Three-Dimensional Radical Covalent Organic Frameworks as Highly Efficient and Stable Catalysts for Selective Oxidation of Alcohols

Article abstract of DOI:10.1002/anie.202108357

With excellent designability, large accessible inner surface, and high chemical stability, covalent organic frameworks (COFs) are promising candidates as metal-free heterogeneous catalysts. Here, we report two 3D radical-based COFs (JUC-565 and JUC-566) in which radical moieties (TEMPO) are uniformly decorated on the channel walls via a bottom-up approach. Based on grafted functional groups and suitable regular channels, these materials open up the application of COFs as highly efficient and selective metal-free redox catalysts in aerobic oxidation of alcohols to relevant aldehydes or ketones with outstanding turn over frequency (TOF) up to 132 h^?1, which has exceeded other TEMPO-modified catalytic materials tested under similar conditions. These stable COF-based catalysts could be easily recovered and reused for multiple runs. This study promotes potential applications of 3D functional COFs anchored with stable radicals in organic synthesis and material science.

Full text of DOI:10.1002/anie.202108357

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How to cite:
International Edition: doi.org/10.1002/anie.202108357
German Edition: doi.org/10.1002/ange.202108357
Covalent Organic Frameworks
Three-Dimensional Radical Covalent Organic Frameworks as Highly
Efficient and Stable Catalysts for Selective Oxidation of Alcohols
Fengqian Chen+, Xinyu Guan+, Hui Li,* Jiehua Ding, Liangkui Zhu, Bin Tang,*
Valentin Valtchev, Yushan Yan, Shilun Qiu, and Qianrong Fang*
Abstract: With excellent designability, large accessible inner
surface, and high chemical stability, covalent organic frame-
works (COFs) are promising candidates as metal-free hetero-
geneous catalysts. Here, we report two 3D radical-based COFs
(JUC-565 and JUC-566) in which radical moieties (TEMPO)
are uniformly decorated on the channel walls via a bottom-up
approach. Based on grafted functional groups and suitable
regular channels, these materials open up the application of
COFs as highly efficient and selective metal-free redox
catalysts in aerobic oxidation of alcohols to relevant aldehydes
or ketones with outstanding turn over frequency (TOF) up to
132 hÀ1, which has exceeded other TEMPO-modified catalytic
materials tested under similar conditions. These stable COF-
based catalysts could be easily recovered and reused for
multiple runs. This study promotes potential applications of 3D
functional COFs anchored with stable radicals in organic
synthesis and material science.
first report of COF-based catalyst (Pd/COF-LZU1) by Wang
and co-workers in 2011,[2a] some related works have been
prepared.[2] For example, we have recently developed a series
of three-dimensional (3D) COF-based catalysts by the func-
tional design,[3] including 3D base-functionalized COFs for
highly selective catalysis in Knoevenagel condensation reac-
tions,[3a] 3D COFs with dual linkages as bifunctional catalysts
for one-pot cascade reactions,[3b] and 3D metal-containing
Salphen COFs as catalytic antioxidants for the removal of
superoxide radicals.[3c] Often the catalytically active compo-
nents (such as metal ions and nanoparticles) of a large portion
of COF catalysts are incorporated through weak interaction,
resulting in potential leaching or aggregation during catalytic
procedures, which limits the stability and reusability of these
materials. Recently, bare COF skeletons as metal-free
heterogeneous chemical catalysts have attracted broad inter-
ests because their active sites were covalently bound to the
backbone.[2i] Among these materials, strong covalent bonds
promise high chemical stability and excellent reusability while
the exposed organic functional groups serve as catalytic
centers. However, the development of such COF-based
catalysts still remains a great challenge.
