Electron spin resonance study of free radicals in Athabasca asphaltene

✍ Scribed by Shigeya Niizuma; C.Truman Steele; Harry E. Gunning; Otto P. Strausz

• Publisher: Elsevier Science
• Year: 1977
• Tongue: English
• Weight: 888 KB
• Volume: 56
• Category: Article
• ISSN: 0016-2361
• DOI: 10.1016/0016-2361(77)90004-7

✦ Synopsis

The asphaltene of the Athabasca bitumen exhibits a single, structureless e.s.r. signal at a g-value of 2.0032 with an intensity corresponding to approximately one free spin per ninety molecules. From the temperature dependence of the absorption it is concluded that absorption is due to stable polycycloaromatic doublet-and triplet-state radicals. Singlet-state e.s.r. inactive biradicals are also present in thermal equilibrium with the triplets which lie 300 cm-' above the singlet level. The internuclear separation of the radical sites in the triplet biradicals is estimated to be greater than 1.7 nm. Oxygen causes a reversible enhancement of the e.s.r. absorption by inducing the conversion of singlet biradicals to doublets. A quantitative treatment of the enhancing effect based on the Langmuir adsorption isotherm made it possible to derive an activation energy of approximately 17 kJ/mol for the dissociation of the oxygen-asphaltene complex and to evaluate the temperature variation of the number of radical pairs which contribute to the intensity variation. The kinetics of the spontaneous relaxation of the Oz-asphaltene system were also investigated and it was possible to show that the rate-controlling step, which is diffusion of oxygen into the interior of the solid asphaltene, has an activation energy of 25-29 kJ/mol.