Influence of Method of Introduction of Cu- and Zn-Based Modifiers on the Properties of Chromia–Alumina Catalysts

Article abstract of DOI:10.1134/S0023158418020118

Three methods of introduction of modifiers based on Cu and Zn compounds into the CrO_x/Al₂O₃ catalysts for dehydrogenation of light paraffin hydrocarbons are considered: Introduction from sol, introduction using successive impregnation technique and introduction of modifiers by impregnation along with precursor of chromium oxide. The obtained samples are studied by a complex of physical-chemical methods (XRD, UV-Vis spectroscopy, temperature-programmed reduction with hydrogen (TPR-H₂), X-ray fluorescent (XRF) spectrometry, low-temperature N₂ sorption). The catalytic properties of the samples are studied in kinetic mode in isobutane dehydrogenation. Cu- and Zn-modifiers are shown to influence on the peculiarities of reduction of Cr⁶⁺ and, hence, specify the state of active surface of CrO_x/Al₂O₃ catalysts formed in the reductive reaction medium. Not only do the states of modifiers influence on the initial activity of the catalyst, but also on its activity after oxidative regeneration. Introduction of modifiers by successive impregnation method results in formation of copper and zinc aluminates or defective spinels on the Al₂O₃ surface. When the active component is introduced, the modified surface of the support promotes formation and stabilization of Cr⁶⁺ sites that can undergo reversible reduction–oxidation and provide high activity and selectivity towards formation of isobutylene (>98%).

Full text of DOI:10.1134/S0023158418020118

ISSN 0023-1584, Kinetics and Catalysis, 2018, Vol. 59, No. 2, pp. 211–217. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © A.A. Merk, M.A. Salaev, O.V. Vodyankina, G.V. Mamontov, 2018, published in Kinetika i Kataliz, 2018, Vol. 59, No. 2, pp. 232–239.
Influence of Method of Introduction of Cu- and Zn-Based Modifiers
on the Properties of Chromia–Alumina Catalysts1
A. A. Merk*, M. A. Salaev, O. V. Vodyankina, and G. V. Mamontov
National Research Tomsk State University, Tomsk, 634050 Russia
*e-mail: merk_a@rambler.ru
Received February 3, 2017
Abstract⎯Three methods of introduction of modifiers based on Cu and Zn compounds into the CrOx/Al2O3
catalysts for dehydrogenation of light paraffin hydrocarbons are considered: Introduction from sol, introduc-
tion using successive impregnation technique and introduction of modifiers by impregnation along with pre-
cursor of chromium oxide. The obtained samples are studied by a complex of physical-chemical methods
(XRD, UV-Vis spectroscopy, temperature-programmed reduction with hydrogen (TPR-H2), X-ray fluores-
cent (XRF) spectrometry, low-temperature N2 sorption). The catalytic properties of the samples are studied
in kinetic mode in isobutane dehydrogenation. Cu- and Zn-modifiers are shown to influence on the pecu-
liarities of reduction of Cr6+ and, hence, specify the state of active surface of CrOx/Al2O3 catalysts formed in
the reductive reaction medium. Not only do the states of modifiers influence on the initial activity of the cat-
alyst, but also on its activity after oxidative regeneration. Introduction of modifiers by successive impregna-
tion method results in formation of copper and zinc aluminates or defective spinels on the Al2O3 surface.
When the active component is introduced, the modified surface of the support promotes formation and sta-
bilization of Cr6+ sites that can undergo reversible reduction–oxidation and provide high activity and selec-
tivity towards formation of isobutylene (>98%).
Keywords: CrOx/Al2O3 catalysts, modification method, Cu- and Zn-based modifiers, state of active compo-
nent, isobutane dehydrogenation
DOI: 10.1134/S0023158418020118
INTRODUCTION
compounds [21, 22]. However, the influence of Cu-
and Zn-based compounds on the surface structure
and state of active component of chromia–alumina
catalysts for isobutane dehydrogenation into isobuty-
lene has been poorly studied.
