Direct conversion of aromatics into a synthetic steamcracker feed using bifunctional zeolite catalysts

Article abstract of DOI:10.1039/b003422n

Aromatics with 6 to 9 carbon atoms can be converted catalytically with hydrogen on bifunctional zeolites, such as Pd/H-ZSM-5, into a high-quality steamcracker feed, thereby opening a direct route for the utilisation of surplus pyrolysis gasoline.

Full text of DOI:10.1039/b003422n

Direct conversion of aromatics into a synthetic steamcracker feed using
bifunctional zeolite catalysts
a
a
a
b
b
Jens Weitkamp,* Andreas Raichle, Yvonne Traa, Martin Rupp and Franz Fuder
a
Institute of Chemical Technology, University of Stuttgart, D-70550 Stuttgart, Germany.
E-mail: jens.weitkamp@po.uni-stuttgart.de
VEBA OEL AG, D-45876 Gelsenkirchen, Germany
b
Received (in Cambridge, UK) 28th April 2000, Accepted 19th May 2000
Aromatics with 6 to 9 carbon atoms can be converted
catalytically with hydrogen on bifunctional zeolites, such as
Pd/H-ZSM-5, into a high-quality steamcracker feed, thereby
opening a direct route for the utilisation of surplus pyrolysis
gasoline.
is converted with a yield of 72.8% into C2+-n-alkanes (cf. Table
1, column 4). It is noteworthy that no aromatics occur in the
product under any reaction conditions, and very little methane is
formed even at 400 °C. For various reasons (high hydrogen
consumption and exothermicity, methane does not form
ethylene or propylene in the steamcracker), methane is a very
undesired product.
In view of the predicted over-supply of aromatics,1 novel
catalytic routes for their conversion into valuable products are
urgently needed. One of the major sources of aromatics is so-
called pyrolysis gasoline, a by-product of steamcracking in
which ethylene and propylene are made from light hydro-
Upon increasing the palladium content of the catalyst from
0.2 to 0.5 and 1.0 wt.%, the yields at a reaction temperature of
400 °C do not change significantly. From this we conclude that,
under the conditions applied in this work, ring opening and the
formation of light alkanes proceed via the bifunctional hydro-
2
3
carbons, such as straight-run naphtha, LPG or ethane. In a
4
13
14
preceding communication we have shown that cycloalkanes
cracking mechanism and Haag–Dessau cracking, rather
(which are readily obtained from aromatics by catalytic ring
than via hydrogenolysis15 on the noble metal.
hydrogenation) can be converted on acidic zeolite catalysts,
such as H-ZSM-5, into a high-quality steamcracker feed
consisting mainly of ethane, propane and n-butane. Here, we
demonstrate that, as an alternative to this two-stage route (ring
hydrogenation followed by ring opening), aromatics can be
directly converted with hydrogen into the same type of synthetic
steamcracker feed, if bifunctional zeolite catalysts, e.g., Pd/H-
ZSM-5, are used. The literature on catalytic ring opening of
aromatics in an excess of hydrogen is very scarce,5 and on the
catalysts used so far, the yields of n-alkanes with two or more
carbon atoms, i.e. the most desirable hydrocarbons for a
synthetic steamcracker feed, are reported to be low.
Table 1 shows results obtained at 400 °C with different feed
hydrocarbons. No products other than alkanes were found with
any feed hydrocarbon under these conditions. The yields of
methane are low throughout (around 3–4%, Table 1, entry 2).
Interestingly, the yields of the desired C2+-n-alkanes (Table 1,
entry 7) do vary significantly with the nature of the aromatic
–8
9
10
Results of the conversion of toluene on 0.2Pd/H-ZSM-5
are displayed in Fig. 1. Whereas the conversion of toluene is
00% throughout the temperature range covered, the types of
1
reaction occurring vary drastically: at 200 °C, hydrogenation to
methylcyclohexane is the predominant reaction, and part of the
methylcyclohexane is isomerised in consecutive reactions via
the well known11 bifunctional mechanism into ethylcyclo-
pentane and dimethylcyclopentanes. With increasing tem-
perature, more and more ring opening occurs. At 320 °C,
virtually all cyclic hydrocarbons are converted into alkanes,12
and increasing the temperature further brings about a diminu-
tion of the yield of isoalkanes which undergo secondary
cracking reactions into propane and ethane. At 400 °C toluene
Fig. 1 Conversion of toluene on 0.2Pd/H-ZSM-5 at various temperatures (X:
conversion; Y: yield; M-CHx: methylcyclohexane; E-CPn: ethylcyclo-
pentane; DM-CPns: dimethylcyclopentanes).
Table 1 Conversion of aromatics on 0.2Pd/H-ZSM-5 at 400 °C
Feed
1
,2,4-Trimethyl-
Entry
Benzene
Toluene
o-Xylene
p-Xylene
Ethylbenzene
benzene
1
2
3
4
5
6
7
8
a
Xa (%)
100
3.7
14.4
62.8
11.9
1.2
100
3.3
7.9
48.6
15.9
0.4
72.8
23.9
100
4.0
15.6
49.1
14.2
3.4
100
4.1
14.9
46.7
15.0
3.8
100
2.5
30.6
49.0
9.5
1.1
90.2
7.3
100
3.0
13.6
49.4
15.2
2.7
b
Y
Y
Y
Y
methane (%)
ethane (%)
propane (%)
n-butane (%)
b
b
b
b
Y 5
Y 2
C
+-n-alkanes (%)
+-n-alkanes (%)
b,c
C
90.3
6.0
82.3
13.7
80.4
15.5
80.9
16.1
b
Yisoalkanes (%)
b
X is the conversion of the aromatic feed hydrocarbon. Y is the yield of the product indicated and defined in the usual manner, e.g., Yn-butane = (4/7) 3
n˙ n-butane, out/ n˙ toluene, in), n˙ being the molar flux. c Sum of entries 3 to 6.
(
DOI: 10.1039/b003422n
Chem. Commun., 2000, 1133–1134
This journal is © The Royal Society of Chemistry 2000
1133

