Platinum catalysed aerobic selective oxidation of cinnamaldehyde to cinnamic acid

Article abstract of DOI:10.1016/j.cattod.2018.02.052

Aerobic selective oxidation of allylic aldehydes offers an atom and energy efficient route to unsaturated carboxylic acids, however suitable heterogeneous catalysts offering high selectivity and productivity have to date proved elusive. Herein, we demonstrate the direct aerobic oxidation of cinnamaldehyde to cinnamic acid employing silica supported Pt nanoparticles under base-free, batch and continuous flow operation. Surface and bulk characterisation of four families of related Pt/silica catalysts by XRD, XPS, HRTEM, CO chemisorption and N₂ porosimetry evidence surface PtO₂ as the common active site for cinnamaldehyde oxidation, with a common turnover frequency of 49,000 ± 600 h^-1; competing cinnamaldehyde hydrogenolysis is favoured over metallic Pt. High area mesoporous (SBA-15 or KIT-6) and macroporous-mesoporous SBA-15 silicas confer significant rate and cinnamic acid yield enhancements versus low area fumed silica, due to superior platinum dispersion. High oxygen partial pressures and continuous flow operation stabilise PtO₂ active sites against in-situ reduction and concomitant deactivation, further enhancing cinnamic acid productivity.

Full text of DOI:10.1016/j.cattod.2018.02.052

Accepted Manuscript
Title: Platinum catalysed aerobic selective oxidation of
cinnamaldehyde to cinnamic acid
Authors: Lee J. Durndell, Costanza Cucuzzella, Christopher
M.A. Parlett, Mark A. Isaacs, Karen Wilson, Adam F. Lee
PII:
S0920-5861(18)30145-7
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.02.052
CATTOD 11277
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
29-11-2017
22-2-2018
27-2-2018
Please cite this article as: Lee J.Durndell, Costanza Cucuzzella, Christopher
M.A.Parlett, Mark A.Isaacs, Karen Wilson, Adam F.Lee, Platinum catalysed
aerobic selective oxidation of cinnamaldehyde to cinnamic acid, Catalysis
Today https://doi.org/10.1016/j.cattod.2018.02.052
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Platinum catalysed aerobic selective oxidation of cinnamaldehyde to cinnamic acid
Lee J. Durndell,a Costanza Cucuzzella,a Christopher M. A. Parlett,a Mark A. Isaacs,a Karen Wilsonb
and Adam. F. Leeb*
aEuropean Bioenergy Research Institute, Aston University, Birmingham, B4 7ET, UK
bSchool of Science, RMIT University, Melbourne, Victoria 3001, Australia
Graphical abstract
Highlights




Silica supported Pt nanoparticles are active and selective for base-free cinnamaldehyde aerobic oxidation
High area, mesoporous silica supports increase Pt dispersion and hence activity and cinnamic acid yield
Surface PtO2 is the active site for selective oxidation of cinnamaldehydecinnamic acid
Continuous flow operation stabilises surface PtO2 thereby significantly enhancing catalyst stability and
acid yield
Abstract
Aerobic selective oxidation of allylic aldehydes offers an atom and energy efficient route to unsaturated carboxylic acids,
however suitable heterogeneous catalysts offering high selectivity and productivity have to date proved elusive. Herein,
we demonstrate the direct aerobic oxidation of cinnamaldehyde to cinnamic acid employing silica supported Pt
nanoparticles under base-free, batch and continuous flow operation. Surface and bulk characterisation of four families of
related Pt/silica catalysts by XRD, XPS, HRTEM, CO chemisorption and N2 porosimetry evidence surface PtO2 as the
common active site for cinnamaldehyde oxidation, with a common turnover frequency of 49,000±600 h-1; competing
cinnamaldehyde hydrogenolysis is favoured over metallic Pt. High area mesoporous (SBA-15 or KIT-6) and
macroporous-mesoporous SBA-15 silicas confer significant rate and cinnamic acid yield enhancements versus low area
1

