Study of the products of benzene transformation in the presence of argon, hydrogen, and propane-butane mixture in barrier discharge

Article abstract of DOI:10.1134/S0965544112010057

The conversion of benzene in a medium of Ar, H₂, and propane-butane mixture in a dielectricbarrier discharge reactor, accompanied by polymerization yielding liquid and solid compounds, has been investigated. The amount of polymerization products reaches 79.7 wt %. Addition of H₂ to benzene reduces the amount of benzene-soluble polymer-like compounds in the products to ～61 wt % and precludes the formation of solid polymerization products. The transformation of benzene with a propane-butane mixture yields alkylbenzenes (up to 38.5 wt %) and liquid alkanes (to 20.5 wt %, mainly with a branched structure). It has been found that an increase in benzene flow rate from 0.08 to 0.4 cm³/min in the case of benzene conversion with the propane-butane mixture can significantly reduce the amount of polymerization products from 56.7 to 9.1 wt %. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0965544112010057

ISSN 0965ꢀ5441, Petroleum Chemistry, 2012, Vol. 52, No. 1, pp. 60–64. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © S.V. Kudryashov, S.A. Perevezentsev, A.Yu. Ryabov, G.S. Shchegoleva, E.E. Sirotkin, 2012, published in Neftekhimiya, 2012, Vol. 52, No. 1, pp. 66–70.
Study of the Products of Benzene Transformation in the Presence
of Argon, Hydrogen, and Propane–Butane Mixture
in Barrier Discharge
S. V. Kudryashov, S. A. Perevezentsev, A. Yu. Ryabov, G. S. Shchegoleva, and E. E. Sirotkin
Institute of Petroleum Chemistry, Siberian Branch, Russian Academy of Sciences, Tomsk, Russia
eꢀmail: ks@ipc.tsc.ru
Received March 27, 2011
Abstract—The conversion of benzene in a medium of Ar, H2, and propane–butane mixture in a dielectricꢀ
barrier discharge reactor, accompanied by polymerization yielding liquid and solid compounds, has been
investigated. The amount of polymerization products reaches 79.7 wt %. Addition of H2 to benzene reduces
the amount of benzeneꢀsoluble polymerꢀlike compounds in the products to ~61 wt % and precludes the forꢀ
mation of solid polymerization products. The transformation of benzene with a propane–butane mixture
yields alkylbenzenes (up to 38.5 wt %) and liquid alkanes (to 20.5 wt %, mainly with a branched structure).
It has been found that an increase in benzene flow rate from 0.08 to 0.4 cm3/min in the case of benzene conꢀ
version with the propane–butane mixture can significantly reduce the amount of polymerization products
from 56.7 to 9.1 wt %.
DOI: 10.1134/S0965544112010057
Studies devoted to reactions of aromatic comꢀ running down the reactor walls for the removal of
pounds in electric discharges are much less in number reaction products from the discharge zone [5]. In this
than those on the transformations of alkanes. Recent study, benzene was used as a hydrocarbon to form the
publications mostly concern processes for postꢀtreatꢀ liquid film on the reactor walls. The experimental
ment of industrial emissions for removal of aromatic setup is sketched in Fig. 1.
compounds (benzene, toluene, xylenes, etc.), which
A gas stream from cylinder
fineꢀtuning valve is directed to mixer
mixed with benzene vapor. Benzene is vaporized in
1
by pipeline
2 through
either undergo deep oxidation to CO, СО2, and Н2О
or polymerize under discharge conditions [1, 2].
Investigations of benzene transformation in electric
discharges of various types [3, 4] revealed as main
products biphenyl and phenylcyclohexadienes in
small quantities and polymerization products making
up to 80% of the total product mass.
