XRD, XAS, and IR Characterization of Copper-Exchanged Y Zeolite

Abstract

Siting of copper ions in CuI−Y zeolite, prepared by gas-phase exchange of NH4−Y with CuCl, has been investigated employing XRPD, XAS and IR spectroscopies. An X-ray powder diffraction study of the zeolite in a vacuum shows that 23.4(2) cuprous ions are located at site I*, 6.1(3) at site II, and 11.5(3) at site II* (sites I* and II* are at the center of the plane of the six-membered ring connecting the hexagonal prism with the sodalite and the sodalite with the supercage, respectively). Addition of CO induces a relevant migration of copper ions from sites II and II* to a more exposed type II. EXAFS analysis shows that CuI ions in the outgassed zeolite are surrounded by 2.8(3) oxygen atoms of the zeolite framework, the average CuI−O distance being 1.99(2) Å. Both X-ray measurements and FTIR spectroscopy show that CO is adsorbed on the zeolite at room temperature with formation of carbonyl adducts. At liquid-nitrogen temperature and low CO pressure, two types of monocarbonyl species are observed, corresponding to CO adsorbed on copper ions located at sites II and II*. On increase of the CO pressure and subsequent formation of polycarbonylic species, cations at site II* move to the more exposed position II, and a single kind of tricarbonyl adducts is observed. IR spectroscopy also provides evidence for the interaction of NO with copper ions located at sites II and II*, which are the first sites able to adsorb up to two molecules of NO, whereas cations at site II*, because of their lower coordinative unsaturation, can only form CuI(NO) adducts. NO proves to be a sensitive probe not only for cuprous but also for cupric ions.