PTFE supported gold nanoparticles as photocatalysts for oxidative esterification of aldehydes

Article abstract of DOI:10.1039/c5nj03451e

Homogeneous, small gold nanoparticles (d = 1.87 nm) have been prepared by photochemical reduction of HAuCl₄ in the presence of Irgacure 2959 under high power UV irradiation at 365 nm produced by a LED source. These particles have been deposited on PTFE microbeads (d = 200 μm) and evaluated for the catalysis of the oxidative esterification of aldehydes in the presence of H₂O₂. Under green light (λ ≈ 530 nm) and repeated addition of H₂O₂, the catalytic system is accelerated and achieves complete conversions in minutes. Furthermore the catalyst can be recycled ten times consecutively without any activity loss by adding a step for AuNP recycling. The mechanism of the reaction follows a zero rate order, and Hammett free energy relationships for substituted benzaldehydes afforded a positive ρ value (ρ = 2.35) demonstrating that the initial hemiacetalisation equilibrium (ρ = 2.31) is the rate determining step. Kinetics data are in agreement with an Eley-Rideal type mechanism and permits proposing a reaction mechanism.

Full text of DOI:10.1039/c5nj03451e

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DOI: 10.1039/C5NJ03451E
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PAPER
PTFE supported gold nanoparticles as photocatalyst for oxidative
esterification of aldehydes.
Maël Penhoat,*a,d Théodore Vanbesien,a,d Adrien Cocud,a,d Ahmed Addad,b,d Hervé Vezin,c,d and
Received 00th January 20xx,
Accepted 00th January 20xx
Christian Rolandoa,d
DOI: 10.1039/x0xx00000x
Homogeneous, small gold nanoparticles (d = 1.87 nm) have been prepared by photochemical reduction of HAuCl4 in the
presence of Irgacure® 2959 under high power UV irradiation at 365 nm produced by a LED source. These particles have been
deposited on PTFE microbeads (d = 200 µm) and evaluated for the catalysis of the oxidative esterification of aldehydes in
the presence of H2O2. Under green light (λ ≈ 530 nm) and repeated addition of H2O2, the catalytic system is accelerated and
achieves complete conversions in minutes. Furthermore the catalyst can be recycled ten times consecutively without any
activity lost by adding a step for AuNPs recycling. The mechanism of the reaction follows a zero rate order, Hammett free
energy relationships for substituted benzaldehydes afforded a positive ρ value (ρ = 2.35) demonstrating that the initial hem-
iacetalisation equilibrium (ρ = 2.31) is the rate determining step. Kinetics data are in agreement with an Eley-Rideal type
mechanism and permits to propose a reaction mechanism.
www.rsc.org/
conditions.9-13 For this purpose, the use of molecular oxygen or
hydrogen peroxide14 in catalyzed oxidation reactions proved to
be particularly effective as co-oxidant for AuNPs catalytic
systems. These new catalytic systems satisfy most of the
demands for a modern, sustainable and ‘‘green’’ oxidation
reaction.
Introduction
Esterification reaction is traditionally a two-step procedure
including, first, the preparation of an activated carboxylic acid
derivative such as acid anhydride, acid chloride or N-
hydroxyester under stoichiometric conditions, then their reaction
with an alcohol to produce the corresponding ester.1, 2 These
protocols usually generate a large amount of waste. Recently,
direct oxidative esterification reactions from alcohols or
aldehydes catalyzed by reusable heterogeneous catalysts under
mild conditions, appeared as attractive alternatives for both
organic synthesis and green chemistry.3
AuNPs exhibit
a surface plasmon resonance (SPR)
absorption band around 520 nm.15 Excitation of the surface
plasmon band was described recently as a strategy of choice in
order to facilitate electronic transfers that occur along redox
17
reactions.16,
This photocatalytic activity of AuNPs has
attracted increasing attention in recent years because it can be
driven with sunlight at ambient temperatures.18 The mechanism
of AuNPs photocatalytic activity has been examined by Zhu et
al. who gave a mechanism different from that for conventional
semiconductor photocatalysts.19
Since the pioneering discovery by Haruta and coworkers who
demonstrated that gold nanoparticles (AuNPs) catalyzes the
oxidation of CO effectively at low temperatures,4 the scope of
gold catalyzed oxidation reactions has expanded tremendously.5-
In the continuity of our research program concerning the
synthesis of gold nanoparticles within microreactors20 and their
immobilization into fused silica capillary tubings for the
development of catalytic microreactors,21 we recently observed
that, submitted to colloidal AuNPs solution, Teflon® coated
magnetic stirrer bars developed a purple–brown color on surface
characteristic of gold nanoparticles deposition. We turned this
observation in a reproducible protocol and we describe here the
preparation and characterization of PTFE supported gold
catalyst. In order to evaluate its catalytic efficiency, the
photocatalyzed oxidative esterification of various substituted
benzaldehydes in the presence of methanol under mild
conditions has been evaluated. Traditional methods for the
preparation of supported catalysts include co-precipitation,
deposition-precipitation, ion-exchange, impregnation, and
successive reduction and calcinations.22 However, these
methodologies have some disadvantages, such as lack of control
7
In particular, the oxidation of alcohols into the corresponding
acids has received significant attention.8 Under similar
conditions, oxidation of aldehydes to esters proceeds under mild
a.Université de Lille, CNRS, USR 3290, MSAP, Miniaturisation pour la Synthèse
l'Analyse et la Protéomique, F-59000 Lille, France - Email : mael.penhoat@univ-
lille1.fr
b.Université de Lille, CNRS, UMR 8207, UMET, Unité Matériaux et Transformations,
F-59000 Lille, France
c. Université de Lille, CNRS, UMR 8516, LASIR Laboratoire de Spectrochimie
Infrarouge et Raman, F-59000 Lille, France
d.
