Al-distribution and acidity of HZSM-5: Theory and practice

Since zeolites as solid acids plays an important role in petrochemical industry, we took HZSM-5 as an example to study the location and distribution as well as the acid property upon the change of Si/Al ratio based on density functional theory computation (GGA-PBE-D3). Two models, the remotely dispersed one for Al atoms at the equivalent sites but in different intersections (up to 4Al), and the aggregated one for Al atoms at different sites but within one intersection (up to 6Al), are used. Based on the adsorption enthalpy of pyridine and ammonia, the acid sites in the remotely dispersed model can be considered as singly isolated. In the aggregated model, the adsorption enthalpies of the firstly adsorbed pyridine and ammonia increase with the increase of Al atoms, revealing the increase of the mutual interaction of the acid sites and therefore the increase of the acid strength, in agreement with the experimentally observed increase of desorption temperatures of pyridine and ammonia. It is noted that for 2-4Al substitution, the aggregated models are more stable than the remotely dispersed models, indicating the thermodynamic preference of the aggregated Al atoms within one intersection. In addition, the adsorption enthalpies of pyridine and ammonia do not correlate with the O-H distances, O-H stretching frequencies, Si-O(H)-Al angle, and the Bader charge of the acidic proton. These results provide a rational basis for exploring the enhanced catalytic activity for HZSM-5 zeolites with multiple Al substitution.