Generation of gold nanoclusters encapsulated in an MCM-22 zeolite for the aerobic oxidation of cyclohexane

Article abstract of DOI:10.1039/c8cc07185c

In this work, we will report the generation of Au clusters in a purely siliceous MCM-22 zeolite. The catalytic properties of these Au clusters have been tested for the selective oxidation of cyclohexane to cyclohexanol and cyclohexanone (KA-oil). The Au clusters encapsulated in the MCM-22 zeolite are highly active and selective for the oxidation of cyclohexane to KA-oil, which is superior to Au nanoparticles on the same support. These results suggest that Au clusters are highly active for the activation of oxygen to produce radical species.

Full text of DOI:10.1039/c8cc07185c

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Generation of gold nanoclusters encapsulated in
an MCM-22 zeolite for the aerobic oxidation of
cyclohexane†
Cite this:DOI: 10.1039/c8cc07185c
Received 6th September 2018,
Accepted 21st November 2018
bcd
a
Lichen Liu,a Raul Arenal,
Debora M. Meirae and Avelino Corma
*
DOI: 10.1039/c8cc07185c
rsc.li/chemcomm
In this work, we will report the generation of Au clusters in a purely Using organometallic Au complexes as precursors, Au clusters
siliceous MCM-22 zeolite. The catalytic properties of these Au can be generated in zeolites.12 This strategy, however, is
clusters have been tested for the selective oxidation of cyclohexane restricted for processes with activation and reaction conditions
to cyclohexanol and cyclohexanone (KA-oil). The Au clusters that do not require higher temperature.
encapsulated in the MCM-22 zeolite are highly active and selective
Considering the low thermal stability of Au particles, the
for the oxidation of cyclohexane to KA-oil, which is superior to Au encapsulation of Au species in zeolites should be a promising
nanoparticles on the same support. These results suggest that Au approach to enhance their stability.13 The well-defined pore
clusters are highly active for the activation of oxygen to produce structures of zeolites can provide effective protection for Au
radical species.
species under reaction conditions and introduce, in some
cases, size-selective catalytic properties. During the past several
Gold catalysts have been intensively studied in recent years years, the encapsulation of Au into zeolite crystallites has been
and they have been shown to exhibit unique and remarkable achieved by several methods. Au nanoparticles (2–3 nm) can be
catalytic performances in many reactions, including selective encapsulated in silicalite-1 with intraparticle voids and meso-
hydrogenation, selective oxidation, organic transformations, pores through a simple impregnation method.14 Recently,
electrocatalysis and photocatalysis.1–4 As is well known, the Iglesia and his co-workers reported a method to generate small
catalytic behavior of Au catalysts is strongly related to their Au nanoparticles (1–2 nm) in Al-containing zeolites with high
particle size. Small Au particles (o2 nm) have been shown to be thermal stability and resistance to poison molecules in catalytic
much more active than larger Au particles in many reactions.5,6 reactions.15 In our recent work, subnanometric Pt single atoms
Nevertheless, when the size of Au decreases to a subnanometric and Pt clusters were encapsulated in MCM-22 through a 2D to
regime, the catalytic properties of Au clusters are quite distinct 3D transformation process. These subnanometric Pt species
to Au nanoparticles due to the size-dependent electronic struc- show excellent stability even at a high temperature (as high
tures of the Au species.7 In our recent works, it has been as 550 1C).16,17
demonstrated that, in some reactions, only Au clusters with a
In this work, by the transformation of a two-dimensional
few atoms are active while neither single Au atoms nor Au zeolite into a three-dimensional zeolite, naked subnanometric
nanoparticles are active.8,9 However, due to their low stability Au clusters can also be generated in purely siliceous MCM-22,
and strong tendency for agglomeration, naked subnanometric Au and the resultant material presents an excellent catalytic activity
clusters with open surface sites have been barely reported.10,11 and selectivity for the selective oxidation of cyclohexane with O2
to produce cyclohexanol and cyclohexanone (KA-oil).
As shown in Fig. 1, the Au@MCM-22 sample was prepared
a
´
´
`
`
Instituto de Tecnologıa Quımica, Universitat Politecnica de Valencia-Consejo
through a similar procedure to that we had reported in our
recent work for the preparation of Pt@MCM-22. Au clusters
dispersed in DMF are prepared firstly as the metal precursor.
´
Superior de Investigaciones Cientıficas (UPV-CSIC), Av. de los Naranjos s/n,
46022 Valencia, Spain. E-mail: acorma@itq.upv.es
b Laboratorio de Microscopias Avanzadas, Instituto de Nanociencia de Aragon,
Universidad de Zaragoza, Mariano Esquillor Edificio I + D, 50018 Zaragoza, Spain Subsequently, the Au clusters are incorporated between the MWW
c ARAID Foundation, 50018 Zaragoza, Spain
layers in ITQ-1 during a swelling process. After the removal of
the organics in the swollen material, Au species are encapsu-
lated by the final MCM-22 zeolite, leading to the formation of
Au@MCM-22-S. However, the Au loading in the Au@MCM-22-S
d Instituto de Ciencias de Materiales de Aragon, CSIC-Universidad de Zaragoza,
C/Pedro Cerbuna 12, 50009 Zaragoza, Spain
e European Synchroton Radiation Facility, 6 Rue Jules Horowitz, Grenoble, BP 156,
F-38042, France
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cc07185c sample was only ca. 0.025 wt%. Then, in order to improve the
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Fig. 2 Electron microscopy characterization on the Au@MCM-22 samples.
