 MPa CO2 pressure, 70 °C
temperature, and 6 h duration.
The initial screening reactions were performed under
solvent-free conditions (Table 1), which displayed no styrene
Table 1. Screening of Various Catalysts for Cycloaddition of
a
Styrene Oxide and CO2
temperature selectivity
yield
(%)
entry
catalyst
(°C)
(%)
1
2
3
4
5
6
7
8
9
none
ZrCl4
NH2−BDC
TBAB
ZrCl4 + NH2−BDC + TBAB
ZrOF
Ni@ZrOF
70
70
70
70
70
70
70
70
70
70
70
120
0
0
98
98
99
99
97
98
99
99
99
25
30
41
45
52
62
98
83
99
ZrOF/TBAB
Ni2+@ZrOF/TBAB
Ni@ZrOF/TBAB
Ni@ZrOF/TBAI
Ni@ZrOF/TBAB
10
11
12
a
Reaction conditions: SO, 30.6 mmol; catalyst, 0.35 mol %;
cocatalyst, 0.11 mol %; CO2 pressure, 1.0 MPa; 70 °C, 6 h.
carbonate (SC) transformation either under catalyst-free
conditions (entry 1) or by using catalyst precursor materials
(entries 2 and 3). On the other hand, ZrOF or Ni@ZrOF, as
sole catalyst, produced only a moderate yield of SC (Table 1,
entries 6 and 7). We also used tetrabutylammonium bromide
(TBAB) as single catalyst that showed only a 25% yield of SC
(Table 1, entry 4), while TBAB mixed with the precursor
materials produced 30% yield (Table 1, entry 5). Surprisingly,
when the reaction was performed in the presence of the Ni@
ZrOF (0.35 mol % MOF) in association with TBAB (0.11 mol
%), SC was obtained in 98% yield (Table 1, entry 10) under
relatively mild reaction conditions (1.0 MPa CO2 pressure, 70
°C temperature, 6 h). To understand whether the amine groups
or the Ni NPs play the more significant role in cyclic carbonate
synthesis, ZrOF/TBAB as well as Ni2+@ZrOF/TBAB systems
were also investigated under similar conditions that produced
52 and 62% SC yields, respectively (Table 1, entries 8 and 9).
It should be noted that cycloaddition of CO2 and SO has
been previously investigated in ZrOF as well as in its
nonaminated version (UiO-66), involving toxic chlorobenzene
solvent that showed inferior results even under harsh reaction
conditions (2.0 MPa, 100 °C).63 The current results manifest
the mutual effects of suitable acidity/basicity, microporosity, in
combination with additional Ni NPs in the solventless catalysis
under relatively milder conditions. Having recognized the
combination of Ni@ZrOF/TBAB as a most effective system for
chemical fixation of CO2, we considered it worthwhile to match
the efficiency of this system with the earlier reported MOFs.
The collective results are furnished in Table S5, which shows
that reactions over ZIFs and MOFs mostly require high-energy
conditions (relatively high temperature and/or CO2 pressure).
Moreover, few catalysts like MOF-5 are prone to moisture,
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Figure 6. Effects of various reaction parameters on the yield and selectivity of SC (a) amount of catalyst, (b) temperature, (c) amount of cocatalyst,
(d) CO2 pressure, (e) time {reaction conditions: SO, 30.6 mmol (for parts a−e); catalyst, Ni@ZrOF 0.35 mol % (for parts b−e); TBAB co-catalyst,
0.11 mol % (for parts a, b, d, e); temperature, 70 °C (for parts a, c, d, e); pressure, 1.0 MPa (for parts a, b, c, e); duration, 6 h (for parts a−d)}, and (f)
recyclability of the catalyst up to five cycles.
Catalyst Recyclability and Substrate Scope. To assess
the recyclability and long-term stability of the catalyst, cycling
experiments were undertaken under optimized conditions for
the representative reaction (vide supra). The catalyst was
separated from the reaction mixture via centrifugation (5000
rpm for 10 min) after each reaction run and reused in the next
cycle. Impressively, gas chromatography (GC) analysis showed
no significant loss in activity in up to five successive catalytic
cycles (Figure 6f). The PXRD analysis on the reused Ni@ZrOF
catalyst was performed after recycling studies, which showed
that characteristic peaks remain unchanged (Figure S8).
Additionally, FTIR analyses (Figure S9) and SEM imaging
(Figure S10) provided further evidence that reused catalyst
maintained the structural integrity and well-defined morphol-
ogy. It is worth pointing out that Ni0 could come out from the
catalyst pore during the reaction, which will eventually make
the entire catalyst less active for the subsequent cycles. To
exclude such possibility, the reaction was continued with the
filtrate in the absence of catalyst for additional 12 h that did not
reveal any increase in the SC formation. At the culmination of
the reaction, ICP-AES analysis was performed on the reaction
mixture filtrate that showed no presence of the Ni and/or Zr.
