In-situ potassium poisoning on cobalt-based catalysts for Fischer-Tropsch synthesis

Abstract

[EN] Nowadays it is a fact that the energy demand is rising, and it is expected to continue. Furthermore, the world is experiencing now an energy crisis, both economic and environmental. Therefore, replacing fossil fuels with alternative energy sources is crucial, which is why biofuels derived from biomass are gaining popularity. For this purpose, the Fischer-Tropsch synthesis over cobalt-based catalysts seems to be promising. Fischer Tropsch is part of The Biomass to Liquid Process. Among the different products, light olefins represent added value compared to fuels, which always will be the main product. Via gasification, the biomass is converted to syngas that can be transformed into liquid hydrocarbons via Fischer-Tropsch. However, the gasification leads to the appearance of numerous impurities. Cleaning this syngas is essential since these impurities can be a poison for the catalysts, leading consequently to their early deactivation. This cleaning process is very expensive, so it is really important to evaluate the potential poisons in order to make it economically viable. Among these impurities, potassium is an essential alkali metal present in most biomass feedstocks. Its behaviour and effect in cobalt-based catalysts has been extensively studied but research over this topic is still uncertain. In this project a study has been carried out on 4 cobalt-based catalysts, promoted by Mn and Re and supported on alumina, titania, and silica. They have been poisoned in-situ with different K-salts: KNO3, KCl and K2SO4. This work has reaffirmed previous research that claimed that potassium causes a drop in catalyst activity and selectivity towards methane, while favouring C5+ products. KNO3 seems to be the K-salt that most influences catalyst behaviour, probably due to the occurrence of side reactions. At high potassium concentrations it is possible that the active sites of the precursor metal are blocked, so the effects on activity and selectivity are irreversible and further addition of K has no major effect. Moreover, the use of different support materials may influence the activity behaviour, probably due to the influence of the particle size. Also, it seems that the different cobalt loading does not make the potassium act differently on the catalyst. However, still further work will be necessary to fully understand the impact of this impurity over cobalt-based catalysts.

[ES] Actualmente es un hecho que la demanda de energía está aumentando mucho, y se espera que continúe con los años. Por esta razón, es evidente la importancia de la búsqueda de nuevas formas de energía que sustituyan a los combustibles fósiles. Es por ello que los biocombustibles derivados de biomasa están ganando popularidad debido a su alta disponibilidad y a su capacidad para reducir las emisiones de efecto invernadero. El objetivo principal del proyecto es el estudio de una serie de catalizadores basados en cobalto diseñados para llevar a cabo el proceso Fischer-Tropsch con énfasis en la selectividad de olefinas a partir de syngas derivado de biomasa. Este proceso parece ideal para producir estos biocombustibles. El estudio incluirá la síntesis de los catalizadores de síntesis Fischer-Tropsch, la caracterización de los materiales y el ensayo de los catalizadores.