Université de Lille, CNRS, FR 2638, Institut Eugène-Michel Chevreul, FR CNRS, F-
59000 Lille, France
Electronic Supplementary Information (ESI) available: Additional data including
characterization of colloidal AuNPs (DLS, UV-Vis), AuNPs/PTFE catalyst (XPS),
catalytic experiments (GC-MS kinetic studies, Eley-Rideal fitting model, 1H NMR
hemiacetal equilibrium constant measurements, Hammett study). Binocular loop
study concerning the evolution of AuNPs/PTFE morphology along catalytic
experiments (with or without recycling procedure) is also included. See
DOI: 10.1039/x0xx00000x
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over size, morphology, especially for catalyst as AuNPs, with spectrophotometer from Varian (Massy, France). DVyienwamArtiiccleLOinglihnet
limited stability. Therefore, the immobilization of pre- Scattering (DLS) analysis was perforDmOeId: 10o.1n039a/C5DNJy0n3a4P5r1oE
synthesized AuNPs has gained attention; since it can lead to NanoStar instrument Wyatt (Toulouse, France) at a thermostated
controllable size, shape and surface properties of nanoparticles.23 temperature of 25 °C using a fixed scattering angle of 173 °.
By choosing the preparation method of the nanoparticles, such TEM analysis has been performed on these samples using a FEI
properties can be tuned as well as their catalytic activities. In the Tecnai G2 20 (Hillsboro, OR, USA) equipped with a LaB6
present study, pre-synthesized uncoordinated, small and filament at 200 kV. The specimens for examination by electron
homogeneous AuNPs (average diameter 1.87 nm), obtained by microscopy were prepared by adding one or two drops of a
photochemical reduction of HAuCl4 in presence of Irgacure® colloïd solution of the nanoparticles onto holey carbon films
2959, were used as precursors for the preparation of PTFE supported on standard copper grids. In order to avoid any
supported gold catalyst. These Teflon® supported particles were agglomeration process along deposition, a silicon dioxide grid
found to be excellent recyclable photocatalyst for the oxidation (20 nm film, from SIMpore Inc., West Henrietta, NY, USA)
of substituted benzaldehydes to their corresponding methyl functionalized
in
surface
with
25
µL
of
esters.
mercaptopropyltriethoxysilane (Sigma, Saint Quantin-Fallavier,
France) for 2 hours has been evaluated in place of standard
copper grids.
Experimental
Materials
AuNPs immobilization on PTFE microbeads
Typical procedure. Fifty mg of PTFE powder (particle diameter:
200 µm) was placed in a 50 mL glass Erlenmeyer. Then, 5 mL
of acetone were poured into the Erlenmeyer. The Erlenmeyer
was closed and mechanically shaken with a CAT S50 (Staufen,
Germany) wrist arm shaker until no PTFE agglomerates
remained. Then 5 mL of AuNPs colloids were added to the PTFE
suspension. The suspension was shaken overnight. The purple-
grey PTFE powder was then filtered under vacuum on a frit,
washed 5 times with 5 mL of dry acetone and let dried over 15
minutes. This experiment has also been performed on larger
quantities (50 mL) using similar ratios.
AuNPs/PTFE characterization. The AuNPs/PTFE has been
characterized by MEB, XPS and EPR. XPS analyses were
obtained on a VG ESCALAB 220XL instrument (Villebon-sur-
Yvette, France). SEM images were obtained with a JEOL JSM-
7800F LV (FEG, low vacuum) instrument (Tokyo, Japan). TEM
images were recorded with a FEI Tecnai G2 20 instrument
(Hillsboro, OR, USA) operating at 200 keV. EPR spectra were
recorded using a Bruker (Wissembourg, France) Elexys E580
spectrometer operating at X-band modulation amplitude and
microwave power were set respectively at 5 Gauss and 1 mW.
Spectra were recorded at 5 K using a Bruker Cryofree system.
All chemicals were used as received without any purification.
Hydrogen tetrachloroaurate (III) hydrate 99.9%, Au 49%
(HAuCl4) was purchased from Strem Chemicals (Bischheim,
France). All other chemical compounds were purchased from
Sigma-Aldrich (Lyon, France). All solutions were prepared from
deionized water produced by a Milli-Q® system from Millipore
(Molsheim, France).
AuNPs photochemical synthesis
The preparation of gold nanoparticles was performed according
to the original procedure reported by Scaiano et al.24-26 Every
reactions have been performed in an optical transparent plastic
or quartz cuvette (12.5 × 12.5 × 45 mm). The irradiation source
Lightning Cure LC-L1 equipped with L9613-200 lens
(illumination diameter = 10 mm) from Hamamatsu (Massy,
France) was placed on the top of the cuvette and used at a
specified intensity (λ = 365 nm). Irradiance was measured in
mW.cm-1 with a Hamamatsu UV power meter (C6080)
positioned at 1 cm from the source and was in accord with the
power indicated by the source.
Typical procedure. All the experiments were performed at
atmospheric pressure without exclusion of air by an inert gas. In
a typical preparation, 2 mL of HAuCl4 solution (6.6 × 10-4 M)
Oxidative esterification of benzaldehydes
are
poured
in
a
cuvette.
Then,
2
mL
of
Typical procedure under room light illumination conditions.
In a typical procedure, 1.3 mmol of base (2 eq.) were dissolved
in 10 ml of dry methanol and poured into a 20 ml flask. The
mixture is stirred until complete dissolution of the base. Then,
0.64 mmol of benzaldehyde are added (1 eq. 65 µL) and the
reaction mixture is let stirred for additional 30 minutes. Finally,
1 mmol (1.5 eq., 50 µL) of hydrogen peroxide (50%w/w in water)
and AuNPs/PTFE catalyst (200 mg, 1%mol) are added to the
mixture which define the starting time of the reaction. The
reaction medium is strongly stirred during the reaction to
disperse the solid catalyst into the solution. Every 20 minutes, 1
mmol (1.5 eq., 50 µL) of hydrogen peroxide (50%w/w in water)
are added. At the end of the reaction, the catalyst is filtered off
on a frit, the solvent is evaporated under reduced pressure and
the crude mixture is analyzed by 1H NMR.
4 - (2 - hydroxyethoxy)phenyl - (2 – hydroxyl – 2 - propyl)ketone
(Irgacure® 2959) solution (2 × 10-3 M) were added to the cuvette.
