Factors influencing the catalytic oxidation of benzyl alcohol using supported phosphine-capped gold nanoparticles

Article abstract of DOI:10.1039/c4cy01168f

Two phosphine-stabilised gold clusters, Au₁₀₁(PPh₃)₂₁Cl₅ and Au₉(PPh₃)₈(NO₃)₃, were deposited and activated on anatase TiO₂ and fumed SiO₂. These catalysts showed an almost complete oxidation of benzyl alcohol (>90%) within 3 hours at 80 °C and 3 bar O₂ in methanol with a high substrate-to-metal molar ratio of 5800 and turn-over frequency of 0.65 s^-1. Factors influencing catalytic activity were investigated, including metal-support interaction, effects of heat treatments, chemical composition of gold clusters, the size of gold nanoparticles and catalytic conditions. It was found that the anions present in gold clusters play a role in determining the catalytic activity in this reaction, with NO₃^- diminishing the catalytic activity. High catalytic activity was attributed to the formation of large gold nanoparticles (>2 nm) that coincides with partial removal of ligands which occurs during heat treatment and catalysis. Selectivity towards the formation of methyl benzoate can be tuned by selection of the reaction temperature. The catalysts were characterised using transmission electron microscopy, UV-vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy.

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ARTICLE TYPE
Factors influencing the catalytic oxidation of benzyl alcohol using
supported phosphine-capped gold nanoparticles
Rohul H. Adnan,a,b Gunther G. Andersson,c Matthew I.J. Polson,a Gregory F. Metha,d and Vladimir B.
Golovko*a,e
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Two phosphine-stabilised gold clusters, Au101(PPh3)21Cl5 and Au9(PPh3)8(NO3)3, were deposited and
activated on anatase TiO2 and fumed SiO2. These catalysts showed almost a complete oxidation of benzyl
alcohol (>90 %) within 3 hours at 80 °C and 3 bar O2 in methanol with a high substrate-to-metal molar
10 ratio of 5800 and turn-over frequency of 0.65 s-1. Factors influencing catalytic activity were investigated,
including metal-support interaction, effects of heat treatments, chemical composition of gold clusters, the
size of gold nanoparticles and catalytic conditions. It was found that the anions present in gold clusters
-
play a role in determining the catalytic activity in this reaction, with NO3 diminishing the catalytic
activity. High catalytic activity was attributed to the formation of large gold nanoparticles (> 2 nm) that
15 coincides with partial removal of ligands which occurs during heat treatment and catalysis. Selectivity
towards the formation of methyl benzoate can be tuned by selection of the reaction temperature. The
catalysts were characterised using transmission electron microscopy, UV-vis diffuse reflectance
spectroscopy and X-ray photoelectron spectroscopy.
opposed to peroxides (e.g. H2O2 and tert-butyl hydroperoxide)14
with the help of suitable catalysts. In recent years, liquid phase
oxidation of alcohols has become popular as a model catalytic
50 test, due to simple setup and handling procedures and relatively
mild reaction conditions.15, 16 Supported platinum group catalysts
are widely used in oxidation of alcohols, yet they often require a
secondary metal as a promoter to increase the selectivity and
stability of such catalysts.17-21 Hence, gold-based catalysts could
55 be advantageous if such systems would not suffer from the
stability and selectivity issues.22
1 Introduction
20 For centuries, gold was considered to be chemically inert and
catalytically inactive, making it valuable in jewellery and as
coinage metal but worthless for applications in catalysis.
However, this perception has changed since the early report by
Haruta et al. that small gold particles were catalytically active in
25 CO oxidation at low temperature1 and discovery by Hutchings et
al. that
a gold-based catalyst could be used in the
hydrochlorination of acetylene.2 These discoveries triggered a
revolution in gold-based catalysis, with numerous follow-up
papers demonstrating the superiority of gold-based catalysts over
30 the more expensive platinum group metals in various catalytic
processes, with improvements in activity, selectivity and stability
A systematic study of the nature of the active sites is crucial in
improving the activity and selectivity of catalysts towards the
formation of the desired partial oxidation products. In the case of
60 heterogeneous Au-based catalysts following factors are often
considered to define the catalytic activity and selectivity: gold
particle size and morphology, oxidation state of gold species,
metal-support interaction and specific parameters of catalytic
testing conditions. The nature of the active site in the liquid phase
against deactivation paving
a
pathway to numerous
commercially-oriented patents.3, The number of studies on
nanoparticulate gold-based catalysts for a broad range of bulk and
35 fine chemical synthesis processes has grown exponentially.1, 5-8
Oxidation reactions, particularly alcohol oxidation, are among
4
65 oxidation of alcohols currently is still unclear, with both ultra-
24
small particles (< 2 nm)23,
and larger particles (> 2 nm)25
the most important and useful reactions used by the chemical
industry and academia.9,
reported as active catalysts. Tsukuda et al. reported that as the
size of gold nanoparticles reduced below 2 nm, the turnover
frequency (TOF) for the aerobic oxidation of p-
70 hydroxybenzylalcohol increased significantly.26 The authors
proposed that the catalytic activity of these small gold
nanoparticles was due to the increased electron density on the
gold core and also charge transfer from the poly(N-vinyl-2-
pyrrolidone) ligands. However, Haider et al. observed that
75 smaller size was not necessarily responsible for higher activity –
10
The most common commercial
methods of alcohol oxidation use stoichiometric oxidants, such as
40 chromates, permanganates or peroxides, that often yield a large
amount of environmentally dangerous waste.11 The use of
harmful organic solvents (e.g. chlorinated solvents) may also
have a negative environmental impact.12,
13
There is an urgent
need to move towards environmentally benign and cost-effective
45 processes involving renewable, environmentally friendly and
cheap oxidants, such as molecular oxygen or atmospheric air as
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the authors reported that the optimum size of the gold
nanoparticles for ethanol oxidation is ca. 7 nm, irrespective of the
supports.25 Zheng et al. also observed a similar trend in ethanol
%), methyl benzoate (99 %), benzoic acid (99.7 %), and anisole
(> 99.5 % anhydrous) were purchased from Sigma Aldrich.
