Synthesis of TEMPO radical decorated hollow porous aromatic frameworks for selective oxidation of alcohols

Article abstract of DOI:10.1039/d0cc06965e

A bottom-up approach was developed to prepare TEMPO radical decorated hollow aromatic frameworks (HPAF-TEMPO) by using TEMPO radical functionalized monomers and SiO₂nanospheres as templates. The accessible inner layer, high density of TEMPO sites, and hybrid micro-/mesopores of the HPAF-TEMPO enable the aerobic oxidation of a broad range of alcohols with high efficiency and excellent selectivity.

Full text of DOI:10.1039/d0cc06965e

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Synthesis of TEMPO radical decorated hollow
porous aromatic frameworks for selective
oxidation of alcohols†
Cite this: Chem. Commun., 2021,
57, 907
Received 20th October 2020,
Accepted 14th December 2020
Yan-Ming Shen,‡a Yun Xue,‡a Mi Yan,a Hui-Ling Mao,a Hu Cheng,a Zhuo Chen,a
ac
Zhi-Wei Sui,*b Shao-Bin Zhu,*c Xiu-Jun Yuad and Jin-Liang Zhuang
*
DOI: 10.1039/d0cc06965e
rsc.li/chemcomm
A bottom-up approach was developed to prepare TEMPO radical
Selective oxidation of alcohols to aldehydes and ketones is one
decorated hollow aromatic frameworks (HPAF-TEMPO) by using of the most fundamental transformations in the fine-chemical
TEMPO radical functionalized monomers and SiO2 nanospheres as and pharmaceutical industry. With the increasing demand of
templates. The accessible inner layer, high density of TEMPO sites, green chemistry, the development of aerobic oxidation of alcohols
and hybrid micro-/mesopores of the HPAF-TEMPO enable the has received tremendous attention.4 Of particular interest in this
aerobic oxidation of a broad range of alcohols with high efficiency field is the use of a family of stable nitroxyl radicals as organo-
and excellent selectivity.
catalysts, named TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy)
radicals and their derivatives, benefiting from their excellent
Porous aromatic frameworks (PAFs) are a class of covalently linked stability, high efficiency, and easy preparation or commercial
organic materials possessing permanent microporosity, high surface availability.5 However, the homogeneous catalytic systems have
areas, and excellent chemical stability.1 These materials have already difficulties in recovery and reuse of TEMPO-based radicals, con-
demonstrated a wide range of applications, such as gas adsorption, sidering the relatively high cost of these compounds. Early studies
energy storage, biosensing, and heterogeneous catalysis.1 From a to address this problem mainly focus on the immobilization of
synthetic point of view, one of the most promising features of the TEMPO onto the surfaces of various supports, including poly-
PAFs is that the functionality of the pore can be engineered either by mers, silicas, carbons, and magnetic particles.6 Nevertheless,
specific predesigned monomers or post-synthetic modification many protocols often suffer from low loading efficiency, partial
(PSM).2 Moreover, the diversity of the building blocks and the degradation of supports, and poor catalytic efficiency. Pioneering
versatile cross-coupling or condensation methods (e.g., Sonaga- work by Kitagawa and co-works demonstrated that the incorpora-
shira–Hagihara coupling, Yamamoto coupling, Buchwald–Hartwig tion of a pyrroline nitroxide moiety into a porous coordination
amination, and cyclotrimerization) offer numerous advantages for polymer (PCP) enables the selective oxidation of alcohols.7 Later
the construction of functional PAFs with different physical–chemical on, TEMPO radical functionalized metal–organic frameworks
properties. In particular, the incorporation of catalytic active (MOFs) and conjugated microporous polymers (CMPs) have
moieties (e.g., organocatalysts and organometallic complexes) into been synthesized with a similar strategy, and displayed excellent
the frameworks has been demonstrated as an efficient way to catalytic activity toward the oxidation of alcohols, benefiting from
synthesize catalytically active PAFs.3
their highly porous nature.8
On the other hand, the size and the morphology of the hetero-
geneous catalysts significantly affects their catalytic performance.
