Palladium hydrogenation catalyst based on a porous carbon material obtained upon the dehydrochlorination of a chloro polymer

Article abstract of DOI:10.1134/S0023158416020063

The applicability of a porous carbon material obtained as a result of the “chemical” dehydrochlorination of chlorinated polyvinyl chloride as a support for palladium hydrogenation catalysts was demonstrated. The efficiency of the catalyst was evaluated in the liquid-phase reactions of nitrobenzene hydrogenation and chlorobenzene hydrodechlorination. Although the specific activity of the catalyst was lower by a factor of 3–4 than that of the samples based on Sibunit and carbon nanotubes, the complete conversion of the initial compounds with the selective formation of end products under mild conditions was achieved at a relatively low palladium content (1.5%).

Full text of DOI:10.1134/S0023158416020063

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2016, Vol. 57, No. 2, pp. 229–233. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © R.M. Mironenko, O.B. Belskaya, V.S. Solodovnichenko, T.I. Gulyaeva, Yu.G. Kryazhev, V.A. Likholobov, 2016, published in Kinetika i Kataliz, 2016,
Vol. 57, No. 2, pp. 223–227.
Palladium Hydrogenation Catalyst Based on a Porous Carbon
Material Obtained upon the Dehydrochlorination
of a Chloro Polymer
R. M. Mironenkoa, b, *, O. B. Belskaya , V. S. Solodovnichenko , T. I. Gulyaeva ,
a, c
a, d
a
a, d
a, d
Yu. G. Kryazhev , and V. A. Likholobov
a
Institute of Hydrocarbons Processing, Siberian Branch, Russian Academy of Sciences, Omsk, 644040 Russia
b
Dostoevsky Omsk State University, Omsk, 644077 Russia
c
Omsk State Technical University, Omsk, 644050 Russia
d
Omsk Scientific Center, Siberian Branch, Russian Academy of Sciences, Omsk, 644024 Russia
*
eꢀmail: chꢀmrm@mail.ru
Received July 1, 2015
Abstract—The applicability of a porous carbon material obtained as a result of the “chemical” dehydrochloꢀ
rination of chlorinated polyvinyl chloride as a support for palladium hydrogenation catalysts was demonꢀ
strated. The efficiency of the catalyst was evaluated in the liquidꢀphase reactions of nitrobenzene hydrogenaꢀ
tion and chlorobenzene hydrodechlorination. Although the specific activity of the catalyst was lower by a facꢀ
tor of 3–4 than that of the samples based on Sibunit and carbon nanotubes, the complete conversion of the
initial compounds with the selective formation of end products under mild conditions was achieved at a relꢀ
atively low palladium content (1.5%).
Keywords: chlorinated PVC, carbon supports, palladium catalysts, nitrobenzene hydrogenation, chlorobenꢀ
zene hydrodechlorination
DOI: 10.1134/S0023158416020063
INTRODUCTION
modal poreꢀsize distribution, which makes it possible to
use them for the separation of small molecules in gas
separation processes [4]. Another method for the proꢀ
duction of porous carbon materials with large
micropore and mesopore volumes is based on the
Polymeric chlorineꢀcontaining materials are in wide
current use because of their performance characterisꢀ
tics. Polyvinyl chloride (PVC), the world output of
which is greater than 35 million tons per year [1], is one
of these materials. Although the products based on PVC
exhibit a prolonged service life, the amount of environꢀ
mentally hazardous wastes obtained from them is growꢀ
ing every year. The problem is acute because all of the
chlorineꢀcontaining organic compounds are xenobiotꢀ
ics, and the unqualified combustion of chlorineꢀbearing
wastes or their spontaneous combustion unavoidably
lead to the formation of highly toxic compounds such as
phosgene and dioxins [1, 2]. Currently, attention has
been focused on the utilization and repeated use of
chlorineꢀcontaining plastics [1–3]. Among many
methods for the conversion of chloro polymers, those
which make it possible to obtain calledꢀfor products
“
chemical” dehydrochlorination of chlorinated carꢀ
bonꢀchain chloro polymers, such as chlorinated PVC
CPVC), under the action of alkalis in organic media.
