13C NMR Determination of Protonated and Nonprotonated Carbons in Model Compounds, Mixtures, and Coal-Derived Liquid Samples

Abstract

The ratio of protonated to nonprotonated carbons provides an estimate of the average ring size in complex polycyclic aromatic hydrocarbon mixtures such as heavy oils and coal-derived liquids. Two model compounds, six mixtures of known composition, and nine heavy oil samples have been studied by 13 C spin lattice relaxation inversion-recovery methods. Spin-lattice relaxation data differentiate protonated and nonprotonated carbons on the basis of relaxation differences arising from direct CH dipolar interactions. The integrated aromatic carbon intensities for a number of partially relaxed spectra were fit to a proposed spin relaxation rate expression. Results indicate that the carbon relaxation rates for both protonated and nonprotonated carbons are consistent, as expected, with the dipolar coupling of carbons to protons. The spin-lattice relaxation rates from nonlinear curve fitting of the composite resonance lines are consistent with experimental data for individual resonance lines in the model compounds and in the mixtures of model compounds involved in this study. The distribution of relaxation times for both carbon types is also estimated from the nonlinear fit of the experimental magnetization data. Simulations of composite magnetization under different conditions and an associated error analysis have been carried out to verify the validity of the composite relaxation expression. Low-temperature experiments were performed to simulate the heterogeneous lines observed in heavy oils and coal-derived liquids and to test the fitting method under heterogeneous line conditions.