Comparison of the Quaternary Aromatic Carbon Contents of a Coal, a Coal Extract, and Its Hydrocracking Products by NMR Methods

Samples of a coal extract and its hydrocracked products have been examined by solution-state and solid-state NMR spectroscopy to establish if these methods could follow the reaction process that is expected to convert bridgehead C atoms into hydrogen-substituted C atoms. The changes have also been followed by size-exclusion chromatography (SEC), elemental analysis, basic -OH titration, and UV-fluorescence spectroscopy. NMR spectra in CDCl ₃ indicated that the products contained more bridgehead C atoms than the extract. Spectra obtained using a better solvent, 1-methyl-2-pyrrolidinone (NMP), did indicate that the extract contained more bridgehead C atoms than the products. The basic -OH measurement indicated rapid removal of the group by hydrocracking; however, this was a relatively minor correction to the overall quaternary carbon concentration. The evidence from SEC and UV-fluorescence suggested that the large molecules that were indicated to be present in the coal extract and the products did not give good NMR spectra, because the solvents did not dissolve them. Therefore, results from solution-state NMR of such coal liquids must be considered as only partial results and not covering the entire sample. Solid-state ¹³C NMR methods showed that the process solvent could not dissolve the most aromatic components of the coal, with a significant loss of aromaticity in the digestion stage. Nonquaternary suppression (NQS) spectra of the microbomb product indicated the formation of methyl groups by the formation of methyl-substituted five-membered rings from tetralin. Solid-state ¹H spectra of the coal extract and products showed a decrease of aromatic to aliphatic hydrogen on reaction, as expected.