Hollow and microporous catalysts bearing Cr(III)-F porphyrins for room temperature CO2 fixation to cyclic carbonates

Article abstract of DOI:10.1039/c7ta06655d

Hollow and microporous metal free/Cr-porphyrin networks were prepared via the Sonogashira coupling of metal-free or Cr-tetra(4-ethynylphenyl) porphyrins and 1,4-diiodobenzene on the surface of silica templates followed by silica etching. Zinc was introduced into a hollow and microporous metal free porphyrin network (H-MPN) through post-synthetic modification to form a H-MZnPN. Hollow and microporous Cr(iii)-F porphyrin networks (H-MCrPNs) showed the best catalytic activities in room temperature CO₂ fixation with epoxides to cyclic carbonates. In addition, the H-MCrPN could be reused at least for five runs, maintaining the original catalytic activity. The good performance of the H-MCrPN is attributed to its microporosity, the shortened diffusion pathways for substrates due to the hollow structure, and the efficient Lewis acidic activity of Cr(iii)-F moieties.

Full text of DOI:10.1039/c7ta06655d

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Hollow and Microporous Catalysts Bearing Cr(III)-F Porphyrins for
Room Temperature CO2 Fixation to Cyclic Carbonates
Myung Hyun Kim,a Taemoon Song,b Ue Ryung Seo,b Ji Eun Park,b Kyoungil Cho,a Sang Moon Lee,c
Hae Jin Kim,c Yoon-Joo Ko,d Young Keun Chung,*b and Seung Uk Son*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Hollow and microporous metal free/Cr-porphyrin networks were prepared via the Sonogashira coupling of
metal-free or Cr- tetra(4-ethynylphenyl) porphyrins and 1,4-diiodobenzene on the surface of silica templates
followed by silica etching. Zinc was introduced to hollow and microporous metal free porphyrin network (H-
MPN) through postsynthetic modification to form H-MZnPN. Hollow and microporous Cr(III)-F porphyrin
networks (H-MCrPN) showed the best catalytic activities in the room temperature CO2 fixation with epoxides
to cyclic carbonates. In addition, the H-MCrPN could be reused at least for five runs, maintaining original
catalytic activity. The good performance of H-MCrPN is attributed to its microporosity, the shortened
diffusion pathways for substrates due to the hollow structure, and the efficient Lewis acidic activity of Cr(III)-
F moieties.
www.rsc.org/
Recently, microporous organic networks (MONs) containing
catalytic sites have been prepared via the networking of preꢀ
Introduction
Chemical CO2 fixation has attracted great attention of
scientists.1 One of the facile CO2 fixation methods is the
reaction of CO2 with epoxides to form cyclic carbonates which
are important polymer precursors, solvent of electrolytes for
secondary batteries, etc.2 Ammonium or phosphonium halides
facilitate this reaction through the ring opening nucleophilic
attack of halides to epoxides.3ꢀ7 The resulting alkoxide
intermediates trap CO2 to form cyclic carbonates. This CO2
fixation process without Lewis acid catalysts usually requires
high reaction temperatures (>100oC) despite being an
exothermic reaction. Lewis acids including Zn or Crꢀ
compounds can accelerate this reaction8ꢀ14 and can therefore
lower the reaction temperature below 100oC.9ꢀ14 Such CO2
fixation catalysts that work at room temperature are most
desirable in the viewpoint of energy efficiency. In order to
allow recycling of Lewis acids, heterogeneous catalysts for CO2
fixations have been developed.15 However, their catalytic
performance at room temperature was less explored.16ꢀ20
designed building blocks, revealing stability and ability of
facile chemical tuning.21ꢀ22 Moreover, MONs bearing Lewis
acids have shown reasonable activities in the CO2 conversion to
cyclic carbonates at reaction temperatures below 100oC.9ꢀ11,14ꢀ21
However, more exploration is required to improve the catalytic
activities of materials at room temperature. The microporosity
of MONs enables substrates to diffuse into the materials.
Hence, the Lewis acid sites in inner MONs can be utilized in
chemical conversions. However, the conventional micron or
submicron sized MONs suffer from the long diffusion pathways
of substrates and the possible clogging of the pathways by
products. Our research group has shown that the MONs can be
engineered to form hollow structures with thin shells (shell
thickness < 50 nm) by template methods.27 The soꢀobtained
reduced diffusion pathways for substrates can improve catalytic
activity.28ꢀ30
In this work, we report the room temperature catalytic
activities of hollow and microporous Cr and Zn porphyrin
networks (HꢀMCrPN and HꢀMZnPN) in the CO2 fixation to
cyclic carbonates.
