In situ spectroscopic investigation of oxidative dehydrogenation and disproportionation of benzyl alcohol

Article abstract of DOI:10.1039/c3cp50710f

In the solvent free oxidation of benzyl alcohol, using supported gold-palladium nanoalloys, toluene is often one of major by-products and it is formed by the disproportionation of benzyl alcohol. Gold-palladium catalysts on acidic supports promote both th

Full text of DOI:10.1039/c3cp50710f

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In situ spectroscopic investigation of oxidative
dehydrogenation and disproportionation of benzyl
alcohol†
Cite this: Phys. Chem. Chem. Phys., 2013,
1
5, 12147
a
b
c
b
Ewa Nowicka, Jan P. Hofmann, Stewart F. Parker, Meenakshisundaram Sankar,
a
a
a
d
Giacomo M. Lari, Simon A. Kondrat, David W. Knight, Donald Bethell,
Bert M. Weckhuysen and Graham J. Hutchings*
b
a
In the solvent free oxidation of benzyl alcohol, using supported gold–palladium nanoalloys, toluene is
often one of major by-products and it is formed by the disproportionation of benzyl alcohol.
Gold–palladium catalysts on acidic supports promote both the disproportionation of benzyl alcohol and
oxidative dehydrogenation to form benzaldehyde. Basic supports completely switch off
disproportionation and the gold–palladium nanoparticles catalyse the oxidative dehydrogenation
reaction exclusively. In an attempt to provide further details on the course of these reactions, we have
utilized in situ ATR-IR, in situ DRIFT and inelastic neutron scattering spectroscopic methods, and in this
article we present the results of these in situ spectroscopic studies.
Received 15th February 2013,
Accepted 2nd May 2013
DOI: 10.1039/c3cp50710f
www.rsc.org/pccp
1
6
aerobic oxidation of benzyl alcohol. In a typical reaction,
besides benzaldehyde, toluene, benzoic acid and benzyl benzoate
Introduction
Supported bimetallic catalysts have received considerable are also formed,
attention because of their enhanced catalytic activities com-
pared to their monometallic analogues, due to a synergistic
2PhCH
2
OH - PhCHO + PhCH
3
+ H
2
O
(I)
1
,2
effect that has been observed between the two metals. They
have been reported as efficient catalysts with superior selectivity
and/or stability for a variety of reactions including selective
lowering the selectivity of benzaldehyde. Among the by-products
detected, toluene is formed in substantial amount and hence it
was necessary to understand the mechanism of its formation.
Toluene has been reported to be formed by the disproportionation
2
–5
6–8
oxidation, hydrogenation/hydrogenolysis, direct synthesis
9
,10
1,11
of hydrogen peroxide
and many more.
Selective oxidation
17,18
of benzyl alcohol [eqn (I)].
of benzyl alcohol to benzaldehyde is one of the most studied
reactions because of the potential applications of benzaldehyde
as an intermediate in the production of fine chemicals and
O2
PhCH2OH ! PhCHO þ H2O
(II)
1
2–15
fragrances.
Achieving high selectivity to benzaldehyde has Benzaldehyde is formed by the oxidative dehydrogenation of
been a long term objective for this reaction. Designing catalysts benzyl alcohol [eqn (II)] and further oxidation of benzaldehyde
to achieve high selectivity needs detailed knowledge about the results in benzoic acid. Previous kinetics studies, on the mecha-
origin and mechanism of the formation of by-products. nism of these two reactions, have revealed that the adsorbed
Recently, we reported that 1% AuPd/TiO , prepared by a sol state for the disproportionation reaction is different from that of
2
1
8
immobilization method, is a catalyst for the solvent-free the oxidative dehydrogenation. In situ spectroscopic methods
are the best tools to characterize these adsorbed species over the
1
9–21
a
b
c
catalyst surface.
However, for the current system, this could
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff,
CF10 3AT, UK. E-mail: hutch@cardiff.ac.uk; Fax: +44 (0)2920-874-030
Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science,
Utrecht University, 3584 CG Utrecht, The Netherlands
not be done earlier because benzaldehyde is formed by both
disproportionation and oxidative dehydrogenation. It is difficult
to distinguish between these two sources of benzaldehyde.
Recently, we reported a method to quantitatively differentiate
(a) oxidative dehydrogenation and (b) disproportionation
ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton,
Didcot Oxon OX11 0QX, UK
d
Department of Chemistry, Liverpool University, Liverpool, L69 7ZD, UK
Electronic supplementary information (ESI) available. See DOI: 10.1039/
2
2
reactions when both reactions are active in the same pot. As
a part of the study, we found that gold–palladium nanoparticles
†
c3cp50710f
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Scheme 1 Two modes of dissociative chemisorption of benzyl alcohol on the metal surface: proposed mechanisms for disproportionation of benzyl alcohol to form
equimolar toluene and benzaldehyde.
supported on basic oxides, like ZnO and MgO, switch-off the interact with the evanescent IR wave ranging a few mm into the
2
2
disproportionation completely. The extent of the oxidation probed medium resulting in molecule specific IR absorption.
reaction does not change with the change in support and hence, Implementing an IR sentinel in the bottom of an autoclave batch
the active site for the oxidation reaction appears to be the reactor allows for measuring in situ under conditions of up to a
metallic sites. However, the extent of the disproportionation is temperature of 350 1C and 200 bar pressure. Diffuse Reflectance
highly dependent on the support and the active site for the Infrared Fourier Transform (DRIFT) spectroscopy is a widely
disproportionation reaction seems to be the metal–support available and well-understood technique for in situ studies of
2
2
26–29
interface. Possible molecular mechanisms for the dispropor- heterogeneous catalysts.
