Tuning the surface properties of novel ternary iron(iii) fluoride-based catalysts using the template effect of the matrix

Abstract

Sol–gel prepared ternary FeF₃–MgF₂ materials have become promising heterogeneous catalysts due to their porosity and surface Lewis/Brønsted acidity (bi-acidity). Despite the good catalytic performance, nanoscopic characterisations of this type of material are still missing and the key factors controlling the surface properties have not yet been identified, impeding both a better understanding and further development of ternary fluoride catalysts. In this study, we characterised the interaction between the bi-acidic component (FeF₃) and the matrix (MgF₂) on the nano-scale. For the first time, the formation pathway of FeF₃–MgF₂ was profiled and the template effect of MgF₂ during the synthesis process was discovered. Based on these new insights two novel materials, FeF₃–CaF₂ and FeF₃–SrF₂, were established, revealing that with decreasing the atomic numbers (from Sr to Mg), the ternary fluorides exhibited increasing surface acidity and surface area but decreasing pore size. These systematic changes gave rise to a panel of catalysts with tuneable surface and bulk properties either by changing the matrix alkaline earth metal fluoride or by adjusting their ratios to Fe or both. The template effect of the alkaline earth metal fluoride matrix was identified as the most probable key factor determining the surface properties and further influencing the catalytic performance in ternary fluoride based catalysts, and paves the way to targeted design of next-generation catalysts with tunable properties.