Spectroscopic Signatures of Internal Hydrogen Bonds of Brønsted‐Acid Sites in the Zeolite H‐MFI
Henning Windeck, Fabian Berger, Joachim Sauer
Abstract
Abstract The location of Brønsted‐acid sites (bridging OH groups, b‐OH) at different crystallographic positions of zeolite catalysts influences their reactivity due to varying confinement. Selecting the most stable b‐OH conformers at each of the 12 T‐sites (T=Si/Al) of H‐MFI, a representative set of 26 conformers is obtained which includes free b‐OH groups pointing into the empty pore space and b‐OH groups forming H‐bonds across five‐ or six‐membered rings of TO 4 tetrahedra. Chemically accurate coupled‐cluster‐quality calculations for periodic models show that the strength of internal H‐bonds and, hence, the OH bond length vary substantially with the framework position. For 11 of the 19 H‐bonded b‐OH groups examined, our predictions fall into the full width at half maximum range of the experimental signals at 3250±175 cm −1 and 7.0±1.4 ppm which supports previously debated assignments of these signals to H‐bonded b‐OH sites.