DRIFTS evidence for the formation of binuclear Cu(I)–dinitrogen complexes upon adsorption of N2 on CuZSM-5

The room temperature adsorption of dinitrogen on CuZSM-5 zeolite was studied using diffuse-reflectance infrared spectroscopy (DRIFTS). In addition to an intense absorption band at 2294 cm−1, a weak band at 2207 cm−1 in the region of NN stretching vibrations was also detected in the spectra. Surprisingly, this latter band increases in intensity when the dinitrogen pressure decreases. This band was assigned to internuclear bridging dinitrogen molecules in copper–dinitrogen complexes formed following the adsorption of dinitrogen on CuZSM-5. Quantum-chemical calculations of model zeolite–2Cu(I)–nN2 compounds were performed to confirm the assignment of the bands in the DRIFT spectra of adsorbed N2. It is suggested that at high dinitrogen pressures the formation of a binuclear zeolite–2Cu(I)–3N2 complex containing both terminal on-top and bridging end-on dinitrogen molecules takes place. At low dinitrogen pressures the terminal molecules are removed and the complex is transformed to one containing only an internuclear dinitrogen ligand. The internuclear coordination could explain the high efficiency of CuZSM-5 for dinitrogen adsorption.