Probing Cu-USY Zeolite Reactivity: Design of a Green Catalyst for the Synthesis of Diynes

Cu-exchanged USY zeolite was synthesized by vapor-phase exchange of parent HUSY with CuCl. Reaction parameters were varied in order to investigate the relationship between the structure of Cu+-promoted FAU zeolites and their catalytic properties in C-C bond homocoupling reaction between two acetylenic molecules (Glaser reaction). A proper choice in the CuCl chemical vaporization duration and temperature led to an increase in extra-framework aluminum species formation and thus to higher Lewis acidity. Moreover, such treatment under nitrogen flow during two days at 623 K kept the material highly crystalline. A drastic influence of the zeolite topology and properties (channels or cages, pore diameter, acidity) was noticed in the conversion of phenylacetylene. Observation of the catalyst before, during, and after the reaction, by multinuclear magic-angle spinning nuclear magnetic resonance, X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and H/D isotope labeling, revealed the necessity for the zeolite to still exhibit both Bronsted and Lewis acidity, after exchange with Cu+ cations. Indeed, the reversible change in copper cations oxidation state from + I to + II was demonstrated by XPS, thus confirming a heterogeneous catalytic process over zeolites to perform terminal alkyne homocoupling reaction. In contrast to its homogeneous Glaser version, this study describes therefore a green route for the quantitative synthesis of diynes via bifunctional heterogeneous catalysis.