Effect of the nature of the active-component precursor on the properties of Pt/MgAlOx catalysts in propane and n-decane dehydrogenation

Article abstract of DOI:10.1134/S0023158417040188

The Pt/MgAlO_x catalysts, in which a mixed aluminum–magnesium oxide obtained by the heat treatment of corresponding layered double hydroxides was used as a support, were studied. The effects of the nature of active-component precursors ([PtCl₆]^2–, [PtCl₄]^2–, and [Pt₃(CO)₆]₆^2-), and the Mg/Al ratio in the support on the anchoring of complexes, the disperse state of platinum, and the properties of the obtained catalysts in the reactions of propane and n-decane dehydrogenation were found.

Full text of DOI:10.1134/S0023158417040188

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 4, pp. 383–391. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © L.N. Stepanova, O.B. Belskaya, V.A. Likholobov, 2017, published in Kinetika i Kataliz, 2017, Vol. 58, No. 4, pp. 400–409.
Effect of the Nature of the Active-Component Precursor
on the Properties of Pt/MgAlO Catalysts
x
1
in Propane and n-Decane Dehydrogenation
a, b,
a, c
a, c, d
L. N. Stepanova *, O. B. Belskaya , and V. A. Likholobov
a
Institute of Hydrocarbons Processing, Siberian Branch, Russian Academy of Sciences, Omsk, 644040 Russia
b
Omsk State University, Omsk, 644077 Russia
Omsk State Technical University, Omsk, 644050 Russia
c
d
Omsk Research Center, Siberian Branch, Russian Academy of Sciences, Omsk, 644024 Russia
*
e-mail: Lchem@yandex.ru
Received September 22, 2016
Abstract⎯ The Pt/MgAlO catalysts, in which a mixed aluminum–magnesium oxide obtained by the heat
x
treatment of corresponding layered double hydroxides was used as a support, were studied. The effects of the
nature of active-component precursors ([PtCl6]2–, [PtCl4]2–, and [Pt (CO) ] ), and the Mg/Al ratio in the
2−
6 6
3
support on the anchoring of complexes, the disperse state of platinum, and the properties of the obtained cat-
alysts in the reactions of propane and n-decane dehydrogenation were found.
Keywords: platinum catalysts, mixed oxides, anchoring of the anionic complexes of platinum, propane dehy-
drogenation, n-decane dehydrogenation
DOI: 10.1134/S0023158417040188
INTRODUCTION
of the adsorption of reagents. In this case, a decrease
in the strength of the adsorption of target products
The dehydrogenation reactions of light and higher
alkanes are of great industrial importance because
they make it possible to obtain alkenes as intermediate
products for their further use in the production of
polymers and synthetic detergents [1–3]. The devel-
opment of new catalytic systems with high selectivity
for the formation of alkenes is of considerable current
interest because of the growing demand for these
products. Among the currently available catalysts, sys-
tems based on platinum in which the active metal is
fixed on a nonacid support are most promising [4–6].
Supports with basic properties are used in order to
decrease the contribution of cracking, isomerization,
and coke formation side reactions, which occur on
(
alkenes) decreases the probability of their subsequent
dehydrogenation with the formation of diene hydro-
carbons and coke-forming products [7–9].
Both the nature of precursor compounds and their
interactions with the support have an effect on the for-
mation and properties of platinum centers. In this
work, we studied platinum catalysts supported onto
mixed aluminum–magnesium oxides MgAlO , which
x
were obtained by the calcination of corresponding lay-
ered double hydroxides (LDHs). These oxide supports
possess moderate basicity and high specific surface
areas [10], and their properties can be easily regulated
acid sites. The formation of an optimum disperse and by changing the composition of their precursors, i.e.,
electronic state of the supported metal is very import- LDHs. The LDHs are inorganic materials whose lay-
ant for the preparation of active and selective catalysts ers include the cations of different-valence metals and
because the target dehydrogenation reaction occurs on anions, which compensate for their positive charge, and
the particles of platinum. An increase in the dispersion water molecules are located between the layers [11].
of Pt particles leads to an increase in the dehydroge- The basic, adsorptive, textural, and structural proper-
nating activity. Furthermore, the disperse and elec- ties of LDHs and the oxides obtained from them can
tronic state of the active metal influences the strength
be regulated by changing the nature of metals in the
layers and a ratio between them and the nature of
interlayer anions. In this work, we synthesized LDHs
with Mg/Al = 2, 3, and 4 for varying the acid–base
and adsorptive properties of the supports.
1
This paper is based on the proceedings of the V All-Russian Sci-
entific Youth School and Conference “Chemistry under the
Sigma Sign: Studies, Innovations, and Technologies” (Omsk,
May 15–20, 2016).
383

3
84
STEPANOVA et al.
