Glycerol Selective Oxidation to Lactic Acid over AuPt Nanoparticles; Enhancing Reaction Selectivity and Understanding by Support Modification

Article abstract of DOI:10.1002/cctc.202000026

A high surface area mesoporous TiO₂ material (110 m²/g) was synthesised using a nanocasting methodology, utilizing SBA-15 as a hard template. This material was subsequently used as a support to prepare a series of 1 wt.% AuPt/TiO₂ catalysts, synthesised by conventional impregnation and sol-immobilisation. Catalysts were tested for the oxidation of glycerol to lactic acid and their performance was compared with corresponding catalysts supported on TiO₂?P25, TiO₂-anatase and TiO₂-rutile. Higher rates of reaction and higher selectivity to lactic acid were observed over nanocast TiO₂ supported catalysts. The increased performance of these catalysts was attributed to the presence of Si on the surface of the support, which likely arose from inefficient etching of the SBA-15 template. The presence of Si in these catalysts was confirmed by X-ray photoelectron spectroscopy and electron energy loss spectroscopy. It was proposed that the residual Si present increases the Br?nsted acidity of the TiO₂ support, which can lead to the formation of Lewis acid sites under reaction conditions; both sites are known to catalyse the dehydration of a primary alcohol in glycerol. Typically, under alkaline conditions, lactic acid is formed by the nucleophilic abstraction of a hydrogen. Thus, we propose that the improved selectivity to lactic acid over the nanocast TiO₂ supported catalyst is attributed to the co-operation of heterogeneous and homogeneous dehydration reactions, as both compete directly with a direct oxidation pathway, which leads to the formation of oxidation products such as glyceric and tartronic acid.

Full text of DOI:10.1002/cctc.202000026

Full Papers
doi.org/10.1002/cctc.202000026
ChemCatChem
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
Glycerol Selective Oxidation to Lactic Acid over AuPt
Nanoparticles; Enhancing Reaction Selectivity and
Understanding by Support Modification
[a]
[a]
[a]
[a]
[b]
Mark Douthwaite, Natasha Powell, Aoife Taylor, Grayson Ford, José Manuel López,
[c]
[a]
[b]
[a]
[a]
Benjamin Solsona, Nating Yang, Olga Sanahuja-Parejo, Qian He, David J. Morgan,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
[b]
[a]
Tomas Garcia,* and Stuart H. Taylor*
2
A high surface area mesoporous TiO material (110 m /g) was
spectroscopy and electron energy loss spectroscopy. It was
proposed that the residual Si present increases the Brønsted
2
synthesised using a nanocasting methodology, utilizing SBA-15
as a hard template. This material was subsequently used as a
support to prepare a series of 1 wt.% AuPt/TiO2 catalysts,
synthesised by conventional impregnation and sol-immobilisa-
tion. Catalysts were tested for the oxidation of glycerol to lactic
acid and their performance was compared with corresponding
catalysts supported on TiO À P25, TiO -anatase and TiO -rutile.
acidity of the TiO support, which can lead to the formation of
2
Lewis acid sites under reaction conditions; both sites are known
to catalyse the dehydration of a primary alcohol in glycerol.
Typically, under alkaline conditions, lactic acid is formed by the
nucleophilic abstraction of a hydrogen. Thus, we propose that
2
2
2
the improved selectivity to lactic acid over the nanocast TiO2
Higher rates of reaction and higher selectivity to lactic acid
were observed over nanocast TiO2 supported catalysts. The
increased performance of these catalysts was attributed to the
presence of Si on the surface of the support, which likely arose
from inefficient etching of the SBA-15 template. The presence
of Si in these catalysts was confirmed by X-ray photoelectron
supported catalyst is attributed to the co-operation of hetero-
geneous and homogeneous dehydration reactions, as both
compete directly with a direct oxidation pathway, which leads
to the formation of oxidation products such as glyceric and
tartronic acid.
Introduction
sustainable routes to liquid fuels and chemical feedstocks.
Indeed, the highly functionalised nature of glycerol means that
numerous different strategies can be employed for its up
grading. For liquid phase valorisation, some of the most notable
developments have utilized heterogeneous catalysis for the
The efficient valorisation of glycerol to high-value products is of
interest to both academics and industrialists.
major by-product formed in the production of first generation
biofuels; an industry which is projected to grow in the coming
years and is driven by the demand for alternative and
[1,2]
Glycerol is a
[3,4]
[5]
hydrogenolysis or oxidation of glycerol, both of which have
displayed promising potential to date.
The selective aerobic oxidation of glycerol over supported
Au containing catalysts has been widely investigated over the
past 20 years. Glycerol is an ideal model substrate to
investigate the physicochemical properties of such catalysts
due to its highly functionalised nature. Much of the early work
investigating supported Au catalysts for liquid phase oxidation
[a] Dr. M. Douthwaite, N. Powell, A. Taylor, G. Ford, Dr. N. Yang, Dr. Q. He,
[6]
Dr. D. J. Morgan, Prof. S. H. Taylor
Cardiff Catalysis Institute
School of Chemistry
Cardiff University
Main Building
Park Place
[
7,8]
reactions was conducted by Prati, Rossi and co-workers.
Cardiff, CF10 3AT (UK)
E-mail: taylorsh@cardiff.ac.uk
b] Dr. J. M. López, O. Sanahuja-Parejo, Dr. T. Garcia
Instituto de Carboquímica (ICB-CSIC)
C/Miguel Luesma Castán
50018 Zaragoza (Spain)
E-mail: tomas@icb.csic.es
c] Dr. B. Solsona
Departament d’Enginyeria Química, ETSE
Universitat de València
Subsequently, many reports have been published investigating
the use of mono-, bi- and even tri-metallic supported
[
[
[9–12]
catalysts,
which has led to a greater understanding of
properties that drive the selectivity in this reaction. In addition,
contributions from Davis and co-workers supplemented this
understanding by conducting a series of experiments, which
À
elegantly confirmed the roles of H O, OH and O2 in this
2
[13]
reaction over Au nanoparticles. To date, it is widely acknowl-
edged that reaction selectivity in this system can be controlled
Av. Universitat
46100 Burjassot, Valencia (Spain)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202000026
by a number of differing catalytic properties, including: the
[14]
noble metal(s) particle size
support.
and the properties of the
©
2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This
[15]
is an open access article under the terms of the Creative Commons Attri-
bution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
Initially, emphasis was placed on the design and synthesis
of heterogeneous catalysts, which could selectively oxidise
ChemCatChem 2020, 12, 1–12
1
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Full Papers
doi.org/10.1002/cctc.202000026
ChemCatChem
glycerol to glyceric (GLA) and tartronic acid (TA), avoiding any
GLA, or dehydration to produce pyruvaldehyde (PALD). Given
that these two pathways are considered to be in competition, it
is somewhat unsurprising that higher yields of LA are typically
observed when the temperature and the pH of the aqueous
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
CÀ C cleavage, which have previously been associated with the
[6,16,17]
formation of H O during the reaction.
Recently however,
2
2
much more attention has been given to the formation of lactic
[18,19]
[22]
acid (LA) from glycerol.
LA has a number of different
solution are increased.
applications, perhaps the most notable is its use as a feedstock
for the formation of polylactic acid. Many prominent strategies
have been adopted for this transformation, which have been
elegantly surveyed in a recent review by Belousov and co-
In addition to the reaction conditions, the physicochemical
properties of the catalyst can have a significant effect on the
[5,6,15]
selectivity of each reaction pathway.
