Laser-driven heterogeneous catalysis: Efficient amide formation catalysed by Au/SiO2 systems

Article abstract of DOI:10.1039/c3gc40166a

A proof of concept of laser-assisted heterogeneously catalysed processes at room temperature using silica supported gold nanoparticles is reported. Au/SiO₂ nanoparticles were laser-irradiated at 532 nm (SPR maximum absorbance) to catalyse the selected reaction, namely the production of 4-benzoylmorpholine from benzaldehyde and morpholine via amide formation, for which quantitative yields to the target amide were obtained after 3-5 h of reaction. The protocol could also be extended to a tandem oxidation/amidation process which shows the potential of the proposed approach for the promotion of liquid-phase organic reactions at room temperature.

Full text of DOI:10.1039/c3gc40166a

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Laser-driven heterogeneous catalysis: efficient amide
formation catalysed by Au/SiO2 systems†
Cite this: DOI: 10.1039/c3gc40166a
Received 21st January 2013,
Accepted 22nd May 2013
Antonio Pineda,a Leyre Gomez,b Alina M. Balu,a Victor Sebastian,b Manuel Ojeda,a
Manuel Arruebo,b Antonio A. Romero,a Jesus Santamariab and Rafael Luque*a
DOI: 10.1039/c3gc40166a
www.rsc.org/greenchem
A proof of concept of laser-assisted heterogeneously catalysed collective oscillation of free electrons in the conduction band.8
processes at room temperature using silica supported gold nano- Incident photons are converted into phonons and generate
particles is reported. Au/SiO2 nanoparticles were laser-irradiated lattice vibrations which are both responsible for heating up
at 532 nm (SPR maximum absorbance) to catalyse the selected the nanoparticles. Hence, this absorption of the incident radi-
reaction, namely the production of 4-benzoylmorpholine from ation generates heat which is rapidly dissipated in the immedi-
benzaldehyde and morpholine via amide formation, for which ate surroundings. This heat can even produce vaporisation of
quantitative yields to the target amide were obtained after 3–5 h the solvent and cavitations in the proximity of nanoparticles.
of reaction. The protocol could also be extended to a tandem oxi- Localised heating at the nanoscale is usually achieved by using
dation/amidation process which shows the potential of the pro- pulsed lasers because the duration of the optical excitation is
posed approach for the promotion of liquid-phase organic shorter than the time scale needed for the heat to diffuse the
reactions at room temperature.
distance of the average particle–particle spacing.9 Continuous
wave lasers can also be used; however, in those cases, heat is
transmitted far away from the nanoparticle.9 This temperature
increase is proportional to the second power of the nanoparti-
cle radius.10 Larger nanoparticles would therefore be prefer-
able to achieve larger heating gradients; however, gold
nanoparticles are catalytically efficient only at reduced sizes
(about 5 nm or less).11 In addition, SPR responsible nanoparti-
cles might undergo acoustic vibration of their lattices after
laser absorption and consequently melting, evaporation and
fragmentation phenomena.12 In this way, there are several
limitations in terms of laser utilisation to be coupled with
nanoparticles to avoid their degradation and activity loss in
the course of laser-promoted chemistries.
For the particular case of nanoparticulated gold (a soft
Lewis acid), its use as a catalyst in organic reactions including
additions, cyclisations, rearrangements, couplings, oxidations,
and hydrogenations has been recently reviewed by Corma
et al.,13 highlighting the extensive range of applications of gold
nanoparticles in catalytic organic chemical processes. The cata-
lytic activity of gold nanoparticles markedly depends on their
size, shape, dispersion, support and the deposition method
used in their preparation as well as on the role of the interface
catalyst–support. In the search for energy efficient processes,
gold nanoparticles can also allow significantly reduced reac-
tion temperatures in catalytic reactions compared to those
utilised in conventional processes.11 Nevertheless, their combi-
Metal nanoparticles can serve as both local nanoscale-heat
sinks and catalysts to drive heterogeneous catalytic reactions at
room temperature in the absence of macroscopic heating. This
could in principle be done by a laser-induced selective exci-
tation of the surface plasmon resonance of nanoparticles at a
particular wavelength. In this way, gas1,2 and liquid3–5 phase
heterogeneous catalytic processes have been reported to be
accelerated by using laser excitation on various nanoparticles
used as catalysts. In view of these principles, Trammel et al.
