Surface plasmon-enhanced zeolite catalysis under light irradiation and its correlation with molecular polarity of reactants

Article abstract of DOI:10.1039/c4cc03225j

Enhanced catalytic performance of zeolites via the plasmonic effect of gold nanoparticles has been discovered to be closely correlated with the molecular polarity of reactants. The intensified polarised electrostatic field of Na⁺ in NaY plays a critical role in stretching the C=O bond of aldehydes to improve the reaction rate.

Full text of DOI:10.1039/c4cc03225j

Showcasing research from Dr Xuebin Ke et al., Queensland
University of Technology, Brisbane, Australia
As featured in:
Surface plasmon-enhanced zeolite catalysis under light
irradiation and its correlation with molecular polarity of
reactants
Basic properties (molecular polarities) of molecules have been
found to be correlated with their reaction behaviors over
photocatalysts of gold nanoparticle decorated-zeolite catalysts.
The zeolite catalysis can be enhanced by means of the plasmonic
effect of gold nanoparticles under visible light.
See Xuebin Ke et al.,
Chem. Commun., 2014, 50, 13893.
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Surface plasmon-enhanced zeolite catalysis under
light irradiation and its correlation with molecular
polarity of reactants†
Cite this: Chem. Commun., 2014,
0, 13893
5
Received 30th April 2014,
Accepted 26th June 2014
a
a
a
b
c
Xingguang Zhang, Aijun Du, Huaiyong Zhu, Jianfeng Jia, Jun Wang and
a
Xuebin Ke*
DOI: 10.1039/c4cc03225j
www.rsc.org/chemcomm
Enhanced catalytic performance of zeolites via the plasmonic effect photoactive sensitizers. Above all, the current zeolite-based photo-
of gold nanoparticles has been discovered to be closely correlated catalysts suffer from weak abilities to harvest visible light, poor
5
with the molecular polarity of reactants. The intensified polarised catalytic performance, and unstable catalyst structures.
electrostatic field of Na in NaY plays a critical role in stretching the
+
Efficient ‘‘antennas’’ are required to enhance the light
absorption efficiency. Recently, plasmonic nanostructures have
been demonstrated to possess remarkable abilities to absorb
CQO bond of aldehydes to improve the reaction rate.
Zeolites are crystalline aluminosilicates and feature a network of visible light owing to the localised surface plasmon resonance
6
molecule-size pores and cages, playing a dominant role in the (LSPR). The plasmonic nanostructures can localise, enhance,
manufacture of everyday chemicals. However, many processes and propagate the electromagnetic field of visible light because
often require high temperatures and pressures, imposing extra of the LSPR effect which features a collective oscillation of free
energy consumptions. Thus it is a great challenge to achieve conduction electrons on the particle surfaces. The excited ‘hot
1
energy-efficient chemical processes.
electrons’ may have a direct or indirect influence on the
6
Photocatalysis offers great promise in the advancement of reaction efficiency. Actually, this LSPR effect can amplify
chemical production, energy storage, environment remediation, the electromagnetic field intensity of incident light (|E |) and
and functional materials. For zeolites, the positive charges of results in an enhanced electromagnetic field (|E|) on the nano-
0
2
7
extra-framework cations balance the negative charges of the frame- metre scale. This enhancing effect is known as the ‘‘electric
work aluminium centres, and these accessible cations have polarised near-field enhancement (ENFE)’’, and the enhancement factor
3
electrostatic fields (PFE) and play an important role in adsorbing and (|E|/|E |) can reach ca. 10 at the surface of an isolated particle
0
6
8
activating reactants. Zeolites have been employed as photocatalysts and ca. 10 at junctions of interparticles (hot spots). These
for the degradation of pollutants under UV light. These UV-exciting unique characteristics of LSPR and ENFE have been successfully
systems have limited applications because UV light only accounts for applied, such as in the surface-enhanced Raman scattering,
3
9
4
% of sunlight. Under visible light irradiation, the selective oxida- photodetectors, photodegradation and photovoltaic solar cells.
tion of hydrocarbons over zeolites has been investigated, whereas the In zeolite photocatalysis, both the LSPR effect and PEF function
4
10
catalytic reaction rate is quite low. To improve the photocatalytic of cations have been examined initially. However, the role of
capacity of zeolite-based catalysts, ion-exchange and host–guest PEF strengthened by the ENFE effect has not been well under-
structures have been developed to take advantage of porous struc- stood as well as the correlation of catalytic performance and the
tures and electron-transfer properties. The host–guest structures, molecular polarity of reactants under irradiation.
however, have low density and poor distribution of encapsulated
This study proposes an innovative plasmonic structure
which is composed of NaY zeolites decorated with gold nano-
particles (Au-NPs) as ‘‘antennas’’ to catalyse the acetalisation of
aromatic aldehydes with methanol. Au/NaY with various
amounts of gold loadings was prepared and the contribution
of this ‘‘relay enhancement’’ from ENFE to PEF was identified.
The enhanced light absorption was discovered to boost the
catalytic activity of zeolites and the enhanced reaction rate
directly correlated with the molecular polarity of reactants.
As illustrated in Scheme 1, Au-NPs exhibit the LSPR effect upon
visible light irradiation, resulting in enhanced electromagnetic
a
School of Chemistry, Physics and Mechanic Engineering, Queensland University of
Technology, Brisbane, QLD 4101, Australia. E-mail: x.ke@qut.edu.au;
Fax: +61 731381804; Tel: +61 731389197
b
School of Materials and Chemical Engineering, Shanxi Normal University,
Linfen, China
c
State Key Laboratory of materials-oriented Chemical Engineering, College of
Chemistry and Chemical Engineering, Nanjing University of Technology,
Nanjing, China
†
Electronic supplementary information (ESI) available: Experimental and calcu-
lation details. See DOI: 10.1039/c4cc03225j
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 13893--13895 | 13893

