A patent application was published this month (Myers, J. (Blue Chip IP) US Patent Application Publication 2024/0018073, 18-Jan-2024) that uses PTC for dehydrochlorination. As will be explained below, we speculate that Aliquat 336 (trademark of BASF) was chosen as the phase-transfer catalyst for two reasons: [1] since Aliquat 336 has negligible partitioning into the aqueous phase with high ionic strength, this particular phase-transfer catalyst will not contaminate aqueous waste streams and will be removed with heavy distillation fractions for incineration and [2] since Aliquat 336 is very effective for PTC dehydrochlorination, it is effective at low levels such as 0.4-0.6 weight% as described in the examples.
First, let’s discuss the background for this PTC dehydrochlorination.
Chloroalkenes are useful intermediates in pharmaceuticals, agrochemicals and other organic chemicals. They are produced by dehydrochlorination of chloroalkanes. Polychloropropenes are particularly useful and produced by dehydrochlorination of polychloropropanes that in turn are produced by a telomerization process between carbon tetrachloride and ethylene that uses metal catalysts such as ferric chloride and promoters such as tributyl phosphate.
During separation of the various polychloropropane isomers after the telomerization process, there are light fractions and heavy fractions. The ferric chloride metal catalyst and the tributyl phosphate promoter are carried forward in the heavy fractions. However, the heavy fractions still contain valuable polychloropropanes, especially 1,1,1,3-tetrachloropentane (HCC-250fb) or 1,1,1,3,3-pentachloropropane (“HCC-240fa”) that together comprise more than 80% by weight of the heavy fractions.
The heavy fraction is dehydrochlorinated in the presence of 0.4-0.6 wt% Aliquat 336 (relative to the total weight of the heavy fraction) in the presence of 13.7% aqueous NaOH for 3.7 hours at 62-68 deg C. The trichloropropenes and tetrachloropropenes can be separated by distillation leaving the final heavies that contain the metal catalyst, the promoter and the Aliquat 336. This final heavy fraction is then sent to incineration.
We speculate that lower molecular weight quat salts such as methyl tributyl ammonium chloride or tetrabutyl ammonium chloride (formed from tetrabutyl ammonium bromide that would cause bromide contamination of the product) would distribute to some degree into the aqueous waste stream and require additional costly waste treatment.
The inventors wanted to prove that the Aliquat 336 was an effective phase-transfer catalyst. They subjected pure 1,1,1,3,3-pentachloropropane to 19.4% aqueous NaOH for 1.5 hours at 66 deg C without PTC and observed 1.3% dehydrochlorination. They then added 0.64 weight% Aliquat 336 to the mixture and after 3 hours at 65 deg C, they observed more than 60% conversion.
This patent application reinforces what we teach in our 2-day course “Industrial Phase-Transfer Catalysis” which is that Aliquat 336 is chosen for PTC reactions that need a combination of high reactivity and separation by certain methods including distilling the product, recrystallizing the product, extracting the product into water and other separation methods that require the phase-transfer catalyst to avoid significant distribution into an aqueous waste stream.
