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.
The AlF 3 catalysts with controlled morphology (cubic, hexahedron and octahedron) were synthesized via hydrothermal method using different solvents. The dehydrofluorination of 1,1,1,3,3-pentafluoropropane (HFC-245fa) was used as the model reaction to evaluate the catalytic performance of the AlF 3 catalysts with different morphology. Results showed that surface area, crystal structure, surface acidic properties together with the chemical composition of the AlF 3 catalyst were substantially influenced by the different morphology. XRD results showed that κ-AlF 3 crystalline structure was formed when using ethanol as solvent while β-AlF 3 crystalline structure was formed when using water as solvent. Among the three prepared AlF 3 samples, the AlF 3 -C catalyst with cubic shape demonstrated not only largest surface area but also most amount of medium-strength surface acid sites, resulting in enhanced reaction activity towards the dehydrofluorination reaction. The characteristic high Miller index crystal planes of (402) and (321) exposed on the cubic shaped AlF 3 -C catalyst is believed to be the main reason accounting for the high catalytic performance as there are equal amount of Al and F atoms exposed. Moreover, a reaction mechanism describing dedehydrofluorination of HFC-245fa on AlF 3 catalyst with equal amount of Al and F atoms exposed on certain crystal planes was proposed. Graphic Abstract
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