Chemical derivatization of Athabasca oil sand asphaltene for analysis of hydroxyl and carboxyl groups via nuclear magnetic resonance spectroscopy

Athabasca oil sand asphaltene was methylated with different base catalyst/solvent combinations in order to find an optimum procedure for analysis of the number and types of hydroxyl and carboxyl functional groups. High resolution carbon-13, fluorine-19, and silicon-29 NMR spectra were used to monitor the degree of methylation, trifluoroacetylation, trimethylsilylation, and aromaticity of asphaltene. Tetra-n-butylammonium hydroxide as phase transfer base catalyst and tetrahydrofuran or dichloromethane as solvent result in enhanced O-methylation of asphaltene. At least two types of acidic oxygen containing functionality have been detected, viz. hydroxyl and carboxyl (aliphatic and aryl). On average there are few, less than or equal to4-8, hydroxyl containing groups (including COOH) per asphaltene molecule. C-13 NMR lineshapes suggest a broad asymmetric distribution of acidic sites. The NMR and elemental analyses allow for oxygen containing functionalities to be included in an average molecular structure. A sludge phase collected from aqueous and hydrochloric acid extractions of asphaltene has also been analyzed. A correlation is observed between the degree of O-methylation and the dielectric permittivity of the solvent and the acidity of the substrate reaction site. (C) 2002 Elsevier Science Ltd. All rights reserved.