The effect of microwave heating on manganese promoted iron based Fischer-Tropsch catalysts
Date
2012-01-18
Authors
Mohiuddin, Ebrahim
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Abstract
A study was performed in order to investigate the effect of preparation method and the
effect of microwave heating on a manganese promoted iron based Fischer-Tropsch
catalyst. The effects of preparation method and microwave heating on the structure and
morphology of the catalyst, its surface area and reduction behavior were investigated
using various techniques such as Transmission electron microscopy (TEM), Powder x-ray
diffraction (PXRD), surface area measurements (BET) and temperature programmed
reduction (TPR). The FTS performance of the catalysts were also studied using a fixed
bed reactor with Fischer-Tropsch Synthesis conditions (270 C, flow rate of 30 ml/min,
H2/CO ratio = 2, pressure of 10 bar). Characterization of the catalysts calcined at 350 C
revealed that manganese enriched the surface of impregnated Mn/Fe catalysts and
suppressed the reduction of the iron catalyst. However, the Mn acted as a structural
promoter in the co-precipitated catalysts and also promoted the reduction of Fe2O3 as the
manganese content increased. The co-precipitated catalyst calcined at 650 C suppressed
the reduction of iron. The impregnated catalysts showed similar conversion (~ 70%) for
catalysts with Mn loadings 5%, 10% and 20%. This suggests Mn promotes the activity of
the iron catalyst since less iron is present in the catalyst as the manganese loading is
increased. The co-precipitated catalysts showed a 10 wt% Mn loading to be the optimum
amount for increased activity and selectivity to C2 – C4 hydrocarbons, lower molecular
weight olefins and a lower selectivity to heavier molecular weight hydrocarbons relative
to Mn loadings of 5, 20 and 50 wt%. Mn loadings in excess of 10 wt% showed a slight
increase in selectivity to heavier weight hydrocarbons. The impregnated catalysts showed
very little difference in activity and selectivity but the co-precipitated catalyst showed a
decrease in activity after the catalyst was microwave heated. A slight increase in
selectivity to lower weight olefins and heavier molecular weight hydrocarbons was noted
after microwave heating. The TPSR (Temperature programmed surface reaction) results
revealed that this may be due to the stronger adsorption of CO on the surface of the
catalyst after microwave heating. A similar trend was observed for catalysts promoted
with 0.1 wt% potassium i.e. a slight increase in selectivity to heavier weight
hydrocarbons after microwave heating.
Description
MSc., Faculty of Science, University of the Witwatersrand, 2011
Keywords
Fischer-Tropsch process, Catalysis, Synthesis, Catalysts, Solid state chemistry