We attempted to eliminate the unwanted process of
polymerization of aromatic compounds, taking benꢀ
zene transformation in dielectricꢀbarrier discharge
(DBD) plasma as an example, by selecting different
reaction conditions, including admixture of comꢀ
pounds intended for advanced use, such as the proꢀ
pane–butane fraction (PBF) of natural or associated
gas, to manufacture valuable chemical feedstock
(alkylbenzenes, branchedꢀchain hydrocarbons).
In this paper, we report the results of experimental
3
4, in which it is
evaporator
lar furnace
5 consisting of metal capillary 6 and tubuꢀ
7
. Benzene is fed into the evaporator by periꢀ
from tank . The evaporator temperature
С is monitored with chromel–alumel thermoꢀ
staltic pump
of 120
8
9
°
couple 10 connected to digital millivoltmeter 11. From
the mixer, the vapor–gas mixture is directed into
plasma reactor 12. The gasꢀdischarge flow reactor is
made of Pyrex glass and has the 10ꢀcm long working
area and the volume of 12.3 cm3. The electrode system
consists of grounded outer electrode 13 and coaxial
highꢀvoltage inner electrode 14. Discharge gap 15
between the dielectric barriers is 1 mm, and the dielecꢀ
tricꢀbarrier thickness is 2 mm. Benzene vapor conꢀ
denses at the reactor walls cooled to –10°С. Further,
the benzene concentration in the vapor–gas mixture is
established on the level of ~6 vol %. The resulting conꢀ
densate containing dissolved reaction products flowed
as a film down to collector ~6 equipped with a reflux
condenser. The reactor walls were temperatureꢀconꢀ
study of the transformations of benzene in an Ar, H2
,
and propane–butane medium in a DBD reactor.
EXPERIMENTAL
trolled with coil 17 connected with thermostat 18
.
One of the reasons for low selectivity of plasmaꢀ Discharge was excited with highꢀvoltage pulses supꢀ
chemical reactions is the lack of an effective channel plied from generator 19. The pulse voltage and disꢀ
for product withdrawal from the discharge zone. Preꢀ charge parameters were recorded by Aktakom ASK
viously, we proposed to use a liquid hydrocarbon film 3106 twoꢀchannel digital oscilloscope 20 with a bandꢀ
60

STUDY OF THE PRODUCTS OF BENZENE TRANSFORMATION
61
14
3
4
2
12
R1
21
17
5
7
18
11
R2
s
19
10
C
13
1
R3
6
22
20
15
8
Gas to analysis
9
16
Fig. 1. Schematics of the experimental setup and measurement of discharge parameters:
) gas cylinder, ( ) pipeline, ( ) gas fineꢀtuning valve, ( ) mixer, ( ) evaporator, ( ) metal capillary, (
staltic pump, ( ) tank with benzene, (10) chromel–alumel thermocouple, (11) digital millivoltmeter, (12) plasma reactor,
13) grounded electrode, (14) highꢀvoltage electrode immersed in a saturated NaCl solution, (15) discharge gap, (16) collector
with a reflux condenser, (17) cooling coil, (18) thermostat, (19) generator of high voltage pulses, (20) digital oscilloscope,
21) voltage divider ( = 1 M = 1 k ), and (22) capacitance ( = 500 pF) and current (R = 5.2 shunts); s is the switch.
(1
2
3
4
5
6
7) tubular furnace, (8) periꢀ
9
(
(
R
Ω
,
R
Ω
C
Ω
1
2
3
width of 100 MHz. The schematic circuit diagram of madzu QP 5050 A GC–MS instrument using a capilꢀ
recording the discharge parameters is shown in Fig. 1. lary column with the DB–5 liquid stationary phase
Figure 2 depicts pulsed voltage and current oscilloꢀ
scope traces and the charge–voltage characteristic of
the discharge.
(
l
= 30 m,
din = 0.25 mm).
The gaseous reaction products were determined in
the isothermal mode on the chromatograph with a therꢀ
mal conductivity detector. Packed columns with
The active discharge power
by the equation:
W
(W) was calculated
molecular sieves 5A (
and Porapak QS 80/100 mesh (
col = 100°C) as sorbents were used.