C
ovalent organic frameworks (COFs),[1] an emerging class
of crystalline porous organic materials, are considered as
prospective platforms for heterogeneous chemical catalysis,
owning to their highly designable organic skeletons, abundant
open active sites, and excellent chemical stability. After the
The stable organic nitroxyl radicals, such as 2,2,6,6-
tetramethylpiperidine-1-oxy (TEMPO), have been widely
studied as highly efficient metal-free redox catalysts.[4]
Individual TEMPO molecules and their derivatives have
been employed in a broad range of chemical transformations,
but the recovery and reuse of catalysts are highly desirable
since TEMPO is considerably expensive. As a result, various
supporting materials have been attempted for the immobili-
zation of TEMPO, such as silica, amorphous polymers, and
metal-organic frameworks (MOFs).[5] However, it was still
difficult to prepare catalysts combining excellent recyclability
and high catalytic activity due to irregular pores or poor
stability in traditional supporting materials. We considered
COFs a promising candidate for TEMPO immobilization,
which can lead to excellent redox catalysis due to their large
exposed inner surface, uniform nano-channels, highly design-
able structures, and good chemical stability. Nevertheless,
only a few reports of TEMPO-decorated COFs were avail-
able up to date,[6] and they were limited to post-modified 2D
frameworks, which has extremely restricted their perform-
ances.
[*] F. Chen,[+] Dr. X. Guan,[+] Dr. H. Li, J. Ding, Dr. L. Zhu, Prof. S. Qiu,
Prof. Q. Fang
State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, Jilin University
Changchun 130012 (China)
E-mail: postlh@jlu.edu.cn
qrfang@jlu.edu.cn
Dr. B. Tang
Deakin University, Institute for Frontier Materials
Geelong, Victoria 3216 (Australia)
E-mail: bin.tang@deakin.edu.au
Prof. V. Valtchev
Qingdao Institute of Bioenergy and Bioprocess Technology
Chinese Academy of Sciences
189 Song Ling Rd, Qingdao, Shandong 266101 (China)
and
Normandie Univ, ENSICAEN, UNICAEN, CNRS,
Laboratoire Catalyse et Spectrochimie
6 Marechal Juin, 14050 Caen (France)
Prof. Y. Yan
Department of Chemical and Biomolecular Engineering, Center for
Catalytic Science and Technology, University of Delaware
Newark, DE 19716 (USA)
Herein, we report two 3D TEMPO-based COFs via
a bottom-up strategy, termed JUC-565 and JUC-566 (JUC =
Jilin University China). Different from previously reported
2D TEMPO-based COFs, TEMPO groups as radical moieties
were uniformly anchored to the channel walls of these 3D
COFs with high density. Since the whole framework was
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202108357.
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connected via strong covalent bonds, these materials showed
high chemical stability in strong basic or acidic aqueous
solutions. These functional COFs have been demonstrated to
be highly selective and reusable metal-free redox catalysts for
the application in aerobic oxidation of alcohols to relevant
aldehydes or ketones with remarkable turn over frequency
(TOF) as high as 132 hÀ1, which is the highest ever reported
under similar conditions. To the best of our knowledge, this
study represents the first case of 3D functional COFs
decorated by TEMPO radicals and their application as
metal-free redox catalysts for aerobic oxidation of alcohols.
The synthesis of 3D radical-based COFs was carried out
by solvothermal condensation of TEMPO-TPDA (TPDA =
[1,1’:4’,1’’-terphenyl]-4,4’’-dicarbaldehyde), and tetra(4-ami-
nophenyl)methane (TAPM) or 1,3,5,7-tetrakis(4-aminophe-
nyl)adamantane (TAPA) in a mixture of dioxane, mesitylene,
and acetic acid at 1208C for 3 days (Scheme 1). A variety of
methods have been utilized for structural confirmation.
Scanning electron microscopy (SEM, Figures S1 and S2)
images revealed isometric nanocrystals building random
aggregates for both COFs. Fourier transform infrared (FT-
IR) spectra showed new peaks at 1658 cmÀ1 for both COFs,
Furthermore, thermogravimetric analysis (TGA) illustrated
good thermal stability (ca. 3008C under N2) of 3D radical-
based COFs (Figures S5 and S6). Outstanding chemical
stability was also verified by their retained powder X-ray
diffraction (PXRD) patterns after immersing in different
organic solvents and strong basic (1 M NaOH) or acidic (1 M
HCl) aqueous solutions for one week (Figures S7–10). More-
over, the radical feature probed by electron paramagnetic
resonance (EPR) spectroscopy at room temperature showed
peaks at g = 2.0096 for JUC-565 and 2.0097 for JUC-566
respectively, which is well consistent with that of TEMPO
radical (g = 2.0097, Figures S12–14).