Complex compounds with different compositions tive equilibrium pressures P/P of nitrogen vapor from
0
and the oxidation states of the central atom were cho- 0.05 to 0.25 based on the adsorption isotherm. In the
sen as the precursors of platinum: anionic chloro com- calculation of S , the molecular area of nitrogen in a
BET
2
–
2
plexes of Pt(IV) and Pt(II), namely, [PtCl6] and
filled monolayer was taken to be 0.162 nm . The
2
−
adsorption pore volume (V ) was measured based on
2–
ads
[
PtCl ] , and the carbonyl complex [Pt (CO) ] ,
4
3 6 6
the adsorption of nitrogen at P/P = 0.990, assuming
0
containing Pt(0). The octahedral chloro complexes of
Pt(IV) and the square-planar chloro complexes of
Pt(II) from aqueous solutions were anchored on the
surface by anion exchange with the interlayer anions of
LDHs (intercalation into the interlayer space). Car-
bonyl complexes were deposited onto the surface of the
preliminarily formed mixed oxides from solutions in
acetone without changes in the support structure
that the adsorbate density was equal to the density of
3
normal liquid, namely, 0.02887 mol/cm . The average
pore diameter (d) was evaluated according to the for-
mula d = 4V /S .
ads sp
The aqueous solutions of hexachloroplatinum acid
were prepared from the crystal hydrate H [PtCl ] ⋅
2
6
6
H O (chemically pure, Aurat). Tetrachloroplatinum
2
(
anchoring on the external surface). Different active-
acid (H [PtCl ]) was obtained by the reduction of
2
4
component precursor compounds and different meth-
ods of their supporting made it possible to ensure differ-
ent strengths of metal–support interactions and to
reveal the effect of the above differences on the forma-
tion and properties of platinum particles.
H [PtCl ] by hydrazine chloride [N H ]Cl [14]. For
2
6
2
6
2
this purpose, a stoichiometric amount of hydrazine
chloride was slowly added to an aqueous solution of
H [PtCl ]; then, the resulting solution was heated in a
2
6
water bath for 1 h until the complete termination of
The aim of this work was to study the effect of the
nature of the complex compounds of platinum immo-
bilized on aluminum–magnesium supports with dif-
effervescence.
2
6
−
The carbonyl complexes of platinum Pt (CO)
[
]
3
6
ferent Mg/Al ratios on the properties of Pt/MgAlO
x
were synthesized by the reduction of a solution of
chloroplatinic acid with carbon monoxide [15]. For
this purpose, 100 mL of an aqueous solution of
H [PtCl ] with a platinum concentration of 2 mg/mL
was placed in a two-neck round-bottom flask with a
volume of 250 mL equipped with a magnetic stirrer
catalysts in the dehydrogenation reactions of propane
and n-decane.
2
6
EXPERIMENTAL
The MgAl-LDH samples were obtained by a tradi- after blowing argon through the flask. Next, the sys-
tional coprecipitation method using the gradual addi- tem was filled with carbon monoxide with intense stir-
tion of a solution containing magnesium and alumi- ring. In the course of the synthesis, the entire system
num nitrates to a solution of sodium carbonate at a was blown with a flow of CO for the removal of the
constant value of pH 10 and a temperature of 60°C resulting CO . The cherry-red precipitate of the car-
2
with the subsequent aging of the suspension at 60°C bonyl complex of platinum was separated from the
for 18 h [12]. LDHs with the predominance of carbon- aqueous solution by filtration; thereafter, it was dis-
ate anions in the interlayer space were obtained after solved in acetone.
washing, filtration, and drying at 80°C. LDHs with
The concentrations of magnesium and aluminum
the predominant occurrence of hydroxide anions in
in the solutions and solid samples were determined by
the interlayer space (an “activated” form) were pre-
inductively coupled plasma atomic emission spectros-
pared in order to exchange the interlayer anions for the
copy on a Varian 710-ES instrument (Varian, United
anions of metal complexes. For this purpose, the syn-
States) and by atomic absorption spectroscopy on an
thesized MgAl-LDH was calcined at 550°C (to obtain
a mixed oxide MgAlO ) and then rehydrated in dis-
AAS 6300 SHIMADZU instrument (Shimadzu,
x
Japan). The platinum content of the solid samples
tilled water [13].
was evaluated after their dissolution in a mixture of
The X-ray diffraction analysis of LDHs was per- acids. Platinum concentrations in solutions were mea-
formed on a D8 Advance diffractometer (Bruker, Ger- sured on a PE-5400VI spectrophotometer (Ekokhim,
many) in parallel CuK radiation in a range of 2θ angles Russia).
from 5° to 80° with a scan step of 0.05° and a signal inte-
α
2–
The chloro complexes of platinum ([PtCl ] and
PtCl ] ) were immobilized on LDHs in an “acti-
4
gration time of 5 s at each step.
