Internal Surface Coating of a Capillary Microreactor for the Selective Hydrogenation of 2-Methyl-3-Butyn-2-Ol Using a PdZn/TiO2 Catalyst. The Effect of the Catalyst’s Activation Conditions on Its Catalytic Properties

Article abstract of DOI:10.1134/S0023158418030163

Finely divided polymer-stabilized PdZn bimetallic nanoclusters are prepared by the polyol method. TiO₂ matrix-impregnated nanoclusters coated on the inner surface of a capillary microreactor are used as catalysts for the selective hydrogenation of 2-methyl-3-butyn-2-ol. The effect of the activation conditions (duration, temperature, and gas medium composition) on the physicochemical and catalytic properties of the coatings is studied. It is shown that their catalytic activities decrease and the reaction’s selectivity increases with an increase in the reaction time and the time of hydrogen reduction at 573 K. The highest selectivity (96.5% at a conversion rate of 99%) is reached on the coatings reduced with hydrogen for 6 h. The coatings remain highly active and stable for 1 month in the continuous flow mode of the reaction. Kinetic simulation showed that a high selectivity level is ensured by a decrease in the rate constants of hydrogenation of 2-methyl-3-buten-2-ol and the ratio of the alkene/alkyne adsorption constants after reductive treatment.

Full text of DOI:10.1134/S0023158418030163

ISSN 0023-1584, Kinetics and Catalysis, 2018, Vol. 59, No. 3, pp. 347–356. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © L.B. Okhlopkova, M.A. Kerzhentsev, Z.R. Ismagilov, 2018, published in Kinetika i Kataliz, 2018, Vol. 59, No. 3, pp. 355–364.
Internal Surface Coating of a Capillary Microreactor for the Selective
Hydrogenation of 2-Methyl-3-Butyn-2-Ol Using a PdZn/TiO2
Catalyst. The Effect of the Catalyst’s Activation Conditions
on Its Catalytic Properties
L. B. Okhlopkovaa, *, M. A. Kerzhentseva, and Z. R. Ismagilova, b
aBoreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
bInstitute of Coal Chemistry and Materials Science, Siberian Branch, Russian Academy of Sciences, Kemerovo, 650000 Russia
*e-mail: mila65@catalysis.ru
Received September 5, 2017
Abstract⎯Finely divided polymer-stabilized PdZn bimetallic nanoclusters are prepared by the polyol method. TiO2
matrix-impregnated nanoclusters coated on the inner surface of a capillary microreactor are used as catalysts for the
selective hydrogenation of 2-methyl-3-butyn-2-ol. The effect of the activation conditions (duration, temperature,
and gas medium composition) on the physicochemical and catalytic properties of the coatings is studied. It is shown
that their catalytic activities decrease and the reaction’s selectivity increases with an increase in the reaction time and
the time of hydrogen reduction at 573 K. The highest selectivity (96.5% at a conversion rate of 99%) is reached on the
coatings reduced with hydrogen for 6 h. The coatings remain highly active and stable for 1 month in the continuous
flow mode of the reaction. Kinetic simulation showed that a high selectivity level is ensured by a decrease in the rate
constants of hydrogenation of 2-methyl-3-buten-2-ol and the ratio of the alkene/alkyne adsorption constants after
reductive treatment.
Keywords: selective hydrogenation of 2-methyl-3-butyn-ol, capillary microreactor, catalytic coating, PdZn
nanoparticles, titanium dioxide, activation
DOI: 10.1134/S0023158418030163
The liquid-phase selective hydrogenation of acety- isophytol [2]. Isophytol, a terpenoid alcohol, is indus-
lenic alcohols is one of the steps taken in the produc- trially produced by the transformation of acetylene,
tion of vitamins [1]. Synthetic vitamin E is produced acetone, diketene, methyl isopropenyl ether, and their
by the condensation of trimethylhydroquinone and derivatives according to Scheme 1:
O
OH
AcO
OH
H
3
Cat
O
3
H2C
O
CH2
O
O
2-Methyl-3-butyn-2-ol
Vitamin K
Vitamin E
HC CH
OH
OH
H2
H2
Cat
OH
Cat
OH
3
3
Linalool
Dehydroisophytol
Isophytol
Scheme 1.
347

348
OKHLOPKOVA et al.
