Liquid-Phase Hydrogenation of Internal and Terminal Alkynes on Pd–Ag/Al2O3 Catalyst

Article abstract of DOI:10.1134/S0023158419050069

Abstract: The structure and catalytic properties of a bimetallic Pd–Ag/α-Al₂O₃ catalyst are studied in the liquid-phase hydrogenation of substituted internal and terminal alkynes using diphenylacetylene, phenylacetylene, 1-phenyl-1-propyne, and 1-phenyl-1-butyne as sample alkynes. IR spectroscopy of adsorbed CO, X?ray diffraction (XRD), and electron microscopy were used to show that the active sites on the Pd–Ag nanoparticle surface are Pd₁ sites. The synthesized Pd–Ag/α-Al₂O₃ catalyst shows a much higher selectivity in the hydrogenation of internal symmetric and asymmetric alkynes compared to the monometallic Pd/α?Al₂O₃ sample. Also, it was found in the hydrogenation of diphenylacetylene and 1-phenyl-1-propyne on Pd–Ag/α-Al₂O₃ that the rate of the stage of desired olefin hydrogenation substantially decreases, which favors the kinetic control of the process.

Full text of DOI:10.1134/S0023158419050069

ISSN 0023-1584, Kinetics and Catalysis, 2019, Vol. 60, No. 5, pp. 642–649. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Kinetika i Kataliz, 2019, Vol. 60, No. 5, pp. 644–651.
Liquid-Phase Hydrogenation of Internal and Terminal Alkynes
on Pd–Ag/Al2O3 Catalyst
A. V. Rassolovа, G. O. Braginaа, G. N. Baevaа, N. S. Smirnovaа, A. V. Kazakovа,
I. S. Mashkovskyа, and A. Yu. Stakheevа, *
аZelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
*е-mail: st@ioc.ac.ru
Received April 18, 2019; revised April 26, 2019; accepted May 6, 2019
Abstract—The structure and catalytic properties of a bimetallic Pd–Ag/α-Al2O3 catalyst are studied in the
liquid-phase hydrogenation of substituted internal and terminal alkynes using diphenylacetylene, phenylacet-
ylene, 1-phenyl-1-propyne, and 1-phenyl-1-butyne as sample alkynes. IR spectroscopy of adsorbed CO,
X-ray diffraction (XRD), and electron microscopy were used to show that the active sites on the Pd–Ag
nanoparticle surface are Pd1 sites. The synthesized Pd–Ag/α-Al2O3 catalyst shows a much higher selectivity
in the hydrogenation of internal symmetric and asymmetric alkynes compared to the monometallic
Pd/α-Al2O3 sample. Also, it was found in the hydrogenation of diphenylacetylene and 1-phenyl-1-propyne
on Pd–Ag/α-Al2O3 that the rate of the stage of desired olefin hydrogenation substantially decreases, which
favors the kinetic control of the process.
Keywords: liquid-phase hydrogenation, bimetallic catalysts, nanoparticles, catalyst structure, terminal
alkynes, internal alkynes, palladium, silver, isolated Pd sites
DOI: 10.1134/S0023158419050069
INTRODUCTION
the structure of the active component were deter-
mined. Acetylene hydrogenation was also studied in
[17], where a catalyst based on a Pd–Ag composition
deposited on carbon nanofibers was used. The main
efforts of researchers are aimed at creating catalysts
with a more uniform structure of active sites. Thus,
Glyzdova et al. [18, 19] studied the properties of palla-
dium catalysts modified by Zn, Ag, and Ga in the liq-
uid-phase hydrogenation of acetylene. It was found
that modification by zinc leads to an increase in the
selectivity to ethylene by 25% compared with other
catalysts of the series. This fact was attributed by the
authors to the formation of a strictly ordered structure
of the Pd–Zn intermetallic compound.
At present, selective catalytic hydrogenation of a
triple carbon–carbon bond in substituted alkynes is
one of the most important chemical processes [1–3].
Olefins formed as the reaction products are widely used
in large-scale processes of the pharmaceutical and petro-
chemical industries and serve as starting materials for the
production of dyes and household chemicals [4–6].