2–
[
6
The textural characteristics of MgAl-LDH were
studied after preliminary heat treatment at 550°C.
The adsorption–desorption isotherms of nitrogen at
vated” form (a fraction of 0.25–0.5 mm) at room tem-
perature from an excess of an aqueous solution with
the ratio support : solution = 1 : 25 by means of anion
exchange between the interlayer hydroxide anions and
the anions of metal complexes [16]. The adsorption
time was 1 h. After the supporting of a complex, the
7
7.4 K were obtained on an ASAP-2020 volumetric
vacuum system (Micromeritics, United States). The
–
3
range of relative equilibrium pressures was from 10
to 0.996 P/P . Specific surface areas (S ) were sample was washed with distilled water and dried in air.
determined by the BET method in a range of the rela- The carbonyl complexes of platinum were anchored on
0
BET
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EFFECT OF THE NATURE OF THE ACTIVE-COMPONENT PRECURSOR
385
the mixed oxides at room temperature from an excess of weight, 0.50 g) at a temperature of 550°C, atmospheric
an acetone solution with support : solution = 1 : 25. The pressure, the molar ratio H /C H = 0.25, and a feed
2
3
8
support was preliminarily evacuated and blown with
argon for the removal of oxygen from the pore space.
The adsorption time was 20 min; thereafter, the sup-
port with the adsorbed precursor was separated from
the solution by filtration, washed with acetone, and
dried in air.
Individual weighed portions with the weight ratio
support : solution = 1 : 40 were used for obtaining the
adsorption isotherms of the chloro complexes of Pt(IV)
and Pt(II) on an “activated” form of MgAl-LDH. In
each particular case, the adsorption time was 24 h. It
was preliminarily found that this time is sufficient for
adsorption equilibration. Then, the support with the
adsorbed complex was separated from the solution by
filtration. The concentration of solutions was varied
from 1 to 40 mmol/L. The platinum content of the
stock solutions and solutions after metal complex
adsorption was monitored by spectrophotometry.
−1
–1
space velocity of 8 g g h . The catalysts were pre-
Cat
liminarily calcined in air at 550°C with the subsequent
reduction in a flow of hydrogen at 550°C. The dura-
tion of each experiment was 5 h. The composition of
products was determined on-line on a Tsvet 500 gas
chromatograph (Russia) equipped with a Pt-Alumina
PLOT column (Restek, United States) with a length of
5
0 m and a flame-ionization detector.
n-Decane was dehydrogenated in an isothermal
flow reactor (catalyst weight, 0.50 g) at a temperature
of 460°С, a pressure of 0.18 MPa, an H /n-decane
2
molar ratio of 7, and a weight hour space velocity of
–1
1
7.2 h . The catalysts were preliminarily calcined in
air at 550°C and reduced in a flow of hydrogen at
00°C. The composition of products was analyzed in
4
the on-line mode by taking samples from the reaction
zone and supplying them to a Tsvet 800 gas chromato-
graph (Russia) equipped with a PONA/PIONA col-
umn (J & W Scientific, United States) and a flame-ion-
ization detector. In the course of n-decane conversion,
the group composition of the products was monitored:
С –С alkanes (cracking products), isodecanes
The electronic diffuse-reflectance spectra (DRES)
of the supported platinum complexes were measured on
a UV-2501PC spectrophotometer (Shimadzu, Japan)
with an ISR-240A attachment in a range of 11000–
1
8
–
1
5
4000 cm . BaSO was used as a reference sample. The
4
(
(
iso-С ), n-decenes (С olefins), and diolefins
10 10
experimental spectra were represented in the Kubelka–
Munk function–wavenumber coordinates.
С dienes). The formation of aromatic hydrocarbons
10
under the reaction conditions was not observed.
The composition of the platinum complexes sup-
ported on LDHs and the metal complex–support
195
RESULTS AND DISCUSSION
Characteristics of the Supports
interactions were evaluated by Pt magic angle spin-
ning NMR (MAS NMR) spectroscopy. The spectra of
solid samples were obtained on an Avance-400 spec-
trometer (Bruker, Germany) with an SB4 (MAS) mul-
For the preparation of platinum catalysts, MgAl-
tinuclear sensor at a Larmor frequency of 86 MHz. LDH with the ratios Mg/Al = 2, 3, and 4 were synthe-
The test samples were placed in 4 mm rotors of zirco- sized. According to X-ray diffraction data, all of the
nium oxide and spun at a magic angle (54°44′) at a fre- samples had an LDH structure. A characteristic set of
4
reflections was present in the diffraction patterns [17]:
a series of basal reflections 003 and 006 and the peaks
of the families {0kl} (012, 015, 018) and {hk0} (110 and
quency of 10 Hz. The pulse duration was 13 μs; the
delay time before digitization was 16 μs; the pulse rep-
etition time was 0.7 s; the window width was 70 kHz;
the number of points per spectrum was 16 000; and the
total number of pulses was 4096. An aqueous solution
1
13) (no. 22-700, ICDD, PDF-2) (Fig. 1, curve 1).