(
A porous carbon material is formed after the carbonizaꢀ
tion of the resulting product under relatively mild conꢀ
ditions and subsequent activation [5]. In this case, the
porous structure of the material obtained can be reguꢀ
lated over a wide range depending on the structure of a
polymer precursor and the conditions of carbon formaꢀ
tion. The porous carbon materials thus obtained are
promising adsorbents, in particular, for medical purꢀ
poses [5].
Carbon materials are frequently used as supports in
upon the utilization of wastes are especially attractive the synthesis of heterogeneous catalysts. The wide
2, 3]. One of these methods is the dehydrochlorination application of carbon supports in catalysis is caused by
[
of chloro polymers with the production of porous carꢀ their large specific surface areas and the presence of a
bon materials. Thus, as a result of the heat treatment of developed pore system, which facilitate the transfer of
the polymers and copolymers of 1,1ꢀdichloroethylene reactants and reaction products. The supports of this
for many hours, their thermal dehydrochlorination type possess tunable surface chemical properties and
occurs with the formation of porous carbon materials chemical inertness, especially in strong acidic and
with an ultramicroporous structure and a narrow uniꢀ basic media [6, 7]. Among them are various forms of
229

230
MIRONENKO et al.
Table 1. Porous structure parameters of the carbon supports
2
3
3
Support
CꢀCPVC
SBET, m /g
V
, cm /g
Vmicro, cm /g
dav, nm
Σ
756
196
354
0.71
1.49
0.60
0.23
0.01
0.03
3.8
30.3
6.7
CNT
Sibunit
porous carbon materials: activated carbons, carbon sorption analyzer (Micromeritics). Before the meaꢀ
black of different grades, carbon nanotubes (CNT), surement of dispersion, the reduced samples were
carbon nanofibers, etc.
pretreated in a flow of a mixture containing 10% Н2
in Ar under temperatureꢀprogrammed conditions in
a temperature range of 303–393 K (rate of heating,
Here, we consider the applicability of a porous carꢀ
bon material obtained by the “chemical” dehydrochloꢀ
rination of CPVC as a palladium catalyst support for the
reactions of nitrobenzene hydrogenation and chloꢀ
robenzene hydrodechlorination. The liquidꢀphase catꢀ
1
0 K/min), and the signals of the thermalꢀconducꢀ
tivity detector were measured.
The liquidꢀphase hydrogenation of nitrobenzene
alytic hydrogenation of nitrobenzene was studied in sufꢀ (10 mmol) was performed in the presence of Pd/C catꢀ
ficient detail. This reaction is an industrial method for alysts (200 mg) in a steel autoclave at a temperature of
the production of aniline, a largeꢀscale product of 323 K and a pressure of 0.5 MPa in an ethanol solution
organic synthesis [8–10]. The hydrodechlorination of (100 mL). Before each particular experiment, the catꢀ
chlorobenzene is frequently used as a model reaction in alyst was activated with hydrogen (323 K, 0.5 MPa) for
the studies of the hydrogenolysis of C–Cl bonds in 0.5 h under conditions of stirring. The course of the
organochlorine compounds. Furthermore, this reacꢀ reaction was judged from the volume of hydrogen conꢀ
tion can be used for the testing of metallic catalysts, sumed in a specific time interval.
which are potentially capable of manifesting high effiꢀ
ciency in hydrogenation processes for the neutralizaꢀ
tion of toxic organochlorine compounds [11–16].
The hydrodechlorination of chlorobenzene
10 mmol) was performed analogously at a temperaꢀ
(
ture of 363 K and a pressure of 0.5 MPa in the presence
of Pd/C (200 mg); however, in this case, an ethanol–
water mixture (1 : 1) with an additive of NaOH (for the
binding of hydrogen chloride formed upon the hydroꢀ
dechlorination) served as a reaction medium.
After the completion of reactions (i.e., after the
cessation of hydrogen consumption), the products
were analyzed by gas chromatography–mass specꢀ
trometry (on an Agilent 5973N/6890N instrument)
and gas–liquid chromatography (on a Hewlett Packꢀ
EXPERIMENTAL
The porous carbon material obtained by the “chemꢀ
ical” dehydrochlorination of CPVC under the action of
KOH in a dimethyl sulfoxide solution followed by heat
treatment was used as a support for a palladium catalyst.
