Exchange of copper species in the cavity of ZSM-5 at low copper loading

Abstract

Density functional theory calculations were performed to examine exchange of Cu[superscript 2+] and Cu[superscript +] and autoreduction of Cu[superscript 2+] to Cu[superscript +] in the cavity of ZSM-5 at T6T12, T8T8, T7T12model1, and T7T12-model2 exchanged site. All geometry optimizations were carried out using B3LYP/6-31G(d,p). To obtain ZSM-5 structures, Molecular Dynamic (MD) Simulations were performed on the MFI structure, in which 2 Si atoms were replaced by 2 Al atoms. Two excess negative charges were balanced by 2 H[superscript +]. The cluster models of 5- and 6-T-ring were extracted from the simulated HZSM-5 structures where all terminal oxygen atoms were saturated with H atoms. Geometrical parameters of the cluster were fixed except the position of H[superscript +], O-H bond lengths (O-H), and T-O-H bond angles ([angle] T-O-H). The results from optimizations yielded O-H distances of 0.96 A and [angle] T-O-H angles of 115.33ํ-118.98ํ. For the cluster model of CuZSM-5, HZSM-5 structures were taken and H[superscript +] were replaced either by 1 Cu[superscript 2+], or 1 Cu[superscript +] for one-ion exchanged CuZSM-5 or 2 Cu[superscript +] for two-ion exchanged CuZSM-5. Only positions of H[superscript +], Cu[superscript +], and Cu[superscript 2+] were optimized. For O-H[superscript +], Cu[superscript 2+]-O, and Cu[superscript +]-O, the distances are between 0.97-1.00 A, 1.92-4.17 A, and 1.75-5.33 A, respectively. For all exchanged sites, Cu species form either mono- or bi- coordination with ZSM-5. When considered exchanged energies (delta E[subscript exch]), all CuZSM-5 exchanged sites have positive delta E[subscript exch]. The most stable exchanged Cu species is Cu[superscript 2+] and the most favorable exchanged site is T6T12 with delta E[subscript exch] of 9.19 kcal/mol. For the autoreduction of Cu[superscript 2+] to Cu[superscript +], lowest autoreduction energy (delta E[subscript exch]) of 148.93 kcal/mol is for the conversion at T7T12-model1. Although delta E[subscript exch] in ZSM-5 is smaller than that in aqueous solution, still it is very high. From the obtained delta E[subscript exch] and delta E[subscript exch], the exchange of bare Cu ion and autoreduction from bare Cu[superscript 2+] would not be preferable at room temperature