Sinter-resistant gold nanoparticles encapsulated by zeolite nanoshell for oxidation of cyclohexane

Article abstract of DOI:10.1039/c5ra20734g

A large surface-to-volume ratio is a prerequisite for highly effective heterogeneous catalysts. Making catalysts in the form of nanoparticles provides a good way to realize this aim. However, agglomeration of such nanoparticles during the preparation and catalytic reaction remains a formidable problem. In the present work we have synthesized gold nanoparticles-coated with different zeolites, ZSM-5 and MCM-22, by hydrothermal route. The method adopted is generic where gold nanoparticles were firstly encapsulated by silica utilizing Stober's process and further these silica coated gold nanoparticles are transformed into Au@MCM-22 and Au@ZSM-5 by modified hydrothermal route. The sinter resistant gold nanoparticle core encapsulated by zeolitic nanoshell showed enhanced conversion for the test reaction of oxidation of cyclohexane to KA-oil, which is an important intermediate in the production of nylon-6 and nylon-6,6. The nano-capsules act as bifunctional catalyst, with the nanoparticles prevented from agglomeration during synthesis or catalytic applications, and the zeolitic-shell enhanced conversion and reusability of the nano-catalyst.

Full text of DOI:10.1039/c5ra20734g

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PAPER
Sinter-resistant gold nanoparticles encapsulated by zeolite
nanoshell for oxidation of cyclohexane
Received 00th January 20xx,
Accepted 00th January 20xx
#
a
# a
a
*a,b
Shilpi Saxena , Rupesh Singh , Raj Ganesh S. Pala and Sri Sivakumar
DOI: 10.1039/x0xx00000x
Abstact: A large surface-to-volume ratio is a prerequisite for highly effective heterogeneous catalysts. Making catalysts in
the form of nanoparticles provides a good way to realize this aim. However, agglomeration of such nanoparticles during
the preparation and catalytic reaction remains a formidable problem. In the present work we have synthesized gold
nanoparticles-coated with different zeolites, ZSM-5 and MCM-22, by hydrothermal route. The method adopted is generic
where gold nanoparticles were firstly encapsulated by silica utilizing Stober’s process and further these silica coated gold
nanoparticles are transformed into Au@MCM-22 and Au@ZSM-5 by modified hydrothermal route. The sinter resistant
gold nanoparticle core encapsulated by zeolitic nanoshell showed enhanced conversion for the test reaction of oxidation
of cyclohexane to KA-oil, which is an important intermediate in the production of nylon-6 and nylon-6,6. The nano-
capsules act as bifunctional catalyst, with the nanoparticles prevented from agglomeration during synthesis or catalytic
applications, and the zeolitic-shell enhanced conversion and reusability of the nano-catalyst.
Keywords: Sinter-resistant gold nanoparticles, Zeolite-nanoshells, ZSM-5, MCM-22, Cyclohexane oxidation.
www.rsc.org/
many reactions, we investigate the oxidation of cyclohexane as
its product KA-oil (a mixture of ketone and alcohol) is an
important intermediate in the production of nylon-6 and
1
. Introduction
Nanocatalysts have distinct activity and selectivity, which can
be achieved by controlling their size, shape, surface
1
7
nylon-6,6 . Among the materials that have well defined pore
1
8
1
-4
structure , zeolites have found wide applicability in oxidation
of cyclohexane. While doping metal ions in zeolites (e.g.Ce-
composition, and electronic structure . In particular, gold
nanoparticles possesses unique catalytic properties when they
are supported on metal oxides/carbon and are utilized for
1
9
17
20
21
MCM-41 , Ti–MCM-41 , Cu(I)-ZSM-5 , and Cr–MCM-41 )
provides a strategy for sinter-resistant cyclohexane oxidation
catalysts, such an approach suffers from leaching of metal
5
,
6
various reactions such as CO oxidation
9
hydrochlorination , hydrogenation , selective oxidation ,
, acetylene
7
8
2
2
1
0
11
ions . On the other hand, gold nanoparticles supported on
hydroamination and epoxidation . However, they may suffer
from sintering and subsequent loss in activity due to harsh
2
3
the surface of ZSM-5 has shown good selectivity, but their
prolonged usage at high temperatures/pressures may lead to
sintering. Additionally, there are few reports available on
3, metal nanoparticles supported on zeolites for other reactions
12
reactions conditions
(e.g. high temperature, pressure,
solvents etc.). This can be circumvented by encapsulating the
1
gold nanoparticles with different shell materials such as silica
1
2
4
25
4
15 16
mesoporous carbon , and polymer shell . Although such
(e.g. hydrodesulfurization , oxidation ) and they may also
suffer from sintering as there is no engineered barrier between
the nanoparticles to prevent sintering. Few reports
,
approaches decrease sintering, the protective shell does not
augment the product selectivity of the composite
2
6
demonstrate that many (>50) nanoparticles (e.g. Pt and Ag)
“
nanoparticle core”-“protective shell” catalysts.
were encapsulated in a single zeolite microcapsule prepared
o
by seed coating technique for alcohol oxidation at 80 C.
