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

High-temperature Fischer-Tropsch synthesis for the production of short-chain olefins over iron catalysts supported on multi-walled carbon nanotubes (CNTs) was investigated under industrially relevant conditions (340 degrees C, 25 bar, H-2/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 degrees C for oxygen functionalization (O-CNTs) and ammonia at 400 degrees 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 degrees C, the Fe/N-CNT and Fe/O-CNT catalysts were applied in FischerTropsch synthesis, in which they showed comparable initial conversion values with an excellent olefin selectivity [S(C3-C6)>85%] and low chain growth probability (alpha <= 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 FischerTropsch 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.