Base-free CO₂ hydrogenation to methyl formate over Re/TiO₂ catalysts: Effects of the crystal phase of TiO₂

The crystal phase of TiO₂ plays a crucial role in tuning the catalytic performance of Re/TiO₂ catalysts for base-free CO₂ hydrogenation to methyl formate (MF). In this study, we investigated Re species supported on TiO₂ with different crystal phases (P25, rutile, and anatase) to elucidate their structure–activity relationships. Among them, P25-supported Re catalyst (denoted as Re/TiO₂-P) exhibited the highest catalytic performance, achieving a turnover frequency (TOF) of 4.8 h^–1 at 150 °C for 2 h, along with excellent stability over five consecutive cycles. By reducing the catalyst dosage, the TOF was further increased to 10.3 h^–1 at 160 °C. Comprehensive characterization revealed that the superior performance of Re/TiO₂-P arises from its relatively weak metal-support interaction, which facilitates the reduction of Re⁷⁺ to Re⁴⁺, the key active species for H₂ activation. These findings highlight the crucial influence of the TiO₂ crystal phase on the oxidation state and catalytic behavior of Re species, offering valuable insights for designing efficient catalysts for base-free CO₂ hydrogenation to MF.