Recent work by Evans
[24]
et al. revealed how influential the support material was for
directing the reaction selectivity under basic conditions. It was
determined that the oxygen adsorption capacity of La contain-
ing perovskite supports for AuPt nanoparticles had a significant
influence on the reaction selectivity. Alloyed supported AuPt
nanoparticles have emerged as promising catalyst for the
aerobic oxidation of glycerol to LA under alkaline conditions.
The high performance of these catalysts has been attributed to
synergistic interactions between the two metals. The incorpo-
ration of Pt is considered to assist with the base catalysed
dehydration of the reaction intermediate, and also improves
[18]
workers. > One such strategy involves the use of supported
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
[20–23]
noble metal catalysts under alkaline conditions,
typically
leading to the formation of the corresponding lactate salt. It is
important to note that any acid products formed in these
reactions under alkaline conditions will likely exist as the
conjugate salt.
Given the abundance of research in this area, there is a very
good understanding of how each of the products are formed;
see Scheme 1. Glycerol first undergoes an oxidative dehydro-
genation reaction to produce either glyceraldehyde (GLD) or
dihydroxyacetone (DHA). Under alkaline conditions, these
substrates are rarely observed, indicating that they are rapidly
consumed in a sequential reaction. The GLD and DHA formed
subsequently either undergo a sequential oxidation to produce
[23]
catalyst stability.
Alternative strategies have also been developed for the
synthesis of LA from glycerol under base-free conditions. Under
such conditions, a strong Bronsted acidic material is typically
Scheme 1. Reaction scheme for the liquid phase aerobic oxidation of glycerol over noble metal supported catalysts. Key: 1. Glycerol; 2. Glyceraldehyde; 3.
Dihydroxyacetone; 4. Glyceric acid; 5. Tartronic acid; 6. Glycolic acid; 7. Formic acid; 8. Oxalic acid; 9. Pyruvaldehyde; 10. Lactic acid; 11. Pyruvic acid; 12. Acetic
acid.
ChemCatChem 2020, 12, 1–12
www.chemcatchem.org
2
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Full Papers
doi.org/10.1002/cctc.202000026
ChemCatChem
[
18]
employed as either a support or as a co-catalyst.
In the
washed, dried for 16 h at 100°C and calcined at 550°C in air for 6 h.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
Secondly, ordered mesoporous TiO was synthesized using the hard
template method reported previously. In a typical preparation,
absence of base, this material serves as a catalyst for both the
dehydration of GLD and intramolecular rearrangement of PALD
to LA. Subsequent work has evidenced that Lewis acidity is
affective at catalysing both the dehydration and rearrangement
2
[32]
titanium(IV) isopropoxide (12 mmol) was dissolved in acetone
(15 mL). After the solution became clear, SBA-15 hard template
(2 g) was added and the mixture was stirred for 2 h, the solvent
[25–27]
reactions.
While this approach is undeniably more desirable
was then evaporated. In order to achieve higher loadings, the
above dried powder was calcined at 200
°
C for 6 h with a heating
from an industrial perspective, it too has limitations. Perhaps
the most pertinent of these is the substantially lower rates of
reaction which are commonly observed under base-free
À 1
ramp rate of 1°Cmin to decompose the metal precursor, and
then the impregnation step was repeated, but the amount of
titanium(IV) isopropoxide was reduced to 8 mmol. The resulting
sample was calcined in air at 500°C for 4 h with the same heating
ramp rate to completely decompose the inorganic precursor.
Finally, the silica template was removed using a NaOH (2 M, 20 mL/
À
conditions. This is because the OH plays a crucial role in the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
activation of the CÀ H bond, widely accepted to be the rate
determining step in the oxidation of glycerol over supported
[13]
g
cat) aqueous solution at 70
°
C. In some cases, three additional
washing steps were conducted on the TiO À NC material; abbre-
heterogeneous catalysts. A number of studies have shown
that improved rates of reaction can be achieved by increasing
2
[
28,29]
viated in the text as TiO À NCÀ W.
2
the reaction temperature and partial pressure of O2.
Heeres
[
30]
and co-workers subsequently developed this further; in the
presence of methanol and a Lewis acidic co-catalyst, the
authors demonstrated that appreciable yields of methyl lactate
could be produced from glycerol.
Au and Pt NP’s were deposited onto the surface of the TiO
supports by conventional impregnation (IMP) and sol-immobilisa-
tion (SI) methods. For preparation by impregnation, HAuCl4
(0.205 mL, 12.25 g/L) and H PtCl (0.513 mL, 4.85 g/L) were added
to a small beaker containing H O (0.782 mL). With stirring, the TiO2
2
2 6
2
In this study, we investigate how the use of a nanocast TiO2
support (0.495 g) was subsequently added to the beaker and the
solution was heated to 80°C. The solution was carefully monitored
support can influence the reaction selectivity of AuPt nano-
particles in the aerobic oxidation of glycerol to LA under
comparatively mild, alkaline conditions. Herein, we demonstrate
and removed from the heat when the consistency of the mixture
resembled that of a thick paste. The paste was then dried for 16 h
at 110°C. The catalyst was then calcined at 200°C (ramp rate=2°C/
that the application of a nanocast mesoporous TiO support for
2
min) in a tubular furnace under flowing air (50 mL/min) for 4 h.
AuPt nanoparticles can result in a higher reaction selectivity to
LA during the oxidation of glycerol, when compared to a
For preparation by sol-immobilisation; HAuCl (0.205 mL, 12.25 g/L)
and H PtCl (0.513 mL, 4.85 g/L) were added to a beaker containing
4
2
6
corresponding AuPt/TiO -catalyst prepared using conventional
2
H O (200 mL). PVA (0.325 mL, from 0.1 g PVA in 10 mL H O 80%
2
2
titania.
hydrolysed) was then added and left to stir for 20 minutes. NaBH4
(
1.964 mL) was added to reduce the metals and stirred for 30
minutes. TiO (0.495 g) was subsequently added and then acidified
to pH 2 with drop-wise addition of concentrated H SO and stirred
2 4
for an additional 1 h. The resulting solution was then filtered under
vacuum and washed with distilled water (500 mL), before the
residue was dried for 16 h at 110°C.
2
Experimental
Chemicals, Source and Purity
Acetic acid (Sigma-Aldrich, �99.7%); Acetone (Scharlau, analytical
grade); Chloroauric acid (Strem, 99.8%); Chloroplatinic acid hexahy-
drate (Sigma Aldrich, ACS reagent, 37.5% Pt basis); Formic acid
Catalyst Characterisation
(
(
Sigma Aldrich, �98%); Glyceric acid (TCI, 40% in water); Glycerol
BET analysis was conducted on a Quantachrome Quadrosorb
instrument; the samples were first degassed at 200°C for 4 h. Once
degassed, 25 point nitrogen adsorption isotherms were collected at
À 196°C, and data analysed using the BET method.
Sigma-Aldrich, �99.5%); Glycolic acid (Sigma-Aldrich, 99%);
Hydrochloric acid (Scharlau, 37% reagent grade); DL-Lactic Acid
(Sigma-Aldrich, 85%); Oxalic acid (Sigma-Aldrich, �99.99%);
Phosphoric Acid (Sigma-Aldrich, (85 wt.% in H O) �99.99%);
2
Pluronic P123 (Mw=5800, EO20PO70EO20) (Sigma-Aldrich); Poly-
Powder X-ray diffraction (XRD) was acquired using a PANalytical
X’Pert Pro system with a Cu Kα X-ray source operated at 40 kV and
40 mA. An X’Celerator detector was used with data collected from
2θ=10° to 80° for 30 minutes collection time.
vinylalcohol (Sigma-Aldrich, 80% Hydrolysed); Sodium Borohydride
(
Sigma-Aldrich, 99.99%); Sodium Hydroxide (Fischer Scientific,
9.3%); Tartronic acid (Sigma-Aldrich, �97%); Tetraethoxysilane
9
(TEOS) (Sigma-Aldrich, >99%); Titania, Anatase (Sigma-Aldrich,
99.7%); Titainia P.25 (Degussa,�99.5%,); Titainia, Rutile (Sigma-
Aldrich, 99.995%); Titanium(IV) isopropoxide (Sigma-Aldrich, 97%);
Water, (Fisher Scientific, HPLC grade).
