Structural Identification of Zn_xZr_yO_z Catalysts for Cascade Aldolization and Self-Deoxygenation Reactions

Here, complementary characterizations, such as nitrogen sorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), visible Raman, scanning transmission electron microscopy (STEM) coupled with elemental mapping, NH₃/CO₂ temperature programmed desorption (NH₃/CO₂-TPD), infrared spectroscopic analysis of adsorbed pyridine (Py-IR), and CO_2-IR, have been employed to identify the structure and surface chemistry (i.e., acid-base) of mixed Zn_xZr_yO_z oxide catalysts of varied ratios of Zn/Zr. Atomically dispersed Zn²⁺ species are present in the framework within a thin surface shell (1.5-2.0 nm) of ZrO₂ particles when the Zn/Zr ratio is smaller than 1/10; when the ratio is above this, both atomically dispersed Zn²⁺ and ZnO clusters coexist in mixed Zn_xZr_yO_z oxide catalysts. The presence of ZnO clusters shows no significant side effect but only a slight increase of selectivity to CO₂, caused by steam reforming. The incorporation of atomic Zn²⁺ into the ZrO₂ framework was found to not only passivate strong Lewis acid sites (i.e., Zr-O-Zr) on ZrO₂, but to also generate new Lewis acid-base site pairs with enhanced Lewis basicity on the bridged O (i.e., _{Zr—o$\frac{\curvearrowleft }{\frac{e}{\text{â}\text{€}\text{”}}}$Zn}). In the mixed ketone (i.e., acetone and methyl ethyl ketone (MEK)) reactions, while the passivation of strong acid sites can be correlated to the inhibition of side reactions, such as ketone decomposition and coking, the new Lewis acid-base pairs introduced enhance the cascade aldolization and self-deoxygenation reactions involved in olefin (C₃⁼-C₆⁼) production. More importantly, the surface acid-base properties change with varying Zn/Zr ratios, which in turn affect the cross- and self-condensation reactivity and subsequent distribution of olefins.