Comparison of H+ and NH4 + forms of zeolites as acid catalysts for HMF etherification

MOR, BEA, MFI (containing or not Al) zeolites in the ammonium and protonic forms are studied in the etherification of HMF (5-hydroxymethylfurfural) in anhydrous ethanol. The conversion of HMF to 5-(ethoxymethyl) furan-2-carbaldehyde (EMF), and ethyl 4-oxopentanoate (EOP) occurs through two parallel reactions, which may be good described kinetically by pseudo-first order rates of formation, being the ethanol the co-reactant and the reaction medium. FTIR data with pyridine and 2,4,6-collidine as probe molecules are used to quantify the amount of Lewis and Bronsted sites in the samples. In the MOR, BEA, MFI (containing or not Al) series of zeolites in acid and ammonium form, a good relationship between the rate constant in EMF formation (k(1)) and the amount of strong Lewis acid sites (LAS) is observed. A correlation between rate constant in EOP formation (k(2)) and the amount of Bronsted acid sites (BAS) is instead observed. While the first correlation is independent on the acid or ammonium form, the second correlation depends on this aspect.