X-ray Photoelectron Spectroscopy (XPS) measurements were per-
formed using
monochromatic Al Kα source (hν=1486.6 eV) operating at 140 W
14 kV and 10 mA). Charge neutralisation was achieved using the
a Kratos Axis Ultra DLD instrument using a
(
Kratos immersion lens system and calibration of binding energies
was conducted against the C1s line for adventitious carbon, taken
to be 284.8 eV. High-resolution and survey scans were performed at
energies of 20 and 160 eV respectively. Data were analysed using
CasaXPS (v2.3.23) using modified Wagner sensitivity factors, as
supplied by the manufacturer.
Catalyst Synthesis
Nanocast TiO (TiO À NC) was synthesised using a hard-templating
2
2
methodology. First, ordered mesoporous silica SBA-15 template
with a rod-like morphology was prepared according to the
[
31]
synthetic process reported in the literature. In a typical synthesis,
P123 (2 g) was dissolved in HCl (75 mL, 2 M) solution with stirring,
followed by addition of TEOS (4 mL) to the homogeneous solution.
This gel was stirred at 40°C for 24 h, and then crystallized at 100°C
for 24 h under static conditions. The resulting solid was filtered,
Ammonia temperature programmed desorption (NH À TPD) experi-
3
ments were conducted on a Quantachrome ChemBET Chemisorp-
tion analyser equipped with a TCD. The titanium supports (100 mg)
were first heated to 130 C (15 C/min) under He (30 mL/min) and
°
°
ChemCatChem 2020, 12, 1–12
www.chemcatchem.org
3
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Full Papers
doi.org/10.1002/cctc.202000026
ChemCatChem
held at temperature for 1 h. After cooling to 30°C, the sample(s)
quantified using an Agilent HPLC system. HPLC analysis was carried
out using refractive index and diode array detectors. Reactants and
products were separated over a Metacarb 67H column at a
temperature of 50°C. An isocratic mobile phase (H PO , 0.1% in
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
were exposed to NH for 0.5 h at a flow rate of 30 mL/min, after
3
which, the samples were heated to 1000 C (10 C/min) under He
(30 mL/min).
°
°
3
4
H O) was used with the flow rate fixed at 0.8 mL/min. Products
2
Microscopy was performed using a Tescan Maia3 field emission gun
scanning electron microscope (FEG-SEM) operating at 30 kV. Images
were acquired using the secondary electron and backscattered
electron detectors. Bright field scanning transmission electron
images (STEM) were obtained using a STEM holder. Samples were
dispersed as a powder onto 300 mesh copper grids coated with
were identified and quantified using an external calibration
method. Response factors were obtained by calibration against
authentic standards.
Holey carbon film. Transmission electron microscopy (TEM) was Results and Discussion
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
carried out using a JEOL JEM-2100 microscope with a LaB6 filament
operating at 200 kV. Powdered catalyst samples were dry dispersed
onto lacy carbon films over a 400 mesh copper grid. High angle
annular dark field scanning transmission electron microscopy
As discussed previously, AuPt/TiO À P.25 catalysts have been
2
demonstrated to be selective for the formation of LA from
[23]
glycerol. Given that the application of mesoporous support
materials can have a significant impact on both selectivity and
(HAADF-STEM) data were acquired using a JEOL ARM200CF micro-
scope in the electron Physical Science Imaging Centre in Diamond.
The instrument was operated at 200 kV and the electron energy
loss spectroscopy was carried out using a Gatan Quantum Dual
EELS spectrometer. Particle size distributions were calculated from
the analysis of 100 particles for a given catalyst, using ImageJ.
Fourier Transform Diffuse Reflectance Infrared (DRIFT) spectroscopic
[34]
activity in liquid phase oxidation reactions, it is important to
establish whether the application of such materials as supports
for noble metal catalysts provide a promotional benefit in this
reaction. As such, SBA-15 was used as a hard template for the
formation of mesoporous-TiO . TEM and N sorption experi-
analysis of these TiO -based catalysts was performed on a VERTEX
2
2
2
7
0 Bruker spectrometer using a reaction chamber. Pyridine was
ments were conducted to probe the structural properties of this
material (Figure 1).
used as a probe molecule for the qualitative and quantitative
determination of the acidity of these materials. For these experi-
ments, each catalyst was first heat-treated at 450°C for 1 h in an N
2
stream to remove any physisorbed compound. Subsequently, the
samples were cooled to 50°C and pyridine vapour was introduced
to the cell for 1 hour. The physically adsorbed pyridine was
removed by flushing N for 1 h. The sample was heated under a N
2
2
flow to different desorption temperatures (50, 100, 200, 300 and
4
00°C). Spectra were collected for each material at these temper-
À 1
À 1
atures in the range of 2000–1350 cm with a resolution of 4 cm ,
obtained by averaging of 128 scans.
À 1
Quantification was performed in the 1600–1400 cm range using
[
33]
the following equations:
2
AB pD
CB ¼
4meB
(1)
(2)
2
C ¼ A
pD
4me
L
L
L
À 1
where A and A are the integrated zone of 1540 and 1450 cm for
B
L
the Brønsted and Lewis sites respectively. ɛ and ɛ are the molar
B
L
extinction coefficients at those wavelengths taken from the
literature, which are respectively 1.67 and 2.22 cm/μmol. D is the
diameter of the powder in the reaction chamber and m is the mass
of the sample used.
Catalyst Testing
A 50 mL Colaver glass reactor was placed in a heated oil bath
containing a stirrer bar (110°C, 800 RPM) and allowed to equilibrate
for 30 minutes. To this, catalyst (substrate: AuPt molar ratio=
4055:1), NaOH (10 mL, 2.4 M), glycerol (10 mL, 0.6 M) and a
magnetic stirrer was added to the reactor, before it was sealed,
purged 5 times and charged with O2 (3 bar). The reaction was
stirred for the desired time (0.5–7.5 h) at 800 rpm. Once the
reaction was complete, the reactor vessel was placed in an ice bath
to cool for 15 minutes before depressurising the reactor. Two
Figure 1. (A) TEM image and diffraction pattern of the TiO
2
À NC material; (B)
0.5 mL samples were subsequently withdrawn and diluted 10 fold
N sorption isotherms and BET SA measurements for the TiO À NC (red) and
2
2
in H O to quench the reaction, after which they were filtered and
TiO
2
À P25 (green).
