Nitrogen‐ and Oxygen‐Functionalized Multiwalled Carbon Nanotubes Used as Support in Iron‐Catalyzed, High‐Temperature Fischer–Tropsch Synthesis

Abstract

AbstractHigh‐temperature Fischer–Tropsch synthesis for the production of short‐chain olefins over iron catalysts supported on multiwalled carbon nanotubes (CNTs) was investigated under industrially relevant conditions (340 °C, 25 bar, H2/CO=1) to elucidate the influence of nitrogen and oxygen functionalization of the CNTs on the activity, selectivity, and long‐term stability. Surface functionalization of the CNTs was achieved by means of a gas‐phase treatment using nitric acid vapor at 200 °C for oxygen functionalization (O‐CNTs) and ammonia at 400 °C for the subsequent nitrogen doping (N‐CNTs). Ammonium iron citrate impregnation followed by calcination was applied for the deposition of iron nanoparticles with particle sizes below 9 nm. Subsequent to reduction in pure H2 at 380 °C, the Fe/N‐CNT and Fe/O‐CNT catalysts were applied in Fischer–Tropsch synthesis, in which they showed comparable initial conversion values with an excellent olefin selectivity [S(C3–C6)>85 %] and low chain growth probability (α≤0.5). TEM analysis of the used catalysts detected particle sizes of 23 and 26 nm on O‐CNTs and N‐CNTs, respectively, and Fe5C2 was identified as the major phase by using XRD, with only traces of Fe3O4. After 50 h time on stream under steady‐state conditions, an almost twofold higher activity compared to the Fe/O‐CNT catalysts had been maintained by the Fe/N‐CNT catalysts, which are considered excellent Fischer–Tropsch catalysts for the production of short‐chain olefins owing to their high activity, high selectivity to olefins, low chain growth probability, and superior long‐term stability.