HPLC grade methanol (99.9 %, Fisher Scientific) was degassed
oxidation in which the most active catalyst contained gold 60 (N2) and dried over pre-calcined alumina prior to use.
5
nanoparticles of around 6 nm.27 There are currently only very few
studies on the effects of the gold nanoparticle precursor (size,
composition etc.) on the performance of resulting catalysts since
the majority of reports in the literature are focused predominantly
Tetrachloroauric acid was prepared using 99.99 % pure gold
following the procedure detailed by
Brauer.51
Triphenylphosphine-stabilised gold nanoparticles with an average
estimated formula Au101(PPh3)21Cl5 (denoted as Au101) have been
on the morphology and size of gold nanoparticles.23, 28-30 Only 65 synthesised according to procedure reported earlier by Weare et
10 recently, several studies on the effects of ligands, such as citrate,
polyvinylpyrrolidone and polyvinyl alcohol, on the performance
of gold-based catalysts have been reported in a wide range of
catalytic reactions - glycerol oxidation,31 hydrogenation of
al.52 while Au9(PPh3)8(NO3)3 was made according to the protocol
reported by Wen et al.53 Anatase (99.5 %, ca. 70 m2/g, 10-30 nm
particles) was purchased from SkySpring Nanomaterials.
Instrument grade H2 (99.98 % with < 20 ppm H2O, BOC) and O2
cinnamaldehyde32 and reduction of p-nitrophenol.33 These 70 (99.2 % with < 10 ppm H2O, Southern Gas Services Ltd.) were
15 findings clearly suggest that the nature of ligands plays a critical
role in determining the performance of a catalytically active gold
particles in a particular reaction, perhaps, similarly to the role of
support.34 Hence, it is important to study the effect of the
composition (size of the gold core, neutral organic ligands and
20 anionic ligands or counter-ions) of precursors used in fabrication
of gold-based catalysts in order to design catalysts with high
activity and improved selectivity.
used for the calcination procedures.
2.2 Preparation of catalysts
The clusters were deposited onto the support using the protocol
75 reported by Zhu et al.38 For the purpose of this study we focused
on materials with 0.17 wt% Au (i.e. ratio of the weight of the Au
due to presence of clusters relative to the weight of support) on
titania and 0.5 wt% Au on silica to give equivalent surface
density of gold particles.47
Supported thiol-capped gold clusters have been widely used in
catalytic studies.35-37 For example, Jin et al. supported
25 Au25(SR)18, Au38(SR)24 and Au99(SPh)42 clusters on various metal
oxides and tested them in selective oxidation and hydrogenation
reactions.38-41 Tsukuda et al. used Au10(SG)10, Au18(SG)14,
Au25(SG)18 and Au39(SG)24 deposited on hydroxyapatite in
styrene oxidation.42 In contrast, very few studies have focussed
30 on using phosphine-capped gold nanoparticles in catalysis.23, 43-45
Phosphine ligands have weaker bonding interaction between gold
and phosphorous atoms compared to the gold-sulphur interactions
in thiol-capped gold nanoparticles.46
80 First, the calculated amount of support (silica or titania) was dried
in the Schlenk tube under vacuum at 200 °C with stirring
overnight (12 h). Upon cooling to room temperature, 5.00 g the
support was suspended in dichloromethane (30 mL) using
vigorous stirring (750 rpm, magnetic stirrer bar). A solution of
85 calculated amount of gold cluster (e.g. 11.2 mg of Au101) in
dichloromethane (10 mL) was added to the suspension of support
under vigorous stirring at room temperature and the mixture was
left stirring overnight (12 h). The mixture was then dried in
vacuum at room temperature and stored at 4 °C (untreated
90 catalysts, labelled as Aux/support-untreated).
Building on our recent X-ray spectroscopy studies of supported
35 triphenylphosphine-stabilised gold nanoparticles,47, 48 we hereby
aim to study the catalytic performance of materials derived from
selected phosphine-capped gold clusters, supported and activated
on titania and silica using liquid phase benzyl alcohol oxidation
as a model catalytic test. Herein, we prepared two phosphine-
40 capped gold clusters: Au101(PPh3)21Cl5 (denoted as Au101 with
mean size of 1.6 0.3 nm) and Au9(PPh3)8(NO3)3 (denoted as
Au9 with mean size of 0.8 nm). These clusters were then
deposited and activated on TiO2 (anatase) and SiO2 (fumed) using
sol-immobilisation method; a well-established method used by
45 Rossi,10 Jin39 and Hutchings49 for immobilising pre-synthesized
gold nanoparticles or clusters onto supports. Sol-immobilisation
method was shown to give minimal aggregation and high
dispersion of gold nanoparticles on the supports.50 The catalysts
were then calcined under different atmospheres: oxygen (O2) and
50 oxygen and subsequently hydrogen (O2-H2) at 200 °C to dislodge
the phosphine ligands and expose the metal core adsorbed on the
support to facilitate access by the substrate.47
Two heat treatments were employed in this work: a)
calcination under pure O2 atmosphere at 200 °C for 2 h and b)
calcination under pure O2 at 200 °C for 2 h followed by
calcination in H2 at 200 °C for 2 h.47 All cluster deposition and
95 activation experiments were performed using glassware wrapped
in Al foil to avoid exposure of materials to light. All prepared
catalysts were stored in vials wrapped in Al foil in a fridge (4 °C).