(a) STEM image of the Au@MCM-22-S sample. (b and c) High-resolution
STEM images of the subnanometric Au clusters observed in the Au@MCM-
22-S sample. (d–f) STEM images of the Au@MCM-22-L sample, showing
the presence of both subnanometric Au clusters and Au nanoclusters
around 1 nm.
To gain more information on the structures of the Au
clusters in the Au@MCM-22 samples, various spectroscopic
characterization tools were used to study them. The coordina-
tion environment and average size of the Au species in the
Au@MCM-22 samples have been studied by X-ray absorption
spectroscopy (XAS). The chemical states of the Au species in the
Au@MCM-22-S and Au@MCM-22-L samples are studied by
X-ray absorption near edge structure (XANES) analysis. As
Fig. 1 Schematic illustration of the incorporation of Au nanoclusters into
a zeolite through the transformation of a 2D zeolite into a 3D structure
(MCM-22). (a) The incorporation of Au clusters in MCM-22 by swelling
of the ITQ-1 with a surfactant and a DMF solution containing Au clusters.
(b) The incorporation of Au clusters in MCM-22 by swelling of the ITQ-1
with a surfactant, 1-octanethiol and a DMF solution containing Au clusters.
Au loading in the final Au@MCM-22 material, 1-octanethiol is displayed in Fig. 3a, both Au@MCM-22-S and Au@MCM-22-L
added here to the swelling mixture (see Fig. 1b), since the show similar XANES spectrum to the reference metallic Au,
strong Au–S bonding interaction may help the incorporation of indicating that the Au nanoclusters exist in the metallic state in
Au species into the MWW layers.18,19 As a result, an Au@MCM- the MCM-22 zeolite. Furthermore, the coordination environ-
22-L sample was obtained with an Au loading of ca. 0.11 wt%. In ment of the Au species has also been studied by extended X-ray
principle, the introduction of 1-octanethiol can also be applied absorption fine structure (EXAFS) analysis. As can be seen in
to the incorporation of other metals into MCM-22 by our Fig. 3b, the fitting results of the EXAFS spectra are shown in
strategy, leading to a higher metal loading and maintenance Table 1. The Au–Au coordination number in Au@MCM-22-S and
of good metal dispersion in the zeolite at the same time.
Au@MCM-22-L is 7.0 and 7.7, respectively. Thus, the average size
Firstly, the Au species in the Au@MCM-22-S and Au@MCM- of the Au species in Au@MCM-22-S is B1 nm and B1.2 nm in
22-L samples have been characterized by electron microscopy. Au@MCM-22-L, which is consistent with the electron micro-
In the low-magnification STEM image of Au@MCM-22-S (see scopy images in Fig. 2, if one takes into account the impact of
Fig. 2a), Au nanoclusters with particle size ranging from 0.5 nm the larger particles on the average Au–Au coordination number
to 1 nm can be seen together with a few Au nanoparticles of
1–2 nm. The atomic structure of the subnanometric Au clusters
was revealed by the high-resolution STEM images (see Fig. 2b
and c). As it can be seen, Au nanoclusters with less than
15 atoms are located in the supercages or surface ‘‘cups’’ of
MCM-22. More high-resolution STEM images of various sub-
nanometric Au species are shown in Fig. S1 (ESI†), including
single Au atom and Au clusters with less than 10 atoms.
Considering the size of the supercages in MCM-22, the maximum
size of the Au clusters is below 1 nm, corresponding to the Au
clusters with less than 15 atoms. In the case of Au@MCM-22-L,
Au nanoclusters around 1 nm can also be observed in the STEM
Fig. 3 (a) XANES spectra of the Au@MCM-22-L and Au@MCM-22-S
images (see Fig. 2d–f). The particle size of the Au species in
the Au@MCM-22-L sample is slightly larger than that in the
Au@MCM-22-S sample.
samples and the Au foil reference. (b) Fourier transform of k2-weighted
EXAFS spectra and the fitting curves for the Au@MCM-22-L and Au@MCM-
22-S samples.
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Table 1 Fitting results of the EXAFS spectra for the Au@MCM-22-L and
2
Au@MCM-22-S samples. S0 = 0.8 and E0 = 5 eV. More fitting details are
shown in the ESI
Sample
CNAu–Au s2 (Å2)
R (Å)
CNref Rref (Å)
2.88470
Au@MCM-22-L 7.7 Æ 0.6 0.008 Æ 0.001 2.853 Æ 0.003 12
Au@MCM-22-S 7.0 Æ 0.6 2.855 Æ 0.004
obtained from the EXAFS fitting results.20,21 Then, by com-
bining the results of electron microscopy and XAS, it can be
deduced that, both Au@MCM-22-L and Au@MCM-22-S consist
of a mixture of subnanometric Au clusters and small Au
nanoparticles (1–2 nm). Furthermore, the percentage of sub-
nanometric Au clusters in the Au@MCM-22-S sample is higher
than that in the Au@MCM-22-L sample according to the EXAFS
fitting results.
