Kinetics and mechanism of oxidative dehydrodisproportionation of methylcyclohexane on modified high-silica zeolite H-TsVM

Article abstract of DOI:10.1023/A:1020948624816

The kinetics of oxidative dehydrodisproportionation of methylcyclohexane in the presence of H-TsVM zeolite (SiO₂:Al₂O₃ = 30) modified with gallium and gadolinium oxides was studied. A kinetic model fairly adequately describing this process in a wide temperature range was proposed. The kinetic constants of some oxidation reactions were determined.

Full text of DOI:10.1023/A:1020948624816

Russian Journal of Applied Chemistry, Vol. 75, No. 8, 2002, pp. 1256 1261. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 8, 2002,
pp. 1282 1287.
Original Russian Text Copyright
2002 by Alimardanov, Velieva, Tairova.
CATALYSIS
Kinetics and Mechanism of Oxidative Dehydrodisproportionation
of Methylcyclohexane on Modified High-Silica Zeolite H-TsVM
Kh. M. Alimardanov, F. M. Velieva, and L. A. Tairova
Institute of Petrochemical Processes, National Academy of Sciences of Azerbaijan, Baku, Azerbaijan
Received December 17, 2001; in final form, April 2002
Abstract The kinetics of oxidative dehydrodisproportionation of methylcyclohexane in the presence of
H-TsVM zeolite (SiO2 : Al2O3 = 30) modified with gallium and gadolinium oxides was studied. A kinetic
model fairly adequately describing this process in a wide temperature range was proposed. The kinetic
constants of some oxidation reactions were determined.
1
In contrast to dehydrogenation, proceeding without
changes in the number of carbon atoms in the mol-
ecule, oxidative dehydrodisproportionation of alicyclic
MCH space velocity of 108 432 h ; the initial partial
pressures of MCH and oxygen were 11.8 33.2 and
12.3 17.5 kPa, respectively. The gas mixture was
hydrocarbons is a poorly studied catalytic process diluted with nitrogen or water vapor. The size of
involving heterogeneously bound oxygen atoms [1 4].
This reaction allows polyalkylbenzenes and naphthenic
hydrocarbons, whose content in straight-run naphtha
is fairly high, to be involved in basic organic syn-
thesis along with low-molecular-weight aromatic hy-
drocarbons [5 9].
catalysis grains was varied within 0.5 0.8 mm; the
plunge pulsation frequency in the reactor was 25
and 50 Hz. Such experimental conditions eliminate
the effect of the outer- and pore-diffusion hindrance
on the reaction rate.
The liquid and gaseous reaction products were anal-
yzed on Tsvet-31 and KhL-3 chromatographic units
using bis(-2-cyanoethyl) sulfide (12 wt %) on Chromo-
sorb W for liquid products or triethylene glycol
butyrate (TEGB, 15 wt %) on INZ 600 brick for
gaseous products.
Single-stage oxidative dehydrodisproportionation
of methyl- and ethylcyclohexane in polyalkylbenzenes
is thermodynamically feasible and yields a limited
number of products [10, 11]. The kinetic data on these
reactions are important for mathematical simulation
and optimization of the process.
The reaction products mainly consisted of xylene
isomers, toluene, benzene, cyclohexane, CO2, water
vapor, and minor amounts of C2 C5 aliphatic hy-
drocarbons. In some runs, isomers of tri- and tetra-
methylbenzene, methylcyclohexene, cyclopentadiene,
CO, and hydrogen were identified. However, due to
the low concentration of these compounds in the cat-
alyzate, their correct quantitative analysis was impos-
sible and thus they were disregarded in the kinetic
simulation of the process. In the catalyzate residue
(after distillation of liquid reaction products and con-
densed water; 0.5 0.8% of the total product weight),
we identified by GLC and IR spectroscopy oxygen-
containing compounds, such as saturated and unsat-
urated ketones, aldehydes, and acids [12].
In this work, we studied the kinetics of oxida-
tive dehydrodisproportionation of methylcyclohexane
(MCH) to form xylene isomers in the presence of
H-TsVM zeolite modified with gallium and gadolini-
um oxides.
