Hydrothermal stability of modified alumina-based catalyst supports under simulated high-conversion Fischer–Tropsch synthesis conditions

The chemical transformation of the widely utilized gamma-alumina (gamma-Al2O3) catalyst support to form boehmite (gamma-AlOOH) under hydrothermal conditions is unfavorable and can contribute to catalyst deactivation. For example, hydrothermal conditions are typically created in the Fischer-Tropsch synthesis (FTS) and can be detrimental to the performance of gamma-Al2O3-supported FTS catalysts that need to operate at high carbon monoxide (CO) conversions. In this study, an unmodified gamma-Al2O3 (Al) catalyst support was evaluated under various water partial pressures (PH2O = 2.0, 2.9, 5.3, 9.0, and 11.8 barg) in a total system pressure of 15.75 barg, simulating the hydrothermal environments realized at different CO conversions (XCO = 30%, 40%, 60%, 80%, and 90%, respectively) in the FTS. Post-exposure to the highest water partial pressure (PH2O = 11.8 barg; XCO = 90%), a decrease in the quantity of gamma-AlOOH detected in the PXRD patterns of the spent modified Al supports was observed, when compared with unmodified Al. This decrease followed the order; Al (48.9 wt%) > MgAl (13.1 wt%) approximate to ZnAl (12.0 wt%) > SiAl (0.0 wt%). ... mehrDespite challenges with quantification, the MnAl spent support contained gamma-AlOOH and MnxOy species after each test, with the presence of MnxOy species indicating the exsolution of Mn from the MnAl support. Overall, the modification of Al improved the hydrothermal stability of the resultant supports by limiting gamma-AlOOH formation under simulated high FTS-CO conversion conditions, with the stability level dependent on the type of dopant present in the support.