This technique has been widely used
tionation process were proposed (Scheme 1), key features of to characterize the chemical species adsorbed on the catalyst
which were the modes of interaction of benzyl alcohol molecules surface and can be used as a complimentary technique to the
with the catalyst/support surfaces (chemisorption on the metal ATR-IR method. Inelastic Neutron Scattering (INS) Spectroscopy is
through C–H or O–H bond cleavage; binding to the support a complementary form of vibrational spectroscopy, where the
involving hydrogen bonding) and the concerted nature of the scattering event is between a neutron and the atomic nucleus,
1
8
bonding changes during reaction. For oxidative dehydrogena- thus the electronic nature of the material – metal, semiconductor,
3
0–32
tion, analogous mechanisms can be drawn to show the reaction or insulator – is irrelevant.
of bound oxygen species with benzyl alcohol attached to the on the incoherent inelastic scattering cross section and the
metal surface either through its benzylic C or O atom.
The scattering intensity depends
1
8
1
amplitude of vibration. For H both of these are large but they
These recent developments have helped us to design experi- are small for all other elements present in this work; consequently
ments, not possible before, where we could perform these two the scattered intensity is dominated by hydrogen motion. We have
reactions separately, without interference from the other reaction. carried out in situ spectroscopic studies over supported gold–
For example, oxidative dehydrogenation of benzyl alcohol could palladium catalysts using these three techniques and in this paper
be performed exclusively by using ZnO or MgO supported gold– we report our findings. Though many attempts have been made to
palladium catalysts under an oxygen atmosphere. Dispropor- study the mechanism of the benzyl alcohol oxidative dehydro-
tionation of benzyl alcohol could be performed exclusively genation, no attempt has been made to study this disproportiona-
using TiO
anaerobic conditions. When TiO
2
or C supported gold–palladium catalysts under tion reaction. To the best of our knowledge, the unique
or C supported catalysts are combination of in situ ATR-IR, in situ DRIFT and INS techniques
2
used under oxygen atmosphere, benzyl alcohol undergoes has never been attempted before.
oxidative dehydrogenation and disproportionation in parallel.
In situ Attenuated Total Reflection Infra-Red (ATR-IR) spectro-
scopy has received considerable attention as a tool for process
Experimental
Materials
23–25
monitoring under reaction conditions.
reaction mixture is probed by an internal reflection element (ATR Benzyl alcohol (Aldrich, anhydrous >99%), TiO
sentinel). Molecules in the proximity of the sentinel’s surface ZnO (Sigma Aldrich), MgO (BDH), Activated C (Darcor G60,
The composition of a
2
(P25 Evonik),
1
2148 Phys. Chem. Chem. Phys., 2013, 15, 12147--12155
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Sigma Aldrich), HAuCl
4
(Sigma Aldrich) and PdCl
2
(Johnson stabilizing agent in catalyst preparation. Therefore, all the
Matthey) were used without further purification. Supported spectra observed were from that of adsorbed species on the
gold–palladium catalysts were prepared by a sol immobilization catalyst surface.
method and the detailed procedure for preparing these cata-
1
8,22
Inelastic neutron scattering (INS) spectroscopy
lysts is reported elsewhere.
These catalysts were used with-
out any further modifications.
INS spectra were recorded with the MAPS spectrometer at ISIS;
the instrument is described in detail elsewhere. In brief, a
3
2
In situ ATR-IR spectroscopy
single energy, E , is selected from the incident, pulsed poly-
i
In situ ATR-IR monitoring of the reactions was conducted in a chromatic beam by a Fermi chopper; after scattering from the
0 mL Parr stainless steel autoclave reactor equipped with sample, the scattered neutrons are detected by a large area
4
2
3
a Mettler Toledo ReactIR 45m ATR-IR instrument using a (B16 m ) of He detectors. For these measurements MAPS was
diamond sentinel and a HgCdTe detector at ꢀ196 1C in the run with the high resolution slit package (B1.5% E ) in the
(600 meV),
i
ꢀ
1 33,34
ꢀ1
spectral range of 4000–650 cm
.
The reaction temperature Fermi chopper. Incident energies of 4033 cm
ꢀ
1
ꢀ1
was monitored and maintained using a thermocouple fitted 2017 cm (250 meV) and 484 cm (60 meV) were used to
into the reactor and connected to the controller. The reaction record the O–H/C–H stretch region, the fingerprint region and
mixture was stirred at 1100 rpm to avoid any mass transfer the region where framework motions (torsions, C–C–C bend)
limitations. The reactor vessel was charged with 15 mL of occur. The catalyst (8 g) was placed inside Conflat sealed, flow
3
5
benzyl alcohol and 0.015 g of catalyst, then was closed and through Inconel cans between quartz wool. Then the inlet line
purged with either pure O or Ar thrice with constant stirring of the can was connected with gas (He) and left for period of 12 h
2
and finally pressurized to 5 bar and 20 bar, respectively. This at a temperature of 120 1C to remove any absorbed species.
pressure was maintained throughout the reaction time with Benzyl alcohol vapour was dosed onto the surface of the catalyst
constant replenishment in the case of O
2
. Heating of the reactor at room temperature (25 1C) for a period of 12 h. The system for
was started with constant stirring and ATR-IR spectra of the dosing benzyl alcohol consists of a He line going through a
reaction mixture were measured every 60 s at a resolution of heated benzyl alcohol (150 1C) reservoir and MAPS sample cell
ꢀ
1
4
cm . However, for clarity, only selected spectra are presented kept at 25 1C. The cans were then loaded into a closed cycle
here. Difference spectra were created by subtraction of a pure refrigerator in MAPS, cooled to oꢀ253 1C and the spectra
benzyl alcohol reference spectrum acquired at 120 1C under recorded for 6–8 h at each of the three incident energies. All
20 bar Ar pressure from the in situ reaction spectrum in liquid species chemisorbed species or strongly adsorbed on to the solid
phase. Time on-line profiles to follow benzyl alcohol consump- catalyst surface are characterized by this technique.