For example, hollow nanostructured catalysts exhibit superior
catalytic activity arising from their accessible inner surface, high
exposed active sites, and reduced diffusion pathways of reactants/
products.9 Template synthesis is one of the most straightforward
and efficient methods to prepare hollow nanostructures, followed by
selective removal of templates.10 Son and co-workers have demon-
strated that by using SiO2 nanospheres as templates, a variety of
(functional) hollow microporous organic networks (HMONs) can be
prepared.11 Nevertheless, the functionality (e.g., –SO3H and pyrro-
lidines) of HMONs is introduced by multiple PSM steps.12
a School of Chemistry and Materials Science, Key Lab for Functional Materials
Chemistry of Guizhou Province, Guizhou Normal University, 116 Baoshan Road
North, Guiyang 550001, P. R. China
b Center for Advanced Measurement Science, National Institute of Metrology,
Beijing, China
c NanoFCM INC., Xiamen Pioneering Park for Overseas Chinese Scholars,
Xiamen, 361005, P. R. China. E-mail: jlzhuang@xmu.edu.cn
d Institute for Inorganic and Analytical Chemistry, University of Frankfurt,
Max-von-Laue-Strasse 7, 60438 Frankfurt/M, Germany
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cc06965e
‡ These authors contributed equally to this work.
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Scheme 1 Synthetic route for TEMPO radical decorated hollow PAFs
(HPAF-TEMPO) by using SiO2 nanospheres as templates.
Herein, we report a bottom-up strategy to prepare TEMPO
radical decorated hollow PAFs (Scheme 1), which show superior
catalytic activity toward the selective oxidation of a broad range of
Fig. 1 TEM images of (a) SiO2@PAF-TEMPO and (b) HPAF-TEMPO.
alcohols. The catalytically active building block 2 was synthesized the 3280 cmÀ1 peak (C–H vibration of alkyne) in the spectrum of
HPAF-TEMPO (3) reveals the successful Sonogashira coupling
reaction. The characteristic peaks of –CH3 (2850–3050 cmÀ1),
–CQO (1670 cmÀ1), and N–Oꢀ (1317 cmÀ1), which arise from the
TEMPO moiety, demonstrate the excellent stability of a TEMPO
radical during the Sonogashira coupling and the HF etching
process. Elemental analysis shows that the HPAF-TEMPO
contains 4.0 wt% of N, 67.7 wt% of C, and 5.0 wt% of
by covalent incorporation of a commercially available amino-
TEMPO radical (pink sphere) into 2,5-dibromobenzoic acid
through an amide bond. Monodispersed SiO2 nanospheres with
a uniform size of 215 nm were prepared by a Stober method.13 In
¨
the presence of SiO2 nanosphere templates, and Pd(0) and CuI
catalysts, the Sonagashira coupling of tetrakis(4-ethynylphenyl)
methane (1) with 2 results in TEMPO radical decorated PAF
coated SiO2 hybrid materials, named SiO2@PAF-TEMPO. Finally, H, respectively. The amorphous nature of the HPAF-TEMPO
was confirmed by powder X-ray diffraction measurements
(Fig. S4, ESI†). Thermogravimetric analysis (TGA) suggested that
the incorporated TEMPO units start to decompose at tempera-
tures higher than 150 1C. The radical nature of the HPAF-TEMPO
was studied by electron paramagnetic resonance (EPR) spectro-
scopy. Compared to the solution state EPR spectrum of the
monomer 2, where a three-line pattern centered at g = 2.008
the removal of the SiO2 templates by diluted HF solution leads
to TEMPO radical functionalized hollow PAFs (3), named
HPAF-TEMPO (see detailed experiments in the ESI†).
The successful coating of PAF-TEMPO on SiO2 nanospheres
was clearly visualized by the TEM images in Fig. 1a. The
thickness of the PAF-TEMPO shells was estimated to be
30 nm after growing for 48 h. The complete removal of SiO2
by HF solution (10% in volume) was evidenced by the disap- was observed, the solid state EPR spectrum of the HPAF-TEMPO
shows a broad peak in this region, indicating the immobilization
of the TEMPO moiety in the pore due to polymerization. Similar
EPR patterns were observed when TEMPO radicals were trapped
in other porous materials.8,14 The N2 isotherm of the HPAF-
TEMPO at 77 K shows a combination of type I and type IV patterns.