2
ChemCatChem 2020, 12, 1–12
www.chemcatchem.org
4
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Full Papers
doi.org/10.1002/cctc.202000026
ChemCatChem
The TEM image displayed in Figure 1(A) confirmed that the
which is typically nanocrystalline in nature. The type (IV)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
nanocast TiO (TiO À NC) did not exhibit a well-defined meso-
character in the sorption isotherm of the TiO À NC support is
2
2
2
porous structure, despite mesopores in SBA-15 being typically 3
to 7 nm in diameter. The material consisted of exceptionally
evidence of mesoporosity, although TEM confirmed that the
replication of the SBA-15 porous structure was not achieved.
small TiO agglomerates; <30 nm in size. The perceived lack of
TiO À P25 consists of both rutile and anatase phases with a
2
2
[35]
an ordered mesoporous structure is likely to be a result of the
interparticle mesoporosity created between the small TiO2
crystallites. Despite this, evidence of mesoporosity can be
ratio of approximately 1:3, respectively. To establish whether
the phase of the TiO support material influenced the catalytic
2
performance, two additional 1 wt.% AuPt catalysts, supported
deduced from the corresponding N sorption isotherm (Figure 1
on TiO anatase (TiO À A) and TiO rutile (TiO À R) were prepared
2
2
2
2
2
(
B)). Whilst the isotherm does exhibit some type (IV) sorption
character, it is poorly defined, albeit exhibiting a very different
by impregnation. The structure of all four support materials
were subsequently investigated by XRD and the corresponding
diffraction patterns for each are displayed in Figure 2. The
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
adsorption and desorption curve to that exhibited by TiO À P25,
2
TiO À NC material predominantly consisted of anatase. However,
2
a diffraction peak at ca. 2θ=27°, which is characteristic of a
rutile (110) reflection, confirmed that the NCÀ TiO contained a
2
small proportion of the rutile phase.
The surface area of all four supports was measured by BET,
which confirmed that there were significant differences in the
specific surface area of the materials (Table S1). The active metal
dispersion in catalysts prepared by conventional impregnation
can be dramatically affected by the total surface area of the
support. Consequently, the dispersion of Au and Pt on each of
the catalysts after impregnation was investigated by SEM
(Figure S1). The particle size distribution (PSD) of metal particles
on the surface of these materials differed significantly. The
mean particle sizes for the AuPt/TiO À NC, AuPt/TiO À P25, AuPt/
2
2
TiO À A and AuPt/TiO À R were determined to be 11.0, 49.3, 38.6
2
2
and 20.6 nm respectively (Table 1, Figure S1).
All four catalysts were subsequently tested for the aerobic
oxidation of glycerol. The results from these experiments are
displayed in Table 1. The reactions were conducted under
previously optimised reaction conditions for LA production
[22]
from glycerol over Pt/CeO2 catalysts.
A highly alkaline
medium (1.2 M) is required, as it promotes the dehydration
pathway to LA; assisting with both the activation of the CÀ H
and OÀ H bonds and promoting the intramolecular rearrange-
ment of PALD. To ensure reactions were conducted under
optimum conditions, the influence of reaction temperature on
lactic acid selectivity was investigated (Table S2). The experi-
ments confirmed that the optimum temperature for LA
formation was 110°C, which aligns with the findings of Heeres
Figure 2. X-ray diffraction over the various TiO
P25À TiO , (b) AÀ TiO , (c) RÀ TiO and (d) NCÀ TiO
*).
2
support materials; (a)
2
. Key: Anatase (~); Rutile
2
2
2
[22]
(
and co-workers, who showed that increasing the reaction
Table 1. The aerobic oxidation of glycerol over a series of supported AuPt catalysts prepared by impregnation. The glycerol conversion (χ), product
selectivity and CMB listed correspond to reactions run for 0.5 and 7 h over each of the catalysts.
Catalyst
Mean Particle Size [nm]
Time [h]
χ Glycerol [%]
Selectivity [%]
CMB [%]
LA
GLA
TA
GLY
OXA
PA
AA
FA
AuPt/TiO
AuPt/TiO
AuPt/TiO
AuPt/TiO
2
À P25
À NC
À A
À R
49.3
11.0
38.6
20.6
0.5
7.0
0.5
6
72
61
84
82
38
31
76
77
14
15
11
6
22
29
8
2
5
1
5
2
3
1
3
7
12
3
0
0
0
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
4
6
2
2
14
15
5
X
103
X
102
X
95
X
82
10
99
3
26
9
2
7.0
4
2
0.5
7.0
0.5
24
22
10
10
2
7.0
92
5
5
98
Reaction Conditions: Reaction solution (20 mL, 0.3 M glycerol, 1.2 M NaOH); 3 bar O
acid), TA (tartronic acid), GLY (glycolic acid), OXA (oxalic acid), PA (pyruvic acid), AA (acetic acid); FA (formic acid); TiO
anatase) and R (rutile).
2
; 110°C; catalyst (0.029 g); time=stated. LA (lactic acid), GLA (glyceric
À X; NC (nanocast), P25 (titania P25), A
2
(
ChemCatChem 2020, 12, 1–12
www.chemcatchem.org
5
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Full Papers
doi.org/10.1002/cctc.202000026
ChemCatChem
temperature increased LA selectivity over a AuPt/CeO catalyst.
The supported metal particle size is another parameter
2
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
[
14]
At temperatures in excess of 110°C, LA is transformed to further
oxidised products and is more susceptible to undergo CÀ C
cleavage.
which can influence reaction selectivity in this reaction. As
metal nanoparticles reduce in size their electronic properties
can change, influencing the binding energy of substrate to the
[36]
The AuPt/TiO À NC catalyst was the most active, exhibiting
active sites on the nanoparticle. This in turn, can effect the
activity and selectivity of a catalyst. It was therefore important
to ensure that the observed differences in reaction selectivity
were not merely attributed to a particle size effect, a parameter
which is typically influenced by the total surface areas of the
support materials. Given that the surface areas of these four
supports differed quite substantially (Table S1), it was important
that an alternative preparation method was used to provide a
similar dispersion of the AuPt nanoparticles. For this reason,
two additional catalysts were prepared by sol immobilisation; a
technique commonly employed to prepare supported metal
catalysts with well-defined and narrow particle size distribu-
tions. This technique relies on the use of a stabilising agent to
control the size of colloidal AuPt particles which are formed
during metal reduction; see the experimental section for further
details. In this study, polyvinyl alcohol (PVA) was used as the
stabilising agent, as it has been demonstrated to provide a
2
glycerol conversions of 10 and 99% after 0.5 and 7 h of
reaction, respectively. Interestingly, the AuPt/TiO À NC material
2
also exhibited a high reaction selectivity to LA (82%) at close to
complete glycerol conversion. Relatively high reaction selectiv-
ity to LA was observed with the AuPt/TiO À R and AuPt/
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
TiO À P25 catalysts; a LA selectivity of 77 and 61% were
2
observed at glycerol conversions of 92 and 82%, respectively.
The AuPt/TiO À A catalyst exhibited the lowest conversion and
2
lowest selectivity to LA. It is therefore interesting that the AuPt/
TiO À NC catalyst, which comprises predominantly of anatase,
2
was selective to LA. For each reaction, the by-products were
representative of those formed from the direct oxidation route
(see Scheme 1). For each of the catalysts, no pyruvic acid (PA),
or acetic acid (AA), was observed, indicating that it was unlikely
that any LA was consumed through sequential reactions.
Given that the catalytic activity was so different after 7 h, it
was important to investigate the reaction selectivity over each
of the catalysts at low conversion. The same trend in catalytic
activity was observed for the series of catalysts after 0.5 h and
the highest reaction selectivity to LA was once again exhibited
[37]
good metal dispersion, regardless of the support. Character-
isation by FEG-SEM (Figure S2) confirmed that the particle size
distribution of these catalysts was indeed more uniform; the
mean particle sizes for the 1 wt.% AuPt/TiO À P.25 and 1 wt.%
2
SI
by the AuPt/TiO À NC catalyst. Indeed, the selectivity to LA over
AuPt/TiO À NC were determined to be 4.4 and 3.4 nm in
2
2
SI
this catalyst at 0.5 h is remarkably similar to the LA selectivity
observed at full conversion, indicating that there is no change
in the rate of each reaction pathway over the duration of the
reaction. For all the catalysts, the proportion of GLA to TA is
higher at low conversion, which is unsurprising given that TA is
a sequential oxidation product of GLA. Perhaps most interest-
ingly, at iso-conversion the selectivity to LA over the AuPt/
diameter, respectively. In order to establish how this change in
particle size from impregnation to sol immobilisation influenced
their catalytic properties, both were tested for the oxidation of
glycerol under the same conditions used previously. The results
from these experiments are displayed in Table 2. To ensure that
any differences in catalytic performance were not attributed to
slight variations in the catalyst metal loading, a known quantity
of each catalyst was digested in aqua regia and the Au and Pt
quantified by MP-AES. The Au and Pt metal loadings were all
considered to be comparable (Table S3).