For the purpose of following discussion catalysts are labelled in
the following manner: “x wt% Auy/support-treatment”, where x
100 corresponds to the weight % of pure Au and y = 9 or 101.
2.3 Catalytic testing
The catalytic oxidation of benzyl alcohol was performed in
stainless steel autoclaves (total internal volume of 57 mL)
105 equipped with Teflon liners and magnetic stirrers, with
temperature control of the reaction achieved by using hotplate
stirrer controlled by a thermocouple immersed into the reaction
mixture. Typically, a mixture of 0.270 g, 2.50 mmol benzyl
alcohol and 0.135 g, 1.25 mmol anisole (internal standard) in
110 methanol (25 mL) were charged into a Teflon liner. Then, 0.345
g, 2.50 mmol K2CO3 and 50 mg of catalyst were added, giving
benzyl alcohol to Au molar ratio of 5800. A purge-vent cycle was
completed 3 times with pure oxygen to ensure high purity of the
2 Experimental
2.1 Materials
55 All reactants were analytical reagent grade and used without
further purification. Benzyl alcohol (> 99 %), benzaldehyde (> 99
2
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atmosphere before the autoclave was pressurised with 5 bar of
over support material, as was emphasised by Hutching et al.54
Hence, no statistical analysis of the particle size distribution was
provided for 0.17 wt% Au9/TiO2-untreated. Although, in the case
oxygen and heated to a specific temperature for the desired
reaction time. At the end of a catalytic test, the reaction was
stopped by cooling the autoclave to 3 °C in an ice-bath. The 60 of Au9 cluster poor contrast did not allow precise particle size
5
reaction mixture was centrifuged at 5000 rpm for 15 minutes to
separate product mixture and the catalyst. The catalyst was
washed with methanol (3 x 15 mL) and dried in vacuo prior to
recycling. Each catalytic test was repeated at least in triplicate
giving lower than 3 % differences in conversion and selectivity.
determination, absence of larger, easily visible in HRTEM
aggregates is evident (Figure 2a). Our earlier study using
synchrotron XPS and NEXAS also confirmed presence of
significant fraction of supported Au9 clusters on TiO2.47 The gold
65 loading of 0.17 wt% on TiO2 was chosen to minimise the
aggregation of gold nanoparticles, yet still to be able to obtain
discernible signals in X-ray photoelectron spectroscopy (XPS)
and UV-vis diffuse reflectance spectroscopy (UV-vis DRS)
studies. In the case of clusters supported on fumed SiO2, the Au
10 Product mixtures were analysed using high performance liquid
chromatography (HPLC) Dionex Ultimate 300 system equipped
with UV detector and fitted with a Luna 5µ C18(2) (250 x 4.60
mm) reversed phase column. The products were eluted using a
mixture of 0.05 v/v% trifluoroacetic acid in water (70 %) and 70 loading was normalised based on the surface area (to be 0.5 wt%)
15 acetonitrile (30%). Gold content was measured using atomic
absorption spectroscopy (AAS) Varian SpectraAA 220 FS with
ASL hollow cathode lamp for the Au element.
in order to obtain equivalent (to TiO2) surface coverage by
clusters.
A careful heat-treatment was employed in order to remove the
capping ligands and expose gold core to the substrate, yet to
75 minimise aggregation. Here, calcination of the catalysts was done
at 200 °C under different environments: under pure oxygen (O2)
and oxygen followed by hydrogen (O2-H2).28, 42 After calcination,
the gold clusters aggregated to form larger nanoparticles (Figures
1 and 2).47 TiO2-based catalysts showed minimal aggregation and
80 narrower particle size distributions as compared to SiO2-based
catalysts, suggesting that gold formed stronger metal-support
interaction with TiO2 cf. SiO2 (Figures 1 and 3). UV-vis DR
spectra (Figures 4 and 5) showed the absence of LSPR bands in
the cases of untreated catalysts allowing to infer that the gold
85 nanoparticles were predominantly with sizes below 2 nm and
retained their non-metallic state. The appearance of an LSPR
band in the UV-vis DR spectra of the TiO2-supported samples
after calcination (Figures 4 and 5) indicated formation of the
plasmonic gold nanoparticles which is indicative of the increase
90 in particle size to greater than 2 nm.55-57 It was previously
reported that the position of the maximum of the LSPR band
moves to longer wavelength as the size of gold particles
increases, although in some cases this general trend broke down
due to the effect of the dielectric constant of the surrounding
95 environment.58-61 Interestingly, the 0.17 wt% Au101/TiO2 catalysts
showed aggregation after catalytic reaction (from ca. 2.0 to ca.
3.6 nm; Table 1). Particle size analysis based on the statistical
evaluation of numerous TEM images (Table 1) confirms that the
increase in particle size after heat-treatments and after catalytic
100 reactions for Aux/TiO2 catalysts (x = 9, 101) was consistent with
the red shift in the peak maxima position of the LSPR bands
observed in UV-vis DR spectra.
2.4 Catalyst characterization
20 High resolution transmission electron microscopy (HRTEM) was
performed using a Philips CM-200 system operating at 200 kV.
