Steady-State and Transient Kinetic Studies of the Acetoxylation of Toluene over Pd-Sb/TiO2

Article abstract of DOI:10.1021/acscatal.6b00646

A combination of steady-state catalytic tests, transient studies with isotopic tracers, and kinetic modeling was used to derive detailed insights into the individual reaction pathways in the course of toluene acetoxylation over a Pd-Sb/TiO₂ catalyst. This reaction can be considered as an environmentally friendly route for the production of benzyl alcohol. Benzyl acetate and benzaldehyde are the only products formed from toluene, while acetic acid gives CO₂ in addition to benzyl acetate. The Arrhenius plots revealed apparent activation energies for formation of benzyl acetate and benzaldehyde of 24.9 and 27.5 kJ mol^-1, respectively, thus, indicating that these products originate from the same surface intermediate, i.e. benzyl cation. The corresponding value for CO₂ formation was 152.9 kJ mol^-1. Transient isotopic studies and their kinetic evaluation demonstrated the participation of lattice oxygen and adsorbed oxygen species in activation of acetic acid, with the latter species favoring oxidation of the acid to CO₂.

Full text of DOI:10.1021/acscatal.6b00646

Research Article
pubs.acs.org/acscatalysis
Steady-State and Transient Kinetic Studies of the Acetoxylation of
Toluene over Pd−Sb/TiO2
Sven Reining, Evgenii V. Kondratenko,* Narayana V. Kalevaru, and Andreas Martin
̈
Leibniz-Institut fur Katalyse e.V., Albert-Einstein-Strasse 29a, 18059 Rostock, Germany
*
S Supporting Information
ABSTRACT: A combination of steady-state catalytic tests,
transient studies with isotopic tracers, and kinetic modeling
was used to derive detailed insights into the individual reaction
pathways in the course of toluene acetoxylation over a Pd−Sb/
TiO catalyst. This reaction can be considered as an
2
environmentally friendly route for the production of benzyl
alcohol. Benzyl acetate and benzaldehyde are the only
products formed from toluene, while acetic acid gives CO2
in addition to benzyl acetate. The Arrhenius plots revealed
apparent activation energies for formation of benzyl acetate
−1
and benzaldehyde of 24.9 and 27.5 kJ mol , respectively, thus, indicating that these products originate from the same surface
−1
intermediate, i.e. benzyl cation. The corresponding value for CO formation was 152.9 kJ mol . Transient isotopic studies and
2
their kinetic evaluation demonstrated the participation of lattice oxygen and adsorbed oxygen species in activation of acetic acid,
with the latter species favoring oxidation of the acid to CO2.
KEYWORDS: acetoxylation, toluene, SSITKA, oxygen isotopic exchange, transient kinetics, Pd
3
1
. INTRODUCTION
approximately 20 h stable operation. The effect of Sb on the
catalytic performance has already been thoroughly investigated
It was demonstrated that the
majority of Sb remains on the surface of TiO as a pentavalent
Alcohols are an important class of compounds in the chemical
industry, in particular, with benzyl alcohol being the most
industrially key aromatic alcohol. It is used as solvent for
8
,12−14
in our previous studies.
1
2
oxide, whereas a small part of Sb is incorporated into Pd
particles (as zerovalent Sb) under reaction conditions. In fact,
Sb incorporation promotes growth of Pd particles. Formation
of large Pd particles (50−100 nm) during the course of the
reaction is necessary for enhancing catalytic performance.
Despite the positive effect of Sb on catalyst stability, the
performance is, still, not sufficient for possible industrial
applications. In order to improve catalyst stability or to develop
novel long-term stable catalysts, it is highly important to
surface-coating materials and resins as well as in the
manufacturing of perfumes and flavors. The global market
1
,2
for the two latter product classes was approximately $6.3 billion
3
in 2006. Its average annual growth rate is estimated to be 4.1%
4
during the years 2014−2019. Benzyl alcohol is mainly
produced through hydrolysis of the harmful intermediate
5
benzyl chloride. From an environmental viewpoint, this
process is not ecofriendly, because HCl is formed as a side
product and additionally chlorine is used to produce benzyl
chloride.
The synthesis of benzyl acetate via acetoxylation of toluene
followed by subsequent hydrolysis can be considered as a
15
understand mechanistic aspects of catalyst functioning. To
the best of our knowledge, no mechanistic studies of toluene
acetoxylation were reported up to now. Several contributions,
however, dealt with the mechanism of ethylene acetoxylation to
“
green and clean” alternative route for the production of benzyl
1
6−25
6
,7
vinyl acetate over supported Pd−Au catalysts,
which is an
alcohol. Our group has intensively investigated acetoxylation
of toluene over supported Pd-containing catalysts with the
purpose of optimizing their composition
conditions. The major drawback of such materials is their
on-stream behavior, i.e. there is an activation phase followed by
a stable operation for a few hours and then deactivation caused
important chemical intermediate used as reactant for polymer-
ization reactions. After the pioneering work of Samanos et
al., it is in general accepted that the reaction proceeds via a ß-
26
8
,9
and reaction
16
3
hydride elimination step. As the main side product, CO is
2
formed by combustion of ethylene in the absence and the
presence of acetic acid in a temperature range of 413−453 K.
The present contribution was aimed to systematically
elucidate possible reaction pathways leading to selective and
10,11
by coke deposition.
