Organosilane oxidation by water catalysed by large gold nanoparticles in a membrane reactor

Article abstract of DOI:10.1039/c3cy00506b

We show that gold nanoparticles catalyse the oxidation of organosilanes using water as oxidant at ambient conditions. Remarkably, monodispersions of small gold particles (3.5 nm diameter) and large ones (6-18 nm diameter) give equally good conversion rates. This is important because separating large nanoparticles is much easier, and can be done using ultrafiltration instead of nanofiltration. We introduce a simple setup, constructed in-house, where the reaction products are extracted through a ceramic membrane under pressure, leaving the gold nanoparticles intact in the vessel. The nominal substrate/catalyst ratios are ca. 1800:1, with typical TONs of 1500-1600, and TOFs around 800 h^-1. But the actual activity of the large nanoparticles is much higher, because most of their gold atoms are "inside", and therefore unavailable. Control experiments confirm that no gold escapes to the membrane permeate. The role of surface oxygen as a possible co-catalyst is discussed. Considering the ease of product separation and the robustness of the ceramic membrane, this approach opens opportunities for actual applications of gold catalysts in water oxidation reactions. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cy00506b

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Organosilane oxidation by water catalysed by
large gold nanoparticles in a membrane reactor†
Cite this: Catal. Sci. Technol., 2014,
4, 2156
ab
a
a
*
*
Vitaly Gitis,
Rolf Beerthuis, N. Raveendran Shiju and Gadi Rothenberga
We show that gold nanoparticles catalyse the oxidation of organosilanes using water as oxidant at ambient
conditions. Remarkably, monodispersions of small gold particles (3.5 nm diameter) and large ones
(6–18 nm diameter) give equally good conversion rates. This is important because separating large
nanoparticles is much easier, and can be done using ultrafiltration instead of nanofiltration. We introduce a
simple setup, constructed in-house, where the reaction products are extracted through a ceramic membrane
under pressure, leaving the gold nanoparticles intact in the vessel. The nominal substrate/catalyst ratios
are ca. 1800 : 1, with typical TONs of 1500–1600, and TOFs around 800 h−1. But the actual activity of
the large nanoparticles is much higher, because most of their gold atoms are “inside”, and therefore
unavailable. Control experiments confirm that no gold escapes to the membrane permeate. The role of
surface oxygen as a possible co-catalyst is discussed. Considering the ease of product separation and the
robustness of the ceramic membrane, this approach opens opportunities for actual applications of gold
catalysts in water oxidation reactions.
Received 16th July 2013,
Accepted 24th April 2014
DOI: 10.1039/c3cy00506b
www.rsc.org/catalysis
Silicon alcohols (silanols) are widely used in organic synthesis,
particularly in cross-coupling reactions, and in industry as
synthons for silicon-based polymeric materials.1 Traditionally,
they are synthesized by hydrolysis of halosilanes, stoichiomet-
ric oxidation of organosilanes, or reactions of siloxanes with
alkali reagents.2 However, these are not clean processes. In
contrast, the catalytic oxidation of silanes with water is envi-
ronmentally benign. It produces silanols with a high selectivity
and hydrogen gas as the only by-product. Catalysing chemical
reactions in water at room temperature is one of the chal-
lenges facing chemists today.3 Oxidizing silanes to silanols is
usually catalysed by noble metals, such as rhenium, ruthenium,
iridium, silver and gold.
at 80 °C. Using a different approach, Asao et al.19 reported
the use of nanoporous gold with pore size around 30 nm
received from a gold–silver alloy for this type of reactions.
