Kinetic and thermodynamic parameters of hydrogen release during the heterogeneous catalytic dehydrogenation of cis- and trans-isomers of perhydro-m-terphenyl

Article abstract of DOI:10.1134/S0036024416100162

Comparative studies on the temperature dependence of the dehydrogenation of cis- and trans-isomers of perhydro-m-terphenyl are performed in a flow catalytic reactor. Rate constants and equilibrium constants of all elementary acts of this reaction are calculated on basis of experimental data using the KINET 0.8 program for the mathematical modeling of the kinetics of complex reactions. The resulting data indicate that perhydro-m-terphenyl cis- and trans-isomers structural differences have no appreciable effect on dehydrogenation.

Full text of DOI:10.1134/S0036024416100162

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2016, Vol. 90, No. 10, pp. 1921–1924. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © A.N. Kalenchuk, S.E. Bogorodskii, V.I. Bogdan, 2016, published in Zhurnal Fizicheskoi Khimii, 2016, Vol. 90, No. 10, pp. 1447–1451.
CHEMICAL KINETICS
AND CATALYSIS
Kinetic and Thermodynamic Parameters of Hydrogen Release
during the Heterogeneous Catalytic Dehydrogenation
of cis- and trans-Isomers of Perhydro-m-terphenyl
A. N. Kalenchuka,b*, S. E. Bogorodskiic, and V. I. Bogdana,b
aDepartment of Chemistry, Moscow State University, Moscow, 119991 Russia
bZelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
cAdvanced Laser Technologies Division, Institute of Laser and Information Technologies,
Russian Academy of Sciences, Troitsk, Moscow oblast, 142092 Russia
*e-mail: akalenchuk@yandex.ru
Received November 16, 2015
Abstract—Comparative studies on the temperature dependence of the dehydrogenation of cis- and trans-iso-
mers of perhydro-m-terphenyl are performed in a flow catalytic reactor. Rate constants and equilibrium con-
stants of all elementary acts of this reaction are calculated on basis of experimental data using the KINET 0.8
program for the mathematical modeling of the kinetics of complex reactions. The resulting data indicate that
perhydro-m-terphenyl cis- and trans-isomers structural differences have no appreciable effect on dehydroge-
nation.
Keywords: dehydrogenation, perhydro-m-terphenyl, catalysis, hydrogen
DOI: 10.1134/S0036024416100162
INTRODUCTION
EXPERIMENTAL
For dehydrogenation, we used perhydro-m-ter-
phenyl prepared in our laboratory via the hydrogena-
tion of commercial grade m-terphenyl (99%, Aldrich).
Samples of the investigated solid substrate (40 g) and
catalyst (4 g) were loaded into a high-pressure PARR-
300 laboratory autoclave, purged with an inert gas, and
then pressed and hydrogenated with stirring at 180°C.
The hydrogen pressure was 70 atm, and the reaction
was conducted until the hydrogen pressure reached a
constant value. Reaction products were identified and
analyzed on a Crystallux-4000М chromatograph
equipped with a ZB-5 capillary column (ZEBRON,
United States) and a flame ionization detector, along
with a FOCUS DSQ II GC-MS equipped with a TR-
5ms capillary column.
Catalysis is becoming an important tool for devel-
oping new technologies needed in the energy industry
and the chemistry of new materials [1, 2]. The search
for new types of fuels with acceptable environmental
characteristics is a matter of special importance. The
introduction of supersonic aircraft once required the
development of new fuels that were characterized by
high heat capacities and chemical and thermal stabili-
ties, compared to the traditional oil and gas fuels that
were used earlier. Such carbon-saturated condensed
and/or aromatic compounds as decalin and terphenyl
largely meet these requirements [3]. Along with the
development of green energy sources based on hydro-
gen fuel cells, it was shown recently that these com-
pounds can serve as the basis for mobile chemical sys-
tems of hydrogen storage that ensure safe and fast
feeds of pure hydrogen via reversible hydrogenation-
dehydrogenation reactions [4]. It is known that the
thermal characteristics of cis- and trans-isomers of
certain cyclic compounds sometimes differ consider-
ably from each other. It has been established (using
decalin as an example) that such differences influence
the rate of hydrogen release [5].
