Supported catalysts based on Pd–In nanoparticles for the liquid- phase hydrogenation of terminal and internal alkynes: 2. catalytic properties

Article abstract of DOI:10.1134/S0023158416050141

Pd–In/Al₂O₃ and Pd–In/MgAl₂O₄ catalysts prepared from dinuclear Pd–In acetate complexes were studied in the hydrogenation of alkyne compounds with different structures. The Pd–In catalysts demonstrate high selectivity in the hydrogenation of internal alkynes comparable with that of the Lindlar catalyst. Similar activity/selectivity characteristics are reached at a significantly lower Pd content. For terminal alkynes, the favorable effect of Indium introduction is considerably less pronounced. An analysis of the In effect on the selectivity and the ratio between the rates of the first and second hydrogenation steps suggests that the reaction selectivity is determined to a large extent by a thermodynamic factor (adsorption–desorption equilibrium between the reactants and the reaction products).

Full text of DOI:10.1134/S0023158416050141

ISSN 0023-1584, Kinetics and Catalysis, 2016, Vol. 57, No. 5, pp. 625–631. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © P.V. Markov, G.O. Bragina, G.N. Baeva, I.S. Mashkovskii, A.V. Rassolov, I.A. Yakushev, M.N. Vargaftik, A.Yu. Stakheev, 2016, published in Kinetika
i Kataliz, 2016, Vol. 57, No. 5, pp. 629–635.
Supported Catalysts Based on Pd–In Nanoparticles for the Liquid-
Phase Hydrogenation of Terminal and Internal Alkynes:
2. Catalytic Properties
P. V. M a r k ov a, G. O. Braginaa, G. N. Baevaa, I. S. Mashkovskiia, A. V. Rassolova,
I. A. Yakushevb, M. N. Vargaftikb, and A. Yu. Stakheeva, *
aZelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
bKurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
*e-mail: st@ioc.ac.ru
Received May 16, 2016
Abstract—Pd–In/Al2O3 and Pd–In/MgAl2O4 catalysts prepared from dinuclear Pd–In acetate complexes
were studied in the hydrogenation of alkyne compounds with different structures. The Pd–In catalysts
demonstrate high selectivity in the hydrogenation of internal alkynes comparable with that of the Lindlar cat-
alyst. Similar activity/selectivity characteristics are reached at a significantly lower Pd content. For terminal
alkynes, the favorable effect of Indium introduction is considerably less pronounced. An analysis of the In
effect on the selectivity and the ratio between the rates of the first and second hydrogenation steps suggests
that the reaction selectivity is determined to a large extent by a thermodynamic factor (adsorption–desorption
equilibrium between the reactants and the reaction products).
Keywords: bimetallic catalysts, Pd, In, liquid-phase hydrogenation, terminal alkynes, internal alkynes, diphe-
nylacetylene, phenylacetylene, 1-phenyl-1-propyne, kinetics, mechanism
DOI: 10.1134/S0023158416050141
INTRODUCTION
get semihydrogenation products in the reaction. One
of the main methods to improve Pd catalysts is by
modifying them with another metal followed by the
formation of bimetallic species possessing a higher
selectivity [7–9].
In recent years, catalysts based on Pd–Ga
nanoparticles have been studied intensively because of
their extremely high selectivity in both gas-phase
hydrogenation and liquid-phase reactions [10–13]. It
is assumed that this is due to the formation of single-
atom Pd sites separated from one another by Ga atoms
on the Pd–Ga nanoparitcle surface [14, 15]. It is this
combination of a geometric factor (formation of iso-
lated Pd sites) and an electronic factor (Pd ↔ Ga
interaction) that favors an increase in the yield of the
target products.