An analysis of the world literature shows that, both
in Russia and abroad, new approaches to the synthesis
of catalysts are being actively developed, which make
it possible to increase the catalyst selectivity, to
broaden the prospects for using heterogeneous cata-
lysts in fine organic synthesis [7, 8]. The most promis-
ing areas are the use of bi- and polymetallic composi-
tions. The two main types of bimetallic catalysts are
most intensively studied: (1) bimetallic alloys with the
structure of a solid substitution solution and (2) inter-
metallic compounds [8, 9].
A very promising area of research developed
recently is associated with the formation of active sites
on the surface of bimetallic particles, which are active
metal atoms separated from each other by a modifying
metal that is inactive in a catalytic reaction. Such cat-
In the development of the selective hydrogenation alysts are called “single-atom alloy catalysts” [20].
of alkynes, Pd-containing systems modified with a This approach can significantly increase the degree of
second usually inactive metal, are of considerable uniformity of the active sites, and thus increase the
interest. Group 6–8 metals are effective modifiers of catalyst selectivity. For instance, a Pd/ZnO catalyst
palladium catalysts [10, 11]. For example, in a number with single Pd atoms on the surface of bimetallic Pd–
of works, the catalytic properties of alloy bimetallic Zn particles was obtained in [21], which showed a high
systems are studied: Pd–Zn [12, 13], Pd–Au [14], Pd– selectivity to ethylene with almost complete hydroge-
Cu [13, 15], Pd–Ag [16]. Optimal conditions for the nation of acetylene. Pei et al. studied the properties of
synthesis of these catalysts and the ratio of metals in single-atom alloy Pd–Ag catalysts in the gas-phase
642

LIQUID-PHASE HYDROGENATION OF INTERNAL AND TERMINAL ALKYNES
643
hydrogenation of acetylene in the excess of ethylene
[22]. The samples were prepared by the method of co-
impregnation from solutions of individual salts fol-
lowed by high-temperature reduction. The authors
showed that an important condition for the high selec-
tivity of catalysts of this type is the great excess of a
modifying metal [16, 22].
EXPERIMENTAL
Catalyst Preparation
For the preparation of the Pd1–Ag2/Al2O3 catalyst,
the method of incipient wetness co-impregnation was
used. The α-Al2O3 support (Alfa Aesar, SBET = 6 m2/g)
preliminarily calcined in a flow of air 550°С was
impregnated with a solution containing a mixture of
palladium and silver nitrates and dried in air at room
temperature. The resulting sample was reduced in a
f low of 5% H2/Ar (Linde Gas, Russia) at 550°С for 3 h
(the rate of temperature increase from room tempera-
ture to 550°С was 4°С/min) and cooled on a flow of
5% H2/Ar to 200°С and in a flow of N2 (purity, 5.0)
(“Linde Gas, Russia) to 20°С. The calculated concen-
tration of metals in the synthesized catalyst was 2 wt %
Pd and 4 wt % Ag. According to the XRD data (see
below), the ratio of metals Ag : Pd in the bimetallic
alloy Pd–Ag was 1.5. The reference sample was mono-
metallic 2% Pd/Al2O3 prepared according to an anal-
ogous technique. The high content of the active com-
ponent in the catalysts was due to the necessity of car-
rying out physicochemical analysis.
In a detailed study of the structure and properties
of the unsupported Pd–Ga intermetallics by Schlögl
et al. [23] the concept of isolated active sites first pro-
posed by Sachtler [24] was used, which is close to the
concept of single atom alloy. Using density functional
theory, the authors showed that the extremely high
selectivity and stability of the catalyst in the gas-phase
acetylene hydrogenation is due to the fact that active
palladium atoms in the catalyst structure are com-
pletely isolated by gallium atoms, which ensured high
homogeneity of the active sites [23]. This conclusion
was supported by in situ XPS and IR spectroscopy of
adsorbed CO. Such a structure complicates or makes
impossible the multipoint adsorption of an alkyne
substrate, which inevitably leads to a change in the
activity/selectivity parameters. However, a significant
drawback of intermetallic catalysts with isolated active
sites is the extremely high oxygen affinity of the inac-
tive component, which makes it unstable when stored
in air and requires special precautions when they are
used. Thus, the formation of oxide phases on the cat-
alyst surface was shown for the Pd–Ga, Pd–Zn, Pd–
Ti, and Pd–In intermetallics [8].
