CHARACTERIZING THE INFLUENCE OF Mn AND K PROMOTERS ON THE CARBURIZATION RATES AND PERFORMANCE PARAMETERS OF Fe-BASED CATALYSTS BY TPR-EXAFS/TPR-XANES AND CATALYTIC TESTING

摘要

Fischer Tropsch synthesis (FTS) is the Fe or Co catalyzed hydrogenation of carbonmonoxide to yield a mixture of hydrocarbons and oxygenates with molecular weights rangingfrom light gases (C1-C4) to heavy waxes (C_(30)+). Small quantities of Cu, Si and K are normallyused as promoters in Fe catalysts to enhance, respectively, its surface area, stability, as wellas the selectivity for high molecular weight product. Among the alternative promoters studied,Mn has also been studied, particularly after Kolbel et al. claimed in a patent [1] that higherselectivities for light products might be obtained on FeMn catalysts (promoted or not with Cuand K) catalysts. According to the authors, even if Fe is the minority component (the authorsmention compositions with at least 50% wt. Mn in their catalysts) high activities with highselectivities for the light products (C2-C4) are obtained with low methane selectivity. Manyauthors have investigated this catalyst, but questions about why Mn would: (a) increaseselectivity for low molecular weight hydrocarbons; or (b) why Mn, being present as an oxide,would not cause activity loss in the catalyst in the same manner as if Al or Si were present inhigh concentrations in the catalyst, remain to be answered. In order to investigate the effectof Mn in the structure/activity of FeMn catalysts, the carburization process of co-precipitatedFe catalysts containing different amounts of Mn, was studied by in-situ XAS from both Fe andMn K-edges. Catalysts containing low (Fe:Mn 100:30) and high Mn (Fe:Mn 100:90) atomicratios were tested for FTS and the results were compared to runs made with unpromoted Fecatalysts. It was found that Mn, present in the carburized catalyst basically in a MnO phase,decreases the amount of iron carbides in the catalyst carburized for 3 h at 300oC. However,the general activities (measured in CO conversion) are unchanged or even higher when Mnis present, even in high content. As far as the selectivities are concerned, higher methaneand water gas shift activities were observed with Mn promoted catalysts. Differences in bothcatalyst activity and selectivity will be correlated with structural information.