Synthesis and study of zeolites modified with cation metals as catalysts for the reaction of oxidative dehydrogenation of naphthenic hydrocarbons

Article abstract of DOI:10.1134/S107042721705010X

A study of the catalytic activity of modified zeolites in the reaction of selective oxidative dehydrogenation of cyclohexane and methyl cyclohexane demonstrated that catalysts prepared based on natural clinoptilolite modified with Cu²⁺, Zn²⁺, Co²⁺, and Cr³⁺ cations showed the highest activity in the reactions considered. Specifcally, the natural zeolite, clinoptilolite, containing 0.5 wt % Co₂₊ and 0.25 wt % Cr₃₊ is an active catalyst for the reaction of oxidative dehydrogenation of methyl cyclohexane into cyclohexadiene-1,3. With the experimental data and the binding energies of the catalyst components with atomic oxygen considered, the active centers in the components of the catalyst for the reaction of oxidative dehydrogenation of cyclohexane, responsible for the formation of cyclohexadiene-1,3-cyclohexene, and benzene can be divided into groups. Principles to be used when selecting high-efficiency catalytic systems for the reaction of oxidative dehydrogenation of alicyclic hydrocarbons to the corresponding dienes are formulated.

Full text of DOI:10.1134/S107042721705010X

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2017, Vol. 90, No. 5, pp. 726−731. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © A.M. Aliev, Z.A. Shabanova, A.I. Kerimov, 2017, published in Zhurnal Prikladnoi Khimii, 2017, Vol. 90, No. 5, pp. 591−597.
CATALYSIS
Synthesis and Study of Zeolites Modified with Cation Metals
as Catalysts for the Reaction of Oxidative Dehydrogenation
of Naphthenic Hydrocarbons
A. M. Aliev, Z. A. Shabanova*, and A. I. Kerimov
Nagiev Institute of Catalysis and Inorganic Chemistry, National Academy of Sciences of Azerbaijan,
Baku, AZ1141 Azerbaijan
*
e-mail: zumrud-042425-@mail.ru
Received April 14, 2016
Abstract—A study of the catalytic activity of modified zeolites in the reaction of selective oxidative dehydro-
genation of cyclohexane and methyl cyclohexane demonstrated that catalysts prepared on the basis of natural
2
+
2+
2+
3+
clinoptilolite modified with Cu , Zn , Co , and Cr cations exhibit the highest activity in the reactions under
consideration. Principles to be used when selecting high-efficiency catalytic systems for the reaction of oxidative
dehydrogenation of alicyclic hydrocarbons to the corresponding dienes are formulated.
DOI: 10.1134/S107042721705010X
Zeolites modified with metal cations are widely used
in practice as catalysts and are intensively studied in the
basic catalysis.An important task is to develop approaches
to optimization of the catalytic activity and selectivity of
zeolite catalysts in the oxidative dehydrogenation of
naphthenic hydrocarbons. Previously, studies have been
carried out in order to synthesize modified ultradispersed
metal-zeolite catalysts for oxidation of lower olefin
hydrocarbons into carbonyl compounds, partial oxidation
of paraffin hydrocarbons, oxidative conversion of
aliphatic alcohols, oxidative coupling of methane, and
oxidative dehydrogenation of naphthenic hydrocarbons
EXPERIMENTAL
The reaction was performed in a flow-through
laboratory installation with a quartz reactor on a fixed
bed of a catalyst under atmospheric pressure in the
temperature range 280–390°C at a volumetric flow rate
–1
of the gas mixture of 1000–3000 h and cyclohexane :
O : N molar ratio of 1 : (0.24–1) : 5.3.
2
2
We used synthetic zeolites NaY (SiO /Al O = λ =
.2), NaX (λ = 2.9), and NaA(λ = 2.0) and natural zeolites
clinoptilolite (λ = 8.68) and mordenite (λ = 9.6) from an
Azerbaijani deposit, modified with various cations of
transition and nontransition elements (Zn, Cu, Co, Cr,
Mn, Fe, Mg, Mo, etc.).
