Dehydrogenation of methanol to methyl formate over supported copper catalysts

Abstract

The catalytic dehydrogenation of methanol over copper-supported (Cu/ MO χ, M=Si, Zr, Mg, Zn and graphite) catalysts has been studied in a fixed bed microreactor in the temperature range of 180–260°C. The copper/silica and copper/zirconium oxide catalysts were found to be very active and selective toward methyl formate (MF) formation. However, the copper/zinc oxide catalyst was less active and the copper/magnesium oxide one much less selective. The exception was the copper/graphite catalyst whose performance approached that of massive copper. The reaction has also been studied on CuM/SiO 2 (M=Na 2O, ZnO, Cr 2O 3) promoted catalysts. Only in the case of copper promotion by 0.1% sodium (copper/sodium=11.2 atomic) was the specific activity found to increase, while the MF selectivity remained unchanged. The state of copper in the catalysts used in the reaction was determined by X-ray photoelectron spectroscopy. With the sole exception of the copper/magnesium oxide catalyst, which was partially oxidised as Cu 2+, the other catalysts showed only reduced copper species. For the used 15.1% copper/silica catalyst the position of the copper L 3VV X-ray induced Auger transition at 913.4 eV confirmed the existence of reduced Cu 0. Additional catalytic experiments carried out on unreduced silica-supported copper catalysts, using carbon dioxide as a carrier feed, showed that a longer time onstream was required before a steady state was achieved, which in turn is typical of catalyst activation by the hydrogen reaction byproduct (eqn. 4). Carbon monoxide poisoning experiments led to the conclusion that metallic copper is very likely the active copper species in this reaction. Furthermore, the infrared spectra of methanol adsorption on the poorly selective 5.7% copper/magnesium oxide catalyst revealed that the MF product undergoes decomposition through carbonate structures at basic sites.