Electron Spin Resonance of Ni(I) in Ni−Containing MCM-41 Molecular Sieves

Abstract

Nickel-containing MCM-41 molecular sieves have been synthesized hydrothermally using cetyltrimethylammonium chloride as a template. The stabilization of Ni(I) in NiMCM-41 is minimal. The various Ni(I) species formed by reduction and adsorbate interactions in NiAlMCM-41 in which Ni(II) is incorporated into a framework position by addition of Ni(II) into the synthesis mixture and in Ni-AlMCM-41 where Ni(II) is incorporated into extraframework positions by liquid state ion exchange were studied by electron spin resonance (ESR) and electron spin−echo modulation. Dehydration at elevated temperature or thermal reduction by hydrogen produces Ni(I) species in Ni-AlMCM-41 which are detectable by ESR. By contrast, in NiAlMCM-41 neither dehydration at high temperature nor hydrogen reduction is effective in producing Ni(I). Ni(I) is obtained in NiAlMCM-41 by γ-irradiation at 77 K. Both NiAlMCM-41 and Ni-AlMCM-41 show differences in their ESR characteristics after reduction and adsorption of D 2 O, CO, and ethylene. D 2 O adsorption forms Ni(I)−(O 2 ) n indicating water decomposition in both materials. CO adsorption onto Ni-AlMCM-41 and NiAlMCM-41 produces Ni(I)−(CO) 3 and Ni(I)−(CO) 2, respectively. The interaction between Ni(I) and ethylene seems weaker in NiAlMCM-41 than in Ni-AlMCM-41. The contrasting characteristics of Ni(I) in NiAlMCM-41 and Ni-AlMCM-41 suggest that Ni(I) is in a framework site in NiAlMCM-41. The differentiation between extraframework and framework sites is confirmed by the effect of sodium ethylenediaminetetraacetate on removing nickel ions from ion-exchanged Ni-AlMCM-41 but not from synthesized NiAlMCM-41.