The crystalline structures were verified by the PXRD and
structural simulations (Figure 1). After a geometrical energy
minimization by the Materials Studio software package[7]
based on 10-fold interpenetrated dia nets and disordered
pendant functional groups,[8] their unit cell parameters were
acquired (a = b = 28.838 ꢀ, c = 7.1132 ꢀ and a = b = g = 908
for JUC-565; a = b = 31.438 ꢀ, c = 7.6704 ꢀ and a = b = g =
908 for JUC-566; Tables S2 and S3). The simulated PXRD
patterns were in good agreement with the experimental ones.
Moreover, the experimental PXRD patterns were analyzed
by full profile pattern matching (Pawley) refinements. Peaks
at 4.31, 6.14, 6.76, 8.68, 9.77 and 13.158 for JUC-565 belong to
the (110), (200), (210), (220), (310), and (330) Bragg peaks of
which is characteristic of the imine bond. At the same time,
À1
=
almost complete disappearance of C O (1696 cm
for
À1
À
TEMPO-TPDA) and N H stretching (ca. 3394 cm for
TAPM and ca. 3332 cmÀ1 for TAPA) proved the transforma-
tion of aldehyde and amine groups (Figures S3 and S4).
¯
space group P4 (No. 81); peaks at 3.97, 5.70, 6.39, 8.05, 9.01
and 12.548 for JUC-566 correspond to the (110), (200), (210),
¯
(220), (310), and (330) Bragg peaks of the space group P4
(No. 81). The calculated results were consistent with the
measured ones with good agreement factors (Rp = 1.15% and
wRp = 1.65% for JUC-565; Rp = 0.73% and wRp = 1.09%
Scheme 1. Strategy for constructing 3D radical-based COFs. Molecular
structures of TEMPO-TPDA (a) as a linear 2-connected building unit
as well as TAPM (b) or TAPA (c) as a tetrahedral 4-connected building
unit. Two novel 3D radical-based COFs, JUC-565 (d) and JUC-566 (e),
prepared by the condensation reaction of TEMPO-TPDA and TAPM or
TAPA. A single dia framework (f) and 10-fold interpenetrated dia net
(g) in 3D radical-based COFs.
Figure 1. PXRD patterns of JUC-565 (a) and JUC-566 (b).
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for JUC-566). Furthermore, we tried alternative structures
with 9- or 11-fold interpenetrated dia topology; however,
there were apparent differences between the simulated and
experimental PXRDs (Figures S15–22). Also, the PXRD
pattern of JUC-565 was in good agreement with that of
LZU-79 with an approximate length of linkers and 10-fold
interpenetrated dia net, which has been proved by its single-
crystal X-ray diffraction (Figure S23).[9] In terms of these
results, the obtained COFs are proposed to be the expected
architectures possessing microporous channels (ca. 0.8 nm for
JUC-565 and ca. 0.9 nm for JUC-566), which are uniformly
decorated by TEMPO radicals (Figure 2 and Figure S24).
The porosity and specific surface areas of 3D radical-
based COFs were determined by N2 adsorption measure-
ments at 77 K. As shown in Figure 3, both COFs exhibited
sharp gas uptake at low relative pressure (below 0.1 P/P0),
which reveals their microporous nature. An inclination of the
isotherm in the 0.8–1.0 P/P0 range and slight desorption
hysteresis can be observed due to the presence of textural
mesopores.[10] The Brunauer–Emmett–Teller (BET) surface
areas were calculated to be 313 m2 gÀ1 for JUC-565 and
482 m2 gÀ1 for JUC-566 (Figures S25 and S26), which are
much higher than those from 2D radical-based COFs, that is,
only 5.2 m2 gÀ1 for [TEMPO]100%-NiP-COF[6a] and 34 m2 gÀ1
for T-COF-100.[6b] The nonlocal density functional theory
(NLDFT) was utilized to estimate their pore size distribu-
tions. JUC-565 and JUC-566 demonstrated microporous
cavities with ca. 0.9 nm and ca. 1.0 nm, respectively
Figure 3. N2 adsorption–desorption isotherms at 77 K for JUC-565 (a)
and JUC-566 (b). Inset: pore-size distribution calculated by fitting on
the NLDFT model to the adsorption data.
