Coordination of Cu+ Ions to Zeolite Frameworks Strongly Enhances Their Ability To Bind NO2. An ab Initio Density Functional Study

Comparison is made of the interaction of NO 2 with Cu + ions in the gas phase and inside zeolites using density functional theory (B3LYP functional). The zeolite is represented by a tritetrahedra model embedded in the periodic structure of zeolite ZSM-5 and by a free space cluster model. Both models yield virtually the same results. Cu + is coordinated to two oxygen atoms of the zeolite framework only. For the complexes with NO 2, several minima and transition structures on the potential energy surfaces are localized. The naked Cu + ion preferentially binds NO 2 in the η 1 -O trans mode, while in zeolites the Cu + site binds NO 2 in a 2 η-O,O coordination. For the 2 η-O,O structure the binding is three to four times stronger in the zeolite (43 kcal/mol) than in the gas phase which is due to a three-body zeolite framework Cu + ion−NO 2 interaction. d 10 −s 1 d 9 promotion leads to a more favorable orbital interaction between Cu + and NO 2 in the 2 A‘‘ state and, due to reduced repulsion, to a stronger electrostatic interaction between Cu + and the zeolite framework.