On the possible causes of enhancement of the heterogeneous catalytic liquid-phase oxidation reaction of m-xylene by microwave radiation

Article abstract of DOI:10.1134/S0965544113020047

The contribution of the heterogeneous component of the total conversion of m-xylene to the process of its heterogeneous catalytic liquid-phase oxidation has been studied, as this contribution is most clearly manifested in the case of microwave treatment. It has been shown that microwave irradiation shortens the induction period of the reaction taken to reach a steady state. It has been suggested that the observed increase in the generation rate of free m-xylyl radicals by microwave treatment is due to the appearance at the hydrocarbons/catalyst interface of local overheating regions whose temperature can exceed the weight-average temperature in the reaction space.

Full text of DOI:10.1134/S0965544113020047

ISSN 0965ꢀ5441, Petroleum Chemistry, 2013, Vol. 53, No. 2, pp. 117–120. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © Yu.N. Litvishkov, V.F. Tret’yakov, R.M. Talyshinskii, N.V. Shakunova, S. M. Zul’fugarova, N.M. Mardanova, Yu.R. Nagdalieva, 2013, published in
Neftekhimiya, 2013, Vol. 53, No. 2, pp. 133–137.
On the Possible Causes of Enhancement of the Heterogeneous
Catalytic LiquidꢀPhase Oxidation Reaction of
by Microwave Radiation
mꢀXylene
Yu. N. Litvishkova, V. F. Tret’yakovb, R. M. Talyshinskiib, N. V. Shakunovaa, S. M. Zul’fugarovaa,
N. M. Mardanovaa, and Yu. R. Nagdalievaa
a Nagiev Institute of Chemical Problems, National Academy of Sciences of Azerbaijan, Baku, Azerbaijan
eꢀmail: yuriylit@rambler.ru
b Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, Russia
Received August 29, 2012
Abstract—The contribution of the heterogeneous component of the total conversion of mꢀxylene to the proꢀ
cess of its heterogeneous catalytic liquidꢀphase oxidation has been studied, as this contribution is most clearly
manifested in the case of microwave treatment. It has been shown that microwave irradiation shortens the
induction period of the reaction taken to reach a steady state. It has been suggested that the observed increase
in the generation rate of free
carbons/catalyst interface of local overheating regions whose temperature can exceed the weightꢀaverage
temperature in the reaction space.
mꢀxylyl radicals by microwave treatment is due to the appearance at the hydroꢀ
Keywords: heterogeneous catalysis, microwave radiation,
geneous component of total ꢀxylene conversion
mꢀxylene, oxidation, radical chain process, heteroꢀ
m
DOI: 10.1134/S0965544113020047
Coming in line with the latest new challenges of ratio of the principal to the side product formation
rates.
Previously [5, 6], we investigated the activity of
Со–Mn/А12О3/А1 framework catalysts in the liquidꢀ
phase oxidation of ꢀxylene to ꢀtoluic acid, which
follows the radical chain heterogeneous–homogeꢀ
neous mechanism. It was shown that in comparison
development of powerꢀsavings processes for heavy
organic and petrochemical syntheses is microwave
chemistry, including microwave chemistry of heteroꢀ
geneous catalysis, which deals with a variety of heterꢀ
ogeneous catalytic transformations of substances
induced by a microwave (MW) electromagnetic field
[1–4].
m
m
with the initiated oxidation of
mꢀxylene and its conꢀ
version in the presence of monolithic Co–Mn samꢀ
ples, higher values of the ratio of the heterogeneous
surface area to the volume of the liquid phase (S/V)
characteristic of supported catalysts can lead to higher
initiation and propagation rates during both the initiꢀ
ation and the reaction propagation periods.
In [7], it was also found that microwave irradiation
of the mꢀxylene–Со–Mn/А12О3/А1 reaction system
of an identical composition noticeably shortens the
reaction time to achieve the maximum yield of the
desired product mꢀtoluic acid as compared with conꢀ
In contrast to the traditional conditions of thermal
treatment of the reaction medium–catalyst system,
feedstock during a process in an MW field has a lower
temperature, since the main component of the reacꢀ
tion system that absorbs radiation and converts MW
energy into heat is the heterogeneous catalyst per se.
In this case, the specifics of microwave treatment will
be most prominently manifested in heterogeneous
catalytic liquidꢀphase processes of hydrocarbon conꢀ
version, in which local temperature above the weightꢀ
average temperature of the reaction system at the
solid/liquid interface can be achieved owing to both
the difference of dielectric losses and a relatively low
thermal conductivity of the reaction medium. This
could lead to prevalence of heterogeneous catalytic
ventional reactor heating with an electric heater.
In this study, we attempted to reveal possible causes
of the enhancement of the aforementioned reaction
assisted by microwave electromagnetic radiation.
routes of
ation of
ylbenzyl hydroperoxide, over the identical homogeꢀ
m
ꢀxylene transformation, such as the generꢀ
EXPERIMENTAL
mꢀxylyl radicals and degradation of ꢀmethꢀ
m
The experiments were carried out in the setup comꢀ
neous routes and, consequently, to a change in the posed of a reaction unit with an outer gas (N2 + O2
)
117