A relatively high long-term stability
could, however, be achieved using Cu as a promoter for Pd due
to the formation of a core−shell structure with a Pd core and a
CuO shell. The activation period was unfortunately established
12,13
to be too long, i.e. around 60 h.
The best compromise in
Received: March 3, 2016
Revised: June 2, 2016
terms of on-stream performance is a Pd−Sb/TiO catalyst,
2
which needs an activation time of around 6 h followed by
©
XXXX American Chemical Society
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nonselective products in the course of toluene acetoxylation
and to determine the nature of oxygen species participating in
these processes. In order to complete the mechanistic picture,
we combined both steady-state catalytic tests and transient
experiments with isotopic tracers, i.e. steady-state isotopic
transient kinetic analysis (SSITKA) and switch experiments
with selected feed components. Primary and secondary
transformations of feed components and reaction products as
well as the rate-limiting step(s) were derived from steady-state
catalytic tests. Surface precursors and their lifetime were
accessed from SSITKA experiments including their kinetic
evaluation. Oxygen isotopic exchange measurements provided
information about (i) the kind (reversible vs irreversible
conversion (X) of feed components and product selectivity (S)
were calculated using eqs 1 and 2 respectively
x
i,out
X = 1 − x
i,in
(1)
(2)
xp,out
Sp,i
=
xi,in − xi,out
where x and x stand for mole fractions of feed components
i
p
and reaction products, respectively. Subscripts in and out are
used to distinguish between the inlet and outlet fractions.
The absence of external mass-transfer limitations was proven
by performing catalytic tests at different total flows but constant
modified residence time as suggested in ref 27. The absence of
mass-transfer limitations inside pores of catalyst particles was
checked experimentally as well. To this end, catalytic tests were
performed with catalyst particles of different sieve fractions,
while the ratio of catalyst amount to total flow was held
(
non)dissociative adsorption) of interaction of gas-phase O2
with Pd−Sb/TiO and (ii) nature of active surface oxygen
2
species. On the basis of the results obtained, a detailed
mechanistic scheme was developed, and some key aspects for
an optimal reactor operation are suggested.
2
. EXPERIMENTAL SECTION
.1. Catalyst Preparation. The acetoxylation catalyst with
27
constant. Importantly, all these experiments were carried out
2
at 503 K, i.e. the highest reaction temperature applied in the
present study. As a result, catalytic tests at lower reaction
temperatures are also free of mass-transfer limitations. Addi-
tionally, the absence of mass and heat-transfer limitations was
proven by theoretical calculations according to Mears, Weisz,
a nominal composition of 10 wt % Pd-16 wt % Sb/TiO was
2
prepared in two steps according to the procedure previously
described in ref 3. To be brief, Sb O was initially deposited
2
3
from aqueous slurry onto the TiO2 support followed by
calcination in air at 673 K for 3 h. Hereafter, the calcined
catalyst precursor was further impregnated with a solution of
28,29
and Prater (Table S1).
.3. Catalytic Tests under Transient Conditions.
2
PdCl in diluted HCl at 343 K followed by removal of excess
2
Various transient experiments were carried out in an in-house
developed setup at 2 bar and 483 K. This setup is equipped
with an online quadrupole mass spectrometer (Balzers Omni
Star) and an online gas chromatograph (Agilent 7890 equipped
with TCD and FID) to analyze gas-phase components with
second and minute resolution, respectively. The mass
spectrometer was also applied to distinguish between isotopi-
cally labeled compounds. A tubular fixed-bed quartz reactor
with an internal diameter of 6 mm was employed. The gaseous
components were dosed by mass flow controllers (Brooks).
The liquid reactants (toluene and acetic acid) were dosed
separately by two HPLC-pumps (Shimadzu), vaporized, and
afterward mixed with the gaseous feed. A 4-way valve with
pneumatic drive was applied for switching between different
feed mixtures within a time, which was significantly shorter than
the response of the catalytic system. Prior to all experiments
described below in detail, the catalyst was activated according
to the same procedure mentioned in the Experimental Section
describing steady-state catalytic tests (section 2.2). The absence
of mass-transfer effects in the transient experiments has been
proven by theoretical calculations as suggested in ref 30. Those
authors also showed that heat-transfer limitations in transient
experiments can be neglected if their absence is already proven
for steady-state catalytic tests.
H O by simple heating of the wet solid at around 423 K. The
2
obtained dry powder material was then calcined at 873 K in a
He flow for 4 h. The concentrations of Pd and Sb determined
by inductively coupled plasma optical emission spectrometry in
the calcined catalyst were 9.6 and 12.5 wt %, respectively. The
loss of Sb is probably due to partial formation of volatile SbCl3
upon catalyst calcination in He. The amount of Pd remained
practically constant during the preparation procedure.
