Organic compounds -- part of the class 532-570 series – Organic compounds – Halogen containing
Reexamination Certificate
2002-04-12
2002-11-19
Siegel, Alan (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Halogen containing
Reexamination Certificate
active
06482996
ABSTRACT:
The present invention relates to a process for the preparation or isolation of 2,6-dichlorotoluene by catalytic hydrodechlorination of mixtures containing polychlorotoluenes.
2,6-Dichlorotoluene is an important intermediate for preparing various active compounds for agrochemicals and pharmaceuticals. The preparation of chlorotoluene usually results in the formation of isomeric mixtures of dichlorotoluenes and trichlorotoluenes from which, however, 2,6-dichlorotoluene has hitherto not been able to be selectively separated and which therefore have to be disposed of at high cost.
It is known in principle from “Catalytic Hydrogenation over Platinum Metals” (P. N. Rylander, Academic Press 1967, pages 405-425) that chlorobenzenes can be hydrodechlorinated in the presence of hydrogen over catalysts based on metals of the platinum group. However, owing to the usually only low selectivity, only partial conversions are generally achieved. It is also stated that reaction rate and conversion of hydrodehalogenations can be increased by the addition of bases such as sodium acetate or alkali metal hydroxide. On page 417 of “Catalytic Hydrogenation over Platinum Metals”, it is indicated that halogen radicals can easily be removed from aromatic rings and that it is very difficult to retain a particular proportion of halogen radicals on the aromatic ring, which is why the formation of only cyclohexane is frequently observed. Activated carbon is reported as support material for the catalysts (see page 425). Furthermore, “Heterogeneous Catalysis for the Synthetic Chemist” (R. L. Augustine, Marcel Dekker Inc. 1995, p. 534-537) discloses that the use of nonpolar solvents such as ethyl acetate, benzene or cyclohexane leads to a reduction in the dechlorination rate. German Offenlegungsschrift 43 34 792 discloses a process for the hydrodehalogenation of halogenated benzenes in which use is made of catalysts obtained by application of one or more salts of Pd and/or Pt and, if desired, a copper salt to an aluminium oxide or titanium dioxide support material. The object of the process is the reconversion of more highly halogenated benzenes into the useful materials benzene and monochlorobenzene. In the description of this process, emphasis is given to the good conversions and the fact that the hydrodehalogenation of dihalogenobenzenes or trihalogenobenzenes forms virtually no cyclohexane or cyclohexane derivatives. The question of selectivity of the hydrodehalogenation is not accorded any importance because of the lack of other non-halogen substituents on the benzene ring.
JP 40-12694 B4 describes a process for the selective preparation of 2,6-dichlorotoluene by hydrodechlorination of 2,3,6-trichlorotoluene, in which a palladium-containing catalyst is used. As an essential condition for this process, it is emphasized that it is carried out in the liquid phase. It is also stated that the catalyst can contain any support material. Explicit mention is made of carbon, activated carbon, carbon black, barium sulphate, silica and diatomaceous earth as supports. In the examples, the palladium-on-activated carbon catalyst is introduced into the starting material mixture, the reaction temperature is set and hydrogen is blown through the resulting melt. The product mixture is subsequently discharged from the reactor in gaseous form together with an excess of hydrogen and is then condensed. Our own studies have shown that this process has the disadvantage that the palladium catalyst described in the examples loses its activity very quickly, i.e. has a short operating life, so that a change of catalyst is necessary at short intervals.
It is therefore an object of the present invention to provide a process which makes it possible to prepare or isolate 2,6-dichlorotoluene with high selectivity and to find a catalyst which at the same time has an improved long-term stability.
The invention provides a process for the preparation or isolation of 2,6-dichlorotoluene by catalytic hydrodechlorination of mixtures containing polychlorotoluenes in the presence of hydrogen, characterized in that the hydrodechlorination is carried out in the presence of a Pd-containing catalyst on an aluminium silicate support.
As mixtures containing polychlorotoluenes, preference is given to using mixtures containing dichlorotoluenes and/or trichlorotoluenes in the process of the invention. Surprisingly, this process makes it possible to hydrodechlorinate such isomer mixtures of various dichlorotoluenes and/or trichlorotoluenes over a long period of operation with excellent selectivity and to isolate 2,6-dichlorotoluene in high purity from the reaction product.
It has been found to be useful to employ dichlorotoluene mixtures containing at least 5% by weight, preferably at least 15% by weight and in particular at least 30% by weight, of 2,6-dichlorotoluene, based on the total dichlorotoluene mixture.
It has also been found useful to employ trichlorotoluene mixtures which contain at least 40% by weight, preferably at least 50% by weight and in particular at least 60% by weight, of 2,x,6-trichlorotoluenes, based on the total trichlorotoluene mixture, where x can represent 3, 4 and/or 5.
An essential aspect of the process of the invention is the use of supported Pd-containing catalysts in which the catalyst support contains an aluminium silicate. Such support materials are known to those skilled in the art. The term aluminium silicate is used as a generic term for compounds having various proportions of Al
2
O
3
and SiO
2
. Aluminium silicates include, for example, zeolites, feldspars and feldspar-like compounds. Aluminium silicates also include the trimorphic group Al
2
SiO
5
(Al
2
O
3
.SiO
2
). Aluminium silicates which do not crystallize well or amorphous aluminium silicates can also be used. Preference is given to using amorphous aluminosilicates and zeolites. The aluminium silicate support materials to be used according to the invention preferably have an SiO
2
:Al
2
O
3
weight ratio of (200-1):(1-200), particularly preferably (20-1):(1-20).
The finding made by the present invention that the use of a specific catalyst support is essential to the activity, selectivity and operating life of the catalyst in the preparation or isolation of 2,6-dichlorotoluene goes significantly beyond the teachings of JP 40-1294 B4, since that reference attaches no importance to the question as to whether the hydrodechlorination is improved at all by the use of a catalyst support and, if yes, by which catalyst support.
It has been found to be useful to employ support materials containing aluminium silicate and having a specific pore volume of greater than 10 ml/kg, preferably in the range 10-1000 ml/kg and in particular in the range 200-800 ml/kg. The BET surface area of the support materials is 5-800 m
2
/g, preferably 10-500 m
2
/g and in particular 50-500 m
2
/g.
The catalysts used contain, as catalytically active metal component, palladium in an amount of 0.1-10% by weight, preferably 0.5-5% by weight, based on the total catalyst. Apart from palladium, the catalysts can also contain further metals which promote the selective hydrodechlorination, e.g. copper in elemental and/or ionic form or else other metals of transition group VIII of the Periodic Table, e.g. ruthenium, rhodium or platinum. If copper is present as further component, the weight ratio of copper to palladium is preferably (0.5-50):100, in particular (1-10):100. If one or more metals of transition group VIII of the Periodic Table is/are present, the weight ratio of these metals to palladium is (0.02-0.5):1, preferably (0.05-0.2):1. Preference is given to using catalysts on aluminium silicate supports which have only palladium as catalytically active metal component.
It is possible to use either catalysts in which the metals are present only in an outer surface zone of the catalyst particle, known as coated catalysts, or catalysts in which the metals are distributed homogeneously through the catalyst particle.
To produce the catalysts, palladium in the form of its compounds such as ammonium he
Eichinger Wolfram
Röhlk Kai
Schulze-Tilling Andreas
Bayer Aktiengesellschaft
Eyl Diderico van
Gil Joseph C.
Siegel Alan
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