Operando characterisation of the products of Fischer-Tropsch synthesis in a fixed-bed reactor studied by magnetic resonance

Operando measurement of catalyst performance presents many difficulties as it is necessary to obtain measurements at reaction temperature and pressure and also at relevant length scales of multiple pellets to replicate the pellet-scale mass transport. Non-invasive measurement under such conditions is challenging. In this study we have developed an NMR/MRI compatible reactor that is of industrially-relevant scale and can operate at typical conditions for Fischer-Tropsch synthesis (FTS). The pilot-scale fixed-bed reactor (20 mm i.d.) used for FTS is operated at 220 degrees C and 37 bar and is packed with 1 wt % Ru/TiO2 catalyst pellets. Multiple magnetic resonance techniques have been developed and applied to achieve quantitative characterisation of product species including hydrocarbons, water, oxygenates and branched alkanes in pores. In addition to spatially-resolved measurements of molecular diffusion coefficients of the product species from which the carbon chain length of the saturated hydrocarbons can be determined, preliminary estimates of the relative amount of oxygenate and water species and branched saturated hydrocarbons present within the pore space of the catalyst are also determined using non spatially-resolved two-dimensional 1H T1-chemical shift and two-dimensional 1H double quantum filtered correlated spectroscopy (DQF-COSY), respectively. The concentrations of water and oxygenates detected inside the pores of catalyst pellets decrease when the feed H2/CO ratio decreases. The amount and extent of branching of saturated hydrocarbon species is observed to be higher than for the effluent wax product, most likely due to the effect of mass transfer limitation within the reactor. This work demonstrates that a toolbox of operando magnetic resonance techniques is available to characterise different product species of FTS from within catalyst pellets. The information provided by these techniques can give insight into how both catalyst and reactor can be optimised.