Separation of cis-Fatty Acids from Saturated and trans-Fatty Acids by Nanoporous Polydicyclopentadiene Membranes

Abstract

This article describes the separation of mixtures of fatty acid salts using a new organic solvent nanofiltration membrane based on polydicyclopentadiene (PDCPD). Mixtures of free fatty acids could not be separated by the membranes because they permeated at similar rates. When triisobutylamine was added to the fatty acids, the cis-fatty acid salts (oleic, petroselinic, vaccenic, linoleic, and linolenic acid) had slower permeation though the membranes than saturated (stearic acid) and trans-fatty acid (elaidic acid) salts. The reason for the difference in permeation was due to the formation of stable salt pairs between the amine and fatty acids that increased their cross-sectional areas. The fatty acid salts derived from saturated and trans-fatty acids were smaller than the critical area cutoff for the PDCPD membranes, so they readily permeated. In contrast, the fatty acid salts derived from the cis-fatty acids had critical areas larger than critical area cutoff of the PDPCD membranes and had slowed permeation. The partitioning coefficients of fatty acids and fatty acid salts were investigated to demonstrate that they were not responsible for the difference in permeation. The use of pressure was investigated to greatly accelerate the permeation through the membranes. For a solvent mixture of 35/65 (v/v) toluene/hexanes, the permeation of solvent was approximately 39 L m(-2) h(-1). This value is similar to values reported for permeation through membranes used in industry. The separation of a mixture of fatty acids based on the composition of soybean oil was investigated using pressure. The saturated fatty acid salts were almost completely removed from the cis-fatty acid salts when iBu(3)N was used as the amine to form the salt pairs. The separation of the cis-fatty acids found in soybean oil was investigated with Pr(3)N as the amine. The oleic acid salt (oleic acid has one cis double bond) preferentially permeated the membrane while the linoleic (two cis double bonds) and linolenic (three cis double bonds) salts were partly retained. The separation of fatty acids using membranes may have real applications in industry to purify fatty acids on a large scale.