Structures and acidities of HZSM-5 with Si/Al ratios of 47 and 95

Abstract

ZSM-5 catalyst is widely used in petrochemical industry. Its catalytic activity depends on its acidity. Investigation on the acidity of H-ZSM-5 with 2 different Si/Al ratios, i.e. 95 (monosubstituted) and 47 (disubstituted) were carried out. Using Molecular Dynamic Simulation (MD), structures of H-ZSM-5 at the Si/Al ratios of 95 and 47 and various substitution patterns were perpared and minimized. The most probable O...H[superscript +] distances were found to be of 0.982 to 0.998 Angstrom for ZSM-5 with the Si/Al ratio of 95, while H-ZSM-5 with the Si/Al ratio of 47 has these distances in the range of 0.986 to 1.000 Aangstrom. The O...H[superscript +] distances obtained for all sites are rather short in the range of O-H bond in water, Thus H[superscript +] forms chemical bond with the zeolitic O. Density functional theory (DFT) B3LYP/6-31G (d,p) calculations were used for determination of proton affinity (PA) of HZSM-5 and thus its acidity. For H-ZSM-5 with the Si/Al ratio of 95, the value of PA is between 388.8 and 414.8 kcal/mol. The T5 substitution has the lowest PA and hence most acidic. While H-ZSM-5 with the Si/Al ratio of 47, the value of PA are between 391.8 and 412.7 kcal/mol. The T9-site in T9,T10 model has lowest PA which makes it highest acidity. Compared acidity between H-ZSM-5 with the Si/Al ratios of 95 and 47, it could be observed that all Al substitutions at Si/Al ratio of 95 have higher acidity (lower PA) than those of 47 ratio. Thus, H-ZSM-5 with higher Si/Al ratio is more acidic in agreement with experiment. However, we have found that T12-site in ZSM-5 with the Si/Al ratio of 47 is more acidic than that with the Si/Al ratio of 95.