Balanced acidity by microwave-assisted ion-exchange of ZSM-5 zeolite as a catalyst for transformation of glucose to levulinic acid

Levulinic acid (LA) is a worthwhile biochemical usable as a building-block for the formation of wide variety of chemicals and fuels. In the present work, a series of ion-exchanged ZSM-5-type zeolites were formulated in order to obtain a bifunctional catalyst with modulated acidity in the aspect of amount, strength, and type (Lewis and Brønsted) for the transformation of glucose as a bio-based model component to LA. In particular, ion-exchanged ZSM-5 were prepared by an aqueous ion-exchange method using NH4Cl and CuCl2 salts and a microwave-assisted solid-state ion-exchange technique using solid transition metal (Mn(II), Fe(II), Fe(III), Co(II), Ni(II), Cu(II)) chlorides. Morphological features and acidity of the catalysts were assessed using XRD, SEM, EDX, TG–DTA, N2-physisorption, NH3-TPD, and FTIR both as such and by adsorbing/desorbing 2,6-dimethylpyridine (2,6-DMP) as a probe molecule. Microwave has been selected as the best ion-exchange technique which led to the improvement of Lewis acidity of ZSM-5 and obtaining a balanced acid catalyst with a high Lewis to Brønsted acid ratio (L/B) and mostly weak and medium acid sites. Among several metal ions used for ion-exchange process, Cu(II) with high charge transfer and by introduction of low amount of acid sites acted as the best option for desired reaction pathway. Therefore, CuZSM5-M prepared by microwave technique with a balanced L/B ratio has shown the best performance in the conversion of glucose to LA with 37% yield. Graphical abstract: [Figure not available: see fulltext.]