The partial hydrogenation of unsaturated hydro-
carbons is widely used in the basic and fine organic
synthesis industries. The selective hydrogenation of
acetylene contained in ethylene is a large-tonnage
process for purification of pyrolysis ethylene per-
formed prior to its polymerization [1]. The selective
hydrogenation of phenylacetylene (PA) is used in the
purification of styrene from trace amounts of PA for
polystyrene production, since even negligible amounts
of PA poison metallocene polymerization catalysts
[2]. The partial hydrogenation of disubstituted
alkynes is one of key steps in the synthesis of different
complex molecules, which involves the formation of
a new C–C bond, for example, through Sonogashira
cross-coupling, i.e., the addition of an organic moiety
of a halide to a terminal alkyne to form a disubstituted
molecule [3] followed by its selective hydrogenation.
A promising alternative to Pd–Ga systems is Pd–
In catalysts [16]. Like gallium, indium is in Group
IIIA of the Periodic Table, so it is an electronic ana-
The most commonly used catalysts for alkyne logue of Ga. Similar to the Pd–Ga system, the Pd–In
hydrogenation are Pd-based systems [4–6]. Monome- catalysts also tend to form isolated Pd sites on the sur-
tallic supported palladium catalysts are highly active face of bimetallic nanoparticles [17]. A potential
and convenient for performing the process, since they advantage of the Pd–In catalysts over the Pd–Ga ones
allow easy separation of the catalyst from reaction consists in their higher stability and resistance to oxi-
products. The main limitation of these catalysts is dation [18]. Even upon a short-term exposure of an
their low selectivity due to the participation of the tar- activated Pd–Ga catalyst to air (for example, during
625

626
MARKOV et al.
loading the catalyst in a reactor), the Ga component is
oxidized into its oxide and bimetallic particles decom-
pose. This results in a decrease in the selectivity of the
process, a serious problem faced with Pd–Ga systems
in liquid-phase hydrogenation [13].
However, the properties of Pd–In catalysts in the
selective liquid-phase hydrogenation of substituted
alkynes have almost not been studied. The aim of the
present work was to study their catalytic characteristics
in the hydrogenation of alkyne compounds with dif-
ferent structures, such as diphenylacetylene (DPA,
symmetric internal alkyne), 1-phenyl-1-propyne
(PhPr, asymmetric internal alkyne), and phenylacety-
lene (PA, terminal alkyne).
Pd–In catalysts were prepared using the acetate
complex PdIn(CH3COO)5, which ensured the forma-
tion of homogeneous bimetallic nanoparticles [17].
Since a support can have a certain effect on the catalytic
properties [19, 20], Al2O3, which possess weakly acidic
properties, and MgAl2O4, which has basic properties,
were chosen for catalyst preparation. A comparison of
their catalytic characteristics revealed a support effect
on the catalytic properties of Pd–In nanoparticles.
The reaction mixture was analyzed by gas chroma-
tography/mass spectrometry (GC/MS) using a Mae-
stro-2 system (Interlab, Russia). Separation was per-
formed on an HP5-MS (5% phenyldimethylsiloxane)
column with a length of 30 m, an inner diameter of
0.25 mm, and a stationary phase film thickness of
0.25 μm (helium as the carrier gas, electron-impact
ionization, 70 eV). Spectra were processed in the
MSD ChemStation program [22] using the NIST14
mass-spectral library [23].
The rates at the first and second steps of hydroge-
nation (r1 and r2, respectively) were determined from
hydrogen uptake vs. reaction time curves by measuring
the slope of their rectilinear segments at an H2/alkyne
ratio of 0.2–0.6 for the first step and 1.1–1.5 for the
second step as the ratio of the H2 absorption rate to the
−1
catalyst weight (mmol H2 min–1
).
mgCat
The catalytic activity of synthesized samples was
also evaluated as the turnover frequency (TOF) of the
catalyst at the first (TOF1) and second (TOF2) steps of
hydrogenation on the basis of the reaction rates at
these steps (see above). In the case of bimetallic cata-
lysts, the number of surface Pd atoms is difficult to
determine from electron microscopy data because part
of the surface can be occupied by atoms of the second
metal. So, the TOF value in this work was calculated
as the ratio of the number of converted substrate mol-
ecules (absorbed Н2 molecules) to the total number of
Pd atoms in the catalyst.
