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

The regioselective nature of the Pd 0 /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%Pd 0 /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%Pd 0 /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 H 2 -TPR, XRD, XPS, TEM, BET-SA, SS NMR and NH 3 and CO 2 TPD techniques. An environmentally benign and highly regioselective catalyst that possesses strong Lewis basicity demonstrated high rates of ynones and flavones under mild reaction conditions in the absence of a base, ligand, and co-catalyst.