EXPERIMENTAL
The initial MCH was prepared by hydrogenation
of chemically pure toluene; according to gas liquid
chromatography (GLC), its purity was 99.3 99.5%,
bp 100 101 C, n2D0 = 1.4244, d240 = 0.7720.
To prepare the required catalyst, the H-TsVM ze-
olite (SiO2 : Al2O3 = 30) was modified with gallium
and gadolinium oxides [10]; the content of metal ox-
ides was 3 wt %.
The main experimental results obtained with a cat-
alyst of steady-state activity are listed in Table 1.
As seen from these data, under the above conditions,
MCH is predominantly involved in oxidative dehy-
The experiments were performed in a laboratory
unit with a nongradient reactor at 773 813 K and
1070-4272/02/7508 1256 $27.00 2002 MAIK Nauka/Interperiodica

KINETICS AND MECHANISM OF OXIDATIVE DEHYDRODISPROPORTIONATION
1257
Table 1. Accumulation rate of reaction products as influenced by the initial partial pressures of MCH and oxygen
i
and by the space velocities v of MCH vapor (Ga Gd O-modified H-TsVM catalyst, pH O = 0.1 kPa)
2
1
T0 , kPa
MCH
i, mol lca1t h
O2
C2 C5
MCH
benzene
toluene
xylene isomers
CO2
1
v = 108 h , T = 793 K
33.2
25.5
20.67
15.0
13.2
11.8
12.3
14.15
15.3
16.67
17.1
17.5
0.1976
0.2118
0.2311
0.2649
0.2840
0.2986
2.4177
0.2024
0.2166
0.2456
0.2746
0.3033
0.3179
1.3685
1.4006
1.4541
1.6138
1.6561
1.6711
0.5156
0.5343
0.5441
0.5973
0.6355
0.6501
0.1349
0.1396
0.1493
0.1831
0.2022
0.2119
2.5066
2.6253
2.9333
3.0836
3.1505
1
v = 144 h , T = 793 K
33.2
25.5
20.67
15.0
13.2
11.8
12.3
14.15
15.3
16.67
17.1
17.5
0.2441
0.2502
0.2825
0.3115
0.3530
0.3467
2.9977
0.2569
0.2695
0.2954
0.3532
0.3659
0.3724
1.6891
1.7194
1.7399
1.7726
1.7907
1.7977
0.6358
0.6737
0.6870
0.7321
0.7374
0.7705
0.1734
0.1925
0.2055
0.2376
0.2503
0.2568
3.1103
3.2247
3.4094
3.5207
3.5452
1
v = 216 h , T = 793 K
33.2
25.5
20.67
15.0
13.2
11.8
12.3
14.15
15.3
16.67
17.1
17.5
0.3083
0.3369
0.3467
0.3950
0.4333
0.4527
3.3695
0.2986
0.3081
0.3371
0.3854
0.4140
4.4334
1.8876
1.8483
1.8874
1.9365
1.9738
2.0034
0.6551
0.6738
0.7225
0.7997
0.8473
0.8765
0.2216
0.2503
0.2696
0.3180
0.3467
0.3564
3.4249
3.5654
3.8332
4.0201
4.3167
1
v = 432 h , T = 793 K
33.2
25.5
20.67
15.0
13.2
11.8
12.3
14.15
15.3
16.67
17.1
17.5
0.5202
0.5583
0.6355
0.7130
0.7510
0.7705
5.7595
0.5587
0.5776
0.6163
0.6552
0.6740
0.6742
3.3525
3.3115
3.2356
3.1793
3.1196
3.0822
1.0019
1.0589
1.1749
1.2717
1.3287
1.3485
0.3275
0.3273
0.3659
0.4239
0.4621
0.4817
5.8486
6.0329
6.2404
6.3456
6.3602
1
v = 432 h , T = 773 K
33.2
25.5
20.67
15.0
13.2
11.8
12.3
14.15
15.3
16.67
17.1
17.5
0.3854
0.4043
0.4621
0.5780
0.6355
0.6742
4.3314
2.4469
2.5414
2.5808
2.3122
2.2723
2.1576
0.4239
0.4428
0.5462
0.5781
0.7124
0.7705
0.7707
0.8278
0.9053
1.0598
1.1362
1.1944
0.3083
0.3273
0.4045
0.4818
0.5199
0.5201
4.5585
4.8387
5.0074
5.2865
5.3199
1
v = 432 h , T = 813 K
33.2
25.5
20.67
15.0
13.2
11.8
12.3
14.15
15.3
16.67
17.1
17.5
0.5780
0.6546
0.7126
0.7900
0.8280
0.8476
9.5712
9.7763
9.8272
10.0748
10.0854
10.2323
1.0983
1.2129
1.2905
1.4066
1.4443
1.4640
4.7398
4.6207
4.4683
4.3740
4.2749
4.2381
2.6974
2.7724
2.8312
2.9481
3.0618
3.1015
0.4624
0.5006
0.5971
0.5588
0.5584
0.5779
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 8 2002

1258
ALIMARDANOV et al.