The obtained pale yellow solution was left under slow
mechanical stirring for 2 minutes. When the solution is irradiated
a red-ruby colloid solution developed with time. After complete
reaction (15 minutes) the gold colloid solution is collected in an
Erlenmeyer flask surrounded with aluminum foil and conserved
at cold temperature (4 °C). The experiment is repeated 10 times
to obtain a volume of about 40 mL for later immobilization on
PTFE microbeads. The study of AuNPs average diameter vs
irradiance of the UV light source has been performed for every
samples according to this procedure by varying the light power
in %.
AuNPs colloid solution Characterization. The UV-vis spectra of
the AuNPs colloids were measured using a Cary UV 100
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Kinetic monitoring. For kinetic monitoring 50 µL of the power UV LED system (Hamamatsu LC-L1, Fig. 1) delivering a
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reaction medium are sampled at different times and introduced focalized irradiation of 350 mW.cm-2, at D3O65I: 1n0m.10.3T9/hCe5NreJ0a3c4ti5o1nE
into a vial containing 2 µL of formic acid and 1 mL of CH2Cl2 to was monitored by UV-Vis spectroscopy and dynamic light
quench the reaction. The sample is then injected without further scattering (DLS) to determine the AuNPs size. The UV-Vis
treatment into a GC-MS apparatus: PolarisQ Thermo Electron kinetic monitoring of the surface plasmon band (530 nm) showed
(electron ionization, EI 70 eV, column: Rxi®-5ms, length: 60 m; that absorbance starts to increase after 3 minutes. The reaction
internal diameter: 0.25 mm; film thickness 0.5 µm). The GC arrived to completion after 13 minutes and followed a first order
method was the following: split 1/50, temperature gradient from rate law, with a kinetic constant of 4.3 × 10-3 s-1 (Fig. 2). This
50 °C to 275 °C at 5 °C.min-1. Every kinetic measurements have result was confirmed by following the disappearance of the
been repeated 2 or 3 times and given values are averaged.
HAuCl4 absorption band at 217 nm. The DLS monitoring
Photocatalytic procedures. For photocatalytic reactions with t- confirmed the UV-Vis observation that the first measurable
BuOK as the base, a green fluorescent tube lamp (LEXMAN, AuNPs appeared after 3 minutes. The AuNPs hydrodynamic
EQSS-3, 15W, 120 mA, E27) is placed in close contact with the diameter steadily increased from 1.10 nm at the beginning to
Pyrex® flask before introducing the catalyst. The irradiation is 1.87 nm at the end of the reaction (Fig. 3b). This results confirms
maintained during all the reaction time and 1 mmol (1.5 eq., 50 that a higher UV intensity led to AuNPs with a smaller diameter.
µL) of hydrogen peroxide (50%w/w in water) are added every 8 DLS analysis showed two distinct populations (Fig. 3a); one
minutes. For Hammett relationship experiments, the same major centered around 1.87 nm (99.8% weight) and a second
procedure has been used with various p-substituted minor centered around 31 nm (0.2% weight) confirming that
benzaldehydes (0.64 mmol). The same previous work-up and AuNPs are nearly monodispersed. A particularly interesting
kinetic monitoring procedure have been used. Every kinetic advantage of this high power UV LED irradiation system
measurements have been repeated 2 or 3 times and given values concerns the possibility to adjust the illumination power at the
are averaged. For reaction under solar illumination conditions, desired value (0-100%). When varying the UV light intensity
the same procedure has been applied by placing the flask under from 350 mW.cm-2 (100%) to 91 mW.cm-2 (25%), we observed
magnetic stirring outside of the building (outside temperature: a linear correlation between the measured average hydrodynamic
31°C). The same previous work-up and kinetic monitoring diameter by DLS and the light intensity. Using this property, it
procedure have been used.
is then possible to directly select the AuNPs diameter by
applying the proper irradiation power according to the following
equation:
Hemiacetalisation equilibrium constants measurements
Typical procedure. In a 15 mL flask, 0.1 mmol of t-BuOK (2 eq.,
11 mg) and 0.05 mmol (1 eq.) of the para-substituted
benzaldehyde are dissolved in CD3OD (0.75 mL) and the
dAuNPs = -0.0384 × Iirr + 14.64
R2 = 0.97
mixture is stirred for 2 hours at room temperature. The mixture where d is the AuNPs average hydrodynamic diameter in
1
is transferred to a NMR tube and analyzed by H NMR with a nanometers and Iirr corresponds to the light source irradiance in
Bruker (Wissembourg, France) Advance 300 Ultrashield mW.cm-2 measured with an UV power meter (Hamamatsu –
spectrometer at room temperature with calibration on the solvent C6080) at 1 cm from the source.
peak. The equilibrium constant is determined by comparison of Despite many efforts, TEM analysis of AuNPs colloid solution
aromatic peaks, hemiacetalic and aldehydic peaks (averaged didn’t furnish acceptable results. Indeed along deposition on
integration).
TEM grids an agglomeration process is observed with the
formation, on the border of the grid, of black vesicles
corresponding to agglomerated 1-2 nm nanoparticles. In every
Catalyst recycling procedure
Typical procedure. In a 25 mL flask, previously used cases only the second minor population (average diameter = 30
AuNPs/PTFE catalyst (250 mg) is introduced and consequently +/- 10 nm) presenting various shapes can be easily observed.
5 mL of acetone and 5 mL of deionized water are added. Then
300 µL of H2O2 (50%w/w in water) are added and the mixture is
stirred for two hours at room temperature. The catalyst is filtered
on a frit, washed with methanol and dried before to be reused in
the following catalytic run. Recycled AuNPs/PTFE catalyst has
then been analysed by XPS and the evolution of morphology is
followed step by step under a stereomicroscope (Nikon, SMZ
1000).
Results and discussion
Gold nanoparticles photochemical synthesis
The HAuCl4/I-2959® samples were typically irradiated in a UV-
Vis cuvette without air exclusion using a new generation of high
Fig. 1
AuNPs photochemical synthesis in the presence of Irgacure® 2959 using
UV LED irradiation system (365 nm, 350 mW.cm-2).
