Location, distribution and acidity of Al substitution in ZSM-5 with different Si/Al ratios – a periodic DFT computation

Abstract

Systematic periodic density functional theory computations including dispersion correction (GGA-PBE-D3) have been carried out to characterize the location, distribution and acidity of Al substitution in ZSM-5 with different Si/Al ratios (up to 8 Al atoms). The model system is the orthorhombic phase, which has a unit cell containing 96 T sites. For one Al substitution, there are two energetically equal sites (T₃ and T₇) as the most stable substitution configurations. On the basis of the computed relative stability of the NH₃ adsorbed states, there are three and two energetically close states at the T₇ and T₃ sites, respectively, indicating the equilibrium and mobility of the chemisorbed NH₃ (or NH₄⁺) among these states. For 2Al substitution, the most stable substitution configuration has Al atoms at the T₃ and T₁₁ sites, and the two most stable acidic O–H groups have the same NH₃ adsorption energies. For 3Al and 4Al substitutions, not all generated O–H acidic sites are available for NH₃ adsorption due to the confinement effect, and higher Al substitutions might not have advantages over lower Al substitutions for ZSM-5. Among the most stable substitution configurations for 1–4 Al substitutions, the T₃ site appears most frequently and should represent the active site for adsorption and reaction.