Mild Supercritical-Gas Extraction from Low-Rank Coals: Separation, Spectroscopy, and Composition of Alkane Products

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Abstract
In the separation by silica-gel chromatography of the. total-alkane fractions of supercritical-gas (SCG) extracts of Turkish lignites, and in the subsequent 5Å-molecular-sieve adsorption to yield the branched/cyclic alkane subfraction, progress may be monitored by gas-liquid chromatography (GLC) and 1H nuclear-magnetic-resonance (NMR) spectroscopy. Saturated aliphatic hydrocarbons identified by gas chromatography/mass spectrometry from one lignite include normals n-C13 to n-C33 inclusive, acyclic isoprenoids C18, C19, and C20, and two C31 pentacyclic (triterpane) and one (C15) dicyclic cycloalkanes. The distributions of n-alkanes depend more on extraction conditions than do the distributions of acyclic isoprenoids and cycloalkanes. Overall, batchwise SCG extraction of low-rank coals with toluene or tetrahydrofuran at about 350°C appears to cause little thermolysis and can yield relatively large quantities of paraffinic hydrocarbons, representative of the original coals, and useful as geochemical markers.