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AuNPs catalysts. If the deposit of gold on PTFE surface is well-
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30
known either by ion beam assisted depoDsiOtiI:o1n0.1o0n39s/uCr5fNaJc0e345o1Er
targeted electrochemical reduction of Au3+ solution31 they are
very few reports concerning the chemical anchoring of AuNPs
on PTFE. Recently Redel et al. described the deposition of
AuNPs on Teflon® surface by the UV irradiation of
-
AuNPs/BMIM+BF4 solution in the presence of PTFE.32 The
authors detect an ESR signal in these AuNPs supported on PTFE
particles. They assign this signal to a charge transfer between the
AuNPs and the PTFE surface and the formation of (F...Au)
interactions that explain the stability of the AuNPs on PTFE
particles. These PTFE immobilized AuNPs have never been
investigated as catalyst. Interestingly, PTFE is a support of
choice for mild catalytic conditions. Indeed, its hydrophobic and
inert character differs from the traditional AuNPs catalyst
supports (TiO2, SiO2, CeO2...) and turn out to be compatible with
most of organic solvents and so assure a high level of catalyst
dispersion in solution. The support is known to have a strong
influence on the reactivity of AuNPs nanoparticles. So we
expected that the very specific interaction between AuNPs and
PTFE will led to new catalytic properties.
Fig. 2
synthesis.
UV-Vis kinetic measurement at 534 (± 2) nm for AuNPs photochemical
The preparation of PTFE immobilized AuNPs catalyst was
preliminary evaluated by mixing commercially available PTFE
microbeads (200 µm diameter) with the colloidal solution (Au
concentration 3.3
×
10-4 mol.L-1) resulting from the
photochemical synthesis. Under such conditions; the
hydrophobic PTFE powder remains on the water surface and did
not disperse into the colloidal gold solution resulting in an
inhomogeneous and extremely slow coloration of the support.
The addition of acetone to the colloidal AuNPs solution
(acetone-water 50/50 vol/vol) drove the microbeads into the
liquid phase, leading to a full decolorizing of the initially purple
solution after 4 hours under mechanical shacking. The catalyst is
finally collected as a purple-grey powder by filtration and UV-
Vis analysis of the filtrate demonstrated the absence of any
absorption in the range of 400-700 nm confirming a complete
immobilization of gold on PTFE surface. Assuming a complete
deposition of AuNPs on PTFE surface lead to an amount of gold
atoms per gram of PTFE powder of 3.3 × 10-5 molAu.g-1PTFE. The
resulting powder has then been characterized by scanning
electron microscopy (SEM) and X-ray photoelectron
spectroscopy (XPS). With classical SEM apparatus, only larger
particles with an average diameter of 30 nm can be observed
immobilized on the PTFE surface. We presently suppose that
AuNPs agglomerates during their immobilization on PTFE
surface resulting in larger particles. Interestingly, SEM pictures
show that larger particles are essentially located in on-surface
roughnesses of the PTFE microbeads (Fig. 4). This particular
localization strongly suggest that a stabilization effect occurs
when the support surface recover AuNPs surface due to larger
Au/F interactions.
Fig. 3
(a) Upper panel. DLS measurement after 15 min. (b) Bottom panel. DLS
monitoring of the major population.
It must be pointed out that most of the TEM analysis of AuNPs
in the nanometer range are AuNPs stabilized by strong ligands
(thiols,27 phosphines,28 amino terminated ionic liquid29) whereas
our AuNPs are only weakly stabilized.
Gold nanoparticles immobilization on PTFE
Preliminary experiments showed that the Teflon coated magnetic
bar used for mixing AuNPs during their photochemical synthesis
becomes purple–brown on surface characteristic of AuNPs
deposition, suggesting that Teflon® could be a new support for
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DOI: 10.1039/C5NJ03451E
Fig. 6
AuNPs/PTFE EPR (5 K) spectra.
This hypothesis was confirmed by oxidation of AuNPs/PTFE in
the presence of H2O2 resulting in a slight increase of the Au(+I)
peak at 353 gauss on the EPR spectrum. Similarly under
reductive conditions in the presence of NaBH4 the EPR spectrum
resulted in a complete disappearance of the Au(+I) peak and the
formation of a unique superparamagnetic peak at 113 gauss
corresponding to Au(0).
Analysis of the carbon region by XPS (Fig. 5 and Table 1,
AuNPs/PTFE – Step 0), gave other fruitful information. PTFE is
characterized by a single peak at 292.5 (+/- 0.1) eV attributed to
CF2 oligomers. When AuNPs are adsorbed on surface, a new
peak at 285.2 (+/- 0.1) eV appears. As previous authors
mentioned in literature this new peak could correspond to CF2
decomposition along AuNPs adsorption with formation of C-C
bonds.35, 36 Such peak is described to increase with the amount
of adsorbed AuNPs suggesting a transformation of PTFE with
AuNPs when anchoring. Here we suppose a fluorine abstraction
with formation of radicals that rearrange on surface to form C-C
bonds as recent literature mention the possibility to generate
radicals from organic halides by halogen abstraction.37, 38
Fig. 4
SEM analysis of the PTFE immobilized AuNPs surface.
XPS analysis shows that compared to PTFE alone, PTFE
immobilized AuNPs (AuNPs/PTFE) presents an absorption band
specific from gold 4f orbital, confirming the absorption of
AuNPs to the PTFE surface (Table 1, AuNPs/PTFE - Step 0). Au
4f7/2 binding energy (BE) of the AuNPs/PTFE is 84.8 eV, which
falls in between Au(0) BE (83.8 eV)33 and Au(+I) BE (85.0 eV),34
suggests the coexistence of Au(+I) and Au(0) in the AuNP core
(Fig. 5). Those results are in agreement with the continuous wave
EPR analysis of AuNPs/PTFE performed at 5 K (Fig. 6). Indeed,
EPR spectrum presents two main peaks corresponding to Au(+I)
at 353 gauss and a second intense superparamagnetic peak at 113
gauss to Au(0) clusters respectively.