Alternatively, the recovered catalyst revealed that the amount of
Ni and Zr in Ni@ZrOF remains almost unchanged. Nitrogen
adsorption isotherms of fresh catalyst and used catalyst showed
no significant change in N2 adsorption (Figure S11), indicating
integrity and porosity of the system are well maintained even
after reaction. These collective results demonstrate that
encapsulated nanoparticles never come out of the cavity during
the reaction.
To ascertain the versatility of this catalytic system for CO2
cycloaddition, we extended the reaction using different other
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Figure 7. Substrate scope for Ni@ZrOF/Bu4NBr catalytic system with yield (red) and selectivity (green). Optimized reaction conditions: epoxide,
30.6 mmol; Ni@ZrOF, 0.35 mol %; Bu4NBr, 0.11 mol %; CO2 pressure, 1.0 MPa; 70 °C, 6 h.
substrates under the optimized conditions. Since the experi-
ments were conducted under solvent-free conditions, liquid
substrates were selected (Figure 7). To the best of our catalytic
system, high yields were preserved for a wide range of aliphatic
as well as aromatic epoxides, producing more than 95% yield
with very high selectivity in every case. These results are
essentially better than those reported previously under
comparable reaction conditions. However, cyclohexene oxide
exhibits reasonably low yield of the analogous cyclic carbonate
(4.5%) even though the reaction selectivity is found to be good.
This inconsistency can be ascribed to the hindrance of anionic
attack due to the steric crowding of the cyclohexene ring, as also
recognized for other catalytic systems.68,69 On the other hand,
larger substrates like 1,2-epoxydodecane, and 2-ethylhexyl
glycidyl ether showed poor yields of just 6 and 5% (Figure
7), respectively. The sharp decrease in product yield suggests
that larger substrates cannot diffuse inside the porous structure
of Ni@ZrOF catalyst. Such size selective cycloaddition
specifically points to the fact that reactions with other substrates
occurred within the framework cavity rather than on the
surface.
active centers. This leads to minimization of the energy barrier
of the reaction and increases the cyclic carbonate yield in
comparison to what has so far been achieved by the sole Zr4+
center in ZrOF. This step triggers electrophilic attack toward
the oxygen atom of the epoxide (Scheme 3), whereby ring
Scheme 3. Proposed Reaction Mechanism for the
Cycloaddition of CO2 and SO Using Synergic Ni@ZrOF/
TBAB Catalyst
Mechanism for Catalytic Reaction. Given that the
catalytic applications in MOF are a versatile and emerging
branch of organic transformation with a heterogeneous manner,
extending the synergic insight among diverse active sites and
their astute utilization will not only provide the comprehensive
mechanistic pathways but also enrich the library of active
catalysts with desirable properties. The high CO2 adsorption
potential together with the efficient catalytic performance of the
Ni@ZrOF system are attributed to its unique hollow structure
with Zr4+ acidic sites, incorporated Ni nanoparticle, and
pendent −NH2 groups that collectively manipulate the inherent
electronic and surface properties of the host material. On the
basis of the above results and previous other studies,70,71 we put
forth a tentative mechanism for the cycloaddition reaction.
Evidently, the catalytic performance of Ni@ZrOF is influenced
by the synergistic effect of both Ni0 and Zr4+ metals, which
promotes activation of the epoxide oxygen atom by distinct
opening takes place by the attack of a Br− ion from the
cocatalyst TBAB on the less hindered carbon atom of the
epoxide.72,73 Subsequently, the O− of the epoxide ring attacks
the carbon atom of the polarized CO2 molecule, while the
oxygen atom of the CO2 molecule attacks the β-carbon of the
ring opened epoxide after elimination of a bromide ion. It is
worth mentioning that inside the autoclave reactor, we
experienced a rapid decline in CO2 pressure, which is indicative
of an extremely supportive environment provided by the MOF
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catalyst. Ring closure in the final step produces SC and
consequently regenerates Ni@ZrOF, which is then transferred
to the next cycle of cycloaddition by coordination with a new
epoxide molecule.
able size selectivity. We put forth a tentative mechanism for the
cycloaddition reaction on the basis of literature reports and
experimental outcome. In a nutshell, the present system
represents a valuable family member of MOFs to future
heterogeneous catalysis in terms of abundant active sites, high
stability, and sufficient reusability and supplements the
suitability in extending its scope from academia and research
to industrial perspectives.