Samples were deposited as a suspension in methanol onto Cu
(300 mesh) grids coated with a holey carbon film and dried in
vacuum immediately prior to TEM study. Statistical evaluation of
25 at least 250 gold particles per sample was undertaken to estimate
mean particle sizes and construct size distribution histograms. X-
ray photoelectron spectroscopy (XPS) study was performed at the
soft X-ray beamline at the Australian Synchrotron using a SPECS
Phoibos 150 hemispherical electron analyser with the photon
30 energy set to 690 eV. The irradiation spot size was ca. 600 x 600
µm, providing an X-ray flux of ca. 1012 photon mm-2 s-1.47 XPS
samples were prepared by suspending catalysts in
dichloromethane at a concentration of ca. 1 mg/mL. A 10 µL
drop of each sample was deposited onto a clean 6 x 6 mm silicon
35 (Si) wafer and dried in vacuum (in the dark) immediately before
analysis. UV-vis diffuse reflectance spectroscopy (UV-vis DRS)
spectra were recorded using Cintra 404 (GBC Scientific
Equipment) spectrophotometer. Thermogravimetric analysis
(TGA) was performed using TGA-DSC Q6000 Universal
40 Analyser. 31P NMR spectra were recorded by taking a small
aliquot of the reaction mixture, dissolving it in CDCl3 solvent and
collecting data using an Agilent Technologies 400 Hz NMR
system.
3 Results and discussion
45 3.1 Catalyst characterization
Au101 nanoparticles and Au9 clusters are smaller than 2 nm, are
highly monodisperse, have a high surface area to volume ratio,
and do not have a localised surface plasmon resonance (LSPR)
due to their non-metallic surface state.46,
52
We prepared
50 heterogeneous gold catalysts derived from Au101 and Au9 clusters
deposited onto the surface of TiO2 (anatase) and SiO2 (fumed)
nanopowders. The 0.17 wt% Au101/TiO2-untreated and 0.17 wt%
Au9/TiO2-untreated catalysts made via sol-immobilisation
method show minimal cluster aggregation (Figure 1a, 2a).
55 Imaging ultra-small clusters on support, such as Au9 on TiO2, was
very difficult due to poor contrast of the ultra-small gold particles
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Fig. 1 Representative TEM images of 0.17 wt% Au101/TiO2 catalysts
before catalytic reaction, a) Au101/TiO2-untreated, b) Au101/TiO2-O2, and
c) Au101/TiO2-O2-H2.
Fig. 2 Representative TEM images of 0.17 wt% Au9/TiO2 catalysts before
catalytic reaction, a) Au9/TiO2-untreated, b) Au9/TiO2-O2, and
c) Au9/TiO2-O2-H2.
5
10
4
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Fig. 3 UV-vis DR spectra of 0.17 wt% Au101/TiO2 catalysts.
5
Fig. 4 UV-vis DR spectra of 0.17 wt% Au9/TiO2 catalysts.
For 0.5 wt% Au101/SiO2 catalyst, a significant agglomeration
of gold cluster occurred upon deposition: the gold clusters in the
0.5 wt% Au101/SiO2-untreated catalyst aggregated to form gold
nanoparticles of mean size 3.6 1.2 nm. The observed particle
10 size is almost twice the size of pristine Au101 cluster and
substantially larger than size of gold particles (2.0 0.4 nm) in
the case of Au101 immobilized on TiO2 at the same surface
coverage. The gold cluster also grew non-uniformly to yield a
wide particle size distribution with the largest particle found
15 under TEM being ca. 8 nm and the smallest was ca. 2 nm
(Figures 5a and S9a ESI). After calcination under O2 and O2
followed by H2 at 200 °C, the agglomeration progressed further
resulting in formation of larger gold nanoparticles and wider
particle size distributions (Figures 5 (b and c) and Figure S9(b
20 and c) ESI). A summary of supported gold particle mean sizes
and positions of the LSPR peak maxima are given in Table 1.
Fig. 5 Representative TEM images of 0.5 wt% Au101/SiO2 catalysts before
catalytic reaction, a) Au101/SiO2-untreated, b) Au101/SiO2-O2, and
c) Au101/SiO2-O2-H2.
25
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Table 1 Summary of the gold particle size and the LSPR peak maximum positions of the supported gold catalysts
Entry Catalyst
Condition
Size (nm)a
2.0 0.4
3.5 0.8
4.4 1.2
3.6 1.2
3.9 1.0
4.6 1.5
4.6 2.5
4.2 1.2
5.0 1.5
2.7 0.6
< 2c
2.4 0.5
2.9 0.9
< 2c
3.6 2.7
4.4 2.5
3.6 1.2
4.7 1.6
5.7 2.9
LSPR peak maximum (nm)b
1
2
0.17% Au101/TiO2-untreated
0.17% Au101/TiO2-O2
Before reaction
Before reaction
Before reaction
After reaction
After reaction
After reaction
After recycle
No peak
533
540
540
541
546
541
547
551
3
4
5
0.17% Au101/TiO2-O2-H2
0.17% Au101/TiO2-untreated
0.17% Au101/TiO2-O2
6
7
8
0.17% Au101/TiO2-O2-H2
0.17% Au101/TiO2-untreated
0.17% Au101/TiO2-O2
After recycle
After recycle
9
0.17% Au101/TiO2-O2-H2
1.3% Au101/TiO2-untreated
0.17% Au9/TiO2-untreated
0.17% Au9/TiO2-O2
0.17% Au9/TiO2-O2-H2
0.17% Au101/SiO2-untreated
0.17% Au101/SiO2-O2
0.17% Au101/SiO2-O2-H2
0.5% Au101/SiO2-untreated
0.5% Au101/SiO2-O2
10
11
12
13
14
15
16
17
18
19
Before reaction
Before reaction
Before reaction
Before reaction
Before reaction
Before reaction
Before reaction
Before reaction
Before reaction
Before reaction
530
No peak
555
558
No peak
529
524
537
525
518
0.5% Au101/SiO2-O2-H2
a Measured using TEM. b Measured using UV-vis DRS. c Estimated based on the absence of LSPR peak in the UV-vis DR spectra.
