Understanding the Role of the Zr-MOF Support Structure on Templated Ternary CO2 Hydrogenation Catalyst Structure and Activity.

Abstract

Depending on catalytic reaction conditions, metal-organic frameworks (MOFs) are excellent supports or templates for catalysts, owing to their ordered porous structures, large surface areas, and degree of thermal and chemical stability. Indeed, the structural diversity afforded (e.g., Zr-node density, pore size, and ligand stability) enables a high degree of control over the chemistry and structure of Zirconium MOF (Zr-MOF)-based or MOF-derived catalysts (MDCs). Here, we synthesize ternary CO2 hydrogenation catalysts from MOF-based precatalysts and examine the effect that the MOF structural features have on the CO2 hydrogenation catalyst structure and activity. This was achieved by preparing Cu/ZnO@Zr-MOF precatalysts with microporous (UiO-66) and mesoporous (MIP-206 and NU-1000) templates. It was found that the Cu/ZnO@Zr-MOF precatalysts underwent in situ structural transitions under reaction conditions that were temperature dependent. Microporous UiO-66 converts to small domains of ZrO2, with Cu dispersion (surface vs interior) dictated by the rate of support conversion at 200, 225, and 250 °C (Cu/ZnO@ZrO2). The mesoporous MOFs (MIP-206 and NU-1000) templated Cu nanoparticles with ZnO clusters on a ZrO2 support (Cu/ZnO@ZrO2) under mild reaction conditions (200/225 °C, 40 bar, 3:1/4:1 H2/CO2), but at higher temperatures, the less stable MIP-206 support converted to small crystalline domains of ZrO2 as well as templating Cu nanoparticles. Indeed, these MDCs displayed varied catalytic activity and selectivity, depending on the MOF template and formation temperature, most notably MIP-206- and UiO-66-based catalysts showing improved activity for methanol formation when prepared at higher temperatures (250 °C, 40 bar, 3:1 H2/CO2; but tested under milder conditions), whereas the NU-1000-derived catalysts gave reduced activity due to pore blockage and poor access to catalyst sites. Our results indicate that higher-performing catalysts can be accessed through careful selection of precursor Zr-MOF with appropriate structure metrics and judicious choice of in situ activation conditions.