Reaction (1) describes oxidative dehydrodispropor-
tionation of MCH into benzene and xylene isomers;
reaction (2) is dehydrogenation of MCH into toluene;
reactions (3), (5), and (6) correspond to oxidative de-
hydrodemethylation of the initial hydrocarbon and re-
sulting alkylaromatic compounds, and reaction (4) de-
scribes destructive oxidation of MCH. These parallel-
consecutive transformations of the initial hydrocarbon
and products of its primary conversion proceed at dif-
ferent active centers of the catalyst [13]. The effect of
the surface heterogeneity of oxide-modified zeolite
catalysts was studied previously in [14].
The above model does not include the reactions of
deep oxidation of MCH and coke formation, be-
cause our calculations showed that the contribution of
these processes is negligibly small.
Fig. 1. Selectivity of the reaction with respect to xylene
isomers vs. MCH conversion . Temperature, C: (1) 753,
(2) 773, (3) 803, (4) 823, and (5) 873.
To determine the formal reaction orders with re-
spect to MCH and oxygen, we plotted the rate of ox-
idative dehydrodisproportionation of MCH against the
partial pressures of hydrocarbon ( pO = 12.3 kPa)
and oxygen (pMCH = 33.2 kPa). The r2esults obtained
show that the formal reaction orders with respect to
MCH and O2 are about 2 and 1, respectively. Hence,
the rate of the main reaction (1) can be given as fol-
lows:
Fig. 2. Rate of oxidative dehydrodisproportionation of MCH
vs. the oxygen partial pressure p . p
= 33.2 kPa,
O2
MCH
rC H = Kp2C H pO
,
(7)
1
v
= 432 h . (
) Rate of accumulation of xylene
8
10
7
14
2
MCH
C8H10
isomers. Temperature, C: (1) 773, (2) 793, (3) 813,
and (4) 843.
where rC H is the rate of reaction (1) and pC H
14
and pO a8re10the partial pressures of MCH and O2,7re-
spectiv2ely.
drodisproportionation, dehydrogenation, dehydrode-
methylation, and destructive oxidation. The oxidative
dehydrodisproportionation of MCH leads to accumula-
tion of benzene and xylene isomers in the catalyzate;
benzene is also formed in oxidative dehydrodemeth-
ylation. The conversion curves (dependences of
the process selectivity on MCH conversion) at various
temperatures (Fig. 1) show that, at MCH conversion
close to zero, the selectivity of the process with re-
spect to xylene isomers is lower that 100%, which
suggests a parallel-consecutive reaction pattern.
With account taken of the stage scheme, in the
steady state, the sum of the active centers with dif-
ferent oxidation states is 1:
(8)
Z + ZO + ZO2 = 1,
where ZO and ZO2 are represented, in accordance
with the stage scheme (Table 2), the by equations
(9)
ZO2 = K [Z] pO ,
2
Based on the experimental data, we included in our
model the following series of independent reaction
pathways:
ZO = K [ZO2]0.5[Z]0.5 = K K [Z] pO0.5
,
(10)
2
where K is the constant of dissociative adsorption
of O2 and K is the equilibrium constant of O2 ad-
sorption.