Our recent XPS study of phosphine-capped Au clusters on 35 The liquid phase oxidation of benzyl alcohol using gold
TiO2 showed that for untreated Au9/TiO2 and Au101/TiO2
a
nanoparticle-based catalysts reported here yielded benzoic acid
and methyl benzoate as the major products, which were identified
using HPLC-MS and quantified using HPLC-UV (using internal
standard and solutions of known concentrations of reference
5
significant fraction of phosphine ligands were dislodged from the
gold cores and formed phosphine oxide-like species by
interaction with oxygen on TiO2 surface, leaving a tiny amount of
phosphine ligands bound to the gold core.47 Calcination under O2 40 compounds for calibration). The untreated catalysts with 0.17
atmosphere detached majority of phosphine ligands from the gold
wt% gold loading were inactive, showing no conversion while
heat treated catalysts showed almost complete (> 92 %)
conversion of benzyl alcohol after 4 hours (see Table 4). The
most active catalysts gave very high catalytic activity with a
10 core by forming phosphine-oxide species. It is worth mentioning
while TGA results show that a complete removal of phosphine
ligands occurred at 250 °C for Au101 clusters (Figure S3, ESI) and
at 240 °C for Au9 cluster (Figure S6, ESI), phosphine ligands 45 molar substrate to metal ratio of 5800 and turnover frequency up
attached to supported Au clusters were not removed as
15 phosphoric acid (H3PO4), but as phosphine-oxide species
probably due to the interaction with the oxygen from TiO2
surfaces (e.g. such oxidized phosphine species remained at the
to 0.65 s-1 (which is orders of magnitude greater than in any of
the related earlier reports, Table S2 ESI). There were several
factors contributing to the high activity of our catalysts. Firstly,
the water soluble base (K2CO3) is known to be a promoter in
surface of the support).47 After a combined O2-H2 calcination, 50 liquid phase oxidation of alcohols.62,63 Rossi proposed that
phosphine ligands were completely dislodged from the gold core
20 for both Au9/TiO2 and Au101/TiO2 catalysts. Our first XPS study
of a wider range of ultra-small gold phosphine clusters suggests
alcohols cannot adsorb directly onto gold and a base was required
to deprotonate the hydroxyl group forming a metal alkoxide prior
to adsorption onto gold.22 The indispensable role of the base was
shown when no conversion of benzyl alcohol was achieved in its
55 absence (Table 1, entries 4-6). There are studies of benzyl alcohol
oxidation using supported-gold catalysts under base-free
conditions, nevertheless, in those studies, high conversions (> 90
%) were rarely achieved.15, 16, 64 In some cases, high conversions
of benzyl alcohol under base-free condition were also reported,
-
that the nature of the anionic species (Cl- and NO3 ) did not
significantly influence the position of the Au 4f7/2 peak (84.7
0.1 eV for Au11 cluster containing Cl- vs 85.1 0.1 eV for both
-
25 Au8 and Au9 clusters containing NO3 ) whereas the size of the
cluster has more pronounced effect (83.9
cluster containing Cl-).48
0.1 eV for Au11
The leaching of metal particles into solution is a common 60 for example, Su et al. reported that gold nanoparticles supported
problem for metal supported heterogeneous catalysts. However,
30 our phosphine-stabilised Au clusters did not show signs of
leaching during our catalytic studies according to Au loading
analysis by AAS (Table S1, ESI).
on the binary mesostructured Ga-Al mixed oxide were able to
catalyse benzyl alcohol under base-free condition at 80 °C with a
high conversion (98%).65 Abad et al. employed Au/CeO2 in the
oxidation of various alcohols and observed high conversions
65 under mild conditions.66 Those authors suggested that the base
was not required to deprotonate alcohols because the support
contained active sites that were able to form cerium alkoxide,
3.2 Catalytic testing
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assisting the oxidation of alcohols.
Secondly, the nature of the support and metal-support
interface. The high activity of heat treated catalysts can be
attributed to stronger metal-support interaction established during
interaction dictated the performance of the catalysts. For 25 calcination. Haruta et al. proposed that during calcination gold
example, catalysts with similar gold loading on different supports
(Table 1S, ESI), Au101 on anatase (0.13 wt% Au by AAS) showed
superior performance (Table 2) when compared with to
analogous catalyst systems made using fumed-SiO2 as a support
nanoparticles melted, rearranged and reconstructed themselves to
achieve stronger interaction with the TiO2 support.75 This
hypothesis is in line with report by Buffat and Borel that 2 nm
gold nanoparticles melt around 600 K,76 which is significantly
5
(with 0.18 wt% Au by AAS). Typically, SiO2 is considered an 30 lower than the melting point of the bulk gold (1337 K). Yiliang et
inert, non-reducible support whereas TiO2 is an activating,
al. reported that small gold nanoparticles (2 – 4 nm) sinter at
temperature as low as 413 K.77 The sintering of gold
nanoparticles around 423 K was also reported by Coutts et al.78
Hence it is experimentally observed that small gold nanoparticles
10 reducible support.67 The higher catalytic activity of Au/TiO2
catalysts over Au/SiO2 was also observed in CO oxidation in
many studies.68-71 Hence, the size effect of gold nanoparticles
alone is not sufficient to explain the activity of supported gold 35 aggregate below the melting point. Analysis of supported-gold
catalysts. Metal-support interactions (MSI) play crucial role in
15 defining reactivity and selectivity of supported gold catalysts.
