Effect of Calcination Atmosphere and Temperature on the Hydrogenolysis Activity and Selectivity of Copper-Zinc Catalysts

A series of CuZn catalysts with a Cu/Zn ratio of 1.6 was prepared by the calcination of a single precursor, CuZn-P consisting of an equimolar mixture of aurichalcite and zincian malachite, in three different calcination atmospheres (air, nitrogen, and hydrogen) at three temperatures (220, 350, and 500 degrees C). All catalysts were characterized by XRD and N-2-physisorption to assess their phase composition, crystallite sizes and textural properties and tested in dimethyl adipate (DMA) hydrogenolysis in a batch reactor at 220 degrees C and 10 MPa H-2. The XRD examination of these catalysts proved that both parameters, calcination temperature and atmosphere, affected the resulting phase composition of the catalysts as well as their crystallite sizes. In an oxidizing atmosphere, CuO and ZnO in intimate contact prevailed whereas in inert or reducing atmosphere both oxides were accompanied by Cu2O and Cu. The crystallite size of Cu2O and Cu was larger than the size of CuO and ZnO thus indicating a less intimate contact between the Cu-phases and ZnO in catalysts calcined in nitrogen and hydrogen. Catalysts prepared by calcination at 220 degrees C and CuZn catalyst calcined in the air at 350 degrees C significantly outperformed the other catalysts in DMA hydrogenolysis with a 59-78% conversion due to the small crystallite size and intimate contact between the CuO and ZnO phases prior to catalyst reduction. Despite the low DMA conversion (<30%), transesterification products were the main reaction products with overall selectivities of >80% over the catalysts calcined in nitrogen or hydrogen at least at 350 degrees C. The obvious change in the preferred reaction pathway because of the atmosphere calcination and temperature shows that there are different active sites responsible for hydrogenolysis and transesterification and that their relative distribution has changed.