2
.2. Steady-State Catalytic Tests. Steady-state catalytic
experiments were performed in a tubular fixed-bed quartz
reactor with an internal diameter of 6 mm. A toluene/acetic
acid/O /(Ne+CH ) = 1/4/3/17 reaction feed was used in all
2
4
catalytic tests at a total pressure of 2 bar and four reaction
temperatures, i.e. 443, 463, 483, and 503 K. Thereby, CH (5
4
mol % in Ne) was used as internal standard. The total molar
−1
flow was adjusted to 100 mmol h , and the amount of catalyst
particles of 400−600 μm) was varied between 0.1 and 0.6 g to
(
achieve different modified residence times and, thus, different
degrees of conversion of feed components. The catalyst was
diluted with corundum particles in a weight ratio of 3. The
toluene/acetic acid mixture and the gaseous feed components
were used without any further purification and dosed by
HPLC-pump (Shimadzu LC-10Ai) and by mass flow
controllers (Bronkhorst), respectively. Before entering the
reactor, the liquid reactants were vaporized and mixed with the
gaseous compounds statically, i.e. by provoking turbulence in
the gaseous stream. The feed components and the reaction
products were quantified by an online gas chromatograph
The measured mass spectroscopic signals were treated
mathematically to get values between zero and one as shown
in eq 3
I(t) − I(0)
F(t) =
(
Shimadzu GC 2010) equipped with a thermal conductivity
I(∞) − I(0)
(3)
detector (to analyze permanent gases) and a flame ionization
detector (to analyze hydrocarbons and oxygenates). Prior to all
measurements, a fresh catalyst sample was activated at 483 K
for 6 h under continuous flow conditions in the reactant feed
where F(t) is the transient response of a compound, whereas
I(t), I(0), and I(∞) are the transient mass spectroscopic signal,
the corresponding signal under steady-state conditions before
switching the valve, and the signal under the new steady-state
achieved after the switching, respectively.
mixture mentioned above. A modified residence time of 24
kgcatalyst s mol−
1
was adjusted for this activation. The
feed
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2
.3.1. Steady-State Isotopic Transient Kinetic Analysis
ing to the time scale of the experimental data. Afterward, the
Levenberg−Marquardt-algorithm was applied to fit the model
to the experimental data using the sum of least-squares as
(
SSITKA). For SSITKA experiments, a quartz reactor was filled
with 0.4 g of catalyst (200−400 μm) diluted by the 3-fold
amount of corundum particles. Toluene/acetic acid/ O /Ar/
He = 1/4/3/3/14 and toluene/acetic acid/ O /He = 1/4/3/
7 mixtures were used as reaction feeds. A modified residence
16
36,37
objective function.
2
1
8
Reasonable starting values were determined as follows: first
of all, the experimental data for each component were fit
respectively to models, which were as simple as possible until a
satisfying description was reached. These different models
represent various adsorbate precursors leading to gas-phase
reaction products, i.e. single pool, two pools in series, and two
pools in parallel and one pool in series to the prior parallel
pools, respectively. The obtained rate constants for the models
describing the transients of the single components were used as
starting values for the overall model.
2
1
−1
time was adjusted to 24 kg s mol . Ar was used for taking the
gas-phase hold-up into account, whereas He was an inert
standard. After reaching the steady-state with the nonlabeled
feed, a switch to the mixture containing labeled 18O was
2
carried out. A similar SSITKA experiment was performed with
.2 g of catalyst without the previous activation procedure and
using O /Ar/He = 5/5/91 and O /He = 5/96 mixtures.
The SSITKA experiments were performed under non-
differential reactor conditions, and, thus, we have taken into
account the concentration profiles along the catalyst bed.
Consequently, we applied a reactor model similar to refs 31 and
2 to evaluate the SSITKA tests with toluene/acetic acid/O2
kinetically (see eqs 4 and 5)
0
16
18
2
2
2.3.2. Transient Response Method. Quartz reactor was filled
with 0.2 g of catalyst diluted by the 3-fold amount of corundum
particles. A modified residence time of 8 kg s mol− was
adjusted. After the activation procedure of 6 h, the catalyst was
heated at atmospheric pressure in a flow (30 mL min− in total)
1
3
1
2
of 5 mol % CH in Ne up to 523 K for 15 min to clean its
4
∂
∂
ci
t
1 ∂ci
τ ∂x
1 ∂ c
ρb
εb
i
=
−
+
+
∑ v r
surface from adsorbates formed during the activation
procedure. Hereafter, the reactor was cooled to 483 K with a
simultaneous increase in pressure to 2 bar followed by
2
ij j
τBo ∂x
j
(4)
(5)
∂
θi,s
switching to the toluene/acetic acid/O /Ar = 1/4/3/17
2
=
∑
v r
ij j
reaction mixture. An additional experiment was performed, in
∂
t
j
which we switched between toluene/acetic acid/O /Ar = 1/4/
2
3
/17 and toluene/acetic acid/(CH +Ne) = 1/4/20 reaction
where τ represents the hydrodynamic residence time in s, Bo is
4
−1
feeds.
the dimensionless Bodenstein number (Bo = uLD ), ρ is the
ax
b
−3
density of the catalyst bed in kg m , ε is the void fraction of
b
the catalyst bed (dimensionless), x is the axial position in the
3. RESULTS AND DISCUSSION
catalyst bed (dimensionless), t is the time in s, c is the
i
−3
3.1. Overall Reaction Pathways in Toluene Acetox-
ylation. For deriving insights into overall reaction pathways of
product formation in the course of toluene acetoxylation,
steady-state catalytic tests were performed at different contact
times to achieve different degrees of conversion of feed
components; the longer the contact time, the higher the
conversion was observed as expected. The purpose of these
tests was to investigate the effect of conversion on the
selectivity to gas-phase products, which were benzyl acetate,
benzaldehyde, and carbon dioxide. The selectivity was
calculated from the corresponding yields and the conversion
of either toluene or acetic acid according to eqs 1 and 2. Such
calculations enable to check if some other not-detected
products (for example surface deposits) were also formed
from feed components.