Towards this goal, the present work explores and
demonstrates the feasibility of structured encapsulants with
well-defined pores to suppress sintering of encapsulated
nanoparticles. While this general approach can be validated by
However, nanoparticles could undergo agglomeration at
higher temperature/pH because of encapsulation of many
nanoparticles inside a single zeolite cavity. Similar effect could
2
7,
also be observed with various metal oxides (e.g. TiO
2
2
3 4
, Fe O
8
a.
Department of Chemical Engineering, Indian Institute of Technology, Kanpur, UP
2
) encapsulated zeolite microcapsule (>50 nanoparticles
08016, India. E-mail: rpala@iitk.ac.in and srisiva@iitk.ac.in
DST Unit of Soft Nanofabrication, Material Science Programme, Centre for
Environmental Science & Engineering, Indian Institute of Technology Kanpur, UP-
encapsulated per zeolite capsule). Recently, gold nanoparticles
were encapsulated inside silicalite through impregnation of Au
b.
2
9
208016, India
These authors have contributed equally
Electronic Supplementary Informaꢀon (ESI) available: Characterization
precursor followed by recrystallization . However, this
approach may not be generic to encapsulate multimetallic
#
†
techniques- TEM images, SEM images, UV-vis spectra, SAED pattern, NH
profiles, and XRD.
See DOI: 10.1039/x0xx00000x
3
-TPD nanoparticles.
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HAuCl
4
solution in water (0.01 mM), with continuous stirring at
o
0 C for 10 min and 20 min to prepare 20 and 35 nm size gold
9
nanoparticles, respectively. The synthesized gold nanoparticles
were separated out through centrifugation.
2
.2.2 Preparation of Au@MCM-22
For silica coating, synthesized gold nanoparticles were first
o
dispersed in 50ml propane-2-ol, at 40 C in round bottom flask.
Under stirring condition 2 ml of water along with 1.0 ml of
liquor NH and 1.21 ml of tetraethyl orthosilicate was added to
3
Scheme 1. Schematic representation of synthesis of single gold nanoparticle the flask. The solution was then kept under stirring condition
encapsulated by zeolite-shell.
for 2h to allow the formation of silica over the gold
nanoparticles. In another beaker 0.031gm of sodium hydroxide
We propose a general hydrothermal based synthetic strategy
was mixed together with 0.34 ml hexamethyleneimine (HMI)
31, 32
to assemble bifunctional catalysts in which a single gold
nanoparticle is encapsulated by MCM-22 and ZSM-5 zeolites-
nanoshell. The zeolite nanoshells are of uniform shell
thickness, morphology, possess high thermal stability and play
the dual role of stabilizing the nanoparticles against sintering
and their distinct pore structure can facilitate shape selective
catalysis. The zeolite nanoshell encapsulating gold
nanoparticles were successfully employed for cyclohexane
oxidation and these catalysts showed better conversion with
increased reusability. The schematic representation for the
and 0.041 gm of sodium aluminate in 8 ml H O
. The
2
mixture thus obtained was added in silica coated gold
nanoparticles. The final mixture was transferred to autoclave
o
and the reaction was carried out at 158 C for 3 days. Once the
reaction is completed the product was washed with D.I water.
o
The obtained solid product was dried at 100 C for 2 hours and
o
ion-exchanged with 1M NH Cl solution 3 times at 80 C for 2
4
o
hours, followed by calcinations at 550 C for 5 hours.
synthesis of gold nanoparticle encapsulated by MCM-22 or 2.2.3 Preparation of Au@ZSM-5
ZSM-5 is presented in Scheme 1 which depicts the synthesis of
For silica coating, synthesized gold nanoparticles were first
o
dispersed in 50ml propane-2-ol, at 40 C in round bottom flask.
gold nanoparticle encapsulated by zeolite nanocapsule, by
two-step process. First gold nanoparticles prepared by
Under stirring condition 2 ml of water along with 15 ml of
3
0
Turkevich method were coated with silica shell using Stöber
method. In the second step, the silica shell on gold particles
was converted into zeolites by modified hydrothermal
synthesis. We have prepared two different zeolites (MCM-22
and ZSM-5) over gold nanoparticles. This method can be
further generalized for encapsulation of different metal/metal
oxides with different zeolite shell.