Note that this hydrotalcite-like structure remained
of H [PtCl ] with a concentration of 0.03 mol/L was upon changes in the composition of interlayer anions in
2
6
used as an external standard.
the course of catalyst synthesis (Fig. 1, curves 2–4).
Thus, the analysis of the structural parameters of
LDHs (as an example, structural data for the ratio
Mg/Al = 2 are given) showed that the parameter c
somewhat decreased (from 22.77 to 22.68 Å) on the
formation of the “activated” form of LDH in the pro-
cess of oxide phase rehydration and, correspondingly,
upon the partial replacement of interlayer carbonate
anions by hydroxide ones. However, the subsequent
anion exchange that occurred on the interaction of
MgAl-LDH-OH with the chloro complexes of Pt(II)
and Pt(IV) led to an increase in the distance between
the layers, which confirms the preferred localization
of complexes in the interlayer space. Because the
The chemisorption of hydrogen was conducted at
room temperature after the preliminary oxidative and
reductive treatment of samples at 550°C. A mixture of
1
0% H + Ar was injected as pulses into a flow of argon
2
at regular time intervals. A detector measured the
amount of the adsorbate that did not react with the
active metal. The injection continued until a constant
TCD signal. The dispersion of the metal (DH2 ) was
calculated as a ratio of the quantity of surface platinum
atoms measured by the chemisorption of hydrogen to
their total quantity with consideration for the H : Pt =
1
: 1 stoichiometry of sorption.
The dehydrogenation of propane was performed in effect of the adsorption of the chloro complexes of
a fixed-bed flow reactor with a catalyst (sample platinum on the structural parameters in the synthesis
KINETICS AND CATALYSIS Vol. 58 No. 4 2017

3
86
STEPANOVA et al.
strength of their interaction with the aluminum–mag-
nesium hydroxide layers was accompanied by a mono-
tonic decrease in the specific surface area and the pore
volume and the average pore size in the resulting oxide
phases with an increasing Mg/Al ratio (Table 1).
1
2
Anchoring of the Anionic Complexes of Platinum
Anchoring of the chloro complexes of Pt(II) and
Pt(IV). The chloro complexes of platinum were
adsorbed from the aqueous solutions of H [PtCl ] and
3
4
2
6
H [PtCl ] onto MgAl-LDH in an “activated” form.
2
4
The influence of the Mg/Al ratio on this process was
studied by analyzing the adsorption isotherms of the
corresponding complexes. The Mg/Al ratio in the
MgAl-LDH samples was 2, 3, or 4 (Fig. 2).
1
0
20
30
40
θ, deg
50
60
70
80
2
Fig. 1. Diffraction patterns of (1) MgAl-LDH-CO ,
3
2
–
(
2) MgAl-LDH-OH, (3) [PtCl ] /MgAl-LDH-OH,
An analysis of the adsorption isotherms of
6
2
–
and (4) [PtCl ] /MgAl-LDH-OH. The Mg/Al ratio is 2;
2–
4
[PtCl ] complexes on MgAl-LDH showed that the
6
the Pt content is 3 wt %.
adsorption (quantity) of immobilized complexes mono-
tonically decreased with the Mg/Al ratio (Fig. 2a). This
is consistent with the previously observed decrease in
the positive charge of layers and a corresponding
decrease in the number of charge-compensating inter-
layer anions [19]. The maximum adsorption varied
from 0.75 to 1.06 mmol/g, which corresponds to 16–
of low-percentage catalysts is insignificant, Fig. 1
shows the diffraction patterns of samples with higher
platinum content to illustrate the effect of intercala-
2–
2–
tion. Upon the adsorption of [PtCl ] or [PtCl ] ,
6
4
the parameter c increased from 22.68 to 22.98 or 23.11 Å,
2
0 wt % adsorbed platinum. The stoichiometric
respectively. At the same time, the sequence of anion
2–
amounts of the interlayer anions [PtCl ] calculated
2
−
6
–
2–
2–
exchanges СО3 → OH → [PtCl ] ([PtCl ] ) did
6
4
from a general formula for LDHs [17] with the
assumption of the immobilization of platinum in the
interlayer space as a result of the exchange of the com-
not cause a substantial change in the cationic compo-
sition of hydroxide layers, and the parameter a
retained its value (3.045–3.050 Å) even upon inter-
action with a sufficiently concentrated solution of
–
plex anions with the interlayer ions OH in the sam-
ples with the ratios Mg/Al = 2, 3, and 4 were 1.45,
1.30, and 1.17 by mmol/g, respectively. These values
are close to the experimental data. It is likely that the
H [PtCl ], when a significant amount of platinum
2
6
(
12 wt %) was introduced.
