Proton mobility in chabazite, faujasite, and ZSM-5 zeolite catalysts, comparison based on ab initio calculations

Ab inito predictions of the proton-transfer reaction rates in chabazite, fa ujasite, and ZSM-5 zeolites are presented. The reaction studied, a proton j ump between neighboring oxygen atoms of the AlO4 tetrahedron, defines proto n mobility in an unloaded catalyst. Classical transition state theory is ap plied, and the potential energy surface is described by the QM-Pot method. The latter combines a quantum mechanical description of the reaction site w ith an interatomic potential function description of the periodic zeolite l attice. At room temperature, the calculated rates vary over a broad range o f 10(-6) to 10(5) s(-1), depending on the zeolite type and the particular p roton jump path within a given zeolite. Proton tunneling effects appear to be negligible above room temperature. The calculated reaction barriers vary between 52 and 106 kJ/mol. While in all three zeolites both low and high b arriers exist, the special structural features of the zeolite frameworks al low the prediction that the proton mobility is generally lower in chabazite and faujasite than in ZSM-5, in agreement with experimental data. Three st ructure factors determine the height of the barriers: (i) stabilization of the proton in the transition structure by interactions with neighboring oxy gen atoms. (ii) local framework flexibility, which allows for narrowing of the O-Al-O angle without too much energy penalty, and (iii) overall flexibi lity of the zeolite lattice. The first two factors explain that proton jump s occurring between oxygen atoms in six-membered aluminosilicate rings have the lowest barriers. Jumps between oxygen atoms in four-membered rings and oxygen atoms in open zeolite channels or cavities have high barriers. The larger overall flexibility of the ZSM-5 lattice makes barriers for jumps oc curring within a ring of a given size in ZSM-5 generally lower than in chab azite and faujasite.