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(
light on) deducts the conversion of Au/NaY (light off), increased
with the molecular polarities of aldehydes. This tendency demon-
strates that photo-excited Au-NPs made more contributions to
enhancing the conversion of reactants with larger molecular pola-
rities. Moreover, without Au-NPs, the conversions on NaY with light
on were similar to those of NaY with light off, showing that light
irradiation alone could not enhance the catalytic performance of
+
NaY and that visible light could not strengthen the PEF of Na
strongly enough to boost the catalytic activity.
Now one may wonder why the photo-excited Au-NPs can
accelerate the acetalisation rate on Au/NaY. The physiochemical
properties of catalysts were investigated. UV/Vis spectra (Fig. 1a)
showed that NaY showed no absorption of visible light, whereas
Au/NaY exhibited evident absorption of visible light at ca. 529 nm
and the light absorption increases with the amounts of gold
loadings because of the LSPR effect. X-ray diffraction patterns
Scheme 1 Proposed mechanism of the acetalisation on Au/NaY under
0
the irradiation of visible light. E refers to the original electromagnetic field
of incident light, and E refers to the intensified electromagnetic field.
3
6
|
0
E|/|E | can be up to 10 –10 .
fields of incident light. Taking Au/NaY for example, the LSPR- (XRD, Fig. 1b and Fig. S1 in ESI,† Section S3) confirmed that NaY
+
induced ENFE can intensify the PEF of Na in zeolites, enabling crystal structures were well preserved after loading of Au-NPs, and
the intensified PEF to stretch the CQO bond of reactants to the gold crystal face of Au(111) and Au(200) can be observed at
10
facilitate their breakage, thus accelerating the reaction rate.
38.21 and 44.51, respectively. X-ray photoelectronic spectra (XPS,
The acetalisation of aromatic aldehydes with methanol can Fig. 1c) identified the metallic state of gold on zeolites: Au 4f
be significantly enhanced by Au-NPs supported on NaY under spectra confirmed that the binding energies of Au 4f and Au 4f5/2
7
1
/2
1
visible light irradiation, and the experimental results are shown electrons were 84.1 eV and 87.8 eV, respectively. TEM images
in Table 1. With light off, Au/NaY and NaY showed no obvious (Fig. 1d) provided direct information about the size, shape and
difference in conversion, indicating that Au-NPs did not distribution of Au-NPs on NaY, showing that Au-NPs existed as
enhance the catalytic performance of NaY without visible light pseudospherical particles and had a narrow size distribution,
irradiation because the Au-NPs were not active centres for the ranging from 3 to 8 nm. The test of reusability of NaY and Au/Y
acetalisation (Experimental details, ESI,† Section S1).
With light on, the conversions on Au/NaY improved significantly runs, showing slight decline in catalytic activity (Fig. 1e).
compared with those on NaY, demonstrating that visible light
On the basis of the experimental and characterization
confirmed that the catalysts could be recycled and reused after five
irradiation plays an essential role in boosting the catalytic activity results, we now tentatively explain the enhancing mechanism.
of acetalisation on Au/NaY. The dipole moments of reactants It has been reported that the existence of external electric fields
corresponding to molecular polarities were calculated by the DFT
method (ESI,† Section S2). It is worth noting that the net increase in
conversion over Au/NaY, which means the conversion of Au/NaY