fumed silica, due to superior platinum dispersion. High oxygen partial pressures and continuous flow operation stabilise
PtO2 active sites against in-situ reduction and concomitant deactivation, further enhancing cinnamic acid productivity.
Keywords: Platinum; Cinnamaldehyde; Oxidation; Porous; Silica
1. Introduction
The aerobic selective oxidation (selox) of aldehydes represents an environmentally benign and atom efficient molecular
transformation for chemical valorisation [1], affording the production of complex α, β-unsaturated acids (and their esters)
under mild conditions [2]. Such acids are valuable chemical products and intermediates, for example cinnamic acid is
important in the fine chemical and pharmaceutical sectors as a food [3] and perfume additive [4], as a precursor to the
sweetener aspartame [5], and also finds application as an anti-fungal [6] and potential anti-malarial agent [7].
Traditionally, stoichiometric amounts of hazardous inorganic oxidants were utilised in selective alcohol and aldehyde
oxidation, but this approach is considered unsustainable and hence alternative catalytic clean technologies are sought [8-
10]. The presence of proximate C=C and C=O functionalities pose an especial challenge to achieving high selectivity to
the unsaturated acid in such aerobic oxidations [11].
Platinum group metals (PGMs) are effective heterogeneous catalysts for aerobic selox of alcohol, aldehydes and
carbohydrates, and the subject of numerous reviews [9, 12-15]. In this regard, α, β-unsaturated acids are typically
prepared by the cascade oxidation of their corresponding alcohols under strongly basic conditions [16-19]. Platinum
nanoparticles dispersed over oxide and carbon supports are widely studied for the aerobic selox of aliphatic [20-22],
allylic [23-25] and benzylic [26, 27] primary and secondary alcohols and carbohydrates [28, 29], but typically utilise p-
block promoters such as Bi, Pd, Sn and Te [19, 23] under basic conditions. Nevertheless, the nature of the active site and
role(s) of promoters and base remain poorly understood, reflecting the complex solid-liquid-gas interface and necessity
for (but attendant difficulties in performing and interpreting) in-situ/operando studies on supported metal catalysts for
aerobic selox [30, 31]. Recent spectroscopic and kinetic studies of cinnamyl alcohol aerobic oxidation over silica
supported Pt nanoparticles identified surface PtO2 as the active site for the selective production of cinnamaldehyde [32].
Hydrogen adatoms liberated during this oxidative dehydrogenation drive in-situ reduction of surface platinum oxide,
with the resulting Pt0 sites favouring styrene formation through hydrogenolysis of the C-C bond, and production of the
saturated 3-phenylpropanoic acid [33]. It is interesting to note that cationic Pt is also proposed as the active species in the
aerobic oxidative dehydrogenation of shikimic acid to dehydroshikimic acid over a platinum tetrasulfophthalocyanine
catalyst [34]. These observations mirror palladium analogues, wherein surface PdO adlayers [35-37] and electron
deficient Pd single atoms [38] are now recognised as the catalytically active species in allylic alcohol selox.
In contrast to alcohol selox, there are scant reports of the direct aerobic oxidation of unsaturated aldehydes over
PGMs, these being restricted to the cascade oxidation of 5-hydroxymethylfurfural, for which Pt catalysts again require
additional inorganic base (liquid or solid [39]) and/or promoters [40, 41] to achieve significant yields of 2,5-
furandicarboxylic acid (2,5-FDCA). There are also few examples of heterogeneously catalysed, liquid phase aerobic
oxidations in continuous flow [42-45], despite the attendant process advantages for atom economical, and scalable,
organic synthesis. In the context of platinum, aerobic flow oxidation of primary and secondary alcohols to their
corresponding carboxylic acids is reported over Pt nanoparticles dispersed in an amphiphilic polymer resin at high
oxygen pressures (40-70 bar) and temperature (100-120 °C), which are undesirable from the perspective of developing a
2