EXPERIMENTAL
Preparation of Catalysts
In the present work, the properties of mono-
(0.5% Pd) and bimetallic (1% Pd–1% In) samples
supported on γ-Al2O3 (Sasol, 150 m2/g) and MgAl2O4
(Sasol, 40 m2/g) were studied. The samples were pre-
pared by incipient-wetness impregnation from solu-
1
tions of [Pd(NH3)4](NO3)2 and PdIn(CH3COO)5 ,
respectively, dried in air at room temperature, and
reduced in a flow of (5 vol % Н2)/Ar at 550°C. These
conditions were found to ensure the formation of
homogeneous bimetallic particles [17]. The reference
catalyst was the commercially available Lindlar cata-
lyst—5% Pd–PbO/CaCO3 (Aldrich).
The selectivity to alkene formation (S=) was deter-
mined using GC/MS data for the reaction mixture:
S= = n= (n= + n−),
where n= and n– are the mole fractions of alkenes and
alkanes, respectively.
Liquid-Phase Hydrogenation
The liquid-phase hydrogenation of alkynes was
carried out in an autoclave-type reactor equipped with
a magnetic stirrer, an electronic pressure sensor, and a
system for gas dosing and liquid phase sampling. The
substrates were DPA (98%), PA (98%), and PhPr
(99%) received from Aldrich. The solvent was n-hex-
ane (98%, Merck). Measurements were performed at
25°C under an initial hydrogen pressure of 10 bar. The
kinetic regime of the process was controlled according
to the procedure described in detail earlier [21].
The efficiency of kinetic control of the process was
estimated by comparing the ratios of alkyne and
alkene hydrogenation rates, r1/r2 [24].
RESULTS AND DISCUSSION
Data analysis was based on the general mechanism
of stepwise alkyne hydrogenation proposed by Bond
[25, 26]. The reaction scheme adapted to the case of
selective liquid-phase hydrogenation is shown below:
1
PdIn(CH COO) was synthesized by the group of Prof.
3
5
M.N. Vargaftik within RFBR project no. 15-03-07652.
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SUPPORTED CATALYSTS BASED
627
(solution)
(solution)
Ph C C R1
Ph C C R1
(II)
(IV)
+ 2H
(I)
+ 2H
(III)
Ph C C R1(ads.)
R1 = Ph, CH3, H
Ph C C R1(ads.)
Ph C C R1(ads.)
According to this mechanism, the route and selec- erably more pronounsed inflection of the kinetic
tivity of hydrogenation are governed by the following curves after consumption of 1 equiv of H2.
two factors:
(1) Kinetic factor (mechanistic factor, according to
For the samples supported on the Mg–Al spinel,
modification with indium leads to a much more sig-
nificant decrease in the activity in the first step of
hydrogenation (TOF1 decreases by a factor of ~8, from
Bond). In case the rate of hydrogenation of the form-
ing alkene intermediate is comparable with the rate of
its desorption into the liquid phase, the selectivity of
the process is determined by the ratio between the
hydrogenation rate constant in step III and the
desorption rate constant in step II. The contribution
from this pathway is especially significant at low con-
versions of the parent alkyne, when the amount of
produced olefin intermediate is not high.
9.8 to 1.2 s–1) and to a significant retardation of the
second step of the process (TOF2 decreases by a factor
of 18, from 1.46 to 0.08 s–1). This increases the r1/r2
ratio from 6.7 to 16.0, which is slightly below the ratio
for the Al2O3-supported samples.
An analysis of the selectivity to alkene formation as
a function of the DPA conversion (Fig. 2a) showed
that at low DPA conversions selectivity approaches
100% for all of the samples. This suggests that the con-
tribution from the direct hydrogenation of the forming
alkene intermediate (step III) to the total process is
insignificant and the forming alkene is predominantly
desorbed to the solution via pathway II.
With an increasing DPA conversion, the selectivity
decreases continuously for all of the catalysts. Accord-
ing to the scheme, this results from the increase of
alkene concentration in solution, which starts to
replace the parent alkyne from the catalyst surface as a
result of competitive adsorption, due to which the
contribution from process III increases, resulting in
the formation of the total hydrogenation product.