Henceforth, this material is referred to as CꢀCPVC.
Baytubes C 150 HP multiwall CNT and a Sibunit
carbon material were also used for the preparation of
catalysts. These supports are radically different in
morphology, and they are frequently used in research
works for the preparation of hydrogenation catalysts.
The porous structure characteristics of the carbon supꢀ
ports, namely, the BET specific surface area (SBET),
ard 5890 Series II instrument). The activity of the catꢀ
alysts was expressed as a turnover frequency (TOF) in
−1
Pd
molH2 mol s–1
.
RESULTS AND DISCUSSION
the total adsorption pore volume (V ), the micropore
Σ
volume (Vmicro), and the average pore diameter ( av
d
),
The study of the carbon supports by the lowꢀtemꢀ
were found by an analysis of the isotherms of nitrogen perature adsorption–desorption of nitrogen showed
adsorption–desorption at 77 K, which were measured that the porous carbon materials chosen for compariꢀ
on an ASAPꢀ2020 surface area and porosimetry anaꢀ son have a mesoporous structure with relatively uniꢀ
lyzer (Micromeritics, United States).
form pores. The difference between CNT and Sibunit
consists in a smaller specific surface area of CNT at a
considerably larger average pore size (Table 1).
The supported palladium catalysts containing
1
.5 wt % Pd were prepared by the incipient wetness
impregnation of carbon supports with aqueous soluꢀ
The carbon material obtained upon the dehydroꢀ
tions of H PdCl4. The preliminary treatment of the chlorination of CPVC was essentially different in texꢀ
2
catalysts included their drying in a flow of argon at a ture from CNT and Sibunit. In contrast to the latter,
temperature of 423 K for 0.5 h and reduction in a CꢀCPVC possessed a much higher specific surface
flow of hydrogen at 523 K for 2 h. The dispersion (
of the supported metal in the reduced Pd/C samples cific surface area of CꢀCPVC is caused by an increased
was determined by the pulse chemisorption of CO at fraction of micropores (more than 30% on an basis)
D
)
area with a small average pore diameter. The high speꢀ
V
Σ
room temperature on an AutoChem II 2920 chemiꢀ (Table 1). Note that the porous structure of a support
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PALLADIUM HYDROGENATION CATALYST
231
Table 2. Catalytic properties of Pd/C samples in the liquidꢀphase reactions of nitrobenzene hydrogenation and chlorobenꢀ
zene hydrodechlorination under a pressure of hydrogen
Nitrobenzene hydrogenation*
Chlorobenzene hydrodechlorination**
Catalyst
D
xNB, %
Sa, %
TOF, s–1
xCB, %
Sb, %
TOF, s–1
1
1
1
.5%Pd/CꢀCPVC
0.22
0.52
100
100
100
98
98
98
1.4
5.5
5.8
100
100
100
99
98
97
1.3
4.2
3.4
.5%Pd/CNT
.5%Pd/Sibunit
0.54
*
Reaction conditions:
T
= 323 K; P = 0.5 MPa; 100 mL of ethanol; 10 mmol of C H NO ; 200 mg of catalyst; C H NO :
6 5 2 6 5 2
Pd = 353 (mol/mol); xNB is the conversion of nitrobenzene; and Sa is the selectivity of aniline formation (based on the chromatoꢀ
graphic analysis data).
*
* Reaction conditions:
T
6
= 363 K; P = 0.5 MPa; 100 mL of a mixture of ethanol–water (1 : 1); 10 mmol of NaOH; 10 mmol of C H Cl;
6 5
5
200 mg of catalyst; C H Cl : Pd = 353 (mol/mol); xCB is the conversion of chlorobenzene; and Sb is the selectivity of benzene forꢀ
mation (based on the chromatographic analysis data).
plays an important role in the formation of metal sites hydrogen was noticeably smaller than that correꢀ
and their stabilization on the support surface and in sponding to the reaction stoichiometry of chlorobenꢀ
the mass transfer of reactants and products within a zene conversion into benzene, especially, on the
catalyst grain in the course of reaction [6, 7].