sustainable process, and for Pt/Al2O3 in a trickle-bed reactor for ethanol and benzyl alcohol oxidation [46] albeit minimal
information was provided on product selectivity.
Herein, we report one of the first examples of Pt catalysed aerobic selox of α, β-unsaturated aldehydes, under
mild and base-free conditions, and highlight the influence of support architecture, Pt oxidation state, and reaction
conditions on the yield of cinnamic acid from cinnamaldehyde. Significant productivity enhancements are also
demonstrated for continuous flow versus batch cinnamaldehyde oxidation to cinnamic acid.
2. Experimental
2.1 Catalyst synthesis
2.1.1 SBA-15 synthesis. 10 g Pluronic P123 was dissolved in 75.5 cm3 water and 291.5 cm3 of 2 M hydrochloric acid
under stirring at 35 °C. 15.5 cm3 Tetraethylorthosilicate was subsequently added and left stirring for 20 h. The resulting
gel was aged for 24 h at 80 °C without agitation. The solid was filtered, washed with 1000 cm3 water, and dried at room
temp before calcination at 500 °C for 6 h in air (ramp 1 °C.min-1). The resulting silica exhibited the expected ordered,
hexagonal (p6mm) arrangements of monodispersed, uniform mesopores.
2.1.2 KIT-6 synthesis. 10g Pluronic F127 was dissolved in 361.6 cm3 water, 12.3 cm3 butan-1-ol and hydrochloric acid
(35%, 16.7 cm3) with stirring at 35 °C. 15.6 cm3 Tetramethoxysilane was added and left for 20 h with agitation. The
resulting gel was aged for 24 h at 80 °C without agitation. The solid was filtered, washed with 1000 cm3 water and dried
at room temperature before calcination at 500 °C for 6 h in air (ramp rate 1 °C.min-1). The resulting silica exhibited the
expected gyroidal pore architecture (Ia3d symmetry) with interconnecting pore channels.
2.1.3 Polystyrene colloidal nanospheres. Polystyrene nanospheres were synthesised by the method of Vaudreuil and co-
worker, employing styrene (Sigma Aldrich, >99 %) as the monomer source and potassium persulphate (Sigma Aldrich,
>99 %) as the initiator, in a 2 L Radleys Reactor Ready jacketed glass reactor at 80 °C. 103 cm3 of styrene was washed
five times with 0.1 M NaOH and deionised water to remove polymerisation inhibitors, and added to 1.275 L of deionised
water previously degassed overnight in the reactor under 1.5 bar flowing N2 (10 cm3.min-1). Subsequently, 0.33 g of
potassium persulphate was dissolved in 50 cm3 deionised water at 70 °C and added to the styrene mixture to initiate
polymerisation, and stirred at 300 rpm under flowing N2 for 22 h. The resultant solid was recovered, filtered and washed
three times with deionised water, and a further three times with ethanol. The polystyrene nanospheres obtained were
dried overnight at 80 °C to yield a final dry mass of approximately 70 g.
2.1.4 Macro-mesoporous SBA-15 synthesis. Hierarchical macroporous-mesoporous SBA-15 was synthesised by a dual
hard-soft templating approach. An organic mesophase was first prepared through True Liquid Crystal Templating by
mixing 2 g Pluronic P123 with 2 g of 2M HCl acidified water (pH 2) and subsequent ultrasonication for 3 h at 40 °C to
produce a homogeneous gel. To this, 4.08 cm3 of aqueous tetramethoxysilane (1:4 molar ratio in H2O) was added and
mixed, yielding a homogeneous liquid. 6 g of polystyrene colloidal nanospheres was subsequently added to this solution,
and the resulting mixture heated at 40 °C in vacuo (100 mbar) overnight to obtain a viscous gel. The gel was aged for 24
h at room temperature to fully condense the silica framework, and then calcined at 500 °C (ramp rate 1°C.min-1) for 6 h
in static air.
2.1.5 Platinum impregnation. Wet impregnation of 2 g each of mesoporous SBA-15, KIT-6 and MM-SBA-15 silicas,
and a commercial fumed silica (Sigma Aldrich S5505, 200 m2.g-1), was performed with 16 cm3 aqueous ammonium
tetrachloroplatinate (II) solution (precursor concentrations adjusted to achieve nominal Pt loadings of 0.05-2 wt%). The
resulting slurries were stirred for 18 h at room temperature before heating to 50 °C. After a further 5 h agitation, the
3

