A novel concept for the mechanism of higher oxygenate formation from synthesis gas over MnO-promoted rhodium catalysts

Based on data from zeolite-supported catalysts, a new model is proposed for the formation of higher oxygenates from syngas over MnO-promoted Rh, Oxygenate precursors, CxHyOz, are formed at MnO sites in close proximity to the Rh-MnO interface. The role of Rh is the formation and delivery of Ch(x) groups and H atoms, FTIR indicates that the CxHyOz complex is probably a surface acetate. Results from ethylene hydroformylation tests at relatively low temperatures, where CO dissociation does not occur, show that the presence of MnO in the catalyst does not enhance the ability of Rh to catalyze CO insertion, At higher temperatures though, where CO dissociation occurs, higher oxygenates are formed only in negligible amounts over unpromoted Rh. The role of MnO in enhancing the formation of higher oxygenates was tested using CHCl3 as a source of CHx groups instead of Rh. Formation of the oxygenate precursor CxHyOz from the surface reaction between CHCl3, CO, and H-2 is detected by FTIR on MnO in the absence of Rh. The same complex is formed from the same reaction over cosupported Rh/MnO and the physical mixture Rh + MnO. However, its hydrogenation to higher oxygenates, occurs only over Rh/MnO, where Rh and MnO particles are in close contact, Furthermore, this hydrogenation is apparently confined to those acetate groups located on MnO sites at the Rh-MnO interface. Summarizing, the MnO promoter basically behaves as a cocatalyst; the formation of higher oxygenates from syngas is another example of bifunctional catalysis with essential steps taking place at the interface of both catalytic functions. (C) 1997 academic Press.