A very interesting and useful patent was issued this month that describes the use and recycle of the phase-transfer catalyst methyl tributyl ammonium chloride (MTBAC, C# = 13, q-value = 1.75).
As we teach in the 2-day course “Industrial Phase-Transfer Catalysis”, careful choice of phase-transfer catalyst, non-polar solvent and high ionic strength of the aqueous phase, can result in the formation of three liquid phases, in which a small volume middle liquid phase is formed that typically consists 30%-70% of phase-transfer catalyst that has two major benefits: (1) greatly enhanced reactivity versus a non-three liquid phase system and (2) suitable for economically advantageous recovery and recycle of the phase-transfer catalyst.
In this patent, the inventors added water to the reaction mixture during workup to expand the volume of what we think is a three liquid phase PTC system. The water in the workup will be discussed later in this review.
Prior to workup during the reaction, there was little free water in the system available for hydration to form an “omega phase” (thin film of saturated aqueous solution of the reacting nucleophile; term coined by Dr. Charles Liotta of Georgia Tech) since the source of water was 50% NaOH that can act as a desiccant for the organic phase due to the high hydrophilicity of sodium and hydroxide ions that are converted to chloride ions as the reaction progresses. The reactivity of such PTC systems are often sensitive to small changes in the water available to form the omega phase which in turn affects the ability of the quat cation to act as an effective phase-transfer catalyst.
We do not know if the reactivity of this particular system was so sensitive to small changes in the amount of water in the system, but hydration level is often be a consideration when optimizing such PTC applications. We show several examples of optimizing water levels in PTC systems in our 2-day course “Industrial Phase-Transfer Catalysis.”
The inventors discuss that one key for the recovery and recycle of the MTBAC was choosing just the right amount of water during workup. That amount of water should be enough to dissolve the NaCl byproduct but not add so much water that the MTBAC, normally soluble in water with low ionic strength, will dissolve in the aqueous phase that would make the MTBAC difficult to recover and recycle. The inventors impressively achieved 84% recovery of the MTBAC by simply separating the middle phase that was well designed for the workup.
The inventors compared various phase-transfer catalysts and they found that 75% methyl tributyl ammonium chloride in water (MTBAC, C# = 13, q-value = 1.75), greatly outperformed the following phase-transfer catalysts used in solid form: benzyl trimethyl ammonium chloride C# = 10, q-value = 3.1; tetraethyl ammonium chloride C# = 8, q-value = 2.0; and tetrapropyl ammonium bromide C# = 12, q-value = 1.33.
As we teach in the 2-day course “Industrial Phase-Transfer Catalysis,” MTBAC is often an optimal phase-transfer catalyst for reactivity and cost for PTC-hydroxide “T-Reactions” (transfer rate limited) with substrates that have a pKa in the range of 16-23 according to the Halpern pKa Guidelines for the evaluation and optimization of PTC applications. The pKa of propargyl alcohol is 13.6. According to the Halpern pKa Guidelines, quat cations with higher C# should work better for PTC reactions of substrates with pKa’s below 16. This is consistent with the order of reactivity of quat salts shown in the patent.
Toluene was used as the non-polar organic solvent. As we teach in our 2-day course “Industrial Phase-Transfer Catalysis,” aromatic and aliphatic hydrocarbons, such as toluene and decane, have been have been studied in detail as solvents to form 3-liquid phase PTC systems.
MTBAC is also one of the lowest cost per mole phase-transfer catalysts used commercially in volumes of more than 100 metric tons per year in a single process.
When your company requires optimization of choice of phase-transfer catalyst for simultaneous high reactivity, low cost, and effective catalyst separation, now contact Marc Halpern of PTC Organics to integrate highly specialized expertise in industrial phase-transfer catalysis with your commercial goals.
