Controlling Hydroxyl Content in Pseudo-Boehmite Precursors for Efficient AlF3 Catalysts in HFC-245fa Dehydrofluorination

The gas-phase catalytic dehydrofluorination of hydrofluorocarbons (HFCs) to hydrofluoroolefins (HFOs) is a critical pathway for HFC resource utilization. Aluminum fluoride (AlF3) derived from Al2O3 fluorination is an efficient catalyst for this process, with its performance strongly dependent on the hydroxyl content and oxygen vacancy concentration of the Al2O3 precursor. In this work, pseudoboehmite (AlOOH) was calcined at 400–600 °C to prepare Al2O3 precursors (AlOOH-T) with tailored hydroxyl contents, followed by fluorination to obtain AlF3 catalysts (AlOOH-T-F). Results showed that increasing the calcination temperature reduced the hydroxyl content and increased the oxygen vacancy concentration in AlOOH-T, leading to significant variations in crystallite size, specific surface area, and surface acid properties of the derived AlF3 catalysts. AlOOH-500-F, derived from AlOOH calcined at 500 °C (optimal hydroxyl and oxygen vacancy levels), exhibited the smallest crystallite size (13.9 nm), the largest specific surface area (21.2 m2·g–1), and abundant medium-strength Lewis acid sites. In the dehydrofluorination of 1,1,1,3,3-pentafluoropropane (HFC-245fa), AlOOH-500-F achieved 75% conversion with no deactivation over 32 h of operation, outperforming catalysts derived from precursors calcined at 400 °C (AlOOH-400-F) and 600 °C (AlOOH-600-F). Further studies on the fluorination degree revealed that excessive fluorination enlarged AlF3 crystallite size and intensified surface acidity, accelerating coke-induced deactivation. Among the tested catalysts, AlOOH-500-5 achieved an HFC-245fa conversion of 80% with no deactivation trend observed over 192 h, demonstrating its excellent stability and potential for industrial applications. This work highlights the critical role of precursor hydroxyl content in tailoring the AlF3 catalyst structure and acidity, providing a practical guideline for designing industrially viable catalysts for HFC conversion.