Spectroscopic Signatures of Internal Hydrogen Bonds of Bronsted-Acid Sites in the Zeolite H-MFI

The location of Bronsted-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 TO4 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.

Automated summary

The location of Bronsted-acid sites at different crystallographic positions of zeolite catalysts influences their reactivity. By selecting the most stable conformers, it is found that the strength of internal H-bonds and the OH bond length vary substantially with the framework position. The experimental results support the previously debated assignments of H-bonded b-OH sites.