Nitrogen-rich nanoporous iron phosphonate as an acid–base bifunctional catalyst for efficient and selective CO₂ conversion without co-catalyst and solvent

Developing efficient and sustainable catalytic systems for carbon dioxide utilization is crucial in tackling the challenges posed by increasing greenhouse gas emissions. This study investigates the co-catalyst- and solvent-free cycloaddition of CO₂ with epoxides using a series of iron-based metal phosphonates, FePPA (iron phenylphosphonate), FeHEDP (iron hydroxyethylidene diphosphonate), and FeEDTMP (iron ethylenediamine tetramethylene phosphonate), with the aim of elucidating the role of acidic-basic sites in enhancing their catalytic performance. These catalysts, prepared using phosphonic acids with various functional groups, offer a platform for examining how structural modifications influence CO₂ fixation efficiency. Among them, FeEDTMP demonstrated superior catalytic performance, attributed to its bifunctional nature. Coordinatively unsaturated iron centers (Fe³⁺/Fe²⁺) provide Lewis acidity and N-containing moieties introduce basicity to activate epoxides and CO₂ synergistically. Unlike many metal phosphonates that require halogen-based co-catalysts, such as tetrabutylammonium bromide (TBAB), which pose environmental concerns and often complicate product separation, this work employs a truly co-catalyst-free and solvent-free system. The reaction mechanism is proposed without invoking external nucleophiles, highlighting the intrinsic bifunctionality of FeEDTMP in facilitating both CO₂ activation and epoxide ring opening. Extensive parameter optimization was performed to study the influence of catalyst loading, temperature, pressure, and reaction time. Under optimized conditions (30 mg of catalyst, 100 °C, 7 bar CO₂, and 24 h), FeEDTMP achieved 99% yield and ∼100% selectivity, with 100% epoxide conversion of CO₂. The catalyst also exhibited broad substrate scope, with steric and electronic factors influencing the reactivity of different epoxides while retaining its structural stability after catalysis. This work provides fundamental insights into structure-activity relationships and offers a promising route for designing green, halogen-free catalytic systems for CO₂ utilization.