Homogeneous and heterogeneous catalytic transformations of carbon dioxide by transition metal derived catalysts

Abstract

In the past five years, the search for alternative sources of petrochemical feedstocks has turned attention toward non-conventional sources of chemical carbon other than carbon monoxide. A renewed interest in the chemistry of carbon dioxide, the most inexpensive and abundant C₁ molecule has thus resulted. The homogeneous systems, group 6 metal anionic complexes or ruthenium carbonyl clusters, were found to be effective catalysts or catalyst precursors for the hydrogenation of CO₂ to small chain alkyl formates (HCO₂R where R = methyl, ethyl or n-propyl) using alcoholic solvents. Also, group 6 metal pentacarbonyl chlorides, (CO)₅MCl⁻ and bridging hydrides, μ-H[M₂(CO)₁₀]⁻, (where M = Cr, Mo or W), were found to be effective homogeneous catalysts for the production of long chain formate esters (HCO₂R where R = n-butyl or n-octyl) from CO₂, H₂ and alkyl halides (RX where X = Cl, Br or I) in the presence of alkali metal salts (NaHCO₃, Na₂CO₃ or K₂CO₃). These homogeneous processes were studied by spectroscopic methods and the most probable mechanisms were proposed in which CO₂ was found to be the source of chemical carbon. Alternatively, CO₂ can be totally hydrogenated to CH₄ using heterogeneous transition metal derived catalysts. Activated supported ruthenium carbonyl clusters have been found to be active catalysts for the methanation of CO₂ under 1 atmosphere of pressure and a CO₂/H₂ ratio of 1:2.5. The catalysts were characterized by diffuse reflectance infrared spectroscopy, surface area determination, transmission electron microscopy and oxygen titration experiments. Upon supporting on partially dehydrogenated alumina, the original nuclearity of the starting carbonyl cluster was retained. Also, the FTIR spectra showed that all the supported activated catalysts originating from low valent ruthenium complexes exhibited the same band pattern in the ν(CO) region. Differences in reactivity of the various ruthenium catalysts are attributed to differences in the dispersion of the metal and/or counterions. In general, the cluster derived catalysts were found to give higher conversions than the conventionally prepared catalyst (from RuCl₃).