a.Department of Chemistry, Sungkyunkwan University, Suwon 16419,
Korea *E-mail: sson@skku.edu
b.Department of Chemistry, College of Natural Sciences, Seoul National
University, Seoul 08826, Korea *E-mail: ykchung@snu.ac.kr
c. Korea Basic Science Institute, Daejeon 34133, Korea
Experimental
General Information
d.Laboratory of Nuclear Magnetic Resonance, The National Center for
Inter-University Research Facilities (NCIRF), Seoul National University,
Seoul 08826, Korea
Transmission electron microscopy (TEM) was conducted
using a JEOL 2100F. Scanning electron microscopy (SEM) and
energy dispersive Xꢀray spectroscopy (EDS) were conducted
using a JSM6700F. N2 adsorptionꢀdesorption isotherm curves
were obtained at 77K using a BELSORP IIꢀmini analyzer. Pore
size distribution of materials was analyzed based on a density
functional theory (DFT) method. Powder Xꢀray diffraction
† Electronic Supplementary Information (ESI) available: Characterization
data of building blocks and model studies, PXRD patterns and XPS
spectra of H-MCrPN, H-MPN, and H-MZnPN, size evaluation of
substrates and products, chemoselectivities for cyclic carbonates
depending on cocatalysts, IR spectra of H-MCrPN before and after
reaction, NMR spectra of cyclic carbonates, and CO2 adsorption
behaviors of H-MCrPN. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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(PXRD) studies were conducted using a Rigaku MAXꢀ2200 ethynylphenyl) Cr(III)ꢀCl porphyrin was isolated by
(CuꢀK
radiation). Solid state 13C nuclear magnetic resonance recrystallization from a mixture of chloroform and hexane,
α
(NMR) spectroscopy was performed using a 500 MHz Bruker washed with hexane, and dried under vacuum. The tetra(4ꢀ
ADVANCE II NMR spectrometer with a mode of crossꢀ ethynylphenyl) Cr(III)ꢀCl porphyrin was characterized by IR
polarization/total side band suppression (CPꢀTOSS) at the spectroscopy, confirming intact terminal alkyne groups. (Fig.
NCIRF of Seoul National University. A 4 mm magic angle S1 in the ESI)
spinning probe was used with a spinning rate of 5 kHz. Solution
For the preparation of SiO2@MCrPN, silica spheres (0.250 g),
state H and 13C NMR spectra were obtained using a 400 MHz (PPh3)2PdCl2 (8.4 mg, 0.012 mmol), and CuI (2.3 mg, 0.012
and 500 MHz Varian spectrometers. Infrared absorption (IR) mmol) were added to a mixture of distilled triethylamine (15
spectroscopy was performed using a Bruker VERTEX 70 FTꢀ mL), distilled toluene (10 mL), and distilled DMF (5 mL) under
IR spectrometer. Diffuse reflectance spectroscopy was argon. The reaction mixture was sonicated at room temperature
conducted using a Shimadzu UNꢀ3600. The UV/vis absorption for 2 h. After tetra(4ꢀethynylphenyl) Cr(II)ꢀCl porphyrin (96
spectroscopy was performed using a JASCO Vꢀ630. Xꢀray mg, 0.12 mmol) and 1,4ꢀdiiodobenzene (79 mg, 0.24 mmol)
photoelectron spectra (XPS) were obtained using a Thermo VG being added, the reaction mixture was heated at 90oC for 48 h
1
and AlꢀKα radiation. Elemental analysis was conducted using a under argon. After cooled to room temperature, the solid
CE EA110 analyzer.
(SiO2@MCrPN) was separated by centrifugation, washed with
methanol, methylene chloride, and acetone, and dried under
vacuum. HF solution (48~51% HF aqueous solution, 5 mL) was
diluted by adding water (25 mL) and methanol (20 mL).
SiO2@MCrPN was treated with a diluted HF solution in a 50
mL Falcon tube. The reaction mixture was stirred at room
temperature for 2 h. The solid (HꢀMCrPN) was separated by
centrifugation, washed with a 3:2 mixture of water and
methanol, and acetone, and dried under vacuum.
Synthetic Procedures for H-MCrPN
Silica spheres with diameters of ~420 nm were prepared by
the synthetic procedures of the literature.31 In our study, the
following procedures were applied. Ethanol (200 mL), water
(23.5 mL), and NH4OH solution (28~30%, 7 mL) were added
to a 250 mL round bottomed flask. The reaction mixture was
stirred at 1150 rpm for 5 min. After tetraethyl orthosilicate
(17.5 mL) was added, the reaction mixture was stirred at room
temperature for 18 h. The silica spheres were separated by Synthetic Procedures for H-MZnPN
centrifugation, washed with hexane, and dried under vacuum.
