Surface intermediate evolution and reaction-network branching in ethanol-to-1,3-butadiene conversion over Zr-β zeolite: An experimental and DFT study

Ethanol-to-1,3-butadiene conversion is a representative cascade reaction involving coupled oxygenate transformation, hydrogen transfer, dehydration, and secondary hydrocarbon-growth reactions. However, the evolution of surface intermediates responsible for reaction-network branching and selectivity loss remains insufficiently understood. Herein, the reaction behavior of ethanol/acetaldehyde and representative intermediates over Zr-β zeolite was investigated by combining online GC-FID product quantification, online GC–MS product identification, extraction-GC–MS identification of retained surface species, in situ DRIFTS, TPSR/TPD-MS experiments, and density functional theory calculations. The results show that acetaldehyde participates in the conventional aldol-condensation pathway toward 1,3-butadiene, while acetaldehyde-derived oxygenates, including ethyl acetate and acetate/acetic acid species, constitute a parallel side branch that can lead to acetone formation through ketonization-related pathways. Intermediate-feeding experiments further indicate that acetone-derived species can be converted to propylene-related intermediates under ETB-relevant conditions. Although propylene is only a minor gas-phase product, it may act as a reactive olefinic intermediate involved in secondary hydrocarbon-growth reactions. Extraction-GC–MS analysis identifies 2-pentene and 5-methyl-1,3-hexadiene as dominant strongly retained surface species, and DFT calculations rationalize their possible formation through Brønsted acid-catalyzed propylene-involved oligomerization pathways. These findings reveal how Lewis-acid-catalyzed oxygenate transformation and Brønsted-acid-catalyzed hydrocarbon growth jointly contribute to reaction-network branching over Zr-β zeolite. This work provides mechanistic guidance for optimizing acid-site distribution, suppressing retained hydrocarbon species, and improving the selectivity and stability of zeolite catalysts in ethanol-to-1,3-butadiene conversion.