Selective bromination of aromatic compounds

Details Selective bromination of aromatic compounds Selective bromination of aromatic compounds

1997-10-27

2002-12-03

Tsang, Cecilia (Department: 1626)

Organic compounds -- part of the class 532-570 series

Organic compounds

Halogen containing

C570S182000, C570S260000, C570S261000

Reexamination Certificate

active

06489524

ABSTRACT:

TECHNICAL FIELD
This invention relates generally 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.
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. 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. 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.
It has now been found that the absorbed water content of the zeolite has a significant effect on the yield and selectivity when producing p-brominated benzene derivatives. In fact, water saturated zeolites favor chain bromination over ring substitution. We have found that maintaining the absorbed water content of the zeolite below a certain level, which varies depending upon the particular zeolite used, permits high selectivity and yields to be achieved using much smaller amounts (about one-half as much) of zeolite than required by Smith et al.
SUMMARY OF INVENTION
In accordance with this invention, there is provided a process for selectively Ar-brominating an aromatic substrate comprising reacting said substrate with a brominating agent in the presence of at least about 0.19 gram per mmol of said substrate of a zeolite catalyst which has an absorbed water content of no greater than about 7.5 weight percent.
The 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 aromatic compounds such as, toluene, fluorobenzene, and isobutylbenzene, faujasites (e.g., NaX, HY, and NaY 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 about 6 to 8 angstroms and preferably from about 6.5 to 7.5 angstroms. The most preferred types of zeolite for use in the process of the invention are NaX and NaY zeolites. Such zeolites are well known—for example, type X zeolites corresponding to the typical formula:
Na
86
(AlO
2
)
86
(SiO
2
)
106
.264H
2
O,
and type Y zeolites corresponding to the typical formula:
Na
56
(AlO
2
)
56
(SiO
2
)
136
.250H
2
O,
where a portion of the sodium cation could be substituted by other cations such as K
+
, Ca
++
, Mg
++
, Ba
++
, Zn
++
, and Fe
++
. Proton form zeolite catalysts, such as HY zeolites, as well as mixtures of cation and proton form zeolites, can also be used.
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 percent water, based on the total weight of zeolite and water. As used in the specification and claims herein, by “absorbed water content” is meant the weight percent of water which can be removed by drying the zeolite at a temperature of from 240-250° C. at a pressure of from 0.1-0.05 mm Hg for 4 hours. The zeolite may also contain additional water which requires higher temperatures for removal. Although it is not possible to distinguish between water of hydration and absorbed water, for the purpose of this invention the residual water content of the zeolites which is not removed by the 240-250° C. drying procedure is considered to be water of hydration. The complete removal of this residual water might further improve the shape selectivity function of the zeolite. However, the presence of the residual water does not prevent the achievement of good yields and selectivity when using much smaller amounts of catalyst than are required by the prior art process. It is important that the dried zeolites be protected against excessive exposure to atmospheric moisture prior to use, because the zeolites may pick up sufficient moisture during handling, or even when kept in a desiccator for a few days, so as to significantly decrease their effectiveness. NaX zeolites appear to be more sensitive to absorbed water content than NaY zeolites.
The zeolites used in the examples below contained varying amounts of absorbed water when received from the vendor. Absorbed water was removed by drying the zeolites at a temperature of 240-250° C. for four hours at a reduced pressure of from about 0.1 to 0.05 mm Hg. Thermogravi-metric analysis (TGA) under nitrogen from 20-900° C. removes not only the absorbed water, but also the water of hydration. Although such high temperature drying conditions could be used, it was found that the drying procedure at 240-250° C. and reduced pressure is sufficient to achieve good yields and selectivity.
In order to illustrate the ready water absorption by zeolites, vendor samples of NaY and NaX zeolites were tested for water content by TGA from 20-900° C. under nitrogen. A sample of NaY zeolite as received from the vendor contained 24-25 wt % water. After drying at 240-250° C. and 0.1-0.05 mm Hg for four hours, analysis by TGA under nitrogen showed a rapid water pick up of about 6.4 wt % at temperatures below 100° C. When again heated to 900° C., the sample lost this water followed by an additional 5.2 wt %, indicating that the “dry” zeolite still contained about 5.2 wt % water.
A sample of NaX zeolite as received from the vendor contained 7.3 wt % water. After drying at 240-250° C. as described above, the sample showed a fast water absorption of 3.1 wt % during handling. When again heated to 900° C., it lost this absorbed water followed by an additional loss of 7.3 wt % indicating that the “dry” zeolite contained 7.3 wt % water.
In order to examine the rate of water absorption by “dry” zeolite, we exposed one gram samples of dried NaY zeolite in dry 20 mL vials to laboratory air and the water gain was measured

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