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

Abstract

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, ≤4–8, hydroxyl containing groups (including COOH) per asphaltene molecule. 13C 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.