Cyclic carbonate synthesis from CO2 and epoxides over diamine-functionalized porous organic frameworks

•Diamine-functionalized porous polymer and its Zn2+/Co2+ complexed forms were prepared.•These catalysts were applied for the CO2 cycloaddition under mild reaction conditions.•High activity, stability, and reusability of the catalysts were demonstrated.•Mechanistic pathway was proposed covering different reaction temperature conditions.

Porous aromatic polymers functionalized with ethylenediamine (CBAP-1(EDA)) and then complexed with Zn2+ or Co2+ ions (CBAP-1(EDA-Zn)) and (CBAP-1(EDA-Co) were synthesized and applied as a catalyst for the solventless synthesis of cyclic carbonates from CO2 and epoxides. The properties of the catalysts were examined by various analytical techniques. CBAP-1(EDA) alone enabled the high catalytic conversion of epoxides (>98%) to five-membered cyclic carbonates at 130 °C under solvent-, metal-, and co-catalyst-free conditions with high selectivity. The catalytic activity of CBAP-1(EDA) was enhanced in the presence of a nucleophile and Lewis acidic metal ion sites. Thus, CBAP-1(EDA) with tetrabutylammonium bromide (TBAB) had shown 98% conversion of epoxide at 80 °C and 1 MPa CO2 in 8 h, whereas CBAP-1(EDA-Zn) and CBAP-1(EDA-Co) with TBAB achieved 96% epoxide conversion in 36 h at room temperature (RT). The stability of the catalysts was confirmed by hot-filtering and reusability tests, which demonstrated that the catalysts could be reused for up to five consecutive runs without any noticeable decline in catalytic activity. The synergism between the basic catalytic sites in CBAP-1(EDA) and TBAB and the Lewis metallic sites was explained by the proposed reaction mechanistic pathway covering both high temperature (metal- and halogen-free) and RT conditions (with metal and halogen).

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