2
2
3
4
[
1–4]. It has been found that the natural clinoptilolite
2
+
2+
2+
3+
modified with Cu , Zn , Co , and Cr cations via ion
exchange is an active catalyst for the reaction of oxidative
dehydrogenation of cyclohexane into cyclohexadiene-1,3,
and certain specific features of the reaction mechanism
have been revealed [4].
The catalysts were synthesized by the ion-exchange
method [2]. Prior to being subjected to ion exchange,
natural; zeolites were treated with 0.5 n HCl. The amount
of elements introduced into a zeolite was determined by
ion-spectral analysis on Agilent ICP-MS. We used cata-
lysts with particle sizes of 0.25–0.63 mm, cyclohexane
of 99.5% purity (MerckCAS-N 110-82-7), and methyl
cyclohexane of 99% purity (Merck, CAS-N 108-87-2).
The goal of our study was to optimize the structure
of ultradispersed metal-zeolite catalysts for the
reaction of oxidative dehydrogenation of naphthenic
hydrocarbons, including the oxidative dehydrogenation
of methyl cyclohexane, and determine the part played by
components of the catalytic system in raising the activity
and selectivity of the catalyst.
The raw materials and reaction products were
analyzed on a gas chromatograph directly connected to
7
26

SYNTHESIS AND STUDY OF ZEOLITES
727
the reaction unit. The reaction products were separated
in a 3-m-long column packed with Porapak-T under the
conditions linearly programmed rise in the temperature
of the thermostat of the chromatograph from 50 to 200°C.
The reaction products were also analyzed with anAgilent
7890 gas chromatograph withAgilent-5975 mass detector
and 30-m-long HP-5 MS column.
RESULTS AND DISCUSSION
The results obtained in testing the catalytic activity of
the metal-zeolites synthesized in the study in the reaction
of cyclohexane dehydrogenation into cyclohexadiene-1,3
are listed in Table 1. It can be seen that natural clinopti-
lolite and its form modified with Zn cations exhibit a low
catalytic activity in the reaction of oxidative dehydroge-
nation of cyclohexane (run nos. 1 and 2). The catalytic
activity of natural clinoptilolite samples modified with
Cr , Cu , and Co cations is high, compared with the
above zinc-containing catalytic systems, especially as
regards the aromatization of cyclohexane (run nos. 3–5).
The yields of benzene are 15.5, 12.5, and 9.8%, respec-
tively. In this case, cyclohexadiene-1,3 is formed in low
Fig. 1. Polyhedral model of clinoptilolite, which reflects a
fragment of the structure with exchange cations [5].
cyclohexane into cyclohexadiene-1,3 (run nos. 1–6). The
highest catalytic activity in this reaction is observed for
3+
2+
2+
2+
3+
natural clinoptilolite modified with Co and Cr cations
2+
(run nos. 11–15). Introduction of the Zn cation into these
catalytic systems makes lower their catalytic activity in
this reaction (run nos. 16, 17). Clinoptilolite containing
Cu, Zn, Co, and Cr cations has a relatively low catalytic
activity in this reaction, compared with clinoptilolite
2
+
3+
2+
yield. Introduction of two cations (Cu , Cr and Co ,
3+
3+
Cr ) into natural clinoptilolite leads to an insignificant
increase in the yield of cyclohexadiene-1,3. However, the
reaction of cyclohexane aromatization (run nos. 7 and 8)
also dominates in this case. Introduction of a third cation
containing Co and Cr cations (run nos. 14 and 18).
Analysis of the data in Table 2 shows that clinoptilolite
2+
3+
containingCo (0.5wt%)andCr (0.25wt%)isanactive
catalyst for the reaction of oxidative dehydrogenation of
methyl cyclohexane into cyclohexadiene-1,3.