3
liquor NH and 2.14 ml of tetraethyl orthosilicate was added to
the flask. The solution was then kept under stirring condition
for 2h to allow the formation of silica over the gold
nanoparticles. In another beaker 0.053 gm of sodium
hydroxide was mixed together with 1.7 ml HMI and 0.15 gm of
33-35
sodium aluminate in 15 ml H
2
O
.
The mixture thus
obtained was added in silica coated gold nanoparticles. The
final mixture was transferred to autoclave and the reaction
o
was carried out at 158 C for 3 days. Once the reaction is
2
. Experimental
completed the product was washed with D.I water. The
o
obtained solid product was dried at 100 C for 2 hours and ion-
o
Cl solution 3 times at 80 C for 2 hours,
exchanged with 1M NH
4
2
.1
Materials
o
followed by calcinations at 550 C for 5 hours.
Sodium aluminate (98%), hexamethyleneimine (99%),
tetraethyl orthosilicate (98%), chloroauric acid (99.99 %) were
purchased from Sigma Aldrich. Liquor NH
3
(25% v/v) and 2.3
Catalytic testing
sodium hydroxide were purchased from Merck’s chemicals
India, trisodium citrate from Fisher Scientific and all the
chemicals were used without further purification. Commercial
ZSM-5 (Com-ZSM-5) was purchased from Alfa Aesar.
To explore the catalytic activity of Au@zeolites, the
synthesized catalysts were utilized for oxidation of
cyclohexane. Oxidation of cyclohexane (92.45 mmol per 50 mg
o
of catalyst) was carried out for 2h in autoclave at 150 C, under
1MPa O pressure in the solvent-free system. The results were
2
also compared with those obtained using bare Au NPs, Com-
ZSM-5, Conv-MCM-22, hollow ZSM-5 and MCM-22 prepared
by current method.
2
.2
Preparation of catalyst
2
.2.1 Preparation of gold nanoparticles
The products were analyzed by gas chromatography (GC).
Gold nanoparticles were prepared by citrate reduction Flame ionization detector (FID) was used for the analysis of the
method. In the typical synthesis procedure, 4 ml of sodium product sample. AT-100 capillary column (O.D. 1/8”, length
citrate solution (38.8 mM in water) was added into 40 ml of =3m) was used for the purpose. The oven temperature was set
2
| J. Name., 2012, 00, 1-3
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3
.
Results and discussion
Figure 1. TEM images of a) Au@ZSM-5, b) Au@MCM-22. SEM images of c)
Au@ZSM-5, d) Au@MCM-22 showing gold nanoparticle encapsulated inside
zeolite shell.
o
o
at 140 C. The injector and detector temperatures were 230 C Figure2.XRD analysis of Au@ZSM-5 and Au@MCM-22.
o
and 240 C respectively.
o
00 C before the experiment to remove any adsorbed water
6
or organic species. The peak area can be correlated with the
2
.4
Characterization
3 3
amount of adsorbed NH based on the pulsed NH injection
X
-Ray Diffraction (XRD) was performed on a PAN analytical experiment.
make X-ray Diffractometer using Ni filtered Kα radiation from a
Cu target (λ = 1.54 Å). The sample was scanned between the
o
angles 5 -80 at a scan rate of 3 /min.
o
o
3.1 Catalyst characteristic
Two different sizes (~ 20 and ~ 35 nm) of gold nanoparticles
were prepared and have been characterized by scanning
electron microscopy (SEM)(Figure S1). Figure S2b shows the
The TEM analysis of bare zeolite and zeolite coated gold
nanoparticle was carried out on FEI make TecnaiTM G2 U-Twin
(200 kV) Transmission Electron Microscope. For TEM analysis,
the samples were dispersed in water. 5µl from the sample was
loaded on a carbon- coated copper grid which was dried under
vacuum. The solid UV analysis for zeolite samples was carried
out in a Varian Cary 5000 UV-Vis-NIR Spectrophotometer
having Deuterium lamp (185-350 nm) halogen lamp (350–3300
nm) with R928 PMT detector.
transmission electron microscopy (TEM) image of Au@SiO2
proving that gold nanoparticles are coated with silica (shell
thickness ~ 400nm), which is the starting material for the
synthesis of different zeolites shell. Figure 1a and 1b displays
TEM images of Au@MCM-22 and Au@ZSM-5 respectively.