tion and benzaldehyde production at 120 1C were constructed
ꢀ
1
by integration of the 1016 and 1039 cm
double band
Results and discussion
In situ ATR-IR spectroscopy
ꢀ1
ꢀ1
(
integration limits 1060–950 cm , baseline 1070–930 cm
and the n(CQO) band at 1703 cm (integration limits 1730–
670 cm , baseline 1740–1660 cm ), respectively. The species The transformations of benzyl alcohol over 1%AuPd/TiO ,
2
)
ꢀ
1
ꢀ
1
ꢀ1
1
present in the liquid phase (mostly) are characterized by this 1%AuPd/MgO and 1%AuPd/C catalysts were monitored using
technique. For ATR-IR studies, we have used 1%AuPd/MgO in situ ATR-IR reactor under both aerobic and anaerobic con-
catalyst whereas for other studies we have used 1%AuPd/ZnO ditions to study (a) disproportionation (b) oxidative dehydro-
catalyst. Our earlier studies have shown that, mechanistically genation reactions. In situ ATR-IR spectroscopic investigation,
these two catalysts behave in a similar way and thus could be using the ATR-IR probe fitted in a stirred autoclave reactor,
swapped for the mechanistic investigation.
yields mostly information about the liquid phase composition
3
3,34
and intermediates during the course of the reaction.
Fig. 1
DRIFT spectroscopy
displays the absorbance (difference) spectra, with respect to
DRIFT spectroscopy studies were performed using a Bruker pure benzyl alcohol (120 1C, 20 bar Ar), during the course of the
Tensor 27 spectrometer fitted with a HgCdTe (MCT) detector reaction using 1%AuPd/TiO , 1%AuPd/MgO and 1%AuPd/C
2
and a Harrick Praying Mantis HVC-DRP-4 cell equipped with catalysts under both anaerobic (120 1C, 20 bar Ar) and aerobic
ZnSe windows. Samples were mixed with benzyl alcohol and (120 1C, 5 bar O ) conditions. Positive band(s) in the spectra
2
added to the cell. An alternative method of adding benzyl indicate an increase in the concentration of the component(s)
alcohol was the use of an in-line saturator heated to 150 1C. corresponding to the band and negative band(s) indicate a
After an initial scan at room temperature the sample was decrease in concentration of the component(s) corresponding
ꢀ1
heated to 280 1C at 1 1C min under an atmosphere of flowing to the band. All spectra show characteristic bands assignable to
ꢀ1
ꢀ1
N2 20 ml min
or O2 20 ml min
(flows regulated by benzyl alcohol (substrate), benzaldehyde and water (products).
Bronkhorst mass flow meters). Spectra were run in the range Vibrational bands corresponding to toluene are largely masked
ꢀ
1
of 4000–600 cm , with 64 accumulation scans, every 10 1C. by both benzyl alcohol and benzaldehyde bands and hence it
Difference spectra were used with catalysts prior to the addition was not possible to observe toluene using this in situ ATR-IR
2
3,24
of benzyl alcohol being used as the background. This removed spectroscopic method.
However, GC and GC-MS studies
interference from bands associated with PVA used as a of the reaction mixtures, reported earlier, clearly indicate the
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Fig. 1 In situ ATR-IR difference spectra for the reaction of benzyl alcohol using 1%AuPd/TiO
2
, 1%AuPd/MgO and 1%AuPd/C catalysts under anaerobic and aerobic
ꢀ1
ꢀ1
conditions in a stirred autoclave reactor. Only 3750–2500 cm [n(O–H) and n(C–H) regions] and 1750–750 cm [fingerprint region] are presented here for clarity.
Pure benzyl alcohol spectrum at 20 bar Ar, 120 1C was used as reference spectrum to calculate the difference spectra. Displayed spectra taken at 120 1C for t = 0
0
0
0
0
0
(
(
black), 30 (dark grey), 60 (grey), 90 (light grey), 120 (red). (a and b) 1%AuPd/TiO
2
, anaerobic; (c and d) 1%AuPd/TiO
2
, aerobic; (e and f) 1%AuPd/MgO, anaerobic;
ꢀ1
ꢀ1
g and h) AuPd/MgO, aerobic; (i and j) 1%AuPd/C, anaerobic and (k and l), aerobic. The bands (1703 cm for benzaldehyde and 1016 cm for benzyl alcohol) used
for integration for the time on-line concentration profile (Fig. 2) are highlighted.