Table 1 Catalytic conversions of acetalisation of aldehydes with methanol
a
on Au/NaY and NaY
Light off
Light on
Dipole
moment
Reactants (Debye)
Net
increase
(%)
(
conversion, %) (conversion, %)
Au/NaY
NaY
50.1
45.8
Au/NaY
96.3
NaY
50.9
49.1
4
3
.81
.30
44.4
38.2
51.9
48.9
87.1
3
3
.29
.25
38.1
35.3
28.2
17.6
28.4
16.9
66.3
52.9
38.1
18.2
a
Fig. 1 (a) UV/Vis patterns of Au/NaY and NaY; (b) XRD patterns of Au/NaY
and NaY; (c) XPS spectra of Au/NaY, (d) the TEM images of Au/NaY of 3 wt%
gold loading and the size distribution of Au-NPs, and (e) test of the
Reaction conditions: reactant, 0.2 g of aldehyde and 10 g of methanol;
.025 g of Au/NaY of 3 wt% gold loading; atmosphere, air; temperature,
0 1C; reaction time 6 h; light intensity, 0.505 W cm ; wavelength
0
6
À2
range (420–800) nm. The selectivity towards diacetal was excellent reusability of catalysts in the acetalisation reaction between benzaldehyde
(
499%) in all reactions.
and methanol.
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of 12–15 V nm obviously helps activate the reactants of CO/O
Communication
À1
2
in enhanced electromagnetic fields of incident light. This LSPR-
+
and enhances the oxidation process on small gold crystals induced ENFE can intensify the PEF of Na in zeolites, enabling
1
2
(
20–30 nm). The reported mechanism of acetalisation demon- the intensified PEF to stretch the CQO bond of reactants to
strates that the metal ions of zeolites break the CQO bond first facilitate their breakage, thus accelerating the reaction rate.
1
3
to initiate the reaction. Moreover, density-function-theory Moreover, reactants with a larger molecular polarity convert
(
DFT) calculations confirmed that the extra electric field much faster because of the stronger interaction between the
À1
(0–12 V nm ) could stretch the CQO bond of aromatic aldehydes, reactant and the intensified PEF. Further studies on exchanged
but did not affect the methanol molecules (ESI,† Section S4). The zeolites of other ions and tuneable sizes of Au-NPs should
strength of the extra electric field employed in DFT calculations is be conducted. This cascade enhancement effect from electro-
+
À1 14
on the same order of the PEF strength of Na in NaY (6.2 V nm ).
magnetic fields of visible light to electrostatic fields of zeolite
+
Therefore, it becomes convincing that the PEF of Na should be extraframe cations may have extensive applications in adsorp-
directly intensified by the LSPR-induced ENFE, so that the tion, separation, and catalysis.
intensified PEF can stretch the CQO bond of aldehydes to boost
The authors gratefully acknowledge financial support from
the reaction rate. Aldehydes with larger molecular polarities can the Australia Research Council. X. Ke is indebted to QUT and the
interact more strongly with the intensified PEF, thereby showing Queensland State Government for a Smart Futures Fellowship.
higher conversions (Fig. S5, ESI†).
Light intensity and wavelength also have an influence on the Notes and references
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Fig. 2 The conversion of the acetalisation between 2-methoxy benzalde-
hyde and methanol varies as the function of light intensity (a), and cut-off
wavelength (b) (e.g. 400 nm means the light wavelength o400 nm is cut
off). Reaction conditions: 2-methoxy benzaldehyde 0.2 g, methanol 10 g,
catalyst 0.025 g, atmosphere air, temperature 60 1C, and time 6 h. The
5
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This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 13893--13895 | 13895