For the preparation of SiO2@MPN, silica spheres (0.25 g),
Tetra(4ꢀethynylphenyl) porphyrin was prepared by the (PPh3)2PdCl2 (8.4 mg, 0.012 mmol), and CuI (2.3 mg, 0.012
synthetic procedures in the literature.32 Tetra(4ꢀethynylphenyl) mmol) were added to a mixture of distilled triethylamine (10
Cr(III)ꢀCl porphyrin was prepared by the following procedures. mL), distilled toluene (20 mL), and distilled THF (10 mL)
Tetra(4ꢀethynylphenyl) porphyrin (0.20 g, 0.28 mmol) was under argon. The reaction mixture was sonicated at room
dissolved in distilled DMF (5 mL) by heating at 170oC for 10 temperature for 2 h. After tetra(4ꢀethynylphenyl) porphyrin (85
min in a flameꢀdried 50 mL two neck Schlenk flask equipped mg, 0.12 mmol) and 1,4ꢀdiiodobenzene (79 mg, 0.24 mmol)
with a reflux condensor under argon.33ꢀ35 CrCl2 (0.124 g, 1.01 being added, the reaction mixture was heated at 90oC for 48 h
mmol) was dissolved in distilled DMF (3 mL) in a 10 mL vial under argon. After being cooled to room temperature, the solid
by sonnication. CrCl2 solution (0.75 mL) was added dropwise (SiO2@MPN) was separated by centrifugation, washed with
to porphyrin solution at 170oC and then, the reaction mixture methanol, methylene chloride, and acetone, and dried under
was stirred at 170oC for 10 min under argon.33ꢀ35 This addition vacuum. HF solution (48~51% in aqueous solution, 5 mL) was
was repeated three more times. The reaction mixture was diluted with water (25 mL) and methanol (20 mL). The
heated at 170oC for 30 min under argon.33ꢀ35 After the reaction SiO2@MPN was treated with the diluted HF solution in a 50
mixture being cooled to room temperature, water (200 mL) was mL Falcon tube at room temperature for 2 h. Violet powder (Hꢀ
added. The reaction mixture was put aside at room temperature MPN) was separated by centrifugation, washed with a 3:2
overnight to induce the formation of precipitates which were mixture of water and methanol, and acetone, and dried under
separated by filtration. The solid was washed with water and vacuum. HꢀMPN (0.21 g) and zinc acetate (0.26 g, 1.4 mmol)
o
hexane and dried in an oven at 90 C. After the solid being were added to a distilled methanol (60 mL) in a flameꢀdried
dissolved in chloroform, the target product was separated by 100 mL one neck Schlenk flask under argon. The reaction
column chromatography (aluminium oxide, eluent: chloroform mixture was heated at 90oC for 24 h under argon. After the
and a 40:1 mixture of chloroform and methanol). The red bands reaction mixture being cooled to room temperature, the powder
contained the tetra(4ꢀethynylphenyl) Cu(II)ꢀporphyrin (formed (HꢀMZnPN) was separated by centrifugation, washed with
by Cu residues) and metalꢀfree porphyrin. The dark green band methanol, THF, and acetone, and dried under vacuum.
contained the tetra(4ꢀethynylphenyl) Cr(III)ꢀCl porphyrin. To
Procedures for Catalytic Reaction
further purify the product, chloroform solution of tetra(4ꢀ
Tetraalkylammonium halide, PPNCl, HꢀMCrPN (0.86 mmol
ethynylphenyl) Cr(III)ꢀCl porphyrin was treated by HCl
solution (12 M) overnight.34 The removal of impurities was
Cr porphyrin/g), and HꢀMZnPN (0.99 mmol Zn porphyrin/g)
were dried at 60oC under vacuum for 12 h. MON catalyst (Hꢀ
confirmed by the disappearance of Q band at 630 nm in UVꢀvis
absorption spectroscopy.34 The product was extracted by
MCrPN: 71 mg, 0.061 mmol Cr porphyrin, HꢀMZnPN: 62 mg,
0.061 Zn porphyrin), ammonium halide (tetrahexylammonium
chloroform and then, the solvent was evaporated. Pure tetra(4ꢀ
2 | J. Name., 2012, 00, 1-3
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bromide, 0.39 g, 0.90 mmol), and epoxide (styrene oxide, 1.42 MHz):
δ =1.62–1.72 (m, 2H), 1.74–1.88 (m, 2H), 2.04–2.26 (m,
mL, 12.5 mmol) were added to a flameꢀdried two neck Schlenk 2H), 5.11(dd, J = 3.6, 1.6Hz, 2H) ppm. 13C NMR (CDCl3, 125
tube under argon. The reaction mixture was bubbled with CO2 MHz):
for 5 min and stirred (200 rpm) at room temperature (~27oC) Table 1: cyclohexaꢀ1,3ꢀdioxolꢀ2ꢀone, 1HꢀNMR (CDCl3, 500
with a CO2 balloon. The conversion of expoxides was analyzed MHz): = 1.38ꢀ1.50 (m, 2H), 1.59ꢀ1.72 (m, 2H), 1.83ꢀ1.99 (m,
by H NMR spectroscopy. For the recycle tests, the catalysts 4H), 4.69 (p, J = 6.1 Hz, 2H) ppm. 13C NMR (CDCl3, 125
δ = 21.6, 33.2, 81.8, 155.5 ppm. Product of Entry 10 in
δ
1
were separated by filtration (syringe filter: SX0001300), MHz):
washed with methanol, methylene chloride, and acetone, dried
δ = 19.2, 26.8, 75.7, 155.3 ppm.
under vacuum at 60oC, and reused for the next run.
