Hydrocarboxylation of methanol to methyl acetate using rhodium and ruthenium nanoparticles supported on titanate nanotubes as catalysts: infrared spectroscopy study

Abstract

Titanate nanotubes (TiNT) were synthesized by the hydrothermal method and used as a support for Rh (Rh/TiNT) and Ru nanoparticles (Ru/TiNT). Both catalysts were found active in the gas-phase hydrocarboxylation of methanol with CO₂ and H₂ as reactants and methyl iodide as a promoter at 1 bar and 150 °C to produce methyl acetate (MA), the Rh/TiNT catalyst being more active in the reaction with a reaction rate of 0.698 mol_MA mol_Rh⁻¹ h⁻¹ compared with 0.272 mol_MA mol_Ru⁻¹ h⁻¹ for the Ru/TiNT catalyst. The structural, textural, and compositional properties of the supported metal catalysts were evaluated by N₂ physisorption, X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). Results of XRD and Raman spectroscopy demonstrated that TiNT consisted of a unique crystalline structure of trititanate (Na₂Ti₃O₇), while the results of TEM confirmed the multilayer nanotubular morphology with an internal diameter of ∼3 nm. Also, the TEM analysis of Rh/TiNT and Ru/TiNT samples revealed small (∼1 nm) and highly dispersed metal particles. Infrared spectroscopy studies performed under reaction conditions allowed the identification of important surface species; in particular, Rh-acetyl species were proposed as key reaction intermediates. This work shows an interesting and promising heterogeneous route for the valorization of CO₂ to produce a value-added compound under moderate conditions.