remaining solids were held at 50 °C for 24 h to obtain dry powders, which were then calcined at 500 °C (ramp rate 1
°C.min-1) for 2 h in static air, prior to reduction at 400 °C (ramp rate 10 °C.min-1) for 2 h under 10 cm3.min-1 flowing
hydrogen. Reference 0.5 wt% Pt catalysts were also prepared by wet impregnation as described above on P25 (Sigma-
Aldrich, 99.5 %) and Degussa C γ-alumina (>99.8 %) supports, to examine the impact of support reducibility.
2.2 Characterisation
Nitrogen porosimetry was undertaken on a Quantachrome Nova 2000e porosimeter using NovaWin version 11 analysis
software. Samples were degassed at 120 °C for 2 h prior to nitrogen physisorption. Adsorption/desorption isotherms were
recorded for parent and Pt impregnated silicas, with BET surface areas calculated over the relative pressure range 0.01-
0.2. Pore diameters and volumes were calculated by applying the BJH method to desorption isotherms for relative
pressures >0.35. Wide angle XRD patterns were recorded on a Bruker D8 Advance diffractometer with a Cu Kα (1.54 Ǻ)
source calibrated against a Si standard, between 2θ = 20-90 ° with a step size of 0.02 °. The Scherrer equation was used
to calculate volume-averaged Pt particle sizes from line broadening. Low angle XRD patterns were recorded for 2θ =
0.5-2.5 ° with a step size of 0.02 °. XPS was performed on a Kratos Axis HSi X-ray photoelectron spectrometer fitted
with a charge neutraliser and magnetic focusing lens employing Al Kα monochromated radiation (1486.7 eV). Spectral
fitting was performed using CasaXPS version 2.3.14, with binding energies corrected to the C 1s peak at 284.6 eV. Pt 4f
XP spectra were fitted using an asymmetric lineshape. Pt nanoparticle dispersion as measured via CO pulse
chemisorption on a Quantachrome ChemBET3000 system. Catalysts were outgassed at 150 °C under 20 cm3.min-
4

nt is the number of mmol cinnamaldehyde at time t, and n0 the initial mmol cinnamaldehyde, and selectivity calculated
from Equation 2 based exclusively on the five major liquid phase products, where nx=i is the mmol of product i
(cinnamic acid, 3-phenylpropan-1-ol, styrene, cinnamyl esters or cinnamyl alcohol) and Σnx denotes the total mmol of all
products. Yield was calculated from Equation 3. Mass-normalised initial rates were calculated from the first hour of
reaction, and Turnover Frequencies (TOFs) calculated from Equation 4 by normalising raw initial rates to the mmol
surface Pt species determined from CO dispersion and XPS.
% Conversion = [(n0 – nt) / (n0)] x 100
% Selectivity = [(nx=i) / (Σnx)] x 100
% Yield = [Conversion x Selectivity) / 100
Equation 1
Equation 2
Equation 3
TOF = mmolCinnamaldehyde converted.h-1 / mmols surface Pt0 or x+ Equation 4
2.3.2 Flow oxidation. Catalyst testing was conducted using a Uniqsis FlowSyn reactor, comprising a GAM II gas-liquid
addition module and packed-bed microreactor with downstream backflow pressure regulator [47]. An integral HPLC
pump delivered a liquid stream of 0.84 M cinnamaldehyde in toluene (with mesitylene as internal standard) at flow rates
between 0.1-2.0 cm3.min-1, while 40 cm3 oxygen gas flow was delivered via an in-line Brooks mass flow controller and
back pressure regulator. Liquid and gas feeds were pre-mixed before introduction to the catalyst bed via the GAM II
module, which features a semi-permeable polymer coil and reactor mandrel heated to 90 ˚C to ensure efficient gas-liquid
mixing and heating to prior to contact with the catalyst. 50 mg of catalyst was diluted with quartz beads (Sigma Aldrich,
mesh size = -325) to minimise back pressure, and packed within a 10 mm i.d. x 100 mm OMNIFIT® glass column to
give a total bed volume of 2.5 cm3, held within the glass reactor by quartz wool plugs. The reactor was vertically
mounted and the gas-saturated liquid stream fed in an up-flow direction to minimise catalyst settling and maximise
permeation through the catalyst bed. Neither fluidisation nor compaction of the catalyst bed was observed. The exit
stream passed through a needle valve and backflow regulator, with excess gas vented prior to sampling for off-line GC
analysis.
3. Results and discussion
3.1 Catalyst characterisation
Four families of Pt catalysts were synthesised employing amorphous, ordered mesoporous (SBA-15 and KIT-6), or
hierarchical microporous-mesoporous (MM-SBA-15) silicas to elucidate the impact of support architecture on their
physicochemical properties, and the subsequent selective aerobic oxidation of cinnamaldehyde. Mesoporous SBA-15
(p6mm) and KIT-6 (Ia3d) architectures were identified through indexing their characteristic low angle powder X-ray
diffraction (XRD) patterns (Figure S1), however macropore incorporation disrupts extended SBA-15 ordering,
preventing the collection of a corresponding (high quality) low angle XRD pattern for the MM-SBA-15 support.
Calculated cell parameters and wall thicknesses for the mesoporous and hierarchical supports (Table S1) are in
accordance with the literature [48-50]. Nitrogen porosimetry confirmed the expected type IV isotherms and type I
hysteresis for the mesoporous and hierarchical supports (Figure S2), and corresponding high BET surface area and
narrow BJH mesopore size distributions (Table S1 and Figure S2). In contrast, the commercial silica exhibited a type II
isotherm, reflecting a disordered pore network, and low BET surface area. High resolution transmission electron
microscopy (HRTEM) in Figure 1 shows hexagonal close-packed mesopores for SBA-15 [49], cubic close-packed
5