Procedures for the Model Studies (The Conversion of
Cr(III)-Cl Porphyrin to Cr(III)-F Porphyrin)
The cyclic carbonates in this study are all known
compounds.36ꢀ42 1H and 13C NMR spectra of the cyclic
carbonate products matched well with those in the literature. 36ꢀ
This conversion was reported in the literature.44 HF solution
(50% in aqueous solution, 5 mL) was diluted with water (25
mL) and methanol (20 mL). Tetra(4ꢀethynylphenyl) Cr(III)ꢀCl
porphyrin (96 mg, 0.12 mmol) was treated with a diluted HF
solution at room temperature for 4 h. The tetra(4ꢀethynylphenyl)
Cr(III)ꢀF porphyrin was isolated by centrifugation, washed with
a mixture (3:2) of water and methanol, acetone, and dried under
vacuum. Tetra(4ꢀethynylphenyl) Cr(III)ꢀF porphyrin was
characterized by UVꢀvisible absorption spectroscopy44 and
EDS. (Fig. S2 in the ESI) After conversion of CrꢀCl to CrꢀF in
porphyrins, the Soret band of Crꢀporphyrins shifted from 451
nm to 441 nm, matching with the observation in the literature.44
The EDS confirmed the conversion of CrꢀCl to CrꢀF.
42
1H and 13C NMR data of the products are as follows.43
Product of Entry 1 in Table 1: 4ꢀphenylꢀ1,3ꢀdioxolanꢀ2ꢀone,
1HꢀNMR (CDCl3, 500 MHz):
δ
= 4.33 (dd, J = 8.6, 7.9 Hz, 1H),
4.81 (t, J = 8.4 Hz, 1H), 5.69 (t, J = 8.0 Hz, 1H), 7.19ꢀ7.52 (m,
5H) ppm. 13C NMR (CDCl3, 125 MHz):
= 71.2, 78.0, 125.9,
129.3, 129.8, 135.8, 154.8 ppm. Product of Entry 2 in Table 1:
4ꢀbutylꢀ1,3ꢀdioxolanꢀ2ꢀone, 1HꢀNMR (CDCl3, 500 MHz):
δ
δ
=
0.85ꢀ0.98 (m, 3H), 1.23ꢀ1.88 (m, 6H), 4.08 (dd, J = 8.4, 7.3 Hz,
1H), 4.56 (t, J = 8.1 Hz, 1H), 4.66ꢀ4.80 (m, 1H) ppm. 13C NMR
(CDCl3, 125 MHz):
Product of Entry 3 in Table 1: 4ꢀhexylꢀ1,3ꢀdioxolanꢀ2ꢀone, Hꢀ
NMR (CDCl3, 500 MHz): = 0.89 (t, J = 6.2 Hz, 3H), 1.22ꢀ
δ = 13.8, 22.3, 26.5, 33.6, 69.4, 155.1 ppm.
1
δ
Result and Discussion
1.88 (m, 10H), 4.07 (t, J = 7.8 Hz, 1H), 4.53 (t, J = 8.1 Hz, 1H),
4.71 (dt, J = 14.5, 7.3 Hz, 1H) ppm. 13C NMR (CDCl3, 125
Fig. 1 shows synthetic routes to HꢀMCrPN and HꢀMZnPN.
MHz):
δ
= 14.0, 22.5, 24.3, 28.8, 31.5, 33.9, 69.4, 155.1 ppm.
in Table 1: 4ꢀ(methoxymethyl)ꢀ1,3ꢀ
dioxolanꢀ2ꢀone, HꢀNMR (CDCl3, 500 MHz): = 3.43 (s, 3H),
Product of Entry
4
1
N
Cl
δ
Model
Study
N
Cr N
N
SiO2
3.58 (dd, J = 11.1, 3.7 Hz, 1H), 3.67 (dd, J = 11.1, 3.4 Hz, 1H),
4.39 (dd, J = 8.4, 6.1 Hz, 1H), 4.53 (t, J = 8.4 Hz, 1H), 4.86 (ddt,
HF
J = 8.6, 6.1, 3.6 Hz, 1H) ppm. 13C NMR (CDCl3, 125 MHz):
δ
X=CrCl, H2
N
X
N
Sonogashira
Coupling
= 59.8, 66.2, 71.5, 75.0, 154.9 ppm. Product of Entry 5 in Table
1: 4ꢀ(hydroxymethyl)ꢀ1,3ꢀdioxolanꢀ2ꢀone, 1HꢀNMR (CDCl3,
N
Cr
N
F
N
N
I
I
+
2
N
N
500 MHz):
δ = 2.74 (t, J = 6.2 Hz, 1H), 3.72 (ddd, J = 12.9, 6.8,
SiO2
Etching
3.5 Hz, 1H), 3.99 (ddd, J = 12.9, 5.5, 3.0 Hz, 1H), 4.39ꢀ4.62 (m,
2H), 4.82 (ddt, J = 8.4, 6.5, 3.3 Hz, 1H) ppm. 13C NMR (CDCl3,
SiO2
Etching
~ 40 nm
HF
125 MHz):
in Table 1: 4ꢀ[(allyloxy)methyl]ꢀ1,3ꢀdioxolanꢀ2ꢀone, HꢀNMR
(CDCl3, 500 MHz): = 3.63 (dd, J = 11.1, 3.7 Hz, 1H), 3.71
δ = 61.7, 65.8, 76.6, 155.3 ppm. Product of Entry 6
HF
1
X=H2
SiO2@MPN
SiO2@MCrPN
H-MPN
δ
H-MCrPN
X=CrF
(dd, J = 11.1, 3.7 Hz, 1H), 3.95ꢀ4.15 (m, 2H), 4.41 (dd, J = 8.4,
6.0 Hz, 1H), 4.53 (t, J = 8.4 Hz, 1H), 4.80ꢀ4.92 (m, 1H), 5.13ꢀ
5.38 (m, 2H), 5.78ꢀ6.02 (m, 1H) ppm. 13C NMR (CDCl3, 125
Zn(OAc)2
N
MHz):
Product of Entry
dioxolanꢀ2ꢀone, 1HꢀNMR (CDCl3, 500 MHz):
δ
= 66.3, 68.9, 72.7, 75.0, 118.0, 133.7, 154.9 ppm.
