Multimode size exclusion chromatography with poly(divinylbenzene)/N-methylpyrrolidinone for the characterization of coal-derived mixtures

Multimode size exclusion chromatography (SEC) was investigated for the characterization of pyridine extracts of coal-derived materials. A system consisting of a poly(divinylbenzene) (PDVB) column with a mobile phase of N-methylpyrrolidinone (NMP) was evaluated. Model compounds covering a wide polarity range were studied to shed light on the separation mechanism. For neutral to moderately polar compounds, elution proceeded in order of decreasing molecular weight. Compounds showing predominantly size-dependent elution include polycyclic aromatic hydrocarbons, polyphenyls and azaarenes. For polar compounds, retention was largely independent of size effects and was governed mainly by polarity. In general, increasing polarity decreased the elution volume. Some polyfunctional polar compounds gave markedly decreased elution volumes compared with their neutral parent compounds. Increasing the extent of alkylation led to increased elution volumes for all compound types. Results are presented for pyridine extracts of Pittsburgh No. 8 bituminous, Montana Rosebud No. 10 subbituminous and Zap ND lignite. Chromatograms of raw coal extracts were characterized by a dominant, early-eluting peak consisting of highly-polar species whose retention was independent of size. The use of NMP as mobile phase permitted the dissolution and separation of highly polar material found in raw coal extracts. Also, it made possible the size-dependent separation of unsubstituted PAC, compounds that are abundant in extensively pyrolysed coal products.