mesopores for KIT-6 [48], and the incorporation of 300 nm macropores between hexagonal close-packed mesopore
channels in MM-SBA-15 [50].
b)
a)
c)
250 nm
40 nm
50 nm
e)
d)
f)
150 nm
40 nm
20 nm
Figure 1. HRTEM dark (top) and bright (bottom) field images of (a,d) SBA-15, (b,e) KIT-6, and (c,f) MM-SBA-15
silicas.
Physicochemical characteristics of the Pt/silica analogues are summarised in Table S2, and are in excellent agreement
with literature reports [32, 51, 52]. XRD, HRTEM and N2 porosimetry confirmed retention of the parent silica pore
structures following platinum impregnation (Table S2, Figure S3-4), but a loss of BET surface area and pore volume
proportional to Pt loading, attributed to partial pore blockage, as previously observed. The smaller magnitude of surface
area decrease observed for the fumed SiO2 catalysts upon Pt impregnation is consistent with nanoparticle deposition
primarily on the external surface. Metal dispersion (determined by CO chemisorption) was inversely proportional to Pt
loading for all supports (Table S2), with particle sizes (determined by a combination of TEM, wide angle XRD (Figure
S5) and CO chemisorption) proportional to the silica surface area and spanning 1.7 nm (0.07 wt% Pt/KIT-6) to 15.6 nm
(2.10 wt%/fumed SiO2). The degree of platinum surface oxidation, as measured by XPS, was directly proportional to Pt
dispersion, as previously reported [32, 52] (Figure S7-8), associated with the formation of PtO2, with a Pt 4f7/2 binding
energies of 74.5 eV [32, 52-54] over smaller nanoparticles. Platinum surface oxidation was support dependent decreasing
in the order: KIT-6 > SBA-15 > MM-SBA-15 > fumed SiO2 (Table S2).
3.2 Batch oxidation of cinnamaldehyde
Cinnamaldehyde aerobic selox was subsequently investigated over the preceding four families of Pt/silica catalysts.
Control experiments showed negligible aldehyde conversion in the absence of supported platinum and/or O2, while hot
filtration tests and elemental analysis of filtrate and spent catalysts revealed negligible platinum leaching (Figure S9).
6

All reactions were performed at 800 rpm, and were zero order with respect to mass-normalised catalyst activity (and
selectivity), evidencing that oxidations were free from external mass-transport limitations (Figure S10 and S11).
Reaction profiles are shown in Figure S12. Initial rates of cinnamaldehyde selox were inversely proportional to Pt
loading for all supports (Figure 2a), with activity proportional to platinum dispersion and hence surface PtO2
concentration (Figure S13): KIT-6 > SBA-15 > MM-SBA-15 > fumed SiO2, consistent with observations for both Pd
and Pt catalysed aerobic selox of cinnamyl alcohol to cinnamaldehyde [32, 55, 56]. Turnover frequencies (TOFs) for
cinnamaldehyde selox normalised per surface PtO2 site were 49,000 h-1, independent of Pt loading or silica support
(Figure 2b): this demonstrates that (i) PtO2 is the active site for cinnamaldehyde aerobic selox (and not Pt metal); and (ii)
the superior activity of the mesoporous and hierarchical catalysts does not arise from enhanced in-pore mass transport,
but is solely associated with higher Pt dispersion.
Surface Pt ()/ mmol (x10-3)
0
0.2
0.4
0.6
0.8
80000
70000
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50000
40000
30000
20000
10000
0
60000
50000
40000
30000
20000
10000
0
40000
35000
30000
25000
20000
15000
10000
5000
Fumed Silica
SBA-15
TLCT-MM-SBA-15
KIT-6
0
0
0.5
1
1.5
2
2.5
0
0.2 0.4 0.6 0.8
1
1.2
Surface PtO2 ( ) / mmol (x10-4)
Pt loading / wt%
Figure 2. (left) Mass-normalised initial rates and (right) corresponding turnover frequencies for cinnamaldehyde aerobic
selox over 0.05-2.1 wt% Pt/silica catalysts. Reaction conditions: 50 mg catalyst, 8.4 mmol cinnamaldehyde in 10 cm3
toluene, 5 cm3.min-1 bubbling O2 at 1 bar, 90 °C, and 800 rpm stirring.
Figure 3 shows the striking performance enhancements attainable through tailoring the support porosity and surface area,
which deliver a three-fold increase in mass-normalised activity and an absolute increase of 11 % in cinnamic acid yield
for 0.15 wt% Pt/silicas by substituting a fumed SiO2 for a KIT-6 support. These Pt/silicas are effective catalysts for the
production of α, β-unsaturated acids, aliphatic and aromatic acids by aerobic oxidation (Table S3).
7

