Direct Synthesis of Dimethyl Ether on Bifunctional Catalysts Based on Cu–ZnO(Al) and Supported H₃PW₁₂O₄₀: Effect of Physical Mixing on Bifunctional Interactions and Activity

We studied different preparation methods to synthesize a series of bifunctional hybrid catalytic systems for the direct synthesis of DME from syngas. The objective was to optimize the contact and interaction between the methanol synthesis catalyst and the methanol dehydration catalyst (Cu/ZnO/Al₂O₃ and H₃PW₁₂O₄₀ supported on TiO₂, respectively) by using different mixing methods (simple mixing, mixing–milling, suspension, and mixing–pressing). It has been found that the close contact between methanol synthesis and acidic functions is highly dependent on the degree of mixing of the two catalysts. In this respect, the hybrid catalyst prepared by mixing–pressing, which represents the closest contact, shows the strongest alterations in both catalytic functions. These modifications are associated with a decrease in copper surface area and a decrease in strong acid sites caused by the physical blocking of active sites or by cation exchange of the Cu²⁺/Zn²⁺ species from Cu–ZnO­(Al) and the H⁺ from the H₃PW₁₂O₄₀ units. The activity results demonstrated that the mixing–pressing method leading to a closer contact between the two catalytic functions led to a very low dimethyl ether time yield compared to the other bifunctional catalysts prepared by less severe mixing methods (19.4 vs 205–335 μmol/min·g_cat). This work clearly indicates the importance of the mixing method in the synthesis of the hybrid catalyst to optimize the distance and interaction between the metal and acid sites.