Two-dimensional gas chromatography-online hydrogenation for improved characterization of petrochemical samples

Abstract

The Fischer-Tropsch (FT) process produces a variety of hydrocarbons over a wide carbon number range and during subsequent product workup a large variety of synthetic fuels and chemicals are produced. The complexity of the product slate obtained from this process is well documented and the high temperature FT (HT-FT) process products are spread over gas, oil and water phases. The characterization of these phases is very challenging even when using comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC-TOF-MS). Despite the increase in separation power, peak coelution still occurs when samples containing isomeric compounds are analysed by comprehensive two dimensional GC. The separation of cyclic and double bond containing compounds and combinations thereof is especially difficult since these compounds elute in a similar position on the GC×GC chromatogram and have identical molecular masses and similar fragmentation patterns. Pre-separation of alkenes and alkanes with HPLC prior to the GC×GC-TOF-MS analysis was used to distinguish between the cyclic and non-cyclic isomers of both these compounds. Subsequently, a method utilizing supercritical fluid chromatography (SFC) as an on-line pre-fractionation step was also developed. These pre-fractionation methods cannot be used to distinguish between classes like dienes and cyclic olefins since both contain alkene double bonds that will form adducts with the silver ions in these methods. On-line hydrogenation after GC×GC separation is a possible way to distinguish between these compounds since the number of rings and alkene double bonds can be determined from the mass spectra of the compounds before and after hydrogenation. This paper describes development of a GC×GC method with post column hydrogenation for the determination of the backbone of cyclic/olefinic structures enabling us to differentiate between classes like dienes and cyclic olefins in complex petrochemical streams.