Pd/CꢀCPVC catalyst, in the presence of which it did
not exceed 54% on a calculated basis (Fig. 1b).
According to the results of chromatographic analyꢀ
sis, all of the 1.5%Pd/C catalysts tested in the reaction
of nitrobenzene hydrogenation under a pressure of
hydrogen ensured the complete conversion of nitrobenꢀ
zene with the selective formation of aniline (Table 2). In
this case, the amount of consumed hydrogen was nearly
stoichiometric for the reaction of nitrobenzene converꢀ
sion into aniline (Fig. 1a). The yield of heavy unidentiꢀ
fied products was about 2%.
The reason for this can be the participation of
hydrogen, which was previously absorbed by the cataꢀ
lyst in the process of its activation before conducting
the experiments, in the reaction. Indeed, in the course
of the preliminary reduction of catalysts under temꢀ
peratureꢀprogrammed conditions, a peak due to the
release of hydrogen with a maximum at 352 K was
detected (Fig. 2). In this case, the amount of hydrogen
The study of the catalysts in the liquidꢀphase reacꢀ desorbed from the Pd/CꢀCPVC sample was considerꢀ
tion of chlorobenzene hydrodechlorination under a ably greater than those desorbed from the Pd/CNT
pressure of hydrogen also showed their high efficiency and Pd/Sibunit samples with the same supported metal
(
Table 2): the complete conversion of chlorobenzene contents. This may be related to the different dispersion
with the selective formation of benzene (cyclohexane of palladium, which was lower for Pd/CꢀCPVC by a
and biphenyl were detected among the byꢀproducts) factor of >2 than that for Pd/CNT and Pd/Sibunit.
was achieved. However, the amount of consumed Coarse palladium particles absorb hydrogen with the
(
а)
(b)
Amount of consumed H2,
mol/(mol chlorobenzene)
Amount of consumed H2,
mol/(mol nitrobenzene)
3
1.0
3.0
2.5
2.0
1.5
1.0
0.5
2
1
2
3
0.8
0.6
0.4
0.2
1
0
10 20 30 40 50 60 70
Time, min
0
5
10
15
20
25 30
Time, min
Fig. 1. Curves of hydrogen consumption in the course of the reactions of (a) nitrobenzene hydrogenation and (b) chlorobenzene
hydrodechlorination in the presence of the following catalysts: ( ) 1.5%Pd/CꢀCPVC, ( ) 1.5%Pd/CNT, and ( ) 1.5%Pd/Sibunit.
1
2
3
KINETICS AND CATALYSIS Vol. 57
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232
MIRONENKO et al.
the test reactions than the samples of Pd/Sibunit and
Pd/CNT, which differ from each other only slightly
in catalytic activity at close metal dispersity values
Thermalꢀconductivity
detector signal, arb. units
3
2
(D = 0.5) (Table 2). The reason for the lowered speꢀ
cific catalytic activity of Pd/CꢀCPVC can be the localꢀ
ization of metal particles in the micropores of the supꢀ
port, as a result of which the working surface area of
the metal decreases and a portion of supported Pd
becomes inaccessible to substrate (nitrobenzene and
chlorobenzene) molecules. The effect of blocking the
supported metal in the narrow pores of the support was
described earlier [19, 20] in a study of Pt/C and Pd/C
catalysts prepared with the use of different carbon supꢀ
ports. Thus, according to Okhlopkova et al. [19],
1
352 К
coarsely dispersed palladium particles (D = 0.19) were
formed on a carbon support with the developed system
of micropores (activated carbon), and the resulting
catalyst manifested specific catalytic activity lower by
a factor of almost 2 than that of a sample based on
mesoporous Sibunit, which was characterized by the
3
10 320 330 340 350 360 370 380 390
Temperature, K
Fig. 2. Desorption of hydrogen under the conditions of the
preliminary reduction of the ( ) 1.5%Pd/CꢀCPVC,
) 1.5%Pd/carbon nanotubes, and ( ) 1.5%Pd/Sibunit
1
value of
D = 0.51, in the reaction of nitrobenzene
(
2
3
catalysts with a mixture of 10% O in Ar in the temperaꢀ
hydrogenation.
2
tureꢀprogrammed mode in a range of 303–393 K.
