Mechanisms of Heptane Degradation and Product Formation in Microwave Discharge

Article abstract of DOI:10.1134/S0018143919040039

Abstract: A mechanism for the degradation of n-heptane and the formation of the products of its plasma-chemical transformation by microwave discharge treatment has been proposed. Chemical reactions resulting in reactive species, namely free radicals that form lower hydrocarbons and polyaromatic structures are presented. The product composition of the gas, liquid, and solid phases has been studied using gas chromatography–mass spectrometry analysis of the precipitate obtained by evaporation of the liquid phase after the treatment of n-heptane.

Full text of DOI:10.1134/S0018143919040039

ISSN 0018-1439, High Energy Chemistry, 2019, Vol. 53, No. 4, pp. 336–340. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Khimiya Vysokikh Energii, 2019, Vol. 53, No. 4, pp. 330–334.
PLASMA CHEMISTRY
Mechanisms of Heptane Degradation and Product Formation
in Microwave Discharge
E. S. Bobkovaa, *, O. A. Stokolosa, and A. R. Garifullina
aGubkin Russian State University of Oil and Gas, Moscow, 119991 Russia
*e-mail: lenabobkova777@gmail.com
Received January 21, 2019; revised February 20, 2019; accepted February 25, 2019
Abstract―A mechanism for the degradation of n-heptane and the formation of the products of its plasma-
chemical transformation by microwave discharge treatment has been proposed. Chemical reactions resulting
in reactive species, namely free radicals that form lower hydrocarbons and polyaromatic structures are pre-
sented. The product composition of the gas, liquid, and solid phases has been studied using gas chromatog-
raphy–mass spectrometry analysis of the precipitate obtained by evaporation of the liquid phase after the
treatment of n-heptane.
Keywords: microwave discharge, heptane, treatment of liquid hydrocarbons, mechanism of product forma-
tion in plasma
DOI: 10.1134/S0018143919040039
INTRODUCTION
understood. The majority of studies dealing with the
products of transformation of aqueous solutions of
organic compounds by the action of dielectric barrier
discharge and direct-current discharge focus on the
environmental aspects [3–5]. Few reports have been
devoted to the study of the conversion of liquid
alkanes. The hydrocarbons described in the literature
on discharge treatment are hexane [6], heptane [7, 8],
n-C5–C8 alkanes and cyclohexane [9], and dodecane
[10]. In this connection, the description of a possible
mechanism for the conversion of n-heptane in a
microwave discharge is a relevant task.
Technologies based on the application of low-tem-
perature plasma are widely used in various branches of
engineering and industry, including petroleum chem-
istry, metallurgy, materials science, microelectronics,
and medicine, and continue to actively develop. The
use of methods of nonthermal activation of chemical
reactions makes it possible to affect molecules more
intensively due to the excitation of internal degrees of
freedom. Therefore, it becomes possible to carry out
reactions that are impossible or difficult to perform
using thermal or thermal catalytic methods [1]. The
interest of researchers in the use of electrical dis-
charges to stimulate chemical reactions in liquids is
due to a high product formation rate and high effi-
ciency of physicochemical processes proceeding in the
course of plasma-induced reactions. The high rate can
be explained by the fact that plasma is created in a gas
bubble inside a liquid, the surface of which is located
near the high-temperature zone [2]. This ensures a
high rate of flow of liquid molecules into the minire-
actor, which is a plasma-containing bubble. Because
of the intensive supply of molecules from the liquid
surface, high concentrations of active species (radi-
cals, excited atoms, and charged particles) involved in
plasma-chemical processes are ensured.
RESULTS AND DISCUSSION
The results of the experiments performed in this
study were obtained using a setup with coaxial intro-
duction of microwave field into the reaction chamber
[11]. The discharge treatment of n-heptane results in
the formation of solid, liquid, and gaseous products.
The solid products were found to contain amorphous
carbon and damaged graphene. The gas phase was
composed of about 98% hydrogen and acetylene. The
liquid products determined by gas chromatographic–
mass spectrometric (GC–MS) analysis of the residue,
obtained by evaporating the liquid phase, are pre-
sented in Table 1. These are mostly bi- and polyaro-
matic compounds, which may be the precursors of
soot formation.
Currently, almost all types of electrical discharges,
including microwave discharges, are used to produce
plasma. However, the use of the microwave discharge
for carrying out plasma-chemical reactions in liquid
media has been studied scarcely. The mechanisms of
In n-heptane subjected to microwave discharge
transformation of liquid hydrocarbons are even less treatment, two competing reactions, the dehydroge-
336