At high alkyne conversion, the introduction of In
into the Pd–In/Al2O3 catalyst results in a significant
increase in selectivity. For example, when the DPA
conversion is 90%, the selectivity of the process over
Pd–In/Al2O3 is ~98%, whereas the selectivity with the
monometallic samples is ~87%. Characteristics of
Pd–In/Al2O3 are comparable with the commercial
Lindlar catalyst and even exceeds it at high DPA con-
versions.
(2) Thermodynamic factor. This factor dominates at
high conversions, when the amount of intermediate
hydrogenation product is large and the selectivity of
the process is determined by competitive
alkyne/alkene adsorption. If the heat of alkyne
adsorption is considerably higher than that of the
forming olefin, the catalyst shows a high selectivity
until the complete conversion of the parent alkyne due
to the fact that the alkyne actually blocks the palla-
dium surface.
Diphenylacetylene Hydrogenation
A comparison of kinetic curves for hydrogen
uptake during the hydrogenation of DPA over the
mono- and bimetallic catalysts showed that the rate of
hydrogenation over the monometallic Pd catalysts is
noticeably higher than the rate of hydrogenation over
the indium-modified samples (Fig. 1a). For both the
monometallic and bimetallic catalysts, the kinetic
curves display a pronounced bend after the consump-
tion of 1 equiv of H2, suggesting a decrease in the rate
of hydrogenation on transition from the first to the
second step of the process. This agrees with the quan-
titative analysis data based on r1/r2 values (Table 1).
For Pd–In/MgAl2O4, as well as for Pd–In/Al2O3,
hydrogenation is mainly suppressed at the second step,
as is evidenced by the r1/r2 ratio for the Pd–In catalysts
being considerably larger than for the monometallic
Pd catalysts. However, although In demonstrates sim-
ilar effects on the kinetics of the processes over these
catalysts, for Pd–In/MgAl2O4, in contrast to Pd–
For the Al2O3-supported catalysts, modification
with indium results in a fourfold decrease in the activ-
ity in the first step of hydrogenation: TOF1 decreases
from 9.7 to 2.4 s–1. The rate of hydrogenation in the
second step falls to a still greater extent: TOF2
decreases by a factor of 27, from 2.5 to 0.09 s–1, and, as In/Al2O3, the increase in selectivity due to modifica-
a consequence, r1/r2 increases from 3.9 to 27.5. The
tion with In is less pronounced (Fig. 2а). Based on the
dominant contribution from the second step to the scheme and data obtained, one can conclude that the
retardation of the process is evidenced by the consid- selectivity of DPA hydrogenation over the bimetallic
KINETICS AND CATALYSIS Vol. 57 No. 5 2016

628
MARKOV et al.
SDPE
(а)
–1
A ,
mol H2 (mol DPA)
(а)
H2
1.0
0.9
0.8
0.7
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Pd/Al2O3
Pd–In/Al2O3
Pd/MgAl2O4
Pd–In/MgAl2O4
Lindlar
Pd/Al2O3
Pd–In/Al2O3
Pd/MgAl2O4
Pd–In/MgAl2O4
Lindlar
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0
10 20 30 40 50 60 70 80 90 100
XDPA
τ, min
(b)
Sphenylpropene
AH , mol H2 (mol PhPr)−1
1.0
2
2.0
(b)
1.8
0.9
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Pd/Al2O3
0.8
0.7
Pd–In/Al2O3
Pd/MgAl2O4
Pd–In/MgAl2O4
Lindlar
Pd/Al2O3
Pd–In/Al2O3
Pd/MgAl2O4
Pd–In/MgAl2O4
Lindlar
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
XPhPr
0
10 20 30 40 50 60 70 80 90 100
(c)
Sstyrene
τ, min
1.0
AH , mol H (mol PA)−1
2
2
2.0
(c)
1.8
0.9
0.8
0.7
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Pd/Al2O3
Pd–In/Al2O3
Pd/MgAl2O4
Pd–In/MgAl2O4
Lindlar
Pd/Al2O3
Pd–In/Al2O3
Pd/MgAl2O4
Pd–In/MgAl2O4
Lindlar
0.6
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
XPA
0
10 20 30 40 50 60 70 80 90 100
τ, min
Fig. 2. Olefin selectivity (S ) as a function of alkyne con-
=
Fig. 1. Hydrogen uptake as a function of reaction time for
the liquid-phase hydrogenation of different substituted
alkynes over Pd-containing catalysts: (a) diphenylacetylene,
version (X) over mono- and bimetallic catalysts in the
selective hydrogenation of terminal and internal alkynes:
(a) diphenylacetylene, (b) 1-phenyl-1-propyne, and
(b) 1-phenyl-1-propyne, and (c) phenylacetylene.
