Elucidation of a preparation method for copper ion-exchanged ZSM-5 samples exhibiting extremely efficient N 2-adsorption at room temperature: effect of counter ions in the exchange solution

Abstract

The adsorption of N 2 on a copper ion-exchanged ZSM-5 sample (CuZSM-5) prepared by ion exchange using an aqueous solution of copper propionate, Cu(C 2H 5COO) 2, was examined at room temperature by measuring the FT-IR spectra, adsorption isotherms and heat of adsorption. This sample was found to be extremely efficient in terms of N 2 adsorption with regard to both the amount and the energy (i.e., heat) of adsorption, compared with samples prepared by a conventional ion-exchange method using an aqueous solution involving Cu 2+ and simple counter ions, Cl − or NO 3 −. To clarify the specificity of the newly-prepared sample, the ion-exchange of ZSM-5 with Cu 2+ was carried out by employing aqueous solutions involving Cu 2+ and various types of counter ions [propionate (C 2H 5COO −), acetate (CH 3COO −), formate (HCOO −), chloride (Cl −) and nitrate (NO 3 −) ions]. When the ion exchange was performed by using a Cu(C 2H 5COO) 2 or Cu(CH 3COO) 2 solution, the Cu 2+ species with propionate or acetate ligand (in the monomer state) were ion-exchanged in ZSM-5, as confirmed by the DR, EPR and FT-IR spectra for CuZSM-5. In contrast, Cu 2+ species were present in the form of aquo-complexes in samples prepared with other solutions. This distinct difference can be ascribed to the difference in the p K a values of the counter ions; carboxylate ions, with a high p K a value, are inclined to form a complex with Cu 2+. Using this newly applied Cu(C 2H 5COO) 2 solution, the present ion-exchange method has the potential to develop new effective materials that possess the specific adsorption and catalytic properties of CuZSM-5.