The Role of Adsorbed Species in 1-Butene Isomerization: Parahydrogen-Induced Polarization NMR of Pd-Au Catalyzed Butadiene Hydrogenation

Isomerization of 1-butene critically influences product distributions in 1,3-butadiene hydrogenation. However, distinguishing between the isomerization and hydrogenation pathways is challenging. Here, we employ parahydrogen-induced polarization (PHIP) NMR spectroscopy to determine the extent of the isomerization pathway when using Pd-Au bimetallic nanoparticles synthesized via a colloidal protocol in the presence or absence of a polyvinylpyrrolidone (PVP) stabilizing ligand and immobilized on TiO2. Residual additives, in particular, sulfur, are observed to considerably influence the pairwise hydrogenation and 1-butene isomerization pathways. PHIP NMR analysis reveals that the PVP ligand can induce strong polarized signals, likely due to restricted proton migration, but minimally impact 1-butene isomerization. In contrast, removing surface sulfur species introduced during catalyst synthesis profoundly enhances 1-butene isomerization by reducing the hydrogen concentration at the nanoparticle surface. This work elucidates how residual species can modulate key reaction pathways such as isomerization during 1,3-butadiene hydrogenation, with implications for rational catalyst design.