For comparison, a sample consisting of Au nanoparticles
supported on MCM-22 (containing 0.1 wt% of Au, denoted as
AuNP/MCM-22) was also prepared by a deposition–precipitation
method (see the ESI† for experimental details). As shown in
Fig. S2 (ESI†), the particle size of the Au nanoparticles in AuNP/
MCM-22 ranges from 1.2 to 2.5 nm.
It is well known that the electronic structures of metal
clusters and nanoparticles are usually associated with their
optical properties. As shown in Fig. S3 (ESI†), the typical surface
plasmon resonance of the Au nanoparticles can be observed in
the UV-vis spectrum of the AuNP/MCM-22 sample at B530 nm.
In the case of the Au@MCM-22 samples containing Au nano-
clusters, we can observe the absorption bands at B615 and
Fig. 4 Catalytic performance of the AuNP/MCM-22 and Au@MCM-22
catalysts for the aerobic oxidation of cyclohexane. (a) Time-profile of the
conversion of cyclohexane with different catalysts. (b) Product distributions
obtained with different catalysts after 2 h of reaction. Reaction conditions:
2 mL cyclohexane, 25 mg solid catalyst, 150 1C, and 10 bar of O2. The
‘‘thermal’’ test was carried out under the same conditions in the absence of
a catalyst.
B670 nm, corresponding to the absorption bands between the nanoparticles are not active for this process, which is consistent
HOMO and the LUMO of Au clusters with different sizes.22 At with the previous works.24
this point, based on the above structural and spectroscopic
characterization results, it is confirmed that the Au nanoclusters both Au@MCM-22-S and Au@MCM-22-L containing subnano-
have been generated and stabilized in MCM-22. metric Au clusters show a shorter induction period (B0.5 h) in
Remarkably, the kinetic curves shown in Fig. 4a show that
To test the catalytic properties of Au clusters, the oxidation the oxidation of cyclohexane and a much higher conversion
of cyclohexane to cyclohexanol and cyclohexanone (KA-oil) has than the previous experiments, indicating the higher activity
been carried out. It has been established in the literature that for the generation of radicals and for the decomposition of
the auto-oxidation of cyclohexane is a radical-chain reaction.23 the hydroperoxide intermediates. As we can see in Fig. 4a, a
Moreover, it has also been proposed that supported Au nano- high conversion of cyclohexane (412%) can be achieved with
particles are actually inert in the oxidation of cyclohexane.24 It Au@MCM-22-S and Au@MCM-22-L after 2 hours and the
has been demonstrated recently by our group that Au clusters selectivity to the desired products (cyclohexanol, cyclohexanone
are efficient catalysts for the activation of O2 and the produc- and cyclohexyl hydroperoxide) on both Au@MCM-22 samples is
tion of radical oxygen species.8,9 Therefore, it is supposed that similar. Au@MCM-22-S shows a slightly higher selectivity than
the Au clusters stabilized in the MCM-22 zeolite may serve as Au@MCM-22-L (see Fig. 4b). The evolution of the product dis-
active species for the initiation of the autocatalytic oxidation of tribution with the reaction time is similar on both Au@MCM-22
cyclohexane to KA-oil.
catalysts (see Fig. S4 and S5, ESI†). The yields of various products
In this work, the oxidation of cyclohexane was performed in at different cyclohexane conversions are shown in Fig. S6 (ESI†). It
a batch reactor without the addition of a radical initiator. As should be noted that, when the same amount of the solid catalyst
presented in Fig. 4a, all the samples show an induction period (25 mg) was used, Au@MCM-22-L shows higher yields of products
in the oxidation of cyclohexane, corresponding to the in situ at the same reaction time. However, if the amount of Au species is
generation of radicals under reaction conditions. The control kept the same for the two catalysts, then Au@MCM-22-S shows
experiments without a solid catalyst (Thermal) and the Au-free better performance for the oxidation of cyclohexane to KA-oil
MCM-22 support show low conversion of cyclohexane after (see Fig. S7 and S8, ESI†). Considering the higher percentage of
2 h, and the major product is cyclohexyl hydroperoxide (see subnanometric Au clusters in the Au@MCM-22-S sample, these
Fig. 4b). Furthermore, the conversion of cyclohexane with the results imply that the Au clusters with low atomicity are probably
AuNP/MCM-22 catalyst is also very low, indicating that Au the active species instead of the ones larger than 1 nm.
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The oxidation of cyclohexane has been proven to be initiated Conflicts of interest
by the formation of cyclohexyl hydroperoxide radicals and sub-
sequently the homolytic cleavage of cyclohexyl hydroperoxide to
There are no conflicts to declare.
form free radicals. Therefore, the role of a catalyst in this
reaction can be related to the acceleration of the generation
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