Recently, carbon nanotube-supported AuNPs (AuCNT) were
reported as highly active for silane oxidation in THF at room
temperature.20 Gold nanoparticles also catalyse selective acti-
vation of alkynes, giving fast hydrogenation or hydration
reactions,21 as well as catalysts for the selective conversion of
biomass into chemicals22 and in fuel cell reactions.23
The particles' size and shape, dictated by the preparation
recipes,24 often determine the catalytic performance. Gener-
ally, the smaller the particle, the higher the catalytic activity,
down to the three-layered Au55 cluster. Indeed, small gold
particles (2–5 nm diameter) catalyse a variety of reactions.25–29
Exceptions are few yet notable, such as the unusually large
38 nm AuNPs reported as catalysts for hydrogen peroxide
decomposition to hydroxyl radicals, enhancing the chemilu-
minescence of the luminol–H2O2 system.30
Although gold is traditionally considered (and prized)
as inert, its nanoparticles (AuNPs) are enjoying a newly
acquired status as catalysts for many reactions.4–17 Kaneda
and co-workers18 showed that hydroxyapatite-supported gold
particles (3.0 0.9 nm) catalysed silane oxidation in water
The problem is that although small AuNPs are exciting as
catalysts, their size hampers practical application. Gold is too
expensive a catalyst to throw away, and separating particles
smaller than 2 nm is no mean feat. Acceptable Au separation
levels often require repeated purification cycles or a combina-
tion of methods, increasing operational costs and decreasing
throughput. One way to solve this is by using pressure-driven
membrane separation, a relatively new technology. State-of-the-
art nanofiltration (NF) membranes can retain even single-nm
particles, but these membranes are expensive, delicate, and have
low flow rates.28 Alternatively, separation using ultrafiltration
a Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science
Park 904, 1098XH Amsterdam, The Netherlands. E-mail: n.r.shiju@uva.nl;
Web: http://hims.uva.nl/hcsc; Tel: +31 20 525 6515
b Unit of Environmental Engineering, Ben-Gurion University of the Negev,
PO Box 653, Beer-Sheva 84105, Israel. E-mail: gitis@bgu.ac.il; Fax: +972 8 6479397;
Tel: +972 8 6479031
† Electronic supplementary information (ESI) available: Complete experimental
procedures for the synthesis of the gold nanoparticles and the catalytic
oxidation of silanes, DLS data for the various nanoparticles, photo and
schematic diagram of the experimental setup and SEM image of the
membrane. See DOI: 10.1039/c3cy00506b
2156 | Catal. Sci. Technol., 2014, 4, 2156–2160
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(UF) membranes demands significantly lower transmembrane
pressures, and the membranes are cheaper and more robust.31
The problem is that UF membranes are only effective for par-
ticles 8–10 nm diameter and larger.
In this paper we show that AuNPs as large as 18 nm can
catalyze the oxidation of silanes to silanols with water, gener-
ating hydrogen as the only by-product (Scheme 1). We dem-
onstrate the effectiveness of these reactions in a dedicated
membrane pressure reactor, using a ceramic membrane for
separating the product mixture from the catalyst. Using large
AuNPs in such a reactor combines the advantages of high
conversion rates and good flux values with full retention of
the catalysts.
We prepared monodispersed gold aquasols of varying sizes
using the citric acid/tannic acid method (Fig. 1 shows TEM
images and gold nanoparticle size distribution; more details
are given in the ESI†). In a typical reaction (Scheme 1), organo-
silanes were oxidized in water in presence of gold nanoparticle
catalyst under 1 bar N2 pressure. A production of correspond-
ing silanols was accompanied by a formation of hydrogen gas
(this was quantified volumetrically, see ESI†). Reactions were
run for up to 24 h in a stainless steel high-pressure reactor,
equipped with a membrane, constructed in-house. The gold
was retained in the reactor by a porous ceramic membrane
placed in the bottom, and the product was isolated and deter-
mined using 1HNMR and gas chromatography (full details
and drawings in the ESI†). The extraction of reaction products
through the membrane was possible using only 4–5 bar addi-
tional nitrogen pressure.
Fig.
1
Transmission electron micrographs of gold nanoparticle
suspensions with different average diameters: 3.1
0.5 nm (II); 9.2
suspensions A–G, determined using dynamic light scattering (IV). The
distribution function analysis is displayed as scattered intensity per
particle size: A (3.1 nm), B (4.8 nm), C (9.2 nm), D (18.3 nm), E (27.5 nm),
F (37.4 nm) and G (45 nm).