TiO À NC catalyst is appreciably higher than that observed even
2
over the AuPt/TiO À R catalyst. The observation that the AuPt/
2
TiO À NC catalyst is more selective to LA, albeit comprising
2
almost entirely of anatase, is of great interest, given that the
Close to full glycerol conversion was observed over both
catalysts after 7 h of reaction. The reaction selectivity to LA over
the AuPt/TiO À NC catalyst remains higher than that observed
corresponding catalyst prepared on TiO À A exhibited such poor
2
selectivity to LA. This indicates that the AuPt/TiO À NC catalyst
2
2
SI
possesses some other physicochemical property which favours
the formation of LA.
over the corresponding catalyst supported on TiO À P25 ;
2
SI
reaction selectivity to LA of 71 and 69% were observed over
Table 2. The aerobic oxidation of glycerol over a series of supported AuPt catalysts prepared by sol immobilisation. W corresponds to an additional pre-
washing step conducted on the support material prior to immobilisation with Au and Pt. The glycerol conversion (χ), product selectivity and CMB listed
correspond to reactions run for 0.5 and 7 h over each of the catalysts.
Catalyst
Mean Particle Size [nm]
Time [h]
χ Glycerol [%]
Selectivity [%]
CMB [%]
LA
GLA
TA
GLY
OXA
PA
AA
FA
AuPt/TiO
AuPt/TiO
AuPt/TiO
AuPt/TiO
2
À P25
À P25À W
À NC
À NCÀ W
4.4
–
0.5
7.0
0.5
18
100
17
100
18
100
18
66
69
67
69
68
71
68
66
25
7
24
1
18
1
15
0
5
5
3
5
6
4
5
3
2
0
1
0
2
0
0
0
2
0
1
0
1
0
0
0
1
0
1
0
2
0
1
0
2
4
1
3
2
4
3
6
8
x
98
x
96
x
99
x
2
7.0
5
2
3.4
–
0.5
7.0
0.5
25
16
22
2
2
1
18
7.0
100
98
Reaction Conditions: Reaction solution (20 mL, 0.3 M glycerol, 1.2 M NaOH); 3 bar O
acid), TA (tartronic acid), GLY (glycolic acid), OXA (oxalic acid), PA (pyruvic acid), AA (acetic acid); FA (formic acid); TiO
exposed to an additional washing procedure with NaOH).
2
; 110°C; catalyst (0.029 g); time=stated. LA (lactic acid), GLA (glyceric
À X; NC (nanocast), W (support material
2
ChemCatChem 2020, 12, 1–12
www.chemcatchem.org
6
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Full Papers
doi.org/10.1002/cctc.202000026
ChemCatChem
these two catalysts respectively. At low conversion (0.5 h of
reaction), the activity of both catalysts appears to be compara-
ble, but the selectivity to LA appears to be slightly higher over
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
the AuPt/TiO À NC catalyst. Given that higher reaction selectiv-
2
SI
ity to LA is observed with the AuPt/TiO À NC catalysts prepared
2
by impregnation and sol-immobilisation, compared to the
corresponding AuPt/TiO À P25 catalysts, it is unlikely that the
2
enhanced LA selectivity is solely attributed to a particle size
effect. Therefore, additional characterisation of these materials
was conducted so that the origin of the effect could be
established.
Both catalysts were studied by XPS, the corresponding
spectra are displayed in Figure 3. Fitting of Au4f and Pt4f
regions confirmed that Au and Pt were present in the metallic
state, which is unsurprising given the excess of reducing agent
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
(
NaBH ) used in their preparation. Inspection of the Si2p region
4
confirmed the presence of Si in the AuPt/TiO -NC catalyst
2
SI
(
Figure 3, Table S3), with a binding energy (BE) of 102.1 eV.
4
+
Typically, Si in the form of SiO (c.f. SBA-15) is evidenced by a
2
peak at a BE of ca. 103.5 eV. A lower BE, as observed here, is
typically attributed to the loss of oxygen coordination of the Si
centre and is attributed to residual SBA-15 or isolated regions
of SiO . For lower concentrations of Si, the BE is observed to
x
shift even lower (up to ca. 0.7 eV) but all other peaks remain
within the limits of experimental uncertainty (ca. 0.2 eV). The
presence of surface Si in the AuPt/TiO -NC catalyst was also
2
evidenced by HAADF-EELS (Figure 4). The AuPt metal particles
can be clearly visualized using the atomic number Z contrast
high angle annular dark field (HAADF) imaging (Figure 4 (a–d)).
Further evidence for the residual Si in the surface layer can be
detected using electron energy loss spectroscopy (EELS). As
Figure 3. X-ray photoelectron spectroscopy data corresponding to Si2p,
Au4f and Pt4f regions of different 1 wt.% AuPt/TiO catalysts; (A) AuPt/
2
NCÀ TiO
2
, (B) AuPt/NCÀ TiO
2
À W, (C) AuPt/P25À TiO
2
and (D) AuPt/P25À TiO À W.
2
W corresponds to the additional washing of the support with NaOH (2 M),
prior to immobilisation with AuPt nanoparticles.
Figure 4. Relatively lower and higher magnification STEM-HAADF images of (a, b) AuPt/TiO NC and (c,d) AuPt/TiO NC W catalysts. (e) Electron energy loss
spectroscopy (EELS) spectra, obtained by parking scanning a small area near the edge of the TiO
2
2
2
particle at locations labeled as A and B shown in images (c)
2
and (d), respectively. A weak Si signal were identified on the AuPt/TiO À NCSI, but not on the AuPt/TiO À NCÀ WSI, which has undergone additional NaOH
washing.
2
ChemCatChem 2020, 12, 1–12
www.chemcatchem.org
7
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Full Papers
doi.org/10.1002/cctc.202000026
ChemCatChem
shown in Figure 4 (e). When scanning the near edge area of the
nanocast TiO2 particles, a week signal of Si K edge at ca.
catalyst is further evidenced by evaluating the reaction
selectivity toward TA and GLA at full conversion. For the AuPt/
TiO À NCÀ W , AuPt/TiO À P25 and AuPt/TiO À P25À W catalysts,
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
1
845 eV can be detected. However, for the AuPt/TiO -NC-W
2 SI
2
SI
2
SI
2
SI
catalyst, after additional washing by NaOH, this signal was no
longer detected. Confirmation that traces of surface Si are
present in this sample is unsurprising, as SBA-15 was used as a
a substantially higher selectivity to TA was observed, at the
expense of GLA. This suggests that the AuPt/TiO À NC catalyst
2
SI
is also not as effective for the sequential oxidation of GLA to TA.
The comparable product distributions observed for the 7 h
reactions over AuPt/TiO À P25 and AuPt/TiO À P25À W indicate
hard template in the production of the TiO -NC material, before
2
being removed by dissolution. Evidently, the etching process
does not result in total removal of the casting template leaving
2
SI
2
SI
that the differences between the NC and NCÀ W supported
catalysts are not merely a function of the additional washing
procedure.
some Si embedded in the TiO surface.