118
LITVISHKOV et al.
1/WOinh
min m2/mol
×
10–4,
consumed (mol),
S is the surface area of a catalyst
sample (m2), 2 is the stoichiometric coefficient of
2
VO , mL
2
inhibition, and h is the length of time (min) cut on
τ
ing
(b)
125
100
75
20
15
10
5
inh
the abscissa by extrapolating the curve in the 1/
WO
–
2
τ
coordinates (Fig. 1).
VO , mL
As the origin of a new time coordinate, the inhibiꢀ
2
tor introduction time (indicated by the dashed arrow)
was taken. The consumption of the inhibitor was monꢀ
itored by following the buildup of its colored oxidation
300
225
150
75
(a)
product
N,N
'ꢀdiꢀ ꢀnaphthylꢀpꢀquinonediimine,
β
50
whose concentration in the catalyzate samples was
determined on an SFꢀ4A spectrophotometer by meaꢀ
suring absorption at a wavelength of λmax = 480 nm
10 20 30 40 50 60
, min
with the molar absorption coefficient of
ε
= 1.1 ×
τ
104 L/(mol cm).
15 30 45 60 75 90 105 120 135
, min
τ
RESULTS AND DISCUSSION
Fig. 1. The kinetics of oxygen uptake (a) in the absence of
–4
It was found that the introduction of the inhibitor
at the onset of the reaction does not result in complete
inhibition of oxygen absorption; however, this is not
due to the ineffectiveness of the inhibitor, rather, it is
associated with the presence of uninhibited heterogeꢀ
inhibitor, (b) with introduced (5.5
ꢀnaphthylꢀ ꢀphenylenediamine (marked by arrow), and
the reciprocal of the oxygen uptake rate in the presence of
the Со Mn/Аl О /Аl catalyst. Magnetron power 600 W.
Weightꢀaverage temperatures, 413 K; the ratio
× 10 mol/L) N,N'ꢀdiꢀ
β
n
⎯
2
3
S/V = 3 ×
6
–1
10
m .
neous catalytic routes of mꢀxylene transformation.
Assuming that the free radical generation process
under the conditions of underdeveloped propagation
of the chain reaction is controlled by the step of heterꢀ
ogeneous catalytic initiation, we can estimate its rate
as the difference of the oxygen uptake rate in the presꢀ
circulation loop and constructed on the basis of a
Panasonic EMꢀG5593V microwave oven having a cavꢀ
ity volume of 23 L, a variable radiation generator
power of 200–800 W, and an operating frequency of
2450 MHz. The temperature in the reaction zone was
monitored using a VA6520 remote noncontact infraꢀ
red pyrometer with a measurement range of –50 to
600оС. Excess heat was withdrawn from the reaction
zone with an effluent stream of the oxygen–nitrogen
mixture. To avoid unexpected overheating of the reacꢀ
tor, a ballast container with circulating distilled water
was mounted in the oven cavity.
ence of the inhibitor ( inh) and the chain inhibition
WO
2
rate (Winh):
inh
=
–
Winh
.
(2)
WR⋅
WO
2
Figures 2 and 3 show the dependence of the initial
rate of heterogeneous catalytic initiation for the chain
oxidation of
mꢀxylene at its conversion of no more
Most components of the liquid phase of the cataꢀ
lyzates were identified by GLC, via matching the
retention volumes of the products with those found for
the relevant commercial chemicals [5]. The amount of
than 8–10% when the effect of change in the hydroꢀ
carbon concentration on the kinetics of the radical
chain process can be ignored.
It is seen that for fixed values of the steadyꢀstate
weightꢀaverage temperature in the reactor, identical
amounts of the catalyst, and the same oxygen concenꢀ
trations, the initial rate of heterogeneous catalytic
m
ꢀmethylbenzyl hydroperoxide in the products was
determined by iodometric titration of the samples
according to the procedure described in [8].
The rate of generation of free radicals, both under
microwave irradiation and in the case of conventional
heating, was measured as the oxygen uptake rate
generation of free radicals ( ) under microwave irraꢀ
WR⋅
diation is greater than that under the conventional
heating conditions. In this case, as the reaction temꢀ
perature achieved by increasing the magnetron power
rises, the difference in the generation rates of free radꢀ
icals becomes more apparent (Fig. 4).
(
inh) in the presence of
WO
N,
N
'ꢀdiꢀ ꢀnaphthylꢀnꢀpheꢀ
β
2
nylenediamine (5.5–6.0 × 10–4 mol/L) as the inhibiꢀ
tor introduced into the system during the reaction and
was calculated by the formula:
The observed proportionality of
to the oxygen
WR⋅
2Δ InH
[
]
concentration and the amount of catalyst introduced
can be explained in terms of the following scheme of
(1)
Winh
=
,
S ⋅ τinh
the formation of free radicals via the interaction of
xylene with the active sites on the heterogeneous cataꢀ
H] is the amount of the inhibitor lyst surface (phase 2 is expected to be slow):
mꢀ
where Winh is the inhibitor consumption rate
(mol/(m2 min)),
[In
Δ
PETROLEUM CHEMISTRY Vol. 53
No. 2
2013