Contrast in the TEM images (Figure S3 and S4) clearly suggest
that single gold nanoparticle is encapsulated by zeolite
nanocapsule (shell thickness ~ 65 nm for ZSM-5 and ~33 nm
for MCM-22). We observed reduction in the shell thickness
from silica encapsulated to zeolite encapsulated nanoparticles
which might be due to the etching of unreacted silica by NaOH
present in the reaction mixture. Further, the TEM images
prove that the gold nanoparticles retain its size/shape even
Elemental analysis for gold in zeolite samples were carried out
using Thermo Fisher Scientific X Series II (Inductively coupled
plasma mass spectrometry) ICP-MS. The elemental
compositions of the prepared zeolite samples were
determined using X-ray fluorescence (XRF). The analysis was
done on a Rigaku-ZSX Primus II XRF spectrometer. For XRF
analysis, the calcined samples were pelletized using a hydraulic
press at a pressure of 15-20 tons. The surface area and pore
o
after calcination at 550 C for 5 h. In addition, the SEM images
clearly suggest that Au@MCM-22 and Au@ZSM-5 catalysts
possess uniform spherical morphology with a size of about
2
size were determined based on the amount of N that was
adsorbed and desorbed at 27 K, using Autosorb-1C (Model:
AS1-C, Quantachrome, USA). The micro and meso pore
volumes in the material were calculated using density
functional theory (DFT). The temperature-programmed
6
00nm and 400nm, respectively (Figure 1c and 1d). In contrast
to gold nanoparticle supported on ZSM-5 (Figure S5), SEM
image of zeolite encapsulated gold nanoparticle (Figure 1c and
1
d) showed no gold nanoparticle on the surface, which proves
3 3
desorption of NH (NH -TPD) was conducted over 0.10 g of
o
each sample from 150 C to 600 C at a heating rate of 5
o
the absence of gold particles on the exterior surface of zeolite
shell. Moreover, the exterior surface morphology of bare
zeolites (Figure S6) matches with that of Au@zeolites. Figure
S7 presents the TEM images of bare zeolite particles. The
hollow cavity (as observed from TEM images) reduces the
o
C/min while monitoring the thermal conductivity detector
o
at 150 C for 30
(TCD) signals and after saturation with NH
3
min, the catalysts were flushed with He for 1h. All of the
samples were treated in He at
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diffusion hindrance for reactants and presence of single gold Table 1. The total acidity of the catalyst
nanoparticle in hollow cavity further prevents sintering during
catalytic reaction.
Weak acidic
site
Strong acidic
site
Conc.
of
total
The X-ray diffraction (XRD) data (Figure 2) of Au@ZSM-5 and
3
Au@MCM-22 possess diffraction peaks
Sample Name
Au@MCM-22
Bare MCM-22
Au@ZSM-5
acidity(mmolNH
3
/gcat.)
2, 33
of corresponding
zeolites matching with previous reports and also with the
JCPDS data (JCPDS files 42-0023 and 47-0242). To check the
effect of heat treatment on crystallinity of ZSM-5, XRD analysis
0.25
0.21
0.34
0.17
0.27
0.17
0.61
0.57
0.52
0.4
0.86
0.78
o
was also carried out for ZSM-5 dried at 200 C (Figure S15). The
o
analysis suggest that calcination at 500 C did not improve the
0.86
crystallinity significantly. In addition, XRD data of commercial
ZSM-5 (Com-ZSM-5) and MCM-22 prepared by using reported
Bare ZSM-5
0.57
3
1
method (Conv-MCM-22) match with Au@zeolites (Figure
S14). In summary, XRD data suggests the formation of
corresponding zeolites. Further, SAED patterns of Au@MCM-
Conv-MCM-22
Com-ZSM-5
0.15
0.22
0.42
2
2, bare MCM-22 and bare ZSM-5 show the diffraction pattern
from planes which match with the corresponding JCPDS data
Figure S12). UV-Vis spectra (Figure S8 and S9) show
0.39
(
absorbance around 520nm due to SPR effect from Au for 22>Au@MCM-22> Bare-MCM-22> Bare-ZSM-5~Com-ZSM-5. In
Au@zeolites, confirming the presence of Au nanoparticle. summary, the catalysts prepared by the current method
Figure S17 demonstrates N adsorption/desorption isotherms possess more acidic sites compared to the commercial
2
of the zeolites prepared by current method which belongs to catalysts.
