Morphology-Reserved Synthesis of Discrete Nanosheets of CuO?SAPO-34 and Pore Mouth Catalysis for One-Pot Oxidation of Cyclohexane

Article abstract of DOI:10.1002/anie.201911749

Discrete nanosheets of silicon-doped AlPO₄ molecular sieves (SAPO-34) with a thickness of ≈7 nm have been prepared through morphology-reserved synthesis with a lamellar aluminum phosphate as precursor. Cages of the nanosheets are in situ incorporated with copper oxide clusters. The CuO?SAPO-34 nanosheets exhibit a large external surface area with a high number of (010) channel pores on the surface. Due to the thin morphology, copper oxide clusters occupy the outmost cages with a probability >50 %. The distinctive configuration facilitates a new concept of pore mouth catalysis, i.e., reactant molecules larger than the pores cannot enter the interior of the molecular sieves but can interact with the CuO clusters at “the mouth” of the pore. In heterogeneous catalysis, CuO?SAPO-34 nanosheets have shown top performance in one-pot oxidation of cyclohexane to adipic acid by O₂, a key compound for the manufacture of nylon-66, which is so far produced using non-green nitric acid oxidation.

Full text of DOI:10.1002/anie.201911749

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Morphology Reserved Synthesis of Discrete Nanosheets of
CuO@SAPO-34 and Pore Mouth Catalysis for One-pot Oxidation
of Cyclohexane
Authors: Weiping Ding, Xiangke Guo, Mengxia Xu, Minyi She, Yan
Zhu, Taotao Shi, Zhaoxu Chen, Luming Peng, Xuefeng Guo,
and Ming Lin
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201911749
Angew. Chem. 10.1002/ange.201911749
Link to VoR: http://dx.doi.org/10.1002/anie.201911749
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10.1002/anie.201911749
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Morphology Reserved Synthesis of Discrete Nanosheets of
CuO@SAPO-34 and Pore Mouth Catalysis for One-pot Oxidation
of Cyclohexane
Xiangke Guo,[a] Mengxia Xu,[a] Minyi She,[a] Yan Zhu,[a] Taotao Shi,[a] Zhaoxu Chen,[a] Luming Peng,[a]
Xuefeng Guo,*[a] Ming Lin,*[b] and Weiping Ding*[a]
Dedicated to the 100th anniversary of the School of Chemistry
and Chemical Engineering, Nanjing University
Abstract: Discrete nanosheets of silicon-doped AlPO4 molecular
sieves (SAPO-34) in thickness of ~7 nm have been prepared
following a mechanism of morphology reserved synthesis with a
lamellar aluminum phosphate as the precursor. Some cages of the
nanosheets are in-situ incorporated with copper oxide clusters
during the preparation. The CuO@SAPO-34 nanosheets have huge
external surface with large numbers of (010) channel pores showing
up on the surface and, on average, the copper oxide clusters occupy
the outmost cages in a possibility larger than 50% due to the super
thin morphology. The distinctive configuration introduces a new
concept of pore mouth catalysis for practical reactions, i.e., a
reactant molecule in a size larger than the pores cannot enter the
interior of molecular sieves but a part of it can interact with the
copper oxide clusters through the pore. As a heterogeneous catalyst,
the CuO@SAPO-34 nanosheet has shown the top catalytic
performance up to date, for one-pot oxidation of cyclohexane by O2
to adipic acid, a key compound for the manufacture of nylon-66
produced in industry so far only using a non-green method of nitric
acid oxidation.