0.5 nm (I); 4.8
0.8 nm (III). Size distribution of gold nanoparticle
Our dead-end membrane reactor enables quick and effi-
cient separation of the AuNPs. This is easily monitored using
UV-visible spectroscopy.32,33 Fig. 3 shows the spectra of
the reaction mixture before and after the membrane. The
Fig.
2 shows the conversion profiles of dimethyl-
phenylsilane 1a (top) and the yield profiles for dimethyl-
phenylsilanol 2a (bottom) vs. time in the presence of AuNPs
of different sizes. The profiles are divided into small particles
and large ones (left and right, respectively) for clarity. For
comparison, we also performed a control reaction in the
absence of any catalyst. These graphs show two important
things. First, there is no significant difference in the conver-
sion when using AuNPs up to 18 nm in diameter. This is
important because the filtering of larger particles (>10 nm) is
much easier, and can be done using standard ultrafiltration
technology. Second, there is a marked difference between sub-
strate conversion and product yield in the absence of AuNPs.
Practically, this means that the formation of the disiloxane
by-product is suppressed in the presence of the catalyst, as
the silane is quickly consumed in the oxidation reaction.
Fig. 2 Conversion profiles of dimethylphenylsilane 1a (top graphs) and
yield profiles for dimethylphenylsilanol 2a (bottom graphs) in the presence
of AuNPs of different diameters (3.1 nm, 4.8 nm, 18.3 nm, 37 nm and 45 nm).
Reaction conditions: 2.93 mmol 1a, 103.3 mM acetone, 222 mM water,
0.31 mg catalyst (1.57 × 10−6 mol gold; 0.05% Au relative to substrate)
and 296 K. All experiments were performed in duplicates.
Scheme
nanoparticles.
1 Oxidation of silanes to silanols with water using Au
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We then examined the scope of the reaction, testing various
aromatic and aliphatic organosilanes in the membrane reactor
(structure 1a–e, Scheme 1). Table 1 shows the conversion,
yield, and nominal TON values. Aliphatic silanes (entries 2, 5)
were more active than the aromatic ones. In all cases, we
observed complete retention of the nanoparticles within the
membrane reactor. Moreover, the membrane could be reused
several times.
We may anticipate that the reaction begins with the inser-
tion of the Si–H bond on the Au nanoparticle to generate a
silyl-metal hydride intermediate. The silyl-metal hydride inter-
mediate is subsequently attacked by a nucleophile derived
from water to generate the silanol (this attack is most likely
2
an SN type, and therefore an inversion would be expected,
since it would be easier for the oxygen to attack trans to the
NP surface). However, this is only a suggestion. In the final
step of the cycle, Au–Hδ+ and Osδ−⋯Hδ+ species should react
to produce H2, regenerating the catalyst surface (Scheme 2).
The critical step of the reaction is most likely the dissocia-
tive activation of water. Our observations show that this step
does not depend on the particle size alone below a certain
size. Recent surface science studies proposed that an oxygen
atom on Au(111) surface (Os) acts as a basic co-catalyst that
promotes various organic reactions at low temperatures.35–38
Cooperation of metal NPs as redox sites and the surface oxy-
gen as a basic co-catalyst leading to enhanced reactivity for
the dissociation of water was also confirmed on Ru, Rh, Pd,
Ir, and Pt surfaces.39 Furthermore, recent theoretical work
showed that the reaction barrier for the water dissociation on
the Pd(111) surface contaminated with Os (0.34 eV) was signifi-
cantly lower than that on the clean Pd(111) surface (0.92 eV).40
This is due to the bonding interaction between the Os and a
hydrogen atom in H2O at the transition state (Osδ−⋯Hδ+⋯OH).