2+
(
Zn ) into these catalytic systems leads to an increase
in the yield of cyclohexadiene-1,3. In this case, the yield
of benzene substantially decreases (run nos. 12 and 13).
It is known that the activity of zeolite catalysts in
different reactions depends on the zeolite structure, nature
of cations, preparation method, and distribution of cations
over the zeolite surface. It has been found previously [4]
that the optimal structure for preparing catalysts for the
oxidative dehydrogenation of naphthenic hydrocarbons is
that of clinoptilolite whose crystal lattice contains three
open channels:A, B, and C. Figure 1 shows a polyhedral
model that reflects a fragment of the clinoptilolite
structure with exchange cations.
The comparatively high yield of cyclohexadiene-1,3 is
2
+
3+
2+
reached in catalytic systems containing Cu , Cr , Co ,
and Zn2 cations (run nos. 19 and 20). It follows from
the data in Table 1 that natural clinoptilolite containing
+
+
2+
2+
3+
(
wt %): Cu2 0.5, Zn 0.2, Co 0.1, and Cr 0.1 (run
no. 19) is an active catalyst for the reaction of oxidative
dehydrogenation of cyclohexane into cyclohexadiene-1,3.
Natural clinoptilolite modified with Fe, Mn, Sn, Mo,
Ni exhibits a comparatively high catalytic activity in the
reaction of oxidative dehydrogenation of cyclohexane
The first two parallel axes c are constituted by ten-
and eight-membered rings. These are intersected by an
eight-membered channel parallel to axis a. The hexagonal
planes in the structure of clinoptilolite are surrounded
by channels A, B, and C in which exchange cations
are localized. The planar molecule of cyclohexane,
which is a six-membered ring, is firmly adsorbed on the
hexagonal planes in the structure of clinoptilolite, which
are surrounded by exchange cations.
(run nos. 9–18) into cyclohexadiene-1,3.
Table 2 lists the results of experimental studies aimed to
select an active catalyst for the reaction of dehydrogenation
of methyl cyclohexane into cyclohexadiene-1,3. It can be
seen that natural clinoptilolite modified with Zn, Cu,
Cr, Co, Fe, and Ni cations exhibits a comparatively low
activity in the reaction of dehydrogenation of methyl
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ALIEV et al.
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SYNTHESIS AND STUDY OF ZEOLITES
729
adsorption on clean surfaces of polycrystalline samples
–
1
of transition metals [9] (kJ mol ): q (Cu) 478, q (Zn)
ads
ads
2
40, q (Co) 418, and q (Cr) 753.
ads ads
Using these data, we can calculate the bonding
energies of the catalyst components with oxygen by
–
1
formula (1) (kJ g at ): q (Cu) 489, q (Zn) 370, q (Co)
0
0
0
4
59, and q (Cr) 612. The force with which hydrogen
0
atoms of cyclohexane are drawn off varies with the
strength of these bonds. Oxygen atoms bound to the Cr3
component of the catalyst show draw off hydrogen atoms
in cyclohexane with the strongest force.
+
Fig. 2. General scheme of the oxidative dehydrogenation of
cyclohexane on modified clinoptilolite. Cli–CuCrCoZn [i is an
exchange cation, i = 1–4; (1) Cr , (2) Cu , (3) Zn , (4) Co ]
Based on the experimental data presented in Table 1
and taking into account the bonding energies of the catalyst
components with atomic oxygen, we can group active
centers in the components of the catalyst for the reaction
of oxidative dehydrogenation of cyclohexane, which are
responsible for the formation of cyclohexadiene-1,3,
cyclohexene, and benzene. The general scheme of the
oxidative dehydrogenation cyclohexane on these active
centers is shown in Fig. 2.
3+
2+
2+
2+
on the active center.
Clinoptilolite has four kinds of places for localization
of exchange cations: M in channel A, M in channel B,
1
2
M in channel C situated along axis a near the center of
3
the six-membered ring, M is the site in channel A near
4
the inversion center. Their number is small; M is situated
3
near M [5].