(1)
(2)
(3)
(4)
(5)
(6)
2C7H14 + 3O2
C6H6 + C8H10 + 6H2O,
C7H8 + 3H2O,
C7H14 + 1.5O2
According to the kinetic data (Figs. 2, 3), at partial
pressures of oxygen and water vapor of 2.0 17.5
and 0.1 10 kPa, respectively, the rate of formation of
xylene isomers decreases with increasing concentra-
tion of both oxygen and water vapor, and this de-
pendence is rather complex. At low partial pressures
of oxygen and water vapor, the rate of formation of
C7H14 + 3O2
C6H6 + CO2 + 4H2O,
C4H8 + C2H4 + CO2 + H2O,
C7H14 + 1.5O2
C7H8 + 1.5O2
C8H10 + 1.5O2
C6H6 + CO2 + H2O,
C7H8 + CO2 + H2O.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 8 2002

KINETICS AND MECHANISM OF OXIDATIVE DEHYDRODISPROPORTIONATION
1259
Table 2. Stage scheme of oxidative dehydrodisproportionation of MCH
Stoichiometric number of stages in indicated pathways
Stage
1
2
3
4
5
6
C7H14 + ZO
C7H13ZOH + ZO
C7H13ZOH
C7H11ZOH + Z + H2O
2
2
2
1
0
0
0
0
0
0
0
0
0
0
3
3
1
0
0
0
1
1
1
0
1
0
0
0
0
0
1.5
1.5
1
0
0
0
1
1
1
0
1
1
1
0
0
0
2
2
0
0
0
0
0
0
0
0
0
0
0
1
1
1
0.5
0.5
0
0
0
0
0
0
0
0
0
1
1
0
0
0
0.5
0.5
0
0
0
0
0
0
0
1
1
0
0
0
0
0
0.5
0.5
C7H ZOH + ZO
C7H ZOH + Z + H2O
11
9
2C7H9ZOH + ZO
C7H13ZOH + ZO
C7H12ZOH + ZO
C6H6 + C8H10 + 2Z + 2H2O
C7H11ZOH + Z + H2O
C7H ZOH + Z + H2O
10
C7H Z + ZO
C7H8Z + Z + H2O
10
C8H + ZO2 + ZO
C7H8Z + CO2 + H2O + Z
10
C7H Z
C7H8 + Z
8
C7H8 + ZO2 + ZO
C6H6Z + CO2 + H2O + Z
C6H6Z
C6H6 + Z
C7H14 + ZO2 + ZO
C4H8Z + C2H4Z + CO2 + H2O
C4H8Z
C2H4Z
Z + O2
Z + ZO2
C4H8 + Z
C2H4 + Z
ZO2
2ZO
xylene isomers increases. However, with increasing
volume fraction of oxygen and water vapor, the rate
of xylene formation sharply decreases. Thus, stoi-
chiometric amounts of oxygen and water vapor are
probably effective agents restoring the active centers
which provide dehydrogenation of the initial hydrocar-
bon and disproportionation of the intermediate carb-
anions. Taking into account the above considerations
and substituting Eqs. (8) (10) in Eq. (7), we obtain
the following kinetic equation:
[k1[ pC H ] + k3][ pC H ][ p0O.5
]
2
7
14
7
14
(14)
r3
=
,
[1 + K p0O.5 + K pO + K pH O
]
2
2
2
k4 [ pC H ] [ pO ]
2
7
14
r4
=
,
(15)
1 + K [ pO0.5] + K [ pO ] + K [ pH O
]
2
2
2
where k1 k6 are the rate constants of the correspond-
ing reactions and r1 r4 are the rates of hydrocarbon
accumulation (r1 = r
, r2 = r
, r3 = r
, and
2
C8H10
C7H8
C6H6
k1 p
p
C7H14 O2
r4 = rC H ) in stoichiometric reactions (1) (4).
rC H
=
10
,
(11)
4
8
8
1 + K0.5pO0.5 + K pO + K pH O
2
2
2
Based on the data of additional experiments and
analysis of the reaction products, the rates of oxidative
demethylation of xylene isomers into toluene and
of toluene into benzene were assumed to be zero.
where rC H is the rate of reaction (1), k1 is the rate
8
constant of10reaction (1) in the steady state, and K is
the equilibrium constant of H2O adsorption.
