Porous noria polymer: a cage-to-network approach toward a robust catalyst for CO2fixation and nitroarene reduction

Article abstract of DOI:10.1039/d0cc07805k

The advantages of the cage-to-network design strategy were demonstrated by knitting a waterwheel-like preporous molecular cage, noria, with a rigid aromatic linker to obtain a highly microporous organic polymer (NPOP,S_BET: 748 ± 25 m²g^?1). The NPOP was employed for the catalytic conversion of CO₂to cyclic carbonates under solvent-free reaction conditions. Furthermore, a silver nanoparticle encapsulated NPOP exhibited remarkable catalytic activity for nitroarene reduction with excellent recyclability.

Full text of DOI:10.1039/d0cc07805k

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Porous noria polymer: a cage-to-network
approach toward a robust catalyst for CO2
fixation and nitroarene reduction†
Cite this: Chem. Commun., 2021,
57, 4404
Received 30th November 2020,
Accepted 19th March 2021
Arkaprabha Giri, ‡ Niraj Nitish Patil‡ and Abhijit Patra
*
DOI: 10.1039/d0cc07805k
rsc.li/chemcomm
The advantages of the cage-to-network design strategy were However, there is a lack of easily scalable preporous building
demonstrated by knitting a waterwheel-like preporous molecular blocks with multiple reacting sites and cost-effective fabrication
cage, noria, with
a
rigid aromatic linker to obtain
a
highly strategies to incorporate these molecular hosts into the polymer
microporous organic polymer (NPOP, SBET: 748 Æ 25 m2
NPOP was employed for the catalytic conversion of CO2 to cyclic
g
À1). The matrix (Fig. S1, ESI†).
In the present study, we employed a molecular host, noria
carbonates under solvent-free reaction conditions. Furthermore, a (literal meaning: a wheel propelled by water current), to develop
silver nanoparticle encapsulated NPOP exhibited remarkable a POP with a high specific surface area through a cage-to-
catalytic activity for nitroarene reduction with excellent recyclability. network design strategy (Fig. 1 and Fig. S1, ESI†). Noria consists
of twenty-four phenolic hydroxyl groups, the maximum number
Porous organic polymers (POPs) with high specific surface of propagating sites in a cage molecule reported so far (Table S1
area, tunable pore size, excellent hydrothermal stability, and and Scheme S1, ESI†), along with six outer cavities, and a
a functionalizable pore wall have emerged as the holy grail in
heterogeneous catalysis.1 Furthermore, the post-synthetic
modification of POPs through the encapsulation of metal
nanoparticles augments the activity due to the distribution of
a large number of catalytic sites across the network.2 On the
other hand, porous organic molecules (POMs), such as macro-
cycles, cavitands, and cages with functionalized hollow
structures, are well-known for selective guest recognition and
enzyme-mimetic catalysis.3,4 Of late, an exciting POM-to-
network design strategy has been demonstrated to amalgamate
the unique features of POMs and POPs.5–8 The benefit of such a
strategy is the facile integration of the dimensional and
functional parameters of the ‘preporous’ building blocks into
the final network.5a Zhang and coworkers demonstrated a
series of macrocycle/cage-based frameworks for selective gas
uptake and catalysis.5a,6 Dai and coworkers employed a Au
nanoparticle loaded b-cyclodextrin-based hypercrosslinked
polymer for the reduction of nitrophenols.7 We reported
C-phenylresorcin[4]arene, a macrocyclic cavitand-based POP,
for molecular separation and heterogeneous catalysis.8
Department of Chemistry, Indian Institute of Science Education and Research
Fig. 1 The hallmark of the present work is the use of rigid aromatic linkers
to knit the waterwheel-like preporous molecular cage, noria, developing a
highly porous organic polymer (POP) for heterogeneous catalysis. Sche-
matic representation illustrating the fabrication of a noria-based porous
organic polymer (NPOP) through the mild base-catalyzed nucleophilic
aromatic substitution reaction.
Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462066, Madhya Pradesh, India.
E-mail: abhijit@iiserb.ac.in
† Electronic supplementary information (ESI) available: Synthesis and character-
ization, catalytic CO2 conversion, nitrophenol reduction, and comparative analy-
sis of multifunctional properties. See DOI: 10.1039/d0cc07805k
‡ These authors contributed equally.