This model was consistent with previous results of the surface
science experiments for water dissociation on clean and oxy-
gen precovered Pd surfaces, in which OH groups were detected
only on the latter surface.41
Fig. 3 UV-visible spectra of the reaction mixture at 0, 1, 2 and 4 h for
18 nm AuNPs, compared with the spectrum of the membrane
permeate. Note that the increase in the absorbance of the plasmon
peak at 520 nm is due to the removal of solvent during the sampling,
which increases the concentration of the NPs.34
permeate shows no absorbance, confirming 100% retention
of the AuNPs. Conversely, the samples taken from the reac-
tion mixture show the typical gold plasmon peak at 520 nm,
which increases as the effective concentration of the particles
increases.32
The AuNP catalysts exhibit high turnover numbers (TONs)
and turnover frequencies (TOFs). Considering that ca. 85%
conversion is reached after 2 h (with 3.1 nm particles) using
a nominal substrate/catalyst ratio of 1870 : 1, typical TONs
are 1500–1600, with TOFs around 800 h−1. But the actual cata-
lytic activity is much higher, especially for the larger AuNPs,
because of their smaller effective surface area. If we consider
that the particles are monodistributed and spherical (see DLS
and TEM analyses), the effective substrate/catalyst molar
ratios are 2500 : 1 (3 nm), 3100 : 1 (6 nm), 10 600 : 1 (18 nm)
and 25 000 : 1 (45 nm). Note that this does not mean that gold
atoms on the surface of large particles are more active than
on smaller ones. Rather, we think that only a small number of
atoms (or facets) are actually active, and the nice part is that
this “window” of activity ranges up to at least 18 nm sized
particles.
Following this, we tested the re-using of the gold NPs in
this reactor system (all these control experiments were run in
duplicate). Here, we first repeated the reaction with 18 nm
particles (Fig. 2c), and after 4 h filtered all the liquids through
the membrane, and washed the remains with acetone. The ace-
tone was filtered and new reactants and solvent were added,
using the same solid catalyst that remained in the vessel. The
results showed 50% conversion and 100% selectivity after 2 h.
STEM analysis of the reaction mixture shows some agglomera-
tion after the reaction (see Fig. S3, ESI†). However, not all par-
ticles were agglomerated, though they were left deliberately in
the reaction mixture for 6 h.
Enhanced reactivity due to adsorbed species on Au surface
is reported for CO oxidation as well. For the vapor phase CO
oxidation, oxide-supported Au particles in the 2–4 nm size
range are the most active.42 Adding water vapor can increase
the CO oxidation rate substantially, presumably due to the
Table
1 Water oxidation of various organosilanes to the
corresponding silanolsa
No
Substrate
Conversion, %
Yield,b
%
TON
1
2
3
4
5
1a, (CH3)2SiHC6H5
1b, (C2H5)3SiH
1c, (C6H5)2SiHCH3
1d, (C6H5)3SiH
80.3
90.0
81.1
79.8
83.4
79.5c
86.8
78.2
76.6
80.8
749
984
520
389
669
1e, [(CH3)2CH]3SiH
a
Reaction conditions: 0.2 g organosilane, 7.6 ml acetone, 10 ml
deionized water, 0.31 mg AuNPs (6 nm) catalyst, reaction time 2 h.
Total reaction volume 17.8 ml, magnetic stirring at 400 rpm, 296 K.
GC yield, corrected for the presence of an external standard.
Isolated yield.
b
c
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larger particles as active as smaller ones.
Conclusions
Water oxidation of silanes to silanols is efficiently catalysed
by “large” gold nanoparticles (6–18 nm in diameter). This cata-
lytic oxidation has several advantages, namely using water as a
clean oxidant, high activity and selectivity for silanols. More-
over, it works well with a number of silanes with different
types of substituents. Combining this reaction with ultrafiltra-
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Acknowledgements
V.G. thanks the European Commission for a Marie Curie
International Exchange Fellowship. We thank A. Duek for
preparing the AuNP suspensions, and Dr. E. Garnett and
M. de Goede (FOM Institute AMOLF) for help with the STEM
experiments.
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