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
To determine whether the presence of surface Si in this
catalyst was responsible for the higher selectivity to LA
observed over the 1 wt.%AuPt/TiO À NC catalyst, the TiO À NC
It has previously been suggested that under alkaline
conditions, the dissociation of the OÀ H bond occurs in solution
2
SI
2
[13]
material was subjected to three additional washing steps with
NaOH (2 M), prior to immobilisation of Au and Pt nanoparticles
prior to adsorption onto the surface of the catalyst. Scheme 2
illustrates how different surface mechanisms, which dictate the
reaction selectivity, occur. Competitive dehydration and oxida-
tion pathways proceed from GLD (2.). For the direct oxidation
pathway, GLD binds to the surface of the catalyst and under-
(see experimental section for further details). This support was
subsequently used to prepare an additional catalyst by SI;
annotated to AuPt/TiO À NCÀ W . This catalyst and a correspond-
2
SI
À
ing P25 supported control catalyst (AuPt/TiO À P25À W ) were
goes nucleophilic attack from OH, leading to the formation of
2
SI
also studied by XPS (Figure 3), and confirmed the additional
washing step had removed a substantial quantity of the Si from
the surface of the NC material (Figure 3, Table S4). Residual
quantities of Si can be observed in the XPS spectra of the other
three catalysts, but the proportion of Si in these three catalysts
was comparable, and most likely originates from Si impurities in
the samples. To establish what influence the surface Si
exhibited on the catalytic performance, the washed catalysts
were also investigated for the aerobic oxidation of glycerol
a germinal diol surface intermediate. Subsequently, abstraction
of the remaining hydrogen atom from the α-carbon by an
À
additional OH anion leads to the formation of GLA (3.). The
remaining primary alcohol in GA can subsequently undergo the
same series of reactions, ultimately resulting in the formation of
TA. Oxygen has an indirect role in this reaction. It has been
demonstrated previously that the partial pressure of O has no
2
influence on the reaction selectivity and only influences the
[22]
rate of glycerol consumption. Davis and co-workers previ-
(Table 2).
ously proposed that the role of O in such systems is to serve as
2
The removal of Si from the surface of the TiO À NC material
an electron scavenger; O is reduced by H O in a parallel surface
2
2
2
[13]
clearly influenced the reaction selectivity. LA selectivity of 71
reaction. This process consumes the electrons generated in
dehydrogenation reactions, liberating actives sites and allowing
additional substrate to interact with the surface. This explains
why the rate of reaction under inert conditions is often
and 66% was observed for the AuPt/TiO À NC and AuPt/
2
SI
TiO À NCÀ W catalysts respectively, after 7 h of reaction. The
2
SI
reduction in selectivity to this pathway over the AuPt/TiO À NC
2
SI
2
Scheme 2. Proposed reaction mechanism for the aerobic oxidation of glycerol over AuPt/TiO catalysts under alkaline conditions. The mechanism provides
insight into the competitive reaction pathways available; the direct oxidation pathway and the acidic and basic dehydration mechanisms. Key: 1. Glycerol; 2.
Glyceraldehyde; 3. Glyceric acid; 4. 2-Hydroxyacrylaldehyde; 5. Pyruvaldehyde; 6. Lactic acid; * Active site vacancy; ORR – Oxygen reduction reaction.
ChemCatChem 2020, 12, 1–12
www.chemcatchem.org
8
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Full Papers
doi.org/10.1002/cctc.202000026
ChemCatChem
significantly reduced and implies that oxygen incorporated into
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
À
the acid products does not come from O but OH.
2
Competing with the formation of GA are three dehydration
routes; a base promoted dehydration reaction, a Brønsted acid
catalysed dehydration reaction, and Lewis acid catalysed
dehydration reaction, which has been reported as being
[27]
effective for this specific dehydration previously.
In the
presence of NaOH, it is likely that many of the Bronsted acid
sites would be neutralised. The result of this neutralisation is
the exchange of the surface proton with a sodium cation,
leading to the formation of a Lewis acid site and a molecule of
water. Considering the dehydration reactions, we propose that
the base catalysed dehydration is dominant. Given that this
reaction occurs in solution and does not require interaction
with the heterogeneous catalyst, it is understandable why
reaction selectivity to LA increases with increasing NaOH
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
[22]
concentration. Each of these reactions lead to the formation
of 2-hydroxyacrylaldehyde (4.), which undergoes a proton
transfer to produce PALD (5.). From here, the PALD undergoes a
base catalysed rearrangement to produce LA (6.).
Figure 5. Pyridine DRIFT spectra for (A) TiO
TiO
À NCÀ W.
2
À P25, (B) TiO À NC and (C)
2
As discussed previously, the presence of Si on the surface of
the support clearly has a promotional effect on the formation of
LA. We propose that the enhanced selectivity to LA over the
2
AuPt/TiO À NC catalysts is a result of an increase in Brønsted
increased from 50°C to 400°C. There is no evidence to suggest
that the TiO À P25 and TiO À NCÀ W materials contain any
2
acid sites on the surface of the catalyst. To establish whether
2
2
this was indeed the case, NH À TPD experiments were con-
Brønsted acid sites. There is however evidence to suggest that
there is a substantial increase in the Lewis acidity in the
3
ducted on the TiO À P25 and TiO À NC materials (Figure S3). A
2
2
peak at ca. 500°C was observed in the experiment over the NC
TiO À NC material after the additional washing steps with NaOH,
2
4
+
material, which wasn’t observed with the TiO À P25. This
likely related to the presence of a higher amount of Ti centres
after Si removal.
2
evidences a difference in the acidic sites present on the
TiO À NC material. To further probe this phenomena, a series of
We propose that these additional acid sites are indicative of
a titanosilicate surface phase, formed from the inefficient
etching of the silicon template. Indeed, a number of previous
studies have confirmed that mixed metal oxides containing Si
2
in situ pyridine DRIFTS experiments were conducted on the
TiO À P25, TiO À NC and TiO À NCÀ W supports. DRIFT spectra for
2
2
2
the TiO À P25, TiO À NC and TiO À NCÀ W materials at 300°C are
2
2
2
[42–44]
displayed in Figure 5.
and Ti can consist of Brønsted acid sites.
Previous studies
À 1
Different bands were observed in the 1350–1700 cm
have confirmed that both Brønsted and Lewis acidic catalysts
can catalyse the dehydration of a primary alcohol in this
range, corresponding to the ring modes of CÀ C and CÀ N. The
[26,27,45]
frequency and sharpness of the strong absorbance appearing at
reaction.
This acid catalysed dehydration reaction also
À 1
1
445 cm are associated with pyridine molecules chemisorbed
competes with the direct oxidation pathway, thereby providing
an explanation to why higher reaction selectivity to LA is
at the Lewis acid sites (LÀ Py). The frequency of this mode is
affected by the interaction between the pair of solitary nitrogen
obtained in reactions over the AuPt/TiO À NC catalyst. Addi-
2
4
+
pyridine molecules and the Ti ions at the substrate surface.
tional washing steps with aqueous NaOH led to the further
À 1
The band at 1530 cm identifies the formation of pyridinium
removal of Si from the surface of the TiO À NC support (Figure 3
2
+
[38,39]
ions (PyH ) on the surface of the catalyst,
associated with
and Figure 4), and a resultant reduction in selectivity to LA
(Table 2), was observed. This provides further evidence for the
À 1
Brønsted acid sites. Finally, the low-intensity band at 1490 cm
is due to the two contributions of both Brønsted and Lewis acid
beneficial role of Si in the AuPt/TiO À NC catalyst. The co-
2
[40,41]
sites.