ON THE POSSIBLE CAUSES OF ENHANCEMENT
119
(
WOinh
–
Winh
)
×
10–3, mol/m2 min
(
WOinh
–
2
Winh
)
10–3, mol/m2 min
×
2
8.0
6.0
4.0
2.0
4
8.0
6.0
4.0
2.0
3
4
3
1
2
2
1
0
2.5
5.0
7.5
10–4, mol/L
СО
×
0
2.5
5.0
7.5
10–4, mol/L
2
СО
×
2
10
20
30
40
10
20
30
40
S
/
V
105, (m–1)
×
S/
V
105, (m–1)
×
Fig. 3. Dependence of the initial rate of heterogeneous
Fig. 2. Dependence of the initial rate of heterogeneous
catalytic chain initiation on the surface of the
catalytic chain initiation on the surface of the
Со
tion at
at CO = 6.5
⎯Mn/Аl О /Аl catalyst upon the oxygen concentraꢀ
Со
tion at
at CO = 6.5
⎯Mn/Аl О /Аl catalyst upon the oxygen concentraꢀ
2 3
2 3
6
–1
6
–1
S/
V
= 3
×
10
–4
m
and the amount of the catalyst
S/V
= 3
×
10
–4
m
and the amount of the catalyst
×
10 mol/L in the modes of (1, 3) convenꢀ
×
10 mol/L in the modes of (1, 3) convenꢀ
2
2
tional heating and (2, 4) microwave heating at a microwave
power of 450 W. The weightꢀaverage temperature is
403 2 K.
tional heating and (2, 4) microwave heating at a microwave
power of 600 W. The weightꢀaverage temperature
is 403 2 K.
for the specific freeꢀradical initiation rate normalized
to the unit area of the catalyst loaded:
k
1
RH + Z
+ O2
R• + Z
(red)
(oxid)
slow
(3)
k
,
2
Z
Z
+ H2O,
(oxid)
(red)
k RH k O
2
[
]
[
]
•
dR
1
2
=
(6)
WR⋅
=
.
where Z(oxid) is an oxidized site and Z(red) is a reduced
site on the catalyst surface.
According to this scheme, the rate of generation of
free radicals on the heterogeneous surface in its quasiꢀ
steady state can be equated to the formation rate of
dτ
k RH + k O
1 2 2
[
]
[
]
Under the experimental conditions when
ӷ
, we get:
]
k RH
k O
[
]
[
1
2
2
Ea
•
−
dR
reoxidation sites Zoxid.
:
WR⋅
= k O
kR•CO
k0e RT CO ,
=
2
=
=
]
2
(7)
[
2
2
dτ
can be considered the specific rate conꢀ
stant of heterogeneous generation of ꢀxylyl radicals,
dZ
dR•
(oxid)
WR• = ꢀꢀꢀꢀꢀꢀꢀ = k1[RH]Θ
= ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ
(oxid)
where
–
kR⋅
dτ
k2Θ(red )[O2] – k1[RH]Θ
dτ
(4)
m
=
,
(oxid)
Еа is the activation energy, and is the preꢀexponenꢀ
k0
tial factor.
where [RH] is the initial concentration of
mꢀxylene
(mol/L); [O2] is the concentration of dissolved oxygen
(
tion and reoxidation of active sites, respectively; and
Θ(oxid) and Θ(red) are respectively the normalizedꢀtoꢀ
unity concentrations of oxidized and reduced sites on
the catalyst surface:
The table lists values for the rate parameters of the
step of heterogeneous generation of mꢀxylyl radicals in
the case of conventional heating of the reactor and
heating by MW irradiation, as calculated from the
10–3 mol/L); k1 and k2 are the rate constants of reducꢀ
Arrhenius plot of kR versus temperature on basis of
•
arrays of experimental data approximated by Eq. (7).
It is seen that the values of the specific rate conꢀ
stants of heterogeneous generation of mꢀxylyl radicals
Θ(oxid)
+
Θ(red) = 1.
(5)
Solving Eq. (4) with respect to Θoxid using simple in the case of microwave irradiation of the reaction
transformations, we obtain the following expression system exceed those for conventional heating, with the
PETROLEUM CHEMISTRY Vol. 53
No. 2
2013

