reverse water-gas-shift over a fischer-tropsch catalyst using entrapped iron nanoparticles in silicalite-1

SSCI-VIDE+ING+JHUInternational audienceAlthough already applied at industrial scale about one century ago, the Fischer-Tropsch process is gaining renewed interests as it is a key step for converting alternative feedstocks, including biomass to transportable fuels. Compared to Co-based catalysts, state of the art Fe catalysts show lower activity (per volume), lower selectivity as it produces a significant and undesirable quantity of CO2 and much faster deactivation. There is a need to develop more active, more selective and more stable Fe Fischer-Tropsch Synthesis (FTS) catalysts. Indeed, Fe particles, especially nanoparticles, sinter to very large particles within the first hours/days on reaction leading to low activity per mass of catalyst. Besides, typical Fe-catalysts contain many different phases, which strongly limit the establishment of structure-activity relationships.We describe here the synthesis of well controlled iron nanoparticles of 3.5 nm encapsulated in the walls of hollow silicalite-11. The encapsulation prevents the particle to sinter under reaction conditions leading to a high and stable Fe dispersion. In addition to the high activity, this catalyst is extremely selective as it does not produce any CO2.High resolution TEM and tomography (figure 1) show silicalite-1 hollow single crystals (~100 nm width) with a narrow distribution of the iron particle size centered at 3.5 nm. After 100 hours under reactions, non-significant sintering was observed. FTS performances of the Fe@silicalite-1 (3.4%Fe) were measured at 250°C, 20 bar and H2/CO = 2. In terms of activity per mass of iron the Fe@silicalite-1 catalyst is 10 times more active than a commercial catalyst. It also shows very high C5+ selectivity. In contrast to all other known Fe-catalysts, this iron ship-in-the-bottle catalyst does not produce any CO2. We may hypothesis that the silicalite-1 nano-shell around the Fe particle acts as a water repellent membrane which prevents WGS reaction to occur (CO + H2O => CO2 + H2).Further analysis of this catalyst shall provide insights into the sites responsible for CO2 production, paving the way to a rational design of iron-based FTS catalysts