CrOx/Al2O3 catalysts are widely used in industry to
produce valuable monomers for chemical synthesis [1,
2]. In the literature a number of methods for prepara-
tion of chromia–alumina catalysts was proposed:
mechanical mixing [3], incipient wetness impregna-
tion [4–6], deposition from solution of precursor salts
of active component and support [7], one-pot synthe-
sis [8], sol–gel synthesis [9], hydrothermal synthesis
[10, 11], etc. The choice of the preparation method is
determined by specified characteristics of the catalyst,
such as structural-mechanical properties, porous
In this work, the effect of method of introduction
of Cu and Zn modifying compounds into CrOx/Al2O3
catalysts on the state of active component and, as a
consequence, on the catalytic properties of the formed
chromium oxide sites in isobutane dehydrogenation
were considered. The impregnation by sol or the solu-
structure, chemical composition, state of active com- tion of modifiers followed by deposition of Cr-con-
ponent, and the opportunity to synthesize it in indus-
try [12]. To increase the activity, selectivity and stabil-
ity of CrOx/Al2O3 catalysts different modifiers based
on compounds of potassium [13], calcium [14], VIII
group metals (first of all, iron, cobalt, nickel) [15],
etc., are widely used. Special interest is brought by the
modifiers based on copper [16–18] and zinc [19, 20]
compounds. Simultaneous introduction of copper and
zinc, including their application as active compo-
nents, is used in catalytic dehydrogenation of organic
taining active component as well as the method of
simultaneous impregnation from solution containing
both the modifiers and chromium oxide precursor
were studied. When choosing the methods of support
preparation and introduction of Cu- and Zn-based
modifiers , we took into account the widespread use of
incipient wetness impregnation method in the indus-
try [3–5] as well as the opportunity to reduce the syn-
thesis time (one-pot impregnation) [8, 23] and forma-
tion of highly disperced phases of the modifiers (sol–
gel synthesis) [24].
1
The article was translated by the authors.
211

212
MERK et al.
eled off in the range of relative pressures P/P0 from
EXPERIMENTAL
0.05 to 0.30. To plot the curves of pore size distribution
the BJH-Desorption method was used and accompa-
nied by the analysis of desorption branches of the
nitrogen adsorption-desorption isotherms. Prior to
the measurements, the sample weighs (100–120 mg)
were outgassed in vacuum at 200°C for 2 h.
The chemical states of chromium and phase com-
position of the catalysts were studied using diffuse
reflectance UV-vis spectroscopy (DRS) on the Evolu-
tion 600 spectrometer (Thermo Scientific, USA)
using a base line spectrum of freshly precipitated
BaSO4 and powder X-ray diffraction (XRD) on the
Miniflex 600 diffractometer (Rigaku Corporation,
Japan) using a monochromatic CuKα radiation (γ =
1.5418 Å). The phase composition was detected using
the PCPDFWIN database.
To evaluate the peculiarities of reduction of copper
and chromium compounds in the catalysts the tempera-
ture-programmed reduction with hydrogen (TPR-H2)
was used. The measurements were carried out using
the ChemiSorb 2750 chemisorption analyzer
(“Micromeritics,” USA) with a thermal-conductive
detector (TCD-signal) at a heating rate of 10°/min and
a flow rate of argon-hydrogen mixture (10 vol % H2) of
20 mL/min.
Synthesis of Supports and Catalysts
Pseudoboehmite was used as a γ-Al2O3 precursor
and obtained by precipitation of sodium aluminate by
the solution of sulphuric acid at pH 7–9 followed by
washing-off from sodium. Cu(NO3)2 ⋅ 3H2O (chemi-
cally pure) and Zn(NO3)2 ⋅ 6H2O (chemically pure) in
amounts of 2.5 wt % in catalysts calculated with
respect to Cu and Zn, respectively, were used as pre-
cursors of the modifiers.
The catalysts were synthesized by three various meth-
ods, with variation of the method of modifier introduc-
tion. The first catalyst (hereinafter CrOx/Cu-Zn/Al2O3)
was prepared by incipient wetness impregnation of the
support with precursors of the modifiers and then with
a precursor of the active component. Pseudoboehmite
was impregnated by water solution of copper and zinc
nitrates followed by drying at 170°С for 12 h and calci-
nation at 500°С for 6 h. To obtain the modified
CrOx/Al2O3 catalysts with Cr2O3 and K2O contents of
4.5 wt % and 0.85 wt % respectively, the support was
impregnated with the aqueous solution of CrO3
(chemically pure) and KNO3 (used as a source of
alkali metal) and then dried at 120°C and calcined in
air at 700°C for 2 h.