It is known that the maximum rate of hydrogen
release is reached at high levels of substrate conver-
sion. At the same time, it is better to use lower levels of
conversion in order to obtain valid kinetic parameters.
Catalysts of different dehydrogenation activity were
used for the sake of comparison: no. 1 was 3 wt %
Pt/С; no. 2 was 1 wt % Pt/С; and no. 3 was 1 wt %
Pt/С*. All catalysts were prepared using aqueous solu-
tions of H2PtCl6 by impregnating carbon supports fol-
The aim of this work was to identify the features
and patterns of the heterogeneous catalytic dehydro- lowed by reduction with hydrogen (except for catalyst
genation of cis- and trans-isomers of perhydro-m-ter- no. 3) at 320°C in a 30 mL/min flow of hydrogen
phenyl in a flow reactor.
for 2 h [6].
1921

1922
KALENCHUK et al.
100
80
100
1
1
(b)
80
60
40
2
60
cis-m-C18H32
trans-m-C18H32
1−cat. no. 1 (3% Pt/C)
2−cat. no. 2 (1% Pt/C)
3−cat. no. 3* (1% Pt/C*)
cis-m-C18H32
2
trans-m-C18H32
(a)
3
1−cat. no. 1 (3% Pt/C)
2−cat. no. 2 (1% Pt/C)
40
260
280
300
320
340
Т, °C
0
10
20
30
40
50
Time, h
Fig. 1. (a) Time and (b) temperature dependences of cis- and trans-perhydro-m-terphenyl conversions upon dehydrogenation for
−1
catalysts nos. 1–3 (V = 1 h , Р = 1 atm).
L
100
80
C18H32
C18H14
C18H26
C18H20
C18H32(cis-)
C18H20(cis-)
C18H26(trans-)
C18H20(trans-)
30
20
10
0
60
40
20
0
(а)
(b)
180
220
260
300
340
Т, °C
180
220
260
300
340
Т, °C
Fig. 2. Temperature dependences of the concentration of the initial perhydro-m-terphenyl and the products of its dehydrogena-
−1
tion for 1 wt % of Pt/C (V = 1 h , Р = 1 atm): (а) total for both isomers; (b) allowing for cis- and trans-isomers.
L
Dehydrogenation was performed in a flow reactor nated m-terphenyl compounds. It is known [8] that
at 260–340°C; the linear flow rate of the substrate was the trans-isomer of perhydro-m-terphenyl has a more
6 mL/h [7]. Volumetric flow rate VL [cm3 h−1 (cm3 Kt)−1]
of the substrate was calculated as the ratio of the linear
flow rate of the substrate in its liquid state at 20°C to
the catalyst volume and was estimated at 1 h−1.
Hydrogenation and dehydrogenation conversions
(K) were calculated using the equation K = (c0 − c)/c0 ×
100%, where c0 and с are the initial and final concen-
trations of the transformed substrate, respectively.
rigid structure and its melting temperature Тm is 62°C,
while the melting temperature of more flexible cis-iso-
mer is 20–25°C. We identified isomers (I) and (II) as
the cis- (I) and trans- (II) isomers of perhydro-m-ter-
phenyl because the melting temperature of the result-
ing substrate was 55–56°C. The time and temperature
dependences of conversion alterations of the cis- and
trans-isomers upon the dehydrogenation of perhydro-
m-terphenyl are presented in Fig. 1. It can be seen in
Fig. 1 that the temperature dependences of the cis-
and trans-perhydro-m-terphenyl conversions were vir-
tually identical if catalyst no. 1 (3 wt % of Pt/C) was
used. Some differences between the curves for cis- and
trans-isomers can be seen if the catalysts containing
1 wt % Pt/C were used.