1
8,36,37
formation of toluene.
limited to non-overlapping bands and their corresponding reaction temperature reaches 120 1C. For the 1%AuPd/TiO
components such as benzyl alcohol and benzaldehyde only. catalyst, under anaerobic conditions, the area corresponding to
For 1%AuPd/TiO , under anaerobic conditions, disproportiona- benzaldehyde increases initially slightly and very slowly
tion of benzyl alcohol is partially observed because of the bands decreases with time; for the same time period there is a slight
In this article the discussion is important to note that the time in the x-axis starts when the
2
2
ꢀ
1
around 1703 cm , corresponding to benzaldehyde (Fig. 1a and b). increase in the area of the band corresponding to benzyl
As mentioned above toluene could not be observed by any alcohol. This observation is different from what has been seen
characteristic bands. Two specific bands, 1016 cm /1039 cm
bands of benzyl alcohol and 1703 cm band of benzaldehyde similar reaction conditions, where we found that there is a
were chosen and their areas under the band, for different steady increase in the amounts of benzaldehyde and toluene
time intervals, were integrated and presented in Fig. 2. It is at the expense of benzyl alcohol. This difference could be
ꢀ
1
ꢀ1
by GC analysis for the disproportionation reaction under
ꢀ
1
1
2150 Phys. Chem. Chem. Phys., 2013, 15, 12147--12155
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using a micro packed bed reactor. This further substantiates
the fact that benzyl alcohol does not undergo any transforma-
tion over 1%AuPd/MgO catalyst in the absence of atmospheric
oxygen. However, under aerobic condition, there is a steady
increase in benzaldehyde concentration and a corresponding
decrease in the benzyl alcohol concentration, but at a much
lower rate compared to 1%AuPd/TiO
reaction that is active under these conditions is oxidative
dehydrogenation whereas in the case of 1%AuPd/TiO both
2
. This is because the only
2
oxidative dehydrogenation and disproportionation reactions
are active. Finally, reactions of benzyl alcohol, over 1%AuPd/C
016 cm (benzyl alcohol) and 1703 cm (n(CQO) of benzaldehyde) from catalyst, were studied under aerobic and anaerobic conditions
Fig. 2 ATR-IR time on line peak area profiles of benzyl alcohol (left) and
benzaldehyde (right) obtained by integrating the peak area of bands at
ꢀ1
ꢀ1
1
2 2
Fig. 1. Key: black squares: 1%AuPd/TiO , anaerobic; red circles: 1%AuPd/TiO ,
and results are presented in Fig. 1 (i, j for anaerobic condition,
k, l for aerobic conditions). Under anaerobic conditions, in
contrast to 1%AuPd/TiO , there is a steady increase in the
2
aerobic; green triangles: 1%AuPd/MgO, anaerobic; blue triangles: 1%AuPd/
MgO, aerobic; magenta diamond: 1%AuPd/C anaerobic and cyan triangles:
1
%AuPd/C aerobic.
concentration of benzaldehyde with a corresponding decrease
in the benzyl alcohol concentration. This is in line with the GC
attributed to the formation of either hemiacetal or acetal from observation that benzyl alcohol undergoes disproportionation
3
7
benzaldehyde and benzyl alcohol over gold–palladium sup- under these conditions. Under aerobic conditions, using the
ported on an acidic support like TiO , however, their presence same catalyst, benzaldehyde is formed at a much higher rate
2
in the reaction mixture could not be verified by either GC or GC- (Fig. 2). From these in situ ATR-IR spectroscopic studies, the
2
2,36
MS.
to form benzene could not be ruled out; the strong absorption 1%AuPd/C (aerobic) > 1%AuPd/TiO
expected for CO could not be detected by the current instru- (aerobic) > 1%AuPd/C (anaerobic) > 1%AuPd/TiO
ment because of the strong absorption of the diamond 1%AuPd/MgO (anaerobic). Except the 1%AuPd/TiO
sentinel in the characteristic CO stretching vibration region reaction, this trend follows the GC based activity trend.
The consumption of benzaldehyde by decarbonylation rate of formation of benzaldehyde obeys the following trend;
(aerobic) > 1%AuPd/MgO
(anaerobic) =
(anaerobic)
2
2
2
3
7
ꢀ
1
(
1850–2150 cm ). For the same catalyst, under aerobic condi-
DRIFT spectroscopy
tions, (Fig. 1c and d), a substantial increase of the n(CQO) band
of benzaldehyde at 1703 cm and another band at 828 cm
was observed. Furthermore, C–H stretching vibrations of the
ꢀ1
ꢀ1
In order to investigate the surface species, that can cast light on
the reaction mechanism, DRIFT spectroscopic studies were
performed on 1%AuPd/TiO and 1%AuPd/ZnO catalysts dosed
2
ꢀ
1
aldehyde group located at 2818, 2736, 2697 cm
intensity. Corresponding decrease in the area of benzyl alcohol
(
gain in
with small quantities of benzyl alcohol. All standards and
bands observed are shown in Table 1 with spectra shown in
the ESI† (S2). Initial room temperature spectra collected from
experiments performed under an inert atmosphere, for both
catalysts, showed bands associated with benzyl alcohol and
also a carbonyl band centred at 1702 cm , which is indicative
of benzaldehyde. The presence of benzaldehyde was also noted
at the start of the reaction in the ATR-IR experiments under
anaerobic conditions. As previously stated, no disproportionation
ꢀ
1
ꢀ1
1016 cm and 1039 cm ) was also observed (Fig. 2). Water
ꢀ
1
formation results in increase of a band at 3650 cm . The rate
of increase in benzaldehyde band or decrease in the benzyl
alcohol band, under aerobic condition, is substantially higher
than that under anaerobic conditions. From previous studies it
has been proven that under anaerobic conditions, the only
pathway for benzyl alcohol consumption and benzaldehyde
formation is disproportionation whereas under anaerobic con-
ditions, benzaldehyde is formed by both disproportionation
and oxidative dehydrogenation. However, with the present set
of data, we could not distinguish between these two pathways.
