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.
