Surface Lewis basic site enabled proton abstraction for the regioselective synthesis of ynones and flavones over the Pd0/Cs-ZSM-5 catalyst: mechanistic understanding and structure–activity correlation

Abstract

The regioselective nature of the Pd⁰/Cs-ZSM-5 catalyst was established by HCOOH-IR and CO chemisorption. Mechanistic studies revealed that CO was activated on the surface Pd(0), and the alkyne proton was abstracted by the surface Lewis basic site, leading to the formation of flavones via the O–Pd–H–π complex. The surface basicity of the 3wt%Pd⁰/H-ZSM-5 catalyst was enhanced by 4-fold due to the alkali metal, with a simultaneous decrease in the acidity. Palladium in close vicinity to the alkali metal was a reason for the shift in T_max of PdO signals to low temperatures. The 3wt%Pd⁰/Cs-ZSM-5 catalyst exhibited excellent rates of the desired products with high functional group tolerance and sustained activity up to four cycles. The role of alkali metals in the activity and selectivity of the Na-, K-, and Cs-exchanged H-ZSM-5 catalysts was substantiated by H₂-TPR, XRD, XPS, TEM, BET-SA, SS NMR and NH₃ and CO₂ TPD techniques.