Figure 1. Schematic show of the formation process of CuO@SAPO-34
nanosheets. (a) Layered alumina phosphate nanoroll precusor with
C12H25NH2 in between the layers; (b) Unfolded nanoroll induced by the
insertion of TEOS and Cu(NO3)2; (c) CuO@SAPO-34 nanosheets formed
after hydrothermal treatment in presence of tetraethylamonium hydroxide
(TEAOH) and triethylamine (TEA).
accessed by a part of a reactant molecule through the pores,
though the reactive molecule cannot enter the interior of the
molecular sieves as a whole, due to the super thin structure.[4]
The particular configuration should be capable of offering
unpredictable catalytic performances. [5]
There have been two important breakthroughs on the
synthesis of super thin molecular sieves respectively reported by
Ryoo et al (2009) and Tsapatsis et al (2017), which are
extremely noticeable to scientific community.[4b,5] And recently a
novel assembly–disassembly–organisation–reassembly (ADOR)
method has been used for the synthesis of 2D zeolites.[5d]
Moreover, it is an extreme challenge to synthesize non-layered
molecular sieves with 2D ultrathin structure for chemists of
molecular sieves and materials because the process of
molecular sieves crystallization and growth is controlled
thermodynamically and 2D ultrathin structure is very unstable in
nature due to the high surface energy.[4b]
The reactive nanoclusters enclosed in and tuned by surrounded
regular porous materials have been paid much research
attention recently, thanks to their excellent performances for
certain catalytic reactions.[1] For such catalysts, the surrounding
materials confine and tune the reactive centers in many aspects
and bring synergistic activities and shape/size selectivities.[2]
The bulk 3D zeolitic framework limits the access of large
molecules to the internal active centers which may locate deeply
in the molecular sieves and also cannot be contacted by
reactant through channel pores.[3] It is a great interest to create a
kind of novel catalytic material which consists of ultrathin and
well-crystallized molecular sieves with larger external surface
and numerous exposed pores. The reactive centers, e.g.,
heteroclusters confined in the zeolitic cages and/or the zeolitic
acidic sites, are limited by the zeolitic framework but easily
Herein, we report a novel topoinvariant synthesis of discrete
nanosheets of CuO@SAPO-34 by an in-situ transformation and
re-crystallization of
a
well-defined super thin aluminum
phosphate. During the preparation, the silicon doping to the
zeolitic framework and the enclosure of CuO clusters in zeolitic
cages can be introduced at the same time. The CuO@SAPO-34
nanosheets have huge external surface with large numbers of
(010) channel pores showing upon the surface and the copper
oxide clusters can be found at the outmost cages in a possibility
large than 50% due to the super thin morphology, and on
average, there are about 1/20 outmost pores can be found to
contain a copper oxide cluster. The sample of CuO@SAPO-34
nanosheets (~7 nm thickness, denoted as NS-CuO@SAPO-34
in the context) shows an excellent catalytic performance for one-
pot oxidation of cyclohexane to adipic acid using O2 as oxidant
(conversion: 42%; selectivity: 74%), which should be the top
results of heterogeneous catalysis reported in the literature up to
[a]
Dr. X.K. Guo, Ms. M.X. Xu, Mr. M.Y. She, Dr. T.T. Shi, Prof. Y. Zhu,
Prof. Z.X. Chen, Prof. L.M. Peng, Prof. X.F. Guo, Prof. W.P. Ding
School of Chemistry and Chemical Engineering
Nanjing University
Nanjing 210023, China
E-mail: guoxf@nju.edu.cn, dingwp@nju.edu.cn
Dr, M. Li
[b]
Institute of Materials Research and Engineering
Agency for Science, Technology and Research (A*STAR)
3 Research Link, Singapore 117602, Singapore
Supporting information including experimental details and Figures
S1~S16 are given via a link at the end of the document.