Selectivity (S) was calculated using the equation
× 100%, where
is the
c(i)
= ∑c(i)/∑c(k)
∑
S(i)
sum of the concentration of part of the products and
is the sum of the concentrations of all prod-
c(k)
∑
ucts, respectively.
The temperature dependences of the concentration
of the initial perhydro-m-terphenyl and the products
of its dehydrogenation on catalyst no. 2 are presented
in Fig. 2.
RESULTS AND DISCUSSION
The hydrogenation of m-terphenyl yielded a sub-
strate consisting of two perhydro-m-terphenyl isomers
Analysis of the product yield of perhydro-m-ter-
(I, 17.3% of isomer; II, 78.0% of isomer), 3.8% of the phenyl dehydrogenation shows that the formation of
initial m-terphenyl, and 0.9% of partially hydroge- final m-terphenyl predominated at temperatures
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KINETIC AND THERMODYNAMIC PARAMETERS
1923
higher than 300°C. At the same time, the content of using the KINET 0.8 mathematical modeling program.
partially hydrogenated trans-isomer compounds is This program optimizes the values of the constants up
greatly superior to the content of cis-isomer derivatives to the best agreement between calculated and experi-
in the total volume over the given range of temperatures. mental kinetic curves. Using the experimental data pre-
The rate constants of all elementary acts of this reaction sented above, the optimized scheme of perhydro-m-ter-
were determined from the obtained experimental data phenyl dehydrogenation takes the form
cis-
C18H32
k1
k−1
k2
k3
k4
k−5 k5
C18H26 + 3H2
C18H20 + 3H2
C18H14 + 3H2.
k−3
k−4
k−2
C H
18 32
trans-
Dehydrogenation proceeds with the simultaneous and values on the inactive catalyst at 300°C and were KI =
mutual transformations of cis- and trans-isomers of
perhydro-m-terphenyl and the formation of the final
m-terphenyl (С18Н14) and the products of partial
hydrogenation, which contain one unsaturated six-
membered ring (С18Н26, dicyclohexylbenzene and
1-cyclohexyl-3-phenylcyclohexane) or two unsatu-
rated rings (С18Н20, 1,3-diphenylcyclohexane and
3-cyclohexylbiphenyl).
It is known that differential equation systems
describing the kinetics of chemical reactions have ana-
lytical solutions in the simplest cases. We therefore
used a simplified model of perhydro-m-terphenyl
dehydrogenation (A) first. All elementary acts of the
reaction were reduced to three: mutual isomerization
of the cis- and trans- perhydro-m-terphenyl (I, cis-
С18H32 ↔ trans-C18H32); 1,3-dehydrogenation of the
1.93 and K−I = 0.52, respectively; KI = 9.38 and K–I
=
0.11 when Т = 320°C. At the same time, KII/KIII ~ 2 at
both temperatures.
Simplified dehydrogenation model (A) considered
above was subsequently complicated by adding experi-
mental data for other products obtained from the reac-
tion. In enhanced model (B), we considered reaction
acts I, cis-С18H32 ↔ trans-C18H32); II, cis-С18H32 ↔
+
Н2); III, trans-C18H32
H2); IV, cis-C18H26
↔
↔
(cis-С18Н26
∑
∑
∑
∑
+
(trans-С18Н26
+ СnHm + H2); and V, trans-C18H26 ↔
(C18H20−14
+ СnHm + H2), with allowance in partic-
(C18H20−14
ular for the formation of two isomers of a partial
hydrogenation product (С18Н26). The comparative
range of equilibrium constants calculated using this
model for catalyst no. 3 at Т = 300°C was KI = 1.69 <
cis-isomer (II, cis-С18H32 ↔
+ СnHm +
(C18H26−14
∑
H2)); and the dehydrogenation of the trans-isomer
KII = 17.15 < KIII = 25.34 ! KV = 230.17 ! KIV
=
(III, trans-C18H32 + СnHm + H2)).
