DIRECT ESTIMATION OF AROMATIC CARBON (Ca) CONTENT OF BASE OILS BY ¹H-NMR SPECTROSCOPY

ABSTRACT

A direct and fast method for the estimation of aromatic carbon content (Ca) in base oils produced through different processes has been developed based on ¹H-NMR spectroscopy. The identity of the complex and overlapped ¹H-NMR region of 2.0−3.5 ppm, which constitutes resonances from α-substituents (−CH, −CH₂ and −CH₃ groups), has been revealed by 2D HetCor spectral analyses. This information has facilitated the estimation of quaternary (Cq) and bridgehead (Cb) aromatic carbons from ¹H-NMR spectrum using the derived equations. The complete and unambiguous assignment of the ¹H-NMR spectrum led to the estimation of Ca content of base oils. The classical Brown-Ladner (BL) equation for Ca estimation has been modified by taking the actual contribution of −CH, CH₂ and CH₃ groups, instead of using average values. The proposed ¹H-NMR, and the modified BL methods correlate very well (R² = 0.97) with the standard ¹³ C-NMR method for Ca estimation.