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Figure 2. Structural characterizations of CuO@SAPO-34 nanosheets: (a)
XRD patterns and (b) Ar sorption/desorption isotherms of NS-CuO@SAPO-34
and bulk-SAPO-34; (c) and (d) TEM images; (e) Elements mapping of the NS-
CuO@SAPO-34.
date for the reaction. The adipic acid is one of the most
important industrial chemicals for the production of nylon-66
polymer and now, it is produced almost using the non-green
method of nitric acid oxidation with very low overall product
yields (<10%) and severe environment problems, such as
corrosion and emission of vast amount of NOx.[7] In the past
decades, great efforts have been devoted to green methods for
production of adipic acid and the most recent significant
progress is reported by Hwang et al, using ozone as oxidant
under irradiation of UV light.[6b] However, the direct oxidation of
cyclohexane using O2 via a heterogeneous process to adipic
acid remains a great challenge at the present time.[7a] Hence, we
envision that the new concept of pore mouth catalysis will be
Figure 3. HR-TEM images of CuO@SAPO-34, obtained with prolonged time
of exposure to very low current (5 electrons/nm2/sec) at 60 kV. (a), (b) and
(c) HRTEM images and (d) SAED pattern of a CuO@SAPO-34 nanosheets
(NS-CuO@SAPO-34). (e) HRTEM image of the micro-porous structure
enlarged from the white square part in (c). (e’) Crystal model corresponding
to the TEM image of (e).
calcination at 550 °C in air. The XRD patterns of the related
samples obtained during the treatments are shown in Figure 2a.
All the peaks are well indexed to CHA type (SAPO-34) without
any observable peaks from a secondary phase other than
SAPO-34. Moreover, the (101) diffraction peak of the
nanosheets exhibits a markedly enhanced intensity (inset, Fig.
2a) than that of the bulk SAPO-34, suggesting the thickness
direction of the nanosheets is along the b axis of the molecular
sieves.[4b] The Ar sorption isotherms of NS-CuO@SAPO-34 have
an uptake step below P/P0=0.02 due to its rich micropores
similar to bulk SAPO-34 and a hysteresis loop at P/P0=0.8~1
indicates the presence of interconnected pores formed by the
aggregation of nanosheets (Figure 2b).[9] Compared with bulk
SAPO-34, NS-CuO@SAPO-34 has a little smaller Density
Functional Theory (DFT) area (382 vs. 448 m2/g), but a much
larger external surface area (129 m2/g, ~18 times larger than
bulk SAPO-34). By calculation, the external surface area of NS-
CuO@SAPO-34 corresponds to the nanosheets inaverage
thickness of ~7 nm, which closes to theoretical external surface
area (147 m2/g) in such thickness. N2 sorption/desorption
isotherms also show the similar results, as show in Figure S4
(SI).
dedicated to open
a new spectrum of high-performance
catalysts for such difficult reactions.
Figure 1 schematically illustrates the fabrication process of
CuO@SAPO-34 nanosheets. Firstly, artificial AlPO4 nanorolls,
with nanosheets rolled up, were synthesized according our
previous work with dodecylamine bilayers in the AlPO4
nanosheet (SI, TEM image shown as Figure S1).[8] Then
tetraethylorthosilicate (TEOS) and Cu(NO3)2 were introduced
into the interlayers of the AlPO4 nanoroll, which were exfoliated
under the synthetic conditions, as revealed by XRD
measurements (SI, Figure S2). Finally, the nanosheets with
CHA crystal structure formed by hydrothermal treatment of the
above composite from AlPO4 nanorolls with co-existence of
tetraethylammonium hydroxide (TEAOH) and triethylamine
(TEA). During the synthesis, the dodecylamine bilayers played a
pivotal role, i.e., ensuring a template-confining structure offered
for the reaction of ultrathin layer of AlPO4 with TEOS and
Cu(NO3)2 to NS-CuO@SAPO-34.
The organic templates in the as-synthesized product, about
40 wt.% as analyzed by thermogravimetric measurement (SI,
Figure S3), can be completely removed by extraction with a
methylamine hydrochloride/ethanol solution followed by
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Figure 4. (a) Cu K-edge XAS spectra of NS-CuO@SAPO-34, CuO and
Cu2O. Inset is radial function of Cu from Fourier transform of EXAFS of the
NS-CuO@SAPO-34 (the fitting curve is a CuO cluster with the size of 0.7
nm and the table of inset lists the parameters about the copper
coordination status); (b) H2-TPR curves of NS-CuO@SAPO-34 and CuO;
(c) Structural model for the skeleton of NS-CuO@SAPO-34; (d) 27Al solid
state NMR spectra of NS-CuO@SAPO-34.