The second catalyst had the same chemical com-
position (hereinafter CrOx–Cu–Zn/Al2O). The cata-
lyst was synthesized by impregnation of pseudoboeh-
mite with water solution simultaneously containing
the precursors of modifiers and active component.
After that the catalyst was dried and calcined under the
same conditions as for the previous sample.
The third catalyst was prepared by introduction of
the modifiers by impregnation of pseudoboehmite
with sol of copper and zinc hydroxides. The sol was
prepared by controllable hydrolysis of the correspond-
ing nitrates at 70°C in the solution of ethylene glycol at
pH 1–2 [25]. The catalyst was hereinafter referred to
as Sol. The mixture obtained was dried using the rota-
tion steamer at 90°C and a 0.5 atm during 4 h. The
modified support was dried in the air flow at 170°C for
12 h and calcined during 6 h at 500°C. After that it was
impregnated with water solution of CrO3 and KNO3
with the given composition, and then the catalyst was
dried and calcined under the same conditions as in the
previously two samples.
Testing of Catalytic Properties of the Samples
Catalytic properties of the samples were investi-
gated in isobutane dehydrogenation into isobutylene
in accordance with technique described in [26]. The
experiments were carried out in the kinetic mode in a
fixed-bed tubular quartz reactor at 540°C. A weigh of
catalyst (500–600 mg) with the particle sizes of 0.1–
0.25 mm was mixed with the inert diluent (quartz) in
the volumetric ratio of 1 : 2. The reaction mix of 15%
i-C4H10 in N2 was passed through the fixed catalyst
bed with a rate of 9 L/h. The testing duration was 3 h
and included air regeneration (rate flow 3 L/h) and
reduction in a flow of H2 + N2 (0.53 and 3 L/h,
respectively).
RESULTS AND DISCUSSION
Characterization of Catalysts
The porous structure of the catalysts was studied
using low-temperature adsorption of nitrogen. In
Fig. 1 the adsorption-desorption isotherms of nitro-
gen and pore size distributions are presented for the
synthesized catalysts and the Al2O3 sample taken as a
Characterization of Supports and Catalysts
Chemical analysis of the catalysts was carried out
using sequential X-ray fluorescence (XRF) wave-
length-dispersive spectroscopy (XRF-1800) (Shi- result of thermal treatment of the initial pseudoboeh-
madzu, Japan). Porous structure of the samples was mite at 500°C. The presence of a hysteresis loop for
studied by low-temperature (–196°C) N2 sorption P/P0 of 0.5–1.0 in all isotherms (Fig. 1a) indicates that
(TriStar 3020 analyzer, Micromeritics, USA). The the synthesized catalysts and alumina support have
specific surface area was determined using the multi- mesoporous structures. In Fig. 1b it can be seen that
point BET method, isotherms of adsorption were lev- the pores within the samples possess the sizes between
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INFLUENCE OF METHOD OF INTRODUCTION
213
2 and 20 nm, with the maximum of distribution being
(a)
at 6–10 nm.
A comparison of the characteristics of the prepared
CrOx/Cu–Zn/Al2O3
CrOx–Cu–Zn/Al2O3
Sol
10
8
catalysts and supports allow tracing how the porous
structure transforms depending on the method of
introduction of modifiers and the active component.
In the isotherm for the Sol catalyst based on the sup-
port modified by sol of copper and zinc hydroxides, a
loop of hysteresis was shifted towards higher values of
P/P0 as compared to isotherms for Al2O3 support. This
suggests decreasing the share of mesopores with sizes
below 5 nm and increasing the share of mesopores
with sizes above 5 nm (Fig. 1b). This can be associated
with localization of particles of sol in the fine pores of
the support.
Al2O3
6
4
2
Adsorption–desorption isotherms for catalysts are
located below the one of the support (Al2O3) in all
range of relative pressures, both for catalysts obtained
by successive introduction and co-introduction of
active components and modifiers. This evidences on
the uniform distribution of the modifiers and active
component inside the support pores. In the curves of
pore size distribution, the volume of pores with size of
2–10 nm in the catalysts is less than in the alumina
obtained by pseudoboehmite calcination at 700°C.