in Table 1: 4ꢀ(phenoxymethyl)ꢀ1,3ꢀ
= 4.14 (dd, J =
N
X
N
N
7
~ 40 nm
δ
X=CrF, Zn
10.6, 3.6 Hz, 1H), 4.18ꢀ4.26 (m, 1H), 4.52 (dd, J = 8.5, 5.9 Hz,
1H), 4.61 (t, J = 8.5 Hz, 1H), 4.97ꢀ5.09 (m, 1H), 6.87ꢀ7.05 (m,
H-MZnPN
Microporous
Metal-Porphyrin Network
(MCrPN, MZnPN)
N
X
N
N
N
3H), 7.22ꢀ7.36 (m, 2H) ppm. 13C NMR (CDCl3, 125 MHz):
δ
=
X=Zn
N
N
X
N
N
66.2, 66.9, 74.3, 114.7, 122.0, 129.7, 154.8, 157.8 ppm. Product
of Entry 8 in Table 1: 4ꢀ(tertꢀbutoxymethyl)ꢀ1,3ꢀdioxolanꢀ2ꢀone,
1HꢀNMR (CDCl3, 500 MHz):
δ = 1.20 (d, J = 4.6 Hz, 9H), 3.53
N
N
N
X
N
(dd, J = 10.3, 3.6 Hz, 1H), 3.62 (dd, J = 10.3, 4.5 Hz, 1H) , 4.39
(dd, J = 8.2, 5.8 Hz, 1H), 4.48 (t, J = 8.3 Hz, 1H), 4.78 (ddd, J =
8.3, 5.6, 4.5 Hz, 1H) ppm. 13C NMR (CDCl3, 125 MHz):
δ =
27.3, 61.3, 66.6, 73.9, 75.1, 155.2 ppm. Product of Entry 9 in Fig. 1 Synthetic schemes for H-MCrPN and H-MZnPN.
Table 1: cyclopentaꢀ1,3ꢀdioxolꢀ2ꢀone, 1HꢀNMR (CDCl3, 500
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For the synthesis of HꢀMCrPN, Cr porphyrin with four 4ꢀ
Journal Name
while HꢀMPN lacked a corresponding peak. (Fig. 3b) The solid
state 13C CP/TOSS NMR of HꢀMPN showed alkyne peaks at
80~90 ppm and aromatic peaks at 110~140 ppm. (Fig. 3c) After
Zn coordination to HꢀMPN, main peaks appeared at 120, 130,
142, and 149 ppm, corresponding to those of Znꢀporphyrin
rings.50ꢀ51
ethynylphenyl groups was prepared as a building block. During
the synthesis of HꢀMZnPN, we found that Zn was etched out of
Znꢀporphyrins by the reaction with HF. We therefore decided to
incorporate Zn to porphyrin networks by a postꢀsynthetic
modification. Thus, metalꢀfree porphyrin having four 4ꢀ
ethynylphenyl groups was prepared as a building block.
Microporous networks were formed on the surface of silica
spheres (~420 nm) via the Sonogashira coupling of the tetra(4ꢀ
ethynylphenyl) Cr(III)ꢀCl porphyrin or metal free tetra(4ꢀ
ethynylphenyl) porphyrin with 2 eq. 1,4ꢀdiiodobenzene. The
inner silica was etched by the treatment with HF solution.
During the etching, the CrꢀCl was converted to CrꢀF.44ꢀ45 The
reaction of hollow and microporous metal free porphyrin
networks (HꢀMPNs) with zinc acetate resulted in HꢀMZnPNs.
SEM and TEM analysis confirmed the hollow structures of
HꢀMCrPN, HꢀMPN, and HꢀMZnPN with diameters of ~500 nm
and shell thicknesses of ~40 nm. (Fig. 2)
(a)
(b)
(c)
(a)
(c)
(f)
(i)
(b)
(e)
(h)
(d)
(g)
Fig. 3 (a) N2 isotherm curves at 77K, pore size distribution diagrams
based on the DFT method and (b) IR spectra of H-MCrPN, H-MPN,
and H-MZnPN. (c) Solid state 13C CP/TOSS NMR spectra of H-MPN
and H-MZnPN.
The coordination of metal to porphyrin rings can be
characterized by UVꢀvis absorption spectroscopy.52 The HꢀMPN
showed unique Qꢀband peaks at 526, 560, 598, and 658 nm. (Fig.