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hydrophobic central void (Fig. 1).9 To date, cross-linked noria- specific BET surface area of the NPOP was found to be 748 Æ
based polymers exhibited very low specific surface areas and 25 m2 gÀ1. High nitrogen uptake by the NPOP at low relative
were not explored for molecular storage or catalysis (Fig. 1).10 pressures (P/P0 o 0.05) with a Type Ib isotherm indicated its
We introduced a rigid F-rich aromatic linker, tetrafluoroter- microporous nature (pore dimension o2 nm, Fig. 2a).12 Pore-
ephthalonitrile (TFN), to knit noria through a cost-effective size distribution (PSD) estimated by the nonlocal density func-
nucleophilic aromatic substitution (SNAr) reaction providing a tional theory (NLDFT) method revealed the presence of bimo-
highly porous and thermally robust NPOP (Fig. 1). The CO2- dal pores at 0.7 and 1.4 nm for the network polymer, similar to
philic NPOP showed excellent catalytic activity with high that of pristine noria (Fig. 2b). The pores at 0.7 nm in the PSD
recyclability toward the conversion of epoxides to cyclic of the NPOP, as well as noria, originated from the central void
carbonates under solvent-free mild reaction conditions. (diameter: B0.67 nm, CCDC no. 1451803) of the cage (Fig. 2b).9
Furthermore, we demonstrated
modification of the NPOP by loading Ag nanoparticles that flat aromatic phenolic core, phloroglucinol, and TFN,
showed a remarkable catalytic reduction of nitroarenes.
possessing a specific surface area of only 15 m2 gÀ1 (Fig. S5,
a
facile post-synthetic
We synthesized an analogous polymer Ph-POP, employing a
Functionalization of reactive phenolic hydroxyl sites of noria ESI†). The nonporous nature of Ph-POP compared to the highly
through a mild base (K2CO3) catalyzed SNAr reaction with TFN microporous NPOP ascertained the importance of the cage-to-
(6 eqv.) in dioxane : DMF (1 : 1 v/v) led to a brown colored solid, network design strategy using the preporous building unit,
NPOP (Fig. 1 and Table S2, ESI†). The broad peaks in the FTIR noria. Morphological analysis using field emission scanning
spectra of the NPOP at 3400–3600 cmÀ1 appeared due to the electron microscopy (FESEM) revealed plate-like particles for
residual phenolic–OH stretching of the noria core (Fig. S2a, the NPOP (Fig. S6a, ESI†) commonly observed in F-rich
ESI†). Similarly, the peaks at 2938 cmÀ1 were assigned for the networks.8 The high-resolution transmission electron micro-
aliphatic C–H stretching that originated from the noria core. scopy (HRTEM) images of the NPOP showed an irregular
Furthermore, the nitrile (CRN) stretching at 2242 cmÀ1 in the porous structure typically observed in amorphous microporous
NPOP confirmed the condensation of TFN and noria in the organic polymers (Fig. S6b, ESI†).8 The broad powder X-ray
network. In the solid-state 13C cross-polarization magic angle diffraction (PXRD) pattern further suggested the amorphous
spinning (CP/MAS) NMR, the peaks at 28 and 35 ppm were nature of the NPOP (Fig. S7, ESI†).
attributed to the –CH– and –CH2– functionalities, respectively
The heteroatom decorated noria cage exhibited a high
(Fig. S2b, ESI†). The peaks at 153 ppm were indicative of C–O affinity for CO2 over N2 [CO2 uptake 7.8 wt%, CO2/N2 selectivity
bond formation.11 The nitrile group (–CN) present in the TFN (IAST model): 24 at 1 bar, 273 K, Fig. S8a, ESI†]. The NPOP
linker resonated at 105 ppm.11 However, the peaks resonating showed a further enhanced CO2 uptake capacity of 9.2 wt% and
at around 100 to 160 ppm signified the aromatic carbons 6 wt% at 273 K and 298 K, respectively at 1 bar with a CO2/N2
present in noria and TFN moieties (Fig. S2b, ESI†). Thermo- selectivity of 30 at 273 K (Fig. S8b, ESI†). Noria and NPOP
gravimetric analysis of the NPOP indicated that the polymer showed considerably high heat of adsorptions of 30.4 and
was highly stable up to B400 1C (Fig. S3, ESI†).
28.9 kJ molÀ1, respectively, for CO2 (Fig. S9, ESI†).12b The
The surface areas of the pristine noria cage and NPOP were conversion of CO2 to value-added chemicals like cyclic organic
determined by N2 sorption isotherms measured at 77 K carbonates has become extremely relevant in the context of
(Fig. 2a). The specific Brunauer–Emmett–Teller (BET) surface environmental remediation.13 Considering the facile activation
area of pristine noria was found to be 218 Æ 3 m2 gÀ1 with a of epoxides by phenolic–OH groups in noria through
Type II isotherm (Table S3 and Fig. S4, ESI†). Conversely, the H-bonding,8 we employed noria and NPOP for the catalytic
conversion of CO2 and epoxides into cyclic carbonates
(Table S5, ESI†).