In Table 1 some parameters of porous structure and
results of chemical analysis of chromia–alumina cata-
lysts and alumina calcined at 500°C is shown. The sur-
face area of the catalysts is lower as compared to the
one of alumina that is connected both with localiza-
tion of active component and modifiers in the pores of
the support and also with higher calcination tempera-
ture of the catalysts (700°C). The sample obtained by
introduction of modifiers through impregnation with
sol has the maximal volume of pores, which is caused
by the increase of the width of the latter (Fig. 1b).
0
0.2
0.4
0.6
0.8
1.0
P/P0
(b)
CrOx/Cu–Zn/Al2O3
CrOx–Cu–Zn/Al2O3
Sol
Al2O3
0.04
0.03
0.02
0.01
0
5
10
15
20
25
Pore diameter, nm
Results of X-ray fluorescence analysis show that
the catalysts have similar chemical composition. Some
elevated level of content of individual components can
be connected with their concentration on the surface
of the sample, which makes them more available for
X-ray beam.
Fig. 1. Isotherms of nitrogen adsorption-desorption (a)
and pore size distribution (b) for support and chromia-alu-
mina catalysts.
face of alumina were investigated by XRD and UV-vis
spectroscopy (DRS). In Fig. 2a XRD patterns for
The phase composition of the catalysts, state of
active component and modifiers supported on the sur- modified catalysts and alumina calcined at 500°C are
Table 1. Characteristics of porous structure of alumina and chromia–alumina catalysts
Pore
volume,
cm3/g
Composition of elements, wt %
Pore
size,
nm
S,
m2/g
Tcalcination
,
Sample
°C
Al
O
Cr
K
Cu
Zn
∑
Al2O3
500
700
700
700
207
138
131
141
0.30
0.28
0.28
0.37
5.7
9.1
8.5
8.6
56.2
49.1
44.9
49.1
43.5
36.9
41.7
38.3
−
−
−
−
99.7
97.4
97.1
99.5
CrOx/Cu–Zn/Al2O3
CrOx–Cu–Zn/Al2O3
Sol
3.6
3.7
3.5
0.8
0.9
0.8
3.4
2.9
3.8
3.6
3.0
4.0
Dashes mean the absence of corresponding elements in composition of sample.
KINETICS AND CATALYSIS Vol. 59 No. 2 2018

214
MERK et al.
of γ-Al2O3 phase (Table 2) increases with introduction
(a)
γ — γ-Al2O3
* — CuAl2O4
Δ — ZnAl2O4
of the modifiers. This allows suggesting that Cu and
Zn form solid solutions with alumina with cubic struc-
ture and spinel phase. The above described reflexes are
more intensive in the spectra of CrOx/Cu–Zn/Al2O3
and Sol catalysts, which can be caused by stabilization
of the main part of modifiers as defective spinel. In the
sample prepared by co-introduction of modifiers and
the active component, copper and zinc, hypotheti-
cally, form joint phases with alumina support as well as
with active component.
γ, *, Δ
γ, *, Δ
γ
γ, *, Δ
γ
γ, *, Δ
γ
CrOx/Cu−Zn/Al2O3
CrOx−Cu−Zn/Al2O3
Sol
In the UV-vis DR spectra (Fig. 2b) of all the cata-
lysts the adsorption bands with maximums at 270 and
370 nm are observed, which can be caused by electron
transfer О → Cr6+ with tetrahedraly coordinated chro-
mium in the strongly- and weakly bonded forms as
well as the band with low intensity with maximum at
445 nm that is typical for dichromates [28–30]. These
absorption bands are the most marked in the spectrum
of the Sol catalyst. Besides, in the spectra for all cata-
lysts, absorption bands in the range of 200–245 nm are
presented and related to electron transfer О → Cu2+
(ligand-to-metal charge transfer) [9, 31].
It is noteworthy that in the spectrum of the Sol cat-
alyst absorption bands in the range of 500–600 nm,
which is more expressed than in the spectra of other
catalysts, is observed and corresponds to Cr3+ state
[29]. The absorption band in the range of 700–800 nm
can be attributed to formation of mixed oxides of
Cu(I) and Cu(II) in the Sol sample. Similar bands
were found in Ref. [31] during the investigation of
Cu/Al2O3.