Table 1. X-ray photoelectron spectroscopy (XPS) quantitative analysis of the AuNPs/PTFE
surface at different stages
AuNPs/PTFE
(Step 0)a
AuNPs/PTFE
(Step 1)b
AuNPs/PTFE
(Step 2)c
Element Orbital
BE (eV)
%
BE (eV)
%
BE (eV)
%
atom
atom
atom
F
1s
1s
1s
1s
689.7 68.40
532.5 0.058
689.5 67.15
689.3 66.48
O
532.9
292.4
285.2
0.25
532.8
292.4
285.2
0.28
292.5
31.56
285.2
C
32.39
33.04
4f7/2
84.8
0.035
88.5
85.2
85.1
88.8
Au
0.020
0.018
4f5/2
88.9
a Step 0: AuNPs/PTFE after immobilization on surface.
b Step 1: AuNPs/PTFE after oxidative esterification.
c Step 2: AuNPs/PTFE after recycling conditions (H2O2/ Acetone / Water).
Fig. 5
AuNPs/PTFE XPS analysis (C(1s) and Au(4f) regions).
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Oxidative esterification of aldehydes
In a next step the reaction was monitored by VGiewCA-rMticlSe.OnWlinee
DOI: 10.1039/C5NJ03451E
observed a break in the kinetic rate after 19 minutes. Both parts
Our catalyst in hand we evaluated it for the oxidative of the kinetics follow an apparent zero order rate law with a
esterification of aromatic aldehydes in methanol. Preliminary reduced rate after 19 minutes. Under green irradiation the rate of
aerobic experiments on benzaldehyde in the presence of oxidation reactions is accelerated and the break time is reduced
AuNPS/PTFE (0.5% mol.) and a base demonstrated that at room to 15 minutes. The reaction is accelerated in the very first
temperature, molecular oxygen was not an appropriate co- minutes and still follow an apparent zero order rate law. In the
oxidant for this reaction. This result is not surprising; as such second part, the curve progress at the same rate for standard and
oxidation conditions usually require high pressure of molecular irradiated conditions (Fig. 7). This reactivity can be explained by
oxygen to go to completion.39
the formation of a reactive specie on AuNPs surface by H2O2
We then decided to study the reaction in the presence of along very first minutes.
aqueous H2O2. A blank test, was first carried out in which the
The second part of the curve can be explained by the presence
benzaldehyde oxidative esterification was performed in the of reactive oxygenated species resulting from H2O2
presence of H2O2 (1.5 eq.) and NaOH (2 eq.) in methanol. The decomposition resulting in a second slower mechanism. The
reaction showed an extremely slow reaction (Table 2, entry 1) involvement of oxygen may be ruled out since the rate is not
with only 1% of methyl ester produced after 48 h. The GC/MS affected when the reaction is performed under a controlled
analysis shows that no Cannizaro reaction occurred in those atmosphere of argon. Successive addition of 1.5 equivalent of
conditions as no benzyl alcohol was detected. In presence of H2O2 every 20 minutes for standard conditions and every 15
AuNPs/PTFE catalyst (Table 2, entry 2) the reaction gave a fair minutes for photoactivated reaction maintained the reaction rate
yield after 24 h. Optimization of the base (Table 2, entries 2-4) observed at the beginning of the reaction. This observation
demonstrated that strong bases such as t-BuOK (pKBH(H2O)
=
confirmed that H2O2 is the key element for generating the
17.0) or NaOH powder/pellets (pKBH(H2O) = 15.7) were preferred oxidative species on AuNPs surface. Under such repeated H2O2
to weaker organic bases such as DBU (pKBH(H2O) = 12.0), for additions conditions, the reaction went faster to completion and
which one no ester formation was observed even after 24 h. kept a constant zero order reaction rate (Table 3). The time to
Strong bases which are able to quantitatively deprotonates completion was reduced to 100 minutes under standard
methanol should accelerate the hemiacetal formation and the conditions and to only 45 minutes under green irradiation
hydroperoxide anion formation required by the catalytic step. compared to 24 hours without repeated H2O2 additions.
Because of its higher activity t-BuOK was chosen for the next
experiments. The reaction appears very sensitive to temperature
conditions (Table 2, entries 5-6). Interestingly yields are
increased at lower temperatures. At the opposite at higher
temperatures, the overall conversion dramatically drops
Table 2. Benzaldehyde oxidative esterification catalyzed by AuNPs/PTFE
traducing an inactivation of the catalytic system.
We observed in every catalytic experiment that gas bubbles
appeared on AuNPs/PTFE surface. This could be explained by
the formation of O2 along H2O2 decomposition catalyzed by
AuNPs as previously described by Scaiano and Garcia according
to following equations:40-43
Ester
conv.
(%)b
1
52
0
56
78
3
67
Acid
Temp.
(°C)
Entrya
Base
%
Light
conv.
(%)b
-
1
0
1
5
0
6
cat.
H2O2 + Au(0)  HO● + HO- + Au+
HO● + H2O2  H2O + HOO●
HOO● + Au+  O2 + H+ + Au(0)
Eq. 1
Eq. 2
Eq. 3
1c
2
3
4
5
NaOH
NaOH
DBU
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
-
25
25
25
25
0
65
25
25
25
No
No
No
No
No
No
No
No
Yes
0.5
0.5
0.5
0.5
0.5
0.5
1
This side reaction which consumes H2O2, is accelerated by
temperature which explains the observed yield decrease. For
compensating H2O2 dismutation we multiplied its amount by
two. The yield of methyl ester was higher but the yield of benzoic
acid was also increased due to the introduction of larger amounts
of water into the reaction medium (Table 2, entry 9). In order to
achieve nearly complete conversions, we first increased the
amount of catalyst (Table 2, entry 11). By doubling its amount
the yield was increased by 20%. Finally we irradiated the mixture
at room temperature with green fluorescent bulb centered on the
plasmon resonance absorption band (LEXMAN EQSS-3 15 W,
 = 525 ± 30 nm) which affords a further 5% increase (Table 2,
entry 12).
