Ring Opening of Naphthene Hydrocarbons under Conditions of Thermal and Microwave Heating

Article abstract of DOI:10.1134/S0036024418120130

Abstract: A comparative study of the activity and selectivity of monometallic Ir-containing catalysts in ring opening reactions under conditions of thermal and microwave heating is performed using the example of cyclohexane. The hydrogenolysis of cyclohexane is shown to be possible with microwave activation. It is found that upon microwave heating, iridium-containing catalysts supported on titanium oxide and activated carbon allow 60% selectivity to be reached with respect to ring opening products at 30% conversion of the initial cyclohexane.

Full text of DOI:10.1134/S0036024418120130

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2018, Vol. 92, No. 12, pp. 2408–2411. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © E.D. Finashina, A.L. Tarasov, 2018, published in Zhurnal Fizicheskoi Khimii, 2018, Vol. 92, No. 12, pp. 1885–1888.
PHYSICAL CHEMISTRY OF HYBRID NANOMATERIALS
AND MULTICOMPONENT SYSTEMS
Ring Opening of Naphthene Hydrocarbons under Conditions
of Thermal and Microwave Heating
E. D. Finashinaa,* and A. L. Tarasova
aZelinsky Institute of Organic Chemistry, Moscow, 119991 Russia
*e-mail: edf@ioc.ac.ru
Received May 23, 2018
Abstract—A comparative study of the activity and selectivity of monometallic Ir-containing catalysts in ring
opening reactions under conditions of thermal and microwave heating is performed using the example of
cyclohexane. The hydrogenolysis of cyclohexane is shown to be possible with microwave activation. It is
found that upon microwave heating, iridium-containing catalysts supported on titanium oxide and activated
carbon allow 60% selectivity to be reached with respect to ring opening products at 30% conversion of the
initial cyclohexane.
Keywords: ring opening of naphthene hydrocarbons, Ir-containing catalysts, microwave heating
DOI: 10.1134/S0036024418120130
INTRODUCTION
erature; in terms of technology, however, the ring
opening process is far from commercial implementa-
tion. At the same time, ring opening reactions are
among the processes used in industrial hydrocracking
units [5–10].
The advantages of microwave treatment of different
materials compared to thermal activation have been
demonstrated in the works of Russian and other
authors. In such treatment, the choice of catalyst is
important. It must be a semiconductor or conductor in
order to absorb the microwave energy and to be heated
up to high temperatures. Only a narrow selection of
supports and catalysts that absorb microwave radiation
are known. These include carbon supports, titanium
oxide, and mixed transition metal oxides.
Catalysis is a promising field that allows us to suc-
cessfully solve problems of transitioning to environ-
mentally friendly and resource-conserving sources of
energy, to increase the efficiency and depth of raw
hydrocarbon refining, and to obtain new ways and
means of storing energy [1–3]. One of the main prob-
lems in refining fuel is removing polycyclic aromatic
and naphthene hydrocarbons, the presence of which
results in the formation of packing products on cata-
lysts and, in the case of diesel fuel, carbon black in
engine exhaust gases. These hydrocarbons also lower
the cetane number. The presence of polycyclic naph-
thenes and aromatic hydrocarbons is also undesirable
in gasoline fuels. The presence of heavy polycyclic
compounds in diesel fuel also raises the freezing point
and degrades the quality of winter fuels. One way of
Russian patent [11] describes the oxidative cou-
pling of methane to C2+ hydrocarbons, conducted in a
solving this problem could be the selective ring open- flow reactor at atmospheric pressure on the catalyst,
ing of aromatic and naphthene hydrocarbons with for- which is heated with microwaves to 750–800°C in a
mation of alkanes with the same number of carbon flow of a methane–air mixture. A catalytic composi-
atoms as in the initial hydrocarbon [4].
tion in the form of a mechanical mixture of the Mn-
Na2WO4/SiO2 catalyst with actively microwave
absorbing carbides of different metals (e.g., W and
Mo) is used.
Despite the apparent simplicity of ring opening
processes, the scheme of transformation even in the
case of model molecules (cyclohexane, benzene)
includes a number of dynamic equilibria, and the final
Yet another current trend is the microwave-acti-
composition of products is governed by the full set of a vated conversion of biomass to yield biofuels and aro-
catalyst’s characteristics (the quantity and strength of matic hydrocarbons [12–14]. The partial oxidation
acid sites, the nature of the metal, and the possibility and carbon dioxide conversion of methane using
of its charging on a support’s surface) and those of the microwave plasma is also being developed extensively
process (temperature, pressure, substrate/hydrogen [15]. An example of using microwave energy in the
ratio, and volumetric rate). Several catalytic systems transesterification for production of biodiesel fuel was
that were promising in terms of subsequent practical described in [16]; the transformation of alcohols on
application have been described in patents and the lit- exposure to microwave radiation was studied in [17].
2408

