Poly(ethyleneimine)-tethered Ir Complex Catalyst Immobilized in Titanate Nanotubes for Hydrogenation of CO2 to Formic Acid.

Production of formic acid (FA) by hydrogenation of CO₂ using a ternary hybrid catalyst, poly(ethyleneimine)-tethered Ir-iminophosphine complex (Ir-PN-PEI) immobilized in titanate nanotubes (TNTs), is reported. On the basis of comprehensive structural analyses, we show that Ir-PN-PEI is tightly immobilized within the cavity space of TNT without affecting the electronic/coordination states of Ir atoms. Liquid-phase CO₂ hydrogenation under pressurized CO₂ and H₂ demonstrates that the Ir-PN-PEI catalyst immobilized in Na⁺-type TNT exhibits the highest FA yields (TON>1000 for 20 h under mild reaction conditions (2.0 MPa, 140 °C)) and improved reusability, which far outperform those of unimmobilized prototype Ir-PN-PEI. The improved catalytic performance is attributed to the ability of TNT to efficiently capture CO₂ and to stabilize PEI, with Na⁺-type TNT with higher basic property providing a more productive effect. The catalyst is reusable over multiple cycles with activity comparable to those of heterogeneous catalysts previously reported, rendering this material suitable for efficient transformation of CO₂ into FA. [ABSTRACT FROM AUTHOR]