=
P
H2
(c) phenylacetylene.
= 10 bar, T = 25°C, [C ] =
P
H2
≡
–1
–1
10 bar, T = 25°C, [C ] = 0.160 mol L , m = 2.5 and
0.160 mol L , m = 2.5 and 5.0 mg (see Table 1); n-hex-
≡
Cat
cat
5 mg (table); n-hexane as a solvent.
ane as a solvent.
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SUPPORTED CATALYSTS BASED
629
Kinetic characteristics of the liquid-phase hydrogenation of different substrates over mono- and bimetallic catalysts
r1 × 103* r2 × 103*
TOF1** TOF2** r1/r2
Substrate
Catalyst
m, mg
0.5%Pd/Al2O3
2.5
2.5
2.5
2.5
5.0
27.38
13.83
27.70
6.85
6.98
0.50
4.10
0.43
0.51
9.71
2.45
9.83
1.22
0.15
2.47
0.09
1.46
0.08
0.02
3.9
27.5
6.7
1%Pd–1%In/Al2O3
0.5%Pd/MgAl2O4
1%Pd–1%In/MgAl2O4
16.0
8.5
5%Pd–PbO/CaCO3
(Lindlar)
4.32
0.5%Pd/Al2O3
2.5
2.5
5.0
5.0
5.0
9.24
7.58
7.36
2.42
2.15
6.14
5.22
4.53
2.00
0.08
3.28
1.34
2.61
0.43
0.07
2.18
0.93
1.61
0.36
0.01
1.5
1.5
1%Pd–1%In/Al2O3
0.5%Pd/MgAl2O4
1%Pd–1%In/MgAl2O4
1.6
CH3
1.2
5%Pd–PbO/CaCO3
(Lindlar)
14.8
0.5%Pd/Al2O3
1.5
2.5
2.5
5.0
5.0
11.33
3.45
8.27
0.97
9.06
16.67
19.09
4.70
3.92
0.61
2.93
0.17
0.32
5.92
3.39
1.67
1.41
0.15
0.67
1%Pd–1%In/Al2O3
0.5%Pd/MgAl2O4
1%Pd–1%In/MgAl2O4
0.2
1.8
0.1
2.1
CH
7.96
5%Pd–PbO/CaCO3
(Lindlar)
4.35
−
–1
–1
* mmol of H min
. ** s .
mgcat
2
Pd–In catalysts is mainly determined by the thermo- sumption. This suggests that the rates of reaction are
dynamic factor rather than the kinetic factor. Proba- similar at both steps of the process.
bly, the introduction of In increases the difference
between the heats of adsorption of the parent alkyne
and the resulting alkene, which favors retention of the
high selectivity of the Pd–In/Al2O3 catalyst up to high
DPA conversions.
Conversely, despite the kinetically favorable
increase in the r1/r2 ratio for the Pd–In/MgAl2O4 cat-
alyst as compared to Pd/MgAl2O4 catalyst, the selec-
tivities of these catalysts are almost identical (Fig. 2a).
It seems that, for the MgAl2O4-supported Pd–In
nanoparticles, the change in the heats of alkyne and
alkene adsorptions as a result of modification with
indium is considerably less pronounced than for Pd–
In/Al2O3; as a consequence, the selectivity of bimetallic
catalyst is similar to that of the monometallic sample.