We shall begin with the discussion of product selectivity
calculated with respect to toluene. It is important to highlight
that irrespective of toluene conversion an overall selectivity to
benzyl acetate and benzaldehyde (both products are definitively
formed from toluene) was around 100%. This strongly suggests
that carbon dioxide is practically not formed from toluene and/
or the above reaction products. It should probably originate
concentration of component i in mol m , and Θ is the
i,s
−1
surface concentration of the intermediate i in mol kg
.
cat
Thereby, the void fraction was set to 0.45, and the density of
the catalyst bed was determined using a graduated cylinder and
−3
set to 2000 kg m . The following initial and boundary
conditions were used to solve the system of (partial) differential
equations.
The following equations show the initial conditions (eq 6)
1
8
and boundary conditions for products (eq 7) and O (eq 8)
2
ci(t = 0) = 0; θi(t = 0) = 0
(6)
∂
∂
2c
i
=
0;
ci
= 0
x=0
2
x
x=1
(7)
(8)
ci
= 6 − exp(−10t)
x=0
An input function was used as boundary condition for 18O2
due to the numerical difficulties applying a Heaviside function,
i.e. an ideal step input. Thereby, the molar concentration of 6
mol m− was used as input for O , which is equivalent to the
3
18
2
oxygen partial pressure of 0.24 bar adjusted in the experiment.
To solve the system of (partial) differential equations, a suitable
procedure for evaluation of transient kinetic data using the
PDEPE routine as suggested in ref 33 was adopted. This
algorithm enables solving partial differential equations in one
space dimension. Thereby, the numerical method of lines is
used, which reduces the system of partial differential equations
to a system of ordinary differential equations by discretization
from acetic acid (see below). Therefore, the selectivity to CO
2
was not used for analyzing the selectivity-conversion plot on
toluene basis in Figure 1a. In a broad conversion range, the
selectivity to benzyl acetate was well above 90% and only
slightly decreased with an increase in the conversion. Since the
selectivity to benzaldehyde increased accordingly, it can be
concluded that benzyl acetate primarily formed from toluene
was further converted to benzaldehyde to a small extent. The
latter product should also be formed directly from toluene
otherwise its selectivity would be zero when toluene conversion
3
4,35
in axial direction.
Due to the different time scales of the
numerical solution and the experimental data, the obtained
solution was interpolated to get the concentrations correspond-
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Figure 1. Selectivity-conversion plots on the basis of toluene (a) and
acetic acid (b) at 483 K (squares) and 503 K (circles).
Figure 2. Arrhenius plot of the rates of formation of main gas-phase
reaction products.
is approaching zero. This direct pathway plays, however, a
minor role in the course of toluene acetoxylation.
form a benzyl cation, which can react with either surface acetate
or the surface hydroxyl-group to yield benzyl acetate and
benzaldehyde, respectively. CO2 should be formed from
Figure 1b shows the selectivity-conversion plot obtained on
acetic acid basis. It is obvious that overall product selectivity is
well below 100%, which gives indications that another
product(s) must be formed from acetic acid. Since no
additional gas-phase components were detected by our online
GC analysis, we suggest that acetic acid might be converted to
another intermediate as concluded from the significantly higher
−1
E value of 152.9 ± 10.9 kJ·mol . Further mechanistic insights
a
into CO formation were derived from transient experiments
2
described and thoroughly discussed in the sections below.
3
.2. Transient Experiments: Mechanistic Insights into
carbon deposits and/or CO ; conversion of toluene and/or
2
Individual Reaction Pathways. The so-called transient
benzyl acetate to these products can be excluded when bearing
response method was used to further elucidate the reaction
in mind the results shown in Figure 1a. CO selectivity did not
42−44
2
mechanism.
In this way, we switched from an inert flow to
change with rising conversion of acetic acid at 483 K. From a
mechanistic viewpoint, we put forward that acetic acid is
a reactive flow containing toluene, acetic acid, and oxygen and
monitored transient responses of feed components and reaction
products by means of an online MS. Such time-resolved
analyses enable the investigation of desired and side reactions
under different coverages by surface species, i.e. from a clean
catalyst surface to that with steady-state concentration of
surface intermediates. As described in the Experimental Section,
the transient responses of feed components and reaction
products were normalized with respect to the corresponding
steady-state concentrations (see eq 3). The normalized
concentration of zero represents the steady-state under inert
flow, and one is related to the new steady-state under the
reaction feed.
decomposed to gas-phase CO and surface carbon-containing
2
19,38−41
species in agreement with previous studies.
The latter
species can be further oxidized to CO as concluded from the
2
fact that CO selectivity at 503 K increased with an increase in
2
acetic acid conversion.
In summary, different reaction pathways of toluene and
acetic acid transformations in the course of toluene
acetoxylation were identified from the analysis of the
dependence of product selectivity on conversion of individual
feed components. Toluene is mainly converted to benzyl
acetate and to a minor extent to benzaldehyde. In addition to
the desired acetoxylation, acetic acid decomposition to CO2
and surface carbonaceous species also occurs to a certain extent.