Haruta et al. and Hassan et al. hypothesized that the oxygen
activation occurs at the perimeter (i.e. interface) of the gold
particle sizes as measured by TEM (Table 1) showed that the
particle size distribution of gold nanoparticles was wider
(indicated by larger standard deviation) on SiO2 support as
compared to TiO2 for the same Au loading (target 0.17 wt%),
nanoparticles and metal oxide supports.72-74 This hypothesis could 40 suggesting that weaker metal-support interaction in Au101/SiO2
explain the lower activity of Au101/SiO2 catalysts as compared to
20 Au101/anatase in benzyl alcohol oxidation observed in this study,
since silica based catalysts with much larger particles will have
much smaller surface area corresponding to the gold-support
catalysts facilitates aggregation resulting in wider particle size
distributions despite the significantly higher surface area of SiO2
support.
45
Table 2 Performance of gold catalysts in the liquid phase oxidation of benzyl alcohol
Entry
Catalyst
Size (nm)
Time (h) Conversion Selectivity to methyl
Selectivity to
benzoic acid (%)
TOF (s-1)
(%)
0
benzoate (%)
1
2
3
4
5
6
7
8
Blanka,b
n/a
n/a
n/a
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
3
3
4
4
4
4
4
4
4
4
4
4
4
0
0
0
0
0
0
0
0
0
0
0
0
Anatasea,b
0
Anatasea
0
0
0.17% Au101/TiO2-untreatedb
2.0 0.4
3.5 0.8
4.4 1.2
2.0 0.4
3.5 0.8
4.4 1.2
3.6 1.2
3.9 1.0
4.6 1.5
< 2e
0
0
b
0.17% Au101/TiO2-O2
0.17% Au101/TiO2-O2-H2
0.17% Au101/TiO2-untreated
0.17% Au101/TiO2-O2
0.17% Au101/TiO2-O2-H2
0.17% Au101/TiO2-untreatedc
0.17% Au101/TiO2-O2
0.17% Au101/TiO2-O2-H2
0.01% Au101/TiO2-untreatedd
0.01% Au101/TiO2-O2
0.01% Au101/TiO2-O2-H2
1.3% Au101/TiO2-untreated
0.17% Au101/TiO2-O2
0.17% Au101/TiO2-O2-H2
0.17% Au9/TiO2-untreated
0.17% Au9/TiO2-O2
0.17% Au9/TiO2-O2-H2
0.17% Au101/SiO2-untreated
0.17% Au101/SiO2-O2
0.17% Au101/SiO2-O2-H2
0.5% Au101/SiO2-untreated
0.5% Au101/SiO2-O2
0.5% Au101/SiO2-O2-H2
0.17% Au101/TiO2-O2 + KNO3
0.17% Au9/TiO2-O2 + KClf
0
0
b
0
0
0
0
0
0
0
0
96
97
29
96
98
0
79
75
65
73
70
0
21
23
15
20
23
0
0
0
13
22
24
0
0
18
0
24
26
16
31
32
21
0
0.51
0.51
0.12
0.51
0.51
0
0
0
0.05
0.65
0.65
0
0
0.08
0
0.25
0.26
0.09
0.13
0.14
0.46
0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
c
c
< 2e
< 2e
0
0
d
d
0
0
2.7 0.6
3.5 0.8
4.4 1.2
< 2e
2.4 0.5
2.9 0.9
< 2e
3.6 2.7
4.4 2.5
3.6 1.2
4.7 1.6
5.7 2.9
3.5 0.8
2.4 0.5
99
93
92
0
87
78
76
0
0
0
20
0
23
0
69
69
63
95
99
85
0
72
70
74
63
65
78
0
f
Reaction conditions: 50 mg catalyst, 2.5 mmol benzyl alcohol, 25 mL methanol (solvent), 1.25 mmol anisole (internal standard), 2.5 mmol K2CO3, 5 bar
O2 pressure, 80 °C. a Without gold catalyst. b Without base (K2CO3). c Recycled catalysts. d 850 mg of a catalyst was used to retain the same total amount
of gold as in the case of 0.17 wt% Au catalyst. e The size of gold nanoparticle was estimated from the absence of LSPR band in UV-vis DR spectrum. f
50 KNO3 (or KCl) was added according to equivalent molar percentage by impregnation method.
For the same Au loading (0.17 wt%) and support (TiO2), Au101 55 analogues, Table 2, entries 19-21 cf. entries 4-6). We hypothesize
cluster-based catalysts showed much higher catalytic activity
compared to Au9-based catalysts even though the gold particles
size were smaller in the case of Au9/TiO2 (cf. Au101/TiO2
that the presence of anionic species within the gold cluster (Cl- in
-
Au101 vs. NO3 in Au9) strongly affected the catalytic activity.