(Figure 3, inset), which are in good agreement with pore
sizes predicted from their crystal structures.
Encouraged by the abundant TEMPO radicals, good
chemical stability, high surface areas and suitable regular
pores of 3D radical-based COFs, JUC-566 was chosen to
evaluate the catalytic performance of a metal-free heteroge-
neous catalyst for aerobic oxidation of hydroxyl to carbonyl.
Typically, the catalytic reaction was carried out in a glass vial
with the COF catalyst (5.0 mol% based on nitroxide radical),
NaNO2 (30.0 mol%) as dioxygen activator, 1,3-dibromo-5,5-
dimethylhydantoin (DBDMH, 7.5 mol%) as co-catalyst, and
anhydrous acetic acid as solvent (AcOH, 0.5 mL) under
oxygen atmosphere (O2 balloon) at room temperature.
As shown in Table 1, successful oxidation of a broad
spectrum of alcohols to the corresponding carbonyl com-
pounds with high yields was achieved. For example, the
reaction of benzyl alcohol could be completed in 9 min with
a 99% yield (Table 1, entry 1). Notably, its TOF was
calculated as high as 132 hÀ1, which has surpassed previous
reported TEMPO-anchored materials under similar reaction
conditions (Table S1), such as MOFs (19.2 hÀ1 for UiO-67-
TEMPO),[11] amorphous polymers (40 hÀ1 for TEMPO-CMP-
4),[12] ordered mesoporous silica (111 hÀ1 for SBA-15-
ABNO),[13] oxide nanoparticles (125 hÀ1 for MNST).[14] The
para-substituted benzyl alcohols could also be transformed to
the corresponding aldehydes in nearly quantitative yields
(Table 1, entries 2–6). Meanwhile, catalytic oxidation of
Figure 2. Extended structure of JUC-565: a) single dia network and
b) 3D porous framework viewed along the c axis. The pink balls
represent TEMPO radicals. C green, H yellow, N blue, O pink.
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Table 1: Aerobic oxidation of alcohols using JUC-566 catalyst.[a]
Entry Alcohol
Product
t [min] Yield [%][b]
1
2
9
99
99
99
99
10
20
30
3
4
5
40
98
6
7
40
40
99
98
8
9
20
60
99
5
10[c]
11[d]
20
30
99
43
[a] Reaction conditions: 0.2 mmol of alcohol, 8.0 mg of JUC-566
(5.0 mol% of nitroxide radical), 4.2 mg of NaNO2 (30.0 mol%), and
4.3 mg of DBDMH (7.5 mol%) in 0.5 mL of AcOH with an O2 balloon at
room temperature. [b] Determined by 1H NMR spectroscopy. [c] Reac-
tion conditions: the catalyst was replaced with TEMPO molecule and all
other conditions remained the same. [d] Reaction conditions: the
absence of DBDMH and all other conditions remained the same.
Figure 4. a) Yield vs. time curve for aerobic oxidation controlled by the
addition or removal of COF crystals. b) Yield vs. time curves for
aerobic oxidation with substrates of different sizes. c) Recyclability
study of aerobic oxidation with 4-nitrobenzyl alcohol as the substrate.
larger substrates required longer reaction time, possibly
caused by the slow diffusion through COF channels. Sub-
sequently, the reaction was extended to some other alcohols,
including secondary alcohols, which produce ketones after
oxidation (Table 1, entries 7 and 8). Noteworthy, the catalytic
oxidation reactions of all selected substrates showed high
yields, illustrating the generality of this catalysis system. To
further confirm the function of JUC-566 in the catalytic
transformation, we attempted to remove it by filtration after
15 mins of reaction, and it was found that the reaction almost
immediately ceased (Figure 4a), which proves the heteroge-
neous nature of catalytic reaction.
