Cu/SiO₂ catalysts for methanol to methyl formate dehydrogenation. A comparative study using different preparation techniques

A comparative study of the different preparation conditions required to produce supported copper catalysts by the wet impregnation (WI) and the ion-exchange (IE) methods has been carried out. Two silicas with different textural characteristics, and one form of naturally occurring pumice were used as supports. In order to obtain specific chemical species of copper in solution as well as different support reactivities, three different pH values (1, 4.5 and 11.5) for the impregnating solution were tested. The catalysts were characterized by means of different techniques, such as XFS, BET, TPR, dissociative chemisorption of N₂O, XRD, TEM, EXAFS and XANES and methanol (MeOH) dehydrogenation. The use of copper solutions with high pH, especially after several hours of contact with the chosen carrier, with high solution volume to pore volume ratios, modifies strongly the support texture decreasing the specific surface areas (SSA) of the final catalyst. Copper particle sizes lower than a critical limit, or that did not fulfill a certain ensemble requirement, were not active for methanol dehydrogenation indicating that below such limit, this might be a structure-sensitive reaction. A catalyst (WI-11 A) prepared by a simple incipient wetness impregnation method at an alkaline pH similar to that used for the IE technique, showed high dispersion and it was simply activated under reaction conditions (MeOH/N₂, 230 °C) giving conversions around 50% and selectivities to methyl formate (MF) in the order of 75-80% with no deactivation observed after 3 h of time on stream.