A semihydrogenation product of 2-methyl-3- titanium dioxide matrix and to perform the kinetic
butyn-2-ol (MBY), 2-methyl-3-buten-2-ol (MBE),
can also be used for the preparation of linalool which
finds application in the synthesis of various com-
pounds (citral, citronellol, ionones, and vitamin A).
interpretation of the data obtained.
EXPERIMENTAL
The current industrial method for the hydrogena-
tion of acetylenic alcohols is based on the use of the
Lindlar catalyst (quinoline-modified Pd/CaCO3),
which provides a selectivity of 94.5% at a conversion
rate of 100% [3]. However, the application of the mod-
ifier results in contamination of the final product and
has a negative environmental impact. The use of new
nanosized multimetallic catalysts allows us to avoid
the above-mentioned drawbacks and, consequently,
to decrease the noble metal consumption and to
improve the quality of the target products and the
environmental friendliness of the production. The
properties of Pd can be improved by the addition of a
second metal (Zn, Cd, Pb, and Ag). For example, the
application of intermetallic PdGa compounds pro-
vides a high selectivity level of acetylene hydrogena-
tion [4]. However, Pd2Ga rapidly oxidizes when the
reaction is performed in a liquid phase [5]. Intermetal-
lic compounds of Pd and Zn are promising hydroge-
nation catalysts [6–8]. In the presence of a stabilizer
(polyvinylpyrrolidone), colloidal PdZn nanoparticles
are resistant to oxidation, undergo no segregation or
agglomeration, which ensures their high selectivity
level [8]. Another important factor to reach a high
selectivity level is the selection of such conditions for
the activation of bimetallic catalysts that favor the for-
mation of the PdZn alloy [9, 10].
The design of continuous-operation microreactors
is a considerable advance in the production of chemi-
cal substances. They allow us to intensify the processes
of chemical synthesis, namely, to increase the product
yield, to maintain milder conditions than conven-
tional reactors, and to perform reactions in the kinetic
mode, as well as comply with strict environmental
requirements. We should also note that scaling up such
devices can be accelerated for the purpose of industrial
production. Despite the considerable advances in this
field, the application of microreactors to carry out dif-
ferent catalytic reactions is encountering certain diffi-
culties, especially in fine organic synthesis, which
requires a high level of formation selectivity of the tar-
get products.
Synthesis of Catalysts and Studying Their
Physicochemical Properties
To deposit coatings on the inner surface of a capil-
lary reactor with a diameter of 500 μm and a length of
10 m, a titanium oxide sol containing Pd–Zn
nanoparticles was prepared. The molar ratio of the
components Ti(O–iPr)4 : Pluronic F127 : C2H5OH :
C4H9OH : H2O : HNO3 in the sol was 1 : 0.009 : 32 :
8 : 1.3 : 0.13. The Pd–Zn nanoparticles with a molar
ratio of Pd : Zn = 4 : 1 were synthesized by the reduc-
tion of Pd(CH3COO)2 (46.5% Pd, Aurate) and ZnCl2
(98%) with ethylene glycol in the presence of polyvin-
ylpyrrolidone as a stabilizer [12]. The resulting colloid
was purified by the addition of acetone and redis-
persed in ethanol. The calculated amount of the solu-
tion obtained (1 wt % of Pd) was added to the titanium
oxide sol. The sol was deposited on the capillary sur-
face by the dip-coating technique at a flow rate of
1 cm/s. All operations were performed in an argon
atmosphere. A fused silica capillary was pretreated
with 1 M NaOH for 30 min at 313 K. The capillary and
the residual sol were dried for 24 at a relative humidity
of 80% and annealed to remove the surfactant at 573 K
at a heating rate of 1 deg/min under a pressure of
13 mbar. The obtained coating was subjected to activa-
tion under different conditions. A total of eight samples
were prepared. Their notations are given in Table 1.
The electron micrographs of the catalysts were
obtained on a JEM-2010 electron microscope (JEOL,
Japan) with a grating resolution of 0.14 nm at an accel-
erating voltage of 200 kV. The samples were deposited
on perforated carbon supports fixed on copper grids.