The Brunauer–Emmett–Teller (BET) surface area of the HPAF-
TEMPO is 245 m2 gÀ1 (Fig. 2c). The total pore volume and the
pearance of the dark inner sphere in the TEM images (Fig. 1b)
as well as a few bowl-shaped nanoparticles in the SEM images
(Fig. S1, ESI†). The almost perfect hollow structures after
HF etching indicate the excellent chemical and mechanical
stability of the HPAF-TEMPO, which is important for catalytic
applications. The amount of SiO2 nanoparticles, the concen-
tration of the monomers, and the reaction temperature play
important roles in the growth of perfect hollow structures. micropore pore volume derived by a t-plot method are 0.46 cm3 gÀ1
and 0.07 cm3 gÀ1, respectively, indicating that the HCMP-TEMPO
Either irregular non-hollow microspheres, or a significant
networks are mainly mesoporous (85%). The pore size distribution
analysis by the nonlocal density functional theory (NLDFT) method
also reveals a hybrid micro-/mesoporous structure (Fig. 2d inset).
We then examined the catalytic activity of the HPAF-TEMPO
for selective oxidation of 5-hydroxymethylfurfural (5-HMF) to
2,5-diformylfuran (DFF). Initially, the reactions were carried out
number of PAF-TEMPO microspheres, or aggregated incom-
plete nanospheres were obtained when the reaction conditions
varied (Table S1 and Fig. S2, ESI†). Moreover, detailed analysis
of the size of SiO2@PAF-TEMPO as well as the cave of the HPAF-
TEMPO, indicates that the diameter of SiO2 nanospheres
slightly decreased (from 215 Æ 3 nm to 183 Æ 2 nm) during
the reaction, which is caused by the basic solvents as confirmed in a closed vial with 0.5 mL solvents (saturated with O2),
0.28 mmol benzyl alcohol, 20 mol% tert-butyl nitrite (TBN),5c
by the controlled experiments (Fig. S3, ESI†).
and 5 mol% catalysts (0.014 mmol based on the calculation of
elemental analysis) under air at 80 1C for 5 h.
The chemical information of the HPAF-TEMPO was studied
by FT-IR spectroscopy and elemental analysis. The absence of
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demonstrated that in the second run no decreased conversion was
observed (entry 7a), while this value drops to 65% in the third run
(entry 7b), presumably caused by the partial blockage of the pores
(Fig. S8, ESI†). The SEM image of the HPAF-TEMPO after catalysis
shows no morphology changes (Fig. S9, ESI†). Extending the
reaction time to 8 h could improve the conversion in the third
run (entry 7c).
To demonstrate the structural advantages of the HPAF-TEMPO
in catalysis, we have synthesized non-hollow PAF-TEMPO micro-
spheres without the use of SiO2 as templates. Almost identical
chemical compositions were found as evidenced by the compar-
ison of FT-IR spectra (Fig. S9, ESI†) and element measurements
(C: 66.8 wt%; H: 5.1 wt%; N: 4.3 wt%). The N2 isotherm suggests
that the PAF-TEMPO possesses mainly micropores with BET
surface areas of 270 m2 gÀ1 (Fig. S8, ESI†). However, these non-
hollow PAF-TEMPO microspheres show lower conversion for the
oxidation of 5-HMF compared to the HPAF-TEMPO (entry 8)
under identical conditions. This effect became more pronounced
when bulky and secondary alcohol, benzhydrol, was examined
(Fig. 3). Full conversion was achieved after 9 h by using
HPAF-TEMPO as a catalyst, while the non-hollow PAF-TEMPO
microspheres only led to 53% conversion under identical
conditions. The reaction kinetics clearly demonstrate that the
HPAF-TEMPO exhibits two times higher efficiency than that of
non-hollow PAF-TEMPO, benefiting from its accessible inner layer,
hybrid pore structures, and reduced diffusion pathway of reagents.
To explore the substrate scope, we applied the HPAF-TEMPO
(Table 2) to oxidize a broad range of alcohols. Under optimized
conditions, primary benzylic alcohols (entries 1–5, with
Fig. 2 (a) FT-IR spectra of monomer 1, monomer 2, and HPAF-TEMPO
(3); (b) TGA curve of HPAF-TEMPO; (c) solution state EPR spectrum of
monomer 2, and solid state EPR spectrum of HPAF-TEMPO (3); (d) N2
isotherm (black: adsorption, red: desorption) and NLDFT pore size
distribution of HPAF-TEMPO (inset).