type IV isotherm suggesting that they possess both micropores
and mesopores. The pore size distributions of all the analysed
3
.2
Catalytic performance
samples were derived from N
2
sorption using density
functional theory (DFT) (Figure S18). All the zeolites prepared The spent catalysts retain their spherical morphology with
by current method possess micropores in the size range of intact zeolite shell as observed from the TEM images (Figure 3
~5.5 Å which is in accordance with the 10-MR channels. a and b). This clearly suggests that Au@zeolites possesses very
Further Au@ ZSM-5, Au@ MCM-22, ZSM-5, and MCM-22 also high stability under severe reaction conditions. To prove the
possess mesopores in size of ~ 30.8, 34.5, 31.5, and 33.1 Å importance of protection by zeolite shell we calcined
respectively. In addition, the surface area of all the zeolites is unsupported gold nanoparticles and gold nanoparticles
in the range of 390-421 m²/g (Table S1). Total pore volumes supported on zeolite surface and in both cases, gold
0
were determined from adsorbed volume at P/P = 0.99 as 0.60 nanoparticles were found to agglomerate (Figure S10 and
3
and 0.51 cm /g for Au@ZSM-5 and Au@MCM-22, respectively. Figure S11). Figure 4 shows the conversion and selectivity of
The zeolites prepared by current method were mesoporous as cyclohexane oxidation using current Au@zeolites and several
NaOH used during synthesis may create mesopores as control samples. Au@MCM-22 (54.5 mmol) and Au@ZSM-5
reported in literature where mesopores ~20nm were (38.9 mmol) showed greater cyclohexane conversion
3
2
created . Additional experiments were performed to compared to their respective control samples. Cyclohexane
investigate the acidity of prepared catalyst using NH -TPD conversion decreases in the following order:Au@MCM-
analysis. Figure S13 depicts temperature programmed 22>gold-nanoparticles>MCM-22>Au@ZSM-5>ZSM-5>Conv-
desorption of NH (NH -TPD) results of Au@Zeolites, bare MCM-22>Com-ZSM-5. It is to be noted that the amount of gold
3
3
3
hollow zeolites (prepared by the proposed method) and used for oxidation of cyclohexane is 1000 times more than the
commercial zeolites. TPD results clearly show two desorption amount of gold in Au@zeolites. Turn over number (TON) was
peaks in which broad peak 1 is attributed to weak acid site calculated as millimole of oxidized products per millimole of
o
which desorbs ammonia around 250 C, whereas peak 2 is strong acidic sites and turn over frequency (TOF) as millimole
o
attributed to strong acid site which is centered at 460 C of oxidized products per millimole of strong acidic sites per
3
2, 36
matching with several reports
. The amount of total acidic hour (Table S3). Au@ MCM-22 exhibits highest TON and TOF
sites was calculated (Table 1) from desorption TPD peak area among the analyzed samples, due to presence of higher
and the total acidity decreases in the following order: concentration of strong acid sites (Table 1). We note that, turn
Au@MCM-22~Au@ZSM-5>Bare-MCM-22>Bare-ZSM-5>Conv-
over frequency for Au@zeolites and bare zeolites (prepared by
MCM-22>Com-ZSM-5. The concentration of stronger acid sites current method) was higher as compared to respective
was also calculated from the high temperature desorption TPD commercial and conventionally prepared zeolites.
peak area which is in the following order: Au@MCM-22>Bare-
MCM-22>Au@ZSM-5>Bare-ZSM-5>Com-ZSM-5>Conv-MCM22.
3
.3 Indicative catalytic mechanisms
Similarly, the concentration of weak acid sites was calculated
from the low temperature desorption TPD peak area and it As reported in literature, oxidation of cyclohexane to
cyclohexanone and cyclohexanol can undergo via 1) Radical-
decreases in the following order: Au@ZSM-5>Conv-MCM-
4
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the formation of carbon centered parent radical which will
again react with O to give peroxyl radicals thus further
ensuring propagation of the reaction. Radical initiators (AIBN
2
3
8
and TBHP) promote the oxidation reaction by decomposing
to carbon centered radicals which can either instigate H
abstraction(step1), or can react further with oxygen present in
the reaction media to form peroxyl radicals(step2). Au is
capable of accelerating the reaction by cleavage of the O–O
3
8
bond of cyclohexyl hydroperoxide(step4) without the need
for initiators. Moreover, Brønsted acid sites present in zeolites
Figure 3. TEM images of spent a) Au@ZSM-5, b) Au@MCM-22 after oxidation of
cyclohexane.
also
enhances
3
the
decomposition
of
cyclohexyl
9
hydroperoxide and in addition Brønsted acid sites enhance
the formation of cyclohexanone, by dehydration of cyclohexyl
4
0
hydroperoxide (step5) . No initiator was used for the current
reaction while most of the experiments from the literature
have initiators.