X-ray Diffraction
XRD patterns of the synthesized catalysts and the
initial support were obtained on a DRON-4 diffrac-
tometer (Burevestnik, Russia) in Bragg–Brentano
geometry using CuKα radiation (Ni filter) in the range
of 15°–95° (2θ) angles, 0.02° increments (2θ) and sig-
nal accumulation time of 1 s. Crystallographic param-
eters were calculated using the Rietan-FT software
[31], which implements the Rietveld method.
The formation of isolated active sites is also possi-
ble on the surface of solid substitution solutions, in
which the characteristics of active and inactive metals
differ slightly, which makes them much less sensitive
to oxygen and much more convenient. Earlier, the for-
mation of isolated Pd atoms on the Pd–Ag/α-Al2O3
surface with the solid substitution solution structure
was detected by IR spectroscopy of adsorbed CO [25–
27]. The main advantage of this type of catalysts is the
possibility of their easy regeneration, for example, by
redox treatment, and therefore, these catalytic systems
can be widely used in industry.
Transmission Electron Microscopy
The microstructure of the samples was studied by
transmission electron microscopy (TEM) on an
HT7700 instrument (Hitachi, Japan). Images were
taken in the bright field mode at an accelerating volt-
age of 100 kV. For the study, powdered samples were
dispersed from the suspension in isopropanol onto
copper gauzes coated with a carbon layer (d = 3 mm).
The microscopic measurements were optimized in the
framework of the previously described methodology
[32]. The average diameter of metal particles and their
size distribution were determined on the basis of mea-
suring 150–180 particles in microscopic images of var-
ious sample regions.
Pd–Ag-based catalysts are intensively studied in
the gas phase hydrogenation of acetylene in ethylene
and demonstrate high efficiency [10, 28–30]. How-
ever, in the liquid-phase hydrogenation of substituted
alkynes of a complex structure their characteristics are
studied insufficiently. Therefore, the purpose of this
work was to obtain a supported Pd–Ag/Al2O3 catalyst
with Pd1 active sites isolated by Ag atoms, and to study
its catalytic properties in the hydrogenation of alkynes
with different positions of a triple carbon bond (inter-
IR Spectroscopy of Adsorbed CO
IR diffuse reflectance spectra of adsorbed CO were
nal or terminal), different structures (symmetric or recorded using a Tensor 27 IR spectrometer (Bruker,
asymmetric), and various substituents at the triple bond Germany) equipped with a Harrick Diffuse Reflec-
(aryl or alkyl): diphenylacetylene (DPA), 1-phenyl-1- tance Kit diffuse reflectance kit (Harrick, UK), which
propyne (PP), 1-phenyl-1-butyne (PB), and phenylacet- allowed for the gas pretreatment of samples in situ. A
ylene (PA).
portion of the ground catalyst (20 mg) was placed in a
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644
RASSOLOV et al.
reaction was carried out in the kinetic mode. To check
if the reaction is controlled by kinetics, the depen-
dence of the reaction rate and TOF on the degree of
grinding the catalyst particles, the weight of the cata-
lyst and the stirring intensity of mixing of the reaction
mixture was studied [33]. Before carrying out the reac-
tion, a flow of high purity argon (~30 mL/min, 3 h)
passed through the solvent to remove impurities of dis-
solved oxygen. The selectivity to an olefin (S=) was
determined according to gas chromatographic analysis
of the reaction mixture on a Crystal 5000 chromato-
graph (Khromatek, Russia) with a flame ionization
detector. The calculation method is given in [34].