For AuPd/MgO supported catalyst, a remarkable feature at
ꢀ
1
ꢀ
1
Table 1 Vibrational modes assignment in the 1800–1300 cm region at 22 1C
1
wt% 1 wt%
Benzoic AuPd/ AuPd/
ꢀ1
3697 cm
showed up under both anaerobic and aerobic Benzyl Benzalde-
alcohol hyde
Toluene acid
TiO
2
N
2
ZnO N
flow
2
conditions (Fig. 1e and g). This is the n(OHMgO) band from
the catalyst support. This assignment of the hydroxyl band is
confirmed by the FT-IR measurement of the fresh 1%AuPd/
(neat) (neat)
(neat) (neat) flow
Assignment
1700
1685
1702
654
1702
n(CQO)
n(CQO)
n(CQC) + d(C–H)
n(CQC) + d(C–H)
d(C–H) + n(CQC)
d(C–H) + n(CQC)
1
ꢀ
1
MgO catalyst that has a sharp band exactly at 3700 cm
1607
1599
1604
1603
1584
1497
1454
1424
1598
1584
1496
1454
1598
1584
1496
1454
1418
(
Fig. S1, ESI†). However, no other bands attributable to species 1586 1584
1496
454
1495
1458
adsorbed on the MgO support could be assigned. Under
anaerobic conditions, there is no increase in the formation of
the benzaldehyde after a few minutes of the start of the
reaction. This initial formation has been attributed to the
oxidative dehydrogenation because of the adsorbed oxygen on
the catalyst surface which has been proven earlier in a study
1
1454
1
424
326
1379
1391
1369
1369
1370
d(O–H)
d(C–H) + n(CQC)
1340
1
1325
1311
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reaction occurs on basic 1%AuPd/ZnO catalysts, but residual electron donation to the group, weakening the CQO bond.
surface absorbed oxygen species on the catalyst surface result This low wavenumber carbonyl band could indicate electron
in some oxidation of benzyl alcohol at the start of the reaction. donation from Au or Pd metal, showing chemisorption of the
As observed in the ATR studies and previous studies in batch species to the metal via the carbon of the carbonyl functional
reactors, total removal of oxygen is difficult and limited oxida- group but might also arise from weak adsorption of benzalde-
ꢀ
1
tion occurs at the start of reactions. It is important to note, hyde to the metal through the carbonyl O-atom [cf. 1667 cm
3
8
3
however, that total removal of oxygen from the 1%AuPd/ZnO for CH CHO on Pt(111) ]. However, it is noted that weak
catalyst surface within a micro reactor set-up has previously adsorption through the carbonyl O-atom is less likely for
shown that no benzyl alcohol oxidation occurs under anaerobic species still bound to the surface at 280 1C. This ambiguity in
3
6
conditions. As oxygen was present even when using inert gas interpretation and the fact that product species are being
flows in the cell, no important information could be elucidated observed means that their structure cannot be used to infer
from changing from inert to oxygen-containing atmospheres how dissociative chemisorption of benzyl alcohol occurs on the
(
spectra from experiments performed under oxygen containing different catalysts. The proposed interaction of the metal with
atmospheres are shown in the ESI† (Fig. S2)). the carbon of the carbonyl functional group seen in the
Heating of the benzyl alcohol dosed catalysts under a flow of 1%AuPd/TiO catalyst might suggest that benzyl alcohol was
2
inert gas resulted in desorption of the benzaldehyde and benzyl chemisorbed by cleavage of a benzylic C–H bond and not the
alcohol by 180 1C. It is important to note the presence of bands O–H bond. The reverse is true if the carbonyl oxygen of the
associated with irreversibly adsorbed species for both 1%AuPd/ product is attached to the metal surface and this is indicative of
TiO and 1%AuPd/ZnO catalyst after heating samples to 280 1C its precursor surface species. Both modes of interaction have
2
1
9
(
Fig. 3). For both catalysts, bands associated with aromatic been previously suggested as possible surface species. As the
CQC and C–H modes (1592–1595 cm and 1493–1496 cm
ꢀ1
ꢀ1
ꢀ1
)
1654 cm
signal was observed only for the 1%AuPd/TiO2
were observed. The 1%AuPd/ZnO catalyst had strong bands at catalyst, which facilitates the disproportionation reaction, it
ꢀ
1
(
1533, 1419 and 1374 cm ), indicative of benzoate species suggests the importance of this chemisorbed species in the
previously observed in in situ ATR infra-red studies of benzyl disproportionation reaction. This is re-enforced by the total
2
3
ꢀ1
alcohol using Pt/Al
were also observed on 1%AuPd/TiO
siderably less intense than that observed on the ZnO catalyst.
2
O
3
catalysts. Bands associated with benzoate absence of the 1655 cm band in the basic ZnO supported
2
, though these were con- catalyst which does not bring about disproportionation.
ꢀ
1
However, in addition to benzoate, a band at 1654 cm with
Inelastic neutron scattering spectroscopy
ꢀ1
a shoulder at 1670 cm was present on the 1%AuPd/TiO
2
ꢀ
1
Fig. 4 and 5 show the adsorption and reaction of benzyl alcohol
on 1%AuPd/TiO and 1%AuPd/ZnO catalyst in the C–H stretch
2
region and Fig. 6 and 7 the corresponding spectra in the
fingerprint region. Reference spectra of pure benzyl alcohol,
benzaldehyde and toluene in the same regions are shown in
catalyst. The band at 1670 cm is indicative of carbonyl bands
associated with a conjugated carboxylic acid group and
2
3
matches that observed for benzoic acid. The lower frequency
is at the low frequency extreme of the range
observed for carbonyl bands and demonstrates significant
ꢀ
1
1654 cm
Fig. 3 DRIFT spectra of benzyl alcohol dosed catalysts after heating the samples
Fig. 4 INS spectra in the C–H stretch region. (a) Bare TiO
2
support after dosing
ꢀ1
to 280 1C (1 1C min ). Key: (a) 1%AuPd/TiO
2
and (b) 1%AuPd/ZnO. Bands
with benzyl alcohol, (b) AuPd/TiO catalyst after 20 h helium purge at 150 1C, (c)
2
labelled are referred to in the text.
after dosing with benzyl alcohol and (d) after heating in flowing helium at 150 1C.