From our XPS investigations we have no evidence that the
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amount and chemical nature of the nitrogen species are changing
catalytic activity of those catalysts. In the case of Au
due to the treatment after the deposition of the Au clusters. To 60 nanoparticles containing residual Cl- species, calcination under
identify the anionic component of the cluster precursor
responsible for quenching of the catalytic activity, we
investigated the role of such anionic component in benzyl alcohol
oxidation. Firstly, we added KCl calculated according to the
H2 atmosphere could remove Cl- in the form of HCl, as proposed
by Haruta et al.82 However, at this stage we could not conclude
the fate of NO3- species after combined O2-H2 calcination because
the available XPS data provide no evidence about the amount and
5
weight percentage of Cl- in Au101(PPh3)21Cl5 (0.08 wt%) to mimic 65 nature of the nitrogen species are changing after such calcination.
the presence of Cl- anion to Au9/TiO2 catalyst and calcined under
The proposed reaction pathway of benzyl alcohol oxidation in
methanol in the presence of base is shown in Figure 6. Benzyl
alcohol is first oxidized to benzaldehyde, which serves as an
intermediate in this catalytic system. In the presence of base,
O2 to resemble Au101/TiO2-O2, and we found no increase in the
10 catalytic activity (Table 2, entry 29 cf. entry 20). When KNO3
-
(amount estimated to mimic the amount of NO3 from Au9, 4.6
wt%) was added to the Au101/TiO2 catalysts, we found noticeable 70 benzaldehyde is preferentially oxidised further to benzoic acid.83,
84
reduction in the catalytic activity (Table 2, entry 28 vs. 8). The
nitrate is an integral part of the Au9 cluster precursor used in
15 catalyst fabrication and would be perfectly positioned to affect
gold particles, causing deactivation of the catalyst (Table 2,
The formation of methyl ester resulted from the reaction
between benzaldehyde and methanol (the solvent). We carried out
a control reaction using benzaldehyde and benzoic acid as
substrates (instead of benzyl alcohol). When similar conditions (5
entries 19-21). For the Au101 based catalyst (Table 2, entry 28), 75 bar O2, 80 °C, 4 hours) were used for the catalytic oxidation of
the nitrate was introduced after the catalyst had been formed,
which could limit the efficiency of the interaction of the nitrate
20 with the gold particles and result in the moderate drop in activity.
Hence, it appears that the presence of NO3 lowers the catalytic
benzaldehyde (2.5 mmol) in methanol (25 mL) using
Au101/anatase catalysts, complete transformation of
a
benzaldehyde to benzoic acid and methyl benzoate with the same
distribution of products (i.e. benzoic acid:methyl benzoate of
activity of Au101-based catalysts and could be the reason for the 80 70:30) was observed. However, we did not observe any reaction
significantly lower activity of Au9-based analogues. To the best
-
of our knowledge this is the first report of the effect of NO3 on
when benzoic acid was used as a substrate. Thus, our
observations excluded the formation of the methyl ester via the
25 activity of gold-based heterogeneous catalysts. While numerous
studies report that ultra-small gold nanoparticles (< 2 nm) are the
key active site,23, 26, 79 our results demonstrate that the size of gold
nanoparticles alone is not sufficient to explain the catalytic
activity of catalysts made using phosphine-capped Au
30 nanoparticles as it is considerably affected by the type of anions
present in the supported gold cluster catalysts, type of support
and activation treatment protocols. Quintanilla et al. investigated
the effect of N-based compounds (dodecylamine vs.
polyvinylpyrrolidone) as stabilizing agents for gold nanoparticles
35 deposited onto γ-Al2O3 in benzyl alcohol oxidation.80 The authors
found that the polyvinylpyrrolidone-capped gold nanoparticles
had higher catalytic activity than the dodecylamine-capped gold
nanoparticles due to the reduced steric hindrance of the gold
surface by the polymer, allowing access of the substrate to the
40 surface of gold nanoparticles. However, in our case, the N-based
benzoic acid route.
O
OH
OH
O
[Au]
O2
[Au]
O2
O
O
[Au]
CH3
CH3OH
O2
85
Fig. 6 Proposed reaction pathway of oxidation of benzyl alcohol in
methanol.
-
compound is present as a counter anion (NO3 ), not as stabilizing
agent. In summary, we found that catalysts fabricated using a
nitrate-containing cluster (Au9) are either inactive or show
noticeably lower conversion compared to analogues made using
45 chloride-containing Au101 while addition of extraneous nitrate to
the Au101-based catalysts diminished their activity.
For the Au101-based catalysts, the catalytic activity arises from
90 formation of the large gold nanoparticles coinciding with removal
of PPh3 ligands. All untreated Au101/TiO2 (0.01 and 0.17 wt%)
and Au101/SiO2 (0.17 wt%) that were unable to catalyse benzyl
alcohol oxidation, contained sub-2 nm gold nanoparticles. In
trying to explain this observation one has to consider effects of
95 metal core properties and the possibility that significant
proportion of an otherwise reactive gold surface could be blocked
by protective phosphine ligands. Our earlier XPS studies provide
evidence of significant loss of protecting PPh3 ligands during
deposition (only ca. 10% of P XPS signal intensity remained to
100 be due to the remaining PPh3 ligands) in the case of 0.17 wt%
Au101/TiO2.47 Although it is impossible to dismiss outright
argument that this residual 10% of ligands could completely
deactivate the catalysts, one should attempt to find an alternative
The effect of different calcination conditions is negligible for
Au101-based catalysts but has significant impact on Au9/TiO2
catalysts (see Table 2). Au101/TiO2-O2 and Au101/TiO2-O2-H2
50 showed similar catalytic activity and selectivity. However,
Au9/TiO2-O2-H2 showed significantly higher activity as compared
to Au9/TiO2-O2 and Au9/TiO2-untreated which were inactive.
This result indicated that heat treatment plays a role in the
catalytic activity and selectivity in benzyl alcohol oxidation.