4a) After Zn coordination to HꢀMPN, the location of the Q band
peaks significantly changed to 559 and 604 nm. Similarly, two
major Q bands were observed at 574 and 612 nm in the
absorption spectrum of HꢀMCrPN. The XPS spectra of Hꢀ
MCrPN and HꢀMZnPN showed the 2p1/2 and 2p3/2 orbital peaks
of Cr at 586.6 and 576.8 eV53 and the 2p1/2 and 2p3/2 orbital peaks
of Zn at 1021.6 and 1044.7 eV51,54, respectively. (Fig. 4b) Also,
One main N 1s oribtal peaks of HꢀMCrPN and HꢀMZnPN
appeared at 398.4 and 398.1 eV, corresponding to CrꢀNꢀ and Znꢀ
Nꢀ, respectively.55ꢀ56 (Fig. S4 in the ESI) In comparison, HꢀMPN
showed no Zn 2p oribtal peaks and instead, showed two main N
1s orbital peaks at 397.8 and 399.6 eV, corresponding to =Nꢀ and
ꢀNHꢀ species of metal free porphyrins, respectively.55ꢀ56 These
observations further confirm Zn coordination to HꢀMPN via postꢀ
synthetic modification. The EDS elemental mapping analysis of
HꢀMCrPN and HꢀMZnPN showed the homogeneous distribution
of Cr(III)ꢀF porphyrin and Znꢀporphyrin moieties in the materials,
respectively. (Fig. 4c) As shown in According to the elemental
analysis of N contents (4.84 and 5.55wt% for HꢀMCrPN and Hꢀ
MZnPN, respectively), the amounts of metal porphyrin moieties
in HꢀMCrPN and HꢀH ZnPN were calculated to be 0.86 and 0.99
mmol/g, respectively.
Fig. 2 SEM images of (a, d) HꢀMCrPN, (b, e) HꢀMPN, and (c, f)
HꢀMZnPN. TEM images of (g) HꢀMCrPN, (h) HꢀMPN, and (i)
HꢀMZnPN.
The analysis of N2 isotherm curves of HꢀMCrPN, HꢀMPN, and
HꢀMZnPN revealed surface areas of 794, 677, and 802 m2/g, and
micropore volumes of 0.24, 0.20, and 0.24 cm3/g, respectively.
(Fig. 3a) The hysteresis loops were observed in the N2 isotherm
curves of the materials, possibly due to the swelling of a nonꢀ
rigid porous structure or the irreversible uptake of N2 in pores.46
The higher surface areas of HꢀMCrPN and HꢀMZnPN than that
of HꢀMPN might result from the suppressed πꢀπ stacking of
polymer chains by metals or the removal of possibly entrapped
impurities during metal coordination. The PXRD showed
amorphous character for HꢀMCrPN, HꢀMPN, and HꢀMZnPN,
matching with the properties of MONs in the literature.47ꢀ48 (Fig.
S3 in the ESI) The IR spectroscopy of HꢀMCrPN and HꢀMZnPN
showed the vibration peaks of metalꢀporphyrin at 999 cmꢀ1,49
4 | J. Name., 2012, 00, 1-3
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(a)
(b)
(a)
(b)
(c)
(c)
(d)
(e)
(f)
Fig. 4 (a) UV-vis absorption, (b) XPS spectra of H-MCrPN, H-MPN,
and H-MZnPN, and (c) EDS elemental mapping of H-MCrPN and H-
MZnPN.
Considering the high surface areas of HꢀMCrPN57 and Hꢀ
MZnPN and excellent dispersion ability in organic solvents
including styrene oxide, we studied their catalytic performance
in the CO2 fixation with styrene oxide to the corresponding
cyclic carbonate at room temperature under ambient CO2
pressure (balloon). Fig. 5 summarizes the results.
As shown in Figs. 5aꢀc, HꢀMCrPN showed superior catalytic
activities compared to HꢀMZnPN, indicating that the CrꢀF
moiety is a better Lewis acid than Zn in the porphyrins. The
effect of alkyl groups in tetraalkylammonium halides was
significant in the CO2 fixation with styrene oxide to cyclic
carbonate. As the chain length of alkyl groups increased from
methyl to butyl and hexyl, the conversion yield after 48 h
gradually increased from 7% to 86 and 93%, respectively. (Fig.
5d) This trend is attributed to the relative solubility of
tetraalkylammonium halides in styrene oxide. Further increase
of the alkyl chain length did not improve the catalytic activity;
Fig. 5 CO2 fixation with styrene oxide to cyclic carbonate depending
on catalysts (a-c) and co-catalysts (d-f). Reaction conditions: styrene
oxide (12.5 mmol), no additional solvent, H-MCrPN or H-MZnPN
(0.49 mol% metal porphyrins, 0.061 mmol), tetraalkylammonium
halide or PPNCl (0.90 mmol), room temperature, and CO2 (balloon).
THABr: (n-hexyl)4NBr, TBABr: (n-butyl)4NBr, TOABr: (n-octyl)4NBr,
TMABr: Me4NBr, PPNCl: [(Ph3P)2N]Cl, TBACl: (n-butyl)4NCl, and TBAI:
(n-butyl)4NI.
