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 propose d for the formation of higher oxygenates from syngas over MnO-promoted Rh, Oxygenate precursors, CxHyOz, are formed at MnO sites in close pr oximity to the Rh-MnO interface. The role of Rh is the formation and d elivery of Ch(x) groups and H atoms, FTIR indicates that the CxHyOz co mplex is probably a surface acetate. Results from ethylene hydroformyl ation tests at relatively low temperatures, where CO dissociation does not occur, show that the presence of MnO in the catalyst does not enh ance the ability of Rh to catalyze CO insertion, At higher temperature s though, where CO dissociation occurs, higher oxygenates are formed o nly in negligible amounts over unpromoted Rh. The role of MnO in enhan cing the formation of higher oxygenates was tested using CHCl3 as a so urce of CHx groups instead of Rh. Formation of the oxygenate precursor CxHyOz from the surface reaction between CHCl3, CO, and H-2 is detect ed by FTIR on MnO in the absence of Rh. The same complex is formed fro m the same reaction over cosupported Rh/MnO and the physical mixture R h + MnO. However, its hydrogenation to higher oxygenates, occurs only over Rh/MnO, where Rh and MnO particles are in close contact, Furtherm ore, this hydrogenation is apparently confined to those acetate groups located on MnO sites at the Rh-MnO interface. Summarizing, the MnO pr omoter basically behaves as a cocatalyst; the formation of higher oxyg enates from syngas is another example of bifunctional catalysis with e ssential steps taking place at the interface of both catalytic functio ns. (C) 1997 academic Press.