We chose styrene oxide (10 mmol, B1.2 g) as the model
substrate to optimize the catalytic conversion of CO2 to cyclic
carbonates. The reaction was performed at 2.5 bar of CO2
pressure at 90 1C using noria/NPOP (30 mg, 0.02 wt%) as a
catalyst and tetra-n-butyl ammonium bromide (TBAB,
0.30 mmol) as cocatalyst under solvent-free conditions for
12 h (Fig. 3a). The % of conversion to styrene carbonate was
found to be 84% and 88% using noria and NPOP, respectively,
as the catalysts (Fig. S29 and S30, ESI†). A very low conversion
(26%) in the absence of the catalyst suggested the role of
H-bond donating noria and NPOP in the activation of epoxides
(Fig. S10 and S28, ESI†).8 Unlike the NPOP, noria was not
recyclable. Thus, we explored the NPOP further for the catalytic
conversion of a range of epoxide derivatives to cyclic carbonates
demonstrating good to excellent conversion (up to 99%, Fig. 3b
and Fig. S32–S37, ESI†). The catalytic performance of noria, as well
Fig. 2 (a) Nitrogen sorption isotherms of noria and NPOP at 77 K (filled
circles: adsorption, hollow circles: desorption); digital photographs of the
respective powder samples are shown. (b) The corresponding pore size
distributions were estimated using the nonlocal density functional theory
(NLDFT) method; inset: zoomed image for the pore size distribution
showing the bimodal pores centered at 0.7 and 1.4 nm. The space-filling
model of noria showing the internal void with a diameter of 0.67 nm
(CCDC no. 1451803).9c
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Fig. 3 (a) CO2 fixation using styrene oxide to the corresponding cyclic
organic carbonate catalyzed by NPOP and noria. (b) Substrate scope for
NPOP-catalyzed cycloaddition of CO2 with diverse epoxides leading to
cyclic carbonates (% of conversion was calculated through 1H NMR
analysis). (c) Recyclability of the NPOP for catalytic conversion of styrene
oxide and CO2 to styrene carbonate. The amount of recovered catalyst
(wt%) in subsequent cycles is represented by red dots; the decrease in % of
conversion is due to the weight loss of the catalyst (B1 mg per cycle)
during the recovery process.
as the NPOP under milder reaction conditions, was comparable or
better than those of many homo- and heterogeneous catalysts
reported in the literature (Table S6, ESI†).13 The NPOP was easily
recovered from the reaction mixture by centrifugation, and the
percentage of conversion was found to be B77%, even after
10 cycles, demonstrating the reusability of the catalyst (Fig. 3c).
The stability of the recovered catalyst was ascertained through
FTIR analysis (Fig. S11b, ESI†). FESEM and HRTEM images of the
NPOP before and after 10 catalytic cycles suggested unaltered
morphology (Fig. S12, ESI†).
The NPOP could easily be post-functionalized through the
loading of metal nanoparticles resulting in nanocomposites
propitious for efficient heterogeneous catalysis. We found facile
reduction of AgNO3 to Ag (0) nanoparticles using the NPOP
without any external reducing agent (Scheme S3, ESI†). The
reducing properties of the NPOP were attributed to the poly-
phenolic noria core analogous to the reducing activity of
natural polyphenols.14 Furthermore, the phenolic –OH groups
Fig. 4 (a) HRTEM image of the Ag@NPOP; inset: image at high magnification
depicting the loading of Ag nanoparticles in the NPOP matrix. (b) XPS
spectrum of Ag(0) 3d in the Ag@NPOP. (c) Ultraviolet-visible spectra for the
conversion of 4-nitrophenol (0.13 mM) to 4-aminophenol catalyzed by the
Ag@NPOP (1 mg mLÀ1, NaBH4: 0.2 M) at successive time interval; inset: digital
images indicating the color change during the conversion, and the kinetic
profile of the reduction illustrating the pseudo-first-order reaction. (d) The %
of conversions of various nitroarenes (0.13 mM) into the corresponding
aromatic amine derivatives with time by the Ag@NPOP (1 mg mLÀ1, NaBH4:
0.2 M); the Ag@NPOP outperformed commercial Ag nanoparticle dispersion
(Ag NPs) under identical reaction conditions. (e) The high % of conversion for
consecutive 15 cycles suggests the recyclability of the Ag@NPOP.
