Passivation and reactivation of catalyst systems for the single step synthesis of dimethyl ether from CO-rich synthesis gas

A methanol catalyst based on Cu/ZnO/Al2O3 (CZA) was prepared and mixed with four different solid acids, a pure g-Al2O3, a SiO2-doped g-Al2O3, an AlPO4-doped g-Al2O3 and the zeolite H-MFI 400, respectively. These admixed catalyst systems were tested in the direct synthesis of dimethyl ether from CO-rich synthesis gas (CO:H2 = 1), which is typical for biomass-derived synthesis gas. Reactions were carried out in a laboratory plant at 250 °C and 51 bar for 120 h time-on-stream followed by catalyst passivation with O2 and regeneration by hydrogenation. After catalyst regeneration, the reaction was run again for another 72 h. The catalyst system containing the zeolite exhibited highest CO-conversion. However, all catalysts showed a drop of activity after the passivation- and regeneration-process and the system with the SiO2-doped g-Al2O3 showed the lowest loss of activity. Investigation of the catalysts after reaction by N2O-pulse chemisorption and XRD revealed that agglomeration of catalyst species took place leading to an increased Cu0-particle size for all systems. This phenomenon was less pronounced in the case of the system containing the SiO2-doped g-Al2O3. Thus, the main cause of catalyst deactivation was found to be sintering of the catalytically active components.