MECHANISMS OF HEPTANE DEGRADATION AND PRODUCT FORMATION
337
Table 1. Results of GC–MS analysis of the residue after evaporation of the liquid phase treatment with n-heptane
Retention time, min
Concentration, %
Component
5.70
6.36
0.48
10.16
0.50
0.40
0.44
31.23
0.31
C10H8
Naphthalene (C10H8)
Biphenyl (C12H10)
C12H10
Acenaphthene (C12H10)
Acenaphthylene (C12H8)
C14H10
C14H10
Phenanthrene (C14H10)
Anthracene (C14H10)
Phenylnaphthalene
(C10H7-C6H5)
13.17
14.03
14.73
15.14
23.35
24.67
26.70
27.11
33.22
1.31
10.77
5.39
0.15
33.56
35.38
36.13
36.77
44.80
47.02
15.47
7.41
3.60
11.98
0.12
Dioctyl phthalate (C6H4(COOC8H17)2)
Fluoranthene (C16H10)
Pyrene (C16H10)
Dibenzopentalene (C16H10)
C18H10
0.32
C18H12
nation and the cleavage (cracking), occur in approxi-
mately equal proportions:
The place of rupture in the molecule is determined
by the bond dissociation energy, with the rupture at
moderate temperature being closer to the middle of
the molecule, i.e. by the weakest bonds:
С7Н16 → СН4 + 3С2Н2 + 3Н2,
С7Н16 → СН4 + 3С2Н4.
The cracking reaction proceeds via the radical
chain mechanism, the bond breaking in the n-heptane
molecule being homolytic:
C
C
C
C
C
C
C
335
335
322
314
314
322
kJ/mol
The cleavage proceeds through the following
stages:
A. Chain initiation.
CH3 +
CH3
CH3
CH3
CH2 CH2 CH2 CH3
CH3 CH2 CH2
CH3 CH2 CH2
CH3 CH2 CH2 CH2
+
H
H
CH3 CH2 CH2 CH2 CH2 CH CH3 +
CH3 CH2 CH2 CH2 CH2 CH CH3
H
+
CH3 CH2 CH2 CH2 CH2 CH2 CH2
B. Chain propagation. Radicals formed with the a homolytic rupture of the C–C bond and the C–H
carbon number of C3 and higher instantaneously
decompose under the action of the discharge. There is carrying the unpaired electron.
bond in the β-position with respect to the carbon atom
CH3 CH2 CH2
CH3 CH CH2
H
H
+
2
+
CH3 CH CH2
CH2 CH2
CH3
HC CH +
HC CH +
+
6H
H
CH4
CH4
+
CH2 CH
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BOBKOVA et al.
The formation of acetylene is possible in two cases:
CH3
CH4
+
H
n the pyrolysis of methane and by the dehydrogena-
tion of the vinyl radical; the hydrogen atom is detached
in both cases.
CH2 CH
CH2 CH2
HC CH
+ H
CH CH
The sequential cleavage of the C–H bonds in the
β-position of the n-heptane molecule leads to the for-
mation of unsaturated organic structures, which par-
ticipate in the cyclization reaction. Almost all cleavage
steps are accompanied by the formation of the hydro-
gen atom, which subsequently forms molecular hydro-
gen, which prevails in the gaseous products and
amounts to 93.5 vol %.
C. Chain termination. The concentration of radi-
cals in the system increases with the progress of crack-
ing reactions, and the probability of their colliding
with each other increases. As a result, in the chain ter-
mination step, low-molecular product molecules
accumulate.
H
+
CH2 CH2 CH2 CH2
CH3 CH2 CH2
CH2 CH2 CH2 CH3
CH2 CH2 CH2 CH2
CH2 CH2 CH CH2
CH3 CH2 CH2
CH3 CH2 CH2
CH3 CH2 CH2
+ H2
CH2 CH2 CH CH2
CH3 CH2 CH2
CH3 CH2 CH2
CH2 CH CH CH2
H
CH CH CH CH2
CH3 CH2 CH
CH CH CH CH2
CH CH CH CH2
CH3 CH2 CH2
−H2
−H
−H
H
CH CH CH CH2
CH3 CH CH
CH3 CH CH
·
−H2
−H2
H
C
CH3
CH
CH
CH CH3
CH CH CH CH
H
CH3
C
CH
−H2
CH
H
H
C
H
A large amount of polyaromatic hydrocarbons are
