Separating fatty acids

Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2013

Researchers develop a less energy-intensive technique for separating fatty acids.

KEY CONCEPTS
• Separation of fatty acids can lead to the development of more oxidatively stable feedstocks for use as raw materials in the preparation of synthetic basestocks.
• A new technique using a polydicyclopentidene membrane has been found to separate the monounsaturated fatty acid, oleic acid from the polyunsaturated fatty acids, linoleic and linolenic acids.
• The use of 90 psi pressure dramatically accelerates the rate of separation.

FOR THOSE IN THE LUBRICANT INDUSTRY, working with a variety of raw materials based on different chemistries is the norm. Some are single components or mixtures of only a few substances, while others such as base oil contain a large number of compounds.

Working with raw materials that contain a blend of many components can be a benefit in some cases, but it can also be a detriment. In a previous TLT article, research was discussed showing how olefins such as ethylene and propylene present in paraffin-rich petroleum waste streams can potentially be separated from their respective paraffins, ethane and propane using ionic liquids that offer a less energy intensive, less expensive process (1). Silver complex-based ionic liquids were found to be a possible candidate because they have an affinity for olefins.

One series of natural products used in lubricants that has a complex structure is triglycerides derived from vegetable oils. Sources such as soybeans produce oils that have a distribution of fatty chains ranging from 16 to 18 carbon atoms. Other vegetable oils have broader distributions that can cover the range between eight and 18 carbon atoms.

The triglycerides are frequently saponified to their corresponding fatty acids with one triglyceride producing three fatty acids. In working with fatty acids with 18 carbon atoms in derivatives such as polyol esters, the addition of multiple carbon-carbon double bonds can have a large impact on performance. For example, 18 carbon fatty acids with more than one double bond (such as linoleic and linolenic acids) are much less oxidatively stable than oleic acid, which has one double bond.

Preparation of an ester with oleic acid produces a more oxidatively stable material than one with linoleic acid. The challenge is to separate these fatty acids from their readily available natural sources.

Ned Bowden, associate professor of chemistry at the University of Iowa in Iowa City, Iowa, says, “There are currently several ways to separate out fatty acids. Distillation can be used, but it is a difficult process because of the energy required and the fact that these fatty acids have similar boiling points. A second option is to use silver ion chromatography that is particularly good at separating out fatty acids with double bonds in a cis orientation. The problem is that this process is not suitable for scale-up to the level required for commercial use.”

A new approach for separating fatty acids is needed that is effective without being energy intensive. Such a process now has been developed.

AMINE SALTS
Bowden and his graduate student, Abhinaba Gupta, have developed a new technique for separating fatty acids through the use of a nanofiltration membrane prepared with polydicyclopentadiene (PDCPD). He says, “We started to use PDCPD as filtration membranes in a separate project because they work to separate diarylamines from triarylamines. Promising results were seen that showed separation could be done based on the size of these molecules.”

Fatty acids can be separated through PDCPD membranes because they have different sizes and shapes based on how many double bonds they have and whether the bonds are cis or trans. PDCPD is well suited for this application because this polymer does not have pores but, rather, contains openings between chains that molecules can slip through if they are small enough, but larger molecules do not pass through the opening and are retained. Thus, the separation is based on the size of the molecules.

The key to separating fatty acids was to prepare salts with specific amines because they possess sufficient size differences to be separated by PDCPD membranes. Bowden says, “We used amines that form closely associated ion pairs with the fatty acids. The amines we used are alkyl-based because they have the proper pka’s to form stable salts.”

Of utmost importance in this approach is the critical area of the molecule and not its molecular weight. Bowden says, “Critical area is the smallest area of an amine salt that permeates through the membrane. This parameter is much more difficult to measure, which we did through the use of the molecular modeling application Spartan.”

The researchers initially found that triisobutylamine can be used to separate fatty acids with cis double bonds (oleic, linoleic and linolenic) from trans double bonds (elaidic) and saturated fatty acids (stearic). Additional experiments showed that oleic can be separated from fatty acids with the same number of carbon atoms and more than one double bond (linoleic and linolenic acids). This feature could lead to the preparation of very pure oleic acid.

Initially, experiments were run at ambient temperature and no pressure. Bowden says, “We found that separation can take as long as three days.” The apparatus used in the experiments is shown in Figure 1.


Figure 1. The apparatus shown was used to separate fatty acids with cis double bonds from trans double bonds and fatty acids with no double bonds. Further experiments have shown that fatty acids with one double bond can be separated from fatty acids with multiple double bonds. (Courtesy of the University of Iowa)

Using 90 psi pressure dramatically improves the rate of separation to 20 minutes. Bowden envisions that this process will be attractive for commercial applications because of the low energy requirements and the fact that the fatty acids can easily be isolated from the amines, which can then be recycled for further use.

Future work will involve optimization of the right amines needed to separate fatty acids and the use of pressure. Bowden would also like to use a more environmentally friendly solvent and is evaluating supercritical carbon dioxide. Additional information can be found in a recent article (2) or by contacting Bowden at ned-bowden@uiowa.edu. Bowden also indicates that patents have been filed on this technology and he is interested in collaborating with industrial partners.

REFERENCES
1. Canter, N. (2011), “Silver Ionic Liquids: Potential New Solvents,” TLT, 67 (8), pp. 12-13.
2. Gupta, A. and Bowden, N. (2013), “Separation of cis-Fatty Acids from Saturated and trans-Fatty Acids by Nanoporous Polydicyclopentadiene Membranes,” ACS Applied Materials & Interfaces, 5 (3), pp. 924-933.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.