Exchange of copper species the cavity of ZSM-5 at high copper loading

Abstract

Copper ion-exchanged ZSM-5 (CuZSM-5) has been found to be active for the direct decomposition of NO[subscript x]. The suitable exchanged sites and copper species in the cavity of ZSM-5 at high copper loading were studied by comparing configurations and exchanged energies of CuOH+ in each exchanged site such as T6 T12, T8 T8, T7 T12 model 1 (M1T7 T12) and T7 T12 model 2 (M2T7 T12) using cluster models. All edges of cluster (all O-H bond lengths and T-O-H bond angles were set same bond length and same bond angle parameter) and positions of H+ were optimized using B3LYP/6-31G(d,p) while freezing other geometrical parameters. The results of partial optimizations gave O-H bond length of 0.96 Angstrom and T-O-H bond angle between 115.0 ํ-119ํ. By replacing H[superscript +] with CuOH[superscript +] and partial optimization of H[superscript +] and CuOH[superscript +], models of CuZSM-5 were prepared. We found Cu-O(Z) distances, Cu to framework oxygen distance, ranging between 1.84-2.15 Angstrom. For one-ion exchanged CuOH[superscript +] at M2T7 T12 and T8 T8 exchanged sites, small positive and negative exchanged energies between -5.36-3.47 kcal/mol were observed. The M1T7 T12 exchanged site possess as the highest exchanged energies. For all two-ion exchanged CuOH[superscript +], positive exchanged energies which ranging from 12.61-75.61 kcal/mol were optained. T6 T12 exchanged sites give the lowest exchanged energies for both one- and two-ion exchanged CuOH[superscript +], -32.47 and +12.61 kcal/mol respectively. It can be concluded that preferable exchanged sites for CuOH[superscript +] is T6 T12 site and there is only one ion exchanged per site. However, the exchanged species are not in the form of CuOH[superscript +] but rather [CuOH[subscript 2]] [superscript 2+] where the repulsion between charge species as minimized. For structures and stabilizations of copper- water complexes, possible structures of [Cu(H[subscript 2]O)[subscript n]] [superscript 2+] for n=1-7 were geometry optimized at HF/6-31G(d,p) level and their energies were more accurately determined at MP2/6-31G(d,p) and B3LYP/6-31G(d,p) level of theory. In addition, for n=6 and 7, single point calculations using HF/6-31++G(d,p), HF/6-311G(d,p) and HF/6-311G(2df,p) were also performed. We found that for n=6 and n=7 only 5-fold and 6-fold coordination complexes were found. The 5-fold consists of square pyramidal (spy) and trigonal bipyramidal (tbp) structures and the 6-fold consists of distorted octahedral (oct) structure. For n=6, at 6-31G(d,p) basis all methods predicted tbp as the most stable from of [Cu(H[subscript 2]O)[subscript n]] [superscript 2+]. However, with HF method but larger basis set, the oct become more stable. For n=7 all methods and basis sets agree that tbp is the most stable and the oct lies slightly above tbp structure. Thus, oct and tbp structures could co-exist in solution but tbp is more abundant.