Lewis and Brönsted acidic sites in M4+-doped zeolites (M = Ti, Zr, Ge, Sn, Pb) as well as interactions with probe molecules: A DFT study

Abstract

Tetravalent-ion (M4+)-doped zeolites show excellent performances for a variety of catalytic processes, including the focusing biomass conversions. In this work, density functional calculations were performed to probe the Lewis and Brönsted acidities of various M4+-doped zeolites as well as to study interactions with probe molecules. The Lewis and Brönsted acidities increase in the orders of Silicalite-1≪Ge<Ti<Pb<Sn<Zr and Silicalite-1≪Ti<Ge<Zr≈B<Pb<Sn<Al, respectively. The Lewis acidities should be defined as the local sites around the M4+ ions, explaining why the adsorption energies give a more consistent order with LUMO energies and absolute electronegativity rather than fukui functions. The formation of Brönsted acidic sites is facilitated by doping with M4+ ions. Albeit the Brönsted acidities of these M4+-doped zeolites change greatly with Sn being the strongest, their strengths are far below that of Al3+. The interactions of five probe molecules of changing basicities with the Brönsted acidic sites indicate that the formations of covalent and/or ionic structures are the proton-competing results: the covalent and ionic structures co-exist only for trimethyphosphine and pyridine of comparable basicity; otherwise, proton transfer will take place and result in only the ionic or covalent structures. The proton affinity fails to predict the Brönsted acidity of Zr and is evidenced, especially by formation of the covalent structure during pyridine adsorption. Thus, this work presents a dynamic image of acid–base interactions and aids our understanding toward the catalysis of solid-state acids.