The lower dispersion of the supported metal in the
sample of Pd/CꢀCPVC can also be explained by the
formation of hydride even at room temperature and a presence of residual chlorine on the support surface.
partial pressure of Н2 of 2.27 kPa [17]. The release of Earlier, it was established [21] that the presence of difꢀ
hydrogen, which was frequently observed under the ferent halide ions on the surface of a carbon support
conditions of the temperatureꢀprogrammed reduction affects both the size of palladium particles formed and
of palladium catalysts at
Т
< 393 K, was related to the the particle size distribution. For example, after the
decomposition of palladium hydride [14, 18]. The preliminary adsorption of chloride ions on activated
much more intense release of previously absorbed carbon, the resulting Pd particles acquired a larger size
hydrogen from the Pd/CꢀCPVC sample (Fig. 2) can be than that without the use of this adsorption.
the reason for the lower degree of Н2 conversion in the
hydrodechlorination reaction. The same effect was also
observed in the hydrogenation of nitrobenzene in the
presence of Pd/CꢀCPVC (Fig. 1a); however, in this
case, it was much smaller (the amount of consumed
CONCLUSIONS
Thus, the CꢀCPVC carbon material obtained by
hydrogen was 87% on a calculated basis) because, at this the dehydrochlorination of CPVC can be used as a
reaction temperature (323 K), it seems likely that the support for the synthesis of palladium catalysts for
desorption of hydrogen absorbed in the course of actiꢀ hydrogenation. A comparatively high specific surface
vation was insignificant.
area and the presence of a considerable fraction of
micropores (more than 30% on a total pore volume
basis) are its special features. The complete conversion
of starting compounds with the selective formation of
target products was achieved on the 1.5%Pd/CꢀCPVC
catalyst in the reactions of nitrobenzene hydrogenaꢀ
tion (323 K, 0.5 MPa) and chlorobenzene hydroꢀ
dechlorination (363 K, 0.5 MPa). In contrast to the
samples of Pd/CNT and Pd/Sibunit, the above catalyst
exhibited lower (by a factor of 3–4) specific catalytic
activity. Furthermore, with the use of Pd/CꢀCPVC, the
deviation of hydrogenation and hydrodechlorination
reactions from stoichiometry was observed; this fact
can be explained by the presence of a considerable
amount of previously accumulated hydrogen in the
We also cannot exclude the solvent effect. The
deviations of hydrogenation and, especially, hydroꢀ
dechlorination reactions from stoichiometry can also
be explained by the participation of ethanol in the
reactions. It is well known that aliphatic alcohols can
serve as the donors of hydrogen, thus facilitating the
reduction of a substrate [12, 15]. Thus, in a study of the
hydrodechlorination of chlorobenzene on a Pd/C catꢀ
alyst in an ethanol solution, Yakovlev et al. [12] found
that an additional quantity of hydrogen, which particꢀ
ipated in the transformations of chlorobenzene, was
generated as a result of the dehydrogenation of ethanol
to the acetate ion.
From an analysis of the results obtained, it follows catalyst or by the participation of ethanol as a donor of
that the catalyst based on CꢀCPVC with the low disꢀ hydrogen in the reaction. In the future, we intend to
persity of supported palladium (D = 0.2) is less active examine the reasons for these special features of the
by a factor of 3–4 (in terms of a surface Pd atom) in Pd/CꢀCPVC system.
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PALLADIUM HYDROGENATION CATALYST
233
ACKNOWLEDGMENTS
9. Kozlov, A.I. and Zbarskii, V.L., Ross. Khim. Zh., 2006,
vol. 50, no. 3, p. 131.
We are grateful to E.N. Kudrya for her assistance in
the identification of hydrogenation products by gas
chromatography–mass spectrometry. The samples
were analyzed and the reaction products were identiꢀ
fied with the use of the equipment of the Center of
Collective Use at the Omsk Scientific Center, Siberian
Branch, Russian Academy of Sciences.
1
0. Willocq, C., Dubois, V., Khimyak, Y.Z., Devillers, M.,
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Gentsler, A.G., Komova, O.V., and Netskina, O.V.,
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Translated by V. Makhlyarchuk
KINETICS AND CATALYSIS Vol. 57
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