L
= 2 m,
d
in = 3 mm,
Tcol = 30°C)
L
= 3 m, din = 3 mm,
W
=
fViq
(1)
T
where
(2 kHz), Vi is the discharge operation voltage, and
the charge transferred per pulse. The value of
f
is the voltage pulse repetition frequency
is
was
Benzeneꢀsoluble polymerꢀlike compounds (SPC)
q
formed by the reaction were isolated by evaporation of
volatile products in an argon stream. Insoluble polyꢀ
merꢀlike compounds (IPC) were removed mechaniꢀ
cally from the reactor walls. The molecular mass of
SPC was determined cryoscopically in naphthalene
according to the procedure given in [7]. The SPC were
investigated by IR (Nicolet 5700 FTIR spectrometer)
and NMR (Avance AV 300 Fourierꢀtransform NMR
spectrometer) spectroscopy.
q
determined from the charge–voltage curve [6]. The
procedure is graphically illustrated in Fig. 2b. The
active discharge power was 21.6 W. The gas (Ar, Н2
,
PBM) flow rate was constant, 15 cm3/min, in all
experiments. The flow rate of benzene was varied
within 0.08–1 cm3/min.
The reaction products were identified and deterꢀ
mined by GLC and GC–MS (gas chromatography–
mass spectrometry). The chromatographic analysis of
the liquid products was performed on a chromatoꢀ
graph with a flameꢀionization detector in the temperꢀ
ature programming mode: isothermal holding at a colꢀ
RESULTS AND DISCUSSION
Table 1 shows the product composition for the
umn temperature of 70оС for 10 min followed by heatꢀ transformation of the С6Н6/Ar mixture in the DBD
ing to 180
capillary column (
°
С
at a heating rate of
5°C/min, on a reactor at different benzene flow rates. The main
l
= 100 m, din = 0.25 mm) coated products are SPC, IPC, biphenyl, and phenylcycloꢀ
with the OVꢀ101 liquid stationary phase using helium hexadienes (phenylcyclohexaꢀ1,3ꢀdiene and phenylꢀ
as the carrier gas. Mass spectra were obtained on a Shiꢀ cyclohexaꢀ1,4ꢀdiene). Monoalkylbenzenes, cycloꢀ
PETROLEUM CHEMISTRY Vol. 52
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62
KUDRYASHOV et al.
, 10–7
Q
C
V, kV
2
20
12
(b)
(а)
15
8
1
10
4
2Vi
5
0
b
0
0
–5
–10
–15
–4
–8
–1
–2
–10
–5
0
5
10
15
, kV
0
1
2
t ×
104, s
V
Fig. 2. Characteristics of barrier discharge: (a) highꢀvoltage pulse and discharge current and (b) charge–voltage curve.
hexadiene, ethynylbenzene, and styrene were also mation of IPC occurs over the entire surface of the
detected in small amounts among the products.
At a benzene flow rate of 0.24 cm3/min, the amount
of SPC is 79.7 wt % and that of IPC is 0.9 wt %. The forꢀ
electrodes. An increase in benzene flow rate to
0.4 ml/min leads to the disappearance of IPC from the
products. The selectivity of formation of SPC is almost
independent of the flow rate of benzene, although the
conversion decreases from 8.8 to 2.7 wt% with the
increasing flow rate.
Ethylene, ethane, and traces of hydrogen were
found among the gaseous products. Since the total
amount of gaseous products in the gas mixture at the
reactor outlet did not exceed 2 vol % in all experiꢀ
ments, we do not present detailed information about
them in this paper. An analysis of the IR and NMR
spectra of the polymeric products showed that they
have a complex crosslinked structure containing fragꢀ
ments of hydrocarbons of various classes. The molecꢀ
ular mass of SPC was found to be ~235 g/mol.