RING OPENING OF NAPHTHENE HYDROCARBONS
2409
conditions of our experiments with thermal heating of
the catalyst were T = 250–450°C, P = 2.0 MPa,
LHSV = 1.5–3.0 h–1, and the H2 feed rate was
60 mL/min.
7
6
5
To conduct the reaction under conditions of
microwave heating, the stainless steel reactor was
replaced with a quartz tube with an inner diameter of
9 mm and a wall thickness of 1 mm. The reaction was
conducted at hydrogen pressures of up to 4 atm. The
other parameters of the process (T, LHSV, and hydro-
gen feed rate) were not changed. A diagram of the
microwave heating unit of the laboratory setup is
shown in Fig. 1.
1
3
2
The catalyst was heated in the resonator of the lab-
oratory microwave setup, which consisted of G4-80
microwave generator 2 with a 2.5–4.0 GHz range of
working frequencies, coupled through waveguides 3
and 5 to power amplifier 1 and resonator 6. The tem-
perature of the catalyst was controlled by varying the
microwave radiation’s frequency and power. The cat-
alyst’s temperature (the absorbed microwave radiation
power) was measured using embedded thermocouple
power meter 4 consisting of an M5-78V receiving ther-
moelectric converter and a DC amplifier.
4
Fig. 1. Laboratory microwave facility: 1 is the power
amplifier, 2 is the frequency generator, 3 is the thermoelec-
tric converter, 4 is the power meter, 5 is the waveguide, 6 is
the resonator, and 7 is the quartz reactor.
So far, however, there are no literature data on the
possibility of ring opening under conditions of micro-
wave heating.
The aim of this work was to perform a comparative
study of titanium oxide- and carbon-supported irid-
ium-containing catalysts under conditions of conven-
tional thermal and microwave heating in the hydroge-
nolysis of cyclohexane.
Analysis of Reaction Mixtures
Online analysis of the reaction mixtures was per-
formed on a CrystalLux-4000 M chromatograph
equipped with a flame ionization detector, using
helium as the carrier gas and a SE-54 (l = 50 m) cap-
illary column. The conditions of analysis were tcol
=
50°C, tevap = 125°C, and tdet = 150°C. The duration of
analysis was 4 min. A typical chromatogram of a reac-
tion mixture containing main reaction products at a
high rate of conversion of the initial cyclohexane is
shown in Fig. 2.
Selectivity with respect to ring opening products
was calculated as the ratio of the amount of products
with the molecular formula C6Н14 (n-hexane, 2- and
3-methylcyclopentanes, 2,3- and 2,2-dimethylbu-
tanes) to the total amount of products that formed.
EXPERIMENTAL
Procedure for Preparing the Catalysts
Our catalysts were prepared via the incipient wet-
ness impregnation of supports (TiO2 Degussa with an
incepient wetness of 0.55 mL/g and SKT-4 carbon
with an incepient wetness of 0.5 mL/g) with an aque-
ous solution of iridium chloride (IrCl3 · 3H2O, Acros
Organics). After the deposition of metal, the Ir/TiO2
catalyst was dried in air for one day at room tempera-
ture, calcined in a stream of dried air for 2 h at 550°С,
and reduced in a stream of hydrogen for 2 h at 400°C.
After the deposition of metal, the Ir/C catalyst was
also dried at room temperature for one day, calcined in
a stream of dried air at 300°C, and reduced in a stream
of hydrogen for 2 h at 350°C. The content of iridium in
the resulting catalysts was 1 wt %.
RESULTS AND DISCUSSION
Iridium-containing catalysts were chosen for com-
parative study, since iridium is one of the most active
and selective metals that catalyze the ring opening of
naphthene hydrocarbons [9, 18, 19]. The choice of
supports was governed by their susceptibility to micro-
wave radiation. It should be noted that carbon sup-
ports are rarely used when ring opening reactions are
conducted under conditions of thermal heating.
Cyclohexane serves as a convenient model substrate
that allows us to determine the contribution from pos-
sible reaction pathways (hydrogenolysis, isomeriza-
tion, dehyhrogenation, and hydrocracking). Even at
Procedure for Catalytic Experiments
The hydrogenolysis of cyclohexane under thermal
heating conditions was conducted in a high-pressure
flow catalytic unit inside a tubular stainless steel fixed-
bed reactor (dIn = 12 mm). The reactor was loaded
with the catalyst (1.5 mL) (0.3–0.6 mm fraction). The low selectivity (upon the simultaneous opening by sev-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 92 No. 12 2018