120
LITVISHKOV et al.
Kinetic parameters of the step of heterogeneous generation of
the reaction system
mꢀxylyl radicals in different modes of thermal treatment of
Parameter
Heating with electric heater
MW irradiation
Weightꢀaverage temperature, K
388
398
403
413
388
398
403
413
k
R, mol/min
a, kJ/mol
log
1.15
2.31
3.69
7.23
2.84
5.23
7.15
18.53
E
122.5
16.3
121.8
16.7
k
0
difference in the values of the constants sharply ence of dielectric loss in the bulk of the heterogeneous
increasing with the increasing reaction temperature.
catalyst and in the bulk of mꢀxylene subjected to oxiꢀ
dation, local overheating spots emerge at the oxidized
hydrocarbon/catalyst interface, so that their temperaꢀ
ture exceeds the weightꢀaverage temperature in the
reaction space. This fact, because of a reduction in the
value of the exponent in Eq. (7), seems to be the real
cause of the observed increase in the specific rate
of initiation of free radicals in the microwaveꢀ
enhanced process of heterogeneous catalytic oxidaꢀ
It is noteworthy that the activation energy values
for the generation of free radicals in both modes of
energy input remain almost unchanged. This invariꢀ
ability suggests the absence of the “nonthermal” effect
in the activation of mꢀxylyl radical generation because
of a relatively low microwave quantum energy comꢀ
pared with the C–H bond energy in the xylene moleꢀ
cule [9].
tion of
mꢀxylene and, under the conditions of develꢀ
oped propagation of the reaction, of the previously
In conclusion, taking into account the above cirꢀ
cumstances, we may assume that owing to the differꢀ observed shortening of the time required to reach the
maximum yield of mꢀtoluic acid [7].
(
WOinh
–
Winh
)
10–3, mol/m2 min
×
2
REFERENCES
8.0
6.0
4.0
2.0
1. D. M. P. Mingos and D. R. Baghurst, Microwave
Enhanced Chemistry, Ed. by H. M. Kingston, and
St. J. Haswell (ACS, Washington (DC), 1997), 3.
2
2. S. S. Berdonosov, Soros. Obraz. Zh. 7 (1), 32 (2001).
3. A. V. Bakhonin, I. Kh. Bikbulatov, R. R. Daminev,
et al., Neftepererab. Neftekhim., No. 2, 19 (2002).
1
4. Yu. N. Litvishkov, in Proceedings of Azerbaijani–Rusꢀ
sian Symposium with International Participation on
Catalysis in Resolving Petroleum Chemistry and Petroꢀ
leum Refining Problems (Baku, 2010) [in Russian].
5. N. V. Shakunova, S. M. Zul’fugarova, Z. F. Aleskerova,
et al., Neftepererab. Neftekhim., No. 12, 21 (2005).
6. Yu. N. Litvishkov, S. M. Zul’fugarova, N. V. Shaꢀ
kunova, and Z. F. Aleskerova, Azerb. Khim. Zh., No. 1,
26 (2006).
390
400
410
420
, К
7. Yu. N. Litvishkov, M. R. Efendiev, E. M. Guseinova,
et al., in Proceedings of Azerbaijani–Russian Symposium
with International Participation on Catalysis in Resolving
Petroleum Chemistry and Petroleum Refining Problems
(Baku, 2010) [in Russian].
Т
Fig. 4. Dependence of the initial rate of heterogeneous catꢀ
alytic chain initiation on the surface of the
Со⎯Mn/Аl О /Аl the catalyst upon the weightꢀaverage
2 3
8. GOST R (Russian State Standard) 53024ꢀ2008: Method
for Determination of the Peroxide Number, 2009 [in Rusꢀ
sian].
temperature at an oxygen concentration of CO = 6.5
–4
×
2
10 mol/L and upon the amount of the catalyst at
S/
V
=
6
–1
9. Quick Reference Handbook of Physicochemical Quantiꢀ
ties,, Ed. by K. P. Mishchenko and A. A. Ravdel’
(Khimiya, Leningrad, 1974) [in Russian].
3
×
10
) microwave heating at input powers of 200, 320, 480 and
640 W.
m
in the modes of (1) conventional heating and
(2
PETROLEUM CHEMISTRY Vol. 53
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2013