Biomass to liquids via Fischer-Tropsch synthesis. Electronic effects on cobalt-based catalysts

Abstract

The transition from fossil fuels to alternative energy sources is one of the main challenges of the XXI century. Several strategies are being studied for decarbonizing difficult sectors as aviation, maritime transport and long−distance trucking. Biomass to liquids via gasification and Fischer−Tropsch synthesis stands out as one of the most promising alternatives. However, numerous challenges, associated with the catalysts, arise from this technology. The aim of this work was to study, with systematic methodology, two challenges from the biomass to liquids process, in order to make small steps towards the feasibility of this technology. First, the investigation of a suitable catalytic system, cobalt−rhenium−manganese, and second, the assessment of phosphorus, a potential poison present in the biomass.

All the Mn-promoted catalysts exhibited an increase in chain growth probability and a decrease in methane selectivity. The origin of the promotion was investigated by operando DRIFTS finding a red−shift in the CO bonds vibrations attributed to an increase in electron density in the cobalt. This increase in electron density weakens the CO bond by an increased π back−donation from the cobalt to the CO. Therefore, the CO dissociation is facilitated, resulting in an increased chain growth probability. This results also pointed to an increase in site−time yield. However, it was only achieved in certain catalysts and support. By means of XPS, it was discovered that Mn migrates to the surface of the catalyst during the reduction procedure, blocking Co particles. With these results, it is therefore concluded that the existence of activity promotion might be in a balance between the decrease in CO bond energy and the blocking of Co active sites by Mn particles.

The effect of phosphorus on the cobalt−based catalyst was also studied. All the catalysts exhibited a decrease in activity, chain growth and olefin selectivity, while CH4 and light paraffins increased. The degree of deactivation varied depending on the type of support employed. The trend follows the strength of the metal−support interactions, γ−Al2O3 being the least affected, while both SiO2 were most affected. The DRIFTS spectra of the P−poisoned catalysts showed a blue−shift in the CO bond vibrations, indicating an increase in bond energy. P may be acting as a strong Lewis acid, accepting electron density and resulting in σ−donation from the CO to the catalysts. Therefore, P addition leads to a higher H2/CO ratio on the surface of the catalyst, resulting in a lower CO conversion and higher degree of hydrogenation of adsorbed intermediates.