Dealumination of the H-BEA Zeolite via the SN2 Mechanism: A Theoretical Investigation

To understand the stability of aluminosilicate zeolites in the presence of steam, a computational study on the dealumination process of the H-BEA zeolite is carried out. High-dimensional free energy profiles characterizing stepwise hydrolysis steps of four Al-O bonds have been mapped out using ab initio molecular dynamics simulations combined with the enhanced sampling methodology. It has been found that the dealumination process of the H-BEA zeolite can be elucidated as the S-N2 mechanism. For each hydrolysis step, the protonation of the Al-O-Al bond is initialized by proton transfer. Then, one water molecule at the antiposition of the protonated O-Al atom attacks the Al atom, resulting in one Al-O-W bond breaking and one Al-O w bond formation simultaneously. Free energy barriers for four hydrolysis steps are calculated to be 16.6, 20.8, 69.3, and 50.0 kJ/mol, respectively. The breakage of the first two Al-O-Al bonds within a small five-member ring is facile while the last two Al-O-Al bonds within a large 12-member ring are hard to break. The leaving Al atom from the zeolitic framework is in the form of Al(OH)(3)(H2O).

Automated summary

A computational study on the dealumination process of H-BEA zeolite in the presence of steam reveals it follows the S-N2 mechanism, with different free energy barriers for each hydrolysis step. The first two steps are easier to break compared to the last two.