Development of Highly Selective, Water Tolerant Cobalt Catalysts for Fischer-Tropsch Synthesis

Gas-to-Liquids (GTL) processes, employing Fischer-Tropsch technology, can be used to monetize remote sources of natural gas by converting the gas into readily transportable hydrocarbon liquid fuels. During times when the oil price is low, the flexibility of the process allows tuning to produce value added products like speciality waxes and high quality base-oil feedstock, which can have a significant product premium over the standard transportation fuels. Cobalt based Fischer-Tropsch synthesis (FTS) catalysts are an integral part of the Gas-To-Liquids (GTL) process. Due to the cost of both cobalt and noble metals, which are often used as promoters, extended catalyst life at high volumetric conversion rates and required minimum selectivity, is required to make the process economically feasible.

Operation of cobalt catalysts at high conversions is advantageous for the efficiency of the FTS process. These conditions leads to an increased reactor water partial pressure as well as a higher H2O:H2 exit partial pressure ratio. It is well known that several deactivation mechanisms such as oxidation, sintering of cobalt, formation of metal support compounds and loss of catalyst integrity are accelerated in presence of high water partial pressures. Thus, the development of more robust, water tolerant FTS catalysts with the desired product selectivity is an important objective.

In this contribution we showcase some of the work done at Sasol Group Technology to develop cobalt catalysts that are stable against high water partial pressures and that can give high selectivity in slurry phase reactors. Various strategies are discussed to tune robustness and performance, which include, (i) tailoring the cobalt particle size distribution, (ii) employing non-traditional hydrothermally resistant support materials such beta-SiC, and (iii) optimization of the physico-chemical properties of the support materials. The learnings from the work have cumulated in the development of a new cobalt-based GTL catalyst, highly selective to hard waxes and base-oil feedstock. The robustness of this catalyst towards water induced deactivation was studied with an in-situ magnetometer which allowed real time monitoring as a function of process conditions. Performance data of this catalyst in pilot scale slurry reactors will be demonstrated and this will be benchmarked against various state-of the art competitor systems.