The kinetic constants were determined by least-
squares method, using the random search procedure
and taking into account the main and side reactions of
hydrocarbon conversion [schemes (1) (6)]. Our ex-
perimental values are fairly adequately described by
the following kinetic equations.
k1 [ pC H ]2[ pO
]
2
7
14
r1
=
,
(12)
(13)
1 + K [ pO
]
0.5 + K [ pO ] + K [ pH O
]
,
2
2
2
[k2 pC H + k6 pC H ][ pO ]
2
Fig. 3. Rates
tion of (2) toluene and (3) xylene isomers vs. the water
vapor partial pressure p . p = 33.2 kPa, T = 813 K.
of (1) MCH conversion and of accumula-
7
14
8
10
r2
=
1 + K [ pO0.5] + K [ pO ] + K [ pH O
]
2
2
2
H2O
MCH
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 8 2002

1260
ALIMARDANOV et al.
The rates of conversion of the initial hydrocarbon and
accumulation of the reaction products by the above
pathways were calculated using the following expres-
sions:
vapor indicate that the limiting stage is the reaction of
two intermediate radicals, followed by their dispro-
portionation, whereas adsorption of MCH and elim-
ination of the hydrogen atom with nucleophilic sur-
face oxygen are fast. In this stage, electron passes
from the hydrocarbon to the catalyst. The carbocations
formed are converted in two competitive reactions to
form toluene (dehydrogenation) and xylene isomers
(dehydrodisproportionation). The first reaction order
with respect to MCH in stage (2) suggests that the de-
hydrogenation rate is rather high.
=
2r1
r2
r3
r6,
= r1 + r3 + r5,
r4,
(16)
(17)
C7H14
= r1
C8H10
(18)
C6H6
C7H8
= r2
r5 + r6,
= 2r4,
(19)
The destructive oxidation of MCH can proceed by
two independent pathways: (1) by heterogeneous
homogeneous conversion of hydrocarbons to form
cycloalkyl radicals and carbocations (after electron
transfer from the radical to the catalyst) on the catalyst
surface, their subsequent passing into the gas phase,
and oxidation with molecular oxygen, and (2) hetero-
geneous conversion of MCH at the active centers of
the catalyst providing oxidative dehydrodispropor-
tionation (strong acid base centers). In this case,
the destructive oxidation on the surface proceeds by
the separate interaction mechanism through dissocia-
tive adsorption of hydrocarbons, accompanied by
the formation of strongly bound hydroxy group from
the oxygen ion with localized negative charge, sta-
bilization of hydrocarbon radical on the metal cation
linked with oxygen, and reactions of the oxygen mol-
ecule from the gas phase with a surface radical. Both
mechanisms of oxidative degradation can operate
simultaneously.
(20)
C2 C4
= r3 + r4 + r5 + r6,
(21)
CO2
where
,
,
,
,
, and
C2 C4
C7H14
C8H10
C6H6
C7H8
are the experimental reaction rates given in
CO2
Table 1.
The best agreement between the rates of oxidative
dehydrodisproportionation of MCH and experimental
values was obtained with the following rate constants
for reactions (1) (6):
k1 = 5.94 103 exp( 97453.146/RT ),
k2 = 3.4572 104 exp( 62211.432/RT ),
k3 = 4.275 103 exp( 70818.8/RT ),
k4 = 7.18 108 exp( 158352.43/RT ),
k5 = k6 = 0,
In the analysis of the reaction mechanism, we used
the stage scheme given in Table 2, involving a num-
ber of stoichiometric reactions whose sequence deter-
mines the rates of realization of independent runs.