6
9d
10
11
87
92
6
7
1
a In a typical procedure, benzaldehyde (0.64 mmol), H2O2 (50% in H2O, 1.5 eq.)
and the base (2 eq.) were dissolved in MeOH (10 mL) for 30 minutes then the
AuNPs/PTFE was added and the reaction was mixed under the desired light and
temperature conditions for 24 h.
b Determined by GC-MS using internal standard.
c The reaction time was extended to 48 h.
d 3 eq. of H2O2 were used.
6 | New J. Chem., 2015, 00, 1-3
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ARTICLE
Table 3. Benzaldehyde oxidative esterification catalyzed by AuNPs/PTFE (1.0%)
with repeated H2O2 additions
When the esterification reaction is fast, no formation of benzoic
DOI: 10.1039/C5NJ03451E
acid is observed until the reaction is complete. At longer times,
View Article Online
under such basic conditions, a slow hydrolysis of methyl ester is
usually observed.
Ester
conv.
(%)
Acid
conv.
(%)
Time
(min)
kobs
Entrya
%
Light
cat.
(mol.L-1.s-1)
Similarly when outer sunlight was used instead of the green
light under optimum solar conditions and repeated additions of
1.5 equivalent of H2O2 every 8 minutes (Table 3, entry 2), the
excellent selectivity for the ester was maintained. Despite higher
temperature conditions (32 °C) a good reactivity was also
conserved compared with the reaction done under fluorescent
tubes room light (Table 3, entry 1), confirming the great potential
of using sunlight instead of an artificial green light.
Then catalyst loading effect on the rate law has been studied
in detail for benzaldehyde oxidative esterification (Table 3,
entries 3-6, Fig. 8) and demonstrated as expected a linear
relationship between the catalyst loading and the observed
kinetic constant as expressed by the following equation:
1
2
3
4
1
Room
Sunb
100
30
92
72
8
-
1.16×10-5
2.55×10-5
4.0×10-5
1
1
Green
Green
30
>99
49
-
0.25
56
-
9.44×10-6
5
6
0.5
Green
Green
32
40
53
-
-
1.83×10-5
3.25×10-5
0.75
>99
a In a typical procedure, benzaldehyde (0.64 mmol), H2O2 (50% in H2O, 1.5 eq.)
and t-BuOK (2 eq.) were dissolved in MeOH (10 mL) during 30 minutes then the
AuNPs/PTFE was added and the reaction was mixed under the desired light
conditions. Standard conditions: 1.5 eq. H2O2 added every 20 min.; Green light
or sunlight conditions: 1.5 eq. H2O2 added every 8 min.
b Reaction temperature = 32 °C
kobs = 4.09 × 105 × %cat.
R2 = 0.99
were kobs is the observed kinetic constant expressed in mol.L-1.s-
and %cat. corresponds to the catalyst loading based on gold
Most of developed AuNPs based photocatalysts are usually
combined with active composites or metal oxide supports44 for
which, based on experimental characterizations,45 a concomitant
electronic transfer from the AuNPs to the support after photon
adsorption, similar to that of a dye-sensitized solar cell, is
envisaged to explain a certain catalytic redox reactivity on the
support itself.46
In our case, considering the inert character of PTFE support,
such mechanism can’t reasonably be envisaged and this
remarkable light effect can be justified by generation of energetic
and resonant electrons on AuNPs surface, after photon
absorption, which strongly interact with the molecules adsorbed
on the Au-NP surface and consequently enhance redox
processes.
Interestingly, the selectivity toward the ester compared to
acid under green irradiation and repeated additions of H2O2 is
greatly enhanced as no trace of benzoic acid is observed. Such
effect on the selectivity has recently been observed by Ke et al.
along AuNPs photocatalyzed reductions.47 These results
highlights important opportunities of light driven catalysis for
the improvement of chemical selectivity and reactivity by
avoiding any by-product formation.
1
atoms in percentage.
Hammett relationships
To study the influence of electronic effects of the substrate
on the reaction rate we established Hammett relationships on
para-substituted benzaldehydes (Table 4). Reaction is slower for
electron donating substituents and high conversions and full
selectivity for the ester, is observed in every case. The observed
rate constants are fitted by the classical σ substituent constants
affording positive ρ Hammett value (ρox = 2.35) with a good
correlation factor (0.99) indicating that a partial negative charge
is built up during the rate-determining step of the overall reaction
pathway (Table 4 and Fig. 9,
 curve). Poor correlations were
observed with Creary’s substituent constants σC● and modified
substituent constants σ- or σ+. This result strongly suggest that
the rate limiting step of the overall reaction pathway is the
hemiacetal formation. Previously, Christensen et al. investigated
such Hammett relationships in the aerobic oxidation of alcohols
49
and aldehydes, catalyzed by Au/TiO2.48,
In the case of
aldehyde oxidative esterification, for which no base was used
compared to our system, the kinetics law was first order and only
●
Creary’s substituent constants for radical reaction (σC ) fitted
with the kinetic data. However, if the p-CF3 is removed, the data
are fitted almost identically by the classical σ substituent
constants.
The reported hemiacetal formation equilibria for substituted
benzaldehydes in the presence of methanoate CH3O- has a large
positive Hammett ρ value (ρHemi = 3.20).50, 51 In the absence of
base, the reported hemiacetal formation equilibrium Hammett
plot has a much lower ρ value (ρHemi = 2.00) close to our
measured value. Those two positive Hammett ρ values strongly
suggest that the rate determining step of the reaction referred to
the hemiacetalisation step.
Fig. 7
standard (
Kinetic of benzoic acid methyl ester formation. Comparison between
), photocatalyzed (), standard with repeated H2O2 additions ()

and photocatalyzed with repeated H2O2 additions ( ) conditions.
