(530g) Heterogeneous Catalyst Stability during Hydrodenitrogenation in Supercritical Water

Algae biocrude is a renewable, energy-dense material produced from the hydrothermal liquefaction (HTL) of microalgae. The biocrude contains 10-20 wt% N, O, and S heteroatoms, which must be removed to (1) prevent the formation of NO_x and SO_x upon combustion, (2) reduce the total acid number, and (3) increase the energy density of the biofuel. Catalysis in supercritical water (SCW) offers a “green” approach for removing these heteroatoms while preserving the energy content of the oil, however, many catalysts are not stable in hot water [1]. A key challenge for applying heterogeneous catalysts for biocrude upgrading in SCW is the limited understanding of the catalyst stability during hydrodenitrogenation (HDN) reactions. In addition to conventional deactivation mechanisms such as poisoning, coking, and sintering, catalysts subjected to hydrothermal reaction conditions can undergo changes in oxidation state, migration and leaching of active metals, and dissolution [1,2]. Temperature, pressure, and concentrations of nitrogen-rich reactants and products can accelerate these deactivation mechanisms by altering the pH and oxygen fugacity of the hydrothermal solution [3].

We constructed oxygen fugacity-pH diagrams to identify a library of catalytic materials that are thermodynamically stable during HDN of propylamine in SCW at 380-500 ^oC and 22-38 MPa. Among the materials assessed with the model, Pt/TiO₂ was chosen for further study as both Pt and TiO₂ are predicted to resist dissolution and changes in oxidation state in the reaction environment. Flow experiments with aqueous feeds of formic acid, ammonia, and propylamine were designed to isolate interactions between the catalyst, the SCW solution, and the reactants and products. When formic acid is in the feed, H₂ forms in-situ and Pt/TiO₂ is active for HDN of propylamine. Without formic acid, propylamine undergoes hydrolysis to propanol. The catalyst does not exhibit any evidence of oxidation or dissolution during the flow experiments. However, when formic acid is present, corrosion of the stainless-steel reactor tubing occurs prior to the catalyst bed and Fe deposits onto the catalyst. Oxygen fugacity-pH diagrams of Fe predict formation of Fe cations in the presence of concentrated formic acid solutions at temperatures below the critical temperature of water (374 ^oC), but these cations are much less soluble at higher temperatures. This thermodynamic analysis provides a key insight into the upstream reactor corrosion and subsequent Fe precipitation onto the catalyst. Overall, the experimentally observed hydrothermal stabilities of the catalyst and reactor materials agree with the stabilities predicted by the oxygen fugacity-pH diagrams. The results and methodology applied in this work can be used to improve the design of catalysts and reactor materials for future hydrothermal HDN reactions.

References

1. M. Yeh, J.G. Dickinson, A. Franck, S. Linic, L.T. Thompson, P.E. Savage, Technol. Biotechnol. 88 (2013) 12-24.
2. Xiong, H.N. Pham, A.K. Datye, Green Chem., 16 (2014) 4627-4643.
3. Kritzer, J. Supercrit. Fluids 29 (2004) 1−29.