As we teach in our 2-day PTC course “Industrial Phase-Transfer Catalysis,” the method of separation of the phase-transfer catalyst from the product is usually determined by the difference in polarity between the catalyst and product and include extraction of the catalyst into water (most common), recrystallization of product, adsorption onto a solid support (clay, silica, alumina) and extraction of the phase-transfer catalyst into a non-polar solvent in which the product is not soluble.
In some cases, phase-transfer catalyst separation is feasible by distilling the product away from the catalyst. This method is used in many commercial PTC processes for the manufacture of fragrances, low molecular weight monomers and halocarbons.
In this patent, the CFC mixture is added to aqueous NaOH and phase-transfer catalyst at 35 C-42 C (near its boiling point), then heated to 70 C. The boiling point of the product (hexafluoro-2-butyne) is -25 C and can be taken overhead as it is formed (collected in a cooling trap, hopefully with low handling losses). PTC enables performing this dehydrochlorination at a low enough temperature to avoid polymerizarion of the alkyne product.
If the organophilic quats tetraoctylammonium or Aliquat 336 would be used for this reaction, one should be able to recycle the phase-transfer catalyst easily. If tetrabutylammonium is used, it may be lost in the aqueous phase during workup. So, if phase-transfer catalyst recycle is required to meet economic targets, we would avoid the use of TBAB.
The patent does not address the recycle of the phase-transfer catalyst and we each this in our 2-day course “Industrial Phase-Transfer Catalysis.”
The patent also mentions the possible addition of NaCl to the reaction mixture, even though NaCl is a byproduct. The inventors note that the alkali metal halide might be stabilizing the phase-transfer catalyst. However, as we teach in our 2-day PTC course “Industrial Phase-Transfer Catalysis,” the addition of NaCl to certain PTC processes is used to salt out phase-transfer catalysts into the organic reaction phase (such as tetrabutylammonium that are not as organophilic as Aliquat 336 or TOAB) which in turn enhances reactivity since the rate determining step is likely to take place in the organic phase according to the Halpern pKa Guidelines (taught in our 2-day PTC course “Industrial Phase-Transfer Catalysis) as applied to PTC dehydrohalogenation.
If your company wants to achieve the lowest cost highest performance green chemistry processes using phase-transfer catalysis, now contact Dr. Marc Halpern of PTC Organics, the industrial phase-transfer catalysis experts.