2410
20507.32
FINASHINA, TARASOV
С1−С4
1.01
2-Methylpentane
17082.85
13655.02
10247.20
6839.369
3431.543
23.716
3-Methylpentane
*2,3-Dimethylbutane
**2,2-Dimethylbutane
n-Hexane
Methylcyclopentane
Isopentane
n-Pentane
**
Di- and trimethyl-
cyclopentanes
*
Cyclohexane
Methylcyclohexane
0
00.28 00.57 00.85 01.14 01.42 01.71 01.99 02.28 02.57 02.85 03.14 03.42 03.71 03.99
t, min
Fig. 2. The chromatogram of the reaction mixture upon strong conversion of cyclohexane.
1
5
1
100
80
60
40
20
0
100
80
60
40
20
0
5
2
1
2
3
2
1
3
4
5
5
4
3
4
3
2
4
Thermal heating, 20 atm
Microwave heating, 4 atm
Thermal heating, 20 atm
Microwave heating, 4 atm
Fig. 3. (1) Conversion of cyclohexane, (2) selectivity with
respect to ring opening products, and (3–5) yields of main
Fig. 4. Conversion of cyclohexane, selectivity with respect
to ring opening products, and yield of the main products
obtained on our Ir/C catalyst under conditions of thermal
products obtained on our Ir/TiO catalyst (C H , C H ,
2
6
14
5
12
and C –C , respectively) under conditions of thermal and
1
4
microwave heating (T = 300°C; H feed rate, 60 mL/min;
and microwave heating (T = 370°C; H feed rate,
2
2
−1
−1
LHSV ≈ 2 h ).
60 mL/min; LHSV ≈ 2 h ); for 1–5, see Fig. 3.
eral pathways), the resulting hydrocarbon mixtures are ucts and n-pentane. The main reaction product upon
not difficult to analyze via chromatography.
microwave heating was n-hexane, while light C1–C4
hydrocarbons predominated upon thermal heating at
the comparison temperature (300°С). In other words,
hydrocracking was the main reaction pathway.
The investigated catalysts were found to convert
cyclohexane under conditions of both thermal and
microwave heating, but the reaction pathways differed
considerably. The diagram (Fig. 3) allows us to com-
pare the conversion of cyclohexane, the selectivity
with respect to ring opening products, and the yields of
n-hexane and n-pentane obtained on Ir/TiO2 catalyst
under the conditions of thermal and microwave heat-
ing. We can see that despite the fairly low conversion
achieved upon microwave activation, the selectivity
with respect to ring opening products reached 60% as
With Ir/C catalysts, the range of products obtained
upon thermal and microwave heating at fairly high
temperatures also differed substantialy (Fig. 4). Only
C1–C4 cracking products were produced upon ther-
mal heating under the considered conditions. Upon
microwave activation, the reaction mixture contained
ring opening products and n-pentane, even though the
a consequence of the low volumes of cracking prod- degree of conversion did not exceed 30%. The selec-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 92
No. 12
2018

RING OPENING OF NAPHTHENE HYDROCARBONS
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Translated by K. Utegenov
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 92 No. 12 2018