↔
(C18H26−14
∑
523.31. The order of the constants obtained for catalyst
no. 2 was the same. A slight increase in equilibrium
constant KI was observed on both nos. 2 and 3 as the
temperature rose. At the same time, ratio KIV/KV grew
significantly, reaching its maximum at Т = 320°C. KII
tended toward zero as the temperature rose and KIII
fell, reaching its maximum at Т = 310°C.
The cis- and trans-isomers of perhydro-m-terphenyl
were considered separately as substrates, and the sums
of all products (С18Н26, С18Н20, and С18Н14) and co-
products (СnHm) of dehydrogenation were (along with
hydrogen, expressed in g/(L h)) considered products
of the reaction. This model allowed us to assess most
accurately the effect cis-trans-isomerization had on
dehydrogenation.
Model (C) includes all products of the reaction
according to the optimized scheme of perhydro-m-ter-
phenyl dehydrogenation given above and can be written
Equilibrium constants were calculated from the
determined rate constants of the elementary acts of the
reaction. The equilibrium constants of the mutual
isomerization of cis- and trans-isomers on catalyst
as I, cis-С18H32 ↔ trans-C18H32; II, cis-С18H32
↔
↔
↔
+
+
Н2); III, trans-C18H32
H2); IV, cis-C18H26
(cis-С18Н26
∑
∑
∑
no. 2 at 260°C were estimated at KI = 7.90 and K−I
=
+
(trans-C18H26
0.013. The equilibrium constants of the dehydrogena-
tion of cis- and trans-isomers were KII = 293.07 and
KIII = 252.61, respectively. The values of all equilibria
constants grew as the temperature was raised to Т =
+ H2); V, trans-C18H26 ↔
(C18H20
(C18H20
∑
H2); and VI, C18H20 ↔
+ СnHm + H2). The
comparative range of the equilibrium constants calcu-
(C18H14
∑
300°C, and were KI = 11.39, KII = 1014.52, and KIII
=
lated using this model at Т = 260°C for catalyst no. 2
492.46, respectively. They remained virtually the same
as the temperature was raised further. The calculated
equilibrium constants of the transformation of cis-per-
was KI = 2.13 < KII = 19.22 < KVI = 31.32 < KIII
=
35.95 < KV = 58.62 ! KIV = 229.38. This range
hydro-m-terphenyl into trans-isomer attained lower changes somewhat as the temperature rises to Т =
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 10 2016

1924
KALENCHUK et al.
ferences between cis- and trans-perhydro-m-terphenyl
lnKp
on the dehydrogenation reaction. We may therefore
conclude that the dehydrogenation of perhydro-m-
terphenyl differs substantially from that of decalin.
25
VI
IV
20
15
10
5
CONCLUSIONS
The dehydrogenation of perhydro-m-terphenyl
allows us to obtain high levels of conversion in a flow
reactor. The rate of dehydrogenation for the more
active cis-isomer is higher than that for the trans-iso-
mer over the considered range of temperatures. How-
ever, we found no significant differences between the
effects the cis- and trans-isomery of perhydro-m-ter-
phenyl have on dehydrogenation. The equilibrium
shifts toward the transformation of trans-perhydro-m-
terphenyl into cis-isomer at lower temperatures.
III
V
0
I
II
−5
1.90
103/T, K−1
1.74
1.78
1.82
1.86
Fig. 3. Perhydro-m-terphenyl dehydrogenation equilib-
rium constants K , K , K , K , K , and K , calculated
ACKNOWLEDGMENTS
This work was supported by the Ministry of Educa-
tion and Science, contract no. RFMEFI61314X0012.
I
II
III
IV
V
VI
using the KINET program.
300°C: KII = 0.15 ≤ KV = 0.32 < KIII = 299 < KI = 427 <
KIV = 25.45 ! KVI = 2.39 × 107. The position of the
calculated equilibrium constants is almost the same
for the constants obtained for catalyst no. 3.
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Translated by P. Vlasov
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