accelerated the catalytic rate of the hydrolysis of methyl para-
thion by a factor of 2 with a simple laser irradiation at 532 nm
of the plasmon absorption band of gold nanoparticles in a Cu-
decorated Au nanoparticles catalyst.6 The selective oxidation of
sec-phenethyl and benzyl alcohols to acetophenone and benz-
aldehyde, respectively, comparing the use of different optical
(laser and light emitting diode) and microwave energy sources,
was also described using gold nanoparticles as catalysts.7
Photonic nanoparticles have a strong surface plasmon reso-
nance (SPR) absorption in the visible region due to the
aDepartment of Organic Chemistry, Marie Curie Building, Ctra Nnal IV-A Km 396,
University of Córdoba, E14014 Córdoba, Spain. E-mail: q62alsor@uco.es
bDepartment of Chemical Engineering and Aragon Institute of Nanoscience (INA),
Campus Río Ebro – Edificio I+D, C/Mariano Esquillor s/n, University of Zaragoza,
50018 Zaragoza, Spain. E-mail: jesus.santamaria@unizar.es
†Electronic supplementary information (ESI) available: Experimental, catalyst
preparation, UV-VIS, HRTEM, videos of the laser-assisted experiments. See DOI:
10.1039/c3gc40166a
nation with
a localised heating achievable under laser
irradiation might enhance even more the energetic efficiency,
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thus accelerating the rates of reaction in catalytic processes function and the logarithmic standard deviation was obtained
even at room temperature. for all the samples. Hydrodynamic sizes were evaluated in a 90
In view of these premises, we describe herein a proof of Plus dynamic light scattering (DLS) apparatus (Brookhaven
concept of laser-assisted heterogeneously catalysed processes Instruments Corp.). An ultraviolet visible spectrophotometer
at room temperature using silica supported gold nanoparti- (Jasco V670) was used to evaluate the extinction (absorption
cles. Au–silica was laser-irradiated at 532 nm (SPR maximum and scattering) bands of the catalysts in the different solvents.
absorbance) to catalyse the selected reaction, namely the pro-
Spherical gold nanoparticles show their characteristic SPR
duction of 4-benzoylmorpholine from benzaldehyde and mor- absorption band at ∼520–550 nm depending on their sizes.
pholine via amide formation. Amide formation avoiding poor For that reason, we used a solid state 532 nm green laser
atom economy reagents was the top priority and most relevant (0.5 W) (Changchun New Industries Optoelectronics Tech. Co.,
transformation for pharmaceutical manufacturers as defined Ltd) operating in continuous wave mode to excite the as
by the ACS GCI Pharmaceutical Round Table in 2007.14 Mor- synthesized plasmonic nanoparticles. The laser was placed at
pholine derivatives are in fact interesting intermediates in the 8 cm from the surface of the solvent rendering a fluence of
synthesis of many pharmaceuticals and they have been 2.1 W cm−2 measured using a photodiode sensor (Ophir
applied as analgesics, local anaesthetics, and respiratory and Optronics).
vasomotor stimulants.15
Amidation reaction
The proposed focused laser-assisted protocol allows a rapid
heating and easy and remote controllability, which can speed
up reaction kinetics without heating all the thermal mass of
the system. In addition to the potential energy savings
achieved with localised laser-driven heating, thermal hetero-
geneities, which are observed in conventional convective heating,
can also be avoided, being such an approach of special interest
for chemical reactions of high energies of activation and slow
reaction rates.
In a typical reaction, 0.025 g of the catalyst was placed in a
pyrex tube together with 19 mL of THF, 0.2 mL of benz-
aldehyde, 0.2 mL of morpholine, 1 mL of H2O2 and 0.01 g of
KOH (under otherwise stated conditions). Then the reaction
mixture was illuminated with the laser source for the desired
period of time, from which samples were periodically with-
drawn, subsequently filtered off and analysed by GC and GC/
MS using an Agilent 6890N GC model equipped with a 7683B
series autosampler, fitted with a DB-5 capillary column and an
FID detector. Response factors of the reaction products were
determined with respect to the original starting materials from
GC analysis using known compounds in calibration mixtures
of specified compositions.
In the case of the tandem process, double quantities of
H2O2 (2 mL) were added dropwise to facilitate the oxidation of
benzyl alcohol (0.2 mL) to benzaldehyde that subsequently
reacted with morpholine to give the desired amide in high
yields.
Reusability experiments performed for the catalysts were
carried out as follows: upon reaction completion in the first
run, the catalyst was filtered off, washed thoroughly with
acetone and oven-dried at 100 °C prior to its reuse in the reac-
tion. For the three uses experiment, double quantities of the
reaction mixture, reagents (0.4 mL benzaldehyde or benzyl
alcohol, 0.4 mL amine) and the catalyst (0.05 g) were utilised
to maximise the quantity of the catalyst available and mini-
mise the effect of catalyst loss after each reuse.