To derive further kinetic insights into toluene acetoxylation,
we performed additional steady-state tests at different reaction
temperatures. The idea behind these tests was to determine
Figure 3 illustrates the obtained transients before and after
switching. Obviously, the normalized CO and H O concen-
2
2
trations strongly increase within the first 10 s in the reaction
feed and then decrease to approach the steady-state values after
5
0 s on stream. The concentration of toluene also passes a
apparent activation energies (E ) of formation of benzyl acetate,
a
maximum, which is, however, significantly less pronounced
benzaldehyde, and carbon dioxide and to check whether these
products are formed from the same intermediate. Since the
tests were performed under integral conditions, we treated all
experimental data according to the below procedure to
calculate initial differential reaction rates. The yield of each
reaction product was plotted as a function of modified
residence time (τ). Afterward, these data were fitted by a
polynomial function of second order (in the case of benzyl
acetate) and linearly (benzaldehyde and CO ). The derivative
2
of the respective function was calculated at τ = 0 to obtain the
initial reaction rate. This procedure was performed for different
temperatures. The obtained Arrhenius plots for each product
are given in Figure 2. The E value determined for benzyl
a
−1
acetate was around 24.9 ± 6.1 kJ·mol and did not significantly
Figure 3. Transient responses after a step input of the toluene/acetic
−1
differ from the value of 27.5 ± 7.8 kJ·mol for benzaldehyde.
This result strongly suggests that these products are formed
with the same rate-limiting step. As will be demonstrated
below, the slow step is the activation of adsorbed toluene to
acid/O /Ar = 1/4/3/17 reaction feed. Reactants: toluene (■), acetic
2
acid (▲), O ( ); products: H O ( , red), CO (Δ, dark blue),
●
◇
2
2
2
benzyl acetate (○, light blue), benzaldehyde (□, gray), T = 483 K, p =
2 bar.
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than for CO and H O. For all other reaction products and feed
which O was removed from the standard toluene acetoxylation
2
2
2
components, their concentration increased continuously from
zero to the steady-state value without passing a maximum with
rising reaction time.
feed and then again cofed after a certain period of time. This
sequence was repeated two times. The total pressure and partial
pressures of acetic acid and toluene were kept constant. Figure
4 shows the normalized concentrations of CO , H O, and O in
It is worth mentioning that the transient of acetic acid is
delayed by approximately 10 s with respect to oxygen (see the
insert in Figure 3) and toluene. This indicates that toluene was
stable, while acetic acid was completely consumed within this
period of time, where the formation of CO and H O reached
2
2
2
2
2
the maximal values. In agreement with the results of steady-
state tests in Figure 1, the results shown in Figure 3 further
support that acetic acid undergoes decarboxylation to CO and
2
H O on the clean catalyst surface. This conclusion is also
2
supported by previous DFT calculations of the mechanism of
38
acetic acid decomposition on Pd(111). Surface acetate was
predicted to form upon interaction of gas-phase acetic acid with
free Pd surface sites. This intermediate turns in parallel to the
0
surface via a free Pd site followed by decomposition to CO ,
Figure 4. Transient responses of CO , H O, and O after switching
2 2 2
2
19,38,39
H O, and coke species.
The experimental results from
from the toluene/acetic acid/O
/Ar = 1/4/3/17 reaction mixture to a
2
2
the present study suggest that this undesired reaction pathways
is hindered in the presence of adsorbates formed from acetic
acid and/or toluene.
We now discuss the responses of benzaldehyde and benzyl
acetate. One can see that the concentration of the former
product increased steeply in comparison with that of benzyl
acetate. Such difference in the temporal evolution of these
transients can be understood as follows. Since acetic acid was
flow of 5 mol % CH in Ne at 483 K and 2 bar.
4
the absence and the presence of the latter in the standard
toluene acetoxylation feed. They were normalized with respect
to the values obtained under steady-state reaction conditions in
the presence of all reactants. In comparison with CO and H O,
2
2
the concentration of O sharply decreases or increases upon
2
removal of O from the reaction feed or by cofeeding of O ,
2
2
decarboxylated to CO and H O within the first seconds on
2
2
thus indicating that gas-phase O weakly interacts with the
2
stream, its surface coverage was low, and, thus, the formation of
benzyl acetate was hindered. Contrarily, high water concen-
tration during this period of time should favor formation of
hydroxyl groups, which can react with the benzyl cation formed
from toluene to yield benzyl alcohol. This alcohol can be easily
oxidized to benzaldehyde as proven in separate experiments on
benzyl alcohol oxidation at 483 K using benzyl alcohol/acetic
catalyst. It is important to note that all reaction products
including benzyl acetate and benzaldehyde were formed in the
absence of gas-phase O but in lower amounts.
2
CO was formed by decarboxylation of acetic acid as already
2
explained above. An overshoot in the CO transients was
2
observed again (Figure 3), but the formation pathways of CO2
must be different in this case due to an increased coverage by
various surface intermediates. We assume an oxidation of
surface coke species as a reason for the overshoot in the CO2
acid/O /inert = 1/4/3/17 feed mixture (Figure S1). Benzyl
2
alcohol was nearly fully converted with a high selectivity to
benzaldehyde of around 95%. In spite of the presence of acetic
acid, only small amounts of benzyl acetate were formed (Figure
S1). In the same way, separate experiments were carried out
using benzaldehyde as feed component instead of toluene. No
conversion could be obtained that is in accordance with the
selectivity-conversion plot shown in Figure 1 (a).
