Pd/Al2O3 catalyst for selective hydrogenation of benzene in benzene-toluene mixture

Article abstract of DOI:10.1016/j.mencom.2009.03.020

Pd/Al₂O₃ demonstrates (in comparison with Pt/Al₂O₃ and Ni/Al₂O₃) higher selectivity toward benzene hydrogenation in competitive hydrogenation of a benzene-toluene mixture, which can be enha

Full text of DOI:10.1016/j.mencom.2009.03.020

Mendeleev
Communications
Mendeleev Commun., 2009, 19, 108–109
Pd/Al O catalyst for selective hydrogenation
2
3
of benzene in benzene–toluene mixture
a
a
a
Igor S. Mashkovsky, Galina N. Baeva, Aleksandr Yu. Stakheev,*
b
b
Timur V. Voskoboynikov and Paul T. Barger
a
b
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
Fax: +7 499 135 5328; e-mail: st@ioc.ac.ru
UOP LLC, A Honeywell Company, Des Plaines, IL 60017-5017, USA
DOI: 10.1016/j.mencom.2009.03.020
Pd/Al O demonstrates (in comparison with Pt/Al O and Ni/Al O ) higher selectivity toward benzene hydrogenation in
2
3
2
3
2
3
competitive hydrogenation of a benzene–toluene mixture, which can be enhanced (up to 76%) by the increase of overall reaction
pressure up to 37 atm.
To limit environmental pollution, many countries all over the
world are developing and enacting very stiff standards for
the acceptable content of harmful substances in the exhaust
gases of automotive vehicles. The most stringent restrictions
are imposed on benzene due to the carcinogenicity of its
combustion materials for humans. The standard EN 228 (Euro-3)
accepted in Europe (2000) limits benzene content in motor
where mbenzene and mtoluene are the molar ratios of benzene and
toluene in aromatics mixture, respectively; convbenzene and convtoluene
are the benzene and toluene conversions, respectively.
Selectivity toward benzene hydrogenation [Sbenzene (%)] was
calculated by the following equation:
mbenzeneconvbenzene
Sbenzene
=
×100.
Aconv
1
gasoline at most 1%. In the USA (1998) benzene content in
2
,3
petroleum restricted to 0.8%. The standard technique to
remove benzene from gasoline is pressure swing adsorption.4–6
Alternatively, the benzene content can be decreased by selective
benzene hydrogenation over a metal catalyst. However, selective
hydrogenation of the benzene ring in gasoline is hindered by
the preferable adsorption of substituted aromatics contained
in the motor fuel on the catalyst surface,7,8 which results in
preferable hydrogenation of the substituted molecules. On
the other hand, it was demonstrated that this effect strongly
The dependences of the selectivity in benzene hydrogena-
tion on the overall aromatics conversion for 0.9% Pt/Al O ,
2
3
1.6% Pd/Al O , and 3.1% Ni/Al O are shown in Figure 1. We
2
3
2
3
used different metal concentrations to equalize the catalytic
activity of these samples and to compare catalytic performance
in the same temperature range. It should be noted that all
catalysts demonstrated very similar activities in hydrogenation
of the model mixture. Comparing the data obtained for Ni/Al O
2
3
and Pt/Al O catalysts in terms of selectivity for benzene hydro-
2
3
9
depends on the electronic structure of the metal. The most
genation, we can conclude that the performance of Ni/Al O is
2 3
favourable results were obtained for Pt and Pd catalysts. There-
fore, in this study we explore the catalytic properties of Pt/Al O ,
very similar to that of the Pt/Al O catalyst. At low aromatics
2 3
conversion, toluene hydrogenation definitely prevails over benzene
hydrogenation. Thus, selectivity towards benzene hydrogena-
tion at low conversion is ~30–35%. With increasing aromatics
conversion, selectivity for benzene hydrogenation increases
gradually. However, over the whole range of aromatics conver-
2
3
Pd/Al O and Ni/Al O in hydrogenation of a benzene–toluene
2
3
2
3
mixture as a model reaction and the influence of the most
important factors, which can affect the selectivity of the process:
(
(
1) reaction temperature, (2) overall aromatics conversion and
3) overall reaction pressure. A choice of Ni was dictated by a
similarity of its electronic structure to Pd and Pt due to their
position in the Periodic Table.
The Pt-, Pd- and Ni-containing catalysts were prepared
by incipient-wetness impregnation with a water solutions of
7
6
5
4
0
0
0
0
[
Pt(NH ) ]Cl , [Pd(NH ) ](NO ) and Ni(NO ) , respectively.
3 4 2 3 4 3 2 3 2
Additional experiments revealed that the nature of the precursor
chloride or nitrate) does not notably affect the catalytic data.
The resulting material was calcined in flowing air at 500 °C
2 h) and reduced in H2 flow at 350 °C (1 h). Competitive
(
(
hydrogenation of a benzene–toluene mixture (1:1 molar ratio)
was carried out at a total aromatic partial pressure ~60 Torr, and
a hydrogen partial pressure of 670 Torr. Catalytic performance
of each catalyst was evaluated at two temperatures: 110 and
30
2
0
0
20
40
60
80
100
–
1
1
60 °C by changing LHSV (from 0.25 to 1 h ) in order to
A
conv (%)
ensure variation of the overall aromatics conversion over a wide
range (2–3% to 90–100%).
Overall aromatics conversion [Aconv (%)] was calculated by
the following expression:
Figure 1 Dependence of selectivity in benzene hydrogenation on the
overall aromatics conversion for Pd/Al O ( , ), Pt/Al O ( , ) and