Al2O3
ZnAl2O4 — PDF 05-0669
CuAl2O4 — PDF 33-0448
γ-Al2O3 — PDF 00-0741
10
20
30
40
2θ, deg
50
60
70
80
(b)
3
370
CrOx/Cu–Zn/Al2O3
CrOx–Cu–Zn/Al2O3
Sol
270
240
2
1
0
445
690
Thus, based on the data of XRD and UV-vis spec-
troscopy, it can be concluded that the active compo-
nent is in a highly-dispersed amorphous state on the
surfaces of the prepared catalysts, mainly in a Cr6+
state. The modifiers are also weakly crystallized and,
in particular, comprise the aluminates of CuAl2O4 and
ZnAl2O4. The Sol sample contains an active compo-
200 300 400 500 600 700 800 900
Wavenumber, nm
Fig. 2. XRD patterns (a) and UV-vis spectra (b) of Al O
support and synthesized catalysts.
2
3
nent in the Cr3+ state. Besides, copper and zinc can be
presented in the form of dispersed oxide. This confirms
that both modifiers and active component change their
state under conditions of sol impregnation.
presented. For all samples, reflexes of γ-Al2O3 phase
typically present and are associated with the use of
pseudoboehmite as a precursor. The absence of
expressed reflexes of Cr-containing phases points at
stabilization of chromia in a highly-dispersed amor-
phous state. Reflexes of phases of Cu and Zn oxides
are not also observed, which evidences on their high
dispersion or incorporation into the support structure.
However, a comparison of X-ray patterns for alumina
and modified catalysts shows that the intensity of
reflexes of the catalysts at 2θ = 31.3° and 36.9° as well
as at 60.5° and 66.6° differ from the intensity of reflexes
for Al2O3 and, according to database PCPDFWIN and
data of [27], are assigned to phases of copper and zinc
The peculiarities of reduction of modified supports
and chromia–alumina catalysts on the basis thereof,
were investigated by TPR-H2. The appearance of TPR
profiles (Fig. 3) shows that hydrogen consumption
basically occurs in a range of 200−430°C. The H2 con-
sumption for CrOx–Cu–Zn/Al2O3 catalyst obtained
by simultaneous introduction of modifiers and active
component into the high-temperature range (the peak
with a maximum at 367°C) can be caused by forma-
tion of mixed phases of copper and chromium, pre-
sumably as copper chromates. The presence of low-
temperature shoulder in the TPR profiles of the
CrOx/Cu–Zn/Al2O3 and Sol catalysts at 257 and
aluminates. Besides, the parameter of cubic lattice, a, 255°C, respectively, probably indicates successive
KINETICS AND CATALYSIS
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2018

INFLUENCE OF METHOD OF INTRODUCTION
215
Table 2. Lattice parameter (a) of γ-Al2O3 phase and amount of hydrogen consumed by the supports and catalysts investi-
gated
H2 consumption**, μmol/g
Catalysts
a*, Å (XRD)
supports
freshly prepared sample sample after reaction
CrOx/Cu–Zn/Al2O3
CrOx–Cu–Zn/Al2O3
Sol
7.9254
7.9381
7.9407
7.9030
269
802
889
584
665
539
163
0 (Al2O3)
243
0 (Al2O3)
1171
493
CrOx/Al2O3
* Determined by XRD method.
** Determined by TPR-H method.
2
reduction of copper compounds (easily reducible alumina occurs for these two samples that leads to
component) from the oxide state [32], and then the
chromium reduction from high valence state, i.e.,
Cr(VI) → Cr(III) transformation [33]. Reduction
peaks with maximum at 325 and 342°C in the TPR
profiles of the supports are referred to reduction of
copper from a highly dispersed phases of surface alu-
minate as it was shown in Ref. [34]. Comparison of
peak areas of hydrogen consumption by the supports
and catalysts shows that impact of hydrogen that is
consumed for the reduction of oxidized forms of cop-
per is 20–34% (Table 2). Thus, one can assume that in
accordance with the results of UV-vis spectroscopy
from 40 to 80% of chromium in the catalysts are pre-
sented as Cr6+. Different location of TPR peaks and
different shape of these hydrogen consumption peaks
for two above noted catalysts can indicate the reduc-
tion of Cr(VI) particles with various sizes from the
same phases [34] or about various interaction of copper
and zinc with the support surface [35]. Smaller reduction
temperature for the Sol sample is attributed to low-tem-
perature reduction of copper (peak at 255°C), which
makes further reduction of Cr6+ easier [36].
changing of the nature of interaction of chromium
with the support.