K
= 5.21 103 exp( 164450/RT ),
= 9.04 10 11 exp(135523.98/RT ),
= 4.21 10 7 exp(119199.546/RT )
According to the stage scheme (Table 2), the stage
of reaction of oxygen from the gas phase with the
K
K
reduced center of the catalyst (Z + O2
assumed to be irreversible, and the stage of dissocia-
tive adsorption of oxygen (Z + ZO2 2ZO), revers-
ZO2) is
1
(R is the universal gas constant, 8.3144 J mol 1 K ).
The root-mean-square deviations of the calculated
rates [Eqs. (6) (21)] from the experiment were 14.5%
for main reaction (1), 18 22% for reactions (2) (4),
and 21 25% for reactions (5) and (6). Zero values of
k5 and k6 suggest that toluene and benzene are pre-
dominantly formed by reactions (1) (3).
ible. Hence, the rate of oxidative dehydrodispropor-
tionation strongly depends on the partial pressure of
MCH (rC H
p2MHC) and is inhibited by oxygen at
7
its content 14exceeding the stoichiometric amount.
Our kinetic model of oxidative dehydrodispropor-
tionation of MCH fairly adequately describes the sys-
tem of chemical transformations proceeding in the re-
action scheme in a wide range of experimental param-
eters and can be used for mathematical simulation of
the process.
The second order of the reaction with respect to
MCH and the maximum in the dependence of the dis-
proportionation rate on the partial pressure of water
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 8 2002

KINETICS AND MECHANISM OF OXIDATIVE DEHYDRODISPROPORTIONATION
1261
CONCLUSIONS
Dokl. Akad. Nauk Az.SSR, 1986, vol. 42, no. 10,
pp. 46 49.
(1) The kinetic features of oxidative dehydrodis-
proportionation of methylcyclohexane in benzene and
xylene isomers in the presence of H-TsVM zeolite
modified with gallium and gadolinium oxides were
studied.
4. Tagiev, D.B. and Dadashev, I.B., Neftekhimiya, 1990,
vol. 30, no. 6, pp. 756 763.
5. Isakov, Ya.I., Isakova, T.A., Minachev, Kh.M., and
Lipkind, B.A., Neftekhimiya, 1986, vol. 26, no. 3,
pp. 335 342.
6. Isakov, Ya.I., Neftekhimiya, 1998, vol. 38, no. 6,
(2) At 773 813 K, methylcyclohexane is involved
in oxidative dehydrogenation, dehydrodisproportiona-
tion, and dehydrodemethylation. The kinetic model of
the process, fairly adequately describing the exper-
imental results in a wide temperature range, was pro-
posed. The rate constants for some reaction pathways
were calculated.
pp. 404 438.
7. Yushchenko, V.V., Wang Syaoyui, and Romanov-
skii, B.V., Neftekhimiya, 1999, vol. 39, no. 3,
pp. 197 202.
8. Ponomareva, O.A., Moskovskaya, I.F., and Romanov-
skii, B.V., Neftekhimiya, 1999, vol. 39, no. 2,
pp. 101 106.
9. Nowak, S., Keil, G., and Zimmermann, G., Hung. J.
Ind. Chem. Veszprem., 1978, vol. 6, no. 1,
pp. 105 109.
10. Alimardanov, Kh.M., Akhverdiev, R.B., Suleimano-
va, E.T., et al., Neftekhimiya, 1997, vol. 37, no. 6,
pp. 523 531.
11. Alimardanov, Kh.M., Abstracts of Papers, Konferen-
tsiya Tonkii organicheskii sintez i kataliz (Conf. on
Fine Organic Synthesis and Catalysis), Baku, 1999,
pp. 19 20.
(3) Reaction of the products of partial dehydro-
genation of methylcyclohexane (namely, methylcy-
clohexyl and methylcyclohexenyl radicals), with their
subsequent disproportionation and dehydrogenation
into benzene and xylene isomers, is the limiting stage
of the process.
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