Ni2+ ion sites in hydrated and dehydrated forms of Ni-exchanged zeolite ferrierite

The location of extra-framework Ni2+ cations in the hydrated and dehydrated nickel ion-exchanged zeolite ferrierite was determined from the analysis o f synchrotron X-ray powder diffraction data. Rietveld refinements were perf ormed in the Immm space group. One Ni2+ was found at the centre of the ferr ierite cage coordinated to six water molecules in a quite regular octahedra l coordination. The Ni(H2O)(6)(2+) octahedron assumes two equally probable statistical orientations, which differ by a rotation of about 45 degrees ar ound the z axis. These two configurations of the Ni(H2O)(6)(2+) polyhedron show strong similarities with the two configurations of the Mg(H2O)(6)(2+) polyhedron found in natural Mg-rich ferrierite. However, whereas the cation site in natural Mg-ferrierite is fully occupied, in the Ni-exchanged form, this site is only half occupied. The unit cell volume of dehydrated Ni-fer rierite was 1.4% smaller than the unit cell volume of the hydrated phase. T wo other Ni2+ extra-framework sites were found, one near the centre of the 10-ring channel, the other not far from the centre of the 8-ring window of the ferrierite cage. Four Ni2+ extra-framework sites were localised in dehy drated Ni-ferrierite. The first is near the centre of the 6-ring window of the ferrierite cage, the second is located on the walls of the cage, the th ird is in the 10-ring channel, and the last is at the intersection of the 1 0- and 8-ring channels. The last Ni2+ site is very close to the Cu(1) site found by Attfield et al. [M.P. Attfield, S.J. Weigel, A.K. Cheetham, J. Cat al. 172 (1997) 274-280] in dehydrated Cu-ferrierite, whereas the other thre e Ni2+ sites resemble the alpha-, beta- and gamma-type sites of Kaucky et a l. [D. Kaucky, J Dedecek, B. Wichterlova, Micropor. Mesopor. Mater. 31 (199 9) 75-87] in dehydrated Co-ferrierite. (C) 2000 Elsevier Science B.V. All r ights reserved.