Experimental
Benzaldehyde (≥99.5%), benzyl alcohol (≥99.5%), morpholine
(≥99.0%), tetrahydrofurane, THF (≥99.9%), (3-aminopropyl)-
triethoxysilane (≥98%), tetraethyl orthosilicate (>98%), gold(III)
chloride hydrate (≥99.999%), a tetrakis(hydroxymethyl)phos-
phonium chloride solution (80% in water) and H2O2 (aqueous
50 wt%, v/v) were purchased from Sigma Aldrich and used as
received.
Catalyst preparation
Silica nanoparticles decorated with gold seeds were prepared
by following the protocol previously described for the seeding
step in the preparation of gold nanoshells by Bardhan et al.16
In brief, silica nanoparticles were initially prepared by follow-
ing the synthesis described by Stöber et al.17 The surface of the
resulting nanoparticles was grafted with amino groups via
silane coupling. Gold nanoparticles were separately prepared
and bound to the amino-functionalized silica. The excess of
gold nanoparticles or unbound seeds was removed by succes-
sive washing steps.
Results and discussion
The synthesized spherical gold nanoparticles comprising mul-
tiply twinned crystals showed a particle-size distribution
Catalyst characterisation
The shape, size, particle size distribution, and microstructure centred at 1.85 0.55 nm. Fig. 1 shows the distribution of the
of the nanoparticles were studied by using a T200 Philips gold seeds on the surface of the dense silica nanoparticles
Tecnai Transmission Electron Microscope (TEM) operated at used as supports. Gold nanoparticles are strongly attached to
200 kV. At least one hundred nanoparticles were measured to the surface of the amino-functionalised silica due to the
evaluate the mean diameter of the particles and the standard chemical bond between the electron-rich nitrogen present in
deviation. The data were fitted with a log–normal distribution the primary amines of the silica and the gold orbitals via their
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Fig. 1 TEM photographs describing the morphology of the catalyst: (a) silica nanoparticles used as a support, (b) Au seeds and (c) Au nanoparticles decorating the
surface of the silica after amino–silane coupling. (d) Histogram (N = 220) showing the particle-size distribution of the Au nanoparticles.
be neglected and the initial rising shoulder at wavelengths
below 500 nm can be attributed to interband transitions.12
Only dipole oscillation contributes to the extinction cross-
section when the diameter of the gold nanoparticle is much
smaller than the wavelength of the radiation. When the nano-
particles are arranged in arrays, the interactions between indi-
vidual dipole plasmons can further increase this extinction.
Fig. 2 also highlights that the solvent utilised in the organic
reaction does not absorb the incident light (532 nm); only
nanoparticles are responsible for the localised heating. Upon
laser irradiation, the temperature of the reaction mixture
remained below 25 °C even after 24 h, indicating that the heat
generated by the gold nanoparticles was rapidly dissipated by
the solvent.
The reaction selected as a proof-of-concept for the proposed
approach was the preparation of 4-benzoylmorpholine
(product A) from benzaldehyde and morpholine (Scheme 1).
Fig. 2 UV-VIS spectra of the Au/SiO2 nanoparticles used as catalysts. For com-
parison the spectrum of the solvent used (THF) is shown to demonstrate its lack
This product was recently reported to be formed by
polymer-incarcerated carbon black Au nanoparticles (PICB-Au)
catalysis at 40 °C in over 12 h of reaction,19 being one of the
most challenging substrates to couple from the reported work.
Fig. 3 summarises the comparative experiments carried out
under the different investigated conditions with Au/SiO2,
demonstrating that 4-benzoylmorpholine could be efficiently
produced at room temperature under laser-assisted conditions
in 4–5 h of reaction. This time reduction compared to the
of absorption at the light wavelength used (532 nm).
lone pairs.18 The hydrodynamic diameter of the catalyst
measured by DLS was centred at ∼350 nm indicating a slight
agglomeration.
Fig. 2 shows the weak absorption of the gold nanoparticles
characteristic of their small size (1.85 0.55 nm). According to
the Mie theory, the scattering for such small nanoparticles can
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Furthermore, while this particular amide formation reaches
conversion levels of ca. 80–85% (at >98% selectivity to benz-
amide) after 1 h at 60 °C but did not progress any further to
completion, conversions in the systems could be pushed to
quantitative yields of N-benzylbenzamide by means of the pro-
posed laser-assisted methodology. For all cases, atom efficien-
cies worked out were between 0.56 (for the reaction between
benzaldehyde and morpholine) and 0.60, taking into account
all reagents including THF as a solvent, hydrogen peroxide
and KOH, which is in the range of 55–60%.
