Balancing activity and stability in POSS supported porphyrin imidazolium catalysts for CO₂ valorization: Evidence of co-activation

Motivated by the ambition to turn CO₂ into value‑added chemicals, four innovative heterogeneous catalysts were designed to probe the promising route of converting CO2 and epoxides into cyclic carbonates. The synthetic strategy relies on a radical copolymerization of octavinylsilsesquioxane (octavynil-POSS), as the inorganic and robust core, with either tetrastyrylporphyrin aluminum chloride (TSP‑AlCl) or tetrastyrylporphyrin zinc (TSP‑Zn) monomers, in the presence of an imidazolium salt bearing chloride, bromide, or iodide as counter‑anions. This strategy yielded four bifunctional catalysts: POSS‑Zn‑Cl, and three aluminum based materials, POSS‑Al‑Cl, POSS‑Al‑Br and POSS‑Al‑I. The different solids were evaluated in the coupling of CO₂ with styrene oxide to form the corresponding cyclic carbonate. The influence of two Lewis‑acidic metal cations (Al3+ and Zn2+), combined with different nucleophilic anions (Cl⁻, Br⁻, I⁻), on the catalytic performance has been systematically investigated. Comprehensive spectroscopic and analytical characterization confirmed their structures and stability. Among the two metal cations, aluminum emerged as the most effective Lewis acid, as highlighted by the superior activity of POSS‑Al‑Cl compared to POSS‑Zn‑Cl. Within the aluminum-based series, POSS‑Al‑Br and POSS‑Al‑I exhibited even higher catalytic activity than POSS‑Al‑Cl but their difficult recoverability limited their practical applicability. Interestingly, POSS‑Al‑Cl achieved an optimal balance of activity, robustness, and recyclability. It was easily recovered from the reaction mixture and reused over four consecutive cycles without requiring any intermediate reactivation. Although, a slight decrease in catalytic activity was observed during the four consecutives runs, full catalytic performance was restored following a simple washing procedure, enabling the catalyst to regain its initial activity in the fifth cycle. In addition, a ²⁷Al ssNMR study was performed on the best-performing material, providing valuable insights into the aluminum coordination environment within the solid framework and clarifying its central role in epoxide activation during catalysis.