HMF etherification using NH4-exchanged zeolites

The properties of BEA, MFI and Silicalite-1 zeolites in the ammonium and protonic forms are studied in the etherification of HMF (5-hydroxymethylfurfural) in anhydrous ethanol and compared with FTIR data on ammonium ion siting and displacement by competitive adsorption, as well as data on ammonium ion dissolution in aqueous solution. For the first time it is demonstrated that ammonium-exchanged zeolites are active and show better performances (particularly for the BEA structure) in the acid-catalyzed etherification reaction. This behavior is associated to a reversible dissociation of NH4+ ions, which is favored by the BEA zeolite structure. A critical condition for enhanced catalytic performances is that dissociated ammonia remains in the zeolite cages, and may be reversibly re-adsorbed. It is thus likely that the dissociated ammonia participates in the reaction or induces a confinement effect.