The amount of hydrogen consumed decreases after
dehydrogenation reaction for all catalysts investigated.
This suggests partial stabilization of chromium sites in
the Cr3+ state (probably, α-Cr2O3) that are not sub-
jected to re-oxidation in the selected regeneration
conditions. As shown in TPR profile of the CrOx–
Cu–Zn/Al2O3 catalyst, this sample consumes maxi-
mum amount of hydrogen. This evidences on the
keeping of larger amount of redox forms of copper and
chromium. In the Sol catalyst, the amount of such
forms decreases considerably after its placement into
redox conditions of the catalytic reaction of isobutane
dehydrogenation.
340
337
(a)
3
2
1
275
Comparison of the amount of hydrogen consumed
by the catalysts before and after the catalytic experi-
ment (two cycles of isobutane dehydrogenation fol-
lowed by re-oxidation in air at 700°C) (Fig. 3, Table 2)
shows that for all catalysts a narrowing of H2 con-
sumption peak is observed that can result from redis-
tribution of modifiers and chromium on the surface of
catalysts with formation of more stable species. Tem-
peratures of TPR peak maximums differ after catalytic
cycles over all three catalysts. Probably, this is caused
by different form of chromium interaction with the
support or different Cr6+ species. Chromium is still
reduced at low temperature (peak at 305°C) in the Sol
catalyst that can be a result of stabilization of copper in
the form of separate oxide phase. Reduction of this
phase (a shoulder at 225°C) simplifies the reduction of
active component. Mechanism of action of modifiers
in the CrOx/Cu–Zn/Al2O3 and CrOx–Cu–Zn/Al2O3
catalysts is different than for the Sol catalyst. Probably,
a stabilization of modifiers in the surface structure of
325
353
367
3
2
1
(b)
(c)
284
305
255
3
2
1
342
100
200
300
400
500
600
Temperature, °C
Fig. 3. TPR-H profiles for catalysts (a) CrO /Cu–
2
x
Zn/Al O , (b) CrO –Cu–Zn/Al O , (c) Sol: (1) sup-
2
3
x
2 3
ports, (2) freshly prepared catalysts on the basis thereof,
(3) same catalysts, after 2 cycles of isobutane dehydroge-
nation and after regeneration cycle.
KINETICS AND CATALYSIS Vol. 59 No. 2 2018

216
MERK et al.
treatments. After slight increase of isobutene conver-
sion at this step, the catalyst activity gradually
decreases.
99
98
The CrOx/Cu–Zn/Al2O3 catalyst obtained by suc-
cessive introduction of modifiers and active compo-
nent, rapidly increases its activity after oxidative and
reductive treatment (isobutane conversion increases
from 10.8 to 13.8%). Then the conversion increases to
up to 14.3%, and then, at the end of dehydrogenation
cycle it decreases to up to ~12%. The selectivity of
isobutylene formation over this catalyst is also higher
than the one over other catalysts.
97
14
12
10
8
6
CrOx/Cu–Zn/Al2O3
CrOx–Cu–Zn/Al2O3
Sol
4
It is noteworthy that the activity (isobutane conver-
sion) and selectivity over the catalysts modified by
copper and zinc compounds are higher than those for
the CrOx/Al2O3 catalyst. Thus, the introduction of the
modifiers makes positive impact on the catalyst activ-
ity, but the activity increase depends on the introduc-
tion method.
2
CrOx/Al2O3
0
6
18
6
18 30 42 54 66
Time of reaction, min
Fig. 4. Dependence of isobutane conversion and selectivity
towards isobutylene formation on time of dehydrogenation
for the prepared chromia-alumina catalysts at 540°C.
