Specialty quat salts, such as quat carboxylates, can be prepared from common quat salts, such as commercially available quat bromides and chlorides by ion exchange. However, there are several factors that must be optimized to make this work.
Three factors that must be taken into account are (1) the relative affinities of the quat cation toward the anion being exchanged and the anion of the original commercial quat salt, (2) whether the ion exchange is performed by liquid-liquid extraction or using an ion exchange resin and (3) the physical characteristics of the ion exchange resin.
The first factor is the relative affinities of the quat cation toward the anion being exchanged and the anion of the original commercial quat salt. As we teach on the famous page 76 of the PTC course manual (this has been the same page in the manual for more than 20 years!), quat cations have different affinities for different anions that differ by many orders of magnitude. For example, the methyl trioctyl ammonium prefers to pair with iodide versus hydroxide by more than 100,000 times, bromide versus hydroxide by about 1,000 times and chloride versus hydroxide by about 100 times.
If one wanted to exchange a halide for hydroxide by liquid-liquid ion exchange, that would take a large number of extractions with large excess of hydroxide or an extremely efficient countercurrent system.
In US Patent Application Publication 2024/0182626 published this month, inventors at BASF reported the preparation of the 2-ethylhexanoate salts of tetrabutylammonium, tetramethylammonium and tributylmethylammonium, in two steps.
The first step uses an ion exchange resin to convert the quat chloride into quat hydroxide. In the second step, the quat hydroxide is neutralized with 2-ethylhexanoic acid to form the quat 2-ethylhexanoate salt.
The inventors wisely chose to perform the ion exchange starting with quat chlorides since quat chlorides are commercially available and it is thermodynamically much easier to exchange chloride for hydroxide than from bromide.
The ion exchange resin chosen in “Reference Example 2” was AMBERSEP 900 in the hydroxide form. AMBERSEP (R) 900 OH is a strongly basic, macroreticular (macroporous), Type I, quaternary ammonium anion exchange resin specially sized for use in Ambersep (R) resin systems. The macroporous resin has large pores formed during the polymerization process by the presence of large inert pore former molecules around which the monomer is polymerized and crosslinked. These large pores are essential to allow the large quat salts to enter and exit the ion exchange resin beads for the chloride-hydroxide ion exchange that provides a large surface area as opposed to accessing only the hydroxide sites on the surface of a non-porous gel resin.
The nature of the ion exchange resin column is such that when passing the quat chloride solution through the length of the column, it acts as a countercurrent ion exchange system.
Once the quat hydroxide is formed, the neutralization with the carboxylic acid is easy and instantaneous.
Thus, the benefit of using the ion exchange resin is to form the thermodynamically disfavored quat hydroxide from the quat chloride in an efficient manner that generates less waste.
Hi Marc
Hope you’re doing fine.
At Tatva Chintan we generate hydroxide quats by replacing all sorts of halides using electrodialysis cells. Although this process requires a lot of energy, we can produce very pure hydroxides on very large scale.
I’m not sure this would also possible with the ion exchange process that you / this patent described.
Best wishes!
Simon