Coordinatively Unsaturated Aluminum Enables Methanol-Selective CO₂ Hydrogenation With Zeolite-Supported Copper Catalysts

We demonstrate the synthesis of Cu(AlO_x)_a(SiO_y)_b clusters under the confines of MER zeolite, which hydrogenate CO₂ to methanol and dimethyl ether with 96% selectivity and a space time yield of 15.0 mmol_C g_Cu⁻¹ h⁻¹ at 250 °C (4:1 H₂:CO₂ and 5 MPa). A crucial aspect of the synthesis involves MER zeolite calcination, leading to framework dealumination and loss of long-range order. These Cu(AlO_x)_a(SiO_y)_b clusters consist of a high density of coordinatively unsaturated aluminum sites, which are lacking in conventional copper catalysts with similar stoichiometry, and stabilize copper in a more oxidic form that is characterized by higher reduction temperatures. Other copper-containing zeolites consisting of stable frameworks that do not dealuminate upon calcination (Cu-Li-FAU and Cu-Li-RHO zeolites) exhibit 99% selectivity to CO under the same reaction conditions. When compared with these catalysts and an industrial CuZnAl catalyst, kinetic analysis shows the Cu(AlO_x)_a(SiO_y)_b clusters are more intrinsically selective for methanol over the reverse water gas shift reaction at low CO₂ converions.