Ultrasound and microwave assisted preparation of high Fe loaded supported catalysts for biosyngas Fischer-Tropsch conversion

Supported Fe- based catalysts have several advantages (greater surface area, better dispersion of the heat developed by the reaction and better mechanical resistance) compared to massive iron catalysts adopted in the current Fischer Tropsch (FT) industrial plants. In particular, the optimized components loading was found to correspond to 30 wt% Fe supported on silica and promoted with K (2.0 wt%) and Cu (3.75 wt%) [1]. In order to study the influence of the preparation procedure, three different kind of catalysts were synthesized according to three different methods: 1) the traditional impregnation (TR) [1]; 2) Ultrasound (US) assisted TR method; 3) Microwave (MW) assisted TR method. All the samples were fully characterized by BET, ICP/OES, XRPD, TG-DTA, FT-IR, TPR, SEM and TEM and tested in a laboratory pilot plant. FT reaction activity tests were carried out in a fixed bed tubular reactor, using 1 g of fresh catalyst mixed with 1 g of diluting material (α-Al2O3, Fluka). All the catalysts were reduced in situ by a flow of H2/CO (molar ratio of 2/1) at 350°C, 500 kPa for 4 h and tested with the standard conditions of 46.8 Nml min-1 flow of syngas (molar ratio of 2/1) plus 5.0 Nml min-1 of N2 as internal standard, at 2MPa and T= 250°C for 90 h, as detailed in [1, 2]. The results of the characterization of the catalysts indicated that the morphology of the samples strongly depends on the method of preparation. Moreover, the samples treated with MW and US have higher surface areas than the traditional ones (Tab. 1). FT catalytic results are displayed in Table 1. The best results in term of C2+ yield (41%), with a good value of selectivity towards heavy hydrocarbons, was obtained using MW, while in terms of CO conversion (58%), using US. The samples prepared with non-traditional methods show better FT results, probably due to a more wide and uniform distribution of Fe achieved during the synthesis. This result was verified by TEM and is in agreement with previous results [3].