Hydrophilic or Hydrophobic? How Byproducts Change the Water Affinity of Fischer–Tropsch Catalysts

The impact of reaction byproducts on the water wettability of Fischer-Tropsch catalysts has been investigated by using supported cobalt and ruthenium catalysts. Water contact angle measurements showed an evolution from 75° to 140° for a supported cobalt on titania catalyst due to catalytic operation, indicating that the catalyst changed from hydrophilic to almost superhydrophobic. MALDI-FT-ICR-MS studies revealed that the dewaxed spent catalyst contained long-chain carboxylates, in line with the presence of carboxylic acids with carbon numbers up to C₁₀₀ in the heavy wax. Using dedicated experiments with alumina, silica, and titania support materials, it was found that alcohols, carboxylic acids, and amines adhered so strongly to the surface that they could not be removed by Soxhlet extraction with xylene. The resulting surface loadings varied from 0.3 to 3 molecules per square nanometer. For amines and carboxylic acids on alumina and titania, it resulted in a similar increase in water contact angle as observed for the spent catalyst. The molecular fingerprint of the organic-support interaction was obtained with ATR-IR, and using TGA data, it was shown that the presence of only 1-2 wt % of carboxylates on titania was sufficient to halve the water uptake capacity. Finally, operando NMR data using a Ru/TiO₂ catalyst provided evidence that during the first weeks of catalytic operation, oxygenates build up with a concomitant decrease in liquid water on the catalyst surface. The profound influence of minor amounts of adsorbed products is of strong relevance for both catalyst design and catalytic evaluation.