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Thermal oxidation of model molecules to reveal vegetable oil polymerization studied by NMR spectroscopy and self-diffusion
  • Mara Barani,
  • Rino Bonetti,
  • Wallace Parker
Mara Barani
ENI San Donato Milanese Oil and Gas Laboratory

Corresponding Author:[email protected]

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Rino Bonetti
ENI San Donato Milanese Oil and Gas Laboratory
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Wallace Parker
ENI San Donato Milanese Oil and Gas Laboratory
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Abstract

Oxidative polymerization of plant oils and lipids is poorly understood yet widely encountered. Oil oxidation is accelerated at high temperatures, typically above 110°C, where tri-acylglycerides are converted into toxic compounds and viscous deleterious polymers. Polymerization of mono-unsaturated oil (210°C, 3h, open to air) was investigated by comparing four similar sized molecules with different functional groups: oleic acid, methyl oleate, trans-7-tetradecene and stearic acid. Non-volatile products identified by NMR spectroscopy are minor ketones for saturated fatty acid (stearic acid), epoxides for acyl chains without acid groups (methyl oleate, tetradecane) and copious oligomerization, through ester cross-links, for acyl chains with acid and olefinic groups (oleic acid). Long range C-H coupling clearly shows ester (not ether) cross-links, contradicting long held beliefs. Chain fragmentation also occurs as revealed by species with methylene groups bonded to oxygen, -CH2-O-C(=O)-R. Large size (slow diffusion) of the first oligomer (trimer) formed by thermal oxidation of oleic acid, (representing hydrolyzed vegetable oil) was evidenced by DOSY (diffusion ordered spectroscopy). Since the first oligomers formed still have reactive groups (olefin, carboxylic acid), poly-ester formation is inevitable at longer oxidation times. Model oil reactions monitored by NMR spectroscopy are important for resolving the complex chemistry of vegetable oil polymerization.
20 Oct 2022Submitted to Journal of the American Oil Chemists' Society
20 Oct 2022Submission Checks Completed
20 Oct 2022Assigned to Editor
23 Oct 2022Review(s) Completed, Editorial Evaluation Pending
01 Nov 2022Reviewer(s) Assigned
06 Dec 2022Editorial Decision: Revise Major
16 Feb 20231st Revision Received
16 Feb 2023Submission Checks Completed
16 Feb 2023Assigned to Editor
16 Feb 2023Review(s) Completed, Editorial Evaluation Pending
21 Feb 2023Reviewer(s) Assigned
08 Apr 2023Editorial Decision: Revise Minor
10 Apr 20232nd Revision Received
10 Apr 2023Submission Checks Completed
10 Apr 2023Assigned to Editor
10 Apr 2023Review(s) Completed, Editorial Evaluation Pending
11 Apr 2023Editorial Decision: Accept