We can clearly observe in Figure 5, that whilst Lewis
operation of the two dehydration routes with the AuPt/TiO À NC
2
acid sites are detected in the TiO À P25, TiO À NC and
catalyst is why higher selectivity to LA is obtained when
2
2
TiO À NCÀ W materials, Brønsted acid sites are only apparent in
compared to the AuPt/TiO À P25 catalyst. To further evidence
2
2
the TiO À NC sample, and they are clearly removed after
this, additional experiments were conducted over the AuPt/
2
additional washing. Therefore, it can be tentatively proposed
that Brønsted acid sites are formed on the support surface due
TiO À NC and AuPt/TiO À P25 catalysts prepared by sol-immobi-
2
2
lisation, with reduced equivalents of NaOH (Table S6). Interest-
ingly, LA selectivity over of the NC supported catalyst appears
to reduce as the pH of the system is lowered, suggesting that a
co-operative effect between NaOH and the SiÀ NC exists. As
such, we must conclude that the improvements in selectivity to
to the interaction of TiO and the remaining Si. The quantity of
2
Brønsted and Lewis acid sites in each material was subsequently
quantified from these spectra and the corresponding data are
displayed in Table S5. The quantity of pyridine adsorbed on to
Lewis and Brønsted acid sites decreases as the temperature
ChemCatChem 2020, 12, 1–12
www.chemcatchem.org
9
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Full Papers
doi.org/10.1002/cctc.202000026
ChemCatChem
Table 3. High performing catalysts used for the aerobic oxidation of glycerol to LA, in the presence of NaOH. The corresponding reaction conditions and
associated references are listed.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
Catalyst
NaOH/Glycerol
mol/mol]
Temp. [°C]
Time [h]
O
2
pP [bar]
χ Glycerol [%]
LA selectivity [%]
Glycerol/metal ratio
[mol/mol]
CMB [%]
[
[
[
23]
21]
AuPt/TiO
AuPt/TiO
2
4
2
4
4
4
4
1.1
1.1
1.1
4
90
60
90
–
2
–
0.5
4
4
3
3
2
1
4
0.2
5
3
30
80
98
99
100
99
99
99
100
99
86
50
83
80
86
63
74
68
94
82
71
–
114
–
680
1000
1000
488
266
0
99ꢀ3
2
95
[46]
Au/CeO
2
94
99–100
–
–
–
–
95
102
99
[
22]
AuÀ Pt/nCeO
2
100
100
90
230
230
200
110
110
[
24]
AuÀ Pt/LaCrO
3
[
47]
Pt
Pt/C
Pd/C
Au
1
Ni
1
O
x
/TiO
2
1
[48]
0.2
0.2
0.2
3
[
48]
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
[
49]
2
Cu
IMP
AuPt/TiO
2
À NC
7
7
4055
4055
S
L
AuPt/TiO
2
À NC °
4
3
100
Key. Carbon mass balance (CMB); impregnation (IMP); sol-immobilisation (SOL); pP (partial pressure); χ (Conversion)
LA may not simply be a result of increased Bronsted and Lewis
is an important characteristic that can be exploited to design
more selective catalysts for this reaction.
acidity in the TiO À NC supported catalyst.
2
The performance of the TiO À NC supported catalysts
2
prepared by both sol- immobilisation and impregnation is
competitive with other high performing catalysts already Acknowledgments
published in the literature (Table 3). They are evidently not the
most selective towards LA, however, the high performance of
these materials is justified when the relatively mild reaction
conditions and comparatively low quantity of catalyst used in
these experiments are considered. Furthermore, our studies
have not concentrated on optimising catalyst performance,
rather they have focused on developing understanding of the
role of the support in the reaction. The observation that
multiple dehydration pathways can function simultaneously,
and the support properties have an influencing role, is the key
observation here and will allow for further developments in
catalyst design.
T. García would like to acknowledge the Spanish MICINN through
its Mobility program for stays of senior researchers and professors
in foreign countries (grant number PRX18/00474) for funding.
O.S.P acknowledges the FPI fellowship (BES-2016-077750) funded
by Spanish MINECO. B.S. also thank MINECO (MAT2017-84118-C2-
1-R project) for funding. We would also like to thank the Cardiff
University TEM facility for the TEM and FEG-SEM electron micro-
scopy data acquired and Diamond Light Source for access and
support in use of the electron Physical Science Imaging Centre
(Instrument E01 and proposal number MG21641 and MG22766)
which contributed to the results presented here.
Conclusions
Conflict of Interest
For glycerol selective oxidation a mesoporous nanocast TiO2
support for AuPt nanoparticles led to enhanced selectivity to
The authors declare no conflict of interest.
LA, compared to a AuPt/TiO À P25 catalyst. Reaction selectivity
2
Keywords: Aerobic Oxidation
Nanocasting · Lactic Acid.
· Dehydration · Glycerol ·
is controlled by the relative competitive rates of dehydration
and oxidation reactions from GLD. The dehydration reaction
predominantly occurs in solution and is catalysed by OH anions.
However, residual quantities of Si remaining on the surface of
the nanocast TiO after etching resulted in the formation of
2
Brønsted acid sites, which (i) may result in an additional
Brønsted acid catalysed dehydration pathway and/or (ii) under-
go dehydration to form Lewis acid sites in-situ due to the
presence of NaOH, which can also promote the dehydration of
primary alcohols. We propose that it is the combination of these
two pathways which ultimately leads to increased reaction
[
[
1] S. Veluturla, N. Archna, D. Subba Rao, N. Hezil, I. S. Indraja, S. Spoorthi,
Biofuels 2018, 9, 305–314.
2] R. A. Sheldon, Green Chem. 2014, 16, 950–963.
[3] Y. Wang, J. Zhou, X. Guo, RSC Adv. 2015, 5, 74611–74628.
4] D. Sun, Y. Yamada, S. Sato, W. Ueda, Appl. Catal. B 2016, 193, 75–92.
[
[5] B. Katryniok, H. Kimura, E. Skrzyńska, J. S. Girardon, P. Fongarland, M.
Capron, R. Ducoulombier, N. Mimura, S. Paul, F. Dumeignil, Green Chem.
2011, 13, 1960–1979.
selectivity to LA with the nanocast AuPt/TiO catalyst. Despite
2
[6] A. Villa, N. Dimitratos, C. E. Chan-Thaw, C. Hammond, L. Prati, G. J.
Hutchings, Acc. Chem. Res. 2015, 48, 1403–1412.
only modest enhancements in selectivity to LA being achieved
herein, this work provides an enhanced understanding on the
mechanistic involvement of the catalyst in this reaction and
provides evidence that surface Brønsted/Lewis support acidity
[
7] L. Prati, M. Rossi, J. Catal. 1998, 176, 552–560.
[8] L. Prati, G. Martra, Gold Bull. 1999, 32, 96–101.
9] W. C. Ketchie, M. Murayama, R. J. Davis, J. Catal. 2007, 250, 264–273.
[10] A. Villa, D. Wang, G. M. Veith, L. Prati, J. Catal. 2012, 292, 73–80.
[
ChemCatChem 2020, 12, 1–12
www.chemcatchem.org
10
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Full Papers
doi.org/10.1002/cctc.202000026
ChemCatChem
[
11] S. A. Kondrat, P. J. Miedziak, M. Douthwaite, G. L. Brett, T. E. Davies, D. J.