The chemical composition of the samples was deter-
mined on an energy-dispersive spectrometer (EDS)
(EDAX Inc., United States) equipped with an Si(Li)
detector having a resolution of 130 eV. The composi-
tion of the nanoparticles and coatings was determined
by inductively coupled plasma atomic emission spec-
troscopy (ICP AES) on an OPTIMA 4300 DV spec-
trometer (Perkin Elmer, United States), for which
purpose a sample aliquot was dissolved in concen-
trated nitric acid and diluted with distilled water. To mea-
sure the amount of the coating, the capillary was washed
with concentrated nitric acid (4 mL) and concentrated
sulfuric acid (4 mL) at a flow rate of 0.5 mL/min. The
palladium content in the catalyst powders was deter-
mined by X-ray fluorescence spectroscopy on a VRA-
30 analyzer (ThermoFisher Scientific, Switzerland)
In our previous work [11], we compared the prop-
erties of the monometallic catalyst which it exhibits in
both the load-type reactor and the microcapillary
reactor. However, the effect of the activation condi-
tions on the properties of the bimetallic catalysts as
coatings on the inner surface of a capillary reactor has
not been studied and this significantly affects their
activities and selectivities. For this reason, the aim of
the present work is to study the activation conditions
and catalytic characteristics of coatings with Pd-con-
taining nanoparticles impregnated into a mesoporous with a Cr anode of the X-ray tube.
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Table 1. Notation of coating samples and their preparation conditions
349
Notation
Preparation (treatment) conditions
Reduction of PdZn/TiO2 by hydrogen for 2 h at 573 K
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 6
Sample 7
Sample 8
Sample 1 after five reaction cycles (35 h)
Sample 2 after hydrogen reduction for 4 h at 573 K
Sample 3 after oxidation in air for 2 h at 573 K and hydrogen reduction for 2 h at 573 K
Sample 4 after oxidation in air for 2 h at 573 K and hydrogen reduction for 2 h at 573 K
Sample 5 after hydrogen reduction for 4 h at 573 K
Sample 6 after additional hydrogen reduction for 4 h at 573 K
Sample 7 after 25 h operation under reaction conditions
Catalytic Tests
The transmission electron microscopy (TEM) of col-
loids showed the mean size of the colloidal nanoparti-
cles to be 2.5 nm with a narrow particle size distribu-
tion. The TEM analysis of d-spacings showed the
presence of different intermetallic phases (Fig. 1a).
The presence of different composition phases explains
some differences in the data on the composition of the
colloids calculated by the ICP AES method and the
energy-dispersive analysis (Table 2).
The coating on the inner surface of the fused silica
capillary was preactivated under different conditions
(see Table 1). The capillary was placed in a tempera-
ture-controlled oven and heated to 313 K. Hydrogen
(99.95 vol %) and a reaction solution (0.2 mol/L of
MBY and 0.1 mol/L of butanol in methanol) were
mixed in a T-blender for which the inner diameters of
the input and output tubes were 250 μm. To determine
the reaction order with respect to hydrogen, the flows
of H2 and inert gas (high-purity argon, 99.998 vol % or
A-type helium, 99.995 vol %) were mixed prior to
feeding them to a T-blender. The liquid flow rate was
varied from 15 to 360 μL/min and the gas flow rate was
0.6 mL/min, which allowed us to realize the annular
flow [13]. The reaction mixture was analyzed on a
Crystal 2000M gas chromatograph (Chromatek, Rus-
sia) equipped with a SKTFT-50X capillary column
(the diameter was 0.22 mm and the length was 30 m)
and a flame ionization detector. The conversion of
MBY (x) and the formation selectivity of products (Si)
were calculated by the following equations:
The TEM images of the nanoparticles after
impregnation in the TiO2 matrix and vacuum activa-
tion at 573 K are characterized by a low contrast ratio;
moreover, the EDS data suggest the presence of metal
nanoparticles in the sample. The low contrast ratio
complicates the calculation of the bar charts. The
images show single metal particles with a mean size of
10 to 15 nm (Fig. 1b). The particle size distribution in
the sample after the reaction has been performed is
shown in Fig. 1d. The X-ray fluorescence analysis sug-
gests the presence of carbon in the sample after their
activation (Table 1). It is obvious that, during the reac-
tion, the carbonaceous deposits resulting from the
decomposition of the template, which according to the
data from the thermal analysis occurs at 453−633 K, are
removed from the catalysts [15]. The mean nanoparti-
cle size after redox activation and reduction of the
sample and carrying out the reaction there becomes
larger than that of the colloidal nanoparticles (Table 1,
Figs. 1c, 1d). The increase in the mean particle size in
the oxidized samples is due to the formation of coarse
particles of 5 to 15 nm. The averaged data from the
EDS measurements and the elemental analysis suggest
an increase in the Pd content after the redox activation
of the sample and a reaction taking place there. The
redox treatment results in the complete removal of the
template.