formation. Interestingly, no overshoot in the H O response was
2
observed, which is in contrast to Figure 3. The reason is the
difference in the reaction pathways of H O formation. In this
2
case, H O is mainly formed as byproduct in the synthesis of
2
benzyl acetate, because the surface is covered by the reactants
in contrast to the results shown in Figure 3 where the surface
was free of adsorbates before switching to a reactive feed. Thus,
Further mechanistic insights can be obtained from the
analysis of the responses at higher reaction times, i.e. higher
coverage of the catalyst surface. In contrast to the response of
CO , the response of H O passed through a minimum (see
H O was mainly formed by complete oxidation of acetic acid
2
which cannot take place in the absence of gas-phase O so that
2
2
2
only the decarboxylation can occur and hence a CH group
x
Figure S2). After passing the minimum at around 125 s (Figure
), the H O response increased in the same way as the
response of the target product benzyl acetate. This indicates
remains on the surface. The responses of benzyl acetate and
benzaldehyde further underline this conclusion because both
3
2
showed similar characteristics as the response of H O did (not
2
two different reaction pathways of H O formation. At low
shown here).
2
surface coverage, H O is formed through the oxidative
The formation of H O even in the absence of gas-phase O
2
2
2
decomposition of acetic acid as discussed above. After passing
can be explained by the participation of lattice oxygen.
Hydrogen abstracted from toluene forms a Pd−H species
which can be removed from the surface by remaining hydroxyl
groups that contain lattice oxygen. The reoxidation of the
catalyst is not possible anymore due to the missing gas-phase
the minimum in H O concentration, the decomposition is
2
hindered because the surface becomes covered by adsorbates
formed from acetic acid and toluene. Therefore, the formation
of benzyl acetate dominates, and H O is mainly formed
2
through the target reaction. Thereby, the latter rate of H O
O which in turn resulted in an exponential decrease in the rate
2
2
formation is much slower in comparison to the first reaction
of H O formation.
2
pathway of H O formation from acetic acid at low surface
coverages.
For clarifying the role of lattice oxygen in the acetoxylation of
toluene, an additional transient experiment was performed, in
3.3. Steady-State Isotopic Transient Kinetic Analysis.
To get insights into the kind (reversible vs irreversible) of
interaction of O with the Pd−Sb/TiO catalyst, we also
2
2
2
investigated oxygen isotopic exchange over fresh or activated
4
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catalyst samples. Irrespective of the catalyst state, O O was
Research Article
1
6
18
Three differently labeled carbon dioxide species were
1
6
18
16
16 18
18
not observed and the responses of O and O were very
observed: C O , C O O, and C O with the latter being
2
2
2
2
16
similar to the response of the inert tracers and overlaid with
formed in traces (Figure 6). The transient of C O fell slightly
2
them (see Figure 5). Thus, gas-phase O was concluded to
within approximately 10 min on stream after switching to
2
18
toluene/acetic acid/ O /He = 1/4/3/17, while those of
2
16
18
18
C O O and C O increased. Hereafter, the concentration
2
of all carbon dioxide isotopes did not practically change with
1
6
rising time-on-stream. C O was always the main isotope.
2
Since 16O was not present in the reaction feed and Pd−Sb/
TiO did not practically exchange its lattice oxygen with gas-
2
2
phase O , the precursor of CO must contain at least one
2
2
oxygen atom. On the basis of the results of steady-state tests in
sections 3.1 and 3.2, acetic acid should be the only precursor in
the formation of CO . This statement is actually supported by
2
1
6
the SSITKA results shown in Figure 6. Similar to C O and
2
16
18
16 16
C O O, the amount of CH C O OH decreases, while the
3
16
18
Figure 5. Transient responses of oxygen (open symbols) and helium
formation of CH C O OH increases within approximately
3
1
6
18
(
solid lines) after switching from O /Ar/He to O /He over the
the first 10 min on stream after switching to toluene/acetic
2
2
18
activated Pd−Sb/TiO catalyst at 483 K.
acid/ O /He = 1/4/3/17 and then their concentrations do not
2
2
alter much with time-on-stream. From a mechanistic point of
view, surface acetate can partially exchange its oxygen followed
by reconstitution to acetic acid and desorption into the gas-
interact with the catalyst weakly and adsorbed irreversibly. A
similar result was obtained for O activation on the Pd/Cd/
2
16
18
24
phase or decomposition to C O O and surface carbon
species. Previous DFT calculations consider a similar
silica/K catalyst used in the acetoxylation of ethylene.
For analyzing a possible role of lattice and adsorbed oxygen
species in CO and H O formation as well as in transformation
38
mechanistic concept for the acetate-Pd(111) system.
2
2
The SSITKA results were kinetically evaluated to estimate
the number of surface intermediates leading to H O and CO
2
of acetic acid, a SSITKA experiment was performed. In this
case, Pd−Sb/TiO was treated at 483 K in toluene/acetic
2
2
16
and their surface residence time as well as to check if different
acid/ O /Ar/He = 1/4/3/3/14 until steady-state operation
2
oxygen species are formed from gas-phase O . In this context,
was achieved. Hereafter, this feed was replaced by a toluene/
2
18
we initially estimated mass transport parameters (Bo and τ in eq
acetic acid/ O /He = 1/4/3/17 feed for approximately 15 min
2
4
) from the transient response of He used as an inert tracer in
followed by switching back to the reaction feed without
16
18
our tests. The Bo and τ values were 8.9 and 11.8 s, respectively.