100 nm
(а)
Given that the hydrogenation of the alkyne sub-
strate occurs consecutively (first to the corresponding
alkene then to alkane) the amount of the complete
hydrogenation product formed in the first stage of
hydrogenation is insignificant. The hydrogenation of
the residual alkyne to alkane in the second stage of the
reaction can also be neglected. Taking into account
these assumptions, the reaction rate r ( g
cat min–1)
molH
2
for each stage of hydrogenation was determined from
the plot of the amount of absorbed hydrogen versus
time of the process [34]. The kinetic selectivity of the
reaction was estimated from the ratio of the rates of
alkyne hydrogenation in the first and second stages
(r1/r2). The specific activity (TOF) of the samples was
calculated as the ratio of the number of molecules of
the converted substrate to the total number of palla-
dium atoms in the catalyst per minute [35].
100 nm
(b)
Fig. 1. TEM images for the catalysts Pd/Al O (a) and Pd–
2
3
Ag/α-Al O (b).
2
3
RESULTS AND DISCUSSION
Transmission Electron Microscopy
thermostatic cell of the chamber with CaF2 windows.
Before measurements, the samples were subjected to
reductive treatment with a mixture of 5% H2/Ar
(30 mL/min) for 1 h at 350°C. Then, the cell was
cooled to 50°C in a flow of argon, CO adsorption was
carried out (0.5 vol % CO/He, 20 min), and the spectra
were recorded (500 scans, a resolution of 4 cm–1).
Figure 1 shows microscopic images of the mono-
metallic Pd/Al2O3 and bimetallic Pd–Ag/Al2O3 cata-
lysts. Both samples are characterized by a relatively
uniform distribution of the particles of the active com-
ponent in the structure of the support. In the case of a
monometallic palladium sample, the maximum of the
histogram of the particle size distribution is 10 nm. For
a bimetallic Pd–Ag catalyst, this value is ~12 nm.
Liquid-Phase Hydrogenation of Substituted Alkynes
In this work the following reagent were used:
diphenylacetylene, 1-phenyl-1-propyne, 1-phenyl-1-
butyne, and phenylacetylene (all had 99% purity,
Sigma-Aldrich); n-hexane (98%, Merck); gases and
gas mixtures (Linde Gas, Balashikha, Russia).
The reaction of liquid-phase hydrogenation was
carried out in an autoclave-type reactor at a tempera-
ture of 25°C and an initial hydrogen pressure of 5 bar
with uninterrupted stirring. A catalyst sample, 6 mL of
the solvent (n-hexane), and 170 mg of alkyne substrate
were placed in the reactor, after which the reactor was
loaded into an autoclave and connected to a gas dosing
unit and an electronic pressure sensor to monitor the
X-Ray Diffraction
The X-ray diffraction patterns of the synthesized
catalysts and the original support are shown in Fig. 2
for the range of angles of 2θ = 36°–50°. The high-
intensity reflections at angles of 37.8° and 43.4° on the
X-ray diffraction pattern of the original support indi-
cate its high crystallinity [36, 37]. The Pd/Al2O3 and
Ag/Al2O3 monometallic catalysts in the indicated
range are characterized by reflections of metallic pal-
ladium (2θ ~ 40.1 (111), 46.7 (200)) and silver (2θ ~
38.1 (111), 44.3 (200)) [38 , 39]. It is clearly seen on the
X-ray diffraction pattern of Pd–Ag/Al2O3 that the sig-
degree of hydrogen absorption. To correctly compare nals of metallic Pd phase shift to smaller angles, which
the catalytic characteristics of the studied samples, the corresponds to a change in the lattice parameters due
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LIQUID-PHASE HYDROGENATION OF INTERNAL AND TERMINAL ALKYNES
645
to the formation of a bimetallic alloy (reflections at
2θ ~ 39° and 45.5°) [39]. At the same time, palladium
Ag
Pd
Ag
Pd
completely transfers to the structure of the bimetallic
alloy, which is evident by the disappearance of the
Pd(111) and Pd(200) signals, while some of the silver
remains in the metallic state (low-intensity reflection
at 2θ ~ 38.1°). The calculation according to Vegard’s
law shows the formation of a bimetallic Pd–Ag alloy
with the ratio Pd : Ag = 1 : 1.5.