found in the products of plasma-chemical conversion
of n-heptane. The formation of high-molecular0-
weight polyaromatic compounds is similar to the cok-
ing reaction, in which aromatic hydrocarbons con-
dense under the action of radical initiators. Prelimi-
narily, the naphthenic cycle is produced from the
aliphatic structure followed by dehydrogenation and
the formation of an aromatic structure.
CH3
+ CH3
+
CH3
CH4
+ H
The mechanism of formation of polyaromatic
hydrocarbons is based on the reaction of polyconden-
sation proceeding through the radical chain mecha-
nism.
The C–H bond in the substrate heptane can break
possible at the primary atom, leading to the formation
of a primary carbon-centered radical followed by
cleavage of the β-C–C bond and the formation of the
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MECHANISMS OF HEPTANE DEGRADATION AND PRODUCT FORMATION
339
i
i
i
lower radicals
and
(R1). Butyl
C4H9
C3H7
C4H9
C2H5i
is sta- cleaved to give the ethyl radical and the
bilized via dehydrogenation along the β-C–H bond to
ethylidene biradical •C2H4•; ethylene (R3) is formed
is when the hydrogen atom is detached.
i
form butadiene (R2). Simultaneously, butyl
C4H9
CH CH2 CH2
(R1)
(R2)
(R3)
CH2 CH2 CH2 CH2
CH3 CH2 CH2 CH2 +
CH3 CH2 CH CH2 CH2 CH CH CH2
2CH2 CH2
CH3 CH2 CH2
3
CH3 CH2 CH2 CH2
CH3 CH2 CH2 CH2
−H2
−H
+ CH2 CH2
CH3 CH2
−H
The butadiene and ethylene formed in the course to the formation of the aromatic hydrocarbon benzene
of reactions (R2) and (R3) can participate in a trans- (R4). Simultaneously, butadiene and cyclohexene as a
formation according to the diene synthesis scheme to dienophile form decalin and then naphthalene (R5)
form cyclohexene. Under the same conditions during via the subsequent dehydrogenation. Thus, the poly-
the microwave discharge treatment, cyclohexene condensation of aromatic structures and the forma-
undergoes β-cleavage of the C–H bonds, which leads tion of β-phenylnaphthalene (R6) occur in this way.
CH2
CH
HC
CH2
CH2
+
+
+ 2H2
(R4)
(R5)
(R6)
CH2
CH2
CH
HC
−4H2
CH2
+
−H
The preparation of Biphenyl can also be produced being a dienophile as well, continues the diene synthe-
via the diene synthesis reaction through the interme- sis with butadiene to produce cyclohexen-3-ylbenzene
diates 4-vinylcyclohexene and styrene (R7). Styrene, followed by dehydrogenation to form biphenyl (R8).
CH2
HC CH CH2
CH2
CH CH2
CH CH2
+
CH
HC
(R7)
(R8)
+
2H2
CH2
H2C
HC
+
CH
CH2
-2H2
CH
H2C
HIGH ENERGY CHEMISTRY Vol. 53 No. 4 2019

340
BOBKOVA et al.
The majority of the detected polyaromatic entities: ment of hydrocarbons; further, based on the results of
the bicyclic (naphthalene and diphenyl) and tricyclic this work, one can find new areas of application of
(acenaphthene, acenaphthylene, phenanthrene, and microwave discharges.
anthracene) structures, are formed via the polycon-
densation type of reaction.
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noting that this paper describes the first cycle of the
experimental study on the microwave discharge treat-
Translated by V. Avdeeva
HIGH ENERGY CHEMISTRY
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2019