in noria and the nitrile groups (–CRN) of TFN prevented spectroscopy (XPS) analysis indicated peaks at 368.3 and
the aggregation of nanoparticles, leading to a homogeneous 374.3 eV (with a spin energy separation of 6 eV) attributed to
distribution of Ag nanoparticles throughout the NPOP matrix Ag(0) 3d5/2 and Ag(0) 3d3/2, respectively, ascertaining the
(Fig. 4a and Fig. S13, ESI†).15 The decrease in surface area formation of silver nanoparticles (Fig. 4b and Fig. S15, ESI†).16
(Ag@NPOP: 100 m2
g
À1) after the metal loading implied Inductively coupled plasma optical emission spectrometry
the occupancy of the accessible surfaces of the NPOP by Ag (ICP-OES) analysis suggested B5.4 wt% of silver loading in
nanoparticles (Fig. S14, ESI†).2b
the Ag@NPOP.
A statistical estimation from the HRTEM images unveiled
We employed the Ag@NPOP as a heterogeneous catalyst for
the size of the Ag nanoparticles in the range of 6–15 nm (Fig. 4a the reduction of 4-nitrophenol to 4-aminophenol in the
and Fig. S13, ESI†). High-angle annular dark-field scanning presence of a hydride donor (NaBH4) at room temperature.
transmission electron microscopy (HAADF-STEM) imaging The progress of the reaction was monitored by UV-Vis spectro-
confirmed the homogeneous distribution of Ag nanoparticles scopy (Scheme S4, ESI†). Initially, the light-yellow color of
into the porous network (Fig. S13e, ESI†). The PXRD pattern of 4-nitrophenol was changed to bright yellow after the deproto-
Ag@NPOP showed the characteristic peaks of Ag (0) at 2y values nation of phenolic–OH by NaBH4 (Fig. 4c, inset). Then, the
27.61, 38.11, 44.21, 64.41, and 77.31 for miller indices (110), solution gradually became colorless upon the addition of the
(111), (200), (220), and (311), respectively, confirming the Ag@NPOP. The absorption peak around 408 nm corresponding
successful incorporation of Ag nanoparticles (JCPDS no. 04-0783) to the bright yellow solution decreased with a concomitant
into the NPOP matrix (Fig. S13f, ESI†).16 X-ray photoelectron increase of the peak at around 315 nm due to the presence of
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4-aminophenol (Fig. 4c). The reaction conditions were Conflicts of interest
optimized by changing the concentration of NaBH4 (0.05–0.2 M)
There are no conflicts to declare.
and catalyst (50–100 mL dispersion of 1 mg mLÀ1) for the reduction
of nitrophenol (0.13 mM, Table S4 and Fig. S16, ESI†). 96%
conversion occurred within 3.5 min using 50 mL of 1 mg mLÀ1
of Ag@NPOP dispersion and 0.2 M of NaBH4 (Fig. 4c and
Fig. S16, ESI†).
Notes and references
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A linear correlation between ln(Ct/Co) [where Ct = the
absorbance of the reaction mixture (B408 nm) at time t and
C0 = the absorbance before the addition of the Ag@NPOP]
against time suggested pseudo-first-order kinetics (Fig. 4c,
inset).17 The rate constant for the catalytic reduction of
4-nitrophenol was estimated to be 1.60 Â 10À2 sÀ1 which was
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catalysts (Table S7, ESI†).18 The Ag@NPOP exhibited faster
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as catalyst showed high conversion (B90%) within 2 h (Fig.
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conversion up to 15 cycles indicated excellent recyclability of
the catalyst (Fig. 4e). Furthermore, FTIR, FESEM, and HRTEM
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under identical reaction conditions, presumably due to a
homogeneous dispersion of nanoparticles throughout the por-
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investigations also revealed a promising catalytic performance
of the Ag@NPOP for the synthesis of propargylamine deriva-
tives through a multicomponent reaction (Fig. S23, S24 and
S41–S44, ESI†).
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Financial support from MHRD STARS/APR2019/CS/560/FS,
SERB/CHM/2017/113 (file no. EMR/2017/000233), infrastruc-
tural support, and FIST supported TEM facility to the
Dept. of Chemistry, IISER Bhopal are gratefully acknowledged.
AG and NP thank UGC and DST-INSPIRE, respectively, for a
fellowship.
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