Relatively low conversion and fast deactivation of
the sample prepared by introducing the modifiers
from the sol can be connected with the fact that the
active component is present in the form of Cr2O3 as shown
by UV-vis and TPR methods (Fig. 2b and Table 2). An
increase of the reduction temperature of chromium
oxides (shift of peak maximums from 284 to 305°C)
and significant decrease of the TPR peak area after
isobutene dehydrogenation over the Sol catalyst can
be a result of relatively low stability of deposited chro-
mium oxides that leads to rapid deactivation of the
sample due to aggregation of active component.
Catalytic Properties of Samples in Isobutane
Dehydrogenation
Figure 4 shows isobutane conversion (X) and selec-
tivity (S) towards isobutylene formation depending on
the time of isobutane dehydrogenation over the pre-
pared modified chromia–alumina catalysts as well as
on the catalyst, obtained by impregnation of Al2O3
support by water solution of CrO3 and KNO3 which
was used as a reference sample. The CrOx/Cu–
Zn/Al2O3 and CrOx–Cu–Zn/Al2O3 catalysts show
comparable activity during the first 18 min of reaction.
Furthermore, isobutane conversion increases that
indicates changing of the state of the active surface
under the influence of reductive reaction mixture and
formation of more active sites. As opposed, the isobu-
tane conversion for the Sol catalyst decreases during
these 18 min. This can be caused by significant chang-
ing of the state of active component, for example, its
aggregation accompanied by formation of low active
α-Cr2O3 phase [1], which is consistent with the TPR
data, or with formation of carbon deposits resulting in
blocking of the main part of active sites [37]. However,
high selectivity towards isobutylene formation (98–99%)
is achieved for all catalysts. This evidences on low proba-
bility of formation of carbon deposits at 540°C.
The sample prepared by one-step introduction of
modifiers and chromium (CrOx–Cu–Zn/Al2O3)
shows higher activity than the Sol catalyst, which can
be attributed to elevated concentration of “strongly
bound” Cr6+ sites, according to the data of UV-vis
spectroscopy (absorption band at 370 nm in Fig. 2b).
After oxidative regeneration and reductive activation
the surface state of these catalysts change, and this
leads to sharp fall of conversion.
The CrOx/Cu–Zn/Al2O3 catalyst prepared by suc-
cessive impregnation is the most active. High values of
conversion and selectivity of isobutylene formation for
this sample are explained by stabilization of active
component predominantly in the form of redox Cr6+
sites (reduction peak at 337°C in Fig. 3a) as well as the
absence or lower content of Cr2O3 (Fig. 2b). The activ-
ity of this catalyst in the dehydrogenation process
increases in the first cycle of dehydrogenation and sig-
nificantly rises after the redox cycle. This is caused by
“favorable” distribution of modifiers, mainly in the
form of surface aluminates that promote stabilization
The catalytic properties change after regeneration
of the samples in the air flow followed by reductive
activation. The isobutane conversion for the Sol cata-
lyst decreases that indicates sample deactivation. In
comparison with the first 18 min, the isobutene con-
version for the CrOx–Cu–Zn/Al2O3 catalyst is much
lower. This confirms significant changing of its surface
state under the influence of oxidative and reductive of active component in the form of redox Cr6+ sites.
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2018

INFLUENCE OF METHOD OF INTRODUCTION
217
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CONCLUSION
Thus, the modifying additives of copper and zinc
compounds increase the activity and selectivity of
chromia-alumina catalysts in isobutane dehydrogena-
tion independently on the method of their introduc-
tion. The method of introduction of modifiers (from
sol or by successive or joint addition of the active com-
ponent with the precursors) influences on their distri-
bution and state in the catalysts obtained, and also
determines the state of active component (chromium
oxides), reactivity and catalytic properties of the sam-
ple. The catalyst prepared by successive introduction
of Cu- and Zn-containing modifiers and chromium
precursor using impregnation method shows the high-
est activity in isobutane dehydrogenation and high
selectivity towards isobutylene formation (98.9%).
15. Beccari, M. and Romano, U., Encyclopaedia of hydro-
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17. US Patent 6362385 B1, 2002.
18. US Patent 20050075243 A1, 2005.
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ACKNOWLEDGMENTS
This work was carried out as a part of a government task
of the Russian Ministry for Education and Science (proj-
ect no. 4.4590.2017/6.7) and the Tomsk State University
competitiveness improvement program (research grant).
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