EXPERIMENTAL SECTION
■
Synthesis of ZrOF. ZrOF was synthesized according to the
previous report with slight modifications. ZrCl4 (233 mg, 1 mmol) and
NH2−H2BDC (181 mg, 1 mmol) were dissolved in 40 mL of DMF,
and a small amount of water (0.150 mL, 8.3 mmol) was added to the
solution. The mixture was sonicated for half an hour and transferred
into a Teflon-lined stainless-steel autoclave. The autoclave was sealed
and heated to 120 °C for 24 h. After cooling to room temperature, the
yellow crystalline powder is collected by filtration and thoroughly
washed with fresh DMF (3 × 10 mL times) and methanol (3 × 10 mL
times) every 12 h. The yield of the yellow crystalline material obtained
after air drying was 330 mg.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00833.
Materials and physical measurements; experimental
section; TGA curves; HRTEM and SEM images;
SAED patterns; Raman and XPS spectra; BET results;
adsorption curves; PXRD, FTIR, and NMR spectra; pore
volume and surface areas on the basis of N2 and CO2
adsorption isotherms; fitted parameters for adsorption
isotherms at 273 K, CO2/N2, and CO2/CH4 adsorption
selectivity; comparison table for CO2 adsorption
capacity, isosteric heat of CO2 adsorption, and Qst for
various MOFs; and comparison of catalytic activities of
Ni@ZrOF catalyst with previously reported MOFs
(PDF)
Synthesis of Ni@ZrOF. As described below, Ni@ZrOF metal
composite was synthesized in two steps. The first step involves the
incorporation of Ni2+ in ZrOF, and the second step is reduction of
embedded metal ions to metal nanoparticles.
Synthesis of Ni2+@ZrOF. Ni2+@ZrOF has been prepared via the
solution impregnation method. In a typical procedure, 1.0 g of
activated ZrOF (obtained upon heating under vacuum at 120 °C for 5
h) was homogeneously dispersed in 100 mL of methanol by
ultrasonicating for 30 min. Then, Ni(NO3)2·6H2O (100 mg) as the
Ni2+ source (in 10 mL of methanol) was added dropwise to the ZrOF
solution under vigorous stirring (to avoid agglomeration of metal
particles) in a 250 mL round-bottom flask. Vigorous stirring was
continued at 800 rpm for 24 h at room temperature. Afterward, Ni2+@
ZrOF was collected by centrifugation, washed three times with
methanol to remove free/excess metal salt from the surface of ZrOF,
and finally dried at 80 °C.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sneogi@csmcri.res.in.
■
Conversion of Ni2+@ZrOF to Ni@ZrOF. Calcination of the as
synthesized Ni2+@ZrOF was carried out in a tubular furnace under a
steam of 10% H2/N2 gas mixture at 200 °C for 2 h at a ramp rate of 5
°C/min. During this process, Ni(II) was reduced and a dark yellow
colored material was formed, which indicated the formation of MNPs
inside ZrOF. The product was thoroughly characterized by means of
several analytic techniques.
ORCID
Manpreet Singh: 0000-0002-0335-6147
Pratik Solanki: 0000-0002-1440-7931
Parth Patel: 0000-0001-9742-2784
Aniruddha Mondal: 0000-0002-5742-4459
Subhadip Neogi: 0000-0002-3838-4180
CONCLUSIONS
■
Notes
In summary, we report for the first time encapsulation of
ultrasmall Ni nanoparticles (NPs) inside the pore of an amine
grafted Zr(IV)-framework. The embedded Ni NPs act as
additional functionality that in turn drastically improves the
adsorption capacity of the major greenhouse gas CO2. The
isosteric heat of CO2 adsorption also experiences a 10 kJ/mol
upsurge than the parent material, highlighting auxiliary
interactions between CO2 molecules and encapsulated NPs.
Further, the postmodified system unveils excellent CO2
selectivity over other gases and demonstrates remarkable
sorption recurrences during multicycle CO2 adsorption. The
collective results render the NP loaded framework as a potential
candidate for practical CO2 sequestration applications. Most
fascinatingly, the unique hollow structure, incorporated Ni NPs,
and pendent basic functionality mutually manipulate the
electronic environment of the pore, which allows outstanding
heterogeneous catalytic performance toward solvent-free cyclo-
addition of CO2 and epoxides under relatively mild conditions.
It is imperative to stress that efficiency of catalyst is superior to
many of the well-known MOFs and it can be recycled several
times without metal leaching or significant loss in activity. The
outstanding yield and selectivity of the catalyst are maintained
for a wide range of aliphatic and aromatic epoxides, while larger
substrates provide negligible conversion, corroborating admir-
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.N. acknowledges DST-SERB (Grant No. ECR/2016/
000156) and CSIR (Grant No. MLP-0028). M.S. acknowledges
UGC, Delhi, for providing a junior research fellowship. A.M.
and P.P. acknowledge CSIR for senior research fellowships. P.S.
acknowledges Manipal University for the M.Sc. internship. The
analytical support from ADCIF is greatly acknowledged. This
Article is CSMCRI Communication No. 003/2019.
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DOI: 10.1021/acs.inorgchem.9b00833
Inorg. Chem. XXXX, XXX, XXX−XXX