Density functional investigations into the siting of Fe and the acidic properties of isomorphously substituted mordenite by B, Al, Ga and Fe

Abstract

High level B3LYP density functional computations identify both T 2 and T 4 positions in the mordenite (MOR) framework for possible Fe substitution on a cluster size of five tetrahedral centers (5T). The charge-compensating proton prefers the O 5 and O 10 oxygens around the T 2 and T 4 centers, respectively. Based on these calculations and a previous work concerning the siting of B, Al, Ga in the MOR framework, we studied the properties of the Brønsted acid sites in B, Al, Ga and Fe isomorphously substituted MOR. The calculated proton affinity, Mulliken charges on the proton and the adsorption energy of NH 3 from clusters containing eight tetrahedral centers (8T) show that the acid strength increases in the order: B-MOR⪡Fe-MOR<Ga-MOR<Al-MOR, paralleling the increasing strength of the Brønsted acidity of B, Al, Ga and Fe substituted ZSM-5. In the NH 3 adsorbed clusters of Al-, Ga- and Fe-MOR, proton transfer occurs and the formed NH 4 + is stabilized by three N–H⋯O hydrogen bonds, as found experimentally, while only a physisorbed NH 3 stabilized by one short and strong N⋯H–O hydrogen bonds and two weaker N–H⋯O bonds is found in B-MOR cluster.