Scheme 1 Laser-driven amide formation between benzaldehyde and morpho-
line at room temperature.
It is worth pointing out at this stage that a series of Au
nanoentities (e.g. hollow and solid Au NPs, Au nanorods) as
well as supported Au NPs on various supports (e.g. core–shell
nanoparticles, etc.) were also investigated in the selected
process, with generally low yields to amides obtained in all
cases. The selected optimum Au/SiO2 catalyst was also found
to be highly stable and reusable under the investigated
conditions, preserving over 90% of its initial activity after 2
re-uses.
The next step forward was the application of this proof of
concept reaction to a tandem oxidation/amidation process
for amide synthesis from benzyl alcohol and morpholine
(Scheme 2).
In a first step, benzyl alcohol was oxidised to benzaldehyde
(promoted by the addition of H2O2 and the bubbling of mole-
Fig. 3 Comparative experiments in the amidation of benzaldehyde and mor- cular oxygen in the system) and then amide formation sub-
pholine at RT conducted under identical conditions. Reaction conditions: 0.2 mL
sequently takes place. The proposed reaction was found to give
ca. 38% conversion in 4 h with >99% selectivity to the target
product. This and related tandem processes are currently
benzaldehyde, 0.2 mL morpholine, 19 mL THF, 1 mL H2O2, 0.01 g KOH and
0.025 g Au–silica catalyst, 4 h of reaction, 25 °C.
under optimisation in our laboratories to select the best con-
previous literature highlights the economical impact of this
energy-saving approach.
ditions to improve conversions in the systems.
In any case, a further approach to the kinetics of the laser-
assisted catalytic system was conducted. Initial and maximum
reaction rates were worked out from conversion data profiles
(Fig. 4) and results have also been included and compared in
Table 1. Unfortunately, the reaction kinetics were rather
complex for a full detailed study at this stage in terms of the
order of reaction, the rate determining step and related kinetic
parameters. Nevertheless, preliminary findings seem to indi-
cate a few interesting points worth further investigation. Gen-
erally, the initial rates of reaction were as expected 10 times
superior in the laser-assisted catalysed process as compared to
those obtained with the Au systems without the laser (Table 1).
The laser-assisted reaction in the absence of catalyst provided
no appreciable rate of reaction. Importantly, the maximum
rates of reaction for the three investigated cases were even
Blank runs (with laser without a catalyst and with a catalyst
without laser) gave almost negligible activity, with only the
laser-absent Au-catalysed reaction at RT providing poor yields
(<30%) after 6 h of reaction (Fig. 3). Interestingly, a convention-
al thermal experiment at 60 °C catalysed by Au–SiO2 provided
fairly similar results to those obtained under laser-assisted
conditions at RT (results not shown), in good agreement with
our proposed theories of laser localised-heating under the
investigated conditions. Interestingly, energy measurements
further proved that the estimated energy consumption of the
laser-assisted process after 5 h (<0.01 kWh) was remarkably
inferior to the obtained 0.1 kWh of a comparable 5 h thermal
reaction (60 °C) experiment run in a 650 W hot plate (for full
details, see the ESI†).
The reaction was also found to be amenable to other
amines, namely alpha-methyl benzylamine (99% conversion,
98% selectivity to the corresponding amide after 3 h of reac-
tion) and benzylamine (97% conversion, 95% selectivity to
N-benzylbenzamide after 5 h of reaction). In the latter case,
N-benzylbenzamide formation was low at RT and was only
efficiently formed thermally from benzylamine (at 60 °C in the
absence of catalyst without the need for the laser-assisted
methodology) or at RT under laser-assisted conditions under
otherwise identical reaction conditions to those of Fig. 3.
Scheme 2 Laser-driven tandem oxidation/amidation reaction. Benzyl alcohol
and benzyl amine to N-benzylbenzamide. Reaction conditions: 0.2 mL benzyl
alcohol, 0.2 mL morpholine, 19 mL THF, 1 mL H2O2, 0.01 g KOH, 2 mL H2O2
(added dropwise) and 0.025 g Au–SiO2 catalyst, 4 h of reaction, 25 °C.
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Fig. 4 Reaction profiles of benzaldehyde consumption and product generation
in the laser-assisted amidation of benzaldehyde and morpholine. Reaction con-
ditions: 0.2 mL benzaldehyde, 0.2 mL morpholine, 19 mL THF, 1 mL H2O2, 0.01 g
KOH and 0.025 g Au–silica catalyst, 25 °C. The proposed induction period can
be clearly observed at the beginning of the reaction (the first 30–60 min, see
the line of 4-benzoylmorpholine formation).