The low value of Bo can be explained by the dead volume
before the reactor. Thus, around 21 s of the measured data
directly after switching the valve were not taken into account
whereby Bo accordingly decreased. Taking the dead volume
into account, a Bo number of 104 was calculated from the jump
in He concentration by the standard procedure known from
isotopically labeled oxygen (Figure 6). No formation of O O
was observed, which is in good agreement with the results
discussed above in the context of Figure 5. These results also
indicate that both feed components and reaction products do
not affect the mechanism of oxygen activation.
4
5−47
chemical reaction engineering textbooks.
For Bo numbers
46
larger than 80, plug flow can be assumed. Thus, the applied
procedure had no influence on the kinetic parameters.
The rate constant of the adsorption of 18O (k ) was
2
ads
determined manually by performing numerical simulations of
the reactor model using different values of k . This procedure
ads
was applied until the experimentally measured O conversion of
2
1
4.1% at the reactor outlet was adjusted in the numerical
simulation. The conversion of toluene and acetic acid was
0.8% and 16.8%, respectively. Thus, a simplified kinetic
evaluation of SSITKA experiments for differential flow
4
48,49
reactors
was not trustworthy. Therefore, an integral reactor
model was applied to kinetically evaluate the acetoxylation
under such high degrees of conversion. As a first step, we used
transient responses of isotopically labeled water, carbon
dioxide, and acetic acid to estimate the number of independent
surface intermediates (pools), from which respective gas-phase
products are formed. The simplest model (one pool) was
initially used to describe the experimental data. If this fitting
was not satisfied, the model complexity was increased by adding
one or several additional pools.
As a result, the model with two pools in series described the
1
8
transient response of C O correctly. Two pools in parallel and
Figure 6. Transient responses of labeled and nonlabeled carbon
dioxide and acetic acid obtained from the SSITKA experiment at 483
K and 2 bar (for details see section 2.3.1).
2
one pool in series to the previous pools were necessary to
describe the C O O and H O responses, and two pools in
18
16
18
2
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ACS Catalysis
Research Article
series could fit the data of labeled acetic acid satisfactorily. The
heterogeneity (several pools) of surface intermediates was
additionally validated by the below theoretical analysis of the
derivative of the experimental responses at t = 0. A positive
derivative indicates a direct formation of the product from one
surface intermediate. A derivative of zero resulting from a
consecutive surface reaction means the necessity of a minimum
of two pools in series. In our case, the derivatives of the
transients at t = 0 showed a value of zero for all the labeled
compounds. Thus, the necessity of at least two pools in series
as mentioned above is confirmed. On the basis of these results,
two models using different oxygen species were developed (see
Figure 7).
Figure 8. Comparison between normalized experimental responses of
16
18
18
16 18
He (○), C O O (◇), H O (□), and CH C O OH (Δ) and their
2
3
simulated counterparts calculated under assumption of participation of
two different (a) or one (b) oxygen species in formation of water,
carbon dioxide, and acetic acid at 483 K and 2 bar.
and one selective oxygen species, which forms the labeled acetic
acid as well as H O as byproduct of the target reaction, i.e. the
2
formation of benzyl acetate. The respective kinetic parameters
determined by data fitting as mentioned in the Experimental
Section are given in Table 1.
Table 1. Kinetic Parameters of Individual Reaction Pathways
in Figure 7(a) Obtained from Fitting SSITKA Experiments
rate constant/s−
1
k1
k2
k3
k4
k5
k6
k7
k8
k9
k10
k11
0.0232
0.0351
0.0176
0.0657
0.336
0.0028
0.4
0.512
0.0129
0.0578
0.167
Figure 7. Mechanistic schemes representing individual reaction steps
leading to labeled acetoxylation products with participation of two
different oxygen species (a) or one oxygen species (b).
The most important point extracted from Table 1 is the fast
insertion of oxygen into the Pd lattice (k ) forming the selective
8
The labeled 18O adsorbs irreversibly on the surface of the
oxygen species. In other words, the surface residence time of
the unselective oxygen species is very low. This result obtained
from kinetic analysis is in accordance with the high selectivity to
benzyl acetate (89.8%) based on toluene conversion measured
in the SSITKA experiment and also in separate catalytic tests
discussed above (see Figure 1). Furthermore, it is worthwhile
2
1
8
catalyst to yield an unselective oxygen species ( O ) which is
s
1
8
then inserted into the lattice ([ O]) in a subsequent step. CO
2
and H O are formed from the unselective oxygen species as a
2
consequence of acetic acid decomposition. H O is also formed
2
from the selective oxygen species as a side product in the
formation of the target product benzyl acetate. For the labeled
acetic acid, a reversible adsorption step (k ) was assumed
to compare the different rate constants of CO formation, i.e.