[30] Z. Tang, S. L. Fiorilli, H. J. Heeres, P. P. Pescarmona, ACS Sustainable
Chem. Eng. 2018, 6, 10923–10933.
[31] D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka,
G. D. Stucky, Science 1998, 279, 548–552.
[32] Y. Wang, B. Li, C. Zhang, L. Cui, S. Kang, X. Li, L. Zhou, Appl. Catal. B
2013, 130–131, 277–284.
[33] T. Barzetti, E. Selli, D. Moscotti, L. Forni, J. Chem. Soc. Faraday Trans.
1996, 92, 1401–1407.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
Morgan, J. K. Edwards, D. W. Knight, C. J. Kiely, S. H. Taylor, G. J.
Hutchings, ChemSusChem 2014, 7, 1326–1334.
[
[
[
[
12] J. Xu, H. Zhang, Y. Zhao, B. Yu, S. Chen, Y. Li, L. Hao, Z. Liu, Green Chem.
2013, 15, 1520–1525.
13] B. N. Zope, D. D. Hibbitts, M. Neurock, R. J. Davis, Science 2010, 330, 74–
78.
14] N. Dimitratos, J. A. Lopez-Sanchez, D. Lennon, F. Porta, L. Prati, A. Villa,
Catal. Lett. 2006, 108, 147–153.
15] A. Villa, S. Campisi, K. M. H. Mohammed, N. Dimitratos, F. Vindigni, M.
Manzoli, W. Jones, M. Bowker, G. J. Hutchings, L. Prati, Catal. Sci. Technol.
[34] N. Pal, A. Bhaumik, RSC Adv. 2015, 5, 24363–24391.
[35] T. Ohno, K. Sarukawa, K. Tokieda, M. Matsumura, J. Catal. 2001, 203, 82–
86.
2
015, 5, 1126–1132.
16] W. C. Ketchie, Y. L. Fang, M. S. Wong, M. Murayama, R. J. Davis, J. Catal.
007, 250, 94–101.
[36] S. Cao, F. F. Tao, Y. Tang, Y. Li, J. Yu, Chem. Soc. Rev. 2016, 45, 4747–
[
4765.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
2
[37] A. Villa, D. Wang, G. M. Veith, F. Vindigni, L. Prati, Catal. Sci. Technol.
2013, 3, 3036–3041.
[38] F. Hemmann, I. Agirrezabal-Telleria, C. Jaeger, E. Kemnitz, RSC Adv.
2015, 5, 89659–89668.
[39] L. F. Isernia, Mater. Res. 2013, 16, 792–802.
[40] B. J. Lee, Y. G. Hur, D. H. Kim, S. H. Lee, K. Y. Lee, Fuel 2019, 253, 449–
[17] A. Villa, G. M. Veith, D. Ferri, A. Weidenkaff, K. A. Perry, S. Campisi, L.
Prati, Catal. Sci. Technol. 2013, 3, 394–399.
18] S. A. Zavrazhnov, A. L. Esipovich, S. M. Danov, S. Y. Zlobin, A. S. Belousov,
[
Kinet. Catal. 2018, 59, 459–471.
[19] M. R. A. Arcanjo, I. J. da Silva, C. L. Cavalcante, J. Iglesias, G. Morales, M.
Paniagua, J. A. Melero, R. S. Vieira, Biofuels, Bioprod. Biorefining 2019, n/
a, bbb.2055.
459.
[41] A. Platon, W. J. Thomson, in Ind. Eng. Chem. Res., 2003, pp. 5988–5992.
[42] Z. Liu, J. Tabora, R. J. Davis, J. Catal. 1994, 149, 117–126.
[43] T. Kataoka, J. A. Dumesic, J. Catal. 1988, 112, 66–79.
[44] X. Gao, I. E. Wachs, Catal. Today 1999, 51, 233–254.
[45] S. Lux, M. Siebenhofer, Chem. Biochem. Eng. Q. 2015, 29, 575–585.
[46] P. Lakshmanan, P. P. Upare, N. T. Le, Y. K. Hwang, D. W. Hwang, U. H.
Lee, H. R. Kim, J. S. Chang, Appl. Catal. A 2013, 468, 260–268.
[47] Y. Li, S. Chen, J. Xu, H. Zhang, Y. Zhao, Y. Wang, Z. Liu, Clean Soil Air
Water 2014, 42, 1140–1144.
[48] M. R. A. Arcanjo, I. J. Silva, E. Rodríguez-Castellón, A. Infantes-Molina,
R. S. Vieira, Catal. Today 2017, 279, 317–326.
[49] L. Shen, X. Zhou, A. Wang, H. Yin, H. Yin, W. Cui, RSC Adv. 2017, 7,
30725–30739.
[
20] T. Komanoya, A. Suzuki, K. Nakajima, M. Kitano, K. Kamata, M. Hara,
ChemCatChem 2016, 8, 1094–1099.
21] E. A. Redina, O. A. Kirichenko, A. A. Greish, A. V. Kucherov, O. P.
Tkachenko, G. I. Kapustin, I. V. Mishin, L. M. Kustov, Catal. Today 2015,
[
246, 216–231.
[22] R. K. P. Purushothaman, J. van Haveren, D. S. van Es, I. Melián-Cabrera,
J. D. Meeldijk, H. J. Heeres, Appl. Catal. B 2014, 147, 92–100.
[
[
23] Y. Shen, S. Zhang, H. Li, Y. Ren, H. Liu, Chem. Eur. J. 2010, 16, 7368–7371.
24] C. D. Evans, S. A. Kondrat, P. J. Smith, T. D. Manning, P. J. Miedziak, G. L.
Brett, R. D. Armstrong, J. K. Bartley, S. H. Taylor, M. J. Rosseinsky, G. .J.
Hutchings, Faraday Discuss. 2016, 188, 427–450.
[
25] M. Tao, D. Zhang, X. Deng, X. Li, J. Shi, X. Wang, Chem. Commun. 2016,
5
2, 3332–3335.
[26] M. Tao, D. Zhang, H. Guan, G. Huang, X. Wang, Sci. Rep. 2016, 6, DOI
0.1038/srep29840.
1
[
27] K. M. A. Santos, E. M. Albuquerque, G. Innocenti, L. E. P. Borges, C.
Sievers, M. A. Fraga, ChemCatChem 2019, 11, 3054–3063.
[28] S. Feng, K. Takahashi, H. Miura, T. Shishido, Fuel Process. Technol. 2020,
97, 106202.
29] H. J. Cho, C. C. Chang, W. Fan, Green Chem. 2014, 16, 3428–3433.
Manuscript received: January 6, 2020
Revised manuscript received: March 26, 2020
Version of record online: ■■■, ■■■■
1
[
ChemCatChem 2020, 12, 1–12
www.chemcatchem.org
11
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
FULL PAPERS
Synopses Nanocasting: Co-operation
between Brønsted and Lewis acid
sites and NaOH may lead to
Dr. M. Douthwaite, N. Powell, A.
Taylor, G. Ford, Dr. J. M. López, Dr. B.
Solsona, Dr. N. Yang, O. Sanahuja-
Parejo, Dr. Q. He, Dr. D. J. Morgan,
Dr. T. Garcia*, Prof. S. H. Taylor*
improved reaction selectivity for the
aerobic oxidation of glycerol to lactic
acid over AuPt supported catalysts
under alkaline conditions. The physi-
cochemical properties of the catalyst
can influence the relative rates of
glyceraldehyde oxidation and dehy-
dration.
1
– 12
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
Glycerol Selective Oxidation to
Lactic Acid over AuPt Nanoparticles;
Enhancing Reaction Selectivity and
Understanding by Support Modifi-
cation