СMBY,0 − СMBY
х =
× 100,
× 100.
СMBY,0
Ci
Si =
CMBY,0 − СMBY
RESULTS AND DISCUSSION
Structural-Compositional Analysis
of Colloids and Catalysts
One of the most common methods for the colloid
preparation of colloids is the chemical reduction of
different metal salts in the presence of a stabilizer [14].
The modified polyol method contemplating stabiliza-
tion of nanoparticles with polyvinylpyrrolidone allows
us to control the composition of bimetallic crystal
grains. The volumetric composition of metal nanopar-
ticles was estimated by the ICP AES method, by dis-
solving a colloid precipitate in HNO3. The data
Hydrogenation of 2-Methyl-3-Butyn-2-Ol
on the PdZn/TiO2 Coating in a Microcapillary Reactor
The kinetic equations describing the change in the
concentrations of the starting reagent and the reaction
products during the hydrogenation of MBY were
obtained agree with the specified composition (Table 2). derived from the liquid-phase composition at the
KINETICS AND CATALYSIS Vol. 59 No. 3 2018

350
Table 2. Composition and mean size of colloids and catalysts
Composition
wt % (ICP AES)
OKHLOPKOVA et al.
Mean size,
nm
at % (EDS)
Zn
Catalyst
TiO2
Pd
81
Zn
19
C
Pd
PdZn
–
–
72
–
18
–
0
2.5
6.5
10.1
Sample 1
Sample 5
0.81
1.36
0.05
0.01
58.1
98.8
41.0
0
100
reactor’s output as a function of the liquid flow rate an increase in the activity and the reaction selectivity due
(Fig. 2). The contact times were calculated by the to the removal of carbonaceous deposits (Figs. 4, 5).
Lockhart–Martinelli–Chisholm equation assuming
that the annular two-phase flow is realized during the
experimental conditions [16].
Intermetallic compounds are usually obtained by
melting metals in an inert atmosphere [22]; however,
this requires very high temperatures (1273−2273 K),
which result in low catalytic activity caused by the
fineness of the samples [23]. The application of a
nanoparticle-based system allows us to solve this
problem and to anneal a metal at a much lower tem-
perature. According to the data of [24], the PdZn
intermetallic phase starts to form in the hydrogen
atmosphere even at temperatures above 273 K and
remains stable up to 973 K. Thus, the alloy’s formation
conditions are governed by the physicochemical char-
acteristics of the system (metal particle size and envi-
ronment). It follows from Fig. 5 that, after further
hydrogen reduction at 573 K, the activity of the coat-
ing decreases and the formation selectivity of MBE
increases. The thermal analysis of the PdZn/TiO2 cat-
alysts suggests an incomplete decomposition of the
stabilizer (polyvinylpyrrolidone) after reduction at the
above-mentioned temperature [8]. Obviously, the
more severe conditions (temperature and duration) of
the reductive treatment are required for the nanopar-
ticles, whose surface is contaminated by the decompo-
sition products of the stabilizer, to form an alloy. The
change in the catalytic properties cannot be caused by
the nanoparticle size effect, since according to the
data of [25] the activity increases with an increase in
the particle size from 6 to 13 nm and the selectivity
does not depend on the selectivity.
Figure 3 shows the concentrations of MBY, MBE,
and 2-methyl-2-butanol (MBA) as a function of the
reaction time. The curve pattern suggests two sequen-
tial steps of the hydrogenation of MBY into MBE and
MBE into MBA. In the presence of MBY, the rate of
the second step is low (Fig. 3b), which is usually
attributed to the strong adsorption of the alkyne mol-
ecules on the Pd surface [17, 18]. The carbon balance
is within 100 2% (Fig. 3c). It should be noted that
the presence of the induction period (prior to the con-
version of 50% of the MBY) is probably due to the for-
mation of active centers [18].
Effect of the Reaction Time and the Conditions
for the Treatment of the Catalytic Coating
on Its Activity and Selectivity
The use of porous materials with a well-controlled
structure (at micro-, meso-, and macrolevels) is a
necessary condition for the implementation of mod-
ern chemical processes in the field of heterogeneous
catalysis. To perform any catalytic process, the active
component is required to be in a finely divided state
and available for a bulky organic substrate and the
transport of the reagents and products should be fast.
