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teflon inert?

Hi. Just wondered if anyone can shed some light on a problem that is puzzling the hell out of me. I work in an ethylene manufacturing plant and have been searching for low level polar impurites in the ethylene, using various techniques. I used to sample the ethylene in stainless steel sample cylinders until I learned about the problem of sulfur compounds adsorbing on stainless steel. So I got myself some teflon lined cylinders and now I keep finding high levels ( well 2 to 6 ppm is high in this business) of ethanol ( positively identified by GC/MS)every time I sample in the teflon cylinders but never in stainless steel cylinders. Other tests confirm there is no ethanol in the ethylene. we also see low levels of butanol and isopropanol which is interesting because the ethyelene also contains levels of propylene and butenes. either their is some residual contamination of ethanol in the cylinders, which is hard to believe because i purge them with copious quantites of nitrogen and then the ethylene sample, or even seemingly less plausible ethanol is being generated somehow through reaction of the teflon and the ethylene. I have no idea what the mechanism could be. the teflon lining in the cylinders , is green in color. Why would that be? The cylinders were purchased new and have not been used for anything other than ethylene gas, ( and as mentioned purged with high purity nitrogen) I would appreciate any ideas thanks terry.

Reply to
d&tm
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Some wild speculation from my side:

"Teflon" is mechanically not very stable, so often instead of pure PTFE composites are used (like the Silverstone coating in household frying pans).

On the other hand perfluorinated compounds are know to easily dissolve large amounts of oxygen (potential uses include blood substitution and doping).

Now you have some potential catalytic material (metal), large concentration of oxygen and ethylene, propylene, butylene... I think the possible consequences are clear.

I did a very quick database search and although I couldn't produce anything which really backs up my thoughts perhaps the following paper justifies part of my reasoning:

Author(s): Tsang, SC; Zhu, J; Steele, AM; Meric, P Title: Partial aerial oxidation of nonpolar alcohols over Teflon-modified noble metal catalysts in supercritical carbon dioxide Source: JOURNAL OF CATALYSIS, 226 (2): 435-442 SEP 10 2004 Abstract: We have reported earlier that modification of commercial graphite Pt-supported catalysts with Teflon fluorinated polymeric coating of a very strong hydrophobic nature can significantly improve catalytic activity for aerial oxidation of water-insoluble alcohols such as anthracene methanol in supercritical carbon dioxide (scCO(2)). Thus, this paper presents some further characterization of these new catalyst materials and the working fluid phase during the catalysis. Using the same Teflon-modified metal catalysts, this paper addresses the oxidation of another water-insoluble alcohol molecule, m-hydrobenzoin in scCO(2). It is found that conversion and product distribution of this diol oxidation critically depend on the temperature and pressure of the scCO(2) used, which suggest the remarkable solvent properties of the scCO(2) under these unconventional oxidation conditions. (C) 2004 Elsevier Inc. All rights reserved.

Regards, Oliver

Reply to
Oliver 'Ojo' Bedford

Thank you for this Oliver. Sounds very interesting indeed. I will get the paper and maybe contact the authors. I was not aware of the ability of Teflon to dissolve oxygen, although my cylinders would rarely see oxygen, but maybe it is still there from the manufacturing process. terry

Reply to
d&tm

Ethylene plus water (acid catalyst) -> ethanol.

Is there water (1-2 ppm) in the ethylene? If the ethylene does not contain any water, bake-out the Teflon lined cylinders in a

100C vac. oven for 24 hours, and fill with N2; never expose to air. Is ethanol still present?

The steel cylinder may have been "primed" (hence the green color) to promote adhesion between the Teflon and steel. Is the "primer" an acidic compound, or is it perhaps a transition metal compound that may catalyze the addition of water to ethanol.

Depending on the manufacturing process used, some fluorine containing polymers may contain a few carboxylic acid end groups. Pefluorocarboxylic acids may catalyze the addition of water to double bonds. If persulfate initiator was used, the polymer may contain a few sulfate end groups. If ammonium persulfate was used, during drying and processing of the polymer, NH3 should vaporize, leading to "sulfuric" acid end groups.

Ernie

Reply to
Ernie

Ernie, no there is very little water present, less than 0.1 ppm. the cylinders dont get exposed to air at all. they have dry break quick connect fittings. the cyclinders are usually left with the last ethylene sample under a pressure of about 20 bar.

I am hoping the manufacturer will give me some more info on this.

Lots of possibilities there, thanks Ernie. So much for Teflon being inert. terry

Reply to
d&tm

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