70000
60000
50000
40000
30000
20000
45
40
35
30
Figure 3. Comparison of initial rates and 5 h cinnamic acid yields for cinnamaldehyde aerobic selox over 0.15 wt%
Pt/silica catalysts as a function of support architecture. Reaction conditions: 50 mg catalyst, 8.4 mmol cinnamaldehyde in
10 cm3 toluene, 5 cm3.min-1 bubbling O2 at 1 bar, 90 °C, and 800 rpm stirring.
Figure 4 shows the product distribution as a function of surface PtO2 concentration, and confirms oxidised platinum as
the active catalytic species for cinnamaldehyde aerobic selox to cinnamic acid, whereas metallic platinum favours
cinnamaldehyde decarbonylation to styrene (or hydrogenation to 3-phenylpropan-1-ol); note we have been previously
shown cinnamic acid does not undergo decarboxylation to styrene over Pt/silica catalysts under identical conditions [32].
Decarbonylation of reactively-formed aldehydes during aerobic selox of alcohols over Pt catalysts, and resulting site
blocking by strongly-bound CO, is reported as a major cause of catalyst deactivation in such systems [57-60].
Complementary studies by Zaera and co-workers over Pt(111) single crystal model catalysts reveal that crotonaldehyde
(a simple allylic aldehyde) binds to the metal surface through the C=C moiety, inducing extensive rehybridization of the
double bond, weakening conjugation and promoting decarbonylation to adsorbed CO and gaseous propene on thermal
treatment [61]. Figures 2-4 suggest that the deactivation and poor selectivity to aldehydes/acids that plagues platinum
selox catalysts reflects their low nanoparticle dispersion and hence metallic surfaces. The importance of maintaining
platinum in an oxidised state was further evidenced by the impact of a mild pre-reduction treatment on the performance
of the 0.52 wt% Pt/SBA-15 catalyst (Figure S14). Pre-reduction suppressed the initial activity and 5 h conversion by 43
% and 25 % respectively for cinnamaldehyde aerobic selox, and lowered the cinnamic acid yield by 37 % at the expense
of increased styrene production. Furthermore, Figure S15 evidences a linear relationship between the surface PtO2
concentration and ratio of cinnamic acid:by-products. Our hypothesis that surface PtO2 is the active and selective site for
cinnamaldehyde oxidation to cinnamic acid is also supported by the significant increase in aldehyde conversion (from
3686 %) and acid selectivity (6786 %) observed on raising the pO2 from 110 bar over 0.52 wt% Pt/SBA-15. XPS
and CHN analysis of fresh and spent 0.52 wt% Pt/silica following 5 h cinnamaldehyde aerobic selox confirm that higher
pO2 stabilise PtO2 and restrict site-blocking by strongly-bound organic products/residues (Figure S16). Analogous
improvements in conversion and selectivity with increasing oxygen pressure are reported in the aerobic selox of glucose
to glucaric acid over Pt/C [62]. Note that it is difficult to develop a quantitative relationship between the oxygen partial
pressure and surface PtO2 concentration, since catalytically active surface oxides formed by platinum under high O2
8

