Phase-Transfer Catalysis
PTC Quat Versus Surfactant Quat

The Industrial
Phase-Transfer
Catalysis Experts

 

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Quats for PTC...Quats for Surfactants
Is There a Difference?
Yes!

by Marc Halpern

PTC Organics, Inc.

900 Briggs Road, Suite 145, Mount Laurel, New Jersey 08054 USA

tel: +1 856-222-1146; send E-mail to Dr. Halpern

 

Introduction

 

From time to time, we see authors of articles refer to quaternary ammonium ("quat") salts as "surfactants" when they should be using the words "phase-transfer catalysts." On other occasions, we observe authors of articles using surfactant quats for phase-transfer catalysis applications. Is there a difference? Yes! And if you are not careful, the difference could be a plant nightmare.

 

What is the difference?

 

One key difference between "PTC quats" and "surfactant quats" is a practical performance issue. There can also be mechanistic differences between the two as well.

 

Simply put, a good PTC quat will catalyze the PTC reaction and when the reaction is over and it's time for workup, the phases will separate shortly after stopping agitation, typically within les than about 60 seconds in a lab reactor. It is always nice to be able to start workup with a nice, quick and clean phase cut.  On the other hand, if you ever performed a PTC reaction in the lab, then poured the contents into a separatory funnel and had to wait an hour or a day for phase separation, you might have been using a surfactant quat. A "good surfactant quat" will reduce the interfacial tension between the phases so much that you may never see phase separation. If you try to scale up the latter, you might get stuck with a bad batch. 

 

One of the worst scenarios is when people use a quat which is on the borderline between being a good PTC quat and being a good surfactant quat and they don't know it. In such cases, you may happily go through an entire process development program and obtain excellent results for reactivity, selectivity and product isolation during workup. all that time, you may have been on the edge of getting an emulsion, but you got lucky due to salt concentration, critical micelle concentration or other factors. All the time, you may be been standing at the edge of a cliff on a windy day and saying "well, I haven't fallen yet!" You can even proceed to scale up and everything continues to work just fine. For years. 

 

Then, one day, management says you must cut costs. A purchasing manager decides to change the supplier of the "phase-transfer catalyst" or a Six Sigma black belt figures out that you can increase productivity by changing a concentration or an operator simply adds the wrong amount of something. All of a sudden you have $50,000 of product in an emulsified batch and you are wondering if the insurance will cover the loss or if you should wait for the phases to separate or if you add a lot of salt and pray for separation.

 

Ask me how I know this.

 

You can learn the hard way, like a colleague of mine, or you can be aware of the pitfall ahead of time.

 

Usually, a good surfactant quat is discovered during development when indeed a stable emulsion is observed. In fact, surfactant quats can often give excellent reactivity by enhancing the reaction at the interface between the organic and aqueous phases, sometimes even by making a real emulsion. The problem usually makes itself known during workup. It doesn't do any good to achieve 100% conversion in 10 minutes if you need 2 days for phase separation. 

 

Is There a Rule of Thumb for Structural Difference?

 

Except for quantum physics, many changes in life lie on a continuum. The same is true when trying to provide a guideline for differentiating between a good PTC quat and a good surfactant quat. Please do not laugh out loud, but there is no sharp boundary between the structures of good PTC quats and good surfactant quats.

 

Nevertheless, we will shall with you some guidelines we use internally at PTC Organics.

 

Quats Which Are Often Risky with PTC:

If the quat has three methyl groups and the 4th chain is C10 or higher (especially C12 or higher), it is likely to be a good surfactant and bad for separation in PTC systems. Hexadecyl trimethyl ammonium chloride (cetrimonium chloride) is a classic quat that falls into this group. These quats often work by a micellar mechanism. 

 

If the quat has two methyl groups and the 3rd and 4th chain are both C14 or higher (especially C16 or higher), it may be a good surfactant and bad for separation in PTC systems. These often work by a vesicle mechanism.

 

If the quat has three ethyl groups and the 4th chain is C14 or higher, it may be a good surfactant and bad for separation in PTC systems.

 

Sometimes, you can get "lucky" and use these quats successfully in PTC systems with very high or saturated salt concentrations. Sometimes the separation problems will start during the second or third water wash when the salt concentration gets very low.

 

All of the quats above may enhance reactivity in PTC systems, they just may make workup impractical.

 

Quats Which Are Often Good for Separation in PTC systems:

 

If the quat has only one methyl group or no methyl groups, it is much less liekly to cause emulsions which result in major separation problems.

 

Quats which are likely to be good for various PTC applications will have no more than 2 methyl groups and will have a total of 12 to 48 carbons.

 

Almost any quat can cause emulsions in some systems. A lot depends on the the solvent and the identities of all of the components in the system. One reason that toluene is a popular solvent in PTC systems is that is is "naturally" less compatible with water so PTC systems using toluene are more "forgiving" when it comes to avoiding emulsions. Toluene is also good for reactivity in many PTC systems becuase it rejects water which can hydrate nucleophiles and bases thereby reducing their reactivity.

 

Examples of ammonium quats which are likely to be good for PTC include:

 

Aliquat 336 (now called Aliquat 134)

Aliquat 175 (methyl tributyl ammonium chloride)

Tetrabutyl ammonium bromide

Didecyl dimethyl ammonium chloride (almost on the borderline for vesicles, but it works in many applications)

Hexadecyl tributyl ammonium bromide 

Tetraoctyl ammonium bromide

 

Examples of ammonium quats which are not likely to be good for PTC include:

 

Tetramethyl ammonium chloride (not organophilic enough)

Cetrimonium chloride (too emulsifying)

 

Summary

 

Life is complicated but that makes it interesting and provides some small measure of job security for process chemists and engineers. It is worthwhile to be aware that not all quats are created equal. Some are better for surfactant applications and some are better for PTC applications. Since you read this article, you are already ahead of the game because you recognize there is a difference. At this very moment, someone somewhere is probably using a surfactant quat while intending to use a PTC quat and hopefully will realize the difference during his/her first workup and make the necessary changes before going to the pilot plant.

 

What kind of changes? The answer must always be given on a case-by-case basis. PTC Organics can often help your company achieve high-performance, low-cost and environmentally-friendly processes for the manufacture of organic chemicals by applying its unmatched highly specialized expertise in industrial phase-transfer catalysis to your commercial goals.  

 

Additional Questions? Contact Marc Halpern of PTC Organics directly at tel +1 856-222-1146 or by E-mail.

 

 

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last modified 28-Jan-2006