An increase in the Mg/Al ratio leads to an increase experimental values are somewhat lower due to the
2
3
−
in the lattice parameter a as a result of a decrease in the
fraction of aluminum cations, which possess a smaller
ionic radius, and to an increase in the parameter c
because of a decrease in the positive charge of hydrox-
ide layers and in the interacting force between the lay-
ers and the interlayer anions (Table 1) [18].
presence of an amount of the interlayer ions СО ,
which are incorporated, as a rule, in the course of
experiments performed in an atmosphere of air.
2
–
Unlike the adsorption of [PtCl ] , inverse depen-
6
2–
dence was observed on the immobilization of [PtCl ]
4
on MgAl-LDH samples with different Mg/Al ratios:
the amount of anchored complexes increased with the
Mg/Al ratio (Fig. 2b). The adsorption capacity of the
supports with Mg/Al = 2 and 3 was 0.91–1.14 mmol/g
The decrease of the positive charge of hydroxide
layers upon a decrease in the fraction of triply-charged
aluminum cations and, because of this, in the quantity
of charge-compensating interlayer anions and the
(16–18 wt % Pt). At Mg/Al = 4, the amount of
adsorbed complexes reached a maximum value of
Table 1. Structural characteristics of the MgAl-LDH sam- 1.37 mmol/g (22 wt % Pt), which exceeds the calcu-
ples and the textural characteristics of MgAlO
lated amount of complex anions to be adsorbed by
anion exchange. An excess amount of adsorbed Pt(II)
complexes upon intercalation, as compared with their
stoichiometric amount, allowed us to assume that a
portion of platinum was unchored by another mecha-
nism. It is likely that, because of a considerably higher
lability of the chloro complexes of Pt(II) in compari-
son with that of Pt(IV) complexes [20], the former
undergo rapid hydrolysis with the formation of plati-
num oxide species on the surface of a basic support. As
x
MgAl-LDH
MgAlOx
Mg/Al
SBET
2
,
Vpore
,
pore
size, Å
a, Å
c, Å
3
m /g
cm /g
2
3
4
3.044
3.064
3.074
22.77
23.46
23.77
241
202
119
1.09
0.81
0.44
181
161
148
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2017

EFFECT OF THE NATURE OF THE ACTIVE-COMPONENT PRECURSOR
а) (b)
387
(
1
.6
1
.2
1.4
3
1
2
1.0
1.2
1.0
2
1
0.8
0.6
0.4
0.2
0
3
0.8
0.6
0.4
0.2
0
5
10
15
20
25
0
10
20
30
40
50
Equilibrium concentration, mmol/L
Equilibrium concentration, mmol/L
Fig. 2. Isotherms of the adsorption of (a) [PtCl ] and (b) [PtCl4]2– complexes on the MgAl-LDH samples with Mg/Al ratios
2–
6
of (1) 2, (2) 3, and (3) 4.
(а)
(b)
27600
38000
1
2
1
20000
27000
17000
21000
2
3
42000
30800
25700
4
20000
30000
Wavenumber, cm–1
40000
50000
15000
20000
25000
30000
Wavenumber, cm–1
2
2–
–
Fig. 3. (a) DRES of the chloro complexes of platinum supported onto MgAl-LDH: (1) [PtCl ] (sample with Mg/Al = 3),
6
2
–
2–
(
2) [PtCl ] (sample with Mg/Al = 4), (3) [PtCl ] (sample with Mg/Al = 3), and (4) [PtCl ] (sample with Mg/Al = 4);
6
4
4
(
b) DRES of the carbonyl complexes of platinum supported onto MgAlO : (1) Mg/Al = 3 and (2) Mg/Al = 4.
x
the support basicity was increased, the amount of the These bands were detected in the DRES spectra of the
2–
hydrolyzed forms of platinum increased with the [PtCl ] /MgAl-LDH supported systems (Fig. 3a,
6
Mg/Al ratio, and this was accompanied by an increase
in the amount of immobilized platinum and by a color
change from pink to brown in the adsorbed precursors.
This assumption is consistent with the results of a
DRES study of the composition of immobilized com-
plexes (Fig. 3).
spectra 1 and 2) regardless of the Mg and Al content of
the support. The intensity ratios of these absorption
bands were almost identical in the spectra of the sys-
tems with different Mg/Al ratios. Consequently, on
2–
the adsorption of the inert complexes [PtCl ] on
6
MgAl-LDH, they did not undergo considerable
According to published data [21], the nonhydro- hydrolysis.