thermic heat in two separate reactors and, hence easier removal
of this heat. It remains to be seen whether the direct route or the
two-stage variant is economically more attractive.
Notes and references
1
W. J. Petzny and C.-P. Hälsig, in DGMK Tagungsbericht 9903, Proc.
DGMK Conference: The Future Role of Aromatics in Refining and
Petrochemistry, October 13-15, 1999, Erlangen, Germany, ed. G. Emig,
M. Rupp and J. Weitkamp, DGMK, Hamburg, 1999, p. 7.
2
3
4
5
6
7
8
Straight-run naphtha is a light petroleum fraction composed essentially
of C and C hydrocarbons.
5 6
3 4
LPG stands for Liquefied Petroleum Gas, the C and C hydrocarbons
from petroleum.
J. Weitkamp, A. Raichle, Y. Traa, M. Rupp and F. Fuder, Chem.
Commun., 2000, 403.
T. Sano, K. Okabe, H. Hagiwara and H. Takaya, J. Mol. Catal., 1987,
Fig. 2 Coke selectivation in the conversion of toluene on 0.2Pd/H-ZSM-5.
During the initial 10 h and from 60 to 70 h on-stream time, the reaction
temperature was 400 °C. In the period between 10 h and 60 h, lower reaction
temperatures in the range from 200 to 350 °C were applied; X: conversion;
Y: yield.)
(
4
0, 113.
J.-K. Chen, A. M. Martin, Y. G. Kim and V. T. John, Ind. Eng. Chem.
Res., 1988, 27, 401.
L. P. Poslovina, V. G. Stepanov, L. V. Malysheva, E. A. Paukshtis, L. A.
Vostrikova and K. G. Ione, Stud. Surf. Sci. Catal., 1997, 105, 997.
A. Chambellan, O. Cairon and T. Chevreau, in Proc. 12th Int. Zeolite
Conf., Warrendale, USA, July 5–10, 1998, ed. M. M. J. Treacy, B. K.
Marcus, M. E. Bisher and J. B. Higgins, Materials Research Society,
Baltimore, 1999, p. 1025.
feed hydrocarbon. This yield is lowest for toluene and above
9
0% for both benzene and ethylbenzene. Note also that very
large amounts of propane are formed from benzene (which can
16,17
9 Toluene is a major constituent of pyrolysis gasoline, other typical
aromatics in pyrolysis gasoline are ethylbenzene, xylenes and ben-
zene.
be rationalized in terms of a so-called type C b-scission
of
hexyl cations), while an unusually high yield of ethane is
observed in the product from ethylbenzene (which we tenta-
tively ascribe to a deethylation reaction).
1
0 Zeolite ZSM-5 (nSi/nAl = 20) was hydrothermally synthesised after S.
Ernst and J. Weitkamp, Chem. Ing. Tech., 1991, 63, 748, ion-exchanged
Regardless of the hydrocarbon used as feed, there was no
significant change in the product yields with time-on-stream. In
Fig. 2, results are shown which were obtained with toluene as
feed on 0.2Pd/H-ZSM-5. During the initial 10 h, the yield of
with aqueous solutions of NH
successively in flows of air (12 h), nitrogen (1 h) and hydrogen (7 h) at
00 °C to yield bifunctional catalysts with mPd/mdry zeolite = 0.2, 0.5 or
4 3 3 4 2
NO and Pd(NH ) Cl and pre-treated
4
1.0% referred to as 0.2Pd/H-ZSM-5, 0.5Pd/H-ZSM-5 and 1.0Pd/H-
ZSM-5, respectively. The experiments were performed in a flow-type
stainless steel apparatus with a fixed-bed reactor. The mass of dry
catalyst (particle size between 0.20 and 0.32 mm), the total pressure
C
2+-n-alkanes amounted to ca. 73%. For the next 50 h (not
shown in Fig. 2) toluene was converted on this catalyst sample
