Experimental and DFT investigations of multiple active sites in Zr/Co-MOFs catalysts for tandem catalysis of styrene to styrene carbonate

The direct synthesis of cyclic carbonates from olefins involves a tandem process of olefin epoxidation and CO₂ cycloaddition, but designing a single catalyst with multiple active sites remains a challenge. In this work, a series of dual ligands quaternized Zr-based MOFs (Zr-Bpydc(Py⁺Br⁻)/CoTCPP-x, x = 1–5) were successfully synthesized by co-assembling 2,2′-bipyridine-5,5′-dicarboxylic acid (Bpydc) and cobalt(II) 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (CoTCPP) with Zr₆ clusters, followed by post-synthetic quaternization to incorporate nucleophilic Br⁻ sites. Among them, Zr-Bpydc(Py⁺Br⁻)/CoTCPP-4 exhibited the best catalytic performance under mild reaction conditions (80 °C, 1.0 MPa, 12 h), achieving styrene conversion of 94.2 % and styrene carbonate (SC) selectivity of 66.5 %, significantly outperforming most previously reported heterogeneous catalytic systems under cocatalyst-free conditions. The excellent catalytic activity was attributed to the synergistic effect among redox-active Co²⁺ sites, Lewis acidic Zr⁴⁺ and Co²⁺ sites, Br⁻ sites, and basic pyridyl sites. Importantly, the catalyst also demonstrated broad substrate applicability and good recyclability, with negligible activity loss over five consecutive cycles. Furthermore, density functional theory (DFT) calculations elucidated the energy profile and reaction pathway of the styrene oxidative carboxylation reaction, revealing the cooperative roles of multiple active sites in both epoxidation and CO₂ cycloaddition reactions. Therefore, this work proposes a strategy based on the synergy of multiple active sites for developing efficient and recyclable heterogeneous MOF catalysts for CO₂ conversion.