(68c) Supported Iron-Based Catalysts for the Fischer-Tropsch Synthesis of Lower Olefins

Lower olefins can be produced from synthesis gas via methanol to olefins process (MTO), via SDTO process (syngas via dimethylether to olefins) or by cracking of Fischer-Tropsch products [1]. These processes involve a large number of steps which are energy and time consuming. One interesting approach is the modification of the Fischer-Tropsch synthesis to directly produce lower olefins.   

The Fischer-Tropsch to olefins (FTO) process has to be carried out at high temperatures in order to drive the selectivity towards short hydrocarbon chains. Under these conditions, undesired side reactions as methanation and the Boudouard reaction (carbon deposition) are also thermodynamically favored [2]. Fe-based catalysts are preferred to perform this type of process due to their higher selectivity to olefins and lower methane selectivity at high temperatures compared with cobalt catalysts. However, carbon buildup on large iron particles may block the active sites in the catalyst surface, leading to deactivation. Moreover, carbon deposits can induce fragmentation of the carbide particles resulting in catalyst attrition [3].

A new type of catalyst has been developed in order to overcome activity and stability problems encountered when using bulk iron catalysts under FTO process conditions. These novel catalysts consist on Fe₂O₃ particles dispersed on an inert support. Here we have used several supports with a restricted reactivity towards iron oxide. More specifically, carbon nanofibers, a-Al₂O₃ and b-SiC have been loaded with 2-10%wt Fe. The support improves the mechanical stability of the catalyst, restricting the mobility of the particles and avoiding aggregation. Smaller Fe₂O₃ particles are less prone to carbon deposition which not only preserves catalytic activity but also avoids catalyst attrition.    

Supported iron catalysts showed 3 to 7 times increase in catalytic activity after 15 h of reaction at 1 bar, 350°C and H₂/CO = 0.5 when compared with an unpromoted bulk iron catalyst. Additionally, C₂-C₄ olefins selectivity was increased from 11 to 52-58%wt and a reduction of methane selectivity from 85%wt to 11-30%wt was observed. The dispersion of Fe₂O₃ particles on an inert support results in improved stability, activity and selectivity. These results were confirmed with high pressure measurements performed at 20 bar.

[1]        Janardanarao, M., Ind. Eng. Chem. Res. 29 (1990) 1735.

[2]        Wang, C. et al., J. Nat. Gas Chem. 12 (2003) 10.

[3]        Shroff, M. D. et al.,  J. Catal. 156 (1995) 185.