Pd/α-Al2O3
Pd–Ag/α-Al2O3
IR Spectroscopy of Adsorbed CO
Ag/α-Al2O3
α-Al2O3
The surface structure of the Pd/Al2O3 and Pd–
Ag/Al2O3 catalysts was studied by IR spectroscopy of
adsorbed CO. For the monometallic palladium cata-
lyst, two broad intense absorption bands are observed
at 2200–1800 cm–1 (Fig. 3, spectrum 1). A small
asymmetric peak with a maximum at 2088 cm–1 can be
attributed to vibrations of linear CO molecules, while
a wide multicomponent band at 2000–1800 cm–1 cor-
responds to multicoordinated CO forms on the metal
surface. Thus, an absorption band with a maximum at
1990 cm–1 presumably refers to the bridging form of
CO adsorbed on the (100) face, and the low-intensity
shoulder (~1938 cm–1) characterizes the bridging
and/or three-point form of CO adsorption on the face
(111) [40–42].
36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
2θ, deg
Fig. 2. XRD patterns of the catalysts Pd/α-Al O , Ag/α-
2
3
Al O , Pd–Ag/α-Al O , and the initial support α-Al O .
2
3
2
3
2 3
2052
2
1
In the Pd–Ag/Al2O3 spectrum, a single absorption
1990
band is detected with a maximum at 2050 cm–1, which
is related to CO molecules adsorbed on the catalyst
surface in a linear form (Fig. 3, spectrum 2). The for-
mation of Pd–Ag on the surface, where only Pd1 sites
are present, confirms the complete absence of absorp-
tion below 2000 cm–1. This is due to the fact that the
formation of Pd1 sites surrounded by Ag atoms eliminates
the possibility of multipoint adsorption of the CO mole-
cule on the surface of Pd–Ag nanoparticles [25].
1938
2088
2200 2150 2100 2050 2000 1950 1900 1850 1800
ν, сm–1
The small width of the absorption band of linearly
adsorbed CO and its symmetry indicate the formation
of highly uniform Pd1 active sites in the catalyst under
study. A significant shift in the linear absorption of CO
by ~35 cm–1 towards the lower wavenumbers relative
to the position of the absorption band in the monome-
tallic catalyst may be due to a decrease in the dipole–
dipole interaction between CO molecules adsorbed on
neighboring palladium atoms isolated from each other
by silver atoms in a bimetallic catalyst. Moreover, as a
result of the formation of bimetallic Pd–Ag particles,
an increase in the electron density on palladium atoms
is possible. This leads to an increase in donating the
electron density to the antibonding π-orbital of the
molecule of adsorbed CO and weakening the C–O
bond [25].
Fig. 3. IR diffuse reflectance spectra of adsorbed CO for
the catalysts Pd/α-Al O (1) and Pd–Ag/α-Al O (2).
2
3
2 3
Analysis of Catalytic Properties
in the Liquid Phase Hydrogenation
Hydrogenation of symmetric internal alkynes.
Diphenylacetylene. The rates of diphenylacetylene
hydrogenation on mono- and bimetallic catalysts are
shown in Table 1. A decrease in the reaction rate is
observed in the transition from the first to the second
hydrogenation stage for both Pd/Al2O3 and Pd–
Ag/Al2O3. It should be noted that for the bimetallic
sample the values of TOF1 and TOF2 (0.64 and
0.05 s–1) are higher than for the monometallic palla-
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RASSOLOV et al.