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3
2
3
Ni/Al O ( , ) catalysts in the hydrogenation of benzene–toluene mixture
2 3
(1:1 molar ratio). Open symbols, reaction at 110 °C; solid symbols,
reaction at 160 °C.
Aconv = (mbenzeneconvbenzene + mtolueneconvtoluene)×100,
©
2009 Mendeleev Communications. All rights reserved.
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Mendeleev Commun., 2009, 19, 108–109
sion, Sbenzene remains lower than 50%. This implies that toluene
hydrogenation prevails.
sample we examined the influence of elevated pressure (1–37 atm)
on the catalyst selectivity in terms of benzene hydrogenation.
The basic idea behind this experiment was the following. As it
was discussed above, the main reason of the low selectivity in
the presence of substituted aromatics lies in the stronger
adsorption of substituted aromatics on the metal surface. Thus,
substituted aromatics block the metal, making the surface
inaccessible for benzene. By increasing the overall reaction
pressure, it is possible to replace a part of aromatics with
adsorbed hydrogen. Thus, there is a chance to liberate a part of
the metal surface from substituted aromatics and make it
accessible for benzene adsorption.
The data on the competitive hydrogenation of the benzene–
toluene mixture at elevated pressure (Figure 2) indicate the
steady increase in the reaction selectivity toward benzene hydro-
genation with the increase in the overall reaction pressure. The
most encouraging results were obtained in the experiments
carried out at 37 atm. As can be seen in Figure 2, even when
overall aromatics conversion reaches 40%, selectivity for benzene
hydrogenation exceeds 70%.
The performance of Pd/Al O differs significantly from the
2
3
performance of Ni- and Pt-containing catalysts. The selectivity
of Pd/Al O for benzene hydrogenation is greater than 50%
2
3
over the entire range of aromatics conversion (Figure 1). This
implies that Pd catalyst shows a tendency to preferably
hydrogenate benzene. This tendency is particularly pronounced
at low conversion (Figure 1). With increasing overall aromatics
conversion, selectivity for benzene hydrogenation diminishes.
However, it remains above 50% over the whole conversion
range. Only cyclohexane and methylcyclohexane were identified
as products. Neither isomerisation nor cracking was observed.
The data obtained can be explained by the difference in the
adsorption coefficients of benzene and substituted aromatics.9
The ratio KT/B of the adsorption coefficients of toluene and
benzene determined from a kinetic analysis of the competitive
hydrogenation of these hydrocarbons differs for the Pt and Pd
9
catalysts. For the Pd-containing catalyst K = 1, which means
T/B
that benzene is as strongly adsorbed as toluene, whereas KT/B = 8
for the Pt-containing catalyst corresponding to a prevailing
Clearly, the selectivity of benzene hydrogenation can be
effectively improved by increasing overall reaction pressure.
This fact can be explained by the increase of the surface
concentration of the adsorbed hydrogen on the metal surface
with an increase in the overall reaction pressure. This leads to a
liberating of the metal surface from strongly adsorbed substi-
tuted aromatics and makes the metal surface more accessible
for adsorption of benzene molecules.10
toluene adsorption on the vacant Pt sites. The K ratios follow
T/B
9
in the range Pd < Pt < Rh < Ir << Os < Ru. In spite of the fact
that KT/B ratio was not calculated for Ni-containing catalyst, our
data indicate that its value is close to that of Pt. Analyzing the
effect of the reaction temperature on the selectivity of benzene
hydrogenation for the Ni/Al O , Pt/Al O and Pd/Al O catalysts
2
3
2
3
2
3
(
Figure 1), we can conclude that Sbenzene is not dependent on the
reaction temperature (at least in the investigated temperature
range). The selectivity for benzene hydrogenation is governed
by the overall aromatics conversion, and the reaction tempera-
Analysis of reaction thermodynamics showed that toluene
hydrogenation is favorable compared to benzene. However, the
reaction is not affected by thermodynamics since the reaction
temperature is significantly lower as compared to temperature
when thermodynamics limitations become significant (> 200 °C
at P = 1 atm, > 350 °C at P = 37 atm).
8
ture does not influence on the dependence of Sbenzene on Aconv
.
Presumably, the observed selectivity independence of tempe-
rature is due to a constancy of KT/B ratio within temperature
range studied.
According to the experimental data obtained, Pd appears to
be the most promising metal in terms of selectivity in benzene
hydrogenation. These data are in a good agreement with the
We are grateful to UOP LLC for the profound interest and
support of this research.
8
literature. However, Poondi and Vannice analyzed the kinetic
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Figure 2 Effect of the overall reaction pressure on the performance of
Pd/Al O catalyst in the hydrogenation of benzene–toluene mixture (1:1 molar
ratio): ( ) 1 atm, ( ) 11 atm, ( ) 23 atm and ( ) 37 atm.
2
3
Received: 8th September 2008; Com. 08/3213
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