MFI zeolite-supported Ru nanoparticles for efficient conversion of pyroglutamic acid to 2-pyrrolidone

Pyroglutamic acid is readily formed through the dehydration–cyclization of glutamic acid, an abundant nonessential amino acid, by heating above 393 K without a catalyst. Herein, we describe the formation of 2-pyrrolidone by the hydrogenation and subsequent decarbonylation of pyroglutamic acid over MFI zeolite-supported Ru nanoparticles (Ru/MFI) under mild reaction conditions (433 K and 2 MPa H2). The yield of 2-pyrrolidone over Ru/MFI was influenced by the pH of the RuCl3 aqueous solution used to impregnate Ru on the MFI zeolite. pH adjustment led to strong adsorption of the cationic Ru precursors on the support and small and dispersed Ru nanoparticles on the MFI zeolite. The resulting catalyst provided a high 2-pyrrolidone yield due to the high rate of decarbonylation into 2-pyrrolidone in the conversion of pyroglutamic acid. In addition, screening of various framework-type zeolites (commercially-available ones) as Ru catalyst supports demonstrated that MFI contains more cationic Ru (Ruδ+) than other zeolites tested, and Ru/MFI provided a higher 2-pyrrolidone yield. The results suggest that Ruδ+ loaded on MFI functions as more active sites for the decarbonylation of an aldehyde intermediate into 2-pyrrolidone compared to Ru0 or other Ru species. This assumption was supported by comparison of the catalytic performance of Ru/MFI catalysts prepared by impregnation and ion exchange.