The application of organic templates allows us to syn-
thesize the mesostructured materials with mono-
The redox treatment has a higher effect on the cat-
disperse mesopores [19]. Certainly, the template alytic characteristics. For example, at 573 and 633 K
removal method influences the characteristics of the and a conversion rate of 99%, the activity and the
mesoporous support and the active metal. The organic selectivity decrease after annealing in air (Figs. 5a, 5c)
surfactants can be removed on exposure to air or pure and, at a conversion rate of 50%, the selectivity
oxygen [20]. The slow vacuum heating of samples at a increases from 91.4 to 97.6% (Fig. 5b). The drop in the
moderate temperature is an efficient removal method activity is due to the increase in the nanoparticle size
which has no negative effect on the morphology and (Fig. 1c) [26] and the increase in the selectivity at a
composition of the support and the active component conversion rate of 50% is explained by the removal of
[21]. However, according to the data from the elemen- carbonaceous deposits and an increase in the avail-
tal analysis and TEM (Table 2), the carbonaceous ability of the active component. The decrease in the
deposits remain on the catalyst’s surface, which can selectivity at high degrees of MBY conversion is due to
have a negative effect on the availability of the active the oxidation of Pd. Upon an increase in the reduction
component after annealing in vacuo under a pressure time from 2 to 6 h, the selectivity at a conversion rate
of 13 mbar at 573 K. In fact, additional washing of the of 99% increased from 43 to 96.5%. It should be noted
catalytic coating by the reaction solution flow causes that sample 3 at such a conversion rate, after the par-
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INTERNAL SURFACE COATING
(a) (b)
351
0.2472 nm
0.1990 nm
0.2347 nm
0.2521 nm
0.2596 nm
0.2392 nm
0.2509 nm
0.2447 nm
0.2077 nm
10 nm
50 nm
Ni/N, %
100
50
0
2
4
6
8
Mean size, nm
(c)
(d)
100 nm
100 nm
Ni/N, %
Ni/N, %
80
60
40
20
60
40
20
0
2
0
5
10
15
6
8
10
Mean size, nm
Mean size, nm
Fig. 1. TEM images (a–d) and bar charts (a, c, d) of particle size: (a) PdZn colloid, (b) PdZn/TiO catalyst after activation at 573 K,
2
(c) sample 5, and (d) sample 1 after reaction.
tial pressure of hydrogen decreased from 1 to 0.5 atm, After the dilution of hydrogen with helium, the selec-
tivity of the reaction on Pd/TiO2 decreased insignifi-
cantly (from 93.4 to 91.7%). The content of the oxygen
impurity in argon was 0.0002 vol % and the content of
the oxygen and argon impurities in helium was 0.0001
vol %. Thus, the decrease in the selectivity in using the
hydrogen–argon mixture is probably due to the oxy-
the selectivity declined from 96.1 to 74%. This effect
did not depend on the composition of the coating and
occurred when using argon as the dilution gas. On the
same coating without zinc (Pd/TiO2), the selectivity
after an analogous decrease in the H2 pressure at a
conversion of 98% decreased from 90.3 to 79.4%. gen impurity in argon.
KINETICS AND CATALYSIS Vol. 59 No. 3 2018

352
OKHLOPKOVA et al.
The plots of the concentrations of MBY, MBE, and sion increases with the rate of the direct hydrogenation
MBA as a function of the contact time on coatings
subjected to the redox treatment are shown in Figs. 6a
and 6b. Figures 6c and 6d show the plots of the rates of
MBY and MBE hydrogenation as a function of the
contact time. On the oxidized coatings, the rate of
MBY hydrogenation is higher than that of MBE
of alkyne into alkane ( ) being constant (Scheme 2).
r
3
Kinetic Model of Hydrogenation
in a Microcapillary Reactor
The hydrogenation of acetylenic alcohols proceeds
along three pathways: reaction (I) is the incomplete
hydrogenation and, after additional hydrogen reduc- hydrogenation to form the target reaction product fol-
lowed by its desorption, reaction (II) is the adsorption
of the alkene alcohol followed by its complete hydro-
genation, and reaction (III) is the adsorption of the
acetylenic alcohol to form an ethylidyne intermediate
followed by direct hydrogenation into alkane.
tion at 573 K, the rate of alkene hydrogenation
decreases. Upon an increase in the ratio of the hydro-
genation rates of the alkyne (r1) and alkene (r2) alco-
hols, the selectivity
at 100% conver-
(r − r2) (r + r )
1
1
3
OH
CH3
OH
CH3
OH
CH3
'
'
CH3
CH3
k1
k2
H3C
HC
H2C
H2
H2
r2
r1
CH3
MBE
MBY
MBA
'
k3
H2
r3
Scheme 2. Hydrogenation pathway of 2-methyl-3-butyn-2-ol.