1
For the reaction of oxidative dehydrogenation of
cyclohexane into cyclohexadiene-1,3, there are two types
of active centers: {M (1), M (2), M (3)} and {M (1),
The part played by components of a catalytic system
in the reaction of dehydrogenation of cyclohexane
can be understood by analyzing the environment of
adsorbed cyclohexane molecules by exchange cations
with dissociatively adsorbed oxygen, with consideration
for their bonding energies, which can be found by the
formula [6]
1
2
3
1
3+
2+
M (4), M (3)}. Because the binding of the Cr , Cu ,
2
3
and Co2+ cations with atomic oxygen are strong, they
draw off hydrogen atoms to a greater extent, with the
resulting cyclohexadiene-1,3 molecules stabilized by
zinc oxide, which has a relatively lower Zn=O bonding
energy. The formation of cyclohexadiene-1,3 is shown
schematically in Fig. 3.
q0 = 1/2(qads + 500),
(1)
where 500 kJ mol–1 is the energy of dissociation of
molecular oxygen into atoms and qads is the heat of oxygen
Analysis of the experimental data and bonding
energies of the catalyst components with atomic oxygen
3+
2+
2+
Fig. 3. Scheme of formation of cyclohexadiene-1,3 on modified clinoptilolite. Cli–CuCrCoZn [(1) Cr , (2) Cu , (3) Zn ] on the active
center.
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ALIEV et al.
–
1
Table 2. Oxidative dehydrogenation of methyl cyclohexane on modified natural clinoptilolite (T = 380°C, V = 2000 h ,
o
C H CH : O : N = 1 : 1 : 5.3)
6
11
3
2
2
Yield of indicated reaction products,
wt %
Methyl
cyclohexane
conversion,
wt %
Selectivity with
respect to methyl
cyclohexadiene,
wt %
Run
no.
Cations in the zeolite
C6H9CH3
C6H7CH3
C6H5CH3
CO2
1
2
3
4
5
6
7
8
9
Zn (0.2)a
7.6
28.9
21.4
19.4
21.4
50.2
34.6
32.5
28.6
18.8
32.7
37.4
41.1
49.8
53.9
45.9
49.4
37.4
7.9
1.03
21.02
19.6
–
0.9
3.8
5.9
6.2
1.5
–
0.6
0.3
4.5
3.8
–
5.2
8.5
0.9
16.3
1.5
Cu (0.5)
Cr (0.1)
9.5
Со (0.1)
7.4
2.0
Fe (0.25)
6.7
13.2
9.9
Ni (0.1)
–
40.3
10.3
15.7
19.8
14.5
13.9
12.3
13.8
15.6
35.7
10.9
21.5
9.9
CuFe (0.5 : 0.25)
CuСо (0.5 : 0.1)
CuCr (0.5 : 0.1)
ZnCr (0.2 : 0.1)
CoCr (0.1 : 0.1)
CoCr (0.5 : 0.1)
CoCr (1.0 : 0.1)
CoCr (0.5 : 0.25)
CoCr (0.5 : 0.5)
ZnCoCr (0.2 : 0.5 : 0.25)
ZnCoCr (0.2 : 0.5 : 0.5)
CuZnCoCr 0.5 : 0.2 : 0.1 : 0.1)
6.4
10.8
7.7
3.2
5.8
3.9
1.9
3.6
5.8
5.2
7.2
1.9
12.5
13.5
5.3
2.2
3.5
2.2
0.8
9.9
10.8
9.6
14.4
6.5
10.7
6.9
8.5
18.9
7.5
2.7
1
0
4.3
1.6
11
30.3
28.9
23.4
28.9
12.1
23.3
13.9
22.7
5.3
1
1
1
1
1
1
1
2
3
4
5
6
7
8
8.5
12.5
12.6
9.8
11.8
7.5
13.7
a
The figures in parentheses show the content of exchange cations in weight percent.