2
18
18 16
k , k , k , and k . The formation of C O and C O O from
1
1
1
4
5
7
2
because acetic acid is a reactant in the reaction system, and,
thus, this assumption is reasonable.
the unselective oxygen species is fast and proceeds with
comparable rate constants values (k , k ), but the release from
5
7
18
The experimental data for the labeled products CO , H O,
the surface is slow in case of C O (k ). The big difference of 1
2
2
2 1
1
8
16
and acetic acid and the respective fits are shown in Figure 8.
order of magnitude between the rate constants of C O O
1
8
For the sake of better clarity, the results for C O are not
formation (k , k ) confirms the assumption of different reaction
2
4
5
shown in Figure 8, but they are given in the Supporting
Information (Figure S3).
pathways. A high activation barrier must be overcome for the
18
16
formation of C O O from labeled surface acetate (k ). This
4
The model presented in Figure 7a describes the experimental
data of all labeled products well. An F-test as applied in ref 50,
for instance, showed on the 95% level that model a) can
describe the data significantly better than model b) (see Figure
result is in agreement with the high activation energy of 152.9
kJ mol− obtained for the formation of nonlabeled CO from
1
2
acetic acid (see Figure 2). Besides the mentioned aspects, the
formation of H218O from lattice oxygen ([ O]) should be
briefly discussed, because this reaction pathway contains a slow
18
8b). Consequently, we assume the participation of two oxygen
species in the acetoxylation of toluene: an unselective oxygen
species, which takes part in the decomposition of acetic acid,
step (k ) compared to other rate constants. As mentioned
above, the H-abstraction from toluene is assumed as rate-
9
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ACS Catalysis
Research Article
limiting step; this slow step is also involved in the formation of
CONCLUSIONS
■
H O due to the removal of the abstracted hydrogen by oxygen.
2
The reaction mechanism of the acetoxylation of toluene over
Thus, the kinetic analysis confirms the assumption of the rate-
limiting step by the slow rate of H O formation (k ).
the Pd−Sb/TiO catalyst was elucidated, for the first time, by
2
2
9
means of steady-state catalytic tests and transient kinetic studies
3.4. Mechanistic Concept for Tuning Catalyst Per-
with isotopic tracers. The breaking C−H bond in the methyl
group of toluene on Pd sites was found to be the rate limiting
step leading to surface benzyl cation. This intermediate reacts
with OH or with surface acetate formed from acetic acid to
yield benzyl alcohol or the main product benzyl acetate,
respectively. The former product is easily oxidized to
benzaldehyde. All these products and toluene were established
formance. On the basis of the present research and our
0
5
1,52
previous studies,
as well as under consideration of literature
19,53
data on acetoxylation of lower olefins
adsorption of acetic acid
and decomposition/
over Pd-containing catalysts, we
38−41
suggest the following mechanistic concept of acetoxylation of
toluene. Metallic Pd and PdO are the two catalytically active
sites participating in the activation of the feed components.
Both these species coexist within one Pd nanoparticle. Metallic
Pd species are responsible for heterolytic splitting of the C−H
bond in the methyl group of toluene resulting in adsorbed
benzyl cation and hydride species, whereas acetic acid
dissociates on PdO sites to yield adsorbed acetate and a
to be stable against oxidation to CO , which originates from
2
acetic acid exclusively. The detailed kinetic evaluation of
isotopically labeled product responses using numerical
simulations suggested that adsorbed oxygen species participate
in CO formation, while lattice oxygen is responsible for the
2
formation of surface acetate which reacts further with the
benzyl cation to the target benzyl acetate. On the basis of the
obtained results, suggestions were deduced for an enhanced
reactor operation.
2
+
2−
proton connected to Pd and O , respectively. The acetate
species mainly reacts with the benzyl cation to the desired
benzyl acetate but can also be converted to CO , H O, and
2
2
coke precursors via an additional active site, i.e. probably
metallic Pd. Another side reaction in the course of toluene
acetoxylation is the coupling of benzyl cation with a
neighboring hydroxyl group to form benzyl alcohol, which is
easily oxidized to benzaldehyde even in the presence of acetic
acid (Figure S1). The formed aldehyde is stable against its
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b00646.
further conversion to benzyl acetate, CO , or carbon deposits.
As concluded from the above-described SSITKA experiments,
the desired benzyl acetate formation takes place with
2
Transient responses of CO , H O, and benzyl acetate at
2
2
1
8
higher times; SSITKA data of C O and the
2
corresponding fits; TPR/TPO cycle of the spent Pd−
Sb/TiO2 catalyst; product distribution while dosing
benzyl alcohol instead of toluene in the reaction feed
mixture; theoretical calculations of transport limitations
participation of lattice oxygen species responsible for H O
2
generation from hydride and hydroxyl species connected to
2+
metallic Pd and Pd , respectively. For charge compensation,
2
+
Pd should be reduced to Pd upon the formation of H O by
2
(PDF)
2
+
taking electrons from the hydride. The Pd /Pd redox cycle has
been previously suggested for acetoxylation of lower olefins.
53
AUTHOR INFORMATION
The detailed mechanism of reoxidation of metallic Pd remains
■
*
unclear, i.e. whether Pd is oxidized by gas-phase O directly or
Corresponding Author
E-mail: evgenii.kondratenko@catalysis.de.
2
by lattice oxygen from the cocomponent Sb which is initially
oxidized by gas-phase O2.
Notes
Bearing in mind the above mechanistic scheme, the
selectivity to benzyl acetate should depend on the rates of
benzyl cation reaction with surface acetate (desired reaction
pathway) or hydroxyl groups (undesired side reaction). Thus, a
high partial pressure of acetic acid should be applied to increase
the coverage by the former surface intermediates. As
demonstrated in this work, acetic acid is not involved in the
rate-limiting step of the desired reaction. Thus, an increased
acetic acid/toluene feed ratio leads to a decrease in the
conversion of acetic acid. The reduced acetic acid conversion is
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Dedicated to Prof. Angelika Bru
60th birthday. Dr. Sergey Sokolov is gratefully acknowledged
for performing the TPR/TPO tests.
̈
ckner on the occasion of her
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