The TEM images (Figures 2c and 2d) and SEM images (SI,
Figure S5) show the nanosheets are thin rectangles in the sizes
of 200-500 nm. As shown in Figure 2e, the elements mapping of
NS-CuO@SAPO-34 shows the elements of Al, P, Si and O are
all uniform distributed throughout the sample and the domains
with Cu enrichment are not detected for their too small sizes. As
seen in Figure 3a, the HR-TEM image reveals the nanosheet
has a single crystalline structure with a lattice fringe of 0.94 nm,
corresponding to the (101) planes of SAPO-34 as the structural
model (SI, Figure S6a). Figure 3b showing the cross-section at
the curl of a nanosheet reveals its thickness of ~7 nm with a d-
spacing of 0.80 nm, corresponding to the (0, 3/2, 0) planes of
SAPO-34, as the structural model listed in Figure S6b. The
thickness of ~7 nm corresponds to ~5 unit cells along the b-axis
(b = 1.3675 nm). Another HRTEM image (Figure 3c) has
provided a further proof that the nanosheet has a single
Figure 5. (a) Catalytic performances for selective oxidation of cyclohexane
to adipic acid measured with NS-CuO@SAPO-34, bulk-CuO/SAPO-34 and
CuO/SiO2 used as the catalyst. (b) Reusability of the NS-CuO@SAPO-34
for the reaction (Error bars represent the standard deviation for three
replicates), and the inset is the Arrhenius’ relation of the reaction. In-situ IR
spectra of cyclohexanone (c) and cyclohexanol (d) adsorbed on NS-
CuO@SAPO-34. (e) The calculation model and results for the IR data, and
the number in the brackets is corrected by a coefficient of 0.95 (the results
were calculated using DFT method with Gaussian software). (f) Schematic
show of catalytic mechanism over NS-CuO@SAPO-34.
crystalline structure with
a
lattice fringe of 0.46 nm,
to the position they located. In addition, as revealed by the
results of hydrogen temperature programmed reduction (H2-
TPR), the NS-CuO@SAPO-34 present a single reduction peak
at 340 °C, much higher than that for the reduction by hydrogen
of the CuO itself, showing the strong interaction of stabilization
of CuO clusters by the molecular sieves enclosing (Figure 4b).[12]
corresponding to the (202) planes of SAPO-34. The selected
area electron diffraction (SAED) (Figure 3d) shows a regular
spot pattern when viewed along the b axis, which further
confirms the single crystalline structure of SAPO-34 nanosheet.
Figure 3e shows HRTEM image of the micropore structure of the
nanosheet, referred to the corresponding crystal model shown
as Figure 3e’. And more HRTEM images (SI, Figure S7) give
similar results that the thickness direction of the nanosheet is
along the (010) direction of the CHA framework, implying the
plane of the nanosheets with many pores is the (010) surface of
the SAPO-34.
Electron paramagnetic resonance (EPR) identifies the
existence of Cu2+ species with axisymmetric octahedrally
coordinated status in the cages of CHA (SI, Figure S8).[10] And
the X-ray absorption near-edge structure (XANES) spectrum
(Figure 4a) indicates the similarity of copper in NS-CuO@SAPO-
34 to CuO.[11] By fitting the Cu radial function, the average
coordination number of Cu-Cu shell, the most important
parameter about the CuO clusters, is found about 2.8±0.2,
implying the size of the CuO clusters is ~ 0.7 nm, in accordance
Summarizing these results,
a
structure model of NS-
CuO@SAPO-34 is proposed and depicted in Figure 4c and
Figure S9 (SI), i.e., CuO clusters (~0.7 nm) with 14 atoms (8 Cu
and 6 O) occupied some cages (t-CHA) of the SAPO-34 and
coordinated the oxygen atoms in framework of t-CHA cages.