Unfortunately, there is a lack of published experiꢀ
mental data on the mechanism of transformation of
hydrocarbons, in particular, benzene directly under
the DBD conditions. However, studies devoted to the
mechanisms of hydrocarbon transformations in nonꢀ
equilibrium electric discharges at low pressures have
been reported. For example, Slovetskii [8], having surꢀ
veyed published data and the results of high own
research, concluded that at a concentration of vaporꢀ
ized hydrocarbons in mixtures with inert gases above
1 vol %, the main contribution to their decomposition
can be due to collisions with electrons that do not lead
to ionization. It was shown that benzene primarily
generates the phenyl radical under conditions of nonꢀ
equilibrium electric discharge. Studying the decomꢀ
Table 1. Product composition for transformation of a benꢀ
zene vapor and argon mixture in the barrierꢀdischarge reacꢀ
tor at different flow rates of benzene
Benzene flow rate, cm3/min
Products
0.24 0.4 0.74
Amount, wt %
1
Toluene
0.5
1.3
0.2
0.3
1.0
0.1
0.5
0.5
6.9
6.8
0.8
1.3
0.2
0.6
1.7
0.2
0.5
0.3
9.0
7.6
1.0
1.0
0.2
0.4
1.9
0.1
0.5
0.3
9.9
8.4
0.7
1.1
0.2
0.4
1.7
0.2
0.5
0.3
8.0
6.6
Ethylbenzene
Cumene
Propylbenzene
Cyclohexadienes
Ethynylbenzene
Styrene
Ethylcyclohexadienes
Phenylcyclohexadienes
Biphenyl
SPC
79.7 76.5 75.1 79.4
IPC
0.9
1.4
–
–
–
Unidentified*
1.2
1.1
0.9
Conversion**, wt %
8.8
5.5
3.6
2.7
Notes: * The total amount of unidentified compounds with a low
intensity of chromatographic peaks.
position products of the
mixture in corona
C6H6/Ar
discharge (close to DBD in properties) by matrix
IR spectroscopy, Bai and Ault [9] also detected the
phenyl radical, along with other benzene degradation
products. Then, dischargeꢀinduced initiation step can
** Hereinafter, the values of the conversion per pass of feed
mixture through the discharge zone of the reactor and
calculated on the basis of the total mass of hydrocarbons
passed through the reactor are given.
PETROLEUM CHEMISTRY Vol. 52
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2012

STUDY OF THE PRODUCTS OF BENZENE TRANSFORMATION
63
Table 2. Product composition for transformation of a mixꢀ
ture of benzene vapor and hydrogen in a barrierꢀdischarge
reactor (benzene flow rate of 0.08 cm3/min)
be represented by the following simplified set of reacꢀ
tions:
H5•
(1)
(2)
С6Н6 + е
С6Н6 + е
С6 + Н• + е,
Products
Amount, wt %
R• + R'•+ е,
Cyclohexadienes
Toluene
1.5
0.7
where e denote discharge electrons and • and • are
R'
R
fragments of the benzene ring.
Ethylbenzene
Ethynylbenzene
Styrene
0.5
The radicals generated at the discharge initiation
step of the reaction subsequently enter into the combiꢀ
nation reaction yielding the products:
0.3
0.4
R• + R'•
R – R'.
(3)
Cumene
0.2
The repeated exposure of the products to DBD
leads to their further transformation into polymerꢀlike
compounds.
We assumed that the replacement of argon by
hydrogen can reduce the yield of the products of benꢀ
zene polymerization in DBD because of the occurꢀ
rence of the competing reaction between atomic
hydrogen, formed by the action of DBD on molecular
hydrogen,
Ethynylbenzene
Propylbenzene
Ethylcyclohexadienes
Phenylcyclohexadienes
Biphenyl
0.1
0.1
0.1
14.3
10.2
61.4
–
IPC
SPC
(4)
H2 + e
and the benzene ring fragment:
R• + H•
RH.