Table 1. Kinetic characteristics of liquid-phase hydrogenation of different substrates in the presence of monometallic
Pd/α-Al2O3 and bimetallic Pd–Ag/α-Al2O3 catalysts
r1 × 103
r2 × 103
TOF1
TOF2
Catalyst
composition
r1/r2
S70%, %
S90%, %
Substrate
−1
s–1
min–1
molН
g
cat
2
Pd
6.57
7.22
0.30
0.58
0.39
0.64
0.02
0.05
21.9
12.4
91.7
94
88.1
91.5
Pd–Ag
Pd
0.34
0.14
0.068
0.005
0.02
0.01
0.0040
0.0004
5.00
79.6
85.6
69.0
79.8
Pd–Ag
28.00
CH3
CH3
Pd
2.98
7.19
1.23
3.37
0.18
0.63
0.07
0.29
2.42
2.13
74.8
85.1
66.0
81.0
Pd–Ag
Pd
15.03
8.62
10.73
11.75
0.89
0.76
0.63
1.04
1.40
0.73
95.8
95.3
94.1
93.2
CH
Pd–Ag
dium catalyst (0.39 and 0.02 s–1). As a result, the ratio
r1/r2 decreases from 21.9 for Pd/Al2O3 to 12.4 for Pd–
Ag/Al2O3, but the latter was more selective to dipheny-
lethylene at high conversions compared to the mono-
metallic sample (Fig. 4a). Thus, at low conversions
(<50%) Pd/Al2O3 and Pd–Ag/Al2O3 demonstrate
similar values of the selectivity below 100%. With
increasing conversion, a predictable decrease in the
selectivity is observed for both samples. At high con-
versions (>90%), the selectivity of Pd–Ag/Al2O3 is
significantly higher (~91.5%) than for Pd/Al2O3
(~88.1%) (Table 1). The observed changes are related
to the fact that the formation of isolated Pd1 sites sig-
nificantly decreases the energy of diphenylethylene
adsorption, as a result its desorption into the solution
becomes preferable rather than further hydrogenation
to diphenylethane.
Comparison of selectivity to the desired alkene
showed that its value is significantly higher for both
the hydrogenation of 1-phenyl-1-propyne and the
hydrogenation of 1-phenyl-1-butyne (Figs. 4b, 4c)
over Pd–Ag catalysts. The high selectivity values were
obtained in almost the entire range of conversions of
the initial substrates in contrast to Pd/Al2O3. This may
be due to the fact that, on bimetallic catalysts,
alkanes—the products of complete hydrogenation—
are formed to a much lesser extent. This indicates an
insignificant contribution of the direct hydrogenation
of adsorbed alkene intermediate as a result of a
decrease in its adsorption on Pd1 sites [43].
Hydrogenation of Terminal Alkynes. Phenylacety-
lene. It is known from the literature that due to the
high steric accessibility of the –C≡C– bond, terminal
alkynes are more reactive compared to internal ones.
The rate of their hydrogenation is more difficult to
control and, as a result, the desired alkene intermedi-
ate is subjected to complete hydrogenation with the
formation of an alkane compound even when using
commercial catalysts [44–46].
Hydrogenation of asymmetric internal alkynes.
1-phenyl-1-propyne and 1-phenyl-1-butyne. As in the
case of diphenylacetylene, in the hydrogenation of
1-phenyl-1-propyne and 1-phenyl-1-butyne,
a
decrease in the catalytic activity is observed during the
transition from hydrogenation of a triple carbon bond
to hydrogenation of a double bond. The values of
TOF1 and TOF2 obtained in the hydrogenation of
Our experimental data on hydrogenation of pheny-
lacetylene are in good agreement with the literature
data. The results of kinetic analysis indicate that the
reaction accelerates during transition from the hydro-
genation of a triple bond to the hydrogenation of a
double bond (Table 1). The quantitative analysis of
TOF values on the first stage of the reaction shows
that, for the Pd–Ag/Al2O3 catalyst, the TOF1 value is
1-phenyl-1-butyne are 0.02 and 0.0040 s–1 for
Pd/Al2O3 and 0.01 and 0.0004 s–1 for Pd–Ag/Al2O3
(Table 1). The values obtained in the hydrogenation of
1-phenyl-1-propyne are 0.18 and 0.07 s–1 and 0.63 and
0.29 s–1, respectively. In the reaction with 1-phenyl-1-
butyne, the value of r1/r2 for Pd–Ag/Al2O3 is almost
six times higher than that for the monometallic palla-
dium catalyst. For 1-phenyl-1-propyne, these values
are comparable: 2.42 for Pd/Al2O3 and 2.13 for Pd–
Ag/Al2O3.
lower (0.76 s–1) compared to the monometallic
Pd/Al2O3 (0.89 s–1). However, at the second stage
(phenylethylene hydrogenation to phenyl ethane),
TOF2 for the palladium catalyst decreases to 0.63 s–1.
For the Pd–Ag sample, it increases to 1.04 s–1, and the
r1/r2 ratio halves (from 1.4 to 0.73 s–1).
