Octa-ion pair-functionalized POSS bridging homogeneous catalysis and heterogeneous recycle for fixation of CO2

Abstract

Bridging the gap of the exact molecular design of homogenous catalyst and the recyclability of heterogenous catalyst was a challenge, especially for the cycloaddition of carbon dioxide to epoxide (CCE) reactions due to the high boiling points of the product cyclic carbonates that exclude evaporation separations. We introduce octa-ion pair-functionalized polyhedral oligomeric silsesquioxanes (IP-POSS) nanoparticle catalyst featured homogenous catalysis and heterogenous recycle for catalytic CCE reactions. Guanidinium phenolate ion pairs covalently anchored on cubic POSS. A typical IP-POSS catalyst bearing 4-pyridinolate (4-OP⁻) and 1,5,7-triazabicyclo[4.4.0]dec-5-enium (TBDH⁺) ion pair, POSS-4-OP⁻·TBDH⁺, was evaluated in CCE reactions by catalyst loading of 0.20 mol% at 120 °C and 1.0 MPa (CO₂); high yield (97 %) and quantitative selectivity (99 %) was achieved by 24 h with a turnover number of 490. The high catalytic performance of the IP-POSS was attributed to the proximity effect. Bifunctional catalytic mechanism was proposed where pyridinolate attacked CO₂ and TBDH⁺ activated epoxide and validated by NMR titrations. The IP-POSS catalyst worked homogenously in CCE reaction mixtures at 120 °C, it aggregated spontaneously as the CO₂ vented and the temperature lowered to ambient by the end of the transformation. The precipitated IP-POSS was simply filtrated and recycled for 7 times with robust recovery (96–98 % yields, 99 % selectivity). A reversible homogenous catalysis to and fro heterogenous recycle of IP-POSS in a closed-loop was explained by the inter-nanoparticle bridging by dimeric ion paring, that aggregated and precipitated the IP-POSS. This work unveiled a practical strategy for developing nanoparticle organocatalysts that worked in homogeneous solvents and recycled in heterogeneous precipitation.