2–
lyzed complexes [PtCl ]
are characterized by
6
At the same time, on the adsorption of the chloro
absorption bands at 21000, 27000 (forbidden d–d complexes of Pt(II), the composition of the adsorbed
–
1
transitions in the central atom) and 38000 cm
complexes changed with the Mg/Al ratio (Fig. 3a,
(
charge transfer between the metal and the ligands). spectra 3 and 4). At Mg/Al = 3, the surface complexes
KINETICS AND CATALYSIS Vol. 58 No. 4 2017

3
88
STEPANOVA et al.
(а)
(b)
1
2
1
2
800
600
400
200
0
500
0
–500 –1000 –1500 –2000 –2500
δ, ppm
δ, ppm
Fig. 4. 1 Pt NMR spectra of (1) the chloro complexes of platinum in solution and (2) the above complexes supported onto
95
2–
2–
MgAl-LDH-OH: (a) [PtCl ] and (b) [PtCl ] . The Mg/Al ratio was 2; the platinum content was 15 wt %.
6
4
–
had the formula [Pt(H O)Cl ] and exhibited char- of the chloro complexes of Pt(II). The spectrum of
2
3
2–
acteristic absorption bands at 42000, 30800, and [PtCl
]
complexes in aqueous solution exhibited a
5700 cm [22], whereas the fraction of more deeply chemical shift at –1600 ppm (Fig. 4b, spectrum 1). The
hydrolyzed forms increased at Mg/Al = 4. This man- NMR signals were absent from the spectra of the
4
–
1
2
2–
ifested itself in a shift of the “ligand” → metal charge [PtCl ] /MgAl-LDH samples regardless of the Mg/Al
4
–1
transfer band (42000 cm ) to the region of higher ratio (Fig. 4b, spectrum 2). It is likely that this system is
wavenumbers with a decrease in its intensity. In this characterized by other metal complex–support interac-
–1
case, the bands at 25600 and 31300 cm were strongly tions. In the case of the intercalation of the square-pla-
smeared.
nar complexes of Pt(II) into the interlayer space, they
–
The 195Pt NMR-spectroscopic study of the
PtCl ] /MgAl-LDH sample also demonstrated the
6
can be additionally coordinated to the OH groups
2–
of the hydroxide layers of MgAl-LDH. The symmetry
of complexes also changed upon the immobilization of
platinum oxide species, which occur on the surface of
the most basic supports.
[
presence of nonhydrolyzed complex species. Spec-
trum 2 in Fig. 4a exhibits only one signal with a chem-
ical shift of about 70 ppm, which corresponds to
2–
[
PtCl ] . Other signals were not detected over a wide
Anchoring of the carbonyl complexes of Pt(0). The
6
195
range. Furthermore, Pt MAS NMR spectroscopy
provides information on the interactions of metal
complexes with the surface. It is well known that the
2−
carbonyl complexes of platinum [Pt (CO) ] were
3
6 6
adsorbed from an acetone solution on the mixed
oxides MgAlO (MgAl-LDHs calcined at 550°C)
x
2–
195
signals of supported [PtCl ] complexes in the Pt
6
under the conditions that prevented the penetration of
air into the system (to prevent the premature destruc-
tion of the complexes). Their adsorption was rapid and
complete, and it was accompanied by the discolor-
ation of the initial olive-green solution. In this case,
the support acquired a garnet red nuance caused by
the presence of the lower nuclearity complexes
NMR spectra are observed only in the case that their
octahedral symmetry remains unchanged or becomes
weakly distorted upon adsorption (for example, as a
result of electrostatic interaction) [23]. NMR signals
were absent upon more specific adsorption on the sup-
port. Because the NMR spectrum exhibited the sig-
nals of the adsorbed chloro complexes of platinum
with chemical shifts in a region close to the region of
their absorption in solution (Fig. 4a, spectrum 1), we
assumed that the anchoring of the chloro complexes of
Pt(IV) onto MgAl-LDH did not cause a distortion in
2
−
[
Pt (CO) ] with a characteristic color on the sup-
3 6 3
port surface [24]. This chemical composition of the
immobilized complexes was confirmed by the results
of the DRES study (Fig. 3b). Absorption bands at
–1
the symmetry of metal complex polyhedrons and it 17800, 20000, and 27600 cm , with are characteristic
was caused by Coulomb interaction between positively
charged aluminum–magnesium hydroxide layers and
negatively charged chloroplatinate ions.
2−
of the [Pt (CO) ] complexes in solution [24], were
3
6 3
present in the electron spectra. The absence of pro-
nounced bands due to the carbonyl complexes of
Fundamentally different results were obtained in the another composition is indicative of the high unifor-
Pt MAS NMR-spectroscopic study of the adsorption mity of an adsorption coating.