Moreover, the size of CuO clusters is confined by the space of
the t-CHA cages.
The solid-state 27Al NMR spectrum of the nanosheets (Figure
4d) shows a strong peak at 40.3 ppm belonged to tetrahedral Al
similar to bulk SAPO-34.[13a] Interestingly, another peak is
observed at -10.7 ppm appeared to be octahedral Al species
which cannot be observed, or very little, on bulk SAPO-34.
According to the unique morphology of the sample NS-
CuO@SAPO-34, the peak is tentatively attributed to the large
amount of open cages at the (010) surface of the nanosheet,
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consulting with the results of HRTEM and XRD. For 31P NMR
spectra (SI, Figure S10), both the samples NS-CuO@SAPO-34
and bulk SAPO-34 show similar 31P-NMR spectra and the main
peak at -26 ppm which indicates the tetrahedral P species. [13b]
The shoulder peak at -15 ppm is more likely from hydroxylated
phosphate. The results imply that the NS-CuO@SAPO-34 has a
relative enrichment of aluminum on the top surface, most likely
on the top (010) surfaces of the nanosheets.
C=O group of cyclohexanone and CuO clusters in the NS-
CuO@SAPO-34. For cyclohexanol, however, no chemical
adsorption can be found by the FT-IR spectrum, which is the
same with liquid cyclohexanol (Figure 5d). Hence, it is proposed
that cyclohexane firstly converts to cyclohexanone as the main
intermediate with minor cyclohexanol as by product, as shown in
Figure 5f. Secondly, the cyclohexanone molecules are
chemically adsorbed on the partially confined CuO cluster sites
and start to be oxidized to adipic acid in the following catalytic
reactions.[7e] The high selectivity of adipic acid should be
attributed to the strong confining effect of the channel mouths on
the interactions between the reactant and CuO clusters which
are the reactive centers under the top surface of the NS-
CuO@SAPO-34. To understand the activation of O2, the
coordinated environment of CuO clusters and free radicals of
oxygen species were detected by EPR, as shown in Figure S16
(SI). We believe it may not be a radical mechanism but a
mechanism that the CuO clusters oxidized cyclohexane
molecular by their lattice oxygen and O2 re-oxidize the CuO
clusters and the turnover frequency of cyclohexane conversion
is 29.4 h-1 per cluster.
In summary, a new kind of hybrid material, CuO@SAPO-34
nanosheet is prepared via a newly demonstrated mechanism of
exfoliation and crystallization of layered AlPO4 precursor with
morphology reserved. The CuO@SAPO-34 nanosheet has
exhibited high crystallinity, large external surface and excellent
hydrothermal stability. It shows an excellent catalytic
performance for the selective oxidation of cyclohexane to adipic
acid using molecular oxygen as oxidant, which holds the
potential as a practical catalyst to produce adipic acid in an
efficient and green way. And also, we envision that the new
concept of pore mouth catalysis extracted from the current
investigation will be dedicated to open a new spectrum of design
for high-performance catalysts.