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Kinetic law
View Article Online
DOI: 10.1039/C5NJ03451E
Kinetics results presented in Table 3 and Figure 7 exhibit an
apparent zero order rate law. Hammett relationships strongly
suggest the hemiacetalisation step as the rate limiting step,
consequently the hemiacetalisation equilibrium constant (KHemi
)
should influence the kinetic law of the reaction. Heterogeneous
catalytic reactions take place on surfaces as described by Eley-
Rideal or Langmuir-Hinshelwood kinetic models.52 Such
mechanisms have already been envisaged in literature for other
heterogeneous catalytic or photocatalytic reactions on AuNPs
surface.53 Eley-Rideal type mechanism describes heterogeneous
catalyzed pathway which assume equilibrated adsorption of one
reactant on catalyst surface followed by a subsequent reaction
with a second reagent in solution.54 In case of a Langmuir-
Hinshelwood type mechanism, this time, equilibrated adsorption
of two reactants on catalyst surface followed by their reactivity
on surface has to be considered.55
Such mechanisms are usually admitted for gas phase
reactions and rarely extended to liquid phase reaction.56 In the
present work an Eley-Rideal kinetic model is proposed in which
hydrogen peroxyde acts as an oxidant in solution and where the
hemiacetal anion is adsorbed on surface:
Fig. 8 Variation of kobs of methyl benzoate formation at different catalyst
concentrations (T = 25 °C, green irradiation).
In order to confirm this hypothesis we measured by 1H NMR
the hemiacetalisation equilibrium constants for the studied
substituted benzaldehydes at the previous concentration (0.064
mol.L-1) in CD3OD and in the presence of 2 equivalents of t-
BuOK as the base (Table 4). As shown in Figure 9, the
corresponding Hammett plot presents a positive ρ value (ρHemi
=
2.31; R2 = 0.98,

curve) similar to the ρox value observed for
KAdsKHemiCBenz
dCBenz
dt
the photocatalyzed oxidative esterification, confirming that the
hemiacetalisation step in this pathway is the rate determining
step.
r0 = -
With:
= krCSCH2O2
Eq. 4
(
1+KAdsKHemiCBenz)
kr: kinetic constant for hemiacetal anion oxydation on surface
퐾퐴푑푠: hemiacetal anion adsorption equilibrium constant
KHemi: hemiacetal anion formation equilibrium constant
CS: surface gold atom concentration
CH2O2: hydrogen peroxide concentration
CBenz: benzaldehyde concentration
Table 4. Hammett study with AuNPs/PTFE (1%) and repeated H2O2 additions, under
green light compared with benzaldehydes hemiacetalisation equilibria.
kobsX
Time
(min)
220
200
30
30
8
6
Ester
conv. (%)
70
77
>99
>99
>99
>99
b
Substituent
σ
Khemixexp
(mol.L-1.s-1) a
9.15×10-6
1.90×10-5
4.07×10-5
6.25×10-5
1.53×10-4
1.33×10-4
p-OMe
p-Me
p-H
p-F
p-Cl
-0.27
-0.17
0.00
0.05
0.22
0.23
5.00×10-2
1.10×10-1
2.90×10-1
3.32×10-1
6.70×10-1
8.69×10-1
Because H2O2 is added continuously to the reaction medium, we
considered its concentration constant along time, the kinetic
model was consequently simplified and resulted in the following
integrated form:
p-Br
a Measured by GC-MS after formic acid quenching. Kinetic constants have
been defined as the average of two or three experiment measurement (see
supplementary material). No trace of benzoic acids have been detected.
b Measured by 1H NMR in CD3OD.
(
)
(
)
KeCBenz + ln CBenz = krCSCH2O2Ket + KeC0Benz - ln C0Benz
Giving:
C0Benz - krCSCH2O2Ket
C0Benz
Ke
Eq. 6
)
CBenz = Keω
{
e(
}
Ke
With: Ke = KAdsKHemi and ω: Lambert function
This model justify apparent zero order reaction rates for low
hemiacetal anion adsorption equilibrium constants (KAds < 10-4)
and high kr values (kr > 103 s-1). The hemiacetal equilibrium step
affecting the overall Ke expression this model correlate with our
preliminary assumption based on hemiacetalisation step as the
rate determining step.
Fig. 9
Hammett plots for experimentally measured log (kobsX/kobsH) = 2.35 × σ +
0.026 () and log (KHemiX/kHemiH) = 2.31 × σ - 0.07 ().
8 | New J. Chem., 2015, 00, 1-3
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Proposed reaction mechanism
Along the second run, the conversion drops to 59%VaiefwteArrtaiclleoOnngliener
DOI: 10.1039/C5NJ03451E
reaction time of 45 minutes and the catalyst appears inactive
Christensen et al. suggest the formation of a radical anion along the third run. Observation of the catalyst after each step
deriving from the aldehyde to explain such correlation with an under a stereomicroscope demonstrated a change in the polymer
H● abstraction occurring with e.g. an Au–O–O- specie in a shape (plates instead of beads). When the catalyst (250 mg) is
radical recombination.
treated for 2 hours with a solution of H2O2 (300 µL, 50%w in
Assuming an Eley-Rideal type mechanism we may propose water) and acetone (5 mL) in water (5 mL), filtrated, and washed
possible steps for the catalytic cycle (Fig. 10). Starting from with methanol, as shown in Figure 11, the methyl ester yields
AuNPs composed of Au(0) and Au(+I) atoms, under such basic remained quantitative and constant for the ten successive cycles.
conditions, hydrogen peroxide anion may oxidize AuNPs surface This result reflects the high stability and reusability of the
generating Au(+I)-OH species and as previously envisaged by regenerated catalyst. Surprisingly, when the catalyst is
Barton along DFT studies Au(+I)-O-O-Au(+I) intermediates.57 reactivated using this procedure, kinetic measurements
This could be justified by the increase of oxygen quantity demonstrated that for every successive run the selectivity is
observed by XPS after the reaction (Table 1, AuNPs/PTFE - Step complete for the ester and that a zero order rate law is still
1) or the increase of the Au(+I) peak at 353 gauss observed by observed. Furthermore the reaction is accelerated and complete
EPR in the presence of H2O2. Then, an Au(+I)-OH activated gold after only 24 minutes compared to 30 minutes under standard
sites may be exchanged with a preformed hemiacetal anion, conditions.