195
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EFFECT OF THE NATURE OF THE ACTIVE-COMPONENT PRECURSOR
389
Table 2. Results of the catalytic tests of Pt/MgAlO samples in the dehydrogenation of propane
x
Mg/Al
Pt, wt %
Precursor
Conversion, %
Selectivity, %
DI*, %
DН2, %
2
3
4
2
3
4
2
3
4
0.24
0.23
0.21
0.17
0.20
0.17
0.21
0.22
0.21
12.2
7.7
98.4
98.0
97.9
97.0
97.4
98.2
95.5
92.9
92.5
48
55
64
46
47
59
23
25
28
44
33
21
58
32
18
95
91
90
H [PtCl ]
2
6
4.4
14.8
11.6
6.6
H [PtCl ]
2
4
19.6
20.4
21.3
2
−
[
Pt (CO) ]
3
6 6
*
1
Deactivation index DI = 100(X
–Xfinal)/Xinitial, where Xinitial and Xfinal are the propane conversions after catalyst operation for
initial
0 min and 5 h, respectively.
The use of mixed oxides and acetone solutions for space of LDHs, which was accompanied by a decrease
the supporting of carbonyl complexes made possible in the positive charge of layers (with a decrease in the
the immobilization of metal complexes directly on the fraction of trivalent cations) and by the corresponding
support surface without the restoration of the laminar weakening of the strength of the interaction of anionic
structure of LDHs (which occurs on the adsorption of complexes with the support. This led to a monotonic
complexes on the mixed oxides from aqueous solu- decrease in the catalyst activity with increasing the
tions [19]). This approach ensured the minimum Mg/Al ratio. The higher activity of the samples based
2–
degree of metal complex–support interactions, and on [PtCl ] can be a consequence of the increased
4
the size of the supported particles of platinum was
determined by only the complex nuclearity and the
porous structure of the oxide support. Unlike the
adsorption of the chloro complexes of platinum, the
dispersion of the portion of platinum formed from the
intercalated complexes. The possibility of the addi-
tional coordination of square-planar complexes
decreases their mobility in the interlayer space and the
probability of the formation of larger platinum parti-
variation of the basicity of the oxide support MgAlO
x
does not exert a substantial effect on the composition cles in the course of heat treatment. This effect was
of the immobilized carbonyl complexes, which was most distinctly pronounced in the case of the sample
2
−
obtained with the use of the least basic support
almost identical in all of the [Pt (CO) ] /MgAlO
x
3
6
6
(
Mg/Al = 2), in which the fraction of platinum parti-
supported systems regardless of the Mg/Al ratio,
according to the DRES data.
cles formed from the hydrolyzed forms of Pt(II) com-
plexes was minimal.
In the catalysts prepared with the use of the car-
bonyl complexes of platinum, the dispersion of plati-
num was the highest (90–95%), and the degree of pro-
pane conversion on them, which weakly depends on
the Mg/Al ratio, reached a maximum.
Catalytic Properties of the Samples
Catalytic properties of the samples in the dehydroge-
nation of propane. The samples with different Mg/Al
ratios (2, 3, and 4) were prepared with the use of plat-
inum precursors of different composition (the chloro
Note that the target product was formed with high
complexes of Pt(IV) and Pt(II) and the carbonyl com- selectivity on the samples in which strong interaction
plexes of platinum) for studying the catalytic proper- between platinum and a basic support occurred on the
ties of the Pt/MgAlO systems in the dehydrogenation intercalation of a metal complex precursor. The rela-
x
reaction of propane. The calculated platinum content tively lower selectivity for the formation of propylene
of the catalysts was 0.2 wt %. Table 2 summarizes the on the samples prepared by the decomposition of car-
results of catalytic tests, specifying the real Pt content bonyl complexes on the surface of a mixed oxide and
and dispersion.
their lowered deactivation was caused by their
increased cracking ability (Fig. 5).
In the samples obtained from the chloride precur-
sors of platinum, a correlation between their catalytic
Catalytic properties of the samples in the dehydroge-
activity and the composition of a support was nation of n-decane. The effect of the composition of an
observed. The less dispersed particles of platinum were active component precursor on the properties of
formed upon the anchoring of the chloro complexes of Pt/MgAlO catalysts was also studied in the dehydro-
x
Pt(II) and Pt(IV) predominantly in the interlayer genation reaction of n-decane. In the conversion pro-
KINETICS AND CATALYSIS Vol. 58 No. 4 2017

3
90
STEPANOVA et al.
Product yield, %
С –С yield, %
1
2
0
.55
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
.50
.45
.40
.35
.30
.25
.20
.15
.10
.05
1
2
3
3
0
0
0
2
1
0
1
2
3
4
5
Cracking Isomerization Olefins
products products
Dienes
Time, h
Fig. 5. Time dependence of the yield of C –C cracking
Fig. 6. Dependence of the yield of n-decane dehydrogena-
1
2
products (methane, ethane, and ethylene) on the
tion products on the composition of active component
2
–
Pt/MgAlOx catalysts prepared with the use of different
precursors after catalyst operation for 3 h: (1) [PtCl ]
,
6
2
–
2–
(2) [PtCl4]2–, and (3) [Pt (CO) ] . The Mg/Al ratio
2
−
active component precursors: (1) [PtCl ] , (2) [PtCl4]
,
6
3 6 6
2
−
and (3) [Pt (CO) ] . The Mg/Al ratio was 2.
was 2.