Overall, we believe that the following factors predominantly
determine the conversion and selectivity of the reaction: 1)
acid sites present in zeolites, 2) Gold nanoparticles, and 3)
Shell thickness and pore size of zeolites.
3
.4 Rationalizing higher conversion of cyclohexane by
Au@Zeolites
We note that since gold and acid sites are capable of
accelerating O–O bond cleavage of cyclohexyl hydroperoxide
and Au@MCM-22 possesses higher concentration of strong
acid sites compared to all the other samples (Table1),
Au@MCM-22 shows highest conversion in comparison to
other catalyst samples. Due to the presence of higher
concentration of strong acid sites, zeolites prepared by our
method showed enhanced cyclohexane conversion in
comparison to commercial zeolites. Moreover, gold is
protected by zeolite nanoshell against sintering and loss of
catalytic activity. It is also noted that we have not added any
initiator in current reaction unlike in most of the published
3
8
reports (e.g. AIBN and TBHP) . We note that the amount
determined by Inductively coupled plasma mass spectrometry
and X-ray fluorescence, Table S2) of Au present in Au@ZSM-5
0.012wt%) and Au@MCM-22 (0.011wt%) is very less
(
Scheme 2. Radical-chain mechanism for oxidation of cyclohexane.
(
compared to bare gold nanoparticle catalysts used control
experiments. If gold and the bare zeolites (prepared by current
method) were acting independently, the conversion due to the
Au@zeolite composite is expected to be additive. This additive
conversion value would have been 31.5mmol for Au@ZSM-5
and 42.8mmol for Au@MCM-22. We note that this additive
conversion value is less than that of experimentally measured
values of 38.9 mmol for Au@ZSM-5 and 54.5 mmol for
Au@MCM-22, suggesting synergistic effect in Au@zeolites
composite catalysts. Apart from the above effects we believe
that thickness of zeolite shell also can affect the conversion
due to mass transfer resistance for the reactive intermediates
towards reacting with Au. Shell thickness of Au@ZSM-5 and
Au@MCM-22 is ~ 65nm and ~ 33nm respectively and hence,
Au@ZSM-5 impose greater mass transfer resistance resulting
in lesser conversion. Apart from cyclohexanol and
cyclohexanone formation, byproducts (cyclohexyl ester, adipic
chain autoxidation 2) Franck-Rabinowitch solvent cage, and 3)
37, 38
Haber-Weiss cycle mechanisms
. Franck-Rabinowitch and
Haber-Weiss cycle mechanism typically occurs in the presence
2
+
2+
2+
of solvent and metal ion (e.g.Co , Mn , and Cr ) substituted
zeolites respectively, and hence, less applicable for the present
study. Hence, we have attempted to rationalize the results by
focusing on the radical- chain autoxidation mechanism
(Scheme2), which has been shown to be the operational
mechanism in most cases. The autoxidation of cyclohexane
proceeds through activation of the C–H bond via abstraction of
an H atom (step1) forming carbon centered radicals which
2
react with O to give peroxyl radicals(step2). The formed
radicals reacts further with cyclohexane to give cyclohexyl
hydroperoxide and another peroxyl radical, thus ensuring
propagation of the reaction(step3). Cyclohexanol can be
obtained by hydrogenation of cyclohexyl alkoxy radical (step6)
after reacting further with cyclohexane. This again results in
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1
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(
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In conclusion, sinter-resistant monodispersed Au@MCM-22
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of Au@silica through hydrothermal route. These gold
nanoparticle encapsulated by zeolite nanoshell catalysts
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2
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Acknowledgements
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We report hydrothermal synthesis of sinter-resistant single gold nanoparticle encapsulated by
different zeolite shells (e.g. ZSM-5 and MCM-22), which serve the role of stabilizing the Au
nanoparticles against sintering during cyclohexane oxidation. The sinter resistant zeolite
encapsulated gold nanoparticle showed enhanced conversion for the oxidation of cyclohexane.