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

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.