According to the Hatta number, the catalytic reac- genation of the triple bond occurs by the Langmuir–
Hinshelwood mechanism [18] with the following
assumptions: (1) the adsorption of the reagents is
competitive; (2) the hydrogenation of MBY proceeds
by the sequential addition of hydrogen atoms, with the
addition of the second hydrogen atom being slow and
the addition of the first atom being a quasi-equilib-
rium step [27]; (3) hydrogen is adsorbed on Pd weaker
than alkyne [28]. Equations (1)−(6) allow us to quan-
titatively describe the observed rate of MBY transfor-
mation. The higher the rate constant the higher the
reaction rate and the higher the corresponding adsorp-
tion constants the lower the reaction rate.
tion in microcapillary reactors proceeds in the kinetic
mode and the limiting step is hydrogenation on the
catalyst surface. It follows from the dependences of the
reaction rate on the initial concentration of MBY and
the partial hydrogen pressure that the reaction has the
first order with respect to hydrogen and the order of
0.4 with respect to MBY. The data obtained corre-
spond to the published data according to which hydro-
x, %
S, %
2
'
k1CMBY KMBY
(1)
(2)
(3)
r =
,
,
,
1
80
80
А
1
'
k2CMBY KMBY
r2 =
r3 =
А
80
0
40
'
k3CMBY KMBY
A
3
where
'
'
k1 = k1KМBY KH CPdCH , k2 = k2KМBЕ KH CPdCH ,
0
2
2
2
2
Н
Н
100
200
300
400
500
k3 = k3K H CPdCH2
,
'
A = (1 + KMBYCMBY + KMBECMBE
+
vliq, µL/min
2
2
КMBAСMBA )2,
Fig. 2. Effect of liquid flow rate v on conversion of MBY
liq
dCМBY
dt
dCМBЕ
(4)
(5)
= −(r + r ),
(1) and selectivities of MBE (2) and MBA (3) formations
1
3
on PdZn/TiO coating (sample 3). Reaction conditions:
2
C
= 0.2 mol/L, gas flow rate was 6.0 mL/min,
MBY, 0
= (r − r2),
1
= 1 atm, 313 K.
P
H2
dt
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INTERNAL SURFACE COATING
353
(a)
(b)
(c)
–1
Pd
dC/dt, mol L–1
100
g
s–1
Carbon balance, %
105
C, mol/L
0.2
2
100
95
1
50
0.1
0
3
2
1
3
2
4
6
8
0
2
4
6
0
2
4
6
t × 10–3, s/gPd
t × 10–3, s/gPd
t × 10–3, s/gPd
Fig. 3. (a) Concentrations of MBY (1, j) and its hydrogenation products MBE (2, d) and MBA (3, m) as a function of contact
time on PdZn/TiO coating (sample 3). (b) Reactions rates as function of contact time. (c) Carbon balance as function of contact
2
time. Experimental data are shown as points and data from calculation by Langmuir–Hinshelwood model are shown as lines.
Reaction conditions: gas flow rate was 6.0 mL/min and
= 1 atm, 313 K.
P
H2
dCМBА
'
than , which suggests a low rate of the direct hydro-
k2
(6)
= (r2 + r ).
3
genation of the MBY. According to the available pub-
lished data, the reaction’s selectivity is governed by the
ratio of the alkene and alkyne adsorption constants
[17, 18, 29, 30]. Since the MBE adsorption constant is
57−95-fold lower than that of the MBY adsorption,
the alkyne content on the catalyst surface should
exceed the alkene content. As a result, the alkene
undergoes no hydrogenation in the presence of the
alkyne and the reaction accelerates at low MBY con-
centrations (Fig. 3), since free radicals for the adsorp-
tion of hydrogen and MBE emerge. The ratio of the
dt
Here, k1, k2, and k3 are the apparent constants of reac-
tions (I), (II), and (III), respectively; KMBY, KMBE, and
KMBA are the adsorption constants;
and
KMBE
KMBY
are the equilibrium constants for the adНdition of the
first hydrogen atom to MBY and MBE, respectively;
and CMBY, CMBE, and CMBA are the concentrations of
alkyne, alkene, and alkane, respectively. To solve the
differential equation system, the reaction rates were
integrated in the MathLab environment using the
ode45 function. The alkene adsorption constant KMBE
was determined from the dependence of the rate of the
MBE hydrogenation at an MBY conversion rate of
20% on the initial concentration of the MBE. The
parameters of the model were optimized using the
Nelder–Mead algorithm for minimizing the function
Н
adsorption constants
treatment increased 1.3-fold. This resulted in a
after reductive
KMBY KMBE
SMBE, %
100
3
(Cexp,t − Csim,t )2,
(7)
S =
∑
t
2
90
where Cexp and Csim are the experimental and simu-
lated concentrations, respectively.
