Location of Brønsted sites in deuterated L-zeolite: a combined neutron powder diffraction and computer modeling study

In this work both concentration and location of Brønsted acid sites (BASs) in a deuterated L-zeolite were determined by combining neutron powder diffraction with ab-initio molecular dynamics modelling. From the structure refinement of the acidic L zeolite, two Brønsted acid sites were identified. The first one, D1, is on the framework oxygen O5 pointing toward the center of the 8-membered ring channel; the second one, D2, located in proximity of the framework oxygen O1 pointing toward the 12-membered ring channel running parallel to the c-axis. On the whole, ∼7.7 acid sites were located in the unit-cell, corresponding to 58% and 14% for D1 and D2, respectively. The average structure of the simulated D-LTL was in good agreement with the experimental structure. The modelling revealed that the D2 BAS shows a large degree of mobility, consistent with the high thermal factor associated with that atomic position. The D1 BAS showed a behaviour consistent with static disorder, while the D2 BAS was characterized by a significant degree of dynamical disorder. In addition to these findings, the study also revealed that the O1 BAS had a higher Brønsted base character compared to the average basicity of the framework oxygens of the 12-membered ring channels.