Coordination of Two N₂ Molecules to One Ni⁺ Site in Ni–ZSM-5: An FTIR Spectroscopy Study

Ni⁺ cations were produced in a Ni–ZSM-5 zeolite by partial reduction with CO, and the ability of the Ni⁺ and Ni²⁺ sites to coordinate ¹⁴N₂, ¹⁵N₂, and/or CO molecules was studied by FTIR spectroscopy. With Ni⁺ cations, CO produces mono-, di-, and tricarbonyl species while only mono- and dicarbonyls are formed with Ni²⁺. Adsorption of ¹⁴N₂ at 100 K results in formation of Ni²⁺–¹⁴N₂ adducts (band at 2343 cm^–1) and geminal Ni⁺(¹⁴N₂)₂ complexes (ν_s at 2287 and ν_as at 2270 cm^–1). The Ni⁺(¹⁴N₂)₂ complexes lose their ligands stepwise during evacuation at 100 K, and two kinds of monoligand Ni⁺–¹⁴N₂ species are formed (2252 and 2238 cm^–1). After ¹⁵N₂ adsorption, the Ni⁺(¹⁵N₂)₂ complexes were observed at 2211 and 2194 cm^–1: the two Ni⁺–¹⁵N₂ species at 2178 and 2163 cm^–1, and the Ni²⁺–¹⁵N₂ adducts at 2264 cm^–1. To prove the geminal structure, coadsorption of ¹⁴N₂ and ¹⁵N₂ was studied. It resulted in formation of Ni⁺(¹⁴N₂)­(¹⁵N₂) species that were characterized by ¹⁴N–¹⁴N and ¹⁵N–¹⁵N stretchings at 2277 and 2201 cm^–1, respectively. These values are in excellent agreement with those calculated on the basis of the approximate force field model. Experiments on coadsorption of ¹⁴N₂ and CO have shown formation of Ni⁺(CO)­(¹⁴N₂) complexes that were characterized by CO and ¹⁴N–¹⁴N stretchings at 2098 and 2305 cm^–1, respectively. The reasons for simultaneous coordination of two ¹⁴N₂ (or one ¹⁴N₂ and one CO) molecules to one Ni⁺ site and the nature of the bonds in the different complexes are discussed

Surprising Coordination Chemistry of Cu⁺ Cations in Zeolites: FTIR Study of Adsorption and Coadsorption of CO, NO, N₂, and H₂O on Cu–ZSM‑5

Cations exchanged in zeolites are generally characterized by a low coordination number and can thus attach simultaneously more than one small guest molecule. For instance, Cu⁺ ions in ZSM-5 can accept, at low temperature, up to three CO and up to two NO molecules. However, only one N₂ molecule can be coordinated to such sites. Although mixed aqua-carbonyl and aqua-dinitrogen complexes are formed, no mixed carbonyl-nitrosyl, carbonyl-dinitrogen, or nitrosyl-dinitrogen species can be produced. Thus, adsorption of NO on CO precovered sample results in segregation of the CO adsorption layer according to the reaction: 2Cu⁺–CO + 2NO → Cu⁺(CO)₂ + Cu⁺(NO)₂. Adsorption of N₂ on NO precovered sample leads to a similar process: 2Cu⁺–NO + N₂ → Cu⁺(NO)₂ + Cu⁺–N₂. No carbonyl-dinitrogen complexes are produced during CO–N₂ coadsorption. The role of the ligand and the nature of the bond in the formation of geminal and mixed-ligand complexes are discussed

Purification of Hydrogen from CO with Cu/ZSM-5 Adsorbents

The transition to a hydrogen economy requires the development of cost-effective methods for purifying hydrogen from CO. In this study, we explore the possibilities of Cu/ZSM-5 as an adsorbent for this purpose. Samples obtained by cation exchange from aqueous solution (AE) and solid-state exchange with CuCl (SE) were characterized by in situ EPR and FTIR, H2-TPR, CO-TPD, etc. The AE samples possess mainly isolated Cu2+ cations not adsorbing CO. Reduction generates Cu+ sites demonstrating different affinity to CO, with the strongest centres desorbing CO at about 350 °C. The SE samples have about twice higher Cu/Al ratios, as one H+ is exchanged with one Cu+ cation. Although some of the introduced Cu+ sites are oxidized to Cu2+ upon contact with air, they easily recover their original oxidation state after thermal treatment in vacuum or under inert gas stream. In addition, these Cu+ centres regenerate at relatively low temperatures. It is important that water does not block the CO adsorption sites because of the formation of Cu+(CO)(H2O)x complexes. Dynamic adsorption studies show that Cu/ZSM-5 selectively adsorbs CO in the presence of hydrogen. The results indicate that the SE samples are very perspective materials for purification of H2 from CO