2H• + е
Unidentified
10.1
Conversion, wt %
4.5
(5)
Table 2 shows the results of experiments on the
conversion of benzene vapor in the presence of hydroꢀ
gen.
Table 3. Product composition for the transformation of a
benzene vapor–PBM mixture in DBD at different benzene
flow rates
Indeed, the formation of IPC is not observed durꢀ
ing the conversion of benzene in the presence of
hydrogen. At as low a benzene flow rate as
0.08 ml/min, the amount of SPC in the reaction prodꢀ
ucts (61.4 wt %) is below that in the case of С6Н6/Ar
mixture at a benzene flow rate of 0.24 cm3/min
(79.7 wt %). The amounts of cyclohexadienes, bipheꢀ
nyl, and alkylbenzenes in the products are 16, 10.2,
and 1.5 wt %, respectively. The degree of conversion is
4.5 wt %.
On the basis of reactions (1)–(3), it might be
expected that the transformation of the benzene–
PBM mixture would lead to the formation of alkylbenꢀ
zenes, valuable preproducts for organic synthesis:
Benzene flow rate, cm3/min
Products
0.08
0.24
0.4
Amount, wt %
Pentanes
Hexanes
Heptanes
Octanes
Cyclohexadiene
Toluene
Ethylbenzene
Cumene
Propylbenzene
Isobutylbenzene
Ethynylbenzene
Styrene
Ethylcyclohexadienes
Propylcyclohexadiene
Isobutylcyclohexadiene
Phenylcyclohexadienes
Diphenyl
2.1
4.4
1.1
0.4
1.6
5.1
4.6
6.9
2.5
1.2
0.7
0.8
2.4
0.6
0.4
1.4
2.6
56.7
–
2.6
2.3
0.8
0.4
2.7
10.2
4.1
1.3
0.5
6.6
2.5
3.3
1.1
1.4
0.1
7.6
7.9
33.0
–
8.6
6.4
5.5
–
6.3
17.2
5.3
11.2
3.8
1.0
0.8
1.2
5.1
1.1
–
RH + e
H5•
R• + H• + e,
(6)
+ R•
C6H5 – R,
(7)
C6
where RH is the alkane molecule.
By analogy with experiments in argon, the experiꢀ
ments in the propane–butane mixture were also conꢀ
ducted at different benzene flow rates. The initial
PBM contained 65.4 vol % propane, 19.5 vol %
butane, and 7.5 vol % isobutane. The results are preꢀ
sented in Table 3.
It is seen that alkanes (to 20.5 wt%, mainly of isoꢀ
meric structure) are produced in addition to alkylbenꢀ
zenes (up to 38.5 wt %). An increase in benzene flow
3.2
8.6
9.1
–
SPC
IPC
Unidentified
4.5
11.6
5.6
Conversion, wt %
3.8
3.1
0.6
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64
KUDRYASHOV et al.
3. M. W. Ranney and W F. O’Connor, J. Am. Chem. Soc.
80, 297 (1969).
4. J. K. Stille, R. L. Sung, and K. J. Vander, J. Org. Chem.
30, 3116 (1965).
5. S. V. Kudryashov, G. S. Shchegoleva, E. E. Sirotkina,
rate reduces the amount of SPC in the products from
56.7 to 9.1 wt %.
In summary, the example of transformation of the
С6Н6–PBM mixture allowed us to find conditions at
which the yield of the traditional polymerization
products formed in dielectricꢀbarrier discharge is sigꢀ
nificantly reduced and to show the feasibility of manꢀ
ufacturing liquid alkylbenzenes and isoalkanes.
and A. Yu. Ryabov, High Energy Chem. 35, 120 (2001).
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Modern Techniques of Petroleum Research, Ed. by
A. I. Bogomolov, M. B. Temyanko, and L. I. Khotyntꢀ
seva (Nedra, Leningrad, 1984) [in Russian].
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PETROLEUM CHEMISTRY Vol. 52
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2012