A computational exploration into isomorphously substituted effects on hydrogen electric field gradient and chemical shielding tensors in the H‐ZSM‐5 zeolite

Abstract

ABSTRACTDensity functional theory calculations have been performed at the B3LYP/6‐31++G** level to investigate the effect of the isomorphously substituted ZSM‐5 zeolite with trivalent elements B, Al, and Ga on the hydrogen atom of the Brönsted acid site via nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) parameters. ZSM‐5 clusters were modeled as ((H)3SiO)3–Si–O(H)–M–(OSi(H)3)3 (M = B, Al, and Ga). Variations of isotropic chemical shifts, δiso, of 1H atom in the Brönsted acid site as well as quadrupole coupling constant, CQ, and asymmetry parameter, ηQ, of 2H atom in the Brönsted acid site are well correlated with the trend in acidic strength of isomorphously substituted H‐ZSM‐5, estimated from the experimental results: B‐ZSM‐5 < Ga‐ZSM‐5 < Al‐ZSM‐5. The CQ value of 2H atom in the Brönsted acid site decreases in the order: B‐ZSM‐5, Ga‐ZSM‐5, Al‐ZSM‐5, respectively; eventually, their ηQ values of 2H atom in the Brönsted acid site increase concluding models with high CQ have a lower Brönsted acidity than those with low CQ. In addition, the calculated chemical shifts of the Brönsted proton increase in the sequence: B‐ZSM‐5 < Ga‐ZSM‐5 < Al‐ZSM‐5. All the results show a high consistency among NQR and NMR parameters and relative acidity of the isomorphously substituted ZSM‐5 zeolite. © 2014 Curtin University of Technology and John Wiley & Sons, Ltd.