resulting from hemiacetalisation step between benzaldehyde and
Interestingly for every runs, a delay of 8 minutes is also
in-situ generated CH3O-. This substitution generating observed before the beginning of the reaction suggesting a pre-
Au(+I)-OCH(OCH3)Ar catalytic intermediate and an equivalent activation of AuNPs surface under such new conditions. This
of hydroxide anion acting later as a strong base along H2O2 delay shows also an exceptional reactivity (only 16 minutes of
deprotonation or even along hemiacetalisation step. The oxidation time) after pre-activation. When the AuNPs/PTFE is
Au(+I)-O-O-Au(+I) active center may then undergo an treated with H2O2/acetone after each reaction step no change in
intramolecular hydride abstraction from adsorbed hemiacetal the shape of the polymer beads is observed even after 10 cycles.
anion to generate by a concomitant electronic transfer on AuNPs This surface activation could be justified by in-situ formation of
surface hydroperoxide anion Au(+I)-OOH intermediate, Au(0) a very reactive diluted triacetonetriperoxide species59 along
and the corresponding desired ester. The formation of the minor reactivation treatment generating unidentified highly reactive
benzoic acid side product can also be explained using this intermediates on AuNPs surface. Nevertheless, XPS analysis of
pathway by formation of an acetal in the presence of produced the treated AuNPs catalyst (Table 1, AuNPs/PTFE - Step 2)
OH- instead of CH3O- and its adsorption/oxidation on AuNPs didn’t show any significant change compared to the AuNPs
surface. To close the catalytic cycle an equivalent of catalyst after reaction without recycling treatment (Table 1,
hydroperoxyde anion may regenerate Au(+I)-O-O-Au(+I) and AuNPs/PTFE - Step 1).
Au(+I)-OH species on AuNPs surface to restart a new catalytic
cycle. Existence of both hydroxide and hydroperoxide anions
(Au(+I)-OH and Au(+I)-OOH) have been observed on AuNPs
surface by XPS analysis by Falaras58 and studied in detail by
DFT calculation along the reaction mechanism of water
formation from H2 and O2.57 The formation of unusual
Au(+I)-O-O-Au(+I) catalytic center is justified in our case by the
highly basic reactive medium.
The only difference in the green light enhanced reaction is
the polarization of AuNPs which enhances the electronic transfer
in the proposed mechanism. In accord with the Hammett ρ value
the rate limiting step in this overall pathway corresponds to the
hemiacetalisation formation. The oxidation step on AuNPs
surface should be considered as fast as proposed by the Eley-
Rideal kinetic model.
Recycling of the catalyst
The AuNPs/PTFE catalyst is perfectly stable after preparation
and may be stored for weeks. We tested the recycling of
AuNPs/PTFE catalyst in the oxidative esterification of
benzaldehyde. After the first benzaldehyde oxidative
esterification, the catalyst was separated from the reaction
Fig. 10. Schematic mechanisms of the reaction pathways in the photocatalytic
mixture by filtration, thoroughly washed with methanol, and then
reused as catalyst for the next run under the same conditions.
oxidative esterification of aromatic aldehydes by AuNPs/PTFE in the presence of
H2O2 and a strong base.
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Parallel to this work, we recently described, theVtireawnAsrtpicolesOitniloinne
of batch reaction into AuNPs immobDilOizIe: 1d0.1c0a3p9i/lCl5aNryJ03f4lo51wE
reactors allowing a better control of the reaction. The design of
AuNPs/PTFE based catalytic microreactors is of high interest,
and will be reported in due course. Further extension of the
present H2O2–AuNPs/PTFE catalytic system to other key
oxidative transformations is currently being explored. In
summary AuNPs supported on PTFE extend the traditional
support of AuNPs and have great potential as photocatalyst for
the development of environmentally benign reactions.
Acknowledgements
This work was supported by the Conseil Régional Nord, Pas-de-
Calais, France. The NMR and Mass Spectrometry facilities used
in this study were funded by the European Community
(FEDER), the Région Nord-Pas de Calais (France), the CNRS,
and the Université de Lille. MP thanks the Region Nord-Pas de
Fig. 11 Recycling of the AuNPs/PTFE catalyst (1%cat.) for the photocatalyzed
oxidative esterification of benzaldehyde under repeated additions of H2O2 every 8
minutes. (
) without reactivation treatment between two runs; () with H2O2
(50%) / Acetone / water treatment between 2 runs. The conversion is measured
at 45 minutes by GC-MS.
By comparison with previously reported oxidative Calais for a Young Investigator Grant funded by the Contrat de
esterification reactions catalyzed by gold nanoparticles Projet État-Région (CPER) to the Institut Michel E. Chevreul
60-
immobilized on various mineral supports in the presence of O2
(FR CNRS 2638). The authors thank Mr. Fredrik Ljungkvist for
64 our catalytic system appears more stable and more active after performing recycling experiments, Dr. Alexandre Mussi for his
the recycling step. Indeed, by comparison with most of mineral help in performing the microscope study, Pr. Patrice Woisel for
supports usually evaluated, the catalyst preparation is usually giving us a generous access to their DLS instrument and Dr.
complex and its activity usually decreases after few cycles. To Maria van Agthoven for her help with editing this manuscript.
our knowledge, from the best recyclable AuNPs catalysts, Au-
K2CO3 reported by Gao65 was reused 8 times without loss of
activity but by comparison our mild catalytic system requires less
energy (15 W light instead of usual high temperatures or high O2
pressures) and appears much faster than most of catalytic
systems using O2 as the co-oxidant.
Notes and references
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Homogeneous unstabilized gold nanoparticles (AuNPs) of 1 to 2
nm diameter were obtained by photochemical reduction of Au(3+)
solution by a high intensity UV LED source. These AuNPs
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ARTICLE
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New Journal of Chemistry
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DOI: 10.1039/C5NJ03451E
Graphical Abstract:
Fast and mild visible light photocatalytic conditions for oxidative esterification of aldehydes using
H2O2 and AuNPs/PTFE catalyst.