Sol-Gel Synthesis and Characterisation of Nanoscopic FeF3-MgF2 Heterogeneous Catalysts with Bi-Acidic Properties.

Nanoscopic metal fluorides with surface hydroxy groups are of broad interest in heterogeneous catalysis. With both Lewis and Brønsted acid sites on the surface, these catalysts can be applied to a wide range of reactions. Having previously synthesised AlF₃- and MgF₂-based catalysts, we report a new transition metal fluoride with bi-acidity. A pre-dehydration procedure was developed to introduce hydroxy groups to a Lewis acid, FeF₃. Subsequently, ternary nanoscopic FeF₃-MgF₂ with enhanced porosity was prepared through a one-step fluorination. The interaction between MgF₂ and FeF₃ was elucidated. Surface characterisation revealed a remarkable increase in the surface area of FeF₃-MgF₂ compared with FeF₃. More importantly, medium-strong Lewis and Brønsted acid sites were detected on the FeF₃-MgF₂ surface. In line with its bi-acidity, FeF₃-MgF₂ was highly active in the model reaction, the isomerisation of citronellal to isopulegol. Finally, we discuss how the porosity and surface acidity jointly determine the activity of FeF₃-MgF₂. Our study demonstrates the feasibility of ternary FeF₃-MgF₂ and opens new possibilities to synthesise bi-acidic fluoride catalysts. [ABSTRACT FROM AUTHOR]