The catalytic results are summarized in Table 1. Solvent
screening experiments suggested that benzotrifluoride and
toluene are the most suitable solvents for the oxidation of
5-HMF (entries 1 and 2); good conversion can be achieved by
using CH3CN and DCE (1,2-dichlorethane) as solvents (entries 3
and 4), while the use of water leads to poor conversion (entry 5).
Under all of the tested conditions, no over-oxidized products,
such as 5-formyl-2-furancarboxylic acid (FFAC) or 2,5-furandi-
carboxylic acid (FDCA), were detected by GC-MS and HNMR
spectrum (Fig. S5, ESI†), indicating the excellent selectivity of the
HPAF-TEMPO. Running the reaction without HPAF-TEMPO led to
no product formation (entry 6). The removal of HPAF-TEMPO after
the reaction was performed for 3 h led to no further increased
conversion of 5-HMF (Fig. S6, ESI†). Moreover, the EPR spectrum of
the filtered reaction solution shows no detectable free TEMPO,
confirming the heterogeneous nature of the reaction and excellent
stability of the HPAF-TEMPO (Fig. S7, ESI†). The cycling experiments
electron-donating or electron-withdrawing substituents),
a
secondary aromatic alcohol (entry 9), and heteroatom substi-
tuted aromatic alcohols (entries 6–8), all afforded excellent
yields. Again, the catalytic activity of the HPAF-TEMPO outper-
forms the non-hollow PAF-TEMPO microspheres (entries 4 and
5, brackets), benefiting from their hollowed nanostructures.
Table 1 Aerobic oxidation of 5-HMF under various conditions
Entry
Catalyst
Solvent
Con. (%)
Sel. (%)
1
2
3
4
5
6
7
8
HPAF-TEMPO
HPAF-TEMPO
HPAF-TEMPO
HPAF-TEMPO
HPAF-TEMPO
—
PhCF3
toluene
CH3CN
DCE
100
95
85
83
499
499
499
499
499
0
H2O
5
0
PhCF3
PhCF3
PhCF3
HPAF-TEMPO
PAF-TEMPO
98a/65b/90c
85
499
499
Fig. 3 Plots of GC conversion (%) versus time for benzhydrol oxidation
catalyzed by HPAF-TEMPO (red squares) and non-hollow PAF-TEMPO
microspheres (black dots), and their corresponding TEM images. Reaction
conditions: benzyl alcohol (0.28 mmol), catalysts (10 mg, 0.014 mmol for
HPAF-TEMPO, 0.015 mmol for non-hollow PAF-TEMPO).
Reaction conditions: benzyl alcohol (0.28 mmol), catalyst (10 mg,
0.014 mmol, 5 mol%), TBN (20 mol%); conversion and selectivity were
determined by GC analysis with p-nitrobenzene as an internal standard.
a
b
c
Second run. Third run. Third run for 8 h.
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Table 2 HPAF-TEMPO catalyzed aerobic oxidation of various alcohols
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However, the oxidation of aliphatic alcohols requires longer
time to achieve moderate yields (entries 10–12), and 34% of
n-hexanoic acid was found in the case of n-hexanol.
In summary, we demonstrate a bottom-up approach to synthe-
size TEMPO radical decorated hollow PAF, HPAF-TEMPO, by
using TEMPO radical functionalized monomers and SiO2 as
templates. The catalytic activity of the HPAF-TEMPO outperforms
the non-hollow PAF-TEMPO microspheres toward the oxidation of
a broad range of alcohols, thanks to their unique hollowed
nanostructures. We are currently exploring the template synthesis
of hollow and bifunctional PAFs and CMPs for enantioselective or
photo-driven organocatalysis by the incorporation of chiral or
photoactive moieties (e.g., L-proline and benzothiadiazole) into
the frameworks.
This work was supported the National Key Research and
Development Program of China (grant 2017YFF0204602), the
National Natural Science Foundation of China (No. 21861013),
Natural Science Foundation of Guizhou Province (No. 20185769),
Department of Education of Guizhou Province (No. YJSCXJH
(2019)045), and State Key Laboratory of Physical Chemistry of
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Conflicts of interest
There are no conflicts to declare.
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