pressures (1-5 bar) are only metastable [63]. This likely explains the non-linear relationship in Figure S16 between the
increase in surface PtO2 oxide determined ex-situ by XPS post-reaction from 7.011.4 wt% upon raising the pO2
employed during oxidation from 110 bar. Accurate quantification of the PtO2 content at elevated pressures would
require a liquid phase operando XAS study, which will be the subject of future investigations.
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
y = 5.4041x + 33.441
R² = 0.9901
Cinnamic acid
Styrene
3-phenylpropan-1-ol
y = 0.4226x + 1.5663
R² = 0.9757
0.1
0.3
0.5
0.7
0.9
0
2
4
6
8
10
Surface PtO2 / mmol (x10-4)
O2 pressure / bar
Figure 4. (left) Influence of surface PtO2 concentration on 5 h product selectivity for cinnamaldehyde aerobic selox over
Pt/silicas at 1 bar O2, and (right) influence of pO2 on 5 h cinnamaldehyde conversion and cinnamic acid:styrene
selectivity over 0.52 wt% Pt/SBA-15 catalyst. Reaction conditions: 50 mg catalyst, 16.8 mmol cinnamaldehyde in 10 cm3
toluene, 90 °C, and 800 rpm stirring.
Having identified PtO2 as the active site in cinnamaldehyde aerobic selox, the question naturally arises as to whether
higher active site densities, and hence activity, might be favoured by more reducible oxide supports which interact more
strongly with platinum (and hence should enhance its dispersion and PtO2 stability). Figure 5 confirms this hypothesis,
with the mass-normalised initial rates of 0.5 wt% Pt wet-impregnated over different oxide supports decreasing in the
order TiO2 > Al2O3 > fumed SiO2, closely mirroring the proportion of surface PtO2 in each catalyst.
9

0.5
0.45
0.4
16000
12000
8000
4000
0
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Titania
Alumina
Fumed
silica
Figure 5. Comparison of mass-normalised initial rates of cinnamaldehyde aerobic selox over 0.52 wt% Pt catalysts as a
function of support, and corresponding degree of surface platinum oxidation determined by XPS of fresh catalysts.
Reaction conditions: 50 mg catalyst, 8.4 mmol cinnamaldehyde in 10 ml toluene, 5 cm3.min-1 bubbling O2 at 1 bar, 90
°C, and 800 rpm stirring.
Cinnamaldehyde aerobic selox over shows a moderate temperature dependence (reflected by an apparent activation
energy of 29±2 kJ.mol-1, Figure S17), with conversion increasing from 2070 % over the 0.52 wt% Pt/SBA-15 catalyst
with increasing reaction temperature from 20-90 °C, however additional heating lowered activity possibly associated
with lower oxygen solubility (Figure 6). Cinnamic acid and styrene production mirrored reactant conversion albeit with
a slightly weaker temperature dependence, and indeed exhibit similar apparent activation energies of 24±2 and 27±2
kJ.mol-1 respectively. The latter is in reasonable agreement with DFT calculations for furfural decarbonylation over a Pt55
cluster of 35 kJ.mol-1 [64]. In contrast, 3-phenylpropan-1-ol production was weakly temperature dependent, increasing
slightly above 90 °C. High reaction temperatures therefore favoured decarbonylation (and C=C/C=O hydrogenation), as
previously observed during alcohol aerobic selox [57, 65], and which we attribute to PtO2 reduction to metallic Pt at
elevated temperature (Figure S16) [66].
10