1
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Fig. 7 INS spectra in the fingerprint region. (a) Bare ZnO support after dosing
with benzyl alcohol, (b) 1%AuPd/ZnO catalyst after helium purge at 150 1C, (c)
after dosing with benzyl alcohol, (d) after heating in flowing helium at 150 1C
Fig. 5 INS spectra in the C–H stretch region. (a) Bare ZnO support after dosing
with benzyl alcohol, (b) AuPd/ZnO catalyst after helium purge at 150 1C, (c) after
dosing with benzyl alcohol, (d) after heating in flowing helium at 150 1C and (e)
2
and (e) after reaction with O .
2
after reaction with O .
ꢀ
1
stretch modes at 2850–2950 cm , the aromatic C–H stretch
the ESI† (Fig. S3 and S4). It can be seen that for both supports
under the conditions used here, there is no adsorption of
benzyl alcohol on the bare support, Fig. 4a, 5a, 6a, and 7a;
the observed features are all assigned to species on the AuPd
nanoparticles. The generally weak intensities observed are
consistent with the low metal loading employed.
ꢀ
1
ꢀ1
modes at 3006–3090 cm and the O–H stretch at 3170 cm
Ab initio calculations of the complete unit cell using the
CASTEP code confirm these assignments and also show that
.
4
0
ꢀ1
2
the CH twist is at 1233 cm and the in-plane C–O–H bending
ꢀ
1
modes are at 1445 and 1507 cm , thus overlapping the CH2
scissors modes.
3
9
From infrared and Raman spectra of solid benzyl alcohol,
ꢀ1
Exposure of the supported catalysts to benzyl alcohol results
in new features in both the C–H stretch and fingerprint regions,
Fig. 4c, 5c, 6c, and 7c. Comparison with the reference spectra,
Fig. S3 and S4 (ESI†), suggests that this is benzyl alcohol.
Overlaying the spectra of pure benzyl alcohol and that adsorbed
the out-of-plane C–O–H bending modes are assigned at 695 cm
,
ꢀ1
2
the CH scissors mode at 1435 cm , the methylene C–H
on 1%AuPd/TiO
apart from an additional weak, broad feature at B3210 cm
2
, Fig. 8, shows that the profile is unchanged
ꢀ1
,
suggesting interaction is between the metal and the alcoholic
Fig. 6 INS spectra in the fingerprint region. (a) Bare TiO
with benzyl alcohol, (b) AuPd/TiO catalyst after 20 h helium purge at 150 1C,
c) after dosing with benzyl alcohol and (d) after heating in flowing helium at
50 1C.
2
support after dosing
2
Fig. 8 Comparison of the INS spectra of (a) benzyl alcohol adsorbed on
(
2
1%AuPd/TiO (red trace) and (b) the pure compound (blue trace). The difference
1
spectrum (black trace) is shown in (c) and is plotted on the same scale as (b).
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oxygen. Curve fitting the C–H stretch region of pure benzyl been undertaken in an attempt to identify intermediates and
alcohol, Fig. S3a (ESI†), shows that the ratio of the areas of the surface-bound species that would throw light on the mecha-
CH , aromatic C–H and O–H stretch modes is 1.9 : 6.6 : 1, close nisms involved. ATR-IR examination yielded signals attributa-
2
3
0,32
to the expected
ratio of 2 : 5 : 1. For benzyl alcohol adsorbed ble only to species in the liquid phase and, under an oxygen
ꢀ
1
on 1%AuPd/TiO , after allowance for the 3210 cm band, the atmosphere, these confirmed the support-dependent differences
2
ratio is: 1.6 : 6 : 1, which given the modest signal-to-noise ratio, in behaviour of the catalysts, observed previously by conven-
indicates that the molecule adsorbs intact, suggesting that the tional catalytic studies. In reactions catalysed by 1%AuPd/TiO
2
C–M and O–M species in Scheme 1 are transition states rather under argon, however, the ATR signals attributed to benzalde-
than intermediates. There is a small shift in the O–H stretch hyde decreased with time at the reaction temperature (120 1C)
ꢀ1
transition energy from 3184 down to 3172 cm which would be with a corresponding increase in the signal used as a measure
consistent with benzyl alcohol hydrogen bonding to the neigh- of the benzyl alcohol remaining in the system. This deviation
bouring benzyl alcohol molecule. Possible assignments for the from expectation did not occur when 1%AuPd/C or 1%AuPd/
ꢀ
1
3
210 cm band are to water or hydroxyls that are formed while MgO were the catalysts. A number of possible explanations of
dosing the sample at 150 1C (a temperature that is required to this paradoxical behaviour have been considered, the favoured
prevent physisorption of the benzyl alcohol on the sample). In one being the analytical complication introduced by the equili-
the case of benzyl alcohol on AuPd/ZnO, the quantity of benzyl brium formation of the benzyl hemiacetal or more probably
alcohol we were able to adsorb was much smaller and the signal- acetal of benzaldehyde on the relationship between signal
to-noise ratio is much poorer; however, the profile matches that intensity and concentration of benzaldehyde and benzyl alcohol.
of pure benzyl alcohol, again indicating that the adsorption is A DRIFTS investigation of the same reaction systems revealed
ꢀ
1
molecular. In this case, the 3210 cm band was absent.