3
6 6
cesses of higher alkanes, the contribution of side reac-
It is likely that the state of platinum produced with
tions is important; therefore, higher requirements are the use of intercalated chloro complexes is more pref-
imposed on the platinum centers responsible for the erable for the selective dehydrogenation of n-decane
to n-decene. In this case, the strong metal–support
interaction with electron density transfer from the
basic support to platinum particles can manifest itself;
as a result of this, the strength of the adsorption of
reagents on platinum decreases to influence the reac-
tion path of the conversion of n-decane molecules and
the desorption of the reaction products.
occurrence of the target reaction than those in the
dehydrogenation of light alkanes. Table 3 and Fig. 6
summarize the results of studying the samples with a
0
.2% Pt content, which were synthesized from the
chloro complexes of Pt(II) and Pt(IV) and the car-
bonyl complexes of Pt(0).
As in the dehydrogenation of propane, the catalyst
prepared from the carbonyl complexes exhibited a
maximum activity; however, the selectivity of the for-
mation of alkenes on it was comparatively low. In spite
CONCLUSIONS
Thus, in this work, we found that the selection of
of its high basicity (because of the use of a chloride- an active component precursor makes it possible to
free precursor), side reactions occurred to a consider- regulate the properties of Pt/MgAlO catalysts in the
x
able degree on the surface of this sample. As compared dehydrogenation reactions of propane and n-decane.
with the catalysts prepared from the complexes Catalysts with different dispersions of supported plat-
PtCl6]2– and [PtCl ] , it possessed noticeable isom-
2–
inum were prepared by the anchoring of various com-
[
4
2–
2–
plex compounds of platinum ([PtCl ] , [PtCl ] , and
erizing and cracking activity. Thus, the amounts of
cracking products and dienes formed on it were greater
6
4
2
−
[
Pt (CO) ] ) on layered aluminum–magnesium
3 6 6
by factors of 13 and 5, respectively, as compared with hydroxides and corresponding oxides with different
those formed on the catalysts synthesized the use of Mg/Al ratios. The highly dispersed catalysts synthe-
the chloro complexes of Pt(IV) and Pt(II) (Fig. 6).
sized with the use of the carbonyl compounds of plat-
Table 3. Results of the catalytic tests of Pt/MgAlO samples in the dehydrogenation reaction of n-decane (the ratio
x
Mg/Al = 3)
Precursor
H [PtCl ]
Pt content, wt %
n-Decane conversion, %
Selectivity, %
DH2, %
0.19
0.19
5.5
6.2
94.0
93.5
26
32
2
6
H [PtCl ]
2
4
2
−
0.18
7.9
62.0
91
[
Pt (CO) ]
3 6 6
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EFFECT OF THE NATURE OF THE ACTIVE-COMPONENT PRECURSOR
391
inum exhibited the greatest activity in alkane conver-
sion processes. At the same time, in contrast to a reac-
tion with the participation of propane, the state of
platinum acquired upon the intercalation of the chloro
complexes of platinum into the interlayer space of
LDHs is optimal for the selective formation of target
alkenes from higher alkanes.
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1. Wang, Q. and O’Hare, D., Chem. Rev., 2012, vol. 112,
Thus, the catalytic system Pt/MgAlO is capable of
x
p. 4124.
ensuring high selectivity for the formation of alkenes in
the dehydrogenation of light and higher alkanes. Fur-
ther research to be performed under the conditions
that ensure an increase in the conversion of reagents
1
2. Wang, J., Wang, L., Chen, X., Lu, Y., and Yang, W.,
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an increase in the reaction temperature and a smaller
1
4. Handbuch der preparativen anorganischen Chemie, von
dilution with hydrogen) with studying the stability and
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compare the currently available platinum catalysts for
dehydrogenation [25] with the samples based on
LDHs examined in this work and to evaluate prospects
for the application of these latter to industrial dehy-
drogenation processes.
Brauer, G., Ed., Stuttgart: Ferdinand Enke, 1981.
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ACKNOWLEDGMENTS
1
This study was carried out in the framework of
research work no. 17120V of Omsk State Technical
University using equipment of the Omsk Shared
Facilities, Siberian Branch, Russian Academy of Sci-
ences.
2
1
9. Bel'skaya, O.B., Gulyaeva, T.I., Leont’eva, N.N.,
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Translated by V. Makhlyarchuk
KINETICS AND CATALYSIS Vol. 58 No. 4 2017