The introduction of indium decreases the rate of
hydrogenation for both Pd–In/Al2O3 and Pd–
In/MgAl2O4, but, in contrast to what is observed in
the hydrogenation of DPA, modification with indium
does not lead to preferential retardation of the second
stage. Both TOF1 and TOF2 decrease by a factor of
2.3–2.5 (see table). As a result, the r1/r2 ratio remains
virtually identical for the monometallic and bimetallic
catalysts, but selectivity to olefin formation for the
Pd–In catalyst is considerably higher than that of the
monometallic samples in a wide range of PhPr conver-
sions (Fig. 2b). According to the model proposed by
Bond (see the scheme), this can be explained and
adapted to the case of the liquid-phase selective
hydrogenation as follows.
At low PhPr conversions, the selectivity of mono-
metallic catalysts is lower than 100%. For example, it
is ~95% for Pd/Al2O3 and ~90% for Pd/MgAl2O4.
Since, at the very begining of the reaction, the PhPr
conversion is low and alkene readsorption (thermody-
namic factor) does not contribute significantly, it can
be assumed that the hydrogenation of the adsorbed
alkene intermediate proceeds via path III (mechanis-
tic factor). For the Pd–In samples, the initial selectiv-
ity approaches 100%, indicating inhibition of the reac-
1-Phenyl-1-Propyne Hydrogenation
Figure 1b plots the amount of hydrogen uptake as a
function of the time of PhPr hydrogenation. In con-
trast to DPA hydrogenation, the kinetics curves have
almost no inflection at the point corresponding to
transition from the first hydrogenation stage (triple
bond hydrogenation) to the second one (double bond
hydrogenation) in the region of equimolar H2 con-
KINETICS AND CATALYSIS Vol. 57 No. 5 2016

630
MARKOV et al.
tion III. Thus, for these samples the rate of alkene trol the process and decrease its selectivity. However,
desorption (pathway II) exceeds the rate of hydrogena- an analysis of the selectivity to phenylacetylene forma-
tion of the adsorbed alkene intermediate (pathway III), tion as a function of PA conversion (Fig. 2c) reveals no
which results in the predominant formation of the tar- significant negative effect of this parameter on the
get product at the early stages of the process.
selectivity. For the Pd–In/Al2O3 catalyst modification
with indium results in a noticeable increase in the ini-
tial selectivity (at low PA conversions). This effect is
also observed for the Pd–In/MgAl2O4 catalyst but to a
Within the high-conversion region, the reaction
selectivity decreases for both the monometallic and
bimetallic catalysts. The decrease in the selectivity is
more gradual for the Pd–In samples and is dramatic considerably lesser extent.
for the monometallic catalysts, which is more pro-
nounced at high alkyne conversions. Evidently, the
thermodynamic factor dominates over the kinetic.
Therefore, the introduction of In significantly effects
the ratio between the adsorption energies of alkyne
and alkene, increasing the alkyne coverage of the
metal surface even at high alkyne conversions.
Note, however, that both Pd–In catalysts, regard-
less of the nature of support, are characterized by a sig-
nificantly increased selectivity (Fig. 2b) exceeding the
selectivity of the commercial Lindlar catalyst. For
example, the selectivity of the Al2O3-supported sam-
ples at a PhPr conversion of 90% increases from ~78
(for monometallic sample) to 90% (for bimetallic cat-
alyst). For the MgAl2O4-supported catalysts, the selec-
tivity at a PhPr conversion of ~90% increases from 80 to
89%, whereas the selectivity of the Lindlar catalyst is
~83%.
At low alkyne conversions, the Lindlar catalyst
possesses the highest selectivity (~93%), while these
parameters for Pd/MgAl2O4 and Pd-In/MgAl2O4 are
almost identical, while for Pd/Al2O3 and Pd–In/Al2O3
catalyst, the monometallic sample has a higher selec-
tivity (~84 and ~75%, respectively) (Fig. 2c). This fact
is in good agreement with the literature. For example,
some researchers showed that, for terminal alkynes the
hydrogenation of the adsorbed alkene intermediate
(pathway III) is dominant because of the too high
reaction rates due to the triple bond geometry in the
hydrocarbon molecule [27, 28].