only one signal that distinguished the behaviour of the
On heating under anaerobic conditions the difference in acidic (TiO ) and basic (ZnO) supports. Using 1%AuPd/TiO a
2
2
behaviour of the two catalysts is readily apparent, in both the carbonyl signal was observed at the unusually low frequency of
ꢀ
1
C–H stretch and fingerprint regions (Fig. 4d, 5d, 6d, and 7d). 1655 cm . It has not proved possible to determine whether
For the TiO supported catalyst, the spectral profiles in both this arises from attachment of the carbonyl moiety to the metal
regions change significantly. In particular, the modes at B1230 through its carbonyl carbon or its carbonyl oxygen atom and
2
ꢀ
1
and 1460 cm , which are associated with the methylene and thus it has not proved possible to discriminate between
hydroxyl functionalities, weaken dramatically as does the signal previously proposed modes of chemisorption of benzyl alcohol
ꢀ
1
at 2900 cm assigned to the symmetric CH stretch; a new on the metal surface and hence on mechanisms for oxidative
2
ꢀ
1
signal appears at 1620 cm . Comparison with the reference dehydrogenation. Inelastic neutron scattering has thrown
spectra shows that the changes are consistent with the loss of somewhat greater light on the processes occurring at the
benzyl alcohol and formation of a mixture of toluene and catalyst surface. Benzyl alcohol appears to adsorb intact on
ꢀ
1
benzaldehyde, although the loss of methylene signals might the metal surface; a weak additional signal at 3210 cm
also be argued as suggestive of chemisorption of benzyl alcohol remains unassigned. It is clear, that the initial interaction of
by benzylic C–H cleavage as well as benzaldehyde via its benzyl alcohol with the metal catalyst is not by benzylic C–H
carbonyl oxygen. For the ZnO supported material, the profiles cleavage, although this process could be involved subsequently,
do not change greatly, indicating that the only effect is to giving a transient surface species that is undetectable by the
remove some of the adsorbate. Heating the 1%AuPd/ZnO in present technique. We have not found any direct spectroscopic
oxygen resulted in reaction of the benzyl alcohol and evidence for the species shown in Scheme 1 for disproportiona-
desorption of most of the products. The very weak remnant is tion; those mechanistic depictions remain merely rationalisa-
clearly aromatic and probably a mixture of toluene and benzal- tions of the organic transformations that are helpful in
dehyde. In contrast to 1%AuPd/ZnO, where all of the reactions understanding the results from earlier isotopic labelling experi-
could be carried out on the same sample, it was necessary to ments. It is clear that even using a range of in situ vibrational
prepare a separate sample for 1%AuPd/TiO2 since reaction spectroscopies it has proved to be difficult to unravel some of
occurs under anaerobic conditions. Benzyl alcohol was the subtle details in this fascinating reaction and those tech-
adsorbed on the catalyst; however, treatment with oxygen did niques using shorter timescales for observation of reaction
not result in any change in the spectrum.
species need to be considered.
Concluding comments
Acknowledgements
It was found that benzyl alcohol undergoes competitive oxida- The authors thank the EPSRC for funding, the STFC Rutherford
tive dehydrogenation, giving benzaldehyde, and disproportio- Appleton Laboratory for access to neutron beam facilities and
nation, to yield equimolar benzaldehyde and toluene, when Dr James Pritchard and Mr Scott Davies for their assistance
treated with oxygen over an AuPd nanoparticulate catalyst during INS measurements. Computing resources (time on the
supported on titania or carbon, but only oxidative dehydro- SCARF computer used to perform the CASTEP calculations) was
genation when the catalyst is supported on MgO or ZnO. provided by STFC’s e-Science facility. MS thanks the European
In situ spectroscopic examination of reaction mixtures have Research Council (ERC) for awarding a Marie Curie Fellowship
1
2154 Phys. Chem. Chem. Phys., 2013, 15, 12147--12155
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Paper
(
IEF) for Career Development (Award No. 275755), while 19 J. D. Grunwaldt and A. Baiker, Phys. Chem. Chem. Phys.,
JPH would like to express gratitude to the German Research
2005, 7, 3526–3539.
Foundation (DFG) (postdoctoral research grant Ho4579/1-1) 20 In Situ Spectroscopy of Catalyst, ed. B. M. Weckhuysen,
and the Dutch National Research School Combination Catalysis
NRSC-Catalysis) for funding.
American Scientific Publishers, California, 2004.
21 B. M. Weckhuysen, Chem. Commun., 2002, 97–110.
(
2
2 M. Sankar, E. Nowicka, R. Tiruvalam, Q. He, S. H. Taylor,
C. J. Kiely, D. Bethell, D. W. Knight and G. J. Hutchings,
Chem.–Eur. J., 2011, 17, 6524–6532.
References
1
2
3
M. Sankar, N. Dimitratos, P. J. Miedziak, P. P. Wells, 23 C. Keresszegi, D. Ferri, T. Mallat and A. Baiker, J. Phys.
C. J. Kiely and G. J. Hutchings, Chem. Soc. Rev., 2012, 41,
099–8139.
Chem. B, 2005, 109, 958–967.
24 D. Ferri and A. Baiker, Top. Catal., 2009, 52, 1323–1333.
8
D. I. Enache, J. K. Edwards, P. Landon, B. Solsona-Espriu, 25 J. M. Andanson and A. Baiker, Chem. Soc. Rev., 2010, 39,
A. F. Carley, A. A. Herzing, M. Watanabe, C. J. Kiely, D. W.
Knight and G. J. Hutchings, Science, 2006, 311, 362–365.
