Nafion Gas Dryers

Compounds Removed by Nafion® Dryers

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Compounds Removed by Nafion® Dryers
Nafion is a copolymer of perfluoro-3,6-dioxa-4-methyl-7octene-sulfonic acid and Teflon®
(polytetrafluoroethylene). This may seem confusing; in simpler terms Nafion contains a Teflon
backbone with occasional side chains of another fluorocarbon added. The fluorocarbon side chain
terminates in a sulfonic acid (-SO3H).
With the exception of the sulfonic acid groups, all of Nafion is a fluorocarbon polymer. Like most
fluoropolymers, it is chemically inert (extremely resistant to chemical attack). Its sulfonic acid
groups are immobilized within the bulk fluorocarbon matrix and cannot be removed, but unlike the
fluorocarbon matrix the sulfonic acid groups do participate in chemical reactions. The presence of
sulfonic acid makes Nafion selectively permeable to compounds that bind to sulfonic acid.
Nafion can function as a cationic exchange resin when exposed to liquids. In the liquid phase, ionic
compounds will dissociate into free ions that can interchange with the sulfonic acid group within
Nafion, and the ions will permeate very readily through the polymer.
Perma Pure dryers are designed for use only with samples in the gas phase. Compounds do not
dissociate into free ions in the gas phase until the temperature exceeds a thousand degrees. This
temperature is far above the operating temperature range of the dryers. Consequently, Nafion does
not permit ionic compounds to permeate in the gas phase unless they specifically complex with
sulfonic acid. Very few gaseous compounds complex (bind) with the sulfonic acid, so Nafion is very
selectively permeable to compounds in the gas phase.
Removal Mechanisms
Nafion removes compounds from a gas stream in three ways:
1. Compounds that bind to the sulfonic acid in Nafion will readily permeate through the polymer.
These compounds are limited essentially to gases that function as bases (in an acid-base
reaction). Not all but most bases contain an hydroxyl group (-OH). For this reason, the following
compounds are all actively removed by Nafion:
a. Water (H-OH)
b. Alcohols (R-OH where R is any general organic group)
c. Ammonia (forms ammonium hydroxide complex with water, NH3 + H2O = NH4-OH)
Primary amines (R-NH2) and secondary amines (R1,R2-NH) are removed by the same
mechanism as ammonia.
2. Nafion functions as an acid catalyst due to the strongly acid properties of the sulfonic acid group
within the Nafion. As a strong acid catalyst, Nafion converts organic compounds susceptible to
acid catalysis into other compounds. Strictly speaking the compound is not removed from the
sample gas, but it is converted into another compound instead. Acid catalysis occurs with
compounds that have:
a. Double or triple bonds between carbon atoms or between carbon and other atoms.
b. Steric stress in the molecule (structural stress that can be relieved by reorganization of the
Organic compounds that have single bonds between the carbons are described as simple
hydrocarbons, or alkanes (methane, ethane, propane, butane, pentane, hexane, heptane, octane,
etc.). These compounds do not undergo acid catalysis and are not removed.
Organic compounds that have double bonds between the carbons are called alkenes (ethylene,
propylene, butylene, etc.). Compounds that have triple bonds between the carbons are called
alkynes. Both alkenes and alkynes under go acid catalysis and may be transformed, depending
upon the specifics of the compound.
Benzene rings are very stable organic structures. Even though it has double bonds between carbons
within the ring structure, benzene does not undergo acid catalysis into another compound. This is
confirmed by EPA Method TO-14, which specifically cites Nafion gas dryers for use with benzene,
toluene, xylene, and various other organic compounds. When another group is attached to the
benzene ring, the other group will determine whether acid catalysis will occur. For example,
benzaldehyde is still an aldehyde, and will still undergo acid catalysis. The presence of a benzene
ring will not cause a compound to undergo acid catalysis, but it will not protect it from undergoing
acid catalysis if another active group is present that will undergo this reaction. Compounds
containing a benzene ring are described as aromatic compounds, and as a class they are not
removed by Nafion.
3. By a combination of methods #1 and #2 above, Nafion converts a compound into an alcohol, then
removes it. Many organic compounds contain a carbonyl group (-C=O). This group will undergo
acid catalysis to form an alcohol. The proper name for the reaction is acid-catalyzed enolization
(enols are another name for alcohols). In this reaction the carbon doubly bonded to oxygen
combines with water to form a diol, a carbon with two hydroxyl groups attached (C=O + H2O
yields HO-C-OH).
Aldehydes, ketones, and organic acids all contain a carbonyl group. All are removed by Nafion after
undergoing acid-catalyzed enolization. These compounds are not merely converted into another
compound, but actually permeate through the Nafion after converting into an alcohol.
Since the compounds removed by this process must first undergo acid-catalyzed enolization, the rate
of removal of the compound depends upon how easily it will undergo acid catalysis. This depends
upon the nature of the organic group R attached to the carbonyl group. For example, formaldehyde
has only a hydrogen in the R group position. Since the hydrogen will not readily accept charge
sharing, formaldehyde resists acid-catalyzed enolization, and little or no formaldehyde is removed by
Nafion from a gas sample. Benzaldehyde, on the other hand, has a benzene ring in the R group
position. A benzene ring will very readily accept charge sharing, so benzaldehyde undergoes acidcatalyzed
enolization and subsequent removal by Nafion.
Other organic compounds that have double or triple bonds between carbon and another atom (oxygen
or nitrogen) may also undergo acid catalysis and subsequent removal. Nitriles are an example of
compounds with a multiple bond between carbon and nitrogen that are removed by Nafion. Dimethyl
sulfoxide (DMSO) and tetrahydrofuran (THF) are two examples of other compounds that are also
removed by Nafion.
Nafion gas dryers directly remove water, alcohols, ammonia, amines, and other compounds that
possess an hydroxyl group or convert to one under acid catalysis. Some compounds are not
removed but are rendered unrecognizable by acid catalysis. Inorganic compounds in general are not
removed (other than water and ammonia).
TOMS RIVER, N.J. 08754
Phone: (800) 33 PERMA / (732) 244-0010
Fax: (732) 244-8140
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