Thus, this study revealed a significant effect of In
on both the activity and selectivity of the Pd catalysts
in the hydrogenation of alkyne compounds. This
effect depends to a considerable extent on the nature
of the substrate. The In-modified catalysts possess a
higher selectivity in hydrogenation of internal alkynes
which is comparable with that of the Lindlar catalyst.
It should be specially noted that the indium-modified
samples show activity/selectivity characteristics simi-
lar to those of the Lindlar catalyst at a significantly
lower Pd content (1 wt % versus 5 wt % Pd). This
makes the Pd–In system a promising catalyst for
industrial and laboratory practices. In the case of ter-
minal alkynes, the favorable effect of the introduction
of In is considerably less pronounced.
Modification of the monometallic Pd sample with
indium did not reveal a direct correlation between the
change in the kinetic parameters of the reaction (ratio
between the rates of first and second hydrogenation
stages) and the selectivity of the process. This possibly
indicates that the selectivity of the reaction is mainly
determined by the thermodynamic factor, namely, the
adsorption–desorption equilibrium between reagents
and reaction products. Note also the considerable
support effect on both the selectivity and activity of
catalysts, which depends on the nature of substrate.
The best characteristics in DPA hydrogenation were
attained with Pd–In/Al2O3; in PhPr hydrogenation, a
higher selectivity is shown by the magnesium alumi-
nate–supported catalyst Pd–In/MgAl2O4.
Phenylacetylene Hydrogenation
A quite different performance is observed in the
hydrogenation of PA. In the case of indium-modified
catalysts, the kinetic curves for hydrogen uptake
(Fig. 1c) display a pronounced bend after the consump-
tion of 1 equiv of H2, indicating an obvious acceleration
of the reaction on passing from the alkyne hydrogena-
tion stage to the alkene hydrogenation stage.
A quantitative analysis of TOF1 and ТОF2 data
(table) shows that modification with indium results in
a more than 6-fold decrease in TOF1 (from 3.9 to
0.6 s–1) for the Al2O3-supported catalyst and a 17-fold
decrease in TOF1 (from 2.93 to 0.17 s–1) for the
MgAl2O4-supported catalyst. Thus, at the triple bond
hydrogenation stage, the effect of modification with
indium is similar to the effects observed in the hydro-
genation of internal alkynes (see above).
However, at the phenylethylene hydrogenation,
modification with indium results only in a slight
decrease in TOF2 (~1.5-fold for the Al2O3-supported
catalyst) or has almost no effect on TOF2 (for the mag-
nesium aluminate–supported catalysts). As a result,
the r1/r2 ratio decreases to 0.2–0.1 (table) for Pd–
In/Al2O3 and Pd–In/MgAl2O4.
ACKNOWLEDGMENTS
In the general case, an increase in the rate of the
The authors are grateful to the International Ana-
second hydrogenation stage relative to the rate of the lytical Center at the Zelinsky Institute of Organic
first one can both make it difficult to kinetically con- Chemistry of the Russian Academy of Sciences
KINETICS AND CATALYSIS
Vol. 57
No. 5
2016

SUPPORTED CATALYSTS BASED
631
13. Wowsnick, G., Teschner, D., Armbrüster, M., Kasat-
kin, I., Girgsdies, F., Grin, Yu., Schlögl, R., and Beh-
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Soc., 2010, vol. 132, p. 14745.
15. Osswald, J., Giedigkeit, R., Jentoft, R.E.,
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Grin, Yu., and Schlögl, R., J. Catal., 2008, vol. 258,
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(ZIOC RAS) for the GC/MS analyses. This work was
performed at the ZIOC RAS and was supported by the
Russian Science Foundation (grant no. 16-13-10530).
The bimetallic complex used as the active component
precursor was synthesized at the Kurnakov Institute of
General and Inorganic Chemistry under financial
support from the Russian Foundation for Basic
Research (grant no. 15-03-07652).
16. Furukawa, S., Endo, M., and Komatsu, T., ACS Catal.,
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Translated by K. Utegenov
KINETICS AND CATALYSIS Vol. 57 No. 5 2016