A novel fluoride-doped aluminium oxide catalyst with tunable Brønsted and Lewis acidity

Abstract

The fluorination of transition aluminas has long been used to modify their properties and tune their catalytic performance. In this study, the preparation of a novel F-doped Al₂O₃ catalyst is demonstrated. The introduced two-step synthesis route enabled adjustment of the amount of fluoride and thus control of the characteristic properties of the catalyst. Structural investigation by XRD, ¹⁹F and ²⁷Al MAS NMR revealed the formation of an F-doped Al₂O₃ phase at high temperatures (700 °C). This new phase shows high similarity to common transition aluminium oxides like γ-Al₂O₃. Contrary to other fluorinated alumina, in this phase fluoride is finely distributed throughout the bulk and surface of the sample. Surface characterisation methods like nitrogen and pyridine adsorption and NH₃-TPD revealed a vital influence of fluoride doping for catalytic purposes. This is evidenced by the increased acidic strength and the introduction of strong Brønsted acid sites. Different from other systems, these Brønsted acid sites are formed despite the absence of a proton-rich atmosphere. The high catalytic potential of this new F-doped Al₂O₃ is additionally demonstrated in a catalytic test reaction which is the cyclisation of citronellal to isopulegol, a key step in the synthetic production of menthol.