at lower temperatures in the range 200–350 °C. Thereafter, the
reaction temperature was again raised to 400 °C for 10 h.
Significantly better yields of the desired C2+-n-alkanes (ca.
(which was virtually equivalent to the partial pressure of hydrogen), the
partial pressure of the aromatic feed hydrocarbon at the reactor inlet and
the weight hourly space velocity (WHSV) amounted to 500 mg, 6.0
8
0%) were attained than with the fresh catalyst. We interpret
21
MPa, 65 kPa and 0.68 h , respectively. Product analysis was achieved
by capillary gas chromatography.
this selectivity gain in terms of a so-called coke selectivation,
i.e. at the lower reaction temperatures between the two runs at
1
1 J. Weitkamp, P. A. Jacobs and S. Ernst, Stud. Surf. Sci. Catal., 1984, 18,
279.
4
00 °C, some dimerization and/or disproportionation reactions
18
of methylcyclohexane or toluene inside the zeolite pores must
have taken place, whereby larger product molecules were
formed which ultimately led to some carbonaceous deposits
with a concomitant narrowing of the pores. Similar effects have
been observed by others, e.g. in the disproportionation of
toluene on H-ZSM-5.19
12 The product yields at 320 °C are: methane 0.5%, ethane 1.0%, propane
41.3%, n-butane 7.4%, n-pentane 1.7%, n-hexane 0.5%, isobutane
4
0.7%, isopentane 2.3%, isohexanes 1.3%, isoheptanes 0.1%, cycloalk-
anes 3.3%.
1
1
3 J. Weitkamp, in Ketjen Catalysts Symposium 1988: Fluid Catcracking,
Hydrocracking, Hydrotreating, Pt-Reforming, ed. H. J. Lovink, Akzo
Chemicals, Amersfoort, 1988, p. G-3 1.
In conclusion, we have demonstrated that aromatics can be
directly converted with hydrogen into a high-quality steam-
cracker feed on bifunctional zeolite catalysts of the Pd/H-ZSM-
4 (a) W. O. Haag and R. M. Dessau, in Proc. 8th Int. Congr. Catal., July
2
–6, 1984, Berlin, Germany, vol. 2, Verlag Chemie, Weinheim, 1984, p.
305; (b) S. Kotrel, H. Knözinger and B. C. Gates, Microporous
Mesoporous Mater., 2000, 35–36, 11.
5
type. This direct route will have to compete with the two-stage
variant consisting of ring hydrogenation in the aromatics over a
15 F. G. Gault, Adv. Catal., 1981, 30, 1.
1
6 b-Scission of alkylcarbenium ions—a scission of the carbon–carbon
bond in the b-position with respect to the positively charged carbon
atom—is the key step in the bifunctional hydrocracking mechanism.
7 J. Weitkamp, P. A. Jacobs and J. A. Martens, Appl. Catal., 1983, 8,
hydrogenation catalyst followed by ring opening of cycloalk-
anes on monofunctional zeolites.4 The main technological
advantage of the direct route described here is a single catalytic
reactor for the manufacture of synthetic steamcracker feed from
pyrolysis gasoline. On the other hand, advantageous features of
the two-stage variant are (i) the possibility to optimise the ring
hydrogenation of aromatics and the ring opening of the resulting
cycloalkanes separately and (ii) the generation of the exo-
1
1
1
23.
8 H. Schulz, J. Weitkamp and H. Eberth, in Proc. 5th Int. Congr.
Catalysis, ed. J. W. Hightower, North-Holland, Amsterdam, 1973, vol.
2, p. 1229.
19 L.-Y. Fang, S.-B. Liu and I. Wang, J. Catal., 1999, 185, 33.
1134
Chem. Commun., 2000, 1133–1134