Facile synthesis of layered spinel ferrite from fly ash waste as a stable and active ketonisation catalyst

Spinel catalysts exhibit superior activity and structural stability across a wide range of catalytic reactions. Nevertheless, few studies have delved into the synthesis of spinels containing more than four metal cations, for which conventional syntheses from pure chemical precursors are costly and generate significant waste. Here we demonstrate a facile, rapid and scalable synthesis of layered spinel ferrite catalysts from fly ash waste that is otherwise detrimental to landfill ecosystems. The optimum waste-derived catalyst primarily comprised MgAl_0.2Fe_1.8O₄, with a distorted structure due to the substitution of various cations (Ca²⁺, Mn²⁺, Mn³⁺, and Ti⁴⁺) at tetrahedral and/or octahedral iron sites, and demonstrates high activity (1.26 mmol⋅g⁻¹⋅min⁻¹) and stability (>100 h) for acetic acid ketonisation at a modest temperature (300 °C). Acidity measurements yield a corresponding turnover frequency of 2.21 min⁻¹. Strong synergies are observed between the different metallic cations and octahedral Fe²⁺ species; XANES and in-situ DRIFTS indicate the latter is the primary active sites for ketonisation in fly ash-derived spinel ferrites, promoting both acetic acid adsorption as bidentate acetate and subsequent C–C coupling to acetone.