1
To explain the observed effects caused by the redox
treatment of the coatings, we compared the adsorption
constants and the reaction rate constants determined
80
70
'
k1
by kinetic simulation (Table 3). The constants
and
'
increased upon oxidation of the coatings. More-
k2
'
over, the constant on the oxidized sample 5 is high,
k2
which resulted in the accelerated hydrogenation of the
MBE into MBA and low reaction selectivity (43%)
(Fig. 5c). After additional reduction, the rate constant
20
40
60
80
100
x, %
'
for the hydrogenation of MBE into MBA increases
k2
Fig. 4. Formation selectivity of MBE as function of its con-
centration on sample coatings 1 (1), 2 (2), and 3 (3). Reac-
tion conditions: C
and the selectivity increases to 94.8%. The rate con-
'
= 0.2 mol/L, gas flow rate was 6.0
MBY, 0
stant for the hydrogenation of MBY into MBA was
k3
L/min,
= 1 atm, 313 K.
P
H2
found to be by at least one order of magnitude lower
KINETICS AND CATALYSIS Vol. 59 No. 3 2018

354
OKHLOPKOVA et al.
(b)
(a)
(c)
SMBE, x = 50, %
Activity, mol L–1
g
s–1
–1
S
MBE, x = 99, %
Pd
100
50
0
20
10
0
100
50
0
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1 2 3 4 5 6 7 8
Sample no.
Sample no.
Sample no.
Fig. 5. Effect of treatment conditions of coating samples 1−8 (see Table 1) on their activity at conversion rate of 20% (a) and selec-
tivity at different conversions x, %: 50 (b) and 99 (c) in hydrogenation of MBY. Reaction conditions: C
= 0.2 mol/L, gas
MBY, 0
flow rate was 6.0 mL/min and
= 1 atm, 313 K.
P
H2
(a)
(b)
C, mol/L
0.3
C, mol/L
0.3
2
3
0.2
0.1
0.2
0.1
1
1
2
3
0
2
4
6
8
10
0
2
4
6
8
10
t × 10–3, s/gPd
t × 10–3, s/gPd
(c)
(d)
–1
Pd
dC/dt, mol L–1
g
s–1
–1
Pd
dC/dt, mol L–1
g
s–1
300
3
2
400
200
100
2
200
1
1
3
0
2
4
6
8
10
0
2
4
6
8
10
t × 10–3, s/gPd
t × 10–3, s/gPd
Fig. 6. Concentrations of MBY (1, j) and products MBE (2, d) and MBA (3, m) as function of contact time (a, b) and reaction rate as
function of contact time (c, d) on coating samples 5 (a, c) and 6 (b, d). Experimental data are shown as points and data from calculation
by Langmuir–Hinshelwood model are shown as lines. Reaction conditions: gas flow rate was 6.0 mL/min and
= 1 atm, 313 K.
P
H2
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2018

INTERNAL SURFACE COATING
355
Table 3. Kinetic parameters of hydrogenation of MBY on
ACKNOWLEDGMENTS
PdZn/TiO2 coatings
This work was financially supported by the Federal
Agency for Scientific Organizations (project no. 0303-
2016-0004).
The authors are grateful to E.Yu. Gerasimov and
I.L. Kraevskaya for their help in studying the samples
by physicochemical methods.
Parameter
Sample 3 Sample 5 Sample 6
'
362
165
661
582
934
k1
−1
mol L−1
L/mol
g
s−1
4690
'
k2
Pd
5 × 10–6 5 × 10–6
15
74
'
k3
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'
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Translated by K. Utegenov
KINETICS AND CATALYSIS
Vol. 59
No. 3
2018