No abstract

aIn the literature, it is widely claimed that during the initial period of Fischer-Tropsch synthesis liquid higher hydrocarbons fill catalyst pores completely, at least for temperatures of less than 250°C at a typical pressure of about 2 MPa. This leads to diffusional restrictions in the porous network for particles with a size for industrial fixed-bed operation (> 1 mm), whereby catalyst effectiveness and product selectivity are strongly affected. However, under industryally relevant reaction conditions, our experimental and theoretical investigations on the interplay of reaction and diffusion in cobalt catalyst particles reveal that the pores are only partly filled with liquid higher hydrocarbons even at a very long time on stream of months or more for a chain growth probability below about 0.8. Experiments were conducted in a magnetic suspension balance using particles of technical size (dp = 3 mm), and a mathematical model was developed describing quite accurately the formation, vaporization and accumulation of liquid products and their C-number distribution in the porous particle.

A promising method for the utilization of CO2 (e.g., captured from the flue gases of gas‐ or coal‐based power plants) is the production of liquid hydrocarbons from CO2 and renewable H2 (power to liquid, PTL). This is a three‐step process and consists of water electrolysis, reverse water–gas shift (RWGS), and Fischer–Tropsch synthesis (FTS). Here, the syngas for the FTS always contains CO2 owing to the incomplete conversion of CO2 in the RWGS reactor because of thermodynamic constraints. Therefore, the influence of not only the main reactants CO and H2 but also CO2 on the kinetics of FTS using a homemade cobalt catalyst was studied. Moreover, under effective conditions (i.e., with particles of millimeter size, as used in fixed‐bed reactors), the FTS is affected by internal mass‐transport limitations, which lead to an increased H2/CO ratio inside the particle, which has an impact on the local reaction rate and selectivity. Therefore, the effect of the H2/CO ratio was studied in a broad range of 0.5 to 40 at temperatures of 210 to 230 °C at a total pressure of approximately 3 MPa. With increasing H2/CO ratio and a surplus of H2, the methane selectivity rises and the selectivity to higher hydrocarbons decreases. As long as a certain (very low) amount of CO is present, CO2 behaves like an inert component. However, for particle sizes of several millimeters and pronounced pore diffusion limitations, the CO concentration decreases towards the particle center and a core region free of CO is formed. At H2/CO ratios >10, CO2 is also converted (but practically solely to methane). The intrinsic kinetic parameters of the reaction rates were evaluated by using Langmuir–Hinshelwood‐type rate expressions. The selectivities were also described by a model from Vervloet et al.1 The used models are in good agreement with the experimental results.

While the production of hydrocarbons by Fischer−Tropsch synthesis (FTS) is a widely recognized, yet technically quite complex way to transform biomass via syngas (mostly from biomass gasification) into liquid fuels, we here present an alternative route transforming biomass first into formic acid (FA) followed by syngas formation by decomposition of FA and finally FTS using regenerative hydrogen (or if needed H 2 from the stored FA) to balance the C:H ratio. The new method builds on the recently developed, selective oxidation of biomass to formic acid using Keggin-type polyoxometalates of the general formula (H 3+x [PV x Mo 12−x O 40 ]) as homogeneous catalysts, oxygen as the oxidant, and water as the solvent. This method is able to transform a wide range of complex and wet biomass mixtures into FA as the sole liquid product at mild reaction conditions (90 °C, 20−30 bar O 2 ). We propose to convert FA with hydrogen from water electrolysisthe electrolysis step producing also the oxygen for the biomass oxidation to FAto green hydrocarbon fuels using a typical Co-based FT catalyst.

The production of liquid hydrocarbons based on CO2 and renewable H2 is a multi‐step process consisting of water electrolysis, reverse water‐gas shift, and Fischer‐Tropsch synthesis (FTS). The syngas will then also contain CO2 and probably sometimes H2O, too. Therefore, the kinetics of FTS on a commercial cobalt catalyst was studied with syngas containing CO, CO2, H2, and H2O. The intrinsic kinetic parameters as well as the influence of pore diffusion (technical particles) were determined. CO2 and H2O showed only negligible or minor influence on the reaction rate. The intrinsic kinetic parameters of the rate of CO consumption were evaluated using a Langmuir‐Hinshelwood (LH) approach. The effectiveness factor describing diffusion limitations was calculated by two different Thiele moduli. The first one was derived by a simplified pseudo first‐order approach, the second one by the LH approach. Only the latter, more complex model is in good agreement with the experimental results.