Organic compounds -- part of the class 532-570 series – Organic compounds – Halogen containing
Reexamination Certificate
2000-09-27
2001-10-23
Siegel, Alan (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Halogen containing
C570S206000
Reexamination Certificate
active
06307113
ABSTRACT:
TECHNICAL FIELD
This invention relates to the selective aromatic bromination (ar-bromination) of aromatic substrates, and more specifically to an improved process for the selective para-bromination of benzene derivatives in the presence of a shape selective zeolite catalyst.
BACKGROUND
The selective para-bromination of toluene, catalyzed by NaY zeolite in the presence of an epoxide, has been reported by F. de la Vega et al.,
J Chem. Soc., Chem. Commun
., 1989, 653. The ratio of p- and o-isomers obtained was 98 to 2, but only about 10 to 13 percent of the toluene was converted. In experiments using no epoxide or in the presence of carbonates. the reaction proceeded to completion but the final para-selectivity was only 67%. The amount of zeolite catalyst used was about 0.09 gram per mmol of toluene. It was stated that adding a fresh batch of catalyst made the reaction resume with the same selectivity, but the final conversions obtained are not reported.
The para-selective bromination of benzene derivatives, such as toluene, using shape-selective zeolite catalysts, for example, NaY, HY, and NaX zeolites, has also been described by K. Smith et al., “Highly efficient para-selective bromination of simple aromatic substrates by means of bromine and a reusable zeolite,”
Chem. Commun
., 1996, 467-468. In contrast to de la Vega, much larger amounts of zeolite were used in order to obtain complete conversion. When NaY zeolite was used in an amount of 0.55 gram per 0.85 mmol of toluene (0.65 gram per mmol), it was reported that 98% absolute yields of p-bromotoluene were obtained. When the reaction was carried out with fluorobenzene and NaY zeolite, only 92% of the desired p-bromofluorobenzene was obtained. The yield of para-brominated products which can be achieved is also indicated to depend upon the type of zeolite used. For example, NaY zeolite gave better conversion (yield) than using the same amount of a NaX zeolite in producing p-bromotoluene. Smith et al. claim that the zeolite catalysts can be regenerated by calcination, but it is not believed that calcination returns the zeolites to their original state. The selectivity incorporated by zeolite in the Smith et al. process does not offer any economical advantage over the available commercial methods (which can produce up to 90% of the desired p-bromofluorobenzene) for the following reasons:
1) Use of a very dilute reaction medium (about 1 wt % of the total reaction mixture) decreases plant throughput;
2) A huge excess of zeolite is used (7 wt %, or a seven-fold excess over the substrate); and
3) Low selectivity for the para-isomer is shown; para/ortho (p/o)=92/8. In any event, the relatively large amount of catalyst required to be regenerated would make this process too expensive to be practical from a commercial standpoint.
Fluorobenzene is a deactivated system towards electrophilic aromatic substitution and requires Lewis acid catalysis for the improvement of the rate of the substitution reaction. However, strong Lewis acid catalysts create fast reactions with lowerp/o ratios. Control of the reaction rate with catalyst and temperature are two methods reported for the improved selectivity. For example, Jacob Oren in EP 0 761 627 A1 describes the effect of temperature on the para-selectivity of the reaction of fluorobenzene with bromine. Selectivities as high as 97-98% were reported at temperatures between 0 to −20° C., and even higher selectivities were achieved at temperatures between −17 and −65° C. Use of such low temperature operations is not a preferred way of operating on a commercial basis.
It would be highly advantageous if a way could be found to produce bromoaromatic compounds in high yield and selectivity by process technology suitable for use on an industrial scale, while at the same time utilizing a shape-selective zeolite catalyst more efficiently than the processes described in the foregoing literature references. In the case of p-bromofluorobenzene production, such process technology would be particularly attractive if product with para/ortho ratios in the vicinity of 98/2 could be achieved without need for any operations conducted at temperatures below room temperature (i.e., below ca. 20° C.). This invention is deemed to fulfill this objective most expeditiously.
SUMMARY OF THE INVENTION
In accordance with one embodiment of this invention, there is provided a process for selectively ar-brominating an aromatic substrate. The process comprises reacting said substrate with bromine in a liquid reaction medium and in the presence of a catalyst composition formed by including in the reaction medium (i) a shape selective HY or HMor zeolite catalyst, and (ii) at least one Lewis acid, typically in a ratio of from 0.005 to 0.5 part by weight of (ii) per part by weight of (i). The amount of such zeolite catalyst is no more than about 10 grams per mole of the aromatic substrate, and thus at most is less than one-fifth of the amount of zeolite described in the above publication of Smith et al. The reaction temperature in the present process is kept between 30° C. and 70° C., and most preferably between 35° C. and 50° C., throughout substantially the entire reaction period.
Among the major advantages achievable by the practice of this invention in the proper manner include (a) higher p/o selectivity, (b) faster conversion rate, and (c) lower cycle times as compared to use of the same amount of HY or HMor zeolite or a metal-containing Lewis acid as the sole catalyst component.
These and other embodiments of this invention will be still further apparent from the ensuing description and appended claims.
Zeolites
With reference to zeolites in general, the aluminum atom in the framework of the aluminosilicate zeolites carries a negative charge which is associated with a cation such as sodium, or a proton, the latter case being the source of the Bronsted acidity of the zeolite. Different families of zeolites provide dramatically different pore structures and sizes. Faujasites (Y zeolites) have intersecting channels with pore size of 7.0-7.4 A. Mordenites have one-dimensional channels and pore sizes of 6.7-6.8 A. Within each family, zeolites with varying Si/Al ratios have been synthesized.
The HY and HMor zeolite catalysts for use in the process of the invention are chosen such that the aromatic substrate will fit into the pores of the zeolite. This permits bromination to occur within the pores where access to sites on the substrate other than the desired position is hindered. Accordingly, such zeolite catalysts are termed as being “shape selective.” For simple mononuclear aromatic compounds such as, toluene, fluorobenzene, and isobutylbenzene, HY zeolites which have a pore (aperture) size of about 7 angstroms give para-brominations in high yield and selectivity provided the zeolites are dried and kept dry. The optimum pore size for the selective bromination of other aromatic substrates is selected depending upon the size and shape of the molecules to be brominated. In general, the pore size should range from 6 to 8 angstroms and preferably from 6.5 to 7.5 angstroms. It is conceivable that by changing temperature, solvent, cation, etc., it may become practical to carry the process with zeolites which have smaller aperture sizes. Suitable HY and HMor zeolites for use in the practice of this invention are available as articles of commerce. For example, suitable HY zeolites are available from Tosoh USA, Inc. as zeolites 320HOA, 360HUA, and 390HUA. Likewise zeolites HSZ-620HOA, HSZ-640HOA, and HSZ-690HOA are examples of HMor zeolites available from Tosoh USA, Inc. that are suitable for use in the practice of this invention.
The pores of the zeolite particles, where the selective bromination process takes place, should be kept substantially free of absorbed water. Accordingly, in practicing the process of the invention, the zeolites should have an absorbed water content of no greater than about 7.5 weight percent, preferably no greater than about 5.0 weight percent, and most preferably less than 5 weight percen
Albemarle Corporation
Siegel Alan
Spielman, Jr. E. E.
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