Pyrolysis‐gas chromatography/mass spectrometry of pitch fractions recovered from preparative size exclusion chromatography: structural differences with increasing molecular size

Abstract

AbstractA sample of high‐temperature coal tar pitch has been fractionated by preparative scale size exclusion chromatography (SEC). Twenty‐four fractions corresponding to narrow molecular mass distributions were collected at sequential elution times of 3 minutes. The preparative scale column is thought to have filtered out most of the very large molecular mass material and the analysis focused on smaller mass components. The recovered fractions were characterised by analytical SEC. Pyrolysis‐gas chromatography/mass spectrometry (pyrolysis‐GC/MS) of the fractions was used to evaluate changes in main structural features with changing molecular size.As expected, analytical SEC of the fractions showed a systematic shift to smaller molecular sizes with increasing elution time. UV‐fluorescence spectra of the samples showed parallel structural shifts to shorter wavelengths and gains in intensity with increasing elution times. Pyrolysis‐GC/MS showed parent polycyclic aromatic components considered typical of pitch. The major fragments detected were pyrenes and fluoranthenes (m/z 202) and chrysenes (m/z 228). Benzopyrenes (m/z 252) and larger aromatics were not prominent amongst the pyrolysis products. Taken together, molecular structures were found to change significantly from fraction to fraction, but no systematic pattern emerged with changing elution time. Some components were identified as having formed during the analytical pyrolysis procedure itself. As aromatic products larger than dibenzopyrene (m/z 302) would not elute from the column, detected fragments are thought to have detached from larger pitch molecules.The pyrolysis of the largest species gave only alkanes and alkenes, with the aromatic fragments thought to be too large to elute through the GC column. The work provides information regarding the aromatic groups that are readily lost from larger molecules. Larger aromatic structures are thought to condense (to char) during the analytical pyrolysis step; the method is limited in not being able to provide information about their structural features. Copyright © 2002 John Wiley & Sons, Ltd.