Figure 5a shows the catalytic results of the NS-CuO@SAPO-
34 for oxidation of cyclohexane to adipic acid by O2 and the
results obtained on Bulk-CuO/SAPO-34 and CuO/SiO2 are also
listed for comparison. The reaction data are obtained by analysis
performed on isolated products and the adipic acid has been
identified by NMR measurements (SI, Figure S11). Excitingly,
the selectivity to adipic acid measured on isolated products
reaches ~74% at the cyclohexane conversion as high as ~42%
are obtained on NS-CuO@SAPO-34 which should be the top
results of heterogeneous catalysis reported in the literature up to
date (SI, Figure S12) and besides adipic acid, some dicyclohexyl
adipate are found as by-product in 20-hour reaction. In sharp
contrast, almost no adipic acid products are detected over the
samples of Bulk-CuO/SAPO-34 or CuO/SiO2 as catalyst and
only small amount of cyclohexanol and cyclohexanone were
detected in the products. We think that the CuO is the reactive
center for the catalytic reaction, for the SAPO-34 itself is inactive
for the reaction. After the reaction, the leached copper species
from the catalyst cannot be detected in the solution using ICP-
MS, reflecting the stability of the catalyst. Moreover, the NS-
CuO@SAPO-34 shows no obvious degeneration in catalytic
performance during the 5 times of reuses in duration of 100
hours (Figure 5b).[14] As shown in inset of Figure 5b and Figure
S13 (SI), we also studied the catalytic kinetics and found the
apparent activation energy is ~84.3 kJ/mol and reaction order of
cyclohexane is ~ 1.5 for the selective oxidation of cyclohexane.
For insight into the mechanism of reaction, cyclohexanol,
cyclohexanone or adipic acid, instead of cyclohexane, was
respectively used as the reactant (SI, Figure S14). It was
discovered that the selectivity to adipic acid reaches ~60% at
conversion of 34% when cyclohexanone was used as reactant.
Interestingly, no any meaningful products were detected when
cyclohexanol or adipic acid was used as the reactant. For
adsorptions of cyclohexane, cyclohexanone or cyclohexanol by
the NS-CuO@SAPO-34 (SI, Figure S15), it is interesting that the
NS-CuO@SAPO-34 catalyst prefers to adsorb cyclohexanone,
compared with cyclohexanol, and the adsorption amount of
cyclohexanone is about 50% higher than cyclohexanol (0.15
mmol/g vs 0.10 mmol/g). The amount of CuO clusters enclosed
in the cages of the sub-surface is 0.17 mmol/g, which is similar
to the adsorption amount of cyclohexanone. Based on these
results, we think that cyclohexanone tends to adsorb at the CuO
clusters through the pores while cyclohexanol tends to adsorb at
the pores without CuO clusters. FT-IR spectra recorded with the
cyclohexanone or cyclohexanol adsorbed samples have
confirmed the conclusion (Figure 5c and 5d). For cyclohexanone
adsorption, the newly appeared peak at 1640 cm-1 is ascribed to
the C=O bond of cyclohexanone chemically adsorbed on the
CuO cluster which is shifted from 1715 cm-1, as proved by DFT
calculations using Gaussian software and B3LYP method
(Figure 5e), which reflects the strong interaction between the
Experimental Section
Experimental Details and Figures S1~S16 see supporting information.
Acknowledgements
This work was financially supported by National Science
Foundation of China (21773110, 21773109, 21932004 and
91745108), the Ministry of Science and Technology of China
(2017YFB0702900) and the Fundamental Research Funds for
the Central Universities and Jiangsu Overseas Visiting Scholar
Program for University Prominent Young
& Middle-aged
Teachers and Presidents. We thank the help from Mr. Jason
Gulbinski and Dr. Wei Fan in UMass Amherst for their proof
reading on the manuscript.
Keywords: molecular sieves • oxide cluster • nanosheet • pore
mouth catalysis • selective oxidation
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This article is protected by copyright. All rights reserved.

10.1002/anie.201911749
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
2D and super-thin CuO@SAPO-4
nanosheets with ~7 nm thickness and
single crystalline structure have been
synthesized and shown an excellent
catalytic performance for the selective
oxidation of cyclohexane to adipic acid
using molecular oxygen as oxidant by a
pore mouth catalysis mechanism.
X. Guo, M. Xu, M. She, Y.
Zhu, T. Shi, Z. Chen, L.
Peng, X. Guo,* M. Lin,* W.
Ding*
Page No. – Page No.
Morphology Reserved
Synthesis of Discrete
Nanosheets of
CuO@SAPO-34 and
Pore Mouth Catalysis for
One-pot Oxidation of
Cyclohexane
This article is protected by copyright. All rights reserved.