The PTC esterification described in this patent has several interesting aspects.
The most notable aspect of the process conditions chosen was the addition of a specific and relatively small amount of water to the reaction. As we teach in our 2-day course “Industrial Phase-Transfer Catalysis,” many PTC applications, especially solid-liquid PTC reactions, can be greatly enhance by adding carefully screened small amounts of water. In one case highlighted in our PTC course manual, maximum reactivity for a PTC reaction using potassium carbonate is achieved when the amount of water added to the reaction is 10 weight% relative to the potassium carbonate. In this case, the amount of water added was 70 weight% relative to the potassium carbonate. Each solid-liquid PTC system must be studied to determine the optimal amount of water. But you have ti be aware to look for this optimum or else, you will never discover it.
One of the projects on which PTC Organics was a consultant and Marc Halpern an inventor on the patent, showed great sensitivity to the exact amount of water added to the reaction that contained potassium carbonate and is described here: http://phasetransfercatalysis.com/ptc_tip/important-effect-of-water-on-profit-in-solid-liquid-ptc-esterification/.
Another interesting aspect of the reaction conditions chosen is the amount of potassium carbonate used. In most PTC-carbonate reactions, the number of moles of potassium carbonate is equal to or greater than the number of deprotonatable protons. The reason for this is that carbonate is about 4 orders of magnitude more basic than hydrogen carbonate. In this case, the inventors used a number of moles of potassium carbonate that was 63% of the number of moles of the carboxylic acid. This works because the pKa of a conjugated carboxylic acid is under 4 whereas the pKa of hydrogen carbonate is above 6. So, bicarbonate can deprotonate the acid.
The inventors chose tetrabutylammonium chloride as the phase-transfer catalyst. That would not be our first choice since TBAC is not only much more expensive than TBAB, TBAC is very hygroscopic and hard to dry. Since this esterification is likely sensitive to the amount of water in the system, we would prefer using a non-hygroscopic quat salt to avoid variability in the commercial process due to variability in the moisture level introduced by a hygroscopic quat salt.
When your company needs to optimize PTC applications, especially solid-liquid PTC applications that are likely sensitive to non-optimal hydration levels, now contact Marc Halpern of PTC Organics to benefit from highly specialized expertise in the most practical aspects PTC process development and optimization.
