Influence of Column Chromatography and Soxhlet Extraction on the Composition of Coal Pyridine-Soluble

Abstract

In order to ascertain the composition differences between extracting by column-chromatography (CC) and by the Soxhlet method, and to look for an economically-friendly solid-sample-extraction method, three coal samples A, B and C were extracted by pyridine solvent at 25°C and 120°C by these two methods, respectively. Size exclusion chromatography (SEC) coupled with photo diode array (PDA) detection was employed to analyze soluble extracts, which was applied to obtain molecular weight (MW) distribution and structural information within the range of 210–400 nm. Electro-spray ionization mass spectrometer (ESI-MS) and synchronous fluorescence spectroscopy with constant wavelength were used to investigate the detailed structural features and poly-aromatic properties. Results showed that the pyridine extraction yield of CC was higher than that of the Soxhlet method although some compounds with ultra-high MW were filtered by the silica gel used in the CC method. The extracts from the CC method gave better S/N (signal to noise) ratio in MS spectra. Five individual compounds were then identified that mainly consist of 3–5 aromatic rings. Nevertheless, extracts from Soxhlet method expressed lots of impurity noise in MS spectra which further interfered with their structural analysis. In conclusion, the CC method is a good substitute for Soxhlet extraction, especially to acquire the compounds with less than 1000 amu MW.