50
40
30
20
10
0
Cinnamaldehyde conversion
3-phenylpropan-1-ol
Cinnamic acid
Styrene
30 45 60 75 90 105 120 135
Temperature / C
Figure 6. Influence of reaction temperature on conversion and 1 h product yield for cinnamaldehyde aerobic selox over
0.52 wt% Pt/SBA-15 catalyst. Reaction conditions: 50 mg catalyst, 8.4 mmol cinnamaldehyde in 10 cm3 toluene, 5
cm3.min-1 bubbling O2 at 1 bar, 90 °C, and 800 rpm stirring.
3.3. Continuous flow oxidation of cinnamaldehyde
The potential for scale-up and impact of continuous versus batch process operation was subsequently assessed for
cinnamaldehyde aerobic selox over the 0.52 wt% Pt/SBA-15 catalyst under the optimum reaction conditions determined
in Section 3.2 (90 °C and 10 bar O2). Cinnamaldehyde conversion (Figure S18) and product selectivity (Figure 7a) were
first determined as a function of residence time (). Negligible cinnamaldehyde conversion was observed for < 5 min,
with longer residence times resulting in a monotonic increase in activity, with single pass conversions reaching a plateau
of 60 % for > 25 min, likely due to limited oxygen availability under these conditions. Cinnamic acid production
displayed a volcano dependence on residence time, reaching a maximum of 84 % for =17 min. In contrast, 3-
phenylpropan-1-ol production exhibited a monotonic decrease with residence time, concomitant with a monotonic
increase with styrene production, suggesting these undesired by-products are formed by competing pathways that share a
common active site (which the preceding batch oxidations indicate is metallic surface Pt). The fall in cinnamic acid
selectivity at the expense of styrene observed at long residence times is attributed oxygen starvation and consequent in-
situ reduction of surface PtO2, which Figure 4a shows is the active site for cinnamaldehyde oxidation. Catalyst stability
was subsequently evaluated for =17 min, the optimum residence time for cinnamic acid production, as a function of
time-on-stream (Figure S19 and S20). Negligible deactivation was observed over 7 h continuous flow operation, in
contrast with a batch reaction under comparable conditions in which 60 % of the initial activity was lost over the same
period, consistent with Figure S16 which shows greater carbon deposition and in-situ reduction of PtO2 in batch; XRD
confirmed deactivation in batch was not due to particle sintering (Figure S21). High selectivity to cinnamic acid (84 %)
was also maintained in continuous flow, whereas deactivation on-stream in batch was accompanied by a slow, but
continuous switchover from cinnamic acid to styrene production. Cumulative 7 h cinnamic acid production in batch and
continuous flow is compared in Figure 7b, revealing a 37 % increase in cinnamaldehydecinnamic acid is attainable by
flow operation.
11

350
325
300
275
250
225
200
100
80
60
40
20
0
Cinnamic acid
Styrene
3-phenylpropan-1-ol
0
5
10 15 20 25 30 35
Batch
Flow
Residence time / min
Figure 7. (left) Influence of residence time on selectivity for cinnamaldehyde aerobic selox over 0.52 wt% Pt/SBA-15 in
continuous flow, and (right) cumulative 7 h cinnamic acid yield for cinnamaldehyde aerobic selox over 0.52 wt%
Pt/SBA-15 in batch versus continuous flow. Reaction conditions: 50 mg catalyst, 16.8 mmol cinnamaldehyde in 10 cm3
toluene (batch)/0.84 M (flow), 10 bar O2 (static batch, 40 cm3/min-1 flow), 90 °C, 800 rpm stirring (batch)/0.2 cm3.min−1
liquid feed (flow).
4. Conclusions
The influence of support architecture and process conditions on the heterogeneously catalysed liquid phase selective
aerobic oxidation of cinnamaldehyde to cinnamic acid was investigated over silica supported Pt nanoparticles. In all
cases, activity was directly proportional to nanoparticle dispersion and the degree of platinum oxidation. Surface PtO2 is
identified as the active site responsible for cinnamic acid production, with in-situ reduction promoting cinnamaldehyde
decarbonylation to styrene (and a minor hydrogenation pathway to 3-phenylpropan-1-ol) over metallic Pt; increasing the
oxygen partial pressure from 110 bar suppresses the later undesirable side-reactions and enhances cinnamic acid
productivity (80 % selectivity and 74 % yield after 5 h at 90 °C). Mesoporous, high area silica supports significantly
increase the concentration of active and selective surface PtO2 catalytic species, and hence cinnamic acid yield, relative
to commercial fumed silica, offering a simple approach to precious metal thrifting [67]. Pt/SBA-15 is also active and
selective for production of aliphatic and aromatic acids from their corresponding aldehydes. Continuous flow operation
improves catalyst stability, completely suppressing on-stream deactivation observed in batch, and thereby maintaining a
high PtO2 concentration (and hence selectivity to the α, β-unsaturated acid of 84 %), resulting in a 37 % increase in net
cinnamic acid productivity over 7 h reaction.
Declarations of interest: none
Acknowledgements
Funding: This work was supported by the EPSRC (EP/G007594/4, EP/K014749/1 and EP/K014676/1). We thank
Professor Richard Palmer and Birmingham University for access to TEM facilities.
12

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