When we screened different anions of tetrabutylammonium
halides, bromide and iodide showed similar conversion yields
(82% and 80% after 48 h, respectively) in the CO2 fixation at
room temperature. (Fig. 5e) The chloride showed a lower
conversion yield (71% after 48 h) than bromide. Thus, the
(
n
ꢀoctyl)4NBr showed a lower conversion yield (72% after 48 h)
than ( ꢀhexyl)4NBr (93% after 48 h), which is attributable to a
n
size effect of the cocatalyst. As the size of tetraalkylammonium
halides increased, the diffusion rate into the pores might
decrease.
optimal combination of HꢀMCrPN with (nꢀhexyl)4NBr (THABr)
showed the best performance (93% conversion yield after 48 h,
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TON=190) in the CO2 fixation with styrene oxide to cyclic The sizes of epoxide substrates were calculated in a range of
carbonate at room temperature. As far as we are aware, this is 4.8~10.6 Å, indicating the possible sizeꢀdependent constrain
one of the best performances of heterogeneous MON catalysts effect in micropores (main pore sizes of 6.8 and 13 Å) of Hꢀ
for the room temperature CO2 fixation with styrene oxide to MCrPN. (Figs. 3a and S6 in the ESI) Epoxides with ether or
cyclic carbonate.16ꢀ20,58 Without HꢀMCrPN or HꢀMZnPN, alcohol groups such as 2ꢀmethoxymethyl)oxirane and glycidol
tetraalkylammonium halides and [(Ph3P)2N]Cl showed poor worked well in the reaction and showed 100% and 88%
conversions of styrene oxide, indicating that the conversion was conversion after 48 h, respectively. (Entries 4ꢀ5 in Table 1)
catalyzed by Cr(III)ꢀF moieties in the HꢀMCrPN. (Fig. 5f) In This indicates that the Lewis acidic activation of epoxides by
control experiments, nonhollow MCrPN materials showed HꢀMCrPN works well with additional ether and alcohol groups.
much pooper catalytic activities than HꢀMCrPN, indicating the However, as the sizes of side substituents increased, conversion
hollow structural effects. (Fig. S5 in the ESI)
became gradually slower. (Entries 6ꢀ8 in Table 1) The 2ꢀ
Next, we screened various epoxides, in which THABr are (allyloxymethyl)oxirane and 2ꢀ(tertꢀbutoxymethyl)oxirane
soluble, as substrates for the room temperature CO2 fixation to showed 83% and 71% conversion after 48 h, respectively. In
cyclic carbonates. Table 1 summarizes the results of catalytic the case of 2ꢀ(phenoxymethyl)oxirane, the partial formation of
performance of HꢀMCrPN.
cyclic carbonate resulted in the solidification of reaction
medium and in a poor conversion yield. (Entry 7 in Table 1)
Epoxides with two tertiary carbons were also tested for the
room temperature CO2 fixation. Relatively bulky cycopentene
oxide and cyclohexene oxide showed slow conversions (72 and
39% after 120 h, respectively). (Entries 9ꢀ10 in Table 1) The
chemoselectivities to cyclic carbonates were perfect in the cases
of most epoxide substrates excepting glycidol (Entry 5 in Table
1, selectivity: 79%) 2ꢀ(allyloxymethyl)oxirane (Entry 6 in
Table 1, selectivity: 92%), and cyclohexene oxide (Entry 10 in
Table 1, selectivity: 75%). Byproducts were homopolymers of
epoxides. When we screened the coꢀcatalysts for the
conversions of glycidol and cyclohexene oxide, THABr showed
the best chemoselectivities. (Fig. S7 in the ESI)
Table 1. Catalytic performance of H-MCrPN in the CO2 fixation
with epoxides to cyclic carbonates.a
O
O
c
Entry 1
Entry 6
b
48 h 83%
72 h 98%
(86%)
48 h 93%
O
O
O
O
O
O
(87%)
O
Ph
Ph
O
O
O
O
O
d
Entry 7
OPh
Entry 2
Entry 3
48 h 70%
72 h 91%
(83%)
48 h 44%
(43%)
O
O
O
O
O
O
O
O
O
O
O
O
OPh
Entry 8
O
48 h 55%
72 h 64%
(61%)
48 h 71%
72 h 84%
(82%)
O
(a)
(b)
O
O
O
O
Entry 4
Entry 9
48 h 100%
(94%)
48 h 47%
72 h 55%
120 h 72%
(65%)
O
O
O
O
O
O
O
O
O
O
f
e
Entry 5
Entry 10
48 h 19%
72 h 27%
120 h 39%
(26%)
48 h 88%
(69%)
O
O
O
O
OH
OH
a Reaction conditions: epoxide (12.5 mmol), no additional solvent, H-
MCrPN (71 mg, 0.86 mmol Cr porphyrin/g, 0.49 mol% Cr porphyrins,
0.061 mmol), THABr (0.90 mmol), room temperature, and CO2
(balloon). The reactions of entries 1-5, 7, and 8 showed no byproducts
(c)
(d)
b
(In these reactions, selectivities for cyclic carbonates were 100%.).
