Insights into Active Sites of Dehydrogenation–Condensation–Dehydration Steps in Ethanol-to-1,3-Butadiene Conversion over Tailored MgO–SiO₂ Catalysts

The conversion of ethanol to 1,3-butadiene over MgO–SiO₂-based catalysts has attracted significant interest. Nevertheless, a precise understanding of the multistep reaction network and the nature of the active sites remains elusive. We herein report a strategically designed series of model catalysts with systematically varied Mg/Si ratios─SiO₂, 0.5MgO–SiO₂, 4MgO–SiO₂, and MgO─enabling the unambiguous identification of active site structures. Through comprehensive characterization and kinetic evaluation using distinct feedstocks (ethanol, acetaldehyde, their mixtures, and crotyl alcohol), we decouple the complex reaction pathway and establish quantitative structure–activity relationships for each key step: dehydrogenation, aldol condensation, and dehydration. The 4MgO–SiO₂ catalyst, comprising amorphous Mg₂SiO₄, possesses the highest density of basic sites and delivers the superior activity for ethanol dehydrogenation to acetaldehyde (0.087 μmol·g^–1·s^–1). In marked contrast, the 0.5MgO–SiO₂ catalyst, characterized by unique Si(OMg)(OH)(OSi)₂ and Si(OMg)(OSi)₃ structures, exhibits the highest concentration of acid sites. This specific acidic architecture drives exceptional performance in both acetaldehyde condensation to crotonaldehyde (0.865 μmol·g^–1·s^–1) and crotyl alcohol dehydration to 1,3-butadiene (7.355 μmol·g^–1·s^–1). Interestingly, even pure SiO₂, with its distinct Si–OH acid sites, shows notable dehydration activity (5.344 μmol·g^–1·s^–1), highlighting that both the acid site identity and density collectively govern dehydration activity. This work provides a novel mechanistic blueprint, demonstrating that ethanol-to-1,3-butadiene conversion follows a precise site-demand sequence: dehydrogenation is favored on strong bases, while condensation and dehydration are accelerated on tailored acid sites. These insights offer a clear strategy for the design of highly selective multifunctional catalysts for complex cascade reactions.