G. L. Brett, Q. He, C. Hammond, P. J. Miedziak,
N. Dimitratos, M. Sankar, A. A. Herzing, M. Conte, J. A.
4571–4584.
26 S. Chilukoti, E. Widjaja, F. Gao, H. Zhang, B. G. Anderson,
J. W. H. Niemantsverdriet and M. Garland, Phys. Chem.
Chem. Phys., 2008, 10, 3535–3547.
Lopez-Sanchez, C. J. Kiely, D. W. Knight, S. H. Taylor and 27 J. G. Highfield, M. Prairie and A. Renken, Catal. Today,
G. J. Hutchings, Angew. Chem., Int. Ed., 2011, 50, 10136–10139. 1991, 9, 39–46.
L. Prati, F. Porta, D. Wang and A. Villa, Catal. Sci. Technol., 28 J. G. Highfield, M. H. Chen, P. T. Nguyen and Z. Chen,
011, 1, 1624–1629. Energy Environ. Sci., 2009, 2, 991–1002.
R. Garcia, M. Besson and P. Gallezot, Appl. Catal., A, 1995, 29 D. Ferri, M. S. Kumar, R. Wirz, A. Eyssler, O. Korsak, P. Hug,
4
5
6
7
8
9
2
1
27, 165–176.
A. Weidenkaff and M. A. Newton, Phys. Chem. Chem. Phys.,
2010, 12, 5634–5646.
30 P. C. H. Mitchell, S. F. Parker, A. J. Ramirez-Cuesta and
J. Tomkinson, Vibrational spectroscopy with neutrons, with
applications in chemistry, biology, materials science and cata-
lysis, Singapore, 2005.
31 S. F. Parker, Handbook of Vibrational Spectroscopy, John
Wiley & Sons, New York, 2006.
32 S. F. Parker, D. Lennon and P. W. Albers, Appl. Spectrosc.,
2011, 65, 1325–1341.
B. Bachiller-Baeza, A. Guerrero-Ruiz, P. Wang and
I. Rodriguez-Ramos, J. Catal., 2001, 204, 450–459.
P. D. Zgolicz, V. I. Rodriguez, I. M. J. Vilella, S. R. de Miguel
and O. A. Scelza, Appl. Catal., A, 2011, 392, 208–217.
A. M. Ruppert, K. Weinberg and R. Palkovits, Angew. Chem.,
Int. Ed., 2012, 51, 2564–2601.
V. Dal Santo, A. Gallo, A. Naldoni, M. Guidotti and R. Psaro,
Catal. Today, 2012, 197, 190–205.
1
0 B. E. Solsona, J. K. Edwards, P. Landon, A. F. Carley,
A. Herzing, C. J. Kiely and G. J. Hutchings, Chem. Mater., 33 J. Zakzeski, R. J. H. Grisel, A. T. Smit and B. M. Weckhuysen,
006, 18, 2689–2695. ChemSusChem, 2012, 5, 430–437.
1 J. M. Parera and J. N. Beltramini, J. Catal., 1988, 112, 357–365. 34 J. Zakzeski, P. C. A. Bruijnincx and B. M. Weckhuysen, Green
2
1
1
2 C. H. Bamford, C. F. H. Tipper and R. G. Compton, Liquid
Phase Oxidation, Elsevier, Oxford, 1980.
3 R. A. Sheldon and J. K. Kochi, Metal-Catalysed Oxidations of
Organic Compounds, Academic Press, New York, 1981.
4 M. Hudlicky, Oxidation in Organic Chemistry, American
Chemical Society, Washington, DC, 1990.
5 T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037–3058.
6 N. Dimitratos, J. A. Lopez-Sanchez, D. Morgan, A. F. Carley,
R. Tiruvalam, C. J. Kiely, D. Bethell and G. J. Hutchings,
Phys. Chem. Chem. Phys., 2009, 11, 5142–5153.
7 G. Kovtun, T. Kameneva, S. Hladyi, M. Starchevsky,
Y. Pazdersky, I. Stolarov, M. Vargaftik and I. Moiseev, Adv.
Synth. Catal., 2002, 344, 957–964.
Chem., 2011, 13, 671–680.
35 I. P. Silverwood, N. G. Hamilton, A. McFarlane,
R. M. Ormerod, T. Guidi, J. Bones, M. P. Dudman,
C. M. Goodway, M. Kibble, S. F. Parker and D. Lennon,
Rev. Sci. Instrum., 2011, 82, 1–7.
36 E. Cao, M. Sankar, E. Nowicka, Q. He, M. Morad,
P. J. Miedziak, S. H. Taylor, D. W. Knight, D. Bethell,
C. J. Kiely, A. Gavriilidis and G. J. Hutchings, Catal. Today,
2012, 203, 146–152.
1
1
1
1
37 M. Sankar, E. Nowicka, R. Tiruvalam, Q. He, S. H. Taylor,
C. J. Kiely, D. Bethell, D. W. Knight and G. J. Hutchings,
Chem.–Eur. J., 2011, 17, 6524–6532.
1
38 H. Zhao, J. Kim and B. E. Koel, Surf. Sci., 2003, 538, 147–159.
1
8 S. Meenakshisundaram, E. Nowicka, P. J. Miedziak, 39 D. A. Prystupa, A. Anderson and B. H. Torrie, J. Raman
G. L. Brett, R. L. Jenkins, N. Dimitratos, S. H. Taylor, Spectrosc., 1994, 25, 175–182.
D. W. Knight, D. Bethell and G. J. Hutchings, Faraday 40 S. K. Nayak, R. Sathishkumar and T. N. G. Row, CrystEng-
Discuss., 2010, 145, 341–356. Comm, 2010, 12, 3112–3118.
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