The reaction time, conversion based on 1H NMR, and isolated yield in
c
parenthesis. The selectivity for cyclic carbonate was 92% and the
byproduct was a homopolymer of epoxide. d The partial formation of
cyclic carbonate resulted in the solidification of reaction medium. Thus,
e
poor conversion could not be improved. The selectivity for cyclic
carbonate was 79% and the byproduct was a homopolymer of epoxide.
f
The selectivity for cyclic carbonate was 75% and the byproduct was a
homopolymer of epoxide.
Fig. 6 (a) Recycle tests of H-MCrPN. Reaction conditions: styrene
oxide (12.5 mmol), no additional solvent, H-MCrPN (71 mg, 0.86
mmol Cr porphyrin/g, 0.49 mol% Cr porphryins, 0.061 mmol Cr
porphyrins), THABr (0.90 mmol), room temperature, 48 h, and CO2
(balloon). Green, orange and violet bars of a graph indicate
conversions of styrene oxide, isolated yields of cyclic carbonate, and
chemoselectivities for cyclic carbonates. (b) UV-vis absorption
spectra, (c) SEM images, and (d) XPS spectra of the original H-
MCrPN and the recovered one after five successive reactions.
Epoxides (12.5 mmol) with phenyl and butyl substituents
worked well in the room temperature CO2 fixation catalyzed by
HꢀMCrPN (0.49 mol% of Cr(III)ꢀF porphyrins) and THABr
(0.90 mmol), showing 93% and 70% conversion after 48 h,
respectively. (Entries 1ꢀ2 in Table 1) When the alkyl chain
length of epoxides was increased from butyl to hexyl, the
conversion decreased to 55% after 48 h. (Entry 3 in Table 1)
6 | J. Name., 2012, 00, 1-3
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10 R. Luo, Y. Chen, Q. He, X. Lin, Q. Xu, X. He, W. Zhang, X.
Zhou and H. Ji, ChemSusChem 2017, 10, 1526ꢀ1533.
11 Y. Chen, R. Luo, Q. Xu, W. Zhang, X. Zhou, H. Ji,
ChemCatChem 2017, 9, 767ꢀ773.
12 C. Maeda, S. Sasaki and T. Ema, ChemCatChem 2017, 9, 746ꢀ
The recyclability of HꢀMCrPN was studied. As shown in Fig.
6a, the HꢀMCrPN maintained good activities throughout five
recycle tests. In the fifth cycle, the recycled HꢀMCrPN showed
94% conversion of styrene oxide, while 93% conversion was
observed in the first reaction. The UVꢀvis and IR absorption
spectroscopy of the HꢀMCrPN recovered after the fifth cycle
showed nearly identical spectrum with those of the original
material. (Figs. 6b and S8 in the ESI) The pyrrole vibration
peak at 999 cmꢀ1 was completely retained in the IR spectrum of
the HꢀMCrPN recovered after the fifth cycle. (Fig. S8 in the
ESI) The SEM image of the HꢀMCrPN recovered after the fifth
cycle showed the complete retention of hollow structure. (Fig.
6c) The XPS studies also supported the retention of CrꢀF
species after five cycles. (Fig. 6d)
949.
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This work shows that efficient heterogeneous catalytic
systems can be developed based on structureꢀengineered MONs
for the room temperature CO2 fixation to cyclic carbonates. The
high surface area and microporosity of MONs enabled the inner
Lewis acid sites to be utilized for the activation of epoxides.
The reduced diffusion pathways of substrates in the HꢀMCrPN
due to the hollow structure and the efficient Lewis acid function
of Cr(III)ꢀF resulted in the efficient CO2 fixation with styrene
oxide with TON 190 at room temperature under ambient
(balloon) pressure. We believe that more diverse Lewis acid
moieties can be incorporated to the hollow and microporous
networks.
835.
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Conflicts of interests
There are no conflicts of interest to declare.
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29 K. Cho, J. Yoo, H. ꢀW. Noh, S. M. Lee, H. J. Kim, Y. ꢀJ. Ko, H. ꢀ
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Acknowledgements
5
This work was supported by “Next Generation Carbon
Upcycling Project” (Project No. 2017M1A2A2042517) through
the National Research Foundation (NRF) funded by the
Ministry of Science and ICT, Republic of Korea.
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2.45 mmol /g at 298 and 273K, respectively and CO2 Qst in a
range of 25~41 kJ/mol. (Fig. S10 in the ESI)
58 In the literature,18 nonhollow microporous polymer catalysts
bearing Cr(III)ꢀCl salens (0.44 mol% Cr salens, 7.2 mol%
ammonium salt) showed a 51.6% conversion yield of styrene
oxide at room temperature after 48 h (TON: 117). In comparison,
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8 | J. Name., 2012, 00, 1-3
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Table of Contents
Hollow and microporous Cr(III)-F porphyrin networks (H-MCrPNs) showed excellent catalytic activities in the room temperature
CO2 fixation with epoxides to cyclic carbonates substrates due to the hollow structure, and the efficient Lewis acidic activity of
Cr(III)-F moieties.