BIOMASS TO LIQUID PROCESS: NEW KIND OF COBALT AND IRON BASED CATALYSTS FOR THE FISCHER-TROPSCH SYNTHESIS

Nowadays it is imperative to develop economical and energy-efficient processes for the sustainable production of fuels and chemicals alternative to the ones deriving from petroleum. Climate change and air quality are major environmental concerns because they directly affect the way we live and breathe. In order to meet the present and future threats generated by emissions to the atmosphere, environmental agencies around the world have issued more stringent regulations. One of them is the control of residual sulfur in diesel fuel and emission standards for particulates from diesel vehicles. All these facts have recently aroused renewed interest in the Fischer–Tropsch Synthesis (FTS) because it can produce super clean diesel oil fraction with high cetane number (typically above 70) without any sulfur and aromatic compounds, using syngas - (mixture of H2, CO, CO2) - from natural gas, CH4, coal or, as a new tendency, from biomass. The essential target of FTS is to produce paraffins and olefins with different molecular weight and to limit the maximum formation of methane and CO2. FTS usually requires catalysts based on iron or cobalt. Iron catalysts are often preferred over cobalt–based ones especially when converting syngas with molar H2/CO ratio lower than 2 (corresponding to the stoichiometry required by the FTS reaction). This is also the typical H2/CO ratio of syngas produced from biomass or coal. In fact, iron-based catalysts are active towards the Water Gas Shift reaction (WGS: CO + H2O-> CO2 + H2), increasing the H2/CO ratio. On the other hand when feeding a syngas mixture with a H2/CO ratio close to 2, cobalt catalysts are preferred due to their high selectivity towards heavy hydrocarbons and their low activity in WGS reaction limiting the CO2 formation. Moreover Co-based catalysts exhibit longer life-time and higher CO conversion compared with Fe based catalyst. In this wide and well-known situation it is nowadays imperative to develop new innovative kind of catalysts in order to make possible the conversion of syngas and biosyngas (i.e. syngas produced from biomass) to useful hydrocarbon by FTS process. Considering very recent research results the aim of the PhD’s research was addressed toward the development of three particular kind of catalysts: The first group of catalysts tested were the Co-based hydrotalcites (HTlc) with different amount of Co in two different pilot plants. HTlc-based materials have been recently reported as good catalysts for several processes in the energy field. Up to now the only study reported in literature on the use of synthetic HTlc as FTS catalysts concerns their use as inert supports for the catalytically active metal. According to this study, hydrotalcite-supported catalysts result in higher activity than Co/Al2O3, even in absence of reduction promoters. The second group of catalysts tested were the Co-based catalysts and bimetallic Co-Ru based catalyst synthesized with the help of ultrasound, because the ultrasound are presented in literature as an innovative way to synthesize new kind of materials. Finally, were synthesized and tested samples of Fe-based catalysts supported on silica with the variation of the H2/CO ratio (with the aim to evaluate the performance of biomass) and with the results propose a kinetic model. Concerning the results obtained in this PhD’s research work, it is clear that all the samples tested have given good results. The Co-based catalysts, synthesized using the traditional impregnation method, with an additional step of ultrasound have given good results in comparison with the results in the current literature. The hydrotalcites have given lower results, if compared with the Co-based catalysts synthesized with the help of ultrasound, but they have opened an alternative and innovative way, that has never been tried before. Iron based catalysts allow a direct conversion of the biosyngas, and the results have shown how our catalysts are active with an H2/CO ratio ≤2. Furthermore, trends have been modeled with success. In conclusion, the PhD’s research work, has given a serious contribution to the current state of the art on catalysis in the Fischer-Tropsch synthesis either with cobalt and iron based catalysts. With cobalt has been optimized a traditional synthesis procedure with the introduction of ultrasound, furthermore has been created a completely new kind of catalyst. With iron has continued an optimization’s work of iron supported with high loading metals, so to develop a suitable kinetic model able to work not only with syngas, but also with biosyngas.