suggest that there are three kinds of active centers for the
reaction of oxidative dehydrogenation of cyclohexane into
benzene: {M (1), M (1), M (1)}, {M (2), M (2), M (2)}
cyclohexadiene-1,3, the following order of introduction
of cations into clinoptilolite is the optimal
1
2
3
1
2
3
Сu2+ (0.5 wt %) → Zn2+ (0.2 wt %)
and {M (4), M (4), M (4)}, and also three types of active
1
2
3
Co2+ (0.2 wt %) → Cr3+ (0.2 wt %) → Cli.
→
centers for the reaction of oxidative dehydrogenation of
cyclohexane into cyclohexane: {M (1), M (3), M (3)},
The experimental data obtained in a study of the
kinetic aspects of the reactions under consideration on
active catalytic system indicate that these reactions do
not occur by the successive mechanism. Figures 4 and
5 show how the conditional contact duration affects the
course of the oxidative dehydrogenation reactions: cy-
clohexane on the CuCrCoZn–clinoptilolite catalyst and
methyl cyclohexane on the CoCr–clinoptilolite catalyst,
respectively.
1
2
3
{
M (2), M (3), M (3)} и {M (4), M (3), M (3)}.
1 2 3 1 2 3
It should be noted that the predominant formation of
a product in the reaction of oxidative dehydrogenation
of cyclohexane depends on the number of active
centers for the corresponding reactions, which, in turn,
depends on the concentrations cations and the order of
their introduction by ion exchange. In order to obtain
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SYNTHESIS AND STUDY OF ZEOLITES
731
X, A, %
X, A, %
τ, h
τ, h
Fig. 5. (1) Conversion X of methyl cyclohexane and the yield
Fig. 4. (1) Conversion X of cyclohexane and the yield A of the
reaction products (2) cyclohexane, (3) cyclohexadiene, and
A of the reaction products (2) methyl cyclohexane, (3) methyl
cyclohexadiene, and (4) toluene vs. the conditional contact
duration τ at C H : O : N molar ratio of 1 : 1 : 5.3 and T =
(
4) benzene vs. the conditional contact duration τ at C H
:
6
12
6
12
2
2
O : N molar ratio of 1 : 1 : 5.3 and T = 380°C.
380°C.
2
2
If we assume that these reactions occur by the
successive mechanism to give, respectively, benzene
and toluene, then cyclohexane and cyclohexadiene-1,3
are intermediate products for the reaction of oxidative
dehydrogenation of cyclohexane, and methyl cyclohexene
and cyclohexadiene-1,3, for the reaction of oxidative
dehydrogenation of methyl cyclohexane.
0.2, Co2+ 0.1, and Cr3+ 0.1 is the active catalyst for the
reaction of oxidative dehydrogenation of cyclohexane
into cyclohexadiene-1,3.
(2) The part played by the components of the catalytic
system in the reaction of oxidative dehydrogenation of
naphthenic hydrocarbons was determined. With the ex-
perimental data and the binding energies of the catalyst
components with atomic oxygen taken into account, the
active centers in the components of the catalyst for the
reaction of oxidative dehydrogenation of cyclohexane,
responsible for the formation of cyclohexadiene-1,3,
cyclohexene, and benzene can be divided into groups.
It was found that there are two kinds of active centers
It can be seen in Figs. 4 and 5 that the runs of the curves
describing the yield of intermediate and final products as
a function of the conditional contact duration disagree
with the successive reaction mechanism. Thus, it can be
concluded on the basis of the experimental data obtained
in the study that the surface of the catalysts has different
active centers constituted by the components responsible
for the formation of the reaction products, in agreement
with the above active centers.
3
+
2+
2+
2+
on the catalyst surface: {Cr , Cu , Zn } and {Co ,
Cu , Zn }.
2
+
2+
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