Fig. 5 TEM photographs of the nanoparticles after laser exposure for (a) 15 h
of reaction and (b) 24 h of reaction (1 use). Inset: STEM–HAADF image (c) after
two successive reactions (2× 24 h) and (d) conventional heating (24 h).
Table 1 Rates of reaction in the amidation of benzaldehyde and morpholine
(from the results depicted in Fig. 4)a
the basis for future applications in different chemistries to
that highlighted in this work as a proof of concept.
The stability of the Au/SiO2 systems under laser exposure
was subsequently investigated in detail. Fig. 5 (see also the
ESI†) shows the morphology of the catalysts after successive
reactions.
Fig. 5a shows the morphology of the nanoparticles after
15 h of reaction (laser exposure) indicating incipient sintering
and gold agglomeration in agreement with previous work.12
After 24 h (Fig. 5b), a large sintering was observed and even
Initial rate
Maximum rate
Conditions
(mmol min−1
)
(mmol min−1
)
Blank (laser, no cat.)
Au–silica (no laser)
Au–silica (laser)
—
2.7 × 10−4
0.07
1.28
1.5 × 10−4
2.5 × 10−3
a Reaction conditions: 0.2 mL benzaldehyde, 0.2 mL morpholine,
19 mL THF, 1 mL H2O2, 0.01 g KOH and 0.025 g Au–silica catalyst
(where appropriate).
more remarkably different (Table 1). A considerable induction bare silica nanoparticles were observed, indicating that the
period in the reaction can therefore be inferred from these gold seeds migrate across the surface and are clumped together.
results which might be plausibly explained as a contribution
As Werner et al.21 demonstrated, nanoparticle fusion was
of two important facts. Firstly, and most importantly, kinetics seriously affected by the irradiation atmosphere showing
seemed to indicate the likely presence of an inhibitor as the larger agglomerations when using oxygen saturated solutions.
cause of the induction period at the beginning of the reaction. In our case, the usage of H2O2 might also accelerate the
The affinity of amines (e.g. pyridine, ethylenediamine, etc.) for coalescence process observed. It is clearly observable that
metal nanoparticles, particularly Pd and Au, is well known.20 silica is partially dissolved after two consecutive reactions
Morpholine (and related investigated amines) may be preferen- (Fig. 5c, see also the ESI†).
tially adsorbed on the Au NP sites and therefore slow the
This result is in agreement with the work by Prasad et al.22
initial reaction kinetics in the process. Upon a local increase which showed silica removal and gold shell disruption after
in temperature on the surface of nanoparticles (achieved upon femto-second laser irradiation for hybrid core/shell SiO2/Au
plasmon excitation), the strongly adsorbed amines may desorb nanoparticles. Control reactions using conventional heating
from the surface leaving uncoordinatively unsaturated sites to showed also nanoparticle coalescence and clumping (Fig. 5d).
start driving the reaction. The room temperature catalysed Au/ Even though the damage caused in the catalysts was clearly
SiO2 reaction in the absence of laser provided very slow observable, the conversion was still very high after two re-uses
maximum rates of reaction, which essentially support the of the same catalyst, indicating that the damage was not
hypothesised mechanism of restricted availability of active extended to all of the catalyst mass. To corroborate this obser-
centers (at low temperatures) due to amine coordination. vation, the inset of Fig. 5b shows that a large number of small
Nevertheless, further studies are currently ongoing to support (<5 nm) gold nanoparticles remain on the catalyst support
the present hypothesis with supplemented results that could even after one use. Also, Liu and Mi23 demonstrated that a
also offer insights into the reaction mechanism and serve as few surface layers of nanoparticles are responsible for the
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Acknowledgements
RL gratefully acknowledges MICINN for the award of a Ramon
y Cajal Contract (ref. RYC-2009-04199) and funding from pro-
jects CTQ2011-28954-C02-02 (MEC) and P10-FQM-6711 (Conse-
jeria de Ciencia e Innovacion, Junta de Andalucía). MA also
gratefully acknowledges the support of the MICINN through
the MAT2011-24988 grant. The authors are indebted to and
greatly appreciate the assistance of Dr Vitaly Budarin from
Green Chemistry at York University (UK) for useful and insight-
ful kinetics calculations and discussions.
Fig. 6 Digital photographs of different reaction times showing the stress wave
generation as well as the bubble/shockwave formation in the course of the
organic reaction during laser excitation.
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