55 Youzhu et al. prepared Au9/TiO2 for CO oxidation.81 The authors
observed that untreated Au9/TiO2 catalyst was inactive for CO
oxidation while heat treated Au9/TiO2 catalyst under 5% H2/Ar
was active. However, they did not comment on the difference of
8
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explanation. An earlier report by Valden et al. demonstrated an
important transition from non-metallic to metallic properties
occurring for ca. Au300 clusters on TiO2.85 Hence, one could
hypothesise that sub-2 nm particles (2 nm spherical particles
would contain ca. 250 atoms)86 could be inactive, as observed in
our reactions, due to their non-metallic state. The presence of
non-metallic particles in these catalysts is further confirmed by
our UV-vis DRS results (Table 1, entries 1 and 14). However,
untreated 1.3 wt% Au101/TiO2 (which has the gold particle mean
5
10 size 2.7 1.6 nm) and untreated 0.5% Au101/SiO2 (which has the
gold particle mean size 3.6 1.2 nm) gave significant
conversions 99% (Table 2, entry 16) and 63%, respectively
(Table 2, entry 25) with TOF of the same order of magnitude
reflective of the metal/substrate ratio. This observation implies
15 that in the case of highly metal-loaded Au101-derived catalysts,
the formation of larger, metallic nanoparticles (which is likely to
coincide with even more pronounced loss of phosphine ligands
during aggregation) could be responsible for the observed
activity. The gold nanoparticles in Au101/TiO2 catalysts
20 agglomerated slightly after the catalytic tests (Table 2). The 31P
NMR spectrum of the reaction mixture showed the presence of
triphenylphosphine oxide, which was formed from the phosphine
ligands dislodged from the gold core, which then oxidised during
the catalytic reaction and dissolved in the catalytic reaction
25 mixture. The Au101/TiO2-untreated catalyst was active upon
recycling (i.e. in the 2nd consecutive catalytic test) and showed
noticeably increased conversion (29%, Table 2, entry 10)
compared to inactive fresh catalyst, and formed considerable
amount of benzaldehyde as compared to other catalytic tests in
30 this study (Figure S11). Again, the increase in activity of 0.17
wt% Au101/TiO2-untreated upon recycling could be attributed to
the increase in size of the gold nanoparticles and the simultaneous
loss of phosphine ligands from the gold core during first catalytic
test. However, comparison between the two catalysts with very
35 similar ca. 3.5 nm particle sizes - 0.17 wt% Au101/TiO2 recycled
(Table 2, entry 10) and activated by calcination under O2 (Table
2, entry 5) shows that particle size is clearly not the only factor. It
should be emphasized that the TEM imaging does not allow easy
quantification of the density/number of Au particles per particle
40 of support and such TEM-based sizing alone could miss the
presence of an inactive population of TEM-invisible smaller
particles. For example, recycled untreated catalyst (Table 2, entry
60 Fig. 7 Time-dependent profile of benzyl alcohol oxidation using 0.17%
Au101/TiO2-O2. Reaction conditions: 2.5 mmol benzyl alcohol, 25 mL
methanol, 1.25 mmol anisole, 2.5 mmol K2CO3, 5 bar O2, 80 °C.
65 Fig.8 Effect of the temperature on the conversion and profile of benzyl
alcohol oxidation products using 0.17% Au101/TiO2-O2. Reaction
conditions: 2.5 mmol benzyl alcohol, 25 mL methanol, 1.25 mmol
anisole, 2.5 mmol K2CO3, 5 bar O2, and 4 hours.
70
The evolution of conversion and selectivity of benzyl alcohol
oxidation as a function of time is presented in Figure 7. The high
conversion of benzyl alcohol (80 - 90 %) was observed after the
first two to three hours. The selectivity towards the formation of
methyl benzoate and benzoic acid are independent of reaction
10) could contain
a lower density/number of large gold
nanoparticles than calcined catalysts (Table 2, entries 8, 9, 11 and
45 12). Yet, a possibly more important factor in explaining observed
differences could be the effect of the higher-temperature
treatment under O2 atmosphere which could form a better contact
between the gold particles and the support, while PPh3 removal
from the gold core is even more pronounced when compared to
50 the recycled catalysts. For example, Haruta et al. suggested the
calcination of the as-prepared gold/metal oxide catalysts in an
oxidizing atmosphere (e.g. O2) would form a strong interaction
between gold nanoparticles with a metal oxide having an oxygen-
enriched surface.87 Both heat treated Au101/TiO2 catalysts under
55 O2 and O2-H2 showed no significant loss of catalytic activity and
selectivity after the first catalytic test.
75 time but primarily dependent on the reaction temperature, as
shown in Figure 8. A considerable amount (ca. 20%) of
benzaldehyde was formed after one hour and then depleted over
time (Figure 7), suggesting that benzaldehyde is the intermediate
in for the formation of benzoic acid and methyl benzoate in
80 benzyl alcohol oxidation.88-90 The effect of the temperature in this
reaction was studied from room temperature (25 °C) up to 80 °C
(Figure 8). Reduction in the temperature of the catalytic reaction
results in the reduction of the catalytic activity. However, the
product selectivity shows non-linear behaviour. The highest
85 selectivity towards methyl benzoate (93 %) was observed at 50
°C with 53 % conversion of benzyl alcohol, suggesting the
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selective formation of methyl benzoate is possible by
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Table of content entry
Novelty (20 words maximum)
The nature of Au cluster precursor and activation treatments affect catalyst activity in aerobic benzyl
alcohol oxidation.
Graphical abstract