Operando Structure Determination of Cu and Zn on Supported MgO/SiO ₂ Catalysts during Ethanol Conversion to 1,3-Butadiene

The electronic structure and reactivity of Cu- and Zn-promoted wet-kneaded MgO/SiO ₂ catalysts was interrogated during ethanol reaction to 1,3-BD. A multimodal nature of characterization, including in situ or operando X-ray, electron, light spectroscopies, and steady state reactivity measurements demonstrated critical information on the temporal evolution of the catalyst active sites including key measurements performed in operando conditions using synchrotron techniques (EXAFS and XANES). In situ DRIFT spectroscopy allowed decoupling of the aldol condensation and dehydrogenation reactive steps due to the promotion with enhanced ability to carry out aldol condensation, as correlated with the steady state reactivity experiments. In situ UV-vis spectroscopy presented a complex picture of the adsorbates with π-π∗ electronic transitions due to the allylic cations, cyclic or aromatic species while also suggesting oligomeric CuO species were formed. Operando X-ray measurements combined with ab initio multiple scattering modeling performed as a function of temperature identified a transient intermediate assigned to a 4-fold coordinate Cu species that was key leading to increase in Cu-Cu pair number. We identified two types of Zn pairs, namely Zn-O and Zn-Mg, during X-ray analysis under operating conditions. With Zn nearly 6-coordinated when in the vicinity of Mg while Zn-O species coordinated to nearly 4 nearest neighbors. The data suggest that such supported catalyst deactivation might proceed not only via carbon coking mechanism but also through the dispersed Cu site diffusion and growth due to the nearest neighbor oxygen atoms loss. The results presented suggest intermediates for segregation/deactivation mechanisms for a broader set of supported Cu and Zn catalysts used for alcohol upgrading catalytic reactions.