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LIQUID-PHASE HYDROGENATION OF INTERNAL AND TERMINAL ALKYNES
647
S=, %
100
(а)
S=, %
100
(b)
95
90
85
80
75
70
65
60
55
50
95
90
85
80
Pd
Pd–Ag
Pd
Pd–Ag
20 30 40 50 60 70 80 90 100
40
50
60
70
80
90
100
X, %
X, %
S=, %
100
S=, %
100
(c)
(d)
95
90
85
80
75
70
65
60
55
50
95
90
85
80
Pd
Pd–Ag
Pd
Pd–Ag
40
50
60
70
80
90
100
X, %
20 30 40 50 60 70 80 90 100
X, %
Fig. 4. The dependence of selectivity in alkene formation (S ) on the conversion of alkyne substrate (X) in the hydrogenation of
=
diphenylacetylene (a), 1-phenyl-1-butyne (b), 1-phenyl-1-propyne (c), and phenylacetylene (d) over the Pd/Al O and Pd–
2
3
Ag/α-Al O catalysts.
2
3
According to the literature, an increase in the
hydrogenation rate at the second stage can not only
hinder the kinetic control of the process, but also lead
to a significant decrease in its selectivity. However, in
the present study, the analysis of the catalytic charac-
teristics did not reveal a significant negative effect of
an increase in the reaction rate on the selectivity: for
Pd/Al2O3 and Pd–Ag/Al2O3, the values of this param-
eter are comparable in the whole range of conversions.
Thus, the selectivity is 95.8 and 95.3% at a conversion
CONCLUSIONS
The data obtained confirm the formation of the
Pd–Ag alloy in the Pd–Ag/Al2O3 catalyst, which is
evident from the shift of the Pd and Ag reflections on
the X-ray diffraction patterns at low and high angles,
respectively. Particles of size 10–12 nm are formed, in
which the Pd : Ag ratio is 1 : 1.5. According to the
results of IR spectroscopy, the structure of the active
sites in the Pd–Ag/Al2O3 catalyst is single Pd1 atoms
isolated by silver atoms. In comparison with the
monometallic palladium analogue, the synthesized
Pd–Ag catalyst has a significantly higher selectivity in
the hydrogenation of internal symmetric and asym-
metric alkynes. In this case, hydrogenation of diphe-
nylacetylene and 1-phenyl-1-propyne is most favor-
able from the process kinetics standpoint. There is a
significant decrease in the rate of hydrogenation at the
of phenylacetylene of 70%, and 94.1 and 93% at ХPA
=
90% for mono- and bimetallic catalysts, respectively
(see Fig. 4d and Table 1). The results obtained allow us
to conclude that the formation of isolated Pd1 sites has
a much smaller effect on the nature of the adsorption
of the terminal alkynes and alkenes on the surface of
the active sites compared with the internal ones.
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RASSOLOV et al.
kov, Yu.E., Gening, M.L., and Nifantiev, N.E., Men-
deleev Commun., 2017, vol. 27, p. 425.
second stage, which greatly facilitates the kinetic con-
trol of the process.
8. Furukawa, S. and Komatsu, T., ACS Catal., 2017,
vol. 7, p. 735.
ABBREVIATIONS AND NOTATION
9. Ellert, O.G., Tsodikov, M.V., Nikolaev, S.A., and No-
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PA
phenylacetylene
Pd1-sites
palladium atoms isolated from each other
by Ag atoms
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r
S=
TEM
TOF
reaction rate
selectivity to an olefin
transmission electronic microscopy
turnover frequency
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XRD
X-ray diffraction
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The studies of the structure and morphology of catalysts
by XRD, IR spectroscopy of adsorbed CO, as well as the
study of the catalytic characteristics in the reaction of liq-
uid-phase hydrogenation of alkyne substrates were sup-
ported by grant no. 16-29-10788 from the Russian Founda-
tion for Basic Research. The method for synthesizing the
Pd–Ag/α-Al2O3 bimetallic catalyst was developed with the
financial support by grant no. 17-13-01526 from the Russian
Science Foundation.
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Translated by Andrey Zeigarnik
KINETICS AND CATALYSIS Vol. 60 No. 5 2019