Comprehensive multidimensional gas chromatography for the analysis of Fischer-Tropsch products

[摘要] ENGLISH ABSTRACT: The analysis of Fischer–Tropsch–derived (FT–derived) synthetic crude and derived productsis very challenging because of the highly complex nature of these products. In this study, theuse of comprehensive multidimensional gas chromatography (GCxGC) with time-of-flightmass spectrometry (TOF-MS) and flame ionisation detection (FID) was investigated for theanalysis of these products and the technique was found to be invaluable for the analysis ofthese complex mixtures.The compositions of FT synthetic crude, produced at low temperature (LT–FT) and hightemperature (HT–FT) processes were compared and the effect that changes in FT reactiontemperature has on product formation was investigated. Results for conventional onedimensionalGC (1D-GC) and GCxGC were compared. It was found that conventional 1D–GCdoes not have sufficient peak capacity to separate the thousands of compounds in the HT FTproducts. GCxGC provides a huge peak capacity of tens-of-thousands to separate highlycomplex mixtures. Structured chromatograms, where groups of compounds with similarproperties are grouped together, aid in peak identification. Moreover, sensitivity at lowmicrogram per milliliter levels is obtained. These attributes enabled accurate analysis ofvarious complex feed and product streams in the FT refinery, and also various final fuelproducts.The use of GCxGC alone was demonstrated, and also combined with high performance liquidchromatography (HPLC), supercritical fluid chromatography (SFC) and nuclear magneticresonance (NMR) when even more separation power was needed. HPLC–GCxGC enabledthe separation of alkene and cyclic alkane compound classes in oligomerisation products.These compound classes have similar mass spectra, elute in adjacent regions and co–eluteeven to some extent on the GCxGC contour plot, making differentiation difficult. SFC is agood replacement for HPLC for these applications because it does not use solvents as mobilephases. CO2 is easily evaporated after the separation and does not interfere with the GCxGCseparation of the analytes. SFC is also a very good technique to separate the compoundclasses of alkanes, alkenes, aromatics and oxygenates, and is therefore highlycomplementary to GCxGC. The combination of GCxGC with NMR data was also found to be very valuable for theidentification of branched alkane isomers in LT–FT diesels. GCxGC provides excellentseparation of individual compounds but the identification of isomers (except for mono–methylbranching) is difficult because the mass spectra of most of these isomers are similar and notall compounds are in the mass spectral libraries. NMR, on the other hand, is able todistinguish between the individual types of branched isomers but has limited separationpower for the complex mixtures. By combining the two techniques, the best of both wasobtained.The study found GCxGC to be invaluable for the analysis of the highly complex FT–derivedproducts, while its combination with other techniques such as HPLC, SFC and NMR providedeven more separation power.

[发布机构] Stellenbosch University