after 60 min under similar conditions for 3,4,5-trimethoxy-
benzyl alcohol with a molecular size of 1.0 ꢁ 1.1 nm (larger
than the aperture of JUC-566 of about 0.9 nm, Table 1 entry 9
and Figure 4b). Meanwhile, a controlled experiment was
performed with the catalysis of TEMPO molecules under
similar conditions and 99% transformation of 3,4,5-trime-
thoxybenzyl alcohol was achieved in 20 mins (Table 1,
entry 10). These results indicated that the catalytic reaction
took place in the COFꢂs channels and that JUC-566 possessed
excellent size selectivity towards substrate dimensions. In
More importantly, thanks to the uniform channels of the
frameworks, JUC-566 exhibited an amazing size selectivity.
For instance, only 5% of the target product was detected even
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Ma, Chem. Soc. Rev. 2020, 49, 708 – 735; h) X. Y. Guan, F. Q.
addition, a controlled experiment without the addition of
DBDMH has been also studied by using 4-nitrobenzyl alcohol
as the substrate (Table 1, entry 11). The catalytic reaction
could still happen without adding DBDMH, but only 43% of
the alcohol was converted into the corresponding aldehyde
after 30 min, which means that DBDMH as a co-catalyst plays
an important auxiliary role.[12] After that, the cycling perfor-
mance of this catalyst was also investigated with 4-nitrobenzyl
alcohol as the substrate. After the reaction was completed,
the catalysts could be recovered by simple filtration, and used
for the next run. After five cycles, a high yield of 95% could
still be observed (Figure 4c). Furthermore, the preserved
PXRD pattern of the recovered JUC-566 confirmed its
structural integrity after recycling tests (Figure S11).
In summary, by using a pre-designed TEMPO-based
monomer, we develop two robust 3D radical-based COFs,
JUC-565 and JUC-566. These materials are provided with
high stability, large surface areas, suitable regular pores, and
abundant accessible TEMPO moieties on the channel walls.
Combining high efficiency and good reusability, JUC-566
exhibit extraordinary selective catalytic performance for
aerobic oxidation of various alcohols to the corresponding
aldehydes or ketones with great TOF up to 132 hÀ1, which is
much higher than those from previous reported TEMPO-
decorated catalytic materials under similar conditions, includ-
ing MOFs, silica, amorphous polymers, and oxide nano-
particles. These results thus open an avenue towards design-
ing 3D functionalized COFs and promote their applications in
organic synthesis and material science.
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Acknowledgements
This work was supported by National Natural Science
Foundation of China (22025504, 21621001, and 21390394),
“111” project (BP0719036 and B17020), China Postdoctoral
Science Foundation (2020TQ0118 and 2020M681034), and
the program for JLU Science and Technology Innovative
Research Team. V.V., Q.F., and S.Q. acknowledge the
collaboration in the framework of China-French joint labo-
ratory “Zeolites”.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: alcohol oxidation · covalent organic frameworks ·
functionalization · radicals · selectivity
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Communications
Chemie
X. Y. Guan, J. L. Li, C. Y. Li, B. Tang, V. Valtchev, Y. S. Yan, S. L.
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Manuscript received: June 23, 2021
Revised manuscript received: August 8, 2021
Accepted manuscript online: August 13, 2021
Version of record online: && &&, &&&&
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www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 1 – 7
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Covalent Organic Frameworks
Three-dimensional covalent organic
frameworks (COFs) decorated with
nitroxyl radicals are successfully prepared
and employed as mild, efficient, and
recyclable catalysts for selective oxidation
of alcohols to aldehydes or ketones.
F. Chen, X. Guan, H. Li,* J. Ding, L. Zhu,
B. Tang,* V. Valtchev, Y. Yan, S. Qiu,
Q. Fang*
&&&& — &&&&
Three-Dimensional Radical Covalent
Organic Frameworks as Highly Efficient
and Stable Catalysts for Selective
Oxidation of Alcohols
Angew. Chem. Int. Ed. 2021, 60, 1 – 7
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
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These are not the final page numbers!